Surface-enhanced Raman scattering and photoemission of C60 on noble-metal surfaces.

Abstract

Surface-enhanced Raman scattering of monolayer ${\mathrm{C}}_{60}$ deposited in UHV on noble metals yields substantial shifts of the high-frequency, ${\mathit{A}}_{\mathit{g}}$(2) pentagonal pinch mode. These shifts, which increase in the series Au, Cu, and Ag, are attributed, in part, to charge transfer to the fullerene. This behavior is consistent with a decrease in noble-metal work function in this sequence. Confirmation of charge transfer in Cu and Ag is directly obtained by He I, ultraviolet photoemission spectroscopy measurements which indicate formation of a lowest-unoccupied-molecular-orbital (LUMO) -derived band similar to that observed in ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$. The form and Fermi edge behavior of the LUMO-derived band further imply the formation of a metallic interfacial layer. X-ray photoemission spectroscopy measurements indicate shifts of the C 1s core level which differ from trends noted in Raman scattering, suggesting that interfacial effects beyond charge transfer may be important.