STRUCTURE, DYNAMICS, AND FORMATION OF CARBON AND ALUMINUM CLUSTERS

Abstract

The results of recent ab initio molecular dynamics studies of C and Al clusters are presented. The simulations have shown that C 60 molecular structure is well preserved in the solid and that the individual C 60 molecules start to rotate at relatively low temperatures. Our results are in very good agreement with NMR, photoemission, and neutron scattering data. At high temperatures C 60 undergoes large amplitude soccerball-rugbyball oscillations, but the cage structure is still preserved. The C 60 isomer containing two pairs of adjacent pentagons has a binding energy only 1.6 eV smaller than that of perfect C 60, but high temperatures and long annealing times are required for the transformation between these two structures. Its activation energy is 5.4 eV. We have also studied the various isomers of C 20, since it could form the smallest possible fullerene. At T=0 , the lowest energy isomer is indeed a dodecohedral structure. However, high temperatures favor the corannulene structure, which is a perfect precursor for the formation of C 60. These results are consistent with the experimental data, since high temperatures are needed for efficient formation of C 60. The atomic and electronic structure and doping properties of semiconducting microtubules have also been investigated. The distortions from the ideal geometries are small in microtubules. Substitutional N and B are effective shallow donors and acceptors, respectively. For Al clusters, we focused on Al 13 and Al 55, which can assume perfect icosahedral and cubic structures. However, the distortions from these ideal structures are substantial. For Al 55, several inequivalent but nearly energetically degenerate structures are found. It is shown that this is due to the short range of the screened interatomic interactions.