Structural And Electronic Properties Of Rare Earth Disilicide Nanowires

Metallic nanostructures show promise for nanocatalyst applications due to quantum size effects that dominate electronic properties at the nanoscale. Large surface area to volume ratios, the availability of a large number of active chemical sites, enhanced resistance to poisoning and enhanced catalytic activity have been observed for an assortment of nanocatalyst systems. This dissertation focuses on the structure and characterization of bimetallic core-shell nanostructures on Si(001) using a unique combination of ultra high vacuum scanning tunneling microscopy, Kelvin Probe force microscopy and density functional theory. Specifically, rare earth disilicide nanowires are used as templates for noble metal nanowire and nanoparticle arrays with monodisperse size and spacing. The commercially available Vienna ab initio Simulation Package was used along with the yttrium pseudo-potential as a substitute for rare earth elements.The nucleation of rare earth disilicide nanowire templates on Si(001) is explored, from low coverage adatom reconstructions to nanowire formation. Experimental and theoretical studies corroborate atomic models for (2×3), (2×4) and (2×7) reconstructions, charge transfer from rare earth adatom to Si and morphology of narrow rare earth disilicide nanowires.Clear understanding of the relationship between electronic structure and chemical activity will aid in the rational design of nanocatalysts. Core (?) shell Au-coated dysprosium and yttrium disilicide nanowires provide a model atomic scale system to understand how charges that transfer across interfaces affect other electronic properties and in turn surface activities toward adsorbates. Scanning tunneling microscopy data demonstrate self-organized growth of Au-coated DySi2 nanowires with a nanometer feature size on Si(001), and Kelvin probe force microscopy data measure a reduction of work function that is explained in terms of charge transfer. Density functional theory calculations predict the preferential adsorption site and segregation path of Au adatoms on Si(001) and YSi2. The chemical properties of Au (?) YSi2 nanowires are then discussed in light of charge density, density of states, and adsorption energy of CO molecules. This work demonstrates the ability to tune work function with material interface and structure size and provides an overall characterization of electronic properties of core-shell nanostructures that can be utilized for nanoscale catalysis studies.