Novel interactions of supported clusters: Contact epitaxy

The study of clusters of `model' metal systems such as Cu and Ag provide a valuable route to explore critical issues in materials epitaxy. Our investigations have led to observations of novel interactions between supported metal clusters in both homo- and heteroepitaxial configurations. In the experiments, clusters of both Cu and Ag were produced by inert gas condensation and deposited on the clean (001)Cu surface under ultrahigh vacuum. Following deposition, the Cu clusters were observed to be of initially random orientation on the substrate surface, undergoing reorientation upon annealing by a mechanism involving sintering and grain growth. In the case of Ag clusters, the formation of a heteroepitaxial layer between the particle and substrate was observed upon initial contact. The phenomenon, which we call `contact epitaxy', may be understood from molecular dynamics simulations of a `soft impact' between the nanoparticle and substrate which indicate that the ordered layers form within picoseconds of impact. The experiments were performed in an ultrahigh vacuum transmission electron microscope equipped with an in-situ nanoparticle sputtering system.