Electronic and geometric structure of transition-metal nanoclusters

A massively-parallel ab initio computer code, which uses Gaussian bases, pseudopotentials, and the local density approximation, permits the study of transition-metal systems with literally hundreds of atoms. We present total energies and relaxed geometries for Ru, Pd, and Ag clusters with N = 55, 135, and 140 atoms; we also used the DMOL code to study 13-atom Pd and Cu clusters, with and without hydrogen. The N = 55 and 135 clusters were chosen because of simultaneous cubo-octahedral (fcc) and icosahedral (icos) sub-shell closings, and we find icos geometries are preferred. Remarkably large compressions of the central atoms are observed for the icos structures (up to 6% compared with bulk interatomic spacings), while small core compressions ({approx} 1 %) are found for the fcc geometry. In contrast, large surface compressive relaxations are found for the fcc clusters ({approx} 2-3% in average nearest neighbor spacing), while the icos surface displays small compressions ({approx} 1%). Energy differences between icos and fcc are smallest for Pd, and for all systems the single-particle densities of states closely resembles bulk results. Calculations with N = 134 suggest slow changes in relative energy with N. Noting that the 135-atom fcc has a much more open surface than the icos, we also compare N = 140 icos and fcc, the latter forming an octahedron with close packed facets. These icos and fcc clusters have identical average coordinations and the octahedron is found to be preferred for Ru and Pd but not for Ag. Finally, we compare Harris functional and LDA energy differences on the N = 140 clusters, and find fair agreement only for Ag

CONNECTION BETWEEN THE LOW TEMPERATURE THERMAL PROPERTIES OF GLASSES AND THEIR GLASS TRANSITION TEMPERATURE

Water doping of nitrate glasses lowers their thermal conductivity. The effect, however, is smaller than expected on the basis of the increased density of states of anomalous states observed in specific heat measurements

Phonon Radiative Heat Transfer and Surface Scattering

Boltzmann transport equation used to derive Fourier's equation fails for nano/microscale heat transport phenomena. An incorporation of the field term into the Boltzmann transport equation with scattering term would be the next logical step for future study of nano/microscale analysis. Additionally, for materials whose structure is not as orderly as a crystal, such as amorphous semiconductors, perhaps the elastic description can be used to solve the phonon-based equations used to approximate quantities such as the heat flux. Acknowledgment