Deposition and diffusion processes of metallic atoms and clusters on graphite

• Main Author: Francis, G. M.
• Published: University of Cambridge 1997
• Subjects: 530.417
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599165

Metal clusters can be regarded as nanometre-scale building blocks for the construction of novel materials, but before this technology can be utilised to its full extent, the processes involved in the creation of nanometre size structures on a surface need to be more fully understood. This thesis describes investigations of the diffusion of metallic atoms and clusters on a graphite surface, on the nanometre scale. In chapter one, I review recent advances in the field of cluster physics, concentrating on the formation of clusters, both from beam sources and by epitaxial growth on a surface. Chapter two details a study of the nucleation and growth of Ag clusters on a highly oriented pyrolytic graphite surface using high resolution scanning electron microscopy (SEM). Atomic clusters of diameters between 4-20nm have been grown by the diffusion and aggregation of Ag atoms across the surface terraces. The growth of three dimensional clusters at the surface steps has also been investigated as a function of the substrate temperature, between the temperatures of 20<SUP>o</SUP>C and 200<SUP>o</SUP>C, and an atomistic theory has been used to calculate the activation energy for diffusion of the atoms along the steps on graphite. At higher temperatures the cluster growth has been confined to the steps, resulting in the formation of quasi one-dimensional cluster wires. The mobilities of atoms and clusters on the surface have also been investigated. Chapter three contains a study of epitaxially grown Ag and Au clusters on graphite surfaces, using a scanning tunnelling microscope (STM) in air. The average size of the clusters observed with the STM has been found to be smaller than that observed with the SEM. This discrepancy has been attributed to an interaction between the STM tip and the larger size clusters, resulting in their movement from the scanning area. Evidence for this process is presented, and a size dependent cluster-surface binding energy has been postulated to explain the results.