Photoemission, near-edge x-ray-absorption spectroscopy, and low-energy electron-diffraction study of C-60 on Au(111) surfaces

Abstract

The geometric and electronic structures of C-60 adsorption on Au(111) surfaces have been studied py low-energy electron diffraction (LEED), angle-resolved photoemission, and near-edge x-ray-absorption spectroscopy. An irreversible structural transition of the C-60 overlayer on Au(111) was observed by LEED upon successive annealing. These structures are 38 x 38 "in phase," R14 degrees and (2 root 3 x 2 root 3)R30 degrees, with the latter phase predominating after annealing to 350 degrees C. Valence-band photoemission spectra reveals a state right below the Fermi level for an annealed, ordered monolayer. This peak disperses across the Fermi energy that indicates the C-60 overlayer becomes metallic. Its intensity shows a resonance that primarily follows the behavior of highest occupied molecular orbitals, identified unambiguously as lowest unoccupied molecular orbitals (LUMO's) filled by charge transfer from the substrate. An asymmetric distribution of LUMO charge is observed. The thermal-desorption energy of the monolayer is estimated from annealing experiments to be 1.9 eV, which is 0.5 eV larger than the desorption energy from multilayers. Comparison with available spectroscopic data indicates that interaction of C-60 with Au(111) is slightly weaker than with Au(110), and much weaker than with Cu(111). The amount of charge transfer estimated from photoemission is 0.8 electrons per C-60 molecule on Au(111), compared to 1.6 electrons on Cu(111). We argue that charge transfer is determined by the bulk sp density of states at the Fermi energy scaled by the size of the C-60 molecule, and also modified by a clean surface electronic structure, and that charge transfer is the dominant interaction in these systems.