温度对GaN(0001)表面吸附生长TiO_2分子的影响

采用基于第一性原理的从头计算分子动力学方法,模拟了在300,400,500,600和700℃5种温度下TiO_2分子在GaN(0001)表面吸附的动力学过程,研究了吸附过程中系统能量、动力学轨迹、Mulliken布居分析、表面成键电子密度分布(ELF)以及扩散系数等性质。结果表明,温度会影响TiO_2分子中两个O原子与GaN表面两个Ga原子成键顺序;500℃时O_2—Ga3成键的时间最早,TiO_2在GaN(0001)表面吸附生长的速率最快。600℃下TiO_2分子在物理吸附阶段的扩散系数比在500℃下扩大了接近100倍,500℃时O—Ga1的局域电荷分布ELF值最大为0.750,说明500℃下O—Ga1键共价性最强。由此可见,TiO_2分子在GaN(0001)吸附生长最佳温度是500℃。     

α-A12O3基片（0001）表面及其对ZnO吸附位置研究

建立了一种2×2α-Al2O3 (0001)基片表面吸附ZnO模型,在周期边界条件下的k空间中,采用基于密度泛函理论的局域密度近似平面波超软赝势法,对α-Al2O3 (0001)基片表面结构及其ZnO分子在表面最初吸附生长位置进行了计算研究.由于较大的表面弛豫,使得氧原子全部暴露于基片最外表面,明显地表现出O原子电子表面态;驰豫后的表面能对ZnO分子产生较强的化学吸附,表面电子结构将发生明显的变化,其表面最优吸附生长点的方位正好偏离α-Al2O3 (0001)表面氧六角对称30°;并计算了这些吸附生长点处Zn与表面O的结合能.     

α-Al2O3基片(0001)表面及其对ZnO吸附位置研究

建立了一种2×2α-Al2O3 (0001)基片表面吸附ZnO模型,在周期边界条件下的k空间中,采用基于密度泛函理论的局域密度近似平面波超软赝势法,对α-Al2O3 (0001)基片表面结构及其ZnO分子在表面最初吸附生长位置进行了计算研究.由于较大的表面弛豫,使得氧原子全部暴露于基片最外表面,明显地表现出O原子电子表面态;驰豫后的表面能对ZnO分子产生较强的化学吸附,表面电子结构将发生明显的变化,其表面最优吸附生长点的方位正好偏离α-Al2O3 (0001)表面氧六角对称30°;并计算了这些吸附生长点处Zn与表面O的结合能.     

AlN在α-Al2O3(0001)表面吸附过程的理论研究

采用基于密度泛函理论的平面波超软赝势法,对α-Al2O3(0001)表面吸附AlN进行了动力学模拟计算,研究了AlN分子在a-Al2O3(0001)表面吸附成键过程、吸附能量与成键方位.计算表明吸附过程经历了物理吸附、化学吸附与稳定态形成的过程,其化学结合能达到4.844eV.吸附后AlN化学键(0.189±0.010nm)与最近邻的表面Al-O键有30°的偏转角度,Al在表面较稳定的化学吸附位置正好偏离表面O六角对称约30°,使得AlN与蓝宝石之间的晶格失配度降低.     

O2分子在Al(110)表面选择吸附性的对比研究

采用密度泛函理论中广义梯度近似(GGA)计算方法对比分析了 O2分子在 Al(110)面3个不同吸附位(顶位、桥位和穴位)的吸附性质,通过吸附能的比较,桥位的吸附能高于顶位和穴位,是最佳的吸附位。吸附质的态密度、吸附物 O2分子轨道电荷分布的变化以及金属表面原子轨道电荷分布的变化揭示了吸附过程中电荷的转移趋势。计算结果表明,通过分析吸附质的原子轨道电荷分布和电子态密度,发现 O2在 Al(110)表面吸附的过程中,主要是 O 原子的2P 轨道和 Al 的3S 轨道的电子相互作用,并展示出较强的化学吸附。     

第一性原理方法探讨氮气在UO2(111)表面的吸附行为

采用第一性原理计算模拟方法对氮气分子及原子在UO2(111)表面的吸附行为进行了系统的研究。计算结果表明,N2在UO2(111)表面倾向于以分子吸附的形式存在,其最稳定的吸附构型为分子中心位于氧原子顶部或三重洞位的情形。两种构型下的N-N键的两端均指向相邻的另外两个高对称吸附位置。对于氮原子的表面吸附,发现其位于第二子层的铀原子的上方时最为稳定,吸附能为-4.792eV,为较强的化学吸附。而对次表面吸附的考察发现,氮原子嵌入最外氧原子层时的稳定性高于表面上吸附的情形。态密度分析表明,对于氮原子吸附,N 2p与U 5f电子态在费米能级附近有非常明显的轨道杂化,表明氮原子主要与铀原子发生了化学作用,N-U键存在共价成分。     

基于第一性原理的Fe掺杂锐钛矿型TiO2(001)表面光催化性研究

基于密度泛函理论的第一性原理对不同掺杂方式和不同含Fe量的Fe/TiO_2(001)禁带宽度及C6H6分子在其表面不同位置的吸附能进行了研究。Fe原子掺杂纳米TiO_2的能带结构和态密度计算表明,表面间隙掺杂比替位掺杂更利于减小TiO_2的禁带宽度,且当掺杂浓度为6.122%时,相比纯TiO_2的禁带宽度减小幅度最大可达59.3%。通过吸附能的比较得出了苯分子在TiO_2(001)表面以水平吸附为主,在研究浓度范围内,随Fe原子掺杂浓度的增加,吸附能并不像带隙一样呈减小的趋势,而是当Fe原子掺杂浓度为4.167%时,吸附能的涨幅最大为63.2%。     

氧原子在Be(0001)表面吸附的第一性原理研究

采用第一性原理的计算方法研究了不同覆盖度下O原子在Be(0001)的表面吸附性质。首先确定O吸附在Be(0001)表面的最稳定吸附构型。通过O原子的吸附能,发现O原子更易吸附在hcp位,并且与Be表面发生氧化反应形成BeO。此外,研究了不同覆盖度下的力学性质,结果表明:不同覆盖度下O原子吸附在Be(0001)表面时,基态下均具有稳定结构;铍的抗断裂和拉伸能力随着O原子覆盖度的增大而增大。     

Si表面吸附GaN的第一性原理研究

为了更好地了解Si表面对CaN的吸附,利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Si(100)和Si(111)弛豫表面分别吸附GaN的电子结构、吸附能大小以及其态密度图.计算结果表明,相对于Si(111)表面,Si(100)表面更容易吸附GaN,在同等实验条件下,在Si(100)表面应更容易沉积GaN薄膜.采用ECR-MOPECVD工艺,于低温下在Si(100)衬底上沉积得到了GaN薄膜,XRD谱图表明该薄膜是一种晶体结构和无定形结构的混合结构.     

热处理GaAs表面氧吸附研究

采用俄歇电子谱研究了热处理GaAs(110)表面与氧的相互作用。热处理GaAs表面吸附氧后,Ga俄歇峰向低能移动表明氧与表面Ga原子形成化学键。通过氧俄歇峰强度与氧暴露量的关系和吸附动力学分析,我们得知740℃热处理表面和清洁解理表面上吸附氧的俄歇信号变化相同,其饱和绝对覆盖量θ_s为0.5单原子层,若热处理温度提高到825℃时,θ_s为1个单原子层,即提供了更多的吸附位置,然而不同温度热处理GaAs(110)表面的氧吸附速度却没有变化并与金属Ga氧吸附速度相同,因此被激励的氧束缚在GaAs(110)表面的Ga原子上。     

直接观察位错结构的扫描电镜电子通道衬度技术

介绍了扫描电镜电子通道衬度技术，该技术可以很好地观察材料中位错结构、用这种技术观察受循环载荷作用的铜单晶中的典型位错结构，与常规的ＴＥＭ方法观察结果基本一致。     

Fabrication and characterization of transparent MEH-PPV/n-GaN (0 0 0 1) heterojunction devices

n-GaN/MEH-PPV thin film heterojunction diode was fabricated by depositing MEH-PPV thin film using spin-coating process on n-GaN (0 0 0 1). The junction properties were evaluated by measuring I-V characteristics. I-V characteristics exhibited well defined rectifying behavior with a barrier height of 0.89 eV and ideality factor of 1.7. The optical band gap of the MEH-PPV film using optical absorption method was found to be 2.2 eV and the fundamental absorption edge in the film is formed by the direct allowed transitions. At higher electric fields, the conductivity mechanism of the film shows a trap charge limited current mechanism. The obtained results indicate that the electronic parameters of the heterojunction diode are affected by properties of MEH-PPV organic film.     

Adsorption of Ni on Si(1 0 0) surface

The chemisorption of one monolayer Ni atoms on ideal Si(1 0 0) surface is studied by using the self-consistent tight binding linear muffin-tin orbital method. Energies of adsorption systems of Ni atoms on different sites are calculated. It is found that Ni atoms can adsorb at fourfold site above the surface and bridge site below the surface. The adsorption of Ni atoms can readily diffuse and penetrate into the subsurface. A Ni, Si mixed layer might exist at the Ni-Si(1 0 0) interface. The layer projected density states are calculated and compared with that of the clean surface. The charge transfers are also investigated.     

Epitaxial growth of transparent tin oxide films on (0 0 0 1) sapphire by pulsed laser deposition

The growth of epitaxial SnO₂ on (0 0 0 1) sapphire using pulsed laser deposition is examined. X-ray diffraction analysis shows that the films are highly a-axis oriented SnO₂ with the rutile structure. Three distinct symmetry-equivalent in-plane epitaxial orientations were observed between the film and substrate. With increasing growth temperature, both the growth rate and surface roughness increase with columnar grain formation. Carrier concentration ranged from 10¹⁷ to 10¹⁹ cm⁻³, with mobility of 0.5-3 cm²/V s. The resistivity of the films increases with increasing growth temperature, suggesting a lower density of oxygen vacancy-related defects formed during high temperature deposition.     

Out-of-plane magnetized Fe-based multilayers on GaAs(0 0 1)

We report on the out-of-plane magnetization in Fe/Ni and Fe/Au bilayers grown on GaAs(0 0 1). For Ni thicknesses of several monolayers, the magnetization of the Fe/Ni bilayer increases linearly with the Fe film thickness, which is typically observed when Fe grows in the BCC-structure. For thicknesses up to 8 ML, Ni is nonmagnetic such as in the Fe/Ni/GaAs(0 0 1) structure. This could be due diffusion of Ga. Up to thickness of 2 ML the Fe overlayer is magnetized out-of-plane, however at low temperatures only. The nature of the out-of-plane spin orientation is discussed, also in view of the crystallographic structure of the Fe/Ni bilayer and by comparison with the Fe/Au bilayer grown on GaAs(0 0 1).     

Ductile-brittle behavior at the (1 1 0)[0 0 1] crack in bcc iron crystals loaded in mode I

Fracture experiments performed at room temperature on four test samples made of Fe-3wt.%Si single crystals with an edge crack (1 1 0)[0 0 1] (crack plane, crack front) showed approximately 45° deflections of the crack from the initial crack plane (1 1 0). This behavior appeared to be independent on loading rate. Fractographic analysis confirmed that the cracks were deviated along {1 0 0} planes and the fracture was accompanied by dislocation slip and by twinning. 3D simulations at 300 K by molecular dynamic technique in bcc iron with edge cracks of equivalent orientation indicated that the crack itself could contribute to understanding of this behavior by three processes: twinning on oblique {1 1 2} planes, which hindered growth of the original crack, and by emission of dislocations on oblique {0 1 1} and {1 2 3} planes, which led to separation of the {1 0 0} planes and might cause decohesion and subsequent cleavage fracture along the mentioned planes.     

Preferentially oriented thin-film growth of CuO(1 1 1) and Cu2O(0 0 1) on MgO(0 0 1) substrate by reactive dc-magnetron sputtering

Copper oxide films deposited on MgO(0 0 1) substrates by reactive magnetron sputtering under the metal-mode condition were studied by X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED) analyses for structural analysis, and X-ray-excited Auger electron spectroscopy (XAES) for chemical bonding analysis. CuO(1 1 1) thin films grew from their initial growth stage maintaining the same crystallinity on MgO(0 0 1) substrates at 400°C. When the substrate temperature was increased to 600 °C, the as-sputtered films comprise Cu(0 0 1), amorphous Cu₂O phase, and Cu₂O(0 0 1) phase. The Cu(0 0 1) phase was observed at initial growth stage. This is probably because O₂ gas molecules could not sufficiently stick to the MgO substrate at 600 °C. Single phase of Cu₂O(0 0 1) was obtained by the cooling of the as-sputtered films in O₂ atmosphere. The growth of single phase Cu₂O(0 0 1) is considered as a solid-phase heteroepitaxial growth on MgO(0 0 1) surface, which was caused by incorporating O₂ gas into the as-sputtered films.     

Growth process of CuO(1 1 1) and Cu2O(0 0 1) thin films on MgO(0 0 1) substrate under metal-mode condition by reactive dc-magnetron sputtering

The growth process of CuO and Cu₂O thin films on MgO(0 0 1) substrates by reactive dc-magnetron sputtering was studied by reflection high-energy electron diffraction (RHEED) and atomic-force microscopy (AFM). The RHEED pattern and AFM image showed that (1) three-dimensional Cu(0 0 1) islands grew on MgO under the nonreactive sputtering condition, (2) CuO(1 1 1) was deposited layer by layer on MgO at 400 °C under the reactive sputtering condition, and (3) the film deposited at 600 °C in the initial growth stage was composed of three-dimensional Cu islands because O₂ gas could not be incorporated into them due to the low sticking coefficient of O₂ on MgO under the reactive sputtering condition. The layer-by-layer CuO(1 1 1) thin-film growth process is discussed from the viewpoint that Cu and oxygen species are supplied in stoichiometry onto the MgO substrate to form CuO thin-film crystals while maintaining minimum interfacial energy between CuO and MgO.     

A SiC bicrystal junction on the (0 0 0 1) plane

A SiC bicrystal junction in which two crystals superpose with a common (0001) plane has been investigated by optical microscopy and X-ray precession camera techniques. The behaviour of such a junction is discussed from the viewpoint of sintering and grain boundaries in SiC using a crystallographic concept of compound tessellation. Six examples of bicrystals are considered with respect to the observed and calculated superposing rotation angles and the densities of the superpositions.