Ion induced modification of size-selected MoO₃ and WO₃ clusters deposited on HOPG

Abstract

The effect of 4 keV Ar+ ions on size-selected (MoO3)n and (WO3)n clusters, where n = 30 ± 1, deposited on highly ordered pyrolytic graphite was studied by x-ray photoelectron spectroscopy and atomic force microscopy. During deposition, both metal oxide clusters collected preferentially at step edges at low coverages, with larger cluster aggregates nucleating on the terraces at higher coverages. Under the influence of 4 keV Ar+ ion bombardment, both clusters initially experienced oxygen loss and corresponding reduction of the transition metal before the remaining species were sputtered from the surface. However, MoO3 was only partially reduced to MoO2, while WO3 was fully reduced to metallic tungsten. As a consequence of these variations in ion induced chemistry, changes in the surface morphology of MoO3 and WO3 following prolonged Ar+ exposure differed significantly. For (MoO3)30±1, the size of the clusters decreased, due to removal of MoO2 fragments. In contrast, for (WO3)30±1, the formation of smaller sized clusters was also accompanied by the appearance of clusters that were much larger in size than the original “as deposited” clusters. The creation of these larger structures is ascribed to the mobility and coalescence of metallic tungsten species produced during the sputtering process. The contrast in the behavior of the molybdenum and tungsten oxide clusters illustrates the interplay that can exist between ion induced changes in speciation and morphology of deposited clusters.