STM-HREELS Investigation of C₆₀ on Cu(111)

Abstract
Using an STM/HREELS system, we have investigated the relationship between the surface morphology and vibrational modes of C₆₀ adsorbed on Cu(111). At 300K, C₆₀ is mobile on the Cu(111) terrace, but is more strongly bound at the step edge. Therefore, any C₆₀ which adsorbs on the terrace will migrate to the step edge where it is immobilized. Thus for very low coverages, the C₆₀ referentially adsorbs at the step edge. Adsorption of the molecule on metal surfaces results in a significant amount of charge transfer from the substrate to the C₆₀ molecule. Two of the four t_lu, infrared active modes are predicted by theory to shift to lower frequency as the amount of charge transfer increases. However, only a softening of the lowest frequency t_1u mode was observed. Furthermore, no further shifting of the frequency of this mode was seen for very low coverages, indicating that any additional charge transfer at the step edge is smaller than one electron per C₆₀ molecule.