Energy band structure of (SN)x chain: Unrestricted Hartree–Fock and charge density wave solutions

The ab initio Hartree–Fock crystal orbital method is used for the calculation of the energy band structure of a one-dimensional model of (SN)_x. Two energy band structures are described corresponding to the self-consistent spin density wave (SDW ) and the self-consistent charge density wave (CDW ) solution, respectively.     

Nanocrystalline Zn1−x Ag x O y thin films evolved through electrodeposition for photoelectrochemical splitting of water

Nanocrystalline Zn_1???x Ag _x O _y (x?=?10^??3???50?×?10^??3) thin films evolved through electrodeposition over ITO substrate have been investigated for photoelectrochemical splitting of water. Samples were characterized by XRD, EDX, SEM, AFM, UV–visible optical absorption, and Mössbauer spectral analysis. Ag incorporation led to a decrease in the band gap energy and alterations in microstructural and semiconductor properties. Raising Ag concentration in samples up to 1 % at. caused a significant reduction in density and electrical resistivity, enhanced absorption along with red shift to the band gap energy, anodic shift in flat band potential, and increased charge carrier density, enabling 1 % at. Ag-incorporated ZnO films most photosensitive by yielding highest short-circuit current, photocurrent density, and applied bias photon to current efficiency. Plausible reasons have been offered.     

Theoretical studies of the band structure and optoelectronic properties of ZnOxS1−x

In the present article, we have revisited the electronic band gap nature of ZnO_xS_1?x (0 ≤ x ≤ 1) with the recently developed modified Becke and Johnson exchange potential and the calculated band gaps are found consistent with the experimental results. We expect that the band gap bowing parameter obtained in the present work will be close to the experimental one. As the optical properties of ZnO_xS_1?x (0 ≤ x ≤ 1) are very important, therefore different optical parameters like dielectric functions, refractive index and reflectivity are also calculated. The results are illustrated in terms of band structures, band gap energy as a function of oxygen composition, total and partial density of states. © 2012 Wiley Periodicals, Inc.     

First principles study on the structural,electronic, and optical properties of Sc-doped AlN

Based on the density functional pseudo-potential method, the structural properties, the band structure, the density of states and the optical properties of the pure and Sc-doped AlN are calculated. The calculation results indicate that the defect of Sc(Al) exists steadily with a certain solubility in the doped system. Sc substitution of the Al site induces effective reduction of the band gap of AlN and the band gap being continuously reduced when increasing Sc concentrations. The existence of the strong hybridization between Sc 3d and N 2p indicates the transport of electrons from Sc atoms to N atoms. Besides, it is shown that the insertion of Sc atom leads to redshift of the optical absorption edge. The intensity of both the imaginary part of the dielectric function and the optical absorption of Al_{1 ? x} Sc _x N are found to decrease with increasing Sc concentrations in the low energy range.     

Investigation of the fermi edge in sodium tungsten bronzes by photoelectron spectroscopy

High-resolution photoelectron spectra of sodium tungsten bronzes Na _xWO₃ (0.26<x< 0.76) reveal a linear variation in the density of states at the Fermi energy as a function of sodium content. This behaviour does not appear to arise from filling of a rigid conduction band. Magnetic susceptibilities calculated from photoemission data using a simple independent-electron model agree with measured values.     

First principle calculations of structural,electronic and thermodynamic properties of Al3(TixV1−x) alloy in D022 and L12 structures

In this work, we employed the FP-LAPW method based on the density functional theory (DFT), and the gradient generalized approximation (GGA) has been used to compute the exchange correlation potential. We calculated the total energy for ternary alloys (Al₃Ti_xV_1−x) in the D0₂₂ and L1₂ structures in relaxed and fully relaxed phases. The total energy calculation shows that the D0₂₂ is the more stable one as it is observed experimentally. The ground states properties such as the lattice parameters, the bulk modulus and the formation enthalpies are determined. The density of states calculation shows that the pseudogap formation is located near the Fermi level. The determination of Gibbs free energy of mixing at different concentrations was used to calculate the T–x diagram which shows the stable, metastable and unstable regions of the alloys.     

Electronic structures and optical properties of wurtzite type LiBSe2 (B=Al, Ga, In): A first-principles study

The electronic structures of three wurtzite type isostructural compounds LiBSe₂ (B=Al, Ga, In) are studied by the density functional theory (DFT). The results reveal that the presence of Li cations has direct influence on neither the band gaps (Eg) nor the bonding levels, but plays an important role in the stabilization of the structures. The band structures and densities of states (DOS) are analyzed in detail, and the band gaps of LiBSe₂ adhere to the following trend Eg_(LiAlSe2)>Eg_(LiGaSe2)>Eg_(LiInSe2), which is in agreement with the decrease of the bond energy of the corresponding Se 4p-B s antibonding orbitals. The role of the active s electrons of B element on the band gaps is also discussed. Finally, the optical properties are predicted, and the results would be a guide to understand the experiments.     

First principles calculations of electronic structure and magnetic properties of Cr-based magnetic semiconductors Al1−xCrxX (X=N, P, As, Sb)

First principles calculations based on the density functional theory (DFT) within the local spin density approximation are performed to investigate the electronic structure and magnetic properties of Cr-based zinc blende diluted magnetic semiconductors Al_1−xCr_xX (X=N, P, As, Sb) for 0≤x≤0.50.The behaviour of magnetic moment of Al_1−xCr_xX at each Cr site as well as the change in the band gap value due to spin down electrons has been studied by increasing the concentration of Cr atom and through changing X from N to Sb. Furthermore, the role of p-d hybridization is analyzed in the electronic band structure and exchange splitting of d-dominated bands. The interaction strength is stronger in Al_1−xCr_xN and becomes weaker in Al_1−xCr_xSb. The band gap due to the spin down electrons decreases with the increased concentration of Cr in Al_1−xCr_xX, and as one moves down along the isoelectronic series in the group V from N to Sb. Our calculations also verify the half-metallic ferromagnetic character in Cr doped AlX.     

Electronic and optical properties of ZnSc2S4 and CdSc2S4 cubic spinels by the modified Becke–Johnson density functional

Structural, electronic and optical properties of the ZnSc₂S₄ and CdSc₂S₄ cubic spinels have been investigated by means of the full-potential (linearized) augmented plane wave plus local orbitals based on density functional theory. The exchange-correlation potential is treated by the GGA–PBEsol [J.P. Perdew, A. Ruzsinszky, G.I. Csonka, O.A. Vydrov, G.E. Scuseria, L.A. Constantin, X. Zhou, K. Burke, Phys. Rev. Lett. 100 (2008) 136406] and the recently proposed modified Becke–Johnson potential approximation (mBJ) [F. Tran, P. Blaha, Phys. Rev. Lett. 102 (2009) 226401], which successfully corrects the band-gap problem found with GGA for a wide range of materials. The obtained structural parameters are in good agreement with the available experimental data. This gives support for the predict properties for ZnSc₂S₄ and CdSc₂S₄. The band structures reveal that both compounds are semiconductor with a direct gap. The obtained gap values show that mBJ is superior for estimating band gap energy. We have calculated the electron and hole effective masses in different directions. The density of states has been analyzed. Based on our electronic structure obtained using the mBJ method we have calculated various optical properties, including the complex dielectric function ɛ(ω), complex index of refraction n(ω), reflectivity coefficient R(ω), absorption coefficient α(ω) and electron energy-loss function L(ω) as functions of the photon energy. We find that the values of zero-frequency limit ɛ₁(0) increase with decreasing the energy band gap in agreement with the Penn model. The origin of the peaks and structures in the optical spectra is determined in terms of the calculated energy band structures.     

Transition between direct gap and indirect gap in two dimensional hydrogenated honeycomb Si x Ge1−x alloys

Using first-principles calculations, we have explored the structural and electronic properties of fully hydrogenated honeycomb Si _x Ge_1?x H alloys. Finite band gaps are opened by hydrogenation for x in the whole range from 0 to 1, while their nature and values can be tuned by x. When x is <0.7, the band gap is direct (from Γ to Γ). And when x is ≥0.7, the gap turns into indirect (from Γ to M). For all the computed compositions, the two kinds of energy differences between valence band and conduction band, Γ–Γ and Γ–M, are described well by two polynomial functions of x. The smaller of the two functions gives a good prediction for the overall band gap at any x. The two curves cross at x = 0.7, leading to the change of band gap type. At PBE level, the values of band gap for different x spread from 1.09 to 2.29 eV. These findings give a new route to tune the electronic properties of these materials and may have potential applications in nanoscale optoelectronics.     

First-principles calculations of Cd-doped ZnO thin films deposited by pulse laser deposition

Zn_1−xCd_xO thin films are deposited on quartz substrate by pulse laser deposition. Their band structure and optical properties are experimentally and theoretically investigated. By varying Cd concentration, the band gap of Zn_1−xCd_xO films can be adjusted in a wide range from 3.219 eV for ZnO to 2.197 eV for Zn_0.5Cd_0.5O, which produces different emissions from ultraviolet to Kelly light in their photoluminescence spectra. Simultaneity, the electronic structure and band gap of Zn_1−xCd_xO are investigated by the density functional theory (DFT) with a combined generalized gradient approximation (GGA) plus Hubbard U approach, which precisely predicts the band-gaps of ZnO and Zn_1−xCd_xO alloys. Both the experimental results and theoretical simulation reveal that with increasing Cd concentration in Zn_1−xCd_xO alloys, their absorption coefficients in visible light range are evidently enhanced. The adjustable photoluminescence emission and enhanced visible light absorption endow Zn_1−xCd_xO alloys potential applications in optoelectronic and photocatalytic fields.     

Electronic Structures and Optical Properties of Ga Doped Single-Layer Indium Nitride

Electronic structures and optical properties of single-layer In_1-xGa_xN are studied by employing Heyd-Scuseria-Ernzerh (HSE) method based on the first-principles. The band structure and density of states (DOS) of single-layer In_1-xGa_xN are calculated, and the band gap ranges from 1.8 eV to 3.8 eV as the ratio x changes, illustrating the potential for the tunability of band gap values via Ga doped. We also have investigated optical properties of single-layer In_1-xGa_xN such as dielectric function, refractive index and absorption coefficient, the main peak of dielectric function spectrum and the absorption edge are found to have a remarkable blue-shift as the concentration of Ga increases. Furthermore, the optical properties of single-layer In_1-xGa_xN are analyzed based on the band structures and DOS analysis. Such unique optical properties have profound application in nanoelectronics and optical devices.     

Electron charge distribution of CaAl2−xZnx: Maximum entropy method combined with Rietveld analysis of high-resolution-synchrotron X-ray powder diffraction data

Using short wavelength X-rays from synchrotron radiation (SPring-8), high-resolution powder diffraction patterns were collected. In order to study both the structural relationship and the mechanism of stability in the CaAl_2−xZn_x system, among the Laves phases (MgCu₂ and MgNi₂ type) and KHg₂-type structures, the charge density distribution of CaAl_2−xZn_x as a function of x was obtained from the diffraction data by Rietveld analysis combined with the maximum entropy method (MEM). In the MEM charge density maps overlapping electron densities were clearly observed, especially in the Kagomé nets of the Laves phases. In order to clarify the charge redistribution in the system, the deformation charge densities from the densities formed by the constituent free atoms are discussed. In the ternary MgNi₂-type phase, partial ordering of Al and Zn atoms is observed, a finding that is supported by ab-initio total energy calculations.     

Density of vibrational states in trans-polyene: comparison with the infrared,Raman and neutron spectra of trans-polyacetylene

The density of states has been calculated for the in-plane and out-of-plane vibrations in the infinite trans polyene chains. Comparison of the density of states spectra with the IR and Raman spectra of trans-polyacetylenes, (CH)_x, (¹³CH)_x and (CD)_x, shows that most of the weak bands as well as the strong bands arise from the trans polyene segments in the polymer films. The IR bands newly assigned to the trans segments include the 740 cm⁻¹ band, which was previously assigned to remnant cis CC bonds. The IR spectra of (¹³CH)_x and (CD)_x clearly indicate that the amount of remnant cis bonds in thermally isomerized trans films is much smaller than that proposed previously. The hydrogen-amplitude-weighted density of states calculated for the infinite polyene chain also shows satisfactory agreement with a neutron inelastic scattering spectrum of trans-(CH)_x.     

Linear and nonlinear optical studies of CdS1−x Se x quantum dots

In this contribution we present and discuss our measurements on CdS_1?x Se _x quantum dots in a glass matrix. In linear absorption measurements we find the typical blue shift of the transitions with decreasing crystallite radius due to quantization. The luminescence shows a significant Stokes shift with respect to absorption, which is interpreted in terms of strong exciton-phonon coupling and allows to deduce the Huang-Rhys factor S. Under high excitation we find an additional emission band on the high energy side, which can be attributed to the recombination of an excited two electron-hole pair state to a one electron-hole pair state in agreement with theory. Pump and probe beam experiments give a bleaching but no hole burning. Finally we discuss some open questions especially concerning the high energy structures in the absorption spectrum.     

Electron energy structure and X-ray spectra of wide-band GaN, AlN, and AlN-GaN semiconductors

The electronic energy structure of GaN, AlN, and AlGaN crystals with the wurzite structure is calculated by the local coherent potential method using the cluster version of the MT-approximation within the framework of the multiple scattering theory. The calculated densities of electron states are compared with XPS spectra of gallium and aluminum, AlL _{II, III} XES, and also with K-spectra of gallium and AlL _{II, III} XAFS absorption. The comparison of the electronic structure of Al_xGa_1?x N crystals and binary GaN and AlN and the interpretation of their features are performed. The concentration dependence of the width of the upper subband of the valence band and the band gap in Al_xGa_1?x N (x = 0, 0.25, 0.5, 0.75, 1) crystals on the content of aluminum is studied and its non-linear character shown.     

Electronic Structures of LixV2O5 (x=0.5 and 1): A Theoretical Study

The electronic properties of α‐Li_xV₂O₅ (x=0.5 and 1) are investigated using first principle calculations based on density functional theory with local density approximation. Different intercalation sites for Li in the V₂O₅ lattices are considered, showing different influences on the electronic structures of Li_xV₂O₅. The lowest total energy is found when Li is only intercalated along the c axis between two bridging oxygen ions of sequential V₂O₅ layers. The intercalation of Li into V₂O₅ does not change the electron transition property of V₂O₅, which is an indirect band gap semiconductor, but leads to a reduction of vanadium ions and an increase of the Fermi level of Li_xV₂O₅ arising from the electron transfer from the Li 2 s orbital to the initially empty conduction band of the V₂O₅ host.     

Antimony-doped tin(IV) oxide: Surface composition and electronic structure

Antimony-doped tin(IV) oxide Sn_1?xSb_xO₂ prepared by a high-temperature (1300 K) solid-state synthetic procedure has been studied over the composition range0 < x < 0.03by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and high-resolution electron-energy-loss spectroscopy (HREELS). Pronounced enrichment by antimony close to the surface is evident from XPS with a heat of segregation approaching 30 kJ/mole. However, no increase in the surface free-carrier concentration is evident from the conduction-to-valence band intensity ratio in UPS or from the surface plasmon frequency in EELS. It is concluded that electrons associated with segregated Sb ions occupy a lone-pair-likesp hybrid surface state whose energy lies well below that of the conduction band.     

Ab-initio study of the structural, linear and nonlinear optical properties of CdAl2Se4 defect-chalcopyrite

The complex density functional theory (DFT) calculations of structural, electronic, linear and nonlinear optical properties for the defect chalcopyrite CdAl₂Se₄ compound have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2k code. We employed the Wu and Cohen generalized gradient approximation (GGA-WC), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure, density of states and the spectral features of the linear and nonlinear optical properties. This compound has a wide direct energy band gap of about 2.927 eV with both the valence band maximum and conduction band minimum located at the center of the Brillouin zone. The ground state quantities such as lattice parameters (a, c, x, y and z), bulk modulus B and its pressure derivative B′ are evaluated. We have calculated the frequency-dependent complex ε(ω), its zero-frequency limit ε₁(0), refractive index n(ω), birefringence Δn(ω), the reflectivity R(ω) and electron energy loss function L(ω). Calculations are reported for the frequency-dependent complex second-order nonlinear optical susceptibilities. We find opposite signs of the contributions of the 2ω and 1ω inter/intra-band to the imaginary part for the dominant component through the wide optical frequency range.