WorldWideScience

Sample records for f-absorbed mos2 monolayers

  1. Rolling Up a Monolayer MoS2 Sheet.

    Science.gov (United States)

    Meng, Jianling; Wang, Guole; Li, Xiaomin; Lu, Xiaobo; Zhang, Jing; Yu, Hua; Chen, Wei; Du, Luojun; Liao, Mengzhou; Zhao, Jing; Chen, Peng; Zhu, Jianqi; Bai, Xuedong; Shi, Dongxia; Zhang, Guangyu

    2016-07-01

    MoS2 nanoscrolls are formed by argon plasma treatment on monolayer MoS2 sheet. The nanoscale scroll formation is attributed to the partial removal of top sulfur layer in MoS2 during the argon plasma treatment process. This convenient, solvent-free, and high-yielding nanoscroll formation technique is also feasible for other 2D transition metal dichalcogenides.

  2. Monolayer MoS2 heterojunction solar cells

    KAUST Repository

    Tsai, Menglin

    2014-08-26

    We realized photovoltaic operation in large-scale MoS2 monolayers by the formation of a type-II heterojunction with p-Si. The MoS 2 monolayer introduces a built-in electric field near the interface between MoS2 and p-Si to help photogenerated carrier separation. Such a heterojunction photovoltaic device achieves a power conversion efficiency of 5.23%, which is the highest efficiency among all monolayer transition-metal dichalcogenide-based solar cells. The demonstrated results of monolayer MoS 2/Si-based solar cells hold the promise for integration of 2D materials with commercially available Si-based electronics in highly efficient devices. © 2014 American Chemical Society.

  3. Monolayer-by-monolayer stacked pyramid-like MoS2 nanodots on monolayered MoS2 flakes with enhanced photoluminescence.

    Science.gov (United States)

    Yuan, Cailei; Cao, Yingjie; Luo, Xingfang; Yu, Ting; Huang, Zhenping; Xu, Bo; Yang, Yong; Li, Qinliang; Gu, Gang; Lei, Wen

    2015-11-07

    The precise control of the morphology and crystal shape of MoS2 nanostructures is of particular importance for their application in nanoelectronic and optoelectronic devices. Here, we describe a single step route for the synthesis of monolayer-by-monolayer stacked pyramid-like MoS2 nanodots on monolayered MoS2 flakes using a chemical vapor deposition method. First-principles calculations demonstrated that the bandgap of the pyramid-like MoS2 nanodot is a direct bandgap. Enhanced local photoluminescence emission was observed in the pyramid-like MoS2 nanodot, in comparison with monolayered MoS2 flakes. The findings presented here provide new opportunities to tailor the physical properties of MoS2via morphology-controlled synthesis.

  4. Conduction quantization in monolayer MoS2

    Science.gov (United States)

    Li, T. S.

    2016-10-01

    We study the ballistic conduction of a monolayer MoS2 subject to a spatially modulated magnetic field by using the Landauer-Buttiker formalism. The band structure depends sensitively on the field strength, and its change has profound influence on the electron conduction. The conductance is found to demonstrate multi-step behavior due to the discrete number of conduction channels. The sharp peak and rectangular structures of the conductance are stretched out as temperature increases, due to the thermal broadening of the derivative of the Fermi-Dirac distribution function. Finally, quantum behavior in the conductance of MoS2 can be observed at temperatures below 10 K.

  5. Thermal conductivity of bulk and monolayer MoS 2

    KAUST Repository

    Gandi, Appala

    2016-02-26

    © Copyright EPLA, 2016. We show that the lattice contribution to the thermal conductivity of MoS2 strongly dominates the carrier contribution in a broad temperature range from 300 to 800 K. Since theoretical insight into the lattice contribution is largely missing, though it would be essential for materials design, we solve the Boltzmann transport equation for the phonons self-consistently in order to evaluate the phonon lifetimes. In addition, the length scale for transition between diffusive and ballistic transport is determined. The low out-of-plane thermal conductivity of bulk MoS2 (2.3 Wm-1K-1 at 300 K) is useful for thermoelectric applications. On the other hand, the thermal conductivity of monolayer MoS2 (131 Wm-1K-1 at 300 K) is comparable to that of Si.

  6. Enhanced absorption of monolayer MoS2 with resonant back reflector

    CERN Document Server

    Liu, Jiang-Tao; Li, Xiao-Jing; Liu, Nian-Hua

    2014-01-01

    By extracting the permittivity of monolayer MoS2 from experiments, the optical absorption of monolayer MoS2 prepared on top of one-dimensional photonic crystal (1DPC) or metal films is investigated theoretically. The 1DPC and metal films act as resonant back reflectors that can enhance absorption of monolayer MoS2 substantially over a broad spectral range due to the Fabry-Perot cavity effect. The absorption of monolayer MoS2 can also be tuned by varying either the distance between the monolayer MoS2 and the back reflector or the thickness of the cover layers.

  7. Monolayer MoS2 self-switching diodes

    Science.gov (United States)

    Al-Dirini, Feras; Hossain, Faruque M.; Mohammed, Mahmood A.; Hossain, Md Sharafat; Nirmalathas, Ampalavanapillai; Skafidas, Efstratios

    2016-01-01

    This paper presents a new molybdenum disulphide (MoS2) nanodevice that acts as a two-terminal field-effect rectifier. The device is an atomically-thin two-dimensional self-switching diode (SSD) that can be realized within a single MoS2 monolayer with very minimal process steps. Quantum simulation results are presented confirming the device's operation as a diode and showing strong non-linear I-V characteristics. Interestingly, the device shows p-type behavior, in which conduction is dominated by holes as majority charge carriers and the flow of reverse current is enhanced, while the flow of forward current is suppressed, in contrast to monolayer graphene SSDs, which behave as n-type devices. The presence of a large bandgap in monolayer MoS2 results in strong control over the channel, showing complete channel pinch-off in forward conduction, which was confirmed with transmission pathways plots. The device exhibited large leakage tunnelling current through the insulating trenches, which may have been due to the lack of passivation; nevertheless, reverse current remained to be 6 times higher than forward current, showing strong rectification. The effect of p-type substitutional channel doping of sulphur with phosphorus was investigated and showed that it greatly enhances the performance of the device, increasing the reverse-to-forward current rectification ratio more than an order of magnitude, up to a value of 70.

  8. Excitonic Stark effect in MoS2 monolayers

    Science.gov (United States)

    Scharf, Benedikt; Frank, Tobias; Gmitra, Martin; Fabian, Jaroslav; Žutić, Igor; Perebeinos, Vasili

    2016-12-01

    We theoretically investigate excitons in MoS2 monolayers in an applied in-plane electric field. Tight-binding and Bethe-Salpeter equation calculations predict a quadratic Stark shift, of the order of a few meV for fields of 10 V/μ m , in the linear absorption spectra. The spectral weight of the main exciton peaks decreases by a few percent with an increasing electric field due to the exciton field ionization into free carriers as reflected in the exciton wave functions. Subpicosecond exciton decay lifetimes at fields of a few tens of V/μ m could be utilized in solar energy harvesting and photodetection. We find simple scaling relations of the exciton binding, radius, and oscillator strength with the dielectric environment and an electric field, which provides a path to engineering the MoS2 electro-optical response.

  9. Hall and Nernst effects in monolayer MoS2

    Science.gov (United States)

    Zhang, Yun-Hai; Zhang, Ming-Hua

    2016-03-01

    We study Hall and Nernst transports in monolayer MoS2 based on Green’s function formalism. We have derived analytical results for spin and valley Hall conductivities in the zero temperature and spin and valley Nernst conductivities in the low temperature. We found that tuning of the band gap and spin-orbit splitting can drive system transition from spin Hall insulator (SHI) to valley Hall insulator (VHI). When the system is subjected to a temperature gradient, the spin and valley Nernst conductivities are dependent on Berry curvature.

  10. Exciton-dominant Electroluminescence from a Diode of Monolayer MoS2

    Science.gov (United States)

    2014-05-14

    injected electrons and holes, is a reliable technique to study exciton recombination processes in monolayer MoS2, including val- ley and spin excitation...We find the emerged feature (labeled NX) peaks at 550 nm with energy of 2.255 eV. In low dimensional system, like monolayer MoS2, Coulomb interactions

  11. A Route to Permanent Valley Polarization in Monolayer MoS2

    KAUST Repository

    Singh, Nirpendra

    2016-10-24

    Realization of permanent valley polarization in Cr-doped monolayer MoS2 is found to be unfeasible because of extended moment formation. Introduction of an additional hole is suggested as a viable solution. V-doped monolayer MoS2 is demonstrated to sustain permanent valley polarization and therefore can serve as a prototype material for valleytronics.

  12. Bandgap engineering of rippled MoS2 monolayer under external electric field

    Science.gov (United States)

    Qi, Jingshan; Li, Xiao; Qian, Xiaofeng; Feng, Ji

    2013-04-01

    In this letter we propose a universal strategy combining external electric field with the ripple of membrane to tune the bandgap of semiconducting atomic monolayer. By first-principles calculations we show that the bandgap of rippled MoS2 monolayer can be tuned in a large range by vertical external electric field, which is expected to have little effect on MoS2 monolayer. This phenomenon can be explained from charge redistribution under external electric field by a simple model. This may open an avenue of optimizing monolayer MoS2 for electronic and optoelectronic applications by surface patterning.

  13. Controlling the Spontaneous Emission Rate of Monolayer MoS$_2$ in a Photonic Crystal Nanocavity

    CERN Document Server

    Gan, Xuetao; Mak, Kin Fai; Yao, Xinwen; Shiue, Ren-Jye; van der Zande, Arend; Trusheim, Matthew; Hatami, Fariba; Heinz, Tony F; Hone, James; Englund, Dirk

    2013-01-01

    We report on controlling the spontaneous emission (SE) rate of a molybdenum disulfide (MoS$_2$) monolayer coupled with a planar photonic crystal (PPC) nanocavity. Spatially resolved photoluminescence (PL) mapping shows strong variations of emission when the MoS$_2$ monolayer is on the PPC cavity, on the PPC lattice, on the air gap, and on the unpatterned gallium phosphide substrate. Polarization dependences of the cavity-coupled MoS$_2$ emission show a more than 5 times stronger extracted PL intensity than the un-coupled emission, which indicates an underlying cavity mode Purcell enhancement of MoS$_2$ SE rate exceeding a factor of 70.

  14. Dual role of monolayer MoS2 in enhanced photocatalytic performance of hybrid MoS2/SnO2 nanocomposite

    Science.gov (United States)

    Ding, Shuang-Shuang; Huang, Wei-Qing; Yang, Yin-Cai; Zhou, Bing-Xin; Hu, Wang-Yu; Long, Meng-Qiu; Peng, P.; Huang, Gui-Fang

    2016-05-01

    The enhanced photocatalytic performance of various MoS2-based nanomaterials has recently been observed, but the role of monolayer MoS2 is still not well elucidated at the electronic level. Herein, focusing on a model system, hybrid MoS2/SnO2 nanocomposite, we first present a theoretical elucidation of the dual role of monolayer MoS2 as a sensitizer and a co-catalyst by performing density functional theory calculations. It is demonstrated that a type-II, staggered, band alignment of ˜0.49 eV exists between monolayer MoS2 and SnO2 with the latter possessing the higher electron affinity, or work function, leading to the robust separation of photoexcited charge carriers between the two constituents. Under irradiation, the electrons are excited from Mo 4d orbitals to SnO2, thus enhancing the reduction activity of latter, indicating that the monolayer MoS2 is an effective sensitizer. Moreover, the Mo atoms, which are catalytically inert in isolated monolayer MoS2, turn into catalytic active sites, making the monolayer MoS2 to be a highly active co-catalyst in the composite. The dual role of monolayer MoS2 is expected to arise in other MoS2-semiconductor nanocomposites. The calculated absorption spectra can be rationalized by available experimental results. These findings provide theoretical evidence supporting the experimental reports and pave the way for developing highly efficient MoS2-based photocatalysts.

  15. First-principles study of the magnetism of Ni-doped MoS2 monolayer

    Science.gov (United States)

    Luo, Min; Hao Shen, Yu; Hao Chu, Jun

    2016-09-01

    The magnetic properties of Ni-doped monolayer MoS2 are investigated using the density function theory. The results show that two Ni-doped systems of the nearest-neighbor configuration are ferromagnetic. The p-d hybridization between the Ni dopant and its neighboring S atoms results in the splitting of energy levels near the Fermi energy. These results suggest the p-d hybridization mechanism for the magnetism of the Ni-doped MoS2 monolayer. The magnetic moment disappears with increasing Ni-Ni distance. Our studies predict the nearest two-Ni-doped MoS2 monolayers to be candidates for thin dilute magnetic semiconductors. Moreover, the formation energy calculations indicate that it would be easier to incorporate Ni atoms into a S-rich MoS2 monolayer in the experiment.

  16. Substrate induced anomalous electrostatic and photoluminescence propeties of monolayer MoS2 edges

    Science.gov (United States)

    Hao, Song; Yang, Bingchu; Yuan, Jingye; Gao, Yongli

    2017-01-01

    Monolayer MoS2 is an emerging two-dimensional semiconductor with wide-ranging potential applications in the next generation electronic and optoelectronic devices. Understanding the influences of the supporting substrates on the physical properties of grown MoS2 is an important step toward its applications. Here we synthesized two typical rhomboid shaped MoS2 on MoO2 and triangle shaped MoS2 on SiO2/Si substrates and characterized them by multiple means of X-Ray Photoemission Spectroscopy, Atomic Force Microscopy, Electrostatic Force Microscopy, Raman and Photoluminescence techniques. We found that triangle shaped MoS2 exhibits different core level spectra compared with rhomboid shaped MoS2, attributed to dissimilar charge transfer with the underlying SiO2 substrate. Interestingly, the triangle shaped MoS2 single crystals exhibit distinct electrostatic and photoluminescence properties at center and edges. The underlying mechanism is proposed that partial decoupling of MoS2 at edges from SiO2 substrate induces different doping level and strain effects, resulting in anomalous physical properties at edges. The results reported here demonstrate that doping and strain effects induced by substrates have a significant influence on physical properties of monolayer triangle shaped MoS2,which can be generally applicable to other transition metal dichalcogenides materials.

  17. Atomic-Monolayer MoS2 Band-to-Band Tunneling Field-Effect Transistor

    KAUST Repository

    Lan, Yann Wen

    2016-09-05

    The experimental observation of band-to-band tunneling in novel tunneling field-effect transistors utilizing a monolayer of MoS2 as the conducting channel is demonstrated. Our results indicate that the strong gate-coupling efficiency enabled by two-dimensional materials, such as monolayer MoS2, results in the direct manifestation of a band-to-band tunneling current and an ambipolar transport.

  18. Stability and Electronic Properties of Hydrogenated MoS2 Monolayer: A First-Principles Study.

    Science.gov (United States)

    Zhang, Weibin; Zhang, Zhijun; Yang, Woochul

    2015-10-01

    First-principles total energy studies are used to investigate the stability of hydrogenated MoS2 monolayer (MoS2-Hx) (x = 1-8), which is a compound with different numbers of H atoms adsorbed on the MoS2 surface. Energetically, the S-top side of the MoS2 is found to be the most favorable for H-adsorption. H2S and graphene are well-known to be stable, and MoS2-Hx is predicted to be even more stable because its binding energy is lower than that of H2S and its formation energy and adsorption energy are lower than those of graphene. The analysis of the electronic density distribution and the orbital hybrid also shows that MoS2-Hx forms stable structures. In addition, the influence of the number of the adsorbed H-atoms in the MoS2-Hx on the electronic structure of the compound is also investigated. The MoS2-Hx band structure exhibits a dispersion and the MoS2-Hx band gap gradually decreases from 1.72 eV to 0 eV as the number of adsorbed H atoms increases. The corresponding work function increases as a result of the strengthening of the dipole moment formed between the H atoms that are adsorbed and the hydrogenated MoS2.

  19. Strain engineering in monolayer WS2, MoS2, and the WS2/MoS2 heterostructure

    KAUST Repository

    He, Xin

    2016-10-27

    Mechanically exfoliated monolayers of WS2, MoS2 and their van der Waals heterostructure were fabricated on flexible substrate so that uniaxial tensile strain can be applied to the two-dimensional samples. The modification of the band structure under strain was investigated by micro-photoluminescence spectroscopy at room temperature as well as by first-principles calculations. Exciton and trion emissions were observed in both WS2 and the heterostructure at room temperature, and were redshifted by strain, indicating potential for applications in flexible electronics and optoelectronics.

  20. Formaldehyde molecule adsorption on the doped monolayer MoS2: A first-principles study

    Science.gov (United States)

    Ma, Dongwei; Ju, Weiwei; Li, Tingxian; Yang, Gui; He, Chaozheng; Ma, Benyuan; Tang, Yanan; Lu, Zhansheng; Yang, Zongxian

    2016-05-01

    Based on first-principles calculations, formaldehyde (H2CO) adsorption on the pristine monolayer MoS2 and that doped with Cl, P, or Si was theoretically studied to explore the potential of the MoS2 sheets as H2CO gas sensors. It is found that under Mo-rich conditions it is viable for Cl to be filled into the S vacancies acting as n-type dopant and for P and Si acting as p-type dopants. The results on the H2CO adsorption on the pristine and the Cl-doped monolayer MoS2 indicate that both are insensitive to H2CO. In contrast, H2CO exhibits strong adsorption on the P or Si-doped monolayer MoS2. And there are large electron transfer from the P or Si-doped monolayer MoS2 to the H2CO and obvious change in the electronic densities of states of both systems induced by the H2CO adsorption. These suggest that P and Si can be appropriate dopants filled into MoS2 sheets for detecting H2CO molecule.

  1. Structural Properties and Phase Transition of Na Adsorption on Monolayer MoS2.

    Science.gov (United States)

    He, Hai; Lu, Pengfei; Wu, Liyuan; Zhang, Chunfang; Song, Yuxin; Guan, Pengfei; Wang, Shumin

    2016-12-01

    First-principles calculations are performed to investigate the structural stability of Na adsorption on 1H and 1T phases of monolayer MoS2. Our results demonstrate that it is likely to make the stability of distorted 1T phase of MoS2 over the 1H phase through adsorption of Na atoms. The type of distortion depends on the concentration of adsorbed Na atoms and changes from zigzag-like to diamond-like with the increasing of adsorbed Na atom concentrations. Our calculations show that the phase transition from 1H-MoS2 to 1T-MoS2 can be obtained by Na adsorption. We also calculate the electrochemical properties of Na adsorption on MoS2 monolayer. These results indicate that MoS2 is one of potential negative electrodes for Na-ion batteries.

  2. Effect of precursor on growth and morphology of MoS2 monolayer and multilayer

    Science.gov (United States)

    Ganorkar, Shraddha; Kim, Jungyoon; Kim, Young-Hwan; Kim, Seong-II

    2015-12-01

    The rise of two-dimensional (2D) material is one of the results of successful efforts of researchers which laid the path to the new era of electronics. One of the most exciting materials is MoS2. Synthesis has been always a major issue as electronic devices need reproducibility along with similar properties for mass productions. Chemical vapor deposition (CVD) is one of the successful methods for 2D materials including graphene. Furthermore, the choice of starting materials for Mo and S source is crucial. The different source has different effects on the layers and morphology of MoS2 films. In this work, we have extensively studied the CVD technique to grow few layers of MoS2 with two precursors MoO3 and MoCl5, show remarkable changes. The MoO3 source gives a triangular shaped MoS2 monolayer while that of MoCl5 can achieve uniform MoS2 without triangle. The absence of geometric shapes with MoCl5 is poorly understood. We tried to explain with MoCl5 precursor, the formation of continuous monolayer of MoS2 without any triangle on the basis of chemical reaction formalism mostly due to one step reaction process and formation of MoS2 from gas phase to the solid phase. The film synthesized by MoCl5 is more continuous and it would be a good choice for device applications.

  3. Ultrafast Transient Terahertz Conductivity of Monolayer MoS 2 and WSe 2 Grown by Chemical Vapor Deposition

    KAUST Repository

    Docherty, Callum J.

    2014-11-25

    We have measured ultrafast charge carrier dynamics in monolayers and trilayers of the transition metal dichalcogenides MoS2 and WSe2 using a combination of time-resolved photoluminescence and terahertz spectroscopy. We recorded a photoconductivity and photoluminescence response time of just 350 fs from CVD-grown monolayer MoS2, and 1 ps from trilayer MoS2 and monolayer WSe2. Our results indicate the potential of these materials as high-speed optoelectronic materials.

  4. Band Alignment and Minigaps in Monolayer MoS2-Graphene van der Waals Heterostructures.

    Science.gov (United States)

    Pierucci, Debora; Henck, Hugo; Avila, Jose; Balan, Adrian; Naylor, Carl H; Patriarche, Gilles; Dappe, Yannick J; Silly, Mathieu G; Sirotti, Fausto; Johnson, A T Charlie; Asensio, Maria C; Ouerghi, Abdelkarim

    2016-07-13

    Two-dimensional layered MoS2 shows great potential for nanoelectronic and optoelectronic devices due to its high photosensitivity, which is the result of its indirect to direct band gap transition when the bulk dimension is reduced to a single monolayer. Here, we present an exhaustive study of the band alignment and relativistic properties of a van der Waals heterostructure formed between single layers of MoS2 and graphene. A sharp, high-quality MoS2-graphene interface was obtained and characterized by micro-Raman spectroscopy, high-resolution X-ray photoemission spectroscopy (HRXPS), and scanning high-resolution transmission electron microscopy (STEM/HRTEM). Moreover, direct band structure determination of the MoS2/graphene van der Waals heterostructure monolayer was carried out using angle-resolved photoemission spectroscopy (ARPES), shedding light on essential features such as doping, Fermi velocity, hybridization, and band-offset of the low energy electronic dynamics found at the interface. We show that, close to the Fermi level, graphene exhibits a robust, almost perfect, gapless, and n-doped Dirac cone and no significant charge transfer doping is detected from MoS2 to graphene. However, modification of the graphene band structure occurs at rather larger binding energies, as the opening of several miniband-gaps is observed. These miniband-gaps resulting from the overlay of MoS2 and the graphene layer lattice impose a superperiodic potential.

  5. Fast and large-area growth of uniform MoS2 monolayers on molybdenum foils

    Science.gov (United States)

    Tai, Guoan; Zeng, Tian; Yu, Jin; Zhou, Jianxin; You, Yuncheng; Wang, Xufeng; Wu, Hongrong; Sun, Xu; Hu, Tingsong; Guo, Wanlin

    2016-01-01

    A controllable synthesis of two-dimensional crystal monolayers in a large area is a prerequisite for potential applications, but the growth of transition metal dichalcogenide monolayers in a large area with spatial homogeneity remains a great challenge. Here we report a novel and efficient method to fabricate large-scale MoS2 monolayers by direct sulfurization of pre-annealed molybdenum foil surfaces with large grain boundaries of more than 50 μm in size at elevated temperatures. Continuous MoS2 monolayers can be formed uniformly by sulfurizing the Mo foils in sulfur vapor at 600 °C within 1 min. At a lower temperature even down to 500 °C, uniform MoS2 monolayers can still be obtained but in a much longer sulfurizing duration. It is demonstrated that the formed monolayers can be nondestructively transferred onto arbitrary substrates by removing the Mo foil using diluted ferric chloride solution and can be successfully fabricated into photodetectors. The results show a novel avenue to efficiently fabricate two-dimensional crystals in a large area in a highly controllable way and should have great potential for the development of large-scale applications of two-dimensional crystals in electrophotonic systems.A controllable synthesis of two-dimensional crystal monolayers in a large area is a prerequisite for potential applications, but the growth of transition metal dichalcogenide monolayers in a large area with spatial homogeneity remains a great challenge. Here we report a novel and efficient method to fabricate large-scale MoS2 monolayers by direct sulfurization of pre-annealed molybdenum foil surfaces with large grain boundaries of more than 50 μm in size at elevated temperatures. Continuous MoS2 monolayers can be formed uniformly by sulfurizing the Mo foils in sulfur vapor at 600 °C within 1 min. At a lower temperature even down to 500 °C, uniform MoS2 monolayers can still be obtained but in a much longer sulfurizing duration. It is demonstrated that the

  6. The Interface between Gd and Monolayer MoS2: A First-Principles Study

    KAUST Repository

    Zhang, Xuejing

    2014-12-08

    We analyze the electronic structure of interfaces between two-, four- and six-layer Gd(0001) and monolayer MoS2 by first-principles calculations. Strong chemical bonds shift the Fermi energy of MoS2 upwards into the conduction band. At the surface and interface the Gd f states shift to lower energy and new surface/interface Gd d states appear at the Fermi energy, which are strongly hybridized with the Mo 4d states and thus lead to a high spin-polarization (ferromagnetically ordered Mo magnetic moments of 0.15 μB). Gd therefore is an interesting candidate for spin injection into monolayer MoS2.

  7. Role of sulphur atoms on stress relaxation and crack propagation in monolayer MoS2

    Science.gov (United States)

    Wang, Baoming; Islam, Zahabul; Zhang, Kehao; Wang, Ke; Robinson, Joshua; Haque, Aman

    2017-09-01

    We present in-situ transmission electron microscopy of crack propagation in a freestanding monolayer MoS2 and molecular dynamic analysis of the underlying mechanisms. Chemical vapor deposited monolayer MoS2 was transferred from sapphire substrate using interfacial etching for defect and contamination minimization. Atomic resolution imaging shows crack tip atoms sustaining 14.5% strain before bond breaking, while the stress field decays at unprecedented rate of 2.15 GPa Å-1. Crack propagation is seen mostly in the zig-zag direction in both model and experiment, suggesting that the mechanics of fracture is not brittle. Our computational model captures the mechanics of the experimental observations on crack propagation in MoS2. While molybdenum atoms carry most of the mechanical load, we show that the sliding motion of weakly bonded sulphur atoms mediate crack tip stress relaxation, which helps the tip sustain very high, localized stress levels.

  8. Tuning the photo-response in monolayer MoS2 by plasmonic nano-antenna

    Science.gov (United States)

    Li, Jiu; Ji, Qingqing; Chu, Saisai; Zhang, Yanfeng; Li, Yan; Gong, Qihuang; Liu, Kaihui; Shi, Kebin

    2016-03-01

    Monolayer molybdenum disulfide (MoS2) has recently attracted intense interests due to its remarkable optical properties of valley-selected optical response, strong nonlinear wave mixing and photocurrent/photovoltaic generation and many corresponding potential applications. However, the nature of atomic-thin thickness of monolayer MoS2 leads to inefficient light-matter interactions and thereby hinders its optoelectronic applications. Here we report on the enhanced and controllable photo-response in MoS2 by utilizing surface plasmonic resonance based on metallic nano-antenna with characteristic lateral size of 40 × 80 nm. Our nano-antenna is designed to have one plasmonic resonance in the visible range and can enhance the MoS2 photoluminescence intensity up to 10 folds. The intensity enhancement can be effectively tuned simply by the manipulation of incident light polarization. In addition, we can also control the oscillator strength ratio between exciton and trion states by controlling polarization dependent hot carrier doping in MoS2. Our results demonstrate the possibility in controlling the photo-response in broad two-dimensional materials by well-designed nano-antenna and facilitate its coming optoelectronic applications.

  9. Large-Scale Synthesis and Systematic Photoluminescence Properties of Monolayer MoS2 on Fused Silica.

    Science.gov (United States)

    Wan, Yi; Zhang, Hui; Zhang, Kun; Wang, Yilun; Sheng, Bowen; Wang, Xinqiang; Dai, Lun

    2016-07-20

    Monolayer MoS2, with fascinating mechanical, electrical, and optical properties, has generated enormous scientific curiosity and industrial interest. Controllable and scalable synthesis of monolayer MoS2 on various desired substrates has significant meaning in both basic scientific research and device application. Recent years have witnessed many advances in the direct synthesis of single-crystalline MoS2 flakes or their polycrystalline aggregates on numerous diverse substrates, such as SiO2-Si, mica, sapphire, h-BN, and SrTiO3, etc. In this work, we used the dual-temperature-zone atmospheric-pressure chemical vapor deposition method to directly synthesize large-scale monolayer MoS2 on fused silica, the most ordinary transparent insulating material in daily life. We systematically investigated the photoluminescence (PL) properties of monolayer MoS2 on fused silica and SiO2-Si substrates, which have different thermal conductivity coefficients and thermal expansion coefficients. We found that there exists a stronger strain on monolayer MoS2 grown on fused silica, and the strain becomes more obvious as temperature decreases. Moreover, the monolayer MoS2 grown on fused silica exhibits the unique trait of a fractal shape with tortuous edges and has stronger adsorbability. The monolayer MoS2 grown on fused silica may find application in sensing, energy storage, and transparent optoelectronics, etc.

  10. MoS2 monolayers on nanocavities: enhancement in light-matter interaction

    Science.gov (United States)

    Janisch, Corey; Song, Haomin; Zhou, Chanjing; Lin, Zhong; Elías, Ana Laura; Ji, Dengxin; Terrones, Mauricio; Gan, Qiaoqiang; Liu, Zhiwen

    2016-06-01

    Two-dimensional (2D) atomic crystals and van der Waals heterostructures constitute an emerging platform for developing new functional ultra-thin electronic and optoelectronic materials for novel energy-efficient devices. However, in most thin-film optical applications, there is a long-existing trade-off between the effectiveness of light-matter interactions and the thickness of semiconductor materials, especially when the materials are scaled down to atom thick dimensions. Consequently, enhancement strategies can introduce significant advances to these atomically thick materials and devices. Here we demonstrate enhanced absorption and photoluminescence generation from MoS2 monolayers coupled with a planar nanocavity. This nanocavity consists of an alumina nanolayer spacer sandwiched between monolayer MoS2 and an aluminum reflector, and can strongly enhance the light-matter interaction within the MoS2, increasing the exclusive absorption of monolayer MoS2 to nearly 70% at a wavelength of 450 nm. The nanocavity also modifies the spontaneous emission rate, providing an additional design freedom to control the interaction between light and 2D materials.

  11. Controlled synthesis of high-quality crystals of monolayer MoS2 for nanoelectronic device application

    DEFF Research Database (Denmark)

    Yang, Xiaonian; Li, Qiang; Hu, Guofeng

    2016-01-01

    Two-dimensional layered materials have attracted significant interest for their potential applications in electronic and optoelectronics devices. Among them, transition metal dichalcogenides (TMDs), especially molybdenum disulfide (MoS2), is extensively studied because of its unique properties....... Monolayer MoS2 so far can be obtained by mechanical exfoliation or chemical vapor deposition (CVD). However, controllable synthesis of large area monolayer MoS2 with high quality needs to be improved and their growth mechanism requires more studies. Here we report a systematical study on controlled...... synthesis of high-quality monolayer MoS2 single crystals using low pressure CVD. Large-size monolayer MoS2 triangles with an edge length up to 405 mu m were successfully synthesized. The Raman and photoluminescence spectroscopy studies indicate high homogenous optical characteristic of the synthesized...

  12. Intrinsic electrical transport and performance projections of synthetic monolayer MoS2 devices

    Science.gov (United States)

    Smithe, Kirby K. H.; English, Chris D.; Suryavanshi, Saurabh V.; Pop, Eric

    2017-03-01

    We demonstrate monolayer (1L) MoS2 grown by chemical vapor deposition (CVD) with transport properties comparable to those of the best exfoliated 1L devices over a wide range of carrier densities (up to ˜1013 cm-2) and temperatures (80-500 K). Transfer length measurements decouple the intrinsic material mobility from the contact resistance, at practical carrier densities (>1012 cm-2). We demonstrate the highest current density reported to date (˜270 μA μm-1 or 44 MA cm-2) at 300 K for an 80 nm long device from CVD-grown 1L MoS2. Using simulations, we discuss what improvements of 1L MoS2 are still required to meet technology roadmap requirements for low power and high performance applications. Such results are an important step towards large-area electronics based on 1L semiconductors.

  13. Ultrafast charge transfer between MoTe2 and MoS2 monolayers

    Science.gov (United States)

    Pan, Shudi; Ceballos, Frank; Bellus, Matthew Z.; Zereshki, Peymon; Zhao, Hui

    2017-03-01

    High quality and stable electrical contact between metal and two-dimensional materials, such as transition metal dichalcogenides, is a necessary requirement that has yet to be achieved in order to successfully exploit the advantages that these materials offer to electronics and optoelectronics. MoTe2, owing to its phase changing property, can potentially offer a solution. A recent study demonstrated that metallic phase of MoTe2 connects its semiconducting phase with very low resistance. To utilize this property to connect other two-dimensional materials, it is important to achieve efficient charge transfer between MoTe2 and other semiconducting materials. Using MoS2 as an example, we report ultrafast and efficient charge transfer between MoTe2 and MoS2 monolayers. In the transient absorption measurements, an ultrashort pump pulse is used to selectively excite electrons in MoTe2. The appearance of the excited electrons in the conduction band of MoS2 is monitored by using a probe pulse that is tuned to the resonance of MoS2. We found that electrons transfer to MoS2 on a time scale of at most 0.3 ps. The transferred electrons give rise to a large transient absorption signal at both A-exciton and B-exciton resonances due to the screening effect. We also observed ultrafast transfer of holes from MoS2 to MoTe2. Our results suggest the feasibility of using MoTe2 as a bridge material to connect MoS2 and other transition metal dichalcogenides, and demonstrate a new transition metal dichalcogenide heterostructure involving MoTe2, which extends the spectral range of such structures to infrared.

  14. A generic tight-binding model for monolayer, bilayer and bulk MoS2

    Directory of Open Access Journals (Sweden)

    Ferdows Zahid

    2013-05-01

    Full Text Available Molybdenum disulfide (MoS2 is a layered semiconductor which has become very important recently as an emerging electronic device material. Being an intrinsic semiconductor the two-dimensional MoS2 has major advantages as the channel material in field-effect transistors. In this work we determine the electronic structures of MoS2 with the highly accurate screened hybrid functional within the density functional theory (DFT including the spin-orbit coupling. Using the DFT electronic structures as target, we have developed a single generic tight-binding (TB model that accurately produces the electronic structures for three different forms of MoS2 - bulk, bilayer and monolayer. Our TB model is based on the Slater-Koster method with non-orthogonal sp3d5 orbitals, nearest-neighbor interactions and spin-orbit coupling. The TB model is useful for atomistic modeling of quantum transport in MoS2 based electronic devices.

  15. Giant Two-Photon Absorption Coefficient and Frequency Up-Converted Luminescence in Monolayer MoS2

    CERN Document Server

    Li, Yuanxin; Zhang, Saifeng; Zhang, Xiaoyan; Feng, Yanyan; Wang, Kangpeng; Zhang, Long; Wang, Jun

    2015-01-01

    Strong two-photon absorption (TPA) in monolayer MoS2 is demonstrated in contrast to saturable absorption (SA) in multilayer MoS2 under the excitation of femtosecond laser pulses in the near infrared region. MoS2 in the forms of monolayer single crystal and multilayer triangular islands are grown on either quartz or SiO2/Si by employing the seeding method through chemistry vapor deposition. The nonlinear transmission measurements reveal that monolayer MoS2 possesses a giant nonsaturation TPA coefficient, larger than that of conventional semiconductors. As a result of TPA, two-photon pumped frequency up-converted luminescence is observed directly in the monolayer MoS2. For the multilayer MoS2, the SA response is demonstrated with the ratio of the excited-state absorption cross section to ground-state cross section of 0.18. In addition, the laser damage threshold of the monolayer MoS2 is 97 GW/cm2, larger than that of the multilayer MoS2 of 78 GW/cm2.

  16. Linear magnetotransport in monolayer MoS2

    Science.gov (United States)

    Wang, C. M.; Lei, X. L.

    2015-09-01

    A momentum balance equation is developed to investigate the magnetotransport properties in monolayer molybdenum disulphide when a strong perpendicular magnetic field and a weak in-plane electric field are applied simultaneously. At low temperature, in the presence of intravalley impurity scattering, Shubnikov-de Haas oscillation shows up accompanied by a beating pattern arising from large spin splitting and its period may halve due to the high-order oscillating term at large magnetic field for samples with ultrahigh mobility. In the case of intervalley disorders, there exists a magnetic-field range where the magnetoresistivity almost vanishes. For a low-mobility layer, a phase inversion of oscillating peaks is acquired in accordance with recent experiment. At high temperature when Shubnikov-de Haas oscillation is suppressed, the magnetophonon resonances induced by both optical phonons (mainly due to homopolar and Fröhlich modes) and acoustic phonons (mainly due to intravalley transverse and longitudinal acoustic modes) emerge for a suspended system with high mobility. For the single layer on a substrate, another resonance due to surface optical phonons may occur, resulting in a complex behavior of the total magnetoresistance. The beating pattern of magnetophonon resonance due to optical phonons can also be observed. However, for a nonsuspended layer with low mobility, the magnetoresistance oscillation almost disappears and the resistivity increases with field monotonically.

  17. Possible doping strategies for MoS 2 monolayers: An ab initio study

    KAUST Repository

    Dolui, Kapildeb

    2013-08-14

    Density functional theory is used to systematically study the electronic properties of doped MoS2 monolayers, where the dopants are incorporated both via S/Mo substitution or as adsorbates. Among the possible substitutional dopants at the Mo site, Nb is identified as suitable p-type dopant, while Re is the donor with the lowest activation energy. When dopants are simply adsorbed on a monolayer we find that alkali metals shift the Fermi energy into the MoS2 conduction band, making the system n type. Finally, the adsorption of charged molecules is considered, mimicking an ionic liquid environment. We find that molecules adsorption can lead to both n- and p-type conductivity, depending on the charge polarity of the adsorbed species. © 2013 American Physical Society.

  18. Robust effective Zeeman energy in monolayer MoS2 quantum dots

    Science.gov (United States)

    Dias, A. C.; Fu, Jiyong; Villegas-Lelovsky, L.; Qu, Fanyao

    2016-09-01

    We report a theoretical investigation on the energy spectrum and the effective Zeeman energy (EZE) in monolayer MoS2 circular quantum dots, subjected to an out-of-plane magnetic field. Interestingly, we observe the emergence of energy-locked modes, depending on the competition between the dot confinement and the applied magnetic field, for either the highest K-valley valence band or the lowest {{K}\\prime} -valley conduction band. Moreover, an unusual dot-size-independent EZE behavior of the highest valence and the lowest conduction bands is found. Although the EZEs are insensitive to the variation of quantum confinement, both of them grow linearly with the magnetic field, similar to that in the monolayer MoS2 material. The EZEs along with their ‘robustness’ against dot confinements open opportunities of a universal magnetic control over the valley degree of freedom, for quantum dots of all sizes.

  19. Research Update: Spin transfer torques in permalloy on monolayer MoS2

    Directory of Open Access Journals (Sweden)

    Wei Zhang

    2016-03-01

    Full Text Available We observe current induced spin transfer torque resonance in permalloy (Py grown on monolayer MoS2. By passing rf current through the Py/MoS2 bilayer, field-like and damping-like torques are induced which excite the ferromagnetic resonance of Py. The signals are detected via a homodyne voltage from anisotropic magnetoresistance of Py. In comparison to other bilayer systems with strong spin-orbit torques, the monolayer MoS2 cannot provide bulk spin Hall effects and thus indicates the purely interfacial nature of the spin transfer torques. Therefore our results indicate the potential of two-dimensional transition-metal dichalcogenide for the use of interfacial spin-orbitronics applications.

  20. Photovoltaic heterojunctions of fullerenes with MoS2 and WS 2 monolayers

    KAUST Repository

    Gan, Liyong

    2014-04-17

    First-principles calculations are performed to explore the geometry, bonding, and electronic structures of six ultrathin photovoltaic heterostructures consisting of pristine and B- or N-doped fullerenes and MoS2 or WS2 monolayers. The fullerenes prefer to be attached with a hexagon parallel to the monolayer, where B and N favor proximity to the monolayer. The main electronic properties of the subsystems stay intact, suggesting weak interfacial interaction. Both the C60/MoS 2 and C60/WS2 systems show type-II band alignments. However, the built-in potential in the former case is too small to effectively drive electron-hole separation across the interface, whereas the latter system is predicted to show good photovoltaic performance. Unfortunately, B and N doping destroys the type-II band alignment on MoS2 and preserves it only in one spin channel on WS2, which is unsuitable for excitonic solar cells. Our results suggest that the C60/WS 2 system is highly promising for excitonic solar cells. © 2014 American Chemical Society.

  1. Tunable redox potential of nonmetal doped monolayer MoS2: First principle calculations

    Science.gov (United States)

    Lu, S.; Li, C.; Zhao, Y. F.; Gong, Y. Y.; Niu, L. Y.; Liu, X. J.

    2016-10-01

    Doping is an effective method to alter the electronic behavior of materials by forming new chemical bonds and bringing bond relaxation. With this aid of first principle calculations, the crystal configuration and electronic properties of monolayer MoS2 have been modulated by the nonmetal (NM) dopants (H, B, C, N, O, F, Si, P, Cl, As, Se, Br, Te and I), and the thermodynamic stability depending on the preparation conditions (Mo-rich and S-rich conditions) were discussed. Results shown that, the NM dopants substituted preferentially for S under Mo-rich condition, the electronic distribution around the dopants and the nearby Mo atoms are changed by the new formed Mo-NM bonds and bands relaxation. Compared to pristine monolayer MoS2, the NM ions with odd chemical valences enhance the oxidation potential and reduce the reduction potential of specimens, but the NM ions with even chemical valences have the opposite effects on the redox potentials. Compared to the NM ions with even chemical valences, the lone pair electrons in NM ions with odd chemical valences can extra interact with the Mo ions and reduces the ECBM and EVBM values of specimens. It offers a simple way to design various monolayer MoS2 based catalysts in order to catalyze different materials by chose the reasonable dopants for stronger oxidation or reduction potential.

  2. Photovoltaic Heterojunctions of Fullerenes with MoS2 and WS2 Monolayers.

    Science.gov (United States)

    Gan, Li-Yong; Zhang, Qingyun; Cheng, Yingchun; Schwingenschlögl, Udo

    2014-04-17

    First-principles calculations are performed to explore the geometry, bonding, and electronic structures of six ultrathin photovoltaic heterostructures consisting of pristine and B- or N-doped fullerenes and MoS2 or WS2 monolayers. The fullerenes prefer to be attached with a hexagon parallel to the monolayer, where B and N favor proximity to the monolayer. The main electronic properties of the subsystems stay intact, suggesting weak interfacial interaction. Both the C60/MoS2 and C60/WS2 systems show type-II band alignments. However, the built-in potential in the former case is too small to effectively drive electron-hole separation across the interface, whereas the latter system is predicted to show good photovoltaic performance. Unfortunately, B and N doping destroys the type-II band alignment on MoS2 and preserves it only in one spin channel on WS2, which is unsuitable for excitonic solar cells. Our results suggest that the C60/WS2 system is highly promising for excitonic solar cells.

  3. An Anomalous Formation Pathway for Dislocation-Sulfur Vacancy Complexes in Polycrystalline Monolayer MoS2.

    Science.gov (United States)

    Yu, Zhi Gen; Zhang, Yong-Wei; Yakobson, Boris I

    2015-10-14

    Two-dimensional (2D) molybdenum disulfide (MoS2) has attracted significant attention recently due to its direct bandgap semiconducting characteristics. Experimental studies on monolayer MoS2 show that S vacancy concentration varies greatly; while recent theoretical studies show that the formation energy of S vacancy is high and thus its concentration should be low. We perform density functional theory calculations to study the structures and energetics of vacancy and interstitial in both grain boundary (GB) and grain interior (GI) in monolayer MoS2 and uncover an anomalous formation pathway for dislocation-double S vacancy (V2S) complexes in MoS2. In this pathway, a (5|7) defect in an S-polar GB energetically favorably converts to a (4|6) defect, which possesses a duality: dislocation and double S vacancy. Its dislocation character allows it to glide into GI through thermal activation at high temperatures, bringing the double vacancy with it. Our findings here not only explain why VS is predominant in exfoliated 2D MoS2 and V2S is predominant in chemical vapor deposition (CVD)-grown 2D MoS2 but also reproduce GB patterns in CVD-grown MoS2. The new pathway for sulfur vacancy formation revealed here provides important insights and guidelines for controlling the quality of monolayer MoS2.

  4. Monolayer MoS2 metal insulator transition based memcapacitor modeling with extension to a ternary device

    OpenAIRE

    Abdul Karim Khan; Byoung Hun Lee

    2016-01-01

    Memcapacitor model based on its one possible physical realization is developed and simulated in order to know its limitation before making a real device. The proposed device structure consists of vertically stacked dielectric layer and MoS2 monolayer between two external metal plates. The Metal Insulator Transition (MIT) phenomenon of MoS2 monolayer is represented in terms of percolation probabilty which is used as the system state. Cluster based site percolation theory is used to mimic the M...

  5. Markedly different adsorption behaviors of gas molecules on defective monolayer MoS2: a first-principles study.

    Science.gov (United States)

    Li, Hongxing; Huang, Min; Cao, Gengyu

    2016-06-01

    Sulfur vacancy (SV) is one of the most typical defects in two-dimensional monolayer MoS2, leading to reactive sites. We presented a systematic study of the adsorption behaviors of gas molecules, CO2, N2, H2O, CO, NH3, NO, O2, H2 and NO2, on monolayer MoS2 with single SV by first-principles calculations. It was found that CO2, N2 and H2O molecules physisorbed at the proximity of single SV. Our adsorption energy calculations and charge transfer analysis showed that the interactions between CO2, N2 and H2O molecules and defective MoS2 are stronger than the cases of CO2, N2 and H2O molecules adsorbed on pristine MoS2, respectively. The defective MoS2 based gas sensors may be more sensitive to CO2, N2 and H2O molecules than pristine MoS2 based ones. CO, NO, O2 and NH3 molecules were found to chemisorb at the S vacancy site and thus modify the electronic properties of defective monolayer MoS2. Magnetism was induced upon adsorption of NO molecules and the defective states induced by S vacancy can be completely removed upon adsorption of O2 molecules, which may provide some helpful information for designing new MoS2 based nanoelectronic devices in future. The H2 and NO2 molecules were found to dissociate at S vacancy. The dissociation of NO2 molecules resulted in O atoms located at the S vacancy site and NO molecules physisorbed on O-doped MoS2. The calculated results showed that NO2 molecules can help heal the S vacancy of the MoS2 monolayer.

  6. Atomic-Scale Spectroscopy of Gated Monolayer MoS2.

    Science.gov (United States)

    Zhou, Xiaodong; Kang, Kibum; Xie, Saien; Dadgar, Ali; Monahan, Nicholas R; Zhu, X-Y; Park, Jiwoong; Pasupathy, Abhay N

    2016-05-11

    The electronic properties of semiconducting monolayer transition-metal dichalcogenides can be tuned by electrostatic gate potentials. Here we report gate-tunable imaging and spectroscopy of monolayer MoS2 by atomic-resolution scanning tunneling microscopy/spectroscopy (STM/STS). Our measurements are performed on large-area samples grown by metal-organic chemical vapor deposition (MOCVD) techniques on a silicon oxide substrate. Topographic measurements of defect density indicate a sample quality comparable to single-crystal MoS2. From gate voltage dependent spectroscopic measurements, we determine that in-gap states exist in or near the MoS2 film at a density of 1.3 × 10(12) eV(-1) cm(-2). By combining the single-particle band gap measured by STS with optical measurements, we estimate an exciton binding energy of 230 meV on this substrate, in qualitative agreement with numerical simulation. Grain boundaries are observed in these polycrystalline samples, which are seen to not have strong electronic signatures in STM imaging.

  7. Activating basal-plane catalytic activity of two-dimensional MoS2 monolayer with remote hydrogen plasma

    KAUST Repository

    Cheng, Chia-Chin

    2016-09-10

    Two-dimensional layered transition metal dichalcogenide (TMD) materials such as Molybdenum disufide (MoS2) have been recognized as one of the low-cost and efficient electrocatalysts for hydrogen evolution reaction (HER). The crystal edges that account for a small percentage of the surface area, rather than the basal planes, of MoS2 monolayer have been confirmed as their active catalytic sites. As a result, extensive efforts have been developing in activating the basal planes of MoS2 for enhancing their HER activity. Here, we report a simple and efficient approach-using a remote hydrogen-plasma process-to creating S-vacancies on the basal plane of monolayer crystalline MoS2; this process can generate high density of S-vacancies while mainly maintaining the morphology and structure of MoS2 monolayer. The density of S-vacancies (defects) on MoS2 monolayers resulted from the remote hydrogen-plasma process can be tuned and play a critical role in HER, as evidenced in the results of our spectroscopic and electrical measurements. The H2-plasma treated MoS2 also provides an excellent platform for systematic and fundamental study of defect-property relationships in TMDs, which provides insights for future applications including electrical, optical and magnetic devices. © 2016 Elsevier Ltd.

  8. Characterization of the second- and third-order nonlinear optical susceptibilities of monolayer MoS$_2$ using multiphoton microscopy

    CERN Document Server

    Woodward, R I; Phelan, C F; de Oliveira, R E P; Runcorn, T H; Kelleher, E J R; Li, S; de Oliveira, E C; Fechine, G J M; Eda, G; de Matos, C J S

    2016-01-01

    We report second- and third-harmonic generation in monolayer MoS$_2$ as a tool for imaging and accurately characterizing the material's nonlinear optical properties under 1560 nm excitation. Using a surface nonlinear optics treatment, we derive expressions relating experimental measurements to second- and third-order nonlinear sheet susceptibility magnitudes, obtaining values of $|\\chi_s^{(2)}|=2\\times10^{-20}$ m$^2$ V$^{-1}$ and for the first time for monolayer MoS$_2$, $|\\chi_s^{(3)}|=2\\times10^{-28}$ m$^3$ V$^{-2}$. Experimental comparisons between MoS$_2$ and graphene are also performed, demonstrating $\\sim$4 times stronger third-order nonlinearity in monolayer MoS$_2$, highlighting the material's potential for nonlinear photonics in the telecommunications C band.

  9. Monolayer MoS2-Graphene Hybrid Aerogels with Controllable Porosity for Lithium-Ion Batteries with High Reversible Capacity.

    Science.gov (United States)

    Jiang, Lianfu; Lin, Binghui; Li, Xiaoming; Song, Xiufeng; Xia, Hui; Li, Liang; Zeng, Haibo

    2016-02-03

    Monolayer MoS2 nanosheets (NSs) are promising anode materials for lithium-ion batteries because all redox reactions take place at the surface without lithium-ion diffusion limit. However, the expanded band gap of monolayer MoS2 NSs (∼1.8 eV) compared to their bulk counterparts (∼1.2 eV) and restacking tendency due to the van der Waals forces result in poor electron transfer and loss of the structure advantage. Here, a facile approach is developed to fabricate the MoS2-graphene aerogels comprising controlled three-dimensional (3D) porous architectures constructed by interconnected monolayer MoS2-graphene hybrid NSs. The robust 3D architectures combining with the monolayer feature of the hybrid NSs not only prevent the MoS2 and graphene NSs from restacking, but also enable fast electrode kinetics due to the surface reaction mechanism and highly conductive graphene matrix. As a consequence, the 3D porous monolayer MoS2-graphene composite aerogels exhibit a large reversible capacity up to 1200 mAh g(-1) as well as outstanding cycling stability and rate performance, making them promising as advanced anode materials for lithium-ion batteries.

  10. Interfacial Properties of Monolayer and Bilayer MoS2 Contacts with Metals: Beyond the Energy Band Calculations.

    Science.gov (United States)

    Zhong, Hongxia; Quhe, Ruge; Wang, Yangyang; Ni, Zeyuan; Ye, Meng; Song, Zhigang; Pan, Yuanyuan; Yang, Jinbo; Yang, Li; Lei, Ming; Shi, Junjie; Lu, Jing

    2016-01-01

    Although many prototype devices based on two-dimensional (2D) MoS2 have been fabricated and wafer scale growth of 2D MoS2 has been realized, the fundamental nature of 2D MoS2-metal contacts has not been well understood yet. We provide a comprehensive ab initio study of the interfacial properties of a series of monolayer (ML) and bilayer (BL) MoS2-metal contacts (metal = Sc, Ti, Ag, Pt, Ni, and Au). A comparison between the calculated and observed Schottky barrier heights (SBHs) suggests that many-electron effects are strongly suppressed in channel 2D MoS2 due to a charge transfer. The extensively adopted energy band calculation scheme fails to reproduce the observed SBHs in 2D MoS2-Sc interface. By contrast, an ab initio quantum transport device simulation better reproduces the observed SBH in 2D MoS2-Sc interface and highlights the importance of a higher level theoretical approach beyond the energy band calculation in the interface study. BL MoS2-metal contacts generally have a reduced SBH than ML MoS2-metal contacts due to the interlayer coupling and thus have a higher electron injection efficiency.

  11. Spin-triplet f-wave symmetry in superconducting monolayer MoS2

    Science.gov (United States)

    Goudarzi, H.; Khezerlou, M.; Sedghi, H.; Ghorbani, A.

    2017-04-01

    The proximity-induced spin-triplet f-wave symmetry pairing in a monolayer molybdenum disulfide-superconductor hybrid features an interesting electron-hole excitations and also effective superconducting subgap, giving rise to a distinct Andreev resonance state. Owing to the complicated Fermi surface and momentum dependency of f-wave pair potential, monolayer MoS2 with strong spin-orbit coupling can be considered an intriguing structure to reveal the superconducting state. Actually, this can be possible by calculating the peculiar spin-valley polarized transport of quasiparticles in a related normal metal/superconductor junction. We theoretically study the formation of effective gap at the interface and resulting normalized conductance on top of a MoS2 under induction of f-wave order parameter, using Blonder-Tinkham-Klapwijk formalism. The superconducting excitations shows that the gap is renormalized by a bias limitation coefficient including dynamical band parameters of monolayer MoS2 , especially, ones related to the Schrodinger-type momentum of Hamiltonian. The effective gap is more sensitive to the n-doping regime of superconductor region. The signature of spin-orbit coupling, topological and asymmetry mass-related terms in the resulting subgap conductance and, in particular, maximum conductance is presented. In the absence of topological term, the effective gap reaches to its maximum value. The unconventional nature of superconducting order leads to the appearance of zero-bias conductance. In addition, the maximum value of conductance can be controlled by tuning the doping level of normal and superconductor regions.

  12. Electric field controlled CO2 capture and CO2/N2 separation on MoS2 monolayers.

    Science.gov (United States)

    Sun, Qiao; Qin, Gangqiang; Ma, Yingying; Wang, Weihua; Li, Ping; Du, Aijun; Li, Zhen

    2017-01-07

    Developing new materials and technologies for efficient CO2 capture, particularly for separation of CO2 post-combustion, will significantly reduce the CO2 concentration and its impacts on the environment. A challenge for CO2 capture is to obtain high performance adsorbents with both high selectivity and easy regeneration. Here, CO2 capture/regeneration on MoS2 monolayers controlled by turning on/off external electric fields is comprehensively investigated through a density functional theory calculation. The calculated results indicate that CO2 forms a weak interaction with MoS2 monolayers in the absence of an electric field, but strongly interacts with MoS2 monolayers when an electric field of 0.004 a.u. is applied. Moreover, the adsorbed CO2 can be released from the surface of MoS2 without any energy barrier once the electric field is turned off. Compared with the adsorption of CO2, the interactions between N2 and MoS2 are not affected significantly by the external electric fields, which indicates that MoS2 monolayers can be used as a robust absorbent for controllable capture of CO2 by applying an electric field, especially to separate CO2 from the post-combustion gas mixture where CO2 and N2 are the main components.

  13. The Effect of Twin Grain Boundary Tuned by Temperature on the Electrical Transport Properties of Monolayer MoS2

    OpenAIRE

    Luojun Du; Hua Yu; Li Xie; Shuang Wu; Shuopei Wang; Xiaobo Lu; Mengzhou Liao; Jianling Meng; Jing Zhao; Jing Zhang; Jianqi Zhu; Peng Chen; Guole Wang; Rong Yang; Dongxia Shi

    2016-01-01

    Theoretical calculation and experimental measurement have shown that twin grain boundary (GB) of molybdenum disulphide (MoS2) exhibits extraordinary effects on transport properties. Precise transport measurements need to verify the transport mechanism of twin GB in MoS2. Here, monolayer molybdenum disulphide with a twin grain boundary was grown in our developed low-pressure chemical vapor deposition (CVD) system, and we investigated how the twin GB affects the electrical transport properties ...

  14. Photoluminescence of monolayer MoS2 on LaAlO3 and SrTiO3 substrates.

    Science.gov (United States)

    Li, Yuanyuan; Qi, Zeming; Liu, Miao; Wang, Yuyin; Cheng, Xuerui; Zhang, Guobin; Sheng, Liusi

    2014-12-21

    In an atomically thin-film/dielectric-substrate heterostructure, the elemental physical properties of the atomically thin-film are influenced by the interaction between the thin-film and the substrate. In this article, utilizing monolayer MoS(2) on LaAlO(3) and SrTiO(3) substrates, as well as SiO2 and Gel-film as reference substrates similar to previously reported work [Nano Res, 2014, 7, 561], we systematically investigate the substrate effect on the photoluminescence of monolayer MoS(2). We observed significantly substrate-dependant photoluminescence of monolayer MoS(2), originating from substrate-to-film charge transfer. We found that SiO2 substrate introduces the most charge doping while SrTiO(3) introduces less charge transfer. Through the selection of desired substrate, we are able to induce different amounts of charge into the monolayer MoS(2), which consequently modifies the neutral exciton and charged exciton (trion) emissions. Finally, we proposed a band-diagram model to elucidate the relation between charge transfer and the substrate Fermi level and work function. Our work demonstrates that the substrate charge transfer exerts a strong influence on the monolayer MoS(2) photoluminescence property, which should be considered during device design and application. The work also provides a possible route to modify the thin-film photoluminescence property via substrate engineering for future device design.

  15. Valley Hall effect in disordered monolayer MoS2 from first principles

    Science.gov (United States)

    Olsen, Thomas; Souza, Ivo

    2015-09-01

    Electrons in certain two-dimensional crystals possess a pseudospin degree of freedom associated with the existence of two inequivalent valleys in the Brillouin zone. If, as in monolayer MoS2, inversion symmetry is broken and time-reversal symmetry is present, equal and opposite amounts of k -space Berry curvature accumulate in each of the two valleys. This is conveniently quantified by the integral of the Berry curvature over a single valley—the valley Hall conductivity. We generalize this definition to include contributions from disorder described with the supercell approach, by mapping ("unfolding") the Berry curvature from the folded Brillouin zone of the disordered supercell onto the normal Brillouin zone of the pristine crystal, and then averaging over several realizations of disorder. We use this scheme to study from first principles the effect of sulfur vacancies on the valley Hall conductivity of monolayer MoS2. In dirty samples the intrinsic valley Hall conductivity receives gating-dependent corrections that are only weakly dependent on the impurity concentration, consistent with side-jump scattering and the unfolded Berry curvature can be interpreted as a k -space resolved side jump. At low impurity concentrations skew scattering dominates, leading to a divergent valley Hall conductivity in the clean limit. The implications for the recently observed photoinduced anomalous Hall effect are discussed.

  16. Atomic defect states in monolayers of MoS2 and WS2

    Science.gov (United States)

    Salehi, Saboura; Saffarzadeh, Alireza

    2016-09-01

    The influence of atomic vacancy defects at different concentrations on electronic properties of MoS2 and WS2 monolayers is studied by means of Slater-Koster tight-binding model with non-orthogonal sp3d5 orbitals and including the spin-orbit coupling. The presence of vacancy defects induces localized states in the bandgap of pristine MoS2 and WS2, which have potential to modify the electronic structure of the systems, depending on the type and concentration of the defects. It is shown that although the contribution of metal (Mo or W) d orbitals is dominant in the formation of midgap states, the sulphur p and d orbitals have also considerable contribution in the localized states, when metal defects are introduced. Our results suggest that Mo and W defects can turn the monolayers into p-type semiconductors, while the sulphur defects make the system a n-type semiconductor, in agreement with ab initio results and experimental observations.

  17. Evidence for two distinct defect related luminescence features in monolayer MoS2

    Science.gov (United States)

    Saigal, Nihit; Ghosh, Sandip

    2016-09-01

    Apart from the defect related emission peak which lies ˜100 meV below the A exciton/trion peak and is labeled D1 here, this study shows that there is another distinct feature D2 lying ˜200 meV below A in the photoluminescence spectrum of the exfoliated monolayer MoS2 on SiO2/Si substrates. The D2 feature is explicitly resolved at low temperature only in few samples. Both D1 and D2 do not show circular polarization anisotropy for 633 nm excitation. Both decay with the increase in temperature in a seemingly activated manner with similar activation energy of ˜50 meV, but D1 decays earlier and therefore D2 dominates at high temperature in all samples. Annealing in vacuum increases both D1 and D2 emission intensities while annealing under sulfur vapour decreases them. Comparison with reported theoretical studies on defects in monolayer MoS2 suggests that these two emissions possibly involve excitons bound to single and double sulphur vacancies, the latter binding excitons more strongly.

  18. Quantum Mechanical Rippling of a MoS2 Monolayer Controlled by Interlayer Bilayer Coupling

    Science.gov (United States)

    Zheng, Yi; Chen, Jianyi; Ng, M.-F.; Xu, Hai; Liu, Yan Peng; Li, Ang; O'Shea, Sean J.; Dumitricǎ, T.; Loh, Kian Ping

    2015-02-01

    Nanoscale corrugations are of great importance in determining the physical properties of two-dimensional crystals. However, the mechanical behavior of atomically thin films under strain is not fully understood. In this Letter, we show a layer-dependent mechanical response of molybdenum disulfide (MoS2 ) subject to atomistic-precision strain induced by 2 H -bilayer island epitaxy. Dimensional crossover in the mechanical properties is evidenced by the formation of star-shaped nanoripple arrays in the first monolayer, while rippling instability is completely suppressed in the bilayer. Microscopic-level quantum mechanical simulations reveal that the nanoscale rippling is realized by the twisting of neighboring Mo—S bonds without modifying the chemical bond length, and thus invalidates the classical continuum mechanics. The formation of nanoripple arrays significantly changes the electronic and nanotribological properties of monolayer MoS2 . Our results suggest that quantum mechanical behavior is not unique for s p2 bonding but general for atomic membranes under strain.

  19. Analytical insight into the lattice thermal conductivity and heat capacity of monolayer MoS2

    Science.gov (United States)

    Saha, Dipankar; Mahapatra, Santanu

    2016-09-01

    We report, a detailed theoretical study on the lattice thermal conductivity of a suspended monolayer MoS2, far beyond its ballistic limit. The analytical approach adopted in this work mainly relies on the use of Boltzmann transport equation (BTE) within the relaxation time approximation (RTA), along with the first-principles calculations. Considering the relative contributions from the various in-plane and out-of-plane acoustic modes, we derive the closed-form expressions of the mode specific heat capacities, which we later use to obtain the phonon thermal conductivities of the monolayer MoS2. Besides finding the intrinsic thermal conductivity, we also analyse the effect of the phonon-boundary scattering, for different dimensions and edge roughness conditions. The viability of the semi-analytic solution of lattice thermal conductivity reported in this work ranges from a low temperature (T∼30 K) to a significantly high temperature (T∼550 K), and the room temperature (RT) thermal conductivity value has been obtained as 34.06 Wm-1K-1 which is in good agreement with the experimental result.

  20. Adsorption of gas molecules on Cu impurities embedded monolayer MoS2: A first- principles study

    Science.gov (United States)

    Zhao, B.; Li, C. Y.; Liu, L. L.; Zhou, B.; Zhang, Q. K.; Chen, Z. Q.; Tang, Z.

    2016-09-01

    Adsorption of small gas molecules (O2, NO, NO2 and NH3) on transition-metal Cu atom embedded monolayer MoS2 was investigated by first-principles calculations based on the density-functional theory (DFT). The embedded Cu atom is strongly constrained on the sulfur vacancy of monolayer MoS2 with a high diffusion barrier. The stable adsorption geometry, charge transfer and electronic structures of these gas molecules on monolayer MoS2 embedded with transition-metal Cu atom are discussed in detail. It is found that the monolayer MoS2 with embedded Cu atom can effectively capture these gas molecules with high adsorption energy. The NH3 molecule acts as electron donor after adsorption, which is different from the other gas molecules (O2, NO, and NO2). The results suggest that MoS2-Cu system may be promising for future applications in gas molecules sensing and catalysis, which is similar to those of the transition-metal embedded graphene.

  1. Monolayer MoS2 metal insulator transition based memcapacitor modeling with extension to a ternary device

    Science.gov (United States)

    Khan, Abdul Karim; Lee, Byoung Hun

    2016-09-01

    Memcapacitor model based on its one possible physical realization is developed and simulated in order to know its limitation before making a real device. The proposed device structure consists of vertically stacked dielectric layer and MoS2 monolayer between two external metal plates. The Metal Insulator Transition (MIT) phenomenon of MoS2 monolayer is represented in terms of percolation probabilty which is used as the system state. Cluster based site percolation theory is used to mimic the MIT of MoS2 which shows slight discontinuous change in MoS2 monolayer conductivity. The metal to insulator transition switches the capacitance of the device in hysterical way. An Ioffe Regel criterion is used to determine the MIT state of MoS2 monolayer. A good control of MIT time in the range of psec is also achieved by changing a single parameter in the model. The model shows memcapacitive behavior with an edge of fast switching (in psec range) over the previous general models. The model is then extended into vertical cascaded version which behaves like a ternary device instead of binary.

  2. Monolayer MoS2 metal insulator transition based memcapacitor modeling with extension to a ternary device

    Directory of Open Access Journals (Sweden)

    Abdul Karim Khan

    2016-09-01

    Full Text Available Memcapacitor model based on its one possible physical realization is developed and simulated in order to know its limitation before making a real device. The proposed device structure consists of vertically stacked dielectric layer and MoS2 monolayer between two external metal plates. The Metal Insulator Transition (MIT phenomenon of MoS2 monolayer is represented in terms of percolation probabilty which is used as the system state. Cluster based site percolation theory is used to mimic the MIT of MoS2 which shows slight discontinuous change in MoS2 monolayer conductivity. The metal to insulator transition switches the capacitance of the device in hysterical way. An Ioffe Regel criterion is used to determine the MIT state of MoS2 monolayer. A good control of MIT time in the range of psec is also achieved by changing a single parameter in the model. The model shows memcapacitive behavior with an edge of fast switching (in psec range over the previous general models. The model is then extended into vertical cascaded version which behaves like a ternary device instead of binary.

  3. Luminescent monolayer MoS2 quantum dots produced by multi-exfoliation based on lithium intercalation

    Science.gov (United States)

    Qiao, Wen; Yan, Shiming; Song, Xueyin; Zhang, Xing; He, Xueming; Zhong, Wei; Du, Youwei

    2015-12-01

    An effective multi-exfoliation method based on lithium (Li) intercalation has been demonstrated for preparing monolayer molybdenum disulfide (MoS2) quantum dots (QDs). The cutting mechanism of MoS2 QDs may involve the complete breakup around the defects and edges during the reaction of LixMoS2 with water and its following ultrasonication process. The multiply exfoliation make the MoS2 fragile and easier to break up. After the third exfoliation, a large number of monolayer MoS2 QDs is formed. The as-prepared MoS2 QDs show photoluminescence (PL) inactive due to the existence of 1T phase. After heating treatment, the PL intensity excited at 300 nm is enhanced by five times. The MoS2 QDs solution has an excitation-dependent luminescence emission which shifts to longer wavelengths when the excitation wavelength changes from 280 nm to 370 nm. The optical properties are explored based on the quantum confinement and edge effect.

  4. Two photon absorption and its saturation of WS2 and MoS2 monolayer and few-layer films

    CERN Document Server

    Zhang, Saifeng; McEvoy, Niall; O'Brien, Maria; Winters, Sinéad; Berner, Nina C; Yim, Chanyoung; Zhang, Xiaoyan; Chen, Zhanghai; Zhang, Long; Duesberg, Georg S; Wang, Jun

    2015-01-01

    The optical nonlinearity of WS2, MoS2 monolayer and few-layer films was investigated using the Z-scan technique with femtosecond pulses from the visible to the near infrared. The dependence of nonlinear absorption of the WS2 and MoS2 films on layer number and excitation wavelength was studied systematically. WS2 with 1~3 layers exhibits a giant two-photon absorption (TPA) coefficient. Saturation of TPA for WS2 with 1~3 layers and MoS2 with 25~27 layers was observed. The giant nonlinearity of WS2 and MoS2 is attributed to two dimensional confinement, a giant exciton effect and the band edge resonance of TPA.

  5. Carrier and polarization dynamics in monolayer MoS2: temperature and power dependence

    Science.gov (United States)

    Urbaszek, Bernhard; Lagarde, D.; Bouet, L.; Amand, T.; Marie, X.; Zhu, C. R.; Liu, B. L.; Tan, P. H.

    2014-03-01

    In monolayer (ML) MoS2 optical transitions across the direct bandgap are governed by chiral selection rules, allowing optical k-valley initialization. Here we present the first time resolved photoluminescence (PL) polarization measurements in MoS2 MLs, providing vital information on the electron valley dynamics. Using quasi-resonant excitation of the A-exciton transitions, we can infer that the PL decays within τ ~= 4ps. The PL polarization of Pc ~ 60 % remains nearly constant in time for experiments from 4K - 300K, a necessary condition for the success of future Valley Hall experiments. τ does not vary significantly over this temperature range. This is surprising when considering the decrease of Pc in continuous wave experiments when going from 4K to 300K reported in the literature. By tuning the laser following the shift of the A-exciton resonance with temperature we are able to recover at 300K ~ 80 % of the polarization observed at 4K. For pulsed laser excitation, we observe a decrease of Pc with increasing laser power at all temperatures.

  6. Plasmonic Gold Nanorods Coverage Influence on Enhancement of the Photoluminescence of Two-Dimensional MoS2 Monolayer

    KAUST Repository

    Lee, Kevin C. J.

    2015-11-17

    The 2-D transition metal dichalcogenide (TMD) semiconductors, has received great attention due to its excellent optical and electronic properties and potential applications in field-effect transistors, light emitting and sensing devices. Recently surface plasmon enhanced photoluminescence (PL) of the weak 2-D TMD atomic layers was developed to realize the potential optoelectronic devices. However, we noticed that the enhancement would not increase monotonically with increasing of metal plasmonic objects and the emission drop after the certain coverage. This study presents the optimized PL enhancement of a monolayer MoS2 in the presence of gold (Au) nanorods. A localized surface plasmon wave of Au nanorods that generated around the monolayer MoS2 can provide resonance wavelength overlapping with that of the MoS2 gain spectrum. These spatial and spectral overlapping between the localized surface plasmon polariton waves and that from MoS2 emission drastically enhanced the light emission from the MoS2 monolayer. We gave a simple model and physical interpretations to explain the phenomena. The plasmonic Au nanostructures approach provides a valuable avenue to enhancing the emitting efficiency of the 2-D nano-materials and their devices for the future optoelectronic devices and systems.

  7. Photoinduced quantum spin and valley Hall effects, and orbital magnetization in monolayer MoS2

    KAUST Repository

    Tahir, M.

    2014-09-22

    We theoretically demonstrate that 100% valley-polarized transport in monolayers of MoS2 and other group-VI dichalcogenides can be obtained using off-resonant circularly polarized light. By tuning the intensity of the off-resonant light the intrinsic band gap in one valley is reduced, while it is enhanced in the other valley, enabling single valley quantum transport. As a consequence, we predict (i) enhancement of the longitudinal electrical conductivity, accompanied by an increase in the spin polarization of the flowing electrons, (ii) enhancement of the intrinsic spin Hall effect, together with a reduction of the intrinsic valley Hall effect, and (iii) enhancement of the orbital magnetic moment and orbital magnetization. These mechanisms provide appealing opportunities to the design of nanoelectronics based on dichalcogenides.

  8. Prediction of two-dimensional diluted magnetic semiconductors: Doped monolayer MoS2 systems

    KAUST Repository

    Cheng, Yingchun

    2013-03-05

    Using first-principles calculations, we propose a two-dimensional diluted magnetic semiconductor: monolayer MoS2 doped by transition metals. Doping of transition metal atoms from the IIIB to VIB groups results in nonmagnetic states, since the number of valence electrons is smaller or equal to that of Mo. Doping of atoms from the VIIB to IIB groups becomes energetically less and less favorable. Magnetism is observed for Mn, Fe, Co, Zn, Cd, and Hg doping, while for the other dopants from these groups it is suppressed by Jahn-Teller distortions. Analysis of the binding energies and magnetic properties indicates that (Mo,X)S2 (X=Mn, Fe, Co, and Zn) are promising systems to explore two-dimensional diluted magnetic semiconductors.

  9. Spin- and valley-polarized transport in a monolayer of MoS2

    Science.gov (United States)

    Krstajić, P. M.; Vasilopoulos, P.; Tahir, M.

    2016-08-01

    We investigate dc and ac transport in a monolayer of MoS2 in the presence of an effective mass asymmetry, a momentum-dependent term, and an out-of plane or in-plane Zeeman term. Analytical results are presented for both dc and ac conductivities in the framework of linear response theory. We show that the spin-Hall conductivity exhibits a strong dependence on the strength of the spin-orbit interaction while both spin- and valley-Hall conductivities show a very weak dependence on the temperature. The sum of the well separated spin-up and spin-down components of the diffusive dc longitudinal conductivity is linear in the electron concentration though the Fermi level is a nonmonotonic function of it. Further, we evaluate the power absorption spectrum and assess its dependence on the spin and valley degrees of freedom as well as on the scattering which is essential at low frequencies.

  10. Trion-Induced Negative Photoconductivity in Monolayer MoS2

    Science.gov (United States)

    Lui, C. H.; Frenzel, A. J.; Pilon, D. V.; Lee, Y.-H.; Ling, X.; Akselrod, G. M.; Kong, J.; Gedik, N.

    2014-10-01

    Optical excitation typically enhances electrical conduction and low-frequency radiation absorption in semiconductors. We, however, observe a pronounced transient decrease of conductivity in doped monolayer molybdenum disulfide (MoS2), a two-dimensional (2D) semiconductor, using ultrafast optical-pump terahertz-probe spectroscopy. In particular, the conductivity is reduced to only 30% of its equilibrium value at high pump fluence. This anomalous phenomenon arises from the strong many-body interactions in the 2D system, where photoexcited electron-hole pairs join the doping-induced charges to form trions, bound states of two electrons and one hole. The resultant increase of the carrier effective mass substantially diminishes the conductivity.

  11. Intense femtosecond photoexcitation of bulk and monolayer MoS2

    CERN Document Server

    Paradisanos, I; Fotakis, C; Kioseoglou, G; Stratakis, E

    2015-01-01

    The effect of femtosecond laser irradiation on bulk and single-layer MoS2 on silicon oxide is studied. Optical, Field Emission Scanning Electron Microscopy (FESEM) and Raman microscopies were used to quantify the damage. The intensity of A1g and E2g1 vibrational modes was recorded as a function of the number of irradiation pulses. The observed behavior was attributed to laser-induced bond breaking and subsequent atoms removal due to electronic excitations. The single-pulse optical damage threshold was determined for the monolayer and bulk under 800nm and 1030nm pulsed laser irradiation and the role of two-photon versus one photon absorption effects is discussed.

  12. Electron velocity saturation and intervalley transfer in monolayer MoS2

    Science.gov (United States)

    Ferry, D. K.

    2016-11-01

    Monolayer MoS2 is a material with a rich history and that is being suggested for many applications in electronics, including novel electron devices. Recent experiments has shown that it has a saturation velocity at high electric fields well below other electronic materials such as Si. This is a very important property that is crucial to high performance electron devices. Here, we study this property with ensemble Monte Carlo simulations of the electron transport. We find that the velocity at high electric fields is larger than the experiments, and does not show a saturation up to 100 kV cm-1. In addition, the transport at high fields is dominated by inter-valley transfer to the T valleys.

  13. Giant valley drifts in uniaxially strained monolayer MoS2

    KAUST Repository

    Zhang, Qingyun

    2013-12-30

    Using first-principles calculations, we study the electronic structure of monolayer MoS2 under uniaxial strain. We show that the energy valleys drift far off the corners of the Brillouin zone (K points), about 12 times the amount observed in graphene. Therefore, it is essential to take this effect into consideration for a correct identification of the band gap. The system remains a direct band gap semiconductor up to 4% uniaxial strain, while the size of the band gap decreases from 1.73 to 1.54 eV. We also demonstrate that the splitting of the valence bands due to inversion symmetry breaking and spin-orbit coupling is not sensitive to strain.

  14. The Effect of Twin Grain Boundary Tuned by Temperature on the Electrical Transport Properties of Monolayer MoS2

    Directory of Open Access Journals (Sweden)

    Luojun Du

    2016-09-01

    Full Text Available Theoretical calculation and experimental measurement have shown that twin grain boundary (GB of molybdenum disulphide (MoS2 exhibits extraordinary effects on transport properties. Precise transport measurements need to verify the transport mechanism of twin GB in MoS2. Here, monolayer molybdenum disulphide with a twin grain boundary was grown in our developed low-pressure chemical vapor deposition (CVD system, and we investigated how the twin GB affects the electrical transport properties of MoS2 by temperature-dependent transport studies. At low temperature, the twin GB can increase the in-plane electrical conductivity of MoS2 and the transport exhibits variable-range hopping (VRH, while at high temperature, the twin GB impedes the electrical transport of MoS2 and the transport exhibits nearest-neighbor hopping (NNH. Our results elucidate carrier transport mechanism of twin GB and give an important indication of twin GB in tailoring the electronic properties of MoS2 for its applications in next-generation electronics and optoelectronic devices.

  15. Characterization of the second- and third-order nonlinear optical susceptibilities of monolayer MoS2 using multiphoton microscopy

    Science.gov (United States)

    Woodward, R. I.; Murray, R. T.; Phelan, C. F.; de Oliveira, R. E. P.; Runcorn, T. H.; Kelleher, E. J. R.; Li, S.; de Oliveira, E. C.; Fechine, G. J. M.; Eda, G.; de Matos, C. J. S.

    2017-03-01

    We report second- and third-harmonic generation in monolayer MoS2 as a tool for imaging and accurately characterizing the material’s nonlinear optical properties under 1560 nm excitation. Using a surface nonlinear optics treatment, we derive expressions relating experimental measurements to second- and third-order nonlinear sheet susceptibility magnitudes, obtaining values of | {χ }{{s}}(2)| =2.0× {10}-20 m2 V-1 and, for the first time for monolayer MoS2, | {χ }{{s}}(3)| =1.7× {10}-28 m3 V-2. These sheet susceptibilities correspond to effective bulk nonlinear susceptibility values of | {χ }{{b}}(2)| =2.9 × {10}-11 m V-1 and | {χ }{{b}}(3)| =2.4× {10}-19 m2 V-2, accounting for the sheet thickness. Experimental comparisons between MoS2 and graphene are also performed, demonstrating ˜3.4 times stronger third-order sheet nonlinearity in monolayer MoS2, highlighting the material’s potential for nonlinear photonics in the telecommunications C band.

  16. Tuning interlayer coupling in large-area heterostructures with CVD-grown MoS2 and WS2 monolayers.

    Science.gov (United States)

    Tongay, Sefaattin; Fan, Wen; Kang, Jun; Park, Joonsuk; Koldemir, Unsal; Suh, Joonki; Narang, Deepa S; Liu, Kai; Ji, Jie; Li, Jingbo; Sinclair, Robert; Wu, Junqiao

    2014-06-11

    Band offsets between different monolayer transition metal dichalcogenides are expected to efficiently separate charge carriers or rectify charge flow, offering a mechanism for designing atomically thin devices and probing exotic two-dimensional physics. However, developing such large-area heterostructures has been hampered by challenges in synthesis of monolayers and effectively coupling neighboring layers. Here, we demonstrate large-area (>tens of micrometers) heterostructures of CVD-grown WS2 and MoS2 monolayers, where the interlayer interaction is externally tuned from noncoupling to strong coupling. Following this trend, the luminescence spectrum of the heterostructures evolves from an additive line profile where each layer contributes independently to a new profile that is dictated by charge transfer and band normalization between the WS2 and MoS2 layers. These results and findings open up venues to creating new material systems with rich functionalities and novel physical effects.

  17. Engineering the Edges of MoS2 (WS2) Crystals for Direct Exfoliation into Monolayers in Polar Micromolecular Solvents.

    Science.gov (United States)

    Hai, Xiao; Chang, Kun; Pang, Hong; Li, Mu; Li, Peng; Liu, Huimin; Shi, Li; Ye, Jinhua

    2016-11-16

    Synthesizing transition metal dichalcogenide (TMDC) monolayers through the liquid exfoliation of bulk crystals in low boiling point polar micromolecular solvents, such as water, is paramount for their practical application. However, the resulting hydrodynamic forces only appear on the crystal edges due to the mismatch in surface tension between the polar micromolecular solvents and the bulk crystals and are insufficient to overcome the strong van der Waals attraction between adjacent microscale layers. Herein, we present the novel strategy of engineering the lateral size of TMDC (MoS2 and WS2) crystals in the nanoscale to increase the fraction of edges, leading to their direct and ready exfoliation in polar micromolecular solvents, even in pure water, to produce monolayer MoS2 and WS2 nanosheets in high yield. To examine one of their important applications, their catalytic hydrogen evolution activities were evaluated when used as cocatalysts with a photoharvester semiconductor (cadmium sulfide, CdS) in a reaction driven by solar energy. These exfoliated MoS2 (WS2) monolayers exhibited superior cocatalytic performance in the photocatalytic hydrogen evolution reaction (HER). Notably, the cocatalytic performance of monolayer WS2 nanosheets is even higher than that of platinum (Pt), which is a state-of-the-art catalyst for catalytic hydrogen evolution. This work elucidates the importance of decreasing the lateral size of layered crystals to significantly enhance their exfoliability, providing a new strategy for the large-scale preparation of nanoscale TMDC monolayers by liquid exfoliation.

  18. Mo-O bond doping and related-defect assisted enhancement of photoluminescence in monolayer MoS2

    Directory of Open Access Journals (Sweden)

    Xiaoxu Wei

    2014-12-01

    Full Text Available In this work, we report a strong photoluminescence (PL enhancement of monolayer MoS2 under different treatments. We find that by simple ambient annealing treatment in the range of 200 °C to 400 °C, the PL emission can be greatly enhanced by a factor up to two orders of magnitude. This enhancement can be attributed to two factors: first, the formation of Mo-O bonds during ambient exposure introduces an effective p-doping in the MoS2 layer; second, localized electrons formed around Mo-O bonds related defective sites where the electrons can be effectively localized with higher binding energy resulting in efficient radiative excitons recombination. Time resolved PL decay measurement showed that longer lifetime of the treated sample consistent with the higher quantum efficiency in PL. These results give more insights to understand the luminescence properties of the MoS2.

  19. Role of Spin-Orbit Interaction and Impurity Doping in Thermodynamic Properties of Monolayer MoS2

    Science.gov (United States)

    Yarmohammadi, Mohsen

    2016-10-01

    Using linear response theory, a tight-binding Hamiltonian model, and the Green's function technique, the influences of spin-orbit interaction (SOI) and impurity doping on the electronic heat capacity (EHC) and magnetic susceptibility (MS) of monolayer MoS2 have been investigated. The effect of scattering on dilute charged impurities is discussed in terms of the self-consistent Born approximation. We have calculated the temperature dependence of the EHC and MS for different values of SOI, concentration, and scattering strength of dopant impurity. The results show that, in the presence of impurities, the heat capacity of MoS2 decreases (increases) before (after) the Schottky anomaly, as does the MS. It is also found that the EHC and MS of the doped MoS2 reduce with the SOI in all temperature ranges.

  20. Protecting the properties of monolayer MoS2 on silicon based substrates with an atomically thin buffer

    Science.gov (United States)

    Man, Michael K. L.; Deckoff-Jones, Skylar; Winchester, Andrew; Shi, Guangsha; Gupta, Gautam; Mohite, Aditya D.; Kar, Swastik; Kioupakis, Emmanouil; Talapatra, Saikat; Dani, Keshav M.

    2016-02-01

    Semiconducting 2D materials, like transition metal dichalcogenides (TMDs), have gained much attention for their potential in opto-electronic devices, valleytronic schemes, and semi-conducting to metallic phase engineering. However, like graphene and other atomically thin materials, they lose key properties when placed on a substrate like silicon, including quenching of photoluminescence, distorted crystalline structure, and rough surface morphology. The ability to protect these properties of monolayer TMDs, such as molybdenum disulfide (MoS2), on standard Si-based substrates, will enable their use in opto-electronic devices and scientific investigations. Here we show that an atomically thin buffer layer of hexagonal-boron nitride (hBN) protects the range of key opto-electronic, structural, and morphological properties of monolayer MoS2 on Si-based substrates. The hBN buffer restores sharp diffraction patterns, improves monolayer flatness by nearly two-orders of magnitude, and causes over an order of magnitude enhancement in photoluminescence, compared to bare Si and SiO2 substrates. Our demonstration provides a way of integrating MoS2 and other 2D monolayers onto standard Si-substrates, thus furthering their technological applications and scientific investigations.

  1. Tuning Optical Properties of MoS2 Bulk and Monolayer Under Compressive and Tensile Strain: A First Principles Study

    Science.gov (United States)

    Kafi, Fariba; Pilevar Shahri, Raheleh; Benam, Mohammad Reza; Akhtar, Arsalan

    2017-06-01

    Knowledge of the optical properties under compressive and tensile strain is highly important in photoelectron devices and the semiconductor industry. In this work, optical properties of bulk and monolayer MoS2 under compressive and tensile strains are investigated by means of density functional theory. The dielectric tensor is derived within the random phase approximation. Calculations indicate that unstrained two-dimensional and bulk MoS2 lead to semiconductors with the gaps of 1.64 eV and 0.84 eV, respectively, whereas the change in the value of the gap by applying tensile or compressive strain depends on the nature of strains. Dielectric function, absorption coefficient, reflectivity, energy loss and the refraction index of the strained and unstrained systems are studied for both parallel (E||x) and perpendicular (E||z) applied electric field polarizations, which are very sensitive to the type and amount of strains. For instance, the reflectivity of a MoS2 monolayer exposed to visible light in the E||z polarization direction, can be tuned from 4% to 10% by introducing strain. Finally, the Abbe number is calculated to characterize the dispersion of the materials under compressive and tensile strain. Bulk MoS2 for E||x shows the highest value of the Abbe number, which shrinks twenty times under the influence of compressive strain.

  2. Thermal conduction across the one-dimensional interface between a MoS2 monolayer and metal electrode

    Institute of Scientific and Technical Information of China (English)

    Xiangjun Liu; Gang Zhang; Yong-Wei Zhang

    2016-01-01

    The thermal conductance across the one-dimensional (1D) interface between a MoS2 monolayer and Au electrode (edge-contact) has been investigated using molecular dynamics simulations.Although the thermal conductivity of monolayer MoS2 is 2-3 orders of magnitude lower than that of graphene,the covalent bonds formed at the interface enable interfacial thermal conductance (ITC) that is comparable to that of a graphene-metal interface.Each covalent bond at the interface serves as an independent channel for thermal conduction,allowing ITC to be tuned linearly by changing the interfacial bond density (controlling S vacandes).In addition,different Au surfaces form different bonding configurations,causing large ITC variations.Interestingly,the S vacancies in the central region of MoS2 only slightly affect the ITC,which can be explained by a mismatch of the phonon vibration spectra.Further,at room temperature,ITC is primarily dominated by phonon transport,and electron-phonon coupling plays a negligible role.These results not only shed light on the phonon transport mechanisms across 1D metal-MoS2 interfaces,but also provide guidelines for the design and optimization of such interfaces for thermal management in MoS2-based electronicdevices.

  3. Tuning Optical Properties of MoS2 Bulk and Monolayer Under Compressive and Tensile Strain: A First Principles Study

    Science.gov (United States)

    Kafi, Fariba; Pilevar Shahri, Raheleh; Benam, Mohammad Reza; Akhtar, Arsalan

    2017-10-01

    Knowledge of the optical properties under compressive and tensile strain is highly important in photoelectron devices and the semiconductor industry. In this work, optical properties of bulk and monolayer MoS2 under compressive and tensile strains are investigated by means of density functional theory. The dielectric tensor is derived within the random phase approximation. Calculations indicate that unstrained two-dimensional and bulk MoS2 lead to semiconductors with the gaps of 1.64 eV and 0.84 eV, respectively, whereas the change in the value of the gap by applying tensile or compressive strain depends on the nature of strains. Dielectric function, absorption coefficient, reflectivity, energy loss and the refraction index of the strained and unstrained systems are studied for both parallel ( E||x) and perpendicular ( E||z) applied electric field polarizations, which are very sensitive to the type and amount of strains. For instance, the reflectivity of a MoS2 monolayer exposed to visible light in the E||z polarization direction, can be tuned from 4% to 10% by introducing strain. Finally, the Abbe number is calculated to characterize the dispersion of the materials under compressive and tensile strain. Bulk MoS2 for E||x shows the highest value of the Abbe number, which shrinks twenty times under the influence of compressive strain.

  4. Passivation of CdSe Quantum Dots by Graphene and MoS2 Monolayer Encapsulation

    Science.gov (United States)

    Zhang, Datong; Wang, Dennis Zi-Ren; Creswell, Richard C.; Lu, Chenguang; Herman, Irving P.

    The encapsulation of a monolayer of CdSe quantum dots (QDs) by one-to-three layer graphene and MoS2 sheets protects the QDs from oxidation. Photoluminescence (PL) from the QD cores shows a much slower decrease in core diameter over time due to slower oxidation in regions where the QDs are covered by van der Waals (vdW) layers than in those where they are not, for chips stored both in the dark and in the presence of light. PL mapping shows that the CdSe QDs under the central part of the vdW sheet age slower than those near its edges, because oxidation of the covered QDs is limited by transport of oxygen from the edges of the vdW sheets and not transport across the vdW layers. This encapsulation effect is also tested with other environments. Preliminary results show that vdW materials could be promising candidates for nano-coating materials for devices operating in extreme environments.

  5. Large Work Function Modulation of Monolayer MoS2 by Ambient Gases.

    Science.gov (United States)

    Lee, Si Young; Kim, Un Jeong; Chung, JaeGwan; Nam, Honggi; Jeong, Hye Yun; Han, Gang Hee; Kim, Hyun; Oh, Hye Min; Lee, Hyangsook; Kim, Hyochul; Roh, Young-Geun; Kim, Jineun; Hwang, Sung Woo; Park, Yeonsang; Lee, Young Hee

    2016-06-28

    Although two-dimensional monolayer transition-metal dichalcogenides reveal numerous unique features that are inaccessible in bulk materials, their intrinsic properties are often obscured by environmental effects. Among them, work function, which is the energy required to extract an electron from a material to vacuum, is one critical parameter in electronic/optoelectronic devices. Here, we report a large work function modulation in MoS2 via ambient gases. The work function was measured by an in situ Kelvin probe technique and further confirmed by ultraviolet photoemission spectroscopy and theoretical calculations. A measured work function of 4.04 eV in vacuum was converted to 4.47 eV with O2 exposure, which is comparable with a large variation in graphene. The homojunction diode by partially passivating a transistor reveals an ideal junction with an ideality factor of almost one and perfect electrical reversibility. The estimated depletion width obtained from photocurrent mapping was ∼200 nm, which is much narrower than bulk semiconductors.

  6. Outstanding mechanical properties of monolayer MoS2 and its application in elastic energy storage.

    Science.gov (United States)

    Peng, Qing; De, Suvranu

    2013-11-28

    The structural and mechanical properties of graphene-like honeycomb monolayer structures of MoS2 (g-MoS2) under various large strains are investigated using density functional theory (DFT). g-MoS2 is mechanically stable and can sustain extra large strains: the ultimate strains are 0.24, 0.37, and 0.26 for armchair, zigzag, and biaxial deformation, respectively. The in-plane stiffness is as high as 120 N m(-1) (184 GPa equivalently). The third, fourth, and fifth order elastic constants are indispensable for accurate modeling of the mechanical properties under strains larger than 0.04, 0.07, and 0.13 respectively. The second order elastic constants, including in-plane stiffness, are predicted to monotonically increase with pressure while the Poisson ratio monotonically decreases with increasing pressure. With the prominent mechanical properties including large ultimate strains and in-plane stiffness, g-MoS2 is a promising candidate of elastic energy storage for clean energy. It possesses a theoretical energy storage capacity as high as 8.8 MJ L(-1) and 1.7 MJ kg(-1), or 476 W h kg(-1), larger than a Li-ion battery and is environmentally friendly.

  7. Rapid visualization of grain boundaries in monolayer MoS2 by multiphoton microscopy

    Science.gov (United States)

    Karvonen, Lasse; Säynätjoki, Antti; Huttunen, Mikko J.; Autere, Anton; Amirsolaimani, Babak; Li, Shisheng; Norwood, Robert A.; Peyghambarian, Nasser; Lipsanen, Harri; Eda, Goki; Kieu, Khanh; Sun, Zhipei

    2017-06-01

    Grain boundaries have a major effect on the physical properties of two-dimensional layered materials. Therefore, it is important to develop simple, fast and sensitive characterization methods to visualize grain boundaries. Conventional Raman and photoluminescence methods have been used for detecting grain boundaries; however, these techniques are better suited for detection of grain boundaries with a large crystal axis rotation between neighbouring grains. Here we show rapid visualization of grain boundaries in chemical vapour deposited monolayer MoS2 samples with multiphoton microscopy. In contrast to Raman and photoluminescence imaging, third-harmonic generation microscopy provides excellent sensitivity and high speed for grain boundary visualization regardless of the degree of crystal axis rotation. We find that the contrast associated with grain boundaries in the third-harmonic imaging is considerably enhanced by the solvents commonly used in the transfer process of two-dimensional materials. Our results demonstrate that multiphoton imaging can be used for fast and sensitive characterization of two-dimensional materials.

  8. Molecular detection on a defective MoS2 monolayer by simultaneous conductance and force simulations

    Science.gov (United States)

    González, C.; Dappe, Y. J.

    2017-06-01

    Based on simultaneous force and conductance simulations, a proof of concept for a potential method of molecular detection is presented. Using density functional theory calculations, a metallic tip has been approached to different small inorganic molecules such as CO, CO2, H2O , NO, N2, or O2. The molecules have been previously chemisorbed on a defect formed by two Mo atoms occupying a S divacancy on a MoS2 monolayer where they are strongly bonded to the topmost substitutional molybdenum. At that site, the fixed molecules can be imaged by a conductive atomic-force-microscopy tip. Due to the differences in atomic composition and electronic configurations, each molecule yields specific conductance/force curves during the tip approach. A molecule-tip contact is established at the force minimum, followed by the formation of a characteristic plateau in the conductance in most of the cases. Focusing our attention on the position and values of such force minimum and conductance maximum, we can conclude that both characteristic properties can give a clear signature of each molecule, proposing a different method of detecting molecules adsorbed on highly reactive sites.

  9. Controllable growth of monolayer MoS2 by chemical vapor deposition via close MoO2 precursor for electrical and optical applications

    Science.gov (United States)

    Xie, Yong; Wang, Zhan; Zhan, Yongjie; Zhang, Peng; Wu, Ruixue; Jiang, Teng; Wu, Shiwei; Wang, Hong; Zhao, Ying; Nan, Tang; Ma, Xiaohua

    2017-02-01

    MoO2 is used as a new source material for the growth of large area and high optical quality monolayer MoS2. However, a systematic study of the growth parameters is still missing and large-area growth of discreet single crystals is still challenging. Hereby, we report the shape evolution of monolayer growth of MoS2 and develop a methodology to achieve centimeter-scaled discrete MoS2 by adopting MoO2 as Mo source material in an atmospheric-pressure chemical vapor deposition process. Our results indicate the growth of monolayer MoS2 could benefit from the precise control of the introduction time of sulfur and the S/MoO2 ratio in experiments. Micro-Raman and photoluminescence spectra confirm the properties of the material. E-beam lithography was utilized to make contact with the as-grown MoS2 located at the selective area. The electrical properties of MoS2 with different morphologies were compared. In the end, the persistent photoconductivity properties of monolayer MoS2 were emphasized and the underlying mechanism was proposed. These studies demonstrate a better understanding of the growth and application of MoS2-based 2D materials.

  10. Unravelling merging behaviors and electrostatic properties of CVD-grown monolayer MoS2 domains.

    Science.gov (United States)

    Hao, Song; Yang, Bingchu; Gao, Yongli

    2016-08-28

    The presence of grain boundaries is inevitable for chemical vapor deposition (CVD)-grown MoS2 domains owing to various merging behaviors, which greatly limits its potential applications in novel electronic and optoelectronic devices. It is therefore of great significance to unravel the merging behaviors of the synthesized polygon shape MoS2 domains. Here we provide systematic investigations of merging behaviors and electrostatic properties of CVD-grown polycrystalline MoS2 crystals by multiple means. Morphological results exhibit various polygon shape features, ascribed to polycrystalline crystals merged with triangle shape MoS2 single crystals. The thickness of triangle and polygon shape MoS2 crystals is identical manifested by Raman intensity and peak position mappings. Three merging behaviors are proposed to illustrate the formation mechanisms of observed various polygon shaped MoS2 crystals. The combined photoemission electron microscopy and kelvin probe force microscopy results reveal that the surface potential of perfect merged crystals is identical, which has an important implication for fabricating MoS2-based devices.

  11. Impact of Contact on the Operation and Performance of Back-Gated Monolayer MoS2 Field-Effect-Transistors.

    Science.gov (United States)

    Liu, Wei; Sarkar, Deblina; Kang, Jiahao; Cao, Wei; Banerjee, Kaustav

    2015-08-25

    Metal contacts to atomically thin two-dimensional (2D) crystal based FETs play a decisive role in determining their operation and performance. However, the effects of contacts on the switching behavior, field-effect mobility, and current saturation of monolayer MoS2 FETs have not been well explored and, hence, is the focus of this work. The dependence of contact resistance on the drain current is revealed by four-terminal-measurements. Without high-κ dielectric boosting, an electron mobility of 44 cm(2)/(V·s) has been achieved in a monolayer MoS2 FET on SiO2 substrate at room temperature. Velocity saturation is identified as the main mechanism responsible for the current saturation in back-gated monolayer MoS2 FETs at relatively higher carrier densities. Furthermore, for the first time, electron saturation velocity of monolayer MoS2 is extracted at high-field condition.

  12. Effect of biaxial strain and external electric field on electronic properties of MoS2 monolayer: A first-principle study

    Science.gov (United States)

    Nguyen, Chuong V.; Hieu, Nguyen N.

    2016-04-01

    In this work, making use of density functional theory (DFT) computations, we systematically investigate the effect of biaxial strain engineering and external electric field applied perpendicular to the layers on the band gaps and electronic properties of monolayer MoS2. The direct-to-indirect band gaps and semiconductor-to-metal transition are observed in monolayer MoS2 when strain and electric field are applied in our calculation. We show that when the biaxial strain and external electric field are introduced, the electronic properties including band gaps of monolayer MoS2 can be reduced to zero. Our results provide many useful insights for the wide applications of monolayer MoS2 in electronics and optoelectronics.

  13. Facile synthesis of MoS2 and MoxW1-xS2 triangular monolayers

    Directory of Open Access Journals (Sweden)

    Zhong Lin

    2014-09-01

    Full Text Available Single- and few-layered transition metal dichalcogenides, such as MoS2 and WS2, are emerging two-dimensional materials exhibiting numerous and unusual physico-chemical properties that could be advantageous in the fabrication of unprecedented optoelectronic devices. Here we report a novel and alternative route to synthesize triangular monocrystals of MoS2 and MoxW1-xS2 by annealing MoS2 and MoS2/WO3 precursors, respectively, in the presence of sulfur vapor. In particular, the MoxW1-xS2 triangular monolayers show gradual concentration profiles of W and Mo whereby Mo concentrates in the islands’ center and W is more abundant on the outskirts of the triangular monocrystals. These observations were confirmed by atomic force microscopy, and high-resolution transmission electron microscopy, as well as Raman and photoluminescence spectroscopy. The presence of tunable PL signals depending on the MoxW1-xS2 stoichiometries in 2D monocrystals opens up a wide range of applications in electronics and optoelectronics.

  14. Grain size effect of monolayer MoS2 transistors characterized by second harmonic generation mapping

    KAUST Repository

    Lin, Chih-Pin

    2015-08-27

    We investigated different CVD-synthesized MoS2 films, aiming to correlate the device characteristics with the grain size. The grain size of MoS2 can be precisely characterized through nondestructive second harmonic generation mapping based on the degree of inversion symmetry. The devices with larger grains at the channel region show improved on/off current ratio, which can be explained by the less carrier scattering caused by the grain boundaries.

  15. Chemical vapor deposition of monolayer MoS2 directly on ultrathin Al2O3 for low-power electronics

    Science.gov (United States)

    Bergeron, Hadallia; Sangwan, Vinod K.; McMorrow, Julian J.; Campbell, Gavin P.; Balla, Itamar; Liu, Xiaolong; Bedzyk, Michael J.; Marks, Tobin J.; Hersam, Mark C.

    2017-01-01

    Monolayer MoS2 has recently been identified as a promising material for high-performance electronics. However, monolayer MoS2 must be integrated with ultrathin high-κ gate dielectrics in order to realize practical low-power devices. In this letter, we report the chemical vapor deposition (CVD) of monolayer MoS2 directly on 20 nm thick Al2O3 grown by atomic layer deposition (ALD). The quality of the resulting MoS2 is characterized by a comprehensive set of microscopic and spectroscopic techniques. Furthermore, a low-temperature (200 °C) Al2O3 ALD process is developed that maintains dielectric integrity following the high-temperature CVD of MoS2 (800 °C). Field-effect transistors (FETs) derived from these MoS2/Al2O3 stacks show minimal hysteresis with a sub-threshold swing as low as ˜220 mV/decade, threshold voltages of ˜2 V, and current ION/IOFF ratio as high as ˜104, where IOFF is defined as the current at zero gate voltage as is customary for determining power consumption in complementary logic circuits. The system presented here concurrently optimizes multiple low-power electronics figures of merit while providing a transfer-free method of integrating monolayer MoS2 with ultrathin high-κ dielectrics, thus enabling a scalable pathway for enhancement-mode FETs for low-power applications.

  16. Adsorption of alkali, alkaline-earth, simple and 3d transition metal, and nonmetal atoms on monolayer MoS2

    Directory of Open Access Journals (Sweden)

    X. D. Li

    2015-05-01

    Full Text Available Single adsorption of different atoms on pristine two-dimensional monolayer MoS2 have been systematically investigated by using density functional calculations with van der Waals correction. The adatoms cover alkali metals, alkaline earth metals, main group metal, 3d-transition metals, coinage metal and nonmetal atoms. Depending on the adatom type, metallic, semimetallic or semiconducting behavior can be found in direct bandgap monolayer MoS2. Additionally, local or long-range magnetic moments of two-dimensional MoS2 sheet can also attained through the adsorption. The detailed atomic-scale knowledge of single adsorption on MoS2 monolayer is important not only for the sake of a theoretical understanding, but also device level deposition technological application.

  17. Order of magnitude enhancement of monolayer MoS2 photoluminescence due to near-field energy influx from nanocrystal films

    Science.gov (United States)

    Guo, Tianle; Sampat, Siddharth; Zhang, Kehao; Robinson, Joshua A.; Rupich, Sara M.; Chabal, Yves J.; Gartstein, Yuri N.; Malko, Anton V.

    2017-02-01

    Two-dimensional transition metal dichalcogenides (TMDCs) like MoS2 are promising candidates for various optoelectronic applications. The typical photoluminescence (PL) of monolayer MoS2 is however known to suffer very low quantum yields. We demonstrate a 10-fold increase of MoS2 excitonic PL enabled by nonradiative energy transfer (NRET) from adjacent nanocrystal quantum dot (NQD) films. The understanding of this effect is facilitated by our application of transient absorption (TA) spectroscopy to monitor the energy influx into the monolayer MoS2 in the process of ET from photoexcited CdSe/ZnS nanocrystals. In contrast to PL spectroscopy, TA can detect even non-emissive excitons, and we register an order of magnitude enhancement of the MoS2 excitonic TA signatures in hybrids with NQDs. The appearance of ET-induced nanosecond-scale kinetics in TA features is consistent with PL dynamics of energy-accepting MoS2 and PL quenching data of the energy-donating NQDs. The observed enhancement is attributed to the reduction of recombination losses for excitons gradually transferred into MoS2 under quasi-resonant conditions as compared with their direct photoproduction. The TA and PL data clearly illustrate the efficacy of MoS2 and likely other TMDC materials as energy acceptors and the possibility of their practical utilization in NRET-coupled hybrid nanostructures.

  18. Electrical transport and persistent photoconductivity in monolayer MoS2 phototransistors

    Science.gov (United States)

    Di Bartolomeo, Antonio; Genovese, Luca; Foller, Tobias; Giubileo, Filippo; Luongo, Giuseppe; Croin, Luca; Liang, Shi-Jun; Ang, L. K.; Schleberger, Marika

    2017-05-01

    We study electrical transport properties in exfoliated molybdenum disulfide (MoS2) back-gated field effect transistors at low drain bias and under different illumination intensities. It is found that photoconductive and photogating effect as well as space charge limited conduction can simultaneously occur. We point out that the photoconductivity increases logarithmically with the light intensity and can persist with a decay time longer than 104 s, due to photo-charge trapping at the MoS2/SiO2 interface and in MoS2 defects. The transfer characteristics present hysteresis that is enhanced by illumination. At low drain bias, the devices feature low contact resistance of 1.4 {{k}}{{Ω }} μ {{{m}}}-1, ON current as high as 1.25 {{nA}} μ {{{m}}}-1, 105 ON-OFF ratio, mobility of ˜1 cm2 V-1 s-1 and photoresponsivity { R} ≈ 1 {{A}} {{{W}}}-1.

  19. Effect of monolayer supports on the electronic structure of single-layer MoS2

    Science.gov (United States)

    Ramirez-Torres, Alfredo; Le, Duy; Rahman, Talat S.

    2015-03-01

    We present results of density functional theory based calculations of the electronic structure of a single-layer of MoS2, as modified by three different single-layer materials used as support: hexagonal boron nitride (h-BN), graphene, and silicene, using the local density approximation (LDA), on the one hand, and a functional that explicitly includes van der Waals interactions (optB88-vdW), on the other hand. Because the lattice mismatch between the primitive cell of MoS2 and those of the supports is large, calculations are performed with as large a supercell as computationally feasible, so as to reduce the incommensurability between lattices. Even though van der Waals interactions are expected to play an important role in the binding between MoS2, and h-BN and graphene, we find that the band structure and related conclusions obtained by optB88-vdW and LDA are quite similar for the three heterostructures considered here. Single-layer MoS2 interacts weakly with h-BN and graphene, while covalent bonds are formed with silicene. Detailed analysis of the electronic density of states also indicates little effect of h-BN and graphene, while silicene severely modifies it by introducing additional states within the band gap. Furthermore, adsorption on graphene brings the conduction bands of MoS2 down to the Fermi level of graphene as a result of charge transfer from graphene to MoS2, while adsorption on silicene shifts both valence and conduction bands towards the Fermi level, in addition to inducing a gap of ~50 meV in silicene itself.

  20. Vibrational and optical properties of MoS2: From monolayer to bulk

    Science.gov (United States)

    Molina-Sánchez, Alejandro; Hummer, Kerstin; Wirtz, Ludger

    2015-12-01

    Molybdenum disulfide, MoS2, has recently gained considerable attention as a layered material where neighboring layers are only weakly interacting and can easily slide against each other. Therefore, mechanical exfoliation allows the fabrication of single and multi-layers and opens the possibility to generate atomically thin crystals with outstanding properties. In contrast to graphene, it has an optical gap of ~1.9 eV. This makes it a prominent candidate for transistor and opto-electronic applications. Single-layer MoS2 exhibits remarkably different physical properties compared to bulk MoS2 due to the absence of interlayer hybridization. For instance, while the band gap of bulk and multi-layer MoS2 is indirect, it becomes direct with decreasing number of layers. In this review, we analyze from a theoretical point of view the electronic, optical, and vibrational properties of single-layer, few-layer and bulk MoS2. In particular, we focus on the effects of spin-orbit interaction, number of layers, and applied tensile strain on the vibrational and optical properties. We examine the results obtained by different methodologies, mainly ab initio approaches. We also discuss which approximations are suitable for MoS2 and layered materials. The effect of external strain on the band gap of single-layer MoS2 and the crossover from indirect to direct band gap is investigated. We analyze the excitonic effects on the absorption spectra. The main features, such as the double peak at the absorption threshold and the high-energy exciton are presented. Furthermore, we report on the the phonon dispersion relations of single-layer, few-layer and bulk MoS2. Based on the latter, we explain the behavior of the Raman-active A1g and E2g1 modes as a function of the number of layers. Finally, we compare theoretical and experimental results of Raman, photoluminescence, and optical-absorption spectroscopy.

  1. The Effect of VMoS3 Point Defect on the Elastic Properties of Monolayer MoS2 with REBO Potentials.

    Science.gov (United States)

    Li, Minglin; Wan, Yaling; Tu, Liping; Yang, Yingchao; Lou, Jun

    2016-12-01

    Structural defects in monolayer molybdenum disulfide (MoS2) have significant influence on the electric, optical, thermal, chemical, and mechanical properties of the material. Among all the types of structural defects of the chemical vapor phase-grown monolayer MoS2, the VMoS3 point defect (a vacancy complex of Mo and three nearby S atoms) is another type of defect preferentially generated by the extended electron irradiation. Here, using the classical molecular dynamics simulation with reactive empirical bond-order (REBO) potential, we first investigate the effect of VMoS3 point defects on the elastic properties of monolayer MoS2 sheets. Under the constrained uniaxial tensile test, the elastic properties of monolayer MoS2 sheets containing VMoS3 vacancies with defect fraction varying from 0.01 to 0.1 are obtained based on the plane anisotropic constitutive relations of the material. It is found that the increase of VMoS3 vacancy concentration leads to the noticeable decrease in the elastic modulus but has a slight effect on Poisson's ratio. The maximum decrease of the elastic modulus is up to 25 %. Increasing the ambient temperature from 10 K to 500 K has trivial influences on the elastic modulus and Poisson's ratio for the monolayer MoS2 without defect and with 5 % VMoS3 vacancies. However, an anomalous parabolic relationship between the elastic modulus and the temperature is found in the monolayer MoS2 containing 0.1 % VMoS3 vacancy, bringing a crucial and fundamental issue to the application of monolayer MoS2 with defects.

  2. The pristine atomic structure of MoS2 monolayer protected from electron radiation damage by graphene

    Science.gov (United States)

    Algara-Siller, Gerardo; Kurasch, Simon; Sedighi, Mona; Lehtinen, Ossi; Kaiser, Ute

    2013-11-01

    Materials can, in principle, be imaged at the level of individual atoms with aberration-corrected transmission electron microscopy. However, such resolution can be attained only with very high electron doses. Consequently, radiation damage is often the limiting factor when characterizing sensitive materials. Here, we demonstrate a simple and an effective method to increase the electron radiation tolerance of materials by using graphene as protective coating. This leads to an improvement of three orders of magnitude in the radiation tolerance of monolayer MoS2. Further on, we construct samples in different heterostructure configurations to separate the contributions of different radiation damage mechanisms.

  3. Origin of the n -type and p -type conductivity of MoS 2 monolayers on a SiO 2 substrate

    KAUST Repository

    Dolui, Kapildeb

    2013-04-02

    Ab initio density functional theory calculations are performed to study the electronic properties of a MoS2 monolayer deposited over a SiO 2 substrate in the presence of interface impurities and defects. When MoS2 is placed on a defect-free substrate, the oxide plays an insignificant role since the conduction band top and the valence band minimum of MoS2 are located approximately in the middle of the SiO2 band gap. However, if Na impurities and O dangling bonds are introduced at the SiO2 surface, these lead to localized states, which modulate the conductivity of the MoS2 monolayer from n- to p-type. Our results show that the conductive properties of MoS2 deposited on SiO 2 are mainly determined by the detailed structure of the MoS 2/SiO2 interface, and suggest that doping the substrate can represent a viable strategy for engineering MoS2-based devices. © 2013 American Physical Society.

  4. Monolayer MoS2 Nanoribbons as a Promising Material for Both n-type and p-type Legs in Thermoelectric Generators

    Science.gov (United States)

    Arab, A.; Davydov, A. V.; Papaconstantopoulos, D. A.; Li, Q.

    2016-10-01

    First-principles calculations have been performed to study the thermoelectric properties of monolayer MoS2 armchair nanoribbons (ACNRs). The electronic behavior of nanoribbons is dominated by the presence of edge states that are dependent on the number of zigzag chains across the nanoribbon. In addition, it is found that the phonon thermal conductance of monolayer MoS2 ACNRs is smaller than monolayer films due to phonon edge scattering. This effect is more pronounced in narrower nanoribbons, which leads to a higher ZT value compared to a monolayer MoS2 sheet. The effects of sulfur vacancy and edge roughness on the thermoelectric properties of MoS2 ACNRs have also been studied. We found that edge roughness decreased ZT values compared to those of perfect nanoribbons, as its impact on electrical conductance is more severe than on phonon thermal conductance. Sulfur vacancy, however, improved ZT in some subbands. It is shown that ZT values as high as 4 for electron-doped and 3 for hole-doped nanoribbons can be achieved at T = 500 K. The ability to achieve high ZT values for both p-type and n-type nanoribbons makes monolayer MoS2 ACNR a promising candidate for future solid-state thermoelectric generators.

  5. Quantum Transport and Observation of Dyakonov-Perel Spin-Orbit Scattering in Monolayer MoS_{2}.

    Science.gov (United States)

    Schmidt, H; Yudhistira, I; Chu, L; Castro Neto, A H; Özyilmaz, B; Adam, S; Eda, G

    2016-01-29

    Monolayers of group 6 transition metal dichalcogenides are promising candidates for future spin-, valley-, and charge-based applications. Quantum transport in these materials reflects a complex interplay between real spin and pseudospin (valley) relaxation processes, which leads to either positive or negative quantum correction to the classical conductivity. Here we report experimental observation of a crossover from weak localization to weak antilocalization in highly n-doped monolayer MoS_{2}. We show that the crossover can be explained by a single parameter associated with electron spin lifetime of the system. At low temperatures and high carrier densities, the spin lifetime is inversely proportional to momentum relaxation time; this indicates that spin relaxation occurs via a Dyakonov-Perel mechanism.

  6. Width-dependent structural stability and magnetic properties of monolayer zigzag MoS2 nanoribbons

    Science.gov (United States)

    Wen, Yan-Ni; Gao, Peng-Fei; Chen, Xi; Xia, Ming-Gang; Zhang, Yang; Zhang, Sheng-Li

    2017-01-01

    First-principles study based on density functional theory has been employed to investigate width-dependent structural stability and magnetic properties of monolayer zigzag MoS2 nanoribbons (ZZ-MoS2 NRs). The width N = 4-6 (the numbers of zigzag Mo-S chains along the ribbon length) are considered. The results show that all studied ZZ-MoS2 NRs are less stable than two-dimensional MoS2 monolayer, exhibiting that a broader width ribbon behaves better structural stability and an inversely proportional relationship between the structural stability (or the ribbon with) and boundary S-Mo interaction. Electronic states imply that all ZZ-MoS2 NRs exhibit magnetic properties, regardless of their widths. Total magnetic moment increases with the increasing width N, which is mainly ascribed to the decreasing S-Mo interaction of the two zigzag edges. In order to confirm this reason, a uniaxial tension strain is applied to ZZ-MoS2 NRs. It has been found that, with the increasing tension strain, the bond length of boundary S-Mo increases, at the same time, the magnetic moment increases also. Our results suggest the rational applications of ZZ-MoS2 NRs in nanoelectronics and spintronics.

  7. Slow cooling and efficient extraction of C-exciton hot carriers in MoS2 monolayer

    Science.gov (United States)

    Wang, Lei; Wang, Zhuo; Wang, Hai-Yu; Grinblat, Gustavo; Huang, Yu-Li; Wang, Dan; Ye, Xiao-Hui; Li, Xian-Bin; Bao, Qiaoliang; Wee, Andrewthye-Shen; Maier, Stefan A.; Chen, Qi-Dai; Zhong, Min-Lin; Qiu, Cheng-Wei; Sun, Hong-Bo

    2017-01-01

    In emerging optoelectronic applications, such as water photolysis, exciton fission and novel photovoltaics involving low-dimensional nanomaterials, hot-carrier relaxation and extraction mechanisms play an indispensable and intriguing role in their photo-electron conversion processes. Two-dimensional transition metal dichalcogenides have attracted much attention in above fields recently; however, insight into the relaxation mechanism of hot electron-hole pairs in the band nesting region denoted as C-excitons, remains elusive. Using MoS2 monolayers as a model two-dimensional transition metal dichalcogenide system, here we report a slower hot-carrier cooling for C-excitons, in comparison with band-edge excitons. We deduce that this effect arises from the favourable band alignment and transient excited-state Coulomb environment, rather than solely on quantum confinement in two-dimension systems. We identify the screening-sensitive bandgap renormalization for MoS2 monolayer/graphene heterostructures, and confirm the initial hot-carrier extraction for the C-exciton state with an unprecedented efficiency of 80%, accompanied by a twofold reduction in the exciton binding energy.

  8. Low Power Monolayer MoS2 Transistors for RF Applications

    Science.gov (United States)

    2015-03-24

    contrast, Raman spectroscopy, and atomic force microscopy (AFM) images. Device active regions were defined using e-beam lithography ( EBL ). Excess MoS...2 was etched using Cl2 plasma. Next, source/drain metal electrodes were defined with EBL . A stack of Ag/Au (20 nm/30 nm) was deposited as low-work...30 nm thick layer of HfOx as the top gate dielectric. The top gate electrode was then defined using a final EBL step. The top gate metal fingers

  9. Spin and valley-dependent electron transport through arrays of ferromagnet on monolayer MoS2

    Science.gov (United States)

    Qiu, X. J.; Cao, Z. Z.; Cheng, Y. F.; Qin, C. C.

    2017-03-01

    We theoretically study ballistic transport of Dirac fermions in MoS2 junction through arrays of barriers, of width L , in the presence of a tunable potential of height U and an exchange field h . The charge conductance as functions of U and h , exhibits more conspicuous and sharpened oscillation as the number of barriers increase, due to the contribution of evanescent modes near the edges of the extremum conductance which are exponentially suppressed or enhanced. Furthermore, we found the valley-resolved conductance exhibits a similar oscillating behavior as the charge conductance for multiple barriers, but with inverse oscillatory phases for {{G}K} and {{G}{{K'}}} , accordingly, a high-efficiency fully valley polarized device is proposed in our system. Also, a perfect 100% spin polarized conductance is observed for 4 barriers and the polarized direction can be switched by changing the direction of exchange field. These findings not only benefit understanding of basic physics in monolayers MoS2, but also provide us a new way to generate a pure and high-efficiency spintronics and valleytronics.

  10. First-principles investigations of metal (V, Nb, Ta)-doped monolayer MoS2: Structural stability, electronic properties and adsorption of gas molecules

    Science.gov (United States)

    Zhu, Jia; Zhang, Hui; Tong, Yawen; Zhao, Ling; Zhang, Yongfan; Qiu, Yuzhi; Lin, Xianning

    2017-10-01

    Two-dimensional (2D) layered materials are at the forefront of research because of their unique structures and promising catalytic abilities. Here, the structural stability, electronic properties and gas adsorption of metal (V, Nb, Ta)-doped monolayer MoS2 have been investigated by density functional theory calculations. Our results show that the metal (V, Nb, Ta)-doped monolayer MoS2 is a stable catalyst under room temperature, due to the strong interaction between the doped metals (V, Nb, Ta) and S vacancy of monolayer MoS2. Compared with the gas adsorption (CO, NO2, H2O, NH3) on pristine monolayer MoS2, doped metal (V, Nb, Ta) can significantly improve the adsorption properties, chemical activity and the sensitivity of that of adsorbed gas molecules. This effect occurs due to the strong overlap between the metal nd orbitals and gas molecule orbitals, result in activation of the adsorbed gas molecules. Analysis of Bader charge shows that, more charge transfer (-0.66 e to -0.72 e) occur from metal (V, Nb, Ta)-doped monolayer MoS2 to the oxidizing gas molecules (NO2) acting as acceptors. While for the adsorption of CO molecules, the relative less electrons (about -0.24 e - -0.35 e) transfer occuring from substrate to the adsorbed gases. Whereas the direction of charge transfers is reversed for the adsorption of the reducing gas (H2O and NH3) behaving as donors, in which small electrons (0.04 e -0.09 e) transfer from adsorbed gas to metal (V, Nb, Ta)-doped monolayer MoS2. Our results suggested that metal (V, Nb, Ta)-doped monolayer MoS2 might be a good candidate for low-cost, highly active, and stable catalysts and gas sensors, providing an avenue to facilitate the design of high active MoS2-based two dimensional catalysts and gas sensors.

  11. Large area growth of monolayer MoS2 film on quartz and its use as a saturable absorber in laser mode-locking

    Science.gov (United States)

    Zhao, Wei-fang; Yu, Hua; Liao, Meng-zhou; Zhang, Ling; Zou, Shu-zhen; Yu, Hai-juan; He, Chao-jian; Zhang, Jing-yuan; Zhang, Guang-yu; Lin, Xue-chun

    2017-02-01

    Monolayer MoS2 film on quartz was fabricated by a home-made three-temperature zone chemical vapor deposition method. The photo, AFM image, Raman spectroscopy and HRTEM image showed that high quality as-grown MoS2 film completely covered the whole quartz substrate of a few cm2. A Nd:YVO4 laser with mode-locking operation was obtained by using the monolayer MoS2 on quartz as the saturable absorber (SA). To the best of our knowledge, this is the first report on large-area growth of high quality monolayer MoS2 film on transparent quartz substrate, and the first time that the CVD MoS2 SA was used in mode-locked solid state lasers. Because of the large area, high transmission, low non-saturable loss and high optical damage threshold of this material, it is very suitable for application in mode-locked solid state lasers.

  12. Growth of monolayer MoS2 films in a quasi-closed crucible encapsulated substrates by chemical vapor deposition

    Science.gov (United States)

    Yang, Yong; Pu, Hongbin; Lin, Tao; Li, Lianbi; Zhang, Shan; Sun, Gaopeng

    2017-07-01

    Monolayer molybdenum disulfide (m-MoS2) has attracted significant interest due to its unique electronic and optical properties. Herein, we report the successful fabrication of high quality and continuous m-MoS2 films in a quasi-closed crucible encapsulated substrates via a three-zone chemical vapor deposition (CVD) system. Quasi-closed crucible lowers the concentration of precursors around substrates and makes the sulfurization rate gentle, which is beneficial for invariable m-MoS2 growth. Characterization results indicate that as-grown m-MoS2 films are of high crystallinity and high quality comparable to the exfoliated MoS2. This approach is also adapted to the growth of other transition metal dichalcogenides.

  13. Spin and valley polarization in MoS2, MoSe2, and WSe2 monolayers (Conference Presentation)

    Science.gov (United States)

    Robert, Cedric; Wang, Gang; Cadiz, Fabian; Balocchi, Andréa.; Glazov, Mikhail M.; Amand, Thierry; Gerber, Iann; Lagarde, Delphine; Urbaszek, Bernhard; Marie, Xavier

    2016-10-01

    The spectacular progress in controlling the electronic properties of graphene has triggered research in alternative atomically thin two-dimensional crystals. Monolayers (ML) of transition-metal dichalcogenides such as MoS2 have emerged as very promising nanostructures for optical and spintronics applications. Inversion symmetry breaking together with the large spin-orbit interaction leads to a coupling of carrier spin and k-space valley physics, i.e., the circular polarization (σ+ or σ-) of the absorbed or emitted photon can be directly associated with selective carrier excitation in one of the two nonequivalent K valleys (K+ or K-, respectively). We have investigated the spin and valley properties for both neutral and charged excitons in transition metal dichalcogenide monolayer MoS2, MoSe2 and WSe2 with cw and time-resolved polarized photoluminescence spectroscopy [1,2]. The key role played by exciton exchange interaction will be presented [3]. We also demonstrate that the optical alignment of excitons ("exciton valley coherence") can be achieved following one or two photon excitation [1,4]. Finally recent results on magneto-photoluminescence spectroscopy on MoSe2 and WSe2 in Faraday configuration up to 9 T will be presented; the results will be discussed in the framework of a k.p theory [5]. [1] G. Wang et al, PRL 114, 97403 (2015) [2] G. Wang et al, Nature Com. 6, 10110 (2015) [3] J. P. Echeverry, ArXiv 1601.07351 (2016) [4] G. Wang et al, PRL 115, 117401 (2015) [5] G. Wang et al, 2D Mat. 2, 34002 (2015)

  14. Enhancement of broad-band light absorption in monolayer MoS2 using Ag grating hybrid with distributed Bragg reflector

    Science.gov (United States)

    Cao, Jintao; Wang, Jin; Yang, Guofeng; Lu, Yann; Sun, Rui; Yan, Pengfei; Gao, Shumei

    2017-10-01

    A hybrid novel structure of monolayer MoS2 with Ag nanograting and DBR on Si substrate has been proposed to obtain broad-band absorption response for two-dimensional (2D) materials. It is effective to reduce light loss and reflect the incident light efficiently for monolayer MoS2 absorption with DBR dielectric layers. Moreover, by combining Ag nanograting with DBR structure, the average absorption achieves as high as 59% within broad wavelength ranging from 420 to 700 nm, which is attributed to the plasmonic resonant effect of metal nanostripes. The absorption would be affected by the duty ratio and period of the Ag nanograting, and shows incident angle dependent characteristics, while an average absorption higher than 60% has been obtained at the incident angle around 40°. These results indicate that 2D MoS2 in combination with DBR and metal nanograting have a promising potential applications for optical nano-devices.

  15. Substrate effect on the growth of monolayer dendritic MoS2 on LaAlO3 (100) and its electrocatalytic applications

    Science.gov (United States)

    Li, Cong; Zhang, Yu; Ji, Qingqing; Shi, Jianping; Chen, Zhaolong; Zhou, Xiebo; Fang, Qiyi; Zhang, Yanfeng

    2016-09-01

    In accommodating the rapid development of two-dimensional (2D) nanomaterials, chemical vapor deposition (CVD) has become a powerful tool for their batch production with desirable characteristics, such as high crystal quality, large domain size, and tunable domain shape. The crystallinity and morphology of the growth substrates usually play a crucial role in the CVD synthesis of high-quality monolayer MoS2, a kind of 2D layered material which has ignited huge interest in nanoelectronics, optoelectronics and energy harvesting, etc. Herein, by utilizing a low-pressure chemical vapor deposition (LPCVD) system, we demonstrate a regioselective synthesis of monolayer MoS2 on the corrugated single-crystal LaAlO3 (100) with twin crystal domains induced by the second-order phase transition. Unique dendritic morphologies with tunable nucleation densities were obtained in different regions of the undulated substrate, presenting a strong substrate modulation effect. Interestingly, the exposure of abundant active edge sites along with the rather high nucleation density makes the monolayer dendritic MoS2 a good electrocatalyst for hydrogen evolution reaction (HER), particularly featured by a rather high exchange current density (70.4 μA cm-2). Furthermore, uniform monolayer MoS2 films can also be obtained and transferred to arbitrary substrates. We believe that this work provides a new growth system for the controllable synthesis of 2D layered materials with unique dendritic morphologies, as well as its great application potential in energy conversion and harvesting.

  16. Exciton diamagnetic shifts and valley Zeeman effects in monolayer WS2 and MoS2 to 65 Tesla

    Science.gov (United States)

    Stier, Andreas V.; McCreary, Kathleen M.; Jonker, Berend T.; Kono, Junichiro; Crooker, Scott A.

    2016-02-01

    In bulk and quantum-confined semiconductors, magneto-optical studies have historically played an essential role in determining the fundamental parameters of excitons (size, binding energy, spin, dimensionality and so on). Here we report low-temperature polarized reflection spectroscopy of atomically thin WS2 and MoS2 in high magnetic fields to 65 T. Both the A and B excitons exhibit similar Zeeman splittings of approximately -230 μeV T-1 (g-factor ~=-4), thereby quantifying the valley Zeeman effect in monolayer transition-metal disulphides. Crucially, these large fields also allow observation of the small quadratic diamagnetic shifts of both A and B excitons in monolayer WS2, from which radii of ~1.53 and ~1.16 nm are calculated. Further, when analysed within a model of non-local dielectric screening, these diamagnetic shifts also constrain estimates of the A and B exciton binding energies (410 and 470 meV, respectively, using a reduced A exciton mass of 0.16 times the free electron mass). These results highlight the utility of high magnetic fields for understanding new two-dimensional materials.

  17. Photonic Band Gap in 1D Multilayers Made by Alternating SiO2 or PMMA with monolayer MoS2 or WS2

    CERN Document Server

    del Valle, Diana Gisell Figueroa; Scotognella, Francesco

    2015-01-01

    Atomically thin molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are very interesting two dimensional materials for optics and electronics. In this work we show the possibility to obtain one-dimensional photonic crystals consisting of low-cost and easy processable materials, as silicon dioxide (SiO2) or poly methyl methacrylate (PMMA), and monolayers of MoS2 or WS2. We have simulated the transmission spectra of the photonic crystals using the transfer matrix method and employing the wavelength dependent refractive indexes of the materials. This study envisages the experimental fabrication of these new types of photonic crystals for photonic and light emission applications.

  18. Electron dynamics and optical properties modulation of monolayer MoS2 by femtosecond laser pulse: a simulation using time-dependent density functional theory

    Science.gov (United States)

    Su, Xiaoxing; Jiang, Lan; Wang, Feng; Su, Gaoshi; Qu, Liangti; Lu, Yongfeng

    2017-07-01

    In this study, we adopted time-dependent density functional theory to investigate the optical properties of monolayer MoS2 and the effect of intense few-cycle femtosecond laser pulses on these properties. The electron dynamics of monolayer MoS2 under few-cycle and multi-cycle laser irradiation were described. The polarization direction of the laser had a marked effect on the energy absorption and electronic excitation of monolayer MoS2 because of anisotropy. Change in the polarization direction of few-cycle pulse changed the absorbed energy by a factor over 4000. Few-cycle pulse showed a higher sensitivity to the electronic property of material than multi-cycle pulse. The modulation of the dielectric properties of the material was observed on the femtosecond time scale. The negative divergence appeared in the real part of the function at low frequencies and photoinduced blue shift occurred due to Burstein-Moss effect. The irradiation of femtosecond laser caused the dielectric response within the infrared region and introduced anisotropy to the in-plane optical properties. Laser-based engineering of optical properties through controlling transient electron dynamics expands the functionality of MoS2 and has potential applications in direction-dependent optoelectronic devices.

  19. The adsorption of CO and NO on the MoS2 monolayer doped with Au, Pt, Pd, or Ni: A first-principles study

    Science.gov (United States)

    Ma, Dongwei; Ju, Weiwei; Li, Tingxian; Zhang, Xiwei; He, Chaozheng; Ma, Benyuan; Lu, Zhansheng; Yang, Zongxian

    2016-10-01

    By performing the first-principles calculation, the adsorption of CO and NO molecules on the Au, Pt, Pd, or Ni doped MoS2 monolayer has been studied. The interaction between CO or NO with the doped MoS2 monolayer is strong and belongs to the chemisorption, as evidenced by the large adsorption energy and the short distance between the adsorbed molecules and the dopants. The charge transfer and the electronic property induced by the molecule adsorption are discussed. It is found that for both CO and NO adsorption, for all the cases charge transfer between the substrates and the adsorbed molecules has been observed. For NO, the adsorption obviously induces new impurity states in the band gap or the redistribution of the original impurity states. These can lead to the change of the transport properties of the doped MoS2 monolayer, by which the adsorbed CO or NO can be detected. The present work shows that introducing appropriate dopants may be a feasible method to improve the performance of MoS2-based gas sensors.

  20. Anomalous lattice vibrations of monolayer MoS 2 probed by ultraviolet Raman scattering

    KAUST Repository

    Liu, Hsiang Lin

    2015-01-01

    We present a comprehensive Raman scattering study of monolayer MoS2 with increasing laser excitation energies ranging from the near-infrared to the deep-ultraviolet. The Raman scattering intensities from the second-order phonon modes are revealed to be enhanced anomalously by only the ultraviolet excitation wavelength 354 nm. We demonstrate theoretically that such resonant behavior arises from a strong optical absorption that forms near the Γ point and of the band structure and an inter-valley resonant electronic scattering by the M-point phonons. These results advance our understanding of the double resonance Raman scattering process in low-dimensional semiconducting nanomaterials and provide a foundation for the technological development of monolayer MoS2 in the ultraviolet frequency range. © the Owner Societies 2015.

  1. Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface

    Science.gov (United States)

    Li, Ming-Yang; Shi, Yumeng; Cheng, Chia-Chin; Lu, Li-Syuan; Lin, Yung-Chang; Tang, Hao-Lin; Tsai, Meng-Lin; Chu, Chih-Wei; Wei, Kung-Hwa; He-Hau, Jr.; Chang, Wen-Hao; Suenaga, Kazu; Li, Lain-Jong

    2015-07-01

    Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS2 and tungsten sulfide WSe2 have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe2-MoS2 heterojunction, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.

  2. Impact and Origin of Interface States in MOS Capacitor with Monolayer MoS2 and HfO2 High-k Dielectric

    Science.gov (United States)

    Xia, Pengkun; Feng, Xuewei; Ng, Rui Jie; Wang, Shijie; Chi, Dongzhi; Li, Cequn; He, Zhubing; Liu, Xinke; Ang, Kah-Wee

    2017-01-01

    Two-dimensional layered semiconductors such as molybdenum disulfide (MoS2) at the quantum limit are promising material for nanoelectronics and optoelectronics applications. Understanding the interface properties between the atomically thin MoS2 channel and gate dielectric is fundamentally important for enhancing the carrier transport properties. Here, we investigate the frequency dispersion mechanism in a metal-oxide-semiconductor capacitor (MOSCAP) with a monolayer MoS2 and an ultra-thin HfO2 high-k gate dielectric. We show that the existence of sulfur vacancies at the MoS2-HfO2 interface is responsible for the generation of interface states with a density (Dit) reaching ~7.03 × 1011 cm−2 eV−1. This is evidenced by a deficit S:Mo ratio of ~1.96 using X-ray photoelectron spectroscopy (XPS) analysis, which deviates from its ideal stoichiometric value. First-principles calculations within the density-functional theory framework further confirms the presence of trap states due to sulfur deficiency, which exist within the MoS2 bandgap. This corroborates to a voltage-dependent frequency dispersion of ~11.5% at weak accumulation which decreases monotonically to ~9.0% at strong accumulation as the Fermi level moves away from the mid-gap trap states. Further reduction in Dit could be achieved by thermally diffusing S atoms to the MoS2-HfO2 interface to annihilate the vacancies. This work provides an insight into the interface properties for enabling the development of MoS2 devices with carrier transport enhancement. PMID:28084434

  3. Impact and Origin of Interface States in MOS Capacitor with Monolayer MoS2 and HfO2 High-k Dielectric

    Science.gov (United States)

    Xia, Pengkun; Feng, Xuewei; Ng, Rui Jie; Wang, Shijie; Chi, Dongzhi; Li, Cequn; He, Zhubing; Liu, Xinke; Ang, Kah-Wee

    2017-01-01

    Two-dimensional layered semiconductors such as molybdenum disulfide (MoS2) at the quantum limit are promising material for nanoelectronics and optoelectronics applications. Understanding the interface properties between the atomically thin MoS2 channel and gate dielectric is fundamentally important for enhancing the carrier transport properties. Here, we investigate the frequency dispersion mechanism in a metal-oxide-semiconductor capacitor (MOSCAP) with a monolayer MoS2 and an ultra-thin HfO2 high-k gate dielectric. We show that the existence of sulfur vacancies at the MoS2-HfO2 interface is responsible for the generation of interface states with a density (Dit) reaching ~7.03 × 1011 cm-2 eV-1. This is evidenced by a deficit S:Mo ratio of ~1.96 using X-ray photoelectron spectroscopy (XPS) analysis, which deviates from its ideal stoichiometric value. First-principles calculations within the density-functional theory framework further confirms the presence of trap states due to sulfur deficiency, which exist within the MoS2 bandgap. This corroborates to a voltage-dependent frequency dispersion of ~11.5% at weak accumulation which decreases monotonically to ~9.0% at strong accumulation as the Fermi level moves away from the mid-gap trap states. Further reduction in Dit could be achieved by thermally diffusing S atoms to the MoS2-HfO2 interface to annihilate the vacancies. This work provides an insight into the interface properties for enabling the development of MoS2 devices with carrier transport enhancement.

  4. Temperature-dependent phonon shifts in monolayer MoS2

    Science.gov (United States)

    Lanzillo, Nicholas A.; Glen Birdwell, A.; Amani, Matin; Crowne, Frank J.; Shah, Pankaj B.; Najmaei, Sina; Liu, Zheng; Ajayan, Pulickel M.; Lou, Jun; Dubey, Madan; Nayak, Saroj K.; O'Regan, Terrance P.

    2013-08-01

    We present a combined experimental and computational study of two-dimensional molybdenum disulfide and the effect of temperature on the frequency shifts of the Raman-active E2g and A1g modes in the monolayer. While both peaks show an expected red-shift with increasing temperature, the frequency shift is larger for the A1g mode than for the E2g mode. This is in contrast to previously reported bulk behavior, in which the E2g mode shows a larger frequency shift with temperature. The temperature dependence of these phonon shifts is attributed to the anharmonic contributions to the ionic interaction potential in the two-dimensional system.

  5. Low-Temperature Ohmic Contact to Monolayer MoS2 by van der Waals Bonded Co/h-BN Electrodes.

    Science.gov (United States)

    Cui, Xu; Shih, En-Min; Jauregui, Luis A; Chae, Sang Hoon; Kim, Young Duck; Li, Baichang; Seo, Dongjea; Pistunova, Kateryna; Yin, Jun; Park, Ji-Hoon; Choi, Heon-Jin; Lee, Young Hee; Watanabe, Kenji; Taniguchi, Takashi; Kim, Philip; Dean, Cory R; Hone, James C

    2017-08-09

    Monolayer MoS2, among many other transition metal dichalcogenides, holds great promise for future applications in nanoelectronics and optoelectronics due to its ultrathin nature, flexibility, sizable band gap, and unique spin-valley coupled physics. However, careful study of these properties at low temperature has been hindered by an inability to achieve low-temperature Ohmic contacts to monolayer MoS2, particularly at low carrier densities. In this work, we report a new contact scheme that utilizes cobalt (Co) with a monolayer of hexagonal boron nitride (h-BN) that has the following two functions: modifies the work function of Co and acts as a tunneling barrier. We measure a flat-band Schottky barrier of 16 meV, which makes thin tunnel barriers upon doping the channels, and thus achieve low-T contact resistance of 3 kΩ.μm at a carrier density of 5.3 × 10(12)/cm(2). This further allows us to observe Shubnikov-de Haas oscillations in monolayer MoS2 at much lower carrier densities compared to previous work.

  6. A theoretical study of the electrical contact between metallic and semiconducting phases in monolayer MoS2

    Science.gov (United States)

    Paz, Wendel S.; Palacios, J. J.

    2017-03-01

    We present a theoretical study of the electrical contact between the two most common crystallographic phases of MoS2 monolayer crystals: the stable semiconducting 2H phase and the metastable metallic 1T phase. A density functional theory (DFT) study of the electronic structure of interface between the two phases shows a higher Schottky barrier for electrons than for holes for the undoped 2H phase. Charge transfer from the 1T to the 2H phase occurs, but, as expected for a one-dimensional contact, the generated dipole potential decays away from the interface and the naive Schottky-Mott band-alignment picture is recovered away from the interface. The decay length of the dipole potential turns out to be larger for the zigzag interface than for the armchair interface due to the different penetration of the edge states into the bulk. Tight-binding quantum transport calculations aided by the DFT results generically confirm a low contact resistance in the range of ≈200-400 Ωμm, as experimentally reported. Furthermore, the contact resistance is predicted to be smaller at the armchair interface for electron injection and, on the contrary, smaller for hole injection at the zigzag interface.

  7. In Situ Monitoring of the Thermal-Annealing Effect in a Monolayer of MoS2

    Science.gov (United States)

    Su, Liqin; Yu, Yifei; Cao, Linyou; Zhang, Yong

    2017-03-01

    We perform in situ two-cycle thermal-annealing studies for a transferred CVD-grown monolayer MoS2 on a SiO2/Si substrate, using spatially resolved micro-Raman and photoluminescence spectroscopy. The evolution in film morphology and film-substrate bonding is continuously monitored by Raman spectroscopy. After the thermal cycling and being annealed at 305 °C twice, the film morphology and film-substrate bonding are significantly modified, which together with the removal of polymer residues causes major changes in the strain and doping distribution over the film, and thus the optical properties. Before annealing, the strain associated with ripples in the transferred film dominates the spatial distributions of the photoluminescence peak position and intensity over the film; after annealing, the variation in film-substrate bonding, affecting both strain and doping, becomes the leading factor. This work reveals that the film-substrate bonding, and thus the strain and doping, is nonstationary under thermal stress, which is important for understanding the substrate effects on the optical and transport properties of the 2D material and their impact on device applications.

  8. Free-standing electronic character of monolayer MoS2 in van der Waals epitaxy

    Science.gov (United States)

    Kim, HoKwon; Dumcenco, Dumitru; Frégnaux, Mathieu; Benayad, Anass; Chen, Ming-Wei; Kung, Yen-Cheng; Kis, Andras; Renault, Olivier

    2016-08-01

    We have evaluated as-grown Mo S2 crystals, epitaxially grown on a monocrystalline sapphire by chemical vapor deposition (CVD), with direct electronic band-structure measurements by energy-filtered k -space photoelectron emission microscopy performed with a conventional laboratory vacuum ultraviolet He I light source under off-normal illumination. The valence states of the epitaxial Mo S2 were mapped in momentum space down to 7 eV below the Fermi level. Despite the high nucleation density within the imaged area, the CVD Mo S2 possesses an electronic structure similar to the free-standing monolayer Mo S2 single crystal, and it exhibits hole effective masses of 2.41 ±0.05 m0 , and 0.81 ±0.05 m0 , respectively, at Γ and K high-symmetry points that are consistent with the van der Waals epitaxial growth mechanism. This demonstrates the excellent ability of the Mo S2 CVD on sapphire to yield a highly aligned growth of well-stitched grains through epitaxial registry with a strongly preferred crystallographic orientation.

  9. Tunable thermoelectricity in monolayers of MoS2 and other group-VI dichalcogenides 2 and other group-VI dichalcogenides

    KAUST Repository

    Tahir, M

    2014-10-31

    We study the thermoelectric properties of monolayers of MoS2 and other group-VI dichalcogenides under circularly polarized off-resonant light. Analytical expressions are derived for the Berry phase mediated magnetic moment, orbital magnetization, as well as thermal and Nernst conductivities. Tuning of the band gap by off-resonant light enhances the spin splitting in both the valence and conduction bands and, thus, leads to a dramatic improvement of the spin and valley thermoelectric properties.

  10. Dual functions of 2D WS2 and MoS2-WS2 monolayers coupled with a Ag3PO4 photocatalyst

    Science.gov (United States)

    Wei, Zeng-Xi; Huang, Wei-Qing; Xu, Liang; Hu, Wangyu; Peng, P.; Huang, Gui-Fang

    2016-09-01

    The photocatalytic performance of semiconductors can be improved by coupling two-dimensional (2D) layered materials. Understanding the underlying mechanism of this phenomenon at the electronic level is important for the development of photocatalysts with a high efficiency. Here, we first present a theoretical elucidation of the dual functions of 2D layered material as a sensitizer and a co-catalyst by performing density functional theory calculations, taking WS2 and a lateral heterogeneous WS2-MoS2 monolayer as examples to couple with a promising photocatalyst Ag3PO4. The band alignment of a staggered type-II is formed between Ag3PO4 and the 2D monolayer with the latter possessing the higher electron affinity, resulting in the robust separation of photoexcited charge carriers between them, and indicating that the 2D monolayer is an effective sensitizer. Interestingly, the W (Mo) atoms, which are catalytically inert in the isolated 2D monolayer, turn into catalytic active sites, making the 2D monolayer a highly active co-catalyst in hybrids. A better photocatalytic performance in the coupled lateral heterogeneous WS2-MoS2 monolayer and Ag3PO4 can be expected. The calculated results can be rationalized by available experiments. These findings provide theoretical evidence supporting the experimental reports and may be used as a foundation for developing highly efficient 2D layered materials-based photocatalysts.

  11. Stability of defects in monolayer MoS2 and their interaction with O2 molecule: A first-principles study

    Science.gov (United States)

    Zhao, B.; Shang, C.; Qi, N.; Chen, Z. Y.; Chen, Z. Q.

    2017-08-01

    The stability of various defects in monolayer MoS2, as well as their interactions with free O2 molecules were investigated by density functional theory (DFT) calculations coupled with the nudged elastic band (NEB) method. The defects including S vacancy (monosulfur and disulfue vacancies), antisite defect (MoS) and external Mo atom can exist steadily in monolayer MoS2, and introduce defect levels in these defective systems, which breaks the surface chemical inertness and significantly enhances the adsorption capacity for free O2. The adsorption energy calculations and electronic properties analysis suggest that there is a strong interaction between O2 molecule and defective system. The adsorbed O2 on the defective surface can dissociate with a lower activation energy barrier, which produce two active oxygen atoms. Especially, two Mo atoms can occupy one Mo lattice site, and adsorbed O2 on the top of the Mo atom can then dissociate directly with the lowest activation energy barrier. Hence, our work may provide useful information to design MoS2-based gas sensor or catalysts.

  12. Adjusting band gap and charge transfer of organometallic complex adsorbed on MoS2 monolayer using vertical electric-field: a first-principles investigation

    Science.gov (United States)

    Bui, Viet Q.; Le, Hung M.; Kawazoe, Yoshiyuki; Kim, Yongho

    2017-01-01

    In this paper, we propose the use of benzene (Bz) to cover the active site on top of a transition metal atom (Cr/Mn/Fe) adsorbed on the MoS2 monolayer. Stable configurations of adatom on the MoS2 surface were predicted using first-principles calculations and their electronic, magnetic properties were investigated. In addition, the influence of vertical electric field on the electronic band structures of the systems was carefully examined. Analyzing the adsorption energies of transition metals given by the PBE calculations, we found that the benzene molecule stabilized the binding of Cr and Mn on the MoS2 surface, but destabilized the Fe binding by approximately 33% of adsorption energy. The attachment of benzene caused modifications on the total magnetizations of the Cr-MoS2 and Mn-MoS2 structures. The Bz-Mn@MoS2 structure was found to exhibit half-metallicity with 100% spin polarization at the Fermi level. The influence of various degrees of vertical electric field was shown to produce a tensile stress, which altered the lattice parameters and led to band gap narrowing and dramatic shifts of the Fermi level.

  13. Monolayer MoS2/GaAs heterostructure self-driven photodetector with extremely high detectivity

    CERN Document Server

    Xu, Zhijuan; Li, Xiaoqiang; Zhang, Shengjiao; Wu, Zhiqian; Xu, Wenli; Lu, Yanghua; Xu, Sen

    2015-01-01

    Two dimensional material/semiconductor heterostructures offer alternative platforms for optoelectronic devices other than conventional Schottky and p-n junction devices. Herein, we use MoS2/GaAs heterojunction as a self-driven photodetector with wide response band width from ultraviolet to visible light, which exhibits high sensitivity to the incident light of 650 nm with responsivity as 446 mA/W and detectivity as 5.9E13 Jones (Jones = cm Hz^1/2 W^-1), respectively. Employing interface design by inserting h-BN and photo-induced doping by covering Si quantum dots on the device, the responsivity is increased to 582 mA/W for incident light of 650 nm. Distinctly, attributing to the low dark current of the MoS2/h-BN/GaAs sandwich structure based on the self-driven operation condition, the detectivity shows extremely high value of 3.2E14 Jones for incident light of 650 nm, which is higher than all the reported values of the MoS2 based photodetectors. Further investigations reveal that the MoS2/GaAs based photodete...

  14. First principles prediction of the magnetic properties of Fe-X 6 (X = S, C, N, O, F) doped monolayer MoS 2

    KAUST Repository

    Feng, Nan

    2014-02-05

    Using first-principles calculations, we have investigated the electronic structure and magnetic properties of Fe-X 6 clusters (X = S, C, N, O, and F) incorporated in 4 4 monolayer MoS 2, where a Mo atom is substituted by Fe and its nearest S atoms are substituted by C, N, O, and F. Single Fe and Fe-F 6 substituions make the system display half-metallic properties, Fe-C 6 and Fe-N 6 substitutions lead to a spin gapless semiconducting behavior, and Fe-O 6 doped monolayer MoS 2 is semiconducting. Magnetic moments of 1.93, 1.45, 3.18, 2.08, and 2.21...? B are obtained for X = S, C, N, O, and F, respectively. The different electronic and magnetic characters originate from hybridization between the X and Fe/Mo atoms. Our results suggest that cluster doping can be an efficient strategy for exploring two-dimensional diluted magnetic semiconductors.

  15. Spin heat capacity of monolayer and AB-stacked bilayer MoS2 in the presence of exchange magnetic field

    Science.gov (United States)

    Hoi, Bui Dinh; Yarmohammadi, Mohsen; Mirabbaszadeh, Kavoos

    2017-04-01

    Dirac theory and Green's function technique are carried out to compute the spin dependent band structures and corresponding electronic heat capacity (EHC) of monolayer (ML) and AB-stacked bilayer (BL) molybdenum disulfide (MoS2) two-dimensional (2D) crystals. We report the influence of induced exchange magnetic field (EMF) by magnetic insulator substrates on these quantities for both structures. The spin-up (down) subband gaps are shifted with EMF from conduction (valence) band to valence (conduction) band at both Dirac points in the ML because of the spin-orbit coupling (SOC) which leads to a critical EMF in the K point and EHC returns to its initial states for both spins. In the BL case, EMF results split states and the decrease (increase) behavior of spin-up (down) subband gaps has been observed at both K and K‧ valleys which is due to the combined effect of SOC and interlayer coupling. For low and high EMFs, EHC of BL MoS2 does not change for spin-up subbands while increases for spin-down subbands.

  16. First principles study on molecule doping in MoS2 monolayer%单层MoS2分子掺杂的第一性原理研究

    Institute of Scientific and Technical Information of China (English)

    刘俊; 梁培; 舒海波; 沈涛; 邢凇; 吴琼

    2014-01-01

    基于密度泛函理论的第一性原理平面波赝势方法的计算,研究了通过吸附不同有机分子对单层MoS2进行化学掺杂。计算结果表明有机分子与MoS2单层衬底间的相互作用主要是范德瓦尔斯作用力。吸附不同有机分子的单层MoS2结构均表现出间接带隙的特征,还表明吸附TTF分子的单层MoS2结构表现出n型半导体的特质,而吸附TCNQ, TCNE两种分子的单层MoS2结构均表现出p型半导体的性质,这些结果表明可以通过改变吸附的分子来实现对单层MoS2的掺杂类型的调控。本文的研究结果将对单层MoS2在晶体管中的应用提供理论基础和指导。%The chemical doping of organic molecules adsorbed on MoS2 monolayers are systematically studied by using plane-wave pseudo-potential method based on the density functional theory. Our results indicate that the interaction between organic molecules and the MoS2 monolayer substrate is of van der Waals’ type of force. Structure of monolayer MoS2 which adsorbs different organic molecules, exhibits indirect bandgap characteristics, and the energy band structure of monolayer MoS2 which adsorbs TTF molecules exhibits n-type conducting characteristics. However, the structures of monolayer MoS2 which adsorbs TCNQ or TCNE molecules would exhibit p-type conductivity characteristics. Thus, the results indicate that the doping type of molecules in monolayer MoS2 can be regulated by adsorbing different molecules. Results of this study may provide a theoretical basis for single-layer MoS2 transistor and guidance for it in the application.

  17. Effects of La, Ce and Nd doping on the electronic structure of monolayer MoS2%La,Ce,Nd掺杂对单层MoS2电子结构的影响

    Institute of Scientific and Technical Information of China (English)

    雷天民; 吴胜宝; 张玉明; 郭辉; 陈德林; 张志勇

    2014-01-01

    To study the effect of rare earth element doping on the electronic structure of monolayer MoS2, the lattice parame-ters, band structures, density of states, and electron density differences of La, Ce and Nd doped and intrinsic monolayer MoS2 are calculated, respectively, using first-principles density functional theory based on the plane wave pseudopoten-tial method in this paper. Calculations indicate that variations of bond length near La impurity are maximum, but they are the minimum near Nd impurity. Analysis points out that lattice distortion in doped monolayer of MoS2 is relative to the magnitude of the covalent radius of doping atom. Analysis of band structure shows that La, Ce and Nd doping can induce three, six and four energy levels, respectively, in the forbidden band of MoS2, and that the properties of impurity levels are analyzed. Rare earth doped monolayer MoS2 make change in electron distribution through the analysis of electron density difference, and especially, the existence of f electrons can induce the electron density difference to exhibit a physical image with a great contrast.%为了研究稀土掺杂对单层MoS2电子结构的影响,文章基于密度泛函理论框架下的第一性原理,采用平面波赝势方法分别计算了本征及La, Ce, Nd掺杂单层MoS2的晶格参数、能带结构、态密度和差分电荷密度。计算发现,稀土掺杂所引起的晶格畸变与杂质原子的共价半径大小有关, La杂质附近的键长变化最大, Nd杂质附近的键长变化最小。能带结构分析表明, La掺杂可以在MoS2的禁带中引入3个能级, Ce掺杂可以形成6个新能级, Nd掺杂可以形成4个能级,并对杂质能级属性进行了初步分析。差分电荷密度分布显示,稀土掺杂可以使单层MoS2中的电子分布发生改变,尤其是f电子的存在会使差分电荷密度呈现出反差极大的物理图象。

  18. The effects of different possible modes of uniaxial strain on the tunability of electronic and band structures in {MoS}_2 monolayer nanosheet via first-principles density functional theory

    Science.gov (United States)

    Dimple; Jena, Nityasagar; Behere, Shounak Dhananjay; De Sarkar, Abir

    2017-07-01

    Ab-initio density functional theory-based calculations have been performed on monolayer (ML) {MoS}_2 nanosheet to study the variation of its electronic properties with the application of uniaxial tensile and compressive strain along its two non-equivalent lattice directions, namely, the zig-zag and the arm-chair directions. Among all the strain types considered in this study, uniaxial tensile strain applied along the zig-zag direction is found to be the most efficacious, inducing a greater tunability in the band gap over a large energy range (from 1.689 to 0.772 eV corresponding to 0-9% of applied strain), followed by uniaxial tensile strain along arm-chair direction. In contrast, the ML- {MoS}_2 nanosheet is found to be less sensitive to the compressive strain applied uniaxially along both the arm-chair as well as zig-zag directions. Moreover, the charges on Mo and S atoms are not found to undergo considerable changes under the application of uniaxial strain, as the atomic motion along the other direction is free from any constraint.

  19. Imaging spectroscopic ellipsometry of MoS2

    Science.gov (United States)

    Funke, S.; Miller, B.; Parzinger, E.; Thiesen, P.; Holleitner, A. W.; Wurstbauer, U.

    2016-09-01

    Micromechanically exfoliated mono- and multilayers of molybdenum disulfide (MoS2) are investigated by spectroscopic imaging ellipsometry. In combination with knife edge illumination, MoS2 flakes can be detected and classified on arbitrary flat and also transparent substrates with a lateral resolution down to 1-2 µm. The complex dielectric functions from mono- and trilayer MoS2 are presented. They are extracted from a multilayer model to fit the measured ellipsometric angles employing an anisotropic and an isotropic fit approach. We find that the energies of the critical points of the optical constants can be treated to be independent of the utilized model, whereas the magnitude of the optical constants varies with the used model. The anisotropic model suggests a maximum absorbance for a MoS2 sheet supported by sapphire of about 14% for monolayer and of 10% for trilayer MoS2. Furthermore, the lateral homogeneity of the complex dielectric function for monolayer MoS2 is investigated with a spatial resolution of 2 µm. Only minor fluctuations are observed. No evidence for strain, for a significant amount of disorder or lattice defects can be found in the wrinkle-free regions of the MoS2 monolayer from complementary µ-Raman spectroscopy measurements. We assume that the minor lateral variation in the optical constants are caused by lateral modification in the van der Waals interaction presumably caused by the preparation using micromechanical exfoliation and viscoelastic stamping.

  20. Co-nucleus 1D/2D Heterostructures with Bi2S3 Nanowire and MoS2 Monolayer: One-Step Growth and Defect-Induced Formation Mechanism.

    Science.gov (United States)

    Li, Yongtao; Huang, Le; Li, Bo; Wang, Xiaoting; Zhou, Ziqi; Li, Jingbo; Wei, Zhongming

    2016-09-27

    Heterostructures constructed by low-dimensional (such as 0D, 1D, and 2D) materials have opened up opportunities for exploring interesting physical properties and versatile (opto)electronics. Recently, 2D/2D heterostructures, in particular, atomically thin graphene and transition-metal dichalcogenides, including graphene/MoS2, WSe2/MoS2, and WS2/WSe2, were efficiently prepared (by transfer techniques, chemical vapor deposition (CVD) growth, etc.) and systematically studied. In contrast, investigation of 1D/2D heterostructures was still very challenging and rarely reported, and the understanding of such heterostructures was also not well established. Herein, we demonstrate the one-step growth of a heterostructure on the basis of a 1D-Bi2S3 nanowire and a 2D-MoS2 monolayer through the CVD method. Multimeans were employed, and the results proved the separated growth of a Bi2S3 nanowire and a MoS2 sheet in the heterostructure rather than forming a BixMo1-xSy alloy due to their large lattice mismatch. Defect-induced co-nucleus growth, which was an important growth mode in 1D/2D heterostructures, was also experimentally confirmed and systematically investigated in our research. Such 1D/2D heterostructures were further fabricated and utilized in (opto)electronic devices, such as field-effect transistors and photodetectors, and revealed their potential for multifunctional design in electrical properties. The direct growth of such nanostructures will help us to gain a better comprehension of these specific configurations and allow device functionalities in potential applications.

  1. 一种简单的化学气相沉积法制备大尺寸单层二硫化钼∗%Synthesis of large size monolayer MoS2 with a simple chemical vap or dep osition

    Institute of Scientific and Technical Information of China (English)

    董艳芳; 何大伟; 王永生; 许海腾; 巩哲

    2016-01-01

    Monolayer molybdenum disulfide (MoS2) has recently aroused the great interest of researchers due to its direct-gap property and potential applications in electronics, catalysis, photovoltaics, and optoelectronics. Chemical vapor deposition (CVD) has been one of the most practical methods of synthesizing large-area and high-quality monolayer MoS2. However, The process of preparation is complex and cumbersome. Here we report that high-quality monolayer MoS2 can be obtained through using sulfurization of MoO3 by a simple and convenient CVD on sapphire substrates. The substrate cleaning is simplified. Substrates are cleaned in detergent solution, deionized water and acetone without sopropanol or piranha solution (H2SO4/H2O2=3:1) in sequence, avoiding their potential dangers . The MoO3 powder (Alfa Aesar, 99.995%, 0.02 g) is placed in an alumina boat, and a sapphire substrate is faced down and is placed 6 cm away from MoO3 powder in the same boat. The sapphire substrate is placed in the center of the heating zone of the furnace. Another alumina boat containing sulfur powder (Alfa Aesar, 99.999%, 0.2 g) is placed upstream with respect to the gas flow direction in the low temperature area. We adopt an atmospheric pressure chemical vapor deposition method, so it does not require a vacuum process. After 30 min of Ar purging, the furnace temperature is directly increased from room temperature to 800 ◦C in 30 min, reducing the heating steps. After 60 min, the furnace is cooled down naturally to room temperature. Optical microscopy (OM) images, Raman spectra and photoluminescence (PL) are all obtained by confocal Raman microscopic system (LabRAM HR Evolution). From the OM images, we can see that isolated islands (triangles) have edge lengths up to 50 µm, which is far larger than that grown by micromechanical exfoliation. The color of the triangles is uniform, which has a strong contrast with the substrate. We can obtain a preliminary result that the sample is a uniform

  2. Synthesis and characterization of MoS2 nanosheets.

    Science.gov (United States)

    Deokar, G; Vignaud, D; Arenal, R; Louette, P; Colomer, J-F

    2016-02-19

    Here, we report on the synthesis of MoS2 nanosheets using a simple two-step additive-free growth technique. The as-synthesized nanosheets were characterized to determine their structure and composition, as well as their optical properties. The MoS2 nanosheets were analyzed by scanning electron microscopy, transmission electron microscopy (TEM), including high-resolution scanning TEM imaging and energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy (XPS), Raman spectroscopy and photoluminescence (PL). The as-produced MoS2 nanosheets are vertically aligned with curved edges and are densely populated. The TEM measurements confirmed that the nanosheets have the 2H-MoS2 crystal structure in agreement with the Raman results. The XPS results revealed the presence of high purity MoS2. Moreover, a prominent PL similar to mechanically exfoliated few and mono-layer MoS2 was observed for the as-grown nanosheets. For the thin (≤50 nm) nanosheets, the PL feature was observed at the same energy as that for a direct band-gap monolayer MoS2 (1.83 eV). Thus, the as-produced high-quality, large-area, MoS2 nanosheets could be potentially useful for various optoelectronic and catalysis applications.

  3. 单层二硫化钼多相性质及相变的第一性原理研究∗%First-principles study on multiphase prop erty and phase transition of monolayer MoS2

    Institute of Scientific and Technical Information of China (English)

    张理勇; 方粮; 彭向阳

    2016-01-01

    Using first principles calculations within density functional theory, we investigate multiphase property and phase transition of monolayer MoS2. All the quantities are calculated using the Vienna ab initio simulation package. Calcula-tions are performed within the generalized gradient approximation with van der Waals corrections (optimized Perdew-Burke-Ernzerhof-vdW). The cutoff energy of plane-wave is set to be 400 eV. The atomic plane and its neighboring image are separated by a 15 Å vacuum layer. The k-meshes for the structure relaxation and post analysis are 11 × 11 × 1 and 19 × 19 × 1 respectively. Firstly, we obtain the geometry configurations of 2H-MoS2, 1T-MoS2 and ZT-MoS2 phases through structure re-laxing. The lattice constants of 2H-MoS2 are a=3.190 Å and b=5.524 Å, and total energy is−39.83 eV which means that it is the most stable phase. The lattice constants of 1T-MoS2 are a=3.191 Å and b=5.528 Å, and total energy is−38.21 eV, which means that it is the most unstable phase. Both 2H-MoS2 and 1T-MoS2 have a three-layer structure with two S layers sandwiching one Mo layer. The difference of 1T-MoS2 from the 2H-MoS2 is the upper S layer shifting. The ZT-MoS2 derives from 1T-MoS2 through lattice distortion. The lattice constants of ZT-MoS2 are a = 3.185 Å and b = 5.725 Å, and total energy is −38.80 eV. The total energy determines the following stability order of three phases: 2H-MoS2>ZT-MoS2>1T-MoS2. Our computed results agree well with the other computed and experimental results. Band structure and density of states confirm that 1T-MoS2 is metallic and ZT-MoS2 is semiconducting. But the bandgap of ZT-MoS2 phase is only 0.01 eV. Then we compute the intrinsic carrier mobility values of 2H-MoS2 and ZT-MoS2 at 300 K with the deformation potential theory. The carrier mobility of 2H-MoS2 is between 100 cm2·V−1·s−1 and 400 cm2·V−1·s−1. Owing to ZT-MoS2 carrier effective mass decreasing obviously, the carrier mobility of ZT phase

  4. Atomic-layer soft plasma etching of MoS2.

    Science.gov (United States)

    Xiao, Shaoqing; Xiao, Peng; Zhang, Xuecheng; Yan, Dawei; Gu, Xiaofeng; Qin, Fang; Ni, Zhenhua; Han, Zhao Jun; Ostrikov, Kostya Ken

    2016-01-27

    Transition from multi-layer to monolayer and sub-monolayer thickness leads to the many exotic properties and distinctive applications of two-dimensional (2D) MoS2. This transition requires atomic-layer-precision thinning of bulk MoS2 without damaging the remaining layers, which presently remains elusive. Here we report a soft, selective and high-throughput atomic-layer-precision etching of MoS2 in SF6 + N2 plasmas with low-energy (etching rates can be tuned to achieve complete MoS2 removal and any desired number of MoS2 layers including monolayer. Layer-dependent vibrational and photoluminescence spectra of the etched MoS2 are also demonstrated. This soft plasma etching technique is versatile, scalable, compatible with the semiconductor manufacturing processes, and may be applicable for a broader range of 2D materials and intended device applications.

  5. Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface

    KAUST Repository

    Li, Ming Yang

    2015-07-30

    Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS2 and tungsten sulfide WSe2 have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe2-MoS2 heterojunction, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.

  6. Well-Hidden Grain Boundary in the Monolayer MoS2 Formed by a Two-Dimensional Core-Shell Growth Mode.

    Science.gov (United States)

    Zhang, Wenting; Lin, Yue; Wang, Qi; Li, Weijie; Wang, Zhifeng; Song, Jiangluqi; Li, Xiaodong; Zhang, Lijie; Zhu, Lixin; Xu, Xiaoliang

    2017-09-22

    Guided by the hexagonal lattice symmetry, triangles and hexagons are the most basic morphological units for two-dimensional (2D) transition metal dichalcogenides (TMDs) synthesized by chemical vapor deposition (CVD). Also, it is widely acknowledged that these units start from the single nucleation site and then grow epitaxially. Accordingly, the triangular monolayer (ML) samples are generally considered as single crystals. Here, we report a 2D core-shell growth mode in the CVD process for ML-MoS2, which leads to one kind of "pseudo" single-crystal triangles containing triangular outline grain boundaries (TO-GBs). It is difficult to be optically distinguished from the "true" single-crystal triangles. The weakening of Raman peaks and the remarkable enhancement of photoluminescence (PL) are found at the built-in TO-GBs, which could be useful for high-performance optoelectronics. In addition, the electrical measurements indicate that the TO-GBs are conductive. Furthermore, TO-GBs and the common grain boundaries (CO-GBs) can coexist in a single flake, whereas their optical visibility and optical modifications (Raman and PL) are quite different. This work is helpful in further understanding the growth mechanism of 2D TMD materials and may also play a significant role in related nanodevices.

  7. Oxidation of atomically thin MoS2 on SiO2

    Science.gov (United States)

    Yamamoto, Mahito; Cullen, William; Einstein, Theodore; Fuhrer, Michael

    2013-03-01

    Surface oxidation of MoS2 markedly affects its electronic, optical, and tribological properties. However, oxidative reactivity of atomically thin MoS2 has yet to be addressed. Here, we investigate oxidation of atomic layers of MoS2 using atomic force microscopy and Raman spectroscopy. MoS2 is mechanically exfoliated onto SiO2 and oxidized in Ar/O2 or Ar/O3 (ozone) at 100-450 °C. MoS2 is much more reactive to O2 than an analogous atomic membrane of graphene and monolayer MoS2 is completely etched very rapidly upon O2 treatment above 300 °C. Thicker MoS2 (> 15 nm) transforms into MoO3 after oxidation at 400 °C, which is confirmed by a Raman peak at 820 cm-1. However, few-layer MoS2 oxidized below 400 °C exhibits no MoO3 Raman mode but etch pits are formed, similar to graphene. We find atomic layers of MoS2 shows larger reactivity to O3 than to O2 and monolayer MoS2 transforms chemically upon O3 treatment even below 100 °C. Work supported by the U. of Maryland NSF-MRSEC under Grant No. DMR 05-20741.

  8. Van der Waals trilayers and superlattices: Modification of electronic structures of MoS2 by intercalation

    OpenAIRE

    Lu, N.; Guo, H. Y.; L. Wang; Wu, X. J.; Zeng, X. C.

    2014-01-01

    We perform a comprehensive first-principles study of the electronic properties of van der Waals (vdW) trilayers via intercalating a two-dimensional (2D) monolayer (ML = BN, MoSe2, WS2, or WSe2) between MoS2 bilayer to form various MoS2/ML/MoS2 sandwich trilayers. We find that the BN monolayer is the most effective sheet to decouple the interlayer vdW coupling of the MoS2 bilayer, and the resulting sandwich trilayer can recover the electronic structures of the MoS2 monolayer, particularly the ...

  9. Photoluminescence quenching in gold - MoS2 hybrid nanoflakes

    Science.gov (United States)

    Bhanu, Udai; Islam, Muhammad R.; Tetard, Laurene; Khondaker, Saiful I.

    2014-01-01

    Achieving tunability of two dimensional (2D) transition metal dichalcogenides (TMDs) functions calls for the introduction of hybrid 2D materials by means of localized interactions with zero dimensional (0D) materials. A metal-semiconductor interface, as in gold (Au) - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science as it constitutes an outstanding platform to investigate plasmonic-exciton interactions and charge transfer. The applied aspects of such systems introduce new options for electronics, photovoltaics, detectors, gas sensing, catalysis, and biosensing. Here we consider pristine MoS2 and study its interaction with Au nanoislands, resulting in local variations of photoluminescence (PL) in Au-MoS2 hybrid structures. By depositing monolayers of Au on MoS2, we investigate the electronic structure of the resulting hybrid systems. We present strong evidence of PL quenching of MoS2 as a result of charge transfer from MoS2 to Au: p-doping of MoS2. The results suggest new avenues for 2D nanoelectronics, active control of transport or catalytic properties. PMID:24992896

  10. 金衬底调控单层二硫化钼电子性能的第一性原理研究%Tuning the electronic property of monolayer MoS2 adsorbed on metal Au substrate: a first-principles study

    Institute of Scientific and Technical Information of China (English)

    张理勇; 方粮; 彭向阳

    2015-01-01

    Using first principles calculations within density functional theory, we investigate the electronic property of a single-layer MoS2 adsorbed on Au. All the quantities are calculated using the Vienna ab initio simulation package. Calculations are performed using the projector augmented wave method with the Perdew-Burke-Ernzerhof functional and a kinetic energy cutoff of 400 eV. The atomic plane and its neighboring image are separated by a 15 ? vacuum layer. The k-meshes for the structure relaxation and post analysis are 9 × 9 × 1 and 19 × 19 × 1, respectively. The spin-orbit coupling is considered in the calculation. The research includes the binding energy, the band structure, density of states (DOS) and electric charge difference density. Three contact modes between MoS2 (0001) and Au (111) are considered. When the atom S layer and the atom Au layer on the contacting interface have the same structure, the minimum binding energy and distance between MoS2 (0001) and Au(111) are 2.2 eV and 2.5 ? respectively. The minimum binding energy confirms that the absorption is unstable. The band structure demonstrates that the MoS2-Au contact nature is of the Schottky-barrier type, and the barrier height is 0.6 eV which is bigger than MoS2-Sc contact. By comparison with other metal contacts such as Ru(0001), Pd(111) and Ir(111), the dependence of the barrier height on the work function difference exhibits a Fermi-level pinning. But the MoS2 is so thin that the Fermi-level pinning must be very small. Maybe there is a metal induced gap state. DOS points out that the Au substrate has no influence on the covalent bond between Mo and S. The influence of the Au substrate is that it shifts the DOS of monolayer MoS2 left on the axis. The change of DOS results in the increases of electron concentration and electric conductivity. Other calculation points out that Ti substrate can excite more electrons. Electric charge density difference demonstrates that there are a few electric

  11. Spectroscopic Signatures for Interlayer Coupling in MoS 2 –WSe 2 van der Waals Stacking

    KAUST Repository

    Chiu, Ming-Hui

    2014-09-23

    Stacking of MoS2 and WSe2 monolayers is conducted by transferring triangular MoS2 monolayers on top of WSe2 monolayers, all grown by chemical vapor deposition (CVD). Raman spectroscopy and photoluminescence (PL) studies reveal that these mechanically stacked monolayers are not closely coupled, but after a thermal treatment at 300 degrees C, it is possible to produce van der Waals solids consisting of two interacting transition metal dichalcogenide (TMD) monolayers. The layer-number sensitive Raman out-of-plane mode A(1g)(2) for WSe2 (309 cm(-1)) is found sensitive to the coupling between two TMD monolayers. The presence of interlayer excitonic emissions and the changes in other intrinsic Raman modes such as E \\'\\' for MoS2 at 286 cm(-1) and A(1g)(2) for MoS2 at around 463 cm(-1) confirm the enhancement of the interlayer coupling.

  12. 单层、双层和多层MoS2合成技术%SYNTHESIS TECHNIQUES OF MONOLAYER,DOUBLE-LAYER,MULTI-LAYER MoS2

    Institute of Scientific and Technical Information of China (English)

    张文钲; 刘燕; 蒋丽娟; 林小辉

    2015-01-01

    当今,科学家利用了化学气相合成法、微机械剥离法、水热合成法和物理气相沉积法等合成出大面积单层、双层和多层二硫化钼,其中以化学气相合成二硫化钼最引人关注. 特别是用五氯化钼和硫在Si/SiO2基材上、蓝宝石基材上和石墨烯基材上合成出大面积单层二硫化钼. 利用水热合成二硫化钼,如在水热溶液中引入氧化石墨烯可生成3~10层的二硫化钼,这种MoS2/RGO杂化物其释氢活性很高,是一种良好的催化剂.%At present , the chemical vapor deposition , micro-mechanical decollement method , hydro-thermal synthe-sis method and physical vapour deposition method were used to synthesize a large area of single , double and multi-layer MoS2 film, which the chemical vapor deposition method has been most noticed .Especially, a large area single layer MoS2 film in substrate of Si/SiO2 , sapphire and graphitized carbon can be synthesized by MoCl 5 and S.The 3~10 layers of MoS 2 film will be formed by hydro-thermal synthesis method which introduces the oxidize graphene into hydro-thermal solution .The hybrid catalyst of this MoS 2/RGO is a good catalyst which exhibited excellent ac-tivity of hydrogen release .

  13. MoS2 spaser

    Science.gov (United States)

    Jayasekara, Charith; Premaratne, Malin; Gunapala, Sarath D.; Stockman, Mark I.

    2016-04-01

    We present a comprehensive analysis of a spaser made of a circular shaped highly doped molybdenum disulfide (MoS2) resonator. "Spaser" is an acronym for "surface plasmon amplification by stimulated emission of radiation"-a nanoscale source of surface plasmons generated by stimulated emission in a plasmonic resonator which receives energy nonradiatively. By considering localized surface plasmon modes, operation characteristics of the model are analysed, and tunability of the design is demonstrated. We find the optimum geometric and material parameters of the spaser that provides efficient outputs and carryout a comparative analysis with a similar circular spaser made of graphene. Owing to physical and chemical properties of MoS2 and the active medium, the proposed design delivers efficient outputs in terms of spaser mode energy, operating thresholds, Q-factor, and electric field amplitude. Lower operating thresholds and higher mode energies are notable advantages of the design. Owing to having many superior features to existing similar designs, this MoS2 spaser may be much suited for applications in nanoplasmonic devices.

  14. Tuning Coupling Behavior of Stacked Heterostructures Based on MoS2, WS2, and WSe2

    Science.gov (United States)

    Wang, Fang; Wang, Junyong; Guo, Shuang; Zhang, Jinzhong; Hu, Zhigao; Chu, Junhao

    2017-03-01

    The interlayer interaction of vertically stacked heterojunctions is very sensitive to the interlayer spacing, which will affect the coupling between the monolayers and allow band structure modulation. Here, with the aid of density functional theory (DFT) calculations, an interesting phenomenon is found that MoS2-WS2, MoS2-WSe2, and WS2-WSe2 heterostructures turn into direct-gap semiconductors from indirect-gap semiconductors with increasing the interlayer space. Moreover, the electronic structure changing process with interlayer spacing of MoS2-WS2, MoS2-WSe2, and WS2-WSe2 is different from each other. With the help of variable-temperature spectral experiment, different electronic transition properties of MoS2-WS2, MoS2-WSe2, and WS2-WSe2 have been demonstrated. The transition transformation from indirect to direct can be only observed in the MoS2-WS2 heterostructure, as the valence band maximum (VBM) at the Γ point in the MoS2-WSe2 and WS2-WSe2 heterostructure is less sensitive to the interlayer spacing than those from the MoS2-WS2 heterostructure. The present work highlights the significance of the temperature tuning in interlayer coupling and advance the research of MoS2-WS2, MoS2-WSe2, and WS2-WSe2 based device applications.

  15. Plasmonic pumping of excitonic photoluminescence in hybrid MoS2-Au nanostructures.

    Science.gov (United States)

    Najmaei, Sina; Mlayah, Adnen; Arbouet, Arnaud; Girard, Christian; Léotin, Jean; Lou, Jun

    2014-12-23

    We report on the fabrication of monolayer MoS2-coated gold nanoantennas combining chemical vapor deposition, e-beam lithography surface patterning, and a soft lift-off/transfer technique. The optical properties of these hybrid plasmonic-excitonic nanostructures are investigated using spatially resolved photoluminescence spectroscopy. Off- and in-resonance plasmonic pumping of the MoS2 excitonic luminescence showed distinct behaviors. For plasmonically mediated pumping, we found a significant enhancement (∼65%) of the photoluminescence intensity, clear evidence that the optical properties of the MoS2 monolayer are strongly influenced by the nanoantenna surface plasmons. In addition, a systematic photoluminescence broadening and red-shift in nanoantenna locations is observed which is interpreted in terms of plasmonic enhanced optical absorption and subsequent heating of the MoS2 monolayers. Using a temperature calibration procedure based on photoluminescence spectral characteristics, we were able to estimate the local temperature changes. We found that the plasmonically induced MoS2 temperature increase is nearly four times larger than in the MoS2 reference temperatures. This study shines light on the plasmonic-excitonic interaction in these hybrid metal/semiconductor nanostructures and provides a unique approach for the engineering of optoelectronic devices based on the light-to-current conversion.

  16. Polarization-dependent photocurrent in MoS2 phototransistor

    Science.gov (United States)

    Li, Jiu; Yu, Wentao; Chu, Saisai; Yang, Hong; Shi, Kebin; Gong, Qihuang

    2015-03-01

    Monolayer or few-layer molybdenum disulfide (MoS2) has attracted increasing interests in studying light-induced electronic effect due to its prominent photo-responsivity at visible spectral range, fast photo-switching rate and high channel mobility. However, the atomically thin layers make the interaction between light and matter much weaker than that in bulk state, hampering its application in two-dimensional material optoelectronics. One of recent efforts was to utilize resonantly enhanced localized surface plasmon for boosting light-matter interaction in MoS2 thin layer phototransistor. Randomly deposited metallic nano-particles were previously reported to modify surface of a back-gated MoS2 transistor for increasing light absorption cross-section of the phototransistor. Wavelength-dependent photocurrent enhancement was observed. In this paper, we report on a back-gated multilayer MoS2 field-effect-transistor (FET), whose surface is decorated with oriented gold nanobar array, of which the size of a single nanobar is 60nm:60nm:120nm. With these oriented nanostructures, photocurrent of the MoS2 FET could be successfully manipulated by a linear polarized incident 633nm laser, which fell into the resonance band of nanobar structure. We find that the drain-source current follows cos2θ relationship with respect to the incident polarization angle. We attribute the polarization modulation effect to the localized enhancement nature of gold nanobar layer, where the plasmon enhancement occurs only when the polarization of incident laser parallels to the longitudinal axis of nanobars and when the incident wavelength matches the resonance absorption of nanobars simultaneously. Our results indicate a promising application of polarization-dependent plasmonic manipulation in two-dimension semiconductor materials and devices.

  17. Large-Area Buckled MoS2 Films on the Graphene Substrate.

    Science.gov (United States)

    Kim, Seon Joon; Kim, Dae Woo; Lim, Joonwon; Cho, Soo-Yeon; Kim, Sang Ouk; Jung, Hee-Tae

    2016-06-01

    In this study, a novel buckled structure of edge-oriented MoS2 films is fabricated for the first time by employing monolayer graphene as the substrate for MoS2 film growth. Compared to typical buckling methods, our technique has several advantages: (1) external forces such as heat and mechanical strain are not applied; (2) uniform and controllable buckling over a large area is possible; and (3) films are able to be transferred to a desired substrate. Dual MoS2 orientation was observed in the buckled film where horizontally aligned MoS2 layers of 7 nm thickness were present near the bottom graphene surface and vertically aligned layers dominated the film toward the outer surface, in which the alignment structure was uniform across the entire film. The catalytic ability of the buckled MoS2 films, measured by performing water-splitting tests in acidic environments, shows a reduced onset potential of -0.2 V versus reversible hydrogen electrode (RHE) compared to -0.32 V versus RHE for pristine MoS2, indicating that the rough surface provided a higher catalytic activity. Our work presents a new method to generate a buckled MoS2 structure, which may be extended to the formation of buckled structures in various 2D materials for future applications.

  18. Electrical transport properties of metal and graphene contacts to MoS2

    Science.gov (United States)

    Luo, Yunqiu (Kelly); Wen, Hua; Zhu, Tiancong

    2014-03-01

    Two-dimensional crystals are an exciting class of materials for novel physics and nanoelectronics. MoS2 and related transition metal dichalcogenides have received tremendous interest due to its native band gap and strong spin orbit coupling. Unlike graphene, the presence of the band gap leads to transistors with high on-off ratios. One important issue is the electrical properties of the contacts to the MoS2. Recent studies have shown the presence of a Schottky barrier and its dependence on the metal workfunction, back gate voltage, and interfacial oxide barriers. In this work, we investigate the interfacial properties of metal to MoS2 contact and graphene to MoS2 contact by studying the junction's Schottky barrier formation and bias dependence. We utilize a polymer based transfer method to precisely position exfoliated graphene flakes onto exfoliated MoS2 flakes. We intensively study various junction combination between monolayer/few-layer graphene and monolayer/few-layer MoS2. Dependence on temperature and back gate will be discussed.

  19. Improving the electrical performance of MoS2 by mild oxygen plasma treatment

    Science.gov (United States)

    Nan, Haiyan; Wu, Zhangting; Jiang, Jie; Zafar, Amina; You, Yumeng; Ni, Zhenhua

    2017-04-01

    Two-dimensional (2D) molybdenum disulfide (MoS2) is considered as a promising candidate for electronic and optoelectronic devices. However, structural defects in MoS2 are widely reported and can greatly degrade its electrical and optical properties. In this work, we investigate the structural defects in MoS2 by low temperature photoluminescence (PL) spectroscopy and study their effects on the electrical performance, i.e. carrier mobility. We also adopt the mild oxygen plasma treatment to repair the structural defects and found that the carrier mobility of monolayer MoS2 can be greatly improved. This work would therefore offer a practical route to improve the performance of 2D dichalcogenide-based electrical and optoelectronic devices.

  20. Advances in MoS2-Based Field Effect Transistors (FETs)

    Institute of Scientific and Technical Information of China (English)

    Xin Tong; Eric Ashalley; Feng Lin; Handong Li; Zhiming M. Wang

    2015-01-01

    This paper reviews the original achievements and advances regarding the field effect transistor (FET) fabricated from one of the most studied transition metal dichalcogenides: two-dimensional MoS2. Not like graphene, which is highlighted by a gapless Dirac cone band structure, Monolayer MoS2 is featured with a 1.9 eV gapped direct energy band thus facilitates convenient electronic and/or optoelectronic modulation of its physical properties in FET structure. Indeed, many MoS2 devices based on FET architecture such as phototransistors, memory devices, and sensors have been studied and extraordinary properties such as excellent mobility, ON/OFF ratio, and sensitivity of these devices have been ex-hibited. However, further developments in FET device applications depend a lot on if novel physics would be involved in them. In this review, an overview on advances and developments in the MoS2-based FETs are presented. Engineering of MoS2-based FETs will be discussed in details for understanding contact physics, formation of gate dielectric, and doping strategies. Also reported are demonstrations of device behaviors such as low-frequency noise and photoresponse in MoS2-based FETs, which is crucial for developing electronic and optoelectronic devices.

  1. Dynamic Memory Cells Using MoS2 Field-Effect Transistors Demonstrating Femtoampere Leakage Currents.

    Science.gov (United States)

    Kshirsagar, Chaitanya U; Xu, Weichao; Su, Yang; Robbins, Matthew C; Kim, Chris H; Koester, Steven J

    2016-09-27

    Two-dimensional semiconductors such as transition-metal dichalcogenides (TMDs) are of tremendous interest for scaled logic and memory applications. One of the most promising TMDs for scaled transistors is molybdenum disulfide (MoS2), and several recent reports have shown excellent performance and scalability for MoS2 MOSFETs. An often overlooked feature of MoS2 is that its wide band gap (1.8 eV in monolayer) and high effective masses should lead to extremely low off-state leakage currents. These features could be extremely important for dynamic memory applications where the refresh rate is the primary factor affecting the power consumption. Theoretical predictions suggest that leakage currents in the 10(-18) to 10(-15) A/μm range could be possible, even in scaled transistor geometries. Here, we demonstrate the operation of one- and two-transistor dynamic memory circuits using MoS2 MOSFETs. We characterize the retention times in these circuits and show that the two-transistor memory cell reveals MoS2 MOSFETs leakage currents as low as 1.7 × 10(-15) A/μm, a value that is below the noise floor of conventional DC measurements. These results have important implications for the future use of MoS2 MOSFETs in low-power circuit applications.

  2. MoS2/MX2 heterobilayers: bandgap engineering via tensile strain or external electrical field

    Science.gov (United States)

    Lu, Ning; Guo, Hongyan; Li, Lei; Dai, Jun; Wang, Lu; Mei, Wai-Ning; Wu, Xiaojun; Zeng, Xiao Cheng

    2014-02-01

    We have performed a comprehensive first-principles study of the electronic and magnetic properties of two-dimensional (2D) transition-metal dichalcogenide (TMD) heterobilayers MX2/MoS2 (M = Mo, Cr, W, Fe, V; X = S, Se). For M = Mo, Cr, W; X = S, Se, all heterobilayers show semiconducting characteristics with an indirect bandgap with the exception of the WSe2/MoS2 heterobilayer which retains the direct-bandgap character of the constituent monolayer. For M = Fe, V; X = S, Se, the MX2/MoS2 heterobilayers exhibit metallic characters. Particular attention of this study has been focused on engineering the bandgap of the TMD heterobilayer materials via application of either a tensile strain or an external electric field. We find that with increasing either the biaxial or uniaxial tensile strain, the MX2/MoS2 (M = Mo, Cr, W; X = S, Se) heterobilayers can undergo a semiconductor-to-metal transition. For the WSe2/MoS2 heterobilayer, a direct-to-indirect bandgap transition may occur beyond a critical biaxial or uniaxial strain. For M (=Fe, V) and X (=S, Se), the magnetic moments of both metal and chalcogen atoms are enhanced when the MX2/MoS2 heterobilayers are under a biaxial tensile strain. Moreover, the bandgap of MX2/MoS2 (M = Mo, Cr, W; X = S, Se) heterobilayers can be reduced by the vertical electric field. For two heterobilayers MSe2/MoS2 (M = Mo, Cr), PBE calculations suggest that the indirect-to-direct bandgap transition may occur under an external electric field. The transition is attributed to the enhanced spontaneous polarization. The tunable bandgaps in general and possible indirect-direct bandgap transitions due to tensile strain or external electric field make the TMD heterobilayer materials a viable candidate for optoelectronic applications.We have performed a comprehensive first-principles study of the electronic and magnetic properties of two-dimensional (2D) transition-metal dichalcogenide (TMD) heterobilayers MX2/MoS2 (M = Mo, Cr, W, Fe, V; X = S, Se). For

  3. Photoelectrochemistry of Pristine Mono- and Few-Layer MoS2.

    Science.gov (United States)

    Velický, Matěj; Bissett, Mark A; Woods, Colin R; Toth, Peter S; Georgiou, Thanasis; Kinloch, Ian A; Novoselov, Kostya S; Dryfe, Robert A W

    2016-03-09

    Two-dimensional crystals are promising building blocks for the new generation of energy materials due to their low volume, high surface area, and high transparency. Electrochemical behavior of these crystals determines their performance in applications such as energy storage/conversion, sensing, and catalysis. Nevertheless, the electrochemistry of an isolated monolayer of molybdenum disulfide, which is one of the most promising semiconducting crystals, has not been achieved to date. We report here on photoelectrochemical properties of pristine monolayer and few-layer basal plane MoS2, namely the electron transfer kinetics and electric double-layer capacitance, supported by an extensive physical and chemical characterization. This enables a comparative qualitative correlation among the electrochemical data, MoS2 structure, and external illumination, although the absolute magnitudes of the electron transfer and capacitance are specific to the redox mediator and electrolyte used in these measurements ([Ru(NH3)6](3+/2+) and LiCl, respectively). Our work shows a strong dependence of the electrochemical properties on the number of MoS2 layers and illumination intensity and proves that an effective interlayer charge transport occurs in bulk MoS2. This highlights the exciting opportunities for tuning of the electrochemical performance of MoS2 through modification of its structure, external environment, and illumination.

  4. Electrical control of the valley Hall effect in bilayer MoS2 transistors.

    Science.gov (United States)

    Lee, Jieun; Mak, Kin Fai; Shan, Jie

    2016-05-01

    The valley degree of freedom of electrons in solids has been proposed as a new type of information carrier, beyond the electron charge and spin. The potential of two-dimensional semiconductor transition metal dichalcogenides in valley-based electronic and optoelectronic applications has recently been illustrated through experimental demonstrations of the optical orientation of the valley polarization and of the valley Hall effect in monolayer MoS2. However, the valley Hall conductivity in monolayer MoS2, a non-centrosymmetric crystal, cannot be easily tuned, which presents a challenge for the development of valley-based applications. Here, we show that the valley Hall effect in bilayer MoS2 transistors can be controlled with a gate voltage. The gate applies an electric field perpendicular to the plane of the material, breaking the inversion symmetry present in bilayer MoS2. The valley polarization induced by the longitudinal electrical current was imaged with Kerr rotation microscopy. The polarization was found to be present only near the edges of the device channel with opposite sign for the two edges, and was out-of-plane and strongly dependent on the gate voltage. Our observations are consistent with symmetry-dependent Berry curvature and valley Hall conductivity in bilayer MoS2.

  5. Large Area Deposition of MoS2 by Pulsed Laser Deposition with In-Situ Thickness Control

    KAUST Repository

    Serna, Martha I.

    2016-05-24

    A scalable and catalyst-free method to deposit stoichiometric Molybdenum Disulfide (MoS2) films over large areas is reported with the maximum area limited by the size of the substrate holder. The method allows deposition of MoS2 layers on a wide range of substrates without any additional surface preparation including single crystals (sapphire and quartz), polycrystalline (HfO2), and amorphous (SiO2). The films are deposited using carefully designed MoS2 targets fabricated with excess of sulfur (S) and variable MoS2 and S particle size. Uniform and layered MoS2 films as thin as two monolayers, with an electrical resistivity of 1.54 × 104 Ω cm-1 were achieved. The MoS2 stoichiometry was as confirmed by High Resolution Rutherford Backscattering Spectrometry (HRRBS). With the method reported here, in situ graded MoS2 films ranging from ~1 to 10 monolayers can also be deposited.

  6. Large-Area Deposition of MoS2 by Pulsed Laser Deposition with In Situ Thickness Control.

    Science.gov (United States)

    Serna, Martha I; Yoo, Seong H; Moreno, Salvador; Xi, Yang; Oviedo, Juan Pablo; Choi, Hyunjoo; Alshareef, Husam N; Kim, Moon J; Minary-Jolandan, Majid; Quevedo-Lopez, Manuel A

    2016-06-28

    A scalable and catalyst-free method to deposit stoichiometric molybdenum disulfide (MoS2) films over large areas is reported, with the maximum area limited by the size of the substrate holder. The method allows deposition of MoS2 layers on a wide range of substrates without any additional surface preparation, including single-crystal (sapphire and quartz), polycrystalline (HfO2), and amorphous (SiO2) substrates. The films are deposited using carefully designed MoS2 targets fabricated with excess sulfur and variable MoS2 and sulfur particle size. Uniform and layered MoS2 films as thin as two monolayers, with an electrical resistivity of 1.54 × 10(4) Ω cm(-1), were achieved. The MoS2 stoichiometry was confirmed by high-resolution Rutherford backscattering spectrometry. With the method reported here, in situ graded MoS2 films ranging from ∼1 to 10 monolayers can be deposited.

  7. Electrical Transport and Low-Frequency Noise in Chemical Vapor Deposited Single-Layer MoS2 Devices

    Science.gov (United States)

    2014-03-18

    noise in graphene devices [18–20], and lately on MoS2 FETs [21]. Particularly for MoS2 devices, there exists a lack of understanding of the dominant...methods Monolayer MoS2 films were grown directly on a SiO2 -coated (285 nm) Si substrate using the procedure described in detail by Najmaei et al [24...passivated and etched devices, respectively. In the case of graphene devices, several groups have reported SID/IDS2 in the range of 10−9 Hz−1 to 10−7 Hz−1 at

  8. Facile Hydrothermal Synthesis of Monodispersed MoS2 Ultrathin Nanosheets Assisted by Ionic Liquid Brij56

    Directory of Open Access Journals (Sweden)

    Guan-Qun Han

    2015-01-01

    Full Text Available Monodispersed MoS2 ultrathin nanosheets have been successfully fabricated by a facile hydrothermal process assisted by ionic liquid Brij56. The effect of Brij56 on the morphology and structure of MoS2 has been obviously observed. XRD shows that the as-prepared MoS2 assisted by Brij56 has the weak and broad peak of (002 planes, which implies the small size and well dispersed structure of MoS2 nanosheets. TEM and SEM images reveal that MoS2 ultrathin nanosheets have small size and few stacking layers with the adding of Brij56. HRTEM images prove that MoS2 appears to have a highly monodispersed morphology and to be monolayer ultrathin nanosheets with the length about 5–8 nm, which can provide more exposed rims and edges as active sites for hydrogen evolution reaction. Brij56 has played a crucial role in preparing monodispersed MoS2 ultrathin nanosheets as excellent electrocatalysts. The growth mechanism of monodispersed MoS2 has been discussed in detail.

  9. Robust Denaturation of Villin Headpiece by MoS2 Nanosheet: Potential Molecular Origin of the Nanotoxicity.

    Science.gov (United States)

    Gu, Zonglin; Yang, Zaixing; Kang, Seung-Gu; Yang, Jerry R; Luo, Judong; Zhou, Ruhong

    2016-06-17

    MoS2 nanosheet, a new two-dimensional transition metal dichalcogenides nanomaterial, has attracted significant attentions lately due to many potential promising biomedical applications. Meanwhile, there is also a growing concern on its biocompatibility, with little known on its interactions with various biomolecules such as proteins. In this study, we use all-atom molecular dynamics simulations to investigate the interaction of a MoS2 nanosheet with Villin Headpiece (HP35), a model protein widely used in protein folding studies. We find that MoS2 exhibits robust denaturing capability to HP35, with its secondary structures severely destroyed within hundreds of nanosecond simulations. Both aromatic and basic residues are critical for the protein anchoring onto MoS2 surface, which then triggers the successive protein unfolding process. The main driving force behind the adsorption process is the dispersion interaction between protein and MoS2 monolayer. Moreover, water molecules at the interface between some key hydrophobic residues (e.g. Trp-64) and MoS2 surface also help to accelerate the process driven by nanoscale drying, which provides a strong hydrophobic force. These findings might have shed new light on the potential nanotoxicity of MoS2 to proteins with atomic details, which should be helpful in guiding future biomedical applications of MoS2 with its nanotoxicity mitigated.

  10. Layer-Controlled Chemical Vapor Deposition Growth of MoS2 Vertical Heterostructures via van der Waals Epitaxy.

    Science.gov (United States)

    Samad, Leith; Bladow, Sage M; Ding, Qi; Zhuo, Junqiao; Jacobberger, Robert M; Arnold, Michael S; Jin, Song

    2016-07-26

    The fascinating semiconducting and optical properties of monolayer and few-layer transition metal dichalcogenides, as exemplified by MoS2, have made them promising candidates for optoelectronic applications. Controllable growth of heterostructures based on these layered materials is critical for their successful device applications. Here, we report a direct low temperature chemical vapor deposition (CVD) synthesis of MoS2 monolayer/multilayer vertical heterostructures with layer-controlled growth on a variety of layered materials (SnS2, TaS2, and graphene) via van der Waals epitaxy. Through precise control of the partial pressures of the MoCl5 and elemental sulfur precursors, reaction temperatures, and careful tracking of the ambient humidity, we have successfully and reproducibly grown MoS2 vertical heterostructures from 1 to 6 layers over a large area. The monolayer MoS2 heterostructure was verified using cross-sectional high resolution transmission electron microscopy (HRTEM) while Raman and photoluminescence spectroscopy confirmed the layer-controlled MoS2 growth and heterostructure electronic interactions. Raman, photoluminescence, and energy dispersive X-ray spectroscopy (EDS) mappings verified the uniform coverage of the MoS2 layers. This reaction provides an ideal method for the scalable layer-controlled growth of transition metal dichalcogenide heterostructures via van der Waals epitaxy for a variety of optoelectronic applications.

  11. Role of interlayer coupling in ultra thin MoS 2

    KAUST Repository

    Cheng, Yingchun

    2012-01-01

    The effects of interlayer coupling on the vibrational and electronic properties of ultra thin MoS 2 were studied by ab initio calculations. For smaller slab thickness, the interlayer distance is significantly elongated because of reduced interlayer coupling. This explains the anomalous thickness dependence of the lattice vibrations observed by Lee et al. (ACS Nano, 2010, 4, 2695). The absence of interlayer coupling in mono-layer MoS 2 induces a transition from direct to indirect band gap behaviour. Our results demonstrate a strong interplay between the intralayer chemical bonding and the interlayer van-der-Waals interaction. This journal is © 2012 The Royal Society of Chemistry.

  12. The effects of surface polarity and dangling bonds on the electronic properties of MoS2 on SiO2

    Science.gov (United States)

    Sung, Ha-Jun; Choe, Duk-Hyun; Chang, Kee Joo

    2015-03-01

    MoS2 has recently attracted much attention due to its intriguing physical phenomena and possible applications for the next generation electronic devices. In pristine monolayer MoS2, strong spin-orbit coupling and inversion symmetry breaking allow for an effective coupling between the spin and valley degrees of freedom, inducing valley polarization at the K valleys. However, the spin-valley coupling disappears in bilayer MoS2 because the inversion symmetry is restored. In this work, we investigate the effects of surface polarity and dangling bonds on the electronic properties of MoS2 on α-quartz SiO2 through first-principles calculations. In monolayer MoS2, a transition can take place from the direct-gap to indirect-gap semiconductor in the presence of O dangling bonds. In bilayer MoS2, O dangling bonds induce dipole fields across the interface and thus break the inversion symmetry, resulting in the valley polarization, similar to that of pristine monolayer MoS2. Based on the results, we discuss the origin of the valley polarization observed in MoS2 deposited on SiO2 This work was supported by National Research Foundation of Korea (NRF) under Grant No. NRF-2005-0093845 and by Samsung Science and Technology Foundation under Grant No. SSTFBA1401-08.

  13. High Luminescence Efficiency in MoS2 Grown by Chemical Vapor Deposition.

    Science.gov (United States)

    Amani, Matin; Burke, Robert A; Ji, Xiang; Zhao, Peida; Lien, Der-Hsien; Taheri, Peyman; Ahn, Geun Ho; Kirya, Daisuke; Ager, Joel W; Yablonovitch, Eli; Kong, Jing; Dubey, Madan; Javey, Ali

    2016-07-26

    One of the major challenges facing the rapidly growing field of two-dimensional (2D) transition metal dichalcogenides (TMDCs) is the development of growth techniques to enable large-area synthesis of high-quality materials. Chemical vapor deposition (CVD) is one of the leading techniques for the synthesis of TMDCs; however, the quality of the material produced is limited by defects formed during the growth process. A very useful nondestructive technique that can be utilized to probe defects in semiconductors is the room-temperature photoluminescence (PL) quantum yield (QY). It was recently demonstrated that a PL QY near 100% can be obtained in MoS2 and WS2 monolayers prepared by micromechanical exfoliation by treating samples with an organic superacid: bis(trifluoromethane)sulfonimide (TFSI). Here we have performed a thorough exploration of this chemical treatment on CVD-grown MoS2 samples. We find that the as-grown monolayers must be transferred to a secondary substrate, which releases strain, to obtain high QY by TFSI treatment. Furthermore, we find that the sulfur precursor temperature during synthesis of the MoS2 plays a critical role in the effectiveness of the treatment. By satisfying the aforementioned conditions we show that the PL QY of CVD-grown monolayers can be improved from ∼0.1% in the as-grown case to ∼30% after treatment, with enhancement factors ranging from 100 to 1500× depending on the initial monolayer quality. We also found that after TFSI treatment the PL emission from MoS2 films was visible by eye despite the low absorption (5-10%). The discovery of an effective passivation strategy will speed the development of scalable high-performance optoelectronic and electronic devices based on MoS2.

  14. Identification of rhenium donors and sulfur vacancy acceptors in layered MoS2 bulk samples

    Science.gov (United States)

    Brandão, F. D.; Ribeiro, G. M.; Vaz, P. H.; González, J. C.; Krambrock, K.

    2016-06-01

    MoS2 monolayers, a two-dimensional (2D) direct semiconductor material with an energy gap of 1.9 eV, offer many opportunities to be explored in different electronic devices. Defects often play dominant roles in the electronic and optical properties of semiconductor devices. However, little experimental information about intrinsic and extrinsic defects or impurities is available for this 2D system, and even for macroscopic 3D samples for which MoS2 shows an indirect bandgap of 1.3 eV. In this work, we evaluate the nature of impurities with unpaired spins using electron paramagnetic resonance (EPR) in different geological macroscopic samples. Regarding the fact that monolayers are mostly obtained from natural crystals, we expect that the majority of impurities found in macroscopic samples are also randomly present in MoS2 monolayers. By EPR at low temperatures, rhenium donors and sulfur vacancy acceptors are identified as the main impurities in bulk MoS2 with a corresponding donor concentration of about 108-12 defects/cm2 for MoS2 monolayer. Electrical transport experiments as a function of temperature are in good agreement with the EPR results, revealing a shallow donor state with an ionization energy of 89 meV and a concentration of 7 × 1015 cm-3, which we attribute to rhenium, as well as a second deeper donor state with ionization energy of 241 meV with high concentration of 2 × 1019 cm-3 and net acceptor concentration of 5 × 1018 cm-3 related to sulfur vacancies.

  15. Electronic Structure and Luminescence of Quasi-Freestanding MoS2 Nanopatches on Au(111)

    Science.gov (United States)

    2016-01-01

    Monolayers of transition metal dichalcogenides are interesting materials for optoelectronic devices due to their direct electronic band gaps in the visible spectral range. Here, we grow single layers of MoS2 on Au(111) and find that nanometer-sized patches exhibit an electronic structure similar to their freestanding analogue. We ascribe the electronic decoupling from the Au substrate to the incorporation of vacancy islands underneath the intact MoS2 layer. Excitation of the patches by electrons from the tip of a scanning tunneling microscope leads to luminescence of the MoS2 junction and reflects the one-electron band structure of the quasi-freestanding layer. PMID:27459588

  16. Electromechanical coupling in atomically thin MoS2 and graphene

    Science.gov (United States)

    Manzeli, Sajedeh; Benameur, Muhammed Malik; Allain, Adrien; Ghadimi, Amirhossein; Tosun, Mahmut; Kis, Andras; Gargiulo, Fernando; Autès, Gabriel; Yazyev, Oleg V.

    Nanoelectromechanical systems (NEMS) based on novel materials such as graphene and MoS2 allow studying their electromechanical characteristics. Here, we incorporate single and bilayer MoS2 and graphene into NEMS and investigated their electromechanical behavior. We observe a Strain-induced bandgap modulation in atomically thin MoS2 membranes with a thickness dependent modulation rate. Finite element modeling is used to extract the piezoresistive gauge factor for MoS2. In the case of graphene, deflection of monolayer graphene nanoribbons results in a linear increase in their electrical resistance where an upper limit is estimated for the gauge factor. Surprisingly, we observe oscillations in the electromechanical response of bilayer graphene. Our numerical simulations indicate that these oscillations arise from quantum mechanical interference in the transition region induced by sliding of individual graphene layers with respect to each other. Our results reveal that atomically thin MoS2 membranes show strong piezoresistive effect, comparable to the state-of-the-art silicon sensors. Moreover, bilayer graphene conceals unexpectedly novel physics allowing the rare observation of room temperature electronic interference phenomena.

  17. Radio Frequency Transistors and Circuits Based on CVD MoS2.

    Science.gov (United States)

    Sanne, Atresh; Ghosh, Rudresh; Rai, Amritesh; Yogeesh, Maruthi Nagavalli; Shin, Seung Heon; Sharma, Ankit; Jarvis, Karalee; Mathew, Leo; Rao, Rajesh; Akinwande, Deji; Banerjee, Sanjay

    2015-08-12

    We report on the gigahertz radio frequency (RF) performance of chemical vapor deposited (CVD) monolayer MoS2 field-effect transistors (FETs). Initial DC characterizations of fabricated MoS2 FETs yielded current densities exceeding 200 μA/μm and maximum transconductance of 38 μS/μm. A contact resistance corrected low-field mobility of 55 cm(2)/(V s) was achieved. Radio frequency FETs were fabricated in the ground-signal-ground (GSG) layout, and standard de-embedding techniques were applied. Operating at the peak transconductance, we obtain short-circuit current-gain intrinsic cutoff frequency, fT, of 6.7 GHz and maximum intrinsic oscillation frequency, fmax, of 5.3 GHz for a device with a gate length of 250 nm. The MoS2 device afforded an extrinsic voltage gain Av of 6 dB at 100 MHz with voltage amplification until 3 GHz. With the as-measured frequency performance of CVD MoS2, we provide the first demonstration of a common-source (CS) amplifier with voltage gain of 14 dB and an active frequency mixer with conversion gain of -15 dB. Our results of gigahertz frequency performance as well as analog circuit operation show that large area CVD MoS2 may be suitable for industrial-scale electronic applications.

  18. MoS2-clad microfibre laser delivering conventional, dispersion-managed and dissipative solitons

    Science.gov (United States)

    Cui, Yudong; Lu, Feifei; Liu, Xueming

    2016-07-01

    Molybdenum disulfide (MoS2), whose monolayer possesses a direct band gap, displays promising applications in optoelectronics, photonics, and lasers. Recent researches have demonstrated that MoS2 has not only a significant broadband saturable absorption performance, but also a higher optical nonlinear response than graphene. However, MoS2 shows much lower optical damage threshold owing to the poorer thermal conductivity and mechanical property. Here, we exploit a MoS2-clad microfibre (MCM) as the saturable absorber (SA) for the generation of ultrashort pulses under different dispersion conditions. The improved evanescent field interaction scheme can overcome the laser-induced thermal damage, as well as take full advantage of the strong nonlinear effect of MoS2. With the MCM SA, conventional, dispersion-managed, and dissipative solitons are generated around 1600 nm in Er-doped fibre lasers with anomalous, near-zero, and normal cavity dispersions, respectively. Our work paves the way for applications of 2D layered materials in photonics, especially in laser sources.

  19. MoS2-clad microfibre laser delivering conventional, dispersion-managed and dissipative solitons

    Science.gov (United States)

    Cui, Yudong; Lu, Feifei; Liu, Xueming

    2016-01-01

    Molybdenum disulfide (MoS2), whose monolayer possesses a direct band gap, displays promising applications in optoelectronics, photonics, and lasers. Recent researches have demonstrated that MoS2 has not only a significant broadband saturable absorption performance, but also a higher optical nonlinear response than graphene. However, MoS2 shows much lower optical damage threshold owing to the poorer thermal conductivity and mechanical property. Here, we exploit a MoS2-clad microfibre (MCM) as the saturable absorber (SA) for the generation of ultrashort pulses under different dispersion conditions. The improved evanescent field interaction scheme can overcome the laser-induced thermal damage, as well as take full advantage of the strong nonlinear effect of MoS2. With the MCM SA, conventional, dispersion-managed, and dissipative solitons are generated around 1600 nm in Er-doped fibre lasers with anomalous, near-zero, and normal cavity dispersions, respectively. Our work paves the way for applications of 2D layered materials in photonics, especially in laser sources. PMID:27456468

  20. Interface designed MoS2/GaAs heterostructure solar cell with sandwich stacked hexagonal boron nitride.

    Science.gov (United States)

    Lin, Shisheng; Li, Xiaoqiang; Wang, Peng; Xu, Zhijuan; Zhang, Shengjiao; Zhong, Huikai; Wu, Zhiqian; Xu, Wenli; Chen, Hongsheng

    2015-10-13

    MoS2 is a layered two-dimensional semiconductor with a direct band gap of 1.8 eV. The MoS2/bulk semiconductor system offers a new platform for solar cell device design. Different from the conventional bulk p-n junctions, in the MoS2/bulk semiconductor heterostructure, static charge transfer shifts the Fermi level of MoS2 toward that of bulk semiconductor, lowering the barrier height of the formed junction. Herein, we introduce hexagonal boron nitride (h-BN) into MoS2/GaAs heterostructure to suppress the static charge transfer, and the obtained MoS2/h-BN/GaAs solar cell exhibits an improved power conversion efficiency of 5.42%. More importantly, the sandwiched h-BN makes the Fermi level tuning of MoS2 more effective. By employing chemical doping and electrical gating into the solar cell device, PCE of 9.03% is achieved, which is the highest among all the reported monolayer transition metal dichalcogenide based solar cells.

  1. Tailoring the electrical properties of MoS2 field effect transistors by depositing Au nanoparticles and alkanethiol molecules.

    Science.gov (United States)

    Cho, Kyungjune; Jeong, Hyunhak; Kim, Tae-Young; Pak, Jinsu; Kim, Jae-Keun; Choi, Barbara Yuri; Lee, Takhee

    2016-05-11

    We fabricated and characterized MoS2 field effect transistors. First, we measured the electrical properties of MoS2 field effect transistors (FETs) that were made with mechanically exfoliated MoS2 flakes. Then, we deposited Au nanoparticles on the MoS2 channel and measured the electrical properties. We observed whether the source-drain current increased or decreased after the Au particles were deposited. The deposited Au particles either formed an extra current path and increased the current or behaved as charge-withdrawing sites and decreased the current. Next, we deposited alkanethiol molecules on the Au particles to reduce the work function of the Au. Alkanethiol molecules are known to form a self-assembled monolayer on the Au surface, and the electric dipole moment of the molecules causes the work function of the Au to decrease. Au particles can capture electrons from the MoS2 channel due to their high work function. However, the decreased work function of the Au particles subjected to alkanethiol treatment could cause captured electrons to be released from the Au particles to MoS2. Therefore, the current increased after alkanethiol treatment. This study may provide useful methods to utilize surface treatments with particles and molecules to tailor the electrical properties of MoS2-based FETs.

  2. Hydrogen intercalation in MoS2

    Science.gov (United States)

    Zhu, Zhen; Peelaers, Hartwin; Van de Walle, Chris G.

    2016-08-01

    We investigate the structure and energetics of interstitial hydrogen and hydrogen molecules in layered 2 H -MoS2, an issue of interest both for hydrogen storage applications and for the use of MoS2 as an (opto)electronic material. Using first-principles density functional theory we find that hydrogen interstitials are deep donors. H2 molecules are electrically inactive and energetically more stable than hydrogen interstitials. Their equilibrium position is the hollow site of the MoS2 layers. The migration barrier of a hydrogen molecule is calculated to be smaller than 0.6 eV. We have also explored the insertion energies of hydrogen molecules as a function of hydrogen concentration in MoS2. For low concentrations, additional inserted H2 molecules prefer to be located in hollow sites (on top of the center of a hexagon) in the vicinity of an occupied site. Once two molecules have been inserted, the energy cost for inserting additional H2 molecules becomes much lower. Once all hollow sites are filled, the energy cost increases, but only by a modest amount. We find that up to 13 H2 molecules can be accommodated within the same interlayer spacing of an areal 3 ×3 supercell.

  3. Growth of centimeter-scale atomically thin MoS2 films by pulsed laser deposition

    Directory of Open Access Journals (Sweden)

    Gene Siegel

    2015-05-01

    Full Text Available We are reporting the growth of single layer and few-layer MoS2 films on single crystal sapphire substrates using a pulsed-laser deposition technique. A pulsed KrF excimer laser (wavelength: 248 nm; pulse width: 25 ns was used to ablate a polycrystalline MoS2 target. The material thus ablated was deposited on a single crystal sapphire (0001 substrate kept at 700 °C in an ambient vacuum of 10−6 Torr. Detailed characterization of the films was performed using atomic force microscopy (AFM, Raman spectroscopy, UV-Vis spectroscopy, and photoluminescence (PL measurements. The ablation of the MoS2 target by 50 laser pulses (energy density: 1.5 J/cm2 was found to result in the formation of a monolayer of MoS2 as shown by AFM results. In the Raman spectrum, A1g and E12g peaks were observed at 404.6 cm−1 and 384.5 cm−1 with a spacing of 20.1 cm−1, confirming the monolayer thickness of the film. The UV-Vis absorption spectrum exhibited two exciton absorption bands at 672 nm (1.85 eV and 615 nm (2.02 eV, with an energy split of 0.17 eV, which is in excellent agreement with the theoretically predicted value of 0.15 eV. The monolayer MoS2 exhibited a PL peak at 1.85 eV confirming the direct nature of the band-gap. By varying the number of laser pulses, bi-layer, tri-layer, and few-layer MoS2 films were prepared. It was found that as the number of monolayers (n in the MoS2 films increases, the spacing between the A1g and E12g Raman peaks (Δf increases following an empirical relation, Δ f = 26 . 45 − 15 . 42 1 + 1 . 44 n 0 . 9 cm − 1 .

  4. Lateral Versus Vertical Growth of Two-Dimensional Layered Transition-Metal Dichalcogenides: Thermodynamic Insight into MoS2.

    Science.gov (United States)

    Shang, Shun-Li; Lindwall, Greta; Wang, Yi; Redwing, Joan M; Anderson, Tim; Liu, Zi-Kui

    2016-09-14

    Unprecedented interest has been spurred recently in two-dimensional (2D) layered transition metal dichalcogenides (TMDs) that possess tunable electronic and optical properties. However, synthesis of a wafer-scale TMD thin film with controlled layers and homogeneity remains highly challenging due mainly to the lack of thermodynamic and diffusion knowledge, which can be used to understand and design process conditions, but falls far behind the rapidly growing TMD field. Here, an integrated density functional theory (DFT) and calculation of phase diagram (CALPHAD) modeling approach is employed to provide thermodynamic insight into lateral versus vertical growth of the prototypical 2D material MoS2. Various DFT energies are predicted from the layer-dependent MoS2, 2D flake-size related mono- and bilayer MoS2, to Mo and S migrations with and without graphene and sapphire substrates, thus shedding light on the factors that control lateral versus vertical growth of 2D islands. For example, the monolayer MoS2 flake in a small 2D lateral size is thermodynamically favorable with respect to the bilayer counterpart, indicating the monolayer preference during the initial stage of nucleation; while the bilayer MoS2 flake becomes stable with increasing 2D lateral size. The critical 2D flake-size of phase stability between mono- and bilayer MoS2 is adjustable via the choice of substrate. In terms of DFT energies and CALPHAD modeling, the size dependent pressure-temperature-composition (P-T-x) growth windows are predicted for MoS2, indicating that the formation of MoS2 flake with reduced size appears in the middle but close to the lower T and higher P "Gas + MoS2" phase region. It further suggests that Mo diffusion is a controlling factor for MoS2 growth owing to its extremely low diffusivity compared to that of sulfur. Calculated MoS2 energies, Mo and S diffusivities, and size-dependent P-T-x growth windows are in good accord with available experiments, and the present data

  5. Active control of surface plasmon resonance in MoS2-Ag hybrid nanostructures

    CERN Document Server

    Zu, Shuai; Gong, Yongji; Ajayan, Pulickel M; Fang, Zheyu

    2016-01-01

    Molybdenum disulfide (MoS2) monolayers have attracted much attention for their novel optical properties and efficient light-matter interactions. When excited by incident laser, the optical response of MoS2 monolayers was effectively modified by elementary photo-excited excitons owing to their large exciton binding energy, which can be facilitated for the optical-controllable exciton-plasmon interactions. Inspired by this concept, we experimentally investigated active light control of surface plasmon resonance (SPR) in MoS2-Ag hybrid nanostructures. The white light spectra of SPR were gradually red-shifted by increasing laser power, which was distinctly different from the one of bare Ag nanostructure. This spectroscopic tunability can be further controlled by near-field coupling strength and polarization state of light, and selectively applied to the control of plasmonic dark mode. An analytical Lorentz model for photo-excited excitons induced modulation of MoS2 dielectric function was developed to explain the...

  6. Perspective: Highly ordered MoS2 thin films grown by multi-step chemical vapor deposition process

    Directory of Open Access Journals (Sweden)

    S. N. Heo

    2016-03-01

    Full Text Available We established a process for growing highly ordered MoS2 thin films. The process consists of four steps: MoO3 thermal evaporation, first annealing, sulfurization, and second annealing. The main feature of this process is that thermally deposited MoO3 thin films are employed as a precursor for the MoS2 films. The first deposition step enabled us to achieve precise control of the resulting thickness of the MoS2 films with high uniformity. The crystalline structures, surface morphologies, and chemical states at each step were characterized by X-ray diffraction, atomic force microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Based on these characterizations and a careful optimization of the growth conditions, we successfully produced a highly oriented MoS2 thin film with a thickness of five monolayers over an entire one-centimeter-square sapphire substrate.

  7. Recent Advancement on the Optical Properties of Two-Dimensional Molybdenum Disulfide (MoS2 Thin Films

    Directory of Open Access Journals (Sweden)

    Mingxiao Ye

    2015-03-01

    Full Text Available The emergence of two-dimensional (2D materials has led to tremendous interest in the study of graphene and a series of mono- and few-layered transition metal dichalcogenides (TMDCs. Among these TMDCs, the study of molybdenum disulfide (MoS2 has gained increasing attention due to its promising optical, electronic, and optoelectronic properties. Of particular interest is the indirect to direct band-gap transition from bulk and few-layered structures to mono-layered MoS2, respectively. In this review, the study of these properties is summarized. The use of Raman and Photoluminescence (PL spectroscopy of MoS2 has become a reliable technique for differentiating the number of molecular layers in 2D MoS2.

  8. First principles calculations of the interface properties of a-Al2O3/MoS2 and effects of biaxial strain

    Science.gov (United States)

    Shi, Li-Bin; Li, Ming-Biao; Xiu, Xiao-Ming; Liu, Xu-Yang; Zhang, Kai-Cheng; Liu, Yu-Hui; Li, Chun-Ran; Dong, Hai-Kuan

    2017-05-01

    An amorphous Al2O3 (a-Al2O3)/MoS2 interface has attracted much attention because of its unique properties. In this study, the interface behaviors under non-strain and biaxial strain are investigated by first principles calculations based on the density functional theory. First of all, the generation process of the a-Al2O3 sample is described by molecular dynamics. The calculated bandgap of a-Al2O3 is 3.66 eV for generalized gradient approximation-Perdew, Burke, and Ernzerhof and 5.26 eV for Heyd-Scuseria-Ernzerhof functional. Then, we give a detailed description of the band alignment for the a-Al2O3/MoS2 interface. The valence band offset and conduction band offset change with the number of MoS2 layers. It is noted that the valence band maximum (VBM) of MoS2 moves upward as the number of MoS2 layers is increased. The leakage current for metal/a-Al2O3/MoS2 MOS is also illustrated. At last, the band structure of monolayer MoS2 under biaxial strain ranging from -6% to 6% is discussed, and the impact of the biaxial strain on the band offset is investigated. The VBM of monolayer MoS2 moves downward as the strain changes from compressive to tensile.

  9. Influence of stoichiometry on the optical and electrical properties of chemical vapor deposition derived MoS2.

    Science.gov (United States)

    Kim, In Soo; Sangwan, Vinod K; Jariwala, Deep; Wood, Joshua D; Park, Spencer; Chen, Kan-Sheng; Shi, Fengyuan; Ruiz-Zepeda, Francisco; Ponce, Arturo; Jose-Yacaman, Miguel; Dravid, Vinayak P; Marks, Tobin J; Hersam, Mark C; Lauhon, Lincoln J

    2014-10-28

    Ultrathin transition metal dichalcogenides (TMDCs) of Mo and W show great potential for digital electronics and optoelectronic applications. Whereas early studies were limited to mechanically exfoliated flakes, the large-area synthesis of 2D TMDCs has now been realized by chemical vapor deposition (CVD) based on a sulfurization reaction. The optoelectronic properties of CVD grown monolayer MoS2 have been intensively investigated, but the influence of stoichiometry on the electrical and optical properties has been largely overlooked. Here we systematically vary the stoichiometry of monolayer MoS2 during CVD via controlled sulfurization and investigate the associated changes in photoluminescence and electrical properties. X-ray photoelectron spectroscopy is employed to measure relative variations in stoichiometry and the persistence of MoOx species. As MoS2-δ is reduced (increasing δ), the field-effect mobility of monolayer transistors increases while the photoluminescence yield becomes nonuniform. Devices fabricated from monolayers with the lowest sulfur content have negligible hysteresis and a threshold voltage of ∼ 0 V. We conclude that the electrical and optical properties of monolayer MoS2 crystals can be tuned via stoichiometry engineering to meet the requirements of various applications.

  10. Effects of Organic Molecules with Different Structures and Absorption Bandwidth on Modulating Photoresponse of MoS2 Photodetector.

    Science.gov (United States)

    Huang, Yanmin; Zheng, Wei; Qiu, Yunfeng; Hu, PingAn

    2016-09-01

    Organic dye molecules possessing modulated optical absorption bandwidth and molecular structures can be utilized as sensitizing species for the enhancement of photodetector performance of semiconductor via photoinduced charge transfer mechanism. MoS2 photodetector were modified by drop-casting of methyl orange (MO), rhodamine 6G (R6G), and methylene blue (MB) with different molecular structures and extinction coefficients, and enhanced photodetector performance in terms of photocurrent, photoresponsity, photodetectivity, and external quantum efficiency were obtained after modification of MO, R6G, and MB, respectively. Furthermore, dyes showed different modulating abilities for photodetector performance after combination with MoS2, mainly due to the variation of molecular structures and optical absorption bandwidth. Among tested dyes, deposition of MB onto monolayer MoS2 grown by CVD resulted in photocurrent ∼20 times as high as pristine MoS2 due to favorable photoinduced charge transfer of photoexcited electrons from flat MB molecules to the MoS2 layer. Meanwhile, the corresponding photoresponsivity, photodetectivity, and an external quantum efficiency are 9.09 A W(1-), 2.2 × 10(11) Jones, 1729% at 610 nm, respectively. Photoinduced electron-transfer measurements of the pristine MoS2 and dye-modified MoS2 indicated the n-doping effect of dye molecules on the MoS2. Additionally, surface-enhanced Raman measurements also confirmed the direct correlation with charge transfer between organic dyes and MoS2 taking into account the chemically enhanced Raman scattering mechanism. Present work provides a new clue for the manipulation of high-performance of two-dimensional layered semiconductor-based photodetector via the combination of organic dyes.

  11. Performance Investigation of Multilayer MoS2 Thin-Film Transistors Fabricated via Mask-free Optically Induced Electrodeposition.

    Science.gov (United States)

    Li, Meng; Liu, Na; Li, Pan; Shi, Jialin; Li, Guangyong; Xi, Ning; Wang, Yuechao; Liu, Lianqing

    2017-03-08

    Transition metal dichalcogenides, particularly MoS2, have recently received enormous interest in explorations of the physics and technology of nanodevice applications because of their excellent optical and electronic properties. Although monolayer MoS2 has been extensively investigated for various possible applications, its difficulty of fabrication renders it less appealing than multilayer MoS2. Moreover, multilayer MoS2, with its inherent high electronic/photonic state densities, has higher output driving capabilities and can better satisfy the ever-increasing demand for versatile devices. Here, we present multilayer MoS2 back-gate thin-film transistors (TFTs) that can achieve a relatively low subthreshold swing of 0.75 V/decade and a high mobility of 41 cm(2)·V(-1)·s(-1), which exceeds the typical mobility value of state-of-the-art amorphous silicon-based TFTs by a factor of 80. Ag and Au electrode-based MoS2 TFTs were fabricated by a convenient and rapid process. Then we performed a detailed analysis of the impacts of metal contacts and MoS2 film thickness on electronic performance. Our findings show that smoother metal contacts exhibit better electronic characteristics and that MoS2 film thickness should be controlled within a reasonable range of 30-40 nm to obtain the best mobility values, thereby providing valuable insights regarding performance enhancement for MoS2 TFTs. Additionally, to overcome the limitations of the conventional fabrication method, we employed a novel approach known as optically induced electrodeposition (OIE), which allows the flexible and precise patterning of metal films and enables rapid and mask-free device fabrication, for TFT fabrication.

  12. Photoluminescence of MoS2 Prepared by Effective Grinding-Assisted Sonication Exfoliation

    Directory of Open Access Journals (Sweden)

    Jing-Yuan Wu

    2014-01-01

    Full Text Available Exfoliation of bulk molybdenum disulfide (MoS2 using sonication in appropriate solvent is a promising route to large-scale preparation of few-layered or monolayered crystals. Grinding-assisted sonication exfoliation was used for preparing monolayered MoS2 nanosheets from natural mineral molybdenite. By controlling the sonication time, larger crystallites could be further exfoliated to smaller as well as thinner nanosheets without damaging their structures. The concentration of 1.6 mg mL−1 of final solution could be achieved. Several microscopic techniques like scanning electron microscopy, transmission electron microscopy, and atomic force microscopy were employed to evaluate the exfoliation results. Strong photoluminescence with the peak centered at 440 nm was also observed in the resulting dispersion which included several small lateral-sized (~3 nm nanostructures.

  13. Electron microscopy studies on MoS2 nanocrystals

    DEFF Research Database (Denmark)

    Hansen, Lars Pilsgaard

    Industrial-style MoS2-based hydrotreating catalysts are studied using electron microscopy. The MoS2 nanostructures are imaged with single-atom sensitivity to reveal the catalytically important edge structures. Furthermore, the in-situ formation of MoS2 crystals is imaged for the first time....

  14. Electron microscopy studies on MoS2 nanocrystals

    DEFF Research Database (Denmark)

    Hansen, Lars Pilsgaard

    Industrial-style MoS2-based hydrotreating catalysts are studied using electron microscopy. The MoS2 nanostructures are imaged with single-atom sensitivity to reveal the catalytically important edge structures. Furthermore, the in-situ formation of MoS2 crystals is imaged for the first time....

  15. Strain and Structure Heterogeneity in MoS2 Atomic Layers Grown by Chemical Vapour Deposition

    Science.gov (United States)

    2014-11-18

    carbon nanotube carpet-based microsupercapacitors with high electrochemical performance. Nano Lett. 13, 72 78 (2012). 19. Zou, X., Liu, Y. & Yakobson, B...through a process widely used for synthesis of this type of single-crystalline material. Sodium molybdate (Na2MoO4) or hexaammonium heptamolybdate...scalable synthesis of uniform, high-quality monolayer and few-layer MoS2 films. Sci. Rep. 3, 1866 (2013). 3. van der Zande, A. M. et al. Grains and

  16. Chemical Vapor Sensing with Monolayer MoS2

    Science.gov (United States)

    2013-01-04

    hydrogen atoms are light gray. Nano Letters Letter dx.doi.org/10.1021/nl3043079 | Nano Lett. XXXX, XXX, XXX−XXXE ■ REFERENCES (1) Geim, A. K.; Novoselov , K...Association: Austin, TX, 2011; http://www. itrs.net. (3) Novoselov , K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Griorieva, I. V...Blake, P.; Katsnelson, M. I.; Novoselov , K. S. Detection of individual gas molecules adsorbed on graphene. Nat. Mater. 2007, 6, 652−655. (15) Dan

  17. Inter-Layer Coupling Induced Valence Band Edge Shift in Mono- to Few-Layer MoS2

    Science.gov (United States)

    Trainer, Daniel J.; Putilov, Aleksei V.; Di Giorgio, Cinzia; Saari, Timo; Wang, Baokai; Wolak, Mattheus; Chandrasena, Ravini U.; Lane, Christopher; Chang, Tay-Rong; Jeng, Horng-Tay; Lin, Hsin; Kronast, Florian; Gray, Alexander X.; Xi, Xiaoxing X.; Nieminen, Jouko; Bansil, Arun; Iavarone, Maria

    2017-01-01

    Recent progress in the synthesis of monolayer MoS2, a two-dimensional direct band-gap semiconductor, is paving new pathways toward atomically thin electronics. Despite the large amount of literature, fundamental gaps remain in understanding electronic properties at the nanoscale. Here, we report a study of highly crystalline islands of MoS2 grown via a refined chemical vapor deposition synthesis technique. Using high resolution scanning tunneling microscopy and spectroscopy (STM/STS), photoemission electron microscopy/spectroscopy (PEEM) and μ-ARPES we investigate the electronic properties of MoS2 as a function of the number of layers at the nanoscale and show in-depth how the band gap is affected by a shift of the valence band edge as a function of the layer number. Green’s function based electronic structure calculations were carried out in order to shed light on the mechanism underlying the observed bandgap reduction with increasing thickness, and the role of the interfacial Sulphur atoms is clarified. Our study, which gives new insight into the variation of electronic properties of MoS2 films with thickness bears directly on junction properties of MoS2, and thus impacts electronics application of MoS2. PMID:28084465

  18. A DFT study on the elastic and plastic properties of MoS2 nanosheet subjected to external electric field

    Science.gov (United States)

    Ansari, R.; Shahnazari, A.; Malakpour, S.; Faghihnasiri, M.; Sahmani, S.

    2016-09-01

    Molybdenum disulfide (MoS2) may be synthesized in a large variety of forms such as particles, monolayer and multilayers nanosheets/nanotubes, ropes and ribbons. Due to such diversity, several applications can be found for MoS2. In this paper, on the basis of density functional theory (DFT) calculations using the generalized gradient approximation (GGA) with the Perdew- Burke-Ernzerhof (PBE) exchange correlation, the elastic properties including Young's and bulk moduli together with plastic properties of MoS2 nanosheet under external electric field with magnitudes within the range of 0 V/ang-1.5 V/ang are determined. It is demonstrated that up to the magnitude of 1 V/ang, the external electric field has a negligible influence on the bulk modulus of MoS2 nanosheet. However, by applying an external electric field equal to 1.3 V/ang, a significant increase in the value of bulk modulus occurs. Additionally, by applying an external electric field equal to 1.5 V/ang, the bulk modulus decreases suddenly, showing the considerable influence of high external electric field on the bulk modulus of MoS2 nanosheet. Also, it is observed that the first and second critical strains of the MoS2 nanosheet subjected to biaxial strain are smaller than those of the MoS2 nanosheet under uniaxial strain. Furthermore, it is revealed that for the both uniaxial and biaxial loading cases, by increasing the magnitude of external electric field, the stability of MoS2 nanosheet decreases.

  19. Covalent functionalization of MoS2

    Directory of Open Access Journals (Sweden)

    Stanislav Presolski

    2016-04-01

    Full Text Available MoS2 nanosheets have been used extensively in catalytic, electronic, optoelectronic and electrochemical research due to their diverse properties that are often determined by the method of fabrication. Fine tuning of the colloidal behaviour, specific interactions and further reactivity of the materials is typically achieved by subsequent surface modifications. Arguably the most permanent of these involve covalent attachment of molecules to either the molybdenum or the sulphur atoms in the lattice. Here we review of the nascent field of transition metal dichalcogenide (TMD covalent functionalization and explore the prospective avenues for hybrid organic-inorganic nanomaterials.

  20. Peculiar half-metallic state in zigzag nanoribbons of MoS2: Spin filtering

    Science.gov (United States)

    Khoeini, F.; Shakouri, Kh.; Peeters, F. M.

    2016-09-01

    Layered structures of molybdenum disulfide (MoS2) belong to a new class of two-dimensional (2D) semiconductor materials in which monolayers exhibit a direct band gap in their electronic spectrum. This band gap has recently been shown to vanish due to the presence of metallic edge modes when MoS2 monolayers are terminated by zigzag edges on both sides. Here, we demonstrate that a zigzag nanoribbon of MoS2, when exposed to an external exchange field in combination with a transverse electric field, has the potential to exhibit a peculiar half-metallic nature and thereby allows electrons of only one spin direction to move. The peculiarity of such spin-selective conductors originates from a spin switch near the gap-closing region, so the allowed spin orientation can be controlled by means of an external gate voltage. It is shown that the induced half-metallic phase is resistant to random fluctuations of the exchange field as well as the presence of edge vacancies.

  1. Ab-initio study of dynamical properties of two dimensional MoS2 under strain

    Directory of Open Access Journals (Sweden)

    Himadri Soni

    2015-10-01

    Full Text Available In this paper, we report the biaxial strain induced modifications in the phonon dispersion curves of monolayer MoS2 using first principles calculations in the framework of density functional perturbation theory. We have used the ultrasoft psedopotentials and the exchange correlation energies have been approximated by the local density approximation. There are significant changes in the phonon dispersion curves under strain. A systematic decrease in the optical phonon modes is observed with strain percentage. The longitudinal and transverse acoustic phonon modes of monolayer MoS2 show linear dependency for both without and with strain cases. However, the out of plane acoustic mode (ZA which behaves quadratically with wave vector k- turns linear under strain. We have also linked ripple in single layer (SL MoS2 by ZA mode with strain using shell elasticity theory. The strain induced softening of linear behaviour of ZA mode with strain indicates the absence of rippling under strain. At a very high strain there is a possibility of structural phase transition as the ZA mode at zone centre turns imaginary above 15% strain. Our results indicate a close relationship between the morphology and properties enabling the device tailoring and bandgap engineering in SL-MoS2 by manipulating the strain.

  2. The Positive Effects of Hydrophobic Fluoropolymers on the Electrical Properties of MoS2 Transistors

    Directory of Open Access Journals (Sweden)

    Somayyeh Rahimi

    2016-08-01

    Full Text Available We report the improvement of the electrical performance of field effect transistors (FETs fabricated on monolayer chemical vapor deposited (CVD MoS2, by applying an interacting fluoropolymer capping layer (Teflon-AF. The electrical characterizations of more than 60 FETs, after applying Teflon-AF cap, show significant improvement of the device properties and reduced device to device variation. The improvement includes: 50% reduction of the average gate hysteresis, 30% reduction of the subthreshold swing and about an order of magnitude increase of the current on-off ratio. These favorable changes in device performance are attributed to the reduced exposure of MoS2 channels to the adsorbates in the ambient which can be explained by the polar nature of Teflon-AF cap. A positive shift in the threshold voltage of all the measured FETs is observed, which translates to the more desirable enhancement mode transistor characteristics.

  3. Phase transition and spin-resolved transport in MoS2 nanoribbons

    Science.gov (United States)

    Heshmati-Moulai, A.; Simchi, H.; Esmaeilzadeh, M.; Peeters, F. M.

    2016-12-01

    The electronic structure and transport properties of monolayer MoS2 are studied using a tight-binding approach coupled with the nonequilibrium Green's function method. A zigzag nanoribbon of MoS2 is conducting due to the intersection of the edge states with the Fermi level that is located within the bulk gap. We show that applying a transverse electric field results in the disappearance of this intersection and turns the material into a semiconductor. By increasing the electric field the band gap undergoes a two stage linear increase after which it decreases and ultimately closes. It is shown that in the presence of a uniform exchange field, this electric field tuning of the gap can be exploited to open low energy domains where only one of the spin states contributes to the electronic conductance. This introduces possibilities in designing spin filters for spintronic applications.

  4. Heterostructures based on graphene and MoS2 layers decorated by C60 fullerenes

    Science.gov (United States)

    Chernozatonskii, Leonid A.; Kvashnin, Alexander G.; Sorokin, Pavel B.

    2016-09-01

    Here we present a comprehensive investigation of various novel composite structures based on graphene (G) and molybdenum disulphide (MoS2) monolayers decorated by C60 fullerenes, which can be successfully applied in photovoltaics as a solar cell unit. Theoretical studies of the atomic structure, stability and electronic properties of the proposed G/C60, MoS2/C60 and G/MoS2/C60/G nanostructures were carried out. We show that making the G/MoS2/C60/G heterostructure from the 2D films considered here will lead to the appearance of particular properties suitable for application in photovoltaics due to the broad energetic region of high electronic density of states.

  5. Spiral growth of few-layer MoS2 by chemical vapor deposition

    Science.gov (United States)

    Dong, X.; Yan, C.; Tomer, D.; Li, C. H.; Li, L.

    2016-08-01

    Growth spirals exhibit appealing properties due to a preferred layer stacking and lack of inversion symmetry. Here, we report spiral growth of MoS2 during chemical vapor deposition on SiO2/Si and epitaxial graphene/SiC substrates, and their physical and electronic properties. We determine the layer-dependence of the MoS2 bandgap, ranging from 2.4 eV for the monolayer to a constant of 1.3 eV beyond the fifth layer. We further observe that spirals predominantly initiate at the step edges of the SiC substrate, based on which we propose a growth mechanism driven by screw dislocation created by the coalescence of two growth fronts at steps.

  6. Efficient spin injection and giant magnetoresistance in Fe / MoS 2 / Fe junctions

    KAUST Repository

    Dolui, Kapildeb

    2014-07-02

    We demonstrate giant magnetoresistance in Fe/MoS2/Fe junctions by means of ab initio transport calculations. We show that junctions incorporating either a monolayer or a bilayer of MoS2 are metallic and that Fe acts as an efficient spin injector into MoS2 with an efficiency of about 45%. This is the result of the strong coupling between the Fe and S atoms at the interface. For junctions of greater thickness, a maximum magnetoresistance of ∼300% is obtained, which remains robust with the applied bias as long as transport is in the tunneling limit. A general recipe for improving the magnetoresistance in spin valves incorporating layered transition metal dichalcogenides is proposed. © 2014 American Physical Society.

  7. Folded MoS2 layers with reduced interlayer coupling

    OpenAIRE

    Castellanos-Gomez, Andres; van der Zant, Herre S. J.; Steele, Gary A.

    2013-01-01

    We study molybdenum disulfide (MoS2) structures generated by folding single- and bilayer MoS2 flakes. We find that this modified layer stacking leads to a decrease in the interlayer coupling and an enhancement of the photoluminescence emission yield. We additionally find that folded single-layer MoS2 structures show a contribution to photoluminescence spectra of both neutral and charged excitons, which is a characteristic feature of single-layer MoS2 that has not been observed in multilayer M...

  8. Enhanced quantum efficiency from a mosaic of two dimensional MoS2 formed onto aminosilane functionalised substrates

    Science.gov (United States)

    Wang, Yichao; Della Gaspera, Enrico; Carey, Benjamin J.; Atkin, Paul; Berean, Kyle J.; Clark, Rhiannon M.; Cole, Ivan S.; Xu, Zai-Quan; Zhang, Yupeng; Bao, Qiaoliang; Ou, Jian Zhen; Daeneke, Torben; Kalantar-Zadeh, Kourosh

    2016-06-01

    Developing scalable methods of growing two dimensional molybdenum disulphide (2D MoS2) with strong optical properties, on any desired substrates, is a necessary step towards industrial uptake of this material for optical applications. In this study, Si/SiO2 substrates were functionalised using self-assembled monolayers of three different aminosilanes with various numbers of amine groups and molecular lengths as underlayers for enhancing the adherence of the molybdenum precursor. The tetrahedral [MoS4]2- anion groups from the molybdenum precursor were bonded on these silanised Si/SiO2 substrates afterwards. The substrates were then treated with a combined thermolysis and sulphurisation step. The results showed that silanisation of the substrates using the longest chains and the largest number of amine groups provided a good foundation to grow quasi 2D MoS2 made from adjacent flakes in a mosaic formation. Microscopy and spectroscopy investigations revealed that these quasi 2D MoS2 formed using this long chain aminosilane resulted in flakes with lateral dimensions in micron and submicron ranges composed of adjoining MoS2 pieces of 20 to 60 nm in lateral dimensions, dominantly made of 3 to 5 MoS2 fundamental layers. The obtained quasi 2D MoS2 shows a high internal quantum efficiency of 2.6% associated with the quantum confinement effect and high stoichiometry of the adjoining nanoflakes that form the structure of the sheets. The synthesis technique in this study is reliable and facile and offers a procedure to form large, scalable and patternable quasi 2D MoS2 sheets on various substrates with enhanced optical properties for practical applications.Developing scalable methods of growing two dimensional molybdenum disulphide (2D MoS2) with strong optical properties, on any desired substrates, is a necessary step towards industrial uptake of this material for optical applications. In this study, Si/SiO2 substrates were functionalised using self-assembled monolayers of

  9. Photoresponse of atomically thin MoS2 layers and their planar heterojunctions.

    Science.gov (United States)

    Kallatt, Sangeeth; Umesh, Govindarao; Bhat, Navakanta; Majumdar, Kausik

    2016-08-18

    MoS2 monolayers exhibit excellent light absorption and large thermoelectric power, which are, however, accompanied by a very strong exciton binding energy - resulting in complex photoresponse characteristics. We study the electrical response to scanning photo-excitation on MoS2 monolayer (1L) and bilayer (2L) devices, and also on monolayer/bilayer (1L/2L) planar heterojunction and monolayer/few-layer/multi-layer (1L/FL/ML) planar double heterojunction devices to unveil the intrinsic mechanisms responsible for photocurrent generation in these materials and junctions. A strong photoresponse modulation is obtained by scanning the position of the laser spot, as a consequence of controlling the relative dominance of a number of layer dependent properties, including (i) the photoelectric effect (PE), (ii) the photothermoelectric effect (PTE), (iii) the excitonic effect, (iv) hot photo-electron injection from metal, and (v) carrier recombination. The monolayer and bilayer devices show a peak photoresponse when the laser is focused at the source junction, while the peak position shifts to the monolayer/few-layer junction in the heterostructure devices. The photoresponse is found to be dependent on the incoming light polarization when the source junction is illuminated, although the polarization sensitivity drastically reduces at the monolayer/few-layer heterojunction. Finally, we investigate the laser position dependent transient response of the photocurrent to reveal that trapping of carriers in SiO2 at the source junction is a critical factor to determine the transient response in 2D photodetectors, and also show that, by a systematic device design, such trapping can be avoided in the heterojunction devices, resulting in a fast transient response. The insights obtained will play an important role in designing a fast 2D TMD based photodetector and related optoelectronic and thermoelectric devices.

  10. MoS2 based dual input logic AND gate

    Science.gov (United States)

    Martinez, Luis M.; Pinto, Nicholas J.; Naylor, Carl H.; Johnson, A. T. Charlie

    2016-12-01

    Crystalline monolayers of CVD MoS2 are used as the active semiconducting channel in a split-gate field effect transistor. The device demonstrates logic AND functionality that is controlled by independently addressing each gate terminal with ±10V. When +10V was simultaneously applied to both gates, the device was conductive (ON), while any other combination of gate voltages rendered the device resistive (OFF). The ON/OFF ratio of the device was ˜ 35 and the charge mobility using silicon nitride as the gate dielectric was 1.2cm2/V-s and 0.1cm2/V-s in the ON and OFF states respectively. Clear discrimination between the two states was observed when a simple circuit containing a load resistor was used to test the device logic AND functionality at 10Hz. One advantage is that split gate technology can reduce the number of devices required in complex circuits, leading to compact electronics and large scale integration based on intrinsic 2-D semiconducting materials.

  11. Magnetoresistance in Co/2D MoS2/Co and Ni/2D MoS2/Ni junctions.

    Science.gov (United States)

    Zhang, Han; Ye, Meng; Wang, Yangyang; Quhe, Ruge; Pan, Yuanyuan; Guo, Ying; Song, Zhigang; Yang, Jinbo; Guo, Wanlin; Lu, Jing

    2016-06-28

    Semiconducting single-layer (SL) and few-layer MoS2 have a flat surface, free of dangling bonds. Using density functional theory coupled with non-equilibrium Green's function method, we investigate the spin-polarized transport properties of Co/2D MoS2/Co and Ni/2D MoS2/Ni junctions with MoS2 layer numbers of N = 1, 3, and 5. Well-defined interfaces are formed between MoS2 and metal electrodes. The junctions with a SL MoS2 spacer are almost metallic owing to the strong coupling between MoS2 and the ferromagnets, while those are tunneling with a few layer MoS2 spacer. Both large magnetoresistance and tunneling magnetoresistance are found when fcc or hcp Co is used as an electrode. Therefore, flat single- and few-layer MoS2 can serve as an effective nonmagnetic spacer in a magnetoresistance or tunneling magnetoresistance device with a well-defined interface.

  12. Enhanced quantum efficiency from a mosaic of two dimensional MoS2 formed onto aminosilane functionalised substrates.

    Science.gov (United States)

    Wang, Yichao; Della Gaspera, Enrico; Carey, Benjamin J; Atkin, Paul; Berean, Kyle J; Clark, Rhiannon M; Cole, Ivan S; Xu, Zai-Quan; Zhang, Yupeng; Bao, Qiaoliang; Ou, Jian Zhen; Daeneke, Torben; Kalantar-Zadeh, Kourosh

    2016-06-16

    Developing scalable methods of growing two dimensional molybdenum disulphide (2D MoS2) with strong optical properties, on any desired substrates, is a necessary step towards industrial uptake of this material for optical applications. In this study, Si/SiO2 substrates were functionalised using self-assembled monolayers of three different aminosilanes with various numbers of amine groups and molecular lengths as underlayers for enhancing the adherence of the molybdenum precursor. The tetrahedral [MoS4](2-) anion groups from the molybdenum precursor were bonded on these silanised Si/SiO2 substrates afterwards. The substrates were then treated with a combined thermolysis and sulphurisation step. The results showed that silanisation of the substrates using the longest chains and the largest number of amine groups provided a good foundation to grow quasi 2D MoS2 made from adjacent flakes in a mosaic formation. Microscopy and spectroscopy investigations revealed that these quasi 2D MoS2 formed using this long chain aminosilane resulted in flakes with lateral dimensions in micron and submicron ranges composed of adjoining MoS2 pieces of 20 to 60 nm in lateral dimensions, dominantly made of 3 to 5 MoS2 fundamental layers. The obtained quasi 2D MoS2 shows a high internal quantum efficiency of 2.6% associated with the quantum confinement effect and high stoichiometry of the adjoining nanoflakes that form the structure of the sheets. The synthesis technique in this study is reliable and facile and offers a procedure to form large, scalable and patternable quasi 2D MoS2 sheets on various substrates with enhanced optical properties for practical applications.

  13. Moiré-related in-gap states in a twisted MoS2/graphite heterojunction

    KAUST Repository

    Lu, Chun-I

    2017-07-21

    This report presents a series of low-temperature (4.5 K) scanning tunneling microscopy and spectroscopy experimental results on monolayer MoS2 deposited on highly oriented pyrolytic graphite using chemical vapor deposition. To reveal the detailed connection between atomic morphology and conductivity in twisted MoS2/graphite heterojunctions, we employ high-sensitivity tunneling spectroscopy measurements by choosing a reduced tip-sample distance. We discern previously unobserved conductance peaks within the band gap range of MoS2, and by comparing the tunneling spectra from MoS2 grains of varying rotation with respect to the substrate, show that these features have small but non-negligible dependence on the moiré superstructure. Furthermore, within a single moiré supercell, atomically resolved tunneling spectroscopy measurements show that the spectra between the moiré high and low areas are also distinct. These in-gap states are shown to have an energy shift attributed to their local lattice strain, matching corresponding behavior of the conduction band edge, and we therefore infer that these features are intrinsic to the density of states, rather than experimental artifacts, and attribute them to the twisted stacking and local strain energy of the MoS2/graphite heterointerface.

  14. Enhancing Photoresponsivity of Self-Aligned MoS2 Field-Effect Transistors by Piezo-Phototronic Effect from GaN Nanowires.

    Science.gov (United States)

    Liu, Xingqiang; Yang, Xiaonian; Gao, Guoyun; Yang, Zhenyu; Liu, Haitao; Li, Qiang; Lou, Zheng; Shen, Guozhen; Liao, Lei; Pan, Caofeng; Lin Wang, Zhong

    2016-08-23

    We report high-performance self-aligned MoS2 field-effect transistors (FETs) with enhanced photoresponsivity by the piezo-phototronic effect. The FETs are fabricated based on monolayer MoS2 with a piezoelectric GaN nanowire (NW) as the local gate, and a self-aligned process is employed to define the source/drain electrodes. The fabrication method allows the preservation of the intrinsic property of MoS2 and suppresses the scattering center density in the MoS2/GaN interface, which results in high electrical and photoelectric performances. MoS2 FETs with channel lengths of ∼200 nm have been fabricated with a small subthreshold slope of 64 mV/dec. The photoresponsivity is 443.3 A·W(-1), with a fast response and recovery time of ∼5 ms under 550 nm light illumination. When strain is introduced into the GaN NW, the photoresponsivity is further enhanced to 734.5 A·W(-1) and maintains consistent response and recovery time, which is comparable with that of the mechanical exfoliation of MoS2 transistors. The approach presented here opens an avenue to high-performance top-gated piezo-enhanced MoS2 photodetectors.

  15. Magnetism by interfacial hybridization and p-type doping of MoS(2) in Fe(4)N/MoS(2) superlattices: a first-principles study.

    Science.gov (United States)

    Feng, Nan; Mi, Wenbo; Cheng, Yingchun; Guo, Zaibing; Schwingenschlögl, Udo; Bai, Haili

    2014-03-26

    Magnetic and electronic properties of Fe4N(111)/MoS2(√3 × √3) superlattices are investigated by first-principles calculations, considering two models: (I) Fe(I)Fe(II)-S and (II) N-S interfaces, each with six stacking configurations. In model I, strong interfacial hybridization between Fe(I)/Fe(II) and S results in magnetism of monolayer MoS2, with a magnetic moment of 0.33 μB for Mo located on top of Fe(I). For model II, no magnetism is induced due to weak N-S interfacial bonding, and the semiconducting nature of monolayer MoS2 is preserved. Charge transfer between MoS2 and N results in p-type MoS2 with Schottky barrier heights of 0.5-0.6 eV. Our results demonstrate that the interfacial geometry and hybridization can be used to tune the magnetism and doping in Fe4N(111)/MoS2(√3 × √3) superlattices.

  16. Magnetism by interfacial hybridization and p-type doping of MoS2 in Fe4N/MoS2 superlattices: A first-principles study

    KAUST Repository

    Feng, Nan

    2014-03-26

    Magnetic and electronic properties of Fe4N(111)/MoS 2(√3 × √3) superlattices are investigated by first-principles calculations, considering two models: (I) FeIFe II-S and (II) N-S interfaces, each with six stacking configurations. In model I, strong interfacial hybridization between FeI/Fe II and S results in magnetism of monolayer MoS2, with a magnetic moment of 0.33 μB for Mo located on top of Fe I. For model II, no magnetism is induced due to weak N-S interfacial bonding, and the semiconducting nature of monolayer MoS2 is preserved. Charge transfer between MoS2 and N results in p-type MoS2 with Schottky barrier heights of 0.5-0.6 eV. Our results demonstrate that the interfacial geometry and hybridization can be used to tune the magnetism and doping in Fe4N(111)/MoS2(√3 × √3) superlattices. © 2014 American Chemical Society.

  17. Vertically Conductive MoS2 Spiral Pyramid.

    Science.gov (United States)

    Ly, Thuc Hue; Zhao, Jiong; Kim, Hyun; Han, Gang Hee; Nam, Honggi; Lee, Young Hee

    2016-09-01

    MoS2 spirals grown by the chemical vapor deposition method, driven by a threading dislocation, has a peculiar rhombohedral-like structure. This threading dislocation can carry helical current in the vertical direction and greatly enhances the vertical conductance in the MoS2 multilayer samples.

  18. The role of electronic coupling between substrate and 2D MoS2 nanosheets in electrocatalytic production of hydrogen

    Science.gov (United States)

    Voiry, Damien; Fullon, Raymond; Yang, Jieun; de Carvalho Castro E Silva, Cecilia; Kappera, Rajesh; Bozkurt, Ibrahim; Kaplan, Daniel; Lagos, Maureen J.; Batson, Philip E.; Gupta, Gautam; Mohite, Aditya D.; Dong, Liang; Er, Dequan; Shenoy, Vivek B.; Asefa, Tewodros; Chhowalla, Manish

    2016-09-01

    The excellent catalytic activity of metallic MoS2 edges for the hydrogen evolution reaction (HER) has led to substantial efforts towards increasing the edge concentration. The 2H basal plane is less active for the HER because it is less conducting and therefore possesses less efficient charge transfer kinetics. Here we show that the activity of the 2H basal planes of monolayer MoS2 nanosheets can be made comparable to state-of-the-art catalytic properties of metallic edges and the 1T phase by improving the electrical coupling between the substrate and the catalyst so that electron injection from the electrode and transport to the catalyst active site is facilitated. Phase-engineered low-resistance contacts on monolayer 2H-phase MoS2 basal plane lead to higher efficiency of charge injection in the nanosheets so that its intrinsic activity towards the HER can be measured. We demonstrate that onset potentials and Tafel slopes of ~-0.1 V and ~50 mV per decade can be achieved from 2H-phase catalysts where only the basal plane is exposed. We show that efficient charge injection and the presence of naturally occurring sulfur vacancies are responsible for the observed increase in catalytic activity of the 2H basal plane. Our results provide new insights into the role of contact resistance and charge transport on the performance of two-dimensional MoS2 nanosheet catalysts for the HER.

  19. Photothermal characterization of MoS2 emission coupled to a microdisk cavity

    Science.gov (United States)

    Reed, Jason C.; Malek, Stephanie C.; Yi, Fei; Naylor, Carl H.; Charlie Johnson, A. T.; Cubukcu, Ertugrul

    2016-11-01

    Integration of emerging two-dimensional direct bandgap semiconductors onto optical microcavities is important for nanophotonic light sources. In most cases, to achieve high quality factors, such microcavity designs require thermally isolated structures leading to pronounced photothermal effects. Here, we report experimental results on spectroscopic and time-domain characterization of photothermal response from MoS2 monolayers coupled to microdisk resonators. We find that judicious utilization of pulsed laser excitation can circumvent irreversible photoabsorption induced material damage. Our results agree well with finite element method based thermal simulations.

  20. Flexible low-power RF nanoelectronics in the GHz regime using CVD MoS2

    Science.gov (United States)

    Yogeesh, Maruthi

    Two-dimensional (2D) materials have attracted substantial interest for flexible nanoelectronics due to the overall device mechanical flexibility and thickness scalability for high mechanical performance and low operating power. In this work, we demonstrate the first MoS2 RF transistors on flexible substrates based on CVD-grown monolayers, featuring record GHz cutoff frequency (5.6 GHz) and saturation velocity (~1.8×106 cm/s), which is significantly superior to contemporary organic and metal oxide thin-film transistors. Furthermore, multicycle three-point bending results demonstrated the electrical robustness of our flexible MoS2 transistors after 10,000 cycles of mechanical bending. Additionally, basic RF communication circuit blocks such as amplifier, mixer and wireless AM receiver have been demonstrated. These collective results indicate that MoS2 is an ideal advanced semiconducting material for low-power, RF devices for large-area flexible nanoelectronics and smart nanosystems owing to its unique combination of large bandgap, high saturation velocity and high mechanical strength.

  1. MoS2 edges and heterophase interfaces: energy, structure and phase engineering

    Science.gov (United States)

    Zhou, Songsong; Han, Jian; Sun, Jianwei; Srolovitz, David J.

    2017-06-01

    The transition metal dichalcogenides exhibit polymorphism; i.e. both 2H and 1T‧ crystal structures, each with unique electronic properties. These two phases can coexist within the same monolayer microstructure, producing 2H/1T‧ interfaces. Here we report a systematic investigation of the energetics of the experimentally most important MoS2 heterophase interfaces and edges. The stable interface and edge structures change with chemical potential (these edges/interfaces are usually non-stoichiometric). Stable edges tend to be those of highest atomic density and the stable interfaces correspond to those with local atomic structure very similar to the 2H crystal. The interfacial energies are lower than those of the edges, and the 1T‧ edges have lower energy than the 2H edges. Because the 1T‧ edges have much lower energy than the 2H edges, a sufficiently narrow 1T‧ ribbon will be more stable than the corresponding 2H ribbon (this critical width is much larger in MoTe2 than in MoS2). Similarly, a large 2H flake have an equilibrium strip of 1T‧ along its edge (again this effect is much larger in MoTe2 than in MoS2). Application of tensile strains can increase the width of the stable 1T‧ strip or the critical thickness below which a ribbon favors the 1T‧ structure. These effects provide a means to phase engineer transition metal dichalcogenide microstructures.

  2. Remote Plasma Oxidation and Atomic Layer Etching of MoS2.

    Science.gov (United States)

    Zhu, Hui; Qin, Xiaoye; Cheng, Lanxia; Azcatl, Angelica; Kim, Jiyoung; Wallace, Robert M

    2016-07-27

    Exfoliated molybdenum disulfide (MoS2) is shown to chemically oxidize in a layered manner upon exposure to a remote O2 plasma. X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and atomic force microscopy (AFM) are employed to characterize the surface chemistry, structure, and topography of the oxidation process and indicate that the oxidation mainly occurs on the topmost layer without altering the chemical composition of underlying layer. The formation of S-O bonds upon short, remote plasma exposure pins the surface Fermi level to the conduction band edge, while the MoOx formation at high temperature modulates the Fermi level toward the valence band through band alignment. A uniform coverage of monolayer amorphous MoO3 is obtained after 5 min or longer remote O2 plasma exposure at 200 °C, and the MoO3 can be completely removed by annealing at 500 °C, leaving a clean ordered MoS2 lattice structure as verified by XPS, LEED, AFM, and scanning tunneling microscopy. This work shows that a remote O2 plasma can be useful for both surface functionalization and a controlled thinning method for MoS2 device fabrication processes.

  3. Optical imaging of the valley Hall effect in MoS2 transistors

    Science.gov (United States)

    Lee, Jieun

    The newly emerged two-dimensional (2D) transition metal dichalcogenides (TMDs) with nonequivalent K and K' valleys have provided an ideal laboratory for exploring the valley degree of freedom of electrons, as well as their potential applications for information processing. Valley Hall effect (VHE), in which a transverse valley current is formed under a longitudinal electrical bias in the absence of a magnetic field, has been predicted in 2D TMDs with broken inversion symmetry. The effect has recently been demonstrated in monolayer MoS2 through a photo-induced anomalous Hall effect, which uses circularly polarized light to preferentially excite electrons into a specific valley. In this talk, we will present our recent results on the development of Kerr rotation microscopy to image the VHE. The valley polarizations of opposite sign accumulated on two opposing edges of MoS2 transistors from the VHE are measured directly. We will also discuss the possibility of electrical control of the VHE in bilayer MoS2, which possesses inversion symmetry. An application of a vertical electric field breaks the inversion symmetry and consequently yields the VHE.

  4. Preparation of Nanocrystalline MoS2 Hollow Spheres

    Institute of Scientific and Technical Information of China (English)

    2003-01-01

    Nanocrystalline MoS2 with hollow spherical morphology has been prepared by the hydrothermal method. The products are characterized by means of X-ray powder diffraction, transmission electron microscopy and high-resolution transmission electron microscopy. The experimental results give the evidence that the sample is consists of hollow spheres 400~600 nm in diameter, and there is much whisker on the surface of MoS2 hollow sphere.

  5. Optical spectrum of MoS2: many-body effects and diversity of exciton states.

    Science.gov (United States)

    Qiu, Diana Y; da Jornada, Felipe H; Louie, Steven G

    2013-11-22

    We present first-principles calculations of the optical response of monolayer molybdenum disulfide employing the GW-Bethe-Salpeter equation (GW-BSE) approach including self-energy, excitonic, and electron-phonon effects. We show that monolayer MoS2 possesses a large and diverse number of strongly bound excitonic states with novel k-space characteristics that were not previously seen experimentally or theoretically. The absorption spectrum is shown to be dominated by excitonic states with a binding energy close to 1 eV and by strong electron-phonon broadening in the visible to ultraviolet range. Our results explain recent experimental measurements and resolve inconsistencies between previous GW-BSE calculations.

  6. Near-unity photoluminescence quantum yield in MoS2

    KAUST Repository

    Amani, Matin

    2015-11-26

    Two-dimensional (2D) transition metal dichalcogenides have emerged as a promising material system for optoelectronic applications, but their primary figure of merit, the room-temperature photoluminescence quantum yield (QY), is extremely low.The prototypical 2D material molybdenum disulfide (MoS2) is reported to have a maximum QYof 0.6%, which indicates a considerable defect density. Herewe report on an air-stable, solution-based chemical treatment by an organic superacid, which uniformly enhances the photoluminescence and minority carrier lifetime of MoS2 monolayers by more than two orders of magnitude.The treatment eliminates defect-mediated nonradiative recombination, thus resulting in a finalQYofmore than 95%, with a longest-observed lifetime of 10.8 0.6 nanoseconds. Our ability to obtain optoelectronic monolayers with near-perfect properties opens the door for the development of highly efficient light-emitting diodes, lasers, and solar cells based on 2D materials.

  7. Novel electronic properties of a new MoS2/TiO2 heterostructure and potential applications in solar cells and photocatalysis

    Science.gov (United States)

    Li, Yanhua; Cai, Congzhong; Gu, Yonghong; Cheng, Wende; Xiong, Wen; Zhao, Chengjun

    2017-08-01

    The structural and electronic properties of two-dimensional (2D) MoS2/TiO2 heterostructure with a special configuration of Moiré pattern have been investigated for the first time using first-principles methods with van der Waals correction. It is found that the new van der Waals heterostructure is of a type-II band alignment between the MoS2 and TiO2 layers, and the electronic structures of monolayer MoS2 and 2D TiO2 are well retained in their respective layers due to a weak interlayer coupling, which suggests that the heterostructure may have potential applications in many fields such as photoelectric devices, photocatalysis, energy conversion and storage, etc. Meanwhile, the heterostructure can also provide an ideal platform of two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) for fundamental research such as spin Hall effect, etc. In addition, it is discovered that the corresponding MoS2/TiO2 superlattice also has similar electronic properties to MoS2/TiO2 bilayer heterostructure. Furthermore, two novel design schemes based on the MoS2/TiO2 heterostructure and superlattice are proposed for a solar cell and photocatalyst, respectively.

  8. Wafer-scale production of highly uniform two-dimensional MoS2 by metal-organic chemical vapor deposition

    Science.gov (United States)

    Kim, TaeWan; Mun, Jihun; Park, Hyeji; Joung, DaeHwa; Diware, Mangesh; Won, Chegal; Park, Jonghoo; Jeong, Soo-Hwan; Kang, Sang-Woo

    2017-05-01

    Semiconducting two-dimensional (2D) materials, particularly extremely thin molybdenum disulfide (MoS2) films, are attracting considerable attention from academia and industry owing to their distinctive optical and electrical properties. Here, we present the direct growth of a MoS2 monolayer with unprecedented spatial and structural uniformity across an entire 8 inch SiO2/Si wafer. The influences of growth pressure, ambient gases (Ar, H2), and S/Mo molar flow ratio on the MoS2 layered growth were explored by considering the domain size, nucleation sites, morphology, and impurity incorporation. Monolayer MoS2-based field effect transistors achieve an electron mobility of 0.47 cm2 V-1 s-1 and on/off current ratio of 5.4 × 104. This work demonstrates the potential for reliable wafer-scale production of 2D MoS2 for practical applications in next-generation electronic and optical devices.

  9. Electron Excess Doping and Effective Schottky Barrier Reduction on the MoS2/h-BN Heterostructure.

    Science.gov (United States)

    Joo, Min-Kyu; Moon, Byoung Hee; Ji, Hyunjin; Han, Gang Hee; Kim, Hyun; Lee, Gwanmu; Lim, Seong Chu; Suh, Dongseok; Lee, Young Hee

    2016-10-12

    Layered hexagonal boron nitride (h-BN) thin film is a dielectric that surpasses carrier mobility by reducing charge scattering with silicon oxide in diverse electronics formed with graphene and transition metal dichalcogenides. However, the h-BN effect on electron doping concentration and Schottky barrier is little known. Here, we report that use of h-BN thin film as a substrate for monolayer MoS2 can induce ∼6.5 × 10(11) cm(-2) electron doping at room temperature which was determined using theoretical flat band model and interface trap density. The saturated excess electron concentration of MoS2 on h-BN was found to be ∼5 × 10(13) cm(-2) at high temperature and was significantly reduced at low temperature. Further, the inserted h-BN enables us to reduce the Coulombic charge scattering in MoS2/h-BN and lower the effective Schottky barrier height by a factor of 3, which gives rise to four times enhanced the field-effect carrier mobility and an emergence of metal-insulator transition at a much lower charge density of ∼1.0 × 10(12) cm(-2) (T = 25 K). The reduced effective Schottky barrier height in MoS2/h-BN is attributed to the decreased effective work function of MoS2 arisen from h-BN induced n-doping and the reduced effective metal work function due to dipole moments originated from fixed charges in SiO2.

  10. Plasmons on the edge of MoS2 nanostructures

    DEFF Research Database (Denmark)

    Andersen, Kirsten; Jacobsen, Karsten Wedel; Thygesen, Kristian Sommer

    2014-01-01

    Using ab initio calculations we predict the existence of one-dimensional (1D), atomically confined plasmons at the edges of a zigzag MoS2 nanoribbon. The strongest plasmon originates from a metallic edge state localized on the sulfur dimers decorating the Mo edge of the ribbon. A detailed analysis...... of the dielectric function reveals that the observed deviations from the ideal 1D plasmon behavior result from single-particle transitions between the metallic edge state and the valence and conduction bands of the MoS2 sheet. The Mo and S edges of the ribbon are clearly distinguishable in calculated spatially...... resolved electron energy loss spectrum owing to the different plasmonic properties of the two edges. The edge plasmons could potentially be utilized for tuning the photocatalytic activity of MoS2 nanoparticles....

  11. Uncovering edge states and electrical inhomogeneity in MoS2 field-effect transistors.

    Science.gov (United States)

    Wu, Di; Li, Xiao; Luan, Lan; Wu, Xiaoyu; Li, Wei; Yogeesh, Maruthi N; Ghosh, Rudresh; Chu, Zhaodong; Akinwande, Deji; Niu, Qian; Lai, Keji

    2016-08-02

    The understanding of various types of disorders in atomically thin transition metal dichalcogenides (TMDs), including dangling bonds at the edges, chalcogen deficiencies in the bulk, and charges in the substrate, is of fundamental importance for TMD applications in electronics and photonics. Because of the imperfections, electrons moving on these 2D crystals experience a spatially nonuniform Coulomb environment, whose effect on the charge transport has not been microscopically studied. Here, we report the mesoscopic conductance mapping in monolayer and few-layer MoS2 field-effect transistors by microwave impedance microscopy (MIM). The spatial evolution of the insulator-to-metal transition is clearly resolved. Interestingly, as the transistors are gradually turned on, electrical conduction emerges initially at the edges before appearing in the bulk of MoS2 flakes, which can be explained by our first-principles calculations. The results unambiguously confirm that the contribution of edge states to the channel conductance is significant under the threshold voltage but negligible once the bulk of the TMD device becomes conductive. Strong conductance inhomogeneity, which is associated with the fluctuations of disorder potential in the 2D sheets, is also observed in the MIM images, providing a guideline for future improvement of the device performance.

  12. Thermal conductivity of MoS2 polycrystalline nanomembranes

    Science.gov (United States)

    Sledzinska, M.; Graczykowski, B.; Placidi, M.; Saleta Reig, D.; El Sachat, A.; Reparaz, J. S.; Alzina, F.; Mortazavi, B.; Quey, R.; Colombo, L.; Roche, S.; Sotomayor Torres, C. M.

    2016-09-01

    Heat conduction in 2D materials can be effectively engineered by means of controlling nanoscale grain structure. A favorable thermal performance makes these structures excellent candidates for integrated heat management units. Here we show combined experimental and theoretical studies for MoS2 nanosheets in a nanoscale grain-size limit. We report thermal conductivity measurements on 5 nm thick polycrystalline MoS2 by means of 2-laser Raman thermometry. The free-standing, drum-like MoS2 nanomembranes were fabricated using a novel polymer- and residue-free, wet transfer, in which we took advantage of the difference in the surface energies between MoS2 and the growth substrate to transfer the CVD-grown nanosheets. The measurements revealed a strong reduction in the in-plane thermal conductivity down to about 0.73 ± 0.25 {{{W}}{{m}}}-1 {{{K}}}-1. The results are discussed theoretically using finite elements method simulations for a polycrystalline film, and a scaling trend of the thermally conductivity with grain size is proposed.

  13. Plasma nanocoating of thiophene onto MoS2 nanotubes

    Science.gov (United States)

    Türkaslan, Banu Esencan; Dikmen, Sibel; Öksüz, Lütfi; Öksüz, Aysegul Uygun

    2015-12-01

    MoS2 nanotubes were coated with conductive polymer thiophene by atmospheric pressure radio-frequency (RF) glow discharge. MoS2 nanotubes were prepared by thermal decomposition of hexadecylamine (HDA) intercalated laminar MoS2 precursor on anodized aluminum oxide template and the thiophene was polymerized directly on surface of these nanotubes as in situ by plasma method. The effect of plasma power on PTh/MoS2 nanocomposite properties has been investigated by means of Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM and EDX), and X-ray diffraction spectroscopy (XRD). The presence of PTh bands in the FTIR spectra of PTh/MoS2 nanotube nanocomposites corresponding XRD results indicates that the polythiophene coating onto MoS2 nanotube. The chemical structure of PTh is not changed when the plasma power of discharge differ from 117 to 360 W. SEM images of nanocomposites show that when the discharge power is increased between 117 and 360 W the average diameter of PTh/MoS2 nanotube nanocomposites are changed and the structure become more uniformly.

  14. Atomic and electronic structure of MoS2 nanoparticles

    DEFF Research Database (Denmark)

    Bollinger, Mikkel; Jacobsen, Karsten Wedel; Nørskov, Jens Kehlet

    2003-01-01

    at the edges. The electronic structure of the edge states is studied and we discuss their influence on the chemical properties of the edges. In particular, we study the reactivity towards hydrogen and show that hydrogen may form stable chemical bonds with both the two low-Miller indexed edges of MoS2. A model...

  15. Simultaneous Hosting of Positive and Negative Trions and the Enhanced Direct Band Emission in MoSe2/MoS2 Heterostacked Multilayers.

    Science.gov (United States)

    Kim, Min Su; Seo, Changwon; Kim, Hyun; Lee, Jubok; Luong, Dinh Hoa; Park, Ji-Hoon; Han, Gang Hee; Kim, Jeongyong

    2016-06-28

    Heterostacking of layered transition-metal dichalcogenide (LTMD) monolayers (1Ls) offers a convenient way of designing two-dimensional exciton systems. Here we demonstrate the simultaneous hosting of positive trions and negative trions in heterobilayers made by vertically stacking 1L MoSe2 and 1L MoS2. The charge transfer occurring between the 1Ls of MoSe2 and MoS2 converted the polarity of trions in 1L MoSe2 from negative to positive, resulting in the presence of positive trions in the 1L MoSe2 and negative trions in the 1L MoS2 of the same heterostacked bilayer. Significantly enhanced MoSe2 photoluminescence (PL) in the heterostacked bilayers compared to the PL of 1L MoSe2 alone suggests that, unlike other previously reported heterostacked bilayers, direct band transition of 1L MoSe2 in heterobilayer was enhanced after the vertical heterostacking. Moreover, by inserting hexagonal BN monolayers between 1L MoSe2 and 1L MoS2, we were able to adjust the charge transfer to maximize the MoSe2 PL of the heteromultilayers and have achieved a 9-fold increase of the PL emission. The enhanced optical properties of our heterostacked LTMDs suggest the exciting possibility of designing LTMD structures that exploit the superior optical properties of 1L LTMDs.

  16. Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS2 bulk crystals

    Science.gov (United States)

    Gehlmann, Mathias; Aguilera, Irene; Bihlmayer, Gustav; Młyńczak, Ewa; Eschbach, Markus; Döring, Sven; Gospodarič, Pika; Cramm, Stefan; Kardynał, Beata; Plucinski, Lukasz; Blügel, Stefan; Schneider, Claus M.

    2016-06-01

    Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be spin-polarized as a whole. However, it has been recently shown that the electronic structure in these crystals can in fact show regions of high spin-polarization, as long as it is probed locally in real and in reciprocal space. In this article we present the first observation of this type of compensated polarization in MoS2 bulk crystals. Using spin- and angle-resolved photoemission spectroscopy (ARPES), we directly observed a spin-polarization of more than 65% for distinct valleys in the electronic band structure. By additionally evaluating the probing depth of our method, we find that these valence band states at the point in the Brillouin zone are close to fully polarized for the individual atomic trilayers of MoS2, which is confirmed by our density functional theory calculations. Furthermore, we show that this spin-layer locking leads to the observation of highly spin-polarized bands in ARPES since these states are almost completely confined within two dimensions. Our findings prove that these highly desired properties of MoS2 can be accessed without thinning it down to the monolayer limit.

  17. Dynamical observations on the crack tip zone and stress corrosion of two-dimensional MoS2

    Science.gov (United States)

    Ly, Thuc Hue; Zhao, Jiong; Cichocka, Magdalena Ola; Li, Lain-Jong; Lee, Young Hee

    2017-01-01

    Whether and how fracture mechanics needs to be modified for small length scales and in systems of reduced dimensionality remains an open debate. Here, employing in situ transmission electron microscopy, atomic structures and dislocation dynamics in the crack tip zone of a propagating crack in two-dimensional (2D) monolayer MoS2 membrane are observed, and atom-to-atom displacement mapping is obtained. The electron beam is used to initiate the crack; during in situ observation of crack propagation the electron beam effect is minimized. The observed high-frequency emission of dislocations is beyond previous understanding of the fracture of brittle MoS2. Strain analysis reveals dislocation emission to be closely associated with the crack propagation path in nanoscale. The critical crack tip plastic zone size of nearly perfect 2D MoS2 is between 2 and 5 nm, although it can grow to 10 nm under corrosive conditions such as ultraviolet light exposure, showing enhanced dislocation activity via defect generation.

  18. New quantum spin Hall insulator in two-dimensional MoS2 with periodically distributed pores.

    Science.gov (United States)

    Liu, Peng-Fei; Zhou, Liujiang; Frauenheim, Thomas; Wu, Li-Ming

    2016-03-01

    MoS2, one of the transition metal dichalcogenides (TMDs), has gained a lot of attention due to its excellent semiconductor characteristics and potential applications. Here, based on density functional theory methods, we predict a novel 2D QSH insulator in the porous allotrope of monolayer MoS2 (g-MoS2), consisting of MoS2 squares and hexagons. g-MoS2 has a nontrivial gap as large as 109 meV, comparable with previously reported 1T'-MoS2 (80 meV) and so-MoS2 (25 meV). We demonstrate that the origin of the 2D QSH effect in g-MoS2 originates from the pure d-d band inversion, different from the conventional band inversion between s-p, p-p or d-p orbitals. The new polymorph greatly enriches the TMD family and its stabilities are confirmed using phonon spectrum analysis. In particular, its porous structure endows it with the potential for efficient gas separation and energy storage applications.

  19. Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS2 bulk crystals.

    Science.gov (United States)

    Gehlmann, Mathias; Aguilera, Irene; Bihlmayer, Gustav; Młyńczak, Ewa; Eschbach, Markus; Döring, Sven; Gospodarič, Pika; Cramm, Stefan; Kardynał, Beata; Plucinski, Lukasz; Blügel, Stefan; Schneider, Claus M

    2016-06-01

    Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be spin-polarized as a whole. However, it has been recently shown that the electronic structure in these crystals can in fact show regions of high spin-polarization, as long as it is probed locally in real and in reciprocal space. In this article we present the first observation of this type of compensated polarization in MoS2 bulk crystals. Using spin- and angle-resolved photoemission spectroscopy (ARPES), we directly observed a spin-polarization of more than 65% for distinct valleys in the electronic band structure. By additionally evaluating the probing depth of our method, we find that these valence band states at the point in the Brillouin zone are close to fully polarized for the individual atomic trilayers of MoS2, which is confirmed by our density functional theory calculations. Furthermore, we show that this spin-layer locking leads to the observation of highly spin-polarized bands in ARPES since these states are almost completely confined within two dimensions. Our findings prove that these highly desired properties of MoS2 can be accessed without thinning it down to the monolayer limit.

  20. Dynamical observations on the crack tip zone and stress corrosion of two-dimensional MoS2

    KAUST Repository

    Ly, Thuc Hue

    2017-01-18

    Whether and how fracture mechanics needs to be modified for small length scales and in systems of reduced dimensionality remains an open debate. Here, employing in situ transmission electron microscopy, atomic structures and dislocation dynamics in the crack tip zone of a propagating crack in two-dimensional (2D) monolayer MoS2 membrane are observed, and atom-to-atom displacement mapping is obtained. The electron beam is used to initiate the crack; during in situ observation of crack propagation the electron beam effect is minimized. The observed high-frequency emission of dislocations is beyond previous understanding of the fracture of brittle MoS2. Strain analysis reveals dislocation emission to be closely associated with the crack propagation path in nanoscale. The critical crack tip plastic zone size of nearly perfect 2D MoS2 is between 2 and 5 nm, although it can grow to 10 nm under corrosive conditions such as ultraviolet light exposure, showing enhanced dislocation activity via defect generation.

  1. Rendering high charge density of states in ionic liquid-gated MoS 2 transistors

    NARCIS (Netherlands)

    Lee, Y.; Lee, J.; Kim, S.; Park, H.S.

    2014-01-01

    We investigated high charge density of states (DOS) in the bandgap of MoS2 nanosheets with variable temperature measurements on ionic liquid-gated MoS2 transistors. The thermally activated charge transport indicates that the electrical current in the two-dimensional MoS 2 nanosheets under high charg

  2. Electrical Transport Properties of Polymorphic MoS2.

    Science.gov (United States)

    Kim, Jun Suk; Kim, Jaesu; Zhao, Jiong; Kim, Sungho; Lee, Jin Hee; Jin, Youngjo; Choi, Homin; Moon, Byoung Hee; Bae, Jung Jun; Lee, Young Hee; Lim, Seong Chu

    2016-08-23

    The engineering of polymorphs in two-dimensional layered materials has recently attracted significant interest. Although the semiconducting (2H) and metallic (1T) phases are known to be stable in thin-film MoTe2, semiconducting 2H-MoS2 is locally converted into metallic 1T-MoS2 through chemical lithiation. In this paper, we describe the observation of the 2H, 1T, and 1T' phases coexisting in Li-treated MoS2, which result in unusual transport phenomena. Although multiphase MoS2 shows no transistor-gating response, the channel resistance decreases in proportion to the temperature, similar to the behavior of a typical semiconductor. Transmission electron microscopy images clearly show that the 1T and 1T' phases are randomly distributed and intervened with 2H-MoS2, which is referred to as the 1T and 1T' puddling phenomenon. The resistance curve fits well with 2D-variable range-hopping transport behavior, where electrons hop over 1T domains that are bounded by semiconducting 2H phases. However, near 30 K, electrons hop over charge puddles. The large temperature coefficient of resistance (TCR) of multiphase MoS2, -2.0 × 10(-2) K(-1) at 300 K, allows for efficient IR detection at room temperature by means of the photothermal effect.

  3. Strain-Gated Field Effect Transistor of a MoS2-ZnO 2D-1D Hybrid Structure.

    Science.gov (United States)

    Chen, Libo; Xue, Fei; Li, Xiaohui; Huang, Xin; Wang, Longfei; Kou, Jinzong; Wang, Zhong Lin

    2016-01-26

    Two-dimensional (2D) molybdenum disulfide (MoS2) is an exciting material due to its unique electrical, optical, and piezoelectric properties. Owing to an intrinsic band gap of 1.2-1.9 eV, monolayer or a-few-layer MoS2 is used for fabricating field effect transistors (FETs) with high electron mobility and on/off ratio. However, the traditional FETs are controlled by an externally supplied gate voltage, which may not be sensitive enough to directly interface with a mechanical stimulus for applications in electronic skin. Here we report a type of top-pressure/force-gated field effect transistors (PGFETs) based on a hybrid structure of a 2D MoS2 flake and 1D ZnO nanowire (NW) array. Once an external pressure is applied, the piezoelectric polarization charges created at the tips of ZnO NWs grown on MoS2 act as a gate voltage to tune/control the source-drain transport property in MoS2. At a 6.25 MPa applied stimulus on a packaged device, the source-drain current can be tuned for ∼25%, equivalent to the results of applying an extra -5 V back gate voltage. Another type of PGFET with a dielectric layer (Al2O3) sandwiched between MoS2 and ZnO also shows consistent results. A theoretical model is proposed to interpret the received data. This study sets the foundation for applying the 2D material-based FETs in the field of artificial intelligence.

  4. An analysis of electrochemical energy storage using electrodes fabricated from atomically thin 2D structures of MoS2, graphene and MoS2/graphene composites

    Science.gov (United States)

    Huffstutler, Jacob D.

    The behavior of 2D materials has become of great interest in the wake of development of electrochemical double-layer capacitors (EDLCs) and the discovery of monolayer graphene by Geim and Novoselov. This study aims to analyze the response variance of 2D electrode materials for EDLCs prepared through the liquid-phase exfoliation method when subjected to differing conditions. Once exfoliated, samples are tested with a series of structural characterization methods, including tunneling electron microscopy, atomic force microscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy. A new ionic liquid for EDLC use, 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate is compared in performance to 6M potassium hydroxide aqueous electrolyte. Devices composed of liquid-phase exfoliated graphene / MoS2 composites are analyzed by concentration for ideal performance. Device performance under cold extreme temperatures for the ionic fluid is presented as well. A brief overview of by-layer analysis of graphene electrode materials is presented as-is. All samples were tested with cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy, with good capacitive results. The evolution of electrochemical behavior through the altered parameters is tracked as well.

  5. Optical Gain in MoS2 via Coupling with Nanostructured Substrate: Fabry-Perot Interference and Plasmonic Excitation.

    Science.gov (United States)

    Jeong, Hye Yun; Kim, Un Jeong; Kim, Hyun; Han, Gang Hee; Lee, Hyangsook; Kim, Min Su; Jin, Youngjo; Ly, Thuc Hue; Lee, Si Young; Roh, Young-Geun; Joo, Won-Jae; Hwang, Sung Woo; Park, Yeonsang; Lee, Young Hee

    2016-09-27

    Despite the direct band gap of monolayer transition metal dichalcogenides (TMDs), their optical gain remains limited because of the poor light absorption in atomically thin, layered materials. Most approaches to improve the optical gain of TMDs mainly involve modulation of the active materials or multilayer stacking. Here, we report a method to enhance the optical absorption and emission in MoS2 simply through the design of a nanostructured substrate. The substrate consisted of a dielectric nanofilm spacer (TiO2) and metal film. The overall photoluminescence intensity from monolayer MoS2 on the nanostructured substrate was engineered based on the TiO2 thickness and amplified by Fabry-Perot interference. In addition, the neutral exciton emission was selectively amplified by plasmonic excitations from the local field originating from the surface roughness of the metal film with spacer thicknesses of less than 10 nm. We further demonstrate that the quality factor of the device can also be engineered by selecting a spacer material with a different refractive index.

  6. Single step, bulk synthesis of engineered MoS2 quantum dots for multifunctional electrocatalysis.

    Science.gov (United States)

    Tadi, Kiran Kumar; Palve, Anil M; Pal, Shubhadeep; Sudeep, P M; Narayanan, Tharangattu N

    2016-07-01

    Bi- or tri- functional catalysts based on atomic layers are receiving tremendous scientific attention due to their importance in various energy technologies. Recent studies on molybdenum disulphide (MoS2) nanosheets revealed that controlling the edge states and doping/modifying with suitable elements are highly important in tuning the catalytic activities of MoS2. Here we report a bulk, single step method to synthesize metal modified MoS2 quantum dots (QDs). Three elements, namely Fe, Mg and Li, are chosen to study the effects of dopants in the catalytic activities of MoS2. Fe and Mg are found to act like dopants in the MoS2 lattice forming respective doped MoS2 QDs, while Li formed an intercalated MoS2 QD. The efficacy and tunability of these luminescent doped QDs towards various electrocatalytic activities (hydrogen evolution reaction, oxygen evolution reaction and oxygen reduction action) are reported here.

  7. Layer-by-layer self-assembly of polyelectrolyte functionalized MoS2 nanosheets.

    Science.gov (United States)

    Shen, Jianfeng; Pei, Yu; Dong, Pei; Ji, Jin; Cui, Zheng; Yuan, Junhua; Baines, Robert; Ajayan, Pulickel M; Ye, Mingxin

    2016-05-01

    Few-layered polyelectrolyte functionalized MoS2 nanosheets were obtained for the first time through in situ polymerization of MoS2 nanosheets with poly(acrylic acid) and poly(acrylamide), both of which demonstrated excellent dispersibility and stability in water. After designing and optimizing the components of this series of polyelectrolyte functionalized MoS2 nanosheets, by exploiting the electrostatic interactions present in the modified MoS2 nanosheets, we further created a series of layer-by-layer (LBL) self-assembling MoS2-based films. To this end, uniform MoS2 nanosheet-based LBL films were precisely deposited on substrates such as quartz, silicon, and ITO. The polyelectrolyte functionalized MoS2 nanosheet assembled LBL film-modified electrodes demonstrated enhanced electrocatalytic activity for H2O2. As such, they are conducive to efficient sensors and advanced biosensing systems.

  8. Facile, substrate-scale growth of mono- and few-layer homogeneous MoS2 films on Mo foils with enhanced catalytic activity as counter electrodes in DSSCs.

    Science.gov (United States)

    Antonelou, Aspasia; Syrrokostas, George; Sygellou, Lamprini; Leftheriotis, George; Dracopoulos, Vassileios; Yannopoulos, Spyros N

    2016-01-29

    The growth of MoS2 films by sulfurization of Mo foils at atmospheric pressure is reported. The growth procedure provides, in a controlled way, mono- and few-layer thick MoS2 films with substrate-scale uniformity across square-centimeter area on commercial foils without any pre- or post-treatment. The prepared few-layer MoS2 films are investigated as counter electrodes for dye-sensitized solar cells (DSSCs) by assessing their ability to catalyse the reduction of I3(-) to I(-) in triiodide redox shuttles. The dependence of the MoS2 catalytic activity on the number of monolayers is explored down to the bilayer thickness, showing performance similar to that of, and stability against corrosion better than, Pt-based nanostructured film. The DSSC with the MoS2-Mo counter electrode yields a photovoltaic energy conversion efficiency of 8.4%, very close to that of the Pt-FTO-based DSSC, i.e. 8.7%. The current results disclose a facile, cost-effective and green method for the fabrication of mechanically robust and chemically stable, few-layer MoS2 on flexible Mo substrates and further demonstrate that efficient counter electrodes for DSSCs can be prepared at thicknesses down to the 1-2 nm scale.

  9. Self-Assembly-Induced Alternately Stacked Single-Layer MoS2 and N-doped Graphene: A Novel van der Waals Heterostructure for Lithium-Ion Batteries.

    Science.gov (United States)

    Zhao, Chenyang; Wang, Xu; Kong, Junhua; Ang, Jia Ming; Lee, Pooi See; Liu, Zhaolin; Lu, Xuehong

    2016-01-27

    In this article, a simple self-assembly strategy for fabricating van der Waals heterostructures from isolated two-dimensional atomic crystals is presented. Specifically, dopamine (DOPA), an excellent self-assembly agent and carbon precursor, was adsorbed on exfoliated MoS2 monolayers through electrostatic interaction, and the surface-modified monolayers self-assembled spontaneously into DOPA-intercalated MoS2. The subsequent in situ conversion of DOPA to highly conductive nitrogen-doped graphene (NDG) in the interlayer space of MoS2 led to the formation of a novel NDG/MoS2 nanocomposite with well-defined alternating structure. The NDG/MoS2 was then studied as an anode for lithium-ion batteries (LIBs). The results show that alternating arrangement of NDG and MoS2 triggers synergistic effect between the two components. The kinetics and cycle life of the anode are greatly improved due to the enhanced electron and Li(+) transport as well as the effective immobilization of soluble polysulfide by NDG. A reversible capacity of more than 460 mAh/g could be delivered even at 5 A/g. Moreover, the abundant voids created at the MoS2-NDG interface also accommodate the volume change during cycling and provide additional active sites for Li(+) storage. These endow the NDG/MoS2 heterostructure with low charge-transfer resistance, high sulfur reservation, and structural robustness, rendering it an advanced anode material for LIBs.

  10. Industrial grade 2D molybdenum disulphide (MoS2): an in vitro exploration of the impact on cellular uptake, cytotoxicity, and inflammation

    Science.gov (United States)

    Moore, Caroline; Movia, Dania; Smith, Ronan J.; Hanlon, Damien; Lebre, Filipa; Lavelle, Ed C.; Byrne, Hugh J.; Coleman, Jonathan N.; Volkov, Yuri; McIntyre, Jennifer

    2017-06-01

    The recent surge in graphene research, since its liquid phase monolayer isolation and characterization in 2004, has led to advancements which are accelerating the exploration of alternative 2D materials such as molybdenum disulphide (MoS2), whose unique physico-chemical properties can be exploited in applications ranging from cutting edge electronic devices to nanomedicine. However, to assess any potential impact on human health and the environment, the need to understand the bio-interaction of MoS2 at a cellular and sub-cellular level is critical. Notably, it is important to assess such potential impacts of materials which are produced by large scale production techniques, rather than research grade materials. The aim of this study was to explore cytotoxicity, cellular uptake and inflammatory responses in established cell-lines that mimic different potential exposure routes (inhalation, A549; ingestion, AGS; monocyte, THP-1) following incubation with MoS2 flakes of varying sizes (50 nm, 117 nm and 177 nm), produced by liquid phase exfoliation. Using high content screening (HCS) and Live/Dead assays, it was established that 1 µg ml-1 (for the three different MoS2 sizes) did not induce toxic effects on any of the cell-lines. Confocal microscopy images revealed a normal cellular morphology in all cases. Transmission electron microscopy (TEM) confirmed the uptake of all MoS2 nanomaterials in all the cell-lines, the MoS2 ultimately locating in single membrane vesicles. At such sub-lethal doses, inflammatory responses are observed, however, associated, at least partially, with the presence of lipopolysaccharide endotoxin in nanomaterial suspensions and surfactant samples. Therefore, the inflammatory response of the cells to the MoS2 or endotoxin contamination was interrogated using a 10-plex ELISA which illustrates cytokine production. The experiments carried out using wild-type and endotoxin hyporesponsive bone marrow derived dendritic cells confirmed that the

  11. Magnetic MoS2 pizzas and sandwiches with Mnn (n = 1–4) cluster toppings and fillings: A first-principles investigation

    Science.gov (United States)

    Zhang, Meng; Huang, Zhongjia; Wang, Xiao; Zhang, Hongyu; Li, Taohai; Wu, Zhaolong; Luo, Youhua; Cao, Wei

    2016-01-01

    The inorganic layered crystal (ILC) MoS2 in low dimensions is considered as one of the most promising and efficient semiconductors. To enable the magnetism and keep intrinsic crystal structures, we carried out a first-principles study of the magnetic and semiconductive monolayer MoS2 adsorbed with the Mnn (n = 1–4) clusters, and bilayer MoS2 intercalated with the same clusters. Geometric optimizations of the Mnn@MoS2 systems show the complexes prefer to have Mnn@MoS2(M) pizza and Mnn@MoS2(B) sandwich forms in the mono- and bi-layered cases, respectively. Introductions of the clusters will enhance complex stabilities, while bonds and charge transfers are found between external Mn clusters and the S atoms in the hosts. The pizzas have medium magnetic moments of 3, 6, 9, 4 μB and sandwiches of 3, 2, 3, 2 μB following the manganese numbers. The pizzas and sandwiches are semiconductors, but with narrower bandgaps compared to their corresponding pristine hosts. Direct bandgaps were found in the Mnn@MoS2(M) (n = 1,4) pizzas, and excitingly in the Mn1@MoS2(B) sandwich. Combining functional clusters to the layered hosts, the present work shows a novel material manipulation strategy to boost semiconductive ILCs applications in magnetics.

  12. Magnetic MoS2 pizzas and sandwiches with Mnn (n = 1-4) cluster toppings and fillings: A first-principles investigation.

    Science.gov (United States)

    Zhang, Meng; Huang, Zhongjia; Wang, Xiao; Zhang, Hongyu; Li, Taohai; Wu, Zhaolong; Luo, Youhua; Cao, Wei

    2016-01-01

    The inorganic layered crystal (ILC) MoS2 in low dimensions is considered as one of the most promising and efficient semiconductors. To enable the magnetism and keep intrinsic crystal structures, we carried out a first-principles study of the magnetic and semiconductive monolayer MoS2 adsorbed with the Mnn (n = 1-4) clusters, and bilayer MoS2 intercalated with the same clusters. Geometric optimizations of the Mnn@MoS2 systems show the complexes prefer to have Mnn@MoS2(M) pizza and Mnn@MoS2(B) sandwich forms in the mono- and bi-layered cases, respectively. Introductions of the clusters will enhance complex stabilities, while bonds and charge transfers are found between external Mn clusters and the S atoms in the hosts. The pizzas have medium magnetic moments of 3, 6, 9, 4 μB and sandwiches of 3, 2, 3, 2 μB following the manganese numbers. The pizzas and sandwiches are semiconductors, but with narrower bandgaps compared to their corresponding pristine hosts. Direct bandgaps were found in the Mnn@MoS2(M) (n = 1,4) pizzas, and excitingly in the Mn1@MoS2(B) sandwich. Combining functional clusters to the layered hosts, the present work shows a novel material manipulation strategy to boost semiconductive ILCs applications in magnetics.

  13. Hierarchical MoS2-rGO nanosheets with high MoS2 loading with enhanced electro-catalytic performance

    Science.gov (United States)

    Zhou, Jing; Xiao, Han; Zhou, Bowen; Huang, Feifan; Zhou, Shoubin; Xiao, Wei; Wang, Dihua

    2015-12-01

    Incorporation of high-loading redox-active materials with small amounts of graphene is a general protocol to achieve high-performance catalysts. Herein, hierarchical MoS2-reduced graphene oxide nanosheet (denoted as MoS2-rGO nanosheets) hybrids with a loading of MoS2 as high as 94 wt% are synthesized. The obtained hierarchical MoS2-rGO nanosheets simultaneously integrate the structural and compositional design rationales for high-efficiency and durable electrocatalysts based on high weight ratio of MoS2 in hybrid composite, highly stable/conducting rGO, well-dispersed two-dimensional ultrathin MoS2 nanosheets, more exposed edge sites and micro/nano hierarchical structure. When evaluated as electrocatalysts for hydrogen evolution and oxygen reduction reactions, the hierarchical MoS2-rGO nanosheets demonstrates enhanced activity and excellent stability, promising their applications in MoS2 based electrochemical, photo-catalytic and photo-elecrocatalytic cells.

  14. Exfoliated MoS2 in Water without Additives.

    Directory of Open Access Journals (Sweden)

    Viviane Forsberg

    Full Text Available Many solution processing methods of exfoliation of layered materials have been studied during the last few years; most of them are based on organic solvents or rely on surfactants and other funtionalization agents. Pure water should be an ideal solvent, however, it is generally believed, based on solubility theories that stable dispersions of water could not be achieved and systematic studies are lacking. Here we describe the use of water as a solvent and the stabilization process involved therein. We introduce an exfoliation method of molybdenum disulfide (MoS2 in pure water at high concentration (i.e., 0.14 ± 0.01 g L-1. This was achieved by thinning the bulk MoS2 by mechanical exfoliation between sand papers and dispersing it by liquid exfoliation through probe sonication in water. We observed thin MoS2 nanosheets in water characterized by TEM, AFM and SEM images. The dimensions of the nanosheets were around 200 nm, the same range obtained in organic solvents. Electrophoretic mobility measurements indicated that electrical charges may be responsible for the stabilization of the dispersions. A probability decay equation was proposed to compare the stability of these dispersions with the ones reported in the literature. Water can be used as a solvent to disperse nanosheets and although the stability of the dispersions may not be as high as in organic solvents, the present method could be employed for a number of applications where the dispersions can be produced on site and organic solvents are not desirable.

  15. Bending response of single layer MoS2.

    Science.gov (United States)

    Xiong, Si; Cao, Guoxin

    2016-03-11

    Using molecular mechanics (or dynamics) simulations, three different approaches, including the targeted molecular mechanics, four-point bending and nanotube methods, are employed to investigate the bending response of single layer MoS2 (SLMoS2), among which four-point bending is the most accurate approach to determine the bending stiffness according to the continuum theory. It is found that when the bending curvature radius is large enough (e.g. >4 nm), three approaches will give the same bending stiffness of SLMoS2 and the bending behavior is isotropic for SLMoS2, whereas the nanotube method with small tubes (e.g. bending stiffness. Compared with the reported result from the MoS2 nanotube calculated by density functional theory, the revised Stillinger-Weber (SW) and reactive empirical bond-order (REBO) potentials can give the reasonable bending stiffness of SLMoS2 (8.7-13.4 eV) as well as the effective deformed conformation. In addition, since the Mo-S bond deformation of SLMoS2 under bending is similar to that under in-plane tension/compression, the continuum bending theory can quite accurately predict the bending stiffness of SLMoS2 if a reasonable thickness of SLMoS2 is given. For SLMoS2, the reasonable thickness should be larger than the distance between its two S atomic planes and lower than the distance between two Mo atomic planes of bulk MoS2 crystal, e.g. 0.375-0.445 nm.

  16. Bending response of single layer MoS2

    Science.gov (United States)

    Xiong, Si; Cao, Guoxin

    2016-03-01

    Using molecular mechanics (or dynamics) simulations, three different approaches, including the targeted molecular mechanics, four-point bending and nanotube methods, are employed to investigate the bending response of single layer MoS2 (SLMoS2), among which four-point bending is the most accurate approach to determine the bending stiffness according to the continuum theory. It is found that when the bending curvature radius is large enough (e.g. >4 nm), three approaches will give the same bending stiffness of SLMoS2 and the bending behavior is isotropic for SLMoS2, whereas the nanotube method with small tubes (e.g. <4 nm) cannot give the correct bending stiffness. Compared with the reported result from the MoS2 nanotube calculated by density functional theory, the revised Stillinger-Weber (SW) and reactive empirical bond-order (REBO) potentials can give the reasonable bending stiffness of SLMoS2 (8.7-13.4 eV) as well as the effective deformed conformation. In addition, since the Mo-S bond deformation of SLMoS2 under bending is similar to that under in-plane tension/compression, the continuum bending theory can quite accurately predict the bending stiffness of SLMoS2 if a reasonable thickness of SLMoS2 is given. For SLMoS2, the reasonable thickness should be larger than the distance between its two S atomic planes and lower than the distance between two Mo atomic planes of bulk MoS2 crystal, e.g. 0.375-0.445 nm.

  17. Monolayer MoSe 2 Grown by Chemical Vapor Deposition for Fast Photodetection

    KAUST Repository

    Chang, Yung-Huang

    2014-08-26

    Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe 2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful for guiding future designs in 2D material-based optoelectronic devices. © 2014 American Chemical Society.

  18. First-principles investigation of the effect of charged unit cell on the electronic structure of two-dimensional MoS2

    Science.gov (United States)

    Shekaari, Ashkan; Abolhassani, Mohammad Reza; Lashgari, Hamed

    2017-01-01

    Density-functional theory has been applied to investigate the effect of charged unit cell on the structural and electronic properties of two-dimensional MoS2 within PBE-GGA. The charge of the unit cell of the monolayer changes from zero to n = ± 4 e with e the absolute value of the elementary electric charge. Variations of the lattice constant, Mo-S bond length, S-Mo-S bond angle, total energy, exchange and correlation contributions, and the Fermi level versus n have been calculated quantitatively, indicating decrease in the stability of the atomic structure of the monolayer with increase in the absolute value of n. It is found that the Fermi level for two-dimensional MoS2 is a function of both the number of electrons in allowed states and the inverse of the volume of the unit cell. The electronic properties of each monolayer have been also calculated via examining the related electronic band structure and density of states. Results broadly support the view that the effect of charged unit cell (n =+ e to - 4 e) on the electronic properties of MoS2 monolayer is manifested in the form of semiconductor-to-metal transition in addition to the Fermi level shift. It is also verified that as the negative charge of the unit cell increases from n = - e to - 4 e , there is an ever-increasing trend in the total number of allowed electronic states at the Fermi level, implying a direct correlation between electrical conductivity and the value of n in a way that the more negative the charge of the unit cell, the higher the electrical conductivity of the monolayer.

  19. Enhanced second harmonic generation of MoS2 layers on a thin gold film.

    Science.gov (United States)

    Zeng, Jianhua; Yuan, Maohui; Yuan, Weiguang; Dai, Qiaofeng; Fan, Haihua; Lan, Sheng; Tie, Shaolong

    2015-08-28

    The linear and nonlinear optical properties of thin MoS2 layers exfoliated on an Au/SiO2 substrate were investigated both numerically and experimentally. It was found that the MoS2 layers with different thicknesses exhibited different colors on the gold film. The reflection spectra of the MoS2 layers with different thicknesses were calculated by using the finite-difference time-domain technique and the corresponding chromaticity coordinates were derived. The electric field enhancement factors at both the fundamental light and the second harmonic were calculated and the enhancement factors for second harmonic generation (SHG) were estimated for the MoS2 layers with different thicknesses. Different from the MoS2 layers on a SiO2/Si substrate where the maximum SHG was observed in the single-layer MoS2, the maximum SHG was achieved in the 17 nm-thick MoS2 layer on the Au/SiO2 substrate. As compared with the MoS2 layers on the SiO2/Si substrate, a significant enhancement in SHG was found for the MoS2 layers on the Au/SiO2 substrate due to the strong localization of the electric field. More interestingly, it was demonstrated experimentally that optical data storage can be realized by modifying the SHG intensity of a MoS2 layer through thinning its thickness.

  20. MoS2 memristor with photoresistive switching

    Science.gov (United States)

    Wang, Wei; Panin, Gennady N.; Fu, Xiao; Zhang, Lei; Ilanchezhiyan, P.; Pelenovich, Vasiliy O.; Fu, Dejun; Kang, Tae Won

    2016-08-01

    A MoS2 nanosphere memristor with lateral gold electrodes was found to show photoresistive switching. The new device can be controlled by the polarization of nanospheres, which causes resistance switching in an electric field in the dark or under white light illumination. The polarization charge allows to change the switching voltage of the photomemristor, providing its multi-level operation. The device, polarized at a voltage 6 V, switches abruptly from a high resistance state (HRSL6) to a low resistance state (LRSL6) with the On/Off resistance ratio of about 10 under white light and smooth in the dark. Analysis of device conductivity in different resistive states indicates that its resistive state could be changed by the modulation of the charge in an electric field in the dark or under light, resulting in the formation/disruption of filaments with high conductivity. A MoS2 photomemristor has great potential as a multifunctional device designed by using cost-effective fabrication techniques.

  1. Large-area MoS2 deposition via MOVPE

    Science.gov (United States)

    Marx, M.; Nordmann, S.; Knoch, J.; Franzen, C.; Stampfer, C.; Andrzejewski, D.; Kümmell, T.; Bacher, G.; Heuken, M.; Kalisch, H.; Vescan, A.

    2017-04-01

    The direct deposition of the 2D transition metal dichalcogenide MoS2 via metal-organic vapour phase epitaxy (MOVPE) is investigated. Growth is performed in a commercial AIXTRON horizontal hot-wall reactor. Molybdenum hexacarbonyl (MCO) and Di-tert-butyl sulfide (DTBS) are used as metal-organic precursors for molybdenum and sulfur, respectively. The successful deposition of MoS2 is demonstrated via Raman spectroscopy on various substrates such as sapphire and Si as well as AlN and GaN templates. The influence of growth time on the evolution of layer morphology is investigated. Variation of carrier gas reveals that a pure nitrogen growth atmosphere and a growth temperature of 750 °C improve layer quality. Additionally, a post-deposition annealing process of the grown samples is examined. It is shown that annealing in a pure nitrogen atmosphere at temperatures between 650 °C and 750 °C strongly increases the Raman intensities.

  2. Large-Area Chemical Vapor Deposited MoS2 with Transparent Conducting Oxide Contacts toward Fully Transparent 2D Electronics

    KAUST Repository

    Dai, Zhenyu

    2017-09-08

    2D semiconductors are poised to revolutionize the future of electronics and photonics, much like transparent oxide conductors and semiconductors have revolutionized the display industry. Herein, these two types of materials are combined to realize fully transparent 2D electronic devices and circuits. Specifically, a large-area chemical vapor deposition process is developed to grow monolayer MoS2 continuous films, which are, for the first time, combined with transparent conducting oxide (TCO) contacts. Transparent conducting aluminum doped zinc oxide contacts are deposited by atomic layer deposition, with composition tuning to achieve optimal conductivity and band-offsets with MoS2. The optimized process gives fully transparent TCO/MoS2 2D electronics with average visible-range transmittance of 85%. The transistors show high mobility (4.2 cm2 V−1 s−1), fast switching speed (0.114 V dec−1), very low threshold voltage (0.69 V), and large switching ratio (4 × 108). To our knowledge, these are the lowest threshold voltage and subthreshold swing values reported for monolayer chemical vapor deposition MoS2 transistors. The transparent inverters show fast switching properties with a gain of 155 at a supply voltage of 10 V. The results demonstrate that transparent conducting oxides can be used as contact materials for 2D semiconductors, which opens new possibilities in 2D electronic and photonic applications.

  3. Synthesis and characterization of vertically standing MoS2 nanosheets

    Science.gov (United States)

    Li, Han; Wu, Huaqiang; Yuan, Shuoguo; Qian, He

    2016-01-01

    Molybdenum disulfide (MoS2) has been attracting much attentions due to its excellent electrical and optical properties. We report here the synthesis of large-scale and uniform MoS2 nanosheets with vertically standing morphology using chemical vapor deposition method. TEM observations clearly reveal the growth mechanism of these vertical structures. It is suggested that the vertical structures are caused by the compression and extrusion between MoS2 islands. More importantly, the vertical morphology of two dimensional (2D) materials hold many promising potential applications. We demonstrate here the as-synthesized vertically standing MoS2 nanosheets could be used for hydrogen evolution reaction, where the exchange current density is about 70 times of bulk MoS2. The field emission performance of vertically standing MoS2 were also improved due to the abundantly exposed edges. PMID:26888690

  4. Microwave irradiation induced band gap tuning of MoS2-TiO2 nanocomposites

    Science.gov (United States)

    Shakya, Jyoti; Mohanty, T.

    2016-05-01

    The MoS2-TiO2 nanocomposites have been synthesized by sol-gel method and characterized by different microscopic and spectroscopic techniques. The crystallinity of these nanocomposites has been confirmed by X-ray diffraction (XRD) analysis. The Raman spectrum of MoS2-TiO2 nanocomposites consists of three distinct peaks (E1 g, E1 2g and A1g) which are associated with TiO2 and MoS2. The morphological study is carried out by scanning electron microscope. The effect of microwave irradiation on the band gap of MoS2-TiO2 nanocomposites has been investigated; it is observed that the microwave irradiation causes decrease in the band gap of MoS2-TiO2 nanocomposites. The microwave treated MoS2-TiO2 thin films offers a novel process route in treating thin films for commercial applications.

  5. Synthesis of hierarchical MoO2/MoS2 nanofibers for electrocatalytic hydrogen evolution

    Science.gov (United States)

    Rheem, Youngwoo; Han, Yosep; Lee, Kyu Hwan; Choi, Sung-Mook; Myung, Nosang V.

    2017-03-01

    Perpendicularly attached MoS2 nanosheets on MoO2 conductive nanofibers were synthesized by combining electrospinning, calcination, and sulfurization processes. Compared to randomly stacked MoS2 nanosheets on MoO2 nanofiber, they show greater hydrogen evolution reaction (HER) performance (i.e., onset potential of ‑180 mV versus normal hydrogen electrode with the Tafel slope of 59 mV dec‑1). HER performance decreases with increasing MoS2 nanocrystal size.

  6. MoS2-modified ZnO quantum dots nanocomposite: Synthesis and ultrafast humidity response

    Science.gov (United States)

    Ze, Lu; Yueqiu, Gong; Xujun, Li; Yong, Zhang

    2017-03-01

    In this work, ZnO quantum dots (QDs), layered MoS2 and MoS2-modified ZnO QDs (MoS2@ZnO QDs) nanocomposite were synthesized and then applied as humidity sensor. The crystal structure, morphology and element distribution of ZnO QDs, MoS2 and MoS2@ZnO QDs were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectrometry, respectively. The humidity sensing characteristics of the MoS2 and MoS2@ZnO QDs against various relative humidity were measured at room temperature. The results show that the MoS2@ZnO QDs sensor exhibits high sensitivity with an impedance variation of three or four orders of magnitude to relative humidity range of 11-95% and it exhibits a short response-recovery time (1 s for adsorption and 20 s for desorption) and excellent repeatability. The mechanisms of the excellent performance for humidity sensing of MoS2@ZnO QDs sensor were discussed based on its impedance properties. Our work could offer guidelines to design higher performance especially ultrafast humidity response sensor utilizing the nanocomposite structure with two dimensional material and QDs.

  7. Formation Energies of the Lithium Intercalations in MoS2

    Institute of Scientific and Technical Information of China (English)

    Aiyu LI; Huiying LIU; Zizhong ZHU; Meichun HUANG; Yong YANG

    2006-01-01

    First-principles calculations have been performed to study the lithium intercalations in MoS2. The formation energies, changes of volumes, electronic structures and charge densities of the lithium intercalations in MoS2 are presented. Our calculations show that during lithium intercalations in MoS2, the lithium intercalation formation energies per lithium atom are between 2.5 eV to 3.0 eV, The volume expansions of MoS2 due to lithium intercalations are relatively small

  8. Effects of plasma treatment on surface properties of ultrathin layered MoS2

    Science.gov (United States)

    Kim, Suhhyun; Choi, Min Sup; Qu, Deshun; Ra, Chang Ho; Liu, Xiaochi; Kim, Minwoo; Song, Young Jae; Jong Yoo, Won

    2016-09-01

    This work investigates the use of oxygen plasma (O2) treatment, applied as an inductively coupled plasma, to control the thickness and work function of a MoS2 layer. Plasma-etched MoS2 exhibited a surface roughness similar to that of the pristine MoS2. The MoS2 field effect transistors fabricated using the plasma-etched MoS2 displayed a higher n-type doping concentration than that of pristine MoS2. The x-ray photoelectron spectroscopy was performed to analyze chemical composition to demonstrate the minimum level of chemical reactions occurred upon plasma treatment. Moreover, Kelvin probe force microscopy measurements were conducted to probe the changes in the work function that could be attributed to the changes in the surface potential. The measured work functions suggest the modification of a band structure and n-doping effect after plasma treatments that depended on the number of MoS2 layers. This study suggests that the O2 plasma can control the layer number of the MoS2 as well as the electronic properties of a MoS2 film.

  9. Interfacial chemical reactions between MoS2 lubricants and bearing materials

    Science.gov (United States)

    Zabinski, J. S.; Tatarchuk, B. J.

    1989-01-01

    XPS and conversion-electron Moessbauer spectroscopy (CEMS) were used to examine iron that was deposited on the basal plane of MoS2 single crystals and subjected to vacuum annealing, oxidizing, and reducing environments. Iron either intercalated into the MoS2 structure or formed oriented iron sulfides, depending on the level of excess S in the MoS2 structure. CEMS data demonstrated that iron sulfide crystal structures preferentially aligned with respect to the MoS2 basal plane, and that alignment (and potentially adhesion) could be varied by appropriate high-temperature annealing procedures.

  10. MoS2 and semiconductors in the flatland

    Directory of Open Access Journals (Sweden)

    Oleg V. Yazyev

    2015-01-01

    Full Text Available The fascinating properties of graphene, the first two-dimensional (2D material, and the accompanying strong activity in the research community have sparked a renewed interest in related layered crystalline materials with unique electronic and optical properties. Their superb mechanical properties, optical transparency, direct band gap and large degree of electrostatic control due to their atomic scale thickness make them interesting inorganic nanosystems for a wide variety of applications. In this review we will present a short history of research in the synthesis, band properties and potential applications of 2D semiconductors with a particular emphasis on MoS2, the prototypical and best-studied material from this family.

  11. The Pore Confinement Effect of FDU-12 Mesochannels on MoS2 Active Phases and Their Hydrodesulfurization Performance

    Directory of Open Access Journals (Sweden)

    Cong Liu

    2016-01-01

    Full Text Available FDU-12 silica with highly ordered face-centered cubic mesoporous structure is developed as support to prepare Mo/FDU-12 catalysts for hydrodesulfurization (HDS of dibenzothiophene (DBT. A series of Mo/FDU-12 catalysts are synthesized by using incipient wetness impregnation method with different MoO3 loadings (6, 8, 10, 12, and 15 wt.%. The objective of this work is to explore the pore confinement effect of FDU-12 mesochannels on the MoS2 morphology with various metal loadings. It is found that, as increasing MoO3 loadings from 6 to 15 wt.%, the MoS2 nanocrystallites transform from monolayer to multilayer and the morphology changes from straight layered to curved and then to ring-like and finally to spherical-like morphology due to the restriction of cage-like pore channels of FDU-12 support. The HDS results show that the catalytic activity increases first and then decreases with the best HDS performance at the MoO3 loading of 10 wt.%. In addition, we compared the HDS activity of Mo catalyst supported on FDU-12 with that on the commercial γ-Al2O3 and SBA-15; the result exhibits that FDU-12 is superior to the other two supports due to its large pore size and ordered three-dimensional open pore channels.

  12. Covalent Modification of MoS2 with Poly(N-vinylcarbazole) for Solid-State Broadband Optical Limiters.

    Science.gov (United States)

    Cheng, Hongxia; Dong, Ningning; Bai, Ting; Song, Yi; Wang, Jun; Qin, Yuanhao; Zhang, Bin; Chen, Yu

    2016-03-18

    New soluble MoS2 nanosheets covalently functionalized with poly(N-vinylcarbazole) (MoS2-PVK) were in situ synthesized for the first time. In contrast to MoS2 and MoS2 /PVK blends, both the solution of MoS2 -PVK in DMF and MoS2-PVK/poly(methyl methacrylate) (PMMA) film show superior nonlinear optical and optical limiting responses. The MoS2-PVK/PMMA film shows the largest nonlinear coefficients (βeff) of about 917 cm GW(-1) at λ=532 nm (cf. 100.69 cm GW(-1) for MoS2/PMMA and 125.12 cm GW(-1) for MoS2/PVK/PMMA) and about 461 cm GW(-1) at λ=1064 nm (cf. -48.92 cm GW(-1) for MoS2/PMMA and 147.56 cm GW(-1) for MoS2/PVK/PMMA). A larger optical limiting effect, with thresholds of about 0.3 GW cm(-2) at λ=532 nm and about 0.5 GW cm(-2) at λ=1064 nm, was also achieved from the MoS2-PVK/PMMA film. These values are among the highest reported for MoS2-based nonlinear optical materials. These results show that covalent functionalization of MoS2 with polymers is an effective way to improve nonlinear optical responses for efficient optical limiting devices.

  13. Impact of N-plasma and Ga-irradiation on MoS2 layer in molecular beam epitaxy

    KAUST Repository

    Mishra, Pawan

    2017-01-03

    Recent interest in two-dimensional materials has resulted in ultra-thin devices based on the transfer of transition metal dichalcogenides (TMDs) onto other TMDs or III-nitride materials. In this investigation, we realized p-type monolayer (ML) MoS2, and intrinsic GaN/p-type MoS2 heterojunction by the GaN overgrowth on ML-MoS2/c-sapphire using the plasma-assisted molecular beam epitaxy. A systematic nitrogen plasma (N∗2N2*) and gallium (Ga) irradiation studies are employed to understand the individual effect on the doping levels of ML-MoS2, which is evaluated by micro-Raman and high-resolution X-Ray photoelectron spectroscopy (HRXPS) measurements. With both methods, p-type doping was attained and was verified by softening and strengthening of characteristics phonon modes E12gE2g1 and A1gA1g from Raman spectroscopy. With adequate N∗2N2*-irradiation (3 min), respective shift of 1.79 cm−1 for A1gA1g and 1.11 cm−1 for E12gE2g1 are obtained while short term Ga-irradiated (30 s) exhibits the shift of 1.51 cm−1 for A1gA1g and 0.93 cm−1 for E12gE2g1. Moreover, in HRXPS valence band spectra analysis, the position of valence band maximum measured with respect to the Fermi level is determined to evaluate the type of doping levels in ML-MoS2. The observed values of valance band maximum are reduced to 0.5, and 0.2 eV from the intrinsic value of ≈1.0 eV for N∗2N2*- and Ga-irradiated MoS2 layers, which confirms the p-type doping of ML-MoS2. Further p-type doping is verified by Hall effect measurements. Thus, by GaN overgrowth, we attained the building block of intrinsic GaN/p-type MoS2 heterojunction. Through this work, we have provided the platform for the realization of dissimilar heterostructure via monolithic approach.

  14. Few-layer MoS2 as nitrogen protective barrier

    Science.gov (United States)

    Akbali, B.; Yanilmaz, A.; Tomak, A.; Tongay, S.; Çelebi, C.; Sahin, H.

    2017-10-01

    We report experimental and theoretical investigations of the observed barrier behavior of few-layer MoS2 against nitrogenation. Owing to its low-strength shearing, low friction coefficient, and high lubricity, MoS2 exhibits the demeanor of a natural N-resistant coating material. Raman spectroscopy is done to determine the coating capability of MoS2 on graphene. Surface morphology of our MoS2/graphene heterostructure is characterized by using optical microscopy, scanning electron microscopy, and atomic force microscopy. In addition, density functional theory-based calculations are performed to understand the energy barrier performance of MoS2 against nitrogenation. The penetration of nitrogen atoms through a defect-free MoS2 layer is prevented by a very high vertical diffusion barrier, indicating that MoS2 can serve as a protective layer for the nitrogenation of graphene. Our experimental and theoretical results show that MoS2 material can be used both as an efficient nanocoating material and as a nanoscale mask for selective nitrogenation of graphene layer.

  15. Hierarchical MoS2@RGO nanosheets for high performance sodium storage

    Science.gov (United States)

    Che, Zongzhou; Li, Yafeng; Chen, Kaixiang; Wei, Mingdeng

    2016-11-01

    The hierarchical MoS2/carbonaceous materials composites have received considerable interest for electrochemical energy storage and conversion. In the present work, the hierarchical MoS2@RGO nanosheets, where ultrathin MoS2 nanosheets perpendicularly anchor on both sides of RGO, are successfully synthesized by a new route. Such hybrid structures not only improve the conductivity of the hybrid structure and effectively prevent the aggregation of MoS2 nanosheets, but also maximize the MoS2 loading in the MoS2@RGO composites to be over 92%. The formation process for the hierarchical MoS2@RGO nanosheets is also investigated. When evaluated as an anode material for sodium-ion batteries, the hierarchical MoS2@RGO nanosheets deliver an excellent cycling performance and a high specific capacity of 420 mA h g-1 at a current density of 100 mA g-1. The strategy can be used to prepare other hierarchical metal sulfides@RGO nanosheets as high performance anode materials for sodium-ion batteries.

  16. Mechanical properties of freely suspended semiconducting graphene-like layers based on MoS2

    NARCIS (Netherlands)

    Castellanos-Gomez, A.; Poot, M.; Steele, G.A.; Van der Zant, H.S.J.; Agrait, N.; Rubio-Bollinger, G.

    2012-01-01

    We fabricate freely suspended nanosheets of molybdenum disulphide (MoS2) which are characterized by quantitative optical microscopy and high-resolution friction force microscopy. We study the elastic deformation of freely suspended nanosheets of MoS2 using an atomic force microscope. The Young’s mod

  17. General Thermal Texturization Process of MoS2 for Efficient Electrocatalytic Hydrogen Evolution Reaction.

    Science.gov (United States)

    Kiriya, Daisuke; Lobaccaro, Peter; Nyein, Hnin Yin Yin; Taheri, Peyman; Hettick, Mark; Shiraki, Hiroshi; Sutter-Fella, Carolin M; Zhao, Peida; Gao, Wei; Maboudian, Roya; Ager, Joel W; Javey, Ali

    2016-07-13

    Molybdenum disulfide (MoS2) has been widely examined as a catalyst containing no precious metals for the hydrogen evolution reaction (HER); however, these examinations have utilized synthesized MoS2 because the pristine MoS2 mineral is known to be a poor catalyst. The fundamental challenge with pristine MoS2 is the inert HER activity of the predominant (0001) basal surface plane. In order to achieve high HER performance with pristine MoS2, it is essential to activate the basal plane. Here, we report a general thermal process in which the basal plane is texturized to increase the density of HER-active edge sites. This texturization is achieved through a simple thermal annealing procedure in a hydrogen environment, removing sulfur from the MoS2 surface to form edge sites. As a result, the process generates high HER catalytic performance in pristine MoS2 across various morphologies such as the bulk mineral, films composed of micron-scale flakes, and even films of a commercially available spray of nanoflake MoS2. The lowest overpotential (η) observed for these samples was η = 170 mV to obtain 10 mA/cm(2) of HER current density.

  18. Patterned Peeling 2D MoS2 off the Substrate.

    Science.gov (United States)

    Zhao, Jing; Yu, Hua; Chen, Wei; Yang, Rong; Zhu, Jianqi; Liao, Mengzhou; Shi, Dongxia; Zhang, Guangyu

    2016-07-01

    The performance of two-dimensional (2D) MoS2 devices depends largely on the quality of the MoS2 itself. Existing fabrication process for 2D MoS2 relies on lithography and etching. However, it is extremely difficult to achieve clean patterns without any contaminations or passivations. Here we report a peel-off pattering of MoS2 films on substrates based on a proper interface engineering. The peel-off process utilizes the strong adhesion between gold and MoS2 and removes the MoS2 film contact with gold directly, leading to clean MoS2 pattern generation without residuals. Significantly improved electrical performances including high mobility ∼17.1 ± 8.3 cm(2)/(V s) and on/off ratio ∼5.6 ± 3.6 × 10(6) were achieved. Such clean fabrication technique paves a way to high quality MoS2 devices for various electrical and optical applications.

  19. Wasp-waisted magnetism in hydrothermally grown MoS2 nanoflakes

    Science.gov (United States)

    Chacko, Levna; Swetha, A. K.; Anjana, R.; Jayaraj, M. K.; Aneesh, P. M.

    2016-11-01

    Two-dimensional semiconducting materials are emanating as a requisite group of materials for future nanoscale electronics and optoelectronics. Particularly, transition-metal dichalcogenides like molybdenum disulphide (MoS2), a semiconducting inorganic counterpart of graphene have intrigued intensive interest as two-dimensional materials due to its novel functionalities. In this work, the utilization of high pressure and low temperature hydrothermal method offers a facile, versatile synthetic tool for MoS2 nanoflakes formation without the addition of any surfactants. Our experimental results resolve the formation of hexagonal phase, well-ordered stacking of S-Mo-S layers, quantum confinement and interlayer interaction. The strong spin-orbit coupling in MoS2 provides enthralling optical and magnetic properties. A large optical absorption in 400-700 nm region and strong luminescence provide evidence for the indirect to direct band gap transition in MoS2. Magnetic measurement results reveal ferromagnetism for all the MoS2 nanoflakes and also indicate an increase in saturation magnetization with increase in duration of growth. In addition, a wasp-waisted hysteresis loop was also observed for the first time in MoS2 nanostructures indicating multimodal population, increased grain growth and MoS2-MoO3 coupling. Our findings provide important insights into the future applications of MoS2 in high-performance nanodevices and spintronics.

  20. Layer-by-layer self-assembly of polyelectrolyte functionalized MoS2 nanosheets

    Science.gov (United States)

    Shen, Jianfeng; Pei, Yu; Dong, Pei; Ji, Jin; Cui, Zheng; Yuan, Junhua; Baines, Robert; Ajayan, Pulickel M.; Ye, Mingxin

    2016-05-01

    Few-layered polyelectrolyte functionalized MoS2 nanosheets were obtained for the first time through in situ polymerization of MoS2 nanosheets with poly(acrylic acid) and poly(acrylamide), both of which demonstrated excellent dispersibility and stability in water. After designing and optimizing the components of this series of polyelectrolyte functionalized MoS2 nanosheets, by exploiting the electrostatic interactions present in the modified MoS2 nanosheets, we further created a series of layer-by-layer (LBL) self-assembling MoS2-based films. To this end, uniform MoS2 nanosheet-based LBL films were precisely deposited on substrates such as quartz, silicon, and ITO. The polyelectrolyte functionalized MoS2 nanosheet assembled LBL film-modified electrodes demonstrated enhanced electrocatalytic activity for H2O2. As such, they are conducive to efficient sensors and advanced biosensing systems.Few-layered polyelectrolyte functionalized MoS2 nanosheets were obtained for the first time through in situ polymerization of MoS2 nanosheets with poly(acrylic acid) and poly(acrylamide), both of which demonstrated excellent dispersibility and stability in water. After designing and optimizing the components of this series of polyelectrolyte functionalized MoS2 nanosheets, by exploiting the electrostatic interactions present in the modified MoS2 nanosheets, we further created a series of layer-by-layer (LBL) self-assembling MoS2-based films. To this end, uniform MoS2 nanosheet-based LBL films were precisely deposited on substrates such as quartz, silicon, and ITO. The polyelectrolyte functionalized MoS2 nanosheet assembled LBL film-modified electrodes demonstrated enhanced electrocatalytic activity for H2O2. As such, they are conducive to efficient sensors and advanced biosensing systems. Electronic supplementary information (ESI) available: SEM, AFM and TEM characterization of PAA-MoS2 and PAM-MoS2 nanocomposites. More characterization and electrochemical properties of LBL films. See DOI: 10.1039/c6nr00583g

  1. Variability of electrical contact properties in multilayer MoS2 thin-film transistors

    Science.gov (United States)

    Kim, Seong Yeoul; Park, Seonyoung; Choi, Woong

    2014-09-01

    We report the variability of electrical properties of Ti contacts in back-gated multilayer MoS2 thin-film transistors based on mechanically exfoliated flakes. By measuring current-voltage characteristics from room temperature to 240 °C, we demonstrate the formation of both ohmic and Schottky contacts at the Ti-MoS2 junctions of MoS2 transistors fabricated using identical electrode materials under the same conditions. While MoS2 transistors with ohmic contacts exhibit a typical signature of band transport, those with Schottky contacts indicate thermally activated transport behavior for the given temperature range. These results provide the experimental evidence of the variability of Ti metal contacts on MoS2, highlighting the importance of understanding the variability of electronic properties of naturally occurring MoS2 for further investigation.

  2. Viable route towards large-area 2D MoS2 using magnetron sputtering

    Science.gov (United States)

    Samassekou, Hassana; Alkabsh, Asma; Wasala, Milinda; Eaton, Miller; Walber, Aaron; Walker, Andrew; Pitkänen, Olli; Kordas, Krisztian; Talapatra, Saikat; Jayasekera, Thushari; Mazumdar, Dipanjan

    2017-06-01

    Structural, interfacial, optical, and transport properties of large-area MoS2 ultra-thin films on BN-buffered silicon substrates fabricated using magnetron sputtering are investigated. A relatively simple growth strategy is demonstrated here that simultaneously promotes superior interfacial and bulk MoS2 properties. Few layers of MoS2 are established using x-ray reflectivity, diffraction, ellipsometry, and Raman spectroscopy measurements. Layer-specific modeling of optical constants show very good agreement with first-principles calculations. Conductivity measurements reveal that few-layer MoS2 films are more conducting than many-layer films. Photo-conductivity measurements reveal that the sputter deposited MoS2 films compare favorably with other large-area methods. Our work illustrates that sputtering is a viable route for large-area device applications using transition metal dichalcogenides.

  3. Thermoelectric performance of restacked MoS2 nanosheets thin-film.

    Science.gov (United States)

    Wang, Tongzhou; Liu, Congcong; Xu, Jingkun; Zhu, Zhengyou; Liu, Endou; Hu, Yongjing; Li, Changcun; Jiang, Fengxing

    2016-07-15

    MoS2 has been predicted to be an excellent thermoelectric material due to its large intrinsic band gap and high carrier mobility. In this work, we exfoliated bulk MoS2 by the assistance of lithium intercalation and fabricated the restacked MoS2 thin-film using a simple filtration technique. These MoS2 thin-films with different thickness showed different thermoelectric performance. It was found that with the increase of thickness, carrier concentration, electrical conductivity and Seebeck coefficient all showed an increasing trend. In particular, the maximum Seebeck coefficient was able to reach 93.5 μV K(-1). This high thermopower indicates that MoS2 will have ideal thermoelectric performance in the future through optimizing its structure. The highest figure of merit (ZT = 0.01) is calculated in this experiment.

  4. Atomically Thin-Layered Molybdenum Disulfide (MoS2) for Bulk-Heterojunction Solar Cells.

    Science.gov (United States)

    Singh, Eric; Kim, Ki Seok; Yeom, Geun Young; Nalwa, Hari Singh

    2017-02-01

    Transition metal dichalcogenides (TMDs) are becoming significant because of their interesting semiconducting and photonic properties. In particular, TMDs such as molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), tungsten diselenide (WSe2), titanium disulfide (TiS2), tantalum sulfide (TaS2), and niobium selenide (NbSe2) are increasingly attracting attention for their applications in solar cell devices. In this review, we give a brief introduction to TMDs with a focus on MoS2; and thereafter, emphasize the role of atomically thin MoS2 layers in fabricating solar cell devices, including bulk-heterojunction, organic, and perovskites-based solar cells. Layered MoS2 has been used as the hole-transport layer (HTL), electron-transport layer (ETL), interfacial layer, and protective layer in fabricating heterojunction solar cells. The trilayer graphene/MoS2/n-Si solar cell devices exhibit a power-conversion efficiency of 11.1%. The effects of plasma and chemical doping on the photovoltaic performance of MoS2 solar cells have been analyzed. After doping and electrical gating, a power-conversion efficiency (PCE) of 9.03% has been observed for the MoS2/h-BN/GaAs heterostructure solar cells. The MoS2-containing perovskites-based solar cells show a PCE as high as 13.3%. The PCE of MoS2-based organic solar cells exceeds 8.40%. The stability of MoS2 solar cells measured under ambient conditions and light illumination has been discussed. The MoS2-based materials show a great potential for solar cell devices along with high PCE; however, in this connection, their long-term environmental stability is also of equal importance for commercial applications.

  5. Laser-Induced Particle Adsorption on Atomically Thin MoS2.

    Science.gov (United States)

    Tran Khac, Bien Cuong; Jeon, Ki-Joon; Choi, Seung Tae; Kim, Yong Soo; DelRio, Frank W; Chung, Koo-Hyun

    2016-02-10

    Atomically thin molybdenum disulfide (MoS2) shows great potential for use in nanodevices because of its remarkable electronic, optoelectronic, and mechanical properties. These material properties are often dependent on the thickness or the number of layers, and hence Raman spectroscopy is widely used to characterize the thickness of atomically thin MoS2 due to the sensitivity of the vibrational spectrum to thickness. However, the lasers used in Raman spectroscopy can increase the local surface temperature and eventually damage the upper layers of the MoS2, thereby changing the aforementioned material properties. In this work, the effects of lasers on the topography and material properties of atomically thin MoS2 were systematically investigated using Raman spectroscopy and atomic force microscopy. In detail, friction force microscopy was used to study the friction characteristics of atomically thin MoS2 as a function of laser powers from 0.5 to 20 mW and number of layers from 1 to 3. It was found that particles formed on the top surface of the atomically thin MoS2 due to laser-induced thermal effects. The degree of particle formation increased as the laser power increased, prior to the thinning of the atomically thin MoS2. In addition, the degree of particle formation increased as the number of MoS2 layers increased, which suggests that the thermal behavior of the supported MoS2 may differ depending on the number of layers. The particles likely originated from the atmosphere due to laser-induced heating, but could be eliminated via appropriate laser powers and exposure times, which were determined experimentally. The outcomes of this work indicate that thermal management is crucial in the design of reliable nanoscale devices based on atomically thin MoS2.

  6. Two-dimensional hybrid layered materials: strain engineering on the band structure of MoS2/WSe2 hetero-multilayers

    Science.gov (United States)

    Gu, Kunming; Yu, Sheng; Eshun, Kwesi; Yuan, Haiwen; Ye, Huixian; Tang, Jiaoning; Ioannou, Dimitris E.; Xiao, Changshi; Wang, Hui; Li, Qiliang

    2017-09-01

    In this paper, we report a comprehensive modeling and simulation study of constructing hybrid layered materials by alternately stacking MoS2 and WSe2 monolayers. Such hybrid MoS2/WSe2 hetero-multilayers exhibited direct bandgap semiconductor characteristics with bandgap energy (E g) in a range of 0.45-0.55 eV at room temperature, very attractive for optoelectronics (wavelength range 2.5-2.75 μm) based on thicker two-dimensional (2D) materials. It was also found that the interlayer distance has a significant impact on the electronic properties of the hetero-multilayers, for example a five orders of magnitude change in the conductance was observed. Three material phases, direct bandgap semiconductor, indirect bandgap semiconductor, and metal were observed in MoS2/WSe2 hetero-multilayers, as the interlayer distance decreased from its relaxed (i.e., equilibrium) value of about 6.73 Å down to 5.50 Å, representing a vertical pressure of about 0.8 GPa for the bilayer and 1.5 GPa for the trilayer. Such new hybrid layered materials are very interesting for future nanoelectronic pressure sensor and nanophotonic applications. This study describes a new approach to explore and engineer the construction and application of tunable 2D semiconductors.

  7. Emerging photoluminescence from bilayer large-area 2D MoS2 films grown by pulsed laser deposition on different substrates

    Science.gov (United States)

    Barvat, Arun; Prakash, Nisha; Satpati, Biswarup; Singha, Shib Shankar; Kumar, Gaurav; Singh, Dilip K.; Dogra, Anjana; Khanna, Suraj P.; Singha, Achintya; Pal, Prabir

    2017-07-01

    We report the growth of continuous large area bilayer films of MoS2 on different substrates by pulsed laser deposition (PLD). The growth parameters for PLD were modified in such a way that results in bilayer 2D-MoS2 films on both c-Al2O3 (0001) (sapphire) and SiO2/Si (SO) substrates. The bilayer large area crystalline nature of growth in the 2 H-phase is determined by Raman spectroscopy. Cross-sectional transmission electron microscopy confirms the distinct thinnest ordered layered structure of MoS2. Chemical analysis reveals an almost stoichiometric 2 H-phase on both the substrates. The photoluminescence intensities of both the films match very well with those of the corresponding exfoliated flakes, as well as chemical vapor deposited (CVD) films as reported in the literature. The in-situ post growth annealing with optimal film thickness acts as a solid phase epitaxy process which provides continuous crystalline layers with a smooth interface and regulates the photoluminescence properties. In contrast, the PLD grown MoS2 monolayer shows poor crystalline quality and non-uniform coverage compared to that with the exfoliated and CVD grown films.

  8. RKKY interaction in triangular MoS2 nanoflakes

    Science.gov (United States)

    Mastrogiuseppe, Diego; Avalos-Ovando, Oscar; Ulloa, Sergio

    Transition-metal dichalcogenides (TMDs), such as MoS2, possess unique electronic and optical properties, making them promising for optospintronics. Exfoliation and CVD growth processes produce nanoflakes of different shapes, often triangular with zigzag edges. Magnetic impurities in this material interact indirectly through the TMD conduction electrons/holes. Using an effective 3-orbital tight-binding model, we study the Ruderman-Kittel-Kasuya-Yosida interaction between magnetic impurities in p-doped triangular flakes with zigzag termination. We analyze the interaction as function of impurity separation along high symmetry directions in the nanoflake, considering hybridization to different Mo orbitals, and different fillings. The interaction is anisotropic for impurities in the interior of the flake. However, when impurities lie on the edges of the crystallite, the effective exchange is Ising-like, reflecting the presence of z2-orbitals associated with edge states. Other interactions are possible by selecting impurity positions and orbital character of the states in their neighborhood. Our results can be tested with local probes, such as spin-polarized STM Supported by NSF DMR-1508325.

  9. Darstellung und Charakterisierung von MoS2 Cokatalysatoren für die H2-Entwicklung

    OpenAIRE

    Djamil, John

    2015-01-01

    Die hergestellten MoS2 Cokatalysatoren wurden hinsichtlich der chemischen Zusammensetzung, der strukturellen und fotokatalytischen Eigenschaften untersucht. MoS2 Cocatalysts were prepared and characterised in their chemical composition, structural and photocatalytic properties.

  10. PL enhancement of MoS2 by Au nanoparticles%Au的金属颗粒对二硫化钼发光增强

    Institute of Scientific and Technical Information of China (English)

    魏晓旭; 程英; 霍达; 张宇涵; 王军转; 胡勇; 施毅

    2014-01-01

    Molybdenum disulphide (MoS2), a layered quasi-two dimensional (2D) chalcogenide material, is a subject of intense research because of its electronic, optical, mechanical and physicochemical properties. Since the monolayer MoS2 is a direct-gap seminconductor, it is widely used in the field of light-emitting area. However, its photoluminescence (PL) efficiency is very low due to excessive doping in monolayer MoS2 and rich non-radiative centers. In this letter, we reportits synthesis using the gold nanoparticles which have a resonance absorption peak around 514 nm. The gold nanoparticles are dispersed on the surface of the MoS2 samples by means of spin-coating. Then, we measure the photoluminescence (PL) of the monolayer, bilayer and multilayer samples before and after the spin-coating, and find a great enhancement in the PL intensity of the monolayer sample. Also, the PL intensities of bi-layer and multiple layer MoS2 samples are slightly enhanced. Our work shows that gold nanoparticles may impose an obvious p-doping effect to the monolayer and bi-layer MoS2 samples to enhance the PL, and a surface plasmon polariton effect of the gold nanoparticles is also a positive factor for the enhancement.%二硫化钼(MoS2)是一种层状的二维过渡金属硫族化合物材料,从块体到单层,禁带由间接带隙变为直接带隙,由于通常机械剥落的单层MoS2是n型掺杂的,使得其发光效率仍然很低。在本文中,采用匀胶机旋涂的方法将共振吸收峰在514 nm附近的纳米金颗粒尽可能均匀的铺在单层、双层以及多层的MoS2样品表面,发现单层和双层样品的光致发光谱(PL谱)分别增强了约30倍和2倍同时伴随着峰位的蓝移,而多层样品的发光强度也略有增强。拉曼特性揭示了纳米金颗粒对单层和双层MoS2样品产生了明显的p型掺杂,从而增强了发光;同时纳米金颗粒的表面等离子激元效应对激发光的天线作用也是增强MoS2的光致发光的一个因素。

  11. Nanostructured MoS2/BiVO4 Composites for Energy Storage Applications

    Science.gov (United States)

    Arora, Yukti; Shah, Amit P.; Battu, Shateesh; Maliakkal, Carina B.; Haram, Santosh; Bhattacharya, Arnab; Khushalani, Deepa

    2016-11-01

    We report the optimized synthesis and electrochemical characterization of a composite of few-layered nanostructured MoS2 along with an electroactive metal oxide BiVO4. In comparison to pristine BiVO4, and a composite of graphene/BiVO4, the MoS2/BiVO4 nanocomposite provides impressive values of charge storage with longer discharge times and improved cycling stability. Specific capacitance values of 610 Fg‑1 (170 mAhg‑1) at 1 Ag‑1 and 166 Fg‑1 (46 mAhg‑1) at 10 Ag‑1 were obtained for just 2.5 wt% MoS2 loaded BiVO4. The results suggest that the explicitly synthesized small lateral-dimensioned MoS2 particles provide a notable capacitive component that helps augment the specific capacitance. We discuss the optimized synthesis of monoclinic BiVO4, and few-layered nanostructured MoS2. We report the discharge capacities and cycling performance of the MoS2/BiVO4 nanocomposite using an aqueous electrolyte. The data obtained shows the MoS2/BiVO4 nanocomposite to be a promising candidate for supercapacitor energy storage applications.

  12. Wettability and friction of water on a MoS2 nanosheet

    Science.gov (United States)

    Luan, Binquan; Zhou, Ruhong

    2016-03-01

    The molybdenum disulfide (MoS2) nanosheet is a promising two-dimensional (2D) material and has recently been used in biological sensing. While the electronic structure of 2D MoS2 sheet has been actively studied, the role of its atomic structure and thus the interfacial interactions with bio-fluids are still elusive. Using Molecular dynamics simulations, we developed MoS2 force field parameters to reproduce the experimentally determined water contact angle of the MoS2 nanosheet and then predicted the slip-length of water that has not been measured in experiment yet. Simulation results suggest that the MoS2 nanosheet is a hydrophobic and low-friction surface, despite its seemingly significant charges of surface atoms and relatively strong strength of van der Waals potentials. We expect that the developed force fields for depicting surface atoms of MoS2 will facilitate future research in understanding biomolecule-MoS2 interactions in MoS2-based biosensors.

  13. Ultra-broadband nonlinear saturable absorption of high-yield MoS2 nanosheets.

    Science.gov (United States)

    Wei, Rongfei; Zhang, Hang; Hu, Zhongliang; Qiao, Tian; He, Xin; Guo, Qiangbing; Tian, Xiangling; Chen, Zhi; Qiu, Jianrong

    2016-07-29

    High-yield MoS2 nanosheets with strong nonlinear optical (NLO) responses in a broad near-infrared range were synthesized by a facile hydrothermal method. The observation of saturable absorption, which was excited by the light with photon energy smaller than the gap energy of MoS2, can be attributed to the enhancement of the hybridization between the Mo d-orbital and S p-orbital by the oxygen incorporation into MoS2. High-yield MoS2 nanosheets with high modulation depth and large saturable intensity generated a stable, passively Q-switched fiber laser pulse at 1.56 μm. The high output power of 1.08 mW can be attained under a very low pump power of 30.87 mW. Compared to recently reported passively Q-switched fiber lasers utilizing exfoliated MoS2 nanosheets, the efficiency of the laser for our passive Q-switching operation is larger and reaches 3.50%. This research may extend the understanding on the NLO properties of MoS2 and indicate the feasibility of the high-yield MoS2 nanosheets to passively Q-switched fiber laser effectively at low pump strengths.

  14. High efficiency photocatalysis for pollutant degradation with MoS2/C3N4 heterostructures.

    Science.gov (United States)

    Li, Qian; Zhang, Ning; Yang, Yong; Wang, Guozhong; Ng, Dickon H L

    2014-07-29

    Porous graphitic carbon nitride was synthesized by controllable thermal polymerization of urea in air. Their textural, electrical, and optical properties were tuned by varying the heating rate. The presence of proper residual oxygen in carbon nitride matrix had enhanced light absorption and inhibited the recombination of charge carriers. Furthermore, the MoS2 nanosheets were coupled into the carbon nitride to form MoS2/C3N4 heterostructures via a facile ultrasonic chemical method. The optimized MoS2/C3N4 heterostructure with 0.05 wt % MoS2 showed a reaction rate constant as high as 0.301 min(-1), which was 3.6 times that of bare carbon nitride. As analyzed by SEM, TEM, UV-vis absorption, PL and photoelectrochemical measurements, intimate contact interface, extended light response range, enhanced separation speed of charge carriers, and high photocurrent density upon MoS2 coupling led to the photocatalytic promotion of the MoS2/C3N4 heterostructures. In this architecture, MoS2 served as electron trapper to extend the lifetime of separated electron-hole pairs. Meanwhile, the accumulated holes on the surface of carbon nitride oxidized the organic dye directly, which was a predominant process in the photodegradation of organic pollutants in water treatment. The promotional mechanisms and principles reported here would have great significance in heterogeneous photocatalysis.

  15. Electrostatic properties of few-layer MoS2 films

    Directory of Open Access Journals (Sweden)

    Guolin Hao

    2013-04-01

    Full Text Available Two-dimensional MoS2-based materials are considered to be one of the most attractive materials for next-generation nanoelectronics. The electrostatic properties are important in designing and understanding the performance of MoS2-based devices. By using Kelvin probe force microscopy, we show that few-layer MoS2 sheets exhibit uniform surface potential and charge distributions on their surfaces but have relatively lower surface potentials on the edges, folded areas as well as defect grain boundaries.

  16. Synthesis of Vertically Standing MoS2 Triangles on SiC

    Science.gov (United States)

    Lan, Feifei; Lai, Zhanping; Xu, Yongkuan; Cheng, Hongjuan; Wang, Zaien; Qi, Chengjun; Chen, Jianli; Zhang, Song

    2016-08-01

    Layered material MoS2 has been attracting much attention due to its excellent electronical properties and catalytic property. Here we report the synthesis of vertically standing MoS2 triangles on silicon carbon(SiC), through a rapid sulfidation process. Such edge-terminated films are metastable structures of MoS2, which may find applications in FinFETs and catalytic reactions. We have confirmed the catalytic property in a hydrogen evolution reaction(HER). The Tafel slope is about 54mV/decade.

  17. Monolayer transition metal disulfide:Synthesis, characterization and applications

    Institute of Scientific and Technical Information of China (English)

    Qi Fu; Bin Xiang

    2016-01-01

    Two-dimensional transition metal dichalcogenides (2D TMDCs) has aroused tremendous attention in recent years, because of their remarkable properties originated from their unique structure. In this re-view we report the synthesis, characterization and applications of monolayer MoS2 and WS2.

  18. Z2 Invariance of Germanene on MoS2 from First Principles

    Science.gov (United States)

    Amlaki, Taher; Bokdam, Menno; Kelly, Paul J.

    2016-06-01

    We present a low energy Hamiltonian generalized to describe how the energy bands of germanene (Ge ¯ ) are modified by interaction with a substrate or a capping layer. The parameters that enter the Hamiltonian are determined from first-principles relativistic calculations for Ge ¯ |MoS2 bilayers and MoS2|Ge ¯ |MoS2 trilayers and are used to determine the topological nature of the system. For the lowest energy, buckled germanene structure, the gap depends strongly on how germanene is oriented with respect to the MoS2 layer(s). Topologically nontrivial gaps for bilayers and trilayers can be almost as large as for a freestanding germanene layer.

  19. Tuning the hysteresis voltage in 2D multilayer MoS2 FETs

    Science.gov (United States)

    Jiang, Jie; Zheng, Zhouming; Guo, Junjie

    2016-10-01

    The hysteresis tuning is of great significance before the two-dimensional (2D) molybdenum disulfide (MoS2) field-effect transistors (FETs) can be practically used in the next-generation nanoelectronic devices. In this paper, a simple and effective annealing method was developed to tune the hysteresis voltage in 2D MoS2 transistors. It was found that high temperature (175 °C) annealing in air could increase the hysteresis voltage from 8.0 V (original device) to 28.4 V, while a next vacuum annealing would reduce the hysteresis voltage to be only 2.0 V. An energyband diagram model based on electron trapping/detrapping due to oxygen adsorption is proposed to understand the hysteresis mechanism in multilayer MoS2 FET. This simple method for tuning the hysteresis voltage of MoS2 FET can make a significant step toward 2D nanoelectronic device applications.

  20. Density functional studies on edge-contacted single-layer MoS2 piezotronic transistors

    Science.gov (United States)

    Liu, Wei; Zhang, Aihua; Zhang, Yan; Wang, Zhong Lin

    2015-08-01

    The piezotronic effect uses strain-induced piezoelectric charges at interfaces and junctions to tune and/or control carrier transport in piezoelectric semiconductor devices. This effect has recently been observed in single-layer 2D MoS2 materials. However, previous work had found that metallic states are generated at the edge of a free-standing MoS2 flat sheet, and these states may screen the piezoelectric charges. Using density functional theory simulations, we found that the metal-MoS2 interface structure plays an important role in enhancing both the piezoelectric and piezotronic effects in MoS2 transistors by breaking the metallic state screening effect at the MoS2 edge. This study not only provides an understanding of the piezoelectric and piezotronic effects based on first principles calculations but also offers guidance for the design of two-dimensional piezotronic devices.

  1. Thickness dependent interlayer transport in vertical MoS2 Josephson junctions

    Science.gov (United States)

    Island, Joshua O.; Steele, Gary A.; van der Zant, Herre S. J.; Castellanos-Gomez, Andres

    2016-09-01

    We report on observations of thickness dependent Josephson coupling and multiple Andreev reflections (MAR) in vertically stacked molybdenum disulfide (MoS2)—molybdenum rhenium (MoRe) Josephson junctions. MoRe, a chemically inert superconductor, allows for oxide free fabrication of high transparency vertical MoS2 devices. Single and bilayer MoS2 junctions display relatively large critical currents (up to 2.5 μA) and the appearance of sub-gap structure given by MAR. In three and four layer thick devices we observe orders of magnitude lower critical currents (sub-nA) and reduced quasiparticle gaps due to proximitized MoS2 layers in contact with MoRe. We anticipate that this device architecture could be easily extended to other 2D materials.

  2. Lubrication with sputtered MoS2 films: Principles, operation, limitations

    Science.gov (United States)

    Spalvins, T.

    1991-01-01

    The present practices, limitations, and understanding of thin sputtered MoS2 films are reviewed. Sputtered MoS2 films can exhibit remarkable tribological properties such as ultralow friction coefficients (0.01) and enhanced wear lives (millions of cycles) when used in vacuum or dry air. To achieve these favorable tribological characteristics, the sputtering conditions during deposition must be optimized for adequate film adherence and appropriate structure (morphology) and composition.

  3. One-dimensional metallic edge states in MoS2

    DEFF Research Database (Denmark)

    Bollinger, Mikkel; Lauritsen, J.V.; Jacobsen, Karsten Wedel;

    2001-01-01

    By the use of density functional calculations it is shown that the edges of a two-dimensional slab of insulating MoS2 exhibit several metallic states. These edge states can be viewed as one-dimensional conducting wires, and we show that they can be observed directly using scanning tunneling...... microscopy for single-layer MoS2 nanoparticles grown on a support....

  4. Flower-like PEGylated MoS2 nanoflakes for near-infrared photothermal cancer therapy

    OpenAIRE

    Wei Feng; Liang Chen; Ming Qin; Xiaojun Zhou; Qianqian Zhang; Yingke Miao; Kexin Qiu; Yanzhong Zhang; Chuanglong He

    2015-01-01

    Photothermal cancer therapy has attracted considerable interest for cancer treatment in recent years, but the effective photothermal agents remain to be explored before this strategy can be applied clinically. In this study, we therefore develop flower-like molybdenum disulfide (MoS2) nanoflakes and investigate their potential for photothermal ablation of cancer cells. MoS2 nanoflakes are synthesized via a facile hydrothermal method and then modified with lipoic acid-terminated polyethylene g...

  5. Morphological, structural and field emission characterization of hydrothermally synthesized MoS2-RGO nanocomposite

    Science.gov (United States)

    Bansode, Sanjeewani R.; Harpale, Kashmira; Khare, Ruchita T.; Walke, Pravin S.; More, Mahendra A.

    2016-11-01

    A few layered MoS2-RGO nanocomposite has been synthesized employing a facile hydrothermal synthesis route. The morphological and structural analysis performed using SEM, TEM, HRTEM and Raman spectroscopy clearly reveal formation of vertically aligned a few layer thick MoS2 sheets on RGO surface. Attempts have been made to reveal the influence of graphite oxide (GO) percentage on morphology of the nanocomposite. Furthermore, field emission (FE) investigations of as-synthesied MoS2-RGO nanocomposite are observed to be superior to the pristine MoS2 emitter. The values of turn-on field, defined at emission current density of 10 μA cm-2, are found to be 2.6 and 4.7 V μm-1 for the MoS2-RGO (5%) nanocomposite and pristine MoS2 emitters, respectively. The value of threshold field, defined at emission current density of 100 μA cm-2, is found to be 3.1 V μm-1 for MoS2-RGO nanocomposite. The emission current stability at the pre-set value of 1 μA over 3 h duration is found to be fairly good, characterized by current fluctuation within ±18% of the average value. The enhanced FE behavior for MoS2-RGO nanocomposite is attributed to a high enhancement factor (β) of 4128 and modulation of the electronic properties. The facile approach adopted herein can be extended to enhance various functionalities of other nanocomposites.

  6. MoS2 Nanosheets Vertically Grown on Graphene Sheets for Lithium-Ion Battery Anodes.

    Science.gov (United States)

    Teng, Yongqiang; Zhao, Hailei; Zhang, Zijia; Li, Zhaolin; Xia, Qing; Zhang, Yang; Zhao, Lina; Du, Xuefei; Du, Zhihong; Lv, Pengpeng; Świerczek, Konrad

    2016-09-27

    A designed nanostructure with MoS2 nanosheets (NSs) perpendicularly grown on graphene sheets (MoS2/G) is achieved by a facile and scalable hydrothermal method, which involves adsorption of Mo7O24(6-) on a graphene oxide (GO) surface, due to the electrostatic attraction, followed by in situ growth of MoS2. These results give an explicit proof that the presence of oxygen-containing groups and pH of the solution are crucial factors enabling formation of a lamellar structure with MoS2 NSs uniformly decorated on graphene sheets. The direct coupling of edge Mo of MoS2 with the oxygen from functional groups on GO (C-O-Mo bond) is proposed. The interfacial interaction of the C-O-Mo bonds can enhance electron transport rate and structural stability of the MoS2/G electrode, which is beneficial for the improvement of rate performance and long cycle life. The graphene sheets improve the electrical conductivity of the composite and, at the same time, act not only as a substrate to disperse active MoS2 NSs homogeneously but also as a buffer to accommodate the volume changes during cycling. As an anode material for lithium-ion batteries, the manufactured MoS2/G electrode manifests a stable cycling performance (1077 mAh g(-1) at 100 mA g(-1) after 150 cycles), excellent rate capability, and a long cycle life (907 mAh g(-1) at 1000 mA g(-1) after 400 cycles).

  7. Large scale MoS2 nanosheet logic circuits integrated by photolithography on glass

    Science.gov (United States)

    Kwon, Hyeokjae; Jeon, Pyo Jin; Kim, Jin Sung; Kim, Tae-Young; Yun, Hoyeol; Lee, Sang Wook; Lee, Takhee; Im, Seongil

    2016-12-01

    We demonstrate 500 × 500 μm2 large scale polygrain MoS2 nanosheets and field effect transistor (FET) circuits integrated using those nanosheets, which are initially grown on SiO2/p+-Si by chemical vapor deposition but transferred onto glass substrate to be patterned by photolithography. In fact, large scale growth of two-dimensional MoS2 and its conventional way of patterning for integrated devices have remained as one of the unresolved important issues. In the present study, we achieved maximum linear mobility of ˜9 cm2 V-1 s-1 from single-domain MoS2 FET on SiO2/p+-Si substrate and 0.5˜3.0 cm2 V-1 s-1 from large scale MoS2 sheet transferred onto glass. Such reduced mobility is attributed to the transfer process-induced wrinkles and crevices, domain boundaries, residue on MoS2, and loss of the back gate-charging effects that might exist due to SiO2/p+-Si substrate. Among 16 MoS2-based FETs, 13 devices successfully work (yield was more than 80%) producing NOT, NOR, and NAND logic circuits. Inverter (NOT gate) shows quite a high voltage gain over 12 at a supply voltage of 5 V, also displaying 60 μs switching speed in kilohertz dynamics.

  8. The intrinsic origin of hysteresis in MoS2 field effect transistors.

    Science.gov (United States)

    Shu, Jiapei; Wu, Gongtao; Guo, Yao; Liu, Bo; Wei, Xianlong; Chen, Qing

    2016-02-07

    We investigate the hysteresis and gate voltage stress effect in MoS2 field effect transistors (FETs). We observe that both the suspended and the SiO2-supported FETs have large hysteresis in their transfer curves under vacuum which cannot be attributed to the traps at the interface between the MoS2 and the SiO2 or in the SiO2 substrate or the gas adsorption/desorption effect. Our findings indicate that the hysteresis we observe comes from the MoS2 itself, revealing an intrinsic origin of the hysteresis besides some extrinsic factors. The fact that the FETs based on thinner MoS2 have larger hysteresis than that with thicker MoS2 suggests that the surface of MoS2 plays a key role in the hysteresis. The gate voltage sweep range, sweep direction, sweep time and loading history all affect the hysteresis observed in the transfer curves.

  9. Environmental Applications of 2D Molybdenum Disulfide (MoS2) Nanosheets.

    Science.gov (United States)

    Wang, Zhongying; Mi, Baoxia

    2017-08-01

    In an era of graphene-based nanomaterials as the most widely studied two-dimensional (2D) materials for enhanced performance of devices and systems in numerous environmental applications, molybdenum disulfide (MoS2) nanosheets stand out as a promising alternative 2D material with many excellent physicochemical, biological, and mechanical properties that differ significantly from those of graphene-based nanomaterials, potentially leading to new environmental phenomena and novel applications. This Critical Review presents the latest advances in the use of MoS2 nanosheets for important water-related environmental applications such as contaminant adsorption, photocatalysis, membrane-based separation, sensing, and disinfection. Various methods for MoS2 nanosheet synthesis are examined, and their suitability for different environmental applications is discussed. The unique structure and properties of MoS2 nanosheets enabling exceptional environmental capabilities are compared with those of graphene-based nanomaterials. The environmental implications of MoS2 nanosheets are emphasized, and research needs for future environmental applications of MoS2 nanosheets are identified.

  10. Vacuum ultraviolet radiation effects on two-dimensional MoS2 field-effect transistors

    Science.gov (United States)

    McMorrow, Julian J.; Cress, Cory D.; Arnold, Heather N.; Sangwan, Vinod K.; Jariwala, Deep; Schmucker, Scott W.; Marks, Tobin J.; Hersam, Mark C.

    2017-02-01

    Atomically thin MoS2 has generated intense interest for emerging electronics applications. Its two-dimensional nature and potential for low-power electronics are particularly appealing for space-bound electronics, motivating the need for a fundamental understanding of MoS2 electronic device response to the space radiation environment. In this letter, we quantify the response of MoS2 field-effect transistors (FETs) to vacuum ultraviolet (VUV) total ionizing dose radiation. Single-layer (SL) and multilayer (ML) MoS2 FETs are compared to identify differences that arise from thickness and band structure variations. The measured evolution of the FET transport properties is leveraged to identify the nature of VUV-induced trapped charge, isolating the effects of the interface and bulk oxide dielectric. In both the SL and ML cases, oxide trapped holes compete with interface trapped electrons, exhibiting an overall shift toward negative gate bias. Raman spectroscopy shows no variation in the MoS2 signatures as a result of VUV exposure, eliminating significant crystalline damage or oxidation as possible radiation degradation mechanisms. Overall, this work presents avenues for achieving radiation-hard MoS2 devices through dielectric engineering that reduces oxide and interface trapped charge.

  11. Two-dimensional MoS2: A promising building block for biosensors.

    Science.gov (United States)

    Gan, Xiaorong; Zhao, Huimin; Quan, Xie

    2017-03-15

    Recently, two-dimensional (2D) layered nanomaterials have trigged intensive interest due to the intriguing physicochemical properties that stem from a quantum size effect connected with their ultra-thin structure. In particular, 2D molybdenum disulfide (MoS2), as an emerging class of stable inorganic graphene analogs with intrinsic finite bandgap, would possibly complement or even surpass graphene in electronics and optoelectronics fields. In this review, we first discuss the historical development of ultrathin 2D nanomaterials. Then, we are concerned with 2D MoS2 including its structure-property relationships, synthesis methods, characterization for the layer thickness, and biosensor applications over the past five years. Thereinto, we are highlighting recent advances in 2D MoS2-based biosensors, especially emphasize the preparation of sensing elements, roles of 2D MoS2, and assay strategies. Finally, on the basis of the current achievements on 2D MoS2 and other ultrathin layered nanomaterials, perspectives on the challenges and opportunities for the exploration of 2D MoS2-based biosensors are put forward.

  12. Two-dimensional lateral heterojunction through bandgap engineering of MoS2 via oxygen plasma

    Science.gov (United States)

    Choudhary, Nitin; Islam, Muhammad R.; Kang, Narae; Tetard, Laurene; Jung, Yeonwoong; Khondaker, Saiful I.

    2016-09-01

    The present study explores the structural, optical (photoluminescence (PL)), and electrical properties of lateral heterojunctions fabricated by selective exposure of mechanically exfoliated few layer two-dimensional (2D) molybdenum disulfide (MoS2) flakes under oxygen (O2)-plasma. Raman spectra of the plasma exposed MoS2 flakes show a significant loss in the structural quality due to lattice distortion and creation of oxygen-containing domains in comparison to the pristine part of the same flake. The PL mapping evidences the complete quenching of peak A and B consistent with a change in the exciton states of MoS2 after the plasma treatment, indicating a significant change in its band gap properties. The electrical transport measurements performed across the pristine and the plasma-exposed MoS2 flake exhibit a gate tunable current rectification behavior with a rectification ratio up to 1.3  ×  103 due to the band-offset at the pristine and plasma-exposed MoS2 interface. Our Raman, PL, and electrical transport data confirm the formation of an excellent lateral heterojunction in 2D MoS2 through its bandgap modulation via oxygen plasma.

  13. Growing and Etching MoS2 on Carbon Nanotube Film for Enhanced Electrochemical Performance

    Directory of Open Access Journals (Sweden)

    Weiyu Xu

    2016-09-01

    Full Text Available In this work we directly synthesized molybdenum disulfide (MoS2 nanosheets on carbon nanotube film (MoS2@CNT via a two-step chemical vapor deposition method (CVD. By etching the obtained MoS2@CNT into 10% wt HNO3, the morphology of MoS2 decorated on CNT bundles was modulated, resulting in more catalytic active MoS2 edges being exposed for significantly enhanced electrochemical performance. Our results revealed that an 8 h acid etching sample exhibited the best performance for the oxygen evolution reaction, i.e., the current density reached 10 mA/cm2 under 375 mV over-potential, and the tafel slope was as low as 94 mV/dec. The enhanced behavior was mainly originated from the more catalytic sites in MoS2 induced by the acid etching treatment and the higher conductivity from the supporting CNT films. Our study provides a new route to produce two-dimensional layers on CNT films with tunable morphology, and thus may open a window for exploring its promising applications in the fields of catalytic-, electronic-, and electrochemical-related fields.

  14. A limited anodic and cathodic potential window of MoS2: limitations in electrochemical applications.

    Science.gov (United States)

    Nasir, Muhammad Zafir Mohamad; Sofer, Zdenek; Ambrosi, Adriano; Pumera, Martin

    2015-02-21

    Molybdenum disulphide has been touted as a good material with diverse possible applications such as an energy storage and sensing platform. However, we demonstrate here the limitation of MoS2 as an analytical sensing platform due to the limited potential window in both the anodic and cathodic regions attributed to the inherent electrochemistry (oxidation of Mo(4+) to Mo(6+)) and the catalytic hydrogen evolution reaction due to H3O(+) reduction on the MoS2 surface, respectively. The electrochemical window of MoS2 lies in the region of ∼-0.6 V to +0.7 V (vs. AgCl). We show that such a limited working potential window characteristic of MoS2 precludes the detection of important analytes such as nitroaromatic explosives, pesticides and mycotoxins which are instead detectable on carbon surfaces. The limited potential window of MoS2 has to be taken into consideration in the construction of electroanalytical devices based on MoS2.

  15. Hydrogen generation via photoelectrochemical water splitting using chemically exfoliated MoS2 layers

    Directory of Open Access Journals (Sweden)

    R. K. Joshi

    2016-01-01

    Full Text Available Study on hydrogen generation has been of huge interest due to increasing demand for new energy sources. Photoelectrochemical reaction by catalysts was proposed as a promising technique for hydrogen generation. Herein, we report the hydrogen generation via photoelectrochecmial reaction using films of exfoliated 2-dimensional (2D MoS2, which acts as an efficient photocatalyst. The film of chemically exfoliated MoS2 layers was employed for water splitting, leading to hydrogen generation. The amount of hydrogen was qualitatively monitored by observing overpressure of a water container. The high photo-current generated by MoS2 film resulted in hydrogen evolution. Our work shows that 2D MoS2 is one of the promising candidates as a photocatalyst for light-induced hydrogen generation. High photoelectrocatalytic efficiency of the 2D MoS2 shows a new way toward hydrogen generation, which is one of the renewable energy sources. The efficient photoelectrocatalytic property of the 2D MoS2 is possibly due to availability of catalytically active edge sites together with minimal stacking that favors the electron transfer.

  16. Excitation mechanism of A1g mode and origin of nonlinear temperature dependence of Raman shift of CVD-grown mono- and few-layer MoS2 films.

    Science.gov (United States)

    Yang, Tianqi; Huang, Xiaoting; Zhou, Hong; Wu, Guangheng; Lai, Tianshu

    2016-05-30

    MoS2 films are grown on SiO2/Si substrates by chemical vapor deposition. The vibrational properties of optical phonons of mono-, bi- and multilayer MoS2 are studied by Raman scattering spectroscopy over temperature range from 90 to 540 K with 514.5 nm and 785 nm lasers. The Raman peaks of E2g1 and A1g modes are observed simultaneously for mono-, bi- and multilayer MoS2 with 514.5 nm laser, but only the Raman peak of E2g1 mode is seen for monolayer MoS2 as 785 nm laser is used, revealing electron-phonon exchange excitation mechanism of A1g mode for the first time. The Raman shifts of E2g1 and A1g modes present obvious nonlinear temperature dependence. A semi-quantitative model is used to fit the nonlinear temperature dependence of Raman shifts which matches well to experimental data. Meanwhile, the fitting results reveal that the nonlinear temperature dependence of Raman shifts of E2g1 mode mainly originates from three-phonon anharmonic effect, while one of A1g mode is contributed by stronger three- and weaker four-phonon anharmonic effects cooperatively but two contributions are comparable in intensity.

  17. First-principles analysis of MoS2/Ti2C and MoS2/Ti2CY2 (Y=F and OH) all-2D semiconductor/metal contacts

    KAUST Repository

    Gan, Liyong

    2013-06-13

    First-principles calculations are used to explore the geometry, bonding, and electronic properties of MoS2/Ti2C and MoS2/Ti2CY2 (Y = F and OH) semiconductor/metal contacts. The structure of the interfaces is determined. Strong chemical bonds formed at the MoS2/Ti2C interface result in additional states next to the Fermi level, which extend over the three atomic layers of MoS2 and induce a metallic character. The interaction in MoS2/Ti2CY2, on the other hand, is weak and not sensitive to the specific geometry, and the semiconducting nature thus is preserved. The energy level alignment implies weak and strong n-type doping of MoS2 in MoS2/Ti2CF2 and MoS2/Ti2C(OH)2, respectively. The corresponding n-type Schottky barrier heights are 0.85 and 0.26 eV. We show that the MoS2/Ti2CF2 interface is close to the Schottky limit. At the MoS2/Ti2C(OH)2 interface, we find that a strong dipole due to charge rearrangement induces the Schottky barrier. The present interfaces are well suited for application in all-two-dimensional devices.

  18. Piezoelectric effect in chemical vapour deposition-grown atomic-monolayer triangular molybdenum disulfide piezotronics.

    Science.gov (United States)

    Qi, Junjie; Lan, Yann-Wen; Stieg, Adam Z; Chen, Jyun-Hong; Zhong, Yuan-Liang; Li, Lain-Jong; Chen, Chii-Dong; Zhang, Yue; Wang, Kang L

    2015-06-25

    High-performance piezoelectricity in monolayer semiconducting transition metal dichalcogenides is highly desirable for the development of nanosensors, piezotronics and photo-piezotransistors. Here we report the experimental study of the theoretically predicted piezoelectric effect in triangle monolayer MoS2 devices under isotropic mechanical deformation. The experimental observation indicates that the conductivity of MoS2 devices can be actively modulated by the piezoelectric charge polarization-induced built-in electric field under strain variation. These polarization charges alter the Schottky barrier height on both contacts, resulting in a barrier height increase with increasing compressive strain and decrease with increasing tensile strain. The underlying mechanism of strain-induced in-plane charge polarization is proposed and discussed using energy band diagrams. In addition, a new type of MoS2 strain/force sensor built using a monolayer MoS2 triangle is also demonstrated. Our results provide evidence for strain-gating monolayer MoS2 piezotronics, a promising avenue for achieving augmented functionalities in next-generation electronic and mechanical-electronic nanodevices.

  19. Piezoelectric effect in chemical vapour deposition-grown atomic-monolayer triangular molybdenum disulfide piezotronics

    KAUST Repository

    Qi, Junjie

    2015-06-25

    High-performance piezoelectricity in monolayer semiconducting transition metal dichalcogenides is highly desirable for the development of nanosensors, piezotronics and photo-piezotransistors. Here we report the experimental study of the theoretically predicted piezoelectric effect in triangle monolayer MoS2 devices under isotropic mechanical deformation. The experimental observation indicates that the conductivity of MoS2 devices can be actively modulated by the piezoelectric charge polarization-induced built-in electric field under strain variation. These polarization charges alter the Schottky barrier height on both contacts, resulting in a barrier height increase with increasing compressive strain and decrease with increasing tensile strain. The underlying mechanism of strain-induced in-plane charge polarization is proposed and discussed using energy band diagrams. In addition, a new type of MoS2 strain/force sensor built using a monolayer MoS2 triangle is also demonstrated. Our results provide evidence for strain-gating monolayer MoS2 piezotronics, a promising avenue for achieving augmented functionalities in next-generation electronic and mechanical–electronic nanodevices.

  20. Structural and electronic properties of germanene/MoS2 monolayer and silicene/MoS2 monolayer superlattices

    OpenAIRE

    Li, Xiaodan; Wu, Shunqing; Zhou, Sen; Zhu, Zizhong

    2014-01-01

    Superlattice provides a new approach to enrich the class of materials with novel properties. Here, we report the structural and electronic properties of superlattices made with alternate stacking of two-dimensional hexagonal germanene (or silicene) and a MoS2 monolayer using the first principles approach. The results are compared with those of graphene/MoS2 superlattice. The distortions of the geometry of germanene, silicene, and MoS2 layers due to the formation of the superlattices are all r...

  1. Scalable Patterning of MoS2 Nanoribbons by Micromolding in Capillaries.

    Science.gov (United States)

    Hung, Yu-Han; Lu, Ang-Yu; Chang, Yung-Huang; Huang, Jing-Kai; Chang, Jeng-Kuei; Li, Lain-Jong; Su, Ching-Yuan

    2016-08-17

    In this study, we report a facile approach to prepare dense arrays of MoS2 nanoribbons by combining procedures of micromolding in capillaries (MIMIC) and thermolysis of thiosalts ((NH4)2MoS4) as the printing ink. The obtained MoS2 nanoribbons had a thickness reaching as low as 3.9 nm, a width ranging from 157 to 465 nm, and a length up to 2 cm. MoS2 nanoribbons with an extremely high aspect ratio (length/width) of ∼7.4 × 10(8) were achieved. The MoS2 pattern can be printed on versatile substrates, such as SiO2/Si, sapphire, Au film, FTO/glass, and graphene-coated glass. The degree of crystallinity of the as-prepared MoS2 was discovered to be adjustable by varying the temperature through postannealing. The high-temperature thermolysis (1000 °C) results in high-quality conductive samples, and field-effect transistors based on the patterned MoS2 nanoribbons were demonstrated and characterized, where the carrier mobility was comparable to that of thin-film MoS2. In contrast, the low-temperature-treated samples (170 °C) result in a unique nanocrystalline MoSx structure (x ≈ 2.5), where the abundant and exposed edge sites were obtained from highly dense arrays of nanoribbon structures by this MIMIC patterning method. The patterned MoSx was revealed to have superior electrocatalytic efficiency (an overpotential of ∼211 mV at 10 mA/cm(2) and a Tafel slope of 43 mV/dec) in the hydrogen evolution reaction (HER) when compared to the thin-film MoS2. The report introduces a new concept for rapidly fabricating cost-effective and high-density MoS2/MoSx nanostructures on versatile substrates, which may pave the way for potential applications in nanoelectronics/optoelectronics and frontier energy materials.

  2. Scalable Patterning of MoS2Nanoribbons by Micromolding in Capillaries

    KAUST Repository

    Hung, Yu-Han

    2016-07-27

    In this study, we report a facile approach to prepare dense arrays of MoS2 nanoribbons by combining procedures of micromolding in capillaries (MIMIC) and thermolysis of thiosalts ((NH4)2MoS4) as the printing ink. The obtained MoS2 nanoribbons had a thickness reaching as low as 3.9 nm, a width ranging from 157 to 465 nm, and a length up to 2 cm. MoS2 nanoribbons with an extremely high aspect ratio (length/width) of ∼7.4 × 108 were achieved. The MoS2 pattern can be printed on versatile substrates, such as SiO2/Si, sapphire, Au film, FTO/glass, and graphene-coated glass. The degree of crystallinity of the as-prepared MoS2 was discovered to be adjustable by varying the temperature through postannealing. The high-temperature thermolysis (1000 °C) results in high-quality conductive samples, and field-effect transistors based on the patterned MoS2 nanoribbons were demonstrated and characterized, where the carrier mobility was comparable to that of thin-film MoS2. In contrast, the low-temperature-treated samples (170 °C) result in a unique nanocrystalline MoSx structure (x ≈ 2.5), where the abundant and exposed edge sites were obtained from highly dense arrays of nanoribbon structures by this MIMIC patterning method. The patterned MoSx was revealed to have superior electrocatalytic efficiency (an overpotential of ∼211 mV at 10 mA/cm2 and a Tafel slope of 43 mV/dec) in the hydrogen evolution reaction (HER) when compared to the thin-film MoS2. The report introduces a new concept for rapidly fabricating cost-effective and high-density MoS2/MoSx nanostructures on versatile substrates, which may pave the way for potential applications in nanoelectronics/optoelectronics and frontier energy materials. © 2016 American Chemical Society.

  3. Observing the semiconducting band-gap alignment of MoS2 layers of different atomic thicknesses using a MoS2/SiO2/Si heterojunction tunnel diode

    NARCIS (Netherlands)

    Nishiguchi, K.; Castellanos-Gomez, A.; Yamaguchi, H.; Fujiwara, A.; Van der Zant, H.S.J.; Steele, G.A.

    2015-01-01

    We demonstrate a tunnel diode composed of a vertical MoS2/SiO2/Si heterostructure. A MoS2 flake consisting four areas of different thicknesses functions as a gate terminal of a silicon field-effect transistor. A thin gate oxide allows tunneling current to flow between the n-type MoS2 layers and p-ty

  4. Attapulgite-CeO2/MoS2 ternary nanocomposite for photocatalytic oxidative desulfurization

    Science.gov (United States)

    Li, Xiazhang; Zhang, Zuosong; Yao, Chao; Lu, Xiaowang; Zhao, Xiaobing; Ni, Chaoying

    2016-02-01

    Novel attapulgite(ATP)-CeO2/MoS2 ternary nanocomposites were synthesized by microwave assisted assembly method. The structures of the nanocomposites were characterized by XRD, FT-IR, UV-vis, XPS and in situ TEM. The photocatalytic activities of ATP-CeO2/MoS2 composites were investigated by degradating dibenzothiophene (DBT) in gasoline under visible light irradiation. The effect of the mass ratio of CeO2 to MoS2 on photocatalytic activity was investigated. The results indicate that the three-dimensional network structure is firmly constructed by ATP skeleton, CeO2 particles and MoS2 nanosheet which effectively increase the surface area of the composites and promote the separation of electrons and holes by resulting electronic transmission channels of multi-channel in space. The degradation rate of DBT can reach 95% under 3 h irradiation when the mass ratio of CeO2/MoS2 is 4/10. A plausible mechanism for the photocatalytic oxidative desulfurization of this nanocomposite is put forward.

  5. Nanoscale inhomogeneity of the Schottky barrier and resistivity in MoS2 multilayers

    Science.gov (United States)

    Giannazzo, F.; Fisichella, G.; Piazza, A.; Agnello, S.; Roccaforte, F.

    2015-08-01

    Conductive atomic force microscopy (CAFM) is employed to investigate the current injection from a nanometric contact (a Pt coated tip) to the surface of MoS2 thin films. The analysis of local current-voltage characteristics on a large array of tip positions provides high spatial resolution information on the lateral homogeneity of the tip /MoS2 Schottky barrier ΦB and ideality factor n , and on the local resistivity ρloc of the MoS2 region under the tip. Here, ΦB=300 ±24 meV , n =1.60 ±0.23 , and ρloc=2.99 ±0.68 Ω cm are calculated from the distributions of locally measured values. A linear correlation is found between the ρloc and ΦB values at each tip position, indicating a similar origin of the ρloc and ΦB inhomogeneities. These findings are compared with recent literature results on the role of sulfur vacancy clusters on the MoS2 surface as preferential paths for current injection from metal contacts. Furthermore, their implications on the behavior of MoS2 based transistors are discussed.

  6. Preparation and tribological properties of MoS2/graphene oxide composites

    Science.gov (United States)

    Song, Haojie; Wang, Biao; Zhou, Qiang; Xiao, Jiaxuan; Jia, Xiaohua

    2017-10-01

    A hydrothermal route is developed for the synthesis of MoS2/graphene oxide (GO) composites based on the hydrothermal reduction of Na2MoO4 and GO sheets with L-cysteine. The MoS2/GO composites in improving friction and wear of the sunshine oil on sliding steel surfaces under low or high applied load were demonstrated. In tests with sliding steel surfaces, the sunshine oil that contains small amounts of MoS2/GO composites exhibited the lowest specific friction coefficient and wear rate under all of the sliding conditions. Scanning electron microscopy and energy dispersive spectrometer performed to analyze the wear scar surfaces after friction confirmed that the outstanding lubrication performance of MoS2/GO composites could be attributed to their good dispersion stability and extremely thin laminated structure, which allow the MoS2/GO composites to easily enter the contact area, thereby preventing the rough surfaces from coming into direct contact.

  7. Excitation dependent bidirectional electron transfer in phthalocyanine-functionalised MoS2 nanosheets.

    Science.gov (United States)

    Nguyen, Emily P; Carey, Benjamin J; Harrison, Christopher J; Atkin, Paul; Berean, Kyle J; Della Gaspera, Enrico; Ou, Jian Zhen; Kaner, Richard B; Kalantar-Zadeh, Kourosh; Daeneke, Torben

    2016-09-15

    Two-dimensional (2D) transition metal chalcogenides such as 2D MoS2 are considered prime candidate materials for the design of next generation optoelectronics. Functionalisation of these materials is considered to be a key step in tailoring their properties towards specific applications and unlocking their full potential. Here we present a van der Waals functionalisation strategy for creating MoS2 nanosheets decorated with free base phthalocyanine chromophores. The semiconducting sheets are found to intimately interact with these optoelectronically active chromophores, resulting in an electronic heterostructure that exhibits enhanced optoelectronic properties and exploitable charge transfer. We show that by utilising laterally confined MoS2 nanosheets, the conduction band of the semiconductor could be positioned between the chromophore's S1 and S2 states. Consequently, bidirectional photoinduced electron transfer processes are observed, with excitation of the functionalised nanosheet's semiconductor transition resulting in electron transfer to the phthalocyanine's LUMO, and excitation of the chromophore's S2 state leading to electron injection into the MoS2 conduction band. However, charge transfer from the dye's S1 transition to the MoS2 nanosheet is found to be thermodynamically unfavourable, resulting in intense radiative recombination. These findings may enable controlling and tuning the charge carrier density of semiconducting nanosheets via optical means through the exploitation of photoinduced electron transfer. Furthermore this work provides access to 2D semiconductor-hybrids with tailored absorption profiles and photoluminescence.

  8. In situ MoS2 Decoration of Laser-Induced Graphene as Flexible Supercapacitor Electrodes.

    Science.gov (United States)

    Clerici, Francesca; Fontana, Marco; Bianco, Stefano; Serrapede, Mara; Perrucci, Francesco; Ferrero, Sergio; Tresso, Elena; Lamberti, Andrea

    2016-04-27

    Herein, we are reporting a rapid one-pot synthesis of MoS2-decorated laser-induced graphene (MoS2-LIG) by direct writing of polyimide foils. By covering the polymer surface with a layer of MoS2 dispersion before processing, it is possible to obtain an in situ decoration of a porous graphene network during laser writing. The resulting material is a three-dimensional arrangement of agglomerated and wrinkled graphene flakes decorated by MoS2 nanosheets with good electrical properties and high surface area, suitable to be employed as electrodes for supercapacitors, enabling both electric double-layer and pseudo-capacitance behaviors. A deep investigation of the material properties has been performed to understand the chemical and physical characteristics of the hybrid MoS2-graphene-like material. Symmetric supercapacitors have been assembled in planar configuration exploiting the polymeric electrolyte; the resulting performances of the here-proposed material allow the prediction of the enormous potentialities of these flexible energy-storage devices for industrial-scale production.

  9. Flower-like PEGylated MoS2 nanoflakes for near-infrared photothermal cancer therapy.

    Science.gov (United States)

    Feng, Wei; Chen, Liang; Qin, Ming; Zhou, Xiaojun; Zhang, Qianqian; Miao, Yingke; Qiu, Kexin; Zhang, Yanzhong; He, Chuanglong

    2015-12-03

    Photothermal cancer therapy has attracted considerable interest for cancer treatment in recent years, but the effective photothermal agents remain to be explored before this strategy can be applied clinically. In this study, we therefore develop flower-like molybdenum disulfide (MoS2) nanoflakes and investigate their potential for photothermal ablation of cancer cells. MoS2 nanoflakes are synthesized via a facile hydrothermal method and then modified with lipoic acid-terminated polyethylene glycol (LA-PEG), endowing the obtained nanoflakes with high colloidal stability and very low cytotoxicity. Upon irradiation with near infrared (NIR) laser at 808 nm, the nanoflakes showed powerful ability of inducing higher temperature, good photothermal stability and high photothermal conversion efficiency. The in vitro photothermal effects of MoS2-PEG nanoflakes with different concentrations were also evaluated under various power densities of NIR 808-nm laser irradiation, and the results indicated that an effective photothermal killing of cancer cells could be achieved by a low concentration of nanoflakes under a low power NIR 808-nm laser irradiation. Furthermore, cancer cell in vivo could be efficiently destroyed via the photothermal effect of MoS2-PEG nanoflakes under the irradiation. These results thus suggest that the MoS2-PEG nanoflakes would be as promising photothermal agents for future photothermal cancer therapy.

  10. Recent advances in MoS2 nanostructured materials for energy and environmental applications - A review

    Science.gov (United States)

    Theerthagiri, J.; Senthil, R. A.; Senthilkumar, B.; Reddy Polu, Anji; Madhavan, J.; Ashokkumar, Muthupandian

    2017-08-01

    Molybdenum disulfide (MoS2), a layered transition metal dichalcogenide with an analogous structure to graphene, has attracted enormous attention worldwide owing to its use in a variety of applications such as energy storage, energy conversion, environmental remediation and sensors. MoS2 and graphene have almost similar functional properties such as high charge carrier transport, high wear resistance and good mechanical strength and friction. However, MoS2 is advantageous over graphene due to its low-cost, abundancy, tailorable morphologies and tuneable band gap with good visible light absorption properties. In this review, we have focussed mainly on recent advances in MoS2 nanostructured materials for the applications in the broad area of energy and environment. Special attention has been paid to their applications in dye-sensitized solar cells, supercapacitor, Li-ion battery, hydrogen evolution reaction, photocatalysis for the degradation of organic pollutants, chemical/bio sensors and gas sensors. Finally, the challenges to design MoS2 nanostructures suitable for energy and environmental applications are also highlighted.

  11. Growth characteristics of MoS2 coatings prepared by unbalanced bipolar DC magnetron sputtering

    Institute of Scientific and Technical Information of China (English)

    WANG Ji-hui; XIA Yang; E.Wieers; L.M.Stals; J.P.Celis

    2005-01-01

    MoS2 coatings were prepared by unbalanced bipolar DC magnetron sputtering under different argon pressures and for different deposition times, and the structure and morphology of MoS2 coatings were determined and observed respectively by X-ray diffractometry and scanning electron microscopy. The results show that at lower argon pressures of 0.15Pa and 0.40Pa, MoS2 coatings are formed with the (002) basal plane parallel to the surface, whereas the coating deposited at the argon pressure above 0.60Pa has the (002) basal plane perpendicular to the surface. Two stages can be classified for the formation of MoS2 coating. At the initial stage of coating formation, the (002) basal plane with S-Mo-S layer structure grows on the substrate whatever the argon pressure is. And then the coating under 0.40Pa argon pressure still grows with (002) laminate structure, but the coatings under 0.88Pa and 1.60Pa argon pressures turn to grow with the mixed basal and edge orientations. The morphology and structure of MoS2 coatings are highly related to their growth rate and the energy of sputtered particles.

  12. Single-layer MoS2 nanopores as nanopower generators

    Science.gov (United States)

    Feng, Jiandong; Graf, Michael; Liu, Ke; Ovchinnikov, Dmitry; Dumcenco, Dumitru; Heiranian, Mohammad; Nandigana, Vishal; Aluru, Narayana R.; Kis, Andras; Radenovic, Aleksandra

    2016-08-01

    Making use of the osmotic pressure difference between fresh water and seawater is an attractive, renewable and clean way to generate power and is known as ‘blue energy’. Another electrokinetic phenomenon, called the streaming potential, occurs when an electrolyte is driven through narrow pores either by a pressure gradient or by an osmotic potential resulting from a salt concentration gradient. For this task, membranes made of two-dimensional materials are expected to be the most efficient, because water transport through a membrane scales inversely with membrane thickness. Here we demonstrate the use of single-layer molybdenum disulfide (MoS2) nanopores as osmotic nanopower generators. We observe a large, osmotically induced current produced from a salt gradient with an estimated power density of up to 106 watts per square metre—a current that can be attributed mainly to the atomically thin membrane of MoS2. Low power requirements for nanoelectronic and optoelectric devices can be provided by a neighbouring nanogenerator that harvests energy from the local environment—for example, a piezoelectric zinc oxide nanowire array or single-layer MoS2 (ref. 12). We use our MoS2 nanopore generator to power a MoS2 transistor, thus demonstrating a self-powered nanosystem.

  13. Large-Area Growth of Uniform Single-Layer MoS2 Thin Films by Chemical Vapor Deposition.

    Science.gov (United States)

    Baek, Seung Hyun; Choi, Yura; Choi, Woong

    2015-12-01

    We report the largest-size thin films of uniform single-layer MoS2 on sapphire substrates grown by chemical vapor deposition based on the reaction of gaseous MoO3 and S evaporated from solid sources. The as-grown thin films of single-layer MoS2 were continuous and uniform in thickness for more than 4 cm without the existence of triangular-shaped MoS2 clusters. Compared to mechanically exfoliated crystals, the as-grown single-layer MoS2 thin films possessed consistent chemical valence states and crystal structure along with strong photoluminescence emission and optical absorbance at high energy. These results demonstrate that it is possible to scale up the growth of uniform single-layer MoS2 thin films, providing potentially important implications on realizing high-performance MoS2 devices.

  14. Fluorescent MoS2 Quantum Dots: Ultrasonic Preparation, Up-Conversion and Down-Conversion Bioimaging, and Photodynamic Therapy.

    Science.gov (United States)

    Dong, Haifeng; Tang, Songsong; Hao, Yansong; Yu, Haizhu; Dai, Wenhao; Zhao, Guifeng; Cao, Yu; Lu, Huiting; Zhang, Xueji; Ju, Huangxian

    2016-02-10

    Small size molybdenum disulfide (MoS2) quantum dots (QDs) with desired optical properties were controllably synthesized by using tetrabutylammonium-assisted ultrasonication of multilayered MoS2 powder via OH-mediated chain-like Mo-S bond cleavage mode. The tunable up-bottom approach of precise fabrication of MoS2 QDs finally enables detailed experimental investigations of their optical properties. The synthesized MoS2 QDs present good down-conversion photoluminescence behaviors and exhibit remarkable up-conversion photoluminescence for bioimaging. The mechanism of the emerging photoluminescence was investigated. Furthermore, superior (1)O2 production ability of MoS2 QDs to commercial photosensitizer PpIX was demonstrated, which has great potential application for photodynamic therapy. These early affording results of tunable synthesis of MoS2 QDs with desired photo properties can lead to application in fields of biomedical and optoelectronics.

  15. Thiol click chemistry on gold-decorated MoS2: elastomer composites and structural phase transitions

    Science.gov (United States)

    Topolovsek, Peter; Cmok, Luka; Gadermaier, Christoph; Borovsak, Milos; Kovac, J.; Mrzel, Ales

    2016-05-01

    We show that gold decorated MoS2 flakes are amenable to thiol chemistry by blending them with a cross-linkable thiolated polysiloxane (PMMS). PMMS prevents restacking of dispersed MoS2 when transforming the metallic to the semiconducting phase. Cross-linking PMMS yields an elastomer of good optical quality, containing individual, mostly single-layer MoS2 flakes.We show that gold decorated MoS2 flakes are amenable to thiol chemistry by blending them with a cross-linkable thiolated polysiloxane (PMMS). PMMS prevents restacking of dispersed MoS2 when transforming the metallic to the semiconducting phase. Cross-linking PMMS yields an elastomer of good optical quality, containing individual, mostly single-layer MoS2 flakes. Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nr01490a

  16. High-Sensitivity, Highly Transparent, Gel-Gated MoS2 Phototransistor on Biodegradable Nanopaper

    KAUST Repository

    Zhang, Qing

    2016-06-21

    Transition metal dichalcogenides hold great promise for a variety of novel electrical, optical and mechanical devices and applications. Among them, molybdenum disulphide (MoS2) is gaining increasing attention as the gate dielectric and semiconductive channel for high-perfomance field effect transistors. Here we report on the first MoS2 phototransistor built on flexible, transparent and biodegradable substrate with electrolyte gate dielectric. We have carried out systematic studies on its electrical and optoelectronic properties. The MoS2 phototransistor exhibited excellent photo responsivity of ~1.5 kA/W, about two times higher compared to typical back-gated devices reported in previous studies. The device is highly transparent at the same time with an average optical transmittance of 82%. Successful fabrication of phototransistors on flexible cellulose nanopaper with excellent performance and transparency suggests that it is feasible to achieve an ecofriendly, biodegradable phototransistor with great photoresponsivity, broad spectral range and durable flexibility.

  17. Structural and Electronic Properties of Germanene on MoS2

    Science.gov (United States)

    Zhang, L.; Bampoulis, P.; Rudenko, A. N.; Yao, Q.; van Houselt, A.; Poelsema, B.; Katsnelson, M. I.; Zandvliet, H. J. W.

    2016-06-01

    To date germanene has only been synthesized on metallic substrates. A metallic substrate is usually detrimental for the two-dimensional Dirac nature of germanene because the important electronic states near the Fermi level of germanene can hybridize with the electronic states of the metallic substrate. Here we report the successful synthesis of germanene on molybdenum disulfide (MoS2), a band gap material. Preexisting defects in the MoS2 surface act as preferential nucleation sites for the germanene islands. The lattice constant of the germanene layer (3.8 ±0.2 Å ) is about 20% larger than the lattice constant of the MoS2 substrate (3.16 Å). Scanning tunneling spectroscopy measurements and density functional theory calculations reveal that there are, besides the linearly dispersing bands at the K points, two parabolic bands that cross the Fermi level at the Γ point.

  18. Probing the interlayer coupling of twisted bilayer MoS2 using photoluminescence spectroscopy.

    Science.gov (United States)

    Huang, Shengxi; Ling, Xi; Liang, Liangbo; Kong, Jing; Terrones, Humberto; Meunier, Vincent; Dresselhaus, Mildred S

    2014-10-08

    Two-dimensional molybdenum disulfide (MoS2) is a promising material for optoelectronic devices due to its strong photoluminescence emission. In this work, the photoluminescence of twisted bilayer MoS2 is investigated, revealing a tunability of the interlayer coupling of bilayer MoS2. It is found that the photoluminescence intensity ratio of the trion and exciton reaches its maximum value for the twisted angle 0° or 60°, while for the twisted angle 30° or 90° the situation is the opposite. This is mainly attributed to the change of the trion binding energy. The first-principles density functional theory analysis further confirms the change of the interlayer coupling with the twisted angle, which interprets our experimental results.

  19. High performance and transparent multilayer MoS2 transistors: Tuning Schottky barrier characteristics

    Directory of Open Access Journals (Sweden)

    Young Ki Hong

    2016-05-01

    Full Text Available Various strategies and mechanisms have been suggested for investigating a Schottky contact behavior in molybdenum disulfide (MoS2 thin-film transistor (TFT, which are still in much debate and controversy. As one of promising breakthrough for transparent electronics with a high device performance, we have realized MoS2 TFTs with source/drain electrodes consisting of transparent bi-layers of a conducting oxide over a thin film of low work function metal. Intercalation of a low work function metal layer, such as aluminum, between MoS2 and transparent source/drain electrodes makes it possible to optimize the Schottky contact characteristics, resulting in about 24-fold and 3 orders of magnitude enhancement of the field-effect mobility and on-off current ratio, respectively, as well as transmittance of 87.4 % in the visible wavelength range.

  20. Edge structures and properties of triangular antidots in single-layer MoS2

    KAUST Repository

    Gan, Li Yong

    2016-08-30

    Density functional theory and experiments are employed to shed light on the edge structures of antidots in O etched single-layer MoS2. The equilibrium morphology is found to be the zigzag Mo edge with each Mo atom bonded to two O atoms, in a wide range of O chemical potentials. Scanning electron microscopy shows that the orientation of the created triangular antidots is opposite to the triangular shape of the single-layer MoS2 samples, in agreement with the theoretical predictions. Furthermore, edges induced by O etching turn out to be p-doped, suggesting an effective strategy to realize p-type MoS2 devices. Published by AIP Publishing.

  1. Microscopic origin of low frequency noise in MoS2 field-effect transistors

    Directory of Open Access Journals (Sweden)

    Subhamoy Ghatak

    2014-09-01

    Full Text Available We report measurement of low frequency 1/f noise in molybdenum di-sulphide (MoS2 field-effect transistors in multiple device configurations including MoS2 on silicon dioxide as well as MoS2-hexagonal boron nitride (hBN heterostructures. All as-fabricated devices show similar magnitude of noise with number fluctuation as the dominant mechanism at high temperatures and density, although the calculated density of traps is two orders of magnitude higher than that at the SiO2 interface. Measurements on the heterostructure devices with vacuum annealing and dual gated configuration reveals that along with the channel, metal-MoS2 contacts also play a significant role in determining noise magnitude in these devices.

  2. Electrohydrodynamic printing for scalable MoS2 flake coating: application to gas sensing device

    Science.gov (United States)

    Lim, Sooman; Cho, Byungjin; Bae, Jaehyun; Kim, Ah Ra; Lee, Kyu Hwan; Kim, Se Hyun; Hahm, Myung Gwan; Nam, Jaewook

    2016-10-01

    Scalable sub-micrometer molybdenum disulfide ({{MoS}}2) flake films with highly uniform coverage were created using a systematic approach. An electrohydrodynamic (EHD) printing process realized a remarkably uniform distribution of exfoliated {{MoS}}2 flakes on desired substrates. In combination with a fast evaporating dispersion medium and an optimal choice of operating parameters, the EHD printing can produce a film rapidly on a substrate without excessive agglomeration or cluster formation, which can be problems in previously reported liquid-based continuous film methods. The printing of exfoliated {{MoS}}2 flakes enabled the fabrication of a gas sensor with high performance and reproducibility for {{NO}}2 and {{NH}}3.

  3. Tribological Performance of MoS2-based Coatings after Deposition and Storage in Humid Air

    Institute of Scientific and Technical Information of China (English)

    JINGYang; LUOJian-bin; PANGSi-qin

    2004-01-01

    MoS2-based composite coatings were deposited with the nano-compound unbalanced plasma plating technique the effects of processing parameters and working enwironments on the tribological properties of the coatings were examined by the drilling experiuments and XPS.the distances between substrate and Ti larget, Ti content and deposition pressure were varied in order to determine the optimun conditions for producing lubricious,long-lasting MoS2-based coatings,IT is found that the tribological performance of Tin-MoS2 roating decreases rqapidly in humid air but the humid resistant property of Tin-MoS2/Ti coating improves evidently it is indicated that the humid-resistantance property and the abrasion durability of MoS2-based coatings can be enhanced markedly by adding Ti with a certain contents.

  4. Effect of MoO3 constituents on the growth of MoS2 nanosheets by chemical vapor deposition

    Science.gov (United States)

    Wang, Xuan; Zhang, Yong Ping; Qian Chen, Zhi

    2016-06-01

    The highly crystalline and uniform MoS2 film was grown on Si substrate by a low-pressure chemical vapor deposition method using S and MoO3 as precursors at an elevated temperature. The structures and properties of MoS2 nanosheets vary greatly with the content of MoO3 constituents in the films. The nanostructured MoS2 film exhibits strong photoluminescence in the visible range. This work may provide a pathway to synthesizing MoS2 nanosheets and facilitate the development of applicable devices.

  5. Three-dimensional nanotubes composed of carbon-anchored ultrathin MoS2 nanosheets with enhanced lithium storage.

    Science.gov (United States)

    Yi, Fenglian; Niu, Yubin; Liu, Sangui; Hou, Junke; Bao, Shujuan; Xu, Maowen

    2016-07-20

    MoS2 nanotubes (denoted as MoS2 NTs) assembled from well-aligned amorphous carbon-modified ultrathin MoS2 nanosheets (denoted as MoS2 NT@C) were successfully fabricated via a facile solvothermal method combined with subsequent annealing treatment. With the assistance of octylamine as a solvent and carbon source, interconnected MoS2 nanosheets (denoted as MoS2 NSs) can assemble into hierarchical MoS2 NTs. Such a hybrid nanostructure can effectively facilitate charge transport and accommodate volume variation upon prolonged charge/discharge cycling for reversible lithium storage. As a result, the MoS2 NT@C composite manifests a very stable high reversible capacity of around 1351 mA h g(-1) at a current density of 100 mA g(-1); even after 150 cycles, the electrode reaches a capacity of 1106 mA h g(-1) and it retains a reversible capacity of 650 mA h g(-1) after the 10th cycle at a current density of 3 A g(-1), all of which indicate that the MoS2 NT@C nanocomposite is a promising negative electrode material for high-energy lithium ion batteries.

  6. Synthesis of fungus-like MoS2 nanosheets with ultrafast adsorption capacities toward organic dyes

    Science.gov (United States)

    Song, HaoJie; You, Shengsheng; Jia, XiaoHua

    2015-11-01

    Fungus-like molybdenum disulfide (MoS2) nanosheets with a thickness of a few nanometers have been successfully synthesized via one-pot hydrothermal method. The as-prepared MoS2 nanosheets with a high surface area of 106.989 m2 g-1 exhibited excellent wastewater treatment performance with high removal capacities toward organic dyes. In addition, the fungus-like MoS2 nanosheets can absorb Congo red completely within 2 min. Successful access to high quality fungus-like MoS2 nanosheets will make it possible for their potential application in catalysis and other fields.

  7. Interface properties of CVD grown graphene transferred onto MoS2(0001).

    Science.gov (United States)

    Coy Diaz, Horacio; Addou, Rafik; Batzill, Matthias

    2014-01-21

    Heterostructures of dissimilar 2D materials are potential building blocks for novel materials and may enable the formation of new (photo)electronic device architectures. Previous work mainly focused on supporting graphene on insulating wide-band gap materials, such as hex-BN and mica. Here we investigate the interface between zero-band gap semiconductor graphene and band-gap semiconductor MoS2 as a potential building block for entirely 2D-material based semiconducting devices. We show that solution transfer results in water trapping at the interface which may be removed by annealing to ~300 °C in a vacuum. After removal of the water, by high temperature annealing, ultraflat graphene is obtained on MoS2 with only a very weak moiré pattern observable in scanning tunneling microscopy images due to lattice mismatch and random alignment of graphene with respect to the MoS2 substrate. Photoemission spectroscopy indicates interface dipole formation, p-type doping of graphene by ~0.09 eV downward shift of the Fermi-level below the Dirac point, and a negative space charge region in bulk MoS2. Interestingly, valence band spectra of the graphene covered MoS2 surface indicate a band gap narrowing of the MoS2 surface by ~0.1 eV. This band gap reduction at the surface is further evidence that interlayer van der Waals interactions critically influence the band structure of 2D-layered dichalcogenides and suggest that interfacing with dissimilar van der Waals materials allows tuning of their electronic properties.

  8. $\\mathbb{Z}_2$ invariance of Germanene on MoS$_2$ from first principles

    OpenAIRE

    Amlaki, Taher; Bokdam, Menno; Paul J. Kelly

    2016-01-01

    We present a low energy Hamiltonian generalized to describe how the energy bands of germanene ($\\rm \\overline{Ge}$) are modified by interaction with a substrate or a capping layer. The parameters that enter the Hamiltonian are determined from first-principles relativistic calculations for $\\rm \\overline{Ge}|$MoS$_2$ bilayers and MoS$_2|\\rm \\overline{Ge} |$MoS$_2$ trilayers and are used to determine the topological nature of the system. For the lowest energy, buckled germanene structure, the g...

  9. Atomic-scale structure of single-layer MoS2 nanoclusters

    DEFF Research Database (Denmark)

    Helveg, S.; Lauritsen, J. V.; Lægsgaard, E.

    2000-01-01

    We have studied using scanning tunneling microscopy (STM) the atomic-scale realm of molybdenum disulfide (MoS2) nanoclusters, which are of interest as a model system in hydrodesulfurization catalysis. The STM gives the first real space images of the shape and edge structure of single-layer MoS2 n...... nanoparticles synthesized on Au(lll), and establishes a new picture of the active edge sires of the nanoclusters. The results demonstrate a way to get detailed atomic-scale information on catalysts in general....

  10. Plasmonic ambient light sensing with MoS2-graphene heterostructures

    Science.gov (United States)

    Radoi, Antonio; Dragoman, Mircea; Dragoman, Daniela

    2017-01-01

    We present experimental results on plasmonic photodetection of ambient light using MoS2-graphene heterostructures illuminated with three very-low-power light emitting diodes (LEDs) radiating in blue, green, and red, respectively. The working principle of this photodetector validates the recent predictions of hot-carrier plasmonic doping of MoS2. The obtained responsivity for each spectral domain is in agreement with photopic standards of the luminosity function. The response time of the detector is less than the eye blinking time.

  11. Efficient many-body calculations for two-dimensional materials using exact limits for the screened potential: Band gaps of MoS2, h -BN, and phosphorene

    Science.gov (United States)

    Rasmussen, Filip A.; Schmidt, Per S.; Winther, Kirsten T.; Thygesen, Kristian S.

    2016-10-01

    Calculating the quasiparticle (QP) band structure of two-dimensional (2D) materials within the GW self-energy approximation has proven to be a rather demanding computational task. The main reason is the strong q dependence of the 2D dielectric function around q =0 that calls for a much denser sampling of the Brillouin zone (BZ) than is necessary for similar three-dimensional solids. Here, we use an analytical expression for the small q limit of the 2D response function to perform the BZ integral over the critical region around q =0 . This drastically reduces the requirements on the q -point mesh and implies a significant computational speedup. For example, in the case of monolayer MoS2, convergence of the G0W0 band gap to within ˜0.1 eV is achieved with 12 ×12 q points rather than the 36 ×36 mesh required with discrete BZ sampling techniques. We perform a critical assessment of the band gap of the three prototypical 2D semiconductors, MoS2, h -BN, and phosphorene, including the effect of self-consistency at the GW0 level. The method is implemented in the open source code gpaw.

  12. In-situ monitoring by Raman spectroscopy of the thermal doping of graphene and MoS2 in O2-controlled atmosphere

    Directory of Open Access Journals (Sweden)

    Aurora Piazza

    2017-02-01

    Full Text Available The effects of temperature and atmosphere (air and O2 on the doping of monolayers of graphene (Gr on SiO2 and Si substrates, and on the doping of MoS2 multilayer flakes transferred on the same substrates have been investigated. The investigations were carried out by in situ micro-Raman spectroscopy during thermal treatments up to 430 °C, and by atomic force microscopy (AFM. The spectral positions of the G and 2D Raman bands of Gr undergo only minor changes during treatment, while their amplitude and full width at half maximum (FWHM vary as a function of the temperature and the used atmosphere. The thermal treatments in oxygen atmosphere show, in addition to a thermal effect, an effect attributable to a p-type doping through oxygen. The thermal broadening of the line shape, found during thermal treatments by in situ Raman measurements, can be related to thermal phonon effects. The absence of a band shift results from the balance between a red shift due to thermal effects and a blue shift induced by doping. This shows the potential of in situ measurements to follow the doping kinetics. The treatment of MoS2 in O2 has evidenced a progressive erosion of the flakes without relevant spectral changes in their central zone during in situ measurements. The formation of MoO3 on the edges of the flakes is observed indicative of the oxygen-activated transformation.

  13. In-situ monitoring by Raman spectroscopy of the thermal doping of graphene and MoS2 in O2-controlled atmosphere

    Science.gov (United States)

    Piazza, Aurora; Giannazzo, Filippo; Buscarino, Gianpiero; Fisichella, Gabriele; Magna, Antonino La; Roccaforte, Fabrizio; Cannas, Marco; Gelardi, Franco Mario

    2017-01-01

    The effects of temperature and atmosphere (air and O2) on the doping of monolayers of graphene (Gr) on SiO2 and Si substrates, and on the doping of MoS2 multilayer flakes transferred on the same substrates have been investigated. The investigations were carried out by in situ micro-Raman spectroscopy during thermal treatments up to 430 °C, and by atomic force microscopy (AFM). The spectral positions of the G and 2D Raman bands of Gr undergo only minor changes during treatment, while their amplitude and full width at half maximum (FWHM) vary as a function of the temperature and the used atmosphere. The thermal treatments in oxygen atmosphere show, in addition to a thermal effect, an effect attributable to a p-type doping through oxygen. The thermal broadening of the line shape, found during thermal treatments by in situ Raman measurements, can be related to thermal phonon effects. The absence of a band shift results from the balance between a red shift due to thermal effects and a blue shift induced by doping. This shows the potential of in situ measurements to follow the doping kinetics. The treatment of MoS2 in O2 has evidenced a progressive erosion of the flakes without relevant spectral changes in their central zone during in situ measurements. The formation of MoO3 on the edges of the flakes is observed indicative of the oxygen-activated transformation. PMID:28326231

  14. The Role of Interfacial Electronic Properties on Phonon Transport in Two-Dimensional MoS2 on Metal Substrates.

    Science.gov (United States)

    Yan, Zhequan; Chen, Liang; Yoon, Mina; Kumar, Satish

    2016-12-07

    We investigate the role of interfacial electronic properties on the phonon transport in two-dimensional MoS2 adsorbed on metal substrates (Au and Sc) using first-principles density functional theory and the atomistic Green's function method. Our study reveals that the different degree of orbital hybridization and electronic charge distribution between MoS2 and metal substrates play a significant role in determining the overall phonon-phonon coupling and phonon transmission. The charge transfer caused by the adsorption of MoS2 on Sc substrate can significantly weaken the Mo-S bond strength and change the phonon properties of MoS2, which result in a significant change in thermal boundary conductance (TBC) from one lattice-stacking configuration to another for same metallic substrate. In a lattice-stacking configuration of MoS2/Sc, weakening of the Mo-S bond strength due to charge redistribution results in decrease in the force constant between Mo and S atoms and substantial redistribution of phonon density of states to low-frequency region which affects overall phonon transmission leading to 60% decrease in TBC compared to another configuration of MoS2/Sc. Strong chemical coupling between MoS2 and the Sc substrate leads to a significantly (∼19 times) higher TBC than that of the weakly bound MoS2/Au system. Our findings demonstrate the inherent connection among the interfacial electronic structure, the phonon distribution, and TBC, which helps us understand the mechanism of phonon transport at the MoS2/metal interfaces. The results provide insights for the future design of MoS2-based electronics and a way of enhancing heat dissipation at the interfaces of MoS2-based nanoelectronic devices.

  15. Graphene decorated with MoS2 nanosheets: a synergetic energy storage composite electrode for supercapacitor applications.

    Science.gov (United States)

    Thangappan, R; Kalaiselvam, S; Elayaperumal, A; Jayavel, R; Arivanandhan, M; Karthikeyan, R; Hayakawa, Y

    2016-02-14

    The two-dimensional (2D) transition metal dichalcogenide nanosheet-carbon composite is an attractive material for energy storage because of its high Faradaic activity, unique nanoconstruction and electronic properties. In this work, a facile one step preparation of a molybdenum disulfide (MoS2) nanosheet-graphene (MoS2/G) composite with the in situ reduction of graphene oxide is reported. The structure, morphology and composition of the pure MoS2 and composites were comparatively analyzed by various characterization techniques. The electrochemical performance of the pure MoS2, graphene oxide and the MoS2/G composite electrode materials was evaluated by cyclic voltammogram, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The MoS2/G composite showed a higher specific capacitance (270 F g(-1) at a current density of 0.1 A g(-1)) compared to the pure MoS2 (162 F g(-1)) in a neutral aqueous electrolyte. Moreover, the energy density of the composite electrode is also higher (12.5 Wh kg(-1)) with a high power density (2500 W kg(-1)) compared to the pure MoS2. In addition, the MoS2/G composite electrode showed excellent cyclic stability even after 1000 cycles. The enhancement in specific capacitance, excellent cyclic stability and high energy density of the composite electrode are mainly due to the interconnected conductive network of the composite as well as the synergetic effect of the pure MoS2 and graphene. The experimental results demonstrated that the MoS2/G composite is a promising electrode material for high-performance supercapacitors.

  16. Structural and electronic properties of germanene/MoS2 monolayer and silicene/MoS2 monolayer superlattices

    Science.gov (United States)

    Li, Xiaodan; Wu, Shunqing; Zhou, Sen; Zhu, Zizhong

    2014-03-01

    Superlattice provides a new approach to enrich the class of materials with novel properties. Here, we report the structural and electronic properties of superlattices made with alternate stacking of two-dimensional hexagonal germanene (or silicene) and a MoS2 monolayer using the first principles approach. The results are compared with those of graphene/MoS2 superlattice. The distortions of the geometry of germanene, silicene, and MoS2 layers due to the formation of the superlattices are all relatively small, resulting from the relatively weak interactions between the stacking layers. Our results show that both the germanene/MoS2 and silicene/MoS2 superlattices are manifestly metallic, with the linear bands around the Dirac points of the pristine germanene and silicene seem to be preserved. However, small band gaps are opened up at the Dirac points for both the superlattices due to the symmetry breaking in the germanene and silicene layers caused by the introduction of the MoS2 sheets. Moreover, charge transfer happened mainly within the germanene (or silicene) and the MoS2 layers (intra-layer transfer), as well as some part of the intermediate regions between the germanene (or silicene) and the MoS2 layers (inter-layer transfer), suggesting more than just the van der Waals interactions between the stacking sheets in the superlattices.

  17. Highly efficient, high speed vertical photodiodes based on few-layer MoS2

    Science.gov (United States)

    Li, Zhen; Chen, Jihan; Dhall, Rohan; Cronin, Stephen B.

    2017-03-01

    Layered transition metal dichalcogenides, such as MoS2, have recently emerged as a promising material system for electronic and optoelectronic applications. The two-dimensional nature of these materials enables facile integration for vertical device design with novel properties. Here, we report highly efficient photocurrent generation from vertical MoS2 devices fabricated using asymmetric metal contacts, exhibiting an external quantum efficiency of up to 7%. Compared to in-plane MoS2 devices, the vertical design of these devices has a much larger junction area, which is essential for achieving highly efficient photovoltaic devices. Photocurrent and photovoltage spectra are measured over the photon energy range from 1.25 to 2.5 eV, covering both the 1.8 eV direct K-point optical transition and the 1.3 eV Σ-point indirect transition in MoS2. Photocurrent peaks corresponding to both direct and indirect transitions are observed in the photocurrent spectra and exhibit different photovoltage-current characteristics. Compared to previous in-plane devices, a substantially shorter photoresponse time of 7.3 μs is achieved due to fast carrier sweeping in the vertical devices, which exhibit a -3 dB cutoff frequency of 48 kHz.

  18. Edge termination of MoS2 and CoMoS catalyst particles

    DEFF Research Database (Denmark)

    Byskov, Line Sjolte; Nørskov, Jens Kehlet; Clausen, B. S.;

    2000-01-01

    The edge termination of MoS2 and CoMoS catalyst particles is studied by density functional calculations. We show that for structures without vacancies Mo-terminated edges have the lowest edge energies. Creation of vacancies, which are believed to be active sites in these catalyst systems, leads...

  19. MoS2 /Carbon Nanotube Core-Shell Nanocomposites for Enhanced Nonlinear Optical Performance.

    Science.gov (United States)

    Zhang, Xiaoyan; Selkirk, Andrew; Zhang, Saifeng; Huang, Jiawei; Li, Yuanxin; Xie, Yafeng; Dong, Ningning; Cui, Yun; Zhang, Long; Blau, Werner J; Wang, Jun

    2017-03-08

    Nanocomposites of layered MoS2 and multi-walled carbon nanotubes (CNTs) with core-shell structure were prepared by a simple solvothermal method. The formation of MoS2 nanosheets on the surface of coaxial CNTs has been confirmed by scanning electron microscopy, transmission electron microscopy, absorption spectrum, Raman spectroscopy, and X-ray photoelectron spectroscopy. Enhanced third-order nonlinear optical performances were observed for both femtosecond and nanosecond laser pulses over a broad wavelength range from the visible to the near infrared, compared to those of MoS2 and CNTs alone. The enhancement can be ascribed to the strong coupling effect and the photoinduced charge transfer between MoS2 and CNTs. This work affords an efficient way to fabricate novel CNTs based nanocomposites for enhanced nonlinear light-matter interaction. The versatile nonlinear properties imply a huge potential of the nanocomposites in the development of nanophotonic devices, such as mode-lockers, optical limiters, or optical switches.

  20. Chemical Vapor Deposition of Atomically-Thin Molybdenum Disulfide (MoS2)

    Science.gov (United States)

    2015-03-01

    photoluminescence. 15. SUBJECT TERMS Chemical vapor deposition (CVD) Nanotechnology Molybdenum disulfide (MoS2) Raman spectroscopy 16...by ANSI Std. Z39.18 UNCLASSIFIED Approved for public release; distribution is unlimited. i CONTENTS Page Introduction 1...UNCLASSIFIED Approved for public release; distribution is unlimited. 1 INTRODUCTION Recently, an explosion of interest in low-dimensional

  1. Intrinsic Ferromagnetism in Mn-Substituted MoS2 Nanosheets Achieved by Supercritical Hydrothermal Reaction.

    Science.gov (United States)

    Tan, Hao; Hu, Wei; Wang, Chao; Ma, Chao; Duan, Hengli; Yan, Wensheng; Cai, Liang; Guo, Peng; Sun, Zhihu; Liu, Qinghua; Zheng, Xusheng; Hu, Fengchun; Wei, Shiqiang

    2017-08-21

    Doping atomically thick nanosheets is a great challenge due to the self-purification effect that drives the precipitation of dopants. Here, a breakthrough is made to dope Mn atoms substitutionally into MoS2 nanosheets in a sulfur-rich supercritical hydrothermal reaction environment, where the formation energy of Mn substituting for Mo sites in MoS2 is significantly reduced to overcome the self-purification effect. The substitutional Mn doping is convincingly evidenced by high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine spectroscopy characterizations. The Mn-doped MoS2 nanosheets show robust intrinsic ferromagnetic response with a saturation magnetic moment of 0.05 µB Mn(-1) at room temperature. The intrinsic ferromagnetism is further confirmed by the reversibility of the magnetic behavior during the cycle of incorporating/removing Li codopants, showing the critical role of Mn 3d electronic states in mediating the magnetic interactions in MoS2 nanosheets. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  2. MoS2 nanocrystals confined in a DNA matrix exhibiting energy transfer.

    Science.gov (United States)

    Goswami, Nirmal; Giri, Anupam; Pal, Samir Kumar

    2013-09-10

    We report the wet chemical synthesis of MoS2 nanocrystals (NCs), a transition-metal dichalcogenide, using DNA as a host matrix. As evidenced from transmission electron microscopy (TEM), the NCs are highly crystalline, with an average diameter of ~5 nm. Ultraviolet-visible (UV-vis) absorption studies along with band gap calculations confirm that NCs are in quantum confinement. A prominent red shift of the optical absorption bands has been observed upon formation of the thin film using hexadecyltrimethylammonium chloride (CTAC), i.e., in the case of MoS2@DNA-CTAC. In the thin film, strong electron-phonon coupling arises because of the resonance effect, which is reflected from the emergence of intense first-, second-, and third-order Raman peaks, whenever excited with the 488 nm line. We have established that our as-synthesized MoS2 NCs quench the fluorescence of a well-known DNA minor groove binding probe, Hoechst 33258. Unprecedented fluorescence quenching (94%) of donor (Hoechst 33258) emission and efficient energy transfer (89%) between Hoechst 33258 and MoS2 NCs (acceptor) are obtained. The donor-acceptor distance of these conjugates has been described by a Förster resonance energy transfer (FRET)-based model. Furthermore, employing a statistical method, we have estimated the probability of the distance distribution between the donor and acceptor. We believe that the study described herein may enable substantial advances in fields of optoelectronics, photovoltaics, catalysis, and many others.

  3. Surface defect passivation of MoS2 by sulfur, selenium, and tellurium

    Science.gov (United States)

    Wang, Ying; Qi, Long; Shen, Lei; Wu, Yihong

    2016-04-01

    Few-layer MoS2 field-effect transistors often show an n-type conduction behavior due to the presence of high-density sulfur vacancies. Here, we investigated the possibility of surface defect passivation of MoS2 by sulfur treatment in (NH4)2S solution or coating with an ultrathin layer of selenium or tellurium. It was found that all three elements investigated are able to induce a p-doping effect through suppressing the residual electron concentration by an amount exceeding 0.5 × 1012 cm-2 in few-layer MoS2. Among them, the sulfur-treatment exhibits the most superior thermal stability that survives thermal annealing at temperatures ≥120 °C for at least 10 h. Tellurium exhibits the strongest p-doping effect due to electron trapping by physisorption-induced gap states near the valence band edge. On the other hand, selenium is highly volatile on MoS2; it evaporates and desorbs easily due to Joule heating during electrical measurements in vacuum. The results of first-principles calculations support the experimental observations.

  4. Water Purification across MoS2 Nano-porous Membranes

    Science.gov (United States)

    Heiranian, Mohammad; Barati Farimani, Amir; Aluru, Narayana R.

    2015-11-01

    A 2D material, molybdenum disulfide (MoS2) , is proposed as a nano-porous membrane for water desalination. By performing detailed molecular dynamics simulations, we find that salt ions are rejected efficiently across a single-layer MoS2 while water permeates at high rates. Depending on the pore area, which ranges from 20 to 60 Å2, the nanopore allows less than 12% of ions to pass through even at theoretically high pressures of 350 MPa. Water permeation across the MoS2 membrane is found to be as high as 12 L/cm2/day/MPa which is at least two orders of magnitude higher than that of other existing nano-porous membranes. Pore chemistry is shown to be one of the important factors leading to large water fluxes. MoS2 pore edges terminated with only molybdenum atoms result in higher fluxes which are about 70% higher than that of graphene nanopores. These findings are explained and supported by the permeation coefficients, energy barriers, water density and velocity distributions in the pores.

  5. Synthetic Fabrication of Nanoscale MoS2-Based Transition Metal Sulfides

    Directory of Open Access Journals (Sweden)

    Jikang Yuan

    2010-01-01

    Full Text Available Transition metal sulfides are scientifically and technologically important materials. This review summarizes recent progress on the synthetic fabrication of transition metal sulfides nanocrystals with controlled shape, size, and surface functionality. Special attention is paid to the case of MoS2 nanoparticles, where organic (surfactant, polymer, inorganic (support, promoter, doping compounds and intercalation chemistry are applied.

  6. Three-dimensional spirals of atomic layered MoS2.

    Science.gov (United States)

    Zhang, Liming; Liu, Kaihui; Wong, Andrew Barnabas; Kim, Jonghwan; Hong, Xiaoping; Liu, Chong; Cao, Ting; Louie, Steven G; Wang, Feng; Yang, Peidong

    2014-11-12

    Atomically thin two-dimensional (2D) layered materials, including graphene, boron nitride, and transition metal dichalcogenides (TMDs), can exhibit novel phenomena distinct from their bulk counterparts and hold great promise for novel electronic and optoelectronic applications. Controlled growth of such 2D materials with different thickness, composition, and symmetry are of central importance to realize their potential. In particular, the ability to control the symmetry of TMD layers is highly desirable because breaking the inversion symmetry can lead to intriguing valley physics, nonlinear optical properties, and piezoelectric responses. Here we report the first chemical vapor deposition (CVD) growth of spirals of layered MoS2 with atomically thin helical periodicity, which exhibits a chiral structure and breaks the three-dimensional (3D) inversion symmetry explicitly. The spirals composed of tens of connected MoS2 layers with decreasing areas: each basal plane has a triangular shape and shrinks gradually to the summit when spiraling up. All the layers in the spiral assume an AA lattice stacking, which is in contrast to the centrosymmetric AB stacking in natural MoS2 crystals. We show that the noncentrosymmetric MoS2 spiral leads to a strong bulk second-order optical nonlinearity. In addition, we found that the growth of spirals involves a dislocation mechanism, which can be generally applicable to other 2D TMD materials.

  7. Magnetic edge states in MoS2 characterized using density-functional theory

    DEFF Research Database (Denmark)

    Vojvodic, Aleksandra; Hinnemann, B.; Nørskov, Jens Kehlet

    2009-01-01

    It is known that the edges of a two-dimensional slab of insulating MoS2 exhibit one-dimensional metallic edge states, the so-called "brim states." Here, we find from density-functional theory calculations that several edge structures, which are relevant for the hydrodesulfurization process, are m...

  8. High performance MoS2 TFT using graphene contact first process

    Directory of Open Access Journals (Sweden)

    Chih-Shiang Chang Chien

    2017-08-01

    Full Text Available An ohmic contact of graphene/MoS2 heterostructure is determined by using ultraviolet photoelectron spectroscopy (UPS. Since graphene shows a great potential to replace metal contact, a direct comparison of Cr/Au contact and graphene contact on the MoS2 thin film transistor (TFT is made. Different from metal contacts, the work function of graphene can be modulated. As a result, the subthreshold swing can be improved. And when Vg

  9. Two-dimensional Layered MoS2 Biosensors Enable Highly Sensitive Detection of Biomolecules

    Science.gov (United States)

    Lee, Joonhyung; Dak, Piyush; Lee, Yeonsung; Park, Heekyeong; Choi, Woong; Alam, Muhammad A.; Kim, Sunkook

    2014-12-01

    We present a MoS2 biosensor to electrically detect prostate specific antigen (PSA) in a highly sensitive and label-free manner. Unlike previous MoS2-FET-based biosensors, the device configuration of our biosensors does not require a dielectric layer such as HfO2 due to the hydrophobicity of MoS2. Such an oxide-free operation improves sensitivity and simplifies sensor design. For a quantitative and selective detection of PSA antigen, anti-PSA antibody was immobilized on the sensor surface. Then, introduction of PSA antigen, into the anti-PSA immobilized sensor surface resulted in a lable-free immunoassary format. Measured off-state current of the device showed a significant decrease as the applied PSA concentration was increased. The minimum detectable concentration of PSA is 1 pg/mL, which is several orders of magnitude below the clinical cut-off level of ~4 ng/mL. In addition, we also provide a systematic theoretical analysis of the sensor platform - including the charge state of protein at the specific pH level, and self-consistent channel transport. Taken together, the experimental demonstration and the theoretical framework provide a comprehensive description of the performance potential of dielectric-free MoS2-based biosensor technology.

  10. Modeling the active sites of Co-promoted MoS2 particles by DFT

    DEFF Research Database (Denmark)

    Šaric, Manuel; Rossmeisl, Jan; Moses, Poul Georg

    2017-01-01

    The atomic-scale structure of the Co-promoted MoS2 catalyst (CoMoS), used for hydrodesulfurization and as a potential replacement for platinum in the acidic hydrogen evolution reaction has been analyzed by modeling its sites using density functional theory and applying thermochemical corrections...

  11. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts

    DEFF Research Database (Denmark)

    Jaramillo, Thomas; Jørgensen, Kristina Pilt; Bonde, Jacob;

    2007-01-01

    The identification of the active sites in heterogeneous catalysis requires a combination of surface sensitive methods and reactivity studies. We determined the active site for hydrogen evolution, a reaction catalyzed by precious metals, on nanoparticulate molybdenum disulfide (MoS2) by atomically...

  12. Ultrathin MoS2 Nanosheets with Superior Extreme Pressure Property as Boundary Lubricants

    Science.gov (United States)

    Chen, Zhe; Liu, Xiangwen; Liu, Yuhong; Gunsel, Selda; Luo, Jianbin

    2015-08-01

    In this paper, a new kind of oil-soluble ultrathin MoS2 nanosheets is prepared through a one-pot process. A superior extreme pressure property, which has not been attained with other nano-additives, is discovered when the nanosheets are used as lubricant additives. The as-synthesized MoS2 nanosheet is only a few atomic layers thick and tens of nanometers wide, and it is surface-modified with oleylamine so it can be well dispersed in oil or lubricant without adscititious dispersants or surfactants. By adding 1 wt% ultrathin MoS2 nanosheets, at the temperature of 120 °C, the highest load liquid paraffin can bear is tremendously improved from less than 50 N to more than 2000 N. Based on the tribological tests and analysis of the wear scar, a lubrication mechanism is proposed. It is believed that the good dispersion and the ultrathin shape of the nanosheets ensure that they can enter the contact area of the opposite sliding surfaces and act like a protective film to prevent direct contact and seizure between them. This work enriches the investigation of ultrathin MoS2 and has potential application in the mechanical industry.

  13. Enhancement of photovoltaic response in multilayer MoS2 induced by plasma doping.

    Science.gov (United States)

    Wi, Sungjin; Kim, Hyunsoo; Chen, Mikai; Nam, Hongsuk; Guo, L Jay; Meyhofer, Edgar; Liang, Xiaogan

    2014-05-27

    Layered transition-metal dichalcogenides hold promise for making ultrathin-film photovoltaic devices with a combination of excellent photovoltaic performance, superior flexibility, long lifetime, and low manufacturing cost. Engineering the proper band structures of such layered materials is essential to realize such potential. Here, we present a plasma-assisted doping approach for significantly improving the photovoltaic response in multilayer MoS2. In this work, we fabricated and characterized photovoltaic devices with a vertically stacked indium tin oxide electrode/multilayer MoS2/metal electrode structure. Utilizing a plasma-induced p-doping approach, we are able to form p-n junctions in MoS2 layers that facilitate the collection of photogenerated carriers, enhance the photovoltages, and decrease reverse dark currents. Using plasma-assisted doping processes, we have demonstrated MoS2-based photovoltaic devices exhibiting very high short-circuit photocurrent density values up to 20.9 mA/cm(2) and reasonably good power-conversion efficiencies up to 2.8% under AM1.5G illumination, as well as high external quantum efficiencies. We believe that this work provides important scientific insights for leveraging the optoelectronic properties of emerging atomically layered two-dimensional materials for photovoltaic and other optoelectronic applications.

  14. Mesoporous Carbon Nanofibers Embedded with MoS2 Nanocrystals for Extraordinary Li-Ion Storage.

    Science.gov (United States)

    Hu, Shan; Chen, Wen; Uchaker, Evan; Zhou, Jing; Cao, Guozhong

    2015-12-07

    MoS2 nanocrystals embedded in mesoporous carbon nanofibers are synthesized through an electrospinning process followed by calcination. The resultant nanofibers are 100-150 nm in diameter and constructed from MoS2 nanocrystals with a lateral diameter of around 7 nm with specific surface areas of 135.9 m(2)  g(-1) . The MoS2 @C nanofibers are treated at 450 °C in H2 and comparison samples annealed at 800 °C in N2 . The heat treatments are designed to achieve good crystallinity and desired mesoporous microstructure, resulting in enhanced electrochemical performance. The small amount of oxygen in the nanofibers annealed in H2 contributes to obtaining a lower internal resistance, and thus, improving the conductivity. The results show that the nanofibers obtained at 450 °C in H2 deliver an extraordinary capacity of 1022 mA h g(-1) and improved cyclic stability, with only 2.3 % capacity loss after 165 cycles at a current density of 100 mA g(-1) , as well as an outstanding rate capability. The greatly improved kinetics and cycling stability of the mesoporous MoS2 @C nanofibers can be attributed to the crosslinked conductive carbon nanofibers, the large specific surface area, the good crystallinity of MoS2 , and the robust mesoporous microstructure. The resulting nanofiber electrodes, with short mass- and charge-transport pathways, improved electrical conductivity, and large contact area exposed to electrolyte, permitting fast diffusional flux of Li ions, explains the improved kinetics of the interfacial charge-transfer reaction and the diffusivity of the MoS2 @C mesoporous nanofibers. It is believed that the integration of MoS2 nanocrystals and mesoporous carbon nanofibers may have a synergistic effect, giving a promising anode, and widening the applicability range into high performance and mass production in the Li-ion battery market. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  15. MoS2/cotton-derived carbon fibers with enhanced cyclic performance for sodium-ion batteries

    Science.gov (United States)

    Li, Xiang; Yang, Yan; Liu, Jiangwen; Ouyang, Liuzhang; Liu, Jun; Hu, Renzong; Yang, Lichun; Zhu, Min

    2017-08-01

    Carbon fibers derived from bio-template are low cost and environmental benign, therefore have attracted much attention in energy storage materials. In this work, we successfully fabricated MoS2/cotton-derived carbon fibers (MoS2/CDCFs) via hydrothermal route followed by carbonization process. In the composite of MoS2/CDCFs, MoS2 nanosheets vertically grow on the carbon fibers which offer fast ways for electron transfer and at the same time act as robust support to buffer the volume changes of MoS2 nanosheets during discharge/charge cycles. As anode materials for sodium-ion batteries, MoS2/CDCFs exhibit good rate performance and markedly enhanced cyclic stability due to the conductive support of CDCFs. At a current density of 0.1 A g-1, the MoS2/CDCFs-1 shows an initial reversible capacity of 504.9 mAh g-1, and maintains 444.5 mAh g-1 after 50 cycles. Even when the current density increases to 0.5 A g-1, it maintains 323.1 mAh g-1 after 150 cycles, which is much higher than the capacity retention of 149.6 mAh g-1 for the bare MoS2 nanosheets. The improved electrochemical performance verifies the effective strategy of using cotton as carbon source to construct hierarchical composites for sodium-ion batteries.

  16. MoS2 nanodot decorated In2S3 nanoplates: a novel heterojunction with enhanced photoelectrochemical performance.

    Science.gov (United States)

    Liu, Fangyang; Jiang, Yan; Yang, Jia; Hao, Mengmeng; Tong, Zhengfu; Jiang, Liangxing; Wu, Zhuangzhi

    2016-01-31

    MoS2 nanodot decorated In2S3 nanoplates were successfully synthesized via a modified one-pot method. The In2S3/MoS2 heterojunction nanocomposite exhibits superior optical and photoelectrochemical performance to the bare ones, owing to the synergistic effect.

  17. Hydrothermal synthesis of layer-controlled MoS2/graphene composite aerogels for lithium-ion battery anode materials

    Science.gov (United States)

    Zhao, Bing; Wang, Zhixuan; Gao, Yang; Chen, Lu; Lu, Mengna; Jiao, Zheng; Jiang, Yong; Ding, Yuanzhang; Cheng, Lingli

    2016-12-01

    Layer-controlled MoS2/graphene aerogels (MoS2/GA) composites are synthesized by a facile hydrothermal route, in which few-layer (5-15 layers) MoS2 nanosheets with high crystalline are decorated on the surface of graphene nanosheets homogeneously and tightly. The number of the MoS2 layers can be easily controlled through adjusting the amount of molybdenum source in the reaction system. Moreover, the growth mechanism of the lay-controlled MoS2/GA composites is proposed. The three-dimensional MoS2/GA with macroporous micro-structure not only shortens the transportation length of electrons and ions, but also restrains the re-stacking of MoS2 effectively, stabilizing the electrode structure during repeated charging/discharging processes. Electrochemical tests demonstrate that this few-layer MoS2/GA composite exhibits a high reversible capacity of 1085.0 mAh g-1 at current density of 100 mA g-1, as well as extraordinarily high cycling stability and rate capability.

  18. Defect assisted coupling of a MoS2/TiO2 interface and tuning of its electronic structure

    Science.gov (United States)

    Chen, Guifeng; Song, Xiaolin; Guan, Lixiu; Chai, Jianwei; Zhang, Hui; Wang, Shijie; Pan, Jisheng; Tao, Junguang

    2016-09-01

    Although MoS2 based heterostructures have drawn increased attention, the van der Waals forces within MoS2 layers make it difficult for the layers to form strong chemical coupled interfaces with other materials. In this paper, we demonstrate the successful strong chemical attachment of MoS2 on TiO2 nanobelts after appropriate surface modifications. The etch-created dangling bonds on TiO2 surfaces facilitate the formation of a steady chemically bonded MoS2/TiO2 interface. With the aid of high resolution transmission electron microscope measurements, the in-plane structure registry of MoS2/TiO2 is unveiled at the atomic scale, which shows that MoS2[1-10] grows along the direction of TiO2[001] and MoS2[110] parallel to TiO2[100] with every six units of MoS2 superimposed on five units of TiO2. Electronically, type II band alignments are realized for all surface treatments. Moreover, the band offsets are delicately correlated to the surface states, which plays a significant role in their photocatalytic performance.

  19. Graphene Oxide-Directed Tunable Assembly of MoS2 Ultrathin Nanosheets for Electrocatalytic Hydrogen Evolution

    DEFF Research Database (Denmark)

    Sun, Hongyu; Zhao, Yanyan; Mølhave, Kristian

    2017-01-01

    Three dimensional (3D) hierarchical architectures based onmolybdenum disulfide (MoS2) an d reduced graphene oxide (rGO) are synthesized through a mixed solvothermal method.By simply increasing the amount of graphene oxide (GO) during the synthesis, the 3D assembly of MoS2 can be tunedfrom...

  20. Formation of a Stable p-n Junction in a Liquid-Gated MoS2 Ambipolar Transistor

    NARCIS (Netherlands)

    Zhang, Y. J.; Ye, J. T.; Yornogida, Y.; Takenobu, T.; Iwasa, Y.

    2013-01-01

    Molybdenum disulfide (MoS2) has gained attention because of its high mobility and circular dichroism. As a crucial step to merge these advantages into a single device, we present a method that electronically controls and locates p-n junctions in liquid-gated ambipolar MoS2 transistors. A bias-indepe

  1. Synthesis of few-layer MoS2 nanosheet-loaded Ag3PO4 for enhanced photocatalytic activity.

    Science.gov (United States)

    Song, Yanhua; Lei, Yucheng; Xu, Hui; Wang, Cheng; Yan, Jia; Zhao, Haozhu; Xu, Yuanguo; Xia, Jiexiang; Yin, Sheng; Li, Huaming

    2015-02-21

    Novel few-layer MoS2/Ag3PO4 composites were fabricated. The results indicated that Ag3PO4 nanoparticles were directly formed on the surface of few-layer MoS2. The physical and chemical properties of the few-layer MoS2/Ag3PO4 composite photocatalysts were tested in order to investigate the effects of few-layer MoS2 on the photocatalytic activity of Ag3PO4. The photocatalytic activity of the few-layer MoS2/Ag3PO4 composites was evaluated by the photocatalytic degradation of Rhodamine B (RhB) and bisphenol A (BPA) under visible light irradiation. The photocatalytic activity of the few-layer MoS2/Ag3PO4 composites was higher than that of pure Ag3PO4. The optimal few-layer MoS2 content for the organic pollutant degradation of the heterojunction structures was determined. The synergic effect between few-layer MoS2 and Ag3PO4 was found to lead to an improved photogenerated carrier separation. The stability and the possible photocatalytic mechanism of the composites were also discussed.

  2. Green Synthesis of Feather-Shaped MoS2/CdS Photocatalyst for Effective Hydrogen Production

    Directory of Open Access Journals (Sweden)

    Yang Liu

    2013-01-01

    Full Text Available MoS2/CdS photocatalyst was fabricated by a hydrothermal method for H2 production under visible light. This method used low toxic thiourea as a sulfur source and was carried out at 200°C. Thus, it was better than the traditional methods, which are based on an annealing process at relatively high temperature (above 400°C using toxic H2S as reducing agent. Scanning electron microscopy and transmission electron microscopy images showed that the morphologies of MoS2/CdS samples were feather shaped and MoS2 layer was on the surface of CdS. The X-ray photoelectron spectroscopy testified that the sample was composed of stoichiometric MoS2 and CdS. The UV-vis diffuse reflectance spectra displayed that the loading of MoS2 can enhance the optical absorption of MoS2/CdS. The photocatalytic activity of MoS2/CdS was evaluated by producing hydrogen. The hydrogen production rate on MoS2/CdS reached 192 μmol·h−1. This performance was stable during three repeated photocatalytic processes.

  3. The Role of Lead (Pb in the High Temperature Formation of MoS2 Nanotubes

    Directory of Open Access Journals (Sweden)

    Olga Brontvein

    2014-06-01

    Full Text Available Recent studies have clearly indicated the favorable effect of lead as a growth promoter for MX2 (M = Mo, W; X = S, Se nanotubes using MX2 powder as a precursor material. The experimental work indicated that the lead atoms are not stable in the molybdenum oxide lattice ion high concentration. The initial lead concentration in the oxide nanowhiskers (Pb:Mo ratio = 0.28 is reduced by one order of magnitude after one year in the drawer. The initial Pb concentration in the MoS2 nanotubes lattice (produced by solar ablation is appreciably smaller (Pb:Mo ratio for the primary samples is 0.12 and is further reduced with time and annealing at 810 °C, without consuming the nanotubes. In order to elucidate the composition of these nanotubes in greater detail; the Pb-“modified” MX2 compounds were studied by means of DFT calculations and additional experimental work. The calculations indicate that Pb doping as well as Pb intercalation of MoS2 lead to the destabilization of the system; and therefore a high Pb content within the MoS2 lattice cannot be expected in the final products. Furthermore; substitutional doping (PbMo leads to p-type semiconducting character; while intercalation of MoS2 by Pb atoms (Pby/MoS2 should cause n-type semiconducting behavior. This study not only sheds light on the role of added lead to the growth of the nanotubes and their role as electron donors; but furthermore could pave the way to a large scale synthesis of the MoS2 nanotubes.

  4. Resonant Raman spectroscopy study of swift heavy ion irradiated MoS2

    Science.gov (United States)

    Guo, Hang; Sun, Youmei; Zhai, Pengfei; Zeng, Jian; Zhang, Shengxia; Hu, Peipei; Yao, Huijun; Duan, Jinglai; Hou, Mingdong; Liu, Jie

    2016-08-01

    Molybdenum disulphide (MoS2) crystal samples were irradiated by swift heavy ions (209Bi and 56Fe). Hillock-like latent tracks were observed on the surface of irradiated MoS2 by atomic force microscopy. The modifications of properties of irradiated MoS2 were investigated by resonant Raman spectroscopy and ultraviolet-visible spectroscopy (UV-Vis). A new peak (E1u2, ∼385.7 cm-1) occurs near the in-plane E2g1 peak (∼383.7 cm-1) after irradiation. The two peaks shift towards lower frequency and broaden due to structural defects and stress with increasing fluence. When irradiated with high fluence, two other new peaks appear at ∼ 190 and ∼ 230 cm-1. The peak at ∼230 cm-1 is disorder-induced LA(M) mode. The presence of this mode indicates defects induced by irradiation. The feature at ∼460 cm-1 is composed of 2LA(M) (∼458 cm-1) and A2u (∼466 cm-1) mode. With increasing fluence, the integrated intensity ratio between 2LA(M) and A2u increases. The relative enhancement of 2LA(M) mode is in agreement with the appearance of LA(M) mode, which both demonstrate structural disorder in irradiated MoS2. The ∼423-cm-1 peak shifts toward lower frequency due to the decrease in exciton energy of MoS2, and this was demonstrated by the results of UV-Vis spectra. The decrease in exciton energy could be due to introduction of defect levels into band gap.

  5. Anomalous photoluminescence thermal quenching of sandwiched single layer MoS_2

    KAUST Repository

    Tangi, Malleswara

    2017-09-22

    We report an unusual thermal quenching of the micro-photoluminescence (µ-PL) intensity for a sandwiched single-layer (SL) MoS2. For this study, MoS2 layers were chemical vapor deposited on molecular beam epitaxial grown In0.15Al0.85N lattice matched templates. Later, to accomplish air-stable sandwiched SL-MoS2, a thin In0.15Al0.85N cap layer was deposited on the MoS2/In0.15Al0.85N heterostructure. We confirm that the sandwiched MoS2 is a single layer from optical and structural analyses using µ-Raman spectroscopy and scanning transmission electron microscopy, respectively. By using high-resolution X-ray photoelectron spectroscopy, no structural phase transition of MoS2 is noticed. The recombination processes of bound and free excitons were analyzed by the power-dependent µ-PL studies at 77 K and room temperature (RT). The temperature-dependent micro photoluminescence (TDPL) measurements were carried out in the temperature range of 77 – 400 K. As temperature increases, a significant red-shift is observed for the free-exciton PL peak, revealing the delocalization of carriers. Further, we observe unconventional negative thermal quenching behavior, the enhancement of the µ-PL intensity with increasing temperatures up to 300K, which is explained by carrier hopping transitions that take place between shallow localized states to the band-edges. Thus, this study renders a fundamental insight into understanding the anomalous thermal quenching of µ-PL intensity of sandwiched SL-MoS2.

  6. Multi-terminal transport measurements of MoS2 using a van der Waals heterostructure device platform

    DEFF Research Database (Denmark)

    Cui, Xu; Lee, Gwan-Hyoung; Kim, Young Duck

    2015-01-01

    Atomically thin two-dimensional semiconductors such as MoS2 hold great promise for electrical, optical and mechanical devices and display novel physical phenomena. However, the electron mobility of mono-and few-layer MoS2 has so far been substantially below theoretically predicted limits, which has...... in performance, including a record-high Hall mobility reaching 34,000 cm2 V-1 s-1 for six-layer MoS2 at low temperature, confirming that low-temperature performance in previous studies was limited by extrinsic interfacial impurities rather than bulk defects in the MoS2. We also observed Shubnikov-de Haas...... scattering, we have developed here a van der Waals heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. Magneto-transport measurements show dramatic improvements...

  7. Zinc Oxide Coating Effect for the Dye Removal and Photocatalytic Mechanisms of Flower-Like MoS2 Nanoparticles.

    Science.gov (United States)

    Tian, Qingyong; Wu, Wei; Yang, Shuanglei; Liu, Jun; Yao, Weijing; Ren, Feng; Jiang, Changzhong

    2017-12-01

    Flower-like MoS2 nanoparticles (NPs) consist of ultra-thin MoS2 nanosheets are synthesized via a facile one-pot hydrothermal method. The MoS2/ZnO p-n heterostructure is formed by coating n-type ZnO on the surface of flower-like MoS2 NPs through the seed-mediate route and post-annealing treatment. The effects for the dye removal and photocatalytic performances after ZnO coating are systematically investigated. The results demonstrated that the coating of ZnO nanoparticles has a positive promotion to the photodegrading properties while negative effect on the adsorption capacity of the MoS2/ZnO heterostructures. The related mechanisms on the relationship of adsorption capacity and photocatalysis are discussed in detail.

  8. MoS2-nanosheet/graphene-oxide composite hole injection layer in organic light-emitting diodes

    Science.gov (United States)

    Park, Minjoon; Nguyen, Thang Phan; Choi, Kyoung Soon; Park, Jongee; Ozturk, Abdullah; Kim, Soo Young

    2017-07-01

    In this work, composite layers comprising two-dimensional MoS2 and graphene oxide (GO) were employed as hole injection layers (HILs) in organic light-emitting diodes (OLEDs). MoS2 was fabricated by the butyllithium (BuLi) intercalation method, while GO was synthesized by a modified Hummers method. The X-ray diffraction patterns showed that the intensity of the MoS2 (002) peak at 14.15° decreased with increase in GO content; the GO (001) peak was observed at 10.07°. In the C 1s synchrotron radiation photoemission spectra, the contributions of the C-O, C=O, and O-C=O components increased with increase in GO content. These results indicated that GO was well mixed with MoS2. The lateral size of MoS2 spanned from a few hundreds of nanometers to 1 μm, while the size of GO was between 400 nm and a few micrometers. Thus, the coverage of the MoS2-GO composite on the ITO surface improved as the GO content increased, owing to the large particle size of GO. Notably, GO with large size could fully cover the indium tin oxide film surface, thus, lowering the roughness. The highest maximum power efficiency (PEmax) was exhibited by the OLED with MoS2-GO 6:4 composite HIL, indicating that similar contents of MoS2 and GO in MoS2-GO composites provide the best results. The OLED with GO HIL showed very high PEmax (4.94 lm W-1) because of very high surface coverage and high work function of GO. These results indicate that the MoS2-GO composites can be used to fabricate HILs in OLEDs.[Figure not available: see fulltext.

  9. Engineering Chemically Exfoliated Large-Area Two-Dimensional MoS2 Nanolayers with Porphyrins for Improved Light Harvesting.

    Science.gov (United States)

    Zhang, Hanyu; Choi, Jungwook; Ramani, Arjun; Voiry, Damien; Natoli, Sean N; Chhowalla, Manish; McMillin, David R; Choi, Jong Hyun

    2016-09-19

    Molybdenum disulfide (MoS2 ) is a promising candidate for electronic and optoelectronic applications. However, its application in light harvesting has been limited in part due to crystal defects, often related to small crystallite sizes, which diminish charge separation and transfer. Here we demonstrate a surface-engineering strategy for 2D MoS2 to improve its photoelectrochemical properties. Chemically exfoliated large-area MoS2 thin films were interfaced with eight molecules from three porphyrin families: zinc(II)-, gallium(III)-, iron(III)-centered, and metal-free protoporphyrin IX (ZnPP, GaPP, FePP, H2 PP); metal-free and zinc(II) tetra-(N-methyl-4-pyridyl)porphyrin (H2 T4, ZnT4); and metal-free and zinc(II) tetraphenylporphyrin (H2 TPP, ZnTPP). We found that the photocurrents from MoS2 films under visible-light illumination are strongly dependent on the interfacial molecules and that the photocurrent enhancement is closely correlated with the highest occupied molecular orbital (HOMO) levels of the porphyrins, which suppress the recombination of electron-hole pairs in the photoexcited MoS2 films. A maximum tenfold increase was observed for MoS2 functionalized with ZnPP compared with pristine MoS2 films, whereas ZnT4-functionalized MoS2 demonstrated small increases in photocurrent. The application of bias voltage on MoS2 films can further promote photocurrent enhancements and control current directions. Our results suggest a facile route to render 2D MoS2 films useful for potential high-performance light-harvesting applications.

  10. Cluster-support interactions and morphology of MoS2 nanoclusters in a graphite-supported hydrotreating model catalyst.

    Science.gov (United States)

    Kibsgaard, Jakob; Lauritsen, Jeppe V; Laegsgaard, Erik; Clausen, Bjerne S; Topsøe, Henrik; Besenbacher, Flemming

    2006-10-25

    Supported MoS(2) nanoparticles constitute the active component of the important hydrotreating catalysts used for industrial upgrading and purification of the oil feedstock for the production of fossil fuels with a low environmental load. We have synthesized and studied a model system of the hydrotreating catalyst consisting of MoS(2) nanoclusters supported on a graphite surface in order to resolve a number of very fundamental questions related to the atomic-scale structure and morphology of the active clusters and in particular the effect of a substrate used in some types of hydrotreating catalysts. Scanning tunneling microscopy (STM) is used to image the atomic-scale structure of graphite-supported MoS(2) nanoclusters in real space. It is found that the pristine graphite (0001) surface does not support a high dispersion of MoS(2), but by introducing a small density of defects in the surface, highly dispersed MoS(2) nanoclusters could be synthesized on the graphite. From high-resolution STM images it is found that MoS(2) nanoclusters synthesized at low temperature in a sulfiding atmosphere preferentially grow as single-layer clusters, whereas clusters synthesized at 1200 K grow as multilayer slabs oriented with the MoS(2)(0001) basal plane parallel to the graphite surface. The morphology of both single-layer and multilayer MoS(2) nanoclusters is found to be preferentially hexagonal, and atom-resolved images of the top facet of the clusters provide new atomic-scale information on the MoS(2)-HOPG bonding. The structure of the two types of catalytically interesting edges terminating the hexagonal MoS(2) nanoclusters is also resolved in atomic detail in STM images, and from these images it is possible to reveal the atomic structure of both edges and the location and coverage of sulfur and hydrogen adsorbates.

  11. The role of charge trapping in MoS2/SiO2 and MoS2/hBN field-effect transistors

    Science.gov (United States)

    Illarionov, Yury Yu; Rzepa, Gerhard; Waltl, Michael; Knobloch, Theresia; Grill, Alexander; Furchi, Marco M.; Mueller, Thomas; Grasser, Tibor

    2016-09-01

    The commonly observed hysteresis in the transfer characteristics of MoS2 transistors is typically associated with charge traps in the gate insulator. Since in Si technologies such traps can lead to severe reliability issues, we perform a combined study of both the hysteresis as well as the arguably most important reliability issue, the bias-temperature instability. We use single-layer MoS2 FETs with SiO2 and hBN insulators and demonstrate that both phenomena are indeed due to traps in the gate insulator with time constants distributed over wide timescales, where the faster ones lead to hysteresis and the slower ones to bias-temperature instabilities. Our data show that the use of hBN as a gate insulator considerably reduces the number of accessible slow traps and thus improves the reliability. However, the reliability of hBN insulators deteriorates with increasing temperature due to the thermally activated nature of charge trapping.

  12. Improving the Photoelectric Characteristics of MoS2 Thin Films by Doping Rare Earth Element Erbium

    Science.gov (United States)

    Meng, Miaofei; Ma, Xiying

    2016-11-01

    We investigated the surface morphologies, crystal structures, and optical characteristics of rare earth element erbium (Er)-doped MoS2 (Er: MoS2) thin films fabricated on Si substrates via chemical vapor deposition (CVD). The surface mopography, crystalline structure, light absorption property, and the photoelectronic characteristics of the Er: MoS2 films were studied. The results indicate that doping makes the crystallinity of MoS2 films better than that of the undoped film. Meanwhile, the electron mobility and conductivity of the Er-doped MoS2 films increase about one order of magnitude, and the current-voltage ( I- V) and the photoelectric response characteristics of the Er:MoS2/Si heterojunction increase significantly. Moreover, Er-doped MoS2 films exhibit strong light absorption and photoluminescence in the visible light range at room temperature; the intensity is enhanced by about twice that of the undoped film. The results indicate that the doping of MoS2 with Er can significantly improve the photoelectric characteristics and can be used to fabricate highly efficient luminescence and optoelectronic devices.

  13. The synthesis of hierarchical nanostructured MoS2/Graphene composites with enhanced visible-light photo-degradation property

    Science.gov (United States)

    Zhao, Yongjie; Zhang, Xiaowei; Wang, Chengzhi; Zhao, Yuzhen; Zhou, Heping; Li, Jingbo; Jin, HaiBo

    2017-08-01

    Novel two-dimensional materials with a layered structure are of special interest for a variety of promising applications. Herein, MoS2 and MoS2/Graphene nanocomposite with hierarchical nanostructure were successfully synthesized employing a one-step hydrothermal method. Photo-degradation of methylene blue (MB) and rhodamine (RHB) were adopted to assess the photo-degradation ability of the products. Comparing with bare MoS2, the hierarchical MoS2/Graphene nanocomposite achieved relatively higher degradation rate of 99% in 28 min for MB as well in 50 min for RHB. These results verified that this proposed hierarchical nanocomposite is a good photo-degradation semiconductor. The excellent performance was mainly ascribed to the synergistic effect of MoS2 and graphene layers. The MoS2 possessing a band gap of 1.9 eV would provide abundant electron-hole pairs. The graphene layers with excellent electro-conductivity could realize the quick transport of electrons via its extended π-conjugation structure, consequently benefiting the separation of photo-generated carriers. These findings indicate that the graphene layer is a promising candidate as a co-catalyst for MoS2 photo-catalyst, and also provide useful information for understanding the observed enhanced photocatalytic mechanism in experiments.

  14. Electrical performance of multilayer MoS2 transistors on high-κ Al2O3 coated Si substrates

    Directory of Open Access Journals (Sweden)

    Tao Li

    2015-05-01

    Full Text Available The electrical performance of MoS2 can be engineered by introducing high-κ dielectrics, while the interactions between high-κ dielectrics and MoS2 need to be studied. In this study, multilayer MoS2 field-effect transistors (FETs with a back-gated configuration were fabricated on high-κ Al2O3 coated Si substrates. Compared with MoS2 FETs on SiO2, the field-effect mobility (μFE and subthreshold swing (SS were remarkably improved in MoS2/Al2O3/Si. The improved μFE was thought to result from the dielectric screening effect from high-κ Al2O3. When a HfO2 passivation layer was introduced on the top of MoS2/Al2O3/Si, the field-effect mobility was further enhanced, which was thought to be concerned with the decreased contact resistance between the metal and MoS2. Meanwhile, the interface trap density increased from 2.4×1012 eV−1cm−2 to 6.3×1012 eV−1cm−2. The increase of the off-state current and the negative shift of the threshold voltage may be related to the increase of interface traps.

  15. Enhanced photocatalytic activity of ZnS nanoparticles loaded with MoS2 nanoflakes by self-assembly approach

    Science.gov (United States)

    Vattikuti, S. V. Prabhakar; Byon, Chan; Jeon, Sora

    2016-12-01

    A hybrid consisting of ZnS nanoparticles supported on layered MoS2-ZnS was synthesized by a hydrothermal method based on self-assembly technique without using a template. XRD, SEM-EDX, TEM, HR-TEM, TG-DTA, XPS, N2 adsorption-desorption, and UV-Vis spectroscopies were used to characterize the structural features, morphology, and composition of the MoS2-ZnS hybrid. The results show that the MoS2-ZnS hybrid is mainly ZnS nanoparticles on layered MoS2 with a thickness of ca. 5-20 nm. The combination of the MoS2 and ZnS hybrid structure is beneficial for enhancing the photocatalytic degradation of rhodamine B (RhB) under visible light irradiation. A possible photoreaction mechanism of the MoS2-ZnS hybrid in the degradation is proposed. The photoexcited electrons from the ZnS could easily transfer to the conduction band of MoS2, thus decreasing the recombination of photoinduced carriers and enabling the degradation of RhB under visible light irradiation.

  16. Effect of contaminations and surface preparation on the work function of single layer MoS2.

    Science.gov (United States)

    Ochedowski, Oliver; Marinov, Kolyo; Scheuschner, Nils; Poloczek, Artur; Bussmann, Benedict Kleine; Maultzsch, Janina; Schleberger, Marika

    2014-01-01

    Thinning out MoS2 crystals to atomically thin layers results in the transition from an indirect to a direct bandgap material. This makes single layer MoS2 an exciting new material for electronic devices. In MoS2 devices it has been observed that the choice of materials, in particular for contact and gate, is crucial for their performance. This makes it very important to study the interaction between ultrathin MoS2 layers and materials employed in electronic devices in order to optimize their performance. In this work we used NC-AFM in combination with quantitative KPFM to study the influence of the substrate material and the processing on single layer MoS2 during device fabrication. We find a strong influence of contaminations caused by the processing on the surface potential of MoS2. It is shown that the charge transfer from the substrate is able to change the work function of MoS2 by about 40 meV. Our findings suggest two things. First, the necessity to properly clean devices after processing as contaminations have a great impact on the surface potential. Second, that by choosing appropriate materials the work function can be modified to reduce contact resistance.

  17. Facile Fabrication of MoS2-Modified SnO2 Hybrid Nanocomposite for Ultrasensitive Humidity Sensing.

    Science.gov (United States)

    Zhang, Dongzhi; Sun, Yan'e; Li, Peng; Zhang, Yong

    2016-06-01

    An ultrasensitive humidity sensor based on molybdenum-disulfide- (MoS2)-modified tin oxide (SnO2) nanocomposite has been demonstrated in this work. The nanostructural, morphological, and compositional properties of an as-prepared MoS2/SnO2 nanocomposite were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectrometry (EDS), nitrogen sorption analysis, and Raman spectroscopy, which confirmed its successful preparation and rationality. The sensing characteristics of the MoS2/SnO2 hybrid film device against relative humidity (RH) were investigated at room temperature. The RH sensing results revealed an unprecedented response, ultrafast response/recovery behaviors, and outstanding repeatability. To our knowledge, the sensor response yielded in this work was tens of times higher than that of the existing humidity sensors. Moreover, the MoS2/SnO2 hybrid nanocomposite film sensor exhibited great enhancement in humidity sensing performances as compared to the pure MoS2, SnO2, and graphene counterparts. Furthermore, complex impedance spectroscopy and bode plots were employed to understand the underlying sensing mechanisms of the MoS2/SnO2 nanocomposite toward humidity. The synthesized MoS2/SnO2 hybrid composite was proved to be an excellent candidate for constructing ultrahigh-performance humidity sensor toward various applications.

  18. Contact and Support Considerations in the Hydrogen Evolution Reaction Activity of Petaled MoS2 Electrodes.

    Science.gov (United States)

    Finn, Shane T; Macdonald, Janet E

    2016-09-28

    Petaled MoS2 electrodes grown hydrothermally from Mo foils are found to have an 800 nm, intermediate, MoSxOy layer. Similar petaled MoS2 films without this intermediate layer are grown on Au. X-ray photoelectron and Raman spectroscopies and transmission electron microscopy indicate the resulting petaled multilayer MoS2 films are frayed and exhibit single-layer, 1T-MoS2 behavior at the edges. We compare the electrocatalytic hydrogen evolution reaction activity via linear sweep voltammetry with Tafel analysis as well as the impedance properties of the electrodes. We find that petaled MoS2/Au and petaled MoS2/Mo exhibit comparable overpotential to 10 mA cm(-2) at -279 vs -242 mV, respectively, and similar Tafel slopes of ∼68 mV/decade indicating a similar rate-determining step. The exchange current normalized to the geometric area of petaled MoS2/Au (0.000921 mA cm(-2)) is 3 times smaller than that of petaled MoS2/Mo (0.00290 mA cm(-2)), and is attributed to the lower petal density on the Au support. However, Au supports increase the turnover frequency per active site of petaled MoS2 to 0.48 H2 Mo(-1) s(-1) from 0.25 H2 Mo(-1) s(-1) on Mo supports. Both petaled MoS2 films have nearly ohmic contacts to their supports with uncompensated resistivity Ru of <2.5 Ω·cm(2).

  19. Thickness-Dependent Binding Energy Shift in Few-Layer MoS2 Grown by Chemical Vapor Deposition.

    Science.gov (United States)

    Lin, Yu-Kai; Chen, Ruei-San; Chou, Tsu-Chin; Lee, Yi-Hsin; Chen, Yang-Fang; Chen, Kuei-Hsien; Chen, Li-Chyong

    2016-08-31

    The thickness-dependent surface states of MoS2 thin films grown by the chemical vapor deposition process on the SiO2-Si substrates are investigated by X-ray photoelectron spectroscopy. Raman and high-resolution transmission electron microscopy suggest the thicknesses of MoS2 films to be ranging from 3 to 10 layers. Both the core levels and valence band edges of MoS2 shift downward ∼0.2 eV as the film thickness increases, which can be ascribed to the Fermi level variations resulting from the surface states and bulk defects. Grainy features observed from the atomic force microscopy topographies, and sulfur-vacancy-induced defect states illustrated at the valence band spectra imply the generation of surface states that causes the downward band bending at the n-type MoS2 surface. Bulk defects in thick MoS2 may also influence the Fermi level oppositely compared to the surface states. When Au contacts with our MoS2 thin films, the Fermi level downshifts and the binding energy reduces due to the hole-doping characteristics of Au and easy charge transfer from the surface defect sites of MoS2. The shift of the onset potentials in hydrogen evolution reaction and the evolution of charge-transfer resistances extracted from the impedance measurement also indicate the Fermi level varies with MoS2 film thickness. The tunable Fermi level and the high chemical stability make our MoS2 a potential catalyst. The observed thickness-dependent properties can also be applied to other transition-metal dichalcogenides (TMDs), and facilitates the development in the low-dimensional electronic devices and catalysts.

  20. Stabilizing MoS2 Nanosheets through SnO2 Nanocrystal Decoration for High-Performance Gas Sensing in Air.

    Science.gov (United States)

    Cui, Shumao; Wen, Zhenhai; Huang, Xingkang; Chang, Jingbo; Chen, Junhong

    2015-05-20

    The unique properties of MoS(2) nanosheets make them a promising candidate for high-performance room temperature sensing. However, the properties of pristine MoS(2) nanosheets are strongly influenced by the significant adsorption of oxygen in an air environment, which leads to instability of the MoS(2) sensing device, and all sensing results on MoS(2) reported to date were exclusively obtained in an inert atmosphere. This significantly limits the practical sensor application of MoS(2) in an air environment. Herein, a novel nanohybrid of SnO(2) nanocrystal (NC)-decorated crumpled MoS(2) nanosheet (MoS(2)/SnO(2)) and its exciting air-stable property for room temperature sensing of NO(2) are reported. Interestingly, the SnO(2) NCs serve as strong p-type dopants for MoS(2), leading to p-type channels in the MoS(2) nanosheets. The SnO(2) NCs also significantly enhance the stability of MoS(2) nanosheets in dry air. As a result, unlike other MoS(2) sensors operated in an inert gas (e.g. N(2)), the nanohybrids exhibit high sensitivity, excellent selectivity, and repeatability to NO(2) under a practical dry air environment. This work suggests that NC decoration significantly tunes the properties of MoS(2) nanosheets for various applications.

  1. Determination of band alignment in the single-layer MoS2/WSe2 heterojunction

    KAUST Repository

    Chiu, Ming-Hui

    2015-07-16

    The emergence of two-dimensional electronic materials has stimulated proposals of novel electronic and photonic devices based on the heterostructures of transition metal dichalcogenides. Here we report the determination of band offsets in the heterostructures of transition metal dichalcogenides by using microbeam X-ray photoelectron spectroscopy and scanning tunnelling microscopy/spectroscopy. We determine a type-II alignment between MoS2 and WSe2 with a valence band offset value of 0.83 eV and a conduction band offset of 0.76 eV. First-principles calculations show that in this heterostructure with dissimilar chalcogen atoms, the electronic structures of WSe2 and MoS2 are well retained in their respective layers due to a weak interlayer coupling. Moreover, a valence band offset of 0.94 eV is obtained from density functional theory, consistent with the experimental determination.

  2. Sulfur bonding in MoS2 and Co-Mo-S structures

    DEFF Research Database (Denmark)

    Byskov, Line Sjolte; Hammer, Bjørk; Nørskov, Jens Kehlet

    1997-01-01

    The structure and bonding in small MoS2 structures with and without Co is studied theoretically using self-consistent density functional theory with a non-local exchange-correlation energy. The structures model the catalysts used extensively in hydrotreating. We study in detail the structure...... and binding energies as a function of the amount of sulfur. The calculations show that extensive reconstructions occur at the two types of MoS2 edges where the sulfur dimerizes and occupies non-lattice positions. These structures are shown to be in good agreement with available experimental data. We also...... study the energy required to form sulfur vacancies, which are believed to be the active sites for many hydrotreating reactions. The presence of Co atoms at the edges is shown to lead to a significant lowering of the metal-sulfur binding energy. This imposes an increase in the concentration of active...

  3. Metallurgical Aspects of Self-lubricating Composites Containing Graphite and MoS2

    Science.gov (United States)

    Furlan, Kaline Pagnan; da Costa Gonçalves, Priscila; Consoni, Deise Rebelo; Dias, Matheus Vinícius Gouvêa; de Lima, Gabriel Araújo; de Mello, José Daniel Biasoli; Klein, Aloisio Nelmo

    2017-02-01

    The performance of dry self-lubricating bulk materials is directly related to microstructural aspects such as solid lubricant chemical composition and distribution. In this paper, dry powder mixtures were prepared from iron powder and 9-16.5 vol.% of solid lubricants (graphite and MoS2), both combined and isolated. The results showed that interactions and reactions occurred during processing, either between the solid lubricants or between the lubricants and the matrix, generating carbides and sulfides. On account of that, the lubricant distribution in the microstructure is greatly altered, and the microhardness, friction coefficient and wear rate are increased. The best results were achieved by adequate powder particle size, solid lubricant content and sintering temperature control. In the composite containing 9%MoS2 + 2.5%C, values of friction coefficient and wear rate lower than 0.08 and 8 × 10-6 mm3 N-1 m-1 were reached.

  4. Hydrothermal synthesis of amorphous MoS2nanofiber bundles via acidification of ammonium heptamolybdate tetrahydrate

    Directory of Open Access Journals (Sweden)

    Tharamani CN

    2007-01-01

    Full Text Available AbstractMoS2nanofiber bundles have been prepared by hydrothermal method using ammonium molybdate with sulfur source in acidic medium and maintained at 180 °C for several hours. The obtained black crystalline products are characterized by powder X-ray diffraction (PXRD, Fourier transform infrared spectrometer (FTIR, X-ray photoelectron spectroscopy (XPS, scanning electron microscopy (SEM and transmission electron microscopy (TEM. The PXRD pattern of the sample can be readily indexed as hexagonal 2H-MoS2. FTIR spectrum of the MoS2shows the band at 480 cm−1corresponds to the γas(Mo-S. SEM/TEM images of the samples exhibit that the MoS2nanofiber exist in bundles of 120–300 nm in diameter and 20–25 μm in length. The effects of temperature, duration and other experimental parameters on the morphology of the products are investigated.

  5. Oxygen Passivation Mediated Tunability of Trion and Excitons in MoS2

    KAUST Repository

    Gogoi, Pranjal Kumar

    2017-08-17

    Using wide spectral range in situ spectroscopic ellipsometry with systematic ultrahigh vacuum annealing and in situ exposure to oxygen, we report the complex dielectric function of MoS2 isolating the environmental effects and revealing the crucial role of unpassivated and passivated sulphur vacancies. The spectral weights of the A (1.92 eV) and B (2.02 eV) exciton peaks in the dielectric function reduce significantly upon annealing, accompanied by spectral weight transfer in a broad energy range. Interestingly, the original spectral weights are recovered upon controlled oxygen exposure. This tunability of the excitonic effects is likely due to passivation and reemergence of the gap states in the band structure during oxygen adsorption and desorption, respectively, as indicated by ab initio density functional theory calculation results. This Letter unravels and emphasizes the important role of adsorbed oxygen in the optical spectra and many-body interactions of MoS2.

  6. Mimicking Neurotransmitter Release in Chemical Synapses via Hysteresis Engineering in MoS2 Transistors.

    Science.gov (United States)

    Arnold, Andrew J; Razavieh, Ali; Nasr, Joseph R; Schulman, Daniel S; Eichfeld, Chad M; Das, Saptarshi

    2017-03-10

    Neurotransmitter release in chemical synapses is fundamental to diverse brain functions such as motor action, learning, cognition, emotion, perception, and consciousness. Moreover, improper functioning or abnormal release of neurotransmitter is associated with numerous neurological disorders such as epilepsy, sclerosis, schizophrenia, Alzheimer's disease, and Parkinson's disease. We have utilized hysteresis engineering in a back-gated MoS2 field effect transistor (FET) in order to mimic such neurotransmitter release dynamics in chemical synapses. All three essential features, i.e., quantal, stochastic, and excitatory or inhibitory nature of neurotransmitter release, were accurately captured in our experimental demonstration. We also mimicked an important phenomenon called long-term potentiation (LTP), which forms the basis of human memory. Finally, we demonstrated how to engineer the LTP time by operating the MoS2 FET in different regimes. Our findings could provide a critical component toward the design of next-generation smart and intelligent human-like machines and human-machine interfaces.

  7. Study on Sulifdation Degree and Morphology of MoS2 Catalyst Derived from Various Molybdate Precursors

    Institute of Scientific and Technical Information of China (English)

    Zhang Le; Li Mingfeng; Nie Hong

    2014-01-01

    The MoS2 catalysts were prepared from various molybdate precursors including inorganic and organic molybdate compounds. The sulifdation degree and morphology of active phases of MoS2 activated by various molybdate precursors in H2S/H2 stream at different temperatures were studied by X-ray photoelectron spectroscopy (XPS) and high-resolution trans-mission electron microscopy (HRTEM). The organic molybdate precursors lead to MoS2 catalysts with higher sulifdation degree and smaller active phases to demonstrate higher catalytic activity during hydrodesulfurizaiton (HDS) of 4,6-DMDBT.

  8. Emergence of intrinsic half-metallicity in MoS2 nano-crystals : A first principles study

    Science.gov (United States)

    Mandal, S. C.; Chatterjee, S.; Taraphder, A.

    2017-05-01

    Using first principles density functional theory we investigate the electronic structure of MoS2 nano-crystals of diameter 1nm. Our calculations suggest that MoS2 nano-crystals are inherently half-metallic, the half-metallicity being robust with respect to the constitution as well as the morphology of the surface and also with respect to the length of the nano-crystal. Thus, MoS2 nano-crystals, may turn out to be very important for application in spintronics based solid state devices.

  9. Large Area Single Crystal (0001) Oriented MoS2 Thin Films

    OpenAIRE

    Laskar, Masihhur R.; Ma, Lu; K, ShanthaKumar; Park, Pil Sung; Krishnamoorthy, Sriram; Nath, Digbijoy N.; Lu, Wu; Wu, Yiying; Rajan, Siddharth

    2013-01-01

    Layered metal dichalcogenide materials are a family of semiconductors with a wide range of energy band gaps and properties, and potential to open up new areas of physics and technology applications. However, obtaining high crystal quality thin films over a large area remains a challenge. Here we show that chemical vapor deposition (CVD) can be used to achieve large area electronic grade single crystal Molybdenum Disulfide (MoS2) thin films with the highest mobility reported in CVD grown films...

  10. Oscillating edge states in one-dimensional MoS2 nanowires

    Science.gov (United States)

    Xu, Hai; Liu, Shuanglong; Ding, Zijing; Tan, Sherman J. R.; Yam, Kah Meng; Bao, Yang; Nai, Chang Tai; Ng, Man-Fai; Lu, Jiong; Zhang, Chun; Loh, Kian Ping

    2016-10-01

    Reducing the dimensionality of transition metal dichalcogenides to one dimension opens it to structural and electronic modulation related to charge density wave and quantum correlation effects arising from edge states. The greater flexibility of a molecular scale nanowire allows a strain-imposing substrate to exert structural and electronic modulation on it, leading to an interplay between the curvature-induced influences and intrinsic ground-state topology. Herein, the templated growth of MoS2 nanowire arrays consisting of the smallest stoichiometric MoS2 building blocks is investigated using scanning tunnelling microscopy and non-contact atomic force microscopy. Our results show that lattice strain imposed on a nanowire causes the energy of the edge states to oscillate periodically along its length in phase with the period of the substrate topographical modulation. This periodic oscillation vanishes when individual MoS2 nanowires join to form a wider nanoribbon, revealing that the strain-induced modulation depends on in-plane rigidity, which increases with system size.

  11. Metallic and ferromagnetic MoS2 nanobelts with vertically aligned edges

    Institute of Scientific and Technical Information of China (English)

    Guanchen Xu[1,2; Xinsheng Wang[1; Yupeng Sun[1; Xiao Chen[2; Jingying Zheng[1; Lifei Sun[1; Liying Jiao[1; Jinghong Li[1

    2015-01-01

    Edge effects are predicted to significantly impact the properties of low dimensional materials with layered structures. The synthesis of low dimensional materials with copious edges is desired for exploring the effects of edges on the band structure and properties of these materials. Here we developed an approach for synthesizing MoS2 nanobelts terminated with vertically aligned edges by sulfurizing hydrothermally synthesized MoO3 nanobelts in the gas phase through a kinetically driven process; we then investigated the electrical and magnetic properties of these metastable materials. These edge-terminated MoS2 nanobelts were found to be metallic and ferromagnetic, and thus dramatically different from the semiconducting and nonmagnetic two-dimensional (2D) and three-dimensional (3D) 2H-MoS2 materials. The transitions in electrical and magnetic properties elucidate the fact that edges can tune the properties of low dimensional materials. The unique structure and properties of this one-dimensional (1D) MoS2 material will enable its applications in electronics, spintronics, and catalysis.

  12. Transition states and modeling for Co9S8/MoS 2 catalysis

    Science.gov (United States)

    Gonzalez, Gabriel Angel

    Transition state computational studies of the sulfur removal from dibenzothiophene (DBT) molecule have herein been performed considering the Co9S8/MoS2 interface existing on unsupported Co/MoS2 catalysts. The linear synchronous transit (LST) and quadratic synchronous transit (QST) methods integrated in a density functional theory (DFT) program such as Dmol3 were used for the calculations of energy barriers of the transition states. Three different configurations present on the Co9S8/MoS2 interface have been envisaged as possible catalytic sites: sulfur-sulfur (S, S) sites, and molybdenum-sulfur (Mo, S) and molybdenum-molybdenum (Mo, Mo) edge sites. This study revealed that the (Mo, Mo) edge site is the most catalytically active site for the hydrodesulfurization (HDS) reaction followed by the (Mo, S) edge site while the (S, S) site shows almost inertness for the HDS reaction. This information allows us to propose clues to design new catalysts based on bulk Co9S8/MoS2 phases with higher efficiency by increasing the proportion of (Mo, Mo) edge sites.

  13. Large-area few-layer MoS 2 deposited by sputtering

    KAUST Repository

    Huang, Jyun-Hong

    2016-06-06

    Direct magnetron sputtering of transition metal dichalcogenide targets is proposed as a new approach for depositing large-area two-dimensional layered materials. Bilayer to few-layer MoS2 deposited by magnetron sputtering followed by post-deposition annealing shows superior area scalability over 20 cm(2) and layer-by-layer controllability. High crystallinity of layered MoS2 was confirmed by Raman, photo-luminescence, and transmission electron microscopy analysis. The sputtering temperature and annealing ambience were found to play an important role in the film quality. The top-gate field-effect transistor by using the layered MoS2 channel shows typical n-type characteristics with a current on/off ratio of approximately 10(4). The relatively low mobility is attributed to the small grain size of 0.1-1 mu m with a trap charge density in grain boundaries of the order of 10(13) cm(-2).

  14. Ideal strength and phonon instability in single-layer MoS2

    Science.gov (United States)

    Li, Tianshu

    2012-06-01

    Ideal tensile stress strain relations for single-layer MoS2 are investigated based on first-principle calculation, for biaxial tension and uniaxial tension along zigzag and armchair directions. The predicted ideal tensile strengths and elastic moduli are in excellent agreement with the very recent experimental measurements of Bertolazzi [ACS Nano1936-085110.1021/nn203879f 5, 9703 (2011)] and Castellanos-Gomez [Adv. Mater.ADVMEW0935-964810.1002/adma.201103965 24, 772 (2012)]. It is identified that the tensile strength of single-layer MoS2 are dictated by out-of-plane soft-mode phonon instability under biaxial tension and uniaxial tension along the armchair direction. This failure mechanism, different from that of the truly two-dimensional material graphene, is attributed to the out-of-plane atomic relaxation upon tensile strain. Investigation of the electronic structures of single-layer MoS2 under tensile strain shows the material becomes an indirect semiconductor at small tensile strain (<2%) and turns into metallic before reaching the ideal tensile strength.

  15. Band engineering of the MoS2/stanene heterostructure: strain and electrostatic gating

    Science.gov (United States)

    Xiong, Wenqi; Xia, Congxin; Du, Juan; Wang, Tianxing; Peng, Yuting; Wei, Zhongming; Li, Jingbo

    2017-05-01

    In a fast developing field, it has been found that van der Waals heterostructures can overcome the weakness of single two-dimensional layered materials and extend their electronic and optoelectronic applications. Through first-principles methods, the studied MoS2/stanene heterostructure preserves high-speed carrier characteristics and opens the direct band gap. Simultaneously, the band alignment shows that the electrons transfer from stanene to MoS2, which forms an internal electric field. As an effective strategy, the out-of-plane strain remarkably changes the band gaps of the heterostructure and enhances its carrier concentration. In addition, the combined effects of the internal and external electric fields can further open the band gaps and induce a direct-to-indirect gap transition in the heterostructure. More interestingly, when the external electric field is equal to the reverse internal one, the heterostructure regains a Dirac cone. Our results show that the MoS2/stanene heterostructure has potential applications in high-speed optoelectronic devices.

  16. Rheological and tribological behaviour of lubricating oils containing platelet MoS2 nanoparticles

    Science.gov (United States)

    Wan, Qingming; Jin, Yi; Sun, Pengcheng; Ding, Yulong

    2014-05-01

    This work concerns rheological and frictional behaviour of lubricating oils containing platelet molybdenum disulfide (MoS2) nanoparticles (average diameter 50 nm; single layer thickness 3 nm). Stable nano-MoS2 lubricants were formulated and measured for their rheological behaviour and tribological performance. Rheological experiments showed that the nano-MoS2 oils were non-Newtonian following the Bingham plastic fluid model. The viscosity data fitted the classic Hinch-Leal (H-L) model if an agglomeration factor of 1.72 was introduced. Tribological experiments indicated that the use of MoS2 nanoparticles could enhance significantly the tribological performance of the base lubricating oil (reduced frictional coefficient, reduced surface wear and increased stability). Scanning electron microscopy, laser confocal microscope and x-ray energy dispersive spectroscopy analyses suggested that the reduced frictional coefficient and surface wear be associated with surface patching effects. Such patching effects were shown to depend on the concentration of MoS2 nanoparticles, and an effective patching required a concentration over approximately 1 wt%. The increased stability could be attributed to the enhanced heat transfer and lubricating oil film strength due to the presence of nanoparticles.

  17. MoS2 Nanosheets Hosted in Polydopamine-Derived Mesoporous Carbon Nanofibers as Lithium-Ion Battery Anodes: Enhanced MoS2 Capacity Utilization and Underlying Mechanism.

    Science.gov (United States)

    Kong, Junhua; Zhao, Chenyang; Wei, Yuefan; Lu, Xuehong

    2015-11-01

    In this work, solid, hollow, and porous carbon nanofibers (SNFs, HNFs, and PNFs) were used as hosts to grow MoS2 nanosheets hydrothermally. The results show that the nanosheets on the surface of SNFs and HNFs are comprised of a few grains stacked together, giving direct carbon-MoS2 contact for the first grain and indirect contact for the rest. In contrast, the nanosheets inside of PNFs are of single-grain size and are distributed evenly in the mesopores of PNFs, providing efficient MoS2-carbon contact. Furthermore, the nanosheets grown on the polydopamine-derived carbon surface of HNFs and PNFs have larger interlayer spacing than those grown on polyacrylonitrile-derived carbon surface. As a result, the MoS2 nanosheets in PNFs possess the lowest charge-transfer resistance, the most accessible active sites for lithiation/delithiation, and can effectively buffer the volume variation of MoS2, leading to its best electrochemical performance as a lithium-ion battery anode among the three. The normalized reversible capacity of the MoS2 nanosheets in PNFs is about 1210 mAh g(-1) at 100 mA g(-1), showing the effective utilization of the electrochemical activity of MoS2.

  18. MoS2夹层化合物的制备与应用%Preparation and Application of MoS2 Intercalation Compounds

    Institute of Scientific and Technical Information of China (English)

    刘佳; 张学斌; 吴海骏; 冀翼; 曹星星; 朱雅君; 凤仪

    2013-01-01

    MoS2为层状结构,层间为较弱的范德华力,因此可将无机、有机离子或基团插入其层间形成具有特定性能和功用的MoS2夹层化合物.综述了MoS2夹层化合物的制备方法.介绍了剥层/重堆法制备MoS2夹层化合物的机理、客体物质的种类及MoS2夹层化合物的性能与应用,并展望了MoS2夹层化合物的发展前景.

  19. Antibacterial activity of two-dimensional MoS2 sheets

    Science.gov (United States)

    Yang, Xi; Li, Jie; Liang, Tao; Ma, Chunyan; Zhang, Yingying; Chen, Hongzheng; Hanagata, Nobutaka; Su, Huanxing; Xu, Mingsheng

    2014-08-01

    Graphene-like two-dimensional materials (2DMats) show application potential in optoelectronics and biomedicine due to their unique properties. However, environmental and biological influences of these 2DMats remain to be unveiled. Here we reported the antibacterial activity of two-dimensional (2D) chemically exfoliated MoS2 (ce-MoS2) sheets. We found that the antibacterial activity of ce-MoS2 sheets was much more potent than that of the raw MoS2 powders used for the synthesis of ce-MoS2 sheets possibly due to the 2D planar structure (high specific surface area) and higher conductivity of the ce-MoS2. We investigated the antibacterial mechanisms of the ce-MoS2 sheets and proposed their antibacterial pathways. We found that the ce-MoS2 sheets could produce reactive oxygen species (ROS), different from a previous report on graphene-based materials. Particularly, the oxidation capacity of the ce-MoS2 sheets toward glutathione oxidation showed a time and concentration dependent trend, which is fully consistent with the antibacterial behaviour of the ce-MoS2 sheets. The results suggest that antimicrobial behaviors were attributable to both membrane and oxidation stress. The antibacterial pathways include MoS2-bacteria contact induced membrane stress, superoxide anion (O2&z.rad;-) induced ROS production by the ce-MoS2, and the ensuing superoxide anion-independent oxidation. Our study thus indicates that the tailoring of the dimension of nanomaterials and their electronic properties would manipulate antibacterial activity.Graphene-like two-dimensional materials (2DMats) show application potential in optoelectronics and biomedicine due to their unique properties. However, environmental and biological influences of these 2DMats remain to be unveiled. Here we reported the antibacterial activity of two-dimensional (2D) chemically exfoliated MoS2 (ce-MoS2) sheets. We found that the antibacterial activity of ce-MoS2 sheets was much more potent than that of the raw MoS2 powders used for the synthesis of ce-MoS2 sheets possibly due to the 2D planar structure (high specific surface area) and higher conductivity of the ce-MoS2. We investigated the antibacterial mechanisms of the ce-MoS2 sheets and proposed their antibacterial pathways. We found that the ce-MoS2 sheets could produce reactive oxygen species (ROS), different from a previous report on graphene-based materials. Particularly, the oxidation capacity of the ce-MoS2 sheets toward glutathione oxidation showed a time and concentration dependent trend, which is fully consistent with the antibacterial behaviour of the ce-MoS2 sheets. The results suggest that antimicrobial behaviors were attributable to both membrane and oxidation stress. The antibacterial pathways include MoS2-bacteria contact induced membrane stress, superoxide anion (O2&z.rad;-) induced ROS production by the ce-MoS2, and the ensuing superoxide anion-independent oxidation. Our study thus indicates that the tailoring of the dimension of nanomaterials and their electronic properties would manipulate antibacterial activity. Electronic supplementary information (ESI) available: Superoxide radical anion production experiments and GSH oxidation experiments by MoS2 materials; Raman peaks of the MoS2 materials; AFM cross-sectional profile analyses of the ce-MoS2 sheets; XPS survey spectra of the MoS2 materials; XRD patterns of the MoS2 materials; digital camera photos of live E.coli DH5α bacteria after they were exposed to ce-MoS2 dispersions with different concentrations for 2 h; glutathione oxidation by monitoring color change; Keynece Microscope images of the cells exposed to ce-MoS2 sheets and raw MoS2 powders. See DOI: 10.1039/c4nr01965b

  20. In situ formation of a MoS2 -based inorganic-organic nanocomposite by directed thermal decomposition.

    Science.gov (United States)

    Djamil, John; Segler, Stefan A W; Bensch, Wolfgang; Schürmann, Ulrich; Deng, Mao; Kienle, Lorenz; Hansen, Sven; Beweries, Torsten; von Wüllen, Leo; Rosenfeldt, Sabine; Förster, Stephan; Reinsch, Helge

    2015-06-08

    Nanocomposites based on molybdenum disulfide (MoS2 ) and different carbon modifications are intensively investigated in several areas of applications due to their intriguing optical and electrical properties. Addition of a third element may enhance the functionality and application areas of such nanocomposites. Herein, we present a facile synthetic approach based on directed thermal decomposition of (Ph4 P)2 MoS4 generating MoS2 nanocomposites containing carbon and phosphorous. Decomposition at 250 °C yields a composite material with significantly enlarged MoS2 interlayer distances caused by in situ formation of Ph3 PS bonded to the MoS2 slabs through MoS bonds and (Ph4 P)2 S molecules in the van der Waals gap, as was evidenced by (31) P solid-state NMR spectroscopy. Visible-light-driven hydrogen generation demonstrates a high catalytic performance of the materials.

  1. Enhancement of photodetection characteristics of MoS2 field effect transistors using surface treatment with copper phthalocyanine

    Science.gov (United States)

    Pak, Jinsu; Jang, Jingon; Cho, Kyungjune; Kim, Tae-Young; Kim, Jae-Keun; Song, Younggul; Hong, Woong-Ki; Min, Misook; Lee, Hyoyoung; Lee, Takhee

    2015-11-01

    Recently, two-dimensional materials such as molybdenum disulfide (MoS2) have been extensively studied as channel materials for field effect transistors (FETs) because MoS2 has outstanding electrical properties such as a low subthreshold swing value, a high on/off ratio, and good carrier mobility. In this study, we characterized the electrical and photo-responsive properties of MoS2 FET when stacking a p-type organic copper phthalocyanine (CuPc) layer on the MoS2 surface. We observed that the threshold voltage of MoS2 FET could be controlled by stacking the CuPc layers due to a charge transfer phenomenon at the interface. Particularly, we demonstrated that CuPc/MoS2 hybrid devices exhibited high performance as a photodetector compared with the pristine MoS2 FETs, caused by more electron-hole pairs separation at the p-n interface. Furthermore, we found the optimized CuPc thickness (~2 nm) on the MoS2 surface for the best performance as a photodetector with a photoresponsivity of ~1.98 A W-1, a detectivity of ~6.11 × 1010 Jones, and an external quantum efficiency of ~12.57%. Our study suggests that the MoS2 vertical hybrid structure with organic material can be promising as efficient photodetecting devices and optoelectronic circuits.Recently, two-dimensional materials such as molybdenum disulfide (MoS2) have been extensively studied as channel materials for field effect transistors (FETs) because MoS2 has outstanding electrical properties such as a low subthreshold swing value, a high on/off ratio, and good carrier mobility. In this study, we characterized the electrical and photo-responsive properties of MoS2 FET when stacking a p-type organic copper phthalocyanine (CuPc) layer on the MoS2 surface. We observed that the threshold voltage of MoS2 FET could be controlled by stacking the CuPc layers due to a charge transfer phenomenon at the interface. Particularly, we demonstrated that CuPc/MoS2 hybrid devices exhibited high performance as a photodetector compared with the pristine MoS2 FETs, caused by more electron-hole pairs separation at the p-n interface. Furthermore, we found the optimized CuPc thickness (~2 nm) on the MoS2 surface for the best performance as a photodetector with a photoresponsivity of ~1.98 A W-1, a detectivity of ~6.11 × 1010 Jones, and an external quantum efficiency of ~12.57%. Our study suggests that the MoS2 vertical hybrid structure with organic material can be promising as efficient photodetecting devices and optoelectronic circuits. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr04836b

  2. On the electrostatic control achieved in transistors based on multilayered MoS2: A first-principles study

    Science.gov (United States)

    Lu, Anh Khoa Augustin; Pourtois, Geoffrey; Luisier, Mathieu; Radu, Iuliana P.; Houssa, Michel

    2017-01-01

    In this work, the electrostatic control in metal-oxide-semiconductor field-effect transistors based on MoS2 is studied, with respect to the number of MoS2 layers in the channel and to the equivalent oxide thickness of the gate dielectric, using first-principles calculations combined with a quantum transport formalism. Our simulations show that a compromise exists between the drive current and the electrostatic control on the channel. When increasing the number of MoS2 layers, a degradation of the device performances in terms of subthreshold swing and OFF currents arises due to the screening of the MoS2 layers constituting the transistor channel.

  3. Analysis of electron beam damage of exfoliated MoS2 sheets and quantitative HAADF-STEM imaging

    Science.gov (United States)

    Garcia, A.; Raya, A.M.; Mariscal, M.M.; Esparza, R.; Herrera, M.; Molina, S.I.; Scavello, G.; Galindo, P.L.; Jose-Yacaman, M.; Ponce, A.

    2014-01-01

    In this work we examined MoS2 sheets by aberration-corrected scanning transmission electron microscopy (STEM) at three different energies: 80, 120 and 200 kV. Structural damage of the MoS2 sheets has been controlled at 80 kV according a theoretical calculation based on the inelastic scattering of the electrons involved in the interaction electron-matter. The threshold energy for the MoS2 material has been found and experimentally verified in the microscope. At energies higher than the energy threshold we show surface and edge defects produced by the electron beam irradiation. Quantitative analysis at atomic level in the images obtained at 80 kV has been performed using the experimental images and via STEM simulations using SICSTEM software to determine the exact number of MoS2 layers. PMID:24929924

  4. Passively Q-switched nanosecond erbium-doped fiber laser with MoS(2) saturable absorber.

    Science.gov (United States)

    Ren, Jun; Wang, Shuxian; Cheng, Zhaochen; Yu, Haohai; Zhang, Huaijin; Chen, Yanxue; Mei, Liangmo; Wang, Pu

    2015-03-09

    Passively Q-switched nanosecond pulsed erbium-doped fiber laser based on MoS(2) saturable absorber (SA) is experimentally demonstrated. The high quality MoS(2) SA deposited on the broadband high-reflectivity mirror with a large modulation depth of 9% was prepared by pulsed laser deposition method. By inserting the MoS(2) SA into an erbium-doped fiber laser, stable Q-switched operation can be achieved with the shortest pulse width of 660 ns, the maximum pulse energy up to 152 nJ and pulse repetition rates varying from 116 to 131 kHz. The experimental results further verify that MoS(2) possesses the potential advantage for stable Q-switched pulse generation at 1.5 μm.

  5. Fabrication of MoS2 nanosheet@TiO2 nanotube hybrid nanostructures for lithium storage

    Science.gov (United States)

    Xu, Xin; Fan, Zhaoyang; Ding, Shujiang; Yu, Demei; Du, Yaping

    2014-04-01

    MoS2 nanosheet@TiO2 nanotube hybrid nanostructures were successfully prepared by a facile two-step method: prefabrication of porous TiO2 nanotubes based on a sol-gel method template against polymeric nanotubes, and then assembly of MoS2 nanoclusters that consist of ultrathin nanosheets through a solvothermal process. These hybrid nanostructures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET) analysis. When evaluated as an electrode material for lithium ion batteries, the results of the electrochemical test show that the unique MoS2 nanosheet@TiO2 nanotube hybrid nanostructures exhibit outstanding lithium storage performances with high specific capacity and excellent rate capability. The smart architecture of the MoS2 nanosheet@TiO2 nanotube hybrid nanostructures makes a prominent contribution to the excellent electrochemical performance.

  6. A theoretical modeling of photocurrent generation and decay in layered MoS2 thin-film transistor photosensors

    Science.gov (United States)

    Hur, Ji-Hyun; Park, Junghak; Jeon, Sanghun

    2017-02-01

    A model that universally describes the characteristics of photocurrent in molybdenum disulphide (MoS2) thin-film transistor (TFT) photosensors in both ‘light on’ and ‘light off’ conditions is presented for the first time. We considered possible material-property dependent carrier generation and recombination mechanisms in layered MoS2 channels with different numbers of layers. We propose that the recombination rates that are mainly composed of direct band-to-band recombination and interface trap-involved recombination change on changing the light condition and the number of layers. By comparing the experimental results, it is shown that the model performs well in describing the photocurrent behaviors of MoS2 TFT photosensors, including the photocurrent generation under illumination and a hugely long time persistent trend of the photocurrent decay in the dark condition, for a range of MoS2 layer numbers.

  7. Nucleation and growth mechanisms of Al2O3 atomic layerdeposition on synthetic polycrystalline MoS2

    Science.gov (United States)

    Zhang, H.; Chiappe, D.; Meersschaut, J.; Conard, T.; Franquet, A.; Nuytten, T.; Mannarino, M.; Radu, I.; Vandervorst, W.; Delabie, A.

    2017-02-01

    Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are of great interest for applications in nano-electronic devices. Their incorporation requires the deposition of nm-thin and continuous high-k dielectric layers on the 2D TMDs. Atomic layer deposition (ALD) of high-k dielectric layers is well established on Si surfaces: the importance of a high nucleation density for rapid layer closure is well known and the nucleation mechanisms have been thoroughly investigated. In contrast, the nucleation of ALD on 2D TMD surfaces is less well understood and a quantitative analysis of the deposition process is lacking. Therefore, in this work, we investigate the growth of Al2O3 (using Al(CH3)3/H2O ALD) on MoS2 whereby we attempt to provide a complete insight into the use of several complementary characterization techniques, including X-ray photo-electron spectroscopy, elastic recoil detection analysis, scanning electron microscopy, and time-of-flight secondary ion mass spectrometry. To reveal the inherent reactivity of MoS2, we exclude the impact of surface contamination from a transfer process by direct Al2O3 deposition on synthetic MoS2 layers obtained by a high temperature sulfurization process. It is shown that Al2O3 ALD on the MoS2 surface is strongly inhibited at temperatures between 125°C and 300°C, with no growth occurring on MoS2 crystal basal planes and selective nucleation only at line defects or grain boundaries at MoS2 top surface. During further deposition, the as-formed Al2O3 nano-ribbons grow in both vertical and lateral directions. Eventually, a continuous Al2O3 film is obtained by lateral growth over the MoS2 crystal basal plane, with the point of layer closure determined by the grain size at the MoS2 top surface and the lateral growth rate. The created Al2O3/MoS2 interface consists mainly of van der Waals interactions. The nucleation is improved by contributions of reversible adsorption on the MoS2 basal planes by using low deposition temperature in combination with short purge times. While this results in a more two-dimensional growth, additional H and C impurities are incorporated in the Al2O3 layers. To conclude, our growth study reveals that the inherent reactivity of the MoS2 basal plane for ALD is extremely low, and this confirms the need for functionalization methods of the TMD surface to enable ALD nucleation.

  8. Mussel inspired preparation of MoS2 based polymer nanocomposites: The case of polyPEGMA

    Science.gov (United States)

    Zeng, Guangjian; Liu, Meiying; Liu, Xinhua; Huang, Qiang; Xu, Dazhuang; Mao, Liucheng; Huang, Hongye; Deng, Fengjie; Zhang, Xiaoyong; Wei, Yen

    2016-11-01

    In this work, we report a facile strategy to prepare PEGylated MoS2 nanosheets through the combination of mussel inspired chemistry and Michael addition reaction. The MoS2 nanosheets were obtained from lithium intercalation and exfoliation method. Meanwhile, the amino-contained poly((polyethylene glycol) methyl ether methacrylate) (PPEGMA) were obtained via chain transfer free radical polymerization using cysteamine hydrochloride as the chain transfer agents and PEGMA as the monomer. To introduce PPEGMA on MoS2 nanosheets, polydopamine (PDA) thin films were first coated on the surface of MoS2 nanosheets through self polymerization of dopamine as the ad-layers, which can react with amino-terminated PPEGMA through Michael addition reaction. The structure, morphology and chemical compositions of MoS2 nanosheets and MoS2-PDA-PPEGMA have been characterized by various characterization techniques. The results demonstrated that the amino-terminated PPEGMA can be successfully immobilized on MoS2 nanosheets via PDA thin films as the ad-layers. More importantly, the strategy described in this work could also be utilized for surface immobilization of various polymers on many other materials and surfaces because of the universal adhesion of PDA and the good monomer applicability of chain transfer free radical polymerization. Taken together, we developed a facile and versatile method to fabricate multifunctional MoS2 based polymer nanocomposites with designable properties and applications via combination of mussel inspired and chain transfer free radical polymerization. The resultant composites are expected to be potentially used for drug delivery and photothermal cancer treatment.

  9. A Microfluidic DNA Sensor Based on Three-Dimensional (3D Hierarchical MoS2/Carbon Nanotube Nanocomposites

    Directory of Open Access Journals (Sweden)

    Dahou Yang

    2016-11-01

    Full Text Available In this work, we present a novel microfluidic biosensor for sensitive fluorescence detection of DNA based on 3D architectural MoS2/multi-walled carbon nanotube (MWCNT nanocomposites. The proposed platform exhibits a high sensitivity, selectivity, and stability with a visible manner and operation simplicity. The excellent fluorescence quenching stability of a MoS2/MWCNT aqueous solution coupled with microfluidics will greatly simplify experimental steps and reduce time for large-scale DNA detection.

  10. The use of UV/ozone-treated MoS2 nanosheets for extended air stability in organic photovoltaic cells.

    Science.gov (United States)

    Le, Quyet Van; Nguyen, Thang Phan; Jang, Ho Won; Kim, Soo Young

    2014-07-14

    MoS2 nanosheets obtained through a simple sonication exfoliation method are employed as a hole-extraction layer (HEL) to improve the efficiency and air stability of organic photovoltaic cells (OPVs). The reduction in the wavenumber difference, appearance of a UV-vis peak, and atomic force microscopy images indicate that MoS2 nanosheets are formed through the sonication method. The OPVs with MoS2 layers show a degraded performance with a power conversion efficiency (PCE) of 1.08%, which is lower than that of OPVs without HEL (1.84%). After performing the UV/ozone (UVO) treatment of the MoS2 surface for 15 min, the PCE value increases to 2.44%. Synchrotron radiation photoelectron spectroscopy data show that the work function of MoS2 increases from 4.6 to 4.9 eV upon UVO treatment, suggesting that the increase in the PCE value is caused by the bandgap alignment. Upon inserting poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) between MoS2 and the active layer, the PCE value of the OPV increases to 2.81%, which is comparable with that of the device employing only PEDOT:PSS. Furthermore, the stability of the OPVs is improved significantly when MoS2/PEDOT:PSS layers are used as the HEL. Therefore, it is considered that the use of UVO-treated MoS2 may improve the stability of OPV cells without degrading the device performance.

  11. Van der Waals Epitaxy of Two-Dimensional MoS2-Graphene Heterostructures in Ultrahigh Vacuum.

    Science.gov (United States)

    Miwa, Jill A; Dendzik, Maciej; Grønborg, Signe S; Bianchi, Marco; Lauritsen, Jeppe V; Hofmann, Philip; Ulstrup, Søren

    2015-06-23

    In this work, we demonstrate direct van der Waals epitaxy of MoS2-graphene heterostructures on a semiconducting silicon carbide (SiC) substrate under ultrahigh vacuum conditions. Angle-resolved photoemission spectroscopy (ARPES) measurements show that the electronic structure of free-standing single-layer (SL) MoS2 is retained in these heterostructures due to the weak van der Waals interaction between adjacent materials. The MoS2 synthesis is based on a reactive physical vapor deposition technique involving Mo evaporation and sulfurization in a H2S atmosphere on a template consisting of epitaxially grown graphene on SiC. Using scanning tunneling microscopy, we study the seeding of Mo on this substrate and the evolution from nanoscale MoS2 islands to SL and bilayer (BL) MoS2 sheets during H2S exposure. Our ARPES measurements of SL and BL MoS2 on graphene reveal the coexistence of the Dirac states of graphene and the expected valence band of MoS2 with the band maximum shifted to the corner of the Brillouin zone at K̅ in the SL limit. We confirm the 2D character of these electronic states via a lack of dispersion with photon energy. The growth of epitaxial MoS2-graphene heterostructures on SiC opens new opportunities for further in situ studies of the fundamental properties of these complex materials, as well as perspectives for implementing them in various device schemes to exploit their many promising electronic and optical properties.

  12. Controlling magnetism of MoS2 sheets by embedding transition-metal atoms and applying strain.

    Science.gov (United States)

    Zhou, Yungang; Su, Qiulei; Wang, Zhiguo; Deng, Huiqiu; Zu, Xiaotao

    2013-11-14

    Prompted by recent experimental achievement of transition metal (TM) atoms substituted in MoS2 nanostructures during growth or saturating existing vacancies (Sun et al., ACS Nano, 2013, 7, 3506; Deepak et al., J. Am. Chem. Soc., 2007, 129, 12549), we explored, via density functional theory, the magnetic properties of a series of 3d TM atoms substituted in a MoS2 sheet, and found that Mn, Fe, Co, Ni, Cu and Zn substitutions can induce magnetism in the MoS2 sheet. The localizing unpaired 3d electrons of TM atoms respond to the introduction of a magnetic moment. Depending on the species of TM atoms, the substituted MoS2 sheet can be a metal, semiconductor or half-metal. Remarkably, the applied elastic strain can be used to control the strength of the spin-splitting of TM-3d orbitals, leading to an effective manipulation of the magnetism of the TM-substituted MoS2 sheet. We found that the magnetic moment of the Mn- and Fe-substituted MoS2 sheets can monotonously increase with the increase of tensile strain, while the magnetic moment of Co-, Ni-, Cu- and Zn-substituted MoS2 sheets initially increases and then decreases with the increase of tensile strain. An instructive mechanism was proposed to qualitatively explain the variation of magnetism with elastic strain. The finding of the magnetoelastic effect here is technologically important for the fabrication of strain-driven spin devices on MoS2 nanostructures, which allows us to go beyond the current scope limited to the spin devices within graphene and BN-based nanostructures.

  13. 2D nanosheet molybdenum disulphide (MoS2) modified electrodes explored towards the hydrogen evolution reaction.

    Science.gov (United States)

    Rowley-Neale, Samuel J; Brownson, Dale A C; Smith, Graham C; Sawtell, David A G; Kelly, Peter J; Banks, Craig E

    2015-11-21

    We explore the use of two-dimensional (2D) MoS2 nanosheets as an electrocatalyst for the Hydrogen Evolution Reaction (HER). Using four commonly employed commercially available carbon based electrode support materials, namely edge plane pyrolytic graphite (EPPG), glassy carbon (GC), boron-doped diamond (BDD) and screen-printed graphite electrodes (SPE), we critically evaluate the reported electrocatalytic performance of unmodified and MoS2 modified electrodes towards the HER. Surprisingly, current literature focuses almost exclusively on the use of GC as an underlying support electrode upon which HER materials are immobilised. 2D MoS2 nanosheet modified electrodes are found to exhibit a coverage dependant electrocatalytic effect towards the HER. Modification of the supporting electrode surface with an optimal mass of 2D MoS2 nanosheets results in a lowering of the HER onset potential by ca. 0.33, 0.57, 0.29 and 0.31 V at EPPG, GC, SPE and BDD electrodes compared to their unmodified counterparts respectively. The lowering of the HER onset potential is associated with each supporting electrode's individual electron transfer kinetics/properties and is thus distinct. The effect of MoS2 coverage is also explored. We reveal that its ability to catalyse the HER is dependent on the mass deposited until a critical mass of 2D MoS2 nanosheets is achieved, after which its electrocatalytic benefits and/or surface stability curtail. The active surface site density and turn over frequency for the 2D MoS2 nanosheets is determined, characterised and found to be dependent on both the coverage of 2D MoS2 nanosheets and the underlying/supporting substrate. This work is essential for those designing, fabricating and consequently electrochemically testing 2D nanosheet materials for the HER.

  14. Fabrication of MgFe2O4/MoS2 Heterostructure Nanowires for Photoelectrochemical Catalysis.

    Science.gov (United States)

    Fan, Weiqiang; Li, Meng; Bai, Hongye; Xu, Dongbo; Chen, Chao; Li, Chunfa; Ge, Yilin; Shi, Weidong

    2016-02-16

    A novel one-dimensional MgFe2O4/MoS2 heterostructure has been successfully designed and fabricated. The bare MgFe2O4 was obtained as uniform nanowires through electrospinning, and MoS2 thin film appeared on the surface of MgFe2O4 after further chemical vapor deposition. The structure of the MgFe2O4/MoS2 heterostructure was systematic investigated by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectrometry (XPS), and Raman spectra. According to electrochemical impedance spectroscopy (EIS) results, the MgFe2O4/MoS2 heterostructure showed a lower charge-transfer resistance compared with bare MgFe2O4, which indicated that the MoS2 played an important role in the enhancement of electron/hole mobility. MgFe2O4/MoS2 heterostructure can efficiently degrade tetracycline (TC), since the superoxide free-radical can be produced by sample under illumination due to the active species trapping and electron spin resonance (ESR) measurement, and the optimal photoelectrochemical degradation rate of TC can be achieved up to 92% (radiation intensity: 47 mW/cm(2), 2 h). Taking account of its unique semiconductor band gap structure, MgFe2O4/MoS2 can also be used as an photoelectrochemical anode for hydrogen production by water splitting, and the hydrogen production rate of MgFe2O4/MoS2 was 5.8 mmol/h·m(2) (radiation intensity: 47 mW/cm(2)), which is about 1.7 times that of MgFe2O4.

  15. Interfacial thermodynamics and kinetics of sorption of diclofenac on prepared high performance flower-like MoS2.

    Science.gov (United States)

    Zhang, Yalei; Yin, Zengfu; Dai, Chaomeng; Zhou, Xuefei; Chen, Wen

    2016-11-01

    Flower-like MoS2 with numerous wrinkled nanosheets was prepared via a facile hydrothermal method. The surface morphology and microstructure of the obtained materials were characterized using X-ray diffraction data (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Additionally, the compositions of the flower-like MoS2 were further revealed by an energy dispersion spectrometer (EDX) and X-ray photoelectron spectrometry (XPS). The obtained MoS2 was used as an adsorbent to remove diclofenac (DCF, C14H10Cl2NO2Na) from aqueous solutions and presented excellent performance for removing DCF. The sorption kinetics, isotherms and effect of solution pH on the sorption were evaluated in batch sorption experiments. The sorption characteristics of the interactions between DCF and MoS2 in water were analyzed using a pseudo-second-order model, an intraparticle diffusion model and Boyd model to determine the sorption rate-determining steps. It was concluded that the sorption of DCF on MoS2 was fitted better by the pseudo-second-order model and that external diffusion governed the sorption process of DCF onto the MoS2. The interfacial interaction free energies between DCF and MoS2 in the sorption process can be calculated based on the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO). The flower-like MoS2 presenting excellent performance for removing DCF, could be a better choice of treating DCF-containing wastewaters.

  16. Location and coordination of promoter atoms in Co- and Ni-promoted MoS2-based hydrotreating catalysts

    DEFF Research Database (Denmark)

    Lauritsen, Jeppe V.; Kibsgaard, Jakob; Olesen, Georg H.

    2007-01-01

    In this study, we used scanning tunneling microscopy (STM) and density functional theory (DFT) to investigate the atomic-scale structure of the active Co- or Ni-promoted MoS2 nanoclusters in hydrotreating catalysts. Co-promoted MoS2 nanoclusters (Co–Mo–S) are found to adopt a hexagonal shape...... atoms. The findings may shed more light on the different selectivities observed for the Co- and Ni-promoted hydrotreating catalysts....

  17. Self-assembled MoS2–carbon nanostructures: influence of nanostructuring and carbon on lithium battery performance

    KAUST Repository

    Das, Shyamal K.

    2012-01-01

    Composites of MoS 2 and amorphous carbon are grown and self-assembled into hierarchical nanostructures via a hydrothermal method. Application of the composites as high-energy electrodes for rechargeable lithium-ion batteries is investigated. The critical roles of nanostructuring of MoS 2 and carbon composition on lithium-ion battery performance are highlighted. © 2012 The Royal Society of Chemistry.

  18. Photo-Promoted Platinum Nanoparticles Decorated MoS2@Graphene Woven Fabric Catalyst for Efficient Hydrogen Generation.

    Science.gov (United States)

    Li, Xiao; Zhang, Li; Zang, Xiaobei; Li, Xinming; Zhu, Hongwei

    2016-05-01

    Hydrogen production from water splitting has been considered as an effective and sustainable method to solve future energy related crisis. Molybdenum sulfides (e.g., MoS2) show promising catalytic ability in hydrogen evolution reaction (HER). Combining MoS2 with conductive carbon-based materials has aroused tremendous research interest recently. In this work, a highly efficient multiple-catalyst is developed for HER by decorating Pt nanoparticles (Pt NPs) on MoS2@graphene protected nickel woven fabrics (NiWF) substrate, which comprises the following components: (i) Graphene protected NiWF acts as the underlying substrate, supporting the whole structure; (ii) MoS2 nanoplates serve as a central and essential photosensitive component, forming a heterostructure with graphene simultaneously; and (iii) on the basis of the intrinsic photoluminescence effect of MoS2, together with the photoelectric response at the MoS2/graphene interface, Pt NPs are successfully deposited on the whole structure under illumination. Particularly and foremost, this work emphasizes on discussion and verification of the underlying mechanism for photopromoted electroless Pt NPs deposition. Due to this assembly approach, the usage amount of Pt is controlled at ∼5 wt % (∼0.59 at. %) with respect to the whole catalyst. MoS2@Substrate with Pt NPs deposited under 643 nm illumination, with the synergistic effect of MoS2 active sites and Pt NPs, demonstrates the most superior electrocatalytic performance, with negligible overpotential and low Tafel slope of 39.4 mV/dec.

  19. High-Performance Hydrogen Evolution from MoS2(1-x) P(x) Solid Solution.

    Science.gov (United States)

    Ye, Ruquan; del Angel-Vicente, Paz; Liu, Yuanyue; Arellano-Jimenez, M Josefina; Peng, Zhiwei; Wang, Tuo; Li, Yilun; Yakobson, Boris I; Wei, Su-Huai; Yacaman, Miguel Jose; Tour, James M

    2016-02-17

    A MoS2(1-x) P(x) solid solution (x = 0 to 1) is formed by thermally annealing mixtures of MoS2 and red phosphorus. The effective and stable electrocatalyst for hydrogen evolution in acidic solution holds promise for replacing scarce and expensive platinum that is used in present catalyst systems. The high performance originates from the increased surface area and roughness of the solid solution.

  20. Defect engineering of single- and few-layer MoS2 by swift heavy ion irradiation

    Science.gov (United States)

    Madauß, Lukas; Ochedowski, Oliver; Lebius, Henning; Ban-d'Etat, Brigitte; Naylor, Carl H.; Johnson, A. T. Charlie; Kotakoski, Jani; Schleberger, Marika

    2017-03-01

    We have investigated the possibility to use swift heavy ion irradiation for nano-structuring supported and freestanding ultra-thin MoS2 samples. Our comprehensive study of the ion-induced morphological changes in various MoS2 samples shows that depending on the irradiation parameters a multitude of extended defects can be fabricated. These range from chains of nano-hillocks in bulk-like MoS2, and foldings in single and bilayer MoS2, to unique nano-incisions in supported and freestanding single layers of MoS2. Our data reveals that the primary mechanism responsible for the incisions in the ultrathin supported samples is the indirect heating by the SiO2 substrate. We thus conclude that an energy of less than 2 keV per nm track length is sufficient to fabricate nano-incisions in MoS2 which is compatible with the use of the smallest accelerators.

  1. Schottky barrier contrasts in single and bi-layer graphene contacts for MoS2 field-effect transistors

    Science.gov (United States)

    Du, Hyewon; Kim, Taekwang; Shin, Somyeong; Kim, Dahye; Kim, Hakseong; Sung, Ji Ho; Lee, Myoung Jae; Seo, David H.; Lee, Sang Wook; Jo, Moon-Ho; Seo, Sunae

    2015-12-01

    We have investigated single- and bi-layer graphene as source-drain electrodes for n-type MoS2 transistors. Ti-MoS2-graphene heterojunction transistors using both single-layer MoS2 (1M) and 4-layer MoS2 (4M) were fabricated in order to compare graphene electrodes with commonly used Ti electrodes. MoS2-graphene Schottky barrier provided electron injection efficiency up to 130 times higher in the subthreshold regime when compared with MoS2-Ti, which resulted in VDS polarity dependence of device parameters such as threshold voltage (VTH) and subthreshold swing (SS). Comparing single-layer graphene (SG) with bi-layer graphene (BG) in 4M devices, SG electrodes exhibited enhanced device performance with higher on/off ratio and increased field-effect mobility (μFE) due to more sensitive Fermi level shift by gate voltage. Meanwhile, in the strongly accumulated regime, we observed opposing behavior depending on MoS2 thickness for both SG and BG contacts. Differential conductance (σd) of 1M increases with VDS irrespective of VDS polarity, while σd of 4M ceases monotonic growth at positive VDS values transitioning to ohmic-like contact formation. Nevertheless, the low absolute value of σd saturation of the 4M-graphene junction demonstrates that graphene electrode could be unfavorable for high current carrying transistors.

  2. Structural and optical properties analysis of MoS2 nanoflakes on quartz substrate as prepared by mechanical exfoliation

    Science.gov (United States)

    Jaka Adilla, Sandy; Nurfani, Eka; Kurniawan, Robi; Dimas Satrya, Christoforus; Darma, Yudi

    2017-07-01

    We study the structural and optical properties of MoS2 nanoflakes on quartz substrate as prepared by mechanical exfoliation method. The structural and morphological properties of MoS2 nanoflakes were characterized by SEM, EDS, and XRD, while the high-resolution spectroscopic ellipsometry (SE) with the photon energy of 1.27 to 6.53 eV is used to study its optical characteristics. As the results, SEM data shows that MoS2 appears to be nanoflakes covering around 25-35% on the surface of quartz substrate, and XRD spectra shows the dominant orientation along c-axis (002). Based on spectroscopic ellipsometry analysis, the average thickness of MoS2 nanoflakes is around 12 nm or about 6-8 layers. By using Tauc plot method, we confirm that MoS2 nanoflakes have a semiconductor characteristic with the optical bandgap as high as 1.68 eV. Our study shows the important of structural and optical properties of MoS2 nanoflakes that can be utilized for future optoelectronic devices and energy-harvesting purposes.

  3. Electric field modulation of the band structure in MoS2/WS2 van der waals heterostructure

    Science.gov (United States)

    Li, Wei; Wang, Tianxing; Dai, Xianqi; Wang, Xiaolong; Zhai, Caiyun; Ma, Yaqiang; Chang, Shanshan; Tang, Yanan

    2017-01-01

    Using density functional theory calculations, we investigate the bandstructure of MoS2/WS2 van der waals heterostructure by applying external electric field perpendicular to the layers. It is demonstrated that the MoS2/WS2 is a type-II heterostructure, and therefore the electrons and holes are spatially separated. The band gap of MoS2/WS2 heterostructure continuously decreases with increasing external electric field, eventually a transition from semiconductor to metal is observed. Applying external electric field along +z direction and -z directions has different effects on the band gap due to the intrinsic spontaneous polarization in MoS2/WS2 heterostructure. The calculated result indicates that the band inversion in MoS2/WS2 heterostructure can be induced by changing the strength of the external electric field. The external electric field can significantly tune the band offsets almost linearly and modify the band alignment between MoS2 and WS2. The present study would open a new avenue for application of such transition-metal dichalcogenides heterostructures in future nano- and optoelectronics.

  4. Band Gap Modulation of Bilayer MoS2 Under Strain Engineering and Electric Field: A Density Functional Theory

    Science.gov (United States)

    Nguyen, Chuong V.; Hieu, Nguyen N.; Ilyasov, Victor V.

    2016-08-01

    In this work, we investigate band-gap tuning in bilayer MoS2 by an external electric field and by applied biaxial strain. Our calculations show that the band gaps of bilayer MoS2 can be tuned by the perpendicular electric field or biaxial strain. The band gaps of bilayer MoS2 decrease with increasing applied electric field or biaxial strain. When the electric field was introduced, electronic levels are split due to the separation of the valence sub-band and the conduction sub-band states. Our calculations also show that the change in the band gap of bilayer MoS2 is due to the separation of electronic levels by electric field via the Stark effect. At the electric field E_{Field} = 5.5 V/nm or biaxial strain ɛ = 15%, bilayer MoS2 becomes metallic. The semiconductor-metal phase transition in bilayer MoS2 plays an important role in its application for nanodevices.

  5. Structure and Electronic Properties of In Situ Synthesized Single-Layer MoS2 on a Gold Surface

    DEFF Research Database (Denmark)

    Sørensen, Signe Grønborg; Füchtbauer, Henrik Gøbel; Tuxen, Anders Kyrme;

    2014-01-01

    When transition metal sulfides such as MoS2 are present in the single-layer form, the electronic properties change in fundamental ways, enabling them to be used, e.g., in two-dimensional semiconductor electronics, optoelectronics, and light harvesting. The change is related to a subtle modification...... of the MoS2 layer resulting in a characteristic moiré pattern. X-ray photoelectron spectroscopy indicates that the system develops the characteristics of n-doped MoS2 due to electron donation. Scanning tunneling spectroscopy furthermore reflects a convolution of MoS2 and Au donor states where the MoS2 band...... structure appears modified at the band gap edges. This electronic effect is further modulated by the moiré periodicity and leads to small substrate-induced electronic perturbations near the conduction band minimum in the band gap of MoS2. The results may be highly relevant in the context of nanopatterned...

  6. MoS2 Quantum Dots with a Tunable Work Function for High-Performance Organic Solar Cells.

    Science.gov (United States)

    Xing, Wang; Chen, Yusheng; Wang, Xinlong; Lv, Lei; Ouyang, Xinhua; Ge, Ziyi; Huang, Hui

    2016-10-12

    An efficient hole extraction layer (HEL) is critical to achieve high-performance organic solar cells (OSCs). In this study, we developed a pinhole-free and efficient HEL based on MoS2 quantum dots (QDs) combined with UV-ozone (UVO) treatment. The optophysical properties and morphology of MoS2 QDs and their photovoltaic performance are investigated. The results showed that MoS2 QDs can form homogeneous films and can be applied as an interfacial layer not only for donors with shallow highest occupied molecular orbital (HOMO) but also for those with deep HOMO energy levels after UVO treatment (O-MoS2 QDs). The solar cells based on O-MoS2 QDs yield a power conversion efficiency (PCE) of 8.66%, which is 71% and 12% higher than those of the OSCs with pristine MoS2 QD and O-MoS2 nanosheets, respectively, and the highest PCEs for OSCs containing MoS2 materials. Furthermore, the stability of solar cells based on MoS2 QDs is greatly improved in comparison with state-of-the-art PEDOT:PSS. These results demonstrate the great potential of O-MoS2 QDs as an efficient HEL for high-performance OSCs.

  7. Broad-Band Photocurrent Enhancement in MoS2 Layers Directly Grown on Light-Trapping Si Nanocone Arrays.

    Science.gov (United States)

    Cho, Yunae; Cho, Byungjin; Kim, Yonghun; Lee, Jihye; Kim, Eunah; Nguyen, Trang Thi Thu; Lee, Ju Hyun; Yoon, Seokhyun; Kim, Dong-Ho; Choi, Jun-Hyuk; Kim, Dong-Wook

    2017-02-22

    There has been growing research interest in realizing optoelectronic devices based on the two-dimensional atomically thin semiconductor MoS2 owing to its distinct physical properties that set it apart from conventional semiconductors. However, there is little optical absorption in these extremely thin MoS2 layers, which presents an obstacle toward applying them for use in high-efficiency light-absorbing devices. We synthesized trilayers of MoS2 directly on SiO2/Si nanocone (NC) arrays using chemical vapor deposition and investigated their photodetection characteristics. The photoresponsivity of the MoS2/NC structure was much higher than that of the flat counterpart across the whole visible wavelength range (for example, it was almost an order of magnitude higher at λ = 532 nm). Strongly concentrated light near the surface that originated from a Fabry-Perot interference in the SiO2 thin layers and a Mie-like resonance caused by the Si NCs boosted the optical absorption in MoS2. Our work demonstrates that MoS2/NC structures could provide a useful means to realize high-performance optoelectronic devices.

  8. Growth and Tunable Surface Wettability of Vertical MoS2 Layers for Improved Hydrogen Evolution Reactions.

    Science.gov (United States)

    Bhimanapati, Ganesh R; Hankins, Trevor; Lei, Yu; Vilá, Rafael A; Fuller, Ian; Terrones, Mauricio; Robinson, Joshua A

    2016-08-31

    Layered materials, especially the transition metal dichalcogenides (TMDs), are of interest for a broad range of applications. Among the class of TMDs, molybdenum disulfide (MoS2) is perhaps the most studied because of its natural abundance and use in optoelectronics, energy storage and energy conversion applications. Understanding the fundamental structure-property relations is key for tailoring the enhancement in the above-mentioned applications. Here, we report a controlled powder vaporization synthesis of MoS2 flower-like structures consisting of vertically grown layers of MoS2 exhibiting exposed edges. This growth is readily achievable on multiple substrates, such as graphite, silicon, and silicon dioxide. The resulting MoS2 flowers are highly crystalline and stoichiometric. Further observations using contact angle indicate that MoS2 flowers exhibit the highest reported contact angle of ∼160 ± 10°, making the material super hydrophobic. This surface wettability was further tuned by changing the edge chemistry of the MoS2 flowers using an ozone etching treatment. Hydrogen evolution reaction (HER) measurements indicate that the surface treated with UV-ozone showed a reduction in the Tafel slope from 185 to 54 mV/dec, suggesting an increase in the amount of reactive surface to generate hydrogen.

  9. Covalent Nitrogen Doping and Compressive Strain in MoS2 by Remote N2 Plasma Exposure.

    Science.gov (United States)

    Azcatl, Angelica; Qin, Xiaoye; Prakash, Abhijith; Zhang, Chenxi; Cheng, Lanxia; Wang, Qingxiao; Lu, Ning; Kim, Moon J; Kim, Jiyoung; Cho, Kyeongjae; Addou, Rafik; Hinkle, Christopher L; Appenzeller, Joerg; Wallace, Robert M

    2016-09-14

    Controllable doping of two-dimensional materials is highly desired for ideal device performance in both hetero- and p-n homojunctions. Herein, we propose an effective strategy for doping of MoS2 with nitrogen through a remote N2 plasma surface treatment. By monitoring the surface chemistry of MoS2 upon N2 plasma exposure using in situ X-ray photoelectron spectroscopy, we identified the presence of covalently bonded nitrogen in MoS2, where substitution of the chalcogen sulfur by nitrogen is determined as the doping mechanism. Furthermore, the electrical characterization demonstrates that p-type doping of MoS2 is achieved by nitrogen doping, which is in agreement with theoretical predictions. Notably, we found that the presence of nitrogen can induce compressive strain in the MoS2 structure, which represents the first evidence of strain induced by substitutional doping in a transition metal dichalcogenide material. Finally, our first principle calculations support the experimental demonstration of such strain, and a correlation between nitrogen doping concentration and compressive strain in MoS2 is elucidated.

  10. The origin of the enhanced performance of nitrogen-doped MoS2 in lithium ion batteries.

    Science.gov (United States)

    Liu, Qiuhong; Weijun, Xia; Wu, Zhenjun; Huo, Jia; Liu, Dongdong; Wang, Qiang; Wang, Shuangyin

    2016-04-29

    MoS2 with a similar layered structure to graphene has been widely applied in various areas including lithium ion batteries. However, low conductivity, capacity fading and poor rate performance are still the main challenges for MoS2 anode materials. In this work, for the first time, we prepared nitrogen-doped MoS2 (N-MoS2) nanosheets through a simple two-step method involving the preparation of MoS2 with defects by the hydrothermal method, followed by sintering in a NH3 atmosphere. Our electrochemical characterizations and density functional theory calculations demonstrated that nitrogen doping could enhance the electron conductivity and showed higher specific capacity than pristine MoS2 as anode materials of lithium ion batteries, which can be attributed to the faster transportation of electrons and ions because of nitrogen doping. This work helps us understand the origin of the enhanced performance of N-doped MoS2 in lithium ion batteries.

  11. Thickness dependence of surface energy and contact angle of water droplets on ultrathin MoS2 films.

    Science.gov (United States)

    Guo, Yanhua; Wang, Zhengfei; Zhang, Lizhi; Shen, Xiaodong; Liu, Feng

    2016-06-01

    We have performed a systematic density functional study of surface energy of MoS2 films as a function of thickness from one to twelve layers with the consideration of van der Waals (vdW) interactions using the vdW-DF and DFT-D2 methods. Both vdW schemes show that the surface energy will increase with the increase of the number of atomic layers and converge to a constant value at about six layers. Based on the calculated surface energies, we further analyze the surface contact angle of water droplets on the MoS2 film surface using Young's equation as a function of thickness in comparison with experiments, from which the water-MoS2 interfacial energy is derived to be independent of MoS2 thickness. Our calculations indicate that the vdW interactions between the MoS2 layers play an important role in determining surface energy, and results in the thickness dependence of the contact angle of water droplets on the MoS2 film surface. Our results explain well the recent wetting experiment [Nano Lett., 2014, 14(8), 4314], and will be useful for future studies of physical and chemical properties of ultrathin MoS2 films.

  12. Structure and Electronic Properties of In Situ Synthesized Single-Layer MoS2 on a Gold Surface

    DEFF Research Database (Denmark)

    Sørensen, Signe Grønborg; Füchtbauer, Henrik Gøbel; Tuxen, Anders Kyrme

    2014-01-01

    with scanning tunneling microscopy and X-ray photoelectron spectroscopy characterization of two-dimensional single-layer islands of MoS2 synthesized directly on a gold single crystal substrate. Thanks to a periodic modulation of the atom stacking induced by the lattice mismatch, we observe a structural buckling...... of the MoS2 layer resulting in a characteristic moiré pattern. X-ray photoelectron spectroscopy indicates that the system develops the characteristics of n-doped MoS2 due to electron donation. Scanning tunneling spectroscopy furthermore reflects a convolution of MoS2 and Au donor states where the MoS2 band......When transition metal sulfides such as MoS2 are present in the single-layer form, the electronic properties change in fundamental ways, enabling them to be used, e.g., in two-dimensional semiconductor electronics, optoelectronics, and light harvesting. The change is related to a subtle modification...

  13. Emergence of Dirac and quantum spin Hall states in fluorinated monolayer As and AsSb

    KAUST Repository

    Zhang, Qingyun

    2016-01-21

    Using first-principles calculations, we investigate the electronic and vibrational properties of monolayer As and AsSb. While the pristine monolayers are semiconductors (direct band gap at the Γ point), fluorination results in Dirac cones at the K points. Fluorinated monolayer As shows a band gap of 0.16 eV due to spin-orbit coupling, and fluorinated monolayer AsSb a larger band gap of 0.37 eV due to inversion symmetry breaking. Spin-orbit coupling induces spin splitting similar to monolayer MoS2. Phonon calculations confirm that both materials are dynamically stable. Calculations of the edge states of nanoribbons by the tight-binding method demonstrate that fluorinated monolayer As is topologically nontrivial in contrast to fluorinated monolayer AsSb.

  14. Facile preparation of MoS2 based polymer composites via mussel inspired chemistry and their high efficiency for removal of organic dyes

    Science.gov (United States)

    Huang, Qiang; Liu, Meiying; Chen, Junyu; Wan, Qing; Tian, Jianwen; Huang, Long; Jiang, Ruming; Wen, Yuanqing; Zhang, Xiaoyong; Wei, Yen

    2017-10-01

    Molybdenum disulfide (MoS2) is a novel type of two-dimensional nanomaterial, which has attracted great research attention for its excellent physicochemical properties and possible applications. In this work, we prepared a novel MoS2 composite (MoS2-PDOPA) through the self-polymerization of levodopa (DOPA) on the surface of MoS2 under a weak alkaline solution. The obtained samples, including pure MoS2 and MoS2-PDOPA composite were characterized by energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) analyses. The potential environmental applications of MoS2-PDOPA were evaluated by using MoS2-PDOPA as adsorbent to remove methylene blue (MB) from aqueous solution. Batch experiments were carried out to investigate the effect of various operational parameters such as contact time, initial MB concentration, solution pH and temperature on the adsorption of MB by MoS2-PDOPA. According to the adsorption kinetics, isotherms and thermodynamics analysis, the MB adsorption onto MoS2-PDOPA follows the intraparticle diffusion model and Langmuir isotherm model, and the MB adsorption process is spontaneous and endothermic. The maximum adsorption capacity of MoS2-PDOPA is calculated to be 244.03 mg/g at 298 K. As compared with unmodified MoS2, the adsorption capacity of MoS2-PDOPA is obviously improved. Taken together, we developed a facile method to prepare MoS2-PDOPA composites based on mussel inspired chemistry. The resultant composites could be utilized as efficient adsorbents with great potential for environmental adsorption applications.

  15. Limits of Exciton-Exciton Annihilation for Light Emission in Transition Metal Dichalcogenide Monolayers

    CERN Document Server

    Yu, Yiling; Xu, Chao; Barrette, Andy; Gundogdu, Kenan; Cao, Linyou

    2015-01-01

    We quantitatively evaluate the exciton-exciton annihilation (EEA) and its effect on light emission properties in monolayer TMDC materials, including WS2, MoS2, and WSe2. The EEA rate is found to be 0.3 cm2/s and 0.1 cm2/s for suspended WS2 and MoS2 monolayers, respectively, and subject to the influence from substrates, being 0.1 cm2/s and 0.05 cm2/s for the supported WS2 and MoS2 on sapphire substrates. It can substantially affect the luminescence efficiency of suspended monolayers even at an exciton concentration as low as 109 cm-2, but plays a milder role for supported monolayers due to the effect of the substrate. However, regardless the presence of substrates or not, the lasing threshold of the monolayer is always predominantly determined by the EEA, which is estimated to be 12-18 MW/cm2 if using 532 nm as the pumping wavelength.

  16. Pressure-dependent optical and vibrational properties of monolayer molybdenum disulfide

    KAUST Repository

    Nayak, Avinash P.

    2015-01-14

    Controlling the band gap by tuning the lattice structure through pressure engineering is a relatively new route for tailoring the optoelectronic properties of two-dimensional (2D) materials. Here, we investigate the electronic structure and lattice vibrational dynamics of the distorted monolayer 1T-MoS2 (1T′) and the monolayer 2H-MoS2 via a diamond anvil cell (DAC) and density functional theory (DFT) calculations. The direct optical band gap of the monolayer 2H-MoS2 increases by 11.7% from 1.85 to 2.08 eV, which is the highest reported for a 2D transition metal dichalcogenide (TMD) material. DFT calculations reveal a subsequent decrease in the band gap with eventual metallization of the monolayer 2H-MoS2, an overall complex structure-property relation due to the rich band structure of MoS2. Remarkably, the metastable 1T′-MoS2 metallic state remains invariant with pressure, with the J2, A1g, and E2g modes becoming dominant at high pressures. This substantial reversible tunability of the electronic and vibrational properties of the MoS2 family can be extended to other 2D TMDs. These results present an important advance toward controlling the band structure and optoelectronic properties of monolayer MoS2 via pressure, which has vital implications for enhanced device applications.

  17. Interface thermal conductance of van der Waals monolayers on amorphous substrates

    Science.gov (United States)

    Correa, Gabriela C.; Foss, Cameron J.; Aksamija, Zlatan

    2017-03-01

    Heterostructures based on atomic monolayers are emerging as leading materials for future energy efficient and multifunctional electronics. Due to the single atom thickness of monolayers, their properties are strongly affected by interactions with the external environment. We develop a model for interface thermal conductance (ITC) in an atomic monolayer van der Waals bonded to a disordered substrate. Graphene on SiO2 is initially used in our model and contrasted against available experimental data; the model is then applied to monolayer molybdenum disulfide (MoS2) on SiO2 substrate. Our findings show the dominant carrier of heat in both graphene and MoS2 in the cross-plane direction is the flexural (ZA) phonon mode, owing to the large overlap between graphene ZA and substrate vibrational density of states. The rate of phonon transfer across the interface depends quadratically on the substrate coupling constant K a , but this interaction also causes a lifting of the lowest flexural phonon modes. As a result, ITC depends roughly linearly on the strength of the coupling between a monolayer and its substrate. We conclude that, in both graphene and MoS2 on SiO2, substrate adhesion plays a strong role in determining ITC, requiring further study of substrate coupling in TMDCs.

  18. Gate-tunable and thickness-dependent electronic and thermoelectric transport in few-layer MoS2

    Science.gov (United States)

    Kayyalha, Morteza; Maassen, Jesse; Lundstrom, Mark; Shi, Li; Chen, Yong P.

    2016-10-01

    Over the past few years, there has been a growing interest in layered transition metal dichalcogenides such as molybdenum disulfide (MoS2). Most studies so far have focused on the electronic and optoelectronic properties of single-layer MoS2, whose band structure features a direct bandgap, in sharp contrast to the indirect bandgap of thicker MoS2. In this paper, we present a systematic study of the thickness-dependent electrical and thermoelectric properties of few-layer MoS2. We observe that the electrical conductivity ( σ) increases as we reduce the thickness of MoS2 and peaks at about two layers, with six-times larger conductivity than our thickest sample (23-layer MoS2). Using a back-gate voltage, we modulate the Fermi energy ( E F ) of the sample where an increase in the Seebeck coefficient ( S ) is observed with decreasing gate voltage ( E F ) towards the subthreshold (OFF state) of the device, reaching as large as 500 μ V / K in a four-layer MoS2. While previous reports have focused on a single-layer MoS2 and measured Seebeck coefficient in the OFF state, which has vanishing electrical conductivity and thermoelectric power factor ( P F = S 2 σ ), we show that MoS2-based devices in their ON state can have P F as large as > 50 /μ W cm K 2 in the two-layer sample. The P F increases with decreasing thickness and then drops abruptly from double-layer to single-layer MoS2, a feature we suggest as due to a change in the energy dependence of the electron mean-free-path according to our theoretical calculation. Moreover, we show that care must be taken in thermoelectric measurements in the OFF state to avoid obtaining erroneously large Seebeck coefficients when the channel resistance is very high. Our study paves the way towards a more comprehensive examination of the thermoelectric performance of two-dimensional (2D) semiconductors.

  19. Thermally Driven Pure Spin and Valley Currents via the Anomalous Nernst Effect in Monolayer Group-VI Dichalcogenides

    DEFF Research Database (Denmark)

    Yu, Xiao-Qin; Zhu, Zhen-Gang; Su, Gang;

    2015-01-01

    The spin and valley-dependent anomalous Nernst effects are analyzed for monolayer MoS2 and other group-VI dichalcogenides. We find that pure spin and valley currents can be generated perpendicular to the applied thermal gradient in the plane of these two-dimensional materials. This effect provide...

  20. MOS 2.0: Modeling the Next Revolutionary Mission Operations System

    Science.gov (United States)

    Delp, Christopher L.; Bindschadler, Duane; Wollaeger, Ryan; Carrion, Carlos; McCullar, Michelle; Jackson, Maddalena; Sarrel, Marc; Anderson, Louise; Lam, Doris

    2011-01-01

    Designed and implemented in the 1980's, the Advanced Multi-Mission Operations System (AMMOS) was a breakthrough for deep-space NASA missions, enabling significant reductions in the cost and risk of implementing ground systems. By designing a framework for use across multiple missions and adaptability to specific mission needs, AMMOS developers created a set of applications that have operated dozens of deep-space robotic missions over the past 30 years. We seek to leverage advances in technology and practice of architecting and systems engineering, using model-based approaches to update the AMMOS. We therefore revisit fundamental aspects of the AMMOS, resulting in a major update to the Mission Operations System (MOS): MOS 2.0. This update will ensure that the MOS can support an increasing range of mission types, (such as orbiters, landers, rovers, penetrators and balloons), and that the operations systems for deep-space robotic missions can reap the benefits of an iterative multi-mission framework.12 This paper reports on the first phase of this major update. Here we describe the methods and formal semantics used to address MOS 2.0 architecture and some early results. Early benefits of this approach include improved stakeholder input and buy-in, the ability to articulate and focus effort on key, system-wide principles, and efficiency gains obtained by use of well-architected design patterns and the use of models to improve the quality of documentation and decrease the effort required to produce and maintain it. We find that such methods facilitate reasoning, simulation, analysis on the system design in terms of design impacts, generation of products (e.g., project-review and software-delivery products), and use of formal process descriptions to enable goal-based operations. This initial phase yields a forward-looking and principled MOS 2.0 architectural vision, which considers both the mission-specific context and long-term system sustainability.

  1. Rapid water disinfection using vertically aligned MoS2 nanofilms and visible light

    Science.gov (United States)

    Liu, Chong; Kong, Desheng; Hsu, Po-Chun; Yuan, Hongtao; Lee, Hyun-Wook; Liu, Yayuan; Wang, Haotian; Wang, Shuang; Yan, Kai; Lin, Dingchang; Maraccini, Peter A.; Parker, Kimberly M.; Boehm, Alexandria B.; Cui, Yi

    2016-12-01

    Solar energy is readily available in most climates and can be used for water purification. However, solar disinfection of drinking water mostly relies on ultraviolet light, which represents only 4% of the total solar energy, and this leads to a slow treatment speed. Therefore, the development of new materials that can harvest visible light for water disinfection, and so speed up solar water purification, is highly desirable. Here we show that few-layered vertically aligned MoS2 (FLV-MoS2) films can be used to harvest the whole spectrum of visible light (∼50% of solar energy) and achieve highly efficient water disinfection. The bandgap of MoS2 was increased from 1.3 to 1.55 eV by decreasing the domain size, which allowed the FLV-MoS2 to generate reactive oxygen species (ROS) for bacterial inactivation in the water. The FLV-MoS2 showed a ∼15 times better log inactivation efficiency of the indicator bacteria compared with that of bulk MoS2, and a much faster inactivation of bacteria under both visible light and sunlight illumination compared with the widely used TiO2. Moreover, by using a 5 nm copper film on top of the FLV-MoS2 as a catalyst to facilitate electron-hole pair separation and promote the generation of ROS, the disinfection rate was increased a further sixfold. With our approach, we achieved water disinfection of >99.999% inactivation of bacteria in 20 min with a small amount of material (1.6 mg l-1) under simulated visible light.

  2. Back-gated Nb-doped MoS2 junctionless field-effect-transistors

    Directory of Open Access Journals (Sweden)

    Gioele Mirabelli

    2016-02-01

    Full Text Available Electrical measurements were carried out to measure the performance and evaluate the characteristics of MoS2 flakes doped with Niobium (Nb. The flakes were obtained by mechanical exfoliation and transferred onto 85 nm thick SiO2 oxide and a highly doped Si handle wafer. Ti/Au (5/45 nm deposited on top of the flake allowed the realization of a back-gate structure, which was analyzed structurally through Scanning Electron Microscopy (SEM and Transmission Electron Microscopy (TEM. To best of our knowledge this is the first cross-sectional TEM study of exfoliated Nb-doped MoS2 flakes. In fact to date TEM of transition-metal-dichalcogenide flakes is extremely rare in the literature, considering the recent body of work. The devices were then electrically characterized by temperature dependent Ids versus Vds and Ids versus Vbg curves. The temperature dependency of the device shows a semiconductor behavior and, the doping effect by Nb atoms introduces acceptors in the structure, with a p-type concentration 4.3 × 1019 cm−3 measured by Hall effect. The p-type doping is confirmed by all the electrical measurements, making the structure a junctionless transistor. In addition, other parameters regarding the contact resistance between the top metal and MoS2 are extracted thanks to a simple Transfer Length Method (TLM structure, showing a promising contact resistivity of 1.05 × 10−7 Ω/cm2 and a sheet resistance of 2.36 × 102 Ω/sq.

  3. Electronic and dielectric properties of MoS2-MoX2 heterostructures

    Science.gov (United States)

    Sharma, Munish; Jamdagni, Pooja; Kumar, Ashok; Ahluwalia, P. K.

    2015-05-01

    We present a comparative study of electronic and dielectric properties of MoS2-MoX2 heteostructures (where X=S, Se, Te) within the framework of density functional theory (DFT). Electronic band structure, real & imaginary part of dielectric function, electron energy loss spectra and static dielectric constant have been calculated for each system and compared with one another. A systematic decrease/increase in band gap/static dielectric constant is observed as the X changes from S to Te. These results provide a physical basis for the potential applications of these heterostructures in optoelectronic devices.

  4. MoS2-Filled PEEK Composite as a Self-Lubricating Material for Aerospace Applications

    Science.gov (United States)

    Theiler, Geraldine; Gradt, Thomas

    2010-01-01

    At BAM, several projects were conducted in the past years dealing with the tribological properties of friction couples at cryogenic temperature and in vacuum environment. Promising candidates for vacuum application are MoS2-filled PEEK/PTFE composites, which showed a friction coefficient as low as 0.03 in high vacuum. To complete the tribological profile of these composites, further tests were performed in ultra-high vacuum (UHV) at room temperature. In this paper, friction and stick slip behavior, as well as outgassing characteristics during the test are presented.

  5. Rotationally Commensurate Growth of MoS2 on Epitaxial Graphene

    Science.gov (United States)

    2015-11-13

    532 nm (with spot size ∼1 μm2) in ambient conditions. The Raman signal was collected using a 100× Olympus objective (NA = 0.9) and dispersed by 1800...Emery, J. D.; Lipson, A. L.; Bedzyk, M. J.; Elam, J. W.; Pellin, M. J.; Hersam, M. C. Seeding Atomic Layer Deposition of High-k Dielectrics on...Amani, M.; Najmaei, S.; Xu, Q.; Zou, X.; Zhou, W.; Yu, T.; Qiu, C.; Birdwell, A. G.; Crowne, F. J.; et al. Strain and Structure Heterogeneity in MoS2

  6. Electronic and photo-electronic transport in sputter deposited MoS2 film

    Science.gov (United States)

    Wasala, Milinda; Ghosh, Sujoy; Zhang, Jie; Richie, Julianna; Mazumdar, Dipanjan; Kar, Swastik; Talapatra, Saikat

    2015-03-01

    Here we report on the electrical transport as well as photo response of large area sputter deposited few-layers of thin MoS2. Temperature dependent (55 K -275K) electronic conductivity measured on these samples show evidence of 2D Variable Range Hopping (2D-VRH) mechanism within 100K-275K. Photoconductivity measurements investigated using a continuous laser of λ = 658nm (E =1.88eV), over a broad range of illuminating laser intensity, P (0.19 μW OISE-0968405.

  7. Silicene on MoS2: role of the van der Waals interaction

    KAUST Repository

    Jiajie Zhu,

    2015-10-13

    We demonstrate for silicene on MoS2 substrate the limitations of the predictive power of first principles calculations based on van der Waals density functional theory. Only the optB86b-vdW functional is found to give reasonable agreement with experimental results on structural properties, while for all other investigated functionals the interlayer interaction is underestimated or the charge redistribution at the interface is not described correctly so that the predicted electronic structure is qualitatively wrong. © 2015 IOP Publishing Ltd.

  8. Titanium plate supported MoS2 nanosheet arrays for supercapacitor application

    Science.gov (United States)

    Wang, Lina; Ma, Ying; Yang, Min; Qi, Yanxing

    2017-02-01

    A promising new concept is to apply binder-free supercapacitor electrode by directly growing active materials on current collectors. However, there are many challenges to be solved, such as fabrication of well quality electronic contact and good mechanical stability films through a simple and feasible method. In this study, MoS2 nanosheet arrays supported on titanium plate has been synthesized by a hydrothermal method without other additives, surface active agents and toxic reagents. As the supercapacitor electrode, a good capacitance of 133 F g-1 is attained at a discharge current density of 1 A g-1. The specific energy density is 11.11 Wh kg-1 at a power density of 0.53 kW kg-1. Moreover, the electrode shows an excellent cyclic stability. The loss of capacity is only 7% even after 1000 cycles. In addition, the formation mechanism is proposed. The facile method of fabricating MoS2 nanosheet arrays on titanium plate affords an green and effective way to prepare other metal sulfides for the application in electrochemical capacitors.

  9. Transport properties of chemically synthesized MoS2 - Dielectric effects and defects scattering

    Science.gov (United States)

    Mongillo, Massimo; Chiappe, Daniele; Arutchelvan, Goutham; Asselberghs, Inge; Perucchini, Marta; Manfrini, Mauricio; Lin, Dennis; Huyghebaert, Cedric; Radu, Iuliana

    2016-12-01

    We report on the electrical characterization of synthetic, large-area MoS2 layers obtained by the sulfurization technique. The effects of dielectric encapsulation and localized defect states on the intrinsic transport properties are explored with the aid of temperature-dependent measurements. We study the effect of dielectric environment by transferring as-grown MoS2 films into different dielectrics such as SiO2, Al2O3, HfO2, and ZrO2 with increasing dielectric permittivity. Electrical data are collected on a statistically-relevant device ensemble and allow to assess device performances on a large scale assembly. Our devices show relative in-sensitiveness of mobility with respect to dielectric encapsulation. We conclude that the device behavior is strongly affected by several scattering mechanisms of different origin that can completely mask any effect related to dielectric mismatch. At low temperatures, conductivity of the devices is thermally activated, a clear footprint of the existence of a mobility edge separating extended states in the conduction band from impurity states in the band-gap.

  10. Charge neutral MoS2 field effect transistors through oxygen plasma treatment

    Science.gov (United States)

    Dhall, Rohan; Li, Zhen; Kosmowska, Ewa; Cronin, Stephen B.

    2016-11-01

    Lithographically fabricated MoS2 field effect transistors suffer from several critical imperfections, including low sub-threshold swings, large turn-on gate voltages (VT), and wide device-to-device variability. The large magnitude and variability of VT stems from unclean interfaces, trapped charges in the underlying substrate, and sulfur vacancies created during the mechanical exfoliation process. In this study, we demonstrate a simple and reliable oxygen plasma treatment, which mitigates the effects of unintentional doping created by surface defect sites, such as S vacancies, and surface contamination. This plasma treatment restores charge neutrality to the MoS2 and shifts the threshold turn-on voltage towards 0 V. Out of the 10 devices measured, all exhibit a shift of the FET turn-on voltage from an average of -18 V to -2 V. The oxygen plasma treatment passivates these defects, which reduces surface scattering, causing increased mobility and improved subthreshold swing. For as-prepared devices with low mobilities (˜0.01 cm2/V s), we observe up to a 190-fold increase in mobility after exposure to the oxygen plasma. Perhaps the most important aspect of this oxygen plasma treatment is that it reduces the device-to-device variability, which is a crucial factor in realizing any practical application of these devices.

  11. Controlled synthesis of 2D transition metal dichalcogenides: from vertical to planar MoS2

    Science.gov (United States)

    Zhang, Fu; Momeni, Kasra; Abu AlSaud, Mohammed; Azizi, Amin; Hainey, Mel F., Jr.; Redwing, Joan M.; Chen, Long-Qing; Alem, Nasim

    2017-06-01

    Among post-graphene two dimensional (2D) materials, transition metal dichalcogenides (TMDs, such as MoS2) have attracted significant attention due to their superior properties for potential electronic, optoelectronic and energy applications. Scalable and controllable powder vapor transport (PVT) methods have been developed to synthesize 2D MoS2 with controllable morphologies (i.e. horizontal and vertical), yet the growth mechanism for the transition from horizontal to vertical orientation is not clearly understood. Here, we combined experimental and numerical modeling studies to investigate the key growth parameters that govern the morphology of 2D materials. The transition from vertical to horizontal growth is achieved by controlling the magnitude and distribution of the precursor concentration by placing the substrate at different orientations and locations relative to the source. We have also shown that the density of as-grown nanostructures can be controlled by the local precursor-containing gas flow rate. This study demonstrates the possibility for engineering the morphology of 2D materials by controlling the concentration of precursors and flow profiles, and provides a new path for controllable growth of 2D TMDs for various applications.

  12. Charge-Transfer Induced High Efficient Hydrogen Evolution of MoS2/graphene Cocatalyst

    Science.gov (United States)

    Li, Honglin; Yu, Ke; Li, Chao; Tang, Zheng; Guo, Bangjun; Lei, Xiang; Fu, Hao; Zhu, Ziqiang

    2015-12-01

    The MoS2 and reduced graphite oxide (rGO) composite has attracted intensive attention due to its favorable performance as hydrogen evolution reaction (HER) catalyst, but still lacking is the theoretical understanding from a dynamic perspective regarding to the influence of electron transfer, as well as the connection between conductivity and the promoted HER performance. Based on the first-principles calculations, we here clearly reveal how an excess of negative charge density affects the variation of Gibbs free energy (ΔG) and the corresponding HER behavior. It is demonstrated that the electron plays a crucial role in the HER routine. To verify the theoretical analyses, the MoS2 and reduced graphite oxide (rGO) composite with well defined 3-dimensional configuration was synthesized via a facile one-step approach for the first time. The experimental data show that the HER performance have a direct link to the conductivity. These findings pave the way for a further developing of 2-dimension based composites for HER applications.

  13. Tunable Molecular MoS2 Edge-Site Mimics for Catalytic Hydrogen Production.

    Science.gov (United States)

    Garrett, Benjamin R; Polen, Shane M; Click, Kevin A; He, Mingfu; Huang, Zhongjie; Hadad, Christopher M; Wu, Yiying

    2016-04-18

    Molybdenum sulfides represent state-of-the-art, non-platinum electrocatalysts for the hydrogen evolution reaction (HER). According to the Sabatier principle, the hydrogen binding strength to the edge active sites should be neither too strong nor too weak. Therefore, it is of interest to develop a molecular motif that mimics the catalytic sites structurally and possesses tunable electronic properties that influence the hydrogen binding strength. Furthermore, molecular mimics will be important for providing mechanistic insight toward the HER with molybdenum sulfide catalysts. In this work, a modular method to tune the catalytic properties of the S-S bond in MoO(S2)2L2 complexes is described. We studied the homogeneous electrocatalytic hydrogen production performance metrics of three catalysts with different bipyridine substitutions. By varying the electron-donating abilities, we present the first demonstration of using the ligand to tune the catalytic properties of the S-S bond in molecular MoS2 edge-site mimics. This work can shed light on the relationship between the structure and electrocatalytic activity of molecular MoS2 catalysts and thus is of broad importance from catalytic hydrogen production to biological enzyme functions.

  14. Nonlinear mode coupling and internal resonances in MoS2 nanoelectromechanical system

    Science.gov (United States)

    Samanta, C.; Yasasvi Gangavarapu, P. R.; Naik, A. K.

    2015-10-01

    Atomically thin two dimensional (2D) layered materials have emerged as a new class of material for nanoelectromechanical systems (NEMS) due to their extraordinary mechanical properties and ultralow mass density. Among them, graphene has been the material of choice for nanomechanical resonator. However, recent interest in 2D chalcogenide compounds has also spurred research in using materials such as MoS2 for the NEMS applications. As the dimensions of devices fabricated using these materials shrink down to atomically thin membrane, strain and nonlinear effects have become important. A clear understanding of the nonlinear effects and the ability to manipulate them is essential for next generation sensors. Here, we report on all electrical actuation and detection of few-layer MoS2 resonator. The ability to electrically detect multiple modes and actuate the modes deep into the nonlinear regime enables us to probe the nonlinear coupling between various vibrational modes. The modal coupling in our device is strong enough to detect three distinct internal resonances.

  15. MoS2 oxygen sensor with gate voltage stress induced performance enhancement

    Science.gov (United States)

    Tong, Yu; Lin, Zhenhua; Thong, John T. L.; Chan, Daniel S. H.; Zhu, Chunxiang

    2015-09-01

    Two-dimensional (2D) materials have recently attracted wide attention and rapidly established themselves in various applications. In particular, 2D materials are regarded as promising building blocks for gas sensors due to their high surface-to-volume ratio, ease in miniaturization, and flexibility in enabling wearable electronics. Compared with other 2D materials, MoS2 is particularly intriguing because it has been widely researched and exhibits semiconducting behavior. Here, we have fabricated MoS2 resistor based O2 sensors with a back gate configuration on a 285 nm SiO2/Si substrate. The effects of applying back gate voltage stress on O2 sensing performance have been systematically investigated. With a positive gate voltage stress, the sensor response improves and the response is improved to 29.2% at O2 partial pressure of 9.9 × 10-5 millibars with a +40 V back-gate bias compared to 21.2% at O2 partial pressure of 1.4 × 10-4 millibars without back-gate bias; while under a negative gate voltage stress of -40 V, a fast and full recovery can be achieved at room temperature. In addition, a method in determining O2 partial pressure with a detectability as low as 6.7 × 10-7 millibars at a constant vacuum pressure is presented and its potential as a vacuum gauge is briefly discussed.

  16. Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets.

    Science.gov (United States)

    Lukowski, Mark A; Daniel, Andrew S; Meng, Fei; Forticaux, Audrey; Li, Linsen; Jin, Song

    2013-07-17

    Promising catalytic activity from molybdenum disulfide (MoS2) in the hydrogen evolution reaction (HER) is attributed to active sites located along the edges of its two-dimensional layered crystal structure, but its performance is currently limited by the density and reactivity of active sites, poor electrical transport, and inefficient electrical contact to the catalyst. Here we report dramatically enhanced HER catalysis (an electrocatalytic current density of 10 mA/cm(2) at a low overpotential of -187 mV vs RHE and a Tafel slope of 43 mV/decade) from metallic nanosheets of 1T-MoS2 chemically exfoliated via lithium intercalation from semiconducting 2H-MoS2 nanostructures grown directly on graphite. Structural characterization and electrochemical studies confirmed that the nanosheets of the metallic MoS2 polymorph exhibit facile electrode kinetics and low-loss electrical transport and possess a proliferated density of catalytic active sites. These distinct and previously unexploited features of 1T-MoS2 make these metallic nanosheets a highly competitive earth-abundant HER catalyst.

  17. Tribological Performance of MoS2 Coatings in Various Environments

    Directory of Open Access Journals (Sweden)

    Thomas Gradt

    2016-09-01

    Full Text Available Molybdenum disulfide (MoS2 is a well-known solid lubricant for tribosystems running in vacuum or dry gases. Problems arise due to its sensitivity to humidity, which is a drawback for its application under ambient conditions. However, by using a physical vapor deposition (PVD process, deposition parameters can be optimized not only to gain a coatings structure with favorable frictional properties but also to minimize the sensitivity to attack by water molecules. Therefore, an improved tribological behavior even under moist conditions can be achieved. MoS2 coatings are also candidates for being applied at cryogenic temperatures. They already have proven their suitability, e.g., for sliding support elements between superconducting magnets of the nuclear fusion-experiment Wendelstein 7-X. However, these coatings were exclusively produced for this particular application and the utilization for more common tribosystems may be precluded due to cost considerations. In view of a wider range of applications, pure and Cr containing PVD-MoS2 coatings with an optimized structure were tested under varying environments including hydrogen gas and cryogenic temperatures. Results of the most promising variant are presented in this paper.

  18. Dynamic Negative Compressibility of Few-Layer Graphene, h-BN, and MoS2

    Science.gov (United States)

    Neves, Bernardo; Barboza, Ana Paula; Chacham, Helio; Oliveira, Camilla; Fernandes, Thales; Martins Ferreira, Erlon; Archanjo, Braulio; Batista, Ronaldo; Oliveira, Alan

    2013-03-01

    We report a novel mechanical response of few-layer graphene, h-BN, and MoS2 to the simultaneous compression and shear by an atomic force microscope (AFM) tip. The response is characterized by the vertical expansion of these two-dimensional (2D) layered materials upon compression. Such effect is proportional to the applied load, leading to vertical strain values (opposite to the applied force) of up to 150%. The effect is null in the absence of shear, increases with tip velocity, and is anisotropic. It also has similar magnitudes in these solid lubricant materials (few-layer graphene, h-BN, and MoS2), but it is absent in single-layer graphene and in few-layer mica and Bi2Se3. We propose a physical mechanism for the effect where the combined compressive and shear stresses from the tip induce dynamical wrinkling on the upper material layers, leading to the observed flake thickening. The new effect (and, therefore, the proposed wrinkling) is reversible in the three materials where it is observed.[2] Financial support from CNPq, Fapemig, Rede Nacional de Pesquisa em Nanotubos de Carbono and INCT-Nano-Carbono

  19. Impurities and Electronic Property Variations of Natural MoS 2 Crystal Surfaces

    KAUST Repository

    Addou, Rafik

    2015-09-22

    Room temperature X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICPMS), high resolution Rutherford backscattering spectrometry (HR-RBS), Kelvin probe method, and scanning tunneling microscopy (STM) are employed to study the properties of a freshly exfoliated surface of geological MoS2 crystals. Our findings reveal that the semiconductor 2H-MoS2 exhibits both n- and p-type behavior, and the work function as measured by the Kelvin probe is found to vary from 4.4 to 5.3 eV. The presence of impurities in parts-per-million (ppm) and a surface defect density of up to 8% of the total area could explain the variation of the Fermi level position. High resolution RBS data also show a large variation in the MoSx composition (1.8 < x < 2.05) at the surface. Thus, the variation in the conductivity, the work function, and stoichiometry across small areas of MoS2 will have to be controlled during crystal growth in order to provide high quality uniform materials for future device fabrication. © 2015 American Chemical Society.

  20. Antibiotic-loaded MoS2 nanosheets to combat bacterial resistance via biofilm inhibition

    Science.gov (United States)

    Zhang, Xu; Zhang, Wentao; Liu, Lizhi; Yang, Mei; Huang, Lunjie; Chen, Kai; Wang, Rong; Yang, Baowei; Zhang, Daohong; Wang, Jianlong

    2017-06-01

    The emergence of antibiotic resistance has resulted in increasing difficulty in treating clinical infections associated with biofilm formation, one of the key processes in turn contributing to enhanced antibiotic resistance. With the rapid development of nanotechnology, a new way to overcome antibiotic resistance has opened up. Based on the many and diverse properties of MoS2 nanosheets that have attracted wide attention, in particular their antibacterial potential, herein, a novel antimicrobial agent to combat resistant gram-positive Staphylococcus aureus and gram-negative Salmonella was prepared using chitosan functionalized MoS2 nanosheets loading tetracycline hydrochloride drugs (abbreviated to CM-TH). The antibacterial and anti-biofilm activities of the CM-TH nanocomposites showed the synergetic effect that the combination of nanomaterials and antibiotics was more efficient than either working alone. In particularly, the minimum inhibitory concentration values generally decreased by a factor of dozens, suggesting that CM-TH may become a possible alternative to traditional antibiotics in disrupting biofilms and overcoming antibiotic resistance in treating medical diseases.

  1. Determination of band offsets at GaN/single-layer MoS2 heterojunction

    KAUST Repository

    Tangi, Malleswararao

    2016-07-25

    We report the band alignment parameters of the GaN/single-layer (SL) MoS2 heterostructure where the GaN thin layer is grown by molecular beam epitaxy on CVD deposited SL-MoS2/c-sapphire. We confirm that the MoS2 is an SL by measuring the separation and position of room temperature micro-Raman E1 2g and A1 g modes, absorbance, and micro-photoluminescence bandgap studies. This is in good agreement with HRTEM cross-sectional analysis. The determination of band offset parameters at the GaN/SL-MoS2 heterojunction is carried out by high-resolution X-ray photoelectron spectroscopy accompanying with electronic bandgap values of SL-MoS2 and GaN. The valence band and conduction band offset values are, respectively, measured to be 1.86 ± 0.08 and 0.56 ± 0.1 eV with type II band alignment. The determination of these unprecedented band offset parameters opens up a way to integrate 3D group III nitride materials with 2D transition metal dichalcogenide layers for designing and modeling of their heterojunction based electronic and photonic devices.

  2. Tuning the electrical property via defect engineering of single layer MoS2 by oxygen plasma

    Science.gov (United States)

    Islam, Muhammad R.; Kang, Narae; Bhanu, Udai; Paudel, Hari P.; Erementchouk, Mikhail; Tetard, Laurene; Leuenberger, Michael N.; Khondaker, Saiful I.

    2014-08-01

    We have demonstrated that the electrical property of single-layer molybdenum disulfide (MoS2) can be significantly tuned from the semiconducting to the insulating regime via controlled exposure to oxygen plasma. The mobility, on-current and resistance of single-layer MoS2 devices were varied by up to four orders of magnitude by controlling the plasma exposure time. Raman spectroscopy, X-ray photoelectron spectroscopy and density functional theory studies suggest that the significant variation of electronic properties is caused by the creation of insulating MoO3-rich disordered domains in the MoS2 sheet upon oxygen plasma exposure, leading to an exponential variation of resistance and mobility as a function of plasma exposure time. The resistance variation calculated using an effective medium model is in excellent agreement with the measurements. The simple approach described here can be used for the fabrication of tunable two-dimensional nanodevices based on MoS2 and other transition metal dichalcogenides.We have demonstrated that the electrical property of single-layer molybdenum disulfide (MoS2) can be significantly tuned from the semiconducting to the insulating regime via controlled exposure to oxygen plasma. The mobility, on-current and resistance of single-layer MoS2 devices were varied by up to four orders of magnitude by controlling the plasma exposure time. Raman spectroscopy, X-ray photoelectron spectroscopy and density functional theory studies suggest that the significant variation of electronic properties is caused by the creation of insulating MoO3-rich disordered domains in the MoS2 sheet upon oxygen plasma exposure, leading to an exponential variation of resistance and mobility as a function of plasma exposure time. The resistance variation calculated using an effective medium model is in excellent agreement with the measurements. The simple approach described here can be used for the fabrication of tunable two-dimensional nanodevices based on MoS2 and other transition metal dichalcogenides. Electronic supplementary information (ESI) available: (1) Resistance versus plasma time for Vg = 20 V. (2) Effect of oxygen plasma on a second single-layer device. (3) Scanning electron microscope image of a plasma-exposed MoS2 flake. (4) Theoretical calculation of resistance. See DOI: 10.1039/c4nr02142h

  3. Graphene intercalated in graphene-like MoS2: A promising cathode for rechargeable Mg batteries

    Science.gov (United States)

    Liu, Yongchang; Fan, Li-Zhen; Jiao, Lifang

    2017-02-01

    In this paper, we report the synthesis of graphene-like MoS2/graphene hybrid by a facile lithium-assisted sonication method and its cathode application for rechargeable Mg batteries. Instrumental analyses elucidate that the composite displays a three-dimensional (3D) porous architecture constructed by exfoliated single or few MoS2 layers, and some graphene is intercalated in the MoS2 gallery with an enlarged interlayer spacing from 0.62 to 0.98 nm. The obtained MoS2/graphene hybrid exhibits high electrochemical performance with a remarkable capacity (115.9 mA h g-1) and good cyclic stability (82.5 mA h g-1 after 50 cycles). This is owing to the synergistic effect between the graphene-like MoS2 and the highly conductive graphene, which can effectively facilitate the Mg2+ ions diffusion and electrons transfer, provide abundant active sites for Mg2+ intercalation, and prevent structural collapse upon prolonged cycling.

  4. Ultrathin MoS2 and WS2 layers on silver nano-tips as electron emitters

    Science.gov (United States)

    Loh, Tamie A. J.; Tanemura, Masaki; Chua, Daniel H. C.

    2016-09-01

    2-dimensional (2D) inorganic analogues of graphene such as MoS2 and WS2 present interesting opportunities for field emission technology due to their high aspect ratio and good electrical conductivity. However, research on 2D MoS2 and WS2 as potential field emitters remains largely undeveloped compared to graphene. Herein, we present an approach to directly fabricate ultrathin MoS2 and WS2 onto Ag nano-tips using pulsed laser deposition at low temperatures of 450-500 °C. In addition to providing a layer of chemical and mechanical protection for the Ag nano-tips, the growth of ultrathin MoS2 and WS2 layers on Ag led to enhanced emission properties over that of pristine nano-tips due to a reduction of the effective barrier height arising from charge injection from Ag to the overlying MoS2 or WS2. For WS2 on Ag nano-tips, the phasic mixture was also an important factor influencing the field emission performance. The presence of 1T-WS2 at the metal-WS2 interface in a hybrid film of 2H/1T-WS2 leads to improvement in the field emission capabilities as compared to pure 2H-WS2 on Ag nano-tips.

  5. Atomically Precise Gold Nanoclusters Accelerate Hydrogen Evolution over MoS2 Nanosheets: The Dual Interfacial Effect.

    Science.gov (United States)

    Zhao, Shuo; Jin, Renxi; Song, Yongbo; Zhang, Hui; House, Stephen D; Yang, Judith C; Jin, Rongchao

    2017-07-24

    Hydrogen generation via electrocatalytic water splitting holds great promise for future energy revolution. It is desirable to design abundant and efficient catalysts and achieve mechanistic understanding of hydrogen evolution reaction (HER). Here, this paper reports a strategy for improving HER performance of molybdenum disulfide (MoS2 ) via introducing gold nanoclusters as a cocatalyst. Compared to plain MoS2 nanosheets, the Au25 (SR)18 /MoS2 nanocomposite exhibits enhanced HER activity with a small onset potential of -0.20 V (vs reversible hydrogen electrode) and a higher current density of 59.3 mA cm(-2) at the potential of -0.4 V. In addition to the interfacial interaction between nanoclusters and MoS2 , the interface between the Au25 core and the surface ligands (thiolate vs selenolate) is also discovered to distinctly affect the catalytic performance. This work highlights the promise of metal nanoclusters in boosting the HER performance via tailoring the interfacial electronic interactions between gold nanoclusters and MoS2 nanosheets, as well as the interface between metal core and surface ligands. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

  6. Theoretical study on the top- and enclosed-contacted single-layer MoS2 piezotronic transistors

    Science.gov (United States)

    Liu, Wei; Zhou, Yongli; Zhang, Aihua; Zhang, Yan; Wang, Zhong Lin

    2016-05-01

    Recently, the piezotronic effect has been observed in two-dimensional single-layer MoS2 materials, which have potential applications in force and pressure triggered or controlled electronic devices, sensors, and human-machine interfaces. However, classical theory faces the difficulty in explaining the mechanism of the piezotronic effect for the top- and enclosed-contacted MoS2 transistors, since the piezoelectric charges are assumed to exist only at the edge of the MoS2 flake that is far from the electronic transport pathway. In the present study, we identify the piezoelectric charges at the MoS2/metal-MoS2 interface by employing both the density functional theory and finite element method simulations. This interface is on the transport pathway of both top- and enclosed-contacted MoS2 transistors, thus it is capable of controlling their transport properties. This study deepens the understanding of piezotronic effect and provides guidance for the design of two-dimensional piezotronic devices.

  7. Dependence of Raman and absorption spectra of stacked bilayer MoS2 on the stacking orientation.

    Science.gov (United States)

    Park, Seki; Kim, Hyun; Kim, Min Su; Han, Gang Hee; Kim, Jeongyong

    2016-09-19

    Stacked bilayer molybdenum disulfide (MoS2) exhibits interesting physical properties depending on the stacking orientation and interlayer coupling strength. Although optical properties, such as photoluminescence, Raman, and absorption properties, are largely dependent on the interlayer coupling of stacked bilayer MoS2, the origin of variations in these properties is not clearly understood. We performed comprehensive confocal Raman and absorption mapping measurements to determine the dependence of these spectra on the stacking orientation of bilayer MoS2. The results indicated that with 532-nm laser excitation, the Raman scattering intensity gradually increased upon increasing the stacking angle from 0° to 60°, whereas 458-nm laser excitation resulted in the opposite trend of decreasing Raman intensity with increasing stacking angle. This opposite behavior of the Raman intensity dependence was explained by the varying resonance condition between the Raman excitation wavelength and C exciton absorption energy of bilayer MoS2. Our work sheds light on the intriguing effect of the subtle interlayer interaction in stacked MoS2 bilayers on the resulting optical properties.

  8. Photoluminescence of MoS2 quantum dots quenched by hydrogen peroxide: A fluorescent sensor for hydrogen peroxide

    Science.gov (United States)

    Gan, Zhixing; Gui, Qingfeng; Shan, Yun; Pan, Pengfei; Zhang, Ning; Zhang, Lifa

    2016-09-01

    By cutting MoS2 microcrystals to quantum dots (QDs) of sizes below 10 nm, the photoluminescence (PL) at ca. 450 nm can be detected easily due to the quantum confinement effects across the 2D planes. The PL is stable under continuous irradiation of UV light but gradually quenches when treated with an increasing concentration of hydrogen peroxide. Time-resolved PL and Raman spectra imply that H2O2 causes the partial oxidation of MoS2 QDs. First-principles calculations reveal that the MoS2 QDs with oxygen impurity are of indirect bandgap structures showing no notable PL. And absorption spectra verify that the PL of MoS2 QDs quenched by H2O2 is attributed to the oxidation. The integrated PL intensity and H2O2 concentration show an exponential relationship in the range of 2-20 μM, suggesting that MoS2 QDs are potential fluorescent probes for hydrogen peroxide sensing in a physiological environment.

  9. Ultrathin MoS2 sheets supported on N-rich carbon nitride nanospheres with enhanced lithium storage properties

    Science.gov (United States)

    Chenrayan, Senthil; Chandra, Kishore S.; Manickam, Sasidharan

    2017-07-01

    Deciphering the structural and volume changes occurring during electrode reactions in lithium-ion batteries is perhaps a boon for high energy density batteries. Here, we report the synthesis of 3D network of dichalcogenide molybdenum disulfide (MoS2) encapsulated over nitrogen rich graphitic carbon nitride nanosphere (g-C3N4) forming an interconnected and uniform g-C3N4/MoS2 scaffolds. The crystallinity, phase purity, morphological features and elemental composition were evaluated through XRD, FESEM, TEM, HRTEM, BET and XPS analyses. The electrochemical properties of N-rich g-C3N4/MoS2 scaffolds were investigated as potential anode materials for lithium-ion batteries. Electrochemical testing of the g-C3N4/MoS2 constructured electrode delivered reversible capacity of 857 mAh g-1at 0.1 C rate after fifty cycles and exhibited a high rate performance with reversible capacity of 383 mAh g-1 at 10 C rate (higher than theoretical capacity of graphite, 372 mAh g-1). The superior electrochemical property of g-C3N4/MoS2 is attributed to N-rich carbon support which favors better electronic conductivity, and affords more sites for Li+ ions. The nitrogen rich carbon nitride accommodates volume changes caused during repeated charge/discharges and maintains high structural integrity and specific capacity.

  10. Synthesis and characterization of large-area and continuous MoS2 atomic layers by RF magnetron sputtering.

    Science.gov (United States)

    Hussain, Sajjad; Shehzad, Muhammad Arslan; Vikraman, Dhanasekaran; Khan, Muhammad Farooq; Singh, Jai; Choi, Dong-Chul; Seo, Yongho; Eom, Jonghwa; Lee, Wan-Gyu; Jung, Jongwan

    2016-02-21

    In this article, we report layer-controlled, continuous and large-area molydenum sulfide (MoS2) growth onto a SiO2/Si substrate by RF sputtering combined with sulfurization. A two-step process was employed to synthesize MoS2 films. In the first step, an atomically thin MoO3 film was deposited by RF magnetron sputtering at 300 °C. Subsequently, the as-sputtered MoO3 film was further subjected to post-annealing and sulfurization processes at 650 °C for 1 hour. It was observed that the number of layers of MoS2 can be controlled by adjusting the sputtering time. The fabricated MoS2 transistors exhibited high mobility values of ∼21 cm(2) V(-1) s(-1) (bilayer) and ∼25 cm(2) V(-1) s(-1) (trilayer), on/off ratios in the range of ∼10(7) (bilayer) and 10(4)-10(5) (trilayer), respectively. We believe that our proposed paradigm can start a new method for the growth of MoS2 in future electronics and optoelectronics applications.

  11. Electronic properties of MoS2/MoOx interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts

    Science.gov (United States)

    K. C., Santosh; Longo, Roberto C.; Addou, Rafik; Wallace, Robert M.; Cho, Kyeongjae

    2016-01-01

    In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations. PMID:27666523

  12. Synthesis and Structural Characterization of Al2O3-Coated MoS2 Spheres for Photocatalysis Applications

    Directory of Open Access Journals (Sweden)

    S. V. Prabhakar Vattikuti

    2015-01-01

    Full Text Available This paper reports the synthesis of novel monodisperse Al2O3-coated molybdenum disulfide nanospheres (i.e., core-shell structures using a one-step facile hydrothermal method. XPS analysis confirmed the purity and stable structure of the Al2O3-coated MoS2 nanospheres. A possible growth mechanism of the core-shell structure is also reported, along with their influence on the photodegradation process of rhodamine B (RhB. The Al2O3-coated MoS2 nanospheres demonstrate good photocatalytic activity and chemical stability compared to MoS2 spheres. TG-DTA analysis provided insight into the decomposition process of the precursor solution and the stability of the nanoparticles. The enhanced photocatalytic activity makes the Al2O3-coated MoS2 nanospheres a promising candidate as a photocatalyst that could be used in place of traditional Al2O3/MoS2 photocatalyst for the removal of pollutants from waste water.

  13. Biocompatible PEGylated MoS2 nanosheets: controllable bottom-up synthesis and highly efficient photothermal regression of tumor.

    Science.gov (United States)

    Wang, Shige; Li, Kai; Chen, Yu; Chen, Hangrong; Ma, Ming; Feng, Jingwei; Zhao, Qinghua; Shi, Jianlin

    2015-01-01

    Two-dimensional transition metal dichalcogenides, particularly MoS2 nanosheets, have been deemed as a novel category of NIR photothermal transducing agent. Herein, an efficient and versatile one-pot solvothermal synthesis based on "bottom-up" strategy has been, for the first time, proposed for the controlled synthesis of PEGylated MoS2 nanosheets by using a novel "integrated" precursor containing both Mo and S elements. This facile but unique PEG-mediated solvothermal procedure endowed MoS2 nanosheets with controlled size, increased crystallinity and excellent colloidal stability. The photothermal performance of nanosheets was optimized via modulating the particulate size and surface PEGylation. PEGylated MoS2 nanosheets with desired photothermal conversion performance and excellent colloidal and photothermal stability were further utilized for highly efficient photothermal therapy of cancer in a tumor-bearing mouse xenograft. Without showing observable in vitro and in vivo hemolysis, coagulation and toxicity, the optimized MoS2-PEG nanosheets showed promising in vitro and in vivo anti-cancer efficacy.

  14. Ab-initio study of gold nanoparticles supported on defect-laden single-layer MoS2

    Science.gov (United States)

    Rawal, Takat B.; Le, Duy; Rahman, Talat S.

    We have investigated the geometry, electronic structure, and catalytic properties of gold nanoparticles on defect-laden single-layer MoS2 using density functional theory (DFT) based calculations with semi-empirical van der Waals interaction (DFT-D3). Our results show that the two-dimensional planar structure, the most favorable one for unsupported Au13 nanoparticle, transforms into a distorted three-dimensional (3D) structure when supported on single-layer MoS2 with single S-vacancy which is more favorable than the icosahedral, decahedron and cuboctahedron forms. The MoS2 support substantially alters the electronic structure of Au13 nanoparticle near the Fermi level, owing to the strong interaction of MoS2 support with Au13 in the presence of an S-vacancy. The modified electronic structure remarkably affects the catalytic activity of the MoS2-supported Au13, offering enhanced activity towards methanol synthesis reaction via CO hydrogenation reaction - a contrast from that of titania-supported Au13 nanoparticlewhich promotes methanol decomposition. This work is supported in part by U.S. Department of Energy (DOE DE-FG02-07ER15842).

  15. Uniform Yolk-Shell MoS2 @Carbon Microsphere Anodes for High-Performance Lithium-Ion Batteries.

    Science.gov (United States)

    Pan, Yunmei; Zhang, Jiajia; Lu, Hongbin

    2017-07-21

    As an electrode material for lithium-ion batteries (LIBs), MoS2 has attracted much attention because of its high capacity and low cost. However, the rational design of a novel electrode structure with a high capacity, fast charge/discharge rate, and long cycling lifetime remains a great challenge. Herein, a environmentally friendly etching strategy is reported for the construction of monodisperse, inner void-controlled yolk-shell MoS2 @carbon microspheres. The resulting anode reveals an initial discharge capacity up to 1813 mAh g(-1) , a high reversible capacity (1016 mAh g(-1) ), excellent cycling stability (200 cycles), and superior rate performance. Such microspheres consist of nanosized MoS2 yolks (≈280 nm), porous carbon shells (≈25 nm) and well-controlled internal voids in between, opening a new pathway for the optimization of the electrochemical properties of MoS2 -based anodes without sacrificing their capacity. In addition, this etching strategy offers a new method for the development of functional, hollow MoS2 -based composites. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

  16. A theoretical prediction of super high-performance thermoelectric materials based on MoS2/WS2 hybrid nanoribbons.

    Science.gov (United States)

    Zhang, Zhongwei; Xie, Yuee; Peng, Qing; Chen, Yuanping

    2016-02-17

    Modern society is hungry for electrical power. To improve the efficiency of energy harvesting from heat, extensive efforts seek high-performance thermoelectric materials that possess large differences between electronic and thermal conductance. Here we report a super high-performance material of consisting of MoS2/WS2 hybrid nanoribbons discovered from a theoretical investigation using nonequilibrium Green's function methods combined with first-principles calculations and molecular dynamics simulations. The hybrid nanoribbons show higher efficiency of energy conversion than the MoS2 and WS2 nanoribbons due to the fact that the MoS2/WS2 interface reduces lattice thermal conductivity more than the electron transport. By tuning the number of the MoS2/WS2 interfaces, a figure of merit ZT as high as 5.5 is achieved at a temperature of 600 K. Our results imply that the MoS2/WS2 hybrid nanoribbons have promising applications in thermal energy harvesting.

  17. Development of a new molecular dynamics method for tribochemical reaction and its application to formation dynamics of MoS 2 tribofilm

    Science.gov (United States)

    Morita, Yusuke; Onodera, Tasuku; Suzuki, Ai; Sahnoun, Riadh; Koyama, Michihisa; Tsuboi, Hideyuki; Hatakeyama, Nozomu; Endou, Akira; Takaba, Hiromitsu; Kubo, Momoji; Del Carpio, Carlos A.; Shin-yoshi, Takatoshi; Nishino, Noriaki; Suzuki, Atsushi; Miyamoto, Akira

    2008-09-01

    Recently we have developed a novel molecular dynamics program NEW-RYUDO-CR, which can deal with chemical reactions. The developed method has been applied to the study of tribochemical reaction dynamics of MoS 2 tribofilm on iron surface. The initially amorphous MoS 2 layer self-organized its structure as result of the tribochemical reactions and formed layered MoS 2 tribofilm. The friction coefficient significantly decreased as the MoS 2 tribofilm was formed. Besides, sliding was observed between sulfur layers of MoS 2 tribofilms which occurred due to repulsive Coulombic interaction forces between sulfur atoms. This indicates that the formation of the layered MoS 2 tribofilm is important to achieve better lubrication properties.

  18. Top-Down and Bottom-Up Approaches in Engineering 1 T Phase Molybdenum Disulfide (MoS2 ): Towards Highly Catalytically Active Materials.

    Science.gov (United States)

    Chua, Chun Kiang; Loo, Adeline Huiling; Pumera, Martin

    2016-09-26

    The metallic 1 T phase of MoS2 has been widely identified to be responsible for the improved performances of MoS2 in applications including hydrogen evolution reactions and electrochemical supercapacitors. To this aim, various synthetic methods have been reported to obtain 1 T phase-rich MoS2 . Here, the aim is to evaluate the efficiencies of the bottom-up (hydrothermal reaction) and top-down (chemical exfoliation) approaches in producing 1 T phase MoS2 . It is established in this study that the 1 T phase MoS2 produced through the bottom-up approach contains a high proportion of 1 T phase and demonstrates excellent electrochemical and electrical properties. Its performance in the hydrogen evolution reaction and electrochemical supercapacitors also surpassed that of 1 T phase MoS2 produced through a top-down approach.

  19. Ammonia intercalated flower-like MoS2 nanosheet film as electrocatalyst for high efficient and stable hydrogen evolution

    Science.gov (United States)

    Wang, F. Z.; Zheng, M. J.; Zhang, B.; Zhu, C. Q.; Li, Q.; Ma, L.; Shen, W. Z.

    2016-08-01

    Ammonia intercalated flower-like MoS2 electrocatalyst film assembled by vertical orientated ultrathin nanosheet on graphite sheethas been successfully synthesized using one-step hydrothermal method. In this strategy, ammonia can effectively insert into the parallel plane of the MoS2 nanosheets, leading to the expansion of lattice and phase transfer from 2H to 1T, generating more active unsaturated sulfur atoms. The flower-like ammoniated MoS2 electrocatalysts with more active sites and large surface area exhibited excellent HER activity with a small Tafel slope and low onset overpotential, resulting a great enhancement in hydrogen evolution. The high efficient activity and recyclable utilization, as well as large-scale, indicate that it is a very promising electrocatalyst to replace Pt in industry application.

  20. MoS2-coated ZnO nanocomposite as an active heterostructure photocatalyst for hydrogen evolution

    Science.gov (United States)

    Zhang, Shibao; Tang, Fumin; Liu, Jinkun; Che, Wei; Su, Hui; Liu, Wei; Huang, Yuanyuan; Jiang, Yong; Yao, Tao; Liu, Qinghua; Wei, Shiqiang

    2017-08-01

    To obtain an efficient photocatalyst for solar energy conversion is a key issue in photocatalytic water splitting. Here, we have synthesized MoS2-coated ZnO nanocomposite photocatalyst and investigated the structure-performance relationship by UV-vis absorption and photoluminescence (PL) spectroscopy. Under the simulated sunlight irradiation, the MoS2-coated ZnO nanocomposite in aqueous Na2SO3 solution achieves a high H2 production rate of 1410 μmol/g/h, 5.8 times higher than that of pristine ZnO nanoparticles. The UV-vis spectra and PL results reveal that the MoS2 coated on the surface of ZnO nanoparticles can significantly facilitate the photoexcited electrons transfer and suppress the recombination of the electrons and holes, thus increasing the H2 evolution activity of the composite system.