Electronic structure and optical properties of a new type of semiconductor material:graphene monoxide

The electronic and optical properties of graphene monoxide,a new type of semiconductor material,are theoretically studied by first-principles density functional theory.The calculated band structure shows that graphene monoxide is a semiconductor with a direct band gap of 0.95 eV.The density of states of graphene monoxide and the partial density of states for C and O are given to understand the electronic structure.In addition,we calculate the optical properties of graphene monoxide,including the complex dielectric function,absorption coefficient, complex refractive index,loss-function,reflectivity and conductivity.These results provide a physical basis for potential application in optoelectronic devices.     

Study of optical and dielectric properties of annealed ZnO nanoparticles in the terahertz regime

The frequency-dependent optical and dielectric properties of annealed ZnO nanoparticles in the range of 0.1 to 0.9 THz are studied by using terahertz time-domain spectroscopy（THz-TDS）.The refractive index,power absorption and complex dielectric constants are obtained and the experimental results are well fit with a simple effective medium theory in conjunction with a pseudo-harmonic model.This study reveals that annealed ZnO nanoparticles exhibit the similar phonon response characteristics to the single ZnO crystal and other ZnO nanostructures,such as tetrapods and nanowires.     

First-principles study of the electronic structures and optical properties of C-F-Be doped wurtzite ZnO

正The electronic structure and optical properties of pure,C-doped,C-F codoped and C-F-Be clusterdoped ZnO with a wurtzite structure were calculated by using the density functional theory with the plane-wave ultrasoft pseudopotentials method.The results indicate that p-type ZnO can be obtained by C incorporation,and the energy level of C_O above the valence band maximum is 0.36 eV.The ionization energy of the complex Zn_(16)O_(14)CF and Zn_(15)BeO_(14)CF can be reduced to 0.23 and 0.21 eV,individually.These results suggest that the defect complex of Zn_(15)BeO_(14)CF is a better candidate for p-type ZnO.To make the optical properties clear,we investigated the imaginary part of the complex dielectric function of undoped and C-F-Be doped ZnO.We found that there is strong absorption in the energy region lower than 2.7 eV for the C-F-Be doped system compared to pure ZnO.     

Preparation and photodetection performance of high crystalline quality and large size β-Ga2O3 microwires

Ultrawide band gap semiconductors are promising solar-blind ultraviolet(UV) photodetector materials due to their suitable bandgap, strong absorption and high sensitivity. Here, β-Ga₂O₃ microwires with high crystal quality and large size were grown by the chemical vapor deposition(CVD) method. The microwires reach up to 1 cm in length and were single crystalline with low defect density. Owing to its high crystal quality, a metal–semiconductor–metal photodetector fabricated f...     

Structural and opto-electrical properties of pyrolized ZnO-CdO crystalline thin films

A series of ZnO-CdO thin films of different molar ratios of Zn and Cd have been deposited on glass substrate at substrate temperature ~ 360 ℃ by the spray pyrolysis technique at an ambient atmosphere. X-ray diffraction (XRD) studies confirmed the polycrystalline nature of the film and modulated crystal structures of wurtzite (ZnO) and cubic (CdO) are formed. The evaluated lattice parameters, and crystallite size are consistent with literature. Dislocation density and strain increased in the film as the grain sizes of ZnO and CdO are decreased. The band gap energy varies from 3.20 to 2.21 eV depending on the Zn/Cd ratios in the film. An incident photon intensity dependent I-V study confirmed that the films are highly photosensitive. Current increased with the increase of the intensity of the light beam. The optical conductivity and the optical constants, such as extinction coefficient, refractive index and complex dielectric constants are evaluated from transmittance and reflectance spectra of the films and these parameters are found to be sensitive to photon energy and displayed intermediate optical properties between ZnO and CdO, making it preferable for applications as the buffer and window layers in solar cells.     

Effects of negative index medium defect layers on the transmission properties of one-dimensional photonic crystal

The photonic band gap structure of 1D photonic crystal with a negative index medium defect layer is studied by using the transfer matrix method. Investigations show that the introdution of negative index medium defect layer and the increase of the negative index value will result in an extension of the band gap. Moreover, by increasing the negative index, the width of defect layer and the numbers of period photonic crystal, the width of defect modes will be narrowed, which is advantaged to obtain optical filters with narrow band. Finally, the effects of absorption on the properties of band gap and on defect modes have been discussed.     

Electronic structure and optical properties of F-doped β-Ga2O3 from first principles calculations

The effects of F-doping concentration on geometric structure, electronic structure and optical property of β-Ga₂O₃ were investigated. All F-doped β-Ga₂O₃ with different concentrations are easy to be formed under Ga-rich conditions, the stability and lattice parameters increase with the F-doping concentration. F-doped β-Ga₂O₃ materials display characteristics of the n-type semiconductor, occupied states contributed from Ga 4s, Ga 4p and O 2p states in the conduction band increase with an increase in F-doping concentration. The increase of F concentration leads to the narrowing of the band gap and the broadening of the occupied states. F-doped β-Ga₂O₃ exhibits the sharp band edge absorption and a broad absorption band. Absorption edges are blue-shifted, and the intensity of broad band absorption has been enhanced with respect to the fluorine content. The broad band absorption is ascribed to the intra-band transitions from occupied states to empty states in the conduction band.     

First principle studies of structural, elastic, electronic and optical properties of Zn-chalcogenides under pressureMuhammad Bilala, M. Shafiqa, Iftikhar Ahmada, Imad Khana,*

Structural, elastic, electronic and optical properties ofzinc-chalcogenides （viz. ZnX, X = S, Se and Te） are studied in zinc-blende structure under hydrostatic pressure using the full-potential linearized augmented plane wave method. Generalized gradient approximation is used for exchange correlation potentials. Pressure-dependent lattice constants and bulk moduli are obtained using the optimization method. Young＇s modulus, Poisson＇s ratio, internal strain parameter and anisotropy are also calculated. The higher values of Young＇s modulus in comparison to the bulk modulus show that these materials are hard to break. Poisson＇s ratio is computed for the first time for these materials to the best of our knowledge and its values show higher ionic contribution in these materials. Modified Becke and Johnson （mBJ） method is used to study band gaps, density of states, dielectric function and refractive index. Electronic study shows direct band gaps convert to indirect band gaps with increasing pressure in the case of ZnS and ZnTe. We compared our results with other theoretical and experimental results. Our results are far better than other theoretical results because mBJ is the best technique to treat Ⅱ-Ⅵ semiconductors.     

Dielectric confinement on exciton binding energy and nonlinear optical properties in a strained Zn1-xinMgx（in）Se/Zn1-x（out）Mgx（out）Se quantum well

正The band offsets for a Zn_(1-Xin)Mg_(Xin)Se/Zn_(1-Xout)Mg_(1-Xout)Se quantum well heterostructure are determined using the model solid theory.The heavy hole exciton binding energies are investigated with various Mg alloy contents.The effect of mismatch between the dielectric constants between the well and the barrier is taken into account.The dependence of the excitonic transition energies on the geometrical confinement and the Mg alloy is discussed.Non-linear optical properties are determined using the compact density matrix approach.The linear,third order non-linear optical absorption coefficient values and the refractive index changes of the exciton are calculated for different concentrations of magnesium.The results show that the occurred blue shifts of the resonant peak due to the Mg incorporation give the information about the variation of two energy levels in the quantum well width.     

The strong absorption characteristics of ternary one-dimensional photonic crystals with silver layer

In order to achieve broadband and efficient optical absorption, the multiple silver nanolayer was introduced into the photonic crystals to form a one-dimensional ternary periodic symmetric structure. The effects of thickness of each layer on the band range, absorption bandwidth, absorbance and absorption energy field distribution of the solar spectrum high absorption band were studied by the transfer matrix method. The absorption band with wavelength range from 724 nm to 1 188 nm, spectral width of 464 nm, and average absorbance of 0.78 was obtained by structural adjustment. The absorbed energy is mainly distributed in the first half of the symmetrical structure of the photonic crystal. When the thickness of the silver layer decreased from 30 nm to 15 nm, the local energy in each period increased significantly. At the same time, the distribution and transfer of energy in silicon and MgF2 layers can be controlled. The results of this paper can be used to improve the absorption of solar radiation, and provide an important basis for the design of photonic crystal and their application in solar energy utilization.     

一种新型半导体材料的电子结构和光学特性：石墨烯一氧化物：

本文首次基于密度泛函理论研究了石墨烯一氧化物一种新型半导体材料的电子结构和光学特性。计算表明石墨烯一氧化物是直接带隙为0.95ev的半导体。通过绘制态密度和分波态密度研究了该材料的能带结构。除此之外,本文给出了该材料的光学特性,这对于材料在光电子器件的潜在应用有着一定的意义。     

四方晶相HfO2电子结构和光学性质研究

采用密度泛函理论(DFT)框架下的局域密度近似(LDA),计算了四方HfO2晶体的电子结构,包括能带结构和态密度.在此基础上计算了四方Hf02晶体的光学线性响应函数,包括复介电函数、吸收光谱、复折射率和光电导谱.通过比较发现,计算结果与实验结果吻合较好,说明采用密度泛函理论的局域密度近似来计算HfO2材料的光学性质是比较可靠的.     

外压调制下ZnO晶体结构与光学性质变化特性的研究

为了研究外压调制对半导体材料ZnO晶体结构和光学性质的影响，利用基于局域密度泛函理论框架下的广义梯度近似平面波超软赝势方法，采用第一性原理对不同外压条件下ZnO晶体的晶格常数、介电函数、复数折射率、吸收系数、反射率等的变化特性进行了模拟分析。计算结果表明：随着压力的增大，晶格常数、晶胞体积缓慢变小，内坐标u值逐渐增大，Zn-O键长缩短，共价性增强，带隙Eg明显展宽。光谱曲线图显示压力对低能段光学性质的影响并不明显，而在高能段，随着压力的增大其光谱发生明显的蓝移。     

Improved Calculation of the Electronic and Optical Properties of Tetragonal Barium Titanate

The electronic and optical properties of tetragonal barium titanate (BaTiO₃) have been investigated by use of first-principles density functional theory on a plane wave basis, by use of norm-conserving pseudopotentials in the localized density approximation. For accuracy, experimental lattice parameters reported in the literature were used. The band structure, total and partial density of states (DOS), and Born effective charges of tetragonal BaTiO₃ were obtained from first-principles calculations. The partial DOS and Born effective charges indicate that the Ti–O bonds are partially covalent and the Ba–O bonds are partially ionic. The complex dielectric functions for ordinary and extraordinary optical polarization were calculated, and were in reasonable agreement with those from previous first-principles calculations. The refractive index, extinction coefficient, reflectivity, and energy-loss spectrum for both types pf polarization were calculated from the complex dielectric function; the results obtained were an improvement on other published results.     

一维光子晶体的全向反射特性

利用传输矩阵方法研究了一维光子晶体的全向反射特性.研究表明,光子晶体的结构参数对全向反射带的产生影响很大.要提高光子晶体的全向反射带带宽,必须增大两种介质的折射率及它们的高低折射率比值,同时使低高折射率层的光学厚度比接近0.85.研究结果对全介质反射器的设计具有指导意义.     

氧化镁单晶在太赫兹波段的介电特性

利用太赫兹时域光谱系统,在0.5~9.5 THz范围内对氧化镁单晶基片的介电特性进行了研究,并获得折射率、吸收系数以及复介电函数信息。实验数据表明,在低频（ 2 THz）范围内,氧化镁单晶透过性较好,折射率在3.12~3.15之间。折射率和吸收系数均随频率增加而增大,且在3.16 THz和8.11 THz两处存在明显的吸收峰。通过经典的赝简谐振动理论很好地拟合了实验结果,分析了晶体中的横向光学声子振动模式,为氧化镁单晶在宽带太赫兹波段的应用提供了有益参考。     

入射角和杂质吸收对一维光子晶体反射镜的影响

利用复折射率的方法和膜系设计软件TFcalc分别研究了入射角和杂质吸收对一维光子晶体反射镜反射谱和透射谱的影响.结果表明:随着入射角的增大,一维光子晶体反射镜的禁带中心位置蓝移,禁带宽度减小.入射角小于60°时,带隙势阱深度几乎不变,大于60°后带隙势阱深度变化较大.当入射角无限接近90°时,P偏振光的带隙几乎消失,S偏振光的带隙几乎保留.杂质吸收对于一维光子晶体的反射谱和透射谱显著影响时的临界消光系数值分别是0.001和0.0003.高折射率介质层的杂质吸收对光谱的影响较小.     

A first-principles comparative study of exchange and correlation potentials for ZnO

Selection of a proper exchange and correlation potential for efficient treatment of excited-state properties such as the electronic band structure is required in density functional theory (DFT). We propose a simple and efficient Engel–Vosko generalized gradient approximation (GGA-EV) for calculating the electronic and optical properties of different phases of ZnO, namely, wurtzite, rock salt, zincblende and the CsCl-type structure. To validate our approach, we compare the results to those obtained using the local density approximation (LDA-PW91) and the parameterized generalized gradient approximation of Perdew et al. (GGA-PBE). We calculated the band structure, density of states, crystal field splitting energy, dielectric function, reflectivity, and absorption coefficient. GGA-EV yielded a wider valence band and narrower d-band in comparison to LDA-PW91 and GGA-PBE. Moreover, GGA-EV resulted in greater separation between Zn d and O p states, which reduced pd repulsion and consequently improved the energy band gap.     

Electronic structure,effective mass,and optical dispersion of 2-mercapto-5-methyl-1,3,4-thiadiazole: density functional theory calculations

Density functional theory was used to calculate the electronic band structure, effective mass, and optical dispersion of 2-mercapto-5-methyl-1,3,4-thiadiazole (MMTD). The all-electron, full-potential, linearized augmented plane wave method was used. The exchange correlation potential was treated using the local density approximation, generalized gradient approximation, and modified Becke Johnson approximation. The calculated band structure shows that MMTD has a direct energy bandgap. The partial density of states revealed strong hybridization between N p, C p, N s, and H s orbitals. The electronic charge density distribution confirmed partial ionic and strong covalent C–N, C–C, C–H, and N–N bonds. We also calculated the optical dielectric function and related optical properties (refractive index, extinction coefficient, absorption coefficient, and reflectivity).     

An ab initio study of the structural,elastic, electronic,optical properties and phonons of the double perovskite oxides Sr2AlXO6 (X=Ta,Nb, V)

We report ab initio density functional theory calculations of the structural, elastic, electronic and optical properties of the double perovskite oxides Sr₂AlXO₆ (X=Ta, Nb, V). We have predicted a direct Г–Г band gap in Sr₂AlXO₆ (X=Ta, Nb) and an indirect Г–X band gap for Sr₂AlVO₆. The fundamental band gap increases linearly when the pressure is enhanced in the range 0–20 GPa. The frequency dependent of complex dielectric function, absorption, reflectivity and electron energy loss function were investigated in the range 0–40 eV. Features such as lattice constant, bulk modulus, elastic constants, band structure, total and local densities of states have been computed.