Clusters are aggregates of atoms (or molecules) containing between three and a few thousand atoms that have properties intermediate between those of the isolated monomer (atom or molecule) and the bulk or solid-state material. The study of such species has been an increasingly active research field since about 1980. This activity is due to the fundamental interest in studying a completely new area that can bridge the gap between atomic and solid-state physics and also shows many analogies to nuclear physics. However, the research is also done for its potential technological interest in areas such as catalysis, photography, and epitaxy.
A characteristic of clusters which is responsible for many of their interesting properties is the large number of atoms at the surface compared to those in the cluster interior. For many kinds of atomic clusters, all atoms are at the surface for sizes of up to 12 atoms. As the clusters grow further in size, the relative number of atoms at the surface scales as approximately 4N⁻1/3, where N is the total number of atoms. Even in a cluster as big as 105 atoms, almost 10% of the atoms are at the surface. Clusters can be placed in the following categories:
- Microclusters have from 3 to 10-13 atoms. Concepts and methods of molecular physics are applicable.
- Small clusters have from 10-13 to about 100 atoms. Many different geometrical isomers exist for a given cluster size with almost the same energies. Molecular concepts lose their applicability.
- Large clusters have from 100 to 1000 atoms. A gradual transition is observed to the properties of the solid state.
- Small particles or nanocrystals have at least 1000 atoms. These bodies display some of the properties of the solid state.
The most favored geometry for rare-gas (neon, argon, and krypton) clusters of up to a few thousand atoms is icosahedral. However, the preferred cluster geometry depends critically on the bonding between the monomers in the clusters. For example, ionic clusters such as those of sodium chloride [(NaCl)N] very rapidly assume the cubic form of the bulk crystal lattice, and for metallic clusters it is the electronic structure rather than the geometric structure which is most important.
There are two main types of sources for producing free cluster beams. In a gas aggregation source, the atoms or molecules are vaporized into a cold, flowing rare-gas atmosphere. In a jet-expansion source, a gas is expanded under high pressure through a small hole into a vacuum.
In most situations, the valence electrons of the atoms making up the clusters can be regarded as being delocalized, that is, not attached to any particular atom but with a certain probability of being found anywhere within the cluster. The simplest and most widely used model to describe the delocalized electrons in metallic clusters is that of a free-electron gas, known as the jellium model. The positive charge is regarded as being smeared out over the entire volume of the cluster, while the valence electrons are free to move within this homogeneously distributed, positively charged background.