Toshinobu Ohno

Synchrotron-radiation and x-ray photoemission studies of the valence states of condensed phase-pure ${\mathrm{C}}_{60}$ showed seventeen distinct molecular fetaures extending \ensuremath{\sim}23 eV below the highest occupied molecular states with intensity variations due to matrix-element effects involving both cluster and free-electron-like final states. Pseudopotential calculations established the orign of these features, and comparison with experiment was excellent. The sharp C 1s main line indicated a single species, and the nine satellite structures were due to shakeup and plasmon features. The 1.9-eV feature reflected transitions to the lowest unoccupied molecular level of the excited state.

Laser vaporization experiments with graphite in a supersonic cluster beam apparatus indicate that the smallest fullerene to form in substantial abundance is C(28). Although ab initio quantum chemical calculations predict that this cluster will favor a tetrahedral cage structure, it is electronically open shell. Further calculations reveal that C(28) in this structure should behave as a sort of hollow superatom with an effective valence of 4. This tetravalence should be exhibited toward chemical bonding both on the outside and on the inside of the cage. Thus, stable closed-shell derivatives of C(28) with large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps should be attainable either by reacting at the four tetrahedral vertices on the outside of the C(28) cage to make, for example, C(28)H(4), or by trapping a tetravalent atom inside the cage to make endothedral fullerenes such as Ti@C(28). An example of this second, inside route to C(28) stabilization is reported here: the laser and carbon-arc production of U@C(28).

Electronic-structure studies of ${\mathrm{C}}_{60}$ condensed on metal surfaces show that the energy levels derived from the fullerene align with the substrate Fermi level, not the vacuum level. For thick layers grown on metals at 300 K, the binding energy of the C 1s main line was 284.7 eV and the center of the band derived from the highest occupied molecular orbital was 2.25 eV below the Fermi level. For monolayer amounts of ${\mathrm{C}}_{60}$ adsorbed on Au and Cr, however, the C 1s line was broadened asymmetrically and shifted to lower binding energy, the shakeup features were less distinct, and a band derived from the lowest unoccupied molecular orbital (LUMO) was shifted toward the Fermi level. These monolayer effects demonstrate partial occupancy of a LUMO-derived state, dipole formation, and changes in screening that are associated with LUMO occupancy. Results for ${\mathrm{C}}_{60}$ monolayers on n-type GaAs(110) show transfer of \ensuremath{\le}0.02 electron per fullerene, as gauged by substrate band bending. For ${\mathrm{C}}_{60}$ on p-type GaAs, however, the bands remained flat because electron redistribution was not possible, and the ${\mathrm{C}}_{60}$-derived energy levels were aligned to the substrate vacuum level.

Photoemission and inverse photoemission studies of thin films of ${\mathrm{C}}_{60}$ and ${\mathit{C}}_{70}$ reveal the distribution of occupied and empty electronic states of these molecular solids. X-ray photoemission results also show the C 1s main line and features related to \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ shakeups, electron energy losses, and plasmons. Potassium doping produces changes that can be related to the occupation of states derived from the lowest unoccupied molecular orbitals of the fullerenes and band-structure effects. Important differences are observed upon K doping of ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$, particularly in states near the Fermi level, and these would be reflected in the electron-phonon coupling, superconductivity, and the phase diagram. Resistivity measurements for ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ show a resistivity minimum for ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and a dependence on stoichiometry that is indicative of dispersed conducting micrograins in an insulating medium. Oxygen-exposure studies demonstrate that ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{60}$ thin films are unstable.