ADSORPTION OF C-60 MOLECULES

A self-consistent real-space scheme for calculating the van der Waals interaction energy between a fullerene molecule and substrate with ato mic surface corrugation is presented. The interaction of a single full erene molecule with various substrates is then considered, to determin e the optimum binding energy, plus the rotational and translational di ffusion barriers. The van der Waals energy is calculated using linear response theory to evaluate the dipole-dipole interactions between the molecule and the substrate. The method is extended beyond the treatme nt of the substrate as a continuous dielectric medium to a discrete st ratified substrate including the atomic nature of the surface. For C-6 0 On graphite the fullerene is always preferentially oriented so as to present a six-membered ring to the surface. The optimum binding energ y is found to be 0.96 eV, with the molecule positioned so as to contin ue the natural stacking of the hexagonal planes. For C-60 on NaCl(001) the most stable position is found to be above a sodium cation with a five-membered ring oriented towards the surface, and a binding energy of 0.42 eV. Unlike the situation for graphite, though, the orientation of the molecule changes with adsorption site. The energy barrier for rotation of an isolated C-60 molecule is of the order of 0.03 eV on bo th surfaces. Luthi et al. [Science 266, 1979 (1994)] recently reported that islands of C-60 deposited on NaCl(001) could be moved by the act ion of the tip of a scanning force microscope, whereas for C-60 On gra phite, collective motion of the islands could not be achieved, instead the islands were disrupted by the tip; These results can be explained in terms of the relative strengths of the C-60-C-60, C-60-graphite, a nd C-60-NaCl interactions and the reduction of the rotational barriers of the interface molecules due to collective effects.