Characterization of wetting layers and nanostructures at surfaces

Wetting layer at surfaces plays a very important role in the growth of thin film and nanostructures as it defines a modified surface layer on which other structures are grown. The wetting layer also changes the physical and electronic properties of the surface and hence it can influence the growth and properties of nanostructures which sit on top. In this thesis, we will characterize the wetting layers and the nanostructures on top using Low Energy Electron Microscopy/Diffraction (LEEM/LEED), and Spectroscopic PhotoEmission and Low Energy Electron Microcopy. We will examine my research on two different metal on semiconductor systems, Pb/Si(111) and Fe/ZnS(001), as well as carbon-based layers on metal surfaces, C60/Pt(111) and graphene/Ru(0001). The main accomplishments of this thesis are summarized as follows.;An exceptionally unusual mass transport behavior has been discovered in the dense Pb wetting layer on the Si(111) surface. Mass transport is studied by observing non-equilibrium coverage profile evolution with LEEM and micro-LEED. During equilibration, an initial coverage step profile produced by laser induced thermal desorption retains its shape as it is displaced linearly in time. This contrasts with the profile broadening and gradual time-dependent evolution that is expected from classical consideration. Equally inexplicable in the classical context is a small but discrete coverage discontinuity in the wetting layer that expands radially outward like a wavefront. Our observations signal the presence of a very novel super-diffusive mass transport mechanism, which is much more efficient at transporting mass across a surface than stochastic atomic hopping processes.;In the study of Fe/ZnS(001), nanowires of high aspect ratio are prepared by epitaxial growth and we observed a complex and strongly temperature dependent nanowire growth behavior. Two kinds of magnetic nanowires have been found, one of which is pure Fe and the other may contain some kind of iron sulfide, probably greigite (Fe3S4), at least on the surface.;C60 on the Pt(111) surface forms two ordered phases: (✓13x✓13)R13.9° and (2✓3x2✓3x)R30° that normally coexist. In the current work, the two phases are isolated experimentally and intensity vs. voltage, I(V), curves are measured. Their structural details are extracted by combined dynamic LEED analysis of I(V) curves and density functional theory calculations. Our results show that both phases involve substrate reconstruction, producing atom vacancies directly under the C60 molecules.;In graphene/Ru(0001), we have found no evidence of broken mirror symmetry in the diffraction peak intensities measured with micro-LEED on a length scale of 250 nm to support the recent report of chirality in this system. Instead, small rotations of the graphene lattice from orientational coincidence with the substrate are evident in moiré-derived diffraction features. Additionally, bias of the lattice rotation on the two different surface terminations of the hcp substrate has been found.