Electronic structure and optical properties of ABP2O7 double phosphates

Luminescence and luminescence excitation under VUV radiation of ${\mathrm{ABP}}_2{\mathrm{O}}_7$ ($\mathrm{A}=\mathrm{Na}$, K, Cs; $\mathrm{B}=\mathrm{Al}$, In) double phosphates are studied. Two emission bands peaking near 330 and 420 nm are common for investigated ${\mathrm{ABP}}_2{\mathrm{O}}_7$ crystals. The band structure and partial densities of electronic states of perfect ${\mathrm{KAlP}}_2{\mathrm{O}}_7$, ${\mathrm{LiInP}}_2{\mathrm{O}}_7$ and ${\mathrm{NaTiP}}_2{\mathrm{O}}_7$ crystals are calculated by the full-potential linear-augmented-plane-wave (FLAPW) method. It is found that the structures of the conduction bands of ${\mathrm{ABP}}_2{\mathrm{O}}_7$ crystals, which have different B cations, are appreciably different. Experimental results are compared with results of calculations of the electronic structure. Assumptions concerning the origin of luminescence in double phosphates are made.     

过渡金属掺杂钛酸纳米管的电子结构和光学性质研究

采用平面波超软赝势方法计算了过渡金属(TM)(Fe,Co或者Ru)掺杂钛酸纳米管的电子结构及光学性质.对TM取代钛酸纳米管层间间隙位H+的几何结构进行优化,发现掺杂对几何结构的影响较大,其中Co或者Ru掺杂的形成能均较低.此外,掺杂的TM与周围的O原子成键,有形成固熔体的趋势.掺杂后的能带结构分析表明:Fe,Co或者Ru掺杂导致钛酸纳米管禁带宽度减小并且于禁带中引入了新的能级,这主要归因于b1g(dx2-y2)及a1g(dz2)态的出现;部分杂质能级处于半填充状态,成为空穴的俘获中心,减少电子和空穴的复合;掺杂后,价带顶向低能方向移动,使价带中形成的空穴氧化性更强.最后,掺杂的钛酸盐纳米管的吸收光谱显示,Ru掺杂的钛酸纳米管导致其在可见光范围内有更强的吸收.     

Electronic structure of semiconducting nanotubes adsorbed on metal surfaces

A total-energy electronic-structure calculation is performed to explore energetics and electronic structures of nanotubes adsorbed on metal surfaces. We find that the charge transfer from metal surfaces to the nanotubes takes place depending on both the electronic structures of the adsorbed nanotubes and the work functions of the metal surfaces. In addition, we also find a substantial hybridization between the electron states of metal atoms and those of the nanotubes, which results in the metal-induced inhomogeneous charge distribution in the nanotubes.     

Electronic and optical properties of Au-doped Cu_2O:A first principles investigation

The Cu2O and Au-doped Cu2O films are prepared on MgO(001) substrates by pulsed laser deposition. The X-ray photoelectron spectroscopy proves that the films are of Au-doped Cu2O. The optical absorption edge decreases by 1.6%after Au doping. The electronic and optical properties of pure and Au-doped cuprite Cu2O films are investigated by the first principles. The calculated results indicate that Cu2O is a direct band-gap semiconductor. The scissors operation of 1.64 eV has been carried out. After correcting, the band gaps for pure and Au doped Cu2O are about 2.17 eV and2.02 eV, respectively, decreasing by 6.9%. All of the optical spectra are closely related to the dielectric function. The optical spectrum red shift corresponding to the decreasing of the band gap, and the additional absorption, are observed in the visible region for Au doped Cu2O film. The experimental results are generally in agreement with the calculated results.These results indicate that Au doping could become one of the more important factors influencing the photovoltaic activity of Cu2O film.     

Electronic and optical properties of Cu, CuO and Cu2O studied by electron spectroscopy

The electronic and optical properties of Cu, CuO and Cu(2)O were studied by x-ray photoelectron spectroscopy (XPS) and reflection electron energy-loss spectroscopy (REELS). We report detailed Cu 2p, Cu LVV, O 1s and O KLL spectra which are in good agreement with previous results. REELS spectra, recorded for primary energies in the range from 150 to 2000 eV, were corrected for multiple inelastically scattered electrons to determine the effective inelastic scattering cross section. The dielectric functions and optical properties were determined by comparing the experimental inelastic electron scattering cross section with a simulated cross section calculated within the semi-classical dielectric response model in which the only input is Im(-1/ε) by using the QUEELS-ε(k,ω)-REELS software package. By Kramers-Kronig transformation of the determined Im(-1/ε), the real and imaginary parts (ε(1) and ε(2)) of the dielectric function, and the refractive index n and extinction coefficient k were determined for Cu, CuO, and Cu(2)O in the 0-100 eV energy range. Observed differences between Cu, CuO and Cu(2)O are mainly due to modifications of the 3d and O 2p electron configurations.     

Electronic and optical properties of ceramic Sc2O3 and Y2O3: Compton spectroscopy and first principles calculations

We present the first-ever experimental Compton profiles (CPs) of Sc₂O₃ and Y₂O₃ using 740 GBq ¹³⁷Cs Compton spectrometer. The experimental momentum densities have been compared with the theoretical CPs computed using linear combination of atomic orbitals (LCAO) within density functional theory (DFT). Further, the energy bands, density of states (DOS) and Mulliken's population (MP) data have been calculated using LCAO method with different exchange and correlation approximations. In addition, the energy bands, DOS, valence charge density (VCD), dielectric function, absorption coefficient and refractive index have also been computed using full potential linearized augmented plane wave (FP-LAPW) method with revised functional of Perdew–Becke–Ernzerhof for solids (PBEsol) and modified Becke Johnson (mBJ) approximations. Both the ab-initio calculations predict wide band gaps in Sc₂O₃ and Y₂O₃. The band gaps deduced from FP-LAPW (with mBJ) are found to be close to available experimental data. The VCD and MP data show more ionic character of Sc₂O₃ than Y₂O₃. The ceramic properties of both the sesquioxides are explained in terms of their electronic and optical properties.     

Al掺杂SnO2 材料电子结构和光学性质

采用基于第一性原理的线性缀加平面波方法(FP-LAPW),研究Al掺杂SnO₂材料Sn_1-xAl_xO₂ (x= 0,0.0625,0.125,0.1875,0.25)的电子结 构和光学性质,包括能带结构、电子态密度、介电函数和其他一些光学性质.计算结果表明,掺杂Al之后价带上部分折叠态增加,价带宽度发生收缩,对导带底起作用的Sn 5s态减少,使得带隙增宽,且态密度整体向高能方向发生移动.随着Al掺杂量的增加带隙越来越宽,Al杂质能级在导带部分与Sn 5p态电子相互作用逐渐增强,虚部谱中的第一介电峰的强度随掺杂Al浓度增大而减弱.同时,吸收谱及其他光学谱线与介电函数虚部谱线相对应,各谱线均发生蓝移现象,对应带隙增宽,从理论上指出了光学性质与电子结构之间的内在关系. 关键词： 能带结构 态密度 光学性质 介电函数     

萤石结构TiO2的电子结构和光学性质

利用第一性原理计算了立方相萤石TiO2的晶胞参数,能带结构和电子态密度.结果显示萤石TiO2属于间接带隙半导体材料,其间接禁带宽度(Γ→X)Eg为2.07 eV,比常见的金红石和锐钛矿TiO2的禁带宽度窄.为了更清楚地了解萤石的光学性质,利用Kramers-Kronig色散关系,分别对萤石和金红石TiO2的复介电常数、吸收率等参数进行了计算,并将二者结果做了比较.其中萤石TiO2的静介电常数为8.31.金红石TiO2的静介电常数表现为各向异性ε1xy(0)=6.01和ε1z(0)=7.07,该计算结果与实验值一致.吸收光谱的对比结果显示萤石结构在51nm和153 nm处增加了新的吸收峰,并且吸收光谱范围已扩大到了可见光区.     

Electronic properties of oxidized carbon nanotubes

The effect of oxygenation on the electronic properties of semiconducting carbon nanotubes is studied from first principles. The O2 is found to bind to a single-walled nanotube with an adsorption energy of about 0.25 eV and to dope semiconducting nanotubes with hole carriers. Weak hybridization between carbon and oxygen is predicted for the valence-band edge states. The calculated density of states shows that weak coupling leads to conducting states near the band gap. The oxygen-induced gap closing for large-diameter semiconducting tubes is discussed as well. The influence of oxygen on the magnetic property is also addressed through a spin-polarized calculation and compared to experiment.     

Electronic properties of double-layer carbon nanotubes

The electronic spectra for double-wall zigzag and armchair nanotubes are found. The influence of nanotube curvatures on the electronic spectra is also calculated. Our finding that the outer shell is hole doped by the inner shell is in the difference between Fermi levels of individual shells which originate from the different hybridization of π orbital. The shift and rotation of the inner nanotube with respect to the outer nanotube are investigated. We found stable semimetal characteristics of the armchair DWNTs in regard of the shift and rotation of the inner nanotube. We predict the shift of k_F towards the bigger wave vectors with decreasing of the radius of the armchair nanotube.     

Formation mechanism of H2Ti3O7 nanotubes

Formation mechanism of H2Ti3O7 nanotubes by single-step reaction of crystalline TiO2 and NaOH has been investigated via transmission electron microscopy examinations of series specimens with different reaction times and extensive ab initio calculations. It was found that the growth mechanism includes several steps. Crystalline TiO2 reacts with NaOH, forming a highly disordered phase, which recrystallized into some H2Ti3O7 thin plates. H-deficiency on the top surface leads to an asymmetrical environment for the surface Ti3O2-7 layer. The calculations of the surface tension, elastic strain energy, interlayer coupling energy, and Coulomb force indicated that the asymmetrical environment is the principal driving force of the cleavage of the single sheets of H2Ti3O7 from the plates and the formation of the multiwall spiral nanotubes.     

CuInSe2电子结构与光学性质的第一性原理计算

从头计算了CuInSe2(CIS)体相的性质,参数设定和性质计算都基于密度泛函理论,交换相关能采用GGA,泛函形式为PBE,原子间相互作用的描述采用超软赝势.计算发现CIS中存在共价键,是一种非典型的离子型晶体,在整个晶体内存在共用电子对,Cu原子和Se原子的作用大于Se原子和In原子.CIS是一种典型的直接带隙半导体,计算得到了光学性质的各项参数,包括折射指数和反射率,吸收系数以及介电函数与光子能量的关系,发现CIS的主要光吸收峰有6个,分别为:3.1,7.6,10.0,16.1,19.0,21.0 eV,理论上最强吸收峰在紫外光区.     

Electronic structure and optical properties of RbPb2Br5

We report on density functional theory (DFT) calculations of the total and partial densities of states of rubidium dilead pentabromide, RbPb₂Br₅, employing the augmented plane wave+local orbitals (APW+lo) method as incorporated in the WIEN2k package. The calculations indicate that the Pb 6s and Br 4p states are the dominant contributors to the valence band: their main contributions are found to occur at the bottom and at the top of the band, respectively. Our calculations reveal that the bottom of the conduction band is formed predominantly from contributions of the unoccupied Pb 6p states. Data of total DOS derived in the present DFT calculations are found to be in agreement with the experimental X-ray photoelectron valence-band spectrum of this compound. The predominant contributions of the Br 4p states at the top of the valence band of rubidium dilead pentabromide are confirmed by comparison on a common energy scale of the X-ray emission band representing the energy distribution of the valence Br p states and the X-ray photoelectron valence-band spectrum of the RbPb₂Br₅ single crystal. Main optical characteristics of RbPb₂Br₅, such as dispersion of the absorption coefficient, real and imaginary parts of dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient and optical reflectivity are explored for RbPb₂Br₅ by the DFT calculations.     

Electronic structures and optical properties of IIIA-doped wurtzite Mg0.25Zn0.75O

The energy band structures, density of states, and optical properties of IliA-doped wurtzite Mg0.25Zn0.75O （IIIA= A1, Ga, In） are investigated by a first-principles method based on the density functional theory. The calculated results show that the optical bandgaps of Mg0.25Zn0.75O：IIIA are larger than those of Mg0.25Zn0.75O because of the Burstein-Moss effect and the bandgap renormalization effect. The electron effective mass values of Mg0.25Zn0.75O：IIIA are heavier than those of Mgo.25Zno.750, which is in agreement with the previous experimental result. The formation energies of MgZnO：Al and MgZnO：Ga are smaller than that of MgZnO：In, while their optical bandgaps are larger, so MgZnO：Al and MgZnO：Ga are suitable to be fabricated and used as transparent conductive oxide films in the ultra-violet （UV） and deep UV optoelectronic devices.     

H2O2 adsorption on the BN and SiC nanotubes: A DFT study

We have performed a comparative density functional theory study on adsorption of hydrogen peroxide (H₂O₂) on the boron nitride and silicon carbide nanotubes (BNNT and SiCNT) in terms of energetic, geometric, and electronic properties. It has been found that the molecule is chemically adsorbed on both of the tubes so that its interaction with SiCNT (adsorption energy ∼−0.97 eV) is much stronger than that with BNNT (adsorption energy ∼−0.47 eV). The H₂O₂ adsorption on BNNT slightly decreases its work function, increasing the field electron emission from the BNNT surface while it may not affect that of the SiCNT. In addition, the adsorption process may increase the electrical conductivity of SiCNT while does not affect that of the BNNT, significantly. We believe that the SiCNT may be a potential candidate for detection of H₂O₂.     

Electronic and optical properties of Ti3+ doped α-Al2O3 crystals: First-principles calculations

Based on density functional theory, the geometric and electronic structures of the Ti doped α-Al₂O₃ system (Ti:α-Al₂O₃) are calculated by using a first-principles method. Due to the Ti-doping, both the lengths and angles of bonds in the local geometries are distorted, from which a trigonal–triclinic structural transition may be expected. With increasing Ti-doping, an insulator–semimetal transition is observed in the Ti:α-Al₂O₃ system, mainly due to a complete spin polarization of electrons at the Fermi level. The dielectric function and absorption edge are further calculated. It is found that the absorption edge is decreased, and is much lower than that of pure Al₂O₃ crystals, which is in good agreement with the experiment. The results indicate the potential applications of the Ti:α-Al₂O₃ optical system.     

Electronic properties of multi-defected zigzag carbon nanotubes

Electronic properties of multi-defected zigzag single-walled carbon nanotubes are investigated by use of the tight-binding Green's function method. The Stone-Wales defects and the vacancies are considered. We find that the conductance sensitively depends on the realistic defect configurations for the metallic zigzag carbon nanotubes. Interestingly, the electronic transport properties of the nanotubes with three vacancies can be considered as the sum effect of two double-vacancies, while those with Stone-Wal...