C60 bonding and energy-level alignment on metal and semiconductor surfaces.

Abstract

Electronic-structure studies of ${\mathrm{C}}_{60}$ condensed on metal surfaces show that the energy levels derived from the fullerene align with the substrate Fermi level, not the vacuum level. For thick layers grown on metals at 300 K, the binding energy of the C 1s main line was 284.7 eV and the center of the band derived from the highest occupied molecular orbital was 2.25 eV below the Fermi level. For monolayer amounts of ${\mathrm{C}}_{60}$ adsorbed on Au and Cr, however, the C 1s line was broadened asymmetrically and shifted to lower binding energy, the shakeup features were less distinct, and a band derived from the lowest unoccupied molecular orbital (LUMO) was shifted toward the Fermi level. These monolayer effects demonstrate partial occupancy of a LUMO-derived state, dipole formation, and changes in screening that are associated with LUMO occupancy. Results for ${\mathrm{C}}_{60}$ monolayers on n-type GaAs(110) show transfer of \ensuremath{\le}0.02 electron per fullerene, as gauged by substrate band bending. For ${\mathrm{C}}_{60}$ on p-type GaAs, however, the bands remained flat because electron redistribution was not possible, and the ${\mathrm{C}}_{60}$-derived energy levels were aligned to the substrate vacuum level.