Doctoral Dissertations

Title

Mechanisms of stabilization in nanocomposites with gold and gallium nitride dispersed phases

January 1997

Keywords

Chemistry, Physical|Chemistry, Polymer

Ph.D.

Abstract

The goal of this work is to synthesize and characterize a novel class of advanced polymer-matrix nanocomposites, particularly poly(methyl methacrylate) - matrix nanocomposites of gold and gallium nitride. Due to their unique interaction with light, these nanocomposites are potential materials for integrated optics. A more fundamental aspect of this study investigated the solid-liquid interface during synthesis of the particles and composites. This study included: synthesis of gold nanoparticles stabilized by unbranched alkanethiols; a synthesis of polymer-stabilized gold nanoparticles within inverse micelles; synthesis of nanostructured gallium nitride; investigation of the surface chemistry of the GaN, which is related to the physical-chemical interactions in the solid-liquid suspension; and optical characterization of the materials obtained.^ Gold nanoparticles were synthesized by a phase-transfer reaction in which zerovalent gold nucleated in the presence of alkanethiols of eight, twelve, and sixteen carbons. The kinetic interplay of two processes was investigated: (i) growth of gold particles from atomic gold; and (ii) coordination of thiol groups to gold. The competition of these processes yielded sterically stabilized particles less than 10 nm in size. By studying the effect of varying the length of the alkyl group, a model was derived to describe the difference between the particle surface and the analogous gold/thiol monolayer at a flat interface. Nanostructured gold was also synthesized in situ within a poly(ethylphenol-co-hydroxythiophenol) matrix using an inverse-micelle technique.^ The first synthesis of zincblende, nanostructured GaN was demonstrated by thermal decomposition of the dimeric precursor $\rm Ga\sb2\lbrack N(CH\sb3)\sb2\rbrack\sb6.$ A method was developed for de-agglomerating this product and preparing a PMMA-matrix nanocomposite with dispersed GaN. A mechanistic investigation of this process using diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) analysis suggested chemisorption plays an important role in de-agglomeration of the powder and stabilization of the resulting suspension proceeds mainly through adsorption of the acetate functionality.^ Composites with dispersed gold had third-order nonlinear optical susceptibility, $\chi\sp{(3)},$ of ${-}1.5\times10\sp{-10}$ e.s.u. Gallium nitride nanocomposites exhibited $\chi\sp{(3)}$ of $+2.6\times10\sp{-11}$ e.s.u. ^

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