Structure and dynamics of Ag clusters on Pt(111)

Abstract

By using the embedded atom method potentials in the form derived by Foiles, Baskes, and Daw [Phys. Rev. B 33, 7983 (1986)], we studied by static computations and molecular-dynamics (MD) simulations structural and dynamical properties of Ag clusters adsorbed on the (111) surface of Pt. Static computations were performed for clusters with up to 37 atoms and diffusion coefficients were evaluated for the adatom, dimer, and trimer at different temperatures. As shown by analyzing the trend of the dissociation energies with cluster size, the most stable structures on the surface are those formed by threefold-coordinated atoms, with dissociation energy ${\mathit{E}}^{\mathit{d}}$≊0.45 eV, while the presence of twofold-coordinated atoms at a given cluster size yields dissociation energies close to ${\mathit{E}}^{\mathit{d}}$≊0.30 eV. In particular, the largest dissociation energy is found for the heptamer, ${\mathit{E}}^{\mathit{d}}$=0.47 eV. Calculated migration barriers are as low as ${\mathit{E}}_1^{\mathit{m}}$=0.05 eV for the adatom, ${\mathit{E}}_2^{\mathit{m}}$=0.1 eV for the dimer, and ${\mathit{E}}_3^{\mathit{m}}$=0.14 eV for the trimer, thereby promoting enhanced mobility of these small structures at low temperatures. An Arrhenius plot of the diffusion coefficients up to T=800 K gives a dynamical diffusion barrier of the adatom which is moderately larger than the static one, i.e., ${\mathit{E}}_1^{\mathit{m}}$ (MD)=(0.060±0.005) eV. Our results are consistent with recent experimental scanning-tunneling-microscopy findings on the early stages of growth of Ag on Pt(111) indicating the rapid formation of large silver islands at temperatures as low as T=200 K.