Scanning tunneling microscopy studies of self-assembled nanostructures on graphite

Abstract

We have used in-situ scanning tunneling microscopy to investigate the growth of nanostructures of various elements such as Sb, Bi, Al, In, Ge and Mn on highly oriented pyrolytic graphite (HOPG) in ultra-high vacuum. Initially, three-dimensional (3D) clusters, islands and crystallites of these elements (except Bi) nucleate and grow at step edges and defect sites of HOPG at room temperature (RT). The clusters of Al, Ge and Mn form chains while Sb and In islands are mostly isolated. The 3D islands of Sb, Al and In have bulk crystalline structure and (111) orientation. In addition to 3D islands, 2D films and 1D nanorods of Sb are observed. At ~ 375 K with a high flux, only 2D and 1D Sb nanostructures are formed, whereas only 3D islands are obtained initially when Sb is deposited with a low flux at RT. The 2D Sb and Bi structures showed bulk lattice structure and (111) orientation whereas the nanorods of Sb and Bi are found in compressed state which is likely obtained under the Laplace pressure that can be quite large in nanostructures. These observations offer the possibility to obtain different shapes and dimensionality of nanostructures by selecting proper growth conditions like flux, exposure time and substrate temperature.Al and In nanostructures grown on single crystal molybdenum disulphide (MoS2) surfaces have also been studied. The shape of Al and In nanostructures are quite different on MoS2 and HOPG. The different growth behaviors of Al and In found on these two substrates indicate that a subtle change in metal-support interaction can alter nanostructural shape significantly.