Ab-Initio Simulation Laboratory

The presence of sulfur atoms on the Pd(100) surface is known to hinder the dissociative adsorption of hydrogen. Using density-functional theory and the full-potential linear augmented plane-wave method, we investigate the potential energy surface (PES) of the dissociative adsorption of H-2 On the sulfur covered Pd(100) surface. The PES is changed significantly compared to the dissociation on the clean Pd(100) surface, particularly for hydrogen close to the S atoms. While the hydrogen dissociation at the clean Pd(100) surface is nonactivated, for the (2 x 2) sulfur adlayer (coverage Theta (S) = 0.25) the dissociation of H-2 is inhibited by energy barriers. Their heights strongly depend on the distance between the hydrogen and sulfur atoms leading to a highly corrugated PES. The largest barriers are in the vicinity of the sulfur atoms due to the strong repulsion between sulfur and hydrogen. Still the hydrogen dissociation on the (2x2) sulfur covered Pd(100) surface is exothermic. Thus the poisoning effect of sulfur adatoms for H-2 dissociation at low sulfur coverage (Theta(S) less than or equal to 0.25) is mainly governed by the formation of energy barriers, not by blocking of the adsorption sites. For the c(2 x 2) sulfur adlayer (Theta(S)= 0.5), the PES for hydrogen dissociation is purely repulsive. This is due to the fact that for all different possible adsorption geometries the hydrogen molecules come too close.to the sulfur adatoms before the dissociation is completed.

The 13-atom metal clusters of fcc elements (Al, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au) were studied by density functional theory calculations. The global minima were searched for by the ab initio random structure searching method. In addition to some new lowest-energy structures for Pd13 and Au13, we found that the effective coordination numbers of the lowest-energy clusters would increase with the ratio of the dimer-to-bulk bond length. This correlation, together with the electronic structures of the lowest-energy clusters, divides the 13-atom clusters of these fcc elements into two groups (except for Au13, which prefers a two-dimensional structure due to the relativistic effect). Compact-like clusters that are composed exclusively of triangular motifs are preferred for elements without d-electrons (Al) or with (nearly) filled d-band electrons (Ni, Pd, Cu, Ag). Non-compact clusters composed mainly of square motifs connected by some triangular motifs (Rh, Ir, Pt) are favored for elements with unfilled d-band electrons.

The adsorption and desorption of stilbene on Ag/Ge(111)-(root 3 x root 3)R30 degrees (Ag/Ge(111)-root 3) were investigated using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and density functional theory (I)FT). Both trans- and cis-stilbenes form a (2 x 1) overlayer structure on Ag/Ge(111)-root 3 at a coverage of similar to 1 ML. The STM images show parallel strips with three equivalent directions, indicating a self-ordered molecular structure. At a coverage of less than I ML, the TPD of cis-stilbene shows only one peak, attributed to submonolayer desorption. The TPD peaks are indistinguishable for desorption of trans-stilbene from the surface submonolayer and multilayer. This is due to the simultaneous desorption and/or thinning of adsorbed multilayers during the TPD process, as determined from the STM analysis of adsorbed trans-stilbene structures before and after annealing. The TPD traces fit the half-order kinetics for molecular desorption of stilbene from Ag/Ge(111)-root 3 with desorption energies of 20.1 (cis-) and 21.3 kcal/mol (trans-), which are comparable with the calculated values using the DFT method. A plausible explanation for the stilbene desorption process on Ag/Ge(111)-root 3 is proposed and discussed.

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Real-time in situ x-ray studies of continuous Pb deposition on Si(111)-(7x7) at 180 K reveal an unusual growth behavior. A wetting layer forms first to cover the entire surface. Then islands of a fairly uniform height of about five monolayers form on top of the wetting layer and grow to fill the surface. The growth then switches to a layer-by-layer mode upon further deposition. This behavior of alternating layer and island growth can be attributed to spontaneous quantum phase separation based on a first-principles calculation of the system energy.

We have calculated the multiple-scattering x-ray photoemission spectroscopy angular profiles of hcp Co(0001). Layer-by-layer emission contributions are presented, and the focusing directions are identified. Angular transformation of the pattern is performed to obtain real-space images of the nearest-neighbor atoms above the emitters.

Using scanning-tunneling microscopy and first-principles total-energy calculations, we have considered the structural properties of the so-called doped clusters formed by depositing additional 0.05 monolayer of In onto the 4x3-periodicity magic-cluster array in the In/Si(100) system. Low-temperature STM observations have revealed that most of the doped clusters have an asymmetric shape. According to the total-energy calculations, these clusters have plausibly Si6In8 composition. In such a cluster, one of the In atoms is mobile and can hop between four equivalent sites within a cluster. The hopping between sites, located in the different 2ax3a halves of the cluster, is characterized by the barrier of about 0.7 eV, and this hopping becomes frozen at 55 K. In contrast, the hopping between the neighboring sites within the same cluster half persists up to very low temperatures, as the barrier height here is an order of magnitude lower. Due to the above structural properties, the doped asymmetric Si6In8 cluster can be treated as a promising switch, logic gate, or memory cell of the atomic-scale size.

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Atomic structures of the reconstructed Si(113) surfaces were studied by using Kikuchi electron holography (KEH). Three-dimensional images show clearly the characteristics of the puckering model for both Si(113)(3x2) and (3x1) surfaces. The KEH results support the puckering model. Based on bur studies, the tetramers are puckering alternatively in the (3x2) surface. Whereas in (3X1) structures, there are two domains, within each of them, tetramers buckled uniformly, but the overall directions are opposite. When doped with H atoms on a (3x2) surface, the asymmetric tetramers change into symmetric form. [S0163-1829(99)51116-8].

We propose all the diffraction patterns can be directly transformed to provide three-dimensional atomic structures for the system studied. Depending on the scattering process, either the holography or Patterson transform scheme is used. For diffraction patterns which are generated from a localized emitter source or dominated by an inelastic-scattering feature like core-level photoelectron or low-energy Kikuchi electron, holography transform is needed. On the other hand, for diffraction patterns which were dominated by elastic-scattering, like grazing-incidence X-ray diffraction, electron correlated thermal diffuse scattering or low-energy electron diffraction curves, Patterson transform is needed. To prove our point, high-fidelity and artifact-free three-dimensional atomic structures obtained by transform of low-energy Kikuchi electron patterns and low-energy electron diffraction curves are presented. The future of these direct methods by transforming diffraction patterns will be discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.

The work function of atomically uniform Ag films grown on Fe(100) is measured as a function of film thickness. It shows layer-resolved variations as a result of quantum confinement of the valence electrons. A first-principles calculation reproduces the observed variations except for very thin films (one and two monolayers), and the differences can be attributed, in part, to strain effects caused by the lattice mismatch between Ag and Fe. These results illustrate the close interaction between interface effects and surface properties.