Date of Award
Doctor of Philosophy
Hanno H. Weitering
James R. Thompson, Zhenyu Zhang, David C. Joy
Nanostructure systems possessing certain desirable features can arise from the self-organization of fundamental building blocks. In this thesis we explore two types of controlled self-assembly mechanisms in hetero-epitaxy: (a) classical assembly of atom vacancies into quasi one-dimensional line structures and (b) quantum-driven assembly of atoms into atomically-smooth two-dimensional thin films. In the classical assembly phenomenon, adatom vacancies, created via elastic strain-relaxation in compressively strained atom chains on a silicon substrate, self-organize into meandering vacancy lines. The average spacing between these line defects can be varied by adjusting the chemical potential μ of the adsorbed atoms. We implemented a lattice model that quantitatively connects density functional theory calculations for perfectly ordered structures to the fluctuating disorder seen in experiment and the experimental control parameter μ. The quantum-mechanical thin-film assembly explored in this thesis has an electronic origin. It is made possible by strong quantum size effects at the nanoscale and can be controlled experimentally by tuning the quantum mechanical boundary conditions and free carrier density of an ultrathin metal film. This is accomplished via atomic-scale template modification and chemical doping, respectively. Our investigations focused on the formation and structure-property relationship of these engineered quantum films, and specifically on the emergence of collective phenomena such as superconductivity and plasmon excitations. We succeeded in growing atomically-smooth Pb1-xGax (x = 0.06) alloy films on a Si(111)-7 × 7 substrate through quantum confinement, a remarkable observation because Pb and Ga are totally immiscible in the bulk. The resulting films exhibit large uniform-depth holes which turn out to be responsible for the exceptionally large critical current density in these films. Remarkably, the critical current density increases with temperature up to 3.25 K, a phenomenon that has not been seen before and that can be attributed to the unusual quantum-growth morphology of this material. The alloying experiments furthermore elucidate the likely origin of the Tc suppression generally observed in thin films. Finally, we demonstrate the existence of quantized plasmon modes in ultrathin metal films. Controlled self-organization experiments thus enable stabilization of novel nanophase materials, which in turn leads to discovery and understanding of novel collective properties.
Moon, Eun Ju, "Controlled Self-organization and Tunable Collective Phenomena in Surface-based Nanostructures. " PhD diss., University of Tennessee, 2009.