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Sample records for band gap energy

  1. Energy bands and gaps near an impurity

    Science.gov (United States)

    Mihóková, E.; Schulman, L. S.

    2016-10-01

    It has been suggested that in the neighborhood of a certain kind of defect in a crystal there is a bend in the electronic band. We confirm that this is indeed possible using the Kronig-Penney model. Our calculations also have implications for photonic crystals.

  2. The calculation of band gap energy in zinc oxide films

    Science.gov (United States)

    Arif, Ali; Belahssen, Okba; Gareh, Salim; Benramache, Said

    2015-01-01

    We investigated the optical properties of undoped zinc oxide thin films as the n-type semiconductor; the thin films were deposited at different precursor molarities by ultrasonic spray and spray pyrolysis techniques. The thin films were deposited at different substrate temperatures ranging between 200 and 500 °C. In this paper, we present a new approach to control the optical gap energy of ZnO thin films by concentration of the ZnO solution and substrate temperatures from experimental data, which were published in international journals. The model proposed to calculate the band gap energy with the Urbach energy was investigated. The relation between the experimental data and theoretical calculation suggests that the band gap energies are predominantly estimated by the Urbach energies, film transparency, and concentration of the ZnO solution and substrate temperatures. The measurements by these proposal models are in qualitative agreements with the experimental data; the correlation coefficient values were varied in the range 0.96-0.99999, indicating high quality representation of data based on Equation (2), so that the relative errors of all calculation are smaller than 4%. Thus, one can suppose that the undoped ZnO thin films are chemically purer and have many fewer defects and less disorder owing to an almost complete chemical decomposition and contained higher optical band gap energy.

  3. CZTS stoichiometry effects on the band gap energy

    Energy Technology Data Exchange (ETDEWEB)

    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.

  4. Urbach's rule derived from thermal fluctuations in the band-gap energy

    DEFF Research Database (Denmark)

    Skettrup, Torben

    1978-01-01

    The exponential absorption edge (known as Urbach's rule) observed in most materials is interpreted in terms of thermal fluctuations in the band-gap energy. The main contribution to the temperature shift of the band-gap energy is due to the temperature-dependent self-energies of the electrons...... and holes interacting with the phonons. Since the phonon number is fluctuating in thermal equilibrium, the band-gap energy is also fluctuating resulting in an exponential absorption tail below the average band-gap energy. These simple considerations are applied to derive Urbach's rule at high temperatures...

  5. Determining the band gap and mean kinetic energy of atoms from reflection electron energy loss spectra

    Energy Technology Data Exchange (ETDEWEB)

    Vos, M. [Atomic and Molecular Physics Laboratories, Research School of Physics and Engineering, Australian National University, Canberra ACT (Australia); Marmitt, G. G. [Atomic and Molecular Physics Laboratories, Research School of Physics and Engineering, Australian National University, Canberra ACT (Australia); Instituto de Fisica da Universidade Federal do Rio Grande do Sul, Avenida Bento Goncalves 9500, 91501-970 Porto Alegre, RS (Brazil); Finkelstein, Y. [Nuclear Research Center — Negev, Beer-Sheva 84190 (Israel); Moreh, R. [Physics Department, Ben-Gurion University of the Negev, Beer-Sheva 84105 (Israel)

    2015-09-14

    Reflection electron energy loss spectra from some insulating materials (CaCO{sub 3}, Li{sub 2}CO{sub 3}, and SiO{sub 2}) taken at relatively high incoming electron energies (5–40 keV) are analyzed. Here, one is bulk sensitive and a well-defined onset of inelastic excitations is observed from which one can infer the value of the band gap. An estimate of the band gap was obtained by fitting the spectra with a procedure that includes the recoil shift and recoil broadening affecting these measurements. The width of the elastic peak is directly connected to the mean kinetic energy of the atom in the material (Doppler broadening). The experimentally obtained mean kinetic energies of the O, C, Li, Ca, and Si atoms are compared with the calculated ones, and good agreement is found, especially if the effect of multiple scattering is taken into account. It is demonstrated experimentally that the onset of the inelastic excitation is also affected by Doppler broadening. Aided by this understanding, we can obtain a good fit of the elastic peak and the onset of inelastic excitations. For SiO{sub 2}, good agreement is obtained with the well-established value of the band gap (8.9 eV) only if it is assumed that the intensity near the edge scales as (E − E{sub gap}){sup 1.5}. For CaCO{sub 3}, the band gap obtained here (7 eV) is about 1 eV larger than the previous experimental value, whereas the value for Li{sub 2}CO{sub 3} (7.5 eV) is the first experimental estimate.

  6. Energy band gap and optical transition of metal ion modified double crossover DNA lattices.

    Science.gov (United States)

    Dugasani, Sreekantha Reddy; Ha, Taewoo; Gnapareddy, Bramaramba; Choi, Kyujin; Lee, Junwye; Kim, Byeonghoon; Kim, Jae Hoon; Park, Sung Ha

    2014-10-22

    We report on the energy band gap and optical transition of a series of divalent metal ion (Cu(2+), Ni(2+), Zn(2+), and Co(2+)) modified DNA (M-DNA) double crossover (DX) lattices fabricated on fused silica by the substrate-assisted growth (SAG) method. We demonstrate how the degree of coverage of the DX lattices is influenced by the DX monomer concentration and also analyze the band gaps of the M-DNA lattices. The energy band gap of the M-DNA, between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), ranges from 4.67 to 4.98 eV as judged by optical transitions. Relative to the band gap of a pristine DNA molecule (4.69 eV), the band gap of the M-DNA lattices increases with metal ion doping up to a critical concentration and then decreases with further doping. Interestingly, except for the case of Ni(2+), the onset of the second absorption band shifts to a lower energy until a critical concentration and then shifts to a higher energy with further increasing the metal ion concentration, which is consistent with the evolution of electrical transport characteristics. Our results show that controllable metal ion doping is an effective method to tune the band gap energy of DNA-based nanostructures.

  7. Strain-Induced Energy Band Gap Opening in Two-Dimensional Bilayered Silicon Film

    Science.gov (United States)

    Ji, Z.; Zhou, R.; Lew Yan Voon, L. C.; Zhuang, Y.

    2016-10-01

    This work presents a theoretical study of the structural and electronic properties of bilayered silicon film (BiSF) under in-plane biaxial strain/stress using density functional theory (DFT). Atomic structures of the two-dimensional (2-D) silicon films are optimized by using both the local-density approximation (LDA) and generalized gradient approximation (GGA). In the absence of strain/stress, five buckled hexagonal honeycomb structures of the BiSF with triangular lattice have been obtained as local energy minima, and their structural stability has been verified. These structures present a Dirac-cone shaped energy band diagram with zero energy band gaps. Applying a tensile biaxial strain leads to a reduction of the buckling height. Atomically flat structures with zero buckling height have been observed when the AA-stacking structures are under a critical biaxial strain. Increase of the strain between 10.7% and 15.4% results in a band-gap opening with a maximum energy band gap opening of ˜0.17 eV, obtained when a 14.3% strain is applied. Energy band diagrams, electron transmission efficiency, and the charge transport property are calculated. Additionally, an asymmetric energetically favorable atomic structure of BiSF shows a non-zero band gap in the absence of strain/stress and a maximum band gap of 0.15 eV as a -1.71% compressive strain is applied. Both tensile and compressive strain/stress can lead to a band gap opening in the asymmetric structure.

  8. Pressure dependence of the band-gap energy in BiTeI

    OpenAIRE

    Güler-Kılıç, Sümeyra; Kılıç, Çetin

    2016-01-01

    The evolution of the electronic structure of BiTeI, a layered semiconductor with a van der Waals gap, under compression is studied by employing semilocal and dispersion-corrected density-functional calculations. Comparative analysis of the results of these calculations shows that the band-gap energy of BiTeI decreases till it attains a minimum value of zero at a critical pressure, after which it increases again. The critical pressure corresponding to the closure of the band gap is calculated,...

  9. Empirical determination of the energy band gap narrowing in highly doped n+ silicon

    Science.gov (United States)

    Yan, Di; Cuevas, Andres

    2013-07-01

    Highly doped regions in silicon devices should be analyzed using Fermi-Dirac statistics, taking into account energy band gap narrowing (BGN). An empirical expression for the BGN as a function of dopant concentration is derived here by matching the modeled and measured thermal recombination current densities J0 of a broad range of n+ dopant concentration profiles prepared by phosphorus diffusion. The analysis is repeated with Boltzmann statistics in order to determine a second empirical expression for the apparent energy band gap narrowing, which is found to be in good agreement with previous work.

  10. Pressure dependence of the band-gap energy in BiTeI

    Science.gov (United States)

    Güler-Kılıç, Sümeyra; Kılıç, ćetin

    2016-10-01

    The evolution of the electronic structure of BiTeI, a layered semiconductor with a van der Waals gap, under compression is studied by employing semilocal and dispersion-corrected density-functional calculations. Comparative analysis of the results of these calculations shows that the band-gap energy of BiTeI decreases till it attains a minimum value of zero at a critical pressure, after which it increases again. The critical pressure corresponding to the closure of the band gap is calculated, at which BiTeI becomes a topological insulator. Comparison of the critical pressure to the pressure at which BiTeI undergoes a structural phase transition indicates that the closure of the band gap would not be hindered by a structural transformation. Moreover, the band-gap pressure coefficients of BiTeI are computed, and an expression of the critical pressure is devised in terms of these coefficients. Our findings indicate that the semilocal and dispersion-corrected approaches are in conflict about the compressibility of BiTeI, which result in overestimation and underestimation, respectively. Nevertheless, the effect of pressure on the atomic structure of BiTeI is found to be manifested primarily as the reduction of the width of the van der Waals gap according to both approaches, which also yield consistent predictions concerning the interlayer metallic bonding in BiTeI under compression. It is consequently shown that the calculated band-gap energies follow qualitatively and quantitatively the same trend within the two approximations employed here, and the transition to the zero-gap state occurs at the same critical width of the van der Waals gap.

  11. Photonic band gap materials

    Science.gov (United States)

    Cassagne, D.

    Photonic band gap materials Photonic band gap materials are periodic dielectric structures that control the propagation of electromagnetic waves. We describe the plane wave method, which allows to calculate the band structures of photonic crystals. By symmetry analysis and a perturbative approach, we predict the appearance of the low energy photonic band gaps of hexagonal structures. We propose new two-dimensional structures called graphite and boron nitride. Using a transfer matrix method, we calculate the transmission of the graphite structure and we show the crucial role of the coupling with external modes. We study the appearance of allowed modes in the photonic band gap by the introduction of localized defects in the periodicity. Finally, we discuss the properties of opals formed by self-organized silica microspheres, which are very promising for the fabrication of three-dimensional photonic crystals. Les matériaux à bandes interdites photoniques sont des structures diélectriques périodiques qui contrôlent la propagation des ondes électromagnétiques. Nous décrivons la méthode des ondes planes qui permet de calculer les structures de bandes des cristaux photoniques. Par une analyse de la symétrie et une approche perturbative, nous précisons les conditions d'existence des bandes interdites de basse énergie. Nous proposons de nouvelles structures bidimensionnelles appelées graphite et nitrure de bore. Grâce à une méthode de matrices de transfert, nous calculons la transmission de la structure graphite et nous mettons en évidence le rôle fondamental du couplage avec les modes extérieurs. Nous étudions l'apparition de modes permis dans la bande interdite grâce à l'introduction de défauts dans la périodicité. Enfin, nous discutons les propriétés des opales constituées de micro-billes de silice auto-organisées, qui sont très prometteuses pour la fabrication de cristaux photoniques tridimensionnels.

  12. Energy Dependence and Scaling Property of Localization Length near a Gapped Flat Band

    CERN Document Server

    Ge, Li

    2015-01-01

    Using a tight-binding model for a one-dimensional Lieb lattice, we show that the localization length near a gapped flat band behaves differently from the typical Urbach tail in a band gap: instead of reducing monotonically as the energy E moves away from the flat band energy E_{FB}, the presence of the flat band causes a nonmonotonic energy dependence of the localization length. This energy dependence follows a scaling property when the energy is within the spread (W) of uniformly distributed diagonal disorder, i.e. the localization length is only a function of (E-E_{FB})/W. Several other lattices are compared to distinguish the effect of the flat band on the localization length, where we eliminate, shift, or duplicate the flat band, without changing the dispersion relations of other bands. Using the top right element of the Green's matrix, we derive an analytical relation between the density of states and the localization length, which shines light on these properties of the latter, including a summation rul...

  13. Effect of Microstructure of TiO2 Thin Films on Optical Band Gap Energy

    Institute of Scientific and Technical Information of China (English)

    TIAN Guang-Lei; HE Hong-Bo; SHAO Jian-Da

    2005-01-01

    @@ TiO2 coatings are prepared on fused silica with conventional electron beam evaporation deposition. After annealed at different temperatures for four hours, the spectra and XRD patterns of TiO2 thin film are obtained. XRD patterns reveal that only anatase phase can be observed in TiO2 coatings regardless of the different annealing temperatures, and with the increasing annealing temperature, the grain size gradually increases. The relationship between the energy gap and microstructure of anatase is determined and discussed. The quantum confinement effect is observed that with the increasing grain size of TiO2 thin film, the band gap energy shifts from 3.4eV to 3.21 eV. Moreover, other possible influence of the TiO2 thin-film microstructure, such as surface roughness and thin film absorption, on band gap energy is also expected.

  14. Nanoscale mapping of optical band gaps using monochromated electron energy loss spectroscopy

    Science.gov (United States)

    Zhan, W.; Granerød, C. S.; Venkatachalapathy, V.; Johansen, K. M. H.; Jensen, I. J. T.; Kuznetsov, A. Yu; Prytz, Ø.

    2017-03-01

    Using monochromated electron energy loss spectroscopy in a probe-corrected scanning transmission electron microscope we demonstrate band gap mapping in ZnO/ZnCdO thin films with a spatial resolution below 10 nm and spectral precision of 20 meV.

  15. Band gap energy and optical transitions in polyenes formed by thermal decomposition of polyvinyl alcohol

    Science.gov (United States)

    Kulak, A. I.; Bondarava, G. V.; Shchurevich, O. A.

    2013-07-01

    The band gap of the ensemble of oligoene clusters formed by thermocatalytic decomposition of polyvinyl alcohol is parametrized using optical absorption spectra. A band gap energy of E gm =1.53 ± 0.02 eV at the end of an infinite polyene chain is found by extrapolating the energies of π → π* transitions in clusters with a number of double bonds varying from 4 to 12. This value is close to the band gap of trans-polyacetylene and the lower bound for the Tauc energy E gT =1.50 eV, which characterizes the minimum interband transition energy. E gT is essentially independent of the concentration of oligoene clusters, which is determined by the concentration of the AlCl3 thermal decomposition catalyst. The Urbach energy determined from the long wavelength edge of the spectrum falls from 2.21 to 0.66 eV as the AlCl3 concentration is raised from 11.1 to 41.7 mmol per mol of polyvinyl alcohol structural units.

  16. Energy Band Gap Study of Semiconducting Single Walled Carbon Nanotube Bundle

    Science.gov (United States)

    Elkadi, Asmaa; Decrossas, Emmanuel; El-Ghazaly, Samir

    2013-01-01

    The electronic properties of multiple semiconducting single walled carbon nanotubes (s-SWCNTs) considering various distribution inside a bundle are studied. The model derived from the proposed analytical potential function of the electron density for an individual s-SWCNT is general and can be easily applied to multiple nanotubes. This work demonstrates that regardless the number of carbon nanotubes, the strong coupling occurring between the closest neighbours reduces the energy band gap of the bundle by 10%. As expected, the coupling is strongly dependent on the distance separating the s-SWCNTs. In addition, based on the developed model, it is proposed to enhance this coupling effect by applying an electric field across the bundle to significantly reduce the energy band gap of the bundle by 20%.

  17. Energy Band Gap Study of Semiconducting Single Walled Carbon Nanotube Bundle

    Science.gov (United States)

    Elkadi, Asmaa; Decrossas, Emmanuel; El-Ghazaly, Samir

    2013-01-01

    The electronic properties of multiple semiconducting single walled carbon nanotubes (s-SWCNTs) considering various distribution inside a bundle are studied. The model derived from the proposed analytical potential function of electron density for na individual s-SWCNT is general and can be easily applied to multiple nanotubes. This work demonstrates that regardless the number of carbon nanotubes, the strong coupling occurring between the closet neighbors reduces the energy band gap of the bundle by 10%. As expected, the coupling is strongly dependent on the distance separating the s-SWCNTs. In addition, based on the developed model, it is proposed to enhance this coupling effect by applying an electric field across the bundle to significantly reduce the energy band gap of the bundle by 20%.

  18. Optical band gap energy and ur bach tail of CdS:Pb2+ thin films

    Energy Technology Data Exchange (ETDEWEB)

    Chavez, M.; Juarez, H.; Pacio, M. [Universidad Autonoma de Puebla, Instituto de Ciencias, Centro de Investigacion en Dispositivos Semiconductores, Av. 14 Sur, Col. Jardines de San Manuel, Ciudad Universitaria, Puebla, Pue. (Mexico); Gutierrez, R.; Chaltel, L.; Zamora, M.; Portillo, O. [Universidad Autonoma de Puebla, Facultad de Ciencias Quimicas, Laboratorio de Materiales, Apdo. Postal 1067, 72001 Puebla, Pue. (Mexico); Mathew, X., E-mail: osporti@yahoo.mx [UNAM, Instituto de Energias Renovables, Temixco, Morelos (Mexico)

    2016-11-01

    Pb S-doped CdS nano materials were successfully synthesized using chemical bath. Transmittance measurements were used to estimate the optical band gap energy. Tailing in the band gap was observed and found to obey Ur bach rule. The diffraction X-ray show that the size of crystallites is in the ∼33 nm to 12 nm range. The peaks belonging to primary phase are identified at 2θ = 26.5 degrees Celsius and 2θ = 26.00 degrees Celsius corresponding to CdS and Pb S respectively. Thus, a shift in maximum intensity peak from 2θ = 26.4 to 28.2 degrees Celsius is clear indication of possible transformation of cubic to hexagonal phase. Also peaks at 2θ = 13.57, 15.9 degrees Celsius correspond to lead perchlorate thiourea. The effects on films thickness and substrate doping on the band gap energy and the width on tail were investigated. Increasing doping give rise to a shift in optical absorption edge ∼0.4 eV. (Author)

  19. Band Gap Energies and Refractive Indices of Epitaxial Pb1-xSrxTe Thin Films

    Institute of Scientific and Technical Information of China (English)

    WENG Bin-Bin; WU Hui-Zhen; SI Jian-Xiao; XU Tian-Ning

    2008-01-01

    Pb1-xSrx Te thin films with different strontium (St) compositions are grown on BaF2 (111) substrates by molecular beam epitaxy (MBE). Using high resolution x-ray diffraction (HRXRD), we obtain Pb1-xSrx Te lattice constants,which vary in the range 6.462-6.492 A. According to the Vegard law and HRXRD data, Sr compositions in Pb1-xSrxTe thin films range from 0.0-8.0%. The Pb1-xSrx Te refractive index dispersions are attained from infrared transmission spectrum characterized by Fourier transform infrared (FTIR) transmission spectroscopy.It is found that refractive index decreases while Sr content increases in Pb1-xSrx Te. We also simulate the Pb1-xSrx Te transmission spectra theoretically to obtain the optical band gap energies which range between0.320 e V and 0.449 e V. The simulated results are in good agreement with the FTIR data. Finally, we determine the relation between Ph1-xSrx Te band gap energies and Sr compositions (Eg = 0.320+0.510x- 0.930x2 + 184x3 (e V) ).

  20. Esaki Diodes in van der Waals Heterojunctions with Broken-Gap Energy Band Alignment.

    Science.gov (United States)

    Yan, Rusen; Fathipour, Sara; Han, Yimo; Song, Bo; Xiao, Shudong; Li, Mingda; Ma, Nan; Protasenko, Vladimir; Muller, David A; Jena, Debdeep; Xing, Huili Grace

    2015-09-09

    van der Waals (vdW) heterojunctions composed of two-dimensional (2D) layered materials are emerging as a solid-state materials family that exhibits novel physics phenomena that can power a range of electronic and photonic applications. Here, we present the first demonstration of an important building block in vdW solids: room temperature Esaki tunnel diodes. The Esaki diodes were realized in vdW heterostructures made of black phosphorus (BP) and tin diselenide (SnSe2), two layered semiconductors that possess a broken-gap energy band offset. The presence of a thin insulating barrier between BP and SnSe2 enabled the observation of a prominent negative differential resistance (NDR) region in the forward-bias current-voltage characteristics, with a peak to valley ratio of 1.8 at 300 K and 2.8 at 80 K. A weak temperature dependence of the NDR indicates electron tunneling being the dominant transport mechanism, and a theoretical model shows excellent agreement with the experimental results. Furthermore, the broken-gap band alignment is confirmed by the junction photoresponse, and the phosphorus double planes in a single layer of BP are resolved in transmission electron microscopy (TEM) for the first time. Our results represent a significant advance in the fundamental understanding of vdW heterojunctions and broaden the potential applications of 2D layered materials.

  1. The Shifts of Band Gap and Binding Energies of Titania/Hydroxyapatite Material

    Directory of Open Access Journals (Sweden)

    Nguyen Thi Truc Linh

    2014-01-01

    Full Text Available The titania/hydroxyapatite (TiO2/HAp product was prepared by precipitating hydroxyapatite in the presence of TiO(OH2 gel in the hydrothermal system. The characteristics of the material were determined by using the measurements such as X-ray photoemission spectroscopy (XPS, X-ray diffraction (XRD, diffuse reflectance spectra (DRS, transmission electron microscopy (TEM, scanning electron microscopy (SEM, and energy dispersive X-ray (EDX. The XPS analysis showed that the binding energy values of Ca (2p1/2, 2p3/2, P (2p1/2, 2p3/2, and O 1s levels related to hydroxyapatite phase whereas those of Ti (2p3/2, 2p1/2 levels corresponded with the characterization of titanium (IV in TiO2. The XRD result revealed that TiO2/HAp sample had hydroxyapatite phase, but anatase or rutile phases were not found out. TEM image of TiO2/HAp product showed that the surface of the plate-shaped HAp particles had a lot of smaller particles which were considered as the compound of Ti. The experimental band gap of TiO2/HAp material calculated by the DRS measurement was 3.6 eV, while that of HAp pure was 5.3 eV and that of TiO2 pure was around 3.2 eV. The shift of the band gap energy of TiO2 in the range of 3.2–3.6 eV may be related to the shifts of Ti signals of XPS spectrum.

  2. Band Gap, Molecular Energy and Electrochromic Characterization of Electrosynthesized Hydroxymethyl 3,4-Ethylenedioxythiophene

    Science.gov (United States)

    Co, Thien Thanh; Tran, Tri Quoc; Le, Hai Viet; Ho, Vu Anh Pham; Tran, Lam Dai

    2017-03-01

    Hydroxymethyl functionalized 3,4-ethylenedioxythiophene (EDOT-MeOH) monomer was synthesized according to a previously reported procedure. Electropolymerization of EDOT-MeOH was performed in acetonitrile (ACN) containing tetrabutylammonium perchlorate (Bu4NClO4) as the supporting electrolyte by chronoamperometry on platinum (Pt) and fluorine-doped tin oxide-coated glass substrates. The resulting conjugated polymer (PEDOT-MeOH) was characterized by cyclic voltammetry and UV-Vis spectrophotometer techniques. The polymer showed a deep HOMO energy level of -5.31 eV with a very low band gap of 1.54 eV. Spectroelectrochemical study revealed that the PEDOT-MeOH has interesting electrochromic properties.

  3. Band Gap, Molecular Energy and Electrochromic Characterization of Electrosynthesized Hydroxymethyl 3,4-Ethylenedioxythiophene

    Science.gov (United States)

    Co, Thien Thanh; Tran, Tri Quoc; Le, Hai Viet; Ho, Vu Anh Pham; Tran, Lam Dai

    2016-12-01

    Hydroxymethyl functionalized 3,4-ethylenedioxythiophene (EDOT-MeOH) monomer was synthesized according to a previously reported procedure. Electropolymerization of EDOT-MeOH was performed in acetonitrile (ACN) containing tetrabutylammonium perchlorate (Bu4NClO4) as the supporting electrolyte by chronoamperometry on platinum (Pt) and fluorine-doped tin oxide-coated glass substrates. The resulting conjugated polymer (PEDOT-MeOH) was characterized by cyclic voltammetry and UV-Vis spectrophotometer techniques. The polymer showed a deep HOMO energy level of -5.31 eV with a very low band gap of 1.54 eV. Spectroelectrochemical study revealed that the PEDOT-MeOH has interesting electrochromic properties.

  4. Determination of band gap energy (Eg) of Cu2ZnSnSe4 thin films: On the discrepancies of reported band gap values

    Science.gov (United States)

    Ahn, SeJin; Jung, Sunghun; Gwak, Jihye; Cho, Ara; Shin, Keeshik; Yoon, Kyunghoon; Park, Doyoung; Cheong, Hyeonsik; Yun, Jae Ho

    2010-07-01

    We demonstrate experimental data to elucidate the reason for the discrepancies of reported band gap energy (Eg) of Cu2ZnSnSe4 (CZTSe) thin films, i.e., 1.0 or 1.5 eV. Eg of the coevaporated CZTSe film synthesized at substrate temperature (Tsub) of 370 °C, which was apparently phase pure CZTSe confirmed by x-ray diffraction (XRD) and Raman spectroscopy, is found to be around 1 eV regardless of the measurement techniques. However, depth profile of the same sample reveals the formation of ZnSe at CZTSe/Mo interface. On the other hand, Eg of the coevaporated films increases with Tsub due to the ZnSe formation, from which we suggest that the existence of ZnSe, which is hardly distinguishable from CZTSe by XRD, is the possible reason for the overestimation of overall Eg.

  5. Band Gap Engineering in a 2D Material for Solar-to-Chemical Energy Conversion.

    Science.gov (United States)

    Hu, Jun; Guo, Zhenkun; Mcwilliams, Peter E; Darges, John E; Druffel, Daniel L; Moran, Andrew M; Warren, Scott C

    2016-01-13

    The electronic structure of 2D semiconductors depends on their thickness, providing new opportunities to engineer semiconductors for energy conversion, electronics, and catalysis. Here we show how a 3D semiconductor, black phosphorus, becomes active for solar-to-chemical energy conversion when it is thinned to a 2D material. The increase in its band gap, from 0.3 eV (3D) to 2.1 eV (2D monolayer), is accompanied by a 40-fold enhancement in the formation of chemical products. Despite this enhancement, smaller flakes also have shorter excited state lifetimes. We deduce a mechanism in which recombination occurs at flake edges, while the "van der Waals" surface of black phosphorus bonds to chemical intermediates and facilitates electron transfer. The unique properties of black phosphorus highlight its potential as a customizable material for solar energy conversion and catalysis, while also allowing us to identify design rules for 2D photocatalysts that will enable further improvements in these materials.

  6. Generalized thermoelastic wave band gaps in phononic crystals without energy dissipation

    Science.gov (United States)

    Wu, Ying; Yu, Kaiping; Li, Xiao; Zhou, Haotian

    2016-01-01

    We present a theoretical investigation of the thermoelastic wave propagation in the phononic crystals in the context of Green-Nagdhi theory by taking thermoelastic coupling into account. The thermal field is assumed to be steady. Thermoelastic wave band structures of 3D and 2D are derived by using the plane wave expansion method. For the 2D problem, the anti-plane shear mode is not affected by the temperature difference. Thermoelastic wave bands of the in-plane x-y mode are calculated for lead/silicone rubber, aluminium/silicone rubber, and aurum/silicone rubber phononic crystals. The new findings in the numerical results indicate that the thermoelastic wave bands are composed of the pure elastic wave bands and the thermal wave bands, and that the thermal wave bands can serve as the low boundary of the first band gap when the filling ratio is low. In addition, for the lead/silicone rubber phononic crystals the effects of lattice type (square, rectangle, regular triangle, and hexagon) and inclusion shape (circle, oval, and square) on the normalized thermoelastic bandwidth and the upper/lower gap boundaries are analysed and discussed. It is concluded that their effects on the thermoelastic wave band structure are remarkable.

  7. Band gap and defect states of MgO thin films investigated using reflection electron energy loss spectroscopy

    Energy Technology Data Exchange (ETDEWEB)

    Heo, Sung [Analytical Engineering Group, Samsung Advanced Institute of Technology, 130, Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do, S. Korea, 443-803 (Korea, Republic of); College of Information and Communication Engineering, Sungkyunkwan University, Cheoncheon-dong 300, Jangan-gu, Suwon 440-746, S. Korea (Korea, Republic of); Cho, Eunseog [CAE Group, Samsung Advanced Institute of Technology, 130, Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do, S. Korea, 443-803 (Korea, Republic of); Lee, Hyung-Ik; Park, Gyeong Su [Analytical Engineering Group, Samsung Advanced Institute of Technology, 130, Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do, S. Korea, 443-803 (Korea, Republic of); Kang, Hee Jae [Department of Physics, Chungbuk National University, Cheongju, S. Korea (Korea, Republic of); Nagatomi, T. [Analysis & Simulation Center, Asahi Kasei Corporation, 2-1 Samejima, Fuji, Shizuoka 416-8501 (Japan); Choi, Pyungho; Choi, Byoung-Deog, E-mail: bdchoi@skku.edu [College of Information and Communication Engineering, Sungkyunkwan University, Cheoncheon-dong 300, Jangan-gu, Suwon 440-746, S. Korea (Korea, Republic of)

    2015-07-15

    The band gap and defect states of MgO thin films were investigated by using reflection electron energy loss spectroscopy (REELS) and high-energy resolution REELS (HR-REELS). HR-REELS with a primary electron energy of 0.3 keV revealed that the surface F center (FS) energy was located at approximately 4.2 eV above the valence band maximum (VBM) and the surface band gap width (E{sub g}{sup S}) was approximately 6.3 eV. The bulk F center (F{sub B}) energy was located approximately 4.9 eV above the VBM and the bulk band gap width was about 7.8 eV, when measured by REELS with 3 keV primary electrons. From a first-principles calculation, we confirmed that the 4.2 eV and 4.9 eV peaks were F{sub S} and F{sub B}, induced by oxygen vacancies. We also experimentally demonstrated that the HR-REELS peak height increases with increasing number of oxygen vacancies. Finally, we calculated the secondary electron emission yields (γ) for various noble gases. He and Ne were not influenced by the defect states owing to their higher ionization energies, but Ar, Kr, and Xe exhibited a stronger dependence on the defect states owing to their small ionization energies.

  8. Band gap and defect states of MgO thin films investigated using reflection electron energy loss spectroscopy

    Directory of Open Access Journals (Sweden)

    Sung Heo

    2015-07-01

    Full Text Available The band gap and defect states of MgO thin films were investigated by using reflection electron energy loss spectroscopy (REELS and high-energy resolution REELS (HR-REELS. HR-REELS with a primary electron energy of 0.3 keV revealed that the surface F center (FS energy was located at approximately 4.2 eV above the valence band maximum (VBM and the surface band gap width (EgS was approximately 6.3 eV. The bulk F center (FB energy was located approximately 4.9 eV above the VBM and the bulk band gap width was about 7.8 eV, when measured by REELS with 3 keV primary electrons. From a first-principles calculation, we confirmed that the 4.2 eV and 4.9 eV peaks were FS and FB, induced by oxygen vacancies. We also experimentally demonstrated that the HR-REELS peak height increases with increasing number of oxygen vacancies. Finally, we calculated the secondary electron emission yields (γ for various noble gases. He and Ne were not influenced by the defect states owing to their higher ionization energies, but Ar, Kr, and Xe exhibited a stronger dependence on the defect states owing to their small ionization energies.

  9. CdSe/CdTe interface band gaps and band offsets calculated using spin-orbit and self-energy corrections

    Energy Technology Data Exchange (ETDEWEB)

    Ribeiro, M. [Centro de Pesquisas Avancadas Wernher von Braun, Av. Alice de Castro P.N. Mattosinho 301, CEP 13098-392 Campinas, SP (Brazil); Ferreira, L.G. [Departamento de Fisica dos Materiais e Mecanica, Instituto de Fisica, Universidade de Sao Paulo, 05315-970 Sao Paulo, SP (Brazil); Fonseca, L.R.C. [Center for Semiconductor Components, State University of Campinas, R. Pandia Calogeras 90, 13083-870 Campinas, SP (Brazil); Ramprasad, R. [Department of Chemical, Materials and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Storrs, CT 06269 (United States)

    2012-09-20

    We performed ab initio calculations of the electronic structures of bulk CdSe and CdTe, and their interface band alignments on the CdSe in-plane lattice parameters. For this, we employed the LDA-1/2 self-energy correction scheme to obtain corrected band gaps and band offsets. Our calculations include the spin-orbit effects for the bulk cases, which have shown to be of importance for the equilibrium systems and are possibly degraded in these strained semiconductors. Therefore, the SO showed reduced importance for the band alignment of this particular system. Moreover, the electronic structure calculated along the transition region across the CdSe/CdTe interface shows an interesting non-monotonic variation of the band gap in the range 0.8-1.8 eV, which may enhance the absorption of light for corresponding frequencies at the interface between these two materials in photovoltaic applications.

  10. A Study of Energy Band Gap Temperature Relationships for Cu2ZnSnS4 Thin Films

    OpenAIRE

    2011-01-01

    The temperature dependent band gap energy of Cu2ZnSnS4 thin film was studied in the temperature range of 77 to 410 K. Various relevant parameters which explain the temperature variation of the fundamental band gap have been calculated using empirical and semi-empirical models. Amongst the models evaluated, the Varshni and P\\"assler models show the best agreement with experimental data in the middle temperature range. However, the Bose-Einstein model fits reasonably well over the entire temper...

  11. Synthesis, characterization and band gap energy of poly(ɛ-caprolactone)/Sr-MSA nano-composite

    Science.gov (United States)

    Kannammal, L.; Palanikumar, S.; Meenarathi, B.; Yelilarasi, A.; Anbarasan, R.

    2014-04-01

    A mercaptosuccinic acid (MSA) decorated Sr nano-particle (NP) was prepared and characterized by using various analytical techniques and was used as a chemical initiator for the ring opening polymerization (ROP) of ɛ-caprolactone (CL). The ROP of CL was carried out at various experimental conditions under N2 atmosphere with mild stirring. The initiating efficiency of MSA-decorated Sr NP was tested in terms of Fourier transform infrared-relative intensity, melting temperature (Tm), degradation temperature (Td) and molecular weight (Mw) of poly(ɛ-caprolactone) (PCL), differential scanning calorimetry, UV-visible spectroscopy, field emission scanning electron microscopy, thermogravimetric analysis and gel permeation chromatography analytical techniques. The nuclear magnetic resonance spectrum confirms the chemical structure of PCL. While increasing the [M/I] ratio, the Mw of PCL was linearly increased. The band gap energy of Sr was determined from the UV-visible spectrum. The reflectance study proves the hydrophobic nature of the Sr-hybrid and its nano-composite formation with PCL.

  12. Band gap opening in strongly compressed diamond observed by x-ray energy loss spectroscopy

    Energy Technology Data Exchange (ETDEWEB)

    Gamboa, E. J. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Fletcher, L. B. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Lee, H. J. [SLAC National Accelerator Lab., Menlo Park, CA (United States); MacDonald, M. J. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Univ. of Michigan, Ann Arbor, MI (United States); Zastrau, U. [High-Energy Density Science Group, Hamburg (Germany); Gauthier, M. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Gericke, D. O. [Univ. of Warwick (United Kingdom); Vorberger, J. [Helmholtz Association of German Research Centres, Dresden (Germany); Granados, E. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Hastings, J. B. [SLAC National Accelerator Lab., Menlo Park, CA (United States); Glenzer, S. H. [SLAC National Accelerator Lab., Menlo Park, CA (United States)

    2016-01-25

    The extraordinary mechanical and optical properties of diamond are the basis of numerous technical applications and make diamond anvil cells a premier device to explore the high-pressure behavior of materials. However, at applied pressures above a few hundred GPa, optical probing through the anvils becomes difficult because of the pressure-induced changes of the transmission and the excitation of a strong optical emission. Such features have been interpreted as the onset of a closure of the optical gap in diamond, and can significantly impair spectroscopy of the material inside the cell. In contrast, a comparable widening has been predicted for purely hydrostatic compressions, forming a basis for the presumed pressure stiffening of diamond and resilience to the eventual phase change to BC8. We here present the first experimental evidence of this effect at geo-planetary pressures, exceeding the highest ever reported hydrostatic compression of diamond by more than 200 GPa and any other measurement of the band gap by more than 350 GPa. We here apply laser driven-ablation to create a dynamic, high pressure state in a thin, synthetic diamond foil together with frequency-resolved x-ray scattering as a probe. The frequency shift of the inelastically scattered x-rays encodes the optical properties and, thus, the behavior of the band gap in the sample. Using the ultra-bright x-ray beam from the Linac Coherent Light Source (LCLS), we observe an increasing direct band gap in diamond up to a pressure of 370 GPa. This finding points to the enormous strains in the anvils and the impurities in natural Type Ia diamonds as the source of the observed closure of the optical window. Our results demonstrate that diamond remains an insulating solid to pressures approaching its limit strength.

  13. Energy Impacts of Wide Band Gap Semiconductors in U.S. Light-Duty Electric Vehicle Fleet.

    Science.gov (United States)

    Warren, Joshua A; Riddle, Matthew E; Graziano, Diane J; Das, Sujit; Upadhyayula, Venkata K K; Masanet, Eric; Cresko, Joe

    2015-09-01

    Silicon carbide and gallium nitride, two leading wide band gap semiconductors with significant potential in electric vehicle power electronics, are examined from a life cycle energy perspective and compared with incumbent silicon in U.S. light-duty electric vehicle fleet. Cradle-to-gate, silicon carbide is estimated to require more than twice the energy as silicon. However, the magnitude of vehicle use phase fuel savings potential is comparatively several orders of magnitude higher than the marginal increase in cradle-to-gate energy. Gallium nitride cradle-to-gate energy requirements are estimated to be similar to silicon, with use phase savings potential similar to or exceeding that of silicon carbide. Potential energy reductions in the United States vehicle fleet are examined through several scenarios that consider the market adoption potential of electric vehicles themselves, as well as the market adoption potential of wide band gap semiconductors in electric vehicles. For the 2015-2050 time frame, cumulative energy savings associated with the deployment of wide band gap semiconductors are estimated to range from 2-20 billion GJ depending on market adoption dynamics.

  14. Nickel(II-oxaloyldihydrazone complexes: Characterization, indirect band gap energy and antimicrobial evaluation

    Directory of Open Access Journals (Sweden)

    Ayman H. Ahmed

    2016-12-01

    Full Text Available A series of oxaloyldihydrazone ligands was prepared essentially by the usual condensation reaction between oxaloyldihydrazide and different aldehydes e.g. salicylaldehyde, 2-hydroxy-1-naphthaldehyde, 2-hydroxyacetophenone and 2-methoxy-benzaldehyde in 1:2 M ratio. The formed compounds were purified to give bis(salicylaldehydeoxaloyldihydrazone (L1, bis(2-hydroxy-1-naphthaldehydeoxaloyldihydrazone (L2, bis(2-hydroxyacetophenoneoxaloyldihydrazone(L3 and bis(2-methoxy-benzaldehydeoxaloyldihydrazone (L4. All the oxaloyldihydrazones (L1–L4 and their relevant solid nickel(II complexes have been prepared and structurally characterized on the basis of the elemental analyses, spectral (UV–vis, IR, mass and 1H NMR, magnetism and thermal (TG measurements. The dihydrazones coordinate to the metal center forming mononuclear complexes with L1, L3 and L4 in addition to binuclear complex with L2. The metal center prefers tetrahedral stereochemistry upon chelation. The optical indirect band gap energy for all compounds underlies the range of semiconductor materials. The prepared ligands and their metal complexes have been assayed for their antimicrobial activity against fungi as well as Gram-positive and Gram-negative bacteria. The resulting data indicate the ability of the investigated compounds to inhibit the growth of some micro-organisms, where L2 showed the highest activity among all the compounds. Minimum inhibitory concentration (MIC of L2 against the growth of five micro-organisms was determined which gives better response against Aspergillus fumigatus and Bacillis subtilis compared with some selected standard drugs.

  15. Valence band gaps and plasma energies for galena, sphalerite, and chalcopyrite natural minerals using differential optical reflectance spectroscopy

    Science.gov (United States)

    Todoran, R.; Todoran, D.; Szakacs, Zs.

    2015-12-01

    The paper presents the determinations of the valence band gaps and plasma energies of the galena, sphalerite and chalcopyrite natural minerals. The work was carried out using differential optical reflectance spectroscopy of the clean mineral surfaces. The determination of the optical properties such as refractive index, real part of the complex dielectric constant and the location of certain van Hove singularities, was carried out using the Kramers-Kronig formalism.

  16. Edge effects on band gap energy in bilayer 2H-MoS{sub 2} under uniaxial strain

    Energy Technology Data Exchange (ETDEWEB)

    Dong, Liang; Wang, Jin; Dongare, Avinash M., E-mail: dongare@uconn.edu [Department of Materials Science and Engineering and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269 (United States); Namburu, Raju [Computational and Information Sciences Directorate, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005 (United States); O' Regan, Terrance P.; Dubey, Madan [Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, Maryland 20783 (United States)

    2015-06-28

    The potential of ultrathin MoS{sub 2} nanostructures for applications in electronic and optoelectronic devices requires a fundamental understanding in their electronic structure as a function of strain. Previous experimental and theoretical studies assume that an identical strain and/or stress state is always maintained in the top and bottom layers of a bilayer MoS{sub 2} film. In this study, a bilayer MoS{sub 2} supercell is constructed differently from the prototypical unit cell in order to investigate the layer-dependent electronic band gap energy in a bilayer MoS{sub 2} film under uniaxial mechanical deformations. The supercell contains an MoS{sub 2} bottom layer and a relatively narrower top layer (nanoribbon with free edges) as a simplified model to simulate the as-grown bilayer MoS{sub 2} flakes with free edges observed experimentally. Our results show that the two layers have different band gap energies under a tensile uniaxial strain, although they remain mutually interacting by van der Waals interactions. The deviation in their band gap energies grows from 0 to 0.42 eV as the uniaxial strain increases from 0% to 6% under both uniaxial strain and stress conditions. The deviation, however, disappears if a compressive uniaxial strain is applied. These results demonstrate that tensile uniaxial strains applied to bilayer MoS{sub 2} films can result in distinct band gap energies in the bilayer structures. Such variations need to be accounted for when analyzing strain effects on electronic properties of bilayer or multilayered 2D materials using experimental methods or in continuum models.

  17. A study of energy band gap versus temperature for Cu 2ZnSnS 4 thin films

    Science.gov (United States)

    Sarswat, Prashant K.; Free, Michael L.

    2012-01-01

    The temperature dependent band gap energy of Cu 2ZnSnS 4 thin film was studied in the temperature range of 77-410 K. Various relevant parameters, which explain the temperature variation of the fundamental band gap, have been calculated using empirical and semi-empirical models. Amongst the models evaluated, the Varshni and Pässler models show the best agreement with experimental data in the middle temperature range. However, the Bose-Einstein model fits reasonably well over the entire temperature range evaluated. The calculated fitting parameters are in good agreement with the estimated value of the Debye temperature calculated using the Madelung-Einstein approximation and the Hailing method.

  18. Microstrip microwave band gap structures

    Indian Academy of Sciences (India)

    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.

  19. Effect of Al Doping on Optical Band Gap Energy of Al-TiO2 Thin Films.

    Science.gov (United States)

    Song, Yo-Seung; Kim, Bae-Yeon; Cho, Nam-Ihn; Lee, Deuk Yong

    2015-07-01

    Al-TiO2 thin films were prepared using a sol-gel derived spin coating by varying the Al/Ti molar ratio from 0 to 0.73 to investigate the effect of Al doping on the optical band gap energy (Eg) of the films. GAXRD results indicated that Al-TiO2 is composed of anatase and FTO phases when the Al/Ti molar ratio was less than 0.18. Above 0.38, no other peaks except FTO were found and transparency of the films was severely deteriorated. Eg of Al-TiO2 decreased from 3.20 eV to 2.07 eV when the Al/Ti ratio was raised from 0 to 0.38. Eg of 2.59 eV was found for the anatase Al-TiO2 films having the Al/Ti ratio of 0.18. The absorption band of Al-TiO2 coatings shifted dramatically from the UV region to the visible region with increasing the amount of Al dopant. The Al doping was mainly attributed to the optical band gap energy of Al-TiO2.

  20. A PHOTONIC BAND GAP FIBRE

    DEFF Research Database (Denmark)

    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...

  1. Sizable band gap in organometallic topological insulator

    Science.gov (United States)

    Derakhshan, V.; Ketabi, S. A.

    2017-01-01

    Based on first principle calculation when Ceperley-Alder and Perdew-Burke-Ernzerh type exchange-correlation energy functional were adopted to LSDA and GGA calculation, electronic properties of organometallic honeycomb lattice as a two-dimensional topological insulator was calculated. In the presence of spin-orbit interaction bulk band gap of organometallic lattice with heavy metals such as Au, Hg, Pt and Tl atoms were investigated. Our results show that the organometallic topological insulator which is made of Mercury atom shows the wide bulk band gap of about ∼120 meV. Moreover, by fitting the conduction and valence bands to the band-structure which are produced by Density Functional Theory, spin-orbit interaction parameters were extracted. Based on calculated parameters, gapless edge states within bulk insulating gap are indeed found for finite width strip of two-dimensional organometallic topological insulators.

  2. Metamorphosis of strain/stress on optical band gap energy of ZAO thin films via manipulation of thermal annealing process

    Energy Technology Data Exchange (ETDEWEB)

    Malek, M.F., E-mail: firz_solarzelle@yahoo.com [NANO-ElecTronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor (Malaysia); NANO-SciTech Centre (NST), Institute of Science (IOS), Universiti Teknologi MARA UiTM, 40450 Shah Alam, Selangor (Malaysia); Mamat, M.H. [NANO-ElecTronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor (Malaysia); Musa, M.Z. [NANO-ElecTronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor (Malaysia); Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM) Pulau Pinang, Jalan Permatang Pauh, 13500 Permatang Pauh, Pulau Pinang (Malaysia); Soga, T. [Department of Frontier Materials, Nagoya Institute of Technology (NITech), Nagoya 466-8555 (Japan); Rahman, S.A. [Low Dimensional Materials Research Centre (LDMRC), Department of Physics, Faculty of Science, Universiti Malaya (UM), 50603 Kuala Lumpur (Malaysia); Alrokayan, Salman A.H.; Khan, Haseeb A. [Department of Biochemistry, College of Science, King Saud University (KSU), Riyadh 11451 (Saudi Arabia); Rusop, M. [NANO-ElecTronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor (Malaysia); NANO-SciTech Centre (NST), Institute of Science (IOS), Universiti Teknologi MARA UiTM, 40450 Shah Alam, Selangor (Malaysia)

    2015-04-15

    We report on the growth of Al-doped ZnO (ZAO) thin films prepared by the sol–gel technique associated with dip-coating onto Corning 7740 glass substrates. The influence of varying thermal annealing (T{sub a}) temperature on crystallisation behaviour, optical and electrical properties of ZAO films has been systematically investigated. All films are polycrystalline with a hexagonal wurtzite structure with a preferential orientation according to the direction 〈0 0 2〉. The metamorphosis of strain/stress effects in ZAO thin films has been investigated using X-ray diffraction. The as growth films have a large compressive stress of 0.55 GPa, which relaxed to 0.25 GPa as the T{sub a} was increased to 500 °C. Optical parameters such as optical transmittance, absorption coefficient, refractive index and optical band gap energy have been studied and discussed with respect to T{sub a}. All films exhibit a transmittance above 80–90% along the visible–NIR range up to 1500 nm and a sharp absorption onset below 400 nm corresponding to the fundamental absorption edge of ZnO. Experimental results show that the tensile stress in the films reveals an incline pattern with the optical band gap energy, while the compressive stress shows opposite relation. - Highlights: • Minimum stress of highly c-axis oriented ZAO was grown at suitable T{sub a} temperature. • The ZAO crystal orientation was influenced by strain/stress of the film. • Minimum stress/strain of ZAO film leads to lower defects. • Bandgap and defects were closely intertwined with strain/stress. • We report additional optical and electrical properties based on T{sub a} temperature.

  3. A detailed analysis of the energy levels configuration existing in the band gap of supersaturated silicon with titanium for photovoltaic applications

    OpenAIRE

    2015-01-01

    The energy levels created in supersaturated n-type silicon substrates with titanium implantation in the attempt to create an intermediate band in their band-gap are studied in detail. Two titanium ion implantation doses (1013cm-2 and 1014cm-2) are studied in this work by conductance transient technique and admittance spectroscopy. Conductance transients have been measured at temperatures of around 100 K. The particular shape of these transients is due to the formation of energy barriers in th...

  4. Photonic band gap in thin wire metamaterials.

    Science.gov (United States)

    Hock, Kai Meng

    2008-03-01

    We investigate the band structure of a class of photonic crystals made from only thin wires. Using a different method, we demonstrate that a complete photonic band gap is possible for such materials. Band gap materials normally consist of space filling dielectric or metal, whereas thin wires occupy a very small fraction of the volume. We show that this is related to the large increase in scattering at the Brillouin zone boundary. The method we developed brings together the calculation techniques in three different fields. The first is the calculation of scattering from periodic, tilted antennas, which we improve upon. The second is the standard technique for frequency selective surface design. The third is obtained directly from low energy electron diffraction theory. Good agreements with experiments for left handed materials, negative materials, and frequency selective surfaces are demonstrated.

  5. Effect of energy band gap in graphene on negative refraction through the veselago lens and electron conductance

    Science.gov (United States)

    Dahal, Dipendra; Gumbs, Godfrey

    2017-01-01

    A remarkable property of intrinsic graphene is that upon doping, electrons and holes travel through the monolayer thick material with constant velocity which does not depend on energy up to about 0.3 eV (Dirac fermions), as though the electrons and holes are massless particles and antiparticles which move at the Fermi velocity vF. Consequently, there is Klein tunneling at a p-n junction, in which there is no backscattering at normal incidence of massless Dirac fermions. However, this process yielding perfect transmission at normal incidence is expected to be affected when the group velocity of the charge carriers is energy dependent and there is non-zero effective mass for the target particle. We investigate how away from normal incidence the combined effect of incident electron energy ɛ and band gap parameter Δ can determine whether a p-n junction would allow focusing of an electron beam by behaving like a Veselago lens with negative refractive index. We demonstrate that there is a specific region in ɛ - Δ space where the index of refraction is negative, i.e., where monolayer graphene behaves as a metamaterial. Outside this region, the refractive index may be positive or there may be no refraction at all. We compute the ballistic conductance across a p-n junction as a function of Δ and ɛ and compare our results with those for a single electrostatic potential barrier and multiple barriers.

  6. Surface band-gap narrowing in quantized electron accumulation layers.

    Science.gov (United States)

    King, P D C; Veal, T D; McConville, C F; Zúñiga-Pérez, J; Muñoz-Sanjosé, V; Hopkinson, M; Rienks, E D L; Jensen, M Fuglsang; Hofmann, Ph

    2010-06-25

    An energy gap between the valence and the conduction band is the defining property of a semiconductor, and the gap size plays a crucial role in the design of semiconductor devices. We show that the presence of a two-dimensional electron gas near to the surface of a semiconductor can significantly alter the size of its band gap through many-body effects caused by its high electron density, resulting in a surface band gap that is much smaller than that in the bulk. Apart from reconciling a number of disparate previous experimental findings, the results suggest an entirely new route to spatially inhomogeneous band-gap engineering.

  7. Temperature-dependent energy band gap variation in self-organized InAs quantum dots

    Energy Technology Data Exchange (ETDEWEB)

    Yeo, Inah [CEA/CNRS/UJF Joint Team ' ' Nanophysics and Semiconductors,' ' Institut Neel-CNRS, BP 166, 25 rue des Martyrs, 38042 Grenoble Cedex 9 (France); Dong Song, Jin; Lee, Jungil [Nanophotonics Research Center, Korea Institute of Science and Technology, Seoul 136-791 (Korea, Republic of)

    2011-10-10

    We investigated the temperature-dependent variation of the photoluminescence emission energy of self-organized InAs/GaAs quantum dots (QDs) grown by conventional Stranski-Krastanov (SK) molecular beam epitaxy and migration-enhanced molecular beam epitaxy (MEMBE) and that of MEMBE InAs QDs in a symmetric and an asymmetric In{sub 0.2}Ga{sub 0.8}As/GaAs well. The temperature-dependent energy variation of each QD is analyzed in low and high temperature regions, including a sigmoidal behavior of conventional SK quantum dots with the well-known Varshni and semi-empirical Fan models.

  8. Tunable Band Gap Energy of Mn-Doped ZnO Nanoparticles Using the Coprecipitation Technique

    Directory of Open Access Journals (Sweden)

    Tong Ling Tan

    2014-01-01

    Full Text Available A simple coprecipitation technique was introduced to form manganese (Mn doped on zinc oxide (ZnO nanoparticles effectively. Based on our morphological studies, it was revealed that mean particle size was increased while bigger agglomeration of nanoparticles could be observed as the amount of concentration of Mn was increased. Interestingly, it was found that the position of the absorption spectra was shifted towards the lower wavelength (UV region as correlated with the increasing of Mn dopants concentration into ZnO nanoparticles. This result inferred that optimum content of Mn doped into the ZnO nanoparticles was crucial in controlling the visible/UV-responsive of samples. In the present study, 3 mol% of Mn dopants into the ZnO nanoparticles exhibited the better UV as well as visible light-responsive as compared to the other samples. The main reason might be attributed to the modification of electronic structure of ZnO nanoparticles via lattice doping of Mn ions into the lattice, whereas excessive Mn dopants doped on ZnO nanoparticles caused the strong UV-responsive due to the more 3d orbitals in the valence band.

  9. Influence of In-N Clusters on Band Gap Energy of Dilute Nitride In x Ga1-x N y As1-y

    Science.gov (United States)

    Zhao, Chuan-Zhen; Guo, Heng-Fei; Chen, Li-Ying; Tang, Chun-Xiao; Lu, Ke-Qing

    2016-05-01

    The In-N clusters form in the dilute nitride InxGa1-xNyAs1-y alloys after annealing. It is found that the formation of the In-N clusters not only raises the N levels lying above the conduction band minimum (CBM) of InGaAs, but also raises the N levels below the CBM of InGaAs, leading to the variation of the impurity-host interaction. The blueshift of the band gap energy is relative to the variation of the impurity-host interaction. In order to describe the blueshift of the band gap energy due to the formation of the In-N clusters, a model is developed. It is found that the model can describe the blueshift of the band gap energy well. In addition, it is found the blueshift of the band gap energy due to the atom interdiffusion at the interface can be larger than that due to the formation of the In-N clusters. Supported by the National Natural Science Foundation of China under Grant No. 61504094, Tinjin Research Program of Application Foundation and Advanced Technology under No. 15JCYBJC16300, and Tianjin City High School Science and Technology Fund Planning Project No. 20120609

  10. Forbidden energy band gap in diluted a-Ge{sub 1-x}Si{sub x}:N films

    Energy Technology Data Exchange (ETDEWEB)

    Guarneros, C.; Rebollo-Plata, B. [Posgrado en Fisica Aplicada, Facultad de Ciencias Fisico-Matematicas, Benemerita Universidad Autonoma de Puebla, Blvd. 14 Sur 6301, Col. San Manuel, 72570, Puebla (Mexico); Lozada-Morales, R., E-mail: rlozada@fcfm.buap.mx [Posgrado en Fisica Aplicada, Facultad de Ciencias Fisico-Matematicas, Benemerita Universidad Autonoma de Puebla, Blvd. 14 Sur 6301, Col. San Manuel, 72570, Puebla (Mexico); Espinosa-Rosales, J.E. [Posgrado en Fisica Aplicada, Facultad de Ciencias Fisico-Matematicas, Benemerita Universidad Autonoma de Puebla, Blvd. 14 Sur 6301, Col. San Manuel, 72570, Puebla (Mexico); Portillo-Moreno, J. [Facultad de Ciencias Quimicas, Benemerita Universidad Autonoma de Puebla, Blvd. 14 Sur 6301, Col. San Manuel, 72570, Puebla (Mexico); Zelaya-Angel, O. [Departamento de Fisica, Centro de Investigacion y de Estudios Avanzados del IPN, PO Box 14-740, Mexico 07360 D.F. (Mexico)

    2012-06-01

    By means of electron gun evaporation Ge{sub 1-x}Si{sub x}:N thin films, in the entire range 0 {<=} x {<=} 1, were prepared on Si (100) and glass substrates. The initial vacuum reached was 6.6 Multiplication-Sign 10{sup -4} Pa, then a pressure of 2.7 Multiplication-Sign 10{sup -2} Pa of high purity N{sub 2} was introduced into the chamber. The deposition time was 4 min. Crucible-substrate distance was 18 cm. X-ray diffraction patterns indicate that all the films were amorphous (a-Ge{sub 1-x}Si{sub x}:N). The nitrogen concentration was of the order of 1 at% for all the films. From optical absorption spectra data and by using the Tauc method the energy band gap (E{sub g}) was calculated. The Raman spectra only reveal the presence of Si-Si, Ge-Ge, and Si-Ge bonds. Nevertheless, infrared spectra demonstrate the existence of Si-N and Ge-N bonds. The forbidden energy band gap (E{sub g}) as a function of x in the entire range 0 {<=} x {<=} 1 shows two well defined regions: 0 {<=} x {<=} 0.67 and 0.67 {<=} x {<=} 1, due to two different behaviors of the band gap, where for x > 0.67 exists an abruptly change of E{sub g}(x). In this case E{sub g}(x) versus x is different to the variation of E{sub g} in a-Ge{sub 1-x}Si{sub x} and a-Ge{sub 1-x}Si{sub x}:H. This fact can be related to the formation of Ge{sub 3}N{sub 4} and GeSi{sub 2}N{sub 4} when x {<=} 0.67, and to the formation of Si{sub 3}N{sub 4} and GeSi{sub 2}N{sub 4} for 0.67 {<=} x. - Highlights: Black-Right-Pointing-Pointer Nitrogen doped amorphous Ge{sub 1-x}Si{sub x} thin films are grown by electron gun technique. Black-Right-Pointing-Pointer Nitrogen atoms on E{sub g} of the a-Ge{sub 1-x}Si{sub x} films in the 0 Pound-Sign x Pound-Sign 1 range are analyzed. Black-Right-Pointing-Pointer Variation in 0 Pound-Sign x Pound-Sign 1 range shows a warped change of E{sub g} in 1.0 - 3.6 eV range. Black-Right-Pointing-Pointer The change in E{sub g}(x) behavior when x {approx} 0.67 was associated with Ge{sub 2}SiN{sub 4

  11. ALD grown nanostructured ZnO thin films: Effect of substrate temperature on thickness and energy band gap

    Directory of Open Access Journals (Sweden)

    Javed Iqbal

    2016-10-01

    Full Text Available Nanostructured ZnO thin films with high transparency have been grown on glass substrate by atomic layer deposition at various temperatures ranging from 100 °C to 300 °C. Efforts have been made to observe the effect of substrate temperature on the thickness of the deposited thin films and its consequences on the energy band gap. A remarkably high growth rate of 0.56 nm per cycle at a substrate temperature of 200 °C for ZnO thin films have been achieved. This is the maximum growth rate for ALD deposited ZnO thin films ever reported so far to the best of our knowledge. The studies of field emission scanning electron microscopy and X-ray diffractometry patterns confirm the deposition of uniform and high quality nanosturtured ZnO thin films which have a polycrystalline nature with preferential orientation along (100 plane. The thickness of the films deposited at different substrate temperatures was measured by ellipsometry and surface profiling system while the UV–visible and photoluminescence spectroscopy studies have been used to evaluate the optical properties of the respective thin films. It has been observed that the thickness of the thin film depends on the substrate temperatures which ultimately affect the optical and structural parameters of the thin films.

  12. Empirical determination of the energy band gap narrowing in p{sup +} silicon heavily doped with boron

    Energy Technology Data Exchange (ETDEWEB)

    Yan, Di, E-mail: di.yan@anu.edu.au; Cuevas, Andres [Research School of Engineering, The Australian National University, Canberra ACT 0200 (Australia)

    2014-11-21

    In the analysis of highly doped silicon, energy band gap narrowing (BGN) and degeneracy effects may be accounted for separately, as a net BGN in conjunction with Fermi-Dirac statistics, or lumped together in an apparent BGN used with Boltzmann statistics. This paper presents an experimental study of silicon highly doped with boron, with the aim of evaluating the applicability of previously reported BGN models. Different boron diffusions covering a broad range of dopant densities were prepared, and their characteristic recombination current parameters J{sub 0} were measured using a contactless photoconductance technique. The BGN was subsequently extracted by matching theoretical simulations of carrier transport and recombination in each of the boron diffused regions and the measured J{sub 0} values. An evaluation of two different minority carrier mobility models indicates that their impact on the extraction of the BGN is relatively small. After considering possible uncertainties, it can be concluded that the BGN is slightly larger in p{sup +} silicon than in n{sup +} silicon, in qualitative agreement with theoretical predictions by Schenk. Nevertheless, in quantitative terms that theoretical model is found to slightly underestimate the BGN in p{sup +} silicon. With the two different parameterizations derived in this paper for the BGN in p{sup +} silicon, both statistical approaches, Boltzmann and Fermi-Dirac, provide a good agreement with the experimental data.

  13. Molecular doping and band-gap opening of bilayer graphene.

    OpenAIRE

    Samuels, AJ; Carey, JD

    2013-01-01

    The ability to induce an energy band gap in bilayer graphene is an important development in graphene science and opens up potential applications in electronics and photonics. Here we report the emergence of permanent electronic and optical band gaps in bilayer graphene upon adsorption of π electron containing molecules. Adsorption of n- or p-type dopant molecules on one layer results in an asymmetric charge distribution between the top and bottom layers and in the formation of an energy gap. ...

  14. Data on energy-band-gap characteristics of composite nanoparticles obtained by modification of the amorphous potassium polytitanate in aqueous solutions of transition metal salts

    Directory of Open Access Journals (Sweden)

    D.A. Zimnyakov

    2016-06-01

    Full Text Available Here we present the data on the energy-band-gap characteristics of composite nanoparticles produced by modification of the amorphous potassium polytitanate in aqueous solutions of different transition metal salts. Band gap characteristics are investigated using diffuse reflection spectra of the obtained powders. Calculated logarithmic derivative quantity of the Kubelka–Munk function reveals a presence of local maxima in the regions 0.5–1.5 eV and 1.6–3.0 eV which correspond to band gap values of the investigated materials. The values might be related to the constituents of the composite nanoparticles and intermediate products of their chemical interaction.

  15. The band-gap enhanced photovoltaic structure

    Science.gov (United States)

    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.

  16. Sub-band-gap laser micromachining of lithium niobate

    DEFF Research Database (Denmark)

    Christensen, F. K.; Müllenborn, Matthias

    1995-01-01

    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...

  17. Low band gap polymers for organic photovoltaics

    DEFF Research Database (Denmark)

    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...

  18. A detailed analysis of the energy levels configuration existing in the band gap of supersaturated silicon with titanium for photovoltaic applications

    Science.gov (United States)

    Pérez, E.; Dueñas, S.; Castán, H.; García, H.; Bailón, L.; Montero, D.; García-Hernansanz, R.; García-Hemme, E.; Olea, J.; González-Díaz, G.

    2015-12-01

    The energy levels created in supersaturated n-type silicon substrates with titanium implantation in the attempt to create an intermediate band in their band-gap are studied in detail. Two titanium ion implantation doses (1013 cm-2 and 1014 cm-2) are studied in this work by conductance transient technique and admittance spectroscopy. Conductance transients have been measured at temperatures of around 100 K. The particular shape of these transients is due to the formation of energy barriers in the conduction band, as a consequence of the band-gap narrowing induced by the high titanium concentration. Moreover, stationary admittance spectroscopy results suggest the existence of different energy level configuration, depending on the local titanium concentration. A continuum energy level band is formed when titanium concentration is over the Mott limit. On the other hand, when titanium concentration is lower than the Mott limit, but much higher than the donor impurity density, a quasi-continuum energy level distribution appears. Finally, a single deep center appears for low titanium concentration. At the n-type substrate, the experimental results obtained by means of thermal admittance spectroscopy at high reverse bias reveal the presence of single levels located at around Ec-425 and Ec-275 meV for implantation doses of 1013 cm-2 and 1014 cm-2, respectively. At low reverse bias voltage, quasi-continuously distributed energy levels between the minimum of the conduction bands, Ec and Ec-450 meV, are obtained for both doses. Conductance transients detected at low temperatures reveal that the high impurity concentration induces a band gap narrowing which leads to the formation of a barrier in the conduction band. Besides, the relationship between the activation energy and the capture cross section values of all the energy levels fits very well to the Meyer-Neldel rule. As it is known, the Meyer-Neldel rule typically appears in processes involving multiple excitations, like

  19. A detailed analysis of the energy levels configuration existing in the band gap of supersaturated silicon with titanium for photovoltaic applications

    Energy Technology Data Exchange (ETDEWEB)

    Pérez, E.; Dueñas, S.; Castán, H.; García, H.; Bailón, L. [Dept. de Electricidad y Electrónica, Universidad de Valladolid, Paseo de Belén 15, 47011 Valladolid (Spain); Montero, D.; García-Hernansanz, R.; García-Hemme, E.; González-Díaz, G. [Dept. de Física Aplicada III (Electricidad y Electrónica), Univ. Complutense de Madrid, 28040 Madrid (Spain); CEI Campus Moncloa, UCM-UPM, 28040 Madrid (Spain); Olea, J. [CEI Campus Moncloa, UCM-UPM, 28040 Madrid (Spain); Instituto de Energía Solar, E.T.S.I. de Telecomunicación, Univ. Politécnica de Madrid, 28040 Madrid (Spain)

    2015-12-28

    The energy levels created in supersaturated n-type silicon substrates with titanium implantation in the attempt to create an intermediate band in their band-gap are studied in detail. Two titanium ion implantation doses (10{sup 13 }cm{sup -2} and 10{sup 14 }cm{sup -2}) are studied in this work by conductance transient technique and admittance spectroscopy. Conductance transients have been measured at temperatures of around 100 K. The particular shape of these transients is due to the formation of energy barriers in the conduction band, as a consequence of the band-gap narrowing induced by the high titanium concentration. Moreover, stationary admittance spectroscopy results suggest the existence of different energy level configuration, depending on the local titanium concentration. A continuum energy level band is formed when titanium concentration is over the Mott limit. On the other hand, when titanium concentration is lower than the Mott limit, but much higher than the donor impurity density, a quasi-continuum energy level distribution appears. Finally, a single deep center appears for low titanium concentration. At the n-type substrate, the experimental results obtained by means of thermal admittance spectroscopy at high reverse bias reveal the presence of single levels located at around E{sub c}-425 and E{sub c}-275 meV for implantation doses of 10{sup 13 }cm{sup −2} and 10{sup 14 }cm{sup −2}, respectively. At low reverse bias voltage, quasi-continuously distributed energy levels between the minimum of the conduction bands, E{sub c} and E{sub c}-450 meV, are obtained for both doses. Conductance transients detected at low temperatures reveal that the high impurity concentration induces a band gap narrowing which leads to the formation of a barrier in the conduction band. Besides, the relationship between the activation energy and the capture cross section values of all the energy levels fits very well to the Meyer-Neldel rule. As it is known

  20. Symmetry-Driven Band Gap Engineering in Hydrogen Functionalized Graphene

    DEFF Research Database (Denmark)

    Jørgensen, Jakob Holm; Grubisic Cabo, Antonija; Balog, Richard;

    2016-01-01

    Band gap engineering in hydrogen functionalized graphene is demonstrated by changing the symmetry of the functionalization structures. Small differences in hydrogen adsorbate binding energies on graphene on Ir(111) allow tailoring of highly periodic functionalization structures favoring one disti...

  1. Solid state dielectric screening versus band gap trends and implications

    Science.gov (United States)

    Ravichandran, Ram; Wang, Alan X.; Wager, John F.

    2016-10-01

    High-frequency (optical) and low-frequency (static) dielectric constant versus band gap trends, as well as index of refraction versus band gap trends are plotted for 107 inorganic semiconductors and insulators. These plots are describable via power-law fitting. Dielectric screening trends that emerge from this analysis have important optical and electronic implications. For example, barrier lowering during Schottky emission, phonon-assisted or Fowler-Nordheim tunneling, or Frenkel-Poole emission from a trap is found to be significantly more pronounced with increasing band gap due to a reduction in the optical dielectric constant with increasing band gap. The decrease in the interface state density with increasing band gap is another optical dielectric constant trend. The tendency for a material with a wider band gap to be more difficult to dope is attributed to an increase in the ionization energy of the donor or acceptor dopant, which in turn, depends on the optical dielectric constant and the effective mass. Since the effective mass for holes is almost always larger than that for electrons, p-type doping is more challenging than n-type doping in a wide band gap material. Finally, the polar optical phonon-limited mobility depends critically upon the reciprocal difference of the optical and the static dielectric constant. Consequently, electron and hole mobility tend to decrease with increasing band gap in a polar material.

  2. Slater half-occupation technique revisited: the LDA-1/2 and GGA-1/2 approaches for atomic ionization energies and band gaps in semiconductors

    Directory of Open Access Journals (Sweden)

    Luiz G. Ferreira

    2011-09-01

    Full Text Available The very old and successful density-functional technique of half-occupation is revisited [J. C. Slater, Adv. Quant. Chem. 6, 1 (1972]. We use it together with the modern exchange-correlation approximations to calculate atomic ionization energies and band gaps in semiconductors [L. G. Ferreira et al., Phys. Rev. B 78, 125116 (2008]. Here we enlarge the results of the previous paper, add to its understandability, and show when the technique might fail. Even in this latter circumstance, the calculated band gaps are far better than those of simple LDA or GGA. As before, the difference between the Kohn-Sham ground state one-particle eigenvalues and the half-occupation eigenvalues is simply interpreted as the self-energy (not self-interaction of the particle excitation. In both cases, that of atomic ionization energies and semiconductor band gaps, the technique is proven to be very worthy, because not only the results can be very precise but the calculations are fast and very simple.

  3. Estimation of the band gap of InPO4

    Science.gov (United States)

    Wager, J. F.; Wilmsen, C. W.; Kazmerski, L. L.

    1983-04-01

    The band gap of a thin layer of InPO4 was estimated to be 4.5 eV using a novel approach employing ultraviolet photoelectron spectroscopy and electron energy loss spectroscopy. The technique measures the conduction-band minimum and valence-band maximum referenced to the In 4d core line energy. Since this technique is highly surface sensitive, it can be used to measure the band gap of a thin layer. This parameter is difficult to measure in such layers using conventional techniques.

  4. One-Dimensional Anisotropic Band Gap Structure

    Institute of Scientific and Technical Information of China (English)

    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.

  5. Multi Band Gap High Efficiency Converter (RAINBOW)

    Science.gov (United States)

    Bekey, I.; Lewis, C.; Phillips, W.; Shields, V.; Stella, P.

    1997-01-01

    The RAINBOW multi band gap system represents a unique combination of solar cells, concentrators and beam splitters. RAINBOW is a flexible system which can readily expand as new high efficiency components are developed.

  6. Band gap narrowing and photocatalytic studies of Nd3+ ion-doped SnO2 nanoparticles using solar energy

    Indian Academy of Sciences (India)

    Dhanya Chandran; Lakshmi S Nair; S Balachandran; K Rajendra Babu; M Deepa

    2016-02-01

    Pure and Nd3+-doped tin oxide (SnO2) nanoparticles have been prepared by the sol–gel method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM, energydispersive spectroscopy and UV–visible spectroscopy. The XRD patterns of all the samples are identified as tetragonal rutile-type SnO2 phase which is further confirmed by TEM analysis. Neodymium doping introduces band gap narrowing in the prepared samples and enhances their absorption towards the visible-light region. The photocatalytic activity of all the samples was evaluated by monitoring the degradation of methylene blue solution under day light illumination and it was found that the photocatalytic activity significantly increases for the samples calcined at 600 than 400°C, which is due to the effective charge separation of photogenerated electron–hole pairs. The efficiency of photocatalysts was found to be related to neodymium doping percentage and calcination temperature.

  7. Band Gap Energy of Chalcopyrite Thin Film Solar Cell Absorbers Determined by Soft X-Ray Emission and Absorption Spectroscopy

    Energy Technology Data Exchange (ETDEWEB)

    Bar, M.; Weinhardt, L.; Pookpanratana, S.; Heske, C.; Nishiwaki, S.; Shafarman, W.; Fuchs, O.; Blum, M.; Yang, W.; Denlinger, J.D.

    2008-05-11

    The chemical and electronic structure of high efficiency chalcopyrite thin film solar cell absorbers significantly differs between the surface and the bulk. While it is widely accepted that the absorber surface exhibits a Cu-poor surface phase with increased band gap (Eg), a direct access to the crucial information of the depth-dependency of Eg is still missing. In this paper, we demonstrate that a combination of x-ray emission and absorption spectroscopy allows a determination of Eg in the surface-near bulk and thus complements the established surface- and bulk-sensitive techniques of Eg determination. As an example, we discuss the determination of Eg for a Cu(In,Ga)Se2 absorber [(1.52 +- 0.20) eV].

  8. Band gap scaling laws in group IV nanotubes

    Science.gov (United States)

    Wang, Chongze; Fu, Xiaonan; Guo, Yangyang; Guo, Zhengxiao; Xia, Congxin; Jia, Yu

    2017-03-01

    By using the first-principles calculations, the band gap properties of nanotubes formed by group IV elements have been investigated systemically. Our results reveal that for armchair nanotubes, the energy gaps at K points in the Brillouin zone decrease as 1/r scaling law with the radii (r) increasing, while they are scaled by ‑1/r 2 + C at Γ points, here, C is a constant. Further studies show that such scaling law of K points is independent of both the chiral vector and the type of elements. Therefore, the band gaps of nanotubes for a given radius can be determined by these scaling laws easily. Interestingly, we also predict the existence of indirect band gap for both germanium and tin nanotubes. Our new findings provide an efficient way to determine the band gaps of group IV element nanotubes by knowing the radii, as well as to facilitate the design of functional nanodevices.

  9. Modification in band gap of zirconium complexes

    Science.gov (United States)

    Sharma, Mayank; Singh, J.; Chouhan, S.; Mishra, A.; Shrivastava, B. D.

    2016-05-01

    The optical properties of zirconium complexes with amino acid based Schiff bases are reported here. The zirconium complexes show interesting stereo chemical features, which are applicable in organometallic and organic synthesis as well as in catalysis. The band gaps of both Schiff bases and zirconium complexes were obtained by UV-Visible spectroscopy. It was found that the band gap of zirconium complexes has been modified after adding zirconium compound to the Schiff bases.

  10. Band gap bowing in quaternary nitride semiconducting alloys

    DEFF Research Database (Denmark)

    Gorczyka, Isabela; Suski, T.; Christensen, Niels Egede;

    2011-01-01

    the composition and atomic arrangements are examined using a supercell geometry. An analytical expression for the band gap is derived for the entire range of compositions. The range of (x, y) values for which InxGayAl1−x−yN is lattice matched to GaN, and the ensuing energy gaps, are given. This range of available...

  11. 光子带隙调制下发光体间的能量传递过程%Photonic band gap effects upon energy transfer in luminophors

    Institute of Scientific and Technical Information of China (English)

    刘震东; 杨正文; 李勃; 周济

    2013-01-01

    The photonic crystals provide unique properties of photonic band gap,photonic localization and so on. The photonic bad gap creates possibilities to produce new photoelectric materials with the modulation of the photophysical process.In this work,the modulation effects on energy transfer were studied both in theory and experiment.The opal and inverse opal photonic crystals with luminophors were fabricated.The energy transfer was investigated by photoluminescence and fluorescence lifetime.The results showed the enhancement of ener-gy transfer when the photonic band gap overlapped the emission wavelength of donor.%光子晶体具有光子带隙和光子局域等特有性质,光物理过程在光子带隙调制下产生新的现象特点为制备新型光电材料提供了可能。通过对光子带隙调制作用的机理探讨,设计制备蛋白石及反蛋白石结构光子晶体。对其中发光体的荧光光谱和给体荧光寿命的实验研究表明,当光子带隙频段与能量传递过程中给体的发射光谱相重叠,能量传递过程将得到增强。

  12. Theoretical analysis of the crystal structure, band-gap energy, polarization, and piezoelectric properties of ZnO-BeO solid solutions

    Science.gov (United States)

    Dong, L.; Alpay, S. P.

    2011-07-01

    The electrical properties, the spontaneous polarization, and the piezoelectric response of ZnO can be tailored by alloying ZnO with BeO for applications such as electrodes in flat panel displays and solar cells, blue and ultraviolet (UV) light emitting devices, and highly sensitive UV detectors. We present here the results of a study that employs density-functional theory to analyze the crystal structure, the band structure, spontaneous polarization, and piezoelectric properties of Zn1-xBexO solid solutions. Our findings indicate that Zn1-xBexO alloys may have a different crystal structure than the end components ZnO and BeO that crystallize in the prototypical wurtzite structure (P63mc). It is shown that orthorhombic lattices with Pmn21, Pna21, or P21 structures may have lower formation energies than the wurtzite lattice at a given Be composition. The band-gap energies of Zn1-xBexO in the wurtzite and the orthorhombic structures are nearly identical and the bowing of the band-gap energy increases with increasing Be concentration. The spontaneous polarization of Zn1-xBexO in the orthorhombic lattice is markedly larger compared to the wurtzite structure while the piezoelectric polarization in the wurtzite and orthorhombic structures varies linearly with the Be concentration.

  13. Structured ZnO films: Effect of copper nitrate addition to precursor solution on topography, band gap energy and photocatalytic activity

    Science.gov (United States)

    Heinonen, S.; Nikkanen, J.-P.; Kaleva, A.; Hyvärinen, L.; Levänen, E.

    2017-02-01

    ZnO is a widely studied semiconductor material with interesting properties such as photocatalytic activity leading to wide range of applications, for example in the field of opto-electronics and self-cleaning and antimicrobial applications. Doping of photocatalytic semiconductor materials has been shown to introduce variation in the band gap energy of the material. In this work, ZnO rods were grown on a stainless steel substrates using hydrothermal method introducing copper nitrate into the precursor solution. Zinc nitrate and hexamethylenetetramine were used as precursor materials and the growth was conducted at 90 °C for 2 h in order to achieve a well-aligned evenly distributed rod structure. Copper was introduced as copper nitrate that was added in the precursor solution in the beginning of the growth. The as-prepared films were then heat-treated at 350 °C and band gap measurements were performed for prepared films. It was found that increase in the copper concentration in the precursor solution decreased the band gap of the ZnO film. Methylene blue discolouration tests were then performed in order to study the effect of the copper nitrate addition to precursor solution on photocatalytic activity of the structured ZnO films.

  14. Band gap opening in graphene: a short theoretical study

    Science.gov (United States)

    Sahu, Sivabrata; Rout, G. C.

    2017-03-01

    Graphene, being a gapless semiconductor, cannot be used in pristine form for nano-electronic applications. Therefore, it is essential to generate a finite gap in the energy dispersion at Dirac point. We present here the tight-binding model Hamiltonian taking into account of various interactions for tuning band gap in graphene. The model Hamiltonian describes the hopping of the π-electrons up to third nearest-neighbours, substrate effects, Coulomb interaction at two sub-lattices, electron-phonon interaction in graphene-on-substrates and high phonon frequency vibrations, besides the bi-layer graphene. We have solved the Hamiltonian using Zubarev's double time single particle Green's function technique. The quasi-particle energies, electron band dispersions, the expression for effective band gap and the density of states (DOS) are calculated numerically. The results are discussed by varying different model parameters of the system. It is observed that the electron DOS and band dispersion exhibit linear energy dependence near Dirac point for nearest-neighbour hopping integral. However, the second and third nearest-neighbour hoppings provide asymmetry in DOS. The band dispersions exhibit wider band gaps with stronger substrate effect. The modified gap in graphene-on-substrate attains its maximum value for Coulomb interaction energy U_{C} = 1.7 t1 . The critical Coulomb interaction is enhanced to U_{C} = 2.5 t1 to produce maximum band gap in the presence of electron-phonon interaction and phonon vibration. The bi-layer graphene exhibits Mexican hat type band gap near Dirac point for transverse gating potential. The other conclusions for the present work are described in the text.

  15. Optical properties of sputtered hexagonal CdZnO films with band gap energies from 1.8 to 3.3 eV

    Energy Technology Data Exchange (ETDEWEB)

    Ma Xiangyang; Chen Peiliang; Zhang Ruijie [State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027 (China); Yang Deren, E-mail: mseyang@zju.edu.cn [State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027 (China)

    2011-06-09

    Highlights: > Cd{sub x}Zn{sub 1-x}O films in a wide range of Cd contents were grown by reactive direct-current magnetron sputtering, which is a simple and widely used method. > The optical band gap energies of Cd{sub x}Zn{sub 1-x}O films were tuned from {approx} 3.3 eV to {approx} 1.8 eV. > The ultraviolet irradiation enhanced photoluminescence from the Cd{sub x}Zn{sub 1-x}O films was observed. - Abstract: Cd{sub x}Zn{sub 1-x}O films in a wide range of Cd contents have been grown by reactive direct-current magnetron sputtering. At x {<=} 0.66, the sputtered Cd{sub x}Zn{sub 1-x}O films are of single hexagonal phase. The optical band gap energies of the Cd{sub x}Zn{sub 1-x}O films decrease from {approx}3.3 eV at x = 0 to {approx}1.8 eV at x = 0.66. Correspondingly, the near-band-edge photoluminescence is tuned in a wide visible region from {approx}378 to 678 nm. Moreover, the ultraviolet irradiation enhanced PL from the Cd{sub x}Zn{sub 1-x}O films has been observed.

  16. Band gap effects of hexagonal boron nitride using oxygen plasma

    Energy Technology Data Exchange (ETDEWEB)

    Sevak Singh, Ram; Leong Chow, Wai [School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore); Yingjie Tay, Roland [School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore); Temasek Laboratories-NTU, 50 Nanyang Avenue, Singapore 639798 (Singapore); Hon Tsang, Siu [Temasek Laboratories-NTU, 50 Nanyang Avenue, Singapore 639798 (Singapore); Mallick, Govind [Temasek Laboratories-NTU, 50 Nanyang Avenue, Singapore 639798 (Singapore); Weapons and Materials Research Directorate, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005 (United States); Tong Teo, Edwin Hang, E-mail: htteo@ntu.edu.sg [School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore); School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 (Singapore)

    2014-04-21

    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.

  17. Molecular doping and band-gap opening of bilayer graphene.

    Science.gov (United States)

    Samuels, Alexander J; Carey, J David

    2013-03-26

    The ability to induce an energy band gap in bilayer graphene is an important development in graphene science and opens up potential applications in electronics and photonics. Here we report the emergence of permanent electronic and optical band gaps in bilayer graphene upon adsorption of π electron containing molecules. Adsorption of n- or p-type dopant molecules on one layer results in an asymmetric charge distribution between the top and bottom layers and in the formation of an energy gap. The resultant band gap scales linearly with induced carrier density though a slight asymmetry is found between n-type dopants, where the band gap varies as 47 meV/10(13) cm(-2), and p-type dopants where it varies as 40 meV/10(13) cm(-2). Decamethylcobaltocene (DMC, n-type) and 3,6-difluoro-2,5,7,7,8,8-hexacyano-quinodimethane (F2-HCNQ, p-type) are found to be the best molecules at inducing the largest electronic band gaps up to 0.15 eV. Optical adsorption transitions in the 2.8-4 μm region of the spectrum can result between states that are not Pauli blocked. Comparison is made between the band gaps calculated from adsorbate-induced electric fields and from average displacement fields found in dual gate bilayer graphene devices. A key advantage of using molecular adsorption with π electron containing molecules is that the high binding energy can induce a permanent band gap and open up possible uses of bilayer graphene in mid-infrared photonic or electronic device applications.

  18. Maximizing band gaps in plate structures

    DEFF Research Database (Denmark)

    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...... periodic plate using Bloch theory, which conveniently reduces the maximization problem to that of a single base cell. Secondly, we construct a finite periodic plate using a number of the optimized base cells in a postprocessed version. The dynamic properties of the finite plate are investigated...

  19. Progress towards fabrication of Th:229-doped high energy band-gap crystals for use as a solid-state optical frequency reference

    CERN Document Server

    Rellergert, Wade G; DeMille, D; Greco, R R; Hehlen, M P; Jackson, R A; Torgerson, J R; Hudson, Eric R

    2010-01-01

    We have recently described a novel method for the construction of a solid-state optical frequency reference based on doping $^{229}$Th into high energy band-gap crystals. Since nuclear transitions are far less sensitive to environmental conditions than atomic transitions, we have argued that the $^{229}$Th optical nuclear transition may be driven inside a host crystal resulting in an optical frequency reference with a short-term stability of $3\\times10^{-17}<\\Delta f/f <1\\times10^{-15}$ at 1 s and a systematic-limited repeatability of $\\Delta f/f \\sim 2 \\times 10^{-16}$. Improvement by $10^2-10^3$ of the constraints on the variability of several important fundamental constants also appears possible. Here we present the results of the first phase of these experiments. Specifically, we have evaluated several high energy band-gap crystals (Th:NaYF, Th:YLF, Th:LiCAF, Na$_2$ThF$_6$, LiSAF) for their suitability as a crystal host by a combination of electron beam microprobe measurements, Rutherford Backscatte...

  20. Band Gap Narrowing in Heavily Doped Silicon.

    Science.gov (United States)

    Gupta, Tapan Kumar

    Two analytic models for transport and band gap narrowing in heavily doped (N_{rm D} > 10^{20} cm^ {-3}) silicon have been set up and verified through measurements on n^{+} -p junction devices. The first model is based on calculation of the ratio of the charge present in the emitter of the n^{+} region of the junction to that of the charge present in the absence of band gap shrinkage. Fermi-Dirac statistics are employed and are found to have a significant effect at this doping level. The second model is based on current transport of minority carriers in the n^{+} region. In this model only two parameters need to be known, the diffusion coefficient and the diffusion length for minority carriers, to calculate the band gap narrowing. An empirical relation between band gap narrowing and donor concentration has also been established based on experimental values of diffusion coefficient and mobility. These models have been verified by several different experimental techniques including surface photovoltage, open circuit voltage decay, photoconductivity decay and modulation reflection spectroscopy. The results indicate that, in the impurity range above about 10^{20} cm^{-3}, Fermi-Dirac statistics must be invoked in order to achieve a satisfactory fit with experimental data.

  1. Plasmonic band gap cavities on biharmonic gratings

    Science.gov (United States)

    Kocabas, Askin; Seckin Senlik, S.; Aydinli, Atilla

    2008-05-01

    In this paper, we have experimentally demonstrated the formation of plasmonic band gap cavities in infrared and visible wavelength range. The cavity structure is based on a biharmonic metallic grating with selective high dielectric loading. A uniform metallic grating structure enables strong surface plasmon polariton (SPP) excitation and a superimposed second harmonic component forms a band gap for the propagating SPPs. We show that a high dielectric superstructure can dramatically perturb the optical properties of SPPs and enables the control of the plasmonic band gap structure. Selective patterning of the high index superstructure results in an index contrast in and outside the patterned region that forms a cavity. This allows us to excite the SPPs that localize inside the cavity at specific wavelengths, satisfying the cavity resonance condition. Experimentally, we observe the formation of a localized state in the band gap and measure the dispersion diagram. Quality factors as high as 37 have been observed in the infrared wavelength. The simplicity of the fabrication and the method of testing make this approach attractive for applications requiring localization of propagating SPPs.

  2. Diluted II-VI oxide semiconductors with multiple band gaps.

    Science.gov (United States)

    Yu, K M; Walukiewicz, W; Wu, J; Shan, W; Beeman, J W; Scarpulla, M A; Dubon, O D; Becla, P

    2003-12-12

    We report the realization of a new mult-band-gap semiconductor. Zn(1-y)Mn(y)OxTe1-x alloys have been synthesized using the combination of oxygen ion implantation and pulsed laser melting. Incorporation of small quantities of isovalent oxygen leads to the formation of a narrow, oxygen-derived band of extended states located within the band gap of the Zn(1-y)Mn(y)Te host. When only 1.3% of Te atoms are replaced with oxygen in a Zn0.88Mn0.12Te crystal the resulting band structure consists of two direct band gaps with interband transitions at approximately 1.77 and 2.7 eV. This remarkable modification of the band structure is well described by the band anticrossing model. With multiple band gaps that fall within the solar energy spectrum, Zn(1-y)Mn(y)OxTe1-x is a material perfectly satisfying the conditions for single-junction photovoltaics with the potential for power conversion efficiencies surpassing 50%.

  3. Diluted magnetic semiconductors with narrow band gaps

    Science.gov (United States)

    Gu, Bo; Maekawa, Sadamichi

    2016-10-01

    We propose a method to realize diluted magnetic semiconductors (DMSs) with p - and n -type carriers by choosing host semiconductors with a narrow band gap. By employing a combination of the density function theory and quantum Monte Carlo simulation, we demonstrate such semiconductors using Mn-doped BaZn2As2 , which has a band gap of 0.2 eV. In addition, we found a nontoxic DMS Mn-doped BaZn2Sb2 , of which the Curie temperature Tc is predicted to be higher than that of Mn-doped BaZn2As2 , the Tc of which was up to 230 K in a recent experiment.

  4. Kohn-Sham potential with discontinuity for band gap materials

    Science.gov (United States)

    Kuisma, M.; Ojanen, J.; Enkovaara, J.; Rantala, T. T.

    2010-09-01

    We model a Kohn-Sham potential with the discontinuity at integer particle numbers starting from the approximation by (GLLB) Gritsenko [Phys. Rev. A 51, 1944 (1995)10.1103/PhysRevA.51.1944]. We evaluate the Kohn-Sham gap and the discontinuity to obtain the quasiparticle gap. This allows us to compare the Kohn-Sham gaps to those obtained by accurate many-body perturbation-theory-based optimized potential methods. In addition, the resulting quasiparticle band gap is compared to experimental gaps. In the GLLB model potential, the exchange-correlation hole is modeled using a generalized gradient approximation (GGA) energy density and the response of the hole-to-density variations is evaluated by using the common-denominator approximation and homogeneous electron-gas-based assumptions. In our modification, we have chosen the PBEsol potential as the GGA to model the exchange hole and add a consistent correlation potential. The method is implemented in the GPAW code, which allows efficient parallelization to study large systems. A fair agreement for Kohn-Sham and the quasiparticle band gaps with semiconductors and other band gap materials is obtained with a potential which is as fast as GGA to calculate.

  5. Preparation of nanofibers consisting of MnO/Mn3O4 by using the electrospinning technique: the nanofibers have two band-gap energies

    Science.gov (United States)

    Barakat, Nasser A. M.; Woo, Kee-Do; Ansari, S. G.; Ko, Jung-Ahn; Kanjwal, Muzafar A.; Kim, Hak Yong

    2009-06-01

    In the present study, nanofibers consisting of manganese monoxide (MnO), which is hard to prepare because of the chemical activity of the manganese metal, and the popular Mn3O4 have been synthesized via the electrospinning technique. The nanofibers were obtained by electrospinning of an aqueous sol-gel consisting of manganese acetate tetra-hydrate and poly(vinyl alcohol). The obtained nanofiber mats were dried in vacuum at 80°C for 24 h and then calcined in argon atmosphere at 900°C for 5 h. According to X-ray diffraction results, the obtained nanofibers contain 65% MnO. Transmission electron microscope analysis reveals good crystallinity of the produced nanofibers. UV-visible spectroscopic analysis has indicated that the produced nanofibers have two band-gap energies, 2 and 3.7 eV, which enhances utilizing of the nanofibers in different applications.

  6. Aluminum doping tunes band gap energy level as well as oxidative stress-mediated cytotoxicity of ZnO nanoparticles in MCF-7 cells.

    Science.gov (United States)

    Akhtar, Mohd Javed; Alhadlaq, Hisham A; Alshamsan, Aws; Majeed Khan, M A; Ahamed, Maqusood

    2015-09-08

    We investigated whether Aluminum (Al) doping tunes band gap energy level as well as selective cytotoxicity of ZnO nanoparticles in human breast cancer cells (MCF-7). Pure and Al-doped ZnO nanoparticles were prepared by a simple sol-gel method. Characterization study confirmed the formation of single phase of Al(x)Zn(1-x)O nanocrystals with the size range of 33-55 nm. Al-doping increased the band gap energy of ZnO nanoparticles (from 3.51 eV for pure to 3.87 eV for Al-doped ZnO). Al-doping also enhanced the cytotoxicity and oxidative stress response of ZnO nanoparticles in MCF-7 cells. The IC50 for undoped ZnO nanoparticles was 44 μg/ml while for the Al-doped ZnO counterparts was 31 μg/ml. Up-regulation of apoptotic genes (e.g. p53, bax/bcl2 ratio, caspase-3 &caspase-9) along with loss of mitochondrial membrane potential suggested that Al-doped ZnO nanoparticles induced apoptosis in MCF-7 cells through mitochondrial pathway. Importantly, Al-doping did not change the benign nature of ZnO nanoparticles towards normal cells suggesting that Al-doping improves the selective cytotoxicity of ZnO nanoparticles toward MCF-7 cells without affecting the normal cells. Our results indicated a novel approach through which the inherent selective cytotoxicity of ZnO nanoparticles against cancer cells can be further improved.

  7. Band gap tuning of amorphous Al oxides by Zr alloying

    DEFF Research Database (Denmark)

    Canulescu, Stela; Jones, N. C.; Borca, C. N.;

    2016-01-01

    minimum changes non-linearly as well.Fitting of the energy band gap values resulted in a bowing parameter of 2 eV. The band gap bowing of themixed oxides is assigned to the presence of the Zr d-electron states localized below the conduction bandminimum of anodized Al2O3.......The optical band gap and electronic structure of amorphous Al-Zr mixed oxides, with Zr content ranging from4.8 to 21.9% were determined using vacuum ultraviolet (VUV) and X-ray absorption spectroscopy (XAS). Thelight scattering by the nano-porous structure of alumina at low wavelengths...... was estimated based on the Miescattering theory. The dependence of the optical band gap of the Al-Zr mixed oxides on Zr content deviatesfrom linearity and decreases from 7.3 eV for pure anodized Al2O3 to 6.45 eV for Al-Zr mixed oxide with Zrcontent of 21.9%. With increasing Zr content, the conduction band...

  8. Electroluminescence from indirect band gap semiconductor ReS2

    Science.gov (United States)

    Gutiérrez-Lezama, Ignacio; Aditya Reddy, Bojja; Ubrig, Nicolas; Morpurgo, Alberto F.

    2016-12-01

    It has been recently claimed that bulk crystals of transition metal dichalcogenide (TMD) ReS2 are direct band gap semiconductors, which would make this material an ideal candidate, among all TMDs, for the realization of efficient opto-electronic devices. The situation is however unclear, because even more recently an indirect transition in the PL spectra of this material has been detected, whose energy is smaller than the supposed direct gap. To address this issue we exploit the properties of ionic liquid gated field-effect transistors (FETs) to investigate the gap structure of bulk ReS2. Using these devices, whose high quality is demonstrated by a record high electron FET mobility of 1100 cm2 V-1 s-1 at 4 K, we can induce hole transport at the surface of the material and determine quantitatively the smallest band gap present in the material, irrespective of its direct or indirect nature. The value of the band gap is found to be 1.41 eV, smaller than the 1.5 eV direct optical transition but in good agreement with the energy of the indirect optical transition, providing an independent confirmation that bulk ReS2 is an indirect band gap semiconductor. Nevertheless, contrary to the case of more commonly studied semiconducting TMDs (e.g., MoS2, WS2, etc) in their bulk form, we also find that ReS2 FETs fabricated on bulk crystals do exhibit electroluminescence when driven in the ambipolar injection regime, likely because the difference between direct and indirect gap is only 100 meV. We conclude that ReS2 does deserve more in-depth investigations in relation to possible opto-electronic applications.

  9. Band gap transmission in periodic bistable mechanical systems

    Science.gov (United States)

    Frazier, Michael J.; Kochmann, Dennis M.

    2017-02-01

    We theoretically and numerically investigate the supratransmission phenomenon in discrete, nonlinear systems containing bistable elements. While linear waves cannot propagate within the band gaps of periodic structures, supratransmission allows large-amplitude waves to transmit energy through the band gap. For systems lacking bistability, the threshold amplitude for such energy transmission at a given frequency in the linear band gap is fixed. We show that the topological transitions due to bistability provide an avenue for switching the threshold amplitude between two well-separated values. Moreover, this versatility is achieved while leaving the linear dispersion properties of the system essentially unchanged. Interestingly, the behavior changes when an elastic chain is coupled to bistable resonators (in an extension of the well-studied linear locally resonant metamaterials). Here, we show that a fraction of the injected energy is confined near the boundary due to the resonators, providing a means of regulating the otherwise unrestrained energy flow due to supratransmission. Together, the results illustrate new means of controlling nonlinear wave propagation and energy transport in systems having multi-stable elements.

  10. Electron Elevator: Excitations across the Band Gap via a Dynamical Gap State.

    Science.gov (United States)

    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.

  11. Effect of MnO2 doping and temperature treatment on optical energy band gap properties in Zn-Bi-Ti-O varistor ceramics

    Science.gov (United States)

    Ghazali, M. S. M.; Abdullah, W. R. W.; Zakaria, A.; Kamari, H. M.; Rizwan, Z.

    2016-11-01

    In this study, the optical band-gap energy (Eg) was investigated with respect to MnO2 and sintering temperatures on ZnO based varistor ceramics. Eg of the ceramic (99-x) mol% ZnO + 0.5 mol% Bi2O3 + 0.5 mol% TiO2 + × MnO2 where × = 0, 0.2, 0.4, 0.6 and 0.8 mol%, were determined using UV-Vis spectrophotometer. The samples was prepared through solid-state route and sintered at the sintering temperature from 1110, 1140 and 1170 °C for 45 and 90 min in open air. At no doping of MnO2, the values of Eg are 2.991 ± 0.001, 2.989 ± 0.001 eV for 45 and 90 min sintering time; respectively. Eg was decreased to 2.192 ± 0.001 eV at 1140 °C at 45 min sintering time. Similar result of Eg was observed at longer heat treatment. Further addition of dopant causing the Eg decreases rapidly to 2.099 and 2.106 ± 0.001 eV at 45 and 90 min sintering time; respectively. XRD analysis indicates that there is hexagonal ZnO and secondary phases, Zn2MnO4, Bi4Ti3O12 and Zn2Ti3O8. The relative density of the sintered ceramics decreased or remain constant with the increase of MnO2 concentration for 45 min sintering time, however, further prolong sintering time; the relative density decreases form 90.25 to 88.35%. This indicates the pores are increasing with the increase of heat treatment. The variation of sintering temperatures to the optical band gap energy of based ZnO varistor doped with MnO2 due to the formation of interface states.

  12. Substrate-induced band gap opening in epitaxial graphene

    Energy Technology Data Exchange (ETDEWEB)

    Zhou, S.Y.; Gweon, G.-H.; Fedorov, A.V.; First, P.N.; de Heer,W.A.; Lee, D.-H.; Guinea, F.; Castro Neto, A.H.; Lanzara, A.

    2007-09-08

    Graphene has shown great application potential as the hostmaterial for next-generation electronic devices. However, despite itsintriguing properties, one of the biggest hurdles for graphene to beuseful as an electronic material is the lack of an energy gap in itselectronic spectra. This, for example, prevents the use of graphene inmaking transistors. Although several proposals have been made to open agap in graphene's electronic spectra, they all require complexengineering of the graphene layer. Here, we show that when graphene isepitaxially grown on SiC substrate, a gap of ~;0.26 eV is produced. Thisgap decreases as the sample thickness increases and eventually approacheszero when the number of layers exceeds four. We propose that the originof this gap is the breaking of sublattice symmetry owing to thegraphene-substrate interaction. We believe that our results highlight apromising direction for band gap engineering of graphene.

  13. Formation of Degenerate Band Gaps in Layered Systems

    Directory of Open Access Journals (Sweden)

    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.

  14. Band gap energy in Zn-rich Zn{sub 1-x} Cd{sub x} Te thin films grown by r.f. sputtering

    Energy Technology Data Exchange (ETDEWEB)

    Becerril, M.; Silva L, H.; Zelaya A, O. [Depto. de Fisica, CINVESTAV, A.P. 14-740, 07360 Mexico D.F. (Mexico)

    2004-07-01

    Ternary Zn{sub 1-x} Cd{sub x} Te semiconducting thin films were grown on 7059 Corning glass substrates at room temperature by co-sputtering from a Zn Te-Cd target. The visible Cd-area onto Zn Te target was varied to cover 0% - 4% of the total area. The optical and structural properties of the films were analysed as a function of the Cd concentration (x) on the layers. The sharp diffraction lines indicate mainly the Zn Te in cubic phase. When the cadmium was incorporated into the Zn Te lattice, the band gap energy (E{sub g}) decreased from 2.2748 eV (x = 0) to 2.2226 eV (x = 0.081). We found a linear relationship between E{sub g} and (x) in the interval of (x) studied, that predicts a value of the bowing parameter b in the E{sub g} (x) = E{sub g0} + ax + bx{sup 2} relationship, which coincides with the value of b calculated by making the same study for Cd-rich Cd Zn Te. This result becomes interesting given the large number of values reported for b and a for E{sub g} (x) of this ternary material. (Author) 17 refs., 1 tab., 6 figs.

  15. Significant Reduction in NiO Band Gap Upon Formation of LixNi1-xO alloys: Applications To Solar Energy Conversion

    Energy Technology Data Exchange (ETDEWEB)

    Alidoust, Nima; Toroker, Maytal Caspary; Keith, John A.; Carter, Emily A.

    2013-11-21

    Long-term sustainable solar energy conversion relies on identifying economical and versatile semiconductor materials with appropriate band structures for photovoltaic and photocatalytic applications (e.g., band gaps of ~1.5–2.0 eV). Nickel oxide (NiO) is an inexpensive yet highly promising candidate. Its charge-transfer character may lead to longer carrier lifetimes needed for higher efficiencies, and its conduction band edge is suitable for driving hydrogen evolution via water-splitting. However, NiO’s large band gap (~4 eV) severely limits its use in practical applications. Our first-principles quantum mechanics calculations show band gaps dramatically decrease to ~2.0 eV when NiO is alloyed with Li2O. In this paper, we show that LixNi1-xO alloys (with x=0.125 and 0.25) are p-type semiconductors, contain states with no impurity levels in the gap and maintain NiO’s desirable charge-transfer character. Lastly, we show that the alloys have potential for photoelectrochemical applications, with band edges well-placed for photocatalytic hydrogen production and CO2 reduction, as well as in tandem dye-sensitized solar cells as a photocathode.

  16. Photovoltaic properties of low band gap ferroelectric perovskite oxides

    Science.gov (United States)

    Huang, Xin; Paudel, Tula; Dong, Shuai; Tsymbal, Evgeny

    2015-03-01

    Low band gap ferroelectric perovskite oxides are promising for photovoltaic applications due to their high absorption in the visible optical spectrum and a possibility of having large open circuit voltage. Additionally, an intrinsic electric field present in these materials provides a bias for electron-hole separation without requiring p-n junctions as in conventional solar cells. High quality thin films of these compounds can be grown with atomic layer precision allowing control over surface and defect properties. Initial screening based on the electronic band gap and the energy dependent absorption coefficient calculated within density functional theory shows that hexagonal rare-earth manganites and ferrites are promising as photovoltaic absorbers. As a model, we consider hexagonal TbMnO3. This compound has almost ideal band gap of about 1.4 eV, very high ferroelectric Curie temperature, and can be grown epitaxially. Additionally hexagonal TbMnO3 offers possibility of coherent structure with transparent conductor ZnO. We find that the absorption is sufficiently high and dominated by interband transitions between the Mn d-bands. We will present the theoretically calculated photovoltaic efficiency of hexagonal TbMnO3 and explore other ferroelectric perovskite oxides.

  17. Band gap determination of Ni–Zn ferrites

    Indian Academy of Sciences (India)

    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.

  18. Ultra-wide acoustic band gaps in pillar-based phononic crystal strips

    Energy Technology Data Exchange (ETDEWEB)

    Coffy, Etienne, E-mail: etienne.coffy@femto-st.fr; Lavergne, Thomas; Addouche, Mahmoud; Euphrasie, Sébastien; Vairac, Pascal; Khelif, Abdelkrim [FEMTO-ST Institute, Université de Franche-Comté, UBFC, CNRS, ENSMM, UTBM, 15B Av. des Montboucons, F-25030 Besançon (France)

    2015-12-07

    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.

  19. Review of wide band-gap semiconductors technology

    Directory of Open Access Journals (Sweden)

    Jin Haiwei

    2016-01-01

    Full Text Available Silicon carbide (SiC and gallium nitride (GaN are typical representative of the wide band-gap semiconductor material, which is also known as third-generation semiconductor materials. Compared with the conventional semiconductor silicon (Si or gallium arsenide (GaAs, wide band-gap semiconductor has the wide band gap, high saturated drift velocity, high critical breakdown field and other advantages; it is a highly desirable semiconductor material applied under the case of high-power, high-temperature, high-frequency, anti-radiation environment. These advantages of wide band-gap devices make them a hot spot of semiconductor technology research in various countries. This article describes the research agenda of United States and European in this area, focusing on the recent developments of the wide band-gap technology in the US and Europe, summed up the facing challenge of the wide band-gap technology.

  20. Bi-directional evolutionary optimization for photonic band gap structures

    Energy Technology Data Exchange (ETDEWEB)

    Meng, Fei [Centre for Innovative Structures and Materials, School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476, Melbourne, VIC 3001 (Australia); School of Civil Engineering, Central South University, Changsha 410075 (China); Huang, Xiaodong, E-mail: huang.xiaodong@rmit.edu.au [Centre for Innovative Structures and Materials, School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476, Melbourne, VIC 3001 (Australia); Key Laboratory of Advanced Technology for Vehicle Body Design & Manufacture, Hunan University, Changsha, 410082 (China); Jia, Baohua [Centre for Micro-Photonics, Faculty of Engineering & Industrial Science, Swinburne University of Technology, PO Box 218, Hawthorn, VIC 3122 (Australia)

    2015-12-01

    Toward an efficient and easy-implement optimization for photonic band gap structures, this paper extends the bi-directional evolutionary structural optimization (BESO) method for maximizing photonic band gaps. Photonic crystals are assumed to be periodically composed of two dielectric materials with the different permittivity. Based on the finite element analysis and sensitivity analysis, BESO starts from a simple initial design without any band gap and gradually re-distributes dielectric materials within the unit cell so that the resulting photonic crystal possesses a maximum band gap between two specified adjacent bands. Numerical examples demonstrated the proposed optimization algorithm can successfully obtain the band gaps from the first to the tenth band for both transverse magnetic and electric polarizations. Some optimized photonic crystals exhibit novel patterns markedly different from traditional designs of photonic crystals.

  1. Understanding Band Gaps of Solids in Generalized Kohn-Sham Theory

    CERN Document Server

    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.

  2. Analysis on Band Gaps of MCM-41 Type of Materials

    Institute of Scientific and Technical Information of China (English)

    HAN Pei-de; LIANG Jian; XU Bing-she; LIU Xu-guang; PENG Lian-mao

    2004-01-01

    The concept and analysis method of photonic crystals and band gaps are introduced into one-dimensional(1D) ordered mesoporous materials. MCM-41 type of materials are treated theoretically as photonic crystals. The formation of band gaps is exhibited and confirmed by a calculation of transfer matrix technique. PBG was found around 9-42 nm in soft X-ray region. The photonic band-gap was predicted to be dependent on incident direction, pore size and lattice constant. The mesoporous materials with different pore sizes and different lattice constants have different band-gap widths.

  3. Origin of multiple band gap values in single width nanoribbons

    Science.gov (United States)

    Goyal, Deepika; Kumar, Shailesh; Shukla, Alok; Kumar, Rakesh

    2016-11-01

    Deterministic band gap in quasi-one-dimensional nanoribbons is prerequisite for their integrated functionalities in high performance molecular-electronics based devices. However, multiple band gaps commonly observed in graphene nanoribbons of the same width, fabricated in same slot of experiments, remain unresolved, and raise a critical concern over scalable production of pristine and/or hetero-structure nanoribbons with deterministic properties and functionalities for plethora of applications. Here, we show that a modification in the depth of potential wells in the periodic direction of a supercell on relative shifting of passivating atoms at the edges is the origin of multiple band gap values in nanoribbons of the same width in a crystallographic orientation, although they carry practically the same ground state energy. The results are similar when calculations are extended from planar graphene to buckled silicene nanoribbons. Thus, the findings facilitate tuning of the electronic properties of quasi-one-dimensional materials such as bio-molecular chains, organic and inorganic nanoribbons by performing edge engineering.

  4. Optical Dispersion Behavior and Band Gap Energy of Relaxor Ferroelectric 0.92Pb(Mg1/3Nb2/3)O3-0.08PbTiO3 Single Crystal

    Institute of Scientific and Technical Information of China (English)

    LIN Yan-Ting; REN Bo; ZHAO Xiang-Yong; WANG Fei-Fei; WANG Yao-Jin; XU Hai-Qing; LIN Di; LUO Hao-Su

    2009-01-01

    Refractive indices and extinction coefficients of 0.92Pb(Mg1/3 Nb2/a )O3-0.08Pb Ti03 (PMN-0.08PT) single crystal are investigated by variable angle spectroscopic ellipsometry (VASE) at different wavelengths. The parameters relative to the energy band structure are obtained by fitting to the single-oscillator dispersion equation, and the band gap energy is also deduced from the Tauc equation. Similar to most oxygen-octahedra ferroelectrics,PMN-0.08PT has the same dispersion behavior described by the refractive-index dispersion parameters.

  5. A model for the band gap energy of the N-rich GaN{sub 1-x}As{sub x} (0

    Energy Technology Data Exchange (ETDEWEB)

    Zhao Chuanzhen, E-mail: as3262001@yahoo.com.cn [School of Electronics and Information Engineering, Tianjin Polytechnic University Tianjin 300387 (China); Li Nana [School of Textiles, Tianjin Polytechnic University, Tianjin 300387 (China); Key Laboratory of Advanced Textile Composites, Ministry of Education of China, Tianjin 300387 (China); Wei Tong [College of Science, Civil Aviation University of China, Tianjin 300300 (China); Wang Shasha; Lu Keqing [School of Electronics and Information Engineering, Tianjin Polytechnic University Tianjin 300387 (China); Xie Zili [Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, Department of Electronic Science and Engineering, Nanjing University, Nanjing 210093 (China)

    2012-12-15

    In this paper, a model for the band gap energy of the N-rich GaNAs (0band gap energy. It is found that the band gap evolution of the N-rich energy is different from that of the As-rich band gap energy. The model may be used to calculate the band gap energy of other dilute group III-N-V nitrides.

  6. The band gap and band offset in ultrathin oxide-semiconductor heterostructures

    Science.gov (United States)

    Schmeißer, D.; Henkel, K.; Bergholz, M.; Tallarida, M.

    2010-03-01

    In ultrathin high- k oxide layers knowledge of the band line up and band gap is essential for modeling the transport properties and to learn about a device's long term stability and reliability. However, such data are hard to determine in such ultrathin layers and usually are extrapolated from values for bulk samples or are taken from the literature. In our in situ approach we use electron energy loss spectroscopy, valence band photoelectron spectroscopy, X-ray absorption spectroscopy, and resonant inelastic X-ray scattering to obtain the loss function and the valence and conduction band densities of states. From such data we derive the values of the band offsets and of the band gap. We discuss the ability of this combination of different techniques for the analysis of such complex ultrathin dielectric systems and discuss in detail the properties of the native oxide in SiO 2/Si(001) and SiO 2/3C-SiC(001).

  7. Narrow Band Gap Lead Sulfide Hole Transport Layers for Quantum Dot Photovoltaics.

    Science.gov (United States)

    Zhang, Nanlin; Neo, Darren C J; Tazawa, Yujiro; Li, Xiuting; Assender, Hazel E; Compton, Richard G; Watt, Andrew A R

    2016-08-24

    The band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies of up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band gap CQDs with different ligands, we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band gap QDs, causing an upshift of valence band position due to 1,2-ethanedithiol (EDT) ligands and a lowering of the Fermi level due to oxidation.

  8. Designer disordered materials with large complete photonic band gaps

    CERN Document Server

    Florescu, Marian; Steinhardt, Paul J; 10.1073/pnas.0907744106

    2010-01-01

    We present designs of 2D isotropic, disordered photonic materials of arbitrary size with complete band gaps blocking all directions and polarizations. The designs with the largest gaps are obtained by a constrained optimization method that starts from a hyperuniform disordered point pattern, an array of points whose number variance within a spherical sampling window grows more slowly than the volume. We argue that hyperuniformity, combined with uniform local topology and short-range geometric order, can explain how complete photonic band gaps are possible without long-range translational order. We note the ramifications for electronic and phononic band gaps in disordered materials.

  9. Strain modulated band gap of edge passivated armchair graphene nanoribbons

    CERN Document Server

    Peng, Xihong

    2011-01-01

    First principles calculations were performed to study strain effects on band gap of armchair graphene nanoribbons (AGNRs)with different edge passivation, including H, O, and OH group. The band gap of the H-passivated AGNRs shows a nearly periodic zigzag variation under strain. For O and OH passivation, the zigzag patterns are significantly shifted by a modified quantum confinement due to the edges. In addition, the band gap of the O-passivated AGNRs experiences a direct-to-indirect transition with sufficient tensile strain (~5%). The indirect gap reduces to zero with further increased strain.

  10. Widely tunable band gaps of graphdiyne: an ab initio study.

    Science.gov (United States)

    Koo, Jahyun; Park, Minwoo; Hwang, Seunghyun; Huang, Bing; Jang, Byungryul; Kwon, Yongkyung; Lee, Hoonkyung

    2014-05-21

    Functionalization of graphdiyne, a two-dimensional atomic layer of sp-sp(2) hybrid carbon networks, was investigated through first-principles calculations. Hydrogen or halogen atoms preferentially adsorb on sp-bonded carbon atoms rather than on sp(2)-bonded carbon atoms, forming sp(2)- or sp(3)-hybridization. The energy band gap of graphdiyne is increased from ~0.5 eV to ~5.2 eV through the hydrogenation or halogenation. Unlike graphene, segregation of adsorbing atoms is energetically unfavourable. Our results show that hydrogenation or halogenation can be utilized for modifying the electronic properties of graphdiyne for applications to nano-electronics and -photonics.

  11. Compositional dependence of the band gap in Ga(NAsP) quantum well heterostructures

    Energy Technology Data Exchange (ETDEWEB)

    Jandieri, K., E-mail: kakhaber.jandieri@physik.uni-marburg.de; Ludewig, P.; Wegele, T.; Beyer, A.; Kunert, B.; Springer, P.; Baranovskii, S. D.; Koch, S. W.; Volz, K.; Stolz, W. [Materials Science Center and Faculty of Physics, Philipps-University Marburg, Marburg (Germany)

    2015-08-14

    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.

  12. Group IV direct band gap photonics: Methods, Challenges and Opportunities

    Directory of Open Access Journals (Sweden)

    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.

  13. Group IV direct band gap photonics: Methods, Challenges and Opportunities

    Science.gov (United States)

    Geiger, Richard; Zabel, Thomas; Sigg, Hans

    2015-07-01

    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.

  14. Band Gaps of an Amorphous Photonic Materials

    Institute of Scientific and Technical Information of China (English)

    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.

  15. Broadening of effective photonic band gaps in biological chiral structures: From intrinsic narrow band gaps to broad band reflection spectra

    Science.gov (United States)

    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.

  16. Optimum design of band-gap beam structures

    DEFF Research Database (Denmark)

    Olhoff, Niels; Niu, Bin; Cheng, Gengdong

    2012-01-01

    -sectional area. To study the band-gap for travelling waves, a repeated inner segment of the optimized beams is analyzed using Floquet theory and the waveguide finite element (WFE) method. Finally, the frequency response is computed for the optimized beams when these are subjected to an external time......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...

  17. Photonic band gap engineering in 2D photonic crystals

    Indian Academy of Sciences (India)

    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.

  18. Band-gap narrowing in heavily doped silicon at 20 and 300 K studied by photoluminescence

    Science.gov (United States)

    Wagner, Joachim

    1985-07-01

    The band-gap shrinkage in heavily doped n- and p-type silicon is studied by photoluminescence both at low temperatures (20 K) and at room temperature (300 K). A line-shape analysis was performed to determine the indirect band-gap energy from the emission spectra. Within the experimental accuracy the same band-gap shift is observed at room temperature as at low temperature. The present results are compared with experimental data from other optical studies and with theoretical calculations.

  19. Analysis of two-dimensional photonic band gap structure with a rhombus lattice

    Institute of Scientific and Technical Information of China (English)

    Limei Qi; Ziqiang Yang; Xi Gao; Zheng Liang

    2008-01-01

    @@ The relative band gap for a rhombus lattice photonic crystal is studied by plane wave expansion method and high frequency structure simulator (HFSS) simulation. General wave vectors in the first Briliouin zone are derived. The relative band gap as a function of air-filling factor and background material is investigated, respectively, and the nature of photonic band gap for different lattice angles is analyzed by the distribution of electric energy. These results would provide theoretical instruction for designing optical integrated devices using photonic crystal with a rhombus lattice.

  20. Band Gap Properties of Magnetoelectroelastic Grid Structures with Initial Stress

    Institute of Scientific and Technical Information of China (English)

    WANG Yi-Ze; LI Feng-Ming

    2012-01-01

    The propagation of elastic waves in magnetoelectroelastic grid structures is studied.Band gap properties are presented and the effects of the magnetoelectroelastic coupling and initial stress are considered. Numerical calculations are performed using the plane-wave expansion method.The results show that the band gap width can be tuned by the initial stress.It is hoped that our results will be helpful for designing acoustic filters with magnetoelectroelastic materials and grid structures.

  1. Amorphous Photonic Lattices: Band Gaps, Effective Mass and Suppressed Transport

    OpenAIRE

    Rechtsman, Mikael; Szameit, Alexander; 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 localiz...

  2. Maximizing the Optical Band Gap in 2D Photonic Crystals

    DEFF Research Database (Denmark)

    Hougaard, Kristian G.; Sigmund, Ole

    Topology optimization is used to find the 2D photonic crystal designs with the largest relative photonic band gaps. Starting points for the topology optimization are found with an exhaustive binary search on a low resolution grid.......Topology optimization is used to find the 2D photonic crystal designs with the largest relative photonic band gaps. Starting points for the topology optimization are found with an exhaustive binary search on a low resolution grid....

  3. Band-gap engineering of functional perovskites through quantum confinement and tunneling

    DEFF Research Database (Denmark)

    Castelli, Ivano Eligio; Pandey, Mohnish; Thygesen, Kristian Sommer

    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...

  4. Carrier concentration dependence of band gap shift in n-type ZnO:Al films

    Science.gov (United States)

    Lu, J. G.; Fujita, S.; Kawaharamura, T.; Nishinaka, H.; Kamada, Y.; Ohshima, T.; Ye, Z. Z.; Zeng, Y. J.; Zhang, Y. Z.; Zhu, L. P.; He, H. P.; Zhao, B. H.

    2007-04-01

    Al-doped ZnO (AZO) thin films have been prepared by mist chemical vapor deposition and magnetron sputtering. The band gap shift as a function of carrier concentration in n-type zinc oxide (ZnO) was systematically studied considering the available theoretical models. The shift in energy gap, evaluated from optical absorption spectra, did not depend on sample preparations; it was mainly related to the carrier concentrations and so intrinsic to AZO. The optical gap increased with the electron concentration approximately as ne2/3 for ne≤4.2×1019 cm-3, which could be fully interpreted by a modified Burstein-Moss (BM) shift with the nonparabolicity of the conduction band. A sudden decrease in energy gap occurred at 5.4-8.4×1019 cm-3, consistent with the Mott criterion for a semiconductor-metal transition. Above the critical values, the band gap increased again at a different rate, which was presumably due to the competing BM band-filling and band gap renormalization effects, the former inducing a band gap widening and the latter an offsetting narrowing. The band gap narrowing (ΔEBGN) derived from the band gap renormalization effect did not show a good ne1/3 dependence predicated by a weakly interacting electron-gas model, but it was in excellent agreement with a perturbation theory considering different many-body effects. Based on this theory a simple expression, ΔEBGN=Ane1/3+Bne1/4+Cne1/2, was deduced for n-type ZnO, as well as p-type ZnO, with detailed values of A, B, and C coefficients. An empirical relation once proposed for heavily doped Si could also be used to describe well this gap narrowing in AZO.

  5. Hypersonic modulation of light in three-dimensional photonic and phononic band-gap materials.

    Science.gov (United States)

    Akimov, A V; Tanaka, Y; Pevtsov, A B; Kaplan, S F; Golubev, V G; Tamura, S; Yakovlev, D R; Bayer, M

    2008-07-18

    The elastic coupling between the a-SiO2 spheres composing opal films brings forth three-dimensional periodic structures which besides a photonic stop band are predicted to also exhibit complete phononic band gaps. The influence of elastic crystal vibrations on the photonic band structure has been studied by injection of coherent hypersonic wave packets generated in a metal transducer by subpicosecond laser pulses. These studies show that light with energies close to the photonic band gap can be efficiently modulated by hypersonic waves.

  6. Reducing support loss in micromechanical ring resonators using phononic band-gap structures

    Science.gov (United States)

    Hsu, Feng-Chia; Hsu, Jin-Chen; Huang, Tsun-Che; Wang, Chin-Hung; Chang, Pin

    2011-09-01

    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.

  7. Reducing support loss in micromechanical ring resonators using phononic band-gap structures

    Energy Technology Data Exchange (ETDEWEB)

    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.

  8. Photonic band gap of 2D complex lattice photonic crystal

    Institute of Scientific and Technical Information of China (English)

    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%.

  9. Multiband Terahertz Photonic Band Gaps of Subwavelength Planar Fractals

    Institute of Scientific and Technical Information of China (English)

    ZHAO Guo-Zhong; TIAN Yan; SUN Hong-Qi; ZHANG Cun-Lin; YANG Guo-Zhen

    2006-01-01

    Optical transmission properties of subwavelength planar fractals in terahertz (THz) frequency regime are studied by means of time-domain spectroscopy. The transmission spectra with multiple pass bands and stop bands are observed. The tunable photonic band gaps are realized by changing the angle between the principle axis of planar fractal and the polarization of THz wave. The possible application of the subwavelength optical component is discussed. We attribute the detected transmittance from subwavelength fractals to localized resonances.

  10. Band Gap Engineering of Two-Dimensional Nitrogene

    Science.gov (United States)

    Li, Jie-Sen; Wang, Wei-Liang; Yao, Dao-Xin

    2016-01-01

    In our previous study, we have predicted the novel two-dimensional honeycomb monolayers of pnictogen. In particular, the structure and properties of the honeycomb monolayer of nitrogen, which we call nitrogene, are very unusual. In this paper, we make an in-depth investigation of its electronic structure. We find that the band structure of nitrogene can be engineered in several ways: controlling the stacking of monolayers, application of biaxial tensile strain, and application of perpendicular electric field. The band gap of nitrogene is found to decrease with the increasing number of layers. The perpendicular electric field can also reduce the band gap when it is larger than 0.18 V/Å, and the gap closes at 0.35 V/Å. A nearly linear dependence of the gap on the electric field is found during the process. Application of biaxial strain can decrease the band gap as well, and eventually closes the gap. After the gap-closing, we find six inequivalent Dirac points in the Brillouin zone under the strain between 17% and 28%, and the nitrogene monolayer becomes a Dirac semimetal. These findings suggest that the electronic structure of nitrogene can be modified by several techniques, which makes it a promising candidate for electronic devices. PMID:27680297

  11. Band Gap Tuning of Armchair Graphene Nanoribbons by Using Antidotes

    Science.gov (United States)

    Zoghi, Milad; Goharrizi, Arash Yazdanpanah; Saremi, Mehdi

    2017-01-01

    The electronic properties of armchair graphene nanoribbons (AGNRs) can be changed by creating antidotes within the pristine ribbons and producing antidote super lattice AGNRs (ASL-AGNRs). In the present work, band gap tuning of ASL-AGNRs is investigated by varying the width of ribbons ( d W) and the distance between antidotes ( d L) for five different antidote topologies. Numerical tight-binding model is applied to obtain the band structure of the ribbons. Based on our results, it is found that the band gap of ASL-AGNRs can be increased or decreased in different cases. Furthermore, changing the width of ribbons generally results in more predictable␣band gap profiles compared to the variation of distance between antidotes. Consequently, by opting appropriate antidote topologies and dimensional parameters ( d W and d L), it is possible to gain a desired band gap size. This can be considered as an alternative solution in design of electronic and optoelectronic devices where tunable band gap values are needed.

  12. Nonideal anion displacement, band gap variation, and valence band splitting in Cu-In-Se compounds

    Energy Technology Data Exchange (ETDEWEB)

    Reena Philip, Rachel [Solid State Physics Laboratory, Department of Physics, Cochin University of Science and Technology, Kochi-682022 Kerala (India)]. E-mail: reenatara@cusat.ac.in; Pradeep, B. [Solid State Physics Laboratory, Department of Physics, Cochin University of Science and Technology, Kochi-682022 Kerala (India)

    2005-01-24

    Polycrystalline thin films of ternary chalcopyrite CuInSe{sub 2} and defect compounds CuIn{sub 3}Se{sub 5} and CuIn{sub 5}Se{sub 8} are prepared in vacuum by three-source coevaporation method. Structural and optical characterizations of the films are done using X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDAX), and optical absorbance spectra measurements. With variation in the composition of CuInSe{sub 2}, a change over from p-type to n-type conductivity is observed (as noted by the hot probe method). The deformation parameters and the anion displacements are calculated from the X-ray diffraction data, and the cation-anion bond lengths are deduced. The dependence of band gap variation on nonideal anion displacement in the ternary compounds and the effect of Se-p-Cu-d repulsion on band gap are studied. The threefold optical structure observed in the fundamental absorption region of the absorption spectra is analysed to extract the valence band splitting parameters. Hopfields quasi-cubic model adapted for chalcopyrites with tetragonal deformation is used to determine the crystal field splittings and spin orbit splittings, and the linear hybridization model is used to calculate the percentage of d-orbital and p-orbital contribution to hybridization in the compounds under consideration.

  13. Quantum electrodynamics near a photonic band-gap

    Science.gov (United States)

    Liu, Yanbing; Houck, Andrew

    Quantum electrodynamics predicts the localization of light around an atom in photonic band-gap (PBG) medium or photonic crystal. Here we report the first experimental realization of the strong coupling between a single artificial atom and an one dimensional PBG medium using superconducting circuits. In the photonic transport measurement, we observe an anomalous Lamb shift and a large band-edge avoided crossing when the artificial atom frequency is tuned across the band-edge. The persistent peak within the band-gap indicates the single photon bound state. Furthermore, we study the resonance fluorescence of this bound state, again demonstrating the breakdown of the Born-Markov approximation near the band-edge. This novel architecture can be directly generalized to study many-body quantum electrodynamics and to construct more complicated spin chain models.

  14. Band gap engineering of N-alloyed Ga2O3 thin films

    Directory of Open Access Journals (Sweden)

    Dongyu Song

    2016-06-01

    Full Text Available The authors report the tuning of band gap of GaON ternary alloy in a wide range of 2.75 eV. The samples were prepared by a two-step nitridation method. First, the samples were deposited on 2-inch fused silica substrates by megnetron sputtering with NH3 and Ar gas for 60 minutes. Then they were annealed in NH3 ambience at different temperatures. The optical band gap energies are calculated from transmittance measurements. With the increase of nitridation temperature, the band gap gradually decreases from 4.8 eV to 2.05 eV. X-ray diffraction results indicate that as-deposited amorphous samples can crystallize into monoclinic and hexagonal structures after they were annealed in oxygen or ammonia ambience, respectively. The narrowing of the band gap is attributed to the enhanced repulsion of N2p -Ga3d orbits and formation of hexagonal structure.

  15. Modeling of Photonic Band Gap Crystals and Applications

    Energy Technology Data Exchange (ETDEWEB)

    El-Kady, Ihab Fathy [Iowa State Univ., Ames, IA (United States)

    2002-01-01

    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

  16. Tuning the band gap in hybrid tin iodide perovskite semiconductors using structural templating.

    Science.gov (United States)

    Knutson, Jeremy L; Martin, James D; Mitzi, David B

    2005-06-27

    Structural distortions within the extensive family of organic/inorganic hybrid tin iodide perovskite semiconductors are correlated with their experimental exciton energies and calculated band gaps. The extent of the in- and out-of-plane angular distortion of the SnI4(2-) perovskite sheets is largely determined by the relative charge density and steric requirements of the organic cations. Variation of the in-plane Sn-I-Sn bond angle was demonstrated to have the greatest impact on the tuning of the band gap, and the equatorial Sn-I bond distances have a significant secondary influence. Extended Hückel tight-binding band calculations are employed to decipher the crystal orbital origins of the structural effects that fine-tune the band structure. The calculations suggest that it may be possible to tune the band gap by as much as 1 eV using the templating influence of the organic cation.

  17. Band-gap and sub-band-gap photoelectrochemical processes at nanocrystalline CdS grown on ZnO by successive ionic layer adsorption and reaction method

    Energy Technology Data Exchange (ETDEWEB)

    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.

  18. Microscopic theoretical model study of band gap opening in AA-stacked bi-layer graphene

    Science.gov (United States)

    Sahu, Sivabrata; Parashar, S. K. S.; Rout, G. C.

    2016-05-01

    We address here a tight-binding theoretical model calculation for AA-stacked bi-layer graphene taking into account of a biased potential between two layers to study the density of states and the band dispersion within the total Brillouin zone. We have calculated the electronic Green's function for electron operator corresponding to A and B sub lattices by Zubarev's Green's function technique from which the electronic density of states and the electron band energy dispersion are calculated. The numerically computed density of states and band energy dispersions are investigated by tuning the biased potential to exhibit the band gap by varying the different physical parameters.

  19. Spectrophotometric method for optical band gap and electronic transitions determination of semiconductor materials

    Science.gov (United States)

    Sangiorgi, Nicola; Aversa, Lucrezia; Tatti, Roberta; Verucchi, Roberto; Sanson, Alessandra

    2017-02-01

    The optical band gap energy and the electronic processes involved are important parameters of a semiconductor material and it is therefore important to determine their correct values. Among the possible methods, the spectrophotometric is one of the most common. Several methods can be applied to determine the optical band gap energy and still now a defined consensus on the most suitable one has not been established. A highly diffused and accurate optical method is based on Tauc relationship, however to apply this equation is necessary to know the nature of the electronic transitions involved commonly related to the coefficient n. For this purpose, a spectrophotometric technique was used and we developed a graphical method for electronic transitions and band gap energy determination for samples in powder form. In particular, the n coefficient of Tauc equation was determined thorough mathematical elaboration of experimental results on TiO2 (anatase), ZnO, and SnO2. The results were used to calculate the band gap energy values and then compared with the information obtained by Ultraviolet Photoelectron Spectroscopy (UPS). This approach provides a quick and accurate method for band gap determination through n coefficient calculation. Moreover, this simple but reliable method can be used to evaluate the nature of electronic transition that occurs in a semiconductor material in powder form.

  20. Uniaxially stressed germanium with fundamental direct band gap

    OpenAIRE

    Geiger, R.; Zabel, T.; Marin, E; Gassenq, A.; Hartmann, J.-M.; Widiez, J.; Escalante, J.; Guilloy, K.; Pauc, N.; Rouchon, D.; Diaz, G. Osvaldo; Tardif, S; Rieutord, F.; Duchemin, I.; Niquet, Y. -M.

    2015-01-01

    We demonstrate the crossover from indirect- to direct band gap in tensile-strained germanium by temperature-dependent photoluminescence. The samples are strained microbridges that enhance a biaxial strain of 0.16% up to 3.6% uniaxial tensile strain. Cooling the bridges to 20 K increases the uniaxial strain up to a maximum of 5.4%. Temperature-dependent photoluminescence reveals the crossover to a fundamental direct band gap to occur between 4.0% and 4.5%. Our data are in good agreement with n...

  1. Wide band gap carbon allotropes: Inspired by zeolite-nets

    Science.gov (United States)

    Wei, Zhi-Jing; Zhao, Hui-Yan; Wang, Jing; Liu, Ying

    2016-10-01

    Based on the topologies proposed for zeolites, six metastable semiconductor carbon allotropes with band gaps of 2.72-3.89 eV are predicted using ab initio density functional calculations. The hardnesses of these allotropes are about 90%-94% that of diamond, indicating that they may be superhard materials. We also present simulated X-ray diffraction spectra of these new carbon allotropes to provide a basis for possible experimental observations and synthesis. These new carbon structures with a range of band gaps and with hardnesses comparable to diamond could be potential targets for the synthesis of hard and transparent materials.

  2. Amorphous Photonic Lattices: Band Gaps, Effective Mass and Suppressed Transport

    CERN Document Server

    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.

  3. Absolute band gaps in two-dimensional graphite photonic crystal

    Institute of Scientific and Technical Information of China (English)

    Gaoxin Qiu(仇高新); Fanglei Lin(林芳蕾); Hua Wang(王华); Yongping Li(李永平)

    2003-01-01

    The off-plane propagation of electromagnetic (EM) waves in a two-dimensional (2D) graphite photoniccrystal structure was studied using transfer matrix method. Transmission spectra calculations indicatethat such a 2D structure has a common band gap from 0.202 to 0.2035 c/a for both H and E polarizationsand for all off-plane angles form 0° up to 90°. The presence of such an absolute band gap implies that 2Dgraphite photonic crystal, which is much easier and more feasible to fabricate, can exhibit some propertiesof a three-dimensional (3D) photonic crystal.

  4. Large area modules based on low band gap polymers

    DEFF Research Database (Denmark)

    Bundgaard, Eva; Krebs, Frederik C

    2010-01-01

    The use of three low band gap polymers in large area roll-to-roll coated modules is demonstrated. The polymers were prepared by a Stille cross coupling polymerization and all had a band gap around 1.6 eV. The polymers were first tested in small area organic photovoltaic devices which showed...... efficiencies from 0.4 to 2 %. Then large area roll-to-roll coated modules were processed and these showed efficiencies up to 0.6 %. It is clear that further study is necessary before this type of polymer is competitive with P3HT in large area modules....

  5. Band-gap narrowing in the space-charge region of heavily doped silicon diodes

    Science.gov (United States)

    Lowney, Jeremiah R.

    1985-02-01

    The densities of states of the valence and conduction bands have been calculated in the space-charge region of a heavily doped linearly graded p- n junction silicon diode. Both the donor and acceptor densities were chosen to be equal to 6.2 × 10 18 cm -3. The results showed the emergence of band tails which penetrated deeply into the energy gap and accounted for the band-gap narrowing observed in such a diode by analysis of capacitance vs voltage measurements of the built-in voltage.

  6. Electronic Band Structure and Sub-band-gap Absorption of Nitrogen Hyperdoped Silicon.

    Science.gov (United States)

    Zhu, Zhen; Shao, Hezhu; Dong, Xiao; Li, Ning; Ning, Bo-Yuan; Ning, Xi-Jing; Zhao, Li; Zhuang, Jun

    2015-05-27

    We investigated the atomic geometry, electronic band structure, and optical absorption of nitrogen hyperdoped silicon based on first-principles calculations. The results show that all the paired nitrogen defects we studied do not introduce intermediate band, while most of single nitrogen defects can introduce intermediate band in the gap. Considering the stability of the single defects and the rapid resolidification following the laser melting process in our sample preparation method, we conclude that the substitutional nitrogen defect, whose fraction was tiny and could be neglected before, should have considerable fraction in the hyperdoped silicon and results in the visible sub-band-gap absorption as observed in the experiment. Furthermore, our calculations show that the substitutional nitrogen defect has good stability, which could be one of the reasons why the sub-band-gap absorptance remains almost unchanged after annealing.

  7. Study of indium nitride and indium oxynitride band gaps

    Directory of Open Access Journals (Sweden)

    M. Sparvoli

    2013-01-01

    Full Text Available This work shows the study of the optical band gap of indium oxynitride (InNO and indium nitride (InN deposited by magnetron reactive sputtering. InNO shows multi-functionality in electrical and photonic applications, transparency in visible range, wide band gap, high resistivity and low leakage current. The deposition processes were performed in a magnetron sputtering system using a four-inches pure In (99.999% target and nitrogen and oxygen as plasma gases. The pressure was kept constant at 1.33 Pa and the RF power (13.56 MHz constant at 250 W. Three-inches diameter silicon wafer with 370 micrometer thickness and resistivity in the range of 10 ohm-centimeter was used as substrate. The thin films were analyzed by UV-Vis-NIR reflectance, photoluminescence (PL and Hall Effect. The band gap was obtained from Tauc analysis of the reflectance spectra and photoluminescence. The band gap was evaluated for both films: for InNO the value was 2.48 eV and for InN, 1.52 eV. The relative quantities obtained from RBS spectra analysis in InNO sample are 48% O, 12% N, 40% In and in InN sample are 8% O, 65% N, 27% In.

  8. Design for maximum band-gaps in beam structures

    DEFF Research Database (Denmark)

    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...

  9. Experimental studies of surface plasmon polariton band gap effect

    DEFF Research Database (Denmark)

    Volkov, V. S.; Bozhevolnyi, S. I.; Leosson, K.;

    2003-01-01

    the dependence of the SPP band gap (SPPBG) effect manifested via the SPP reflection and guiding (along line defects) on the parameters of the surface structures (period, filling factor and lattice orientation). We find that the SPPBG effect is stronger along &ggr;K direction for all investigated periodic...

  10. Analysis of photonic band-gap structures in stratified medium

    DEFF Research Database (Denmark)

    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...

  11. Strain sensitivity of band gaps of Sn-containing semiconductors

    DEFF Research Database (Denmark)

    Li, Hong; Castelli, Ivano Eligio; Thygesen, Kristian Sommer

    2015-01-01

    Tuning of band gaps of semiconductors is a way to optimize materials for applications within photovoltaics or as photocatalysts. One way to achieve this is through applying strain to the materials. We investigate the effect of strain on a range of Sn-containing semiconductors using density...

  12. Topological Design of Cellular Phononic Band Gap Crystals

    Directory of Open Access Journals (Sweden)

    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.

  13. Lewis acid adducts of narrow band gap conjugated polymers.

    Science.gov (United States)

    Welch, Gregory C; Bazan, Guillermo C

    2011-03-30

    We report on the interaction of Lewis acids with narrow band gap conjugated copolymers containing donor and acceptor units. Examination of the widely used poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b']dithiophene)-2,6-(diyl-alt-benzo[2,1,3]thiadiazole)-4,7-diyl] (1) shows weaker binding with B(C(6)F(5))(3) when compared with a small molecule that contains a cyclopenta-[2,1-b:3,4-b']dithiophene (CDT) unit flanked by two benzo[2,1,3]thiadiazole (BT) fragments. Studies on model compounds representative of 1, together with a comparison between B(C(6)F(5))(3) and BBr(3), indicate that the propensity for Lewis acid coordination is decreased because of steric encumbrance surrounding the BT nitrogen sites. These observations led to the design of chromophores that incorporate an acceptor unit with a more basic nitrogen site, namely pyridal[2,1,3]thiadiazole (PT). That this strategy leads to a stronger B-N interaction was demonstrated through the examination of the reaction of B(C(6)F(5))(3) with two small molecules bis(4,4-bis(hexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-4,7-pyridal[2,1,3]thiadiazole (8) and bis{2-thienyl-(4,4-bis(hexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)}-4,7-pyridal[2,1,3]thiadiazole (9) and two polymer systems (poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b']dithiophene)-2,6-diyl-alt-([1,2,5]thiadiazolo[3,4-c]pyridine)-4,7-diyl] (10) and poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b']dithiophene)-2,6-diyl-alt-(4',7'-bis(2-thienyl)-[1,2,5]thiadiazolo[3,4-c]pyridine)-5,5-diyl] (11). From a materials perspective, it is worth pointing out that through the binding of B(C(6)F(5))(3), new NIR-absorbing polymers can be generated with band gaps from 1.31 to 0.89 eV. A combination of studies involving ultraviolet photoemission spectroscopy and density functional theory shows that the narrowing of the band gap upon borane coordination to the pyridal nitrogen on PT is a result of lowering the energies of both the highest occupied molecular

  14. Dielectric functions and energy band gap variation studies of manganese doped Bi{sub 3.25}La{sub 0.75}Ti{sub 3}O{sub 12} thin films using spectroscopic ellipsometry

    Energy Technology Data Exchange (ETDEWEB)

    Gautam, Prikshit, E-mail: pgautam.phy.du@gmail.com [Department of Physics and Astrophysics, University of Delhi (DU), Delhi 110007 (India); Department of Physics Kirori Mal College, University of Delhi, Delhi 110007 (India); Sachdeva, Anupama [Department of Physics and Astrophysics, University of Delhi (DU), Delhi 110007 (India); Singh, Sushil K. [Functional Materials Division, SSPL, Timarpur, New Delhi 110054 (India); Tandon, R.P., E-mail: ram_tandon@hotmail.com [Department of Physics and Astrophysics, University of Delhi (DU), Delhi 110007 (India)

    2014-12-25

    Highlights: • Mn Doped Bi{sub 3.25}La{sub 0.75}Ti{sub 3}O{sub 12} (BLT) thin films prepared by chemical solution deposition technique. • Raman spectroscopy of these films shows that Mn{sup 3+} is well substituted at Ti{sup 4+} site. • The optical properties of BLT and Mn modified BLT thin films were investigated by using spectroscopic ellipsometry. • A double Tauc–Lorentz (DTL) dispersion relation was successfully used to model the dielectric functions. • The direct optical band gap (Eg{sup d}) is found to decrease with increase in Mn content. - Abstract: Single phase polycrystalline Mn-modified Bi{sub 3.25}La{sub 0.75}Ti{sub 3}O{sub 12} (BLT) thin films were prepared by chemical solution deposition method using spin coating technique on Pt/Ti/SiO{sub 2}/Si (1 0 0) substrates. Raman spectroscopy of these films shows that Mn{sup 3+} is well substituted at Ti{sup 4+} site. The optical properties of BLT and Mn modified BLT thin films were investigated at room temperature by using spectroscopic ellipsometry (SE) in the energy range 0.72–6.2 eV. A double Tauc–Lorentz (DTL) dispersion relation was successfully used to model the dielectric functions of these films where a shift to the lower energy side with Mn doping is seen. The full width at half maxima (FWHM) (Γ) of dielectric function is found to increase with Mn doping. This increase in FWHM may be attributed to the increase in the trap density in forbidden band which consequently decreases the value of direct optical band gap (Eg{sup d}). The direct optical band gap (Eg{sup d}) is found to decrease with increase in Mn content in the studied composition range. This decrease in Eg{sup d} with doping may be attributed to the variation in the defect concentration present in the structure.

  15. Low Loss Plastic Terahertz Photonic Band-Gap Fibres

    Institute of Scientific and Technical Information of China (English)

    GENG You-Fu; TAN Xiao-Ling; ZHONG Kai; WANG Peng; YAO Jian-Quan

    2008-01-01

    We report a numerical investigation on terahertz wave propagation in plastic photonic band-gap fibres which are characterized by a 19-unit-cell air core and hexagonal air holes with rounded corners in cladding. Using the finite element method, the leakage loss and absorption loss are calculated and the transmission properties are analysed.The lowest loss of 0.268 dB/m is obtained. Numerical results show that the fibres could liberate the constraints of background materials beyond the transparency region in terahertz wave band, and efficiently minimize the effect of absorption by background materials, which present great advantage of plastic photonic band-gap fibres in long distance terahertz delivery.

  16. Synthesis of copper quantum dots by chemical reduction method and tailoring of its band gap

    Directory of Open Access Journals (Sweden)

    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.

  17. Experimental Work With Photonic Band Gap Fiber: Building A Laser Electron Accelerator

    Energy Technology Data Exchange (ETDEWEB)

    Lincoln, Melissa; Ischebeck, Rasmus; Nobel, Robert; Siemann, Robert; /SLAC

    2006-09-29

    In the laser acceleration project E-163 at the Stanford Linear Accelerator Center, work is being done toward building a traveling wave accelerator that uses as its accelerating structure a length of photonic band gap fiber. The small scale of the optical fiber allows radiation at optical wavelengths to be used to provide the necessary accelerating energy. Optical wavelength driving energy in a small structure yields higher accelerating fields. The existence of a speed-of-light accelerating mode in a photonic band gap fiber has been calculated previously [1]. This paper presents an overview of several of the experimental challenges posed in the development of the proposed photonic band gap fiber accelerator system.

  18. Robust band gap and half-metallicity in graphene with triangular perforations

    Science.gov (United States)

    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.

  19. Synthesis and characterization of a mixture of CoFe2O4 and MgFe2O4 from layered double hydroxides: Band gap energy and magnetic responses

    Science.gov (United States)

    Agú, Ulises A.; Oliva, Marcos I.; Marchetti, Sergio G.; Heredia, Angélica C.; Casuscelli, Sandra G.; Crivello, Mónica E.

    2014-11-01

    A mixture of nanocrystals of cobalt ferrite and magnesium ferrite was obtained from Layered Double Hydroxides (LDH) through a co-precitation method with a theoretical molar ratio M2+:Fe3+=3:1, where M2+represents Mg2+ and/or Co2+. The molar ratios between Co2+:Fe3+ were 0.0 (0Co), 0.2 (5Co), and 0.4 (10Co). In order to assess the effect on the properties of the LDH and their oxides, the molar percentages were 0, 5 and 10%. Two different synthesis methods were evaluated; (i) ageing at room temperature (rt), and (ii) hydrothermal ageing at 200 °C in autoclave (ht), both methods needed 15 h of ageing. Then, these LDH were calcined in air atmosphere at 550 °C for 10 h. The calcined materials were characterized by X-ray diffraction (XRD), thermogravymetric analysis (TGA), temperature-programmed reduction (TPR), infrared spectroscopy with Fourier transform (FTIR), Diffuse Reflectance UV-visible spectroscopy (UV-vis-DRS), Mössbauer spectroscopy and inductively coupled plasma optical emission spectroscopy (ICP-OES). The magnetic response was analyzed using a vibrating sample magnetometer (VSM). The band gap energy of the iron oxides was determined through the UV-vis-DRS analysis. Through these studies it was possible to identify the presence of a mixture of cobalt ferrite and magnesium ferrite. Samples did not show hematite and cobalt oxides, but the presence of MgO in the periclase phase was determined. This magnesium oxide promoted a good dispersion of the ferrites. Moreover, when a single ferrite phase of Co or Mg was formed, a diminution of the crystal size with consequent enlarged values of band gap energy was observed. Thus, materials synthesized by room temperature ageing promoted the superparamagnetic behaviour of samples, attributed to the content of the cobalt ferrite structure in nanocrystals. In regard to the estimated band gap energy, all samples exhibited low levels. These results indicate that these solids would be suitable for photocatalysts use in all

  20. Slow light and band gaps in metallodielectric cylinder arrays.

    Science.gov (United States)

    Shainline, Jeffrey M; Xu, Jimmy

    2009-05-25

    We consider two-dimensional three-component photonic crystals wherein one component is modeled as a drude-dispersive metal. It is found that the dispersion relation of light in this environment depends critically on the configuration of the metallic and dielectric components. In particular, for the case of an incident electromagnetic wave with electric field vector parallel to the axis of the cylinders it is shown that the presence of dielectric shells covering the metallic cylinders leads to a closing of the structural band gap with increased filling factor, as would be expected for a purely dielectric photonic crystal. For the same polarization, the photonic band structure of an array of metallic shell cylinders with dielectric cores do not show the closing of the structural band gap with increased filling factor of the metallic component. In this geometry, the photonic band structure contains bands with very small values of group velocity with some bands having a maximum of group velocity as small as .05c.

  1. The strain induced band gap modulation from narrow gap semiconductor to half-metal on Ti2CrGe: A first principles study

    Directory of Open Access Journals (Sweden)

    Jia Li

    2015-11-01

    Full Text Available The Heusler alloy Ti2CrGe is a stable L21 phase with antiferromagnetic ordering. With band-gap energy (∼ 0.18 eV obtained from a first-principles calculation, it belongs to the group of narrow band gap semiconductor. The band-gap energy decreases with increasing lattice compression and disappears until a strain of −5%; moreover, gap contraction only occurs in the spin-down states, leading to half-metallic character at the −5% strain. The Ti1, Ti2, and Cr moments all exhibit linear changes in behavior within strains of −5%– +5%. Nevertheless, the total zero moment is robust for these strains. The imaginary part of the dielectric function for both up and down spin states shows a clear onset energy, indicating a corresponding electronic gap for the two spin channels.

  2. Anomalous Temperature Dependence of the Band Gap in Black Phosphorus.

    Science.gov (United States)

    Villegas, Cesar E P; Rocha, A R; Marini, Andrea

    2016-08-10

    Black phosphorus (BP) has gained renewed attention due to its singular anisotropic electronic and optical properties that might be exploited for a wide range of technological applications. In this respect, the thermal properties are particularly important both to predict its room temperature operation and to determine its thermoelectric potential. From this point of view, one of the most spectacular and poorly understood phenomena is indeed the BP temperature-induced band gap opening; when temperature is increased, the fundamental band gap increases instead of decreases. This anomalous thermal dependence has also been observed recently in its monolayer counterpart. In this work, based on ab initio calculations, we present an explanation for this long known and yet not fully explained effect. We show that it arises from a combination of harmonic and lattice thermal expansion contributions, which are in fact highly interwined. We clearly narrow down the mechanisms that cause this gap opening by identifying the peculiar atomic vibrations that drive the anomaly. The final picture we give explains both the BP anomalous band gap opening and the frequency increase with increasing volume (tension effect).

  3. Hollow-Core Photonic Band Gap Fibers for Particle Acceleration

    Energy Technology Data Exchange (ETDEWEB)

    Noble, Robert J.; Spencer, James E.; /SLAC; Kuhlmey, Boris T.; /Sydney U.

    2011-08-19

    Photonic band gap (PBG) dielectric fibers with hollow cores are being studied both theoretically and experimentally for use as laser driven accelerator structures. The hollow core functions as both a longitudinal waveguide for the transverse-magnetic (TM) accelerating fields and a channel for the charged particles. The dielectric surrounding the core is permeated by a periodic array of smaller holes to confine the mode, forming a photonic crystal fiber in which modes exist in frequency pass-bands, separated by band gaps. The hollow core acts as a defect which breaks the crystal symmetry, and so-called defect, or trapped modes having frequencies in the band gap will only propagate near the defect. We describe the design of 2-D hollow-core PBG fibers to support TM defect modes with high longitudinal fields and high characteristic impedance. Using as-built dimensions of industrially-made fibers, we perform a simulation analysis of the first prototype PBG fibers specifically designed to support speed-of-light TM modes.

  4. Quasiparticle self-consistent GW theory of III-V nitride semiconductors: Bands, gap bowing, and effective masses

    DEFF Research Database (Denmark)

    Svane, Axel; Christensen, Niels Egede; Gorczyca, I.

    2010-01-01

    on the basis of the local approximation to density functional theory, although generally overestimated by 0.2–0.3 eV in comparison with experimental gap values. Details of the electronic energies and the effective masses including their pressure dependence are compared with available experimental information....... The band gap of InGaN2 is considerably smaller than what would be expected by linear interpolation implying a significant band gap bowing in InGaN alloys....

  5. Germanium under high tensile stress: nonlinear dependence of direct band gap vs. strain

    OpenAIRE

    Guilloy, K.; Pauc, N.; Gassenq, A.; Niquet, Y. M.; Escalante, J. M.; Duchemin, I.; Tardif, S; Dias, G. Osvaldo; Rouchon, D.; Widiez, J.; Hartmann, J.M.; Geiger, R.; Zabel, T.; Sigg, H; Faist, J.

    2016-01-01

    Germanium is a strong candidate as a laser source for silicon photonics. It is widely accepted that the band structure of germanium can be altered by tensile strain so as to reduce the energy difference between its direct and indirect band gaps. However, the conventional deformation potential model most widely adopted to describe this transformation happens to have been investigated only up to 1 % uniaxially loaded strains. In this work, we use a micro-bridge geometry to uniaxially stress ger...

  6. Single-Crystal Semiconductors with Narrow Band Gaps for Solar Water Splitting.

    Science.gov (United States)

    Wang, Tuo; Gong, Jinlong

    2015-09-07

    Solar water splitting provides a clean and renewable approach to produce hydrogen energy. In recent years, single-crystal semiconductors such as Si and InP with narrow band gaps have demonstrated excellent performance to drive the half reactions of water splitting through visible light due to their suitable band gaps and low bulk recombination. This Minireview describes recent research advances that successfully overcome the primary obstacles in using these semiconductors as photoelectrodes, including photocorrosion, sluggish reaction kinetics, low photovoltage, and unfavorable planar substrate surface. Surface modification strategies, such as surface protection, cocatalyst loading, surface energetics tuning, and surface texturization are highlighted as the solutions.

  7. Crystal structure and band gap of AlGaAsN

    Science.gov (United States)

    Munich, D. P.; Pierret, R. F.

    1987-09-01

    Quantum dielectric theory is applied to the quaternary alloy Al xGa 1- xAs 1- yN y to predict its electronic properties as a function of Al and N mole fractions. Results are presented for the expected crystal structure, minimum electron energy band gap, and direction in k-space of the band gap minimum for all x and y values. The results suggest that, for a proper choice of x and y, Al xGa 1- xAs 1- yN y could exhibit certain advantages over Al xGa 1- xAs when utilized in field-effect transistor structures.

  8. Band gap and conductivity evaluation of carbon nanotube with hematite for green ammonia synthesis

    Science.gov (United States)

    Rehman, Zia Ur; Yahya, Noorhana; Shafie, A'fza; Soleimani, Hassan; Alqasim, Bilal Hassan; Irfan, Muhammad; Qureshi, Saima

    2016-11-01

    To understand the change in number of electrons, band gap and total energy in the catalyst simulation was performed using Cambridge Serial Total Energy Package (CASTEP). Two catalyst were taken into consideration namely carbon nanotubes (CNTs) and hematite adjacent with CNTs. The simulation based study of the adsorption of hydrogen and nitrogen with reference to change in number of electron and band-gap of carbon nano tubes and hematite mixed with carbon nanotubes was not reported in literature. For this reason carbon nanotubes band gap for different chirality and number of walls was calculated through simulation. After that simulation for number of electrons, band gap and average total energy of CNTs alone and a mixture hematite with CNTs was performed before and after adsorption of hydrogen and nitrogen. From simulation the number of electrons were found to be doubled for hematite mixed with CNTs and average total energy was also increased as compared to similar parameter for CNTs without hematite. In conclusion the hematite with carbon nanotubes is preferred candidate for ammonia synthesis using magnetic induction method. Ammonia synthesis was done using MIM. Ammonia yield was quantified by Kjaldal method.

  9. On acoustic band gaps in homogenized piezoelectric phononic materials

    Directory of Open Access Journals (Sweden)

    Rohan E.

    2010-07-01

    Full Text Available We consider a composite medium made of weakly piezoelectric inclusions periodically distributed in the matrix which ismade of a different piezoelectricmaterial. Themediumis subject to a periodic excitation with an incidence wave frequency independent of scale ε of the microscopic heterogeneities. Two-scale method of homogenization is applied to obtain the limit homogenized model which describes acoustic wave propagation in the piezoelectric medium when ε → 0. In analogy with the purely elastic composite, the resulting model allows existence of the acoustic band gaps. These are identified for certain frequency ranges whenever the so-called homogenized mass becomes negative. The homogenized model can be used for band gap prediction and for dispersion analysis for low wave numbers. Modeling such composite materials seems to be perspective in the context of Smart Materials design.

  10. Engineering the hypersonic phononic band gap of hybrid Bragg stacks.

    Science.gov (United States)

    Schneider, Dirk; Liaqat, Faroha; El Boudouti, El Houssaine; El Hassouani, Youssef; Djafari-Rouhani, Bahram; Tremel, Wolfgang; Butt, Hans-Jürgen; Fytas, George

    2012-06-13

    We report on the full control of phononic band diagrams for periodic stacks of alternating layers of poly(methyl methacrylate) and porous silica combining Brillouin light scattering spectroscopy and theoretical calculations. These structures exhibit large and robust on-axis band gaps determined by the longitudinal sound velocities, densities, and spacing ratio. A facile tuning of the gap width is realized at oblique incidence utilizing the vector nature of the elastic wave propagation. Off-axis propagation involves sagittal waves in the individual layers, allowing access to shear moduli at nanoscale. The full theoretical description discerns the most important features of the hypersonic one-dimensional crystals forward to a detailed understanding, a precondition to engineer dispersion relations in such structures.

  11. Band gap structure modification of amorphous anodic Al oxide film by Ti-alloying

    DEFF Research Database (Denmark)

    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 located in a TiO2 unit in the oxide layer, but rather in a mixed Ti-Al oxide layer. The optical band gap energy of the anodic oxide layers was determined by vacuum ultraviolet spectroscopy in the energy range from 4.1 to 9.2 eV (300–135 nm). The results indicate that amorphous anodic Al2O3 has a direct...

  12. Direct band gap silicon crystals predicted by an inverse design method

    Science.gov (United States)

    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).

  13. Band gap engineering of zinc selenide thin films through alloying with cadmium telluride.

    Science.gov (United States)

    Al-Kuhaili, M F; Kayani, A; Durrani, S M A; Bakhtiari, I A; Haider, M B

    2013-06-12

    This work investigates band gap engineering of zinc selenide (ZnSe) thin films. This was achieved by mixing ZnSe with cadmium telluride (CdTe). The mass ratio (x) of CdTe in the starting material was varied in the range x = 0-0.333. The films were prepared using thermal evaporation. The chemical composition of the films was investigated through energy dispersive spectroscopy and Rutherford backscattering spectrometry. Structural analysis was carried out using X-ray diffraction and atomic force microscopy. Normal incidence transmittance and reflectance were measured over the wavelength range 300-1300 nm. The absorption coefficients and band gaps were determined from these spectrophotometric measurements. The band gap monotonically decreased from 2.58 eV (for x = 0) to 1.75 eV (for x = 0.333). Photocurrent measurements indicated that the maximum current density was obtained for films with x = 0.286. A figure of merit, based on crystallinity, band gap, and photocurrent, was defined. The optimum characteristics were obtained for the films with x = 0.231, for which the band gap was 2.14 eV.

  14. Microscopic theory of photonic band gaps in optical lattices

    CERN Document Server

    Samoylova, M; Bachelard, R; Courteille, Ph W

    2013-01-01

    We propose a microscopic model to describe the scattering of light by atoms in optical lattices. The model is shown to efficiently capture Bragg scattering, spontaneous emission and photonic band gaps. A connection to the transfer matrix formalism is established in the limit of a one-dimensional optical lattice, and we find the two theories to yield results in good agreement. The advantage of the microscopic model is, however, that it suits better for studies of finite-size and disorder effects.

  15. One-dimensional photonic band gaps in optical lattices

    CERN Document Server

    Samoylova, Marina; Holynski, Michael; Courteille, Philippe Wilhelm; Bachelard, Romain

    2013-01-01

    The phenomenon of photonic band gaps in one-dimensional optical lattices is reviewed using a microscopic approach. Formally equivalent to the transfer matrix approach in the thermodynamic limit, a microscopic model is required to study finite-size effects, such as deviations from the Bragg condition. Microscopic models describing both scalar and vectorial light are proposed, as well as for two- and three-level atoms. Several analytical results are compared to experimental data, showing a good agreement.

  16. Band gap engineering in polymers through chemical doping and applied mechanical strain

    Science.gov (United States)

    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.

  17. Perovskite-perovskite tandem photovoltaics with optimized band gaps

    Science.gov (United States)

    Eperon, Giles E.; Leijtens, Tomas; Bush, Kevin A.; Prasanna, Rohit; Green, Thomas; Wang, Jacob Tse-Wei; McMeekin, David P.; Volonakis, George; Milot, Rebecca L.; May, Richard; Palmstrom, Axel; Slotcavage, Daniel J.; Belisle, Rebecca A.; Patel, Jay B.; Parrott, Elizabeth S.; Sutton, Rebecca J.; Ma, Wen; Moghadam, Farhad; Conings, Bert; Babayigit, Aslihan; Boyen, Hans-Gerd; Bent, Stacey; Giustino, Feliciano; Herz, Laura M.; Johnston, Michael B.; McGehee, Michael D.; Snaith, Henry J.

    2016-11-01

    We demonstrate four- and two-terminal perovskite-perovskite tandem solar cells with ideally matched band gaps. We develop an infrared-absorbing 1.2-electron volt band-gap perovskite, FA0.75Cs0.25Sn0.5Pb0.5I3, that can deliver 14.8% efficiency. By combining this material with a wider-band gap FA0.83Cs0.17Pb(I0.5Br0.5)3 material, we achieve monolithic two-terminal tandem efficiencies of 17.0% with >1.65-volt open-circuit voltage. We also make mechanically stacked four-terminal tandem cells and obtain 20.3% efficiency. Notably, we find that our infrared-absorbing perovskite cells exhibit excellent thermal and atmospheric stability, not previously achieved for Sn-based perovskites. This device architecture and materials set will enable “all-perovskite” thin-film solar cells to reach the highest efficiencies in the long term at the lowest costs.

  18. Two novel silicon phases with direct band gaps.

    Science.gov (United States)

    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.

  19. Band gap engineering of indium zinc oxide by nitrogen incorporation

    Energy Technology Data Exchange (ETDEWEB)

    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}.

  20. Electronic materials with a wide band gap: recent developments

    Directory of Open Access Journals (Sweden)

    Detlef Klimm

    2014-09-01

    Full Text Available The development of semiconductor electronics is reviewed briefly, beginning with the development of germanium devices (band gap Eg = 0.66 eV after World War II. A tendency towards alternative materials with wider band gaps quickly became apparent, starting with silicon (Eg = 1.12 eV. This improved the signal-to-noise ratio for classical electronic applications. Both semiconductors have a tetrahedral coordination, and by isoelectronic alternative replacement of Ge or Si with carbon or various anions and cations, other semiconductors with wider Eg were obtained. These are transparent to visible light and belong to the group of wide band gap semiconductors. Nowadays, some nitrides, especially GaN and AlN, are the most important materials for optical emission in the ultraviolet and blue regions. Oxide crystals, such as ZnO and β-Ga2O3, offer similarly good electronic properties but still suffer from significant difficulties in obtaining stable and technologically adequate p-type conductivity.

  1. Characterization of all-glass photonic band gap fiber

    Science.gov (United States)

    Buczynski, Ryszard; Kujawa, Ireneusz; Lusawa, Marzenna; Pysz, Dariusz; Martynkien, Tadeusz; Berghmans, Francis; Nasilowski, Tomasz; Thienpont, Hugo; Stepien, Ryszard

    2008-12-01

    In this paper we report on the fabrication and characterization of a double glass micro-structured fiber with low index core and photonic cladding made of high index micro-rods. Micro rods are made of lead-oxide F2 commercially available glass (SCHOTT Inc.) with a refractive index nD=1.619, while as background we use a borosilicate NC21 glass with a refractive index nD=1.533. The fiber cladding is composed of 8 rings of F2 glass micro rods ordered in hexagonal lattice. A core is created by replacement of seven F2 rods with NC21 rods. A fabricated fiber has a linear filling factor of 0.75 and micro rods diameter of 1.2 μm. A core has a diameter of 3.7 μm while cladding and total fiber diameter are 42,6μm and 120 μm, respectively. Using supercontinuum source we have measured transmission properties of the fabricated fiber. Based on measurements of the fiber samples of 18-80 cm long we have identified two photonic band gaps. Fist band gap is localized in visible range at 610 nm central wavelength. The second broadband photonic band gap is localized in near infrared and it is 80 nm wide at 840 nm central wavelength.

  2. Plasmonic photosensitization of a wide band gap semiconductor: converting plasmons to charge carriers.

    Science.gov (United States)

    Mubeen, Syed; Hernandez-Sosa, Gerardo; Moses, Daniel; Lee, Joun; Moskovits, Martin

    2011-12-14

    A fruitful paradigm in the development of low-cost and efficient photovoltaics is to dope or otherwise photosensitize wide band gap semiconductors in order to improve their light harvesting ability for light with sub-band-gap photon energies.(1-8) Here, we report significant photosensitization of TiO2 due to the direct injection by quantum tunneling of hot electrons produced in the decay of localized surface-plasmon polaritons excited in gold nanoparticles (AuNPs) embedded in the semiconductor (TiO2). Surface plasmon decay produces electron-hole pairs in the gold.(9-15) We propose that a significant fraction of these electrons tunnel into the semiconductor's conduction band resulting in a significant electron current in the TiO2 even when the device is illuminated with light with photon energies well below the semiconductor's band gap. Devices fabricated with (nonpercolating) multilayers of AuNPs in a TiO2 film produced over 1000-fold increase in photoconductance when illuminated at 600 nm over what TiO2 films devoid of AuNPs produced. The overall current resulting from illumination with visible light is ∼50% of the device current measured with UV (ℏω>Eg band gap) illumination. The above observations suggest that plasmonic nanostructures (which can be fabricated with absorption properties that cover the full solar spectrum) can function as a viable alternative to organic photosensitizers for photovoltaic and photodetection applications.

  3. An efficient method of DFT/LDA band-gap correction

    Science.gov (United States)

    Scharoch, Pawel; Winiarski, Maciej

    2013-12-01

    It has been shown that the underestimated by DFT/LDA(GGA) band-gap can be efficiently corrected by an averaging procedure of transition energies over a region close to the direct band-gap transition, which we call the Δ(EIG) method (the differences in the Kohn-Sham eigenvalues). For small excitations the averaging appears to be equivalent to the Δ(SCF) approach (differences in the self-consistent energies), which is a consequence of Janak’s theorem and has been confirmed numerically. The Gaussian distribution in k-space for electronic excitation has been used (occupation numbers in the Δ(SCF) or eigenenergy sampling in the Δ(EIG)). A systematic behavior of the k-space localization parameter σk correcting the band-gap has been observed in numerical experiments. On that basis some sampling schemes for band-gap correction have been proposed and tested in the prediction of the band-gap behavior in InxGa(1-x)N semiconducting alloy, and a very good agreement with independent calculations has been obtained. In the context of the work the issue of electron localization in the r-space has been discussed which, as it has been predicted by Mori-Sánchez et al. [P. Mori-Sánchez, A.J. Cohen, W. Yang, Phys. Rev. Lett. 100 (2008) 146401], should reduce the effect of the convex behavior of the LDA/GGA functionals and improve the band-gap prediction within DFT/LDA(GGA). A scheme for electron localization in r-space has been suggested.

  4. Hydrogen production by tuning the photonic band gap with the electronic band gap of TiO₂.

    Science.gov (United States)

    Waterhouse, G I N; Wahab, A K; Al-Oufi, M; Jovic, V; Anjum, D H; Sun-Waterhouse, D; Llorca, J; Idriss, H

    2013-10-10

    Tuning the photonic band gap (PBG) to the electronic band gap (EBG) of Au/TiO2 catalysts resulted in considerable enhancement of the photocatalytic water splitting to hydrogen under direct sunlight. Au/TiO2 (PBG-357 nm) photocatalyst exhibited superior photocatalytic performance under both UV and sunlight compared to the Au/TiO2 (PBG-585 nm) photocatalyst and both are higher than Au/TiO2 without the 3 dimensionally ordered macro-porous structure materials. The very high photocatalytic activity is attributed to suppression of a fraction of electron-hole recombination route due to the co-incidence of the PBG with the EBG of TiO2 These materials that maintain their activity with very small amount of sacrificial agents (down to 0.5 vol.% of ethanol) are poised to find direct applications because of their high activity, low cost of the process, simplicity and stability.

  5. Two-dimensional boron-nitrogen-carbon monolayers with tunable direct band gaps

    Science.gov (United States)

    Zhang, Miao; Gao, Guoying; Kutana, Alex; Wang, Yanchao; Zou, Xiaolong; Tse, John S.; Yakobson, Boris I.; Li, Hongdong; Liu, Hanyu; Ma, Yanming

    2015-07-01

    The search for new candidate semiconductors with direct band gaps of ~1.4 eV has attracted significant attention, especially among the two-dimensional (2D) materials, which have become potential candidates for next-generation optoelectronics. Herein, we systematically studied 2D Bx/2Nx/2C1-x (0 optimization method (CALYPSO) in conjunction with density functional theory. Furthermore, we examine more stoichiometries by the cluster expansion technique based on a hexagonal lattice. The results reveal that all monolayer Bx/2Nx/2C1-x stoichiometries adopt a planar honeycomb character and are dynamically stable. Remarkably, electronic structural calculations show that most of Bx/2Nx/2C1-x phases possess direct band gaps within the optical range, thereby they can potentially be used in high-efficiency conversion of solar energy to electric power, as well as in p-n junction photovoltaic modules. The present results also show that the band gaps of Bx/2Nx/2C1-x can be widely tuned within the optical range by changing the concentration of carbon, thus allowing the fast development of band gap engineered materials in optoelectronics. These new findings may enable new approaches to the design of microelectronic devices.The search for new candidate semiconductors with direct band gaps of ~1.4 eV has attracted significant attention, especially among the two-dimensional (2D) materials, which have become potential candidates for next-generation optoelectronics. Herein, we systematically studied 2D Bx/2Nx/2C1-x (0 optimization method (CALYPSO) in conjunction with density functional theory. Furthermore, we examine more stoichiometries by the cluster expansion technique based on a hexagonal lattice. The results reveal that all monolayer Bx/2Nx/2C1-x stoichiometries adopt a planar honeycomb character and are dynamically stable. Remarkably, electronic structural calculations show that most of Bx/2Nx/2C1-x phases possess direct band gaps within the optical range, thereby they can

  6. A Model Describing the Band Gap Energy of the Strained In x Ga1- x N y Sb z As1- y- z Alloy (0 < x ≤ 0.5, 0 < y ≤ 0.05, 0 < z ≤ 0.1)

    Science.gov (United States)

    Zhao, Chuan-Zhen; Fu, Qiang; Wei, Tong; Wang, Sha-Sha; Lu, Ke-Qing

    2017-03-01

    The physical mechanism for the band gap evolution of the strained In x Ga1- x N y Sb z As1- y- z alloy is investigated. It is found that In x Ga1- x N y Sb z As1- y- z alloy with small N and Sb contents can be considered as an alloy formed by adding N and Sb atoms in the host material In x Ga1- x As. Under this condition, the band gap evolution of In x Ga1- x N y Sb z As1- y- z is due to three factors. One is the intraband coupling interactions within the conduction band and separately within the valence band of the host material, another is the coupling interaction between the Sb level and the D valence band maximum of the host material, and the other is the coupling interaction between the N level and the D conduction band minimum of the host material. Based on the physical mechanism for the band gap evolution of In x Ga1- x N y Sb z As1- y- z , a model is developed. The model can describe the band gap energy of the strained In x Ga1- x N y Sb z As1- y- z alloy well.

  7. Effect of hydrogenation on the band gap of graphene nano-flakes

    Energy Technology Data Exchange (ETDEWEB)

    Tachikawa, Hiroto, E-mail: hiroto@eng.hokudai.ac.jp; Iyama, Tetsuji; Kawabata, Hiroshi

    2014-03-03

    The effects of hydrogenation on the band gap of graphene have been investigated by means of density functional theory method. It is generally considered that the band gap increases with increasing coverage of hydrogen atom on the graphene. However, the present study shows that the band gap decreases first with increasing hydrogen coverage and reaches the lowest value at finite coverage (γ = 0.3). Next, the band gap increases to that of insulator with coverage from 0.3 to 1.0. This specific feature of the band gap is reasonably explained by broken symmetry model and the decrease of pi-conjugation. The electronic states of hydrogenated graphene are discussed. - Highlights: • Density functional theory calculations were carried out for hydrogen on graphene • Effects of hydrogenation on the band gap of graphene were examined. • The band gap showed a minimum at a finite coverage. • Mechanism of specific band gap feature was discussed.

  8. A study of potential high band-gap photovoltaic materials for a two step photon intermediate technique in fission energy conversion. Final report

    Energy Technology Data Exchange (ETDEWEB)

    Prelas, M.A.

    1996-01-24

    This report describes progress made to develop a high bandgap photovoltaic materials for direct conversion to electricity of excimer radiation produced by fission energy pumped laser. This report summarizes the major achievements in sections. The first section covers n-type diamond. The second section covers forced diffusion. The third section covers radiation effects. The fourth section covers progress in Schottky barrier and heterojunction photovoltaic cells. The fifth section covers cell and reactor development.

  9. Method of construction of composite one-dimensional photonic crystal with extended photonic band gaps.

    Science.gov (United States)

    Tolmachev, V; Perova, T; Moore, R

    2005-10-17

    A method of photonic band gap extension using mixing of periodic structures with two or more consecutively placed photonic crystals with different lattice constants is proposed. For the design of the structures with maximal photonic band gap extension the gap map imposition method is utilised. Optimal structures have been established and the gap map of photonic band gaps has been calculated at normal incidence of light for both small and large optical contrast and at oblique incidence of light for small optical contrast.

  10. Relaxation of femtosecond photoexcited electrons in a polar indirect band-gap semiconductor nanoparticle

    Indian Academy of Sciences (India)

    Navinder Singh

    2005-01-01

    A model calculation is given for the energy relaxation of a non-equilibrium distribution of hot electrons (holes) prepared in the conduction (valence) band of a polar indirect band-gap semiconductor, which has been subjected to homogeneous photoexcitation by a femtosecond laser pulse. The model assumes that the pulsed photoexcitation creates two distinct but spatially interpenetrating electron and hole non-equilibrium subsystems that initially relax non-radiatively through the electron (hole)–phonon processes towards the conduction (valence) band minimum (maximum), and finally radiatively through the phonon-assisted electron–hole recombination across the band-gap, which is a relatively slow process. This leads to an accumulation of electrons (holes) at the conduction (valence) band minimum (maximum). The resulting peaking of the carrier density and the entire evolution of the hot electron (hole) distribution has been calculated. The latter may be time resolved by a pump-probe study. The model is particularly applicable to a divided (nanometric) polar indirect band-gap semiconductor with a low carrier concentration and strong electron–phonon coupling, where the usual two-temperature model [1–4] may not be appropriate.

  11. Synthesis and characterization of a mixture of CoFe{sub 2}O{sub 4} and MgFe{sub 2}O{sub 4} from layered double hydroxides: Band gap energy and magnetic responses

    Energy Technology Data Exchange (ETDEWEB)

    Agú, Ulises A., E-mail: uagu@scdt.frc.utn.edu.ar [CITeQ—CONICET, Universidad Tecnológica Nacional—Facultad Regional Córdoba, Córdoba (Argentina); Oliva, Marcos I. [IFEG—CONICET, Universidad Nacional de Córdoba, Córdoba (Argentina); Marchetti, Sergio G. [CINDECA—CONICET, Universidad Nacional de La Plata, La Plata, Buenos Aires (Argentina); Heredia, Angélica C.; Casuscelli, Sandra G.; Crivello, Mónica E. [CITeQ—CONICET, Universidad Tecnológica Nacional—Facultad Regional Córdoba, Córdoba (Argentina)

    2014-11-15

    A mixture of nanocrystals of cobalt ferrite and magnesium ferrite was obtained from Layered Double Hydroxides (LDH) through a co-precitation method with a theoretical molar ratio M{sup 2+}:Fe{sup 3+}=3:1, where M{sup 2+}represents Mg{sup 2+} and/or Co{sup 2+}. The molar ratios between Co{sup 2+}:Fe{sup 3+} were 0.0 (0Co), 0.2 (5Co), and 0.4 (10Co). In order to assess the effect on the properties of the LDH and their oxides, the molar percentages were 0, 5 and 10%. Two different synthesis methods were evaluated; (i) ageing at room temperature (rt), and (ii) hydrothermal ageing at 200 °C in autoclave (ht), both methods needed 15 h of ageing. Then, these LDH were calcined in air atmosphere at 550 °C for 10 h. The calcined materials were characterized by X-ray diffraction (XRD), thermogravymetric analysis (TGA), temperature-programmed reduction (TPR), infrared spectroscopy with Fourier transform (FTIR), Diffuse Reflectance UV–visible spectroscopy (UV–vis-DRS), Mössbauer spectroscopy and inductively coupled plasma optical emission spectroscopy (ICP–OES). The magnetic response was analyzed using a vibrating sample magnetometer (VSM). The band gap energy of the iron oxides was determined through the UV–vis-DRS analysis. Through these studies it was possible to identify the presence of a mixture of cobalt ferrite and magnesium ferrite. Samples did not show hematite and cobalt oxides, but the presence of MgO in the periclase phase was determined. This magnesium oxide promoted a good dispersion of the ferrites. Moreover, when a single ferrite phase of Co or Mg was formed, a diminution of the crystal size with consequent enlarged values of band gap energy was observed. Thus, materials synthesized by room temperature ageing promoted the superparamagnetic behaviour of samples, attributed to the content of the cobalt ferrite structure in nanocrystals. In regard to the estimated band gap energy, all samples exhibited low levels. These results indicate that these solids

  12. LSE investigation of the thermal effect on band gap energy and thermodynamic parameters of BInGaAs/GaAs Single Quantum Well

    Science.gov (United States)

    Hidouri, T.; Saidi, F.; Maaref, H.; Rodriguez, Ph.; Auvray, L.

    2016-12-01

    In this paper, we report on the experimental and theoretical study of BInGaAs/GaAs Single Quantum Well elaborated by Metal Organic Chemical Vapor Deposition (MOCVD). We carried out the photoluminescence (PL) peak energy temperature-dependence over a temperature range of 10-300 K. It shows the S-shaped behavior as a result of a competition process between localized and delocalized states. We simulate the peak evolution by the empirical model and modified models. The first one is limited at high PL temperature. For the second one, a correction due to the thermal redistribution based on the Localized State Ensemble model (LSE). The new fit gives a good agreement between theoretical and experimental data in the entire temperature range. Furthermore, we have investigated an approximate analytical expressions and interpretation for the entropy and enthalpy of formation of electron-hole pairs in quaternary BInGaAs/GaAs SQW.

  13. Band gap tuning of nickelates for photovoltaic applications

    Science.gov (United States)

    Chang, Lei; Wang, Le; You, Lu; Zhou, Yang; Fang, Liang; Wang, Shiwei; Wang, Junling

    2016-11-01

    Hybrid perovskites have achieved tremendous success as a light absorber in solar cells during the past few years. However, the stability issue casts shadow on their practical applications. Perovskite oxides may offer an alternative. In this study, the metal-insulator transition in perovskite neodymium nickelates (NdNiO3) is systematically tuned by adjusting the oxygen partial pressure during film growth. Room temperature insulating films with different band gaps are obtained. Testing photovoltaic cells have been prepared by combining the nickelates with Nb-doped SrTiO3, and photovoltaic performance has been optimized. Our study offers a new route for designing novel photovoltaic materials.

  14. Photonic band gap spectra in Octonacci metamaterial quasicrystals

    Science.gov (United States)

    Brandão, E. R.; Vasconcelos, M. S.; Albuquerque, E. L.; Fulco, U. L.

    2017-02-01

    In this work we study theoretically the photonic band gap spectra for a one-dimensional quasicrystal made up of SiO2 (layer A) and a metamaterial (layer B) organized following the Octonacci sequence, where its nth-stage Sn is given by the inflation rule Sn =Sn - 1Sn - 2Sn - 1 for n ≥ 3 , with initial conditions S1 = A and S2 = B . The metamaterial is characterized by a frequency dependent electric permittivity ε(ω) and magnetic permeability μ(ω) . The polariton dispersion relation is obtained analytically by employing a theoretical calculation based on a transfer-matrix approach. A quantitative analysis of the spectra is then discussed, stressing the distribution of the allowed photonic band widths for high generations of the Octonacci structure, which depict a self-similar scaling property behavior, with a power law depending on the common in-plane wavevector kx .

  15. Photonic band gap enhancement in frequency-dependent dielectrics.

    Science.gov (United States)

    Toader, Ovidiu; John, Sajeev

    2004-10-01

    We illustrate a general technique for evaluating photonic band structures in periodic d -dimensional microstructures in which the dielectric constant epsilon (omega) exhibits rapid variations with frequency omega . This technique involves the evaluation of generalized electromagnetic dispersion surfaces omega ( k--> ,epsilon) in a (d+1) -dimensional space consisting of the physical d -dimensional space of wave vectors k--> and an additional dimension defined by the continuous, independent, variable epsilon . The physical band structure for the photonic crystal is obtained by evaluating the intersection of the generalized dispersion surfaces with the "cutting surface" defined by the function epsilon (omega) . We apply this method to evaluate the band structure of both two- and three-dimensional (3D) periodic microstructures. We consider metallic photonic crystals with free carriers described by a simple Drude conductivity and verify the occurrence of electromagnetic pass bands below the plasma frequency of the bulk metal. We also evaluate the shift of the photonic band structure caused by free carrier injection into semiconductor-based photonic crystals. We apply our method to two models in which epsilon (omega) describes a resonant radiation-matter interaction. In the first model, we consider the addition of independent, resonant oscillators to a photonic crystal with an otherwise frequency-independent dielectric constant. We demonstrate that for an inhomogeneously broadened distribution of resonators impregnated within an inverse opal structure, the full 3D photonic band gap (PBG) can be considerably enhanced. In the second model, we consider a coupled resonant oscillator mode in a photonic crystal. When this mode is an optical phonon, there can be a synergetic interplay between the polaritonic resonance and the geometrical scattering resonances of the structured dielectric, leading to PBG enhancement. A similar effect may arise when resonant atoms that are

  16. Lamb wave band gaps in locally resonant phononic crystal strip waveguides

    Energy Technology Data Exchange (ETDEWEB)

    Yao, Yuanwei, E-mail: yaoyw@scut.edu.cn [Department of Physics, Guangdong University of Technology, Guangzhou 510006 (China); Wu, Fugen [Experiment and Educational Center, Guangdong University of Technology, Guangzhou 510006 (China); Zhang, Xin [Department of Physics, Guangdong University of Technology, Guangzhou 510006 (China); Hou, Zhilin [Department of Physics, South China University of Technology, Guangzhou 510640 (China)

    2012-01-09

    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.

  17. Enlargement of Photonic Band Gaps and Physical Picture of Photonic Band Structures

    Institute of Scientific and Technical Information of China (English)

    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.

  18. First Principles Study of Band Structure and Band Gap Engineering in Graphene for Device Applications

    Science.gov (United States)

    2015-03-20

    successfully to realise the full applications of graphene? What is the current status of the graphene based devices or Electronics ? How the graphene...gap value has increased to 1.5eV. It is reflected in the density of states (Fig.20c). The nitrogen atoms are at a distance of 5Ȧ. And there is a...completely modified.The calculation of surface doping of graphene with S is repeated with 96 atom simulation cell. The band gap value is 0.7 eV. The value of

  19. Graded band gap GaInNAs solar cells

    Energy Technology Data Exchange (ETDEWEB)

    Langer, F.; Perl, S.; Kamp, M. [Technische Physik, Physikalisches Institut and Wilhelm Conrad Röntgen Research Center for Complex, Material Systems, University of Würzburg, Am Hubland, Würzburg D97074 (Germany); Höfling, S. [Technische Physik, Physikalisches Institut and Wilhelm Conrad Röntgen Research Center for Complex, Material Systems, University of Würzburg, Am Hubland, Würzburg D97074 (Germany); SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS (United Kingdom)

    2015-06-08

    Dilute nitride GaInN(Sb)As with a band gap (E{sub g}) of 1.0 eV is a promising material for the integration in next generation multijunction solar cells. We have investigated the effect of a compositionally graded GaInNAs absorber layer on the spectral response of a GaInNAs sub cell. We produced band gap gradings (ΔE{sub g}) of up to 39 meV across a 1 μm thick GaInNAs layer. Thereby, the external quantum efficiency—compared to reference cells—was increased due to the improved extraction of photo-generated carriers from 34.0% to 36.7% for the wavelength range from 900 nm to 1150 nm. However, this device figure improvement is accompanied by a small decrease in the open circuit voltage of about 20 mV and the shift of the absorption edge to shorter wavelengths.

  20. Narrow band gap conjugated polymers for emergent optoelectronic technologies

    Science.gov (United States)

    Azoulay, Jason D.; Zhang, Benjamin A.; London, Alexander E.

    2015-09-01

    Conjugated organic molecules effectively produce and harvest visible light and find utility in a variety of emergent optoelectronic technologies. There is currently interest in expanding the scope of these materials to extend functionality into the infrared (IR) spectral regions and endow functionality relevant in emergent technologies. Developing an understanding of the interplay between chemical and electronic structure in these systems will require control of the frontier orbital energetics (separation, position, and alignment), ground state electronic configurations, interchain arrangements, solid-state properties, and many other molecular features with synthetic precision that has yet to be demonstrated. Bridgehead imine substituted 4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) structural units, in combination with strong acceptors with progressively delocalized π-systems, afford modular donor-acceptor copolymers with broad and long wavelength absorption that spans technologically relevant wavelength (λ) ranges from 0.7 < λ < 3.2 μm.1 Here we demonstrate that electronic and structural manipulation play a major role in influencing the energetics of these systems and ultimately controlling the band gap of the materials. These results bear implication in the development of very narrow band gap systems where precise control will be necessary for achieving desired properties such as interactions with longer wavelength light.

  1. OPTICAL BAND GAP AND CONDUCTIVITY MEASUREMENTS OF POLYPYRROLE-CHITOSAN COMPOSITE THIN FILMS

    Institute of Scientific and Technical Information of China (English)

    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.

  2. Steric engineering of metal-halide perovskites with tunable optical band gaps.

    Science.gov (United States)

    Filip, Marina R; Eperon, Giles E; Snaith, Henry J; Giustino, Feliciano

    2014-12-15

    Owing to their high energy-conversion efficiency and inexpensive fabrication routes, solar cells based on metal-organic halide perovskites have rapidly gained prominence as a disruptive technology. An attractive feature of perovskite absorbers is the possibility of tailoring their properties by changing the elemental composition through the chemical precursors. In this context, rational in silico design represents a powerful tool for mapping the vast materials landscape and accelerating discovery. Here we show that the optical band gap of metal-halide perovskites, a key design parameter for solar cells, strongly correlates with a simple structural feature, the largest metal-halide-metal bond angle. Using this descriptor we suggest continuous tunability of the optical gap from the mid-infrared to the visible. Precise band gap engineering is achieved by controlling the bond angles through the steric size of the molecular cation. On the basis of these design principles we predict novel low-gap perovskites for optimum photovoltaic efficiency, and we demonstrate the concept of band gap modulation by synthesising and characterising novel mixed-cation perovskites.

  3. Uniaxial stress influence on lattice, band gap and optical properties of n-type ZnO:first-principles calculations

    Institute of Scientific and Technical Information of China (English)

    Yang Ping; Li Pei; Zhang Li-Qiang; Wang Xiao-Liang; Wang Huan; Song Xi-Fu; Xie Fang-Wei

    2012-01-01

    The lattice,the band gap and the optical properties of n-type ZnO under uniaxial stress are investigated by firstprinciples calculations.The results show that the lattice constants change linearly with stress.Band gaps are broadened linearly as the uniaxial compressive stress increases.The change of band gap for n-type ZnO comes mainly from the contribution of stress in the c-axis direction,and the reason for band gap of n-type ZnO changing with stress is also explained.The calculated results of optical properties reveal that the imaginary part of the dielectric function decreases with the increase of uniaxial compressive stress at low energy.However,when the energy is higher than 4.0 eV,the imaginary part of the dielectric function increases with the increase of stress and a blueshift appears.There are two peaks in the absorption spectrum in an energy range of 4.0-13.0 eV.The stress coefficient of the band gap of n-type ZnO is larger than that of pure ZnO,which supplies the theoretical reference value for the modulation of the band gap of doped ZnO.

  4. Light-gated single CdSe nanowire transistor: photocurrent saturation and band gap extraction

    Energy Technology Data Exchange (ETDEWEB)

    Zhang, Yang, E-mail: yangzh08@gmail.com; Chakraborty, Ritun; Kudera, Stefan; Krahne, Roman, E-mail: roman.krahne@iit.it [Istituto Italiano di Tecnologia, Nanochemistry department (Italy)

    2015-11-15

    CdSe nanowires are popular building blocks for many optoelectronic devices mainly owing to their direct band gap in the visible range of the spectrum. Here we investigate the optoelectronic properties of single CdSe nanowires fabricated by colloidal synthesis, in terms of their photocurrent–voltage characteristics and photoconductivity spectra recorded at 300 and 18 K. The photocurrent is identified as the secondary photocurrent, which gives rise to a photoconductive gain of ∼35. We observe a saturation of the photocurrent beyond a certain voltage bias that can be related to the finite drift velocity of electrons. From the photoconductivity spectra, we determine the band gap energy of the nanowires as ∼1.728 eV, and we resolve low-energy peaks that can be associated with sub-bandgap states.Graphical Abstract.

  5. Band gap engineering of silicene zigzag nanoribbons with perpendicular electric fields: a theoretical study.

    Science.gov (United States)

    Liang, Yunye; Wang, Vei; Mizuseki, Hiroshi; Kawazoe, Yoshiyuki

    2012-11-14

    The electronic properties of silicene zigzag nanoribbons with the presence of perpendicular fields are studied by using first-principles calculations and the generalized nearest neighboring approximation method. In contrast to the planar graphene, in silicene the Si atoms are not coplanar. As a result, by applying perpendicular fields to the two-dimensional silicene sheet, the on-site energy can be modulated and the band gap at the Dirac point is open. The buckled structure also creates a height difference between the two edges of the silicene zigzag nanoribbons. We find that the external fields can modulate the energies of spin-polarized edge states and their corresponding band gaps. Due to the polarization in the plane, the modulation effect is width dependent and becomes much more significant for narrow ribbons.

  6. Polarization field gradient effects in inhomogeneous metal-ferroelectric bilayers: Optical response and band gap tunability

    Science.gov (United States)

    Vivas C., H.; Vargas-Hernández, C.

    2012-06-01

    Optical constants, reflectivity response and direct band gap energy (Egd) were calculated and simulated by developing an electrodynamic-based model for a three medium system, namely vacuum/ferroelectric film/metallic substrate. Depolarization effects due to the contact between the metallic substrate and the FE film, as well as the spatially dependent profile of the dielectric susceptibility ε(z) enter into the formalism by adapting the phenomenological Landau-Ginzburg-Devonshire theory (LGD). Absorption coefficient is obtained from the Lambert-Beer-Bouguer (LBB) approximation and the direct band gap energy as a function of the characteristic length is calculated by using the general Tauc power law. Numerical simulations lead to range of values for tunable Egd from 2.6 to 2.8 eV for characteristic lengths up to 30% the thickness of the film, in concordance with recent reports.

  7. Image-potential band-gap narrowing at a metal/semiconductor interface

    OpenAIRE

    2001-01-01

    GW approximation is used to systematically revisit the image-potential band-gap narrowing at metal/semiconductor interfaces proposed by Inkson in the 1970's. Here we have questioned how the narrowing as calculated from quasi-particle energy spectra for the jellium/Si interface depends on $r_s$ of the jellium. The gap narrowing is found to only weakly depend on $r_s$ (i.e., narrowing $\\simeq 0.3$ eV even for a large $r_s = 6)$. Hence we can turn to smaller polarizability in the semiconductor s...

  8. Band Gap Engineering and Layer-by-Layer Band Gap Mapping of Selenium-doped Molybdenum Disulfide

    Energy Technology Data Exchange (ETDEWEB)

    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.

  9. Hydrogen production by Tuning the Photonic Band Gap with the Electronic Band Gap of TiO2

    KAUST Repository

    Waterhouse, G. I. N.

    2013-10-10

    Tuning the photonic band gap (PBG) to the electronic band gap (EBG) of Au/TiO2 catalysts resulted in considerable enhancement of the photocatalytic water splitting to hydrogen under direct sunlight. Au/TiO2 (PBG-357 nm) photocatalyst exhibited superior photocatalytic performance under both UV and sunlight compared to the Au/TiO2 (PBG-585 nm) photocatalyst and both are higher than Au/TiO2 without the 3 dimensionally ordered macro-porous structure materials. The very high photocatalytic activity is attributed to suppression of a fraction of electron-hole recombination route due to the co-incidence of the PBG with the EBG of TiO2 These materials that maintain their activity with very small amount of sacrificial agents (down to 0.5 vol.% of ethanol) are poised to find direct applications because of their high activity, low cost of the process, simplicity and stability.

  10. Unfolding the band structure of non-crystalline photonic band gap materials.

    Science.gov (United States)

    Tsitrin, Samuel; Williamson, Eric Paul; Amoah, Timothy; Nahal, Geev; Chan, Ho Leung; Florescu, Marian; Man, Weining

    2015-08-20

    Non-crystalline photonic band gap (PBG) materials have received increasing attention, and sizeable PBGs have been reported in quasi-crystalline structures and, more recently, in disordered structures. Band structure calculations for periodic structures produce accurate dispersion relations, which determine group velocities, dispersion, density of states and iso-frequency surfaces, and are used to predict a wide-range of optical phenomena including light propagation, excited-state decay rates, temporal broadening or compression of ultrashort pulses and complex refraction phenomena. However, band calculations for non-periodic structures employ large super-cells of hundreds to thousands building blocks, and provide little useful information other than the PBG central frequency and width. Using stereolithography, we construct cm-scale disordered PBG materials and perform microwave transmission measurements, as well as finite-difference time-domain (FDTD) simulations. The photonic dispersion relations are reconstructed from the measured and simulated phase data. Our results demonstrate the existence of sizeable PBGs in these disordered structures and provide detailed information of the effective band diagrams, dispersion relation, iso-frequency contours, and their angular dependence. Slow light phenomena are also observed in these structures near gap frequencies. This study introduces a powerful tool to investigate photonic properties of non-crystalline structures and provides important effective dispersion information, otherwise difficult to obtain.

  11. Large acoustic band gaps created by rotating square rods in two-dimensional periodic composites

    CERN Document Server

    Li Xiao Ling; Hu He Fei; Zhong Shao; Liu You Yan

    2003-01-01

    Effects of orientations of square rods on the acoustic band gaps in two-dimensional periodic arrays of rigid solid rods embedded in air are studied. The acoustic band gaps will be opened and enlarged greatly by increasing the rotation angle. For any filling fraction F, the maximum acoustic band gaps appear at the same rotation angle theta = 45 deg. for the cases of F<=0.50, otherwise they will appear at different limit values theta sub c and the largest band gap is achieved at a filling fraction of about F=0.85. This gap-tuning effect will be stronger with increase in filling fraction. This tuning mechanism of band gap suggests a new way to design band gaps of two-dimensional phononic crystals. (rapid communication)

  12. II-VI wide band gap semiconductors under hydrostatic pressure

    Science.gov (United States)

    Baquero, R.; Decoss, R.; Olguin, D.

    1993-08-01

    We set an analytical expression for the gap as a function of hydrostatic deformation, E(sub g)(epsilon), by diagonalizing in Gamma the corresponding empirical tight-binding Hamiltonian (ETBH). In the ETBH we use the well known d(exp -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 E(sub g)(epsilon) for wide band gap II-VI semiconductors with zincblende crystal structure for deformations under pressure up to -5 percent. Results are in 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. Within the approximation we always find a positive second derivative of E(sub g)(epsilon) with respect to epsilon, independent of the material. Furthermore, the inclusion of deviations from HSL appear to be unimportant to this problem.

  13. Band gap modulation of transition-metal dichalcogenide MX2 nanosheets by in-plane strain

    Science.gov (United States)

    Su, Xiangying; Ju, Weiwei; Zhang, Ruizhi; Guo, Chongfeng; Yong, Yongliang; Cui, Hongling; Li, Xiaohong

    2016-10-01

    The electronic properties of quasi-two-dimensional honeycomb structures of MX2 nanosheets (M=Mo, W and X=S, Se) subjected to in-plane biaxial strain have been investigated using first-principles calculations. We demonstrate that the band gap of MX2 nanosheets can be widely tuned by applying tensile or compressive strain, and these ultrathin materials undergo a universal reversible semiconductor-metal transition at a critical strain. Compared to WX2, MoX2 need a smaller critical tensile strain for the band gap close, and MSe2 need a smaller critical compressive strain than MS2. Taking bilayer MoS2 as an example, the variation of the band structures was studied and the semiconductor-metal transition involves a slightly different physical mechanism between tensile and compressive strain. The ability to tune the band gap of MX2 nanosheets in a controlled fashion over a wide range of energy opens up the possibility for its usage in a range of application.

  14. Direct optical band gap measurement in polycrystalline semiconductors: A critical look at the Tauc method

    Science.gov (United States)

    Dolgonos, Alex; Mason, Thomas O.; Poeppelmeier, Kenneth R.

    2016-08-01

    The direct optical band gap of semiconductors is traditionally measured by extrapolating the linear region of the square of the absorption curve to the x-axis, and a variation of this method, developed by Tauc, has also been widely used. The application of the Tauc method to crystalline materials is rooted in misconception-and traditional linear extrapolation methods are inappropriate for use on degenerate semiconductors, where the occupation of conduction band energy states cannot be ignored. A new method is proposed for extracting a direct optical band gap from absorption spectra of degenerately-doped bulk semiconductors. This method was applied to pseudo-absorption spectra of Sn-doped In2O3 (ITO)-converted from diffuse-reflectance measurements on bulk specimens. The results of this analysis were corroborated by room-temperature photoluminescence excitation measurements, which yielded values of optical band gap and Burstein-Moss shift that are consistent with previous studies on In2O3 single crystals and thin films.

  15. Experimental and theoretical investigation of relative optical band gaps in graphene generations

    Science.gov (United States)

    Bhatnagar, Deepika; Singh, Sukhbir; Yadav, Sriniwas; Kumar, Ashok; Kaur, Inderpreet

    2017-01-01

    Size and chemical functionalization dependant optical band gaps in graphene family nanomaterials were investigated by experimental and theoretical study using Tauc plot and density functional theory (DFT). We have synthesized graphene oxide through a modified Hummer’s method using graphene nanoplatelets and sequentially graphene quantum dots through hydrothermal reduction. The experimental results indicate that the optical band gap in graphene generations was altered by reducing the size of graphene sheets and attachment of chemical functionalities like epoxy, hydroxyl and carboxyl groups plays a crucial role in varying optical band gaps. It is further confirmed by DFT calculations that the π orbitals were more dominatingly participating in transitions shown by projected density of states and the molecular energy spectrum represented the effect of attached functional groups along with discreteness in energy levels. Theoretical results were found to be in good agreement with experimental results. All of the above different variants of graphene can be used in native or modified form for sensor design and optoelectronic applications.

  16. Optical Properties of One-dimensional Three-component Photonic Band Gap Structure

    Institute of Scientific and Technical Information of China (English)

    2003-01-01

    Theoretical study of the optical properties of one-dimensional three-component photonic band gap structure, which is composed of three alternating dielectric layers of different refractive indices and thickness in a unit cell, is performed. This one-dimensional photonic band gap structure exhibits the transparency band and forbidden band. We find that there are several mini-bands of the allowed transmission to be created within the photonic band gap region of the structure if a defect designed specially is introduced inside the structure. This characteristic is very important for some practical applications.

  17. Cherenkov oscillator operating at the second band gap of leakage waveguide structures

    Science.gov (United States)

    Jang, Kyu-Ha; Park, Seong Hee; Lee, Kitae; Jeong, Young Uk

    2016-10-01

    An electromagnetic wave source operating around second band gaps of metallic grating structures is presented. The considered metallic grating structures are not perfect periodic but inhomogeneously structured within a period to have a second band gap where the wavelength is equal to the period of the structures. The radiation mechanism by an electron beam in the structures is different from the well-known Smith-Purcell radiation occurring in perfect periodic grating structures. That is, the radiating wave has a single frequency and the radiation is unidirectional. When the energy of the electron beam is synchronized at the standing wave point in the dispersion curves, strong interaction happens and coherent radiation perpendicular to the grating surface is generated with relatively lower starting oscillation current.

  18. Cherenkov oscillator operating at the second band gap of leakage waveguide structures

    Directory of Open Access Journals (Sweden)

    Kyu-Ha Jang

    2016-10-01

    Full Text Available An electromagnetic wave source operating around second band gaps of metallic grating structures is presented. The considered metallic grating structures are not perfect periodic but inhomogeneously structured within a period to have a second band gap where the wavelength is equal to the period of the structures. The radiation mechanism by an electron beam in the structures is different from the well-known Smith-Purcell radiation occurring in perfect periodic grating structures. That is, the radiating wave has a single frequency and the radiation is unidirectional. When the energy of the electron beam is synchronized at the standing wave point in the dispersion curves, strong interaction happens and coherent radiation perpendicular to the grating surface is generated with relatively lower starting oscillation current.

  19. Negative capacitance switching via VO2 band gap engineering driven by electric field

    Science.gov (United States)

    He, Xinfeng; Xu, Jing; Xu, Xiaofeng; Gu, Congcong; Chen, Fei; Wu, Binhe; Wang, Chunrui; Xing, Huaizhong; Chen, Xiaoshuang; Chu, Junhao

    2015-03-01

    We report the negative capacitance behavior of an energy band gap modulation quantum well with a sandwich VO2 layer structure. The phase transition is probed by measuring its capacitance. With the help of theoretical calculations, it shows that the negative capacitance changes of the quantum well device come from VO2 band gap by continuously tuning the temperature or voltage. Experiments reveal that as the current remains small enough, joule heating can be ignored, and the insulator-metal transition of VO2 can be induced by the electric field. Our results open up possibilities for functional devices with phase transitions induced by external electric fields other than the heating or electricity-heat transition.

  20. Research on the elastic wave band gaps of curved beam of phononic crystals

    Energy Technology Data Exchange (ETDEWEB)

    Shaogang, Liu; Shidan, Li; Haisheng, Shu, E-mail: shuhaisheng@hrbeu.edu.cn; Weiyuan, Wang; Dongyan, Shi; Liqiang, Dong; Hang, Lin; Wei, Liu

    2015-01-15

    Based on wave equations of Timoshenko curved beam, the theoretical derivation and numerical calculation of the behavior of in-plane and out-of-plane wave propagating in curved beam of phononic crystals (CBPC) are carried out using transfer matrix method combined with the Bloch theorem. Finite CBPC is also simulated by FEM method. It is shown that both in-plane and out-of-plane elastic waves band gaps exist in CBPC. Compared with equivalent straight beam of phononic crystals (SBPC), CBPC has some unique characteristics, such as the first complete in-plane band gap, special in-plane coupling band gap, and out-of-plane coupling band gap. In those band gaps, CBPC has a better property of vibration reduction than the equivalent SBPC in some ways. Furthermore, effects of curvature of CBPC on the in-plane and out-of-plane band gaps are discussed.

  1. Band gaps, ionization potentials, and electron affinities of periodic electron systems via the adiabatic-connection fluctuation-dissipation theorem

    Science.gov (United States)

    Trushin, Egor; Betzinger, Markus; Blügel, Stefan; Görling, Andreas

    2016-08-01

    An approach to calculate fundamental band gaps, ionization energies, and electron affinities of periodic electron systems is explored. Starting from total energies obtained with the help of the adiabatic-connection fluctuation-dissipation (ACFD) theorem, these physical observables are calculated according to their basic definition by differences of the total energies of the N -, (N -1 ) -, and (N +1 ) -electron system. The response functions entering the ACFD theorem are approximated here by the direct random phase approximation (dRPA). For a set of prototypical semiconductors and insulators it is shown that even with this quite drastic approximation the resulting band gaps are very close to experiment and of a similar quality to those from the computationally more involved G W approximation. By going beyond the dRPA in the future the accuracy of the calculated band gaps may be significantly improved further.

  2. A generation/recombination model assisted with two trap centers in wide band-gap semiconductors

    Science.gov (United States)

    Yamaguchi, Ken; Kuwabara, Takuhito; Uda, Tsuyoshi

    2013-03-01

    A generation/recombination (GR) model assisted with two trap centers has been proposed for studying reverse current on pn junctions in wide band-gap semiconductors. A level (Et1) has been assumed to be located near the bottom of the conduction band and the other (Et2) to be near the top of the valence band. The GR model has been developed by assuming (1) a high-electric field; F, (2) a short distance; d, between trap centers, (3) reduction in an energy-difference; Δeff = |Et1 - Et2| - eFd, and (4) hopping or tunneling conductions between trap centers with the same energy-level (Δeff ≈ 0). The GR rate has been modeled by trap levels, capture cross-sections, trap densities, and transition rate between trap centers. The GR rate, about 1010 greater than that estimated from the single-level model, has been predicted on pn junctions in a material with band-gap of 3.1 eV. Device simulations using the proposed GR model have been demonstrated for SiC diodes with and without a guard ring. A reasonable range for reverse current at room temperature has been simulated and stable convergence has been obtained in a numerical scheme for analyzing diodes with an electrically floating region.

  3. Band-inverted gaps in InAs/GaSb and GaSb/InAs core-shell nanowires

    Science.gov (United States)

    Luo, Ning; Huang, Guang-Yao; Liao, Gaohua; Ye, Lin-Hui; Xu, H. Q.

    2016-12-01

    The [111]-oriented InAs/GaSb and GaSb/InAs core-shell nanowires have been studied by the 8 × 8 Luttinger-Kohn Hamiltonian to search for non-vanishing fundamental gaps between inverted electron and hole bands. We focus on the variations of the band-inverted fundamental gap, the hybridization gap, and the effective gap with the core radius and shell thickness of the nanowires. The evolutions of all the energy gaps with the structural parameters are shown to be dominantly governed by the effect of quantum confinement. With a fixed core radius, a band-inverted fundamental gap exists only at intermediate shell thicknesses. The maximum band-inverted gap found is ~4.4 meV for GaSb/InAs and ~3.5 meV for InAs/GaSb core-shell nanowires, and for the GaSb/InAs core-shell nanowires the gap persists over a wider range of geometrical parameters. The intrinsic reason for these differences between the two types of nanowires is that in the shell the electron-like states of InAs is more delocalized than the hole-like state of GaSb, while in the core the hole-like state of GaSb is more delocalized than the electron-like state of InAs, and both favor a stronger electron-hole hybridization.

  4. Reduction of Refractive Index Contrast Threshold for Photonic Band-Gap in Square Lattices

    Institute of Scientific and Technical Information of China (English)

    WANG Jian-Feng; HUANG Yi-Dong; ZHANG Wei; PENG Jiang-De

    2005-01-01

    @@ The threshold of refractive index contrast (RIC) to open a photonic band gap can be reduced by symmetry breaking. For the case of square lattice composed by dielectric cylinders, the absolute band gap is demonstrated by inserting small rods in the centre of the lattices, and the threshold RIC is reduced to 3.8. As for the square lattices composed by air holes in dielectric, the minimal RIC required for an absolute band gap decreases to 2.20.

  5. Non-Band-Gap Photoexcitation of Hydroxylated TiO2.

    Science.gov (United States)

    Zhang, Yu; Payne, Daniel T; Pang, Chi L; Fielding, Helen H; Thornton, Geoff

    2015-09-03

    The photochemistry of TiO2 has been studied intensively since it was discovered that TiO2 can act as a photocatalyst. Nevertheless, it has proven difficult to establish the detailed charge-transfer processes involved, partly because the excited states involved are difficult to study. Here we present evidence of the existence of hydroxyl-induced excited states in the conduction band region. Using two-photon photoemission, we show that stepwise photoexcitation from filled band gap states lying 0.8 eV below the Fermi level of rutile TiO2(110) excites hydroxyl-induced states 2.73 eV above the Fermi level that has an onset energy of ∼3.1 eV. The onset is shifted to lower energy by the coadsorption of molecular water, which suggests a means of tuning the energy of the excited state.

  6. Band gap tuning of armchair silicene nanoribbons using periodic hexagonal holes

    Energy Technology Data Exchange (ETDEWEB)

    Mehdi Aghaei, Sadegh; Calizo, Irene, E-mail: icalizo@fiu.edu [Department of Electrical and Computer Engineering, Florida International University, Miami, Florida 33174 (United States)

    2015-09-14

    The popularity of graphene owing to its unique and exotic properties has triggered a great deal of interest in other two-dimensional nanomaterials. Among them silicene shows considerable promise for electronic devices with a carrier mobility comparable to graphene, flexible buckled structure, and expected compatibility with silicon electronics. Using first-principle calculations based on density functional theory, the electronic properties of armchair silicene nanoribbons perforated with periodic nanoholes (ASiNRPNHs) are investigated. Two different configurations of mono-hydrogenated (:H) and di-hydrogenated (:2H) silicene edges are considered. Pristine armchair silicene nanoribbons (ASiNRs) can be categorized into three branches with width W = 3P − 1, 3P, and 3P + 1, P is an integer. The order of their energy gaps change from “E{sub G} (3P − 1) < E{sub G} (3P) < E{sub G} (3P + 1)” for W-ASiNRs:H to “E{sub G} (3P + 1) < E{sub G} (3P − 1) < E{sub G} (3P)” for W-ASiNRs:2H. We found the band gaps of W-ASiNRs:H and (W + 2)-ASiNRs:2H are slightly different, giving larger band gaps for wider ASiNRs:2H. ASiNRPNHs' band gaps changed based on the nanoribbon's width, nanohole's repeat periodicity and position relative to the nanoribbon's edge compared to pristine ASiNRs because of changes in quantum confinement strength. ASiNRPNHs:2H are more stable than ASiNRPNHs:H and their band gaps are noticeably greater than ASiNRPNHs:H. We found that the value of energy band gap for 12-ASiNRPNHs:2H with repeat periodicity of 2 is 0.923 eV. This value is about 2.2 times greater than pristine ASiNR:2H and double that of the 12-ASiNRPNHs:H with repeat periodicity of 2.

  7. Conductors with small Fermi energies and small gap energies

    Energy Technology Data Exchange (ETDEWEB)

    Thorn, R.J.

    1993-09-01

    If the Fermi energy is of the order of meV`s, the usual treatment of the density of free electrons is not valid, but use can be made of an averaged density of states that depends weakly on temperature, so that the temperature variation of the conductivity can be expressed by the equation: {sigma} {congruent} CT{sup (1-s)} 1n{l_brace}[(exp({beta}E{sub f}) + 1)/2][exp({minus}{beta}(E{sub g} {minus} E{sub f})) + 1)]{r_brace} in which E{sub f} is the Fermi energy, E{sub g} is the top of the energy gap for thermal activation, s is the exponent of the temperature-dependent scattering. This equation serves to define a class of solids consisting of a microcomposite with a narrow conduction band for which E{sub f} of the order of ceV`s or less and a thermal activated conduction for which E{sub g} is of the order of ceV`s. It describes quantitatively the conductivity, {sigma}(T;{Delta}, for YBa{sub 2}Cu{sub 3}O{sub 7-{Delta}} and {sigma}(T;p) as the hydrostatic pressure p is varied for {kappa}-(BEDT-TTF){sub 2}CuN(CN){sub 2}Br.

  8. Differences of Band Gap Characteristics of Square and Triangular Lattice Photonic Crystals in Terahertz Range

    Institute of Scientific and Technical Information of China (English)

    Jie Zha; Zhi-Yong Zhong; Huai-Wu Zhang; Qi-Ye Wen; Yuan-Xun Li

    2009-01-01

    Band gap characteristics of the photonic crystals in terahertz range with square lattice and triangular lattice of GaAs cylinders are comparatively studied by means of plane wave method (PWM). The influence of the radius on the band gap width is analyzed and the critical values where the band gap appears are put forward. The results show that themaximum band gap width of photonic crystal with triangular lattice of GaAs cylinders is much wider than that of photonic crystal with square lattice. The research provides a theoretic basis for the development of terahertz (THz) devices.

  9. Band gap engineering in silicene: A theoretical study of density functional tight-binding theory

    Science.gov (United States)

    Zaminpayma, Esmaeil; Nayebi, Payman

    2016-10-01

    In this work, we performed first principles calculations based on self-consistent charge density functional tight-binding to investigate different mechanisms of band gap tuning of silicene. We optimized structures of silicene sheet, functionalized silicene with H, CH3 and F groups and nanoribbons with the edge of zigzag and armchair. Then we calculated electronic properties of silicene, functionalized silicene under uniaxial elastic strain, silicene nanoribbons and silicene under external electrical fields. It is found that the bond length and buckling value for relaxed silicene is agreeable with experimental and other theoretical values. Our results show that the band gap opens by functionalization of silicene. Also, we found that the direct band gap at K point for silicene changed to the direct band gap at the gamma point. Also, the functionalized silicene band gap decrease with increasing of the strain. For all sizes of the zigzag silicene nanoribbons, the band gap is near zero, while an oscillating decay occurs for the band gap of the armchair nanoribbons with increasing the nanoribbons width. At finally, it can be seen that the external electric field can open the band gap of silicene. We found that by increasing the electric field magnitude the band gap increases.

  10. Low Band Gap Polymers for Roll-to-Roll Coated Polymer Solar Cells

    DEFF Research Database (Denmark)

    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 expl......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...

  11. Band gap calculation and photo catalytic activity of rare earths doped rutile TiO2

    Institute of Scientific and Technical Information of China (English)

    BIAN Liang; SONG Mianxin; ZHOU Tianliang; ZHAO Xiaoyong; DAI Qingqing

    2009-01-01

    The density of states (DOS) of 17 kinds of rare earths (RE) doped futile TiO2 was by using fast-principles density functional the-ory (DFF) calculation. The band gap widths of RE doped rutile TiO2 were important factors for altering their absorbing wavelengths. The results show that RE ions could obviously reduce the band gap widths and form of energy of rutile TiO2 except Lu, Y, Yb and Sc, and the order of absorbing wavelengths of RE doped rutile TiO2 were the same as that of the results of calculation. The ratio of RE dopant was an-other important factor for the photo catalytic activity of RE doped rutile TiO2, and there was an optimal ratio of dopant. There was a constant for predigesting the calculation difficulty, respectively, which were 0.5mol.% and 100 mol-1 under supposition. The band gap widths of RE doped rutile TiO2 by DFT calculation were much larger than that by experiment. Finally, by transferring the calculation values to experiment values, it could be found and predicted that RE enlarged obviously the absorbing wavelengh of futile TiO2. In addition, the degree of RE ions edging out the Ti atom using the parameters of RE elements was computed.

  12. Tuning the electronic band-gap of fluorinated 3C-silicon carbide nanowires

    Science.gov (United States)

    Miranda Durán, Álvaro; Trejo Baños, Alejandro; Pérez, Luis Antonio; Cruz Irisson, Miguel

    The possibility of control and modulation of the electronic properties of silicon carbide nanowires (SiCNWs) by varying the wire diameter is well known. SiCNWs are particularly interesting and technologically important, due to its electrical and mechanical properties, allowing the development of materials with specific electronic features for the design of stable and robust electronic devices. Tuning the band gap by chemical surface passivation constitutes a way for the modification of the electronic band gap of these nanowires. We present, the structural and electronic properties of fluorinated SiCNWs, grown along the [111] crystallographic direction, which are investigated by first principles. We consider nanowires with six diameters, varying from 0.35 nm to 2.13 nm, and eight random covering schemes including fully hydrogen- and fluorine terminated ones. Gibbs free energy of formation and electronic properties were calculated for the different surface functionalization schemes and diameters considered. The results indicate that the stability and band gap of SiCNWs can be tuned by surface passivation with fluorine atoms This work was supported by CONACYT infrastructure project 252749 and UNAM-DGAPA-PAPIIT IN106714. A.M. would like to thank for financial support from CONACyT-Retención. Computing resources from proyect SC15-1-IR-27 of DGTIC-UNAM are acknowledged.

  13. Photonic-Band-Gap Traveling-Wave Gyrotron Amplifier

    Science.gov (United States)

    Nanni, E. A.; Lewis, S. M.; Shapiro, M. A.; Griffin, R. G.; Temkin, R. J.

    2014-01-01

    We report the experimental demonstration of a gyrotron traveling-wave-tube amplifier at 250 GHz that uses a photonic band gap (PBG) interaction circuit. The gyrotron amplifier achieved a peak small signal gain of 38 dB and 45 W output power at 247.7 GHz with an instantaneous −3 dB bandwidth of 0.4 GHz. The amplifier can be tuned for operation from 245–256 GHz. The widest instantaneous −3 dB bandwidth of 4.5 GHz centered at 253.25 GHz was observed with a gain of 24 dB. The PBG circuit provides stability from oscillations by supporting the propagation of transverse electric (TE) modes in a narrow range of frequencies, allowing for the confinement of the operating TE03-like mode while rejecting the excitation of oscillations at nearby frequencies. This experiment achieved the highest frequency of operation for a gyrotron amplifier; at present, there are no other amplifiers in this frequency range that are capable of producing either high gain or high output power. This result represents the highest gain observed above 94 GHz and the highest output power achieved above 140 GHz by any conventional-voltage vacuum electron device based amplifier. PMID:24476286

  14. Pattern reconfigurable antenna using electromagnetic band gap structure

    Science.gov (United States)

    Ismail, M. F.; Rahim, M. K. A.; Majid, H. A.; Hamid, M. R.; Yusoff, M. F. M.; Dewan, R.

    2017-01-01

    In this paper, a single rectangular patch antenna incorporated with an array of electromagnetic band gap (EBG) structures is proposed. The proposed antenna features radiation pattern agility by means of connecting the shorting pin vias to the EBG unit cells. The proposed design consists of 32 mm × 35.5 mm rectangular patch antenna and 10.4-mm-square mushroom-like EBG unit cells. The EBGs are placed at both sides of the antenna radiating patch and located on the thicker substrate of thickness, h. The copper tape which represents the PIN diode is used to control the connection between the EBG's via and the ground plane as reconfigurable mechanism of the antenna. The simulated result shows by switching the ON and OFF EBG structures in either sides or both, the directional radiation pattern can be tilted from 0 to +14°. The proposed antenna exhibits 7.2 dB realized gain at 2.42 GHz. The parametric study on EBG and antenna is also discussed.

  15. The study of response of wide band gap semiconductor detectors using the Geant4

    Directory of Open Access Journals (Sweden)

    Hussain Riaz

    2014-01-01

    Full Text Available The energy dependence on the intrinsic efficiency, absolute efficiency, full energy peak absolute efficiency and peak-to-total ratio have been studied for various wide band gap semiconductor detectors using the Geant4 based Monte Carlo simulations. The detector thickness of 1-4 mm and the area in 16-100 mm2 range were considered in this work. In excellent agreement with earlier work (Rybka et al., [20], the Geant4 simulated values of detector efficiencies have been found to decrease with incident g-ray energy. Both for the detector thickness and the detector area, the increasing trends have been observed for total efficiency as well as for full-energy peak efficiency in 0.1 MeV-50 MeV range. For Cd1-xZnxTe, the detector response remained insensitive to changes in relative proportions of Zn. For various wide band gap detectors studied in this work, the detection efficiency of TlBr was found highest over the entire range of energy, followed by the HgI2, CdTe, and then by CZT.

  16. Increased visible-light photocatalytic activity of TiO2 via band gap manipulation

    Science.gov (United States)

    Pennington, Ashley Marie

    Hydrogen gas is a clean burning fuel that has potential applications in stationary and mobile power generation and energy storage, but is commercially produced from non-renewable fossil natural gas. Using renewable biomass as the hydrocarbon feed instead could provide sustainable and carbon-neutral hydrogen. We focus on photocatalytic oxidation and reforming of methanol over modified titanium dioxide (TiO2) nanoparticles to produce hydrogen gas. Methanol is used as a model for biomass sugars. By using a photocatalyst, we aim to circumvent the high energy cost of carrying out endothermic reactions at commercial scale. TiO2 is a semiconductor metal oxide of particular interest in photocatalysis due to its photoactivity under ultraviolet illumination and its stability under catalytic reaction conditions. However, TiO2 primarily absorbs ultraviolet light, with little absorption of visible light. While an effective band gap for absorbance of photons from visible light is 1.7 eV, TiO2 polymorphs rutile and anatase, have band gaps of 3.03 eV and 3.20 eV respectively, which indicate ultraviolet light. As most of incident solar radiation is visible light, we hypothesize that decreasing the band gap of TiO2 will increase the efficiency of TiO2 as a visible-light active photocatalyst. We propose to modify the band gap of TiO2 by manipulating the catalyst structure and composition via metal nanoparticle deposition and heteroatom doping in order to more efficiently utilize solar radiation. Of the metal-modified Degussa P25 TiO2 samples (P25), the copper and nickel modified samples, 1%Cu/P25 and 1%Ni/P25 yielded the lowest band gap of 3.05 eV each. A difference of 0.22 eV from the unmodified P25. Under visible light illumination 1%Ni/P25 and 1%Pt/P25 had the highest conversion of methanol of 9.9% and 9.6%, respectively.

  17. Nanoscale Imaging of Band Gap and Defects in Polycrystalline CdTe Photovoltaic Devices

    Science.gov (United States)

    Zhitenev, Nikolai; Yoon, Yohan; Chae, Jungseok; Katzenmeyer, Aaron; Yoon, Heayoung; An, Sangmin; Shumacher, Joshua; Centrone, Andrea

    To further increase the power efficiency of polycrystalline thin film photovoltaic (PV) technology, a detailed understanding of microstructural properties of the devices is required. In this work, we investigate the microstructure of CdTe PV devices using two optical spectroscopies. Sub-micron thickness lamella samples were cut out from a PV device, either in cross-section or in-plane, by focused ion beam. The first technique is the photothermal induced resonance (PTIR) used to obtain absorption spectra over a broad range of wavelengths. In PTIR, a wavelength tunable pulsed laser is combined with an atomic force microscope to detect the local thermal expansion of lamella CdTe sample induced by light absorption. The second technique based on a near-field scanning optical microscope maps the local absorption at fixed near-IR wavelengths with energies at or below CdTe band-gap energy. The variation of the band gap throughout the CdTe absorber determined from PTIR spectra is ~ 20 meV. Both techniques detect strong spatial variation of shallow defects over different grains. The spatial distribution of mid-gap defects appears to be more uniform. The resolution, the sensitivity and the applicability of these two approaches are compared.

  18. Band gap engineering of ZnO by doping with Mg

    Science.gov (United States)

    Rana, N.; Chand, Subhash; Gathania, Arvind K.

    2015-08-01

    Mg-doped zinc oxide (MgxZn1-xO (0 ≤ x ≤ 0.20)) samples were synthesized by polymeric precursor method. The structural and optical properties were investigated by x-ray diffractometer (XRD), field emission scanning electron microscope (FE-SEM), UV-visible spectroscopy, Fourier transform infrared (FTIR) and Raman spectroscopy. XRD patterns reveal that synthesized samples have a wurtzite structure. Lattice parameters, the degree of distortion of the samples were calculated from the XRD. SEM images show that the synthesized samples contain the elongated spherical shaped grains. The Raman scattering investigation and FTIR spectra authenticate the presence of Mg in the system and also show phase segregation at the higher Mg doping concentration. Optical band gap energy is determined from the Tauc relation. It is interesting to know that optical band energy exhibits blue shift with the increase of Mg doping concentration up to 16 mole %.

  19. {ital In Situ} Band Gap Engineering of Carbon Nanotubes

    Energy Technology Data Exchange (ETDEWEB)

    Crespi, V.H.; Cohen, M.L. [Department of Physics, University of California at Berkeley, Berkeley, California 94720, and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 (United States); Rubio, A. [Departamento de Fisica Teorica, Universidad de Valladolid, E-47011 Valladolid (Spain)

    1997-09-01

    Bond rotation defects close the gap in large-gap nanotubes, open the gap in small-gap nanotubes, and increase the density of states in metallic nanotubes. Not only are these defects likely to be present in as-grown nanotubes, but they could be introduced locally into intact nanotubes, thereby opening a new road towards device applications. {copyright} {ital 1997} {ital The American Physical Society}

  20. Controllable Synthesis of Band Gap-Tunable and Monolayer Transition Metal Dichalcogenide Alloys

    Directory of Open Access Journals (Sweden)

    Sheng-Han eSu

    2014-07-01

    Full Text Available The electronic and optical properties of transition metal dichalcogenide (TMD materials are directly governed by their energy gap; thus, the band gap engineering has become an important topic recently. Theoretical and some experimental results have indicated that these monolayer TMD alloys exhibit direct-gap properties and remain stable at room temperature, making them attractive for optoelectronic applications. Here we systematically compared the two approaches of forming MoS2xSe2(1-x monolayer alloys: selenization of MoS2 and sulfurization of MoSe2. The optical energy gap of as-grown CVD MoS2 can be continuously modulated from 1.86 eV (667 nm to 1.57 eV (790 nm controllable by the reaction temperature. Spectroscopic and microscopic evidences show that the Mo-S bonds can be replaced by the Mo-Se bonds in a random and homogeneous manner. By contrast, the replacement of Mo-Se by Mo-S does not randomly occur in the MoSe2 lattice, where the reaction preferentially occurs along the crystalline orientation of MoSe2 and thus the MoSe2/MoS2 biphases are easily observed in the alloys, which makes the optical band gap of these alloys distinctly different. Therefore, the selenization of metal disulfide is preferred and the proposed synthetic strategy opens up a simple route to control the atomic structure as well as optical properties of monolayer TMD alloys.

  1. High-power picosecond pulse delivery through hollow core photonic band gap fibers

    DEFF Research Database (Denmark)

    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...

  2. Local density of optical states of an asymmetric waveguide grating at photonic band gap resonant wavelength

    Science.gov (United States)

    Alatas, Husin; Sumaryada, Tony I.; Ahmad, Faozan

    2015-01-01

    We have investigated the characteristics of local density of optical states (LDOS) at photonic band gap resonant wavelength of an asymmetric waveguide grating based on Green's function formulation. It is found that the LDOS of the considered structure exhibits different characteristics in its localization between the upper and lower resonant wavelengths of the corresponding photonic band gap edges.

  3. True photonic band-gap mode-control in VCSEL structures

    DEFF Research Database (Denmark)

    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....

  4. Spectroscopy of photonic band gaps in mesoporous one-dimensional photonic crystals based on aluminum oxide

    Science.gov (United States)

    Gorelik, V. S.; Voinov, Yu. P.; Shchavlev, V. V.; Bi, Dongxue; Shang, Guo Liang; Fei, Guang Tao

    2016-12-01

    Mesoporous one-dimensional photonic crystals based on aluminum oxide have been synthesized by electrochemical etching method. Reflection spectra of the obtained mesoporous samples in a wide spectral range that covers several band gaps are presented. Microscopic parameters of photonic crystals are calculated and corresponding reflection spectra for the first six band gaps are presented.

  5. Maximizing phononic band gaps in piezocomposite materials by means of topology optimization.

    Science.gov (United States)

    Vatanabe, Sandro L; Paulino, Glaucio H; Silva, Emílio C N

    2014-08-01

    Phononic crystals (PCs) can exhibit phononic band gaps within which sound and vibrations at certain frequencies do not propagate. In fact, PCs with large band gaps are of great interest for many applications, such as transducers, elastic/acoustic filters, noise control, and vibration shields. Previous work in the field concentrated on PCs made of elastic isotropic materials; however, band gaps can be enlarged by using non-isotropic materials, such as piezoelectric materials. Because the main property of PCs is the presence of band gaps, one possible way to design microstructures that have a desired band gap is through topology optimization. Thus in this work, the main objective is to maximize the width of absolute elastic wave band gaps in piezocomposite materials designed by means of topology optimization. For band gap calculation, the finite element analysis is implemented with Bloch-Floquet theory to solve the dynamic behavior of two-dimensional piezocomposite unit cells. Higher order frequency branches are investigated. The results demonstrate that tunable phononic band gaps in piezocomposite materials can be designed by means of the present methodology.

  6. Theoretical study of relative width of photonic band gap for the 3-D dielectric structure

    Indian Academy of Sciences (India)

    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.

  7. Atypically small temperature-dependence of the direct band gap in the metastable semiconductor copper nitride Cu3N

    Science.gov (United States)

    Birkett, Max; Savory, Christopher N.; Fioretti, Angela N.; Thompson, Paul; Muryn, Christopher A.; Weerakkody, A. D.; Mitrovic, I. Z.; Hall, S.; Treharne, Rob; Dhanak, Vin R.; Scanlon, David O.; Zakutayev, Andriy; Veal, Tim D.

    2017-03-01

    The temperature-dependence of the direct band gap and thermal expansion in the metastable anti-ReO3 semiconductor Cu3N are investigated between 4.2 and 300 K by Fourier-transform infrared spectroscopy and x-ray diffraction. Complementary refractive index spectra are determined by spectroscopic ellipsometry at 300 K . A direct gap of 1.68 eV is associated with the absorption onset at 300 K , which strengthens continuously and reaches a magnitude of 3.5 ×105cm-1 at 2.7 eV , suggesting potential for photovoltaic applications. Notably, the direct gap redshifts by just 24 meV between 4.2 and 300 K , giving an atypically small band-gap temperature coefficient d Eg/d T of -0.082 meV /K . Additionally, the band structure, dielectric function, phonon dispersion, linear expansion, and heat capacity are calculated using density functional theory; remarkable similarities between the experimental and calculated refractive index spectra support the accuracy of these calculations, which indicate beneficially low hole effective masses and potential negative thermal expansion below 50 K . To assess the lattice expansion contribution to the band-gap temperature-dependence, a quasiharmonic model fit to the observed lattice contraction finds a monotonically decreasing linear expansion (descending past 10-6K-1 below 80 K ), while estimating the Debye temperature, lattice heat capacity, and Grüneisen parameter. Accounting for lattice and electron-phonon contributions to the observed band-gap evolution suggests average phonon energies that are qualitatively consistent with predicted maxima in the phonon density of states. As band-edge temperature-dependence has significant consequences for device performance, copper nitride should be well suited for applications that require a largely temperature-invariant band gap.

  8. Simultaneous existence of phononic and photonic band gaps in periodic crystal slabs.

    Science.gov (United States)

    Pennec, Y; Djafari Rouhani, B; El Boudouti, E H; Li, C; El Hassouani, Y; Vasseur, J O; Papanikolaou, N; Benchabane, S; Laude, V; Martinez, A

    2010-06-21

    We discuss the simultaneous existence of phononic and photonic band gaps in a periodic array of holes drilled in a Si membrane. We investigate in detail both the centered square lattice and the boron nitride (BN) lattice with two atoms per unit cell which include the simple square, triangular and honeycomb lattices as particular cases. We show that complete phononic and photonic band gaps can be obtained from the honeycomb lattice as well as BN lattices close to honeycomb. Otherwise, all investigated structures present the possibility of a complete phononic gap together with a photonic band gap of a given symmetry, odd or even, depending on the geometrical parameters.

  9. Depth-resolved band gap in Cu(In,Ga)(S,Se)2 thin films

    Energy Technology Data Exchange (ETDEWEB)

    Bar, M.; Nishiwaki, S.; Weinhardt, L.; Pookpanratana, S.; Fuchs, O.; Blum, M.; Yang, W.; Denlinger, J. D.; Shafarman, W.; Heske, C.

    2008-06-24

    The surface composition of Cu(In,Ga)(S,Se)2 (?CIGSSe?) thin films intrinsically deviates from the corresponding bulk composition, which also modifies the electronic structure and thus the optical properties.We have used a combination of photon and electron spectroscopies with different information depths to gain depth-resolved information on the band gap energy (Eg) in CIG(S)Se thin films. We find an increasing Eg with decreasing information depth, indicating the formation of a surface region with significantly higher Eg. This Eg-widened surface region extends further into the bulk of the sulfur-free CIGSe thin film compared to the CIGSSe thin film.

  10. Depth-resolved band gap in Cu(In,Ga)(S,Se)2 thin films

    Science.gov (United States)

    Bär, M.; Nishiwaki, S.; Weinhardt, L.; Pookpanratana, S.; Fuchs, O.; Blum, M.; Yang, W.; Denlinger, J. D.; Shafarman, W. N.; Heske, C.

    2008-12-01

    The surface composition of Cu(In,Ga)(S,Se)2 ("CIGSSe") thin films intrinsically deviates from the corresponding bulk composition, which also modifies the electronic structure and thus the optical properties. We used a combination of photon and electron spectroscopies with different information depths to gain depth-resolved information on the band gap energy (Eg) in CIG(S)Se thin films. We find an increasing Eg with decreasing information depth, indicating the formation of a surface region with significantly higher Eg. This Eg-widened surface region extends further into the bulk of the sulfur-free CIGSe thin film compared to the CIGSSe thin film.

  11. A Large-Area Transferable Wide Band Gap 2D Silicon Dioxide Layer.

    Science.gov (United States)

    Büchner, Christin; Wang, Zhu-Jun; Burson, Kristen M; Willinger, Marc-Georg; Heyde, Markus; Schlögl, Robert; Freund, Hans-Joachim

    2016-08-23

    An atomically smooth silica bilayer is transferred from the growth substrate to a new support via mechanical exfoliation at millimeter scale. The atomic structure and morphology are maintained perfectly throughout the process. A simple heating treatment results in complete removal of the transfer medium. Low-energy electron diffraction, Auger electron spectroscopy, scanning tunneling microscopy, and environmental scanning electron microscopy show the success of the transfer steps. Excellent chemical and thermal stability result from the absence of dangling bonds in the film structure. By adding this wide band gap oxide to the toolbox of 2D materials, possibilities for van der Waals heterostructures will be broadened significantly.

  12. Robust band gap and half-metallicity in graphene with triangular perforations

    DEFF Research Database (Denmark)

    Gregersen, Søren Schou; Power, Stephen; Jauho, Antti-Pekka

    2016-01-01

    . 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...... 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......, and furthermore, of exploiting the strong spin dependence of the system for spintronic applications....

  13. Grain size dependent optical band gap of CdI2 films

    Indian Academy of Sciences (India)

    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.

  14. Photonic band gap of one-dimensional periodic structure containing dispersive left-handed metamaterials

    Institute of Scientific and Technical Information of China (English)

    Zhanshan Wang; Tian Sang; Fengli Wang; Yonggang Wu; Lingyan Chen

    2008-01-01

    Band structures of one-dimensional(1D)photonic crystals(PCs)containing dispersive left-handed metamaterials are studied theoretically.The results show that the structure possesses a type of photonic band gap originating from total internal reflection(TIR).In contrast to photonic band gaps corresponding to zero average refractive index and zero phase.the TIR gap exhibits sharp angular effect and has no polarization effect.It should also be noted that band structures of transverse electric(TE) and transverse magnetic(TM) mode waves are exactly the same in the PCs we studied.

  15. Density of States for Warped Energy Bands

    Science.gov (United States)

    Mecholsky, Nicholas A.; Resca, Lorenzo; Pegg, Ian L.; Fornari, Marco

    2016-02-01

    Warping of energy bands can affect the density of states (DOS) in ways that can be large or subtle. Despite their potential for significant practical impacts on materials properties, these effects have not been rigorously demonstrated previously. Here we rectify this using an angular effective mass formalism that we have developed. To clarify the often confusing terminology in this field, “band warping” is precisely defined as pertaining to any multivariate energy function E(k) that does not admit a second-order differential at an isolated critical point in k-space, which we clearly distinguish from band non-parabolicity. We further describe band “corrugation” as a qualitative form of band warping that increasingly deviates from being twice differentiable at an isolated critical point. These features affect the density-of-states and other parameters ascribed to band warping in various ways. We demonstrate these effects, providing explicit calculations of DOS and their effective masses for warped energy dispersions originally derived by Kittel and others. Other physical and mathematical examples are provided to demonstrate fundamental distinctions that must be drawn between DOS contributions that originate from band warping and contributions that derive from band non-parabolicity. For some non-degenerate bands in thermoelectric materials, this may have profound consequences of practical interest.

  16. Temperature induced band gap shrinkage in Cu 2GeSe 3: Role of electron-phonon interaction

    Science.gov (United States)

    Sarkar, Bimal Kumar; Verma, Ajay Singh; Deviprasad, P. S.

    2011-07-01

    The ternary semiconducting compound Cu 2GeSe 3 has been investigated for optical properties with photoacoustic spectroscopy. Optical absorption spectra of Cu 2GeSe 3 is obtained in the range of 0.76-0.81 eV photon-energy at temperatures between 80 and 300 K. The thermal variation of band gap energy has been examined from the optical absorption spectra at different temperatures. The temperature induced band gap shrinkage has been explained on the basis of electron-phonon interaction. Varshni's empirical relation in conjunction with Vina and Passler model is taken into consideration for data fitting. The Debye temperature was calculated approximately as 240 K. The acoustic phonons with a characteristic temperature as 160 K corresponding to effective mean frequency have been attributed to the thermal variation of the energy gap.

  17. Simplification of QPLDA: A practical method to the correction for the LDA band gap problem

    CERN Document Server

    Kikuchi, Akihito

    2012-01-01

    It is necessary to employ quasi-particle calculations to correct band gap problems in LDA. As an expedient way for the reliable but massive GWA, Quasi-Particle-LDA (QPLDA) is proposed by several authors, where the total computational cost scales with N. Historically, Sham and Kohn introduced the idea of the local mass operator based on local wavenumber similar to WKB, but did not execute actual numerical calculations. They took into account exchange interaction only, from which we could not expect precise treatments anyway. Later, Pickett and Wang had proposed a more qualitative method and had shown its potentiality in examples of semiconductors, such as silicon, diamond and GaP. They used a model analytic formula for the dialectic function and adopted a model energy dispersion, which is free-electron like one, except that, being accompanied with an artificial band-gap discontinuity. The latter method has two shortcomings. First, to execute the calculation, several parameters are needed, such as the macroscop...

  18. Strain Engineering of the Band Gap of HgTe Quantum Wells Using Superlattice Virtual Substrates

    Science.gov (United States)

    Leubner, Philipp; Lunczer, Lukas; Brüne, Christoph; Buhmann, Hartmut; Molenkamp, Laurens W.

    2016-08-01

    The HgTe quantum well (QW) is a well-characterized two-dimensional topological insulator (2D TI). Its band gap is relatively small (typically on the order of 10 meV), which restricts the observation of purely topological conductance to low temperatures. Here, we utilize the strain dependence of the band structure of HgTe QWs to address this limitation. We use CdTe-Cd 0.5Zn0.5Te strained-layer superlattices on GaAs as virtual substrates with adjustable lattice constant to control the strain of the QW. We present magnetotransport measurements, which demonstrate a transition from a semimetallic to a 2D-TI regime in wide QWs, when the strain is changed from tensile to compressive. Most notably, we demonstrate a much enhanced energy gap of 55 meV in heavily compressively strained QWs. This value exceeds the highest possible gap on common II-VI substrates by a factor of 2-3, and extends the regime where the topological conductance prevails to much higher temperatures.

  19. Band Gap Narrowing and Widening of ZnO Nanostructures and Doped Materials.

    Science.gov (United States)

    Kamarulzaman, Norlida; Kasim, Muhd Firdaus; Rusdi, Roshidah

    2015-12-01

    Band gap change in doped ZnO is an observed phenomenon that is very interesting from the fundamental point of view. This work is focused on the preparation of pure and single phase nanostructured ZnO and Cu as well as Mn-doped ZnO for the purpose of understanding the mechanisms of band gap narrowing in the materials. ZnO, Zn0.99Cu0.01O and Zn0.99Mn0.01O materials were prepared using a wet chemistry method, and X-ray diffraction (XRD) results showed that all samples were pure and single phase. UV-visible spectroscopy showed that materials in the nanostructured state exhibit band gap widening with respect to their micron state while for the doped compounds exhibited band gap narrowing both in the nano and micron states with respect to the pure ZnO materials. The degree of band gap change was dependent on the doped elements and crystallite size. X-ray photoelectron spectroscopy (XPS) revealed that there were shifts in the valence bands. From both UV-visible and XPS spectroscopy, it was found that the mechanism for band gap narrowing was due to the shifting of the valance band maximum and conduction band minimum of the materials. The mechanisms were different for different samples depending on the type of dopant and dimensional length scales of the crystallites.

  20. Ionization of Rydberg H atoms at band-gap metal surfaces via surface and image states

    CERN Document Server

    So, E; Softley, T P

    2015-01-01

    Wavepacket propagation calculations are reported for the interaction of a Rydberg hydrogen atom ($n=2-8)$ with Cu(111) and Cu(100) surfaces (represented by a Chulkov potential), in comparison with a Jellium surface. Both copper surfaces have a projected band gap at the surface in the energy range degenerate with some or all of the Rydberg energies. The charge transfer of the Rydberg electron to the surface is found to be enhanced for $n$ values at which there is a near-degeneracy between the Rydberg energy level and an image state or a surface state of the surface. The enhancement is facilitated by the strong overlap of the surface image-state orbital lying outside the surface and the orbital of the incoming Rydberg atom. These calculations point to the possibility of using Rydberg-surface collisions as a probe of surface electronic structure.

  1. Using Wannier functions to improve solid band gap predictions in density functional theory

    Science.gov (United States)

    Ma, Jie; Wang, Lin-Wang

    2016-04-01

    Enforcing a straight-line condition of the total energy upon removal/addition of fractional electrons on eigen states has been successfully applied to atoms and molecules for calculating ionization potentials and electron affinities, but fails for solids due to the extended nature of the eigen orbitals. Here we have extended the straight-line condition to the removal/addition of fractional electrons on Wannier functions constructed within the occupied/unoccupied subspaces. It removes the self-interaction energies of those Wannier functions, and yields accurate band gaps for solids compared to experiments. It does not have any adjustable parameters and the computational cost is at the DFT level. This method can also work for molecules, providing eigen energies in good agreement with experimental ionization potentials and electron affinities. Our approach can be viewed as an alternative approach of the standard LDA+U procedure.

  2. Band gap bowing and electron localization of (GaxIn1-x)N

    Energy Technology Data Exchange (ETDEWEB)

    Lee, Byounghak; Wang, Lin-Wang

    2006-05-09

    The band gap bowing and the electron localization ofGaxIn1-xN are calculated using both the local density approximation (LDA)and screened-exchange local density functional (sX-LDA) methods. Thecalculated sX-LDA band gaps are in good agreement with the experimentallyobserved values, with errors of -0.26 and 0.09 eV for bulk GaN and InN,respectively. The LDA band gap errors are 1.33 and 0.81 eV for GaN andInN, in order. In contrast to the gap itself, the band gap bowingparameter is found to be very similar in sX-LDA and LDA. We identify thelocalization of hole states in GaxIn1-xN alloys along In-N-In chains. Thepredicted localizationis stronger in sX-LDA.

  3. Reversible tuning of ZnO optical band gap by plasma treatment

    Energy Technology Data Exchange (ETDEWEB)

    Lee, Szetsen, E-mail: slee@cycu.edu.tw [Department of Chemistry and Center for Nano-technology, Chung Yuan Christian University, Jhongli, Taoyuan 32023, Taiwan (China); Peng, Jr-Wei [Department of Chemistry and Center for Nano-technology, Chung Yuan Christian University, Jhongli, Taoyuan 32023, Taiwan (China); Ho, Ching-Yuan [Department of Mechanical Engineering, Chung Yuan Christian University, Jhongli, Taoyuan 32023, Taiwan (China)

    2011-12-15

    Highlights: Black-Right-Pointing-Pointer The ZnO optical band gap blue-shifts with hydrogen plasma treatment. Black-Right-Pointing-Pointer The ZnO optical band gap red-shifts with oxygen plasma treatment. Black-Right-Pointing-Pointer 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.

  4. Compositional dependence of optical band gap and refractive index in lead and bismuth borate glasses

    Energy Technology Data Exchange (ETDEWEB)

    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.

  5. X-Band Photonic Band-Gap Accelerator Structure Breakdown Experiment

    Energy Technology Data Exchange (ETDEWEB)

    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.

  6. Structure, band gap, and Mn-related mid-gap states in epitaxial single crystal (Zn1-xMgx)1-yMnyO thin films

    Science.gov (United States)

    Zhu, Dapeng; Liu, Guolei; Xiao, Shuqin; Yan, Shishen; He, Shumin; Cai, Li; Li, Qinghao; Cao, Qiang; Hu, Shujun; Chen, Yanxue; Kang, Shishou; Mei, Liangmo

    2013-05-01

    Epitaxial (Zn1-xMgx)1-yMnyO thin films were grown on c-Al2O3 substrates by radio frequency oxygen plasma assisted molecular beam epitaxy. Single crystal structure of the (Zn1-xMgx)1-yMnyO films was revealed by reflection high energy electron diffraction and X-ray diffraction. The band gap of the films can be tuned dramatically with increasing the Mg concentration, while the onset energy of Mn-related mid-gap absorption band only shows a small blue shift. Photoconductivity measurements indicate the Mn-related mid-gap states in (Zn1-xMgx)1-yMnyO films can create free carriers and contribute to charge transfer transitions. The conduction band offset ΔEC = 0.13 eV and valence band offset ΔEV = 0.1 eV were obtained for ZnO/Zn0.8Mg0.2O heterostructures, which increase to ΔEC = 0.21 eV and ΔEV = 0.14 eV for ZnO/Zn0.7Mg0.3O heterostructures.

  7. Observation of band gaps in the gigahertz range and deaf bands in a hypersonic aluminum nitride phononic crystal slab

    Science.gov (United States)

    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.

  8. Synthesis and Characterization of Small Band-gap Conjugated Polymers - Poly(pyrrolyl methines)

    Institute of Scientific and Technical Information of China (English)

    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.

  9. Localization and characterization of the metallic band gaps in a ternary metallo-dielectric photonic crystal

    Science.gov (United States)

    Alejo-Molina, Adalberto; Romero-Antequera, David L.; Sánchez-Mondragón, José J.

    2014-02-01

    In this work, we demonstrate the existence of structural metallic band gaps in a ternary material, dielectric-dielectric-metal, and we show analytical equations for their computation. We show the existence of metallic band gaps not only in the lowest band but also for high frequencies. These gaps are structural ones but different and additional to the dielectric ones in the dielectric photonic crystal substrate. Therefore, as the desire properties of both, the dielectric and metallic photonic crystals, are present the applications for this particular structure are straightforward.

  10. Optimization of band gaps of 2D photonic crystals by the rapid generic algorithm

    Institute of Scientific and Technical Information of China (English)

    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.

  11. Monolithic phononic crystals with a surface acoustic band gap from surface phonon-polariton coupling.

    Science.gov (United States)

    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.

  12. Modelling and design of complete photonic band gaps in two-dimensional photonic crystals

    Indian Academy of Sciences (India)

    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.

  13. Study of periodic band gap structure of the magnetized plasma photonic crystals

    Institute of Scientific and Technical Information of China (English)

    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.

  14. Large Frequency Range of Photonic Band Gaps on Porous Silicon Heterostructures for Infrared Applications

    CERN Document Server

    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.

  15. Engineering the electronic structure and band gap of boron nitride nanoribbon via external electric field

    Science.gov (United States)

    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.

  16. Flexural vibration band gaps in thin plates with two-dimensional binary locally resonant structures

    Institute of Scientific and Technical Information of China (English)

    Yu Dian-Long; Wang Gang; Liu Yao-Zong; Wen Ji-Hong; Qiu Jing

    2006-01-01

    The complete flexural vibration band gaps are studied in the thin plates with two-dimensional binary locally resonant structures, i.e. the composite plate consisting of soft rubber cylindrical inclusions periodically placed in a host material. Numerical simulations show that the low-frequency gaps of flexural wave exist in the thin plates. The width of the first gap decreases monotonically as the matrix density increases. The frequency response of the finite periodic thin plates is simulated by the finite element method, which provides attenuations of over 20dB in the frequency range of the band gaps. The findings will be significant in the application of phononic crystals.

  17. Band gaps and cavity modes in dual phononic and photonic strip waveguides

    Directory of Open Access Journals (Sweden)

    Y. Pennec

    2011-12-01

    Full Text Available We discuss theoretically the simultaneous existence of phoxonic, i.e., dual phononic and photonic, band gaps in a periodic silicon strip waveguide. The unit-cell of this one-dimensional waveguide contains a hole in the middle and two symmetric stubs on the sides. Indeed, stubs and holes are respectively favorable for creating a phononic and a photonic band gap. Appropriate geometrical parameters allow us to obtain a complete phononic gap together with a photonic gap of a given polarization and symmetry. The insertion of a cavity inside the perfect structure provides simultaneous confinement of acoustic and optical waves suitable to enhance the phonon-photon interaction.

  18. Temperature dependence of band gaps in semiconductors: electron-phonon interaction

    Energy Technology Data Exchange (ETDEWEB)

    Kremer, Reinhard K.; Cardona, M.; Lauck, R. [MPI for Solid State Research, Stuttgart (Germany); Bhosale, J.; Ramdas, A.K. [Physics Dept., Purdue University, West Lafayette, IN (United States); Burger, A. [Fisk University, Dept. of Life and Physical Sciences, Nashville, TN (United States); Munoz, A. [MALTA Consolider Team, Dept. de Fisica Fundamental II, Universidad de La Laguna, Tenerife (Spain); Instituto de Materiales y Nanotecnologia, Universidad de La Laguna, Tenerife (Spain); Romero, A.H. [CINVESTAV, Dept. de Materiales, Unidad Queretaro, Mexico (Mexico); MPI fuer Mikrostrukturphysik, Halle an der Saale (Germany)

    2013-07-01

    We investigate the temperature dependence of the energy gap of several semiconductors with chalcopyrite structure and re-examine literature data and analyze own high-resolution reflectivity spectra in view of our new ab initio calculations of their phonon properties. This analysis leads us to distinguish between materials with d-electrons in the valence band (e.g. CuGaS{sub 2}, AgGaS{sub 2}) and those without d-electrons (e.g. ZnSnAs{sub 2}). The former exhibit a rather peculiar non-monotonic temperature dependence of the energy gap which, so far, has resisted cogent theoretical description. We demonstrate it can well be fitted by including two Bose-Einstein oscillators with weights of opposite sign leading to an increase at low-T and a decrease at higher T's. We find that the energy of the former correlates well with characteristic peaks in the phonon density of states associated with low-energy vibrations of the d-electron constituents.

  19. The optical band gap investigation of PVP-capped ZnO nanoparticles synthesized by sol-gel method

    Science.gov (United States)

    Yuliah, Yayah; Bahtiar, Ayi; Fitrilawati, Siregar, Rustam E.

    2016-02-01

    ZnO Nanoparticles (NPs) has unique natures on their crystal structure, direct band gap and high exciton binding energy, consequently applied in optoelectronic devices such as solar cells, optical wave guide and light emitting diodes (LED). However the drawback was ZnO NPs tend to agglomerate and turn to nano-structured materials with poor properties. Effort to avoid agglomerations generally resolved by surface modification of ZnO NPs to obtain well-dispersed suspension. However changes in the surface of ZnO NPs may change the electronic structure and density of states of ZnO NPs, in turn may change the optical band gap. Thus, the objective of current research is investigation of optical band gap of ZnO NPs due to surface modification by capping agent of poly-4-vinylpyrrolidone (PVP) molecules. Uncapped and PVP-capped ZnO nanoparticles were prepared by sol-gel method. The characteristics of surface modifications were investigated by UV-Vis and Photo Luminescence spectroscopy and Transmission Electron Microscope (TEM). The results shows the surface modification has change the band gap of ZnO NPs obtained at second precipitated stage. In contrast, the change of the optical band gap did not observe due to the surface modification of ZnO NPs obtained at the first stage. It was concluded that PVP capping on ZnO NPs did not affect on the band gap when the capping was performed on first stage. It is emphasized that this statement also supported by TEM images observations.

  20. Systematic analysis of the unique band gap modulation of mixed halide perovskites.

    Science.gov (United States)

    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.

  1. Residual stress dependant anisotropic band gap of various (hkl) oriented BaI2 films

    Science.gov (United States)

    Kumar, Pradeep; Gulia, Vikash; Vedeshwar, Agnikumar G.

    2013-11-01

    The thermally evaporated layer structured BaI2 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 me*=0.66m0 and the hole effective mass mh*=0.53m0 have been determined from the calculated band structure.

  2. Band gap tuning of epitaxial SrTiO{sub 3-δ}/Si(001) thin films through strain engineering

    Energy Technology Data Exchange (ETDEWEB)

    Cottier, Ryan J.; Steinle, Nathan A.; Currie, Daniel A.; Theodoropoulou, Nikoleta, E-mail: ntheo@txstate.edu [Physics Department, Texas State University, San Marcos, Texas 78666 (United States)

    2015-11-30

    We investigate the effect of strain and oxygen vacancies (V{sub O}) on the crystal and optical properties of oxygen deficient, ultra-thin (4–30 nm) films of SrTiO{sub 3-δ} (STO) grown heteroepitaxially on p-Si(001) substrates by molecular beam epitaxy. We demonstrate that STO band gap tuning can be achieved through strain engineering and show that the energy shift of the direct energy gap transition of SrTiO{sub 3-δ}/Si films has a quantifiable dimensional and doping dependence that correlates well with the changes in crystal structure.

  3. Optimization of Band Gap and Thickness for the Development of Efficient n-i-p+ Solar Cell

    Directory of Open Access Journals (Sweden)

    A. Belfar

    2015-06-01

    Full Text Available By using an electrical-optical AMPS-1D program (One Dimensional Analysis of Microelectronic and Photonic structures, a n-i-p type solar cell, based on hydrogenated amorphous silicon (a-Si : H and hydrogenated nanocrystalline silicon oxide (nc-SiOx : H has been investigated and simulated. The numerical analysis describes the modeling of the external cell performances, like, the short-circuit current (JSC, the open circuit voltage (VOC, the fill factor (FF and efficiency (Eff with the oxygen content in the p-nc-SiOx : H window layer by varying its mobility band gap (Eg associated simultaneously to the effect of the absorber layer (i-a-Si : H thickness. Also, the i-a-Si : H absorber layer band gap was optimized. The simulation result shows that the VOC depend strongly on the band offset (ΔEV in valence band of p-side. But, VOC does not depend on the thickness of the intrinsic layer. However, VOC increases when the energy band gap of the intrinsic layer is higher. It is demonstrated that the highest efficiency of 10.44 % (JSC = 11.67 mA/cm2; FF = 0.829; VOC = 1070 mV has been obtained when values of p-nc-SiOx : H window layer band gap, i-a-Si : H absorber layer band gap and i-a-Si : H absorber layer thickness are 2.10 eV, 1.86 eV, and 550 nm, respectively.

  4. Modeling the band gap of CdS quantum well structures

    Science.gov (United States)

    Harris, R. A.; Terblans, J. J.

    2016-10-01

    Within the framework of the effective mass approximation, an excited electron is studied in a cadmium sulfide (CdS) quantum well with varying well widths. The envelope function approximation is employed involving a three parameter variational calculation wherein one of these parameters is the distance between the electron and the hole. The relative change in the electron's energy (relative to its energy when it is in the valence band; in the hole) is investigated as a function of the electron-hole distance. Results from numerical calculations are presented and the non-linear behavior of different sized CdS quantum wells are discussed. Comparisons between experimentally measured CdS band gap energies (as a function of well-width) and the simulation data are made. A good agreement between the current model and experimental data exists. Density functional theory (DFT) calculations are done on crystallites of extremely small sizes to compare the current model's bandgap energies to DFT-predicted bandgap values at these extremes.

  5. Energy gap in concentrated spin glasses

    Energy Technology Data Exchange (ETDEWEB)

    Liao, S.B.; Bhagat, S.M.; Manheimer, M.A.; Moorjani, K.

    1988-04-15

    Using magnetic resonance data at several frequencies, we have obtained the temperature dependence of the field-induced magnetization (M) in three concentrated spin glasses. At every frequency, M is independent of T for Tapprox. gap energy with values close to the corresponding Curie--Weiss temperatures theta/sub p/ but much larger than the respective spin-glass transition temperatures T/sub SG/.

  6. Calculation of effective band gap narrowing in heavily-doped and compensated silicon

    Science.gov (United States)

    Polsky, B. S.; Rimshans, J. S.

    1991-06-01

    The effective band gap narrowing in heavily-doped and compensated silicon for different values of impurity concentration is calculated within the semiclassical approximation. The calculated and known measured data are compared.

  7. Molecular design for improved photovoltaic efficiency: band gap and absorption coefficient engineering

    KAUST Repository

    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.

  8. Vibration band-gap properties of three-dimensional Kagome lattices using the spectral element method

    Science.gov (United States)

    Wu, Zhi-Jing; Li, Feng-Ming; Zhang, Chuanzeng

    2015-04-01

    The spectral element method (SEM) is extended to investigate the vibration band-gap properties of three-dimensional (3D) Kagome lattices. The dynamic stiffness matrix of the 3D element which contains bending, tensional and torsional components is derived. The spectral equations of motion of the whole 3D Kagome lattice are then established. Comparing with frequency-domain solutions calculated by the finite element method (FEM), the accuracy and the feasibility of the SEM solutions are verified. It can be shown that the SEM is suitable for analyzing the vibration band-gap properties. Due to the band-gap characteristics, the periodic 3D Kagome lattice has the performance of vibration isolation. The influences of the structural and material parameters on the vibration band-gaps are discussed and a new type of 3D Kagome lattice is designed to obtain the improved vibration isolation capability.

  9. Tunable Photonic Band Gaps In Photonic Crystal Fibers Filled With a Cholesteric Liquid Crystal

    Institute of Scientific and Technical Information of China (English)

    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.

  10. Accurate evaluation of lowest band gaps in ternary locally resonant phononic crystals

    Institute of Scientific and Technical Information of China (English)

    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.

  11. Manipulating full photonic band gaps in two dimensional birefringent photonic crystals.

    Science.gov (United States)

    Proietti Zaccaria, Remo; Verma, Prabhat; Kawaguchi, Satoshi; Shoji, Satoru; Kawata, Satoshi

    2008-09-15

    The probability to realize a full photonic band gap in two-dimensional birefringent photonic crystals can be readily manipulated by introducing symmetry reduction or air holes in the crystal elements. The results lie in either creation of new band gaps or enlargement of existing band gaps. In particular, a combination of the two processes produces an effect much stronger than a simple summation of their individual contributions. Materials with both relatively low refractive index (rutile) and high refractive index (tellurium) were considered. The combined effect of introduction of symmetry reduction and air holes resulted in a maximum enlargement of the band gaps by 8.4% and 20.2%, respectively, for the two materials.

  12. Shape optimization of phononic band gap structures using the homogenization approach

    CERN Document Server

    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...

  13. Study on the vibration band gap and vibration attenuation property of phononic crystals

    Institute of Scientific and Technical Information of China (English)

    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.

  14. Study on the vibration band gap and vibration attenuation property of phononic crystals

    Institute of Scientific and Technical Information of China (English)

    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.

  15. Band gap narrowing models tested on low recombination phosphorus laser doped silicon

    Science.gov (United States)

    Dahlinger, Morris; Carstens, Kai

    2016-10-01

    This manuscript discusses bandgap narrowing models for highly phosphorus doped silicon. We simulate the recombination current pre-factor J0,phos in PC1Dmod 6.2 of measured doping profiles and apply the theoretical band gap narrowing model of Schenk [J. Appl. Phys. 84, 3684 (1998)] and an empirical band gap narrowing model of Yan and Cuevas [J. Appl. Phys. 114, 044508 (2013)]. The recombination current pre-factor of unpassivated and passivated samples measured by the photo conductance measurement and simulated J0,phos agrees well, when the band gap narrowing model of Yan and Cuevas is applied. With the band gap narrowing model of Schenk, the simulation cannot reproduce the measured J0,phos. Furthermore, the recombination current pre-factor of our phosphorus laser doped silicon samples are comparable with furnace diffused samples. There is no indication of recombination active defects, thus no laser induced defects in the diffused volume.

  16. Isotropic properties of the photonic band gap in quasicrystals with low-index contrast

    CERN Document Server

    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.

  17. Correlation between the band gap expansion and melting temperature depression of nanostructured semiconductors

    Energy Technology Data Exchange (ETDEWEB)

    Li, Jianwei, E-mail: jwl189@163.com; Zhao, Xinsheng [Laboratory for Quantum Design of Functional Material, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116 (China); Liu, Xinjuan [Center for Coordination Bond and Electronic Engineering, College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018 (China); Zheng, Xuejun [School of Mechanical Engineering, Xiangtan University, Xiangtan, Hunan 411105 (China); Yang, Xuexian [Department of Physics, Jishou University, Jishou 416000, Hunan (China); Zhu, Zhe [School of Materials Science and Engineering, Xiangtan University, Xiangtan, Hunan 411105 (China)

    2015-09-28

    The band gap and melting temperature of a semiconductor are tunable with the size and shape of the specimen at the nanometer scale, and related mechanisms remain as yet unclear. In order to understand the common origin of the size and shape effect on these two seemingly irrelevant properties, we clarify, correlate, formulate, and quantify these two properties of GaAs, GaN, InP, and InN nanocrystals from the perspectives of bond order-length-strength correlation using the core-shell configuration. The consistency in the theoretical predictions, experimental observations, and numerical calculations verify that the broken-bond-induced local bond contraction and strength gain dictates the band gap expansion, while the atomic cohesive energy loss due to bond number reduction depresses the melting point. The fraction of the under-coordinated atoms in the skin shell quantitatively determines the shape and size dependency. The atomic under-coordination in the skin down to a depth of two atomic layers inducing a change in the local chemical bond is the common physical origin.

  18. Minority carrier blocking to enhance the thermoelectric figure of merit in narrow-band-gap semiconductors

    Science.gov (United States)

    Bahk, Je-Hyeong; Shakouri, Ali

    2016-04-01

    We present detailed theoretical predictions on the enhancement of the thermoelectric figure of merit by minority carrier blocking with heterostructure barriers in bulk narrow-band-gap semiconductors. Bipolar carrier transport, which is often significant in a narrow-band-gap material, is detrimental to the thermoelectric energy conversion efficiency as it suppresses the Seebeck coefficient and increases the thermal conductivity. When the minority carriers are selectively prevented from participating in conduction while the transport of majority carriers is relatively unaffected by one-sided heterobarriers, the thermoelectric figure of merit can be drastically enhanced. Thermoelectric transport properties such as Seebeck coefficient, electrical conductivity, and electronic thermal conductivity including the bipolar term are calculated with and without the barriers based on the near-equilibrium Boltzmann transport equations under the relaxation time approximation to investigate the effects of minority carrier barriers on the thermoelectric figure of merit. For this, we provide details of carrier transport modeling and fitting results of experimental data for three important material systems, B i2T e3 -based alloys, M g2S i1 -xS nx , and S i1 -xG ex , that represent, respectively, near-room-temperature (300 K-500 K), midtemperature (600 K-900 K), and high-temperature (>1000 K ) applications. Theoretical maximum enhancement of thermoelectric figure of merit that can be achieved by minority carrier blocking is quantified and discussed for each of these semiconductors.

  19. Band-gap narrowing in heavily doped silicon: A comparison of optical and electrical data

    Science.gov (United States)

    Wagner, Joachim; del Alamo, Jesús A.

    1988-01-01

    The band-gap narrowing in heavily doped silicon has been studied by optical techniques—namely, photoluminescence and photoluminescence excitation spectroscopy—and by electrical measurements on bipolar transistors. The optical experiments give a consistent set of data for the band-gap narrowing in n- and p-type material at low temperatures as well as at room temperature. A good agreement is found between the optical and electrical data removing the discrepancies existing so far in the literature.

  20. Loss properties of all-solid photonic band gap fibers with an array of rings

    Institute of Scientific and Technical Information of China (English)

    GENG You-fu; LI Xue-jin; TAN Xiao-ling; YAO Jian-quan

    2010-01-01

    @@ The confinement loss and bend loss properties of all-solid photonic band gap fibers with an array of rings doped with highindex material are investigated.The calculated results show that for a specific structure,the confinement loss and the critical bend radius are reduced simultaneously in some band gaps by increasing the inner diameter of ring,which provides a useful guide and a theoretical basis for designing large mode area fibers with low loss.

  1. Variable band-gap semiconductors as the basis of new solar cells

    Energy Technology Data Exchange (ETDEWEB)

    Morales-Acevedo, Arturo [Centro de Investigacion y de Estudios Avanzados del IPN, Electrical Engineering Department, Avenida IPN No. 2508, 07360 Mexico, DF (Mexico)

    2009-09-15

    Some basic concepts related to variable band-gap absorbing semiconductors in solar cell structures, such as the associated quasi-electric field, will be discussed. The effects of this quasi-electric field upon the minority carrier drift-diffusion length and the back surface recombination velocity may induce a larger generated carrier collection at the junction with the corresponding increase of the illumination current density. It will also be shown that an additional improvement of the open-circuit voltage is possible when the band-gap is reduced within the space charge region so that the dark saturation current density is reduced there. Our estimation is that in the case of a solar cell where the band-gap is changed about 0.5 eV within the space charge region, an increase of the open-circuit voltage around 115 mV will be observed with respect to the single minimum band-gap absorbing material case. A similar band-gap variation in the bulk of the material will cause an increase of the minority carrier drift-diffusion length by a factor of 10 with respect to the single band-gap material. Therefore, based on these physical concepts, two possible structures with variable band-gap layers are proposed in order to have higher efficiencies than for cells without any band-gap grading. It will be shown that these concepts can be applied to II-VI, III-V chalcopyrite and even amorphous semiconductor solar cells. (author)

  2. Study on band gap structure of Fibonacci quantum superlattices by using the transfer matrix method

    Science.gov (United States)

    Ferrando, V.; Castro-Palacio, J. C.; Marí, B.; Monsoriu, J. A.

    2014-02-01

    The scattering properties of particles in a one-dimensional Fibonacci sequence based potential have been analyzed by means of the Transfer Matrix Method. The electronic band gaps are examined comparatively with those obtained using the corresponding periodic potentials. The reflection coefficient shows self-similar properties for the Fibonacci superlattices. Moreover, by using the generalized Bragg's condition, the band gaps positions are derived from the golden mean involved in the design of the superlattice structure.

  3. Wide band gap tunability in complex transition metal oxides by site-specific substitution

    OpenAIRE

    Choi, Woo Seok; Chisholm, Matthew F.; Singh, David J.; Choi, Taekjib; Jellison Jr, Gerald E.; Lee, Ho Nyung

    2012-01-01

    Fabricating complex transition metal oxides with a tuneable band gap without compromising their intriguing physical properties is a longstanding challenge. Here we examine the layered ferroelectric bismuth titanate and demonstrate that, by site-specific substitution with the Mott insulator lanthanum cobaltite, its band gap can be narrowed as much as one electron volt, while remaining strongly ferroelectric. We find that when a specific site in the host material is preferentially substituted, ...

  4. Robust room temperature ferromagnetism and band gap tuning in nonmagnetic Mg doped ZnO films

    Science.gov (United States)

    Quan, Zhiyong; Liu, Xia; Qi, Yan; Song, Zhilin; Qi, Shifei; Zhou, Guowei; Xu, Xiaohong

    2017-03-01

    Mg doped ZnO films with hexagonal wurtzite structure were deposited on c-cut sapphire Al2O3 substrates by pulsed laser deposition. Both room temperature ferromagnetism and band gap of the films simultaneously tuned by the concentration of oxygen vacancies were performed. Our results further reveal that the singly occupied oxygen vacancies should be responsible for the room temperature ferromagnetism and band gap narrowing. Singly occupied oxygen vacancies having the localized magnetic moments form bound magnetic polarons, which results in a long-range ferromagnetic ordering due to Mg doping. Moreover, band gap narrowing of the films is probably due to the formation of impurity band in the vicinity of valence band, originating from singly occupied oxygen vacancies. These results may build a bridge to understand the relationship between the magnetic and optical properties in oxide semiconductor, and are promising to integrate multiple functions in one system.

  5. Influence of process parameters on band gap of AI-doped ZnO film

    Institute of Scientific and Technical Information of China (English)

    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.

  6. Origins of electronic band gap reduction in Cr/N codoped TiO2.

    Science.gov (United States)

    Parks Cheney, C; Vilmercati, P; Martin, E W; Chiodi, M; Gavioli, L; Regmi, M; Eres, G; Callcott, T A; Weitering, H H; Mannella, N

    2014-01-24

    Recent studies indicated that noncompensated cation-anion codoping of wide-band-gap oxide semiconductors such as anatase TiO2 significantly reduces the optical band gap and thus strongly enhances the absorption of visible light [W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)]. We used soft x-ray spectroscopy to fully determine the location and nature of the impurity levels responsible for the extraordinarily large (∼1 eV) band gap reduction of noncompensated codoped rutile TiO2. It is shown that Cr/N codoping strongly enhances the substitutional N content, compared to single element doping. The band gap reduction is due to the formation of Cr 3d3 levels in the lower half of the gap while the conduction band minimum is comprised of localized Cr 3d and delocalized N 2p states. Band gap reduction and carrier delocalization are critical elements for efficient light-to-current conversion in oxide semiconductors. These findings thus raise the prospect of using codoped oxide semiconductors with specifically engineered electronic properties in a variety of photovoltaic and photocatalytic applications.

  7. Acoustic band gaps in two-dimensional square arrays of semi-hollow circular cylinders

    Institute of Scientific and Technical Information of China (English)

    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.

  8. Optical band gap tuning of Sb-Se thin films for xerographic based applications

    Science.gov (United States)

    Kaur, Ramandeep; Singh, Palwinder; Singh, Kulwinder; Kumar, Akshay; Thakur, Anup

    2016-10-01

    In the present paper we have studied the effect of Sb addition on the optical band gap tuning of thermally evaporated SbxSe100-x (x = 0, 5, 20, 50 and 60) thin films. The structural investigations revealed that all thin films were amorphous in nature. Transmission spectrum was taken in the range 400-2500 nm shows that all films are highly transparent in the near infrared region. The fundamental absorption edge shifts towards longer wavelength with Sb incorporation. The optical band gap decreases with addition of antimony in a-Se thin films. A good correlation has been drawn between experimentally estimated and theoretically calculated optical band gap. The decrease in optical band gap of thin films has been explained using chemical bond approach and density of states model. Decrease in optical band gap with Sb addition increases the concentration of electron deep traps which increases the X-ray sensitivity of Sb-Se thin films. Thus by tuning the optical band gap of Sb-Se alloy, it could be utilized for xerographic based applications.

  9. The Miscibility of PCBM in Low Band-Gap Conjugated Polymers in Organic Photovoltaics

    Science.gov (United States)

    Chen, Huipeng; You, Wei; Peet, Jeff; Azoulay, Jason; Bazan, Guillermo; Dadmun, Mark

    2012-02-01

    Understanding the morphology of the photoactive layer in organic photovoltaics (OPVs) is essential to optimizing conjugated polymer-based solar cells to meet the targeted efficiency of 10%. The miscibility and interdiffusion of components are among the key elements that impact the development of morphology and structure in OPV active layers. This study uses neutron reflectivity to correlate the structure of low band gap polymers to their miscibility with PCBM. Several low band gap polymers that exhibit power conversion efficiencies exceeding 7%, including PBnDT-DTffBT were examined. The intermixing of low band-gap polymer and PCBM bilayers was monitored by neutron reflectivity before and after thermal annealing, providing quantification of the miscibility and interdiffusion of PCBM within the low band gap polymer layer. These results indicate that the miscibility of PCBM ranges from 3% to 26% with the low band-gap polymers studied. The correlation between low band gap polymer structure and miscibility of PCBM will also be discussed.

  10. Eastern Band of Cherokee Strategic Energy Plan

    Energy Technology Data Exchange (ETDEWEB)

    Souther Carolina Institute of energy Studies-Robert Leitner

    2009-01-30

    The Eastern Band of Cherokee Indians was awarded a grant under the U.S. Department of Energy Tribal Energy Program (TEP) to develop a Tribal Strategic Energy Plan (SEP). The grant, awarded under the “First Steps” phase of the TEP, supported the development of a SEP that integrates with the Tribe’s plans for economic development, preservation of natural resources and the environment, and perpetuation of Tribal heritage and culture. The Tribe formed an Energy Committee consisting of members from various departments within the Tribal government. This committee, together with its consultant, the South Carolina Institute for Energy Studies, performed the following activities: • Develop the Tribe’s energy goals and objectives • Establish the Tribe’s current energy usage • Identify available renewable energy and energy efficiency options • Assess the available options versus the goals and objectives • Create an action plan for the selected options

  11. Band gap narrowing in nitrogen-doped La2Ti2O7 predicted by density-functional theory calculations.

    Science.gov (United States)

    Zhang, Junying; Dang, Wenqiang; Ao, Zhimin; Cushing, Scott K; Wu, Nianqiang

    2015-04-14

    In order to reveal the origin of enhanced photocatalytic activity of N-doped La2Ti2O7 in both the visible light and ultraviolet light regions, its electronic structure has been studied using spin-polarized conventional density functional theory (DFT) and the Heyd-Scuseria-Ernzerhof (HSE06) hybrid approach. The results show that the deep localized states are formed in the forbidden band when nitrogen solely substitutes for oxygen. Introducing the interstitial Ti atom into the N-doped La2Ti2O7 photocatalyst still causes the formation of a localized energy state. Two nitrogen substitutions co-exist stably with one oxygen vacancy, creating a continuum energy band just above the valence band maximum. The formation of a continuum band instead of mid-gap states can extend the light absorption to the visible light region without increasing the charge recombination, explaining the enhanced visible light performance without deteriorating the ultraviolet light photocatalytic activity.

  12. Temperature dependence of active photonic band gap in bragg-spaced quantum wells

    Energy Technology Data Exchange (ETDEWEB)

    Hu Zhiqiang; Wang Tao; Yu Chunchao; Xu Wei, E-mail: huzhiqianghzq@163.com [Wuhan National Laboratory for Optoelectronics, College of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei (China)

    2011-02-01

    A novel all-optical polarization switch of active photonic band gap structure based on non-resonant optical Stark effect bragg-spaced quantum wells was investigated and it could be compatible with the optical communication system. The theory is based on InGaAsP/InP Bragg-spaced quantum wells (BSQWs). Mainly through the design of the InGaAsP well layer component and InP barrier thickness to make the quantum-period cycle meet the bragg condition and the bragg frequency is equal to re-hole exciton resonance frequency. When a spectrally narrow control pulse is tuned within the forbidden gap, such BSQWs have been shown to exhibit large optical nonlinearities and ps recovery times, which can form T hz switch. However, the exciton binding energy of InGaAsP will be automatically separate at room temperature, so the effect of all-optical polarization switching of active photonic band gap bragg structure quantum wells can only be studied at low temperature. By a large number of experiments, we tested part of the material parameters of BSQWs in the temperature range 10-300K. On this basis, the InGaAsP and InP refractive index changes with wavelength, InP thermal expansion coefficient are studied and a relationship equation is established. Experimental results show that the bragg reflection spectra with temperature mainly is effected by InP refractive index changes with temperature. Our theoretical study and experiment are an instruction as a reference in the designs and experiments of future practical optical switches.

  13. Analysis of photonic band-gap (PBG) structures using the FDTD method

    DEFF Research Database (Denmark)

    Tong, M.S.; Cheng, M.; Lu, Y.L.

    2004-01-01

    In this paper, a number of photonic band-gap (PBG) structures, which are formed by periodic circuit elements printed oil transmission-line circuits, are studied by using a well-known numerical method, the finite-difference time-domain (FDTD) method. The results validate the band-stop filter...

  14. Anomalous composition dependence of the band gap pressure coefficients in In-containing nitride semiconductors

    DEFF Research Database (Denmark)

    Gorczyca, I.; Kamińska, A.; Staszczak, G.;

    2010-01-01

    The pressure-induced changes in the electronic band structures of In-containing nitride alloys, InxGa1-xN and InxAl1-xN are examined experimentally as well as by ab initio calculations. It is found that the band gap pressure coefficients, dEg/dp, exhibit very large bowing with x, and calculations...

  15. Incomplete photonic band gap as inferred from the speckle pattern of scattered light waves.

    Science.gov (United States)

    Apalkov, V M; Raikh, M E; Shapiro, B

    2004-06-25

    Motivated by recent experiments on intensity correlations of the waves transmitted through disordered media, we demonstrate that the speckle pattern from disordered photonic crystal with incomplete band gap represents a sensitive tool for determination of the stop-band width. We establish the quantitative relation between this width and the angular anisotropy of the intensity correlation function.

  16. Experimental indication for band gap widening of chalcopyrite solar cell absorbers after potassium fluoride treatment

    Energy Technology Data Exchange (ETDEWEB)

    Pistor, P., E-mail: paul.pistor@physik.uni-halle.de [Martin-Luther-Universität Halle, Von-Danckelmann-Platz 3, 06120 Halle (Germany); Greiner, D.; Kaufmann, C. A.; Brunken, S.; Gorgoi, M.; Steigert, A.; Calvet, W.; Lauermann, I.; Klenk, R.; Unold, T.; Lux-Steiner, M.-C. [Helmholtz-Zentrum Berlin, Hahn-Meitner-Platz 1, 14109 Berlin (Germany)

    2014-08-11

    The implementation of potassium fluoride treatments as a doping and surface modification procedure in chalcopyrite absorber preparation has recently gained much interest since it led to new record efficiencies for this kind of solar cells. In the present work, Cu(In,Ga)Se{sub 2} absorbers have been evaporated on alkali containing Mo/soda-lime glass substrates. We report on compositional and electronic changes of the Cu(In,Ga)Se{sub 2} absorber surface as a result of a post deposition treatment with KF (KF PDT). In particular, by comparing standard X-ray photoelectron spectroscopy and synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES), we are able to confirm a strong Cu depletion in the absorbers after the KF PDT which is limited to the very near surface region. As a result of the Cu depletion, we find a change of the valence band structure and a shift of the valence band onset by approximately 0.4 eV to lower binding energies which is tentatively explained by a band gap widening as expected for Cu deficient compounds. The KF PDT increased the open circuit voltage by 60–70 mV compared to the untreated absorbers, while the fill factor deteriorated.

  17. Design Analysis of An Electromagnetic Band Gap Microstrip Antenna

    Directory of Open Access Journals (Sweden)

    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.

  18. Band gap tunability of magneto-elastic phononic crystal

    Science.gov (United States)

    Bou Matar, O.; Robillard, J. F.; Vasseur, J. O.; Hladky-Hennion, A.-C.; Deymier, P. A.; Pernod, P.; Preobrazhensky, V.

    2012-03-01

    The possibility of control and tuning of the band structures of phononic crystals offered by the introduction of an active magnetoelastic material and the application of an external magnetic field is studied. Two means to obtain large elastic properties variations in magnetoelastic material are considered: Giant magnetostriction and spin reorientation transition effects. A plane wave expansion method is used to calculate the band structures. The magnetoelastic coupling is taken into account through the consideration of an equivalent piezomagnetic material model with elastic, piezomagnetic, and magnetic permeability tensors varying as a function of the amplitude and orientation of the applied magnetic field. Results of contactless tunability of the absolute bandgap are presented for a two-dimensional phononic crystal constituted of Terfenol-D square rod embedded in an epoxy matrix.

  19. Fe-substituted indium thiospinels: New intermediate band semiconductors with better absorption of solar energy

    Science.gov (United States)

    Chen, Ping; Chen, Haijie; Qin, Mingsheng; Yang, Chongyin; Zhao, Wei; Liu, Yufeng; Zhang, Wenqing; Huang, Fuqiang

    2013-06-01

    The indium thiospinels In2S3 and MgIn2S4 are promising host for the intermediated band (IB) photovoltaic materials due to their ideal band gap value. Here, the optical properties and electronic structure of Fe-doped In2S3 and MgIn2S4 have been investigated. All the Fe-substituted semiconductors exhibit two additional absorption bands at about 0.7 and 1.25 eV, respectively. The results of first-principles calculations revealed that the Fe substituted at the octahedral In site would introduce a partially filled IB into the band gap. Thanks to the formation of IB, the Fe-substituted semiconductors have the ability to absorb the photons with energies below the band gap. With the wide-spectrum absorption of solar energy, these materials possess potential applications in photovoltaic domain.

  20. First-principle study of energy band structure of armchair graphene nanoribbons

    Science.gov (United States)

    Ma, Fei; Guo, Zhankui; Xu, Kewei; Chu, Paul K.

    2012-07-01

    First-principle calculation is carried out to study the energy band structure of armchair graphene nanoribbons (AGNRs). Hydrogen passivation is found to be crucial to convert the indirect band gaps into direct ones as a result of enhanced interactions between electrons and nuclei at the edge boundaries, as evidenced from the shortened bond length as well as the increased differential charge density. Ribbon width usually leads to the oscillatory variation of band gaps due to quantum confinement no matter hydrogen passivated or not. Mechanical strain may change the crystal symmetry, reduce the overlapping integral of C-C atoms, and hence modify the band gap further, which depends on the specific ribbon width sensitively. In practical applications, those effects will be hybridized to determine the energy band structure and subsequently the electronic properties of graphene. The results can provide insights into the design of carbon-based devices.

  1. Synergistic effects on band gap-narrowing in titania by codoping from first-principles calculations

    OpenAIRE

    2010-01-01

    The large intrinsic band gap in TiO2 has hindered severely its potential application for visible-light irradiation. In this study, we have used a passivated approach to modify the band edges of anatase-TiO2 by codoping of X (N, C) with transition metals (TM=W, Re, Os) to extend the absorption edge to longer visible-light wavelengths. It was found that all the codoped systems can narrow the band gap significantly; in particular, (N+W)-codoped systems could serve as remarkably better photocatal...

  2. Band Gap and Waveguide States in Two-Dimensional Disorder Phononic Crystals

    Institute of Scientific and Technical Information of China (English)

    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.

  3. Band gap of two-dimensional fiber-air photonic crystals

    Energy Technology Data Exchange (ETDEWEB)

    Yang, Shu, E-mail: yangshu5678@163.com; Li, Masha

    2016-04-15

    A two-dimensional photonic crystal (PC) composed of textile fiber and air is initially discussed in this paper. Textile materials are so called soft materials, which are different from the previous PCs composed of rigid materials. The plain wave expansion method is used to calculate band structure of different PCs by altering component properties or structural parameters. Results show that the dielectric constant of textile fibers, fiber filling ratio and lattice arrangement are effective factors which influence PCs' band gap. Yet lattice constant and fiber diameter make inconspicuous influence on the band gap feature.

  4. Band Gap Variation of CdInSe and CdZnS Fabricated by High Throughput Combinatorial Growth Technique

    Science.gov (United States)

    Ma, Z. X.; Hao, H. Y.; Xiao, P.; Oehlerking, L. J.; Liu, D. F.; Zhang, X. J.; Yu, K.-M.; Walukiewicz, W.; Mao, S. S.; Yu, P. Y.; Liu, Lei; Yu, Peter Y.

    2011-12-01

    High energy radiation detector operating at room temperature requires the semiconductors having band-gap energies in the range of 1.35 ˜ 2.5 eV, high Z and high carrier mobility-lifetime (μτ) product. We report here the screening of the band-gap energies of compound semiconductor CdIn2Se4 and ZnCdS doped with Sn and In, prepared by high throughput combinatorial growth technique. It is found that the band-gap energies decrease as [Cd] decreases in Cd1-xIn2+2xSe4+2x, and as In or Sn elements are incorporated in ZnxCd1-xS. For both libraries, the μτ can reach a value on the order of 10-4 cm2/V. These results have demonstrated the strong capability of the combinatorial growth technique in rapid material discovery for room temperature radiation detector applications.

  5. Effect of particle size on band gap and DC electrical conductivity of TiO2 nanomaterial

    Science.gov (United States)

    Avinash, B. S.; Chaturmukha, V. S.; Jayanna, H. S.; Naveen, C. S.; Rajeeva, M. P.; Harish, B. M.; Suresh, S.; Lamani, Ashok R.

    2016-05-01

    Materials reduced to the Nano scale can exhibit different properties compared to what they exhibit on a micro scale, enabling unique applications. When TiO2 is reduced to Nano scale it shows unique properties, of which the electrical aspect is highly important. This paper presents increase in the energy gap and decrease in conductivity with decrease in particle size of pure Nano TiO2 synthesized by hydrolysis and peptization of titanium isopropoxide. Aqueous solution with various pH and peptizing the resultant suspension will form Nano TiO2 at different particle sizes. As the pH of the solution is made acidic reduction in the particle size is observed. And it is confirmed from XRD using Scherer formula and SEM, as prepared samples are studied for UV absorbance, and DC conductivity from room temperature to 400°C. From the tauc plot it was observed, and calculated the energy band gap increases as the particle size decreases and shown TiO2 is direct band gap. From Arrhenius plot clearly we encountered, decrease in the conductivity for the decrease in particle size due to hopping of charge carriers and it is evident that, we can tailor the band gap by varying particle size.

  6. Design of a 3D photonic band gap cavity in a diamond-like inverse woodpile photonic crystal

    CERN Document Server

    Woldering, Léon A; Vos, Willem L

    2014-01-01

    We theoretically investigate the design of cavities in a three-dimensional (3D) inverse woodpile photonic crystal. This class of cubic diamond-like crystals has a very broad photonic band gap and consists of two perpendicular arrays of pores with a rectangular structure. The point defect that acts as a cavity is centred on the intersection of two intersecting perpendicular pores with a radius that differs from the ones in the bulk of the crystal. We have performed supercell bandstructure calculations with up to $5 \\times 5 \\times 5$ unit cells. We find that up to five isolated and dispersionless bands appear within the 3D photonic band gap. For each isolated band, the electric-field energy is localized in a volume centred on the point defect, hence the point defect acts as a 3D photonic band gap cavity. The mode volume of the cavities resonances is as small as 0.8 $\\lambda^{3}$ (resonance wavelength cubed), indicating a strong confinement of the light. By varying the radius of the defect pores we found that o...

  7. Oxygen vacancy induced band gap narrowing of ZnO nanostructures by an electrochemically active biofilm.

    Science.gov (United States)

    Ansari, Sajid Ali; Khan, Mohammad Mansoob; Kalathil, Shafeer; Nisar, Ambreen; Lee, Jintae; Cho, Moo Hwan

    2013-10-07

    Band gap narrowing is important and advantageous for potential visible light photocatalytic applications involving metal oxide nanostructures. This paper reports a simple biogenic approach for the promotion of oxygen vacancies in pure zinc oxide (p-ZnO) nanostructures using an electrochemically active biofilm (EAB), which is different from traditional techniques for narrowing the band gap of nanomaterials. The novel protocol improved the visible photocatalytic activity of modified ZnO (m-ZnO) nanostructures through the promotion of oxygen vacancies, which resulted in band gap narrowing of the ZnO nanostructure (Eg = 3.05 eV) without dopants. X-ray diffraction, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, Raman spectroscopy, photoluminescence spectroscopy and high resolution transmission electron microscopy confirmed the oxygen vacancy and band gap narrowing of m-ZnO. m-ZnO enhanced the visible light catalytic activity for the degradation of different classes of dyes and 4-nitrophenol compared to p-ZnO, which confirmed the band gap narrowing because of oxygen defects. This study shed light on the modification of metal oxide nanostructures by EAB with a controlled band structure.

  8. Modifying the band gap and optical properties of Germanium nanowires by surface termination

    Science.gov (United States)

    Legesse, Merid; Fagas, Giorgos; Nolan, Michael

    2017-02-01

    Semiconductor nanowires, based on silicon (Si) or germanium (Ge) are leading candidates for many ICT applications, including next generation transistors, optoelectronics, gas and biosensing and photovoltaics. Key to these applications is the possibility to tune the band gap by changing the diameter of the nanowire. Ge nanowires of different diameter have been studied with H termination, but, using ideas from chemistry, changing the surface terminating group can be used to modulate the band gap. In this paper we apply the generalised gradient approximation of density functional theory (GGA-DFT) and hybrid DFT to study the effect of diameter and surface termination using -H, -NH2 and -OH groups on the band gap of (001), (110) and (111) oriented germanium nanowires. We show that the surface terminating group allows both the magnitude and the nature of the band gap to be changed. We further show that the absorption edge shifts to longer wavelength with the -NH2 and -OH terminations compared to the -H termination and we trace the origin of this effect to valence band modifications upon modifying the nanowire with -NH2 or -OH. These results show that it is possible to tune the band gap of small diameter Ge nanowires over a range of ca. 1.1 eV by simple surface chemistry.

  9. A Monte Carlo simulation for bipolar resistive memory switching in large band-gap oxides

    Energy Technology Data Exchange (ETDEWEB)

    Hur, Ji-Hyun, E-mail: jhhur123@gmail.com, E-mail: jeonsh@korea.ac.kr [Department of Applied Physics, Korea University, Sejong 2511, Sejong 339-700 (Korea, Republic of); Compound Device Laboratory, Samsung Advanced Institute of Technology, Nongseo-dong, Giheung-gu, Yongin-si, Gyeonggi-Do 446-712 (Korea, Republic of); Lee, Dongsoo [Compound Device Laboratory, Samsung Advanced Institute of Technology, Nongseo-dong, Giheung-gu, Yongin-si, Gyeonggi-Do 446-712 (Korea, Republic of); Jeon, Sanghun, E-mail: jhhur123@gmail.com, E-mail: jeonsh@korea.ac.kr [Department of Applied Physics, Korea University, Sejong 2511, Sejong 339-700 (Korea, Republic of)

    2015-11-16

    A model that describes bilayered bipolar resistive random access memory (BL-ReRAM) switching in oxide with a large band gap is presented. It is shown that, owing to the large energy barrier between the electrode and thin oxide layer, the electronic conduction is dominated by trap-assisted tunneling. The model is composed of an atomic oxygen vacancy migration model and an electronic tunneling conduction model. We also show experimentally observed three-resistance-level switching in Ru/ZrO{sub 2}/TaO{sub x} BL-ReRAM that can be explained by the two types of traps, i.e., shallow and deep traps in ZrO{sub 2}.

  10. Photoconductivity of composites based on CdSe quantum dots and low-band-gap polymers

    Science.gov (United States)

    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.

  11. The band gap of Cu2ZnSnSe4: Effect of order-disorder

    Science.gov (United States)

    Rey, G.; Redinger, A.; Sendler, J.; Weiss, T. P.; Thevenin, M.; Guennou, M.; El Adib, B.; Siebentritt, S.

    2014-09-01

    The order-disorder transition in kesterite Cu2ZnSnSe4 (CZTSe), an interesting material for solar cell, has been investigated by spectrophotometry, photoluminescence (PL), and Raman spectroscopy. Like Cu2ZnSnS4, CZTSe is prone to disorder by Cu-Zn exchanges depending on temperature. Absorption measurements have been used to monitor the changes in band gap energy (Eg) of solar cell grade thin films as a function of the annealing temperature. We show that ordering can increase Eg by 110 meV as compared to fully disordered material. Kinetics simulations show that Eg can be used as an order parameter and the critical temperature for the CZTSe order-disorder transition is 200 ± 20 °C. On the one hand, ordering was found to increase the correlation length of the crystal. But on the other hand, except the change in Eg, ordering did not influence the PL signal of the CZTSe.

  12. Low frequency band gaps below 10 Hz in radial flexible elastic metamaterial plate

    Science.gov (United States)

    Gao, Nansha; Hou, Hong; Wu, Jiu Hui; Cheng, Baozhu

    2016-11-01

    This paper presents the low frequency acoustic properties of a new proposed elastic metamaterial, which is arranged in the axial coordinate. The band structures, transmission spectra, and eigenmode displacement fields of this metamaterial are different from previous elastic metamaterial structures. Numerical calculation results show that the first order band gap of the radial flexible elastic metamaterial plate is below 10 Hz. A multiple-vibration coupling mechanism is proposed to explain the low frequency band gaps. By changing the geometrical dimensions h 1, h 2, b 1, and b 1 of the centre part, the location and width of the low frequency band gaps can be varied easily. The effects of density and Young’s modulus are also discussed in detail. In summary, the radial flexible elastic metamaterial plate can restrain low frequency vibration, owing to which it can potentially be used to protect infrasound, generate filters, and design acoustic devices.

  13. Two-Dimensional Phononic-Photonic Band Gap Optomechanical Crystal Cavity

    Science.gov (United States)

    Safavi-Naeini, Amir H.; Hill, Jeff T.; Meenehan, Seán; Chan, Jasper; Gröblacher, Simon; Painter, Oskar

    2014-04-01

    We present the fabrication and characterization of an artificial crystal structure formed from a thin film of silicon that has a full phononic band gap for microwave X-band phonons and a two-dimensional pseudo-band gap for near-infrared photons. An engineered defect in the crystal structure is used to localize optical and mechanical resonances in the band gap of the planar crystal. Two-tone optical spectroscopy is used to characterize the cavity system, showing a large coupling (g0/2π≈220 kHz) between the fundamental optical cavity resonance at ωo/2π =195 THz and colocalized mechanical resonances at frequency ωm/2π ≈9.3 GHz.

  14. Fabrication of 3-D Photonic Band Gap Crystals Via Colloidal Self-Assembly

    Science.gov (United States)

    Subramaniam, Girija; Blank, Shannon

    2005-01-01

    The behavior of photons in a Photonic Crystals, PCs, is like that of electrons in a semiconductor in that, it prohibits light propagation over a band of frequencies, called Photonic Band Gap, PBG. Photons cannot exist in these band gaps like the forbidden bands of electrons. Thus, PCs lend themselves as potential candidates for devices based on the gap phenomenon. The popular research on PCs stem from their ability to confine light with minimal losses. Large scale 3-D PCs with a PBG in the visible or near infra red region will make optical transistors and sharp bent optical fibers. Efforts are directed to use PCs for information processing and it is not long before we can have optical integrated circuits in the place of electronic ones.

  15. Simultaneous band-gap narrowing and carrier-lifetime prolongation of organic-inorganic trihalide perovskites.

    Science.gov (United States)

    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.

  16. Simultaneous band-gap narrowing and carrier-lifetime prolongation of organic–inorganic trihalide perovskites

    Science.gov (United States)

    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

    2016-01-01

    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

  17. Simultaneous band-gap narrowing and carrier-lifetime prolongation of organic–inorganic trihalide perovskites

    Energy Technology Data Exchange (ETDEWEB)

    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

    2016-07-21

    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.

  18. Dynamics of Below-Band-Gap Carrier in Highly Excited GaN

    Institute of Scientific and Technical Information of China (English)

    郭冰; 黄锦圣; 叶志镇; 江红星; 林景瑜

    2003-01-01

    Femtosecond time-resolved reflectivity was used to investigate below-band-gap (3.1 eV) carrier dynamics in a nominally undoped GaN epilayer under high excitation. A 2.5-ps rising process can be observed in the transient trace. This shot rising time results from the hot phonon effects which can cause a delayed energy relaxation of the initial photocarriers toward the band edge. From the density dependence of the carrier dynamics, the Mott density was estimated to be 1.51-1.56 × 1019 cm-3. Below the Mott density, the initial probed carrier dynamics was explained to the effect of acoustic phonon-assisted tunnelling for localized states, where a significant excitation density dependence of the tunnelling probability was observed due to the optically induced bandtail extension to lower energies. Above the Mott density, the measured carrier dynamics reflected the relaxation of an electron-hole plasma, in which a distinct fast decay component of 2.3 ps was observed due to the onset of nonlinear relaxation processes such Auger recombination.

  19. First-principles determination of band-to-band electronic transition energies in cubic and hexagonal AlGaInN alloys

    Directory of Open Access Journals (Sweden)

    F. L. Freitas

    2016-08-01

    Full Text Available We provide approximate quasiparticle-corrected band gap energies for quaternary cubic and hexagonal AlxGayIn1–x–yN semiconductor alloys, employing a cluster expansion method to account for the inherent statistical disorder of the system. Calculated values are compared with photoluminescence measurements and discussed within the currently accepted model of emission in these materials by carrier localization. It is shown that bowing parameters are larger in the cubic phase, while the range of band gap variation is bigger in the hexagonal one. Experimentally determined transition energies are mostly consistent with band-to-band excitations.

  20. k.p theory of freestanding narrow band gap semiconductor nanowires

    Science.gov (United States)

    Luo, Ning; Liao, Gaohua; Xu, H. Q.

    2016-12-01

    We report on a theoretical study of the electronic structures of freestanding nanowires made from narrow band gap semiconductors GaSb, InSb and InAs. The nanowires are described by the eight-band k.p Hamiltonians and the band structures are computed by means of the finite element method in a mixture basis consisting of linear triangular elements inside the nanowires and constrained Hermite triangular elements near the boundaries. The nanowires with two crystallographic orientations, namely the [001] and [111] orientations, and with different cross-sectional shapes are considered. For each orientation, the nanowires of the three narrow band gap semiconductors are found to show qualitatively similar characteristics in the band structures. However, the nanowires oriented along the two different crystallographic directions are found to show different characteristics in the valence bands. In particular, it is found that all the conduction bands show simple, good parabolic dispersions in both the [001]- and [111]-oriented nanowires, while the top valence bands show double-maximum structures in the [001]-oriented nanowires, but single-maximum structures in the [111]-oriented nanowires. The wave functions and spinor distributions of the band states in these nanowires are also calculated. It is found that significant mixtures of electron and hole states appear in the bands of these narrow band gap semiconductor nanowires. The wave functions exhibit very different distribution patterns in the nanowires oriented along the [001] direction and the nanowires oriented along the [111] direction. It is also shown that single-band effective mass theory could not reproduce all the band state wave functions presented in this work.

  1. Design of a three-dimensional photonic band gap cavity in a diamondlike inverse woodpile photonic crystal

    Science.gov (United States)

    Woldering, Léon A.; Mosk, Allard P.; Vos, Willem L.

    2014-09-01

    We theoretically investigate the design of cavities in a three-dimensional (3D) inverse woodpile photonic crystal. This class of cubic diamondlike crystals has a very broad photonic band gap and consists of two perpendicular arrays of pores with a rectangular structure. The point defect that acts as a cavity is centered on the intersection of two intersecting perpendicular pores with a radius that differs from the ones in the bulk of the crystal. We have performed supercell band structure calculations with up to 5×5×5 unit cells. We find that up to five isolated and dispersionless bands appear within the 3D photonic band gap. For each isolated band, the electric-field energy is localized in a volume centered on the point defect, hence the point defect acts as a 3D photonic band gap cavity. The mode volume of the cavities resonances is as small as 0.8 λ3 (resonance wavelength cubed), indicating a strong confinement of the light. By varying the radius of the defect pores we found that only donorlike resonances appear for smaller defect radius, whereas no acceptorlike resonances appear for greater defect radius. From a 3D plot of the distribution of the electric-field energy density we conclude that peaks of energy are found in sharp edges situated at the point defect, similar to how electrons collect at such features. This is different from what is observed for cavities in noninverted woodpile structures. Since inverse woodpile crystals can be fabricated from silicon by CMOS-compatible means, we project that single cavities and even cavity arrays can be realized, for wavelength ranges compatible with telecommunication windows in the near infrared.

  2. Acoustic band gaps of the woodpile sonic crystal with the simple cubic lattice

    Energy Technology Data Exchange (ETDEWEB)

    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.

  3. Band gap design of graphene photocathode%石墨烯光阴极带隙设计

    Institute of Scientific and Technical Information of China (English)

    李世龙; 石峰; 张太民; 刘照路; 张番; 李丹; 任兆玉

    2015-01-01

    In order to achieve graphene photocathode photoelectric conversion function, hexagonal boron nitride was doped in graphene in the form of hybrid superlattices nanoribbons. As can be seen from the band structure which was obtained by applying first-principles methods, the band gap of the superlattices was effectively regulated in a wide range (0-2.5 eV) by this means. The mechanism of band gap regulation was analyzed by the energy band structure and the charge density distribution. Furthermore, the present results were coincidence with the conclusion of Kronig-Penney model. With the increase of the h-BN proportion, the band gap engineering of graphene materials in this way, the band gap increases both zigzag edges superlattices nanoribbons and armchair edges superlattices nanoribbons. Besides, the band gap is almost independent of the width of nanoribbons, thus the size of the material can be more miniaturized. Moreover, the graphene photocathode with the gradient band gap characteristic can be made based on this approach, it can respond to different spectral ranges.%为了使石墨烯光阴极实现光电转化功能,以超晶格形式掺杂六角氮化硼到石墨烯中,形成杂化纳米带.通过基于第一性原理的计算,从能带结构可以看出,这种方法可以在一个很大的范围内(0~2.5 eV)调控带隙大小.结合能带结构和电荷密度分布分析了带隙调控的机理,此外,运用K-P模型理论分析也得到了一致的结果.以这种方式调控石墨烯材料的带隙,锯齿型边缘和扶手椅型边缘的六角氮化硼/石墨烯(h-BN/graphene)超晶格纳米带,其带隙大小均随着其中h-BN所占比例的增加而变大,而且其带隙大小几乎不受纳米带宽度的影响,这样一来材料的尺寸可以做到更加微型化.再者,基于此方法可以制成渐变带隙结构,进而实现同一光阴极对不同范围光谱的响应.

  4. Determination of the band gap of TiO{sub 2}-Al{sub 2}O{sub 3} films as a function of processing parameters

    Energy Technology Data Exchange (ETDEWEB)

    Barajas-Ledesma, E., E-mail: edgar_acuario112@hotmail.com [Instituto de Investigaciones Metalurgicas, Edificio ' U' , Ciudad Universitaria, UMSNH, Morelia, Michoacan, C.P. 58000 (Mexico); Garcia-Benjume, M.L. [Instituto de Investigaciones Metalurgicas, Edificio ' U' , Ciudad Universitaria, UMSNH, Morelia, Michoacan, C.P. 58000 (Mexico); Espitia-Cabrera, I. [Facultad de Ingenieria Quimica, Edificio ' M' , Ciudad Universitaria, UMSNH, Morelia, Michoacan, C.P. 58000 (Mexico); Ortiz-Gutierrez, M. [Facultad de Ciencias Fisico Matematicas, Edificio ' L' , Ciudad Universitaria, UMSNH, Morelia, Michoacan, C.P. 58000 (Mexico); Espinoza-Beltran, F.J. [CINVESTAV-Queretaro Libramiento Norponiente 2000, Fracc. Real de Juriquilla, Santiago de Queretaro, Queretaro, C.P. 76230 (Mexico); Mostaghimi, J. [Faculty of Applied Science and Engineering, University of Toronto, 5 King' s College Road, Toronto, Ontario, M5S 3G8 (Canada); Contreras-Garcia, M.E. [Instituto de Investigaciones Metalurgicas, Edificio ' U' , Ciudad Universitaria, UMSNH, Morelia, Michoacan, C.P. 58000 (Mexico)

    2010-10-25

    In this work the study of band gap is based on the processing parameters and was calculated using the Indirect Transition Model. An experimental design was done, in order to have a sequence of 18 samples to analyze. The alumina doped titania thin films were prepared by combining electrophoretic deposition (EPD) with sputtering. The addition of alumina to the titania was with the purpose to reduce the band gap of the semiconductor. Several researches have tried to dope titania with other materials, because it has photocatalytic activity only in the UV spectrum. Then, reducing the band gap of the titania, it will have activity in the entire visible spectrum, and its applications increase considerably. Comparing with the adsorption line in the ultraviolet region for all the samples, the results show the adsorption edge for samples doped with fewer amounts of alumina shifts a little toward a lower energy region, leading to a band gap reduction.

  5. Band-gap control in phosphorene/BN structures from first-principles calculations

    Science.gov (United States)

    Marsoner Steinkasserer, Lukas Eugen; Suhr, Simon; Paulus, Beate

    2016-09-01

    Using both DFT as well as G0W0 calculations, we investigate static and dynamic effects on the phosphorene band gap upon deposition and encapsulation on/in BN multilayers. We demonstrate how competing long- and short-range effects cause the phosphorene band gap to increase at low P -BN interlayer spacings, while the band gap is found to drop below that of isolated phosphorene in the BN/P bilayer at intermediate distances around 4 Å. Subsequent stacking of BN layers, i.e., BN/BN/P and BN/BN/BN/P is found to have a negligible effect at the DFT level while at the G0W0 level, increased screening lowers the band gap as compared to the BN/P bilayer. Encapsulation between two BN layers, on the other hand, is found to further increase the phosphorene band gap with respect to the BN/P bilayer. Lastly we investigate the use of the GLLB-SC functional as a starting point for G0W0 calculations showing it to, in the case of phosphorene, yield results close to those obtained from G W0@P B E .

  6. The band gap of II-Vi ternary alloys in a tight-binding description

    Energy Technology Data Exchange (ETDEWEB)

    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.

  7. Ultrafast internal conversion in a low band gap polymer for photovoltaics: experimental and theoretical study.

    Science.gov (United States)

    Fazzi, Daniele; Grancini, Giulia; Maiuri, Margherita; Brida, Daniele; Cerullo, Giulio; Lanzani, Guglielmo

    2012-05-14

    Ultrafast dynamics upon photoexcitation in a low band gap polymer for photovoltaics is investigated both experimentally and theoretically. Our work sheds light on the excess energy relaxation processes occurring immediately after photon absorption and responsible for dissipation in the photovoltaic process of light harvesting and energy storage. A peculiar non-adiabatic decay path through a conical intersection (CI) between the higher excited state S(2) and the first singlet state S(1) is identified by ultrafast spectroscopy and theoretical calculations. Ultrafast twisting of the initially flat conformation in S(2) drives the system to the CI connecting the two potential energy surfaces, actually eliciting an internal conversion within 60 femtoseconds, followed by planarization along the adiabatic surface in S(1). Relaxed potential energy profiles (PEPs) of ground and lowest excited states along a dihedral coordinate, calculated within the time dependent density functional theory (TDDFT) approach, support the S(2)/S(1) CI mechanism. Furthermore a screening of the widely used hybrid and range separated exchange-correlation (XC) DFT functionals has been carried out finding different descriptions of S(2)/S(1) PEPs and good agreement between experimental data and long-range corrected DFT.

  8. Low band gap frequencies and multiplexing properties in 1D and 2D mass spring structures

    Science.gov (United States)

    Aly, Arafa H.; Mehaney, Ahmed

    2016-11-01

    This study reports on the propagation of elastic waves in 1D and 2D mass spring structures. An analytical and computation model is presented for the 1D and 2D mass spring systems with different examples. An enhancement in the band gap values was obtained by modeling the structures to obtain low frequency band gaps at small dimensions. Additionally, the evolution of the band gap as a function of mass value is discussed. Special attention is devoted to the local resonance property in frequency ranges within the gaps in the band structure for the corresponding infinite periodic lattice in the 1D and 2D mass spring system. A linear defect formed of a row of specific masses produces an elastic waveguide that transmits at the narrow pass band frequency. The frequency of the waveguides can be selected by adjusting the mass and stiffness coefficients of the materials constituting the waveguide. Moreover, we pay more attention to analyze the wave multiplexer and DE-multiplexer in the 2D mass spring system. We show that two of these tunable waveguides with alternating materials can be employed to filter and separate specific frequencies from a broad band input signal. The presented simulation data is validated through comparison with the published research, and can be extended in the development of resonators and MEMS verification.

  9. The improvement of hole transport property and optical band gap for amorphous Cu{sub 2}O films

    Energy Technology Data Exchange (ETDEWEB)

    Huang, Qin; Li, Jin; Bi, Xiaofang

    2015-10-25

    This work presents an interesting observation that the suppression of crystallization for p-type Cu{sub 2}O facilitates the transition of transport behaviors from variable-range-hopping (VRH) to Arrhenius-like mechanism and further lead to a great reduction of thermal activation energy. Raman spectroscopy analysis shows a distortion of symmetrical O–Cu–O crosslink structure in the amorphous Cu{sub 2}O. The disruption of symmetry is revealed to increase dispersion of upper valence band and reduce Fermi as well, which results in possible intrusion of the Fermi level into a band tail state adjacent to the upper valence band level. Meanwhile, the amorphous Cu{sub 2}O film shows an optical band gap of 2.7 eV, much larger than 2.0 eV for the crystalline counterparts. The blue shift is consistent with the variation of energy band structure with the film changing from crystalline to amorphous state, suggesting that the O-mediated d–d interaction can be weakened with the nonsymmetrical structure in amorphous phase. - Graphical abstract: Suppression of crystallization for p-type Cu{sub 2}O is observed to facilitate the transition of transport behaviors from variable-range-hopping to the Arrhenius-like behavior based on the band tail transport mode. The amorphous Cu{sub 2}O film also shows a blue shift as compared to its crystalline counterpart. The effect of amorphous structure on the performances is discussed in combination with Raman spectroscopy and band structure calculation. - Highlights: • Amorphous Cu{sub 2}O films show Arrhenius-like p-type conductivity. • Raman spectroscopy is analyzed on the change of crystallization. • Physical origin of the transport behavior is clarified with electronic structure. • Optical band gap can be widened by suppressing crystallization of Cu{sub 2}O.

  10. Band gap tuning and optical absorption in type-II InAs/GaSb mid infrared short period superlattices: 14 bands K Dot-Operator p study

    Energy Technology Data Exchange (ETDEWEB)

    AbuEl-Rub, Khaled M. [Department of Applied Physical Sciences, Jordan University of Science and Technology Irbid, 21141 (Jordan)

    2012-09-06

    The MBE growth of short-period InAs/GaSb type-II superlattice structures, varied around 20.5 A InAs/24 A GaSb were [J. Applied physics, 96, 2580 (2004)] carried out by Haugan et al. These SLs were designed to produce devices with an optimum mid-infrared photoresponse and a sharpest photoresponse cutoff. We have used a realistic and reliable 14-band k.p formalism description of the superlattice electronic band structure to calculate the absorption coefficient in such short-period InAs/GaSb type-II superlattices. The parameters for this formalism are known from fitting to independent experiments for the bulk materials. The band-gap energies are obtained without any fitting parameters, and are in good agreement with experimental data.

  11. Band gap tuning and optical absorption in type-II InAs/GaSb mid infrared short period superlattices: 14 bands Kṡp study

    Science.gov (United States)

    AbuEl-Rub, Khaled M.

    2012-09-01

    The MBE growth of short-period InAs/GaSb type-II superlattice structures, varied around 20.5 Å InAs/24 Å GaSb were [J. Applied physics, 96, 2580 (2004)] carried out by Haugan et al. These SLs were designed to produce devices with an optimum mid-infrared photoresponse and a sharpest photoresponse cutoff. We have used a realistic and reliable 14-band k.p formalism description of the superlattice electronic band structure to calculate the absorption coefficient in such short-period InAs/GaSb type-II superlattices. The parameters for this formalism are known from fitting to independent experiments for the bulk materials. The band-gap energies are obtained without any fitting parameters, and are in good agreement with experimental data.

  12. Investigations on crystalline structure and optical band gap of nearly stoichiometric LiNbO3 nanoparticles

    Science.gov (United States)

    Debnath, C.; Kar, S.; Verma, S.; Bartwal, K. S.

    2014-11-01

    The structural and optical characteristics of nearly stoichiometric lithium niobate, LiNbO3 nanoparticles have been studied. The results are very different compared to the bulk LiNbO3 single crystals. The nanoparticles were synthesized by citrate gel method and the phase was confirmed by powder X-ray diffraction. The size and size distribution of the nanoparticles were obtained by XRD, SEM, TEM and DLS experiments. The particles were in the range of 50-200 nm and most of the particles are about 100 nm of size. The lattice parameters obtained from selected area electron diffraction are aH = 5.213 Å and cH = 14.026 Å for hexagonal system which are slightly larger than the other reported values (JCPDS). The optical properties were obtained from optical absorption spectroscopy in UV-vis.-NIR and IR (FTIR) range, the electronic band gap structure were determined from the fundamental absorption edge in the UV region. The indirect band gap was of 4.78 eV where as the direct gap was of 6.0 eV which are much larger than the other experimental values. The absorption features in the UV range indicate the discrete nature of conduction band and the allowed energy levels in the forbidden gap appeared due to surface defects.

  13. Exact two-component relativistic energy band theory and application

    Energy Technology Data Exchange (ETDEWEB)

    Zhao, Rundong; Zhang, Yong; Xiao, Yunlong; Liu, Wenjian, E-mail: liuwj@pku.edu.cn [Beijing National Laboratory for Molecular Sciences, Institute of Theoretical and Computational Chemistry, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, and Center for Computational Science and Engineering, Peking University, Beijing 100871 (China)

    2016-01-28

    An exact two-component (X2C) relativistic density functional theory in terms of atom-centered basis functions is proposed for relativistic calculations of band structures and structural properties of periodic systems containing heavy elements. Due to finite radial extensions of the local basis functions, the periodic calculation is very much the same as a molecular calculation, except only for an Ewald summation for the Coulomb potential of fluctuating periodic monopoles. For comparison, the nonrelativistic and spin-free X2C counterparts are also implemented in parallel. As a first and pilot application, the band gaps, lattice constants, cohesive energies, and bulk moduli of AgX (X = Cl, Br, I) are calculated to compare with other theoretical results.

  14. Crystal growth and characterization of the narrow-band-gap semiconductors OsPn₂ (Pn = P, As, Sb).

    Science.gov (United States)

    Bugaris, Daniel E; Malliakas, Christos D; Shoemaker, Daniel P; Do, Dat T; Chung, Duck Young; Mahanti, Subhendra D; Kanatzidis, Mercouri G

    2014-09-15

    Using metal fluxes, crystals of the binary osmium dipnictides OsPn2 (Pn = P, As, Sb) have been grown for the first time. Single-crystal X-ray diffraction confirms that these compounds crystallize in the marcasite structure type with orthorhombic space group Pnnm. The structure is a three-dimensional framework of corner- and edge-sharing OsPn6 octahedra, as well as [Pn2(4-)] anions. Raman spectroscopy shows the presence of P-P single bonds, consistent with the presence of [Pn2(-4)] anions and formally Os(4+) cations. Optical-band-gap and high-temperature electrical resistivity measurements indicate that these materials are narrow-band-gap semiconductors. The experimentally determined Seebeck coefficients reveal that nominally undoped OsP2 and OsSb2 are n-type semiconductors, whereas OsAs2 is p-type. Electronic band structure using density functional theory calculations shows that these compounds are indirect narrow-band-gap semiconductors. The bonding p orbitals associated with the Pn2 dimer are below the Fermi energy, and the corresponding antibonding states are above, consistent with a Pn-Pn single bond. Thermopower calculations using Boltzmann transport theory and constant relaxation time approximation show that these materials are potentially good thermoelectrics, in agreement with experiment.

  15. Quantum speedup of an atom coupled to a photonic-band-gap reservoir

    Science.gov (United States)

    Wu, Yu-Nan; Wang, Jing; Zhang, Han-Zhuang

    2017-01-01

    For a model of an atom embedded in a photonic-band-gap reservoir, it was found that the speedup of quantum evolution is subject to the atomic frequency changes. In this work, we propose different points of view on speeding up the evolution. We show that the atomic embedded position, the width of the band gap and the defect mode also play an important role in accelerating the evolution. By changing the embedded position of the atom and the coupling strength with the defect mode, the speedup region lies even outside the band-gap region, where the non-Markovian effect is weak. The mechanism for the speedup is due to the interplay of atomic excited population and the non-Markovianity. The feasible experimental system composed of quantum dots in the photonic crystal is discussed. These results provide new degree of freedoms to depress the quantum speed limit time in photonic crystals.

  16. Wavelet-based method for computing elastic band gaps of one-dimensional phononic crystals

    Institute of Scientific and Technical Information of China (English)

    YAN; ZhiZhong; WANG; YueSheng

    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.

  17. Effective permittivity and permeability of one-dimensional dielectric photonic crystal within a band gap

    Institute of Scientific and Technical Information of China (English)

    Guo Ji-Yong; Chen Hong; Li Hong-Qiang; Zhang Ye-Wen

    2008-01-01

    We take a finite dielectric photonic crystal as a homogeneous slab and have extracted the effective parameters. Our systematic study shows that the effective permittivity or permeability of dielectric photonic crystal is negative within a band gap region. This means that the band gap might act as ε-negative materials (ENMs) with ε0, or μ-negative materials (MNMs) with ε>0 and μ<0. Moreover the effective parameters sensitively rely on size, surface termination, symmetry, etc. The effective parameters can be used to design full transmission tunnelling modes and amplify evanescent wave. Several cases are studied and the results show that dielectric photonic band gap can indeed mimic a single negative material (ENM or MNM) under some restrictions.

  18. Band gap of β-PtO2 from first-principles

    Directory of Open Access Journals (Sweden)

    Yong Yang

    2012-06-01

    Full Text Available We studied the band gap of β-PtO2 using first-principles calculations based on density functional theory (DFT. The results are obtained within the framework of the generalized gradient approximation (GGA, GGA+U, GW, and the hybrid functional methods. For the different types of calculations, the calculated band gap increases from ∼0.46 eV to 1.80 eV. In particular, the band gap by GW (conventional and self-consistent calculation shows a tendency of converging to ∼1.25 ± 0.05 eV. The effect of on-site Coulomb interaction on the bonding characteristics is also analyzed.

  19. All-optical ultrafast switching of Si woodpile photonic band gap crystals

    CERN Document Server

    Euser, T G; Fleming, J G; Gralak, B; Polman, Albert; Vos, W L; Euser, Tijmen G.; Molenaar, Adriaan J.; Gralak, Boris; Polman, Albert; Vos, Willem L.

    2006-01-01

    We present ultrafast all-optical switching measurements of Si woodpile photonic band gap crystals at telecom frequencies. The crystals are homogeneously excited by a two-photon process. We probe the switching by measuring reflectivity over broad frequency ranges as a function of time. At short delay times, we observe that the photonic gap becomes narrower than in the unswitched case. After 1 ps, the complete gap has shifted to higher frequencies. This intricate behavior is the result of competing refractive index changes due to the electronic Kerr effect and to optically excited free carriers. The frequency shift of the band gap as a function of pump intensity agrees well with Fourier modal method calculations with no freely adjustable parameters.

  20. Robust topology optimization of three-dimensional photonic-crystal band-gap structures

    CERN Document Server

    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...

  1. Acoustic band gaps in two-dimensional square arrays of semi-hollow circular cylinders

    Institute of Scientific and Technical Information of China (English)

    LU TianJian; GAO GuoQin; MA ShouLin; JIN Feng; 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 per-formance 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 cyl-inder/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 meas-urements on square arrays of hollow AI cylinders in air and semi-hollow AI cylinders in air are carried out. The results demonstrate that the semi-hollow circular cylinder/air system has the best overall band gap performance.

  2. Determination of the optical band gap for amorphous and nanocrystalline copper oxide thin films prepared by SILAR technique

    Science.gov (United States)

    Abdel Rafea, M.; Roushdy, N.

    2009-01-01

    Amorphous copper oxide films were deposited using the SILAR technique. Both Cu2O and CuO crystallographic phases exist in deposited and annealed films. Crystallization and growth processes by annealing at temperatures up to 823 K form grains with nano- and micro-spherical shapes. The calculated crystallite size from the XRD measurement was found to be in the range 14-21 nm while nano-spheres in the diameter range 50-100 nm were observed by SEM micrographs. The band gap for amorphous film was found to be 2.3 eV which increased slowly to 2.4 eV by annealing the film at 373 K. This was explained by defect redistribution in amorphous films. Annealing in the temperature range 373-673 K decreased the band gap gradually to 1.85 eV. The decrease of the band gap with annealing temperature in the range 373-673 K agrees well with the Brus model of the energy gap confinement effect in nanostructured semiconducting materials. Annealing in the temperature range 673-823 K decreases the band gap slowly to 1.7 eV due to the smaller contribution of the confinement effect. Below 573 K, Cu2O is the most probable crystalline phase in the film, while Cu2O and CuO crystalline phases may coexist at annealing temperatures above 573 K due to further oxidation of Cu2O. A wider transmittance spectral window in the visible region was obtained by controlling the annealing conditions of the amorphous copper oxide film and its applicability to the window layer of solar cell was suggested.

  3. Determination of the optical band gap for amorphous and nanocrystalline copper oxide thin films prepared by SILAR technique

    Energy Technology Data Exchange (ETDEWEB)

    Abdel Rafea, M; Roushdy, N [Electronic Materials Department, Advanced Technologies and New Materials Institute, Mubarak City for Scientific Research and Technology Applications, PO Box 21934, New Borg El-Arab City, Alexandria (Egypt)], E-mail: m.abdelrafea@mucsat.sci.eg

    2009-01-07

    Amorphous copper oxide films were deposited using the SILAR technique. Both Cu{sub 2}O and CuO crystallographic phases exist in deposited and annealed films. Crystallization and growth processes by annealing at temperatures up to 823 K form grains with nano- and micro-spherical shapes. The calculated crystallite size from the XRD measurement was found to be in the range 14-21 nm while nano-spheres in the diameter range 50-100 nm were observed by SEM micrographs. The band gap for amorphous film was found to be 2.3 eV which increased slowly to 2.4 eV by annealing the film at 373 K. This was explained by defect redistribution in amorphous films. Annealing in the temperature range 373-673 K decreased the band gap gradually to 1.85 eV. The decrease of the band gap with annealing temperature in the range 373-673 K agrees well with the Brus model of the energy gap confinement effect in nanostructured semiconducting materials. Annealing in the temperature range 673-823 K decreases the band gap slowly to 1.7 eV due to the smaller contribution of the confinement effect. Below 573 K, Cu{sub 2}O is the most probable crystalline phase in the film, while Cu{sub 2}O and CuO crystalline phases may coexist at annealing temperatures above 573 K due to further oxidation of Cu{sub 2}O. A wider transmittance spectral window in the visible region was obtained by controlling the annealing conditions of the amorphous copper oxide film and its applicability to the window layer of solar cell was suggested.

  4. Effect of stress on optical band gap of ZnO thin films with substrate temperature by spray pyrolysis

    Energy Technology Data Exchange (ETDEWEB)

    Rao, T. Prasada, E-mail: prasadview@gmail.co [Department of physics, National Institute of Technology, Tiruchirappalli 620015 (India); Kumar, M.C. Santhosh, E-mail: santhoshmc@yahoo.co [Department of physics, National Institute of Technology, Tiruchirappalli 620015 (India); Angayarkanni, S. Anbumozhi; Ashok, M. [Department of physics, National Institute of Technology, Tiruchirappalli 620015 (India)

    2009-10-19

    Zinc oxide (ZnO) thin films have been deposited with various substrate temperatures by spray pyrolysis method onto glass substrates. The effects of substrate temperature on the crystallization behavior and optical properties of the films have been studied. The evolution of strain and stress effects in ZnO thin films on glass substrate has been studied using X-ray diffraction. The films deposited at low substrate temperature have large compressive stress of 1.77 GPa, which relaxed to 1.47 GPa as the substrate temperature increased to 450 deg. C. Optical parameters such as optical transmittance, reflectance, dielectric constant, refractive index and energy band gap have been studied and discussed with respect to substrate temperature. All films exhibit a transmittance of about 85% in the visible region. It was found that the compressive stress in the films causes a decrease in the optical band gap.

  5. Effect of thermal annealing on structure and optical band gap of amorphous Se72Te25Sb3 thin films

    Science.gov (United States)

    Dwivedi, D. K.; Pathak, H. P.; Kumar, Vipin; Shukla, Nitesh

    2014-04-01

    Thin films of a-Se72Te25Sb3 were prepared by vacuum evaporation technique in a base pressure of 10-6 Torr on to well cleaned glass substrate. a-Se72Te25Sb3 thin films were annealed at different temperatures below their crystallization temperatures for 2h. The structural analysis of the films has been investigated using X-ray diffraction technique. The optical band gap of as prepared and annealed films as a function of photon energy in the wavelength range 400-1100 nm has been studied. It has been found that the optical band gap decreases with increasing annealing temperatures in the present system.

  6. Energy gap of novel edge-defected graphene nanoribbons

    Science.gov (United States)

    Yuan, Weiqing; Wen, Zhongquan; Li, Min; Chen, Li; Chen, Gang; Ruan, Desheng; Gao, Yang

    2016-08-01

    Herein, the effects of width and boundary defects on the energy gap of graphene nanoribbons (GNRs) have been explored and theoretically investigated by means of semi-empirical atomic basis Extended Hückel method. Due to the existence of boundary defects, the energy gap of GNRs is mainly determined by the width of graphene nanoribbons for armchair graphene nanoribbons (AGNRs) or zigzag graphene nanoribbons (ZGNRs). Interestingly, the energy gap of AGNRs with a 120° V-type defect displays the monotone decreasing tendency when the width reaches to 2 nm, while the energy gap of intrinsic AGNRs is oscillatory. At the same time, the energy gap of U-type defected ZGNRs is opened, which differs from the zero energy gap characteristics of the intrinsic zigzag graphene. Furthermore, the size of energy gap of the defected AGNRs and ZGNRs with the same width is proved to be very close. Calculation results demonstrate that the energy gap of GNRs is just inversely proportional to the width and has little to do with the crystallographic direction. All the findings above provide a basis for energy gap engineering with different edge defects in GNRs and signify promising prospects in graphene-based semiconductor electronic devices.

  7. Band gap and conductivity variations of ZnO thin films by doping with Aluminium

    Science.gov (United States)

    Vattappalam, Sunil C.; Thomas, Deepu; T, Raju Mathew; Augustine, Simon; Mathew, Sunny

    2015-02-01

    Zinc Oxide thin films were prepared by Successive Ionic layer adsorption and reaction technique(SILAR). Aluminium was doped for different doping concentrations from 3 at.% to 12 at.% in steps of 3 at.%. Conductivity of the samples were taken at different temperatures. UV Spectrograph of the samples were taken and the band gap of each sample was found from the data. It was observed that as the doping concentration of Aluminium increases, the band gap of the samples decreases and concequently conductivity of the samples increases.

  8. Band gap and conductivity variations of ZnO nano structured thin films annealed under Vacuum

    Science.gov (United States)

    Vattappalam, Sunil C.; Thomas, Deepu; T, Raju Mathew; Augustine, Simon; Mathew, Sunny

    2015-02-01

    Zinc Oxide thin films were prepared by Successive Ionic layer adsorption and reaction technique(SILAR). The samples were annealed under vacuum and conductivity of the samples were taken at different temperatures. UV Spectrograph of the samples were taken and the band gap of each sample was found from the data. All the results were compared with that of the sample annealed under air. It was observed that the band gap decreases and concequently conductivity of the samples increases when the samples are annealed under vacuum.

  9. Lattice reconfiguration and phononic band-gap adaptation via origami folding

    Science.gov (United States)

    Thota, M.; Li, S.; Wang, K. W.

    2017-02-01

    We introduce a framework of utilizing origami folding to redistribute the inclusions of a phononic structure to achieve significant phononic band-gap adaptation. Cylindrical inclusions are attached to the vertices of a Miura-Ori sheet, whose 1 degree-of-freedom rigid folding can enable fundamental reconfigurations in the underlying periodic architecture via switching between different Bravais lattice types. Such a reconfiguration can drastically change the wave propagation behavior in terms of band gap and provide a scalable and practical means for broadband wave tailoring.

  10. Band Gap Narrowing and Widening of ZnO Nanostructures and Doped Materials

    OpenAIRE

    2015-01-01

    Band gap change in doped ZnO is an observed phenomenon that is very interesting from the fundamental point of view. This work is focused on the preparation of pure and single phase nanostructured ZnO and Cu as well as Mn-doped ZnO for the purpose of understanding the mechanisms of band gap narrowing in the materials. ZnO, Zn0.99Cu0.01O and Zn0.99Mn0.01O materials were prepared using a wet chemistry method, and X-ray diffraction (XRD) results showed that all samples were pure and single phase....

  11. Optical band gap of Sn0.2Bi1.8Te3 thin films

    Indian Academy of Sciences (India)

    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.

  12. Enhanced third-harmonic generation in photonic crystals at band-gap pumping

    Science.gov (United States)

    Yurchenko, Stanislav O.; Zaytsev, Kirill I.; Gorbunov, Evgeny A.; Yakovlev, Egor V.; Zotov, Arsen K.; Masalov, Vladimir M.; Emelchenko, Gennadi A.; Gorelik, Vladimir S.

    2017-02-01

    More than one order enhancement of third-harmonic generation is observed experimentally at band-gap pumping of globular photonic crystals. Due to a lateral modulation of the dielectric permittivity in two- and three-dimensional photonic crystals, sharp peaks of light intensity (light localization) arise in the media at the band-gap pumping. The light localization enhances significantly the nonlinear light conversion, in particular, third-harmonic generation, in the near-surface volume of photonic crystal. The observed way to enhance the nonlinear conversion can be useful for creation of novel compact elements of nonlinear and laser optics.

  13. Complete Band-Gap in Two-Dimensional Quasiperiod Photonic Crystals with Hollow Cylinders

    Institute of Scientific and Technical Information of China (English)

    FENG Zhi-Fang; FENG Shuai; REN Kun; LI Zhi-Yuan; CHENG Bing-Ying; ZHANG Dao-Zhong

    2005-01-01

    @@ The transmission properties of quasiperiodic photonic crystals (QPCs) based on the random square-triangle tilingsystem are investigated by the multiple scattering method. The hollow cylinders are introduced in our calculation. It is found that QPCs with hollow cylinders also possess a complete band gap common to s- and p-polarized waves when the inner radius of hollow cylinders is larger than a certain value. The QPCs possessing the complete band gap can be applied to the fields of light emitting, wave-guides, optical filters, high-Q resonators and antennas.

  14. Tuning the light emission properties by band gap engineering in hybrid lead halide perovskite.

    Science.gov (United States)

    D'Innocenzo, Valerio; Srimath Kandada, Ajay Ram; De Bastiani, Michele; Gandini, Marina; Petrozza, Annamaria

    2014-12-24

    We report about the relationship between the morphology and luminescence properties of methylammonium lead trihalide perovskite thin films. By tuning the average crystallite dimension in the film from tens of nanometers to a few micrometers, we are able to tune the optical band gap of the material along with its photoluminescence lifetime. We demonstrate that larger crystallites present smaller band gap and longer lifetime, which correlates to a smaller radiative bimolecular recombination coefficient. We also show that they present a higher optical gain, becoming preferred candidates for the realization of CW lasing devices.

  15. Signature of a three-dimensional photonic band gap observed on silicon inverse woodpile photonic crystals

    CERN Document Server

    Huisman, Simon R; Woldering, Léon A; Leistikow, Merel D; Mosk, Allard P; Vos, Willem L

    2010-01-01

    We have studied the reflectivity of CMOS-compatible three-dimensional silicon inverse woodpile photonic crystals at near-infrared frequencies. Polarization-resolved reflectivity spectra were obtained from two orthogonal crystal surfaces corresponding to 1.88 pi sr solid angle. The spectra reveal broad peaks with high reflectivity up to 67 % that are independent of the spatial position on the crystals. The spectrally overlapping reflectivity peaks for all directions and polarizations form the signature of a broad photonic band gap with a relative bandwidth up to 16 %. This signature is supported with stopgaps in plane wave bandstructure calculations and with the frequency region of the expected band gap.

  16. Semiconducting graphene nanoribbon retains band gap on amorphous or crystalline SiO_2

    OpenAIRE

    Hossain, M. Zubaer

    2011-01-01

    Electronic properties of a semiconducting armchair graphene nanoribbon on SiO_2 are examined using first-principles calculations and taking into account the van der Waals interaction. Unlike semiconducting carbon nanotubes, which exhibit variations in band gap on SiO_2, the nanoribbon is found to retain its band gap on SiO_2, regardless of the separation distance or the dielectric’s surface type—crystalline or amorphous. The interfacial interaction leads to electron-transfer from the nanor...

  17. Annealing-induced optical and sub-band-gap absorption parameters of Sn-doped CdSe thin films

    Science.gov (United States)

    Kaur, Jagdish; Tripathi, S. K.

    2016-01-01

    Thin films of Sn-doped CdSe were prepared by thermal evaporation onto glass substrates in an argon gas atmosphere and annealed at different temperatures. Structural evaluation of the films was carried out using X-ray diffraction and their stoichiometry studied by energy-dispersive X-ray analysis. The films exhibit a preferred orientation along the hexagonal direction of CdSe. The optical transmittance of the films shows a red shift of the absorption edge with annealing. The fundamental absorption edge corresponds to a direct energy gap with a temperature coefficient of 3.34 × 10-3 eV K-1. The refractive index, optical conductivity and real and imaginary parts of the dielectric constants were found to increase after annealing. The sub-band gap absorption coefficient was evaluated using the constant photocurrent method. It varies exponentially with photon energy. The Urbach energy, the density of defect states, and the steepness of the density of localized states were evaluated from the sub-band-gap absorption.

  18. Analysis of Photonic Band Gaps in a Two-Dimensional Triangular Lattice with Superconducting Hollow Rods

    Science.gov (United States)

    Diaz-Valencia, B. F.; Calero, J. M.

    2017-02-01

    In this work, we use the plane wave expansion method to calculate photonic band structures in two-dimensional photonic crystals which consist of high-temperature superconducting hollow rods arranged in a triangular lattice. The variation of the photonic band structure with respect to both, the inner radius and the system temperature, is studied, taking into account temperatures below the critical temperature of the superconductor in the low frequencies regime and assuming E polarization of the incident light. Permittivity contrast and nontrivial geometry of the hollow rods lead to the appearance of new band gaps as compared with the case of solid cylinders. Such band gaps can be modulated by means of the inner radius and system temperature.

  19. The scaling of the effective band gaps in indium-arsenide quantum dots and wires.

    Science.gov (United States)

    Wang, Fudong; Yu, Heng; Jeong, Sohee; Pietryga, Jeffrey M; Hollingsworth, Jennifer A; Gibbons, Patrick C; Buhro, William E

    2008-09-23

    Colloidal InAs quantum wires having diameters in the range of 5-57 nm and narrow diameter distributions are grown from Bi nanoparticles by the solution-liquid-solid (SLS) mechanism. The diameter dependence of the effective band gaps (DeltaE(g)s) in the wires is determined from photoluminescence spectra and compared to the experimental results for InAs quantum dots and rods and to the predictions of various theoretical models. The DeltaE(g) values for InAs quantum dots and wires are found to scale linearly with inverse diameter (d(-1)), whereas the simplest confinement models predict that DeltaE(g) should scale with inverse-square diameter (d(-2)). The difference in the observed and predicted scaling dimension is attributed to conduction-band nonparabolicity induced by strong valence-band-conduction-band coupling in the narrow-gap InAs semiconductor.

  20. Band gap bowing in NixMg1-xO

    Science.gov (United States)

    Niedermeier, Christian A.; Råsander, Mikael; Rhode, Sneha; Kachkanov, Vyacheslav; Zou, Bin; Alford, Neil; Moram, Michelle A.

    2016-08-01

    Epitaxial transparent oxide NixMg1-xO (0 ≤ x ≤ 1) thin films were grown on MgO(100) substrates by pulsed laser deposition. High-resolution synchrotron X-ray diffraction and high-resolution transmission electron microscopy analysis indicate that the thin films are compositionally and structurally homogeneous, forming a completely miscible solid solution. Nevertheless, the composition dependence of the NixMg1-xO optical band gap shows a strong non-parabolic bowing with a discontinuity at dilute NiO concentrations of x band structure and the density of states demonstrate that deep Ni 3d levels are introduced into the MgO band gap, which significantly reduce the fundamental gap as confirmed by optical absorption spectra. These states broaden into a Ni 3d-derived conduction band for x > 0.074 and account for the anomalously large band gap narrowing in the NixMg1-xO solid solution system.

  1. Local density of optical states in the band gap of a finite photonic crysta

    CERN Document Server

    Yeganegi, Elahe; Mosk, Allard P; Vos, Willem L

    2014-01-01

    We study the local density of states (LDOS) in a finite photonic crystal, in particular in the frequency range of the band gap. We propose a new point of view on the band gap, which we consider to be the result of vacuum fluctuations in free space that tunnel in the forbidden range in the crystal. As a result, we arrive at a model for the LDOS that is in two major items modified compared to the well-known expression for infinite crystals. Firstly, we modify the Dirac delta functions to become Lorentzians with a width set by the crystal size. Secondly, building on characterization of the fields versus frequency and position we calculated the fields in the band gap. We start from the fields at the band edges, interpolated in space and position, and incorporating the exponential damping in the band gap. We compare our proposed model to exact calculations in one dimension using the transfer matrix method and find very good agreement. Notably, we find that in finite crystals, the LDOS depends on frequency, on posi...

  2. Dipole-induced band-gap reduction in an inorganic cage.

    Science.gov (United States)

    Lv, Yaokang; Cheng, Jun; Steiner, Alexander; Gan, Lihua; Wright, Dominic S

    2014-02-10

    Metal-doped polyoxotitanium cages are a developing class of inorganic compounds which can be regarded as nano- and sub-nano sized molecular relatives of metal-doped titania nanoparticles. These species can serve as models for the ways in which dopant metal ions can be incorporated into metal-doped titania (TiO2 ), a technologically important class of photocatalytic materials with broad applications in devices and pollution control. In this study a series of cobalt(II)-containing cages in the size range ca. 0.7-1.3 nm have been synthesized and structurally characterized, allowing a coherent study of the factors affecting the band gaps in well-defined metal-doped model systems. Band structure calculations are consistent with experimental UV/Vis measurements of the Tix Oy absorption edges in these species and reveal that molecular dipole moment can have a profound effect on the band gap. The observation of a dipole-induced band-gap decrease mechanism provides a potentially general design strategy for the formation of low band-gap inorganic cages.

  3. Robust topology optimization of three-dimensional photonic-crystal band-gap structures.

    Science.gov (United States)

    Men, H; Lee, K Y K; Freund, R M; Peraire, J; Johnson, S G

    2014-09-22

    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 techniques such as a simple approach to impose symmetry constraints. We also demonstrate a technique for robust topology optimization, in which some uncertainty is included in each voxel and we optimize the worst-case gap, and we show that the resulting band gaps have increased robustness to systematic fabrication errors.

  4. Relationship between band gap and bond length alternation in organic conjugated polymers

    Science.gov (United States)

    Bredas, J. L.

    1985-04-01

    A description is given of calculations of the evolution of the band gap as a function of geometry in conjugated polymers based on aromatic rings; polyparaphenylene, polypyrrole, polythiophene. The results demonstrate that the gap decreases as a function of increasing quinoid character of the backbone and is thus not minimal in the case of zero bond length alternation, in contrast to the situation found in polyacetylene-like compounds. The consequences of these results are stressed for the understanding of the effects of doping and for the design of new organic polymers with small gaps.

  5. Ultrawide low frequency band gap of phononic crystal in nacreous composite material

    Energy Technology Data Exchange (ETDEWEB)

    Yin, J.; Huang, J.; Zhang, S., E-mail: zhangs@dlut.edu.cn; Zhang, H.W.; Chen, B.S.

    2014-06-27

    The band structure of a nacreous composite material is studied by two proposed models, where an ultrawide low frequency band gap is observed. The first model (tension-shear chain model) with two phases including brick and mortar is investigated to describe the wave propagation in the nacreous composite material, and the dispersion relation is calculated by transfer matrix method and Bloch theorem. The results show that the frequency ranges of the pass bands are quite narrow, because a special tension-shear chain motion in the nacreous composite material is formed by some very slow modes. Furthermore, the second model (two-dimensional finite element model) is presented to investigate its band gap by a multi-level substructure scheme. Our findings will be of great value to the design and synthesis of vibration isolation materials in a wide and low frequency range. Finally, the transmission characteristics are calculated to verify the results. - Highlights: • A Brick-and-Mortar structure is used to discuss wave propagation through nacreous materials. • A 1D Bloch wave solution of nacreous materials with a tension-shear chain model is obtained. • The band structure and transmission characteristics of nacreous materials with the FE model are examined. • An ultrawide low frequency band gap is found in nacreous materials with both theory and FE model.

  6. Effect of band gap engineering in anionic-doped TiO2 photocatalyst

    Science.gov (United States)

    Samsudin, Emy Marlina; Abd Hamid, Sharifah Bee

    2017-01-01

    A simple yet promising strategy to modify TiO2 band gap was achieved via dopants incorporation which influences the photo-responsiveness of the photocatalyst. The mesoporous TiO2 was successfully mono-doped and co-doped with nitrogen and fluorine dopants. The results indicate that band gap engineering does not necessarily requires oxygen substitution with nitrogen or/and fluorine, but from the formation of additional mid band and Ti3+ impurities states. The formation of oxygen vacancies as a result of modified color centres and Ti3+ ions facilitates solar light absorption and influences the transfer, migration and trapping of the photo-excited charge carriers. The synergy of dopants in co-doped TiO2 shows better optical properties relative to single N and F doped TiO2 with c.a 0.95 eV band gap reduction. Evidenced from XPS, the synergy between N and F in the co-doped TiO2 uplifts the valence band towards the conduction band. However, the photoluminescence data reveals poorer electrons and holes separation as compared to F-doped TiO2. This observation suggests that efficient solar light harvesting was achievable via N and F co-doping, but excessive defects could act as charge carriers trapping sites.

  7. Electronic structure of the valence band of II--VI wide band gap semiconductor interfaces

    OpenAIRE

    1996-01-01

    In this work we present the electronic band structure for (001)--CdTe interfaces with some other II--VI zinc blende semiconductors. We assume ideal interfaces. We use tight binding Hamiltonians with an orthogonal basis ($s p^3 s^*$). We make use of the well--known Surface Green's Function Matching method to calculate the interface band structure. In our calculation the dominion of the interface is constituted by four atomic layers. We consider here anion--anion interfaces only. We have includ...

  8. Experimental investigation of photonic band gap in one-dimensional photonic crystals with metamaterials

    Energy Technology Data Exchange (ETDEWEB)

    Chen, Yihang, E-mail: eon.chen@yahoo.com.cn [Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong (China); Laboratory of Quantum Information Technology, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou (China); Wang, Xinggang [Laboratory of Quantum Information Technology, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou (China); Yong, Zehui; Zhang, Yunjuan [Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong (China); Chen, Zefeng [Laboratory of Quantum Information Technology, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou (China); He, Lianxing; Lee, P.F.; Chan, Helen L.W.; Leung, Chi Wah [Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong (China); Wang, Yu, E-mail: apywang@inet.polyu.edu.hk [Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong (China)

    2012-03-19

    Composite right/left-handed transmission lines with lumped element series capacitors and shunt inductors are used to experimentally realize the one-dimensional photonic crystals composed of single-negative metamaterials. The simulated and experimental results show that a special photonic band gap corresponding to zero-effective-phase (zero-φ{sub eff}) may appear in the microwave regime. In contrast to the Bragg gap, by changing the length ratio of the two component materials, the width and depth of the zero-φ{sub eff} gap can be conveniently adjusted while keeping the center frequency constant. Furthermore, the zero-φ{sub eff} gap vanishes when both the phase-matching and impedance-matching conditions are satisfied simultaneously. These transmission line structures provide a good way for realizing microwave devices based on the zero-φ{sub eff} gap. -- Highlights: ► 1D photonic crystals with metamaterials were investigated experimentally. ► Both Bragg gap and zero-φ{sub eff} gap were observed in the microwave regime. ► The width and depth of the zero-φ{sub eff} gap were experimentally adjusted. ► Zero-φ{sub eff} gap was observed to be close when two match conditions were satisfied.

  9. Electron tunneling in single layer graphene with an energy gap

    Institute of Scientific and Technical Information of China (English)

    Xu Xu-Guang; Zhang Chao; Xu Gong-Jie; Cao Jun-Cheng

    2011-01-01

    When a single layer graphene is epitaxially grown on silicon carbide, it will exhibit a finite energy gap like a conventional semiconductor, and its energy dispersion is no longer linear in momentum in the low energy regime. In this paper, we have investigated the tunneling characteristics through a two-dimensional barrier in a single layer graphene with an energy gap. It is found that when the electron is at a zero angle of incidence, the transmission probability as a function of incidence energy has a gap. Away from the gap the transmission coefficient oscillates with incidence energy which is analogous to that of a conventional semiconductor. The conductance under zero temperature has a gap. The properties of electron transmission may be useful for developing graphene-based nano-electronics.

  10. Photonic Band Gaps in Two-Dimensional Crystals with Fractal Structure

    Institute of Scientific and Technical Information of China (English)

    刘征; 徐建军; 林志方

    2003-01-01

    We simulate the changes of the photonic band structure of the crystal in two dimensions with a quasi-fractal structure when it is fined to a fractal. The result shows that when the dielectric distribution is fined, the photonic band structure will be compressed on the whole and the ground photonic band gap (PBG) closed while the next PBGs shrunk, in conjunction with their position declining in the frequency spectrum. Furthermore, the PBGs in the high zone are much more sensitive than those in low zones.

  11. Tuning characteristic of band gap and waveguide in a multi-stub locally resonant phononic crystal plate

    Directory of Open Access Journals (Sweden)

    Xiao-Peng Wang

    2015-10-01

    Full Text Available In this paper, the tuning characteristics of band gaps and waveguides in a locally resonant phononic crystal structure, consisting of multiple square stubs deposited on a thin homogeneous plate, are investigated. Using the finite element method and supercell technique, the dispersion relationships and power transmission spectra of those structures are calculated. In contrast to a system of one square stub, systems of multiple square stubs show wide band gaps at lower frequencies and an increased quantity of band gaps at higher frequencies. The vibration modes of the band gap edges are analyzed to clarify the mechanism of the generation of the lowest band gap. Additionally, the influence of the stubs arrangement on the band gaps in multi-stub systems is investigated. The arrangements of the stubs were found to influence the band gaps; this is critical to understand for practical applications. Based on this finding, a novel method to form defect scatterers by changing the arrangement of square stubs in a multi-stub perfect phononic crystal plate was developed. Defect bands can be induced by creating defects inside the original complete band gaps. The frequency can then be tuned by changing the defect scatterers’ stub arrangement. These results will help in fabricating devices such as acoustic filters and waveguides whose band frequency can be modulated.

  12. High-power picosecond pulse delivery through hollow core photonic band gap fibers

    DEFF Research Database (Denmark)

    Michieletto, Mattia; Johansen, Mette Marie; Lyngsø, Jens Kristian;

    2016-01-01

    We demonstrated robust and bend insensitive fiber delivery of high power laser with diffraction limited beam quality for two different kinds of hollow core band gap fibers. The light source for this experiment consists of ytterbium-doped double clad fiber aeroGAIN-ROD-PM85 in a high power amplifier...

  13. Transmission measurement of the photonic band gap of GaN photonic crystal slabs

    NARCIS (Netherlands)

    Caro, J.; Roeling, E.M.; Rong, B.; Nguyen, H.M.; Van der Drift, E.W.J.M.; Rogge, S.; Karouta, F.; Van der Heijden, R.W.; Salemink, H.W.M.

    2008-01-01

    A high-contrast-ratio (30 dB) photonic band gap in the near-infrared transmission of hole-type GaN two-dimensional photonic crystals (PhCs) is reported. These crystals are deeply etched in a 650 nm thick GaN layer grown on sapphire. A comparison of the measured spectrum with finite difference time d

  14. Surface plasmon polariton band gap structures: implications to integrated plasmonic circuits

    DEFF Research Database (Denmark)

    Bozhevolnyi, S. I.; Volkov, V. S.; Østergaard, John Erland;

    2001-01-01

    Conventional photonic band gap (PBG) structures are composed of regions with periodic modulation of refractive index that do not allow the propagation of electromagnetic waves in a certain interval of wavelengths, i.e., that exhibit the PBG effect. The PBG effect is essentially an interference ph...

  15. A novel benzodipyrrolidone-based low band gap polymer for organic solar cells

    DEFF Research Database (Denmark)

    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 donor–acceptor conjugated polymers for PSCs....

  16. Photonic band gaps in materials with triply periodic surfaces and related tubular structures

    NARCIS (Netherlands)

    Michielsen, K; Kole, JS

    2003-01-01

    We calculate the photonic band gap of triply periodic bicontinuous cubic structures and of tubular structures constructed from the skeletal graphs of triply periodic minimal surfaces. The effect of the symmetry and topology of the periodic dielectric structures on the existence and the characteristi

  17. Tunable band gap photoluminescence from atomically thin transition-metal dichalcogenide alloys.

    Science.gov (United States)

    Chen, Yanfeng; Xi, Jinyang; Dumcenco, Dumitru O; Liu, Zheng; Suenaga, Kazu; Wang, Dong; Shuai, Zhigang; Huang, Ying-Sheng; Xie, Liming

    2013-05-28

    Band gap engineering of atomically thin two-dimensional (2D) materials is the key to their applications in nanoelectronics, optoelectronics, and photonics. Here, for the first time, we demonstrate that in the 2D system, by alloying two materials with different band gaps (MoS2 and WS2), tunable band gap can be obtained in the 2D alloys (Mo(1-x)W(x)S(2) monolayers, x = 0-1). Atomic-resolution scanning transmission electron microscopy has revealed random arrangement of Mo and W atoms in the Mo(1-x)W(x)S(2) monolayer alloys. Photoluminescence characterization has shown tunable band gap emission continuously tuned from 1.82 eV (reached at x = 0.20) to 1.99 eV (reached at x = 1). Further, density functional theory calculations have been carried out to understand the composition-dependent electronic structures of Mo(1-x)W(x)S(2) monolayer alloys.

  18. Photonic Band Gap Structures with Periodically Arranged Atoms in a Two-Dimensional Photonic Crystal

    Institute of Scientific and Technical Information of China (English)

    LI Zhi-Yu; CHEN Fang; ZHOU Jian-Ying

    2005-01-01

    @@ Linear transmission, reflection and absorption spectra for a new two-dimensional photonic crystal with periodically arranged resonant atoms are examined. Numerical results show that a twin-gap structure with forbidden bands displaced from a non-doped bandgap structure can be produced as a result of atomic polarization. The absorption spectrum is also significantly altered compared to the single atom entity.

  19. Experimental investigation of hollow-core photonic crystal fibers with five photonic band-gaps

    Institute of Scientific and Technical Information of China (English)

    YUAN Jin-hui; HOU Lan-tian; WEI Dong-bin; WANG Hai-yun; ZHOU Gui-yao

    2008-01-01

    The hollow-core photonic crystal fibers (HC-PCFs) with integrity structure have been fabricated with an improved twice stack-and-draw technique. The transmission spectrum shows that five photonic band-gaps within 450-1100 nm have been obtained.And the green light transmission in the HC-PCFs'has been observed remarkably.

  20. Photonic band gap structures for long-range surface plasmon polaritons

    DEFF Research Database (Denmark)

    Bozhevolnyi, Sergey I.; Boltasseva, Alexandra; Søndergaard, Thomas;

    2005-01-01

    -size thickness variations result in the pronounced band gap effect, and obtain very good agreement between measured and simulated (transmission and reflection) spectra. This effect is exploited to realize a compact wavelength add-drop filter with the bandwidth of ~20 nm centered at 1550 nm. The possibilities...

  1. Fused thiophene/quinoxaline low band gap polymers for photovoltaic's with increased photochemical stability

    DEFF Research Database (Denmark)

    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 ...

  2. Band gap engineering in penta-graphene by substitutional doping: first-principles calculations

    Science.gov (United States)

    Berdiyorov, G. R.; Dixit, G.; Madjet, M. E.

    2016-11-01

    Using density functional theory, we study the structure, electronic properties and partial charges of a new carbon allotrope—penta-graphene (PG)—substitutionally doped by Si, B and N. We found that the electronic bandgap of PG can be tuned down to 0.2 eV due to carbon substitutions. However, the value of the band gap depends on the type and location of the dopants. For example, the strongest reduction of the band gap is obtained for Si substitutions on the top (bottom) plane of PG, whereas the substitution in the middle plane of PG has a smaller effect on the band gap of the material. Surface termination with fluorine and hydroxyl groups results in an increase of the band gap together with considerable changes in electronic and atomic partial charge distribution in the system. Our findings, which are robust against the use of different exchange-correlation functionals, indicate the possibility of tuning the bandgap of the material to make it suitable for optoelectronic and photovoltaic applications.

  3. Size of the Organic Cation Tunes the Band Gap of Colloidal Organolead Bromide Perovskite Nanocrystals.

    Science.gov (United States)

    Mittal, Mona; Jana, Atanu; Sarkar, Sagar; Mahadevan, Priya; Sapra, Sameer

    2016-08-18

    A few approaches have been employed to tune the band gap of colloidal organic-inorganic trihalide perovskites (OTPs) nanocrystals by changing the halide anion. However, to date, there is no report of electronic structure tuning of perovskite NCs upon changing the organic cation. We report here, for the first time, the room temperature colloidal synthesis of (EA)x(MA)1-xPbBr3 nanocrystals (NCs) (where, x varies between 0 and 1) to tune the band gap of hybrid organic-inorganic lead perovskite NCs from 2.38 to 2.94 eV by varying the ratio of ethylammonium (EA) and methylammonium (MA) cations. The tuning of band gap is confirmed by electronic structure calculations within density functional theory, which explains the increase in the band gap upon going toward larger "A" site cations in APbBr3 NCs. The photoluminescence quantum yield (PLQY) of these NCs lies between 5% to 85% and the average lifetime falls in the range 1.4 to 215 ns. A mixture of MA cations and its higher analog EA cations provide a versatile tool to tune the structural as well as optoelectronic properties of perovskite NCs.

  4. Effects of weak nonlinearity on dispersion relations and frequency band-gaps of periodic structures

    DEFF Research Database (Denmark)

    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...

  5. MoS2-WSe2 Hetero Bilayer: Possibility of Mechanical Strain Induced Band Gap Engineering

    Science.gov (United States)

    Sharma, Munish; Kumar, Ashok; Ahluwalia, P. K.

    2014-03-01

    The tunability of band gap in two-dimensional (2D) hetero-bilayers of MoS2-WSe2 with applied mechanical strains (in-plane and out-of-plane) in two different types of stackings (AA and AB) have been investigated in the framework of density functional theory (DFT). The in-plane biaxial tensile strain is found to reduce electronic band gap monotonically and rendered considered bilayer into metal at 6% of applied strain. The transition pressure required for complete semiconductor-to-metal transition is found to be of 7.89 GPa while tensile strength of the reported hetero-bilayer has been calculated 10 GPa at 25% strain. In case of vertical compression strain, 16 GPa pressure has been calculated for complete semiconductor-to-metal transition. The band-gap deformation potentials and effective masses (electron and hole) have been found to posses strong dependence on the type of applied strain. Such band gap engineering in controlled manner (internal control by composition and external control by applied strain) makes the considered hetero-bilayer as a strong candidate for the application in variety of nano scale devices.

  6. H-shaped oligothiophenes with low band gaps and amphoteric redox properties

    KAUST Repository

    Luo, Jing

    2010-12-17

    H-shaped bridged oligothiophenes HT-1 and HT-2 were synthesized by two different approaches. Different from normal oligothiophenes, HT-1 and HT-2 showed low band gaps and amphoteric redox behaviors due to intramolecular charge transfer, which is further supported by time-dependent DFT calculations. © 2010 American Chemical Society.

  7. Atomically thin arsenene and antimonene: semimetal-semiconductor and indirect-direct band-gap transitions.

    Science.gov (United States)

    Zhang, Shengli; Yan, Zhong; Li, Yafei; Chen, Zhongfang; Zeng, Haibo

    2015-03-01

    The typical two-dimensional (2D) semiconductors MoS2, MoSe2, WS2, WSe2 and black phosphorus have garnered tremendous interest for their unique electronic, optical, and chemical properties. However, all 2D semiconductors reported thus far feature band gaps that are smaller than 2.0 eV, which has greatly restricted their applications, especially in optoelectronic devices with photoresponse in the blue and UV range. Novel 2D mono-elemental semiconductors, namely monolayered arsenene and antimonene, with wide band gaps and high stability were now developed based on first-principles calculations. Interestingly, although As and Sb are typically semimetals in the bulk, they are transformed into indirect semiconductors with band gaps of 2.49 and 2.28 eV when thinned to one atomic layer. Significantly, under small biaxial strain, these materials were transformed from indirect into direct band-gap semiconductors. Such dramatic changes in the electronic structure could pave the way for transistors with high on/off ratios, optoelectronic devices working under blue or UV light, and mechanical sensors based on new 2D crystals.

  8. Phononic band gaps and vibrations in one- and two-dimensional mass-spring structures

    DEFF Research Database (Denmark)

    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 dampin...

  9. Optical study of the band structure of wurtzite GaP nanowires

    KAUST Repository

    Assali, S.

    2016-07-25

    We investigated the optical properties of wurtzite (WZ) GaP nanowires by performing photoluminescence (PL) and time-resolved PL measurements in the temperature range from 4 K to 300 K, together with atom probe tomography to identify residual impurities in the nanowires. At low temperature, the WZ GaP luminescence shows donor-acceptor pair emission at 2.115 eV and 2.088 eV, and Burstein-Moss band-filling continuum between 2.180 and 2.253 eV, resulting in a direct band gap above 2.170 eV. Sharp exciton α-β-γ lines are observed at 2.140–2.164–2.252 eV, respectively, showing clear differences in lifetime, presence of phonon replicas, and temperature-dependence. The excitonic nature of those peaks is critically discussed, leading to a direct band gap of ∼2.190 eV and to a resonant state associated with the γ-line ∼80 meV above the Γ8C conduction band edge.

  10. Effect of Pd ion doping in the band gap of SnO{sub 2} nanoparticles: structural and optical studies

    Energy Technology Data Exchange (ETDEWEB)

    Nandan, Brajesh; Venugopal, B. [Pondicherry University, Centre for Nanoscience and Technology (India); Amirthapandian, S.; Panigrahi, B. K. [Indira Gandhi Centre for Atomic Research, Ion Beam and Computer Simulation Section, Materials Science Group (India); Thangadurai, P., E-mail: thangadurai.p@gmail.com [Pondicherry University, Centre for Nanoscience and Technology (India)

    2013-10-15

    Pd ion doping has influenced the band gap of SnO{sub 2} nanoparticles. Undoped and Pd ion-doped SnO{sub 2} nanoparticles were synthesized by chemical co-precipitation method. A tetragonal phase of SnO{sub 2} with a grain size range of 7-13 nm was obtained (studied by X-ray diffraction and transmission electron microscopy). A decreasing trend in the particle size with increasing doping concentration was observed. The presence of Pd in doped SnO{sub 2} was confirmed by chemical analysis carried out by energy-dispersive spectroscopy in the transmission electron microscope. Diffuse reflectance spectra showed a blue shift in absorption with increasing palladium concentration. Band gap of SnO{sub 2} nanoparticles was estimated from the diffuse reflectance spectra using Kubelka-Munk function and it was increasing with the increase of Pd ion concentration from 3.73 to 4.21 eV. The variation in band gap is attributed predominantly to the lattice strain and particle size. All the samples showed a broad photoluminescence emission centered at 375 nm when excited at 270 nm. A systematic study on the structural and optical properties of SnO{sub 2} nanoparticles is presented.

  11. Effect of Thermal Annealing on the Band GAP and Optical Properties of Chemical Bath Deposited ZnSe Thin Films

    Science.gov (United States)

    Ezema, F. I.; Ekwealor, A. B. C.; Osuji, R. U.

    2006-05-01

    Zinc selenide (ZnSe) thin films were deposited on glass substrate using the chemical bath deposition method at room temperature from aqueous solutions of zinc sulphate and sodium selenosulfate in which sodium hydroxide was employed as complexing agents. The `as-deposited' ZnSe thin films are red in color and annealed in oven at 473 K for 1 hour and on a hot plate in open air at 333 K for 5 minutes, affecting the morphological and optical properties. Optical properties such as absorption coefficient a and extinction coefficient k, were determined using the absorbance and transmission measurement from Unico UV-2102 PC spectrophotometer, at normal incidence of light in the wavelength range of 200-1000 nm. The films have transmittance in VIS-NIR regions that range between 26 and 87%. From absorbance and transmittance spectra, the band gap energy determined ranged between 1.60 eV and 1.75 for the `as deposited' samples, and the annealed samples exhibited a band gap shift of 0.15 eV. The high transmittance of the films together with its large band gap made them good materials for selective coatings for solar cells.

  12. Anomalous dispersion and band gap reduction in UO2+x and its possible coupling to the coherent polaronic quantum state

    Science.gov (United States)

    Conradson, Steven D.; Andersson, David A.; Bagus, Paul S.; Boland, Kevin S.; Bradley, Joseph A.; Byler, Darrin D.; Clark, David L.; Conradson, Dylan R.; Espinosa-Faller, Francisco J.; Lezama Pacheco, Juan S.; Martucci, Mary B.; Nordlund, Dennis; Seidler, Gerald T.; Valdez, James A.

    2016-05-01

    Hypervalent UO2, UO2(+x) formed by both addition of excess O and photoexcitation, exhibits a number of unusual or often unique properties that point to it hosting a polaronic Bose-Einstein(-Mott) condensate. A more thorough analysis of the O X-ray absorption spectra of UO2, U4O9, and U3O7 shows that the anomalous increase in the width of the spectral features assigned to predominantly U 5f and 6d final states that points to increased dispersion of these bands occurs on the low energy side corresponding to the upper edge of the gap bordered by the conduction or upper Hubbard band. The closing of the gap by 1.5 eV is more than twice as much as predicted by calculations, consistent with the dynamical polaron found by structural measurements. In addition to fostering the excitation that is the proposed mechanism for the coherence, the likely mirroring of this effect on the occupied, valence side of the gap below the Fermi level points to increased complexity of the electronic structure that could be associated with the Fermi topology of BEC-BCS crossover and two band superconductivity.

  13. Emission of direct-gap band in germanium with Ge-GeSn layers on one-dimensional structure

    Science.gov (United States)

    Huang, Zhong-Mei; Huang, Wei-Qi; Liu, Shi-Rong; Dong, Tai-Ge; Wang, Gang; Wu, Xue-Ke; Qin, Cao-Jian

    2016-04-01

    In our experiment, it was observed that the emission of direct-gap band in germanium with Ge-GeSn layers on one-dimensional (1D) structure. The results of experiment and calculation demonstrate that the uniaxial tensile strain in the (111) and (110) direction can efficiently transform Ge to a direct bandgap material with the bandgap energy useful for technological application. It is interested that under the tensile strain from Ge-GeSn layers on 1D structure in which the uniaxial strain could be obtained by curved layer (CL) effect, the two bandgaps EΓg and ELg in the (111) direction become nearly equal at 0.83 eV related to the emission of direct-gap band near 1500 nm in the experiments. It is discovered that the red-shift of the peaks from 1500 nm to 1600 nm occurs with change of the uniaxial tensile strain, which proves that the peaks come from the emission of direct-gap band.

  14. Stable Band-Gaps in Phononic Crystals by Harnessing Hyperelastic Transformation Media

    CERN Document Server

    Liu, Yan; Feng, Xi-Qiao

    2016-01-01

    The band structure in phononic crystals (PCs) is usually affected by the deformations of their soft components. In this work, hyperelastic transformation media is proposed to be integrated in the PCs'design, to achieve stable elastic band-gaps which is independent with finite mechanical deformations. For a one-dimensional (1D) PC, we demonstrate the semi-linear soft component can keep all elastic wave bands unchanged with the external deformation field. While for neo-Hookean soft component, only S-wave bands can be precisely retained. The change of the P-wave bands can be predicted by using a lumped mass method. Numerical simulations are performed to validate our theory predictions and the robustness of the proposed PCs.

  15. Photonic-band-gap engineering for volume plasmon polaritons in multiscale multilayer hyperbolic metamaterials

    DEFF Research Database (Denmark)

    Zhukovsky, Sergei; Orlov, Alexey A.; Babicheva, Viktoriia E.;

    2014-01-01

    ) on a larger, wavelength scale, the propagation of volume plasmon polaritons in the resulting multiscale hyperbolic metamaterials is subject to photonic-band-gap phenomena. A great degree of control over such plasmons can be exerted by varying the superstructure geometry. When this geometry is periodic, stop...... bands due to Bragg reflection form within the volume plasmonic band. When a cavity layer is introduced in an otherwise periodic superstructure, resonance peaks of the Fabry-Perot nature are present within the stop bands. More complicated superstructure geometries are also considered. For example......, fractal Cantor-like multiscale metamaterials are found to exhibit characteristic self-similar spectral signatures in the volume plasmonic band. Multiscale hyperbolic metamaterials are shown to be a promising platform for large-wave-vector bulk plasmonic waves, whether they are considered for use as a kind...

  16. Inter-band phase fluctuations in macroscopic quantum tunneling of multi-gap superconducting Josephson junctions

    Energy Technology Data Exchange (ETDEWEB)

    Asai, Hidehiro, E-mail: hd-asai@aist.go.jp [Electronics and Photonics Research Institute (ESPRIT), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568 (Japan); Ota, Yukihiro [CCSE, Japan Atomic Energy Agency, Kashiwa, Chiba 277-8587 (Japan); Kawabata, Shiro [Electronics and Photonics Research Institute (ESPRIT), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568 (Japan); Nori, Franco [CEMS, RIKEN, Wako-shi, Saitama 351-0198 (Japan); Physics Department, University of Michigan, Ann Arbor, MI 48109-1040 (United States)

    2014-09-15

    Highlights: • We study MQT in Josephson junctions composed of multi-gap superconductors. • We derive a formula of the MQT escape rate for multiple phase differences. • We investigate the effect of inter-band phase fluctuation on MQT. • The MQT escape rate is significantly enhanced by the inter-band phase fluctuation. - Abstract: We theoretically investigate macroscopic quantum tunneling (MQT) in a hetero Josephson junction formed by a conventional single-gap superconductor and a multi-gap superconductor. In such Josephson junctions, phase differences for each tunneling channel are defined, and the fluctuation of the relative phase differences appear which is referred to as Josephson–Leggett’s mode. We take into account the effect of the fluctuation in the tunneling process and calculate the MQT escape rate for various junction parameters. We show that the fluctuation of relative phase differences drastically enhances the escape rate.

  17. Tuning of band gaps for a two-dimensional piezoelectric phononic crystal with a rectangular lattice

    Institute of Scientific and Technical Information of China (English)

    Yize Wang; Fengming Li; Yuesheng Wang; Kikuo Kishimoto; Wenhu Huang

    2009-01-01

    In this paper, the elastic wave propagation in a two-dimensional piezoelectric phononic crystal is studied by considering the mechanic-electric coupling. The gener-alized eigenvalue equation is obtained by the relation of the mechanic and electric fields as well as the Bloch-Floquet the-orem. The band structures of both the in-plane and anti-plane modes are calculated for a rectangular lattice by the plane-wave expansion method. The effects of the lattice constant ratio and the piezoelectricity with different filling fractions are analyzed. The results show that the largest gap width is not always obtained for a square lattice. In some situations, a rectangular lattice may generate larger gaps. The band gap characteristics are influenced obviously by the piezoelectric-ity with the larger lattice constant ratios and the filling frac-tions.

  18. The Development of Layered Photonic Band Gap Structures Using a Micro-Transfer Molding Technique

    Energy Technology Data Exchange (ETDEWEB)

    Sutherland, Kevin Jerome [Iowa State Univ., Ames, IA (United States)

    2001-01-01

    Photonic band gap (PBG) crystals are periodic dielectric structures that manipulate electromagnetic radiation in a manner similar to semiconductor devices manipulating electrons. Whereas a semiconductor material exhibits an electronic band gap in which electrons cannot exist, similarly, a photonic crystal containing a photonic band gap does not allow the propagation of specific frequencies of electromagnetic radiation. This phenomenon results from the destructive Bragg diffraction interference that a wave propagating at a specific frequency will experience because of the periodic change in dielectric permitivity. This gives rise to a variety of optical applications for improving the efficiency and effectiveness of opto-electronic devices. These applications are reviewed later. Several methods are currently used to fabricate photonic crystals, which are also discussed in detail. This research involves a layer-by-layer micro-transfer molding ({mu}TM) and stacking method to create three-dimensional FCC structures of epoxy or titania. The structures, once reduced significantly in size can be infiltrated with an organic gain media and stacked on a semiconductor to improve the efficiency of an electronically pumped light-emitting diode. Photonic band gap structures have been proven to effectively create a band gap for certain frequencies of electro-magnetic radiation in the microwave and near-infrared ranges. The objective of this research project was originally two-fold: to fabricate a three dimensional (3-D) structure of a size scaled to prohibit electromagnetic propagation within the visible wavelength range, and then to characterize that structure using laser dye emission spectra. As a master mold has not yet been developed for the micro transfer molding technique in the visible range, the research was limited to scaling down the length scale as much as possible with the current available technology and characterizing these structures with other methods.

  19. Vibration band gaps for elastic metamaterial rods using wave finite element method

    Science.gov (United States)

    Nobrega, E. D.; Gautier, F.; Pelat, A.; Dos Santos, J. M. C.

    2016-10-01

    Band gaps in elastic metamaterial rods with spatial periodic distribution and periodically attached local resonators are investigated. New techniques to analyze metamaterial systems are using a combination of analytical or numerical method with wave propagation. One of them, called here wave spectral element method (WSEM), consists of combining the spectral element method (SEM) with Floquet-Bloch's theorem. A modern methodology called wave finite element method (WFEM), developed to calculate dynamic behavior in periodic acoustic and structural systems, utilizes a similar approach where SEM is substituted by the conventional finite element method (FEM). In this paper, it is proposed to use WFEM to calculate band gaps in elastic metamaterial rods with spatial periodic distribution and periodically attached local resonators of multi-degree-of-freedom (M-DOF). Simulated examples with band gaps generated by Bragg scattering and local resonators are calculated by WFEM and verified with WSEM, which is used as a reference method. Results are presented in the form of attenuation constant, vibration transmittance and frequency response function (FRF). For all cases, WFEM and WSEM results are in agreement, provided that the number of elements used in WFEM is sufficient to convergence. An experimental test was conducted with a real elastic metamaterial rod, manufactured with plastic in a 3D printer, without local resonance-type effect. The experimental results for the metamaterial rod with band gaps generated by Bragg scattering are compared with the simulated ones. Both numerical methods (WSEM and WFEM) can localize the band gap position and width very close to the experimental results. A hybrid approach combining WFEM with the commercial finite element software ANSYS is proposed to model complex metamaterial systems. Two examples illustrating its efficiency and accuracy to model an elastic metamaterial rod unit-cell using 1D simple rod element and 3D solid element are

  20. Effect of Sn on the optical band gap determined using absorption spectrum fitting method

    Energy Technology Data Exchange (ETDEWEB)

    Heera, Pawan, E-mail: sramanb70@mailcity.com [Department of Physics, Himachal Pradesh University, Shimla, INDIA, 171005 (India); Govt. College Amb, Himachal Pradesh, INDIA,177203 (India); Kumar, Anup, E-mail: kumar.anup.sml@gmail.com [Department of Physics, Himachal Pradesh University, Shimla, INDIA, 171005 (India); Physics Department, Govt. College, Kullu, H. P., INDIA, 175101 (India); Sharma, Raman, E-mail: pawanheera@yahoo.com [Department of Physics, Himachal Pradesh University, Shimla, INDIA, 171005 (India)

    2015-05-15

    We report the preparation and the optical studies on tellurium rich glasses thin films. The thin films of Se{sub 30}Te{sub 70-x} Sn{sub x} system for x= 0, 1.5, 2.5 and 4.5 glassy alloys prepared by melt quenching technique are deposited on the glass substrate using vacuum thermal evaporation technique. The analysis of absorption spectra in the spectral range 400nm–4000 nm at room temperature obtained from UV-VIS-NIR spectrophotometer [Perkin Elmer Lamda-750] helps us in the optical characterization of the thin films under study. The absorption spectrum fitting method is applied by using the Tauc’s model for estimating the optical band gap and the width of the band tail of the thin films. The optical band gap is calculated and is found to decrease with the Sn content.

  1. Solid-State Nanopore Confinement for Band Gap Engineering of Metal-Halide Perovskites

    CERN Document Server

    Demchyshyn, Stepan; Groiss, Heiko; Heilbrunner, Herwig; Ulbricht, Christoph; Apaydin, Dogukan; Rütt, Uta; Bertram, Florian; Hesser, Günter; Scharber, Markus; Nickel, Bert; Sariciftci, Niyazi Serdar; Bauer, Siegfried; Głowacki, Eric Daniel; Kaltenbrunner, Martin

    2016-01-01

    Tuning the band gap of semiconductors via quantum size effects launched a technological revolution in optoelectronics, advancing solar cells, quantum dot light-emitting displays, and solid state lasers. Next generation devices seek to employ low-cost, easily processable semiconductors. A promising class of such materials are metal-halide perovskites, currently propelling research on emerging photovoltaics. Their narrow band emission permits very high colour purity in light-emitting devices and vivid life-like displays paired with low-temperature processing through printing-compatible methods. Success of perovskites in light-emitting devices is conditional upon finding reliable strategies to obtain tunability of the band gap. So far, colour can be tuned chemically by mixed halide stoichiometry, or by synthesis of colloidal particles. Here we introduce a general strategy of controlling shape and size of perovskite nanocrystallites (less than 10 nm) in domains that exhibit strong quantum size effects. Without ma...

  2. Band Gap Tuning and Defect Tolerance of Atomically Thin Two- Dimensional Organic-Inorganic Halide Perovskites

    DEFF Research Database (Denmark)

    Pandey, Mohnish; Jacobsen, Karsten Wedel; Thygesen, Kristian Sommer

    2016-01-01

    Organic−inorganic halide perovskites have proven highly successful for photovoltaics but suffer from low stability, which deteriorates their performance over time. Recent experiments have demonstrated that low dimensional phases of the hybrid perovskites may exhibit improved stability. Here we...... report first-principles calculations for isolated monolayers of the organometallic halide perovskites (C4H9NH3)2MX2Y2, where M = Pb, Ge, Sn and X,Y = Cl, Br, I. The band gaps computed using the GLLB-SC functional are found to be in excellent agreement with experimental photoluminescence data...... for the already synthesized perovskites. Finally, we study the effect of different defects on the band structure. We find that the most common defects only introduce shallow or no states in the band gap, indicating that these atomically thin 2D perovskites are likely to be defect tolerant....

  3. Global Evolutionary Algorithms in the Design of Electromagnetic Band Gap Structures with Suppressed Surface Waves Propagation

    Directory of Open Access Journals (Sweden)

    P. Kovacs

    2010-04-01

    Full Text Available The paper is focused on the automated design and optimization of electromagnetic band gap structures suppressing the propagation of surface waves. For the optimization, we use different global evolutionary algorithms like the genetic algorithm with the single-point crossover (GAs and the multi-point (GAm one, the differential evolution (DE and particle swarm optimization (PSO. The algorithms are mutually compared in terms of convergence velocity and accuracy. The developed technique is universal (applicable for any unit cell geometry. The method is based on the dispersion diagram calculation in CST Microwave Studio (CST MWS and optimization in Matlab. A design example of a mushroom structure with simultaneous electromagnetic band gap properties (EBG and the artificial magnetic conductor ones (AMC in the required frequency band is presented.

  4. The energy-service gap: What does it mean?

    Energy Technology Data Exchange (ETDEWEB)

    Backlund, Sandra; Thollander, Patrik (Department of Management and Engineering, Division of Energy Systems, Linkoeping University (Sweden))

    2011-07-01

    Through the formulation of the 2020-targets, the EU has set as an objective to reduce the use of primary energy with 20 % by 2020. The target is supposed to be reached through increased energy efficiency. Despite a large potential for energy efficiency, cost effective measures are not always implemented which is explained by market failures and barriers to energy efficiency. This difference between potential energy-efficiency and what is actually implemented, is referred to as the energy-efficiency gap. Energy service companies (ESCOs) have been put forth as a potential means of overcoming this gap to energy-efficiency. Well-functioning markets for ESCOs are therefore addressed as one of the key elements in the Energy Services Directive (ESD), a tool for the economy to move towards increased energy efficiency and sustainability. In other words, the development of the energy service market is of crucial importance if a Member State is to achieve the ambitious 2020-target. The aim of this article is to analyse the market for energy services towards industrial small- and medium sized Enterprises (SMEs). Focus will be on the Swedish market, however general conclusions may be drawn from this example. A large part of the potential for energy services is not being implemented today - this is identified as the energy-service gap. The gap is explained by transaction cost economics; relatively high transaction costs for consulting ESCOs inhibit further market development. The ESCO market in Sweden is estimated, by the Swedish state, to still be immature but have potential for further development. A government report does not identify the market barriers on the energy service market as market failures. By introducing market development mechanisms (e.g. standardized contracts and an accreditation system) the state could decrease the transaction and thus the energy service gap. Reducing the energy-service gap could be a cost effective way of reducing the energy efficiency gap

  5. Electron concentration dependence of optical band gap shift in Ga-doped ZnO thin films by magnetron sputtering

    Energy Technology Data Exchange (ETDEWEB)

    Wang, Yaqin [State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054 (China); Tang, Wu, E-mail: tang@uestc.edu.cn [State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054 (China); Zhang, Lan [State Key Laboratory for Mechanical Behavior of Materials, Xi' an Jiaotong University, Xi' an 710049 (China); Zhao, Junliang [Shanghai Juntech Co. Ltd., 1378 Xingxian Road, Shanghai 201815 (China)

    2014-08-28

    Ga-doped ZnO (GZO) thin films were deposited on glass substrates by a radio frequency magnetron sputtering technique. The optical properties of the deposited GZO films were evaluated using an optical transmission measurement. The optical band gap increased from 3.32 eV to 3.45 eV with the increasing carrier density from 2.0 × 10{sup 20} cm{sup −3} to 3.24 × 10{sup 20} cm{sup −3}. Based on the experimental results, the optical band gap as a function of carrier density is systematically investigated with four available theoretical models taken into consideration. The blueshift of the optical band gap in GZO films can be well interpreted with a complex model which combines the Burstein–Moss effect, the band gap renormalization effect and the nonparabolic nature of conduction band. In addition, the BM contribution is almost offset by the BGR effect in both conduction band and valence band due to the approximate equality between electron and hole effective masses in GZO films with a nonparabolic conduction band. The tunability of optical band gap in GZO thin films by carrier density offers a number of potential advantages in the development of semiconductor optoelectronic devices. - Highlights: • The effects of electron concentration on optical band gap were analyzed. • The measured optical band gap corresponded well with the calculated ones. • The Burstein–Moss (BM) and band gap renormalization (BGR) effects were considered. • Nonparabolic conduction band parameters were used in theoretical analysis. • The BM effect was offset by the BGR effect in both conduction band and valence band.

  6. Zero permeability and zero permittivity band gaps in 1D metamaterial photonic crystals

    Energy Technology Data Exchange (ETDEWEB)

    Depine, Ricardo A. [Grupo de Electromagnetismo Aplicado, Departamento de Fisica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellon I, C1428EHA Buenos Aires (Argentina); Martinez-Ricci, Maria L. [Grupo de Electromagnetismo Aplicado, Departamento de Fisica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellon I, C1428EHA Buenos Aires (Argentina); Monsoriu, Juan A. [Departamento de Fisica Aplicada, Universidad Politecnica de Valencia, 46022 Valencia (Spain)]. E-mail: jmonsori@fis.upv.es; Silvestre, Enrique [Departamento de Optica, Universidad de Valencia, 46100 Burjassot (Spain); Andres, Pedro [Departamento de Optica, Universidad de Valencia, 46100 Burjassot (Spain)

    2007-04-30

    We consider layered heterostructures combining ordinary positive index materials and dispersive metamaterials. We show that these structures can exhibit a new type of photonic gap around frequencies where either the magnetic permeability {mu} or the electric permittivity {epsilon} of the metamaterial is zero. Although the interface of a semi-infinite medium with zero refractive index (a condition attained either when {mu}=0 or when {epsilon}=0) is known to give full reflectivity for all incident polarizations, here we show that a gap corresponding to {mu}=0 occurs only for TE polarized waves, whereas a gap corresponding to {epsilon}=0 occurs only for TM polarized waves. These band gaps are scale-length invariant and very robust against disorder, although they may disappear for the particular case of propagation along the stratification direction.

  7. Synthesis and Band Gap Control in Three-Dimensional Polystyrene Opal Photonic Crystals

    Institute of Scientific and Technical Information of China (English)

    LIU Ye; ZHENG Zhong-Yu; QIN Fei; ZHOU Fei; ZHOU Chang-Zhu; ZHANG Dao-Zhong; MENG Qing-Bo; LI Zhi-Yuan

    2008-01-01

    High-quality three-dimensional polystyrene opal photonic crystals are fabricated by vertical deposition method.The transmission properties with different incident angles and different composite refractive index contrasts are experimentally and theoretically studied. Good agreement between the experiment and theory is achieved. We find that with the increasing incident angle, the gap position shifts to the short wavelength (blue shift) and the gap becomes shallower; and with the increase of refractive index of the opal void materials and decrease the contrast of refractive index, the gap position shifts to the long wavelength (red shift). At the same time, we observe the swelling effects when the sample is immerged in the solutions with different refractive indices, which make the microsphere diameter in solution become larger than that in air. The understanding of band gap shift behaviour may be helpful in designing optical sensors and tunable photonic crystal ultrafast optical switches.

  8. Photonic band gap of a graphene-embedded quarter-wave stack

    CERN Document Server

    Fan, Yuancheng; Li, Hongqiang; Chen, Hong; Soukoulis, Costas M

    2013-01-01

    Here, we present a mechanism for tailoring the photonic band structure of a quarter-wave stack without changing its physical periods by embedding conductive sheets. Graphene is utilized and studied as a realistic, two-dimensional conductive sheet. In a graphene-embedded quarter-wave stack, the synergic actions of Bragg scattering and graphene conductance contributions open photonic gaps at the center of the reduced Brillouin zone, that nonexistent in conventional quarter-wave stacks. Such photonic gaps show giant, loss-independent density of optical states at the fixed lower-gap-edges, of even-multiple characteristic frequency of the quarter-wave stack. The novel conductive sheets induced photonic gaps provide a new platform for the enhancement of light-matter interactions.

  9. Band gap states in nanocrystalline WO3 thin films studied by soft x-ray spectroscopy and optical spectrophotometry

    Science.gov (United States)

    Johansson, M. B.; Kristiansen, P. T.; Duda, L.; Niklasson, G. A.; Österlund, L.

    2016-11-01

    Nanocrystalline tungsten trioxide (WO3) thin films prepared by DC magnetron sputtering have been studied using soft x-ray spectroscopy and optical spectrophotometry. Resonant inelastic x-ray scattering (RIXS) measurements reveal band gap states in sub-stoichiometric γ-WO3-x with x  =  0.001-0.005. The energy positions of these states are in good agreement with recently reported density functional calculations. The results were compared with optical absorption measurements in the near infrared spectral region. An optical absorption peak at 0.74 eV is assigned to intervalence transfer of polarons between W sites. A less prominent peak at energies between 0.96 and 1.16 eV is assigned to electron excitation of oxygen vacancies. The latter results are supported by RIXS measurements, where an energy loss in this energy range was observed, and this suggests that electron transfer processes involving transitions from oxygen vacancy states can be observed in RIXS. Our results have implications for the interpretation of optical properties of WO3, and the optical transitions close to the band gap, which are important in photocatalytic and photoelectrochemical applications.

  10. Band gap states in nanocrystalline WO3 thin films studied by soft x-ray spectroscopy and optical spectrophotometry.

    Science.gov (United States)

    Johansson, M B; Kristiansen, P T; Duda, L; Niklasson, G A; Österlund, L

    2016-11-30

    Nanocrystalline tungsten trioxide (WO3) thin films prepared by DC magnetron sputtering have been studied using soft x-ray spectroscopy and optical spectrophotometry. Resonant inelastic x-ray scattering (RIXS) measurements reveal band gap states in sub-stoichiometric γ-WO3-x with x  =  0.001-0.005. The energy positions of these states are in good agreement with recently reported density functional calculations. The results were compared with optical absorption measurements in the near infrared spectral region. An optical absorption peak at 0.74 eV is assigned to intervalence transfer of polarons between W sites. A less prominent peak at energies between 0.96 and 1.16 eV is assigned to electron excitation of oxygen vacancies. The latter results are supported by RIXS measurements, where an energy loss in this energy range was observed, and this suggests that electron transfer processes involving transitions from oxygen vacancy states can be observed in RIXS. Our results have implications for the interpretation of optical properties of WO3, and the optical transitions close to the band gap, which are important in photocatalytic and photoelectrochemical applications.

  11. High-power picosecond pulse delivery through hollow core photonic band gap fibers

    Science.gov (United States)

    Michieletto, Mattia; Johansen, Mette M.; Lyngsø, Jens K.; Lægsgaard, Jesper; Bang, Ole; Alkeskjold, Thomas T.

    2016-03-01

    We demonstrated robust and bend insensitive fiber delivery of high power laser with diffraction limited beam quality for two different kinds of hollow core band gap fibers. The light source for this experiment consists of ytterbium-doped double clad fiber aeroGAIN-ROD-PM85 in a high power amplifier setup. It provided 22ps pulses with a maximum average power of 95W, 40MHz repetition rate at 1032nm (~2.4μJ pulse energy), with M2 power output for a 5 meters fiber. The damage threshold for a 19-cell hollow core photonic bandgap fiber exceeded the maximum power provided by the light source and up to 76W average output power was demonstrated for a 1m fiber. In both cases, no special attention was needed to mitigate bend sensitivity. The fibers were coiled on 8 centimeters radius spools and even lower bending radii were present. In addition, stimulated rotational Raman scattering arising from nitrogen molecules was measured through a 42m long 19 cell hollow core fiber.

  12. Theoretical investigation into tunable band gaps of an Euler- Bernoulli beam with 2DOF LR structures

    Science.gov (United States)

    Xingqian, Zhao; Changgeng, Shuai; Yan, Gao; Rustighi, Emiliano

    2016-09-01

    This paper is concerned with an intelligent phonotic crystals (IPC) consisting of an Euler-Bernoulli beam attached with 2DOF locally resonant (LR) structures. The novel design of the dielectric electroactive polymer (DEAP) rings acting as the springs of oscillators is presented that could be employed to control the transmission of flexural waves on the beam. Tunable band gaps (BGs) can be realized by changing the stiffness of each oscillator driven by the external electric field, where the DEAPs transform electric energy directly into mechanical work under the applied voltage. Discrete copper (Cu) strips are then attached to the DEAP to allow the deformation of DEAP rings. The transfer matrix (TM) theory is adopted to assist readers to better understand the formation of the BG. Simulation results show that this particular configuration is effective for attenuating the flexural waves at low frequencies below 1000Hz where the tunable BGs may occur. Moreover, it is found that a wider BG can be achieved and shifts towards higher frequencies by increasing the applied voltages.

  13. Efficient photovoltaic cells from low band-gap fluorene-based copolymer

    Institute of Scientific and Technical Information of China (English)

    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.

  14. Polarization-induced electrical conductivity in ultra-wide band gap AlGaN alloys

    Science.gov (United States)

    Armstrong, Andrew M.; Allerman, Andrew A.

    2016-11-01

    Unintentionally doped (UID) AlGaN epilayers graded over Al compositions of 80%-90% and 80%-100% were grown by metal organic vapor phase epitaxy and were electrically characterized using contactless sheet resistance (Rsh) and capacitance-voltage (C-V) measurements. Strong electrical conductivity in the UID graded AlGaN epilayers resulted from polarization-induced doping and was verified by the low resistivity of 0.04 Ω cm for the AlGaN epilayer graded over 80%-100% Al mole fraction. A free electron concentration (n) of 4.8 × 1017 cm-3 was measured by C-V for Al compositions of 80%-100%. Average electron mobility ( μ ¯ ) was calculated from Rsh and n data for three ranges of Al composition grading, and it was found that UID AlGaN graded from 88%-96% had μ ¯ = 509 cm2/V s. The combination of very large band gap energy, high μ ¯ , and high n for UID graded AlGaN epilayers make them attractive as a building block for high voltage power electronic devices such as Schottky diodes and field effect transistors.

  15. An operational method to model carrier degeneracy and band gap narrowing

    Science.gov (United States)

    Adler, M. S.

    1983-05-01

    In this paper an operational method of modeling heavily doped silicon to include effects of carrier degeneracy and band gap narrowing is presented. The issue of carier degeneracy on majority carrier flow is discussed together with the question of the ambiguity in the electrostatic potential associated with identifying which band edge is narrowed. Using an exact numerical analysis of a bipolar transistor as an example it is shown that when modeling carrier flow in quasi-neutral regions, classical statistics can be used for the majority carrier and the ambiguity in the electrostatic potential can be ignored. Overall, it is shown that for the same quasi-neutral heavily doped regions the effects of carrier degeneracy and band gap narrowing are accurately modeled within the context of classical statistics by adding the quasi field term to the minority carrier transport equation that is based on the commonly used "band gap narrowing" data available from measurements of minority carrier transport in heavily doped regions. While it is recognized that this is not rigorously correct the result of this paper is to establish the accuracy for the operational method most commonly used to model heavy doping effects.

  16. Enhanced thermoelectric performance in the Rashba semiconductor BiTeI through band gap engineering.

    Science.gov (United States)

    Wu, Lihua; Yang, Jiong; Zhang, Tiansong; Wang, Shanyu; Wei, Ping; Zhang, Wenqing; Chen, Lidong; Yang, Jihui

    2016-03-01

    Rashba semiconductors are of great interest in spintronics, superconducting electronics and thermoelectrics. Bulk BiTeI is a new Rashba system with a giant spin-split band structure. 2D-like thermoelectric response has been found in BiTeI. However, as optimizing the carrier concentration, the bipolar effect occurs at elevated temperature and deteriorates the thermoelectric performance of BiTeI. In this paper, band gap engineering in Rashba semiconductor BiTeI through Br-substitution successfully reduces the bipolar effect and improves the thermoelectric properties. By utilizing the optical absorption and Burstein-Moss-effect analysis, we find that the band gap in Rashba semiconductor BiTeI increases upon bromine substitution, which is consistent with theoretical predictions. Bipolar transport is mitigated due to the larger band gap, as the thermally-activated minority carriers diminish. Consequently, the Seebeck coefficient keeps increasing with a corresponding rise in temperature, and thermoelectric performance can thus be enhanced with a ZT  =  0.5 at 570 K for BiTeI0.88Br0.12.

  17. Photonic bands, gap maps, and intrinsic losses in three-component 2D photonic crystal slabs

    Institute of Scientific and Technical Information of China (English)

    Hongjun Shen; Huiping Tian; Yuefeng Ji

    2009-01-01

    We obtain the photonic bands and intrinsic losses for the triangular lattice three-component two-dimensional (2D) photonic crystal (PhC) slabs by expanding the electromagnetic field on the basis of waveguide modes of an effective homogeneous waveguide. The introduction of the third component into the 2D PhC slabs influences the photonic band structure and the intrinsic losses of the system. We ex-amine the dependences of the band gap width and gap edge position on the interlayer dielectric constant and interlayer thickness. It is found that the gap edges shift to lower frequencies and the intrinsic losses of each band decrease with the increasing interlayer thickness or dielectric constant. During the design of the real PhC system, the effect of unintentional native oxide surface layer on the optical properties of 2D PhC slabs has to be taken into consideration. At the same time, intentional oxidization of macroporous PhC structure can be utilized to optimize the design.

  18. Enhanced thermoelectric performance in the Rashba semiconductor BiTeI through band gap engineering

    Science.gov (United States)

    Wu, Lihua; Yang, Jiong; Zhang, Tiansong; Wang, Shanyu; Wei, Ping; Zhang, Wenqing; Chen, Lidong; Yang, Jihui

    2016-03-01

    Rashba semiconductors are of great interest in spintronics, superconducting electronics and thermoelectrics. Bulk BiTeI is a new Rashba system with a giant spin-split band structure. 2D-like thermoelectric response has been found in BiTeI. However, as optimizing the carrier concentration, the bipolar effect occurs at elevated temperature and deteriorates the thermoelectric performance of BiTeI. In this paper, band gap engineering in Rashba semiconductor BiTeI through Br-substitution successfully reduces the bipolar effect and improves the thermoelectric properties. By utilizing the optical absorption and Burstein-Moss-effect analysis, we find that the band gap in Rashba semiconductor BiTeI increases upon bromine substitution, which is consistent with theoretical predictions. Bipolar transport is mitigated due to the larger band gap, as the thermally-activated minority carriers diminish. Consequently, the Seebeck coefficient keeps increasing with a corresponding rise in temperature, and thermoelectric performance can thus be enhanced with a ZT  =  0.5 at 570 K for BiTeI0.88Br0.12.

  19. Role of Fe doping in tuning the band gap of TiO2 for the photo-oxidation-induced cytotoxicity paradigm.

    Science.gov (United States)

    George, Saji; Pokhrel, Suman; Ji, Zhaoxia; Henderson, Bryana L; Xia, Tian; Li, LinJiang; Zink, Jeffrey I; Nel, André E; Mädler, Lutz

    2011-07-27

    UV-light-induced electron-hole (e(-)/h(+)) pair generation with free radical production in TiO(2)-based nanoparticles is a major conceptual paradigm for biological injury. However, to date, this hypothesis has been difficult to experimentally verify due to the high energy of UV light that is intrinsically highly toxic to biological systems. Here, a versatile flame spray pyrolysis (FSP) synthetic process has been exploited to synthesize a library of iron-doped (0-10 wt%) TiO(2) nanoparticles. These particles have been tested for photoactivation-mediated cytotoxicity using near-visible light exposure. The reduction in TiO(2) band gap energy with incremental levels of Fe loading maintained the nanoparticle crystalline structure in spite of homogeneous Fe distribution (demonstrated by XRD, HRTEM, SAED, EFTEM, and EELS). Photochemical studies showed that band gap energy was reciprocally tuned proportional to the Fe content. The photo-oxidation capability of Fe-doped TiO(2) was found to increase during near-visible light exposure. Use of a macrophage cell line to evaluate cytotoxic and ROS production showed increased oxidant injury and cell death in parallel with a decrease in band gap energy. These findings demonstrate the importance of band gap energy in the phototoxic response of the cell to TiO(2) nanoparticles and reflect the potential of this material to generate adverse effects in humans and the environment during high-intensity light exposure.

  20. Development of ultra-narrow gap welding with constrained arc by flux band

    Institute of Scientific and Technical Information of China (English)

    Zhu Liang; Zheng Shaoxian; Chen Jianhong

    2006-01-01

    Narrow gap welding has merits of lower residual stress and distortion, and superior mechanical properties of joints.A major problem of this process is the lack of fusion in sidewalls, hence many methods of weaving arc have been developed to increase heating effect of arc to the sidewalls. In this work, a new approach without weaving arc is attempted to ensure the penetration of sidewall, and ultra-narrow gap welding with the gap of less than 5 mm was executed successfully. In this approach, the width of gap is decreased further, so that the sidewalls are made within range of arc heating to obtain the enough heat. Inorder to prevent the arc from being attracted by sidewall and going up alongthe sidewalls, two pieces of flux bands consisting of the specified aggregates are adhered to the sidewalls to constrain the arc. In addition, when flux band being heated by the arc, slag and gases are formed to shield the arc and the weld pool. This technique was tested on the welding experiment of pipeline steel with thickness of 20 mm. The involved welding parameters were obtained, that is, the width of gap is 4 mm, the welding current 250 A, and the heat input 0. 5 k J/mm, the width of heat-affected zone is 1 - 2mm.

  1. Tuning band gaps of BN nanosheets and nanoribbons via interfacial dihalogen bonding and external electric field.

    Science.gov (United States)

    Tang, Qing; Bao, Jie; Li, Yafei; Zhou, Zhen; Chen, Zhongfang

    2014-08-07

    Density functional theory computations with dispersion corrections (DFT-D) were performed to investigate the dihalogen interactions and their effect on the electronic band structures of halogenated (fluorinated and chlorinated) BN bilayers and aligned halogen-passivated zigzag BN nanoribbons (BNNRs). Our results reveal the presence of considerable homo-halogen (FF and ClCl) interactions in bilayer fluoro (chloro)-BN sheets and the aligned F (Cl)-ZBNNRs, as well as substantial hetero-halogen (FCl) interactions in hybrid fluoro-BN/chloro-BN bilayer and F-Cl-ZBNNRs. The existence of interfacial dihalogen interactions leads to significant band-gap modifications for the studied BN nanosystems. Compared with the individual fluoro (chloro)-BN monolayers or pristine BNNRs, the gap reduction in bilayer fluoro-BN (B-FF-N array), hybrid fluoro-BN/chloro-BN bilayer (N-FCl-N array), aligned Cl-ZBNNRs (B-ClCl-N alignment), and hybrid F-Cl-ZBNNRs (B-FCl-N alignment) is mainly due to interfacial polarizations, while the gap narrowing in bilayer chloro-BN (N-ClCl-N array) is ascribed to the interfacial nearly-free-electron states. Moreover, the binding strengths and electronic properties of the interactive BN nanosheets and nanoribbons can be controlled by applying an external electric field, and extensive modulation from large-gap to medium-gap semiconductors, or even metals can be realized by adjusting the direction and strength of the applied electric field. This interesting strategy for band gap control based on weak interactions offers unique opportunities for developing BN nanoscale electronic devices.

  2. Investigation of optical band gap and device parameters of rubrene thin film prepared using spin coating technique

    Energy Technology Data Exchange (ETDEWEB)

    Tuğluoğlu, Nihat, E-mail: tugluo@gmail.com [Department of Technology, Sarayköy Nuclear Research and Training Center, 06983 Saray, Ankara (Turkey); Barış, Behzad; Gürel, Hatice [Department of Physics, Faculty of Arts and Sciences, Giresun University, Gazipaşa Campus, Giresun 28100 (Turkey); Karadeniz, Serdar [Department of Technology, Sarayköy Nuclear Research and Training Center, 06983 Saray, Ankara (Turkey); Yüksel, Ömer Faruk [Department of Physics, Faculty of Science, Selçuk University, Campus Konya 42075 (Turkey)

    2014-01-05

    Highlights: • Thin film of rubrene has been deposited by spin coating technique. • The band gap properties of the film were investigated in the range 200–700 nm. • The analysis of the absorption coefficient revealed indirect allowed transition. • The parameters such as barrier height and ideality factor were determined. -- Abstract: Rubrene thin film has been deposited by spin coating technique. The optical band gap properties of rubrene thin film have been investigated in the spectral range 200–700 nm. The results of the absorption coefficient (α) were analyzed in order to determine the optical band gap and Urbach energy of the film. The absorption spectra recorded in the UV–vis region shows two peaks at 250 nm and 300 nm. The analysis of the spectral behavior of the absorption coefficient (α) in the absorption region revealed indirect allowed transition with corresponding energy 2.31 eV. The value of Urbach energy (E{sub U}) was determined to be 1.169 eV. The current–voltage (I–V) characteristics and electrical conduction properties of rubrene/n-Si device fabricated by spin coating method have also been investigated. The I–V characteristic in dark was showed the rectification effect due to the formation of Schottky barrier at rubrene/silicon interface. From analyzing the I-V measurement for the device, the basic device parameters such as barrier height, ideality factor and series resistance were determined. At the low-voltage region, the current conduction in Au/rubrene/n-Si device is ohmic type. The charge transport phenomenon appears to be space charge limited current (SCLC) at higher-voltage regions.

  3. Photonic band-gap engineering for volume plasmon polaritons in multiscale multilayer hyperbolic metamaterials

    CERN Document Server

    Zhukovsky, Sergei V; Babicheva, Viktoriia E; Lavrinenko, Andrei V; Sipe, J E

    2013-01-01

    We theoretically study the propagation of large-wavevector waves (volume plasmon polaritons) in multilayer hyperbolic metamaterials with two levels of structuring. We show that when the parameters of a subwavelength metal-dielectric multilayer ("substructure") are modulated ("superstructured") on a larger, wavelength scale, the propagation of volume plasmon polaritons in the resulting multiscale hyperbolic metamaterials is subject to photonic band gap phenomena. A great degree of control over such plasmons can be exerted by varying the superstructure geometry. When this geometry is periodic, stop bands due to Bragg reflection are shown to form within the volume plasmonic band. When a cavity layer is introduced in an otherwise periodic superstructure, resonance peaks of the Fabry-P\\'erot nature are shown to be present within the stop bands. More complicated superstructure geometries are also considered. For example, fractal Cantor-like multiscale metamaterials are found to exhibit characteristic self-similar s...

  4. Low-frequency band gaps in chains with attached non-linear oscillators

    DEFF Research Database (Denmark)

    Lazarov, Boyan Stefanov; Jensen, Jakob Søndergaard

    2007-01-01

    in structures with periodic or random inclusions are located mainly in the high frequency range, as the wavelength has to be comparable with the distance between the alternating parts. Band gaps may also exist in structures with locally attached oscillators. In the linear case the gap is located around......The aim of this article is to investigate the wave propagation in one-dimensional chains with attached non-linear local oscillators by using analytical and numerical models. The focus is on the influence of non-linearities on the filtering properties of the chain in the low frequency range...

  5. Strong interaction of a transmon qubit with 1D band-gap medium

    Science.gov (United States)

    Liu, Yanbing; Sadri, Darius; Houck, Andrew; Bronn, Nicholas; Chow, Jerry; Gambetta, Jay

    2015-03-01

    The spontaneous emission of an atom will be enhanced or suppressed in a structured vacuum, commonly known as Purcell effect. Moreover, in a frequency gap medium, an atom-photon bound state is predicted to exist in the band gap, causing the localization of light. Here using the technology of circuit quantum electrodynamics, we experimentally explore this mechanism by fabricating a microwave step-impedance filter strongly coupled to a transmon qubit. Standard transmission and spectroscopy measurements support the existence of atom-photon bound states in the system. Correlation measurement shows that the atom-photon interaction induces strong correlation of the transmitted light through the system. Thanks support from IARPA

  6. Two-dimensional microwave band-gap structures of different dielectric materials

    Indian Academy of Sciences (India)

    E D V Nagesh; G Santosh Babu; V Subramanian; V Sivasubramanian; V R K Murthy

    2005-12-01

    We report the use of low dielectric constant materials to form two-dimensional microwave band-gap structures for achieving high gap-to-midgap ratio. The variable parameters chosen are the lattice spacing and the geometric structure. The selected geometries are square and triangular and the materials chosen are PTFE ( = 2.1), PVC ( = 2.38) and glass ( = 5.5). Using the plane-wave expansion method, proper lattice spacing is selected for each structure and material. The observed experimental results are analyzed with the help of the theoretical prediction.

  7. Anisotropic evolution of energy gap in Bi2212 superconductor

    Science.gov (United States)

    Durajski, A. P.

    2016-10-01

    We present a systematic analysis of the energy gap in underdoped Bi2212 superconductor as a function of temperature and hole doping level. Within the framework of the theoretical model containing the electron-phonon and electron-electron-phonon pairing mechanism, we reproduced the measurement results of modern ARPES experiments with very high accuracy. We showed that the energy-gap amplitude is very weakly dependent on the temperature but clearly dependent on the level of doping. The evidence for a non-zero energy gap above the critical temperature, referred to as a pseudogap, was also obtained.

  8. Theoretical aspects of photonic band gap in 1D nano structure of LN: MgLN periodic layer

    Energy Technology Data Exchange (ETDEWEB)

    Sisodia, Namita, E-mail: namitasisodiya@gmail.com [Department of Physics, Holkar Science Collage, Indore-452017 (India)

    2015-06-24

    By using the transfer matrix method, we have analyzed the photonic band gap properties in a periodic layer of LN:MgLN medium. The Width of alternate layers of LN and MgLN is in the range of hundred nanometers. The birefringent and ferroelectric properties of the medium (i.e ordinary, extraordinary refractive indices and electric dipole moment) is given due considerations in the formulation of photonic band gap. Effect of electronic transition dipole moment of the medium on photonic band gap is also taken into account. We find that photonic band gap can be modified by the variation in the ratio of the width of two medium. We explain our findings by obtaining numerical values and the effect on the photonic band gap due to variation in the ratio of alternate medium is shown graphically.

  9. Kaolinite: Defect defined material properties – A soft X-ray and first principles study of the band gap

    Energy Technology Data Exchange (ETDEWEB)

    Pietzsch, A., E-mail: annette.pietzsch@helmholtz-berlin.de [Institute for Methods and Instrumentation in Synchrotron Radiation Research G-ISRR, Helmholtz-Zentrum für Materialien und Energie GmbH, Albert-Einstein-Str. 15, 12489 Berlin (Germany); Nisar, J. [Pakistan Atomic Energy Commission (PAEC), P.O. Box 2151, Islamabad (Pakistan); Jämstorp, E. [Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI (Switzerland); Gråsjö, J. [Department of Pharmacy, Uppsala University, Box 580, 75123 Uppsala (Sweden); Århammar, C. [Coromant R& D, S-126 80 Stockholm (Sweden); Ahuja, R.; Rubensson, J.-E. [Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala (Sweden)

    2015-07-15

    Highlights: • The respective electronic structure of synthetic and natural kaolinite is compared. • The size of the band gap and thus many important material properties are defined by defect states in the band gap. • The oxygen-based defect states are identified and analyzed. • The band gap of kaolinite decreases significantly due to the forming of defects. - Abstract: By combining X-ray absorption spectroscopy and first principles calculations we have determined the electronic structure of synthetic and natural kaolinite as a model system for engineered and natural clay materials. We have analyzed defect states in the band gap and find that both natural and synthetic kaolinite contain defects where oxygen replaces hydrogen in one of the Al (0 0 1)-hydroxyl groups of the kaolinite clay sheets. The band gap of both synthetic and natural kaolinite is found to decrease by about 3.2 eV as this defect is formed.

  10. Effect of a Two-Dimensional Periodic Dielectric Background on Complete Photonic Band Gap in Complex Square Lattices

    Institute of Scientific and Technical Information of China (English)

    ZHANG Yan; SHI Jun-Jie

    2008-01-01

    A two-dimensional photonic crystal model with a periodic square dielectric background is proposed.The photonic band modulation effects due to the two-dimensional periodic background are investigated jn detail.It is found that periodic modulation of the dielectric background greatly alters photonic band structures,especially for the Epolarization modes.The number,width and position of the photonic band gaps sensitively depend on the dielectric constants of the two-dimensional periodic background.Complete band gaps are found,and the dependence of the widths of these gaps on the structural and material parameters of the two alternating rods/holes is studied.

  11. Absolute band gaps of a two-dimensional triangular-lattice dielectric photonic crystal with different shapes

    Institute of Scientific and Technical Information of China (English)

    2010-01-01

    Absolute band gaps of a two-dimensional triangular-lattice photonic crystal are calculated with the finite-difference time-domain method in this paper.Through calculating the photonic band structures of the triangular-lattice photonic crystal consisting of Ge rods immersed in air with different shapes,it is found that a large absolute band gap of 0.098 (2c/a) can be obtained for the structures with hollow triangular Ge rods immersed in air,corresponding to 19.8% of the middle frequency.The influence of the different factors on the width of the absolute band gaps is also discussed.

  12. Chiral doublet bands and energy-level crossing

    Institute of Scientific and Technical Information of China (English)

    QI Bin; MENG Jie; ZHANG Shuang-Quan; WANG Shou-Yu; PENG Jing

    2009-01-01

    Different definitions for chiral doublet bands based on excitation energies, B(E2) and B(M1) respectively are discussed in the triaxial particle rotor model. For the ideal chiral geometry, the selection rules of the electromagnetic transitions in different band definitions are illustrated. It is also shown that the energy-level crossings between chiral doublet bands may occur.

  13. Temperature and magnetization-dependent band-gap renormalization and optical many-body effects in diluted magnetic semiconductors

    OpenAIRE

    2005-01-01

    We calculate the Coulomb interaction induced density, temperature and magnetization dependent many-body band-gap renormalization in a typical diluted magnetic semiconductor GaMnAs in the optimally-doped metallic regime as a function of carrier density and temperature. We find a large (about 0.1 eV) band gap renormalization which is enhanced by the ferromagnetic transition. We also calculate the impurity scattering effect on the gap narrowing. We suggest that the temperature, magnetization, an...

  14. Thermal tuning the reversible optical band gap of self-assembled polystyrene photonic crystals

    Science.gov (United States)

    Vakili Tahami, S. H.; Pourmahdian, S.; Shirkavand Hadavand, B.; Azizi, Z. S.; Tehranchi, M. M.

    2016-11-01

    Nano-sized polymeric colloidal particles could undergo self-organization into three-dimensional structures to produce desired optical properties. In this research, a facile emulsifier-free emulsion polymerization method was employed to synthesize highly mono-disperse sub-micron polystyrene colloids. A high quality photonic crystal (PhC) structure was prepared by colloidal polystyrene. The reversible thermal tuning effect on photonic band gap position as well as the attenuation of the band gap was investigated in detail. The position of PBG can be tuned from 420 nm to 400 nm by varying the temperature of the PhC structure, reversibly. This reversible effect provides a reconfigurable PhC structure which could be used as thermo-responsive shape memory polymers.

  15. Magneto-resistive property study of direct and indirect band gap thermoelectric Bi-Sb alloys

    Science.gov (United States)

    Das, Diptasikha; Malik, K.; Bandyopadhyay, S.; Das, D.; Chatterjee, S.; Banerjee, Aritra

    2014-08-01

    We report magneto-resistive properties of direct and indirect band gap Bismuth-Antimony (Bi-Sb) alloys. Band gap increases with magnetic field. Large positive magnetoresistance (MR) approaching to 400% is observed. Low field MR experiences quadratic growth and at high field it follows a nearly linear behavior without sign of saturation. Carrier mobility extracted from low field MR data depicts remarkable high value of around 5 m2V-1s-1. Correlation between MR and mobility is revealed. We demonstrate that the strong nearly linear MR at high field can be well understood by classical method, co-build by Parish and Littlewood, Nature 426, 162 (2003) and Phys. Rev. B 72, 094417 (2005).

  16. Very low band gap thiadiazoloquinoxaline donor-acceptor polymers as multi-tool conjugated polymers.

    Science.gov (United States)

    Steckler, Timothy T; Henriksson, Patrik; Mollinger, Sonya; Lundin, Angelica; Salleo, Alberto; Andersson, Mats R

    2014-01-29

    Here we report on the synthesis of two novel very low band gap (VLG) donor-acceptor polymers (Eg ≤ 1 eV) and an oligomer based on the thiadiazoloquinoxaline acceptor. Both polymers demonstrate decent ambipolar mobilities, with P1 showing the best performance of ∼10(-2) cm(2) V(-1) s(-1) for p- and n-type operation. These polymers are among the lowest band gap polymers (≲0.7 eV) reported, with a neutral λmax = 1476 nm (P2), which is the farthest red-shifted λmax reported to date for a soluble processable polymer. Very little has been done to characterize the electrochromic aspects of VLG polymers; interestingly, these polymers actually show a bleaching of their neutral absorptions in the near-infrared region and have an electrochromic contrast up to 30% at a switching speed of 3 s.

  17. Room Temperature Direct Band Gap Emission from Ge p-i-n Heterojunction Photodiodes

    Directory of Open Access Journals (Sweden)

    E. Kasper

    2012-01-01

    Full Text Available Room temperature direct band gap emission is observed for Si-substrate-based Ge p-i-n heterojunction photodiode structures operated under forward bias. Comparisons of electroluminescence with photoluminescence spectra allow separating emission from intrinsic Ge (0.8 eV and highly doped Ge (0.73 eV. Electroluminescence stems from carrier injection into the intrinsic layer, whereas photoluminescence originates from the highly n-doped top layer because the exciting visible laser wavelength is strongly absorbed in Ge. High doping levels led to an apparent band gap narrowing from carrier-impurity interaction. The emission shifts to higher wavelengths with increasing current level which is explained by device heating. The heterostructure layer sequence and the light emitting device are similar to earlier presented photodetectors. This is an important aspect for monolithic integration of silicon microelectronics and silicon photonics.

  18. Intrinsic magnetism and spontaneous band gap opening in bilayer silicene and germanene.

    Science.gov (United States)

    Wang, Xinquan; Wu, Zhigang

    2017-01-18

    It has been long sought to create magnetism out of simple non-magnetic materials, such as silicon and germanium. Here we show that intrinsic magnetism exists in bilayer silicene and germanene with no need to cut, etch, or dope. Unlike bilayer graphene, strong covalent interlayer bonding formed in bilayer silicene and germanene breaks the original π-bonding network of each layer, leaving the unbonded electrons unpaired and localized to carry magnetic moments. These magnetic moments then couple ferromagnetically within each layer while antiferromagnetically across two layers, giving rise to an infinite magnetic sheet with structural integrity and magnetic homogeneity. Furthermore, this unique magnetic ordering results in fundamental band gaps of 0.55 eV and 0.32 eV for bilayer silicene and germanene, respectively. The integration of intrinsic magnetism and spontaneous band gap opening makes bilayer silicene and germanene attractive for future nanoelectronics as well as spin-based computation and data storage.

  19. LC Filter Design for Wide Band Gap Device Based Adjustable Speed Drives

    DEFF Research Database (Denmark)

    Vadstrup, Casper; Wang, Xiongfei; Blaabjerg, Frede

    2014-01-01

    the LC filter with a higher cut off frequency and without damping resistors. The selection of inductance and capacitance is chosen based on capacitor voltage ripple and current ripple. The filter adds a base load to the inverter, which increases the inverter losses. It is shown how the modulation index......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...... affects the capacitor capacitor and the inverter current....

  20. Crystal thickness and sphere dispersion dependence of the photonic band gap of silica colloidal crystals

    Institute of Scientific and Technical Information of China (English)

    Yongjun He(何拥军); Zhongchao Wei(韦中超); Yongchun Zhong(钟永春); Jianwei Diao(刁建伟); Hezhou Wang(汪河洲)

    2004-01-01

    Experimental results demonstrate that the band gap of colloidal suspension crystal changes with both the thickness of crystal and the dispersity of micro-spheres.As the thickness decreases,a red shift of band gap is observed,and there is a maximum of red shift.The values of the maximum red shifts are dependent on the standard deviations of micro-spheres.The experimental results are consistent with theoretical calculation.As the colloidal suspension crystal is assembled from micro-spheres with a standard deviation of 8.4% in a thick cell,an incident angles independent broadband is observed,which is explained as an amorphous structure.Two amorphous models are discussed.

  1. Fabrication of Ceramic Layer-by-Layer Infrared Wavelength Photonic Band Gap Crystals

    Energy Technology Data Exchange (ETDEWEB)

    Kang, Henry Hao-Chuan [Iowa State Univ., Ames, IA (United States)

    2004-12-19

    Photonic band gap (PBG) crystals, also known as photonic crystals, are periodic dielectric structures which form a photonic band gap that prohibit the propagation of electromagnetic (EM) waves of certain frequencies at any incident angles. Photonic crystals have several potential applications including zero-threshold semiconductor lasers, the inhibition of spontaneous emission, dielectric mirrors, and wavelength filters. If defect states are introduced in the crystals, light can be guided from one location to another or even a sharp bending of light in micron scale can be achieved. This generates the potential for optical waveguide and optical circuits, which will contribute to the improvement in the fiber-optic communications and the development of high-speed computers.

  2. Coupled-mode theory for photonic band-gap inhibition of spatial instabilities.

    Science.gov (United States)

    Gomila, Damià; Oppo, Gian-Luca

    2005-07-01

    We study the inhibition of pattern formation in nonlinear optical systems using intracavity photonic crystals. We consider mean-field models for singly and doubly degenerate optical parametric oscillators. Analytical expressions for the new (higher) modulational thresholds and the size of the "band gap" as a function of the system and photonic crystal parameters are obtained via a coupled-mode theory. Then, by means of a nonlinear analysis, we derive amplitude equations for the unstable modes and find the stationary solutions above threshold. The form of the unstable mode is different in the lower and upper parts of the band gap. In each part there is bistability between two spatially shifted patterns. In large systems stable wall defects between the two solutions are formed and we provide analytical expressions for their shape. The analytical results are favorably compared with results obtained from the full system equations. Inhibition of pattern formation can be used to spatially control signal generation in the transverse plane.

  3. On the wavelength dependence of femtosecond laser interactions inside band gap solids

    Science.gov (United States)

    Leyder, S.; Grojo, D.; Delaporte, Ph.; Lebugle, M.; Marine, W.; Sanner, N.; Sentis, M.; Utéza, O.

    2013-03-01

    3D laser microfabrication inside narrow band gap solids like semiconductors will require the use of long wavelength intense pulses. We perform an experimental study of the multiphoton-avalanche absorption yields and thresholds with tightly focused femtosecond laser beams at wavelengths: 1.3μm and 2.2μm. For comparisons, we perform the experiments in two very different materials: silicon (semiconductor, ˜1.1 eV indirect bandgap) and fused silica (dielectric, ˜9 eV direct bandgap). For both materials, we find only moderate differences while the number of photons required to cross the band gap changes from 2 to 3 in silicon and from 10 to 16 in fused silica.

  4. Optical Band Gap and Thermal Diffusivity of Polypyrrole-Nanoparticles Decorated Reduced Graphene Oxide Nanocomposite Layer

    Directory of Open Access Journals (Sweden)

    Amir Reza Sadrolhosseini

    2016-01-01

    Full Text Available 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 between 3.580 eV and 3.853 eV, and thermal diffusivity was increased with increasing the layer thickness from 2.873 cm2/s to 12.446 cm2/s.

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

    Science.gov (United States)

    Shakya, Jyoti; Mohanty, T.

    2016-05-01

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

  6. On the Integration of Wide Band-gap Semiconductors in Single Phase Boost PFC Converters

    DEFF Research Database (Denmark)

    Hernandez Botella, Juan Carlos

    Power semiconductor technology has dominated the evolution of switched mode power supplies (SMPS). Advances in silicon (Si) technology, as the introduction of metal oxide field effect transistor (MOSFET), isolated gate bipolar transistors (IGBT), superjunction vertical structures and Schottky...... compared to Si semiconductors. Moreover, both semiconductor materials are particularly interesting for high temperature operation. These characteristics makes integration of SiC and GaN devices as the next logical step to further increase efficiency and power density in SMPS. This work is part of the Ph......D project “Single phase PFC converter using wide band-gap devices” and focuses on attainable advantages by introducing wide band-gap semiconductors, and more particularly GaN devices in power factor correction circuits (PFC). First, an overview of current state-of-the-art semiconductor technology in the 600...

  7. Multiband frequency-reconfigurable antenna using metamaterial structure of electromagnetic band gap

    Science.gov (United States)

    Dewan, Raimi; Rahim, M. K. A.; Himdi, Mohamed; Hamid, M. R.; Majid, H. A.; Jalil, M. E.

    2017-01-01

    A metamaterial of electromagnetic band gap (EBG) is incorporated to an antenna for frequency reconfigurability is proposed. The EBG consists of two identical unit cells that provide multiple band gaps at 1.88-1.94, 2.25-2.44, 2.67-2.94, 3.52-3.54, and 5.04-5.70 GHz with different EBG configurations. Subsequently, the antenna is incorporated with EBG. The corresponding incorporated structure successfully achieves various reconfigurable frequencies at 1.60, 1.91, 2.41, 3.26, 2.87, 5.21, and 5.54 GHz. The antenna has the potential to be implemented for Bluetooth, Wi-Fi, WiMAX, LTE, and cognitive radio applications.

  8. Electrostatic tuning of Kondo effect in a rare-earth-doped wide-band-gap oxide

    KAUST Repository

    Li, Yongfeng

    2013-04-29

    As a long-lived theme in solid-state physics, the Kondo effect reflects the many-body physics involving the short-range Coulomb interactions between itinerant electrons and localized spins in metallic materials. Here we show that the Kondo effect is present in ZnO, a prototypical wide-band-gap oxide, doped with a rare-earth element (Gd). The localized 4f electrons of Gd ions do not produce remanent magnetism, but interact strongly with the host electrons, giving rise to a saturating resistance upturn and negative magnetoresistance at low temperatures. Furthermore, the Kondo temperature and resistance can be electrostatically modulated using electric-double-layer gating with liquid ionic electrolyte. Our experiments provide the experimental evidence of tunable Kondo effect in ZnO, underscoring the magnetic interactions between localized and itinerant electrons and the emergent transport behaviors in such doped wide-band-gap oxides.

  9. Probing optical band gaps at nanoscale from tetrahedral cation vacancy defects and variation of cation ordering in NiCo2O4 epitaxial thin films

    Science.gov (United States)

    Dileep, K.; Loukya, B.; Silwal, P.; Gupta, A.; Datta, R.

    2014-10-01

    High resolution electron energy loss spectroscopy (HREELS) is utilized to probe the optical band gaps at the nanoscale in epitaxial NiCo2O4 (NCO) thin films with different structural order (cation/charge). The structure of NCO deviates from the ideal inverse spinel (non-magnetic and insulating) for films grown at higher temperatures (>500 °C) towards a mixed cation structure (magnetic with metallic conductivity) at lower deposition temperatures (<450 °C). This significantly modifies the electronic structure as well as the nature of the band gap of the material. Nanoscale regions with unoccupied tetrahedral A site cations are additionally observed in all the samples and direct measurement from such areas reveals considerably lower band gap values as compared to the ideal inverse spinel and mixed cation configurations. Experimental values of band gaps have been found to be in good agreement with the theoretical mBJLDA exchange potential based calculated band gaps for various cation ordering and consideration of A site cation vacancy defects. The origin of rich variation in cation ordering observed in this system is discussed.

  10. Theoretical study on the two-band degenerate-gaps superconductors: Application to SrPt3P

    Science.gov (United States)

    Huang, Hai; Hou, Li-Chao; Zhao, Bin-Peng

    2016-09-01

    We study the magnetic properties of two-band degenerate-gaps superconductors with two-band isotropic Ginzburg-Landau theory. The exact solutions of upper critical field and London penetration depth are obtained, and the calculations reproduce the experimental data of the recently observed superconducting crystal SrPt3P in a broad temperature range. It directly underlies that SrPt3P is a multi-band superconductor with equal gaps in two Fermi surface sheets.

  11. Correlation between Bonding Geometry and Band Gap States at Organic -- inorganic interfaces: Catechol on Rutile TiO2 (110)

    Science.gov (United States)

    Diebold, Ulrike; Li, Shao-Chun; Wang, Jian-Guo; Jacobson, Peter; Gong, Xue-Qing; Selloni, Annabella

    2009-03-01

    Adsorbate-induced band gap states in semiconductors are of particular interest due to the potential of increased light absorption and photoreactivity. A combined theoretical (DFT) and experimental (STM, photoemission) study of the molecular-scale factors involved in the formation of gap states in TiO2 is presented. Using the organic catechol on rutile TiO2(110) as a model system it is found that the bonding geometry strongly affects the molecular electronic structure. At saturation catechol forms an ordered 4 x 1 overlayer. This structure is attributed to catechol adsorbed on rows of surface Ti atoms with the molecular plane tilted from the surface normal by about ±27 in an alternating fashion. In the lowest-energy structure one of the two terminal OH groups at each catechol dissociates and the O binds to a surface Ti atom in a monodentate configuration, while the other OH group forms a H-bond to the next catechol neighbor. Through proton exchange with the surface this structure transforms into one where both OH groups dissociate and the catechol is bound to two surface Ti in a bidentate configuration. Only bidendate catechol introduces states in the band gap of TiO2.

  12. Sub-band gap photo-enhanced secondary electron emission from high-purity single-crystal chemical-vapor-deposited diamond

    Science.gov (United States)

    Yater, J. E.; Shaw, J. L.; Pate, B. B.; Feygelson, T. I.

    2016-02-01

    Secondary-electron-emission (SEE) current measured from high-purity, single-crystal (100) chemical-vapor-deposited diamond is found to increase when sub-band gap (3.06 eV) photons are incident on the hydrogenated surface. Although the light does not produce photoemission directly, the SEE current increases by more than a factor of 2 before saturating with increasing laser power. In energy distribution curves (EDCs), the emission peak shows a corresponding increase in intensity with increasing laser power. However, the emission-onset energy in the EDCs remains constant, indicating that the bands are pinned at the surface. On the other hand, changes are observed on the high-energy side of the distribution as the laser power increases, with a well-defined shoulder becoming more pronounced. From an analysis of this feature in the EDCs, it is deduced that upward band bending is present in the near-surface region during the SEE measurements and this band bending suppresses the SEE yield. However, sub-band gap photon illumination reduces the band bending and thereby increases the SEE current. Because the bands are pinned at the surface, we conclude that the changes in the band levels occur below the surface in the electron transport region. Sample heating produces similar effects as observed with sub-band gap photon illumination, namely, an increase in SEE current and a reduction in band bending. However, the upward band bending is not fully removed by either increasing laser power or temperature, and a minimum band bending of ˜0.8 eV is established in both cases. The sub-band gap photo-excitation mechanism is under further investigation, although it appears likely at present that defect or gap states play a role in the photo-enhanced SEE process. In the meantime, the study demonstrates the ability of visible light to modify the electronic properties of diamond and enhance the emission capabilities, which may have potential impact for diamond-based vacuum electron

  13. Effective absorption coefficient for graded band-gap semiconductors and the expected photocurrent density in solar cells

    Energy Technology Data Exchange (ETDEWEB)

    Morales-Acevedo, Arturo [CINVESTAV del IPN, Electrical Engineering Department, Avenida IPN No. 2508, 07360 Mexico, D. F. (Mexico)

    2009-01-15

    A simple model for the generation of carriers by photons incident on a (linearly) decreasing band-gap material, such as has been described in recent CIGS solar cells, is developed. The model can be generalized for different cases such as increasing band-gap grading or for having a more complex band-gap profile. The model developed for direct band semiconductors such as CIGS or AlGaAs allows us to define an effective absorption coefficient, so that the ideal photocurrent density can be calculated in a similar manner as for solar cells with non-graded band-gap materials. We show that this model gives completely different results as those expected from intuitive approaches for calculating this ideal photocurrent density. We also show that grading of the band-gap of the absorbing material in solar cells makes the photocurrent less sensitive to the total band-gap change, in such a way that the design of the band-gap variation can be more flexible in order to have other advantages such as higher built-in voltage or higher back surface field in the device structure. (author)

  14. Short pulse equations and localized structures in frequency band gaps of nonlinear metamaterials

    Energy Technology Data Exchange (ETDEWEB)

    Tsitsas, N.L. [School of Applied Mathematical and Physical Sciences, National Technical University of Athens, Zografos, Athens 15773 (Greece); Horikis, T.P. [Department of Mathematics, University of Ioannina, Ioannina 45110 (Greece); Shen, Y.; Kevrekidis, P.G.; Whitaker, N. [Department of Mathematics and Statistics, University of Massachusetts, Amherst, MA 01003-4515 (United States); Frantzeskakis, D.J., E-mail: dfrantz@phys.uoa.g [Department of Physics, University of Athens, Panepistimiopolis, Zografos, Athens 157 84 (Greece)

    2010-03-01

    We consider short pulse propagation in nonlinear metamaterials characterized by a weak Kerr-type nonlinearity in their dielectric response. Two short-pulse equations (SPEs) are derived for the high- and low-frequency 'band gaps' (where linear electromagnetic waves are evanescent) with linear effective permittivity epsilon<0 and permeability mu>0. The structure of the solutions of the SPEs is also briefly discussed, and connections with the soliton solutions of the nonlinear Schroedinger equation are made.

  15. Band gap tunning in BN-doped graphene systems with high carrier mobility

    KAUST Repository

    Kaloni, T. P.

    2014-02-17

    Using density functional theory, we present a comparative study of the electronic properties of BN-doped graphene monolayer, bilayer, trilayer, and multilayer systems. In addition, we address a superlattice of pristine and BN-doped graphene. Five doping levels between 12.5% and 75% are considered, for which we obtain band gaps from 0.02 eV to 2.43 eV. We demonstrate a low effective mass of the charge carriers.

  16. Optical band gap and magnetic properties of unstrained EuTiO3 films

    Science.gov (United States)

    Lee, J. H.; Ke, X.; Podraza, N. J.; Kourkoutis, L. Fitting; Heeg, T.; Roeckerath, M.; Freeland, J. W.; Fennie, C. J.; Schubert, J.; Muller, D. A.; Schiffer, P.; Schlom, D. G.

    2009-05-01

    Phase-pure, stoichiometric, unstrained, epitaxial (001)-oriented EuTiO3 thin films have been grown on (001) SrTiO3 substrates by reactive molecular-beam epitaxy. Magnetization measurements show antiferromagnetic behavior with TN=5.5 K, similar to bulk EuTiO3. Spectroscopic ellipsometry measurements reveal that EuTiO3 films have a direct optical band gap of 0.93±0.07 eV.

  17. Quantum information processing in localized modes of light within a photonic band-gap material

    CERN Document Server

    Vats, N; John, S; Vats, Nipun; Rudolph, Terry; John, Sajeev

    1999-01-01

    The single photon occupation of a localized field mode within an engineered network of defects in a photonic band-gap (PBG) material is proposed as a unit of quantum information (qubit). Qubit operations are mediated by optically-excited atoms interacting with these localized states of light as the atoms traverse the connected void network of the PBG structure. We describe conditions under which this system can have independent qubits with controllable interactions and very low decoherence, as required for quantum computation.

  18. Designer disordered complex media : hyperuniform photonic and phononic band gap materials.

    OpenAIRE

    Amoah, T. K.

    2016-01-01

    In this thesis we investigate designer disordered complex media for photonics and phononics applications. Initially we focus on the photonic properties and we analyse hyperuniform disordered structures (HUDS) using numerical simulations. Photonic HUDS are a new class of photonic solids, which display large, isotropic photonic band gaps (PBG) comparable in size to the ones found in photonic crystals (PC). We review their complex interference properties, including the origin of PBGs and potent...

  19. Dispersion characteristics of a slow wave structure with a modified photonic band gap

    Institute of Scientific and Technical Information of China (English)

    Gao Xi; Yang Zi-Qiang; Cao Wei-Ping; Jiang Yan-Nan

    2011-01-01

    This paper studies the dispersion characteristics of a modified photonic band-gap slow-wave structure with an open boundary by simulation and experiment. A mode launcher with a wheel radiator and a coupling probe is presented to excite a pure TM01-like mode. The cold test and simulation results show that the TM01-like mode is effectively excited and no parasitic modes appear. The dispersion characteristics obtained from the cold test are in good agreement with the calculated results.

  20. Room Temperature Direct Band Gap Emission from Ge p-i-n Heterojunction Photodiodes

    OpenAIRE

    2012-01-01

    Room temperature direct band gap emission is observed for Si-substrate-based Ge p-i-n heterojunction photodiode structures operated under forward bias. Comparisons of electroluminescence with photoluminescence spectra allow separating emission from intrinsic Ge (0.8 eV) and highly doped Ge (0.73 eV). Electroluminescence stems from carrier injection into the intrinsic layer, whereas photoluminescence originates from the highly n-doped top layer because the exciting visible laser wavelength is ...

  1. Engineering of the band gap and optical properties of thin films of yttrium hydride

    Energy Technology Data Exchange (ETDEWEB)

    You, Chang Chuan; Mongstad, Trygve; Maehlen, Jan Petter; Karazhanov, Smagul, E-mail: smagulk@ife.no [Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller (Norway)

    2014-07-21

    Thin films of oxygen-containing yttrium hydride show photochromic effect at room temperature. In this work, we have studied structural and optical properties of the films deposited at different deposition pressures, discovering the possibility of engineering the optical band gap by variation of the oxygen content. In sum, the transparency of the films and the wavelength range of photons triggering the photochromic effect can be controlled by variation of the deposition pressure.

  2. Simulation Analysis of a Strip Dipole Excited Electromagnetic Band-Gap (EBG) Structure

    Science.gov (United States)

    2015-07-01

    that the phase of the scattered near fields at the EBG surface is more applicable to characterizing the EBG for antenna applications. A new set of...number of unit cells is also demonstrated. 15. SUBJECT TERMS electromagnetic band gap, strip dipole, reflection phase, vias, near fields, bandwidth...Image Theory Approximation 10 2.4 The Periodic Boundary Condition (PBC) Approach 11 2.5 The Phase of the Near Electric Field (NEF) 12 3. Analysis of an

  3. Band-gaps electromagnéticos con celdas unidad de tres dieléctricos

    OpenAIRE

    Gómez Gómez, Álvaro; Herrán Planchuelo, Jaime; Cordobés Gallo, David; Vegas García, Ángel; Saiz Ipiña, Juan Antonio; Solano Vérez, Miguel Ángel; Lakhtakia, Akhlesh

    2004-01-01

    Electromagnetic band-gap structures (EBG) with Kronig-Penney morphology implemented inside rectangular waveguides are theoretically and experimentally examined using a unit cell with three dielectrics. Filtering properties of these structures are analysed, invoking the Floquet theorem for ideal structures (infinite in the propagation direction), and by means of the scattering matrix technique for real structures (i.e., of finite length). Measurements of the transmis...

  4. Thiophene-fused tetracene diimide with low band gap and ambipolar behavior

    KAUST Repository

    Ye, Qun

    2011-11-18

    The first tetracene diimide derivative fused with four thiophene rings, TT-TDI, was synthesized by an FeCl3 mediated oxidative cyclodehydrogenation reaction. TT-TDI exhibited a low band gap of 1.52 eV and amphoteric redox behavior. TT-TDI also showed a liquid crystalline property and ambipolar charge transport in thin film field-effect transistors. © 2011 American Chemical Society.

  5. Halogenated MOF-5 variants show new configuration, tunable band gaps and enhanced optical response in the visible and near infrared.

    Science.gov (United States)

    Yang, Li-Ming; Fang, Guo-Yong; Ma, Jing; Pushpa, Raghani; Ganz, Eric

    2016-11-30

    Inspired by recent experimental fabrication of mono-halogenated versions of Metal-Organic Framework MOF-5 (i.e., X-MOF-5, X = F to I) and some experimentally known fully halogenated MOF compounds, we systematically studied frameworks incorporating full halogenation of the BDC linkers of the prototypical Iso-Reticular Metal-Organic Framework (IRMOF) series, exemplified by MOF-5. Using quantum chemistry calculations, we find that halogenation leads to a 90° rotation of the aryl group, which is mainly ascribed to overcrowding between halogen atoms and the carboxyl and benzene ring and strong repulsion among in-plane atoms/groups. The 90° configuration decreases the repulsion, and maximizes the stabilization energy, and is therefore more stable than 0° configuration. We find that the band gap can be tuned from 4.1 to 1.5 eV as we go from F, Cl, Br, to I. This extends the optical response of these experimentally accessible materials through the visible and infrared region. We have also considered a broader range of new materials that substitute various metals for Zn. Totally, 70 materials were systematically examined computationally including (M4O)(BDC-Z4)3 (M = Zn, Cd, Be, Mg, Ca, Sr, Ba; Z = H, F, Cl, Br, I). For the full range of materials, we calculate band gaps of 4.2 to 1.0 eV, corresponding to a threshold of absorption of 290-1240 nm. Four selected materials were tested for stability using short 5 ps molecular dynamics simulations up to 600 K. The new materials with the smallest band gaps could potentially be used in near-infrared (NIR) light-emitting devices. Other properties, e.g., bulk moduli, formation energy, chemical bonding, and optical properties, were also investigated. The present results may provide new materials for use as novel photocatalysts, photoactive materials for photovoltaic cells, or functional devices in nanoelectronics and optoelectronics.

  6. Nonlinear sub-cyclotron resonance as a formation mechanism for gaps in banded chorus

    CERN Document Server

    Fu, Xiangrong; Dong, Chuanfei; Gary, S Peter

    2015-01-01

    An interesting characteristic of magnetospheric chorus is the presence of a frequency gap at $\\omega \\simeq 0.5\\Omega_e$, where $\\Omega_e$ is the electron cyclotron angular frequency. Recent chorus observations sometimes show additional gaps near $0.3\\Omega_e$ and $0.6\\Omega_e$. Here we present a novel nonlinear mechanism for the formation of these gaps using Hamiltonian theory and test-particle simulations in a homogeneous, magnetized, collisionless plasma. We find that an oblique whistler wave with frequency at a fraction of the electron cyclotron frequency can resonate with electrons, leading to effective energy exchange between the wave and particles.

  7. Microwave band gap and cavity mode in spoof-insulator-spoof waveguide with multiscale structured surface

    CERN Document Server

    Zhang, Qiang; Han, Dezhuan; Qin, Fei Fei; Zhang, Xiao Ming; Yao, Yong

    2015-01-01

    We propose a multiscale spoof-insulator-spoof (SIS) waveguide by introducing periodic geometry modulation in the wavelength scale to a SIS waveguide made of perfect electric conductor. The MSIS consists of multiple SIS subcells. The dispersion relationship of the fundamental guided mode of the spoof surface plasmon polaritons (SSPPs) is studied analytically within the small gap approximation. It is shown that the multiscale SIS possesses microwave band gap (MBG) due to the Bragg scattering. The "gap maps" in the design parameter space are provided. We demonstrate that the geometry of the subcells can efficiently adjust the effective refraction index of the elementary SIS and therefore further control the width and the position of the MBG. The results are in good agreement with numerical calculations by the finite element method (FEM). For finite-sized MSIS of given geometry in the millimeter scale, FEM calculations show that the first-order symmetric SSPP mode has zero transmission in the MBG within frequency...

  8. Tunable band gaps in acoustic metamaterials with periodic arrays of resonant shunted piezos

    Science.gov (United States)

    Chen, Sheng-Bing; Wen, Ji-Hong; Wang, Gang; Wen, Xi-Sen

    2013-07-01

    Periodic arrays of resonant shunted piezoelectric patches are employed to control the wave propagation in a two-dimensional (2D) acoustic metamaterial. The performance is characterized by the finite element method. More importantly, we propose an approach to solving the conventional issue of the nonlinear eigenvalue problem, and give a convenient solution to the dispersion properties of 2D metamaterials with periodic arrays of resonant shunts in this article. Based on this modeling method, the dispersion relations of a 2D metamaterial with periodic arrays of resonant shunted piezos are calculated. The results show that the internal resonances of the shunting system split the dispersion curves, thereby forming a locally resonant band gap. However, unlike the conventional locally resonant gap, the vibrations in this locally resonant gap are unable to be completely localized in oscillators consisting of shunting inductors and piezo-patches.

  9. Effect of band gap narrowing on GaAs tunnel diode I-V characteristics

    Science.gov (United States)

    Lebib, A.; Hannanchi, R.; Beji, L.; EL Jani, B.

    2016-12-01

    We report on experimental and theoretical study of current-voltage characteristics of C/Si-doped GaAs tunnel diode. For the investigation of the experimental data, we take into account the band-gap narrowing (BGN) effect due to heavily-doped sides of the tunnel diode. The BGN of the n- and p-sides of tunnel diode was measured by photoluminescence spectroscopy. The comparison between theoretical results and experimental data reveals that BGN effect enhances tunneling currents and hence should be considered to identify more accurately the different transport mechanisms in the junction. For C/Si-doped GaAs tunnel diode, we found that direct tunneling is the dominant transport mechanism at low voltages. At higher voltages, this mechanism is replaced by the rate-controlling tunneling via gap states in the forbidden gap.

  10. Tunable band gaps in acoustic metamaterials with periodic arrays of resonant shunted piezos

    Institute of Scientific and Technical Information of China (English)

    Chen Sheng-Bing; Wen Ji-Hong; Wang Gang; Wen Xi-Sen

    2013-01-01

    Periodic arrays of resonant shunted piezoelectric patches are employed to control the wave propagation in a twodimensional (2D) acoustic metamaterial.The performance is characterized by the finite element method.More importantly,we propose an approach to solving the conventional issue of the nonlinear eigenvalue problem,and give a convenient solution to the dispersion properties of 2D metamaterials with periodic arrays of resonant shunts in this article.Based on this modeling method,the dispersion relations of a 2D metamaterial with periodic arrays of resonant shunted piezos are calculated.The results show that the internal resonances of the shunting system split the dispersion curves,thereby forming a locally resonant band gap.However,unlike the conventional locally resonant gap,the vibrations in this locally resonant gap are unable to be completely localized in oscillators consisting of shunting inductors and piezo-patches.

  11. Quantum confinement induced band gaps in MgB2 nanosheets

    Science.gov (United States)

    Xu, Bo Z.; Beckman, Scott P.

    2016-09-01

    The discovery of two-dimensional semiconducting materials, a decade ago, spawned an entire sub-field within solid-state physics that is focused on the development of nanoelectronics. Here we present a new class of semiconducting two-dimensional material based on hexagonal MgB2. Although MgB2 is a semimetal, similar to the other well-studied transition metal diborides, we demonstrate that, unlike the transition metal diborides, thinning MgB2, to create nanosheets, opens a band gap in the density of states. We predict that a 7 Å thick MgB2 nanosheet will have a band gap of 0.51 eV. MgB2 nanosheets differ from other two-dimensional semiconductors in that the band gap is introduced by (001) surfaces and is opened by the quantum confinement effect. The implications of these findings are that nanostructured MgB2 is not merely a new composition, but also has intrinsic mechanisms for tuning its electronic properties, which may facilitate the development of nanoelectronics.

  12. Band gap engineered nano perforated graphene microstructures for field effect transistor

    Science.gov (United States)

    Palla, Penchalaiah; Tiwari, Durgesh Laxman; Ansari, Hasan Raza; Babu, Taraprasanna Saha; Ethiraj, Anita Sagadevan; Raina, J. P.

    2016-05-01

    To make use of exceptional properties of graphene in Field effect Transistor (FETs) for switching devices a band gap must be introduced in order to switch -off the device. Through periodic nano perforations a semi-metallic graphene is converted into semiconducting graphene. To understand the device physics behind the reported experiments theoretical simulations has been carried out. The present paper illustrates nano perforated semiconducting graphene Field effect Transistor (FETs) with micron scale dimensions. The simulation has been performed using drift-diffusion semi-classical and tight-binding based non-equilibrium green's function (NEGF) methods. The obtained simulation results are compared with previously reported experimental work. The device dimensions considered for simulations and the experiment are similar with neck width, hole periodicity and channel length of 6.3 nm, 16.3 nm and 1 µm respectively. The interesting and new finding in this work is the p-type I-V characteristics for small band gap devices and n-type behavior for large band gap devices.

  13. Analogy of transistor function with modulating photonic band gap in electromagnetically induced grating.

    Science.gov (United States)

    Wang, Zhiguo; Ullah, Zakir; Gao, Mengqin; Zhang, Dan; Zhang, Yiqi; Gao, Hong; Zhang, Yanpeng

    2015-09-09

    Optical transistor is a device used to amplify and switch optical signals. Many researchers focus on replacing current computer components with optical equivalents, resulting in an optical digital computer system processing binary data. Electronic transistor is the fundamental building block of modern electronic devices. To replace electronic components with optical ones, an equivalent optical transistor is required. Here we compare the behavior of an optical transistor with the reflection from a photonic band gap structure in an electromagnetically induced transparency medium. A control signal is used to modulate the photonic band gap structure. Power variation of the control signal is used to provide an analogy between the reflection behavior caused by modulating the photonic band gap structure and the shifting of Q-point (Operation point) as well as amplification function of optical transistor. By means of the control signal, the switching function of optical transistor has also been realized. Such experimental schemes could have potential applications in making optical diode and optical transistor used in quantum information processing.

  14. Crystal structure, conformation, vibration and optical band gap analysis of bis[ rac-propranolol nitrate

    Science.gov (United States)

    Franklin, S.; Balasubramanian, T.; Nehru, K.; Kim, Youngmee

    2009-06-01

    The crystal structure of the title rac-propranolol salt, CHNO2+·NO3-, consists of two protonated propranolol residues and nitrate anions. Three virtually flat fragments, characteristics of most of the β-adrenolytics with oxy-methylene bridge are present in both the cations (A and B). The plane of the propranolol chain is twisted with respect to the plane of the aromatic ring in both the cations. Present study investigates the conformation and hydrogen bonding interactions, which play an important role in biological functions. A gauche conformation is observed for the oxo-methylene bridge of cation A, while a trans conformation prevails in cation B. These conformations are found in majority of β-blockers. Presence of twenty intermolecular hydrogen bonds mediating through the anions stabilizes the crystal packing. Vibration analysis and earlier theoretical predictions complement the structure analysed. From the UV-Vis spectral analysis for the crystal, the optical band gap is found to be Eg = 5.12 eV, where as the chloride salt has Eg = 3.81 eV. The increase in the band gap may be attributed by the increase in the number of intermolecular hydrogen bonds. Good optical transmittance in the entire visible region and the direct band gap property suggest that it is a suitable candidate for optical applications in UV region.

  15. Analysis of band gap of non-bravais lattice photonic crystal fiber

    Institute of Scientific and Technical Information of China (English)

    Yichao MA; Heming CHEN

    2009-01-01

    This article designs a novel type ofnon-bravais lattice photonic crystal fiber. To form the nesting complexperiod with positive and negative refractive index materials respectively, a cylinder with the same radius and negative refractive index is introduced into the center of each lattice unit cell in the traditional square lattice air-holes photonic crystal fiber. The photonic band-gap of the photonic crystal fiber is calculated numerically by the plane wave expansion method. The result shows that compared with the traditional square photonic band-gap fiber (PBGF),when R/A is 0.35, the refractive index of the substrate, airhole, and medium-column are 1.30, 1.0, and -1.0,respectively. This new PBGF can transmit signal by the photonic band-gap effect. When the lattice constant Λvaries from 1.5 μm to 3.0 μm, the range of the wavelength ranges from 880 nm to 2300 nm.

  16. Mechanical Properties of a Library of Low-Band-Gap Polymers

    DEFF Research Database (Denmark)

    Roth, Bérenger; Savagatrup, Suchol; de los Santos, Nathaniel V.

    2016-01-01

    The mechanical properties of low-band-gap polymers are important for the long-term survivability of roll to-roll processed organic electronic devices. Such devices, e.g., solar cells, displays, and thin-film transistors, must survive the rigors of roll-to-roll coating and also thermal and mechani......The mechanical properties of low-band-gap polymers are important for the long-term survivability of roll to-roll processed organic electronic devices. Such devices, e.g., solar cells, displays, and thin-film transistors, must survive the rigors of roll-to-roll coating and also thermal...... and mechanical forces in the outdoor environment and in stretchable and ultraflexible form factors. This paper measures the stiffness (tensile modulus), ductility (crack-onset strain), or both of a combinatorial library of 51 low-band-gap polymers. The purpose of this study is to systematically screen a library...... of an isolated molecule only partially determines the mechanical properties another important codeterminant is the packing structure some general trends can be identified. (1) Fused rings tend to increase the modulus and decrease the ductility. (2) Branched side chains have the opposite effect. Despite...

  17. Photonic band-gap properties for two-component slow light

    CERN Document Server

    Ruseckas, J; Juzeliunas, G; Unanyan, R G; Otterbach, J; Fleischhauer, M

    2011-01-01

    We consider two-component "spinor" slow light in an ensemble of atoms coherently driven by two pairs of counterpropagating control laser fields in a double tripod-type linkage scheme. We derive an equation of motion for the spinor slow light (SSL) representing an effective Dirac equation for a massive particle with the mass determined by the two-photon detuning. By changing the detuning the atomic medium acts as a photonic crystal with a controllable band gap. If the frequency of the incident probe light lies within the band gap, the light tunnels through the sample. For frequencies outside the band gap, the transmission probability oscillates with increasing length of the sample. In both cases the reflection takes place into the complementary mode of the probe field. We investigate the influence of the finite excited state lifetime on the transmission and reflection coefficients of the probe light. We discuss possible experimental implementations of the SSL using alkali atoms such as Rubidium or Sodium.

  18. Band-gap tunable dielectric elastomer filter for low frequency noise

    Science.gov (United States)

    Jia, Kun; Wang, Mian; Lu, Tongqing; Zhang, Jinhua; Wang, Tiejun

    2016-05-01

    In the last decades, diverse materials and technologies for sound insulation have been widely applied in engineering. However, suppressing the noise radiation at low frequency still remains a challenge. In this work, a novel membrane-type smart filter, consisting of a pre-stretched dielectric elastomer membrane with two compliant electrodes coated on the both sides, is presented to control the low frequency noise. Since the stiffness of membrane dominates its acoustic properties, sound transmission band-gap of the membrane filter can be tuned by adjusting the voltage applied to the membrane. The impedance tube experiments have been carried out to measure the sound transmission loss (STL) of the filters with different electrodes, membrane thickness and pre-stretch conditions. The experimental results show that the center frequency of sound transmission band-gap mainly depends on the stress in the dielectric elastomer, and a large band-gap shift (more than 60 Hz) can be achieved by tuning the voltage applied to the 85 mm diameter VHB4910 specimen with pre-stretch {λ }0=3. Based on the experimental results and the assumption that applied electric field is independent of the membrane behavior, 3D finite element analysis has also been conducted to calculate the membrane stress variation. The sound filter proposed herein may provide a promising facility to control low frequency noise source with tonal characteristics.

  19. Numerical modelling of Mars supersonic disk-gap-band parachute inflation

    Science.gov (United States)

    Gao, Xinglong; Zhang, Qingbin; Tang, Qiangang

    2016-06-01

    The transient dynamic behaviour of supersonic disk-gap-band parachutes in a Mars entry environment involving fluid structure interactions is studied. Based on the multi-material Arbitrary Lagrange-Euler method, the coupling dynamic model between a viscous compressible fluid and a flexible large deformation structure of the parachute is solved. The inflation performance of a parachute with a fixed forebody under different flow conditions is analysed. The decelerating parameters of the parachute, including drag area, opening loads, and coefficients, are obtained from the supersonic wind tunnel test data from NASA. Meanwhile, the evolution of the three-dimensional shape of the disk-gap-band parachute during supersonic inflation is presented, and the structural dynamic behaviour of the parachute is predicted. Then, the influence of the presence of the capsule on the flow field of the parachute is investigated, and the wake of unsteady fluid and the distribution of shock wave around the supersonic parachute are presented. Finally, the structural dynamic response of the canopy fabric under high-pressure conditions is comparatively analysed. The results show that the disk-gap-band parachute is well inflated without serious collapse. As the Mach numbers increase from 2.0 to 2.5, the drag coefficients gradually decrease, along with a small decrease in inflation time, which corresponds with test results, and proves the validity of the method proposed in this paper.

  20. Modulation of the Band Gaps of Phononic Crystals with Thermal Effects

    Science.gov (United States)

    Aly, Arafa H.; Mehaney, Ahmed

    2015-11-01

    Band gaps of elastic waves, both in-plane and shear waves, propagating through one-dimensional perfect/defect phononic crystals (PnCs) that involve thermal effects are studied in this paper. Based on the transfer matrix method and Bloch theory, the expressions of the reflection coefficients and dispersion relation are presented. Elastic waves localization is obtained by immersing a defect layer through a perfect structure. Compared with the periodic structure, we observed that defected PnCs introduced localized modes or peaks within the phononic band gaps. Hence, Numerical simulations are performed to investigate the influences of the defect layer thickness and type on the number and intensity of the localized modes. Moreover, we have observed that temperature changes have prominent effects on the localized modes and band gaps width, especially at plane wave propagation. Such effects could change thermal properties of the PnCs structure such as thermal conductivity and could control the thermal emission contributed by phonons in many engineering structures.