Surface Morphologies of Size-Selected Mo_100±2.5and (MoO₃)_67±1.5Clusters Soft-Landed onto HOPG

Mass-selected, cluster anion beams of Mo 100±2.5 – and (MoO 3 ) 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, Mo 100±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 Mo 100±2.5 and (MoO 3 ) 67±1.5 clusters were interpreted in terms of differences in the interactions of metal and metal oxide clusters with carbonaceous substrates.