Size-selected clusters on solid surfaces

The present critical review examines the status of the research on supported, size-selected clusters with specific emphasis on their size-dependent physical and chemical proper-ties. We describe the most relevant experimental methods with their advantages and drawbacks for the production of size-selected clusters and look critically at the present evidence for soft-landing of the clusters onto solid surfaces. Growth and characterization of cluster support materials are surveyed and the important role of surface defects for the properties of the supported clusters is considered. Recent advances in the characterization of the supported, size-selected clusters with local and non-local surface science analytical tools are presented and a few illustrative examples for size effects are given. Of specific and possibly broad impact on heterogeneous catalysis, we consider the discovery of the size-dependence of chemical reactions on very small supported clusters where recent experimental and theoretical evidence indicates charge transfer from the support to the cluster. This charge transfer turns inactive clusters into active ones, opening new perspectives to tune catalytic processes on the nanoscale.