Controlling Gold Nanoparticle Assembly through Particle-Particle and Particle-Surface Interactions

Date of Award

2018

Degree Name

Ph.D. in Materials Engineering

Department

Department of Chemical, Materials and Bioengineering

Advisor/Chair

Advisor: Erick Vasquez

Second Advisor

Advisor: Richard Vaia

Abstract

Two-dimensional assemblies of colloidal gold nanoparticles were deposited via electrostatic self-assembly onto silicon substrates modified with aminopropyltriethoxysilane. Assemblies were tuned by systematically adjusting the pH and ionic strength of the nanoparticle solutions and the fraction of adsorbed aminosilane molecules on the silicon surfaces. The nanoparticles were characterized by their size distribution, solution stability and electrokinetic properties. The resulting two-dimensional assemblies varied in particle surface coverage, interparticle separation and lateral organization. Increasing solution pH intensified interparticle repulsions and reduced the charge density of the aminosilane substrate, thus decreasing the fractional monolayer coverage of particles. Additionally, increasing ionic strength reduced interparticle separations, which were described by radial distribution functions, and consequently produced denser particle assemblies. At long adsorption times, surface coverage approaches a maximum which was constrained by the extent of interparticle repulsion and particle-surface interactions. With strong surface attraction of the pure aminosilane surface, the particles were incapable of lateral rearrangement during the adsorption process and, at best, organized into liquid-like structures, in agreement with the random sequential adsorption model for colloidal monolayers. In an effort to circumvent this issue, non-binding alkylsilanes were incorporated into the modified surfaces, thereby reducing the aminosilane surface density and weakening the attractive potential of the surface. These mixed silane surfaces were characterized to reveal their chemical and interfacial energetic properties. At a particular threshold of reduced aminosilane density, nanoparticle coverage fell considerably and two-dimensional order degraded. The local geometries of particle assemblies were evaluated by Voronoi tessellation which provided indication of structural transformations with changing solution and surface conditions. As a result, optimal processing parameters were described for obtaining monolayers of gold nanoparticles with varying degrees of surface coverage and two-dimensional arrangement. The results from this study expands the understanding of the underlying chemical and physical mechanisms behind colloidal stability and particle adsorption. This progresses towards the realization of arrays of highly-ordered and densely packed nanoparticles of diverse chemistries largely assembled in parallel onto assorted surfaces using minimal processing.

Keywords

Materials Science, gold nanoparticle, electrostatic self-assembly, self-assembled monolayer, amino silane, mixed silane, random sequential adsorption, radial distribution function, Voronoi tessellation

Rights Statement

Copyright © 2018, author

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