Identifier

etd-11162005-160340

Degree

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Document Type

Dissertation

Abstract

Ag nanostructures of multilayer coverages (< 30 monolayers) epitaxially self-assembled on Cu(110) and Ni(110) have been explored by scanning tunneling microscopy (STM), angle-resolved photoelectron emission spectroscopy (ARPES), and low energy electron diffraction (LEED). We have studied varied nanostructure morphologies self-assembled depending on different deposition/annealing processes and coverages, their atomic structures, growth behaviors and mechanisms, and the electronic structures of nanowires. At nominal coverages of 1.2 ML < ¦È < ~10 ML, there are two epitaxial structures on Cu(110) and Ni(110). One is a Ag(110) multilayer film, which has a superstructure with lateral periodic units of eight and three/four substrate lattice constants along [¯110] and along [001] respectively. Another is that of Ag(110) nanowires surrounded by pseudohexagonal Ag(111) monolayer. The Ag(110) nanowires are triangular in cross section. The two side surfaces are faceted and the long axis is atomically straight along [¯110]. Typical lengths are within the range of 100 ~ 5000 Å, widths 70 ~ 300 Å, side slopes 10 ~ 30º, and heights 5 ~ 60 Å. The Ag nanowires present extraordinary anisotropy with observed aspect ratios (length:width) of up to 20:1. The Ag(110) nanowires are in-registry with the substrate along [001], but not along [¯110]. At coverages of ~ 10 ML < ¦È < ~25 ML, there also exist two different nanostructures, the nanowires and a Ag(110) atomically-flat film with some pits as deep as down to the substrate and a one-dimensional quasiperiodic superstructure along [001]. There are two basic separations of the superstructural stripes: one is three lattices wide (~11 Å) and the other is two lattices wide (~7 Å). Both of nanostructures are stable at temperature up to at least 200 ¡ãC and not inter-transformable. The growth of the nanowires is driven by the elastic strain mechanism, but the growth of the atomically-flat film is driven by electronic growth mechanism originated from the electron quantum confinement in the vertical direction of the film. The ARPES of the nanowires shows dispersion in the vertical and the [¯110] directions, but no dispersion in the [001] direction because of the limited width (~ 200 Å).

Date

2005

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Phillip T. Sprunger

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