Nucleation and growth of cobalt nanostructures on highly oriented pyrolytic graphite

Cobalt in the form of three-dimensional (3D) hemispherical clusters (size similar to 10-30 nm) were observed to grow on pristine graphite surfaces via a Volmer-Weber growth mode. X-Ray photoelectron spectroscopy (XPS) reveals that these clusters are physisorbed on the surface. In the presence of minute surface contamination, the morphology of Co changes into a mixture of irregular and hemispherical three-dimensional islands. The formation of irregular islands appears to be mediated by the chemical interactions between Co and the surface contaminants as evidenced from analysis of the carbon pi-pi* transitions. Further analysis of size distribution of Co nanoclusters grown on pristine surfaces shows a critical nucleus size of i* = 1, i.e. a Co dimer forms the smallest stable cluster on a pristine graphite surface.