Gary Adams

Energetics of large fullerenes : balls, tubes, and capsules

First-principles calculations were performed to compare the energies of 29 different fullerene structures, with mass number from 60 to 240, and of eight nonhelical graphite tubes of different radii. A quantity called the planarity, which indicates the completeness of the π-bonding, is the single most important parameter determining the energetics of these structures. Empirical equations were constructed for the energies of nonhelical tubes and for those fullerene structures that may be described as balls or capsules. For a given mass number, bail-shaped fullerenes are energetically favored over capsular (tube-like) fullerenes.

Jahn-Teller distortions in solid C20 and other fullerene structures

Abstract We have used first principles molecular dynamics to determine the energies of Jahn-Teller distortions in a variety of neutral and negatively charged fullerene structures. We find that smaller structures have larger Jahn-Teller distortion energies. Solid C 28 and solid C 20 , which have covalent bonds connecting the individual fullerene balls, are found to have energies which are much lower than the energies of their molecular components. The charged state of solid C 20 is found to have a total distortion energy which is about six times larger than that of C − 60 .

Duals of Frank-Kasper structures as C, Si and Ge clathrates: Energetics and structure

The four-connected clathrate structures derived as the duals of some of the tetrahedrally close-packed intermetallic structures (Frank-Kasper structures) are considered as possible structures for elemental C, Si and Ge. The minimumenergy structures are found using empirical potentials. The type II structure is found to have the lowest energy, but other clathrate structures have only a slightly higher energy within a few kT of the energy of the diamond form. Explicit coordinates are given for some Si structures.

Evidence for solid-state effects in the electronic structure of C60 films: a resonance-Raman study

Abstract The Raman excitation profiles for C 60 films grown on silicon reveal the existence of optical transitions near 2.4 eV, well below the lowest dipole-allowed absorption line predicted for isolated icosahedral molecules. These transitions are not observed in C 60 dissolved in CS 2 . We propose an explanation in terms of a breakdown of the icosahedral inversion symmetry in C 60 crystals.

The influence of internal surfaces on the (2 × 1) shuffle and glide cleavage reconstructions for Si(111)

The surface reconstructions which occur when a parallel-sided gap opens up between the (111) planes of silicon are computed using the ab initio total-energy and lattice dynamics method of Sankey and Niklewski (1989). This allows the time-evolution of the surface structures to be observed. The shuffle and glide terminations are compared, and found to differ in energy by 0.17 eV/(surface atom), with the shuffle termination having lower energy. The Haneman (2 × 1) buckled row reconstruction is found to occur at a certain critical gap dc = 0.46 nm, and acts as a precursor for the Pandey (2 × 1) π-bonded chain model, which occurs at larger gaps. Charge-density maps are shown, and the results related to cleavage in real crystals. The surface energy of (2 × 1) reconstructed Si(111) is calculated to be 1340 erg/cm2 (1.07 eV per surface atom) in good agreement with experiment.

Neutron diffraction and structural models of RbC60 phases

Abstract Pair distribution functions (PDFs), obtained from powder neutron diffraction measurements have been used to construct structural models for the body centered orthorhombic (bco) and fcc phases of RbC 60 . The PDF exhibits small differences, primarily due to the shortened inter-fullerene distance in the bco phase. The bco-RbC 60 system is well fit to a model derived from first-principles quantum molecular dynamics in which the C-C interfullerene distance is 1.57 A. While the geometry of the interfullerene linkages is similar to a previously proposed model, the present results imply less distortion of the C 60 molecules and larger interfullerene distances. The PDF analysis also indicates significant orientational disorder between chains of linked molecules.

Quantum molecular dynamics calculations and experimental Raman spectra confirm the proposed structure of the odd-numbered dimeric fullerene C119

A first-principles quantum molecular dynamics (QMD) method and a bond polarizability model, whose parameters were optimized on the basis of C60 data, have been used to calculate theoretical Raman spectra for four possible low-energy isomers of the odd-numbered dimeric fullerene C119 produced by thermolysis of C60 oxides. Comparison of the calculated and experimentally determined spectra provides strong evidence that the structure obtained by thermolysis is indeed the thermodynamically most stable isomer with C2 symmetry, as proposed earlier on the basis of semiempirical molecular modeling and 13C-NMR spectroscopy. This isomer has the structure originally predicted for C119 on the basis of QMD simulations.

Theoretical Studies of Raman Spectra for Planar Polymerized C60

We have used an approximate first-principles method together with a bond-polarizability model to calculate theoretical Raman spectra for the orthorhombic (O), tetragonal (T), and rhombohedral (R) phases of pressure-polymerized C 60 solid. We have also calculated Raman spectra for a sequence of C 60 polymer structures which include all possible two-connected C 60 balls, to help elucidate the yet unknown structure of the room-temperature C 60 photopolymer. To quantitatively determine the contribution of the I h C 60 vibrational modes to each polymer mode, we have expanded the polymer eigenvectors in the eigenvectors of I h C 60 . Our calculated O-polymer Raman spectrum is compared with the available experimental spectrum measured under off-resonance conditions. For the sequence of two-connected-ball polymers, we present our calculated spectra in the frequency region of the I h C 60 Ag(2) vibrational mode, and relate these spectra to measured spectra for the room-temperature photopolymer. We find the simplest agreement with experiment for the triangular C 60 trimer.

A first principles molecular dynamics study of the fullerene derivative C119

Abstract We have used local-orbital-based first principles molecular dynamics to predict the structure of the C 119 molecule which is found in the mass spectra of C 60 —ozone reactions. Our simulation assumes that the C 119 molecule is formed by joining an intact C 60 molecule with a C 59 (a C 60 with one atom removed by reaction with ozone). The predicted C 119 configuration has C 2 symmetry, three four-coordinated atoms, and two seven-membered rings. For this predicted configuration, we report calculated structural properties, electronic eigenvalues, and Raman spectra.

Applications of abinitio quantum molecular dynamical relaxation: Silicon(111)‐5×5 surface reconstruction and aluminum deposited on silicon(100)

We present two applications of a first principles quantum molecular dynamics technique. In the first, we have determined the relaxed atomic geometry and the corresponding electronic structure of the Si(111) surface in the 5×5 dimer–adatom‐stacking fault structure. We find a unique low temperature behavior for this surface, namely, corrugation of the adatoms on each side of the 5×5 unit cell, which we verify by the use of perturbation theory. In a second application, we have determined the relaxed atomic geometry and the corresponding electronic structure of the Al:Si(100) surface in both the 2×2 and the 2×3 reconstructions. In particular, we find that the Al dimers on this surface are parallel to, rather than perpendicular to, the Si dimer direction.