Charge transfer of C₆₀ on copper surfaces

Using Scanning Tunneling Microscopy (STM) to monitor the surface morphology, and High Resolution Electron Energy Loss Spectroscopy (HREELS) to measure the vibrational spectrum, we have systematically investigated the relationship between the surface morphology and charge transfer from Cu(111) and Cu(110) to C₆₀. Since at 300K, C₆₀ is mobile on the Cu(111) terrace, but is more strongly bound at the step edge, any C₆₀ which adsorbs on the terrace will migrate to the step edge where it is immobilized. Because of this, at very low coverages, the C₆₀ preferentially adsorbs at the step edge. The amount of charge transfer from the substrate to the C₆₀ can be determined by measuring the amount of shift of the T_1u (IR active) modes. For C₆₀ adsorbed on the terrace, we determined a transfer of approximately 1.6 electrons/C₆₀ molecule. No additional frequency shift of the vibrational modes was seen for C₆₀ adsorbed at the step edge, indicating that any additional charge transfer at the step edge is small - less than one electron/C₆₀ molecule. In contrast, on Cu(110) C₆₀ is not mobile on the terrace, but forms well ordered islands upon annealing. C₆₀ molecules with three different levels of brightness were observed in the STM images, indicating that C₆₀ with different charge states were present on the surface. The HREELS spectrum confirmed this since the single peak observed at 66 meV in the multilayer film split into one shifted and one unshifted component upon annealing to remove the multilayer.