Step edge selection during ion erosion of Cu(001)

Upon Ar+ ion erosion of a Cu(001) surface, we observed with HR-LEED faceted structures on the surface oriented preferentially along <100> at 200 K and along <110> at 330 K. These results are evaluated with a geometric rule, relating under kinetic conditions the orientation of the surviving steps to their advance rates. Application of this generic rule demonstrates that the <100> steps advance slowest. This reveals enhanced interlayer mass transport across these steps. We also show that a transition from kinetically favored step orientation into the thermodynamically favored one occurs at very short time scales even near room temperature.     

Abnormal ripple patterns with enhanced regularity and continuity in a bulk metallic glass induced by femtosecond laser irradiation

Two types of abnormal ripple patterns: classical ripples (C-ripples) with continuous ridges and unclassical ripples (U-ripples) with regular micropores were investigated in a Zr-based bulk metallic glass (Zr-BMG) after femtosecond laser beam irradiation. U-ripples with a period of $\sim $ 1,600 nm and the orientation nearly parallel to the polarization of laser beam were observed to form gradually on the top of C-ripples with the effective pulse number. Micropores created by the superposition of C-ripples and U-ripples had an average diameter of $\sim $ 750 nm nearly equal to the period of C-ripples ( $\sim $ 720 nm) and ran along parallel lines in the grooves of U-ripples. Both U-ripples and C-ripples in Zr-BMG showed significant microstructural differences comparing with those in a conventional Zr-based alloy with the same composition and processing conditions, including much more regular and continuous ridges. The results indicate that the amorphous structure in Zr-BMG plays a key role in the uniform morphology of laser-induced structures.     

The influence of oxygen on the growth of silver on Cu(110)

The influence of the (2 × 1)O reconstruction on the growth of Ag on a Cu(110) surface was studied by scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). On the bare Cu(110) surface, Stranski–Krastanov growth of silver is observed at sample temperatures between 277 K and 500 K: The formation of a Ag wetting layer is followed by the growth of three-dimensional Ag wires. In contrast, on the oxygen-precovered Cu(110) surface, the growth of silver depends heavily on the substrate temperature. Upon Ag deposition at room temperature, a homogeneous, polycrystalline Ag layer is observed, whereas at 500 K, three-dimensional wires separated by (2 × 1)O reconstructed areas are formed. The behavior of a deposited Ag layer upon annealing is also influenced greatly by the presence of oxygen. On the bare surface, annealing does not change the Ag wetting layer and gives rise to Ostwald ripening of the Ag wires. On the oxygen-precovered surface, however, the initial polycrystalline Aglayer first transforms into Ag wires at around 500 K. Above this temperature, the depletion of the (2 × 1)O reconstructed areas due to Ag-induced O desorption is balanced by the formation of a Ag wetting layer. On both, the bare and the oxygen-precovered Cu(110) surface, the deposited silver diffuses into the Cu bulk at temperatures above 700 K.     

Molecular adsorption and growth of naphthalene films on Ag(1 0 0)

The adsorption of naphthalene, vacuum deposited on a Ag(1 0 0) surface, was comprehensively investigated by means of low-energy electron diffraction (LEED), temperature-programmed thermal desorption (TPD) spectroscopy, X-ray photoelectron spectroscopy (XPS), and polarization-dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the mono- and multilayer regime. A growth of long-range ordered monolayer at 140 K is observed with LEED. The polarization-dependent C 1s NEXAFS shows that the naphthalene molecules in the monolayer lie almost parallel to the Ag(1 0 0) surface. With increasing film thickness, the molecular orientation turns to upright position. Furthermore, NEXAFS measurements show that in the multilayer regime the molecular orientation depends on the substrate temperature during deposition.     

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.     

Growth of CuO chains on Ag(110) surfaces

The (AgO) chains grown along the 〈001〉 direction on Ag(110) disappeared by the reaction with Cu atoms at room temperature, and new one-dimensional chains grew in the 〈110〉 direction. The STM images at different bias potentials suggest the growth of (CuO) chains in the 〈110〉 direction on the Ag(110) surface. The (CuO) chains on the Ag(110) surface disappeared by heating to 470 K, but they reappeared by exposing to O₂ at room temperature.     

A study of the induced magnetic hyperfine fields in Fe/Ag multilayers

Near perpendicular magnetic hyperfine fields in ^110mAg have been observed in Fe/Ag multilayers. These fields are studied by low temperature nuclear orientation (LTNO) in multilayers [Ag(x ML)/Fe(y ML)]₂₀, with (x,y) monolayer (ML) values of (2,10), (3,9), (5,10) and (3,18). The ^110mAg γ-ray anisotropy was measured as a function of applied magnetic field parallel to the multilayer. The average induced hyperfine field of Ag is significantly influenced by the quality of the multilayer as measured by X-ray diffraction. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ag-induced atomic structures on the Si(110) surface

We report the results of STM investigation of the initial stage of Ag adsorption on an Si(110) surface. At 0.21 ML Ag coverage, the size and orientation of the unit cell correspond to the parameters of a 16 × 2 unit cell of clean Si(110) surface. With increasing of the Ag coverage up to 0.42 ML, the type of surface reconstruction changes to a 4 × 1-Si(110)-Ag structure. The text was submitted by the authors in English.     

Misfit driven azimuthal orientation of NaCl domains on Ag(1 0 0)

We investigated the growth of thin NaCl films on Ag(1 0 0) by spot-profile-analysis low energy electron diffraction (SPA-LEED), varying extensively the growth temperature (200–500 K) and the film thickness (0.5–14 ML). The incommensurate growth of NaCl on Ag(1 0 0) yields (1 0 0)-terminated epitaxial NaCl domains, which are preferentially oriented with their [0 1 0] axis parallel to that of the substrate. At 300 K, the NaCl domains exhibit an azimuthal mosaicity by 14° around this orientation and the NaCl unit cell is laterally contracted in the first layers by 0.9% with respect to the bulk. At higher growth temperatures, the azimuthal mosaic distribution sharpens and additional distinct orientations appear, presumably due to a higher-order commensurability. The evolution of the azimuthal mosaic distribution with increasing temperature can be ascribed to both the NaCl thermal expansion and higher diffusion rates of NaCl on Ag(1 0 0). The best epitaxy, i.e. that with the highest selectivity of a specific azimuthal domain orientation, is achieved by growing NaCl films at low deposition rate (0.1 ML min⁻¹) on the Ag(1 0 0) substrate at constant high temperature (450–500 K). The observations made here can probably be applied more generally to other heterogeneous interfaces and, in particular, be used to improve the quality of thin insulating films.     

Size effect of the longitudinal magnetoresistance of thin films of n- type Ge

In a study of the longitudinal magnetoresistance as a function of the magnetic field in samples of n-type Ge of various thicknesses at 300°K and 77°K, the surface of the samples was oriented parallel to the (001) plane. The directions of the current and the magnetic field were along either the <100> direction or the <110> direction. As the thickness of the samples was reduced, regardless of the orientation of the current and the magnetic field with respect to the crystallographic directions, a decrease was observed in the magnetoresistance over the entire range of magnetic fields studied, 1–280 kOe. The experimental results are attributed to a screening-length size effect and to “sliding” orbits in the enriched surface layers, which lead to an increase in the electron mobility in a strong magnetic field.     

Evidence for two-step disordering of the Au(110)−:1 × 2 reconstructed surface

We present the results of a synchrotron X-ray scattering study of the thermal disordering of the Au(110)−1 × 2 reconstructed surface. Observing the temperature dependence of the in-plane superlattice and integral-order surface peaks, as well as the (ll0) specular reflectivity, we find that at T_c = 735 K the 1 × 2 surface undergoes a reversible deconstruction transition characterized by a proliferation of compact antiphase defects with no measurable change in the density of surface steps. This transition is described by critical exponents close to those characterizing a two-dimensional Ising transition. We also find that by 784 K there has been a significant increase in the density of surface steps of the type associated with surface roughening. This suggests that the Au(110) surface disorders in a two-step process, deconstruction followed by roughening, with a difference in the two transition temperatures of less than 50 K.     

偏振对飞秒激光辐照LiF晶体的影响

飞秒激光聚焦到LiF晶体内部, 晶体的加工形貌随偏振改变. 实验表明, 偏振方向平行于<110> 晶向时, 加工起点到表面的距离是<100>偏振下的1.08 倍; 而<110>偏振下加工终点到表面的距离是<100> 偏振下的1.01 倍. 为了解释加工形貌的偏振依赖, 建立了逆韧致辐射、雪崩电离和无辐射跃迁的模型, 首先, 价带电子通过强场电离和雪崩电离, 从激光中吸收能量跃迁到导带, 该过程用电子密度演化方程和傍轴非线性薛定谔方程描述, 求解方程得到导带电子密度; 其次, 导带电子通过无辐射跃迁过程释放能量给晶格, 由能量守恒计算出晶格温度沿激光传播方向的分布; 最后, 晶格温度超过熔点以上的区域被加工. 模拟结果显示, <110>偏振下加工起点到表面的距离是<100> 偏振下的1.03倍, 而<110>偏振下加工终点到表面的距离是<100>偏振下的0.981 倍, 与实验结果基本一致. 虽然Z扫描技术测量的非线性折射率随偏振方向变化, 但是非线性折射率的变化趋势与实验结果相反. 模拟和实验证明逆韧致辐射导致加工形貌随偏振变化.     

镍单晶膜上氧化镍的配位生长取向关系

对在膜面为(110),(001)和(111)的镍单晶膜上生成的氧化镍取向进行了电子衍射分析,除在(111)和(001)镍膜上肯定了前人已发现的氧化镍与镍的平行取向关系外,还在(110)和(001)镍单晶膜上发现了(111)_NiO∥(001)_Ni,〈110〉_NiO∥〈110〉_Ni取向关系以及〈110〉_NiO∥〈110〉_Ni氧化镍纤维织构。在所有氧化镍与镍的取向关系中均有〈110     

Laser deposition of iron on graphite substrates

Pulsed laser deposition of iron atoms on graphite substrates was performed to produce iron carbide films. Mössbauer spectra of the sample revealed that iron carbide was produced on the substrate surface and that an α-Fe layer was produced above the iron carbide layer. When the substrate temperature was maintained at 300 K, the iron carbide layer had a hyperfine magnetic distribution because it contained high density of defects. Laser deposition of Fe at 570 K produced cementite Fe₃C with fewer defects due to enhancement of thermal reactions or annealing of the films. The orientation of hyperfine field of the Fe₃C film was parallel to the substrate surface.     

NEXAFS study of HCOO/Ag(110): Evidence for dynamic bending

The structure of the surface formate on Ag(110) at 300 K was studied by near-edge X-ray absorption fine structure (NEXAFS) at the carbon K-edge. Interpretation of the NEXAFS spectra of the formate intermediate with a localized bond picture is inconsistent with the findings of previous vibrational studies. Rather, the resonances must be assigned to transitions to delocalized molecular orbitals, whereupon the results agree with the bidentate configuration deduced from the vibrational experiments. A bonding geometry is determined with respect to the surface normal in which the O-O direction lies parallel to the plane of the surface; no azimuthal ordering was detected. The molecular plane appears to be inclined at an angle of 30 ± 15°. This apparent tilt may be due to a dynamical motion of the species about the surface normal in which the molecular plane oscillates and rotates about the surface normal. These results indicate different bonding geometries for formate on Ag(110) and Cu(110).     

不同取向下单晶铁纳米线拉伸行为的模拟研究

本文利用分子动力学模拟的方法研究了不同取向、尺寸和温度因素对单晶体心立方铁纳米线的拉伸变形行为的影响.铁纳米线轴向初始取向分别为<001>、<110>、<111>、<102>、<112>,模拟了不同温度(10～700 K)和不同尺寸范围(1.5～5 nm)下的变形机制.研究结果表明取向、尺寸和温度会显著影响单晶体心立方铁纳米线的拉伸变形行为.分子动力学模拟结果表明,直径为2 nm的<001>铁纳米线在300 K的拉伸载荷下,主要通过孪晶的模式发生变形,最后拉伸取向转变为<110>.而在700 K下,<001>铁纳米线的拉伸变形模式由滑移主导.不同初始取向在不同温度和尺寸下其变形机制截然不同,这导致了铁纳米线不同的力学性能.本文系统性地研究了在不同取向下的铁纳米线变形机制随尺寸和温度变化发生的转变.     

Self-organization of ripples on Ti irradiated with focused ion beam

30 keV focused Ga⁺ ions were used to raster the metallographically polished surface of commercially pure Ti (CP Ti) at various FIB incidence angles over a wide range of doses (10¹⁶-10¹⁸ ions/cm²) at room temperature. The sputtered surfaces were observed in situ using FIB imaging and later carefully characterized ex situ under scanning electron microscope (SEM) and atomic force microscope (AFM). Ripples were observed on the irradiated surfaces even at the normal FIB incidence angle. The ripple evolution is analyzed as functions of surface diffusion, surface crystallographic orientation, ion dose and incidence angle. It is found that the ripple orientation was progressively influenced by the ion beam direction with incidence angle increasing and in some cases curved ripples or fragmented rods viewed from different angles occurred at high ion doses. The morphological evolution from the well-developed straight ripples to the curved ones is never observed. The formation of ripples is attributed to the competition between the formation of ripples due to anisotropic surface diffusion and the formation of incidence-angle dependent ripples determined by Bradley-Harper (BH) model.     

Noncollinear magnetic hyperfine fields in the Ag spacers of Fe/Ag multilayers

Nearly perpendicular magnetic hyperfine fields have been observed for the first time in the Ag "spacers" of Fe/Ag multilayers using low temperature nuclear orientation of (110)Ag(m) at 6 mK. At the same time, vibrating sample magnetometry measurements at temperatures down to 4 K have shown the magnetic anisotropy of the Fe to be in plane. The direction of the Ag hyperfine field is thus noncollinear (nearly orthogonal) to the Fe anisotropy. These results are compared with full potential linearized augmented plane wave calculations using the wien97 code.     

Imaging surface electronic structure of NiAl(110) using low-temperature scanning tunneling microscopy

The surface electronic structure of NiAl(110) is examined by means of scanning tunneling microscopy and spectroscopy at a temperature of 4 K. Topography and conductance images for a wide range of bias voltages reveal wavelike patterns around steps and defects. Fourier transforms of conductance images are used to map the surface electronic structure of NiAl(110). We interpret the patterns in the Fourier transforms in terms of surface resonances, and analyze the details of its dispersion relation E(k_). A comparison with density-functional-based calculations and photoemission experiments is presented, and alternative explanations for the appearance of structures in Fourier transforms of conductance images are discussed.     

In situ measurement of the surface stress evolution during magnetron sputter-deposition of Ag thin film

We measured the evolution of in situ surface stress of Ag thin film during the magnetron sputter deposition. The measurement of force per width of Ag thin film showed that both the surface state and surface stress of Ag layer can be controlled through the variation of the deposition conditions such as the deposition temperature and rate. At room temperature, the force per width curve of Ag film deposited to 1 Å/s showed a typical curve consisting of three stages of surface stress. A brief presence of initial compressive stage and broad tensile maximum resulting in a compressive state had a tendency to disappear with increasing the deposition temperature. Meanwhile, a development of final compressive stage was more at higher temperature. Similar effect was observed but less obvious on increasing the deposition rate.