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Sample records for band gap micro-cavities

  1. Photonic band gap materials

    SciTech Connect

    Soukoulis, C.M. |

    1993-12-31

    An overview of the theoretical and experimental efforts in obtaining a photonic band gap, a frequency band in three-dimensional dielectric structures in which electromagnetic waves are forbidden, is presented.

  2. Investigation on spectral response of micro-cavity structure by symmetrical tapered fiber tips

    NASA Astrophysics Data System (ADS)

    Liu, Yan; Li, Yang; Yan, Xiaojun; Li, Weidong

    2016-06-01

    We proposed and experimentally demonstrated a micro-cavity structure made of symmetrical tapered fiber tips. The waist of a conventional fiber taper fabricated from heating and stretching technique is symmetrically cleaved, and the aligned fiber tips with air gap constitute a Fabry-Perot micro-cavity due to the reflection at the tip facet. The spectral responses of such micro-cavity structure have been investigated both in beam propagation models and experiments. The multibeam interference in the micro-cavity and the impact of the waist diameter and cavity length on the spectral response has been successfully demonstrated. And a micro-cavity structure with 45 μm waist diameter was experimentally achieved, the measured spectra agree well with the simulation ones, indicating that the spectral response of the micro-cavity structure is contributed by both the multibeam interference and the Fabry-Perot micro-cavity.

  3. Photonic band gap structure simulator

    DOEpatents

    Chen, Chiping; Shapiro, Michael A.; Smirnova, Evgenya I.; Temkin, Richard J.; Sirigiri, Jagadishwar R.

    2006-10-03

    A system and method for designing photonic band gap structures. The system and method provide a user with the capability to produce a model of a two-dimensional array of conductors corresponding to a unit cell. The model involves a linear equation. Boundary conditions representative of conditions at the boundary of the unit cell are applied to a solution of the Helmholtz equation defined for the unit cell. The linear equation can be approximated by a Hermitian matrix. An eigenvalue of the Helmholtz equation is calculated. One computation approach involves calculating finite differences. The model can include a symmetry element, such as a center of inversion, a rotation axis, and a mirror plane. A graphical user interface is provided for the user's convenience. A display is provided to display to a user the calculated eigenvalue, corresponding to a photonic energy level in the Brilloin zone of the unit cell.

  4. Direct Band Gap Wurtzite Gallium Phosphide Nanowires

    PubMed Central

    2013-01-01

    The main challenge for light-emitting diodes is to increase the efficiency in the green part of the spectrum. Gallium phosphide (GaP) with the normal cubic crystal structure has an indirect band gap, which severely limits the green emission efficiency. Band structure calculations have predicted a direct band gap for wurtzite GaP. Here, we report the fabrication of GaP nanowires with pure hexagonal crystal structure and demonstrate the direct nature of the band gap. We observe strong photoluminescence at a wavelength of 594 nm with short lifetime, typical for a direct band gap. Furthermore, by incorporation of aluminum or arsenic in the GaP nanowires, the emitted wavelength is tuned across an important range of the visible light spectrum (555–690 nm). This approach of crystal structure engineering enables new pathways to tailor materials properties enhancing the functionality. PMID:23464761

  5. Array servo scanning micro EDM of 3D micro cavities

    NASA Astrophysics Data System (ADS)

    Tong, Hao; Li, Yong; Yi, Futing

    2010-12-01

    Micro electro discharge machining (Micro EDM) is a non-traditional processing technology with the special advantages of low set-up cost and few cutting force in machining any conductive materials regardless of their hardness. As well known, die-sinking EDM is unsuitable for machining the complex 3D micro cavity less than 1mm due to the high-priced fabrication of 3D microelectrode itself and its serous wear during EDM process. In our former study, a servo scanning 3D micro-EDM (3D SSMEDM) method was put forward, and our experiments showed it was available to fabricate complex 3D micro-cavities. In this study, in order to improve machining efficiency and consistency accuracy for array 3D micro-cavities, an array-servo-scanning 3D micro EDM (3D ASSMEDM) method is presented considering the complementary advantages of the 3D SSMEDM and the array micro electrodes with simple cross-section. During 3D ASSMEDM process, the array cavities designed by CAD / CAM system can be batch-manufactured by servo scanning layer by layer using array-rod-like micro tool electrodes, and the axial wear of the array electrodes is compensated in real time by keeping discharge gap. To verify the effectiveness of the 3D ASSMEDM, the array-triangle-micro cavities (side length 630 μm) are batch-manufactured on P-doped silicon by applying the array-micro-electrodes with square-cross-section fabricated by LIGA process. Our exploratory experiment shows that the 3D ASSMEDM provides a feasible approach for the batch-manufacture of 3D array-micro-cavities of conductive materials.

  6. Array servo scanning micro EDM of 3D micro cavities

    NASA Astrophysics Data System (ADS)

    Tong, Hao; Li, Yong; Yi, Futing

    2011-05-01

    Micro electro discharge machining (Micro EDM) is a non-traditional processing technology with the special advantages of low set-up cost and few cutting force in machining any conductive materials regardless of their hardness. As well known, die-sinking EDM is unsuitable for machining the complex 3D micro cavity less than 1mm due to the high-priced fabrication of 3D microelectrode itself and its serous wear during EDM process. In our former study, a servo scanning 3D micro-EDM (3D SSMEDM) method was put forward, and our experiments showed it was available to fabricate complex 3D micro-cavities. In this study, in order to improve machining efficiency and consistency accuracy for array 3D micro-cavities, an array-servo-scanning 3D micro EDM (3D ASSMEDM) method is presented considering the complementary advantages of the 3D SSMEDM and the array micro electrodes with simple cross-section. During 3D ASSMEDM process, the array cavities designed by CAD / CAM system can be batch-manufactured by servo scanning layer by layer using array-rod-like micro tool electrodes, and the axial wear of the array electrodes is compensated in real time by keeping discharge gap. To verify the effectiveness of the 3D ASSMEDM, the array-triangle-micro cavities (side length 630 μm) are batch-manufactured on P-doped silicon by applying the array-micro-electrodes with square-cross-section fabricated by LIGA process. Our exploratory experiment shows that the 3D ASSMEDM provides a feasible approach for the batch-manufacture of 3D array-micro-cavities of conductive materials.

  7. Narrow band gap amorphous silicon semiconductors

    DOEpatents

    Madan, A.; Mahan, A.H.

    1985-01-10

    Disclosed is a narrow band gap amorphous silicon semiconductor comprising an alloy of amorphous silicon and a band gap narrowing element selected from the group consisting of Sn, Ge, and Pb, with an electron donor dopant selected from the group consisting of P, As, Sb, Bi and N. The process for producing the narrow band gap amorphous silicon semiconductor comprises the steps of forming an alloy comprising amorphous silicon and at least one of the aforesaid band gap narrowing elements in amount sufficient to narrow the band gap of the silicon semiconductor alloy below that of amorphous silicon, and also utilizing sufficient amounts of the aforesaid electron donor dopant to maintain the amorphous silicon alloy as an n-type semiconductor.

  8. Optically tuneable blue phase photonic band gaps

    SciTech Connect

    Liu, H.-Y.; Wang, C.-T.; Hsu, C.-Y.; Lin, T.-H.; Liu, J.-H.

    2010-03-22

    This study investigates an optically switchable band gap of photonic crystal that is based on an azobenzene-doped liquid crystal blue phase. The trans-cis photoisomerization of azobenzene deforms the cubic unit cell of the blue phase and shifts the photonic band gap. The fast back-isomerization of azobenzene was induced by irradiation with different wavelengths light. The crystal structure is verified using Kossel diffraction diagram. An optically addressable blue phase display, based on Bragg reflection from the photonic band gap, is also demonstrated. The tunable ranges are around red, green, and blue wavelengths and exhibit a bright saturated color.

  9. Semiconductor band gap localization via Gaussian function

    NASA Astrophysics Data System (ADS)

    Ullrich, B.; Brown, G. J.; Xi, H.

    2012-10-01

    To determine the band gap of bulk semiconductors with transmission spectroscopy alone is considered as an extremely difficult task because in the higher energy range, approaching and exceeding the band gap energy, the material is opaque yielding no useful data to be recorded. In this paper, by investigating the transmission of industrial GaSb wafers with a thickness of 500 µm, we demonstrate how these obstacles of transmission spectroscopy can be overcome. The key is the transmission spectrums’ derivative, which coincides with the Gaussian function. This understanding can be used to transfer Beers’ law in an integral form opening the pathway of band gap determinations based on mathematical parameters only. The work also emphasizes the correlation between the thermal band gap variation and Debye temperature.

  10. Multi Band Gap High Efficiency Converter (RAINBOW)

    NASA Technical Reports Server (NTRS)

    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.

  11. Modification in band gap of zirconium complexes

    NASA Astrophysics Data System (ADS)

    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.

  12. Bulk band gaps in divalent hexaborides

    SciTech Connect

    Denlinger, Jonathan; Clack, Jules A.; Allen, James W.; Gweon, Gey-Hong; Poirier, Derek M.; Olson, Cliff G.; Sarrao, John L.; Bianchi, Andrea D.; Fisk, Zachary

    2002-08-01

    Complementary angle-resolved photoemission and bulk-sensitive k-resolved resonant inelastic x-ray scattering of divalent hexaborides reveal a >1 eV X-point gap between the valence and conduction bands, in contradiction to the band overlap assumed in several models of their novel ferromagnetism. This semiconducting gap implies that carriers detected in transport measurements arise from defects, and the measured location of the bulk Fermi level at the bottom of the conduction band implicates boron vacancies as the origin of the excess electrons. The measured band structure and X-point gap in CaB6 additionally provide a stringent test case for proper inclusion of many-body effects in quasi-particle band calculations.

  13. Nonreciprocal microwave band-gap structures.

    PubMed

    Belov, P A; Tretyakov, S A; Viitanen, A J

    2002-07-01

    An electrically controlled nonreciprocal electromagnetic band-gap material is proposed and studied. The new material is a periodic three-dimensional regular lattice of small magnetized ferrite spheres. In this paper, we consider plane electromagnetic waves in this medium and design an analytical model for the material parameters. An analytical solution for plane-wave reflection from a planar interface is also presented. In the proposed material, a new electrically controlled stop band appears for one of the two circularly polarized eigenwaves in a frequency band around the ferrimagnetic resonance frequency. This frequency can be well below the usual lattice band gap, which allows the realization of rather compact structures. The main properties of the material are outlined. PMID:12241501

  14. Robust photonic band gap from tunable scatterers

    PubMed

    Zhang; Lei; Wang; Zheng; Tam; Chan; Sheng

    2000-03-27

    We show theoretically and experimentally that photonic band gaps can be realized using metal or metal-coated spheres as building blocks. Robust photonic gaps exist in any periodic structure built from such spheres when the filling ratio of the spheres exceeds a threshold. The frequency and the size of the gaps depend on the local order rather than on the symmetry or the global long range order. Good agreement between theory and experiment is obtained in the microwave regime. Calculations show that the approach can be scaled up to optical frequencies even in the presence of absorption. PMID:11018959

  15. Sculpting the band gap: a computational approach

    PubMed Central

    Prasai, Kiran; Biswas, Parthapratim; Drabold, D. A.

    2015-01-01

    Materials with optimized band gap are needed in many specialized applications. In this work, we demonstrate that Hellmann-Feynman forces associated with the gap states can be used to find atomic coordinates that yield desired electronic density of states. Using tight-binding models, we show that this approach may be used to arrive at electronically designed models of amorphous silicon and carbon. We provide a simple recipe to include a priori electronic information in the formation of computer models of materials, and prove that this information may have profound structural consequences. The models are validated with plane-wave density functional calculations. PMID:26490203

  16. Sculpting the band gap: a computational approach.

    PubMed

    Prasai, Kiran; Biswas, Parthapratim; Drabold, D A

    2015-01-01

    Materials with optimized band gap are needed in many specialized applications. In this work, we demonstrate that Hellmann-Feynman forces associated with the gap states can be used to find atomic coordinates that yield desired electronic density of states. Using tight-binding models, we show that this approach may be used to arrive at electronically designed models of amorphous silicon and carbon. We provide a simple recipe to include a priori electronic information in the formation of computer models of materials, and prove that this information may have profound structural consequences. The models are validated with plane-wave density functional calculations. PMID:26490203

  17. Phase Modulation of Photonic Band Gap Signal

    PubMed Central

    Wang, Zhiguo; Gao, Mengqin; Mahesar, Abdul Rasheed; Zhang, Yanpeng

    2016-01-01

    We first investigate the probe transmission signal (PTS) and the four wave mixing band gap signal (FWM BGS) modulated simultaneously by the relative phase and the nonlinear phase shift in the photonic band gap (PBG) structure. The switch between the absorption enhancement of PTS and the transmission enhancement of PTS with the help of changing the relative phase and the nonlinear phase shift is obtained in inverted Y-type four level atomic system experimentally and theoretically. The corresponding switch in PTS can be used to realize all optical switches. On other hand, the relative phase and the nonlinear phase shift also play the vital role to modulate the intensity of FWM BGS reflected from the PBG structure. And it can be potentially used to realize the optical amplifier. PMID:27323849

  18. Phase Modulation of Photonic Band Gap Signal.

    PubMed

    Wang, Zhiguo; Gao, Mengqin; Mahesar, Abdul Rasheed; Zhang, Yanpeng

    2016-01-01

    We first investigate the probe transmission signal (PTS) and the four wave mixing band gap signal (FWM BGS) modulated simultaneously by the relative phase and the nonlinear phase shift in the photonic band gap (PBG) structure. The switch between the absorption enhancement of PTS and the transmission enhancement of PTS with the help of changing the relative phase and the nonlinear phase shift is obtained in inverted Y-type four level atomic system experimentally and theoretically. The corresponding switch in PTS can be used to realize all optical switches. On other hand, the relative phase and the nonlinear phase shift also play the vital role to modulate the intensity of FWM BGS reflected from the PBG structure. And it can be potentially used to realize the optical amplifier. PMID:27323849

  19. Fabrication of photonic band gap materials

    DOEpatents

    Constant, Kristen; Subramania, Ganapathi S.; Biswas, Rana; Ho, Kai-Ming

    2002-01-15

    A method for forming a periodic dielectric structure exhibiting photonic band gap effects includes forming a slurry of a nano-crystalline ceramic dielectric or semiconductor material and monodisperse polymer microspheres, depositing a film of the slurry on a substrate, drying the film, and calcining the film to remove the polymer microspheres therefrom. The film may be cold-pressed after drying and prior to calcining. The ceramic dielectric or semiconductor material may be titania, and the polymer microspheres may be polystyrene microspheres.

  20. Fabrication of Photonic band gap Materials

    SciTech Connect

    Constant, Kristen; Subramania, Ganapathi S.; Biswas, Rana; Ho, Kai-Ming

    2000-01-05

    A method for forming a periodic dielectric structure exhibiting photonic band gap effects includes forming a slurry of a nano-crystalline ceramic dielectric or semiconductor material and monodisperse polymer microsphere, depositing a film of the slurry on a substrate, drying the film, and calcining the film to remove the polymer microsphere there from. The film may be cold-pressed after drying and prior to calcining. The ceramic dielectric or semiconductor material may be titania, and the polymer microsphere may be polystyrenemicrosphere.

  1. Elucidating the Band Gap of Niobium Dioxide

    NASA Astrophysics Data System (ADS)

    O'Hara, Andrew; Vigil-Fowler, Derek; Louie, Steven G.; Demkov, Alexander A.

    2015-03-01

    Like VO2, niobium dioxide (NbO2) belongs to the family of transition metal oxides with a temperature-driven metal-to-insulator transition. However, NbO2 has received considerably less attention, and several open questions about the material remain. One such question, of both practical and fundamental importance, is the nature and size of the band gap in the low-temperature, distorted rutile phase with a range reported for the gap of 0.5 eV to 1.2 eV. In this work, we investigate the low-temperature phase, utilizing several methodologies - density functional theory within the standard local density approximation (LDA), LDA +U, hybrid functional, and the GW approximation, to better understand the physics of the band gap in NbO2. Comparisons of the calculations are made to recent experimental work on NbO2 utilizing photoemission spectroscopy and ellipsometry. This work is supported by DOE under the SciDAC program, the NSF, and SRC.

  2. Optical band gaps of organic semiconductor materials

    NASA Astrophysics Data System (ADS)

    Costa, José C. S.; Taveira, Ricardo J. S.; Lima, Carlos F. R. A. C.; Mendes, Adélio; Santos, Luís M. N. B. F.

    2016-08-01

    UV-Vis can be used as an easy and forthright technique to accurately estimate the band gap energy of organic π-conjugated materials, widely used as thin films/composites in organic and hybrid electronic devices such as OLEDs, OPVs and OFETs. The electronic and optical properties, including HOMO-LUMO energy gaps of π-conjugated systems were evaluated by UV-Vis spectroscopy in CHCl3 solution for a large number of relevant π-conjugated systems: tris-8-hydroxyquinolinatos (Alq3, Gaq3, Inq3, Al(qNO2)3, Al(qCl)3, Al(qBr)3, In(qNO2)3, In(qCl)3 and In(qBr)3); triphenylamine derivatives (DDP, p-TTP, TPB, TPD, TDAB, m-MTDAB, NPB, α-NPD); oligoacenes (naphthalene, anthracene, tetracene and rubrene); oligothiophenes (α-2T, β-2T, α-3T, β-3T, α-4T and α-5T). Additionally, some electronic properties were also explored by quantum chemical calculations. The experimental UV-Vis data are in accordance with the DFT predictions and indicate that the band gap energies of the OSCs dissolved in CHCl3 solution are consistent with the values presented for thin films.

  3. Highly dispersive photonic band-gap prism.

    PubMed

    Lin, S Y; Hietala, V M; Wang, L; Jones, E D

    1996-11-01

    We propose the concept of a photonic band-gap (PBG) prism based on two-dimensional PBG structures and realize it in the millimeter-wave spectral regime. We recognize the highly nonlinear dispersion of PBG materials near Brillouin zone edges and utilize the dispersion to achieve strong prism action. Such a PBG prism is very compact if operated in the optical regime, ~20 mm in size for lambda ~ 700 nm, and can serve as a dispersive element for building ultracompact miniature spectrometers. PMID:19881796

  4. The band-gap enhanced photovoltaic structure

    NASA Astrophysics Data System (ADS)

    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.

  5. Electronic Band Gap of SrSe at High Pressure

    SciTech Connect

    Atkinson,T.; Chynoweth, K.; Cervantes, P.

    2006-01-01

    The electronic band gap of SrSe, in the CsCl-stuctured phase, was measured to 42 GPa via optical absorption studies. The indirect electronic band gap was found to close monotonically with pressure for the range of pressures studied. The change in band gap with respect to pressure, dE{sub gap}/dP, was determined to be -6.1(5)x10{sup -3} eV/GPa. By extrapolation of our line fit, we estimate band gap closure to occur at 180(20) GPa.

  6. Effect of size of silica microspheres on photonic band gap

    SciTech Connect

    Dhiman, N. Sharma, A. Gathania, A. K.; Singh, B. P.

    2014-04-24

    In present work photonic crystals of different size of silica microspheres have been fabricated. The optical properties of these developed photonic crystals have been studied using UV-visible spectroscopy. UV-visible spectroscopy shows that they have photonic band gap that can be tuned in visible and infrared regime by changing the size of silica microspheres. The photonic band gap structures of these photonic crystals have been calculated using MIT photonic band gap package. It also reveals that with the increase in size of silica microspheres the photonic band gap shifts to lower energy region.

  7. Designer disordered materials with large, complete photonic band gaps

    PubMed Central

    Florescu, Marian; Torquato, Salvatore; Steinhardt, Paul J.

    2009-01-01

    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 band 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. PMID:19918087

  8. Controlling the band gap energy of cluster-assembled materials.

    PubMed

    Mandal, Sukhendu; Reber, Arthur C; Qian, Meichun; Weiss, Paul S; Khanna, Shiv N; Sen, Ayusman

    2013-11-19

    Cluster-assembled materials combine the nanoscale size and composition-dependent properties of clusters, which have highly tunable magnetic and electronic properties useful for a great variety of potential technologies. To understand the emergent properties as clusters are assembled into hierarchical materials, we have synthesized 23 cluster-assembled materials composed of As7(3-)-based motifs and different countercations and measured their band gap energies. We found that the band gap energy varies from 1.09 to 2.21 eV. In addition, we have carried out first principles electronic structure studies to identify the physical mechanisms that enable control of the band gap edges of the cluster assemblies. The choice of counterion has a profound effect on the band gap energy in ionic cluster assemblies. The top of the valence band is localized on the arsenic cluster, while the conduction band edge is located on the alkali metal counterions. Changing the counterion changes the position of the conduction band edge, enabling control of the band gap energy. We can also vary the architecture of the ionic solid by incorporating cryptates as counterions, which provide charge but are separated from the clusters by bulky ligands. Higher dimensionality typically decreases the band gap energy through band broadening; however band gap energies increased upon moving from zero-dimensional (0D) to two-dimensional (2D) assemblies. This is because internal electric fields generated by the counterion preferentially stabilize the adjacent lone pair orbitals that mark the top of the valence band. Thus, the choice of the counterion can control the position of the conduction band edge of ionic cluster assemblies. In addition, the dimensionality of the solid via internal electric fields can control the valence band edge. Through covalently linking arsenic clusters into composite building blocks, we have also been able to tune the band gap energy. We used a theoretical description based on

  9. Highly dispersive photonic band-gap-edge optofluidic biosensors

    NASA Astrophysics Data System (ADS)

    Xiao, S.; Mortensen, N. A.

    2006-11-01

    Highly dispersive photonic band-gap-edge optofluidic biosensors are studied theoretically. We demonstrate that these structures are strongly sensitive to the refractive index of the liquid, which is used to tune dispersion of the photonic crystal. The upper frequency band-gap edge shifts about 1.8 nm for δ n=0.002, which is quite sensitive. Results from transmission spectra agree well with those obtained from the band structure theory.

  10. Band gap in tubular pillar phononic crystal plate.

    PubMed

    Shu, Fengfeng; Liu, Yongshun; Wu, Junfeng; Wu, Yihui

    2016-09-01

    In this paper, a phononic crystal (PC) plate with tubular pillars is presented and investigated. The band structures and mode displacement profiles are calculated by using finite element method. The result shows that a complete band gap opens when the ratio of the pillar height to the plate thickness is about 1.6. However, for classic cylinder pillar structures, a band gap opens when the ratio is equal or greater than 3. A tubular pillar design with a void room in it enhances acoustic multiple scattering and gives rise to the opening of the band gap. In order to verify it, a PC structure with double tubular pillars different in size (one within the other) is introduced and a more than 2times band gap enlargement is observed. Furthermore, the coupling between the resonant mode and the plate mode around the band gap is characterized, as well as the effect of the geometrical parameters on the band gap. The behavior of such structure could be utilized to design a pillar PC with stronger structural stability and to enlarge band gaps. PMID:27376841

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

    NASA Astrophysics Data System (ADS)

    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.

  12. Resolution of the Band Gap Prediction Problem for Materials Design.

    PubMed

    Crowley, Jason M; Tahir-Kheli, Jamil; Goddard, William A

    2016-04-01

    An important property with any new material is the band gap. Standard density functional theory methods grossly underestimate band gaps. This is known as the band gap problem. Here, we show that the hybrid B3PW91 density functional returns band gaps with a mean absolute deviation (MAD) from experiment of 0.22 eV over 64 insulators with gaps spanning a factor of 500 from 0.014 to 7 eV. The MAD is 0.28 eV over 70 compounds with gaps up to 14.2 eV, with a mean error of -0.03 eV. To benchmark the quality of the hybrid method, we compared the hybrid method to the rigorous GW many-body perturbation theory method. Surprisingly, the MAD for B3PW91 is about 1.5 times smaller than the MAD for GW. Furthermore, B3PW91 is 3-4 orders of magnitude faster computationally. Hence, B3PW91 is a practical tool for predicting band gaps of materials before they are synthesized and represents a solution to the band gap prediction problem. PMID:26944092

  13. Locally resonant periodic structures with low-frequency band gaps

    NASA Astrophysics Data System (ADS)

    Cheng, Zhibao; Shi, Zhifei; Mo, Y. L.; Xiang, Hongjun

    2013-07-01

    Presented in this paper are study results of dispersion relationships of periodic structures composited of concrete and rubber, from which the frequency band gap can be found. Two models with fixed or free boundary conditions are proposed to approximate the bound frequencies of the first band gap. Studies are conducted to investigate the low-frequency and directional frequency band gaps for their application to engineering. The study finds that civil engineering structures can be designed to block harmful waves, such as earthquake disturbance.

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

    SciTech Connect

    Sevak Singh, Ram; Leong Chow, Wai; Yingjie Tay, Roland; Hon Tsang, Siu; Mallick, Govind; Tong Teo, Edwin Hang

    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.

  15. Tunable band gap of boron nitride interfaces under uniaxial pressure

    NASA Astrophysics Data System (ADS)

    Moraes, Elizane E.; Manhabosco, Taíse M.; de Oliveira, Alan B.; Batista, Ronaldo J. C.

    2012-11-01

    In this work we show, by means of a density functional theory formalism, that the interaction between hydrogen terminated boron nitride surfaces gives rise to a metallic interface with free carriers of opposite sign at each surface. A band gap can be induced by decreasing the surface separation. The size of the band gap changes continuously from zero up to 4.4 eV with decreasing separation, which is understood in terms of the interaction between surface states. Due to the high thermal conductivity of cubic boron nitride and the coupling between band gap and applied pressure, such tunable band gap interfaces may be used in highly stable electronic and electromechanical devices. In addition, the spatial separation of charge carriers at the interface may lead to photovoltaic applications.

  16. Tunable Band Gap of Boron Nitride Interfaces under Uniaxial Pressure

    NASA Astrophysics Data System (ADS)

    Moraes, Elizane; Manhabosco, Taise; de Oliveira, Alan; Batista, Ronaldo

    2013-03-01

    In this work we show, by means of a density functional theory formalism, that the interaction between hydrogen terminated boron nitride surfaces gives rise to a metallic interface with free carries of opposite sign at each surface. A band gap can be induced by decreasing the surface separation. The size of the band gap changes continuously from zero up to 4.4 eV with decreasing separation, which is understood in terms of the interaction between surface states.Due to the high thermal conductivity of cubic boron nitride and the coupling between band gap and applied pressure, such tunable band gap interfaces may be used in high stable electronic and electromechanical devices. In addition, the spacial separation of charge carries at the interface may lead to photovoltaic applications. The authors thank tha brazilian agencies Fapemig, CNPq and Capes

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

    PubMed

    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%. PMID:14683137

  18. Effect of disorder on photonic band gaps

    NASA Astrophysics Data System (ADS)

    Sigalas, M. M.; Soukoulis, C. M.; Chan, C. T.; Biswas, R.; Ho, K. M.

    1999-05-01

    We study the transmission of electromagnetic waves propagating in three-dimensional disordered photonic crystals that are periodic on the average with a diamond symmetry. The transmission has been calculated using the transfer matrix method. We study two different geometries for the scatterers: spheres and rods connecting nearest neighbors. We find that the gaps of the periodic structure survive to a higher amount of disorder in the rods' case than in the spheres' case. We argue that this is due to the connectivity of the rod structure that exists for any amount of disorder.

  19. Tunable band gap in biased rhombohedral-stacked trilayer graphene

    NASA Astrophysics Data System (ADS)

    Mihiri Shashikala, H. B.; Wang, Xiao-Qian

    2012-03-01

    We have employed dispersion-corrected density-functional calculations to investigate the electronic characteristics of Bernal-stacked trilayer (ABA) and rhombohedral-stacked (ABC) trilayer graphene. In contrast to semimetallic behavior for Bernal-stacked trilayer, rhombohedral-stacked trilayer leads to a band gap opening with the applications of a perpendicular electric bias. The induced gap is shown to be attributed to the avoiding of level crossing among even and odd parity states that depends on the stacking pattern. The tunable band gap suggests a sensitive and effective way to tailor properties of trilayer graphene for future applications in nanoscale devices.

  20. Band gap engineering strategy via polarization rotation in perovskite ferroelectrics

    SciTech Connect

    Wang, Fenggong Grinberg, Ilya; Rappe, Andrew M.

    2014-04-14

    We propose a strategy to engineer the band gaps of perovskite oxide ferroelectrics, supported by first principles calculations. We find that the band gaps of perovskites can be substantially reduced by as much as 1.2 eV through local rhombohedral-to-tetragonal structural transition. Furthermore, the strong polarization of the rhombohedral perovskite is largely preserved by its tetragonal counterpart. The B-cation off-center displacements and the resulting enhancement of the antibonding character in the conduction band give rise to the wider band gaps of the rhombohedral perovskites. The correlation between the structure, polarization orientation, and electronic structure lays a good foundation for understanding the physics of more complex perovskite solid solutions and provides a route for the design of photovoltaic perovskite ferroelectrics.

  1. Complete band gaps and deaf bands of triangular and honeycomb water-steel phononic crystals

    NASA Astrophysics Data System (ADS)

    Hsiao, Fu-Li; Khelif, Abdelkrim; Moubchir, Hanane; Choujaa, Abdelkrim; Chen, Chii-Chang; Laude, Vincent

    2007-02-01

    Phononic crystals with triangular and honeycomb lattices are investigated experimentally and theoretically. They are composed of arrays of steel cylinders immersed in water. The measured transmission spectra reveal the existence of complete band gaps but also of deaf bands. Band gaps and deaf bands are identified by comparing band structure computations, obtained by a periodic-boundary finite element method, with transmission simulations, obtained using the finite difference time domain method. The appearance of flat bands and the polarization of the associated eigenmodes is also discussed. Triangular and honeycomb phononic crystals with equal cylinder diameter and smallest spacing are compared. As previously obtained with air-solid phononic crystals, it is found that the first complete band gap opens for the honeycomb lattice but not for the triangular lattice, thanks to symmetry reduction.

  2. Quantum electrodynamics near a photonic band-gap

    NASA Astrophysics Data System (ADS)

    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.

  3. Wide band gap ferromagnetic semiconductors and oxides

    NASA Astrophysics Data System (ADS)

    Pearton, S. J.; Abernathy, C. R.; Overberg, M. E.; Thaler, G. T.; Norton, D. P.; Theodoropoulou, N.; Hebard, A. F.; Park, Y. D.; Ren, F.; Kim, J.; Boatner, L. A.

    2003-01-01

    Recent advances in the theory and experimental realization of ferromagnetic semiconductors give hope that a new generation of microelectronic devices based on the spin degree of freedom of the electron can be developed. This review focuses primarily on promising candidate materials (such as GaN, GaP and ZnO) in which there is already a technology base and a fairly good understanding of the basic electrical and optical properties. The introduction of Mn into these and other materials under the right conditions is found to produce ferromagnetism near or above room temperature. There are a number of other potential dopant ions that could be employed (such as Fe, Ni, Co, Cr) as suggested by theory [see, for example, Sato and Katayama-Yoshida, Jpn. J. Appl. Phys., Part 2 39, L555 (2000)]. Growth of these ferromagnetic materials by thin film techniques, such as molecular beam epitaxy or pulsed laser deposition, provides excellent control of the dopant concentration and the ability to grow single-phase layers. The mechanism for the observed magnetic behavior is complex and appears to depend on a number of factors, including Mn-Mn spacing, and carrier density and type. For example, in a simple Ruderman-Kittel-Kasuya-Yosida carrier-mediated exchange mechanism, the free-carrier/Mn ion interaction can be either ferromagnetic or antiferromagnetic depending on the separation of the Mn ions. Potential applications for ferromagnetic semiconductors and oxides include electrically controlled magnetic sensors and actuators, high-density ultralow-power memory and logic, spin-polarized light emitters for optical encoding, advanced optical switches and modulators and devices with integrated magnetic, electronic and optical functionality.

  4. The Band Gap of AlGaN Alloys

    SciTech Connect

    Biefeld, R.M.; Crawford, M.H.; Han, J.; Lee, S.R.; Petersen, G.A.; Wright, A.F.

    1999-01-29

    The band gap of AlXGal.XN is measured for the composition range 0s<0.45; the resulting bowing parameter, b=+O.69 eV, is compared to 20 previous works. A correlation is found between the measured band gaps and the methods used for epitaxial growth of the AlXGal_XN: directly nucleated or buffered growths of AlXGal-XN initiated at temperatures T>800 C on sapphire usually lead to stronger apparent bowing (b> +1.3 eV); while growths initiated using low-temperature buffers on sapphire, followed by high-temperature growth, lead to weaker bowing (b<+ 1.3 eV). Extant data suggests that the correct band-gap bowing parameter for AlXGal-XN is b=+O.62 (N.45) eV.

  5. Amorphous copper tungsten oxide with tunable band gaps

    SciTech Connect

    Chen Le; Shet, Sudhakar; Tang Houwen; Wang Heli; Yan Yanfa; Turner, John; Al-Jassim, Mowafak; Ahn, Kwang-soon

    2010-08-15

    We report on the synthesis of amorphous copper tungsten oxide thin films with tunable band gaps. The thin films are synthesized by the magnetron cosputtering method. We find that due to the amorphous nature, the Cu-to-W ratio in the films can be varied without the limit of the solubility (or phase separation) under appropriate conditions. As a result, the band gap and conductivity type of the films can be tuned by controlling the film composition. Unfortunately, the amorphous copper tungsten oxides are not stable in aqueous solution and are not suitable for the application of photoelectrochemical splitting of water. Nonetheless, it provides an alternative approach to search for transition metal oxides with tunable band gaps.

  6. Energy band gaps in graphene nanoribbons with corners

    NASA Astrophysics Data System (ADS)

    Szczȩśniak, Dominik; Durajski, Artur P.; Khater, Antoine; Ghader, Doried

    2016-05-01

    In the present paper, we study the relation between the band gap size and the corner-corner length in representative chevron-shaped graphene nanoribbons (CGNRs) with 120° and 150° corner edges. The direct physical insight into the electronic properties of CGNRs is provided within the tight-binding model with phenomenological edge parameters, developed against recent first-principle results. We show that the analyzed CGNRs exhibit inverse relation between their band gaps and corner-corner lengths, and that they do not present a metal-insulator transition when the chemical edge modifications are introduced. Our results also suggest that the band gap width for the CGNRs is predominantly governed by the armchair edge effects, and is tunable through edge modifications with foreign atoms dressing.

  7. Band gap modulation of functionalized metal-organic frameworks.

    PubMed

    Musho, Terence; Li, Jiangtan; Wu, Nianqiang

    2014-11-21

    Metal-organic frameworks (MOFs) have been envisioned as alternatives to planar metallic catalysts for solar-to-fuel conversion. This is a direct result of their porous structure and the ability to tailor their optical absorption properties. This study investigates the band gap modulation of Zr-UiO-66 MOFs from both the computational and experimental points of view for three linker designs that include benzenedicarboxylate (BDC), BDC-NO2, and BDC-NH2. Emphasis in this study was aimed at understanding the influence of the bonding between the aromatic ring and the functional group. A ground state density functional theory (DFT) calculation was carried out to investigate the projected density of states and the origins of the modulation. A time-dependent density functional theory (TDDFT) calculation of the hydrogen terminated linkers confirmed the modulation and accounted for the electron charge transfer providing comparable optical band gap predictions to experimental results. Computational results confirmed the hybridization of the carbon-nitrogen bond in conjunction with the donor state resulting from the NH2 functionalization. The NO2 functionalization resulted in an acceptor configuration with marginal modification to the valence band maximum. The largest modulation was BDC-NH2 with a band gap of 2.75 eV, followed by BDC-NO2 with a band gap of 2.93 eV and BDC with a band gap of 3.76 eV. The electron effective mass was predicted from the band structure to be 8.9 me for all MOF designs. PMID:25269595

  8. HAC: Band Gap, Photoluminescence, and Optical/Near-Infrared Absorption

    NASA Technical Reports Server (NTRS)

    Witt, Adolf N.; Ryutov, Dimitri; Furton, Douglas G.

    1996-01-01

    We report results of laboratory measurements which illustrate the wide range of physical properties found among hydrogenated amorphous carbon (HAC) solids. Within this range, HAC can match quantitatively the astronomical phenomena ascribed to carbonaceous coatings on interstellar grains. We find the optical band gap of HAC to be well correlated with other physical properties of HAC of astronomical interest, and conclude that interstellar HAC must be fairly hydrogen-rich with a band gap of E(sub g) is approx. greater than 2.0 eV.

  9. On band gap predictions for multiresonant metamaterials on plates.

    PubMed

    Yoritomo, John Y; Weaver, Richard L; Roux, Philippe; Rupin, Matthieu; Williams, Earl G

    2016-03-01

    Recently wide frequency band gaps were observed in an experimental realization of a multiresonant metamaterial for Lamb waves propagating in thin plates. The band gaps rose from hybridization between the flexural plate (A0 Lamb waves) and longitudinal resonances in rods attached perpendicularly. Shortly thereafter a theory based on considering a one-dimensional periodic array of rods and the scattering matrix for a single rod successfully described the observations. This letter presents an alternative simpler theory, arguably accurate at high rod density, that treats the full two-dimensional array of rods and makes no assumption of periodicity. This theory also fits the measurements. PMID:27036264

  10. Special purpose modes in photonic band gap fibers

    SciTech Connect

    Spencer, James; Noble, Robert; Campbell, Sara

    2013-04-02

    Photonic band gap fibers are described having one or more defects suitable for the acceleration of electrons or other charged particles. Methods and devices are described for exciting special purpose modes in the defects including laser coupling schemes as well as various fiber designs and components for facilitating excitation of desired modes. Results are also presented showing effects on modes due to modes in other defects within the fiber and due to the proximity of defects to the fiber edge. Techniques and devices are described for controlling electrons within the defect(s). Various applications for electrons or other energetic charged particles produced by such photonic band gap fibers are also described.

  11. Below-Band-Gap Laser Ablation Of Diamond For TEM

    NASA Technical Reports Server (NTRS)

    George, Thomas; Foote, Marc C.; Vasquez, Richard P.; Fortier, Edward P.; Posthill, John B.

    1995-01-01

    Thin, electron-transparent layers of diamond for examination in transmission electron microscope (TEM) fabricated from thicker diamond substrates by using laser beam to ablate surface of substrate. Involves use of photon energy below band gap. Growing interest in use of diamond as bulk substrate and as coating material in variety of applications has given rise to increasing need for TEM for characterization of diamond-based materials. Below-band-gap laser ablation method helps to satisfy this need. Also applied in general to cutting and etching of diamonds.

  12. New insights into the opening band gap of graphene oxides

    NASA Astrophysics Data System (ADS)

    Tran, Ngoc Thanh Thuy; Lin, Shih-Yang; Lin, Ming-Fa

    Electronic properties of oxygen absorbed few-layer graphenes are investigated using first-principle calculations. They are very sensitive to the changes in the oxygen concentration, number of graphene layer, and stacking configuration. The feature-rich band structures exhibit the destruction or distortion of the Dirac cone, opening of band gap, anisotropic energy dispersions, O- and (C,O)-dominated energy dispersions, and extra critical points. The band decomposed charge distributions reveal the π-bonding dominated energy gap. The orbital-projected density of states (DOS) have many special structures mainly coming from a composite energy band, the parabolic and partially flat ones. The DOS and spatial charge distributions clearly indicate the critical orbital hybridizations in O-O, C-O and C-C bonds, being responsible for the diversified properties. All of the few-layer graphene oxides are semi-metals except for the semiconducting monolayer ones.

  13. Tuning the electronic band gap of graphene by oxidation

    SciTech Connect

    Dabhi, Shweta D.; Jha, Prafulla K.

    2015-06-24

    Using plane wave pseudo potential density functional theory, we studied the electronic properties of graphene with different C:O ratio. In this work, we discussed the changes that occur in electronic band structure of graphene functionalized with different amount of epoxy group. Electronic band gap depends on C:O ratio in graphene oxide containing epoxy group. The present work will have its implication for making devices with tunable electronic properties by oxidizing graphene.

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

    PubMed Central

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

    2015-01-01

    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. PMID:26012369

  15. Band-Gap and Band-Edge Engineering of Multicomponent Garnet Scintillators from First Principles

    NASA Astrophysics Data System (ADS)

    Yadav, Satyesh K.; Uberuaga, Blas P.; Nikl, Martin; Jiang, Chao; Stanek, Christopher R.

    2015-11-01

    Complex doping schemes in R3 Al5 O12 (where R is the rare-earth element) garnet compounds have recently led to pronounced improvements in scintillator performance. Specifically, by admixing lutetium and yttrium aluminate garnets with gallium and gadolinium, the band gap is altered in a manner that facilitates the removal of deleterious electron trapping associated with cation antisite defects. Here, we expand upon this initial work to systematically investigate the effect of substitutional admixing on the energy levels of band edges. Density-functional theory and hybrid density-functional theory (HDFT) are used to survey potential admixing candidates that modify either the conduction-band minimum (CBM) or valence-band maximum (VBM). We consider two sets of compositions based on Lu3 B5O12 where B is Al, Ga, In, As, and Sb, and R3Al5 O12 , where R is Lu, Gd, Dy, and Er. We find that admixing with various R cations does not appreciably affect the band gap or band edges. In contrast, substituting Al with cations of dissimilar ionic radii has a profound impact on the band structure. We further show that certain dopants can be used to selectively modify only the CBM or the VBM. Specifically, Ga and In decrease the band gap by lowering the CBM, while As and Sb decrease the band gap by raising the VBM, the relative change in band gap is quantitatively validated by HDFT. These results demonstrate a powerful approach to quickly screen the impact of dopants on the electronic structure of scintillator compounds, identifying those dopants which alter the band edges in very specific ways to eliminate both electron and hole traps responsible for performance limitations. This approach should be broadly applicable for the optimization of electronic and optical performance for a wide range of compounds by tuning the VBM and CBM.

  16. Dipole-allowed direct band gap silicon superlattices

    NASA Astrophysics Data System (ADS)

    Oh, Young Jun; Lee, In-Ho; Kim, Sunghyun; Lee, Jooyoung; Chang, Kee Joo

    2015-12-01

    Silicon is the most popular material used in electronic devices. However, its poor optical properties owing to its indirect band gap nature limit its usage in optoelectronic devices. Here we present the discovery of super-stable pure-silicon superlattice structures that can serve as promising materials for solar cell applications and can lead to the realization of pure Si-based optoelectronic devices. The structures are almost identical to that of bulk Si except that defective layers are intercalated in the diamond lattice. The superlattices exhibit dipole-allowed direct band gaps as well as indirect band gaps, providing ideal conditions for the investigation of a direct-to-indirect band gap transition. The fact that almost all structural portions of the superlattices originate from bulk Si warrants their stability and good lattice matching with bulk Si. Through first-principles molecular dynamics simulations, we confirmed their thermal stability and propose a possible method to synthesize the defective layer through wafer bonding.

  17. Dipole-allowed direct band gap silicon superlattices

    PubMed Central

    Oh, Young Jun; Lee, In-Ho; Kim, Sunghyun; Lee, Jooyoung; Chang, Kee Joo

    2015-01-01

    Silicon is the most popular material used in electronic devices. However, its poor optical properties owing to its indirect band gap nature limit its usage in optoelectronic devices. Here we present the discovery of super-stable pure-silicon superlattice structures that can serve as promising materials for solar cell applications and can lead to the realization of pure Si-based optoelectronic devices. The structures are almost identical to that of bulk Si except that defective layers are intercalated in the diamond lattice. The superlattices exhibit dipole-allowed direct band gaps as well as indirect band gaps, providing ideal conditions for the investigation of a direct-to-indirect band gap transition. The fact that almost all structural portions of the superlattices originate from bulk Si warrants their stability and good lattice matching with bulk Si. Through first-principles molecular dynamics simulations, we confirmed their thermal stability and propose a possible method to synthesize the defective layer through wafer bonding. PMID:26656482

  18. Photonic Band Gap structures: A new approach to accelerator cavities

    SciTech Connect

    Kroll, N. |; Smith, D.R.; Schultz, S.

    1992-12-31

    We introduce a new accelerator cavity design based on Photonic Band Gap (PGB) structures. The PGB cavity consists of a two-dimensional periodic array of high dielectric, low loss cylinders with a single removal defect, bounded on top and bottom by conducting sheets. We present the results of both numerical simulations and experimental measurements on the PGB cavity.

  19. Multi-band gap and new solar cell options workshop

    NASA Technical Reports Server (NTRS)

    Hutchby, J.; Timmons, M.; Olson, J. M.

    1993-01-01

    Discussions of the multi-band gap (MBG) and new solar cell options workshop are presented. Topics discussed include: greater than 2 terminal cells; radiation damage preventing development of MBG cells for space; lattice matching; measurement of true performance; future of II-VI materials in MBG devices; and quaternaries.

  20. Anomalous dependence of band gaps of binary nanotubes on diameters

    NASA Astrophysics Data System (ADS)

    Adhikari, Kapil; Huda, Muhammad; Ray, Asok

    2012-02-01

    Using cluster approximation, AlN, BN, GaN, SiGe, SiC, and GeC armchair type 1 nanotubes have been spin optimized using the hybrid functional B3LYP, a double ζ basis set and the GAUSSIAN 03 software. The electronic structures of group III nitride and group IV-IV nanotubes indicate that the band gap increases with tube diameter contrary to behavior expected from quantum size effects. A detailed study indicates that, in a class of binary nanotubes with partial ionic contributions in the bonds, for example, AlN, BN, GaN, GeC, and SiC, ionicity of the bonds decreases as diameter decreases due to increased sp^3 contribution. This causes the band gap to increase with diameter. But in nanotubes with covalent bonding, for example SiGe, the gap decreases with diameter. A general trend for a class of binary nanotubes is established.

  1. Surface band gaps and superexchange interaction in transition metal oxides

    SciTech Connect

    Pothuizen, J.J.M.; Cohen, O.; Sawatzky, G.A.

    1996-11-01

    In this paper, the authors discuss the change in the band gap of charge transfer insulators for different surface terminations. They have calculated the Madelung potential of the unreconstructed (100) surfaces of the rocksalt structured TM-O compounds (TM = Mn, Fe, Co and Ni). They also considered possible step defects on a (100) surface. The presented results are calculated in both the purely ionic case (TM{sup 2+}O{sup 2{minus}}) and in the strong ligand p - cation 4s,p hybridization (TM{sup 1+}O{sup 1{minus}}) case. In both cases the charge transfer gap, {Delta}, for the surface is reduced compared to the bulk value. As a consequence of this reduction there is a large increase of the surface superexchange interaction, J{sub sur}, and a decrease of the band gap.

  2. Band gap anomaly and topological properties in lead chalcogenides

    NASA Astrophysics Data System (ADS)

    Simin, Nie; Xiao, Yan Xu; Gang, Xu; Zhong, Fang

    2016-03-01

    Band gap anomaly is a well-known issue in lead chalcogenides PbX (X = S, Se, Te, Po). Combining ab initio calculations and tight-binding (TB) method, we have studied the band evolution in PbX, and found that the band gap anomaly in PbTe is mainly related to the high on-site energy of Te 5s orbital and the large s-p hopping originated from the irregular extended distribution of Te 5s electrons. Furthermore, our calculations show that PbPo is an indirect band gap (6.5 meV) semiconductor with band inversion at L point, which clearly indicates that PbPo is a topological crystalline insulator (TCI). The calculated mirror Chern number and surface states double confirm this conclusion. Project supported by the National Natural Science Foundation of China (Grant No. 11204359), the National Basic Research Program of China (Grant No. 2013CB921700), and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB07020100).

  3. Continuously controlled optical band gap in oxide semiconductor thin films

    DOE PAGESBeta

    Herklotz, Andreas; Rus, Stefania Florina; Ward, Thomas Zac

    2016-02-02

    The optical band gap of the prototypical semiconducting oxide SnO2 is shown to be continuously controlled through single axis lattice expansion of nanometric films induced by low-energy helium implantation. While traditional epitaxy-induced strain results in Poisson driven multidirectional lattice changes shown to only allow discrete increases in bandgap, we find that a downward shift in the band gap can be linearly dictated as a function of out-of-plane lattice expansion. Our experimental observations closely match density functional theory that demonstrates that uniaxial strain provides a fundamentally different effect on the band structure than traditional epitaxy-induced multiaxes strain effects. In conclusion, chargemore » density calculations further support these findings and provide evidence that uniaxial strain can be used to drive orbital hybridization inaccessible with traditional strain engineering techniques.« less

  4. Robust band gap of TiS3 nanofilms.

    PubMed

    Kang, Jun; Wang, Lin-Wang

    2016-06-01

    First-principles calculations have been performed on the band structure of mono- and few-layer TiS3 nanofilms. It is found that the band gap character of the TiS3 films is quite robust, almost independent of layer thickness, vertical strain and stacking order, which is in sharp contrast to most other two-dimensional materials, such as MoS2. The robustness of the band gap originates from the location of the CBM and VBM states, which are at the center atoms of TiS3, and are thus unaffected by the layer-layer coupling. Such a property of TiS3 nanofilms promises good application potential in nanoelectronics and optoelectronics, and also makes TiS3 a good platform to study the electronic properties of a material in the two-dimensional limit. PMID:27029227

  5. Band gaps of two-dimensional antiferromagnetic photonic crystal

    NASA Astrophysics Data System (ADS)

    Song, Yu-Ling; Ta, Jin-Xing; Wang, Xuan-Zhang

    2011-07-01

    In an external magnetic field, the band structure of a two-dimensional photonic crystal (PC) composed of parallel antiferromagnetic cylinders in a background dielectric is investigated with a Green's function method. The cylinders with two resonant frequencies form a square lattice and are characterized by a magnetic permeability tensor. In our numerical calculation, we find that this method allows fast convergence and is available in both the resonant and non-resonant frequency ranges. In the non-resonant range, the PC is similar in band structure to an ordinary dielectric PC. Two electromagnetic band gaps, however, appear in the resonant frequency region, and their frequency positions and widths are governed by the external field. The dependence of the electromagnetic gaps on the cylinder radius also is discussed.

  6. Continuously Controlled Optical Band Gap in Oxide Semiconductor Thin Films.

    PubMed

    Herklotz, Andreas; Rus, Stefania Florina; Ward, Thomas Zac

    2016-03-01

    The optical band gap of the prototypical semiconducting oxide SnO2 is shown to be continuously controlled through single axis lattice expansion of nanometric films induced by low-energy helium implantation. While traditional epitaxy-induced strain results in Poisson driven multidirectional lattice changes shown to only allow discrete increases in bandgap, we find that a downward shift in the band gap can be linearly dictated as a function of out-of-plane lattice expansion. Our experimental observations closely match density functional theory that demonstrates that uniaxial strain provides a fundamentally different effect on the band structure than traditional epitaxy-induced multiaxes strain effects. Charge density calculations further support these findings and provide evidence that uniaxial strain can be used to drive orbital hybridization inaccessible with traditional strain engineering techniques. PMID:26836282

  7. Band-gap and band-edge engineering of multicomponent garnet scintillators from first principles

    DOE PAGESBeta

    Yadav, Satyesh K.; Uberuaga, Blas P.; Nikl, Martin; Jiang, Chao; Stanek, Christopher R.

    2015-11-24

    Complex doping schemes in R3Al5O12 (where R is the rare-earth element) garnet compounds have recently led to pronounced improvements in scintillator performance. Specifically, by admixing lutetium and yttrium aluminate garnets with gallium and gadolinium, the band gap is altered in a manner that facilitates the removal of deleterious electron trapping associated with cation antisite defects. Here, we expand upon this initial work to systematically investigate the effect of substitutional admixing on the energy levels of band edges. Density-functional theory and hybrid density-functional theory (HDFT) are used to survey potential admixing candidates that modify either the conduction-band minimum (CBM) or valence-bandmore » maximum (VBM). We consider two sets of compositions based on Lu3B5O12 where B is Al, Ga, In, As, and Sb, and R3Al5O12, where R is Lu, Gd, Dy, and Er. We find that admixing with various R cations does not appreciably affect the band gap or band edges. In contrast, substituting Al with cations of dissimilar ionic radii has a profound impact on the band structure. We further show that certain dopants can be used to selectively modify only the CBM or the VBM. Specifically, Ga and In decrease the band gap by lowering the CBM, while As and Sb decrease the band gap by raising the VBM, the relative change in band gap is quantitatively validated by HDFT. These results demonstrate a powerful approach to quickly screen the impact of dopants on the electronic structure of scintillator compounds, identifying those dopants which alter the band edges in very specific ways to eliminate both electron and hole traps responsible for performance limitations. Furthermore, this approach should be broadly applicable for the optimization of electronic and optical performance for a wide range of compounds by tuning the VBM and CBM.« less

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

    PubMed

    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). PMID:27428304

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

    SciTech Connect

    Noble, Robert J.; Spencer, James E.; 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.

  10. Approach to band gap alignment in confined semiconductors

    NASA Astrophysics Data System (ADS)

    Goldbach, Andreas; Saboungi, Marie-Louise; Iton, Lennox E.; Price, David L.

    2001-12-01

    Se has been sorbed into the pore system of Cu2+-ion exchanged Y zeolite to investigate the effect of transition metal countercations on the structure and electronic properties of the incorporated semiconductor. Results from anomalous x-ray scattering experiments at the Se K-absorption edge and Raman measurements show the formation of isolated, strongly disordered Sex chains (x→∞) with an intrachain bond length of 2.39 Å. The difference pair correlation function TSe(r) exhibits a short-range pair correlation at 3.30 Å, which is attributed to specific Cu2+-Sex interactions. The optical spectrum of the composite is characterized by a strong absorption band around 400 nm with a shoulder at 450 nm and an optical band gap of 2.09 eV. The strong Cu2+-Sex correlation is explained by charge transfer from the Sex chains to the cations, and the formation of band gap states is postulated to account for the unusually long intrachain bond length and the remarkably small band gap.

  11. Ultrafast band-gap oscillations in iron pyrite

    SciTech Connect

    Kolb, B; Kolpak, AM

    2013-12-20

    With its combination of favorable band gap, high absorption coefficient, material abundance, and low cost, iron pyrite, FeS2, has received a great deal of attention over the past decades as a promising material for photovoltaic applications such as solar cells and photoelectrochemical cells. Devices made from pyrite, however, exhibit open circuit voltages significantly lower than predicted, and despite a recent resurgence of interest in the material, there currently exists no widely accepted explanation for this disappointing behavior. In this paper, we show that phonons, which have been largely overlooked in previous efforts, may play a significant role. Using fully self-consistent GW calculations, we demonstrate that a phonon mode related to the oscillation of the sulfur-sulfur bond distance in the pyrite structure is strongly coupled to the energy of the conduction-band minimum, leading to an ultrafast (approximate to 100 fs) oscillation in the band gap. Depending on the coherency of the phonons, we predict that this effect can cause changes of up to +/- 0.3 eV relative to the accepted FeS2 band gap at room temperature. Harnessing this effect via temperature or irradiation with infrared light could open up numerous possibilities for novel devices such as ultrafast switches and adaptive solar absorbers.

  12. Band-Gap Engineering of Carbon Nanotubes with Grain Boundaries

    SciTech Connect

    Wang, Zhiguo; Zhou, Yungang; Zhang, Yanwen; Gao, Fei

    2012-01-26

    Structure and electronic properties of carbon nanotubes (CNTs) with grain boundaries (GBs) are investigated using density-functional calculations, where the GBs parallel and perpendicular to the tube axis are considered. Simulation results show that the GBs have a great effect on the electronic properties of the CNTs. For the GBs along the tube axis, the CNTs are narrow or zero band gap (<0.16 eV) materials, independent of the misorientation angle and diameter. For the GBs perpendicular to the tube axis, localized electronic states appear within the GBs regions, leading to a larger band gap of up to 0.6 eV. It is convenient to transport and localize the electrons and holes by engineering the GBs. These findings are of great significance for developing carbon-based nanomaterials and electronic devices.

  13. Band-Gap Engineering of Carbon Nanotubes with Grain Boundaries

    SciTech Connect

    Wang, Zhiguo; Zhou, Yungang; Zhang, Yanwen; Gao, Fei

    2011-01-01

    Structure and electronic properties of carbon nanotubes (CNTs) with grain boundaries (GBs) are investigated using density-functional calculations, where the GBs parallel and perpendicular to the tube axis are considered. Simulation results show that the GBs have a great effect on the electronic properties of the CNTs. For the GBs along the tube axis, the CNTs are narrow or zero band gap (<0.16 eV) materials, independent of the misoritentaion angle and diameter. For the GBs perpendicular to the tube axis, localized electronic states appear within the GBs regions, leading to a larger band gap of up to 0.6 eV. It is convenient to transport and localize the electrons and holes by engineering the GBs. These findings are of great significance for developing carbon-based nanomaterials and electronic devices.

  14. Band gap engineering of MoS2 upon compression

    NASA Astrophysics Data System (ADS)

    López-Suárez, Miquel; Neri, Igor; Rurali, Riccardo

    2016-04-01

    Molybdenum disulfide (MoS2) is a promising candidate for 2D nanoelectronic devices, which shows a direct band-gap for monolayer structure. In this work we study the electronic structure of MoS2 upon both compressive and tensile strains with first-principles density-functional calculations for different number of layers. The results show that the band-gap can be engineered for experimentally attainable strains (i.e., ±0.15). However, compressive strain can result in bucking that can prevent the use of large compressive strain. We then studied the stability of the compression, calculating the critical strain that results in the on-set of buckling for free-standing nanoribbons of different lengths. The results demonstrate that short structures, or few-layer MoS2, show semi-conductor to metal transition upon compressive strain without bucking.

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

    PubMed

    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. PMID:22506610

  16. Direct band gap carbon superlattices with efficient optical transition

    NASA Astrophysics Data System (ADS)

    Oh, Young Jun; Kim, Sunghyun; Lee, In-Ho; Lee, Jooyoung; Chang, K. J.

    2016-02-01

    We report pure carbon-based superlattices that exhibit direct band gaps and excellent optical absorption and emission properties at the threshold energy. The structures are nearly identical to that of cubic diamond except that defective layers characterized by five- and seven-membered rings are intercalated in the diamond lattice. The direct band gaps lie in the range of 5.6-5.9 eV, corresponding to wavelengths of 210-221 nm. The dipole matrix elements of direct optical transition are comparable to that of GaN, suggesting that the superlattices are promising materials as an efficient deep ultraviolet light emitter. Molecular dynamics simulations show that the superlattices are thermally stable even at a high temperature of 2000 K. We provide a possible route to the synthesis of superlattices through wafer bonding of diamond (100) surfaces.

  17. Photonic Band Gap resonators for high energy accelerators

    SciTech Connect

    Schultz, S.; Smith, D.R.; Kroll, N. |

    1993-12-31

    We have proposed that a new type of microwave resonator, based on Photonic Band Gap (PBG) structures, may be particularly useful for high energy accelerators. We provide an explanation of the PBG concept and present data which illustrate some of the special properties associated with such structures. Further evaluation of the utility of PBG resonators requires laboratory testing of model structures at cryogenic temperatures, and at high fields. We provide a brief discussion of our test program, which is currently in progress.

  18. Twisted bilayer blue phosphorene: A direct band gap semiconductor

    NASA Astrophysics Data System (ADS)

    Ospina, D. A.; Duque, C. A.; Correa, J. D.; Suárez Morell, Eric

    2016-09-01

    We report that two rotated layers of blue phosphorene behave as a direct band gap semiconductor. The optical spectrum shows absorption peaks in the visible region of the spectrum and in addition the energy of these peaks can be tuned with the rotational angle. These findings makes twisted bilayer blue phosphorene a strong candidate as a solar cell or photodetection device. Our results are based on ab initio calculations of several rotated blue phosphorene layers.

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

    PubMed

    Klimm, Detlef

    2014-09-01

    The development of semiconductor electronics is reviewed briefly, beginning with the development of germanium devices (band gap E g = 0.66 eV) after World War II. A tendency towards alternative materials with wider band gaps quickly became apparent, starting with silicon (E g = 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 E g 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. PMID:25295170

  20. Two novel silicon phases with direct band gaps.

    PubMed

    Fan, Qingyang; Chai, Changchun; Wei, Qun; Yang, Yintang

    2016-05-14

    Due to its abundance, silicon is the preferred solar-cell material despite the fact that many silicon allotropes have indirect band gaps. Elemental silicon has a large impact on the economy of the modern world and is of fundamental importance in the technological field, particularly in the solar cell industry. Looking for direct band gap silicon is still an important field in material science. Based on density function theory with the ultrasoft pseudopotential scheme in the frame of the local density approximation and the generalized gradient approximation, we have systematically studied the structural stability, absorption spectra, electronic, optical and mechanical properties and minimum thermal conductivity of two novel silicon phases, Cm-32 silicon and P21/m silicon. These are both thermally, dynamically and mechanically stable. The absorption spectra of Cm-32 silicon and P21/m silicon exhibit significant overlap with the solar spectrum and thus, excellent photovoltaic efficiency with great improvements over Fd3[combining macron]m Si. These two novel Si structures with direct band gaps could be applied in single p-n junction thin-film solar cells or tandem photovoltaic devices. PMID:27104737

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

    PubMed Central

    Klimm, Detlef

    2014-01-01

    The development of semiconductor electronics is reviewed briefly, beginning with the development of germanium devices (band gap E g = 0.66 eV) after World War II. A tendency towards alternative materials with wider band gaps quickly became apparent, starting with silicon (E g = 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 E g 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. PMID:25295170

  2. Implications of mercury interactions with band-gap semiconductor oxides

    SciTech Connect

    Granite, E.J.; King, W.P.; Stanko, D.C.; Pennline, H.W.

    2008-09-01

    Titanium dioxide is a well-known photooxidation catalyst. It will oxidize mercury in the presence of ultraviolet light from the sun and oxygen and/or moisture to form mercuric oxide. Several companies manufacture self-cleaning windows. These windows have a transparent coating of titanium dioxide. The titanium dioxide is capable of destroying organic contaminants in air in the presence of ultraviolet light from the sun, thereby keeping the windows clean. The commercially available self-cleaning windows were used to sequester mercury from oxygen–nitrogen mixtures. Samples of the self-cleaning glass were placed into specially designed photo-reactors in order to study the removal of elemental mercury from oxygen–nitrogen mixtures resembling air. The possibility of removing mercury from ambient air with a self-cleaning glass apparatus is examined. The intensity of 365-nm ultraviolet light was similar to the natural intensity from sunlight in the Pittsburgh region. Passive removal of mercury from the air may represent an option in lieu of, or in addition to, point source clean-up at combustion facilities. There are several common band-gap semiconductor oxide photocatalysts. Sunlight (both the ultraviolet and visible light components) and band-gap semiconductor particles may have a small impact on the global cycle of mercury in the environment. The potential environmental consequences of mercury interactions with band-gap semiconductor oxides are discussed. Heterogeneous photooxidation might impact the global transport of elemental mercury emanating from flue gases.

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

    PubMed

    Lim, A; Foulkes, W M C; Horsfield, A P; Mason, D R; Schleife, A; Draeger, E W; Correa, A A

    2016-01-29

    We use time-dependent density functional theory to study self-irradiated Si. We calculate the electronic stopping power of Si in Si by evaluating the energy transferred to the electrons per unit path length by an ion of kinetic energy from 1 eV to 100 keV moving through the host. Electronic stopping is found to be significant below the threshold velocity normally identified with transitions across the band gap. A structured crossover at low velocity exists in place of a hard threshold. An analysis of the time dependence of the transition rates using coupled linear rate equations enables one of the excitation mechanisms to be clearly identified: a defect state induced in the gap by the moving ion acts like an elevator and carries electrons across the band gap. PMID:26871327

  4. Band-gap and band-edge engineering of multicomponent garnet scintillators from first principles

    SciTech Connect

    Yadav, Satyesh K.; Uberuaga, Blas P.; Nikl, Martin; Jiang, Chao; Stanek, Christopher R.

    2015-11-24

    Complex doping schemes in R3Al5O12 (where R is the rare-earth element) garnet compounds have recently led to pronounced improvements in scintillator performance. Specifically, by admixing lutetium and yttrium aluminate garnets with gallium and gadolinium, the band gap is altered in a manner that facilitates the removal of deleterious electron trapping associated with cation antisite defects. Here, we expand upon this initial work to systematically investigate the effect of substitutional admixing on the energy levels of band edges. Density-functional theory and hybrid density-functional theory (HDFT) are used to survey potential admixing candidates that modify either the conduction-band minimum (CBM) or valence-band maximum (VBM). We consider two sets of compositions based on Lu3B5O12 where B is Al, Ga, In, As, and Sb, and R3Al5O12, where R is Lu, Gd, Dy, and Er. We find that admixing with various R cations does not appreciably affect the band gap or band edges. In contrast, substituting Al with cations of dissimilar ionic radii has a profound impact on the band structure. We further show that certain dopants can be used to selectively modify only the CBM or the VBM. Specifically, Ga and In decrease the band gap by lowering the CBM, while As and Sb decrease the band gap by raising the VBM, the relative change in band gap is quantitatively validated by HDFT. These results demonstrate a powerful approach to quickly screen the impact of dopants on the electronic structure of scintillator compounds, identifying those dopants which alter the band edges in very specific ways to eliminate both electron and hole traps responsible for performance limitations. Furthermore, this approach should be broadly applicable for the optimization of electronic and optical performance for a wide range of compounds by tuning the VBM and CBM.

  5. Band gap engineering via doping: A predictive approach

    SciTech Connect

    Andriotis, Antonis N.; Menon, Madhu

    2015-03-28

    We employ an extension of Harrison's theory at the tight binding level of approximation to develop a predictive approach for band gap engineering involving isovalent doping of wide band gap semiconductors. Our results indicate that reasonably accurate predictions can be achieved at qualitative as well as quantitative levels. The predictive results were checked against ab initio ones obtained at the level of DFT/SGGA + U approximation. The minor disagreements between predicted and ab initio results can be attributed to the electronic processes not incorporated in Harrison's theory. These include processes such as the conduction band anticrossing [Shan et al., Phys. Rev. Lett. 82, 1221 (1999); Walukiewicz et al., Phys. Rev. Lett. 85, 1552 (2000)] and valence band anticrossing [Alberi et al., Phys. Rev. B 77, 073202 (2008); Appl. Phys. Lett. 92, 162105 (2008); Appl. Phys. Lett. 91, 051909 (2007); Phys. Rev. B 75, 045203 (2007)], as well as the multiorbital rehybridization. Another cause of disagreement between the results of our predictive approach and the ab initio ones is shown to be the result of the shift of Fermi energy within the impurity band formed at the edge of the valence band maximum due to rehybridization. The validity of our approach is demonstrated with example applications for the systems GaN{sub 1−x}Sb{sub x}, GaP{sub 1−x}Sb{sub x}, AlSb{sub 1−x}P{sub x}, AlP{sub 1−x}Sb{sub x}, and InP{sub 1−x}Sb{sub x}.

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

    NASA Astrophysics Data System (ADS)

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

    2013-10-01

    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.

  7. Substrate effect on the band gap of semiconducting atomic wires

    NASA Astrophysics Data System (ADS)

    Simbeck, Adam J.; Nayak, Saroj K.

    2014-03-01

    The electronic structure of free-standing and supported semiconducting atomic wires is investigated using a combination of first-principles density functional theory (DFT) and many-body perturbation theory (MBPT). The band gaps predicted from DFT for SiH2 and GeH2 atomic wires are unaffected by the presence of the substrate, whereas the gaps calculated using MBPT under the GW approximation are reduced by about 1eV when the wires are supported. The reduction in the band gap is attributed to a change in the electronic correlation energy, which can be understood as a screened Coulomb interaction. These results highlight the importance of the role played by the substrate in manipulating the electronic and optical properties of quantum confined Si and Ge systems. Work supported by the Interconnect Focus Center (MARCO program), State of New York, NSF IGERT Program, Grant no. 0333314, NSF Petascale Simulations and Analysis (PetaApps) program, Grant No. 0749140, and computing resources of the CCNI at RPI.

  8. Identical band gaps in structurally re-entrant honeycombs.

    PubMed

    Zhu, Zhu-Wei; Deng, Zi-Chen

    2016-08-01

    Structurally re-entrant honeycomb is a sort of artificial lattice material, characterized by star-like unit cells with re-entrant topology, as well as a high connectivity that the number of folded sheets jointing at each vertex is at least six. In-plane elastic wave propagation in this highly connected honeycomb is investigated through the application of the finite element method in conjunction with the Bloch's theorem. Attention is devoted to exploring the band characteristics of two lattice configurations with different star-like unit cells, defined as structurally square re-entrant honeycomb (SSRH) and structurally hexagonal re-entrant honeycomb (SHRH), respectively. Identical band gaps involving their locations and widths, interestingly, are present in the two considered configurations, attributed to the resonance of the sketch folded sheets, the basic component elements for SSRH and SHRH. In addition, the concept of heuristic models is implemented to elucidate the underlying physics of the identical gaps. The phenomenon of the identical bandgaps is not only beneficial for people to further explore the band characteristics of lattice materials, but also provides the structurally re-entrant honeycombs as potential host structures for the design of lattice-based metamaterials of interest for elastic wave control. PMID:27586722

  9. The Band-Gap and TRUE Band-Gap in Nominally Metallic Carbon Nanotubes: the Tight-Binding Study on Corrugation Effect

    NASA Astrophysics Data System (ADS)

    Lu, Hongxia; Wu, Jianbao; Wang, Jizhen; Shi, Shaocong; Zhang, Weiyi

    2014-11-01

    In this paper, the band-gap and true band-gap are analyzed for the corrugated structures of various types of single wall carbon nanotubes (SWCNTs) within the tight binding approximation. We show that corrugation, combined with curvature effect, yields naturally the true small band-gap in all SWCNTs with small radius. The more stable corrugated structures of SWCNTs are backed by the abinitio total energy calculations for nominally metallic armchair SWCNTs.

  10. Electronic band gaps and transport in Cantor graphene superlattices

    NASA Astrophysics Data System (ADS)

    Xu, Yi; He, Ying; Yang, Yanfang; Zhang, Huifang

    2015-04-01

    The electronic band gap and transport in Cantor graphene superlattices are investigated theoretically. It is found that such fractal structure can possess an unusual Dirac point located at the energy corresponding to the zero-averaged wave number (zero- k ‾) . The location of the Dirac point shifts to lower energy with the increase of order number. The zero- k ‾ gap is robust against the lattice constants and less sensitive to the incidence angle. Moreover, multi-Dirac-points may appear by adjusting the lattice constants and the order, and an expression for their location is derived. The control of electron transport in such fractal structure may lead to potential applications in graphene-based electronic devices.

  11. Phononic and photonic band gap structures: modelling and applications

    NASA Astrophysics Data System (ADS)

    Armenise, Mario N.; Campanella, Carlo E.; Ciminelli, Caterina; Dell'Olio, Francesco; Passaro, Vittorio M. N.

    2010-01-01

    Photonic crystals (PhCs) are artificial materials with a permittivity which is a periodic function of the position, with a period comparable to the wavelength of light. The most interesting characteristic of such materials is the presence of photonic band gaps (PBGs). PhCs have very interesting properties of light confinement and localization together with the strong reduction of the device size, orders of magnitude less than the conventional photonic devices, allowing a potential very high scale of integration. These structures possess unique characteristics enabling to operate as optical waveguides, high Q resonators, selective filters, lens or superprism. The ability to mould and guide light leads naturally to novel applications in several fields. Band gap formation in periodic structures also pertains to elastic wave propagation. Composite materials with elastic coefficients which are periodic functions of the position are named phononic crystals. They have properties similar to those of photonic crystals and corresponding applications too. By properly choosing the parameters one may obtain phononic crystals (PhnCs) with specific frequency gaps. An elastic wave, whose frequency lies within an absolute gap of a phononic crystal, will be completely reflected by it. This property allows realizing non-absorbing mirrors of elastic waves and vibration-free cavities which might be useful in high-precision mechanical systems operating in a given frequency range. Moreover, one can use elastic waves to study phenomena such as those associated with disorder, in more or less the same manner as with electromagnetic waves. The authors present in this paper an introductory survey of the basic concepts of these new technologies with particular emphasis on their main applications, together with a description of some modelling approaches.

  12. Behaviour of hydrogen in wide band gap oxides

    SciTech Connect

    Li, H.; Robertson, J.

    2014-05-28

    The defect formation energies and atomic geometries of interstitial hydrogen in its different charge states in a number of wide band gap oxides are calculated by the Heyd, Scuseria, Ernzerhof hybrid functional. As in semiconductors, two behaviours are found, it acts either as an amphoteric defect or as a shallow donor. There are large scale lattice relaxations between the different charge states for the case of the amphoteric defect. Interestingly, we find that the +/− transition level does have a good alignment below the vacuum level, as was found previously for tetrahedral semiconductors.

  13. Behaviour of hydrogen in wide band gap oxides

    NASA Astrophysics Data System (ADS)

    Li, H.; Robertson, J.

    2014-05-01

    The defect formation energies and atomic geometries of interstitial hydrogen in its different charge states in a number of wide band gap oxides are calculated by the Heyd, Scuseria, Ernzerhof hybrid functional. As in semiconductors, two behaviours are found, it acts either as an amphoteric defect or as a shallow donor. There are large scale lattice relaxations between the different charge states for the case of the amphoteric defect. Interestingly, we find that the +/- transition level does have a good alignment below the vacuum level, as was found previously for tetrahedral semiconductors.

  14. Nonlinear Bloch waves in metallic photonic band-gap filaments

    SciTech Connect

    Kaso, Artan; John, Sajeev

    2007-11-15

    We demonstrate the occurrence of nonlinear Bloch waves in metallic photonic crystals (PCs). These periodically structured filaments are characterized by an isolated optical pass band below an effective plasma gap. The pass band occurs in a frequency range where the metallic filament exhibits a negative, frequency-dependent dielectric function and absorption loss. The metallic losses are counterbalanced by gain in two models of inhomogeneously broadened nonlinear oscillators. In the first model, we consider close-packed quantum dots that fill the void regions of a two-dimensional (2D) metallic PC, and whose inhomogeneously broadened emission spectrum spans the original optical pass band of the bare filament. In the second model, we consider thin (10-50 nm) layers of inhomogeneously broadened two-level resonators, with large dipole oscillator strength, that cover the interior surfaces of 2D metallic (silver and tungsten) PCs. These may arise from localized surface plasmon resonances due to small metal particles or an otherwise rough metal surface. For simplicity, we treat electromagnetic modes with electric field perpendicular to the plane of metal periodicity. In both models, a pumping threshold of the resonators is found, above which periodic nonlinear solutions of Maxwell's equations with purely real frequency within the optical pass band emerge. These nonlinear Bloch waves exhibit a laserlike input pumping to output amplitude characteristic. For strong surface resonances, these nonlinear waves may play a role in light emission from a hot tungsten (suitably microstructured) filament.

  15. Plasmon-pole models affect band gaps in GW calculations

    NASA Astrophysics Data System (ADS)

    Larson, Paul; Wu, Zhigang

    2013-03-01

    Density functional theory calculations have long been known to underestimate the band gaps in semiconductors. Significant improvements have been made by using GW calculations that uses the self energy, defined as the product of the Green function (G) and screened Coulomb exchange (W). However, many approximations are made in the GW method, specifically the plasmon-pole approximation. This approximation replaces the integration necessary to produce W with a simple approximation to the inverse dielectric function. Four different plasmon-pole approximations have been tested using the tight-binding program ABINIT: Godby-Needs, Hybertsen-Louie, von der Linden-Horsch, and Engel-Farid. For many materials, the differences in the GW band gaps for the different plasmon-pole models are negligible, but for systems with localized electrons, the difference can be larger than 1 eV. The plasmon-pole approximation is generally chosen to best agree with experimental data, but this is misleading in that this ignores all of the other approximations used in the GW method. Improvements in plasmon-pole models in GW can only come about by trying to reproduce the results of the numerical integration rather than trying to reproduce experimental results.

  16. Graded band gap GaInNAs solar cells

    SciTech Connect

    Langer, F.; Perl, S.; Kamp, M.; Höfling, S.

    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.

  17. Uncover the electroluminescence in wide band gap polymers

    NASA Astrophysics Data System (ADS)

    Qiao, B.; Teyssedre, G.; Laurent, C.

    2015-10-01

    Due to the rapidly increasing demand of electric power, insulating materials used in electrical components are pushed up to their limits, where their electronic properties are of fundamental importance. Electroluminescence provides an elegant way to investigate electronic properties, high field effects and electrical ageing of polymers although the emission spectrum is still poorly understood. Unlike in organic semi-conductors, electroluminescence spectra of large band gap polymers exhibit specific spectral features that cannot be interpreted on the basis of the photo-physical properties of the material. By irradiating polypropylene thin films with electrons up to a few keV and by analyzing the emitted light, we were able to isolate the elementary components of the emission and to reconstruct the electroluminescence spectrum. For the first time, a comprehensive study of electroluminescence in polymers is provided and the underlying mechanisms of the emission are discussed. The results herein provide an univocal demonstration that the electroluminescence from wide band gap polymers results in part from chemical reactions, opening the way to the diagnosis and prognosis of polymeric materials under electrical stress.

  18. Computational investigation on tunable optical band gap in armchair polyacenes

    SciTech Connect

    Das, Mousumi

    2015-08-14

    Polyacenes in their armchair geometry (phenacenes) have recently been found to possess appealing electronic and optical properties with higher chemical stability and comparatively larger band gap as compared to linear polyacenes. They also behave as high-temperature superconductors upon alkali metal doping. Moreover, the optical properties of crystalline picene can be finely tuned by applying external pressure. We investigated the variation of optical gap as a function of altering the interplanar distances between parallel cofacial phenacene dimers. We employed both time-dependent density functional theory and density matrix renormalization group (DMRG) technique to investigate the lowest singlet excitations in phenacene dimer. Our study showed that the lowest singlet excitation in these systems evolved as a function of interplanar separation. The optical excitation energy gap decreases as a function of inverse interplanar separation of the phenacene dimer. The distant dependent variation of optical absorption at the dimer level may be comparable with experimental observation in picene crystal under pressure. DMRG study also demonstrates that besides picene, electronic properties of higher phenacenes can also be tunable by altering interplanar separation.

  19. Transport band gap opening at metal–organic interfaces

    SciTech Connect

    Haidu, Francisc Salvan, Georgeta; Zahn, Dietrich R. T.; Smykalla, Lars; Hietschold, Michael; Knupfer, Martin

    2014-07-01

    The interface formation between copper phthalocyanine (CuPc) and two representative metal substrates, i.e., Au and Co, was investigated by the combination of ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy. The occupied and unoccupied molecular orbitals and thus the transport band gap of CuPc are highly influenced by film thickness, i.e., molecule substrate distance. Due to the image charge potential given by the metallic substrates the transport band gap of CuPc “opens” from (1.4 ± 0.3) eV for 1 nm thickness to (2.2 ± 0.3) eV, and saturates at this value above 10 nm CuPc thickness. The interface dipoles with values of 1.2 eV and 1.0 eV for Au and Co substrates, respectively, predominantly depend on the metal substrate work functions. X-ray photoelectron spectroscopy measurements using synchrotron radiation provide detailed information on the interaction between CuPc and the two metal substrates. While charge transfer from the Au or Co substrate to the Cu metal center is present only at sub-monolayer coverages, the authors observe a net charge transfer from the molecule to the Co substrate for films in the nm range. Consequently, the Fermi level is shifted as in the case of a p-type doping of the molecule. This is, however, a competing phenomenon to the energy band shifts due to the image charge potential.

  20. Preprophase band formation and cortical division zone establishment: RanGAP behaves differently from microtubules during their band formation

    PubMed Central

    Yabuuchi, Takatoshi; Nakai, Tomonori; Sonobe, Seiji; Yamauchi, Daisuke; Mineyuki, Yoshinobu

    2015-01-01

    Correct positioning of the division plane is a prerequisite for plant morphogenesis. The preprophase band (PPB) is a key intracellular structure of division site determination. PPB forms in G2 phase as a broad band of microtubules (MTs) that narrows in prophase and specializes few-micrometer-wide cortical belt region, named the cortical division zone (CDZ), in late prophase. The PPB comprises several molecules, some of which act as MT band organization and others remain in the CDZ marking the correct insertion of the cell plate in telophase. Ran GTPase-activating protein (RanGAP) is accumulated in the CDZ and forms a RanGAP band in prophase. However, little is known about when and how RanGAPs gather in the CDZ, and especially with regard to their relationships to MT band formation. Here, we examined the spatial and temporal distribution of RanGAPs and MTs in the preprophase of onion root tip cells using confocal laser scanning microscopy and showed that the RanGAP band appeared in mid-prophase as the width of MT band was reduced to nearly 7 µm. Treatments with cytoskeletal inhibitors for 15 min caused thinning or broadening of the MT band but had little effects on RanGAP band in mid-prophase and most of late prophase cells. Detailed image analyses of the spatial distribution of RanGAP band and MT band showed that the RanGAP band positioned slightly beneath the MT band in mid-prophase. These results raise a possibility that RanGAP behaves differently from MTs during their band formation. PMID:26237087

  1. Two-Dimensional Phosphorus Porous Polymorphs with Tunable Band Gaps.

    PubMed

    Zhuo, Zhiwen; Wu, Xiaojun; Yang, Jinlong

    2016-06-01

    Exploring stable two-dimensional (2D) crystalline structures of phosphorus with tunable properties is of considerable importance partly due to the novel anisotropic behavior in phosphorene and potential applications in high-performance devices. Here, 21 new 2D phosphorus allotropes with porous structure are reported based on topological modeling method and first-principles calculations. We establish that stable 2D phosphorus crystals can be obtained by topologically assembling selected phosphorus monomer, dimer, trimer, tetramer, and hexamer. Nine of reported structures are predicted to be more stable than white phosphorus. Their dynamic and thermal stabilities are confirmed by the calculated vibration spectra and Born-Oppenheimer molecular dynamic simulation at temperatures up to 1500 K. These phosphorus porous polymorphs have isotropic mechanic properties that are significantly softer than phosphorene. The electronic band structures calculated with the HSE06 method indicate that new structures are semiconductors with band gaps ranging widely from 0.15 to 3.42 eV, which are tuned by the basic units assembled in the network. Of particular importance is that the position of both conduction and valence band edges of some allotropes matches well with the chemical reaction potential of H2/H(+) and O2/H2O, which can be used as element photocatalysts for visible-light-driven water splitting. PMID:27171121

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

    SciTech Connect

    Gong, Yongji; Liu, Zheng; Lupini, Andrew R; Lin, Junhao; Pantelides, Sokrates T; Pennycook, Stephen J; Zhou, Wu; Ajayan, Pullikel M

    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.

  3. Inter-band optoelectronic properties in quantum dot structure of low band gap III-V semiconductors

    SciTech Connect

    Dey, Anup; Maiti, Biswajit; Chanda, Debasree

    2014-04-14

    A generalized theory is developed to study inter-band optical absorption coefficient (IOAC) and material gain (MG) in quantum dot structures of narrow gap III-V compound semiconductor considering the wave-vector (k{sup →}) dependence of the optical transition matrix element. The band structures of these low band gap semiconducting materials with sufficiently separated split-off valance band are frequently described by the three energy band model of Kane. This has been adopted for analysis of the IOAC and MG taking InAs, InSb, Hg{sub 1−x}Cd{sub x}Te, and In{sub 1−x}Ga{sub x}As{sub y}P{sub 1−y} lattice matched to InP, as example of III–V compound semiconductors, having varied split-off energy band compared to their bulk band gap energy. It has been found that magnitude of the IOAC for quantum dots increases with increasing incident photon energy and the lines of absorption are more closely spaced in the three band model of Kane than those with parabolic energy band approximations reflecting the direct the influence of energy band parameters. The results show a significant deviation to the MG spectrum of narrow-gap materials having band nonparabolicity compared to the parabolic band model approximations. The results reflect the important role of valence band split-off energies in these narrow gap semiconductors.

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

    PubMed Central

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

    2015-01-01

    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. PMID:26289434

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

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

    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.

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

    PubMed

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

    2015-01-01

    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. PMID:26289434

  7. Measurement of photonic band diagram in non-crystalline photonic band gap (PBG) materials

    NASA Astrophysics Data System (ADS)

    Man, Weining; Williamson, Eric; Hashemizad, Seyed; Yadak, Polin; Florescu, Marian

    2011-03-01

    Non-crystalline PBG materials have received increasing attention recently and sizeable PBGs have been reported in quasi-crystalline structures or even in disordered structures. Band calculations for periodic structures produce accurate dispersion relations in them and refraction properties at their surfaces. However, band calculations for non-periodic structures employ large super-cells of N >100 building blocks, and provide little useful information other than the PBG frequency and width. Since band is folded into N bands, within the first Brillouin zone of the supper-cell. Using stereolithography, we construct various quasi-crystalline or disordered PBG materials and perform transmission measurements. The dispersion relations of EM wave (band diagrams) are reconstructed from the measured phase data. Our experiments not only verify the existence of sizeable PBGs in these structures, but also provide detailed information of the effective band diagrams, dispersion relation, group velocity vector, and their angular dependence. Slow light phenomena are also observed in these structures near gap frequencies. This study presents a powerful tool to investigate photonic properties of non-crystalline structures and provides important dispersion information, which is otherwise impossible to obtain.

  8. Single-junction solar cells with the optimum band gap for terrestrial concentrator applications

    DOEpatents

    Wanlass, Mark W.

    1994-01-01

    A single-junction solar cell having the ideal band gap for terrestrial concentrator applications. Computer modeling studies of single-junction solar cells have shown that the presence of absorption bands in the direct spectrum has the effect of "pinning" the optimum band gap for a wide range of operating conditions at a value of 1.14.+-.0.02 eV. Efficiencies exceeding 30% may be possible at high concentration ratios for devices with the ideal band gap.

  9. Recent progress on photonic band gap accelerator cavities

    SciTech Connect

    Smith, D.R.; Li, D.; Vier, D.C.

    1997-02-01

    We report on the current status of our program to apply Photonic Band Gap (PBG) concepts to produce novel high-energy, high-intensity accelerator cavities. The PBG design on which we have concentrated our initial efforts consists of a square array of metal cylinders, terminated by conducting or superconducting sheets, and surrounded by microwave absorber on the periphery of the structure. A removed cylinder from the center of the array constitutes a site defect where a localized electromagnetic mode can occur. In previous work, we have proposed that this structure could be utilized as an accelerator cavity, with advantageous properties over conventional cavity designs. In the present work, we present further studies, including MAFIA-based numerical calculations and experimental measurements, demonstrating the feasibility of using the proposed structure in a real accelerator application.

  10. Half-oxidized phosphorene: band gap and elastic properties modulation

    NASA Astrophysics Data System (ADS)

    Drissi, L. B.; Sadki, S.; Sadki, K.

    2016-04-01

    Based on a first principles approach, we study structural, electronic and elastic properties, as well as stabilities of all possible half-oxidized phosphorene conformers. Stability analysis reveals that oxygen chemisorption is an exothermic process in the six configurations despite the formation of interstitial oxygen bridges in three of them. Electronic structure calculations show that oxidation induces a band gap modulation ranging between 0.54 and 1.57 eV in the generalized gradient approximation corrected to 1.19 and 2.88 eV using GW. The mechanical response of the conformers is sensitively dependent on direction and indicates that the new derivatives are incompressible materials and one configuration has an auxetic behavior. The present results provide a basis for tailoring the electronic and elastic properties of phosphorene via half oxidation.

  11. Introducing Defects in Photonic Band-Gap (PBG) Crystals

    SciTech Connect

    Johnson, Elliott C.; /North Dakota State U. /SLAC

    2007-11-07

    Photonic Band-Gap (PBG) fibers are a periodic array of optical materials arranged in a lattice called a photonic crystal. The use of PBG fibers for particle acceleration is being studied by the Advanced Accelerator Research Department (AARD) at SLAC. By introducing defects in such fibers, e.g. removing one or more capillaries from a hexagonal lattice, spatially confined modes suitable for particle acceleration may be created. The AARD has acquired several test samples of PBG fiber arrays with varying refractive index, capillary size, and length from an external vendor for testing. The PBGs were inspected with a microscope and characteristics of the capillaries including radii, spacing, and errors in construction were determined. Transmission tests were performed on these samples using a broad-range spectrophotometer. In addition, detailed E-field simulations of different PBG configurations were done using the CUDOS and RSOFT codes. Several accelerating modes for different configurations were found and studied in detail.

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

    PubMed

    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. PMID:24691588

  13. Voltage-matched, monolithic, multi-band-gap devices

    DOEpatents

    Wanlass, Mark W.; Mascarenhas, Angelo

    2006-08-22

    Monolithic, tandem, photonic cells include at least a first semiconductor layer and a second semiconductor layer, wherein each semiconductor layer includes an n-type region, a p-type region, and a given band-gap energy. Formed within each semiconductor layer is a sting of electrically connected photonic sub-cells. By carefully selecting the numbers of photonic sub-cells in the first and second layer photonic sub-cell string(s), and by carefully selecting the manner in which the sub-cells in a first and second layer photonic sub-cell string(s) are electrically connected, each of the first and second layer sub-cell strings may be made to achieve one or more substantially identical electrical characteristics.

  14. Colloidal dual-band gap cell for photocatalytic hydrogen generation.

    PubMed

    Li, Wei; O'Dowd, Graeme; Whittles, Thomas J; Hesp, David; Gründer, Yvonne; Dhanak, Vinod R; Jäckel, Frank

    2015-10-28

    We report that the internal quantum efficiency for hydrogen generation in spherical, Pt-decorated CdS nanocrystals can be tuned by quantum confinement, resulting in higher efficiencies for smaller than for larger nanocrystals (17.3% for 2.8 nm and 11.4% for 4.6 nm diameter nanocrystals). We attribute this to a larger driving force for electron and hole transfer in the smaller nanocrystals. The larger internal quantum efficiency in smaller nanocrystals enables a novel colloidal dual-band gap cell utilising differently sized nanocrystals and showing larger external quantum efficiencies than cells with only one size of nanocrystals (9.4% for 2.8 nm particles only and 14.7% for 2.8 nm and 4.6 nm nanocrystals). This represents a proof-of-principle for future colloidal tandem cell. PMID:26415524

  15. Half-oxidized phosphorene: band gap and elastic properties modulation.

    PubMed

    Drissi, L B; Sadki, S; Sadki, K

    2016-04-13

    Based on a first principles approach, we study structural, electronic and elastic properties, as well as stabilities of all possible half-oxidized phosphorene conformers. Stability analysis reveals that oxygen chemisorption is an exothermic process in the six configurations despite the formation of interstitial oxygen bridges in three of them. Electronic structure calculations show that oxidation induces a band gap modulation ranging between 0.54 and 1.57 eV in the generalized gradient approximation corrected to 1.19 and 2.88 eV using GW. The mechanical response of the conformers is sensitively dependent on direction and indicates that the new derivatives are incompressible materials and one configuration has an auxetic behavior. The present results provide a basis for tailoring the electronic and elastic properties of phosphorene via half oxidation. PMID:26964522

  16. Voltage-Matched, Monolithic, Multi-Band-Gap Devices

    DOEpatents

    Wanlass, M. W.; Mascarenhas, A.

    2006-08-22

    Monolithic, tandem, photonic cells include at least a first semiconductor layer and a second semiconductor layer, wherein each semiconductor layer includes an n-type region, a p-type region, and a given band-gap energy. Formed within each semiconductor layer is a string of electrically connected photonic sub-cells. By carefully selecting the numbers of photonic sub-cells in the first and second layer photonic sub-cell string(s), and by carefully selecting the manner in which the sub-cells in a first and second layer photonic sub-cell string(s) are electrically connected, each of the first and second layer sub-cell strings may be made to achieve one or more substantially identical electrical characteristics.

  17. Colloidal dual-band gap cell for photocatalytic hydrogen generation

    NASA Astrophysics Data System (ADS)

    Li, Wei; O'Dowd, Graeme; Whittles, Thomas J.; Hesp, David; Gründer, Yvonne; Dhanak, Vinod R.; Jäckel, Frank

    2015-10-01

    We report that the internal quantum efficiency for hydrogen generation in spherical, Pt-decorated CdS nanocrystals can be tuned by quantum confinement, resulting in higher efficiencies for smaller than for larger nanocrystals (17.3% for 2.8 nm and 11.4% for 4.6 nm diameter nanocrystals). We attribute this to a larger driving force for electron and hole transfer in the smaller nanocrystals. The larger internal quantum efficiency in smaller nanocrystals enables a novel colloidal dual-band gap cell utilising differently sized nanocrystals and showing larger external quantum efficiencies than cells with only one size of nanocrystals (9.4% for 2.8 nm particles only and 14.7% for 2.8 nm and 4.6 nm nanocrystals). This represents a proof-of-principle for future colloidal tandem cell.We report that the internal quantum efficiency for hydrogen generation in spherical, Pt-decorated CdS nanocrystals can be tuned by quantum confinement, resulting in higher efficiencies for smaller than for larger nanocrystals (17.3% for 2.8 nm and 11.4% for 4.6 nm diameter nanocrystals). We attribute this to a larger driving force for electron and hole transfer in the smaller nanocrystals. The larger internal quantum efficiency in smaller nanocrystals enables a novel colloidal dual-band gap cell utilising differently sized nanocrystals and showing larger external quantum efficiencies than cells with only one size of nanocrystals (9.4% for 2.8 nm particles only and 14.7% for 2.8 nm and 4.6 nm nanocrystals). This represents a proof-of-principle for future colloidal tandem cell. Electronic supplementary information (ESI) available: Full experimental details and additional experimental results. See DOI: 10.1039/c5nr04950d

  18. Two-dimensional tricycle arsenene with a direct band gap.

    PubMed

    Ma, ShuangYing; Zhou, Pan; Sun, L Z; Zhang, K W

    2016-03-28

    Based on a comprehensive investigation including ab initio phonon and finite-temperature molecular dynamics calculations, we find that two-dimensional tricycle-shaped arsenene (T-As) is robust and even stable under high temperature. T-As is energetically comparable to previously reported chair-shaped arsenene (C-As) and more stable than stirrup-shaped arsenene (S-As). In contrast to C-As and S-As, the monolayer T-As is a direct band gap semiconductor with an energy gap of 1.377 eV. Our results indicate that the electronic structure of T-As can be effectively modulated by stacking, strain, and patterning, which shows great potential of T-As in future nano-electronics. Moreover, by absorbing H or F atoms on the surface of T-As along a specific direction, nanoribbons with desired edge type and even width can be obtained, which is suitable for the fabrication of nano-devices. PMID:26954607

  19. Wake-field studies on photonic band gap accelerator cavities

    NASA Astrophysics Data System (ADS)

    Li, Derun; Kroll, N.; Smith, D. R.; Schultz, S.

    1997-03-01

    We have studied the wake-field of several metal Photonic Band Gap (PBG) cavities which consist of either a square or a hexagonal array of metal cylinders, bounded on top and bottom by conducting or superconducting sheets, surrounded by placing microwave absorber at the periphery or by replacing outer rows of metal cylinders with lossy dielectric ones, or by metallic walls. A removed cylinder from the center of the array constitutes a site defect where a localized electromagnetic mode can occur. While both monopole and dipole wake-fields have been studied, we confine our attention here mainly to the dipole case. The dipole wake-field is produced by modes in the propagation bands which tend to fill the entire cavity more or less uniformly and are thus easy to damp selectively. MAFIA time domain simulation of the transverse wake-field has been compared with that of a cylindrical pill-box comparison cavity. Even without damping the wake-field of the metal PBG cavity is substantially smaller than that of the pill-box cavity and may be further reduced by increasing the size of the lattice. By introducing lossy material at the periphery we have been able to produce Q factors for the dipole modes in the 40 to 120 range without significantly degrading the accelerating mode.

  20. Hypersonic crystal band gaps in Ni/Cu superlattice nanowire arrays

    NASA Astrophysics Data System (ADS)

    Hu, Jia-Guang; Shen, Tie

    2016-03-01

    The hexagonal and tetragonal ordered arrays were prepared by Ni/Cu superlattice nanowires on the porous anodic alumina membrane template, and their phonon band structures were calculated by using the plane wave expansion method. Numerical results show that the hypersonic band gaps can be acquired by adjusting the structural parameters. Along the different wave-vector directions, the width and position of band gap would vary. If the nanowires'filling fraction is increased continuously, the width of the first band gap firstly increases and then decreases within a certain range. The height of superlattice nanowire elementary unit can only affect the width of band gap within a quite narrow range. When the height of elementary unit remains unchanged, the decrease of the Cu-component ratio can contribute to the formation of a wider band gap. Additionally, the wide band gap is more easily formed in tetragonal structure than in hexagonal structure.

  1. Modeling of Photonic Band Gap Crystals and Applications

    SciTech Connect

    Ihab Fathy El-Kady

    2002-08-27

    In this work, the authors have undertaken a theoretical approach to the complex problem of modeling the flow of electromagnetic waves in photonic crystals. The focus is to address the feasibility of using the exciting phenomena of photonic gaps (PBG) in actual applications. The authors start by providing analytical derivations of the computational electromagnetic methods used in their work. They also present a detailed explanation of the physics underlying each approach, as well as a comparative study of the strengths and weaknesses of each method. The Plane Wave expansion, Transfer Matrix, and Finite Difference time Domain Methods are addressed. They also introduce a new theoretical approach, the Modal Expansion Method. They then shift the attention to actual applications. They begin with a discussion of 2D photonic crystal wave guides. The structure addressed consists of a 2D hexagonal structure of air cylinders in a layered dielectric background. Comparison with the performance of a conventional guide is made, as well as suggestions for enhancing it. The studies provide an upper theoretical limit on the performance of such guides, as they assumed no crystal imperfections and non-absorbing media. Next, they study 3D metallic PBG materials at near infrared and optical wavelengths. The main objective is to study the importance of absorption in the metal and the suitability of observing photonic band gaps in such structures. They study simple cubic structures where the metallic scatters are either cubes or interconnected metallic rods. Several metals are studied (aluminum, gold, copper, and silver). The effect of topology is addressed and isolated metallic cubes are found to be less lossy than the connected rod structures. The results reveal that the best performance is obtained by choosing metals with a large negative real part of the dielectric function, together with a relatively small imaginary part. Finally, they point out a new direction in photonic crystal

  2. High-Power Fiber Lasers Using Photonic Band Gap Materials

    NASA Technical Reports Server (NTRS)

    DiDomenico, Leo; Dowling, Jonathan

    2005-01-01

    High-power fiber lasers (HPFLs) would be made from photonic band gap (PBG) materials, according to the proposal. Such lasers would be scalable in the sense that a large number of fiber lasers could be arranged in an array or bundle and then operated in phase-locked condition to generate a superposition and highly directed high-power laser beam. It has been estimated that an average power level as high as 1,000 W per fiber could be achieved in such an array. Examples of potential applications for the proposed single-fiber lasers include welding and laser surgery. Additionally, the bundled fibers have applications in beaming power through free space for autonomous vehicles, laser weapons, free-space communications, and inducing photochemical reactions in large-scale industrial processes. The proposal has been inspired in part by recent improvements in the capabilities of single-mode fiber amplifiers and lasers to produce continuous high-power radiation. In particular, it has been found that the average output power of a single strand of a fiber laser can be increased by suitably changing the doping profile of active ions in its gain medium to optimize the spatial overlap of the electromagnetic field with the distribution of active ions. Such optimization minimizes pump power losses and increases the gain in the fiber laser system. The proposal would expand the basic concept of this type of optimization to incorporate exploitation of the properties (including, in some cases, nonlinearities) of PBG materials to obtain power levels and efficiencies higher than are now possible. Another element of the proposal is to enable pumping by concentrated sunlight. Somewhat more specifically, the proposal calls for exploitation of the properties of PBG materials to overcome a number of stubborn adverse phenomena that have impeded prior efforts to perfect HPFLs. The most relevant of those phenomena is amplified spontaneous emission (ASE), which causes saturation of gain and power

  3. Micro-cavity lasers with large device size for directional emission

    NASA Astrophysics Data System (ADS)

    Yan, Chang-ling; Li, Peng; Shi, Jian-wei; Feng, Yuan; Hao, Yong-qin; Zhu, Dongda

    2014-10-01

    Optical micro-cavity structures, which can confine light in a small mode volume with high quality factors, have become an important platform not only for optoelectronic applications with densely integrated optical components, but also for fundamental studies such as cavity quantum electrodynamics and nonlinear optical processes. Micro-cavity lasers with directional emission feature are becoming a promising resonator for the compact laser application. In this paper, we presented the limason-shaped cavity laser with large device size, and fabricated this type of micro-cavity laser with quantum cascade laser material. The micro-cavity laser with large device size was fabricated by using InP based InGaAs/InAlAs quantum cascade lasers material at about 10um emitting wavelength, and the micro-cavity lasers with the large device size were manufactured and characterized with light output power, threshold current, and the far-field pattern.

  4. Multi-flexural band gaps in an Euler-Bernoulli beam with lateral local resonators

    NASA Astrophysics Data System (ADS)

    Wang, Ting; Sheng, Mei-Ping; Qin, Qing-Hua

    2016-02-01

    Flexural vibration suppression in an Euler-Bernoulli beam with attached lateral local resonators (LLR) is studied theoretically and numerically. Hamilton's principle and Bloch's theorem are employed to derive the dispersion relation which reveals that two band gaps are generated. Within both band gaps, the flexural waves are partially transformed into longitudinal waves through a four-link-mechanism and totally blocked. The band gaps can be flexibly tuned by changing the geometry parameter of the four-link-mechanism and the spring constants of the resonators. Frequency response function (FRF) from finite element analysis via commercial software of ANSYS shows large flexural wave attenuation within the band gaps and the effect of damping from the LLR substructures which helps smooth and lower the response peaks at the sacrifice of the band gap effect. The existence of the multi-flexural band gaps can be exploited for the design of flexural vibration control of beams.

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

    SciTech Connect

    Coffy, Etienne Lavergne, Thomas; Addouche, Mahmoud; Euphrasie, Sébastien; Vairac, Pascal; Khelif, Abdelkrim

    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.

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

    NASA Astrophysics Data System (ADS)

    Coffy, Etienne; Lavergne, Thomas; Addouche, Mahmoud; Euphrasie, Sébastien; Vairac, Pascal; Khelif, Abdelkrim

    2015-12-01

    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.

  7. Optical properties of silver nanocomposites and photonic band gap - Pressure dependence

    NASA Astrophysics Data System (ADS)

    Ramanujam, N. R.; Wilson, K. S. Joseph

    2016-06-01

    We theoretically investigate the effect of photonic band gaps in one dimensional photonic crystals based on nanocomposite of silver nanoparticles. The dielectric permittivity is computed based on the pressure dependence of plasma frequency and damping constant of silver nanoparticle. It leads to the tuning of photonic band gap. We have also investigated the change in photonic band gap due to the influence of filling factor and the size of the nanoparticles. Our results provide a guideline for designing potential photonic devices.

  8. Tensile-strain effect of inducing the indirect-to-direct band-gap transition and reducing the band-gap energy of Ge

    SciTech Connect

    Inaoka, Takeshi Furukawa, Takuro; Toma, Ryo; Yanagisawa, Susumu

    2015-09-14

    By means of a hybrid density-functional method, we investigate the tensile-strain effect of inducing the indirect-to-direct band-gap transition and reducing the band-gap energy of Ge. We consider [001], [111], and [110] uniaxial tensility and (001), (111), and (110) biaxial tensility. Under the condition of no normal stress, we determine both normal compression and internal strain, namely, relative displacement of two atoms in the primitive unit cell, by minimizing the total energy. We identify those strain types which can induce the band-gap transition, and evaluate the critical strain coefficient where the gap transition occurs. Either normal compression or internal strain operates unfavorably to induce the gap transition, which raises the critical strain coefficient or even blocks the transition. We also examine how each type of tensile strain decreases the band-gap energy, depending on its orientation. Our analysis clearly shows that synergistic operation of strain orientation and band anisotropy has a great influence on the gap transition and the gap energy.

  9. Infrared detectors and lasers operating in the 3-12 μm range using band-gap engineered structures with type II band-gap alignment

    NASA Astrophysics Data System (ADS)

    Swaminathan, Venkataraman; Little, John W.; Tober, Richard L.

    2006-02-01

    The Type II broken band-gap alignment in semiconductor structures wherein the conduction band minimum is in one semiconductor (e.g., InAs) and the valence band maximum is in another (e.g., GaInSb) offers certain unique advantages which can be utilized to realize band-gap engineered novel quantum electro-optic devices such as lasers and detectors. The advantages of the type II structures include reduced Auger recombination, extending the effective band-gap energy of materials wherein type I band-gap alignment would give rise to difficulties such as miscibility gap. In this paper we describe the work carried out at the Army Research Laboratory on type II semiconductor quantum electro-optic devices such as IR lasers and detectors operating in the 3-12 μm range. Specifically we will cover the progress made in GaSb based type II strained layer superlattice IR detectors and Interband Cascade IR Lasers. We will also present our recent work in self-assembled quantum dots which have type II band-gap alignment with the matrix material in which the dots are embedded.

  10. Tuning of full band gap in anisotropic photonic crystal slabs using a liquid crystal

    NASA Astrophysics Data System (ADS)

    Khalkhali, T. Fathollahi; Rezaei, B.; Ramezani, A. H.

    2012-11-01

    We analyze the tunability of full band gap in photonic crystal slabs created by square and triangular lattices of air holes in anisotropic tellurium background, considering that the regions above and below the slab are occupied by SiO2 and the holes are infiltrated with liquid crystals. Using the supercell method based on plane wave expansion, we study the variation of full band gap by changing the optical axis orientation of liquid crystal. Our results demonstrate the existence and remarkable tunability of full band gap in both square and triangular lattices, largest band gap and tunability being obtained for the triangular lattice.

  11. Miniaturization of electromagnetic band gap structures for mobile applications

    NASA Astrophysics Data System (ADS)

    Goussetis, G.; Feresidis, A. P.; Palikaras, G. K.; Kitra, M.; Vardaxoglou, J. C.

    2005-12-01

    It is well known that interference of the human body affects the performance of the antennas in mobile phone handsets. In this contribution, we investigate the use of miniaturized metallodielectric electromagnetic band gap (MEBG) structures embedded in the case of a mobile handset as a means of decoupling the antenna from the user's hand. The closely coupled MEBG concept is employed to achieve miniaturization of the order of 15:1. Full wave dispersion relations for planar closely coupled MEBG arrays are presented and are validated experimentally. The performance of a prototype handset with an embedded conformal MEBG is assessed experimentally and is compared to a similar prototype without the MEBG. Reduction in the detuning of the antenna because of the human hand by virtue of the MEBG is demonstrated. Moreover, the efficiency of the handset when loaded with a human hand model is shown to improve when the MEBG is in place. The improvements are attributed to the decoupling of the antenna from the user's hand, which is achieved by means of suppressing the fields in the locality of the hand.

  12. Granular micromechanics based micromorphic model predicts frequency band gaps

    NASA Astrophysics Data System (ADS)

    Misra, Anil; Poorsolhjouy, Payam

    2016-03-01

    Granular materials are typically characterized by complex structure and composition. Continuum modeling, therefore, remains the mainstay for describing properties of these material systems. In this paper, we extend the granular micromechanics approach by considering enhanced kinematic analysis. In this analysis, a decomposition of the relative movements of interacting grain pairs into parts arising from macro-scale strain as well as micro-scale strain measures is introduced. The decomposition is then used to formulate grain-scale deformation energy functions and derive inter-granular constitutive laws. The macro-scale deformation energy density is defined as a summation of micro-scale deformation energy defined for each interacting grain pair. As a result, a micromorphic continuum model for elasticity of granular media is derived and applied to investigate the wave propagation behavior. Dispersion graphs for different cases and different ratios between the microscopic stiffness parameters have been presented. It is seen that the model has the capability to present band gaps over a large range of wave numbers.

  13. A 250 GHz Photonic Band Gap Gyrotron Amplifier

    NASA Astrophysics Data System (ADS)

    Nanni, Emilio A.; Lewis, Samantha M.; Shapiro, Michael A.; Temkin, Richard J.

    2012-10-01

    Initial results for a high power 250 GHz gyrotron traveling wave tube (gyro-TWT) amplifier will be presented. The amplifier uses a novel photonic band gap (PBG) interaction circuit that confines the TE03-like mode for operation. Stability from oscillations in lower order modes is provided by the PBG circuit. At 26.6 kV and 0.25 A the gyro-TWT operates with peak small signal gain of 27.3 dB at 251 GHz. The instantaneous -3 dB bandwidth of the amplifier at peak gain is 0.4 GHz. The amplifier can be tuned for operation from 245-254 GHz. A peak output power of 7.5 W has been measured. Experimental results taken over a wide range of parameters, 15-30 kV and 0.25-0.5 A, show good agreement with a theoretical model in the small signal gain regime. The theoretical model incorporates cold test measurements for the transmission line, input coupler, PBG waveguide and mode converter.

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

    PubMed Central

    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

  15. Photonic-band-gap traveling-wave gyrotron amplifier.

    PubMed

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

    2013-12-01

    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

  16. Acoustic band gaps of three-dimensional periodic polymer cellular solids with cubic symmetry

    NASA Astrophysics Data System (ADS)

    Chen, Yanyu; Yao, Haimin; Wang, Lifeng

    2013-07-01

    The band structure and sound attenuation of the triply periodic co-continuous composite materials with simple cubic lattice, body-centered cubic lattice, and face-centered cubic lattice consisting of PMMA and air are investigated using finite element method. Complete band gaps are found in these structures and the width of band gaps is depending on volume fraction. It is shown that the width of band gaps along different directions in the first irreducible Brillouin zone enlarges as the volume fraction increases from 0.2 to 0.7. The largest complete band gap widths of the three types of co-continuous structures are 0.29, 0.54, and 0.55, respectively. As the complete band gaps appear in audible range of frequencies, these triply periodic co-continuous composite materials can be utilized to control noise.

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

    PubMed

    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. PMID:27421066

  18. Wind Tunnel Testing of Various Disk-Gap-Band Parachutes

    NASA Technical Reports Server (NTRS)

    Cruz, Juan R.; Mineck, Raymond E.; Keller, Donald F.; Bobskill, Maria V.

    2003-01-01

    Two Disk-Gap-Band model parachute designs were tested in the NASA Langley Transonic Dynamics Tunnel. The purposes of these tests were to determine the drag and static stability coefficients of these two model parachutes at various subsonic Mach numbers in support of the Mars Exploration Rover mission. The two model parachute designs were designated 1.6 Viking and MPF. These model parachute designs were chosen to investigate the tradeoff between drag and static stability. Each of the parachute designs was tested with models fabricated from MIL-C-7020 Type III or F-111 fabric. The reason for testing model parachutes fabricated with different fabrics was to evaluate the effect of fabric permeability on the drag and static stability coefficients. Several improvements over the Viking-era wind tunnel tests were implemented in the testing procedures and data analyses. Among these improvements were corrections for test fixture drag interference and blockage effects, and use of an improved test fixture for measuring static stability coefficients. The 1.6 Viking model parachutes had drag coefficients from 0.440 to 0.539, while the MPF model parachutes had drag coefficients from 0.363 to 0.428. The 1.6 Viking model parachutes had drag coefficients 18 to 22 percent higher than the MPF model parachute for equivalent fabric materials and test conditions. Model parachutes of the same design tested at the same conditions had drag coefficients approximately 11 to 15 percent higher when manufactured from F-111 fabric as compared to those fabricated from MIL-C-7020 Type III fabric. The lower fabric permeability of the F-111 fabric was the source of this difference. The MPF model parachutes had smaller absolute statically stable trim angles of attack as compared to the 1.6 Viking model parachutes for equivalent fabric materials and test conditions. This was attributed to the MPF model parachutes larger band height to nominal diameter ratio. For both designs, model parachutes

  19. Opening Loads Analyses for Various Disk-Gap-Band Parachutes

    NASA Technical Reports Server (NTRS)

    Cruz, J. R.; Kandis, M.; Witkowski, A.

    2003-01-01

    Detailed opening loads data is presented for 18 tests of Disk-Gap-Band (DGB) parachutes of varying geometry with nominal diameters ranging from 43.2 to 50.1 ft. All of the test parachutes were deployed from a mortar. Six of these tests were conducted via drop testing with drop test vehicles weighing approximately 3,000 or 8,000 lb. Twelve tests were conducted in the National Full-Scale Aerodynamics Complex 80- by 120-foot wind tunnel at the NASA Ames Research Center. The purpose of these tests was to structurally qualify the parachute for the Mars Exploration Rover mission. A key requirement of all tests was that peak parachute load had to be reached at full inflation to more closely simulate the load profile encountered during operation at Mars. Peak loads measured during the tests were in the range from 12,889 to 30,027 lb. Of the two test methods, the wind tunnel tests yielded more accurate and repeatable data. Application of an apparent mass model to the opening loads data yielded insights into the nature of these loads. Although the apparent mass model could reconstruct specific tests with reasonable accuracy, the use of this model for predictive analyses was not accurate enough to set test conditions for either the drop or wind tunnel tests. A simpler empirical model was found to be suitable for predicting opening loads for the wind tunnel tests to a satisfactory level of accuracy. However, this simple empirical model is not applicable to the drop tests.

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

    PubMed

    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. PMID:25096084

  1. Stacking orders induced direct band gap in bilayer MoSe2-WSe2 lateral heterostructures

    PubMed Central

    Hu, Xiaohui; Kou, Liangzhi; Sun, Litao

    2016-01-01

    The direct band gap of monolayer semiconducting transition-metal dichalcogenides (STMDs) enables a host of new optical and electrical properties. However, bilayer STMDs are indirect band gap semiconductors, which limits its applicability for high-efficiency optoelectronic devices. Here, we report that the direct band gap can be achieved in bilayer MoSe2-WSe2 lateral heterostructures by alternating stacking orders. Specifically, when Se atoms from opposite layers are stacked directly on top of each other, AA and A’B stacked heterostructures show weaker interlayer coupling, larger interlayer distance and direct band gap. Whereas, when Se atoms from opposite layers are staggered, AA’, AB and AB’ stacked heterostructures exhibit stronger interlayer coupling, shorter interlayer distance and indirect band gap. Thus, the direct/indirect band gap can be controllable in bilayer MoSe2-WSe2 lateral heterostructures. In addition, the calculated sliding barriers indicate that the stacking orders of bilayer MoSe2-WSe2 lateral heterostructures can be easily formed by sliding one layer with respect to the other. The novel direct band gap in bilayer MoSe2-WSe2 lateral heterostructures provides possible application for high-efficiency optoelectronic devices. The results also show that the stacking order is an effective strategy to induce and tune the band gap of layered STMDs. PMID:27528196

  2. Stacking orders induced direct band gap in bilayer MoSe2-WSe2 lateral heterostructures.

    PubMed

    Hu, Xiaohui; Kou, Liangzhi; Sun, Litao

    2016-01-01

    The direct band gap of monolayer semiconducting transition-metal dichalcogenides (STMDs) enables a host of new optical and electrical properties. However, bilayer STMDs are indirect band gap semiconductors, which limits its applicability for high-efficiency optoelectronic devices. Here, we report that the direct band gap can be achieved in bilayer MoSe2-WSe2 lateral heterostructures by alternating stacking orders. Specifically, when Se atoms from opposite layers are stacked directly on top of each other, AA and A'B stacked heterostructures show weaker interlayer coupling, larger interlayer distance and direct band gap. Whereas, when Se atoms from opposite layers are staggered, AA', AB and AB' stacked heterostructures exhibit stronger interlayer coupling, shorter interlayer distance and indirect band gap. Thus, the direct/indirect band gap can be controllable in bilayer MoSe2-WSe2 lateral heterostructures. In addition, the calculated sliding barriers indicate that the stacking orders of bilayer MoSe2-WSe2 lateral heterostructures can be easily formed by sliding one layer with respect to the other. The novel direct band gap in bilayer MoSe2-WSe2 lateral heterostructures provides possible application for high-efficiency optoelectronic devices. The results also show that the stacking order is an effective strategy to induce and tune the band gap of layered STMDs. PMID:27528196

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

    PubMed

    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. PMID:20588565

  4. Carrier Plasmon Induced Nonlinear Band Gap Renormalization in Two-Dimensional Semiconductors

    NASA Astrophysics Data System (ADS)

    Liang, Yufeng; Yang, Li

    2015-02-01

    In reduced-dimensional semiconductors, doping-induced carrier plasmons can strongly couple with quasiparticle excitations, leading to a significant band gap renormalization. However, the physical origin of this generic effect remains obscure. We develop a new plasmon-pole theory that efficiently and accurately captures this coupling. Using monolayer MoS2 and MoSe2 as prototype two-dimensional (2D) semiconductors, we reveal a striking band gap renormalization above 400 meV and an unusual nonlinear evolution of their band gaps with doping. This prediction significantly differs from the linear behavior that is observed in one-dimensional structures. Notably, our predicted band gap renormalization for MoSe2 is in excellent agreement with recent experimental results. Our developed approach allows for a quantitative understanding of many-body interactions in general doped 2D semiconductors and paves the way for novel band gap engineering techniques.

  5. Modulating the band gap of germanane nanoribbons for quantum well devices.

    PubMed

    Zhou, Yungang; Li, Xuemei; Wang, Zhiguo; Li, Sean; Zu, Xiaotao

    2014-09-01

    The effective modulation of the band gaps in nanostructures is of both fundamental and technological interest because a tunable band gap gives great flexibility in the design and optimization of nanodevices. Using density functional theory calculations, we have shown that germanane nanoribbons of various widths or under various strains can provide rich band gaps. Width- and strain-induced changes in the band gaps of germanane nanoribbons result from a reduction in quantum confinement with width and the weakening of sp(3) hybridization with strain, respectively. Both changes represent a monotonous relationship. To utilize such a monotonous change in band gap, we designed a quantum well based on germanane nanoribbons in which photoexcited electrons and holes occupy the same spatial region, resulting in a desirable light-emitting device. PMID:25051154

  6. Direct Observation of Electrostatically Driven Band Gap Renormalization in a Degenerate Perovskite Transparent Conducting Oxide.

    PubMed

    Lebens-Higgins, Z; Scanlon, D O; Paik, H; Sallis, S; Nie, Y; Uchida, M; Quackenbush, N F; Wahila, M J; Sterbinsky, G E; Arena, Dario A; Woicik, J C; Schlom, D G; Piper, L F J

    2016-01-15

    We have directly measured the band gap renormalization associated with the Moss-Burstein shift in the perovskite transparent conducting oxide (TCO), La-doped BaSnO_{3}, using hard x-ray photoelectron spectroscopy. We determine that the band gap renormalization is almost entirely associated with the evolution of the conduction band. Our experimental results are supported by hybrid density functional theory supercell calculations. We determine that unlike conventional TCOs where interactions with the dopant orbitals are important, the band gap renormalization in La-BaSnO_{3} is driven purely by electrostatic interactions. PMID:26824566

  7. Single-junction solar cells with the optimum band gap for terrestrial concentrator applications

    DOEpatents

    Wanlass, M.W.

    1994-12-27

    A single-junction solar cell is described having the ideal band gap for terrestrial concentrator applications. Computer modeling studies of single-junction solar cells have shown that the presence of absorption bands in the direct spectrum has the effect of ''pinning'' the optimum band gap for a wide range of operating conditions at a value of 1.14[+-]0.02 eV. Efficiencies exceeding 30% may be possible at high concentration ratios for devices with the ideal band gap. 7 figures.

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

    NASA Astrophysics Data System (ADS)

    Jandieri, K.; Ludewig, P.; Wegele, T.; Beyer, A.; Kunert, B.; Springer, P.; Baranovskii, S. D.; Koch, S. W.; Volz, K.; Stolz, W.

    2015-08-01

    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.

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

    SciTech Connect

    Jandieri, K. Ludewig, P.; Wegele, T.; Beyer, A.; Kunert, B.; Springer, P.; Baranovskii, S. D.; Koch, S. W.; Volz, K.; Stolz, W.

    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.

  10. Measurements of the energy band gap and valence band structure of AgSbTe2

    NASA Astrophysics Data System (ADS)

    Jovovic, V.; Heremans, J. P.

    2008-06-01

    The de Haas-van Alphen effect, galvanomagnetic and thermomagnetic properties of high-quality crystals of AgSbTe2 are measured and analyzed. The transport properties reveal the material studied here to be a very narrow-gap semiconductor (Eg≈7.6±3meV) with ˜5×1019cm-3 holes in a valence band with a high density of states and thermally excited ˜1017cm-3 high-mobility (2200cm2/Vs) electrons at 300 K. The quantum oscillations are measured with the magnetic field oriented along the ⟨111⟩ axis. Taken together with the Fermi energy derived from the transport properties, the oscillations confirm the calculated valence band structure composed of 12 half-pockets located at the X -points of the Brillouin zone, six with a density-of-states effective mass mda∗≫0.21me and six with mdb∗≫0.55me , giving a total density-of-states effective mass, including Fermi pocket degeneracy, of md∗≈1.7±0.2me ( me is the free electron mass). The lattice term dominates the thermal conductivity, and the electronic contribution in samples with both electrons and holes present is in turn dominated by the ambipolar term. The low thermal conductivity and very large hole mass of AgSbTe2 make it a most promising p -type thermoelectric material.

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

    PubMed

    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. PMID:26319225

  12. A parametric study on the PD pulses activity within micro-cavities

    NASA Astrophysics Data System (ADS)

    Ganjovi, Alireza A.

    2016-03-01

    A two-dimensional kinetic model has been used to parametric investigation of the spark-type partial discharge pulses inside the micro-cavities. The model is based on particle-in-cell methods with Monte Carlo Collision techniques for modeling of collisions. Secondary processes like photo-emission and cathode-emission are considered. The micro-cavity may be sandwiched between two metallic conductors or two dielectrics. The discharge within the micro-cavity is studied in conjunction with the external circuit. The model is used to successfully simulate the evolution of the discharge and yield useful information about the build-up of space charge within the micro-cavity and the consequent modification of the applied electric field. The phase-space scatter plots for electrons, positive, and negative ions are obtained in order to understand the manner in which discharge progresses over time. The rise-time and the magnitude of the discharge current pulse are obtained and are seen to be affected by micro-cavity dimensions, gas pressure within the micro-cavity, and the permittivity of surrounding dielectrics. The results have been compared with existing experimental, theoretical, and computational results, wherever possible. An attempt has been made to understand the nature of the variations in terms of the physical processes involved.

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

    SciTech Connect

    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.

  14. Tunable and Sizable Band Gap of Single Layer Graphene Sandwiched between Hexagonal Boron Nitride

    NASA Astrophysics Data System (ADS)

    Zheng, Jiaxin; Qu, Heruge; Liu, Qihang; Qin, Rui; Zhou, Jing; Yu, Dapeng; Gao, Zhengxiang; Lu, Jing; Luo, Guangfu; Nagase, Shigeru; Mei, Wai-Ning

    2012-02-01

    It is a big challenge to open a tunable and sizable band gap of single layer graphene without big loss in structural integrity and carrier mobility. By using density functional theory calculations, we show that the band gap of single layer graphene can be opened to 0.16 (without electrical field) and 0.34 eV (with a strong electrical field) when sandwiched between two hexagonal boron nitride single layers in a proper way. The zero-field band gaps are increased by about 50% when many-body effects are included. Ab initio quantum transport simulation of a dual-gated FET out of such a sandwich structure further confirms an electrical field-enhanced transport gap. The tunable and sizeable band gap and structural integrity render this sandwich structure a promising candidate for high-performance single layer graphene field effect transistors.

  15. Towards Tunable Band Gap and Tunable Dirac Point in Bilayer Graphene with Molecular Doping

    PubMed Central

    Yu, Woo Jong; Liao, Lei; Chae, Sang Hoon; Lee, Young Hee; Duan, Xiangfeng

    2011-01-01

    The bilayer graphene has attracted considerable attention for potential applications in future electronics and optoelectronics because of the feasibility to tune its band gap with a vertical displacement field to break the inversion symmetry. Surface chemical doping in bilayer graphene can induce an additional offset voltage to fundamentally affect the vertical displacement field and band-gap opening in bilayer graphene. In this study, we investigate the effect of chemical molecular doping on band-gap opening in bilayer graphene devices with single or dual gate modulation. Chemical doping with benzyl viologen molecules modulates the displacement field to allow the opening of a transport band gap and the increase of the on/off ratio in the bilayer graphene transistors. Additionally, Fermi energy level in the opened gap can be rationally controlled by the amount of molecular doping to obtain bilayer graphene transistors with tunable Dirac points, which can be readily configured into functional devices such as complementary inverters. PMID:21985035

  16. Calculations of band gaps in polyaniline from theoretical studies of oligomers

    SciTech Connect

    Kwon, O.; McKee, M.L.

    2000-03-02

    Geometries and band gaps of polyaniline oligomers up to decamer have been systematically calculated and analyzed using various computational techniques such as molecular mechanics, semiempirical, and ab initio methods. On the basis of fully optimized geometries of neural and charged forms of polyaniline oligomers, excitation energies are calculated at the semiempirical ZINDO (INDO/S) level and extrapolated to the band gap value of the infinite chain. Band gaps are also approximately by extrapolating the HOMO/LUMO difference calculated at the density functional level (B3LYP/6--31G*). The SINDO//AM1 band gaps in the reduced and oxidized form of polyaniline (4.3 and 2.7 eV) are in good agreement with experimental values (3.8 {+-} 2 and 1.8 {+-} 3 eV, respectively). The doped form of polyaniline (two positive charges per four aniline units) has been computed with a spin-unrestricted method (UAM1) and the band gap approximated from an extrapolation of the tetramer and octamer. The calculated band gap of 1.3 eV (UZINDO//UAM1) is in good agreement with experiment (1.5 eV). The influence of ring torsional angle and interchain interaction on the band gap of the polyaniline system are also discussed.

  17. Electrically controlled band gap and topological phase transition in two-dimensional multilayer germanane

    NASA Astrophysics Data System (ADS)

    Qi, Jingshan; Li, Xiao; Qian, Xiaofeng

    2016-06-01

    Electrically controlled band gap and topological electronic states are important for the next-generation topological quantum devices. In this letter, we study the electric field control of band gap and topological phase transitions in multilayer germanane. We find that although the monolayer and multilayer germananes are normal insulators, a vertical electric field can significantly reduce the band gap of multilayer germananes owing to the giant Stark effect. The decrease of band gap eventually leads to band inversion, transforming them into topological insulators with nontrivial Z2 invariant. The electrically controlled topological phase transition in multilayer germananes provides a potential route to manipulate topologically protected edge states and design topological quantum devices. This strategy should be generally applicable to a broad range of materials, including other two-dimensional materials and ultrathin films with controlled growth.

  18. Tunability of Band Gap in Multilayer Phosphorene by External Electric Fields and Electron Dopings

    NASA Astrophysics Data System (ADS)

    Baik, Seung Su; Choi, Hyoung Joon

    2015-03-01

    Black phosphorus (BP) and its two-dimensional derivative phosphorene are rapidly emerging nanoelectronic materials with potential applicability to field effect transistors and optoelectronic devices. Unlike the gapless semiconductor graphene, multilayer BP has a substantial band gap of ~ 0.2 eV and the band-gap size is reportedly varied by external electric fields. To explore the extensibility of such band-gap modulation, we have investigated electronic band structures of multilayer BP by using the first-principles density-functional method as implemented in the SIESTA code. By controlling the electron doping concentrations and the resultant electric fields therefrom, we examine the manageability of the band-gap size and the anisotropic carrier mobility. This work was supported by NRF of Korea (Grant No. 2011-0018306) and KISTI supercomputing center (Project No. KSC-2013-C3-062).

  19. Band gap states of V and Cr in 6H-silicon carbide

    NASA Astrophysics Data System (ADS)

    Achtziger, N.; Grillenberger, J.; Witthuhn, W.

    Band gap states of Ti, V and Cr in n-type 6H-SiC were investigated by radiotracer deep level transient spectroscopy (DLTS). Doping with the radioactive isotopes 48V and 51Cr was done by recoil implantation followed by annealing (1600 K). Repeated DLTS measurements during the elemental transmutation of these isotopes to 48Ti and 51V respectively revealed the corresponding concentration changes of band gap states. Thus, three levels were identified in the band gap: a Cr level at 0.54 eV and two V levels at 0.71 and 0.75 eV below the conduction band edge. There are no deep levels of Ti in the upper part of the band gap.

  20. Low-frequency band gap mechanism of torsional vibration of lightweight elastic metamaterial shafts

    NASA Astrophysics Data System (ADS)

    Li, Lixia; Cai, Anjiang

    2016-07-01

    In this paper, the low-frequency band gap mechanism of torsional vibration is investigated for a kind of light elastic metamaterial (EM) shafts architecture comprised of a radial double-period element periodically as locally resonant oscillators with low frequency property. The dispersion relations are calculated by a method combining the transfer matrix and a lumped-mass method. The theoretical results agree well with finite method simulations, independent of the density of the hard material ring. The effects of the material parameters on the band gaps are further explored numerically. Our results show that in contrast to the traditional EM shaft, the weight of our proposed EM shaft can be reduced by 27% in the same band gap range while the vibration attenuation is kept unchanged, which is very convenient to instruct the potential engineering applications. Finally, the band edge frequencies of the lower band gaps for this light EM shaft are expressed analytically using physical heuristic models.

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

    SciTech Connect

    Marsh, Roark A.; Shapiro, Michael A.; Temkin, Richard J.; 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.

  2. Multilayer-split-tube resonators with low-frequency band gaps in phononic crystals

    NASA Astrophysics Data System (ADS)

    Jing, Li; Wu, Jiu Hui; Guan, Dong; Gao, Nansha

    2014-09-01

    In this paper, low-frequency band gaps in two-dimensional Helmholtz resonant phononic crystals (PCs) composed of multilayer-split-tube resonators are investigated. The band structures, transmission spectra, and pressure field of the acoustic modes of these PCs are calculated by using a finite element method (FEM). The numerical results show that the first band gap of the structure is from 88 to 140 Hz. The transmission spectra are in accordance with those of the dispersion relation calculations. The acoustic modes of the bands are analyzed to reveal the nature of this phenomenon. It is found that the interaction between the local resonance and the traveling wave modes in proposed structure is responsible for the formation of the first band gap. The influences of the structural parameters on the band gaps are investigated by using FEM and the electrical circuit analogy. Numerical results show that the band gaps can be modulated in an even wider frequency range by changing the structural parameters, such as the rotation angle, the number of tubes, and the radius of the outer tube. The structural design results provide an effective way for phononic crystals to obtain the low-frequency band gaps, which have potential application in the low-frequency noise reduction.

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

    SciTech Connect

    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.

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

    NASA Astrophysics Data System (ADS)

    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.

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

    NASA Astrophysics Data System (ADS)

    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.

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

    PubMed

    Yudistira, D; Boes, A; Djafari-Rouhani, B; Pennec, Y; Yeo, L Y; Mitchell, A; Friend, J R

    2014-11-21

    We theoretically and experimentally demonstrate the existence of complete surface acoustic wave band gaps in surface phonon-polariton phononic crystals, in a completely monolithic structure formed from a two-dimensional honeycomb array of hexagonal shape domain-inverted inclusions in single crystal piezoelectric Z-cut lithium niobate. The band gaps appear at a frequency of about twice the Bragg band gap at the center of the Brillouin zone, formed through phonon-polariton coupling. The structure is mechanically, electromagnetically, and topographically homogeneous, without any physical alteration of the surface, offering an ideal platform for many acoustic wave applications for photonics, phononics, and microfluidics. PMID:25479504

  7. Band-Gap Engineering at a Semiconductor-Crystalline Oxide Interface

    SciTech Connect

    Jahangir-Moghadam, Mohammadreza; Ahmadi-Majlan, Kamyar; Shen, Xuan; Droubay, Timothy; Bowden, Mark; Chrysler, Matthew; Su, Dong; Chambers, Scott A.; Ngai, Joseph H.

    2015-02-09

    The epitaxial growth of crystalline oxides on semiconductors provides a pathway to introduce new functionalities to semiconductor devices. Key to integrating the functionalities of oxides onto semiconductors is controlling the band alignment at interfaces between the two materials. Here we apply principles of band gap engineering traditionally used at heterojunctions between conventional semiconductors to control the band offset between a single crystalline oxide and a semiconductor. Reactive molecular beam epitaxy is used to realize atomically abrupt and structurally coherent interfaces between SrZrxTi1-xO₃ and Ge, in which the band gap of the former is enhanced with Zr content x. We present structural and electrical characterization of SrZrxTi1-xO₃-Ge heterojunctions and demonstrate a type-I band offset can be achieved. These results demonstrate that band gap engineering can be exploited to realize functional semiconductor crystalline oxide heterojunctions.

  8. Band-Gap Engineering at a Semiconductor-Crystalline Oxide Interface

    DOE PAGESBeta

    Jahangir-Moghadam, Mohammadreza; Ahmadi-Majlan, Kamyar; Shen, Xuan; Droubay, Timothy; Bowden, Mark; Chrysler, Matthew; Su, Dong; Chambers, Scott A.; Ngai, Joseph H.

    2015-02-09

    The epitaxial growth of crystalline oxides on semiconductors provides a pathway to introduce new functionalities to semiconductor devices. Key to integrating the functionalities of oxides onto semiconductors is controlling the band alignment at interfaces between the two materials. Here we apply principles of band gap engineering traditionally used at heterojunctions between conventional semiconductors to control the band offset between a single crystalline oxide and a semiconductor. Reactive molecular beam epitaxy is used to realize atomically abrupt and structurally coherent interfaces between SrZrxTi1-xO₃ and Ge, in which the band gap of the former is enhanced with Zr content x. We presentmore » structural and electrical characterization of SrZrxTi1-xO₃-Ge heterojunctions and demonstrate a type-I band offset can be achieved. These results demonstrate that band gap engineering can be exploited to realize functional semiconductor crystalline oxide heterojunctions.« less

  9. Enlarged band gap and electron switch in graphene-based step-barrier structure

    SciTech Connect

    Lu, Wei-Tao Ye, Cheng-Zhi; Li, Wen

    2013-11-04

    We study the transmission through a step-barrier in gapped graphene and propose a method to enlarge the band gap. The step-barrier structure consists of two or more barriers with different strengths. It is found that the band gap could be effectively enlarged and controlled by adjusting the barrier strengths in the light of the mass term. Klein tunneling at oblique incidence is suppressed due to the asymmetry of step-barrier, contrary to the cases in single-barrier and superlattices. Furthermore, a tunable conductance channel could be opened up in the conductance gap, suggesting an application of the structure as an electron switch.

  10. Vibrational renormalisation of the electronic band gap in hexagonal and cubic ice

    SciTech Connect

    Engel, Edgar A. Needs, Richard J.; Monserrat, Bartomeu

    2015-12-28

    Electron-phonon coupling in hexagonal and cubic water ice is studied using first-principles quantum mechanical methods. We consider 29 distinct hexagonal and cubic ice proton-orderings with up to 192 molecules in the simulation cell to account for proton-disorder. We find quantum zero-point vibrational corrections to the minimum electronic band gaps ranging from −1.5 to −1.7 eV, which leads to improved agreement between calculated and experimental band gaps. Anharmonic nuclear vibrations play a negligible role in determining the gaps. Deuterated ice has a smaller band-gap correction at zero-temperature of −1.2 to −1.4 eV. Vibrations reduce the differences between the electronic band gaps of different proton-orderings from around 0.17 eV to less than 0.05 eV, so that the electronic band gaps of hexagonal and cubic ice are almost independent of the proton-ordering when quantum nuclear vibrations are taken into account. The comparatively small reduction in the band gap over the temperature range 0 − 240 K of around 0.1 eV does not depend on the proton ordering, or whether the ice is protiated or deuterated, or hexagonal, or cubic. We explain this in terms of the atomistic origin of the strong electron-phonon coupling in ice.

  11. Effect of interfacial lattice mismatch on bulk carrier concentration and band gap of InN

    SciTech Connect

    Kuyyalil, Jithesh; Tangi, Malleswararao; Shivaprasad, S. M.

    2012-10-15

    The issue of ambiguous values of the band gap (0.6 to 2 eV) of InN thin film in literature has been addressed by a careful experiment. We have grown wurtzite InN films by PA-MBE simultaneously on differently modified c-plane sapphire substrates and characterized by complementary structural and chemical probes. Our studies discount Mie resonances caused by metallic In segregation at grain boundaries as the reason for low band gap values ( Almost-Equal-To 0.6 eV) and also the formation of Indium oxides and oxynitrides as the cause for high band gap value ( Almost-Equal-To 2.0 eV). It is observed that polycrystallinity arising from azimuthal miss-orientation of c-oriented wurtzite InN crystals increases the carrier concentration and the band gap values. We have reviewed the band gap, carrier concentration, and effective mass of InN in literature and our own measurements, which show that the Moss-Burstein relation with a non-parabolic conduction band accounts for the observed variation of band gap with carrier concentration.

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

    PubMed

    Kim, Jongseob; Lee, Sung-Hoon; Chung, Choong-Heui; Hong, Ki-Ha

    2016-02-14

    Solar cells based on organic-inorganic hybrid metal halide perovskites have been proven to be one of the most promising candidates for the next generation thin film photovoltaic cells. Mixing Br or Cl into I-based perovskites has been frequently tried to enhance the cell efficiency and stability. One of the advantages of mixed halides is the modulation of band gap by controlling the composition of the incorporated halides. However, the reported band gap transition behavior has not been resolved yet. Here a theoretical model is presented to understand the electronic structure variation of metal mixed-halide perovskites through hybrid density functional theory. Comparative calculations in this work suggest that the band gap correction including spin-orbit interaction is essential to describe the band gap changes of mixed halides. In our model, both the lattice variation and the orbital interactions between metal and halides play key roles to determine band gap changes and band alignments of mixed halides. It is also presented that the band gap of mixed halide thin films can be significantly affected by the distribution of halide composition. PMID:26791587

  13. Band gap engineering of FeS2 under biaxial strain: a first principles study.

    PubMed

    Xiao, Pin; Fan, Xiao-Li; Liu, Li-Min; Lau, Woon-Ming

    2014-11-28

    The promising photovoltaic activity of pyrite (FeS2) is attributed to its excellent optical absorptivity and earth abundance, but its band gap, 0.95 eV, is slightly lower than the optimum value of 1.3 eV. Here we report the first investigation of strained FeS2, whose band gap can be increased by ∼0.3 eV. The influence of uniaxial and biaxial strains on the atomic structure as well as the electronic and optical properties of bulk FeS2 is systematically examined by the first principles calculations. We found that the biaxial strain can effectively increase the band gap with respect to uniaxial strain. Our results indicate that the band gap increases with increasing tensile strain to its maximum value at 6% strain, but under the increasing compressive strain, the band gap decreases almost linearly. Moreover, the low intensity states at the bottom of the conduction band disappear and a sharp increase in the intensity appears at the lower energy level under the tensile strain, which causes the red shift of the absorption edge and enhances the overall optical absorption. With the increase of the band gap and enhanced optical absorption, FeS2 will make a better photovoltaic material. PMID:25308322

  14. Hybrid phononic crystal plates for lowering and widening acoustic band gaps.

    PubMed

    Badreddine Assouar, M; Sun, Jia-Hong; Lin, Fan-Shun; Hsu, Jin-Chen

    2014-12-01

    We propose hybrid phononic-crystal plates which are composed of periodic stepped pillars and periodic holes to lower and widen acoustic band gaps. The acoustic waves scattered simultaneously by the pillars and holes in a relevant frequency range can generate low and wide acoustic forbidden bands. We introduce an alternative double-sided arrangement of the periodic stepped pillars for an enlarged pillars' head diameter in the hybrid structure and optimize the hole diameter to further lower and widen the acoustic band gaps. The lowering and widening effects are simultaneously achieved by reducing the frequencies of locally resonant pillar modes and prohibiting suitable frequency bands of propagating plate modes. PMID:24996255

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

    PubMed

    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. PMID:18764257

  16. Kekule-induced band-gap opening in graphene in contact with ZrO2

    NASA Astrophysics Data System (ADS)

    Goh, Jung Suk; Choi, Hyoung Joon

    2013-03-01

    We have studied pressure-dependent atomic and electronic structure of graphene in contact with (111) surface of zirconium dioxide (ZrO2) using first-principles calculations. The atomic structures are optimized by relaxation, and we found that the lowest-energy configuration shows a band gap at the Dirac point at ambient pressure and the band gap increases as pressure increases. Our analysis shows that the band-gap opening is due to overlap of wavefunctions, change in potential energy, and in-plane distortion of graphene lattice. This in-plane distortion of graphene is found to be the Kekule distortion, which generates intervalley coupling. As pressure increases, the Kekule distortion in graphene increases and the band gap at the Dirac point is proportional to the size of the distortion. This work was supported by the NRF of Korea (Grant No. 2011-0018306) and KISTI Supercomputing Center (Project No. KSC-2012-C2-14).

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

    NASA Astrophysics Data System (ADS)

    Song, Dongyu; Li, Li; Li, Bingsheng; Sui, Yu; Shen, Aidong

    2016-06-01

    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.

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

    NASA Astrophysics Data System (ADS)

    Priya Rose, T.; di Gennaro, E.; Abbate, G.; Andreone, A.

    2011-09-01

    We report on the formation and development of the photonic band gap in two-dimensional 8-, 10-, and 12-fold symmetry quasicrystalline lattices of low-index contrast. Finite-size structures made of dielectric cylindrical rods were studied and measured in the microwave region, and their properties were compared with a conventional hexagonal crystal. Band-gap characteristics were investigated by changing the direction of propagation of the incident beam inside the crystal. Various angles of incidence from 0∘ to 30∘ were used to investigate the isotropic nature of the band gap. The arbitrarily high rotational symmetry of aperiodically ordered structures could be practically exploited to manufacture isotropic band-gap materials, which are perfectly suitable for hosting waveguides or cavities.

  19. Magnonic band gaps in two-dimension magnonic crystals with diffuse interfaces

    NASA Astrophysics Data System (ADS)

    Wang, Qi; Zhang, Huaiwu; Ma, Guokun; Tang, Xiaoli; Liao, Yulong; Zhong, Zhiyong

    2014-03-01

    In this paper, the plane wave method is extended to include the diffuse interface in the calculation of the dispersion of spin waves in two-dimension magnonic crystals. The diffuse interfaces with linear and sinusoidal profiles of variation in the spontaneous magnetization and exchange constant are considered and the effects of the thicknesses and profiles of diffuse interfaces on the magnonic band gaps are investigated. The results show that the thicknesses and profiles of diffuse interfaces are clearly seen to play a significant role in determining the size and position of the magnonic band gaps in the both square and triangular lattices in the exchange interaction regime. The smooth (linear or sinusoidal) interface does not lead to disappearance of the band gaps, instead it may lead to larger band gaps than those in the model with sharp (infinitely thin) diffuse interface under certain conditions.

  20. A New Silicon Allotrope with a Direct Band Gap for Optoelectronic Applications

    NASA Astrophysics Data System (ADS)

    Guo, Yaguang; Wang, Qian; Kawazoe, Yoshiyuki; Jena, Puru; Peking University Team; Kawazoe Collaboration; Jena Collaboration

    Silicon structures with direct band gaps have been hotly pursued for solar cell applications. To effectively harvest the sunlight in the whole frequency region, it is a good strategy to use arrays consisting of Si structures with different direct band gaps. However, the structure with a direct band gap about 0.6 eV has been missing according to current progress made in the direction. Here we report our findings that the missing structure can be constructed by using Si triangles as the building blocks, which is stable dynamically and thermally, not only exhibiting the desirable band gap, but also showing high intrinsic mobility and low mass density. These advantages over the existing Si structures would motivate new experimental effort in this direction.

  1. Further improvements in program to calculate electronic properties of narrow band gap materials

    NASA Technical Reports Server (NTRS)

    Patterson, James D.

    1991-01-01

    Research into the properties of narrow band gap materials during the period 15 Jun. to 15 Dec. 1991 is discussed. Abstracts and bibliographies from papers presented during this period are reported. Graphs are provided.

  2. Thermophotovoltaic conversion using selective infrared line emitters and large band gap photovoltaic devices

    DOEpatents

    Brandhorst, Jr., Henry W.; Chen, Zheng

    2000-01-01

    Efficient thermophotovoltaic conversion can be performed using photovoltaic devices with a band gap in the 0.75-1.4 electron volt range, and selective infrared emitters chosen from among the rare earth oxides which are thermally stimulated to emit infrared radiation whose energy very largely corresponds to the aforementioned band gap. It is possible to use thermovoltaic devices operating at relatively high temperatures, up to about 300.degree. C., without seriously impairing the efficiency of energy conversion.

  3. Enhanced thermoelectric performance of a BiCuSeO system via band gap tuning.

    PubMed

    Liu, Yong; Lan, Jinle; Xu, Wei; Liu, Yaochun; Pei, Yan-Ling; Cheng, Bo; Liu, Da-Bo; Lin, Yuan-Hua; Zhao, Li-Dong

    2013-09-21

    Upon 20% Te substitution, the band gap decreases from 0.8 eV to 0.65 eV. Rising temperature promotes minority carrier jumps across the band gap, thereby improving electrical conductivity. With low thermal conductivity and large Seebeck coefficients, a remarkable ZT of 0.71 at 873 K is achieved for BiCuSe0.94Te0.06O. PMID:23912639

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

    NASA Astrophysics Data System (ADS)

    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.

  5. One-dimensional electromagnetic band gap structures formed by discharge plasmas in a waveguide

    SciTech Connect

    Arkhipenko, V. I.; Simonchik, L. V. Usachonak, M. S.; Callegari, Th.; Sokoloff, J.

    2014-09-28

    We demonstrate the ability to develop one-dimensional electromagnetic band gap structure in X-band waveguide solely by using the positive columns of glow discharges in neon at the middle pressure. Plasma inhomogeneities are distributed uniformly along a typical X-band waveguide with cross section of 23×10 mm². It is shown that electron densities larger than 10¹⁴ cm ⁻³ are needed in order to create an effective one-dimensional electromagnetic band gap structure. Some applications for using the one-dimensional electromagnetic band gap structure in waveguide as a control of microwave (broadband filter and device for variation of pulse duration) are demonstrated.

  6. Local band gap measurements by VEELS of thin film solar cells.

    PubMed

    Keller, Debora; Buecheler, Stephan; Reinhard, Patrick; Pianezzi, Fabian; Pohl, Darius; Surrey, Alexander; Rellinghaus, Bernd; Erni, Rolf; Tiwari, Ayodhya N

    2014-08-01

    This work presents a systematic study that evaluates the feasibility and reliability of local band gap measurements of Cu(In,Ga)Se2 thin films by valence electron energy-loss spectroscopy (VEELS). The compositional gradients across the Cu(In,Ga)Se2 layer cause variations in the band gap energy, which are experimentally determined using a monochromated scanning transmission electron microscope (STEM). The results reveal the expected band gap variation across the Cu(In,Ga)Se2 layer and therefore confirm the feasibility of local band gap measurements of Cu(In,Ga)Se2 by VEELS. The precision and accuracy of the results are discussed based on the analysis of individual error sources, which leads to the conclusion that the precision of our measurements is most limited by the acquisition reproducibility, if the signal-to-noise ratio of the spectrum is high enough. Furthermore, we simulate the impact of radiation losses on the measured band gap value and propose a thickness-dependent correction. In future work, localized band gap variations will be measured on a more localized length scale to investigate, e.g., the influence of chemical inhomogeneities and dopant accumulations at grain boundaries. PMID:24690441

  7. Quasiparticle Band Gaps of Graphene and Graphone on Hexagonal Boron Nitride Substrate

    NASA Astrophysics Data System (ADS)

    Kharche, Neerav; Nayak, Saroj

    2012-02-01

    Graphene holds great promise for post-silicon electronics; however, it faces two main challenges: opening up a band gap and finding a suitable substrate material. Graphene on hexagonal boron nitride (hBN) substrate provides a potential system to overcome these challenges. While theoretical studies suggested a possibility of a finite band gap of graphene on hBN, recent experimental studies find no band gap. We have studied graphene-hBN system using the first-principles density functional method and the many-body perturbation theory within GW approximation [1]. A Bernal stacked graphene on hBN has a band gap on the order of 0.1 eV, which disappears when graphene is misaligned with respect to hBN. The latter is the likely scenario in realistic devices. In contrast, if graphene supported on hBN is hydrogenated, the resulting system (graphone) exhibits band gaps larger than 2.5 eV. The graphone band gap is due to chemical functionalization and is robust in the presence of misalignment, however, it reduces by about 1 eV due to the polarization effects at the graphone/hBN interface.[4pt] [1] N. Kharche and S. K. Nayak, Nano Lett., DOI: 10.1021/nl202725w, (2011).

  8. Origin of band gaps in graphene on hexagonal boron nitride

    PubMed Central

    Jung, Jeil; DaSilva, Ashley M.; MacDonald, Allan H.; Adam, Shaffique

    2015-01-01

    Recent progress in preparing well-controlled two-dimensional van der Waals heterojunctions has opened up a new frontier in materials physics. Here we address the intriguing energy gaps that are sometimes observed when a graphene sheet is placed on a hexagonal boron nitride substrate, demonstrating that they are produced by an interesting interplay between structural and electronic properties, including electronic many-body exchange interactions. Our theory is able to explain the observed gap behaviour by accounting first for the structural relaxation of graphene’s carbon atoms when placed on a boron nitride substrate, and then for the influence of the substrate on low-energy π-electrons located at relaxed carbon atom sites. The methods we employ can be applied to many other van der Waals heterojunctions. PMID:25695638

  9. Gap state analysis in electric-field-induced band gap for bilayer graphene

    PubMed Central

    Kanayama, Kaoru; Nagashio, Kosuke

    2015-01-01

    The origin of the low current on/off ratio at room temperature in dual-gated bilayer graphene field-effect transistors is considered to be the variable range hopping in gap states. However, the quantitative estimation of gap states has not been conducted. Here, we report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method. An energy gap of ~250 meV is obtained at the maximum displacement field of ~3.1 V/nm, where the current on/off ratio of ~3 × 103 is demonstrated at 20 K. The density of states for the gap states are in the range from the latter half of 1012 to 1013 eV−1cm−2. Although the large amount of gap states at the interface of high-k oxide/bilayer graphene limits the current on/off ratio at present, our results suggest that the reduction of gap states below ~1011 eV−1cm−2 by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications. PMID:26511395

  10. Gap state analysis in electric-field-induced band gap for bilayer graphene.

    PubMed

    Kanayama, Kaoru; Nagashio, Kosuke

    2015-01-01

    The origin of the low current on/off ratio at room temperature in dual-gated bilayer graphene field-effect transistors is considered to be the variable range hopping in gap states. However, the quantitative estimation of gap states has not been conducted. Here, we report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method. An energy gap of ~ 250 meV is obtained at the maximum displacement field of ~ 3.1 V/nm, where the current on/off ratio of ~ 3 × 10(3) is demonstrated at 20 K. The density of states for the gap states are in the range from the latter half of 10(12) to 10(13) eV(-1) cm(-2). Although the large amount of gap states at the interface of high-k oxide/bilayer graphene limits the current on/off ratio at present, our results suggest that the reduction of gap states below ~ 10(11) eV(-1) cm(-2) by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications. PMID:26511395

  11. Band gap engineering for single-layer graphene by using slow Li(+) ions.

    PubMed

    Ryu, Mintae; Lee, Paengro; Kim, Jingul; Park, Heemin; Chung, Jinwook

    2016-08-01

    In order to utilize the superb electronic properties of graphene in future electronic nano-devices, a dependable means of controlling the transport properties of its Dirac electrons has to be devised by forming a tunable band gap. We report on the ion-induced modification of the electronic properties of single-layer graphene (SLG) grown on a SiC(0001) substrate by doping low-energy (5 eV) Li(+) ions. We find the opening of a sizable and tunable band gap up to 0.85 eV, which depends on the Li(+) ion dose as well as the following thermal treatment, and is the largest band gap in the π-band of SLG by any means reported so far. Our Li 1s core-level data together with the valence band suggest that Li(+) ions do not intercalate below the topmost graphene layer, but cause a significant charge asymmetry between the carbon sublattices of SLG to drive the opening of the band gap. We thus provide a route to producing a tunable graphene band gap by doping Li(+) ions, which may play a pivotal role in the utilization of graphene in future graphene-based electronic nano-devices. PMID:27345294

  12. Efficient Excitonic Photoluminescence in Direct and Indirect Band Gap Monolayer MoS2.

    PubMed

    Steinhoff, A; Kim, J-H; Jahnke, F; Rösner, M; Kim, D-S; Lee, C; Han, G H; Jeong, M S; Wehling, T O; Gies, C

    2015-10-14

    We discuss the photoluminescence (PL) of semiconducting transition metal dichalcogenides on the basis of experiments and a microscopic theory. The latter connects ab initio calculations of the single-particle states and Coulomb matrix elements with a many-body description of optical emission spectra. For monolayer MoS2, we study the PL efficiency at the excitonic A and B transitions in terms of carrier populations in the band structure and provide a quantitative comparison to an (In)GaAs quantum well-structure. Suppression and enhancement of PL under biaxial strain is quantified in terms of changes in the local extrema of the conduction and valence bands. The large exciton binding energy in MoS2 enables two distinctly different excitation methods: above-band gap excitation and quasi-resonant excitation of excitonic resonances below the single-particle band gap. The latter case creates a nonequilibrium distribution of carriers predominantly in the K-valleys, which leads to strong emission from the A-exciton transition and a visible B-peak even if the band gap is indirect. For above-band gap excitation, we predict a strongly reduced emission intensity at comparable carrier densities and the absence of B-exciton emission. The results agree well with PL measurements performed on monolayer MoS2 at excitation wavelengths of 405 nm (above) and 532 nm (below the band gap). PMID:26322814

  13. Band gap engineering for single-layer graphene by using slow Li+ ions

    NASA Astrophysics Data System (ADS)

    Ryu, Mintae; Lee, Paengro; Kim, Jingul; Park, Heemin; Chung, Jinwook

    2016-08-01

    In order to utilize the superb electronic properties of graphene in future electronic nano-devices, a dependable means of controlling the transport properties of its Dirac electrons has to be devised by forming a tunable band gap. We report on the ion-induced modification of the electronic properties of single-layer graphene (SLG) grown on a SiC(0001) substrate by doping low-energy (5 eV) Li+ ions. We find the opening of a sizable and tunable band gap up to 0.85 eV, which depends on the Li+ ion dose as well as the following thermal treatment, and is the largest band gap in the π-band of SLG by any means reported so far. Our Li 1s core-level data together with the valence band suggest that Li+ ions do not intercalate below the topmost graphene layer, but cause a significant charge asymmetry between the carbon sublattices of SLG to drive the opening of the band gap. We thus provide a route to producing a tunable graphene band gap by doping Li+ ions, which may play a pivotal role in the utilization of graphene in future graphene-based electronic nano-devices.

  14. Communication: excitation band modulation with high-order photonic band gap in PMMA:Eu(TTA)3(TPPO)2 opals.

    PubMed

    Xu, Wen; Bai, Xue; Zhu, Yongsheng; Liu, Tong; Xu, Sai; Dong, Biao; Song, Hongwei

    2013-05-14

    Changes in the excitation spectra of luminescent species inserted in photorefractive crystals as a function of changes in the high-order photonic band gap (PBG) have not been previously observed. In this communication, we present our results monitoring the excitation band of Eu(TTA)3(TPPO)2 inserted in the PMMA opal photonic crystals as a function of the changes in the high-order PBG of the crystals. We find shifts in the complex excitation band and changes in the integrated emission intensity that correlates with shifts in the high-order PBG through coupling to the excitation transition. PMID:23676019

  15. Communication: Excitation band modulation with high-order photonic band gap in PMMA:Eu(TTA)3(TPPO)2 opals

    NASA Astrophysics Data System (ADS)

    Xu, Wen; Bai, Xue; Zhu, Yongsheng; Liu, Tong; Xu, Sai; Dong, Biao; Song, Hongwei

    2013-05-01

    Changes in the excitation spectra of luminescent species inserted in photorefractive crystals as a function of changes in the high-order photonic band gap (PBG) have not been previously observed. In this communication, we present our results monitoring the excitation band of Eu(TTA)3(TPPO)2 inserted in the PMMA opal photonic crystals as a function of the changes in the high-order PBG of the crystals. We find shifts in the complex excitation band and changes in the integrated emission intensity that correlates with shifts in the high-order PBG through coupling to the excitation transition.

  16. Small band gap superlattices as intrinsic long wavelength infrared detector materials

    NASA Technical Reports Server (NTRS)

    Smith, Darryl L.; Mailhiot, C.

    1990-01-01

    Intrinsic long wavelength (lambda greater than or equal to 10 microns) infrared (IR) detectors are currently made from the alloy (Hg, Cd)Te. There is one parameter, the alloy composition, which can be varied to control the properties of this material. The parameter is chosen to set the band gap (cut-off wavelength). The (Hg, Cd)Te alloy has the zincblend crystal structure. Consequently, the electron and light-hole effective masses are essentially inversely proportional to the band gap. As a result, the electron and light-hole effective masses are very small (M sub(exp asterisk)/M sub o approx. M sub Ih/M sub o approx. less than 0.01) whereas the heavy-hole effective mass is ordinary size (M sub hh(exp asterisk)/M sub o approx. 0.4) for the alloy compositions required for intrinsic long wavelength IR detection. This combination of effective masses leads to rather easy tunneling and relatively large Auger transition rates. These are undesirable characteristics, which must be designed around, of an IR detector material. They follow directly from the fact that (Hg, Cd)Te has the zincblend crystal structure and a small band gap. In small band gap superlattices, such as HgTe/CdTe, In(As, Sb)/InSb and InAs/(Ga,In)Sb, the band gap is determined by the superlattice layer thicknesses as well as by the alloy composition (for superlattices containing an alloy). The effective masses are not directly related to the band gap and can be separately varied. In addition, both strain and quantum confinement can be used to split the light-hole band away from the valence band maximum. These band structure engineering options can be used to reduce tunneling probabilities and Auger transition rates compared with a small band gap zincblend structure material. Researchers discuss the different band structure engineering options for the various classes of small band gap superlattices.

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

    NASA Astrophysics Data System (ADS)

    Yang, Shu; Li, Masha

    2016-04-01

    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.

  18. Superlattices of group IV elements, a new possibility to produce direct band gap material

    NASA Astrophysics Data System (ADS)

    Kasper, E.

    1991-01-01

    Consideration is given to the diamond lattice type group IV semiconductors C, SiC, Si and Ge, which exhibit an indirect band gap with the conduction band minimum outside the Brillouin zone center. Ultrathin superlattices are predicted to convert the indirect band gap into a quasi-direct one under certain circumstances. Attention is also given to the growth of Si/Ge strained monolayer superlattices (SMS) by molecular beam epitaxy and experimental results obtained with these structures. The existing investigations of Si/Ge SMS have shown folded quasi-direct conditions in group IV superlattices.

  19. Impurity-Band Model for GaP1-xNx

    SciTech Connect

    Fluegel, B.; Zhang, Y.; Geisz, J. F.; Mascarenhas, A.

    2005-11-01

    Low-temperature absorption studies on free-standing GaP1-xNx films provide direct experimental evidence that the host conduction-band minimum (CBM) near X1C does not plunge downward with increased nitrogen doping, contrary to what has been suggested recently; rather, it remains stationary for x up to 0.1%. This fact, combined with the results of earlier studies of the CBM at ..GAMMA.. and conduction-band edge near L, confirms that the giant bandgap lowering observed in GaP1-xNx results from a CBM that evolves purely from nitrogen impurity bands.

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

    NASA Astrophysics Data System (ADS)

    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.

  1. Analysis of longitudinal vibration band gaps in periodic carbon nanotube intramolecular junctions using finite element method

    NASA Astrophysics Data System (ADS)

    Li, Jiaqian; Shen, Haijun

    2015-12-01

    The longitudinal vibration band gaps in periodic (n, 0)-(2n, 0) single-walled carbon nanotube(SWCNT) intramolecular junctions(IMJs) are investigated based on the finite element calculation. The frequency ranges of band gaps in frequency response functions(FRF) simulated by finite element method (FEM) show good agreement with those in band structure obtained by simple spring-mass model. Moreover, a comprehensive parametric study is also conducted to highlight the influences of the geometrical parameters such as the size of unit cell, component ratios of the IMJs and diameters of the CNT segments as well as geometric imperfections on the first band gap. The results show that the frequency ranges and the bandwidth of the gap strongly depend on the geometrical parameters. Furthermore, the influences of geometrical parameters on gaps are nuanced in IMJs with different topological defects. The existence of vibration band gaps in periodic IMJs lends a new insight into the development of CNT-based nano-devices in application of vibration isolation.

  2. Band gap and electronic structure of MgSiN{sub 2}

    SciTech Connect

    Quirk, J. B. Råsander, M.; McGilvery, C. M.; Moram, M. A.; Palgrave, R.

    2014-09-15

    Density functional theory calculations and electron energy loss spectroscopy indicate that the electronic structure of ordered orthorhombic MgSiN{sub 2} is similar to that of wurtzite AlN. A band gap of 5.7 eV was calculated for both MgSiN{sub 2} (indirect) and AlN (direct) using the Heyd-Scuseria-Ernzerhof approximation. Correction with respect to the experimental room-temperature band gap of AlN indicates that the true band gap of MgSiN{sub 2} is 6.2 eV. MgSiN{sub 2} has an additional direct gap of 6.3 eV at the Γ point.

  3. Strain-induced band-gap engineering of graphene monoxide and its effect on graphene

    NASA Astrophysics Data System (ADS)

    Pu, H. H.; Rhim, S. H.; Hirschmugl, C. J.; Gajdardziska-Josifovska, M.; Weinert, M.; Chen, J. H.

    2013-02-01

    Using first-principles calculations we demonstrate the feasibility of band-gap engineering in two-dimensional crystalline graphene monoxide (GMO), a recently reported graphene-based material with a 1:1 carbon/oxygen ratio. The band gap of GMO, which can be switched between direct and indirect, is tunable over a large range (0-1.35 eV) for accessible strains. Electron and hole transport occurs predominantly along the zigzag and armchair directions (armchair for both) when GMO is a direct- (indirect-) gap semiconductor. A band gap of ˜0.5 eV is also induced in graphene at the K' points for GMO/graphene hybrid systems.

  4. Band gap engineering for graphene by using Na{sup +} ions

    SciTech Connect

    Sung, S. J.; Lee, P. R.; Kim, J. G.; Ryu, M. T.; Park, H. M.; Chung, J. W.

    2014-08-25

    Despite the noble electronic properties of graphene, its industrial application has been hindered mainly by the absence of a stable means of producing a band gap at the Dirac point (DP). We report a new route to open a band gap (E{sub g}) at DP in a controlled way by depositing positively charged Na{sup +} ions on single layer graphene formed on 6H-SiC(0001) surface. The doping of low energy Na{sup +} ions is found to deplete the π* band of graphene above the DP, and simultaneously shift the DP downward away from Fermi energy indicating the opening of E{sub g}. The band gap increases with increasing Na{sup +} coverage with a maximum E{sub g}≥0.70 eV. Our core-level data, C 1s, Na 2p, and Si 2p, consistently suggest that Na{sup +} ions do not intercalate through graphene, but produce a significant charge asymmetry among the carbon atoms of graphene to cause the opening of a band gap. We thus provide a reliable way of producing and tuning the band gap of graphene by using Na{sup +} ions, which may play a vital role in utilizing graphene in future nano-electronic devices.

  5. Indirect-to-direct band gap crossover in few-layer MoTe₂.

    PubMed

    Lezama, Ignacio Gutiérrez; Arora, Ashish; Ubaldini, Alberto; Barreteau, Céline; Giannini, Enrico; Potemski, Marek; Morpurgo, Alberto F

    2015-04-01

    We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temperature microreflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions associated with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconductor and with trilayers having nearly identical direct and indirect gaps. This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides for which monolayers are found to be direct band gap semiconductors, and thicker layers have indirect band gaps that are significantly smaller (by hundreds of meV) than the direct gap. We discuss the relevance of our findings for experiments of fundamental interest and possible future device applications. PMID:25803208

  6. Band gap states of Ti, V, and Cr in 4H-silicon carbide

    NASA Astrophysics Data System (ADS)

    Achtziger, Norbert; Witthuhn, Wolfgang

    1997-07-01

    Band gap states of Ti, V, and Cr in n-type 4H-SiC were investigated by radiotracer deep level transient spectroscopy (DLTS). Doping with the radioactive isotopes 48V and 51Cr was done by recoil implantation followed by annealing (1600 K). Repeated DLTS measurements during the elemental transmutation of these isotopes to 48Ti and 51V, respectively, reveal the corresponding concentration changes of band gap states. Thus, six levels are identified in the band gap: Cr levels at 0.15, 0.18, and 0.74 eV, one V level at 0.97 eV, and two Ti levels at 0.13 and 0.17 eV below the conduction band edge.

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

    PubMed

    Kong, Lingping; Liu, Gang; Gong, Jue; Hu, Qingyang; Schaller, Richard D; Dera, Przemyslaw; Zhang, Dongzhou; Liu, Zhenxian; Yang, Wenge; Zhu, Kai; Tang, Yuzhao; Wang, Chuanyi; Wei, Su-Huai; Xu, Tao; Mao, Ho-Kwang

    2016-08-01

    The organic-inorganic hybrid lead trihalide perovskites have been emerging as the most attractive photovoltaic materials. As regulated by Shockley-Queisser theory, a formidable materials science challenge for improvement to the next level requires further band-gap narrowing for broader absorption in solar spectrum, while retaining or even synergistically prolonging the carrier lifetime, a critical factor responsible for attaining the near-band-gap photovoltage. Herein, by applying controllable hydrostatic pressure, we have achieved unprecedented simultaneous enhancement in both band-gap narrowing and carrier-lifetime prolongation (up to 70% to ∼100% increase) under mild pressures at ∼0.3 GPa. The pressure-induced modulation on pure hybrid perovskites without introducing any adverse chemical or thermal effect clearly demonstrates the importance of band edges on the photon-electron interaction and maps a pioneering route toward a further increase in their photovoltaic performance. PMID:27444014

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

    NASA Technical Reports Server (NTRS)

    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.

  9. Pentamode metamaterials with tunable acoustics band gaps and large figures of merit

    NASA Astrophysics Data System (ADS)

    Wang, Zhaohong; Cai, Chengxin; Li, Qingwei; Li, Jing; Xu, Zhuo

    2016-07-01

    In this paper, we propose a class of pentamode metamaterials for which the frequency range of the acoustics band gaps can be tuned and large figures of merit can be obtained. The band structures of the pentamode metamaterials are calculated systematically by using the finite element method. The numerical results show that the lower edge frequency of the first acoustics band gaps of pentamode metamaterials can be tuned between 3.72 kHz and 10.6 kHz by changing the diameters of the bottom and top touch cones slightly, and the relative bandwidth of the first acoustics band gaps can also be expanded. In addition, compared with the results seen in the previous research in this area, the volume filling fraction of pentamode metamaterials can be decreased by 15.7%-24.4% and the maximum figure of merit can be increased by 39.2%.

  10. Band-Gap Modulation of GeCH3 Nanoribbons Under Elastic Strain: A Density Functional Theory Study

    NASA Astrophysics Data System (ADS)

    Ma, ShengQian; Li, Feng; Jiang, ChunLing

    2016-06-01

    Using the density functional theory method, we researched the band-gap modulation of GeCH3 nanoribbons under uniaxial elastic strain. The results indicated that the band gap of GeCH3 nanoribbons could be tuned along two directions, namely, stretching or compressing ribbons when ɛ was changed from -10% to 10% in 6-zigzag, 10-zigzag, 13-armchair, and 17-armchair nanoribbons, respectively. The band gap greatly changed with strain. In the case of tension, the amount of change in the band gap was bigger. But in the case of compression, the gradient was steeper. The band gap had a nearly linear relationship when ɛ ranges from 0% to 10%. We also investigated if the band gap is changed with widths. The results showed variation of the band gap did not rely on widths. Therefore, the GeCH3 nanoribbons had the greatest potential application in strain sensors and optical electronics at the nanoscale.

  11. Nonlinear band gap transmission in optical waveguide arrays.

    PubMed

    Khomeriki, Ramaz

    2004-02-13

    The effect of nonlinear transmission in coupled optical waveguide arrays is theoretically investigated and a realistic experimental setup is suggested. The beam is injected in a single boundary waveguide, linear refractive index of which (n(0)) is larger than refractive indexes (n) of other identical waveguides in the array. Particularly, the effect holds if omega(n(0)-n)/c>2Q, where Q is a linear coupling constant between array waveguides, omega is a carrier wave frequency, and c is a light velocity. Numerical experiments show that the energy transfers from the boundary waveguide to the waveguide array above a certain threshold intensity of the injected beam. This effect is due to the creation and the propagation of gap solitons in full analogy with a similar phenomenon in sine-Gordon lattice [Phys. Rev. Lett. 89, 134102 (2002)

  12. Hydrostatic pressure sensor based on micro-cavities developed by the catastrophic fuse effect

    NASA Astrophysics Data System (ADS)

    Domingues, M. F.; Paixão, T.; Mesquita, E.; Alberto, N.; Antunes, P.; Varum, H.; André, P. S.

    2015-09-01

    In this work, an optical fiber hydrostatic pressure sensor based in Fabry-Perot micro-cavities is presented. These micro structures were generated by the recycling of optical fiber previously damaged by the fiber fuse effect, resulting in a cost effective solution when compared with the traditional methods used to produce similar micro-cavities. The developed sensor was tested for pressures ranging from 20.0 to 190.0 cmH2O and a sensitivity of 53.7 +/- 2.6 pm/cmH2O for hydrostatic pressures below to 100 cmH2O was achieved.

  13. Near band edge anisotropic optical transitions in wide band gap semiconductor Cu2ZnSiS4

    NASA Astrophysics Data System (ADS)

    Levcenco, S.; Dumcenco, D.; Huang, Y. S.; Arushanov, E.; Tezlevan, V.; Tiong, K. K.; Du, C. H.

    2010-10-01

    In this study, anisotropic near band edge transitions of Cu2ZnSiS4 single crystals grown by chemical vapor transport were characterized by using polarization-dependent absorption, piezoreflectance (PzR) and surface photovoltage (SPV) spectroscopy techniques at room temperature. The measurements were carried out on the as grown basal plane with the normal along [2 1 0] and the axis c parallel to the long edge of the crystal platelet. Analysis of absorption and SPV spectra reveal indirect allowed transitions for the absorption edge of Cu2ZnSiS4. The estimated values of indirect band gap are 2.97 eV and 3.07 eV, respectively, for E ⊥c and E ∥c polarization configurations. The polarization-dependent PzR and SPV spectra in the vicinity of the direct band gap of Cu2ZnSiS4 reveal features E⊥ex and E∥ex at around 3.32 eV and 3.41 eV for E ⊥c and E ∥c polarizations, respectively. Both features E⊥ex and E∥ex are associated with the interband excitonic transitions at point Γ and can be explained by crystal-field splitting of valence band. Based on the experimental observations, a plausible band structure near band edge of Cu2ZnSiS4 is proposed.

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

    NASA Astrophysics Data System (ADS)

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

    2016-06-01

    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.

  15. Direct band gaps in group IV-VI monolayer materials: Binary counterparts of phosphorene

    NASA Astrophysics Data System (ADS)

    Kamal, C.; Chakrabarti, Aparna; Ezawa, Motohiko

    2016-03-01

    We perform systematic investigation on the geometric, energetic, and electronic properties of group IV-VI binary monolayers (XY ), which are the counterparts of phosphorene, by employing density functional theory based electronic structure calculations. For this purpose, we choose the binary systems X Y consisting of equal numbers of group IV (X = C, Si, Ge, Sn) and group VI elements (Y = O, S, Se, Te) in three geometrical configurations, the puckered, buckled and planar structures. The results of binding energy calculations show that all the binary systems studied are energetically stable. It is observed that, the puckered structure, similar to that of phosphorene, is the energetically most stable geometric configuration. Moreover, the binding energies of buckled configuration are very close to those of the puckered configuration. Our results of electronic band structure predict that puckered SiO and CSe are direct band semiconductors with gaps of 1.449 and 0.905 eV, respectively. Band structure of CSe closely resembles that of phosphorene. Remaining group IV-VI binary monolayers in the puckered configuration and all the buckled monolayers are also semiconductors, but with indirect band gaps. Importantly, we find that the difference between indirect and direct band gaps is very small for many puckered monolayers. Thus there is a possibility of making these systems undergo transition from indirect to direct band gap semiconducting state by a suitable external influence. Indeed, we show in the present work that seven binary monolayers, namely, SnS, SiSe, GeSe, SnSe, SiTe, GeTe, and SnTe become direct band gap semiconductors when they are subjected to a small mechanical strain (≤3 % ). This makes nine out of sixteen binary monolayers studied in the present work direct band gap semiconductors. Thus there is a possibility of utilizing these binary counterparts of phosphorene in future light-emitting diodes and solar cells.

  16. TiS3 nanoribbons: Width-independent band gap and strain-tunable electronic properties

    NASA Astrophysics Data System (ADS)

    Kang, Jun; Sahin, Hasan; Ozaydin, H. Duygu; Senger, R. Tugrul; Peeters, François M.

    2015-08-01

    The electronic properties, carrier mobility, and strain response of TiS3 nanoribbons (TiS3 NRs) are investigated by first-principles calculations. We found that the electronic properties of TiS3 NRs strongly depend on the edge type (a or b). All a-TiS3 NRs are metallic with a magnetic ground state, while b-TiS3 NRs are direct band gap semiconductors. Interestingly, the size of the band gap and the band edge position are almost independent of the ribbon width. This feature promises a constant band gap in a b-TiS3 NR with rough edges, where the ribbon width differs in different regions. The maximum carrier mobility of b-TiS3 NRs is calculated by using the deformation potential theory combined with the effective mass approximation and is found to be of the order 103cm2V-1s-1 . The hole mobility of the b-TiS3 NRs is one order of magnitude lower, but it is enhanced compared to the monolayer case due to the reduction in hole effective mass. The band gap and the band edge position of b-TiS3 NRs are quite sensitive to applied strain. In addition we investigate the termination of ribbon edges by hydrogen atoms. Upon edge passivation, the metallic and magnetic features of a-TiS3 NRs remain unchanged, while the band gap of b-TiS3 NRs is increased significantly. The robust metallic and ferromagnetic nature of a-TiS3 NRs is an essential feature for spintronic device applications. The direct, width-independent, and strain-tunable band gap, as well as the high carrier mobility, of b-TiS3 NRs is of potential importance in many fields of nanoelectronics, such as field-effect devices, optoelectronic applications, and strain sensors.

  17. Single Material Band Gap Engineering in GaAs Nanowires

    SciTech Connect

    Spirkoska, D.; Abstreiter, G.; Efros, A.; Conesa-Boj, S.; Morante, J. R.; Arbiol, J.; Fontcuberta i Morral, A.

    2011-12-23

    The structural and optical properties of GaAs nanowire with mixed zinc-blende/wurtzite structure are presented. High resolution transmission electron microscopy indicates the presence of a variety of shorter and longer segments of zinc-blende or wurtzite crystal phases. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV. The downward shift of the emission peaks can be understood by carrier confinement at the wurtzite/zinc-blende heterojunction, in quantum wells and in random short period superlattices existent in these nanowires, assuming the theoretical staggered band-offset between wurtzite and zinc-blende GaAs.

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

    NASA Astrophysics Data System (ADS)

    Wu, Liang-Yu; Chen, Lien-Wen

    2011-02-01

    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 Γ-X and Γ-X' directions are also presented. The calculated results are compared with the experimental results.

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

    NASA Astrophysics Data System (ADS)

    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).

  20. Direct Band Gap Gallium Antimony Phosphide (GaSbxP1−x) Alloys

    PubMed Central

    Russell, H. B.; Andriotis, A. N.; Menon, M.; Jasinski, J. B.; Martinez-Garcia, A.; Sunkara, M. K.

    2016-01-01

    Here, we report direct band gap transition for Gallium Phosphide (GaP) when alloyed with just 1–2 at% antimony (Sb) utilizing both density functional theory based computations and experiments. First principles density functional theory calculations of GaSbxP1−x alloys in a 216 atom supercell configuration indicate that an indirect to direct band gap transition occurs at x = 0.0092 or higher Sb incorporation into GaSbxP1−x. Furthermore, these calculations indicate band edge straddling of the hydrogen evolution and oxygen evolution reactions for compositions ranging from x = 0.0092 Sb up to at least x = 0.065 Sb making it a candidate for use in a Schottky type photoelectrochemical water splitting device. GaSbxP1−x nanowires were synthesized by reactive transport utilizing a microwave plasma discharge with average compositions ranging from x = 0.06 to x = 0.12 Sb and direct band gaps between 2.21 eV and 1.33 eV. Photoelectrochemical experiments show that the material is photoactive with p-type conductivity. This study brings attention to a relatively uninvestigated, tunable band gap semiconductor system with tremendous potential in many fields. PMID:26860470

  1. Band-gap measurements of direct and indirect semiconductors using monochromated electrons

    SciTech Connect

    Gu Lin; Srot, Vesna; Sigle, Wilfried; Koch, Christoph; Aken, Peter van; Ruehle, Manfred; Scholz, Ferdinand; Thapa, Sarad B.; Kirchner, Christoph; Jetter, Michael

    2007-05-15

    With the development of monochromators for transmission electron microscopes, valence electron-energy-loss spectroscopy (VEELS) has become a powerful technique to study the band structure of materials with high spatial resolution. However, artifacts such as Cerenkov radiation pose a limit for interpretation of the low-loss spectra. In order to reveal the exact band-gap onset using the VEELS method, semiconductors with direct and indirect band-gap transitions have to be treated differently. For direct semiconductors, spectra acquired at thin regions can efficiently minimize the Cerenkov effects. Examples of hexagonal GaN (h-GaN) spectra acquired at different thickness showed that a correct band-gap onset value can be obtained for sample thicknesses up to 0.5 t/{lambda}. In addition, {omega}-q maps acquired at different specimen thicknesses confirm the thickness dependency of Cerenkov losses. For indirect semiconductors, the correct band-gap onset can be obtained in the dark-field mode when the required momentum transfer for indirect transition is satisfied. Dark-field VEEL spectroscopy using a star-shaped entrance aperture provides a way of removing Cerenkov effects in diffraction mode. Examples of Si spectra acquired by displacing the objective aperture revealed the exact indirect transition gap E{sub g} of 1.1 eV.

  2. Band gaps in InN/GaN superlattices: Nonpolar and polar growth directions

    SciTech Connect

    Gorczyca, I. Skrobas, K.; Suski, T.; Christensen, N. E.; Svane, A.

    2013-12-14

    The electronic structures of nonpolar short-period InN/GaN superlattices (SLs) grown in the wurtzite a- and m-directions have been calculated and compared to previous calculations for polar superlattices (grown in the c-direction). The variation of the band gaps with the composition (m, n) of the mInN/nGaN unit cells of the superlattices was examined. The band structures were obtained by self-consistent calculations based on the local density approximation to the density functional theory using the Linear-Muffin-Tin-Orbital method with a semi-empirical correction for the band gaps. The calculated band gaps and their pressure coefficients for nonpolar superlattices are similar to those calculated for bulk InGaN alloys with an equivalent In/Ga concentration ratio. This is very different from what has been found in polar superlattices where the band gaps are much smaller and vanish when the number m of InN layers in the unit cell exceeds three. A strong internal electric field is responsible for this behavior of polar structures. Experimental photoluminescence data for polar SLs agree very well with gaps calculated for the nonpolar structures. It is suggested that this is caused by screening of the electric field in the polar structures by carriers originating from unintentional defects.

  3. Residual stress dependant anisotropic band gap of various (hkl) oriented BaI{sub 2} films

    SciTech Connect

    Kumar, Pradeep; Gulia, Vikash; Vedeshwar, Agnikumar G. E-mail: agvedeshwar@gmail.com

    2013-11-21

    The thermally evaporated layer structured BaI{sub 2} grows in various completely preferred (hkl) film orientations with different growth parameters like film thickness, deposition rate, substrate temperature, etc. which were characterized by structural, morphological, and optical absorption measurements. Structural analysis reveals the strain in the films and the optical absorption shows a direct type band gap. The varying band gaps of these films were found to scale linearly with their strain. The elastic moduli and other constants were also calculated using Density Functional Theory (DFT) formalism implemented in WIEN2K code for converting the strain into residual stress. Films of different six (hkl) orientations show stress free anisotropic band gaps (2.48–3.43 eV) and both positive and negative pressure coefficients. The negative and positive pressure coefficients of band gap are attributed to the strain in I-I (or Ba-Ba or both) and Ba-I distances along [hkl], respectively. The calculated band gaps are also compared with those experimentally determined. The average pressure coefficient of band gap of all six orientations (−0.071 eV/GPa) found to be significantly higher than that calculated (−0.047 eV/GPa) by volumetric pressure dependence. Various these issues have been discussed with consistent arguments. The electron effective mass m{sub e}{sup *}=0.66m{sub 0} and the hole effective mass m{sub h}{sup *}=0.53m{sub 0} have been determined from the calculated band structure.

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

    NASA Astrophysics Data System (ADS)

    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.

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

    SciTech Connect

    Canulescu, S. Schou, J.; Rechendorff, K.; Pleth Nielsen, L.; Borca, C. N.; Jones, N. C.; Hoffmann, S. V.; Bordo, K.; Ambat, R.

    2014-03-24

    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 TiO{sub 2} 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 Al{sub 2}O{sub 3} has a direct band gap of 7.3 eV, which is about ∼1.4 eV lower than its crystalline counterpart (single-crystal Al{sub 2}O{sub 3}). Upon Ti-alloying, extra bands appear within the band gap of amorphous Al{sub 2}O{sub 3}, mainly caused by Ti 3d orbitals localized at the Ti site.

  6. Reliability of the Tran-Blaha functional in predicting band gaps and widths

    NASA Astrophysics Data System (ADS)

    Rignanese, Gian-Marco; David, Waroquiers; Lherbier, Aurélien; Miglio, Anna; Stankovski, Martin; Ponce, Samuel; Oliveira, Micael; Giantomassi, Matteo; Gonze, Xavier

    2013-03-01

    For a set of oxides and semiconductors, we compute the electronic band structures (gaps and widths) within Density-Functional theory (DFT) using the Tran-Blaha (TB09) functional [Phys. Rev. Lett. 102, 226401 (2009)]. We compare them with those obtained from (i) DFT using the local-density approximation (LDA), (ii) many-body perturbation theory (MBPT), and (iii) experiments. TB09 leads to band gaps in much better agreement with experiment than the LDA. However, the valence (and conduction) band widths are often underestimated (noticeably more than in LDA). MBPT corrections are obtained peforming one-shot GW calculations using DFT eigenenergies and wavefunctions as starting point (both LDA and TB09 are considered). These corrections lead to a much better agreement with experimental data for the band widths. The MBPT band gaps obtained starting TB09 are close to those from quasi-particle self-consistent GW calculations, at a much reduced cost. Finally, we explore the possibility to tune a semi-empirical parameter present in the TB09 functional aiming to obtain simultaneously better gaps and band widths. We find that these requirements are conflicting.

  7. Experimental and Theoretical Studies of Photonic Band gaps in Artificial Opals

    NASA Astrophysics Data System (ADS)

    Wang, Lei; Yin, Ming; Arammash, Fouzi; Datta, Timir

    2014-03-01

    Photonic band structure and band gap were numerically computed for a number of closed packed simple cubic and Hexagonal arrangements of non-conducting spheres using ``Finite Difference Time Domain Method''. Photonic gaps were found to exist in the simple cubic overlapping spheres with index of refraction (n) >3.2. Gap increased linearly from 0.117- 0.161 (1/micron) as lattice constant decreased from 0.34 to 0.18 (micron). For less than 3.2 no gap was obtained. Also, no gaps were obtained for hexagonal packing. UV-VIS reflectivity and transmission measurements of polycrystalline bulk artificial opals of silica (SiO2) spheres, ranging from 250nm to 300nm in sphere diameter indicate a reflection peak in the 500-600 nm regimes. Consistent with photonic band gap behavior we find that reflectivity is enhanced in the same wavelength where transmission is reduced. To the best of our knowledge this is the first observation of photonic gap in the visible wave length under ambient conditions. The wave length at the reflectance peak increases with the diameter of the SiO2 spheres, and is approximately twice the diameter following Bragg reflection. DOD Award No 60177-RT-H from ARO.

  8. Effects of corrugation shape on frequency band-gaps for longitudinal wave motion in a periodic elastic layer.

    PubMed

    Sorokin, Vladislav S

    2016-04-01

    The paper concerns determining frequency band-gaps for longitudinal wave motion in a periodic waveguide. The waveguide may be considered either as an elastic layer with variable thickness or as a rod with variable cross section. As a result, widths and locations of all frequency band-gaps are determined by means of the method of varying amplitudes. For the general symmetric corrugation shape, the width of each odd band-gap is controlled only by one harmonic in the corrugation series with its number being equal to the number of the band-gap. Widths of even band-gaps, however, are influenced by all the harmonics involved in the corrugation series, so that the lower frequency band-gaps can emerge. These are band-gaps located below the frequency corresponding to the lowest harmonic in the corrugation series. For the general non-symmetric corrugation shape, the mth band-gap is controlled only by one, the mth, harmonic in the corrugation series. The revealed insights into the mechanism of band-gap formation can be used to predict locations and widths of all frequency band-gaps featured by any corrugation shape. These insights are general and can be valid also for other types of wave motion in periodic structures, e.g., transverse or torsional vibration. PMID:27106336

  9. Zero-coupling-gap degenerate band edge resonators in silicon photonics.

    PubMed

    Burr, Justin R; Reano, Ronald M

    2015-11-30

    Resonances near regular photonic band edges are limited by quality factors that scale only to the third power of the number of periods. In contrast, resonances near degenerate photonic band edges can scale to the fifth power of the number periods, yielding a route to significant device miniaturization. For applications in silicon integrated photonics, we present the design and analysis of zero-coupling-gap degenerate band edge resonators. Complex band diagrams are computed for the unit cell with periodic boundary conditions that convey characteristics of propagating and evanescent modes. Dispersion features of the band diagram are used to describe changes in resonance scaling in finite length resonators. Resonators with non-zero and zero coupling gap are compared. Analysis of quality factor and resonance frequency indicates significant reduction in the number of periods required to observe fifth power scaling when degenerate band edge resonators are realized with zero-coupling-gap. High transmission is achieved by optimizing the waveguide feed to the resonator. Compact band edge cavities with large optical field distribution are envisioned for light emitters, switches, and sensors. PMID:26698725

  10. Enhancement of band gap and photoconductivity in gamma indium selenide due to swift heavy ion irradiation

    SciTech Connect

    Sreekumar, R.; Jayakrishnan, R.; Sudha Kartha, C.; Vijayakumar, K. P.; Khan, S. A.; Avasthi, D. K.

    2008-01-15

    {gamma}-In{sub 2}Se{sub 3} thin films prepared at different annealing temperatures ranging from 100 to 400 deg. C were irradiated using 90 MeV Si ions with a fluence of 2x10{sup 13} ions/cm{sup 2}. X-ray diffraction analysis proved that there is no considerable variation in structural properties of the films due to the swift heavy ion irradiation. However, photosensitivity and sheet resistance of the samples increased due to irradiation. It was observed that the sample, which had negative photoconductivity, exhibited positive photoconductivity, after irradiation. The negative photoconductivity was due to the combined effect of trapping of photoexcited electrons, at traps 1.42 and 1.26 eV, above the valence band along with destruction of the minority carriers, created during illumination, through recombination. Photoluminescence study revealed that the emission was due to the transition to a recombination center, which was 180 meV above the valence band. Optical absorption study proved that the defects present at 1.42 and 1.26 eV were annealed out by the ion beam irradiation. This allowed photoexcited carriers to reach conduction band, which resulted in positive photoconductivity. Optical absorption study also revealed that the band gap of the material could be increased by ion beam irradiation. The sample prepared at 400 deg. C had a band gap of 2 eV and this increased to 2.8 eV, after irradiation. The increase in optical band gap was attributed to the annihilation of localized defect bands, near the conduction and valence band edges, on irradiation. Thus, by ion beam irradiation, one could enhance photosensitivity as well as the optical band gap of {gamma}-In{sub 2}Se{sub 3}, making the material suitable for applications such as window layer in solar cells.

  11. Enhancement of band gap and photoconductivity in gamma indium selenide due to swift heavy ion irradiation

    NASA Astrophysics Data System (ADS)

    Sreekumar, R.; Jayakrishnan, R.; Sudha Kartha, C.; Vijayakumar, K. P.; Khan, S. A.; Avasthi, D. K.

    2008-01-01

    γ-In2Se3 thin films prepared at different annealing temperatures ranging from 100to400°C were irradiated using 90MeV Si ions with a fluence of 2×1013ions/cm2. X-ray diffraction analysis proved that there is no considerable variation in structural properties of the films due to the swift heavy ion irradiation. However, photosensitivity and sheet resistance of the samples increased due to irradiation. It was observed that the sample, which had negative photoconductivity, exhibited positive photoconductivity, after irradiation. The negative photoconductivity was due to the combined effect of trapping of photoexcited electrons, at traps 1.42 and 1.26eV, above the valence band along with destruction of the minority carriers, created during illumination, through recombination. Photoluminescence study revealed that the emission was due to the transition to a recombination center, which was 180meV above the valence band. Optical absorption study proved that the defects present at 1.42 and 1.26eV were annealed out by the ion beam irradiation. This allowed photoexcited carriers to reach conduction band, which resulted in positive photoconductivity. Optical absorption study also revealed that the band gap of the material could be increased by ion beam irradiation. The sample prepared at 400°C had a band gap of 2eV and this increased to 2.8eV, after irradiation. The increase in optical band gap was attributed to the annihilation of localized defect bands, near the conduction and valence band edges, on irradiation. Thus, by ion beam irradiation, one could enhance photosensitivity as well as the optical band gap of γ-In2Se3, making the material suitable for applications such as window layer in solar cells.

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

    NASA Astrophysics Data System (ADS)

    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.

  13. Local strain effect on the band gap engineering of graphene by a first-principles study

    SciTech Connect

    Gui, Gui; Booske, John; Ma, Zhenqiang E-mail: mazq@engr.wisc.edu; Morgan, Dane; Zhong, Jianxin E-mail: mazq@engr.wisc.edu

    2015-02-02

    We have systematically investigated the effect of local strain on electronic properties of graphene by first-principles calculations. Two major types of local strain, oriented along the zigzag and the armchair directions, have been studied. We find that local strain with a proper range and strength along the zigzag direction results in opening of significant band gaps in graphene, on the order of 10{sup −1 }eV; whereas, local strain along the armchair direction cannot open a significant band gap in graphene. Our results show that appropriate local strain can effectively open and tune the band gap in graphene; therefore, the electronic and transport properties of graphene can also be modified.

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

    SciTech Connect

    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.

  15. General band gap condition in one-dimensional resonator-based acoustic metamaterial

    NASA Astrophysics Data System (ADS)

    Liu, Yafei; Hou, Zhilin; Fu, Xiujun

    2016-03-01

    A one-dimensional model for resonator-based acoustic metamaterials is introduced. The condition for band gap in such kind of structure is obtained. According to this condition, the dispersion relation is in general a result of the scattering phase and propagating phase. The phenomenon that the band gap is less dependent on lattice structure appears only in the special system in which the coupling between the resonators and the host medium is weak enough. For strong coupled systems, the dispersion of wave can be significantly adjusted by the propagating phase. Based on the understanding, a general guide for band gap optimization is given and the mechanism for structures with the defect states at subwavelength scale is revealed.

  16. Bragg band gaps tunability in an homogeneous piezoelectric rod with periodic electrical boundary conditions

    NASA Astrophysics Data System (ADS)

    Degraeve, S.; Granger, C.; Dubus, B.; Vasseur, J. O.; Pham Thi, M.; Hladky-Hennion, A.-C.

    2014-05-01

    An homogeneous piezoelectric rod is shown to exhibit Bragg band gaps when an electrical boundary condition is applied periodically with the help of metallic electrodes. An analytical model is developed which formulation depends on the applied electric boundary condition and reveals that Bragg band gaps occurring in this very peculiar phononic crystal are related to the electric charge located on the electrodes. Moreover, via an accurate boundary condition (electrodes connected in short circuit, in open circuit, or through an external capacitance), full tunability of the Bragg band gaps can be achieved. Measurements of ultrasonic transmission present an overall excellent agreement with the theoretical results. This phononic crystal can be easily manufactured and presents many potential applications as frequency filters especially for radio frequency telecommunications.

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

    NASA Astrophysics Data System (ADS)

    Prabhash, P. G.; Nair, Swapna S.

    2016-05-01

    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.

  18. A New Silicon Phase with Direct Band Gap and Novel Optoelectronic Properties.

    PubMed

    Guo, Yaguang; Wang, Qian; Kawazoe, Yoshiyuki; Jena, Puru

    2015-01-01

    Due to the compatibility with the well-developed Si-based semiconductor industry, there is considerable interest in developing silicon structures with direct energy band gaps for effective sunlight harvesting. In this paper, using silicon triangles as the building block, we propose a new silicon allotrope with a direct band gap of 0.61 eV, which is dynamically, thermally and mechanically stable. Symmetry group analysis further suggests that dipole transition at the direct band gap is allowed. In addition, this new allotrope displays large carrier mobility (~10(4) cm/V · s) at room temperature and a low mass density (1.71 g/cm(3)), making it a promising material for optoelectronic applications. PMID:26395926

  19. A New Silicon Phase with Direct Band Gap and Novel Optoelectronic Properties

    NASA Astrophysics Data System (ADS)

    Guo, Yaguang; Wang, Qian; Kawazoe, Yoshiyuki; Jena, Puru

    2015-09-01

    Due to the compatibility with the well-developed Si-based semiconductor industry, there is considerable interest in developing silicon structures with direct energy band gaps for effective sunlight harvesting. In this paper, using silicon triangles as the building block, we propose a new silicon allotrope with a direct band gap of 0.61 eV, which is dynamically, thermally and mechanically stable. Symmetry group analysis further suggests that dipole transition at the direct band gap is allowed. In addition, this new allotrope displays large carrier mobility (~104 cm/V · s) at room temperature and a low mass density (1.71 g/cm3), making it a promising material for optoelectronic applications.

  20. A New Silicon Phase with Direct Band Gap and Novel Optoelectronic Properties

    PubMed Central

    Guo, Yaguang; Wang, Qian; Kawazoe, Yoshiyuki; Jena, Puru

    2015-01-01

    Due to the compatibility with the well-developed Si-based semiconductor industry, there is considerable interest in developing silicon structures with direct energy band gaps for effective sunlight harvesting. In this paper, using silicon triangles as the building block, we propose a new silicon allotrope with a direct band gap of 0.61 eV, which is dynamically, thermally and mechanically stable. Symmetry group analysis further suggests that dipole transition at the direct band gap is allowed. In addition, this new allotrope displays large carrier mobility (~104 cm/V · s) at room temperature and a low mass density (1.71 g/cm3), making it a promising material for optoelectronic applications. PMID:26395926

  1. Method of manufacturing flexible metallic photonic band gap structures, and structures resulting therefrom

    DOEpatents

    Gupta, Sandhya; Tuttle, Gary L.; Sigalas, Mihail; McCalmont, Jonathan S.; Ho, Kai-Ming

    2001-08-14

    A method of manufacturing a flexible metallic photonic band gap structure operable in the infrared region, comprises the steps of spinning on a first layer of dielectric on a GaAs substrate, imidizing this first layer of dielectric, forming a first metal pattern on this first layer of dielectric, spinning on and imidizing a second layer of dielectric, and then removing the GaAs substrate. This method results in a flexible metallic photonic band gap structure operable with various filter characteristics in the infrared region. This method may be used to construct multi-layer flexible metallic photonic band gap structures. Metal grid defects and dielectric separation layer thicknesses are adjusted to control filter parameters.

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

    PubMed

    Lanzillo, Nicholas A; Breneman, Curt M

    2016-08-17

    We report simulations based on density functional theory and many-body perturbation theory exploring the band gaps of common crystalline polymers including polyethylene, polypropylene and polystyrene. Our reported band gaps of 8.6 eV for single-chain polyethylene and 9.1 eV for bulk crystalline polyethylene are in excellent agreement with experiment. The effects of chemical doping along the polymer backbone and side-groups are explored, and the use mechanical strain as a means to modify the band gaps of these polymers over a range of several eV while leaving the dielectric constant unchanged is discussed. This work highlights some of the opportunities available to engineer the electronic properties of polymers with wide-reaching implications for polymeric dielectric materials used for capacitive energy storage. PMID:27324304

  3. Identification of zero density of states domains in band gap fibers using a single binary function.

    PubMed

    Li, Guangrui; Schmidt, Markus A

    2016-07-11

    Here we introduce a new calculation method to find the domains of zero density of states for photonic band gap guiding fibers consisting of arrays of high refractive index strands in a low refractive index cladding. We find an analytic expression that associates any combination of geometric parameter, effective index, material and wavelength with a single binary function which allows direct determination whether the density of cladding states is zero or not. The method neither requires the typically used root finding procedure for dispersion tracking nor simulation volume discretization. We verify the validity of our approach on well-established results and reveal as example that band gap regions are mainly determined by the two lowest order Bessel function orders. Our method allows for extensive parameter scans and evaluation of photonic band gap structures against structural and material inaccuracies with substantially reduced simulation effort. PMID:27410888

  4. Tuning the band gap and magnetic properties of BN sheets impregnated with graphene flakes

    NASA Astrophysics Data System (ADS)

    Kan, M.; Zhou, J.; Wang, Q.; Sun, Q.; Jena, P.

    2011-11-01

    The BN sheet is a nonmagnetic wide-band-gap semiconductor. Using density functional theory, we show that these properties can be fundamentally altered by embedding graphene flakes. Not only do graphene flakes preserve the two-dimensional (2D) planar structure of the BN sheet, but by controlling their shape and size, unexpected electronic and magnetic properties also emerge. The electronic band structure can be tuned from a direct gap to an indirect gap, the energy gap can be further modulated by changing the bonding patterns, and both hole injecting or electron injecting can be achieved by tailoring the triangular embedding pattern. Furthermore, the Lieb theorem still holds, and the embedded triangular graphene flakes become ferromagnetic with full spin polarizations of the introduced electrons or holes, opening the door to their use as spin filters. The study sheds new light on hybrid single-atomic-layer engineering for unprecedented applications of 2D nanomaterials.

  5. Towards direct-gap silicon phases by the inverse band structure design approach.

    PubMed

    Xiang, H J; Huang, Bing; Kan, Erjun; Wei, Su-Huai; Gong, X G

    2013-03-15

    Diamond silicon (Si) is the leading material in the current solar cell market. However, diamond Si is an indirect band gap semiconductor with a large energy difference (2.3 eV) between the direct gap and the indirect gap, which makes it an inefficient absorber of light. In this work, we develop a novel inverse band structure design approach based on the particle swarming optimization algorithm to predict the metastable Si phases with better optical properties than diamond Si. Using our new method, we predict a cubic Si(20) phase with quasidirect gaps of 1.55 eV, which is a promising candidate for making thin-film solar cells. PMID:25166584

  6. Photoemission and density functional theory study of Ir(111); energy band gap mapping

    NASA Astrophysics Data System (ADS)

    Pletikosić, I.; Kralj, M.; Šokčević, D.; Brako, R.; Lazić, P.; Pervan, P.

    2010-04-01

    We have performed combined angle-resolved photoemission spectroscopy (ARPES) experiments and density functional theory (DFT) calculations of the electronic structure of the Ir(111) surface, with the focus on the existence of energy band gaps. The investigation was motivated by the experimental results suggesting Ir(111) as an ideal support for the growth of weakly bonded graphene. Therefore, our prime interest was electronic structure around the \\bar {\\mathrm {K}} symmetry point. In accordance with DFT calculations, ARPES has shown a wide energy band gap with the shape of a parallelogram centred around the \\bar {\\mathrm {K}} point. Within the gap three surface states were identified; one just below the Fermi level and two spin-orbit split surface states at the bottom of the gap.

  7. The electronic structures of vanadate salts: Cation substitution as a tool for band gap manipulation

    NASA Astrophysics Data System (ADS)

    Dolgos, Michelle R.; Paraskos, Alexandra M.; Stoltzfus, Matthew W.; Yarnell, Samantha C.; Woodward, Patrick M.

    2009-07-01

    The electronic structures of six ternary metal oxides containing isolated vanadate ions, Ba 3(VO 4) 2, Pb 3(VO 4) 2, YVO 4, BiVO 4, CeVO 4 and Ag 3VO 4 were studied using diffuse reflectance spectroscopy and electronic structure calculations. While the electronic structure near the Fermi level originates largely from the molecular orbitals of the vanadate ion, both experiment and theory show that the cation can strongly influence these electronic states. The observation that Ba 3(VO 4) 2 and YVO 4 have similar band gaps, both 3.8 eV, shows that cations with a noble gas configuration have little impact on the electronic structure. Band structure calculations support this hypothesis. In Pb 3(VO 4) 2 and BiVO 4 the band gap is reduced by 0.9-1.0 eV through interactions of (a) the filled cation 6 s orbitals with nonbonding O 2 p states at the top of the valence band, and (b) overlap of empty 6 p orbitals with antibonding V 3 d-O 2 p states at the bottom of the conduction band. In Ag 3VO 4 mixing between filled Ag 4 d and O 2 p states destabilizes states at the top of the valence band leading to a large decrease in the band gap ( Eg=2.2 eV). In CeVO 4 excitations from partially filled 4 f orbitals into the conduction band lower the effective band gap to 1.8 eV. In the Ce 1-xBi xVO 4 (0≤ x≤0.5) and Ce 1-xY xVO 4 ( x=0.1, 0.2) solid solutions the band gap narrows slightly when Bi 3+ or Y 3+ are introduced. The nonlinear response of the band gap to changes in composition is a result of the localized nature of the Ce 4 f orbitals.

  8. Fundamental Role of Oxygen Stoichiometry in Controlling the Band Gap and Reactivity of Cupric Oxide Nanosheets.

    PubMed

    Fishman, Zachary S; Rudshteyn, Benjamin; He, Yulian; Liu, Bolun; Chaudhuri, Subhajyoti; Askerka, Mikhail; Haller, Gary L; Batista, Victor S; Pfefferle, Lisa D

    2016-08-31

    CuO is a nonhazardous, earth-abundant material that has exciting potential for use in solar cells, photocatalysis, and other optoelectronic applications. While progress has been made on the characterization of properties and reactivity of CuO, there remains significant controversy on how to control the precise band gap by tuning conditions of synthetic methods. Here, we combine experimental and theoretical methods to address the origin of the wide distribution of reported band gaps for CuO nanosheets. We establish reaction conditions to control the band gap and reactivity via a high-temperature treatment in an oxygen-rich environment. SEM, TEM, XRD, and BET physisorption reveals little to no change in nanostructure, crystal structure, or surface area. In contrast, UV-vis spectroscopy shows a modulation in the material band gap over a range of 330 meV. A similar trend is found in H2 temperature-programmed reduction where peak H2 consumption temperature decreases with treatment. Calculations of the density of states show that increasing the oxygen to copper coverage ratio of the surface accounts for most of the observed changes in the band gap. An oxygen exchange mechanism, supported by (18)O2 temperature-programmed oxidation, is proposed to be responsible for changes in the CuO nanosheet oxygen to copper stoichiometry. The changes induced by oxygen depletion/deposition serve to explain discrepancies in the band gap of CuO, as reported in the literature, as well as dramatic differences in catalytic performance. PMID:27454546

  9. Characterization of wide band gap semiconductors and multiferroic materials

    NASA Astrophysics Data System (ADS)

    Cai, Bo

    Structural, optical and electrical properties of zinc oxide (ZnO), aluminum nitride (AlN), and lutetium ferrite (LuFe2O4) have been investigated. Temperature dependent Hall Effect measurements were performed between 80 and 800 K for phosphorus (P) and arsenic (As) doped ZnO thin films grown on c-plane sapphire substrate by RF magnetron sputtering. These samples exhibited n-type conductivity throughout the temperature range with carrier concentration of 3.85 x 1016 cm-3 and 3.65 x 10 17 cm-3 at room temperature for P-doped and As-doped ZnO films, respectively. The Arrhenius plots of free electron concentration of those doped samples showed double thermal activation processes with a small activation energy of about 0.04 eV due to shallow donors and a large activation energy of about 0.8 eV due to deep donors. The deep donor level could be related to oxygen vacancy. For undoped ZnO layer, growth condition was optimized to use as low background electron buffer layer. Hall Effect measurements showed that the resistivity and background electron concentration of the films decreases as the substrate temperature increases. The film deposited at 900 oC has more than two orders less background electron concentration than that deposited at 300 °C. Based on photoluminescence and Transmission Electron Microscopy (TEM) analysis, the ZnO grown under this condition is formed to be a greatly reduced density of stacking faults. Transmission electron microscopy (TEM) was employed to investigate dislocations in aluminum nitride (AlN) epilayers grown on sapphire substrate using three-step growth method by metal organic chemical vapor deposition (MOCVD). AlN epilayers grown by this method have smooth surfaces, narrow width of X-ray rocking curves, and strong band edge photoluminescence (PL) emissions with low impurity emissions. Transmission electron microscopy revealed that most of the threading dislocations are annihilated within 300 nm. Stacking faults are greatly reduced in the

  10. Polarization-induced Zener tunnel junctions in wide-band-gap heterostructures.

    PubMed

    Simon, John; Zhang, Ze; Goodman, Kevin; Xing, Huili; Kosel, Thomas; Fay, Patrick; Jena, Debdeep

    2009-07-10

    The large electronic polarization in III-V nitrides allows for novel physics not possible in other semiconductor families. In this work, interband Zener tunneling in wide-band-gap GaN heterojunctions is demonstrated by using polarization-induced electric fields. The resulting tunnel diodes are more conductive under reverse bias, which has applications for zero-bias rectification and mm-wave imaging. Since interband tunneling is traditionally prohibitive in wide-band-gap semiconductors, these polarization-induced structures and their variants can enable a number of devices such as multijunction solar cells that can operate under elevated temperatures and high fields. PMID:19659229