Microclusters

Divide and subdivide a solid and the traits of its solidity fade away one by one, like the features of the Cheshire cat, to be replaced by characteristics that are not those of liquids or gases. They belong instead to a new phase of matter, the microcluster. Microcluster consist of tiny aggregates comprising from two to several hundred atoms. They pose questions that lie at the heart of solid-state physics and chemistry and the related field of materials science: How small must an aggregate of particles become before the character of the substance they once formed is lost? How might the atoms reconfigure themselves if freed from the influence of the matter that surrounds them? If the substance is a metal, how small must its cluster of atoms be to avoid the characteristic sharing of free electrons that underlies conductivity? Do growing clusters proceed gradually from one stable structure to another, largely through the simple addition of atoms, or do they undergo radical transformation as they grow? Many cluster properties are determined by the fact that a cluster is mostly surface. A closely packed cluster of 20 atoms has only one atom in its interior; a cluster made up of 100 atoms may have only 20. Other properties stem from clusters unfilled electronic bonding capability, which leaves them naked and hence extremely reactive. This reactivity makes them effective tools for the study of the solid state and, potentially, for such industrial processes as the growing of crystals, selective chemical catalysis and the creation of entirely new materials with made-to-order electronic, magnetic and optical properties. Such materials, in turn, could enhance the performance of products as diverse as lasers, photographic film, electrosensitive phosphors, magnetic disks and supercomputers