Ar+-induced surface defects on HOPG and their effect on the nucleation, coalescence and growth of evaporated copper

We have used a component peak of the C1s XPS spectrum to evaluate surface defects of HOPG induced by keV Ar+ radiation: we previously showed that a component 1.0 eV higher than the main C1s peak at 284.6 eV, whose intensity is strongly correlated with the extent of Ar+ irradiation, is due to less extensive sp2 electron delocalization caused by the breaking of surface bonds. We show here that, for evaporated Cu deposited onto the Ar+-treated HOPG surface, both the number density and average size of the Cu clusters formed are well correlated with the surface defect density. Our results also indicate that the nucleation of Cu clusters takes place at these defect sites. Further, the Cu cluster must overcome an energy barrier to diffuse and coalesce. The coalescence process follows the universal d=ktα, where d is the average cluster size, and k and α are two coalescence parameters influenced by the interaction between cluster and substrate surface. The Cu cluster coalescence coefficient is strongly dependent on the surface defect density and the initial size of the Cu cluster. This is used to suggest electrostatic interactions between deposited atoms and small clusters, and the defect sites.