C60-terminated Si surfaces:: Charge transfer, bonding, and chemical passivation

The interaction of C-60 with the Si(111)-(7X7) and Si(100)-(2X1) surfaces has been investigated using synchrotron radiation core-level and valence-band photoelectron spectroscopy. C-60 induces distinct spectral changes in the Si-2p core-level emission from both surfaces, indicative of charge transfer to the adsorbed fullerene molecules. Our results suggest that C-60 adsorption on Si(111) induces a redistribution of charge within the (7x7) unit cell involving electron transfer from rest atom to adatom dangling bonds. For a one monolayer coverage [on both Si(111) and Si(100)], broad C-60-induced chemically shifted components are present in the core-level spectra. Valence-band spectra, however, show no evidence for a high degree of electron occupation of the C-60 lowest unoccupied molecular orbital. We present core-level data which illustrate that adsorption of a C-60 monolayer inhibits ambient oxidation of the Si surface.