Scanning Tunneling Microscopy Studies of Metal Clusters Supported on Graphene and Silica Thin Film
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The understanding of nucleation and growth of metals on a planar support at the atomic level is critical for both surface science research and heterogeneous catalysis studies. In this dissertation, two planar substrates, including graphene and ultra-thin silica film were employed for supported model catalysts studies. The structure and stability of several catalytically important metals supported on these two substrates were thoroughly investigated using scanning tunneling microscopy (STM) coupled with other traditional surface science techniques. In the study of the graphene/Ru(0001) system, the key factors that govern the growth and distribution of metals on the graphene have been studied based on different behaviors of five transition metals, namely Pt, Rh, Pd, Co, and Au supported on the template of a graphene moire pattern formed on Ru(0001). Both metal-carbon (M-C) bond strength and metal cohesive energies play significant roles in the cluster formation process and the M-C bond strength is the most important factor that affects the morphology of clusters at the initial stages of growth. Interestingly, Au exhibits two-dimensional (2-D) structures that span several moire unit cells. Preliminary data obtained by dosing molecular oxygen onto CO pre-covered Au islands suggest that the 2-D Au islands catalyze the oxidation of CO. Moreover, graphene/Ru(0001) system was modified by introducing transition metals, oxygen or carbon at the interface between the graphene and Ru(0001). Our STM results reveal that the geometric and/or electronic structure of graphene can be adjusted correspondingly. In the study of the silica thin film system, the structure of silica was carefully investigated and our STM images favor for the [SiO4] cluster model rather than the network structure. The nucleation and adsorption of three metals, namely Rh, Pt and Pd show that the bond strength between the metal atom and Si is the key factor that determines the nucleation sites at the initial stages of metal deposition. The annealing effect studies reveal that Rh and Pt atoms diffuse beneath the silica film and form the 2-D islands that are covered with a silica thin film. In contrast, the formation of Pd silicide was observed upon annealing to high temperatures.