Chang, C. M., Wei C. M., & Chen S. P.
(2000).
Self-diffusion of small clusters on fcc metal (111) surfaces.
Physical Review Letters. 85, 1044-1047., Jul, Number 5
AbstractWe 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.
Chang, C. Y., Chou Y. C., & Wei C. M.
(1999).
Atomic structures and phase transitions of Si(113) reconstructed surfaces: Kikuchi electron holography studies.
Physical Review B. 59, 10453-10456., Apr, Number 16
AbstractAtomic 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].
Chang, J. C., Wei C. M., Kuo T. Y., & Huang K. N.
(1994).
THEORETICAL THRESHOLD CROSS-SECTION OF ELECTRON-IMPACT IONIZATION OF THE HYDROGEN-ATOM.
Journal of Physics B-Atomic Molecular and Optical Physics. 27, 4715-4733., Oct, Number 19
AbstractSingle differential and total cross sections of the electron-impact ionization of the hydrogen atom are calculated numerically in the two-potential distorted-wave approximation for excess energies from 0 to 1 eV. Partial-wave contributions to the cross sections are also investigated. The near-threshold cross section is parametrized by the power gamma and the proportionality constant a(0) for models with various asymptotic charges, and the dependence of a(0) on the asymptotic charge is studied. The validity range of the threshold power law is also discussed.
Chang, C. M., Wei C. M., & Hafner J.
(2001).
Self-diffusion of adatoms on Ni(100) surfaces.
Journal of Physics-Condensed Matter. 13, L321-L328., Apr, 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.
Chang, C. Y., Hong I. H., Chou Y. C., & Wei C. M.
(2001).
Atomic structures by direct transform of diffraction patterns.
Journal of Physics and Chemistry of Solids. 62, 1777-1788., Sep-Oct, Number 9-10
AbstractWe 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.
Chang, C. Y., Lin Z. C., Chou Y. C., & Wei C. M.
(1999).
Direct three-dimensional Patterson inversion of low-energy electron diffraction I(E) curves.
Physical Review Letters. 83, 2580-2583., Sep, Number 13
AbstractA Patterson-like scheme is proposed for direct inversion of the conventional low-energy electron diffraction (LEED) intensity versus energy I(E) curves, which is in contrast with the previously suggested holographic scheme. Using the Si(111)-(7 X 7) and Si(113)-(3 X 2) surfaces as examples, high quality three-dimensional images, with a resolution better than 0.5 Angstrom, of both surface atoms and bulk atoms are obtained from the direct Patterson inversion of LEED-I(E) curves with the integral-energy phase-summing method.
Chang, C. Y., Hong I. H., Chou Y. C., & Wei C. M.
(2001).
Surface structures by direct transform of electron diffraction patterns.
Journal of Physics-Condensed Matter. 13, 10709-10728., Nov, 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.
Chang, C. M., Wei C. M., & Chen S. P.
(1996).
Modeling of Ir adatoms on Ir surfaces.
Physical Review B. 54, 17083-17096., Dec, Number 23
AbstractWe used the embedded-atom method potential to study the structures, adsorption energies, binding energies, migration paths, and energy barriers of the Ir adatom and small clusters on fcc Ir (100), (110), and (111) surfaces. We found that the barrier for single-adatom diffusion is lowest on the (111) surface, higher on the (110) surface, and highest on the (100) surface. The exchange mechanisms of adatom diffusion on (100) and (110) surfaces are energetically favored. On all three Ir surfaces, Ir-2 dimers with nearest-neighbor spacing are the most stable. On the (110) surface, the Ir-2 dimer diffuses collectively along the (110) channel, while motion perpendicular to the channel walls is achieved by successive one-atom and correlated jumps. On (111) surface, the Ir-2 dimer diffuses in a zigzag motion on hcp and fee sites without breaking into two single atoms. On the (100) surface, diffusion of the Ir-2 dimer is achieved by successive one-atom exchange with the substrate atom accompanying by a 90 degrees rotation of the Ir-2 dimer. This mechanism has a surprisingly low activation energy of 0.65 eV, which is 0.14 eV lower than the energy for single adatom exchange on the (100) surface. Trimers were found to have a one-dimensional (1D) structure on (100) and (110) surfaces, and a 2D structure on the (111) surface. The observed abrupt drop of the diffusion barrier of tetramer, I-gamma 4 on the Ir (111) surface was confirmed theoretically.
Chang, C. S., Su W. B., Wei C. M., & Tsong T. T.
(1999).
Large Fermi density waves on the reconstructed Pt(100) surface.
Physical Review Letters. 83, 2604-2607., Sep, Number 13
AbstractSeveral long-range superstructures have been observed with the scanning tunneling microscopy on the reconstructed Pt(100) surface at room temperature. They are present in strained domains and involve both the Fermi electrons and the concomitant lattice distortions. A first-principles calculation shows that the top layer expanded similar to 18% on average and the Fermi surface for a single hexagon layer displays some nesting portions, which can be related to the wave vectors of the observed superstructures. Thus, these superstructures existing in the local domains of the reconstructed surface have the likely origin of incipent charge density waves.
Chiniwar, S., Huang A., Chen T. - Y., Lin C. - H., Hsing C. - R., Chen W. - C., Cheng C. - M., Jeng H. - T., Wei C. M., Pai W. W., & Tang S. - J.
(2019).
Substrate-mediated umklapp scattering at the incommensurate interface of a monatomic alloy layer1.
PHYSICAL REVIEW B. 99, 155408., {APR 8}, Number {15}
Abstract
Chou, J. P., Pai W. W., Kuo C. C., Lee J. D., Lin C. H., & Wei C. M.
(2009).
Promotion of CO Oxidation on Bimetallic Au-Ag(110) Surfaces: A Combined Microscopic and Theoretical Study.
Journal of Physical Chemistry C. 113, 13151-13159., Jul, Number 30
AbstractThe clean-off reaction of AgO added rows by CO on Ag(110) and Au/Ag(110) bimetallic surfaces was studied by scanning tunneling microscopy (STM) and compared with density functional theory (DFT). This combined study of a model system illustrated the complexity of catalytic enhancement in bimetallic systems. By analyzing in situ time-lapsed STM image series, we found that CO oxidation on a Au-enriched Ag(110) surface leads to an exponential depletion of oxygen with time and a reaction rate that is synergistically enhanced by the presence of Au. First principles calculations indicate that the local atomic configuration around the active reaction sites at the chain ends and the preference of An atom substitution into the subsurface second Ag layer are of critical importance. By calculating CO adsorption energies and reaction barriers for plausible reaction pathways, a detailed description of the CO oxidation reaction emerges, For the optimal reaction pathway, a large (similar to 0.09 eV) barrier reduction and a small barrier of similar to 0.01 eV were found for the Eley-Rideal (ER) mechanism. In contrast, a small (similar to 0.03 eV) barrier reduction and a moderate barrier of similar to 0.23 eV were obtained for the Langmuir-Hinshelwood (LH) mechanism. The ER transitional state was also found to be lower in energy. We conclude that, irrespective of whether the ER mechanism is actually rate dominating, it is definitively enhanced.