Title Page

Contents

Abstract 18

Chapter 1. Introduction of 2D piezoelectric materials 20

1.1. Piezoelectric effect 20

2.1. Piezoelectricity in 2D materials 25

1.2.1. In-plane 2D piezoelectric materials 28

1.2.2. Out-of-plane 2D piezoelectric materials 33

1.2.3. Modulated 2D piezoelectric materials 35

2.2. Preparation of 2D piezoelectric materials 39

1.2.4. Exfoliated methods 39

1.2.5. Chemical vapor deposition 40

1.2.6. Phase transition growth 41

References 44

Chapter 2. Controllable charge transfer by local strain gradient 47

2.3. Introduction 47

2.4. Experimental section 52

2.4.1. Materials 52

2.4.2. AFM Measurements 53

2.4.3. Fabrication 54

2.5. Results and discussion 55

2.5.1. Force dependent electron transfer 55

2.5.2. Mechanism 58

2.5.3. Verification in other materials 64

2.5.4. Verification in practical TENGs 71

2.6. Conclusion 74

References 75

Chapter 3. Boosting Output Performance of MoS₂ Monolayer-based Piezoelectric Nanogenerator by Artificial Strain Concentration 81

3.1. Introduction 81

3.2. Experimental details 84

3.2.1. Synthesis of MoS₂ monolayer 84

3.3.2. Fabrication of MoS₂ monolayer-based PENG with wavy structure 85

3.3.3. Setup for measuring the piezoelectric output performance 89

3.3.4. Characteration and simulation 91

3.3. Results and discussion 92

3.3.1. Concept overview 92

3.3.5. Characterization of MoS₂-based PENG with wavy structure 96

3.3.6. Pre-strain distribution of MoS₂ monolayer with wary structure 99

3.3.7. Strain distribution under bending of MoS₂ monolayer with wavy structure 108

3.3.8. Output performance of MoS₂ monolayer based CS-PENGs 113

3.4. Conclusion 120

References 121

Chapter 4. Observation of ultra-high out-of-plane piezoelectric response of MoS₂ bilayers induced by local strain 125

4.1. Introduction 125

4.2. Experimental details 128

4.2.1. Sample preparation 128

4.3.2. Characterization and measurement 130

4.4. Results and Discussion 131

4.4.1. Concept overview 131

4.4.2. Layer dependence of d₃₃ piezoelectric effect 134

4.4.3. Strain dependence of d₃₃ piezoelectric effect 139

4.4.4. Van der Waals interaction effect 143

4.4.5. DFT simulation 145

4.5. Conclusion 152

References 153

Chapter 5. Summary and perspective 157

논문요약 159

Table 1-1. Centrosymmetric and non-centrosymmetric point groups in crystals. 24

Figure 1-1. Schematic of direct piezoelectric effect: (a) piezoelectric material; electrical charge generation under (b) tension and (c) compression 22

Figure 1-2. A variety of representative 2D materials and their properties. 26

Figure 1-3. Predicted piezoelectric coefficients and crystal structures of TMDCs and Janus TMDCs monolayer 27

Figure 1-4. Top view and side view of the 2H-MoS₂ 30

Figure 1-5. SH intensity for different layered h-BN 32

Figure 1-6. Layered graphene nitride sheets and corrected vertical PFM amplitudes 32

Figure 1-7. Cross-view of α-In₂Se₃ structure 34

Figure 1-8. (a), Schematic diagram of the interface interaction between monolayer graphene and SiO₂ substrate. (b),(c), Schematic of the PFM... 36

Figure 1-9. (a), Schematic diagram of different absorbed atoms on graphene. (b), Strain dependent on different absorbed atoms on graphene. (c), strain dependent... 36

Figure 1-10. Schematic of defects in graphene and the polarization dependent on the external strain of graphene 38

Figure 1-11. (a) Schematic of the sputtering system. (b) Sulfurization of amorphous MoSxOy by thermal annealing in a H₂S gas atmosphere and the phase...[이미지참조] 43

Figure 2-1. a) Surface potential images after rubbing with different normal forces on TiO₂ thin film. Scale bar is 2 μm. b) Corresponding surface potential profiles, c)... 57

Figure 2-2. a,d) Schematics of tip-sample contact under a) low and d) high normal force and b,e) corresponding (top) representative work function images and... 61

Figure 2-3. The distribution of a-e, strain gradient of a,d, εzxx and b,e, ε zzz and c,f, flexoelectric field Ez under (a-c) 50 nN, (d-f) 400nN[이미지참조] 62

Figure 2-4. Calculated (a) tip-radius dependent and (b) normal-force dependent flexoelectric surface potentials. Normal force of 500 nN and tip radius of 100 nm... 63

Figure 2-5. a,b) Surface potential profiles after rubbing with different normal forces in systems of a) Pt-coated tip and TiO₂ thin film, and b) conductive... 68

Figure 2-6. Surface potential images after rubbing with different normal forces on TiO₂ thin film by Pt-coated tip (Multi75E-G). Numbers indicate measurement... 69

Figure 2-7. Surface potential images after rubbing with different normal forces on SiO₂ thin film by conductive diamond-coated tip (CDT-FMR). Numbers indicate... 69

Figure 2-8. a) Schematic diagrams of triboelectric devices: i) pyramid-featured Pt and ii) flat Pt. b) Peak-to-peak voltage as function of applied force (1 kgf＝9.8 N)... 73

Figure 3-1. A schematic illustration of fabrication progress of concentrated strain-applied piezoelectric nanogenerators based on 2D materials (CS-PENGs) 86

Figure 3-2. The force diagram of wavy on CS-PENG under different pre-strain 87

Figure 3-3. The neutral axis (N.A.) of (a) PMMA/MoS₂ and (b) PMMA layer 87

Figure 3-4. The experiment condition for measuring the piezoelectric output performance 90

Figure 3-5. (a) Schematic illustration of CS-PENG and the inset of the fabrication method (b) A photograph of MoS₂ monolayer-based PENG. The red square... 95

Figure 3-6. (a) Raman spectrum with peak positions of synthesized MoS₂ monolayer. Inserted image: A photograph of synthesized MoS₂ monolayer with a... 98

Figure 3-7. (a) The optical image of CS-PENG (15%) (upper image) and two-dimensional schematic illustration of the single wavy structure in the yellow arrow... 100

Figure 3-8. The SEM image of CS-PENG, b the thickness of PMMA and PDMS ce the SEM image of CS-PENG under different pre-strained condition (5, 10, 15, 20%) 103

Figure 3-9. The morphological cross-section profile of CS-PENG under different pre-strained conditions (a-5%, b-10%, c-15%, d-20%). The inset is optical... 104

Figure 3-10. The strain distribution of CS-PENG by informing wavy structure a the initial state of CS-PENG and contact area condition before removing pre-... 105

Figure 3-11. Photoluminescence intensity variation of one wavy structure in MoS₂ monolayer along the red line to the blue line in the lower image of Figure 10a. The... 107

Figure 3-12. (a) P-PENG and (b) CS-PENG (c) The bottom side strain distribution of one wavy. The yellow and purple dashed box shows the deformation... 110

Figure 3-13. The strain distribution of CS-PENG a. when it was bent upwards and strain distribution, and schematic of one wavy for x and y axis, and for x and z axis.... 111

Figure 3-14. The strain distribution of CS-PENG when it is bent with downward the 3D FEM models for the CS-PENG (15%) The strain distribution of CS-PENG... 112

Figure 3-15. (a) Current-Voltage plots of MoS₂ monolayer with different wavy structures obtained from pre-strained PDMS. The plot shows the characteristics... 115

Figure 3-16. Current-Voltage plots of MoS₂ monolayer with wavy structure made by (a) 5%, (b) 10% and (c) 20 % pre-strained PDMS, respectively 116

Figure 3-17. (a), (b) Voltage and current responses of CS-PENG at △L of 4 cm, respectively. Wavy structures made by 15% pre-strained PDMS led to generation... 118

Figure 3-18. The output performance of a-b P-PENG and CS-PENG (c-d 5%, e-f 10%, g-h 20%) with different length changes (△L＝1cm, 2cm, 3cm, 4cm) 119

Figure 4-1. Fabrication processing of MoS₂ wrinkle array 129

Figure 4-2. Optic microscopy images of (a) polystyrene beads array on Si substrate and (b) MoS2/Au on polystyrene beads 129

Figure 4-3. (a) Schematic of MoS₂/Au array on beads, (b) PFM tip measure on the wrinkles, (c) Atomic structure before and after strain, (d) vdWs interaction... 133

Figure 4-4. (a) Raman spectroscopy of MoS₂ with different layers. (b) Raman shift difference of MoS₂ with different layers 135

Figure 4-5. (a) Surface topography and (b) PFM amplitude of MoS₂/Au on the polystyrene beads array, the bead size is 15 μm 138

Figure 4-6. Piezoelectric coefficient of MoS₂ with different layers on the polystyrene beads array, the bead size is 15 μm 138

Figure 4-7. (a) Surface topography and (b) PFM amplitude of MoS₂ bilayer/Au on the polystyrene beads array with the different bead size 140

Figure 4-8. Piezoelectric coefficient of MoS₂ bilayer on the polystyrene beads array with different sizes 140

Figure 4-9. Raman mapping of MoS₂ bilayer on the polystyrene beads array with different sizes 142

Figure 4-10. (a) Surface topography and (b) KPFM data of MoS₂ bilayer/Au on the polystyrene beads array 142

Figure 4-11. (a) Surface topography of WS₂ bilayer/Au on the polystyrene beads array with the different bead size 144

Figure 4-12. Piezoelectric coefficient of WS₂ bilayer on the polystyrene beads array with different sizes 144

Figure 4-13. Atomic modeling of wrinkled MoS₂ in (a) top view and (b) side view. (c) formation of wrinkled MoS₂ on periodically arranged PS beads 147

Figure 4-14. Linear profile of the wrinkles with different strain 148

Figure 4-15. Gaussian function for atomic modeling of wrinkled MoS₂: amplitudedependent with k＝4 148

Figure 4-16. Electric property of MoS₂. (a) Electric polarization of wrinkled MoS₂. (b) Partial charges of upper and lower S atoms in wrinkled MoS₂. The electrostatic... 151