Chang, CY, Hong IH, Chou YC, Wei CM.
2001.
Surface structures by direct transform of electron diffraction patterns, Nov. Journal of Physics-Condensed Matter. 13:10709-10728., Number 47
AbstractWe 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.
Ahn, H, Wu CL, Gwo S, Wei CM, Chou YC.
2001.
Structure determination of the Si3N4/Si(111)-(8 x 8) surface: A combined study of Kikuchi electron holography, scanning tunneling microscopy, and ab initio calculations, Mar. Physical Review Letters. 86:2818-2821., Number 13
AbstractA 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.
Chang, CM, Wei CM, Hafner J.
2001.
Self-diffusion of adatoms on Ni(100) surfaces, Apr. Journal of Physics-Condensed Matter. 13:L321-L328., Number 17
AbstractUsing 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.
] 
