Optimal electron doping of a C60 monolayer on Cu(111) via interface reconstruction.

We demonstrate the charge state of C60 on a Cu(111) surface can be made optimal, i.e., forming C60(3-) as required for superconductivity in bulk alkali-doped C60, purely through interface reconstruction rather than with foreign dopants. We link the origin of the C60(3-) charge state to a reconstructed interface with ordered (4x4) 7-atom vacancy holes in the surface. In contrast, C60 adsorbed on unreconstructed Cu(111) receives a much smaller amount of electrons. Our results illustrate a definitive interface effect that affects the electronic properties of molecule-electrode contact.