Surface Atomic Scale Engineering by Deposition of Mass Selected Clusters: STM and Helium Scattering Analysis

Abstract

Wide effort is actually carried to use metallic clusters for technological applications like nanosized structured new materials, thin films, surface coatings, etc. Prepared in the gas phase, the clusters are generally deposited on surfaces, either as building blocks in a growth process [1], or to function as a tool to modify the characteristics of the surface [2, 3]. No size selection has been applied so far to the clusters in these applications. However, clusters formed and mass selected in the gas phase and subsequently deposited in a controlled way, is a promising alternative in nanostructure formation on surfaces. Clusters are systems containing typically from 2 to 2000 atoms or molecules. They have been studied for their specific properties, which are size dependent and different from both the atoms (or molecules) and the bulk material [4], mostly due to their large surface to volume ratio. Cluster deposition is of fundamental interest as it differs radically from conventional thermal atom deposition, a field which is now well established [5]. The difference arises mainly from the fact that new parameters, such as the cluster size and the deposition energy, may be used to tailor the collision outcome [6]. These input parameters open new perspectives in the controlled growth of such structures, with new phenomena not accessible within conventional atom deposition or atomic manipulation by scanning probe methods. For instance, the size or equivalently the number of atoms constituting the clusters to be deposited can be tuned for a specific inherent property. In that sense, one could prepare the systems as free clusters before deposition and then deposit them. An important issue for future technological applications of cluster deposition is the relation between the size of the incident cluster’s and the size of the islands obtained on the substrate. Experimental evidence for the importance of the precise definition of the cluster size can be found in the catalytic activity of clusters on oxide substrates [7] and the minimum size of a silver cluster to form an image speck in the photographic process [8].