Introduction to Clusters

Clusters, or nanoparticles, are mesoscopically small pieces of a given material, typically consisting of 3 to 10⁶ atoms or molecules of the same type. It took time to physicists to identify these "small particles", as they were called before, as objects with specific properties, between large molecules and small pieces of bulk.

Clusters around us

Clusters, however, have been used since antiquity by artcraft workers. For instance, Romans used to add gold powder (thus dispersed particles of gold) in glass, producing red stained-glass.

An amazing example is given by the Lycurgus cup, a Roman vase from the fourth century A.D.: viewed in reflected light (as during the day), the cup appears green. However, in transmitted light, that is, with a light source in it, it appears red.

Clusters played also a major role in photography: tiny clusters of Silver bromide AgBr on films, exposed to light, produced clusters of Silver Ag. The longer the exposure, the larger the number of silver clusters and the "darker" the regions of the negative film.

More recently, the discovery of the C₆₀, the so-called fullerene, has opened a wide field in cluster physics, in theory as well as in experiment. Other fullerenes and nanotubes, which rapidly followed this discovery, exhibit exceptional mechanical and electrical properties and look promising for many applications in industry.

On the left are few examples of fullerenes and nanotubes, with various helicities.

Neither a molecule nor a piece of bulk

A cluster differs quantitatively from a large molecule in the sense that a molecule has usually a small number of isomers (that is, stable spatial configurations for the same number of constituents), whereas a cluster typically exhibits a large number of isomers. For instance, various theoretical models have demonstrated hundreds of isomers of the cluster Ar₁₃ .

The difference between a cluster and a small piece of bulk is also significant: the ratio of atoms on the surface to those in the volume is generally not negligible. Indeed finite volume effects are often preponderant in cluster physics and are sources of complexity in the theoretical description of cluster dynamics.

One could consider that cluster physics lies between molecular and solid state physics. This field, well identified since the last quarter of the 20th century, now booms, in close relation with quantum chemistry.

How to produce a cluster in a laboratory?

The first method consists in exposing a material to an external environment (vapor or salt) and inducing the exchange of atoms between the environment and the bulk or the surface. This is a way of manufacturing embedded clusters in glass or deposited on a surface.

Since the 1980's, we know how to produce free clusters by a fast expansion of supersonic atomic jets. This experimental method has allowed a great development of cluster physics. Indeed, embedded and deposited clusters are much more involved theoretically than free clusters. This method can also be the first step in the production of deposited and embedded clusters, by colliding free clusters and a matrix.

Another typical feature in cluster production is the large scalability in the number of constituents. This allows specific studies with respect to the size of the cluster.