STRONG AND WEAK INTERLAYER INTERACTIONS OF TWO-DIMENSIONAL MATERIALS AND THEIR ASSEMBLIES Tyler William Farnsworth A dissertation submitted to the faculty at the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry. Chapel Hill 2018 Approved by: Scott C. Warren James F. Cahoon Wei You Joanna M. Atkin Matthew K. Brennaman © 2018 Tyler William Farnsworth ALL RIGHTS RESERVED ii ABSTRACT Tyler William Farnsworth: Strong and weak interlayer interactions of two-dimensional materials and their assemblies (Under the direction of Scott C. Warren) The ability to control the properties of a macroscopic material through systematic modification of its component parts is a central theme in materials science. This concept is exemplified by the assembly of quantum dots into 3D solids, but the application of similar design principles to other quantum-confined systems, namely 2D materials, remains largely unexplored. Here I demonstrate that solution-processed 2D semiconductors retain their quantum-confined properties even when assembled into electrically conductive, thick films. Structural investigations show how this behavior is caused by turbostratic disorder and interlayer adsorbates, which weaken interlayer interactions and allow access to a quantum- confined but electronically coupled state. I generalize these findings to use a variety of 2D building blocks to create electrically conductive 3D solids with virtually any band gap. I next introduce a strategy for discovering new 2D materials. Previous efforts to identify novel 2D materials were limited to van der Waals layered materials, but I demonstrate that layered crystals with strong interlayer interactions can be exfoliated into few-layer or monolayer materials. The strategy relies on a mechanistic similarity between mechanical exfoliation and scratching in layered materials: both involve crack propagation between layers. I therefore use the Mohs hardness scale, a measure of scratch resistance, to identify promising layered materials, and I test these predictions using mechanical exfoliation. We find that a Mohs hardness of five is a threshold below which mechanical iii exfoliation occurs. To understand why, we examined 1,000 crystals and find an intuitive correlation between Mohs hardness and the nature of interlayer bonding. Finally, we show how our approach can be extended to computational searches of large databases of material properties to find additional 2D materials that can be used as building blocks for new 3D solids with custom-designed properties. iv To my wife, Katie – my constant encouragement, biggest supporter, and love of my life. v ACKNOWLEDGEMENTS I would like to thank my advisor, Professor Scott Warren, for his guidance, mentoring, and support during my PhD research. I owe my ability to think critically and carefully about scientific ideas to him, and I’m grateful for his pushing me to be the best that I can be. Thank you to my committee members, Professor Wei You, Professor Jim Cahoon, Professor Joanna Atkin, and Dr. Matthew (Kyle) Brennaman, for taking the time to serve on my committee. Each of them has provided stimulating discussions about my research at various points over the past five years, and I’m thankful for each of their contributions. I would also like to thank the Warren Lab for the thought-provoking discussions of scientific research and the fun we have had over the years. You are each so very gifted and talented in your abilities as scientists and I know that each of you have a bright future ahead. I’m looking forward to hearing about your upcoming accomplishments! Special shout-out here to Adam – aka my Chess coach – I couldn’t have asked for a better partner in crime as we built up the lab, and now we are “exit buddies” as we move on to our next paths. Best of luck in all your endeavors! I would also be remiss to not mention the many undergraduate students who worked with me over the years. Thank you to Emily, Jon, Elle, and Rebekah for your hard work and uncanny ability to give me a hard time. I extend a special thanks to my wife, Katie, for her constant encouragement and support as I’ve completed this dissertation. You mean the world to me and I honestly wouldn’t have made it to the end of this PhD without you. I’m grateful as well to my family vi and friends who have encouraged me throughout the PhD. You are each so important to me and I’m grateful for your support. Thank you, Mom, for the time and energy that you invested into my homeschool education, K5-12th. Thank you for teaching me to work hard and well and that I could accomplish anything if I put my mind to it. Thank you also to my former advisor, Professor Christopher Bender at the University of South Carolina Upstate, who showed me how cool chemistry is and convinced me to become a Chemistry major. I am grateful to the National Science Foundation for their support over the past three years through the Graduate Research Fellowship Program. It is a privilege to be called an NSF Fellow, and I will strive to live up to the honor of this award during my scientific career. Finally, I give thanks to God for his faithfulness to me. All that I have is a gift from God, and I continue to be amazed that I have an opportunity to study and learn more about the complexity of our world and its reflection of the Creator. To Him be the glory. vii TABLE OF CONTENTS LIST OF TABLES ................................................................................................................... xi LIST OF FIGURES ................................................................................................................ xii LIST OF ABBREVIATIONS ................................................................................................ xvi CHAPTER ONE – INTRODUCTION ..................................................................................... 1 1.1 2D nanomaterials as unique material class ..................................................................... 1 1.2 Extension of confined-yet-coupled design to 2D materials ............................................ 3 1.3 Opportunities to advance the discovery of new 2D materials ......................................... 6 REFERENCES ...................................................................................................................... 8 CHAPTER TWO – METHODS ............................................................................................. 14 Introduction ......................................................................................................................... 14 2.1 2D phosphorus preparation ........................................................................................... 14 2.1.1 Black phosphorus synthesis .................................................................................... 14 2.1.2 2D phosphorus exfoliation...................................................................................... 20 2.2 2D MoS2 preparation ..................................................................................................... 24 2.2.1 Method 1: Scaled-up MoS2 liquid-phase exfoliation ............................................. 24 2.2.2 Solvent transfer procedure ...................................................................................... 26 2.2.3 Method 2: n-butyllithium MoS2 exfoliation ........................................................... 28 2.2.4 Conversion of 1T nBuLi-MoS2 to 2H phase .......................................................... 30 2.3 Thin film deposition of 2D materials ............................................................................ 33 2.3.1 Vial interface method ............................................................................................. 33 viii 2.3.2 Buchner interface method ....................................................................................... 34 2.3.3 Langmuir-Blodgett assembly ................................................................................. 36 2.4. Spectroscopy of 2D dispersions and thin films ............................................................ 50 2.5 Diamond anvil cell high pressure measurements .......................................................... 61 2.5.1 The diamond anvils ................................................................................................ 63 2.5.2 The gasket ............................................................................................................... 66 2.5.3 The pressure medium.............................................................................................. 72 2.5.4 The pressure measurement ..................................................................................... 73 2.5.5 General advice ........................................................................................................ 77 2.5.6 Sample preparation of LB Trough Film for DAC .................................................. 77 2.6. Preparation of KBr pellet for FTIR solid sample analysis ........................................... 82 2.7 Data mining of minerals ................................................................................................ 84 2.8 Mechanical exfoliation and characterization of layered minerals ................................. 85 REFERENCES .................................................................................................................... 87 CHAPTER THREE