A POTASSIUM-DOPED MU-M THICK C-60 FILM - DIRECT OBSERVATIONS OF THRESHOLD DOPING AND PHASES EVOLUTION

The evolution of the valence band spectra of K doped mu m thick C-60 f ilm is presented. The data shows directly that there exists a threshol d concentration before the first stable phase, K3C60, starts to appear . Below the threshold concentration, no Fermi emission is seen and the K atoms distributed randomly in the bulk retain their 4s electron who se binding energy is at 0.9 eV below the Fermi level. This finding dif fers from all the previous experiments, which reported the immediate a ppearance of the K3C60 phase with a distinct Fermi cutoff upon K dopin g. A second stable phase, K6C60, evolves upon further doping with two characteristic states at 0.5 and 2.0 eV and coexists with the K3C60 ph ase. Our work also demonstrates for the first time that the superfulle ride phase which arises from the filling of the t(1g) band can be obta ined at high doping. This phase is found on the surface. (C) 1998 Else vier Science Ltd. All rights reserved.