Controllable Formation of Zero-twisted 2D Layers via Atomic Reconstruction

Abstract

반데르발스(vdW) 층상 구조의 모아레 초격자는 새로운 전자 및 광학 현상을 연구하기 위해 광범위하게 연구되어 왔습니다. 완벽한 제로 트위스트 (zero-twisted) 이중층은 에너지적으로 가장 안정적이지만, 실험적 한계로 인해 적층된 2D 층을 완벽하게 정렬하기는 불가능하고, 자발적인 원자 재구성으로 인해 필연적으로 불균일한 도메인 구조를 형성할 수밖에 없습니다. 본 학위논문에서는 열적으로 유도된 원자 재구성을 통해 뒤틀린 전이금속칼코겐화물 (transition metal dichalcogenide, TMD) 이중층에서의 결맞음 (fully commensurate, FC) 구조의 형성에 대해 연구하였습니다. 이 연구는 결정 방향이 정렬되고 제로 트위스트 TMD 층을 달성하는 새로운 방법을 탐구합니다. 이 연구는 TMD 동종 및 이종 이중층에서 FC 구조의 제작하여 구조의 변화에 따른 광학적, 전기적 특성을 분석하고, 적층 방향에 따른 FC 구조의 원자구조의 제어가 가능함을 연구했습니다. 원자 재구성은 격자 불일치가 큰 이종접합 TMD 이중층에서도 가능하며, in-situ 주사 터널링 전자 현미경(STEM)을 통해 원자 수준의 정렬 메커니즘을 규명합니다. 또한 3R-TMD 이중층의 FC 구조에서 강유전 특성의 향상에 대해 조사했습니다. 본 연구는 TMD 층의 FC 구조 형성에 대한 이해와 새로운 현상 및 응용에 대한 시사점에 기여합니다.

Moiré superlattices in twisted van der Waals (vdW) layered structures have been studied widely to investigate unique and unprecedented electronic and optical phenomena. Despite perfectly zero-twisted bilayers are most energetically stable, achieving perfect alignment of stacked 2D layers remains challenging due to the limitations of mechanical manipulators, resulting in the inevitable formation of incommensurate domain boundaries around the commensurate reconstructed domains. This study investigates the formation of fully commensurate (FC) structures in twisted transition metal dichalcogenide (TMD) bilayers through thermally induced atomic reconstruction. The research focuses on a novel method for achieving zero-twisted TMD layers by utilizing encapsulation annealing to induce atomic rearrangement, regardless of initial twist angles and lattice mismatches. The resulting FC structures exhibit perfectly aligned crystalline orientations and opposite strains in adjacent layers. The study demonstrates the fabrication of FC structures in TMD homo- and hetero-bilayers using encapsulation annealing. This approach enables better control over interfacial properties and opens opportunities for fundamental studies and diverse applications. The optical properties of FC structures are also examined. Investigating the effect of stacking types on FC structure formation in TMD hetero-bilayers reveals distinct mesoscopic and microscopic structures based on R-stack and H-stack configurations. This approach is validated to large lattice-mismatched hetero-bilayers, such as MoS2/MoSe2 and WS2/WSe2, which presents a challenge for achieving self-reconstruction and atomic alignment. The impact of lattice mismatch on the atomic rearrangement process is investigated, providing insights into achieving FC structures in such systems. Moreover, I elucidate the atomic-scale mechanisms through in-situ scanning tunneling electron microscopy (STEM). Observations reveal sequential rotational ordering of crystal lattices, formation of nanoscale-aligned domains through atomic rearrangement, and the presence of defect pairs at moiré pattern boundaries. Furthermore, the improvement of ferroelectric properties in 3R-TMD bilayers, particularly in FC structures, is investigated. Structural homogeneity in FC structures enhances interfacial sliding ferroelectricity compared to near-zero stacked TMD bilayers with separated domains and topological defects. Overall, this thesis advances the understanding of FC structure formation in TMD layers and its implications for novel physical phenomena and potential applications. The findings highlight the significance of thermally induced atomic reconstruction in achieving fully commensurate structures with controlled properties, paving the way for advancements in two-dimensional materials research.