PHOTOEMISSION-STUDY OF K DOPING ON A MONOLAYER OF C-60 ON CLEAN SI(001)-(2X1) SURFACE

We present the first valence band photoemission study of a monolayer K xC60 on a clean Si(001)-(2 x 1) surface. The monolayer C-60 which show s weak interaction with the silicon surface reveals clear, but broaden ed, structures corresponding to bulk C-60 Upon K exposure, the work fu nction drops rapidly due to charge polarization toward the Si surface, considerably affecting then the rate of the LUMO filling. Its centroi d initially shown at 0.6 eV shifts to higher binding energy with highe r concentration. Moreover, the LUMO always separates 1.5 +/- 0.1 eV fr om the HOMO. Features associated with the many-body effect do not appe ar in the spectre. The Fermi cutoff has never been observed, indicatin g the insulating nature of the KxC60 surface.