Slowing down adatom diffusion by an adsorbate: Co on Pt(111) with and without preadsorbed CO

Submonolayer deposition of cobalt on Pt(111) was studied by scanning tunneling microscopy at room temperature. It was found that deposition on a surface with adsorbed carbon monoxide (saturation coverage) leads to an island density almost ten times as high as that resulting from deposition on the clean platinum surface. Based on nucleation theory, we explain this fact as the result of a reduction of the diffusion coefficient of the Co adatoms in the presence of CO by more than two orders of magnitude. This effect is attributed to the displacement and/or rearrangement of the CO molecules necessary when Co adatoms diffuse over the “crowded” surface.