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Layer controlled atomic spalling of van der Waals materials

초록/요약

Two-dimensional materials (2DMs) have garnered significant attention and research interest as a promising solution to overcome current technological limitations. These materials exhibit diverse physical properties that depend on their type and layer number. Due to quantum effects at the nanoscale, they demonstrate optical, electrical, and physical properties that are not present in conventional bulk materials. In this dissertation, I will discuss a straightforward and efficient method to obtain high-quality 2DMs with precise control over the number of layers on a large scale. Our approach begins with the exfoliation of graphene from graphite, enabling us to obtain a desired number of controllable layers. The interfacial toughness between the metal and graphene varies depending on the metal type, allowing us to exert control over the exfoliated graphene layers. Through experiments, we discovered that the crack depth, representing the thickness of the exfoliated graphene, is directly influenced by the interfacial toughness of the metal. For instance, when utilizing Au stressor films with a binding energy comparable to van der Waals forces, cracks propagate parallel to the surface, selectively delaminating monolayer graphene instead of perpendicular to it. Conversely, metals such as Pd, Ni, and Co, possessing higher binding energies than Au, promote deeper crack propagation, enabling the controlled exfoliation of graphene with layer numbers ranging from bilayer to 40 layers. To achieve precise control over crack depth in 2DMs, we focused on regulating the internal stress of the stressor material. By precisely tuning the internal stress of a metal film on the surface of the 2D crystal, using controlled stress release times, we successfully attained atomic-level control over crack depth. The depth of cracks varied atomically depending on the internal stress level of the film, allowing us to selectively obtain 2DMs with mono-, bi-, and trilayer. Lastly, we made a significant breakthrough by discovering a new stressor material based on semiconductor materials. Unlike conventional metal stressors that require an acid-based solution for removal, this novel material, Ge film, is water-soluble. Consequently, it can be easily removed using water alone, eliminating the need for acid-based processes. This advantage is particularly beneficial when peeling various types of 2DMs, particularly those susceptible to instability.

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목차

1. Introduction 1
1-1 General Introduction of 2D materials 1
1-1-1 Brief overview of 2D materials 1
1-1-2 Graphene 5
1-1-3 Transition Metal Dichalcogenides (TMDCs) 8
1-2 Research objective and approach 10

2. Literature Survey 12
2-1 Layer and Stacked order dependence of 2D materials 12
2-1-1 General Introduction 12
2-1-2 Layer dependence of 2D materials 12
2-1-3 Stacking order dependence of 2D materials 15
2-2 Preparation of 2D materials 16
2-2-1 General Introduction 16
2-2-2 Bottom-up approach 19
2-2-3 Top-down approach 20
2-3 Controlled Spalling 25
2-3-1 General Introduction 25
2-3-2 Spalling in bulk materials 30
2-3-3 Spalling in 2D or low dimensional materials 33

3. Layer engineered atomic spalling of Graphene via adjusting interfacial toughness of stressor 38
3-1 Introduction 39
3-2 Experimental 42
3-3 Result and Discussion 47
3-3-1 Layer engineered exfoliation (LEE) of graphene 47
3-3-2 Spectroscopy analysis of LEE-graphene 63
3-3-3 Electron transport properties in LEE-graphene 72
3-4 Conclusion 77

4. Layer engineered atomic spalling of TMDCs via modulating residual stress of stressor film 80
4-1 Introduction 81
4-2 Experimental 86
4-3 Result and Discussion 90
4-3-1 Layer engineered spalling of MoS2 90
4-3-2 Crack propagation depth control at atomic scale 101
4-3-3 6 × 6 matrix photodetector array based on combination of spalled materials 107
4-3-4 Layer engineered spalling of MoSe2 and WSe2 119
4-3-5 Analytic study of atomic spalling 123
4-4 Conclusion 134

5. Non-metal mediated atomic spalling of monolayer TMDCs 135
5-1 Introduction 137
5-2 Experimental 140
5-3 Result and Discussion 143
5-3-1 Analytic model for Ge-mediated atomic spalling 143
5-3-2 Atomic spalling of monolayer MoS2 146
5-3-3 Atomic spalling of monolayer MoTe2 155
5-3-4 Electrical properties in spalled monolayer MoTe2 163
5-4 Conclusion 166

References 167

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