Photoemission study of K doping on a monolayer of C60 on clean Si(001)-(2 x 1) surface

We present the first valence band photoemission study of a monolayer KxC60 on a clean Si(001)-(2 x 1) surface. The monolayer C-60 which shows weak interaction with the silicon surface reveals clear, but broadened, structures corresponding to bulk C-60 Upon K exposure, the work function drops rapidly due to charge polarization toward the Si surface, considerably affecting then the rate of the LUMO filling. Its centroid initially shown at 0.6 eV shifts to higher binding energy with higher concentration. Moreover, the LUMO always separates 1.5 +/- 0.1 eV from the HOMO. Features associated with the many-body effect do not appear in the spectre. The Fermi cutoff has never been observed, indicating the insulating nature of the KxC60 surface.