Band Structure Engineering and Thermoelectric Properties of Charge-Compensated Filled Skutterudites

NASA Astrophysics Data System (ADS)

Shi, Xiaoya; Yang, Jiong; Wu, Lijun; Salvador, James R.; Zhang, Cheng; Villaire, William L.; Haddad, Daad; Yang, Jihui; Zhu, Yimei; Li, Qiang

2015-10-01

Thermoelectric properties of semiconductors are intimately related to their electronic band structure, which can be engineered via chemical doping. Dopant Ga in the cage-structured skutterudite Co4Sb12 substitutes Sb sites while occupying the void sites. Combining quantitative scanning transmission electron microscopy and first-principles calculations, we show that Ga dual-site occupancy breaks the symmetry of the Sb-Sb network, splits the deep triply-degenerate conduction bands, and drives them downward to the band edge. The charge-compensating nature of the dual occupancy Ga increases overall filling fraction limit. By imparting this unique band structure feature, and judiciously doping the materials by increasing the Yb content, we promote the Fermi level to a point where carriers are in energetic proximity to these features. Increased participation of these heavier bands in electronic transport leads to increased thermopower and effective mass. Further, the localized distortion from Ga/Sb substitution enhances the phonon scattering to reduce the thermal conductivity effectively.

Quantum-confinement effects on conduction band structure of rectangular cross-sectional GaAs nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tanaka, H., E-mail: tanaka@semicon.kuee.kyoto-u.ac.jp; Morioka, N.; Mori, S.

2014-02-07

The conduction band structure and electron effective mass of GaAs nanowires with various cross-sectional shapes and orientations were calculated by two methods, a tight-binding method and an effective mass equation taking the bulk full-band structure into account. The effective mass of nanowires increases as the cross-sectional size decreases, and this increase in effective mass depends on the orientations and substrate faces of nanowires. Among [001], [110], and [111]-oriented rectangular cross-sectional GaAs nanowires, [110]-oriented nanowires with wider width along the [001] direction showed the lightest effective mass. This dependence originates from the anisotropy of the Γ valley of bulk GaAs. Themore » relationship between effective mass and bulk band structure is discussed.« less

Nitrogen-related intermediate band in P-rich GaN xP yAs 1-x-y alloys

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zelazna, K.; Gladysiewicz, M.; Polak, M. P.

The electronic band structure of phosphorus-rich GaN xP yAs 1-x-y alloys (x ~ 0.025 and y ≥ 0.6) is studied experimentally using optical absorption, photomodulated transmission, contactless electroreflectance, and photoluminescence. Here, it is shown that incorporation of a few percent of N atoms has a drastic effect on the electronic structure of the alloys. The change of the electronic band structure is very well described by the band anticrossing (BAC) model in which localized nitrogen states interact with the extended states of the conduction band of GaAsP host. The BAC interaction results in the formation of a narrow intermediate bandmore » (E - band in BAC model) with the minimum at the Γ point of the Brillouin zone resulting in a change of the nature of the fundamental band gap from indirect to direct. The splitting of the conduction band by the BAC interaction is further confirmed by a direct observation of the optical transitions to the E + band using contactless electroreflectance spectroscopy.« less

Nitrogen-related intermediate band in P-rich GaN xP yAs 1-x-y alloys

DOE PAGES

Zelazna, K.; Gladysiewicz, M.; Polak, M. P.; ...

2017-11-16

The electronic band structure of phosphorus-rich GaN xP yAs 1-x-y alloys (x ~ 0.025 and y ≥ 0.6) is studied experimentally using optical absorption, photomodulated transmission, contactless electroreflectance, and photoluminescence. Here, it is shown that incorporation of a few percent of N atoms has a drastic effect on the electronic structure of the alloys. The change of the electronic band structure is very well described by the band anticrossing (BAC) model in which localized nitrogen states interact with the extended states of the conduction band of GaAsP host. The BAC interaction results in the formation of a narrow intermediate bandmore » (E - band in BAC model) with the minimum at the Γ point of the Brillouin zone resulting in a change of the nature of the fundamental band gap from indirect to direct. The splitting of the conduction band by the BAC interaction is further confirmed by a direct observation of the optical transitions to the E + band using contactless electroreflectance spectroscopy.« less

Characterization and Physics-Based Modeling of Electrochemical Memristors

DTIC Science & Technology

2015-11-16

conducting films that result from electrical or optical stress. Model parameters and electrical characteristics were obtained from and validated...x- ray scattering, Conductive Bridge Random Access Memory 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT 18. NUMBER OF PAGES 19a. NAME...Calculated DOS for GeSe2 in valence band and (b) conduction band .................. 43  Figure 45. DFT band structure for crystalline GeSe2

Conduction band edge effective mass of La-doped BaSnO{sub 3}

DOE Office of Scientific and Technical Information (OSTI.GOV)

James Allen, S., E-mail: allen@itst.ucsb.edu; Law, Ka-Ming; Raghavan, Santosh

2016-06-20

BaSnO{sub 3} has attracted attention as a promising material for applications requiring wide band gap, high electron mobility semiconductors, and moreover possesses the same perovskite crystal structure as many functional oxides. A key parameter for these applications and for the interpretation of its properties is the conduction band effective mass. We measure the plasma frequency of La-doped BaSnO{sub 3} thin films by glancing incidence, parallel-polarized resonant reflectivity. Using the known optical dielectric constant and measured electron density, the resonant frequency determines the band edge electron mass to be 0.19 ± 0.01. The results allow for testing band structure calculations and transport models.

Anisotropy of the nitrogen conduction states in the group III nitrides studied by polarized x-ray absorption

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lawniczak-Jablonska, K.; Liliental-Weber, Z.; Gullikson, E.M.

1997-04-01

Group III nitrides (AlN, GaN, and InN) consist of the semiconductors which appear recently as a basic materials for optoelectronic devices active in the visible/ultraviolet spectrum as well as high-temperature and high-power microelectronic devices. However, understanding of the basic physical properties leading to application is still not satisfactory. One of the reasons consists in unsufficient knowledge of the band structure of the considered semiconductors. Several theoretical studies of III-nitrides band structure have been published but relatively few experimental studies have been carried out, particularly with respect to their conduction band structure. This motivated the authors to examine the conduction bandmore » structure projected onto p-states of the nitrogen atoms for AlN, GaN and InN. An additional advantage of their studies is the availability of the studied nitrides in two structures, hexagonal (wurtzite) and cubic (zincblende). This offers an opportunity to gain information about the role of the anisotropy of electronic band states in determining various physical properties.« less

Facile fabrication of high-performance InGaZnO thin film transistor using hydrogen ion irradiation at room temperature

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ahn, Byung Du; Park, Jin-Seong; Chung, K. B., E-mail: kbchung@dongguk.edu

Device performance of InGaZnO (IGZO) thin film transistors (TFTs) are investigated as a function of hydrogen ion irradiation dose at room temperature. Field effect mobility is enhanced, and subthreshold gate swing is improved with the increase of hydrogen ion irradiation dose, and there is no thermal annealing. The electrical device performance is correlated with the electronic structure of IGZO films, such as chemical bonding states, features of the conduction band, and band edge states below the conduction band. The decrease of oxygen deficient bonding and the changes in electronic structure of the conduction band leads to the improvement of devicemore » performance in IGZO TFT with an increase of the hydrogen ion irradiation dose.« less

Role of oxygen functionality on the band structure evolution and conductance of reduced graphene oxide

NASA Astrophysics Data System (ADS)

Roy, Rajarshi; Thapa, Ranjit; Chakrabarty, Soubhik; Jha, Arunava; Midya, Priyanka R.; Kumar, E. Mathan; Chattopadhyay, Kalyan K.

2017-06-01

Here we report, structural and electrical transport properties of reduced graphene oxide as a function of oxygen bonding configuration. We find that mainly epoxy (Csbnd Osbnd C) and carbonyl (Cdbnd O) functional groups remain as major residual components after reduction using three different reducing agents. We calculate the band structure in the presence of epoxy and carbonyl groups and defects. Finally, we calculate the theoretical band mobility and find that it is less for the carbonyl with epoxy system. We correlate the distortion of linear dispersion and opening of bandgap at K-point with conductance for different graphene system in presence of oxygen moieties.

The electronic structure of lithium metagallate.

PubMed

Johnson, N W; McLeod, J A; Moewes, A

2011-11-09

Herein we present a study of the electronic structure of lithium metagallate (LiGaO(2)), a material of interest in the field of optoelectronics. We use soft x-ray spectroscopy to probe the electronic structure of both the valence and conduction bands and compare our measurements to ab initio density functional theory calculations. We use several different exchange-correlation functionals, but find that no single theoretical approach used herein accurately quantifies both the band gap and the Ga 3d(10) states in LiGaO(2). We derive a band gap of 5.6 eV, and characterize electron hybridization in both the valence and conduction bands. Our study of the x-ray spectra may prove useful in analysing spectra from more complicated LiGaO(2) heterostructures. © 2011 IOP Publishing Ltd

Holmium Doped Solid State Laser Resonantly Pumped and Q-Switched by Novel GaSb-Based Photonic Devices

DTIC Science & Technology

2011-08-31

dominant role of inter valence band absorption [7]. Details of the conduction band structure of the particular 0 20 40 60 80 100 0 10 20 30 CW 30s...here the n-cladding composition resulted into material with three valleys in conduction band to have almost the same energy minimum so no inter...emitting GaSb -based diode lasers was improved by utilization of the waveguide structure with asymmetric claddings. The AlGaAsSb p-cladding contained

Measuring the band structures of periodic beams using the wave superposition method

NASA Astrophysics Data System (ADS)

Junyi, L.; Ruffini, V.; Balint, D.

2016-11-01

Phononic crystals and elastic metamaterials are artificially engineered periodic structures that have several interesting properties, such as negative effective stiffness in certain frequency ranges. An interesting property of phononic crystals and elastic metamaterials is the presence of band gaps, which are bands of frequencies where elastic waves cannot propagate. The presence of band gaps gives this class of materials the potential to be used as vibration isolators. In many studies, the band structures were used to evaluate the band gaps. The presence of band gaps in a finite structure is commonly validated by measuring the frequency response as there are no direct methods of measuring the band structures. In this study, an experiment was conducted to determine the band structure of one dimension phononic crystals with two wave modes, such as a bi-material beam, using the frequency response at only 6 points to validate the wave superposition method (WSM) introduced in a previous study. A bi-material beam and an aluminium beam with varying geometry were studied. The experiment was performed by hanging the beams freely, exciting one end of the beams, and measuring the acceleration at consecutive unit cells. The measured transfer function of the beams agrees with the analytical solutions but minor discrepancies. The band structure was then determined using WSM and the band structure of one set of the waves was found to agree well with the analytical solutions. The measurements taken for the other set of waves, which are the evanescent waves in the bi-material beams, were inaccurate and noisy. The transfer functions at additional points of one of the beams were calculated from the measured band structure using WSM. The calculated transfer function agrees with the measured results except at the frequencies where the band structure was inaccurate. Lastly, a study of the potential sources of errors was also conducted using finite element modelling and the errors in the dispersion curve measured from the experiments were deduced to be a result of a combination of measurement noise, the different placement of the accelerometer with finite mass, and the torsional mode.

First-principle calculation of the electronic structure, DOS and effective mass TlInSe2

NASA Astrophysics Data System (ADS)

Ismayilova, N. A.; Orudzhev, G. S.; Jabarov, S. H.

2017-05-01

The electronic structure, density of states (DOS), effective mass are calculated for tetragonal TlInSe2 from first principle in the framework of density functional theory (DFT). The electronic structure of TlInSe2 has been investigated by Quantum Wise within GGA. The calculated band structure by Hartwigsen-Goedecker-Hutter (HGH) pseudopotentials (psp) shows both the valence band maximum and conduction band minimum located at the T point of the Brillouin zone. Valence band maximum at the T point and the surrounding parts originate mainly from 6s states of univalent Tl ions. Bottom of the conduction band is due to the contribution of 6p-states of Tl and 5s-states of In atoms. Calculated DOS effective mass for holes and electrons are mDOS h∗ = 0.830m e, mDOS h∗ = 0.492m e, respectively. Electron effective masses are fairly isotropic, while the hole effective masses show strong anisotropy. The calculated electronic structure, density of states and DOS effective masses of TlInSe2 are in good agreement with existing theoretical and experimental results.

Widely tunable band gap in a multivalley semiconductor SnSe by potassium doping

NASA Astrophysics Data System (ADS)

Zhang, Kenan; Deng, Ke; Li, Jiaheng; Zhang, Haoxiong; Yao, Wei; Denlinger, Jonathan; Wu, Yang; Duan, Wenhui; Zhou, Shuyun

2018-05-01

SnSe, a group IV-VI monochalcogenide with layered crystal structure similar to black phosphorus, has recently attracted extensive interest due to its excellent thermoelectric properties and potential device applications. Experimental electronic structure of both the valence and conduction bands is critical for understanding the effects of hole versus electron doping on the thermoelectric properties, and to further reveal possible change of the band gap upon doping. Here, we report the multivalley valence bands with a large effective mass on semiconducting SnSe crystals and reveal single-valley conduction bands through electron doping to provide a complete picture of the thermoelectric physics. Moreover, by electron doping through potassium deposition, the band gap of SnSe can be widely tuned from 1.2 eV to 0.4 eV, providing new opportunities for tunable electronic and optoelectronic devices.

Origin of and tuning the optical and fundamental band gaps in transparent conducting oxides: The case of M2O3(M =Al ,Ga ,In )

NASA Astrophysics Data System (ADS)

Sabino, Fernando P.; Besse, Rafael; Oliveira, Luiz Nunes; Wei, Su-Huai; Da Silva, Juarez L. F.

2015-11-01

Good transparent conducting oxides (TCOs), such as In2O3 :Sn (ITO), usually combine large optical band gaps, essential for high transparency, with relatively small fundamental band gaps due to low conduction-band minima, which favor n -type doping and enhance the electrical conductivity. It has been understood that the optical band gaps are wider than the fundamental band gaps because optical transitions between the band-edge states are forbidden. The mechanism blocking such transitions, which can play a crucial role in the designing of alternative TCOs, nonetheless remains obscure. Here, based on first-principles density functional theory calculations and symmetry analysis of three oxides, M2O3 (M =Al ,Ga ,In ), we identify the physical origin of the gap disparities. Three conditions are necessary: (1) the crystal structure must have global inversion symmetry; (2) in order to belong to the Ag or A1 g irreducible representations, the states at the conduction-band minimum must have cation and oxygen s character; (3) in order to have g parity, the oxygen p orbitals constituting the states near the valence-band maximum must be strongly coupled to the cation d orbitals. Under these conditions, optical excitations across the fundamental gap will be forbidden. The three criteria explain the trends in the M2O3 (M =Al,Ga,In) sequence, in particular, explaining why In2O3 in the bixbyite structure yields the highest figure of merit. Our study provides guidelines expected to be instrumental in the search for new TCO materials.

A Unifying Perspective on Oxygen Vacancies in Wide Band Gap Oxides.

PubMed

Linderälv, Christopher; Lindman, Anders; Erhart, Paul

2018-01-04

Wide band gap oxides are versatile materials with numerous applications in research and technology. Many properties of these materials are intimately related to defects, with the most important defect being the oxygen vacancy. Here, using electronic structure calculations, we show that the charge transition level (CTL) and eigenstates associated with oxygen vacancies, which to a large extent determine their electronic properties, are confined to a rather narrow energy range, even while band gap and the electronic structure of the conduction band vary substantially. Vacancies are classified according to their character (deep versus shallow), which shows that the alignment of electronic eigenenergies and CTL can be understood in terms of the transition between cavity-like localized levels in the large band gap limit and strong coupling between conduction band and vacancy states for small to medium band gaps. We consider both conventional and hybrid functionals and demonstrate that the former yields results in very good agreement with the latter provided that band edge alignment is taken into account.

Shape effects on the electronic structure and the optical gain of InAsN/GaAs nanostructures: From a quantum lens to a quantum ring

NASA Astrophysics Data System (ADS)

Chen, J.; Fan, W. J.; Xu, Q.; Zhang, X. W.; Li, S. S.; Xia, J. B.

2012-10-01

The electronic structures of self-assembled InAs1-xNx/GaAs nanostructures from quantum lens to quantum rings (QRs) are calculated using the 10-band k.p method and the valence force field (VFF) method. With the variation of shape of the nanostructure and nitrogen (N) content, it shows that the N and the strains can significantly affect the energy levels especially the conduction band because the N resonant state has repulsion interaction with the conduction band due to the band anticrossing (BAC). The structures with N and greater height have smaller transition energy, and the structures with N have greater optical gain due to its overwhelming greater value of factor f+f-1. After analyzing the shape effect, we suggested that the nanostructures with volcano shape are preferred because the maximum optical gain occurs for quantum volcano. With our simulation result, researchers could select quantum dots (QDs) structures to design laser with better performance.

Effects of electronic and lattice polarization on the band structure of delafossite transparent conductive oxides.

PubMed

Vidal, Julien; Trani, Fabio; Bruneval, Fabien; Marques, Miguel A L; Botti, Silvana

2010-04-02

We use hybrid functionals and restricted self-consistent GW, state-of-the-art theoretical approaches for quasiparticle band structures, to study the electronic states of delafossite Cu(Al,In)O2, the first p-type and bipolar transparent conductive oxides. We show that a self-consistent GW approximation gives remarkably wider band gaps than all the other approaches used so far. Accounting for polaronic effects in the GW scheme we recover a very nice agreement with experiments. Furthermore, the modifications with respect to the Kohn-Sham bands are strongly k dependent, which makes questionable the common practice of using a scissor operator. Finally, our results support the view that the low energy structures found in optical experiments, and initially attributed to an indirect transition, are due to intrinsic defects in the samples.

Enhancing the performance of tungsten doped InZnO thin film transistors via sequential ambient annealing

NASA Astrophysics Data System (ADS)

Park, Hyun-Woo; Song, Aeran; Kwon, Sera; Choi, Dukhyun; Kim, Younghak; Jun, Byung-Hyuk; Kim, Han-Ki; Chung, Kwun-Bum

2018-03-01

This study suggests a sequential ambient annealing process as an excellent post-treatment method to enhance the device performance and stability of W (tungsten) doped InZnO thin film transistors (WIZO-TFTs). Sequential ambient annealing at 250 °C significantly enhanced the device performance and stability of WIZO-TFTs, compared with other post-treatment methods, such as air ambient annealing and vacuum ambient annealing at 250 °C. To understand the enhanced device performance and stability of WIZO-TFT with sequential ambient annealing, we investigate the correlations between device performance and stability and electronic structures, such as band alignment, a feature of the conduction band, and band edge states below the conduction band. The enhanced performance of WIZO-TFTs with sequential ambient annealing is related to the modification of the electronic structure. In addition, the dominant mechanism responsible for the enhanced device performance and stability of WIZO-TFTs is considered to be a change in the shallow-level and deep-level band edge states below the conduction band.

High-order harmonic generation from a two-dimensional band structure

NASA Astrophysics Data System (ADS)

Jin, Jian-Zhao; Xiao, Xiang-Ru; Liang, Hao; Wang, Mu-Xue; Chen, Si-Ge; Gong, Qihuang; Peng, Liang-You

2018-04-01

In the past few years, harmonic generation in solids has attracted tremendous attention. Recently, some experiments of two-dimensional (2D) monolayer or few-layer materials have been carried out. These studies demonstrated that harmonic generation in the 2D case shows a strong dependence on the laser's orientation and ellipticity, which calls for a quantitative theoretical interpretation. In this work, we carry out a systematic study on the harmonic generation from a 2D band structure based on a numerical solution to the time-dependent Schrödinger equation. By comparing with the 1D case, we find that the generation dynamics can have a significant difference due to the existence of many crossing points in the 2D band structure. In particular, the higher conduction bands can be excited step by step via these crossing points and the total contribution of the harmonic is given by the mixing of transitions between different clusters of conduction bands to the valence band. We also present the orientation dependence of the harmonic yield on the laser polarization direction.

Opposing effects of stacking faults and antisite domain boundaries on the conduction band edge in kesterite quaternary semiconductors

NASA Astrophysics Data System (ADS)

Park, Ji-Sang; Kim, Sunghyun; Walsh, Aron

2018-01-01

We investigated stability and the electronic structure of extended defects including antisite domain boundaries and stacking faults in the kesterite-structured semiconductors, Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe). Our hybrid density functional theory calculations show that stacking faults in CZTS and CZTSe induce a higher conduction band edge than the bulk counterparts, and thus the stacking faults act as electron barriers. Antisite domain boundaries, however, accumulate electrons as the conduction band edge is reduced in energy, having an opposite role. An Ising model was constructed to account for the stability of stacking faults, which shows the nearest-neighbor interaction is stronger in the case of the selenide.

The effects of the chemical composition and strain on the electronic properties of GaSb/InAs core-shell nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ning, Feng; Wang, Dan; Tang, Li-Ming, E-mail: lmtang@hnu.edu.cn

2014-09-07

The effects of the chemical composition and strain on the electronic properties of [111] zinc-blende (ZB) and [0001] wurtzite (WZ) GaSb/InAs core-shell nanowires (NWs) with different core diameters and shell thicknesses are studied using first-principles methods. The band structures of the [111] ZB GaSb/InAs core-shell NWs underwent a noticeable type-I/II band alignment transition, associated with a direct-to-indirect band gap transition under a compressive uniaxial strain. The band structures of the [0001] WZ GaSb/InAs core-shell NWs preserved the direct band gap under either compressive or tensile uniaxial strains. In addition, the band gaps and the effective masses of the carriers couldmore » be tuned by their composition. For the core-shell NWs with a fixed GaSb-core size, the band gaps decreased linearly with an increasing InAs-shell thickness, caused by the significant downshift of the conduction bands. For the [111] ZB GaSb/InAs core-shell NWs, the calculated effective masses indicated that the transport properties could be changed from hole-dominated conduction to electron-dominated conduction by changing the InAs-shell thickness.« less

Thin SOI lateral IGBT with band-to-band tunneling mechanism

NASA Astrophysics Data System (ADS)

Fu, Qiang; Tang, Zhaohuan; Tan, Kaizhou; Wang, Zhikuan; Mei, Yong

2017-06-01

In this paper, a novel 200V lateral IGBT on thin SOI layer with a band-to-band tunneling junction near the anode is proposed. The structure and the operating mechanism of the proposed IGBT are described and discussed. Its main feature is that the novel IGBT structure has a unique abrupt doped p++/n++ tunneling junction in the side of the anode. By utilizing the reverse bias characteristics of the tunneling junction, the proposed IGBT can achieve excellent reverse conducting performance. Numerical simulations suggest that a low reverse conduction voltage drop VR=-1.6V at a current density of 100A/cm2 and a soft factor S=0.63 of the build-in diode are achieved.

Active Narrow-Band Vibration Isolation of Large Engineering Structures

NASA Technical Reports Server (NTRS)

Rahman, Zahidul; Spanos, John

1994-01-01

We present a narrow-band tracking control method using a variant of the Least Mean Squares (LMS) algorithm to isolate slowly changing periodic disturbances from engineering structures. The advantage of the algorithm is that it has a simple architecture and is relatively easy to implement while it can isolate disturbances on the order of 40-50 dB over decades of frequency band. We also present the results of an experiment conducted on a flexible truss structure. The average disturbance rejection achieved is over 40 dB over the frequency band of 5 Hz to 50 Hz.

Study of Electronic Structure, Thermal Conductivity, Elastic and Optical Properties of α, β, γ-Graphyne

PubMed Central

Hou, Xun; Xie, Zhongjing; Li, Chunmei; Li, Guannan; Chen, Zhiqian

2018-01-01

In recent years, graphyne was found to be the only 2D carbon material that has both sp and sp2 hybridization. It has received significant attention because of its great potential in the field of optoelectronics, which arises due to its small band gap. In this study, the structural stability, electronic structure, elasticity, thermal conductivity and optical properties of α, β, γ-graphynes were investigated using density functional theory (DFT) systematically. γ-graphyne has the largest negative cohesive energy and thus the most stable structure, while the β-graphyne comes 2nd. Both β and γ-graphynes have sp-sp, sp-sp2 and sp2-sp2 hybridization bonds, of which γ-graphyne has shorter bond lengths and thus larger Young’s modulus. Due to the difference in acetylenic bond in the structure cell, the effect of strain on the electronic structure varies between graphynes: α-graphyne has no band gap and is insensitive to strain; β-graphyne’s band gap has a sharp up-turn at 10% strain, while γ-graphyne’s band gap goes up linearly with the strain. All the three graphynes exhibit large free carrier concentration and these free carriers have small effective mass, and both free carrier absorption and intrinsic absorption are found in the light absorption. Based on the effect of strain, optical properties of three structures are also analyzed. It is found that the strain has significant impacts on their optical properties. In summary, band gap, thermal conductivity, elasticity and optical properties of graphyne could all be tailored with adjustment on the amount of acetylenic bonds in the structure cell. PMID:29370070

Study of Electronic Structure, Thermal Conductivity, Elastic and Optical Properties of α, β, γ-Graphyne.

PubMed

Hou, Xun; Xie, Zhongjing; Li, Chunmei; Li, Guannan; Chen, Zhiqian

2018-01-25

In recent years, graphyne was found to be the only 2D carbon material that has both sp and sp² hybridization. It has received significant attention because of its great potential in the field of optoelectronics, which arises due to its small band gap. In this study, the structural stability, electronic structure, elasticity, thermal conductivity and optical properties of α, β, γ-graphynes were investigated using density functional theory (DFT) systematically. γ-graphyne has the largest negative cohesive energy and thus the most stable structure, while the β-graphyne comes 2nd. Both β and γ-graphynes have sp-sp, sp-sp² and sp²-sp² hybridization bonds, of which γ-graphyne has shorter bond lengths and thus larger Young's modulus. Due to the difference in acetylenic bond in the structure cell, the effect of strain on the electronic structure varies between graphynes: α-graphyne has no band gap and is insensitive to strain; β-graphyne's band gap has a sharp up-turn at 10% strain, while γ-graphyne's band gap goes up linearly with the strain. All the three graphynes exhibit large free carrier concentration and these free carriers have small effective mass, and both free carrier absorption and intrinsic absorption are found in the light absorption. Based on the effect of strain, optical properties of three structures are also analyzed. It is found that the strain has significant impacts on their optical properties. In summary, band gap, thermal conductivity, elasticity and optical properties of graphyne could all be tailored with adjustment on the amount of acetylenic bonds in the structure cell.

CdO as the archetypical transparent conducting oxide. Systematics of dopant ionic radius and electronic structure effects on charge transport and band structure.

PubMed

Yang, Yu; Jin, Shu; Medvedeva, Julia E; Ireland, John R; Metz, Andrew W; Ni, Jun; Hersam, Mark C; Freeman, Arthur J; Marks, Tobin J

2005-06-22

A series of yttrium-doped CdO (CYO) thin films have been grown on both amorphous glass and single-crystal MgO(100) substrates at 410 degrees C by metal-organic chemical vapor deposition (MOCVD), and their phase structure, microstructure, electrical, and optical properties have been investigated. XRD data reveal that all as-deposited CYO thin films are phase-pure and polycrystalline, with features assignable to a cubic CdO-type crystal structure. Epitaxial films grown on single-crystal MgO(100) exhibit biaxial, highly textured microstructures. These as-deposited CYO thin films exhibit excellent optical transparency, with an average transmittance of >80% in the visible range. Y doping widens the optical band gap from 2.86 to 3.27 eV via a Burstein-Moss shift. Room temperature thin film conductivities of 8,540 and 17,800 S/cm on glass and MgO(100), respectively, are obtained at an optimum Y doping level of 1.2-1.3%. Finally, electronic band structure calculations are carried out to systematically compare the structural, electronic, and optical properties of the In-, Sc-, and Y-doped CdO systems. Both experimental and theoretical results reveal that dopant ionic radius and electronic structure have a significant influence on the CdO-based TCO crystal and band structure: (1) lattice parameters contract as a function of dopant ionic radii in the order Y (1.09 A) < In (0.94 A) < Sc (0.89 A); (2) the carrier mobilities and doping efficiencies decrease in the order In > Y > Sc; (3) the dopant d state has substantial influence on the position and width of the s-based conduction band, which ultimately determines the intrinsic charge transport characteristics.

Electrical and optical properties of Si-doped Ga2O3

NASA Astrophysics Data System (ADS)

Li, Yin; Yang, Chuanghua; Wu, Liyuan; Zhang, Ru

2017-05-01

The charge densities, band structure, density of states, dielectric functions of Si-doped β-Ga2O3 have been investigated based on the density functional theory (DFT) within the hybrid functional HSE06. The heavy doping makes conduction band split out more bands and further influences the band structure. It decreases the band gap and changes from a direct gap to an indirect gap. After doping, the top of the valence bands is mainly composed by the O-2p states, Si-3p states and Ga-4p states and the bottom of the conduction bands is almost formed by the Si-3s, Si-3p and Ga-4s orbits. The anisotropic optical properties have been investigated by means of the complex dielectric function. After the heavy Si doping, the position of absorption band edges did not change much. The slope of the absorption curve descends and indicates that the absorption became more slow for Si-doped β-Ga2O3 than undoped one due to the indirect gap of Si-doped β-Ga2O3.

First principle study of electronic structures and optical properties of Ce-doped SiO2

NASA Astrophysics Data System (ADS)

Cong, Wei-Yan; Lu, Ying-Bo; Zhang, Peng; Guan, Cheng-Bo

2018-05-01

Electronic structures and optical properties of Silicon dioxide (SiO2) systems with and without cerium(Ce) dopant were calculated using the density functional theory. We find that after the Ce incorporation, a new localized impurity band appears between the valance band maximum (VBM) and the conduction band minimum (CBM) of SiO2 system, which is induced mainly by the Ce-4f orbitals. The localized impurity band constructs a bridge between the valence band and the conduction band, making the electronic transition much easier. The calculated optical properties show that in contrast from the pure SiO2 sample, absorption in the visible-light region is found in Ce-doped SiO2 system, which originates from the transition between the valence band and Ce-4f dominated impurity band, as well as the electronic transition from Ce-4f states to Ce-5d states. All calculated results indicate that Ce doping is an effective strategy to improve the optical performance of SiO2 sample, which is in agreement with the experimental results.

Electronic and transport properties of fluorite structure of La2Ce2O7

NASA Astrophysics Data System (ADS)

Mahida, H. R.; Singh, Deobrat; Gupta, Sanjeev K.; Sonvane, Yogesh; Thakor, P. B.

2017-05-01

In this paper, we have symmetrically investigated the structural, electronic and transport properties of fluorite structure of lanthanum cerate oxide (La2Ce2O7) using density functional theory (DFT). The electronic band structure of La2Ce2O7 show semiconducting in nature with band gap of 1.54 eV (indirect at R-X points) and 1.71 eV (direct at R points). We have also calculated the susceptibility, hall resistance, electrical, and thermal conductivity by using Boltztrap equation. The electrical conductivity decreases where as thermal conductivity increases with increase in the temperature. Our result shows that La2Ce2O7 has application in Proton exchange membrane (PEM) fuel cells applications.

Band gap and conductivity variations of ZnO nano structured thin films annealed under Vacuum

NASA Astrophysics Data System (ADS)

Vattappalam, Sunil C.; Thomas, Deepu; T, Raju Mathew; Augustine, Simon; Mathew, Sunny

2015-02-01

Zinc Oxide thin films were prepared by Successive Ionic layer adsorption and reaction technique(SILAR). The samples were annealed under vacuum and conductivity of the samples were taken at different temperatures. UV Spectrograph of the samples were taken and the band gap of each sample was found from the data. All the results were compared with that of the sample annealed under air. It was observed that the band gap decreases and concequently conductivity of the samples increases when the samples are annealed under vacuum.

Systematic research on Ag2X (X = O, S, Se, Te) as visible and near-infrared light driven photocatalysts and effects of their electronic structures

NASA Astrophysics Data System (ADS)

Jiang, Wei; Wu, Zhaomei; Zhu, Yingming; Tian, Wen; Liang, Bin

2018-01-01

Four silver chalcogen compounds, Ag2O, Ag2S, Ag2Se and Ag2Te, can be utilized as visible-light-driven photocatalysts. In this research, the electronic structures of these compounds were analyzed by simulation and experiments to systematically reveal the relationship between photocatalytic performance and energetic structure. All four chalcogenides exhibited interesting photocatalytic activities under ultraviolet, visible and near-infrared light. However, their photocatalytic performances and stability significantly depended on the band gap width, and the valence band and conduct band position, which was determined by their composition. Increasing the X atomic number from O to Te resulted in the upward movement of the valence band top and the conduct band bottom, which resulted in narrower band gaps, a wider absorption spectrum, a weaker photo-oxidization capacity, a higher recombination probability of hole and electron pairs, lower quantum efficiency, and worse stability. Among them, Ag2O has the highest photocatalytic performance and stability due to its widest band gap and lowest position of VB and CB. The combined action of photogenerated holes and different radicals, depending on the different electronic structures, including anion ozone radical, hydroxide radical, and superoxide radical, was observed and understood. The results of experimental observations and simulations of the four silver chalcogen compounds suggested that a proper electronic structure is necessary to obtain a balance between photocatalytic performance and absorbable light region in the development of new photocatalysts.

Infrared and Terahertz Lasers on SI Using Novel Group-IV Alloys

DTIC Science & Technology

2011-11-30

gain at 2,883 nm is comparable with those of many conventional III-V semiconductor lasers. On the other hand, a waveguide design was also presented ...other conduction-band valleys (", X) are above the L-valley band edge of the Ge0.76Si0.19Sn0.05 barrier. This band alignment presents a desirable...the QCL structure based upon Ge/ Ge0.76Si0.19Sn0.05 QWs. Only L-valley conduction- band lineups are shown in the potential diagram under an applied

Effect of conduction band non-parabolicity on the optical gain of quantum cascade lasers based on the effective two-band finite difference method

NASA Astrophysics Data System (ADS)

Cho, Gookbin; Kim, Jungho

2017-09-01

We theoretically investigate the effect of conduction band non-parabolicity (NPB) on the optical gain spectrum of quantum cascade lasers (QCLs) using the effective two-band finite difference method. Based on the effective two-band model to consider the NPB effect in the multiple quantum wells (QWs), the wave functions and confined energies of electron states are calculated in two different active-region structures, which correspond to three-QW single-phonon and four-QW double-phonon resonance designs. In addition, intersubband optical dipole moments and polar-optical-phonon scattering times are calculated and compared without and with the conduction band NPB effect. Finally, the calculation results of optical gain spectra are compared in the two QCL structures having the same peak gain wavelength of 8.55 μm. The gain peaks are greatly shifted to longer wavelengths and the overall gain magnitudes are slightly reduced when the NPB effect is considered. Compared with the three-QW active-region design, the redshift of the peak gain is more prominent in the four-QW active-region design, which makes use of higher electronic states for the lasing transition.

A Chemical Understanding of the Band Convergence in Thermoelectric CoSb 3 Skutterudites: Influence of Electron Population, Local Thermal Expansion, and Bonding Interactions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hanus, Riley; Guo, Xingyu; Tang, Yinglu

2017-01-13

N-Type skutterudites, such as Yb xCo 4Sb 12, have recently been shown to exhibit high valley degeneracy with possible band convergence, explaining the excellent thermoelectric efficiency of these materials. Using a combined theoretical and experimental approach involving temperature-dependent synchrotron diffraction, molecular orbital diagrams, and computational studies, the chemical nature of critical features in the band structure is highlighted. We identify how n-type doping on the filler site induces structural changes that are observed in both the diffraction data and computational results. Additionally, we show how chemical n-type doping slightly alters the electronic band structure, moving the high-valley degeneracy secondary conductionmore » band closer to the primary conduction band and thus inducing band convergence.« less

Electronic structure and optical properties of iron based chalcogenide FeX2 (X = S, Se, Te) for photovoltaic applications: a first principle study

NASA Astrophysics Data System (ADS)

Ghosh, Anima; Thangavel, R.

2017-11-01

In present work, the electronic structure and optical properties of the FeX2 (X = S, Se, Te) compounds have been evaluated by the density functional theory based on the scalar-relativistic full potential linear augmented plane wave method via Wien2K. From the total energy calculations, it has been found that all the compounds have direct band nature, which determined by iron 3 d states at valance band edge and anion p dominated at conduction band at Γ-point and the fundamental band gap between the valence band and conduction band are estimated 1.40, 1.02 and 0.88 eV respectively with scissor correction for FeS2, FeSe2 and FeTe2 which are close to the experimental values. The optical properties such as dielectric tensor components and the absorption coefficient of these materials are determined in order to investigate their usefulness in photovoltaic applications.

Transparent, conducting films based on metal/dielectric photonic band gaps

NASA Astrophysics Data System (ADS)

Bloemer, Mark J.; Scalora, Michael; D'Aguanno, G.; Bowden, Charles M.; Baglio, Salvatore; Sibilia, Concita; Centini, Marco; Bertolotti, Mario

1999-07-01

A transparent conductor has been developed based on 1D metal/dielectric photonic band gap structures. Laminated metal/dielectric filters containing 100 nm of silver have been fabricated with > 50% transmittance. Applications for transparent, conducting films include antennas embedded in windshields, electrodes on flat panel displays, electromagnetic shielding, and solar window panes.

Doping effects in InN/GaN short-period quantum well structures-Theoretical studies based on density functional methods

NASA Astrophysics Data System (ADS)

Strak, Pawel; Kempisty, Pawel; Sakowski, Konrad; Krukowski, Stanislaw

2014-09-01

Density functional theory studies were conducted to determine an influence of the carrier concentration on the optical and electronic properties of InN/GaN superlattice system. The oscillator strength values, energy gaps and the band profiles were obtained. The band profiles were found to be strongly affected for technically possible heavy n-type doping while for p-type doping the carrier influence, both screening and band shift, is negligible. Blue shift of the transition energy between conduction band minima and valence band maxima was observed for high concentrations of both type carriers.

Electronic structure of hydrogenated diamond: Microscopical insight into surface conductivity

NASA Astrophysics Data System (ADS)

Iacobucci, S.; Alippi, Paola; Calvani, P.; Girolami, M.; Offi, F.; Petaccia, L.; Trucchi, D. M.

2016-07-01

We have correlated the surface conductivity of hydrogen-terminated diamond to the electronic structure in the Fermi region. Significant density of electronic states (DOS) in proximity of the Fermi edge has been measured by photoelectron spectroscopy (PES) on surfaces exposed to air, corresponding to a p -type electric conductive regime, while upon annealing a depletion of the DOS has been achieved, resembling the diamond insulating state. The surface and subsurface electronic structure has been determined, exploiting the different probing depths of PES applied in a photon energy range between 7 and 31 eV. Ab initio density functional calculations including surface charge depletion and band-bending effects favorably compare with electronic states measured by angular-resolved photoelectron spectroscopy. Such states are organized in the energy-momentum space in a twofold structure: one, bulk-derived, band disperses in the Γ -X direction with an average hole effective mass of (0.43 ±0.02 ) m0 , where m0 is the bare electron mass; a second flatter band, with an effective mass of (2.2 ±0.9 ) m0 , proves that a hole gas confined in the topmost layers is responsible for the conductivity of the (2 ×1 ) hydrogen-terminated diamond (100 ) surface.

Quasi one-dimensional band dispersion and surface metallization in long-range ordered polymeric wires

DOE PAGES

Vasseur, Guillaume; Fagot-Revurat, Yannick; Sicot, Muriel; ...

2016-01-04

We study the electronic structure of an ordered array of poly(para-phenylene) chains produced by surface-catalyzed dehalogenative polymerization of 1,4-dibromobenzene on copper (110). The quantization of unoccupied molecular states is measured as a function of oligomer length by scanning tunnelling spectroscopy, with Fermi level crossings observed for chains longer than ten phenyl rings. Angle-resolved photoelectron spectroscopy reveals a quasi-one-dimensional valence band as well as a direct gap of 1.15 eV, as the conduction band is partially filled through adsorption on the surface. Tight-binding modelling and ab initio density functional theory calculations lead to a full description of the organic band-structure, includingmore » the k-dispersion, the gap size and electron charge transfer mechanisms, highlighting a strong substrate-molecule interaction that drives the system into a metallic behaviour. In summary, we have fully characterized the band structure of a carbon-based conducting wire. This model system may be considered as a fingerprint of -conjugation of surface organic frameworks.« less

Discrete Electronic Bands in Semiconductors and Insulators: Potential High-Light-Yield Scintillators

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shi, Hongliang; Du, Mao-Hua

Bulk semiconductors and insulators typically have continuous valence and conduction bands. In this paper, we show that valence and conduction bands of a multinary semiconductor or insulator can be split to narrow discrete bands separated by large energy gaps. This unique electronic structure is demonstrated by first-principles calculations in several quaternary elpasolite compounds, i.e., Cs 2NaInBr 6, Cs 2NaBiCl 6, and Tl 2NaBiCl 6. The narrow discrete band structure in these quaternary elpasolites is due to the large electronegativity difference among cations and the large nearest-neighbor distances in cation sublattices. We further use Cs 2NaInBr 6 as an example tomore » show that the narrow bands can stabilize self-trapped and dopant-bound excitons (in which both the electron and the hole are strongly localized in static positions on adjacent sites) and promote strong exciton emission at room temperature. The discrete band structure should further suppress thermalization of hot carriers and may lead to enhanced impact ionization, which is usually considered inefficient in bulk semiconductors and insulators. Finally, these characteristics can enable efficient room-temperature light emission in low-gap scintillators and may overcome the light-yield bottleneck in current scintillator research.« less

Discrete Electronic Bands in Semiconductors and Insulators: Potential High-Light-Yield Scintillators

DOE PAGES

Shi, Hongliang; Du, Mao-Hua

2015-05-12

Bulk semiconductors and insulators typically have continuous valence and conduction bands. In this paper, we show that valence and conduction bands of a multinary semiconductor or insulator can be split to narrow discrete bands separated by large energy gaps. This unique electronic structure is demonstrated by first-principles calculations in several quaternary elpasolite compounds, i.e., Cs 2NaInBr 6, Cs 2NaBiCl 6, and Tl 2NaBiCl 6. The narrow discrete band structure in these quaternary elpasolites is due to the large electronegativity difference among cations and the large nearest-neighbor distances in cation sublattices. We further use Cs 2NaInBr 6 as an example tomore » show that the narrow bands can stabilize self-trapped and dopant-bound excitons (in which both the electron and the hole are strongly localized in static positions on adjacent sites) and promote strong exciton emission at room temperature. The discrete band structure should further suppress thermalization of hot carriers and may lead to enhanced impact ionization, which is usually considered inefficient in bulk semiconductors and insulators. Finally, these characteristics can enable efficient room-temperature light emission in low-gap scintillators and may overcome the light-yield bottleneck in current scintillator research.« less

Band-like transport in highly crystalline graphene films from defective graphene oxides.

PubMed

Negishi, R; Akabori, M; Ito, T; Watanabe, Y; Kobayashi, Y

2016-07-01

The electrical transport property of the reduced graphene oxide (rGO) thin-films synthesized from defective GO through thermal treatment in a reactive ethanol environment at high temperature above 1000 °C shows a band-like transport with small thermal activation energy (Ea~10 meV) that occurs during high carrier mobility (~210 cm(2)/Vs). Electrical and structural analysis using X-ray absorption fine structure, the valence band photo-electron, Raman spectra and transmission electron microscopy indicate that a high temperature process above 1000 °C in the ethanol environment leads to an extraordinary expansion of the conjugated π-electron system in rGO due to the efficient restoration of the graphitic structure. We reveal that Ea decreases with the increasing density of states near the Fermi level due to the expansion of the conjugated π-electron system in the rGO. This means that Ea corresponds to the energy gap between the top of the valence band and the bottom of the conduction band. The origin of the band-like transport can be explained by the carriers, which are more easily excited into the conduction band due to the decreasing energy gap with the expansion of the conjugated π-electron system in the rGO.

Band-like transport in highly crystalline graphene films from defective graphene oxides

NASA Astrophysics Data System (ADS)

Negishi, R.; Akabori, M.; Ito, T.; Watanabe, Y.; Kobayashi, Y.

2016-07-01

The electrical transport property of the reduced graphene oxide (rGO) thin-films synthesized from defective GO through thermal treatment in a reactive ethanol environment at high temperature above 1000 °C shows a band-like transport with small thermal activation energy (Ea~10 meV) that occurs during high carrier mobility (~210 cm2/Vs). Electrical and structural analysis using X-ray absorption fine structure, the valence band photo-electron, Raman spectra and transmission electron microscopy indicate that a high temperature process above 1000 °C in the ethanol environment leads to an extraordinary expansion of the conjugated π-electron system in rGO due to the efficient restoration of the graphitic structure. We reveal that Ea decreases with the increasing density of states near the Fermi level due to the expansion of the conjugated π-electron system in the rGO. This means that Ea corresponds to the energy gap between the top of the valence band and the bottom of the conduction band. The origin of the band-like transport can be explained by the carriers, which are more easily excited into the conduction band due to the decreasing energy gap with the expansion of the conjugated π-electron system in the rGO.

Probing the magnetic field dependence of the light hole transition in GaAs/AlGaAs quantum wells using optically pumped NMR

NASA Astrophysics Data System (ADS)

Willmering, Matthew M.; Sesti, Erika L.; Hayes, Sophia E.; Wood, Ryan M.; Bowers, Clifford R.; Thapa, Sunil K.; Stanton, Christopher J.; Reyes, Arneil P.; Kuhns, Philip; McGill, Stephen

2018-02-01

Optically pumped NMR (OPNMR) of the NMR-active Ga/7169 species has been shown to be a unique method to probe electronic energy bands in GaAs, with sensitivity to the light hole-to-conduction band transition. This transition is often obscured in other optical measurements such as magnetoabsorption. Using OPNMR, we exploit the hyperfine interaction between conduction band electrons (and their spin states) and nuclear spins, which are detected through phase-sensitive radio-frequency (NMR) spectroscopy. Measurements were made over a range of external magnetic fields (B0) in two different labs with separate experimental setups to obtain the magnetic field dependence of the light hole-to-conduction band transition energy. In addition, k .p theory was used to interpret the experimental results, mapping out this specific transition's magnetic field dependence in an AlGaAs/GaAs quantum well. The combination of theory and experiment point to a mixing of valence bands at a field of approximately B0=4.7 T, swapping the dominant character of the absorption transition and, thus, explaining the magnetic field dependence. Lastly, the experimental dependence of the light hole-to-conduction band transition energy on B0 is found to be less steep compared to the calculated trend, indicating that inclusion of additional effects may be necessary to accurately model the spin-split band structure. The additional insight gained by Ga/7169 OPNMR about the light hole states will facilitate future testing of more complex band structure models.

Interacting quasi-band theory for electronic states in compound semiconductor alloys: Wurtzite structure

NASA Astrophysics Data System (ADS)

Kishi, Ayaka; Oda, Masato; Shinozuka, Yuzo

2016-05-01

This paper reports on the electronic states of compound semiconductor alloys of wurtzite structure calculated by the recently proposed interacting quasi-band (IQB) theory combined with empirical sp3 tight-binding models. Solving derived quasi-Hamiltonian 24 × 24 matrix that is characterized by the crystal parameters of the constituents facilitates the calculation of the conduction and valence bands of wurtzite alloys for arbitrary concentrations under a unified scheme. The theory is applied to III-V and II-VI wurtzite alloys: cation-substituted Al1- x Ga x N and Ga1- x In x N and anion-substituted CdS1- x Se x and ZnO1- x S x . The obtained results agree well with the experimental data, and are discussed in terms of mutual mixing between the quasi-localized states (QLS) and quasi-average bands (QAB): the latter bands are approximately given by the virtual crystal approximation (VCA). The changes in the valence and conduction bands, and the origin of the band gap bowing are discussed on the basis of mixing character.

Band gap tuning of amorphous Al oxides by Zr alloying

DOE Office of Scientific and Technical Information (OSTI.GOV)

Canulescu, S., E-mail: stec@fotonik.dtu.dk; Schou, J.; Jones, N. C.

2016-08-29

The optical band gap and electronic structure of amorphous Al-Zr mixed oxides with Zr content ranging from 4.8 to 21.9% were determined using vacuum ultraviolet and X-ray absorption spectroscopy. The light scattering by the nano-porous structure of alumina at low wavelengths was estimated based on the Mie scattering theory. The dependence of the optical band gap of the Al-Zr mixed oxides on the Zr content deviates from linearity and decreases from 7.3 eV for pure anodized Al{sub 2}O{sub 3} to 6.45 eV for Al-Zr mixed oxides with a Zr content of 21.9%. With increasing Zr content, the conduction band minimum changes non-linearlymore » as well. Fitting of the energy band gap values resulted in a bowing parameter of ∼2 eV. The band gap bowing of the mixed oxides is assigned to the presence of the Zr d-electron states localized below the conduction band minimum of anodized Al{sub 2}O{sub 3}.« less

Real-structure effects: Band gaps of Mg_xZn_{1-x}O, Cd_xZn_{1-x}O, and n-type ZnO from ab-initio calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schleife, A; Bechstedt, F

2012-02-15

Many-body perturbation theory is applied to compute the quasiparticle electronic structures and the optical-absorption spectra (including excitonic effects) for several transparent conducting oxides. We discuss HSE+G{sub 0}W{sub 0} results for band structures, fundamental band gaps, and effective electron masses of MgO, ZnO, CdO, SnO{sub 2}, SnO, In{sub 2}O{sub 3}, and SiO{sub 2}. The Bethe-Salpeter equation is solved to account for excitonic effects in the calculation of the frequency-dependent absorption coefficients. We show that the HSE+G{sub 0}W{sub 0} approach and the solution of the Bethe-Salpeter equation are very well-suited to describe the electronic structure and the optical properties of various transparentmore » conducting oxides in good agreement with experiment.« less

Towards band structure and band offset engineering of monolayer Mo(1-x)W(x)S2 via Strain

NASA Astrophysics Data System (ADS)

Kim, Joon-Seok; Ahmad, Rafia; Pandey, Tribhuwan; Rai, Amritesh; Feng, Simin; Yang, Jing; Lin, Zhong; Terrones, Mauricio; Banerjee, Sanjay K.; Singh, Abhishek K.; Akinwande, Deji; Lin, Jung-Fu

2018-01-01

Semiconducting transition metal dichalcogenides (TMDs) demonstrate a wide range of optoelectronic properties due to their diverse elemental compositions, and are promising candidates for next-generation optoelectronics and energy harvesting devices. However, effective band offset engineering is required to implement practical structures with desirable functionalities. Here, we explore the pressure-induced band structure evolution of monolayer WS2 and Mo0.5W0.5S2 using hydrostatic compressive strain applied in a diamond anvil cell (DAC) apparatus and theoretical calculations, in order to study the modulation of band structure and explore the possibility of band alignment engineering through different compositions. Higher W composition in Mo(1-x)W(x)S2 contributes to a greater pressure-sensitivity of direct band gap opening, with a maximum value of 54 meV GPa-1 in WS2. Interestingly, while the conduction band minima (CBMs) remains largely unchanged after the rapid gap increase, valence band maxima (VBMs) significantly rise above the initial values. It is suggested that the pressure- and composition-engineering could introduce a wide variety of band alignments including type I, type II, and type III heterojunctions, and allow to construct precise structures with desirable functionalities. No structural transition is observed during the pressure experiments, implying the pressure could provide selective modulation of band offset.

Electronic transport properties of Ti-impurity band in Si

NASA Astrophysics Data System (ADS)

Olea, J.; González-Díaz, G.; Pastor, D.; Mártil, I.

2009-04-01

In this paper we show that pulsed laser melted high dose implantation of Ti in Si, above the Mott transition, produces an impurity band (IB) in this semiconductor. Using the van der Pauw method and Hall effect measurements we find strong laminated conductivity at the implanted layer and a temperature dependent decoupling between the Ti implanted layer (TIL) and the substrate. The conduction mechanism from the TIL to the substrate shows blocking characteristics that could be well explained through IB theory. Using the ATLAS code we can estimate the energetic position of the IB at 0.36 eV from the conduction band, the density of holes in this band which is closely related to the Ti atomic density and the hole mobility in this band. Band diagrams of the structure at low and high temperatures are also simulated in the ATLAS framework. The simulation obtained is fully coherent with experimental results.

Fine structure of the red luminescence band in undoped GaN

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reshchikov, M. A., E-mail: mreshchi@vcu.edu; Usikov, A.; Saint-Petersburg National Research University of Information Technologies, Mechanics and Optics, 49 Kronverkskiy Ave., 197101 Saint Petersburg

2014-01-20

Many point defects in GaN responsible for broad photoluminescence (PL) bands remain unidentified. Their presence in thick GaN layers grown by hydride vapor phase epitaxy (HVPE) detrimentally affects the material quality and may hinder the use of GaN in high-power electronic devices. One of the main PL bands in HVPE-grown GaN is the red luminescence (RL) band with a maximum at 1.8 eV. We observed the fine structure of this band with a zero-phonon line (ZPL) at 2.36 eV, which may help to identify the related defect. The shift of the ZPL with excitation intensity and the temperature-related transformation of the RLmore » band fine structure indicate that the RL band is caused by transitions from a shallow donor (at low temperature) or from the conduction band (above 50 K) to an unknown deep acceptor having an energy level 1.130 eV above the valence band.« less

Electronic structures of [001]- and [111]-oriented InSb and GaSb free-standing nanowires

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liao, Gaohua; Department of Applied Physics and Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha 410082; Luo, Ning

We report on a theoretical study of the electronic structures of InSb and GaSb nanowires oriented along the [001] and [111] crystallographic directions. The nanowires are described by atomistic, tight-binding models, including spin-orbit interaction. The band structures and the wave functions of the nanowires are calculated by means of a Lanczos iteration algorithm. For the [001]-oriented InSb and GaSb nanowires, the systems with both square and rectangular cross sections are considered. Here, it is found that all the energy bands are doubly degenerate. Although the lowest conduction bands in these nanowires show good parabolic dispersions, the top valence bands showmore » rich and complex structures. In particular, the topmost valence bands of the nanowires with a square cross section show a double maximum structure. In the nanowires with a rectangular cross section, this double maximum structure is suppressed, and the top valence bands gradually develop into parabolic bands as the aspect ratio of the cross section is increased. For the [111]-oriented InSb and GaSb nanowires, the systems with hexagonal cross sections are considered. It is found that all the bands at the Γ-point are again doubly degenerate. However, some of them will split into non-degenerate bands when the wave vector moves away from the Γ-point. Although the lowest conduction bands again show good parabolic dispersions, the topmost valence bands do not show the double maximum structure. Instead, they show a single maximum structure with its maximum at a wave vector slightly away from the Γ-point. The wave functions of the band states near the band gaps of the [001]- and [111]-oriented InSb and GaSb nanowires are also calculated and are presented in terms of probability distributions in the cross sections. It is found that although the probability distributions of the band states in the [001]-oriented nanowires with a rectangular cross section could be qualitatively described by one-band effective mass theory, the probability distributions of the band states in the [001]-oriented nanowires with a square cross section and the [111]-oriented nanowires with a hexagonal cross section show characteristic patterns with symmetries closely related to the irreducible representations of the relevant double point groups and, in general, go beyond the prediction of a simple one-band effective mass theory. We also investigate the effects of quantum confinement on the band structures of the [001]- and [111]-oriented InSb and GaSb nanowires and present an empirical formula for the description of quantization energies of the band edge states in the nanowires, which could be used to estimate the enhancement of the band gaps of the nanowires as a result of quantum confinement. The size dependencies of the electron and hole effective masses in these nanowires are also investigated and discussed.« less

Electronic structures of [001]- and [111]-oriented InSb and GaSb free-standing nanowires

NASA Astrophysics Data System (ADS)

Liao, Gaohua; Luo, Ning; Yang, Zhihu; Chen, Keqiu; Xu, H. Q.

2015-09-01

We report on a theoretical study of the electronic structures of InSb and GaSb nanowires oriented along the [001] and [111] crystallographic directions. The nanowires are described by atomistic, tight-binding models, including spin-orbit interaction. The band structures and the wave functions of the nanowires are calculated by means of a Lanczos iteration algorithm. For the [001]-oriented InSb and GaSb nanowires, the systems with both square and rectangular cross sections are considered. Here, it is found that all the energy bands are doubly degenerate. Although the lowest conduction bands in these nanowires show good parabolic dispersions, the top valence bands show rich and complex structures. In particular, the topmost valence bands of the nanowires with a square cross section show a double maximum structure. In the nanowires with a rectangular cross section, this double maximum structure is suppressed, and the top valence bands gradually develop into parabolic bands as the aspect ratio of the cross section is increased. For the [111]-oriented InSb and GaSb nanowires, the systems with hexagonal cross sections are considered. It is found that all the bands at the Γ-point are again doubly degenerate. However, some of them will split into non-degenerate bands when the wave vector moves away from the Γ-point. Although the lowest conduction bands again show good parabolic dispersions, the topmost valence bands do not show the double maximum structure. Instead, they show a single maximum structure with its maximum at a wave vector slightly away from the Γ-point. The wave functions of the band states near the band gaps of the [001]- and [111]-oriented InSb and GaSb nanowires are also calculated and are presented in terms of probability distributions in the cross sections. It is found that although the probability distributions of the band states in the [001]-oriented nanowires with a rectangular cross section could be qualitatively described by one-band effective mass theory, the probability distributions of the band states in the [001]-oriented nanowires with a square cross section and the [111]-oriented nanowires with a hexagonal cross section show characteristic patterns with symmetries closely related to the irreducible representations of the relevant double point groups and, in general, go beyond the prediction of a simple one-band effective mass theory. We also investigate the effects of quantum confinement on the band structures of the [001]- and [111]-oriented InSb and GaSb nanowires and present an empirical formula for the description of quantization energies of the band edge states in the nanowires, which could be used to estimate the enhancement of the band gaps of the nanowires as a result of quantum confinement. The size dependencies of the electron and hole effective masses in these nanowires are also investigated and discussed.

Ba 2TeO as an optoelectronic material: First-principles study

DOE PAGES

Sun, Jifeng; Shi, Hongliang; Du, Mao-Hua; ...

2015-05-21

The band structure, optical and defects properties of Ba 2TeO are systematically investigated using density functional theory with a view to understanding its potential as an optoelectronic or transparent conducting material. Ba 2TeO crystallizes with tetragonal structure (space group P4/nmm) and with a 2.93 eV optical band gap1. We find relatively modest band masses for both electrons and holes suggesting applications. Optical properties show a infrared-red absorption when doped. This could potentially be useful for combining wavelength filtering and transparent conducting functions. Furthermore, our defect calculations show that Ba 2TeO is intrinsically p-type conducting under Ba-poor condition. However, the spontaneousmore » formation of the donor defects may constrain the p-type transport properties and would need to be addressed to enable applications.« less

Distribution of Chern number by Landau level broadening in Hofstadter butterfly

NASA Astrophysics Data System (ADS)

Yoshioka, Nobuyuki; Matsuura, Hiroyasu; Ogata, Masao

2015-04-01

We discuss the relationship between the quantum Hall conductance and a fractal energy band structure, Hofstadter butterfly, on a square lattice under a magnetic field. At first, we calculate the Hall conductance of Hofstadter butterfly on the basis of the linear responce theory. By classifying the bands into some groups with a help of continued fraction expansion, we find that the conductance at the band gaps between the groups accord with the denominators of fractions obtained by aborting the expansion halfway. The broadening of Landau levels is given as an account of this correspondance.

Band Anticrossing in Highly Mismatched Compound Semiconductor Alloys

NASA Technical Reports Server (NTRS)

Yu, Kin Man; Wu, J.; Walukiewicz, W.; Ager, J. W.; Haller, E. E.; Miotkowski, I.; Su, Ching-Hua; Curreri, Peter A. (Technical Monitor)

2001-01-01

Compound semiconductor alloys in which metallic anions are partially replaced with more electronegative isoelectronic atoms have recently attracted significant attention. Group IIIN(sub x)V(sub 1-x) alloys with a small amount of the electronegative N substituting more metallic column V elements has been the most extensively studied class of such Highly Mismatched Alloys (HMAs). We have shown that many of the unusual properties of the IIIN(sub x)V(sub 1-x) alloys can be well explained by the Band Anticrossing (BAC) model that describes the electronic structure in terms of an interaction between highly localized levels of substitutional N and the extended states of the host semiconductor matrix. Most recently the BAC model has been also used to explain similar modifications of the electronic band structure observed in Te-rich ZnS(sub x)Te(sub 1-x) and ZnSe(sub y)Te(sub 1-y) alloys. To date studies of HMAs have been limited to materials with relatively small concentrations of highly electronegative atoms. Here we report investigations of the electronic structure of ZnSe(sub y)Te(sub 1-y) alloys in the entire composition range, y between 0 and 1. The samples used in this study are bulk ZnSe(sub y)Te(sub 1-y) crystals grown by either a modified Bridgman method or by physical vapor transport. Photomodulated reflection (PR) spectroscopy was used to measure the composition dependence of optical transitions from the valence band edge and from the spin-orbit split off band to the conduction band. The pressure dependence of the band gap was measured using optical absorption in a diamond anvil cell. We find that the energy of the spin-orbit split off valence band edge does not depend on composition and is located at about 3 eV below the conduction band edge of ZnSe. On the Te-rich side the pressure and the composition dependence of the optical transitions are well explained by the BAC model which describes the downward shift of the conduction band edge in terms of the interaction between localized Se states and the conduction band. On the other hand we show that the large band gap reduction observed on the Se-rich side of the alloy system is a result of an interaction between the localized Te level and the valence bands. This interaction leads to the formation of a Te-like valence band edge that strongly interacts with the light hole valence band. Calculations based on a modified k(sup dot)p model account for the reduction of the band gap and the large increase of the spin-orbit splitting observed in Se-rich ZnSe(sub y)Te(sub 1-y) alloys. We will also discuss the importance of these new results for understanding of the electronic structure and band offsets in other highly mismatched alloy systems.

Band Anticrossing in Highly Mismatched Compound Semiconductor Alloys

NASA Technical Reports Server (NTRS)

Yu, Kin Man; Wu, J.; Walukiewicz, W.; Ager, J. W.; Haller, E. E.; Miotkowski, I.; Ramdas, A.; Su, Ching-Hua; Whitaker, Ann F. (Technical Monitor)

2001-01-01

Compound semiconductor alloys in which metallic anions are partially replaced with more electronegative isoelectronic atoms have recently attracted significant attention. Group IIIN(x)V(1-x), alloys with a small amount of the electronegative N substituting more metallic column V elements has been the most extensively studied class of such Highly Mismatched Alloys (HMAs). We have shown that many of the unusual properties of the IIIN(x),V(1-x) alloys can be well explained by the Band Anticrossing (BAC) model that describes the electronic structure in terms of an interaction between highly localized levels of substitutional N and the extended states of the host semiconductor matrix. Most recently the BAC model has been also used to explain similar modifications of the electronic band structure observed in Te-rich ZnS(x)Te(l-x) and ZnSe(Y)Te(1-y) alloys. To date studies of HMAs have been limited to materials with relatively small concentrations of highly electronegative atoms. Here we report investigations of the electronic structure of ZnSe(y)Te(1-y) alloys in the entire composition range, 0 less than or equal to y less than or equal to 1. The samples used in this study are bulk ZnSe(y)Te(1-y) crystals grown by either a modified Bridgman method or by physical vapor transport. Photomodulated reflection (PR) spectroscopy was used to measure the composition dependence of optical transitions from the valence band edge and from the spin-orbit split off band to the conduction band. The pressure dependence of the band gap was measured using optical absorption in a diamond anvil cell. We find that the energy of the spin-orbit split off valence band edge does not depend on composition and is located at about 3 eV below the conduction band edge of ZnSe. On the Te-rich side the pressure and the composition dependence of the optical transitions are well explained by the BAC model which describes the downward shift of the conduction band edge in terms of the interaction between localized Se states and the conduction band. On the other hand we show that the large band gap reduction observed on the Se-rich side of the alloy system is a result of an interaction between the localized Te level and the valence bands. This interaction leads to the formation of a Te-like valence band edge that strongly interacts with the light hole valence band. Calculations based on a modified k p model account for the reduction of the band gap and the large increase of the spin-orbit splitting observed in Se-rich ZnSe(y)Te(l-y) alloys. We will also discuss the importance of these new results for understanding of the electronic structure and band offsets in other highly mismatched alloy systems.

Hybrid functional study of band structures of GaAs1-xNx and GaSb1-xNx alloys

NASA Astrophysics Data System (ADS)

Virkkala, Ville; Havu, Ville; Tuomisto, Filip; Puska, Martti J.

2012-02-01

Band structures of GaAs1-xNx and GaSb1-xNx alloys are studied in the framework of the density functional theory within the hybrid functional scheme (HSE06). We find that the scheme gives a clear improvement over the traditional (semi)local functionals in describing, in a qualitative agreement with experiments, the bowing of electron energy band gap in GaAs1-xNx alloys. In the case of GaSb1-xNx alloys, the hybrid functional used makes the study of band structures possible ab initio without any empirical parameter fitting. We explain the trends in the band gap reductions in the two materials that result mainly from the positions of the nitrogen-induced states with respect to the bottoms of the bulk conduction bands.

Camel-back band-induced power factor enhancement of thermoelectric lead-tellurium from Boltzmann transport calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, X. G., E-mail: wang2006@mail.ustc.edu.cn; Wang, L., E-mail: sqtb@mail.ustc.edu.cn; Liu, J., E-mail: jingliu@mail.ustc.edu.cn

2014-03-31

Band structures of PbTe can be abnormally bended via dual-doping on both the cationic and anionic sites to form camel-back multivalley energy band structures near the band edge. As a result, additional carrier pockets and strong intervalley scattering of carriers are introduced. Boltzmann transport calculations indicate that their contradictory effects yield remarkably enhanced power factor due to the improved thermopower and almost unchanged electrical conductivity in low temperature and high carrier concentration ranges. These findings prove dual-doping-induced band bending as an effective approach to improve the thermoelectric properties of PbTe and other similar materials.

Evidence of ion intercalation mediated band structure modification and opto-ionic coupling in lithium niobite

NASA Astrophysics Data System (ADS)

Shank, Joshua C.; Tellekamp, M. Brooks; Doolittle, W. Alan

2015-01-01

The theoretically suggested band structure of the novel p-type semiconductor lithium niobite (LiNbO2), the direct coupling of photons to ion motion, and optically induced band structure modifications are investigated by temperature dependent photoluminescence. LiNbO2 has previously been used as a memristor material but is shown here to be useful as a sensor owing to the electrical, optical, and chemical ease of lithium removal and insertion. Despite the high concentration of vacancies present in lithium niobite due to the intentional removal of lithium atoms, strong photoluminescence spectra are observed even at room temperature that experimentally confirm the suggested band structure implying transitions from a flat conduction band to a degenerate valence band. Removal of small amounts of lithium significantly modifies the photoluminescence spectra including additional larger than stoichiometric-band gap features. Sufficient removal of lithium results in the elimination of the photoluminescence response supporting the predicted transition from a direct to indirect band gap semiconductor. In addition, non-thermal coupling between the incident laser and lithium ions is observed and results in modulation of the electrical impedance.

Density-functional studies of tungsten trioxide, tungsten bronzes, and related systems

NASA Astrophysics Data System (ADS)

Ingham, B.; Hendy, S. C.; Chong, S. V.; Tallon, J. L.

2005-08-01

Tungsten trioxide adopts a variety of structures which can be intercalated with charged species to alter the electronic properties, thus forming “tungsten bronzes.” Similar effects are observed upon removing oxygen from WO3 . We present a computational study of cubic and hexagonal alkali bronzes and examine the effects on cell size and band structure as the size of the intercalated ion is increased. With the exception of hydrogen (which is predicted to be unstable as an intercalate), the behavior of the bronzes are relatively consistent. NaWO3 is the most stable of the cubic systems, although in the hexagonal system the larger ions are more stable. The band structures are identical, with the intercalated atom donating its single electron to the tungsten 5d valence band. A study of fractional doping in the NaxWO3 system (0⩽x⩽1) showed a linear variation in cell parameter and a systematic shift in the Fermi level into the conduction band. In the oxygen-deficient WO3-x system the Fermi level undergoes a sudden jump into the conduction band at around x=0.2 . Lastly, three compounds of a layered WO4•α,ω -diaminoalkane hybrid series were studied and found to be insulating, with features in the band structure similar to those of the parent WO3 compound that relate well to experimental UV-visible spectroscopy results.

Tuning optical properties of transparent conducting barium stannate by dimensional reduction

DOE PAGES

Li, Yuwei; Zhang, Lijun; Ma, Yanming; ...

2015-01-30

We report calculations of the electronic structure and optical properties of doped n-type perovskite BaSnO 3 and layered perovskites. While doped BaSnO 3 retains its transparency for energies below the valence to conduction band onset, the doped layered compounds exhibit below band edge optical conductivity due to transitions from the lowest conduction band. This gives absorption in the visible for Ba 2SnO 4. It is important to minimize this phase in transparent conducting oxide (TCO) films. Ba 3Sn 2O 7 and Ba 4Sn 3O 10 have strong transitions only in the red and infrared, respectively. Thus, there may be opportunitiesmore » for using these as wavelength filtering TCO.« less

Effects of B site doping on electronic structures of InNbO4 based on hybrid density functional calculations

NASA Astrophysics Data System (ADS)

Lu, M. F.; Zhou, C. P.; Li, Q. Q.; Zhang, C. L.; Shi, H. F.

2018-01-01

In order to improve the photocatalytic activity under visible-light irradiation, we adopted first principle calculations based on density functional theory (DFT) to calculate the electronic structures of B site transition metal element doped InNbO4. The results indicated that the complete hybridization of Nb 4d states and some Ti 3d states contributed to the new conduction band of Ti doped InNbO4, barely changing the position of band edge. For Cr doping, some localized Cr 3d states were introduced into the band gap. Nonetheless, the potential of localized levels was too positive to cause visible-light reaction. When it came to Cu doping, the band gap was almost same with that of InNbO4 as well as some localized Cu 3d states appeared above the top of VB. The introduction of localized energy levels benefited electrons to migrate from valence band (VB) to conduction band (CB) by absorbing lower energy photons, realizing visible-light response.

Low-temperature growth and electronic structures of ambipolar Yb-doped zinc tin oxide transparent thin films

NASA Astrophysics Data System (ADS)

Oh, Seol Hee; Ferblantier, Gerald; Park, Young Sang; Schmerber, Guy; Dinia, Aziz; Slaoui, Abdelilah; Jo, William

2018-05-01

The compositional dependence of the crystal structure, optical transmittance, and surface electric properties of the zinc tin oxide (Zn-Sn-O, shortened ZTO) thin films were investigated. ZTO thin films with different compositional ratios were fabricated on glass and p-silicon wafers using radio frequency magnetron sputtering. The binding energy of amorphous ZTO thin films was examined by a X-ray photoelectron spectroscopy. The optical transmittance over 70% in the visible region for all the ZTO films was observed. The optical band gap of the ZTO films was changed as a result of the competition between the Burstein-Moss effect and renormalization. An electron concentration in the films and surface work function distribution were measured by a Hall measurement and Kelvin probe force microscopy, respectively. The mobility of the n- and p-type ZTO thin films have more than 130 cm2/V s and 15 cm2/V s, respectively. We finally constructed the band structure which contains band gap, work function, and band edges such as valence band maximum and conduction band minimum of ZTO thin films. The present study results suggest that the ZTO thin film is competitive compared with the indium tin oxide, which is a representative material of the transparent conducting oxides, regarding optoelectronic devices applications.

Conductance modulation in Weyl semimetals with tilted energy dispersion without a band gap

NASA Astrophysics Data System (ADS)

Yesilyurt, Can; Siu, Zhuo Bin; Tan, Seng Ghee; Liang, Gengchiau; Jalil, Mansoor B. A.

2017-06-01

We investigate the tunneling conductance of Weyl semimetal with tilted energy dispersion by considering electron transmission through a p-n-p junction with one-dimensional electric and magnetic barriers. In the presence of both electric and magnetic barriers, we found that a large conductance gap can be produced with the aid of tilted energy dispersion without a band gap. The origin of this effect is the shift of the electron wave-vector at barrier boundaries caused by (i) the pseudo-magnetic field induced by electrical potential, i.e., a newly discovered feature that is only possible in the materials possessing tilted energy dispersion, (ii) the real magnetic field induced by a ferromagnetic layer deposited on the top of the system. We use a realistic barrier structure applicable in current nanotechnology and analyze the temperature dependence of the tunneling conductance. The new approach presented here may resolve a major problem of possible transistor applications in topological semimetals, i.e., the absence of normal backscattering and gapless band structure.

Electronic structures of filled tetrahedral semiconductors LiMgN and LiZnN: conduction band distortion

NASA Astrophysics Data System (ADS)

Yu, L. H.; Yao, K. L.; Liu, Z. L.

2004-12-01

The band structures of the filled tetrahedral semiconductors LiMgN and LiZnN, viewed as the zinc-blende (MgN) - and (ZnN) - lattices partially filled with He-like Li + ion interstitials, were studied using the full-potential linearized augmented plane wave method (FP-LAPW) within density functional theory. The conduction band distortions of LiMgN and LiZnN, compared to their “parent” zinc-blende analog AlN and GaN, are discussed. It was found that the insertion of Li + ions at the interstitial sites near the cation or anion pushes the conduction band minimum of the X point in the Brillouin zone upward, relative to that of the Γ point, for both (MgN) - and (ZnN) - lattices (the valence band maximum is at Γ for AlN, GaN, LiMgN, and LiZnN), which provides a method to convert a zinc-blende indirect gap semiconductor into a direct gap material, but the conduction band distortion of the β phase (Li + near the cation) is quite stronger than that of the α phase (Li + near the anion). The total energy calculations show the α phase to be more stable than the β phase for both LiMgN and LiZnN. The Li-N and Mg-N bonds exhibit a strong ionic character, whereas the Zn-N bond has a strong covalent character in LiMgN and LiZnN.

Hydrothermal temperature effect on crystal structures, optical properties and electrical conductivity of ZnO nanostructures

NASA Astrophysics Data System (ADS)

Dhafina, Wan Almaz; Salleh, Hasiah; Daud, Mohd Zalani; Ghazali, Mohd Sabri Mohd; Ghazali, Salmah Mohd

2017-09-01

ZnO is an wide direct band gap semiconductor and possess rich family of nanostructures which turned to be a key role in the nanotechnology field of applications. Hydrothermal method was proven to be simple, robust and low cost among the reported methods to synthesize ZnO nanostructures. In this work, the properties of ZnO nanostructures were altered by varying temperatures of hydrothermal process. The changes in term of morphological, crystal structures, optical properties and electrical conductivity were investigated. A drastic change of ZnO nanostructures morphology and decreases of 002 diffraction peak were observed as the hydrothermal temperature increased. The band gap of samples decreased as the size of ZnO nanostructure increased, whereas the electrical conductivity had no influence on the band gap value but more on the morphology of ZnO nanostructures instead.

Energies of the X- and L-valleys in In{sub 0.53}Ga{sub 0.47}As from electronic structure calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Greene-Diniz, Gabriel; Greer, J. C.; Fischetti, M. V.

2016-02-07

Several theoretical electronic structure methods are applied to study the relative energies of the minima of the X- and L-conduction-band satellite valleys of In{sub x}Ga{sub 1−x}As with x = 0.53. This III-V semiconductor is a contender as a replacement for silicon in high-performance n-type metal-oxide-semiconductor transistors. The energy of the low-lying valleys relative to the conduction-band edge governs the population of channel carriers as the transistor is brought into inversion, hence determining current drive and switching properties at gate voltages above threshold. The calculations indicate that the position of the L- and X-valley minima are ∼1 eV and ∼1.2 eV, respectively, higher in energymore » with respect to the conduction-band minimum at the Γ-point.« less

Ab - initio study of rare earth magnesium alloy: TbMg

NASA Astrophysics Data System (ADS)

Kumari, Meena; Yadav, Priya; Nautiyal, Shashank; Verma, U. P.

2018-05-01

The structural, electronic and magnetic properties of TbMg were analyzed by using full-potential linearized augmented plane wave method. This intermetallic is stable in structure CsCl (B2 phase) with space group Pm-3m. In electronic properties, we show the electronic band structure and density of states plots. These plots show that this alloy have metallic character because there is no band gap between the valance band and conduction band at Fermi level. The structural properties, i.e. equilibrium lattice constant, bulk modulus and its pressure derivative, energy and volume show good agreement with available data. In this paper, we also present the total magnetic moment along with the magnetic moment on the atomic and interstitial sites of TbMg intermetallic in B2 phase.

Electronic Structure of ABC-stacked Multilayer Graphene and Trigonal Warping:A First Principles Calculation

NASA Astrophysics Data System (ADS)

Yelgel, Celal

2016-04-01

We present an extensive density functional theory (DFT) based investigation of the electronic structures of ABC-stacked N-layer graphene. It is found that for such systems the dispersion relations of the highest valence and the lowest conduction bands near the K point in the Brillouin zone are characterised by a mixture of cubic, parabolic, and linear behaviours. When the number of graphene layers is increased to more than three, the separation between the valence and conduction bands decreases up until they touch each other. For five and six layer samples these bands show flat behaviour close to the K point. We note that all states in the vicinity of the Fermi energy are surface states originated from the top and/or bottom surface of all the systems considered. For the trilayer system, N = 3, pronounced trigonal warping of the bands slightly above the Fermi level is directly obtained from DFT calculations.

Band gap narrowing in n-type and p-type 3C-, 2H-, 4H-, 6H-SiC, and Si

NASA Astrophysics Data System (ADS)

Persson, C.; Lindefelt, U.; Sernelius, B. E.

1999-10-01

Doping-induced energy shifts of the conduction band minimum and the valence band maximum have been calculated for n-type and p-type 3C-, 2H-, 4H-, 6H-SiC, and Si. The narrowing of the fundamental band gap and of the optical band gap are presented as functions of ionized impurity concentration. The calculations go beyond the common parabolic treatments of the ground state energy dispersion by using energy dispersion and overlap integrals from band structure calculations. The nonparabolic valence band curvatures influence strongly the energy shifts especially in p-type materials. The utilized method is based on a zero-temperature Green's function formalism within the random phase approximation with local field correction according to Hubbard. We have parametrized the shifts of the conduction and the valence bands and made comparisons with recently published results from a semi-empirical model.

Tunnel optical radiation in In{sub x}Ga{sub 1−x}N

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alexandrov, Dimiter; Skerget, Shawn

2014-02-21

An investigation of tunnel optical radiation in epitaxial layers of n-type In{sub x}Ga{sub 1−x}N grown on p-type GaN by novel plasma based migration enhanced epitaxy is presented. Experimental results of electro-luminescence spectra for In{sub x}Ga{sub 1−x}N/p−GaN hetero-junctions were obtained and they show two well expressed optical bands - one in range 500-540 nm and other in range 550-610 nm. An interesting detail is that each band begins and ends by sharp drops of the radiation, which nearly approach zero. A theoretical investigation of the unusual behavior of these spectra was done using LCAO electron band structure calculations. The optical rangesmore » of these bands show that the radiation occurs in the In{sub x}Ga{sub 1−x}N region. In fact, substitutions of In atoms in Ga sites creates defects in the structure of In{sub x}Ga{sub 1−x}N and the corresponding LCAO matrix elements are found on this basis. The LCAO electron band structures are calculated considering the interactions between nearest-neighbor orbitals. Electron energy pockets are found in both the conduction and the valence bands at the Γ point of the electron band structures. Also it is found that these pockets are separated by distances, for which there is overlapping between the electron wave functions describing localized states belonging to the pockets, and as a result tunnel optical radiation can take place. This type of electron transition - between such a pocket in the conduction band and a pocket in the valence band - occurs in In{sub x}Ga{sub 1−x}N, causing the above described optical bands. This conclusion concurs with the fact that the shapes of these bands change with change of the applied voltage.« less

D Haas-Van Alphen Oscillations in the Diluted Magnetic Semiconductor MERCURY(1-X)IRON(X)SELENIUM.

NASA Astrophysics Data System (ADS)

Miller, Michael Montgomery

de Haas-van Alphen measurements are performed in oriented single crystals of Hg_{ rm 1-x}Fe_{rm x}Se in the range 0.0 <=q x <=q 0.05 for 0.5 < T < 4.2K for the magnetic field range 0.2 T < H < 1.0 T. These data can be interpreted in terms of a closed orbit magnetic breakdown model. The effect of Fe on the conduction band is explored in some detail. It is found that the presence of Fe lowers the Dingle temperature in a non-monotonic fashion, i.e., there is a minimum in the Dingle temperature for x ~ 0.001. This effect cannot be attributed to a gross modification of the band structure. Effective mass measurements are in good agreement with those expected for HgSe. However, the presence of Fe is seen to have a subtle effect on the band structure. The overall symmetry of the band structure may be modified by the addition of Fe. Furthermore, the presence of Fe tends to decrease the inversion asymmetry splitting of the conduction band as evidenced in the low-field beating.

Elucidation of band structure of charge storage in conducting polymers using a redox reaction.

PubMed

Contractor, Asfiya Q; Juvekar, Vinay A

2014-07-01

A novel technique to investigate charge storage characteristics of intrinsically conducting polymer films has been developed. A redox reaction is conducted on a polymer film on a rotating disk electrode under potentiostatic condition so that the rate of charging of the film equals the rate of removal of the charge by the reaction. The voltammogram obtained from the experiment on polyaniline film using Fe(2+)/Fe(3+) in HCl as the redox system shows five distinct linear segments (bands) with discontinuity in the slope at specific transition potentials. These bands are the same as those indicated by electron spin resonance (ESR)/Raman spectroscopy with comparable transition potentials. From the dependence of the slopes of the bands on concentration of ferrous and ferric ions, it was possible to estimate the energies of the charge carriers in different bands. The film behaves as a redox capacitor and does not offer resistance to charge transfer and electronic conduction.

Effect of thermal stresses on frequency band structures of elastic metamaterial plates

NASA Astrophysics Data System (ADS)

Wu, Ying; Yu, Kaiping; Yang, Linyun; Zhao, Rui; Shi, Xiaotian; Tian, Kuo

2018-01-01

We investigate the effect of thermal stresses on the band structure of elastic metamaterial plates by developing a useful finite-element based method. The thermal field is assumed to be uniform throughout the whole plate. Specifically, we find that the stiffness matrix of plate element is comprised of elastic and thermal stresses parts, which can be regarded as a linear function of temperature difference. We additionally demonstrate that the relative magnitudes between elastic properties and thermal stresses will lead to nonlinear effects on frequency band structures based on two different types of metamaterial plates made of single and double inclusions of square plates, respectively. Then, we validate the proposed approach by comparing the band structures with the frequency response curves obtained in finite periodic structures. We conduct sensitivity analysis and discuss in-depth the sensitivities of band structures with respect to temperature difference to quantitatively investigate the effect of thermal stresses on each band. In addition, the coupled effects of thermal stresses and temperature-dependent material properties on the band structure of Aluminum/silicone rubber plate have also been discussed. The proposed method and new findings in this paper extends the ability of existing metamaterial plates by enabling tunability over a wide range of frequencies in thermal environments.

Electronic Structure and Thermoelectric Properties of Transition Metal Monosilicides

NASA Astrophysics Data System (ADS)

Pshenay-Severin, D. A.; Ivanov, Yu. V.; Burkov, A. T.; Novikov, S. V.; Zaitsev, V. K.; Reith, H.

2018-06-01

We present theoretical and experimental results on electronic structure and thermoelectric properties of cobalt monosilicide (CoSi) and of Co1- x M x Si diluted alloys (M = Fe and Ni) at temperatures from 2 K to 800 K. CoSi crystallizes into a non-centrosymmetric cubic B20 structure, which suggests the possibility of a topologically non-trivial electronic structure. We show that the electronic structure of CoSi exhibits linear band crossings in close vicinity to Fermi energy, confirming the possibility of non-trivial topology. The proximity of the linear-dispersion bands to Fermi energy implies their important contribution to the electronic transport. Calculation of thermopower of CoSi, using ab initio band structure and the constant relaxation time approximation, is carried out. It reveals that many body corrections to the electronic spectrum are important in order to obtain qualitative agreement of theoretical and experimental temperature dependences of thermopower. Phonon dispersion and lattice thermal conductivity are calculated. The phonons give a major contribution to the thermal conductivity of the compound below room temperature.

Boltzmann transport properties of ultra thin-layer of h-CX monolayers

NASA Astrophysics Data System (ADS)

Kansara, Shivam; Gupta, Sanjeev K.; Sonvane, Yogesh

2018-04-01

Structural, electronic and thermoelectric properties of monolayer h-CX (X= Al, As, B, Bi, Ga, In, P, N, Sb and Tl) have been computed using density functional theory (DFT). The structural, electronic band structure, phonon dispersion curves and thermoelectric properties have been investigated. h-CGa and h-CTl show the periodically lattice vibrations and h-CB and h-CIn show small imaginary ZA frequencies. Thermoelectric properties are obtained using BoltzTrap code with the constant relaxation time (τ) approximation such as electronic, thermal and electrical conductivity calculated for various temperatures. The results indicate that h-CGa, h-CIn, h-CTl and h-CAl have direct band gaps with minimum electronic thermal and electrical conductivity while h-CB and h-CN show the high electronic thermal and electrical conductivity with highest cohesive energy.

Electronic Structures of Purple Bronze KMo6O17 Studied by X-Ray Photoemission Spectra

NASA Astrophysics Data System (ADS)

Qin, Xiaokui; Wei, Junyin; Shi, Jing; Tian, Mingliang; Chen, Hong; Tian, Decheng

X-ray photoemission spectroscopy study has been performed for the purple bronze KMo6O17. The structures of conduction band and valence band are analogous to the results of ultraviolet photoemission spectra and are also consistent with the model of Travaglini et al., but the gap between conduction and valence band is insignificant. The shape of asymmetric and broadening line of O-1s is due to unresolved contributions from the many inequivalent oxygen sites in this crystal structure. Mo 3d core-level spectrum reveals that there are two kinds of valence states of Molybdenum (Mo+5 and Mo+6). The calculated average valence state is about +5.6, which is consistent with the expectation value from the composition of this material. The tail of Mo-3d spectrum toward higher binding energy is the consequence of the excitation of electron-hole pairs with singularity index of 0.21.

Efficient band structure modulations in two-dimensional MnPSe3/CrSiTe3 van der Waals heterostructures

NASA Astrophysics Data System (ADS)

Pei, Qi; Wang, Xiaocha; Zou, Jijun; Mi, Wenbo

2018-05-01

As a research upsurge, van der Waals (vdW) heterostructures give rise to numerous combined merits and novel applications in nanoelectronics fields. Here, we systematically investigate the electronic structure of MnPSe3/CrSiTe3 vdW heterostructures with various stacking patterns. Then, particular attention of this work is paid on the band structure modulations in MnPSe3/CrSiTe3 vdW heterostructures via biaxial strain or electric field. Under a tensile strain, the relative band edge positions of heterostructures transform from type-I (nested) to type-II (staggered). The relocation of conduction band minimum also brings about a transition from indirect to direct band gap. Under a compressive strain, the electronic properties change from semiconducting to metallic. The physical mechanism of strain-dependent band structure may be ascribed to the shifts of the energy bands impelled by different superposition of atomic orbitals. Meanwhile, our calculations manifest that band gap values of MnPSe3/CrSiTe3 heterostructures are insensitive to the electric field. Even so, by applying a suitable intensity of negative electric field, the band alignment transition from type-I to type-II can also be realized. The efficient band structure modulations via external factors endow MnPSe3/CrSiTe3 heterostructures with great potential in novel applications, such as strain sensors, photocatalysis, spintronic and photoelectronic devices.

Efficient band structure modulations in two-dimensional MnPSe3/CrSiTe3 van der Waals heterostructures.

PubMed

Pei, Qi; Wang, Xiaocha; Zou, Jijun; Mi, Wenbo

2018-05-25

As a research upsurge, van der Waals (vdW) heterostructures give rise to numerous combined merits and novel applications in nanoelectronics fields. Here, we systematically investigate the electronic structure of MnPSe 3 /CrSiTe 3 vdW heterostructures with various stacking patterns. Then, particular attention of this work is paid on the band structure modulations in MnPSe 3 /CrSiTe 3 vdW heterostructures via biaxial strain or electric field. Under a tensile strain, the relative band edge positions of heterostructures transform from type-I (nested) to type-II (staggered). The relocation of conduction band minimum also brings about a transition from indirect to direct band gap. Under a compressive strain, the electronic properties change from semiconducting to metallic. The physical mechanism of strain-dependent band structure may be ascribed to the shifts of the energy bands impelled by different superposition of atomic orbitals. Meanwhile, our calculations manifest that band gap values of MnPSe 3 /CrSiTe 3 heterostructures are insensitive to the electric field. Even so, by applying a suitable intensity of negative electric field, the band alignment transition from type-I to type-II can also be realized. The efficient band structure modulations via external factors endow MnPSe 3 /CrSiTe 3 heterostructures with great potential in novel applications, such as strain sensors, photocatalysis, spintronic and photoelectronic devices.

Shot noise and Fano factor in tunneling in three-band pseudospin-1 Dirac-Weyl systems

NASA Astrophysics Data System (ADS)

Zhu, Rui; Hui, Pak Ming

2017-06-01

Tunneling through a potential barrier of height V0 in a two-dimensional system with a band structure consisting of three bands with a flat band intersecting the touching apices of two Dirac cones is studied. Results of the transmission coefficient at various incident angles, conductivity, shot noise, and Fano factor in this pseudospin-1 Dirac-Weyl system are presented and contrasted with those in graphene which is typical of a pseudospin-1/2 system. The pseudospin-1 system is found to show a higher transmission and suppressed shot noise in general. Significant differences in the shot noise and Fano factor due to the super Klein tunneling effect that allows perfect transmission at all incident angles under certain conditions are illustrated. For Fermi energy EF =V0 / 2, super Klein tunneling leads to a noiseless conductivity that takes on the maximum value 2e2 DkF / (πh) for 0 ≤EF ≤V0. This gives rise to a minimum Fano factor, in sharp contrast with that of a local maximum in graphene. For EF =V0, the band structure of pseudospin-1 system no longer leads to a quantized value of the conductivity as in graphene. Both the conductivity and the shot noise show a minimum with the Fano factor approaching 1/4, which is different from the value of 1/3 in graphene.

Room Temperature Interband Cascade Lasers

DTIC Science & Technology

2010-01-01

interband cascade laser (ICL) [6]-[8] is particularly convenient, since the semimetallic band alignment between InAs and GaSb allows the electrons that...superlattice claddings and a non-ohmic part that may be associated with various band discontinuities in the structure . While the latter can probably be...made a radiative transition to the valence band to be transferred readily back to the conduction band via elastic or nearly elastic processes. Our

Quasiparticle band gap in the topological insulator Bi2Te3

NASA Astrophysics Data System (ADS)

Nechaev, I. A.; Chulkov, E. V.

2013-10-01

We present a theoretical study of dispersion of states that form the bulk band-gap edges in the three-dimensional topological insulator Bi2Te3. Within density functional theory, we analyze the effect of atomic positions varied within the error range of the available experimental data and approximation chosen for the exchange-correlation functional on the bulk band gap and k-space location of valence- and conduction-band extrema. For each set of the positions with different exchange-correlation functionals, we show how many-body corrections calculated within a one-shot GW approach affect the mentioned characteristics of electronic structure of Bi2Te3. We thus also illustrate to what degree the one-shot GW results are sensitive to the reference one-particle band structure in the case of bismuth telluride. We found that for this topological insulator the GW corrections enlarge the fundamental band gap and for certain atomic positions and reference band structure bring its value in close agreement with experiment.

X-ray absorption and reflection as probes of the GaN conduction bands: Theory and experiments

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lambrecht, W.R.L.; Rashkeev, S.N.; Segall, B.

1997-04-01

X-ray absorption measurements are a well-known probe of the unoccupied states in a material. The same information can be obtained by using glancing angle X-ray reflectivity. In spite of several existing band structure calculations of the group III nitrides and previous optical studies in UV range, a direct probe of their conduction band densities of states is of interest. The authors performed a joint experimental and theoretical investigation using both of these experimental techniques for wurtzite GaN.

Electronic structure modifications and band gap narrowing in Zn0.95V0.05O

NASA Astrophysics Data System (ADS)

Ahad, Abdul; Majid, S. S.; Rahman, F.; Shukla, D. K.; Phase, D. M.

2018-04-01

We present here, structural, optical and electronic structure studies on Zn0.95V0.05O, synthesized using solid state method. Rietveld refinement of x-ray diffraction pattern indicates no considerable change in the lattice of doped ZnO. The band gap of doped sample, as calculated by Kubelka-Munk transformed reflectance spectra, has been found reduced compared to pure ZnO. Considerable changes in absorbance in UV-Vis range is observed in doped sample. V doping induced decrease in band gap is supported by x-ray absorption spectroscopy measurements. It is experimentally confirmed that conduction band edge in Zn0.95V0.05O has shifted towards Fermi level than in pure ZnO.

Band crossing in isovalent semiconductor alloys with large size mismatch

NASA Astrophysics Data System (ADS)

Deng, Hui-Xiong; Wei, Su-Huai

2012-02-01

Mixing isovalent compounds AC with BC to form alloys A1-xBxC has been an effective way in band structure engineering to enhance the availability of material properties. In most cases, the mixed isovalent atoms A and B, such as Al and Ga in Al1-xGaxAs or As and Sb in GaAs1-xSbx are similar in their atomic sizes and chemical potentials; therefore, the physical properties of A1-xBxC change smoothly from AC to BC. However, in some cases when the chemical and size differences between the isovalent atoms A and B are large, adding a small amount of B to AC or vice versa can lead to a discontinuous change in the electronic band structure. These large size- and chemicalmismatched (LSCM) systems often show unusual and abrupt changes in the alloys' material properties, which provide great potential in material design for novel device applications. In this report, based on first-principles band-structure calculations we show that for LSCM GaAs1-xNx and GaAs1-xBix alloys at the impurity limit the N (Bi)-induced impurity level is above (below) the conduction-(valence-) band edge of GaAs. These trends reverse at high concentration, i.e., the conduction-band edge of GaAs1-xNx becomes an N-derived state and the valence-band edge of GaAs1-xBix becomes a Bi-derived state, as expected from their band characters. We show that this band crossing phenomenon cannot be described by the popular BAC model but can be naturally explained by a simple band broadening picture.

Understanding conductivity anomalies in Cu(I)-based delafossite transparent conducting oxides: Theoretical insights.

PubMed

Scanlon, David O; Godinho, Kate G; Morgan, Benjamin J; Watson, Graeme W

2010-01-14

The Cu(I)-based delafossite structure, Cu(I)M(III)O(2), can accommodate a wide range of rare earth and transition metal cations on the M(III) site. Substitutional doping of divalent ions for these trivalent metals is known to produce higher p-type conductivity than that occurring in the undoped materials. However, an explanation of the conductivity anomalies observed in these p-type materials, as the trivalent metal is varied, is still lacking. In this article, we examine the electronic structure of Cu(I)M(III)O(2) (M(III)=Al,Cr,Sc,Y) using density functional theory corrected for on-site Coulomb interactions in strongly correlated systems (GGA+U) and discuss the unusual experimental trends. The importance of covalent interactions between the M(III) cation and oxygen for improving conductivity in the delafossite structure is highlighted, with the covalency trends found to perfectly match the conductivity trends. We also show that calculating the natural band offsets and the effective masses of the valence band maxima is not an ideal method to classify the conduction properties of these ternary materials.

Investigation on Fluorescence Quenching Mechanism of Perylene Diimide Dyes by Graphene Oxide.

PubMed

Zhao, Yuzhen; Li, Kexuan; He, Zemin; Zhang, Yongming; Zhao, Yang; Zhang, Haiquan; Miao, Zongcheng

2016-11-30

Perylene diimide derivatives were used as probes to investigate the effect of the molecular structures on the fluorescence quenching mechanism in a perylene diimide/graphene oxide system. The electrons transferred from the excited state of dyes to the conductive band of graphene oxide with different concentrations were determined by fluorescence spectra. The results indicated that the quenching efficiency of perylene diimides by graphene oxide was not only dependent on the difference between the lowest unoccupied molecular orbital level of dyes and the conduction band of the graphene oxide, but also mainly on the difference in the molecular structures.

Local Bonding Influence on the Band Edge and Band Gap Formation in Quaternary Chalcopyrites.

PubMed

Miglio, Anna; Heinrich, Christophe P; Tremel, Wolfgang; Hautier, Geoffroy; Zeier, Wolfgang G

2017-09-01

Quaternary chalcopyrites have shown to exhibit tunable band gaps with changing anion composition. Inspired by these observations, the underlying structural and electronic considerations are investigated using a combination of experimentally obtained structural data, molecular orbital considerations, and density functional theory. Within the solid solution Cu 2 ZnGeS 4- x Se x , the anion bond alteration parameter changes, showing larger bond lengths for metal-selenium than for metal-sulfur bonds. The changing bonding interaction directly influences the valence and conduction band edges, which result from antibonding Cu-anion and Ge-anion interactions, respectively. The knowledge of the underlying bonding interactions at the band edges can help design properties of these quaternary chalcopyrites for photovoltaic and thermoelectric applications.

Charge-transport in tin-iodide perovskite CH3NH3SnI3: origin of high conductivity.

PubMed

Takahashi, Yukari; Obara, Rena; Lin, Zheng-Zhong; Takahashi, Yukihiro; Naito, Toshio; Inabe, Tamotsu; Ishibashi, Shoji; Terakura, Kiyoyuki

2011-05-28

The structural and electrical properties of a metal-halide cubic perovskite, CH(3)NH(3)SnI(3), have been examined. The band structure, obtained using first-principles calculation, reveals a well-defined band gap at the Fermi level. However, the temperature dependence of the single-crystal electrical conductivity shows metallic behavior down to low temperatures. The temperature dependence of the thermoelectric power is also metallic over the whole temperature range, and the large positive value indicates that charge transport occurs with a low concentration of hole carriers. The metallic properties of this as-grown crystal are thus suggested to result from spontaneous hole-doping in the crystallization process, rather than the semi-metal electronic structure. The present study shows that artificial hole doping indeed enhances the conductivity.

From lattice Hamiltonians to tunable band structures by lithographic design

NASA Astrophysics Data System (ADS)

Tadjine, Athmane; Allan, Guy; Delerue, Christophe

2016-08-01

Recently, new materials exhibiting exotic band structures characterized by Dirac cones, nontrivial flat bands, and band crossing points have been proposed on the basis of effective two-dimensional lattice Hamiltonians. Here, we show using atomistic tight-binding calculations that these theoretical predictions could be experimentally realized in the conduction band of superlattices nanolithographed in III-V and II-VI semiconductor ultrathin films. The lithographed patterns consist of periodic lattices of etched cylindrical holes that form potential barriers for the electrons in the quantum well. In the case of honeycomb lattices, the conduction minibands of the resulting artificial graphene host several Dirac cones and nontrivial flat bands. Similar features, but organized in different ways, in energy or in k -space are found in kagome, distorted honeycomb, and Lieb superlattices. Dirac cones extending over tens of meV could be obtained in superlattices with reasonable sizes of the lithographic patterns, for instance in InAs/AlSb heterostructures. Bilayer artificial graphene could be also realized by lithography of a double quantum-well heterostructure. These new materials should be interesting for the experimental exploration of Dirac-based quantum systems, for both fundamental and applied physics.

Three-dimensional nature of the band structure of ZrTe 5 measured by high-momentum-resolution photoemission spectroscopy [3D nature ZrTe 5 band structure measured by high-momentum-resolution photoemission spectroscopy

DOE PAGES

Xiong, H.; Sobota, J. A.; Yang, S. -L.; ...

2017-05-10

Here, we have performed a systematic high-momentum-resolution photoemission study on ZrTe 5 using 6-eV photon energy. We have measured the band structure near the Γ point, and quantified the gap between the conduction and valence band as 18 ≤ Δ ≤ 29 meV. We have also observed photon-energy-dependent behavior attributed to final-state effects and the three-dimensional (3D) nature of the material's band structure. Our interpretation indicates the gap is intrinsic and reconciles discrepancies on the existence of a topological surface state reported by different studies. The existence of a gap suggests that ZrTe 5 is not a 3D strong topologicalmore » insulator nor a 3D Dirac semimetal. Therefore, our experiment is consistent with ZrTe 5 being a 3D weak topological insulator.« less

Three-dimensional nature of the band structure of ZrTe 5 measured by high-momentum-resolution photoemission spectroscopy [3D nature ZrTe 5 band structure measured by high-momentum-resolution photoemission spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xiong, H.; Sobota, J. A.; Yang, S. -L.

Here, we have performed a systematic high-momentum-resolution photoemission study on ZrTe 5 using 6-eV photon energy. We have measured the band structure near the Γ point, and quantified the gap between the conduction and valence band as 18 ≤ Δ ≤ 29 meV. We have also observed photon-energy-dependent behavior attributed to final-state effects and the three-dimensional (3D) nature of the material's band structure. Our interpretation indicates the gap is intrinsic and reconciles discrepancies on the existence of a topological surface state reported by different studies. The existence of a gap suggests that ZrTe 5 is not a 3D strong topologicalmore » insulator nor a 3D Dirac semimetal. Therefore, our experiment is consistent with ZrTe 5 being a 3D weak topological insulator.« less

Tunable electronic structure in stained two dimensional van der Waals g-C2N/XSe2 (X = Mo, W) heterostructures

NASA Astrophysics Data System (ADS)

Zheng, Z. D.; Wang, X. C.; Mi, W. B.

2017-10-01

The electronic structure of the strained g-C2N/XSe2 (X=Mo, W) van der Waals heterostructures are investigated by first-principles calculations. The g-C2N/MoSe2 heterostructure is an indirect band gap semiconductor at a strain from 0% to 8%, where its band gap is 0.66, 0.61, 0.73, 0.60 and 0.33 eV. At K point, the spin splitting is 186, 181, 39, 13 and 9 meV, respectively. For g-C2N/WSe2 heterostructures, the band gap is 0.32, 0.37, 0.42, 0.45 and 0.36 eV, and the conduction band minimum is shifted from Г-M region to K-Г region as the strain increases from 0% to 8%. Its spin splitting monotonically decreases as a strain raises to 8%, which is 445, 424, 261, 111 and 96 meV, respectively. Moreover, at a strain less than 4%, the conduction band mainly comes from g-C2N, but it comes from XSe2 (X=Mo, W) above 6%. Our results show that the g-C2N/XSe2 heterostructures have tunable electronic structures, which makes it a potential candidate for novel electronic devices.

Structural and electronic properties of low-index stoichiometric Cu2ZnSnS4 surfaces

NASA Astrophysics Data System (ADS)

Jia, Zhan-Ju; Wang, Yu-An; Zhao, Zong-Yan; Liu, Qing-Ju

2018-05-01

Over the past few years, quaternary Cu2ZnSnS4 (CZTS) has attracted a great deal of attention as the most promising photovoltaic absorber layer, due to its abundance and non-toxic properties. However, the significant surface structures and properties for photo-catalytic absorption layers have not yet been studied in detail for CZTS. Hence, the surface structure and electronic properties of low-index stoichiometric CZTS surfaces are calculated based on density functional theory. The relaxation is much large for the (001), (100), (101) and (112) surfaces. Moreover, more surface states appear at the bottom of conduction band and the top of valence band. The conduction band is mainly composed of S-3p and Sn-5p orbits. The valence band top is mainly composed of S-3p and Cu-3d orbits. The band gap values of five surfaces do not vary greatly. The dangling bond density for the (112) surfaces is minimal, resulting in minimum surface energy. Finally, the equilibrium morphology of CZTS is constructed by the Wulff rule. It is found that the {101} surface is the dominant surface (72.6%). These results will help us to better understand the surface properties of absorption layer that is related to CZTS surface and provide theoretical support for future experimental studies.

Plasmon Excitations of Multi-layer Graphene on a Conducting Substrate

PubMed Central

Gumbs, Godfrey; Iurov, Andrii; Wu, Jhao-Ying; Lin, M. F.; Fekete, Paula

2016-01-01

We predict the existence of low-frequency nonlocal plasmons at the vacuum-surface interface of a superlattice of N graphene layers interacting with conducting substrate. We derive a dispersion function that incorporates the polarization function of both the graphene monolayers and the semi-infinite electron liquid at whose surface the electrons scatter specularly. We find a surface plasmon-polariton that is not damped by particle-hole excitations or the bulk modes and which separates below the continuum mini-band of bulk plasmon modes. The surface plasmon frequency of the hybrid structure always lies below , the surface plasmon frequency of the conducting substrate. The intensity of this mode depends on the distance of the graphene layers from the conductor’s surface, the energy band gap between valence and conduction bands of graphene monolayer and, most importantly, on the number of two-dimensional layers. For a sufficiently large number of layers the hybrid structure has no surface plasmon. The existence of plasmons with different dispersion relations indicates that quasiparticles with different group velocity may coexist for various ranges of wavelengths determined by the number of layers in the superlattice. PMID:26883086

Theoretical studies on band structure and optical gain of GaInAsN/GaAs /GaAs cylindrical quantum dot

NASA Astrophysics Data System (ADS)

Mal, Indranil; Samajdar, Dip Prakash; John Peter, A.

2018-07-01

Electronic band structure, effective masses, band offsets and optical gain of Ga0.661In0.339N0.0554As0.9446/GaAs quantum dot systems are investigated using 10 band k·p Hamiltonian for various nitrogen and indium concentrations. The calculations include the effects of strain generated due to the lattice mismatch and the effective band gap of GaInAsN/GaAs heterostructures. The variation of conduction band, light hole and heavy hole band offsets with indium and nitrogen compositions in the alloy are obtained. The band structure of Ga0.661In0.339N0.0554As0.9446/GaAs quantum dot is found in the crystal directions Δ (100) and Λ (111) using 10 band k·p Hamiltonian. The optical gain of the cylindrical quantum dot structures as functions of surface carrier concentration and the dot radius is investigated. Our results show that the tensile strain of 1.34% generates a band gap of 0.59 eV and the compressive strain of 2.2% produces a band gap of 1.28 eV and the introduction of N atoms has no effect on the spin orbit split off band. The variation of optical gain with the dot size and the carrier concentration indicates that the optical gain increases with the decrease in the radius of the quantum dot. The results may be useful for the potential applications in optical devices.

Electronic structure and optical properties of Cs2HgI4: Experimental study and band-structure DFT calculations

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Shkumat, P. N.; Myronchuk, G. L.; Khvyshchun, M.; Fedorchuk, A. O.; Parasyuk, O. V.; Khyzhun, O. Y.

2015-04-01

High-quality single crystal of cesium mercury tetraiodide, Cs2HgI4, has been synthesized by the vertical Bridgman-Stockbarger method and its crystal structure has been refined. In addition, electronic structure and optical properties of Cs2HgI4 have been studied. For the crystal under study, X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion irradiated surfaces have been measured. The present X-ray photoelectron spectroscopy (XPS) results indicate that the Cs2HgI4 single crystal surface is very sensitive with respect to Ar+ ion-irradiation. In particular, Ar+ bombardment of the single crystal surface alters the elemental stoichiometry of the Cs2HgI4 surface. To elucidate peculiarities of the energy distribution of the electronic states within the valence-band and conduction-band regions of the Cs2HgI4 compound, we have performed first-principles band-structure calculations based on density functional theory (DFT) as incorporated in the WIEN2k package. Total and partial densities of states for Cs2HgI4 have been calculated. The DFT calculations reveal that the I p states make the major contributions in the upper portion of the valence band, while the Hg d, Cs p and I s states are the dominant contributors in its lower portion. Temperature dependence of the light absorption coefficient and specific electrical conductivity has been explored for Cs2HgI4 in the temperature range of 77-300 K. Main optical characteristics of the Cs2HgI4 compound have been elucidated by the first-principles calculations.

Tuning the optical bandgap in multi-cation compound transparent conducting-oxides: The examples of In2ZnO4 and In4Sn3O12

NASA Astrophysics Data System (ADS)

Sabino, Fernando P.; Oliveira, Luiz N.; Wei, Su-Huai; Da Silva, Juarez L. F.

2018-02-01

Transparent conducting oxides such as the bixbyite In2O3 and rutile SnO2 systems have large disparities between the optical and fundamental bandgaps, ΔEgO F , because selection rules forbid dipolar transitions from the top of the valence band to the conduction-band minimum; however, the optical gaps of multi-cation compounds with the same chemical species often coincide with their fundamental gaps. To explain this conundrum, we have employed density-functional theory to compute the optical properties of multi-cation compounds, In2ZnO4 and In4Sn3O12, in several crystal structures. We show that a recently proposed mechanism to explain the disparity between the optical and fundamental gaps of M2O3 (M = Al, Ga, and In) applies also to other binary systems and to multi-compounds. Namely, a gap disparity will arise if the following three conditions are satisfied: (i) the crystal structure has inversion symmetry; (ii) the conduction-band minimum is formed by the cation and O s-orbitals; and (iii) there is strong p-d coupling and weak p-p in the vicinity of the valence-band maximum. The third property depends critically on the cationic chemical species. In the structures with inversion symmetry, Zn (Sn) strengthens (weakens) the p-d coupling in In2ZnO4 (In4Sn3O12), enhancing (reducing) the gap disparity. Furthermore, we have also identified a In4Sn3O12 structure that is 31.80 meV per formula unit more stable than a recently proposed alternative model.

First-principle study of effect of variation of `x' on the band alignment in CZTS1-xSex

NASA Astrophysics Data System (ADS)

Ghemud, Vipul; Kshirsagar, Anjali

2018-04-01

The present work concentrates on the electronic structure study of CZTS1-xSex alloy with x ranging from 0 to 1. For the alloy study, we have carried out first-principles calculations employing generalized gradient approximation for structural optimization and further hybrid functional approach to compare the optical band gap with that obtained from the experiments. A systematic increase in the lattice parameters with lowering of band gap from 1.52eV to 1.04eV is seen with increasing Se concentration from 0 to 100%, however the lowering of valence band edge and conduction band edge is not linear with the concentration variation. Our results indicate that the lowering of band gap is a result increased Cu:d and Se:p hybridization with increasing `x'.

Tuning oxidation level, electrical conductance and band gap structure on graphene sheets by cyclic atomic layer reduction technique

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gu, Si-Yong; Hsieh, Chien-Te; Lin, Tzu-Wei

The present work develops an atomic layer reduction (ALR) method to accurately tune oxidation level, electrical conductance, band-gap structure, and photoluminescence (PL) response of graphene oxide (GO) sheets. The ALR route is carried out at 200 °C within ALR cycle number of 10–100. The ALR treatment is capable of striping surface functionalities (e.g., hydroxyl, carbonyl, and carboxylic groups), producing thermally-reduced GO sheets. The ALR cycle number serves as a controlling factor in adjusting the crystalline, surface chemistry, electrical, optical properties of GO sheets. With increasing the ALR cycle number, ALR-GO sheets display a high crystallinity, a low oxidation level, anmore » improved electrical conductivity, a narrow band gap, and a tunable PL response. Finally, on the basis of the results, the ALR technique offers a great potential for accurately tune electrical and optical properties of carbon materials through the cyclic removal of oxygen functionalities, without any complicated thermal and chemical desorption processes.« less

Effect of electronic structure of the diamond surface on the strength of the diamond-metal interface

NASA Technical Reports Server (NTRS)

Pepper, S. V.

1981-01-01

A diamond surface undergoes a transformation in its electronic structure by a vacuum anneal at approximately 900 C. The polished surface has no electronic states in the band gap, whereas the annealed surface has both occupied and unoccupied states in the and gap and exhibits some electrical conductivity. The effect of this transformation on the strength of the diamond metal interface was investigated by measuring the static friction force of an atomically clean meta sphere on a diamond flat in ultrahigh vacuum. It was found that low friction (weak bonding) is associated with the diamond surface devoid of gap states whereas high friction (strong bonding) is associated with the diamond surface with gap states. Exposure of the annealed surface to excited hydrogen also leads to weak bonding. The interfacial bond is discussed in terms of interaction of the metal conduction band electrons with the band gap states on the diamond surface. Effects of surface electrical conductivity on the interfacial bond are also be considered.

Tuning oxidation level, electrical conductance and band gap structure on graphene sheets by cyclic atomic layer reduction technique

DOE PAGES

Gu, Si-Yong; Hsieh, Chien-Te; Lin, Tzu-Wei; ...

2018-05-12

The present work develops an atomic layer reduction (ALR) method to accurately tune oxidation level, electrical conductance, band-gap structure, and photoluminescence (PL) response of graphene oxide (GO) sheets. The ALR route is carried out at 200 °C within ALR cycle number of 10–100. The ALR treatment is capable of striping surface functionalities (e.g., hydroxyl, carbonyl, and carboxylic groups), producing thermally-reduced GO sheets. The ALR cycle number serves as a controlling factor in adjusting the crystalline, surface chemistry, electrical, optical properties of GO sheets. With increasing the ALR cycle number, ALR-GO sheets display a high crystallinity, a low oxidation level, anmore » improved electrical conductivity, a narrow band gap, and a tunable PL response. Finally, on the basis of the results, the ALR technique offers a great potential for accurately tune electrical and optical properties of carbon materials through the cyclic removal of oxygen functionalities, without any complicated thermal and chemical desorption processes.« less

Novel natural super-lattice materials with low thermal conductivity for thermoelectric applications: A first principles study

NASA Astrophysics Data System (ADS)

Sreeparvathy, P. C.; Kanchana, V.

2017-12-01

A systematic study which reveals the low thermal conductivity and high thermopower on a series of natural superlattice structures in the form BaXFCh (X: Cu, Ag, Ch: S, Se, Te), LaXSO (X: Cu, Ag) and SrCuTeF are presented. Low thermal conductivity is predicted by combining elastic constants and few well established models. The electronic properties reveal the highly two dimensional nature of band structure in the valence band, and this is confirmed through effective mass calculations. The huge difference in effective mass along different crystallographic directions in valence band introduces anisotropy in the transport properties for hole doping, and 'a' axis is found to be more favourable. In addition to these, the parameter A (S2σ/τT/κe /τ), which can decouple the relaxation time is also calculated, and it reveals the possibility of good thermoelectric properties in these compounds. Our results are comparable with prototype thermoelectric materials, and show better values than traditional TE materials.

Compositional bowing of band energies and their deformation potentials in strained InGaAs ternary alloys: A first-principles study

NASA Astrophysics Data System (ADS)

Khomyakov, Petr A.; Luisier, Mathieu; Schenk, Andreas

2015-08-01

Using first-principles calculations, we show that the conduction and valence band energies and their deformation potentials exhibit a non-negligible compositional bowing in strained ternary semiconductor alloys such as InGaAs. The electronic structure of these compounds has been calculated within the framework of local density approximation and hybrid functional approach for large cubic supercells and special quasi-random structures, which represent two kinds of model structures for random alloys. We find that the predicted bowing effect for the band energy deformation potentials is rather insensitive to the choice of the functional and alloy structural model. The direction of bowing is determined by In cations that give a stronger contribution to the formation of the InxGa1-xAs valence band states with x ≳ 0.5, compared to Ga cations.

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

DTIC Science & Technology

2015-03-20

In the bandstructure of graphene which is dominated by Dirac description, valence and conduction bands cross the Fermi level at a single point (K...of energy bands and appearance of Dirac cones near the ‘K’ point and Fermi level the electrons behave like massless Dirac fermions. For applications...results. Introduction Graphene, the super carbon , is now accepted as wonder material with new physics and it has caused major

Observation of spontaneous spin-splitting in the band structure of an n-type zinc-blende ferromagnetic semiconductor

PubMed Central

Anh, Le Duc; Hai, Pham Nam; Tanaka, Masaaki

2016-01-01

Large spin-splitting in the conduction band and valence band of ferromagnetic semiconductors, predicted by the influential mean-field Zener model and assumed in many spintronic device proposals, has never been observed in the mainstream p-type Mn-doped ferromagnetic semiconductors. Here, using tunnelling spectroscopy in Esaki-diode structures, we report the observation of such a large spontaneous spin-splitting energy (31.7–50 meV) in the conduction band bottom of n-type ferromagnetic semiconductor (In,Fe)As, which is surprising considering the very weak s-d exchange interaction reported in several zinc-blende type semiconductors. The mean-field Zener model also fails to explain consistently the ferromagnetism and the spin-splitting energy of (In,Fe)As, because we found that the Curie temperature values calculated using the observed spin-splitting energies are much lower than the experimental ones by a factor of 400. These results urge the need for a more sophisticated theory of ferromagnetic semiconductors. PMID:27991502

Kohn-Sham Band Structure Benchmark Including Spin-Orbit Coupling for 2D and 3D Solids

NASA Astrophysics Data System (ADS)

Huhn, William; Blum, Volker

2015-03-01

Accurate electronic band structures serve as a primary indicator of the suitability of a material for a given application, e.g., as electronic or catalytic materials. Computed band structures, however, are subject to a host of approximations, some of which are more obvious (e.g., the treatment of the exchange-correlation of self-energy) and others less obvious (e.g., the treatment of core, semicore, or valence electrons, handling of relativistic effects, or the accuracy of the underlying basis set used). We here provide a set of accurate Kohn-Sham band structure benchmarks, using the numeric atom-centered all-electron electronic structure code FHI-aims combined with the ``traditional'' PBE functional and the hybrid HSE functional, to calculate core, valence, and low-lying conduction bands of a set of 2D and 3D materials. Benchmarks are provided with and without effects of spin-orbit coupling, using quasi-degenerate perturbation theory to predict spin-orbit splittings. This work is funded by Fritz-Haber-Institut der Max-Planck-Gesellschaft.

Observation of Wakefield Suppression in a Photonic-Band-Gap Accelerator Structure

DOE PAGES

Simakov, Evgenya I.; Arsenyev, Sergey A.; Buechler, Cynthia E.; ...

2016-02-10

We report experimental observation of higher order mode (HOM) wakefield suppression in a room-temperature traveling-wave photonic band gap (PBG) accelerating structure at 11.700 GHz. It has been long recognized that PBG structures have potential for reducing long-range wakefields in accelerators. The first ever demonstration of acceleration in a room-temperature PBG structure was conducted in 2005. Since then, the importance of PBG accelerator research has been recognized by many institutions. However, the full experimental characterization of the wakefield spectrum and demonstration of wakefield suppression when the accelerating structure is excited by an electron beam has not been performed to date. Wemore » conducted an experiment at the Argonne Wakefield Accelerator (AWA) test facility and observed wakefields excited by a single high charge electron bunch when it passes through a PBG accelerator structure. Lastly, excellent HOM suppression properties of the PBG accelerator were demonstrated in the beam test.« less

Electronic and thermoelectric analysis of phases in the In 2O 3(ZnO) k system

DOE PAGES

Hopper, E. Mitchell; Zhu, Qimin; Song, Jung-Hwan; ...

2011-01-01

The high-temperature electrical conductivity and thermopower of several compounds in the In 2O 3(ZnO) k system (k = 3, 5, 7, and 9) were measured, and the band structures of the k = 1, 2, and 3 structures were predicted based on first-principles calculations. These phases exhibit highly dispersed conduction bands consistent with transparent conducting oxide behavior. Jonker plots (Seebeck coefficient vs. natural logarithm of conductivity) were used to obtain the product of the density of states and mobility for these phases, which were related to the maximum achievable power factor (thermopower squared times conductivity) for each phase by Ioffemore » analysis (maximum power factor vs. Jonker plot intercept). With the exception of the k = 9 phase, all other phases were found to have maximum predicted power factors comparable to other thermoelectric oxides if suitably doped.« less

Single Crystal Growth, Resistivity, and Electronic Structure of the Weyl Semimetals NbP and TaP

DOE PAGES

Sapkota, Deepak; Mukherjee, Rupam; Mandrus, David

2016-12-06

We have successfully synthesized niobium monophosphide and tantalum monophosphide crystals by a chemical vapor transport technique. We report resistivity vs. temperature of both materials in the temperature range from 2 K to 300 K. We have also performed electronic structure calculations and present the band structure and density of states of these two compounds. The calculations show that both compounds are semimetals, as their conduction and valence bands overlap near the Fermi energy.

disorder effect on quantum transport properties of ultra thin Fe film

NASA Astrophysics Data System (ADS)

Zhang, Xiaotian; Nakamura, Kohji; Shindou, Ryuichi

2015-03-01

Ferromagnetic ultrathin films are experimentally known to often exhibit perpendicular magnetic anisotropy, when being placed on certain substrates. Based on reported ab-initio band calculations of free-standing Fe-monolayer and that on MgO substrate, we will introduce an effective tight-binding model, which capture a part of an electronic structure near Fermi level for both cases. We will show that the model supports electronic bands with non-zero Chern number and chiral edge modes which cross a direct band gap on the order of 50meV. Unluckily, however, the direct band gap is also masked by another dispersive bands which have non-zero Berry's curvature in the k-space. To demonstrate how disorder kills conducting characters of the latter bulk bands while leave intact those of the chiral edge modes, we will clarify behaviors of localization length and conductance in the effective model with on-site disorders.

A Green's function formulation of the k→ ·p→ theory in the presence of spin-orbit interaction and magnetic field: Application to the electronic structure and related properties of w-GaN

NASA Astrophysics Data System (ADS)

Shadangi, Subrat K.; Mishra, Sambit R.; Tripathi, Gouri S.

2018-01-01

We use a Green's function perturbation formalism in the presence of an applied magnetic field and spin-orbit effects in the effective mass representation (EMR). The lack of lattice translational symmetry of the vector potential in the presence of the magnetic field is considered by redefining the Green's function in terms of the Peierls' phase factor. The equation of motion of the Green's function as a function of a magnetic wave vector was solved using perturbation theory, leading to expressions for the effective mass and the g-factor. We study the electronic structure of wurtzite GaN theoretically using the resulting k→ ·π→ method, where k→ is the electronic wave vector and π→ is the relativistic momentum operator by considering the conduction band edge and three valence bands. The k→ ·π→ Hamiltonians for the conduction band edge and the valence bands are diagonalized, considering the conduction band and one valence band at a time. We obtain electron and hole dispersions. Effects of other bands are considered by using perturbation theory. Resulting dispersions agree with the results of other calculations. In order to study the effective mass and the g-factor, we use the eigenvalues and eigenfunctions obtained after the diagonalization. Our results for the effective masses and the g-factors agree fairly well with available theoretical and experimental results, Temperature dependence of both the electronic effective mass and g-factor is studied and trends obtained agree with the existing experimental data.

Study of surface and bulk electronic structure of II-VI semiconductor nanocrystals using Cu as a nanosensor.

PubMed

Grandhi, G Krishnamurthy; Tomar, Renu; Viswanatha, Ranjani

2012-11-27

Efficiency of the quantum dots based solar cells relies on charge transfer at the interface and hence on the relative alignment of the energy levels between materials. Despite a high demand to obtain size specific band offsets, very few studies exist where meticulous methods like photoelectron spectroscopy are used. However, semiconductor charging during measurements could result in indirect and possibly inaccurate measurements due to shift in valence and conduction band position. Here, in this report, we devise a novel method to study the band offsets by associating an atomic like state with the conduction band and hence obtaining an internal standard. This is achieved by doping copper in semiconductor nanocrystals, leading to the development of a characteristic intragap Cu-related emission feature assigned to the transition from the conduction band to the atomic-like Cu d state. Using this transition we determine the relative band alignment of II-VI semiconductor nanocrystals as a function of size in the below 10 nm size regime. The results are in excellent agreement with the available photoelectron spectroscopy data as well as the theoretical data. We further use this technique to study the excitonic band edge variation as a function of temperature in CdSe nanocrystals. Additionally, surface electronic structure of CdSe nanocrystals have been studied using quantitative measurements of absolute quantum yield and PL decay studies of the Cu related emission and the excitonic emission. The role of TOP and oleic acid as surface passivating ligand molecules has been studied for the first time.

Conduction- and Valence-Band Energies in Bulk InAs(1-x)Sb(x) and Type II InAs(1-x) Sb(x)/InAs Strained-Layer Superlattices

DTIC Science & Technology

2013-03-08

tions in the studied SLS structures . The fit of the dependence of the valence- band energy of unstrained InAs1!xSbx on the composition x with a... band . STRUCTURES Bulk InAsSb epilayers on metamorphic buffers and InAsSb/InAs strained-layer superlattices (SLS) were grown on GaSb substrates by solid...meV in InAs and Ev = 0 meV in InSb. For InAsSb with 22.5% Sb grown on GaSb , an unstrained valence- band energy of Ev = !457 meV was obtained. For the

Designing Semiconductor Heterostructures Using Digitally Accessible Electronic-Structure Data

NASA Astrophysics Data System (ADS)

Shapera, Ethan; Schleife, Andre

Semiconductor sandwich structures, so-called heterojunctions, are at the heart of modern applications with tremendous societal impact: Light-emitting diodes shape the future of lighting and solar cells are promising for renewable energy. However, their computer-based design is hampered by the high cost of electronic structure techniques used to select materials based on alignment of valence and conduction bands and to evaluate excited state properties. We describe, validate, and demonstrate an open source Python framework which rapidly screens existing online databases and user-provided data to find combinations of suitable, previously fabricated materials for optoelectronic applications. The branch point energy aligns valence and conduction bands of different materials, requiring only the bulk density functional theory band structure. We train machine learning algorithms to predict the dielectric constant, electron mobility, and hole mobility with material descriptors available in online databases. Using CdSe and InP as emitting layers for LEDs and CH3NH3PbI3 and nanoparticle PbS as absorbers for solar cells, we demonstrate our broadly applicable, automated method.

Ionic liquid gating reveals trap-filled limit mobility in low temperature amorphous zinc oxide

NASA Astrophysics Data System (ADS)

Bubel, S.; Meyer, S.; Kunze, F.; Chabinyc, M. L.

2013-10-01

In low-temperature solution processed amorphous zinc oxide (a-ZnO) thin films, we show the thin film transistor (TFT) characteristics for the trap-filled limit (TFL), when the quasi Fermi energy exceeds the conduction band edge and all tail-states are filled. In order to apply gate fields that are high enough to reach the TFL, we use an ionic liquid tape gate. Performing capacitance voltage measurements to determine the accumulated charge during TFT operation, we find the TFL at biases higher than predicted by the electronic structure of crystalline ZnO. We conclude that the density of states in the conduction band of a-ZnO is higher than in its crystalline state. Furthermore, we find no indication of percolative transport in the conduction band but trap assisted transport in the tail-states of the band.

Wide-field Infrared Polarimetry of the ρ Ophiuchi Cloud Core

NASA Astrophysics Data System (ADS)

Kwon, Jungmi; Tamura, Motohide; Hough, James H.; Nakajima, Yasushi; Nishiyama, Shogo; Kusakabe, Nobuhiko; Nagata, Tetsuya; Kandori, Ryo

2015-09-01

We conducted wide and deep simultaneous JHKs-band imaging polarimetry of the ρ Ophiuchi cloud complex. Aperture polarimetry in the JHKs band was conducted for 2136 sources in all three bands, of which 322 sources have significant polarizations in all the JHKs bands and have been used for a discussion of the core magnetic fields. There is a positive correlation between degrees of polarization and H - Ks color up to H - Ks ≈ 3.5. The magnetic field structures in the core region are revealed up to at least AV ≈ 47 mag and are unambiguously defined in each sub-region (core) of Oph-A, Oph-B, Oph-C, Oph-E, Oph-F, and Oph-AC. Their directions, degrees of polarization, and polarization efficiencies differ but their changes are gradual; thus, the magnetic fields appear to be connected from core to core, rather than as a simple overlap of the different cloud core components. Comparing our results with the large-scale field structures obtained from previous optical polarimetric studies, we suggest that the magnetic field structures in the core were distorted by the cluster formation in this region, which may have been induced by shock compression due to wind/radiation from the Scorpius-Centaurus association.

Structural and optical properties of co-precipitated copper doped zinc oxide

NASA Astrophysics Data System (ADS)

Pandey, Devendra K.; Modi, Anchit; Pandey, Padmini; Gaur, N. K.

2018-05-01

We have synthesized pure and copper doped zinc oxide Zn1-xO:Cux (x = 0, 0.03) powder by wet chemical co-precipitation method followed by sintering of the co-precipitated amorphous phase powder at 450°C for 4 hours. The experiment is performed to recognize the effect of nominal doping of transition metal over the structural, morphological and optical properties. The structural parameters are observed by using Rietveld refinement of X-ray diffraction data which clearly represents that Cu ion is perfectly incorporated at the Zn site with minimal distortions within the lattice. The crystallite size is estimated by Debye-Scherrer and Hall-Williamson formulation. The particle morphology and size is determined with scanning electron microscopic (SEM) technique. The band gap and optical measurements are carried out with UV-visible absorption and photoluminescence (PL) spectroscopic technique, respectively. Enhanced PL spectral response is observed for ZnO:Cu along with non-radiative transitions from conduction band to valence band. The energy levels near the conduction band that are commonly involved in the optoelectronic transitions in the UV-region are traced by using absorption and luminescence spectral graphs.

Electronic Structure of I-M8Ga16Sn30 (M = Ba, Sr, Yb) by First-Principles Calculation

NASA Astrophysics Data System (ADS)

Wang, Jin-song; Liu, Hong-xia; Deng, Shuping; Li, De-cong; Shen, Lan-xian; Cheng, Feng; Deng, Shu-kang

2017-05-01

Sn-based clathrates possess excellent thermoelectric properties ascribed to their higher Seebeck coefficient and lower thermal conductivity. Guest atoms significantly modulate the thermoelectric properties of Sn-based calculates because of their diverse atomic radius and interactions with framework atoms. Thus, we explored the electronic structure of I-M8Ga16Sn30 (M = Ba, Sr, Yb) by first-principles calculation. Results revealed significant differences between Yb8Ga16Sn30 and M8Ga16Sn30 (M = Ba, Sr,). In particular, the Yb-filled compound substitution possesses lowest formation energy and the off-center distance of the Yb atom is the largest compared with the other structures. I-M8Ga16Sn30 (M = Ba, Sr, Yb) is an indirect band gap semiconductor, and the enhanced hybridization effect between the guest and framework atoms' orbits exists because the Yb f orbit results in a decrease in band gap. Ba- and Sr-filled clathrates have similar valence bands but slightly different conduction bands; however, Yb8Ga16Sn30 possess the spiculate density of states near the Fermi level that reveals excellent thermoelectric properties.

The structural, electronic and optical properties of Nd doped ZnO using first-principles calculations

NASA Astrophysics Data System (ADS)

Wen, Jun-Qing; Zhang, Jian-Min; Chen, Guo-Xiang; Wu, Hua; Yang, Xu

2018-04-01

The density functional theory calculations using general gradient approximation (GGA) applying Perdew-Burke-Ernzerhof (PBE) as correlation functional have been systematically performed to research the formation energy, the electronic structures, band structures, total and partial DOS, and optical properties of Nd doping ZnO with the content from 6.25% to 12.5%. The formation energies are negative for both models, which show that two structures are energetically stable. Nd doping ZnO crystal is found to be a direct band gap semiconductor and Fermi level shifts upward into conduction band, which show the properties of n-type semiconductor. Band structures are more compact after Nd doping ZnO, implying that Nd doping induces the strong interaction between different atoms. Nd doping ZnO crystal presents occupied states at near Fermi level, which mainly comes from the Nd 4f orbital. The calculated optical properties imply that Nd doping causes a red-shift of absorption peaks, and enhances the absorption of the visible light.

Evolution of Defect Structures and Deep Subgap States during Annealing of Amorphous In-Ga-Zn Oxide for Thin-Film Transistors

NASA Astrophysics Data System (ADS)

Jia, Junjun; Suko, Ayaka; Shigesato, Yuzo; Okajima, Toshihiro; Inoue, Keiko; Hosomi, Hiroyuki

2018-01-01

We investigate the evolution behavior of defect structures and the subgap states in In-Ga-Zn oxide (IGZO) films with increasing postannealing temperature by means of extended x-ray absorption fine-structure (EXAFS) measurements, positron annihilation lifetime spectroscopy (PALS), and cathodoluminescence (CL) spectroscopy, aiming to understand the relationship between defect structures and subgap states. EXAFS measurements reveal the varied oxygen coordination numbers around cations during postannealing and confirm two types of point defects, namely, excess oxygen around Ga atoms and oxygen deficiency around In and/or Zn atoms. PALS suggests the existence of cation-vacancy (VM )-related clusters with neutral or negative charge in both amorphous and polycrystalline IGZO films. CL spectra show a main emission band at approximately 1.85 eV for IGZO films, and a distinct shoulder located at about 2.15 eV for IGZO films postannealed above 600 °C . These two emission bands are assigned to a recombination between the electrons in the conduction band and/or in the shallow donor levels near the conduction band and the acceptors trapped above the valence-band maximum. The shallow donors are attributed to the oxygen deficiency, and the acceptors are thought to possibly arise from the excess oxygen or the VM-related clusters. These results open up an alternative route for understanding the device instability of amorphous IGZO-based thin-film transistors, especially the presence of the neutral or negatively charged VM-related clusters in amorphous IGZO films.

Graphene oxide quantum dot-sensitized porous titanium dioxide microsphere: Visible-light-driven photocatalyst based on energy band engineering.

PubMed

Zhang, Yu; Qi, Fuyuan; Li, Ying; Zhou, Xin; Sun, Hongfeng; Zhang, Wei; Liu, Daliang; Song, Xi-Ming

2017-07-15

We report a novel graphene oxide quantum dot (GOQD)-sensitized porous TiO 2 microsphere for efficient photoelectric conversion. Electro-chemical analysis along with the Mott-Schottky equation reveals conductivity type and energy band structure of the two semiconductors. Based on their energy band structures, visible light-induced electrons can transfer from the p-type GOQD to the n-type TiO 2 . Enhanced photocurrent and photocatalytic activity in visible light further confirm the enhanced separation of electrons and holes in the nanocomposite. Copyright © 2017 Elsevier Inc. All rights reserved.

Electronic structure in 1T-ZrS2 monolayer by strain

NASA Astrophysics Data System (ADS)

Xin, Qianqian; Zhao, Xu; Ma, Xu; Wu, Ninghua; Liu, Xiaomeng; Wei, Shuyi

2017-09-01

We report electronic structure of 1T-ZrS2 monolayer with biaxial strain from -10% to 15%, basing the first principles calculations. Our calculation results indicate that the band structure of ZrS2 monolayer was changed clearly. The location of conduction band minimum (CBM) and valence band maximum (VBM) changed with the variation of isotropic strain. At compressive strain, the location of CBM and VBM retains at M and Γ point, respectively. The band gap of ZrS2 monolayer decreases from 1.111 eV to 0 eV when compressive strain increases from 0% to -8%, which means that the ZrS2 monolayer turns to metal at -8% compressive strain. Under the tensile strain, the ZrS2 monolayer also retains be an indirect band gap semiconductor. The location of CBM moves from M to Γ point and the location of VBM moves along Γ-A-K-Γ direction. The band gap of ZrS2 monolayer firstly increases and then decreases and the biggest band gap is 1.577 eV at tensile strain 6%. We can see the compression strain is more effective than tensile strain in modulating band gap of 1T-ZrS2 monolayer.

Nanodopant-Induced Band Modulation in AgPbmSbTe2+m-Type Nanocomposites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Yi; Ke, Xuezhi; Chen, Changfeng

2011-01-01

We elucidate the fundamental physics of nanoscale dopants in narrow band-gap thermoelectric nanocomposites XPbmYTe2+m (X=Ag,Na; Y=Sb,Bi) using first-principles calculations. Our re- sults unveil distinct band-structure modulations, most notably a sizable band-gap widening driven by nanodopant-induced lattice strain and a band split-off at the conduction band minimum caused by the spin-orbit interaction of the dopant Sb or Bi atoms. Boltzmann transport calculations demon- strate that these band modulations have significant but competing effects on high-temperature elec- tron transport behavior. These results offer insights for understanding recent experimental findings and suggest principles for optimizing thermoelectric properties of narrow band-gap semiconductors.

Ultrafast laser-induced modifications of energy bands of non-metal crystals

NASA Astrophysics Data System (ADS)

Gruzdev, Vitaly

2009-10-01

Ultrafast laser-induced variations of electron energy bands of transparent solids significantly influence ionization and conduction-band electron absorption driving the initial stage of laser-induced damage (LID). The mechanisms of the variations are attributed to changing electron functions from bonding to anti-bonding configuration via laser-induced ionization; laser-driven electron oscillations in quasi-momentum space; and direct distortion of the inter-atomic potential by electric field of laser radiation. The ionization results in the band-structure modification via accumulation of broken chemical bonds between atoms and provides significant contribution to the overall modification only when enough excited electrons are accumulated in the conduction band. The oscillations are associated with modification of electron energy by pondermotive potential of the oscillations. The direct action of radiation's electric field leads to specific high-frequency Franz-Keldysh effect (FKE) spreading the allowed electron states into the bands of forbidden energy. Those processes determine the effective band gap that is a laser-driven energy gap between the modified electron energy bands. Among those mechanisms, the latter two provide reversible band-structure modification that takes place from the beginning of the ionization and are, therefore, of special interest due to their strong influence on the initial stage of the ionization. The pondermotive potential results either in monotonous increase or oscillatory variations of the effective band gap that has been taken into account in some ionization models. The classical FKE provides decrease of the band gap. We analyzing the competition between those two opposite trends of the effective-band-gap variations and discuss applications of those effects for considerations of the laser-induced damage and its threshold in transparent solids.

Recent Developments in Quantum-Well Infrared Photodetectors

NASA Technical Reports Server (NTRS)

Gunapala, S. D.; Bandara, K. M. S. V.

1995-01-01

Intrinsic infrared (IR) detectors in the long wavelength range (8-20 Am) are based on an optically excited interband transition, which promotes an electron across the band gap (E(sub g)) from the valence band to the conduction band as shown. These photoelectrons can be collected efficiently, thereby producing a photocurrent in the external circuit. Since the incoming photon has to promote an electron from the valence band to the conduction band, the energy of the photon (h(sub upsilon)) must be higher than the E(sub g) of the photosensitive material. Therefore, the spectral response of the detectors can be controlled by controlling the E(sub g) of the photosensitive material. Examples for such materials are Hg(1-x), Cd(x), Te, and Pb(1-x), Sn(x), Te, in which the energy gap can be controlled by varying x. This means detection of very-long-wavelength IR radiation up to 20 microns requires small band gaps down to 62 meV. It is well known that these low band gap materials, characterized by weak bonding and low melting points, are more difficult to grow and process than large-band gap semiconductors such as GaAs. These difficulties motivate the exploration of utilizing the intersub-band transitions in multiquantum well (MQW) structures made of more refractory large-band gap semiconductors. The idea of using MQW structures to detect IR radiation can be explained by using the basic principles of quantum mechanics. The quantum well is equivalent to the well-known particle in a box problem in quantum mechanics, which can be solved by the time independent Schroudiner equation.

Contactless electroreflectance study of the Fermi level pinning on GaSb surface in n-type and p-type GaSb Van Hoof structures

NASA Astrophysics Data System (ADS)

Kudrawiec, R.; Nair, H. P.; Latkowska, M.; Misiewicz, J.; Bank, S. R.; Walukiewicz, W.

2012-12-01

Contactless electroreflectance (CER) has been applied to study the Fermi-level position on GaSb surface in n-type and p-type GaSb Van Hoof structures. CER resonances, followed by strong Franz-Keldysh oscillation of various periods, were clearly observed for two series of structures. This period was much wider (i.e., the built-in electric field was much larger) for n-type structures, indicating that the GaSb surface Fermi level pinning position is closer to the valence-band than the conduction-band. From analysis of the built-in electric fields in undoped GaSb layers, it was concluded that on GaSb surface the Fermi-level is located ˜0.2 eV above the valence band.

Electron band structure of the high pressure cubic phase of AlH3

NASA Astrophysics Data System (ADS)

Shi, Hongliang; Zarifi, Niliffar; Yim, Wai-Leung; Tse, J. S.

2012-07-01

The electronic band structure of the cubic Pm3n phase of AlH3 stable above 100 GPa is examined with semi-local, Tran-Blaha modified Becke-Johnson local density approximation (TB-mBJLDA), screened hybrid density functionals and GW methods. The shift of the conduction band to higher energy with increasing pressure is predicted by all methods. However, there are significant differences in detail band structure. In the pressure range from 90 to160 GPa, semi-local, hybrid functional and TB-mBJLDA calculations predicted that AlH3 is a poor metal. In comparison, GW calculations show a gap opening at 160 GPa and AlH3 becomes a small gap semi-conductor. From the trends of the calculated band shifts, it can be concluded that the favourable conditions leading to the nesting of Fermi surfaces predicted by semi-local calculation have disappeared if the exchange term is included. The results highlight the importance of the correction to the exchange energy on the band structure of hydrogen dominant dense metal hydrides at high pressure hydrides and may help to rationalize the absence of superconductivity in AlH3 from experimental measurements.

First principles investigation of SiC/AlGaN(0001) band offset

NASA Astrophysics Data System (ADS)

Kojima, E.; Endo, K.; Shirakawa, H.; Chokawa, K.; Araidai, M.; Ebihara, Y.; Kanemura, T.; Onda, S.; Shiraishi, K.

2017-06-01

We are attempting to develop a new type of vertical MOSFET with SiC/AlGaN heterojunction. Toward the realization of the vertical MOSFET, the control of conduction-band offset is one of the crucial subjects. We investigated the conduction-band offset of 4H-SiC/AlxGa1-xN interface by the first-principles electronic structure calculations. We found that the offset of the interface with 40% Al content becomes almost zero. Therefore, 4H-SiC/Al0.4Ga0.6N interface is one of the most promising candidates for the vertical MOSFET in future power conversion devices.

Quadratic band touching points and flat bands in two-dimensional topological Floquet systems

NASA Astrophysics Data System (ADS)

Du, Liang; Zhou, Xiaoting; Fiete, Gregory A.

2017-01-01

In this paper we theoretically study, using Floquet-Bloch theory, the influence of circularly and linearly polarized light on two-dimensional band structures with Dirac and quadratic band touching points, and flat bands, taking the nearest neighbor hopping model on the kagome lattice as an example. We find circularly polarized light can invert the ordering of this three-band model, while leaving the flat band dispersionless. We find a small gap is also opened at the quadratic band touching point by two-photon and higher order processes. By contrast, linearly polarized light splits the quadratic band touching point (into two Dirac points) by an amount that depends only on the amplitude and polarization direction of the light, independent of the frequency, and generally renders dispersion to the flat band. The splitting is perpendicular to the direction of the polarization of the light. We derive an effective low-energy theory that captures these key results. Finally, we compute the frequency dependence of the optical conductivity for this three-band model and analyze the various interband contributions of the Floquet modes. Our results suggest strategies for optically controlling band structure and interaction strength in real systems.

Controlling the optical properties of polyaniline doped by boric acid particles by changing their doping agent and initiator concentration

NASA Astrophysics Data System (ADS)

Cabuk, Mehmet; Gündüz, Bayram

2017-12-01

In this study, polyaniline doped by boric acid (PAni:BA) conducting polymers were chemically synthesized by oxidative polymerization method using (NH4)2S208 (APS) as initiator. Pani:BA conducting polymers were synthesized by using two different APS/aniline molar ratios as 1:1 and 2:1. Their results were compared with PAni doped by HCl (PAni) conducting polymer. Structural properties of the PAni, PAni:BA (1:1) and PAni:BA (2:1) conducting polymers were characterized by using FTIR, SEM, TGA, particle size and apparent density measurements. Effects of doping agents and initiator concentrations on optical properties were investigated in detail. The optoelectronic parameters such as absorption band edge, molar extinction coefficient, direct allowed band gap, refractive index, optical conductance and electrical conductance of the PAni, PAni:BA (1:1) and PAni:BA (2:1) were determined. The absorption band edge and direct allowed band gap of PAni were decreased with doping BA and increasing APS ratio. Also, the refractive index values of the materials were calculated from experimental results and compared with obtained results from Moss, Ravindra, Herve-Vandamme, Reddy and Kumar-Singh relations.

Electronic band structures and optical properties of type-II superlattice photodetectors with interfacial effect.

PubMed

Qiao, Peng-Fei; Mou, Shin; Chuang, Shun Lien

2012-01-30

The electronic band structures and optical properties of type-II superlattice (T2SL) photodetectors in the mid-infrared (IR) range are investigated. We formulate a rigorous band structure model using the 8-band k · p method to include the conduction and valence band mixing. After solving the 8 × 8 Hamiltonian and deriving explicitly the new momentum matrix elements in terms of envelope functions, optical transition rates are obtained through the Fermi's golden rule under various doping and injection conditions. Optical measurements on T2SL photodetectors are compared with our model and show good agreement. Our modeling results of quantum structures connect directly to the device-level design and simulation. The predicted doping effect is readily applicable to the optimization of photodetectors. We further include interfacial (IF) layers to study the significance of their effect. Optical properties of T2SLs are expected to have a large tunable range by controlling the thickness and material composition of the IF layers. Our model provides an efficient tool for the designs of novel photodetectors.

Crystal growth, electronic structure and optical properties of Sr2Mg(BO3)2

NASA Astrophysics Data System (ADS)

Lv, Xianshun; Wei, Lei; Wang, Xuping; Xu, Jianhua; Yu, Huajian; Hu, Yanyan; Zhang, Huadi; Zhang, Cong; Wang, Jiyang; Li, Qinggang

2018-02-01

Single crystals of Sr2Mg(BO3)2 (SMBO) were grown by Kyropoulos method. X-ray powder diffraction (XRD) analysis, transmission spectrum, thermal properties, band structure, density of states and charge distribution as well as Raman spectra of SMBO were described. The as-grown SMBO crystals show wide transparency range with UV cut-off below 180 nm. A direct band gap of 4.66 eV was obtained from the calculated electronic structure results. The calculated band structure and density of states results indicated the top valence band is determined by O 2p states whereas the low conduction band mainly consists of Sr 5s states. Twelve Raman peaks were observed in the experimental spectrum, fewer than the number predicted by the site group analysis. Raman peaks of SMBO were assigned combining first-principle calculation and site group analysis results. The strongest peak at 917 cm-1 in the experimental spectrum is assigned to symmetric stretching mode A1‧(ν1) of free BO3 units. SMBO is a potential Raman crystal which can be used in deep UV laser frequency conversion.

First-principles study of nitrogen-doped CuAlO2

NASA Astrophysics Data System (ADS)

Xu, Ying; Ao, Zhi Min; Yuan, Ding Wang

2012-08-01

The electronic structure and formation energies of N-doped CuAlO2 are studied using first-principles calculations. It is found that, when a N atom is doped into CuAlO2, the N atom prefers to substitute an O atom rather than to occupy an interstitial site of the Cu layer. The NO acts as a shallow accepter while the Ni acts as a deep accepter. The results of the electronic structure show that the N-doping doesn't alter the band gap of CuAlO2 for the both cases. In the substitutional case, the N impurity states occur at the top of valance band maximum (VBM), which provides holes and increases the p-type conductivity. However, in the interstitial case, the N impurity states occur in the middle of the band gap, which are more localized and this indicates that it is not good for p-type conductivity.

Raman spectra boron doped amorphous carbon thin film deposited by bias assisted-CVD

NASA Astrophysics Data System (ADS)

Ishak, A.; Fadzilah, A. N.; Dayana, K.; Saurdi, I.; Malek, M. F.; Nurbaya, Z.; Shafura, A. K.; Rusop, M.

2018-05-01

Boron doped amorphous carbon thin film carbon was deposited at 200°C-350°C by bias assisted-CVD using palm oil as a precursor material. The structural boron doped amorphous carbon films were discussed by Raman analysis through the evolution of D and G bands. The spectral evolution observed showed the increase of upward shift of D and G peaks as substrate deposition temperatures increased. These structural changes were further correlated with optical gap and the results obtained are discussed and compared. The estimated optical band gap is found to be 1.9 to 2.05 eV and conductivity is to be in the range of 10-5 Scm-1 to 10-4 Scm-1. The decrease of optical band gap is associated to conductivity increased which change the characteristic parameters of Raman spectra including the position of G peak, full width at half maximum of G peak, and ID/IG.

Single crystal growth, electronic structure and optical properties of Cs2HgBr4

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Shkumat, P. N.; Parasyuk, O. V.; Fedorchuk, A. O.; Khyzhun, O. Y.

2015-10-01

We report on successful synthesis of high-quality single crystal of cesium mercury tetrabromide, Cs2HgBr4, by using the vertical Bridgman-Stockbarger method as well as on studies of its electronic structure. For the Cs2HgBr4 crystal, we have recorded X-ray photoelectron spectra for both pristine and Ar+ ion-bombarded surfaces. Our data indicate that the Cs2HgBr4 single crystal surface is rather sensitive with respect to Ar+ ion-bombardment. In particular, such a treatment of the Cs2HgBr4 single crystal surface alters its elemental stoichiometry. To explore peculiarities of the energy distribution of total and partial densities of states within the valence band and the conduction band of Cs2HgBr4, we have made band-structure calculations based on density functional theory (DFT) employing the augmented plane wave+local orbitals (APW+lo) method as incorporated in the WIEN2k package. The APW+lo calculations allow for concluding that the Br 4p states make the major contributions in the upper portion of the valence band, while its lower portion is dominated by contributors of the Hg 5d and Cs 5p states. Further, the main contributors to the bottom of the conduction band of Cs2HgBr4 are the unoccupied Br p and Hg s states. In addition, main optical characteristics of Cs2HgBr4 such as dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity have been explored from the first-principles band-structure calculations.

Electronic and optical properties of the LiCdX (X = N, P, As and Sb) filled-tetrahedral compounds with the Tran–Blaha modified Becke–Johnson density functional

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bouhemadou, A., E-mail: a_bouhemadou@yahoo.fr; Bin-Omran, S.; Department of Physics, Faculty of Science & Humanitarian Studies, Salman Bin Abdalaziz University, Alkharj 11942

Highlights: • Electronic and optical properties of the LiCdX compounds have been predicted. • Tran–Blaha-modified Becke–Johnson functional significantly improves the band gap. • We predict a direct band gap in all of the considered LiCdX compounds. • Origin of the peaks in the optical spectra is determined. - Abstract: The structural, electronic and optical properties of the LiCdN, LiCdP, LiCdAs and LiCdSb filled-tetrahedral compounds have been explored from first-principles. The calculated structural parameters are consistent with the available experimental results. Since DFT with the common LDA and GGA underestimates the band gap, we use a new developed functional able tomore » accurately describe the electronic structure of semiconductors, namely the Tran–Blaha-modified Becke–Johnson potential. The four investigated compounds demonstrate semiconducting behavior with direct band gap ranging from about 0.32 to 1.65 eV. The charge-carrier effective masses are evaluated at the topmost valence band and at the bottommost conduction band. The evolution of the value and nature of the energy band gap under pressure effect is also investigated. The frequency-dependent complex dielectric function and some macroscopic optical constants are estimated. The microscopic origins of the structures in the optical spectra are determined in terms of the calculated energy band structures.« less

Interacting quasi-band model for electronic states in compound semiconductor alloys: Zincblende structure

NASA Astrophysics Data System (ADS)

Shinozuka, Yuzo; Oda, Masato

2015-09-01

The interacting quasi-band model proposed for electronic states in simple alloys is extended for compound semiconductor alloys with general lattice structures containing several atoms per unit cell. Using a tight-binding model, a variational electronic wave function for quasi-Bloch states yields a non-Hermitian Hamiltonian matrix characterized by matrix elements of constituent crystals and concentration of constituents. Solving secular equations for each k-state yields the alloy’s energy spectrum for any type of randomness and arbitrary concentration. The theory is used to address III-V (II-VI) alloys with a zincblende lattice with crystal band structures well represented by the sp3s* model. Using the resulting 15 × 15 matrix, the concentration dependence of valence and conduction bands is calculated in a unified scheme for typical alloys: Al1-xGaxAs, GaAs1-xPx, and GaSb1-xPx. Results agree well with experiments and are discussed with respect to the concentration dependence, direct-indirect gap transition, and band-gap-bowing origin.

Probing the electronic structure of liquid water with many-body perturbation theory

NASA Astrophysics Data System (ADS)

Pham, Tuan Anh; Zhang, Cui; Schwegler, Eric; Galli, Giulia

2014-03-01

We present a first-principles investigation of the electronic structure of liquid water based on many-body perturbation theory (MBPT), within the G0W0 approximation. The liquid quasiparticle band gap and the position of its valence band maximum and conduction band minimum with respect to vacuum were computed and it is shown that the use of MBPT is crucial to obtain results that are in good agreement with experiment. We found that the level of theory chosen to generate molecular dynamics trajectories may substantially affect the electronic structure of the liquid, in particular, the relative position of its band edges and redox potentials. Our results represent an essential step in establishing a predictive framework for computing the relative position of water redox potentials and the band edges of semiconductors and insulators. Work supported by DOE/BES (Grant No. DE-SC0008938). Work at LLNL was performed under Contract DE-AC52-07NA27344.

Electrons and phonons in layered and monolayer vanadium pentoxide

NASA Astrophysics Data System (ADS)

Lambrecht, Walter R. L.

Vanadium pentoxide (V2O5) is a layered material with the potential for interesting new properties when made in 2D mono- or few-layer form. Its band structure is characterized by a split-off conduction band. The lowest conduction band is separated from the rest of the conduction bands by about 1 eV and consists of V-dxy orbitals, non-bonding to the oxygens by symmetry. This narrow band has dispersion essentially along the direction of chains occurring in the layer. When this band becomes half-filled by doping, spin-splitting occurs accompanied by an antiferromagnetic coupling between nearest neighbors along the chain direction. This situation is well known to occur in the so-called ladder compound NaV2O5 , which was extensively studied in the late 90s as a potential spin-Peierls or charge ordering compound. However, the monolayer form of V2O5 may allow for other ways to control the doping by gating, removing vanadyl oxygens, adsorption of alkali metals, nanoribbon formation, etc. Our calculations predict a switch from antiferromagnetic to ferromagnetic coupling for doping slightly less than half filling of the split-off band. In this talk we will discuss our recent work on the electronic band structure of both bulk and monolayer V2O5 as well as the phonons. We find that the quasi-particle self-consistent GW method strongly overestimates the band gap. Lattice polarization corrections of the screening are required because of the large LO/TO phonon frequency ratios. Excitonic effects may also be expected to be fairly large. We find that some of the vibrational modes, notably the vanadyl-oxygen bond stretch perpendicular to the layer, unexpectedly shows a strong blue shift. This is explained in terms of reduced screening affecting the long-range dipole components of the force constants. Supported by AFOSR and DOE. Work done with Churna Bhandari, Mark van Schilfgaarde and Andre Schleiffe.

Interplay of Coulomb interactions and disorder in three-dimensional quadratic band crossings without time-reversal symmetry and with unequal masses for conduction and valence bands

NASA Astrophysics Data System (ADS)

Mandal, Ipsita; Nandkishore, Rahul M.

2018-03-01

Coulomb interactions famously drive three-dimensional quadratic band crossing semimetals into a non-Fermi liquid phase of matter. In a previous work [Nandkishore and Parameswaran, Phys. Rev. B 95, 205106 (2017), 10.1103/PhysRevB.95.205106], the effect of disorder on this non-Fermi liquid phase was investigated, assuming that the band structure was isotropic, assuming that the conduction and valence bands had the same band mass, and assuming that the disorder preserved exact time-reversal symmetry and statistical isotropy. It was shown that the non-Fermi liquid fixed point is unstable to disorder and that a runaway flow to strong disorder occurs. In this paper, we extend that analysis by relaxing the assumption of time-reversal symmetry and allowing the electron and hole masses to differ (but continuing to assume isotropy of the low energy band structure). We first incorporate time-reversal symmetry breaking disorder and demonstrate that there do not appear any new fixed points. Moreover, while the system continues to flow to strong disorder, time-reversal-symmetry-breaking disorder grows asymptotically more slowly than time-reversal-symmetry-preserving disorder, which we therefore expect should dominate the strong-coupling phase. We then allow for unequal electron and hole masses. We show that whereas asymmetry in the two masses is irrelevant in the clean system, it is relevant in the presence of disorder, such that the `effective masses' of the conduction and valence bands should become sharply distinct in the low-energy limit. We calculate the RG flow equations for the disordered interacting system with unequal band masses and demonstrate that the problem exhibits a runaway flow to strong disorder. Along the runaway flow, time-reversal-symmetry-preserving disorder grows asymptotically more rapidly than both time-reversal-symmetry-breaking disorder and the Coulomb interaction.

Observations of Multi-band Structures in Double Star TC-1 PEACE Electron and HIA Ion Data

NASA Astrophysics Data System (ADS)

Mohan Narasimhan, K.; Fazakerley, A. N.; Grimald, S.; Dandouras, I. S.; Mihaljcic, B.; Kistler, L. M.; Owen, C. J.

2015-12-01

Several authors have reported inner magnetosphere observations of proton distributions confined to narrow energy bands in the range 1 - 25 keV (Smith and Hoffman (1974), etc). These structures have been described as "nose structures", with reference to their appearance in energy-time spectrograms and are also known as "bands" if they occur for extended periods of time. Multi-nose structures have been observed if 2 or more noses appear at the same time (Vallat et al., 2007). Gaps between "noses" (or "bands") have been explained in terms of the competing corotation, convection and magnetic gradient drifts. Charge exchange losses in slow drift paths for steady state scenarios and the role of substorm injections have also been considered (Li et al., 2000; Ebihara et al., 2004). We analyse observations of electron and ion multi-band structures frequently seen in Double-Star TC1 PEACE and HIA data. We present results from statistical surveys conducted using data from the duration of the mission. Furthermore, using a combination of both statistics and simulations, we test previous theories as to possible formation mechanisms and explore other possible explanations.

Theoretical investigation of the structural, electronic, and thermodynamic properties of CdS1-xSex alloys

NASA Astrophysics Data System (ADS)

Long, Debing; Li, Mingkai; Meng, Dongxue; Ahuja, Rajeev; He, Yunbin

2018-03-01

In this work, the structural, electronic, and thermodynamic properties of wurtzite (WZ) and zincblende (ZB) CdS1-xSex alloys are investigated using the density functional theory (DFT) and the cluster expansion method. A special quasirandom structure containing 16 atoms is constructed to calculate the band structures of random alloys. The band gaps of CdS1-xSex alloys are direct and decrease as the Se content increases. The delta self-consistent-field method is applied to correct band gaps that are underestimated by DFT. The band offsets clearly reflect the variation in valence band maxima and conduction band minima, thus providing information useful to the design of relevant quantum well structures. The positive formation enthalpies of both phases imply that CdS1-xSex is an immiscible system and tends to phase separate. The influence of lattice vibrations on the phase diagram is investigated by calculating the phonon density of states. Lattice vibration effects can reduce the critical temperature Tc and increase alloy solid solubilities. This influence is especially significant in the ZB structure. When only chemical interactions are present, the Tc values for WZ- and ZB-CdS1-xSex are 260 K and 249 K, respectively. The lattice vibration enthalpy and entropy lower the Tc to 255 K and 233 K, respectively.

E+ Transition in GaAs1-xNx and GaAs1-xBix Due to Isoelectronic-Impurity-Induced Perturbation of the Conduction Band

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fluegel, B.; Mascarenhas, A.; Ptak, A. J.

2007-01-01

An above-band-gap transition E{sub +} is experimentally observed in the dilute GaAs{sub 1-x}Bi{sub x} alloy. Precise measurements at very low dilutions are made of the above-band-gap transition E{sub +} that is observed in GaAs{sub 1-x}N{sub x}, making it possible to compare the behavior of the different isoelectronic traps Bi and N in the common host GaAs with respect to their perturbation to the host electronic structure. We suggest that the origin of the E{sub +} level observed in GaAs is not the isolated isoelectronic impurity level N{sub x}, as is presumed in the band-anticrossing model, but rather the isoelectronic-impurity-induced perturbationmore » of the conduction band L{sub 6}{sup c}.« less

Band Structures and Transport Properties of High-Performance Half-Heusler Thermoelectric Materials by First Principles.

PubMed

Fang, Teng; Zhao, Xinbing; Zhu, Tiejun

2018-05-19

Half-Heusler (HH) compounds, with a valence electron count of 8 or 18, have gained popularity as promising high-temperature thermoelectric (TE) materials due to their excellent electrical properties, robust mechanical capabilities, and good high-temperature thermal stability. With the help of first-principles calculations, great progress has been made in half-Heusler thermoelectric materials. In this review, we summarize some representative theoretical work on band structures and transport properties of HH compounds. We introduce how basic band-structure calculations are used to investigate the atomic disorder in n-type M NiSb ( M = Ti, Zr, Hf) compounds and guide the band engineering to enhance TE performance in p-type Fe R Sb ( R = V, Nb) based systems. The calculations on electrical transport properties, especially the scattering time, and lattice thermal conductivities are also demonstrated. The outlook for future research directions of first-principles calculations on HH TE materials is also discussed.

Band Structures and Transport Properties of High-Performance Half-Heusler Thermoelectric Materials by First Principles

PubMed Central

Fang, Teng; Zhao, Xinbing

2018-01-01

Half-Heusler (HH) compounds, with a valence electron count of 8 or 18, have gained popularity as promising high-temperature thermoelectric (TE) materials due to their excellent electrical properties, robust mechanical capabilities, and good high-temperature thermal stability. With the help of first-principles calculations, great progress has been made in half-Heusler thermoelectric materials. In this review, we summarize some representative theoretical work on band structures and transport properties of HH compounds. We introduce how basic band-structure calculations are used to investigate the atomic disorder in n-type MNiSb (M = Ti, Zr, Hf) compounds and guide the band engineering to enhance TE performance in p-type FeRSb (R = V, Nb) based systems. The calculations on electrical transport properties, especially the scattering time, and lattice thermal conductivities are also demonstrated. The outlook for future research directions of first-principles calculations on HH TE materials is also discussed. PMID:29783759

Microscopic Observations of Adiabatic Shear Bands in Three Different Steels

DTIC Science & Technology

1988-09-01

low thermal conductivity, and a high thermal softening rate. Examples include alloys of titanium. aluminum, copper , as well as steels [5-221... steels : 1 (1) an AISI 1018 cold rolled steel , (2) a high strength low alloy structural steel , and deformation in shear was impo.ed to produce shear bands...stecls: (1) an AISI 1018 cold rolled steel , (2) a high strength low alloy structural steel , and (3) an AISI 4340 VAR steel tempered

Silicon quantum dots embedded in a SiO2 matrix: From structural study to carrier transport properties

NASA Astrophysics Data System (ADS)

Garcia-Castello, Nuria; Illera, Sergio; Guerra, Roberto; Prades, Joan Daniel; Ossicini, Stefano; Cirera, Albert

2013-08-01

We study the details of electronic transport related to the atomistic structure of silicon quantum dots embedded in a silicon dioxide matrix using ab initio calculations of the density of states. Several structural and composition features of quantum dots (QDs), such as diameter and amorphization level, are studied and correlated with transport under transfer Hamiltonian formalism. The current is strongly dependent on the QD density of states and on the conduction gap, both dependent on the dot diameter. In particular, as size increases, the available states inside the QD increase, while the QD band gap decreases due to relaxation of quantum confinement. Both effects contribute to increasing the current with the dot size. Besides, valence band offset between the band edges of the QD and the silica, and conduction band offset in a minor grade, increases with the QD diameter up to the theoretical value corresponding to planar heterostructures, thus decreasing the tunneling transmission probability and hence the total current. We discuss the influence of these parameters on electron and hole transport, evidencing a correlation between the electron (hole) barrier value and the electron (hole) current, and obtaining a general enhancement of the electron (hole) transport for larger (smaller) QD. Finally, we show that crystalline and amorphous structures exhibit enhanced probability of hole and electron current, respectively.

Effect of potassium doping on electronic structure and thermoelectric properties of topological crystalline insulator

DOE Office of Scientific and Technical Information (OSTI.GOV)

Roychowdhury, Subhajit; Biswas, Kanishka, E-mail: kanishka@jncasr.ac.in; Sandhya Shenoy, U.

2016-05-09

Topological crystalline insulator (TCI), Pb{sub 0.6}Sn{sub 0.4}Te, exhibits metallic surface states protected by crystal mirror symmetry with negligibly small band gap. Enhancement of its thermoelectric performances needs tuning of its electronic structure particularly through engineering of its band gap. While physical perturbations tune the electronic structure of TCI by breaking of the crystal mirror symmetry, chemical means such as doping have been more attractive recently as they result in better thermoelectric performance in TCIs. Here, we demonstrate that K doping in TCI, Pb{sub 0.6}Sn{sub 0.4}Te, breaks the crystal mirror symmetry locally and widens electronic band gap, which is confirmed bymore » direct electronic absorption spectroscopy and electronic structure calculations. K doping in Pb{sub 0.6}Sn{sub 0.4}Te increases p-type carrier concentration and suppresses the bipolar conduction via widening a band gap, which collectively boosts the thermoelectric figure of merit (ZT) to 1 at 708 K.« less

Influence of nitrogen-doping concentration on the electronic structure of CuAlO2 by first-principles studies

NASA Astrophysics Data System (ADS)

Liu, Wei-wei; Chen, Hong-xia; Liu, Cheng-lin; Wang, Rong

2017-02-01

Effect of N doping concentration on the electronic structure of N-doped CuAlO2 was investigated by density functional theory based on generalized-gradient approximation plus orbital potential. Lattice parameters a and c both increase with increasing N-doping concentration. Formation energies increase with increasing N doping concentration and all N-doped CuAlO2 were structurally stable. The calculated band gaps for N-doped CuAlO2 narrowed compared to pure CuAlO2, which was attributed to the stronger hybridization between Cu-3d and N-2p states and the downward shift of Cu-3p states in conduction bands. The higher the N-doping concentration is, the narrower the band gap. N-doped CuAlO2 shows a typical p-type semiconductor. The band structure changed from indirect to direct after N doping which will benefit the application of the CuAlO2 materials in optoelectronic and electronic devices.

Dual roles of f electrons in mixing Al 3 p character into d -orbital conduction bands for lanthanide and actinide dialuminides

DOE PAGES

Altman, Alison B.; Pemmaraju, C. D.; Alayoglu, Selim; ...

2018-01-15

Correlated electron phenomena in lanthanide and actinide materials are driven by a complex interplay between the f and d orbitals. Here in this study, aluminum K-edge x-ray absorption spectroscopy and density functional theory calculations are used to evaluate the electronic structure of the dialuminides, MAl 2 (M = Ce, Sm, Eu, Yb, Lu, U, and Pu). The results show how the energy and occupancy of the 4f or 5f orbitals impacts mixing of Al 3p character into the 5d or 6d conduction bands, which has implications for understanding the magnetic and structural properties of correlated electron systems.

Origins of Fermi-level pinning on GaN and InN polar and nonpolar surfaces

NASA Astrophysics Data System (ADS)

Segev, D.; Van de Walle, C. G.

2006-10-01

Using band structure and total energy methods, we study the atomic and electronic structures of the polar (+c and - c plane) and nonpolar (a and m plane) surfaces of GaN and InN. We identify two distinct microscopic origins for Fermi-level pinning on GaN and InN, depending on surface stoichiometry and surface polarity. At moderate Ga/N ratios unoccupied gallium dangling bonds pin the Fermi level on n-type GaN at 0.5 0.7 eV below the conduction-band minimum. Under highly Ga-rich conditions metallic Ga adlayers lead to Fermi-level pinning at 1.8 eV above the valence-band maximum. We also explain the source of the intrinsic electron accumulation that has been universally observed on polar InN surfaces. It is caused by In-In bonds leading to occupied surface states above the conduction-band minimum. We predict that such a charge accumulation will be absent on the nonpolar surfaces of InN, when prepared under specific conditions.

Theoretical Study of α-V2O5 -Based Double-Wall Nanotubes.

PubMed

Porsev, Vitaly V; Bandura, Andrei V; Evarestov, Robert A

2015-10-05

First-principles calculations of the atomic and electronic structure of double-wall nanotubes (DWNTs) of α-V2 O5 are performed. Relaxation of the DWNT structure leads to the formation of two types of local regions: 1) bulk-type regions and 2) puckering regions. Calculated total density of states (DOS) of DWNTs considerably differ from that of single-wall nanotubes and the single layer, as well as from the DOS of the bulk and double layer. Small shoulders that appear on edges of valence and conduction bands result in a considerable decrease in the band gaps of the DWNTs (up to 1 eV relative to the single-layer gaps). The main reason for this effect is the shift of the inner- and outer-wall DOS in opposite directions on the energetic scale. The electron density corresponding to shoulders at the conduction-band edges is localized on vanadium atoms of the bulk-type regions, whereas the electron density corresponding to shoulders at the valence-band edges belongs to oxygen atoms of both regions. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Photoemission and Auger-electron spectroscopic study of the Chevrel-phase compound FexMo6S8

NASA Astrophysics Data System (ADS)

Fujimori, A.; Sekita, M.; Wada, H.

1986-05-01

The electronic structure of the Chevrel-phase compound FexMo6S8 has been studied by photoemission and Auger-electron spectroscopy. Core-level shifts suggest a large charge transfer from the Fe atoms to the Mo6S8 clusters and a small Mo-to-S charge transfer within the cluster. Line-shape asymmetry in the core levels indicates that the density of states (DOS) at the Fermi level has a finite S 3p component as well as the dominant Mo 3d character. Satellite structure and exchange splitting in the Fe core levels point to weak Fe 3d-S 3p hybridization in spite of the short Fe-S distances comparable to that in FeS. The x-ray and ultraviolet valence-band photoemission spectra and the Mo 4d partial DOS obtained by deconvoluting the Mo M4,5VV Auger spectrum are compared with existing band-structure calculations, and the Mo 4d-S 3p bonding character, the structure of the Mo 4d-derived conduction band etc., are discussed. In particular, it is shown that the conduction-band structure is sensitive to the noncubic distortion of the crystal through changes in the intercluster Mo 4d-S 3p hybridization. A pronounced final-state effect is found in the Mo M4,5N2,3V Auger spectrum and is attributed to strong 4p-4d intershell coupling.

Determination of the energy structure of recombination centers in heavily doped AlxGa1-xN:Si epitaxial layers with x > 0.5

NASA Astrophysics Data System (ADS)

Osinnykh, I. V.; Malin, T. V.; Zhuravlev, K. S.

2018-03-01

The photoluminescence properties of the intensive defect-related emission in heavily doped Al x Ga l-x N:Si layers with x > 0.5 have been investigated by photoluminescence (PL) spectroscopy. The PL band in AlN was attributed to donor-acceptor (DA) transitions. At the lowest Al content, the impurity band merges with the conduction band and DA transitions are replaced by electron-acceptor transitions involving the same acceptor. The energy structure of recombination centers was obtained using the model of configuration coordinates for Al0.67Ga0.33N.

Floquet band structure of a semi-Dirac system

NASA Astrophysics Data System (ADS)

Chen, Qi; Du, Liang; Fiete, Gregory A.

2018-01-01

In this work we use Floquet-Bloch theory to study the influence of circularly and linearly polarized light on two-dimensional band structures with semi-Dirac band touching points, taking the anisotropic nearest neighbor hopping model on the honeycomb lattice as an example. We find that circularly polarized light opens a gap and induces a band inversion to create a finite Chern number in the two-band model. By contrast, linearly polarized light can either open up a gap (polarized in the quadratically dispersing direction) or split the semi-Dirac band touching point into two Dirac points (polarized in the linearly dispersing direction) by an amount that depends on the amplitude of the light. Motivated by recent pump-probe experiments, we investigated the nonequilibrium spectral properties and momentum-dependent spin texture of our model in the Floquet state following a quench in the absence of phonons, and in the presence of phonon dissipation that leads to a steady state independently of the pump protocol. Finally, we make connections to optical measurements by computing the frequency dependence of the longitudinal and transverse optical conductivity for this two-band model. We analyze the various contributions from interband transitions and different Floquet modes. Our results suggest strategies for optically controlling band structures and experimentally measuring topological Floquet systems.

Lightning protection design external tank /Space Shuttle/

NASA Technical Reports Server (NTRS)

Anderson, A.; Mumme, E.

1979-01-01

The possibility of lightning striking the Space Shuttle during liftoff is considered and the lightning protection system designed by the Martin Marietta Corporation for the external tank (ET) portion of the Shuttle is discussed. The protection system is based on diverting and/or directing a lightning strike to an area of the spacecraft which can sustain the strike. The ET lightning protection theory and some test analyses of the system's design are reviewed including studies of conductivity and thermal/stress properties in materials, belly band feasibility, and burn-through plug grounding and puncture voltage. The ET lightning protection system design is shown to be comprised of the following: (1) a lightning rod on the forward most point of the ET, (2) a continually grounded, one inch wide conductive strip applied circumferentially at station 371 (belly band), (3) a three inch wide conductive belly band applied over the TPS (i.e. the insulating surface of the ET) and grounded to a structure with eight conductive plugs at station 536, and (4) a two inch thick TPS between the belly bands which are located over the weld lands.

Band Offsets at the Interface between Crystalline and Amorphous Silicon from First Principles

NASA Astrophysics Data System (ADS)

Jarolimek, K.; Hazrati, E.; de Groot, R. A.; de Wijs, G. A.

2017-07-01

The band offsets between crystalline and hydrogenated amorphous silicon (a -Si ∶H ) are key parameters governing the charge transport in modern silicon heterojunction solar cells. They are an important input for macroscopic simulators that are used to further optimize the solar cell. Past experimental studies, using x-ray photoelectron spectroscopy (XPS) and capacitance-voltage measurements, have yielded conflicting results on the band offset. Here, we present a computational study on the band offsets. It is based on atomistic models and density-functional theory (DFT). The amorphous part of the interface is obtained by relatively long DFT first-principles molecular-dynamics runs at an elevated temperature on 30 statistically independent samples. In order to obtain a realistic conduction-band position the electronic structure of the interface is calculated with a hybrid functional. We find a slight asymmetry in the band offsets, where the offset in the valence band (0.29 eV) is larger than in the conduction band (0.17 eV). Our results are in agreement with the latest XPS measurements that report a valence-band offset of 0.3 eV [M. Liebhaber et al., Appl. Phys. Lett. 106, 031601 (2015), 10.1063/1.4906195].

Mapping the Coulomb Environment in Interference-Quenched Ballistic Nanowires.

PubMed

Gutstein, D; Lynall, D; Nair, S V; Savelyev, I; Blumin, M; Ercolani, D; Ruda, H E

2018-01-10

The conductance of semiconductor nanowires is strongly dependent on their electrostatic history because of the overwhelming influence of charged surface and interface states on electron confinement and scattering. We show that InAs nanowire field-effect transistor devices can be conditioned to suppress resonances that obscure quantized conduction thereby revealing as many as six sub-bands in the conductance spectra as the Fermi-level is swept across the sub-band energies. The energy level spectra extracted from conductance, coupled with detailed modeling shows the significance of the interface state charge distribution revealing the Coulomb landscape of the nanowire device. Inclusion of self-consistent Coulomb potentials, the measured geometrical shape of the nanowire, the gate geometry and nonparabolicity of the conduction band provide a quantitative and accurate description of the confinement potential and resulting energy level structure. Surfaces of the nanowire terminated by HfO 2 are shown to have their interface donor density reduced by a factor of 30 signifying the passivating role played by HfO 2 .

Design of Multilayer Dual-Band BPF and Diplexer with Zeros Implantation Using Suspended Stripline

NASA Astrophysics Data System (ADS)

Ho, Min-Hua; Hsu, Wei-Hong

In this paper, a dual-band bandpass filter (BPF) of multilayer suspended stripline (SSL) structure and an SSL diplexer composed of a low-pass filter (LPF) and a high-pass filter (HPF) are proposed. Bandstop structure creating transmission zeros is adopted in the BPF and diplexer, enhancing the signal selectivity of the former and increasing the isolation between the diverting ports of the latter. The dual-band BPF possesses two distinct bandpass structures and a bandstop circuit, all laid on different metallic layers. The metallic layers together with the supporting substrates are vertically stacked up to save the circuit dimension. The LPF and HPF used in the diplexer structure are designed by a quasi-lumped approach, which the LC lumped-elements circuit models are developed to analyze filters' characteristics and to emulate their frequency responses. Half-wavelength resonating slots are employed in the diplexer's structure to increase the isolation between its two signal diverting ports. Experiments are conducted to verify the multilayer dual-band BPF and the diplexer design. Agreements are observed between the simulation and the measurement.

Modeling of the absorption properties of Ga1-xInxAs1-yNy/GaAs quantum well structures for photodetection applications

NASA Astrophysics Data System (ADS)

Aissat, A.; Bestam, R.; Alshehri, B.; Vilcot, J. P.

2015-06-01

This work reports on theoretical studies on the GaInNAs material properties (bandgap, lattice mismatch, absorption coefficient) as grown on GaAs substrate. The Band Anti-Crossing (BAC) kṡp 8 × 8 model has been used to determine the influence of indium and nitrogen concentrations on the position of conduction and valence bands. The incorporation of nitrogen at a level lower than 5% causes the split of the conduction band. For indium and nitrogen concentrations of 38% and 3.5%, respectively, the strained bandgap energy is 0.70 eV and the absorption coefficient of indium and nitrogen-rich compounds increases significantly.

Mapping the conduction band edge density of states of γ-In2Se3 by diffuse reflectance spectra

NASA Astrophysics Data System (ADS)

Kumar, Pradeep; Vedeshwar, Agnikumar G.

2018-03-01

It is demonstrated that the measured diffuse reflectance spectra of γ-In2Se3 can be used to map the conduction band edge density of states through Kubelka-Munk analysis. The Kubelka-Munk function derived from the measured spectra almost mimics the calculated density of states in the vicinity of conduction band edge. The calculation of density of states was carried out using first-principles approach yielding the structural, electronic, and optical properties. The calculations were carried out implementing various functionals and only modified Tran and Blaha (TB-MBJ) results tally closest with the experimental result of band gap. The electronic and optical properties were calculated using FP-LAPW + lo approach based on the Density Functional Theory formalism implementing only TB-mBJ functional. The electron and hole effective masses have been calculated as me * = 0.25 m 0 and mh * = 1.11 m 0 , respectively. The optical properties clearly indicate the anisotropic nature of γ-In2Se3.

Spectrally resolved localized states in GaAs 1– xBi x

DOE PAGES

Christian, Theresa M.; Alberi, Kirstin; Beaton, Daniel A.; ...

2017-02-01

In this study, the role of localized states and their influence on the broader band structure remains a crucial question in understanding the band structure evolution in GaAs 1-xBi x. Here in this work, we present clear spectroscopic observations of recombination at several localized states in GaAs 1-xBi x. Sharp and recognizable photoluminescence features appear in multiple samples and redshift as a function of GaBi fraction between x = 0.16% and 0.4% at a linearized rate of 34 meV per % Bi, weaker than the redshift associated with band-to-band recombination. Interpreting these results in terms of radiative recombination between localizedmore » holes and free electrons sheds light on the relative movement of the conduction band minimum and the characteristics of localized bismuth-related trap states in GaAs 1-xBi x alloys.« less

Thermal tuning on band gaps of 2D phononic crystals considering adhesive layers

NASA Astrophysics Data System (ADS)

Zhou, Xiaoliang; Chen, Jialin; Li, Yuhang; Sun, Yuxin; Xing, Yufeng

2018-02-01

Phononic crystals are very attractive in many applications, such as noise reduction, filters and vibration isolation, due to their special forbidden band gap structures. In the present paper, the investigation of tunable band gaps of 2D phononic crystals with adhesive layers based on thermal changing is conducted. Based on the lumped-mass method, an analytical model of 2D phononic crystals with relatively thin adhesive layers is established, in which the in-plane and out-of-plane modes are both in consideration. The adhesive material is sensitive to temperature so that the band structure can be tuned and controlled by temperature variation. As temperature increases from 20 °C-80 °C, the first band gap shifts to the frequency zone around 10 kHz, which is included by the audible frequency range. The results propose an important guideline for applications, such as noise suppression using the 2D phononic crystals.

Band gap of corundumlike α -Ga2O3 determined by absorption and ellipsometry

NASA Astrophysics Data System (ADS)

Segura, A.; Artús, L.; Cuscó, R.; Goldhahn, R.; Feneberg, M.

2017-07-01

The electronic structure near the band gap of the corundumlike α phase of Ga2O3 has been investigated by means of optical absorption and spectroscopic ellipsometry measurements in the ultraviolet (UV) range (400-190 nm). The absorption coefficient in the UV region and the imaginary part of the dielectric function exhibit two prominent absorption thresholds with wide but well-defined structures at 5.6 and 6.3 eV which have been ascribed to allowed direct transitions from crystal-field split valence bands to the conduction band. Excitonic effects with large Gaussian broadening are taken into account through the Elliott-Toyozawa model, which yields an exciton binding energy of 110 meV and direct band gaps of 5.61 and 6.44 eV. The large broadening of the absorption onset is related to the slightly indirect character of the material.

Cermet based metamaterials for multi band absorbers over NIR to LWIR frequencies

NASA Astrophysics Data System (ADS)

Pradhan, Jitendra K.; Behera, Gangadhar; Agarwal, Amit K.; Ghosh, Amitava; Ramakrishna, S. Anantha

2017-06-01

Cermets or ceramic-metals are known for their use in solar thermal technologies for their absorption across the solar band. Use of cermet layers in a metamaterial perfect absorber allows for flexible control of infra-red absorption over the short wave infra-red, to long wave infra-red bands, while keeping the visible/near infra-red absorption properties constant. We design multilayered metamaterials consisting of a conducting ground plane, a low metal volume fraction cermet/ZnS as dielectric spacer layers, and a top structured layer of an array of circular discs of metal/high volume metal fraction cermet that give rise to specified absorption bands in the near-infra-red (NIR) frequencies, as well as any specified band at SWIR-LWIR frequencies. Thus, a complete decoupling of the absorption at optical/NIR frequencies and the infra-red absorption behaviour of a structured metamaterial is demonstrated.

Determination of band structure parameters and the quasi-particle gap of CdSe quantum dots by cyclic voltammetry.

PubMed

Inamdar, Shaukatali N; Ingole, Pravin P; Haram, Santosh K

2008-12-01

Band structure parameters such as the conduction band edge, the valence band edge and the quasi-particle gap of diffusing CdSe quantum dots (Q-dots) of various sizes were determined using cyclic voltammetry. These parameters are strongly dependent on the size of the Q-dots. The results obtained from voltammetric measurements are compared to spectroscopic and theoretical data. The fit obtained to the reported calculations based on the semi-empirical pseudopotential method (SEPM)-especially in the strong size-confinement region, is the best reported so far, according to our knowledge. For the smallest CdSe Q-dots, the difference between the quasi-particle gap and the optical band gap gives the electron-hole Coulombic interaction energy (J(e1,h1)). Interband states seen in the photoluminescence spectra were verified with cyclic voltammetry measurements.

Electronic properties of graphene and effect of doping on the same

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nag, Abhinav, E-mail: abhinavn76@gmail.com; Kumar, Jagdish, E-mail: jagdishphysicist@gmail.com; Sastri, O. S. K. S., E-mail: sastri.osks@gmail.com

2015-05-15

The electronic structure of pure and doped two dimensional crystalline material graphene have been computed and analyzed. Density functional theory has been employed to perform calculations. The electronic exchange and correlations are considered using local density approximation (LDA). The doped material is studied within virtual crystal approximation (VCA) upto 0.15e excess as well as deficient charge per unit cell. Full Potential Linear Augmented Plane Wave basis as implemented in ELK code has been used to perform the calculations. To ensures the monolayer of graphene, distance after which energy is almost constant when interlayer seperation is varied, is taken as separatingmore » distance between the layers. The obtained density of states and band structure is analyzed. Results show that there is zero band gap in undoped graphene and conduction and valence band meets at fermi level at symmetry point K. PDOS graph shows that near the fermi level the main contribution is due to 2p{sub z} electrons. By using VCA, calculations for doped graphene are done and the results for doped graphene are compared with undoped graphene. We found that by electron or hole doping, the point where conduction and valence bands meet can shift below or above the fermi level. The shift in bands seems almost as per rigid band model upto doping concentration studied.« less

Luminescence properties and electronic structure of Ce{sup 3+}-doped gadolinium aluminum garnet

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dotsenko, V.P., E-mail: ssclab@ukr.net; Berezovskaya, I.V.; Voloshinovskii, A.S.

2015-04-15

Highlights: • The luminescence properties of Ce{sup 3+} ions in (Y, Gd){sub 3}Al{sub 5}O{sub 12} are analyzed. • The Gd{sup 3+} → Y{sup 3+} substitution leads to increasing of Ce{sup 3+} noncubic crystal field splitting parameter. • The excitation spectra for the Ce{sup 3+} emission in GdAG contain bands at 6.67, 7.75, and 9.76 eV. • These features are due to the Ce{sup 3+}-bound exciton formation and O 2p → Al 3s, 3p transitions. • Contributions from Al atoms to the conduction-band density of states are quite essential. - Abstract: Yttrium-gadolinium aluminum garnets (YGdAG) doped with Ce{sup 3+} ions havemore » been prepared by co-precipitation method. The luminescent properties of Ce{sup 3+} ions in Gd{sub 3(1−x)}Ce{sub 3x}Al{sub 5}O{sub 12} (x = 0.01) have been studied upon excitation in the 2–20 eV region. The substitution of Gd{sup 3+} for Y{sup 3+} in the garnet structure results in broadening the emission band and shifting its maximum towards the longer wavelengths. It was found that in addition to the 4f → 5d excitation bands of Ce{sup 3+} ions, the excitation spectra for the Ce{sup 3+} emission contain bands at 6.67, 7.75, and 9.76 eV. These bands are attributed to the Ce{sup 3+}-bound exciton formation and O 2p → Al 3s, 3p transitions, respectively. Although gadolinium states dominate near the bottom of the conduction band of Gd{sub 3}Al{sub 5}O{sub 12}, contributions from Al{sub tetr} and Al{sub oct} atoms to the conduction-band density of states are evaluated as quite essential.« less

Quadratic band touching points and flat bands in two-dimensional topological Floquet systems

NASA Astrophysics Data System (ADS)

Du, Liang; Zhou, Xiaoting; Fiete, Gregory; The CenterComplex Quantum Systems Team

In this work we theoretically study, using Floquet-Bloch theory, the influence of circularly and linearly polarized light on two-dimensional band structures with Dirac and quadratic band touching points, and flat bands, taking the nearest neighbor hopping model on the kagome lattice as an example. We find circularly polarized light can invert the ordering of this three band model, while leaving the flat-band dispersionless. We find a small gap is also opened at the quadratic band touching point by 2-photon and higher order processes. By contrast, linearly polarized light splits the quadratic band touching point (into two Dirac points) by an amount that depends only on the amplitude and polarization direction of the light, independent of the frequency, and generally renders dispersion to the flat band. The splitting is perpendicular to the direction of the polarization of the light. We derive an effective low-energy theory that captures these key results. Finally, we compute the frequency dependence of the optical conductivity for this 3-band model and analyze the various interband contributions of the Floquet modes. Our results suggest strategies for optically controlling band structure and interaction strength in real systems. We gratefully acknowledge funding from ARO Grant W911NF-14-1-0579 and NSF DMR-1507621.

Electronic structure of antifluorite Cu2X (X = S, Se, Te) within the modified Becke-Johnson potential plus an on-site Coulomb U.

PubMed

Zhang, Yubo; Wang, Youwei; Xi, Lili; Qiu, Ruihao; Shi, Xun; Zhang, Peihong; Zhang, Wenqing

2014-02-21

The traditional photon absorbers Cu2-xX (X = S, Se, and Te) have regained significant research attention in the search of earth-abundant photovoltaic materials. These moderate- and narrow-gap materials have also been shown to exhibit excellent thermoelectric properties recently. However, semimetallic band structures with inverted band orderings are predicted for antifluorite structure Cu2X using density functional theory with the local density approximation or the generalized gradient approximation. We find that semiconducting band structures and normal band orderings can be obtained using the modified Becke-Johnson potential plus an on-site Coulomb U (the mBJ+U approach), which is consistent with our earlier finding for diamond-like Cu-based multinary semiconductors [Y. Zhang, J. Zhang, W. Gao, T. A. Abtew, Y. Wang, P. Zhang, and W. Zhang, J. Chem. Phys. 139, 184706 (2013)]. The trend of the chemical bonding of Cu2X is analyzed, which shows that the positions of the valence band maximum and conduction band minimum are strongly affected by the inter-site pd and intra-site sp hybridizations, respectively. The calculated gaps of Cu2S and Cu2Se still seem to be underestimated compared with experimental results. We also discuss the effects of different structural phases and Cu disordering and deficiency on the bandgaps of these materials.

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jandieri, K., E-mail: kakhaber.jandieri@physik.uni-marburg.de; Ludewig, P.; Wegele, T.

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.

Silicon micromechanical sensors model of piezoresistivity

NASA Astrophysics Data System (ADS)

Lysko, Jan M.

2001-08-01

Application of the piezo resistivity model to estimate valence and conduction bands shifts induced by the mechanical stress is presented. Parameters of the silicon pressure and acceleration sensor, which are under development in the ITE, Warsaw, were used. Geometrical and technological data were used in calculations of the silicon energy band structure and longitudinal coefficient of the piezo resistivity.(pi) L.

Synthesis, structural and optical properties of (ALa)(FeMn)O6 (A = Ba and Sr) double perovskites

NASA Astrophysics Data System (ADS)

Kumar, Dinesh; Sudarshan, V.; Singh, Akhilesh Kumar

2018-05-01

Here, we report structural and optical properties of ALaFeMnO6 (A = Ba and Sr) double perovskite synthesized via auto-combustion followed by calcinations process. Rietveld refinement of structure using x-ray diffraction data reveals that BaLaFeMnO6 crystallizes into cubic crystal structure with space group Pm-3m while SrLaFeMnO6 crystallizes into rhombohedral crystal structure having space group R-3c. The absorption spectrum measurement using UV-Vis spectroscopy reveals that these samples are prefect insulator having energy band gap between conduction and valence band of the order of 6 eV.

Thermoelectric properties of AgSbTe₂ from first-principles calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rezaei, Nafiseh; Akbarzadeh, Hadi; Hashemifar, S. Javad, E-mail: hashemifar@cc.iut.ac.ir

2014-09-14

The structural, electronic, and transport properties of AgSbTe₂ are studied by using full-relativistic first-principles electronic structure calculation and semiclassical description of transport parameters. The results indicate that, within various exchange-correlation functionals, the cubic Fd3⁻m and trigonal R3⁻m structures of AgSbTe₂ are more stable than two other considered structures. The computed Seebeck coefficients at different values of the band gap and carrier concentration are accurately compared with the available experimental data to speculate a band gap of about 0.1–0.35 eV for AgSbTe₂ compound, in agreement with our calculated electronic structure within the hybrid HSE (Heyd-Scuseria-Ernzerhof) functional. By calculating the semiclassical Seebeckmore » coefficient, electrical conductivity, and electronic part of thermal conductivity, we present the theoretical upper limit of the thermoelectric figure of merit of AgSbTe₂ as a function of temperature and carrier concentration.« less

Energy band structure and electrical properties of Ga-oxide/GaN interface formed by remote oxygen plasma

NASA Astrophysics Data System (ADS)

Yamamoto, Taishi; Taoka, Noriyuki; Ohta, Akio; Truyen, Nguyen Xuan; Yamada, Hisashi; Takahashi, Tokio; Ikeda, Mitsuhisa; Makihara, Katsunori; Nakatsuka, Osamu; Shimizu, Mitsuaki; Miyazaki, Seiichi

2018-06-01

The energy band structure of a Ga-oxide/GaN structure formed by remote oxygen plasma exposure and the electrical interface properties of the GaN metal–oxide–semiconductor (MOS) capacitors with the SiO2/Ga-oxide/GaN structures with postdeposition annealing (PDA) at various temperatures have been investigated. Reflection high-energy electron diffraction and X-ray photoelectron spectroscopy clarified that the formed Ga-oxide layer is neither a single nor polycrystalline phase with high crystallinity. We found that the energy band offsets at the conduction band minimum and at the valence band maximum between the Ga-oxide layer and the GaN surface were 0.4 and 1.2 ± 0.2 eV, respectively. Furthermore, capacitance–voltage (C–V) characteristics revealed that the interface trap density (D it) is lower than the evaluation limit of Terman method without depending on the PDA temperatures, and that the SiO2/Ga-oxide stack can work as a protection layer to maintain the low D it, avoiding the significant decomposition of GaN at the high PDA temperature of 800 °C.

Electronic structures of C u 2 O , C u 4 O 3 , and CuO: A joint experimental and theoretical study

DOE PAGES

Wang, Y.; Lany, S.; Ghanbaja, J.; ...

2016-12-14

We present a joint experimental and theoretical study for the electronic structures of copper oxides including Cu 2O, CuO, and the metastable mixed-valence oxide Cu 4O 3. The optical band gap is determined by experimental optical absorption coefficient, and the electronic structure in valence and conduction bands is probed by photoemission and electron energy loss spectroscopies, respectively. Furthermore, we compare our experimental results with many-body GW calculations utilizing an additional on-site potential for d-orbital energies that facilitates tractable and predictive computations. The side-by-side comparison between the three oxides, including a band insulator (Cu2O) and two Mott/charge-transfer insulators (CuO, Cu 4Omore » 3) leads to a consistent picture for the optical and band-structure properties of the Cu oxides, strongly supporting indirect band gaps of about 1.2 and 0.8 eV in CuO and Cu 4O 3, respectively. This comparison also points towards surface oxidation and reduction effects that can complicate the interpretation of the photoemission spectra.« less

Interfacial band alignment and structural properties of nanoscale TiO2 thin films for integration with epitaxial crystallographic oriented germanium

NASA Astrophysics Data System (ADS)

Jain, N.; Zhu, Y.; Maurya, D.; Varghese, R.; Priya, S.; Hudait, M. K.

2014-01-01

We have investigated the structural and band alignment properties of nanoscale titanium dioxide (TiO2) thin films deposited on epitaxial crystallographic oriented Ge layers grown on (100), (110), and (111)A GaAs substrates by molecular beam epitaxy. The TiO2 thin films deposited at low temperature by physical vapor deposition were found to be amorphous in nature, and high-resolution transmission electron microscopy confirmed a sharp heterointerface between the TiO2 thin film and the epitaxially grown Ge with no traceable interfacial layer. A comprehensive assessment on the effect of substrate orientation on the band alignment at the TiO2/Ge heterointerface is presented by utilizing x-ray photoelectron spectroscopy and spectroscopic ellipsometry. A band-gap of 3.33 ± 0.02 eV was determined for the amorphous TiO2 thin film from the Tauc plot. Irrespective of the crystallographic orientation of the epitaxial Ge layer, a sufficient valence band-offset of greater than 2 eV was obtained at the TiO2/Ge heterointerface while the corresponding conduction band-offsets for the aforementioned TiO2/Ge system were found to be smaller than 1 eV. A comparative assessment on the effect of Ge substrate orientation revealed a valence band-offset relation of ΔEV(100) > ΔEV(111) > ΔEV(110) and a conduction band-offset relation of ΔEC(110) > ΔEC(111) > ΔEC(100). These band-offset parameters are of critical importance and will provide key insight for the design and performance analysis of TiO2 for potential high-κ dielectric integration and for future metal-insulator-semiconductor contact applications with next generation of Ge based metal-oxide field-effect transistors.

XANES: observation of quantum confinement in the conduction band of colloidal PbS quantum dots

NASA Astrophysics Data System (ADS)

Demchenko, I. N.; Chernyshova, M.; He, X.; Minikayev, R.; Syryanyy, Y.; Derkachova, A.; Derkachov, G.; Stolte, W. C.; Piskorska-Hommel, E.; Reszka, A.; Liang, H.

2013-04-01

The presented investigations aimed at development of inexpensive method for synthesized materials suitable for utilization of solar energy. This important issue was addressed by focusing, mainly, on electronic local structure studies with supporting x-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis of colloidal galena nano-particles (NPs) and quantum dots (QDs) synthesized using wet chemistry under microwave irradiation. Performed x-ray absorption near edge structure (XANES) analysis revealed an evidence of quantum confinement for the sample with QDs, where the bottom of the conduction band was shifted to higher energy. The QDs were found to be passivated with oxides at the surface. Existence of sulfate/sulfite and thiosulfate species in pure PbS and QDs, respectively, was identified.

Observation of Dirac-like energy band and ring-torus Fermi surface associated with the nodal line in topological insulator CaAgAs

NASA Astrophysics Data System (ADS)

Takane, Daichi; Nakayama, Kosuke; Souma, Seigo; Wada, Taichi; Okamoto, Yoshihiko; Takenaka, Koshi; Yamakawa, Youichi; Yamakage, Ai; Mitsuhashi, Taichi; Horiba, Koji; Kumigashira, Hiroshi; Takahashi, Takashi; Sato, Takafumi

2018-01-01

One of key challenges in current material research is to search for new topological materials with inverted bulk-band structure. In topological insulators, the band inversion caused by strong spin-orbit coupling leads to opening of a band gap in the entire Brillouin zone, whereas an additional crystal symmetry such as point-group and nonsymmorphic symmetries sometimes prohibits the gap opening at/on specific points or line in momentum space, giving rise to topological semimetals. Despite many theoretical predictions of topological insulators/semimetals associated with such crystal symmetries, the experimental realization is still relatively scarce. Here, using angle-resolved photoemission spectroscopy with bulk-sensitive soft-x-ray photons, we experimentally demonstrate that hexagonal pnictide CaAgAs belongs to a new family of topological insulators characterized by the inverted band structure and the mirror reflection symmetry of crystal. We have established the bulk valence-band structure in three-dimensional Brillouin zone, and observed the Dirac-like energy band and ring-torus Fermi surface associated with the line node, where bulk valence and conducting bands cross on a line in the momentum space under negligible spin-orbit coupling. Intriguingly, we found that no other bands cross the Fermi level and therefore the low-energy excitations are solely characterized by the Dirac-like band. CaAgAs provides an excellent platform to study the interplay among low-energy electron dynamics, crystal symmetry, and exotic topological properties.

Split-Channel Ballistic Transport in an InSb Nanowire.

PubMed

Estrada Saldaña, Juan Carlos; Niquet, Yann-Michel; Cleuziou, Jean-Pierre; Lee, Eduardo J H; Car, Diana; Plissard, Sébastien R; Bakkers, Erik P A M; De Franceschi, Silvano

2018-04-11

We report an experimental study of one-dimensional (1D) electronic transport in an InSb semiconducting nanowire. A total of three bottom gates are used to locally deplete the nanowire, creating a ballistic quantum point contact with only a few conducting channels. In a magnetic field, the Zeeman splitting of the corresponding 1D sub-bands is revealed by the emergence of conductance plateaus at multiples of e 2 /h, yet we find a quantized conductance pattern largely dependent on the configuration of voltages applied to the bottom gates. In particular, we can make the first plateau disappear, leaving a first conductance step of 2 e 2 / h, which is indicative of a remarkable 2-fold sub-band degeneracy that can persist up to several tesla. For certain gate voltage settings, we also observe the presence of discrete resonant states producing conductance features that can resemble those expected from the opening of a helical gap in the sub-band structure. We explain our experimental findings through the formation of two spatially separated 1D conduction channels.

Impact of Wide-Ranging Nanoscale Chemistry on Band Structure at Cu(In, Ga)Se 2 Grain Boundaries

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stokes, Adam; Al-Jassim, Mowafak; Diercks, David

The relative chemistry from grain interiors to grain boundaries help explain why grain boundaries may be beneficial, detrimental or benign towards device performance. 3D Nanoscale chemical analysis extracted from atom probe tomography (APT) (10’s of parts-per-million chemical sensitivity and sub-nanometer spatial resolution) of twenty grain boundaries in a high-efficiency Cu(In, Ga)Se 2 solar cell shows the matrix and alkali concentrations are wide-ranging. The concentration profiles are then related to band structure which provide a unique insight into grain boundary electrical performance. Fluctuating Cu, In and Ga concentrations result in a wide distribution of potential barriers at the valence band maximummore » (VBM) (-10 to -160 meV) and the conduction band minimum (CBM) (-20 to -70 meV). Furthermore, Na and K segregation is not correlated to hampering donors, (In, Ga) Cu and V Se, contrary to what has been previously reported. In addition, Na and K are predicted to be n-type dopants at grain boundaries. An overall band structure at grain boundaries is presented.« less

Impact of Wide-Ranging Nanoscale Chemistry on Band Structure at Cu(In, Ga)Se 2 Grain Boundaries

DOE PAGES

Stokes, Adam; Al-Jassim, Mowafak; Diercks, David; ...

2017-10-26

The relative chemistry from grain interiors to grain boundaries help explain why grain boundaries may be beneficial, detrimental or benign towards device performance. 3D Nanoscale chemical analysis extracted from atom probe tomography (APT) (10’s of parts-per-million chemical sensitivity and sub-nanometer spatial resolution) of twenty grain boundaries in a high-efficiency Cu(In, Ga)Se 2 solar cell shows the matrix and alkali concentrations are wide-ranging. The concentration profiles are then related to band structure which provide a unique insight into grain boundary electrical performance. Fluctuating Cu, In and Ga concentrations result in a wide distribution of potential barriers at the valence band maximummore » (VBM) (-10 to -160 meV) and the conduction band minimum (CBM) (-20 to -70 meV). Furthermore, Na and K segregation is not correlated to hampering donors, (In, Ga) Cu and V Se, contrary to what has been previously reported. In addition, Na and K are predicted to be n-type dopants at grain boundaries. An overall band structure at grain boundaries is presented.« less

Thomson backscattering diagnostics of nanosecond electron bunches in high space charge regime

NASA Astrophysics Data System (ADS)

Plachinda, Pavel

The trend over the last 50 years of down-scaling the silicon transistor to achieve faster computations has led to doubling of the number of transistors and computation speed over about every two years. However, this trend cannot be maintained due to the fundamental limitations of silicon as the main material for the semiconducting industry. Therefore, there is an active search for exploration of alternate materials. Among the possible candidates that can may be able to replace silicon is graphene which has recently gained the most attention. Unique properties of graphene include exceedingly high carrier mobility, tunable band gap, huge optical density of a monolayer, anomalous quantum Hall effect, and many others. To be suitable for microelectronic applications the material should be semiconductive, i.e. have a non-zero band gap. Pristine graphene is a semimetal, but by the virtue of doping the graphene surface with different molecules and radicals a band gap can be opened. Because the electronic properties of all materials are intimately related to their atomic structure, characterization of molecular and electronic structure of functionalizing groups is of high interest. The ab-inito (from the first principles) calculations provide a unique opportunity to study the influence of the dopants and thus allow exploration of the physical phenomena in functionalized graphene structures. This ability paves the road to probe the properties based on the intuitive structural information only. A great advantage of this approach lies in the opportunity for quick screening of various atomic structures. We conducted a series of ab-inito investigations of graphene functionalized with covalently and hapticly bound groups, and demonstrated possible practical usage of functionalized graphene for microelectronic and optical applications. This investigation showed that it is possible produce band gaps in graphene (i.e., produce semiconducting graphene) of about 1 eV, without degrading the carrier mobility. This was archived by considering the influence of those adducts on electronic band structure and conductivity properties.

Electronic structure and optical properties of CsI, CsI(Ag), and CsI(Tl)

NASA Astrophysics Data System (ADS)

Zhang, Zheng; Zhao, Qiang; Li, Yang; Ouyang, Xiao-Ping

2016-05-01

The band structure, electronic density of states and optical properties of CsI and of CsI doped with silver or thallium are studied by using a first-principles calculation based on density functional theory (DFT). The exchange and the correlation potentials among the electrons are described by using the generalized gradient approximation (GGA). The results of our study show that the electronic structure changes somewhat when CsI is doped with silver or thallium. The band gaps of CsI(Ag) and CsI(Tl) are smaller than that of CsI, and the width of the conduction band of CsI is increased when CsI is doped with thallium or silver. Two peaks located in the conduction band of CsI(Ag) and CsI(Tl) are observed from their electronic densities of states. The absorption coefficients of CsI, CsI(Ag), and CsI(Tl) are zero when their photon energies are below 3.5 eV, 1.5 eV, and 3.1 eV, respectively. The results show that doping can improve the detection performance of CsI scintillators. Our study can explain why doping can improve the detection performance from a theoretical point of view. The results of our research provide both theoretical support for the luminescent mechanisms at play in scintillator materials when they are exposed to radiation and a reference for CsI doping from the point of view of the electronic structure.

Doping-dependent correlation effects in (Sr1-xLax) 3Ir2O7

NASA Astrophysics Data System (ADS)

Affeldt, Gregory; Hogan, Tom; Denlinger, Jonathan D.; Vishwanath, Ashvin; Wilson, Stephen D.; Lanzara, Alessandra

2018-03-01

We have measured the signatures of electronic energy scales and their doping evolution in the band structure of (Sr1-xLax) 3Ir2O7 using angle-resolved photoemission spectroscopy. While band splittings and positions corresponding to the bilayer splitting and spin-orbit coupling undergo only small changes, the Mott gap and effective mass of both the lower Hubbard band and conduction band exhibit strong variations with doping. These changes correspond to similar observations in the cuprate superconductors, and are likely connected to the changing effective Coulomb interaction upon addition of itinerant carriers.

A simplified approach to the band gap correction of defect formation energies: Al, Ga, and In-doped ZnO

NASA Astrophysics Data System (ADS)

Saniz, R.; Xu, Y.; Matsubara, M.; Amini, M. N.; Dixit, H.; Lamoen, D.; Partoens, B.

2013-01-01

The calculation of defect levels in semiconductors within a density functional theory approach suffers greatly from the band gap problem. We propose a band gap correction scheme that is based on the separation of energy differences in electron addition and relaxation energies. We show that it can predict defect levels with a reasonable accuracy, particularly in the case of defects with conduction band character, and yet is simple and computationally economical. We apply this method to ZnO doped with group III elements (Al, Ga, In). As expected from experiment, the results indicate that Zn substitutional doping is preferred over interstitial doping in Al, Ga, and In-doped ZnO, under both zinc-rich and oxygen-rich conditions. Further, all three dopants act as shallow donors, with the +1 charge state having the most advantageous formation energy. Also, doping effects on the electronic structure of ZnO are sufficiently mild so as to affect little the fundamental band gap and lowest conduction bands dispersion, which secures their n-type transparent conducting behavior. A comparison with the extrapolation method based on LDA+U calculations and with the Heyd-Scuseria-Ernzerhof hybrid functional (HSE) shows the reliability of the proposed scheme in predicting the thermodynamic transition levels in shallow donor systems.

Nonreciprocity of spin waves in magnonic crystals created by surface acoustic waves in structures with yttrium iron garnet

NASA Astrophysics Data System (ADS)

Kryshtal, R. G.; Medved, A. V.

2015-12-01

Experimental results of investigations of nonreciprocity for surface magnetostatic spin waves (SMSW) in the magnonic crystal created by surface acoustic waves (SAW) in yttrium iron garnet films on a gallium gadolinium garnet substrate as without metallization and with aluminum films with different electrical conductivities (thicknesses) are presented. In structures without metallization, the frequency of magnonic gaps is dependent on mutual directions of propagation of the SAW and SMSW, showing nonreciprocal properties for SMSW in SAW - magnonic crystals even with the symmetrical dispersion characteristic. In metalized SAW - magnonic crystals the shift of the magnonic band gaps frequencies at the inversion of the biasing magnetic field was observed. The frequencies of magnonic band gaps as functions of SAW frequency are presented. Measured dependencies, showing the decrease of magnonic gaps frequency and the expansion of the magnonic band gap width with the decreasing of the metal film conductivity are given. Such nonreciprocal properties of the SAW - magnonic crystals are promising for signal processing in the GHz range.

Band alignment and p -type doping of ZnSnN2

NASA Astrophysics Data System (ADS)

Wang, Tianshi; Ni, Chaoying; Janotti, Anderson

2017-05-01

Composed of earth-abundant elements, ZnSnN2 is a promising semiconductor for photovoltaic and photoelectrochemical applications. However, basic properties such as the precise value of the band gap and the band alignment to other semiconductors are still unresolved. For instance, reported values for the band gap vary from 1.4 to 2.0 eV. In addition, doping in ZnSnN2 remains largely unexplored. Using density functional theory with the Heyd-Scuseria-Ernzerhof hybrid functional, we investigate the electronic structure of ZnSnN2, its band alignment to GaN and ZnO, and the possibility of p -type doping. We find that the position of the valence-band maximum of ZnSnN2 is 0.39 eV higher than that in GaN, yet the conduction-band minimum is close to that in ZnO, which suggests that achieving p -type conductivity is likely as in GaN, yet it may be difficult to control unintentional n -type conductivity as in ZnO. Among possible p -type dopants, we explore Li, Na, and K substituting on the Zn site. We show that while LiZn is a shallow acceptor, NaZn and KZn are deep acceptors, which we trace back to large local relaxations around the Na and K impurities due to the atomic size mismatches.

Conduction quantization in monolayer MoS2

NASA Astrophysics Data System (ADS)

Li, T. S.

2016-10-01

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

Interface band alignment in high-k gate stacks

NASA Astrophysics Data System (ADS)

Eric, Bersch; Hartlieb, P.

2005-03-01

In order to successfully implement alternate high-K dielectric materials into MOS structures, the interface properties of MOS gate stacks must be better understood. Dipoles that may form at the metal/dielectric and dielectric/semiconductor interfaces make the band offsets difficult to predict. We have measured the conduction and valence band densities of states for a variety MOS stacks using in situ using inverse photoemission (IPE) and photoemission spectroscopy (PES), respectively. Results obtained from clean and metallized (with Ru or Al) HfO2/Si, SiO2/Si and mixed silicate films will be presented. IPE indicates a shift of the conduction band minimum (CBM) to higher energy (i.e. away from EF) with increasing SiO2. The effect of metallization on the location of band edges depends upon the metal species. The addition of N to the dielectrics shifts the CBM in a way that is thickness dependent. Possible mechanisms for these observed effects will be discussed.

Swift heavy ion irradiation effects on structural, optical properties and ac conductivity of polypyrrole nanofibers

NASA Astrophysics Data System (ADS)

Hazarika, J.; Kumar, A.

2016-12-01

Polypyrrole (PPy) nanofibers have been synthesized by interfacial polymerization method and irradiated with 160 MeV Ni12+ ions under vacuum with fluences in the range of 1010-1012 ions/cm2. High-resolution transmission electron microscopy results show that upon swift heavy ion (SHI) irradiation the PPy nanofibers become denser. The crystallinity of PPy nanofibers increases upon SHI irradiation, while their d-spacing decreases. Upon SHI irradiation, the polaron absorption band gets red-shifted indicating reduction in the optical band gap energy of the irradiated PPy nanofibers. The indirect optical band gap energy is decreased as compared to corresponding direct optical band gap energy. The number of carbon atoms per conjugation length (N) and carbon atoms per cluster (M) of the SHI-irradiated PPy nanofibers increase with increasing the irradiation fluence. Fourier transform infrared spectra reveal the enhancement in intensity of some characteristic vibration bands upon SHI irradiation. The thermal stability of the PPy nanofibers is enhanced on SHI irradiation. The charge carriers in both pristine and irradiated PPy nanofibers follow the correlated barrier hopping mechanism. Scaling of ac conductivity reveals that the conduction mechanism is independent of the SHI irradiation fluence.

Electronic structure of monolayer 1T'-MoTe2 grown by molecular beam epitaxy

NASA Astrophysics Data System (ADS)

Tang, Shujie; Zhang, Chaofan; Jia, Chunjing; Ryu, Hyejin; Hwang, Choongyu; Hashimoto, Makoto; Lu, Donghui; Liu, Zhi; Devereaux, Thomas P.; Shen, Zhi-Xun; Mo, Sung-Kwan

2018-02-01

Monolayer transition metal dichalcogenides (TMDCs) in the 1T' structural phase have drawn a great deal of attention due to the prediction of quantum spin Hall insulator states. The band inversion and the concomitant changes in the band topology induced by the structural distortion from 1T to 1T' phases are well established. However, the bandgap opening due to the strong spin-orbit coupling (SOC) is only verified for 1T'-WTe2 recently and still debated for other TMDCs. Here we report a successful growth of high-quality monolayer 1T'-MoTe2 on a bilayer graphene substrate through molecular beam epitaxy. Using in situ angle-resolved photoemission spectroscopy (ARPES), we have investigated the low-energy electronic structure and Fermi surface topology. The SOC-induced breaking of the band degeneracy points between the valence and conduction bands is clearly observed by ARPES. However, the strength of SOC is found to be insufficient to open a bandgap, which makes monolayer 1T'-MoTe2 on bilayer graphene a semimetal.

Electronic structure of monolayer 1T'-MoTe 2 grown by molecular beam epitaxy

DOE PAGES

Tang, Shujie; Zhang, Chaofan; Jia, Chunjing; ...

2017-11-14

Monolayer transition metal dichalcogenides (TMDCs) in the 1T' structural phase have drawn a great deal of attention due to the prediction of quantum spin Hall insulator states. The band inversion and the concomitant changes in the band topology induced by the structural distortion from 1T to 1T' phases are well established. However, the bandgap opening due to the strong spin-orbit coupling (SOC) is only verified for 1T'-WTe 2 recently and still debated for other TMDCs. Here we report a successful growth of high-quality monolayer 1T'-MoTe 2 on a bilayer graphene substrate through molecular beam epitaxy. Using in situ angle-resolved photoemissionmore » spectroscopy (ARPES), we have investigated the low-energy electronic structure and Fermi surface topology. The SOC-induced breaking of the band degeneracy points between the valence and conduction bands is clearly observed by ARPES. However, the strength of SOC is found to be insufficient to open a bandgap, which makes monolayer 1T'-MoTe 2 on bilayer graphene a semimetal.« less

Effective mass and Fermi surface complexity factor from ab initio band structure calculations

NASA Astrophysics Data System (ADS)

Gibbs, Zachary M.; Ricci, Francesco; Li, Guodong; Zhu, Hong; Persson, Kristin; Ceder, Gerbrand; Hautier, Geoffroy; Jain, Anubhav; Snyder, G. Jeffrey

2017-02-01

The effective mass is a convenient descriptor of the electronic band structure used to characterize the density of states and electron transport based on a free electron model. While effective mass is an excellent first-order descriptor in real systems, the exact value can have several definitions, each of which describe a different aspect of electron transport. Here we use Boltzmann transport calculations applied to ab initio band structures to extract a density-of-states effective mass from the Seebeck Coefficient and an inertial mass from the electrical conductivity to characterize the band structure irrespective of the exact scattering mechanism. We identify a Fermi Surface Complexity Factor: Nv*K* from the ratio of these two masses, which in simple cases depends on the number of Fermi surface pockets (Nv* ) and their anisotropy K*, both of which are beneficial to high thermoelectric performance as exemplified by the high values found in PbTe. The Fermi Surface Complexity factor can be used in high-throughput search of promising thermoelectric materials.

Transition temperature from band to hopping direct current conduction in crystalline semiconductors with hydrogen-like impurities: Heat versus Coulomb attraction

NASA Astrophysics Data System (ADS)

Poklonski, N. A.; Vyrko, S. A.; Poklonskaya, O. N.; Zabrodskii, A. G.

2011-12-01

For nondegenerate bulk semiconductors, we have used the virial theorem to derive an expression for the temperature Tj of the transition from the regime of "free" motion of electrons in the c-band (or holes in the υ-band) to their hopping motion between donors (or acceptors). Distribution of impurities over the crystal was assumed to be of the Poisson type, while distribution of their energy levels was assumed to be of the Gaussian type. Our conception of the virial theorem implementation is that the transition from the band-like conduction to hopping conduction occurs when the average kinetic energy of an electron in the c-band (hole in the υ-band) is equal to the half of the absolute value of the average energy of the Coulomb interaction of an electron (hole) with the nearest neighbor ionized donor (acceptor). Calculations of Tj according to our model agree with experimental data for crystals of Ge, Si, diamond, etc. up to the concentrations of a hydrogen-like impurity, at which the phase insulator-metal transition (Mott transition) occurs. Under the temperature Th ≈ Tj /3, when the nearest neighbor hopping conduction via impurity atoms dominates, we obtained expressions for the electrostatic field screening length Λh in the Debye-Hückel approximation, taking into account a nonzero width of the impurity energy band. It is shown that the measurements of quasistatic capacitance of the semiconductor in a metal-insulator-semiconductor structure in the regime of the flat bands at the temperature Th allow to determine the concentration of doping impurity or its compensation ratio by knowing Λh.

Electron and hole photoemission detection for band offset determination of tunnel field-effect transistor heterojunctions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Wei; Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871; Zhang, Qin

2014-11-24

We report experimental methods to ascertain a complete energy band alignment of a broken-gap tunnel field-effect transistor based on an InAs/GaSb hetero-junction. By using graphene as an optically transparent electrode, both the electron and hole barrier heights at the InAs/GaSb interface can be quantified. For a Al{sub 2}O{sub 3}/InAs/GaSb layer structure, the barrier height from the top of the InAs and GaSb valence bands to the bottom of the Al{sub 2}O{sub 3} conduction band is inferred from electron emission whereas hole emissions reveal the barrier height from the top of the Al{sub 2}O{sub 3} valence band to the bottom ofmore » the InAs and GaSb conduction bands. Subsequently, the offset parameter at the broken gap InAs/GaSb interface is extracted and thus can be used to facilitate the development of predicted models of electron quantum tunneling efficiency and transistor performance.« less

The structural, electronic and magnetic properties of CoS2 under pressure

NASA Astrophysics Data System (ADS)

Feng, Zhong-Ying; Yang, Yan; Zhang, Jian-Min

2018-05-01

The structural, electronic and magnetic properties of CoS2 under pressure have been investigated by the first-principles calculations. The lattice constant and volume decrease with increasing pressure. The CoS2 is stable and behaves a brittle characteristic under the pressures of 0-5 GPa. The CoS2 presents metallic characteristic under the pressures of 1-5 GPa although it is nearly half-metal (HM) under the pressure of 0 GPa. The lowest conduction bands for spin-up and spin-down channels shift towards higher and lower energy region, respectively, with the pressure increasing from 0 to 5 GPa. In spin-up channel the conduction band minimum (CBM) is mainly contributed by Co-3d(eg) orbitals at R point but the valence band maximum (VBM) is contributed by Co-3d(t2g) orbitals near M point. While in spin-down channel the CBM is contributed by S-3p orbitals at Γ point but the VBM is contributed by Co-3d(t2g) orbitals near X point. The CoS2 is still suitable to be used in the supercapacitor under the environmental pressures of 0-5 GPa due to the high conductivity.

Crystal and electronic structure of copper sulfides

NASA Astrophysics Data System (ADS)

Lukashev, Pavel

Copper sulfides with different copper concentration exist in mineral form ranging from CuS to Cu2S. Among these, chalcosite Cu 2S, and digenite Cu1.8S were the subject of extensive research for decades mainly because of their use as the absorber in photovoltaic cells. Yet; their electronic structure is poorly understood because their crystal structure is complex. Most of the results published so far report the semiconducting nature of these compounds with the energy band gap being in the range of 0.84 to 1.9 eV. The crystal structure consists of a close-packed lattice of S with mobile Cu occupying various types of interstitial sites with a statistical distribution depending on temperature. In this thesis we present the first computational study of their electronic band structure. Initially, we investigated the simpler antifluorite structure. Both local density approximation (LDA) and self-consistent quasiparticle GW calculations with the full-potential linearized muffin-tin orbital method give a semimetallic band structure. Inspection of the nature of the bands shows that the lowest conduction band is mainly Cu-s-like except right near the center of the Brillouin zone where a Cu-s-like state lies about 1 eV below the valence band maximum. Significantly, in GW calculations, this state shifts up by several 0.1 eV but not sufficiently to open a gap. A random distortion of the Cu atoms from the perfect antifluorite positions is found to break the degeneracy of the d state at the Gamma-point and thus opens up a small gap of about 0.1 eV in LDA. As our next step we constructed supercell models for the cubic and hexagonal phases with the Cu positions determined by a weighted random number generator. The low temperature monoclinic phase was also studied. The computed total energies of these structures follow the same order as the reported phases with increasing temperatures. All these models gave similar small band gaps of order 0.1-0.2 eV. However, their conduction band is now mainly s-like and addition of an expected Cu-s level shift opens the gap to about 0.5 eV. Some simpler hexagonal model structures gave slightly larger band gap but were found to be unrealistic. The optical absorption data all show a strong intraband absorption with a minimum in absorption at about 1 eV. Our calculations suggest a significantly lower gap of order 0.5 eV with low absorption cross section, the true nature of which is masked by the free carrier absorption. As part of our study of the related Cu-compounds, we analyzed the quasiparticle effects beyond LDA obtained from a GW calculation on the effective masses and Kohn-Luttinger hamiltonian parameters for CuBr.

Effect of band filling on anomalous Hall conductivity and magneto-crystalline anisotropy in NiFe epitaxial thin films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shi, Zhong; Jiang, Hang-Yu; Zhou, Shi-Ming, E-mail: shiming@tongji.edu.cn

2016-01-15

The anomalous Hall effect (AHE) and magneto-crystalline anisotropy (MCA) are investigated in epitaxial Ni{sub x}Fe{sub 1−x} thin films grown on MgO (001) substrates. The scattering independent term b of anomalous Hall conductivity shows obvious correlation with cubic magneto-crystalline anisotropy K{sub 1}. When nickel content x decreasing, both b and K{sub 1} vary continuously from negative to positive, changing sign at about x = 0.85. Ab initio calculations indicate Ni{sub x}Fe{sub 1−x} has more abundant band structures than pure Ni due to the tuning of valence electrons (band fillings), resulting in the increased b and K{sub 1}. This remarkable correlation betweenmore » b and K{sub 1} can be attributed to the effect of band filling near the Fermi surface.« less

WIDE-FIELD INFRARED POLARIMETRY OF THE ρ OPHIUCHI CLOUD CORE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kwon, Jungmi; Tamura, Motohide; Kusakabe, Nobuhiko

2015-09-15

We conducted wide and deep simultaneous JHK{sub s}-band imaging polarimetry of the ρ Ophiuchi cloud complex. Aperture polarimetry in the JHK{sub s} band was conducted for 2136 sources in all three bands, of which 322 sources have significant polarizations in all the JHK{sub s} bands and have been used for a discussion of the core magnetic fields. There is a positive correlation between degrees of polarization and H − K{sub s} color up to H − K{sub s} ≈ 3.5. The magnetic field structures in the core region are revealed up to at least A{sub V} ≈ 47 mag andmore » are unambiguously defined in each sub-region (core) of Oph-A, Oph-B, Oph-C, Oph-E, Oph-F, and Oph-AC. Their directions, degrees of polarization, and polarization efficiencies differ but their changes are gradual; thus, the magnetic fields appear to be connected from core to core, rather than as a simple overlap of the different cloud core components. Comparing our results with the large-scale field structures obtained from previous optical polarimetric studies, we suggest that the magnetic field structures in the core were distorted by the cluster formation in this region, which may have been induced by shock compression due to wind/radiation from the Scorpius–Centaurus association.« less

SLD-MOSCNT: A new MOSCNT with step-linear doping profile in the source and drain regions

NASA Astrophysics Data System (ADS)

Tahne, Behrooz Abdi; Naderi, Ali

2017-01-01

In this paper, a new structure, step-linear doping MOSCNT (SLD-MOSCNT), is proposed to improve the performance of basic MOSCNTs. The basic structure suffers from band to band tunneling (BTBT). We show that using SLD profile for source and drain regions increases the horizontal distance between valence and conduction bands at gate to source/drain junction which reduces BTBT probability. SLD performance is compared with other similar structures which have recently been proposed to reduce BTBT such as MOSCNT with lightly-doped drain and source (LDDS), and with double-light doping in source and drain regions (DLD). The obtained results using a nonequilibrium Green’s function (NEGF) method show that the SLD-MOSCNT has the lowest leakage current, power consumption and delay time, and the highest current ratio and voltage gain. The ambipolar conduction in the proposed structure is very low and can be neglected. In addition, these structures can improve short-channel effects. Also, the investigation of cutoff frequency of the different structures shows that the SLD has the highest cutoff frequency. Device performance has been investigated for gate length from 8 to 20 nm which demonstrates all discussions regarding the superiority of the proposed structure are also valid for different channel lengths. This improvement is more significant especially for channel length less than 12 nm. Therefore, the SLD can be considered as a candidate to be used in the applications with high speed and low power consumption.

Spin-polarized structural, elastic, electronic and magnetic properties of half-metallic ferromagnetism in V-doped ZnSe

NASA Astrophysics Data System (ADS)

Monir, M. El Amine.; Baltache, H.; Murtaza, G.; Khenata, R.; Ahmed, Waleed K.; Bouhemadou, A.; Omran, S. Bin; Seddik, T.

2015-01-01

Based on first principles spin-polarized density functional theory, the structural, elastic electronic and magnetic properties of Zn1-xVxSe (for x=0.25, 0.50, 0.75) in zinc blende structure have been studied. The investigation was done using the full-potential augmented plane wave method as implemented in WIEN2k code. The exchange-correlation potential was treated with the generalized gradient approximation PBE-GGA for the structural and elastic properties. Moreover, the PBE-GGA+U approximation (where U is the Hubbard correlation terms) is employed to treat the "d" electrons properly. A comparative study between the band structures, electronic structures, total and partial densities of states and local moments calculated within both GGA and GGA+U schemes is presented. The analysis of spin-polarized band structure and density of states shows the half-metallic ferromagnetic character and are also used to determine s(p)-d exchange constants N0α (conduction band) and N0β (valence band) due to Se(4p)-V(3d) hybridization. It has been clearly evidence that the magnetic moment of V is reduced from its free space change value of 3 μB and the minor atomic magnetic moment on Zn and Se are generated.

Spectroscopic evidence for temperature-dependent convergence of light- and heavy-hole valence bands of PbQ (Q = Te, Se, S)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhao, J.; Malliakas, C. D.; Wijayaratne, K.

2017-01-01

We have conducted a temperature- dependent angle-resolved photoemission spectroscopy (ARPES) study of the electronic structures of PbTe, PbSe and PbS. Our ARPES data provide direct evidence for the light-hole upper valence bands (UVBs) and hitherto undetected heavy-hole lower valence bands (LVBs) in these materials. An unusual temperature-dependent relative movement between these bands leads to a monotonic decrease in the energy separation between their maxima with increasing temperature, which is known as band convergence and has long been believed to be the driving factor behind extraordinary thermoelectric performances of these compounds at elevated temperatures.

Spectroscopic evidence for temperature-dependent convergence of light- and heavy-hole valence bands of PbQ (Q = Te, Se, S)

NASA Astrophysics Data System (ADS)

Zhao, J.; Malliakas, C. D.; Wijayaratne, K.; Karlapati, V.; Appathurai, N.; Chung, D. Y.; Rosenkranz, S.; Kanatzidis, M. G.; Chatterjee, U.

2017-01-01

We have conducted a temperature-dependent angle-resolved photoemission spectroscopy (ARPES) study of the electronic structures of PbTe, PbSe and PbS. Our ARPES data provide direct evidence for the light-hole upper valence bands (UVBs) and hitherto undetected heavy-hole lower valence bands (LVBs) in these materials. An unusual temperature-dependent relative movement between these bands leads to a monotonic decrease in the energy separation between their maxima with increasing temperature, which is known as band convergence and has long been believed to be the driving factor behind extraordinary thermoelectric performances of these compounds at elevated temperatures.

Thermal conductivity of graphene nanoribbons under shear deformation: A molecular dynamics simulation

PubMed Central

Zhang, Chao; Hao, Xiao-Li; Wang, Cui-Xia; Wei, Ning; Rabczuk, Timon

2017-01-01

Tensile strain and compress strain can greatly affect the thermal conductivity of graphene nanoribbons (GNRs). However, the effect of GNRs under shear strain, which is also one of the main strain effect, has not been studied systematically yet. In this work, we employ reverse nonequilibrium molecular dynamics (RNEMD) to the systematical study of the thermal conductivity of GNRs (with model size of 4 nm × 15 nm) under the shear strain. Our studies show that the thermal conductivity of GNRs is not sensitive to the shear strain, and the thermal conductivity decreases only 12–16% before the pristine structure is broken. Furthermore, the phonon frequency and the change of the micro-structure of GNRs, such as band angel and bond length, are analyzed to explore the tendency of thermal conductivity. The results show that the main influence of shear strain is on the in-plane phonon density of states (PDOS), whose G band (higher frequency peaks) moved to the low frequency, thus the thermal conductivity is decreased. The unique thermal properties of GNRs under shear strains suggest their great potentials for graphene nanodevices and great potentials in the thermal managements and thermoelectric applications. PMID:28120921

Effect of weight fraction of carbon black and number of plies of E-glass fiber to reflection loss of E-glass/ripoxy composite for radar absorbing structure (RAS)

NASA Astrophysics Data System (ADS)

Widyastuti, Ramadhan, Rizal; Ardhyananta, Hosta; Zainuri, Mochamad

2013-09-01

Nowadays, studies on investigating radar absorbing structure (RAS) using fiber reinforced polymeric (FRP) composite materials are becoming popular research field because the electromagnetic properties of FRP composites can be tailored effectively by just adding some electromagnetic powders, such as carbon black, ferrite, carbonyl iron, and etc., to the matrix of composites. The RAS works not only as a load bearing structure to hold the antenna system, but also has the important function of absorbing the in-band electromagnetic wave coming from the electromagnetic energy of tracking systems. In this study, E-glass fiber reinforced ripoxy resin composite was fabricated by blending the conductive carbon black (Ketjenblack EC300J) with the binder matrix of the composite material and maximizing the coefficient of absorption more than 90% (more than -10 dB) within the X-band frequency (8 - 12 GHz). It was measured by electrical conductivity (LCR meter) and vector network analyzer (VNA). Finally, the composite RAS with 0.02 weight fraction of carbon black and 4 plies of E-glass fiber showed thickness of 2.1 mm, electrical conductivity of 8.33 × 10-6 S/m, and maximum reflection loss of -27.123 dB, which can absorb more than 90% of incident EM wave throughout the entire X-band frequency range, has been developed.

Structural, optical, physical and electrical properties of V2O5.SrO.B2O3 glasses.

PubMed

Sindhu, S; Sanghi, S; Agarwal, A; Seth, V P; Kishore, N

2006-05-01

The present work aims to study the structure and variation of optical band gap, density and dc electrical conductivity in vanadium strontium borate glasses. The glass systems xV2O5.(40-x)SrO.60B2O3 and xV2O5.(60-x)B2O3.40SrO with x varying from 0 to 20 mol% were prepared by normal melt quench technique. Structural studies were made by recording IR transmission spectra. The fundamental absorption edge for all the glasses was analyzed in terms of the theory proposed by Davis and Mott. The position of absorption edge and hence the value of the optical band gap was found to depend on the semiconducting glass composition. The absorption in these glasses is believed to be associated with indirect transitions. The origin of Urbach energy is associated with the phonon-assisted indirect transitions. The change in both density and molar volume was discussed in terms of the structural modifications that take place in the glass matrix on addition of V2O5. dc conductivity of the glass systems is also reported. The change of conductivity and activation energy with composition indicates that the conduction process varies from ionic to polaronic one.

Study of cobalt effect on structural and optical properties of Dy doped ZnO nanoparticles

NASA Astrophysics Data System (ADS)

Kumar, Pawan; Pandey, Praveen C.

2018-05-01

The present study has been carried out to investigate the effect of Co doping on structural and optical properties of Dy doped ZnO nanoparticles. We have prepared pure Zinc oxide, Dy (1%) doped ZnO and Dy (1%) doped ZnO co-doped with Co(2%) with the help of simple sol-gel combustion method. The structural analysis carried out using X-ray diffraction spectra (XRD) indicates substitution of Dy and Co at Zn site of ZnO crystal structure and hexagonal crystal structure without any secondary phase formation in all the samples. The surface morphology was analyzed by transmission electron microscopy (TEM). Absorption study indicates that Dy doping causes a small shift in band edge, while Co co-doping results significant change is absorption edge as well as introduce defect level absorption in the visible region. The band gap of samples decreases due to Dy and Co doping, which can be attributed to defect level formation below the conduction band in the system.

Electronic structure of antifluorite Cu{sub 2}X (X = S, Se, Te) within the modified Becke-Johnson potential plus an on-site Coulomb U

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Yubo; Wang, Youwei; Xi, Lili

The traditional photon absorbers Cu{sub 2−x}X (X = S, Se, and Te) have regained significant research attention in the search of earth-abundant photovoltaic materials. These moderate- and narrow-gap materials have also been shown to exhibit excellent thermoelectric properties recently. However, semimetallic band structures with inverted band orderings are predicted for antifluorite structure Cu{sub 2}X using density functional theory with the local density approximation or the generalized gradient approximation. We find that semiconducting band structures and normal band orderings can be obtained using the modified Becke-Johnson potential plus an on-site Coulomb U (the mBJ+U approach), which is consistent with our earliermore » finding for diamond-like Cu-based multinary semiconductors [Y. Zhang, J. Zhang, W. Gao, T. A. Abtew, Y. Wang, P. Zhang, and W. Zhang, J. Chem. Phys. 139, 184706 (2013)]. The trend of the chemical bonding of Cu{sub 2}X is analyzed, which shows that the positions of the valence band maximum and conduction band minimum are strongly affected by the inter-site pd and intra-site sp hybridizations, respectively. The calculated gaps of Cu{sub 2}S and Cu{sub 2}Se still seem to be underestimated compared with experimental results. We also discuss the effects of different structural phases and Cu disordering and deficiency on the bandgaps of these materials.« less

Electronic structure and optical properties of Cs2HgCl4: DFT calculations and X-ray photoelectron spectroscopy measurements

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Parasyuk, O. V.; Fedorchuk, A. O.; Khyzhun, O. Y.

2016-10-01

A high-quality single crystal of cesium mercury tetrabromide, Cs2HgCl4, was synthesized by using the vertical Bridgman-Stockbarger method and its electronic structure was studied from both experimental and theoretical viewpoints. In particular, X-ray photoelectron spectra were measured for both pristine and Ar+ ion-bombarded Cs2HgCl4 single crystal surfaces. The present XPS measurements indicate that the Cs2HgCl4 single crystal surface is sensitive with respect to Ar+ ion-bombardment: such a treatment changes substantially its elemental stoichiometry. With the aim of exploring total and partial densities of states within the valence band and conduction band regions of the Cs2HgCl4 compound, band-structure calculations based on density functional theory (DFT) using the augmented plane wave + local orbitals (APW + lo) method as incorporated within the WIEN2k package are performed. The calculations indicate that the Cl 3p states are the principal contributors in the upper portion of the valence band, while the Hg 5d and Cs 5p states dominate in its lower portion. In addition, the calculations allow for concluding that the unoccupied Cl p and Hg s states are the main contributors to the bottom of the conduction band. Furthermore, main optical characteristics of Cs2HgCl4, namely dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity, are elucidated based on the DFT calculations.

Catalyst-free fabrication of novel ZnO/CuO core-Shell nanowires heterojunction: Controlled growth, structural and optoelectronic properties

NASA Astrophysics Data System (ADS)

Khan, Muhammad Arif; Wahab, Yussof; Muhammad, Rosnita; Tahir, Muhammad; Sakrani, Samsudi

2018-03-01

Development of controlled growth and vertically aligned ZnO/CuO core-shell heterojunction nanowires (NWs) with large area by a catalyst free vapor deposition and oxidation approach has been investigated. Structural characterization reveals successful fabrication of a core ZnO nanowire having single crystalline hexagonal wurtzite structure along [002] direction and CuO nanostructure shell with thickness (8-10 nm) having polycrystalline monoclinic structure. The optical property analysis suggests that the reflectance spectrum of ZnO/CuO heterostructure nanowires is decreased by 18% in the visible range, which correspondingly shows high absorption in this region as compared to pristine ZnO nanowires. The current-voltage (I-V) characteristics of core-shell heterojunction nanowires measured by conductive atomic force microscopy (C-AFM) shows excellent rectifying behavior, which indicates the characteristics of a good p-n junction. The high-resolution transmission electron microscopy (HRTEM) has confirmed the sharp junction interface between the core-shell heterojunction nanowire arrays. The valence band offset and conduction band offset at ZnO/CuO heterointerfaces are measured to be 2.4 ± 0.05 and 0.23 ± 0.005 eV respectively, using X-ray photoelectron spectroscopy (XPS) and a type-II band alignment structure is found. The results of this study contribute to the development of new advanced device heterostructures for solar energy conversion and optoelectronics applications.

Unoccupied electronic structure of Ni 2MnGa ferromagnetic shape memory alloy

DOE PAGES

Maniraj, M.; D׳Souza, S. W.; Rai, Abhishek; ...

2015-08-20

Momentum resolved inverse photoemission spectroscopy measurements show that the dispersion of the unoccupied bands of Ni 2MnGa is significant in the austenite phase. Furthermore, in the martensite phase, it is markedly reduced, which is possibly related to the structural transition to an incommensurate modulated state in the martensite phase. Finally, based on the first principle calculations of the electronic structure of Ni–Mn–Ga, we show that the modification of the spectral shape with surface composition is related to change in the hybridization between the Mn 3d and Ni 3d-like states that dominate the unoccupied conduction band.

Unoccupied electronic structure of Ni2MnGa ferromagnetic shape memory alloy

NASA Astrophysics Data System (ADS)

Maniraj, M.; D`Souza, S. W.; Rai, Abhishek; Schlagel, D. L.; Lograsso, T. A.; Chakrabarti, Aparna; Barman, S. R.

2015-11-01

Momentum resolved inverse photoemission spectroscopy measurements show that the dispersion of the unoccupied bands of Ni2MnGa is significant in the austenite phase. In the martensite phase, it is markedly reduced, which is possibly related to the structural transition to an incommensurate modulated state in the martensite phase. Based on the first principle calculations of the electronic structure of Ni-Mn-Ga, we show that the modification of the spectral shape with surface composition is related to change in the hybridization between the Mn 3d and Ni 3d-like states that dominate the unoccupied conduction band.

An all-perovskite p-n junction based on transparent conducting p -La 1-x Sr x CrO 3 epitaxial layers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Du, Yingge; Li, Chen; Zhang, Kelvin H. L.

2017-08-07

Transparent, conducting p -La 1-x Sr x CrO 3 epitaxial layers were deposited on Nb-doped SrTiO 3(001) by oxygen-assisted molecular beam epitaxy to form structurally coherent p-n junctions. X-ray photoelectron spectroscopy reveals a type II or “staggered” band alignment, with valence and conduction band offsets of 2.0 eV and 0.9 eV, respectively. Diodes fabricated from these heterojunctions exhibit rectifying behavior, and the I-V characteristics are different from those for traditional semiconductor p-n junctions. A rather large ideality factor is ascribed to the complex nature of the interface.

Design of triple-band polarization controlled terahertz metamaterial absorber

NASA Astrophysics Data System (ADS)

Wang, Ben-Xin; Xie, Qin; Dong, Guangxi; Huang, Wei-Qing

2018-02-01

A kind of triple-band polarization tunable terahertz absorber based on a metallic mirror and a metallic patch structure with two indentations spaced by an insulating medium layer is presented. Results prove that three near-perfect absorption peaks with average absorption coefficients of 98.25% are achieved when the polarization angle is equal to zero, and their absorptivities gradually decrease (and even disappear) by increasing the angle of polarization. When the polarization angle is increased to 90°, three new resonance modes with average absorption rates of 96.59% can be obtained. The field distributions are given to reveal the mechanisms of the triple-band absorption and the polarization tunable characteristics. Moreover, by introducing photosensitive silicon materials (its conductivity can be changed by the pump beam) in the indentations of the patch structure, the number of resonance peaks of the device can be actively tuned from triple-band to dual-band. The presented absorbers have potential applications, such as controlling thermal emissivity, and detection of polarization direction of the incident waves.

Effect of doping on electronic properties of HgSe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nag, Abhinav, E-mail: abhinavn76@gmail.com; Sastri, O. S. K. S., E-mail: sastri.osks@gmail.com; Kumar, Jagdish, E-mail: jagdishphysicist@gmail.com

2016-05-23

First principle study of electronic properties of pure and doped HgSe have been performed using all electron Full Potential Linearized Augmented Plane Wave (FP-LAPW) method using ELK code. The electronic exchange and co-relations are considered using Generalized Gradient Approach (GGA). Lattice parameter, Density of States (DOS) and Band structure calculations have been performed. The total energy curve (Energy vs Lattice parameter), DOS and band structure calculations are in good agreement with the experimental values and those obtained using other DFT codes. The doped material is studied within the Virtual Crystal Approximation (VCA) with doping levels of 10% to 25% ofmore » electrons (hole) per unit cell. Results predict zero band gap in undopedHgSe and bands meet at Fermi level near the symmetry point Γ. For doped HgSe, we found that by electron (hole) doping, the point where conduction and valence bands meet can be shifted below (above) the fermi level.« less

AB INITIO STUDY OF OPTOELECTRONIC PROPERTIES OF SPINEL ZnAl2O4 BEYOND GGA AND LDA

NASA Astrophysics Data System (ADS)

Yousaf, Masood; Saeed, M. A.; Isa, Ahmad Radzi Mat; Rahnamaye Aliabad, H. A.; Noor, N. A.

2012-12-01

Electronic band structure and optical parameters of ZnAl2O4 are investigated by first-principles technique based on a new potential approximation, known as modified Becke-Johnson (mBJ). This method describes the excited states of insulators and semiconductors more accurately The recent direct band gap result by EV-GGA is underestimated by about 15% compared to our band gap value using mBJ-GGA. The value of the band gap of ZnAl2O4 decreases as follows: Eg(mBJ-GGA/LDA) > Eg(GGA) > Eg(LDA). The band structure base optical parametric quantities (dielectric constant, index of refraction, reflectivity and optical conductivity) are also calculated, and their variations with energy range are discussed. The first critical point (optical absorption's edge) in ZnAl2O4 occurs at about 5.26 eV in case of mBJ. This study about the optoelectronic properties indicates that ZnAl2O4 can be used in optical devices.

Interplay of Cu and oxygen vacancy in optical transitions and screening of excitons in ZnO:Cu films

NASA Astrophysics Data System (ADS)

Darma, Yudi; Seng Herng, Tun; Marlina, Resti; Fauziah, Resti; Ding, Jun; Rusydi, Andrivo

2014-02-01

We study room temperature optics and electronic structures of ZnO:Cu films as a function of Cu concentration using a combination of spectroscopic ellipsometry, photoluminescence, and ultraviolet-visible absorption spectroscopy. Mid-gap optical states, interband transitions, and excitons are observed and distinguishable. We argue that the mid-gap states are originated from interactions of Cu and oxygen vacancy (Vo). They are located below conduction band (Zn4s) and above valence band (O2p) promoting strong green emission and narrowing optical band gap. Excitonic states are screened and its intensities decrease upon Cu doping. Our results show the importance of Cu and Vo driving the electronic structures and optical transitions in ZnO:Cu films.

A dual-band THz absorber based on graphene sheet and ribbons

NASA Astrophysics Data System (ADS)

Xing, Rui; Jian, Shuisheng

2018-03-01

A dual-band graphene absorber is proposed and investigated in this paper. The absorber consists of the gold substrate, the graphene sheet sandwiched by dielectric layers and the array of graphene ribbon placed on the top. The two absorption peaks of the dual-band are 99.8% at 4.95 THz and 99.6% at 9.2 THz, respectively. Due to the characteristic of tunable surface conductivity of graphene, the absorption can be controlled by adjusting the chemical potential of graphene. We also investigate the dependence of the absorption curve of the proposed absorber on the structure parameters. In addition, the structure of the absorber is very simple and it can be manufactured by chemical vapor deposition (CVD).

Tuning the Electronic and Optical Properties of Two-Dimensional Graphene-like C_2N Nanosheet by Strain Engineering

NASA Astrophysics Data System (ADS)

Phuc, Huynh V.; Tuan, Vu V.; Hieu, Nguyen N.; Ilyasov, Victor V.; Fedorov, Igor A.; Hoi, Bui D.; Phuong, Le T. T.; Hieu, Nguyen V.; Feddi, Elmustapha; Nguyen, Chuong V.

2018-05-01

Using density functional theory, we have studied the structural, electronic and optical properties of two-dimensional graphene-like C_2N nanosheet under in-plane strains. Our results indicate that the C_2N nanosheet is a semiconductor with a direct band gap of 1.70 eV at the equilibrium state opening between the highest valence band and lowest conduction band located at the Γ point. The band gap of the C_2N nanosheet decreases with the increasing of both uniaxial/biaxial strains. In the presence of the strain, we found band shift and band splitting of the occupied and unoccupied energy states of the valence and conduction bands, resulting in a decrease of the band gap. Furthermore, the absorption and reflectance spectra for the C_2N nanosheet have a broad peak around 2.6 eV, where a maximum absorption value is up to 3.2 × 10^{-5} cm^{-1} and reflectance is about 0.27%. Moreover, our calculations also show that the optical properties of the C_2N nanosheets can be controlled by applying the biaxial and uniaxial strains. The obtained results might provide potential applications for the C_2N nanosheets in nanoelectronics and optoelectronics.

Experimental and Theoretical Evidences of p-Type Conductivity in Nickel Carbodiimide Nanoparticles with a Delafossite Structure Type.

PubMed

Jiang, Tengfei; Polteau, Baptiste; Farré, Yoann; Cario, Laurent; Latouche, Camille; Pellegrin, Yann; Boujtita, Mohammed; Odobel, Fabrice; Tessier, Franck; Cheviré, François; Jobic, Stéphane

2017-07-17

Nickel carbodiimide (NiCN 2 ) was synthesized using a two-step precipitation-decomposition route leading to a brown powder with gypsum-flower-like morphology and a large specific surface area (75 m 2 /g). This layered material crystallizes in the 2H structure type of delafossite (space group P6 3 /mmc), which is built upon infinite 2 / ∞ [NiN 2 ] layers connected by linear carbodiimide ([N═C═N] 2- ) bridges. An X-ray diffraction Rietveld refinement and thermal analyses pointed out some nickel deficiencies in the material, and band structure calculations carried out on the defect compound predicted p-type conductivity in relation to a slight amount of N 2- . This p-type conductivity was demonstrated by electrochemical impedance spectroscopy measurements, and a flat band potential of 0.90 V vs SCE at pH 9.4 was measured. This value, which is more positive than those of CuGaO 2 and CuCrO 2 delafossite oxides and NiO, prompted us to test NiCN 2 nanoparticles as a photocathode in p-type dye-sensitized solar cells.

Hole conduction pathways in transparent amorphous tin oxides

NASA Astrophysics Data System (ADS)

Wahila, Matthew; Lebens-Higgins, Zachary; Quackenbush, Nicholas; Piper, Louis; Butler, Keith; Hendon, Christopher; Walsh, Aron; Watson, Graeme

P-type transparent amorphous oxide semiconductors (TAOS) have yet to be sufficiently demonstrated or commercialized, severely limiting the possible device architecture of transparent and flexible oxide electronics. The lack of p-type amorphous oxide candidates mainly originates from the directional oxygen 2 p character of their topmost valence states. Previous attempts to create p-type oxides have involved hybridization of the O 2 p with metal orbitals, such as with CuAlO2 and its Cu 3 d - O 2 p hybridization. However, the highly directional nature of the utilized orbitals means that structural disorder inhibits hybridization and severely disrupts hole-conduction pathways. Crystalline stannous oxide (SnO) and other lone-pair active post-transition metal oxides can have reduced localization at the valence band edge due to complex hybridization between the O 2 p, metal p, and spherical metal s-orbitals. I will discuss our investigation of structural disorder in SnO. Using a combination of synchrotron spectroscopy, and atomistic calculations, our investigation elucidates the important interplay between atomistic and electronic structure in establishing continuous hole conduction pathways at the valence band edge of transparent amorphous oxides.

Band gap bowing in NixMg1−xO

PubMed Central

Niedermeier, Christian A.; Råsander, Mikael; Rhode, Sneha; Kachkanov, Vyacheslav; Zou, Bin; Alford, Neil; Moram, Michelle A.

2016-01-01

Epitaxial transparent oxide NixMg1−xO (0 ≤ x ≤ 1) thin films were grown on MgO(100) substrates by pulsed laser deposition. High-resolution synchrotron X-ray diffraction and high-resolution transmission electron microscopy analysis indicate that the thin films are compositionally and structurally homogeneous, forming a completely miscible solid solution. Nevertheless, the composition dependence of the NixMg1−xO optical band gap shows a strong non-parabolic bowing with a discontinuity at dilute NiO concentrations of x < 0.037. Density functional calculations of the NixMg1−xO band structure and the density of states demonstrate that deep Ni 3d levels are introduced into the MgO band gap, which significantly reduce the fundamental gap as confirmed by optical absorption spectra. These states broaden into a Ni 3d-derived conduction band for x > 0.074 and account for the anomalously large band gap narrowing in the NixMg1−xO solid solution system. PMID:27503808

Semiconducting transition metal oxides.

PubMed

Lany, Stephan

2015-07-22

Open shell transition metal oxides are usually described as Mott or charge transfer insulators, which are often viewed as being disparate from semiconductors. Based on the premise that the presence of a correlated gap and semiconductivity are not mutually exclusive, this work reviews electronic structure calculations on the binary 3d oxides, so to distill trends and design principles for semiconducting transition metal oxides. This class of materials possesses the potential for discovery, design, and development of novel functional semiconducting compounds, e.g. for energy applications. In order to place the 3d orbitals and the sp bands into an integrated picture, band structure calculations should treat both contributions on the same footing and, at the same time, account fully for electron correlation in the 3d shell. Fundamentally, this is a rather daunting task for electronic structure calculations, but quasi-particle energy calculations in GW approximation offer a viable approach for band structure predictions in these materials. Compared to conventional semiconductors, the inherent multivalent nature of transition metal cations is more likely to cause undesirable localization of electron or hole carriers. Therefore, a quantitative prediction of the carrier self-trapping energy is essential for the assessing the semiconducting properties and to determine whether the transport mechanism is a band-like large-polaron conduction or a small-polaron hopping conduction. An overview is given for the binary 3d oxides on how the hybridization between the 3d crystal field symmetries with the O-p orbitals of the ligands affects the effective masses and the likelihood of electron and hole self-trapping, identifying those situations where small masses and band-like conduction are more likely to be expected. The review concludes with an illustration of the implications of the increased electronic complexity of transition metal cations on the defect physics and doping, using as an example the diversity of possible atomic and magnetic configurations of the O vacancy in TiO(2), and the high levels of hole doping in Co(2)ZnO(4) due to a self-doping mechanism that originates from the multivalence of Co.

Influence of Nb-doped TiO2 blocking layers as a cascading band structure for enhanced photovoltaic properties

NASA Astrophysics Data System (ADS)

Koo, Bon-Ryul; Oh, Dong-Hyeun; Ahn, Hyo-Jin

2018-03-01

Nb-doped TiO2 (Nb-TiO2) blocking layers (BLs) were developed using horizontal ultrasonic spray pyrolysis deposition (HUSPD). In order to improve the photovoltaic properties of the dye-sensitized solar cells (DSSCs), we optimized the Nb doping level of the Nb-TiO2 BLs by controlling the Nb/Ti molar ratio (0, 5, 6, and 7) of the precursor solution for HUSPD. Compared to bare TiO2 BLs, the Nb-TiO2 BLs formed a cascading band structure using the positive shift of the conduction band minimum of the Nb-TiO2 positioned between fluorine-doped tin oxide (FTO) and TiO2. This results in the increase of the potential current and the suppression of the electron recombination. Hence, it led to the improvement of the electrical conductivity, due to the increased electron concentration by the Nb doping into TiO2. Therefore, the DSSC fabricated with the Nb-TiO2 BLs at a Nb/Ti molar ratio of 6 showed superior photoconversion efficiency (∼7.50 ± 0.20%) as a result of the improved short-circuit current density. This is higher than those with the other Nb-TiO2 BLs and without BL. This improvement of the photovoltaic properties for the DSSCs can be attributed to the synergistic effects of uniform and compact BL relative to the prevention of the backward electron transport at the FTO/electrolyte interface, efficient electron transport at interfaces relative to a cascading band structure of FTO/Nb-TiO2/TiO2 multilayers and the facilitated electron transport at the BLs relative to the increased electrical conductivity of the optimized Nb-TiO2 BLs.

Excitation Dependent Phosphorous Property and New Model of the Structured Green Luminescence in ZnO

NASA Astrophysics Data System (ADS)

Ye, Honggang; Su, Zhicheng; Tang, Fei; Wang, Mingzheng; Chen, Guangde; Wang, Jian; Xu, Shijie

2017-02-01

The copper induced green luminescence (GL) with two sets of fine structures in ZnO crystal has been found for several decades (i.e., R. Dingle, Phys. Rev. Lett. 23, 579 (1969)), but the physical origin of the doublet still remains as an open question up to now. In this paper, we provide new insight into the mechanism of the structured GL band in terms of new experimental findings and theoretical calculations. It is found, for the first time, that the GL signal exhibits persistent afterglow for tens of minutes after the switch-off of below-band-gap excitation light but it cannot occur under above-band-gap excitation. Such a phosphorous property may be interpreted as de-trapping and feeding of electrons from a shallow trapping level via the conduction band to the Cu-related luminescence centers where the Cu3+ ion is proposed to work as the final state of the GL emission. From first-principles calculation, such a Cu3+ ion in wurtzite ZnO prefers a high spin 3d8 state with two non-degenerated half-filled orbitals due to the Jahn-Teller effect, probably leading to the double structures in photoluminescence spectrum. Therefore, this model gives a comprehensively new understanding on the mechanism of the structured GL band in ZnO.

Thermoelectric properties of p-type cubic and rhombohedral GeTe

DOE Office of Scientific and Technical Information (OSTI.GOV)

Xing, Guangzong; Sun, Jifeng; Li, Yuwei

Here, we investigate the electronic and thermoelectric properties of GeTe in both cubic and rhombohedral phases. We find that cubic GeTe has an electronic structure with a narrow band gap that is unfavorable at high temperature, where the cubic phase is normally stable. However, cubic GeTe has electronic features that may lead to p-type performance superior to the normal rhombohedral phase at lower temperature. This is explained in part by the combination of light and heavy band character that is very effective in obtaining high thermopower and conductivity. In addition, the valence band edge carrier pockets in cubic GeTe possessmore » the largest anisotropy among cubic IV-VI analogs. These effects are stronger than the effect of band convergence in the rhombohedral structure. The results suggest further study of stabilized cubic GeTe as a thermoelectric.« less

Thermoelectric properties of p-type cubic and rhombohedral GeTe

DOE PAGES

Xing, Guangzong; Sun, Jifeng; Li, Yuwei; ...

2018-05-21

Here, we investigate the electronic and thermoelectric properties of GeTe in both cubic and rhombohedral phases. We find that cubic GeTe has an electronic structure with a narrow band gap that is unfavorable at high temperature, where the cubic phase is normally stable. However, cubic GeTe has electronic features that may lead to p-type performance superior to the normal rhombohedral phase at lower temperature. This is explained in part by the combination of light and heavy band character that is very effective in obtaining high thermopower and conductivity. In addition, the valence band edge carrier pockets in cubic GeTe possessmore » the largest anisotropy among cubic IV-VI analogs. These effects are stronger than the effect of band convergence in the rhombohedral structure. The results suggest further study of stabilized cubic GeTe as a thermoelectric.« less

Electronic modification of Cu-based chalcopyrite semiconductors induced by lattice deformation and composition alchemy

NASA Astrophysics Data System (ADS)

Jiang, F. D.; Feng, J. Y.

2008-02-01

Using first principles calculation, we systematically investigate the electronic modification of Cu-based chalcopyrite semiconductors induced by lattice deformation and composition alchemy. It is shown that the optical band gap Eg is remarkably sensitive to the anion displacement μ, resulting from the opposite shifts of conduction band minimum and valence band maximum. Meanwhile, the dependence of structural parameters of alloyed compounds on alloy composition x is demonstrated for both cation and anion alloying. The d orbitals of group-III cations are found to be of great importance in the calculation. Abnormal changes in the optical band gap Eg induced by anion alloying are addressed.

Multicolor emission from intermediate band semiconductor ZnO 1-xSe x

DOE Office of Scientific and Technical Information (OSTI.GOV)

Welna, M.; Baranowski, M.; Linhart, W. M.

Photoluminescence and photomodulated reflectivity measurements of ZnOSe alloys are used to demonstrate a splitting of the valence band due to the band anticrossing interaction between localized Se states and the extended valence band states of the host ZnO matrix. A strong multiband emission associated with optical transitions from the conduction band to lower E - and upper E + valence subbands has been observed at room temperature. The composition dependence of the optical transition energies is well explained by the electronic band structure calculated using the kp method combined with the band anticrossing model. The observation of the multiband emissionmore » is possible because of relatively long recombination lifetimes. Longer than 1 ns lifetimes for holes photoexcited to the lower valence subband offer a potential of using the alloy as an intermediate band semiconductor for solar power conversion applications.« less

Multicolor emission from intermediate band semiconductor ZnO 1-xSe x

DOE PAGES

Welna, M.; Baranowski, M.; Linhart, W. M.; ...

2017-03-13

Photoluminescence and photomodulated reflectivity measurements of ZnOSe alloys are used to demonstrate a splitting of the valence band due to the band anticrossing interaction between localized Se states and the extended valence band states of the host ZnO matrix. A strong multiband emission associated with optical transitions from the conduction band to lower E - and upper E + valence subbands has been observed at room temperature. The composition dependence of the optical transition energies is well explained by the electronic band structure calculated using the kp method combined with the band anticrossing model. The observation of the multiband emissionmore » is possible because of relatively long recombination lifetimes. Longer than 1 ns lifetimes for holes photoexcited to the lower valence subband offer a potential of using the alloy as an intermediate band semiconductor for solar power conversion applications.« less

Bands dispersion and charge transfer in β-BeH2

NASA Astrophysics Data System (ADS)

Trivedi, D. K.; Galav, K. L.; Joshi, K. B.

2018-04-01

Predictive capabilities of ab-initio method are utilised to explore bands dispersion and charge transfer in β-BeH2. Investigations are carried out using the linear combination of atomic orbitals method at the level of density functional theory. The crystal structure and related parameters are settled by coupling total energy calculations with the Murnaghan equation of state. Electronic bands dispersion from PBE-GGA is reported. The PBE-GGA, and PBE0 hybrid functional, show that β-BeH2 is a direct gap semiconductor with 1.18 and 2.40 eV band gap. The band gap slowly decreases with pressure and beyond l00 GPa overlap of conduction and valence bands at the r point is observed. Charge transfer is studied by means of Mullikan population analysis.

One-Dimensional Nature of InAs/InP Quantum Dashes Revealed by Scanning Tunneling Spectroscopy.

PubMed

Papatryfonos, Konstantinos; Rodary, Guillemin; David, Christophe; Lelarge, François; Ramdane, Abderrahim; Girard, Jean-Christophe

2015-07-08

We report on low-temperature cross-sectional scanning tunneling microscopy and spectroscopy on InAs(P)/InGaAsP/InP(001) quantum dashes, embedded in a diode-laser structure. The laser active region consists of nine InAs(P) quantum dash layers separated by the InGaAsP quaternary alloy barriers. The effect of the p-i-n junction built-in potential on the band structure has been evidenced and quantified on large-scale tunneling spectroscopic measurements across the whole active region. By comparing the tunneling current onset channels, a consistent energy shift has been measured in successive quantum dash or barrier layers, either for the ground state energy of similar-sized quantum dashes or for the conduction band edge of the barriers, corresponding to the band-bending slope. The extracted values are in good quantitative agreement with the theoretical band structure calculations, demonstrating the high sensitivity of this spectroscopic measurement to probe the electronic structure of individual nanostructures, relative to local potential variations. Furthermore, by taking advantage of the potential gradient, we compared the local density of states over successive quantum dash layers. We observed that it does not vanish while increasing energy, for any of the investigated quantum dashes, in contrast to what would be expected for discrete level zero-dimensional (0D) structures. In order to acquire further proof and fully address the open question concerning the quantum dash dimensionality nature, we focused on individual quantum dashes obtaining high-energy-resolution measurements. The study of the local density of states clearly indicates a 1D quantum-wirelike nature for these nanostructures whose electronic squared wave functions were subsequently imaged by differential conductivity mapping.

Electrical conduction at domain walls in multiferroic BiFeO3

NASA Astrophysics Data System (ADS)

Seidel, Jan; Martin, Lane; He, Qing; Zhan, Qian; Chu, Ying-Hao; Rother, Axel; Hawkridge, Michael; Maksymovych, Peter; Yu, Pu; Gajek, Martin; Balke, Nina; Kalinin, Sergei; Gemming, Sybille; Wang, Feng; Catalán, Gustau; Scott, James; Spaldin, Nicola; Orenstein, Joseph; Ramesh, Ramamoorthy

2009-03-01

We report the observation of room temperature electronic conductivity at ferroelectric domain walls in BiFeO3. The origin and nature of the observed conductivity is probed using a combination of conductive atomic force microscopy, high resolution transmission electron microscopy and first-principles density functional computations. We show that a structurally driven change in both the electrostatic potential and local electronic structure (i.e., a decrease in band gap) at the domain wall leads to the observed electrical conductivity. We estimate the conductivity in the wall to be several orders of magnitude higher than for the bulk material. Additionally we demonstrate the potential for device applications of such conducting nanoscale features.

Investigation of thermoelectricity in KScSn half-Heusler compound

NASA Astrophysics Data System (ADS)

Shrivastava, Deepika; Acharya, Nikita; Sanyal, Sankar P.

2018-05-01

The electronic and transport properties of KScSn half-Heusler (HH) compound have been investigated using first-principles density functional theory and semi classical Boltzmann transport theory. The electronic band structure and density of states (total and partial) show semiconducting nature of KScSn with band gap 0.48 eV which agree well with previously reported results. The transport coefficient such as electrical conductivity, Seebeck coefficient, electronic thermal conductivity and power factor as a function of chemical potential are evaluated. KScSn has high power factor for p-type doping and is a potential candidate for thermoelectric applications.

A high-performance wave guide cryogenic thermal break

NASA Astrophysics Data System (ADS)

Melhuish, S. J.; McCulloch, M. A.; Piccirillo, L.; Stott, C.

2016-10-01

We describe a high-performance wave guide cryogenic thermal break. This has been constructed both for Ka band, using WR28 wave guide, and Q band, using WR22 wave guide. The mechanical structure consists of a hexapod (Stewart platform) made from pultruded carbon fibre tubing. We present a tentative examination of the cryogenic Young's modulus of this material. The thermal conductivity is measured at temperatures above the range explored by Runyan and Jones, resulting in predicted conductive loads through our thermal breaks of 3.7 mW to 3 K and 17 μK to 1 K.

Structural defects and recombination behavior of excited carriers in Cu(In,Ga)Se2 solar cells

NASA Astrophysics Data System (ADS)

Yang, J.; Du, H. W.; Li, Y.; Gao, M.; Wan, Y. Z.; Xu, F.; Ma, Z. Q.

2016-08-01

The carriers' behavior in neutral region (NTR) and space charged region (SCR) of Cu(In,Ga)Se2 thin film based solar cells has been investigated by temperature dependent photoluminescence (PL-T), electroluminescence (EL-T) and current-voltage (IV-T) from 10 to 300 K. PL-T spectra show that three kinds of defects, namely VSe, InCu and (InCu+VCu), are localized within the band gap of NTR and SCR of CIGS layer, corresponding to the energy levels of EC-0.08, EC-0.20 and EC-0.25 eV, respectively. The InCu and (InCu+VCu) deep level defects are non-radiative recombination centers at room temperature. The IV-T and EL-T analysis reveals that the injection modes of electrons from ZnO conduction band into Cu(In,Ga)Se2 layer are tunneling, thermally-excited tunneling and thermionic emission under 10-40, 60-160, and 180-300 K, respectively. At 10-160 K, the electrons tunnel into (InCu+VCu) and Vse defect levels in band gap of SCR and the drifting is involved in the emission bands at 0.96 and 1.07 eV, which is the direct evidence for a tunneling assisted recombination. At 180-300 K, the electrons are directly injected into the Cu(In,Ga)Se2 conduction band, and the emission of 1.13 eV are ascribed to the transitions from the conduction band to the valence band.

Structure and properties of a model conductive filament/host oxide interface in HfO2-based ReRAM

NASA Astrophysics Data System (ADS)

Padilha, A. C. M.; McKenna, K. P.

2018-04-01

Resistive random-access memory (ReRAM) is a promising class of nonvolatile memory capable of storing information via its resistance state. In the case of hafnium oxide-based devices, experimental evidence shows that a conductive oxygen-deficient filament is formed and broken inside of the device by oxygen migration, leading to switching of its resistance state. However, little is known about the nature of this conductive phase, its interface with the host oxide, or the associated interdiffusion of oxygen, presenting a challenge to understanding the switching mechanism and device properties. To address these problems, we present atomic-scale first-principles simulations of a prototypical conductive phase (HfO), the electronic properties of its interface with HfO2, as well as stability with respect to oxygen diffusion across the interface. We show that the conduction-band offset between HfO and HfO2 is 1.3 eV, smaller than typical electrode-HfO2 band offsets, suggesting that positive charging and band bending should occur at the conductive filament-HfO2 interface. We also show that transfer of oxygen across the interface, from HfO2 into HfO, costs around 1.2 eV per atom and leads to a gradual opening of the HfO band gap, and hence disruption of the electrical conductivity. These results provide invaluable insights into understanding the switching mechanism for HfO2-based ReRAM.

Electron doping through lithium intercalation to interstitial channels in tetrahedrally bonded SiC

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sakai, Yuki; Center for Computational Materials, Institute for Computational Engineering and Sciences, The University of Texas at Austin, Austin, Texas 78712; Oshiyama, Atsushi

2015-11-07

We report on first-principles calculations that clarify the effect of lithium atom intercalation into zinc blende 3C-silicon carbide (3C-SiC) on electronic and structural properties. Lithium atoms inside 3C-SiC are found to donate electrons to 3C-SiC that is an indication of a new way of electron doping through the intercalation. The electrons doped into the conduction band interact with lithium cations and reduce the band spacing between the original valence and conduction bands. We have also found that a silicon monovacancy in 3C-SiC promotes the lithium intercalation, showing that the vacancy generation makes SiC as a possible anode material for lithium-ionmore » battery.« less

First-principles study on half-metallic ferromagnetic properties of Zn1- x V x Se ternary alloys

NASA Astrophysics Data System (ADS)

Khatta, Swati; Tripathi, S. K.; Prakash, Satya

2017-09-01

The spin-polarised density functional theory along with self-consistent plane-wave pseudopotential is used to investigate the half-metallic ferromagnetic properties of ternary alloys Zn1- x V x Se. The generalized gradient approximation is used for exchange-correlation potential. The equilibrium lattice constants, bulk modulus, and its derivatives are calculated. The calculated spin-polarised energy-band structures reveal that these alloys are half-metallic for x = 0.375 and 0.50 and nearly half-metallic for other values of x. The estimated direct and indirect bandgaps may be useful for the magneto-optical absorption experiments. It is found that there is strong Zn 4s, Se 4p, and V 3d orbital hybridization in the conduction bands of both the spins, while Se 4p and V 3d orbital hybridization predominates in the valence bands of both the spins. The s, p-d, and p-d orbital hybridization reduces the local magnetic moment of V atoms and small local magnetic moments are produced on Zn and Se atoms which get coupled with V atoms in ferromagnetic and antiferromagnetic phases, respectively. The conduction and valence-band-edge splittings and exchange constants predict the ferromagnetism in these alloys. The conduction band-impurity (s and p-d) exchange interaction is more significant for ferromagnetism in these alloys than the valence band-impurity (p-d) exchange interaction.

Electronic structure and linear optical properties of ZnSe and ZnSe:Mn.

PubMed

Su, Kang; Wang, Yuhua

2010-03-01

As an important wide band-gap II-VI semiconductor, ZnSe has attracted much attention for its various applications in photo-electronic devices such as blue light-emitting diodes and blue-green diode lasers. Mn-doped ZnSe is an excellent quantum dot material. The electronic structures of the sphalerite ZnSe and ZnSe:Mn were calculated using the Vienna ab initio Simulation Package with ultra-soft pseudo potentials and Material Studio. The calculated equilibrium lattice constants agree well with the experimental values. Using the optimized equilibrium lattice constants, the densities of states and energy band structures were further calculated. By analyzing the partial densities of states, the contributions of different electron states in different atoms were estimated. The p states of Zn mostly contribute to the top of the valence band, and the s states of Zn and the s states of Se have major effects on the bottom of the conduction band. The calculated results of ZnSe:Mn show the band gap was changed from 2.48 to 1.1 eV. The calculated linear optical properties, such as refractive index and absorption spectrum, are in good agreement with experimental values.

Valence-band-edge shift due to doping in p + GaAs

NASA Astrophysics Data System (ADS)

Silberman, J. A.; de Lyon, T. J.; Woodall, J. M.

1991-05-01

Accurate knowledge of the shifts in valence- and conduction-band edges due to heavy doping effects is crucial in modeling GaAs device structures that utilize heavily doped layers. X-ray photoemission spectroscopy was used to deduce the shift in the valence-band-edge induced by carbon (p type) doping to a carrier density of 1×1020 cm-3 based on a determination of the bulk binding energy of the Ga and As core levels in this material. Analysis of the data indicates that the shift of the valence-band maximum into the gap and the penetration of the Fermi level into the valence bands exactly compensate at this degenerate carrier concentration, to give ΔEv =0.12±0.05 eV.

Spectroscopic evidence for temperature dependent relative movement of light and heavy hole valence bands of PbQ (Q=Te,Se,S)

NASA Astrophysics Data System (ADS)

Chatterjee, Utpal; Zhao, Junjing; Kanatzidis, Mercouri; Malliakas, Christos

We have conducted temperature dependent Angle Resolved Photoemission Spectroscopy (ARPES) studies of the electronic structures of PbTe, PbSe and PbS. Our ARPES measurements provide direct evidences for the light hole upper valence bands (UVBs) and the so-called heavy hole lower valence bands (LVBs), and an unusual temperature dependent relative movement between their band maxima leading to a monotonic decrease in the energy separation between LVBs and UVBs with increase in temperature. This enables convergence of these valence bands and consequently an effective increase in the valley degeneracy in PbQ at higher temperatures, which has long been believed to be the driving factor behind their extraordinary thermoelectric performance.

Multiwalled carbon nanotubes and dispersed nanodiamond novel hybrids: Microscopic structure evolution, physical properties, and radiation resilience

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gupta, S.; Farmer, J.

2011-01-01

We report the structure and physical properties of novel hybrids of multiwalled carbon nanotubes (MWCNTs) and ultradispersed diamond (UDD) forming nanocomposite ensemble that were subjected to 50, 100, and 10{sup 3} kGy gamma ray doses and characterized using various analytical tools to investigate hierarchical defects evolution. This work is prompted by recent work on single-walled CNTs and UDD ensemble [Gupta et al., J. Appl. Phys. 107, 104308 (2010)] where radiation-induced microscopic defects seem to be stabilized by UDD. The present experiments show similar effects where these hybrids display only a minimal structural modification under the maximum dose. Quantitative analyses ofmore » multiwavelength Raman spectra revealed lattice defects induced by irradiation assessed through the variation in prominent D, G, and 2D bands. A minimal change in the position of D, G, and 2D bands and a marginal increase in intensity of the defect-induced double resonant Raman scattered D and 2D bands are some of the implications suggesting the radiation coupling. The in-plane correlation length (L{sub a}) was also determined following Tunistra-Koenig relation from the ratio of D to G band (I{sub D}/I{sub G}) besides microscopic stress. However, we also suggest the following taking into account of intrinsic defects of the constituents: (a) charge transfer arising at the interface due to the difference in electronegativity of MWCNT C sp{sup 2} and UDD core (C sp{sup 3}) leading to phonon and electron energy renormalization; (b) misorientation of C sp{sup 2} at the interface of MWCNT and UDD shell (C sp{sup 2}) resulting in structural disorder; (c) softening or violation of the q{approx}0 selection rule leading to D band broadening and a minimal change in G band intensity; and (d) normalized intensity of D and G bands with 2D band help to distinguish defect-induced double resonance phenomena. The MWCNT when combined with nanodiamond showed a slight decrease in their conductance further affected by irradiation pointing at relatively good interfacial contact. Furthermore, owing to high thermal and electrical conductivity properties, they can facilitate potentially efficient heat-transfer applications and some results deduced using Nielsen's model is provided.« less

Probing the band structure and local electronic properties of low-dimensional semiconductor structures

NASA Astrophysics Data System (ADS)

Walrath, Jenna Cherie

Low-dimensional semiconductor structures are important for a wide variety of applications, and recent advances in nanoscale fabrication are paving the way for increasingly precise nano-engineering of a wide range of materials. It is therefore essential that the physics of materials at the nanoscale are thoroughly understood to unleash the full potential of nanotechnology, requiring the development of increasingly sophisticated instrumentation and modeling. Of particular interest is the relationship between the local density of states (LDOS) of low-dimensional structures and the band structure and local electronic properties. This dissertation presents the investigation of the band structure, LDOS, and local electronic properties of nanostructures ranging from zero-dimensional (0D) quantum dots (QDs) to two-dimensional (2D) thin films, synthesizing computational and experimental approaches including Poisson-Schrodinger band structure calculations, scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and scanning thermoelectric microscopy (SThEM). A method is presented for quantifying the local Seebeck coefficient (S) with SThEM, using a quasi-3D conversion matrix approach to directly convert temperature gradient-induced voltages S. For a GaAs p-n junction, the resulting S-profile is consistent with that computed using the free carrier concentration profile. This combined computational-experimental approach is expected to enable nanoscale measurements of S across a wide variety of heterostructure interfaces. The local carrier concentration, n, is profiled across epitaxial InAs/GaAs QDs, where SThEM is used to profile the temperature gradient-induced voltage, which is converted to a profile of the local S and finally to an n profile. The S profile is converted to a conduction band-edge profile and compared with Poisson-Schrodinger band-edge simulations. The combined computational-experimental approach suggests a reduced n in the QD center in comparison to that of the 2D alloy layer. The surface composition and band structure of ordered horizontal Sb2Te3 nanowires induced by femtosecond laser irradiation of a thin film are investigated, revealing a band gap modulation between buried Sb2Te3 nanowires and the surrounding insulating material. Finally, STM and STS are used to investigate the band structure of BiSbTe alloys at room temperature, revealing both the Fermi level and Dirac point located inside the bulk bandgap, indicating bulk-like insulating behavior with accessible surface states.

Conduction band position tuning and Ga-doping in (Cd,Zn)S alloy thin films

DOE PAGES

Baranowski, Lauryn L.; Christensen, Steven; Welch, Adam W.; ...

2017-02-13

In recent years, the number of novel photovoltaic absorber materials under exploration has rapidly increased. However, to reap the most benefit from these new absorbers, alternative device structures and components must also be considered. In particular, the choice of a heterojunction partner, or contact layer, is critical to device optimization. In this work, we explore alternative n-type contact layer candidates that could be widely applicable to a variety of new absorbers. We use theory to calculate the band edge tuning provided by a variety of II-VI alloy systems, and select the (Cd,Zn)S system as one that affords a wide rangemore » of conduction band tuning. The synthesis of (Cd,Zn)S alloys is explored using atomic layer deposition, which afforded precise compositional control and produced crystalline thin films. The predicted tuning of the band gap and conduction band minimum is confirmed through X-ray photoelectron spectroscopy and optical absorption measurements. In addition, we investigated Ga-doping in Cd 0.6Zn 0.4S films to decrease their series resistance when used as contact layers in photovoltaic devices. In conclusion, this study provides a framework for exploring and optimizing alternative contact layer materials, which will prove critical to the success of new PV absorbers.« less

p × n-Type Transverse Thermoelectrics: A Novel Type of Thermal Management Material

NASA Astrophysics Data System (ADS)

Tang, Yang; Cui, Boya; Zhou, Chuanle; Grayson, Matthew

2015-06-01

In this paper we review the recently identified p × n-type transverse thermoelectrics and study the thermoelectric properties of the proposed candidate materials. Anisotropic electron and hole conductivity arise from either an artificially engineered band structure or from appropriately anisotropic crystals, and result in orthogonal p-type and n-type directional Seebeck coefficients, inducing a non-zero off-diagonal transverse Seebeck coefficient with appropriately oriented currents. Such materials have potential for new applications of thermoelectric materials in transverse Peltier cooling and transverse thermal energy harvesting. In this paper we review general transverse thermoelectric phenomena to identify advantages of p × n-type transverse thermoelectrics compared with previously studied transverse thermoelectric phenomena. An intuitive overview of the band structure of one such p × n-material, the InAs/GaSb type-II superlattice, is introduced, and the plot of thermoelectric performance as a function of superlattice structure is calculated, as an example of how band structures can be optimized for the best transverse thermoelectric performance.

Ab initio study on half-metallic, electronic and thermodynamic attributes of LaFeO3

NASA Astrophysics Data System (ADS)

Tariq, Saad; Saad, Saher; Jamil, M. Imran; Sohail Gilani, S. M.; Mahmood Ramay, Shahid; Mahmood, Asif

2018-03-01

By using the density functional theory (DFT) the systematic study of the structural, electronic and thermodynamic properties of lanthanum ferrite (LaFeO3) has been conducted. The elastic stability criterion and structural tolerance factor reveal that LaFeO3 exists in the cubic phase and is found to be stable under the ambient conditions. In electronic properties, the optical spectrum of the compound has been found to fall in the range of 488 to 688nm which has been calculated from the electronic band gap values by using the PBE-GGA and mBJ-GGA techniques. The light between 488 to 688nm would cause the valence electrons to jump in the conduction band showing the photoconductivity. The pronounced half-metallic character has been discussed by using the projected electronic density of states. The ferromagnetic response has been observed which may be attributed to the Fe-O bonding situation. The compound exhibits ductile, indirect band gap and half-metallic traits in the bulk phase. We expect the compound to be felicitous for the novel spintronic applications.

Optical nonlinear properties and dynamics of interband transitions in multilayer MoS2 structures under femtosecond excitation at a wavelength of 514 nm

NASA Astrophysics Data System (ADS)

Khudyakov, D. V.; Borodkin, A. A.; Mazin, D. D.; Lobach, A. S.; Vartapetov, S. K.

2018-02-01

The optical nonlinear absorption and bleaching of aqueous suspensions of multilayer MoS2 sheets (structural modification 2H) under excitation by a 400-fs pulse at a wavelength of 514 nm is investigated using longitudinal scanning. The sample exhibits nonlinear absorption at intensities up to 15 GW cm-2, while a further increase in intensity to 70 GW cm-2 causes nonlinear bleaching with a relative change in transmission to 14%. The dynamics of interband transitions in the picosecond range is studied by femtosecond laser photolysis. The relaxation time of photoexcited excitons is measured to be 20 ± 2 ps. The transition dynamics is calculated in the three-level approximation, and the absorption cross sections of photoinduced electron transitions from the valence band to the conduction band and from the first to the second conduction band are estimated. It is shown that the optical nonlinear properties of suspensions of multilayer 2H MoS2 sheets are mainly determined by the dynamics of single-photon interband transitions.

Electronic and Thermoelectric Properties of Ternary Chalcohalide Semiconductors: First Principles Study

NASA Astrophysics Data System (ADS)

Khan, Wilayat; Hussain, Sajjad; Minar, Jan; Azam, Sikander

2018-02-01

Ternary chalcohalides have been widely utilized for different device applications. The thermoelectric properties of SbSI, SbSeI and SbSBr have been investigated by theoretical simulations, and the findings have been performed using BoltzTraP code, based on semi-classical Boltzmann transport theory. In this study, we simulated the electronic structures using the Englo-Vosko generalized gradient approximation employed in the WIEN2k program. From the electronic band structures, we found a combination of light and heavy bands around the Fermi level in the valence band, which strongly affect the effective masses of the carriers. The entire thermoelectric parameters, like the electrical, the electronic part of the thermal conductivities, the Seebeck coefficient and the power factor have been analysed as functions of temperature and chemical potential. The correlation between the effective masses and the thermoelectric properties is also included in the discussion because the effective mass reveals the mobility of the carriers which in turn affect the thermoelectric properties. The substitution of sulfur reveals high electrical conductivity and a smaller Seebeck coefficient based on effective mass leads to the increase in the power factor.

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

NASA Astrophysics Data System (ADS)

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

2017-04-01

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

Optical and electronic properties of doped p -type CuI: Explanation of transparent conductivity from first principles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, Yuwei; Sun, Jifeng; Singh, David J.

In this paper, we report the properties of the reported transparent conductor CuI, including the effect of heavy p-type doping. The results, based on first-principles calculations, include an analysis of the electronic structure and calculations of optical and dielectric properties. We find that the origin of the favorable transparent conducting behavior lies in the absence in the visible of strong interband transitions between deeper valence bands and states at the valence-band maximum that become empty with p-type doping. Instead, strong interband transitions to the valence-band maximum are concentrated in the infrared with energies below 1.3 eV. This is contrast tomore » the valence bands of many wide-band-gapmaterials. Turning to the mobility,we find that the states at the valence-band maximum are relatively dispersive. This originates from their antibonding Cu d–I p character. We find a modest enhancement of the Born effective charges relative to nominal values, leading to a dielectric constant ε(0) = 6.3. This is sufficiently large to reduce ionized impurity scattering, leading to the expectation that the properties of CuI can still be significantly improved through sample quality.« less

Optical and electronic properties of doped p -type CuI: Explanation of transparent conductivity from first principles

DOE PAGES

Li, Yuwei; Sun, Jifeng; Singh, David J.

2018-03-26

In this paper, we report the properties of the reported transparent conductor CuI, including the effect of heavy p-type doping. The results, based on first-principles calculations, include an analysis of the electronic structure and calculations of optical and dielectric properties. We find that the origin of the favorable transparent conducting behavior lies in the absence in the visible of strong interband transitions between deeper valence bands and states at the valence-band maximum that become empty with p-type doping. Instead, strong interband transitions to the valence-band maximum are concentrated in the infrared with energies below 1.3 eV. This is contrast tomore » the valence bands of many wide-band-gapmaterials. Turning to the mobility,we find that the states at the valence-band maximum are relatively dispersive. This originates from their antibonding Cu d–I p character. We find a modest enhancement of the Born effective charges relative to nominal values, leading to a dielectric constant ε(0) = 6.3. This is sufficiently large to reduce ionized impurity scattering, leading to the expectation that the properties of CuI can still be significantly improved through sample quality.« less

Direct evidence of hidden local spin polarization in a centrosymmetric superconductor LaO0.55 F0.45BiS2.

PubMed

Wu, Shi-Long; Sumida, Kazuki; Miyamoto, Koji; Taguchi, Kazuaki; Yoshikawa, Tomoki; Kimura, Akio; Ueda, Yoshifumi; Arita, Masashi; Nagao, Masanori; Watauchi, Satoshi; Tanaka, Isao; Okuda, Taichi

2017-12-04

Conventional Rashba spin polarization is caused by the combination of strong spin-orbit interaction and spatial inversion asymmetry. However, Rashba-Dresselhaus-type spin-split states are predicted in the centrosymmetric LaOBiS 2 system by recent theory, which stem from the local inversion asymmetry of active BiS 2 layer. By performing high-resolution spin- and angle-resolved photoemission spectroscopy, we have investigated the electronic band structure and spin texture of superconductor LaO 0.55 F 0.45 BiS 2 . Here we present direct spectroscopic evidence for the local spin polarization of both the valence band and the conduction band. In particular, the coexistence of Rashba-like and Dresselhaus-like spin textures has been observed in the conduction band. The finding is of key importance for fabrication of proposed dual-gated spin-field effect transistor. Moreover, the spin-split band leads to a spin-momentum locking Fermi surface from which superconductivity emerges. Our demonstration not only expands the scope of spintronic materials but also enhances the understanding of spin-orbit interaction-related superconductivity.

Hubbard physics in the PAW GW approximation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Booth, J. M., E-mail: jamie.booth@rmit.edu.au; Smith, J. S.; Russo, S. P.

It is demonstrated that the signatures of the Hubbard Model in the strongly interacting regime can be simulated by modifying the screening in the limit of zero wavevector in Projector-Augmented Wave GW calculations for systems without significant nesting. This modification, when applied to the Mott insulator CuO, results in the opening of the Mott gap by the splitting of states at the Fermi level into upper and lower Hubbard bands, and exhibits a giant transfer of spectral weight upon electron doping. The method is also employed to clearly illustrate that the M{sub 1} and M{sub 2} forms of vanadium dioxidemore » are fundamentally different types of insulator. Standard GW calculations are sufficient to open a gap in M{sub 1} VO{sub 2}, which arise from the Peierls pairing filling the valence band, creating homopolar bonds. The valence band wavefunctions are stabilized with respect to the conduction band, reducing polarizability and pushing the conduction band eigenvalues to higher energy. The M{sub 2} structure, however, opens a gap from strong on-site interactions; it is a Mott insulator.« less

Work Function Engineering of Graphene

PubMed Central

Garg, Rajni; Dutta, Naba K.; Roy Choudhury, Namita

2014-01-01

Graphene is a two dimensional one atom thick allotrope of carbon that displays unusual crystal structure, electronic characteristics, charge transport behavior, optical clarity, physical & mechanical properties, thermal conductivity and much more that is yet to be discovered. Consequently, it has generated unprecedented excitement in the scientific community; and is of great interest to wide ranging industries including semiconductor, optoelectronics and printed electronics. Graphene is considered to be a next-generation conducting material with a remarkable band-gap structure, and has the potential to replace traditional electrode materials in optoelectronic devices. It has also been identified as one of the most promising materials for post-silicon electronics. For many such applications, modulation of the electrical and optical properties, together with tuning the band gap and the resulting work function of zero band gap graphene are critical in achieving the desired properties and outcome. In understanding the importance, a number of strategies including various functionalization, doping and hybridization have recently been identified and explored to successfully alter the work function of graphene. In this review we primarily highlight the different ways of surface modification, which have been used to specifically modify the band gap of graphene and its work function. This article focuses on the most recent perspectives, current trends and gives some indication of future challenges and possibilities. PMID:28344223

Work Function Engineering of Graphene.

PubMed

Garg, Rajni; Dutta, Naba K; Choudhury, Namita Roy

2014-04-03

Graphene is a two dimensional one atom thick allotrope of carbon that displays unusual crystal structure, electronic characteristics, charge transport behavior, optical clarity, physical & mechanical properties, thermal conductivity and much more that is yet to be discovered. Consequently, it has generated unprecedented excitement in the scientific community; and is of great interest to wide ranging industries including semiconductor, optoelectronics and printed electronics. Graphene is considered to be a next-generation conducting material with a remarkable band-gap structure, and has the potential to replace traditional electrode materials in optoelectronic devices. It has also been identified as one of the most promising materials for post-silicon electronics. For many such applications, modulation of the electrical and optical properties, together with tuning the band gap and the resulting work function of zero band gap graphene are critical in achieving the desired properties and outcome. In understanding the importance, a number of strategies including various functionalization, doping and hybridization have recently been identified and explored to successfully alter the work function of graphene. In this review we primarily highlight the different ways of surface modification, which have been used to specifically modify the band gap of graphene and its work function. This article focuses on the most recent perspectives, current trends and gives some indication of future challenges and possibilities.

Hole effective masses and subband splitting in type-II superlattice infrared detectors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ting, David Z., E-mail: David.Z.Ting@jpl.nasa.gov; Soibel, Alexander; Gunapala, Sarath D.

We explore band structure effects to help determine the suitability of n-type type-II superlattice (T2SL) absorbers for infrared detectors. It is often assumed that the exceedingly large growth-direction band-edge curvature hole effective mass in n-type long wavelength infrared (LWIR) T2SL would lead to low hole mobility and therefore low detector collection quantum efficiency. We computed the thermally averaged conductivity effective mass and show that the LWIR T2SL hole conductivity effective mass along the growth direction can be orders of magnitude smaller than the corresponding band-edge effective mass. LWIR InAs/GaSb T2SL can have significantly smaller growth-direction hole conductivity effective mass thanmore » its InAs/InAsSb counterpart. For the InAs/InAsSb T2SL, higher Sb fraction is more favorable for hole transport. Achieving long hole diffusion length becomes progressively more difficult for the InAs/InAsSb T2SL as the cutoff wavelength increases, since its growth-direction hole conductivity effective mass increases significantly with decreasing band gap. However, this is mitigated by the fact that the splitting between the top valence subbands also increases with the cutoff wavelength, leading to reduced inter-subband scattering and increased relaxation time.« less

Electrical transport and optical band gap of NiFe2Ox thin films

NASA Astrophysics Data System (ADS)

Bougiatioti, Panagiota; Manos, Orestis; Klewe, Christoph; Meier, Daniel; Teichert, Niclas; Schmalhorst, Jan-Michael; Kuschel, Timo; Reiss, Günter

2017-12-01

We fabricated NiFe2Ox thin films on MgAl2O4(001) by reactive dc magnetron co-sputtering varying the oxygen partial pressure. The fabrication of a material with a variable oxygen deficiency leads to controllable electrical and optical properties which are beneficial for the investigations of the transport phenomena and could, therefore, promote the use of such materials in spintronic and spin caloritronic applications. We used several characterization techniques to investigate the film properties, focusing on their structural, magnetic, electrical, and optical properties. From the electrical resistivity, we obtained the conduction mechanisms that govern the systems in the high and low temperature regimes. We further extracted low thermal activation energies which unveil extrinsic transport mechanisms. The thermal activation energy decreases in the less oxidized samples revealing the pronounced contribution of a large amount of electronic states localized in the band gap to the electrical conductivity. The Hall coefficient is negative and decreases with increasing conductivity as expected for n-type conduction, while the Hall- and the drift mobilities show a large difference. The optical band gaps were determined via ultraviolet-visible spectroscopy. They follow a similar trend as the thermal activation energies, with lower band gap values in the less oxidized samples.

Ab initio study of II-(VI)2 dichalcogenides.

PubMed

Olsson, P; Vidal, J; Lincot, D

2011-10-12

The structural stabilities of the (Zn,Cd)(S,Se,Te)(2) dichalcogenides have been determined ab initio. These compounds are shown to be stable in the pyrite phase, in agreement with available experiments. Structural parameters for the ZnTe(2) pyrite semiconductor compound proposed here are presented. The opto-electronic properties of these dichalcogenide compounds have been calculated using quasiparticle GW theory. Bandgaps, band structures and effective masses are proposed as well as absorption coefficients and refraction indices. The compounds are all indirect semiconductors with very flat conduction band dispersion and high absorption coefficients. The work functions and surface properties are predicted. The Te and Se based compounds could be of interest as absorber materials in photovoltaic applications.

A soft X-ray spectroscopic perspective of electron localization and transport in tungsten doped bismuth vanadate single crystals.

PubMed

Jovic, Vedran; Rettie, Alexander J E; Singh, Vijay R; Zhou, Jianshi; Lamoureux, Bethany; Buddie Mullins, C; Bluhm, Hendrik; Laverock, Jude; Smith, Kevin E

2016-11-23

Doped BiVO 4 is a promising photoelectrochemical water splitting anode, whose activity is hampered by poor charge transport. Here we use a set of X-ray spectroscopic methods to probe the origin and nature of localized electron states in W:BiVO 4 . Furthermore, using the polarized nature of the X-rays, we probe variations in the electronic structure along the crystal axes. In this manner, we reveal aspects of the electronic structure related to electron localization and observations consistent with conductivity anisotropy between the ab-plane and c-axis. We verify that tungsten substitutes as W 6+ for V 5+ in BiVO 4 . This is shown to result in the presence of inter-band gap states related to electrons at V 4+ sites of e symmetry. The energetic position of the states in the band gap suggest that they are highly localized and may act as recombination centres. Polarization dependent X-ray absorption spectra reveal anisotropy in the electronic structure between the ab-plane and c-axis. Results show the superior hybridization between V 3d and O 2p states, higher V wavefunction overlap and broader conduction bands in the ab-plane than in the c-axis. These insights into the electronic structure are discussed in the context of existing experimental and theoretical reports regarding charge transport in BiVO 4 .

Analysis of InxGa1-xN/Si p-n heterojunction solar cells and the effects of spontaneous and piezoelectric polarization charges

NASA Astrophysics Data System (ADS)

Zheng, Yangdong; Mihara, Akihiro; Yamamoto, Akio

2013-10-01

The band structures, current-voltage characteristics under solar illumination, and photovoltaic (PV) properties of InxGa1-xN/Si p-n heterojunction solar cells (SCs), as well as the effects of spontaneous and piezoelectric polarization (Psp-PPZ) induced charges are investigated theoretically and numerically. We find that the energy peaks on the conduction and valence bands could exponentially reduce the diffusion currents and photocurrents, thus profoundly affect the PV properties of the SCs. Except for large values, the Psp-PPZ induced interface charges have little influence on the band structures and the PV properties. These results should be useful in analysis and design for multijunction tandem InxGa1-xN/Si SC devices.

Optically detected cyclotron resonance investigations on 4H and 6H SiC: Band-structure and transport properties

NASA Astrophysics Data System (ADS)

Meyer, B. K.; Hofmann, D. M.; Volm, D.; Chen, W. M.; Son, N. T.; Janzén, E.

2000-02-01

We present experimental data on the band-structure and high-mobility transport properties of 6H and 4H-SiC epitaxial films based on optically detected cyclotron resonance investigations. From the orientational dependence of the electron effective mass in 6H-SiC we obtain direct evidence for the camels back nature of the conduction band between the M and L points. The broadening of the resonance signal in 4H-SiC as a function of temperature is used to extract information on electron mobilities and to conclude on the role of the different scattering mechanisms. Under high microwave powers an enhancement of the electron effective mass is found which is explained by a coupling of the electrons with longitudinal optical phonons.

Effect of hydrostatic pressure on the structural and electronic properties of Cd{sub 0.75}Cr{sub 0.25}S

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rani, Anita; Kaur, Kulwinder; Kumar, Ranjan

In this paper we present the results obtained from first principle calculations of the effect of hydrostatic pressure on the structural and electronic properties of Cd{sub 1-x}Cr{sub x}S diluted magnetic semiconductor in Zinc Blende (B3) phase at x=0.25. High pressure behavior of Cd{sub 1-x}Cr{sub x}S has been investigated between 0 GPa to 100 GPa The calculations have been performed using Density functional theory as implemented in the Spanish Initiative for Electronic Simulations with Thousands of Atoms code using local density approximation as exchange-correlation (XC) potential. Calculated electronic band structures of Cd{sub 1-x}Cr{sub x}S are discussed in terms of contribution ofmore » Cr 3d{sup 5} 4s{sup 1}, Cd 4d{sup 10} 5s{sup 2}, S 3s{sup 2} 3p{sup 4} orbital’s. Study of band structures shows half-metallic ferromagnetic nature of Cd{sub 0.75}Cr{sub 0.25}S with 100% spin polarization. Under application of external pressure, the valence band and conduction band are shifted upward which leads to modification of electronic structure.« less

Understanding of sub-band gap absorption of femtosecond-laser sulfur hyperdoped silicon using synchrotron-based techniques

PubMed Central

Limaye, Mukta V.; Chen, S. C.; Lee, C. Y.; Chen, L. Y.; Singh, Shashi B.; Shao, Y. C.; Wang, Y. F.; Hsieh, S. H.; Hsueh, H. C.; Chiou, J. W.; Chen, C. H.; Jang, L. Y.; Cheng, C. L.; Pong, W. F.; Hu, Y. F.

2015-01-01

The correlation between sub-band gap absorption and the chemical states and electronic and atomic structures of S-hyperdoped Si have been extensively studied, using synchrotron-based x-ray photoelectron spectroscopy (XPS), x-ray absorption near-edge spectroscopy (XANES), extended x-ray absorption fine structure (EXAFS), valence-band photoemission spectroscopy (VB-PES) and first-principles calculation. S 2p XPS spectra reveal that the S-hyperdoped Si with the greatest (~87%) sub-band gap absorption contains the highest concentration of S2− (monosulfide) species. Annealing S-hyperdoped Si reduces the sub-band gap absorptance and the concentration of S2− species, but significantly increases the concentration of larger S clusters [polysulfides (Sn2−, n > 2)]. The Si K-edge XANES spectra show that S hyperdoping in Si increases (decreased) the occupied (unoccupied) electronic density of states at/above the conduction-band-minimum. VB-PES spectra evidently reveal that the S-dopants not only form an impurity band deep within the band gap, giving rise to the sub-band gap absorption, but also cause the insulator-to-metal transition in S-hyperdoped Si samples. Based on the experimental results and the calculations by density functional theory, the chemical state of the S species and the formation of the S-dopant states in the band gap of Si are critical in determining the sub-band gap absorptance of hyperdoped Si samples. PMID:26098075

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mitkova, Maria; Butt, Darryl; Kozicki, Michael

For many decades, various radiation detecting material have been extensively researched, to find a better material or mechanism for radiation sensing. Recently, there is a growing need for a smaller and effective material or device that can perform similar functions of bulkier Geiger counters and other measurement options, which fail the requirement for easy, cheap and accurate radiation dose measurement. Here arises the use of thin film chalcogenide glass, which has unique properties of high thermal stability along with high sensitivity towards short wavelength radiation. The unique properties of chalcogenide glasses are attributed to the lone pair p-shell electrons, whichmore » provide some distinctive optical properties when compared to crystalline material. These qualities are derived from the energy band diagram and the presence of localized states in the band gap. Chalcogenide glasses have band tail states and localized states, along with the two band states. These extra states are primarily due to the lone pair electrons as well as the amorphous structure of the glasses. The localized states between the conductance band (CB) and valence band (VB) are primarily due to the presence of the lone pair electrons, while the band tail states are attributed to the Van der Waal's forces between layers of atoms [1]. Localized states are trap locations within the band gap where electrons from the valence band can hop into, in their path towards the conduction band. Tail states on the other hand are locations near the band gap edges and are known as Urbach tail states (Eu). These states are occupied with many electrons that can participate in the various transformations due to interaction with photons. According to Y. Utsugi et. al.[2], the electron-phonon interactions are responsible for the generation of the Urbach tails. These states are responsible for setting the absorption edge for these glasses and photons with energy near the band gap affect these states. We have studied the effect of x-rays and γ-rays, on thin film chalcogenide glasses and applied them in conjunction with film incorporating a silver source in a new type of radiation sensor for which we have an US patent application [3]. In this report, we give data about our studies regarding our designed radiation sensor along with the testing and performance at various radiation doses. These studies have been preceded by materials characterization research related to the compositional and structural characteristics of the active materials used in the radiation sensor design. During the work on the project, we collected a large volume of material since every experiment was repeated many times to verify the results. We conducted a comprehensive material research, analysis and discussion with the aim to understand the nature of the occurring effects, design different structures to harness these effects, generated models to aid in the understanding the effects, built different device structures and collected data to quantify device performance. These various aspects of our investigation have been detailed in previous quarterly reports. In this report, we present our main results and emphasize on the results pertaining to the core project goals materials development, sensor design and testing and with an emphasis on classifying the appropriate material and design for the optimal application. The report has three main parts: (i) Presentation of the main data; (ii) Bulleted summary of the most important results; (iii) List of the patent, journal publications, conference proceedings and conferences participation, occurring as a result of working on the project.« less

Ultrafast dynamics of an unoccupied surface resonance state in B i2T e2Se

NASA Astrophysics Data System (ADS)

Munisa, Nurmamat; Krasovskii, E. E.; Ishida, Y.; Sumida, K.; Chen, Jiahua; Yoshikawa, T.; Chulkov, E. V.; Kokh, K. A.; Tereshchenko, O. E.; Shin, S.; Kimura, Akio

2018-03-01

Electronic structure and electron dynamics in the ternary topological insulator B i2T e2Se are studied with time- and angle-resolved photoemission spectroscopy using optical pumping. An unoccupied surface resonance split off from the bulk conduction band previously indirectly observed in scanning tunneling measurements is spectroscopically identified. Furthermore, an unoccupied topological surface state (TSS) is found, which is serendipitously located at about 1.5 eV above the occupied TSS, thereby facilitating direct optical transitions between the two surface states at ℏ ω =1.5 eV in an n -type topological insulator. An appreciable nonequilibrium population of the bottom of the bulk conduction band is observed for longer than 15 ps after the pump pulse. This leads to a long recovery time of the lower TSS, which is constantly populated by the electrons coming from the bulk conduction band. Our results demonstrate B i2T e2Se to be an ideal platform for designing future optoelectronic devices based on topological insulators.

Energy band engineering and controlled p-type conductivity of CuAlO2 thin films by nonisovalent Cu-O alloying

NASA Astrophysics Data System (ADS)

Yao, Z. Q.; He, B.; Zhang, L.; Zhuang, C. Q.; Ng, T. W.; Liu, S. L.; Vogel, M.; Kumar, A.; Zhang, W. J.; Lee, C. S.; Lee, S. T.; Jiang, X.

2012-02-01

The electronic band structure and p-type conductivity of CuAlO2 films were modified via synergistic effects of energy band offset and partial substitution of less-dispersive Cu+ 3d10 with Cu2+ 3d9 orbitals in the valence band maximum by alloying nonisovalent Cu-O with CuAlO2 host. The Cu-O/CuAlO2 alloying films show excellent electronic properties with tunable wide direct bandgaps (˜3.46-3.87 eV); Hall measurements verify the highest hole mobilities (˜11.3-39.5 cm2/Vs) achieved thus far for CuAlO2 thin films and crystals. Top-gate thin film transistors constructed on p-CuAlO2 films were presented, and the devices showed pronounced performance with Ion/Ioff of ˜8.0 × 102 and field effect mobility of 0.97 cm2/Vs.

Electronic Band Structure Tuning of Highly-Mismatched-Alloys for Energy Conversion Applications

NASA Astrophysics Data System (ADS)

Ting, Min

Highly-mismatched alloys: ZnO1-xTe x and GaN1-xSb x are discussed within the context of finding the suitable material for a cost-effective Si-based tandem solar cell (SBTSC). SBTSC is an attractive concept for breaking through the energy conversion efficiency theoretical limit of a single junction solar cell. Combining with a material of 1.8 eV band gap, SBTSC can theoretically achieve energy conversion efficiency > 45%. ZnO and GaN are wide band gap semiconductors. Alloying Te in ZnO and alloying Sb in GaN result in large band gap reduction to < 2 eV from 3.3 eV and 3.4 eV respectively. The band gap reduction is majorly achieved by the upward shift of valence band (VB). Incorporating Te in ZnO modifies the VB of ZnO through the valence-band anticrossing (VBAC) interaction between localized Te states and ZnO VB delocalized states, which forms a Te-derived VB at 1 eV above the host VB. Similar band structure modification is resulted from alloying Sb in GaN. Zn1-xTex and GaN 1-xSbx thin films are synthesized across the whole composition range by pulsed laser deposition (PLD) and low temperature molecular beam epitaxy (LT-MBE) respectively. The electronic band edges of these alloys are measured by synchrotron X-ray absorption, emission, and the X-ray photoelectron spectroscopies. Modeling the optical absorption coefficient with the band anticrossing (BAC) model revealed that the Te and Sb defect levels to be at 0.99 eV and 1.2 eV above the VB of ZnO and GaN respectively. Electrically, Zn1-xTex is readily n-type conductive and GaN1-xSbx is strongly p-type conductive. A heterojunction device of p-type GaN 0.93Sb0.07 with n-type ZnO0.77Te0.93 upper cell (band gap at 1.8 eV) on Si bottom cell is proposed as a promising SBTSC device.

Prospective high thermoelectric performance of the heavily p -doped half-Heusler compound CoVSn

DOE PAGES

Shi, Hongliang; Ming, Wenmei; Parker, David S.; ...

2017-05-11

The electronic structure and transport properties of the half-Heusler compound CoVSn are studied in this paper systematically by combining first-principles electronic structure calculations and Boltzmann transport theory. The band structure at the valence-band edge is complex with multiple maxima derived from hybridized transition element d states. The result is a calculated thermopower larger than 200 μV /Κ within a wide range of doping concentrations and temperatures for heavily doped p-type CoVSn. The thermoelectric properties additionally benefit from the corrugated shapes of the hole pockets in our calculated isoenergy surfaces. Our calculated power factor S 2σ/τ (with respect to an averagemore » unknown scattering time) of CoVSn is comparable to that of FeNbSb. A smaller lattice thermal conductivity can be expected from the smaller group velocities of acoustical modes compared to FeNbSb. Finally, overall, good thermoelectric performance for CoVSn can be expected by considering the electronic transport and lattice thermal conductivity.« less

Thermoelectric properties of doped BaHfO{sub 3}

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dixit, Chandra Kr., E-mail: ckparadise@gmail.com, E-mail: sharmarameshfgiet@gmail.com; Bhamu, K. C.; Sharma, Ramesh, E-mail: ckparadise@gmail.com, E-mail: sharmarameshfgiet@gmail.com

2016-05-06

We have studied the structural stability, electronic structure, optical properties and thermoelectric properties of doped BaHfO{sub 3} by full potential linearized augmented plane wave (FP-LAPW) method. The electronic structure of BaHfO{sub 3} doped with Sr shows enhances the indirect band gaps of 3.53 eV, 3.58 eV. The charge density plots show strong ionic bonding in Ba-Hf, and ionic and covalent bonding between Hf and O. Calculations of the optical spectra, viz., the dielectric function, refractive index and extinction coefficient are performed for the energy range are calculated and analyzed. Thermoelectric properties of semi conducting are also reported first time. Themore » doped BaHfO{sub 3} is approximately wide band gap semiconductor with the large p-type Seebeck coefficient. The power factor of BaHfO{sub 3} is increased with Sr doping, decreases because of low electrical resistivity and thermal conductivity.« less

Prospective high thermoelectric performance of the heavily p -doped half-Heusler compound CoVSn

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shi, Hongliang; Ming, Wenmei; Parker, David S.

The electronic structure and transport properties of the half-Heusler compound CoVSn are studied in this paper systematically by combining first-principles electronic structure calculations and Boltzmann transport theory. The band structure at the valence-band edge is complex with multiple maxima derived from hybridized transition element d states. The result is a calculated thermopower larger than 200 μV /Κ within a wide range of doping concentrations and temperatures for heavily doped p-type CoVSn. The thermoelectric properties additionally benefit from the corrugated shapes of the hole pockets in our calculated isoenergy surfaces. Our calculated power factor S 2σ/τ (with respect to an averagemore » unknown scattering time) of CoVSn is comparable to that of FeNbSb. A smaller lattice thermal conductivity can be expected from the smaller group velocities of acoustical modes compared to FeNbSb. Finally, overall, good thermoelectric performance for CoVSn can be expected by considering the electronic transport and lattice thermal conductivity.« less

A reconfigurable frequency-selective surface for dual-mode multi-band filtering applications

NASA Astrophysics Data System (ADS)

Majidzadeh, Maryam; Ghobadi, Changiz; Nourinia, Javad

2017-03-01

A reconfigurable single-layer frequency-selective surface (FSS) with dual-mode multi-band modes of operation is presented. The proposed structure is printed on a compact 10 × 10 mm2 FR4 substrate with the thickness of 1.6 mm. A simple square loop is printed on the front side while another one along with two defected vertical arms is deployed on the backside. To realise the reconfiguration, two pin diodes are embedded on the backside square loop. Suitable insertion of conductive elements along with pin diodes yields in dual-mode multi-band rejection of applicable in service frequency ranges. The first operating mode due to diodes' 'ON' state provides rejection of 2.4 GHz WLAN in 2-3 GHz, 5.2/5.8 GHz WLAN and X band in 5-12 GHz, and a part of Ku band in 13.9-16 GHz. In diodes 'OFF' state, the FSS blocks WLAN in 4-7.3 GHz, X band in 8-12.7 GHz as well as part of Ku band in 13.7-16.7 GHz. As well, high attenuation of incident waves is observed by a high shielding effectiveness (SE) in the blocked frequency bands. Also, a stable behaviour against different polarisations and angles of incidence is obtained. Comprehensive studies are conducted on a fabricated prototype to assess its performance from which encouraging results are obtained.

Modification of band gaps and optoelectronic properties of binary calcium chalcogenides by means of doping of magnesium atom(s) in rock-salt phase- a first principle based theoretical initiative

NASA Astrophysics Data System (ADS)

Debnath, Bimal; Sarkar, Utpal; Debbarma, Manish; Bhattacharjee, Rahul; Chattopadhyaya, Surya

2018-02-01

The band gaps and optoelectronic properties of binary calcium chalcogenide semiconductors have been modified theoretically by doping magnesium atom(s) into their respective rock-salt unit cells at some specific concentrations x = 0.0, 0.25, 0.50, 0.75 and 1.0 and confirmed such modifications by studying their structural, electronic and optical properties using DFT based FP-LAPW approach. The WC-GGA functional is used to calculate structural properties, while mBJ, B3LYP and WC-GGA are used for calculating electronic and optical properties. The concentration dependences of lattice parameter, bulk modulus and fundamental band gap for each alloy system exhibit nonlinearity. The atomic and orbital origin of different electronic states in the band structure of each compound are explored from its density of states (DOS). The microscopic origin of band gap bowing for each of the alloy systems is explored in terms of volume deformation, charge exchange and structural relaxation. The chemical bonds between the constituent atoms in each compound are found as ionic in nature. Optical properties of each specimen are calculated from its computed spectra of dielectric function, refractive index, extinction coefficient, normal incidence reflectivity, optical conductivity, optical absorption and energy loss function. Several calculated results have been compared with available experimental and other theoretical data.

Codoping in SnTe: Enhancement of Thermoelectric Performance through Synergy of Resonance Levels and Band Convergence

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tan, Gangjian; Shi, Fengyuan; Hao, Shiqiang

2015-04-22

We report a significant enhancement of the thermoelectric performance of p-type SnTe over a broad temperature plateau with a peak ZT value of similar to 1.4 at 923 K through In/Cd codoping and a CdS nanostructuring approach. Indium and cadmium play different but complementary roles in modifying the valence band structure of SnTe. Specifically, In-doping introduces resonant levels inside the valence bands, leading to a considerably improved Seebeck coefficient at low temperature. Cd-doping, however, increases the Seebeck coefficient of SnTe remarkably in the mid- to high-temperature region via a convergence of the light and heavy hole bands and an enlargementmore » of the band gap. Combining the two dopants in SnTe yields enhanced Seebeck coefficient and power factor over a wide temperature range due to the synergy of resonance levels and valence band convergence, as demonstrated by the Pisarenko plot and supported by first-principles band structure calculations. Moreover, these codoped samples can be hierarchically structured on all scales (atomic point defects by doping, nanoscale precipitations by CdS nanostructuring, and mesoscale grains by SPS treatment) to achieve highly effective phonon scattering leading to strongly reduced thermal conductivities. In addition to the high maximum ZT the resultant large average ZT of similar to 0.8 between 300 and 923 K makes SnTe an attractive p-type material for high-temperature thermoelectric power generation.« less

Sputtered deposited nanocrystalline ZnO films: A correlation between electrical, optical and microstructural properties

NASA Astrophysics Data System (ADS)

Lee, J.; Gao, W.; Li, Z.; Hodgson, M.; Metson, J.; Gong, H.; Pal, U.

2005-05-01

Zinc oxide thin films were prepared by dc (direct current) and rf (radio frequency) magnetron sputtering on glass substrates. ZnO films produced by dc sputtering have a high resistance, while the films produced using rf sputtering are significantly more conductive. While the conductive films have a compact nodular surface morphology, the resistive films have a relatively porous surface with columnar structures in cross section. Compared to the dc sputtered films, rf sputtered films have a microstructure with smaller d spacing, lower internal stress, higher band gap energy and higher density. Dependence of conductivity on the deposition technique and the resulting d spacing , stress, density, band gap, film thickness and Al doping are discussed. Correlations between the electrical conductivity, microstructural parameters and optical properties of the films have been made.

First-principle simulations of electronic structure in semicrystalline polyethylene

NASA Astrophysics Data System (ADS)

Moyassari, A.; Unge, M.; Hedenqvist, M. S.; Gedde, U. W.; Nilsson, F.

2017-05-01

In order to increase our fundamental knowledge about high-voltage cable insulation materials, realistic polyethylene (PE) structures, generated with a novel molecular modeling strategy, have been analyzed using first principle electronic structure simulations. The PE structures were constructed by first generating atomistic PE configurations with an off-lattice Monte Carlo method and then equilibrating the structures at the desired temperature and pressure using molecular dynamics simulations. Semicrystalline, fully crystalline and fully amorphous PE, in some cases including crosslinks and short-chain branches, were analyzed. The modeled PE had a structure in agreement with established experimental data. Linear-scaling density functional theory (LS-DFT) was used to examine the electronic structure (e.g., spatial distribution of molecular orbitals, bandgaps and mobility edges) on all the materials, whereas conventional DFT was used to validate the LS-DFT results on small systems. When hybrid functionals were used, the simulated bandgaps were close to the experimental values. The localization of valence and conduction band states was demonstrated. The localized states in the conduction band were primarily found in the free volume (result of gauche conformations) present in the amorphous regions. For branched and crosslinked structures, the localized electronic states closest to the valence band edge were positioned at branches and crosslinks, respectively. At 0 K, the activation energy for transport was lower for holes than for electrons. However, at room temperature, the effective activation energy was very low (˜0.1 eV) for both holes and electrons, which indicates that the mobility will be relatively high even below the mobility edges and suggests that charge carriers can be hot carriers above the mobility edges in the presence of a high electrical field.

Modeling Ballistic Current Flow in Carbon Nanotube Wires

NASA Technical Reports Server (NTRS)

Anantram, M. P.; Biegel, Bryan (Technical Monitor)

2001-01-01

Experiments have shown carbon nanotubes (CNT) to be almost perfect conductors at small applied biases. The features of the CNT band structure, large velocity of the crossing subbands and the small number of modes that an electron close to the band center / Fermi energy can scatter into, are the reasons for the near perfect small bias conductance. We show that the CNT band structure does not help at large applied biases - electrons injected into the non crossing subbands can either be Bragg reflected or undergo Zener-type tunneling. This limits the current carrying capacity of CNT. We point out that the current carrying capacity of semiconductor quantum wires in the ballistic limit is different, owing to its band structure. The second aspect addressed is the relationship of nanotube chirality in determining the physics of metal-nanotube coupling. We show that a metallic-zigzag nanotube couples better than an armchair nanotube to a metal contact. This arises because in the case of armchair nanotubes, while the pi band couples well, the pi* band does not couple well to the metal. In the case of zigzag nanotube both crossing modes couple reasonably well to the metal. Many factors such as the role of curvature, strain and defects will play a role in determining the suitability of nanotubes as nanowires. From the limited view point of metal-nanotube coupling, we feel that metallic-zigzag nanotubes are preferable to armchair nanotubes.

Valley-dependent band structure and valley polarization in periodically modulated graphene

NASA Astrophysics Data System (ADS)

Lu, Wei-Tao

2016-08-01

The valley-dependent energy band and transport property of graphene under a periodic magnetic-strained field are studied, where the time-reversal symmetry is broken and the valley degeneracy is lifted. The considered superlattice is composed of two different barriers, providing more degrees of freedom for engineering the electronic structure. The electrons near the K and K' valleys are dominated by different effective superlattices. It is found that the energy bands for both valleys are symmetric with respect to ky=-(AM+ξ AS) /4 under the symmetric superlattices. More finite-energy Dirac points, more prominent collimation behavior, and new crossing points are found for K' valley. The degenerate miniband near the K valley splits into two subminibands and produces a new band gap under the asymmetric superlattices. The velocity for the K' valley is greatly renormalized compared with the K valley, and so we can achieve a finite velocity for the K valley while the velocity for the K' valley is zero. Especially, the miniband and band gap could be manipulated independently, leading to an increase of the conductance. The characteristics of the band structure are reflected in the transmission spectra. The Dirac points and the crossing points appear as pronounced peaks in transmission. A remarkable valley polarization is obtained which is robust to the disorder and can be controlled by the strain, the period, and the voltage.

Electronic properties of ZnPSe3-MoS2 Van der Waals heterostructure

NASA Astrophysics Data System (ADS)

Sharma, Munish; Kumar, Ashok; Ahluwalia, P. K.

2018-04-01

We present a comparative study of electronic properties of ZnPSe3-MoS2 heterostructure using GGA-PBE functional and DFT-D2 method within the framework of density functional theory (DFT). Electronic band structure for the considered heterostructure shows a direct band gap semiconducting character. A decrease in band gap is observed with the heterostructuring as compared to their constituent pristine monolayers. The alignment of valance band maxima and conduction band minima on different layers in heterostructure indicate the physical separation of charge carriers. A work function of 5.31 eV has been calculated for ZnPSe3-MoS2 heterostructure. These results provide a physical basis for the potential applications of these ZnPSe3-MoS2 heterostructure in optoelectronic devices.

Band Gaps for Elastic Wave Propagation in a Periodic Composite Beam Structure Incorporating Microstructure and Surface Energy Effects

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, G. Y.; Gao, X. -L.; Bishop, J. E.

Here, a new model for determining band gaps for elastic wave propagation in a periodic composite beam structure is developed using a non-classical Bernoulli–Euler beam model that incorporates the microstructure, surface energy and rotational inertia effects. The Bloch theorem and transfer matrix method for periodic structures are employed in the formulation. The new model reduces to the classical elasticity-based model when both the microstructure and surface energy effects are not considered. The band gaps predicted by the new model depend on the microstructure and surface elasticity of each constituent material, the unit cell size, the rotational inertia, and the volumemore » fraction. To quantitatively illustrate the effects of these factors, a parametric study is conducted. The numerical results reveal that the band gap predicted by the current non-classical model is always larger than that predicted by the classical model when the beam thickness is very small, but the difference is diminishing as the thickness becomes large. Also, it is found that the first frequency for producing the band gap and the band gap size decrease with the increase of the unit cell length according to both the current and classical models. In addition, it is observed that the effect of the rotational inertia is larger when the exciting frequency is higher and the unit cell length is smaller. Furthermore, it is seen that the volume fraction has a significant effect on the band gap size, and large band gaps can be obtained by tailoring the volume fraction and material parameters.« less

Band Gaps for Elastic Wave Propagation in a Periodic Composite Beam Structure Incorporating Microstructure and Surface Energy Effects

DOE PAGES

Zhang, G. Y.; Gao, X. -L.; Bishop, J. E.; ...

2017-11-20

Here, a new model for determining band gaps for elastic wave propagation in a periodic composite beam structure is developed using a non-classical Bernoulli–Euler beam model that incorporates the microstructure, surface energy and rotational inertia effects. The Bloch theorem and transfer matrix method for periodic structures are employed in the formulation. The new model reduces to the classical elasticity-based model when both the microstructure and surface energy effects are not considered. The band gaps predicted by the new model depend on the microstructure and surface elasticity of each constituent material, the unit cell size, the rotational inertia, and the volumemore » fraction. To quantitatively illustrate the effects of these factors, a parametric study is conducted. The numerical results reveal that the band gap predicted by the current non-classical model is always larger than that predicted by the classical model when the beam thickness is very small, but the difference is diminishing as the thickness becomes large. Also, it is found that the first frequency for producing the band gap and the band gap size decrease with the increase of the unit cell length according to both the current and classical models. In addition, it is observed that the effect of the rotational inertia is larger when the exciting frequency is higher and the unit cell length is smaller. Furthermore, it is seen that the volume fraction has a significant effect on the band gap size, and large band gaps can be obtained by tailoring the volume fraction and material parameters.« less

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.

Electromagnetic waves in a topological insulator thin film stack: helicon-like wave mode and photonic band structure.

PubMed

Inoue, Jun-ichi

2013-09-09

We theoretically explore the electromagnetic modes specific to a topological insulator superlattice in which topological and conventional insulator thin films are stacked periodically. In particular, we obtain analytic formulas for low energy mode that corresponds to a helicon wave, as well as those for photonic bands. We illustrate that the system can be modeled as a stack of quantum Hall layers whose conductivity tensors alternately change signs, and then we analyze the photonic band structures. This subject is a natural extension of a previous study by Tselis et al., which took into consideration a stack of identical quantum Hall layers but their discussion was limited into a low energy mode. Thus we provide analytic formulas for photonic bands and compare their features between the two systems. Our central findings in the topological insulator superlattice are that a low energy mode corresponding to a helicon wave has linear dispersion instead of the conventional quadratic form, and that a robust gapless photonic band appears although the system considered has spacial periodicity. In addition, we demonstrate that the photonic bands agree with the numerically calculated transmission spectra.

Aspects of Dzyaloshinskii-Moriya Interaction in Two Dimensional Magnetic Structures

NASA Astrophysics Data System (ADS)

Kundu, Anirban

Research on topologically protected chiral magnetic structures such as magnetic domain walls (DWs) and skyrmions, have gained extensive interest because of their possible applications in magnetic data storage industries. The recently observed chiral DW structures in ultrathin ferromagnetic lms with perpendicular magnetic anisotropy has been attributed to the presence of a strong Dzyaloshinskii-Moriya interaction (DMI). In this thesis, the DMI mediated by the conduction electrons in two dimensional magnetic systems such as magnetic thin lms or at the interfaces between two magnetic materials has been studied. I calculate the Ruderman-Kittel- Kasuya-Yosida (RKKY) type indirect exchange coupling between two magnetic moments at nite temperature using the free electron band. At high temperature, the coupling strength decays with distance faster than the coupling at zero temperature but the period of oscillation remains same. However, the free electron band alone could not produce DMI. In the next step, I show addition of Rashba spin-orbit coupling (RSOC) with the spin-polarized conduction electron band produces the DMI between two magnetic ions. The essential feature of this DMI is: the coupling strength increases with the strength of RSOC, but decreases signi cantly with the Heisenberg exchange coupling. The DMI calculated with this model well explains the possibility of preferred Neel or Bloch DW structures with specifc chirality. In addition: I study switching of magnetization with ultrafast laser pulse by inverse Faraday e ect (IFE) where an optically induced non-equilibrium orbital momentum generates an e ective magnetic eld via spin-orbit coupling for magnetization switching. I calculate the magnitude of induced orbital moment for the generic itinerant band and show that magnitude is not large enough to make the switching by a single pulse, however, switching could be possible if multiple pulses are applied to the material.

Effects of post-deposition annealing ambient on band alignment of RF magnetron-sputtered Y2O3 film on gallium nitride

PubMed Central

2013-01-01

The effects of different post-deposition annealing ambients (oxygen, argon, forming gas (95% N2 + 5% H2), and nitrogen) on radio frequency magnetron-sputtered yttrium oxide (Y2O3) films on n-type gallium nitride (GaN) substrate were studied in this work. X-ray photoelectron spectroscopy was utilized to extract the bandgap of Y2O3 and interfacial layer as well as establishing the energy band alignment of Y2O3/interfacial layer/GaN structure. Three different structures of energy band alignment were obtained, and the change of band alignment influenced leakage current density-electrical breakdown field characteristics of the samples subjected to different post-deposition annealing ambients. Of these investigated samples, ability of the sample annealed in O2 ambient to withstand the highest electric breakdown field (approximately 6.6 MV/cm) at 10−6 A/cm2 was related to the largest conduction band offset of interfacial layer/GaN (3.77 eV) and barrier height (3.72 eV). PMID:23360596

How the laser-induced ionization of transparent solids can be suppressed

NASA Astrophysics Data System (ADS)

Gruzdev, Vitaly

2013-12-01

A capability to suppress laser-induced ionization of dielectric crystals in controlled and predictable way can potentially result in substantial improvement of laser damage threshold of optical materials. The traditional models that employ the Keldysh formula do not predict any suppression of the ionization because of the oversimplified description of electronic energy bands underlying the Keldysh formula. To fix this gap, we performed numerical simulations of time evolution of conduction-band electron density for a realistic cosine model of electronic bands characteristic of wide-band-gap cubic crystals. The simulations include contributions from the photo-ionization (evaluated by the Keldysh formula and by the formula for the cosine band of volume-centered cubic crystals) and from the avalanche ionization (evaluated by the Drude model). Maximum conduction-band electron density is evaluated from a single rate equation as a function of peak intensity of femtosecond laser pulses for alkali halide crystals. Results obtained for high-intensity femtosecond laser pulses demonstrate that the ionization can be suppressed by proper choice of laser parameters. In case of the Keldysh formula, the peak electron density exhibits saturation followed by gradual increase. For the cosine band, the electron density increases with irradiance within the low-intensity multiphoton regime and switches to decrease with intensity approaching threshold of the strong singularity of the ionization rate characteristic of the cosine band. Those trends are explained with specific modifications of band structure by electric field of laser pulses.

Tuning of electronic band gaps and optoelectronic properties of binary strontium chalcogenides by means of doping of magnesium atom(s)- a first principles based theoretical initiative with mBJ, B3LYP and WC-GGA functionals

NASA Astrophysics Data System (ADS)

Debnath, Bimal; Sarkar, Utpal; Debbarma, Manish; Bhattacharjee, Rahul; Chattopadhyaya, Surya

2018-02-01

First principle based theoretical initiative is taken to tune the optoelectronic properties of binary strontium chalcogenide semiconductors by doping magnesium atom(s) into their rock-salt unit cells at specific concentrations x = 0.0, 0.25, 0.50, 0.75 and 1.0 and such tuning is established by studying structural, electronic and optical properties of designed binary compounds and ternary alloys employing WC-GGA, B3LYP and mBJ exchange-correlation functionals. Band structure of each compound is constructed and respective band gaps under all the potential schemes are measured. The band gap bowing and its microscopic origin are calculated using quadratic fit and Zunger's approach, respectively. The atomic and orbital origins of electronic states in the band structure of any compound are explored from its density of states. The nature of chemical bonds between the constituent atoms in each compound is explored from the valence electron density contour plots. Optical properties of any specimen are explored from the computed spectra of its dielectric function, refractive index, extinction coefficient, normal incidence reflectivity, optical conductivity optical absorption and energy loss function. Several calculated results are compared with available experimental and earlier theoretical data.

Growth of non-polar and semi-polar gallium nitride with plasma assisted molecular beam epitaxy: Relatonships between film microstructure, reciprocal lattice and transport properties

NASA Astrophysics Data System (ADS)

McLaurin, Melvin Barker

2007-12-01

The group-III nitrides exhibit significant spontaneous and piezoelectric polarization parallel to the [0001] direction, which are manifested as sheet charges at heterointerfaces. While polarization can be used to engineer the band-structure of a device, internal electric fields generated by polarization discontinuities can also have a number of negative consequences for the performance and design of structures utilizing heterojunctions. The most direct route to polarization free group-III nitride devices is growth on either one of the "non-polar" prismatic faces of the crystal (m-plane (1010) or a-plane (1120)) where the [0001] direction lies in the plane of any heterointerfaces. This dissertation focuses on the growth of non-polar and semi-polar GaN by MBE and on how the dominant feature of the defect structure of non-polar and semi-polar films, basal plane stacking faults, determines the properties of the reciprocal lattice and electrical transport of the films. The first part is a survey of the MBE growth of the two non-polar planes (10 10) and (1120) and three semi-polar planes (1011), (1013) and {11 22} investigated in this work. The relationship between basal plane stacking faults and broadening of the reciprocal lattice is discussed and measured with X-ray diffraction using a lateral-variant of the Williamson-Hall analysis. The electrical properties of m-plane films are investigated using Hall-effect and TLM measurements. Anisotropic mobilities were observed for both electrons and holes along with record p-type conductivities and hole concentrations. By comparison to both inversion-domain free c-plane films and stacking-fault-free free-standing m-plane GaN wafers it was determined that basal plane stacking faults were the source of both the enhanced p-type conductivity and the anisotropic carrier mobilities. Finally, we propose a possible source of anisotropic mobilities and enhanced p-type conduction in faulted films is proposed. Basal plane stacking faults are treated as heterostructures of the wurtzite and zincblende polytypes of GaN. The band parameter and polarization differences between the polytypes result in large offsets in both the conduction and valence band edges at the stacking faults. Anisotropy results from scattering from the band-edge offsets and enhanced mobility from screening due to charge accumulation at these band edge offsets.

Synthesis and first-principle calculations of the structural and electronic properties of Ge-substituted type-VIII Ba8Ga16Sn30 clathrate

NASA Astrophysics Data System (ADS)

Shen, Lanxian; Li, Decong; Liu, Hongxia; Liu, Zuming; Deng, Shukang

2016-12-01

In this study, the structural and electronic structural properties of Ba8Ga16Sn30-xGex (0≤x≤30) are determined by the first-principle method on the basis of density functional theory. Consistent with experimental findings, calculated results reveal that Ge atoms preferentially occupy the 2a and 24g sites in these compounds. As the content of Ge in Ge-substituted clathrate is increased, the lattice parameter is decreased, and the structural stability is enhanced. The bandgaps of the compound at 1≤x≤10 are smaller than those of Ba8Ga16Sn30. By contrast, the bandgaps of the compound at x>10 are larger than those of Ba8Ga16Sn30. The substitution of Ge for Sn affects p-type conductivity but not n-type conductivity. As Ge content increases, the whole conduction band moves to the direction of high energy, and the density of states of valence-band top decreases. The calculated potential energy versus displacement of Ba indicates that the vibration energy of this atom increases as cage size decreases. Because Ge substitution also affects clathrate structural symmetry, the distance of Ba atom deviation from the center of the cage initially increases and subsequently decreases as the Ge content increases.

Distribution of resistive and conductive structures in Nankai accretionary wedge reveals contrasting stress paths

NASA Astrophysics Data System (ADS)

Conin, Marianne; Bourlange, Sylvain; Henry, Pierre; Boiselet, Aurelien; Gaillot, Philippe

2014-01-01

In this article, we study the characteristics and spatial distribution of the deformation structures along the Kumano transect of the Nankai accretionary wedge, and use this information to interpret the stress path followed by the sediments. Deformation structures are identified from logging while drilling (LWD) resistivity images of the materials surrounding the drill hole and from 3-dimensional X-ray CT-images of cores acquired during the IODP NanTroSEIZE project. The relative resistivity of the structures identified on logs and the strike, dip, and density of structures identified on CT scan images are measured. The analysis of dip and strike of structures indicates that most of the resistive structures identified on logging data correspond to compactive shear bands. Results also indicate that conductive structures predominate at the toe of the prism and above the main out of sequence thrust, in locations where past and recent erosion occurred. We propose several mechanisms that could explain the relation between erosion and the absence of compactive shear bands. We conclude that sediments followed different stress paths depending on their location within the wedge, and that those differences explain the distribution of deformation structures within the wedge. We also show the coexistence of dilatant and compactant structures in fault zones including the frontal thrust and mega splay fault, and we interpret the coexistence of these structures as a possible consequence of a transient fluid pressure.

Growth and Characterization of the p-type Semiconductors Tin Sulfide and Bismuth Copper Oxy Selenide

NASA Astrophysics Data System (ADS)

Francis, Jason

BiCuOSe and SnS are layered, moderate band gap (epsilon G ≈ 1 eV) semiconductors that exhibit intrinsic p type conductivity. Doping of BiCuOSe with Ca results in a slight expansion of the lattice and an increase of the hole concentration from 10 18 cm--3 to greater than 1020 cm --3. The large carrier density in undoped films is the result of copper vacancies. Mobility is unaffected by doping, remaining constant at 1.5 cm2V--1s--1 in both undoped and doped films, because the Bi-O layers serve as the source of carriers, while transport occurs within the Cu-Se layers. Bi possesses a 6s2 lone pair that was expected to hybridize with the oxygen p states at the top of the valence band, resulting in high hole mobility as compared to similar materials such as LaCuOSe, which lack this lone pair. However, both LaCuOSe and BiCuOSe have similar hole mobility. X-ray absorption and emission spectroscopy, combined with density functional theory calculations, reveal that the Bi 6 s states contribute deep within the valence band, forming bonding and anti-bonding states with O 2p at 11 eV and 3 eV below the valence band maximum, respectively. Hence, the Bi lone pair does not contribute at the top of the valence band and does not enhance the hole mobility. The Bi 6p states contribute at the bottom of the conduction band, resulting in a smaller band gap for BiCuOSe than LaCuOSe (1 eV vs. 3 eV). SnS is a potential photovoltaic absorber composed of weakly coupled layers stacked along the long axis. This weak coupling results in the formation of strongly oriented films on amorphous substrates. The optical band gap is 1.2 eV, in agreement with GW calculations. Absorption reaches 105 cm--1 within 0.5 eV of the band gap. The p type conduction arises from energetically favorable tin vacancies. Variation of growth conditions yields carrier densities of 1014 -- 1016 cm--3 and hole mobility of 7 -- 15 cm2V--1s--1. SnS was alloyed with rocksalt CaS, which was predicted to form a rocksalt structure when the calcium content is increased past 18%. Films of Sn1--x CaxS with x from 0.4 to 0.9 adopt the rocksalt structure with a band gap of 1.1-1.3 eV, with absorption greater than 105 cm--1 within about 0.7 eV of the band gap. The lattice contracts as the calcium content of the films is increased, reaching 5.7 A when x = 0.93. Films are highly insulating, but Seebeck measurements do indicate p type conduction.

First-principles study of band gap engineering via oxygen vacancy doping in perovskite ABB'O₃ solid solutions

DOE PAGES

Qi, Tingting; Curnan, Matthew T.; Kim, Seungchul; ...

2011-12-15

Oxygen vacancies in perovskite oxide solid solutions are fundamentally interesting and technologically important. However, experimental characterization of the vacancy locations and their impact on electronic structure is challenging. We have carried out first-principles calculations on two Zr-modified solid solutions, Pb(Zn 1/3Nb 2/3)O₃ and Pb(Mg 1/3Nb 2/3)O₃, in which vacancies are present. We find that the vacancies are more likely to reside between low-valent cation-cation pairs than high-valent cation-cation pairs. Based on the analysis of our results, we formulate guidelines that can be used to predict the location of oxygen vacancies in perovskite solid solutions. Our results show that vacancies canmore » have a significant impact on both the conduction and valence band energies, in some cases lowering the band gap by ≈0.5 eV. The effects of vacancies on the electronic band structure can be understood within the framework of crystal field theory.« less

Wavefunction Properties and Electronic Band Structures of High-Mobility Semiconductor Nanosheet MoS2

NASA Astrophysics Data System (ADS)

Baik, Seung Su; Lee, Hee Sung; Im, Seongil; Choi, Hyoung Joon; Ccsaemp Team; Edl Team

2014-03-01

Molybdenum disulfide (MoS2) nanosheet is regarded as one of the most promising alternatives to the current semiconductors due to its significant band-gap and electron-mobility enhancement upon exfoliating. To elucidate such thickness-dependent properties, we have studied the electronic band structures of bulk and monolayer MoS2 by using the first-principles density-functional method as implemented in the SIESTA code. Based on the wavefunction analyses at the conduction band minimum (CBM) points, we have investigated possible origins of mobility difference between bulk and monolayer MoS2. We provide formation energies of substitutional impurities at the Mo and S sites, and discuss feasible electron sources which may induce a significant difference in the carrier lifetime. This work was supported by NRF of Korea (Grant Nos. 2009-0079462 and 2011-0018306), Nano-Material Technology Development Program (2012M3a7B4034985), and KISTI supercomputing center (Project No. KSC-2013-C3-008). Center for Computational Studies of Advanced Electronic Material Properties.

Band structure calculations of CuAlO2, CuGaO2, CuInO2, and CuCrO2 by screened exchange

NASA Astrophysics Data System (ADS)

Gillen, Roland; Robertson, John

2011-07-01

We report density functional theory band structure calculations on the transparent conducting oxides CuAlO2, CuGaO2, CuInO2, and CuCrO2. The use of the hybrid functional screened-exchange local density approximation (sX-LDA) leads to considerably improved electronic properties compared to standard LDA and generalized gradient approximation (GGA) approaches. We show that the resulting electronic band gaps compare well with experimental values and previous quasiparticle calculations, and show the correct trends with respect to the atomic number of the cation (Al, Ga, In). The resulting energetic depths of Cu d and O p levels and the valence-band widths are considerable improvements compared to LDA and GGA and are in good agreement with available x-ray photoelectron spectroscopy data. Lastly, we show the calculated imaginary part of the dielectric function for all four systems.

First-Principles Materials Design of High-Performing Bulk Photovoltaics with the Li Nb O 3 Structure

DOE PAGES

Young, Steve M.; Zheng, Fan; Rappe, Andrew M.

2015-11-18

Here, the bulk photovoltaic effect is a long-known but poorly understood phenomenon. Recently, however, the multiferroic bismuth ferrite has been observed to produce strong photovoltaic response to visible light, suggesting that the effect has been underexploited as well. Here we present three polar oxides in the LiNbOmore » $$_3$$ structure that we predict to have band gaps in the 1-2 eV range and very high bulk photovoltaic response: PbNiO$$_3$$, Mg$$_{1/2}$$Zn$$_{1/2}$$PbO$$_3$$, and LiBiO$$_3$$. All three have band gaps determined by cations with $$d^{10}s^0$$ electronic configurations, leading to conduction bands composed of cation $s$-orbitals and O $p$-orbitals. This both dramatically lowers the band gap and increases the bulk photovoltaic response by as much as an order of magnitude over previous materials, demonstrating the potential for high-performing bulk photovoltaics.« less

On the electronic structure and thermoelectric properties of BiTeBr and BiTeI single crystals and of BiTeI with the addition of BiI{sub 3} and CuI

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kulbachinskii, Vladimir A., E-mail: kulb@mig.phys.msu.ru; Kytin, Vladimir G.; Kudryashov, Alexey A.

The electronic structures were calculated for BiTeBr and BiTeI using the density-functional theory approach and accounting for the strong spin-orbital interaction. Qualitatively, the band structures for two compounds are similar, showing strong mixing of the p states of all elements in vicinity of the Fermi level, with the band gaps of 0.595 and 0.478 eV for BiTeBr and BiTeI, respectively. The optimized crystal structures show a tendency for the Bi-X (X=Br, I) bond elongation compared to the Bi-Te one. Both compounds are intrinsic n-type semiconductors but display a metallic-like conductivity coupled to rather large thermopower, which is rationalized within themore » frames of the acoustic phonons scattering model. Because of larger thermopower BiTeBr exhibits a twice higher thermoelectric figure-of-merit near room temperature, ZT=0.17, compared to BiTeI. The addition of 1 mass% of BiI{sub 3} or CuI to BiTeI decreases the mobility of electrons by two orders of magnitude, leading to significantly lower electrical conductivity, but at the same time effectively reduces the thermal conductivity. The prospects of further enhancing the thermoelectric efficiency are briefly discussed. - Graphical abstract: View of the crystal structure of BiTeBr is shown in the figure The optimized crystal structures show a tendency for the Bi-X (X=Br, I) bond elongation compared to the Bi-Te one. The electronic structures were calculated for BiTeBr and BiTeI using the density-functional theory approach and accounting for the strong spin-orbital interaction. Qualitatively, the band structures for two compounds are similar, showing strong mixing of the p states of all elements in vicinity of the Fermi level, with the band gaps of 0.595 and 0.478 eV for BiTeBr and BiTeI, respectively. Both compounds are intrinsic n-type semiconductors but display a metallic-like conductivity coupled to rather large thermopower, which is rationalized within the frames of the acoustic phonons scattering model. The addition of 1 mass% of BiI{sub 3} or CuI to BiTeI effectively reduces the thermal conductivity. The prospects of further enhancing the thermoelectric efficiency are briefly discussed. Highlights: Black-Right-Pointing-Pointer BiTeBr and BiTeI feature mixing of p states of Bi, Te, and halogen near Fermi level. Black-Right-Pointing-Pointer BiTeBr has thermoelectric figure-of-merit ZT=0.17, two times that of BiTeI. Black-Right-Pointing-Pointer 1% CuI or BiI{sub 3} decrease dramatically electron mobility in BiTeI. Black-Right-Pointing-Pointer 1% CuI decreases thermal conductivity of BiTeI by a factor of 4, reaching 0.5 W m{sup -1} K.« less

Correlation and nuclear distortion effects of Cr-substituted ZnSe.

PubMed

Tablero, C

2007-04-28

There is a great deal of interest in the effect of the correlation and effect of the atomic distortion in materials with a metallic intermediate band. This band, situated within the semiconductor band gaps, would be split, thus creating two bands, a full one below the Fermi energy and an empty one above it, i.e., a metal-insulator transition. This basic electronic band structure corresponds to intermediate band materials and is characteristic of transparent-conducting oxides, up and down converters, and intermediate band solar cells. A sufficiently high density of Cr in ZnSe substituting the Zn atoms leads to a microscopic intermediate band, in which these effects will be analyzed. A Hubbard term has been included to improve the description of the many-body effect. This term modifies the bandwidth of the intermediate band, the Fermi energy, and breaks the orbital-occupation degeneracy. From the results, the intermediate band is not split within the range of Hubbard term values analyzed and for Cr substituting Zn from 0.463% to 3.125% of Cr atomic concentration.

Absolute band structure determination on naturally occurring rutile with complex chemistry: Implications for mineral photocatalysis on both Earth and Mars

NASA Astrophysics Data System (ADS)

Li, Yan; Xu, Xiaoming; Li, Yanzhang; Ding, Cong; Wu, Jing; Lu, Anhuai; Ding, Hongrui; Qin, Shan; Wang, Changqiu

2018-05-01

Rutile is the most common and stable form of TiO2 that ubiquitously existing on Earth and other terrestrial planets like Mars. Semiconducting mineral such as rutile-based photoredox reactions have been considered to play important roles in geological times. However, due to the inherent complexity in chemistry, the precision determination on band structure of natural rutile and the theoretical explanation on its solar-driven photochemistry have been hardly seen yet. Considering the multiple minor and trace elements in natural rutile, we firstly obtained the single-crystal crystallography, mineralogical composition and defects characteristic of the rutile sample by using both powder and single crystal X-ray diffraction, electron microprobe analysis and X-ray photoelectron spectroscopy. Then, the band gap was accurately determined by synchrotron-based O K-edge X-ray absorption and emission spectra, which was firstly applied to natural rutile due to its robustness on compositions and defects. The absolute band edges of the rutile sample was calculated by considering the electronegativity of the atoms, band gap and point of zero charge. Besides, after detecting the defect energy levels by photoluminescence spectra, we drew the schematic band structure of natural rutile. The band gap (2.7 eV) of natural rutile was narrower than that of synthetic rutile (3.0 eV), and the conduction and valence band edges of natural rutile at pH = pHPZC were determined to be -0.04 V and 2.66 V (vs. NHE), respectively. The defect energy levels located at nearly the middle position of the forbidden band. Further, we used theoretical calculations to verify the isomorphous substitution of Fe and V for Ti gave rise to the distortion of TiO6 octahedron and created vacancy defects in natural rutile. Based on density functional theory, the narrowed band gap was interpreted to the contribution of Fe-3d and V-3d orbits, and the defect energy state was formed by hybridization of O-2p and Fe/V/Ti-3d orbits in the forbidden band. Therefore, excitons can be created under visible light. The conduction band electrons and valence band holes enabled the photoreduction of CO2 to organic molecules (e.g., acetic acid and CH4) and photooxidative generation of oxidants (e.g., radOH, O2 and ClO4-) via rutile photocatalysis, respectively. This study underlies the capability of natural semiconducting minerals in solar energy utilization and the implications of their photocatalysis in both the origin of primitive life on Earth and formation of modern environments on Mars.

Optical and electrical properties of mechanochemically synthesized nanocrystalline delafossite CuAlO2.

PubMed

Prakash, T; Prasad, K Padma; Ramasamy, S; Murty, B S

2008-08-01

Nanocrystalline p-type semiconductor copper aluminum oxide (CuAlO2) has been synthesized by mechanical alloying using freshly prepared Cu2O and alpha-AlO2O3 nanocrystals in toluene medium. A study on structural property performed with different alloying and post annealing durations, by X-ray diffraction (XRD) reveals the formation of single phase with average crystallite size approximately 45 nm. Optical absorbance onset at 364.5 nm confirms its wide band gap nature (E(g) = 3.4 eV) and the fluorescence emission behaviour (390 nm) confirms its direct band type transition. The activation energy for electrical conduction has been calculated by Arrhenius plots using impedance measurement. Both grain and grain boundary conductivity takes place with almost equal activation energies of approximately 0.45 eV. The paper discusses synthesis, structural, optical and electrical properties of delafossite CuAlO2 in detail.

Band gap engineering of NaTaO3 using density functional theory: a charge compensated codoping strategy.

PubMed

Modak, Brindaban; Srinivasu, K; Ghosh, Swapan K

2014-08-28

In this theoretical study, we employ a codoping strategy to reduce the band gap of NaTaO3 aimed at improving the photocatalytic activity under visible light. The systematic study includes the effects of metal (W) and nonmetal (N) codoping on the electronic structure of NaTaO3 in comparison to the effect of individual dopants. The feasibility of the introduction of N into the NaTaO3 crystal structure is found to be enhanced in the presence of W, as indicated by the calculated formation energy. This codoping leads to formation of a charge compensated system, beneficial for the minimization of vacancy related defect formation. The electronic structure calculations have been carried out using a hybrid density functional for an accurate description of the proposed system. The introduction of W in place of Ta leads to the appearance of donor states below the conduction band, while N doping in place of oxygen introduces isolated acceptor states above the valence band. The codoping of N and W also passivates undesirable discrete midgap states. This feature is not observed in the case of (Cr, N) codoped NaTaO3 in spite of its charge compensated nature. We have also studied charge non-compensated codoping using several dopant pairs, including anion-anion and cation-anion pairs. However, this non-compensated codoping introduces localized states in between the valence band and the conduction band, and hence may not be effective in enhancing the photocatalytic properties of NaTaO3. The optical spectrum shows that the absorption curve for the (W, N)-codoped NaTaO3 is extended to the visible region due to narrowing of the band gap to 2.67 eV. Moreover, its activity for the photo decomposition of water to produce both H2 and O2 remains intact. Hence, based on the present investigation we can propose (W, N) codoped NaTaO3 as a promising photocatalyst for visible light driven water splitting.

Radiative transitions in highly doped and compensated chalcopyrites and kesterites: The case of Cu2ZnSnS4

NASA Astrophysics Data System (ADS)

Teixeira, J. P.; Sousa, R. A.; Sousa, M. G.; da Cunha, A. F.; Fernandes, P. A.; Salomé, P. M. P.; Leitão, J. P.

2014-12-01

The theoretical models of radiative recombinations in both CuIn1 -xGaxSe2 chalcopyrite and Cu2ZnSnS4 kesterite, and related compounds, were revised. For heavily doped materials, electrons are free or bound to large donor agglomerates which hinders the involvement of single donors in the radiative recombination channels. In this work, we investigated the temperature and excitation power dependencies of the photoluminescence of Cu2ZnSnS4-based solar cells in which the absorber layer was grown through sulphurization of multiperiod structures of precursor layers. For both samples the luminescence is dominated by an asymmetric band with peak energy at ˜1.22 eV, which is influenced by fluctuating potentials in both conduction and valence bands. A value of ˜60 meV was estimated for the root-mean-square depth of the tails in the conduction band. The radiative transitions involve the recombination of electrons captured by localized states in tails of the conduction band with holes localized in neighboring acceptors that follow the fluctuations in the valence band. The same acceptor level with an ionization energy of ˜280 meV was identified in both absorber layers. The influence of fluctuating potentials in the electrical performance of the solar cells was discussed.

A computational study on the photoelectric properties of various Bi2O3 polymorphs as visible-light driven photocatalysts.

PubMed

Wang, Fang; Cao, Kun; Zhang, Qian; Gong, Xuedong; Zhou, Ying

2014-11-01

This paper presents first-principle studies on the photoelectric properties of various Bi2O3 polymorphs. The intrinsic reason of different photocatalytic activities was revealed by electronic structures and optical features. Results showed that for α, β, and γ-Bi2O3, the top of valence bands were mainly constructed by Bi6s and O2p orbitals, and the bottom of conduction bands were dominantly composed by Bi6p orbital. However, two intermediate bands were found at the Fermi level for γ-Bi2O3, which leads to a two-step transition from the top of valence band to the bottom of conduction band and facilitates electron transition under irradiation. Absent forbidden gap was found in δ-Bi2O3, resulting in a semimetallic character due to its intrinsic oxygen vacancy and high ionic conductivity. Moreover, the optical properties of α, β, and γ-Bi2O3 were investigated by absorption spectrum, dielectric constant function, and energy loss spectroscopy. We concluded that the photocatalytic activities followed in the order of γ-Bi2O3 > β-Bi2O3 > α-Bi2O3, in accord with the experimental report. Calculation results illustrated the experimental observations and provided a useful guidance in exploring promising visible-light semiconductor photocatalysts.

Side-looking airborne radar image interpretation and geological mapping: Problems and results

NASA Technical Reports Server (NTRS)

Scanvic, J. Y.; Soubari, E. H.

1980-01-01

Geological experiments and surveys conducted by BRGM and GDTA members to evaluate interest in SLAR image interpretation are summarized. Two surveys were selected for presentation: Les Vans (Massif central, France) and Guyana (South America). They have permitted a comparison between different types of SLAR: Goodyear, Motorola, JPL, and Vigie in term of lithological and structural applications. On the Les Vans test site conclusions reached concern radiometry, which is better on L-band imagery, polarization, HV being more useful than HH for geological mapping in an L-band system, wavelength and illuminations. Over Guyana, the use of Goodyear X-band SLAR enables satisfactory geological and structural mapping under heavy equatorial forest with cloud cover conditions. A differential program was developed for fracture filtering and image enhancement with a coherent light laser, and significant results were obtained.

The Electronic Structure and Optical Properties of Anatase TiO₂ with Rare Earth Metal Dopants from First-Principles Calculations.

PubMed

Xie, Kefeng; Jia, Qiangqiang; Wang, Yizhe; Zhang, Wenxue; Xu, Jingcheng

2018-01-24

The electronic and optical properties of the rare earth metal atom-doped anatase TiO₂ have been investigated systematically via density functional theory calculations. The results show that TiO₂ doped by Ce or Pr is the optimal choice because of its small band gap and strong optical absorption. Rare earth metal atom doping induces several impurity states that tune the location of valence and conduction bands and an obvious lattice distortion that should reduce the probability of electron-hole recombination. This effect of band change originates from the 4 f electrons of the rare earth metal atoms, which leads to an improved visible light absorption. This finding indicates that the electronic structure of anatase TiO₂ is tuned by the introduction of impurity atoms.

The Electronic Structure and Optical Properties of Anatase TiO2 with Rare Earth Metal Dopants from First-Principles Calculations

PubMed Central

Xie, Kefeng; Jia, Qiangqiang; Wang, Yizhe; Zhang, Wenxue; Xu, Jingcheng

2018-01-01

The electronic and optical properties of the rare earth metal atom-doped anatase TiO2 have been investigated systematically via density functional theory calculations. The results show that TiO2 doped by Ce or Pr is the optimal choice because of its small band gap and strong optical absorption. Rare earth metal atom doping induces several impurity states that tune the location of valence and conduction bands and an obvious lattice distortion that should reduce the probability of electron–hole recombination. This effect of band change originates from the 4f electrons of the rare earth metal atoms, which leads to an improved visible light absorption. This finding indicates that the electronic structure of anatase TiO2 is tuned by the introduction of impurity atoms. PMID:29364161

Ab Initio Calculations of Transport Properties of Vanadium Oxides

NASA Astrophysics Data System (ADS)

Lamsal, Chiranjivi; Ravindra, N. M.

2018-04-01

The temperature-dependent transport properties of vanadium oxides have been studied near the Fermi energy using the Kohn-Sham band structure approach combined with Boltzmann transport equations. V2O5 exhibits significant thermoelectric properties, which can be attributed to its layered structure and stability. Highly anisotropic electrical conduction in V2O5 is clearly manifested in the calculations. Due to specific details of the band structure and anisotropic electron-phonon interactions, maxima and crossovers are also seen in the temperature-dependent Seebeck coefficient of V2O5. During the phase transition of VO2, the Seebeck coefficient changes by 18.9 µV/K, which is close to (within 10% of) the observed discontinuity of 17.3 µV/K.

Microstructure and conductance-slope of InAs/GaSb tunnel diodes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Iutzi, Ryan M., E-mail: iutzi@mit.edu; Fitzgerald, Eugene A.

2014-06-21

InAs/GaSb and similar materials systems have generated great interest as a heterojunction for tunnel field effect transistors (TFETs) due to favorable band alignment. However, little is currently understood about how such TFETs are affected by materials defects and nonidealities. We present measurements of the conductance slope for various InAs/GaSb heterojunctions via two-terminal electrical measurements, which removes three-terminal parasitics and enables direct study on the effect of microstructure on tunnelling. Using this, we can predict how subthreshold swings in TFETs can depend on microstructure. We also demonstrate growth and electrical characterization for structures grown by metalorganic chemical vapor deposition (MOCVD)—a generallymore » more scalable process compared with molecular beam epitaxy (MBE). We determine that misfit dislocations and point defects near the interface can lead to energy states in the band-gap and local band bending that result in trap-assisted leakage routes and nonuniform band alignment across the junction area that lower the steepness of the conductance slope. Despite the small lattice mismatch, misfit dislocations still form in InAs on GaSb due to relaxation as a result of large strain from intermixed compositions. This can be circumvented by growing GaSb on InAs, straining the GaSb underlayer, or lowering the InAs growth temperature in the region of the interface. The conductance slope can also be improved by annealing the samples at higher temperatures, which we believe acts to annihilate point defects and average out major fluctuations in band alignment across the interface. Using a combination of these techniques, we can greatly improve the steepness of the conductance slope which could result in steeper subthreshold swings in TFETs in the future.« less

Characterization of Lorenz number with Seebeck coefficient measurement

DOE PAGES

Kim, Hyun -Sik; Gibbs, Zachary M.; Tang, Yinglu; ...

2015-04-01

In analyzing zT improvements due to lattice thermal conductivity (κ L ) reduction, electrical conductivity (σ) and total thermal conductivity (κ Total ) are often used to estimate the electronic component of the thermal conductivity (κ E ) and in turn κ L from κ L = ~ κ Total - LσT. The Wiedemann-Franz law, κ E = LσT, where L is Lorenz number, is widely used to estimate κ E from σ measurements. It is a common practice to treat L as a universal factor with 2.44 × 10⁻⁸ WΩK⁻² (degenerate limit). However, significant deviations from the degenerate limitmore » (approximately 40% or more for Kane bands) are known to occur for non-degenerate semiconductors where L converges to 1.5 × 10⁻⁸ WΩK⁻² for acoustic phonon scattering. The decrease in L is correlated with an increase in thermopower (absolute value of Seebeck coefficient (S)). Thus, a first order correction to the degenerate limit of L can be based on the measured thermopower, |S|, independent of temperature or doping. We propose the equation: (where L is in 10⁻⁸ WΩK⁻² and S in μV/K) as a satisfactory approximation for L. This equation is accurate within 5% for single parabolic band/acoustic phonon scattering assumption and within 20% for PbSe, PbS, PbTe, Si₀.₈Ge₀.₂ where more complexity is introduced, such as non-parabolic Kane bands, multiple bands, and/or alternate scattering mechanisms. The use of this equation for L rather than a constant value (when detailed band structure and scattering mechanism is not known) will significantly improve the estimation of lattice thermal conductivity. L = 1.5 + exp [-|S|116]« less

Crystal growth and structure, electrical, and optical characterization of the semiconductor Cu2SnSe3

NASA Astrophysics Data System (ADS)

Marcano, G.; Rincón, C.; de Chalbaud, L. M.; Bracho, D. B.; Pérez, G. Sánchez

2001-08-01

X-ray powder diffraction by p-type Cu2SnSe3, prepared by the vertical Bridgman-Stockbarger technique, shows that this material crystallizes in a monoclinic structure, space group Cc, with unit cell parameters a=6.5936(1) Å, b=12.1593(4) Å, c=6.6084(3) Å, and β=108.56(2)°. The temperature variation of the hole concentration p obtained from the Hall effect and electrical resistivity measurements from about 160 to 300 K, is explained as due to the thermal activation of an acceptor level with an ionization energy of 0.067 eV, whereas below 100 K, the conduction in the impurity band dominates the electrical transport process. From the analysis of the p vs T data, the density-of-states effective mass of the holes is estimated to be nearly of the same magnitude as the free electron mass. In the valence band, the temperature variation of the hole mobility is analyzed by taking into account the scattering of charge carriers by ionized and neutral impurities, and acoustic phonons. In the impurity band, the mobility is explained as due to the thermally activated hopping transport. From the analysis of the optical absorption spectra at room temperature, the fundamental energy gap was determined to be 0.843 eV. The photoconductivity spectra show the presence of a narrow band gap whose main peak is observed at 0.771 eV. This band is attributed to a free-to-bound transition from the defect acceptor level to the conduction band. The origin of this acceptor state, consistent with the chemical composition of the samples and screening effects, is tentatively attributed to selenium interstitials.

Electronic, optical and photocatalytic behavior of Mn, N doped and co-doped TiO{sub 2}: Experiment and simulation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhao, Ya Fei; Li, Can, E-mail: canli1983@gmail.com; Lu, Song

2016-03-15

The crystal phase structure, surface morphology, chemical states and optical properties of Mn, N mono-doped and co-doped TiO{sub 2} nanoparticles were investigated by X-ray powder diffractometry, Raman spectra, scanning electron microscopy, X-ray photoelectron spectroscopy and UV–vis diffuse reflectance spectroscopy. Meanwhile, geometry structures, formation energies, electronic and optical properties of all systems have been also analyzed by density functional theory. The results showed that the band gap values and the carrier mobility in the valence band, conduction band and impurity levels have a synergetic influence on the visible-light absorption and photocatalytic activity of the doped TiO{sub 2}. The number and themore » carrier mobility of impurity level jointly influence the photocatalytic activity of catalyst under visible-light. Especially, the photocatalytic activity of Mn-2N co-doped TiO{sub 2} beyond three-fold than that of pure TiO{sub 2} under visible-light. - Graphical abstract: The ILs formed by N-2p orbital in N single doped specimen lie above the VB, while the ILs formed by Mn-3d orbital in Mn single doped specimen appear below the CB. However, a large amount of ILs formed by N-2p orbital and Mn-3d orbital in N and Mn codoped specimens. The band gap values and the carrier mobility in the valence band, conduction band and impurity levels have a synergetic influence on the visible-light absorption and photocatalytic activity of the doped TiO{sub 2}. The number and the carrier mobility of impurity level jointly influence the photocatalytic activity of catalyst under visible-light.« less

Thermoelectric properties of layered NaSbSe2.

PubMed

Putatunda, Aditya; Xing, Guangzong; Sun, Jifeng; Li, Yuwei; Singh, David J

2018-06-06

We investigate ordered monoclinic NaSbSe 2 as a thermoelectric using first principles calculations. We find that from an electronic point of view, ordered and oriented n-type NaSbSe 2 is comparable to the best known thermoelectric materials. This phase has a sufficiently large band gap for thermoelectric and solar absorber applications in contrast to the disordered phase which has a much narrower gap. The electronic structure shows anisotropic, non-parabolic bands. The results show a high Seebeck coefficient in addition to direction dependent high conductivity. The electronic structure quantified by an electron fitness function is very favorable, especially in the n-type case.

Electronic structure, transport, and phonons of SrAg ChF ( Ch = S,Se,Te): Bulk superlattice thermoelectrics

DOE PAGES

Gudelli, Vijay Kumar; Kanchana, V.; Vaitheeswaran, G.; ...

2015-07-15

Here, we report calculations of the electronic structure, vibrational properties, and transport for the p-type semiconductors, SrAg ChF ( Ch = S, Se, and Te). We find soft phonons with low frequency optical branches intersecting the acoustic modes below 50 cm –1, indicative of a material with low thermal conductivity. The bands at and near the valence-band maxima are highly two-dimensional, which leads to high thermopowers even at high carrier concentrations, which is a combination that suggests good thermoelectric performance. These materials may be regarded as bulk realizations of superlattice thermoelectrics.

Thermoelectric properties of layered NaSbSe2

NASA Astrophysics Data System (ADS)

Putatunda, Aditya; Xing, Guangzong; Sun, Jifeng; Li, Yuwei; Singh, David J.

2018-06-01

We investigate ordered monoclinic NaSbSe2 as a thermoelectric using first principles calculations. We find that from an electronic point of view, ordered and oriented n-type NaSbSe2 is comparable to the best known thermoelectric materials. This phase has a sufficiently large band gap for thermoelectric and solar absorber applications in contrast to the disordered phase which has a much narrower gap. The electronic structure shows anisotropic, non-parabolic bands. The results show a high Seebeck coefficient in addition to direction dependent high conductivity. The electronic structure quantified by an electron fitness function is very favorable, especially in the n-type case.

Direct imaging of band profile in single layer MoS2 on graphite: quasiparticle energy gap, metallic edge states, and edge band bending.

PubMed

Zhang, Chendong; Johnson, Amber; Hsu, Chang-Lung; Li, Lain-Jong; Shih, Chih-Kang

2014-05-14

Using scanning tunneling microscopy and spectroscopy, we probe the electronic structures of single layer MoS2 on graphite. The apparent quasiparticle energy gap of single layer MoS2 is measured to be 2.15 ± 0.06 eV at 77 K, albeit a higher second conduction band threshold at 0.2 eV above the apparent conduction band minimum is also observed. Combining it with photoluminescence studies, we deduce an exciton binding energy of 0.22 ± 0.1 eV (or 0.42 eV if the second threshold is use), a value that is lower than current theoretical predictions. Consistent with theoretical predictions, we directly observe metallic edge states of single layer MoS2. In the bulk region of MoS2, the Fermi level is located at 1.8 eV above the valence band maximum, possibly due to the formation of a graphite/MoS2 heterojunction. At the edge, however, we observe an upward band bending of 0.6 eV within a short depletion length of about 5 nm, analogous to the phenomena of Fermi level pinning of a 3D semiconductor by metallic surface states.

Spin- and Valley-Dependent Electronic Structure in Silicene Under Periodic Potentials

NASA Astrophysics Data System (ADS)

Lu, Wei-Tao; Li, Yun-Fang; Tian, Hong-Yu

2018-03-01

We study the spin- and valley-dependent energy band and transport property of silicene under a periodic potential, where both spin and valley degeneracies are lifted. It is found that the Dirac point, miniband, band gap, anisotropic velocity, and conductance strongly depend on the spin and valley indices. The extra Dirac points appear as the voltage potential increases, the critical values of which are different for electron with different spins and valleys. Interestingly, the velocity is greatly suppressed due to the electric field and exchange field, other than the gapless graphene. It is possible to achieve an excellent collimation effect for a specific spin near a specific valley. The spin- and valley-dependent band structure can be used to adjust the transport, and perfect transmissions are observed at Dirac points. Therefore, a remarkable spin and valley polarization is achieved which can be switched effectively by the structural parameters. Importantly, the spin and valley polarizations are greatly enhanced by the disorder of the periodic potential.

Quantum spin Hall state in monolayer 1T '-WTe 2

DOE PAGES

Tang, Shujie; Zhang, Chaofan; Wong, Dillon; ...

2017-06-26

A quantum spin Hall (QSH) insulator is a novel two-dimensional quantum state of matter that features quantized Hall conductance in the absence of a magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin–orbit coupling. By investigating the electronic structure of epitaxially grown monolayer 1T '-WTe 2 using angle-resolved photoemission (ARPES) and first-principles calculations, we observe clear signatures of topological band inversion and bandgap opening, which are the hallmarks of a QSH state. Scanning tunnelling microscopy measurements further confirm the correct crystal structure and the existence of a bulkmore » bandgap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Our results establish monolayer 1T '-WTe 2 as a new class of QSH insulator with large band gap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs).« less

Quantum spin Hall state in monolayer 1T '-WTe 2

DOE PAGES

Tang, Shujie; Zhang, Chaofan; Wong, Dillon; ...

2017-06-26

A quantum spin Hall (QSH) insulator is a novel two-dimensional quantum state of matter that features quantized Hall conductance in the absence of a magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin–orbit coupling. By investigating the electronic structure of epitaxially grown monolayer 1T '-WTe 2 using angle-resolved photoemission (ARPES) and first-principles calculations, we observe clear signatures of topological band inversion and bandgap opening, which are the hallmarks of a QSH state. Scanning tunnelling microscopy measurements further confirm the correct crystal structure and the existence of a bulkmore » bandgap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Finally, our results establish monolayer 1T '-WTe 2 as a new class of QSH insulator with large band gap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs).« less

Strain engineering on electronic structure and carrier mobility in monolayer GeP3

NASA Astrophysics Data System (ADS)

Zeng, Bowen; Long, Mengqiu; Zhang, Xiaojiao; Dong, Yulan; Li, Mingjun; Yi, Yougen; Duan, Haiming

2018-06-01

Using density functional theory coupled with the Boltzmann transport equation with relaxation time approximation, we have studied the strain effect on the electronic structure and carrier mobility of two-dimensional monolayer GeP3. We find that the energies of valence band maximum and conduction band minimum are nearly linearly shifted with a biaxial strain in the range of  ‑4% to 6%, and the band structure experiences a remarkable transition from semiconductor to metal with the appropriate compression (‑5% strain). Under biaxial strain, the mobility of the electron and hole in monolayer GeP3 reduces and increases by more than one order of magnitude, respectively. It is suggested that it is possible to perform successive transitions from an n-type semiconductor (‑4% strain) to a good performance p-semiconductor (+6% strain) by applying strain in monolayer GeP3, which is potentially useful for flexible electronics and nanosized mechanical sensors.

The metallic state in neutral radical conductors: dimensionality, pressure and multiple orbital effects.

PubMed

Tian, Di; Winter, Stephen M; Mailman, Aaron; Wong, Joanne W L; Yong, Wenjun; Yamaguchi, Hiroshi; Jia, Yating; Tse, John S; Desgreniers, Serge; Secco, Richard A; Julian, Stephen R; Jin, Changqing; Mito, Masaki; Ohishi, Yasuo; Oakley, Richard T

2015-11-11

Pressure-induced changes in the solid-state structures and transport properties of three oxobenzene-bridged bisdithiazolyl radicals 2 (R = H, F, Ph) over the range 0-15 GPa are described. All three materials experience compression of their π-stacked architecture, be it (i) 1D ABABAB π-stack (R = Ph), (ii) quasi-1D slipped π-stack (R = H), or (iii) 2D brick-wall π-stack (R = F). While R = H undergoes two structural phase transitions, neither of R = F, Ph display any phase change. All three radicals order as spin-canted antiferromagnets, but spin-canted ordering is lost at pressures <1.5 GPa. At room temperature, their electrical conductivity increases rapidly with pressure, and the thermal activation energy for conduction Eact is eliminated at pressures ranging from ∼3 GPa for R = F to ∼12 GPa for R = Ph, heralding formation of a highly correlated (or bad) metallic state. For R = F, H the pressure-induced Mott insulator to metal conversion has been tracked by measurements of optical conductivity at ambient temperature and electrical resistivity at low temperature. For R = F compression to 6.2 GPa leads to a quasiquadratic temperature dependence of the resistivity over the range 5-300 K, consistent with formation of a 2D Fermi liquid state. DFT band structure calculations suggest that the ease of metallization of these radicals can be ascribed to their multiorbital character. Mixing and overlap of SOMO- and LUMO-based bands affords an increased kinetic energy stabilization of the metallic state relative to a single SOMO-based band system.

Tunability of the fractional quantum Hall states in buckled Dirac materials

NASA Astrophysics Data System (ADS)

Apalkov, Vadym M.; Chakraborty, Tapash

2014-12-01

We report on the fractional quantum Hall states of germanene and silicene where one expects a strong spin-orbit interaction. This interaction causes an enhancement of the electron-electron interaction strength in one of the Landau levels corresponding to the valence band of the system. This enhancement manifests itself as an increase of the fractional quantum Hall effect gaps compared to that in graphene and is due to the spin-orbit induced coupling of the Landau levels of the conduction and valence bands, which modifies the corresponding wave functions and the interaction within a single level. Due to the buckled structure, a perpendicular electric field lifts the valley degeneracy and strongly modifies the interaction effects within a single Landau level: in one valley the perpendicular electric field enhances the interaction strength in the conduction band Landau level, while in another valley, the electric field strongly suppresses the interaction effects.

An impurity intermediate band due to Pb doping induced promising thermoelectric performance of Ca5In2Sb6.

PubMed

Feng, Zhenzhen; Wang, Yuanxu; Yan, Yuli; Zhang, Guangbiao; Yang, Jueming; Zhang, Jihua; Wang, Chao

2015-06-21

Band engineering is one of the effective approaches for designing ideal thermoelectric materials. Introducing an intermediate band in the band gap of semiconducting thermoelectric compounds may largely increase the carrier concentration and improve the electrical conductivity of these compounds. We test this hypothesis by Pb doping in Zintl Ca5In2Sb6. In the current work, we have systematically investigated the electronic structure and thermoelectric performances of substitutional doping with Pb on In sites at a doping level of 5% (0.2 e per cell) for Ca5In2Sb6 by using density functional theory combined with semi-classical Boltzmann theory. It is found that in contrast to Zn doping, Pb doping introduces a partially filled intermediate band in the band gap of Ca5In2Sb6, which originates from the Pb s states by weakly hybridizing with the Sb p states. Such an intermediate band dramatically increases the electrical conductivity of Ca5In2Sb6 and has little detrimental effect on its Seebeck coefficient, which may increase its thermoelectric figure of merit, ZT. Interestingly, a maximum ZT value of 2.46 may be achieved at 900 K for crystalline Pb-doped Ca5In2Sb6 when the carrier concentration is optimized. Therefore, Pb-doped Ca5In2Sb6 may be a promising thermoelectric material.

Lifetime enhancement for multiphoton absorption in intermediate band solar cells

NASA Astrophysics Data System (ADS)

Bezerra, Anibal T.; Studart, Nelson

2017-08-01

A semiconductor structure consisting of two coupled quantum wells embedded into the intrinsic region of a p-i-n junction is proposed as an intermediate band solar cell with a photon ratchet state, which would lead to increasing the cell efficiency. The conduction subband of the right-hand side quantum well works as the intermediated band, whereas the excited conduction subband of the left-hand side quantum well operates as the ratchet state. The photoelectrons in the intermediate band are scattered through the thin wells barrier and accumulated into the ratchet subband. A rate equation model for describing the charge transport properties is presented. The efficiency of the current generation is analyzed by studying the occupation of the wells subbands, taking into account the charge dynamic behavior provided by the electrical contacts connected to the cell. The current generation efficiency depends essentially from the relations between the generation, recombination rates and the scattering rate to the ratchet state. The inclusion of the ratchet states led to both an increase and a decrease in the cell current depending on the transition rates. This suggests that the coupling between the intermediate band and the ratchet state is a key point in developing an efficient solar cell.

Electrical conductivity of a locked fault: investigation of the Ganos segment of the North Anatolian Fault using three-dimensional magnetotellurics

NASA Astrophysics Data System (ADS)

Karaş, Mustafa; Tank, Sabri Bülent; Özaydın, Sinan

2017-08-01

This study attempts to reveal the fault zone characteristics of the locked Ganos Fault based on electrical resistivity studies including audio-frequency (AMT: 10,400-1 Hz) and wide-band (MT: 360-0.000538 Hz) magnetotellurics near the epicenter of the last major event, that is, the 1912 Mürefte Earthquake ( M w 7.4). The AMT data were collected at twelve stations, closely spaced from north to south, to resolve the shallow resistivity structure to 1 km depth. Subsequently, 13 wide-band MT stations were arranged to form a grid enclosing the AMT profile to decipher the deeper structure. Three-dimensional inverse modeling indicates highly conductive anomalies representing fault zone conductors along the Ganos Fault. Subsidiary faults around the Ganos Fault, which are conductive structures with individual mechanically weak features, merge into a greater damage zone, creating a wide fluid-bearing environment. This damage zone is located on the southern side of the fault and defines an asymmetry around the main fault strand, which demonstrates distributed conduit behavior of fluid flow. Ophiolitic basement occurs as low-conductivity block beneath younger formations at a depth of 2 km, where the mechanically weak to strong transition occurs. Resistive structures on both sides of the fault beneath this transition suggest that the lack of seismicity might be related to the absence of fluid pathways in the seismogenic zone.[Figure not available: see fulltext.

Investigation of the electronic, magnetic and optical properties of {\\sf Co}_{\\sf 2}{\\sf CrZ} (Z = Si, Ge) under pressure—a density functional theory study

NASA Astrophysics Data System (ADS)

Seema, K.; Kumar, Ranjan

2014-01-01

The structural, electronic, magnetic and optical properties of Co-based Heusler compounds, Co2CrZ (Z = Si, Ge), are studied using first-principle density functional theory. The calculations are performed within the generalized gradient approximation. Our calculated structural parameters at 0 GPa agree well with previous available results. The calculated magnetic moment agrees well with the Slater-Pauling (SP) rule. We have studied the effect of pressure on the electronic and magnetic properties of Co2CrSi and Co2CrGe. With an increase in applied pressure, a decrease in cell volume is observed. Under application of external pressure, the valence band and conduction band are shifted downward which leads to a modification of electronic structure. There exists an indirect band gap along Γ-X for both the alloys. Co2CrSi and Co2CrGe retain 100% spin polarization up to 60 and 50 GPa, respectively. The local magnetic moments of the Co and Si (Ge) atoms increase with an increase in pressure whereas the local magnetic moment of the Cr atom decreases. In addition, the optical properties such as dielectric function, absorption spectra, optical conductivity and energy loss function of these alloys have also been investigated. To our knowledge this is the first theoretical prediction of the pressure dependence of the structural, electronic, magnetic and optical properties of Co2CrSi and Co2CrGe.

Robust indirect band gap and anisotropy of optical absorption in B-doped phosphorene.

PubMed

Wu, Zhi-Feng; Gao, Peng-Fei; Guo, Lei; Kang, Jun; Fang, Dang-Qi; Zhang, Yang; Xia, Ming-Gang; Zhang, Sheng-Li; Wen, Yu-Hua

2017-12-06

A traditional doping technique plays an important role in the band structure engineering of two-dimensional nanostructures. Since electron interaction is changed by doping, the optical and electrochemical properties could also be significantly tuned. In this study, density functional theory calculations have been employed to explore the structural stability, and electronic and optical properties of B-doped phosphorene. The results show that all B-doped phosphorenes are stable with a relatively low binding energy. Of particular interest is that these B-doped systems exhibit an indirect band gap, which is distinct from the direct one of pure phosphorene. Despite the different concentrations and configurations of B dopants, such indirect band gaps are robust. The screened hybrid density functional HSE06 predicts that the band gap of B-doped phosphorene is slightly smaller than that of pure phosphorene. Spatial charge distributions at the valence band maximum (VBM) and the conduction band minimum (CBM) are analyzed to understand the features of an indirect band gap. By comparison with pure phosphorene, B-doped phosphorenes exhibit strong anisotropy and intensity of optical absorption. Moreover, B dopants could enhance the stability of Li adsorption on phosphorene with less sacrifice of the Li diffusion rate. Our results suggest that B-doping is an effective way of tuning the band gap, enhancing the intensity of optical absorption and improving the performances of Li adsorption, which could promote potential applications in novel optical devices and lithium-ion batteries.

Sizable band gap in organometallic topological insulator

NASA Astrophysics Data System (ADS)

Derakhshan, V.; Ketabi, S. A.

2017-01-01

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

Best Manufacturing Practices Survey Conducted at Litton Data Systems Division, Van Nuys, California

DTIC Science & Technology

1988-10-01

Hardware and Software ................................ 10 DESIGN RELEASE Engineering Change Order Processing and Analysis...structured using bridges to isolate local traffic. Long term plans call for a wide-band network. ENGINEERING CHANGE ORDER PROCESSING AND ANALYSIS

Pressure-Dependent Electronic and Transport Properties of Bulk Platinum Oxide by Density Functional Theory

NASA Astrophysics Data System (ADS)

Kansara, Shivam; Gupta, Sanjeev K.; Sonvane, Yogesh; Nekrasov, Kirill A.; Kichigina, Natalia V.

2018-02-01

The structural, electronic, and vibrational properties of bulk platinum oxide (PtO) at compressive pressures in the interval from 0 GPa to 35 GPa are investigated using the density functional theory. The calculated electronic band structure of PtO shows poor metallicity at very low density of states on the Fermi level. However, the hybrid pseudopotential calculation yielded 0.78 eV and 1.30 eV direct band and indirect gap, respectively. Importantly, our results predict that PtO has a direct band gap within the framework of HSE06, and it prefers equally zero magnetic order at different pressures. In the Raman spectra, peaks are slightly shifted towards higher frequency with the decrease in pressure. We have also calculated the thermoelectric properties, namely the electronic thermal conductivity and electrical conductivity, with respect to temperature and thermodynamic properties such as entropy, specific heat at constant volume, enthalpy and Gibbs free energy with respect to pressure. The result shows that PtO is a promising candidate for use as a catalyst, in sensors, as a photo-cathode in water electrolysis, for thermal decomposition of inorganic salt and fuel cells.

Quantum sized gold nanoclusters with atomic precision.

PubMed

Qian, Huifeng; Zhu, Manzhou; Wu, Zhikun; Jin, Rongchao

2012-09-18

Gold nanoparticles typically have a metallic core, and the electronic conduction band consists of quasicontinuous energy levels (i.e. spacing δ ≪ k(B)T, where k(B)T is the thermal energy at temperature T (typically room temperature) and k(B) is the Boltzmann constant). Electrons in the conduction band roam throughout the metal core, and light can collectively excite these electrons to give rise to plasmonic responses. This plasmon resonance accounts for the beautiful ruby-red color of colloidal gold first observed by Faraday back in 1857. On the other hand, when gold nanoparticles become extremely small (<2 nm in diameter), significant quantization occurs to the conduction band. These quantum-sized nanoparticles constitute a new class of nanomaterial and have received much attention in recent years. To differentiate quantum-sized nanoparticles from conventional plasmonic gold nanoparticles, researchers often refer to the ultrasmall nanoparticles as nanoclusters. In this Account, we chose several typical sizes of gold nanoclusters, including Au(25)(SR)(18), Au(38)(SR)(24), Au(102)(SR)(44), and Au(144)(SR)(60), to illustrate the novel properties of metal nanoclusters imparted by quantum size effects. In the nanocluster size regime, many of the physical and chemical properties of gold nanoparticles are fundamentally altered. Gold nanoclusters have discrete electronic energy levels as opposed to the continuous band in plasmonic nanoparticles. Quantum-sized nanoparticles also show multiple optical absorption peaks in the optical spectrum versus a single surface plasmon resonance (SPR) peak at 520 nm for spherical gold nanocrystals. Although larger nanocrystals show an fcc structure, nanoclusters often have non-fcc atomic packing structures. Nanoclusters also have unique fluorescent, chiral, and magnetic properties. Due to the strong quantum confinement effect, adding or removing one gold atom significantly changes the structure and the electronic and optical properties of the nanocluster. Therefore, precise atomic control of nanoclusters is critically important: the nanometer precision typical of conventional nanoparticles is not sufficient. Atomically precise nanoclusters are represented by molecular formulas (e.g. Au(n)(SR)(m) for thiolate-protected ones, where n and m denote the respective number of gold atoms and ligands). Recently, major advances in the synthesis and structural characterization of molecular purity gold nanoclusters have made in-depth investigations of the size evolution of metal nanoclusters possible. Metal nanoclusters lie in the intermediate regime between localized atomic states and delocalized band structure in terms of electronic properties. We anticipate that future research on quantum-sized nanoclusters will stimulate broad scientific and technological interests in this special type of metal nanomaterial.

Strain-induced enhancement of thermoelectric performance of TiS2 monolayer based on first-principles phonon and electron band structures

NASA Astrophysics Data System (ADS)

Li, Guanpeng; Yao, Kailun; Gao, Guoying

2018-01-01

Using first-principle calculations combined with Boltzmann transport theory, we investigate the biaxial strain effect on the electronic and phonon thermal transport properties of a 1 T (CdI2-type) structural TiS2 monolayer, a recent experimental two-dimensional (2D) material. It is found that the electronic band structure can be effectively modulated and that the band gap experiences an indirect-direct-indirect transition with increasing tensile strain. The band convergence induced by the tensile strain increases the Seebeck coefficient and the power factor, while the lattice thermal conductivity is decreased under the tensile strain due to the decreasing group velocity and the increasing scattering chances between the acoustic phonon modes and the optical phonon modes, which together greatly increase the thermoelectric performance. The figure of merit can reach 0.95 (0.82) at an 8 percent tensile strain for the p-type (n-type) doping, which is much larger than that without strain. The present work suggests that the TiS2 monolayer is a good candidate for 2D thermoelectric materials, and that biaxial strain is a powerful tool with which to enhance thermoelectric performance.

Band structure and spin texture of Bi2Se3 3 d ferromagnetic metal interface

NASA Astrophysics Data System (ADS)

Zhang, Jia; Velev, Julian P.; Dang, Xiaoqian; Tsymbal, Evgeny Y.

2016-07-01

The spin-helical surface states in a three-dimensional topological insulator (TI), such as Bi2Se3 , are predicted to have superior efficiency in converting charge current into spin polarization. This property is said to be responsible for the giant spin-orbit torques observed in ferromagnetic metal/TI structures. In this work, using first-principles and model tight-binding calculations, we investigate the interface between the topological insulator Bi2Se3 and 3 d -transition ferromagnetic metals Ni and Co. We find that the difference in the work functions of the topological insulator and the ferromagnetic metals shift the topological surface states down about 0.5 eV below the Fermi energy where the hybridization of these surface states with the metal bands destroys their helical spin structure. The band alignment of Bi2Se3 and Ni (Co) places the Fermi energy far in the conduction band of bulk Bi2Se3 , where the spin of the carriers is aligned with the magnetization in the metal. Our results indicate that the topological surface states are unlikely to be responsible for the huge spin-orbit torque effect observed experimentally in these systems.

Strain-induced enhancement of thermoelectric performance of TiS2 monolayer based on first-principles phonon and electron band structures.

PubMed

Li, Guanpeng; Yao, Kailun; Gao, Guoying

2018-01-05

Using first-principle calculations combined with Boltzmann transport theory, we investigate the biaxial strain effect on the electronic and phonon thermal transport properties of a 1 T (CdI 2 -type) structural TiS 2 monolayer, a recent experimental two-dimensional (2D) material. It is found that the electronic band structure can be effectively modulated and that the band gap experiences an indirect-direct-indirect transition with increasing tensile strain. The band convergence induced by the tensile strain increases the Seebeck coefficient and the power factor, while the lattice thermal conductivity is decreased under the tensile strain due to the decreasing group velocity and the increasing scattering chances between the acoustic phonon modes and the optical phonon modes, which together greatly increase the thermoelectric performance. The figure of merit can reach 0.95 (0.82) at an 8 percent tensile strain for the p-type (n-type) doping, which is much larger than that without strain. The present work suggests that the TiS 2 monolayer is a good candidate for 2D thermoelectric materials, and that biaxial strain is a powerful tool with which to enhance thermoelectric performance.

2016 International Workshop on Nitride Semiconductors (IWN 2016)

DTIC Science & Technology

2017-01-01

Doping Structure & Photoluminescence Properties of Flower-Like Spiral AIN Micro-Crystal Array Thermal Conductivity of Bulk AIN Direct Determination of...5.03 Optical and Electronic Properties HVPE GaN Wafers with Improved Crystallinity 5:00pm Michael Slomski 01.5.04 Thermal Conductivity of Bulk GaN...Broad-Band Emission Effect of lnter1ayers on the Vertical Electrical Conductivity of Si-Doped AIN/GaN DBRs Grown by PA-MBE Thermal Analys is of

Electric Conduction in Semiconductors: A Pedagogical Model Based on the Monte Carlo Method

ERIC Educational Resources Information Center

Capizzo, M. C.; Sperandeo-Mineo, R. M.; Zarcone, M.

2008-01-01

We present a pedagogic approach aimed at modelling electric conduction in semiconductors in order to describe and explain some macroscopic properties, such as the characteristic behaviour of resistance as a function of temperature. A simple model of the band structure is adopted for the generation of electron-hole pairs as well as for the carrier…

Conductive multi-walled boron nitride nanotubes by catalytic etching using cobalt oxide.

PubMed

Kim, Do-Hyun; Jang, Ho-Kyun; Kim, Min-Seok; Kim, Sung-Dae; Lee, Dong-Jin; Kim, Gyu Tae

2017-01-04

Boron nitride nanotubes (BNNTs) are ceramic compounds which are hardly oxidized below 1000 °C due to their superior thermal stability. Also, they are electrically almost insulators with a large band gap of 5 eV. Thus, it is a challenging task to etch BNNTs at low temperature and to convert their electrical properties to a conductive behavior. In this study, we demonstrate that BNNTs can be easily etched at low temperature by catalytic oxidation, resulting in an electrically conductive behavior. For this, multi-walled BNNTs (MWBNNTs) impregnated with Co precursor (Co(NO 3 ) 2 ·6H 2 O) were simply heated at 350 °C under air atmosphere. As a result, diverse shapes of etched structures such as pits and thinned walls were created on the surface of MWBNNTs without losing the tubular structure. The original crystallinity was still kept in the etched MWBNNTs in spite of oxidation. In the electrical measurement, MWBNNTs with a large band gap were converted to electrical conductors after etching by catalytic oxidation. Theoretical calculations indicated that a new energy state in the gap and a Fermi level shift contributed to MWBNNTs being conductive.

Design and Realization of a Planar Ultrawideband Antenna with Notch Band at 3.5 GHz

PubMed Central

2014-01-01

A small antenna with single notch band at 3.5 GHz is designed for ultrawideband (UWB) communication applications. The fabricated antenna comprises a radiating monopole element and a perfectly conducting ground plane with a wide slot. To achieve a notch band at 3.5 GHz, a parasitic element has been inserted in the same plane of the substrate along with the radiating patch. Experimental results shows that, by properly adjusting the position of the parasitic element, the designed antenna can achieve an ultrawide operating band of 3.04 to 11 GHz with a notched band operating at 3.31–3.84 GHz. Moreover, the proposed antenna achieved a good gain except at the notched band and exhibits symmetric radiation patterns throughout the operating band. The prototype of the proposed antenna possesses a very compact size and uses simple structures to attain the stop band characteristic with an aim to lessen the interference between UWB and worldwide interoperability for microwave access (WiMAX) band. PMID:25133245

BiVO4 -TiO2 Composite Photocatalysts for Dye Degradation Formed Using the SILAR Method.

PubMed

Odling, Gylen; Robertson, Neil

2016-09-19

Composite photocatalyst films have been fabricated by depositing BiVO4 upon TiO2 via a sequential ionic layer adsorption reaction (SILAR) method. The photocatalytic materials were investigated by XRD, TEM, UV/Vis diffuse reflectance, inductively coupled plasma optical emission spectrometry (ICP-OES), XPS, photoluminescence and Mott-Schottky analyses. SILAR processing was found to deposit monoclinic-scheelite BiVO4 nanoparticles onto the surface, giving successive improvements in the films' visible light harvesting. Electrochemical and valence band XPS studies revealed that the prepared heterojunctions have a type II band structure, with the BiVO4 conduction band and valence band lying cathodically shifted from those of TiO2 . The photocatalytic activity of the films was measured by the decolourisation of the dye rhodamine 6G using λ>400 nm visible light. It was found that five SILAR cycles was optimal, with a pseudo-first-order rate constant of 0.004 min(-1) . As a reference material, the same SILAR modification has been made to an inactive wide-band-gap ZrO2 film, where the mismatch of conduction and valence band energies disallows charge separation. The photocatalytic activity of the BiVO4 -ZrO2 system was found to be significantly reduced, highlighting the importance of charge separation across the interface. The mechanism of action of the photocatalysts has also been investigated, in particular the effect of self-sensitisation by the model organic dye and the ability of the dye to inject electrons into the photocatalyst's conduction band. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Structural defects and recombination behavior of excited carriers in Cu(In,Ga)Se{sub 2} solar cells

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, J.; Du, H. W.; Li, Y.

2016-08-15

The carriers’ behavior in neutral region (NTR) and space charged region (SCR) of Cu(In,Ga)Se{sub 2} thin film based solar cells has been investigated by temperature dependent photoluminescence (PL-T), electroluminescence (EL-T) and current-voltage (IV-T) from 10 to 300 K. PL-T spectra show that three kinds of defects, namely V{sub Se}, In{sub Cu} and (In{sub Cu}+V{sub Cu}), are localized within the band gap of NTR and SCR of CIGS layer, corresponding to the energy levels of E{sub C}-0.08, E{sub C}-0.20 and E{sub C}-0.25 eV, respectively. The In{sub Cu} and (In{sub Cu}+V{sub Cu}) deep level defects are non-radiative recombination centers at room temperature.more » The IV-T and EL-T analysis reveals that the injection modes of electrons from ZnO conduction band into Cu(In,Ga)Se{sub 2} layer are tunneling, thermally-excited tunneling and thermionic emission under 10-40, 60-160, and 180-300 K, respectively. At 10-160 K, the electrons tunnel into (In{sub Cu}+V{sub Cu}) and V{sub se} defect levels in band gap of SCR and the drifting is involved in the emission bands at 0.96 and 1.07 eV, which is the direct evidence for a tunneling assisted recombination. At 180-300 K, the electrons are directly injected into the Cu(In,Ga)Se{sub 2} conduction band, and the emission of 1.13 eV are ascribed to the transitions from the conduction band to the valence band.« less

Ultra-thin narrow-band, complementary narrow-band, and dual-band metamaterial absorbers for applications in the THz regime

NASA Astrophysics Data System (ADS)

Astorino, Maria Denise; Frezza, Fabrizio; Tedeschi, Nicola

2017-02-01

In this paper, ultra-thin narrow-band, complementary narrow-band, and dual-band metamaterial absorbers (MMAs), exploiting the same electric ring resonator configuration, are investigated at normal and oblique incidence for both transverse electric (TE) and transverse magnetic (TM) polarizations, and with different physical properties in the THz regime. In the analysis of the ultra-thin narrow-band MMA, the limit of applicability of the transmission line model has been overcome with the introduction of a capacitance which considers the z component of the electric field. These absorbing structures have shown a wide angular response and a polarization-insensitive behavior due to the introduction of a conducting ground plane and to the four-fold rotational symmetry of the resonant elements around the propagation axis. We have adopted a retrieval procedure to extract the effective electromagnetic parameters of the proposed MMAs and we have compared the simulated and analytical results through the interference theory.

Unprecedented ultrahigh photocatalytic activity of δ-Bi2O3 for cylindrospermopsin decomposition

NASA Astrophysics Data System (ADS)

Sudrajat, Hanggara

2017-11-01

The delta phase of bismuth oxide (δ-Bi2O3) is an important metal oxide due to its highest conductivity of any oxide material. However, it is only stable over a narrow high temperature range, and thus, incorporation of small, high-valence cation is a prerequisite for stabilizing its cubic structure to room temperature. The δ-Bi2O3 is also known to have low photocatalytic activity because of its low conduction band edge. As a consequence, the conduction band electrons cannot be consumed by the dissolved oxygen to produce superoxide radicals. Herein, for the first time, the δ-Bi2O3 has been successfully synthesized through a facile hydrothermal route without addition of any dopant. The as-synthesized δ-Bi2O3 shows ultrahigh photocatalytic activity for cylindrospermopsin decomposition. Within only 20 min of UV irradiation, the degradation efficiency for cylindrospermopsin by 0.5 g/L of the δ-Bi2O3 with a cylindrospermopsin concentration of 5 mg/L reaches 98%. Restricted charge carrier recombination and effective consumption of the conduction band electrons are behind such an unprecedented high photocatalytic activity of the δ-Bi2O3. [Figure not available: see fulltext.

HOM-Free Linear Accelerating Structure for e+ e- Linear Collider at C-Band

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kubo, Kiyoshi

2003-07-07

HOM-free linear acceleration structure using the choke mode cavity (damped cavity) is now under design for e{sup +}e{sup -} linear collider project at C-band frequency (5712 MHz). Since this structure shows powerful damping effect on most of all HOMs, there is no multibunch problem due to long range wakefields. The structure will be equipped with the microwave absorbers in each cells and also the in-line dummy load in the last few cells. The straightness tolerance for 1.8 m long structure is closer than 30 {micro}m for 25% emittance dilution limit, which can be achieved by standard machining and braising techniques.more » Since it has good vacuum pumping conductance through annular gaps in each cell, instabilities due to the interaction of beam with the residual-gas and ions can be minimized.« less

Measurements of Local Strain Variation in Si(1-x)Ge(x)/Si Heterostructures

NASA Technical Reports Server (NTRS)

Bell, L. D.; Kaiser, W. J.; Manion, S. J.; Milliken, S. J.; Pike, W. T.; Fathauer, R. W.

1995-01-01

The energy splitting of the conduction-band minimum of Si(1-x), Ge(x), due to strain has been directly measured by the application of ballistic-electron-emission microscope (BEEM) spectroscopy to Ag/Si(1-x), Ge(x) structures. Experimental values for this conduction-band splitting agree well with calculations. For Au/Si(1-x), Ge(x), however, heterogeneity in the strain of the Si(1-x), Ge(x) layer is introduced by deposition of the Au. This variation is attributed to species interdiffusion, which produces a rough Si(1-x)Ge(x) surface. Preliminary modeling indicates that the observed roughness is consistent with the strain variation measured by BEEM.

One-step synthesis and patterning of aligned polymer nanowires on a substrate

DOEpatents

Wang, Zhong L [Marietta, GA; Wang, Xudong [Atlanta, GA; Morber, Jenny R [Atlanta, GA; Liu, Jin [Danbury, CT

2011-11-08

In a method of making a polymer structure on a substrate a layer of a first polymer, having a horizontal top surface, is applied to a surface of the substrate. An area of the top surface of the polymer is manipulated to create an uneven feature that is plasma etched to remove a first portion from the layer of the first polymer thereby leaving the polymer structure extending therefrom. A light emitting structure includes a conductive substrate from which an elongated nanostructure of a first polymer extends. A second polymer coating is disposed about the nanostructure and includes a second polymer, which includes a material such that a band gap exists between the second polymer coating and the elongated nanostructure. A conductive material coats the second polymer coating. The light emitting structure emits light when a voltage is applied between the conductive substrate and the conductive coating.

Theory of extrinsic and intrinsic heterojunctions in thermal equilibrium

NASA Technical Reports Server (NTRS)

Von Ross, O.

1980-01-01

A careful analysis of an abrupt heterojunction consisting of two distinct semiconductors either intrinsic or extrinsic is presented. The calculations apply to a one-dimensional, nondegenerate structure. Taking into account all appropriate boundary conditions, it is shown that the intrinsic Fermi level shows a discontinuity at the interface between the two materials which leads to a discontinuity of the valence band edge equal to the difference in the band gap energies of the two materials. The conduction band edge stays continuous however. This result is independent of possible charged interface states and in sharp contrast to the Anderson model. The reasons for this discrepancy are discussed.

Effects of impurity adsorption on topological surface states of Bi2Te3

NASA Astrophysics Data System (ADS)

Shati, Khaqan; Arshad Farhan, M.; Selva Chandrasekaran, S.; Shim, Ji Hoon; Lee, Geunsik

2017-08-01

Electronic structures of Bi2Te3 with adsorption of Rb, In, Ga and Au atoms are studied by using the first-principle method, focusing on the effect of non-magnetic impurities on the topologically protected surface states. Upon monolayer formation, the bulk conduction band is moved down to the Fermi level with a significant Rashba splitting due to n-doping behavior with band modification details depending on the adatom chemistry. Our study shows the robustness of the intrinsic spin-momentum coupled surface band and emergence of a new similar one, which could provide helpful insight for developing novel spintronic devices.

Optical band gap tuning and electrical properties of polyaniline and its nanocomposites for hybrid solar cell application

NASA Astrophysics Data System (ADS)

Singh, R.; Choudhary, R. B.; Kandulna, R.

2018-05-01

Hcl doped conducting polyaniline-CdS nanocomposite has been prepared via In-situ polymerization in which cadmium nitrate was used as a source for cadmium. The structural morphology was investigated using FESEM and the presence of fibrous polyaniline and CdS nanoparticles. The synthesis of CdS and polyaniline was confirmed using the XRD analysis. I-V characteristic was used to explore the electrical behavior of PANI and its nanocoposites. Optical properties were studied and minimum band gap with highest band absorbance was found for synergistic concentration PANI-CdS (10%) for solar cells application.

Measurement of InAsSb bandgap energy and InAs/InAsSb band edge positions using spectroscopic ellipsometry and photoluminescence spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Webster, P. T.; Riordan, N. A.; Liu, S.

2015-12-28

The structural and optical properties of lattice-matched InAs{sub 0.911}Sb{sub 0.089} bulk layers and strain-balanced InAs/InAs{sub 1−x}Sb{sub x} (x ∼ 0.1–0.4) superlattices grown on (100)-oriented GaSb substrates by molecular beam epitaxy are examined using X-ray diffraction, spectroscopic ellipsometry, and temperature dependent photoluminescence spectroscopy. The photoluminescence and ellipsometry measurements determine the ground state bandgap energy and the X-ray diffraction measurements determine the layer thickness and mole fraction of the structures studied. Detailed modeling of the X-ray diffraction data is employed to quantify unintentional incorporation of approximately 1% Sb into the InAs layers of the superlattices. A Kronig-Penney model of the superlattice miniband structure ismore » used to analyze the valence band offset between InAs and InAsSb, and hence the InAsSb band edge positions at each mole fraction. The resulting composition dependence of the bandgap energy and band edge positions of InAsSb are described using the bandgap bowing model; the respective low and room temperature bowing parameters for bulk InAsSb are 938 and 750 meV for the bandgap, 558 and 383 meV for the conduction band, and −380 and −367 meV for the valence band.« less

Structural and electrical properties of nickel substituted cadmium ferrite

NASA Astrophysics Data System (ADS)

Chethan, B.; Raj Prakash, H. G.; Vijayakumari, S. C.; Ravikiran, Y. T.

2018-05-01

Spinal nano-sized Cadmium ferrite (CD) and Nickel substituted cadmium ferrite (NSCF) were fabricated by sol-gel auto combustion method. The formation of spinal structure of ferrite materials was confirmed by X-ray diffraction (XRD) analysis. The crystallites size of CF and NSCF as determined by Scherrer's formula were found to be 24.73 nm and 17.70 nm respectively. comparative study of Fourier transform infrared spectroscopy (FTIR) of CF and NSCF revealed tetrahedral absorption bands shifted slightly towards higher frequency where as octahedral bands shifted towards lower frequency side confirming interfacial interaction between Ni and CF. The AC conductivity (σ), loss tangent (tan δ) and complex plane impedance plots for both CF and NSCF are determined at various frequencies ranging from 50 kHz to 5 MHz and comparatively analyzed. The increase in AC conductivity of the NSCF nano particles as compared to CF was explained in the light of hopping model. The impedance measurement of NSCF show presence of a semi-circle corresponding to the grain boundary resistance and hence shows that the conductivity takes place largely through grain boundaries.

High-temperature luminescence in an n-GaSb/n-InGaAsSb/p-AlGaAsSb light-emitting heterostructure with a high potential barrier

DOE Office of Scientific and Technical Information (OSTI.GOV)

Petukhov, A. A., E-mail: andrey-rus29@rambler.ru; Zhurtanov, B. E.; Kalinina, K. V.

2013-09-15

The electroluminescent properties of an n-GaSb/n-InGaAsSb/p-AlGaAsSb heterostructure with a high potential barrier in the conduction band (large conduction-band offset) at the n-GaSb/n-InGaAsSb type-II heterointerface ({Delta}E{sub c} = 0.79 eV) are studied. Two bands with peaks at 0.28 and 0.64 eV at 300 K, associated with radiative recombination in n-InGaAsSb and n-GaSb, respectively, are observed in the electroluminescence (EL) spectrum. In the entire temperature range under study, T = 290-480 K, additional electron-hole pairs are formed in the n-InGaAsSb active region by impact ionization with hot electrons heated as a result of the conduction-band offset. These pairs contribute to radiative recombination,more » which leads to a nonlinear increase in the EL intensity and output optical power with increasing pump current. A superlinear increase in the emission power of the long-wavelength band is observed upon heating in the temperature range T = 290-345 K, and a linear increase is observed at T > 345 K. This work for the first time reports an increase in the emission power of a light-emitting diode structure with increasing temperature. It is shown that this rise is caused by a decrease in the threshold energy of the impact ionization due to narrowing of the band gap of the active region.« less

Theoretical studies of optics and charge transport in organic conducting oligomers and polymers: Rational design of improved transparent and conducting polymers

NASA Astrophysics Data System (ADS)

Hutchison, Geoffrey Rogers

Theoretical studies on a variety of oligo- and polyheterocycles elucidate their optical and charge transport properties, suggesting new, improved transparent conductive polymers. First-principles calculations provide accurate methodologies for predicting both optical band gaps of neutral and cationic oligomers and intrinsic charge transfer rates. Multidimensional analysis reveals important motifs in chemical tailorability of oligoheterocycle optical and charge transport properties. The results suggest new directions for design of novel materials. Using both finite oligomer and infinite polymer calculations, the optical band gaps in polyheterocycles follow a modified particle-in-a-box formalism, scaling approximately as 1/N (where N is the number of monomer units) in short chains, saturating for long chains. Calculations demonstrate that band structure changes upon heteroatom substitution, (e.g., from polythiophene to polypyrrole) derive from heteroatom electron affinity. Further investigation of chemical variability in substituted oligoheterocycles using multidimensional statistics reveals the interplay between heteroatom and substituent in correlations between structure and redox/optical properties of neutral and cationic species. A linear correlation between band gaps of neutral and cationic species upon oxidation of conjugated oligomers, shows redshifts of optical absorption for most species and blueshifts for small band gap species. Interstrand charge-transport studies focus on two contributors to hopping-style charge transfer rates: internal reorganization energy and the electronic coupling matrix element. Statistical analysis of chemical variability of reorganization energies in oligoheterocycles proves the importance of reorganization energy in determining intrinsic charge transfer rates (e.g., charge mobility in unsubstituted oligothiophenes). Computed bandwidths across several oligothiophene crystal packing motifs show similar electron and hole bandwidths, and show that well-known tilted and herringbone motifs in oligothiophenes are driven by electrostatic repulsion. Tilted stacks exhibit intrinsic charge-transfer rates smaller than cofacial stacks, but with lower packing energy. Given similar electron and hole bandwidths, a charge injection model explains substitution-modulated majority carrier changes in n- and p-type oligothiophene field-effect transistors.

Revisiting Photoemission and Inverse Photoemission Spectra of Nickel Oxide from First Principles: Implications for Solar Energy Conversion

PubMed Central

2015-01-01

We use two different ab initio quantum mechanics methods, complete active space self-consistent field theory applied to electrostatically embedded clusters and periodic many-body G0W0 calculations, to reanalyze the states formed in nickel(II) oxide upon electron addition and ionization. In agreement with interpretations of earlier measurements, we find that the valence and conduction band edges consist of oxygen and nickel states, respectively. However, contrary to conventional wisdom, we find that the oxygen states of the valence band edge are localized whereas the nickel states at the conduction band edge are delocalized. We argue that these characteristics may lead to low electron–hole recombination and relatively efficient electron transport, which, coupled with band gap engineering, could produce higher solar energy conversion efficiency compared to that of other transition-metal oxides. Both methods find a photoemission/inverse-photoemission gap of 3.6–3.9 eV, in good agreement with the experimental range, lending credence to our analysis of the electronic structure of NiO. PMID:24689856

Theoretical studies on anisotropic electrical conductivity of trans-polyacetylene doped with n-type dopants

NASA Astrophysics Data System (ADS)

Wang, Cunguo; Wang, Rongshun

2000-12-01

Based on energy band theory of solid states, extended Hückel molecular orbital methods (EHMO/CO) were used to calculate the two-dimensional (2D) energy band structures of highly oriented trans-polyacetylene (PA) undoped and doped with n-type dopant (Li, Na, K). The band gaps ( Eg) of undoped PA in directions parallel and perpendicular to the oriented direction were 1.195 and 3.040 eV, respectively. When PA was doped with n-type dopant, the corresponding band gaps Eg1 and Eg2 decreased significantly. Based on the calculated results, we could successfully account for the changes of electrical anisotropy of PA from the undoped state to the doped form. The conductivity anisotropy ratio σ1/ σ2 decreased when PA was doped with n-type dopant, because the PA chains and the dopant showed a strong interchain coupling. It was the interchain coupling that acted as a bridge between two neighboring chains, and made the charge-carrier transport easier between the interchains. The theoretical results for undoped and doped PA are in good agreement with the experiment.

A photo-excited broadband to dual-band tunable terahertz prefect metamaterial polarization converter

NASA Astrophysics Data System (ADS)

Zhu, Jianfeng; Yang, Yang; Li, Shufang

2018-04-01

A new and simple design of photo-excited broadband to dual-band tunable terahertz (THz) metamaterial cross polarization converter is proposed in this paper. The tunable converter is a sandwich structure with the center-cut cross-shaped metallic patterned structure as a resonator, the middle dielectric layer as a spacer and the bottom metallic film as the ground. The conductivity of the photoconductive semiconductor (Silicon) filled in the gap of the cross-shaped metallic resonator can be tuned by the incident pump power, leading to an easy modulation of the electromagnetic response of the proposed converter. The results show that the proposed cross-polarization converter can be tuned from a broadband with polarization conversion ratio (PCR) beyond 95% (1.86-2.94 THz) to dual frequency bands (fl = 1 . 46 THz &fh = 2 . 9 THz). The conversion peaks can reach 99.9% for the broadband and, 99.5% (fl) and 99.7% (fh) for the dual-band, respectively. Most importantly, numerical simulations demonstrate that the broadband/dual-band polarization conversion mechanism of the converter originates from the localized surface plasmon modes, which make the design simple and different from previous designs. With these good features, the proposed broadband to dual-band tunable polarization converter is expected to be used in widespread applications.

Three-dimensional graphdiyne as a topological nodal-line semimetal

NASA Astrophysics Data System (ADS)

Nomura, Takafumi; Habe, Tetsuro; Sakamoto, Ryota; Koshino, Mikito

2018-05-01

We study the electronic band structure of three-dimensional ABC-stacked (rhombohedral) graphdiyne, which is a new planar carbon allotrope recently fabricated. Using first-principles calculation, we show that the system is a nodal-line semimetal, in which the conduction band and valence band cross at a closed ring in the momentum space. We derive the minimum tight-binding model and the low-energy effective Hamiltonian in a 4 ×4 matrix form. The nodal line is protected by a nontrivial winding number, and it ensures the existence of the topological surface state in a finite-thickness slab. The Fermi surface of the doped system exhibits a peculiar, self-intersecting hourglass structure, which is quite different from the torus or pipe shape in the previously proposed nodal semimetals. Despite its simple configuration, three-dimensional graphdiyne offers unique electronic properties distinct from any other carbon allotropes.

Fabrication of reduced graphene oxide nanosheets doped PVA composite films for tailoring their opto-mechanical properties

NASA Astrophysics Data System (ADS)

Aslam, Muhammad; Kalyar, Mazhar Ali; Raza, Zulfiqar Ali

2017-06-01

Laminar graphene nanosheets have raised passionate attention due to their incredible physico-chemical properties. Its wide-scale, high-yield production at low-cost has made it possible to produce top class promising versatile polymer nanocomposites. Reduced graphene oxide (RGO) nanosheets were incorporated to prepare optically tunable and high mechanical strength polymer nanocomposite films. RGO-doped poly(vinyl alcohol) (PVA) nanocomposite films were prepared via solution casting. Low level RGO doping significantly altered the structural, optical and mechanical properties of pure PVA films. Most of the band structure parameters like direct/indirect band gap, band tail, refractive index, dielectric constant, optical conductivity and dispersion parameters were investigated in detail for the first time. Tauc's, Wemple-DiDomenico, Helpin-Tsai and mixture rule models were employed to investigate optical and mechanical parameters. The applied models reinforced the experimental results in the present study. Advanced analytical techniques were engaged to characterize the nanocomposites films.

Spectromicroscopy measurements of surface morphology and band structure of exfoliated graphene

NASA Astrophysics Data System (ADS)

Knox, Kevin; Locatelli, Andrea; Cvetko, Dean; Mentes, Tevfik; Nino, Miguel; Wang, Shancai; Yilmaz, Mehmet; Kim, Philip; Osgood, Richard; Morgante, Alberto

2011-03-01

Monolayer-thick crystals, such as graphene, are an area of intense interest in condensed matter research. ~However, crystal deformations in these 2D systems are known to adversely affect conductivity and increase local chemical reactivity. Additionally, surface roughness in graphene complicates band-mapping and limits resolution in techniques such as angle resolved photoemission spectroscopy (ARPES), the theory of which was developed for atomically flat surfaces. Thus, an understanding of the surface morphology of graphene is essential to making high quality devices and important for interpreting ARPES results. In this talk, we will describe a non-invasive approach to examining the corrugation in exfoliated graphene using a combination of low energy electron microscopy (LEEM) and micro-spot low energy electron diffraction (LEED). We will also describe how such knowledge of surface roughness can be used in the analysis of ARPES data to improve resolution and extract useful information about the band-structure.

Band lineup of lattice mismatched InSe/GaSe quantum well structures prepared by van der Waals epitaxy: Absence of interfacial dipoles

NASA Astrophysics Data System (ADS)

Lang, O.; Klein, A.; Pettenkofer, C.; Jaegermann, W.; Chevy, A.

1996-10-01

Epitaxial growth of the strongly lattice mismatched (6.5%) layered chalcogenides InSe and GaSe on each other is obtained with the concept of van der Waals epitaxy as proven by low-energy electron diffraction and scanning tunnel microscope. InSe/GaSe/InSe and GaSe/InSe/GaSe quantum well structures were prepared by molecular beam epitaxy and their interface properties were characterized by soft x-ray photoelectron spectroscopy. Valence and conduction band offsets are determined to be 0.1 and 0.9 eV, respectively, and do not depend on deposition sequence (commutativity). As determined from the measured work functions the interface dipole is 0.05 eV; the band lineup between the two materials is correctly predicted by the Anderson model (electron affinity rule).

Optoelectronics and defect levels in hydroxyapatite by first-principles.

PubMed

Avakyan, Leon A; Paramonova, Ekaterina V; Coutinho, José; Öberg, Sven; Bystrov, Vladimir S; Bugaev, Lusegen A

2018-04-21

Hydroxyapatite (HAp) is an important component of mammal bones and teeth, being widely used in prosthetic implants. Despite the importance of HAp in medicine, several promising applications involving this material (e.g., in photo-catalysis) depend on how well we understand its fundamental properties. Among the ones that are either unknown or not known accurately, we have the electronic band structure and all that relates to it, including the bandgap width. We employ state-of-the-art methodologies, including density hybrid-functional theory and many-body perturbation theory within the dynamically screened single-particle Green's function approximation, to look at the optoelectronic properties of HAp. These methods are also applied to the calculation of defect levels. We find that the use of a mix of (semi-)local and exact exchange in the exchange-correlation functional brings a drastic improvement to the band structure. Important side effects include improvements in the description of dielectric and optical properties not only involving conduction band (excited) states but also the valence. We find that the highly dispersive conduction band bottom of HAp originates from anti-bonding σ* states along the ⋯OH-OH-⋯ infinite chain, suggesting the formation of a conductive 1D-ice phase. The choice of the exchange-correlation treatment to the calculation of defect levels was also investigated by using the OH-vacancy as a testing model. We find that donor and acceptor transitions obtained within semi-local density functional theory (DFT) differ from those of hybrid-DFT by almost 2 eV. Such a large discrepancy emphasizes the importance of using a high-quality description of the electron-electron interactions in the calculation of electronic and optical transitions of defects in HAp.

Optoelectronics and defect levels in hydroxyapatite by first-principles

NASA Astrophysics Data System (ADS)

Avakyan, Leon A.; Paramonova, Ekaterina V.; Coutinho, José; Öberg, Sven; Bystrov, Vladimir S.; Bugaev, Lusegen A.

2018-04-01

Hydroxyapatite (HAp) is an important component of mammal bones and teeth, being widely used in prosthetic implants. Despite the importance of HAp in medicine, several promising applications involving this material (e.g., in photo-catalysis) depend on how well we understand its fundamental properties. Among the ones that are either unknown or not known accurately, we have the electronic band structure and all that relates to it, including the bandgap width. We employ state-of-the-art methodologies, including density hybrid-functional theory and many-body perturbation theory within the dynamically screened single-particle Green's function approximation, to look at the optoelectronic properties of HAp. These methods are also applied to the calculation of defect levels. We find that the use of a mix of (semi-)local and exact exchange in the exchange-correlation functional brings a drastic improvement to the band structure. Important side effects include improvements in the description of dielectric and optical properties not only involving conduction band (excited) states but also the valence. We find that the highly dispersive conduction band bottom of HAp originates from anti-bonding σ* states along the ⋯OH-OH-⋯ infinite chain, suggesting the formation of a conductive 1D-ice phase. The choice of the exchange-correlation treatment to the calculation of defect levels was also investigated by using the OH-vacancy as a testing model. We find that donor and acceptor transitions obtained within semi-local density functional theory (DFT) differ from those of hybrid-DFT by almost 2 eV. Such a large discrepancy emphasizes the importance of using a high-quality description of the electron-electron interactions in the calculation of electronic and optical transitions of defects in HAp.

Electronic, thermoelectric and transport properties of cesium cadmium trifluoride: A DFT study

NASA Astrophysics Data System (ADS)

Abraham, Jisha Annie; Pagare, G.; Sanyal, Sankar P.

2018-04-01

The full potential linearized augmented plane wave method based on density functional theory is employed to investigate the electronic structure of CsCdF3. The electronic properties of this compound have been studied from the band structure plot and density of states. The presence of indirect energy gap reveals its insulating nature. Using constant relaxation time, the electrical conductivity, electronic thermal conductivity, Seebeck coefficient and figure of merit are calculated by using Boltzmann transport theory. We have also studied the temperature dependence of thermoelectric properties of this compound.

Impact of Fe doping on the electronic structure of SrTiO3 thin films determined by resonant photoemission

NASA Astrophysics Data System (ADS)

Kubacki, J.; Kajewski, D.; Goraus, J.; Szot, K.; Koehl, A.; Lenser, Ch.; Dittmann, R.; Szade, J.

2018-04-01

Epitaxial thin films of Fe doped SrTiO3 have been studied by the use of resonant photoemission. This technique allowed us to identify contributions of the Fe and Ti originating electronic states to the valence band. Two valence states of iron Fe2+ and Fe3+, detected on the base of x-ray absorption studies spectra, appeared to form quite different contributions to the valence band of SrTiO3. The electronic states within the in-gap region can be attributed to Fe and Ti ions. The Fe2+ originating states which can be connected to the presence of oxygen vacancies form a broad band reaching binding energies of about 0.5 eV below the conduction band, while Fe3+ states form in the gap a sharp feature localized just above the top of the valence band. These structures were also confirmed by calculations performed with the use of the FP-LAPW/APW+lo method including Coulomb correlations within the d shell. It has been shown that Fe doping induced Ti originating states in the energy gap which can be related to the hybridization of Ti and Fe 3d orbitals.

Electronic band structure and Shubnikov-de Haas effect in two-dimensional semimetallic InAs/GaSb nanostructure superlattice

NASA Astrophysics Data System (ADS)

Boutramine, Abderrazak; Nafidi, Abdelhakim; Barkissy, Driss; El-Frikhe, Es-Said; Charifi, Hicham; Elanique, Abdellatif; Chaib, Hassan

2016-02-01

We have investigated the band structure E( d = d 1 + d 2), E( k z) and E( k p), respectively, as a function of the SL period, d, in the growth direction and in plan of InAs( d 1 = 160 Å)/GaSb( d 2 = 105 Å) type II superlattice, performed in the envelope function formalism with the valence band offset, Λ, of 510 meV at 4.2 K. For the ratio d 1/ d 2 = 1.52, d and Λ dependence of the SL energy band gap show that the semiconductor-to-semimetal transition takes place at d c = 173 Å and Λ c = 463 meV. Therefore, this sample is semimetallic. The position of the Fermi level, E F = 500.2 meV, indicates n type conductivity. The spectra of energy, E( k z, k p), show a negative band gap of -48.3 meV. The cutoff wavelength | λ c| = 25.7 µm indicates that this sample can be used as a far-infrared detector. Further, we have interpreted the minima of the magnetoresistance oscillations, Shubnikov-de Haas effect, observed by D. M. Symons et al.

Electronic and mechanic properties of trigonal boron nitride by first-principles calculations

NASA Astrophysics Data System (ADS)

Mei, Hua Yue; Pang, Yong; Liu, Ding Yu; Cheng, Nanpu; Zheng, Shaohui; Song, Qunliang; Wang, Min

2018-07-01

A new boron nitride allotrope with 6 atoms in a unit cell termed as trigonal BN (TBN), which belongs to P3121 space group, is theoretically investigated. Electronic structures, mechanic properties, phonon spectra and other properties were calculated by using first-principles based on density functional theory (DFT). The elastic constants reveal that TBN is mechanically stable. Furthermore, phonon dispersion indicates that TBN is dynamically stable. The calculated bulk modulus and shear modulus of TBN are 323 and 342 GPa, respectively. The calculated Young's modulus are Ex = Ey = 760 GPa, Ez = 959 GPa, indicating that TBN is a super-hard and brittle material. The universal anisotropy index, which is only 0.296, shows its weak anisotropy. Band structure states clearly that TBN is an indirect semiconductor with a band gap of 3.87 eV. The valence bands are mainly composed of N 2p states, and the conduction bands are mainly contributed by B 2p states. Simulated X-ray diffraction patterns (XRD) and Raman spectra were also provided for future experimental characterizations. Due to its band gap and super-hard properties, TBN may possess potential in super-hard, optical and electronic applications.

Temperature-dependent optical band gap of the metastable zinc-blende structure [beta]-GaN

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ramirez-Flores, G.; Navarro-Contreras, H.; Lastras-Martinez, A.

1994-09-15

The temperature-dependent (10--300 K) optical band gap [ital E][sub 0]([ital T]) of the epitaxial metastable zinc-blende-structure [beta]-GaN(001)4[times]1 has been determined by modulated photoreflectance and used to interpret low-temperature photoluminescence spectra. [ital E][sub 0] in [beta]-GaN was found to vary from 3.302[plus minus]0.004 eV at 10 K to 3.231[plus minus]0.008 eV at 300 K with a temperature dependence given by [ital E][sub 0]([ital T]) =3.302--6.697[times]10[sup [minus]4][ital T][sup 2]/([ital T]+600) eV. The spin-orbit splitting [Delta][sub 0] in the valence band was determined to be 17[plus minus]1 meV. The oscillations in the photoreflectance spectra were very sharp with a broadening parameter [Gamma] ofmore » only 10 meV at 10 K. The dominant transition observed in temperature-dependent photoluminescence was attributed to radiative recombination between a shallow donor, at [congruent]11 meV below the conduction-band edge and the valence band.« less

Effect of strain on the electronic structure and optical properties of germanium

NASA Astrophysics Data System (ADS)

Wen, Shumin; Zhao, Chunwang; Li, Jijun; Hou, Qingyu

2018-05-01

The effects of biaxial strain parallel to the (001) plane on the electronic structures and optical properties of Ge are calculated using the first-principles plane-wave pseudopotential method based on density functional theory. The screened-exchange local-density approximation function was used to obtain more reliable band structures, while strain was changed from ‑4% to +4%. The results show that the bandgap of Ge decreases with the increase of strain. Ge becomes a direct-bandgap semiconductor when the tensile strain reaches to 2%, which is in good agreement with the experimental results. The density of electron states of strained Ge becomes more localized. The tensile strain can increase the static dielectric constant distinctly, whereas the compressive strain can decrease the static dielectric constant slightly. The strain makes the absorption band edge move toward low energy. Both the tensile strain and compressive strain can significantly increase the reflectivity in the range from 7 eV to 14 eV. The tensile strain can decrease the optical conductivity, but the compressive strain can increase the optical conductivity significantly.

Impact of high-κ dielectric and metal nanoparticles in simultaneous enhancement of programming speed and retention time of nano-flash memory

NASA Astrophysics Data System (ADS)

Pavel, Akeed A.; Khan, Mehjabeen A.; Kirawanich, Phumin; Islam, N. E.

2008-10-01

A methodology to simulate memory structures with metal nanocrystal islands embedded as floating gate in a high-κ dielectric material for simultaneous enhancement of programming speed and retention time is presented. The computational concept is based on a model for charge transport in nano-scaled structures presented earlier, where quantum mechanical tunneling is defined through the wave impedance that is analogous to the transmission line theory. The effects of substrate-tunnel dielectric conduction band offset and metal work function on the tunneling current that determines the programming speed and retention time is demonstrated. Simulation results confirm that a high-κ dielectric material can increase programming current due to its lower conduction band offset with the substrate and also can be effectively integrated with suitable embedded metal nanocrystals having high work function for efficient data retention. A nano-memory cell designed with silver (Ag) nanocrystals embedded in Al 2O 3 has been compared with similar structure consisting of Si nanocrystals in SiO 2 to validate the concept.

Band-structure tailoring and surface passivation for highly efficient near-infrared responsive PbS quantum dot photovoltaics

NASA Astrophysics Data System (ADS)

Zhou, Ru; Niu, Haihong; Ji, Fengwei; Wan, Lei; Mao, Xiaoli; Guo, Huier; Xu, Jinzhang; Cao, Guozhong

2016-11-01

PbS is a promising light harvester for near-infrared (NIR) responsive quantum dot (QD) photovoltaics due to its narrow bulk band gap (0.41 eV) and large exciton Bohr radius (18 nm). However, the relatively low conduction band (CB) and high-density surface defects of PbS as two major drawbacks for its use in solar cells severely hamper the photovoltaic performance enhancement. In this work, a modified solution-based successive ionic layer adsorption and reaction (SILAR) utilizing mixed cationic precursors of Pb2+ and Cd2+ is explored, and such a scheme offers two benefits, band-structure tailoring and surface passivation. In-situ deposited CdS suppresses the excessive growth of PbS in the mesopores, thereby facilitating the favorable electron injection from PbS to TiO2 in view of the up-shifted CB level of QDs; the intimate interpenetration of two sulfides with each other leads to superior passivation of trap state defects on PbS, which suppresses the interfacial charge recombination. With the construction of photovoltaics based on such a hybrid (Pb,Cd)S/CdS configuration, impressive power conversion efficiency up to 4.08% has been reached, outperforming that of the conventional PbS/CdS pattern (2.95%). This work highlights the great importance of band-structure tailoring and surface passivation for constructing highly efficient PbS QD photovoltaics.

Complex band structures of transition metal dichalcogenide monolayers with spin-orbit coupling effects

NASA Astrophysics Data System (ADS)

Szczęśniak, Dominik; Ennaoui, Ahmed; Ahzi, Saïd

2016-09-01

Recently, the transition metal dichalcogenides have attracted renewed attention due to the potential use of their low-dimensional forms in both nano- and opto-electronics. In such applications, the electronic and transport properties of monolayer transition metal dichalcogenides play a pivotal role. The present paper provides a new insight into these essential properties by studying the complex band structures of popular transition metal dichalcogenide monolayers (MX 2, where M  =  Mo, W; X  =  S, Se, Te) while including spin-orbit coupling effects. The conducted symmetry-based tight-binding calculations show that the analytical continuation from the real band structures to the complex momentum space leads to nonlinear generalized eigenvalue problems. Herein an efficient method for solving such a class of nonlinear problems is presented and yields a complete set of physically relevant eigenvalues. Solutions obtained by this method are characterized and classified into propagating and evanescent states, where the latter states manifest not only monotonic but also oscillatory decay character. It is observed that some of the oscillatory evanescent states create characteristic complex loops at the direct band gap of MX 2 monolayers, where electrons can directly tunnel between the band gap edges. To describe these tunneling currents, decay behavior of electronic states in the forbidden energy region is elucidated and their importance within the ballistic transport regime is briefly discussed.

Crystal structure, stability, and optoelectronic properties of the organic-inorganic wide-band-gap perovskite CH3NH3BaI3 : Candidate for transparent conductor applications

NASA Astrophysics Data System (ADS)

Kumar, Akash; Balasubramaniam, K. R.; Kangsabanik, Jiban; Vikram, Alam, Aftab

2016-11-01

Structural stability, electronic structure, and optical properties of CH3NH3BaI3 hybrid perovskite are examined from theory as well as experiment. Solution-processed thin films of CH3NH3BaI3 exhibited a high transparency in the wavelength range of 400-825 nm (1.5-3.1 eV for which the photon current density is highest in the solar spectrum) which essentially justifies a high band gap of 4 eV obtained by theoretical estimation. Also, the x-ray diffraction patterns of the thin films match well with the {00 l } peaks of the simulated pattern obtained from the relaxed unit cell of CH3NH3BaI3 , crystallizing in the I 4 /m c m space group, with lattice parameters, a =9.30 Å, c =13.94 Å. Atom projected density of state and band structure calculations reveal the conduction and valence band edges to be comprised primarily of barium d orbitals and iodine p orbitals, respectively. The larger band gap of CH3NH3BaI3 compared to CH3NH3PbI3 can be attributed to the lower electronegativity coupled with the lack of d orbitals in the valence band of Ba2 +. A more detailed analysis reveals the excellent chemical and mechanical stability of CH3NH3BaI3 against humidity, unlike its lead halide counterpart, which degrades under such conditions. We propose La to be a suitable dopant to make this compound a promising candidate for transparent conductor applications, especially for all perovskite solar cells. This claim is supported by our calculated results on charge concentration, effective mass, and vacancy formation energies.

Dynamic analysis of clamp band joint system subjected to axial vibration

NASA Astrophysics Data System (ADS)

Qin, Z. Y.; Yan, S. Z.; Chu, F. L.

2010-10-01

Clamp band joints are commonly used for connecting circular components together in industry. Some of the systems jointed by clamp band are subjected to dynamic load. However, very little research on the dynamic characteristics for this kind of joint can be found in the literature. In this paper, a dynamic model for clamp band joint system is developed. Contact and frictional slip between the components are accommodated in this model. Nonlinear finite element analysis is conducted to identify the model parameters. Then static experiments are carried out on a scaled model of the clamp band joint to validate the joint model. Finally, the model is adopted to study the dynamic characteristics of the clamp band joint system subjected to axial harmonic excitation and the effects of the wedge angle of the clamp band joint and the preload on the response. The model proposed in this paper can represent the nonlinearity of the clamp band joint and be used conveniently to investigate the effects of the structural and loading parameters on the dynamic characteristics of this type of joint system.

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

DOE Office of Scientific and Technical Information (OSTI.GOV)

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

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 polycrystallinitymore » 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.« less

Morphology-dependent optical absorption and conduction properties of photoelectrochemical photocatalysts for H2 production: A case study

NASA Astrophysics Data System (ADS)

Huda, Muhammad N.; Turner, John A.

2010-06-01

Efficient photoelectrochemical H2 production by solar irradiation depends not only on the photocatalyst's band gap and its band-edge positions but also on the detailed electronic nature of the bands, such as the localization or delocalization of the band edges and their orbital characteristics. These determine the carrier transport properties, reactivity, light absorption strength, etc. and significantly impact the material's efficiency as a photoconverter. The localization or delocalization of the band edges may arise either due to the orbital nature of the bands or the structural morphology of the material. A recent experimental report on a photocatalyst based on s /p orbitals showed very poor performance for H2 production despite the delocalized nature of the s /p bands as compared to the d-bands of transition metal oxides. It is then important to examine whether this poor performance is inherent to these materials or rather arises from some experimental limitations. A theoretical analysis by first-principle methods is well suited to shed light on this question.

Prototypal dithiazolodithiazolyl radicals: synthesis, structures, and transport properties.

PubMed

Beer, Leanne; Britten, James F; Brusso, Jaclyn L; Cordes, A Wallace; Haddon, Robert C; Itkis, Mikhail E; MacGregor, Douglas S; Oakley, Richard T; Reed, Robert W; Robertson, Craig M

2003-11-26

New synthetic routes to 1,2,3-dithiazolo-1,2,3-dithiazolylium salts, based on double Herz condensations of N-alkylated 2,6-diaminopyridinium salts with sulfur monochloride, have been developed. The two prototypal 1,2,3-dithiazolo-1,2,3-dithiazolyl radicals HBPMe and HBPEt have been prepared and characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G) gas-phase disproportionation enthalpies favor a low on-site Coulombic repulsion energy U in the solid state. The crystal structures of HBPR (R = Me, Et) have been determined by X-ray crystallography (at 293 K). Both consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S- - -S contacts. Magnetic, conductivity, and optical measurements have been performed and the results interpreted in light of extended Hückel band calculations. The crystalline materials are paramagnetic above 100 K, with room-temperature conductivities sigma(RT) of 10(-5)-10(-6) S cm(-1); the slightly greater conductivity of the R = Et compound can be associated with a more well developed band structure. We suggest a Mott-Hubbard insulator ground state for these materials, with an on-site Coulomb repulsion energy U of about 1.0 eV.

The optoelectronic properties and role of Cu concentration on the structural and electrical properties of Cu doped ZnO nanoparticles

NASA Astrophysics Data System (ADS)

Omri, K.; Bettaibi, A.; Khirouni, K.; El Mir, L.

2018-05-01

In the current study, we synthesized a Cu-doped ZnO (CZO) nanoparticles material using a sol-gel method with different doping concentrations of Cu (0, 2, 3 and 4 at.%). The control of the Cu concentration on structural, electrical and optical properties of CZO nanoparticles was investigated in detail. The XRD analysis of the CZO nanoparticles reveals the formation of ZnO hexagonal wurtzite structure for all samples which confirm the incorporation of Cu2+ ions into the ZnO lattice by substitution. Furthermore, CZO nanoparticles showed a small red shift of absorption band with the incorporation of Cu from 0 to 4 at.%; i.e. a decreased band gap value from 3.34 eV to 3.27 eV with increasing of Cu doping content. The frequency dispersion of the electric conductivity were studied using the Jonscher universal power law, according to relation σ(ω) = σDC + A ωs(T). Alternative current conductivity increases with increasing Cu content in spite of the decrease the activation energy with copper loading. It was found that the conductivity reached its maximum value for critical Cu concentration of 3 at.%. The frequency relaxation phenomenon was also investigated and all results were discussed in term of the copper doping concentration.

Structural, electronic, mechanical, and thermoelectric properties of a novel half Heusler compound HfPtPb

NASA Astrophysics Data System (ADS)

Kaur, Kulwinder; Rai, D. P.; Thapa, R. K.; Srivastava, Sunita

2017-07-01

We explore the structural, electronic, mechanical, and thermoelectric properties of a new half Heusler compound HfPtPb, an all metallic heavy element, recently proposed to be stable [Gautier et al., Nat. Chem. 7, 308 (2015)]. In this work, we employ density functional theory and semi-classical Boltzmann transport equations with constant relaxation time approximation. The mechanical properties, such as shear modulus, Young's modulus, elastic constants, Poisson's ratio, and shear anisotropy factor, have been investigated. The elastic and phonon properties reveal that this compound is mechanically and dynamically stable. Pugh's ratio and Frantsevich's ratio demonstrate its ductile behavior, and the shear anisotropic factor reveals the anisotropic nature of HfPtPb. The band structure predicts this compound to be a semiconductor with a band gap of 0.86 eV. The thermoelectric transport parameters, such as Seebeck coefficient, electrical conductivity, electronic thermal conductivity, and lattice thermal conductivity, have been calculated as a function of temperature. The highest value of Seebeck coefficient is obtained for n-type doping at an optimal carrier concentration of 1.0 × 1020 e/cm3. We predict the maximum value of figure of merit (0.25) at 1000 K. Our investigation suggests that this material is an n-type semiconductor.

Observation of Landau quantization and standing waves in HfSiS

NASA Astrophysics Data System (ADS)

Jiao, L.; Xu, Q. N.; Qi, Y. P.; Wu, S.-C.; Sun, Y.; Felser, C.; Wirth, S.

2018-05-01

Recently, HfSiS was found to be a new type of Dirac semimetal with a line of Dirac nodes in the band structure. Meanwhile, Rashba-split surface states are also pronounced in this compound. Here we report a systematic study of HfSiS by scanning tunneling microscopy/spectroscopy at low temperature and high magnetic field. The Rashba-split surface states are characterized by measuring Landau quantization and standing waves, which reveal a quasilinear dispersive band structure. First-principles calculations based on density-functional theory are conducted and compared with the experimental results. Based on these investigations, the properties of the Rashba-split surface states and their interplay with defects and collective modes are discussed.

DFT study on the interfacial properties of vertical and in-plane BiOI/BiOIO3 hetero-structures.

PubMed

Dai, Wen-Wu; Zhao, Zong-Yan

2017-04-12

Composite photocatalysts with hetero-structures usually favor the effective separation of photo-generated carriers. In this study, BiOIO 3 was chosen to form a hetero-structure with BiOI, due to its internal polar field and good lattice matching with BiOI. The interfacial properties and band offsets were focused on and analyzed in detail by DFT calculations. The results show that the charge depletion and accumulation mainly occur in the region near the interface. This effect leads to an interfacial electric field and thus, the photo-generated electron-hole pairs can be easily separated and transferred along opposite directions at the interface, which is significant for the enhancement of the photocatalytic activity. Moreover, according to the analysis of band offsets, the vertical BiOI/BiOIO 3 belongs to the type-II hetero-structure, while the in-plane BiOI/BiOIO 3 belongs to the type-I hetero-structure. The former type of hetero-structure has more favorable effects to enhance the photocatalytic activity of BiOI than that of the latter type of hetero-structure. In the case of the vertical BiOI/BiOIO 3 hetero-structure, photo-generated electrons can move from the conduction band of BiOI to that of BiOIO 3 , while holes can move from the valence band of BiOIO 3 to that of BiOI under solar radiation. In addition, the introduced internal electric field functions as a selector that can promote the separation of photo-generated carriers, resulting in the higher photocatalytic quantum efficiency. These findings illustrate the underlying mechanism for the reported experiments, and can be used as a basis for the design of novel highly efficient composite photocatalysts with hetero-structures.

The Role of Work Function and Band Gap in Resistive Switching Behaviour of ZnTe Thin Films

NASA Astrophysics Data System (ADS)

Rowtu, Srinu; Sangani, L. D. Varma; Krishna, M. Ghanashyam

2018-02-01

Resistive switching behavior by engineering the electrode work function and band gap of ZnTe thin films is demonstrated. The device structures Au/ZnTe/Au, Au/ZnTe/Ag, Al/ZnTe/Ag and Pt/ZnTe/Ag were fabricated. ZnTe was deposited by thermal evaporation and the stoichiometry and band gap were controlled by varying the source-substrate distance. Band gap could be varied between 1.0 eV to approximately 4.0 eV with the larger band gap being attributed to the partial oxidation of ZnTe. The transport characteristics reveal that the low-resistance state is ohmic in nature which makes a transition to Poole-Frenkel defect-mediated conductivity in the high-resistance states. The highest R off-to- R on ratio achieved is 109. Interestingly, depending on stoichiometry, both unipolar and bipolar switching can be realized.

Quasiparticle band structures and interface physics of SnS and GeS

NASA Astrophysics Data System (ADS)

Malone, Brad; Kaxiras, Efthimios

2013-03-01

Orthorhombic SnS and GeS are layered materials made of earth-abundant elements which have the potential to play a useful role in the massive scale up of renewable power necessary by 2050 to avoid unmanageable levels of climate change. We report on first principles calculations of the quasiparticle spectra of these two materials, predicting the type and magnitude of the fundamental band gap, a quantity which shows a strong degree of scatter in the experimental literature. Additionally, in order to evaluate the possible role of GeS as an electron-blocking layer in a SnS-based photovoltaic device, we investigate the band offsets of the interfaces between these materials along the three principle crystallographic directions. We find that while the valence-band offsets are similar along the three principle directions, the conduction-band offsets display a substantial amount of anisotropy.

Inverse problem of the vibrational band gap of periodically supported beam

NASA Astrophysics Data System (ADS)

Shi, Xiaona; Shu, Haisheng; Dong, Fuzhen; Zhao, Lei

2017-04-01

The researches of periodic structures have a long history with the main contents confined in the field of forward problem. In this paper, the inverse problem is considered and an overall frame is proposed which includes two main stages, i.e., the band gap criterion and its optimization. As a preliminary investigation, the inverse problem of the flexural vibrational band gap of a periodically supported beam is analyzed. According to existing knowledge of its forward problem, the band gap criterion is given in implicit form. Then, two cases with three independent parameters, namely the double supported case and the triple one, are studied in detail and the explicit expressions of the feasible domain are constructed by numerical fitting. Finally, the parameter optimization of the double supported case with three variables is conducted using genetic algorithm aiming for the best mean attenuation within specified frequency band.

Isoelectronic tungsten doping in monolayer MoSe 2 for carrier type modulation

DOE PAGES

Li, Xufan; Lin, Ming -Wei; Basile, Leonardo; ...

2016-07-06

Doping and alloying are effective ways to engineer the band structure and modulate the optoelectronic functionality of monolayer transition metal dichalcogenides (TMDs). In this work, we explore the synthesis and electronic properties of monolayer Mo 1-xW xSe 2 (0 < x < 0.18) alloys with almost 100% alloying degree. The isoelectronic substitutional doping of tungsten for molybdenum in the monolayer MoSe 2 is shown to suppress its intrinsically n-type conduction behavior, with p-type conduction gradually emerging to become dominant with increasing W concentration in the alloys. Atomic resolution Z-contrast electron microscopy show that W is shown to substitute directly formore » Mo without the introduction of noticeable vacancy or interstitial defects, however with randomly-distributed W-rich regions ~2 nm in diameter. Scanning tunneling microscopy/spectroscopy measurements reveal that these W-rich regions exhibit a local band structure with the valence band maximum (VBM) closer to the Fermi level as compared with the Mo-rich regions in the monolayer Mo 1-xW xSe 2 crystal. These localized upshifts of the VBM in the local band structure appear responsible for the overall p-type behavior observed for the monolayer Mo 1-xW xSe 2 crystals. Stacked monolayers of n-type MoSe 2 and p-type Mo 1-xW xSe 2 were demonstrated to form atomically thin, vertically stacked p n homojunctions with gate-tunable characteristics, which appear useful for future optoelectronic applications. Lastly, these results indicate that alloying with isoelectronic dopant atoms appears to be an effective and advantageous alternate strategy to doping or alloying with electron donors or acceptors in two-dimensional TMDs.« less

Effect of Isovalent Substitution on the Electronic Structure and Thermoelectric Properties of the Solid Solution α-As2Te3-xSex (0 ≤ x ≤ 1.5).

PubMed

Vaney, Jean-Baptiste; Delaizir, Gaëlle; Wiendlocha, Bartlomiej; Tobola, Janusz; Alleno, Eric; Piarristeguy, Andrea; Gonçalves, Antonio Pereira; Gendarme, Christine; Malaman, Bernard; Dauscher, Anne; Candolfi, Christophe; Lenoir, Bertrand

2017-02-20

We report on the influence of Se substitution on the electronic band structure and thermoelectric properties (5-523 K) of the solid solution α-As 2 Te 3-x Se x (0 ≤ x ≤ 1.5). All of the polycrystalline compounds α-As 2 Te 3-x Se x crystallize isostructurally in the monoclinic space group C2/m (No. 12, Z = 4). Regardless of the Se content, chemical analyses performed by scanning electron microscopy and electron probe microanalysis indicate a good chemical homogeneity, with only minute amounts of secondary phases for some compositions. In agreement with electronic band structure calculations, neutron powder diffraction suggests that Se does not randomly substitute for Te but exhibits a site preference. These theoretical calculations further predict a monotonic increase in the band gap energy with the Se content, which is confirmed experimentally by absorption spectroscopy measurements. Increasing x up to x = 1.5 leaves unchanged both the p-type character and semiconducting nature of α-As 2 Te 3 . The electrical resistivity and thermopower gradually increase with x as a result of the progressive increase in the band gap energy. Despite the fact that α-As 2 Te 3 exhibits very low lattice thermal conductivity κ L , the substitution of Se for Te further lowers κ L to 0.35 W m -1 K -1 at 300 K. The compositional dependence of the lattice thermal conductivity closely follows classical models of phonon alloy scattering, indicating that this decrease is due to enhanced point-defect scattering.

Characterization and Mitigation of Resistive Losses in a Large Area Laser Power Converter

DTIC Science & Technology

2014-03-27

level lies between the valence and conduction band such that relatively few electrons are thermally excited into the conduction band. Pure crystalline...have an equal number of electrons in the conduction band and holes in the valence band when it is in thermal equilibrium. That is, the electron...easily be thermally excited into the conduction band and act as a mobile charge carrier within the material, now considered n-type for it contains a

Crystal structures, Hirshfeld surface analysis, thermal behavior and dielectric properties of a new organic-inorganic hybrid [C6H10(NH3)2]Cu2Cl8

NASA Astrophysics Data System (ADS)

Salah, Najet; Hamdi, Besma; Bouzidia, Nabaa; Salah, Abdelhamid Ben

2017-12-01

A novel organic-inorganic hybrid sample [C6H10(NH3)2]Cu2Cl8 has been prepared under mild hydrothermal conditions and characterized by single crystal X-ray diffraction, Hirshfeld surface analysis, FT-IR,NMR and UV-Vis spectroscopies, differential scanning calorimetric and dielectric measurement. It is crystallized in the monoclinic system with P21/c space group. The cohesion and stabilization of the structure are provided by the hydrogen bond interactions, (Nsbnd H⋯Cl and Csbnd H⋯Cl), between [C6H10(NH3)2]2+ cation and [Cu2Cl8]2- anion. The Hirschfeld surface analysis has been performed to explore the behavior of these weak interactions. The presence of different functional groups and the nature of their vibrations were identified by FT-IR and Solid state NMR. The thermal study revealed that this compound undergoes two structural phase transitions around 353 and 376 K. Electrical measurements of our compounds have been investigated using complex impedance spectroscopy (CIS) in the frequency and temperature range 331-399 K and 200 Hz-5 MHz, respectively. The AC conductivity is explained using the correlated barrier hopping model (CBH) conduction mechanism. The nature of DC conductivity variation suggests Arrhenius type of electrical conductivity. A relationship between crystal structure and ionic conductivity was established and discussed. Finally, the real and imaginary parts of the permittivity constant are analyzed with the Cole-Cole formalism and the optical spectra indicate that the compound has a direct band gap (3.14 eV) due to direct transition. The wide band gap is due to low defect concentration in the grown crystal, which is more useful for the laser/optical applications.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Hyewon; Cheong, S.W.; Kim, Bog G., E-mail: boggikim@pusan.ac.kr

We have studied the properties of SnO{sub 6} octahedra-containing perovskites and their derived structures using ab initio calculations with different density functionals. In order to predict the correct band gap of the materials, we have used B3LYP hybrid density functional, and the results of B3LYP were compared with those obtained using the local density approximation and generalized gradient approximation data. The calculations have been conducted for the orthorhombic ground state of the SnO{sub 6} containing perovskites. We also have expended the hybrid density functional calculation to the ASnO{sub 3}/A'SnO{sub 3} system with different cation orderings. We propose an empirical relationshipmore » between the tolerance factor and the band gap of SnO{sub 6} containing oxide materials based on first principles calculation. - Graphical abstract: (a) Structure of ASnO{sub 3} for orthorhombic ground state. The green ball is A (Ba, Sr, Ca) cation and the small (red) ball on edge is oxygen. SnO{sub 6} octahedrons are plotted as polyhedron. (b) Band gap of ASnO{sub 3} as a function of the tolerance factor for different density functionals. The experimental values of the band gap are marked as green pentagons. (c) ASnO{sub 3}/A'SnO{sub 3} superlattices with two types cation arrangement: [001] layered structure and [111] rocksalt structure, respectively. (d) B3LYP hybrid functional band gaps of ASnO{sub 3}, [001] ordered superlattices, and [111] ordered superlattices of ASnO{sub 3}/A'SnO{sub 3} as a function of the effective tolerance factor. Note the empirical linear relationship between the band gap and effective tolerance factor. - Highlights: • We report the hybrid functional band gap calculation of ASnO{sub 3} and ASnO{sub 3}/A'SnO{sub 3}. • The band gap of ASnO{sub 3} using B3LYP functional reproduces the experimental value. • We propose the linear relationship between the tolerance factor and the band gap.« less

The effect of simultaneous substitution on the electronic band structure and thermoelectric properties of Se-doped Co3SnInS2 with the Kagome lattice

NASA Astrophysics Data System (ADS)

Fujioka, Masaya; Shibuya, Taizo; Nakai, Junya; Yoshiyasu, Keigo; Sakai, Yuki; Takano, Yoshihiko; Kamihara, Yoichi; Matoba, Masanori

2014-12-01

The thermoelectric properties and electronic band structures for Se-doped Co3SnInS2 were examined. The parent compound of this material (Co3Sn2S2) has two kinds of Sn sites (Sn1 and Sn2 sites). The density functional theory (DFT) calculations show that the indium substitution at the Sn2 site induces a metallic band structure, on the other hand, a semiconducting band structure is obtained from substitution at the Sn1 site. However, according to the previous reports, since the indium atom prefers to replace the tin atom at the Sn1 site rather than the Sn2 site, the resistivity of Co3SnInS2 shows semiconducting-like behavior. In this study we have demonstrated that metallic behavior and a decrease in resistivity for Se-doped Co3SnInS2 occurs without suppression of the Seebeck coefficient. From the DFT calculations, when the selenium content is above 0.5, the total crystallographic energy shows that a higher indium occupancy at Sn2 site is more stable. Therefore, it is suggested that the selenium doping suppress the site preference for indium substitution. This is one of the possible explanations for the metallic conductivity observed in Se-doped Co3SnInS2

Design and analysis of InN - In0.25Ga0.75N single quantum well laser for short distance communication wavelength

NASA Astrophysics Data System (ADS)

Polash, Md. Mobarak Hossain; Alam, M. Shah; Biswas, Saumya

2018-03-01

A single quantum well semiconductor laser based on wurtzite-nitride is designed and analyzed for short distance communication wavelength (at around 1300 nm). The laser structure has 12 Å well layer of InN, 15 Å barrier layer of In0.25Ga0.75N, and 54 Å separate confinement heterostructure layer of GaN. To calculate the electronic characteristics of the structure, a self-consistent method is used where Hamiltonian with effective mass approximation is solved for conduction band while six-bands Hamiltonian matrix with k · p formalism including the polarization effect, valence-band mixing effect, and strain effect is solved for valence band. The interband optical transition elements, optical gain, differential gain, radiative current density, spontaneous emission rate, and threshold characteristics have been calculated. The wave function overlap integral is found to be 45.93% for TE-polarized structure. Also, the spontaneous emission rate is found to be 6.57 × 1027 s - 1 cm - 3 eV - 1 at 1288.21 nm with the carrier density of 5 × 1019 cm - 3. Furthermore, the radiative current density and the radiative recombination rate are found to be 121.92 A cm - 2 and 6.35 × 1027 s - 1 cm - 3, respectively, while the TE-polarized optical gain of the structure is 3872.1 cm - 1 at 1301.7 nm.

Luminescence of delafossite-type CuAlO2 fibers with Eu substitution for Al cations

PubMed Central

Liu, Yin; Gong, Yuxuan; Mellott, Nathan P.; Wang, Bu; Ye, Haitao; Wu, Yiquan

2016-01-01

Abstract CuAlO2 has been examined as a potential luminescent material by substituting Eu for Al cations in the delafossite structure. CuAlO2:Eu3+ nanofibers have been prepared via electrospinning for the ease of mitigating synthesis requirements and for future optoelectronics and emerging applications. Single-phase CuAlO2 fibers could be obtained at a temperature of 1100 °C in air. The Eu was successfully doped in the delafossite structure and two strong emission bands at ~405 and 610 nm were observed in the photoluminescence spectra. These bands are due to the intrinsic near-band-edge transition of CuAlO2 and the f-f transition of the Eu3+ activator, respectively. Further electrical characterization indicated that these fibers exhibit semiconducting behavior and the introduction of Eu could act as band-edge modifiers, thus changing the thermal activation energies. In light of this study, CuAlO2:Eu3+ fibers with both strong photoluminescence and p-type conductivity could be produced by tailoring the rare earth doping concentrations. PMID:27877870

Luminescence of delafossite-type CuAlO2 fibers with Eu substitution for Al cations

NASA Astrophysics Data System (ADS)

Liu, Yin; Gong, Yuxuan; Mellott, Nathan P.; Wang, Bu; Ye, Haitao; Wu, Yiquan

2016-01-01

CuAlO2 has been examined as a potential luminescent material by substituting Eu for Al cations in the delafossite structure. CuAlO2:Eu3+ nanofibers have been prepared via electrospinning for the ease of mitigating synthesis requirements and for future optoelectronics and emerging applications. Single-phase CuAlO2 fibers could be obtained at a temperature of 1100 °C in air. The Eu was successfully doped in the delafossite structure and two strong emission bands at 405 and 610 nm were observed in the photoluminescence spectra. These bands are due to the intrinsic near-band-edge transition of CuAlO2 and the f-f transition of the Eu3+ activator, respectively. Further electrical characterization indicated that these fibers exhibit semiconducting behavior and the introduction of Eu could act as band-edge modifiers, thus changing the thermal activation energies. In light of this study, CuAlO2:Eu3+ fibers with both strong photoluminescence and p-type conductivity could be produced by tailoring the rare earth doping concentrations.

Luminescence of delafossite-type CuAlO2 fibers with Eu substitution for Al cations.

PubMed

Liu, Yin; Gong, Yuxuan; Mellott, Nathan P; Wang, Bu; Ye, Haitao; Wu, Yiquan

2016-01-01

CuAlO 2 has been examined as a potential luminescent material by substituting Eu for Al cations in the delafossite structure. CuAlO 2 :Eu 3+ nanofibers have been prepared via electrospinning for the ease of mitigating synthesis requirements and for future optoelectronics and emerging applications. Single-phase CuAlO 2 fibers could be obtained at a temperature of 1100 °C in air. The Eu was successfully doped in the delafossite structure and two strong emission bands at ~405 and 610 nm were observed in the photoluminescence spectra. These bands are due to the intrinsic near-band-edge transition of CuAlO 2 and the f-f transition of the Eu 3+ activator, respectively. Further electrical characterization indicated that these fibers exhibit semiconducting behavior and the introduction of Eu could act as band-edge modifiers, thus changing the thermal activation energies. In light of this study, CuAlO 2 :Eu 3+ fibers with both strong photoluminescence and p-type conductivity could be produced by tailoring the rare earth doping concentrations.

Ab-Initio Calculation of Electronic Structure of Lead Halide Perovskites with Formamidinium Cation as an Active Material for Perovskite Solar Cells

NASA Astrophysics Data System (ADS)

Indari, E. D.; Wungu, T. D. K.; Hidayat, R.

2017-07-01

Organic lead halide perovskite material based solar cells show impressive power conversion efficiencies, which can reach above 19 percent for perovskite solar cell with methyl-ammonium cations. These efficiencies are originated from efficient photoexcitation and charge carrier transport and not observed in conventional perovskite crystals. In this preliminary research work, we therefore performed Density Functional Theory (DFT) calculation of formamidinium lead iodide (FAPI), an alternative to methyl-ammonium lead iodide (MAPI), to predict their electronic structure and density of state (DOS). The calculation result at the most stable lattice parameters show a good agreement with the experiment results. The obtained band gap energy is 1.307 eV. The valence band is dominantly formed by the 5p orbitals of I- anions, while the conduction band is dominantly formed by the 6p orbitals of Pb2+ cations. The DOS of valence band of this perovskite seems smaller compared to the case of methyl-ammonium lead iodide perovskite, which then may explain the observation of smaller power conversion efficiencies in perovskite solar cells with this formamidinium cations.

Spatially Resolved Imaging on Photocarrier Generations and Band Alignments at Perovskite/PbI2 Heterointerfaces of Perovskite Solar Cells by Light-Modulated Scanning Tunneling Microscopy.

PubMed

Shih, Min-Chuan; Li, Shao-Sian; Hsieh, Cheng-Hua; Wang, Ying-Chiao; Yang, Hung-Duen; Chiu, Ya-Ping; Chang, Chia-Seng; Chen, Chun-Wei

2017-02-08

The presence of the PbI 2 passivation layers at perovskite crystal grains has been found to considerably affect the charge carrier transport behaviors and device performance of perovskite solar cells. This work demonstrates the application of a novel light-modulated scanning tunneling microscopy (LM-STM) technique to reveal the interfacial electronic structures at the heterointerfaces between CH 3 NH 3 PbI 3 perovskite crystals and PbI 2 passivation layers of individual perovskite grains under light illumination. Most importantly, this technique enabled the first observation of spatially resolved mapping images of photoinduced interfacial band bending of valence bands and conduction bands and the photogenerated electron and hole carriers at the heterointerfaces of perovskite crystal grains. By systematically exploring the interfacial electronic structures of individual perovskite grains, enhanced charge separation and reduced back recombination were observed when an optimal design of interfacial PbI 2 passivation layers consisting of a thickness less than 20 nm at perovskite crystal grains was applied.

Coexistence of type-II Dirac point and weak topological phase in Pt 3 Sn

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Minsung; Wang, Cai -Zhuang; Ho, Kai -Ming

Intriguing topological phases may appear in both insulating and semimetallic states. Topological insulators exhibit topologically nontrivial band inversion, while topological Dirac/Weyl semimetals show “relativistic” linear band crossings. Here, we report an unusual topological state of Pt 3Sn, where the two topological features appear simultaneously. Based on first-principles calculations, we show that Pt 3Sn is a three-dimensional weak topological semimetal with topologically nontrivial band inversion between the valence and conduction bands, where the band structure also possesses type-II Dirac points at the boundary of two electron pockets. The formation of the Dirac points can be understood in terms of the representationsmore » of relevant symmetry groups and the compatibility relations. The topological surface states appear in accordance with the nontrivial bulk band topology. As a result, the unique coexistence of the two distinct topological features in Pt 3Sn enlarges the material scope in topological physics, and is potentially useful for spintronics.« less

Coexistence of type-II Dirac point and weak topological phase in Pt 3 Sn

DOE PAGES

Kim, Minsung; Wang, Cai -Zhuang; Ho, Kai -Ming

2017-11-06

Intriguing topological phases may appear in both insulating and semimetallic states. Topological insulators exhibit topologically nontrivial band inversion, while topological Dirac/Weyl semimetals show “relativistic” linear band crossings. Here, we report an unusual topological state of Pt 3Sn, where the two topological features appear simultaneously. Based on first-principles calculations, we show that Pt 3Sn is a three-dimensional weak topological semimetal with topologically nontrivial band inversion between the valence and conduction bands, where the band structure also possesses type-II Dirac points at the boundary of two electron pockets. The formation of the Dirac points can be understood in terms of the representationsmore » of relevant symmetry groups and the compatibility relations. The topological surface states appear in accordance with the nontrivial bulk band topology. As a result, the unique coexistence of the two distinct topological features in Pt 3Sn enlarges the material scope in topological physics, and is potentially useful for spintronics.« less

Analysis of forest structure using thematic mapper simulator data

NASA Technical Reports Server (NTRS)

Peterson, D. L.; Westman, W. E.; Brass, J. A.; Stephenson, N. J.; Ambrosia, V. G.; Spanner, M. A.

1986-01-01

The potential of Thematic Mapper Simulator (TMS) data for sensing forest structure information has been explored by principal components and feature selection techniques. In a survey of forest structural properties conducted for 123 field sites of the Sequoia National Park, the canopy closure could be well estimated (r = 0.62 to 0.69) by a variety of channel bands and band ratios, without reference to the forest type. Estimation of the basal area was less successful (r = 0.51 or less) on the average, but could be improved for certain forest types when data were stratified by floristic composition. To achieve such a stratification, individual sites were ordinated by a detrended correspondence analysis based on the canopy of dominant species. The analysis of forest structure in the Sequoia data suggests that total basal area can be best predicted in stands of lower density, and in younger even-aged managed stands.

Engineering the electronic band structures of novel cubic structured germanium monochalcogenides for thermoelectric applications

NASA Astrophysics Data System (ADS)

Ul Haq, Bakhtiar; AlFaify, S.; Ahmed, R.; Butt, Faheem K.; Laref, A.; Goumri-Said, Souraya; Tahir, S. A.

2018-05-01

Germanium mono-chalcogenides have received considerable attention for being a promising replacement for the relatively toxic and expensive chalcogenides in renewable and sustainable energy applications. In this paper, we explore the potential of the recently discovered novel cubic structured (π-phase) GeS and GeSe for thermoelectric applications in the framework of density functional theory coupled with Boltzmann transport theory. To examine the modifications in their physical properties, the across composition alloying of π-GeS and π-GeSe (such as π-GeS1-xSex for x =0, 0.25, 0.50, 0.75, and 1) has been performed that has shown important effects on the electronic band structures and effective masses of charge carriers. An increase in Se composition in π-GeS1-xSex has induced a downward shift in their conduction bands, resulting in the narrowing of their energy band gaps. The thermoelectric coefficients of π-GeS1-xSex have been accordingly influenced by the evolution of the electronic band structures and effective masses of charge carriers. π-GeS1-xSex features sufficiently larger values of Seebeck coefficients, power factors and figures of merit (ZTs), which experience further improvement with an increase in temperature, revealing their potential for high-temperature applications. The calculated results show that ZT values equivalent to unity can be achieved for π-GeS1-xSex at appropriate n-type doping levels. Our calculations for the formation enthalpies indicate that a π-GeS1-xSex alloying system is energetically stable and could be synthesized experimentally. These intriguing characteristics make π-GeS1-xSex a promising candidate for futuristic thermoelectric applications in energy harvesting devices.

Electronic structure and optical properties of noncentrosymmetric LiGaSe2: Experimental measurements and DFT band structure calculations

NASA Astrophysics Data System (ADS)

Lavrentyev, A. A.; Gabrelian, B. V.; Vu, V. T.; Ananchenko, L. N.; Isaenko, L. I.; Yelisseyev, A. P.; Khyzhun, O. Y.

2017-04-01

We report on measurements of X-ray photoelectron (XP) spectra for pristine and Ar+ ion-irradiated surfaces of LiGaSe2 single crystal grown by Bridgman-Stockbarger method. Electronic structure of the LiGaSe2 compound is studied from a theoretical and experimental viewpoint. In particular, total and partial densities of states of LiGaSe2 are investigated by density functional theory (DFT) calculations employing the augmented plane wave + local orbitals (APW + lo) method and they are verified by data of X-ray spectroscopy measurements. The DFT calculations indicate that the main contributors to the valence band of LiGaSe2 are the Se 4p states, which contribute mainly at the top and in the upper portion of the valence band, with also essential contributions of these states in the lower portion of the band. Other substantial contributions to the valence band of LiGaSe2 emerge from the Ga 4s and Ga 4p states contributing mainly at the lower ant upper portions of the valence band, respectively. With respect to the conduction band, the calculations indicate that its bottom is composed mainly from contributions of the unoccupied Ga s and Se p states. The present calculations are confirmed experimentally when comparing the XP valence-band spectrum of the LiGaS2 single crystal on a common energy scale with the X-ray emission bands representing the energy distribution of the Ga 4p and Se 4p states. Measurements of the fundamental absorption edges at room temperature reveal that bandgap value, Eg, of LiGaSe2 is equal to 3.47 eV and the Eg value increases up to 3.66 eV when decreasing temperature to 80 K. The main optical characteristics of the LiGaSe2 compound are clarified by the DFT calculations.

Direct measurement of Dirac point energy at the graphene/oxide interface.

PubMed

Xu, Kun; Zeng, Caifu; Zhang, Qin; Yan, Rusen; Ye, Peide; Wang, Kang; Seabaugh, Alan C; Xing, Huili Grace; Suehle, John S; Richter, Curt A; Gundlach, David J; Nguyen, N V

2013-01-09

We report the direct measurement of the Dirac point, the Fermi level, and the work function of graphene by performing internal photoemission measurements on a graphene/SiO(2)/Si structure with a unique optical-cavity enhanced test structure. A complete electronic band alignment at the graphene/SiO(2)/Si interfaces is accurately established. The observation of enhanced photoemission from a one-atom thick graphene layer was possible by taking advantage of the constructive optical interference in the SiO(2) cavity. The photoemission yield was found to follow the well-known linear density-of-states dispersion in the vicinity of the Dirac point. At the flat band condition, the Fermi level was extracted and found to reside 3.3 eV ± 0.05 eV below the bottom of the SiO(2) conduction band. When combined with the shift of the Fermi level from the Dirac point, we are able to ascertain the position of the Dirac point at 3.6 eV ± 0.05 eV with respect to the bottom of the SiO(2) conduction band edge, yielding a work function of 4.5 eV ± 0.05 eV which is in an excellent agreement with theory. The accurate determination of the work function of graphene is of significant importance to the engineering of graphene-based devices, and the measurement technique we have advanced in this Letter will have significant impact on numerous applications for emerging graphene-like 2-dimensional material systems.

Design of ternary alkaline-earth metal Sn(II) oxides with potential good p-type conductivity

DOE PAGES

Du, Mao -Hua; Singh, David J.; Zhang, Lijun; ...

2016-04-19

Oxides with good p-type conductivity have been long sought after to achieve high performance all-oxide optoelectronic devices. Divalent Sn(II) based oxides are promising candidates because of their rather dispersive upper valence bands caused by the Sn-5s/O-2p anti-bonding hybridization. There are so far few known Sn(II) oxides being p-type conductive suitable for device applications. Here, we present via first-principles global optimization structure searches a material design study for a hitherto unexplored Sn(II)-based system, ternary alkaline-earth metal Sn(II) oxides in the stoichiometry of MSn 2O 3 (M = Mg, Ca, Sr, Ba). We identify two stable compounds of SrSn 2O 3 andmore » BaSn 2O 3, which can be stabilized by Sn-rich conditions in phase stability diagrams. Their structures follow the Zintl behaviour and consist of basic structural motifs of SnO 3 tetrahedra. Unexpectedly they show distinct electronic properties with band gaps ranging from 1.90 (BaSn 2O 3) to 3.15 (SrSn 2O 3) eV, and hole effective masses ranging from 0.87 (BaSn 2O 3) to above 6.0 (SrSn 2O 3) m0. Further exploration of metastable phases indicates a wide tunability of electronic properties controlled by the details of the bonding between the basic structural motifs. Lastly, this suggests further exploration of alkaline-earth metal Sn(II) oxides for potential applications requiring good p-type conductivity such as transparent conductors and photovoltaic absorbers.« less

Development of Seismic Isolation Systems Using Periodic Materials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yan, Yiqun; Mo, Yi-Lung; Menq, Farn-Yuh

Advanced fast nuclear power plants and small modular fast reactors are composed of thin-walled structures such as pipes; as a result, they do not have sufficient inherent strength to resist seismic loads. Seismic isolation, therefore, is an effective solution for mitigating earthquake hazards for these types of structures. Base isolation, on which numerous studies have been conducted, is a well-defined structure protection system against earthquakes. In conventional isolators, such as high-damping rubber bearings, lead-rubber bearings, and friction pendulum bearings, large relative displacements occur between upper structures and foundations. Only isolation in a horizontal direction is provided; these features are notmore » desirable for the piping systems. The concept of periodic materials, based on the theory of solid-state physics, can be applied to earthquake engineering. The periodic material is a material that possesses distinct characteristics that prevent waves with certain frequencies from being transmitted through it; therefore, this material can be used in structural foundations to block unwanted seismic waves with certain frequencies. The frequency band of periodic material that can filter out waves is called the band gap, and the structural foundation made of periodic material is referred to as the periodic foundation. The design of a nuclear power plant, therefore, can be unified around the desirable feature of a periodic foundation, while the continuous maintenance of the structure is not needed. In this research project, three different types of periodic foundations were studied: one-dimensional, two-dimensional, and three-dimensional. The basic theories of periodic foundations are introduced first to find the band gaps; then the finite element methods are used, to perform parametric analysis, and obtain attenuation zones; finally, experimental programs are conducted, and the test data are analyzed to verify the theory. This procedure shows that the periodic foundation is a promising and effective way to mitigate structural damage caused by earthquake excitation.« less

Bandgap engineering through nanocrystalline magnetic alloy grafting on reduced graphene oxide.

PubMed

De, D; Chakraborty, M; Majumdar, S; Giri, S

2014-09-28

High conductivity and the absence of ferromagnetism in pristine graphene fail to satisfy primary criteria for possible technological application in spintronics. Opening of the bandgap in graphene is primarily desirable for such applications. We report a simplified and novel approach of controlled grafting of a magnetic alloy on reduced graphene oxide. This eventually leads to ferromagnetism of the stable hybrid material at room temperature, with a large moment (∼1.2 μB) and a remarkable decrease in conductivity (∼10 times) compared to highly ordered pyrolytic graphite. Our model band-structure calculation indicates that the combined effect of controlled vacancies and impurities attributed to the nanocrystalline alloy grafting leads to a promising step toward band gap engineering.

Creation of quasi-Dirac points in the Floquet band structure of bilayer graphene.

PubMed

Cheung, W M; Chan, K S

2017-06-01

We study the Floquet quasi-energy band structure of bilayer graphene when it is illuminated by two laser lights with frequencies [Formula: see text] and [Formula: see text] using Floquet theory. We focus on the dynamical gap formed by the conduction band with Floquet index  =  -1 and the valence band with Floquet index  =  +1 to understand how Dirac points can be formed. It is found that the dynamical gap does not have rotation symmetry in the momentum space, and quasi-Dirac points, where the conduction and valence bands almost touch, can be created when the dynamical gap closes along some directions with suitably chosen radiation parameters. We derive analytical expressions for the direction dependence of the dynamical gaps using Lowdin perturbation theory to gain a better understanding of the formation of quasi-Dirac points. When both radiations are circularly polarized, the gap can be exactly zero along some directions, when only the first and second order perturbations are considered. Higher order perturbations can open a very small gap in this case. When both radiations are linearly polarized, the gap can be exactly zero up to the fourth order perturbation and more than one quasi-Dirac point is formed. We also study the electron velocity around a dynamical gap and show that the magnitude of the velocity drops to values close to zero when the k vector is near to the gap minimum. The direction of the velocity also changes around the gap minimum, and when the gap is larger in value the change in the velocity direction is more gradual. The warping effect does not affect the formation of a Dirac point along the k x axis, while it prevents its formation when there is phase shift between the two radiations.

The electronic structure and thermoelectric properties of BiTl{sub 9}Te{sub 6} and SbTl{sub 9}Te{sub 6}: First-principles calculations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Guo, Li Bin; Ye, Lingyun; Wang, Yuan Xu, E-mail: wangyx@henu.edu.cn

2015-12-21

The electronic structure and thermoelectric properties of MTl{sub 9}Te{sub 6} (M = Bi, Sb) were studied using density functional theory and the semiclassical Boltzmann theory. It is found that the band gaps of BiTl{sub 9}Te{sub 6} and SbTl{sub 9}Te{sub 6} are equal to 0.59 eV and 0.72 eV, respectively. The relative large band gap and strong coupling between Sb s and Te p are helpful to the thermoelectric properties of SbTl{sub 9}Te{sub 6}. Near the bottom of the conduction bands, the number of band valleys of SbTl{sub 9}Te{sub 6} is four and is larger than that of BiTl{sub 9}Te{sub 6} (two band valleys),more » which will increase its Seebeck coefficient. Although BiTl{sub 9}Te{sub 6} has a larger electrical conductivity relative to relaxation time (σ/τ) along the z-direction than that of SbTl{sub 9}Te{sub 6}, the results show that the transport properties of SbTl{sub 9}Te{sub 6} are better than those of BiTl{sub 9}Te{sub 6} possibly due to its large Seebeck coefficient. The maximum value of power factor relative to relaxation time (S{sup 2}σ/τ) for SbTl{sub 9}Te{sub 6} reaches 4.30 × 10{sup 11 }W/K{sup 2} m s at 900 K, that is, originated from its relatively large Seebeck coefficient, suggesting its promising thermoelectric performance at high temperature.« less

Giant Stark effect in double-stranded porphyrin ladder polymers

NASA Astrophysics Data System (ADS)

Pramanik, Anup; Kang, Hong Seok

2011-03-01

Using the first-principles calculations, we have investigated the stability and the electronic structure of two types of recently synthesized one-dimensional nanoribbons, i.e., double-stranded zinc(II) porphyrin ladder polymer (LADDER) arrays. First, electronic structure calculations were used to show that the LADDER is a semiconductor. Most importantly, the application of a transverse electric field significantly reduces the band gap of the LADDER, ultimately converting the LADDER to a metal at a field strength of 0.1 V/Å. The giant Stark effect in this case is almost as strong as that in boron nitride nanotubes and nanoribbons. In the presence of an electric field, hole conduction and electronic conduction will occur entirely through spatially separated strands, rendering these materials useful for nanoelectronic devices. Second, the substitution of hydrogen atoms in the porphyrin units or that of zinc ions with other kinds of chemical species is found to increase the binding strength of the LADDER and reduce the band gap.

Synthesis and thermal conductivity of type II silicon clathrates

NASA Astrophysics Data System (ADS)

Beekman, M.; Nolas, G. S.

2006-08-01

We have synthesized and characterized polycrystalline Na 1Si 136 and Na 8Si 136, compounds possessing the type II clathrate hydrate crystal structure. Resistivity measurements from 10 to 300 K indicate very large resistivities in this temperature range, with activated temperature dependences indicative of relatively large band gap semiconductors. The thermal conductivity is very low; two orders-of-magnitude lower than that of diamond-structure silicon at room temperature. The thermal conductivity of Na 8Si 136 displays a temperature dependence that is atypical of crystalline solids and more indicative of amorphous materials. This work is part of a continuing effort to explore the many different compositions and structure types of clathrates, a class of materials that continues to be of interest for scientific and technological applications.

Pass-Band Characteristics of an L-Shaped Waveguide in a Diamond Structure Photonic Crystal

NASA Astrophysics Data System (ADS)

Chen, Shibin; Ma, Jingcun; Yao, Yunshi; Liu, Xin; Lin, Ping

2018-06-01

The conduction characteristics of a L-shaped waveguide in a diamond structure photonic crystal is investigated in this paper. The waveguides were fabricated with titanium dioxide ceramic via 3-D printing and sintering. The effects of the position and size of line defects on the transmission characteristics are first simulated using a finite-difference time-domain method. The simulated results show that, when the length of the rectangular defect equals the lattice constant, multiple extended modes are generated. When the centers of the single unit cell of the diamond structure and the line defect waveguide coincide, higher transmission efficiency in the line defect can be achieved. In addition, the corner of the L-shaped waveguide was optimized to reduce reflection loss at the turning point using the arc transition of the large diameter. Our experimental results indicate that L-shaped waveguides with an optimized photonic band gap structure and high-K materials can produce a pass-band between 13.8 GHz and 14.4 GHz and increase transmission efficiency. The computed results agree with the experimental results. Our results may help the integration of microwave devices in the future and possibly enable new applications of photonic crystals.

X-ray diffraction, crystal structure, and spectral features of the optical susceptibilities of single crystals of the ternary borate oxide lead bismuth tetraoxide, PbBiBO4.

PubMed

Reshak, Ali Hussain; Kityk, I V; Auluck, S; Chen, Xuean

2009-05-14

The all-electron full-potential linearized augmented plane-wave method has been used for an ab initio theoretical study of the band structure, the spectral features of the optical susceptibilities, the density of states, and the electron charge density for PbBiBO4. Our calculations show that the valence-band maximum (VBM) and conduction-band minimum (CBM) are located at the center of the Brillouin zone, resulting in a direct energy gap of about 3.2 eV. We have synthesized the PbBiBO4 crystal by employing a conventional solid-state reaction method. The theoretical calculations in this work are based on the structure built from our measured atomic parameters. We should emphasize that the observed experimental X-ray diffraction (XRD) pattern is in good agreement with the theoretical one, confirming that our structural model is valid. Our calculated bond lengths show excellent agreement with the experimental data. This agreement is attributed to our use of full-potential calculations. The spectral features of the optical susceptibilities show a small positive uniaxial anisotropy.

Effect of species structure and dielectric constant on C-band forest backscatter

NASA Technical Reports Server (NTRS)

Lang, R. H.; Landry, R.; Kilic, O.; Chauhan, N.; Khadr, N.; Leckie, D.

1993-01-01

A joint experiment between Canadian and USA research teams was conducted early in Oct. 1992 to determine the effect of species structure and dielectric variations on forest backscatter. Two stands, one red pine and one jack pine, in the Petawawa National Forestry Institute (PNFI) were utilized for the experiment. Extensive tree architecture measurements had been taken by the Canada Centre for Remote Sensing (CCRS) several months earlier by employing a Total Station surveying instrument which provides detailed information on branch structure. A second part of the experiment consisted of cutting down several trees and using dielectric probes to measure branch and needle permittivity values at both sites. The dielectric and the tree geometry data were used in the George Washington University (GWU) Vegetation Model to determine the C band backscattering coefficients of the individual stands for VV polarization. The model results show that backscatter at C band comes mainly from the needles and small branches and the upper portion of the trunks acts only as an attenuator. A discussion of variation of backscatter with specie structure and how dielectric variations in needles for both species may affect the total backscatter returns is provided.

Potential variations around grain boundaries in impurity-doped BaSi₂ epitaxial films evaluated by Kelvin probe force microscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tsukahara, D.; Baba, M.; Honda, S.

2014-09-28

Potential variations around the grain boundaries (GBs) in antimony (Sb)-doped n-type and boron (B)-doped p-type BaSi₂ epitaxial films on Si(111) were evaluated by Kelvin probe force microscopy. Sb-doped n-BaSi₂ films exhibited positively charged GBs with a downward band bending at the GBs. The average barrier height for holes was approximately 10 meV for an electron concentration n ≈ 10¹⁷ cm⁻³. This downward band bending changed to upward band bending when n was increased to n = 1.8 × 10¹⁸cm⁻³. In the B-doped p-BaSi₂ films, the upward band bending was observed for a hole concentration p ≈ 10¹⁸cm⁻³. The average barriermore » height for electrons decreased from approximately 25 to 15 meV when p was increased from p = 2.7 × 10¹⁸ to p = 4.0 × 10¹⁸ cm⁻³. These results are explained under the assumption that the position of the Fermi level E{sub f} at GBs depends on the degree of occupancy of defect states at the GBs, while E{sub f} approached the bottom of the conduction band or the top of the valence band in the BaSi₂ grain interiors with increasing impurity concentrations. In both cases, such small barrier heights may not deteriorate the carrier transport properties. The electronic structures of impurity-doped BaSi₂ are also discussed using first-principles pseudopotential method to discuss the insertion sites of impurity atoms and clarify the reason for the observed n-type conduction in the Sb-doped BaSi₂ and p-type conduction in the B-doped BaSi₂.« less

Correlations of charge neutrality level with electronic structure and p-d hybridization

PubMed Central

Das, Arkaprava; Gautam, Subodh K.; Shukla, D. K.; Singh, Fouran

2017-01-01

The formation of charge neutrality level (CNL) in highly conducting Cadmium oxide (CdO) thin films is demonstarted by the observed variation in the band gap upon annealing and doping. It may be explained by the observation that Tin (Sn) doping breaks the perfect periodicity of CdO cubic crystal structure and creates virtual gap states (ViGS). The level of local CNL resides at the branch point of ViGS, making the energy at which native defect’s character changes from predominantly donor-like below CNL to predominantly acceptor-like above the CNL and a schematic band diagram is developed to substantiate the same. Further investigations using soft x-ray absorption spectroscopy (SXAS) at Oxygen and Cadmium edges show the reduction of Sn4+ to Sn2+. The analysis of the spectral features has revealed an evidence of p-d interaction between O 2p and Cd 4d orbitals that pushes the valence band minima at higher energies which is symmetry forbidden at г point and causing a positive valance band dispersion away from the zone centre in the г ~ L, K direction. Thus, origin of the CNL is attributed to the high density of the Oxygen vacancies as confirmed by the change in the local electronic structure and p-d hybridization of orbitals. PMID:28102312

First principles study on the electronic structures and transport properties of armchair/zigzag edge hybridized graphene nanoribbons

NASA Astrophysics Data System (ADS)

Yi, Xiuying; Long, Mengqiu; Liu, Anhua; Li, Mingjun; Xu, Hui

2018-05-01

Graphene nanoribbons (GNRs) can be mainly classified into armchair graphene nanoribbons (aGNRs) and zigzag graphene nanoribbons (zGNRs) by different edge chiral directions. In this work, by introducing Stone-Wales defects on the edges of the V-shaped aGNRs, we propose a kind of armchair/zigzag edge hybridized GNRs (a/zHGNRs) and using the density functional theory and the nonequilibrium Green's function method, the band structures and electronic transport properties of the a/zHGNRs have been calculated. Our results show that an indirect bandgap appears in the band structures of the a/zHGNRs, which is very different from the direct bandgap of aGNRs and gapless of zGNRs. We also find that the valance band is mainly derived from the armchair partial atoms on the hybridized edge, while the conduction band comes mainly from the zigzag partial atoms of the hybridized edge. Meanwhile, the bandgap also oscillates with a period of three when the ribbon width increases. In addition, our quantum transport calculations show that there is a remarkable transition between the semiconductor and the metal with different ribbon widths in the a/zHGNRs devices, and the corresponding physical analysis is given.

Correlations of charge neutrality level with electronic structure and p-d hybridization

NASA Astrophysics Data System (ADS)

Das, Arkaprava; Gautam, Subodh K.; Shukla, D. K.; Singh, Fouran

2017-01-01

The formation of charge neutrality level (CNL) in highly conducting Cadmium oxide (CdO) thin films is demonstarted by the observed variation in the band gap upon annealing and doping. It may be explained by the observation that Tin (Sn) doping breaks the perfect periodicity of CdO cubic crystal structure and creates virtual gap states (ViGS). The level of local CNL resides at the branch point of ViGS, making the energy at which native defect’s character changes from predominantly donor-like below CNL to predominantly acceptor-like above the CNL and a schematic band diagram is developed to substantiate the same. Further investigations using soft x-ray absorption spectroscopy (SXAS) at Oxygen and Cadmium edges show the reduction of Sn4+ to Sn2+. The analysis of the spectral features has revealed an evidence of p-d interaction between O 2p and Cd 4d orbitals that pushes the valence band minima at higher energies which is symmetry forbidden at г point and causing a positive valance band dispersion away from the zone centre in the г ~ L, K direction. Thus, origin of the CNL is attributed to the high density of the Oxygen vacancies as confirmed by the change in the local electronic structure and p-d hybridization of orbitals.

Valence-Band Electronic Structures of High-Pressure-Phase PdF2-type Platinum-Group Metal Dioxides MO2 (M = Ru, Rh, Ir, and Pt)

NASA Astrophysics Data System (ADS)

Soda, Kazuo; Kobayashi, Daichi; Mizui, Tatsuya; Kato, Masahiko; Shirako, Yuichi; Niwa, Ken; Hasegawa, Masashi; Akaogi, Masaki; Kojitani, Hiroshi; Ikenaga, Eiji; Muro, Takayuki

2018-04-01

The valence-band electronic structures of high-pressure-phase PdF2-type (HP-PdF2-type) platinum-group metal dioxides MO2 (M = Ru, Rh, Ir, and Pt) were studied by synchrotron radiation photoelectron spectroscopy and first-principles calculations. The obtained photoelectron spectra for HP-PdF2-type RuO2, RhO2, and IrO2 agree well with the calculated valence-band densities of states (DOSs) for these compounds, indicating their metallic properties, whereas the DOS of HP-PdF2-type PtO2 (calculated in the presence and absence of spin-orbit interactions) predicts that this material may be metallic or semimetallic, which is inconsistent with the electric conductivity reported to date and the charging effect observed in current photoelectron measurements. Compared with the calculated results, the valence-band spectrum of PtO2 appears to have shifted toward the high-binding-energy side and reveals a gradual intensity decrease toward the Fermi energy EF, implying a semiconductor-like electronic structure. Spin-dependent calculations predict a ferromagnetic ground state with a magnetization of 0.475 μB per formula unit for HP-PdF2-type RhO2.

High thermoelectric performances of monolayer SnSe allotropes.

PubMed

Hu, Zi-Yu; Li, Kai-Yue; Lu, Yong; Huang, Yan; Shao, Xiao-Hong

2017-10-26

α-SnSe is one of the most promising thermoelectric materials with low thermal conductivity and a high power factor. Since the thermoelectric properties of a material have a strong dependence on its crystal structure, we study the energetic and thermoelectric properties of four new monolayer phases of SnSe (β, γ, δ and ε) together with α-SnSe using the ab initio density functional theory method. The calculated electronic structures show that all five phases are semiconductors with different band gaps. The α, β, γ, and δ phases have an indirect band gap with the hybridization of sp 2 orbitals, whereas the ε phase has a direct band with the hybridization of sp 3 orbitals. The thermoelectric transport properties and coefficients are obtained from the electronic structure using semi-classical Boltzmann theory, and the results indicate that the four new phases of SnSe (β, γ, δ and ε) all have better thermoelectric properties compared with the reported α phase. The predicted ZT value for the β-SnSe phase is 2.06 at 300 K, suggesting that it has great potential for novel thermoelectric applications.

Experimental and theoretical study of the electronic structure of single-crystal BaBiO3

NASA Astrophysics Data System (ADS)

Balandeh, Shadi; Green, Robert J.; Foyevtsova, Kateryna; Chi, Shun; Foyevtsov, Oleksandr; Li, Fengmiao; Sawatzky, George A.

2017-10-01

High quality single crystals of BaBiO3 were grown by congruent melting technique and characterized with x-ray diffraction, x-ray photoemission, and transport property studies. The perovskite oxide BaBiO3 is a negative charge transfer gap high Tc oxide parent superconducting compound exhibiting self-doping of holes into the oxygen 2 p band. We study the low energy scale valence and conduction bands in detail from both a theoretical perspective as well as through x ray, absorption/emission, and photoelectron spectroscopies. X-ray spectroscopy verifies the results of density functional theory (DFT) regarding the overall band structure featuring strong O 2 p character of the empty antibonding combination of the hybridized Bi 6 s and O 2 p states. From the analysis of the core level line shapes we conclude that the dominant O 2 p -Bi 6 s hybridization energy scale determines the low energy scale electronic structure. This analysis provides further insight into the importance of self-doped oxygen 2 p states in this high Tc family of oxides.

High-frequency response and the possibilities of frequency-tunable narrow-band terahertz amplification in resonant tunneling nanostructures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kapaev, V. V., E-mail: kapaev@sci.lebedev.ru; Kopaev, Yu. V.; Savinov, S. A.

2013-03-15

The characteristics of the high-frequency response of single- and double-well resonant tunneling structures in a dc electric field are investigated on the basis of the numerical solution of a time-dependent Schroedinger equation with open boundary conditions. The frequency dependence of the real part of high frequency conductivity (high-frequency response) in In{sub 0.53}Ga{sub 0.47}As/AlAs/InP structures is analyzed in detail for various values of the dc voltage V{sub dc} in the negative differential resistance (NDR) region. It is shown that double-well three-barrier structures are promising for the design of terahertz-band oscillators. The presence of two resonant states with close energies in suchmore » structures leads to a resonant (in frequency) response whose frequency is determined by the energy difference between these levels and can be controlled by varying the parameters of the structure. It is shown that, in principle, such structures admit narrow-band amplification, tuning of the amplification frequency, and a fine control of the amplification (oscillation) frequency in a wide range of terahertz frequencies by varying a dc electric voltage applied to the structure. Starting from a certain width of the central intermediate barrier in double-well structures, one can observe a collapse of resonances, where the structure behaves like a single-well system. This phenomenon imposes a lower limit on the oscillation frequency in three-barrier resonant tunneling structures.« less

Characterization of the Reverberation Chamber at the NASA Langley Structural Acoustics Loads and Transmission (SALT) Facility

NASA Technical Reports Server (NTRS)

Grosveld, Ferdinand W.

2013-01-01

In 2011 the noise generating capabilities in the reverberation chamber of the Structural Acoustic Loads and Transmission (SALT) facility at NASA Langley Research Center were enhanced with two fiberglass reinforced polyester resin exponential horns, each coupled to Wyle Acoustic Source WAS-3000 airstream modulators. This report describes the characterization of the reverberation chamber in terms of the background noise, diffusivity, sound pressure levels, the reverberation times and the related overall acoustic absorption in the empty chamber and with the acoustic horn(s) installed. The frequency range of interest includes the 80 Hz to 8000 Hz one-third octave bands. Reverberation time and sound pressure level measurements were conducted and standard deviations from the mean were computed. It was concluded that a diffuse field could be produced above the Schroeder frequency in the 400 Hz one-third octave band and higher for all applications. This frequency could be lowered by installing panel diffusers or moving vanes to improve the acoustic modal overlap in the chamber. In the 80 Hz to 400 Hz one-third octave bands a successful measurement will be dependent on the type of measurement, the test configuration, the source and microphone locations and the desired accuracy. It is recommended that qualification measurements endorsed in the International Standards be conducted for each particular application.

Evidence of Type-II Band Alignment in III-nitride Semiconductors: Experimental and theoretical investigation for In0.17Al0.83N/GaN heterostructures

PubMed Central

Wang, Jiaming; Xu, Fujun; Zhang, Xia; An, Wei; Li, Xin-Zheng; Song, Jie; Ge, Weikun; Tian, Guangshan; Lu, Jing; Wang, Xinqiang; Tang, Ning; Yang, Zhijian; Li, Wei; Wang, Weiying; Jin, Peng; Chen, Yonghai; Shen, Bo

2014-01-01

Type-II band alignment structure is coveted in the design of photovoltaic devices and detectors, since it is beneficial for the transport of photogenerated carriers. Regrettably, for group-III-nitride wide bandgap semiconductors, all existing devices are limited to type-I heterostructures, owing to the unavailable of type-II ones. This seriously restricts the designing flexibility for optoelectronic devices and consequently the relevant performance of this material system. Here we show a brandnew type-II band alignment of the lattice-matched In0.17Al0.83N/GaN heterostructure from the perspective of both experimental observations and first-principle theoretical calculations. The band discontinuity is dominated by the conduction band offset ΔEC, with a small contribution from the valence band offset ΔEV which equals 0.1 eV (with being above). Our work may open up new prospects to realize high-performance III-Nitrides optoelectronic devices based on type-II energy band engineering. PMID:25283334

Evidence of type-II band alignment in III-nitride semiconductors: experimental and theoretical investigation for In 0.17 Al 0.83 N/GaN heterostructures.

PubMed

Wang, Jiaming; Xu, Fujun; Zhang, Xia; An, Wei; Li, Xin-Zheng; Song, Jie; Ge, Weikun; Tian, Guangshan; Lu, Jing; Wang, Xinqiang; Tang, Ning; Yang, Zhijian; Li, Wei; Wang, Weiying; Jin, Peng; Chen, Yonghai; Shen, Bo

2014-10-06

Type-II band alignment structure is coveted in the design of photovoltaic devices and detectors, since it is beneficial for the transport of photogenerated carriers. Regrettably, for group-III-nitride wide bandgap semiconductors, all existing devices are limited to type-I heterostructures, owing to the unavailable of type-II ones. This seriously restricts the designing flexibility for optoelectronic devices and consequently the relevant performance of this material system. Here we show a brandnew type-II band alignment of the lattice-matched In 0.17 Al 0.83 N/GaN heterostructure from the perspective of both experimental observations and first-principle theoretical calculations. The band discontinuity is dominated by the conduction band offset ΔEC, with a small contribution from the valence band offset ΔEV which equals 0.1 eV (with E(AlInN(VBM) being above E(GaN)(VBM)). Our work may open up new prospects to realize high-performance III-Nitrides optoelectronic devices based on type-II energy band engineering.

Verification of band offsets and electron effective masses in GaAsN/GaAs quantum wells: Spectroscopic experiment versus 10-band k·p modeling

NASA Astrophysics Data System (ADS)

Ryczko, K.; Sek, G.; Sitarek, P.; Mika, A.; Misiewicz, J.; Langer, F.; Höfling, S.; Forchel, A.; Kamp, M.

2013-06-01

Optical transitions in GaAs1-xNx/GaAs quantum wells (QWs) have been probed by two complementary techniques, modulation spectroscopy in a form of photoreflectance and surface photovoltage spectroscopy. Transition energies in QWs of various widths and N contents have been compared with the results of band structure calculations based on the 10-band k.p Hamiltonian. Due to the observation of higher order transitions in the measured spectra, the band gap discontinuities at the GaAsN/GaAs interface and the electron effective masses could be determined, both treated as semi-free parameters to get the best matching between the theoretical and experimental energies. We have obtained the chemical conduction band offset values of 86% for x = 1.2% and 83% for x = 2.2%, respectively. For these determined band offsets, the electron effective masses equal to about 0.09 mo in QWs with 1.2% N and 0.15 mo for the case of larger N content of 2.2%.

Few-layer 1T‧ MoTe2 as gapless semimetal with thickness dependent carrier transport

NASA Astrophysics Data System (ADS)

Song, Peng; Hsu, Chuanghan; Zhao, Meng; Zhao, Xiaoxu; Chang, Tay-Rong; Teng, Jinghua; Lin, Hsin; Loh, Kian Ping

2018-07-01

Semimetal MoTe2 can be a type II Weyl semimetal in the bulk, but monolayer of this material is predicted to be quantum spin hall insulators. This dramatic change in electronic properties with number of layers is an excellent example of the dimensional effects of quantum transport. However, a detailed experimental study of the carrier transport and band structure of ultrathin semimetal MoTe2 is lacking so far. We performed magneto-transport measurements to study the conduction behavior and quantum phase coherence of 1T‧ MoTe2 as a function of its thickness. We show that due to a unique two-band transport mechanism (synergetic contribution from electron conduction and hole conduction), the conduction behavior of 1T‧ MoTe2 changes from metallic to p-type unipolar, and finally to ambipolar as the thickness decreases, suggesting that this effect can be used in devices by effectively controlling the thickness. Our transport studies, optical measurements and first-principles electronic structure calculations reveal that 1T‧ MoTe2 remains gapless down to a few (~2–3) layers. Despite being gapless, 1T‧ MoTe2 exhibits metal-insulator transition at 3-layer thickness, due to enhanced carrier localization effect.

Properties of transparent conducting tin monoxide(SnO) thin films prepared by chemical spray pyrolysis method

NASA Astrophysics Data System (ADS)

Eqbal, Ebitha; Anila, E. I.

2018-01-01

Transparent conducting Stannous Oxide (SnO) thin films were obtained by chemical spray pyrolysis method on glass substrates for 0.1 M and 0.25 M concentration of precursor solutions. Their structural, morphological, optical and electrical properties were investigated. X-ray diffraction (XRD) study shows polycrystalline nature of the films with orthorhombic crystal structure. The morphological analysis was carried out by Scanning electron microscopy (SEM) and elemental analysis was done by Energy dispersive X-ray spectroscopy (EDX). The band gap of 0.1 M and 0.25 M thin film samples were found to be 3.58eV with 82% transmission and 3 eV with 30% transmission respectively. The film thickness, refractive index (n) and extinction coefficient (k) of the films were obtained by ellipsometric technique. Hall effect measurements reveal p-type conduction with mobility 7.8 cm2V-1s-1 and 15 cm2V-1s-1 and conductivity of 8.5 S/cm and 17.1 S/cm respectively for the 0.1 M and 0.25 M samples. Photoluminescence (PL) spectrum of the samples show a broad emission which covers near band edge (NBE) as well as deep level emission (DLE) in the region 380 nm-620 nm.

CsSnI[subscript 3]: Semiconductor or Metal? High Electrical Conductivity and Strong Near-Infrared Photoluminescence from a Single Material. High Hole Mobility and Phase-Transitions

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chung, In; Song, Jung-Hwan; Im, Jino

CsSnI{sub 3} is an unusual perovskite that undergoes complex displacive and reconstructive phase transitions and exhibits near-infrared emission at room temperature. Experimental and theoretical studies of CsSnI{sub 3} have been limited by the lack of detailed crystal structure characterization and chemical instability. Here we describe the synthesis of pure polymorphic crystals, the preparation of large crack-/bubble-free ingots, the refined single-crystal structures, and temperature-dependent charge transport and optical properties of CsSnI{sub 3}, coupled with ab initio first-principles density functional theory (DFT) calculations. In situ temperature-dependent single-crystal and synchrotron powder X-ray diffraction studies reveal the origin of polymorphous phase transitions of CsSnI{submore » 3}. The black orthorhombic form of CsSnI{sub 3} demonstrates one of the largest volumetric thermal expansion coefficients for inorganic solids. Electrical conductivity, Hall effect, and thermopower measurements on it show p-type metallic behavior with low carrier density, despite the optical band gap of 1.3 eV. Hall effect measurements of the black orthorhombic perovskite phase of CsSnI{sub 3} indicate that it is a p-type direct band gap semiconductor with carrier concentration at room temperature of {approx} 10{sup 17} cm{sup -3} and a hole mobility of {approx} 585 cm{sup 2} V{sup -1} s{sup -1}. The hole mobility is one of the highest observed among p-type semiconductors with comparable band gaps. Its powders exhibit a strong room-temperature near-IR emission spectrum at 950 nm. Remarkably, the values of the electrical conductivity and photoluminescence intensity increase with heat treatment. The DFT calculations show that the screened-exchange local density approximation-derived band gap agrees well with the experimentally measured band gap. Calculations of the formation energy of defects strongly suggest that the electrical and light emission properties possibly result from Sn defects in the crystal structure, which arise intrinsically. Thus, although stoichiometric CsSnI{sub 3} is a semiconductor, the material is prone to intrinsic defects associated with Sn vacancies. This creates highly mobile holes which cause the materials to appear metallic.« less

Wide-Band Monolithic Acoustoelectric Memory Correlators.

DTIC Science & Technology

1982-11-01

piezoelectric and non- earlier analysis of thin- oxide varactors . The new analysis ex- conducting. Tapped structures which satisfy this criterion are plains...for tapped LiNbO3/metal- oxide - important realization. The logical consequence is that only silicon [26] structures is, in fact, not applicable here. It...Clarke, "The GaAs SAW depletion layer of’ the diode array. A more complex structure, diode storage correlalor," in 1980 Ultrasonics Synp. Proc., pp a GaAs

Disorder induced magnetism and electrical conduction in La doped Ca2FeMoO6 double perovskite

NASA Astrophysics Data System (ADS)

Poddar, Asok; Bhowmik, R. N.; Muthuselvam, I. Panneer

2010-11-01

We report the magnetism and electrical transport properties of La doped Ca2FeMoO6 double perovskite. Reduction in magnetic moment, nonmonotonic variation in magnetic ordering temperature (TC), increasing magnetic hardness, low temperature resistivity upturn, and loss of metallic conductivity are some of the major changes that we observed due to La doping induced disorder in double perovskite structure. The increase in magnetic disorder in La doped samples and its effect on TC is more consistent with the mean field theory. The modification in electronic band structure due to La doping is understood by establishing a correlation between the temperature dependence of electrical conductivity and thermoelectric power.

Optical and electrical properties of heavily indium-doped CdS around the semiconductor-metal phase transition

NASA Astrophysics Data System (ADS)

Broser, I.; Broser, R.; Birkicht, E.

1990-04-01

Heavily indium-doped CdS crystals are studied by comparing their optical and electrical properties. It is shown that in the near infrared spectral region for highly conducting crystals the correlation of electrical conductivity and optical effects can still be understood in the frame of the classical Drude-Lorentz model. Even for high doping the relaxation time τ and the effective mass m ∗ of the electrons are not markedly different from the room temperature values of "pure" crystals. At photon energies near the band gap, however, optical spectra in transmission, reflectivity, and emission show clearly the existence of band-tails and screening effects. A different situation holds for a highly compensated specimen: They are in a wide temperature region highly isolating, show activated photoconductivity and special structures in the optical spectra near the band gap. Their properties can be explained by assuming a meandering bandbending due to the combined action of donors and acceptors and the assumption of spatially isolated electron and hole droplets [6].

Observation of nodal line in non-symmorphic topological semimetal InBi

DOE PAGES

Ekahana, Sandy Adhitia; Wu, Shu-Chun; Jiang, Juan; ...

2017-05-30

Topological nodal semimetal (TNS), characterized by its touching conduction and valence bands, is a newly discovered state of quantum matter which exhibits various exotic physical phenomena. Recently, a new type of TNS called topological nodal line semimetal (TNLS) is predicted where its conduction and valence band form a degenerate one-dimension line which is further protected by its crystal symmetry. In this work, we systematically investigated the bulk and surface electronic structure of the non-symmorphic, TNLS in InBi (which is also a type II Dirac semimetal) with strong spin–orbit coupling by using angle resolved photoemission spectroscopy. By tracking the crossing points of the bulk bands at the Brillouin zone boundary, we discovered the nodal-line feature along themore » $${{k}}_{{z}}$$ direction, in agreement with the ab initio calculations and confirmed it to be a new compound in the TNLS family. Our discovery provides a new material platform for the study of these exotic topological quantum phases and paves the way for possible future applications.« less

Electronic localization in an extreme 1-D conductor: the organic salt (TTDM-TTF) [Au(mnt)

NASA Astrophysics Data System (ADS)

Lopes, E. B.; Alves, H.; Ribera, E.; Mas-Torrent, M.; Auban-Senzier, P.; Canadell, E.; Henriques, R. T.; Almeida, M.; Molins, E.; Veciana, J.; Rovira, C.; Jérome, D.

2002-09-01

This article reports the investigation of a new low-dimensional organic salt, (TTDM-TTF)2 [ Au(mnt)2] , by single crystal X-ray diffraction, static magnetic susceptibility, EPR, thermopower, electrical resistivity measurements under pressure up to 25 kbar and band structure calculations. The crystal structure consists in a dimerized head to tail stacking of TTDM-TTF molecules separated by layers of orthogonal Au(mnt)2 anions. The absence of overlap between neighboring chains coming from this particular crystal structure leads to an extreme one-dimensionality (1-D) for which the carriers of the half-filled conduction band become strongly localized in a Mott-Hubbard insulating state. This material is the first 1-D conductor in which the Mott-Hubbard insulating character cannot be suppressed under pressure.

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.

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

DOE PAGES

Yadav, Satyesh K.; Uberuaga, Blas P.; Nikl, Martin; ...

2015-11-24

Complex doping schemes in R 3Al 5O 12 (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 minimummore » (CBM) or valence-band maximum (VBM). We consider two sets of compositions based on Lu 3B 5O 12 where B is Al, Ga, In, As, and Sb, and R 3Al 5O 12, 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

Multiscale calculations of thermoelectric properties of n-type Mg2Si1-xSnx solid solutions

NASA Astrophysics Data System (ADS)

Tan, X. J.; Liu, W.; Liu, H. J.; Shi, J.; Tang, X. F.; Uher, C.

2012-05-01

The band structure of Mg2Si1-xSnx solid solutions with 0.250 ⩽ x ⩽ 0.875 is calculated using the first-principles pseudopotential method. It is found that the low-lying light and heavy conduction bands converge and the effective mass reaches a maximum value near x = 0.625. Using the semiclassical Boltzmann transport theory and relaxation-time approximation, we find that the system with x = 0.625 exhibits both higher Seebeck coefficient and higher electrical conductivity than other solid solutions at intermediate temperatures. By fitting first-principles total energy calculations, a modified Morse potential is constructed, which is used to predicate the lattice thermal conductivity via equilibrium molecular dynamics simulations. Due to relatively higher power factor and lower thermal conductivity, the Mg2Si0.375Sn0.625 is found to exhibit enhanced thermoelectric performance at 800 K, and additional Sb doping is considered in order to make a better comparison with experiment results.

Characterization of Lorenz number with Seebeck coefficient measurement

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kim, Hyun-Sik; Materials Research Center, Samsung Advanced Institute of Technology, Samsung Electronics, Suwon 443-803; Gibbs, Zachary M.

2015-04-01

In analyzing zT improvements due to lattice thermal conductivity (κ{sub L}) reduction, electrical conductivity (σ) and total thermal conductivity (κ{sub Total}) are often used to estimate the electronic component of the thermal conductivity (κ{sub E}) and in turn κ{sub L} from κ{sub L} = ∼ κ{sub Total} − LσT. The Wiedemann-Franz law, κ{sub E} = LσT, where L is Lorenz number, is widely used to estimate κ{sub E} from σ measurements. It is a common practice to treat L as a universal factor with 2.44 × 10{sup −8} WΩK{sup −2} (degenerate limit). However, significant deviations from the degenerate limit (approximatelymore » 40% or more for Kane bands) are known to occur for non-degenerate semiconductors where L converges to 1.5 × 10{sup −8} WΩK{sup −2} for acoustic phonon scattering. The decrease in L is correlated with an increase in thermopower (absolute value of Seebeck coefficient (S)). Thus, a first order correction to the degenerate limit of L can be based on the measured thermopower, |S|, independent of temperature or doping. We propose the equation: L=1.5+exp[−(|S|)/(116) ] (where L is in 10{sup −8} WΩK{sup −2} and S in μV/K) as a satisfactory approximation for L. This equation is accurate within 5% for single parabolic band/acoustic phonon scattering assumption and within 20% for PbSe, PbS, PbTe, Si{sub 0.8}Ge{sub 0.2} where more complexity is introduced, such as non-parabolic Kane bands, multiple bands, and/or alternate scattering mechanisms. The use of this equation for L rather than a constant value (when detailed band structure and scattering mechanism is not known) will significantly improve the estimation of lattice thermal conductivity.« less

Effects of Conductor Expressivity on Secondary School Band Members' Performance and Attitudes toward Conducting

ERIC Educational Resources Information Center

Silvey, Brian A.; Koerner, Bryan D.

2016-01-01

We investigated the effects of expressive and unexpressive conducting on secondary school band members' and experts' audio evaluations of band performance expressivity. A conductor, who demonstrated either expressive or unexpressive conducting techniques, led both an eighth-grade and a high school band in four separate "run-throughs" of…

Hopping and band mobilities of pentacene, rubrene, and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) from first principle calculations.

PubMed

Kobayashi, Hajime; Kobayashi, Norihito; Hosoi, Shizuka; Koshitani, Naoki; Murakami, Daisuke; Shirasawa, Raku; Kudo, Yoshihiro; Hobara, Daisuke; Tokita, Yuichi; Itabashi, Masao

2013-07-07

Hopping and band mobilities of holes in organic semiconductors at room temperature were estimated from first principle calculations. Relaxation times of charge carriers were evaluated using the acoustic deformation potential model. It is found that van der Waals interactions play an important role in determining accurate relaxation times. The hopping mobilities of pentacene, rubrene, and 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) in bulk single crystalline structures were found to be smaller than 4 cm(2)∕Vs, whereas the band mobilities were estimated between 36 and 58 cm(2)∕Vs, which are close to the maximum reported experimental values. This strongly suggests that band conductivity is dominant in these materials even at room temperature.

Effects of an additional conduction band on the singlet-antiferromagnet competition in the periodic Anderson model

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hu, Wenjian; Scalettar, Richard T.; Huang, Edwin W.

The competition between antiferromagnetic (AF) order and singlet formation is a central phenomenon of the Kondo and periodic Anderson Hamiltonians and of the heavy fermion materials they describe. In this paper, we explore the effects of an additional conduction band on magnetism in these models, and, specifically, on changes in the AF-singlet quantum critical point (QCP) and the one particle and spin spectral functions. To understand the magnetic phase transition qualitatively, we first carry out a self-consistent mean field theory (MFT). The basic conclusion is that, at half filling, the coupling to the additional band stabilizes the AF phase tomore » larger f d hybridization V in the PAM. We also explore the possibility of competing ferromagnetic phases when this conduction band is doped away from half filling. Here, we next employ quantum Monte Carlo (QMC) which, in combination with finite size scaling, allows us to evaluate the position of the QCP using an exact treatment of the interactions. This approach confirms the stabilization of AF order, which occurs through an enhancement of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. QMC results for the spectral function A (q,ω) and dynamic spin structure factor χ (q,ω) yield additional insight into the AF-singlet competition and the low temperature phases.« less

Effects of an additional conduction band on the singlet-antiferromagnet competition in the periodic Anderson model

DOE PAGES

Hu, Wenjian; Scalettar, Richard T.; Huang, Edwin W.; ...

2017-06-12

The competition between antiferromagnetic (AF) order and singlet formation is a central phenomenon of the Kondo and periodic Anderson Hamiltonians and of the heavy fermion materials they describe. In this paper, we explore the effects of an additional conduction band on magnetism in these models, and, specifically, on changes in the AF-singlet quantum critical point (QCP) and the one particle and spin spectral functions. To understand the magnetic phase transition qualitatively, we first carry out a self-consistent mean field theory (MFT). The basic conclusion is that, at half filling, the coupling to the additional band stabilizes the AF phase tomore » larger f d hybridization V in the PAM. We also explore the possibility of competing ferromagnetic phases when this conduction band is doped away from half filling. Here, we next employ quantum Monte Carlo (QMC) which, in combination with finite size scaling, allows us to evaluate the position of the QCP using an exact treatment of the interactions. This approach confirms the stabilization of AF order, which occurs through an enhancement of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. QMC results for the spectral function A (q,ω) and dynamic spin structure factor χ (q,ω) yield additional insight into the AF-singlet competition and the low temperature phases.« less

Structural, optical and electronic properties of K2Ba(NO3)4 crystal

NASA Astrophysics Data System (ADS)

Isaenko, L. I.; Korzhneva, K. E.; Goryainov, S. V.; Goloshumova, A. A.; Sheludyakova, L. A.; Bekenev, V. L.; Khyzhun, O. Y.

2018-02-01

Nitrate crystals reveal nonlinear optical properties and could be considered as converters of laser radiation in the short-wave region. The conditions for obtaining and basic properties of K2Ba(NO3)4 double nitrate crystals were investigated. Crystal growth was implemented by slow cooling in the temperature range of 72-49 °C and low rate evaporation. The structural analysis of K2Ba(NO3)4 formation on the basis of two mixed simple nitrate structures is discussed. The main groups of oscillations in K2Ba(NO3)4 crystal were revealed using Raman and IR spectroscopy, and the table of vibrations for this compound was compiled. The electronic structure of K2Ba(NO3)4 was elucidated in the present work from both experimental and theoretical viewpoints. In particular, X-ray photoelectron spectroscopy (XPS) was employed in the present work to measure binding energies of the atoms constituting the titled compound and its XPS valence-band spectrum for both pristine and Ar+ ion-bombarded surfaces. Further, total and partial densities of states of constituent atoms of K2Ba(NO3)4 have been calculated. The calculations reveal that the O 2p states dominate in the total valence-band region of K2Ba(NO3)4 except of its bottom, where K 3p and Ba 5p states are the principal contributors, while the bottom of the conduction band is composed mainly of the unoccupied O 2p states, with somewhat smaller contributions of the N 2p∗ states as well. With respect to the occupation of the valence band by the O 2p states, the present band-structure calculations are confirmed by comparison on a common energy scale of the XPS valence-band spectrum and the X-ray emission O Kα band for the K2Ba(NO3)4 crystal under study. Furthermore, the present calculations indicate that the K2Ba(NO3)4 compound is a direct-gap material.

New rare earth hafnium oxynitride perovskites with photocatalytic activity in water oxidation and reduction.

PubMed

Black, Ashley P; Suzuki, Hajime; Higashi, Masanobu; Frontera, Carlos; Ritter, Clemens; De, Chandan; Sundaresan, A; Abe, Ryu; Fuertes, Amparo

2018-02-06

RHfO 2 N perovskites with R = La, Nd and Sm show a GdFeO 3 -type structure and are semiconductors with band gaps of 3.35, 3.40 and 2.85 eV and relative dielectric constants of 30, 16 and 28 respectively. These compounds have adequate reduction and oxidation potentials to conduct the overall water splitting reaction, and the analogous perovskite LaZrO 2 N with a band gap of 2.8 eV shows photocatalytic activity under visible light irradiation for O 2 evolution.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kononov, A.; Egorov, S. V.; Kvon, Z. D.

We experimentally investigate spin-polarized electron transport between a permalloy ferromagnet and the edge of a two-dimensional electron system with band inversion, realized in a narrow, 8 nm wide, HgTe quantum well. In zero magnetic field, we observe strong asymmetry of the edge potential distribution with respect to the ferromagnetic ground lead. This result indicates that the helical edge channel, specific for the structures with band inversion even at the conductive bulk, is strongly coupled to the ferromagnetic side contact, possibly due to the effects of proximity magnetization. This allows selective and spin-sensitive contacting of helical edge states.

Energy band and transport properties in magnetic aperiodic graphene superlattices of Thue-Morse sequence

NASA Astrophysics Data System (ADS)

Yin, Yiheng; Niu, Yanxiong; Zhang, Huiyun; Zhang, Yuping; Liu, Haiyue

2016-02-01

Utilizing the transfer matrix method, we develop the electronic band structure and transport properties in Thue-Morse aperiodic graphene superlattices with magnetic barriers. It is found that the normal transmission is blocked and the position of the Dirac point can be shifted along the wavevector axis by changing the height and width ratio of magnetic barriers, which is intrinsic different from electronic field modulated superlattices. In addition, the angular threshold property of the transmission spectra and the oscillatory property of the conductance have been studied.

Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption

NASA Astrophysics Data System (ADS)

Rahman, Altaf Ur; Rahman, Gul; Kratzer, Peter

2018-05-01

The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (NGa), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (FS or FGa) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for FGa and F adsorption at the Ga site, a strong F–Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, FGa induces p-type conductivity in GaS, whereas FS induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N2 molecules. While NGa induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic NS impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices.

Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption.

PubMed

Ur Rahman, Altaf; Rahman, Gul; Kratzer, Peter

2018-05-16

The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (N Ga ), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (F S or F Ga ) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for F Ga and F adsorption at the Ga site, a strong F-Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, F Ga induces p-type conductivity in GaS, whereas F S induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N 2 molecules. While N Ga induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic N S impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices.

Electronic Structure and Band Gap of Fullerenes on Tungsten Surfaces: Transition from a Semiconductor to a Metal Triggered by Annealing.

PubMed

Monazami, Ehsan; McClimon, John B; Rondinelli, James; Reinke, Petra

2016-12-21

The understanding and control of molecule-metal interfaces is critical to the performance of molecular electronics and photovoltaics devices. We present a study of the interface between C 60 and W, which is a carbide-forming transition metal. The complex solid-state reaction at the interface can be exploited to adjust the electronic properties of the molecule layer. Scanning tunneling microscopy/spectroscopy measurements demonstrate the progression of this reaction from wide band gap (>2.5 eV) to metallic molecular surface during annealing from 300 to 800 K. Differential conduction maps with 10 4 scanning tunneling spectra are used to quantify the transition in the density of states and the reduction of the band gap during annealing with nanometer spatial resolution. The electronic transition is spatially homogeneous, and the surface band gap can therefore be adjusted by a targeted annealing step. The modified molecules, which we call nanospheres, are quite resistant to ripening and coalescence, unlike any other metallic nanoparticle of the same size. Densely packed C 60 and isolated C 60 molecules show the same transition in electronic structure, which confirms that the transformation is controlled by the reaction at the C 60 -W interface. Density functional theory calculations are used to develop possible reaction pathways in agreement with experimentally observed electronic structure modulation. Control of the band gap by the choice of annealing temperature is a unique route to tailoring molecular-layer electronic properties.

Configurations, band structures and photocurrent responses of 4-(4-oxopyridin-1(4H)-yl)phthalic acid and its metal-organic frameworks

NASA Astrophysics Data System (ADS)

Yan, Xingxiu; Qiu, Xiandeng; Yan, Zhishuo; Li, Hongjiang; Gong, Yun; Lin, Jianhua

2016-05-01

4-(4-oxopyridin-1(4 H)-yl)phthalic acid (H2L) and three H2L-based metal-organic frameworks (MOFs) formulated as ZnL(DPE)(H2O)·H2O (DPE=(E)-1, 2-di(pyridine -4-yl)ethene) (1), CdL(H2O)2 (2) and CdL (3) were synthesized and structurally characterized by single-crystal X-ray diffraction. The free H2L ligand shows an enol-form and the L2- ligand in the three MOFs exists as the keto-form. Density functional theory (DFT) calculations indicate H2L and the three MOFs possess different band structures. Due to the existence of the N-donor, DPE in MOF 1, the conduction band (CB) minimum and band gap of MOF 1 are much lower than those of H2L. And MOF 1 yielded much larger photocurrent density than H2L upon visible light illumination. Electrochemical impedance spectroscopy (EIS) shows the interfacial charge transfer impedance in the presence of MOF 1 is lower than that in the presence of H2L. The hydrous MOF 2 and the anhydrous MOF 3 are both constructed by Cd(II) and L2-, and they can be reversibly transformed to each other. However, MOFs 2 and 3 possess different CB minimums and VB maximums, and their band gaps are much larger than that of MOF 1.

Electrical and optical transport properties of single layer WSe2

NASA Astrophysics Data System (ADS)

Tahir, M.

2018-03-01

The electronic properties of single layer WSe2 are distinct from the famous graphene due to strong spin orbit coupling, a huge band gap and an anisotropic lifting of the degeneracy of the valley degree of freedom under Zeeman field. In this work, band structure of the monolayer WSe2 is evaluated in the presence of spin and valley Zeeman fields to study the electrical and optical transport properties. Using Kubo formalism, an explicit expression for the electrical Hall conductivity is examined at finite temperatures. The electrical longitudinal conductivity is also evaluated. Further, the longitudinal and Hall optical conductivities are analyzed. It is observed that the contributions of the spin-up and spin-down states to the power absorption spectrum depend on the valley index. The numerical results exhibit absorption peaks as a function of photon energy, ℏ ω, in the range ∼ 1.5 -2 eV. Also, the optical response lies in the visible frequency range in contrast to the conventional two-dimensional electron gas or graphene where the response is limited to terahertz regime. This ability to isolate carriers in spin-valley coupled structures may make WSe2 a promising candidate for future spintronics, valleytronics and optical devices.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Du, Mao -Hua; Singh, David J.; Zhang, Lijun

Oxides with good p-type conductivity have been long sought after to achieve high performance all-oxide optoelectronic devices. Divalent Sn(II) based oxides are promising candidates because of their rather dispersive upper valence bands caused by the Sn-5s/O-2p anti-bonding hybridization. There are so far few known Sn(II) oxides being p-type conductive suitable for device applications. Here, we present via first-principles global optimization structure searches a material design study for a hitherto unexplored Sn(II)-based system, ternary alkaline-earth metal Sn(II) oxides in the stoichiometry of MSn 2O 3 (M = Mg, Ca, Sr, Ba). We identify two stable compounds of SrSn 2O 3 andmore » BaSn 2O 3, which can be stabilized by Sn-rich conditions in phase stability diagrams. Their structures follow the Zintl behaviour and consist of basic structural motifs of SnO 3 tetrahedra. Unexpectedly they show distinct electronic properties with band gaps ranging from 1.90 (BaSn 2O 3) to 3.15 (SrSn 2O 3) eV, and hole effective masses ranging from 0.87 (BaSn 2O 3) to above 6.0 (SrSn 2O 3) m0. Further exploration of metastable phases indicates a wide tunability of electronic properties controlled by the details of the bonding between the basic structural motifs. Lastly, this suggests further exploration of alkaline-earth metal Sn(II) oxides for potential applications requiring good p-type conductivity such as transparent conductors and photovoltaic absorbers.« less

Electronic structure and dynamics of thin Ge/GaAs(110) heterostructures

NASA Astrophysics Data System (ADS)

Haight, R.; Silberman, J. A.

1990-10-01

Using angle-resolved picosecond laser photoemission we have investigated both occupied and transiently excited empty states at the surface of Ge grown epitaxially on GaAs(110). We observe a normally unoccupied, Ge layer derived state whose separation from the valence-band maximum of the system is 700±50 meV at six monolayers Ge coverage. The evolution of the electronic structure is followed as a function of coverage and correlated with low-energy electron diffraction. The time dependence of the transiently occupied Ge signal is compared with that of the clean GaAs(110) surface and shows that electrons are prevented from diffusing into the GaAs bulk by the conduction-band offset of 330±40 meV.

Determination of band offset using continuous-wave two-photon excitation in a ZnSe quantum-well waveguide structure

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wagner, H. P.; Kuhnelt, M.; Wenisch, H.

2001-06-15

We investigate exciton subband transitions in a ZnSe/Zn{sub 1{minus}x}Mg{sub x}S{sub y}Se{sub 1{minus}y} multiple-quantum-well grown by molecular beam epitaxy waveguide structure by photoluminescence excitation and two-photon excitation spectroscopy. A continuous-wave two-photon absorption is realized by an efficient waveguide coupling scheme within the cryostat. From the energetic position of the 1s and 2p exciton transitions exciton binding energies of 33 and 38 meV are deduced for heavy and light-hole excitons, respectively. With these values we are able to determine the strain free and dimensionless conduction-band-offset parameter to Q{sub c}=0.3{+-}0.1.

FAST TRACK COMMUNICATION: Electronic structure of a graphene/hexagonal-BN heterostructure grown on Ru(0001) by chemical vapor deposition and atomic layer deposition: extrinsically doped graphene

NASA Astrophysics Data System (ADS)

Bjelkevig, Cameron; Mi, Zhou; Xiao, Jie; Dowben, P. A.; Wang, Lu; Mei, Wai-Ning; Kelber, Jeffry A.

2010-08-01

A significant BN-to-graphene charge donation is evident in the electronic structure of a graphene/h-BN(0001) heterojunction grown by chemical vapor deposition and atomic layer deposition directly on Ru(0001), consistent with density functional theory. This filling of the lowest unoccupied state near the Brillouin zone center has been characterized by combined photoemission/k vector resolved inverse photoemission spectroscopies, and Raman and scanning tunneling microscopy/spectroscopy. The unoccupied σ*(Γ1 +) band dispersion yields an effective mass of 0.05 me for graphene in the graphene/h-BN(0001) heterostructure, in spite of strong perturbations to the graphene conduction band edge placement.

Elucidating the electronic structure of supported gold nanoparticles and its relevance to catalysis by means of hard X-ray photoelectron spectroscopy

DOE PAGES

Reinecke, Benjamin N.; Kuhl, Kendra P.; Ogasawara, Hirohito; ...

2015-12-31

We report on the electronic structure of Au (gold) nanoparticles supported onto TiO 2 with a goal of elucidating the most important effects that contribute to their high catalytic activity. We synthesize and characterize with high resolution transmission electron microscopy (HRTEM) 3.4, 5.3, and 9.5 nm diameter TiO 2-supported Au nanoparticles with nearly spherical shape and measure their valence band using Au 5d subshell sensitive hard X-ray photoelectron spectroscopy (HAXPES) conducted at Spring-8. Based on density functional theory (DFT) calculations of various Au surface structures, we interpret the observed changes in the Au 5d valence band structure as a functionmore » of size in terms of an increasing percentage of Au atoms at corners/edges for decreasing particle size. Finally, this work elucidates how Au coordination number impacts the electronic structure of Au nanoparticles, ultimately giving rise to their well-known catalytic activity.« less

Orientation dependence of temporal and spectral properties of high-order harmonics in solids [Orientation dependence of high-harmonic temporal and spectral properties in solids

DOE PAGES

Wu, Mengxi; You, Yongsing; Ghimire, Shambhu; ...

2017-12-18

We investigate the connection between crystal symmetry and temporal and spectral properties of high-order harmonics in solids. We calculate the orientation-dependent harmonic spectrum driven by an intense, linearly polarized infrared laser field, using a momentum-space description of the generation process in terms of strong-field-driven electron dynamics on the band structure. We show that the orientation dependence of both the spectral yield and the subcycle time profile of the harmonic radiation can be understood in terms of the coupling strengths and relative curvatures of the valence band and the low-lying conduction bands. In particular, we show that in some systems thismore » gives rise to a rapid shift of a quarter optical cycle in the timing of harmonics in the secondary plateau as the crystal is rotated relative to the laser polarization. Here, we address recent experimental results in MgO and show that the observed change in orientation dependence for the highest harmonics can be interpreted in the momentum space picture in terms of the contributions of several different conduction bands.« less

The Fowler-Nordheim behavior and mechanism of photo-sensitive field from SnS{sub 2} nanosheets

DOE Office of Scientific and Technical Information (OSTI.GOV)

Suryawanshi, Sachin R.; Chaudhari, Nilima S.; Warule, Sambhaji S.

2015-06-24

Here in, we report photo-sensitive field emission measurements of SnS{sub 2} nanosheets at base pressure of ∼1×10{sup −8} mbar are reported. The nonlinear Fowler-Nordheim (F-N) plot is elucidate according to a (F-N) model of calculation based on shift in a saturation of conduction band current density after light illumination and prevalence of valence band current density at high electric field values. The model of calculation suggests that the slope variation before and after visible light illumination of the F-N plot, in the high-field and low-field regions, does not depend on the magnitude of saturation but also depend on charge carriermore » (electron) concentration get increased in conduction band. The F-N model of calculation is important for the fundamental understanding of the photo-sensitive field emission mechanism of semiconducting SnS{sub 2}. The replicate F-N plots exhibit similar features to those observed experimentally. The model calculation suggests that the nonlinearity of the F-N plot is a characteristic of the photo-enhanced energy band structure of the photo-sensitive semiconductor material.« less

Orientation dependence of temporal and spectral properties of high-order harmonics in solids [Orientation dependence of high-harmonic temporal and spectral properties in solids

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wu, Mengxi; You, Yongsing; Ghimire, Shambhu

We investigate the connection between crystal symmetry and temporal and spectral properties of high-order harmonics in solids. We calculate the orientation-dependent harmonic spectrum driven by an intense, linearly polarized infrared laser field, using a momentum-space description of the generation process in terms of strong-field-driven electron dynamics on the band structure. We show that the orientation dependence of both the spectral yield and the subcycle time profile of the harmonic radiation can be understood in terms of the coupling strengths and relative curvatures of the valence band and the low-lying conduction bands. In particular, we show that in some systems thismore » gives rise to a rapid shift of a quarter optical cycle in the timing of harmonics in the secondary plateau as the crystal is rotated relative to the laser polarization. Here, we address recent experimental results in MgO and show that the observed change in orientation dependence for the highest harmonics can be interpreted in the momentum space picture in terms of the contributions of several different conduction bands.« less

The thermopower in the temperature range T(sub c)-1000K and the bank spectrum of Bi-based superconductors

NASA Technical Reports Server (NTRS)

Gasumyants, V. E.; Vladimirskaya, E. V.; Smirnov, V. I.; Kazanskiy, S. V.

1995-01-01

The temperature dependencies of thermopower, S, in the range T = T(sub c)-1000K as well as of resistivity and Hall coefficient in the range T = T(sub c)-300K for the single-phase ceramic samples Bi2Sr2Ca(1-x)Nd(x)Cu2O(y) have been measured. It was found that the S(T) dependencies in normal phase have three characteristic regions. Despite the fact that the S(T) dependencies in Bi-based high-T(sub c) superconductors (HTSC) differ essentially from ones in Y-based HTSC at T = T(sub c)-300K, the main feature of theirs (S(T) = const at high temperatures) retains in samples investigated at T is greater than 620K. The results obtained have been analyzed on the basis of the narrow-band model with the use of assumption of slight asymmetry of the conductive band. The band spectrum parameters of the samples studied have been calculated. An analysis of the tendencies in these parameters changes with samples composition varying enables to make the conclusion about the similarity of the main features of the conductive band structure in Y- and Bi-based HTSC.

Orientation dependence of temporal and spectral properties of high-order harmonics in solids

NASA Astrophysics Data System (ADS)

Wu, Mengxi; You, Yongsing; Ghimire, Shambhu; Reis, David A.; Browne, Dana A.; Schafer, Kenneth J.; Gaarde, Mette B.

2017-12-01

We investigate the connection between crystal symmetry and temporal and spectral properties of high-order harmonics in solids. We calculate the orientation-dependent harmonic spectrum driven by an intense, linearly polarized infrared laser field, using a momentum-space description of the generation process in terms of strong-field-driven electron dynamics on the band structure. We show that the orientation dependence of both the spectral yield and the subcycle time profile of the harmonic radiation can be understood in terms of the coupling strengths and relative curvatures of the valence band and the low-lying conduction bands. In particular, we show that in some systems this gives rise to a rapid shift of a quarter optical cycle in the timing of harmonics in the secondary plateau as the crystal is rotated relative to the laser polarization. We address recent experimental results in MgO [Y. S. You et al., Nat. Phys. 13, 345 (2017)., 10.1038/nphys3955] and show that the observed change in orientation dependence for the highest harmonics can be interpreted in the momentum space picture in terms of the contributions of several different conduction bands.

Effect of chlorination on the TlBr band edges for improved room temperature radiation detectors: Effect of chlorination on the TlBr band edges for radiation detectors

DOE PAGES

Varley, J. B.; Conway, A. M.; Voss, L. F.; ...

2015-02-09

Thallium bromide (TlBr) crystals subjected to hydrochloric acid (HCl) chemical treatments have been shown to advantageously affect device performance and longevity in TlBr-based room temperature radiation detectors, yet the exact mechanisms of the improvements remain poorly understood. Here in this paper, we investigate the influence of several HCl chemical treatments on device-grade TlBr and describe the changes in the composition and electronic structure of the surface. Composition analysis and depth profiles obtained from secondary ion mass spectrometry (SIMS) identify the extent to which each HCl etch condition affects the detector surface region and forms of a graded TlBr/TlBr 1-xCL xmore » surface heterojunction. Using a combination of X-ray photoemission spectroscopy (XPS) and hybrid density functional calculations, we are able to determine the valence band offsets, band gaps, and conduction band offsets as a function of Cl content over the entire composition range of TIBr 1-xC1 X. This study establishes a strong correlation between device process conditions, surface chemistry, and electronic structure with the goal of further optimizing the long-term stability and radiation response of TlBr-based detectors.« less

Synthesis, Structure, and Optical Properties of Antiperovskite-Derived Ba2MQ3X (M = As, Sb; Q = S, Se; X = Cl, Br, I) Chalcohalides.

PubMed

Wang, Ruiqi; Zhang, Xian; He, Jianqiao; Bu, Kejun; Zheng, Chong; Lin, Jianhua; Huang, Fuqiang

2018-02-05

Six isostructural antiperovskite-derived chalcohalides, Ba 2 MQ 3 X (M = As, Sb; Q = S, Se; X = Cl, Br, I), crystallizing in the space group Pnma, have been synthesized by solid-state reactions. The crystal structure features a 3D framework with the [XBa 5 ] 9+ disordered square pyramids as building blocks and [MQ 3 ] 3- units filling the interspace. [XBa 5 ] 9+ disordered square pyramids are edge-sharing along [010], derived from the fusing of the two pyramids in octahedral [XBa 6 ] 11+ . Surprisingly, Ba 2 AsS 3 X (X = Cl, Br, I) show almost the same optical band gap of 2.80 eV, and Ba 2 AsSe 3 X (X = Br, I) also have a similar band gap of 2.28 eV. The optical band gap of Ba 2 SbS 3 I is 2.64 eV. First-principles calculations reveal that the optical absorption is attributed to the transitions between Q np at the valence band maximum (VBM) and M np-Q np at the conduction band minimum (CBM). These compounds also possess interesting photoluminescence properties with splitting emission peaks on excitation at 200 nm.

k - dependent Jeff=1/2 band splitting and the electron-hole asymmetry in SrIrO3

NASA Astrophysics Data System (ADS)

Singh, Vijeta; Pulikkotil, J. J.

2017-02-01

The Ir ion in Srn+1 IrnO 3 n + 1 series of compounds is octahedrally coordinated. However, unlike Sr2IrO4 (n=1) and Sr3Ir2O7 (n=2) which are insulating due to spin-orbit induced Jeff splitting of the t2g bands, SrIrO3 (n= ∞) is conducting. To explore whether such a splitting is relevant in SrIrO3, and if so to what extent, we investigate the electronic structure of orthorhombic SrIrO3 using density functional theory. Calculations reveal that the crystal field split Ir t2 g bands in SrIrO3 are indeed split into Jeff=3/2 and and Jeff=1/2 states. However, the splitting is found to be strongly k - dependent with its magnitude determined by the Ir - O orbital hybridization. Besides, we find that the spin-orbit induced pseudo-gap, into which the Fermi energy is positioned, is composed of both light electron-like and heavy hole-like bands. These features in the band structure of SrIrO3 suggest that variations in the carrier concentration control the electronic transport properties in SrIrO3, which is consistent with the experiments.

Anomalous Hall conductivity and electronic structures of Si-substituted Mn2CoAl epitaxial films

NASA Astrophysics Data System (ADS)

Arima, K.; Kuroda, F.; Yamada, S.; Fukushima, T.; Oguchi, T.; Hamaya, K.

2018-02-01

We study anomalous Hall conductivity (σAHC) and electronic band structures of Si-substituted Mn2CoAl (Mn2CoAl1 -xSix ). First-principles calculations reveal that the electronic band structure is like a spin-gapless system even after substituting a quaternary element of Si for Al up to x =0.2 in Mn2CoAl1 -xSix . This means that the Si substitution enables the Fermi-level shift without largely changing the electronic structures in Mn2CoAl . By using molecular beam epitaxy techniques, Mn2CoAl1 -xSix epitaxial films can be grown, leading to the systematic control of x (0 ⩽x ⩽0.3 ). In addition to the electrical conductivity, the values of σAHC for the Mn2CoAl1 -xSix films are similar to those in Mn2CoAl films shown in previous reports. We note that a very small σAHC of ˜1.1 S/cm is obtained for x = 0.225, and the sign of σAHC is changed from positive to negative at around x = 0.25. We discuss the origin of the sign reversal of σAHC as a consequence of the Fermi-level shift in Mn2CoAl . Considering the presence of the structural disorder in the Mn2CoAl1 -xSix films, we can conclude that the small value and sign reversal of σAHC are not related to the characteristics of spin-gapless semiconductors.

Exploring function activated chlorins using MCD spectroscopy and DFT methods: design of a chlorin with a remarkably intense, red Q band.

PubMed

Zhang, Angel; Stillman, Martin J

2018-05-09

The electronic structures of three previously synthesized Ni-coordinated chlorins with β-substituents of thioketone, fluorene, and ketone were investigated using magnetic circular dichroism spectroscopy (MCD) and density functional theory (DFT) for potential application as sensitizers for dye-sensitized solar cells (DSSCs). Computational studies on modeled Zn-coordinated chlorins allowed identification of charge transfer and d-d transitions of the Ni2+ coordinated chlorins. Two fictive Zn chlorins, M1 and M2, were designed with thiophene units based on the fluorene substituted chlorin. Substitution with thiophene altered the typical arrangement of the four Gouterman molecular orbitals (MOs) and red-shifted and greatly intensified the lowest energy absorption band (the Q band). The introduction of the thiophene-based MO as the LUMO below the usual Gouterman LUMO is predicted to increase the efficiency of electron transfer from the dye to the conduction band of the semiconductor in DSSCs. The addition of a donor group on the opposite pyrrole (M2) red-shifted the Q band further and introduced a donor-based MO between the typical Gouterman HOMO and HOMO-1. Despite the relatively small ΔHOMO, M1 and M2 exhibited remarkably intense Q bands. M2 would be a possible candidate for application in DSSCs due to its panchromatic absorption, intense and red-shifted Q band, and the presence of the substituent based MO properties. Another indicator of a successful dye is the alignment of the ground state and excited state oxidation potentials (GSOP and ESOP, respectively) with respect to the conduction band of the semiconductor. The GSOP for M2 lies 0.55 eV below the I-/I3- redox potential and the ESOP lies 0.48 eV above the TiO2 conduction band. The impact of the thiophene dominance in the LUMO also supports the prediction of efficient sensitization properties. The remarkably intense Q band of M2 predicted to be at 777 nm with a ΔHOMO of just 1.04 eV provides a synthetic route to tetrapyrroles with extremely intense, red Q bands without the need for aza nitrogens of the phthalocyanines. This study illustrates the value of guided synthesis using MCD spectral analysis and computational methods for optimizing the design of porphyrin dyes.

Rhombohedral to Cubic Conversion of GeTe via MnTe Alloying Leads to Ultralow Thermal Conductivity, Electronic Band Convergence, and High Thermoelectric Performance.

PubMed

Zheng, Zheng; Su, Xianli; Deng, Rigui; Stoumpos, Constantinos; Xie, Hongyao; Liu, Wei; Yan, Yonggao; Hao, Shiqiang; Uher, Ctirad; Wolverton, Chris; Kanatzidis, Mercouri G; Tang, Xinfeng

2018-02-21

In this study, a series of Ge 1-x Mn x Te (x = 0-0.21) compounds were prepared by a melting-quenching-annealing process combined with spark plasma sintering (SPS). The effect of alloying MnTe into GeTe on the structure and thermoelectric properties of Ge 1-x Mn x Te is profound. With increasing content of MnTe, the structure of the Ge 1-x Mn x Te compounds gradually changes from rhombohedral to cubic, and the known R3m to Fm-3m phase transition temperature of GeTe moves from 700 K closer to room temperature. First-principles density functional theory calculations show that alloying MnTe into GeTe decreases the energy difference between the light and heavy valence bands in both the R3m and Fm-3m structures, enhancing a multiband character of the valence band edge that increases the hole carrier effective mass. The effect of this band convergence is a significant enhancement in the carrier effective mass from 1.44 m 0 (GeTe) to 6.15 m 0 (Ge 0.85 Mn 0.15 Te). In addition, alloying with MnTe decreases the phonon relaxation time by enhancing alloy scattering, reduces the phonon velocity, and increases Ge vacancies all of which result in an ultralow lattice thermal conductivity of 0.13 W m -1 K -1 at 823 K. Subsequent doping of the Ge 0.9 Mn 0.1 Te compositions with Sb lowers the typical very high hole carrier concentration and brings it closer to its optimal value enhancing the power factor, which combined with the ultralow thermal conductivity yields a maximum ZT value of 1.61 at 823 K (for Ge 0.86 Mn 0.10 Sb 0.04 Te). The average ZT value of the compound over the temperature range 400-800 K is 1.09, making it the best GeTe-based thermoelectric material.

Photoluminescence from narrow InAs-AlSb quantum wells

NASA Technical Reports Server (NTRS)

Brar, Berinder; Kroemer, Herbert; Ibbetson, James; English, John H.

1993-01-01

We report on photoluminescence spectra from narrow InAs-AlSb quantum wells. Strong, clearly resolved peaks for well widths from 2 to 8 monolayers were observed. Transmission electron micrographs show direct evidence for the structural quality of the quantum well structures. The transition energies of the narrowest wells suggest a strong influence of the AlSb X-barrier on the electronic states in the conduction band.

External electric field driven modification of the anomalous and spin Hall conductivities in Fe thin films on MgO(001)

NASA Astrophysics Data System (ADS)

Pradipto, Abdul-Muizz; Akiyama, Toru; Ito, Tomonori; Nakamura, Kohji

2018-01-01

The effects of applying external electric fields to the anomalous and spin Hall conductivities in Fe thin-film models with different layer thicknesses on MgO(001) are investigated by using first-principles calculations. We observe that, for the considered systems, the application of positive electric field associated with the accumulation of negative charges on the Fe side generally decreases (increases) the anomalous (spin) Hall conductivities. The mapping of the Hall conductivities within the two-dimensional Brillouin zone shows that the electric-field-induced modifications are related to the modification of the band structures of the atoms at the interface with the MgO substrate. In particular, the external electric field affects the Hall conductivities via the modifications of the dx z,dy z orbitals, in which the application of positive electric field pushes the minority-spin states of the dx z,dy z bands closer to the Fermi level. Better agreement with the anomalous Hall conductivity for bulk Fe and a more realistic scenario for the electric field modification of Hall conductivities are obtained by using the thicker layers of Fe on MgO (Fe3/MgO and Fe5/MgO).

The influence of surface functionalization on thermal transport and thermoelectric properties of MXene monolayers.

PubMed

Sarikurt, Sevil; Çakır, Deniz; Keçeli, Murat; Sevik, Cem

2018-05-10

The newest members of a two-dimensional material family, involving transition metal carbides and nitrides (called MXenes), have garnered increasing attention due to their tunable electronic and thermal properties depending on the chemical composition and functionalization. This flexibility can be exploited to fabricate efficient electrochemical energy storage (batteries) and energy conversion (thermoelectric) devices. In this study, we calculated the Seebeck coefficients and lattice thermal conductivity values of oxygen terminated M2CO2 (where M = Ti, Zr, Hf, Sc) monolayer MXene crystals in two different functionalization configurations (model-II (MD-II) and model-III (MD-III)), using density functional theory and Boltzmann transport theory. We estimated the thermoelectric figure-of-merit, zT, of these materials by two different approaches, as well. First of all, we found that the structural model (i.e. adsorption site of oxygen atom on the surface of MXene) has a paramount impact on the electronic and thermoelectric properties of MXene crystals, which can be exploited to engineer the thermoelectric properties of these materials. The lattice thermal conductivity κl, Seebeck coefficient and zT values may vary by 40% depending on the structural model. The MD-III configuration always has the larger band gap, Seebeck coefficient and zT, and smaller κl as compared to the MD-II structure due to a larger band gap, highly flat valence band and reduced crystal symmetry in the former. The MD-III configuration of Ti2CO2 and Zr2CO2 has the lowest κl as compared to the same configuration of Hf2CO2 and Sc2CO2. Among all the considered structures, the MD-II configuration of Hf2CO2 has the highest κl, and Ti2CO2 and Zr2CO2 in the MD-III configuration have the lowest κl. For instance, while the band gap of the MD-II configuration of Ti2CO2 is 0.26 eV, it becomes 0.69 eV in MD-III. The zTmax value may reach up to 1.1 depending on the structural model of MXene.

Control of Ge1-x-ySixSny layer lattice constant for energy band alignment in Ge1-xSnx/Ge1-x-ySixSny heterostructures

NASA Astrophysics Data System (ADS)

Fukuda, Masahiro; Watanabe, Kazuhiro; Sakashita, Mitsuo; Kurosawa, Masashi; Nakatsuka, Osamu; Zaima, Shigeaki

2017-10-01

The energy band alignment of Ge1-xSnx/Ge1-x-ySixSny heterostructures was investigated, and control of the valence band offset at the Ge1-xSnx/Ge1-x-ySixSny heterointerface was achieved by controlling the Si and Sn contents in the Ge1-x-ySixSny layer. The valence band offset in the Ge0.902Sn0.098/Ge0.41Si0.50Sn0.09 heterostructure was evaluated to be as high as 330 meV, and its conduction band offset was estimated to be 150 meV by considering the energy bandgap calculated from the theoretical prediction. In addition, the formation of the strain-relaxed Ge1-x-ySixSny layer was examined and the crystalline structure was characterized. The epitaxial growth of a strain-relaxed Ge0.64Si0.21Sn0.15 layer with the degree of strain relaxation of 55% was examined using a virtual Ge substrate. Moreover, enhancement of the strain relaxation was demonstrated by post-deposition annealing, where a degree of strain relaxation of 70% was achieved after annealing at 400 °C. These results indicate the possibility for enhancing the indirect-direct crossover with a strained and high-Sn-content Ge1-xSnx layer on a strain-relaxed Ge1-x-ySixSny layer, realizing preferable carrier confinement by type-I energy band alignment with high conduction and valence band offsets.

Acceptor-modulated optical enhancements and band-gap narrowing in ZnO thin films

NASA Astrophysics Data System (ADS)

Hassan, Ali; Jin, Yuhua; Irfan, Muhammad; Jiang, Yijian

2018-03-01

Fermi-Dirac distribution for doped semiconductors and Burstein-Moss effect have been correlated first time to figure out the conductivity type of ZnO. Hall Effect in the Van der Pauw configuration has been applied to reconcile our theoretical estimations which evince our assumption. Band-gap narrowing has been found in all p-type samples, whereas blue Burstein-Moss shift has been recorded in the n-type films. Atomic Force Microscopic (AFM) analysis shows that both p-type and n-type films have almost same granular-like structure with minor change in average grain size (˜ 6 nm to 10 nm) and surface roughness rms value 3 nm for thickness ˜315 nm which points that grain size and surface roughness did not play any significant role in order to modulate the conductivity type of ZnO. X-ray diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS) have been employed to perform the structural, chemical and elemental analysis. Hexagonal wurtzite structure has been observed in all samples. The introduction of nitrogen reduces the crystallinity of host lattice. 97% transmittance in the visible range with 1.4 × 107 Ω-1cm-1 optical conductivity have been detected. High absorption value in the ultra-violet (UV) region reveals that NZOs thin films can be used to fabricate next-generation high-performance UV detectors.

Photodetectors based on single-walled carbon nanotubes and thiamonomethinecyanine J-aggregates on flexible substrates

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fedorov, I. V., E-mail: i-v-fedorov@mail.ru; Emel’yanov, A. V.; Romashkin, A. V.

2015-09-15

The present paper is devoted to observations of the photoresistive effect in multilayer structures with a sensitive layer of J-aggregates of thiamonomethinecyanine polymethine dye and a transparent electrode of a conductive carbon-nanotube network on a flexible polyethylenenaphtalate substrate. The effect of narrow-band emission with a wavelength of 465 nm on a change in the conductivity of the fabricated structures is studied. The prepared samples are studied by atomic-force microscopy, Raman spectroscopy, and spectrophotometry methods. It is shown that these structures are photosensitive to the indicated spectral region, and the dye layer is a film of dye J-aggregates. The change inmore » the sample conductivity upon exposure to light one hundred times exceeds the dark conductivity. In general, the principal possibility of developing a photoresistive detector based on J-aggregates of cyanine dyes on flexible supports on account of the use of transparent and conductive carbon-nanotube layers is shown.« less

Insight into the optoelectronic and thermoelectric properties of Ca-based Zintl phase CaCd2X2 (X = P, As) from first principles calculation

NASA Astrophysics Data System (ADS)

Belfarh, T.; Batouche, M.; Seddik, T.; Uğur, G.; Omran, S. Bin; Bouhemadou, A.; Sandeep; Wang, Xiaotian; Sun, Xiao-Wei; Khenata, R.

2018-06-01

We have studied the structural, optical, electronic and thermoelectric properties of the CaCd2X2 (X = P, As) compounds by using the full-potential augmented plane wave plus local orbitals method (FP-APW + lo). The exchange-correlation potential was treated using both the gradient generalized approximation (WC-GGA) and local density approximation (LDA). The estimated structural parameters, including the lattice parameters and internal coordinates agree well with the available experimental data. Our computed band structure shows that both studied compounds are semiconductors, with direct band gaps (Γ-Γ) of approximately 1.78 eV and 1.2 eV for CaCd2P2 and CaCd2As2, respectively, using GGA-TB-mBJ approach. The calculated optical spectra reveal a strong response of these materials in the energy range between the visible light and extreme UV regions, making them a good candidate for optoelectronic devices. Thermoelectric parameters, such as thermal conductivity, electrical conductivity, Seebeck coefficient, power factor and figure of merit were calculated. We note that both the CaCd2P2 and CaCd2As2 compounds show promising thermoelectric properties.

DFT calculation and experimental study on structural, optical and magnetic properties of Co-doped SrTiO3

NASA Astrophysics Data System (ADS)

Sikam, Pornsawan; Moontragoon, Pairot; Sararat, Chayanin; Karaphun, Attaphol; Swatsitang, Ekaphan; Pinitsoontorn, Supree; Thongbai, Prasit

2018-07-01

SrTiO3 (STO) is an attractive material that offers a wide range of technological applications, e.g., ferroelectricity, solar cell and photocatalysis. An application that the STO might be utilized is diluted magnetic semiconductors. Here, we would like to improve magnetic property of the STO by Ti site substitution using Co atoms. In this work, we present the structural, optical and magnetic properties of perfect and oxygen defect structures of STO and Co-doped SrTiO3 via experimental and theoretical aspects. In first-principles calculation, the structural properties, electronic band structure and magnetic properties of undoped STO and Co-doped STO supercells have been investigated by density functional theory using GGA with Hubbard model scheme (GGA+U) on Vienna Ab initio Simulation Package (VASP). In calculation detail, pure phase of STO with nanometer scale size of undoped STO and Co-doped STO have been synthesized using hydrothermal technique. The findings obtained from DFT computation reveal that the new states in gap between the valence band and conduction band of the STO were induced after Co atom was doped into the host structure. These impurity states narrow the band gap corresponding to experimental results. In addition, band splitting was observed on O defect and dopant systems, indicating that missing O and doping Co on STO could induce magnetization on none-magnetic material of STO. In case of synthesized powder, ferromagnetic behaviors are determined in the dopant system annealed in Ar. Additionally, another appreciated point of Co doping is that surface area of the STO is improved. Thus, it is expected that the surface activity, such as photocatalytic performance, of the STO will be enhanced. From all referred results, they introduce that the Co-doped STO might be a potential candidate to be a photocatalyst for the high photocatalytic performance under visible light radiation and the diluted magnetic semiconductor in spintronic devices.

Thermal evaporation and condensation synthesis of metallic Zn layered polyhedral microparticles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Khan, Waheed S.; Cao, Chuanbao, E-mail: cbcao@bit.edu.cn; Usman, Zahid

2011-12-15

Highlights: Black-Right-Pointing-Pointer Zn polyhedral microparticles prepared by thermal evaporation and condensation route. Black-Right-Pointing-Pointer Vapour-solid process based growth model governs the formation of Zn microparticles. Black-Right-Pointing-Pointer A strong PL emission band is observed at 369 nm in UV region. Black-Right-Pointing-Pointer Radiative recombination of electrons in the s, p conduction band and the holes in the d bands causes this emission. -- Abstract: Metallic zinc layered polyhedral microparticles have been fabricated by thermal evaporation and condensation technique using zinc as precursor at 750 Degree-Sign C for 120 min and NH{sub 3} as a carrier gas. The zinc polyhedral microparticles with oblate sphericalmore » shape are observed to be 2-9 {mu}m in diameter along major axes and 1-7 {mu}m in thickness along minor axes. The structural, compositional and morphological characterizations were performed by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). A vapour-solid (VS) mechanism based growth model has been proposed for the formation of Zn microparticles. Room temperature photoluminescence (PL) emission spectrum of the product exhibited a strong emission band at 369 nm attributed to the radiative recombination of electrons in the s, p conduction band near Fermi surface and the holes in the d bands generated by the optical excitation.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Argondizzo, Adam; Cui, Xuefeng; Wang, Cong

We investigate the spectroscopy and photoinduced electron dynamics within the conduction band of reduced rutile TiO2(110) surface by multiphoton photoemission (mPP) spectroscopy with wavelength tunable ultrafast (!20 fs) laser pulse excitation. Tuning the mPP photon excitation energy between 2.9 and 4.6 eV reveals a nearly degenerate pair of new unoccupied states located at 2.73 ± 0.05 and 2.85 ± 0.05 eV above the Fermi level, which can be analyzed through the polarization and sample azimuthal orientation dependence of the mPP spectra. Based on the calculated electronic structure and optical transition moments, as well as related spectroscopic evidence, we assign thesemore » resonances to transitions between Ti 3d bands of nominally t2g and eg symmetry, which are split by crystal field. The initial states for the optical transition are the reduced Ti3+ states of t2g symmetry populated by formation oxygen vacancy defects, which exist within the band gap of TiO2. Furthermore,we studied the electron dynamics within the conduction band of TiO2 by three-dimensional time-resolved pump-probe interferometric mPP measurements. The spectroscopic and time-resolved studies reveal competition between 2PP and 3PP processes where the t2g-eg transitions in the 2PP process saturate, and are overtaken by the 3PP process initiated by the band-gap excitation from the valence band of TiO2.« less

Improved photovoltaic properties of ZnTeO-based intermediate band solar cells

NASA Astrophysics Data System (ADS)

Tanaka, Tooru; Saito, Katsuhiko; Guo, Qixin; Yu, Kin Man; Walukiewicz, Wladek

2018-02-01

Highly mismatched ZnTe1-xOx (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC) because a narrow, O-derived intermediate band IB (E-) is formed well below the conduction band CB (E+) edge of the ZnTe. We have previously demonstrated the generation of photocurrent induced by two-step photon absorption (TSPA) in ZnTeO IBSCs using n-ZnO window layer. However, because of the large conduction band offset (CBO) between ZnTe and ZnO, only a small open circuit voltage (Voc) was observed in this structure. Here, we report our recent progress on the development of ZnTeO IBSCs with n-ZnS window layer. ZnS has a large direct band gap of 3.7 eV with an electron affinity of 3.9 eV that can realize a smaller CBO with ZnTe. We have grown n-type ZnS thin films on ZnTe substrates by molecular beam epitaxy (MBE), and demonstrated ZnTe solar cells and ZnTeO IBSCs using n-ZnS window layer with an improved VOC. Especially, a n-ZnS/i-ZnTe/p-ZnTe solar cell showed an improved Voc of 0.77 V, a large short-circuit current density of 6.7 mA/cm2 with a fill factor of 0.60, yielding the power conversion efficiency of 3.1 %, under 1 sun illumination.

First-principles study of structural, electronic, linear and nonlinear optical properties of Ga{2}PSb ternary chalcopyrite

NASA Astrophysics Data System (ADS)

Ouahrani, T.; Reshak, A. H.; de La Roza, A. Otero; Mebrouki, M.; Luaña, V.; Khenata, R.; Amrani, B.

2009-12-01

We report results from first-principles density functional calculations using the full-potential linear augmented plane wave (FP-LAPW) method. The generalized gradient approximation (GGA) and the Engel-Vosko-generalized gradient approximation (EV-GGA) were used for the exchange-correlation energy of the structural, electronic, linear and nonlinear optical properties of the chalcopyrite Ga2PSb compound. The valence band maximum (VBM) is located at the Γv point, and the conduction band minimum (CBM) is located at the Γc point, resulting in a direct band gap of about 0.365 eV for GGA and 0.83 eV for EV-GGA. In comparison with the experimental one (1.2 eV) we found that EV-GGA calculation gives energy gap in reasonable agreement with the experiment. The spin orbit coupling has marginal influence on the optical properties. The ground state quantities such as lattice parameters (a, c and u), bulk modules B and its pressure derivative B^primeare evaluated.

Electronic transport in disordered MoS2 nanoribbons

NASA Astrophysics Data System (ADS)

Ridolfi, Emilia; Lima, Leandro R. F.; Mucciolo, Eduardo R.; Lewenkopf, Caio H.

2017-01-01

We study the electronic structure and transport properties of zigzag and armchair monolayer molybdenum disulfide nanoribbons using an 11-band tight-binding model that accurately reproduces the material's bulk band structure near the band gap. We study the electronic properties of pristine zigzag and armchair nanoribbons, paying particular attention to the edges states that appear within the MoS2 bulk gap. By analyzing both their orbital composition and their local density of states, we find that in zigzag-terminated nanoribbons these states can be localized at a single edge for certain energies independent of the nanoribbon width. We also study the effects of disorder in these systems using the recursive Green's function technique. We show that for the zigzag nanoribbons, the conductance due to the edge states is strongly suppressed by short-range disorder such as vacancies. In contrast, the local density of states still shows edge localization. We also show that long-range disorder has a small effect on the transport properties of nanoribbons within the bulk gap energy window.

Role of Boron Element on the Electronic Properties of α-Nb5Si3: A First-Principle Study

NASA Astrophysics Data System (ADS)

Pan, Yong; Lin, Yuanhua

2018-03-01

Transition metal silicides (TMSis) are attracting increasing interest from the microelectronics and nanoelectronic industries. In this paper, we use the first-principles method to investigate the B-doped mechanism and the influence of B on the electronic properties of α-Nb5Si3. The calculated results show that B-doped Nb5Si3 is thermodynamically stable at the ground state. The calculated electronic structure shows that the thermodynamically stable B-doped Nb5Si3 is attributed to the 3D-network B-Si bonds and B-Nb bond. In particular, B element prefers to occupy B -IT4 site in comparison to other sites. Moreover, the calculated band structure indicates that Nb5Si3 exhibits metallic behavior at the ground state. We find that B-doping can improve charge overlap between conduction band and the valence band, which effectively improves the electronic properties of Nb5Si3.

Remarkable optical red shift and extremely high optical absorption coefficient of V-Ga co-doped TiO2

NASA Astrophysics Data System (ADS)

Deng, Quanrong; Han, Xiaoping; Gao, Yun; Shao, Guosheng

2012-07-01

A first attempt has been made to study the effect of codoping of transition metal and sp metal on the electronic structure and associated optical properties of TiO2, through V-Ga codoped thin films. V-Ga codoped rutile TiO2 films were fabricated on fused quartz substrates using pulsed laser ablation, followed by heat treatment at high temperatures. Gigantic redshift in the optical absorption edge was observed in V-Ga co-doped TiO2 materials, from UV to infrared region with high absorption coefficient. Through combined structural characterization and theoretical modeling, this is attributed to the p-d hybridization between the two metals. This leads to additional energy bands to overlap with the minimum of the conduction band, leading to remarkably narrowed band gap free of mid-gap states. The direct-gap of the co-doped phase is key to the remarkably high optical absorption coefficient of the coped titania.

RF design for the TOPGUN photogun: A cryogenic normal conducting copper electron gun

DOE PAGES

Cahill, A. D.; Fukasawa, A.; Pakter, R.; ...

2016-08-31

Some recent studies of rf breakdown physics in cryogenic copper X-band accelerating structures have shown a dramatic increase in the operating gradient while maintaining low breakdown rates. The TOPGUN project, a collaboration between UCLA, SLAC, and INFN, will use this improvement in gradient to create an ultra-high brightness cryogenic normal conducting photoinjector [16]. The brightness is expected to be higher by a factor of 25 relative to the LCLS photogun [9]. This improvement in the brightness will lead to increased performance of X-Ray free electron lasers (FELs) and ultrafast electron diffraction devices [16]. Here, we present the rf design formore » this S-band photogun, which will be a drop-in replacement for the current LCLS photogun.« less