UC Berkeley

Moiré superlattices, formed by vertically stacking atomically thin van der Waals layers with a slight interlayer rotation and/or lattice constant difference, are a powerful platform for modulating the physicochemical behavior of two-dimensional solids. While the optical, electronic, and magnetic properties of moiré materials can be intentionally tuned by changing the extent of crystallographic mismatch between constituent layers, structural perturbations such as lattice reconstruction, strain, and disorder also have a substantial impact on observed behavior. Therefore, directly measuring intrinsic structural deformations in moiré superlattices, learning how to dynamically deform moiré structures, and efforts toward correlative structure–property measurements are critical to understanding and controlling the emergent properties of these unique materials.

In this dissertation, Chapter 1 first provides an introductory overview of recent developments in the field of two-dimensional materials and how the properties of these materials can be modified, including through construction of moiré superlattices. This discussion is followed by a comprehensive look at the fundamentals of moiré engineering, the role that structural deformations play in affecting moiré properties, and the appeal of a diffraction-based imaging approach for linking the structure of moiré architectures to observed properties and current theoretical models. Chapter 2 then describes the development of Bragg interferometry, a 4D-STEM-based imaging methodology for mapping moiré structures, and the insights afforded by the methodology regarding the spontaneous lattice deformations driving reconstruction in twisted bilayer graphene, the effects of these deformations on flat band formation, and the impact of extrinsic heterostrain on reconstruction-induced strain fields. Chapter 3 explores the extension of Bragg interferometry to transition metal dichalcogenide (TMD) systems, providing evidence of distinct reconstruction mechanisms in twisted bilayer TMDs and heterobilayer TMDs. The compatibility of Bragg interferometry with different heterostructure geometries is also exploited to illuminate the effects of encapsulation layers on in-plane and out-of-plane reconstruction. Chapter 4 demonstrates the application of Bragg interferometry to functional devices for the first time, specifically for mapping the spatial arrangement of polar stacking domains in twisted trilayer WSe2. This information is then complemented by operando dark-field TEM imaging that uncovers a variety of electric field-driven structural responses in different twisted trilayer polytypes. Lastly, Chapter 5 provides a summary of the reported work and an outlook for future endeavours.