First-principles Calculations On The Structures And Electronic Properties Of Aluminum Clusters

Atomic clusters are the aggregates of finite number of atoms with relative stability. As the bridge between microscopic atoms or molecules and macroscopic condensed matters, clusters exhibit plenty of size-dependent novel physical and chemical properties so that one can design new materials using clusters as the building blocks. In the study of metal clusters, there have been considerable interests in the aluminum clusters, because the pure aluminum solid is a nearly perfect free-electron metal. Moreover, in the fast-developing spaceflight technology, the amount of the spaceplanes going into the space has increased rapidly. While they are in the outer space which contains the atomic oxygen, the spaceplanes will be corroded. Since most of the spaceplanes are made of aluminum alloy, so it is important to study aluminum clusters and the chemical absorption with the oxygen atom.The calculations in this work were done with density functional theory within PW91 exchange-correlation functional, as implemented in the DMol software. Different isomer structures for Al2 to Al15 were optimized, and the lowest energy structures were obtained. The structures and properties of several typical isomers with high symmetry (Ih, D3h, D5h, Oh) for Al13 were studied, and the lowest-energy configurations were obtained for 13-atom Al clusters with different charge states (Al13, Al13- and Al13+ ). We found that the low-energy structures of Al13 and Al13-are all icosahedra, whereas Al13+ is a decahedron. The energy of the Oh isomer is higher, that is, the structure of Al13 cluster is far from a piece of bulk fcc aluminum.The clusters of larger sizes were also calculated including Al55 and Al147. The minimum-energy structure of Al55 is a distorted icosahedron, and also does the Al55 clusters with charges. The minimum-energy structure of Al147 appears to be a slightly distorted cuboctahedron. The structural transition from the icosahedron to the bulklike structure seems to occur earlier than n=147.Based on the minimum-energy structures of the Al13 clusters, the chemisorptions of atomic oxygen on the Al13 and Al13- clusters at different sites have been computed, and compared with oxygen adsorption on the Al (111) surface. It was found that the magic-numbered Al13- cluster is less reactive to atomic oxygen than the neutral Al13 cluster as well as the Al (111) surface.Several magic-numbered clusters Al12X (X=C, Si, Ge) with forty valence electrons were studied by the density functional theory. The neutral Al12C seems to have the most compact structure, and also has the lowest binding energy among the neutral clusters. The chemisorptions of atomic oxygen on the Al12X clusters were also calculated, and we found that the best absorption site is the hollow site, that is different from the Al13 clusters with the bridge site. The results showed that, the oxygen absorption energies of Al12X clusters are lower than the pure Al13 clusters.In summary, the study of aluminum clusters is a hot topic in cluster physics. The present results provide useful insight in understanding the structures and properties of pure and doped aluminum clusters.