Synthesis of atomically thin transition metal disulfides for photovoltaics and water splitting catalysts

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2018. 2. 장호원.As a new class of material, two-dimensional (2D) atomic crystals have attracted enormous research interest in the last decade that has led to a number of breakthroughs in physics owing to the confined charge, spin and heat transport within the 2D planes. The most outstanding one of these materials is graphene, as its exceptional electronic, optical and mechanical properties may hold great promise for a variety of future applications. The chapter 3 will cover the application of 2D transition metal disulfide thin films to charge transport layers and p-n junction material with p-Si wafer. Transition metal disulfides (MeS2) such as MoS2 and WS2 were used as charge transport layers in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells in order to enhance the stability in air comparing to poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). MeS2 layers with a polycrystalline structure were synthesized by a chemical deposition method using uniformly spin-coated (NH4)MoS4 and (NH4)WS4 precursor solutions. The ultraviolet-ozone (UV-O3) treatment on MeS2 leads to the removal of the surface contaminants produced by the transfer process, resulting in a uniform surface and an increase of the work function. Furthermore, the strong light absoption of single-layer of MoS2 could be utilized by light absorption layer in p-type Si-based photovoltaic cells. Specifically, a single semiconducting 0.6-nm-thick MoS2 can absorb as much sunlight as 50 nm of Si and generate photocurrents as similar as 12-nm-thick GaAs semiconductor. The MoS2 thin films could be the one of promising light absorption layer and have potential to make p-n heterojunction photovoltaic cells with p-Si substrate. We synthesize the wafer-scale molybdenum disulfide thin-films by thermolysis of solution precursor based method. After that, the thin films are transferred to p-Si and formed a heterojunction with p-Si. In order to maximize and fully utilize the excellent property of the n-MoS2, Transparent Au nanomesh electrode (Sheet resistance ≈ 6 Ω/sq. at 90% transmittance) fabricated from UV-O3 treated polymeric nanofiber templates is integrated to n-MoS2/p-Si heterojunction. The n-MoS2/p-Si heterojunction with Au nanomesh electrodes exhibit a power conversion efficiency of 4.69%. After antireflection coating, the device shows the efficiency of 5.96% at 0.44 cm2 of the active area. Hydrogen appears as a next-generation clean energy source to replace fossil fuels. One of the most promising ways to produce hydrogen is photoelectrochemical (PEC) water splitting. However, the existing photoelectrodes such as Si with noble metal catalysts still suffer from low efficiency and poor stability and the extremely high cost of the noble metal catalysts limits the wide use of water splitting photoelectrodes. Therefore, a novel approach is necessary to make a breakthrough for highly efficient PEC water splitting. This thesis contains that the demonstration of wafer-scale, transferable, and transparent thin-film catalysts based on MoS2, which consists of cheap and earth abundant elements, can provide the low onset potential of 1 mA/cm2 at 0.17 V versus a reversible hydrogen electrode and the high photocurrent density of 24.6 mA/cm2 at 0 V for a p-type Si photocathode. c-Domains with vertically stacked (100) planes in the transferable 2H-MoS2 thin films, which are grown by a thermolysis method, act as active sites for the hydrogen evolution reaction, and photogenerated electrons are efficiently transported through the n-MoS2/p-Si heterojunction. Moreover, in chapter 4.3, the anion-engineered MoS2 thin-films display the higher catalytic activity compared to the partially vertical-aligned MoS2 thin films, due to its many inherent dangling bonds on their surface and the metallic nature. The transferrable and transparent anion-engineered molybdenum disulfide thin-film catalysts synthesized through simple thermolysis method by using [(NH4)2MoS4] solution and powder precursors with different sulfur/phosphorus weight ratios. The synthesized sulfur-doped molybdenum phosphide (S:MoP) thin film changed from two-dimensional van der Waals structure to three-dimensional hexagonal structure by introduction of phosphorus atoms in the MoS2 thin film. The S:MoP thin film catalyst, which is composed of cheap and earth abundant elements, could provide the lowest onset potential and the highest photocurrent density for planar p-type Si photocathode. The density functional theory calculations indicate that the surface of S:MoP thin film absorb hydrogen better than that of MoS2 thin film. The structurally engineered thin film catalyst facilitates the easy transfer of photogenerated electrons from p-Si light absorber to electrolyte. Anion-engineering of MoS2 thin film catalyst would be an efficient way to enhance the catalytic activity for photoelectrochemical water splitting.Chapter 1 1 1.1. Scope and objective of the thesis 2 1.2. Transition Metal Dichalcogenides (TMDs) 4 1.2.1. Crystal structure 6 1.2.2. Physical properties 9 1.3. Synthetic methods 12 1.3.1. Mechanical exfoliation 13 1.3.2. Liquid exfoliation 14 1.3.3. Chemical vapor deposition 17 1.4. References 22 Chapter 2 24 2. Application to various devices 25 2.1. Application to electronic devices 26 2.2. Application to water splitting catalysts 29 2.3. Application to gas sensors 32 2.4. Reference 34 Chapter 3 35 3.1. Charge transport layers in organic-based optoelectronics 36 3.1.1. Introduction 36 3.1.2. Experimental procedures 39 3.1.3. Results and Discussion 42 3.1.4. Conclusion 61 3.1.5. References 62 3.2. Charge transport layers in perovskite-based organic photovoltaic cells 64 3.2.1. Introduction 64 3.2.2. Experimental procedures 67 3.2.3. Results and Discussion 72 3.2.4. Conclusion 81 3.2.5. References 82 3.3. p-n junction photovoltaic layers 85 3.3.1. Introduction 85 3.3.2. Experimental procedures 88 3.3.3. Results and Discussion 91 3.3.4. Conclusion 112 3.3.5. References 113 Chapter 4 115 4.1. Partially vertical-aligned molybdenum disulfides 116 4.1.1. Introduction 116 4.1.2. Experimental procedures 119 4.1.3. Results and Discussion 122 4.1.4. Conclusion 147 4.1.5. References 148 4.2. Tungsten disulfide thin films 151 4.2.1. Introduction 151 4.2.2. Experimental procedures 154 4.2.3. Results and Discussion 157 4.2.4. Conclusion 168 4.2.5. References 169 4.3. Sulfur-doped molybdenum phosphide thin films 171 4.3.1. Introduction 171 4.3.2. Experimental procedures 175 4.3.3. Results and Discussion 179 4.3.4. Conclusion 204 4.3.5. References 205 Chapter 5 208 Summary 208 국문초록 211 List of Publications 214Docto

동티모르 UN PKO 군사작전 분석 : 한국군과 호주군의 소프트파워 중심으로

Thesis(masters)--서울대학교 국제대학원 :국제학과(국제협력전공),2007.Maste

Stock Selection Strategies through Multiple Filtering Method

학위논문 (석사)-- 서울대학교 대학원 : 협동과정 계산과학전공, 2014. 2. 김태완.주식 투자에 있어서 첫 단계는 투자할 주식을 선택하는 것이다. 재무제표를 기반으로 기업을 분석하고 가치지표를 통하여 고평가, 저평가를 판단하여 주식을 선택하는 전략을 연구한다. 재무지표의 특성에 부합함의 여부에 따라 주식을 수익형, 안정형, 성장형, 활동형 주식으로 분류한다. 초기 구성을 유지하는 보유형과 주기적으로 평가하여 편입과 퇴출을 통하여 종목을 변경하는 매매형 모두에서 수익형, 활동형 주식의 성과가 높게 나옴을 확인한다. 본 논문에서 제안하는 다중 필터링 방법을 활용하여 투자할 주식을 선택한다면, 좋은 수익을 얻을 수 있을 것으로 기대된다.Stock selection is the first step in stock investing. In this study, corporations are analyzed based on financial statements and judged to over-evaluation or under-evaluation by value indicators. Stocks are categorized into profitable-type, stable-type, growing-type, active-type groups. Among them, profitable-type and active-type groups show high performance. Stock selection strategies through multiple filtering method is effective to stock investment.초 록 i 목 차 ii List of Tables iii List of Figures v 제 1 장 서론 1 제 1 절 연구의 동기 1 제 2 절 선행 연구 1 제 3 절 논문의 구조 2 제 2 장 재무지표와 가치지표를 이용한 투자 3 제 1 절 재무지표와 가치지표 3 제 2 절 주식 종목의 평가를 위한 기본 요인 파악 5 제 3 절 기업경영분석을 통한 기업의 가치 분석 6 제 3 장 다중 필터링 시스템 구현 10 제 1 절 시스템 설계 10 제 2 절 데이터 수집 11 제 3 절 포트폴리오 구성 25 제 4 절 포트폴리오 평가 26 제 4 장 다중 필터링 시스템을 이용한 주식 선택 전략 시뮬레이션 30 제 1 절 실험 설계 30 제 2 절 실험 과정 33 제 3 절 실험 결과 43 제 5 장 결론 및 향후 연구계획 51 참고 문헌 53 Abstract 54Maste

Improving Air stability in Organic Opto-electronic Devices Using MoS2 and WS2 Synthesized by Chemical Vapor Depostion

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