Oxygen adsorption on stepped Pd(100) surfaces

We use scanning tunneling microscopy (STM) and high-resolution core-level spectroscopy (XPS) measurements to study the initial oxidation of vicinal Pd(100) surfaces exhibiting close-packed (111) steps. The XPS data analysis is supported by detailed surface-core level shift calculations based on density-functional theory. Both STM images and the XPS spectra are found to be perfectly consistent with a characteristic zigzag O decoration of the Pd steps predicted by a preceding cluster-expansion based theoretical study [Y. Zhang and K. Reuter, Chem. Phys. Lett. 465, 303 (2008)]. Continued oxygen uptake leads to the additional stabilization of a p(2 × 2)-O overlayer on the Pd(100) terraces, and ultimately to step bunching with the resulting large Pd(100) terraces covered by the PdO(101) surface oxide.     

Theoretical studies of hydrogen molecule adsorption on flat and stepped platinum surfaces

Theoretical calculations have been applied to the adsorption of hydrogen molecule on flat and stepped platinum surfaces. The method of large unit cell is modified to deal with the stepped surface as well as the flat one. This method is free from the boundary effect which is inevitable in the cluster method. The results calculated for the d-band width and the highest occupied level are in good agreement with the experiments. For the dissociative adsorption, the bottom of the step site is the most favorable, and the result is attributed to the extra orbital interactions at this site.     

Molecular dynamics simulation of manipulation of metallic nanoclusters on stepped surfaces

Molecular dynamics simulations are carried out to investigate the manipulation of metallic clusters on stepped surfaces. Five surface forms are considered in the simulations. The system parts are made of pure transition metals and Sutton-Chen many-body potential is used as interatomic potential. The conditions which are subjected to change in the tests include: materials used for particles and substrate, and surface step conditions. In addition to qualitative observations, two criteria which represent the particle deformation and substrate abrasion are utilized as evaluation tools and are computed for each case. Simulation results show the effect of the aforementioned working conditions on the particle behavior as well as changes in the pushing forces. Obtaining this sort of knowledge is highly beneficial for further experiments in order to be able to plan the conditions and routines which guarantee better success in the manipulation process.     

Diffraction from stepped surfaces: I. Reversible surfaces

In electron or atom diffraction experiments on surfaces, the angular shapes of the diffracted beams depend upon the distribution of steps over the surface. In this paper we analyze diffracted beam profiles from stepped surfaces that are reversible. A reversible surface is one in which the pair correlation function over the surface is symmetric with respect to positive and negative directions. We show that the intensity profile across a diffracted beam can be separated into a sharp central spike due to the limit of the correlation function at large separation plus wings or shoulders due to the finite extent of the step disorder. Simple functional expressions for these angular profiles are obtained by a Markov method of treating a one-dimensional geometric distribution of steps. The result explicitly displays the deep structure found for the general case. The method reduces the calculation to a simple eigenvalue problem so that even the continuously changing step distributions that occur in epitaxial growth can be treated easily. As in the general case, the resulting intensity profile is a sharp central spike plus a step-broadened term which now is a sum of Lorentzians. The widths of the Lorentzians are the logarithms of the eigenvalues of the matrix of probabilities which describe the step distribution over the surface. This matrix method, which treats the surface as a Markov chain, also points the correct way to account for correlations between surface atoms for two-dimensional distributions of steps. For a two-dimensional surface one must consider a Markov Random Field as opposed to a simple multiplication of two one-dimensional results. We compare the results of the general calculation to the Si epitaxy experiments of Gronwald and Henzler. The coverage and momentum transfer dependencies of the shapes of the calculated profiles agree with their measurements. The calculation is also applied to the RHEED measurements of Van Hove et al. during GaAs MBE. The measured intensity oscillations can be accounted for by a cyclically changing one-dimensional geometric distribution of steps among three layers in which the third-layer scattering increases with time.     

Kinetics of adsorption on stepped surfaces and the determination of surface diffusion constants

Nitrogen adsorption on stepped W(110) surfaces is examined to illustrate a theory of surface kinetics. Experimental findings by Besockeet al. have shown that nitrogen chemisorbs dissociatively only at the step corner sites of a W(110) surface. Thus the rate of dissociation reveals the mobility of nitrogen and its interaction with the surface. Using continuous-time-random-walk theory, we obtain the probability that molecules reach the step corner sites as a function of time. A kinetic model of nitrogen dissociation is proposed to calculate a coverage function that is in good agreement with experiment. The surface diffusion constant of nitrogen molecules is obtained and is in accordance with previous observations that nitrogen molecules are first weakly physisorbed on the W(110) terrace. Finally, the coverage functions for different step densities are predicted.     

Density functional slab model studies of water adsorption on flat and stepped Cu surfaces

Density functional and periodic slab model calculations are performed to study adsorption of water on various Cu surfaces, focusing on monomers and dimers at the planar Cu surfaces and monomers at stepped ones. Single water molecules tend to weakly bind to atop positions with the molecular plane basically parallel to the substrate surface on the planar surfaces or the step plane on the stepped surfaces with negligible structural deformation of water. The experimental adsorption energies of water on the (1 1 1) and (1 0 0) surfaces are about twice as large as the theoretical values of monomerically adsorbed water. This phenomenon is demonstrated to be due to formation of water clusters and/or existence of surface defects. It is revealed that the most favorable hexagonal ring superstructure on Cu(1 1 0) is a four-layer structure, not the commonly accepted bi-layer configuration. We found that the adsorption energy of monomeric water correlates linearly with following quantities, respectively: the bond length and the stretching frequency of the Cu-O bond, the coordination number of the surface Cu atom, the surface work function of the clean surface and the 1b₁ MO energy shift with respect to the value in the gas phase.     

Monte Carlo simulation of hydrogen adsorption on Ni surfaces

In the present paper the adsorption kinetics of the hydrogen molecule on the (111) and (100) surfaces have been studied with the model proposed by Panczyk and the grand canonical Monte Carlo simulation method. The equilibrium adsorption isotherms are calculated at five different temperatures ranging from 314 K to 376 K and compared with the experimental equilibrium adsorption isotherms. The effects of temperature and pressure on coverage are also analyzed.      

DFT study of CO and NO adsorption on low index and stepped surfaces of gold

Adsorption energies and vibrational frequencies of CO and NO adsorbed on gold (1 1 1), (1 0 0), (1 1 0) and (3 1 0) surfaces, as well as on adatoms on Au(1 0 0) have been calculated using density functional theory. The results clearly show that the adsorption energy of the molecules increases considerably with increasing the degree of coordinative unsaturation of the gold atoms to which the molecules bind, and thus support the view that defects, steps and kinks on the surface determine the activity of gold catalysts.     

LEED studies on stepped W surfaces

Steps on single crystal surfaces have been shown to cause changes of various physical properties and to influence the behaviour towards chemical reactions. A proper knowledge of the step structure is required for the understanding of these phenomena. The following investigation concentrates on a detailed evaluation of the LEED patterns of various stepped tungsten surfaces. A formula is given for determining the terrace width of ordered step arrays from any diffraction order. Step height, step orientation, terrace width and the sample orientation have been deduced from the LEED patterns and the experimental errors involved are being discussed. The step height can be determined within 1% and the terrace width within 1 to 3% depending on the step density. It is concluded that the determination of the sample orientation as obtained from the LEED pattern is at least as precise as by using the Laue X-ray back reflection technique.     

Structural phase transitions on stepped surfaces

We predict a new class of structural phase transitions which occur on a microscopic length scale on stepped surfaces. These include commensurate and incommensurate “step-step” transitions. They can occur either on clean equilibrium surfaces or can be adsorbate induced. We also present a Landau-Lifshitz classification of some of these structural phase transitions. Critical exponents characterizing these transitions can be experimentally determined by LEED measurements.     

Segregation at stepped surfaces

A theoretical model of segregation at a (120) stepped surface with ($\text{1}$10) monoatomic steps and terraces on the (110) plane of a body centered cubic alloy is presented. The model is based on pairwise interactions between nearest neighbours only and includes long and short range order effects. Results are presented for the concentration at different sites on the stepped surface as well as for the short range order parameters as functions of temperature.     

A model calculation on adsorption of CO on the flat,stepped and kinked nickel surfaces

The adsorption of CO on Ni was investigated by quantum chemical calculations using the CNDO/2 tight binding method. The surfaces used as models are the (111), 4(111) × (111), 3(111) × (110) and 3(111) × (100) surfaces. The CO bond is weakened in this sequence of surfaces. The active sites for the CO bond fission are the trench regions of the step and kink structures. The Ni 3d orbitals play an important role for the weakening of the CO bond, though their contribution is small for the Ni-C bond formation.     

Modeling of adsorption and phase diagrams for stepped surfaces: Transfer matrix approach

The simplest model of stepped surface has been constructed and scrutinized with different values of model parameters. The transfer matrix method was shown to be a very effective approach. Phase diagrams and local isotherms have been obtained. It was shown that the local coverage can be the non-monotonous function of the total coverage or the chemical potential.     

ZnO 极性表面及其N原子吸附机理的第一性原理研究

基于密度泛函理论的第一性原理赝势法计算了ZnO极性表面的几何结构和电子结构特性,对比分析了ZnO(0001)和ZnO(0001)表面结构弛豫、能带结构、电子态密度及N吸附ZnO极性表面的形成能情况.计算结果表明: 相对于ZnO(0001)表面,ZnO(0001)表面受结构弛豫影响更加明显,而ZnO(0001)表面完整性更好.相对于体相ZnO结构,ZnO(0001)表面的能带带隙变窄,同时价带顶附近能级非局域性增强使晶体表面的导电性能变得更好;而ZnO(0001)表面的能带带隙变宽,由于O^{-关键词： 密度泛函理论 第一性原理 ZnO极性表面 N吸附}     

Atomistic structure of stepped surfaces

Atomistic computer simulation with embedded atom method (EAM) interatomic forces was used to study the structure of surface steps on the {111} unreconstructed surface in fcc metallic materials. The energetics and local atomic relaxation behavior of ledges parallel to the 110 direction were studied using a potential describing lattice properties of Au. The vacancy formation energies in the stepped surfaces was also studied, and it was found that the energy of formation of a vacancy in a terrace is the same as that in the perfect unstepped surface. This value is 30% lower than that of the bulk. The vacancy formation energy in the ledge is reduced by a factor of two with respect to that of the terraces. The structure of the “up ledge” (A step) is different from the “down ledge” (B step). These differences do not significantly affect the energy of the ledges, although they do affect the vacancy formation energies in sites in the second surface layer near the ledge. The implications of the results for the formation of kinks and the general structure of high index surfaces are discussed.     

Dissociation of carbon monoxide on stepped nickel surfaces

Field emission microscopy with photometric measurements has been applied to investigate the adsorption of carbon monoxide on various crystal planes of a nickel field emitter. Upon heating the CO-covered surface, the work function drastically decreased during desorption of CO molecules into the gas phase and exhibited almost the value of the clean surface at 450 K. However, part of the CO molecules adsorbed on the stepped planes such as (510) and (310) were found to dissociate upon heating at 450–470 K, which was accompanied by an increase of the work function of ～0.2 eV.