Quantitative description of C60 diffusion on an insulating surface

The diffusion of C-60 molecules on large, atomically flat terraces of the CaF2(111) surface is studied under ultrahigh vacuum conditions at various substrate temperatures below room temperature. The weak molecule-substrate interaction on this insulating surface makes a direct observation of hopping events difficult. Therefore, to determine a quantitative value of the diffusion barrier, we employ the so-called onset method. This method is based on the analysis of spatial properties of islands created by nucleation of diffusing C-60 molecules, as measured by noncontact atomic force microscopy. We first determine the critical cluster size to be i* = 1 from coverage-dependent island size distributions prepared at a fixed substrate temperature. The diffusion barrier of E-d=(214 +/- 16) meV and an attempt frequency of nu(0)=1.4 X 10(12 +/- 0.6) s(-1) are then obtained by analyzing the island densities at different substrate temperatures.