Dissertation/Thesis Abstract

Theoretical and Computational Approaches to Vapor-Liquid-Solid Nanowire Growth
by Schwalbach, Edwin Joseph, Ph.D., Northwestern University, 2011, 220; 3456698
Abstract (Summary)

There has been significant research on the synthesis of semiconducting nanowires in recent years due to their potential applications in fields as diverse as integrated circuit technology, photonics, energy generation and storage, and biological sensing. However, there are still many open questions in the field and control over wire morphology and composition are crucial for the fabrication of functional nanowire devices. The present work employs theoretical and computational techniques and focuses on several important aspects of the popular Vapor-Liquid-Solid (VLS) growth mechanism which influence these vital nanowire characteristics.

Nanowires have high surface area to volume ratios and thus bulk phase diagrams must be corrected for size dependent effects. These thermodynamic corrections explain the presence of liquid droplets well below bulk eutectic temperatures. Finite system size effects alter the undercooling necessary for nucleation and give limits on supersaturation of liquid droplets during growth.

The elastic energy stored in core-shell nanowires can be significant due to both lattice misfit and surface-stress effects. Calculations identify the arrangement of phases that gives the lowest elastic energy and establish conditions where misfit or surface-stresses dominate the total energy.

Doping semiconductor nanowires during VLS growth is required to synthesize devices such as field effect transistors. A model for steady-state VLS growth of ternary semiconductorcatalyst- dopant nanowires predicts that the relative flux of dopant atoms through the liquid controls the mole fraction of dopant in the solid wire, which in turn influences its electrical properties. The ternary phase diagram can be used to determine an upper limit on the dopant composition in the solid, and comparison of the model with measurements of dopant distributions in experimentally grown nanowires gives important thermodynamic quantities.

The morphology of nanowires is another key aspect which must be controlled in order to produce functional devices. Kinking during growth has been linked to liquid droplet stability. A phase-field model is used to examine the stability of different liquid configurations and simulations identify the pathways that liquids take during morphological transformations. Simulations also give estimates for the time scales for these transitions.

Indexing (document details)
Advisor: Voorhees, Peter W.
Commitee: Davis, Stephen H., Lauhon, Lincoln J., Wolverton, Christopher M.
School: Northwestern University
Department: Materials Science and Engineering
School Location: United States -- Illinois
Source: DAI-B 72/08, Dissertation Abstracts International
Subjects: Materials science
Keywords: Doping, Fluid flow, Nanowires, Phase diagrams, Phase fields, Vapor-liquid-solid growth
Publication Number: 3456698
ISBN: 9781124662879