First principles study on the structural, electronic and optical properties of diluted magnetic semiconductors Zn1-xCoxX （X=S, Se, Te）

The electronic and optical properties of zincblende ZnX（X=S, Se, Te） and ZnX：Co are studied from density functional theory （DFT） based first principles calculations. The local crystal structure changes around the Co atoms in the lattice are studied after Co atoms are doped. It is shown that the Co-doped materials have smaller lattice constant （about 0.6%-0.9%）. This is mainly due to the shortened Co-X bond length. The （partial） density of states （DOS） is calculated and differences between the pure and doped materials are studied. Results show that for the Co-doped materials, the valence bands are moving upward due to the existence of Co 3d electron states while the conductance bands are moving downward due to the reduced lattice constants. This results in the narrowed band gap of the doped materials. The complex dielectric indices and the absorption coefficients are calculated to examine the influences of the Co atoms on the optical properties. Results show that for the Co-doped materials, the absorption peaks in the high wavelength region are not as sharp and distinct as the undoped materials, and the absorption ranges are extended to even higher wavelength region.     

Bulk Modulus and Electronic Band Structure of ZnGa2Xa （X=S,Se）： a First-Principles Study

First-principles local density functional calculations are presented for the compounds ZnGa2X4 （X = S, Se）. We investigate the bulk moduli and electronic band structures in a defect chalcopyrite structure. The lattice constants and internal parameters are optimized. The electronic structures are analysed with the help of total and partial density of states. The relation between the cohesive energy and the unit cell volume is obtained by fully relaxed structures. We derive the bulk modulus of ZnGa2Xa by fitting the Birch-Murnaghan＇s equation of state. The extended Cohen＇s empirical formula agrees well with our ab initio results.     

Phase Transition and Phonon Spectrum of Zinc-Blende Structure ZnX （X = S,Se, Te）

Calculations have been performed to investigate the pressure-induced solid-solid phase transitions and the mechanical stability for three zinc-blende II-VI semiconductor compounds： ZnS, ZnSe, ZnTe by ab initio plane-wave pseudopotential density functional theory （DFT）. Using the generalized gradient approximation （GGA） for exchange and correlation in the scheme of Perdew-Wang 1991 （P Wgl ）, the ground state properties and equation of state are obtained, which are well consistent with the experimental data available and other calculations. On the basis of the forth-order Birch-Murnaghan equation of states, the transition pressures Pt are determined through the analysis of enthalpy variation with pressure. A linear-response approach is used to calculate the frequencies of the phonon dispersion. Finally, by the calculations of phonon frequencies, some thermodynamic properties such as the vibrational contribution to the Helmholtz free energy （F）, enthedpy （H）, entropy （S）, and the heat capacity （Cv ） are also successfully obtained.     

First-principles study on the structure,electronic and magnetic properties of HoSin （n=1-12, 20） clusters

The structure, electronic and magnetic properties of HoSin（n= 1 - 12, 20） clusters have been widely investigated by first-principles calculation method based on density flmctional theory （DFT）. From our calculation results, we find that for HoSin（n=1- 12） clusters except n = 7.10, the most stable structures are a replacement of Si atom in the corresponding pure Sin＋1 clusters by Ho atom. The doping of Ho atom makes the stability of Si clusters enhance remarkably, and HoSin（n = 2, 5, 8, 11） clusters are more stable than their neighboring clusters. The magnetic moment of Ho atom in HoSin （n = 1 - 12, 20） clusters mainly comes from of electron of tto, and never quenches.     

Comparisons of ZnO codoped by group ⅢA elements （Al,Ga, In） and N： a first-principle study

The electronic structures and effective masses of the N mono-doped and Al N, Ga-N, In-N codoped ZnO system have been calculated by a first-principle method, and comparisons among different doping cases are made. According to the results, the impurity states in the codoping cases are more delocalised compared to the N mono-doping case, which means a better conductive behaviour can be obtained by codoping. Besides, compared to the Al-N and Ga-N codoping cases, the hole effective mass of In-N codoped system is much smaller, indicating the p-type conductivity can be more enhanced by In N codoping     

Structures,stabilities, and electronic properties of F-doped Sin （n=1～12） clusters：Density functional theory investigation

The geometries, stabilities, and electronic properties of FSin （n=1～12） clusters are systematically investigated by using first-principles calculations based on the hybrid density-functional theory at the B3LYP/6-311G level. The geometries are found to undergo a structural change from two-dimensional to three-dimensional structure when the cluster size n equals 3. On the basis of the obtained lowest-energy geometries, the size dependencies of cluster properties, such as averaged binding energy, fragmentation energy, second-order energy difference, HOMO–LUMO （highest occupied molecular orbital–lowest unoccupied molecular orbital） gap and chemical hardness, are discussed. In addition, natural population analysis indicates that the F atom in the most stable FSin cluster is recorded as being negative and the charges always transfer from Si atoms to the F atom in the FSin clusters.     

First-principles study of the structural,mechanical and electronic properties of ZnX204 （X=Al,Cr and Ga）

This paper performs first-principles calculations to study the structural, mechanical and electronic properties of the spinels ZnA1204, ZnGa2O4 and ZnCr2O4, using density functional theory with the plane-wave pseudopotential method. Our calculations are in good agreement with previous theoretical calculations and the available experimental data. The studies in this paper focus on the evolution of the mechanical properties of ZnAl2O4, ZnGa2O4 and ZnCr2O4 under hydrostatic pressure. The results show that the cubic phases of ZnAl2O4, ZnCa2O4 and ZnCr2O4 become unstable at about 50 GPa, 40 GPa and 25 GPa, respectively. From analysis of the band structure of the three compounds at equilibrium volume, it obtains a direct band gap of 4.35 eV for ZnA1204 and 0.89 cV for ZnCr2O4, while ZnGa2O4 has an indirect band gap of 2.73 eV.     

First-Principles Study of Geometries,Stabilities and Electronic Properties of Ca2Sin （n=1-11） Clusters

The equilibrium geometries, relative stabilities, and electronic properties of Ca2Sin （n = 1-11） clusters have been systematically investigated by using the density function theory at the 6-311G （d） level The optimized geometries indicate that the most stable isomers have three-dimensional structures for n = 3-11. The electronic properties of Ca2 Sin （n = 1-11） dusters axe obtained through the analysis of the natural charge population, natural electron configuration, vertical ionization potential, and vertical electron affinity. The results show that the charges in corresponding Ca2Sin clusters transfer from the Ca atoms to the Sin host. Based on the obtained lowest-energy geometries, the size dependence of cluster properties, such as averaged binding energies, fragmentation energies, second-order energy differences, HOMO- LUMO gaps and chemical hardness, are deeply discussed.     

Electronic structure and optical properties of Mg_xZn_(1-x)S bulk crystal using first-principles calculations

We perform the first-principles calculations within the framework of density functional theory to determine the elec- tronic structure and optical properties of MgxZnl-xS bulk crystal. The results indicate that the electronic structure and optical properties of MgxZnl_xS bulk crystal are sensitive to the Mg impurity composition. In particular, the MgxZnl-xS bulk crystal displays a direct band structure and the band gap increases from 2.05 eV to 2.91 eV with Mg dopant compo- sition value x increasing from 0 to 0.024. The S 3p electrons dominate the top of valence band, while the Zn 4s electrons and Zn 3p electrons occupy the bottom of conduction band in MgxZnl_xS bulk crystal. Moreover, the dielectric constant decreases and the optical absorption peak obviously has a blue shift. The calculated results provide important theoretical guidance for the applications of MgxZn1-xS bulk crystal in optical detectors.     

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     

Uniaxial strain-modulated electronic structures of CdX (X = S,Se, Te) from first-principles calculations:A comparison between bulk and nanowires

Using first-principles calculations based on density functional theory, we systematically study the structural deformation and electronic properties of wurtzite CdX(X = S, Se, Te) bulk and nanowires(NWs) under uniaxial [0001] strain. Due to the intrinsic shrinking strain induced by surface contraction, large NWs with {10ˉ10} facets have heavy hole(HH)-like valence band maximum(VBM) states, while NWs with {11ˉ20} facets have crystal hole(CH)-like VBM states. The external uniaxial strain induces an HH–CH band crossing at a critical strain for both bulk and NWs, resulting in nonlinear variations in band gap and hole effective mass at VBM. Unlike the bulk phase, the critical strain of NWs highly depends on the character of the VBM state in the unstrained case, which is closely related to the size and facet of NWs. The critical strain of bulk is at compressive range, while the critical strain of NWs with HH-like and CH-like VBM appears at compressive and tensile strain, respectively. Due to the HH–CH band crossing, the charge distribution of the VBM state in NWs can also be tuned by the external uniaxial strain. Despite the complication of the VBM state, the electron effective mass at conduction band minimum(CBM) of NWs shows a linear relation with the CBM–HH energy difference, the same as the bulk material.     

Structural,electronic, and optical properties of ZnO_(1-x)Se_x alloys using first-principles calculations

The structural, electronic, and optical properties of binary ZnO, ZnSe compounds, and their ternary ZnO 1-x Se x alloys are computed using the accurate full potential linearized augmented plane wave plus local orbital (FP-LAPW + lo) method in the rocksalt (B1) and zincblende (B3) crystallographic phases. The electronic band structures, fundamental energy band gaps, and densities of states for ZnO 1-x Se x are evaluated in the range 0 ≤ x ≤ 1 using Wu-Cohen (WC) generalized gradient approximation (GGA) for the exchange-correlation potential. Our calculated results of lattice parameters and bulk modulus reveal a nonlinear variation for pseudo-binary and their ternary alloys in both phases and show a considerable deviation from Vegard's law. It is observed that the predicted lattice parameter and bulk modulus are in good agreement with the available experimental and theoretical data. We establish that the composition dependence of band gap is semi-metallic in B1 phase, while a direct band gap is observed in B3 phase. The calculated density of states is described by taking into account the contribution of Zn 3d, O2p, and Se 4s, and the optical properties are studied in terms of dielectric functions, refractive index, reflectivity, and energy loss function for the B3 phase and are compared with the available experimental data.     

Fe, S共掺杂SnO2材料第一性原理分析

本文采用基于第一性原理的全电势线性缀加平面波(FP-LAPW)法, 计算了Fe, S两种元素共掺杂SnO₂材料的电子结构和光学性质. 结果表明: 材料仍为直接禁带半导体, 体系呈现半金属性; Fe, S共掺可以窄化带隙, 且随S浓度增加, 态密度向低能方向移动, 带隙减小; 共掺体系电荷密度重新分布, 随S浓度增加, Fe原子极化程度增强, 原子间键合能力增强. 共掺后介电函数虚部谱与光学吸收谱各峰随S浓度增加而发生红移, 光学吸收边减小.     

Prediction of 2D Li2X (X=Se,Te) monolayer semiconductors by first principles calculations

Two dimensional monolayer materials play important roles in new generation of electronic and optical devices in nano scale. In this paper, by using first principles calculations, the existence of 2D Li₂X (X=Se, Te) monolayer materials are theoretically predicted. Through cohesive energy calculation and phonon dispersion simulation, it is proved that the proposed 2D Li₂Se and Li₂Te monolayer materials are energetically and dynamically stable suggests their potential experimental realization. Our study shows that these newly predicted compounds are direct semiconductors and have strain tunable wide band gaps. As direct semiconductors, these new monolayers may have many applications in electronics and optoelectronics devices.     

幻数尺寸Li-n-1,Lin,Li+ n+1(n=20,40)团簇的几何结构、电子与光学性质的第一性原理研究

基于密度泛函理论,采用第一性原理分子动力学模拟退火方法,对Li^-_n-1,Li_n,Li⁺_n+1 (n=20,40)的最低能量结构进行了全局搜索. 发现锂团簇的生长模式是以单个或多个嵌套的正多面体为核心,其余原子以五角锥为基本单元围绕核心生长. 基于最低能量结构的第一性原理电子结构计算得到锂团簇的分子轨道能级分布与无结构凝胶模型给出的电子壳层完全一致. 在总电 关键词： 团簇 电子结构 极化率 光吸收     

Electronic and thermoelectric properties of Mg_2Ge_xSn-_(1-x)(x=0.25,0.50,0.75) solid solutions by first-principles calculations

The electronic structure and thermoelectric(TE) properties of Mg_2Ge_xSn_(1-x)(x = 0.25, 0.50, 0.75) solid solutions are investigated by first-principles calculations and semi-classical Boltzmann theory. The special quasi-random structure(SQS) is used to model the solid solutions, which can produce reasonable band gaps with respect to experimental results.The n-type solid solutions have an excellent thermoelectric performance with maximum zT values exceeding 2.0, where the combination of low lattice thermal conductivity and high power factor(PF) plays an important role. These values are higher than those of pure Mg_2Sn and Mg_2Ge. The p-type solid solutions are inferior to the n-type ones, mainly due to the much lower PF. The maximum zT value of 0.62 is predicted for p-type Mg_2Ge_(0.25)Sn_(0.75) at 800K. The results suggest that the n-type Mg_2Ge_xSn_(1-x) solid solutions are promising mid-temperature TE materials.     

Ab-initio calculations of bandgap tuning of In1-xGaxY (Y = N,P) alloys for optoelectronic applications

The Ⅲ-V alloys and doping to tune the bandgap for solar cells and other optoelectronic devices has remained a hot topic of research for the last few decades. In the present article, the bandgap tuning and its influence on optical properties of In_1-xGa_xN/P, where (x = 0.0, 0.25, 0.50, 0.75, and 1.0) alloys are comprehensively analyzed by density functional theory based on full-potential linearized augmented plane wave method (FP-LAPW) and modified Becke and Johnson potentials (TB-mBJ). The direct bandgaps turn from 0.7 eV to 3.44 eV, and 1.41 eV to 2.32 eV for In_1-xGa_xN/P alloys, which increases their potentials for optoelectronic devices. The optical properties are discussed such as dielectric constants, refraction, absorption, optical conductivity, and reflection. The light is polarized in the low energy region with minimum reflection. The absorption and optical conduction are maxima in the visible region, and they are shifted into the ultraviolet region by Ga doping. Moreover, static dielectric constant ε₁(0) is in line with the bandgap from Penn's model.