Electronic vacancy states in silicon by the chemical pseudopotential method

The electronic states of a neutral vacancy in Si are studied through the chemical pseudopotential method by creating a vacancy in a large crystal unit cell containing up to 54 atoms. A localized vacancy state is found in the forbidden gap and its energy is shown to be convergent with respect to the size of the cell. The density of states of the valence band is modified by the presence of the vacancy with additional peaks which give charge localization on the vacany nearest neighbour atoms.     

Electronic energy structure of vacancy and divacancy in SiO2

We investigated electronic energy structure of vacancy and divacancy in SiO₂, and found that oxygen vacancy and divacancy give rise to bound-states near the edge of the conduction band, whereas localized states related to the silicon vacancy occur in the valence band. Our results demonstrate that the doubly occupied oxygen vacancy state yields electrons to silicon in Si-SiO₂ junction and serves as a fixed oxide charge.     

NiAl的几何与电子结构

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Relative energies of surface and defect states: ab initio calculations for the MgO (001) surface

We present the results of calculations of the energy levels of defects at the (001) surface of MgO relative to the top of the valence band and values of defect ionisation potentials and electron affinities. The calculations were made using an embedded cluster method in which a cluster of several tens of ions treated quantum mechanically is embedded in a finite array of polarisable and point ions modelling the crystalline potential and the classical polarisation of the host lattice. The calculated ionisation potential of the ideal surface, which fixes the position of the top of the valence band with respect to the vacuum level, is about 6.7 eV. This value is used as a reference for positioning the energy levels of three charge states of a surface anion vacancy, which are also calculated as ionisation energies with respect to the vacuum level. The surface and vacancy electron affinities are calculated using the same method. As a prototype low-coordinated surface site, we have considered a cube corner. Our calculations predict the splitting of the corner states from the top of the surface valence band by about 1.0 eV. Both unrelaxed and relaxed holes are strongly localised at the corner oxygen ion. The ionisation energies and electron affinities of the corner anion vacancy are calculated. The electrons in the F and F⁺ centres at the corner are shown to be significantly delocalised over surrounding Mg ions.     

Electronic Structures of Wurtzite GaN with Ga and N Vacancies

The electronic band structures of wurtzite GaN with Ga and N vacancy defects are investigated by means of the first-principles total energy calculations in the neutral charge state. Our results show that the band structures can be significantly modified by the Ga and N vacancies in the GaN samples. Generally, the width of the valence band is reduced and the band gap is enlarged. The defect-induced bands can be introduced in the band gap of GMV due to the Ga and N vacancies. Moreover, the GaN with high density of N vacancies becomes an indirect gap semiconductor. Three defect bands due to Ga vacancy defects are created within the band gap and near the top of the valence band. In contrast, the N vacancies introduce four defect bands within the band gap. One is in the vicinity of the top of the valence band, and the others are near the bottom of the conduction band. The physical origin of the defect bands and modification of the band structures due to the Ga and N vacancies are analysed in depth.     

Understanding the Effects of Point Defects on the Electronic Properties of Titania Nanoparticles: An Ab Initio Study

First-principles calculations are used to study the electronic structure of the TiO₂(101) surface. The effect of oxygen vacancies and interstitial titanium ions on the electronic structure is investigated, and models for optical single-electron charge transfer transitions in the structure are proposed. We found that the addition of an uncharged oxygen vacancy leads to a shift of the total density of states toward lower energy, and the bandgap increases. Therefore, interstitial titanium ion incorporation induces donor states above the valence band and increases the bandgap. These results can be used to explain the observed blueshift in nanoscale TiO_2.     

Oxygen vacancy in N-doped Cu₂ O crystals:A density functional theory study

The N-doping effects on the electronic properties of Cu2O crystals are investigated using density functional theory.The calculated results show that N-doped Cu2O with or without oxygen vacancy exhibits different modifications of electronic band structure.In N anion-doped Cu2O,some N 2p states overlap and mix with the O2p valence band,leading to a slight narrowing of band gap compared with the undoped Cu2O.However,it is found that the coexistence of both N impurity and oxygen vacancy contributes to band gap widening which may account for the experimentally observed optical band gap widening by N doping.     

硫钒铜矿化合物电子结构和电致变色特性的第一原理研究

基于密度泛函的第一原理赝势平面波方法,计算晶体结构、电子结构和光学性质,研究硫钒铜矿化合物Cu₃VS₄、Cu₃NbS₄和Cu₃TaS₄的电子输运及电致变色特性,探讨作为透明半导体材料应用于太阳能电池和电致变色器件的可能性.电子结构的计算表明这类化合物是间接带隙半导体,其电子能带的导带底和价带顶分别位于布里渊区的X点和R点.价带顶的电子本征态主要来自于Cu原子的d电子轨道,而导带底电子态主要来源于VB族元素原子的d电子轨道.能带结构、电荷布居分析、电子局域化函数和光吸收及反射谱的计算表明这些硫钒铜矿化合物属于极性共价半导体,具有较高的电荷迁移率和优良的电致变色特性,可应用于高效电致变色器件.     

Vacancy at the Si(111) unreconstructed surface: Electron states and absence of the Jahn-Teller distortion

The electron states of a vacancy at the Si(111) surface are calculated by means of a tight-binding scheme. The results for a vacancy at the surface layer show one state of A_l symmetry below the surface dangling bond band, and another doubly degenerate state of E symmetry above it. The Fermi energy at an isolated vacancy remains fixed by the surface. This allows to derive two important consequences: i) The vacancy state is a neutral one as can be shown by integrating the local density of states up to the Fermi energy. ii) The electronic charge around the vacancy has got the whole surface point symmetry and therefore a Jahn-Teller effect is not induced.     

A study by XPS and XRS of the participation in chemical bonding of the 3d electrons of copper,zinc and gallium

The participation of 3d electrons in chemical bonds and their part in the formation of valence bands was studied by X-ray photoelectron- and X-ray-spectroscopy for Cu, Zn and Ga phosphides, sulphides and oxides. Incomplete screening of (n + 1)s,p electrons through the nd shell leads to non-systematic changes of orbital energies and radii of the valence electrons in the first, second and third Group elements. It is reflected in the electronic structure of the respective compounds. A qualitative interpretation of XPS and XRS data for Cu, Zn, Ga phosphides is given. Possible reasons for phosphorus s band splitting for zinc and copper phosphides are considered. The interaction of 3d metal states and 3s, p third Period element states considerably affects the valence band of compounds, and this interaction should be taken into account when considering electronic structures of Cu, Zn and Ga compounds.     

Electronic Structure and Optical Property Calculation of an Oxygen Vacancy in NH_4H_2PO_4 Crystals

The electronic structure of perfect ammonium dihydrogen phosphate(ADP) and defective ADP with an oxygen(O) vacancy are calculated by screened-exchange hybrid density functional HSE06. The optimized structural parameters of the defective ADP crystal are analyzed. The PO_4 tetrahedron with an O vacancy is distorted and its symmetry is broken. The band gap of the defective ADP with an O vacancy is about 1.5 eV lower than the perfect ADP, which is due to the new O vacancy defect states near the valence band maximum. Moreover, more peaks appear in the low-energy region(lower than 6 eV) in the curves of the linear optical properties for the defective ADP. The results indicate that the O vacancy will significantly influence the laser damage performance of ADP crystals.     

空位缺陷对β-AgVO_3电子结构和光吸收性能的影响

基于密度泛函的第一性原理研究了Ag空位、O空位和Ag-O双空位对β-AgVO_3的电子结构及光学性质的影响.采用广义梯度近似平面波超软赝势GGA+U方法,对不同缺陷体系的形成能、能带结构、电子态密度、差分电荷密度和吸收光谱进行了计算和分析.通过比较不同Ag空位和O空位的形成能,确定了β-AgVO_3中主要形成Ag3空位和O1空位,并且Ag空位较O空位更容易形成.Ag3空位和O1空位的存在都使得β-AgVO_3带隙有一定程度的减小;Ag3空位使β-AgVO_3呈现p-型半导体性质,而O1空位和Ag3-O1双空位使β-AgVO_3呈现n-型半导体性质.Ag3和O1空位对晶体在可见光范围内的光吸收影响较小.     

分子空位缺陷对环三亚甲基三硝胺含能材料几何结构、电子结构及振动特性的影响

含能材料中的微观缺陷是导致“热点”形成并相继引发爆轰的重要因素. 然而, 由于目前人们对材料内部微观缺陷的认识不足, 限制了对含能材料中“热点”形成微观机理的理解, 进而阻碍了含能材料的发展和应用. 为了洞悉含能材料内部微观缺陷特性及探索缺陷引发“热点”的形成机理, 利用第一性原理方法研究了分子空位缺陷对环三亚甲基三硝胺(RDX) 含能材料的几何结构、电子结构及振动特性的影响, 探讨了微观缺陷对初始“热点”形成的基本机理. 采用周期性模型分析了分子空位缺陷对RDX几何结构、电子能带结构、电子态密度及前线分子轨道的影响. 采用团簇模型分析了分子空位缺陷对RDX振动特性的影响. 结果发现, 分子空位缺陷的存在使其附近的N–N键变长, 分子结构变得松弛; 使导带中很多简并的能级发生分离, 电子态密度减小, 并使由N-2p和O-2p轨道形成的导带底和价带顶均向费米面方向移动, 降低了能带隙值, 增加了体系活性. 前线分子轨道及红外振动光谱的计算分析表明, 分子缺陷使最高已占分子轨道电荷主要集中在缺陷附近的分子上, 且分子中C–H键和N–N键能减弱. 这些特性表明, 分子空位缺陷的存在使体系能带隙变小, 并使缺陷附近的分子结构松弛, 电荷分布增多, 反应活性增强; 在外界能量激发下, 缺陷附近分子将变得不稳定, 分子中的C–H键或N–N键较易先发生断裂, 发生化学反应释放能量, 进而成为形成“热点”的根源.     

Influence of nitrogen doping on the defect formation and surface properties of TiO2 rutile and anatase

Nitrogen doping-induced changes in the electronic properties, defect formation, and surface structure of TiO2 rutile(110) and anatase(101) single crystals were investigated. No band gap narrowing is observed, but N doping induces localized N 2p states within the band gap just above the valence band. N is present in a N(III) valence state, which facilitates the formation of oxygen vacancies and Ti 3d band gap states at elevated temperatures. The increased O vacancy formation triggers the 1 x 2 reconstruction of the rutile (110) surface. This thermal instability may degrade the catalyst during applications.     

Oxygen vacancy in N-doped Cu2O crystals: A density functional theory study

The N-doping effects on the electronic properties of Cu2O crystals are investigated using density functional theory. The calculated results show that N-doped Cu2O with or without oxygen vacancy exhibits different modifications of electronic band structure. In N anion-doped Cu2O, some N 2p states overlap and mix with the O 2p valence band, leading to a slight narrowing of band gap compared with the undoped Cu2O. However, it is found that the coexistence of both N impurity and oxygen vacancy contributes to band gap widening which may account for the experimentally observed optical band gap widening by N doping.     

Ce和O空位共掺杂TiO_2的电子结构与光学性质

采用基于密度泛函理论加U的计算方法,研究了Ce和O空位单(共)掺杂锐钛矿相TiO_2的电子结构和光吸收性质.计算结果表明,Ce和O空位共掺杂TiO_2的带隙中出现了杂质能级,且带隙窄化为2.67 eV,明显比纯TiO_2和Ce,O空位单掺杂TiO_2的要小,因而可提高TiO_2对可见光的响应能力,使TiO_2的光吸收范围增加.光吸收谱显示,掺杂后TiO_2的光吸收边发生了显著红移;在400.0—677.1 nm的可见光区,共掺杂体系的光吸收强度显著高于纯TiO_2和Ce单掺杂TiO_2,而略低于O空位单掺杂TiO_2.此外,Ce掺杂TiO_2中引入O空位后,TiO_2的导带边从-0.27 eV变化为-0.32 eV,这表明TiO_2的导带边的还原能力得到了加强.计算结果为Ce和O空位共掺杂TiO_2在可见光光解水方面的进一步研究提供了有力的理论依据.     

Electronic structures of hydrogen in perovskite-type oxide,SrTiO3

The electronic structures of hydrogen in SrTiO₃ are simulated by the DV-Xα molecular orbital method. In pure SrTiO₃, there is a band gap of about 3.5 eV between the O-2p valence band and the Ti-3d conduction band, in agreement with experiments. When Sc is doped into SrTiO₃, an acceptor level appears just above the valence band. On the other hand, when hydrogen is introduced into SrTiO₃, a donor level appears below the conduction band. The molecular orbital of the donor level is composed mostly of the Ti-3d and O-2p electrons, but still there is a small occupancy (6%∼12%) of the H-1s electrons in it. When both Sc and hydrogen coexist in SrTiO₃, charge transfer takes place from the donor level to the acceptor level. As a result of this charge compensation, the effective ionicity of hydrogen becomes about +0.17∼+0.24, the value of which is dependent on the hydrogen positions in the crystal lattice. Also, the chemical bond strengths between constituent ions are modified largely by dopants. For example, the Sc doping tends to strengthen the chemical bond between hydrogen and oxygen ions, but instead to weaken the chemical bond between the oxygen ion and the surrounding metal ions. In addition, it is shown that an oxygen ion vacancy makes the defect level below the conduction band in the Sc-doped oxide.     

Bi,Sn and Sb valence states in highT c (Bi,Pb/Sn,Sb)2Sr2Ca2Cu3Oy

The valence of Bi and its substituents Sn and Sb are investigated in the high temperature superconductor (Bi_0.8X_0.15Sb_0.05)₂Sr₂Ca₂Cu₃O_y (often denoted 2223) where X=Pb or Sn. Pb and Sn are isoelectronic with the valence states 2+ and 4+; Bi and Sb are as well with valence states 3+ and 5+. The valence state of Sn and Sb has been obtained from Mössbauer spectroscopy: they exist in the high charge states 4+ and 5+ respectively. Sn, Sb, and presumably Pb, take on the high valence state, and so furnish electrons to the conduction band. This is probably the reason why, although the (Pb, Sb)-doping aids in stabilizing the 2223 crystal structure, it is detremental to the electronic properties which lead to the superconducting phase. The valence state of Bi has been studied using X-ray photoelectron spectroscopy (XPS). These results show clearly that the Bi-based superconductor has a metal-like density of states at the Fermi level, and that the valence in the (Pb, Sb)-doped compounds is less than 3. This very surprising change in valence will have a profound effect on the superconducting properties, and is probably associated with the high valent states of Pb, Sn and Sb.     

Alkali metal intercalation into SnS2

The intercalation of sodium and potassium into the layered semiconductor SnS₂ has been investigated by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and ion scattering spectroscopy (ISS). After deposition of the alkali metals onto (0001) cleavage planes of SnS₂ in ultra high vacuum (UHV), semiconducting intercalation phases were formed. They seem to be homogeneous and disordered under the given experimental conditions. The valence electrons of the alkali metals are transferred into electronic states of the host lattice, whose valence band density of states changes significantly during intercalation. The underlying changes of the binding properties of the host lattice are discussed. The course of intercalation can be separated into three phases. During an induction period the concentration of the alkali metal on the surface remains very small, the electronic states of the substrate are shifted by band bending. During an intercalation period the topotactic reaction proceeds. After reaching saturation compositions of the intercalation phase at the surface, the alkali metal diffuses into the bulk. Crystal or surface defects seem to have a significant influence on the kinetics of intercalation and on the stoichiometry of the intercalation compounds.     

LiF中空位形成能的第一性原理计算

采用平面波展开和基于密度泛函理论框架下的第一性原理赝势法,计算了102GPa下LiF化合物中Li空位和F空位的形成能及空间周围的原子弛豫,讨论了空位形成时电荷密度的重新分布,相应的电子态密度以及能带结构等性质.结果表明:LiF晶体中F空位的形成能比大于Li空位的形成能;F空位对LiF晶体的电子结构等性质的影响要比Li空位的大.