Ca2Si电子结构和光学性质的研究

采用第一性原理赝势平面波方法系统的计算了Ca2Si电子结构和光学性质，其中包括能带、态密度、介电函数、复折射率、吸收系数、光电导率和能量损失函数。计算结果显示Ca2Si是典型的半导体，正交相结构有一个直接的带隙，并且光学性质显示出各向异性。Ca2Si立方相的计算结果也显示是直接带隙半导体，并且有很高的振子强度。从能带和态密度的计算结果判断出它们的光学性质主要由Si的3p态电子向Ca的3d态的带间跃迁所决定。     

K掺杂立方相Ca2Si电子结构及光学性能的研究

基于第一性原理的密度泛函理论赝势平面波方法,采用广义梯度近似（GGA）对掺K的立方相Ca2Si的电子结构和光学性能,包括能带结构、态密度、介电函数、折射率、反射率、吸收系数、光电导率及能量损失函数进行理论计算,结果表明,掺K后立方相Ca2Si的能带向高能方向发生了偏移,形成直接带隙的P型半导体,禁带宽度为0.6230eV,光学带隙变宽,价带主要是Si的3p、Ca的4s、3d以及K的3p、4s态的贡献;静态介电函数ε1（0）=14.4;折射率n0=3.8;吸收系数最大峰值为3.47×105cm-1。通过掺杂调制材料电子结构和光电性能,为Ca2Si材料光电性能的开发与应用提供了理论依据。     

(001)面双轴应变锗材料的能带调控

本文基于形变势理论构建(001)面双轴应变Ge材料的能带结构模型。计算结果表明(001)面双轴应变可以将Ge的能带从以L能谷为导带底的间接带半导体调控到以Δ4能谷为导带底的间接带半导体或者以Г能谷为导带底的直接带半导体。同时室温下Ge的带隙与应变的关系可用四段函数来表示:当压应变将Ge材料调控为以Г能谷为导带底的间接带半导体后,每增加1%的压应变,禁带宽度将线性减小约78.63meV;当张应变将Ge材料调控为直接带半导体后,张应变每增加1%,禁带宽度将线性减小约177.98meV;应变介于-2.06%和1.77%时,Ge将被调控为以L能谷为导带底的间接带半导体,禁带宽度随着压应变每增加1%而增加11.66meV,随着张应变每增加1%而线性减小约88.29meV。该量化结果可为研究和设计双轴应变Ge材料及其器件提供理论指导和实验依据。     

硅基外延Mn_4Si_7薄膜电子结构与光学性质研究

基于第一性原理的密度泛函理论赝势平面波方法,对外延关系Mn4Si7(001)//Si(001),取向关系Mn4Si7[001]//Si[001]的Mn4Si7平衡体系下的电子结构和光学性质进行了理论计算,计算结果表明:当Mn4Si7晶格常数选取为a=b=0.5431nm、c=1.747nm时,Mn4Si7为带隙宽度为0.834eV的直接带隙半导体。Mn4Si7费米面附近的价带主要由Mn的3d5态电子构成,导带主要由Mn的3d5态电子及Si的3p态电子构成。静态介电常数ε1(0)=14.48,折射率n0=3.8056。     

以第一性原理研究立方相CeO2的弹性、电子结构和光学性质

运用基于密度泛函理论(DFT)框架下的超软赝势平面波(USPP)方法,计算了立方相CeO2的几何结构、弹性性质、电子结构和光学性质,并采用HSE06杂化泛函矫正了带隙.所得晶格参数及体模量与先前文献报道数据基本吻合.计算出了二阶弹性常数及德拜温度值,并给出了能带结构、态密度、差分电荷密度的分布情况.最后,为了阐明CeO2的光学跃迁机制,计算并分析了其复介电常数、折射率、吸附光谱、反射光谱等光学性质.     

新型稀磁半导体(La_(0.75)Ba_(0.25))(Ag_(0.75)Mn_(0.25))SO电子结构与光学性质的第一性原理计算

采用基于密度泛函理论的第一性原理赝势平面波方法,对新型稀磁半导体(La_(0.75)Ba_(0.25))(Ag_(0.75)Mn_(0.25))SO的电子能带结构、自旋极化态密度和光学性质进行了计算与分析。计算结果表明La_(0.75)Ba_(0.25)Ag_(0.75)Mn_(0.25)SO为直接带隙半导体,能隙约为1.2 eV,掺杂后体系带隙明显减少;费米面附近态密度发生劈裂,出现了明显的自旋有序排列,呈明显的铁磁性,磁性的产生是通过Mn3d和相邻S3p原子之间的p-d杂化作用实现的;La_(0.75)Ba_(0.25)Ag_(0.75)-Mn_(0.25)SO反射主峰与吸收曲线峰值都在紫外波段,而且集中在100~300 nm的范围。但在近红外和可见光区,此材料反射和吸收都较低,可用于可见光高透明的领域。     

基于Si掺杂增强光吸收提升Li2SnO3 光催化降解四环素的研究

在Li₂SnO₃中掺杂Si,研究了Si掺杂Li₂SnO₃对四环素的光催化降解性能。结果表明,对Li₂SnO₃进行等电子Si掺杂使其光学吸收带隙减小和光吸收系数增大,提高了对四环素的光催化降解效率。等电子Si掺杂Li₂SnO₃为纯相不规则块状固体,随着Si掺杂量的增加其晶格参数呈减小的趋势。Si掺杂使样品的光催化性能显著提高。Si掺杂量为10%的样品,在紫外光照射25 min后光催化降解效率为75.8%,约为母体的2倍。Si掺杂Li₂SnO₃的光催化降解行为满足赝一级动力学模型,拟合速率常数为0.02464 min^-1。在Si掺杂Li₂SnO₃的价带顶形成的Si-O键减少了光学吸收带隙,使其光吸收能力增强。Si掺杂Li₂SnO₃的光催化降解机制,属于空穴主导型。     

利用第一性原理研究Ge:Si电子结构与光学性质

采用基于密度泛函理论的平面渡超软赝势方法和广义梯度近似,计算了掺杂Ge前后单晶Si中Si-Ge键的布居值、键长以及能带结构和态密度.计算结果表明,Ge掺杂后体系晶格常数发生变化,Ge-Si键变长,布居值及带隙宽度减小.还进一步研究了掺杂Ge后的光学性质,掺杂后静态介电常数值与纯Si相比有所增大,且吸收带宽变窄、吸收带边明显红移,并对这些掺杂诱导的材料物性变化进行了解释.     

以第一性原理研究立方相CeO_2的弹性、电子结构和光学性质（英文）

运用基于密度泛函理论(DFT)框架下的超软赝势平面波(USPP)方法,计算了立方相CeO2的几何结构、弹性性质、电子结构和光学性质,并采用HSE06杂化泛函矫正了带隙。所得晶格参数及体模量与先前文献报道数据基本吻合。计算出了二阶弹性常数及德拜温度值,并给出了能带结构、态密度、差分电荷密度的分布情况。最后,为了阐明CeO2的光学跃迁机制,计算并分析了其复介电常数、折射率、吸附光谱、反射光谱等光学性质。     

方柱复式正方晶格固/气型声子晶体的带隙研究

在方柱简正方晶格每个原胞的中心插入另一个方柱形成复式结构固/气型声子晶体,采用平面波法计算了其弹性波带隙结构.结果表明,与简单晶格相比,复式结构能有效地降低晶格的对称性,使能带的简并态分离,有利于获得完全带隙,而且填充率越高,能带曲线的变化越平滑,带隙数目越多.此外,在相同的填充率下,最低带隙的宽度及中心频率都随着插入体尺寸的增大而增加,当插入体尺寸与原柱体相同时,可获得最宽的带隙.     

Structural and optical characterization of strained free-standing InP nanowires

The structural and optical properties of high-quality crystalline strained InP nanowires are reported in this article. The nanowires were produced by the vapor-liquid-solid growth method in a chemical-beam epitaxy reactor, using 20 nm gold nanoparticles as catalysts. Polarization-resolved photoluminescence experiments were carried out to study the optical properties of the InP nanowires. These experiments revealed a large blue shift of 74 meV of the first electron-to-heavy hole optical transition in the nanowires, which cannot be solely explained by quantum size effects. The blue shift is mainly attributed to the presence of biaxial compressive strain in the inward radial direction of the InP nanowires. High-resolution transmission electron microscopy Electron and selected area electron diffraction experiments show that the nanowires have high crystal quality and grow along a [001] axes. These experiments also confirmed the presence of 1.8% compressive radial strain and 2% tensile longitudinal strain in the nanowires. A simple theoretical model including both quantum confinement and strain effects consistently describes the actual energy position of the InP nanowires optical emission.     

体硅上外延超薄Si1-xGex用于制备SGOI材料

采用减压化学气相沉积的方法在Si衬底上制备了高质量的Si0.75Ge0.25/Si/Si0.86Ge0.14叠层材料,通过TEM、光学显微镜和XRD分析表明,外延的SiGe薄膜具有完好的晶格结构,平整的表面质量,SiGe薄膜处于完全应变状态.通过与Si上外延渐变缓冲层制备的SiGe材料比较发现,使用这种超薄的全应变Si...     

Strain-driven electronic band structure modulation of si nanowires

One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.     

Size-Dependent Bandgap Modulation of ZnO Nanowires by Tensile Strain

We quantified the size-dependent energy bandgap modulation of ZnO nanowires under tensile strain by an in situ measurement system combining a uniaxial tensile setup with a cathodoluminescence spectroscope. The maximal strain and corresponding bandgap variation increased by decreasing the size of the nanowires. The adjustable bandgap for the 100 nm nanowire caused by a strain of 7.3% reached approximately 110 meV, which is nearly double the value of 59 meV for the 760 nm nanowire with a strain of 1.7%. A two-step linear feature involving bandgap reduction caused by straining and a corresponding critical strain was identified in ZnO nanowires with diameters less than 300 nm. The critical strain moved toward the high strain level with shrunken nanowires. The distinct size effect of strained nanowires on the bandgap variation reveals a competition between core-dominated and surface-dominated bandgap modulations. These results could facilitate potential applications involving nanowire-based optoelectronic devices and band-strain engineering.     

Variation of Electronic Structure with C Content in Si1-x-yGexCy-5/Si(001) System

The electronic structures of strained Si1-x-yGexCy (y£0.09) alloys on Si(001) have been studied by the ab initio pseudopotential method within the local density functional theory. The variations of the minimum band gap, the valence-band offset and the strain properties in the heterojunction interface are calculated together with the average bond energy theory. It is found that the dependences of the minimum band gap and the valence-band offset on the alloy composition change around the point of zero lattice mismatch. A comparison between our theoretical results and the available experimental data indicates that some of the contradictions from different research groups can be reasonably explained.     

Effects of tensile and compressive stresses on the passive layers formed on a type 302 stainless steel in a normal sulphuric acid bath

Type 302 stainless steel samples were subjected to a U-bend test in a normal sulphuric acid bath at room temperature. The passive layers which subsequently grew on both sides of the sample, were studied by Auger electron spectroscopy. Each strained sample was immersed and passivated in a sulphuric acid solution under a voltage of +450 mV/SCE and the anodic current density was studied as a function of the immersion time. The results obtained show that the anodic current density is higher for the strained specimens than for the unstrained specimen. Thus, passivation occurs more rapidly for an unstrained sample than for strained samples. There is also clear evidence that the decrease in the anodic current density is more rapid for an elastically strained sample than for a plastically strained one. The influence of the tensile and compressive stresses on the composition of the passive films was studied by comparing the elemental Auger depth profiles. The results obtained show that oxidation is more important on the tensile stressed side than on the compressive stressed one, whether the samples were elastically or plastically strained. Similarly, the width of the region affected by the chromium enrichment was enhanced by tensile straining and reduced by compressive straining, whether the samples were elastically or plastically strained.     

The role of SiGe buffer in growth and relaxation of Ge on free-standing Si(001) nano-pillars

We study the growth and relaxation processes of Ge nano-clusters selectively grown by chemical vapor deposition on free-standing 90?nm wide Si(001) nano-pillars with a thin Si(0.23)Ge(0.77) buffer layer. We found that the dome-shaped SiGe layer with a height of about 28?nm as well as the Ge dot deposited on top of it partially relaxes, mainly by elastic lattice bending. The Si nano-pillar shows a clear compliance behavior-an elastic response of the substrate on the growing film-with the tensile strained top part of the pillar. Additional annealing at 800?°C leads to the generation of misfit dislocation and reduces the compliance effect significantly. This example demonstrates that despite the compressive strain generated due to the surrounding SiO(2) growth mask it is possible to realize an overall tensile strain in the Si nano-pillar and following a compliant substrate effect by using a SiGe buffer layer. We further show that the SiGe buffer is able to improve the structural quality of the Ge nano-dot.