Quantitative description ofC60diffusion on an insulating surface

Abstract

The diffusion of ${\text{C}}_{60}$ molecules on large, atomically flat terraces of the ${\text{CaF}}_{2}(111)$ 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 ${\text{C}}_{60}$ molecules, as measured by noncontact atomic force microscopy. We first determine the critical cluster size to be ${i}^{\ensuremath{\ast}}=1$ from coverage-dependent island size distributions prepared at a fixed substrate temperature. The diffusion barrier of ${E}_{d}=(214\ifmmode\pm\else\textpm\fi{}16)\text{ }\text{meV}$ and an attempt frequency of ${\ensuremath{\nu}}_{0}=1.4\ifmmode\times\else\texttimes\fi{}{10}^{12\ifmmode\pm\else\textpm\fi{}0.6}\text{ }{\text{s}}^{\ensuremath{-}1}$ are then obtained by analyzing the island densities at different substrate temperatures.