The Pennsylvania State University The Graduate School FUNCTIONALIZATION OF TWO-DIMENSIONAL TRANSITION METAL DICHALCOGENIDES A Dissertation in Chemistry by He Liu 2021 He Liu Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy May 2021 ii The dissertation of He Liu was reviewed and approved by the following: Mauricio Terrones Verne M. Willaman Professor of Physics Professor of Chemistry, and Materials Science & Engineering Dissertation Advisor and Chair of Committee Raymond E. Schaak DuPont Professor of Materials Chemistry and Professor of Chemical Engineering Kenneth Knappenberger Jr. Professor of Chemistry Joshua Robinson Professor of Materials Science and Engineering Philip Bevilacqua Distinguished Professor of Chemistry, Biochemistry and Molecular Biology Head of the Department of Chemistry iii ABSTRACT Chemical surface functionalization has been widely used to tune the electronic, optical, and catalytic properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). This thesis focuses on applying novel chemical functionalization methods to functionalize 2D TMD materials to tune its physical and chemical properties. The controlled tuning of these properties can then be utilized to improve the performance of 2D TMD-based devices and other applications such as sensing and catalysis. In chapter one, this thesis introduces current research fronts of 2D materials including graphene, TMDs and hexagonal boron nitride (hBN). The common synthesis and functionalization methods are also categorized and discussed in this chapter. Chapters two to four focus on individual research projects where I played a leading role and are summarized below. In chapter two, we demonstrate the spontaneous chemical functionalization via coordination of Au/Ag single atoms on monolayer MoS2. In this work, we developed an innovative route to functionalize monolayers of MoS2 with individual Au atoms via the formation of S-Au-Cl coordination complexes ([Au(MoS2)Clx]) on the TMD surface. The [Au(MoS2)Clx] coordination complexes were synthesized by taking advantage of the lone pair electrons of the S atoms present in the MoS2 lattice. In chapter three, we continue studying the coordination reaction between transition metals that include Fe, Co, Ni, Cu, Zn and MoS2. We studied the formation of these coordination complexes on MoS2 monolayers and correlate their properties with classical coordination complexes. Chapter four discusses surface enhanced Raman spectroscopy (SERS) using Au nanoparticles (Au NPs) functionalized MoS2. In this work, monolayer MoS2 is transferred on top of a monolayer of Au nanoparticles in order to achieve constructive interference of electrochemical enhancement and charge-transfer-based chemical enhancement. In the Appendix, we describe defect engineering to create vacancies and exposed edges in MoS2. We demonstrate that defect engineering via cryo-milling can be utilized to activate the inert sites in these materials to improve their Hydrogen evolution reaction (HER) catalytic performances. Chapter 2 is adapted from a published article which I am the first author. Chapters 3 and 4 are adapted from manuscripts in preparation which I am also the first author or co-first author. iv TABLE OF CONTENTS List of Figures .......................................................................................................................... vi List of Tables ........................................................................................................................... xvii Acknowledgements .................................................................................................................. xviii Chapter 1 Introduction to Two-Dimensional Transition Metal Dichalcogenides .................... 1 1.1 Two-dimensional materials ........................................................................................ 1 1.1.1 Graphene ......................................................................................................... 2 1.1.2 Transition metal dichalcogenides .................................................................... 3 1.1.3 Hexagonal Boron Nitride ................................................................................ 5 1.2 Preparation of 2D TMD flakes ................................................................................... 6 1.2.1 Preparation of TMD Flakes in Dispersion ...................................................... 6 1.2.2 Preparation of Solid-state TMD Flakes on substrates ..................................... 10 1.3 Surface Functionalization of TMDs ........................................................................... 13 1.3.1 Chemisorption of thiol-terminating ligands .................................................... 14 1.3.2 Coordination based functionalization .............................................................. 16 1.3.3 Physisorption of small molecules .................................................................... 17 1.3.4 Functionalization of 2D materials by Defect Engineering .............................. 18 1.3.5 Nanoparticle functionalization on 2D Materials ............................................. 19 1.4 Objectives and Thesis outline .................................................................................... 21 1.5 References .................................................................................................................. 21 Chapter 2 Spontaneous chemical functionalization via coordination of Au single atoms on monolayer MoS2.......................................................................................................... 34 2.1 Introduction ................................................................................................................ 34 2.2 Synthesis and characterization of [Au(MoS2)Clx] complexes.................................... 36 2.2.1 Materials Synthesis ......................................................................................... 36 2.2.2 Materials Characterization .............................................................................. 37 2.3 Results ........................................................................................................................ 39 2.3.1 Single Au and Ag atoms on MoS2 ................................................................... 39 2.3.2 Experimental and Theoretical Evidence of the [Au(MoS2)Clx] coordination complex ....................................................................................... 44 2.3.3 Fermi-level tuning through AuClx coordination .............................................. 54 2.3.4 Thermal boundary conductance measurements............................................... 59 2.4 Discussion .................................................................................................................. 61 1.5 Conclusion and Outlook ............................................................................................. 63 1.6 Acknowledgements .................................................................................................... 64 1.7 References .................................................................................................................. 65 Chapter 3 Coordination Chemistry Trends between Transition Metals and MoS2 .................. 69 3.1 Introduction ................................................................................................................ 69 3.2 Synthesis and characterization of Transition Metal-MoS2 complexes ....................... 70 3.2.1 Materials synthesis .......................................................................................... 71 v 3.2.2 Materials characterization ............................................................................... 72 3.3 Results and Discussion ............................................................................................... 73 3.3.2 Experimental characterization of Metal-MoS2 coordination complexes ......... 76 3.3.3 Photoluminescence tuning through TM coordination ..................................... 82 3.4 Conclusion and Outlook ............................................................................................. 86 3.5 Acknowledgements .................................................................................................... 87 3.6 References .................................................................................................................. 87 Chapter 4 Au Nanoparticle Functionalization of MoS2 for Surface Enhanced Raman Spectroscopy .................................................................................................................... 90 4.1 Introduction ................................................................................................................ 90 4.1.1 Introduction to 2D materials as SERS substrates ............................................ 91 4.1.2 Introduction to the Au NP and MoS2 heterostructure ..................................... 92 4.2 Synthesis and characterization of Au nanoparticle-MoS2 heterostructures ............... 93 4.2.1 Materials synthesis .......................................................................................... 93 4.2.2 Materials characterization ............................................................................... 94 4.3 Results and Discussion ............................................................................................... 95 4.3.1 Construction of