ELECTRON-DIFFRACTION AND PHOTOELECTRON-SPECTROSCOPY STUDIES OF FULLERENE AND ALKALI-METAL FULLERIDE FILMS

Photoelectron spectroscopy and low-energy electron diffraction (LEED) have been used to examine the electronic structure and crystallinity o f thin films of A(X)C60 where A = Na, K, Rb, and Cs and 0 less-than-or -equal-to x less-than-or-equal-to 6. For undoped C60 films, temperatur e-dependent LEED studies show changes that correspond to the lattice t ransformation from the simple-cubic to the face-centered-cubic structu re. For doped C60 films, the LEED results show a decrease in the quali ty of the LEED pattern upon the nucleation of the body-centered A-C60 phases. Spectroscopic studies of these fullerides indicate that the ef fects of electron correlation are always important. In particular, the A1C60 phases of Rb and Cs are characterized by an occupied-valence-ba nd feature approximately 0.5 eV wide centered approximately 0.25 eV be low E(F) that is derived from the lowest unoccupied molecular orbitals (LUMO) of C60. The much greater width relative to band calculations i s attributed to electron correlation. For these phases, there is also emission at the Fermi level, despite the fact that transport studies i ndicate insulating character. This implies that the electronic states at E(F) are localized. The A3C60 phase of K and Rb exhibit a metallic Fermi-level cutoff. Spectroscopic features 0.3 and 0.7 eV below EF are also observed that are not reproduced in one-electron band-structure calculations. The A4C60 phases of K, Rb, and Cs all exhibit insulating character with a split LUMO band. All of the A6C60 phases are insulat ors with a filled LUMO band. For Na-C60, the valence-band spectra show no emission at E(F) for any Na concentration. Finally, photoemission results showed partial occupation of the (LUMO+1)-derived levels, corr esponding to C60(8-) for isolated C60 molecules deposited onto multila yers of Na, K, and Rb at 40 K.