J. H. Weaver

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).

Carbon nanotubes produced in arcs have been found to have the form of multiwalled fullerenes, at least over short lengths. Sintering of the tubes to each other is the predominant source of defects that limit the utility of these otherwise perfect fullerene structures. The use of a water-cooled copper cathode minimized such defects, permitting nanotubes longer than 40 micrometers to be attached to macroscopic electrodes and extracted from the bulk deposit. A detailed mechanism that features the high electric field at (and field-emission from) open nanotube tips exposed to the arc plasma, and consequent positive feedback effects from the neutral gas and plasma, is proposed for tube growth in such arcs.

The reflectivity and ${\ensuremath{\epsilon}}_{2}$ spectra of the fcc metals Rh, Pd, Ir, and Pt are reviewed and discussed between about 0.1 and 30 eV. Structures in ${\ensuremath{\epsilon}}_{2}$ are interpreted in terms of recent band calculations. Calculations of the joint density of states and ${\ensuremath{\epsilon}}_{2}$, based on the energy distribution of the joint-density-of-states results of Smith, are presented and compared with experiment. The low-energy dielectric functions are observed to be sensitive to the bands near the symmetry point $L$, notably the spin-orbit-split $d$-like states which shift relative to the $\mathrm{sp}$-like ${L}_{6}^{\ensuremath{-}}$ level and affect transitions below about 2 eV. Structure near 2.8 eV in Rh is interpreted in terms of transitions between bands 3 and 5 along $Q$. Like the bcc and hcp transition metals, the fcc metals display minima in ${\ensuremath{\epsilon}}_{2}$ near 10 eV and, at higher energies, characteristic structures are observed which can be understood qualitatively in terms of transitions to high-lying bands. The fcc metals, like the bcc and hcp transition metals, are shown to have two volume and two surface plasmons. In the discussion, shortcomings and uncertainties in the optical data are pointed out and appeals for further study are made.

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.