Acomparative study of al, Ge and Sb selfassembled nanostructures on graphite

In situ scanning tunneling microscopy (STM) investigations of the nucleation, growth, aggregation and coarsening of nanoparticles on an inert substrate, such as graphite, reveal many intrinsic thermodynamic and kinetic properties of nanoparticles important to nanostructural self-assembly and applications. We performed systematic in situ STM studies of Al, Ge and Sb growth on highly oriented pyrolytic graphite (HOPG). At room temperature (RT), three dimensional (3D) clusters of all three elements nucleate and grow at step edges and defect sites of HOPG. The clusters of Al and Ge form chains, while Sb islands are mostly isolated. With increasing deposition at RT, Al clusters grow and coarsen into crystallites with (111) facets on top, which coalesce further into flat islands with craters on the top. In contrast, due to a low sticking probability of Ge atoms on graphite and little coarsening among Ge clusters, single- and double-layer cluster chains as well as ramified islands are observed. When deposited or annealed at T ≥ 450 K, Ge forms crystallites but with randomly oriented high-index facets. As spherical Sb islands grow beyond certain size, (111) facets appear on the top. In addition to 3D islands, 2D crystalline Sb films and 1D nanorods are observed. At T ≈ 375 K and a high flux, only 2D and 1D Sb islands are formed, whereas only 3D islands are formed initially when Sb is deposited with a low flux at RT. This selectivity of different dimensional Sb nano-assembly is explained in terms of Sb4 diffusion and dissociation kinetics. The Sb nanorods start with a simple cubic lattice structure, which is observed under high pressure for bulk Sb crystal. These different growth behaviors reflect the unique nature of interaction among the atoms (molecules), clusters and crystallites of each element, as well as with HOPG substrate.