CONTROLLING GOLD NANOPARTICLE ASSEMBLY THROUGH PARTICLE- PARTICLE AND PARTICLE-SURFACE INTERACTIONS Dissertation Submitted to The School of Engineering of the UNIVERSITY OF DAYTON In Partial Fulfillment of the Requirements for The Degree of Doctor of Philosophy in Engineering By John Joseph Kelley, M.S. Dayton, Ohio August, 2018 CONTROLLING GOLD NANOPARTICLE ASSEMBLY THROUGH PARTICLE-PARTICLE AND PARTICLE-SURFACE INTERACTIONS Name: Kelley, John Joseph APPROVED BY: ___________________________ ___________________________ Erick S. Vasquez, Ph.D. Donald Klosterman, Ph.D. Advisory Committee Chairman Committee Member Assistant Professor, Department of Associate Professor, Department of Chemical and Materials Engineering Chemical and Materials Engineering ___________________________ ___________________________ Andrey Voevodin, Ph.D. P. Terrence Murray, Ph.D. Committee Member Committee Member Adjunct Professor, Department of Adjunct Professor, Department of Chemical and Materials Engineering Chemical and Materials Engineering ___________________________ Richard A. Vaia, Ph.D. Research Advisor Technical Director, Air Force Research Laboratory ___________________________ ___________________________ Robert J. Wilkens, Ph.D., P.E. Eddy M. Rojas, Ph.D., M.A., P.E. Associate Dean for Research and Innovation Dean, School of Engineering Professor School of Engineering ii ABSTRACT CONTROLLING GOLD NANOPARTICLE ASSEMBLY THROUGH PARTICLE- PARTICLE AND PARTICLE-SURFACE INTERACTIONS Name: Kelley, John Joseph University of Dayton Advisor: Dr. Erick S. Vasquez 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 iii 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. iv DEDICATION Dedicated to my support group. For all that we have endured together. This accomplishment would not have been possible without you. v ACKNOWLEDGEMENTS Above all, I must thank my family: Caryn, Jack, Sam, my parents, my in-laws and anyone else who suffered yet still supported me during this arduous journey. Whether it was through gentle words of encouragement or punitive threats, you did what was necessary to light a fire under my butt and push me across the finish line. You were the inspiration for me to finish that I might not have had on my own. I would also like to thank my friends who have encouraged me along the way, but more importantly, provided me with necessary distractions to help maintain my sanity, whether it be a couple jokes, a few (or more) beers or a good old gripe session. You definitely helped to keep me grounded and not to take life too seriously. I am also grateful for having Jen DeCerbo as my partner-in-crime throughout grad school. Thank you for keeping me awake during class and making the journey as enjoyable as possible. Furthermore, I am grateful to those who have contributed to my research, whether through technical assistance, imparted knowledge or much-needed guidance. At the front of the pack is Rich Vaia, who has educated and guided me substantially for over a decade. He has also shown considerable patience with me while working on this dissertation, probably more than I deserve. I would also like to thank Hilmar Koerner for being my first mentor at AFRL and helping to establish my skillset in materials research. I also greatly valued the mentorship and friendship of Mike Jespersen, who showed me the ropes with vi nanoparticle assemblies and surface modifications (it’s all your fault!). Additionally, Mike supported my research with useful XPS data, even when he had more important things to do. I also appreciate the assistance and interactions from all of the researchers and technicians at the AFRL Materials and Manufacturing directorate that I’ve had the pleasure to work with at some capacity, a list which is surely too long to include. I would also like to thank all of my professors at UD as well as my committee: Erick Vasquez, Terry Murray, Andrey Voevodin and Don Klosterman. Their knowledge and support have been quite beneficial during my time at UD. Lastly, I would like to thank myself for persevering and seeing this dissertation through to the end, despite all the grief it has imparted over the years. You are my rock. You are my inspiration. You are my hero. vii TABLE OF CONTENTS ABSTRACT ……………………………………………………………………………iii DEDICATION .....................................................................................................................v ACKNOWLEDGEMENTS ................................................................................................vi LIST OF FIGURES ............................................................................................................xi LIST OF TABLES .......................................................................................................... xviii LIST OF ABBREVIATIONS AND NOTATIONS ..........................................................xix CHAPTER 1. INTRODUCTION ....................................................................................... 1 1.1 The Rise of Nanotechnology .................................................................................. 1 1.2 Nanoparticle Assemblies for Technology ............................................................... 2 1.3 Dissertation Overview ............................................................................................ 7 1.4 Research Objectives ................................................................................................ 8 CHAPTER 2. BACKGROUND ....................................................................................... 10 2.1 Nanoscale Structures ............................................................................................. 10 2.2 Functional Surfaces for Nanoparticle Assembly .................................................. 11 2.2.1 Self-assembled Monolayers on Two-dimensional Surfaces ........................ 12 2.2.2 Self-assembled Monolayers on SiO2 Surfaces ............................................. 13 2.3 Gold Nanoparticles ............................................................................................... 26 2.3.1 History and Properties.................................................................................. 26 2.3.2 Synthesis and Purification............................................................................ 29 2.4 Particle Interactions and Stability ......................................................................... 34 2.4.1 Interaction Potentials ................................................................................... 35 2.4.2 Zeta Potential ............................................................................................... 38 2.4.3 Acid-base Chemistry .................................................................................... 39 2.4.4 Ionic Environment ....................................................................................... 43 2.5 Electrostatic Adsorption of Nanoparticles ............................................................ 44 2.5.1 Theory and Simulations ..............................................................................