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 of thin films of ${\mathit{A}}_{\mathit{X}}$${\mathrm{C}}_{60}$ where A=Na, K, Rb, and Cs and 0\ensuremath{\le}x=6. For undoped ${\mathrm{C}}_{60}$ films, temperature-dependent LEED studies show changes that correspond to the lattice transformation from the simple-cubic to the face-centered-cubic structure. For doped ${\mathrm{C}}_{60}$ films, the LEED results show a decrease in the quality of the LEED pattern upon the nucleation of the body-centered A-${\mathrm{C}}_{60}$ phases. Spectroscopic studies of these fullerides indicate that the effects of electron correlation are always important. In particular, the ${\mathit{A}}_{1}$${\mathrm{C}}_{60}$ phases of Rb and Cs are characterized by an occupied-valence-band feature \ensuremath{\sim}0.5 eV wide centered \ensuremath{\sim}0.25 eV below ${\mathit{E}}_{\mathit{F}}$ that is derived from the lowest unoccupied molecular orbitals (LUMO) of ${\mathrm{C}}_{60}$. The much greater width relative to band calculations is attributed to electron correlation. For these phases, there is also emission at the Fermi level, despite the fact that transport studies indicate insulating character. This implies that the electronic states at ${\mathit{E}}_{\mathit{F}}$ are localized. The ${\mathit{A}}_{3}$${\mathrm{C}}_{60}$ phase of K and Rb exhibit a metallic Fermi-level cutoff. Spectroscopic features 0.3 and 0.7 eV below ${\mathit{E}}_{\mathit{F}}$ are also observed that are not reproduced in one-electron band-structure calculations. The ${\mathit{A}}_{4}$${\mathrm{C}}_{60}$ phases of K, Rb, and Cs all exhibit insulating character with a split LUMO band. All of the ${\mathit{A}}_{6}$${\mathrm{C}}_{60}$ phases are insulators with a filled LUMO band. For Na-${\mathrm{C}}_{60}$, the valence-band spectra show no emission at ${\mathit{E}}_{\mathit{F}}$ for any Na concentration. Finally, photoemission results showed partial occupation of the (LUMO+1)-derived levels, corresponding to ${\mathrm{C}}_{60}^{8\mathrm{\ensuremath{-}}}$ for isolated ${\mathrm{C}}_{60}$ molecules deposited onto multilayers of Na, K, and Rb at 40 K.