Supported metal clusters and cluster-substrate interaction studied by scanning tunneling microscopy

Abstract

This PhD dissertation is concerned with an experimental study of supported metal clusters and cluster-substrate interaction. The geometric and electronic structures of platinum and cobalt clusters on graphite, the effects of clusters on the graphite substrate, and the influences from the substrate to clusters were investigated by scanning tunneling microscopy. A topographic study of adatoms and small clusters of platinum and cobalt on highly-oriented pyrolytic graphite was performed both in air and in ultra-high vacuum. The samples were prepared by vapor deposition on the graphite surface. Stable images of adatoms and small clusters on graphite were obtained with atomic resolution showing their atomic structures. Taking the graphite lattice as reference, the adsorption sites of adatoms and clusters on the surface as well as the bond lengths and bond angles of clusters were determined. Various superstructures on graphite were found near the adsorbed clusters, or generally near defects. The superstructures are localized in small areas near the clusters and decay within a distance of 2 - 5 nm into the graphite lattice. Detailed analyses show that they were due to periodic charge-density modulations superimposed onto the graphite lattice. The different periodic modulations have the same period of 1.5a (a =0.245 nm, is the lattice constant of graphite) and exist in three possible directions, each rotated 30° relative to the graphite lattice. They are generated by adsorbed clusters which act as a scattering center and perturb the surface charge-density. Among many graphite substrates being studied, anomalous giant lattices were observed on three of the samples. They exhibited hexagonal symmetry with lattice constants of 1.7 nm, 2.8 nm, 3.8 nm and 6.6 nm. Atomic resolution of graphite was obtained simultaneously. By introducing small rotations of the top graphite layer relative to the underlying single crystal, a complete description is developed to account for all the features displayed by the STM images. In addition to the giant and atomic lattices, a supergiant lattice was observed. Cobalt particles on the surface were also imaged and were found on the top sites of the giant lattice.