Atomic-Scale Characterization and Manipulation of Freestanding Graphene Using Adapted Capabilities of a Scanning Tunneling Microscope

Graphene was the first two-dimensional material ever discovered, and it exhibits many unusual phenomena important to both pure and applied physics. To ensure the purest electronic structure, or to study graphene’s elastic properties, it is often suspended over holes or trenches in a substrate. The aim of the research presented in this dissertation was to develop methods for characterizing and manipulating freestanding graphene on the atomic scale using a scanning tunneling microscope (STM). Conventional microscopy and spectroscopy techniques must be carefully reconsidered to account for movement of the extremely flexible sample. First, the acquisition of atomic-scale images of freestanding graphene using the STM and the ability to pull the graphene perpendicular to its plane by applying an electrostatic force with the STM tip are demonstrated. The atomic-scale images contained surprisingly large corrugations due to the electrostatic attractive force varying in registry with the local density of states. Meanwhile, a large range of control over the graphene height at a point was obtained by varying the tip bias voltage, and the application to strain engineering of graphene’s so-called pseudomagnetic field is examined. Next, the effect of the tunneling current was investigated. With increasing current, the graphene sample moves away from the tip rather than toward it. It was determined that this must be due to local heating by the electric current, causing the graphene to contract because it has a negative coefficient of thermal expansion. Finally, by imaging a very small area, the STM can monitor the height of one location over long time intervals. Results sometimes exhibit periodic behavior, with a frequency and amplitude that depend on the tunneling current. These fluctuations are interpreted as lowfrequency flexural phonon modes within elasticity theory. All of these findings set the foundation for employing a STM in the study of freestanding graphene. ACKNOWLEDGEMENTS I would like to first thank my research advisor Dr. Paul Thibado for his guidance and encouragement over the last few years. I feel very fortunate to have been part of his group, and I share the credit for our success with him, Dr. Peng Xu, and my fellow hard-working students Dejun Qi, Matt Ackerman, Kevin Schoelz, Gobind Basnet, Cameron Cook, and Josh Thompson. We have also been supported by a number of invaluable collaborators on various projects. My special thanks to Drs. Salvador Barraza-Lopez, Yurong Yang, and Laurent Bellaiche at the University of Arkansas, Dr. Lifeng Dong at Missouri State University, and Drs. Mehdi NeekAmal and Francois Peeters at the University of Antwerp. I have had an excellent dissertation committee whom I appreciate greatly for taking the time to review my progress and results. In addition to Dr. Thibado, I thank Drs. Julio GeaBanacloche, Surendra Singh, and Huaxiang Fu from the Department of Physics and Dr. Hameed Naseem from the Department of Electrical Engineering. Moreover, all my teachers have my gratitude for investing their time to instill the love of learning in me. I furthermore gratefully acknowledge the financial aid provided by several sources. Grants from the National Science Foundation and the Office of Naval Research have helped fund our operating costs and my two-year research assistantship. The University of Arkansas Graduate School generously awarded me a four-year Doctoral Academy Fellowship in addition to the teaching assistantship offered by the physics department. I especially express thanks to Mrs. Jane Hughes for sponsoring the Raymond H. Hughes Graduate Fellowship, which I received in my last semester of study. Finally, I could not have made it to this point without my parents, who always believed in me, and my loving wife, for whom I thank God every day. This is for you and because of you. TABLE OF CONTENTS