Building up to be a metal.

Building up to be a metal

It sounds like a riddle: How many atoms make a metal? According to present-day theory, the atoms in a chunk of metal sit in an orderly, three-dimensional array, but each atom typically contributes one or two electrons that are free to roam throughout the lattice. Together, these itinerant electrons form a kind of electronic "sea' that gives a material its metallic quality.

The physical question is whether clusters containing only a few dozen atoms still display the magnetic, electrical and optical properties shown by the corresponding bulk metal. At some point, as the number of atoms in a cluster decreases, these tiny chunks of matter must lose their metallic character, and the electrons should no longer be free.

In the March 21 NATURE, Peter P. Edwards and his colleagues at the University of Cambridge in England and Cornell University in Ithaca, N.Y., report that certain magnetic properties characteristic of the bulk metal begin to appear when as few as 10 metal atoms are present. Although this result isn't necessarily true for other properties like electrical conductivity, it is an important step in probing the evolution of metallic characteristics.

The Cornell-Cambridge group investigated a set of molecular cluster compounds that consist of a clump of osmium atoms surrounded by a protective sheath of carbon monoxide molecules. In effect, each cluster has a tiny piece of metal at its center, while the sheath prevents the metal cores from aggregating to form larger particles of bulk metal. By measuring the magnetic susceptibility of these clusters, the researchers discovered that as the unmber of osmium atoms in a cluster goes from three to 10, the material increasingly takes on the magnetic properties expected of osmium metal.

The researchers now plan to extend their studies to larger clusters containing up to 40 metal atoms. This could reveal the stage at which electrons, initially bound to particular atoms, are actually set free within a material. Studies of large clusters may also help industrial chemists, for example, get a better idea of how big a metal particle must be before it acts effectively and selectively as a catalyst in a chemical reaction.

"There are certainly lots of systems in which metallic particles are used extensively,' says Edwards. "At the moment, the physics and chemistry of these particles is not clear.' In the future, by specifying the number of metal atoms needed within constituent particles, it may be possible to custom design improved catalysts, photographic emulsions, magnetic recording media, pigments and other products.