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Title Page

ABSTRACT

Contents

LIST OF ABBREVIATIONS 13

CHAPTER Ⅰ. GENERAL INTRODUCTION 15

1.1. Transition metal dichalcogenides 15

1.1.1. Atomic structure of TMDs 15

1.1.2. Intrinsic properties of TMDs 16

1.1.3. TMDs-based functional applications 16

1.2. Synthesis of 2D semiconductors 18

1.2.1. Mechanical and liquid phase exfoliation 18

1.2.2. Chalcogenization of transition metal oxides thin film 18

1.2.3. Thermal decomposition of thiosalts 18

1.2.4. Vapor phase reaction of transition metal oxides with chalcogen 18

1.3. Bibliography 20

CHAPTER Ⅱ. CONTROLLABLE SUBSTITUTIONAL DOPING IN WAFER-SCALE 2D SEMICONDUCTORS 22

2.1. Introduction 22

2.2. Experimental Details 24

2.2.1. V-MoS₂ synthesis 24

2.2.2. FET device fabrication 24

2.2.3. Characterization 24

2.3. Results and Discussions 25

2.4. Conclusions 31

2.5. Bibliography 32

CHAPTER Ⅲ. EPITAXIAL GROWTH OF HIGHLY ORIENTED 2D SEMICONDUCTORS 34

3.1. Introduction 34

3.2. Experimental Details 36

3.2.1. Highly oriented TMDs synthesis 36

3.2.2. CFD simulations 36

3.2.3. FET device fabrication 37

3.2.4. Characterization 37

3.3. Results and Discussions 38

3.4. Conclusions 49

3.5. Bibliography 50

CHAPTER Ⅳ. WAFER-SCALE SYNTHESIS OF PATTERNABLE 2D SEMICONDUCTORS 52

4.1. Introduction 52

4.2. Experimental Details 54

4.2.1. Pre-patterned Ni layer fabrication 54

4.2.2. Patterned WS₂ synthesis 54

4.2.3. FET device fabrication 54

4.2.4. Characterization 54

4.3. Results and Discussions 56

4.4. Conclusions 63

4.5. Bibliography 64

CHAPTER Ⅴ. ULTRASENSITIVE MOLECULAR SENSING PLATFORMS FROM 2D HETEROSTRUCTURE 66

5.1. Introduction 66

5.2. Experimental Details 69

5.2.1. ReOxSy synthesis[이미지참조] 69

5.2.2. Graphene/ReOxSy synthesis[이미지참조] 69

5.2.3. Flexible SERS substrate 69

5.2.4. SERS measurements 69

5.2.5. DFT calculations 69

5.2.6. Characterization 70

5.3. Results and Discussions 71

5.4. Conclusions 84

5.5. Bibliography 85

CHAPTER Ⅵ. SINGLE-ATOM DOPED 2D SEMICONDUCTORS FOR MOLECULAR SENSING 87

6.1. Introduction 87

6.2. Experimental Details 89

6.2.1. VSADReSe₂ synthesis[이미지참조] 89

6.2.2. DFT simulation 89

6.2.3. DFT simulation 89

6.2.4. Characterization 90

6.3. Results and Discussions 91

6.4. Conclusions 99

6.5. Bibliography 100

CHAPTER Ⅶ. SUMMARY 102

CURRICULUM VITAE 106

List of Tables

Table 5.1. Growth stage for the synthesis of ReOxSy-LT/MT/HT.[이미지참조] 72

Table 5.2. Atomic percentages of Re, O, and S atoms in the as-synthesized ReOxSy-LT/MT/HT.[이미지참조] 75

List of Figures

Figure 1.1. (a) MX₂ structure of TMDs consisting of a transition metal and two chalcogens. (b) Polytype structure of TMDs (1T, 2H, and 3R). 15

Figure 1.2. Diverse intrinsic properties of TMDs. 16

Figure 1.3. Diverse functional applications of TMDs. 17

Figure 1.4. (a) Mechanical and (b) liquid phase exfoliation for fabrication of TMDs flakes. (c) Chalcogenization process of transition metal oxides thin film and thermal decomposition process of... 19

Figure 2.1. (a) Schematic of the synthesis process of V-MoS₂ via the liquid precursor-assisted CVD process. (b-d) OM images of as-synthesized V-MoS₂ with different potassium iodide concentrations (V-... 26

Figure 2.2. (a-c) HAADF-STEM images of as-synthesized MoS₂ under the different growth conditions (scale bars: 2nm). Yellow dashed circles indicate the sites of substitutionally doped V atoms in the MoS₂... 27

Figure 2.3. XPS spectra of (a) Mo 3d, (b) S 2p, and (c) V 2p for the as-synthesized MoS₂ with different growth conditions. 28

Figure 2.4. (a) Digital image and (b) OM images of wafer-scale V-MoS₂/P2 film. (c) 2D mapping of Raman signal intensity, (d) Rv peak position and intensity, and (e) PL spectra from the randomly selected...[이미지참조] 29

Figure 2.5. (a) OM image and schematic of V-MoS₂/P2 film-based FET arrays. (b) output and (c) transfer characterizations for pristine MoS₂. (d) output and (e) transfer characterizations for V-MoS₂/P2.... 30

Figure 3.1. (a) OM image and (b) orientation histogram of MoS₂ synthesized on the pre-annealed sapphire substrate at 1,000 ℃. 39

Figure 3.2. (a) OM image and (b) orientation histogram of MoS₂ synthesized at high growth temperature (900 ℃). 39

Figure 3.3. (a) OM image and (b) orientation histogram of MoS₂ synthesized with high sulfur-to-molybdenum ratio. 39

Figure 3.4. (a) Schematic of synthesis of MoS₂ by controlling the chalcogen vapor pressure under liquid precursor-mediated CVD process. (b) Growth behavior of MoS₂ with two different chalcogen vapor... 40

Figure 3.5. (a,b) OM images and (c) orientation histogram of MoS₂ synthesized under the two different chalcogen vapor pressure distribution. 41

Figure 3.6. (a) Raman spectrum, (b) PL spectrum, and (c) Raman and PL mapping images of highly oriented MoS₂. Raman and PL peaks corresponding to the sapphire substrate are highlighted by star... 42

Figure 3.7. (a) Carrier-gas profile and (b) vapor pressure of sulfur in the conventional CVD system. (c) Carrier-gas profile and (d) vapor pressure of sulfur in the proposed CVD system. 43

Figure 3.8. (a) Schematic of the growth kinetics of MoS2 under the two different chalcogen vapor pressure profiles. Yellow and black arrows show the migration of sulfur atoms and growth of MoS₂,... 44

Figure 3.9. Orientation mapping profile of MoS₂ synthesized with various temperatures and different Mo concentrations under the quasi-static distribution of the chalcogen vapor pressure. 45

Figure 3.10. Orientation histogram of MoS₂ synthesized with 5 mM Mo precursor at different growth temperature. 45

Figure 3.11. Orientation histogram of MoS₂ synthesized with 25 mM Mo precursor at different growth temperature. 45

Figure 3.12. Orientation histogram of MoS₂ synthesized with 50 mM Mo precursor at different growth temperatures. 46

Figure 3.13. (a) Schematic of aligned MoS₂ based FET device. (b) output characteristics with varying gate biases and (c) transfer characteristics at a Vsd of 2 V for MoS₂ FET. (d) Distribution of electrical...[이미지참조] 47

Figure 3.14. (a) SEM image, (b) orientation histogram, (c) Raman spectrum, and (d) PL spectrum of aligned MoSe₂ synthesized under the quasi-static distribution of the sulfur vapor pressure. 48

Figure 3.15. (a) SEM image, (b) orientation histogram, (c) Raman spectrum, and (d) PL spectrum of aligned WS₂ synthesized under the quasi-static distribution of the sulfur vapor pressure. 48

Figure 4.1. (a) OM image, (b) SEM image, and (c) XRD pattern of polycrystalline Ni layer deposited on a SiO₂ substrate. (d) Schematic of intergranular diffusion of W atoms through the grain boundaries... 57

Figure 4.2. (a) Synthesis procedure for the patterned WS₂ layer and corresponding OM images. XPS S 2p and W 4f spectra of the patterned (b) W layer and (c) WS₂ layer. 58

Figure 4.3. (a) OM image and (b) Raman spectra of the patterned WS₂ layer (red and black circles represent the WS₂ and SiO₂ regions). (c) Raman intensity mapping profiles from the blue box in (a). 59

Figure 4.4. (a,c) OM images and (b,d) Raman spectra for the WS₂ synthesized with other precursors. 59

Figure 4.5. (a) OM image and (b) Raman spectrum of the directly patterned MoS₂ layer. 59

Figure 4.6. OM images of (a) pre-patterned Ag layer and (b) patterned MoS₂ using sacrificial Ag layer. (c,d) Raman spectra of the patterned MoS₂ layer (blue and red circles represent the SiO₂ and MoS₂... 60

Figure 4.7. (a) Digital image, (b) OM images, and (c) peak positions and intensity of E¹₂g and A₁g of the uniformly patterned WS₂ layer synthesized on a 2-inch SiO₂/Si substrate (scale bars: 20 μm). (d)...[이미지참조] 61

Figure 4.8. (a) Schematic and (b) OM image of the fabricated WS₂ FET device. (c) Transfer and (d) output characteristics of the WS₂ FET device. (e) Photoresponsivity of the WS₂ under monochromatic... 62

Figure 5.1. (a) The atomic structure of distorted 1T ReS₂. The diamond-shaped Re 4-clusters form Re chains along a specific direction. (b) Schematic of the liquid precursor-mediated synthesis process of... 71

Figure 5.2. Growth sequence of ReOxSy in the liquid precursor-assisted synthesis (Stages A-E). By modulating growth stage D, the oxygen concentration in the ReOxSy thin film can be controlled.[이미지참조] 72

Figure 5.3. Temperature profiles and gas composition in the growth of ReOxSy thin film via liquid precursor-assisted CVD process (A: ramping process, B: annealing process, C: ramping process, D:...[이미지참조] 72

Figure 5.4. (a) OM image and (b) Raman spectrum of ReOxSy-HT thin film with high uniformity over a large area and high degree of sulfurization. (c) GIWAXS 2D image of the ReOxSy-HT film revealing...[이미지참조] 73

Figure 5.5. (a) AFM image and (b) thickness of ReOxSy-HT thin film on sapphire substrate.[이미지참조] 74

Figure 5.6. (a,b) OM images, (b) thickness, and (c) Raman spectra for the ReOxSy-MT/LT.[이미지참조] 74

Figure 5.7. XPS spectra of the (a) Re 4f, (b) S 2p, and (c) O 1s peaks in ReOxSy films with different growth temperatures.[이미지참조] 75

Figure 5.8. (a) synthesis process of graphene/ReOxSy vertical heterostructure via liquid precursor-assisted CVD process. (b) OM image of 2D vertical heterostructure on the SiO₂/Si substrate. (c) AFM...[이미지참조] 76

Figure 5.9. (a) Raman spectrum of graphene/ReOxSy vertical heterostructure. (b) Raman mapping of G mode for graphene and A₁g mode for ReOxSy from 2D vertical heterostructure.[이미지참조] 77

Figure 5.10. XPS spectra of the (a) Re 4f, (b) S 2p, and (c) C 1s peaks from the 2D vertical heterostructure. 77

Figure 5.11. (a) Illustration of Raman enhancement effect for R6G on the ReOxSy-HT/MT/LT thin films and molecular structure of R6G. (b) SERS effect of R6G (10⁻⁴ M) on the ReOxSy synthesized with...[이미지참조] 78

Figure 5.12. Charge transfer resonance in (a) R6G-graphene and (b) R6G-ReS₂. 78

Figure 5.13. Charge transfer resonance (a) from molecule to semiconductor and (b) from semiconductor to molecule, which is coupled with molecular and exciton resonances. 79

Figure 5.14. (a) Density of states of the ReOxSy-HT/MT/LT. (b) Energy diagram and diverse resonances between R6G and ReOxSy-HT/MT/LT. (c) Charge density plot of ReOxSy-MT.[이미지참조] 80

Figure 5.15. (a) Schematic of SERS effect in the 2D vertical heterostructure. (b) SERS spectra and (c) analytical enhancement factors of R6G (10⁻⁴ M) on the different as-synthesized 2D materials. (d)... 81

Figure 5.16. (a) Digital image and (b) SERS signals of the flexible SERS substrate with bending test. (c) SERS spectra and (d) SERS signal intensity of R6G with different concentrations on the 2D vertical... 83

Figure 6.1. (a) Schematic of the VSADReSe₂ growth via liquid precursor-assisted CVD process. (b,c) Projected and total DOS of pristine ReSe₂ and VSADReSe₂.[이미지참조] 91

Figure 6.2. (a) Two position types for the substitutional doping of vanadium atoms in the VSADReSe₂ lattice. (b) Atomic structure of VSADReSe₂ for position I. (c) Projected DOS of the vanadium atom and...[이미지참조] 92

Figure 6.3. (a,b) OM image and thickness of pristine ReSe₂ and VSADReSe₂ (scale bars: 10 μm). (c,d) Intensity of the Ag mode of as-synthesized ReSe₂ as a function of the polarization angle. (e,f) XPS...[이미지참조] 93

Figure 6.4. (a) Raman spectra and (b) XPS spectra of V 2p obtained from pristine ReSe₂ and VSADReSe₂.[이미지참조] 94

Figure 6.5. (a) Atomic structure, (b) experimental HAADF-STEM, and (c) simulated HAADF-STEM images of pristine 1T' ReSe₂. (scale bars: 2 nm). (d) Atomic structure, (e) experimental HAADF-STEM,... 95

Figure 6.6. (a) Schematic of the Raman enhancement effect of the VSADReSe₂-based SERS substrate for R6G molecules. (b,c) PICT resonance effect in ReSe₂/R6G and VSADReSe₂/R6G systems. (d) SERS...[이미지참조] 97

Figure 6.7. (a) SERS profiles of R6G on VSADReSe₂ at different concentrations in the range of 10⁻⁴ to 10⁻¹⁸ M. (b) SERS intensity at 614 cm⁻¹ for different R6G concentrations on VSADReSe₂. The inset...[이미지참조] 98

Figure 6.8. (a-c) SERS profiles of diverse probe molecules (RhB, CV, and MB) on pristine ReSe₂ and VSADReSe₂ (probe concentration: 10⁻⁴ M; excitation wavelength: 532 nm).[이미지참조] 98

초록보기

 2D transition metal dichalcogenides (TMDs) have been emerged as promising candidates for next-generation high-performance functional applications owing to their outstanding intrinsic properties. The TMDs-based functional devices exhibit unique phenomena and high-performance characteristics in a wide range of applications including electronic and optoelectronic devices, energy storage, catalysis, photothermal and thermoelectric application, and neuromorphic computing. To implement high-performance TMDs-based functional devices, the top-down method and bottom-up synthesis process have been steadily developed to high-quality TMDs thin film. However, there are still some issues for the high-quality wafer-scale TMDs thin film synthesis. Thus, the reliable growth approaches should be developed to synthesize the uniform wafer-scale TMDs thin film with highly crystalline. Furthermore, it is important to in-situ modulate the intrinsic properties of TMDs without structural defects to significantly improve the performance of functional devices.

The purpose of this thesis is to develop the reliable growth process for the high-crystalline wafer-scale TMDs thin films with in-situ controllable intrinsic properties. The developed liquid precursor-assisted chemical vapor deposition (CVD) process was introduced to solve some issues of the conventional growth method, enabling the synthesis of high-quality TMDs thin films. The growth dynamics and intrinsic properties of TMDs were further investigated to successfully synthesize the TMDs with desired electronic properties for various applications. Moreover, in this thesis, the TMDs-based ultrasensitive molecular sensing platforms were demonstrated through the in-situ modulation of electronic structure and surface properties of TMDs. The thesis is consisted of the following chapters:

In Chapter 1, the general information of TMDs such as the atomic structure, intrinsic properties, and diverse functional applications was introduced. In addition, the diverse synthesis approaches of TMDs including top-down and bottom-up methods were discussed.

In Chapter 2, a novel growth approach for synthesizing the wafer-scale heavily doped TMDs thin films was introduced. The liquid precursors-assisted CVD process with reaction promoter effectively improves the doping concentration and promote the lateral growth of TMDs, enabling the synthesis of uniform wafer-scale TMDs monolayer film with high doping concentration.

In Chapter 3, a facile growth model based on the liquid precursor-assisted CVD process was proposed for the successful synthesis of the highly oriented TMDs. It was found that the growth dynamics and orientations of TMDs were significantly influenced by the chalcogen vapor pressure. In the quasi-static distribution during the growth process, the sulfur atoms can be steadily migrated into the energetically favorable sites of sapphire substrate with low potential energies, resulting in the synthesis of highly oriented TMDs.

In Chapter 4, a novel strategy for directly synthesizing patternable TMDs with controllable thickness at the wafer scale was introduced. The uniformly patterned large-area TMDs films can be directly grown through an intergranular diffusion-assisted CVD process with liquid-phase transition metal precursors and a sacrificial metal layer.

In Chapter 5, the facile synthesis process for the uniform large-area ReOxSy thin film using the liquid precursor without post-treatment was proposed. In addition, a graphene/ReOxSy van der Waals heterostructure was demonstrated as an ultrasensitive CM-based SERS platform. For the graphene/ReOxSy vertical heterostructure, the charge-transfer and exciton resonances are simultaneously enhanced between SERS substrate and probe molecules, enabling the femtomolar level detection.

In Chapter 6, a substitutional single-atom transition metal-doped 1T' semiconducting TMDs was demonstrated as an ultrasensitive SERS platform without relying on LSPR. The introduction of vanadium atoms creates a new gap state between the valence and conduction bands in ReSe₂, resulting in enhanced PICT resonance with the probe molecules, improving SERS effects. Furthermore, the single-atom vanadium-doped ReSe2 platform shows ultrahigh detection sensitivity, excellent signal uniformity and reproducibility, and broad applicability for various probe molecules, making it a promising platform for practical SERS applications.

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