Surface heterogeneity of C-60 as studied by infrared spectroscopy of adsorbed CO and adsorption potential calculations

In our recent investigation of the IR spectrum of CO physically adsorbed on C-60 films, two well-resolved absorption bands at 2135 and 2128 cm(-1) were found, suggesting that the molecule is adsorbed on two different sites. To determine the nature of these adsorption sites, calculations of adsorption potentials and spectral shifts for the CO/C-60 system were performed. The calculations were done for the fee (100), fee (111) hcp (001), and hcp (111) surface planes. In the calculations the 6-exponential and the Lennard-Jones potentials were used. A number of adsorption sites were chosen. These included the void space between four, three, and two neighboring C-60 molecules and the center of the hexagon and the pentagon on the C-60 surface. The calculated potentials and spectral shifts clearly indicate that adsorption sites in the voids between the C-60 molecules are energetically preferred over sites on top of single C-60 molecules. Comparison is made between results obtained with the two potentials and with results obtained previously with the two other carbon allotropes: graphite and diamond.