CHARGE-TRANSFER OF C-60 ON COPPER SURFACES

Using Scanning Tunneling Microscopy (STM) to monitor the surface morph ology, and High Resolution Electron Energy Loss Spectroscopy (HREELS) to measure the vibrational spectrum, we have systematically investigat ed the relationship between the surface morphology and charge transfer from Cu(111) and Cu(110) to C-60. Since at 300K, C-60 is mobile on th e Cu(111) terrace, but is more strongly bound at the step edge, any C- 60 which adsorbs on the terrace will migrate to the step edge where it is immobilized. Because of this, at very low coverages, the C-60 pref erentially adsorbs at the step edge. The amount of charge transfer fro m the substrate to the C-60 can be determined by measuring the amount of shift of the T-1u (IR active) modes. For C-60 adsorbed on the terra ce, we determined a transfer of approximately 1.6 electrons/C-60 molec ule. No additional frequency shift of the vibrational modes was seen f or C-60 adsorbed at the step edge, indicating that any additional char ge transfer at the step edge is small - less than one electron/C-60 mo lecule. In contrast, on Cu(110) C-60 is not mobile on the terrace, but forms well ordered islands upon annealing. C-60 molecules with three different levels of brightness were observed in the STM images, indica ting that C-60 With different charge states were present on the surfac e. The HREELS spectrum confirmed this since the single peak observed a t 66 meV in the multilayer film split into one shifted and one unshift ed component upon annealing to remove the multilayer.