稀土金属Y掺杂锐钛矿TiO2(101)表面的改性

为了掌握Y原子掺杂在锐钛矿TiO₂(101)表面的稳定吸附位置和电子结构变化,提高其表面光催化活性,本文利用基于密度泛函理论的第一性原理计算研究了Y原子掺杂在完美的、带有亚表层氧空位和带有表层氧空位的锐钛矿TiO₂(101)表面的结构稳定性和电子性能。结构优化和电荷密度结果表明,Y原子可以稳定吸附在三种不同的表面上。在完美表面吸附时,Y原子最稳定的吸附位置是两个三配位O原子之间的空位;与完美表面类似,在带有亚表层氧空位表面吸附时,Y原子最稳定吸附位置是与氧空位邻近的两个三配位O原子之间的空位;而在带有表层氧空位表面吸附时,Y原子则停留于氧空位邻近的四配位Ti原子位置上最稳定。电荷密度计算结果也表明Y原子与这三种表面结合非常稳固。电子态密度计算结果表明,在带有表层氧空位的锐钛矿TiO₂(101)表面引入Y原子会在费米面附近的带隙中引入缺陷态,带隙从1.67 eV降至1.44 eV,这有可能引起电子的分级跃迁,提高表面光催化能力。本文的研究为利用单原子Y掺杂提高TiO₂(101)表面光催化能力提供了理论...     

Si(100)表面吸附Sr的建模与第一性原理计算

为了研究Sr在Si(100)表面的稳定吸附结构和吸附特性,采用基于密度泛函理论的第一性原理平面波赝势法,通过改变Sr在Si(100)表面的覆盖度和吸附位置,计算了Si(100)表面吸附Sr的吸附能、电子态密度、电子布居、电荷密度和差分电荷密度.计算结果表明,三种吸附位置中,空位的吸附能最低,Sr与Si(100)表面的作用力最大,结构最稳定;覆盖度越低,吸附能越小,Sr与Si(100)表面的作用力越大,吸附结构越稳定.Sr、Si原子间的作用力主要由Sr的3d轨道电子和Si的3s、3p轨道电子杂化耦合作用(d-sp3杂化)贡献,包括共价键和离子键.共价键和离子键的强度均随覆盖度增大而减弱,这可能是由于Sr与Sr之间的排斥力减弱了Sr与Si之间的作用力,并且这种排斥力随覆盖度增大而增大.     

六方Fe2Ge合金块体及其(001)表面电子结构和磁性的第一性原理

采用基于密度泛函理论第一性原理的赝势平面波方法,计算了块体Fe₂Ge及其(001)表面的电子结构和磁性。考虑了两种类型的终端(001)表面:Ge(Ⅰ)-(001)表面和Ge(Ⅱ)-(001)表面。电子结构方面,不同类型的Fe₂Ge(001)表面都表现出金属特性,这与块体的金属性保持一致。通过计算它们的自旋极化率,得出Ge(Ⅰ)-(001)表面的自旋极化程度最高。磁性方面,在块体和Ge(Ⅱ)-(001)表面的Ge原子是铁磁自旋有序的,而在Ge(Ⅰ)-(001)表面第一层的Ge原子是亚铁磁自旋有序的。此外,Ge(Ⅱ)-(001)表面Ge原子的自旋磁矩优于块体中和Ge(Ⅰ)-(001)表面Ge原子的自旋磁矩。这些结果与Fe的d态和Ge的p态电子的杂化有关,本文中通过分析它们的态密度进行了讨论。     

六钛酸钾(K2Ti6O13)晶须几何构型、能量及电子结构的第一性原理计算

本文采用基于密度泛函理论的第一性原理计算方法,对具有单斜结构六钛酸钾( K2Ti6O13)晶体的几何构型、能量及电子结构进行系统研究.计算所得K2Ti6 O13晶体的晶格常数、原子位置等晶胞参数均与实验值吻合,且形成热与结合能的计算结果显示该晶体具有较高的相结构稳定性;进一步的电子结构分析表明,K2Ti6O13晶体呈现具有间接带隙的半导体性质,在K2Ti6O13内部,Ti-O间成键作用明显强于K-O,而K-Ti间却未明显成键,且Ti(d)与O(p)轨道电子间较强的共价键相互作用直接决定了K2Ti6O13晶体的相结构稳定性.     

SiC中间层对金刚石涂层硬质合金刀具膜 基界面结合性能的影响

基于第一性原理分别计算了WC-Co/Graphite/Diamond、WC-Co/SiCC-Si/Diamond和WC-Co/SiCSi-C/Diamond界面模型的粘附功、断裂韧性,分析了电子结构和态密度.结果表明:Graphite/Diamond界面粘附功极小,金刚石在石墨基面上成核不良;WC-Co/Graphite界面处Co与C(Graphite)原子具有同种电荷而相斥,添加SiC中间层改变了界面处原子的电荷分配与成键方式,WC-Co/SiC界面Co与C(或Si)原子具有异种电荷而相吸,且Co-Si(SiC)键强于Co-C(SiC)键;SiC/Diamond界面C(SiC)-C(Diamond)键强于Si(SiC)-C(Diamond)键.因此,三种界面模型中各界面的粘附功SiCSi-C/Diamond>SiCC-Si/Diamond>WC-Co/SiCSi-C>WC-Co/SiCC-Si>WC-Co/Graphite>Graphite/Diamond.总之,添加SiC中间层提高了金刚石涂层硬质合金刀具膜基界面结合性能.     

CdTe(110)表面原子与电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理计算了CdTe(110)表面的原子和电子性质.结果表明,CdTe(110)理想表面在禁带中出现两个明显的表面态,弛豫后表层Cd原子和Te原子p态电子发生转移,Cd原子趋向于sp2平面杂化构型,Te原子趋向p3杂化的锥形构型.经过表面弛豫大大降低了表面能,增大了表面功函数,表面占据态和表面空态分别被推进价带顶之下和导带底之上,导致弛豫表面没有明显的表面态.     

Ba掺杂Si 笼状化合物结构和电子性质的研究

利用第一性原理研究了Ba_8Si_(46)和Ba_8Ag_6Si_(40)笼状化合物的电子性质.对于Ba_8Si_(46),化合物中Ba原子5d轨道与Si原子3s,特别是3p轨道杂化,一方面导致了Ba_8Si_(46)在费米能级附近有很高的态密度,另一方面说明了笼内Ba原子和笼上Si原子之间存在耦合.对于Ba_8Ag_6Si_(40)分析结果表明,由于Ag的5s轨道向Si的3p轨道转移电子,导致Ag与Si原子间成键趋向离子键.带结构显示这两种化合物具有弱金属特征,但Ba_8Ag_6Si_(40)化合物与费米能级相交的带宽较大,预期这种化合物具有更好的导电性能.     

SiH4在Si(001)-(2×1)表面吸附的第一性原理研究

用第一性原理密度泛函理论研究SiH4在Si(001)-(2×1)表面的分裂吸附及吸附后的结构、能量和成键特性,计算了反应物、过渡态及生成物三个状态的能量,吸附能和反应能.计算结果表明:SiH4在Si(001)-(2×1)重构表面吸附的可能反应路径是由于SiH4中Si-H键的拉长和二聚体键的断裂导致的,形成产物Si(001)-(2×1)(SiH3:H);由SiH4分裂的能量势垒0.78 eV说明所推测的反应路径是合理的.     

MgH2脱氢动力学的表面缺陷效应研究

采用基于密度泛函理论的第一性原理计算方法,研究了完整、含Mg原子空位及Pd原子掺杂三种MgH2(110)表面的氢脱附行为及其动力学,并从电子结构角度给出了表面空位/掺杂两类缺陷对其脱氢动力学的影响机制.结果显示:MgH2(110)表面六重配位的Mg原子位置是形成Mg空位或Pd掺杂的优先位置;相对于完整表面而言,Mg空位或Pd掺杂均极大地降低了MgH2(110)表面的氢脱附能垒,在一定程度上解释了MgH2纳米结构调制与催化掺杂可明显改善体系脱氢动力学的实验现象;电子结构分析发现,表面空位/掺杂缺陷的存在致使MgH2表面在费米能级附近能隙变窄、低能级区成键电子数减少,进而导致近表面的原子层稳定性降低,从而使得表面Mg-H间相互作用减弱.     

锑烯表面碱金属原子吸附和扩散行为研究

锑烯是一种新兴的具有多种新奇特性的二维材料.本文基于密度泛函理论的第一性原理方法,主要研究了Li、Na、K原子在锑烯表面的吸附和扩散行为.结果显示,Li、Na、K原子在锑烯表面吸附时,具有较大的吸附能,分别为-2.36 eV,-1.84 eV和-1.60 eV.通过引入垂直于衬底指向吸附原子的外电场,发现它们的吸附能和转移电荷都随着外加电场的增加而增加,其中对K原子的吸附能和转移电荷影响最显著.采用Climbing Image-NudgedElastic Band (CI-NEB)的方法研究得出Li、Na和K在锑烯表面的扩散行为都具有较小的扩散势垒,其中Li为0.09eV,Na为0.08 eV,而K仅有0.04 eV,这有利于碱金属原子在锑烯表面的扩散.研究结果为进一步了解锑烯的性质和应用推广提供了基础理论支持.     

基于POLO钝化接触结构的晶硅电池技术及其研究进展

多晶硅氧化物(POLO)结构是在晶硅表面依次生长一层极薄的界面氧化层与多晶硅层所形成的钝化接触结构。基于POLO结构的钝化接触技术不仅能够获得优异的表面钝化特性,而且避免了金属与晶硅表面的直接接触,极大地降低了金属与晶硅表面的接触复合。目前应用POLO钝化接触结构制作的小面积晶硅太阳能电池转换效率高达26.1%,制作的大面积晶硅太阳能电池产业化效率已经超过24.5%。同时POLO钝化接触技术应用于晶硅电池的制作可以承受高温工艺,兼容现有的晶硅电池产业化设备,是未来极具产业化潜力的钝化接触技术方案。本文主要综述了POLO钝化接触结构中载流子的传输机理及相应的量化参数表征方法;对比了POLO结构制备中界面氧化层生长、多晶硅层的沉积、掺杂及氢化处理的方法;总结了多晶硅层的寄生吸收效应、晶硅表面形貌结构、掺杂浓度分布对POLO结构钝化接触特性的影响;简述了POLO钝化接触技术的研究进展及当前POLO电池制作面临的技术难点。     

A scanning tunnelling microscopy study of the deposition of Si on GaAs(001); Implications for Si δ-doping

Atomic resolution scanning tunnelling microscopy (STM) has been used to study the adsorption of Si on GaAs(001) surfaces, grown in situ by molecular beam epitaxy (MBE), with a view to understanding the incorporation of Si in δ-doped GaAs structures. Under the low-temperature deposition conditions chosen, the clean GaAs surface is characterized by a well-defined c(4×4) reflection high-energy electron diffraction (RHEED) pattern, a structure involving termination with two layers of As. Filled states STM images of this surface indicate that the basic structural unit, when complete, consists of rectangular blocks of six As atoms with the As-As bond in the surface layer aligned along the [110] direction. Deposition of <0.05 ML of Si at 400°C onto this surface shows significant disruption of the underlying structure. A series of dimer rows are formed on the surface which, with increasing coverage, form anisotropic "needle-like" islands which show no tendency to coalesce even at relatively high coverages (0.5 ML). The formation of these islands accompanies the splitting of the 1/2 order rods in the RHEED pattern along [110]. As the Si is known to occupy only Ga sites, the Si atoms displace the top layer As atoms of the c(4×4) structure, with the displaced As atoms forming dimers in a new top layer. The results are consistent with a recently proposed site exchange model and subsequent island formation for surfactant mediated epitaxial growth.     

An EXAFS study of the local structure of Ce---Si---O amorphous thin films

An EXAFS study has been made on the structure of three composition ranges of Ce---Si---O amorphous thin films prepared by RF sputtering. The measurements, carried out on the K edge of silicon and the L₃ edge of cerium, reveal that in the stoichiometric oxygen films of the general formula (Ce, Si) O₂, both cerium and silicon are four-coordinated by oxygen regardless of the O : Si ratio. In the oxygen-deficient films cerium remains four-coordinated by oxygen, but, around silicon, the oxygen atoms are progressively replaced by silicon as the oxygen content of the films is reduced. In silicon-rich films which are very deficient in oxygen, the oxygen atoms prefer to ramain coordinated with cerium, rather than silicon.

A definite decrease in the Si---O distance with increase in Si---O coordination has been found. The effect is attributed to an increase in the charge of silicon with oxygen coordination, and supports a randomly bonded model for the structure.

The total oxygen coordination, derived from a consideration of bond conservation, indicates that the film structures are probably SiO₂-type continuous random networks.     

X-ray spectroscopic investigation of the structure of silica,silicates and oxides in the crystalline and vitreous state

The Si Kα_1,2 emission lines and their satellite lines α′, α₃ and α₄ were measured for several samples of vitreous silica (Suprasil W, Infrasil, Suprasil), sodium silicate glasses (8, 15, 20 and 25 wt% Na₂O), and crystalline Mg₂SiO₄ (Forsterite). The observed shifts of the peak positions indicate a systematic increase of the electron density on the silicon atoms with increasing break-up of the SiO₂ network by OH^? or alkali ions. These results are compared with information from the corresponding Si Kβ and O K emission bands and also with the O K emission bands from quartz, MgO and Al₂O₃. They are discussed on the basis of the MO theory and are compared with the characteristic physical properties and structure of silica and silicate glasses. Both the O K and Si Kβ emission bands are closely related to the electronegativities of the relevant metal atoms of the oxides and glasses.     

Effect of ex-situ hydrogen passivation on the structural properties of e-beam evaporated amorphous silicon thin film for solar cell applications

Thin-film photovoltaics greatly reduce the semiconductor material content in the finished product, using 150–200 times less material as compared with conventional Si wafer based cells. Electron beam evaporation (e-beam), a non-ultra-high vacuum technique has the potential for being inexpensive, and simpler process for a-Si deposition. It offers specific advantages such as high Si deposition rate (up to 1 μm/min), excellent Si source material usage, avoidance of toxic gases, and simple sample preparation conditions. In this work, we report the growth of a-Si films using e-beam at a growth rate exceeding 30 Å/s (1–5 Å/s for conventional PECVD process). We report the effect of hydrogen passivation on amorphous silicon network and on silicon-bonded hydrogen configuration under ex-situ hydrogenation in hydrogen plasma. The hydrogen concentration and silicon-hydrogen bonding configuration was evaluated using nuclear reaction analysis (NRA) and Fourier transform infrared spectroscopy (FTIR). Hydrogen plasma treatment shows an increase in the monohydride bond concentration with substrate temperature, and is corroborated by our FTIR investigation, in addition to reducing clustered monohydride bonds or polyhydride bonds in a-Si:H film. Raman analysis indicates reduction in silicon bond angle as well as the bond distance, both leading to significant structural improvement in short-range and medium range order in the amorphous phase. Thus, ex-situ hydrogenation clearly demonstrates the possibility of comparable hydrogen passivation in e‐beam evaporated a-Si films with high growth rate. One can easily extrapolate this result to microcrystalline film growth, assuming the structural improvement of the silicon network preceding the microcrystalline nucleation, where ex-situ passivation is most effective. Thus ex-situ hydrogenation opens up new possibilities in minutely tailoring the a:Si film properties especially for solar cell applications.     

The effect of the translational symmetry of crystalline silicon on the structure of amorphous germanium in the interfacial region

The structure of an amorphous Ge layer near an interface with a Si(111) crystal was studied by quantitative high-resolution electron microscopy. It was found that the translational symmetry of a Si crystal leads to the crystal-like order in the positions of Ge atoms in the interfacial region, the width of which is about 1.4 nm. In this region, the average orientation of interatomic bonds tilted with respect to the interface compensates for the difference in the bond lengths in crystalline Si and amorphous Ge and is responsible for the tetragonal distortion of the most likely atomic positions.     

Smooth silicon oxide nanowires under supercritically hydrothermal conditions

Large-scale amorphous silicon oxide nanowires (SiONWs) have been synthesized from silicon monoxide powder under supercritically hydrothermal conditions. The SiONWs with smooth surface can reach tens of microns long. Selected area electron diffraction (SAED) shows that the samples are completely amorphous. Energy-dispersive X-ray spectrometry (EDX) analysis reveals that the SiONWs consist of Si and O elements in atomic ratio approximately to 1:1.5. Photoluminescence (PL) shows luminescence in both blue and visible. During the growth process, the H⁺ and OH⁻ blocks the defects located on the surface of the SiONWs and lead to the constant growth of the SiONWs.     

Effective local environment model for vibrational states in pure amorphous germanium and silicon

A detailed comparison between the measured infrared absorption (IR) spectra of pure amorphous germanium and silicon and the calculated density of vibrational states (DVS) and IR spectra for several models shows the need for an improved model of this system. A small (17 atom) crystalline cluster model is proposed, in which the imbedding of the cluster in the bulk amorphous medium is simulated by nearest-neighbor forces on the surface atoms of the cluster. The surface force constants are adjustable parameters of the model. A DVS is obtained which has the distinct three-peaked form of the measured IR, and systematic variation of the surface parameters show the range in which these may be adjusted to achieve precise agreement with the peak frequencies of the observed IR.     

Electronic redistribution around oxygen atoms in silicate melts by ab initio molecular dynamics simulation

The structure around oxygen atoms of four silicate liquids (silica, rhyolite, a model basalt and enstatite) is evaluated by ab initio molecular dynamics simulation. Thanks to the use of maximally localized Wannier orbitals to represent the electronic ground state of the simulated system, one is able to quantify the redistribution of electronic density around oxygen atoms as a function of the cationic environment and melt composition. It is shown that the structure of the melt in the immediate vicinity of the oxygen atoms modulates the distribution of the Wannier orbitals associated with oxygen atoms. In particular the evaluation of the distances between the oxygen-core and the orbital Wannier centers and their evolution with the nature of the cation indicates that the Al―O bond in silicate melts is certainly less covalent than the Si―O bond while for the series Mg―O, Ca―O, Na―O and K―O the covalent character of the M―O bond diminishes rapidly to the benefit of the ionic character. Furthermore it is found that the distribution of the oxygen dipole moment coming from the electronic polarization is only weakly dependent on the melt composition, a finding which could explain why some empirical force fields can exhibit a high degree of transferability with melt composition.     

Order and disorder in edge-supported pure amorphous Si and pure amorphous Si on Si(001)

We report results from an investigation into hidden anisotropy in pure fully-dense amorphous silicon. For amorphous silicon in intimate contact with a crystalline Si(001) substrate, one can reasonably expect that the interface with the substrate may impose anisotropy in the form of distorted ordering within the film. Indeed, we found four-fold periodic intensity variations, with bimodal intensity centered along the substrate c-Si < 110> directions, in the X-ray scattering from a-Si on Si(001). These well-defined intensity variations disappeared entirely in X-ray scattering from edge-supported a-Si films, where there was no detectable anisotropy.