Scanning tunneling microscopy studies of metal clusters supported on TiO2 (110): Morphology and electronic structure

A brief review of our laboratory's recent scanning tunneling microscopy (ST M) studies on nanoclusters supported on TiO2(110) is presented. Particular emphasis is placed on the system Au/TiO2(110). The nucleation and growth of the clusters, which were vapor-deposited on TiO2(110) under ultra high vac uum (UHV) conditions, were investigated using STM. It was found that Au, Pd , and Ag clusters all grow in a three-dimensional (3D) (Volmer-Weber) fashi on on TiO2(110), but that at low coverages, quasi-two dimensional (quasi-2D ) Au and Pd clusters were observed. These quasi-2D clusters are characteriz ed by heights of 1-2 atomic layers. Annealing studies show that Au and Pd c lusters form large microcrystals with well-defined hexagonal shapes. Al clu sters, which have a strong interaction with the substrate, are oxidized upo n deposition, "wetting" the surface and forming small clusters. In addition to the topographic studies, the local electronic properties of these clust ers have been studied using scanning tunneling spectroscopy (STS) to measur e the cluster band gaps. The electronic structure was found to be cluster s ize-dependent, as seen by the appearance of a band gap as the cluster size decreased. More specifically, the onset of cluster metallic properties corr elates with the transition from quasi-2D to 3D cluster growth.