Low band gap polymers for organic photovoltaics
Bundgaard, Eva; Krebs, Frederik C
2007-01-01
Low band gap polymer materials and their application in organic photovoltaics (OPV) are reviewed. We detail the synthetic approaches to low band gap polymer materials starting from the early methodologies employing quinoid homopolymer structures to the current state of the art that relies...
Low Band Gap Polymers for Roll-to-Roll Coated Polymer Solar Cells
2010-01-01
We present the synthesis of a low band gap copolymer based on dithienothiophene and dialkoxybenzothiadiazole (poly(dithienothiophene-co-dialkoxybenzothiadiazole), PDTTDABT). The optical properties of the polymer showed a band gap of 1.6 eV and a sky-blue color in solid films. The polymer was...
Investigations into low band-gap, semiconducting polymers
The physical and electronic characteristics of the low band-gap polymers, poly(4-dicyanomethylene-4H-cyclopenta[1,2-b;3,4-b']dithiophene) (PCDM) and poly(7-benzo[1,3]dithiol-2-ylidine-7H-3,4-dithia-cyclopenta[a]pentalene) (PBDT), have been examined. The polymers were studied using microscopic and spectroscopic methods to determine the chemical and morphological properties of the polymer films. Micrographs of the polymer surfaces revealed PCDM to consist of a relatively smooth surface covered by large aggregations. PBDT had a much rougher surface with a large surface area, possibly suitable for the production of a polymeric battery. Polymer band-gaps were estimated using UV-vis spectra to be 1.38 eV and 1.29 eV respectively for PCDM and PBDT. XPS measurements allied with the PCDM and PBDT band-gaps have been used to produce preliminary band diagrams for ITO/polymer/Al diodes. Diodes of PCDM and PBDT were fabricated to allow study of the electrical behaviour of the polymers using DC current-voltage (I-V) measurements and AC impedance spectroscopy. Equivalent RC circuits of ITO/PCDM/Al diodes have been used to model the presence of 2 dispersions in the admittance plots and to explain the effects of increasing temperature, polymer film thickness and contact material. The DC electrical measurements for PCDM reveal almost symmetrical current-voltage characteristics. A hysteresis effect dependent on the direction of the voltage sweep observed in the DC characteristics has a potential application in the production of a polymeric ''memory element''. For sweep voltages starting above approximately ±4 V, ''high'' and ''low'' conductance states occur when sweeping from negative to positive and positive to negative bias respectively. The ''low'' state is stable for many months whereas the ''high'' state decays exponentially with time constants of approximately 2000 s. Characteristic values for PCDM films have been estimated using the experimental results. An investigation of
Uncover the electroluminescence in wide band gap polymers
Qiao, B.; Teyssedre, G.; Laurent, C.
2015-10-01
Due to the rapidly increasing demand of electric power, insulating materials used in electrical components are pushed up to their limits, where their electronic properties are of fundamental importance. Electroluminescence provides an elegant way to investigate electronic properties, high field effects and electrical ageing of polymers although the emission spectrum is still poorly understood. Unlike in organic semi-conductors, electroluminescence spectra of large band gap polymers exhibit specific spectral features that cannot be interpreted on the basis of the photo-physical properties of the material. By irradiating polypropylene thin films with electrons up to a few keV and by analyzing the emitted light, we were able to isolate the elementary components of the emission and to reconstruct the electroluminescence spectrum. For the first time, a comprehensive study of electroluminescence in polymers is provided and the underlying mechanisms of the emission are discussed. The results herein provide an univocal demonstration that the electroluminescence from wide band gap polymers results in part from chemical reactions, opening the way to the diagnosis and prognosis of polymeric materials under electrical stress.
Synthesis and Characterization of Small Band-gap Conjugated Polymers - Poly(pyrrolyl methines)
无
2002-01-01
A kind of small band-gap conjugated polymers-poly (pyrrolyl methines) and their precursors-(poly pyrrolyl methanes) have been synthesized by a simple method and characterized by 1HNMR, FT-IR, TGA and UV-Vis. These polymers can be dissolved in high polar solvents such as DMSO, DMF or NMP. The results reveals that the band-gap of the synthesized conjugated polymers are in the range of 0.96～1.14 eV and they all belong to the small band-gap polymers. The conductivity of doped products with iodine is in the range of semiconductor.
Band gap engineering in polymers through chemical doping and applied mechanical strain.
Lanzillo, Nicholas A; Breneman, Curt M
2016-08-17
We report simulations based on density functional theory and many-body perturbation theory exploring the band gaps of common crystalline polymers including polyethylene, polypropylene and polystyrene. Our reported band gaps of 8.6 eV for single-chain polyethylene and 9.1 eV for bulk crystalline polyethylene are in excellent agreement with experiment. The effects of chemical doping along the polymer backbone and side-groups are explored, and the use mechanical strain as a means to modify the band gaps of these polymers over a range of several eV while leaving the dielectric constant unchanged is discussed. This work highlights some of the opportunities available to engineer the electronic properties of polymers with wide-reaching implications for polymeric dielectric materials used for capacitive energy storage. PMID:27324304
Band gap engineering in polymers through chemical doping and applied mechanical strain
Lanzillo, Nicholas A.; Breneman, Curt M.
2016-08-01
We report simulations based on density functional theory and many-body perturbation theory exploring the band gaps of common crystalline polymers including polyethylene, polypropylene and polystyrene. Our reported band gaps of 8.6 eV for single-chain polyethylene and 9.1 eV for bulk crystalline polyethylene are in excellent agreement with experiment. The effects of chemical doping along the polymer backbone and side-groups are explored, and the use mechanical strain as a means to modify the band gaps of these polymers over a range of several eV while leaving the dielectric constant unchanged is discussed. This work highlights some of the opportunities available to engineer the electronic properties of polymers with wide-reaching implications for polymeric dielectric materials used for capacitive energy storage.
Low band gap polymers for organic solar cells
Bundgaard, Eva; Krebs, Frederik C
2008-01-01
The synthesis of copolymers based on thiophene, benzothiadiazole and benzo-bis-thiadiazole are described. The polymers were obtained by employing Stille cross coupling polymerization. The polymers were characterized by NMR, size exclusion chromatography, UV-vis and ultraviolet photoelectron spect...
Incorporation of Furan into Low Band-Gap Polymers for Efficient Solar Cells
Woo, Claire H.
2010-11-10
The design, synthesis, and characterization of the first examples of furan-containing low band-gap polymers, PDPP2FT and PDPP3F, with substantial power conversion efficiencies in organic solar cells are reported. Inserting furan moieties in the backbone of the conjugated polymers enables the use of relatively small solubilizing side chains because of the significant contribution of the furan rings to overall polymer solubility in common organic solvents. Bulk heterojunction solar cells fabricated from furan-containing polymers and PC71BM as the acceptor showed power conversion efficiencies reaching 5.0%. © 2010 American Chemical Society.
Zhang, Haichang
2013-01-01
In this work new monomers and polymers with donor-acceptor ability and polymers containing diketopyrrolo[3,2-b]pyrrole (isoDPP), benzodione and naphthodione chromophores in the main chain with deep colour, broad absorption and low band gap are prepared and investigated. The compounds might be suitable for electronic applications, especially in solar cell devices. Characteristic properties are studied using spectroscopic methods (UV/vis, fluorescecne, NMR spectroscopy), gel permeation chromato...
A novel benzodipyrrolidone-based low band gap polymer for organic solar cells
Yue, Wei; Huang, Xiaodong; Yuan, Jianyu;
2013-01-01
A low band gap polymer PBDPDP-DTP, with alternating benzodipyrrolidone (BDP) unit and dithienopyrrole, was synthesized and characterized. A PCE of 2.60%and a Voc of up to 0.74 V were realized in PSCs, which demonstrated the strong potential of BDP as the electron deficient unit in the design of...
Bundgaard, Eva; Hagemann, Ole; Jørgensen, Mikkel;
2011-01-01
In this paper we present the design and synthesis of 25 new low band gap polymers. The polymers were characterized by UV-vis spectroscopy which showed optical band gaps of 2.0–0.9 eV. The polymers which were soluble enough were applied in organic photovoltaics, both small area devices with a spin...... coated active layer and in large area modules where all layers including the active layer were either roll-to-roll coated or printed. These experiments showed that the design of polymers compatible with roll-toroll coating is not straightforward and that there are various issues such as donor...
Carlé, Jon Eggert; Jørgensen, Mikkel; Manceau, Matthieu;
2011-01-01
We investigate a family of low band-gap polymers based on the common acceptor moiety 2,3-bis-(3-octyloxyphenyl)quinoxaline (Q) combined with thiophene (T) or the fused thiophene systems: benzo[2,1-b:3,4-b′]-dithiophene (BDT) or dithieno[3,2-b,2′,3′-d]-thiophene (DTT). The photochemical stability of...... the three polymers was examined and compared to P3HT. They were found to be substantially more robust than P3HT with a ranking of DTTQ>BDTQ>TQ1P3HT, indicating that the fused ring systems of DTT and BDT impart a large degree of photochemical stability than thiophene. Furthermore devices with normal...
The effects of gamma (γ) irradiation on dielectric and optical properties of polymer electrolyte film were investigated. The dielectric constant and ac conductivity increases with γ dose. Also optical band gap decreased from 4.23 to 3.78ev after irradiation. A large dependence of the polymer properties on the irradiation dose was noticed. This suggests that there is a possibility of improving polymer electrolyte properties on gamma irradiation
Shear banding in entangled polymers in the micron scale gap: a confocal-rheoscopic study.
Boukany, Pouyan E; Wang, Shi-Qing; Ravindranath, Sham; Lee, L James
2015-11-01
Recent shear experiments in well-entangled polymer solutions demonstrated that interfacial wall slip is the only source of shear rate loss and there is no evidence of shear banding in the micron scale gap. In this work, we experimentally elucidate how molecular parameters such as slip length, b, influence shear inhomogeneity of entangled polybutadiene (PBD) solutions during shear in a small gap H ∼ 50 μm. Simultaneous rheometric and velocimetric measurements are performed on two PBD solutions with the same level of entanglements (Z = 54) in two PBD solvents with molecular weights of 1.5 kg mol(-1) and 10 kg mol(-1) that possess different levels of shear inhomogeneity (2bmax/H = 17 and 240). For the PBD solution made with a low molecular weight PBD solvent of 1.5 kg mol(-1), wall slip is the dominant response within the accessible range of the shear rate, i.e., up to the nominal Weissenberg number (Wi) as high as 290. On the other hand, wall slip is minimized using a high molecular-weight PBD solvent of 10 kg mol(-1) so that bulk shear banding is observed to take place in the steady state for Wi > 100. Finally, these findings and previous results are in good agreement with our recently proposed phase diagram in the parameter space of apparent Wi versus 2bmax/H suggesting that shear banding develops across the micron scale gap when the imposed Wi exceeds 2bmax/H [Wang et al., Macromolecules, 2011, 44, 183]. PMID:26377827
Photoconductivity of composites based on CdSe quantum dots and low-band-gap polymers
Dayneko, Sergey; Linkov, Pavel; Martynov, Igor; Tameev, Alexey; Tedoradze, Marine; Samokhvalov, Pavel; Nabiev, Igor; Chistyakov, Alexander
2016-05-01
Photoconductivity of thin layers prepared by spin coating of blends of CdSe quantum dots (QDs) and a low-band-gap polymer PCDTBT or PTB7 has been studied. It has been found that photocurrent in the composites containing QDs of 10-nm in size is significantly higher than in those of containing 5-nm QDs. Analysis of the results showed that the photoresponse of the thin layers is mainly determined by the relative positions of the frontier energy levels of the materials used, organic semiconductors and QDs. Therefore, the ability to tune the relative positions of these levels by varying the QD size is of special importance, thus allowing the optimization of photodetectors and photovoltaic cells.
Wang, Suxiao; Singh, Amita; Walsh, Nichola; Redmond, Gareth
2016-06-17
Stable, aqueous dispersions of nanoparticles based on the low band gap polymers poly [2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (APFO-3) and poly [N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) were prepared, using a flexible, surfactant-free reprecipitation method, and characterized by a variety of optical techniques. Light scattering measurements indicated average nanoparticle hydrodynamic diameters of approximately 40 nm. The particles presented wide-bandwidth absorption and photoluminescence excitation spectra with high absorption cross-sections on the order of 10(-12) cm(2). Nanoparticle emission spectra were significantly red-shifted, with decreased emission quantum yields and lifetimes, consistent with increased inter-polymer chain interactions in the condensed phase. Single particle photoluminescence studies highlighted the multi-chromophoric nature of the polymer nanoparticles and confirmed their favorable photostabilities. When the nanoparticles were doped with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the correspondence of photoluminescence emission quenching, quantum yield decreases, emission lifetime shortening, and increased non-radiative rates with increasing PCBM concentration suggested efficient photo-induced donor-to-acceptor charge transfer between the conjugated polymers and the fullerene dopants co-localized in the nanoparticle cores. Taken together, the data suggest that these surfactant-free hybrid nanomaterials may be useful for integration with future nanostructured organic photovoltaics technologies. PMID:27159927
Wang, Suxiao; Singh, Amita; Walsh, Nichola; Redmond, Gareth
2016-06-01
Stable, aqueous dispersions of nanoparticles based on the low band gap polymers poly [2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4‧,7‧-di-2-thienyl-2‧,1‧,3‧-benzothiadiazole)] (APFO-3) and poly [N-9‧-heptadecanyl-2,7-carbazole-alt-5,5-(4‧,7‧-di-2-thienyl-2‧,1‧,3‧-benzothiadiazole)] (PCDTBT) were prepared, using a flexible, surfactant-free reprecipitation method, and characterized by a variety of optical techniques. Light scattering measurements indicated average nanoparticle hydrodynamic diameters of approximately 40 nm. The particles presented wide-bandwidth absorption and photoluminescence excitation spectra with high absorption cross-sections on the order of 10‑12 cm2. Nanoparticle emission spectra were significantly red-shifted, with decreased emission quantum yields and lifetimes, consistent with increased inter-polymer chain interactions in the condensed phase. Single particle photoluminescence studies highlighted the multi-chromophoric nature of the polymer nanoparticles and confirmed their favorable photostabilities. When the nanoparticles were doped with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the correspondence of photoluminescence emission quenching, quantum yield decreases, emission lifetime shortening, and increased non-radiative rates with increasing PCBM concentration suggested efficient photo-induced donor-to-acceptor charge transfer between the conjugated polymers and the fullerene dopants co-localized in the nanoparticle cores. Taken together, the data suggest that these surfactant-free hybrid nanomaterials may be useful for integration with future nanostructured organic photovoltaics technologies.
Aqueous processing of low-band-gap polymer solar cells using roll-to-roll methods.
Andersen, Thomas R; Larsen-Olsen, Thue T; Andreasen, Birgitta; Böttiger, Arvid P L; Carlé, Jon E; Helgesen, Martin; Bundgaard, Eva; Norrman, Kion; Andreasen, Jens W; Jørgensen, Mikkel; Krebs, Frederik C
2011-05-24
Aqueous nanoparticle dispersions of a series of three low-band-gap polymers poly[4,8-bis(2-ethylhexyloxy)benzo(1,2-b:4,5-b')dithiophene-alt-5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)(2,1,3-benzothiadiazole)-5,5'-diyl] (P1), poly[(4,4'-bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (P2), and poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl] (P3) were prepared using ultrasonic treatment of a chloroform solution of the polymer and [6,6]-phenyl-C(61)-butyric acid methyl ester ([60]PCBM) mixed with an aqueous solution of sodium dodecylsulphate (SDS). The size of the nanoparticles was established using small-angle X-ray scattering (SAXS) of the aqueous dispersions and by both atomic force microscopy (AFM) and using both grazing incidence SAXS (GISAXS) and grazing incidence wide-angle X-ray scattering (GIWAXS) in the solid state as coated films. The aqueous dispersions were dialyzed to remove excess detergent and concentrated to a solid content of approximately 60 mg mL(-1). The formation of films for solar cells using the aqueous dispersion required the addition of the nonionic detergent FSO-100 at a concentration of 5 mg mL(-1). This enabled slot-die coating of high quality films with a dry thickness of 126 ± 19, 500 ± 25, and 612 ± 22 nm P1, P2, and P3, respectively for polymer solar cells. Large area inverted polymer solar cells were thus prepared based on the aqueous inks. The power conversion efficiency (PCE) reached for each of the materials was 0.07, 0.55, and 0.15% for P1, P2, and P3, respectively. The devices were prepared using coating and printing of all layers including the metal back electrodes. All steps were carried out using roll-to-roll (R2R) slot-die and screen printing methods on flexible substrates. All five layers were processed using environmentally friendly methods and solvents. Two of the layers were processed entirely from water (the electron transport layer and the active
An overview of the theoretical and experimental efforts in obtaining a photonic band gap, a frequency band in three-dimensional dielectric structures in which electromagnetic waves are forbidden, is presented
Mondal, Rajib
2010-01-01
Thiophene enriched fused-aromatic thieno[3,4-b]pyrazine systems were designed and employed to produce low band gap polymers (Eg = 1.0-1.4 eV) when copolymerized with fluorene and cyclopentadithiophene. The copolymers are mainly investigated for organic thin film transistor and organic photovoltaic applications. Molecular packing in the thin films of these polymers was investigated using Grazing incidence X-ray Scattering. Although both fluorene and cyclopentadithiophene polymers follow similar face to face π-π stacking, the latter polymers show much smaller lamellar d-spacings due to side-chain interdigitation between the lamellae. This lead to the higher charge carrier mobilities in cyclopentadithiophene polymers (up to 0.044 cm2/V.s) compared to fluorene polymers (up to 8.1 × 10-3 cm2/V.s). Power conversion efficiency of 1.4% was achieved using fluorene copolymer in solar cells with a fullerene derivative as an acceptor. Although the cyclopentadithiophene polymers show lower band gaps with higher absorption coefficients compared to fluorene copolymers, but the power conversion efficiencies in solar cells of these polymers are low due to their low ionization potentials. © The Royal Society of Chemistry 2010.
Xu, Lirong; Yu, Yanxia; Lin, Jianbin; Zhou, Xin; Tian, Wei Quan; Nieckarz, Damian; Szabelski, Pawel; Lei, Shengbin
2016-04-01
Two-dimensional polymers are of great interest for many potential applications in nanotechnology. The preparation of crystalline 2D polymers with a tunable band gap is critical for their applications in nano-electronics and optoelectronics. In this work, we try to tune the band gap of 2D imine polymers by expanding the conjugation of the backbone of aromatic diamines both laterally and longitudinally. STM characterization reveals that the regularity of the 2D polymers can be affected by the existence of lateral bulky groups. Density functional theory (DFT) simulations discovered a significant narrowing of the band gap of imine 2D polymers upon the expansion of the conjugation of the monomer backbone, which has been confirmed experimentally by UV absorption measurements. Monte Carlo simulations help us to gain further insight into the controlling factors of the formation of regular 2D polymers, which demonstrated that based on the all rigid assumption, the coexistence of different conformations of the imine moiety has a significant effect on the regularity of the imine 2D polymers.Two-dimensional polymers are of great interest for many potential applications in nanotechnology. The preparation of crystalline 2D polymers with a tunable band gap is critical for their applications in nano-electronics and optoelectronics. In this work, we try to tune the band gap of 2D imine polymers by expanding the conjugation of the backbone of aromatic diamines both laterally and longitudinally. STM characterization reveals that the regularity of the 2D polymers can be affected by the existence of lateral bulky groups. Density functional theory (DFT) simulations discovered a significant narrowing of the band gap of imine 2D polymers upon the expansion of the conjugation of the monomer backbone, which has been confirmed experimentally by UV absorption measurements. Monte Carlo simulations help us to gain further insight into the controlling factors of the formation of regular 2D
Hu, Xiaolian; Zuo, Lijian; Fu, Weifei;
2012-01-01
To increase the open circuit voltage (VOC) of polymer solar cells based on diketopyrrolopyrrole (DPP) containing polymers, the weakly electron-withdrawing thiophene-3,4-dicarboxylate unit was introduced into the polymer backbone. Two ester group functionalized DPP containing polymers, PCTDPP with a....... The PSC based on a PCTDPP:PCBM blend shows a power conversion efficiency (PCE) up to 3.52%, with a VOC of 0.66 V, a short circuit current (ISC) of 8.53 mA cm−2, and a fill factor (FF) of 0.63. For the PDCTDPP:PCBM blend, the highest VOC reaches a value of 0.84 V, and a final PCE (0.92%) is limited by...
Xu, Lirong; Yu, Yanxia; Lin, Jianbin; Zhou, Xin; Tian, Wei Quan; Nieckarz, Damian; Szabelski, Pawel; Lei, Shengbin
2016-04-28
Two-dimensional polymers are of great interest for many potential applications in nanotechnology. The preparation of crystalline 2D polymers with a tunable band gap is critical for their applications in nano-electronics and optoelectronics. In this work, we try to tune the band gap of 2D imine polymers by expanding the conjugation of the backbone of aromatic diamines both laterally and longitudinally. STM characterization reveals that the regularity of the 2D polymers can be affected by the existence of lateral bulky groups. Density functional theory (DFT) simulations discovered a significant narrowing of the band gap of imine 2D polymers upon the expansion of the conjugation of the monomer backbone, which has been confirmed experimentally by UV absorption measurements. Monte Carlo simulations help us to gain further insight into the controlling factors of the formation of regular 2D polymers, which demonstrated that based on the all rigid assumption, the coexistence of different conformations of the imine moiety has a significant effect on the regularity of the imine 2D polymers. PMID:27049517
Brandt, Rasmus G.; Sveegaard, Steffen G.; Xiao, Manjun;
2016-01-01
In this communication, we investigate the direct and still conjugated intramolecular +N=B- interactions in novel high band gap borylated carbazole containing polymers, namely, poly(3,6-(N-di(2,4,6-trimethyl)-phenylboryl-carbazole)-alt- 4,8-di(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b: 4,5-b'] di...
Aqueous Processing of Low-Band-Gap Polymer Solar Cells Using Roll-to-Roll Methods
Andersen, Thomas Rieks; Larsen-Olsen, Thue Trofod; Andreasen, Birgitta;
2011-01-01
. This enabled slot-die coating of high quality films with a dry thickness of 126 ± 19, 500 ± 25, and 612 ± 22 nm P1, P2, and P3, respectively for polymer solar cells. Large area inverted polymer solar cells were thus prepared based on the aqueous inks. The power conversion efficiency (PCE) reached for each...... films. The aqueous dispersions were dialyzed to remove excess detergent and concentrated to a solid content of approximately 60 mg mL–1. The formation of films for solar cells using the aqueous dispersion required the addition of the nonionic detergent FSO-100 at a concentration of 5 mg mL–1...
Ashraf, Raja Shahid
2015-01-28
The design, synthesis, and characterization of a series of diketopyrrolopyrrole-based copolymers with different chalcogenophene comonomers (thiophene, selenophene, and tellurophene) for use in field-effect transistors and organic photovoltaic devices are reported. The effect of the heteroatom substitution on the optical, electrochemical, and photovoltaic properties and charge carrier mobilities of these polymers is discussed. The results indicate that by increasing the size of the chalcogen atom (S < Se < Te), polymer band gaps are narrowed mainly due to LUMO energy level stabilization. In addition, the larger heteroatomic size also increases intermolecular heteroatom-heteroatom interactions facilitating the formation of polymer aggregates leading to enhanced field-effect mobilities of 1.6 cm2/(V s). Bulk heterojunction solar cells based on the chalcogenophene polymer series blended with fullerene derivatives show good photovoltaic properties, with power conversion efficiencies ranging from 7.1-8.8%. A high photoresponse in the near-infrared (NIR) region with excellent photocurrents above 20 mA cm-2 was achieved for all polymers, making these highly efficient low band gap polymers promising candidates for use in tandem solar cells. (Graph Presented).
Microstrip microwave band gap structures
V Subramanian
2008-04-01
Microwave band gap structures exhibit certain stop band characteristics based on the periodicity, impedance contrast and effective refractive index contrast. These structures though formed in one-, two- and three-dimensional periodicity, are huge in size. In this paper, microstrip-based microwave band gap structures are formed by removing the substrate material in a periodic manner. This paper also demonstrates that these structures can serve as a non-destructive characterization tool for materials, a duplexor and frequency selective coupler. The paper presents both experimental results and theoretical simulation based on a commercially available finite element methodology for comparison.
Sodium ion conducting polymer blend electrolyte films, based on polyethylene oxide (PEO) and polyvinyl pyrrolidone (PVP) complexed with NaF salt, were prepared using solution casting technique. The complexation of the salt with the polymer blend was confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and UV-vis spectroscopy. Electrical conductivity of the films was measured with impedance analyzer in the frequency range of 1 Hz to 1 MHz and in the temperature range of 303-348 K. It was observed that the magnitude of conductivity increased with the increase in the salt concentration as well as the temperature. UV-vis absorption spectra in wavelength region of 200-800 nm were used to evaluate the optical properties like direct and indirect optical energy band gaps, optical absorption edge. The optical band gaps decreased with the increase in Na+ ion concentration. This suggests that NaF, as a dopant, is a good choice to improve the electrical properties of PEO/PVP polymer blend electrolytes.
Direct band gap silicon allotropes.
Wang, Qianqian; Xu, Bo; Sun, Jian; Liu, Hanyu; Zhao, Zhisheng; Yu, Dongli; Fan, Changzeng; He, Julong
2014-07-16
Elemental silicon has a large impact on the economy of the modern world and is of fundamental importance in the technological field, particularly in solar cell industry. The great demand of society for new clean energy and the shortcomings of the current silicon solar cells are calling for new materials that can make full use of the solar power. In this paper, six metastable allotropes of silicon with direct or quasidirect band gaps of 0.39-1.25 eV are predicted by ab initio calculations at ambient pressure. Five of them possess band gaps within the optimal range for high converting efficiency from solar energy to electric power and also have better optical properties than the Si-I phase. These Si structures with different band gaps could be applied to multiple p-n junction photovoltaic modules. PMID:24971657
1999-01-01
An optical fibre having a periodicidal cladding structure provididing a photonic band gap structure with superior qualities. The periodical structure being one wherein high index areas are defined and wherein these are separated using a number of methods. One such method is the introduction of...
Ions in a plasma may be radially separated according to mass using a combination of an axial magnetic field and either a radial or azimuthal electric field. The separation is qualitatively different from that obtained by a plasma centrifuge and the characteristics of confined and unconfined ion orbits are analogous to the phenomenon of band gaps in semiconductors
Dadmun, Mark; Chen, Huipeng; Hsiao, Yu-Che; Hu, Bin
2014-03-01
One of the most important challenges facing our society is the development of technologies for renewable energy conversion. Polymeric bulk-heterojunction (BHJ) photovoltaics, based on conjugated polymers and fullerenes, are an economically viable option for low cost renewable power generation. The most promising conjugated polymer:fullerene active layers in organic photovoltaics now utilize low band-gap (LBG) copolymers. Unfortunately, for most of these LBG devices, the as-cast film is not usually optimal, and there are few further treatment available after film deposition to optimize the morphology. To address this problem, we have exploited the selective solubility of the LBG:fullerene nanocomposite components to direct the assembly of these mixtures by annealing in the vapor of a selective solvent. Our recent work demonstrates that annealing in a solvent that is selective to the fullerene forms a sample with fullerene aggregation, while annealing in a solvent vapor that is selective to the polymer forms a thin film with polymer precipitation. There is also a direct correlation between the resultant morphology and OPV performance, increasing PCE by 190%. These results indicate that solvent annealing and solvent choice provides a unique tool to precisely tune the morphology of CP:Fullerene BHJ systems, optimizing the morphology and performance of the active layer.
Huh, Yoon Ho; Park, Byoungchoo
2013-01-14
We herein report on the improved photovoltaic (PV) effects of using a polymer bulk-heterojunction (BHJ) layer that consists of a low-band gap electron donor polymer of poly(N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)) (PCDTBT) and an acceptor of [6,6]-phenyl C₇₁ butyric acid methyl ester (PCBM₇₀), doped with an interface-engineering surfactant additive of poly(oxyethylene tridecyl ether) (PTE). The presence of an interface-engineering additive in the PV layer results in excellent performance; the addition of PTE to a PCDTBT:PCBM₇₀ system produces a power conversion efficiency (PCE) of 6.0%, which is much higher than that of a reference device without the additive (4.9%). We attribute this improvement to an increased charge carrier lifetime, which is likely to be the result of the presence of PTE molecules oriented at the interfaces between the BHJ PV layer and the anode and cathode, as well as at the interfaces between the phase-separated BHJ domains. Our results suggest that the incorporation of the PTE interface-engineering additive in the PCDTBT:PCBM₇₀ PV layer results in a functional composite system that shows considerable promise for use in efficient polymer BHJ PV cells. PMID:23389265
Wolf, Jannic
2015-05-27
We report on a wide band-gap polymer donor composed of benzo[1,2-b:4,5-b\\']dithiophene (BDT) and 3,4-difluorothiophene ([2F]T) units (Eopt ~2.1 eV), and show that the fluorinated analog PBDT[2F]T performs significantly better than its non-fluorinated counterpart PBDT[2H]T in BHJ solar cells with PC71BM. While control P3HT- and PBDT[2H]T-based devices yield PCEs of ca. 4% and 3% (Max.) respectively, PBDT[2F]T-based devices reach PCEs of ca. 7%, combining a large Voc of ca. 0.9 V and short-circuit current values (ca. 10.7 mA/cm2) comparable to those of the best P3HT-based control devices.
Helgesen, Martin; Sørensen, Thomas J.; Manceau, Matthieu;
2011-01-01
indicating greater interchain packing when the side chains are situated on the thienyl groups compared to on the benzothiadiazole unit. The best photovoltaic devices based on blends of polymer and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were prepared with polymers based on the DBT1 unit giving...
Mondal, Rajib
2011-01-01
A strategic side-chain engineering approach leads to the two orders of magnitude enhancement of charge carrier mobility in phenanthrene based fused aromatic thienopyrazine polymers. Hole carrier mobility up to 0.012 cm 2/Vs can be obtained in thin film transistor devices. Polymers were also utilized to fabricate bulk heterojunction photovoltaic devices and the maximum PCE obtained in these OPV\\'s was 1.15%. Most importantly, performances of the devices were correlated with thin morphological analysis performed by atomic force microscopy and grazing incidence X-ray scattering. © 2011 The Royal Society of Chemistry.
Photonic band gap structure simulator
Chen, Chiping; Shapiro, Michael A.; Smirnova, Evgenya I.; Temkin, Richard J.; Sirigiri, Jagadishwar R.
2006-10-03
A system and method for designing photonic band gap structures. The system and method provide a user with the capability to produce a model of a two-dimensional array of conductors corresponding to a unit cell. The model involves a linear equation. Boundary conditions representative of conditions at the boundary of the unit cell are applied to a solution of the Helmholtz equation defined for the unit cell. The linear equation can be approximated by a Hermitian matrix. An eigenvalue of the Helmholtz equation is calculated. One computation approach involves calculating finite differences. The model can include a symmetry element, such as a center of inversion, a rotation axis, and a mirror plane. A graphical user interface is provided for the user's convenience. A display is provided to display to a user the calculated eigenvalue, corresponding to a photonic energy level in the Brilloin zone of the unit cell.
Semiconductor resonator solitons above band gap
Taranenko, V. B.; Weiss, C. O.; Stolz, W.
2001-01-01
We show experimentally the existence of bright and dark spatial solitons in semiconductor resonators for excitation above the band gap energy. These solitons can be switched on, both spontaneously and with address pulses, without the thermal delay found for solitons below the band gap which is unfavorable for applications. The differences between soliton properties above and below gap energy are discussed.
Schroeder, Bob C.; Huang, Zhenggang; Ashraf, Raja Shahid; Durrant, James R.; McCulloch, Iain [Department of Chemistry and Centre for Plastic Electronics, Imperial College London (United Kingdom); Smith, Jeremy; D' Angelo, Pasquale; Anthopoulos, Thomas D. [Department of Physics and Centre for Plastic Electronics, Imperial College London (United Kingdom); Watkins, Scott E. [CSIRO Materials Science and Engineering, Melbourne, VIC (Australia)
2012-04-24
Silaindacenodithiophene is copolymerized with benzo[c][1,2,5]thiadiazole (BT) and 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT), respectively their fluorinated counter parts 5,6-difluorobenzo[c][1,2,5]thiadiazole (2FBT) and 5,6-difluoro-4,7-di(thiophen-2-yl) benzo[c][1,2,5]thiadiazole (2FDTBT). The influence of the thienyl spacers and fluorine atoms on molecular packing and active layer morphology is investigated with regard to device performances. bulk heterojunction (BHJ) solar cells based on silaindacenodithiophene donor-acceptor polymers achieved PCE's of 4.5% and hole mobilities of as high as 0.28 cm{sup 2}/(V s) are achieved in an organic field-effect transistor (OFET). (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)
Band gaps of primary metallic carbon nanotubes
Kleiner, Alex; Eggert, Sebastian
2000-01-01
Primary metallic, or small gap semiconducting nanotubes, are tubes with band gaps that arise solely from breaking the bond symmetry due to the curvature. We derive an analytic expression for these gaps by considering how a general symmetry breaking opens a gap in nanotubes with a well defined chiral wrapping vector. This approach provides a straightforward way to include all types of symmetry breaking effects, resulting in a simple unified gap equation as a function of chirality and deformati...
Maximizing band gaps in plate structures
Halkjær, Søren; Sigmund, Ole; Jensen, Jakob Søndergaard
2006-01-01
Band gaps, i.e., frequency ranges in which waves cannot propagate, can be found in elastic structures for which there is a certain periodic modulation of the material properties or structure. In this paper, we maximize the band gap size for bending waves in a Mindlin plate. We analyze an infinite...
Mondal, Rajib
2009-01-01
Removing the adjacent thiophene groups around the acceptor core in low band gap polymers significantly enhances solar cell efficiency through increasing the optical absorption and raising the ionization potential of the polymer. © 2009 The Royal Society of Chemistry.
One-Dimensional Anisotropic Band Gap Structure
无
2000-01-01
The band gap structure of one-dimensional anisotropic photonic crystal has been studied by means of the transfer matrix formalism. From the analytic expressions and numeric calculations we see some general characteristics of the band gap structure of anisotropic photonic crystals, each band separates into two branches and the two branches react to polarization sensitively. In the practical case of oblique incidence, gaps move towards high frequency when the angle of incidence increases. Under some special conditions, the two branches become degenerate again.
Optimum band gap of a thermoelectric material
Transport properties of direct-gap semiconductors are calculated in order to find the best thermoelectrics. Previous calculations on semiconductors with indirect band gaps found that the best thermoelectrics had gaps equal to nkBT, where n=6-10 and T is the operating temperature of the thermoelectric device. Here we report similar calculations on direct-gap materials. We find that the optimum gap is always greater than 6kBT, but can be much larger depending on the specific mechanism of electron scattering
Bulk band gaps in divalent hexaborides
Denlinger, Jonathan; Clack, Jules A.; Allen, James W.; Gweon, Gey-Hong; Poirier, Derek M.; Olson, Cliff G.; Sarrao, John L.; Bianchi, Andrea D.; Fisk, Zachary
2002-08-01
Complementary angle-resolved photoemission and bulk-sensitive k-resolved resonant inelastic x-ray scattering of divalent hexaborides reveal a >1 eV X-point gap between the valence and conduction bands, in contradiction to the band overlap assumed in several models of their novel ferromagnetism. This semiconducting gap implies that carriers detected in transport measurements arise from defects, and the measured location of the bulk Fermi level at the bottom of the conduction band implicates boron vacancies as the origin of the excess electrons. The measured band structure and X-point gap in CaB6 additionally provide a stringent test case for proper inclusion of many-body effects in quasi-particle band calculations.
Highlights: • Three D-A type polymers based on 2,3-di(2-furyl) quinoxaline were synthesized and characterized. • The structure of substitution influences electrochromic properties of the polymers • All three polymers are both p- and n-type dopable and show excellent electrochromic properties. - Abstract: Three donor–acceptor type π-conjugated polymers were synthesized electrochemically:poly[2,3-di(2-furyl)-5,8-bis (2-(3,4-ethylenedioxythiophene)) quinoxaline] (PFETQ), poly[2,3-di(2-furyl)-5,8-bis(2-thienyl) quinoxaline] (PFTQ) and poly[2,3-di(2-furyl)-5,8-bis(2-(3-methoxythiophene)) quinoxaline] (PFMTQ). All of the synthesized polymers, contained the 2,3-di(2-furyl) quinoxaline moiety in the backbone as the acceptor unit and different thiophene derivatives as the donor units. The electroactivity of the monomers and the electrochemical properties of their polymers were investigated by cyclic voltammetry. The presence of the strong electron-donating ethylenedioxy and methoxy groups on the aromatic structure increased the electron density. Thus, the oxidation potential of FETQ and FMTQ shifted to a lower value than that of FTQ. The optical properties of the polymers were investigated by UV–vis–NIR spectroscopy. Both PFETQ and PFMTQ reveal two distinct absorption bands in the red and blue regions of the visible spectrum, while PFTQ has only one dominant wavelength at 596 nm in the visible region. The colorimetry analysis revealed that while PFTQ has a light blue color, PFETQ and PFMTQ are green in the neutral state. The optical band gaps, defined as the onset of the π–π* transition, were found to be 1.15 eV for PFETQ, 1.2 eV for PFMTQ and 1.34 eV for PFTQ. Moreover, all three polymers showed both n-doping and fast switching times
Wide band-gap nanostructure based devices
Chen, Xinyi; 陈辛夷
2012-01-01
Wide band gap based nanostructures have being attracting much research interest because of their promise for application in optoelectronic devices. Among those wide band gap semiconductors, gallium nitride (GaN) and zinc oxide (ZnO) are the most commonly studied and optoelectronic devices based on GaN and ZnO have been widely investigated. This thesis concentrates on the growth, optical and electrical properties of GaN and ZnO nanostructures, plus their application in solar cells and light e...
Phononic band gap structures as optimal designs
Jensen, Jakob Søndergaard; Sigmund, Ole
2003-01-01
In this paper we use topology optimization to design phononic band gap structures. We consider 2D structures subjected to periodic loading and obtain the distribution of two materials with high contrast in material properties that gives the minimal vibrational response of the structure. Both in...
Sculpting the band gap: a computational approach.
Prasai, Kiran; Biswas, Parthapratim; Drabold, D A
2015-01-01
Materials with optimized band gap are needed in many specialized applications. In this work, we demonstrate that Hellmann-Feynman forces associated with the gap states can be used to find atomic coordinates that yield desired electronic density of states. Using tight-binding models, we show that this approach may be used to arrive at electronically designed models of amorphous silicon and carbon. We provide a simple recipe to include a priori electronic information in the formation of computer models of materials, and prove that this information may have profound structural consequences. The models are validated with plane-wave density functional calculations. PMID:26490203
Proceedings of wide band gap semiconductors
This book contains the proceedings of wide band gap semiconductors. Wide band gap semiconductors are under intense study because of their potential applications in photonic devices in the visible and ultraviolet part of the electromagnetic spectrum, and devices for high temperature, high frequency and high power electronics. Additionally, due to their unique mechanical, thermal, optical, chemical, and electronic properties many wide band gap semiconductors are anticipated to find applications in thermoelectric, electrooptic, piezoelectric and acoustooptic devices as well as protective coatings, hard coatings and heat sinks. Material systems covered in this symposium include diamond, II-VI compounds, III-V nitrides, silicon carbide, boron compounds, amorphous and microcrystalline semiconductors, chalcopyrites, oxides and halides. The various papers addressed recent experimental and theoretical developments. They covered issues related to crystal growth (bulk and thin films), structure and microstructure, defects, doping, optoelectronic properties and device applications. A theoretical session was dedicated to identifying common themes in the heteroepitaxy and the role of defects in doping, compensation and phase stability of this unique class of materials. Important experimental milestones included the demonstrations of bright blue injection luminescence at room temperatures from junctions based on III-V nitrides and a similar result from multiple quantum wells in a ZnSe double heterojunction at liquid nitrogen temperatures
Optical band gaps of organic semiconductor materials
Costa, José C. S.; Taveira, Ricardo J. S.; Lima, Carlos F. R. A. C.; Mendes, Adélio; Santos, Luís M. N. B. F.
2016-08-01
UV-Vis can be used as an easy and forthright technique to accurately estimate the band gap energy of organic π-conjugated materials, widely used as thin films/composites in organic and hybrid electronic devices such as OLEDs, OPVs and OFETs. The electronic and optical properties, including HOMO-LUMO energy gaps of π-conjugated systems were evaluated by UV-Vis spectroscopy in CHCl3 solution for a large number of relevant π-conjugated systems: tris-8-hydroxyquinolinatos (Alq3, Gaq3, Inq3, Al(qNO2)3, Al(qCl)3, Al(qBr)3, In(qNO2)3, In(qCl)3 and In(qBr)3); triphenylamine derivatives (DDP, p-TTP, TPB, TPD, TDAB, m-MTDAB, NPB, α-NPD); oligoacenes (naphthalene, anthracene, tetracene and rubrene); oligothiophenes (α-2T, β-2T, α-3T, β-3T, α-4T and α-5T). Additionally, some electronic properties were also explored by quantum chemical calculations. The experimental UV-Vis data are in accordance with the DFT predictions and indicate that the band gap energies of the OSCs dissolved in CHCl3 solution are consistent with the values presented for thin films.
Highly dispersive photonic band-gap prism
We propose the concept of a photonic band-gap (PBG) prism based on two-dimensional PBG structures and realize it in the millimeter-wave spectral regime. We recognize the highly nonlinear dispersion of PBG materials near Brillouin zone edges and utilize the dispersion to achieve strong prism action. Such a PBG prism is very compact if operated in the optical regime, ∼20μm in size for λ∼700nm, and can serve as a dispersive element for building ultracompact miniature spectrometers. copyright 1996 Optical Society of America
Highly dispersive photonic band-gap prism.
Lin, S Y; Hietala, V M; Wang, L; Jones, E D
1996-11-01
We propose the concept of a photonic band-gap (PBG) prism based on two-dimensional PBG structures and realize it in the millimeter-wave spectral regime. We recognize the highly nonlinear dispersion of PBG materials near Brillouin zone edges and utilize the dispersion to achieve strong prism action. Such a PBG prism is very compact if operated in the optical regime, ~20 mm in size for lambda ~ 700 nm, and can serve as a dispersive element for building ultracompact miniature spectrometers. PMID:19881796
The band-gap enhanced photovoltaic structure
Tessler, Nir
2016-05-01
We critically examine the recently suggested structure that was postulated to potentially add 50% to the photo-conversion efficiency of organic solar cells. We find that the structure could be realized using stepwise increase in the gap as long as the steps are not above 0.1 eV. We also show that the charge extraction is not compromised due to an interplay between the contact's space charge and the energy level modification, which result in a flat energy band at the extracting contact.
Photonic-Band-Gap Resonator Gyrotron
We report the design and experimental demonstration of a gyrotron oscillator using a photonic-band-gap (PBG) structure to eliminate mode competition in a highly overmoded resonator. The PBG cavity supports a TE041 -like mode at 140GHz and is designed to have no competing modes over a minimum frequency range δω/ω of 30% about the design mode. Experimental operation of a PBG gyrotron at 68kV and 5A produced 25kW of peak power in the design mode. No other modes were observed over the full predicted operating range about the design mode. PBG cavities show great promise for applications in vacuum electron devices in the millimeter- and submillimeter-wave bands
Size effects in band gap bowing in nitride semiconducting alloys
Gorczyca, I.; Suski, T.; Christensen, Niels Egede;
2011-01-01
Chemical and size contributions to the band gap bowing of nitride semiconducting alloys (InxGa1-xN, InxAl1-xN, and AlxGa1-xN) are analyzed. It is shown that the band gap deformation potentials of the binary constituents determine the gap bowing in the ternary alloys. The particularly large gap...
Formation of Degenerate Band Gaps in Layered Systems
Alexey P. Vinogradov
2012-06-01
Full Text Available In the review, peculiarities of spectra of one-dimensional photonic crystals made of anisotropic and/or magnetooptic materials are considered. The attention is focused on band gaps of a special type—the so called degenerate band gaps which are degenerate with respect to polarization. Mechanisms of formation and properties of these band gaps are analyzed. Peculiarities of spectra of photonic crystals that arise due to the linkage between band gaps are discussed. Particularly, it is shown that formation of a frozen mode is caused by linkage between Brillouin and degenerate band gaps. Also, existence of the optical Borrmann effect at the boundaries of degenerate band gaps and optical Tamm states at the frequencies of degenerate band gaps are analyzed.
Electronic band gap of SrSe at high pressure
David Atkinson, Timothy; Chynoweth, Katie Mae; Cervantes, Phillip
2006-08-01
The electronic band gap of SrSe, in the CsCl-stuctured phase, was measured to 42 GPa via optical absorption studies. The indirect electronic band gap was found to close monotonically with pressure for the range of pressures studied. The change in band gap with respect to pressure, dE gap/dP, was determined to be -6.1(5)×10 -3 eV/GPa. By extrapolation of our line fit, we estimate band gap closure to occur at 180(20) GPa.
Limitations to band gap tuning in nitride semiconductor alloys
Gorczyca, I.; Suski, T.; Christensen, Niels Egede;
2010-01-01
Relations between the band gaps of nitride alloys and their lattice parameters are presented and limits to tuning of the fundamental gap in nitride semiconductors are set by combining a large number of experimental data with ab initio theoretical calculations. Large band gap bowings obtained...
Yiu, Alan T.
2012-02-01
The solution-processability of conjugated polymers in organic solvents has classically been achieved by modulating the size and branching of alkyl substituents appended to the backbone. However, these substituents impact structural order and charge transport properties in thin-film devices. As a result, a trade-off must be found between material solubility and insulating alkyl content. It was recently shown that the substitution of furan for thiophene in the backbone of the polymer PDPP2FT significantly improves polymer solubility, allowing for the use of shorter branched side chains while maintaining high device efficiency. In this report, we use PDPP2FT to demonstrate that linear alkyl side chains can be used to promote thin-film nanostructural order. In particular, linear side chains are shown to shorten π-π stacking distances between backbones and increase the correlation lengths of both π-π stacking and lamellar spacing, leading to a substantial increase in the efficiency of bulk heterojunction solar cells. © 2011 American Chemical Society.
Strain sensitivity of band gaps of Sn-containing semiconductors
Li, Hong; Castelli, Ivano Eligio; Thygesen, Kristian Sommer;
2015-01-01
functional theory and many-body perturbation theory calculations. We find that the band gaps of bulk Sn oxides with SnO6 octahedra are highly sensitive to volumetric strain. By applying a small isotropic strain of 2% (-2%), a decrease (increase) of band gaps as large as 0.8 to 1.0 eV are obtained. We...
Modeling charged defects inside density functional theory band gaps
Density functional theory (DFT) has emerged as an important tool to probe microscopic behavior in materials. The fundamental band gap defines the energy scale for charge transition energy levels of point defects in ionic and covalent materials. The eigenvalue gap between occupied and unoccupied states in conventional DFT, the Kohn–Sham gap, is often half or less of the experimental band gap, seemingly precluding quantitative studies of charged defects. Applying explicit and rigorous control of charge boundary conditions in supercells, we find that calculations of defect energy levels derived from total energy differences give accurate predictions of charge transition energy levels in Si and GaAs, unhampered by a band gap problem. The GaAs system provides a good theoretical laboratory for investigating band gap effects in defect level calculations: depending on the functional and pseudopotential, the Kohn–Sham gap can be as large as 1.1 eV or as small as 0.1 eV. We find that the effective defect band gap, the computed range in defect levels, is mostly insensitive to the Kohn–Sham gap, demonstrating it is often possible to use conventional DFT for quantitative studies of defect chemistry governing interesting materials behavior in semiconductors and oxides despite a band gap problem
Band-gap and band-edge engineering of multicomponent garnet scintillators from first principles
Yadav, S.K.; Uberuaga, B.P.; Nikl, Martin; Jiang, C.; Stanek, C.R.
2015-01-01
Roč. 4, č. 5 (2015), "054012-1"-"054012-9". ISSN 2331-7019 R&D Projects: GA ČR GAP204/12/0805 Institutional support: RVO:68378271 Keywords : scintillator * electronic band gap structure * garnets * band gap engineering Subject RIV: BM - Solid Matter Physics ; Magnetism
Band Gaps of an Amorphous Photonic Materials
WANG Yi-Quan; FENG Zhi-Fang; HU Xiao-Yong; CHENG Bing-Ying; ZHANG Dao-Zhong
2004-01-01
@@ A new kind of amorphous photonic materials is presented. Both the simulated and experimental results show that although the disorder of the whole dielectric structure is strong, the amorphous photonic materials have two photonic gaps. This confirms that the short-range order is an essential factor for the formation of the photonic gaps.
Vargas, W. E.; Hernández-Jiménez, M.; Libby, E.; Azofeifa, D. E.; Solis, Á.; Barboza-Aguilar, C.
2015-09-01
Under normal illumination with non-polarized light, reflection spectra of the cuticle of golden-like and red Chrysina aurigans scarabs show a structured broad band of left-handed circularly polarized light. The polarization of the reflected light is attributed to a Bouligand-type left-handed chiral structure found through the scarab's cuticle. By considering these twisted structures as one-dimensional photonic crystals, a novel approach is developed from the dispersion relation of circularly polarized electromagnetic waves traveling through chiral media, to show how the broad band characterizing these spectra arises from an intrinsic narrow photonic band gap whose spectral position moves through visible and near-infrared wavelengths.
Resolution of the Band Gap Prediction Problem for Materials Design.
Crowley, Jason M; Tahir-Kheli, Jamil; Goddard, William A
2016-04-01
An important property with any new material is the band gap. Standard density functional theory methods grossly underestimate band gaps. This is known as the band gap problem. Here, we show that the hybrid B3PW91 density functional returns band gaps with a mean absolute deviation (MAD) from experiment of 0.22 eV over 64 insulators with gaps spanning a factor of 500 from 0.014 to 7 eV. The MAD is 0.28 eV over 70 compounds with gaps up to 14.2 eV, with a mean error of -0.03 eV. To benchmark the quality of the hybrid method, we compared the hybrid method to the rigorous GW many-body perturbation theory method. Surprisingly, the MAD for B3PW91 is about 1.5 times smaller than the MAD for GW. Furthermore, B3PW91 is 3-4 orders of magnitude faster computationally. Hence, B3PW91 is a practical tool for predicting band gaps of materials before they are synthesized and represents a solution to the band gap prediction problem. PMID:26944092
Sub-band-gap laser micromachining of lithium niobate
Christensen, F. K.; Müllenborn, Matthias
1995-01-01
Laser processing of insulators and semiconductors is usually realized using photon energies exceeding the band-gap energy. This makes laser processing of insulators difficult since high photon energies typically require either a pulsed laser or a frequency-doubled continuous-wave laser. A new...... method is reported which enables us to do laser processing of lithium niobate using sub-band-gap photons. Using high scan speeds, moderate power densities, and sub-band-gap photon energies results in volume removal rates in excess of 106µm3/s. This enables fast micromachining of small piezoelectric...
Design for maximum band-gaps in beam structures
Olhoff, Niels; Niu, Bin; Cheng, Gengdong
2012-01-01
This paper aims to extend earlier optimum design results for transversely vibrating Bernoulli-Euler beams by determining new optimum band-gap beam structures for (i) different combinations of classical boundary conditions, (ii) much larger values of the orders n and n-1 of adjacent upper and lower...... eigenfrequencies of maximized band-gaps, and (iii) different values of a minimum cross-sectional area constraint. The periodicity of the optimum beams and the attenuation of their band-gaps are also discussed....
Replication technology for photonic band gap applications
Grigaliunas, V.; Kopustinskas, V.; Meskinis, S.; Margelevicius, M.; Mikulskas, I.; Tomasiunas, R.
2001-06-01
Replication technology was applied for photonic structure fabrication in silicon substrate. It was revealed, that thin thermoplastic polymer layers on silicon substrates may be patterned by hot embossing technique for dry etching masking. Ni mold used for plain hot embossing into polymer layers was fabricated by Ni electrochemical deposition on the reference silicon surface structure, which was obtained by direct electron beam (EB) writing and SF 6/N 2 reactive ion etching (RIE) technique. It is shown that the shape of replicated photonic structures is determined by RIE parameters.
Band gap effects of hexagonal boron nitride using oxygen plasma
Tuning of band gap of hexagonal boron nitride (h-BN) has been a challenging problem due to its inherent chemical stability and inertness. In this work, we report the changes in band gaps in a few layers of chemical vapor deposition processed as-grown h-BN using a simple oxygen plasma treatment. Optical absorption spectra show a trend of band gap narrowing monotonically from 6 eV of pristine h-BN to 4.31 eV when exposed to oxygen plasma for 12 s. The narrowing of band gap causes the reduction in electrical resistance by ∼100 fold. The x-ray photoelectron spectroscopy results of plasma treated hexagonal boron nitride surface show the predominant doping of oxygen for the nitrogen vacancy. Energy sub-band formations inside the band gap of h-BN, due to the incorporation of oxygen dopants, cause a red shift in absorption edge corresponding to the band gap narrowing
Tunable Band Gap of Boron Nitride Interfaces under Uniaxial Pressure
Moraes, Elizane; Manhabosco, Taise; de Oliveira, Alan; Batista, Ronaldo
2013-03-01
In this work we show, by means of a density functional theory formalism, that the interaction between hydrogen terminated boron nitride surfaces gives rise to a metallic interface with free carries of opposite sign at each surface. A band gap can be induced by decreasing the surface separation. The size of the band gap changes continuously from zero up to 4.4 eV with decreasing separation, which is understood in terms of the interaction between surface states.Due to the high thermal conductivity of cubic boron nitride and the coupling between band gap and applied pressure, such tunable band gap interfaces may be used in high stable electronic and electromechanical devices. In addition, the spacial separation of charge carries at the interface may lead to photovoltaic applications. The authors thank tha brazilian agencies Fapemig, CNPq and Capes
Photonic band gap engineering in 2D photonic crystals
Yogita Kalra; R K Sinha
2006-12-01
The polarization-dependent photonic band gaps (TM and TE polarizations) in two-dimensional photonic crystals with square lattices composed of air holes in dielectric and vice versa i.e., dielectric rods in air, using the plane-wave expansion method are investigated. We then study, how the photonic band gap size is affected by the changing ellipticity of the constituent air holes/dielectric rods. It is observed that the size of the photonic band gap changes with changing ellipticity of the constituent air holes/dielectric rods. Further, it is reported, how the photonic band gap size is affected by the change in the orientation of the constituent elliptical air holes/dielectric rods in 2D photonic crystals.
Luminescence from wide band gap materials and their applications
Shinde, S. L.; Senapati, S.; Nanda, K. K.
2015-03-01
We demonstrate ZnO and In2O3 microcrystals as an optical probe for wide range thermometry. Both ZnO and In2O3 microcrystals exhibit a monotonic decrease in luminescence intensities with increase in temperature. The variation has been explored to develop a thermometer in a wide temperature range. We also demonstrate enhanced brightness from broad-luminescent-wide band gap materials when sensitized with low band gap CdTe quantum dots. Wide band gap materials act as acceptors, while CdTe act as donors. One of the major implications is the designing of weak-luminescent-wide-band gap materials as bright white light emitting phosphors that can convert the ultraviolet into visible light. Invited talk at the 7th International Workshop on Advanced Materials Science and Nanotechnology IWAMSN2014, 2-6 November, 2014, Ha Long, Vietnam.
Band Gap Engineering of Two-Dimensional Nitrogene
Lee, Jason; Wang, Wei-Liang; Yao, Dao-Xin
2016-01-01
Recently, two-dimensional honeycomb monolayers of pnictogen have been predicted. In particular, the honeycomb monolayer of nitrogen has been studied, and we call it nitrogene. In this paper, we investigate the band structure of nitrogene under various conditions: stacking of monolayers, biaxial tensile strain, and perpendicular electric field. The band gap of nitrogene is found to decrease with increasing number of layers. The perpendicular electric field can also decrease the energy gap, and...
An active viscoelastic metamaterial with enhanced band gap properties
Reynolds, Matthew; Gao, Yan; Daley, Stephen
2013-01-01
Metamaterials have been the subject of significant interest over the past decade due to their ability to produce novel acoustic behaviour beyond that seen in naturally occurring media. Of particular interest is the appearance of band gaps which lead to very high levels of attenuation across the material within narrow frequency ranges. Unlike traditional periodic materials which have been employed at high frequencies, the resonant elements within metamaterials allow band gaps to form within th...
Photonic band gap of 2D complex lattice photonic crystal
GUAN Chun-ying; YUAN Li-bo
2009-01-01
It is of great significance to present a photonic crystal lattice structure with a wide photonic bandgap. A two-dimension complex lattice photonic crystal is proposed. The photonic crystal is composed of complex lattices with triangular structure, and each single cell is surrounded by six scatterers in an hexagon. The photonic band gaps are calculated based on the plane wave expansion (PWE) method. The results indicate that the photonic crystal has tunable large TM polarization band gap, and a gap-midgap ratio of up to 45.6%.
Band-gap engineering at a semiconductor - crystalline oxide interface
Moghadam, J.; Ahmadi-Majlan, K.; Shen, X; Droubay, T.; Bowden, M; Chrysler, M.; Su, D.; Chambers, S. A.; Ngai, J. H.
2014-01-01
The epitaxial growth of crystalline oxides on semiconductors provides a pathway to introduce new functionalities to semiconductor devices. Key to electrically coupling crystalline oxides with semiconductors to realize functional behavior is controlling the manner in which their bands align at interfaces. Here we apply principles of band gap engineering traditionally used at heterojunctions between conventional semiconductors to control the band offset between a single crystalline oxide and a ...
Optimum design of band-gap beam structures
Olhoff, Niels; Niu, Bin; Cheng, Gengdong
2012-01-01
of a single, linearly elastic material without damping. Numerical results are presented for different combinations of classical boundary conditions, prescribed orders of the upper and lower natural frequencies of maximized natural frequency gaps, and a given minimum constraint value for the beam......The design of band-gap structures receives increasing attention for many applications in mitigation of undesirable vibration and noise emission levels. A band-gap structure usually consists of a periodic distribution of elastic materials or segments, where the propagation of waves is impeded or...... significantly suppressed for a range of external excitation frequencies. Maximization of the band-gap is therefore an obvious objective for optimum design. This problem is sometimes formulated by optimizing a parameterized design model which assumes multiple periodicity in the design. However, it is shown in...
Band gap engineering strategy via polarization rotation in perovskite ferroelectrics
We propose a strategy to engineer the band gaps of perovskite oxide ferroelectrics, supported by first principles calculations. We find that the band gaps of perovskites can be substantially reduced by as much as 1.2 eV through local rhombohedral-to-tetragonal structural transition. Furthermore, the strong polarization of the rhombohedral perovskite is largely preserved by its tetragonal counterpart. The B-cation off-center displacements and the resulting enhancement of the antibonding character in the conduction band give rise to the wider band gaps of the rhombohedral perovskites. The correlation between the structure, polarization orientation, and electronic structure lays a good foundation for understanding the physics of more complex perovskite solid solutions and provides a route for the design of photovoltaic perovskite ferroelectrics
Energy band gaps in graphene nanoribbons with corners
Szczȩśniak, Dominik; Durajski, Artur P.; Khater, Antoine; Ghader, Doried
2016-05-01
In the present paper, we study the relation between the band gap size and the corner-corner length in representative chevron-shaped graphene nanoribbons (CGNRs) with 120° and 150° corner edges. The direct physical insight into the electronic properties of CGNRs is provided within the tight-binding model with phenomenological edge parameters, developed against recent first-principle results. We show that the analyzed CGNRs exhibit inverse relation between their band gaps and corner-corner lengths, and that they do not present a metal-insulator transition when the chemical edge modifications are introduced. Our results also suggest that the band gap width for the CGNRs is predominantly governed by the armchair edge effects, and is tunable through edge modifications with foreign atoms dressing.
Amorphous Photonic Lattices: Band Gaps, Effective Mass and Suppressed Transport
Rechtsman, Mikael; Dreisow, Felix; Heinrich, Matthias; Keil, Robert; Nolte, Stefan; Segev, Mordechai
2010-01-01
We present, theoretically and experimentally, amorphous photonic lattices exhibiting a band-gap yet completely lacking Bragg diffraction: 2D waveguides distributed randomly according to a liquid-like model responsible for the absence of Bragg peaks as opposed to ordered lattices containing disorder, which always exhibit Bragg peaks. In amorphous lattices the bands are comprised of localized states, but we find that defect states residing in the gap are more localized than the Anderson localization length. Finally, we show how the concept of effective mass carries over to amorphous lattices.
On band gap predictions for multiresonant metamaterials on plates.
Yoritomo, John Y; Weaver, Richard L; Roux, Philippe; Rupin, Matthieu; Williams, Earl G
2016-03-01
Recently wide frequency band gaps were observed in an experimental realization of a multiresonant metamaterial for Lamb waves propagating in thin plates. The band gaps rose from hybridization between the flexural plate (A0 Lamb waves) and longitudinal resonances in rods attached perpendicularly. Shortly thereafter a theory based on considering a one-dimensional periodic array of rods and the scattering matrix for a single rod successfully described the observations. This letter presents an alternative simpler theory, arguably accurate at high rod density, that treats the full two-dimensional array of rods and makes no assumption of periodicity. This theory also fits the measurements. PMID:27036264
HAC: Band Gap, Photoluminescence, and Optical/Near-Infrared Absorption
Witt, Adolf N.; Ryutov, Dimitri; Furton, Douglas G.
1996-01-01
We report results of laboratory measurements which illustrate the wide range of physical properties found among hydrogenated amorphous carbon (HAC) solids. Within this range, HAC can match quantitatively the astronomical phenomena ascribed to carbonaceous coatings on interstellar grains. We find the optical band gap of HAC to be well correlated with other physical properties of HAC of astronomical interest, and conclude that interstellar HAC must be fairly hydrogen-rich with a band gap of E(sub g) is approx. greater than 2.0 eV.
Surface Plasmon-Induced Band Gap in the Photocurrent Response of Organic Solar Cells
Ribal Georges Sabat
2010-01-01
Full Text Available A 260 nm layer of organic bulk heterojunction blend of the polymer poly(3-hexylthiophene (P3HT and the fullerene [6,6]-phenyl C61-butyric (PCBM was spin-coated in between aluminum and gold electrodes, respectively, on top of a laser inscribed azo polymer surface-relief diffraction grating. Angle-dependent surface plasmons (SPs with a large band gap were observed in the normalized photocurrent by the P3HT-PCBM layer as a function of wavelength. The SP-induced photocurrents were also investigated as a function of the grating depth and spacing.
Band-Gap and Band-Edge Engineering of Multicomponent Garnet Scintillators from First Principles
Yadav, Satyesh K.; Uberuaga, Blas P.; Nikl, Martin; Jiang, Chao; Stanek, Christopher R.
2015-11-01
Complex doping schemes in R3 Al5 O12 (where R is the rare-earth element) garnet compounds have recently led to pronounced improvements in scintillator performance. Specifically, by admixing lutetium and yttrium aluminate garnets with gallium and gadolinium, the band gap is altered in a manner that facilitates the removal of deleterious electron trapping associated with cation antisite defects. Here, we expand upon this initial work to systematically investigate the effect of substitutional admixing on the energy levels of band edges. Density-functional theory and hybrid density-functional theory (HDFT) are used to survey potential admixing candidates that modify either the conduction-band minimum (CBM) or valence-band maximum (VBM). We consider two sets of compositions based on Lu3 B5O12 where B is Al, Ga, In, As, and Sb, and R3Al5 O12 , where R is Lu, Gd, Dy, and Er. We find that admixing with various R cations does not appreciably affect the band gap or band edges. In contrast, substituting Al with cations of dissimilar ionic radii has a profound impact on the band structure. We further show that certain dopants can be used to selectively modify only the CBM or the VBM. Specifically, Ga and In decrease the band gap by lowering the CBM, while As and Sb decrease the band gap by raising the VBM, the relative change in band gap is quantitatively validated by HDFT. These results demonstrate a powerful approach to quickly screen the impact of dopants on the electronic structure of scintillator compounds, identifying those dopants which alter the band edges in very specific ways to eliminate both electron and hole traps responsible for performance limitations. This approach should be broadly applicable for the optimization of electronic and optical performance for a wide range of compounds by tuning the VBM and CBM.
Photonic Band Gap structures: A new approach to accelerator cavities
Kroll, N. [California Univ., San Diego, La Jolla, CA (United States). Dept. of Physics]|[Stanford Linear Accelerator Center, Menlo Park, CA (United States); Smith, D.R.; Schultz, S. [California Univ., San Diego, La Jolla, CA (United States). Dept. of Physics
1992-12-31
We introduce a new accelerator cavity design based on Photonic Band Gap (PGB) structures. The PGB cavity consists of a two-dimensional periodic array of high dielectric, low loss cylinders with a single removal defect, bounded on top and bottom by conducting sheets. We present the results of both numerical simulations and experimental measurements on the PGB cavity.
Photonic Band Gap structures: A new approach to accelerator cavities
We introduce a new accelerator cavity design based on Photonic Band Gap (PGB) structures. The PGB cavity consists of a two-dimensional periodic array of high dielectric, low loss cylinders with a single removal defect, bounded on top and bottom by conducting sheets. We present the results of both numerical simulations and experimental measurements on the PGB cavity
Topological Design of Cellular Phononic Band Gap Crystals
Yang Fan Li
2016-03-01
Full Text Available This paper systematically investigated the topological design of cellular phononic crystals with a maximized gap size between two adjacent bands. Considering that the obtained structures may sustain a certain amount of static loadings, it is desirable to ensure the optimized designs to have a relatively high stiffness. To tackle this issue, we conducted a multiple objective optimization to maximize band gap size and bulk or shear modulus simultaneously with a prescribed volume fraction of solid material so that the resulting structures can be lightweight, as well. In particular, we first conducted the finite element analysis of the phononic band gap crystals and then adapted a very efficient optimization procedure to resolve this problem based on bi-directional evolutionary structure optimization (BESO algorithm in conjunction with the homogenization method. A number of optimization results for maximizing band gaps with bulk and shear modulus constraints are presented for out-of-plane and in-plane modes. Numerical results showed that the optimized structures are similar to those obtained for composite case, except that additional slim connections are added in the cellular case to support the propagation of shear wave modes and meanwhile to satisfy the prescribed bulk or shear modulus constraints.
Band gap bowing in quaternary nitride semiconducting alloys
Gorczyka, Isabela; Suski, T.; Christensen, Niels Egede;
2011-01-01
Structural properties of InxGayAl1−x−yN alloys are derived from total-energy minimization within the local-density approximation (LDA). The electronic properties are studied by band structure calculations including a semiempirical correction for the “LDA gap error.” The effects of varying the...
Photonic band gap materials: design, synthesis, and applications
Full text: Unlike semiconductors which facilitate the coherent propagation of electrons, photonic band gap (PBG) materials execute their novel functions through the coherent localization of photons. I review and discuss our recent synthesis of a large scale three-dimensional silicon photonic crystal with a complete photonic band gap near 1.5 microns. When a PBG material is doped with impurity atoms which have an electronic transition that lies within the gap, spontaneous emission of light from the atom is inhibited. Inside the gap, the photon forms a bound state to the atom. Outside the gap, radiative dynamics in the colored vacuum is highly non Markovian. I discuss the influence of these memory effects on laser action. When spontaneous emission is absent, the next order radiative effect (resonance dipole dipole interaction between atoms) must be incorporated leading to anomalous nonlinear optical effects which occur at a much lower threshold than in ordinary vacuum. I describe the collective switching of two-level atoms near a photonic band edge, by external laser field, from a passive state to one exhibiting population inversion. This effect is forbidden in ordinary vacuum. However, in the context of a PBG material, this effect may be utilized for an all-optical transistor. Finally, I discuss the prospects for a phase sensitive, single atom quantum memory device, onto which information may be written by an external laser pulse
Robust band gap of TiS3 nanofilms.
Kang, Jun; Wang, Lin-Wang
2016-06-01
First-principles calculations have been performed on the band structure of mono- and few-layer TiS3 nanofilms. It is found that the band gap character of the TiS3 films is quite robust, almost independent of layer thickness, vertical strain and stacking order, which is in sharp contrast to most other two-dimensional materials, such as MoS2. The robustness of the band gap originates from the location of the CBM and VBM states, which are at the center atoms of TiS3, and are thus unaffected by the layer-layer coupling. Such a property of TiS3 nanofilms promises good application potential in nanoelectronics and optoelectronics, and also makes TiS3 a good platform to study the electronic properties of a material in the two-dimensional limit. PMID:27029227
Continuously Controlled Optical Band Gap in Oxide Semiconductor Thin Films.
Herklotz, Andreas; Rus, Stefania Florina; Ward, Thomas Zac
2016-03-01
The optical band gap of the prototypical semiconducting oxide SnO2 is shown to be continuously controlled through single axis lattice expansion of nanometric films induced by low-energy helium implantation. While traditional epitaxy-induced strain results in Poisson driven multidirectional lattice changes shown to only allow discrete increases in bandgap, we find that a downward shift in the band gap can be linearly dictated as a function of out-of-plane lattice expansion. Our experimental observations closely match density functional theory that demonstrates that uniaxial strain provides a fundamentally different effect on the band structure than traditional epitaxy-induced multiaxes strain effects. Charge density calculations further support these findings and provide evidence that uniaxial strain can be used to drive orbital hybridization inaccessible with traditional strain engineering techniques. PMID:26836282
OPTICAL BAND GAP AND CONDUCTIVITY MEASUREMENTS OF POLYPYRROLE-CHITOSAN COMPOSITE THIN FILMS
Mahnaz M.Abdi; H.N.M.Ekramul Mahmud; Luqman Chuah Abdullah; Anuar Kassim; Mohamad Zaki Ab.Rahman; Josephine Liew Ying Chyi
2012-01-01
Electrical conductivity and optical properties of polypyrrole-chitosan (PPy-CHI) conducting polymer composites have been investigated to determine the optical transition characteristics and energy band gap of composite films.The two electrode method and Ⅰ-Ⅴ characteristic technique were used to measure the conductivity of the PPy-CHI thin films,and the optical band gap was obtained from their ultraviolet absorption edges.Depending upon experimental parameter,the optical band gap (Eg) was found within 1.30-2.32 eV as estimated from optical absorption data.The band gap of the composite films decreased as the CHI content increased.The room temperature electrical conductivity of PPy-CHI thin films was found in the range of 5.84 × 10-7-15.25 × 10-7 S.cm-1 depending on the chitosan content.The thermogravimetry analysis (TGA)showed that the CHI can improve the thermal stability of PPy-CHI composite films.
Dual-band electromagnetic band gap structure for noise isolation in mixed signal SiP
Rotaru, M. D.; Sykulski, J. K.
2010-01-01
A compact dual-band electromagnetic band-gap (EBG) structure is proposed. It is shown through numerical simulation using 3D electromagnetic finite element modelling that by adding a slit to the classical mushroom shape an extra resonance is introduced and thus dual-band EBG structures can be built by cascading these new elements. It is also demonstrated that this novel approach can be used to isolate noise in a system such as a dual band transceiver integrated into a mixed signal system in a ...
Band gap engineering of MoS2 upon compression
López-Suárez, Miquel; Neri, Igor; Rurali, Riccardo
2016-04-01
Molybdenum disulfide (MoS2) is a promising candidate for 2D nanoelectronic devices, which shows a direct band-gap for monolayer structure. In this work we study the electronic structure of MoS2 upon both compressive and tensile strains with first-principles density-functional calculations for different number of layers. The results show that the band-gap can be engineered for experimentally attainable strains (i.e., ±0.15). However, compressive strain can result in bucking that can prevent the use of large compressive strain. We then studied the stability of the compression, calculating the critical strain that results in the on-set of buckling for free-standing nanoribbons of different lengths. The results demonstrate that short structures, or few-layer MoS2, show semi-conductor to metal transition upon compressive strain without bucking.
Band gap determination of Ni–Zn ferrites
G P Joshi; N S Saxena; R Mangal; A Mishra; T P Sharma
2003-06-01
Nanocomposites of Ni–Zn with copolymer matrix of aniline and formaldehyde in presence of varying concentrations of zinc ions have been studied at room temperature and normal pressure. The energy band gap of these materials are determined by reflection spectra in the wavelength range 400–850 nm by spectrophotometer at room temperature. From the analysis of reflection spectra, nanocomposites of copolymer of aniline and formaldehyde with Ni$_{1–x}$Zn$_x$Fe2O4 ( = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) have been found to have direct band gaps ranging from 1.50–1.66 eV.
Band gap engineering of indium zinc oxide by nitrogen incorporation
Highlights: • IZON thin films were deposited by RF reactive sputtering at room temperature. • The effects of nitrogen on physical properties of IZO were analyzed. • Optical properties of IZON were studied by SE and UV–vis spectroscopy. • Adachi and classical parameters were quantitative and qualitatively congruent. • Nitrogen induces a gradual narrowing band gap from 3.5 to 2.5 eV on IZON films. - Abstract: The effects of nitrogen incorporation in indium zinc oxide films, as grown by RF reactive magnetron sputtering, on the structural, electrical and optical properties were studied. It was determined that the variation of the N2/Ar ratio, in the reactive gas flux, was directly proportional to the nitrogen percentage measured in the sample, and the incorporated nitrogen, which substituted oxygen in the films induces changes in the band gap of the films. This phenomenon was observed by measurement of absorption and transmission spectroscopy in conjunction with spectral ellipsometry. To fit the ellipsometry spectra, the classical and Adachi dispersion models were used. The obtained optical parameters presented notable changes related to the increment of the nitrogen in the film. The band gap narrowed from 3.5 to 2.5 eV as the N2/Ar ratio was increased. The lowest resistivity obtained for these films was 3.8 × 10−4 Ω cm with a carrier concentration of 5.1 × 1020 cm−3
Band gap engineering of indium zinc oxide by nitrogen incorporation
Ortega, J.J., E-mail: jjosila@hotmail.com [Unidad Académica de Física, Universidad Autónoma de Zacatecas, Calzada Solidaridad esq. Paseo la Bufa, Fracc. Progreso, C.P. 98060 Zacatecas (Mexico); Doctorado Institucional de Ingeniería y Ciencia de Materiales, Universidad Autónoma de San Luis Potosí, Av. Salvador Nava, Zona Universitaria, C.P. 78270 San Luis Potosí (Mexico); Aguilar-Frutis, M.A.; Alarcón, G. [Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada del Instituto Politécnico Nacional, Unidad Legaría, Calz. Legaría No. 694, Col. Irrigación, C.P. 11500 México D.F. (Mexico); Falcony, C. [Departamento de Física, Centro de Investigación y Estudios Avanzados del Instituto Politécnico Nacional campus Zacatenco, Av. Instituto Politécnico Nacional 2508, Col. San Pedro Zacatenco, C.P. 07360 México D.F. (Mexico); and others
2014-09-15
Highlights: • IZON thin films were deposited by RF reactive sputtering at room temperature. • The effects of nitrogen on physical properties of IZO were analyzed. • Optical properties of IZON were studied by SE and UV–vis spectroscopy. • Adachi and classical parameters were quantitative and qualitatively congruent. • Nitrogen induces a gradual narrowing band gap from 3.5 to 2.5 eV on IZON films. - Abstract: The effects of nitrogen incorporation in indium zinc oxide films, as grown by RF reactive magnetron sputtering, on the structural, electrical and optical properties were studied. It was determined that the variation of the N{sub 2}/Ar ratio, in the reactive gas flux, was directly proportional to the nitrogen percentage measured in the sample, and the incorporated nitrogen, which substituted oxygen in the films induces changes in the band gap of the films. This phenomenon was observed by measurement of absorption and transmission spectroscopy in conjunction with spectral ellipsometry. To fit the ellipsometry spectra, the classical and Adachi dispersion models were used. The obtained optical parameters presented notable changes related to the increment of the nitrogen in the film. The band gap narrowed from 3.5 to 2.5 eV as the N{sub 2}/Ar ratio was increased. The lowest resistivity obtained for these films was 3.8 × 10{sup −4} Ω cm with a carrier concentration of 5.1 × 10{sup 20} cm{sup −3}.
Two novel silicon phases with direct band gaps.
Fan, Qingyang; Chai, Changchun; Wei, Qun; Yang, Yintang
2016-05-14
Due to its abundance, silicon is the preferred solar-cell material despite the fact that many silicon allotropes have indirect band gaps. Elemental silicon has a large impact on the economy of the modern world and is of fundamental importance in the technological field, particularly in the solar cell industry. Looking for direct band gap silicon is still an important field in material science. Based on density function theory with the ultrasoft pseudopotential scheme in the frame of the local density approximation and the generalized gradient approximation, we have systematically studied the structural stability, absorption spectra, electronic, optical and mechanical properties and minimum thermal conductivity of two novel silicon phases, Cm-32 silicon and P21/m silicon. These are both thermally, dynamically and mechanically stable. The absorption spectra of Cm-32 silicon and P21/m silicon exhibit significant overlap with the solar spectrum and thus, excellent photovoltaic efficiency with great improvements over Fd3[combining macron]m Si. These two novel Si structures with direct band gaps could be applied in single p-n junction thin-film solar cells or tandem photovoltaic devices. PMID:27104737
Electron Elevator: Excitations across the Band Gap via a Dynamical Gap State.
Lim, A; Foulkes, W M C; Horsfield, A P; Mason, D R; Schleife, A; Draeger, E W; Correa, A A
2016-01-29
We use time-dependent density functional theory to study self-irradiated Si. We calculate the electronic stopping power of Si in Si by evaluating the energy transferred to the electrons per unit path length by an ion of kinetic energy from 1 eV to 100 keV moving through the host. Electronic stopping is found to be significant below the threshold velocity normally identified with transitions across the band gap. A structured crossover at low velocity exists in place of a hard threshold. An analysis of the time dependence of the transition rates using coupled linear rate equations enables one of the excitation mechanisms to be clearly identified: a defect state induced in the gap by the moving ion acts like an elevator and carries electrons across the band gap. PMID:26871327
Mechanism of photonic band gap, optical properties, tuning and applications
Mechanism of occurrence of Photonic Band Gap (PBG) is presented for 3-D structure using close packed face centered cubic lattice. Concepts and our work, specifically optical properties of 3-D photonic crystal, relative width, filling fraction, effective refractive index, alternative mechanism of photonic band gap scattering strength and dielectric contrast, effect of fluctuations and minimum refractive index contrast, are reported. The temperature tuning and anisotropy of nematic and ferroelectric liquid crystal infiltrated opal for different phase transitions are given. Effective dielectric constant with filling fraction using Maxwell Garnet theory (MG), multiple modified Maxwell Garnet (MMMG) and Effective Medium theory (EM) and results are compared with experiment to understand the occurrence of PBG. Our calculations of Lamb shifts including fluctuations are given and compared with those of literature values. We have also done band structure calculations including anisotropy and compared isotropic characteristic of liquid crystal. A possibility of lowest refractive index contrast useful for the fabrication of PBG is given. Our calculations for relative width as a function of refractive index contrast are reported and comparisons with existing theoretical and experimental optimal values are briefed. Applications of photonic crystals are summarized. The investigations conducted on PBG materials and reported here may pave the way for understanding the challenges in the field of PBG. (author)
Group IV direct band gap photonics: Methods, Challenges and Opportunities
Richard eGeiger
2015-07-01
Full Text Available The concept of direct band gap group IV materials offers a paradigm change for Si-photonics concerning the monolithic implementation of light emitters: The idea is to integrate fully compatible group IV materials with equally favorable optical properties as the chemically incompatible group III-V-based systems. The concept involves either mechanically applied strain on Ge or alloying of Ge with Sn and permits to drastically improve the insufficient radiative efficiency of Ge. The favorable optical properties result from a modified band structure transformed from an indirect to a direct one. The first demonstration of such a direct band gap laser, accomplished in GeSn, exemplifies the capability of this new concept. These systems may permit a qualitative as well as a quantitative expansion of Si-photonics into traditional but also new areas of applications, provided they can be operated energy efficiently, under ambient conditions and integrated with current Si technologies. This review aims to discuss the challenges along this path in terms of fabrication, characterization and fundamental understanding, and will elaborate on evoking opportunities of this new class of group IV-based laser materials.
Charged defects in highly emissive organic wide-band-gap semiconductors
A combined photoluminescence (PL) -detected magnetic-resonance (PLDMR) and thermally stimulated current (TSC) study of defects in wide-band-gap para-phenylene-type semiconductors is described. As TSC probes the density of mobile charge carriers after detrapping and PLDMR reveals the influence of trapped charges on the PL, their combination yields the concentration of traps, their energetic position, and their contribution to PL quenching. The reported trap densities, which are 2x1016 for the polymer and 1x1014 cm-3, for the oligomer, are the lowest reported for para-phenylene-type materials. (c) 2000 American Institute of Physics
Liu, Z.Q. [Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576 (Singapore); Chim, W.K., E-mail: elecwk@nus.edu.sg [Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576 (Singapore); Chiam, S.Y., E-mail: chiamsy@imre.a-star.edu.sg [Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602 (Singapore); Pan, J.S. [Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602 (Singapore); Ng, C.M. [GLOBALFOUNDRIES Singapore Pte. Ltd, 60 Woodlands Street 2, Singapore 738406 (Singapore)
2013-05-01
We studied the effects of adding yttrium (Y) in bulk lanthanum aluminate (LaAlO{sub 3} or LAO) by investigating the quaternary compound oxide, lanthanum yttrium aluminum oxide La{sub 0.3}Y{sub 0.7}AlO{sub 3} (LYAO), on silicon (Si). It is found that the inclusion of Y to LAO increases the band gap by ∼ 0.9 eV without compromising the dielectric constant. The enhancement in the band gap results in larger band offsets in LYAO and we also observe a decrease in leakage current at low voltage accumulation bias for Al/LYAO/Si as compared to Al/LAO/Si. In addition, the interface trap density of the Al/LYAO/Si structure remains comparable to that of Al/LAO/Si. Our findings show that LYAO is an attractive high dielectric constant material for use in next-generation low standby power devices. - Highlights: • We studied the inclusion of yttrium in LaAlO{sub 3} by investigating La{sub 0.3}Y{sub 0.7}AlO{sub 3} on Si. • Inclusion of yttrium in LaAlO{sub 3} (LAO) increases the band gap by 0.9 eV. • Enhancement in the band gap results in larger band offsets in La{sub 0.3}Y{sub 0.7}AlO{sub 3} (LYAO). • Decrease in leakage current at low voltage for Al/LYAO/Si compared to Al/LAO/Si. • LYAO is an attractive high-k material for low standby power devices.
We studied the effects of adding yttrium (Y) in bulk lanthanum aluminate (LaAlO3 or LAO) by investigating the quaternary compound oxide, lanthanum yttrium aluminum oxide La0.3Y0.7AlO3 (LYAO), on silicon (Si). It is found that the inclusion of Y to LAO increases the band gap by ∼ 0.9 eV without compromising the dielectric constant. The enhancement in the band gap results in larger band offsets in LYAO and we also observe a decrease in leakage current at low voltage accumulation bias for Al/LYAO/Si as compared to Al/LAO/Si. In addition, the interface trap density of the Al/LYAO/Si structure remains comparable to that of Al/LAO/Si. Our findings show that LYAO is an attractive high dielectric constant material for use in next-generation low standby power devices. - Highlights: • We studied the inclusion of yttrium in LaAlO3 by investigating La0.3Y0.7AlO3 on Si. • Inclusion of yttrium in LaAlO3 (LAO) increases the band gap by 0.9 eV. • Enhancement in the band gap results in larger band offsets in La0.3Y0.7AlO3 (LYAO). • Decrease in leakage current at low voltage for Al/LYAO/Si compared to Al/LAO/Si. • LYAO is an attractive high-k material for low standby power devices
Enlargement of the band gaps of water waves over one-dimensional combination bottom structures
The phenomena of band gaps and propagation of water waves over one-dimensional periodic bottoms and combination bottoms is investigated by the transfer matrix method. For the periodic bottoms (PBs), the effect of the steps' numbers, their height and width on the band gaps are discussed, respectively. For two kinds of combination periodic-bottoms (CPBs), namely height-CPBs and width-CPBs, both corresponding whole band gaps are gained, respectively. Each of whole band gaps is the juxtaposition of the gaps of two kind of PBs, without covering. The numerical results show that the band gaps could be enlarged effectively by choosing the steps' height or width properly
CZTS stoichiometry effects on the band gap energy
Malerba, Claudia, E-mail: claudia.malerba-1@ing.unitn.it [University of Trento, Department of Civil, Environmental and Mechanical Engineering, via Mesiano 77, 38123 Trento (Italy); ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma (Italy); Biccari, Francesco [ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma (Italy); Azanza Ricardo, Cristy Leonor [University of Trento, Department of Civil, Environmental and Mechanical Engineering, via Mesiano 77, 38123 Trento (Italy); Valentini, Matteo [Sapienza – University of Rome, Department of Physics, p.le A. Moro 5, 00185 Roma (Italy); ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma (Italy); Chierchia, Rosa [ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma (Italy); Müller, Melanie [University of Trento, Department of Civil, Environmental and Mechanical Engineering, via Mesiano 77, 38123 Trento (Italy); Max Planck Institute for Solid State Research, Heisenberg str. 1, 70569 Stuttgart (Germany); Santoni, Antonino [ENEA, Frascati Research Center, via E. Fermi 45, 00044 Frascati (Italy); Esposito, Emilia [ENEA, Portici Research Center, Piazzale E. Fermi, 80055 Portici (Napoli) (Italy); Mangiapane, Pietro [ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma (Italy); Scardi, Paolo [University of Trento, Department of Civil, Environmental and Mechanical Engineering, via Mesiano 77, 38123 Trento (Italy); Mittiga, Alberto [ENEA, Casaccia Research Center, via Anguillarese 301, 00123 Roma (Italy)
2014-01-05
Highlights: • CZTS films with different compositions were grown from stacked-layer precursors. • The band-gap energy varies from 1.48 to 1.63 eV as the [Sn]/[Cu] ratio increases. • The Zn content seems not to be a critical parameter for the optical properties. • PDS data show an increase of the sub-gap absorption as the Sn content is reduced. • Formation of defects at low Sn content was proposed to explain the Eg variation. -- Abstract: The considerable spread of Cu{sub 2}ZnSnS{sub 4} (CZTS) optical properties reported in the literature is discussed in terms of material stoichiometry. To this purpose, kesterite thin films were prepared by sulfurization of multilayered precursors of ZnS, Cu and Sn, changing the relative amounts to obtain CZTS layers with different compositions. X-Ray Diffraction (XRD), Energy Dispersive X-Ray (EDX) spectroscopy, X-Ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy were used for structural and compositional analysis. XRD quantitative phase analysis provides the amount of spurious phases and information on Sn-site occupancy. The optical properties were investigated by spectrophotometric and Photothermal Deflection Spectroscopy (PDS) measurements to assess the absorption coefficient of samples with different compositions. The PDS data show an increase of the sub-band absorption as the Sn content decreases. The results are interpreted assuming the formation of additional defects as the tin content is reduced. Those defects can also be responsible for the decrease of the band gap energy value as the Sn/Cu ratio is decreased.
Rubber Bands as Model Polymers in Couette Flow
Dunstan, Dave E.
2008-01-01
We present a simple device for demonstrating the essential aspects of polymers in flow in the classroom. Rubber bands are used as a macroscopic model of polymers to allow direct visual observation of the flow-induced changes in orientation and conformation. A transparent Perspex Couette cell, constructed from two sections of a tube, is used to…
Analysis of photonic band-gap structures in stratified medium
Tong, Ming-Sze; Yinchao, Chen; Lu, Yilong;
2005-01-01
Purpose - To demonstrate the flexibility and advantages of a non-uniform pseudo-spectral time domain (nu-PSTD) method through studies of the wave propagation characteristics on photonic band-gap (PBG) structures in stratified medium Design/methodology/approach - A nu-PSTD method is proposed in...... the occasions where the spatial distributions contain step of up to five times larger than the original size, while simultaneously the flexibility of non-uniform sampling offers further savings on computational storage. Research limitations/implications - Research has been mainly limited to the simple...
Band gap engineering in simultaneous phononic and photonic crystal slabs
Djafari Rouhani, B.; Pennec, Y.; Vasseur, J.O.; Hassouani, Y.El; Li, C.; Akjouj, A. [Universite de Lille1 Sciences et Technologies, Cite Scientifique, Institut d' Electronique, de Microelectronique et de Nanotechnologie, UMR CNRS 8520, Villeneuve d' Ascq (France); Boudouti, E.H.El; Bria, D. [Universite de Lille1 Sciences et Technologies, Cite Scientifique, Institut d' Electronique, de Microelectronique et de Nanotechnologie, UMR CNRS 8520, Villeneuve d' Ascq (France); Universite d' Oujda, Laboratoire de Dynamique et d' Optique des Materiaux, Faculte des Sciences, Oujda (Morocco)
2011-06-15
We discuss the simultaneous existence of phononic and photonic band gaps in two types of phononic crystals slabs, namely periodic arrays of nanoholes in a Si membrane and of Si nanodots on a SiO{sub 2} membrane. In the former geometry, we investigate in detail both the boron nitride lattice and the square lattice with two atoms per unit cell (these include the square, triangular and honeycomb lattices as particular cases). In the latter geometry, some preliminary results are reported for a square lattice. (orig.)
Photonic band gap resonators for high energy accelerators
The authors have proposed that a new type of microwave resonator, based on Photonic Band Gap (PBG) structures, may be particularly useful for high energy accelerators. The authors provide an explanation of the PBG concept and present data which illustrate some of the special properties associated with such structures. Further evaluation of the utility of PBG resonators requires laboratory testing of model structures at cryogenic temperatures, and at high fields. They provide a brief discussion of their test program, which is currently in progress
Flexible design of band gaps in the biopolymer photonic crystals
One-dimensional photonic crystals (PC) are fabricated in dichromate-sensitized biopolymer as volume holograms. The flexibility of the PC band gap (BG) parameters was investigated. The spectral position of a BG can be varied by changing the exposure for two concentrations of sensitizer during the fabrication process. The spectral measurements show that the BG centre shifts towards longer wavelengths with decreasing exposure and concentration of the sensitizer. A tuning of the position of the BG for about 120 nm was obtained.
Optimization of Beam Properties with Respect to Maximum Band-Gap
Halkjær, Søren; Sigmund, Ole
2004-01-01
We study numerically the frequency band-gap phenomenon for bending waves in an infinite periodic beam. The outcome of the analysis is then subjected to an optimization problem in order to maximize these band-gaps. The band-gap maximization may be performed with respect to material parameters and...
Enlargement of Photonic Band Gaps and Physical Picture of Photonic Band Structures
ZHANG Yan; SHI Jun-Jie
2006-01-01
@@ Light propagation in a one-dimensional photonic crystal (PC), consisting of alternative slabs with refractive indices (layer thicknesses) n1 (a) and n2 (b), is investigated. An important optimal parameter matching condition,n1a ≈ n2b, is obtained for the largest photonic band gap (PBG). Moreover, we find that the exact analytical solutions for the electric/magnetic field eigenmodes at the band edges are standing waves with odd or even symmetry about the centre of each layer. The electric/magnetic field eigenfunctions at the top and bottom of the nth band have n and n - 1 nodes in one period of PC, respectively. The PBG arises from the symmetric differences of the field eigenfunctions at the band edges.
Lamb wave band gaps in locally resonant phononic crystal strip waveguides
Using finite element method, we have made a theoretically study of the band structure of Lamb wave in a locally resonant phononic crystal strip waveguide with periodic soft rubber attached on the two sides of epoxy main plate. The numerical results show that the Lamb wave band gap based on local resonant mechanism can be opened up in the stub strip waveguides, and the width of the local resonant band gap is narrower than that based on the Bragg scattering mechanism. The results also show that the stub shape and width have influence on the frequency and width of the Lamb wave band gap. -- Highlights: ► The local resonant Lamb wave band gap can be opened up in a stub strip waveguides. ► The width of the local resonant band gap is narrower than that Bragg scattering band gap. ► The shape and width of the stub have strongly influence on the local resonant band gap.
II-VI wide band gap semiconductors under hydrostatic pressure
We set an analytical expression for the gap as a function of hydrostatic deformation, Eg(ε), by diagonalizing in Γ the corresponding Empirical Tight-binding Hamiltonian (ETBH). In our ETBH we use the well known d-2 Harrison Scaling Law (HSL) to adjust the TB parameter (TBP) to the changes in interatomic distances. We do not consider cation-anion charge transfer. We calculate Eg(ε) for wide band gap II-VI semiconductors with zincblende crystal structure for deformations under pressure up to -5%. Our results are in quite good agreement with experiment for the compounds of lower ionicity but deviate as the ionicity of the compound increases. This is due to the neglect of charge transfer which should be included self-consistently as done, for example, by Bertho et al. starting from an Empirical Tight-binding description of the bands similar as the one presented here. They sometimes get nevertheless a negative second derivative of Eg(ε) with respect to ε which is never gotten in the experiments. Within our approximation we always find a positive sing independently of the material. It seems that the procedure in which self-consistency is achieved is the source of this wrong sign. Furthermore, the inclusion of deviations from HSL appear to be unimportant to this problem. (author). 15 refs, 4 figs, 2 tabs
Transmission and photonic band gaps in Fibonacci superlattices
J. Garus
2013-12-01
Full Text Available Purpose: The purpose of the article was to broaden the knowledge about the behavior of Fibonacci superlattices as filters electromagnetic waves. Design/methodology/approach: Simulations of multi-layer systems is usually carried out by using two complementary methods. The first, matrix method which allows the study of the properties of structures using transmission maps and the second method used is the Finite-Difference Time Domain (FDTD algorithm allows on the study of electromagnetic wave propagation in the structure. Findings: It can be seen that the lighting of the filter with monochromatic light in the wavelength range of the band gap filter at the output causes propagation of low intensity in the range other than the wavelength of the incident beam. Research limitations/implications: The simulation was not considered the impact of losses in the material. Practical implications: Present clear differences depending on the polarization allow the use of superlattices as polarizers for specific ranges of wavelengths and angles of incidence. Originality/value: Fibonacci superlattices have been pre-tested in. The purpose of the article was to broaden the knowledge about the behavior of these structures as filters electromagnetic waves with a wavelength range from the near infrared, the effect of the material surrounding the transmission and increasing knowledge of the formation of band gaps.
Computational investigation on tunable optical band gap in armchair polyacenes
Polyacenes in their armchair geometry (phenacenes) have recently been found to possess appealing electronic and optical properties with higher chemical stability and comparatively larger band gap as compared to linear polyacenes. They also behave as high-temperature superconductors upon alkali metal doping. Moreover, the optical properties of crystalline picene can be finely tuned by applying external pressure. We investigated the variation of optical gap as a function of altering the interplanar distances between parallel cofacial phenacene dimers. We employed both time-dependent density functional theory and density matrix renormalization group (DMRG) technique to investigate the lowest singlet excitations in phenacene dimer. Our study showed that the lowest singlet excitation in these systems evolved as a function of interplanar separation. The optical excitation energy gap decreases as a function of inverse interplanar separation of the phenacene dimer. The distant dependent variation of optical absorption at the dimer level may be comparable with experimental observation in picene crystal under pressure. DMRG study also demonstrates that besides picene, electronic properties of higher phenacenes can also be tunable by altering interplanar separation
The optical band gap and surface free energy of polyethylene modified by electron beam irradiations
Abdul-Kader, A. M.
2013-04-01
In this study, investigations have been carried out on electron beam irradiated ultra high molecular weight polyethylene (UHMWPE). Polyethylene samples were irradiated with 1.5 MeV electron beam at doses ranging from 50 to 500 kGy. Modifications in optical properties and photoluminescence behavior of the polymer were evaluated by UV-vis and photoluminescence techniques. Changes of surface layer composition of UHMWPE produced by electron irradiations were studied by Rutherford back scattering spectrometry (RBS). The change in wettability and surface free energy induced by irradiations was also investigated. The optical absorption studies reveal that both optical band gap and Urbach's energy decreases with increasing electron dose. A correlation between energy gap and the number of carbon atoms in clusters is discussed. Photoluminescence spectra were reveal remarkable decrease in the integrated luminescence intensity with increasing irradiation dose. Contact angle measurements showed that wettability and surface free energy increases with increasing the irradiation dose.
Hydrogen production by Tuning the Photonic Band Gap with the Electronic Band Gap of TiO2
Waterhouse, G. I. N.
2013-10-10
Tuning the photonic band gap (PBG) to the electronic band gap (EBG) of Au/TiO
Band Gap Engineering and Layer-by-Layer Band Gap Mapping of Selenium-doped Molybdenum Disulfide
Gong, Yongji [Rice University; Liu, Zheng [Rice University; Lupini, Andrew R [ORNL; Lin, Junhao [ORNL; Pantelides, Sokrates T [ORNL; Pennycook, Stephen J [ORNL; Zhou, Wu [ORNL; Ajayan, Pullikel M [Rice University
2014-01-01
Ternary two-dimensional dichalcogenide alloys exhibit compositionally modulated electronic structure and hence, control of dopant concentration within each layer of these layered compounds provides a powerful way to modify their properties. The challenge then becomes quantifying and locating the dopant atoms within each layer in order to better understand and fine-tune the desired properties. Here we report the synthesis of selenium substitutionally doped molybdenum disulfide atomic layers, with a broad range of selenium concentrations, resulting in band gap modulations of over 0.2 eV. Atomic scale chemical analysis using Z-contrast imaging provides direct maps of the dopant atom distribution in individual MoS2 layers and hence a measure of the local band gaps. Furthermore, in a bilayer structure, the dopant distribution of each layer is imaged independently. We demonstrate that each layer in the bilayer contains similar doping levels, randomly distributed, providing new insights into the growth mechanism and alloying behavior in two-dimensional dichalcogenide atomic layers. The results show that growth of uniform, ternary, two-dimensional dichalcogenide alloy films with tunable electronic properties is feasible.
Inter-band optoelectronic properties in quantum dot structure of low band gap III-V semiconductors
A generalized theory is developed to study inter-band optical absorption coefficient (IOAC) and material gain (MG) in quantum dot structures of narrow gap III-V compound semiconductor considering the wave-vector (k→) dependence of the optical transition matrix element. The band structures of these low band gap semiconducting materials with sufficiently separated split-off valance band are frequently described by the three energy band model of Kane. This has been adopted for analysis of the IOAC and MG taking InAs, InSb, Hg1−xCdxTe, and In1−xGaxAsyP1−y lattice matched to InP, as example of III–V compound semiconductors, having varied split-off energy band compared to their bulk band gap energy. It has been found that magnitude of the IOAC for quantum dots increases with increasing incident photon energy and the lines of absorption are more closely spaced in the three band model of Kane than those with parabolic energy band approximations reflecting the direct the influence of energy band parameters. The results show a significant deviation to the MG spectrum of narrow-gap materials having band nonparabolicity compared to the parabolic band model approximations. The results reflect the important role of valence band split-off energies in these narrow gap semiconductors
Peculiarities of Tamm states formed in degenerate photonic band gaps
Merzlikin, A.M., E-mail: merzlikin_a@mail.r [Institution for Theoretical and Applied Electromagnetics, Russian Academy of Science, 13/19 Izhorskaya ul., 125412 Moscow (Russian Federation); Vinogradov, A.P.; Lagarkov, A.N. [Institution for Theoretical and Applied Electromagnetics, Russian Academy of Science, 13/19 Izhorskaya ul., 125412 Moscow (Russian Federation); Levy, M. [Department of Physics, Michigan Technological University, 1400 Townsend Drive Houghton, MI 49931-1295 (United States); Bergman, D.J. [Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978 (Israel); Strelniker, Y.M. [Department of Physics, Bar-Ilan University, IL-52900 Ramat-Gan (Israel)
2010-07-15
The structure of the Tamm state localized at the interface between anisotropic magnetophotonic crystal (anisotropic MPC) and a photonic crystal (PC) made of isotropic dielectrics is studied. It is shown that if the frequency of this state appears within the degenerate band gap then its structure qualitatively differs from the structure of a well-known Tamm state localized at the interface between two one-dimensional PC made of isotropic materials. Since inside the degenerate BG the real part of the Bloch wavenumber differs from the Brillouin value, two Bloch waves with different signs in the real part of the wavenumber and the same sign in the imaginary part have different input impedance values. Moreover, contrary to the case of a PC made of isotropic materials the impedance of each Bloch wave is a tensor. As a consequence to construct a surface state at least three evanescent Bloch waves are required. The conditions that determine the Tamm state frequency also change.
Peculiarities of Tamm states formed in degenerate photonic band gaps
The structure of the Tamm state localized at the interface between anisotropic magnetophotonic crystal (anisotropic MPC) and a photonic crystal (PC) made of isotropic dielectrics is studied. It is shown that if the frequency of this state appears within the degenerate band gap then its structure qualitatively differs from the structure of a well-known Tamm state localized at the interface between two one-dimensional PC made of isotropic materials. Since inside the degenerate BG the real part of the Bloch wavenumber differs from the Brillouin value, two Bloch waves with different signs in the real part of the wavenumber and the same sign in the imaginary part have different input impedance values. Moreover, contrary to the case of a PC made of isotropic materials the impedance of each Bloch wave is a tensor. As a consequence to construct a surface state at least three evanescent Bloch waves are required. The conditions that determine the Tamm state frequency also change.
Half-oxidized phosphorene: band gap and elastic properties modulation
Based on a first principles approach, we study structural, electronic and elastic properties, as well as stabilities of all possible half-oxidized phosphorene conformers. Stability analysis reveals that oxygen chemisorption is an exothermic process in the six configurations despite the formation of interstitial oxygen bridges in three of them. Electronic structure calculations show that oxidation induces a band gap modulation ranging between 0.54 and 1.57 eV in the generalized gradient approximation corrected to 1.19 and 2.88 eV using GW. The mechanical response of the conformers is sensitively dependent on direction and indicates that the new derivatives are incompressible materials and one configuration has an auxetic behavior. The present results provide a basis for tailoring the electronic and elastic properties of phosphorene via half oxidation. (paper)
Khan, Hamna; Gahfoor, Bilal; Mehmood, Malik Sajjad; Ahmad, Manzoor; Yasin, Tariq; Ikram, Masroor
2015-12-01
Muller matrix spectro-polarimeter has been used to study the absorption behavior of pristine and e-beam irradiated (30, 65,100 kGy) ultra-high molecular weight polyethylene (UHMWPE) over the visible spectral range i.e. 400-800 nm. As a result, significant changes occur in the absorption behavior of irradiated samples due to radiation induced physical and chemical changes. To analyze these (radiation induced) changes in polymer matrix, Urbach edge method is employed for the calculation of optical activation energy. In addition to this, direct and indirect energy band gaps along the number of carbon atoms in C=C unsaturation have been determined by using modified Urbach formula and Tauc's equation, respectively. The results obtained during study reveal that Urbach energy decreases with radiation treatment and has a lower value for 100 kGy sample i.e. Eu=71.63 meV. The values of direct and indirect energy band gaps are also following the decreasing trend with e-beam irradiation. Moreover, indirect energy gaps are found to have lower values as compared to direct energy gaps. The number of carbon atoms in clusters (as estimated from modified Tauc's equation) has been found to vary from ∼6 to 8 for direct energy band gaps and from ∼9 to 11 for indirect energy band gaps.
Two-dimensional tricycle arsenene with a direct band gap.
Ma, ShuangYing; Zhou, Pan; Sun, L Z; Zhang, K W
2016-03-16
Based on a comprehensive investigation including ab initio phonon and finite-temperature molecular dynamics calculations, we find that two-dimensional tricycle-shaped arsenene (T-As) is robust and even stable under high temperature. T-As is energetically comparable to previously reported chair-shaped arsenene (C-As) and more stable than stirrup-shaped arsenene (S-As). In contrast to C-As and S-As, the monolayer T-As is a direct band gap semiconductor with an energy gap of 1.377 eV. Our results indicate that the electronic structure of T-As can be effectively modulated by stacking, strain, and patterning, which shows great potential of T-As in future nano-electronics. Moreover, by absorbing H or F atoms on the surface of T-As along a specific direction, nanoribbons with desired edge type and even width can be obtained, which is suitable for the fabrication of nano-devices. PMID:26954607
Study of optical band gap of zinc-borate glasses
The present study deals with binary xZnO-(100-x)B/sub 2/O/sub 3) (x=50-60%) and ternary yV/sub 2/O/sub 5/-50ZnO-(50-y)B/sub 2/O/sub 3/ (y=5-10%) glass systems. The mass density, oxygen packing density, molar volume and optical energy gap of these glasses were measured at room temperature. The density varies in the range of 2.92 - 3.57 g cm/sup 03/. The absorption spectra of these glasses were recorded in the UV-visible range. The optical band gap energies for these glasses were estimated from absorption data using the Mott and Davis relation and found to be in the range of 1.14 approx. equal to 2.92 eV. These results show that E/sub opt/ decreases with decreasing concentration of ZnO in binary glasses. In ternary glasses E/sub opt/ decreases with increasing concentration of V/sub 2/O/sub 5/ for a fixed amount of ZnO.(author)
Hypersonic crystal band gaps in Ni/Cu superlattice nanowire arrays
Hu, Jia-Guang; Shen, Tie
2016-03-01
The hexagonal and tetragonal ordered arrays were prepared by Ni/Cu superlattice nanowires on the porous anodic alumina membrane template, and their phonon band structures were calculated by using the plane wave expansion method. Numerical results show that the hypersonic band gaps can be acquired by adjusting the structural parameters. Along the different wave-vector directions, the width and position of band gap would vary. If the nanowires'filling fraction is increased continuously, the width of the first band gap firstly increases and then decreases within a certain range. The height of superlattice nanowire elementary unit can only affect the width of band gap within a quite narrow range. When the height of elementary unit remains unchanged, the decrease of the Cu-component ratio can contribute to the formation of a wider band gap. Additionally, the wide band gap is more easily formed in tetragonal structure than in hexagonal structure.
Phononic Band Gaps in 2D Quadratic and 3D Cubic Cellular Structures
Franziska Warmuth
2015-12-01
Full Text Available The static and dynamic mechanical behaviour of cellular materials can be designed by the architecture of the underlying unit cell. In this paper, the phononic band structure of 2D and 3D cellular structures is investigated. It is shown how the geometry of the unit cell influences the band structure and eventually leads to full band gaps. The mechanism leading to full band gaps is elucidated. Based on this knowledge, a 3D cellular structure with a broad full band gap is identified. Furthermore, the dependence of the width of the gap on the geometry parameters of the unit cell is presented.
Modeling of Photonic Band Gap Crystals and Applications
Ihab Fathy El-Kady
2002-08-27
In this work, the authors have undertaken a theoretical approach to the complex problem of modeling the flow of electromagnetic waves in photonic crystals. The focus is to address the feasibility of using the exciting phenomena of photonic gaps (PBG) in actual applications. The authors start by providing analytical derivations of the computational electromagnetic methods used in their work. They also present a detailed explanation of the physics underlying each approach, as well as a comparative study of the strengths and weaknesses of each method. The Plane Wave expansion, Transfer Matrix, and Finite Difference time Domain Methods are addressed. They also introduce a new theoretical approach, the Modal Expansion Method. They then shift the attention to actual applications. They begin with a discussion of 2D photonic crystal wave guides. The structure addressed consists of a 2D hexagonal structure of air cylinders in a layered dielectric background. Comparison with the performance of a conventional guide is made, as well as suggestions for enhancing it. The studies provide an upper theoretical limit on the performance of such guides, as they assumed no crystal imperfections and non-absorbing media. Next, they study 3D metallic PBG materials at near infrared and optical wavelengths. The main objective is to study the importance of absorption in the metal and the suitability of observing photonic band gaps in such structures. They study simple cubic structures where the metallic scatters are either cubes or interconnected metallic rods. Several metals are studied (aluminum, gold, copper, and silver). The effect of topology is addressed and isolated metallic cubes are found to be less lossy than the connected rod structures. The results reveal that the best performance is obtained by choosing metals with a large negative real part of the dielectric function, together with a relatively small imaginary part. Finally, they point out a new direction in photonic crystal
Multi-flexural band gaps in an Euler-Bernoulli beam with lateral local resonators
Wang, Ting; Sheng, Mei-Ping; Qin, Qing-Hua
2016-02-01
Flexural vibration suppression in an Euler-Bernoulli beam with attached lateral local resonators (LLR) is studied theoretically and numerically. Hamilton's principle and Bloch's theorem are employed to derive the dispersion relation which reveals that two band gaps are generated. Within both band gaps, the flexural waves are partially transformed into longitudinal waves through a four-link-mechanism and totally blocked. The band gaps can be flexibly tuned by changing the geometry parameter of the four-link-mechanism and the spring constants of the resonators. Frequency response function (FRF) from finite element analysis via commercial software of ANSYS shows large flexural wave attenuation within the band gaps and the effect of damping from the LLR substructures which helps smooth and lower the response peaks at the sacrifice of the band gap effect. The existence of the multi-flexural band gaps can be exploited for the design of flexural vibration control of beams.
Ultra-wide acoustic band gaps in pillar-based phononic crystal strips
An original approach for designing a one dimensional phononic crystal strip with an ultra-wide band gap is presented. The strip consists of periodic pillars erected on a tailored beam, enabling the generation of a band gap that is due to both Bragg scattering and local resonances. The optimized combination of both effects results in the lowering and the widening of the main band gap, ultimately leading to a gap-to-midgap ratio of 138%. The design method used to improve the band gap width is based on the flattening of phononic bands and relies on the study of the modal energy distribution within the unit cell. The computed transmission through a finite number of periods corroborates the dispersion diagram. The strong attenuation, in excess of 150 dB for only five periods, highlights the interest of such ultra-wide band gap phononic crystal strips
Photonic band gaps with layer-by-layer double-etched structures
Periodic layer-by-layer dielectric structures with full three-dimensional photonic band gaps have been designed and fabricated. In contrast to previous layer-by-layer structures the rods in each successive layer are at an angle of 70.5 degree to each other, achieved by etching both sides of a silicon wafer. Photonic band-structure calculations are utilized to optimize the photonic band gap by varying the structural geometry. The structure has been fabricated by double etching Si wafers producing millimeter wave photonic band gaps between 300 and 500 GHz, in excellent agreement with band calculations. Overetching this structure produces a multiply connected geometry and increases both the size and frequency of the photonic band gap, in very good agreement with experimental measurements. This new robust double-etched structure doubles the frequency possible from a single Si wafer, and can be scaled to produced band gaps at higher frequencies. copyright 1996 American Institute of Physics
True photonic band-gap mode-control in VCSEL structures
Romstad, F.; Madsen, M.; Birkedal, Dan;
2003-01-01
Photonic band-gap mode confinement in novel nano-structured large area VCSEL structures is confirmed by the amplified spontaneous emission spectrum. Both guide and anti-guide VCSEL structures are experimentally characterised to verify the photonic band-gap effect.......Photonic band-gap mode confinement in novel nano-structured large area VCSEL structures is confirmed by the amplified spontaneous emission spectrum. Both guide and anti-guide VCSEL structures are experimentally characterised to verify the photonic band-gap effect....
One-dimensional tunable magnetic photonic band-gap materials at microwave frequency
The microwave transmission characteristics of one-dimensional magnetic photonic band-gap (MPBG) materials, in which a ferromagnetic composite layer is sandwiched by periodic dielectric layers, are studied. The magnetic photonic band gaps (MBPGs) are obtained related to the existence of ferromagnetic resonance (FMR) in the vicinity of the band-gap frequency. We investigate the effects of period structure and the applied magnetic field on the MPBGs as well as the ferromagnetic resonance. The photonic band gaps of the TE mode shift to lower frequencies. The MPBG effect is strongly dependent on the periodic structure of the MPBG materials. While the FMR effect is dominated by the applied magnetic field
Efficient photovoltaic cells from low band-gap fluorene-based copolymer
Tian Ren-Yu; Yang Ren-Qiang; Peng Jun-Biao; Cao Yong
2005-01-01
Polymer photovoltaic cells based on low band-gap copolymer, poly [2,7-(9,9-dioctyl) fluorene-co-5,5'-(4,7-diselenophenyl)-2,2'-yl-2,1,3-benzothiadiazole] (PFSeBT) are investigated, focusing on the effects of cathode and blend concentration on device performance. The best device, with active layer from PFSeBT:PCBM=1:2 blend and with LiF/Al as cathode, achieves an open-circuit voltage of 1.00V, a short-circuit current density of 4.42mA/cm2, and energy conversion efficiency of 1.67% under AM1.5 illumination (100mW/cm2).The short-circuit current density indicates the dependence of power law on the incident light intensity with a power index of 0.887. All devices have a spectral response up to 680nm. The results indicate that PFSeBT is a potential polymer functioning as an electron donor in polymer photovoltaic cells.
Miniaturization of electromagnetic band gap structures for mobile applications
Goussetis, G.; Feresidis, A. P.; Palikaras, G. K.; Kitra, M.; Vardaxoglou, J. C.
2005-12-01
It is well known that interference of the human body affects the performance of the antennas in mobile phone handsets. In this contribution, we investigate the use of miniaturized metallodielectric electromagnetic band gap (MEBG) structures embedded in the case of a mobile handset as a means of decoupling the antenna from the user's hand. The closely coupled MEBG concept is employed to achieve miniaturization of the order of 15:1. Full wave dispersion relations for planar closely coupled MEBG arrays are presented and are validated experimentally. The performance of a prototype handset with an embedded conformal MEBG is assessed experimentally and is compared to a similar prototype without the MEBG. Reduction in the detuning of the antenna because of the human hand by virtue of the MEBG is demonstrated. Moreover, the efficiency of the handset when loaded with a human hand model is shown to improve when the MEBG is in place. The improvements are attributed to the decoupling of the antenna from the user's hand, which is achieved by means of suppressing the fields in the locality of the hand.
Photonic band gap materials: Technology, applications and challenges
Last century has been the age of Artificial Materials. One material that stands out in this regard is the semiconductor. The revolution in electronic industry in the 20th century was made possible by the ability of semiconductors to microscopically manipulate the flow of electrons. Further advancement in the field made scientists suggest that the new millennium will be the age of photonics in which artificial materials will be synthesized to microscopically manipulate the flow of light. One of these will be Photonic Band Gap material (PBG). PBG are periodic dielectric structures that forbid propagation of electromagnetic waves in a certain frequency range. They are able to engineer most fundamental properties of electromagnetic waves such as the laws of refraction, diffraction, and emission of light from atoms. Such PBG material not only opens up variety of possible applications (in lasers, antennas, millimeter wave devices, efficient solar cells photo-catalytic processes, integrated optical communication etc.) but also give rise to new physics (cavity electrodynamics, localization, disorder, photon-number-state squeezing). Unlike electronic micro-cavity, optical waveguides in a PBG microchip can simultaneously conduct hundreds of wavelength channels of information in a three dimensional circuit path. In this article we have discussed some aspects of PBG materials and their unusual properties, which provided a foundation for novel practical applications ranging from clinical medicine to information technology. (author)
Optical band gap and Urbach tail width of HCl and CSA doped polyaniline (PAni) nanofibers and the ion beam induced modifications in the band gap and Urbach's tail of the samples have been studied employing UV-Vis absorption spectroscopy. All the major bands appearing in the FTIR spectra exhibit a decrease in intensity and broadening in their band widths upon interaction with the highly energetic ion beams. This suggests that SHI irradiation induces chain-scissioning events in the PAni nanofibers. An interesting result that comes out from the FTIR analysis is a transition from the benzenoid to quinoid states in the PAni chains, which reveals that there is a decrease in the degree of conjugation in the polymer upon irradiation. Optical absorption studies indicate three direct allowed transitions at ∼2.64, 3.61 and 4.08 eV for HCl doped PAni nanofibers and at ∼2.62, 3.49 and 4.02 eV for the CSA doped PAni nanofibers. The optical band gap is found to increase with increasing ion fluence which may be attributed to the reduction in the fiber diameters upon irradiation, which is corroborated by TEM analysis. Increase in the optical band gap also points out to a decrease in the conjugation length due to the larger torsion angles between the adjacent phenyl rings of the polymer with respect to the plane of the nitrogen atoms, which is also supported by FTIR results. The Urbach tail width decreases with increasing ion fluence indicating that structural disorders are annealed out of the PAni nanofibers which is also observed from the plots of (αhν)2 against photon energy (hν) for HCl doped PAni nanofibers. The quantum confinement effect is confirmed by fact that a band gap exhibits a linear dependence on the inverse of the square of the radius of the PAni nanofibers. Infact, the increase in the optical band gap may be a combined effect of the decrease in the Urbach band width and the quantum confinement effect.
Effects of inclusion shapes on the band gaps in two-dimensional piezoelectric phononic crystals
In this paper, the elastic wave propagation in piezoelectric phononic crystals with several inclusion shapes is investigated by taking the electromechanical coupling into account. The band structures for five different shapes of scatterers (regular triangle, square, hexagon, circle, and oval) with square lattice are calculated using the plane-wave expansion method. The effects of the inclusion shapes on the normalized band width are discussed. The largest complete band gap is obtained by selecting the scatterers with the same symmetry of lattice for the first band gap, but this rule is not valid for the second band gap
Theoretical study of relative width of photonic band gap for the 3-D dielectric structure
G K Johri; Akhilesh Tiwari; Saumya Saxena; Rajesh Sharma; Kuldeep Srivastava; Manoj Johri
2002-03-01
Calculations for the relative width (/0) as a function of refractive index and relative radius of the photonic band gap for the fcc closed packed 3-D dielectric microstructure are reported and comparison of experimental observations and theoretical predictions are given. This work is useful for the understanding of photonic crystals and occurrence of the photonic band gap.
LC Filter Design for Wide Band Gap Device Based Adjustable Speed Drives
Vadstrup, Casper; Wang, Xiongfei; Blaabjerg, Frede
This paper presents a simple design procedure for LC filters used in wide band gap device based adjustable speed drives. Wide band gap devices offer fast turn-on and turn-off times, thus producing high dV/dt into the motor terminals. The high dV/dt can be harmful for the motor windings and bearings...
High-power picosecond pulse delivery through hollow core photonic band gap fibers
Michieletto, Mattia; Johansen, Mette Marie; Lyngsø, Jens Kristian;
2015-01-01
We demonstrated robust and bend insensitive fiber delivery of high power pulsed laser with diffraction limited beam quality for two different kind of hollow core photonic band gap fibers......We demonstrated robust and bend insensitive fiber delivery of high power pulsed laser with diffraction limited beam quality for two different kind of hollow core photonic band gap fibers...
Reversible band gap tuning of metal oxide films using hydrogen and oxygen plasmas
We report an approach to the reversible tuning of the band gaps of metal oxide (MO) films. ZnO and CuO, synthesized by hydrothermal methods, were treated with hydrogen and oxygen plasmas. From UV–visible transmittance spectra, we have found that the optical band gaps of MO films blue-shifted with hydrogen plasma treatment, but red-shifted with oxygen plasma treatment. By alternating the treatment sequences of hydrogen and oxygen plasmas, the MO optical band gap values can be reversibly fine-tuned with the tunable ranges of 80 and 550 meV for ZnO and CuO, respectively. The mechanism for reversible tuning of optical band gaps is proposed based on the results of optical emission, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy characterization. Compared to conventional metal ion doping and high temperature annealing methods, the use of room temperature hydrogen and oxygen plasmas for tuning band gaps is more environmentally friendly. - Highlights: ► Metal oxide band gap blue-shifts with hydrogen plasma treatment. ► Metal oxide band gap red-shifts with oxygen plasma treatment. ► Metal oxide band gap can be reversibly fine-tuned
Phononic First Band Gap of Quaternary Layered Periodic Structure with the Lumped-Mass Method
Chen Yuan
2014-01-01
Full Text Available Existing band gap analysis is mostly focused on the binary structure, while the researches on the quaternary layered periodic structure are still lacking. In this paper, the unidimensional lumped-mass method in the phonic crystal theory is firstly improved so that the material viscoelasticity can be taken into consideration. Then, the binary layered periodic structure is converted into a quaternary one and band gaps appear at low frequency range. Finally, the effects of density, elastic modulus, damping ratio, and the thickness of single material on the first band gap of the quaternary layered periodic structure are analyzed after the algorithm is promoted. The research findings show that effects of density, elastic modulus, and thickness of materials on the first band gap are considerable but those of damping ratio are not so distinct. This research provides theoretical bases for band gap design of the quaternary layered periodic structure.
Modulating the band gap of germanane nanoribbons for quantum well devices.
Zhou, Yungang; Li, Xuemei; Wang, Zhiguo; Li, Sean; Zu, Xiaotao
2014-09-01
The effective modulation of the band gaps in nanostructures is of both fundamental and technological interest because a tunable band gap gives great flexibility in the design and optimization of nanodevices. Using density functional theory calculations, we have shown that germanane nanoribbons of various widths or under various strains can provide rich band gaps. Width- and strain-induced changes in the band gaps of germanane nanoribbons result from a reduction in quantum confinement with width and the weakening of sp(3) hybridization with strain, respectively. Both changes represent a monotonous relationship. To utilize such a monotonous change in band gap, we designed a quantum well based on germanane nanoribbons in which photoexcited electrons and holes occupy the same spatial region, resulting in a desirable light-emitting device. PMID:25051154
Understanding Band Gaps of Solids in Generalized Kohn-Sham Theory
Perdew, John P; Burke, Kieron; Yang, Zenghui; Gross, Eberhard K U; Scheffler, Matthias; Scuseria, Gustavo E; Henderson, Thomas M; Zhang, Igor Ying; Ruzsinszky, Adrienn; Peng, Haowei; Sun, Jianwei
2016-01-01
The fundamental energy gap of a periodic solid distinguishes insulators from metals and characterizes low-energy single-electron excitations. But the gap in the band-structure of the exact multiplicative Kohn-Sham (KS) potential substantially underestimates the fundamental gap, a major limitation of KS density functional theory. Here we give a simple proof of a new theorem: In generalized KS theory (GKS), the band gap equals the fundamental gap for the approximate functional if the GKS potential operator is continuous and the density change is delocalized when an electron or hole is added. Our theorem explains how GKS band gaps from meta-generalized gradient approximations (meta-GGAs) and hybrid functionals can be more realistic than those from GGAs or even from the exact KS potential, It also follows from earlier work. The band edges in the GKS one-electron spectrum are also related to measurable energies. A linear chain of hydrogen molecules provides a numerical illustration.
Grain size dependent optical band gap of CdI2 films
Pankaj Tyagi; A G Vedeshwar
2001-06-01
The thermally evaporated stoichiometric CdI2 films show good -axis alignment normal to substrate plane for film thickness up to 200 nm. The optical absorption data indicate an allowed direct interband transition across a gap of 3.6 eV in confirmation with earlier band structure calculations. However, part of the absorption data near band edge can be fitted to an indirect band gap of 3 eV. The dependence of band gap on film thickness (> 200 nm) can be explained qualitatively in terms of decreasing grain boundary barrier height with grain size.
Compositional dependence of the band gap in Ga(NAsP) quantum well heterostructures
We present experimental and theoretical studies of the composition dependence of the direct band gap energy in Ga(NAsP)/GaP quantum well heterostructures grown on either (001) GaP- or Si-substrates. The theoretical description takes into account the band anti-crossing model for the conduction band as well as the modification of the valence subband structure due to the strain resulting from the pseudomorphic epitaxial growth on the respective substrate. The composition dependence of the direct band gap of Ga(NAsP) is obtained for a wide range of nitrogen and phosphorus contents relevant for laser applications on Si-substrate
Preliminary results from our charge self-consistent LCAO band structure (CSCBS) calculations with Bloch sums as the basis reveal that a noncorrosive reduced band gap electrode for photoelectrochemical solar cells may be produced from a (1:1) mixture of β-PbO2 and TiO2 (both rutile). The band gaps for the constituents (β-PbO2 and TiO2) and the 1:1 mixture are calculated and a detailed characterization of the valence and the conduction bands is undertaken to offer a possible mechanism for the reduction of the band gap of the mixture. The band gap for the perovskite PbTiO3 is also calculated to offer a guideline for selecting from the competing pathways to the fabrication of noncorrosive photoelectrochemical electrodes
Band-gap measurements of bulk and nanoscale hematite by soft x-ray spectroscopy
Gilbert, B.; Frandsen, Cathrine; Maxey, E.R.;
2009-01-01
Chemical and photochemical processes at semiconductor surfaces are highly influenced by the size of the band gap, and ability to control the band gap by particle size in nanomaterials is part of their promise. The combination of soft x-ray absorption and emission spectroscopies provides band......-gap determination in bulk and nanoscale itinerant electron semiconductors such as CdS and ZnO, but this approach has not been established for materials such as iron oxides that possess band-edge electronic structure dominated by electron correlations. We performed soft x-ray spectroscopy at the oxygen K-edge to...... reveal band-edge electronic structure of bulk and nanoscale hematite. Good agreement is found between the hematite band gap derived from optical spectroscopy and the energy separation of the first inflection points in the x-ray absorption and emission onset regions. By applying this method to two sizes...
Design and analysis of microstrip photonic band gap filter without etching ground plane
R. N. Baral; P. K. Singhal
2009-01-01
A design of microstrip photonic band gap (PBG) filter is presented in this paper. The band reject filter is compact in structure and provides accurate band gap. The proposed filter has a periodic configuration; the impedance distribution over the length of each period is varied according to a designated sinusoidal function. Both theoretical and experimental results indicate that a bandstop filter with an extremely wide bandwidth can be achieved. Detailed analysis of the frequency response of ...
Plasma photonic band gaps have been observed in a two-dimensional microplasma array, and we have characterized their properties by both experimental and theoretical results. Microplasma columns ignited in helium near atmospheric pressure formed crystal-like structures in a square lattice with a lattice constant from 1.5 to 2.5 mm. Microwaves in the millimeter range transmitting through the array region attenuated at frequencies of photonic band gap in the Γ-X direction, as predicted by the modified plane-wave expansion method. Frequency dependence around the band gap was clarified in the numerical analysis of electromagnetic wave propagation and agreed with experimental results. Electron density in microplasmas was estimated to be 1x1013 cm-3 from the attenuation rate at the band gap in the Γ-X direction. Variation of the lattice constant induced frequency shift of the band gap in the millimeter and subterahertz regions, and so plasma photonic crystal can perform as a dynamically controllable band-stop filter
Photonic band gap of superconductor-medium structure: Two-dimensional triangular lattice
Liu, Wan-guo; Pan, Feng-ming, E-mail: fmpan@nuaa.edu.cn; Cai, Li-wei
2014-05-15
Highlights: • Plane wave expansion is generalized to superconductor-medium periodic structure. • A wider band gap appears than that in conventional photonic crystals. • Part of original energy levels are rearranged upon consideration of the superconductivity. • Band gap width decreases monotonically with penetration length, but not with the filling factor. • Band gaps can be partially shut down or opened by adjusting filling factor. - Abstract: Based on London theory a general form of wave equation is formulated for both dielectric medium and superconductor. Using the wave equation and applying plane wave expansion, we have numerically calculated the band structures and density of states of a photonic crystal, whose intersection is constructed by a two-dimensional triangular lattice of superconductor padding in dielectric medium. Results indicate a wider band gap in the superconductor-medium photonic crystal than that in conventional photonic crystals. And part of original energy levels are found to be rearranged upon consideration of the superconductivity. The dependence of band gap on penetration length and filling factor is also discussed. Band gap width decreases monotonically with the penetration length, but not with the filling factor. Band gaps can be partially shut down or opened by adjusting filling factor.
A thermodynamic study of shear banding in polymer solutions
Hooshyar, Soroush; Germann, Natalie
2016-06-01
Although shear banding is a ubiquitous phenomenon observed in soft materials, the mechanisms that give rise to shear-band formation are not always the same. In this work, we develop a new two-fluid model for semi-dilute entangled polymer solutions using the generalized bracket approach of nonequilibrium thermodynamics. The model is based on the hypothesis that the direct coupling between polymer stress and concentration is the driving mechanism of steady shear-band formation. To obtain smooth banded profiles in the two-fluid framework, a new stress-diffusive term is added to the time evolution equation for the conformation tensor. The advantage of the new model is that the differential velocity is treated as a state variable. This allows a straightforward implementation of the additional boundary conditions arising from the derivative diffusive terms with respect to this new state variable. To capture the overshoot of the shear stress during the start of a simple shear flow, we utilize a nonlinear Giesekus relaxation. Moreover, we include an additional relaxation term that resembles the term used in the Rouse linear entangled polymer model to account for convective constraint release and chain stretch to generate the upturn of the flow curve at large shear rates. Numerical calculations performed for cylindrical Couette flow confirm the independency of the solution from the deformation history and initial conditions. Furthermore, we find that stress-induced migration is the responsible diffusive term for steady-state shear banding. Because of its simplicity, the new model is an ideal candidate for the use in the simulation of more complex flows.
Malashchonak, M.V., E-mail: che.malasche@gmail.com [Belarusian State University, Nezalezhnastsi Av. 4, Minsk 220030 (Belarus); Streltsov, E.A., E-mail: streltea@bsu.by [Belarusian State University, Nezalezhnastsi Av. 4, Minsk 220030 (Belarus); Mazanik, A.V. [Belarusian State University, Nezalezhnastsi Av. 4, Minsk 220030 (Belarus); Kulak, A.I., E-mail: kulak@igic.bas-net.by [Institute of General and Inorganic Chemistry, National Academy of Sciences of Belarus, Surganova str., 9/1, Minsk 220072 (Belarus); Poznyak, S.K. [Belarusian State University, Nezalezhnastsi Av. 4, Minsk 220030 (Belarus); Stroyuk, O.L., E-mail: stroyuk@inphyschem-nas.kiev.ua [L.V. Pysarzhevsky Institute of Physical Chemistry of National Academy of Sciences of Ukraine, 31 prosp. Nauky, 03028 Kyiv (Ukraine); Kuchmiy, S.Ya. [L.V. Pysarzhevsky Institute of Physical Chemistry of National Academy of Sciences of Ukraine, 31 prosp. Nauky, 03028 Kyiv (Ukraine); Gaiduk, P.I. [Belarusian State University, Nezalezhnastsi Av. 4, Minsk 220030 (Belarus)
2015-08-31
Cadmium sulfide nanoparticle (NP) deposition by the successive ionic layer adsorption and reaction (SILAR) method on the surface of mesoporous ZnO micro-platelets with a large specific surface area (110 ± 10 m{sup 2}g{sup −1}) results in the formation of ZnO/CdS heterostructures exhibiting a high incident photon-to-current conversion efficiency (Y) not only within the region of CdS fundamental absorption (Y{sub max} = 90%; 0.1 M Na{sub 2}S + 0.1 M Na{sub 2}SO{sub 3}), but also in the sub-band-gap (SBG) range (Y{sub max} = 25%). The onset potentials of SBG photoelectrochemical processes are more positive than the band-gap (BG) onset potential by up to 100 mV. A maximum incident photon-to-current conversion efficiency value for SBG processes is observed at larger amount of deposited CdS in comparison with the case of BG ones. The Urbach energy (E{sub U}) of CdS NPs determined from the photocurrent spectra reaches a maximal value on an early deposition stage (E{sub U} = 93 mV at SILAR cycle number N = 5), then lowers somewhat (E{sub U} = 73 mV at N = 10) and remains steady in the range of N from 20 to 300 (E{sub U} = 67 ± 1 mV). High efficiency of the photoelectrochemical SBG processes are interpreted in terms of light scattering in the ZnO/CdS heterostructures. - Highlights: • ZnO/CdS films demonstrate high quantum efficiency (25%) for sub-band-gap transitions. • Onset photocurrent potentials for sub-band-gap processes differ than those for band-gap ones. • Sub-band-gap transitions are caused by band-tail states in CdS nanoparticles.
Robust band gap and half-metallicity in graphene with triangular perforations
Gregersen, Søren Schou; Power, Stephen; Jauho, Antti-Pekka
2016-01-01
disorders. Here we study a rectangular array of triangular antidots with zigzag edge geometries and show that their band gap behavior qualitatively differs from the standard behavior which is exhibited, e.g., by rectangular arrays of armchair-edged triangles. In the spin unpolarized case, zigzag......-edged antidots give rise to large band gaps compared to armchair-edged antidots, irrespective of the rules which govern the existence of gaps in armchair-edged antidot lattices. In addition the zigzag-edged antidots appear more robust than armchair-edged antidots in the presence of geometrical disorder. The......, reducing the band gaps compared to the unpolarized case. This behavior is also found to be robust in the presence of disorder. Our results highlight the possibilities of using triangular perforations in graphene to open electronic band gaps in systems with experimentally realistic levels of disorder, and...
Multi-large low-frequency band gaps in a periodic hybrid structure
Wang, T.; Sheng, M. P.; Guo, H. B.
2016-03-01
A hybrid structure composed of a local resonance mass and an external oscillator is proposed in this paper for restraining the elastic longitudinal wave propagation. Theoretical model has been established to investigate the dispersion relation and band gaps of the structure. The results show that the hybrid structure can produce multi-band gaps wider than the multi-resonator acoustic metamaterials. It is much easier for the hybrid structure to yield wide and low band gaps by adjusting the mass and stiffness of the external oscillator. Small series spring constant ratio results in low-frequency band gaps, in which the external oscillator acts as a resonator and replaces the original local resonator to hold the band gaps in low frequency range. Compared with the one-dimensional phononic crystal (PC) lattice, a new band gap emerges in lower frequency range in the hybrid structure because of the added local resonance, which will be a significant assistance in low-frequency vibration and noise reduction. Further, harmonic response analysis using finite element method (FEM) has been performed, and results show that elastic longitudinal waves are efficiently forbidden within the band gaps.
Reversible tuning of ZnO optical band gap by plasma treatment
Highlights: ► The ZnO optical band gap blue-shifts with hydrogen plasma treatment. ► The ZnO optical band gap red-shifts with oxygen plasma treatment. ► The ZnO optical band gap can be reversibly fine-tuned. - Abstract: Zinc oxide (ZnO) films synthesized by reacting zinc nitrate with hexamethylenetetramine were treated with hydrogen and oxygen plasmas. From UV–visible absorption and optical emission inspection, we have found that the optical band gap of ZnO films blue-shifted with hydrogen plasma treatment, but red-shifted with oxygen plasma treatment. By alternating the treatment sequence of hydrogen and oxygen plasmas, the ZnO optical band gap can be reversibly fine-tuned with the tunable range up to 80 meV. Scanning electron microscopy characterization indicates that the variation of the optical band gap is attributed to the competition between amorphous and crystalline forms of ZnO. The mechanism of reversible optical band gap tuning is discussed.
Low-frequency band gap mechanism of torsional vibration of lightweight elastic metamaterial shafts
Li, Lixia; Cai, Anjiang
2016-07-01
In this paper, the low-frequency band gap mechanism of torsional vibration is investigated for a kind of light elastic metamaterial (EM) shafts architecture comprised of a radial double-period element periodically as locally resonant oscillators with low frequency property. The dispersion relations are calculated by a method combining the transfer matrix and a lumped-mass method. The theoretical results agree well with finite method simulations, independent of the density of the hard material ring. The effects of the material parameters on the band gaps are further explored numerically. Our results show that in contrast to the traditional EM shaft, the weight of our proposed EM shaft can be reduced by 27% in the same band gap range while the vibration attenuation is kept unchanged, which is very convenient to instruct the potential engineering applications. Finally, the band edge frequencies of the lower band gaps for this light EM shaft are expressed analytically using physical heuristic models.
X-Band Photonic Band-Gap Accelerator Structure Breakdown Experiment
Marsh, Roark A.; /MIT /MIT /NIFS, Gifu /JAERI, Kyoto /LLNL, Livermore; Shapiro, Michael A.; Temkin, Richard J.; /MIT; Dolgashev, Valery A.; Laurent, Lisa L.; Lewandowski, James R.; Yeremian, A.Dian; Tantawi, Sami G.; /SLAC
2012-06-11
In order to understand the performance of photonic band-gap (PBG) structures under realistic high gradient, high power, high repetition rate operation, a PBG accelerator structure was designed and tested at X band (11.424 GHz). The structure consisted of a single test cell with matching cells before and after the structure. The design followed principles previously established in testing a series of conventional pillbox structures. The PBG structure was tested at an accelerating gradient of 65 MV/m yielding a breakdown rate of two breakdowns per hour at 60 Hz. An accelerating gradient above 110 MV/m was demonstrated at a higher breakdown rate. Significant pulsed heating occurred on the surface of the inner rods of the PBG structure, with a temperature rise of 85 K estimated when operating in 100 ns pulses at a gradient of 100 MV/m and a surface magnetic field of 890 kA/m. A temperature rise of up to 250 K was estimated for some shots. The iris surfaces, the location of peak electric field, surprisingly had no damage, but the inner rods, the location of the peak magnetic fields and a large temperature rise, had significant damage. Breakdown in accelerator structures is generally understood in terms of electric field effects. These PBG structure results highlight the unexpected role of magnetic fields in breakdown. The hypothesis is presented that the moderate level electric field on the inner rods, about 14 MV/m, is enhanced at small tips and projections caused by pulsed heating, leading to breakdown. Future PBG structures should be built to minimize pulsed surface heating and temperature rise.
Band-gap engineering of functional perovskites through quantum confinement and tunneling
Castelli, Ivano Eligio; Pandey, Mohnish; Thygesen, Kristian Sommer; Jacobsen, Karsten Wedel
2015-01-01
An optimal band gap that allows for a high solar-to-fuel energy conversion efficiency is one of the key factors to achieve sustainability. We investigate computationally the band gaps and optical spectra of functional perovskites composed of layers of the two cubic perovskite semiconductors BaSnO3...... and BaTaO2N. Starting from an indirect gap of around 3.3 eV for BaSnO3 and a direct gap of 1.8 eV for BaTaO2N, different layerings can be used to design a direct gap of the functional perovskite between 2.3 and 1.2 eV. The variations of the band gap can be understood in terms of quantum confinement...
Compositional dependence of optical band gap and refractive index in lead and bismuth borate glasses
Mallur, Saisudha B.; Czarnecki, Tyler; Adhikari, Ashish; Babu, Panakkattu K.
2015-08-15
Highlights: • Refractive indices increase with increasing PbO/Bi{sub 2}O{sub 3} content. • Optical band gap arises due to direct forbidden transition. • Optical band gaps decrease with increasing PbO/Bi{sub 2}O{sub 3} content. • New empirical relation between the optical band gap and the refractive index. - Abstract: We prepared a series of lead and bismuth borate glasses by varying PbO/Bi{sub 2}O{sub 3} content and studied refractive index and optical band gap as a function of glass composition. Refractive indices were measured very accurately using a Brewster’s angle set up while the optical band gaps were determined by analyzing the optical absorption edge using the Mott–Davis model. Using the Lorentz–Lorentz method and the effective medium theory, we calculated the refractive indices and then compared them with the measured values. Bismuth borate glasses show better agreement between the calculated values of the refractive index and experimental values. We used a differential method based on Mott–Davis model to obtain the type of transition and optical band gap (E{sub opt}) which in turn was compared with the value of E{sub opt} obtained using the extinction coefficient. Our analysis shows that in both lead and bismuth borate glasses, the optical band gap arises due to direct forbidden transition. With increasing PbO/Bi{sub 2}O{sub 3} content, the absorption edge shifts toward longer wavelengths and the optical band gap decreases. This behavior can be explained in terms of changes to the Pb−O/Bi−O chemical bonds with glass composition. We obtained a new empirical relation between the optical band gap and the refractive index which can be used to accurately determine the electronic oxide polarizability in lead and bismuth oxide glasses.
Compositional dependence of optical band gap and refractive index in lead and bismuth borate glasses
Highlights: • Refractive indices increase with increasing PbO/Bi2O3 content. • Optical band gap arises due to direct forbidden transition. • Optical band gaps decrease with increasing PbO/Bi2O3 content. • New empirical relation between the optical band gap and the refractive index. - Abstract: We prepared a series of lead and bismuth borate glasses by varying PbO/Bi2O3 content and studied refractive index and optical band gap as a function of glass composition. Refractive indices were measured very accurately using a Brewster’s angle set up while the optical band gaps were determined by analyzing the optical absorption edge using the Mott–Davis model. Using the Lorentz–Lorentz method and the effective medium theory, we calculated the refractive indices and then compared them with the measured values. Bismuth borate glasses show better agreement between the calculated values of the refractive index and experimental values. We used a differential method based on Mott–Davis model to obtain the type of transition and optical band gap (Eopt) which in turn was compared with the value of Eopt obtained using the extinction coefficient. Our analysis shows that in both lead and bismuth borate glasses, the optical band gap arises due to direct forbidden transition. With increasing PbO/Bi2O3 content, the absorption edge shifts toward longer wavelengths and the optical band gap decreases. This behavior can be explained in terms of changes to the Pb−O/Bi−O chemical bonds with glass composition. We obtained a new empirical relation between the optical band gap and the refractive index which can be used to accurately determine the electronic oxide polarizability in lead and bismuth oxide glasses
Gorisse, M.; Benchabane, S.; Teissier, G.; Billard, C.; Reinhardt, A.; Laude, V.; Defaÿ, E.; Aïd, M.
2011-06-01
We report on the observation of elastic waves propagating in a two-dimensional phononic crystal composed of air holes drilled in an aluminum nitride membrane. The theoretical band structure indicates the existence of an acoustic band gap centered around 800 MHz with a relative bandwidth of 6.5% that is confirmed by gigahertz optical images of the surface displacement. Further electrical measurements and computation of the transmission reveal a much wider attenuation band that is explained by the deaf character of certain bands resulting from the orthogonality of their polarization with that of the source.
Yudistira, D; Boes, A; Djafari-Rouhani, B; Pennec, Y; Yeo, L Y; Mitchell, A; Friend, J R
2014-11-21
We theoretically and experimentally demonstrate the existence of complete surface acoustic wave band gaps in surface phonon-polariton phononic crystals, in a completely monolithic structure formed from a two-dimensional honeycomb array of hexagonal shape domain-inverted inclusions in single crystal piezoelectric Z-cut lithium niobate. The band gaps appear at a frequency of about twice the Bragg band gap at the center of the Brillouin zone, formed through phonon-polariton coupling. The structure is mechanically, electromagnetically, and topographically homogeneous, without any physical alteration of the surface, offering an ideal platform for many acoustic wave applications for photonics, phononics, and microfluidics. PMID:25479504
Study of periodic band gap structure of the magnetized plasma photonic crystals
ZHANG Hai-feng; MA Li; LIU Shao-bin
2009-01-01
The characteristics of the periodic band gaps of the one dimension magnetized plasma photonic crystals are studied with the piecewise linear current density recursive convolution (PLCDRC) finite-differential time-domain (FDTD) method. In fre-quency-domain, the transmission coefficients of electromagnetic Gaussian pulses are computed, and the effects of the periodic structure constant, plasma layer thickness and parameters of plasma on the properties of periodic band gaps of magnetized photonic crystals are analyzed. The results show that the periodic band gaps depend strongly on the plasma parameters.
Modelling and design of complete photonic band gaps in two-dimensional photonic crystals
Yogita Kalra; R K Sinha
2008-01-01
In this paper, we investigate the existence and variation of complete photonic band gap size with the introduction of asymmetry in the constituent dielectric rods with honeycomb lattices in two-dimensional photonic crystals (PhC) using the plane-wave expansion (PWE) method. Two examples, one consisting of elliptical rods and the other comprising of rectangular rods in honeycomb lattices are considered with a view to estimate the design parameters for maximizing the complete photonic band gap. Further, it has been shown that complete photonic band gap size changes with the variation in the orientation angle of the constituent dielectric rods.
Optimization of band gaps of 2D photonic crystals by the rapid generic algorithm
SUN Yun-tao
2011-01-01
@@ Based on the rapid genetic algorithm (RGA), the band gap structures of square lattices with square scatters are optimized.In the optimizing process, gene codes are used to express square scatters and the fitting function adopts the relative values of the largest absolute photonic band gaps (PBGs).By changing the value of filling factor, three cell forms with large photonic band gaps are obtained.In addition, the comparison between the rapid genetic algorithm and the general genetic algorithm (GGA) is analyzed.
Chegel, Raad
2016-06-01
By using the third nearest neighbor modified tight binding (3NN-TB) method, the electronic structure and band gap of BNNRs under transverse electric fields are explored. The band gap of the BNNRs has a decreasing with increasing the intensity of the applied electric field, independent on the ribbon edge types. Furthermore, an analytic model for the dependence of the band gap in armchair and zigzag BNNRs on the electric field is proposed. The reduction of E g is similar for some N a armchair and N z zigzag BNNRs independent of their edges.
Manzanares-Martinez, J; Archuleta-Garcia, R; Moctezuma-Enriquez, D
2010-01-01
In this work we show theoretically that it is possible to design a large band gap in the infrared range using a one-dimensional Photonic Crystal heterostructure made of porous silicon. Stacking together multiple photonic crystal substructures of the same contrast index, but of different lattice periods, it is possible to broad the narrow forbidden band gap that can be reached by the low contrast index of the porous silicon multilayers. The main idea in this work is that we can construct a Giant Photonic Band Gap -as large as desired- by combining a tandem of photonic crystals substructures by using a simple analytical rule to determine the period of each substructure.
Kim, Jimin; Baik, Seung Su; Ryu, Sae Hee; Sohn, Yeongsup; Park, Soohyung; Park, Byeong-Gyu; Denlinger, Jonathan; Yi, Yeonjin; Choi, Hyoung Joon; Kim, Keun Su
2015-08-14
Black phosphorus consists of stacked layers of phosphorene, a two-dimensional semiconductor with promising device characteristics. We report the realization of a widely tunable band gap in few-layer black phosphorus doped with potassium using an in situ surface doping technique. Through band structure measurements and calculations, we demonstrate that a vertical electric field from dopants modulates the band gap, owing to the giant Stark effect, and tunes the material from a moderate-gap semiconductor to a band-inverted semimetal. At the critical field of this band inversion, the material becomes a Dirac semimetal with anisotropic dispersion, linear in armchair and quadratic in zigzag directions. The tunable band structure of black phosphorus may allow great flexibility in design and optimization of electronic and optoelectronic devices. PMID:26273052
The Development of Layered Photonic Band Gap Structures Using a Micro-Transfer Molding Technique
Kevin Jerome Sutherland
2001-06-27
Over the last ten years, photonic band gap (PBG) theory and technology have become an important area of research because of the numerous possible applications ranging from high-efficiency laser diodes to optical circuitry. This research concentrates on reducing the length scale in the fabrication of layered photonic band gap structures and developing procedures to improve processing consistency. Various procedures and materials have been used in the fabrication of layered PBG structures. This research focused on an economical micro transfer molding approach to create the final PBG structure. A poly dimethylsiloxane (PDMS) rubber mold was created from a silicon substrate. It was filled with epoxy and built layer-by-layer to create a 3-D epoxy structure. This structure was infiltrated with nanoparticle titania or a titania sol-gel, then fired to remove the polymer mold, leaving a monolithic ceramic inverse of the epoxy structure. The final result was a lattice of titania rolds that resembles a face-centered tetragonal structure. The original intent of this research was to miniaturize this process to a bar size small enough to create a photonic band gap for wavelengths of visible electro-magnetic radiation. The factor limiting progress was the absence of a silicon master mold of small enough dimensions. The Iowa State Microelectronics Research Center fabricated samples with periodicities of 2.5 and 1.0 microns with the existing technology, but a sample was needed on the order of 0.3 microns or less. A 0.4 micron sample was received from Sandia National Laboratory, which was made through an electron beam lithography process, but it contained several defects. The results of the work are primarily from the 2.5 and 1.0 micron samples. Most of the work focused on changing processing variables in order to optimize the infiltration procedure for the best results. Several critical parameters were identified, ranging from the ambient conditions to the specifics of the
Numerical study of the effect of permeability on square and triangular microwave band gap structures
We report the theoretical work on the photonic band gap structures suitable for microwave frequency region formed by magnetic materials (ε=9.87 and μ=2.17) using plane wave expansion method. The structures under analysis are two-dimensional square and triangular lattices. The calculated band gap between 10 and 20GHz region is anlaysed for the effect due to lattice spacing and the property of the material. The results are also compared with that of pure dielectric case. Obtained results indicate that both impedance and effective refractive index are responsible for the gap width and mid-gap frequency
Vibrational renormalisation of the electronic band gap in hexagonal and cubic ice
Electron-phonon coupling in hexagonal and cubic water ice is studied using first-principles quantum mechanical methods. We consider 29 distinct hexagonal and cubic ice proton-orderings with up to 192 molecules in the simulation cell to account for proton-disorder. We find quantum zero-point vibrational corrections to the minimum electronic band gaps ranging from −1.5 to −1.7 eV, which leads to improved agreement between calculated and experimental band gaps. Anharmonic nuclear vibrations play a negligible role in determining the gaps. Deuterated ice has a smaller band-gap correction at zero-temperature of −1.2 to −1.4 eV. Vibrations reduce the differences between the electronic band gaps of different proton-orderings from around 0.17 eV to less than 0.05 eV, so that the electronic band gaps of hexagonal and cubic ice are almost independent of the proton-ordering when quantum nuclear vibrations are taken into account. The comparatively small reduction in the band gap over the temperature range 0 − 240 K of around 0.1 eV does not depend on the proton ordering, or whether the ice is protiated or deuterated, or hexagonal, or cubic. We explain this in terms of the atomistic origin of the strong electron-phonon coupling in ice
Vibrational renormalisation of the electronic band gap in hexagonal and cubic ice
Engel, Edgar A., E-mail: eae32@cam.ac.uk; Needs, Richard J. [TCM Group, Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE (United Kingdom); Monserrat, Bartomeu [TCM Group, Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE (United Kingdom); Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854-8019 (United States)
2015-12-28
Electron-phonon coupling in hexagonal and cubic water ice is studied using first-principles quantum mechanical methods. We consider 29 distinct hexagonal and cubic ice proton-orderings with up to 192 molecules in the simulation cell to account for proton-disorder. We find quantum zero-point vibrational corrections to the minimum electronic band gaps ranging from −1.5 to −1.7 eV, which leads to improved agreement between calculated and experimental band gaps. Anharmonic nuclear vibrations play a negligible role in determining the gaps. Deuterated ice has a smaller band-gap correction at zero-temperature of −1.2 to −1.4 eV. Vibrations reduce the differences between the electronic band gaps of different proton-orderings from around 0.17 eV to less than 0.05 eV, so that the electronic band gaps of hexagonal and cubic ice are almost independent of the proton-ordering when quantum nuclear vibrations are taken into account. The comparatively small reduction in the band gap over the temperature range 0 − 240 K of around 0.1 eV does not depend on the proton ordering, or whether the ice is protiated or deuterated, or hexagonal, or cubic. We explain this in terms of the atomistic origin of the strong electron-phonon coupling in ice.
Systematic analysis of the unique band gap modulation of mixed halide perovskites.
Kim, Jongseob; Lee, Sung-Hoon; Chung, Choong-Heui; Hong, Ki-Ha
2016-02-14
Solar cells based on organic-inorganic hybrid metal halide perovskites have been proven to be one of the most promising candidates for the next generation thin film photovoltaic cells. Mixing Br or Cl into I-based perovskites has been frequently tried to enhance the cell efficiency and stability. One of the advantages of mixed halides is the modulation of band gap by controlling the composition of the incorporated halides. However, the reported band gap transition behavior has not been resolved yet. Here a theoretical model is presented to understand the electronic structure variation of metal mixed-halide perovskites through hybrid density functional theory. Comparative calculations in this work suggest that the band gap correction including spin-orbit interaction is essential to describe the band gap changes of mixed halides. In our model, both the lattice variation and the orbital interactions between metal and halides play key roles to determine band gap changes and band alignments of mixed halides. It is also presented that the band gap of mixed halide thin films can be significantly affected by the distribution of halide composition. PMID:26791587
Modification of Band Gap of β-SiC by N-Doping
LIU Hong-Sheng; FANG Xiao-Yong; SONG Wei-Li; HOU Zhi-Ling; LU Ran; YUAN Jie; CAO Mao-Sheng
2009-01-01
The geometrical and electronic structures of nitrogen-doped β-SiC are investigated by employing the first principles of plane wave ultra-soft pseudo-potential technology based on density functional theory.The structures of SiC1-xNx (x=0,1/32,1/16,1/8,1/4) with different doping concentrations are optimized.The results reveal that the band gap of β-SiC transforms from an indirect band gap to a direct band gap with band gap shrinkage after carbon atoms are replaced by nitrogen atoms.The Fermi level shifts from valence band top to conduction band by doping nitrogen in pure β-SiC,and the doped β-SiC becomes metallic.The degree of Fermi levels entering into the conduction band increases with the increment of doping concentration;however,the band gap becomes narrower.This is attributed to defects with negative electricity occurring in surrounding silicon atoms.With the increase of doping concentration,more residual electrons,more easily captured by the 3p orbit in the silicon atom,will be provided by nitrogen atoms to form more defects with negative electricity.
Phononic band gaps and vibrations in one- and two-dimensional mass-spring structures
Jensen, Jakob Søndergaard
2003-01-01
The vibrational response of finite periodic lattice structures subjected to periodic loading is investigated. Special attention is devoted to the response in frequency ranges with gaps in the band structure for the corresponding infinite periodic lattice. The effects of boundaries, viscous damping......, and imperfections are studied by analyzing two examples; a 1-D filter and a 2-D wave guide. In 1-D the structural response in the band gap is shown to be insensitive to damping and small imperfections. In 2-D the similar effect of damping is noted for one type of periodic structure, whereas for...... another type the band gap effect is nearly eliminated by damping. In both 1-D and 2-D it is demonstrated how the free structural boundaries affect the response in the band gap due to local resonances. Finally, 2-D wave guides are considered by replacing the periodic structure with a homogeneous structure...
Thermal emissivity for finite three-dimensional photonic band gap crystals
Stimpson, Andrew J.; Dowling, Jonathan P.
2002-01-01
We discuss the results of computer model for the thermal emissivity of a three-dimensional photonic band gap (PBG) crystal, specifically an inverted opal structure. The thermal emittance for a range of frequencies and angles is calculated.
Accurate evaluation of lowest band gaps in ternary locally resonant phononic crystals
Wang Gang; Shao Li-Hui; Liu Yao-Zong; Wen Ji-Hong
2006-01-01
Based on a better understanding of the lattice vibration modes, two simple spring-mass models are constructed in order to evaluate the frequencies on both the lower and upper edges of the lowest locally resonant band gaps of the ternary locally resonant phononic crystals. The parameters of the models are given in a reasonable way based on the physical insight into the band gap mechanism. Both the lumped-mass methods and our models are used in the study of the influences of structural and the material parameters on frequencies on both edges of the lowest gaps in the ternary locally resonant phononic crystals. The analytical evaluations with our models and the theoretical predictions with the lumped-mass method are in good agreement with each other. The newly proposed heuristic models are helpful for a better understanding of the locally resonant band gap mechanism, as well as more accurate evaluation of the band edge frequencies.
Shape optimization of phononic band gap structures using the homogenization approach
Vondřejc, Jaroslav; Heczko, Jan
2016-01-01
The paper deals with optimization of the acoustic band gaps computed using the homogenized model of strongly heterogeneous elastic composite which is constituted by soft inclusions periodically distributed in stiff elastic matrix. We employ the homogenized model of such medium to compute intervals - band gaps - of the incident wave frequencies for which acoustic waves cannot propagate. It was demonstrated that the band gaps distribution can be influenced by changing the shape of inclusions. Therefore, we deal with the shape optimization problem to maximize low-frequency band gaps; their bounds are determined by analyzing the effective mass tensor of the homogenized medium. Analytic transformation formulas are derived which describe dispersion effects of resizing the inclusions. The core of the problem lies in sensitivity of the eigenvalue problem associated with the microstructure. Computational sensitivity analysis is developed, which allows for efficient using of the gradient based optimization methods. Num...
Study on the vibration band gap and vibration attenuation property of phononic crystals
2008-01-01
Phononic crystals (PCs) are functional materials with periodic structures and elas- tic wave (vibration) band gaps, where propagation of vibrations with frequencies within band gaps is forbidden. PCs with finite periods can restrain the propagation of vibrations with frequencies in band gaps and thus has vibration attenuation property. Worldwide, many institutions and researchers are engaged in the re- search of PCs, however, studies on the vibration attenuation property of PCs are still limited. In this paper, we report our study of band gaps and vibration attenua- tion properties of 1) a simplified PC—periodic mass-spring structures, 2) longitu- dinal vibration of one-dimensional (1D-), 2D-, 3D-PCs, and 3) the flexural vibration of 1D- and 2D-PCs. These studies provide a foundation for the applications of PCs in vibration attenuation.
Isotropic properties of the photonic band gap in quasicrystals with low-index contrast
Priya Rose, T.; di Gennaro, E.; Abbate, G.; Andreone, A.
2011-09-01
We report on the formation and development of the photonic band gap in two-dimensional 8-, 10-, and 12-fold symmetry quasicrystalline lattices of low-index contrast. Finite-size structures made of dielectric cylindrical rods were studied and measured in the microwave region, and their properties were compared with a conventional hexagonal crystal. Band-gap characteristics were investigated by changing the direction of propagation of the incident beam inside the crystal. Various angles of incidence from 0∘ to 30∘ were used to investigate the isotropic nature of the band gap. The arbitrarily high rotational symmetry of aperiodically ordered structures could be practically exploited to manufacture isotropic band-gap materials, which are perfectly suitable for hosting waveguides or cavities.
Isotropic properties of the photonic band gap in quasicrystals with low-index contrast
Rose, Priya; Abbate, G; Andreone, A
2011-01-01
We report on the formation and development of the photonic band gap in two-dimensional 8-, 10- and 12-fold symmetry quasicrystalline lattices of low index contrast. Finite size structures made of dielectric cylindrical rods were studied and measured in the microwave region, and their properties compared with a conventional hexagonal crystal. Band gap characteristics were investigated by changing the direction of propagation of the incident beam inside the crystal. Various angles of incidence from 0 \\degree to 30\\degree were used in order to investigate the isotropic nature of the band gap. The arbitrarily high rotational symmetry of aperiodically ordered structures could be practically exploited to manufacture isotropic band gap materials, which are perfectly suitable for hosting waveguides or cavities.
Study on the vibration band gap and vibration attenuation property of phononic crystals
无
2008-01-01
Phononic crystals (PCs) are functional materials with periodic structures and elastic wave (vibration) band gaps, where propagation of vibrations with frequencies within band gaps is forbidden. PCs with finite periods can restrain the propagation of vibrations with frequencies in band gaps and thus has vibration attenuation property. Worldwide, many institutions and researchers are engaged in the research of PCs, however, studies on the vibration attenuation property of PCs are still limited. In this paper, we report our study of band gaps and vibration attenuation properties of 1) a simplified PC-periodic mass-spring structures, 2) longitudinal vibration of one-dimensional (1D-), 2D-, 3D-PCs, and 3) the flexural vibration of 1D- and 2D-PCs. These studies provide a foundation for the applications of PCs in vibration attenuation.
Tunable Photonic Band Gaps In Photonic Crystal Fibers Filled With a Cholesteric Liquid Crystal
Thomas; Tanggaard; Larsen; David; Sparre; Hermann; Anders; Bjarklev
2003-01-01
A photonic crystal fiber has been filled with a cholesteric liquid crystal. A temperature sensitive photonic band gap effect was observed, which was especially pronounced around the liquid crystal phase transition temperature.
Band structure in the polymer quantization of the harmonic oscillator
We discuss the detailed structure of the spectrum of the Hamiltonian for the polymerized harmonic oscillator and compare it with the spectrum in the standard quantization. As we will see the non-separability of the Hilbert space implies that the point spectrum consists of bands similar to the ones appearing in the treatment of periodic potentials. This feature of the spectrum of the polymeric harmonic oscillator may be relevant for the discussion of the polymer quantization of the scalar field and may have interesting consequences for the statistical mechanics of these models. (paper)
Printable, wide band-gap chalcopyrite thin films for power generating window applications
Sung Hwan Moon; Se Jin Park; Yun Jeong Hwang; Doh-Kwon Lee; Yunae Cho; Dong-Wook Kim; Byoung Koun Min
2014-01-01
Printable, wide band-gap chalcopyrite compound films (CuInGaS2, CIGS) were synthesized on transparent conducting oxide substrates. The wide band-gap and defective nature of the films reveal semi-transparent and bifacial properties that are beneficial for power generating window applications. Importantly, solar cell devices with these films demonstrate a synergistic effect for bifacial illumination resulting in a 5.4–16.3% increase of the apparent power conversion efficiency compared to the si...
Amir Reza Sadrolhosseini; Suraya Abdul Rashid; Noor, A. S. M.; Alireza Kharazmi; Lim, H. N.; Mohd Adzir Mahdi
2016-01-01
A polypyrrole-nanoparticles reduced graphene oxide nanocomposite layer was prepared using electrochemical method. The prepared samples were characterized using Fourier transform infrared spectroscopy, field emission scanning electron microscopy, and UV-visible spectroscopy. The band gap of nanocomposite layers was calculated from UV-visible spectra and the thermal diffusivity of layers was measured using a photoacoustic technique. As experimental results, the optical band gap was in the range...
Omnidirectional photonic band gap of one-dimensional ternary plasma photonic crystals
An omnidirectional photonic band gap (PBG) is presented in one-dimensional ternary plasma photonic crystal (PPC). In contrast to the omnidirectional PBG coming from a zero- n-tilde gap or single negative (SNG) (permittivity- or permittivity-negative) gap, this gap originates from a Bragg gap. It has been shown that the ternary PPC, compared with the usual binary photonic crystal (PC), has a superior feature in the enhancement of the high-reflectance range (HRR) and the modulation of the omnidirectional PBG. Omnidirectional PBGs can be utilized as omnidirectional or large incident angle filters or reflectors in microwave devices
Reducing support loss in micromechanical ring resonators using phononic band-gap structures
Hsu, Feng-Chia; Huang, Tsun-Che; Wang, Chin-Hung; Chang, Pin [Industrial Technology Research Institute-South, Tainan 709, Taiwan (China); Hsu, Jin-Chen, E-mail: fengchiahsu@itri.org.t, E-mail: hsujc@yuntech.edu.t [Department of Mechanical Engineering, National Yunlin University of Science and Technology, Douliou, Yunlin 64002, Taiwan (China)
2011-09-21
In micromechanical resonators, energy loss via supports into the substrates may lead to a low quality factor. To eliminate the support loss, in this paper a phononic band-gap structure is employed. We demonstrate a design of phononic-crystal (PC) strips used to support extensional wine-glass mode ring resonators to increase the quality factor. The PC strips are introduced to stop elastic-wave propagation by the band-gap and deaf-band effects. Analyses of resonant characteristics of the ring resonators and the dispersion relations, eigenmodes, and transmission properties of the PC strips are presented. With the proposed resonator architecture, the finite-element simulations show that the leaky power is effectively reduced and the stored energy inside the resonators is enhanced simultaneously as the operating frequencies of the resonators are within the band gap or deaf bands. Realization of a high quality factor micromechanical ring resonator with minimized support loss is expected.
Influence of process parameters on band gap of AI-doped ZnO film
Diqiu HUANG; Xiangbin ZENG; Yajuan ZHENG; Xiaojin WANG; Yanyan YANG
2013-01-01
This paper presents the influence of process parameters, such as argon （Ar） flow rate, sputtering power and substrate temperature on the band gap of Al-doped ZnO film, Al-doped ZnO thin films were fabricated by radio frequency （RF） magnetron sputtering technology and deposited on polyimide and glass substrates. Under different Ar flow rates varied from 30 to 70 sccm, the band gap of thin films were changed from 3.56 to 3.67 eV. As sputtering power ranged from 125 to 200 W, the band gap was varied from 3.28 to 3.82 eV; the band gap was between 3.41 and 3.88 eV as substrate temperature increases from 150℃ to 300℃. Furthermore, the correlation between carrier concentration and band gap was investigated by HALL. These results demonstrate that the band gap of the Al-doped ZnO thin film can be adjusted by changing the Ar flow rate, sputtering power and substrate temperature, which can improve the performance of semiconductor devices related to Al-doped ZnO thin film.
Acoustic band gaps in two-dimensional square arrays of semi-hollow circular cylinders
T.; Kim
2009-01-01
Concave surfaces focus sound while convex surfaces disperse sound. It is therefore interesting to know if it is possible to make use of these two opposite characteristics to enhance the band gap performance of periodic arrays of solid cylinders in air. In this paper, the band gap characteristics of a 2-D square array of semi-hollow circular cylinders embedded in air are investigated, both experimentally and theoretically. In comparison with the types of inclusion studied by previous researchers, a semi-hollow circular cylinder is unique in the sense that it has concave inner surfaces and convex outer surfaces. The finite difference time domain (FDTD) method is employed to study the propagation behavior of sound across the new phononic crystal of finite extent, and the influences of sample size and inclusion orientation on band gap characteristics are quantified in order to obtain the maximum band gap. For reference, the band gap behaviors of solid circular cylinder/air and hollow circular cylinder/air systems are considered and compared with those of semi-hollow circular cylinder/air systems. In addition to semi-hollow circular cylinders, other inclusion topologies such as semi-hollow triangular and square cylinders are also investigated. To validate the theoretical predictions, experimental measurements on square arrays of hollow Al cylinders in air and semi-hollow Al cylinders in air are carried out. The results demonstrate that the semi-hollow circular cylinder/air system has the best overall band gap performance.
Quasiparticle Band Gaps of Graphene and Graphone on Hexagonal Boron Nitride Substrate
Kharche, Neerav; Nayak, Saroj
2012-02-01
Graphene holds great promise for post-silicon electronics; however, it faces two main challenges: opening up a band gap and finding a suitable substrate material. Graphene on hexagonal boron nitride (hBN) substrate provides a potential system to overcome these challenges. While theoretical studies suggested a possibility of a finite band gap of graphene on hBN, recent experimental studies find no band gap. We have studied graphene-hBN system using the first-principles density functional method and the many-body perturbation theory within GW approximation [1]. A Bernal stacked graphene on hBN has a band gap on the order of 0.1 eV, which disappears when graphene is misaligned with respect to hBN. The latter is the likely scenario in realistic devices. In contrast, if graphene supported on hBN is hydrogenated, the resulting system (graphone) exhibits band gaps larger than 2.5 eV. The graphone band gap is due to chemical functionalization and is robust in the presence of misalignment, however, it reduces by about 1 eV due to the polarization effects at the graphone/hBN interface.[4pt] [1] N. Kharche and S. K. Nayak, Nano Lett., DOI: 10.1021/nl202725w, (2011).
Gap state analysis in electric-field-induced band gap for bilayer graphene.
Kanayama, Kaoru; Nagashio, Kosuke
2015-01-01
The origin of the low current on/off ratio at room temperature in dual-gated bilayer graphene field-effect transistors is considered to be the variable range hopping in gap states. However, the quantitative estimation of gap states has not been conducted. Here, we report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method. An energy gap of ~ 250 meV is obtained at the maximum displacement field of ~ 3.1 V/nm, where the current on/off ratio of ~ 3 × 10(3) is demonstrated at 20 K. The density of states for the gap states are in the range from the latter half of 10(12) to 10(13) eV(-1) cm(-2). Although the large amount of gap states at the interface of high-k oxide/bilayer graphene limits the current on/off ratio at present, our results suggest that the reduction of gap states below ~ 10(11) eV(-1) cm(-2) by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications. PMID:26511395
Band gap engineering for single-layer graphene by using slow Li+ ions
Ryu, Mintae; Lee, Paengro; Kim, Jingul; Park, Heemin; Chung, Jinwook
2016-08-01
In order to utilize the superb electronic properties of graphene in future electronic nano-devices, a dependable means of controlling the transport properties of its Dirac electrons has to be devised by forming a tunable band gap. We report on the ion-induced modification of the electronic properties of single-layer graphene (SLG) grown on a SiC(0001) substrate by doping low-energy (5 eV) Li+ ions. We find the opening of a sizable and tunable band gap up to 0.85 eV, which depends on the Li+ ion dose as well as the following thermal treatment, and is the largest band gap in the π-band of SLG by any means reported so far. Our Li 1s core-level data together with the valence band suggest that Li+ ions do not intercalate below the topmost graphene layer, but cause a significant charge asymmetry between the carbon sublattices of SLG to drive the opening of the band gap. We thus provide a route to producing a tunable graphene band gap by doping Li+ ions, which may play a pivotal role in the utilization of graphene in future graphene-based electronic nano-devices.
Band gap engineering for single-layer graphene by using slow Li(+) ions.
Ryu, Mintae; Lee, Paengro; Kim, Jingul; Park, Heemin; Chung, Jinwook
2016-08-01
In order to utilize the superb electronic properties of graphene in future electronic nano-devices, a dependable means of controlling the transport properties of its Dirac electrons has to be devised by forming a tunable band gap. We report on the ion-induced modification of the electronic properties of single-layer graphene (SLG) grown on a SiC(0001) substrate by doping low-energy (5 eV) Li(+) ions. We find the opening of a sizable and tunable band gap up to 0.85 eV, which depends on the Li(+) ion dose as well as the following thermal treatment, and is the largest band gap in the π-band of SLG by any means reported so far. Our Li 1s core-level data together with the valence band suggest that Li(+) ions do not intercalate below the topmost graphene layer, but cause a significant charge asymmetry between the carbon sublattices of SLG to drive the opening of the band gap. We thus provide a route to producing a tunable graphene band gap by doping Li(+) ions, which may play a pivotal role in the utilization of graphene in future graphene-based electronic nano-devices. PMID:27345294
Design Analysis of An Electromagnetic Band Gap Microstrip Antenna
M. S. Alam
2011-01-01
Full Text Available Problem statement: Wideband compact antenna is highly demandable due to the dynamic development in the wireless technology. Approach: A simple, compact EBG microstrip antenna is proposed in this study that covers a wideband of 250 GHz and the design is conformal with the 2.45 GHz ISM band (WLAN, IEEE 802.11b and g/Bluetooth/RFID applications. Results: A 6×6 array of square unit cell formed the EBG structure which is incorporated with the radiating patch to enhance the antenna performances. This design achieved an impedance bandwidth of 10.14% (2.34-2.59 GHz at -10 dB return loss and VSWR ≤ 2. Simulated radiation pattern is almost omnideirectional. Conclusion/Recommendations: The simulated results prove the compatibility of the EBG antenna with the 2.45 GHz ISM band applications. Further enhancement of the antenna performance with improved design is under consideration.
Tight-binding parameterization for photonic band gap materials
Lidorikis, E.; Sigalas, M. M.; Soukoulis, C. M.; Economou, E. N.
1998-01-01
The ideas of the linear combination of atomic orbitals (LCAO) method, well known from the study of electrons, is extended to the classical wave case. The Mie resonances of the isolated scatterer in the classical wave case, are analogous to the localized eigenstates in the electronic case. The matrix elements of the two-dimensional tight-binding (TB) Hamiltonian are obtained by fitting to ab initio results. The transferability of the TB model is tested by reproducing accurately the band struct...
Band gap structure modification of amorphous anodic Al oxide film by Ti-alloying
Canulescu, Stela; Rechendorff, K.; Borca, C. N.;
2014-01-01
The band structure of pure and Ti-alloyed anodic aluminum oxide has been examined as a function of Ti concentration varying from 2 to 20 at. %. The band gap energy of Ti-alloyed anodic Al oxide decreases with increasing Ti concentration. X-ray absorption spectroscopy reveals that Ti atoms are not...
Band Gap and Waveguide States in Two-Dimensional Disorder Phononic Crystals
LI Xiao-Chun; LIU Zheng-You; LIANG Hong-Yu; XIAO Qing-Wu
2006-01-01
@@ The influences of the configurational disorders on phononic band gaps and on waveguide modes are investigated for the two-dimensional phononic crystals consisting of water cylinders periodically arrayed in mercury. Two types of conflgurational disorders, relevant to the cylinder position and cylinder size respectively, are taken into account. It is found that the phononic band gap and the guide band are sensitive to the disorders, and generally become narrower with the increasing disorders. It is also found that the waveguide side walls without disorder can significantly prevent the guide modes in the waveguide from influence by the disorders in the crystals to a large amount.
Robust band gap and half-metallicity in graphene with triangular perforations
Gregersen, Søren Schou; Power, Stephen R.; Jauho, Antti-Pekka
2016-06-01
Ideal graphene antidot lattices are predicted to show promising band gap behavior (i.e., EG≃500 meV) under carefully specified conditions. However, for the structures studied so far this behavior is critically dependent on superlattice geometry and is not robust against experimentally realistic disorders. Here we study a rectangular array of triangular antidots with zigzag edge geometries and show that their band gap behavior qualitatively differs from the standard behavior which is exhibited, e.g., by rectangular arrays of armchair-edged triangles. In the spin unpolarized case, zigzag-edged antidots give rise to large band gaps compared to armchair-edged antidots, irrespective of the rules which govern the existence of gaps in armchair-edged antidot lattices. In addition the zigzag-edged antidots appear more robust than armchair-edged antidots in the presence of geometrical disorder. The inclusion of spin polarization within a mean-field Hubbard approach gives rise to a large overall magnetic moment at each antidot due to the sublattice imbalance imposed by the triangular geometry. Half-metallic behavior arises from the formation of spin-split dispersive states near the Fermi energy, reducing the band gaps compared to the unpolarized case. This behavior is also found to be robust in the presence of disorder. Our results highlight the possibilities of using triangular perforations in graphene to open electronic band gaps in systems with experimentally realistic levels of disorder, and furthermore, of exploiting the strong spin dependence of the system for spintronic applications.
Li, Jiaqian; Shen, Haijun
2015-12-01
The longitudinal vibration band gaps in periodic (n, 0)-(2n, 0) single-walled carbon nanotube(SWCNT) intramolecular junctions(IMJs) are investigated based on the finite element calculation. The frequency ranges of band gaps in frequency response functions(FRF) simulated by finite element method (FEM) show good agreement with those in band structure obtained by simple spring-mass model. Moreover, a comprehensive parametric study is also conducted to highlight the influences of the geometrical parameters such as the size of unit cell, component ratios of the IMJs and diameters of the CNT segments as well as geometric imperfections on the first band gap. The results show that the frequency ranges and the bandwidth of the gap strongly depend on the geometrical parameters. Furthermore, the influences of geometrical parameters on gaps are nuanced in IMJs with different topological defects. The existence of vibration band gaps in periodic IMJs lends a new insight into the development of CNT-based nano-devices in application of vibration isolation.
Jiaqian Li
2015-12-01
Full Text Available The longitudinal vibration band gaps in periodic (n, 0–(2n, 0 single-walled carbon nanotube(SWCNT intramolecular junctions(IMJs are investigated based on the finite element calculation. The frequency ranges of band gaps in frequency response functions(FRF simulated by finite element method (FEM show good agreement with those in band structure obtained by simple spring-mass model. Moreover, a comprehensive parametric study is also conducted to highlight the influences of the geometrical parameters such as the size of unit cell, component ratios of the IMJs and diameters of the CNT segments as well as geometric imperfections on the first band gap. The results show that the frequency ranges and the bandwidth of the gap strongly depend on the geometrical parameters. Furthermore, the influences of geometrical parameters on gaps are nuanced in IMJs with different topological defects. The existence of vibration band gaps in periodic IMJs lends a new insight into the development of CNT-based nano-devices in application of vibration isolation.
Strain-induced band-gap engineering of graphene monoxide and its effect on graphene
Pu, H. H.; Rhim, S. H.; Hirschmugl, C. J.; Gajdardziska-Josifovska, M.; Weinert, M.; Chen, J. H.
2013-02-01
Using first-principles calculations we demonstrate the feasibility of band-gap engineering in two-dimensional crystalline graphene monoxide (GMO), a recently reported graphene-based material with a 1:1 carbon/oxygen ratio. The band gap of GMO, which can be switched between direct and indirect, is tunable over a large range (0-1.35 eV) for accessible strains. Electron and hole transport occurs predominantly along the zigzag and armchair directions (armchair for both) when GMO is a direct- (indirect-) gap semiconductor. A band gap of ˜0.5 eV is also induced in graphene at the K' points for GMO/graphene hybrid systems.
Band gap and electronic structure of MgSiN2
Density functional theory calculations and electron energy loss spectroscopy indicate that the electronic structure of ordered orthorhombic MgSiN2 is similar to that of wurtzite AlN. A band gap of 5.7 eV was calculated for both MgSiN2 (indirect) and AlN (direct) using the Heyd-Scuseria-Ernzerhof approximation. Correction with respect to the experimental room-temperature band gap of AlN indicates that the true band gap of MgSiN2 is 6.2 eV. MgSiN2 has an additional direct gap of 6.3 eV at the Γ point.
Band gap and electronic structure of MgSiN{sub 2}
Quirk, J. B., E-mail: james.quirk09@imperial.ac.uk; Råsander, M.; McGilvery, C. M.; Moram, M. A. [Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ (United Kingdom); Palgrave, R. [Department of Chemistry, University College London, Gordon Street WC1H 0AJ (United Kingdom)
2014-09-15
Density functional theory calculations and electron energy loss spectroscopy indicate that the electronic structure of ordered orthorhombic MgSiN{sub 2} is similar to that of wurtzite AlN. A band gap of 5.7 eV was calculated for both MgSiN{sub 2} (indirect) and AlN (direct) using the Heyd-Scuseria-Ernzerhof approximation. Correction with respect to the experimental room-temperature band gap of AlN indicates that the true band gap of MgSiN{sub 2} is 6.2 eV. MgSiN{sub 2} has an additional direct gap of 6.3 eV at the Γ point.
Design of nanostrip magnonic crystal waveguides with a single magnonic band gap
A novel planar structure of magnonic-crystal waveguide (MCW) with periodic rectangular-shaped holes embedded in a magnetic nanostrip film was designed. The effects of the distance between rectangular-shaped holes in the width direction of MCW on magnonic band structures were studied by micromagnetic simulations. The results show that a MCW with a single magnonic band gap can be obtained by adjusting the distance to meet the condition of Bragg reflection of spin waves in the width direction of MCW. Moreover, the center frequency and width of magnonic gap can be regulated by changing the period and length of rectangular-shaped holes. - Highlights: • Design a novel planar structure of magnonic-crystal waveguide. • The physical origin of a single magnonic band gap. • Study of the center frequency and width of magnonic gap
Acoustic band gaps in 2D liquid phononic crystals of rectangular structure
We present band structure results for a new two dimensional (2D) rectangular array geometry of water (mercury) cylinders of square cross section in a mercury (water) host. The results show that the water/mercury system, consisting of low-density cylinders in a high-density host, is the most favourable configuration for obtaining large acoustic gaps. Otherwise, only very small stop gaps can be found for the mercury/water systems. For a given cylinder width value, the lowest band gap may not always have the maximum width, but at some value in both systems the lowest band gap will always have the largest width. The differences in the case of circular cylinders are also discussed. (author)
Pentamode metamaterials with tunable acoustics band gaps and large figures of merit
Wang, Zhaohong; Cai, Chengxin; Li, Qingwei; Li, Jing; Xu, Zhuo
2016-07-01
In this paper, we propose a class of pentamode metamaterials for which the frequency range of the acoustics band gaps can be tuned and large figures of merit can be obtained. The band structures of the pentamode metamaterials are calculated systematically by using the finite element method. The numerical results show that the lower edge frequency of the first acoustics band gaps of pentamode metamaterials can be tuned between 3.72 kHz and 10.6 kHz by changing the diameters of the bottom and top touch cones slightly, and the relative bandwidth of the first acoustics band gaps can also be expanded. In addition, compared with the results seen in the previous research in this area, the volume filling fraction of pentamode metamaterials can be decreased by 15.7%-24.4% and the maximum figure of merit can be increased by 39.2%.
Kong, Lingping; Liu, Gang; Gong, Jue; Hu, Qingyang; Schaller, Richard D.; Dera, Przemyslaw; Zhang, Dongzhou; Liu, Zhenxian; Yang, Wenge; Zhu, Kai; Tang, Yuzhao; Wang, Chuanyi; Wei, Su-Huai; Xu, Tao; Mao, Ho-kwang
2016-08-09
The organic-inorganic hybrid lead trihalide perovskites have been emerging as the most attractive photovoltaic materials. As regulated by Shockley-Queisser theory, a formidable materials science challenge for improvement to the next level requires further band-gap narrowing for broader absorption in solar spectrum, while retaining or even synergistically prolonging the carrier lifetime, a critical factor responsible for attaining the near-band-gap photovoltage. Herein, by applying controllable hydrostatic pressure, we have achieved unprecedented simultaneous enhancement in both band-gap narrowing and carrier-lifetime prolongation (up to 70% to -100% increase) under mild pressures at -0.3 GPa. The pressure-induced modulation on pure hybrid perovskites without introducing any adverse chemical or thermal effect clearly demonstrates the importance of band edges on the photon-electron interaction and maps a pioneering route toward a further increase in their photovoltaic performance.
Kong, Lingping; Liu, Gang; Gong, Jue; Hu, Qingyang; Schaller, Richard D; Dera, Przemyslaw; Zhang, Dongzhou; Liu, Zhenxian; Yang, Wenge; Zhu, Kai; Tang, Yuzhao; Wang, Chuanyi; Wei, Su-Huai; Xu, Tao; Mao, Ho-Kwang
2016-08-01
The organic-inorganic hybrid lead trihalide perovskites have been emerging as the most attractive photovoltaic materials. As regulated by Shockley-Queisser theory, a formidable materials science challenge for improvement to the next level requires further band-gap narrowing for broader absorption in solar spectrum, while retaining or even synergistically prolonging the carrier lifetime, a critical factor responsible for attaining the near-band-gap photovoltage. Herein, by applying controllable hydrostatic pressure, we have achieved unprecedented simultaneous enhancement in both band-gap narrowing and carrier-lifetime prolongation (up to 70% to ∼100% increase) under mild pressures at ∼0.3 GPa. The pressure-induced modulation on pure hybrid perovskites without introducing any adverse chemical or thermal effect clearly demonstrates the importance of band edges on the photon-electron interaction and maps a pioneering route toward a further increase in their photovoltaic performance. PMID:27444014
Band-Gap Modulation of GeCH3 Nanoribbons Under Elastic Strain: A Density Functional Theory Study
Ma, ShengQian; Li, Feng; Jiang, ChunLing
2016-06-01
Using the density functional theory method, we researched the band-gap modulation of GeCH3 nanoribbons under uniaxial elastic strain. The results indicated that the band gap of GeCH3 nanoribbons could be tuned along two directions, namely, stretching or compressing ribbons when ɛ was changed from -10% to 10% in 6-zigzag, 10-zigzag, 13-armchair, and 17-armchair nanoribbons, respectively. The band gap greatly changed with strain. In the case of tension, the amount of change in the band gap was bigger. But in the case of compression, the gradient was steeper. The band gap had a nearly linear relationship when ɛ ranges from 0% to 10%. We also investigated if the band gap is changed with widths. The results showed variation of the band gap did not rely on widths. Therefore, the GeCH3 nanoribbons had the greatest potential application in strain sensors and optical electronics at the nanoscale.
Sorokin, Vladislav
2016-01-01
one, the mth, harmonic in the corrugation series. The revealed insights into the mechanism of band-gap formation can be used to predict locations and widths of all frequency band-gaps featured by any corrugation shape. These insights are general and can be valid also for other types of wave motion in......The paper concerns determining frequency band-gaps for longitudinal wave motion in a periodic waveguide. The waveguide may be considered either as an elastic layer with variable thickness or as a rod with variable cross section. As a result, widths and locations of all frequency band-gaps are...... determined by means of the method of varying amplitudes. For the general symmetric corrugation shape, the width of each odd band-gap is controlled only by one harmonic in the corrugation series with its number being equal to the number of the band-gap. Widths of even band-gaps, however, are influenced by all...
Wide band-gap materials for high power electronics
The wide gap semiconductors are the basis for the third generation of microelectronics and specially for the high end of the temperature range. In this presentation we will review the prospects and status of two members of this group: Diamond and Silicon Carbide (SiC). The two are at different stages of technological development and their respective modes of application at present are quite different. SiC devices can operate at up to 105 deg C. High power and high frequency devices have been demonstrated. Diamond is not yet ready for real electronic devices but its many extreme properties find their applications in several cases. The prospects of the future applications will be described in view of the semiconducting characteristics of these materials
Near band edge anisotropic optical transitions in wide band gap semiconductor Cu2ZnSiS4
Levcenco, S.; Dumcenco, D.; Huang, Y. S.; Arushanov, E.; Tezlevan, V.; Tiong, K. K.; Du, C. H.
2010-10-01
In this study, anisotropic near band edge transitions of Cu2ZnSiS4 single crystals grown by chemical vapor transport were characterized by using polarization-dependent absorption, piezoreflectance (PzR) and surface photovoltage (SPV) spectroscopy techniques at room temperature. The measurements were carried out on the as grown basal plane with the normal along [2 1 0] and the axis c parallel to the long edge of the crystal platelet. Analysis of absorption and SPV spectra reveal indirect allowed transitions for the absorption edge of Cu2ZnSiS4. The estimated values of indirect band gap are 2.97 eV and 3.07 eV, respectively, for E ⊥c and E ∥c polarization configurations. The polarization-dependent PzR and SPV spectra in the vicinity of the direct band gap of Cu2ZnSiS4 reveal features E⊥ex and E∥ex at around 3.32 eV and 3.41 eV for E ⊥c and E ∥c polarizations, respectively. Both features E⊥ex and E∥ex are associated with the interband excitonic transitions at point Γ and can be explained by crystal-field splitting of valence band. Based on the experimental observations, a plausible band structure near band edge of Cu2ZnSiS4 is proposed.
TiS3 nanoribbons: Width-independent band gap and strain-tunable electronic properties
Kang, Jun; Sahin, Hasan; Ozaydin, H. Duygu; Senger, R. Tugrul; Peeters, François M.
2015-08-01
The electronic properties, carrier mobility, and strain response of TiS3 nanoribbons (TiS3 NRs) are investigated by first-principles calculations. We found that the electronic properties of TiS3 NRs strongly depend on the edge type (a or b). All a-TiS3 NRs are metallic with a magnetic ground state, while b-TiS3 NRs are direct band gap semiconductors. Interestingly, the size of the band gap and the band edge position are almost independent of the ribbon width. This feature promises a constant band gap in a b-TiS3 NR with rough edges, where the ribbon width differs in different regions. The maximum carrier mobility of b-TiS3 NRs is calculated by using the deformation potential theory combined with the effective mass approximation and is found to be of the order 103cm2V-1s-1 . The hole mobility of the b-TiS3 NRs is one order of magnitude lower, but it is enhanced compared to the monolayer case due to the reduction in hole effective mass. The band gap and the band edge position of b-TiS3 NRs are quite sensitive to applied strain. In addition we investigate the termination of ribbon edges by hydrogen atoms. Upon edge passivation, the metallic and magnetic features of a-TiS3 NRs remain unchanged, while the band gap of b-TiS3 NRs is increased significantly. The robust metallic and ferromagnetic nature of a-TiS3 NRs is an essential feature for spintronic device applications. The direct, width-independent, and strain-tunable band gap, as well as the high carrier mobility, of b-TiS3 NRs is of potential importance in many fields of nanoelectronics, such as field-effect devices, optoelectronic applications, and strain sensors.
Silicon-based optical waveguide polarizer using photonic band gap
Based on different photonic band structures of TE and TM polarization modes in periodic multilayers, a method to realize the waveguide polarizer is proposed. The waveguide structure contains a SiO2 core layer sandwiched between two multilayers of alternately stacked poly-Si and SiO2, and the whole structure can be grown on a Si substrate. Its propagation characteristics are studied theoretically. High extinction ratio over 40 dB at a light wavelength of 1.3 μm is expected in the waveguide of only 40 μm long, accompanied with very low propagation loss of the passive TE mode. These characteristics are very suitable for the applications in integrated optics. The fabrication of this polarizer structure by using the magnetron sputtering method is demonstrated
Single Material Band Gap Engineering in GaAs Nanowires
The structural and optical properties of GaAs nanowire with mixed zinc-blende/wurtzite structure are presented. High resolution transmission electron microscopy indicates the presence of a variety of shorter and longer segments of zinc-blende or wurtzite crystal phases. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV. The downward shift of the emission peaks can be understood by carrier confinement at the wurtzite/zinc-blende heterojunction, in quantum wells and in random short period superlattices existent in these nanowires, assuming the theoretical staggered band-offset between wurtzite and zinc-blende GaAs.
Acoustic band gaps of the woodpile sonic crystal with the simple cubic lattice
Wu, Liang-Yu; Chen, Lien-Wen, E-mail: chenlw@mail.ncku.edu.t [Department of Mechanical Engineering, National Cheng Kung University, Tainan 70101, Taiwan (China)
2011-02-02
This study theoretically and experimentally investigates the acoustic band gap of a three-dimensional woodpile sonic crystal. Such crystals are built by blocks or rods that are orthogonally stacked together. The adjacent layers are perpendicular to each other. The woodpile structure is embedded in air background. Their band structures and transmission spectra are calculated using the finite element method with a periodic boundary condition. The dependence of the band gap on the width of the stacked rods is discussed. The deaf bands in the band structure are observed by comparing with the calculated transmission spectra. The experimental transmission spectra for the {Gamma}-X and {Gamma}-X' directions are also presented. The calculated results are compared with the experimental results.
Large complete band gap in two-dimensional phononic crystal slabs with elliptic inclusions
Phononic band structure with periodic elliptic inclusions for the square lattice is investigated based on the plane wave expansion method. The numerical results show the systems composed of tungsten (W) elliptic rods embedded in a silicon (Si) matrix can exhibit a larger complete band gap than the conventional circular phononic crystal (PC) slabs. The phononic band structure of the plate-mode waves and the width of the first complete band gap can be tuned by varying the ratio of the minor axis and the major axis, the orientation angle of the elliptic rods and the thickness of the PC slabs. We also study the band structure of plate-mode waves propagating in two-dimensional (2D) slabs with periodic elliptic inclusions coated on uniform substrate.
Hybrid functional band gap calculation of SnO6 containing perovskites and their derived structures
We have studied the properties of SnO6 octahedra-containing perovskites and their derived structures using ab initio calculations with different density functionals. In order to predict the correct band gap of the materials, we have used B3LYP hybrid density functional, and the results of B3LYP were compared with those obtained using the local density approximation and generalized gradient approximation data. The calculations have been conducted for the orthorhombic ground state of the SnO6 containing perovskites. We also have expended the hybrid density functional calculation to the ASnO3/A'SnO3 system with different cation orderings. We propose an empirical relationship between the tolerance factor and the band gap of SnO6 containing oxide materials based on first principles calculation. - Graphical abstract: (a) Structure of ASnO3 for orthorhombic ground state. The green ball is A (Ba, Sr, Ca) cation and the small (red) ball on edge is oxygen. SnO6 octahedrons are plotted as polyhedron. (b) Band gap of ASnO3 as a function of the tolerance factor for different density functionals. The experimental values of the band gap are marked as green pentagons. (c) ASnO3/A'SnO3 superlattices with two types cation arrangement: [001] layered structure and [111] rocksalt structure, respectively. (d) B3LYP hybrid functional band gaps of ASnO3, [001] ordered superlattices, and [111] ordered superlattices of ASnO3/A'SnO3 as a function of the effective tolerance factor. Note the empirical linear relationship between the band gap and effective tolerance factor. - Highlights: • We report the hybrid functional band gap calculation of ASnO3 and ASnO3/A'SnO3. • The band gap of ASnO3 using B3LYP functional reproduces the experimental value. • We propose the linear relationship between the tolerance factor and the band gap
The band gap of II-Vi ternary alloys in a tight-binding description
Olguin, Daniel; Blanquero, Rafael [Instituto Politecnico Nacional, Mexico, D.F (Mexico); De Coss, Romeo [Instituto Politecnico Nacional, Yucatan (Mexico)
2001-02-01
We present tight-binding calculations for the band gap of II-Vi pseudobinary ternary alloys. We use an sp{sup 3} s* tight-binding Hamiltonian which include spin-orbit coupling. The band gap composition dependence is calculated using a extended version of the virtual crystal approximation, which introduce an empirical correction factor that takes into account the non-linear dependence of the band gap with the composition. The results compare quite well with the experimental data, both for the ternary alloys with wide band gap and for the narrow band gap ones. [Spanish] Presentamos el calculo de la banda de energia prohibida de aleaciones ternarias de compuestos II-VI. El calculo, que incluye interaccion espin-orbita, se hace con el metodo de enlace fuerte, utilizando una base ortogonal de cinco orbitales atomicos por atomo (sp{sup 3} s*), en conjunto con la aproximacion del cristal virtual. En la aproximacion del cristal virtual, incluimos un factor de correccion que toma en cuenta la no linealidad de la banda de energia prohibida como funcion de la concentracion. Con esta correccion nuestros resultados reproducen aceptablemente los datos experimentales hallados en la literatura.
Direct band gap silicon crystals predicted by an inverse design method
Oh, Young Jun; Lee, In-Ho; Lee, Jooyoung; Kim, Sunghyun; Chang, Kee Joo
2015-03-01
Cubic diamond silicon has an indirect band gap and does not absorb or emit light as efficiently as other semiconductors with direct band gaps. Thus, searching for Si crystals with direct band gaps around 1.3 eV is important to realize efficient thin-film solar cells. In this work, we report various crystalline silicon allotropes with direct and quasi-direct band gaps, which are predicted by the inverse design method which combines a conformation space annealing algorithm for global optimization and first-principles density functional calculations. The predicted allotropes exhibit energies less than 0.3 eV per atom and good lattice matches, compared with the diamond structure. The structural stability is examined by performing finite-temperature ab initio molecular dynamics simulations and calculating the phonon spectra. The absorption spectra are obtained by solving the Bethe-Salpeter equation together with the quasiparticle G0W0 approximation. For several allotropes with the band gaps around 1 eV, photovoltaic efficiencies are comparable to those of best-known photovoltaic absorbers such as CuInSe2. This work is supported by the National Research Foundation of Korea (2005-0093845 and 2008-0061987), Samsung Science and Technology Foundation (SSTF-BA1401-08), KIAS Center for Advanced Computation, and KISTI (KSC-2013-C2-040).
Band gaps in InN/GaN superlattices: Nonpolar and polar growth directions
Gorczyca, I., E-mail: iza@unipress.waw.pl; Skrobas, K.; Suski, T. [Institute of High Pressure Physics, UNIPRESS, 01-142 Warsaw (Poland); Christensen, N. E.; Svane, A. [Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C (Denmark)
2013-12-14
The electronic structures of nonpolar short-period InN/GaN superlattices (SLs) grown in the wurtzite a- and m-directions have been calculated and compared to previous calculations for polar superlattices (grown in the c-direction). The variation of the band gaps with the composition (m, n) of the mInN/nGaN unit cells of the superlattices was examined. The band structures were obtained by self-consistent calculations based on the local density approximation to the density functional theory using the Linear-Muffin-Tin-Orbital method with a semi-empirical correction for the band gaps. The calculated band gaps and their pressure coefficients for nonpolar superlattices are similar to those calculated for bulk InGaN alloys with an equivalent In/Ga concentration ratio. This is very different from what has been found in polar superlattices where the band gaps are much smaller and vanish when the number m of InN layers in the unit cell exceeds three. A strong internal electric field is responsible for this behavior of polar structures. Experimental photoluminescence data for polar SLs agree very well with gaps calculated for the nonpolar structures. It is suggested that this is caused by screening of the electric field in the polar structures by carriers originating from unintentional defects.
Residual stress dependant anisotropic band gap of various (hkl) oriented BaI{sub 2} films
Kumar, Pradeep; Gulia, Vikash; Vedeshwar, Agnikumar G., E-mail: agni@physics.du.ac.in, E-mail: agvedeshwar@gmail.com [Thin Film Laboratory, Department of Physics and Astrophysics, University of Delhi, Delhi-110007 (India)
2013-11-21
The thermally evaporated layer structured BaI{sub 2} grows in various completely preferred (hkl) film orientations with different growth parameters like film thickness, deposition rate, substrate temperature, etc. which were characterized by structural, morphological, and optical absorption measurements. Structural analysis reveals the strain in the films and the optical absorption shows a direct type band gap. The varying band gaps of these films were found to scale linearly with their strain. The elastic moduli and other constants were also calculated using Density Functional Theory (DFT) formalism implemented in WIEN2K code for converting the strain into residual stress. Films of different six (hkl) orientations show stress free anisotropic band gaps (2.48–3.43 eV) and both positive and negative pressure coefficients. The negative and positive pressure coefficients of band gap are attributed to the strain in I-I (or Ba-Ba or both) and Ba-I distances along [hkl], respectively. The calculated band gaps are also compared with those experimentally determined. The average pressure coefficient of band gap of all six orientations (−0.071 eV/GPa) found to be significantly higher than that calculated (−0.047 eV/GPa) by volumetric pressure dependence. Various these issues have been discussed with consistent arguments. The electron effective mass m{sub e}{sup *}=0.66m{sub 0} and the hole effective mass m{sub h}{sup *}=0.53m{sub 0} have been determined from the calculated band structure.
Observation of large photonic band gaps and defect modes in one-dimensional networked waveguides
The photonic band structures and transmission spectra of serial loop structures (SLSs), made of loops pasted together with segments of finite length, are investigated experimentally and theoretically. These monomode structures, composed of one-dimensional dielectric materials, may exhibit large stop bands where the propagation of electromagnetic waves is forbidden. The width of these band gaps depends on the geometrical and compositional parameters of the structure and may be drastically increased in a tandem geometry made up of several successive SLSs which differ in their physical characteristics. These SLSs may have potential applications as ultrawide-band filters
Herring, Helen M.
2008-01-01
Various solid polymers, polymer-based composites, and closed-cell polymer foam are being characterized to determine their mechanical properties, using low-load test methods. The residual mechanical properties of these materials after environmental exposure or extreme usage conditions determines their value in aerospace structural applications. In this experimental study, four separate polymers were evaluated to measure their individual mechanical responses after thermal aging and moisture exposure by dynamic mechanical analysis. A ceramic gap filler, used in the gaps between the tiles on the Space Shuttle, was also tested, using dynamic mechanical analysis to determine material property limits during flight. Closed-cell polymer foam, used for the Space Shuttle External Tank insulation, was tested under low load levels to evaluate how the foam's mechanical properties are affected by various loading and unloading scenarios.
Large band gaps in two-dimensional phononic crystal composed of periodic cross-boards in air
Using the finite-element method, the band structure and the transmission coefficient spectrum of a new two-dimensional phononic crystal (PC) composed of periodic cross-boards in air were investigated. The effects of the geometry parameters on band gaps are discussed. Numerical results show that this PC can yield large band gaps in the low-frequency range. The location and width of the band gaps can be modulated by geometry parameters such as the margin width and cross-angle. Furthermore, the acoustic modes on the edge of the band gaps are calculated to find the role of the cross-board in the formation of band gaps. Results show that the edges of the band gaps are determined, respectively, by the acute or obtuse air regions divided by the cross-board.
Sorokin, Vladislav S
2016-04-01
The paper concerns determining frequency band-gaps for longitudinal wave motion in a periodic waveguide. The waveguide may be considered either as an elastic layer with variable thickness or as a rod with variable cross section. As a result, widths and locations of all frequency band-gaps are determined by means of the method of varying amplitudes. For the general symmetric corrugation shape, the width of each odd band-gap is controlled only by one harmonic in the corrugation series with its number being equal to the number of the band-gap. Widths of even band-gaps, however, are influenced by all the harmonics involved in the corrugation series, so that the lower frequency band-gaps can emerge. These are band-gaps located below the frequency corresponding to the lowest harmonic in the corrugation series. For the general non-symmetric corrugation shape, the mth band-gap is controlled only by one, the mth, harmonic in the corrugation series. The revealed insights into the mechanism of band-gap formation can be used to predict locations and widths of all frequency band-gaps featured by any corrugation shape. These insights are general and can be valid also for other types of wave motion in periodic structures, e.g., transverse or torsional vibration. PMID:27106336
Zero-coupling-gap degenerate band edge resonators in silicon photonics.
Burr, Justin R; Reano, Ronald M
2015-11-30
Resonances near regular photonic band edges are limited by quality factors that scale only to the third power of the number of periods. In contrast, resonances near degenerate photonic band edges can scale to the fifth power of the number periods, yielding a route to significant device miniaturization. For applications in silicon integrated photonics, we present the design and analysis of zero-coupling-gap degenerate band edge resonators. Complex band diagrams are computed for the unit cell with periodic boundary conditions that convey characteristics of propagating and evanescent modes. Dispersion features of the band diagram are used to describe changes in resonance scaling in finite length resonators. Resonators with non-zero and zero coupling gap are compared. Analysis of quality factor and resonance frequency indicates significant reduction in the number of periods required to observe fifth power scaling when degenerate band edge resonators are realized with zero-coupling-gap. High transmission is achieved by optimizing the waveguide feed to the resonator. Compact band edge cavities with large optical field distribution are envisioned for light emitters, switches, and sensors. PMID:26698725
Wavelet-based method for computing elastic band gaps of one-dimensional phononic crystals
无
2007-01-01
A wavelet-based method was developed to compute elastic band gaps of one-dimensional phononic crystals. The wave field was expanded in the wavelet basis and an equivalent eigenvalue problem was derived in a matrix form involving the adaptive computation of integrals of the wavelets. The method was then applied to a binary system. For comparison, the elastic band gaps of the same one-di- mensional phononic crystals computed with the wavelet method and the well- known plane wave expansion (PWE) method are both presented in this paper. The numerical results of the two methods are in good agreement while the computation costs of the wavelet method are much lower than that of PWE method. In addition, the adaptability of wavelets makes the method possible for efficient band gap computation of more complex phononic structures.
Degraeve, S.; Granger, C.; Dubus, B.; Vasseur, J. O.; Pham Thi, M.; Hladky-Hennion, A.-C.
2014-05-01
An homogeneous piezoelectric rod is shown to exhibit Bragg band gaps when an electrical boundary condition is applied periodically with the help of metallic electrodes. An analytical model is developed which formulation depends on the applied electric boundary condition and reveals that Bragg band gaps occurring in this very peculiar phononic crystal are related to the electric charge located on the electrodes. Moreover, via an accurate boundary condition (electrodes connected in short circuit, in open circuit, or through an external capacitance), full tunability of the Bragg band gaps can be achieved. Measurements of ultrasonic transmission present an overall excellent agreement with the theoretical results. This phononic crystal can be easily manufactured and presents many potential applications as frequency filters especially for radio frequency telecommunications.
Effects of weak nonlinearity on dispersion relations and frequency band-gaps of periodic structures
Sorokin, Vladislav; Thomsen, Jon Juel
2015-01-01
The analysis of the behaviour of linear periodic structures can be traced back over 300 years, to Sir Isaac Newton, and still attracts much attention. An essential feature of periodic struc-tures is the presence of frequency band-gaps, i.e. frequency ranges in which waves cannot propagate....... Determination of band-gaps and the corresponding attenuation levels is an im-portant practical problem. Most existing analytical methods in the field are based on Floquet theory; e.g. this holds for the classical Hill’s method of infinite determinants, and the method of space-harmonics. However, application of...... accounted for. The present work deals with analytically predicting dynamic responses for nonlinear continuous elastic periodic structures. Specifically, the effects of weak nonlinearity on the dispersion re-lation and frequency band-gaps of a periodic Bernoulli-Euler beam performing bending os...
Band gaps of wurtzite ScxGa1−xN alloys
Optical transmittance measurements on epitaxial, phase-pure, wurtzite-structure ScxGa1−xN films with 0 ≤ x ≤ 0.26 showed that their direct optical band gaps increased from 3.33 eV to 3.89 eV with increasing x, in agreement with theory. These films contained I1- and I2-type stacking faults. However, the direct optical band gaps decreased from 3.37 eV to 3.26 eV for ScxGa1−xN films, which additionally contained nanoscale lamellar inclusions of the zinc-blende phase, as revealed by aberration-corrected scanning transmission electron microscopy. Therefore, we conclude that the apparent reduction in ScxGa1−xN band gaps with increasing x is an artefact resulting from the presence of nanoscale zinc-blende inclusions
Synthesis of copper quantum dots by chemical reduction method and tailoring of its band gap
P. G. Prabhash
2016-05-01
Full Text Available Metallic copper nano particles are synthesized with citric acid and CTAB (cetyltrimethylammonium bromide as surfactant and chlorides as precursors. The particle size and surface morphology are analyzed by High Resolution Transmission Electron Microscopy. The average size of the nano particle is found to be 3 - 10 nm. The optical absorption characteristics are done by UV-Visible spectrophotometer. From the Tauc plots, the energy band gaps are calculated and because of their smaller size the particles have much higher band gap than the bulk material. The energy band gap is changed from 3.67 eV to 4.27 eV in citric acid coated copper quantum dots and 4.17 eV to 4.52 eV in CTAB coated copper quantum dots.
General band gap condition in one-dimensional resonator-based acoustic metamaterial
Liu, Yafei; Hou, Zhilin; Fu, Xiujun
2016-03-01
A one-dimensional model for resonator-based acoustic metamaterials is introduced. The condition for band gap in such kind of structure is obtained. According to this condition, the dispersion relation is in general a result of the scattering phase and propagating phase. The phenomenon that the band gap is less dependent on lattice structure appears only in the special system in which the coupling between the resonators and the host medium is weak enough. For strong coupled systems, the dispersion of wave can be significantly adjusted by the propagating phase. Based on the understanding, a general guide for band gap optimization is given and the mechanism for structures with the defect states at subwavelength scale is revealed.
Acoustic band gaps of two-dimensional three-component composite
ZHANG Shu; CHENG Jianchun
2003-01-01
Numerical analyses of the band-gaps in the two-dimensional three-component periodic composite are presented in this paper. The three-component composite is composed of an array of coated cylinders embedded in a resin. The coated cylinder consists of a hard inner core and a soft coating, which has much smaller wave velocity and mass density than the matrix and the inner material. The calculated band gaps appear in a lower frequency domain in contrast to the two-component counterpart composite because of the localized resonance in the elastically soft coating ring. The influence of the properties of the different coating materials and the geometry of the crystal on the band gaps is also investigated.
Robust topology optimization of three-dimensional photonic-crystal band-gap structures
Men, Han; Freund, Robert M; Peraire, Jaime; Johnson, Steven G
2014-01-01
We perform full 3D topology optimization (in which "every voxel" of the unit cell is a degree of freedom) of photonic-crystal structures in order to find optimal omnidirectional band gaps for various symmetry groups, including fcc (including diamond), bcc, and simple-cubic lattices. Even without imposing the constraints of any fabrication process, the resulting optimal gaps are only slightly larger than previous hand designs, suggesting that current photonic crystals are nearly optimal in this respect. However, optimization can discover new structures, e.g. a new fcc structure with the same symmetry but slightly larger gap than the well known inverse opal, which may offer new degrees of freedom to future fabrication technologies. Furthermore, our band-gap optimization is an illustration of a computational approach to 3D dispersion engineering which is applicable to many other problems in optics, based on a novel semidefinite-program formulation for nonconvex eigenvalue optimization combined with other techniq...
Robust topology optimization of three-dimensional photonic-crystal band-gap structures
Men, Han; Lee, Karen Y. K.; Freund, Robert M.; Peraire, Jaime; Johnson, Steven G.
2014-01-01
We perform full 3D topology optimization (in which "every voxel" of the unit cell is a degree of freedom) of photonic-crystal structures in order to find optimal omnidirectional band gaps for various symmetry groups, including fcc (including diamond), bcc, and simple-cubic lattices. Even without imposing the constraints of any fabrication process, the resulting optimal gaps are only slightly larger than previous hand designs, suggesting that current photonic crystals are nearly optimal in thi...
Role of excited states in Shockley-Read-Hall recombination in wide-band-gap semiconductors
Alkauskas, Audrius; Dreyer, Cyrus E.; Lyons, John L.; Van de Walle, Chris G.
2016-05-01
Defect-assisted recombination is an important limitation on efficiency of optoelectronic devices. However, since nonradiative capture rates decrease exponentially with the energy of the transition, the mechanisms by which such recombination can take place in wide-band-gap materials are unclear. Using electronic structure calculations we uncover the crucial role of electronic excited states in nonradiative recombination processes. The impact is elucidated with examples for the group-III nitrides, for which accumulating experimental evidence indicates that defect-assisted recombination limits efficiency. Our work provides insights into the physics of nonradiative recombination, and the mechanisms are suggested to be ubiquitous in wide-band-gap semiconductors.
Band gap engineering of early transition-metal-doped anatase TiO₂: first principles calculations.
Li, C; Zhao, Y F; Gong, Y Y; Wang, T; Sun, C Q
2014-10-21
The thermal stability and electronic structures of anatase TiO2 doped with early transition metals (TM) (group III-B = Sc, Y and La; group IV-B = Zr and Hf; group V-B = V, Nb and Ta) have been studied using first principles calculations. It was found that all doped systems are thermodynamically stable, and their band gaps were reduced by 1-1.3 eV compared to pure TiO2. Doping with transition metals affects the strength of the hybrid orbital of TM-O bonding, and the band gap increases approximately linearly with the MP value of TM-O bonding. PMID:25183457
Optical band gap of Sn0.2Bi1.8Te3 thin films
P H Soni; M V Hathi; C F Desai
2003-12-01
Sn0.2Bi1.8Te3 thin films were grown using the thermal evaporation technique on a (001) face of NaCl crystal as a substrate at room temperature. The optical absorption was measured in the wave number range 500–4000 cm-1. From the optical absorption data the band gap was evaluated and studied as a function of film thickness and deposition temperature. The data indicate absorption through direct interband transition with a band gap of around 0.216 eV. The detailed results are reported here.
S.V. Kryuchkov
2015-03-01
Full Text Available The power of the elliptically polarized electromagnetic radiation absorbed by band-gap graphene in presence of constant magnetic field is calculated. The linewidth of cyclotron absorption is shown to be non-zero even if the scattering is absent. The calculations are performed analytically with the Boltzmann kinetic equation and confirmed numerically with the Monte Carlo method. The dependence of the linewidth of the cyclotron absorption on temperature applicable for a band-gap graphene in the absence of collisions is determined analytically.
Low-frequency band gaps in one-dimensional thin phononic crystal plate with periodic stubbed surface
Using supercell plane wave expansion method, the Lamb wave band structure of one-dimensional thin plate with periodic stubs is investigated. The numerical results show that flat bands will appear and band gap can exist in a low-frequency domain. The position of the flat bands and width of the low-frequency Lamb wave band gap can be tuned by the stub height, plate thickness and filling fraction. The band gap is obtained by opening the folding points of the same plate modes not the crossing point of different plate mode when the stub height is small.
Dual Band Gap Coupled Antenna Design with DGS for Wireless Communications
Abhishek Kandwal
2014-02-01
Full Text Available A novel gap coupled dual band multiple ring antenna with a defected ground structure (DGS has been successfully implemented. A different technique is used in this communication where both gap coupling and defected ground are applied to obtain better results for wireless applications. The designed antenna operates in two different frequency bands. The antenna shows a wideband in C-band and also resonates in the X-band. The main parameters like return loss, impedance bandwidth, radiation pattern and gain are presented and discussed. The gain is increased and the side lobe level is considerably reduced to a good extent. Designed antenna is tested and the results show that the simulation and experimental results are in good agreement with each other.
Wave propagation in single column woodpile phononic crystals: Formation of tunable band gaps
Kim, Eunho; Yang, Jinkyu
2014-11-01
We study the formation of frequency band gaps in single column woodpile phononic crystals composed of orthogonally stacked slender cylinders. We focus on investigating the effect of the cylinders' local vibrations on the dispersion of elastic waves along the stacking direction of the woodpile phononic crystals. We experimentally verify that their frequency band structures depend significantly on the bending resonant behavior of unit cells. We propose a simple theoretical model based on a discrete element method to associate the behavior of locally resonant cylindrical rods with the band gap formation mechanism in woodpile phononic crystals. The findings in this work imply that we can achieve versatile control of frequency band structures in phononic crystals by using woodpile architectures. The woodpile phononic crystals can form a new type of vibration filtering devices that offer an enhanced degree of freedom in manipulating stress wave propagation.