Modeling of Ir adatoms on Ir surfaces.

Abstract

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, ${\mathrm{Ir}}_{2}$ dimers with nearest-neighbor spacing are the most stable. On the (110) surface, the ${\mathrm{Ir}}_{2}$ 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 ${\mathrm{Ir}}_{2}$ dimer diffuses in a zigzag motion on hcp and fcc sites without breaking into two single atoms. On the (100) surface, diffusion of the ${\mathrm{Ir}}_{2}$ dimer is achieved by successive one-atom exchange with the substrate atom accompanying by a 90\ifmmode^\circ\else\textdegree\fi{} rotation of the ${\mathrm{Ir}}_{2}$ 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, ${\mathit{I}}_{{\ensuremath{\gamma}}_{4}}$ on the Ir (111) surface was confirmed theoretically. \textcopyright{} 1996 The American Physical Society.