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Scanning Tunneling Microscopy of Two-Dimensional Materials Scanning Tunneling Microscopy of Two-Dimensional Materials DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Grady Gambrel Graduate Program in Physics The Ohio State University 2017 Dissertation Committee: Professor Jay A. Gupta, Advisor Professor Roland Kawakami Professor Mohit Randeria Professor Louis F. DiMauro Copyrighted by Grady Alexander Gambrel 2017 Abstract Two-dimensional materials are of fundamental and practical interest due to the unique properties they exhibit. As they are only one layer or one atom thick, their physical and electronic properties are sensitive to their local environment, and may be manipulated by engineering variations within the materials. Incorporating these materials into electronic devices requires that their interactions are well understood so that they behave as predicted. For this reason, the study of defects, likely contaminants, and engineered structures is worthwhile. The scanning tunneling microscope is of particular use in such experiments as it can be used to characterize the atomic structure and locally measure the electronic properties of samples. Graphene islands were grown on Cu(111) in ultrahigh vacuum to study graphene/copper interactions with minimal surface adsorbates. Using in situ low-temperature scanning tunneling microscopy (STM), both the physical and electronic structure were measured with atomic spatial resolution and compared with those of clean copper regions. These measurements indicate a decrease in the work function relative to clean Cu(111) and reduced coupling of surface electronic states to the bulk. Other measurements find that both the band edge and effective mass of Shockley surface state electrons are influenced by the graphene layer. Similarly, single-layer germanene was grown on and measured on Cu(111). A competing alloy process which forms Cu2Ge restricts notable germanene coverage to a substrate temperature range ii between 40ºC and 150ºC. Atomic resolution imaging and tunneling spectroscopy are used to distinguish germanene and Cu2Ge regions, and probe the interaction of both with the Cu(111) substrate. Germanene was found to be weakly coupled to the Cu(111), maintaining an incommensurate 3.8 Å lattice but lack any clear signatures of the Dirac point. In contrast, a 4.4 Å lattice is observed on Cu2Ge that is commensurate with a √3 x √3R30˚ reconstruction of Cu(111), and distinct unoccupied electronic states are observed well above the sample Fermi level. iii Acknowledgments I would like to thank my advisor, Jay A. Gupta for his patience and support. I doubt I would have persevered in many other research groups. I would also like to thank Shawna M. Hollen and Nancy M. Santagata for their encouragement and perspectives. iv Vita May 2011 .......................................................B.S. summa cum laude in Physics, State University of New York at Buffalo May 2014 .......................................................M.S. Physics, The Ohio State University September 2011 – August 2012 .....................Fowler Fellow, Department of Physics, The Ohio State University September 2012 – August 2013 .....................Graduate Teaching Associate, Department of Physics, The Ohio State University September 2013 – December 2016 ................Graduate Research Associate, Department of Physics, The Ohio State University January 2017 – present...................................Graduate Teaching Associate, Department of Physics, The Ohio State University v Publications “Characterization of germanene islands grown on Cu(111).” G. A. Gambrel, S. J. Tjung, and J. A. Gupta. Submitted for peer review, (2017). “Crystalline hydrogenation of graphene by STM tip-induced field dissociation of H2.” S. J. Tjung, S. M. Hollen, G. A. Gambrel, N. M. Santagata, E. Johnston-Halperin, J. A. Gupta.: Submitted to arXiv, (2016). “Native defects in ultra-high vacuum grown graphene islands on Cu(111).” S. M. Hollen, S. J. Tjung, K. R. Mattioli, G. A. Gambrel, N. M. Santagata, E. Johnston-Halperin, and J. A. Gupta. J. Phys.: Matter 28 (2016) 034003 (8pp). “Modification of electronic surface states by graphene islands on Cu(111).” S. M. Hollen, G. A. Gambrel, S. J. Tjung, N. M. Santagata, E. Johnston-Halperin, and J. A. Gupta. Phys. Rev. B 195425 (2015). vi Fields of Study Major Field: Physics STM studies of two-dimensional materials, Advisor: Jay A. Gupta vii Table of Contents Abstract ............................................................................................................................... ii Acknowledgments.............................................................................................................. iv Vita ...................................................................................................................................... v Publications ........................................................................................................................ vi Fields of Study .................................................................................................................. vii Table of Contents ............................................................................................................. viii List of Figures .................................................................................................................... xi Chapter 1 : Introduction to Two-Dimensional Materials .................................................... 1 Chapter 2 : Experimental Methods ..................................................................................... 9 2.1 STM Chamber ...................................................................................................... 9 2.2 Auger Electron Spectroscopy ............................................................................. 12 2.2 STM Theory ....................................................................................................... 15 2.3 Topographic Imaging ......................................................................................... 19 2.4 Image Processing................................................................................................ 21 2.5 dI/dz: probe of local work function .................................................................... 24 viii 2.6 dI/dV Spectroscopy ............................................................................................ 25 2.7 dI/dV Mapping ................................................................................................... 27 2.8 Tip Preparation ................................................................................................... 28 2.9 Summary ............................................................................................................ 29 Chapter 3 : Scanning Tunneling Microscopy Studies of Graphene Grown on Cu(111) .. 30 3.1 Preparation of Graphene..................................................................................... 31 3.2 STM Topography ............................................................................................... 35 3.3 Defect Characterization in Graphene ................................................................. 41 3.4 Modification of Surface States in Graphene/Cu(111) ........................................ 49 Chapter 4 : Scanning Tunneling Microscopy Studies of Germanene and Cu2Ge Grown on Cu(111) ............................................................................................................................. 71 4.1 Preparation of Germanene .................................................................................. 74 4.2 Topography of Ge Structures ............................................................................. 77 4.3 Germanene Characterization .............................................................................. 80 4.4 Cu2Ge Characterization ...................................................................................... 86 4.5 Multi-layer Characterization .............................................................................. 94 Chapter 5 : Conclusions and Outlook ............................................................................. 102 5.1 Outlook ............................................................................................................. 104 Appendix A: Simultaneous Electrical Measurements and STM of Graphene Devices .. 106 ix Appendix B: STM studies of Silicene ............................................................................ 112 References ....................................................................................................................... 115 x List of Figures Figure 1.1: Graphene crystal structure ................................................................................ 2 Figure 1.2: Band structure of graphene ............................................................................... 3 Figure 1.3: Model of the germanene lattice.[36] ................................................................ 5 Figure 1.4: Bandgap of germanene influenced by electric field ......................................... 6 Figure 1.5: Germanene grown on various substrates .......................................................... 7 Figure 2.1: The CreaTec SPECS LT-STM System .......................................................... 11 Figure 2.2: STM Stage with Sample Holder ....................................................................
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