Ab-Initio Simulation Laboratory

Surface-supported planar clusters can sprout active research and create numerous applications in the realm of nanotechnology. Exploitation of these clusters will be more extended if their properties on a supported substrate are thoroughly apprehended, and if they can be fabricated in a controllable way. Here we report finding the magic numbers in two-dimensional Ag clusters grown on Pb quantum islands. We demonstrate, with the images and energy spectra of atomic precision, the transition from electronic origin to a geometric one within the same system. Applying the magic nature, we can also produce a large array of planar clusters with well-defined sizes and shapes.

We present a method for spatially resolved imaging of energy-dependent photoelectron diffraction. Energy-dependent photoelectron-diffraction spectra are individually Fourier transformed to three-dimensional vector space. The complex transformed amplitudes are summed over a span of phi angles or over a span of polar angles. The images are, respectively, well resolved in the radial and azimuthal directions, or in the radial and polar directions. The intersections of these real-space maps fix the atomic coordinates. We show how the intensity loci from single and multiple scattering paths are separately resolved and how most multiple scattering contributions are eliminated. By varying the collection angles, atoms in different regions relative to the emitter, e.g., surface or bulk atoms, are imaged. One can also use the photon's A vector to enhance the near-pi backscattering geometry. We compare this method with another direct method: extended x-ray-absorption fine structure.

We 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.

We propose in this paper a theoretical model to investigate surface self-diffusion of single adatoms on the face-centered-cubic metals. Calculations are performed on the channeled (110), densely packed (111) and loosely packed (001) surfaces of iridium at temperature T = 800 K. Three realistic model potentials, Embedded Atom method, Sutton-Chen and Rosato-Guillope-Legrand potentials, are applied to describe the interatomic interaction of the adatom/substrate systems. These potentials all involve a few empirical fittings of bulk properties of solid which incorporate with many-body effects. With these potentials, conventional molecular dynamics (MD) is employed to obtain trajectories of the atoms. On the (111) plane, via the Einstein relation, the estimated random walk exponential prefactors and activation energies do exhibit Arrhenius behavior, which are in reasonably good agreement with the experimental results. On the (001) and (110) faces, a number of theoretical evidences for atomic diffusion by exchange mechanism of the adatom with a surface atom are presented, which are again in fairly good agreement with the experiments. In addition, an examination of the exchange diffusion characteristics on several systems (Cu, Rh and Pt) is also presented.