Thermal diffusion of C60 molecules and Clusters on Au(111)

The structure and dynamics of partial monolayers of C-60 on the Au(111) surface were observed by scanning tunneling microscopy (STM) under ambient conditions. At submonolayer coverages, C-60 molecules group into islands, and while the molecules in the interior of islands are quite stable, the C(60)s at island edges are less so, and their motions can be observed in real time using STM. The motion of surface-adsorbed C-60 is predominantly the result of thermal diffusion; the extent of perturbation by the STM tip is determined to be minimal, using a quantitative analysis of a time series of STM images. Further analysis shows that motion of single C-60 molecules is relatively uncommon, and that most diffusion of C-60 on the surface occurs through the correlated or cooperative motion of molecular clusters ranging in size from 2 to 8 C-60 molecules. Cluster diffusion is explained by a proposed "anchoring" mechanism, in which the stability of each C-60 varies according to its position and orientation relative to the surface and to neighboring molecules.