Self-assembly of Ge clusters on highly oriented pyrolytic graphite surfaces

The self-assembly of Ge clusters on highly oriented pyrolytic graphite (HOPG) was investigated by depositing Ge at various temperatures, using in situ-scanning electron microscopy under an ultrahigh vacuum. At the first stage of Ge deposition, Ge clusters were formed along the steps on the HOPG surfaces. With an increase in the amount of Ge deposited, nanostructures of Ge self-assembled depending on the deposition temperature used. At room temperature, the Ge clusters merged together, resulting in the formation of dendritic-shaped structures. At high temperatures (200-600 °C), chain structures of Ge clusters were formed along the steps on the HOPG surfaces. The density of the Ge clusters comprising the chains along the chain direction decreased with an increase in the deposition temperature. This can be explained by the diffusion length of the Ge atoms along the steps during chain formation. From this result, an activation energy of ~ 0.12 eV was determined for the diffusion of the Ge atoms along the HOPG steps. Ostwald ripening of the Ge clusters was also observed by annealing the chain structure of the Ge clusters. By analyzing the change in the Ge cluster density in the chains, an activation energy of ~ 0.68 eV was obtained for a movement of Ge cluster periphery by attachment and detachment of Ge atoms.