Cr doping induced structural, phonon and excitonic properties of ZnO nanoparticles

We report systematic study of structural, phonon and optical properties of Cr-doped ZnO nanoparticles. These particles are synthesized through simple sol–gel technique. Structural studies carried out by X-ray diffraction method, confirm that the prepared particles are in hexagonal wurtzite structure and lattice parameters change considerably while increasing the doping. Raman and Fourier transform infrared spectral studies show that the intensity of the phonon modes decreased and also blue shift due to ion doping, respectively. Apart from this, transmission electron microscopic studies show reduction in particle size where the particle diameters reduced from 36 to 11 nm. Optical absorption spectral measurements show a blue shift in the band-gap and increment in excitonic oscillator strength. Photoluminescence studies show doping altered the near-band edge emission but there is no change in the other emission bands which is due to oxygen vacancy, surface defects and surface dangling bonds.     

The influence of atomic steps on the photoelectric properties of clean cleaved silicon as derived from the surface photovoltage

The correlation of structural and electrical properties of clean silicon surfaces cleaved in UHV was investigated quantitatively by the surface photovoltage, using light with an energy larger than the band gap of silicon. The surface photovoltage, which is a function of band bending and recombination probability, depends strongly on the appearance of atomic steps. The additional surface states vary with density and crystallographic orientation of the steps as well as with adsorption of oxygen. The experimental facts can be explained by accepting a shift of the Fermi level at the surface towards the valence band due to edge atoms. By measuring the change of sign of the surface photovoltage of crystals with various dopings an exponential temperature dependence of the ratio of the recombination probabilities r_v/r_c for transitions from and into the Surface states has been derived.     

First-principles study of CaTiO3 oxygen-vacancies (0 0 1) surface

The structural and electronic properties of fully relaxed CaTiO₃ oxygen-vacancies (0 0 1) surface with CaO and TiO₂ terminations are investigated by first-principles plane waves ultrasoft pseudopotential method based on local density approximation. The present results show that the direction and the magnitudes of the atomic relaxations for oxygen-vacancies surface are different from that of the perfect surface. Compared with the TiO₂-terminated oxygen-vacancy surface, the CaO-terminated oxygen-vacancy surface are likely to be observed in oxygen environment conditions. Much different from the perfect surface, the oxygen-vacancy surface becomes metallic caused by some states in the conduction band are lowered and pulled down in the band-gap region.     

Spin-orbit symmetries of conduction electrons in silicon

We derive a spin-dependent Hamiltonian that captures the symmetry of the zone edge states in silicon. We present analytical expressions of the spin-dependent states and of spin relaxation due to electron-phonon interactions in the multivalley conduction band. We find excellent agreement with experimental results. Similar to the usage of the Kane Hamiltonian in direct band-gap semiconductors, the new Hamiltonian can be used to study spin properties of electrons in silicon.     

扶手椅型硅纳米管的电学性质研究

本文采用密度泛函理论的第一性原理方法，对手性指数m=n=K（K为3～15的整数）的扶手型硅纳米管的能带结构和态密度进行了研究。计算结果表明，扶手型（3，3）硅纳米管为间接带隙结构，其余均为直接带隙结构；随着手性指数的增加，硅纳米管的直径增大，硅纳米管的禁带宽度逐渐减小，且导带逐渐下移，总态密度图峰值强度增大；扶手型（3，3）硅纳米管的禁带宽度最大；扶手型（13，13）硅纳米管的禁带宽度最小，说明其导电性优于其他手性指数的扶手椅型硅纳米管；同时，扶手型（4，4）硅纳米管的导带和价带出现重叠，说明扶手型（4，4）硅纳米管为金属性纳米管；态密度图表明扶手型（9，9）硅纳米管的价带顶主要由Si-3p电子态构成，导带底由Si-3p态电子和Si-3s态电子共同构成。     

Surface passivant effects on electronic states of the band edge in Si-nanocrystals

We studied the effect of the surface passivants fluorine (F), chlorine (Cl), oxygen (O) and oxygen-related OH on the energy band edge states of clusters with the same Si₂₉ and Si₄₇ core by means of the atomic cluster model and density functional theory (DFT). The results confirm that the electronic states of the band edge in clusters are sensitive to these passivants, and the passivant O that may form double bonded structure affects the band edge states most strongly. The results may be helpful for understanding and controlling the electrical and optical properties of nanocrystalline silicon.     

锐钛矿二氧化钛体相缺陷的准粒子能带结构

Quasiparticle band structures of the defective anatase TiO₂ bulk with O vacancy, Ti interstitial and H interstitial are investigated by the GW method within many-body Green''s function theory. The computed direct band gap of the perfect anatase bulk is 4.3 eV, far larger than the experimental optical absorption edge (3.2 eV). We found that this can be ascribed to the inherent defects in anatase which drag the conduction band (CB) edge down. The occupied band-gap states induced by these defects locate close to the CB edge, excluding the possible contribution of these bulk defects to the deep band-gap state below CB as observed in experiments.     

Remarkable effects of surface dihydride configurations in electronic properties of 110 silicon nanowires

Based on density functional theoretical calculations, we present remarkable differences in the electronic properties of 110 silicon nanowires (SiNWs) with symmetric and canted dihydrides (SiH₂) on (100) facets which are, however, energetically very competitive. It is found that surface terminations with the canted SiH₂ result in dramatic widening of the band gap, with an increment as large as 20%. The valence band maximum diffuses in the surface layers, which enhances the electronic activity of surface defects. The revealed significant effects of the surface multistates could be important for the surface functionization of SiNWs.     

Origins of visible-light emissions in hydrogen-coated silicon nanocrystals: Role of passivating coating

We present a theoretical investigation of the electronic and optical properties of hydrogen-coated silicon nanocrystals (Si:H NCs). On one hand, the density-functional theory (DFT) is used to both calculate the total energy and relax the NCs. On a second hand, the tight-binding method, which includes the minimal sp³-basis set within the second-nearest-neighbor interaction scheme, is applied to calculate the electronic structures, oscillator strength (OS) and recombination rate (RR) versus the NC size, coating and atomic relaxation. Three main findings are reported: (i) The quantum confinement in these NCs do follow similar rule to the case of a single-particle in a box, where the confinement energy decays in power-law with the increasing NC's size. (ii) The coating is shown to play the essential role in creation of large band-gap energy lying within the visible-light energy spectrum. (iii) The surface atomic relaxation is found to reduce the band-gap energy by about 150 meV and enhance both OS and RR. Our claims are corroborated by the available experimental data.     

Energy band and band-gap properties of deformed single-walled silicon nanotubes

The fantastic physical properties of single-walled silicon nanotubes (SWSiNTs) under mechanical strain make them promising materials for fabricating nanoscale electronic devices or transducers. Here we investigate the energy band and band-gap properties of the SWSiNTs calculated from the tight-binding model approximation. The results show that the band-gap properties are very sensitive to the deformation degree and the helicity of the SWSiNTs. The results can be employed to guide the design of nanoelectronic devices based on silicon nanotubes.     

Energy band and band-gap properties of deformed single-walled silicon nanotubes

The fantastic physical properties of single-walled silicon nanotubes (SWSiNTs) under mechanical strain make them promising materials for fabricating nanoscale electronic devices or transducers. Here we investigate the energy band and band-gap properties of the SWSiNTs calculated from the tight-binding model approximation. The results show that the band-gap properties are very sensitive to the deformation degree and the helicity of the SWSiNTs. The results can be employed to guide the design of nanoelectronic devices based on silicon nanotubes.     

P掺杂硅纳米管电子结构与光学性质的研究

采用基于密度泛函理论的第一性原理计算,研究了P掺杂对单壁扶手型硅纳米管电子结构和光学性质的影响.结果表明:经过P掺杂,单壁扶手型硅纳米管的能带结构从间接带隙变为直接带隙,其价带顶主要由Si-3p态电子构成,导带底主要由Si-3p态电子和Si-3s态电子共同决定;同时通过P掺杂,使单壁扶手型硅纳米管的禁带宽度变窄,导电性增强,吸收光谱产生红移.研究结果为硅纳米管在光电器件方面的应用提供了理论基础.     

Quasiparticle bandstructures of covalent semiconductors and their surfaces

The exchange-correlation self-energies and quasiparticle shifts are calculated for band states of covalent materials (diamond, silicon) and their (001) 2×1 surface in order to solve the bulk and surface band-gap problem of the LDA. The screening properties are described by a model dielectric function taking into account the spatial nonlocality in the surface case assuming specular electron reflection. The wave functions are expanded in terms of localized orbitals. The quasiparticle bandstructures obtained are in reasonable agreement with experimental results.     

Tuning the chemical functionality of a gas sensitive material: Water adsorption on SnO2(1 0 1)

Photoemission and density functional theory studies show that water adsorbs dissociatively on the SnO₂(1 0 1) surface in the presence of terminating oxygen atoms and molecularly if these surface oxygen atoms are removed. The different chemical surface responses of these two bulk terminations of SnO₂ also change the water induced band bending and consequently the conductivity of the gas sensing material.     

硅基超晶格Si1-xSnx/Si的能带结构

由于Si基发光材料能与现有的Si微电子工艺兼容,其应用前景被广泛看好. 设计具有直接带隙的Si基材料,备受实验和理论研究者的关注. 本文根据芯态效应、电负性差效应和对称性效应设计了Si基超晶格Si_1－xSn_x/Si. 其中Si_0.875Sn_0.125/Si为直接带隙材料. 在密度泛函框架内,采用平面波赝势法计算表明,Si_0.875Sn_{0.125相似文献}   

Temperature-dependent optical properties of GaSe layered single crystals

Optical properties of GaSe single crystals have been investigated using temperature-dependent transmission and room temperature reflection measurements in the wavelength range of 380–1100 nm. The analysis of the absorption data at room temperature showed the existence of indirect transitions in the crystal with energy band gap of 1.98 eV. Temperature dependence of the transmission measurements revealed the shift of the absorption edge toward lower energy as temperature is increased from 10 to 280 K. The rate of change of the indirect band gap was found as γ = ?6.6 × 10^?4 eV/K from the analysis of experimental data under the light of theoretical relation giving the band gap energy as a function of temperature. The absolute zero value of the band gap energy and Debye temperature were calculated from the same analysis. The Wemple–DiDomenico single-effective-oscillator model applied to refractive index dispersion data was used to determine the oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index values.     

N掺杂Cu2O薄膜的光学性质及第一性原理分析

采用射频磁控溅射技术, 在不同温度下制备了N掺杂Cu₂O薄膜．透射光谱分析发现, N掺杂导致Cu₂O成为允许的带隙直接跃迁半导体, 并使Cu₂O的光学禁带宽度增加．不同温度下沉积的薄膜光学禁带宽度E_g=2.52± 0.03 eV．第一性原理计算表明, N掺杂导致Cu₂O的禁带宽度增加了约25%, 主要与价带顶下移和导带底上移有关, 与实验报道基本符合．N的2p电子态分布不同于O原子, 在价带顶附近具有较大的态密度是N掺杂Cu₂O变成允许的带隙直接跃迁半导体的根本原因．     

Anomalous oxidation properties of the ZnSe (100) surface

We have observed a unique, pressure-dependent adsorption isotherm of oxygen on the ZnSe (100) surface, which consists of an unmeasurable uptake followed by an irreversible, step-like uptake for pressures exceeding a critical value of ～ 0.08 torr at room temperature. A partial depletion of Se accompanies this adsorption process. For incomplete oxidation, oxygen induced (2 × 1) and (3 × 1) surface reconstructions may be generated, the first such structures to be observed for semiconductors. The electron-energy-loss spectra for these surfaces and for the clean ZnSe (100)c(2 × 2) surface are presented. The clean surface exhibits a dangling-bond-derived empty surface state ～ 1 eV above the conductor band edge, and filled surface states near 3.2, 6.5, and 15 eV below the valence band edge.     

(001)面任意方向单轴应变硅材料能带结构

首先计算了(001)晶面单轴应变张量,在此基础上采用结合形变势理论的K ·P微扰法建立了在(001)晶面内受任意方向的单轴压/张应力作用时,应变硅材料的能带结构与应力(类型、大小)及晶向的关系模型,进而分析了不同单轴应力(类型、大小)及晶向对应变硅材料导带带边、价带带边、导带分裂能、价带分裂能、禁带宽度的影响.研究结果可为单轴应变硅器件应力及晶向的选择设计提供理论依据. 关键词： 单轴应变硅 K ·P法 能带结构     

Optical constants and band gap of PbTe from thin film studies between 25 and 300°K

Optical constants of PbTe have been derived in the fundamental gap region between 25 and 300°K. It is shown in this paper by a direct compatibility check through Kramers-Krönig inversion, that properly prepared thin films have the optical properties of homogeneous material. A physical model is used to relate the optical edge and the refractive index signularity to the band gap and a value of the low-temperature dispersion of the refractive index is given, for the low-energy side of the singularity where photoelectronic phenomena take place. The precise shape of the refractive index singularity, and its relation to band tailing are discussed. The values of the optical band-gap temperature coefficient are compared with results from other authors.