Ab-Initio Simulation Laboratory

Using ab initio density-functional theory, the self-diffusion of adatom and dimer on fcc(100) metal surfaces are studied. For adatom diffusion, we find that the exchange mechanism is favored for Al, Ir, Ni, Pd, Pt and Au, while the hopping mechanism is favored for Rh, Cu, and Ag. Except for Ir/Ir(100), the exchange diffusion energy has a surprising large size-effect and decreases as the surface unit cell increases. This is due to the long-ranged strain-field created at the exchange transition state, which needs a larger cell to relax. The hopping diffusion energy, on the other hand, has a very small size-effect and keeps approximately the same value for various surface unit cells. For self-diffusion on lr(100), the formation of covalent bonds are found at the exchange transition state, and thus the exchange diffusion energy has a little size-effect. Our results also indicate that the exchange mechanism is energetically more favorable for dimer diffusion on fcc(100) surface whenever it is favored for adatom diffusion on fcc(100) surface.

Using ab initio calculations, we fmd that the calculated energy barrier for exchange diffusion of Ni adatoms on Ni(100) surfaces shows a surprisingly large dependence on the size of the surface unit cell. It decreases from 1.39 to 0.78 eV when the cell size changes from (2 x 2) to (6 x 6). This is due to the long-ranged strain field created by the transition state for atomic exchange, which needs a larger cell to relax. The hopping diffusion energy, on the other hand, shows only a very small size effect and remains approximately constant at 0.82-0.86 eV, independently of the cell size. Our results indicate that Ni diffusion on Ni(100) occurs by the exchange mechanism and this is consistent with recent experiments. Previous results obtained using (3 x 3) or (4 x 4) unit cells did not converge sufficiently well to yield correct conclusions.

We use ab initio density-functional theory supplemented with the embedded-atom method to study the self-diffusion of small clusters on the (111) surface of eight fee metals. A zigzag motion is found to be important in the dimer and tetramer diffusions. The dimer diffuses by a zigzag and concerted motion. The trimer diffuses by a concerted three-atom motion. The tetramer diffuses through a zigzag motion where only two atoms move simultaneously in each step. Thus, instead of increasing, the migration energy is lowered (or stays constant) for the tetramer as compared to that for the trimer. This novel break of the upwards trend in migration energy is predicted to be a general phenomenon.

Previous energetic considerations have led to the belief that carbon nanotubes (CNTs) of 4 Angstrom in diameter are the smallest stable CNTs. Using high-resolution transmission electron microscopy, we find that a stable 3 Angstrom CNT can be grown inside a multiwalled carbon nanotube. Density functional calculations indicate that the 3 Angstrom CNT is the armchair CNT(2,2) with a radial breathing mode at 787 cm(-1). Each end can be capped by half of a C(12) cage (hexagonal prism) containing tetragons.

Self-assembly of atoms or molecules on a crystal surface is considered one of the most promising methods to create molecular devices. Here we report a stepwise self-assembly of C60 molecules into islands with unusual shapes and preferred sizes on a gold–indium-covered Si(111) surface. Specifically, 19-mer islands prefer a non-compact boomerang shape, whereas hexagonal 37-mer islands exhibit extraordinarily enhanced stability and abundance. The stepwise self-assembly is mediated by the moiré interference between an island with its underlying lattice, which essentially maps out the adsorption-energy landscape of a C60 on different positions of the surface with a lateral magnification factor and dictates the probability for the subsequent attachment of C60 to an island’s periphery. Our discovery suggests a new method for exploiting the moiré interference to dynamically assist the self-assembly of particles and provides an unexplored tactic of engineering atomic scale moiré magnifiers to facilitate the growth of monodispersed mesoscopic structures.

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Trimer is the smallest cluster that can have a one-dimensional or two-dimensional structure on surfaces, and it can diffuse and transform between these structures. Using first-principles density-functional theory (DFT) calculations, the structural and dynamical behaviors of Al trimer on Al(111) surface have been studied in detail. Al trimer on Al(111) surface has three different kinds of structure conformations (groups with similar configurations): close-packed (compact) triangular trimers, non-compact triangular trimers, and linear trimers. The close-packed triangular trimers are more stable than the non-compact triangular trimers and the linear trimers, while most of the non-compact triangular trimers are as stable as the linear trimers. For the dynamics of Al trimer on Al(111) surface, there are three different kinds of diffusion mechanisms: (1) concerted translations and rotation of compact triangular trimers (the highest energy barrier by DFT calculation

Basing on the results of the scanning tunneling microscopy (STM) observations and density functional theory (DFT) calculations, the structural model for the Cu magic clusters formed on Si(1 1 1)7 x 7 surface has been proposed. Using STM, composition of the Cu magic clusters has been evaluated from the quantitative analysis of the Cu and Si mass transport occurring during magic cluster converting into the Si(1 1 1)’5.5 x 5.5’-Cu reconstruction upon annealing. Evaluation yields that Cu magic cluster accommodates similar to 20 Cu atoms with similar to 20 Si atoms being expelled from the corresponding 7 x 7 half unit cell (HUC). In order to fit these values, it has been suggested that the Cu magic clusters resemble fragments of the Cu2Si-silicide monolayer incorporated into the rest-atom layer of the Si(1 1 1)7 x 7 HUCs. Using DFT calculations, stability of the nineteen models has been tested of which five models appeared to have formation energies lower than that of the original Si(1 1 1)7 x 7 surface. The three of five models having the lowest formation energies have been concluded to be the most plausible ones. They resemble well the evaluated composition and their counterparts are found in the experimental STM images. (C) 2009 Elsevier B.V. All rights reserved.

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Trimer is the smallest cluster that can have a one-dimensional or a two-dimensional structure on fee (111) surface. Using first-principles density-functional-theory calculations, the structural and dynamical properties of Al trimer on Al(111) surface have been studied in detail. Al trimer on Al(111) surface has four close-packed (compact) triangular configurations, two linear configurations, and some other noncompact triangular configurations. The close-packed triangular trimers are more stable than the noncompact triangular trimers as well as the linear trimers. For the dynamics of Al trimer on Al(111) surface, the diffusion processes are much more complicated than the adatom and dimer diffusions. There are three different kinds of diffusion mechanisms: concerted translations and rotation of compact triangular trimers (the energy barrier

The structure of a type of surface magic cluster is determined by a combination of scanning tunneling microscopy, density-functional calculations, and dynamical low energy electron diffraction. The diffraction method is applicable because these clusters created through hierarchical self-organization of Ga deposited onto a Si(111)-7x7 surface have identical size and structure and form an ordered array with exact translational symmetry. The unprecedented detailed structure information provided by the diffraction measurement is consistent with direct microscopic imaging and theoretical calculations.

A comprehensive atomic model for the reconstructed surface of Si3N4 thin layer grown on Si(lll) is presented. Kikuchi electron holography images clearly show the existence of adatoms on the Si3N4(0001)/Si(111)-(8 x 8) surface. Compared with the nb initio calculations, more than 30 symmetry-inequivalent atomic pairs in the outmost layers are successfully identified. Scanning tunneling microscopy (STM) images show diamond-shaped unit cells and nine adatoms in each cell. High-resolution STM images reveal extra features and are in good agreement with the partial charge density distribution obtained from total-energy calculations.

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Energy- and angle-dependent photoelectron cross sections from surface and bulk W(110) 4f7/2 core levels are measured and compared with dynamical multiple scattering calculations. The agreement between experimental and theoretical results is found to be significantly better than corresponding previous studies, permitting a determination of the first layer atomic plane distance: d12 = 2.26 +/- 0.05 angstrom. Forward-scattering enhancements along bond directions are observed under selected scattering conditions. In all cases, final-state multiple scattering accounts for the principal energy and angle dependencies in the cross section. Typical variation of bulk and surface 4f photoelectron intensities with kinetic energy or emission angle resulting from final-state effects is observed to be a factor of 2. This result suggests that previous core-level spectra for stepped W(110) surfaces have been incorrectly interpreted.

We obtained highly resolved and artifact-free 3D holographic images reconstructed from measured Kikuchi electron (quasi-elastic electron) diffraction patterns with contributions from different emitters. Direct inversion of Kikuchi patterns with glancing incidence geometry shows clear images of the surface dimer and the bulk atoms of Si(001)(2 x 1) surface. This observation demonstrates the applicability of electron-emission holography to complicated systems that contain more than one emitter. This work also demonstrates the surface sensitivity of Kikuchi electron holography.

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We find that electron diffraction patterns can be directly inverted to provide three-dimensional atomic structures for the system studied. Depending on the scattering process, either holography or a Patterson inversion scheme is used. For diffraction patterns which were generated from a localized emitter source or predominantly by an inelastic-scattering feature like low-energy Kikuchi electrons, holography inversion is needed. The information obtained from Kikuchi electron holography includes the building blocks on the surface and their relative position to the atoms below the surface layer. On the other hand, for diffraction patterns generated predominantly by an inelastic-scattering feature like low-energy electron diffraction (LEED), a Patterson inversion is needed. The information obtained from the Patterson transform of the LEED I(E) curves is the relative positions of surface atoms to the atoms in underlying layers; no intra-layer information can be extracted with this method. High-fidelity and artifact-free three-dimensional atomic structures obtained by inversion of low-energy Kikuchi electron patterns and low-energy electron diffraction curves are presented. The results from the two inversion methods are complementary and can be used to construct or to discriminate the surface atomic structural models. The future of these direct methods by inverting diffraction patterns is discussed.

Whether photoemission probes surface or bulk properties has long been a topic of interest and debate. This work employs angle-resolved photoemission to map the electronic structure of Ag films of varying thicknesses prepared on Si(111). As expected, the discrete quantum-well states or subbands observed at small thicknesses merge into a continuum as the film thickness approaches the bulk limit. However, a number of discrete states remain isolated within gaps or pockets in the bulk continuum. While these Ag surface states have been predicted previously by calculations, most are experimentally identified herein only for the first time. Copyright (C) EPLA, 2009

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Direct inversion of measured multiple-energy Kikuchi electron patterns from a Si(001) (2 x 1) surface with glancing and normal-incidence geometry shows clear images of the surface dimer and the bulk atoms. The three-dimensional artifact-free real-space images of the atoms contributed from different local emitters are resolved clearly. The observations demonstrate that Kikuchi electron holography has the surface sensitivity and can reveal the atomic structures of complicated multiemitter systems. By changing the collecting angle of Kikuchi electrons, one can selectively image the atoms behind the emitter in the backward direction; thus the surface and the bulk information can be obtained with different collecting angles. Therefore, the potential of Kikuchi electron holography to solve the local atomic structure of the unknown surfaces is high.

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