Electronic structure of fullerene and alkali-fulleride films
Photoemission and inverse photoemission spectroscopy have been used with low energy electron diffraction (LEED) and resistivity measurements to examine the electronic structure, transport properties, and crystallinity of A[sub x]C[sub 60] thin films where A = Na, K, Rb, and Cs and 0 [le] x [le] 6. These measurements demonstrate the highly molecular character of fullerene and fulleride films. For undoped C[sub 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 C[sub 60] films, the LEED results show a decrease in the quality of the LEED pattern upon the nucleation of the body-centered A-C[sub 60] phases. Alkali doping produces changes in the emission spectra that can be related to the occupation of bands derived from the lowest unoccupied molecular orbital of the fullerenes. The effects of electron correlation are important in each fulleride phase. The A[sub 1]C[sub 60] phases of Rb and Cs are characterized by an occupied valence band feature that is [approximately]0.5 eV wide centered [approximately]0.25 eV below E[sub F] derived from the lowest unoccupied molecular orbitals (LUMO) of C[sub 60]. For these phases there is also emission at the Fermi level. The A[sub 3]C[sub 60] phase of K and Rb exhibit a metallic Fermi level cutoff. Spectroscopic features 0.3 and 0.7 eV below E[sub F] are observed that are not reproduced in one-electron band structure calculations. The A[sub 4]C[sub 60] phases of K, Rb, and Cs all exhibit insulating character with a split LUMO band. All of the A[sub 4]C[sub 60] phases are insulators with a filled LUMO band. For Na-C[sub 60], the valence band spectra show no emission at E[sub F] for any Na concentration. Resistivity measurements for K[sub x]C[sub 60] show a resistivity minimum for K[sub 3]C[sub 60] and a dependence on stoichiometry indicative of dispersed conducting micrograins in an insulating medium.
- Research Organization:
- Minnesota Univ., Minneapolis, MN (United States)
- OSTI ID:
- 7112255
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
360600 -- Other Materials
664500* -- Special Atoms & Molecules-- (1992-)
74 ATOMIC AND MOLECULAR PHYSICS
ALKALI METAL COMPOUNDS
CARBON
COHERENT SCATTERING
CRYSTAL DOPING
CRYSTAL STRUCTURE
CRYSTAL-PHASE TRANSFORMATIONS
DIFFRACTION
ELECTRIC CONDUCTIVITY
ELECTRICAL PROPERTIES
ELECTRON DIFFRACTION
ELECTRONIC STRUCTURE
ELEMENTS
EMISSION
FULLERENES
NONMETALS
PHASE TRANSFORMATIONS
PHOTOEMISSION
PHYSICAL PROPERTIES
SCATTERING
SECONDARY EMISSION
TEMPERATURE DEPENDENCE