Ulsan National Institute of Science and Technology

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Doctoral Thesis

Junghyun Lee

Department of Materials Science and Engineering

Ulsan National Institute of Science and Technology

2022

Modulation of Material Properties in Two-

Dimensional Semiconductors via

Defect Engineering

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Junghyun Lee

Department of Materials Science and Engineering

Ulsan National Institute of Science and Technology

Modulation of Material Properties in Two-

Dimensional Semiconductors via

Defect Engineering

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A thesis/dissertation submitted to

Ulsan National Institute of Science and Technology in partial fulfillment of the

requirements for the degree of Doctor of Philosophy

Junghyun Lee

Hyesung Park

Modulation of Material Properties in Two-

Dimensional Semiconductors via Defect Engineering

12.15.2021 of submission Approved by

Advisor

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Abstract

With its outstanding physical, chemical, and electrical properties, two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs) have been received extensive global research interest. A wide range of potential applications such as electrocatalyst, electronic devices, optical devices, and energy storage devices (rechargeable batteries) has been explored based on their intriguing physicochemical properties. However, the lack of physicochemical properties modulation in TMDs limited the best of the material properties which inevitably restricted their performance in target- specific applications.

Point defects, zero-dimensional (0D) defects, such as atomic vacancy and molecular doping which were formed during the initial growth stage or created by post-process are ubiquitous in a wide range of TMDs. Despite the atomic structure of TMDs was not largely affected, a wide range of intrinsic properties including chemical, electrical, and optical properties could be significantly modulated, potentially have applicable functions used in diverse applications.

In this thesis, we utilized diverse point defects including chalcogen vacancy and molecular doping to optimize the physicochemical properties of TMDs in target applications. Initial two chapters covered the synthetic methods for one-step chalcogen vacancy generation in TMDs lattice and the modulation of the physicochemical properties of TMDs through the defect engineering to enhance the electrochemical activity. Firstly, vacancy-induced molybdenum diselenides (MoSe2) was one-step synthesized by hydrogen reactivity control during the solid-phase CVD approach. The synthesized vacancy-induced MoSe2 exhibited unprecedented electrochemical activity with an exceptionally low Tafel slope. These results present a new perspective for developing high-performance TMDs-based electrocatalysts. The next chapter is devoted to a synthetic method for vacancy-modulated MoS2 thin film through the liquid-phase organometallic reaction. The organometallic approach is based on a liquid reaction, the synthesized MoS2 film showed large-scale synthesis capability with high uniformity and high reproducibility regardless of vacancy concentrations. It is further found that the electrical, chemical, and physical properties were optimized for hydrogen evolution reaction by introducing the chalcogen vacancy. Following chapter dealt with the effect of molecular doping to control the electrical properties of TMDs. Molecular doping could successfully control the carrier concentration in TMDs, leading to enhancing the overall photovoltaic effect. Finally, the vacancy-induced TMDs was used as a the host materials for suppression of Li dendritic growth. The strong binding energy between vacancy-induced MoS2 and Li-ion effectively induced lateral growth of Li-metal, leading to enhancing the stability of the Li-metal battery.

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List of Figures

Figure 1.1. Classification of 2D materials according to electrical properties and their potential applications in diverse fields ... 1 Figure 2.1. Classification of various defects in 2D semiconductors according to their dimensionality.

(Reprinted from Ref. 1, Copyright@2021 IOP publishing) ... 6 Figure 3.1. Schematic illustration of vacancy-engineered MoSe2 synthesis according to hydrogen concentrations. Hydrogen plays a dual role in decrease the Se vacancy by promoting the reduction of MoO3 and increase the Se vacancy by etching the Se atom at the MoSe2 lattice.

... 17 Figure 3.2. Synthesis profile of in-situ vacancy-modulated MoSe2. Chalcogen vacancy in MoSe2 lattice was modulated by varing the carrier gas ratio (H2/Ar) to control the reactivity of hydrogen during the growth step. Cooling process was carried out under Ar atmosphere to prevent additional Se etching by hydrogen gas. ... 17 Figure 3.3. Surface morphology of optimized MoSe2 film (MoSe2-30). The AFM image shows the the synthesized MoSe2 film has monolayer characteristics with clean surface. ... 18 Figure 3.4. Elemental analysis of representative MoSe2 film (MoSe2-30). STEM-EDS image and relative elemental mapping image demonstrates uniform distribution of Mo and Se atoms..

... 18 Figure 3.5. SEM analysis of MoSe2-10 after the intermediate growth shows the irregular shaped flakes with MoO3-x impurities. ... 19 Figure 3.6. SEM-EDS elemental analysis indicates the formation of MoO3-x clusters at low H2

concentration (white box in the inset image)... 19

Figure 3.7. SEM analysis of MoSe2-30 and MoSe2-50 flakes. ... 19 Figure 3.8. XPS Mo 3d and Se 3d XPS specta of MoSe2-0 indicate that MoSe2 is not synthesized under the absence of hydrogen. ... 20 Figure 3.9. XPS Mo 3d spectra of (a) MoSe2-10, (b) MoSe2-20, (c) MoSe2-30, (d) MoSe2-40, and (e) MoSe2-50 exhibit that the defect-related peaks (Mo⁶⁺ 3d, magenta line) vary with the

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hydrogen concentration (green line and dotted line indictated Mo⁴⁺ 3d peak and original XPS data, respectively). ... 21 Figure 3.10. XPS Se 3d spectra of (a) MoSe2-10, (b) MoSe2-20, (c) MoSe2-30, (d) MoSe2-40, and (e) MoSe2-50 exhibit that the defect-related peak (SeO2, magenta line) varies with the hydrogen concentration (green line and dotted line indictated Se^2─ 3d peak and original XPS data, respectively). ... 21 Figure 3.11. XPS Se 3d spectra of (a) MoSe2-10, (b) MoSe2-20, (c) MoSe2-30, (d) MoSe2-40, and (e) MoSe2-50 exhibit that the defect-related peak (SeO2, magenta line) varies with the hydrogen concentration (green line and dotted line indictated Se^2─ 3d peak and original XPS data, respectively). ... 22 Figure 3.12. Schematic diagram of lattice structure changes according to Se vacancy in MoSe2 lattice.

... 23 Figure 3.13. (a) Atomic-resolution ADF-STEM and (b) corresponding atomic position mapping of stoichiometry MoSe2 (MoSe2-30). ... 23 Figure 3.14. (a) Atomic-resolution ADF-STEM and (b) corresponding atomic position mapping of non- stoichiometry MoSe2 (MoSe2-50).. ... 23 Figure 3.15. Raman spectra of vacancy-modulated MoSe2. (a) Raman spectra and (b) comparison of Raman peak position and intensity with varying H2 concentration. ... 24 Figure 3.16. (a) Atomic-resolution ADF-STEM and (b) corresponding atomic position mapping of non- stoichiometry MoSe2 (MoSe2-50). ... 25 Figure 3.17. PL mapping of MoSe2-10, MoSe2-30, and MoSe2-50. PL mapping images show that the as-synthesized MoSe2 has high uniformity. ... 25 Figure 3.18. Relative peak shift of XPS Mo and Se 3d peaks from H2 30%. XPS (a) Mo⁴⁺, Mo⁶⁺, (b) Se²⁻, and (c) SeO2 3d peaks shift toward higher binding energies as H2 concentration either increases or decreases from 30%, indicating that the formation of Se-vacancy induces n- doping effect in MoSe2. ... 25 Figure 3.19. HER activity of vacancy-modulated MoSe2. (a) HER polarization curves and (b) corresponding Tafel plots according to hydrogen concentations and Pt/C. ... 27 Figure 3.20. Calculation models and H adsorption sites. Top views of (a) pristine MoSe2, (b) 1V-MoSe2, (c) AV-MoSe2, and (d) CV-MoSe2. Red dotted circle represents H adsorption site. ... 28

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Figure 3.21. Volcano plot for pure metals and vacancy-induced MoSe2 exhibitng that the relative hydrogen adsorption free energy and exchange current density of vacancy-MoSe2 are comparable to those of high-performance noble metals. Inset table indicates the ΔGH and Log j of pristine-MoSe2. The values for the representative metals are taken from the literature [39]. ... 28 Figure 3.22. Electronic band structures of MoSe2 with different Se vacancy concentrations. Valence and conduction bands are represented with red lines. Partial charge density of valence band in inset shows the localization of electrons around the Se vacancy site. ... 29 Figure 3.23. Comparison of electrocatalytic activity of diverse TMDs illustrates that our optimized vacancy-modulated MoSe2 shows superior HER activity. ... 30

Figure 3.24. Schematic illustration of HER pathway in AV- and CV-MoSe2, respectively... 30 Figure 3.25. Electrochemical stability test. (a) HER polarization curves obtained at the first scan and after 100 consecutive scans at a scan rate of 10 mV s⁻¹ for MoSe2-50 in 0.5 M H2SO4. (b) The chronopotentiometric reduction profile of MoSe2-50 measured by a rotating disk electrode at a current density of 10 mA cm⁻². The potential notation was converted to reversible hydrogen electrode and iR drop was not corrected. ... 31 Figure 3.26. Diffusion on Se-vacancy-MoSe2 and schematic illustration of Volmer-Tafel reaction. (a) Top and (b) Oblique views of H^* diffusion path in AV-MoSe2. (c) Top view of H^* diffusion path in CV-MoSe2. ... 32 Figure 3.27. H^* diffusion path in the five coalesced Se vacancy. When five Se vacancy are coalesced, the most stable hydrogen adsorption site changes from Se vacancy site to hollow site and the H^* diffusion barrier is dramatically reduced to 0.19 eV. ... 32 Figure 3.28. Schematic illustration of Volmer-Tafel reaction pathway of HER consisting of H⁺

adsorption, H^* diffusion, and H2 desorption steps ... 32 Figure 4.1. Digital images showing the representative synthesis steps for the preparation of a Mo(TDT)x

precursor solution shown with the corresponding chemical structures. ... 42 Figure 4.2. Schematic of the sulfur vacancy-modulated MoS2 synthesis routes via molar ratio control of S to Mo in the MoS2 precursor solution, and the corresponding AFM images and atomic structures of the high-crystalline HV-MoS2 and LV-MoS2. ... 43 Figure 4.3. Temperature profiles and gas compositions of the synthesis of the crystallinity and vacancy

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controllable MoS2. ... 43

Figure 4.4. Raman spectra of MoCl5, Mo(TDT)x intermediates, and MoS2 obtained from the representative synthesis stages of the low-vacancy MoS2(HC). ... 44

Figure 4.5. Raman spectra of crystallinity-modulated LV-MoS2. ... 45

Figure 4.6. Absorption spectra of crystallinity-modulated LV-MoS2. ... 46

Figure 4.7. SEM images of high-crystalline HV-MoS2 and LV-MoS2 films. ... 46

Figure 4.8. Raman and XPS spectra of MoSe2(HC), WS2(HC), and WSe2(HC) with minimal chalcogen vacancy. ... 46

Figure 4.9. (a) Mo 3d and (b) S 2p XPS spectra with different sulfur vacancy concentrations in MoS2(HC). (c) Mo⁴⁺ and Mo⁶⁺ 3d XPS peak ratio and (d) atomic ratio of S and Mo derived from the XPS spectra for each of the conditioned samples. ... 47

Figure 4.10. GIWAXS patterns of MoS2(HC) films on a Si substrate with different sulfur vacancy concentrations. ... 47

Figure 4.11. Raman spectra of MoS2(HC) at different sulfur vacancy concentrations. ... 48

Figure 4.12. Absorption spectra of MoS2(HC) at different sulfur vacancy concentrations. ... 48

Figure 4.13.Atomic resolution ADF-STEM images of high-crystalline (a) HV-MoS2 and (b) LV-MoS2. Green, yellow, and red spheres indicate molybdenum, sulfur, and single sulfur vacancies, respectively. (c) Atomic arrangement profile along the two marked dashed lines in (a) and (b). ... 49

Figure 4.14. Large-scale synthesis of vacancy-modulated MoS2 thin films. (a) Digital images of high- crystalline HV-MoS2 and LV-MoS2 synthesized on a 4-inch SiO2/Si wafer. (b) Corresponding optical microscopy images obtained at different positions as indicated in (a).. ... 49

Figure 4.15. Raman and XPS Mo 3d spectra of high-crystalline (a, b) HV-MoS2 and (c, d) LV-MoS2 films, respectively, at different locations marked in Figure 4.14a. The blue shaded region in the XPS Mo 3d spectra indicates defect-related peaks. ... 50

Figure 4.16. UPS spectra of the high-crystalline HV-MoS2, MV-MoS2, and LV-MoS2 with the corresponding Fermi energy levels.. ... 50

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Figure 4.17. (a) Model system of MoS2 monolayer. (b) The structures with the global minimum energy of MoS2-x (x = 0.4, 0.6). The dotted circles filled in orange and gray represent the upper and lower sulfur vacancy, respectively. (c) The relative energies of all possible configurations of MoS2-x (9 and 36 cases for MoS1.6 and MoS1.4, respectively). The inset shows the labeled S atoms whose distance from the pivot point varies (Table 4.1). The possible combinations and their relative energies of MoS1.6 and MoS1.4 are listed in Table 4.1 and 4.2, respectively.

... 52 Figure 4.18 Projected density of states (PDOS) of MoS2-x (x = 0.6, 0.4, and 0.0). The donor states of MoS1.4 and MoS1.6 labeled in the order of low energy level are indicated with the gray and red arrows, respectively. E=0 is the valence band maximum.. ... 53 Figure 4.19. Isosurface plot of the orbitals (isovalue: 0.025 a.u.) corresponding to the defect states of MoS1.4 and MoS1.6 and optimized structure of MoS2 monolayer system. The dotted circles filled in orange and gray represent upper and lower sulfur atom vacancies and the gray- filled dotted circles denote lower sulfur vacancies. ... 53 Figure 4.20. HER polarization curves of MoS2(HC), MoS2(MC), and MoS2(LC) with minimal sulfur vacancy.. ... 54

Figure 4.21. EIS of HER in MoS2(HC), MoS2(MC), and MoS2(LC) with minimal sulfur vacancy... . 55 Figure 4.22. (a) LSV curves for MoS2(HC) at different sulfur vacancy concentrations. (b) Corresponding Tafel plots obtained from the polarization curves.... ... 56 Figure 4.23. Free energies for HER on MoS2 and MoS1.4 representing LV- and HV-MoS2, respectively.

... 56 Figure 4.24. Summary of the overpotential (at 10 mA cm⁻²) and Tafel slope of vacancy-modulated MoS2 electrocatalysts. ... 57

Figure 4.25. EIS of HER in MoS2(HC) at different sulfur vacancy concentrations.. ... 57 Figure 4.26. Transfer characteristics of MoS2(HC) transistor with different sulfur vacancy concentration. ... 58 Figure 4.27. Electrochemical stability tests of high-crystalline HV-MoS2 after 1,000 CV cycles in 0.5 M H2SO4 aqueous electrolyte solutions. (Inset graph indicates the chronoamperometric stability profile of the HER test for 50 hr at a working voltage of 0.2 V)... ... 58 Figure 4.28. Principle of HER activity enhancement in band structure modulated MoS2(HC) through

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the introduction of sulfur vacancy. ... 59

Figure 5.1. (a) OM image of MoS2 flake and (b) corresponding Raman spectrum. ... 69

Figure 5.2. (a) OM image of WSe2 flake and (b) corresponding Raman spectrum. ... 69

Figure 5.3. Schematic of the fabricated TMDs FET device. ... 70

Figure 5.4. (a) Output characteristics of the MoS2 FET device under various gate biases (Vbg= −40 V to 40 V). (b) Transfer characteristics of the MoS2 FET device at constant applied bias voltage of Vsd=1 V.. ... 70

Figure 5.5. (a) Output characteristics of the MoS2 FET device under various gate biases (Vbg= −40 V to 40 V). (b) Transfer characteristics of the MoS2 FET device at constant applied bias voltage of Vsd=1 V. ... 71

Figure 5.6. (a) OM image of MoS2/WSe2 heterostructure. (b) Raman and (c) PL spectra of MoS2 region (blue line), WSe2 region (green line), and overlapped region (black line), respectively. Raman intensity mapping image of (d) MoS2 A1g, (e) WSe2 A1g, and (f) combined of MoS2 and WSe2 A1g peak... 71

Figure 5.7. Electrical properties of MoS2/WSe2 heterostructure.. ... 72

Figure 5.8. Photoswitching behavior of MoS2/WSe2 heterostructure. ... 72

Figure 5.9. Photovoltaic effect of MoS2/WSe2 heterostructure. ... 73

Figure 5.10. Transfer characteristics of MoO3 and AuCl3-doped WSe2, respectively. ... 73

Figure 5.11. Photovoltaic effect of MoS2/MoO3-doped WSe2 heterostructure. ... 74

Figure 6.1. SEM image of CNT and CNT@MoS2 anodes for Li-metal battery. ... 81

Figure 6.2. Raman spectrum of CNT@MoS2 anodes. ... 81

Figure 6.3. TEM images of CNT@MoS2 and corresponding TEM-EDS elemental mapping. ... 82

Figure 6.4. Molecular structure of PC, EMC, DOL, DME, DEGDME, and TEGDME. ... 83

Figure 6.5. Voltage profile for galvanostatic lithium plating/stripping at a current density of 1 mA cm⁻² with different electrolyte including PC, EMC, DOL, DME, DEGDME, and TEGDME. 83 Figure 6.6. SEM images of Li-metal anode of CNT@MoS2 after the litihium plating according to electrolytes.. ... 84

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Figure 6.7. Raman spectra of CNT@MoS2 anodes after electrochemical testing with differenet electrolyte conditions.. ... 85 Figure 6.8. Voltage profile for galvanostatic lithium plating/stripping at a current density of 1 mA cm⁻²

of CNT and CNT@MoS2 (1M LiTFSI in TEGDME).. ... 86 Figure 6.9. STEM images of 2H MoS2 and 1T LixMoS2.. ... 86

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List of Tables

Table 3.1. Quantitative analysis of the Se vacancy according to hydrogen concentrations. ... 22 Table 3.2. XPS Mo and Se 3d peak positions with varying hydrogen concentrations. ... 26 Table 3.3. Comparison of the HER activity of diverse MoSe2-based electrocatalysts. ... 30 Table 4.1. The distance from the sulfur atom at pivot point to each sulfur atom (A~I) and the relative energies (∆ERel) of the MoS1.6 systems in which the corresponding two sulfur atoms were replaced to sulfur vacancies in MoS2 monolayer. ... 52 Table 4.2. The relative energies (∆ERel) of MoS1.4 models where the sulfur atom at the pivot point and the other two sulfur atoms were eliminated in MoS2 monolayer. The numbering #1~8 indicates the data collection that overlaps in Figure 4.17. ... 53 Table 4.3. Comparison of the HER performances for various types of MoS2-based electrocatalysts.. 57

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Abbreviation

Abbreviation Description

2D two-dimensional

h-BN hexagonal boron nitrides

CVD chemical vapor deposition

TCE transparent conducting electrode

THF tetrahydrofuran

CNT carbon nanotubes

HER hydrogen evolution reaction

PV photovoltaics

PCE power conversion efficiecy

VOC open-circuit voltage

JSC short-circuit current density

FF fill factor

FWHM full width at half maximum

FET field-effect transistor

GIWAXS grazing-incidence wide-angle X-ray scattering

OM optical microscopy

AFM atomic force microscopy

TEM transmission electron microscopy

SEM scanning electron microscopy

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

IPA isopropyl alcohol

UV ultraviolet

DFT density functional theory

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TDT tert-dodecylmercaptan

PMMA poly(methyl methacrylate)

AMO ammonium molybdenum oxide

ATO ammonium tungsten oxide

RHE reversible hydrogen electrode

SHE standard hydrogen electrode

PC propylene carbonate

DOL 1,3-dioxolane

EMC ethyl methyl carbonate

DME dimethoxyethane

DEGDME diethylene glycol dimethyl ether

TEGDME triethylene glycol dimethyl ether

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1

CHAPTER 1. Introduction

efects that could significantly affect intrinsic properties were inevitably generated in real materials owing to the kinetics of processing and thermal equilibrium.¹ Therefore, many efforts have been devoted to modulating the physicochemical properties by engineering diverse defects.² A representative example of physicochemical properties modulation through defect engineering is controlling the transport characteristics of silicon-based electronic devices via the chemical doping process.³ In this regard, precise control of the defects in crystalline materials is one of the critical factors that can efficiently determine physicochemical properties.

Figure 1.1. Classification of 2D materials according to electrical properties and their potential applications in diverse fields. (Reprinted with permission from Nano Lett. 2020, 20, 1620. Copyright 2021 American Chemical Society / Reprinted with permission from Sol. RRL 2019, 1900420. Copyright 2021 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim / Reprinted with permission from Nat.

Recently, 2D van der Waals layered materials show unique band structure, strong light-matter interaction, negligible dangling bonds, and high specific surface area which is not observed in 3D bulk states.⁴ 2D materials are classified into three types according to their electrical properties from the

D

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conductor (graphene) to insulator (hexagonal boron nitride, h-BN) as shown in Figure 1.1. Graphene is the first discovered 2D material which composed of all carbon atoms with honeycomb lattice structures. With its outstanding physicochemical properties, graphene has been investigated as an emerging nanomaterial with diverse potential applications including transparent conducting electrode (TCE), membrane filters, and drug delivery.^5,6 Hexagonal boron nitride is also one of the 2D materials family which has a honeycomb lattice composed of boron and nitrogen atom. Unlike graphene which exhibits semi-metallic properties, the h-BN shows insulating properties owing to its large bandgap (Eg

> 5 eV) and applies to the dielectric layer for advanced electronics and tunneling layers.⁷ Transition metal dichalcogenides (TMDs) are composed of X–M–X sandwiched structure, where M is a transition metal (= Mo, W) and X is a chalcogen (= S, Se, and Te) with the stoichiometry MX2. TMDs possess outstanding physicochemical properties, including high carrier mobility, high electrocatalytic activity, high chemical and mechanical stability, and bandgap tunability.⁸ Based on these outstanding intrinsic properties, many previous studies demonstrated that TMDs have potential application in diverse fields such as electrocatalysts, electronic devices, and secondary ion batteries.⁹

However, in most cases, the best performance of TMDs in target-specific applications is not yet realized. To achieve the best performance of TMDs, the physicochemical properties should be effectively modulated according to target-specific applications. To this end, diverse approaches such as heterostructure formation,¹⁰ introducing mechanical strain,¹¹ and chemical or molecular doping^12-14 are investigated to maximize the intrinsic properties of TMDs.

In this thesis, introducing vacancy and chemical/molecular doping approaches were utilized to control the physicochemical properties of TMDs. In addition, the intrinsic properties modulated TMDs was applied to electrocatalyst for hydrogen production, electronic devices, and secondary ion battery to demonstrate the feasibility of activation of desired properties according to target applications through defect engineering. The following chapters provide the background of the material properties modulation via defect engineering of TMDs for a better understanding of the thesis.

Chapter 2 provides information on the classification of defects according to their dimensions, methodology to the generation of defects, and the effects of the defects on the physicochemical properties of TMDs. In addition, we will also cover in detail how defects, especially point defects, can be utilized to target-specific applications by controlling the physicochemical properties of 2D semiconductors.

Chapter 3 demonstrates the synthetic methods of the vacancy-induced molybdenum diselenides (MoSe2) by hydrogen reactivity control during the CVD process without any post-treatment. The one- step synthesized vacancy-MoSe2 exhibited coalesced vacancy in their lattice, which resulted in enhanced overall electrocatalytic, one of the best values reported for TMDs to date in our knowledge.

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3

In this chapter, we introduce the possibility of controlling the electrocatalytic reaction mechanism by modulating physicochemical properties of TMDs-based electrocatalysts for hydrogen production which is induced by introducing the chalcogen vacancy.

Chapter 4 provides that wafer-scale in-situ vacancy-tunable TMDs thin film was realized via a facile solution-phase synthesis route. The chalcogen vacancy in the TMDs lattice enabled the modulation of electronic band structure, electrochemical, and optical properties through the generation of donor states and inducing doping. The as-prepared vacancy-induced TMDs as the electrocatalyst exhibited enhanced hydrogen production capability owing to the preferred intrinsic properties as the electrocatalysts through defect engineering. In this chapter, the role and utilization of the chalcogen vacancy as doping agents will be introduced.

Chapter 5 is devoted to modulating the electrical property of 2D semiconductors via the implementation of molecular doping. Though the molecular doping (impurities) on TMDs surface, leading to modulate the charge carrier density of TMDs. The electrical property-modulated TMDs exhibited enhanced diode characteristics when fabricated PN junction electronic devices. As a result, the fabricated electronic devices exhibit the improvement of overall photovoltaic effects. In this chapter, the way to maximize the performance of TMDs-based electronic devices through the doping process is presented.

Chapter 6 presents the applicability of vacancy-induced TMDs as the host materials for stable Li- metal batteries. The CNT@TMDs composite core-shell anodes were synthesized through the organometallic reaction-based liquid-phase appaches. The phase transition and activation of the piezoelectric properties in the vacancy-induced TMDs induced the lateral growth of Li-metal by enhancing Li-ion flux and increase the binding energy of Li-ion and TMDs, effectively inhibiting the dendritic growth of Li-metal. In this chapter, we present the feasibility of controlling Li-ion kinetics in energy storage devices through defect engineers of host materials.

The conclusion and perspective on the field are presented in the last chapter.

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References

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(10) Liu, Y.; Weiss, N. O.; Duan, X.; Cheng, H.-C.; Huang, Y.; Duan, X. Van der Waals heterostructures and devices. Nat. Rev. Mater. 2016, 1, 16042.

(11) Johari, P.; Shenoy, V. B. Tuning the Electronic Properties of Semiconducting Transition Metal Dichalcogenides by Applying Mechanical Strains. ACS Nano 2012, 6, 5449-5456.

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Sinnott, S. B.; Terrones, M. Carbon Doping of WS2 Monolayers: Bandgap Reduction and p-type Doping Transport. Sci. Adv. 2019, 5, eaav5003.

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Kc, S.; Dubey, M.; Cho, K.; Wallace, R. M.; Lee, S.-C.; He, J.-H.; Ager, J. W.; Zhang, X.;

Yablonovitch, E.; Javey, A. Near-unity photoluminescence quatum yield in MoS2. Science 2015, 350, 1065-1068.

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(14) Yoo, H.; Hong, S.; On, S.; Ahn, H.; Lee, H.-K.; Hong, Y. K.; Kim, S.; Kim, J.-J. Chemical Doping Effects in Multilayer MoS2 and its Application in Complementary Inverter. ACS Appl. Mater.

Interfaces 2018, 10, 23270-23276.

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CHAPTER 2. Defects in 2D Semiconductors: Classification, Generation, and Their Effects on the Material Properties

2.1. Classification of Defects

2.2. Generation of Chalcogen Vacancy

2.3. Effect of Chalcogen Vacancy on the Materials Properties of TMDs 2.4. Effect of Chemical Doping on the Materials Properties of TMDs 2.5. References

This chapter provides backgrounds of the defect engineering in 2D semiconductors on which there is little discussion in each chapter. It covers from the classification of defects to their effect on the material properties of 2D semiconductors.

2.1 Classification of Defects

Figure 2.1. Classification of various defects in 2D semiconductors according to their dimensionality.

(Reprinted from Ref. 1, Copyright@2021 IOP publishing)

Owing to the second law of thermodynamics and kinetic process 2D semiconductors inevitably possess diverse types of defects in their crystal structure. These defects were classified into three types according to their dimension from two-dimensional (2D) to zero-dimensional (0D) defects as shown in Figure 2.1.¹

Theoretically, 2D materials become unstable under thermal fluctuation, but as confirmed in graphene, the formation of ripples, one of the 2D defects, was stabilized the 2D materials.² These ripples induce mechanical strains to 2D materials, leading to a change in the electronic structure of 2D materials.³ Formation of van der Waals vertical 2D heterostructures (i.e., MoS2/WS2, graphene/h-BN, MoS2/graphene) also significantly affects to electronic and optical properties at their interfaces, regarding as 2D defects.^4,5 The interfaces formed by vertical heterostructures exhibited intriguing

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properties such as periodic Moire patterns which change in optoelectronic properties.⁶

Topological defects (i.e., grain boundary), edge, and line vacancy are regarded as 1D defects. When 2D materials were exposed to high energy circumstances such as electron bombardment, agglomerated vacancy was formed in its lattice.⁷ In this case, single- or double-line vacancy was normally formed along with the zig-zag direction. This result was attributed to the zig-zag direction having lower formation energy than other forms of line vacancy, confirmed by DFT simulation.⁸ Edge is one of the prominent defects in TMDs flake which is normally observed by OM and SEM analysis. This edge was easily controlled by modulating synthetic conditions (i.e., hydrogen concentrations, chalcogen vapor pressure) in the CVD process.^9,10 The grain boundary is another type of 1D defect that is normally observed in TMDs films when TMDs flakes merge. In addition, the shape and properties of grain boundaries are dependent on the merged angle.^11,12

The 0D defects such as atomic vacancy, substitution, or molecular doping were most abundant in TMDs. In the case of atomic vacancy, it can be classified according to the composition of the released atoms. These atomic vacancies induced the formation of chalcogen- or transition metal-rich conditions in TMDs which led to effective control of physicochemical properties of TMDs.¹³ Substitutional or molecular doping (impurities) are other types of 0D defects. In the case of substitutional doping, foreign atoms replaced the crystalline lattice atoms. Therefore, when considering substitutional doping, diverse factors including relative atomic size, electronegativity, and crystal structure should be considered.¹ In molecular doping, p- or n-type dopant was deposited onto the TMDs lattice. The properties of TMDs have easily modulated through varying the doping molecule of control of doping concentrations.¹⁴

In this thesis, we focus on 0D defects especially sulfur vacancy and molecular doping to modulate the physicochemical properties of TMDs. In chapters 3 and 4, the effect of chalcogen vacancy on TMDs intrinsic properties will be introduced. Modulating electrical properties of TMDs via doping approach is presented in chapter 5. At last chapter 6, the vacancy-modulated TMDs was applied in Li-metal battery anodes as the protective layer for suppression of Li dendritic growth.

2.2 Generation of Chalcogen Vacancy

In this subsection, diverse approaches to generate 0D defects in TMDs lattice are described.

Especially, we focus on the generation of chalcogen vacancy in the chemical vapor deposition (CVD) approach.

The most widely adopted approaches for generating chalcogen vacancy in TMDs lattice are post- treatments to the synthesized TMDs. Owing to chalcogen vacancy having relatively low formation energy, chalcogen vacancy was easily introduced under reactive conductions such as hydrogen

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annealing, electron beam irradiations, and reactive ion etching (RIE).^15-17

At the hydrogen annealing process, the as-synthesized TMDs was annealed under high temperature flowing with hydrogen gas to generate the chalcogen vacancy. This approach effectively generates the chalcogen vacancy in TMDs lattice, but unwanted multi-dimensional defects (i.e., cracks, hole, line defects) still remain a limitation of the hydrogen annealing process.¹⁸

Exposure to highly reactive sources such as electron beam irradiation and RIE are also commonly used to generate the chalcogen vacancy. However, in the case of the electron beam irradiation approach, it could not be proper approaches for large-scale synthesis of vacancy-modulated TMDs film due to the directionality of the electron beam. RIE approach is also effectively introduced the chalcogen vacancy in TMDs. However, the generation of undesirable defects due to the high reactivity of plasma and the concentration gradient of defects in TMDs lattice due to the directionality of plasma remain issues to be solved.¹⁹

Recently, a method of introducing chalcogen vacancies into synthesized TMDs through electrochemical treatment has been proposed.¹⁹ This approach suppressed the formation of unwanted defects by controlling the applied voltage. In addition, the electrochemical approach has an advantage in uniform and large-area synthesis but the complexity of the synthesis step due to the additional process is still a bottleneck. Therefore, one-step in-situ synthetic methods to synthesis vacancy-modulated TMDs with high uniformity and scalability are required. In chapters 3 and 4, we present a synthetic method of introducing in-situ vacancy via solid-phase and liquid-phase CVD methods.

2.3 Effect of Chalcogen Vacancy on the Materials Properties of TMDs

In this subsection, we demonstrate how defects could affect the physicochemical properties of TMDs.

Chalcogen vacancy in TMDs lattice could limit or widen the potential applications of TMDs. Herein, we focus a description on the positive effects of chalcogen vacancy on target-specific applications.

For the electrical properties, chalcogen vacancy is normally considered as a degrading factor, especially in lateral transports.²⁰ However, in certain conditions, such as vertical transport, chalcogen vacancy acts as an enhanced factor for carrier transports.²¹ Furthermore, selective formation of vacancy- induced TMDs at the region in contact with the metal electrodes could effectively reduce the contact resistance.²² The chalcogen vacancy induced n-type doping to host TMDs, leading to the formation of heavily-doped TMDs. This heavily-doped TMDs exhibited metallic properties, resulting in the formation of Schottky contacts between TMDs and contact metals.²² The electrical properties analysis through the fabrication of field-effect transistor (FET) devices also supported the semiconducting to metallic property transition in vacancy-induced MoS2.²³

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In the case of the electrochemical properties, the chalcogen vacancy was utilized to activate the inert basal plane of TMDs. The formation of the localized electrons at vacancy sites led to introduce the strain in the MoS2 lattice.²⁴ The strain in MoS2 could increase the active sites of electrochemical reaction for hydrogen evolution reaction (HER) by activating its basal plane.²⁴ The Volmer reaction (H⁺+e^-  H^* (adsorbed hydrogen ion)) which is related to the hydrogen adsorption reaction in HER is promoted by exposing the Mo site in the MoS2 lattice.²⁵ Recently, Lee et al. demonstrated that the Tafel reaction (H^*+H^*H2), related to hydrogen desorption, could be activated in the vacancy MoSe2, leading to enhance overall electrocatalytic activity.²⁶ These results suggest that chalcogen vacancy can be used in a variety of chemical reaction promoters by modulating the intrinsic properties.

The piezoelectric property was also enhanced by inducing the vacancy in TMDs lattice. The computational analysis confirmed that the asymmetrically MoS2 induced by generating the vacancy could improve the piezoelectric response.²⁷ The enhanced piezoelectric response in defective MoS2 is attributed to the break of out-of-plane centrosymmetry formed biased charge distribution in MoS2

lattice.²⁷ In addition, recently, the piezoelectric properties in asymmetric MoS2 were experimentally demonstrated. The strength of piezoelectric was enhanced according to increase the asymmetricity, induced by introducing more defects.²⁸

2.4 Effect of Chemical Doping on the Materials Properties of TMDs

In this subsection, we introduce the modulation of materials properties of TMDs via simple chemical doping approaches.

The chemical doping approach is commonly used to control the electrical properties of TMDs.

Chemical doping is commonly carried out by evaporating or spin-coating the chemical dopant on their surface. The chemical dopant acts as electron donors or electron acceptors of the host materials, leading to a change of charge carrier density in TMDs. Gold chlorides (AuCl3) and molybdenum oxides (MoO3) are commonly used to synthesize p-doped TMDs.^29,30 Otherwise, aluminum oxides (AlOx) and zinc oxides (ZnO) were normally used as n-type dopants.^31,32 In this thesis, we modulated the electrical properties of WSe2 via chemical doping to enhance the performance of PN junction diode characteristics.

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2.5 References

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Defect engineering of two-dimensional transition metal dichalcogenides. 2D Mater. 2016, 3, 022002.

(2) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. The structure of suspended graphene sheets. Nature 2007, 446, 60-63.

(3) Luo, S.; Hao, G.; Fan, Y.; Kou, L.; He, C.; Qi, X.; Tang, C.; Li, J.; Huang, K.; Zhong, J. Formation of ripples in atomically thin MoS2 and local strain engineering of electrostatic properties.

Nanotechnology 2015, 26, 105705.

(4) Geim, A. K.; Grigoieva, I. V. Van der Waals heterostructures. Nature 2013, 499, 419-425.

(5) Lin, Y.-C.; Lu, N.; Perea-Lopez, N.; Li, J.; Lin, Z.; Peng, X.; Lee, C.-H.; Sun, C.; Calderin, L.;

Browning, P. N.; Bresnehan, M. S.; Kim, M. J.; Mayer, T. S.; Terrones, M.; Robinson, J. A. Direct Synthesis of van der Waals Solids. ACS Nano 2014, 8, 3715-3723.

(6) Kang, J.; Li, J.; Li, S.-S.; Xia, J.-B.; Wang, L.-W. Electronic Structural Moire Pattern Effects on MoS2/MoSe2 2D Heterostructures. Nano Lett. 2013, 13, 5485-5490.

(7) Komsa, H.-P.; Kurasch, S.; Lehrinen, O.; Kaiser, U.; Krasheninnikov, A. V. From point to extended defects in two-dimensional MoS2: Evolution of atomic structure under electron irradiation. Phys.

Rev. B 2013, 88, 035301.

(8) Han, Y.; Hu, T.; Li, R.; Zhou, J.; Dong, J. Stabilities and electronic properties of monolayer MoS2

with one or two sulfur line vacancy defects. Phys. Chem. Chem. Phys. 2015, 17, 3813-3819.

(9) Chen, J.; Liu, B.; Liu, Y.; Tang, W.; Nai, C. T.; Li, L.; Zheng, J.; Gao, L.; Zheng, Y.; Sin, H. S.;

Loh, K. P. Chemical Vapor Deposition of Large-Sized Hexagonal WSe2 Crystals on Dielectric Substrates. Adv. Mater. 2015, 27, 6722-6727.

(10) Withanage, S. S.; Kalita, H.; Chung, H.-S.; Roy, T.; Jung, Y.; Khodaker, S. I. Uniform Vapor- Pressure-Based Chemical Vapor Deposition Growth of MoS2 Using MoO3 Thin Film as a Precursor for Coevaporation. ACS Omege 2018, 3, 18943-18949.

(11) Lin, J.; Pantelides, S. T.; Zhou, W. Vacancy-Induced Formation and Growth of Inversion Domains in Transition-Metal Dichalcogenide Monolayer. ACS Nano 2015, 9, 5189-5197.

(12) Azizi, A.; Zou, X.; Ercius, P.; Zhang, Z.; Elias, A. L.; Perea-Lopez, N.; Stone, G.; Terrones, M.;

Yakobson, B. I.; Alem, N. Dislocation motion and grain boundary migration in two-dimensional tungsten disulphide. Nat. Commum. 2014, 5, 4867.

(13) Liang, Q.; Zhang, Q.; Zhao, X.; Liu, M.; Wee, A. T. S. Defect Engineering of Two-Dimensional Transition-Metal Dichalcogenides: Applications, Challenges, and Opportunities. ACS Nano 2021, 15, 2165-2181.

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(14) Yoo, H.; Heo, K.; Ansari, M. H. R.; Cho, S. Recent Advances in Electrical Doping of 2D Semiconductor Materials: Methods, Analyses, and Applications. Nanomaterials 2021, 11, 832.

(15) Ye, G.; Gong, Y.; Lin, J.; Li, B.; He, Y.; Pantelides, S. T.; Zhou, W.; Vajtai, R.; Ajayan, P. M. Defects Engineered Monolayer MoS2 for Improved Hydrogen Evolution Reaction. Nano Lett. 2016, 16, 1097-1103.

(16) Stanford, M. G.; Pudasaini, P. R.; Belianinov, A.; Cross, N.; Noh, J. H.; Koehler, M. R.; Mandrus, D. G.; Duscher, G.; Rondinone, A. J.; Ivanov, I. N.; Ward, T. Z.; Rack, P. D. Focused helium-ion beam irradiation effects on electrical transport properties of few-layer WSe2: enabling nanoscale direct write homo-junctions. Sci. Rep. 2016, 6, 27276.

(17) Tosun, M.; Chan, L.; Amani, M.; Roy, T.; Ahn, G. H.; Taheri, P.; Carraro, C.; Ager, J. W.;

Maboudian, R.; Javey, A. Air-Stable n-Doping of WSe2 by Anion Vacancy Formation with Mild Plasma Treatment. ACS Nano 2016, 10, 6853-6860.

(18) Wang, X.; Zhang, Y.; Si, H.; Zhang, Q.; Wu, J.; Gao, L.; Wei, X.; Sun, Y.; Liao, Q.; Zhang, Z.;

Ammarah, K.; Gu, L.; Kang, Z.; Zhang, Y. Single-Atom Vacancy Defect to Trigger High-Efficiency Hydrogen Evolution of MoS2. J. Am. Chem. Soc. 2020, 142, 4298−4308.

(19) Tsai, C.; Li, H.; Park, S.; Park, J.; Han, H. S.; Nørskov, J. K.; Zheng, X.; Ablid-Pedersen, F.

Electrochemical Generation of Sulfur Vacancies in the Basal Plane of MoS2 for Hydrogen Evolution. Nat. Commun. 2017, 8, 15113.

(20) Ma, D.; Wang, Q.; Li, T.; He, C.; Ma, B.; Tang, Y.; Lu, Z.; Yang, Z. Repairing sulfur vacancies in the MoS2 monolayer by using CO, NO and NO2 molecules. J. Mater. Chem. C 2016, 4, 7093-7101.

(21) Liu, Y.; Gao, Z.; Tan, Y.; Chen, F. Enhancement of Out-of-Plane Charge Transport in a Vertically Stacked Two-Dimensional Heterostructure Using Point Defects. ACS Nano 2018, 12, 10529-10536.

(22) Shen, P.-C.; Su, C.; Lin, Y.; Chou, A.-S.; Cheng, C.-C.; Park, J.-H.; Chiu, M.-H., Lu, A.-Y.; Tang, H.-L.; Tavakoli, M. M.; Pitner, G.; Ji, X.; Cai, Z.; Mao, N.; Wang, J.; Tung, V.; Li, J.; Bokor, J.;

Zettl, A.; Wu, C.-I.; Palacios, T.; Li, L.-J.; Kong, J. Ultralow contact resistance between semimetal and monolayer semiconductors. Nature 2021, 593, 211-217.

(23) Yang, J.; Wang, Y.; Lagos, M. J.; Manichev, V.; Fullon, R.; Song, X.; Voiry, D.; Chakraborty, S.;

Zhang, W.; Batson, P. E.; Feldman, L.; Gustafsson, T.; Chhowalla, M. Single Atomic Vacancy Catalysis. ACS Nano 2019, 13, 9958−9964.

(24) Li, H.; Tsai, C.; Koh, A. L.; Cai, L.; Contryman, A. W.; Fragapane, A. H.; Zhao, J.; Han, H. S.;

Manoharan, H. C.; Abild-Pedersen, F.; Nørskov, J. K.; Zheng, X. Activating and Optimizing MoS2

Basal Planes for Hydrogen Evolution through the Formation of Strained Sulphur Vacancies. Nat.

Mater. 2016, 15, 48−54.

(25) Huang, Y.; Nielsen, R. J.; Ill, W. A.G. Reaction Mechanism for the Hydrogen Evolution Reaction

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on the Basal Plane Sulfur Vacancy Site of MoS2 Using Grand Canonical Potential Kinetics. J. Am.

Chem. Soc. 2018, 140, 16773-16782.

(26) Lee, J.; Kim, C.; Choi, K.; Seo, J.; Choi, Y.; Choi, W.; Kim, Y.-M.; Jeong, H. Y.; Lee, J. H.; Kim, G.; Park, H. in-Situ Coalesced Vacancies on MoSe2 Mimicking Noble Metal: Unprecedented Tafel Reaction in Hydrogen Evolution. Nano Energy 2019, 63, 103846.

(27) Ng, L.-R.; Chen, G.-F.; Lin, S.-H. Generating large out-of-plane piezoelectric properties of atomically thin MoS2 via defect engineering. Phys. Chem. Chem. Phys. 2021, 23, 23945-23952.

(28) Choi, W.; Kim, J.; Lee, E.; Mehta, G.; Prasad, V. Asymmetric 2D MoS2 for Scalable and High- Performance Piezoelectric Sensors. ACS Appl. Mater. Interfaces 2021, 13, 13596-13603.

(29) Liu, X.; Qu, D.; Ryu, J.; Ahmed, F.; Yang, Z.; Lee, D.; Yoo, W. J. P-Type Polar Transition of Chemically Doped Multilayer MoS2 Transistor. Adv. Mater. 2016, 28, 2345-2351.

(30) Nazif, K. N.; Kumar, A.; Hong, J.; Lee, N.; Islam, R.; Mclellan, C. J.; Karni, O.; Croep, J.; Heinz, T. F.; Pop, E.; Brongersma, M. L.; Saraswat, K. C. High-Performance p–n Junction Transition Metal Dichalcogenide Photovoltaic Cells Enabled by MoOx Doping and Passivation. Nano Lett.

2021, 21, 3443-3450.

(31) McClellan, C. J.; Yalon, E.; Smithe, K. K. H.; Suryavanshi, S. V.; Pop E. High Current Density in Monolayer MoS2 Doped by AlOx. ACS Nano 2021, 15, 1587-1596.

(32) Kang, D.-H.; Hong, S.-T.; Oh, A.; Kim, S.-H.; Yu, H.-Y.; Park, J.-H. Nondegenerate n-type doping phenomenon on molybdenum disulfide (MoS2) by zinc oxide (ZnO). Mater. Res. Bull. 2016, 82, 26-30.

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CHATER 3. In Situ Coalesced Vacancies on MoSe

2

Mimicking Noble Metal:

Unprecedented Tafel Reaction in Hydrogen Evolution

3.1. Research Background 3.2. Experimental Details 3.3. Results and Discussion 3.4. Conclusion

3.5. References

3.1 Research Background

Recently, 2D semiconductor transition metal dichalcogenides (TMDs) have received widely attentions due to their outstanding physicochemical properties such as wide bandgap tunability, high carrier mobility, and high catalytic activity, leading to apply diverse potential applications.^1-6 In particularly, high mechanical, chemical stability and oustanding electrochemical properties make TMDs a promising alternative to noble metal-based electrochemical catalysts (i.e., Pt, Ru, Ir).^7-13 However, the electrocatalytic activity such as hydrogen evolution reaction (HER), including on-set potential and Tafel slope, of pristine 2H-phase TMDs (trigonal prismatic structure) still much lower than noble metal-based electrocatalysts owing to limitation of their active sites such as edge of TMDs.¹⁴ To this end, several strategies such as nanostructure formation, chemical doping, and phase engineering were demonstrated to enhance the catalytic activity of TMDs by increasing the active sites.^14-21 Although, formation of TMDs nanostructure is enhanced the overall HER activity by increase the active sites, the complicated synthesis process remains a bottleneck.¹⁴ Activation of TMDs basal plane through the phase transition (2H  1T) or heteroatom doping approaches improves the HER activity, however, have limitation in practical application owing to low stability of 1T-phase TMDs.^15-19 Recently, tensile strain and sulfur vacancies in TMDs lattice can optimize HER reaction free energy difference, leading to activate the basal plane.²¹ In this respect, diverse post-treatment processes such as plasma etching, hydrogen annealing, electrochemical etching were effectively generated the defect sites (i.e., chalcogen vacancy) to enhance the HER activity.^19-21 Although diverse approaches were demonstrated to improve the HER activity of TMDs-based electrocatalysts, the activation of Volmer-Tafel reaction in this system has not been demonstrated.

In this work, in-situ chalcogen vacancy engineered molybdenum diselenides (MoSe2) was synthesized through the reactivity control of hydrogen during the chemical vapor deposition (CVD) process. Such vacancy-engineered MoSe2 exhibited outstanding overall HER activity with an exceptionally low Tafel slope for hydrogen production. In addition, the concentrations of defects (Se

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vacancy) in as-synthesized MoSe2 are strongly influenced by the concentrations of hydrogen at synthesis step. In this step, the hydrogen gas has dual role as a reaction promoter or etching agent which is improved the substitution reaction of MoO3 and Se (decrease the Se vacancy concentrations) or etching the Se atom in MoSe2 lattice (increase the Se vacancy concentrations), respectively.^22-23 The vacancy-modulated MoSe2 which has coalesced Se vacancy in their lattice exhibited improved overall HER activity with unprecedented Tafel slope at high overpotential region, mainly attributing to the not only activation of additional Tafel reaction but also contribution of Heyrovsky reaction for hydrogen production. Moreover, the density functional theory (DFT) calculations reveal that the hydrogen adsorption free energy for Volmer reaction and hydrogen diffusion barrier (ED) for activation of Tafel reaction were optimized by introducing Se vacancy in MoSe2 lattice, leading to enhance overall HER activity.

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3.2 Experimental Details

Vacancy-modulated MoSe2 synthesis: Singlelayer MoSe2 was synthesized by CVD method under ambient pressure. For MoSe2 synthesis, MoO3 (99.998%, Alfa Aesar) powder was located in ceramic boat at the center of furnace and sapphire substrate was placed at the downstream of MoO3 source. Se powder (99.999%, Alfa Aesar) was placed upstream with independent heating belt and carried by Ar and H2 mixture gases. At first, to remove any residual moisture MoO3 powder was annealed at 200 °C for 20 min, after that the furnace was ramped upto 700 °C. To achieve the simultaneous supply of MoO3

and Se precursors, Se powder was ramped to 300 °C when the chamber temperature of MoO3 source reached at 400 °C. All the MoSe2 samples synthesized with diverse hydrogen concentrations from 0%

to 50% were prepared under the same conditions as mentioned above. After synthesis of vacancy- engineered MoSe2, the furnace was cooled down to room temperature under the flow with only Ar to prevent additional Se etching by H2.

Electrochemical analysis: Vacancy-modulated MoSe2 film was transferred through conventional poly(methyl methacrylate) (PMMA)-assisted wet transfer method. After coating the PMMA on MoSe2/sapphire film, sapphire substrate was detached by treating the buffered oxide etch (BOE, J.T.

Baker) solution and washed with D.I. water. After transferring the PMMA/MoSe2 film onto the glassy carbon electrode, PMMA layer was removed by treating the acetone solution. To characterize the electrochemical properties, a graphite electrode wire was used and Ag/AgCl (saturated KCl filled) was used as a counter electrode and reference electrode, respectively. A N2-saturated 0.5M H2SO4 solution was used as the electrolyte. The electrochemical performance was analyzed by using a computercontrolled potentiostat (Biologic VMP3) with a rotating disk electrode system (RRDE-3A, ALS Co.) at a scan rate of 10 mV sec⁻¹. A calibration in reversible hydrogen electrode (RHE) was experimentally determined in 0.5M H2SO4 with H2-saturated at a scan rate of 1 mV s⁻¹, where the Ag/AgCl as the reference electrode and platinum wire was used as the counter and working electrode.

For half-cell configuration, the charge transfer resistance of the 0.5M H2SO4 solution was measured to apply iR correction. Tafel slopes were obtained through the linear fitting of polarization curve of RDE around the onset potential region (@1 mA cm⁻²) and the activation-controlled region (@10 mA cm⁻²).

The Tafel slope was obtained by followed equation: η=a + b log| i | (b=0.059/nα), where η is potential, b is Tafel slope, i is current density, n is the number of electrons involved, and α is transfer coefficient.

DFT calculation: Diverse DFT calculations were carried out using Vienna ab initio Simulation Package.^24,25 The exchange correlation functional is described by revised Perdew-Burke-Ernzerhof (RPBE) functional with the general gradient approximation (GGA) method.^26,27 Pseudo potentials

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generated under projector-augmented wave (PAW) method was used.²⁸ The cut-off energy for plane- wave basis was set to 500 eV and the force criteria for optimizing the structure was set to 0.01 eV/Å . The lattice constant of periodic cell was 16.7×16.7×16 Å³, which corresponds to 5×5 supercell. Climb- Image Nudged Elastic Band (CI-NEB) method was applied to calculate the hydrogen diffusion barrier energy.²⁹ The hydrogen adsorption free energy was obtained by using the following relation, Ea=E(MoSe2+H)–(E(MoSe2)+1/2E(H2)). The adsorption free energy is calculated by using the following equation, ΔGH=Ea + ZPE - TΔS, where Ea is adsorption energy, T is temperature, S is entropy, and ZPE is zero point energy, respectively. The thermodynamic corrections terms, ZPE-TΔS, for metals and TMDs were adopted from the previous studies with 0.29 and 0.24 eV, respectively.^30,31

Characterization: Morphology of vacancy-modulated MoSe2 was carried out using Dimension AFM (Veeco, DI-3100) with tapping mode and FE-SEM (Hitachi, S-4800). Raman and PL spectroscopy were obtained by using confocal Raman (WITec, alpha300R) with 532 nm monochromatic wavelength.

Chemical composition and defect density analyses were investigated by XPS (Thermo Fisher Scientific, ESCALAB 250XI). Atomic-resolution ADF-STEM images were obtained at 200 kV using FEI Titan3 G2 60–300 with a double-side spherical aberration (Cs) corrector. The probe convergence semi-angle was set to be ~25 mrad. ADF-STEM images were acquired from 50 to 200 mrad range. EDS elemental mapping images were obtained by using a JEOL 2100F microscope with an accelerating voltage of 200 kV.

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3.3 Results and Discussion

Figure 3.1. Schematic illustration of vacancy-engineered MoSe2 synthesis according to hydrogen concentrations. Hydrogen plays a dual role in decrease the Se vacancy by promoting the reduction of MoO3 and increase the Se vacancy by etching the Se atom at the MoSe2 lattice.

In this work, monolayer MoSe2 film was synthesized by CVD method using Se and MoO3 powder as precursors with argon and hydrogen as carrier gases, and the concentrations of Se-vacancy were one- step modulated by controlling the concentration of hydrogen gas during the growth process. The samples are referred to as MoSe2-X, where X=0, 10, 20, 30, 40, and 50 indicates the relative hydrogen concentration with argon in the carrier gas. Figure 3.1 schematically illustrates the growth mechanism of synthesizing Se vacancy-modulated MoSe2: i) Hydrogen gas reduce stable MoO3 to reactive MoO3−x, leading to enhance the reactivity between Mo precursor and Se precursor, ii) Substitution reaction between selenium and oxygen in Moby selenization reaction of MoO3−x to form MoSe2 vapor, iii) Seeding and growth of MoSe2 on the targeted substrate, and iv) Hydrogen-assisted Se atom etching in MoSe2 lattice by forming H2Se gas. Detailed parameters for vacancy-modulated MoSe2 synthesis are provided in Figure 3.2.

Figure 3.2. Synthesis profile of in-situ vacancy-modulated MoSe2. Chalcogen vacancy in MoSe2 lattice was modulated by varing the carrier gas ratio (H2/Ar) to control the reactivity of hydrogen during the growth step. Cooling process was carried out under Ar atmosphere to prevent additional Se etching by hydrogen gas.