Electronic and geometric structure of C60 on Al(111) and Al(110)

Two new ordered monolayer phases of C-60 on Al surfaces have been studied using electron spectroscopies and low-energy electron diffraction (LEED). On Al(111), in addition to the previously reported (6x6) phase formed by evaporating with T-sample=620 K, a metastable (2 root 3x2 root 3)R30 degrees phase can be produced with T-sample=300 K. This phase exhibits the first LEED pattern reported for an unannealed C-60 overlayer. On Al(110), when evaporations are also made with T-sample=620 K, LEED shows the presence of a monolayer with a pseudo-c(4x4) structure. Al 2p photoemission for C-60/Al(110) and for (2 root 3x2 root 3)R30 degrees C-60/Al(111) reveals no evidence of strong substrate reconstruction. The perturbation of the geometric and electronic structure of the C-60 molecule due to the bonding interaction with the Al surface increases in the order C-60/Al(110), (2 root 3x2 root 3)R30 degrees C-60/Al(111), (6x6) C-60/Al(111), as demonstrated using element-specific probing of the valence band with x-ray absorption and C Is shakeup. The bond has covalent character in all three cases. Symmetry-induced splitting in the 5h(u)-derived level is observed using valence photoemission, and is particularly clear for C-60/Al(110). The stability of the equilibrium structures can be qualitatively understood from considerations of the energetics of the overlayer compression and the chemical bond between adsorbate and substrate. Work-function measurements for these and other C-60 overlayer systems cannot, in general, be understood within a simple description involving the addition of a dipole to the surface potential.