Aj. Maxwell et al., ELECTRONIC AND GEOMETRIC STRUCTURE OF C-60 ON AL(111) AND AL(110), Physical review. B, Condensed matter, 57(12), 1998, pp. 7312-7326
Two new ordered monolayer phases of C-60 on Al surfaces have been stud
ied 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 r
oot 3)R30 degrees phase can be produced with T-sample=300 K. This phas
e exhibits the first LEED pattern reported for an unannealed C-60 over
layer. 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) structur
e. 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 reconstr
uction. The perturbation of the geometric and electronic structure of
the C-60 molecule due to the bonding interaction with the Al surface i
ncreases in the order C-60/Al(110), (2 root 3x2 root 3)R30 degrees C-6
0/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. Th
e bond has covalent character in all three cases. Symmetry-induced spl
itting in the 5h(u)-derived level is observed using valence photoemiss
ion, and is particularly clear for C-60/Al(110). The stability of the
equilibrium structures can be qualitatively understood from considerat
ions of the energetics of the overlayer compression and the chemical b
ond between adsorbate and substrate. Work-function measurements for th
ese 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.