Interaction of C60 with Ge(111) in the 3?3×3?3R30° phase: A (2×2) model

The adsorption of a close-packed C60 single layer on the clean c(2×8)-Ge(111) surface leads to the formation of a 3?3×3?3R30° low-energy electron diffraction pattern upon annealing at 450-500 °C. We have investigated this phase using synchrotron-radiation photoemission. The Ge3d core-level spectra show the quenching of the surface features related to the adatoms/rest-atoms complex of the original Ge(111)-c(2×8) reconstruction, suggesting that the observed 3?3×3?3R30° ordering is not due to a Ge(111) reconstruction induced by the C60 adsorption, as instead previously argued on the basis of scanning tunneling microscopy measurements. Moreover, no filling of the lowest unoccupied molecular orbital derived bands has been observed. The C 1s core level and the valence-band photoemission spectra point to the presence of a localized covalent bond between the C60 molecules and the Ge atoms. This bond induces the splitting of the C60 highest occupied molecular orbital with a feature appearing at about 2.3 eV.