The oxidation of Ni(100) studied by medium energy ion scattering

We have used Rutherford backscattering (RBS) in combination with channeling and blocking to study the initial stages of oxidation of the Ni(100) surface at 325 and 415 K. Not only the oxygen coverage as function of exposure has been measured, but also the amount of nickel taking part in the oxidation process, both quantitatively. RBS results for the oxidation at both temperatures are consistent with the lateral growth of NiO islands.     

Silver sulphide growth on Ag(111): A medium energy ion scattering study

The interaction of S₂ with Ag(111) under ultra-high vacuum conditions has been investigated by medium energy ion scattering (MEIS). 100 keV He⁺ MEIS measurements provide a direct confirmation of a previous report, based on thermal desorption, that the growth of multilayer films of Ag₂S occurs through a continuous corrosion process. These films show a commensurate (√7 × √7)R19° unit mesh in low energy electron diffraction, consistent with the epitaxial growth of (111) layers of the high-temperature F-cubic phase of Ag₂S. The substantial range of co-existing film thicknesses found indicates that the growth must be in the form of variable-thickness islands. The use of 100 keV H⁺ incident ions leads to a very rapid decrease in the sulphide film thickness with increasing exposure that we attribute to an unusual chemical leaching, with implanted H atoms interacting with S atoms and desorption of H₂S from the surface.     

低温下Au959团簇负载于MgO(100)表面后结构变化的分子动力学研究

采用基于Chen-Mbius反演方法,从金属/金属氧化物界面第一性原理计算的粘结能结果中推导出的Au/MgO原子间相互作用势的正则系综（NVT）分子动力学,模拟了在10 K条件下,Au₉₅₉团簇负载于MgO（100）表面后团簇结构的变化.根据原子对分析技术和对分布函数的分析表明,由于团簇界面处原子间距与载体原子间距相匹配,置于载体上的Au团簇经过一个变形过程后,较其孤立自由表面时的团簇体积变大. 关键词： 团簇 分子动力学 计算机模拟 表面     

低温下Au959团簇负载于MgO(100)表面后结构变化的分子动力学研究

采用基于Chen-Mbius反演方法,从金属/金属氧化物界面第一性原理计算的粘结能结果中推导出的Au/MgO原子间相互作用势的正则系综(NVT)分子动力学,模拟了在10 K条件下,Au959团簇负载于MgO(100)表面后团簇结构的变化.根据原子对分析技术和对分布函数的分析表明,由于团簇界面处原子间距与载体原子间距相匹配,置于载体上的Au团簇经过一个变形过程后,较其孤立自由表面时的团簇体积变大.     

The structure of epitaxial V2O3 films and their surfaces: A medium energy ion scattering study

Medium energy ion scattering, using 100 keV H⁺ incident ions, has been used to investigate the growth of epitaxial films, up to thicknesses of ~ 200 Å, of V₂O₃ on both Pd(111) and Au(111). Scattered-ion energy spectra provide a measure of the average film thickness and the variations in this thickness, and show that, with suitable annealing, the crystalline quality is good. Plots of the scattering yield as a function of scattering angle, so-called blocking curves, have been measured for two different incidence directions and have been used to determine the surface structure. Specifically, scattering simulations for a range of different model structures show poor agreement with experiment for half-metal (….V′O₃V) and vanadyl (….V′O₃V=O) terminations, with and without surface interlayer relaxations. However, good agreement with experiment is found for the modified oxygen-termination structure, first proposed by Kresse et al., in which a subsurface V half-metal layer is moved up into the outermost V buckled metal layer to produce a VO₂ overlayer on the underlying V₂O₃, with an associated layer structure of ….O₃VV′′V ′O₃. This result is consistent with the predictions of thermodynamic equilibrium at the surface under the surface preparation conditions, but is at variance with the conclusions of earlier studies of this system that have favoured the vanadyl termination. The results of these previous studies are re-evaluated in the light of the new result.     

Multilayer relaxations of Ni(110): New medium energy ion scattering results

Recent ion scattering and LEED measurements of the clean Ni(110) surface structure show a significant difference in the magnitude of the first layer relaxation. Ion scattering has measured the first layer contraction to be 4.5% while LEED I–V analyses have resulted in measurement of an 8.5% contraction. In an attempt to resolve this apparent discrepancy we have undertaken a careful reexamination of the clean Ni(110) surface with medium energy ion scattering using the technique of channeling and blocking. The experiment was conducted in a UHV chamber with a toroidal electrostatic analyzer using 110 keV protons. Our analysis was based on comparisons of measured surface blocking curves in two scattering geometries with full crystal Monte Carlo simulations. These simulations include variations of the interlayer spacing of the outer three surface layers and of the surface Debye temperature. Our measurement shows a first layer contraction, D₁₂ = −9.0% ± 1.0% and a second layer expansion, D₂₃ = +3.5% ± 1.5% (measured as percent of the bulk interlayer spacing) with a surface Debye temperature of 395 K. The present result is in excellent agreement with two recent LEED measurements.     

Improvement of interface structure and magnetic properties of Co on Si (100) by surfactant (Sb) mediated growth

In this study thin Co films were grown on Si (100): (1) with one monolayer of Sb as surfactant and (2) without any surfactant. The Co film, its interface with the Si substrate and the behavior of the Sb surfactant layer were investigated during the growth by high-resolution Rutherford backscattering. By the use of Sb, the evaporated cobalt grows in a layer-by-layer mode and the mixing of Co and Si at the interface is strongly reduced. During the evaporation of Co, Sb floats on the surface for all Co coverages with some incorporation in the grown Co film only for higher coverages. The improvement of the interface quality is also reflected in the magnetic properties of the Co film.     

An investigation of the kinetics of structural changes during the early oxidation stages of a Ni(100) surface using low energy ion scattering (LEIS)

Low energy ion scattering has been used to investigate the early stages of the oxidation of a Ni(100) surface. This technique allows simultaneous study of the oxygen uptake in the surface and the development of surface structures. Bombardment induced surface damages was minimised by performing the experiments with low ion doses, while keeping the target at 200–300°C. The measured kinetics of the oxygen uptake are in good agreement with recent work, using different techniques. It is concluded that during the early chemisorption, a two stage process takes place: an initial oxygen adsorption during which the O atoms probably reside within the fourfold surface hollows, followed by a reconstruction process, caused by the combined action of at least two nearest neighbour O atoms, trapping mobile Ni adatoms, after which the O atoms stabilise at a site in or close to the reconstructed 〈001̄〉 row. Observed structural changes at higher exposures are compatible with a transition into a (3 × 1) structure and subsequently NiO, but cannot, as yet be positively identified.     

Electronic States and Schottky Barrier Height at Metal/MgO(100) Interfaces

Using an ab initio total energy approach, we study the electronic structure of metal/MgO(100) interfaces. By considering simple and transition metals, different adsorption sites and different interface separations, we analyze the influence of the character of metal and of the detailed interfacial atomic structure. We calculate the interface density of states, electron transfer, electric dipole, and the Schottky barrier height. We characterize three types of electronic states: states due to chemical bonding which appear at well defined energies, conventional metal-induced gap states associated to a smooth density of states in the MgO gap region, and metal band distortions due to polarization by the electrostatic field of the ionic substrate. We point out that, with respect to the extended Schottky limit, the interface formation yields an electric dipole mainly determined by the substrate characteristics. Indeed, the metal-dependent contributions (interfacial states and electron transfer) remain small with respect to the metal polarization induced by the substrate electrostatic field.     

Relaxation effects and thermal vibrations in a Pt(111) surface measured by medium energy ion scattering

Medium energy ion scattering, in conjunction with channeling and blocking has been applied to search for differences in the planar spacing of the first two layers of a Pt(111) crystal surface as compared to the spacing in the bulk. We find a small expansion of 1.5 ± 1%. The measurements have yielded a value for the transverse rms thermal vibration amplitude of a surface atom relative to its neighbour along the [001] direction and likewise for the [110] axis. The results are giving evidence for an enhanced amplitude for the surface atom perpendicular to the surface. A strong blocking effect has been observed along the [110] axis, which may be due to correlation effects, although the role of surface defects cannot be ruled out.     

Magnetic phase transition in Co/Cu/Ni/Cu(100) and Co/Fe/Ni/Cu(100)

Magnetic phase transitions in coupled magnetic sandwiches of Cu/Co/Cu/Ni/Cu(100) and Cu/Co/Fe/Ni/Cu(100) are investigated by photoemission electron microscopy. Element-specific magnetic domains are taken at room temperature to reveal the critical thickness at which the magnetic phase transition occurs. The results show that a coupled magnetic sandwich undergoes three types of magnetic phase transitions depending on the two ferromagnetic films' thickness. A phase diagram is constructed and explained in the process of constructing Monte Carlo simulations, which corroborate the experimental results.     

Structure study of oxygen-adsorbed Ni(111) surface by high energy ion scattering

Surface atomic structures of clean, oxygen-adsorbed, and oxidized (111) nickel have been studied quantitatively by using MeV ion scattering in combination with AES and MEED. We show that; the clean (111) nickel surface has the bulk-like structure with reconstruction or relaxation less than 0.02 Å, the surface thermal vibration amplitude is enhanced by ~20% compared to the bulk value, adsorbed oxygen results in surface lattice expansion by ~0.15 Å which is closely correlated to the p(2 × 2) or (√3 × √3) R30° superstructure, and oxidation at room temperature saturates at a stage which incorporates ~ 3 monolayers of nickel in a stoichiometric amorphous film of NiO whereas at temperatures higher than ~200° C thicker oxide films are produced. Our study indicates that each oxygen atom adsorbed on the Ni(111) surface interacts with and relaxes three nearest neighbor nickel atoms, and after saturation of the relaxation, oxidation of three monolayers takes place abruptly after which the oxide layer on the surface apparently blocks further reaction.     

Local spin density total energy study of surface magnetism: V (100)

Results of self-consistent all-electron local (spin) density functional studies of the electronic and magnetic properties of vanadium (100) 1-, 3-, 5- and 7-layers films are reported using our full-potential linearized augmented plane wave (FLAPW) method. The calculated work function, 4.2 eV, agrees very well with the experimental value of 4.12 eV. From both Stoner factor analyses and spin-polarized total energy calculations, it is concluded that V(100) undergoes a ferromagnetic phase transition only for the monolayer system. The magnetic moment is found to be 3.09μ_B per atom of this monolayer film and to have a total energy 57 mRy below that of the paramagnetic structure. For multilayer V(001) systems, the sharp surface density-of-states peak which is characteristic of the occurrence of surface magnetism in the 3d transition metals is located 0.3 eV above the Fermi level. As a result, the paramagnetic state is stable. In addition, no enhancement of the exchange-correlation integral is found for the surface atoms compared with the bulk value. The lower energy of the paramagnetic structure is further supported by total energy investigations of the multilayer relaxation of V(100) — the calculated interlayer spacings for the paramagnetic surface with a 9% contraction of the topmost interlayer spacing and a 1% expansion of the second interlayer spacing with respect to its bulk value are in good agreement with LEED measurements. It is suggested that the surface magnetism of V(100) may be associated with surface oxygen or caused by impurity induced surface reconstructions.