低能电子显微术及其应用

低能电子显微术是新发展起来的一种显微探测技术。它的特点是利用低能（１－３０ｅＶ）电子的弹性背散射使表面实空间实时成像，具有高的横向（１５ｎｍ）和纵向（原子级）分辩率，且易与低能电子衍射及其他电子显微术相结合。近年来它已有效地应用于金属和半导体表面的形貌观测、表面相变、吸附、反应及生长过程的研究。     

氮离子轰击钼(001)和钼(110)表面形成的有序结构

用能量为1千电子伏,束流为6微安的氮离子轰击含有痕量碳和氧的钼(001)和钼(110)表面10至15分钟,在俄歇能谱中出现了很强的氮的俄歇峰。从室温直到350℃退火,低能电子衍射观察表明,表面是无序层。样品加热到530℃和650℃之间,在钼(001)表面上得到c(2×2)-氮,p(2×2)-氮和(4(2^1/2)×2^1/2)R45°-氮、氧三种结构的低能电子衍射图;在密堆的钼(110)面得到单一结构的c(7×3)-氮的低能电子衍射图。低能电子衍射图与热脱附密切相关 关键词：     

用中能电子向前散射图像研究表面原子结构

从中能电子向前散射产生的实空间图像研究了Cu（111）1×1，Si（111）（3^1/2×3^1/2）R30°-In及Ge（111）（3^1/2×3^1/2）R30°-Ag的表面结构．Cu（111）1×1的图像不仅说明表面有三重旋转轴对称性，而且还表明fcc结构一直保持到最表面一层原子．Si（111）（3^1/2×3^1/2）R30°-In表面的图像说明In原子占据T4位，而不是H3位．Ge（111）（3^1/2×3^1/2）R30°-Ag的图像说明HCT模型是正确的．这些成功的应用说明从中能电子向前散射图像可以直观而且快捷地获得表面结构类型的可靠信息，而无需类似于低能电子衍射（LEED）表面结构分析那样的复杂计算 关键词：     

氧化锌纳米带中的面缺陷

面缺陷是纳米带中非常普遍和非常重要的一类缺陷。在有些情况下，面缺陷对于高表面能指数面的出现起着决定性的作用。同时。它们可以诱导纳米带沿着特殊的方向生长。面缺陷可以是孪晶或双晶，层错和由杂质原子聚集在特定原子面所形成的间隙原子层。在本文中。利用透射电子显微术，我们将介绍氧化锌纳米带中被发现的几种面缺陷。我们确认了两种孪晶／双晶结构，它们的孪晶面分别是（01^-13）和（^-2112）面。基面层错有I1和I2两种。在大尺寸的纳米带中，I1基面层错可以折叠到（2^110）面形成棱面层错。当少量的In离子掺入氧化锌纳米带后，我们发现伴随着杂质In在基面的聚集，形成了两种倒反畴界。     

Ag在Si(111)表面二维生长有序化动力学研究

利用自制的高分辨率衍射斑角分布测量仪对低能电子衍射斑进行测量,研究Ag在Si(111)表面的有序化规律.结果表明,Ag3^1/2畴在生长过程中,满足标度不变性,是一种自相似的生长过程,在生长初期,它的生长指数近似为1/2,并提出了Ag从Ag(111)岛向外扩散形成Ag3^1/2畴的生长机制.还提出利用光助退火以在较低温度下获得完善的表面结构.     

湿法刻蚀对Si基片孔点阵及ZnO外延薄膜周期形貌的影响

利用射频等离子体辅助分子束外延法,在刻有周期性孔点阵结构的Si衬底上生长了ZnO二维周期结构薄膜,系统研究了湿法化学刻蚀对孔形点阵Si(100),Si(111)基片表面形貌的影响,以及两种初底上ZnO外延薄膜的结晶质量与周期形貌的差异.X射线衍射及扫描电子显微测试结果表明：Si (111)衬底上生长出的ZnO二维周期结构薄膜具有较好的结晶质量与较好的周期性表面形貌.该研究结果为二维周期结构的制备提供了一种新颖的方法. 关键词： ZnO 分子束外延 Si 湿法刻蚀     

二维伊辛模型的畴壁动力学

表面可以改变纳米磁性薄膜的结构和相变温度，畴壁动力学由此成为研究的重点。本文采用动力学蒙特卡罗模拟方法，对二维Ising模型磁畴界面的非平衡动力学展开数值研究。系统初态设为半正半负，即由完全有序但自旋取向完全相反的两部分组成，其间的磁畴壁随时间生长。通过对磁化标度形式的分析，发现畴壁内外的动力学标度形式虽然相同，但临界指数在数值上却存在很大差异，相差一个 =1，这是由初始条件导致的。     

依赖能量的光电子衍射曲线Fourier变换分析方法的研究(Ⅱ) 以Ni(111)为衬底的系统

本文研究了氧族元素s和Se在Ni(111)衬底上形成的p(2×2)和(3~(1/2)×3~(1/2))两种重构吸附系统依赖能量的光电子衍射曲线的直接Fourier变换分析方法,由对不同的原子吸附位和不同的电子波束的计算,分析了它们对Fourier峰位及相对应的层距修正值Δ_n的影响,并详细讨论了它们与表面原子间距之间的关系。     

二维平面光栅角色散公式的完整解析形式及分析

在矢量衍射理论基础上给出了任意入射状态下二维平面光栅的光栅方 程，推导出二维平面光栅角色散公式的完整解析形式，确立了二维平面光栅衍射极角 角色散（第一类角色散）和衍射方位角角色散（第二类角色散）的概念. 通过二维平面光栅 衍射极角和衍射方位角角色散的理论分析和数值计算，阐明了当入射波矢偏离光栅法线时， 衍射光斑阵列发生畸变的机理，并明确指出了引起衍射方位角角色散的突变、导致衍射光斑 阵列剧烈变化的条件，因而具有理论上的意义. 在实际应用方面，以晶体和量子点 的x射线衍射、低能电子衍射和反射式高能电子衍射为 关键词： 二维平面光栅 衍射极角角色散 衍射方位角角色散 晶体电子衍射     

3C-SiC(001)-(2×1)表面原子与电子结构研究

采用广义梯度近似的密度泛函理论方法计算了3C-SiC（001）-（2×1）表面的原子及电子结构．计算结果表明,3C-SiC（001）-（2×1）表面为非对称性的Si二聚体模型,其二聚体的Si原子间键长为0.232 nm．电子结构的计算结果表明,在费米能级处有明显的态密度,因此3C-SiC（001）-（2×1）表面呈金属性．在带隙附近存在四个表面态带,一个位于费米能级附近,一个位于费米能级以上5 eV处,另外两个位于费米能级以下的价带中． 关键词： 碳化硅 密度泛函理论计算 原子结构 电子结构     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

Oxygen adsorption on clean Mo (100) surfaces

Oxygen adsorption on clean Mo (100) surfaces has been studied by LEED, AES, work function changes and energy loss spectroscopy. At room temperature, the oxygen uptake as determined by AES is linear up to one third of the saturation value. Data obtained with CO adsorption have been used to determine the oxygen coverage. With increasing oxygen exposure LEED shows three stages: a c (2 × 2) phase growing simultaneously with a (6 × 2) structure, a stage with (110) microfacets covered by two-dimensional structures and finally a p (3×1) structure together with a p (1×1) structure, probably due to an oxide phase. Even in the low temperature range (370–500 K) remarkable effects are observed: adsorption at 370 K produces a disordered c (4×4) structure which is followed by a (√5 × √5)?R 26° 33 structure. The same occurs when the inital c (2 × 2) structure formed at 295 K is heated above 370 K. Measurements of the work function indicate a minimum at the end of the c (2×2) structure, then a rapid increase and at saturation a value of about 1.5 V above that of the clean surface. Energy loss spectroscopy measurements point to an increase of the surface plasmon energy during the faceting stage. New transitions are observed which are due to new electronic levels induced by the adsorption. They are comparable with photoemission results on W and Mo.     

A kinetic study of the initial oxidation of the Ni(110) surface by RHEED and X-ray emission

Nickel (110) surfaces, prepared by a combination of high temperature oxidation, argon ion bombardment and hydrogen reduction, were oxidized in pure O₂. The structural aspects of this interaction were studied by reflection high energy electron diffraction (RHEED) and the corresponding kinetics determined by electron excited X-ray emission spectroscopy. On exposure to oxygen the surface was observed to go through three ordered two-dimensional structures. These were a (2 × 1), a (3 × 1) and finally a (9 × 4) structure containing 0.015 microg/cm² of oxygen. From this surface NiO was produced in a (001) epitaxy which was compressed 4.5% in the plane of the surface. With oxidation beyond 0.060 microg/cm² the oxide strain was relieved and a complex oxide epitaxy developed which has been tentatively identified as a (117) oxide parallel to the nickel (110) surface. The kinetics have been explained on the basis of three distinct processes. (i) An initial chemisorption stage (0 to 0.015 microg/cm²) associated with the two-dimensional structures, (ii) Oxide nucleation and spreading to cover the surface, associated with the (001)-NiO epitaxy (0.015 to 0.06 microg/cm²), (iii) Logarithmic film thickening above 0.060 0.06 microg/cm² associated with the development of (117) epitaxy.     

Initial growth of Cu films on oxygen covered W(110)

The growth of Cu films on the p(2 × 1)O-W(110) and (1 × 1)O-W(110) surfaces has been monitored using specular He scattering. Deposition at temperatures below 200 K leads to rough films on both surfaces. Deposition at T > 300 K leads to phase separation of Cu and O on the p(2 × 1)O-W(110) surface. Deposition at 300 K on the higher oxygen coverage surface leads to films which are nearly as smooth as these deposited on clean W(110). Deposition at 800 K leads to needle-like crystallite formation without an initial continuous film.     

Surface crystallography of W(110) and W(110) p (2×1)-O: The position of W atoms

The position of W atoms in the surface layers of clean W (110) and W (110) p (2 × 1)-O is studied using Constant-Momentum-Transfer Averaging of LEED intensities. It is shown that the clean surface is not relaxed to an uncertainty of < 0.06 Å. Analysis of superstructure beam intensity averages from W(110) p (2 × 1)-O indicates that oxygen does not reconstruct W(110) at these coverages and below 1000 ° K. An upper limit of 0.05 Å can be put on the out-of-plane displacement of W atoms by the oxygen. Substrate beam averages from W (110) p (2 × 1)-O verify the non-reconstruction. The use of CMTA for adsorbed-layer crystallography in general is briefly discussed.     

O在预覆盖K的Ag(110)表面的共吸附及其性质

本文用X射线光电子能谱(XPS),紫外光电子能谱(UPS),电子能量损失谱(EELS)和低能电子衍射(LEED)研究了O与预覆盖K的Ag(110)表面相互作用及其性质。在低覆盖度K下,发现有两种O的吸附态,经鉴别为溶解到表面下的O^2-和表面上吸附的O^x-增加K的覆盖度,出现分子状态的吸附物O₂^δ-,它与表面下存在的K相联系。XPS和UPS均清楚地显示出对应于三种不同吸附态的光电子发射峰。Ag(110)表面预覆盖K后的粘滞系数大大增加。K和O的共吸附引起它们彼此向Ag(110)表面下的溶解。LEED实验结果表明,清洁Ag(110)表面覆盖单层K原子后衍射图形从(1×1)变到(1×2),再吸附O后表面吸附层结构变为(2×1)。另外,结合UPS和EELS测量初步考察了O/K/Ag(110)共吸附系统的电子结构。本文还提出了一个共吸附模型来解释这些现象。 关键词：     

Surface structural transition of adsorption of oxygen on Ag(100)

High resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED) were used to study the oxygen adsorption on Ag(100). An ordered c(2 × 2) superstructure occurs after low temperature adsorption, in which the stretching mode at 37 meV was observed. The energy loss at 30 meV is attributed to the ordered p(1 × 1) structure after the adsorption at room temperature. The structure transition from the c(2 × 2) to the p(1 × 1) has been observed when heating the adsorbed surface from low temperature to room temperature. Cooling of the adsorbed surface at room temperature down to 180 K results in the surface transition from the p(1 × 1) structure to the coexistence of the p(1 × 1) and c(2 × 2) structures.     

The structure of the c(2 × 2) oxygen overlayer on the unreconstructed (110) surface of iridium

An Ir(110)-c(2 × 2)O structure has been prepared by adsorbing a half-monolayer of oxygen at room temperature on an unreconstructed (1 × 1)Ir surface stabilized by a quarter-monolayer of randomly adsorbed oxygen. Results of the low energy electron diffraction structural analysis indicate that the ordered oxygen atoms are residing on the short-bridged sites on the (110) surface. The Ir-O interlayer spacing is 1.37 ± 0.05 Å, and the bond length is 1.93 ± 0.07 Å. The topmost substrate interlayer spacing is found to be 1.33 ± 0.07 Å rather than 1.26 ± 0.07 Å which is the topmost interlayer spacing of the unreconstructed (1× 1)Ir surface.     

Oxygen adsorption on (110) silver

The adsorption of oxygen on a (110)Ag surface is investigated by means of Auger electron spectroscopy, LEED and low energy helium ion scattering (IS). With LEED two ordered structures, i.e. (3×1) and (2×1) were observed at oxygen exposures of 1700 L and 7000 L respectively. The oxygen signal observed by AES and IS increases monotonically with oxygen exposure. The signals can be related to absolute coverage by comparison with Δφ measurements and by the use of the LEED data. With this calibration and with theoretical scattering cross-sections the IS measurements allow the position of the adsorbed oxygen to be estimated. The observation of a strong azimuthal anisotropy of the IS signal, e.g. a large oxygen signal if the plane of scattering is parallel to the [110] direction and a relatively small oxygen signal in the [100] direction, leads to the conclusion that the oxygen is adsorbed in a bridge position between two Ag atoms of the [110] surface channels, its centre being slightly below the centres of the Ag atoms.     

Adsorption and desorption of oxygen on stepped tungsten surfaces

The adsorption and desorption of oxygen on stepped tungsten surfaces with orientations close to the (110) orientation and steps parallel to the most densely packed crystal direction ([111]) is studied with low energy electron diffraction, Auger electron spectroscopy, work function measurements and thermal desorption spectroscopy. With increasing deviation from the (110) orientation, an increasing preference for the formation of the p(2 × 1) domain with the densely packed direction parallel to the steps is noted. The adsorption kinetics does not differ markedly from that on the flat (110) surface, however the desorption behaviour at low coverages (θ < 0.3) is quite different. The results are interpreted in terms of the dissociation of a mobile precursor at terrace and step sites, the competition between the two domains during their growth and a step-induced premature transition to the complex structure observed on flat (110) surfaces at $\text{θ ? 8}$. The steps are believed to play also a significant role in desorption.