The diffusion of ${\text{C}}_{60}$ molecules on large, atomically flat terraces of the ${\text{CaF}}_2\left(111\right)$ 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 $i^{\ast }=1$ from coverage-dependent island size distributions prepared at a fixed substrate temperature. The diffusion barrier of $E_d=\left(214\pm 16\right)\text{meV}$ and an attempt frequency of ${\nu }_0=1.4\times {10}^{12\pm 0.6}{\text{s}}^{-1}$ are then obtained by analyzing the island densities at different substrate temperatures.

• Received 11 August 2010

DOI:https://doi.org/10.1103/PhysRevB.82.155428