Electronic structure of K-doped C60 monolayer films adsorbed on Si(0 0 1)-(2×1) and Si(1 1 1)-(7×7) surfaces

We have measured the valence spectra of K-doped C60 monolayer films adsorbed on Si(0 0 1)-(2×1) and Si(1 1 1)-(7×7) surfaces. No density of states is observed at the Fermi level for the undoped C60 monolayer films on both Si surfaces. After a small K doping, a density of states is clearly observed at the Fermi level on a Si(0 0 1)-(2×1) surface. On the other hand, no density of states is observed at the Fermi level at any amount of K doping on a Si(1 1 1)-(7×7) surface. The difference observed on both Si surfaces indicates that the symmetry of C60 molecules is almost the same as that in solid phase on a Si(0 0 1)-(2×1) surface, and is broken due to a stronger C60-substrate and/or C60–C60 interaction on the Si(1 1 1)-(7×7) one.