Covalent Functionalization of Germanane DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Shishi Jiang Graduate Program in Chemistry The Ohio State University 2016 Dissertation Committee: Dr. Joshua E. Goldberger, Advisor Dr. Anne Co Dr. Patrick Woodward Copyright by Shishi Jiang 2016 Abstract The group IV graphane analogues represent a new family of single atom-thick two-dimensioanl materials, in which the structure, stability and properties can be tailored for covalent ligand termination. These materials have attracted considerable interest as electronic, optoelectronic and topological materials. Here, we have established the synthesis, properties and surface functionalization of germananes. For the first time, we have reported gram-scale, millimeter-size hydrogen- and methyl-terminated germanane (GeH and GeCH3), via the topotactic deintercaltion of CaGe2 with HCl and CH3I, respectively. While the surface is oxidizing over time, the bulk of both GeH and GeCH3 are resilient to air for several weeks. Compared with GeH’s band gap at ~1.6 eV, the band gap of GeCH3 is increased by ~0.1 eV to 1.7 eV, producing a strong band edge photoluminescence with little layer dependence. Additionally, the thermal stability is o o improved from GeH at 75 C to GeCH3 at 250 C. Furthermore, germanane samples with various ratios between methyl and hydrogen termination (Ge(CH3)xH1-x) were created from a co-deintercaltion of CaGe2 in the mixture of HCl and CH3I solution. The structure and property of Ge(CH3)xH1-x changes pseudo-linearly with the x value, in a similar fashion as Vegard’s law. Finally, this alkylation method was successfully extended to various organohalides, producing a library of germanane terminated with different ii organic ligands, which allows us to understand the influence of ligand size and electronegativity on electronic structure. Ligands that have a greater size and are more electronegative will expand the Ge-Ge bond length, thereby lowering the band gap. Simply by changing the identity of the organic ligand, the band gap of germanane can be tuned by ~15%, highlighting the power of surface chemistry to manipulate the properties of single-atom thick materials. iii Acknowledgments I would like to express my deepest appreciation to my advisor, Dr. Joshua Goldberger, for his intelligence, motivation and patience as a mentor and for offering me the opportunity to work on an interesting and exciting project. His enthusiasm and persistence in research has inspired me and will continue beyond my graduate study. I also would like to thank Dr. Anne Co and Dr. Patrick Woodward for serving on my committee and for being encouraging and supportive. I hold many thanks to my collaborates, Dr. Leonard Brillson, Dr. David McComb, Dr. Ezekiel Johnston-Halperin, Dr. Ronald Kawakami, Dr. Wolfgang Windl and their graduate students for all the stimulating meetings we had, the feedback I received and for helping me to develop the background in physics and material science. My sincere thanks extend to everyone in Goldberger’s group, previously and currently, for the inspiring discussions, the late night hangout in the lab and all the fun we have had. I also want to thank all my dear friends scattering all over the world, for all the great conversations we had during the up and downs. Last but not least, my special thanks goes to my big family, especially my mother, and my boyfriend, for your endless support, love and caring. iv Vita 2004-2007 ..............................Wenling High School, China 2007-2011 ..............................B.S. with Honors, Physical Science; B.S., Chemistry University of Science and Technology of China, China Publications 1. E. Bianco, S. Butler, S. Jiang, O. Restrepo, W. Windl, J. E. Goldberger, “Stability and Exfoliation of Germanane: A Germanium Graphane Analogue.” ACS Nano 7, 4414-4421 (2013). 2. S. Jiang, S. Butler, E. Bianco, O. Restrepo, W. Windl, J. E. Goldberger, “Improving the Stability and Optoelectronic Properties of Germanane via One- Step Covalent Methyl-Termination.” Nat. Commun. 5, 3389 (2014). 3. S. Jiang, E. Bianco, J. E. Goldberger, “The Structure and Amorphization of Germanane.” J. Mater. Chem. C 2, 3185-3188 (2014). 4. Y. Ma, Y. Chen, Y. Ma, S. Jiang, J. E. Goldberger, T. Vogt, Y. Lae, “Inter- and Intra-Layer Compression of Germanane.” J. Phys. Chem. C 118, 28196-28201, (2014). v 5. M. Arguilla, S. Jiang, B. Chitara, J. E. Goldberger, “Synthesis and Stability of Two-Dimensional Ge/Sn Graphane Alloys.” Chem. Mater. 26, 6941-6945 (2014). 6. S. Jiang,‡ M. Arguilla,‡ N. Cultrara,‡ J. E. Goldberger, “Covalently-Controlled Properties by Design in Group IV Graphane Analogues.” Acc. Chem. Res. 48, 144-151 (2015). 7. J. R Young, B. Chitara, N. D. Cultrara, M. Q. Arguilla, S. Jiang, F. Fan, E. Johnston- Halperin, J. E. Goldberger, “Water Activated Doping and Transport in Multilayered Germanane Crystals.” J. Phys.: Condens. Matter, 3, 034001 (2016). Fields of Study Major Field: Chemistry vi Table of Contents Abstract ............................................................................................................................... ii Acknowledgments.............................................................................................................. iv Vita ...................................................................................................................................... v Table of Contents .............................................................................................................. vii List of Tables ..................................................................................................................... xi List of Figures ................................................................................................................... xii Chapter 1: Introduction ...................................................................................................... 1 1.1 Two-Dimensional Materials ...................................................................................... 2 1.1.1 Materials from van der Waals Solids ................................................................. 2 1.1.2 MXenes ............................................................................................................. 11 1.1.3 Germanene, Silicene and Stanene .................................................................... 12 1.2 Covalent-Functionalization of 2D Materials ........................................................... 15 1.3 Functionalization on Si/Ge (111) Surface ............................................................... 20 1.4 Zintl Phase and Deintercalation Chemistry ............................................................. 23 1.5 References for Chapter1 .......................................................................................... 26 Chapter 2: Hydrogen-Terminated Germanane (GeH) ...................................................... 41 vii 2.1 Introduction ............................................................................................................. 41 2.2 Experimental ........................................................................................................... 42 2.2.1 Materials ........................................................................................................... 42 2.2.2 Sample Synthesis and Preparation .................................................................... 43 2.2.3 Instrumentation ................................................................................................. 44 2.3 Results and Discussion ............................................................................................ 45 2.3.1 Structural Characterization ............................................................................... 45 2.3.2 Optical Properties and Band Structure ............................................................. 56 2.3.3 Air Stability ...................................................................................................... 61 2.3.4 Thermal Stability .............................................................................................. 62 2.4 Conclusion ............................................................................................................... 70 2.5 References for Chapter 2 ......................................................................................... 70 Chapter 3: Methyl-Terminated Germanane (GeCH3) ....................................................... 76 3.1 Introduction ............................................................................................................. 76 3.2 Experimental ........................................................................................................... 77 3.2.1 Materials ........................................................................................................... 77 3.2.2 Sample Synthesis and Preparation .................................................................... 77 3.2.3 Instrumentation ................................................................................................. 79 3.3 Results and Discussion ............................................................................................ 80 viii 3.3.1 Structural Characterization ............................................................................... 80 3.3.2 Optical Properties and Band Structure ............................................................