ABSTRACT
Thin films of MoO3 were prepared by deposition of size-selected ligand-free Mo clusters under high vacuum conditions and subsequent exposure to air. The growth pattern is highly dependent on the cluster size. At low coverage, small clusters (Mo51) form a continuous monolayer of fused particles. On top of this monolayer, additional clusters survive as individual entities. Medium sized clusters (Mo251 and Mo1253) do not coalesce and form a monolayer of clusters. Close examination using in situ scanning tunneling microscopy reveals a local order of the particles. At higher coverage a new pattern of large 3-dimensional aggregations of clusters (pylons) appears. The pylons are not formed under high vacuum conditions. Their formation is most likely caused by the air exposure. For the largest clusters (Mo3349) studied here, no monolayer is formed. Instead, the clusters are randomly distributed as expected for particles with zero mobility. These results demonstrate the high potential of cluster deposition for the production of new types of nanostructured surfaces, thin films and nanomaterials.
ABSTRACT
Research on homogeneous and heterogeneous catalysis is indeed convergent and finds subnanometric particles to be at the heart of catalytically active species. Here, monodisperse gold clusters are deposited from the gas phase onto porous titania generating well-defined model systems and the resulting composite materials exhibit a sharp size-dependency on the number of gold atoms per cluster and exceptionally high-turnovers toward the bromination of 1,4-dimethoxybenzene are observed. This indicates that the deliberate generation of active centres is of utmost importance for the creation of the most "gold-efficient" catalysts.
