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Title Page

Contents

ABSTRACT 20

초록 21

I. Introduction 22

1. Transition metal dichalcogenides (TMDs) 22

2. Excitons in TMDs 27

3. TMD heterostructure 30

4. Raman spectroscopy 36

5. Second harmonic generation (SHG) 45

5.1. Polarization dependence of SHG 47

5.2. Wavelength dependence of SHG 52

II. Experimental 57

1. Sample preparations 57

2. Raman spectroscopy 59

3. Second harmonic generation 61

4. Reflectance contrast difference spectroscopy 64

III. Results 65

1. Determination of the crystallographic orientations in 2-dimensional heterostructures using resonant SHG 65

1.1. Introduction 65

1.2. Polarization dependence of SHG in TMD 66

1.3. Wavelength dependence of SHG 70

1.4. Wavelength- and polarization-dependent SHG in TMD 76

1.5. Summary 79

2. Effect of interlayer interaction on structural and electronic band modulations in TMD heterostructures 80

2.1. Introduction 80

2.2. Characterization of coupled MoS₂/WSe₂ heterostructure using Raman and SHG measurement 82

2.3. Twist angle dependent phonon variations in coupled heterostructure 85

2.4. Direct observation of atomic reconstruction in heterostructure with small twist angle 92

2.5. Twist angle dependent band changes in heterostructure 94

2.6. Appendix 99

IV. Conclusion 122

V. References 123

Figure I-1. The periodic table that the transition metals and the three chalcogen atoms... 22

Figure I-2. Transition metal atom coordination for (a) trigonal prismatic (1H) and (b)... 24

Figure I-3. (a) and (b) show the top views of the primitive unit cells for TMD materials... 24

Figure I-4. Primitive unit cell and symmetry operations of the 2H polytype 26

Figure I-5. (a) Real-space representation of electrons and holes bound into excitons for... 27

Figure I-6. Schematic illustration of the A and B excitons in TMD. 28

Figure I-7. Different exciton types in TMD 29

Figure I-8. (a) Illustration of the interlayer exciton where electrons and holes are... 30

Figure I-9. (a) Different local atomic alignments occur in a MoSe₂/WSe₂ vertical... 32

Figure I-10. TEM dark field images measured on (a) twisted bilayer graphene, (b)... 33

Figure I-11. Diagram of Raman scattering. 38

Figure I-12. Example of diatomic crystal phonon dispersion. 39

Figure I-13. Raman spectra of MoS₂ for thicknesses from monolayer (1L) to 7 layers... 40

Figure I-14. Raman active phonon modes of typical 2H-type TMD such as MoS₂ 42

Figure I-15. (a) Geometry of SHG. (b) Energy-level diagram describing second... 46

Figure I-16. Two strategies for enhancing the process of SHG. (a) The one-photon... 52

Figure II-1. Schematic of fabricating TMD heterostructure using stamping method. 58

Figure II-2. Schematic of the polarized Raman setup. 60

Figure II-3. Schematic of the polarization dependent SHG measurement setup. 62

Figure II-4. Schematic of the SHG spectroscopy setup. 63

Figure II-5. (a) Schematic of the setup for reflectance contrast difference spectroscopy.... 64

Figure III-1. The schematic of polarization geometry in the lattice structure. Red and... 67

Figure III-2. Polarization dependences of SHG measured from monolayer MoS₂ (a... 68

Figure III-3. Schematic for SHG when 1-(a) and 2-(b)incident photons are resonant with... 70

Figure III-4. Reflectance contrast difference spectrum measured from monolayer MX₂... 72

Figure III-5. Optical image of a MoS₂/WSe₂ heterostructure on a SiO₂/Si substrate.... 73

Figure III-6. Results of SHG spectroscopy measured on heterostructure (blue),... 74

Figure III-7. The polarization dependence of SHG measured from... 76

Figure III-8. The polarization dependences of SHG on the heterostructure as a function... 78

Figure III-9. MoS₂/WSe₂ heterostructure (HS). (a) Optical image of the HS sample.... 82

Figure III-10. Circularly polarized Raman spectra of MoS₂/WSe₂ heterostructure with... 84

Figure III-11. Twist angle dependence of Raman spectra. (a) Twist angle... 85

Figure III-12. SAED pattern obtained from heterostructure. (a) The SAED pattern of a... 93

Figure III-13. Twist-angle dependent exciton energy shift in heterostructure. (a)... 94

Figure III-14. (a) Moiré wavelength (red lines) with respect to the twist angle ø. The... 102

Figure III-15. (a) Optical phonon modes for 1L WSe₂ (upper) and 1L MoS₂ (lower).... 104

Figure III-16. (a) Projected phonon wave functions 〈φ1L,E2g|ψHS〉 of the model...[이미지참조] 106

Figure III-17. Projected phonon wave functions 〈φ1L,A1g|ψHS〉 of the heterostructure...[이미지참조] 107

Figure III-18. Lorentzian-broadened E2g modes of the model heterostructures...[이미지참조] 108

Figure III-19. Lorentzian-broadened A1g modes of the model heterostructure...[이미지참조] 109

Figure III-20. Out-of-plane and in-plane distortions. (a) Top view of moiré... 110

Figure III-21. z coordinates of all the Sbottom (red) and Setop (blue) atoms projected on...[이미지참조] 112

Figure III-22. In-plane displacement vector dv (red arrows) at each layer in MoS₂ of the... 115

Figure III-23. (a) The unfolded band structures of PBE+SOC calculations along the unit... 117

Figure III-24. (a) Band alignment of the model heterostructure at the twist angle of 20˚.... 119

초록보기

전이금속 칼코겐화합물 이종접합 구조는 각기 다른 차원 물질인 전이금속 칼코겐화합물이 수직으로 적층된 구조를 갖는다. 이종접합 구조는 동종으로 이루어진 2 차원 물질과 마찬가지로 반데르발스 힘에 의해 각 층이 상호작용하고 이러한 층간 상호작용은 이종접합 구조의 전자밴드 구조에 영향을 미친다. 더불어 이종접합 구조에서는 격자상수 차이로 인해 생성되는 모아레 무늬에 의한 모아레 퍼텐셜 또한 이종접합 구조의 전자구조밴드 구조에 영향을 미친다. 모아레 무늬의 주기는 이종접합 구조 각층의 격자상수 차이뿐만 아니라 접합각도에 따라서도 달라지기 때문에 모아레 퍼텐셜 역시 접합 각도에 따라 달라진다. 따라서 접합각도는 이종접합 구조의 전자구조를 포함한 물리적 특성을 결정하는 데 매우 중요한 역할을 한다. 본 논문에서는 여러가지 접합 각도를 갖는 전이금속 칼코겐 화합물 이종접합 구조를 제작한 후. 분광학적 방법을 이용하여 수행한 접합 각도에 따른 물리적 특성 연구 결과를 보고한다.

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