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Complex Refractive Index Modeling and Nanoscale Patterning of Solid-State Colloidal Quantum Dots for Nanophotonic Applications A DISSERTATION SUBMITTED TO THE FACULTY OF THE UNIVERSITY OF MINNESOTA BY Dana Brooke Dement In Partial Fulfilment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Vivian E. Ferry, Advisor May, 2019 Dana Brooke Dement © 2019 Acknowledgements This work would not have been possible without the many people who have helped and encouraged me along the way. First, I would like to thank my advisor, Vivian Ferry, for her guidance and support. Her drive and enthusiasm for research have been inspirational. I am extremely grateful for the time she devoted to giving quick and detailed feedback on the many abstracts, papers, talks and posters I have sent her over the years, no matter how hectic the week. More importantly, I want to thank her for helping me believe in my data and for giving me the freedom to explore new ideas and experiments in the lab. I would also like to thank Eray Aydil and his student, Whitney Wenger, for their help in my initial few years of graduate school. I first worked with Eray as an undergraduate summer student, and my interaction with him and his group that summer, especially Melissa Johnson and Mike Manno, were a large part of why I decided to attend the University of Minnesota. Once I returned as a graduate student, he continued to provide support by allowing me to use the Aydil group wet lab while our labs were still being set- up. During this time Whitney taught me how to perform quantum dot synthesis and since then she has always been happy to engage in helpful discussions related to quantum dots or otherwise. A tremendous thank you to all of my colleagues in the Ferry group. Specifically, thank you to Ryan Connell who started this journey with me and was always willing to answer questions, help me troubleshoot Lumerical and MSI, or remind me about various dates and deadlines coming up. Thank you to Matt Quan who spent many hours simulating my quantum dot structures and who always had a helping hand and a pun at the ready. Thank you to Mayank Puri who spent many hours synthesizing materials for my first paper. More importantly, he also taught me many wet lab techniques and gave me the courage to try new experiments in the lab. Also thank you to Pavlos Pachidis, not only for stimulating intellectual discussions, but also for the many moments of laughter and support. Additionally, thank you to the other members of the Ferry group including Ian Slauch, John Caputo, Christian Pinnell, Bryan Cote, John Keil, Eleanor Mayes and the many wonderful undergraduates I have had the privilege of working with. They have all contributed to i making the group a fun and collaborative place. I am also grateful to the other members of my CEMS class who have acted as a strong support group ever since our first-semester thermodynamics class. I would also like to thank the people who helped me on my journey before I started graduate school. I have had the privilege of being taught by a number of great teachers and professors during school. One teacher who particularly stands out is Mr. Phil Hammond, who volunteered to teach three different advanced courses that were previously not available at my high school. Despite teaching so many different classes, he always found a way to make learning fun and rewarding. During my undergraduate career, I also had the privilege of attending a summer program at the University of Arizona where Shiva Shahin and Dr. Norwood taught me how to perform research and introduced me to the field of optics. At my home institution, Rice University, I was able to further my curiosity in optical materials by working with Yen-Hao Lin and Dr. Verduzco who taught me the basics of chemical synthesis. Finally, I would like to thank my family. Thank you to my sister, Megan, who has been a consistent source of support and laugher in the last few years. Thank you to my mom, who always knows how to encourage me or cheer me up and to my dad, for providing many enjoyable discussions of materials science in the everyday world. Both of them always found ways to encourage my scientific interests growing up, whether through scientific shows, summer programs, or thought-provoking questions and I could not have gotten this far without them. ii Abstract The small size of photoluminescent, nanocrystal quantum dots (QDs) leads to a variety of unique optical properties that are well-suited to many optoelectronic devices and nanophotonic studies. Here, we demonstrate techniques to further improve the design of solid-state QD structures. A problem for many applications is that predicting the optical behavior of QD solids is difficult because the complex refractive index of QD solids is a composite quantity that is dependent on size, ligand chain length, and the deposition process of the QDs. To address this problem, we show that the intrinsic refractive index of neat CdSe/CdS QDs can be extracted from solution-state absorption data. We then show how this information can be used with effective medium approximations to describe the effective refractive index of QD films associated with a variety of QD sizes and packing fractions. Our predictions are verified experimentally by spectroscopic ellipsometry. With our modeling tool, we can also understand packing variations between QD films and predict the absorption in solid-state QD structures, leading to significant savings in both time and materials. Using the same QD materials, we next address the need for accurate patterning of QD solids at the nanoscale. We have found that direct electron beam lithography is a straightforward patterning process that does not require ligand exchange and results in structures that retain bright photoluminescence. We demonstrate that feature sizes as narrow as 30 nm with many QD layers can be patterned. These structures can withstand sonication in a variety of solvents, show no distortion, and can be placed within 20 nm of their intended location nearly 100% of the time. Combining our nanofabrication technique with the ability to measure the refractive index of the QD pattern, we find that edge effects arising from the finite shape of the QD nanostructure lead to substantial absorption enhancement when compared to an equivalent volume region taken from a continuous QD film. Finally, we explore more complex structures by patterning QD arrays, multilayer QD structures, and QD disks inside plasmonic resonators. We believe that the work presented here lays important groundwork to improve the modeling of QD solids and reveals new ways QDs can be incorporated into devices and nanophotonic designs. iii Table of Contents Abstract ................................................................................................................... iii Table of Contents ................................................................................................... iv List of Tables ........................................................................................................... vi List of Figures ........................................................................................................ vii List of Publications ................................................................................................ xii Chapter 1 – An Introduction to Quantum Dot Solids .......................................... 1 1.1. What is a Quantum Dot? ........................................................................... 2 1.2. Commercial Quantum Dot Applications ................................................... 3 1.3. Quantum Dots for Nanophotonics............................................................. 5 1.4. Thesis Overview ........................................................................................ 6 Chapter 2 – Colloidal Quantum Dot Synthesis .................................................... 8 2.1. The Heat-up Synthesis of CdSe Quantum Dots ...................................... 10 2.2. Shelling of CdSe Cores ........................................................................... 13 2.3. CdSe Nanoplatelets ................................................................................. 18 2.4. Quantum Dot Ligands ............................................................................. 20 Chapter 3 – Quantum Dots in the Solid-State .................................................... 23 3.1. Solid-State Quantum Dot Films .............................................................. 23 3.1.1. QDs in Encapsulate Matrices ............................................................. 23 3.1.2. Neat Quantum Dot Films .................................................................... 24 3.2. Film Deposition Methods ........................................................................ 25 3.2.1. Processing Tools and Film Deposition ............................................... 25 3.2.2. Solid-State Ligand Exchange ............................................................. 29 3.3. Ligand Crosslinking in Quantum Dot Solids .......................................... 33 Chapter 4 – The Complex Refractive Index of Quantum Dot Materials ......... 35 4.1. Introduction ............................................................................................. 35 4.1.1. Overview of Spectroscopic Ellipsometry ........................................... 36 4.2. Motivation: The Refractive Index of Composite QD Films ................... 39 4.3. Determination of the Refractive Index

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