DIET in the bulk: evidence for hot electron cleavage of Si---H bonds in SiO2 films

The observed increase in leakage current through SiO₂ films after hot electron exposure is ascribed to dissociation induced by electronic transitions (“DIET”) of bulk Si---H bonds, producing mobile hydrogen. We use ab initio supercell bandstructure calculations at the local density functional level to locate features produced by hydrogen-containing defects in -SiO₂. The edge of the Si---H σ* resonance is found to be about 2.7 eV above the conduction band rise, in good agreement with the observed threshold for hot electron induced damage in amorphous SiO₂ films grown on Si substrates. The O---H σ* resonance is almost 4 eV higher. Removing H from O---H in the supercell does not affect the gap region (---O⁻ forms); however, removing H from Si---H produces a mid-gap state, suggesting leakage current by hopping conductivity between Si dangling bonds. A Morse potential model is used to explore the dynamics of bond scission by short-lived (<1 fs) hot electron σ* capture. Supercell calculations on interstitial atomic hydrogen indicate the energy cost to break an embedded Si---H bond is about 0.6 eV less than in the gas phase. The DIET yield is substantially increased by reducing both ground and electron-attached state binding by this amount. While uncertainty over the displaced equilibrium in the electron-attached excited state remains, the computed DIET cross-section for reasonable parameters is ≈10⁻¹⁸ cm², and is in agreement with the semi-empirically derived value for trap creation. Comparisons are made to surface DIET processes involving Si---H bonds.     

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.     

NEUTRON SCATTERING AND LATTICE DYNAMICAL STUDIES OF THE HIGH-PRESSURE PHASE ICE (II)

Lattice dynamical calculations have been carried out for ice II based on the force field constructed for ice Ih. In order to fully understand ice II inelastic neutron scattering spectra, the decomposed phonon density of states was shown mode by mode. Calculated results have shown that the hydrogen bond force constant between the six-molecule rings is significantly weaker, 75eV/nm², compared with the force constant, 220eV/nm², within the rings. Inelastic neutron scattering spectra of clathrate hydrate H₂O+He are almost the same as ice II. This means that the absorption of He atoms cannot affect the bond strengths of the ice II host lattice. Based on the force field model for ice II, the van der Waals interactions between water molecules and helium atoms are considered. The results obtained are consistent with experimental data. Lattice dynamical calculations have been carried out for ice II using seven rigid pairwise potentials. It was found that MCY makes the stretching and bending interactions in ice II too weak and makes the O-O bond length too long (～5%), thus its lattice densities are obviously lower than other potential lattices or experimental values.     

强激光照射对2H-SiC晶体电子特性的影响

使用基于密度泛函微扰理论的第一原理赝势法, 计算了纤锌矿结构2H-SiC晶体在强激光照射下的电子特性, 分析了其能带结构和电子态分布. 计算结果表明: 2H-SiC平衡晶格参数a 和c随电子温度T_e的升高逐渐增大; 电子温度在0–2.25 eV范围内时, 2H-SiC仍然是间接带隙的半导体晶体, 当T_e超过2.25 eV达到2.5 eV以上时, 2H-SiC变为直接带隙的半导体晶体; 随着电子温度升高, 导带底和价带顶向高能量或低能量方向发生了移动, 当电子温度T_e大于3.5 eV以后, 价带顶穿越费米能级; 电子温度T_e在0–2.0 eV变化时, 带隙随电子温度升高而增大; T_e在2.0–3.5 eV范围变化时, 带隙随电子温度升高而快速地减少, 表明2H-SiC晶体的金属性随电子温度T_e的继续升高而增强. 在T_e =0, 5.0 eV 处, 计算了2H-SiC晶体总的电子态密度和分波态密度. 电子结构表明T_e =0 eV 时, 2H-SiC 是一个带隙为2.3 eV的半导体; 在T_e =5.0 eV时, 带隙已经消失而呈现出金属特性, 表明当电子温度升高时晶体的共价键变弱、金属键增强, 晶体经历了一个熔化过程, 过渡到金属状态.     

1,1-二氨基二硝基乙烯晶体的密度泛函理论研究

对 1,1 二氨基二硝基乙烯晶体进行了DFT B3LYP水平计算 .计算所得晶格能为 -10 5 .81kJ/mol,与文献值相近 .晶体的前线能带较为平坦 ,表明分子轨道能态受分子晶体场的影响较小 .电荷的分布决定了晶体中DADNE以“头 尾”方式通过分子间氢键相连形成层状结构 ,而层与层之间相互作用较弱 .从带隙 4.0eV推知DADNE的导电性介于半导体和绝缘体之间 .前线轨道由C -NO2 的原子轨道所组成 ,说明分子的强共轭性 ,也表明C -NO2 为化学反应活性部位 .C -NO2 键的布居数远小于其它键 ,提示该键为起爆引发键     

高压下NH_4ClO_4结构、电子及弹性性质的第一性原理研究

研究高压下NH_4ClO_4的结构和性质对于NH_4ClO_4在固体推进剂和炸药的安全应用具有重要意义.采用基于色散校正密度泛函理论的第一性原理方法,研究了0—15 GPa静水压力下NH_4ClO_4的晶体结构、分子结构、电子性质和弹性性质,计算结果与实验值具有较好的一致性.在压强为1,4和9 GPa时,NH_4ClO_4的晶体参数、键长和分子构型等均出现不连续变化,说明了在压强作用下结构发生变化.随着压强增加,氢键增多且作用增强,由分子内氢键向分子内和分子间的氢键转变;导带态密度峰值增加,电子局域性增强,晶体内N-H和Cl-O共价键作用增强,带隙增大,不同相变区域内带隙呈线性关系.0—15 GPa条件下NH_4ClO_4的弹性常数满足力学稳定性标准,采用Voigt-Reuss-Hill方法计算了体积模量B,剪切模量G和杨氏模量E,根据Cauchy压力和B/G值,说明NH_4ClO_4属于韧性材料,随着压强增加韧性增强.     

Mg3MNi2 (M=A1, Ti) 的晶体结构与电子性能原子模拟研究

利用元素取代Mg2Ni合金的部分Mg后得到Mg3MNi2(M=Al,Ti)合金。计算得到的Mg2Ni和Mg3MNi2(M=Al,Ti)的晶格常数和实验一致,研究了Mg3MNi2(M=A1,Ti)的电子性能和储氢性能的关系,结果表明：Mg2Ni合金中Al和Ti部分取代Mg形成新的合金Mg3MNi2(M=A1,Ti)后,Mg和Al、Ti之间形成共价键。从晶体结构特征和电子性能等方面分析预测了Mg3MNi2 (M=A1,Ti) 合金比Mg2Ni合金具有较低的吸、放氢温度。     

H,F修饰单层GeTe的电子结构与光催化性质

采用基于密度泛函理论的第一性原理平面波赝势方法,计算了单层GeTe、表面氢化及氟化单层GeTe的晶体结构、稳定性、电子结构和光学性质.计算结果表明,经过修饰后, GeTe的晶格常数、键角、键长增大,且均具有较好的稳定性.电子结构分析表明,单层GeTe为间接带隙半导体,全氢化修饰、全氟化修饰以及氢氟共修饰(F, Ge同侧;H, Te同侧)则转变为直接带隙半导体,且修饰后的能隙均不同程度减小.载流子有效质量表明,全氢化、全氟化以及氢氟共修饰GeTe (F, Ge同侧;H, Te同侧)的有效质量减小,其载流子迁移率增强.带边势分析结果显示,单层GeTe能够光裂解水制氢和析氧,而修饰后的GeTe的价带带边势明显下移,其氧化性明显增强,能够光裂解水析O2, H2O2, O3以及OH·等产物.光学性质表明,修饰后的GeTe对可见光区和红、紫外区的光谱吸收效果明显增强,表明其在光催化领域有着广阔的应用前景.     

Dynamical stability,electronic and thermoelectric properties of quaternary ZnFeTiSi Heusler compound

We investigated the dynamical stability, electronic and thermoelectric properties of the ZnFeTiSi Heusler compound by combining the first-principles calculations and semi-classical Boltzmann transport theory. The phonon dispersion indicates the dynamical stability and the calculated formation energy is negative which confirm the stability of ZnFeTiSi in the Heusler structure. The calculated electronic structures show that ZnFeTiSi is a semiconductor with an indirect band gap of about 0.573 eV using GGA and 0.643 eV by mBJ-GGA potentials at equilibrium lattice parameter (5.90 Å). Seebeck coefficient, electrical conductivity and electronic thermal conductivity were calculated to describe the thermoelectric properties of the ZnFeTiSi compound. It is found that it exhibits high Seebeck coefficient and power factor, making it promising for future thermoelectric applications.     

The influence of hydrogen impurities on the atomic and electronic structures of carbyne crystals

An ab initio DFT study of atomic and electronic structure of carbyne crystals was carried out. The influence of hydrogen impurities on carbyne structure was investigated. Calculations with atomic relaxations showed that carbon chains in the carbyne crystal structure are bow-like curved; free-energy calculations showed that the most probable lengths of those chains are four and six atoms, which is in a good agreement with experiments. Carbyne-crystal electronic-structure analysis showed that there is a small gap of 0.09 eV near the Fermi level in four-atomic carbyne, while there is no such gap in six-atomic carbyne. In studying of the hydrogen impurity influence on the atomic and electronic structure of carbyne crystals, hydrogen atoms were embedded in two directions: across and along carbon chains in the crystal. As a result we found that the crystal structure is not distorted in the case of hydrogen embedded across the chains, while the type of bonding between carbon atoms in carbon chains in the carbyne crystal structure depended on the impurity concentration. The crystal structure was distorted when hydrogen was embedded along the chains. The concentration of impurities influences the conductivity of a carbyne crystal.     

The first principle studies of the structural and vibrational properties of solid β-HMX under compression

The structural, vibrational and electronic properties of β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (β-HMX) crystal under high pressure up to 40 GPa have been studied using density functional theory (DFT). The pressure dependences on the cell volume, lattice constants, and molecular geometry of solid β-HMX are presented and discussed. It is found that the N–N bonds are significantly reduced under compression, which may be of importance for initial decomposition. Based on the optimized crystal structures, the vibrational frequencies for the internal and lattice modes of the β-HMX crystal at ambient and high pressures are computed, and the pressure-induced frequency shifts of these modes are discussed.     

苝四甲酸二酐的真空升华提纯及其光谱测试与分析

纯度为9.75%的有机半导体材料苝四甲酸二酐(PTCDA),在其升华点进行了真空升华提纯,其纯度可达99.9%。利用质谱、红外光谱及X光电子能谱对这种高纯材料进行了测试并详细分析了其分子结构、化学键的形成、原子在晶格平衡位置的振动模式、电子的组态和原子的结合能的变化。由红外光谱分析得出,苝四甲酸二酐的分子结构是中央5个C构所组成的苝核基团及位于苝核两端的两个酸酐组成,它们主要以共价键结合。晶格上的C原子在其平衡位置主要以伸缩振动为主。其分子中有大量可以自由移动的π电子;分子间离域大π键的交叠决定了苝四甲酸二酐的导电性能。由XPS谱分析得出,高纯度的苝四甲酸二酐中有结合能不同的两种C原子,结合能分别为：285.3和288.7 eV。它们对应于苝环及酸酐上的C原子。另外,有两种类型的O原子,即CO和C—O—C,其结合能分别为531.3和533.1 eV。     

On some hydrogen bond correlations at high pressures

Structural data from in situ high pressure neutron diffractions studies are analysed to show that the measured lengths of O?H and H---O pairs in hydrogen bonds follow the correlation between them established from 0.1 MPa data on different chemical compounds. In particular, the accepted view in literature that the high pressure data on ice VIII differ from it is not supported. For compounds in which the O?H stretching frequencies red shift under pressure, it is shown that wherever structural data is available, they follow the stretching frequency versus H---O (or O---O) distance correlation. For compounds displaying blue shifts with pressure, an analogy appears to exist with ‘improper’ hydrogen bonds. Another pressure effect expected is the symmetrization of a hydrogen bond with evolution of the double-welled, hydrogen bond potential into a single-well potential. Using the results from some recent ab initio electronic structure studies, we show that the bulk modulus shows a discontinuity at this point and this can be taken as signature for hydrogen bond symmetrization. This is supported by analysing a few available experimental data.     

Electronic structure and optical properties of CdWO4 with oxygen vacancy studied from first principles

The electronic structures, dielectric functions and absorption coefficient of both perfect CdWO₄ crystal (CWO) and the CWO crystal containing oxygen vacancy (CWO: V _O) have been studied using the CASTEP code with the lattice structure optimized. The calculated total density of states (TDOS) of CWO: V _O indicates that the oxygen vacancy would introduce a new electronic state within the band gap compared with that of perfect CWO. The dielectric functions are calculated since the imaginary part of the dielectric function can reduce the optical absorption of a certain crystal, and then the absorption coefficient is calculated. The calculated absorption spectra show that CWO: V _O exhibits two absorption bands in the ultraviolet and visible region, peaking at about 3.0 eV (413 nm) and 3.5 eV (354 nm), respectively, which are in agreement with the experimental results showing that the yellow CWO has two optical absorption bands in this region peaking at around 350 nm and 400 nm respectively. It can be concluded that oxygen vacancy causes these two absorption bands. The calculations also indicate that the optical properties of CWO exhibit anisotropy, and can be explained by the anisotropy of the crystal lattice.     

Effect of intense laser irradiation on the lattice stability of semiconductors and metals

The effect of intense ultrashort irradiation on interatomic forces, crystal stability, and possible melting transition of the underlying lattice is not completely elucidated. By using ab initio linear response to compute the phonon spectrum of gold, silicon, and aluminum, we found that silicon and gold behave in opposite ways when increasing radiation intensity: whereas a weakening of the silicon bond induces a lattice instability, gold undergoes a sharp increase of its melting temperature, while a significantly smaller effect is observed for aluminum for electronic temperatures up to 6 eV.     

First-principles calculation of structural and electronic properties of pyrochlore Lu2Sn2O7

A Density functional theory method within generalized gradient approximation has been performed to obtain the static lattice parameters, oxygen positional parameter, bond length and bond angle and electronic properties of ideal Lu₂Sn₂O₇ pyrochlore. The results are in excellent agreement with available experimental measurements. Density of states (DOS) of this compound was presented and analysed. We also notice the presence of the hybridization between oxygen and Lu metal. The band structure calculations show that the compound has direct band gap of 2.67 eV at the Γ point in the Brillouin zone and this indicates that the material has a semi-conducting feature.     

Structures and elastic properties of paraelectric and ferroelectric KTa0.5Nb0.5O3 from first-principles calculation

The structures, elastic properties and intrinsic hardness of B-O bonds of KTa_0.5Nb_0.5O₃ crystal in paraelectric and ferroelectric phase structures have been investigated by means of the density functional theory. Both structures are found to be elastically stable and in good agreement with available results. The elastic properties including the bulk modulus, shear modulus and Young’s modulus change largely during phase transition. The paraelectric KTa_0.5Nb_0.5O₃ crystal is more incompressible and harder than ferroelectric phase. The hardness of KTa_0.5Nb_0.5O₃ crystal is mostly determined by Nb-O bonds and the modifications of the bond strength affect the hardness of the crystal. Charge density contours indicate that the electronic distributions between B-O bonds play an important role in the formation of elastic properties.     

有机分子PTCDA在P型Si单晶(100)晶面的生长机理

对PTCDA的分子结构及其化学键的形成进行了分析,并讨论了晶面指数（100）Si单晶的晶格结构。在此基础上,评述了PTCDA分子在P-Si单晶（100）晶面上生长的机理,并制备了样品PTCDA/P-Si（100）。利用XRD对样品测试得出,在P-Si（100）晶面上沉积的PTCDA薄膜中仅存在α物相。利用XPS对样品测试得出,在其界面层中PTCDA酸酐中的4个羟基O原子与C原子结合,其结合能为532.4 eV;苝核基团外围的8个C、H原子以共价键结合,其结合能为289.0 eV;在界面处,悬挂键上的Si原子与PTCDA酸酐中的C、O原子结合,形成C-Si-O键及C-Si键,构成了界面层的稳定结构。     

Cs2XF6 (X=Si,Ge) compounds: Common and different features as uncovered by the first-principles calculations

Results of ab initio calculations of the electronic, optical and elastic constants for Cs₂GeF₆ and Cs₂SiF₆ are reported for the first time. Both compounds are the direct band gap dielectrics, with the calculated band gaps 6.926 eV (Cs₂SiF₆) and 6.417 eV (Cs₂GeF₆). Analysis of the calculated elastic constants and Cauchy condition shows both crystals as slightly covalent compounds. However, with increased pressure chemical bonds in Cs₂GeF₆ turn to be more ionic, whereas in Cs₂SiF₆ the covalent character of chemical bonds is enhanced. Pressure dependence of the chemical bond lengths and lattice constants was determined and represented as the second power functions of external pressure. Different trends in the values of the Mulliken charges for both compounds were found. The obtained results are applicable to the analysis of the luminescence properties of impurity ions at varying pressure or to the microscopic studies of crystal field effects in these crystals.     

The H atom in CaTiO 3 : Structure and electronic properties

In the present work we explore the effects that an H impurity produces upon the geometry and electronic structure of the CaTiO 3 crystal considering the cubic and orthorhombic crystallographic lattices of the material. A quantum-chemical method based on the Hartree-Fock formalism and the periodic large-unit-cell model is used throughout the work. The analysis of the results shows that the interstitial H impurity binds to one of the O atoms forming the so-called O-H group. At equilibrium, the O-H distances are found to be equal to 0.89 and 1.04 Å for cubic and orthorhombic lattices respectively. Atomic displacements and relaxation energies are analysed comparing the obtained results in the cubic lattice with those in the orthorhombic lattice. In the cubic phase the computed relaxation energy of vicinity of the O-H group is found to be equal to 1.1 eV and the atomic displacements generally obey the Coulomb law. So, the negatively charged O atoms move outwards from the defective region by about 0.09 Å while the positively charged Ti and Ca atoms move towards the defective region by about 0.05 and 0.01 Å respectively. A similar effect is observed in the orthorhombic lattice of CaTiO 3 doped with an H atom. It is necessary to mention that different O positions in the orthorhombic structure are considered for the O-H bond creation. The computed relaxation energies of the atomic displacements in this structure are found to be equal to 2.3 and 2.1 eV depending on the crystallographic type of the bonding O atom.