Surface morphologies of size-selected Mo_100±2.5 and (MoO₃)_67±1.5 clusters soft-landed onto HOPG

Abstract

Mass-selected, cluster anion beams of Mo100±2.5– and (MoO3)67±1.5– were prepared with a magnetron source and soft-landed onto highly ordered pyrolytic graphite (HOPG) under UHV conditions. These two clusters were selected because they possess essentially the same masses and consequently could be soft-landed with the same low kinetic energies. The chemical composition of the deposited clusters was analyzed using in situ Auger electron spectroscopy and ex situ X-ray photoelectron spectroscopy, while their surface morphologies were characterized with in situ scanning tunneling microscopy (STM) and ex situ atomic force microscopy (AFM). Both STM and AFM results indicated a high mobility for the metal atom clusters on HOPG at room temperature. At low coverages, Mo100±2.5 clusters nucleated preferentially at step-edges. As their coverage increased, cluster aggregates formed on the terraces until a fully saturated overlayer was created. By contrast, deposited metal oxide clusters produced a stochastic array of adsorbed clusters for all coverages. Differences in the behavior of deposited Mo100±2.5 and (MoO3)67±1.5 clusters were interpreted in terms of differences in the interactions of metal and metal oxide clusters with carbonaceous substrates.