Open Thesis He Liu-V9.Pdf

The Pennsylvania State University

The Graduate School

FUNCTIONALIZATION OF TWO-DIMENSIONAL TRANSITION METAL

DICHALCOGENIDES

A Dissertation in

Chemistry

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

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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.

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

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 Au nanoparticle and MoS2 heterostructures ...... 95 4.3.2 SERS studies of Au NP film and MoS2 monolayers ...... 98 4.3.3 Ultra low detection of R6G and CuPc molecules via Au NP-MoS2 heterostructures ...... 100 4.3.4 Possible mechanism studies of AuNP-MoS2 heterostructures ...... 101 4.4 Conclusion and Outlook ...... 104 4.5 Acknowledgements ...... 106 4.6 References ...... 106

Chapter 5 Conclusion and Perspectives ...... 111

5.1 Summary of contributions ...... 111 5.2 Perspectives ...... 113 5.3 References ...... 115

Appendix: Defect Engineering to activate MoS2 for hydrogen evolution reaction ...... 116

LIST OF FIGURES

Figure 1-1. Graphene crystal structure and electronic structure. (A) Graphene crystal structure displaying hexagonal arrangement of carbon atoms with two unique positions per unit cell in green and blue.(5) (B) Electronic structure of pristine and doped graphene. In pristine graphene, the conduction and valence bands meeting at the Dirac point. In electron doped graphene, the Fermi level is displaced to higher energy to the Dirac point. In hole doped graphene, Fermi level is displaced to higher energy to the Dirac point. Figure adapted from Reference (5) with permission from The Royal Society of Chemistry. 3

Figure 1-2. Metal coordination, top view and stacking sequences of TMD structural unit cells for 1T, 2H and 3R phases. Figure adapted from reference (17) with permission from The Royal Society of Chemistry. 4

Figure 1-3. (A) Electronic structure of Bilayer and monolayer MoS2. The MoS2 transits from an indirect bandgap semiconductor into a direct bandgap semiconductor when thinned to the monolayer. The band gap is called "direct" if the momentum of electrons and holes is the same in both the conduction band and the valence band. Figure adapted with permission from reference (22). Copyright (2010) American Chemical Society. (B) The emerging photoluminescence of

MoS2 when thinned down to monolayer. Figure adapted with permission from reference (2). Copyright (2010) by the American Physical Society. 5

Figure 1-4. (A) Crystal structure of hBN showing the B and N atoms bonded via covalent bonds within each BN layer and van der Waals forces hold the layers together to form the bulk material. Figure adapted from reference (24), Copyright (2020), with permission from Elsevier.(B) Oriental control of epitaxial MoS2 on hBN assisted by defects. Figure adapted with permission from reference (26). Copyright (2019) by the American Physical Society. (C) Orientation control of

Epitaxial WSe2 on hBN. Figure adapted with permission from reference (27). Copyright (2019) American Chemical Society. 6

Figure 1-5. Overview of common liquid exfoliation techniques and mechanisms. (A) Crystal structure and photograph of a bulk MoS2 powder. (B) Schematic description showing two main

vii liquid exfoliation techniques: direct ultrasonication and ion intercalation. (C) Crystal structure and photograph of exfoliated MoS2 dispersion prepared by ultrasonication in N-Methyl-2- pyrrolidone. Figure adapted from reference (38) with permission from The Royal Society of Chemistry. 7

Figure 1-6. TEM gallery of solution-synthesized colloidal TMD nanosheets, including 1T-WS2,

2H-WS2, MoSe2, WSe2, 1T′-MoTe2 and WTe2. Figure adapted from reference (42) by permission from Springer Nature, Copyright (2020). 9

Figure 1-7. Comparison between Mechanical exfoliation and Vapor deposition in the synthesis of high crystalline monolayer MoS2 (A) Optical image of a MoS2 flake deposited on SiO2/Si via mechanical exfoliation.(51) (B) Optical image of a single-layer MoS2 crystal grown on SiO2/Si via chemical vapor deposition.(51) MoS2 single crystals prepared via CVD can normally reach larger sizes compared to mechanically exfoliated MoS2 flakes. Figure adapted from reference (51) with permission from The IOP Publishing. Copyright (2015). 10

Figure 1-8. Optical library of 32 different 2D binary compounds synthesized via CVD, containing: Mo (MoS2, MoSe2, MoTe2), W (WS2, WSe2, WTe2), Re (ReS2, ReSe2), Ti (TiS2,

TiSe2, TiTe2), Zr (ZrS2, ZrSe2, ZrTe2), Hf (HfS2, HfSe2, HfTe2), V (VS2, VSe2, VTe2), Nb (NbS2,

NbSe2, NbTe2), Ta (TaS2, TaSe2, TaTe2), Pt (PtS2, PtSe2, PtTe2), Pd (PdS2, PdSe2) or Fe (FeSe). Figure adapted from reference (58) by permission from Springer Nature, Copyright (2018). 12

Figure 1-9. (A) Schematic of the ligand conjugation process between ce-MoS2 sheets and PEG ligands in solution. (B) ζ-potential before and after ligand conjugation. (C) FT-IR spectra before and after ligand conjugation. Figure adapted with permission from reference (72). Copyright (2013) American Chemical Society. 16

Figure 1-10. (A) schematic of Ti ion with empty orbitals approaching the InSe surface, the lone pair electrons from Se enter the empty orbitals of the metallic ion and form coordination bonds (B) HAADF images and Z-contrast mapping of pristine InSe. (C) HAADF images and Z-contrast mapping of Ti functionalized InSe. Figure adapted from reference (79) by permission from Springer Nature, Copyright (2016). 17

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Figure 1-11. (A) PL spectra of monolayer MoS2 before and after being treated with p-type molecules (TCNQ and F4TCNQ). (81) (B) PL spectra of monolayer MoS2 before and after being treated with an n-type dopant (NADH). (81) (C) Schematic of relative potentials (vs SHE) of monolayer MoS2 and p- and n-type dopants. Figure adapted with permission from reference (81). Copyright (2013) American Chemical Society. 18

Figure 1-12. Classification of the structural defects in 2D materials based on dimensionality: (A) 0D defects such as vacancies, adatoms, substitutional dopants, squares, and octagons; (B) 1D defects, such as grain boundaries, edges, and in plane heterostructures; (c) 2D defects such as stacking, folding, wrinkling, scrolling, rippling, and Van der Waals heterostructures. Figure adapted from reference (110) with permission from The IOP Publishing. Copyright (2016). 19

Figure 1-13. (A) (SEM) image showing patterned silver nanoparticle arrays directly stacked on top of a MoS2 flake.(120) (B) Device schematic showing the geometrical factors of the bowtie array: gap separation (g), thickness of the metal deposition (h), side length of a triangle (s), and unit cell dimension or pitch (p = (px, py)). (120) (C) PL enhancements of MoS2 observed for the four patterns. (i) s = 100 nm, p = (400 nm, 500 nm); (ii) s = 100 nm, p = (400 nm, 300 nm); (iii) s = 100 nm, p = (300 nm, 200 nm); (iv) s = 170 nm, p = (500 nm, 800 nm). Figure adapted with permission from reference (120). Copyright (2015) American Chemical Society. 20

Figure 2-1. Synthesis of Au single atoms on monolayer MoS2 flakes. (A) High resolution STEM images of [Au(MoS2)Clx] featuring the single Au atoms on S sites. Insets: The structure model of

Au-MoS2 used for STEM image simulation (top right) and the higher magnification STEM image of an Au aggregate found on MoS2 (bottom left).(B) and (D) HAADF images and Z-contrast line scan (along the vertical grey line in (B)) of one Au single atom on MoS2, revealing the position of the Au atom to be directly on top of the S atom. (C) and (E) TEM simulation of one Au atom directly on top of one S atom in MoS2 lattice and Z-contrast line scan of the simulation image. The simulated TEM image and line scan match exactly with experimental data, confirming the atomic structure of Au single atoms directly bonded on S atoms on the MoS2 surface. 40

Figure 2-2. High resolution STEM images of Ag-MoS2 complex. The inset of the image highlights the single Ag atoms on S sites. 41

Figure 2-3. STEM simulations of [Au(MoS2)Clx] structures. (A)STEM Simulations of a Au atom on MoS2 with no chlorine, one chlorine atom and three chlorine atoms. (B) Z contrast line scan of the three structures and comparison with an experimental line scan. 42

Figure 2-4 (A) Time lapse images of Au single atoms on MoS2. The Au single atoms can move on the MoS2 plane under the E-beam irradiation during STEM characterization. The full video can be viewed in the SI. The grey curves show the trail of one Au atom moving on MoS2. (B) The statistical analysis of the position of one Au atom and displacement distance between each movement. 43

1+ unreacted HAuCl4 precursor, whereas the Au from the reduction reaction with MoS2. The purple curve is the sum of the blue and red curves. (B) S 2p orbit of pristine and Au functionalized MoS2. The green curve is the original spectrum after C1s(248.8eV) calibration. The blue and red curves are fittings which show the S 2p1/2 and 2p3/2 peaks respectfully. The S 2p orbitals shift to higher energy due to the loss of outer electrons to Au-S bonds. 45

Figure 2-6. Peak area analysis of the XPS spectrum of the [Au(MoS2)Clx] complex using CasaXPS. Peak area is calculated in the green box with the red curve as base line. (A) Au 4f spectra peak area analysis (peak area=617.91). (B) Cl 2p spectra peak area analysis (peak area=250.81). 45

Figure 2-7. Spectroscopic studies of the Ag-MoS2 complex. (A) X-ray photoelectron

+ spectroscopy (XPS) spectra of Ag 3d orbit on Ag-MoS2. The Ag reference peak of 368.1 eV was extracted from Ag2S spectra from reference 23. (B) S 2p orbit of pristine and Ag functionalized

MoS2. The Ag-MoS2 XPS measurements used a different batch of MoS2 samples from the Au-

MoS2 XPS measurements. They are also measured in different batches which leads to different sample charging effects causing shifts in the pristine S 2p binding energy. 46

Figure 2-8. (A) Photoluminescence spectrum of pristine and functionalized MoS2 monolayers. The A exciton is deconvoluted into the trion (X-) (blue curve) and exciton (X) (red curve) peaks through Lorentzian functions. After doping, the trion intensity decreased while the exciton intensity increased, which corresponds well with the p-type doping effect of the Au1+. (B) Exciton to trion intensity ratio of pristine and functionalized MoS2 with different precursor Au concentrations. 47

Figure 2-9. Band structure studies of the [Au(MoS2)Clx] complex. (A) DFT optimized structure for the [Au(MoS2)Cl3] complex. Chlorine atoms were considered to complete the Au coordination sphere. (B) Charge transfer map of the [Au(MoS2)Cl3] complex showing a higher hole concentration along most of the MoS2 sheet (blue surface), and a pronounced electron concentration on the Mo and S atoms near the Au atom (red surface), commensurate with a p- type doping effect of the Au1+. 48

Figure 2-10. DFT optimized structure showing the bond distance and angles for the Au-MoS2 complex, considering the chlorine ions on the Au coordination sphere. 49

Figure 2-11. DFT optimized structure showing the bond distance and angles for the Ag-MoS2, bond distance and angles are shown on Table 2-1. 49

Figure 2-12. Charge transfer map of [Au(MoS2)Cl] and [Au(MoS2)] complexes showing the hole concentration as blue surfaces and the electron concentration as red surfaces. 50

Figure 2-13 (A) Electronic band structure of pristine MoS2 and of the [Au(MoS2)Cl3] complex, both calculated without considering the spin-orbit coupling. Electronic transitions are indicated by vertical arrows. In the [Au(MoS2)Cl3] complex, besides the pristine MoS2 excitons A and B, charge transfer transitions are expected between the MoS2 state (blue line) and the Au localized states (red line). (B) Room temperature absorbance spectra of the [Au(MoS2)Clx] complex (dotted line) and of pristine MoS2 (black). A slight blue shift of the A and B excitons and new features associated with charge transfer transitions at energies lower than 1.7eV are observed, as expected from the electronic band structure of the [Au(MoS2)Cl3] complex shown in (A). 51

Figure 2-14. Micro-absorption measurements of the [Au(MoS2)Clx] Complex. (A) Optical images

5 different MoS2 flakes were the absorption spectra were measured. The yellow circle shows the size and position of the laser spot on each flake. (B) Room temperature absorption spectra of pristine MoS2. (C) Room temperature absorption spectra of [Au(MoS2)Clx] complex. The spectra were measured on the same flakes before and after the functionalization process. 52

Figure 2-15. Tauc plot of absorption spectroscopy of pristine and AuClx functionalized MoS2. 52

Figure 2-16. Electronic band structures of pristine MoS2, [Au(MoS2)], [Au(MoS2)Cl] , and

[Au(MoS2)Cl3] complexes, calculated without considering spin-orbit coupling. 54

Figure 2-17. (A) Schematic of a back-gated field effect transistor involving coordination of gold on the MoS2 channel. (B) Optical image of the fabricated devices showing the lateral structure of 8 devices. 55

Figure 2-18. (A) Back-gated transfer characteristics of the MoS2 FET. (B) Output characteristics of the MoS2 FET. 55

Figure 2-19. Fermi level tuning using gold doping. (A-D) Drain current (IDS) vs back-gate voltage

-10 -1 -9 -1 - (VBG) characteristics with a drain bias of 1 V. (A) Pristine (B) 10 mol L (C) 10 mol L (D) 10

6 -1 mol L concentrations of HAuCl4 in ethanol. 56

Figure 2-20. Analytical Studies of AuClx complexes on MoS2. (A) Threshold change for different

Au concentrations with error bar included. (B) Number of AuClx complexes calculated through threshold shift for varies Au concentrations. 58

Figure 2-21. Transfer characteristics i.e. drain current (퐼) versus back gate voltage (푉) at drain bias, VDS = 1 V for MoS2 FETs with AuClx coordination doping (A) before and (B) after a year. 59

xii picosecond acoustics response at earlier time delays. (C) Results for the thermal boundary conductance at Al/Au-MoS2/SiO2 interfaces. 60

Figure 2-23. Thermodynamic cycle for oxidation-reduction and complexation reactions of Au complexes considered to understand the ligand effect on the Au redox potential. 63

Figure 2-24. Characterization of monolayer MoS2. (A) Optical image (B) fluorescence image (C) Raman spectra. 63

Figure 3-1. DFT theoretical analyses of M-MoS2 complexes. Optimized structure pristine MoS2 showing the available coordination sites (a) and for Ni-MoS2 (b). Adsorption energy (c) and metal-sulfur bond length (d) as function of the transition metal electronic configuration for M-

MoS2 complexes, where M is a 3d transition metals. 74

Figure 3-2. Electronic Band Dispersion for (A) Fe functionalized MoS2, (B) Co functionalized

MoS2, (C) Ni functionalized MoS2 and (D) pristine MoS2 .The blue curves are MoS2 states and the red curve localized states of the metal. The TM-MoS2 remains a direct bandgap semiconductor, yet the states near the Fermi level are drastically changed after functionalization. 76

Figure 3-3. XPS spectra of TM 3d orbit after functionalization. (A) Cr 2p orbit of Cr-MoS2. The

3+ black curve is the original spectra after C1s calibration. The blue curves are fitted to Cr 2p1/2 and

2p3/2 peaks and they match perfectly with the original spectra. (B) Mn 2p orbit of Mn-MoS2. The

2+ blue curves show the fitted Mn peaks. In addition to the fitted Mn 2p1/2 and 2p3/2 peaks, a Mn satellite peak is also visible in the green curve. (C) and (D)Co and Ni 2p orbit of Co-MoS2 and

2+ 2+ Ni-MoS2. Similar satellite peaks could be fitted in addition to the Co and Ni peaks. (E) Cu 2p

1+ 2+ orbit of Cu-MOS2. Here the spectra is fitted into 3 groups. The Cu 2p peaks in red, the Cu 2p peaks in blue and the Cu2+ satellite peaks in green. The emergence of the Cu1+ peaks indicates a

2+ reduction reaction between the Cu precursor and MoS2. Similar to that of Au and MoS2. (F)Zn

2+ 2p orbit of Zn-MoS2 showing a perfect fit of Zn 2p1/2, 2p3/2 and the original spectrum. 78

Figure 3-4. X-ray photoelectron spectroscopy studies of M-MoS2 complexes. (a)XPS spectra of

Mo 3d orbit on pristine and doped MoS2. The curves are the original spectrum after C1s

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(248.8 eV) calibration. (b) S 2p orbit of pristine and M functionalized MoS2, showing the S 2p1/2 and 2p3/2 peaks. (d) Deconvolution of Mo 3d spectra for pristine, Cr-MoS2, Ni-MoS2 and Cu-

4+ 5+ MoS2.The blue curves are the Mo 3d5/2 and 3d3/2, purple curves are the. Mo 3d5/2 and 3d3/2

6+ peaks and the red curves are the Mo 3d5/2 and 3d3/2 peaks. The green curves are the S 2s peak.

(d) Binding energy shift for S 2p3/2 (blue curve) and Mo 3d5/2 peak as function of M 3d electronic distribution for M-MoS2 samples. 80

Figure 3-5: Energy Level Diagram for 3d transition metals and MoS2 valance band and conduction band. 81

Fig. 3-6 STEM images of (A) PtCl4 functionalized MoS2 and (B) NiCl2 factionalized MoS2. 82

Figure 3-7 PL spectra analysis of TM functionalized MoS2. (A) Photoluminescence spectrum of

- pristine and functionalized MoS2 monolayers. The A exciton is deconvoluted into the trion (X ) (blue curve) and exciton (X) (red curve) peaks through Lorentzian functions. After functionalization, the trion intensity increased while the exciton intensity decreased, which corresponds well with the n-type doping effect of the Ni functionalization. (B)

Photoluminescence spectrum of pristine and Cu functionalized MoS2 monolayers. After functionalization, the trion intensity decreased while the exciton intensity increased, which corresponds well with the p-type doping effect of the Cu functionalization. (B-D and F-H)

Exciton to trion intensity ratio of pristine and functionalized MoS2 for different TM precursor: Cr (B), Mn (C), Co (D), Ni (F), Cu (G) and Zn (H) at different concentrations. 84

Figure 3-8. Normalized exciton to trion intensity changes showing the percent change in exciton to trion ratio for various TM functionalized MoS2 samples at different TM concentrations The TMs here can be separated into 3 groups. One is Cu, which is showing an increase to the exciton to trion ratio, indicating p-type doping on the MoS2. The second group is Zn, Mn and Cr where we see a small decrease in the ratio, indicating weak n-type doping of MoS2. The last group is Co and Ni where we see a large decrease in the ratio that corresponds to strong n-type doping of

MoS2. 85

Figure 3-9. Figure 3-9. Preliminary REELS measurements of TM-MoS2 complexes. 87

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Figure 4-1. Theoretical summary of four types of SERS enhancement mechanism. Image adapted from ref (3). 91

The MoS2 is first grown on SiO2 via CVD and is transferred to the Au nanoparticles functionalized substrate via PMMA transfer. 96

Figure 4-3 Raman and PL characterization of monolayer MoS2 grown via PTAS assisted CVD.

-1 (A) Raman spectrum of monolayer MoS2 excited via 488 nm laser. The E’ mode at 382 cm and

-1 the A1’ mode at 401 cm confirm its monolayer nature. (B) Photoluminescence spectrum of monolayer MoS2. 97

Figure 4-4. (A) SEM image of Au nanoparticle film deposited on top of amino silane treated

SiO2/Si substrates. (B) UV-VIS absorption spectrum of as synthesized Au nanoparticles in solution. 97

Figure 4-5. (A) Schematic of the Au nanoparticle and MoS2 heterostructure. (B) SEM image of the Au nanoparticle and MoS2 heterostructure. 98

Figure 4-6. Raman performance comparison between Au NP and MoS2 heterostructure with pristine MoS2 and pristine Au NPs using the R6G dye as probe molecule. (A)The Raman spectra collected from the three substrates with a R6G concentration of 1x10-5M. The R6G modes at 613cm-1, 1364cm-1, 1577 cm-1 and 1651cm-1 could be clearly identified on all three substrates, yet the peaks are much sharper for the Au NP and MoS2 heterostructure. (B) Raman spectra collected with a R6G concentration of 1x10-8M. The Raman modes disappear for the pristine MoS2 flake and the Au NPs. The R6G modes remain clearly visible for the heterostructure. 99

Figure 4-7. Detection limit studies of the Au-MoS2 heterostructure as SERS substrates. (A)

-8 Raman spectra of R6G deposited on Au-MoS2 substrates at low concentrations of 1x10 M to 1x10-10M. The signature R6G bands would be identified in at all three concentrations, yet we see a clear decrease in intensity as the concentration of R6G decreases. The lowest detectable

xv concentration for R6G is 1x10-10M. (B) Raman spectra of CuPc at 1x10-8M and 1x10-9M on the

-8 Au NP-MoS2 substrate. The signature CuPc peaks would still be clearly identified at 1x10 M but becomes too low to clearly recognize at 1x10-9M. 100

Figure 4-8. (A) Raman spectra comparison of pristine MoS2, Au-MoS2 heterostructure and Au-

MoS2+dyes. The peak position of MoS2 modes E’ and A1’ did not change when the MoS2 was transferred on top of the Au nanoparticle film as well as when dye molecules were deposited on top of the MoS2. The relative peak intensity(A1’/E’) did not change. The peak intensity relative to Si peak changes slightly due to not being able to measure on same flake. (B) PL spectrum of the same MoS2 flake before and after being transferred to the Au nanoparticle film. The shift in the exciton emission is likely due to the strain release in the transfer process.(35) There is a large enhancement in the PL intensity due to the EM enhancement due to the plasmonic effect of the Au nanoparticles. 102

Figure 4-9. Energy diagram of dye molecules and MoS2 illustrating the possible charge transfer process between the Fermi level of MoS2 to the HOMO level of R6G and CuPc. The HOMO and

LUMO gap of R6G is shown in red and CuPc in blue. The band structure of monolayer 2H MoS2 is shown in green. The values extracted from refs (11, 29, 42). 103

Figure 4-10. The Absorbance spectra of MoS2 and Au-MoS2 when deposited with dye molecules.

(A) The absorbance spectra of R6G deposited on MoS2 and Au-MoS2 substrates. The red curve is the R6G molecule on SiO2. The light green and olive colored curves are pristine MoS2 and

MoS2+R6G. The light blue and navy curves are Au-MoS2 and Au-MoS2+R6G. The dash line is the max absorbance of the R6G. A clear shoulder could be seen on the Au-MoS2+R6G sample.

(B) The absorbance spectra of CuPc on MoS2 and Au-MoS2 substrates. 104

Figure A-1. (A) Powder XRD patterns of cryomilled MoS2. The peaks are normalized to the 002 peak of MoS2. The relative intensity of the 002-peak compared to other peaks decreases with cryomilling time. (B) Close up analysis of the 002 peak. The full width half maximum (FWHM) increases 118

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Figure A-2. (A) N2 gas absorption isotherms of pristine, 30min, 60min and 90min cryomilled

MoS2. (B) Raman spectra of pristine, 45min and 90min cryomilled MoS2 excited under the 633nm laser. The 2LA(M) mode at 466cm-1 significantly increases with increased cryomilling time, indicating the increase in defect density during the cryomilling process with cryomilling time. 118

Figure A-3. STEM images of 90min cryomilled MoS2. (A)High resolution STEM image showing

Mo vacancies and exposed edges forming at the edge of the MoS2 particle. (B)Low magnification

STEM image showing reduced particle size of the MoS2 after cryomilling and displaying large amounts of exposed edges. 119

Figure A-4. HER performance of cryomilled MoS2. (A) The HER polarization curves. (B) The Tafel slopes. 120

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LIST OF TABLES

Table 1-1. Summary of surface functionalization with regards to functionalization types, species, materials, and applications. 14

Table 2-1 Calculation details (bond length, bond energy etc.) on [Au(MoS2)Cl3] and Ag-MoS2 complexes. 50

Table 2-2. Threshold voltage, SS, mobility and ON/OFF ratio statistics of the ten representative devices for various concentrations of HAuCl4. 56

Table 3-1. Calculated adsorption site, adsorption energy and metal MoS2 bond length for all 3d metals. 74

Table 4-1 Comparison of the performance of different MoS2 samples as SERS substrates for molecular sensing based on this work and other results reported in the literature. Sensing materials, together with its calculated HOMO-LUMO gap, type of MoS2 used, laser excitation energy used and detection level for each molecule are recorded. 101

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ACKNOWLEDGEMENTS

First, I wish to thank my advisor, Dr. Mauricio Terrones, for his help and support during my PhD research career, as well as his kind mentoring and broad horizon that has helped me progress in my research. His guidance, patience and kindness made this dissertation possible. His enthusiasm and optimism have led me to appreciate and enjoy conducting scientific research. Besides Dr. Terrones, I would also like to thank my committee members, Dr. Ray Schaak, Dr. Ken Knappenberger Jr. and Dr. Joshua Robinson for their support and knowledgeable suggestions. The work in this dissertation was also helped by my collaborators, Dr. Daniel Grasseschi, Dr. Saptarshi Das, Dr. Patrick Hopkins, Dr. Rodolfo Cruz Silva and Akhil Dodda, David Olson and many others. I would like to express my deepest gratitude to my fellow lab mates in the Terrones group. Special thanks to Dr. Kazunori Fujisawa, Dr. Ana Laura Elias Arriaga, Dr. Nestor Perea-Lopez and Dr. Yu Lei for their kind help and support for my time in the group. I would not have been able to complete this thesis without your support. Lastly, to my parents and my friends, I am forever in your debt for your endless and selfless support to me throughout my PhD study in Penn State. I would like to express specially thanks to my wife Duo Pan for her unconditional love, support, sacrifice and understanding. My life in State College would be much less enjoyable without your long-lasting encouragements.

Acknowledgement of Federal Funding This work in this thesis was supported by the Air Force Office of Scientific Research (AFOSR) through grant No. FA9550-18-1-0072 and the NSF:I/UCRC ATOMIC program for support (award #1540018). The findings and conclusions of this work do not necessarily reflect the view of the funding agencies.

Chapter 1 Introduction to Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional materials (2D materials) have been extensively studied since the successful isolation of graphene from graphite in 2004.(1) Over the past decade, the family of 2D materials has been expanded to many other classes of materials beyond graphene, such as layered transition metal dichalcogenides (TMDs), insulating hexagonal boron nitride (h-BN), phosphorene and etc. When thinned down to mono- or few-layers, these layered materials exhibit a series of novel physical properties that are absent in its bulk counterpart due to the confinement of phonons and charge carriers.(2) This offers great scientific and industrial opportunities for exploring novel physical, chemical, electronic, optical and catalytic properties which leads to promising applications in various fields including electronics, energy storage, biomedicine, electrochemistry and catalysis.(3) The further development of these applications brings up the need for better understanding and control over the intrinsic properties of the 2D materials via functionalization. In this chapter, we will introduce three classic 2D materials including graphene, TMDs and hexagonal Boron Nitride (hBN). This is followed by a detailed introduction to the synthesis and surface functionalization techniques applied to TMDs for better application purposes. In the end there is an outline of the entire thesis.

1.1 Two-dimensional materials

Two-dimensional (2D) materials are atomically thin layers that exhibit covalent bonding confined within a few- or single-atom thin plane and contains no dangling bonds in the third direction. The 2D flakes are held together by Van der Waals forces to form the bulk crystals. Foremost among these is graphene and recent reports have expanded the library of 2D materials to include numerous insulators, semiconductors, metals, and semimetals. In this section we will go over the basic information about graphene, TMDs and hBN.

1.1.1 Graphene

The chemical element carbon (C) can form various allotropes based on the hybridization of its orbitals, such as 0-dimensional fullerenes, 1-dimensional carbon nanotubes, 2-dimensional graphene and 3-dimensional graphite or diamond.(4) Graphene is an atomically thin single layered material composed of sp2 hybridized carbon atoms arranged in a honeycomb lattice (see Fig 1-1 A). It has been widely investigated both experimentally and theoretically since Novoselov and Geim successfully exfoliated graphene from graphite in 2004. (1) In the absence of doping, graphene is a zero-gap semiconductor because its conduction and valance bands touch at the Dirac point. (5) Due to its gapless nature, graphene can also be viewed as a semimetal with zero density of state at the Fermi level. This can be shifted into the valence or conduction band with a corresponding negative or positive backgate voltage as shown in Fig 1-1 B. (5) This means both electrons and holes can be utilized as charge carriers and that the carrier concentration and mobility of graphene can reach up to 1013 cm-2 and 20 000 cm2 V-1 s-1, which is about 2 orders of magnitudes greater than that of silicon.(6) These extraordinary electronic properties make graphene a promising candidate for various applications in electronic devices, sensors, energy storage and catalysts.(7) However, lacking a sizable band gap (1-2eV) limits the performance of graphene in certain electronic and optoelectronic devices such as showing relatively low On/Off ratio in field effect transistors (FETs). This can be overcome by functionalization through the means of Nitrogen or Boron doping(8–11), chemical functionalization(12) and strain engineering(13).

Figure 1-1. Graphene crystal structure and electronic structure. (A) Graphene crystal structure displaying hexagonal arrangement of carbon atoms with two unique positions per unit cell in green and blue.(5) (B) Electronic structure of pristine and doped graphene. In pristine graphene, the conduction and valence bands meeting at the Dirac point. In electron doped graphene, the Fermi level is displaced to higher energy than the Dirac point. In hole doped graphene, Fermi level is displaced to lower energy than the Dirac point. Figure adapted from Reference (5) with permission from The Royal Society of Chemistry.

1.1.2 Transition metal dichalcogenides

Layered transition metal dichalcogenides (TMDs) are an emerging group of two- dimensional materials which have recently attracted considerable research attention due to the emergence of a series of novel physical properties when thinned down to few- or single-layer. Unlike graphene which is a single layer of carbon atoms, monolayer TMD materials are three atoms thick with a formula of MX2 (M = Mo, W and X = S, Se) where a layer of transition metal atoms (M) is sandwiched by two layers of chalcogen atoms (X) in monolayer TMDs.(14) TMD sheets held together by weak van der Waals forces form the bulk material. Three main polytypes of the bulk crystals: 2H, 1T and 3R (see Fig 1-2) can be formed based on the different coordination of chalcogen atoms and the stacking order of the layers.(15, 16) The 2H phase is when the transition atom is coordinated in a trigonal prismatic lattice and the layers follow the ABAB… stacking order. The 1T phase has a tetragonal symmetry where the transition atom is coordinated in an octahedral lattice. Meanwhile the 3R phase has the trigonal prismatic coordination but follow the ABCABC… stacking pattern, displaying rhombohedral symmetry.

In general, the 2H polytype is more stable and behaves as an indirect band-gap semiconductor. (14) Remarkably, in contrast to the gapless graphene, semiconducting TMDs like

MoS2 and WS2 undergo a transition from an indirect band gap material in their bulk or multilayer form, to a direct band gap material in their monolayer form.(2) This direct band gap feature of semiconducting TMD monolayers (see Fig 1-3) make them more suitable for functional electronics and optoelectronics where a sizable band gap is needed.(16) In addition, semiconducting TMDs such as MoS2 can also be used as a catalyst for the hydrogen evolution reaction (HER) due to its highly reactive edges which exhibit similar ΔGH as Pt in the Volcano plot.(18–21) Because of the especially high specific surface area and the reduced dielectric screening in the 2D limit, a series of structural, electrical, chemical and optical properties of TMDs are extremely sensitive to defects and surface functionalization.

Figure 1-3. (A) Electronic structure of bilayer and monolayer MoS2. The MoS2 transits from an indirect bandgap semiconductor into a direct bandgap semiconductor when thinned to the monolayer. The band gap is called "direct" if the momentum of electrons and holes is the same in both the conduction band and the valence band. Figure adapted with permission from reference (22). Copyright (2010) American Chemical Society. (B) The emerging photoluminescence of MoS2 when thinned down to monolayer. Figure adapted with permission from reference (2). Copyright (2010) by the American Physical Society.

1.1.3 Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) is another structural analogue of layered graphene. It is an electrically insulating 2D material with a large bandgap of 5.5eV.(23) Within each layer, Boron and Nitrogen atoms are bound by covalent σ bonds in a honeycomb arrangement (see Figure 1- 4A), whereas the layers are held together by weak van der Waals forces. (23, 24) The absence of delocalized  bonds results in the large bandgap and little absorbance of visible light, making monolayer and few layer hBN colorless and bulk hBN a white powder.(23) The electric insulating, chemically inert and optically transparent nature of hBN has led to its wide applications in dielectric layers, anti-corrosion coating and catalysis supports(25). Recently, hBN has also attracted much attention in its ability to form van der Waals heterostructures and promote epitaxial growth. Zhang et al. demonstrated that MoS2(26) and WSe2(27) monolayer domains

6 grown on hBN sheets (see Fig 1-4B and C) display preferred orientation growth and that the process is controllable via defect engineering of the hBN sheets.

1.2 Preparation of 2D TMD flakes

The preparation route for TMD materials greatly affects their sizes and existing states (dispersions or solid-state), which further dictates their properties and fields of applications. Based on the existing state of as-prepared TMD materials, the preparation methodologies can be categorized into two types: TMD dispersion preparation and solid-state TMD flakes preparation. In this section, we will introduce primary preparation methods of TMDs as well as some recent advancements in this field.

1.2.1 Preparation of TMD Flakes in Dispersion

1.2.1.1 Liquid exfoliation via sonication The preparation of TMD dispersions usually relies on the top-down exfoliation process, in which bulk TMD crystals are exfoliated into few-layer or monolayer materials in the liquid phase by applying external forces.(28) This method has the capability of producing a large quantity of thin layered TMDs. In addition, the exfoliated TMD dispersions can be further

7 modified with different functional groups and materials to engineer their functionalities.(29, 30) So far, this method has shown advantages for applications such as catalysis and sensing. The exfoliation of TMDs is enabled by weakening the interlayer van der Waals interaction through molecule intercalation or applying shear force along the in-plane direction. To date, many top- down strategies have been developed to prepare TMDs dispersions, such as lithium intercalation(31), ultra-sonication(32, 33), chemical exfoliation(34) and biomolecule-assisted exfoliation.(35–37) Figure 1-5 illustrates the ultra-sonication and ion intercalation processes.

Coleman et al. exfoliated TMDs such as MoS2, WS2 by sonication in organic solvents like N- methyl-2-pyrrrolidone and dimethylformamide.(32) During the sonication process, the solvent or surfactant was able to break the interlayer interactions and stabilize the nanosheets. Zhou et al. realized the exfoliation of TMDs in ethanol and water solutions by using a mixed solvent strategy.(33) This method dramatically lowered the boiling point of the solvent which gave rise to lower toxicity and better biocompatibility. It is worth noting that pure sonication generally takes 4-12 hours of sonication time. The average size of the exfoliated nanosheets vary from 100 nm to a few micrometers and are mostly few layers. (33)

Figure 1-5. Overview of common liquid exfoliation techniques and mechanisms. (A) Crystal structure and photograph of the bulk MoS2 powder. (B) Schematic description showing two main liquid exfoliation techniques: ultrasonication and ion intercalation. (C) Crystal structure and photograph of exfoliated MoS2 dispersion prepared by ultrasonication in N-Methyl-2-pyrrolidone. Figure adapted from reference (38) with permission from The Royal Society of Chemistry.

1.2.1.2 Liquid exfoliation by Ion intercalation Another effective liquid exfoliation method is alkali-metal ion intercalation. When the layered materials are immersed in n-butyllithium or other alkali metal solutions, the Li ions can intercalate into the layered materials which lowers the stability of the structure, making their

8 exfoliation easier through mild sonication. Zeng et al. first demonstrated the high-yield exfoliation of monolayer MoS2, WS2, TiS2, TaS2, ZrS2 and graphite in large amounts.(31) It was later proven that Na+ and K+ can also be used to exfoliate layered materials using this method.(39) The key feature of the Li intercalation method is that the produced TMDs are in the metallic 1T phase which is caused by the charge transfer from Li ions to TMDs.(14) In addition, biomolecules such as single-stranded DNA (ssDNA) and certain proteins have also been reported to produce TMDs nanosheets through a novel biomolecule-assisted exfoliation process. (35–37, 40) These bio polymers possess hydrophilic-hydrophobic domains where it could easily bound to TMD surfaces with the hydrophobic group. Bang et al. demonstrated the enhanced exfoliation of

WS2 nanosheets due to the adsorption of ssDNA on the WS2 surface which caused electrostatic repulsion from the sugars on the ssDNA backbone.(40) The application of ssDNA was reported to have a higher yield of WS2 nanosheets compared to previous aqueous exfoliation.(41) The

WSe2-ssDNA nanosheets also displayed antibacterial activity against Escherichia coli K-12 MG1655 cells.(40) Guan et al. reported the application of bovine serum albumin (BSA), a serum albumin protein derived from cows, as an effective exfoliation agent for producing monolayer

MoS2 nanosheets.(36) The exfoliation was achieved in an aqueous solution containing BSA through ultrasonication. Another protein molecule, the silk fibroin (produced by Bombyx mori silkworms), could also be used as an exfoliation agent of TMDs during the sonication process.(37) Huang et al. recently reported the efficient exfoliation of MoSe2 into mono- and few layers by mixing silk fibroin aqueous solution with bulk MoSe2 powder for ultrasonication. The silk fibroin-MoSe2 nanosheets also displayed effective wound disinfection and healing efficacy with low amounts of H2O2 in vivo, which could lead to more research towards applying TMDs for clinical applications. (37)

Figure 1-6. TEM gallery of solution-synthesized colloidal TMD nanosheets, including 1T-WS2, 2H-WS2, MoSe2, WSe2, 1T′-MoTe2 and WTe2. Figure adapted from reference (42) by permission from Springer Nature, Copyright (2020).

1.2.1.3 Colloidal synthesis Colloidal synthesis is another synthetic method that has been proven useful for the controlled growth of conventional semiconductor nanocrystals and heterostructures with controlled morphology and size-dependent properties (see Fig. 1-6). (42) Despite of this, few reports exist on colloidally synthesized layered materials.(43) Mahler et al. first demonstrated the synthesis of 1T- and 2H-phase WS2 nanosheets. The addition of hexamethyldisilazane (HMDS) to the reaction vessel was found to change the growth morphology from sheets to flowers.(44) The many exposed edges and decoupled basal planes of this morphology enhance the utility of the crystals for electrochemical applications and make them a possible platform for chemical functionalization. Flowers of Mo-W TMD alloys have been synthesized, as well as the semi- metallic 1T’ phase of MoTe2.(45–47) In addition to HMDS, other choices of passivating ligands can affect growth, with oleic acid resulting in monolayer MoSe2 and WSe2 sheets.(48) Recently, Sun et al. demonstrated that colloidal synthesized TMD nanoflowers are capable of reacting with Ag and Au salts in solution which results in distinct classes of 0D-2D and 2D-2D metal-TMD hybrids. (42)

1.2.2 Preparation of Solid-state TMD Flakes on substrates

1.2.2.1 Mechanical Exfoliation In the section, we focus on the preparation of TMDs that are deposited on solid substrates. The first reported method for preparing such solid-state flakes is the mechanical exfoliation via Scotch tape, which was inspired by the exfoliation of graphene. Novoselov et al. first prepared mono- and few-layer MoS2 samples via this method. (49) This method yields monolayer samples ranged from 25 to 200 μm in size, (2) with high channel mobility (200 cm2V-1s-1) and high current on/off ratio (1×108).(50) Since high quality TMDs crystals can be acquired by mechanical exfoliation, this method is ideal for the fabrication of high-performance electronic devices.(3)

Figure 1-7. Comparison between mechanical exfoliation and vapor deposition in the synthesis of high crystalline monolayer MoS2 (A) Optical image of a MoS2 flake deposited on SiO2/Si via mechanical exfoliation.(51) (B) Optical image of a single-layer MoS2 crystal grown on SiO2/Si via chemical vapor deposition.(51) MoS2 single crystals prepared via CVD can normally reach larger sizes compared to mechanically exfoliated MoS2 flakes. Figure adapted from reference (51) with permission from The IOP Publishing. Copyright (2015).

1.2.2.2 Chemical Vapor Deposition (CVD) Other than mechanical exfoliation, many researchers have explored the bottom-up synthesis approaches due to its capability in large-scale production of high-quality TMDs materials. The current bottom-up synthesis of TMD materials including chemical vapor deposition (CVD)(52), powder vaporization (PV)(53), molecular beam epitaxy (MBE)(54), atomic layer deposition (ALD)(55), and etc. Among them, vapor transport methods such as CVD and PV have been extensively studied due to its potential in synthesizing high-quality 2D TMDs.(56) During the vapor transport synthesis process, various parameters can be fine-tuned to affect the final properties of TMDs. These parameters including precursor, temperature, carrier gas flow, seeding promoter and substrate.(56) Due to the fast development of CVD and PV, a large variety of TMDs can now be readily synthesized with large single crystal size(57), a wide variety of compositions(58), controlled alloying or hetero-stacking(59, 60), controlled crystal orientation(61), controlled heteroatom doping(62) as well as engineered morphology and edge terminations.(63–65) In addition, efforts have also been made to grow wafer-scale 2D TMDs films with well-defined layer numbers.(52) The recent breakthroughs in vapor transport synthesis of TMDs relies on the halide-assisted synthesis and metal-organic chemical vapor deposition (MOCVD). In 2015, Li et al. first reported the usage of alkali metal halides (such as NaCl, NaBr and KCl) as promoters for synthesizing monolayer WS2 and WSe2.(66) Since then, there are increasing number of reports using halides to promote the synthesis of a variety of TMDs.(67, 68) It is considered that the addition of halides can increase the vapor pressure of transition metal source by forming volatile intermediates with transition metal precursors, thus efficiently increasing the nucleation and growth rate of TMDs and yield large-size TMDs single crystals. Recently, Zhou et al. reported that this method has a striking universality that can be applied to a broad range of TMDs including 32 different binary compounds (see Fig. 1-8), 13 alloys and 2 heterostructures.(58) In addition, as reported by Li et al., with the mediation of NaCl salt, the vapor-liquid-solid (VLS) growth mode of MoS2 can also be achieved, resulting in MoS2 nanoribbons with tens to thousands of nanometers.(69) Thus, it is believed that the development of halide-promoted growth can greatly facilitate the synthesis of various novel TMDs materials with engineered morphologies and trigger the exploration of many exotic properties.

Figure 1-8. Optical library of 32 different 2D binary compounds synthesized via CVD, containing: Mo (MoS2, MoSe2, MoTe2), W (WS2, WSe2, WTe2), Re (ReS2, ReSe2), Ti (TiS2, TiSe2, TiTe2), Zr (ZrS2, ZrSe2, ZrTe2), Hf (HfS2, HfSe2, HfTe2), V (VS2, VSe2, VTe2), Nb (NbS2, NbSe2, NbTe2), Ta (TaS2, TaSe2, TaTe2), Pt (PtS2, PtSe2, PtTe2), Pd (PdS2, PdSe2) or Fe (FeSe). Figure adapted from reference (58) by permission from Springer Nature, Copyright (2018).

It is worth noting that in recent years, the MOCVD technique has also enabled many breakthroughs in the wafer-scale synthesis of TMDs. In a typical MOCVD process, only gas- phase precursors are used for metal and sulfur source (such as Mo(CO)6 and H2S). As a result, compared with PV method, precursors in MOCVD process can be supplied in a more controlled fashion by regulating their partial pressures. Because this advantage, MOCVD techniques can potentially have better control over the TMD growth rate and uniformity. Over past years, MOCVD methods has been employed to controllably synthesize wafer-scale uniform monolayer TMDs and the in-plane superlattices of different TMDs.(52, 70) The as-synthesized wafer-scale film of different TMDs can be further assembled layer-by-layer into designed heterostructures by

13 a programmed vacuum stack (PVS) process.(71) In all, the development of scalable growth of wafer-scale uniform 2D TMDs and the assembly of their heterostructures have accelerated the process toward the ultimate commercialization of TMDs-based electronics and optoelectronics.

1.3 Surface Functionalization of TMDs

Due to the high specific surface area and reduced dielectric screening in the 2D limit, a series of structural, electrical, and optical properties of 2D TMD materials are extremely sensitive to defects and surface perturbations.(14, 110, 111) Therefore, precise control of the electronic surface states of 2D materials can increase their versatility and widen their applicability in electronics and sensing. As shown in Table 1-1, many chemical and physical surface functionalizations has been used to adjust the electronic, optical, chemical, and sensing properties of 2D TMD materials. During synthesis, various structural defects with different dimensionalities can natively occur in TMDs.(110) Atomic vacancies and topological defects, such as grain boundaries and exposed edges, can serve as favorable active sites for the absorption of molecules, functional groups and nanostructures, enabling various types of chemical and physical interactions. In this section, we will briefly introduce and categorize the various functionalization methods of TMDs based on the chemical interaction between the functionalizing molecule and the TMD material.

Table 1-1. Summary of surface functionalization with regards to functionalization types, species, materials, and applications.

Functionalization Functionalizing Species Material Effect/Application Type

Chemisorption poly(ethylene glycol) (PEG),(72) MoS2,(72– sensing,(73) thionin,(73) {2-mercaptoethylamine, 75) n/p type doping,(74)

1H,1H,2H,2H-perfluorodecanethiol MoS2,(77) dispersion solubility(72, 75) and 2,2,2-trifluoroethanthiol},(74) InSe,(79) p-doping(76) Diazonium Salts(75, 76), M(OAc)2 photodetector(79), (M=Ni, Cu, Zn),(77) {Ti4+(78), Al3+, improved stability(78) B3+,Sn+4}(79)

Physisorption/ {F4TCNQ, TCNQ,(80) NADH},(81) MoS2(81, PL tuning,(81, 82) Charge Transfer {adenine (A), guanine (G), cytosine 86, 90, 92, n/p type doping,(83–96, 98)

(C), and thymine (T)},(82) K,(83, 84) 93, 97, 98) Resistance reduction(97), Band

Cu,(85) CsCO3,(86–88) Benzyl WS2,(82, gap modulation(84) Viologen,(89) Cl,(90) 90)

Triphenylphosphine,(91) poly(vinyl- MoSe2(96)

alcohol),(92) Artificial DNA and M- WSe2(87,

DNA,(93) Au, MoO3,(94) MoOx,(95) 91, 93, 94),

Mo(NMe2)4, triphenylphosphine

(PPh3),(96) amine-rich aliphatic polymer,(97)

Nanoparticle Pt,(99) Ag,(100–102) Au(99, 100, MoS2,(100– PL tuning,(102–104) SERS,(105) Decoration 103–109) 108) water splitting,(106) solar cells,(107) photocatalysis,(109) phase transition,(108) sensors(99)

1.3.1 Chemisorption of thiol-terminating ligands

Functionalization in the form of thiol-terminating ligands that interact with defects (mostly S vacancies) on liquid exfoliated TMD flakes was among the first forms of functionalization reported due to the high defect density in chemically exfoliated (ce)-TMDs. This type of ligand- surface interaction was first reported by Dravid and co-workers.(72) They reacted chemically

15 exfoliated 1T- MoS2 with different bifunctional poly(ethylene glycol) (PEG) molecules (Fig 1-9 A), and observed a change of surface properties via large changes in the zeta potential of these functionalized materials(Fig 1-9 B). These results were supported by x-ray photoelectron spectroscopy (XPS) analysis, which demonstrated the presence of carbon and oxygen atoms on the material’s surface, indicating the incidence of polyether functional groups. Fourier Transform Infrared spectroscopy (FTIR) in Fig 1-9 C further suggested that thiol groups have reacted with the surface, as it was observed that the S–H stretch at 2563 cm−1 was lost after functionalization. (72) Another report of chemisorption used the similarly sized thiol-bearing ligand thionin, which was applied to MoS2 during sonication (with subsequent gradient centrifugation). (73) The organometallic complex formed was then used to detect double-stranded DNA. This work indicated that edge-based adsorption was limited only by the availability of defective regions, and speculated that a higher defect density at the edge sites might lead to higher efficacy unless otherwise capped by the degradation of the material or the steric hindrance of ligand-ligand interactions. Since then, many thiol-terminating ligands and other organic molecules have been reported to conjugate with TMDs (see Table 1-1 for details), such as 2-mercaptoethylamine, 1H,1H,2H,2H-perfluorodecanethiol and 2,2,2-trifluoroethanthiol,(74) 1-methyl-2- pyrrolidinone,(112) and diazonium salts on MoS2.(75)

1.3.2 Coordination based functionalization

Another class of chemical functionalization on TMDs takes advantage of the lone pair electrons on the TMD surface and functionalizes the TMDs via coordination chemistry. Backes et al. first reported the functionalization of 2H-MoS2 via formation of MoS2-M(OAc)2 materials

(M=Ni, Cu, Zn).(77) By mixing sonication-exfoliated MoS2 with M(OAc)2 in IPA, Backes et al. demonstrated the first case of forming metal-chalcogen bonds on the TMD surface. Lei et al. showed theoretically and experimentally that the presence of nonbonding electron pairs on the surface of the InSe (see Fig 1-10) can also be exploited by applying Lewis acid-base concepts.(79) Thus, the formation of complexes with Lewis acids, such as Ti4+, Al3+, and 2D

17 materials could be achieved by simple acid-base reactions. Depending on the strength of the acid, it is possible to control the material’s Fermi level and the bandgap.

1.3.3 Physisorption of small molecules

Another form of functionalization is through the physisorption of small molecules on the surface of TMDs which induces charge transfer and results in n- or p-type doping of TMDs.(81, 113–115) Mouri et al. demonstrated the tunable PL of monolayer TMDs (see Fig 1-11) by solution-based molecular doping.(81) As-prepared MoS2 usually shows an n-type behavior due to the low formation energy of sulfur vacancies, which leads to the domination of negative trion

(X-) emission in the PL of MoS2. By drop-casting p-type dopants (F4TCNQ and TCNQ), the PL emission is enhanced due to the switch of dominant PL process from X- to X0 emission. (81) On the contrary, the PL intensity is weakened by drop-casting n-type dopants (NADH), as a consequence of further suppression of X0 by excess electron injection. Feng et al. reported the

0 tunability of the PL of WS2 via similar strategy. Enhanced X emission peak as well as total PL intensity are observed by introducing F4TCNQ and H2O as p-type dopants.(82) The functionalization of TMDs by physisorption charge transfer has also been studied in field effect transistors (FETs). Many molecules (See Table 1-1) have been investigated for the p-type (90, 93–96, 116, 117) and n-type(83, 86, 87, 90–92) doping of TMDs as well as investigations regarding resistance(97) and transfer characteristics.(98)

1.3.4 Functionalization of 2D materials by Defect Engineering

Defect Engineering is another powerful tool to functionalize 2D materials. The controlled introduction of defects into 2D materials greatly alter its electric, optical and catalytic properties, thus improving the performance of the pristine 2D material in many applications.(110) Structural

19 defects in 2D materials can be categorized into 3 categories based on defect dimensionality (see Fig1-12) : 1) zero dimensional defects such as vacancies, dopants and adatoms; 2) one dimensional defects such as grain boundaries, edges, line vacancies and in-plane heterojunctions; 3) two dimensional defects such as hetero-layer stacking, scrolling, rippling and folding. (110) Surface chemical functionalization via defect engineering mostly focuses on utilizing vacancies, introducing dopants and adatoms as well as increasing exposed edges in 2D TMDs for catalytic applications. (19–21)

1.3.5 Nanoparticle functionalization on 2D Materials

Formation of nanostructures composed of TMDs and plasmonic metal components (such as Au nanoparticles) have also been extensively studied.(118, 119) Because of the strong plasmonic effect induced by the metal nanomaterials in the hybrid nanostructures, some properties of the 2D material component, e.g. photocatalysis, PL, and optoelectronics, will be altered and enhanced by the proximity of the metal. Lee et al. deposited bowtie structured Ag nanoarrays on top of CVD

20 grown MoS2 and found a dramatic enhancement in the PL of the MoS2. (120) The PL enhancement factored can be controlled via tailoring the parameters of the Ag nanoarray (see Fig 1-13). To date, hybrid nanostructures of metal nanomaterials/2D materials have been explored in various applications, including plasmon-enhanced optical signals (i.e., PL, SERS, and SPR sensors)(100–105), plasmon-enhanced photocatalytic reactions (i.e., water splitting),(106, 121) plasmon-enhanced optoelectronic devices (i.e., solar cells),(107) electrochemical sensors.(99) However, the plasmonic resonance of the metal component will be greatly tailored after hybridization with the 2D material component because plasmonic resonance is highly dependent on the dielectric function of the surrounding medium. In addition, a plasmon- exciton interaction triggers ‘‘hot electron’’ transfer from the metal NPs to the 2D materials, which will dampen the plasmonic resonance of metal nanostructures or change the absorption of the 2D materials.(108, 121)

1.4 Objectives and Thesis outline

As stated above, chemical surface functionalization has been widely used to tune the electronic, optical, and catalytic properties of 2D TMDs. Therefore, this thesis will focus on applying novel chemical functionalization methods to functionalize 2D TMD materials to tune its physical and chemical properties. In chapter one, we have introduced the basic properties of 2D TMD materials as well as summarized their common synthesis and functionalization methods.

In chapter two, we will demonstrate the spontaneous chemical functionalization of MoS2via coordination of Au/Ag single atoms. In this work, we synthesized the [Au(MoS2)Clx]

- coordination complex on the TMD surface and used the AuClx concentration to control the optical, electric, and thermal properties of MoS2 monolayers. In chapter three, we will continue exploring the coordination-based reactions between other transition metals (Cr, Mn, Fe, Co, Ni, Cu and Zn) and MoS2. We studied the formation of these coordination complexes on MoS2 monolayers and correlate their properties with classical coordination complexes.

In chapter four, this thesis will discuss the SERS performance of Au NP functionalized MoS2 via constructing a Au NP-MoS2 heterostructure to achieve the constructive interference of electrochemical enhancement and charge-transfer based chemical enhancement of SERS. In chapter five, this thesis will summarize its achievements and contributions to the field.

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Chapter 2 Spontaneous chemical functionalization via coordination of Au

single atoms on monolayer MoS2

This chapter is based on a published article which I am the first author and it is reproduced with permission from H. Liu, D. Grasseschi, A. Dodda, K. Fujisawa, D. Olson, E. Kahn, F. Zhang, T. Zhang, Y. Lei, R. B. N. Branco, A. L. Elías, R. C. Silva, Y.-T. Yeh, C. M. Maroneze, L. Seixas, P. Hopkins, S. Das, C. J. S. de Matos, M. Terrones, Spontaneous chemical functionalization via coordination of Au single atoms on monolayer MoS2. Sci. Adv. 6, eabc9308 (2020). The theoretical calculations in this published work were performed by our collaborator Dr. Daniel Grasseschi, Dr. Camila M. Maroneze, Dr. Leandro Seixas and Dr.Christiano J.S. de Matos. As discussed in Chapter 1, surface functionalization of metallic and semiconducting 2D transition metal dichalcogenides (TMDs) have mostly relied on physi- and chemi-sorption at defect sites which can diminish the potential applications of the decorated 2D materials, as structural defects can have significant drawbacks on the electronic and optoelectronic characteristics. Here we demonstrate a spontaneous defect-free functionalization method consisting of attaching Au single atoms to monolayers of semiconducting MoS2(1H) via S-Au-Cl coordination complexes. This strategy offers an effective and controllable approach for tuning the Fermi level and excitation spectra of MoS2 via p-type doping as well as enhancing the thermal boundary conductance of monolayer MoS2, thus promoting heat dissipation. The coordination-based method offers an effective and damage-free route of functionalizing TMDs and can be applied to other metals and used in single atom catalysis (SAC), quantum information devices, optoelectronics and enhanced sensing.

2.1 Introduction

Chemical surface functionalization has been used to tune the electronic, optical, and catalytic properties of two dimensional (2D) transition metal dichalcogenides (TMD).(1–3) Chemical functionalization commonly relies on the presence of lattice defects and physisorption methods to circumvent the chemically inertness of pristine semiconducting TMDs. (4) Unfortunately, such routes inevitably modify the surface characteristics as well as the optical,

35 thermal and transport properties of the atomically thin layers.(5–9) Most of the proposed MoS2 functionalization routes are based on liquid exfoliated flakes, which have reduced and more reactive lateral sizes. In addition this exfoliation route is not ideal for scaling up processes in the electronics and optoelectronics industries.(10) For example, Voiry et al. reported covalent functionalization using chemically exfoliated MoS2, WS2 and MoSe2 with organohalides and its impact on their photoluminescence (PL) emission.(6) Although this functionalization did not entirely rely on defect engineering, it is restricted to TMDs in the 1T phase, and the exfoliation procedure involves harsh chemical treatments, such as butyllithium. Efforts have also been made to functionalize chemical vapor deposition (CVD) grown TMDs. For example, Ding et. al. reported that thiol functionalization of CVD-grown MoS2 led to an increase in the PL intensity due to the passivation of S vacancies by the ligand, thus relying on the presence of inherent defects to achieve functionalization.(2) Yet, in this case the thiol does not provide perfect healing to the crystalline structure, thus the charge mobility of a pristine crystal is not reestablished/recovered. To the best of our knowledge, chemically bonding single atoms, molecules, or clusters to the surface of pristine crystalline TMD monolayers remains a challenge. For metal monochalcogenides, Lei et. al. proposed the formation of coordination complexes on the surface InSe and showed that the presence of nonbonding electron pairs can be exploited by applying Lewis acid-base concepts in order to form coordination bonds with metal ions.(11) As a result, the electronic properties of InSe were tailored and p-type doping was observed through this functionalization approach.

In this chapter, we have developed a novel 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. To the best of our knowledge, the [Au(MoS2)Clx] coordination complexes have been synthesized for the first time by taking advantage of the lone pair electrons of the S atoms present in the MoS2 lattice. Unlike substitutional doping or defect passivation, the formation of the metal-MoS2 coordination complexes does not rely on the presence of additional defects on the 1H-MoS2, but induces significant changes in the optical, thermal and electrical properties. Density functional theory (DFT) calculations confirmed that single AuClx complex would bond to S atoms, rather than randomly absorbed on top of Mo atoms or over the vacant spots of the hexagonal lattice, and scanning transmission electron microscopy (STEM) imaging revealed the presence of Au single atoms fixed on top of S atoms. Remarkably, during the Au functionalization, the Au3+ precursor was spontaneously reduced to Au1+ on the TMD surface prior to forming the Au-S coordination bonds. This Au-S bond results in an efficient transfer of

36 electrons from the MoS2 valence band with high contribution of S 3p orbitals to the Au 5d and 6s orbitals, thus tuning the Fermi level of MoS2 monolayers. By treating MoS2 with different Au concentrations, we found an effective way to tune the Fermi level of MoS2 via controlled p-type doping, as measured in field effect transistors (FET). Additionally, Au functionalization leads to high exciton to trion ratios in the PL spectra of monolayer MoS2. The stability of the S-Au-Cl coordination sphere leads to the existence of [Au(MoS2)Clx] complexes that results in a major enhancement on the thermal boundary conductance across MoS2 monolayers. This coordination method could also be used to synthesize Ag single atoms on MoS2. The isolation and anchorage of single noble atoms via a solution phase chemical approach pave the way to large-scale manipulation of single atoms, leading to high-performance catalysis and quantum information applications.(12)

2.2 Synthesis and characterization of [Au(MoS2)Clx] complexes

2.2.1 Materials Synthesis

2.2.1.1 Synthesis of monolayer MoS2

Monolayer MoS2 was synthesized by a salt-assisted CVD method, similar to previous publications.(13),(14)NaBr (Alfa Aesar, 99%) was ground into a fine powder with a mortar and pestle, then mixed with MoO2 in a 10:1 ratio by weight. For growth of monolayer films, 2 mg of the salt/oxide mixture was placed in the bottom of a ceramic boat and a piece of SiO2 (300 nm)/Si substrate was placed facing-down over the mixture, with ca. 2 mm of space between the mixture and substrate. 100 mg of sulfur powder (Alfa Aesar, 95%, 300 mesh) were used as the sulfur source. The growth substrate was placed at the center of a 1-inch diameter horizontal tube furnace (Lindberg/Blue M), while the sulfur powder was placed upstream outside of the furnace and 30 cm away from the growth substrates. Prior the synthesis experiments, the tube was flushed with 400 sccm of Ar for 20 min, and then the flow was reduced to 100 sccm. The furnace was then heated to 800 oC in 20 min and then held for 5 min. The sulfur powder was separately heated to 220 oC in 5 min and held for 5 min, while the furnace was kept at 800 oC.

2.2.1.2 Synthesis of the [Au(MoS2)Clx] complex

The functionalization was performed by dipping the Si/SiO2 substrate with the CVD

-2 -9 -1 MoS2 into an ethanol solution of HAuCl4 with concentrations between 1×10 to 1×10 mol L for 10 min. The functionalized [Au(MoS2)Clx] sample was then immersed in isopropanol (IPA) for a few seconds in order to remove excess HAuCl4, followed by N2 drying. Finally, the sample was kept in vacuum for 10 min prior the measurements. To exclude the effect of ethanol or IPA adsorption on the flake surface, pristine samples were also washed with ethanol and IPA, dried with N2 and kept in vacuum for 10 minutes. 2.2.1.3 FET Device Fabrication

Back-gated field effect transistors were fabricated using a 50nm alumina (Al2O3) film as

++ the gate oxide and a stack of Pt/TiN/p Si as the back gate electrode. First, CVD grown MoS2 were transferred onto the alumina sample, then the sample was spin coated with A6 PMMA, followed by electron-beam (e-beam) lithography to specify the 2 µm channels and then separating them out by sulfur hexafluoride (SF6) etch under 5 °C for 30s. After each step, the sample was rinsed in acetone for 30 min followed by a rinse in IPA. To define the source and drain contacts, the sample was then spin coated with methyl methacrylate (MMA) followed by A3 PMMA. Using electron-beam lithography source and drain contacts were then patterned and further developed by using 1:1 mixture of 4-methyl -2-pentanone (MIBK) and IPA for 60s. 40nm of Ni and 30 nm of Au were deposited onto the patterns using electron beam (E-beam) evaporation. Lift-off of the evaporated materials was carried out by immersing the sample in acetone for 30 min followed by an IPA final rinse.

2.2.2 Materials Characterization

2.2.2.1 Scanning Transmission Electron Microscopy (STEM) STEM imaging was carried out in a FEI Titan3 G2 60/300 operated at 80 kV to reduce irradiation damage. A high-angle annular dark field (HAADF) detector was used to collect ADF signal. A Gaussian blur filter was applied using the ImageJ software to reduce the noise and enhance the visibility of the detailed structure, but raw images were used for acquiring the line profile of the ADF intensity. STEM-ADF image simulations were conducted using the QSTEM package. Simulation parameters such as acceleration voltage, spherical aberration (C3 and C5), convergence angle, and inner/outer angle for the HAADF detector were set according to

38 experimental conditions. It is worth noting that prior to the TEM imaging, the CVD grown MoS2

-6 -1 was first transferred to a TEM grid and then functionalized by 1×10 HAuCl4 mol L in ethanol solution. 2.2.2.2 Electrical Characterization Electrical Characterization of the fabricated devices and coordinated devices was performed in a Lake Shore CRX-VF probe station under high vacuum condition using a Keysight B1500A parameter analyzer. 2.2.2.3 Thermal boundary conductance measurements A nominally 80 nm Al film was deposited on specimens via E-beam evaporation to serve as the metal transducer in our experiment. The 808.5 nm wavelength beam of a 80 MHz Ti: Sapphire oscillator was spectrally separated into pump and probe paths. The pump path was amplitude-modulated via an electro-optic modulator at 8.4 MHz, creating a frequency-dependent heating event at the surface of the Al-coated MoS2. The probe was mechanically delayed in time and monitored the thermoreflectance at the sample surface due to the pump-induced heating event. With a 10x objective, the 1/e2 diameters of the pump and probe were 14 and 11 μm, respectively. We fitted the data to the radially symmetric heat diffusion equation for the conductance, hK, of the Al/MoS2/SiO2 interfaces and the thermal conductivity of the underlying thermal oxide. Additional information regarding the specific analyses of TDTR can be found elsewhere.(15–17) Single crystal MoS2 flakes were located by mounting our samples on a 2-axis stage, which was oriented to minimize changes in the pump/probe radii over the measurement range. 2.2.2.4 XPS measurements XPS measurements were conducted in a high-resolution Thermo Scientific XPS with monochromatic Al Kα X-ray source. The binding energies were calibrated with C1s binding energy of 284.8 eV. The analysis of peak fitting was performed on XPSPEAK41 software. 2.2.2.5 Optical absorbance measurements Optical absorption measurements were performed using a homemade setup. For excitation, a supercontinuum white light source (NKT Photonics), ranging from 400-2400 nm, was coupled to a 9 μm core diameter optical fiber and the output collimated using a 5x Newport objective (0.10 NA). The beam was then reflected, using a Thorlabs visible non-polarizing cube beam splitter (400 - 700nm), to a Nikon TU Plan Fluor EPI 50x objective (NA 0.8) which focused the beam on the sample at normal incidence. The reflected signal was collected and collimated by the same 50x objective. After transmission through the cube beam splitter, the

39 beam was focused onto the input of a 500 µm core diameter Thorlabs multimode fiber, using a 10x Newport objective (0.25 NA), connected to a Yokogawa Optical Spectrum Analyzer (OSA) operation in the 350-1700 nm range. In this setup, the position where the spot was focused on the sample was determined by directing the reflected beam to a Thorlabs USB CCD Camera (1024 x 768 resolution) on which an image was formed using a Thorlabs infinity corrected lens (focus = 200mm). For imaging, another visible beam splitter was placed before the camera lens and a diffused white light LED source was used to illuminate the sample. To obtain absorbance spectra, the bare substrate was used as a reference. 2.2.2.6 First principles calculations Density-functional calculations were performed using the SIESTA code(18) with a DZP localized basis set, energy shift of 0.03 eV and mesh cutoff of 400 Ry. The exchange-correlation functional was described by the gradient generalized approximation with a Perdew-Burke- Ernzerhof parameterization.(19) We used a 4 x 4 diagonal supercell for the electronic band structure, and 3 x 2 orthorhombic supercells for real-space charge density plots. Brillouin zone sampling was based on Γ-centered Monkhorst-Pack method with 4 x 4 grid for supercells.(20) A vacuum spacing of 20 Å was added to avoid interaction among periodic layers. Structural optimization was performed with a force tolerance of 0.010 eV/Å.

2.3 Results

2.3.1 Single Au and Ag atoms on MoS2

MoS2 monolayers were synthesized by CVD on Si/SiO2 substrates using powder precursors (see Chapter 1.2 for details). Optical microscopy, Raman spectroscopy and PL spectroscopy were carried out to characterize the MoS2 flakes (see Fig. 2-24). The coordination reaction between the MoS2 monolayers with HAuCl4 was performed by dipping the as grown

MoS2 into an ethanol solution of HAuCl4 exhibiting various concentrations for 10 minutes, following washing and drying.

Figure 2-1. Synthesis of Au single atoms on monolayer MoS2 flakes. (A) High resolution STEM images of [Au(MoS2)Clx] featuring the single Au atoms on S sites. Insets: The structure model of Au-MoS2 used for STEM image simulation (top right) and the higher magnification STEM image of an Au aggregate found on MoS2 (bottom left).(B) and (D) HAADF images and Z-contrast line scan (along the vertical grey line in (B)) of one Au single atom on MoS2, revealing the position of the Au atom to be directly on top of the S atom. (C) and (E) TEM simulation of one Au atom directly on top of one S atom in MoS2 lattice and Z-contrast line scan of the simulation image. The simulated TEM image and line scan match exactly with experimental data, confirming the atomic structure of Au single atoms directly bonded on S atoms on the MoS2 surface.

Figure 2-2. High resolution STEM images of Ag-MoS2 complex. The inset of the image highlights the single Ag atoms on S sites

High-resolution STEM imaging using a high-angle annular dark-field (ADF) detector was conducted to study the atomic structure of the Au atoms on MoS2. The ADF intensity changes depending on the Z-number of atoms (~Z1.6–1.9),(21) thus the higher Z-number Au atoms stand out in the MoS2 lattice. As shown in Fig. 2-1A, bright single Au atoms on top of the MoS2 lattice can be observed. Interestingly, these Au atoms remain isolated, rather than aggregated. The largest Au aggregate found by STEM is shown in the inset of Fig. 2-1A, where 4 Au atoms occupy neighboring S sites. It is worth noting that we consider this as a planar aggregate of Au single atoms, rather than a regular Au cluster, as the atomic spacing corresponds to that of the S in the MoS2 lattice in a planar view, rather than the predicted 1.34 Å interatomic distance between Au atoms in planar clusters.(22, 23) Atomic resolution STEM-ADF images confirmed the presence of Au single atoms as well as their preferred position on the MoS2 monolayer, which is directly on top of the S sites (see Fig. 2-1A and 2-1B). The DFT simulated STEM-ADF image (Fig. 1-1C) was obtained based on the model shown in Fig. 2-1A inset and exhibits an excellent agreement with the experimental image. Single atom Ag could also be synthesized via AgNO3 ethanol solution displaying similar Ag-MoS2 structures (see Fig. 2-2). An ADF intensity line scan

42 was also performed across the single Au atom shown in Figs. 2-1B and 2-1C (vertical gray line in the Figs). In the experimentally acquired ADF intensity line profile (Fig. 2-1D), besides observing the ADF intensity peak at the Mo site and 2S sites, an intense peak corresponding to the Au-2S-site was found matching the simulated ADF intensity at the Au-2S-site (Fig. 2-1E and Fig. 2-3).

We believe that Cl atoms are bonded to the Au single ions to complete the Au coordination sphere and balance the charge on the Au atoms in the form of [Au(MoS2)Cl] and

[Au(MoS2)Cl3] complexes (see below for details). However, STEM simulations conducted for

[Au(MoS2)], [Au(MoS2)Cl] and [Au(MoS2)Cl3] structures (Fig. 2-3) indicate the absence of Cl atoms on our experimental STEM images. We believe this is due to the e-beam irradiation at 80 keV that may be displacing Cl atoms during imaging.

High-resolution STEM was also used to observe changes in the position of individual Au atoms on the MoS2 surface over time. Fig. 2-4A shows selected frames of a video showing the movement of a single Au atom on MoS2 under e-beam irradiation. The beam provides energy to the Au atom, leading to a dynamic atom displacement. It can be observed that the Au atom is capable of hopping from one S coordination site to another. A clear trajectory (gray curve) was determined for the atom movement. For the first 100 seconds of the referred video, the tracked Au atom stayed directly above the same S atom and it then jumped towards a neighboring S site, occupying that for several more seconds. The displacement distance of each hop is shown in Fig. 2-4B. The Au atom moves to the nearest S atom most of the time but can cover a distance of 2 nm

44 during one frame (10 secs per frame). The displacement distance covered by the Au atom shows that the atom stays almost static when it is on top of S while it moves much more actively when it is on top of Mo. We could not find an Au atom that would stay above the Mo atom or at any position other than above a S site for several seconds, thus we conclude that those positions are only transitory and not stable configurations. This observation indicates that stronger interactions between the Au and S atoms make S sites energetically favorable. The movement of single Au atoms, if deterministically manipulated, could be further exploited in future quantum information devices.(12)

2.3.2 Experimental and Theoretical Evidence of the [Au(MoS2)Clx] coordination complex

To further understand the chemical nature of the functionalization and to characterize the Au oxidation state, X-ray photoelectron spectroscopy (XPS) was carried out. Fig. 2-5A shows the

Au 4f core-level spectrum of the [Au(MoS2)Clx] complex (green curve). The spectrum can be well fitted by two-sets of doublet peaks (red and blue curves) that can be assigned to Au3+, from

1+ the HAuCl4 precursor, and Au , indicating spontaneous Au reduction on MoS2. Specifically, the

3+ 1+ Au signature corresponds to the 4f5/2 (4f7/2) peak at 91.0 (87.4) eV while the Au signature relates to the 4f5/2 (4f7/2) peak at 88.2 (84.5) eV, which are comparable to other studies reporting Au ions with S-containing ligands.(24, 25) Huang et al. have reported that gold can be spontaneously reduced from Au3+ to Au0 nanoparticles in a solution of chemically exfoliated

3+ 1+ MoS2.(26) In this work, we demonstrate that Au can be reduced into Au in the form of single atoms due to the formation of a coordination bond to the non-defective surface of CVD grown

3+ 1+ MoS2 monolayers. Based on the standard reduction potentials for the Au /Au pair (0.926 V)

3+ 3+ and MoS2 (-0.090 V), electrons from the MoS2 monolayer can be donated to Au so that Au is reduced to Au1+.(27) In addition, according to Pearson, in hard-soft acid and base theory(28, 29), soft ligands, such as S, stabilize linear Au1+ complexes.(30)

Figure 2-5. Spectroscopic studies of the [Au(MoS2)Clx] complex. (A) X-ray photoelectron spectroscopy (XPS) spectra of Au 4f orbit on Au-MoS2. The green curve is the original spectrum after C1s(248.8eV) calibration. The blue and red curves are fittings of the original data which show the presence of Au3+ as well Au1+. We believe the presence of Au3+comes from the 1+ unreacted HAuCl4 precursor, whereas the Au from the reduction reaction with MoS2. The purple curve is the sum of the blue and red curves. (B) S 2p orbit of pristine and Au functionalized MoS2. The blue and red curves are fittings which show the S 2p1/2 and 2p3/2 peaks respectfully. The S 2p orbitals shift to higher energy due to the loss of outer electrons to Au-S bonds.

The formation of Au-S bonds was also confirmed by the S 2p core-level spectra shown in

Fig. 2-5B. In pristine MoS2 monolayers, the S shows a doublet peak corresponding to a 2p1/2 peak at 163.0 eV and a 2p3/2 peak at 161.9 eV. After functionalization, the doublet peak shifted to a higher binding energy (0.8 eV shift), thus confirming the formation of Au-S bonds. The donation

46 of lone pair electrons from S to Au atoms results in a higher electrostatic attraction on the inner shells of S, causing XPS peaks with higher binding energy.(25) The shift further confirms the formation of Au-S bonds. Therefore, the formation of Au1+-S bonds contribute to the stabilization, partial reduction and formation of single Au ions on the MoS2 surface. Additionally, the absence of water inhibits the Au1+ disproportionation reaction that results in the formation of Au0 and Au3+, which in turn prevents the formation of gold clusters or nanoparticles.(30) Additionally, XPS data reveals the presence of Cl with a Au:Cl ratio of 1:3, as shown in Fig. 2-6, which could be due to the presence of unreacted Au precursor HAuCl4 and/or from AuClx complexes coordinated on MoS2 surface (see discussion below). Similar peak shifting of S 2p orbitals and additional peak splitting were observed for Ag-MoS2 functionalizations demonstrating the formation of Ag-S bonds. (31)(see Fig. 2-7)

Figure 2-7. Spectroscopic studies of the Ag-MoS2 complex. (A) X-ray photoelectron + spectroscopy (XPS) spectra of Ag 3d orbit on Ag-MoS2. The Ag reference peak of 368.1 eV was extracted from Ag2S spectra from reference 23. (B) S 2p orbit of pristine and Ag functionalized MoS2. The Ag-MoS2 XPS measurements used a different batch of MoS2 samples from the Au- MoS2 XPS measurements. They are also measured in different batches which leads to different sample charging effects causing shifts in the pristine S 2p binding energy.

To understand the effect of the single AuClx complex functionalization on the optical properties of monolayer MoS2, PL spectroscopy measurements were carried out. As seen in Fig.

2-8A, PL studies reveal the presence of a pristine MoS2 monolayer characteristic signature, due to its direct band gap at the K(K’) point. The peak shown in Fig. 2-8A can be deconvoluted, via fitting with two Lorentzians, into a neutral exciton (X, red) at 1.83 eV and a negative trion (X–, blue) located at 1.79 eV. It can also be seen that the X- trion was dramatically quenched while the neutral exciton X intensity was enhanced after the Au functionalization. The formation of the coordination bonds between the S and Au atoms results in MoS2 electrons being transferred to the Au atoms, thus leading to a decrease in the X- trion intensity and an increase in the neutral exciton intensity in MoS2.(32) Therefore, we believe that the recorded changes in the PL after functionalization are caused by the p-type doping of MoS2 via AuClx coordination. The p-type doping level of MoS2 via AuClx coordination can be carefully controlled by exposing the MoS2 monolayers to different concentrations of HAuCl4 solutions. Fig. 2-8B shows the exciton-trion intensity ratio as a function of the Au concentration. It can be clearly seen that this ratio increases with the Au concentration, offering an effective approach to sharpen the PL spectra of MoS2. A more detailed discussion on the p-type doping effect is available below (Fig. 2-9). Raman spectra of monolayer MoS2 before and after the Au functionalization exhibit the characteristic in-plane E’

48 and out-of-plane A’1 modes, without noticeable frequency nor intensity differences (Fig. 2-5). As shown in Table. S1, no significant changes can be observed either in the Mo-S bond lengths and the Mo-S-Mo bond angles, for the optimized Au coordination in the 1H phase MoS2.

The formation of the Au-S bond was further investigated by DFT calculations, which were performed to identify the most energy favorable structure for the [Au(MoS2)Clx] complex (shown in Figs. 2-9A and 2-9B). Cl atoms were added to the Au coordination sphere to balance the charge on the Au atoms and complete its coordination sphere, which is also confirmed by XPS (Fig.2-6). Thus, three different structures were evaluated, with an isolated Au atom

([Au(MoS2)]), with one Cl atom ([Au(MoS2)Cl]) and, with three Cl atoms ([Au(MoS2)Cl3]). After the optimization simulations, three available coordination sites, known as the H- (hollow), Mo- and 2S-sites, were identified. In the H-site, the Au is located at the center of the hexagon formed by the lattice projection on a plane (Fig. 2-10); in the Mo-site, the Au sits on top of one Mo atom and is bound to the 3 adjacent S atoms. Finally, the 2S-site, where the Au atom is positioned on top of a S atom, was found to be the most stable site by an energy difference of 550 meV (Figs. 2-

9A and 2-9B). Illustration of the H- and S-site of Ag-MoS2 complexes can be found in Fig. 2-11 and Table 2-1, where the S-site is found to be the most favorable for Ag single atoms. These results are in good agreement with the HR-STEM images depicted in Figs 1B, 2A and S2, where Au and Ag locate preferentially over S atoms.

Figure 2-10. DFT optimized structure showing the bond distance and angles for the Au-MoS2 complex, considering the chlorine ions on the Au coordination sphere.

Figure 2-11. DFT optimized structure showing the bond distance and angles for the Ag-MoS2, bond distance and angles are shown on Table 2-1.

Table 2-1 Calculation details (bond length, bond energy etc.) on [Au(MoS2)Cl3] and Ag-MoS2 complexes Material M Electronic Site Eads Charge d M-S d Mo-S Angle Configuration (eV) (Metal) (Å) (Å) S-Mo- S

Pristine MoS2 ------2.47 82.48

10 1 o Ag-MoS2 4d 5s S -0.96 +0.25 2.84 2.41 81.87

10 1 o Ag-MoS2 4d 5s H -0.87 +0.30 2.63 2.40 81.39

10 1 o [Au(MoS2)Cl3] 5d 6s S -1.41 +0.80 2.63 2.47 82.50

10 1 o [Au(MoS2)Cl] 5d 6s S -1.06 +0.40 2.57 2.47 82.43

10 2 o [Au(MoS2)] 5d 6s H -0.86 +0.30 2.70 2.47 82.27

Fig. 2-12. Charge transfer map of [Au(MoS2)Cl] and [Au(MoS2)] complexes showing the hole concentration as blue surfaces and the electron concentration as red surfaces.

Furthermore, to understand the spontaneous reduction of single AuClx complexes, the charge distribution of the three systems was calculated and analyzed using the Voronoi deformation density (VDD) method. The Au atom charge in the [Au(MoS2)Cl3] and

[Au(MoS2)Cl] optimized geometries were found to be +0.80 and 0.40 eV, respectively, indicating that the charge transfer from the S atoms leads to the spontaneous reduction of Au3+ to Au1+ during the Au-S bond formation. The [Au(MoS2)Cl3] projected charge transfer map shown in Fig. 2-9B indicates a higher electron concentration in the atoms near the Au-S bond. On the contrary, the MoS2 regions far from the AuCl3 exhibited a higher hole concentration, which can be understood as p-type of doping. For the [Au(MoS2)Cl], the projected charge transfer map shown in Fig. 2-12 indicates a very small hole concentration on MoS2, whereas for Au-MoS2 we found a

51 high electron concentration on the MoS2 and an n-type of doping, showing Cl atoms bound to Au in order to provide the p-type doping observed experimentally. The [Au(MoS2)Cl] and

[Au(MoS2)Cl3] data are in good agreement with XPS, where the Au 4f core-level spectra confirm the Au1+ formation, and the Cl 2p spectra indicates the presence of Cl on the surface. In addition, the exciton to trion intensity ratio changes observed in the PL spectra (Fig. 2-8A and 2-8B) show a clear p-type doping of MoS2.

Figure 2-14. Micro-absorption measurements of the [Au(MoS2)Clx] Complex. (A) Optical images 5 different MoS2 flakes were the absorption spectra were measured. The yellow circle shows the size and position of the laser spot on each flake. (B) Room temperature absorption spectra of pristine MoS2. (C) Room temperature absorption spectra of [Au(MoS2)Clx] complex. The spectra were measured on the same flakes before and after the functionalization process.

Figure 2-15. Tauc plot of absorption spectroscopy of pristine and AuClx functionalized MoS2.

From the calculated electronic band structure, displayed in Fig. 2-13A, we notice that the

MoS2 remained a direct gap semiconductor after functionalization. However, the bands near the

1+ Fermi level were drastically affected by the presence of single Au ions coordinated on the MoS2 monolayer, with the corresponding wavefunctions overlapping with Au orbitals. A localized state is also apparent in the functionalized structure above the Fermi level corresponding to Au d orbitals. Clearly, the mentioned changes in the band structure are expected to affect the optical properties of MoS2. In this context, optical absorbance measurements conducted in MoS2, before and after functionalization, provide some valuable information that can be correlated to band structure. Fig. 2-13B depicts the average spectra of 10 pristine (black) and [Au(MoS2)Clx] (pink) samples, some of the individual spectra can be found in Fig. 2-14. After Au coordination, there is a small blue shift in the A (1.83 to 1.88 eV) and B (1.98 to 2.02 eV), exciton absorption peaks, as predicted by the DFT calculations (blue arrow on Fig. 2-13A). The Tauc plots and its representative fitting of absorption edge show that the MoS2 preserved a direct gap after the functionalization.(33) (Fig. 2-15) Additionally, a new broad absorption band located at a low energy, between 1.5 and 1.7eV, can be observed in the Au-MoS2 absorption spectra, which can be assigned to charge transfer transitions from localized states in the Au ions to the conduction band, with major contributions from MoS2 electronic states (red arrow in Fig. 2-13A). Another charge transfer transition can be seen at 1.35 eV, which can be assigned to an electronic transition from states on the valence band showing a mixture of Au and MoS2 to the conduction band (green) arrow in Fig. 2-13A. It is worth mentioning that the band structure calculated for [Au(MoS2)Cl] (Fig. 2-16) could also explain the electronic transitions observed experimentally. Since both

[Au(MoS2)Cl] and [Au(MoS2)Cl3] lead to p-type doping, both complexes could be well formed during the functionalization process, however the [Au(MoS2)Cl3] has a better fit with the experimental data, due the higher binding energy and p-type doping induced on MoS2.

Figure 2-16. Electronic band structures of pristine MoS2, [Au(MoS2)], [Au(MoS2)Cl] , and [Au(MoS2)Cl3] complexes, calculated without considering spin-orbit coupling.

2.3.3 Fermi-level tuning through AuClx coordination

To further investigate the influence of AuClx complex coordination on the electronic properties of monolayer MoS2, back gated field effect transistors (BGFETs) were designed and fabricated (Fig. 2-17B). Fig. 2-17A shows the schematic of the monolayer MoS2 BGFET. Figs 2-

18A and 2-18B indicate the transfer characteristics i.e. drain current (IDS) versus back gate voltage (VBG) and output characteristics i.e. drain current (IDS) versus drain bias (VDS), respectively, of a representative pristine MoS2 BGFET. The field effect mobility(FE) extracted from the peak transconductance was found to be ~ 25 cm2/V-s, which is comparable to the mobility values reported for exfoliated single crystal material(34). This confirms the superior quality of the as grown monolayer MoS2. The ON state current, which is proportional to the µFE and the charge in the channel, i.e. COX*(VBG - VT), where VT is the threshold voltage of the device and COX is the oxide capacitance, was found to be ~ 120 μA/μm at VDS = 8V and VBG = 10V, further confirming the high performance of the monolayer MoS2 BGFET.

Figure 2-18. (A) Back-gated transfer characteristics of the MoS2 FET. (B) Output characteristics of the MoS2 FET.

Figure 2-19. Fermi level tuning using gold doping. (A-D) Drain current (IDS) vs back-gate voltage -10 -1 -9 -1 - (VBG) characteristics with a drain bias of 1 V. (A) Pristine (B) 10 mol L (C) 10 mol L (D) 10 6 -1 mol L concentrations of HAuCl4 in ethanol.

Table 2-2. Threshold voltage, SS, mobility and ON/OFF ratio statistics of the ten representative devices for various concentrations of HAuCl4 Gold Threshold Subthreshold Mobility (µ ON/OFF Coordination Voltage slope (cm2/V-s) ratio

(mol/L-1) 푉(푉) (SS) (mV/dec) Pristine -4.4 ± 2.5 360 ± 150 15 ±10 106 - 107 10-10 -3.0 ± 1.0 370 ± 150 15 ± 5 106 - 107 10-9 -2.0 ± 0.8 340 ± 140 15 ± 5 106 - 107 10-6 -0.42 ± 1.0 325 ± 150 15 ± 5 106 - 107

The subthreshold slope (SS) of the device was found to be 350 mV/decade, which indicates a reasonably clean interface with the back gate despite the wet transfer process carried out on the MoS2 during device fabrication. Fig. 2-19A shows device to device variations in pristine MoS2 BGFETs. These same devices were then treated with ethanol solution with various

-10 -9 -6 -1 concentration of HAuCl4 such as 10 , 10 and 10 molL . Figs. 2-19B, 2-19C, and 2-19D display the transfer characteristics at VDS = 1V for 10 representative devices after functionalization. It should be noted that MoS2 BGFETs were treated with the lowest concentration of HAuCl4 at first (Fig. 2-19B), measured and then subsequently treated with increasing concentrations (Figs. 2-19C and 2-19D). For higher HAuCl4 concentrations, the threshold voltage becomes increasingly positive i.e. it shifts towards the right indicating that the Au coordination acts as a p-dopant. From TEM, XPS and DFT results, we can then infer that the

AuClx complexes coordinate to the S atoms, forming a fixed charge on the channel which influences the threshold voltage, based on the equation given below.

훥푄 훥푉 = (1) 푐

Here, ΔVT corresponds to the change in threshold voltage while ΔQF is the change in the fixed charge. As the fixed charge on the semiconducting material changes, the flat band voltage increases, meaning that the Fermi level moves closer to the valence band. Table 2-2 shows the statistics for threshold voltage, SS and field effect mobilities for 10 representative devices corresponding to different concentrations of HAuCl4. The threshold voltage shift in the devices is extracted for the electron branch using the iso-current method for a current value of 100 nA/µm for VD=1V. As the concentration of HAuCl4 increases, the coordination of AuClx complexes to the S also increases, which leads to an increased fixed charge on the channel and, ultimately, a shift in the threshold voltage.

Figure 2-20. Analytical Studies of AuClx complexes on MoS2. (A) Threshold change for different Au concentrations with error bar included. (B) Number of AuClx complexes calculated through threshold shift for varies Au concentrations.

Fig. 2-20 shows the error bar plot of threshold voltage variations for different doping concentrations. Mobility values and SS from the statistics suggest that neither the ON state nor the OFF-state devices performance are significantly degraded due to the Au doping. It is also observed that mobility and SS values remain almost constant throughout for various Au concentrations. Based on the threshold voltage shift, we further computed the number of AuClx complexes coordinated to the MoS2 monolayer using the equation below. 푐 훥푉 푛= (2) 푞 Here, q is the electronic charge. Fig. S13B shows the error bar plot of the number of

2 coordinated AuClx complexes versus concentration for a 10x10 nm area. It should be noted that this particular area was chosen to compare the number of Au atoms coordinated to S through equation (2), and then compared with the number obtained from HR-STEM. The results obtained for the 10-6 mol L-1 concentration (ca. 8 single complexes per 100nm2) match perfectly with the number of Au atoms coordinated to S shown in the HR-STEM image of Fig. 2-1A.

Figure 2-21.Transfer characteristics i.e. drain current (퐼) versus back gate voltage (푉) at drain bias, VDS = 1 V for MoS2 FETs with AuClx coordination doping (A) before and (B) after a year.

The stability of the single AuClx complex functionalization is further confirmed via FET measurements carried out one year after the functionalization process (Fig. 2-21). It was observed that the devices did not degrade over time, ensuring that the devices and the functionalization method to be highly reliable. Therefore, we can conclude that our novel Au functionalization technique clearly results in a precise tuning of the Fermi level positions by adjusting the HAuCl4 solution concentration.

2.3.4 Thermal boundary conductance measurements

To further understand the effects of the single AuClx complex functionalization of MoS2, thermal boundary conductance measurements were carried out via time-domain thermoreflectance (TDTR). Previous measurements via Frequency Domain Thermoreflectance (FDTR) have demonstrated that the thermal conductance at graphene contacts can be extracted via a multi-modulation frequency approach.(35) Recently, Brown et al. have shown that thermal conductance values at the metal/MoSe2 interfaces can be extracted via a time-domain thermoreflectance TDTR mapping technique.(36) In order to capture the appropriate conductance at the Al/MoS2/SiO2 interface (see Methods), we have used the magnitude of the thermoreflectance signal to locate single crystals of MoS2, and subsequently performed full

TDTR measurements near the center of the crystals. Fig. 5A depicts a representative micrograph where the thermoreflectance overall magnitude was used to find a MoS2 single crystal. The uniformity of the thermoreflectance magnitude in this region suggests that the thermal conductance across the interface is relatively uniform. The TDTR curve and best fit for the conductance, hK, are presented in Fig. 5B for the pristine MoS2 monolayer, with the inset showing the early picosecond acoustic response used to extract the thickness of the Al layer.(37, 38) A summary of the hK results obtained for AuClx functionalized MoS2 monolayers at various metal concentrations is presented in Fig. 5C.

Figure 2-22. Thermal conductance enhancement from single AuClx complexes. (A) Time-domain thermoreflectance (TDTR) magnitude mapping of a pristine single-crystal MoS2 flake. (B) TDTR model and best fit for the conductance at the Al/pristine MoS2/SiO2 interface. The inset shows the picosecond acoustics response at earlier time delays. (C) Results for the thermal boundary conductance at Al/Au-MoS2/SiO2 interfaces.

In general, we observed that the thermal conductance at the Al/MoS2/SiO2 interfaces is commensurate with the Au concentration. In this context, chemical functionalization of graphene via oxygen plasma treatment has previously reported to increase the conductance by 50% and 100% at these interfaces when Au and Al are chosen as the metal contact, respectively.(39, 40)

As in these works, we attribute the enhanced conductance at the Al/MoS2/SiO2 interface to the

61 enhanced reactivity of the MoS2 caused by chemical functionalization. As Al is required to be in contact with some fraction of Au ions, the reasons for the enhanced conductance could be attributed to the additional pathway of conduction via the electronic system, or through an improvement of the bonding state at the Al/MoS2 interfacial region through functionalization.

Additionally, functionalization of MoS2 monolayers will inherently change the localized vibrational density of states, thus offering additional pathways of conduction. Various reports have previously examined the thermal conductance at the MoS2/SiO2 interfaces via Raman spectroscopy.(41, 42) There, a conductance value of ~15 MW m-2 K-1 was extracted, which is similar to those presented here. However, a direct comparison between the two approaches is not possible, as the conductance at the Al/MoS2/SiO2 interface is inherently measured in our experiment, whereas just the MoS2/SiO2 interface without a metal topcoat is measured in the referred studies. (39, 40) The major enhancement on the thermal boundary conductance of monolayer MoS2, obtained upon Au functionalization, can be applied to increase the heat dissipation rate in vertically stacked 2D transistors.

2.4 Discussion

The formation of chemical bonds is crucial for an effective and non-destructive functionalization of 2D TMD materials. In this work, we demonstrated the formation of the

[Au(MoS2)Clx] complex, which results in the transfer of electrons from MoS2 valence band formed by Mo 4d and S 3p orbitals into the Au valence orbitals (6s and 5d), as indicated by the high mixing of Mo, S and Au states on the bands near the Fermi Level shown in Fig. 2-13. In the STEM images shown in Fig. 2-1 we observe that the distribution of Au single atoms is fixed on top of S sites rather than randomly dispersed. The movie trajectory image (Fig. 2-4) further confirm that Au atoms prefer to sit on S sites even when moving. The displacement distance analysis also concludes that the Au atoms stay static on S, and move immediately to adjacent S when found on top of Mo. These pieces of evidence confirm that the Au atoms have a much higher affinity towards S atoms, indicating chemical interactions between Au and S atoms. STEM simulations conducted for [Au(MoS2)], [Au(MoS2)Cl] and [Au(MoS2)Cl3] structures (Fig. 2-3) indicate the absence of Cl atoms in our experimental STEM images. It is due to the e-beam irradiation at 80 kV, which might have displaced Cl atoms prior to image acquisition. The

[Au(MoS2)Clx] complex is further studied via XPS. The shift of S 2p spectra (Fig. 2-5B) that indicate electron transfer from S to Au, agrees with the formation of a covalent bond through Au coordination with S atoms, where MoS2 acts as ligands donating electrons to Au atoms. XPS data for the Au and Cl core level depicted in Fig. 2-6 show the presence of Cl atoms on the

[Au(MoS2)Clx] sample with a Au:Cl ratio of 1:3. We constructed three different structures with isolated Au ion, Au with one Cl and Au with three Cl on MoS2 via DFT calculations. For

[Au(MoS2)Cl] and [Au(MoS2)Cl3] complexes, the electrons are transferred from the MoS2 to Au, promoting the Au reduction and a p-type doping on the MoS2, thus matching our experimental observations. In contrast, for [Au(MoS2)], electrons are transferred from Au atoms to MoS2, leading to n-type doping. When considering 3Cl as additional ligand, the Au-S binding energy (- 1.4 eV) is higher than the Au-S binding energy (-0.86 eV) without Cl, which could facilitate the formation of single AuClx complexes and prevent aggregation. Thus, the simulations indicate the need of Cl ligands to complete gold's coordination sphere and balance the charges.

The Au 4f spectrum also reveals two states of Au present on the MoS2 surface, namely

1+ 3+ 3+ the Au and Au . We believe that the presence of Au comes from the unreacted HAuCl4

1+ precursor, while the Au comes from the reduction with MoS2. The standard reduction potentials

3+ 1+ - - difference between the Au /Au pair (AuCl4 /AuCl2 = 0.926V) and MoS2 (-0.090V) causes the

3+ 1+ spontaneous reduction of Au to Au by MoS2. In addition, and according to Pearson hard-soft acid and base theory, the reduction from Au3+ to Au1+ makes Au atoms softer, thus promoting a strong bonding to S with a more covalent character, since S atoms are soft ligands known to

1+ stabilize linear Au complexes. Due to the filled 3p S orbitals, MoS2 acts as a pi donor ligand.

1+ This way, when Au ion forms coordination bonds with MoS2, the Au redox potential should decrease considerably, preventing further reduction into its elemental state. For example,

- - [Au(SCN)2] complexes have a redox potential of +0.66 V, whereas [AuCl2] complexes have a much higher redox potential (E0 = +1.15 V), as shown in Fig. 2-23. The absence of elemental Au atoms also prevents the formation of Au clusters and particles since both Au3+ and Au1+ are positively charged ions that show electrostatic repulsion. The repulsion and coordination bonding thus resulted in the stable Au single ions we see in Fig. 2-1.

Figure 2-23. Thermodynamic cycle for oxidation-reduction and complexation reactions of Au complexes considered to understand the ligand effect on the Au redox potential.

Figure 2-24. Characterization of monolayer MoS2. (A) Optical image (B) fluorescence image (C) Raman spectra.

1.5 Conclusion and Outlook

In this work, we successfully prepared [Au(MoS2)Clx] and Ag-MoS2 coordination complexes where Au and Ag single atoms were bonded to S atoms via coordination bonds. This novel approach does not rely on defects, such as vacancies, in the MoS2 crystalline layers, but does have a significant impact on the optical, electrical, and thermal properties of the functionalized monolayer MoS2. The formation of the coordination complex [Au(MoS2)Clx] led to the transfer of electrons from MoS2 to the noble metal, which introduced p-type doping to the

64 functionalized atomically thin semiconductor. Moreover, the degree of p-type doping can be fine- tuned by varying the Au precursor (HAuCl4) concentrations, thus controlling the Fermi level of

MoS2 and the exciton to trion relative population. Furthermore, the MoS2 surface functionalization with single noble metal atoms creates midgap states in the electronic structure of MoS2 and yields band displacements near the Fermi level, which effectively modified the absorbance spectra in the visible range and are responsible for shifts in the PL excitonic emissions. The single noble metal atom functionalization has resulted in a major enhancement of the thermal boundary conductance across monolayer MoS2 interfaces, which could be applied to increase the heat dissipation rate of vertically stacked 2D transistors based on this and other semiconducting TMDs. In addition, the synthesis of single atoms introduced in this work could also be exploited in other applications such as single-atom catalysis (SAC), quantum information devices, optoelectronics, and enhanced sensing. Furthermore, we believe this coordination-based functionalization strategy is general and could be applied to other metals and TMD systems. Many transition metals such as Fe, Ni, and Pt have empty d orbitals that have been well known for forming coordination complexes with S- based ligands.(43–45) Since S is considered a soft base in Pearson hard-soft acid-base theory, we would expect soft atoms such as Pt and Ni to form stronger coordination bonds when compared to Fe and Ti. The coordination sphere will also depend on the ligands. In addition, since Se and Te are even softer than S, we believe that the interaction between Au or Ag with selenides or tellurides would be stronger. The controlled doping of Se and Te into MoS2 systems may offer a way to control the location of Au single atoms since Au-Se bonds should be more favorable.

1.6 Acknowledgements

This work was supported by the Air Force Office of Scientific Research (AFOSR) through grant No. FA9550-18-1-0072. The findings and conclusions of this work do not necessarily reflect the view of the funding agency.

1.7 References

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Chapter 3 Coordination Chemistry Trends between Transition Metals and MoS2

This chapter is based on a manuscript in preparation that I am the first author and it is reproduced with permission from “Coordination Chemistry Trends between Transition Metals and MoS2”, to be submitted (2021) The theoretical calculations in this published work were performed by our collaborator Dr. Daniel Grasseschi and group members. As discussed in Chapter 2, surface functionalization via coordination bonding of metal ions to semiconducting 2D transition metal dichalcogenides (TMDs) offers an effective and controllable approach for tuning the Fermi level and excitation spectra of MoS2 via p-type doping as well as enhancing the thermal boundary conductance of monolayer MoS2, thus promoting heat dissipation. Here we move to further study the coordination reaction between first transition metals Cr, Mn, Co, Ni, Cu, Zn with MoS2 and focus on the periodic trends these metals imply on the electric and optical properties of MoS2.

3.1 Introduction

In 2D materials, functionalization and doping are accomplished through the induction of defects in the crystalline lattice, or through the formation of substitutional solutions, where atoms with a greater or lesser number of electrons are placed as dopants for the creation of semiconductors of type n or p, or still through the formation of covalent bonds between organic molecules and the atoms at the edges of the materials.(1–3) However, these types of interactions tend to be unstable and significantly alter the crystalline structure of the 2D materials, drastically modifying their properties and stability. Recently it was theoretically proposed that the physical adsorption of organic molecules or metallic atoms as a functionalization method can be an effective and damage free way of altering the properties of 2D materials.(4–6) Lei et al. show theoretically and experimentally that the presence of non-bonding electron pairs on the surface of the InSe can be exploited by applying Lewis acid-base concepts.(7) Thus, the formation of complexes with Lewis acids, such as Ti4+ and 2D materials could be achieved by simple acid- base reactions. Depending on the strength of the acid, it is possible to control the material’s Fermi level and the bandgap. (7) In addition, the authors showed that this approach can be extended and that the metal atom can be exploited as a bridge for interaction with different types of molecules,

70 such as Ru-based dyes commonly used in dye-sensitized solar cells. (7) This opens a huge range of possible applications for these materials in the fabrication of photovoltaic devices. In phosphorene e.g., the electron configuration of the P atoms leads to a hybridization with a pair of non-bonding electrons in the orbital perpendicular to the surface, thus phosphorene can be considered a Lewis base. Ding et al. showed by DFT that the adsorption of metal atoms from the first transition series on the surface of the phosphorene is favorable, except for Zn.(8) This leads to the formation of electronic states between phosphorene valence and conduction band, changing the bandgap energy value. Additionally, they showed that the adsorption of Co leads to the creation of a magnetic moment. The interaction of transition metals with phosphorene, therefore, allows a control of the optical and magnetic properties of the material. (8) Arqum et al. showed theoretically that transition metals coordinate at a "triangular" site on the surface of phosphorene and the geometry resembles that of an octahedral coordination complex, when viewed through the axis of rotation. A more careful inspection of the adsorption energy value of the different metals clearly shows that it follows the energy stabilization profile of the crystal field for weak-field complexes. This explains why the interaction with Zn is not favorable.(9) A similarity between the surface chemistry of 2D materials and the analog classical coordination complexes is expected. The application of coordination chemistry and hard-soft acid-base concepts(10–12) can lead to greater understanding and control of the chemical, electric and optical properties of 2D materials. Here we studied the CVD synthesis and functionalization of MoS2 with different transition metal ions (Cr, Mn, Co, Ni, Cu and Zn).We focus on the metal- TMD interaction to evaluate the formation of coordination complexes on surface and correlate their properties with classical coordination complexes, thus facilitating the rationalization of functionalization and the control of its optical and chemical properties with minimal damage to the crystal structure.

3.2 Synthesis and characterization of Transition Metal-MoS2 complexes

In this section, we will go over the detailed methods related to the synthesis and characterization of the TM functionalization process. The MOS2 used in this chapter is monolayer

MoS2 synthesized via CVD. The CVD MoS2 is soaked in various ethanol solutions containing

TMs for 10min to allow the coordination reaction to occur. The TM-MoS2 complexes are

71 characterized via XPS, PL, STEM, REELs and studied via DFT to explain the trends induced to the MoS2 after functionalization.

3.2.1 Materials synthesis

3.2.1.1 Synthesis of monolayer MoS2

Synthesis of MoS2 was carried out by a salt-assisted CVD method, similar to previous publications. (13, 14) NaBr (Alfa Aesar, 99%) was ground into a fine powder with a mortar and pestle, then mixed with MoO2 in a 10:1 ratio by weight. For growth of monolayer films, 2 mg of the salt/oxide mixture was placed in the bottom of a ceramic boat and a piece of SiO2 (300nm)/Si substrate was placed facing-down over the mixture, with ~2 mm of space between the mixture and substrate. 100 mg of sulfur powder (Alfa Aesar, 95%, 300 mesh) were used as the sulfur source. The growth substrate was placed at the center of a 1-inch diameter horizontal tube furnace (Lindberg/Blue M), while the sulfur powder was placed upstream outside of the furnace and 30 cm away from the growth substrates. Prior the synthesis experiments, the tube was flushed with 400 sccm of Ar for 20 min, and then the flow was reduced to 100 sccm. The furnace was then heated to 800 oC in 20 min and then held for 5 min. The sulfur powder was independently heated to 220 oC in 5 min and held for 5 min, while the furnace was kept at 800 oC.

3.2.1.2 Functionalization of MoS2.

The TM-MoS2 complex formation were performed by dipping the Si/SiO2 substrate with the CVD MoS2 into an ethanol solution of chromium acetate, manganese acetate, Iron(III) chloride , Nickle chloride , cooper sulphate or zinc chloride with concentrations between 1×10-2

-9 -1 to 1×10 mol L for 10 min. The functionalized TM-MoS2 sample was then immersed in isopropanol (IPA) for a few seconds in order to remove excess of transition metal salts, followed by N2 drying. Finally, the sample was kept in vacuum for 10 min prior the measurements. To exclude the effect of ethanol or IPA adsorption on the flake surface, pristine samples were also washed with ethanol and IPA, dried with N2 and kept in vacuum for 10 minutes.

3.2.2 Materials characterization

3.2.2.1 Raman and photoluminescence (PL) spectroscopy Raman and PL spectroscopy were performed by excitation at 488 nm in a Microscope- based Renishaw INVia Spectrometer with thermoelectric CCD. For measurements in function of transition metal concentration we followed approximately the same region of the same flake. At least 5 different flakes were measured for each M-MoS2 complexe and M concentration. Raman and PL mapping were obtained using an Alpha 300R Witec confocal Raman microscope with a highly sensitive EMS detector, at 488 nm excitation wavelength and 2mW laser power. 3.2.2.2 X-ray photoelectron spectroscopy (XPS) and Reflection Electron Energy Loss Spectroscopy (REELS) measurements XPS and REELS were conducted in a high-resolution Thermo Scientific ESCALAB 250Xi spectrometer equipped with an electron energy hemispherical analyzer using monochromatized Al K line (1486.6 eV) excitation. The spectra were energy referenced to the C1s signal of aliphatic C atoms at the binding energy of 248.80 eV. XPS spectra were collected using X-ray beam spot size = 650 μm with an emission angle of 90o with respect to the sample surface. High-resolution spectra were acquired with 25 eV pass energy. REELS spectra were acquired with the electron source operating at 1 keV. 3.2.2.3 The computational details To study the equilibrium structure, stability, and electronic structure of transition metal doped MoS2 is based on density functional theory (DFT) as implemented in SIESTA package. We used DZP localized basis, norm-conserved pseudopotentials with Troullier-Martins parametrization, mesh cutoff energy of 350 Ry, and k points sampling of Brillouin zone in Monkhorst-Pack algorithm with 10 x 10 x 1 grid. The exchange-correlation functional used is based on PBE generalized-gradient approximation. The calculations were performed within a supercell framework with 3 x 3 unit cells. 3.2.2.4 Scanning Transmission Electron Microscopy (STEM) STEM imaging was carried out in a FEI Titan3 G2 60/300 operated at 80 kV to reduce irradiation damage. A high-angle annular dark field (HAADF) detector was used to collect ADF signal. A Gaussian blur filter was applied using the ImageJ software to reduce the noise and enhance the visibility of the detailed structure, but raw images were used for acquiring the line profile of the ADF intensity. STEM-ADF image simulations were conducted using the QSTEM package. Simulation parameters such as acceleration voltage, spherical aberration (C3 and C5),

73 convergence angle, and inner/outer angle for the HAADF detector were set according to experimental conditions. It is worth noting that prior to the TEM imaging, the CVD grown MoS2 was first transferred to a TEM grid and then functionalized by 1×10-6 mol L-1 TM precursor in ethanol solution.

3.3 Results and Discussion

In this section, we will go over the theoretical and experimental characterization results and discuss how the different TMs affect the properties of the MoS2. We will first use DFT calculations to help predict the reactivity and coordination of various first series transition metals with MoS2. Then we will measure the XPS, REELs and PL of the TM-MoS2 complexes and try to correlate the trends with classical coordination chemistry theory.

3.3.1 Prediction of Metal-MoS2 complexes via DFT First, a screening experiment was done to evaluate the general trend of the coordination of different transition metals on the MoS2 surface and correlate these trends with the classical coordination chemistry theory to rationalize the properties of chemically functionalized M-MoS2 complexes. Figure 3-1 shows the optimized structure for the semiconductor 2H phase of MoS2 and the coordination of Co on its surface. There are three available coordination sites named here: H, M and S sites. On the H site, the metal is located on the center of the hexagon and bound to 3 S atoms. In the M site, the coordinated metal is located on top of one Mo atom and bound to 3 S atoms, as shown in Figure 3-1B. When the metal is coordinated in the S site it will be directly on top of one sulfur atom and bound just to that atom, as case of Au atoms discussed in a previous work of our group.(12) Table 3-1 summarizes the results for all 3d transition metal (TM)-MoS2 coordination complexes. The Mo site is most common coordination site for 3d TMs, with the exception for Sc and Co that absorb at the H sites. All the 3d TMs show negative adsorption energy (Eads), showing that the coordination process is exothermic and thermally favorable. To observe a clear trend, we plotted the Eads modulus as function of the number of electrons in the 3d orbitals. Figure 3-1C shows a clear trend of the adsorption energy that resembles the one observed for the crystal field stabilization energy and the hydration enthalpy for weak-field 3d TM complexes. Closed shell (d10) and the half shell (d5) have smaller adsorption energy due the spherical symmetry of this electronic configuration. For other configurations, the non-symmetrical electronic

74 distribution, and the degeneracy loss of 3d orbitals leads to a stability gain upon the coordination, increasing the adsorption energy. As Ni with a d8 electronic configuration, Figure 3-1C shows the highest adsorption energy due the complete occupancy of the low energy ligand orbitals, same as classical 3d coordination complexes.

Figure 3-1. DFT theoretical analyses of M-MoS2 complexes. Optimized structure pristine MoS2 showing the available coordination sites (A) and for Ni-MoS2 (B). Adsorption energy (C) and metal-sulfur bond length (D) as function of the transition metal electronic configuration for M- MoS2 complexes, where M is a 3d transition metals.

Table 3-1. Calculated adsorption site, adsorption energy and metal MoS2 bond length for all 3d metals

For all the calculations we began with a neutral transition metal and MoS2. Analyzing the charge on the adsorbed metal after the geometry optimization we noticed that metals with a harder acid character (Sc, Ti and V) tend to donate more electrons and stabilize with a higher positive charge. This way, the metal-sulfur interaction should have some ionic character. This is related to the small ionization energy of these metals which leads to greater tendency to lose the 4s and 3d electron leaving the atom with a closed shell and higher positive charges. For Zn, considering its hard acid character, the presence of a close shell orbital and the increase in the effective nuclear charge make it less prone to lose electrons to decreasing the ionic contribution to the bond energy, as can be seen by the low adsorption energy, small charge on Zn, and the long Zn-S and Zn-Mo distance. In contrast, for softer metals, as Co and Ni, there is a tendency to form stronger covalent bonds, increasing the charge delocalization and decreasing the charge on the metal. Since sulfur atoms have a soft base character the interaction with soft metals have a higher adsorption energy due to its more covalent character, as can be seen by the high Eads for Co and Ni on Figure 3-1C and Table 3-1. Looking at the M-S bond length in Table 3-1 and Figure 3-1D, the same trend is observed, atoms with closed and half shell electronic configuration have the longest M-S bond, and the others TMs follow the ligand-field stabilization energy profile for weak-field coordination complexes, with some deviations that could be related to the different TM-Mo interactions.

In the electronic band structure, we noticed that all the 3d TM functionalized MoS2 remained a direct gap semiconductor. However, the Fermi level energy, the states near the Fermi level and the spin polarization of these states were drastically affected by the TM’s coordination functionalization(Figure 3-2). As shown in Figure 3-2b, the adsorption of Co atoms creates new states near the Fermi level localized on the Co with different spin polarization, which could be used to control the materials’ magnetic properties. For Ni atoms (Figure 3-2C), the new levels localized near the Fermi level could be accessed electrochemically, improving the efficiency of electrocatalysis. The coordination of Fe atoms leads to the creation of localized states above the Fermi level and near the conduction band (Figure 3-2A) that can modify the linear and non-linear optical properties of MoS2 flakes. In this way, the chemical, optical and magnetic properties of TMDs flakes can be tuned by the carefully choosing the coordinated TM. Another important fact is that these controlled changes can be performed with minimal changes on crystalline structure, as shown in Figure 3-1B where there is no significant alteration of the Mo-S bond length and Mo- S-Mo bond angle after TM functionalization. This is a clear advantage in comparison with other types of doping strategies, such controlling the defects density or interstitial/substitutional doping

76 where high doping levels cannot be achieved without compromising materials stability and mechanical properties.(15) Based on the DFT calculations and predictions we preformed the functionalization of MoS2 samples with 3d TM such as Cr, Mn, Co, Ni, and Zn to correlate the trends on Eads calculated by DFT, with the photoluminescence, XPS and REEL spectra.

Figure 3-2. Electronic Band Dispersion for (A) Fe functionalized MoS2, (B) Co functionalized MoS2, (C) Ni functionalized MoS2 and (D) pristine MoS2 .The blue curves are MoS2 states and the red curve localized states of the metal. The TM-MoS2 remains a direct bandgap semiconductor, yet the states near the Fermi level are drastically changed after functionalization.

3.3.2 Experimental characterization of Metal-MoS2 coordination complexes

TM coordination on MoS2 was performed by exploring straight forward acid-base reaction between a Lewis acid, the TM precursor, and a Lewis base, the MoS2. During the reaction, sulfur’s 3p valence electrons are donated to TM’s 3d valence orbital, forming a MoS2-

TM coordination bond. The functionalizations were performed on CVD grown MoS2 monolayers as described in the previous section. The flakes were transferred to a new Si/SiO2 substrate to remove the excess Na salt and MoO3 used in the growth process. The formation of MoS2-TM complexes were performed by dipping the Si/SiO2 substrate with the CVD MoS2 into ethanol solution of Cr, Mn, Fe, Co, Ni, Cu and Zn salts, with concentrations ranging between 10 - 3 and 10 - 10 mol L-1 . TM chlorides, acetates and sulfates were chosen since they form labile complexes with TM in aqueous and ethanol solutions, favoring ligand exchange reactions on

TM’s coordination sphere. To confirm the TM presence on the MoS2 surface, XPS measurements

-4 -1 were conducted on MoS2-TM samples functionalized in a TM solution of 1x10 molL . Figure 3-4 depicts the XPS results and indicates that TM functionalization was effective, since we observed shifts on Mo 3d and S 2p core levels. The high-resolution spectra in specific regions of

77 each TM (Figure 3-3) shows the presence of the studied metals, and the chemical shift for each one can be related to its chemical environment and the chemical state. To determine the oxidation state of each TM, the core level spectra were compared with known species referent to each TM as shown in Figure 3-3. For Cr, its 2p core level spectra are clearly comparable that of Cr2O3.(16) Cr 2p orbit presents a multi-plet splitting with binding

3+ 3+ energy of 576.7 eV for Cr 2p3/2 and 586.5 eV for Cr 2p1/2 and peak separation of 9.8 eV

3+ (Figure 3-3a), indicating the presence of Cr on the MoS2 surface. For Mn, the presence of a satellite peak indicates an Mn+2 oxidation state and the spectra is comparable to that of MnO, with peaks at 641.2 and 653.2 eV for Mn 2p3/2 and 2p1/2, with slightly red shift of 0.2 eV (Figure 3-

3b).(16) Co 2p3/2 781.1 and 2p1/2 797.1 eV peaks presented a blueshift of 1.4 eV and is comparable to that of CoO, with split spin-orbit of 16.0 eV, and characteristics “satellites”, confirming the

2+ presence of Co .(16) For Ni, we observed peak at 856.0 and 873.6 eV, assigned to 2p3/2 and 2p1/2, receptively. The Ni 2p core level spectra are comparable to Ni(OH)2, with great blueshift and split spin-orbit of 17.6 eV, although Ni 2p3/2 peak seems to be asymmetric and comparable to

2+ NiO, indicating a mixture of species, with Ni state.(16) For Cu the XPS spectra shows a mixture

2+ 1+ of Cu and Cu , since there is a satellite peak (938.0 to 947.1 eV) between the Cu 2p1/2 (952.7 eV) and 2p3/2 (932.9 eV) peaks, although this peak is broad and may have contribution of both Cu2+ and Cu1+ species.(16) Thus, Cu 2p core level XPS spectra indicate a reduction of Cu2+ to

1+ Cu during the MoS2 functionalization. Further results indicate that this reduction takes place spontaneously on MoS2 surface.(see below for details). Zn spectra has peaks at 1022.4 and 1045.4

2+ eV which are assigned to Zn 2p3/2 and 2p1/2, indicating a Zn oxidation state. It is in good agreement with that of ZnO showing on shifts. For all TMs studied used here, their oxidation states remained unchanged before and after the functionalization, with an exception for Cu, where a reduction on the oxidation state was observed. The shifts observed on the TM XPS spectra can be explained by the charge transfers between the TM and the MoS2. Figure 3-4a shows the spectra of Mo 3d core level of pristine and doped MoS2. The deconvolution of pristine MoS2 spectrum (Figure 3-4C) shows the

4+ characteristic Mo 3d5/2 peak at 230.0 eV that will be the reference for the further discussion.

When the MoS2 is functionalized with a TM, charge transfer may occur, leading to new charge

4+ densities on Mo and TM atoms, therefore a shift in Mo 3d5/2 peak is expected. In this context, a blueshift, a shift to higher energies, indicates a charge transfer from Mo to the TM, leading to a more negative character on the TM and a more positive character (p-type doping) on MoS2, increasing the binding energy of Mo electrons. Moreover, a redshift, a shift for lower energies,

78 indicates a charge transference from TM to Mo, leading to a more positive character in TM and a more negative character (n-type doping) in MoS2, thus decreasing the binding energy of its electrons.

Figure 3-3. XPS spectra of TM 3d orbit after functionalization. (A) Cr 2p orbit of Cr-MoS2. The 3+ black curve is the original spectra after C1s calibration. The blue curves are fitted to Cr 2p1/2 and 2p3/2 peaks and they match perfectly with the original spectra. (B) Mn 2p orbit of Mn-MoS2. The 2+ blue curves show the fitted Mn peaks. In addition to the fitted Mn 2p1/2 and 2p3/2 peaks, a Mn satellite peak is also visible in the green curve. (C) and (D)Co and Ni 2p orbit of Co-MoS2 and 2+ 2+ Ni-MoS2. Similar satellite peaks could be fitted in addition to the Co and Ni peaks. (E) Cu 2p 1+ 2+ orbit of Cu-MOS2. Here the spectra can be fitted into 3 groups. The Cu 2p peaks in red, the Cu 2p peaks in blue and the Cu2+ satellite peaks in green. The emergence of the Cu1+ peaks indicates 2+ a reduction reaction between the Cu precursor and MoS2. Similar to that of Au and MoS2. (F)Zn 2+ 2p orbit of Zn-MoS2 showing a perfect fit of Zn 2p1/2, 2p3/2 and the original spectrum.

Figures 3-4A and D show the charge transfer behavior of 3d TM between MoS2 during functionalization. For Cr and Mn, we observe an increase in the binding energy of Mo4+ 3d electrons. For Co and Ni, a decrease was observed with ΔBE of -0.3 and -0.4 eV, respectively, compared to the pristine MoS2 peaks. For Zn, no considerable shift is observed on Mo 3d peaks. Figure 3-4B shows S 2p core level spectra, indicating the same behavior observed in the Mo 3d spectra for all 3d TM studied here. This is expected, since the top of the valence and the bottom of conduction bands in the MoS2 show a high degree of mixing of Mo and S atomic orbitals. Therefore, both S and Mo core level spectra should exhibit similar shifts upon TM coordination.

4+ The Mo 3d core level spectra for the MoS2-Cu samples indicates the oxidation of Mo to higher oxidation states. The deconvolution of Mo 3d spectra in Figure 3-4C points to the presence

79 of Mo5+ peaks at 231.1 and 234.2 eV. Thus, there is a spontaneous oxidation of Mo4+ to Mo5+

2+ 1+ followed by the reduction of Cu to Cu on MoS2-Cu samples, as indicated by the Cu 2p spectra.

In Figure 3D the deconvolution of Mo 3d spectra of pristine, MoS2-Cr, and MoS2-Ni samples are also presented, showing that the oxidation of Mo4+ is observed only for Cu samples. The presence of Mo6+ in some samples is originated from the synthesis and it is present in the pristine and functionalized samples. The XPS results agrees well with the PL measurements of the TM functionalized MOS2. We see a significant decrease in the exciton to trion ratio of MoS2, matching that of n-type doping of MoS2, when it is functionalized with Ni and Co, In contrast, we see an increase in the exciton to trion ratio in the case of Cu functionalization and very little change in the case of Zn and Mn. The details of PL tuning are described below in section 3.3 and Figures 3- 7 and 3-8.

Figure 3-4. X-ray photoelectron spectroscopy studies of M-MoS2 complexes. (A)XPS spectra of Mo 3d orbit on pristine and doped MoS2. The curves are the original spectrum after C1s (248.8 eV) calibration. (B) S 2p orbit of pristine and M functionalized MoS2, showing the S 2p1/2 and 2p3/2 peaks. (C) Deconvolution of Mo 3d spectra for pristine, Cr-MoS2, Ni-MoS2 and Cu- 4+ 5+ MoS2.The blue curves are the Mo 3d5/2 and 3d3/2, purple curves are the. Mo 3d5/2 and 3d3/2 6+ peaks and the red curves are the Mo 3d5/2 and 3d3/2 peaks. The green curves are the S 2s peak. (D) Binding energy shift for S 2p3/2 (blue curve) and Mo 3d5/2 peak as function of M 3d electronic distribution for M-MoS2 samples.

According to Pearson acid-base hard-soft theory, when the acid’s LUMO level is closest to the base’s HOMO level, the acid-base coordination bond has a more covalent character and charge transference may occur between the species’ frontier orbitals. On the other hand, when the acid’s and base’s valence orbitals have a large energy difference the interaction between them has a more ionic character. In the case of MoS2-TM complexes we can estimate the TM LUMO by its electron affinity and the base’s HOMO level by the top of MoS2 valence band (VB). Figure 3-5 shows the energy level diagram created using the HOMO and LUMO orbitals of each TM, estimated by their ionization potential and electron affinity of neutral TM, the TM2+/TM1+ redox potentials, and the absolute energies of MoS2 valence (VB) and conduction (CB) bands.

Figure 3-5: Energy Level Diagram for 3d transition metals and MoS2 valance band and conduction band.

MoS2 is considered a soft base, that will interact better with a soft-acid due to the small energy difference between VB and CB. When the TM LUMO is closer to MoS2 VB, this leads to a more covalent bond and a higher probability of a charge transference from MoS2 to the TM.

This behavior can be observed for Cr, Mn, Co, and Ni. This indicates the formation a MoS2-TM coordination complexes with strong sigma covalent bonds, where S acts as a sigma donor ligand. Mn has an intermediary acid character (Absolute Hardness – η = 3.72) and induces a decrease in the electron density of MoS2, shifting the Mo 3d peaks towards higher energies. On the other hand, Cr is a softer acid (η = 3.05) and tends to form a more covalent bond with S atoms and the electrons on MoS2-Cr bond should be more delocalized. Additionally, S can act as p donor ligand, increasing the charge transfer to the TM and the observed blueshift on the XPS Mo spectra. Meanwhile, Co and Ni show a more donating character that can be attributed to their higher HOMO values. Co and Ni are acids with intermediary hardness (η of 3.6 and 3.2, respectively) and tend to form strong covalent bonds with S ligands due to their high ligand-field stabilization energy. Due to their filled/half-filled 3d orbitals, Co and Ni complexes are known to form p backbonds with soft ligands. In the MoS2-Ni and MoS2-Co cases, there is a sigma donation from

S to TM and a pi backbonding involving the 3dxy, xz, yz TM orbitals to empty orbitals on MoS2 CB. The redshift observed on the Mo 3d core level indicates that the charge transfer from Ni or Co to the MoS2 due to the back-donation is more efficient.

Zn is the hardest TM in this series (η = 4.72), so there is a poor overlapping with MoS2 orbitals, resulting in no variation of Mo 3d binding energy on the XPS spectra. Additionally, due to Zn electronica configuration of 3d10 Zn complexes do not show ligand-field stabilization

82 energy. This results in poor interaction with softer ligands, such as MoS2. For MoS2-Cu complexes, by comparing Cu HOMO-LUMO levels with MoS2 VB-CB energies, we expected a more covalent interaction. However, due to the Jahn-Teller effect , a smaller Eads is shown in Table 3-1. Looking to the TM redox potential, Cu has the more negative value. Thus, electrons on

2+ 1+ 3+ MoS2 VB can be transferred to Cu and reducing it to Cu , similar to that of Au reduced to Au1+ in our previous report. Cu1+ do not have ligand-field stabilization energy due to its 3d10 configuration but the absence of Jahn-Teller distortion and its softer character causes the MoS2- Cu bond energy to be higher than that of Cu2+. High-resolution STEM imaging using a high-angle annular dark-field (ADF) detector was conducted to study the atomic structure of the TM atoms on MoS2. The ADF intensity changes depending on the Z-number of atoms (~Z1.6–1.9),(17) thus the higher Z-number Pt and Ni atoms stand out in the MoS2 lattice. As shown in Fig. 3-6A and B, bright single Pt and Ni atoms on top of the MoS2 lattice can be observed, similar to our previously published results on AuClx functionalized MoS2. The imaging of other transition metals studied in this chapter such as Co and Cr were also imaged but due to their low Z number, we were not able to identify these atoms on top of the MoS2 lattice.

Fig. 3-6 STEM images of (A) PtCl4 functionalized MoS2 and (B) NiCl2 factionalized MoS2.

3.3.3 Photoluminescence tuning through TM coordination

MoS2 is a layered semiconductor that shows the transition from indirect to direct bandgap when it reaches a monolayer. Optically generated electron-hole pairs in monolayer form stable exciton states even at room temperature because of the extremely large Coulomb interactions in

83 atomically thin 2D materials. The neutral exciton plays an important role in the optical properties of monolayer. Figure 3-7A shows the PL spectra for a typical monolayer with the main excitonic peak at 1.83 eV (exciton A) related to the direct bandgap transition at K and a small shoulder at 2.0 eV (exciton B) related to the transition at K’ point with its energy being related to the level splinting due spin-orbit coupling. Raman spectra of this sample is shown in Figure 2-5. To study the effect of the TM coordination functionalizations on the optical properties of MoS2, PL spectroscopy measurements were carried out for TM-MoS2 complexes at various TM concentrations ranging from 1x10-9 M/L to 1x10-6 M/L. Figure 3-7A shows the PL spectra of pristine and Ni functionalized MoS2 monolayers. The PL intensity and shape of the A exciton is significantly altered after Ni functionalization and changes as a function of TM concentration. To understand these spectral changes, The PL peak can be deconvoluted and fitted into two Lorentzians, the neutral exciton(X) and the trion (X-). It can also be seen that the X- trion was enhanced while the neutral exciton X intensity was quenched after the Ni functionalization. The formation of the coordination pi backbonds between the S and Ni atoms results in Ni electrons

- being transferred to the MoS2, thus leading to an increase in the X trion intensity and an decrease in the neutral exciton intensity in MoS2. This results in an overall quenching and widening of the

PL spectra and is a result of n doping MoS2, which agrees well with the XPS and DFT data above and published results.(18) The exact opposite effect is seen for Cu functionalization as shown in Figure 3-7G. After Cu functionalization, we note an increase in exciton intensity and a decrease in trion intensity, similar to that of Au functionalization, resulting from p-type doping of

MoS2.(12) Figure 3-7 further shows the exciton to trion ratio for other TM functionalized MoS2. After TM functionalization, some metal ions such as, Cr, Co and Ni significantly quenches the neutral exciton and enhances the trion intensity. For Mn and Zn, we observe only a small quenching effect in the PL which can be explained by their half shell and full shell 3d configuration. And in the case of Cu, we see a small increase in the exciton to trion ratio.

Figure 3-7 PL spectra analysis of TM functionalized MoS2. (A) Photoluminescence spectrum of - pristine and functionalized MoS2 monolayers. The A exciton is deconvoluted into the trion (X ) (blue curve) and exciton (X) (red curve) peaks through Lorentzian functions. After functionalization, the trion intensity increased while the exciton intensity decreased, which corresponds well with the n-type doping effect of the Ni functionalization. (B) Photoluminescence spectrum of pristine and Cu functionalized MoS2 monolayers. After functionalization, the trion intensity decreased while the exciton intensity increased, which corresponds well with the p-type doping effect of the Cu functionalization. (B-D and F-H) Exciton to trion intensity ratio of pristine and functionalized MoS2 for different TM precursor: Cr (B), Mn (C), Co (D), Ni (F), Cu (G) and Zn (H) at different concentrations.

Figure 3-8. Normalized exciton to trion intensity changes showing the percent change in exciton to trion ratio for various TM functionalized MoS2 samples at different TM concentrations. The TMs here can be separated into 3 groups. One is Cu, which is showing an increase to the exciton to trion ratio, indicating p-type doping on the MoS2. The second group is Zn, Mn and Cr where we see a small decrease in the ratio, indicating weak n-type doping of MoS2. The last group is Co and Ni where we see a large decrease in the ratio that corresponds to strong n-type doping of MoS2

To further verify the trends in PL spectra as function of functionalization with the different metals, we plotted the normalized exciton to trion ratio percentage change as function 3d metal concentration, as shown in Figure 3-8. A high PL quench, like that induced by Ni coordination, was also observed for Cr and Co doping. In contrast, for low concentrations(1x10- 9M-1x10-7M) of Zn practically no PL changes were observed, we notice just a small intensity (less than 20% change) decrease for Zn and Mn functionalizations, probably due salt deposition the surface. These results show a remarkable resemblance to the trend of theoretical adsorption energy shown in Table 3-1. As expected, Mn and Zn showed the smallest effect due the half and closed shell configuration, respectively, this leading to a lower stabilization energy for the 3d orbitals upon their coordination to the sulfur atoms on the surface. In contrast, Ni and Co showed the highest effect on the PL spectra, both in terms of intensity and exciton/trion populations. These results suggest that the effects of Ni coordination on were maximized by the formation of a stronger covalent Ni-S bound, which increases the charge delocalization and the possibility of

86 ligand-metal charge transfers. It is also worth noting that the change in PL is of positive correlation between the concentration of TM ions in all cases, regardless of n or p doping types.

3.4 Conclusion and Outlook

In this work, we successfully prepared TM-MoS2 coordination complexes for 3d transition metals Cr, Mn, Co, Ni, Cu, Zn and Pt and demonstrated that Pt and Ni form single atoms that are bonded to S atoms via coordination bonds. This novel approach does not rely on defects and yet implies significant impact on the optical and electrical properties of the functionalized monolayer MoS2. The formation of the coordination complexes led to the transfer of electrons between MoS2 and the transition metal, which can introduce both n- and p-type doping to MoS2 depending on the transition metal used. Moreover, the degree of n and p-type doping can be fine-tuned by choosing the transition metal and varying the transition metal precursor concentrations, thus controlling electronic band structure of MoS2 as well as the exciton to trion relative population. In addition, the properties of the TM functionalized MoS2 displays a trend based on the 3d electron configuration of the transition metal that matches the periodic trend of well-studied coordination compounds. This trend can serve as a guide for future chemical functionalizations of monolayer TMD materials.

To further study the electronic properties of TM-MoS2 complexes, REELS measurements will be measured to give us band gap values for these systems. By Tauc-Plot method, one can observe that TM functionalization does not change the direct band-gap character, indicated by the linear regression on the first transition presented. Figure 3-9 shows preliminary REELS data and band-gap variation for the doping species related to pristine MoS2. As we can see, all TMs remained a direct bandgap semiconductor, matching that of our PL data. In several cases, such as for Cr and Co, the functionalization lowered the bandgap of MoS2, which indicates the creation of new states between VB and CB because this bandgap decrease was not seen in the PL spectra of the MoS2. Additional DFT calculations of the band structure are needed to fully explain this trend. In addition to the coordination trends introduced in this chapter, the synthesis of single atoms introduced could also be exploited in other applications such as single-atom catalysis (SAC), quantum information devices, optoelectronics, and enhanced sensing. More details of proposed experiments and some preliminary data on SAC experiments can be found in chapter 5 of this thesis.

Figure 3-9. Preliminary REELS measurements of TM-MoS2 complexes.

3.5 Acknowledgements

This work was supported by the Air Force Office of Scientific Research (AFOSR) through grant No. FA9550-18-1-0072 and the NSF:I/UCRC ATOMIC program for support (award #1540018). The findings and conclusions of this work do not necessarily reflect the view of the funding agencies.

3.6 References

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Chapter 4 Au Nanoparticle Functionalization of MoS2 for Surface Enhanced Raman Spectroscopy

This chapter is based on a manuscript in preparation that I am the first author and it is reproduced with permission from “Au Nanoparticle Functionalization of MoS2 for Surface Enhanced Raman Spectroscopy”, to be submitted (2021) As discussed in Chapter 1, surface functionalization via nanoparticle functionalization to semiconducting 2D transition metal dichalcogenides (TMDs) offers an effective and controllable approach for tuning the optical properties of MoS2. In this chapter, we will construct heterostructures of Au nanoparticles and MoS2 monolayers which shows extraordinary SERS performance in the detection of R6G and CuPc molecules.

4.1 Introduction

Surface-enhanced Raman spectroscopy (SERS) is a very sensitive and selective analytical technique used to detect molecules adsorbed on specially designed surfaces. (1–4) Although the exact nature of SERS is still under debate, it is widely accepted that SERS comes from two major contributions: the electromagnetic enhancement mechanism(EM) and the chemical mechanism. Jensen et.al has proposed from a theoretical standpoint that four situations can occur when the SERS signal is enhanced. Figure 4-1 summaries the four effects: (1) Ground state chemical interaction between the molecule and substrate. (2) Resonant Raman scattering when the laser excitation energy matches the HOMO-LUMO gap of the molecule. (3) Charge transfer resonance effect when the laser excitation energy matches the substrate to molecule transition. (4) Electrochemical effect when the laser excitation matches the plasmon excitations of the substrate (nanoparticles). The first three cases are grouped together to form the chemical mechanism. Conventional SERS substrates such as roughed metal surfaces and metal nanoparticles mainly utilizes the electromagnetic (EM) field from the localized surface plasmon resonances (SPR) generated at the surface of noble metal nanostructures upon laser excitation to achieve greatly enhanced Raman signals.(5, 6) This is a long-range effect and requires the substrate to be rough. Therefore, traditional SERS substrates rely heavily on the shape and size control of the noble metal nanoparticles and usually lack in stability and uniformity.

Figure 4-1. Theoretical summary of four types of SERS enhancement mechanism. Figure adapted from reference (3) with permission from The Royal Society of Chemistry.

4.1.1 Introduction to 2D materials as SERS substrates

Recently, alternative SERS platforms adopting active and stable materials such as graphene (7–10), molybdenum dichalcogenides MoX2 (X = S, Se) (11–13), have also been studied as possible candidates for SERS effects. The SERS effect of 2D materials such as graphene and MoS2 mainly rely on the chemically enhanced mechanism through a charge transfer effect between the probe molecules and the substrate. This mechanism is normally considered to be much weaker than the EM field effect which results in relatively weaker Raman signals from 2D SERS substrates.(3) Recently, Feng et al. demonstrated that a strong chemical enhancement via an enhanced charge effect can be achieved when the Fermi level of the 2D material aligns with the LUMO of the probe molecule.(14) Among 2D material SERS substrates, transition metal dichalcogenides (MoX2 (X = S, Se)) have started to attracted research attention to their tunable electronic band structure and good biocompatibility. (12, 15) When compared to noble metal nanostructured SERS substrates, 2D materials show higher uniformity and lower noise in the Raman collection process.(16) In addition, the charge transfer mechanism also aids in the quenching of fluorescence of the probe molecules and reduces variation due to photo bleaching. These factors make them ideal SERS substrates in the detection of florescent molecules and complex bio molecules. However, their relatively weak intrinsic SERS effect limits their effectiveness in the detection of trace amounts of molecules.(12, 17)

4.1.2 Introduction to the Au NP and MoS2 heterostructure

The functionalization of TMDs with plasmonic metal components (such as Au nanoparticles) might be a promising strategy and has attracted much research attention in recent years.(18–20) Because of the strong plasmonic effect induced by the metal nanomaterials in the hybrid nanostructures, some properties of the 2D material component, e.g. photocatalysis, PL, and optoelectronics, will be altered and enhanced by the proximity of the metal. To date, hybrid nanostructures of metal nanomaterials/2D materials have been explored in plasmon-enhanced optical signals (21–26) but these works mainly focus on liquid exfoliated or colloidal synthesized

TMD systems. Lee et al. deposited bowtie structured Ag nanoarrays on top of CVD grown MoS2 and found a dramatic enhancement in the PL of the MoS2 which showed that CVD grown MoS2 can also be functionalized with metal nanoparticles for optical signal enhancement.(27) However, in this case the Au nanoparticles were deposited on top of the MoS2 flake and its SERS performance was not investigated.

In this chapter, we have constructed a Au nanoparticle and MoS2 heterostructure (Au NP-

MoS2) where monolayer MoS2 flakes are placed on top of a uniformly aligned Au nanoparticle film. In this heterostructure, we believe both the strong EM field effect from the Au nanoparticles and the charge transfer mechanism from the TMD itself can work constructively to enhance Raman signals. A comprehensive SERS comparison between the Au nanoparticle film, the pristine monolayer MoS2 and the Au NP-MoS2 heterostructure showed that forming the heterostructure greatly enhances the substrate’s effectiveness in the detection of trace amounts of Rhodamine 6G (R6G) and Cupper phthalocyanine (CuPc) molecules. Interestingly, we also observed PL enhancement from the MoS2 on top of the Au nanoparticle film as well as shifts in the absorbance spectra of MoS2 after dye molecule deposition. These results indicate that both the plasmonic EM effect and the charge-transfer CM effect can be activated in our Au NP-MoS2 heterostructure which results in the superior SERS performance of forming the heterostructure.

-10 With the Au nanoparticle functionalization, the MoS2 can detect R6G at 1x10 M and CuPc at

-8 1x10 M which is much higher than previously reported values for MoS2.(28, 29)

4.2 Synthesis and characterization of Au nanoparticle-MoS2 heterostructures

In this section, we will go over the detailed methods of how the Au NP-MoS2 heterostructures are synthesized as well as a description of the characterization details. The Au

NP film is synthesized on a SiO2/Si substrate via amino-silane treatment. The silane functionalization forms exposed NH2 groups on the surface and can be used to anchor a uniform layer of Au NPs. The MoS2 monolayers are then transferred on top of the Au NP film to form the heterostructure.

4.2.1 Materials synthesis

4.2.1.1 Synthesis of spherical Au NPs The Au nanoparticles are synthesized using the Turkevich method.(30) For precaution, all the glassware was previous washed with aqua regia (3HCl:1HNO3). 50mL of HAuCl4 aqueous solution (0.01% by weight) is first heated to a slight boil at stirred at 500rpm. Then 0.8mL of sodium citrate solution (1% by weight) is slowly added to the boiling solution. The mixture is kept boiling for another 5min and slowly cooled down to room temperature, yielding a bright red solution.

4.2.1.2 Synthesis of monolayer MoS2 via CVD

MoS2 flakes were prepared by CVD using elemental sulfur and MnO3 as precursors. Both reactants were placed on a Al2O3 boat. The boat with sulfur was put inside a quartz tube near the gas inlet and the boat with MnO3 was put in the middle of the quartz tube. A Si/SiO2 substrate was placed on top of the Al2O3 boat. The substrate was previous cleaned with acetone and IPA ultrasonic bath an oxygen plasma. Perylene-3,4,9,10-tetracarboxylic acid tetra potassium salt(PTAS) was drop-casted on the Si/SiO2 substrate before conventional CVD growth to act as a nucleation agent promoting a seed assisted growth process.(31, 32) The temperature is ramped up to 650C in 20min and the growth time was set to 10 min. During all the process the Ar flux was held at approximately 20 sccm. After the growth the substrate was cleaned in an IPA ultrasonic bath for 10 s and dried in N2 flux.

4.2.1.3 Au NP deposition on SiO2 substrates

The assembly of Au nanoparticle was immobilized on a Silicon oxide surface functionalized with 3-Aminopropyltrimethoxysilane, according to the following procedure. A clean SiO2 (300 nm)/Si substrate was dipped into a solution containing 5% (v/v) of 3-

Aminopropyltrimethoxysilane in CHCl3 for 5 minutes and washed 5 times with ultra-pure water. Then, the amino-silane coated-glass was sintered at 180 °C for 2 hours. After cooling down at room temperature, it was further cleaned with a N2 flux. For the nanoparticle deposition, the amino-silane coated glass was dipped into the solution containing citrate stabilized AuNPs, for 1 h, and dried under a N2 flux.

4.2.1.4 Formation of Au NP-MoS2 heterostructures via wet transfer

After synthesis, the MoS2 monolayers are stacked on top of the Au nanoparticle film via PMMA wet chemical transfer.(33) A thin layer of Poly-methyl methacrylate (PMMA) A6 solution is drop casted onto the CVD grown monolayers and spin coated at 3000rpm for 2 minutes. After spin coating, the sample is heated on a hotplate at 150C for curing. When the film is cured and cooled down, the sample is etched in a NaOH solution at 80C until the PMMA film separates from the original SiO2/Si substrate. The film is then rinsed with deionized water for 3 times via successive water bathes. After rinsing, the film is fished out of the water bath with the new Au nanoparticle film functionalized substrate and dried overnight. Prior to characterization, the prepared AuNP-MoS2 hybrid samples were soaked in acetone to dissolve away the PMMA layer on top of the MoS2. The samples were then rinsed in isopropanol alcohol and immediately dried under a N2 flow.

4.2.2 Materials characterization

4.2.2.1 Raman and PL The Raman and PL measurements were performed with a Renishaw InVia confocal micro Raman spectrometer with 1800/mm grating and with the laser spot size around 1mm. Three laser excitation wavelengths (488nm, 514nm and 633nm) are used to collect the spectra. The 100x objective lens is used to focus the laser beam and collect the spectra. The typical spectrum is collected with 5% power, 10s accumulation time and 1 accumulation. 4.2.2.2 Surface characterization

The surface morphology of the samples was analyzed by field emission scanning electron microscopy (FESEM) using a Zeiss Merlin instrument at 10 kV. The optical and fluorescent images were taken using Carl Zeiss Axio Imager Microscope at 100x objective lens. 4.2.2.3 UV-Vis absorption The UV-Vis absorbance spectra were acquired using a Perkin-Elmer Lambda 950 with a universal reflectance accessory. 4.2.2.4 Reflectance spectroscopy The optical absorption was measured on an iHR320 spectrometer with a 100×objective lens. The substrate was illuminated by Tungsten-Halogen lamp in reflection mode. The absorbance (A) was calculated as A= ln (Isam - Isub) / I0, where Isub and Isam are the light intensity reflected on substrate and sample, respectively, and I0 is the light intensity reflected by a Ag mirror.

4.3 Results and Discussion

In this section, we will go over the detailed test results of the Au-MoS2 heterostructures and discuss a tentative mechanism behind the enhanced SERS effect. We first synthesize monolayer MoS2 via CVD and synthesize spherical Au nanoparticle via the Turkevich method.

Monolayer MoS2 is then transferred onto the Au NP film via wet chemical transfer methods. The

SERS capabilities of the Au NP-MoS2 heterostructure studied and compared with pristine MoS2 and pristine Au NP films.

4.3.1 Construction of Au nanoparticle and MoS2 heterostructures

The process to stack the Au NP-MoS2 heterostructure on Si/SiO2 substrates is shown in

Figure 4-2. Au nanoparticles is first prepared via the Turkevich method where HAuCl4 aqueous solution was mixed with sodium citrate solution at boiling.(30) A SiO2/Si substrate is then treated with Amino-propyl trimethoxy silane to attach exposed -NH2 groups on the surface, which can act as anchoring points for Au nanoparticles. The silane treated SiO2/Si substrate is soaked in the Au nanoparticle solution to form a uniform layer of Au nanoparticles. (Figure 4-2A and Figure 4-

4) Monolayer MoS2 is stacked on top of Au NP functionalized Si/SiO2 via wet chemical transfer(33) to form the heterostructure.(Figure 4-2B) The Monolayer MoS2 samples were prepared via PTAS assisted chemical vapor deposition(CVD) and characterized via Raman and photoluminescence(PL) spectroscopy (Figure 4-3). The two phonon modes E’ and A1’ were found at 382cm-1 and 401 cm-1, respectively. The separation between them are about 19cm-1, confirming its monolayer nature.(34) The PL spectrum is shown in Figure 4-3B, the strong PL at

1.78eV indicates the high quality of the monolayer MoS2 flakes.(35)

Figure 4-2. Schematic of the SiO2 functionalization and wet transfer process to form the Au nanoparticle and MoS2 heterostructure. (A) A clean SiO2/Si substrate is treated with Amino-silane and then soaked in Au NP solution to form a uniform layer of Au NPs on the SiO2 surface. (B) The MoS2 is first grown on SiO2 via CVD and is transferred to the Au nanoparticles functionalized substrate via PMMA transfer.

Figure 4-3. Raman and PL characterization of monolayer MoS2 grown via PTAS assisted CVD. -1 (A) Raman spectrum of monolayer MoS2 excited via 488 nm laser. The E’ mode at 382 cm and -1 the A1’ mode at 401 cm confirm its monolayer nature. (B) Photoluminescence spectrum of monolayer MoS2.

Figure 4-4. (A) SEM image of Au nanoparticle film deposited on top of amino silane treated SiO2/Si substrates. The inset is the (B) UV-VIS absorption spectrum of as synthesized Au nanoparticles in solution

Figure 4-4A shows the surface morphology of the Au nanoparticle film on SiO2 via Scanning electron spectroscopy (SEM). A uniform layer of 20nm spherical nanoparticles are anchored to the SiO2 surface. Though some small aggregates could be found, most of them are horizontally stacked Au nanoparticles and do not affect the overall morphology of the Au nanoparticle film. A size distribution histogram is of 120 NPs is shown in the inset of Figure 4A and the majority of Au NPs are within 20-25nm in diameter. The absorbance of the Au nanoparticles was also characterized via UV-VIS absorption. (Figure 4-4B) The sharp maximum absorption peak at around 528nm further indicates the uniformity of the Au nanoparticles and

98 agrees well with previous reports of spherical nanoparticles.(30, 36) After transfer, the morphology of the Au-NP and MoS2 heterostructure is illustrated in Figure 4-5. Figure 4-5A shows the schematic of Au NP and MoS2 heterostructure. The sample is studied via SEM and be seen in Fig 4-5B. The MoS2 triangles directly sit on top of the Au nanoparticle film. The cracks in the MoS2 flakes are introduced in the transfer process and can be seen in transferred MoS2 on

SiO2 as well. The detailed synthesis and characterization details can be found in section 4.2 of this chapter.

Figure 4-5. (A) Schematic of the Au nanoparticle and MoS2 heterostructure. (B) SEM image of the Au nanoparticle and MoS2 heterostructure.

4.3.2 SERS studies of Au NP film and MoS2 monolayers

To study the SERS effect, different fluorescent dyes such as R6G and CuPc were used as probe molecules. The dyes were dissolved in ethanol to obtain a solution ranging from 1x10-5M

-10 to 1x10 M. The Au nanoparticle and MoS2 heterostructure was soaked into each solution for

30min, after which the samples were rinsed with ethanol and dried under N2 purge. We first study the Raman spectroscopy of the R6G dye at a high concentration of 1x10-5M under the resonant

532nm laser excitation.(37) Figure 4-6A shows the SERS performance of pristine MoS2, pristine

Au nanoparticle film and the Au nanoparticle and MoS2 heterostructure. All three substrates are capable of generating the signature R6G Raman modes at around 613cm-1, 1364cm-1, 1577 cm-1 and 1651cm-1 which can be assigned to the in plane xanthene ring deformations, the in plane C-H

99 bend, the in plane N-H bend and the xanthene ring stretch, respectfully, of the R6G molecule.(37) Though all three samples showed the signature R6G peaks, it can be clearly seen that the relative peak intensities are much higher for the Au-MoS2 heterostructure. We then decreased the R6G concentration to 1x10-8M and the results are shown in Figure 4-6B. At this relatively lower concentration, it can be clearly seen that only the Au nanoparticle and MoS2 heterostructure can generate the signature R6G bands while the pristine MoS2 and pristine Au nanoparticles show no bands at all. This shows the superior SERS capabilities of forming the Au nanoparticle and MoS2 heterostructure as the heterostructure completely outperforms its individual components. Similar effects can also be found for CuPc where the Au-MoS2 heterostructure can detect CuPc at low

-8 -5 concentrations of 1x10 M and pristine Au NPs and MoS2 can hardly detect CuPc at 1x10 M. More details can be found in the next section when we address the detection limits of R6G and CuPc.

Figure 4-6. Raman performance comparison between Au NP and MoS2 heterostructure with pristine MoS2 and pristine Au NPs using the R6G dye as probe molecule. (A)The Raman spectra collected from the three substrates with a R6G concentration of 1x10-5M. The R6G modes at 613cm-1, 1364cm-1, 1577 cm-1 and 1651cm-1 could be clearly identified on all three substrates, yet the peaks are much sharper for the Au NP and MoS2 heterostructure. (B) Raman spectra collected -8 with a R6G concentration of 1x10 M. The Raman modes disappear for the pristine MoS2 flake and the Au NPs. The R6G modes remain clearly visible for the heterostructure.

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4.3.3 Ultra low detection of R6G and CuPc molecules via Au NP-MoS2 heterostructures

We have shown experimentally that forming the Au-MoS2 heterostructure can significantly enhance the SERS performance of MoS2. Here we look at the detection limits which can be reached using this SERS substrate in the detection of dye molecules R6G and CuPc.

Figure 4-7A shows the Raman spectra of R6G on top of Au-MoS2 with concentrations ranging from 1x10-8M to 1x10-10M. The R6G Raman modes at 1364cm-1, 1512cm-1, 1577cm-1 and 1651 cm-1 (37) are still clearly seen even when the R6G concentration is as low as 1x10-10M. Figure 4-

-8 7B shows the Raman spectra of CuPc on Au-MoS2 substrates with concentrations of 1x10 M and 1x10-9M. The CuPc modes are also detectable at a low concentration of 1x10-8M. The detection limits reported here is significantly lower than previously reported values for pristine MoS2.

Table 4-1 summaries the recent reports of using MoS2 based SERS substrates for the detection of R6G and CuPc molecules. Our detection limits are significantly lower than previously reported values for pristine MoS2 and thus offers an effective way to enhance the SERS capabilities of monolayer MoS2.

Figure 4-7. Detection limit studies of the Au-MoS2 heterostructure as SERS substrates. (A) -8 Raman spectra of R6G deposited on Au-MoS2 substrates at low concentrations of 1x10 M to 1x10-10M. The signature R6G bands would be identified in at all three concentrations, yet we see a clear decrease in intensity as the concentration of R6G decreases. The lowest detectable concentration for R6G is 1x10-10M. (B) Raman spectra of CuPc at 1x10-8M and 1x10-9M on the -8 Au NP-MoS2 substrate. The signature CuPc peaks would still be clearly identified at 1x10 M but becomes too low to clearly recognize at 1x10-9M.

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Au NP-MoS2 This 1x10-10 mol/L 2.41 heterostructure work Mechanically 1x10-6 mol/L 2.33 (28) R6G 2.3(7) exfoliated MoS2 Sonication 1x10-6 mol/L 2.33 (29) exfoliated MoS2

-8 1T phase MoS2 1x10 mol/L 2.33 (29)

Au NP-MoS2 This 1x10-8 mol/L 1.96 CuPc 1.7(38) heterostructure work

CVD grown MoS2 4Å 1.96 (17)

4.3.4 Possible mechanism studies of AuNP-MoS2 heterostructures

To further study the mechanism behind the superior SERS performance of the Au-MoS2 heterostructures. Raman, PL, and Absorbance spectra of the Au-MoS2 heterostructure (with dye molecules) is conducted. Figure 4-8A shows the Raman spectra comparison of MoS2, MoS2+dyes and Au-MoS2+dyes. We neither observe a change in the peak positions of the MoS2 E’ and A1’ modes nor in their relative intensity when the MoS2 is transferred on top of the Au nanoparticle film. The indicates that forming the Au-MoS2 heterostructure does not change the intrinsic properties and structure of the MoS2.(34) We also observe no change in the Raman modes when the dye molecules are deposited on the Au-MoS2 heterostructure. The superior SERS effect from the Au NP-MoS2 heterostructure could be explained by the combined effect of the EM from the

Au NPs and the CM charge transfer from the MoS2. The EM plasmonic effect of Au NPs comes from the strong local electric field generated when the excitation wavelength is resonant with the

102 plasmon excitations in the metal nanoparticle. (3) Figure 4-8B shows the PL spectra of the same

MoS2 flake before and after forming the heterostructure with Au nanoparticle film. We note that there is a large enhancement in the PL emission intensity of the MoS2 on top of the Au nanoparticles which can be explained by the electromagnetic enhancement mechanism of the plasmonic Au nanoparticles and agrees well with previous reports. (27) The plasmon excitations in the metal nanoparticles can produce a strong local electric field which enhances the intensity of the PL emission light. This PL increase and the intrinsic SERS performance of the Au NP film shown in Figure 4-7A confirms that the plasmonic EM effect is in fact utilized in the Au-MoS2 heterostructure and it is partially responsible for the SERS effect we observe in the detection of dye molecules.

Figure 4-8. (A) Raman spectra comparison of pristine MoS2, Au-MoS2 heterostructure and Au- MoS2+dyes. The peak position of MoS2 modes E’ and A1’ did not change when the MoS2 was transferred on top of the Au nanoparticle film as well as when dye molecules were deposited on top of the MoS2. The relative peak intensity(A1’/E’) did not change. The peak intensity relative to Si peak changes slightly due to not being able to measure on same flake. (B) PL spectrum of the same MoS2 flake before and after being transferred to the Au nanoparticle film. The shift in the exciton emission is likely due to the strain release in the transfer process.(39) There is a large enhancement in the PL intensity due to the EM enhancement due to the plasmonic effect of the Au nanoparticles.

The CM effect of the MoS2 can be explained via the charge transfer effect between the

MoS2 Fermi level (EF=4.5eV) and the HOMO of both dye molecules.(EHOMO,R6G=-5.7eV and

EHOMO,CuPc=-5.3eV).(29, 40) The energy diagram is shown in Figure 4-9. Since the Valance band energy of MoS2 is close to the HOMO of both dyes, the charge transfer from VB to the HOMO may occur during the Raman spectroscopy process. However, due to the requirement of additional energy, this charge transfer effect may not be very strong which results in the rather

103 weak SERS effect of pristine MoS2. Figure 4-10 shows the absorbance spectra of MoS2 and Au-

MoS2 heterostructures before and after dye deposition. The absorbance (A) was calculated as A= ln [(Isam - Isub) / I0], where Isub and Isam are the light intensity reflected on substrate and sample, respectively, and I0 is the light intensity reflected by a Ag mirror. The red curve is R6G deposited directly on SiO2. Due to the poor absorption of the dyes on SiO2 and the lack of interaction between SiO2 and the molecules, it’s absorption peaks in the visible range is barely observed on

SiO2. In contrast, upon contact with MoS2 and AuNP-MoS2, the R6G exhibit its absorbance peak at the dashed line around 570nm, where a clear shoulder could be seen on the Au-MoS2 sample with R6G. We can also observe a small shift in the absorbance spectra peaks of the MoS2 (exciton A and exciton B) towards lower energies after the deposition of R6G. Though direct charge transfer between the dye and MoS2 is not observed, energy diagram and the absorption spectra agrees with past reports on the charge transfer mechanism of MoS2 and graphene with probe molecules which introduces chemical enhancement to the SERS signals.(12, 41) Figure 4-10B also shows the same measurement done for CuPc. However, because the CuPc absorbance peak falls in the same range as the MoS2 absorbance peaks, we only see a broad peak after CuPc deposition.

Figure 4-9. Energy diagram of dye molecules and MoS2 illustrating the possible charge transfer process between the Fermi level of MoS2 to the HOMO level of R6G and CuPc. The HOMO and LUMO gap of R6G is shown in red and CuPc in blue. The band structure of monolayer 2H MoS2 is shown in green. The values extracted from refs (11, 29, 42).

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Figure 4-10. The Absorbance spectra of MoS2 and Au-MoS2 when deposited with dye molecules. (A) The absorbance spectra of R6G deposited on MoS2 and Au-MoS2 substrates. The red curve is the R6G molecule on SiO2. The light green and olive colored curves are pristine MoS2 and MoS2+R6G. The light blue and navy curves are Au-MoS2 and Au-MoS2+R6G. The dash line is the max absorbance of the R6G. A clear shoulder could be seen on the Au-MoS2+R6G sample. (B) The absorbance spectra of CuPc on MoS2 and Au-MoS2 substrates.

4.4 Conclusion and Outlook

We have demonstrated that it is possible to construct heterostructures of Au NP film and monolayer MoS2 flakes. This heterostructure was further studied as SERS substrates to probe dye molecules R6G and CuPc. We found that the Au NP-MoS2 heterostructure exhibited extraordinary sensing properties when compared to both Au NP film and pristine MoS2 flakes. The detection limit for R6G under resonant laser excitation can be as low as 1x10-10M, which is significantly lower than previously reported values for TMD materials. We observed a PL enhancement in the excitonic emission of MoS2 after forming the heterostructure with Au nanoparticles which confirms the plasmonic EM effect of the Au nanoparticles. We also observed shifts in the MoS2 absorbance spectra after dye deposition which can be corelated to the charge transfer between the dye molecules and MoS2. This indicates a possible mechanism where both the EM effect of the plasmonic Au nanoparticles and the CM charge transfer effect of the MoS2 are in effect during the SERS process We believe the formation of the Au-NP heterostructure can effectively utilize both SERS mechanisms which results in its superior SERS performance. These results open new avenues for developing SERS substrates and the Au NP-TMD system can be

105 further exploited in the detection of other molecules and even biological samples due to the high bio combability of TMD surfaces. However, we do admit the complexity of SERS on 2D materials and believe our proposed mechanism could be one of many possibilities. Though the plasmonic effect of the Au NPs have been well studied and understood, the CM SERS effect of TMDs remains controversial. We are currently in collaboration with the Jensen group to further study the mechanism in more detail. We believe more theoretical calculation results and direct experimental evidence of charge transfer via pump probe measurements is needed to fully understand the interactions between MoS2 and the dye molecules in terms of knowing whether the CM charge transfer effect is the only effect happening on the surface of the 2D MoS2. More experiment carried out with varying metal NP types, sizes and shapes as well as other TMDs including semiconducting WS2, WSe2 and metallic NbS2, 1T’-MoS2 will also be done to further confirm the mechanism. For future work, one could also put spacers such as the insulating hBN in between the Au

NP film and the MoS2 on top. This could effectively tune the plasmonic field on the surface of the

MoS2 and fine tune the SERS effect of the heterostructure. Another way is to use Au NPs of different sizes and shapes for the detection of different molecules. In this work, our 20nm Au spheres have a max absorption at 528nm, and this can be excited with the 514nm laser effectively. However, for the 785nm laser, the excitation is expected to be much worse. Therefore, to detect biomolecules using the 785nm laser, larger nanoparticles with a max absorption in the ~700nm range would be more favorable. In addition, the doping and functionalization of TMD materials could effectively shift its Fermi level which could also affect the SERS performances of the material.(43, 44) Feng et al. have shown that N doping of graphene causes the Fermi level of graphene to align better with dye molecules. We believe similar phenomenon should also work on doped and functionalized semiconducting TMD materials and could be studied in the future.

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4.5 Acknowledgements

This work was supported by the the NSF:I/UCRC ATOMIC program for support (award #1540018). The findings and conclusions of this work do not necessarily reflect the view of the funding agencies.

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32. X. Ling, Y. H. Lee, Y. Lin, W. Fang, L. Yu, M. S. Dresselhaus, J. Kong, Role of the seeding promoter in MoS2 growth by chemical vapor deposition. Nano Lett. (2014), doi:10.1021/nl4033704. 33. A. Gurarslan, Y. Yu, L. Su, Y. Yu, F. Suarez, S. Yao, Y. Zhu, M. Ozturk, Y. Zhang, L. Cao, Surface-energy-assisted perfect transfer of centimeter-scale monolayer and few-layer MoS2 films onto arbitrary substrates. ACS Nano (2014), doi:10.1021/nn5057673. 34. H. Li, Q. Zhang, C. C. R. Yap, B. K. Tay, T. H. T. Edwin, A. Olivier, D. Baillargeat, From bulk to monolayer MoS 2: Evolution of Raman scattering. Adv. Funct. Mater. (2012), doi:10.1002/adfm.201102111. 35. K. F. Mak, C. Lee, J. Hone, J. Shan, T. F. Heinz, Atomically Thin ${MoS}_{2}$ : A New Direct-Gap Semiconductor. Phys. Rev. Lett. (2010). 36. M. A. Uppal, A. Kafizas, M. B. Ewing, I. P. Parkin, The effect of initiation method on the size, monodispersity and shape of gold nanoparticles formed by the Turkevich method. New J. Chem. (2010), doi:10.1039/c0nj00505c. 37. L. Jensen, G. C. Schatz, Resonance Raman scattering of rhodamine 6G as calculated using time-dependent density functional theory. J. Phys. Chem. A (2006), doi:10.1021/jp0610867. 38. S. Huang, X. Ling, L. Liang, Y. Song, W. Fang, J. Zhang, J. Kong, V. Meunier, M. S. Dresselhaus, Molecular selectivity of graphene-enhanced raman scattering. Nano Lett. (2015), doi:10.1021/nl5045988. 39. H. J. Conley, B. Wang, J. I. Ziegler, R. F. Haglund, S. T. Pantelides, K. I. Bolotin, Bandgap engineering of strained monolayer and bilayer MoS2. Nano Lett. (2013), doi:10.1021/nl4014748. 40. D. Pierucci, H. Henck, J. Avila, A. Balan, C. H. Naylor, G. Patriarche, Y. J. Dappe, M. G. Silly, F. Sirotti, A. T. C. Johnson, M. C. Asensio, A. Ouerghi, Band alignment and minigaps in monolayer MoS2-graphene van der Waals heterostructures. Nano Lett. (2016), doi:10.1021/acs.nanolett.6b00609. 41. H. Sun, M. Yao, Y. Song, L. Zhu, J. Dong, R. Liu, P. Li, B. Zhao, B. Liu, Pressure- induced SERS enhancement in a MoS2/Au/R6G system by a two-step charge transfer process. Nanoscale (2019), doi:10.1039/c9nr07098b. 42. X. Ling, W. Fang, Y. H. Lee, P. T. Araujo, X. Zhang, J. F. Rodriguez-Nieva, Y. Lin, J. Zhang, J. Kong, M. S. Dresselhaus, Raman enhancement effect on two-dimensional layered materials: Graphene, h-BN and MoS2. Nano Lett. (2014), doi:10.1021/nl404610c.

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43. T. Zhang, K. Fujisawa, F. Zhang, M. Liu, M. C. Lucking, R. N. Gontijo, Y. Lei, H. Liu, K. Crust, T. Granzier-Nakajima, H. Terrones, A. L. Elías, M. Terrones, Universal In Situ Substitutional Doping of Transition Metal Dichalcogenides by Liquid-Phase Precursor-Assisted Synthesis. ACS Nano (2020), doi:10.1021/acsnano.9b09857. 44. H. Liu, D. Grasseschi, A. Dodda, K. Fujisawa, D. Olson, E. Kahn, F. Zhang, T. Zhang, Y. Lei, R. B. Nogueira Branco, A. L. Elías, R. C. Silva, Y. T. Yeh, C. M. Maroneze, L. Seixas, P. Hopkins, S. Das, C. J. S. de Matos, M. Terrones, Spontaneous chemical functionalization via coordination of Au single atoms on monolayer MoS2. Sci. Adv. (2020), doi:10.1126/sciadv.abc9308.

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Chapter 5 Conclusion and Perspectives

In summary, this thesis provided a comprehensive understanding of the synthesis, characterization, and functionalization of 2D TMDs. This thesis focused on applying novel chemical functionalization routes to functionalize 2D TMDs to tune its physical, chemical, and electronic properties.

5.1 Summary of contributions

Chemical surface functionalization has been long used to tune the electronic, optical, and catalytic properties of 2D TMDs but many functionalization routes rely on the presence of lattice defects and physisorption methods which inevitably modify the surface characteristics as well as the optical, thermal and transport properties of the atomically thin layers. In chapter 2 of this thesis, I have developed a new a new chemical functionalization method via coordination of

Au/Ag single atoms on monolayer MoS2. In this work, I developed a novel 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. We found that single Au and Ag single atoms can be bonded to Sulfur atoms of monolayer MoS2 with minimal impact to its crystalline integrity. We applied Aberration corrected high-resolution scanning transmission electron microscopy (AC-HRSTEM) in conjunction with X-ray photo spectroscopy (XPS) and DFT calculations to confirm the presence of covalently bonded Au single atoms on MoS2. In addition, we demonstrated that AuClx coordinated MoS2 monolayers exhibit tunable optical, electronic, and thermal transport properties based on the Au concentration. For example, PL measurements indicated that the exciton to trion ratio can be increased from 2 to 6 to provide a sharper emission. Thermal boundary conductance can be increased from 15 MW m-2 K-1 to 25 MW m-2 K-1 to promote better heat dissipation. We also achieved the controlled Fermi level tuning via Au coordination, exhibiting shifts in the threshold voltage while maintaining carrier mobility, corresponding to the p-type doping of MoS2. The results reported in this chapter are pioneering on the chemical functionalization of MoS2 as we report the first case of coordinating single

Au/Ag atoms onto MoS2 and demonstrate that this coordination functionalization could effectively tailor the physicochemical properties of 2D TMD materials. Sun et al. have recently shown that TMDs spontaneously reduce noble metal salts into metal nanoparticles (e.g. Au3+ to

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Au0) of different morphology and demonstrated that this process is dependent on the phase, Fermi level and type of each TMD material.(1) In contrast, our results show that CVD grown MoS2 can

3+ 1+ 1+ spontaneously reduce Au to Au and stabilize the Au through the [Au(MoS2)Clx complex instead of reducing Au3+ to Au 0 nanoparticles. We believe the formation of the coordination complex reduces the reduction potential of Au1+ and thus stopes further reduction of the metal. The absence of elemental Au atoms prevents the formation of Au clusters and particles since both Au3+ and Au1+ are positively charged ions that show electrostatic repulsion. We believe the repulsion and coordination bonding thus resulted in the stable Au single ions we see in chapter 2 of this thesis. The coordination functionalization was further investigated in chapter three, where I continued to study the coordination reaction between transition metals such as Cr, Co, Ni, Cu, Zn and MoS2 monolayers. We studied the formation of these coordination complexes and correlated their properties with classical coordination complexes. We have demonstrated the formation these coordination complexes lead to the transfer of electrons between MoS2 and the transition metal, which can introduce both n and p type doping to MoS2 depending on the transition metal used. Moreover, the degree of n- and p-type doping can be fine-tuned by choosing the transition metal and varying the transition metal precursor concentrations, thus controlling electronic band structure of MoS2 as well as the exciton to trion relative population. In addition, the properties of the transition metal functionalized MoS2 displays a trend based on the 3d electron configuration of the transition metal that matches the periodic trend of well-studied coordination compounds.

We have also compared the EF level of MoS2 with the reduction potential of these transition

2+ 0 2+ metals and found the EF to be higher than that of Cu to Cu . This indicates Cu can be reduced

2+ 1+ by MoS2. Similar to the case of Au, we found via XPS that the MoS2 reduced the Cu to Cu instead. This agrees well with our previous discussion on Au that forming the coordination complex can stabilize soft ions such as Au1+ and Cu1+. We believe these trends can serve as a guide for future chemical functionalizations of monolayer semiconducting TMD materials. On another direction, I have also investigated using Au nanoparticles to functionalize

MoS2 for enhanced SERS effects. Pristine MoS2 has many advantages as a SERS substrate but has a rather weak enhancement effect due to its lack of EM enhancements.(2, 3) In chapter 4, I constructed heterostructures of a uniform Au NP film and monolayer MoS2 flakes. The Au NP-

MoS2 heterostructure showed far superior SERS performances compared to pristine MoS2. We also proposed an alternative route where the formation of the heterostructure can achieve the constructive interference of electrochemical enhancement and charge-transfer based chemical

113 enhancement of SERS. We also proposed a tentative mechanism behind the SERS enhancement, yet additional experiments and theoretical calculations are needed to fully clarify the mechanism and more details can be found in section 5.2 below. In the appendix of this thesis, I studied the application of defect engineering to create vacancies and exposed edges in MoS2. We demonstrated 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. Since this project is not yet complete, it was not written as a chapter but rather the preliminary results were summarized in the appendix. In summary, I have explored new ways to functionalize TMDs and applied these functionalization routes to tailor or enhance the optical, chemical, electrical, and thermal properties of the TMDs.

5.2 Perspectives

Even though this thesis summarized several effective methods to apply chemical functionalization to tune or control various properties of 2D TMDs, the extend of chemical functionalization is yet to be researched and there is still more work needed to be done.

For the coordination-based functionalization of MoS2 with Au single atoms. We have demonstrated that the functionalization effectively tunes the properties of MoS2. Yet this functionalization technique also offers a way to synthesize Au, Ag and Pt single atoms which can be further utilized in many impacting applications such as in single atom catalysis (SAC).(4, 5) In chapter 2 and 3, we have synthesized single atoms of Au and Pt sit on top of 2D MoS2 and this can be seen as a 2D/SAC system where the MoS2 is a 2D catalyst support and the single atoms are the SAC. Novel in situ characterization techniques such as X-ray absorption spectroscopy (XAS), Raman and Fourier Transform infrared spectroscopy (FTIR) and ambient pressure XPS (APXPS), will need be employed to understand key fundamental properties of these 2D/SACs to identify the byproducts and monitor the catalytic reaction pathways. Due to the high reactivity of SACs during reactions, ex situ experiment cannot provide the information of catalyst evolution during reactions. For instance, in situ Raman spectroscopy, which is sensitive to electron/hole doping, could be used to monitor the charge transfer and phase changes during a catalytic reaction. In addition, in situ FTIR spectroscopy can be used to provide important information on surface adsorbates during catalytic reactions. Molecules such as CO can provide the location and

114 bonding capability of the catalyst, which is an excellent probe to check the active site of catalyst surfaces. Moreover, in situ APXPS can be used to directly probe the adsorbed molecules, binding energy and Fermi level of 2D/SACs under reaction conditions. APXPS measured in ultrahigh vacuum (UHV) and ambient pressure conditions can be used to address how the 2D/SACs are influenced by the presence of CO, CO2 and CH4 at different temperatures which gives information of the chemisorption of these gases on the single metals.

Controlling where the single atoms go/form on the MoS2 is another interesting topic worth studying. Currently we have little control over where the single atoms go on the MoS2 surface. Potentially there could be ways to direct the single atoms to go to desired locations and form certain patterns on the MoS2 which can be developed into the next generation of atomic resolution lithography. One possible solution is the controlled doping of Se and Te into sulfides to control the location of Au single atoms since Au-Se bonds should be more favorable. Another possible solution is through the controlled defect engineering of the 2D material. Though the formation of the single atoms does not rely on defects, the presence of defects will ultimately affect the formation of these single atoms. A comprehensive study of how different defects affect the formation of the single atoms will need to be done first. In chapter 3, we have expanded the coordination functionalization to 3d transition metals such as Cr, Co, Ni and Cu. Yet there are many more transition metals such are the rare earth metals which can form coordination compounds with S based ligands. These other metals can also be studied in the coordination functionalization of 2D TMDs and can introduce many new properties to the materials.

In the case of Au NP-MoS2 heterostructures, we demonstrated that forming heterostructures of 0D metal particles and 2D materials can significantly enhance the SERS performances. We also proposed a possible mechanism behind this enhancement where we think the EM of Au NPs and CM of MoS2 is working constructively. However, a more comprehensive study of the mechanism from both experimental and theoretical standpoints need to be conducted. While the plasmonic effect of the Au NPs have been well studied and understood, the CM SERS effect of TMDs remains controversial and requires further research. We are currently in collaboration with the Jensen group to further study this mechanism in more detail. We believe more theoretical calculation results and direct experimental evidence of charge transfer via pump probe measurements is needed to fully understand the interactions between MoS2 and the dye molecules in terms of knowing whether the CM charge transfer effect is the only effect happening on the surface of the 2D MoS2. More experiment carried out with varying metal NP types, sizes and shapes as well as other TMDs including semiconducting WS2, WSe2 and metallic NbS2, 1T’-

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MoS2 will also be done to further confirm the mechanism. The new SERS substrates can also be applied to identify and differentiate more complex molecules such as viruses so that Raman Spectroscopy can be utilized as a fast and damage free method for virus detection.

In the appendix of this thesis, we showed that defect engineering of bulk MoS2 powders can be used to introduce defects into the “inert” MoS2 and greatly enhance its performances as an HER catalyst. To finish this project, more characterization such as X-ray absorption (XAFS), proton induced X-ray emission (PIXE) and electron spin response (ESR) measurements need to be done to comprehensively study the reactive defects. The mechanism of the HER reaction also needs to be investigated with theoretical calculations. These reactive defects can also be explored in the spontaneous reduction reaction with metal ions. It would be interesting to know if the cryomilled MoS2 can reduce Au or Pt ions into nanoparticles or even single atoms.

Finally, this thesis focuses on the functionalization of 2H MoS2. Yet the functionalization techniques discussed here can be applied to other 2D systems such as hBN and graphene. Alloys and doped systems of TMDs and graphene should also be tested to see if these functionalization methods work or not.

5.3 References

1. Y. Sun, Y. Wang, J. Y. C. Chen, K. Fujisawa, C. F. Holder, J. T. Miller, V. H. Crespi, M. Terrones, R. E. Schaak, Interface-mediated noble metal deposition on transition metal dichalcogenide nanostructures. Nat. Chem. (2020), doi:10.1038/s41557-020-0418-3. 2. X. Ling, W. Fang, Y. H. Lee, P. T. Araujo, X. Zhang, J. F. Rodriguez-Nieva, Y. Lin, J. Zhang, J. Kong, M. S. Dresselhaus, Raman enhancement effect on two-dimensional layered materials: Graphene, h-BN and MoS2. Nano Lett. (2014), doi:10.1021/nl404610c. 3. S. Huang, X. Ling, L. Liang, Y. Song, W. Fang, J. Zhang, J. Kong, V. Meunier, M. S. Dresselhaus, Molecular Selectivity of Graphene-Enhanced Raman Scattering. Nano Lett. 15, 2892–2901 (2015). 4. X. F. Yang, A. Wang, B. Qiao, J. Li, J. Liu, T. Zhang, Single-atom catalysts: A new frontier in heterogeneous catalysis. Acc. Chem. Res. (2013), doi:10.1021/ar300361m. 5. A. Wang, J. Li, T. Zhang, Heterogeneous single-atom catalysis. Nat. Rev. Chem. (2018), , doi:10.1038/s41570-018-0010-1.

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Appendix: Defect Engineering to activate MoS2 for hydrogen evolution reaction

As discussed in Chapter 1, defect engineering of 2D transition metal dichalcogenides (TMDs) offers an effective and controllable approach for tuning the electronic and catalytic properties of the TMD material. Here we use cryomilling to introduce defects into MoS2 and demonstrate that the defective MoS2 display extraordinary catalytic performances on the hydrogen evolution reaction. Since this project is still unfinished, we summarize the preliminary results here in the Appendix and not as a chapter of the thesis.

Introduction

Hydrogen has been long considered as the ideal alternative for traditional fossil fuels in the future and huge efforts has been made towards a sustainable and effective method to manufacture H2. The electrolysis of water containing the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is one of the most important pathways to produce hydrogen efficiently. (1) During the HER reaction, H+ ions absorb electrons from the electrode and is reduced to H2 when an overpotential is applied. This overpotential is a huge waste of energy and can be reduced with a catalyst.(2) The most effective HER electrocatalysts up to now are based on platinum due to its near zero overpotential. (3, 4) However, the high price and limited resource of Pt largely prevent their utilization in practice. Therefore, developing cost effective and energy effective HER electrocatalysts still remains a large challenge. During the past decade, transition metal chalcogenides have gained much attention posing as promising alternatives for platinum due to their high abundance and low cost.(5–7)

Historically, bulk TMD materials such as MoS2 have been considered a poor HER catalyst due to its inert basal planes.(8) However, recent research has shown both experimentally and theoretically that the HER activity of semiconducting MoS2 greatly relies on the amount of exposed catalytically active defect sites and edge sites. (9–15) Hence, defect engineer of MoS2 has become an effective strategy to obtain a high performing MoS2 HER electrocatalysts.(16) For instance, Kong et al. prepared MoS2 films with vertically aligned layers that maximally expose the edges on the film surface via chemical vapor deposition (CVD). (10) Xie et al. produced defect rich MoS2 nanosheets with additional active sites via solvent thermal.(17) However, these methods reported up to now are of limited use in practice due to their poor controllability and

117 scalability. Recently, Yu et al. reported that a new method, cryomilling, can be applied to engineer defects into hexagonal boron nitride (hBN) which can reduce and stabilize single Pt atoms and AgPt sub nanoclusters.

In this section, we applied cryomilling techniques onto MoS2 and produced bulk amounts

(5 grams per run) of defective MoS2 powders with high defect density and large quantities of exposed reactive edges. These defects have the capability of reducing noble metal salts into nanoparticles which are anchored into its lattice. The defective MoS2 is characterized via STEM, Raman Spectroscopy, BET gas absorption and X-ray diffraction to confirm its high defect density and high surface area. The 90min cryomilled MoS2 powders exhibit superior HER performance with an onset potential of 141mV and Tafel slope of 34.4 mV/dec, much higher than that of previously reported defective MoS2 samples.

Defect engineering of MoS2 for HER catalyst

To create defects, MoS2 powders were cryo-milled (CM) at -196 °C for 15-90min (see previous section for details). At -196 °C, the oxidation and powder agglomeration are significantly suppressed, so that structural defects including vacancies and reactive edges could be retained. To study the effect of cryomilling on the crystal structure of MoS2, Powder X-Ray diffraction (XRD) was measured on the cryomilled MoS2 powders. Figure A-3A shows the diffraction pattern of cryomilled MoS2 ranging from 15min to 75min. The XRD patterns are normalized to the MoS2 002 peak and it can be clearly seen in Figure A-3A that the relative intensity of the 002 peak decreases with cryomilling time. Figure A-3B shows the close-up analysis of the 002 band and its peak width or full width half maximum increases with cryomilling time. This shows that a gradual disordering process happens when the cryomilling time increases, which agrees well with the Raman and STEM results shown below. The broadening of the XRD peaks also shows that the grain size of the MoS2 decreases which also agrees with our low-mag STEM image in Figure A-1B. The N2 absorption measurements were also conducted on the cryomilled MoS2 samples and the isotherms are shown in Figure A-2A. These isotherms are classified and type II, indicating macropore solids.(20) The surface area of the cryomilled MoS2 samples are calculated using the Brunauer-Emmer-Teller (BET) (21) surface area formula. The BET surface area increases from 10.93 m2/g to 19.26 m2/g, 20.19 m2/g

2 and 24.86 m /g pristine, 30min, 60min and 90 min cryomilled MoS2 ,respectfully. A steady increase can be seen with cryomilling time. The effects of cryomilling was further studied by

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Raman spectroscopy using 633nm laser which is the resonance wavelength for the longitudinal acoustic(LA) mode of the MoS2. The LA mode has been well studied and the intensity of this mode can be directly related to the defect density in MoS2.(18, 19) Figure A-2B shows the Raman

-1 spectra of pristine, 45min and 90min cryomilled MoS2. The 2LA mode at 466 cm could be clearly identified and its intensity increases with increasing cryomilling time, which further confirms the creation of defects in the cryomilling process.

Figure A-2. (A) N2 gas absorption isotherms of pristine, 30min, 60min and 90min cryomilled MoS2. (B) Raman spectra of pristine, 45min and 90min cryomilled MoS2 excited under the 633nm laser. The 2LA(M) mode at 466cm-1 significantly increases with increased cryomilling time, indicating the increase in defect density during the cryomilling process.

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High-resolution STEM (HR-STEM) imaging using a high-angle annular dark-field

(ADF) detector was also conducted to study the atomic structure of the cryomilled MoS2 powders. The MoS2 powders remain a multi-layer particle after cryomilling and the particle size can be reduced to as small as 40nm as shown in Figure A-3B. This size reduction leads to the significant increase of reactive edges which have been previously proven to be HER active compared to the rather inert basal plane. The atomic resolution STEM-ADF image (Figure A-3A) further shows the creation of Mo vacancies and highly defective regions around the edges of the

MoS2 particles.

Figure A-3. STEM images of 90min cryomilled MoS2. (A)High resolution STEM image showing Mo vacancies and exposed edges forming at the edge of the MoS2 particle. (B)Low magnification STEM image showing reduced particle size of the MoS2 after cryomilling and displaying large amounts of exposed edges.

The HER performance of the cryomilled MoS2 and was investigated in a 0.5M H2SO4 solution. As shown in Figure A-4A, significant HER activities were for the cryomilled MoS2 samples. We report 241mV, 174mV, 157mV, 141mV for 15, 30, 60 and 90min cryomilled MoS2, respectively, when the current density is at 10 mA/cm2. Figure A-4B shows the corresponding

Tafel plots and its linear fitting yielding 34.4 mV/dec Tafel slope for 90min MoS2. The onset potential and Tafel slope is still higher than that of Pt/C electrodes (~30mV/dec) because of the different reaction mechanism. The Pt follows the Volmer reaction while MoS2 follows the Heyrovsky reaction. However, our reported values are significantly better than previous reports of defective MoS2 and 1T MoS2. In contrast, the cryomilling approach also produces defective MoS2 at a much larger scale than conventional defect engineering such as irradiation.

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Figure A-4. HER performance of cryomilled MoS2. (A) The HER polarization curves. (B) The Tafel slopes.

In conclusion, we have applied a novel strategy, cryomilling, to realize the controllable and scalable defect engineering in bulk amounts of MoS2. The high defect density and surface area as well as the large quantity of exposed edges in the cryomilled MoS2 powders results in superior HER performances with a small onset potential of 141mV and small Tafel slope and

34.4 mV/dec, outperforming previous reports of pure defective MoS2.The success of using cryomilling to achieve the controlled defect engineering for HER catalysts may pave the way towards producing more effective catalysts in the future. The cryomilling of TMD mixtures can also be expected to have even better performances.

Methods

In a typical experiment, 5 g MoS2 powder (Sigma Aldrich) was cryo-milled in a polycarbonate encapsulated cell (SPEX 6875D Freezer/Mill) at -196 °C. The polycarbonate capsulated compactor (SPEX 6761)) hit the two ends of the cell with a frequency of 10 cycles per second (cps). The powders were cryomilled in 3min On and 2min Off cycles so that they don’t overheat. The cryomilling time mentioned in this chapter only considers the On time. The samples were prepared with different cryomilling times ranging from 30min-90min. Raman measurements were performed at room temperature using a Renishaw InVia confocal Raman spectrometer using 633 nm laser excitation. X-ray diffraction was performed using a PANalytical Empryean X-Ray Diffractometer. Gas adsorption measurements were carried out at 77K with an Accelerated Surface Area and Porosimetry Analyzer (ASAP 2020;

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Micrometritics Instrument Corp.). The surface areas were calculated using the Brunauer, Emmett and Teller (BET) equation. Prior to the measurement, the sample was degassed at 353 K for 24 h under 4μm Hg vacuum. STEM was carried out by FEI Titan3 G2 S/TEM operating at 80 kV. The d-MoS2 samples were first sonicated in ethanol for 1 hour and then drop casted onto a Quantifoil TEM grid. A high-angle annular dark field (HAADF) detector was used for STEM-ADF imaging. A Gaussian blur filter was applied by the ImageJ program to reduce noise and enhance the visibility of structural details.

The electrodes are fabricated with 25 mg d-MoS2 mixed with 5 mg carbon powder and 100 mL Nafion, 200 mL DI water, and 800 mL isopropanol. The mixture was then sonicated for 1h to form a dense slurry. 5 microliters of slurry were drop casted on the glassy carbon electrode (3 mm diameter) in 5 successive 1 microliter droplets. The electrolyte used for HER measurement is 0.5 M H2SO4 with a graphite rod as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode using a Versa STAT 4 potentiostat with a Basi rotating stage. The scan rate of linear sweep voltammetry (LSV) was set at 2 mV s-1 with IR-compensation and the rotating stage is set at 3000rpm so that the electrolyte is well mixed during the measurement. To stabilize the electrode surface, the electrode was cycled three times before data collection. Each sample was tested 3 times to get a statistic result.

References

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VITA He Liu (Moses)

Education The Pennsylvania State University Ph.D. Candidate in Chemistry Aug. 2015–May 2021 Lanzhou University Bachelor of Science, Chemistry Sep. 2011–Jun. 2015

Honors and Awards The Dan H. Waugh Memorial Teaching Award Penn State Department of Chemistry Oct.2016

Selected publications 1. He Liu, Daniel Grasseschi, Akhil Dodda, Mauricio Terrones. Spontaneous Chemical Functionalization via Coordination of Single Atoms on Monolayer MoS2. Science Advances. 2020 6 : eabc9308 2. He Liu et al. Coordination chemistry trends between 3d transition metals and MoS2. (In preparation) 3. He Liu et al. Au Nanoparticle Functionalization of MoS2 for Surface Enhanced Raman Spectroscopy (In preparation) 4. Yu Lei, Srimanta Pakhira, Kazunori Fujisawa, He Liu, Jose L. Mendoza-Cortes, Mauricio Terrones. Room Temperature Spontaneous Pt Reduction on Defective BN for Single Atom Catalysis (submitted to Nature Materials) 5. A. Quintanilla, G. Vega, J. Carbajo, J.A. Casas, Y. Lei, K. Fujisawa, H. Liu, R. Cruz-Silva, M. Terrones, P. Miranzo, M.I. Osendi, M. Belmonte, J. Fernández Sanz. Understanding the active sites of boron nitride for CWPO: an experimental and computational approach. Chemical Engineering Journal.Volume 406, 15 February 2021, 126846 6. Ethan Kahn, Mingzu Liu, Tianyi Zhang, He Liu, Kazunori Fujisawa, George Bepete, Mauricio Terrones. Functional Hetero-interfaces in Atomically Thin Materials. Materials Today. 2020, 37, 74-92 7. Yin-Ting Yeh, Yijing Zhou, Donghua Zou, He Liu, Haiyang Yu, Huaguang Lu, Venkataraman Swaminathan, Yingwei Mao, Mauricio Terrones. Rapid Size-based Isolation of Extracellular Vesicles by 3-dimensional Carbon Nanotube Arrays. ACS Appl. Mater. Interfaces 2020, 12(11), 13134-13139 8. Tianyi Zhang, Kazunori Fujisawa, Fu Zhang, Mingzu Liu, Michael C. Lucking, Rafael N. Gontijo, Yu Lei, He Liu, Mauricio Terrones. Universal In-Situ Substitutional Doping of Transition Metal Dichalcogenides by Liquid Precursor-Based Chemical Vapor Deposition. ACS nano. 2020, 14(4), 4326-4335 9. Alexander Silver, Hikari Kitadai, He Liu, Tomotaroh Granzier-Nakajima, Mauricio Terrones, Xi Ling, Shengxi Huang. Chemical and Bio Sensing Using Graphene-Enhanced Raman Spectroscopy. Nanomaterials. 2019, 9(4), 516 10. Mingsong Wang, Zilong Wu, Alex Krasnok, Tianyi Zhang, Mingzu Liu, He Liu, Leonardo Scarabelli, Luis M. Liz-Marzan, Mauricio Terrones, Andrea Alu, Yuebing Zheng . Dark-Exciton-Mediated Fano Resonance from a Single Gold Nanostructure Deposited on MonolayerWS2 at Room Temperature. Small. 2019, 15(31) 11. Yifan Sun, Albert J. Darling, Yawei Li, Kazunori Fujisawa, Cameron F. Holder, He Liu, Michael J. Janik, Mauricio Terrones, Raymnd E. Schaak. Defect-mediated selective hydrogenation of nitroarenes on nanostructuredWS2 Chem. Sci., 2019,10, 10310-10317 12. Amritanand Sebastian, Fu Zhang, Akhil Dodda, Dan May-Rawding, He Liu, Tianyi Zhang, Mauricio Terrones, Saptarshi Das. Electrochemical Polishing of Two-Dimensional Materials. ACS nano. 2018, 13(1), 78-86 13. Archi Dasgupta, Hiroyuki Muramatsu, Yuji Ono, Viviana González, He Liu, Mauricio Terrones and Juan Matos. Nanostructured Carbon Materials for Enhanced Nitrobenzene Adsorption: Physical vs. Chemical Surface Properties. Carbon. 2018,139, 833-844 14. Sergey Lepeshov, Mingsong Wang, Alex Krasnok, Oleg Kotov, Tianyi Zhang, He Liu, Taizhi Jiang, Brian Korgel, Mauricio Terrones, Yuebing Zheng, Andrea Alú. Tunable Resonance Coupling in Single Si Nanoparticle–Monolayer WS2 Structures. ACS. Appl. Mater. Interfaces. 2018, 10(19), 16690–16697 15. Mingsong Wang, Alex Krasnok, Tianyi Zhang, Leonardo Scarabelli, He Liu, Luis M. Liz-Marzán, Mauricio Terrones, Andrea Alù, Yuebing Zheng. Fano Resonances: Tunable Fano Resonance and Plasmon–Exciton Coupling in Single Au Nanotriangles on MonolayerWS2 at Room Temperature. Advanced Materials. 2018, 30(22), 1705779