Structural,electronic and optical properties of cubic SrTiO3 and KTaO3: Ab initio and GW calculations

We present first-principles VASP calculations of the structural, electronic, vibrational, and optical properties of paraelectric SrTiO₃ and KTaO₃. The ab initio calculations are performed in the framework of density functional theory with different exchange-correlation potentials. Our calculated lattice parameters, elastic constants, and vibrational frequencies are found to be in good agreement with the available experimental values. Then, the bandstructures are calculated with the GW approximation, and the corresponding band gap is used to obtain the optical properties of SrTiO₃ and KTaO₃.     

Magnetic and optical properties of Cu-doped ZnO nanosheet: First-principles calculations

We performed first-principles calculations within density-functional theory to study the magnetic and optical properties of Cu-doped ZnO nanosheet (NS). We found that Cu atom prefers to substitute for Zn site and can induce a local magnetic moment of 1.00 μ_B per unit in ZnO NS. When two Zn atoms are substituted by two Cu dopants, they tend to form a cluster and ferromagnetic (FM) ordering becomes energetically more favorable. In addition, localized states appear within the band gap due to the introduction of Cu dopant to ZnO NS. With increasing Cu concentrations, both the imaginary part of dielectric function and the absorption spectrum exhibit a red-shift behavior, which are in good agreement with the recent experimental results. The ferromagnetic coupling can be attributed to the p–d hybridization mechanism. The intriguing properties of Cu-doped ZnO NS may be promising for designing novel multifunctional nanodevice.     

Electronic and optical properties of V doped AlN nanosheet: DFT calculations

Electronic and optical properties of pure and V-doped AlN nanosheet have been investigated using density functional theory, and the dielectric tensor is calculated using the random phase approximation (RPA). The results of structural calculations show that the V atoms tend to replace instead of aluminum atoms with the lowest formation energy. In addition, study of the electronic properties shows that pure AlN nanosheet is a p-type semiconductor that by increasing one V atom, it possesses the metallic properties and magnetic moment becomes Zero. Moreover, by replacing two V atoms, the half-metallic behavior with 100% spin polarization can be found, and each supercell gains a net magnetic moment of 3.99 µ_B. Optical properties like the dielectric function, the energy loss function, the absorption coefficients, the refractive index are calculated for both parallel and perpendicular electric field polarizations, and the results show that the optical spectra are anisotropic.     

Quasiparticle band structure and optical properties of NH3BH3

The quasiparticle band structure of the low temperature orthorhombic phase of NH₃BH₃ is studied by using the GW approximation. It is found that NH₃BH₃ is an insulator with a value of the band gap of 5.90 eV with GGA and of 9.60 eV with the GW approximation. Then, the optical properties of NH₃BH₃ are obtained by the calculation of the dielectric function, corrected by a scissor shift operation corresponding to the GW correction on the band gap. Also, the optical anisotropy in NH₃BH₃ is analyzed through the refractive index and static dielectric constants along the different crystallographic directions. Finally, it is found that the energy loss function has a prominent peak at 22.26 eV; at these frequencies (above 22.26 eV) NH₃BH₃ becomes transparent. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystalline perfection, optical and dielectric studies on l-histidine nitrate: A nonlinear optical material

Single crystals of l-histidine nitrate (LHN), a recently investigated nonlinear optical material, were grown by conventional solution technique. Crystal structure and vibrational modes of the grown crystals were confirmed by powder X-ray diffractometry and FT-Raman spectrometry, respectively. Crystalline perfection of the grown crystals was evaluated by employing an in-house developed high-resolution X-ray diffractometer (HRXRD) and it was found that the grown crystals were free from structural grain boundaries and the perfection was reasonably good. However, HRXRD could reveal the fact that the crystals contain predominantly the interstitial point defects. The birefringence was measured over a range of wavelength between 5480 and 5630 Å and it was found that its value is nearly constant and 10 times higher than that of KDP. The optical band gap was found to be ∼3.73 eV. The photoluminescence excitation and emission spectra for single crystals were recorded. The SHG efficiencies of LHN samples of different particle sizes were measured by the Kurtz and Perry technique and they removed the ambiguity in the values reported differently in the literature. Dielectric properties were studied as a function of temperature over a wide range of frequency. The optical and dielectric studies along with the crystalline perfection reveal that the LHN crystal could be a good candidate for nonlinear optical devices.     

高压下c-BN的力学、电子结构及光学性质研究

为了研究不同压力下c-BN的力学性质、电子结构以及光学性质的变化,基于密度泛函理论构建了不同压力下c-BN的晶体模型。发现c-BN在不同压力下均力学稳定,随着压力增加,c-BN的弹性常数逐渐增大,材料的可压缩性逐渐变差;c-BN在零压下的禁带宽度为4.367 eV,表明c-BN是间接宽带隙半导体,随着压力增加,c-BN的禁带宽度逐渐增大,态密度谱图变化不明显;对Milliken 布居分布在不同压力下进行分析,表明随着压力增加,B、N原子杂化后形成的B-N共价性增强。对c-BN的复介电函数、折射率、反射率等进行分析,发现随着压力增大,它们都产生一定蓝移,且在整个可见光谱范围以及红外与紫外（约从204nm开始）光谱的很大范围内都透明。研究结果对高压下c-BN的应用有一定参考价值。     

Effects of aluminum vacancies on electronic structure and optical properties of Ta4AlC3: A first principles study

We investigated the effect of aluminum vacancies (V_Al) on the structural, electronic and optical properties of Ta₄Al_1−xC₃ (x=0, 0.25, 0.5, 0.75) based on the first-principle calculation using density functional theory. We found that the lattice constant a remains almost unchanged with the variation of V_Al concentration, while c and c/a ratio decrease with increasing V_Al concentration. Moreover V_Al induced local distortions have significant influence on the electronic and optical properties of Ta₄AlC₃, especially beyond the critical V_Al concentration (x=0.5). On the other hand, the presence of V_Al can improve the dielectric properties of Ta₄AlC₃. From the optical properties analysis, we predicted that Ta₄Al_1−xC₃ is not suitable as a coating material to avoid solar heating.     

Structures and electronic properties of stoichiometric hydrogenated aluminum clusters

The lowest-energy geometries and electronic-structure properties have been obtained for Al_nH_n (n=1-10) clusters within the density-functional theory using the generalized gradient approximation for the exchange correlation potential. The resulting geometries show that the hydrogen atoms tend to occupy outside positions and no hollow positions are found. The subunit Al_n of Al_nH_n (n=1-5) have little distortion, in comparison with corresponding pure Al_n cluster, whereas the subunit Al_n have large distortion from n=6. The stability has been investigated by analyzing the binding energy per atom and the second difference in energy, indicating that Al₈H₈ exhibit higher stability than others. The bonding property has been analyzed by calculating the Mulliken charges and Al–H distances. The calculated energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO), the vertical ionization potential, and the vertical electron affinity also confirm that Al₈H₈ is a stable cluster. The density of states (DOS) shows that Al_nH_n exhibit changes from molecular-like (Al₁H₁) to band-like structure (Al₁₀H₁₀) as n increases.     

Calculated structural, electronic and optical properties of Ga-based semiconductors under pressure

The structural, electronic and optical properties of GaP, GaAs and GaSb at ambient and under hydrostatic pressure have been calculated using the full potential linear augmented plane wave (FP-LAPW) method. The calculated lattice constant, bulk modulus and its pressure derivative are compared with available experimental data. The first and second order pressure coefficients for the (Γ-Γ) energy gaps and hydrostatic deformation potential shows agreement with measurement. The linear pressure coefficients of the transition (Γ-Γ) increases significantly as anion atomic number increases (GaP→GaAs→GaSb). The magnitude of linear pressure coefficient of the transition (Γ-X) are small and usually negative. The variation of linear pressure coefficient of the transition (Γ-L) are relatively small and follow similar trend as (Γ-Γ). Overall the calculated linear and nonlinear pressure coefficients show good agreement with the experimental data. The obtained dielectric function, refractive index, extinction coefficient and reflectivity are compared with measured data and show qualitatively good agreement.     

高压下LiNbO3晶体电子结构与光学性质的第一性原理研究

利用基于密度泛函理论的第一性原理超软赝势平面方法研究了外界压强对LiNbO3晶体波态密度,能带结构,电荷密度以及光学性质的影响.能带结构计算表明,价带顶主要由O-2p和Nb-4d态电子贡献,导带底主要由Nb-4d态电子贡献,且带隙随着压强的增加而线性增大.利用复介电函数计算了LiNbO3晶体在不同压强下光学性质的折射率、反射率、吸收系数,能量损失函数以及光电导率. 研究发现：外界压强大于10Gpa时,静态折射率保持不变,随外界压强的增加,反射率、吸收函数以及光电导率区间有一定程度的拓宽,损失函数峰发生“蓝移”.研究表明,高压可以有效调控LiNbO3晶体的电子结构和光学性质,为LiNbO3晶体的高压应用提供了有益的理论依据.     

高压下LiNbO_3晶体电子结构与光学性质的第一性原理研究

利用基于密度泛函理论的第一性原理超软赝势平面方法研究了外界压强对LiNbO_3晶体态密度,能带结构,电荷密度以及光学性质的影响.能带结构计算表明,价带顶主要由O-2p和Nb-4d态电子贡献,导带底主要由Nb-4d态电子贡献,且带隙随着压强的增加而线性增大.利用复介电函数计算了LiNbO_3晶体在不同压强下光学性质的折射率、反射率、吸收函数,能量损失函数以及光电导率.研究发现:外界压强大于10GPa时,静态折射率保持不变,随外界压强的增加,反射率、吸收函数以及光电导率区间有一定程度的拓宽,损失函数峰发生"蓝移".研究表明,外界高压可以有效调控LiNbO_3晶体的电子结构和光学性质,为LiNbO_3晶体的高压应用提供了有益的理论依据.     

热效应对tP10-FeB4的电子结构与光学性质的影响

本文采用密度泛函理论,深入研究了温度对tP10-FeB4的电子结构和吸收系数、反射率等光学性质并得出了如下几点结论： tP10-FeB4的价带顶部和导带底部主要由Fe的3d和B的2p轨道构成。随着温度的升高,带隙略微减小, 其吸收谱和反射谱出现红移,这与带隙的变化规律一致。tP10-FeB4在紫外区间有良好的吸收谱,是潜在的紫外吸收材料。     

立方KCaF3电子、弹性和光学性质的第一性原理研究

基于密度泛函理论和赝势平面波近似法计算研究了立方钙钛矿KCaF3的弹性、电子和光学性质。基态时,KCaF3平衡晶格常数、体积弹性模量和其他计实验和算值一致。根据Hooke定律和Christoffel方程,研究了KCaF3弹性常数Cij、体积弹性模量B、各向同性波速和弹性各向性异性因子随压力的变化关系。从电子能带理论出发,计算得到了KCaF3电子能带、态密度和Milliken电荷布居数,并对其电子性质进行了详细分析。结果显示：立方钙钛矿KCaF3为直接带隙绝缘体材料,其禁带宽度为6.22eV;电荷主要从Ca和K原子向F原子转移;立方钙钛矿KCaF3属于纯粹的共价型化合物。同时,本文还计算研究了KCaF3的光学介电函数、吸收系数、复折射率、能量损失谱和反射系数等光学性质。     

立方KCaF_3电子、弹性和光学性质的第一性原理研究

基于密度泛函理论和赝势平面波近似法计算研究了立方钙钛矿KCaF_3的弹性、电子和光学性质.基态时,KCaF_3平衡晶格常数、体积弹性模量和实验及其他计算值一致.根据Hooke定律和Christoffel方程,研究了KCaF_3弹性常数Cij、体积弹性模量B、各向同性波速和弹性各向性异性因子随压力的变化关系.从电子能带理论出发,计算得到了KCaF_3电子能带、态密度和Milliken电荷布居数,并对其电子性质进行了详细分析.结果显示:立方钙钛矿KCaF_3为直接带隙绝缘体材料,其禁带宽度为6.22 e V;电荷主要从Ca和K原子向F原子转移;立方钙钛矿KCaF_3属于纯粹的共价型化合物.同时,本文还计算研究了KCaF_3的光学介电函数、吸收系数、复折射率、能量损失谱和反射系数等光学性质.     

Vacancy-defect effect on the electronic and optical properties of Pmm2 BC2N: A first-principles study

Orthorhombic-Pmm2-BC₂N as a superhard photocatalyst simulates great interests in the researches of materials-design and application. To promote the studies of Pmm2 BC₂N as a multifunctional material with both great hardness and good optical properties, we investigated the electronic and optical properties of Pmm2 BC₂N with various vacancy-defects by the systematic first-principles density functional theory (DFT) calculations in this work. The absorption, refractivity, reflectivity, and photoconductivity of considered structures were calculated and explored. The various characteristics of the optical properties were analyzed based on relative computed density of states (DOS).     

Ab-initio study of electronic and optical properties of InN in wurtzite and cubic phases

We have performed systematic first principle calculations for the electronic and optical properties of a narrow band gap semiconductor InN in cubic and wurtzite phases by ‘state-of-the-art’ DFT calculations within generalized gradient approximation (GGA) and Engel-Vosko's corrected generalized gradient approximation (EVGGA) using full potential linear augmented plane wave (FPLAPW) method as implemented in WIEN2k code. The total energy for the wurtzite phase of InN was found to be smaller by 0.0184 Ry/molecule by cubic phase which confirms the greater stability of the wurtzite structure than the cubic one. Band structure, effective masses, density of states, valence charge densities, and dielectric functions are computed and presented in detail. The critical points are extracted out of calculated dielectric function, compared with available measured data and are explained in terms of transitions occurred in the band structure along different symmetry and antisymmetry lines. The valence band maxima and conduction band minima are strongly dominated by N-2p states and located at the Γ-symmetrical line which predicts its direct band gap nature in both phases.     

立方钙钛矿RbZnF3电子、弹性和光学性质的第一性原理研究

基于密度泛函理论和赝势平面波方法研究了立方钙钛矿RbZnF3的电子结构和光学性质;利用静水有限应变技术计算研究了RbZnF3弹性常数Cij、体积弹性模量B和剪切模量G随压力的变化关系。基态下,RbZnF3晶格常数a和体积弹性模量B0计算值与实验值以及其他理论值一致。根据能带结构、总态密度以及分波态密度分析可知：基态下立方钙钛矿结构RbZnF3为间接带隙半导体材料,带隙为3.57eV,与其他计算结果比较,本文计算结果偏低,这是由于局域密度近似(LAD)或广义梯度近似(GGA)交换关联函数的局限性所致。基态下RbZnF3的Mulliken电荷分布和集居数说明：RbZnF3属于共价键和离子键所形成的混合键化合物;RbZnF3的电荷总数主要来源于Rb 4s和4p轨道,Zn 3d轨道,以及F 2s和2p轨道。电荷主要从Rb, Zn原子向F原子转移。同时,本文还计算研究了RbZnF3的光学介电函数、吸收系数、复折射率、能量损失谱和反射系数等光学性质。     

Effect of La doping on the electronic structure and optical properties of ZnO

The electronic structure and optical properties of ZnO doped with La have been investigated using density functional theory based on first-principles ultrasoft pseudopotential method. The calculated results show that the La doping increases the bandgap of ZnO, in agreement with the experimental results; while the Fermi level shifts into the conduction band, revealing the so-called Burstein-Moss effect. In comparison to pure ZnO, a new peak appears in the imaginary part of dielectric function in the system doped with La and the optical absorption edge has been obviously changed. Moreover, the covalent property of Zn_1−xLa_xO is found to weaken with the increase of La concentration.     

应力效应对tP10-FeB_4的电子结构与光学性质的影响

由于具有高强度、超硬度、良好的热稳定性和半导体性质等特点,tP10-FeB_4可以在高温、高压的条件下使用,所以本文采用密度泛函理论,深入研究高压下tP10-FeB_4的电子结构和吸收系数、反射率、折射率等光学性质并得出了如下几点结论:随着压强的增大,电子的重叠增大,进而导致轨道杂化、能带宽度变宽,压缩带隙. tP10-FeB_4的价带顶部和导带底部主要由Fe的3d、B的2p和2s轨道构成. tP10-FeB_4在紫外区间有良好的吸收谱,是潜在的紫外吸收材料.而在可见光区吸收较小且对可见光的折射率比较大,因此可以考虑用于制作光导纤维.     

First-principles study on electronic and optical properties of S,N single-doped and S-N co-doped ZnO

The electronic and optical properties of undoped, N single-doped, S single-doped, and S-N co-doped ZnO were systematically investigated by first-principles calculations. The lattice parameters of S single-doped and S-N co-doped ZnO clearly increased. After N-doping, a strongly localized impurity energy level of N was formed near the Fermi level at the top of valence band (VB). In S-N co-doped ZnO, the localization of N weakened, and the Fermi level went deeper into the VB, indicating that the acceptor energy level of N formed in S-N co-doped system became shallower due to the effect of 3p state of S. Therefore, S-N co-doping is beneficial to obtain p-type ZnO with a higher hole concentration than N single-doping. Compared with undoped ZnO, the static dielectric constant, absorption coefficient, refractive index, energy loss function, and reflectivity of N single-doped, S single-doped, and S-N co-doped ZnO exhibited an increase in low-energy area.