Spherosiloxane- and alkylsilane-based monolayers: A combined DFT, STM and transport simulation study.

This dissertation provides a contribution on the study of spherosiloxane- and alkylsilane-based monolayers on Si/Au surfaces at the molecular level from a modeling and simulation perspective. The work includes density functional theory (DFT) calculations, scanning tunneling microscopy (STM) simulations, and transport simulations, supplemented with surface X-ray absorption near edge structure (XANES), extended X-ray adsorption fine structure (EXAFS) experiments. The adsorption of alkylsilanes and alkylgermanes on the Au(111) surface was investigated using DFT within a generalized gradient approximation (GGA). The objective of this study is to evaluate the possible adsorption sites and ultimate local bonding geometries of alkylsilanes/alkylgermanes on Au(111). We find that there are two favorable chemisorption sites: the hollow site and an atop site with an ejected gold substrate atom. Comparisons are made with experimental results from reflection-absorption infrared spectroscopy (RAIRS), X-ray photoemission spectroscopy (XPS) and STM data. The differences between silane- and germane-based monolayers are also addressed. Experiments with XANES/EXAFS for the alkylsilane monolayer on the Au surface are also reported. We attempted to achieve unambiguous structural information for the bonding geometry of alkylsilanes on the gold surface by measuring the Si K-edge X-ray adsorption spectroscopy. A second part of the dissertation examines discrepancies in the apparent vertical and lateral dimensions relative to geometric molecular dimensions in STM images of molecular adsorbates. The Tersoff-Hamann approximation and Bardeen's perturbation model are employed to simulate STM images of the interfacial systems investigated. The results provide an understanding of H8Si 8O12 cluster features on Si(100) and the apparent vertical and lateral dimensions observed in experimental STM data. They also explain how STM images alkylsilane SAMS, which have a large HOMO-LUMO gap, and are dominated by the Si/Au states. The last part of the dissertation examines the electrical transport properties of an H8Si8O12 cluster. A quantum transport simulation based on the Green function method is applied at the molecular level to explore such a microscopic mechanism. We describe the details of the simulation model and propose a hypothesis for the possible two-phase dynamics of HSQ/DSQ metal-insulator-metal (MIM) devices. These properties are postulated to be caused by two molecular conformations.Ph.D.Condensed matter physicsPhysical chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/124645/2/3150176.pd