LiGaX2(X=S,Se, Te)的电子结构,光学和弹性性质的第一性原理计算

采用基于第一性原理的密度泛函理论赝势平面波方法,对LiGaX₂(X=S, Se, Te)的能带结构、态密度、光学以及弹性性质进行了理论计算. 能带结构计算表明LiGaS₂ 的禁带宽度为4.146 eV, LiGaSe₂ 的禁带宽度为3.301 eV, LiGaTe₂ 的禁带宽度为2.306 eV; 其价带主要由Ga-4p 层电子和X- np 层电子的能态密度决定; 同时也对LiGaX< 关键词： 电子结构 光学性质 弹性性质 LGX     

DFT study on electronic structure and optical properties of N-doped,S-doped,and N/S co-doped SrTiO3

The electronic structures and optical properties of N-doped, S-doped and N/S co-doped SrTiO₃ have been investigated on the basis of density functional theory (DFT) calculations. Through band structure calculation, the top of the valence band is made up of the O 2p states for the pure SrTiO₃. When N and S atoms were introduced into SrTiO₃ lattice at O site, the electronic structure analysis shows that the doping of N and S atoms could substantially lower the band gap of SrTiO₃ by the presence of an impurity state of N 2p on the upper edge of the valence band and S 2p states hybrid with O 2p states, respectively. When the N/S co-doped, the energy gap has further narrowing compared with only N or S doped SrTiO₃. The calculations of optical properties also indicate a high photo response for visible light for N/S co-doped SrTiO₃. Besides, we find a new impurity state which separates from the O 2p states could improve the photocatalytic efficiency and we also propose a model for light electron-hole transportation which can explain the experiment results well. All these conclusions are in agreement with the recent experimental results.     

Valence band photoemission study of the copper chalcogenide compounds, Cu2S, Cu2Se and Cu2Te

The electronic structures of the copper chalcogenide compounds, Cu₂S, Cu₂Se and Cu₂Te have been investigated by taking photoemission data with synchrotron photon sources. The band calculations are done using the full-potential linear-muffin-tin-orbital method. Since the crystal structures are not clarified well, several simplified structure models are used. The calculated densities of states are compared with the observed spectra. The analysis shows that a sharp peak at −3.5 eV is due to the Cu 3d states, and that the tails at the high and low energy sides of the Cu 3d peak are due to the chalcogen p states.     

Theoretical optoelectronic analysis of intermediate-band photovoltaic material based on ZnY1-xOx （Y = S,Se, Te） semiconductors by first-principles calculations

The structural, energetic, and electronic properties of lattice highly mismatched ZnY1-xOx （Y = S, Se, Te） ternary alloys with dilute O concentrations are calculated from first principles within the density functional theory. We demonstrate the formation of an isolated intermediate electronic band structure through diluted O-substitute in zinc-blende ZnY （Y = S, Se, Te） at octahedral sites in a semiconductor by the calculations of density of states （DOS）, leading to a significant absorption below the band gap of the parent semiconductor and an enhancement of the optical absorption in the whole energy range of the solar spectrum. It is found that the intermediate band states should be described as a result of the coupling between impurity O 2p states with the conduction band states. Moreover, the intermediate bands （IBs） in ZnTeO show high stabilization with the change of O concentration resulting from the largest electronegativity difference between O and Te compared with in the other ZnSO and ZnSeO.     

S掺杂对锐钛矿相TiO2电子结构与光催化性能的影响

采用基于第一性原理的平面波超软赝势方法研究了掺杂不同价态S的锐钛矿相TiO₂的晶体结构、杂质形成能、电子结构及光学性质.计算结果表明硫在掺杂体系中的存在形态与实验中的制备条件有关;掺杂后晶格发生畸变、原子间的键长及原子的电荷量也发生了变化,导致晶体中的八面体偶极矩增大; S 3p态与O 2p态、Ti 3d态杂化而使导带位置下移、价带位置上移及价带宽化,从而导致TiO₂的禁带宽度变窄、光吸收曲线红移到可见光区.这些结果很好地解释了S掺杂锐钛矿相TiO₂在可见光下具有优良的光催化性能的内在原因.根据计算结果分析比较了硫以不同离子价态掺杂对锐钛矿相TiO₂电子结构和光催化性能影响的差别. 关键词： 2')" href="#">锐钛矿相TiO₂ S掺杂 第一性原理 光催化性能     

Origin of visible-light-induced photocatalytic properties of S-doped anatase TiO2 by first-principles investigation

Electronic structures and optical absorption spectra of S-doped anatase TiO₂ are calculated by means of the first-principles pseudopotential total energy method. Electronic structure analyses demonstrate that isolated S 3p states which appear on the upper edge of the valence band cause the narrowing of the band gap of S-doped TiO₂. These changes of electronic structure exert great influence on the optical responses of S-doped TiO₂. Our results are in line with recent experimental observations that the redshift of optical absorption could be attributed to these isolated S 3p states on the upper edge of the valence band of S-doped TiO₂.     

Electronic structure and structural phase stability of CuAlX2 (X=S, Se, Te) under pressure

The electronic and structural properties of chalcopyrite compounds CuAlX₂ (X=S, Se, Te) have been studied using the first principle self-consistent Tight Binding Linear Muffin-Tin Orbital (TBLMTO) method within the local density approximation. The present study deals with the ground state properties, structural phase transition, equations of state and pressure dependence of band gap of CuAlX₂ (S, Se, Te) compounds.Electronic structure and hence total energies of these compounds have been computed as a function of reduced volume. The calculated lattice parameters are in good agreement with the available experimental results. At high pressures, structural phase transition from bct structure (chalcopyrite) to cubic structure (rock salt) is observed. The pressure induced structural phase transitions for CuAlS₂, CuAlSe₂, and CuAlTe₂ are observed at 18.01, 14.4 and 8.29 GPa, respectively. Band structures at normal as well as for high-pressure phases have been calculated. The energy band gaps for the above compounds have been calculated as a function of pressure, which indicates the metallic character of these compounds at high-pressure fcc phase. There is a large downshift in band gaps due to hybridatization of the noble-metal d levels with p levels of the other atoms.     

Ab initio band structure calculations of the low-temperature phases of Ag2Se, Ag2Te and Ag3AuSe2

Ab initio band structure calculations were performed for the low-temperature modifications of the silver chalcogenides β-Ag₂Se, β-Ag₂Te and the ternary compound β-Ag₃AuSe₂ by the local spherical wave (LSW) method. Coordinates of the atoms of β-Ag₂Se and β-Ag₃AuSe₂ were obtained from refinements using X-ray powder data. The structures are characterized by three, four and five coordinations of silver by the chalcogen, a linear coordination of gold by Se, and by metal-metal distances only slightly larger than in the metals. The band structure calculations show that β-Ag₃AuSe₂ is a semiconductor, while β-Ag₂Se and β-Ag₂Te are semimetals with an overlap of about 0.1-0.2 eV. The Ag 4d and Au 5d states are strongly hybridized with the chalcogen p states all over the valence bands. β-Ag₂Se and β-Ag₂Te have a very low DOS in the energy range from about −0.1 to +0.5 eV. The calculated effective mass β-Ag₂Se is about 0.1-0.3 m_e for electrons and 0.75 m_e for holes, respectively.     

Many-body effects on the width of the band gap in Bi2Te2X (X = Te, Se, S) topological insulators

The spectrum of quasiparticles of Bi₂Te₂X (X = Te, Se, S) three-dimensional topological insulators has been theoretically studied in the GW approximation with the inclusion of the spin-orbit interaction in the construction of the Green’s function and self-energy. It has been shown that many-body corrections to the Kohn-Sham states in Bi₂Te₂X increase the fundamental band gap similar to conventional semiconductors. However, the band gap at the Γ point decreases in this case. Gaps in the quasiparticle spectrum obtained in agreement with the experimental data correspond to the difference between the minimum of the conduction band, which is located on the Γ-Z line, and the maximum of the valence band, which lies beyond the symmetric directions in the mirror plane.     

Electronic structure of CdTe probed by Cd and Te M4,5 X-ray emission spectra

We have investigated the bulk electronic structure of CdTe focusing on the Cd 5p and Te 5p valence states by X-ray emission spectroscopy (XES). Despite the very low fluorescence yields the Cd and Te M_4,5 (5p → 3d_3/2,5/2) spectra have been recorded successfully. A good correspondence has been found between the valence band XES and X-ray photoelectron spectra (XPS) by comparison on a common binding energy scale. We also performed a density functional theory calculation of the CdTe valence band, obtaining the Cd 4d, 5s, 5p and Te 5s, 5p local partial densities of states. The experimental Cd 5p and Te 5p derived from the X-ray emission spectra are in good agreement with the calculation. The intensity ratio of the Cd M_4,5 to the Te M_4,5 spectrum is obtained to be 0.25, in agreement with the ratio of the calculated Cd 5p to the Te 5p density of states in the CdTe upper valence band (0.22).     

Electronic structure of MgS and MgYb2S4: Electron Energy-Loss Spectroscopy and self-consistent multiple scattering calculations

The electronic structure of MgS and MgYb₂S₄ have been studied using the fine structure of the Mg-K, S-K, Mg-L_2,3, S-L_2,3 and Yb-N₅ edges measured by electron energy-loss spectroscopy (EELS). Our experimental results are compared with real-space full multiple scattering calculations as incorporated in the FEFF9.6 code. All edges are very well reproduced. Total and partial densities of states have been calculated. The calculated densities of states of Mg and S are similar in both compounds. The energy distribution of these states suggests a covalent nature for both materials. For MgYb₂S₄ a band gap smaller than for MgS is predicted. In this compound the top of the valence band and the bottom of the conduction band are dominated by Yb states.     

A high-resolution X-ray photoemission study of the total valence-band densities of states of GaSe and BiI3

The total valence band denstiy of states spectra for the semiconducting layered compounds GaSe and BiI₃ were obtained by X-ray photoemission spectroscopy. The results for GaSe were used to test recent band structure calculations and are compared with earlier photoemission results. The BiI₃ data are the first experimental determination of the total valence band density of states for this compound. Since no calculations exist for BiI₃, tentative assignments were made.     

Band gap anomaly and topological properties in lead chalcogenides

Band gap anomaly is a well-known issue in lead chalcogenides Pb X(X = S, Se, Te, Po). Combining ab initio calculations and tight-binding(TB) method, we have studied the band evolution in Pb X, and found that the band gap anomaly in Pb Te is mainly related to the high on-site energy of Te 5s orbital and the large s–p hopping originated from the irregular extended distribution of Te 5s electrons. Furthermore, our calculations show that Pb Po is an indirect band gap(6.5 me V) semiconductor with band inversion at L point, which clearly indicates that Pb Po is a topological crystalline insulator(TCI). The calculated mirror Chern number and surface states double confirm this conclusion.     

Electronic properties and Compton profiles of molybdenum dichalcogenides

We present the first ever experimental Compton profiles of molybdenum dichalcogenides (MoX₂; X=S and Te) using 20 Ci ¹³⁷Cs Compton spectrometer. To interpret our experimental data, we have computed the theoretical profiles, energy bands and density of states using linear combination of atomic orbitals method in the framework of density functional theory and its hybridisation with Hartree Fock. The energy bands and density of states using full potential linearised augmented plane wave method have also been computed. Both theories show the existence of the indirect band gap. In addition, the relative nature of bonding is explained in terms of equal-valence-electron-density profiles and valence band charge densities.     

Benefits of oxygen in CuInSe2 and CuGaSe2 containing Se-rich grain boundaries

Using density functional theory calculation, we show that oxygen (O) exhibits an interesting effect in CuInSe₂ and CuGaSe₂. The Se atoms with dangling bonds in a Se-rich Σ3 (114) grain boundary (GB) create deep gap states due to strong interaction between Se atoms. However, when such a Se atom is substituted by an O atom, the deep gap states can be shifted into valence band, making the site no longer a harmful non-radiative recombination center. We find that O atoms prefer energetically to substitute these Se atoms and induce significant lattice relaxation due to their smaller atomic size and stronger electronegativity, which effectively reduces the anion–anion interaction. Consequently, the deep gap states are shifted to lower energy regions close or even below the top of the valence band.     

First-principles study of electronic and optical properties of BaS,BaSe and BaTe

The optimized crystal structure, energy band structures, density of states (DOS) and optical properties of BaX (X=S, Se and Te) were investigated by the full potential linearized augmented plane wave plus local orbitals method (FP-LAPW+lo). The exchange-correlation potential was treated using the generalized gradient approximation (GGA). We have used also Engel and Vosko GGA (EV-GGA) formalism to improve the band gap results. The calculated results such as band gaps, dielectric constants and reflectivity spectra showed good agreement with the experimental data. The effect of the spin-orbit coupling (SOC) on the optical properties was also studied and found to be very small, especially in the low-energy region.     

Fermi level pinning at CdS/Cu(In,Ga)(Se,S)2 interfaces: effect of chalcopyrite alloy composition

We introduce a quantitative model for the band diagram of ZnO/CdS/Cu(In,Ga)(Se,S)₂ heterostructures and for carrier recombination at the CdS/chalcopyrite interface. We derive analytical expressions for the open circuit voltage and the Fermi energy position at the active interface. The open circuit voltage under interface recombination is almost independent from the band gap energy of the chalcopyrite when the valence band edge of the absorber remains at the same energy position. The analytical calculations are in relatively good agreement with numerical simulations. Experimental current–voltage analysis indicates that devices prepared from Cu-poor Cu(In,Ga)(Se,S)₂ chalcopyrites are dominated by recombination in the bulk of the absorber while interface recombination prevails if the absorbers are prepared under Cu-excess. In the latter case, the experimentally determined interface barriers reveal that the interface Fermi energy position shifts upward on the energy scale upon increasing the Ga content into the absorber and remains at a relatively low energy value under S/Se alloying.     

Ab initio calculations of band structure and thermophysical properties for SnS2 and SnSe2

The electronic band structure and elastic constants of SnS₂ and SnSe₂ have been calculated by using density-functional theory (DFT). The calculated band structures show that SnS₂ and SnSe₂ are both indirect band gap semiconductors. The upper valence bands originate mainly from S_p and Sn_d electrons, while the lowest conduction bands are mainly from (S, Se) _p and Sn_s states. The calculated elastic constants indicate that the bonding strength along the [100] and [010] direction is stronger than that along the [001] direction and the shear elastic properties of the (010) plane are anisotropic for SnS₂ and SnSe₂. Both compounds exhibit brittle behavior due to their low B/G ratio. Relationships among volumes, the heat capacity, thermal expansion coefficients, entropy, vibrational energy, internal energy, Gibbs energy and temperature at various pressures are also calculated by using the Debye mode in this work.     

Theoretical orbital energy spectra of electrons in chemisorbed systems: Chalcogens on Ni(001)

Theoretical energy level spectra calculated by the localised orbital pseudopotential method are presented for chemisorbed O, S, Se and Te monolayers on Ni(001). Adsorption-induced features are a band of bonding states 2–3 eV wide centred ≈ 4 eV below E_f and a band of states centred near E_f. Supporting experimental evidence is discussed.     

Electronic structure of ZrTiO4 and HfTiO4: Self-consistent cluster calculations and X-ray spectroscopy studies

Total and partial densities of states of the constituent atoms of ZrTiO₄ and HfTiO₄ titanates have been calculated using a self-consistent cluster method as incorporated in the FEFF8 code. The calculations reveal the similarity of the electronic structure of both titanates and indicate that the valence band of the compounds under consideration is dominated by contributions of O 2p states. These states contribute throughout the whole valence-band region; however their maximum contributions occur in the upper portion of the band. Other significant contributors in the valence-band region are Ti 3d and Zr 4d states in ZrTiO₄ and Ti 3d and Hf 5d states in HfTiO₄. All the above d-like states contribute throughout the whole valence-band region of the titanates; however maximum contributions of the Ti 3d states occur in the upper portion, whilst those of the Zr 4d (Hf 5d) states are in the central portions of the valence band. The FEFF8 calculations render that the bottom of the conduction band of ZrTiO₄ and HfTiO₄ is dominated by contributions of Ti 3d^? states, with also smaller contributions of Zr 4d^?/Hf 5d^? and O 2p^? states. To verify the above FEFF8 data, the X-ray emission bands, representing the energy distributions of mainly O 2p, Ti 3d and Zr 4d states, were measured and compared on a common energy scale. These experimental data are found to be in agreement with the theoretical FEFF8 results for the electronic structure of ZrTiO₄ and HfTiO₄ titanates. Additionally, X-ray photoelectron valence-band and core-level spectra were recorded for the constituent atoms of the titanates under study.