Title Page

Contents

ABSTRACT 18

Chapter 1. Introduction 20

1.1. Research Motivation and Purpose 20

1.2. Related Terms 22

1.3. List of Abbreviations 24

Chapter 2. Background Reviews 25

2.1. X-ray Detector 25

2.2. Scintillator 27

2.2.1. Solid Scintillator 28

2.2.2. Liquid Scintillator 29

2.3. Materials on Study 31

2.3.1. 2,5-Diphenyloxazole 31

2.3.2. Perovskite Materials 32

2.4. Comparison with Semiconductor Nanocrystals 39

2.5. Quantum Confinement Effect 43

2.6. Theoretical Approaches to Describe Growth 48

2.6.1. Nucleation 48

2.6.2. LaMer's Mechanism 52

2.7. Perovskite Nanocrystals Applications 55

2.7.1. Photovoltaic 55

2.7.2. Light Emitting Diodes (LEDs) 57

2.7.3. White LEDs 59

2.8. Conventional Synthesis of Perovskite Nanocrystals 61

2.8.1. Hot-injection Method 61

2.8.2. Ligand-assisted Reprecipitation Process 63

2.8.4. One-pot Heating Up Method 64

2.8.5. Microwave-assisted Synthesis Method 65

2.9. Structural and Optical Analysis 67

2.9.1. X-Ray Diffraction 67

2.9.2. X-Ray Photoelectron Spectroscopy 69

2.9.3. Photoluminescence Quantum Yield 71

2.9.4. Ultraviolet-Visible Spectroscopy 71

2.9.5. Photoluminescence (PL) 72

2.9.6. Density Functional Theoiy (DFT) 74

Chapter 3. Experimental Details 76

3.1. Introduction Synthesis Process 76

3.2. Chemical Information 83

3.3. Conventional Microwave-assisted Synthesis 83

3.4. Synthesis of CsPbBr₃ Nanocrystals via Microwave-Assisted Bath Process (MABP) 84

3.4.1. Preparation of Cs-oleate 84

3.4.2. Preparation of Pb Precursor 85

3.4.3. Synthesis of CsPbBr₃ Perovskite Nanocrystals 85

3.5. Anion Exchange 85

3.6. Density Functional Theory (DFT) 86

3.6.1. Band Structure and Density of State 86

3.6.2. Total Energy 87

3.7. Phase Transformation of Cs₄PbBr₆ to CsPbBr₃ 88

3.8. Fabrication of Photodetector 89

3.9. Method for Nanohybrid Liquid Scintillator (PPO+CsPbBr₃ NCs) 89

3.10. Characterization 90

Chapter 4. Results and Discussion 92

4.1. Comparison with Conventional Microwave-assisted Synthesis 92

4.2. Microwave-assisted Bath Process 100

4.2.1. Thermal Stability of Oleic Acid 100

4.2.2. Solubility of Cesium Carbonate, CS₂CO₃ 104

4.2.3. Solubility of Lead Precursor, PbBr₂ 106

4.2.4. Phase Transformation of Cs₄PbBr₆ to CsPbBr₃ 119

4.2.5. Insight of Synthesis Mechanism 126

4.3. Anion Exchange Depending on Other Halides 162

4.3.1. Density Functional Theory 162

4.3.2. Optical Properties 168

4.4. Performance of CsPbX₃ / MoS₂ Photodetector 172

4.4.1. Molybdenum Disulfide, MoS₂ 172

4.4.2. Interface of MoS₂/CsPbBr₃ Van der Waals Hetero-structures. 175

4.4.3. Measurements of Electrical and Photoresponsive Characteristics 177

4.5. Performance of the Hybrid CsPbBr₃+PPO Scintillator 183

Chapter 5. Conclusion 185

국문요약 187

RESEARCH ACHIEVEMENTS 189

REFERENCE 190

Table 2.1. Fluorescence quantum yield, absorbance, and emission peak of the commonly used solvent and wavelength shifter in organic liquid systems. 30

Table 3.1. Commonly used organic solvents and their heating efficiency (tanδ) in the microwave field. 81

Table 3.2. Commonly used organic solvents classified according to their boiling point (BP). 81

Table 4.1. Total energies of each material were calculated on each optimized structure using Density Functional Theory. The change of the total... 122

Table 4.2. Summary of the experimental details according to the amount and the ratio ligands in CsPbBr₃ NCs with a molar ratio of Cs₂CO₃ : PbBr₂ of 1 : 3,... 130

Table 4.3. Summary of the experimental details according to the amount and the ratio ligands in CsPbBr₃ NCs with a molar ratio of Cs₂cO₃ : PbBr₂ of 1 : 4,... 132

Table 4.4. Summary of the experimental details according to the amount and the ratio ligands in CsPbBr₃ NCs with a molar ratio of Cs₂CO₃ : PbBr₂ of 1 : 5,... 134

Table 4.5. Summary of the experimental details according to the amount and the ratio ligands in CsPbBr₃ NCs with various molar ratio Cs-oleate and... 155

Table 4.6. Calculated band gaps (in eV) and lattice constants (in Å) of CsPbX₃ NCs (with X ＝ Cl, Br, and I) using the different types of... 167

Table 4.7. The position of emission and absorption peaks of CsPbBr₃ NCs after reacting anion exchange with different amount of other halide precursors. 170

Table 4.8. Summary of the experimental details of hybrid CsPbBr₃ NCs+PPO scintillator fabrication. 184

Fig. 2.1. Schematic of different detection types for x-ray image. 26

Fig. 2.2. The schematic and principle of solid scintillator. 28

Fig. 2.3. In a liquid scintillator system, the absorption band and emission peak of the solvent, primary and secondary wavelength shifter scintillators... 30

Fig. 2.4. The chemical structure of the 2,5-diphenyloxazole (PPO). 31

Fig. 2.5. Unit cell of the different metal halide perovskite ABX₃ ideal cubic structure. 32

Fig. 2.6. Schematic of the quantum confinement effect. The bandgap size of NCs increase with decreasing their diameter. 33

Fig. 2.7. Schematic of the perovskite nanocrystals CsPbX₃ (X＝Cl, Br and I) NCs according to halide composition ratio. 33

Fig. 2.8. Applications for nanocrystals in optoelectronics fields. 35

Fig. 2.9. (a) Tolerance factor (t) indicating the stability of the XPbBr₃ structure according to the A-cation. The XPbBr₃ structure is stable when 0.8... 37

Fig. 2.10. The simulated crystal structure of (a) cubic CsPbBr₃, (b) orthorhombic CsPbBr₃ (c) tetragonal CsPbBr₃, (d) tetragonal CsPb₂Br₅, (e)... 38

Fig. 2.11. Representation of energy structure of PNCs yielding the unique defect-tolerant electronic and optical characteristics in comparison... 42

Fig. 2.12. Schematic of the energy level structure of a bulk (a), and various semiconductor structures (b-d) with reduced dimensionality.... 44

Fig. 2.13. Each electronic energy level of bulk, nanocrystal, Molecule, and atom materials. The discrete energy levels of atoms are combined... 47

Fig. 2.14. The dependence of the cluster free energy, △G, on the cluster radius, r according to the classical nucleation theory. The... 50

Fig. 2.15. The principle of nanoparticle nucleation due to LaMer's mechanism of nucleation derived from CNT. The qualitative curve... 54

Fig. 2.16. Perovskite solar cells have increased in power conversion efficiency at a phenomenal rate compared to other types of photovoltaic. 56

Fig. 2.17. The Schematic of the device structure and Operation process of PeLED 58

Fig. 2.18. Three methods of producing white light. a) RGB LEDs, b) RGB phosphor on UV LED, c) yellow phosphor on blue LED. 60

Fig. 2.19. Representation of the hot-injection method employed to prepare monodisperse nanocrystals. 62

Fig. 2.20. Representation of the LARP method to synthesize monodisperse PNCs 63

Fig. 2.21. Representation of the one-pot heating up method to synthesize monodisperse PNCs. 65

Fig. 2.22. Representation of Microwave-assisted method to synthesize monodisperse PNCs. 66

Fig. 2.23. Principle of X-ray diffraction is characterized by Bragg's law of reflection. 68

Fig. 2.24. Schematic of the principle of X-ray photoelectron spectroscopy. The photoemission process involved in XPS surface chemical analysis. The... 70

Fig. 2.25. Schematic of the energy diagram showing fluorescence and phosphorescence processes. 73

Fig. 3.1. The schematic of the Vessel for the Microwave-assisted Bath Process (MABP). 80

Fig. 3.2. Illustration of the Microwave oven used for the MABP (LG co. MW25B model) 80

Fig. 3.3. Schematic of the heat-up synthesis mechanism (a), and the modified microwave assisted synthesis mechanism (b). 82

Fig. 4.1. (a) The digital photograph of the obtained CsPbBr₃ NCs obtained using a various ratio of Cs₂CO₃ and PbBr₂. (b) Emission peaks of each... 93

Fig. 4.2. XRD data of PNCs as the amount of Cs reactive species increases. (The molar ratio of Cs₂CO₃ and PbBr₂ is 1:4 ①, 2:4 ②, and 3:4 ③) 94

Fig. 4.3. (a) Emission peaks as functions of OAm and OA densities of synthesized CsPbBr₃ NCs samples, (b) The digital photograph of the obtained... 96

Fig. 4.4. (a) Emission peaks under UV-light of each synthesized CsPbBr₃ NCs samples under UV-light. (b) The digital photograph of the obtained CsPbBr₃ NCs samples... 97

Fig. 4.5. Effect of microwave power and duration. (a) PL peak energy of the synthesized PNCs as functions of power and duration. (b) the digital... 99

Fig. 4.6. (a) PL spectrum of CsPbBr₃ NCs through large-scale synthesis using the microwave-assisted method. (b) XRD data, (c) digital photograph... 99

Fig. 4.7. Photo-oxidation and auto-oxidation of oleic acid and formation of hydroperoxide. 101

Fig. 4.8. Effect of (a) ultraviolet light exposure, (b) volume ratio (mL oleic acid / mL 1-octadecene) (c) the presence of oxygen, (d) metal, (e)... 101

Fig. 4.9. Precipitation of PbBr₂ by thermal decomposition of oleic acid. (a) digital photograph of PbBr₂ precipitated on the bottom of the vial, (b) XRD data. 103

Fig. 4.10. Weight % of precipitates in Cs-oleate prepared by Cs₂CO₃. 105

Fig. 4.11. Maximum temperature as a additional ligand concentration; PbBr₂ salts precipitate from the reaction medium above a critical reaction... 109

Fig. 4.12. PbBr₂ solubility according to the amount of OA and OAm added to PbBr₂ precursors. 109

Fig. 4.13. Photograph of PbBr₂ precursors according to reaction temperature or the amount of ligand. When the reaction temperature is high... 110

Fig. 4.14. (a) Photograph of the PbSr₂ precursor solution according to the ratio of ligand (OA / OAm). (b) Solubility is increased in amine-based... 111

Fig. 4.15. Schematic diagram of lead reactive species according to the amount and ratio of the ligand. 118

Fig. 4.16. The X-ray Diffraction pattern of (a) CsPbBr₃ and (c)Cs₄PbBr₆. The absorption spectra of (b) CsPbBr₃ and (d) Cs₄PbBr₆. 120

Fig. 4.17. TEM images Cs₄PbBr₆ (a) and CsPbBr₃ (c). The high resolution TEM images of Cs₄PbBr₆ (b), and CsPbBr₃ (d) 121

Fig. 4.18. The schematic of the phase transition of Cs-Pb-Br compound extracting CsBr (Physical approach) 123

Fig. 4.19. The phase transition from Cs₄PbBr₆ to CsPbBr₃ using thermal annealing. 124

Fig. 4.20. The experimental model for synthesizing CsPbBr₃ NCs with excellent optical properties and pure morphology. As the reaction... 126

Fig. 4.21. (a) Representation of an experimental model applying CsPbBr₃ NCs under specified condition in table 4.2. Physical characterization and optical... 131

Fig. 4.22. (a) Representation of an experimental model applying CsPbBr₃ NCs under specified condition in table 4.3. Physical characterization and optical... 133

Fig. 4.23. (a) Representation of an experimental model applying CsPbBr₃ NCs under specified condition in table 4.4. Physical characterization and optical... 135

Fig. 4.24. The photograph of the synthesis process by reaction time according to the molar ratio of the ligand. When the molar ratio of the ligand was 1 : 2,... 136

Fig. 4.25. (a) Transmission electron microscopy (TEM) image of the CsPbBr₃ NCs (SP08). The inset shows a size distribution of PNCs. (b) a high-resolution... 137

Fig. 4.26. Schematic representation of the surface chemistry of before- (a) and after-(b) purification in excess oleic acid environment. 140

Fig. 4.27. Photographs of the CsPbBr₃ NCs dispersions in OA/cyclohexane mixtures during storage in the dark. Images were captured under daylight and 365... 141

Fig. 4.28. Schematic representation of the surface chemistry of before- (a) and after-(b) purification in excess oleylamine environment. 142

Fig. 4.29. Photographs of the CsPbBr₃ NCs dispersions in OAm/cyclohexane mixtures during storage in the dark. Images were captured under daylight and... 143

Fig. 4.30. The absorption spectra over time after oleylamine addition to as-prepared CsPbBr₃ NCs. 145

Fig. 4.31. Photographs of the CsPbBr₃ NCs dispersions in OA+OAm/cyclohexane mixtures during storage in the dark. Images were captured under daylight and 365... 147

Fig. 4.32. Photographs under vdoite light (a), and UV irradiation (b) of CsPbBr₃ NCs colloidal solution according to total ligand amount and their molar ratio. The vertical... 150

Fig. 4.33. Representation of the PL emission peak position of CsPbBr₃ NCs according to the total amount of ligand and their molar ratio. The vertical axis... 151

Fig. 4.34. XPS survey spectra of CsPbBr3 (a). CS3d (b), N1s (c), BR3d (d), and Pb4f (e) spectra of CsPbBr₃ NCs.[이미지참조] 153

Fig. 4.35. (a) Representation of an experimental model applying CsPbBr₃ NCs under the specified condition in table 4.5. Physical characterization of as-prepared... 157

Fig. 4.36. (a) Representation of an experimental model applying CsPbBr₃ NCs under the specified condition in table 4.5. Physical characterization of as-prepared... 158

Fig. 4.37. The TEM images of CsPbBr₃ NCs according to the amount of total ligand. SP08 (34 mmol), SP09 (40 mmol), and SP10 (45 mmol). The inset shows... 159

Fig. 4.38. The photograph of the total process procedures of Microwave-Assisted Bath Process (MABP) process. 161

Fig. 4.39. Crystal structure of perovskite nanocrystals in the cubic phase. The primitive cell of cubic perovskite CsPbX₃ is consists of corner-linked... 163

Fig. 4.40. The energy band gap of CsPbBr₃ NCs is calculated by different pseudo potential types. 163

Fig. 4.41. Calculated band structures and density of states (DOS) of CsPbCl₃ (a), CsPbBr₃ (b), CsPbI₃ (c) using the PBE-PAW pseudo potentials... 164

Fig. 4.42. Relaxed crystal structure of CsPbCl₃ (a), CsPbBr₃ (b), CsPbI₃ (c) and the calculated band structure for CsPbCl₃ (d), CsPbBr₃ (e), CsPbI₃... 164

Fig. 4.43. Anion-exchange reactions in CsPbX₃ NC systems. The digital photograph of the CsPbX₃ colloidal solution after anion-exchange reaction... 171

Fig. 4.44. The Band Structure of the MoS₂ bulk (left), bilayer (middle), monolayer (right) using DFT Calculation at high-symmetric points in the... 174

Fig. 4.45. Schematic chemical structure of MoS₂ (a). The band diagram of PNCS/MoS₂ heterojunction was determined to form a type II (Staggered band... 176

Fig. 4.46. PL and absorption spectrum for each CsPbCl₃ (left), CsPbBr₃ (middle), and CsPbI₃ (right). 177

Fig. 4.47. Emission Spectrum of CsPbI₃ (left), CsPbBr₃ (middle), and CsPbCI₃ (right) depending on density. 177

Fig. 4.48. The device schematic of the PNCs/MOS₂ photodetector. 178

Fig. 4.49. Optoelectronics performance and the schematic of the channel current transport mechanism. (a) equilibrium without bias and no illumination, (b) low... 179

Fig. 4.50. Output characteristic curves (IDS versus VDS) of the device uisng pristine MoS₂ (a, c, e) and MoS₂/CsPbI₃ NCs (b), MoS₂/CsPbBr₃ NCs...[이미지참조] 182

Fig. 4.51. Photoswitching properties of the pristine MoS₂ and PNCs/MoS₂ photodetector. 182

Fig. 4.52. Radioluminescence of the hybrid PPO + CsPbBr₃ NCs scintillators. The photograph of hybrid PNCs + PPO scintillators in daylight, λ₁ (365 nm),... 184

 최근 많은 응용 분야에서 고해상도 X-선 센서에 대한 수요가 증가함에 따라 신틸레이터 소재에 대한 광범위한 연구가 진행되고 있다. 금속 할라이드 페로브스카이트는 독특한 구조적 및 광학적 특성으로 인해 광소재 분야에서 가장 흥미로운 재료로 촉망받고 있다. 높은 광안정성, 높은 발광 양자수율 (PLQY, ＞90%), 좁은 방출 반치폭 (FWHM,＜ 20nm)을 갖는 CsPbX₃ (Cl, Br, 및 I) 페로브스카이트 나노결정은 X-선 및 광전자 응용 분야의 잠재적인 재료로 간주된다. 최근 콜로이드 금속 할로겐화물 페로브스카이트 나노결정과 유기분자의 나노화합물로 구성된 새로운 고효율 저비용 액체 신틸레이터 소재가 보고되었다. 신틸레이터 소재의 다양한 특성 중에서 양자 수율은 검출기의 효율성 및 분해능과 가장 밀접한 관련이 있다. 양자 수율은 다양한 정도의 에너지를 가진 입자와 광자의 특성에 따라 달라지기 때문에 응용 분야 유형에 따라 적절한 신틸레이터 물질이 선택된다. 뛰어난 광특성을 가지는 금속 할로겐화 페로브스카이트 나노결정은 이러한 요구사항을 모두 충족시킨다. 그러나 고품질의 페로브스카이트 나노결정을 대량으로 합성하는 것은 여전히 도전 과제로 남아있다.

본 논문에서는 확장 가능하고 비용 효율적인 나노결정의 대량 생산을 위한 Microwave-Assisted Bath Process (MABP) 전략을 제안한다. MABP 방법은 기존의 합성방법과는 다른 전략으로 마이크로파를 이용하여 임계 반응온도까지 빠르게 가열하고 반응 혼합물에 균일하게 전달할 수 있다. 그 결과, 기존 합성법의 약 20배 이상의 합성 수율을 가지며, 높은 농도에서 521.6 nm (25 mg/ml)의 방출 피크와 16.8 nm의 좁은 반치폭, 평균 크기 9.06 nm의 우수한 크기 균일도를 갖는 페로브스카이트 나노결정을 합성하는데 성공하였다. 더 나아가, 반응 혼합물을 구성하는 전구체와 리간드의 양을 조절하여 고농도의 투명하고 깨끗한 콜로이드 용액과 균일한 나노큐브 형태의 페로브스카이트 나노결정을 얻기 위한 실험적 모델을 수립하였다. 본 논문에서 제안하는 합성전략은 향후 광전자 분야에서 페로브스카이트 나노결정의 상용화를 촉진할 것으로 예상된다.