Clusters as new materials

Over the past two decades methods have been developed to produce clusters with an exactly determined number of atoms. Due to their finite size these small particles have totally different structures and `materials propertiesʹ than their bulk crystalline counterparts. Even more, these properties sometimes change drastically whenever a single atom is added to or removed from the cluster. This opens the pathway for a whole new world of tailor made materials in the future. In this article we describe the present state of the knowledge of the properties of clusters of atoms which in their bulk form conventional metals or semiconductors. The questions addressed include the development of the electronic structure as a function of cluster size and for example what remains of the `metallicʹ properties of the bulk solid in these very small clusters. Technological advances are expected using clusters on a specific support material in the areas of catalysis, magnetic storage media or electronic materials, and even solids assembled totally from clusters. Examples from each of these fields will be discussed in the context of this article.