Clusters
Clusters Sources
Clusters can be formed from any element on the periodic table. They can be homogeneous or heterogeneous, metallic or non-metallic even neutral or charged. What all clusters have in common however is that they fall between the territories of the isolated atom and the bulk phase.
Lying as they do between the quantum regime of small molecules and the classical regime of condensed matter you cannot predict the properties of clusters by extrapolating from models which describe either isolated or bulk systems. Indeed, it has been found that clusters frequently exhibit chemical and physical properties not found in the bulk phase. It is these unique properties which scientists hope to exploit in a wide number of areas.
The cluster regime is broadly accepted as being anything from two to several hundred thousand atoms. There is much debate on where the boundary between the cluster and bulk regime lies - particularly since it often depends on which aspect of the cluster you are looking at. However, at the upper end of this regime, the clusters tend to have bulk properties. For medium sized clusters it has been found that the cluster properties tend to be a smoothly varying function of their size whereas for small clusters the properties are more dependent on the structure of the molecule.
This trend can be understood if you consider the surface atoms of a cluster. Compared to the bulk phase, the atoms in a cluster are far more likely to be on or near the surface. Even for a cluster composed of a thousand atoms, about one quarter of these will lie on the surface. As a result of these surface effects then all of the physical properties of a cluster, whether electronic, optical or thermodynamic for example, become essentially size dependant. More specifically, they become dependant on the surface to volume ratio of a cluster.
To fully understand a clusters properties it is necessary to get accurate information on their geometric and electronic structures. However, in order to do this it is first necessary to produce the cluster you wish to investigate.
The difficulty in the generation of significant numbers of clusters of the required form has meant that in cluster science the most significant advances have been made as a result of technical developments in the production of these clusters. Over the next few pages you can have a look at some of the different sources which have been developed.