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Towards Angle-Controlled Van Der Waals Heterostructures TOWARDS ANGLE-CONTROLLED VAN DER WAALS HETEROSTRUCTURES A Dissertation Presented to the Faculty of the Graduate School of Cornell University In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy by Lola Brown August 2015 This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/4.0/. 2015, Lola Brown TOWARDS ANGLE-CONTROLLED VAN DER WAALS HETEROSTRUCTURES Lola Brown, Ph. D. Cornell University 2015 Two-dimensional (2D) materials such as graphene exhibit a combination of unique electronic, mechanical, and optical properties that have drawn significant attention over the past decade. While there has been extensive investigation into individual 2D materials, the burgeoning field of 2D heterostructures offers an even richer array of desirable properties. This led to increasing efforts to controllably manipulate these materials and to tailor them toward potential applications. An important step toward the realization of functional 2D heterostructures is the fabrication and characterization of high quality bilayers with a uniform rotation angle (θ) between the constituent layers. The rotation angle represents a new degree of freedom capable of tuning both optical and electrical properties and is therefore a critical component in designing heterostructures for specific applications. In this thesis, we discuss the fabrication of bilayer graphene with a controlled rotation angle. To accomplish this, we first develop an application of transmission electron microscopy (TEM) capable of imaging the structure and atomic registry between graphene layers in bilayer and trilayer structures. We then introduce a new method for creating graphene and hexagonal boron nitride (h-BN) single layers with crystallographic alignment over large scales, and we characterize the structural and electronic uniformity of these films using a variety of techniques, including TEM, low-energy electron diffraction (LEED), and angle-resolved photoemission spectroscopy. We conclude by showing that these large-scale aligned films can be used as building blocks for 2D layered structures with a controllable rotation angle and uniform optical properties. These findings provide a framework for imaging and fabricating angle-controlled heterostructures that is extensible beyond graphene and h- BN to a variety of 2D materials, thereby opening the door to a virtually limitless combination of 2D heterostructures with uniquely tailored properties. BIOGRAPHICAL SKETCH Lola Brown was born in 1982 and grew up in a small town near Jerusalem, Israel. She completed her military service, and graduated from the Adi Lautman program for outstanding students at Tel Aviv University. After earning her M.Sc. degree in Chemistry in 2010 from Tel Aviv University, she entered the Chemistry and Chemical Biology doctoral program at Cornell University, where she received a Fulbright Doctoral Fellowship. She completed her Ph.D. under the supervision of Professor Jiwoong Park. v To Ima, Aba, Dor, and Guy vi ACKNOWLEDGEMENT This work would not have been possible without the support of many dear friends and colleagues. First and foremost, I owe the greatest thanks to my family. Ima, Aba, Dor, and Guy, you are the wind under my wings. You have always supported my wildest projects, and offered advice, help and encouragement, even when there was an ocean between us. Your good nature, enthusiasm, and creativity inspire me to be the best person I can be. I can't thank you enough for the great love you've surrounded me with, and I dedicate this thesis to you with love. I wouldn’t be here today without much love and support from my extended family. Savta Chana has encouraged my academic development and supported me throughout boarding school. I know she would have been proud of her granddaughter’s PhD. I also spent many happy days with Savta Sylvia and Saba David, who taught me how to appreciate time spent together, and how to build things and nurture kittens and roses. Aunt Zoe and uncle Shalom and their children, Hadas, Zohar, and Elisha, have surrounded me with love and with delicious foods (and made uncle Shalom’s famous Fish Patties every time I came to visit). I hope to keep you in my life always. The choice of joining the Park group has shaped my time at Cornell. Jiwoong, your sharp wit and your commitment to the scientific process have been an inspiration to me. You demanded creativity, independence, and deep conviction in my work, and never settled for anything less than adequate. Even though the process was, at times, frustrating, I am now an incredibly better scientist than when I started. You have also encouraged me to explore my true passion, and for that I am eternally grateful. I would also like to thank the other members of my committee, Professor David Muller and Professor William Dichtel. Your insights and advice have been vii invaluable through the years. I am also thankful for the opportunity to work with a number of other Professors and experts through collaborations, including Kyle Shen, Rodney Ruoff, Abhay Pasupathy, Maria Asensio, José Avila, and Paul McEuen. I owe much of my success in graduate school to my choice of wonderful collaborators. The work done in this thesis could not have been written without the ongoing collaboration with Robert Hovden and Edward Lochocki. Robby and Ed, thank you for sharing your expertise with such patience and enthusiasm. The pleasure, as always, was mine. Furthermore, I was fortunate to collaborate with a large group of talented graduate students and postdocs, with special thanks to Pinshane Huang, Daniel Shai, Eric Monkman, Haofei Wei, ShanShan Chen, and Christopher Gutiérrez. I am also thankful for the technical support provided by John Grazul on the TEM and Nathan Ellis in the Machine Shop, as well as many of the CNF and CCMR staff. Things would not have gone as smoothly without you. The collaborative spirit of the Park group is another thing I am profoundly grateful for. I have benefited from the wisdom and companionship of many of the Park group members, with special thanks to Robin Havener, Mark Levendorf, Wei Tsen, Cheol-Joo Kim, Yui Ogawa, Michal Wojcik, Nicholas Rolston, and Dong-Ki Kim. I also enjoyed scientific discussions and time spent with Carlos Ruiz-Vargas, Lujie Huang, Matt Graham, Kin Fai Mak, Zenghui Wang, Chibeom Park, Lihong Herman, Daniel Joh, Michael Segal, Sang-Yong Ju, Joel Sleppy, Saien Xie, Kibum Kang, Yimo Han, Kan-heng Lee, Hui Gao, Zack Ziegler, and Marcos Guimarães. This thesis was mostly written on Daniel Shai’s dining table while he provided me with writing space and delicious snacks. Daniel, you deserve an extremely special thank-you for your sense of humor and your clear thinking, which light up my days and frequently serve to get me out of a rut. Spending time away from the lab, through rare, was particularly joyous thanks viii to several dear friends. Robin, Mark, Cheol-Joo, Wei, and Carlos, they say the best friendships are forged on the battlefield – thank you for sharing the PhD battlefield with me in many parties, cookouts, and travels. I hope our gang will come together for many more happy occasions. Robert Hovden and Kara Church, your house has become my second home, thank you for graciously providing room on your couch and many reasons to celebrate. A big thank-you goes to Lennessa Shantaya for her gentle teaching and her kindness. I am eternally grateful for the company of my girlfriends – Robin Havener, Sasha Devore, Sisca Harya, Kara Church, Shira Kritchman, Noa Marom, Tauhira Hoossainy, Hyeri Anna Lee, and Elva Cha – you walked into my life precisely when I needed you. I am so blessed to have found the company of such wonderful friends throughout my time in Ithaca. Thank you all! Lola Brown August 2015 ix TABLE OF CONTENTS Chapter 1 : INTRODUCTION ....................................................................................... 1! 1.1! Overview ........................................................................................................... 1! 1.2! Introduction to 2D Materials ............................................................................. 4! 1.3! Methods for Graphene Fabrication .................................................................. 10! 1.4! Bilayer and Trilayer Graphene ........................................................................ 16! 1.5! Summary and Outlook ..................................................................................... 20! References ................................................................................................................ 22! Chapter 2 : IMAGING AND CHARACTERIZATION OF GRAPHENE .................. 27! 2.1! Introduction ..................................................................................................... 27! 2.2! Optical Microscopy Imaging ........................................................................... 29! 2.3! Raman Spectroscopy and Wide-Field Raman ................................................. 35! 2.4! Electron Microscopy Techniques .................................................................... 38! 2.5! Angle Resolved Photoemission Electron Spectroscopy .................................. 48! 2.6! Scanning Probe Techniques ............................................................................ 50! 2.7! Conclusion ......................................................................................................
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