Atomically thin van der Waals (vdW) materials are sensitive to their surrounding environment, and the properties they exhibit can be significantly affected by changing this environment. By placing two atomically thin layers together with different work functions, charge will transfer between the two materials. Appreciable hole doping (on the order of 1013 holes/cm2) is realized in graphene, MoS2, and WSe2 interfaced with α-RuCl3 as a result of this charge transfer. α-RuCl3 has a relatively large work function that allows it to accept a large density of electrons. The charge transfer to α-RuCl3 can be modified by increasing the distance between the vdW material and α-RuCl3 with a hBN spacer layer.The electrochemical intercalation of lithium ions in graphene and TMD heterostructures is shown to change significantly in the presence of α-RuCl3. Raman spectroscopy and electrical transport measurements are used to monitor the intercalation reactions in situ. The main finding is that α-RuCl3 significantly lowers the driving force needed to electrochemically insert lithium ions into vdW heterostructures. It is hypothesized that the hole doping induced by α-RuCl3 increases the favorability of electron reduction in the other layer that is concomitant with lithium intercalation. In addition, the intercalation driving force and the intercalation amount can be tuned with the doping in these heterostructures using hBN spacers. The control α-RuCl3 offers for ionic charge accumulation could potentially inform future battery technologies. The control over electronic charge accumulation in α-RuCl3 heterostructures also has interesting applications. The transport of graphene interfaced with monolayer α-RuCl3 is shown to exhibit an anomalous, dynamic response to gate voltages when observed by low temperature magnetotransport measurements. While the origin of this behavior is unknown, the characteristics of the transport in the presented graphene/α-RuCl3 devices could be useful for next generation electronic devices. When TMDs are interfaced with α-RuCl3, their trans- port properties are also significantly impacted. WSe2 doped with α-RuCl3 is metallic and its pristine photoluminescence (PL) response is quenched, which could have significant implications if utilized in optoelectronic devices. In addition, lateral heterojunctions of pristine WSe2 and WSe2 on α-RuCl3 exhibit pP type diode junction behavior, with direction dependent current responses. This study demonstrates the versatility of α-RuCl3 heterostructures, and documents the progress we have made towards honing our ability to control electronic and ionic charge in vdW heterostructures.

Charge density waves (CDWs) are periodic distortions in a crystal lattice which arise due to the instability of low-dimensional systems, including two-dimensional (2D) van der Waals (vdW) materials. 2D materials can be tuned through a variety of techniques: the creation of vdW heterostructures by stacking together different 2D materials can finely tune properties of the constituent materials and lead to novel behaviors; and electrochemical intercalation, whereby ions are inserted into the vdW gap, can allowed for controlled tuning of a material’s charge carrier density. Both of these techniques for modifying 2D materials offer potential routes towards manipulation of their emergent CDW phases.Chapter 1 of this dissertation contextualizes the study of CDW materials in terms of broader motivation, theoretical background, and a brief literature review. The integration of CDW materials into computer hardware could someday offer a solution to challenges in computing efficiency. Additionally, manipulation of the correlated electronic phenomena observed in CDW materials could also offer a route to high temperature superconductivity. The mechanisms underlying CDW formation include electron-electron coupling and electron-phonon coupling, which lead to periodic atomic displacements. CDWs can be seen in a variety of 1D and 2D materials, and external perturbations to these materials can be used to manipulate and suppress CDW transitions. Chapter 2 discusses the creation of van der Waals heterostructures which interface the CDW material 1T-TaS2 and and the non-CDW material 1H-WSe2. Temperature-dependent electronic transport measurements show a suppression of the CDW transition temperature in 1H-WSe2/1T-TaS2 heterostructures compared to 1T-TaS2 and reveal the TaS2 thickness- dependence of this effect. Raman spectroscopy measurements demonstrate that interfacial interactions in 1T-TaS2/1H-WSe2 lead to a significant decrease in intensity of signal from vibrational modes associated with the CDW lattice of 1T-TaS2, as well as non-uniform shifts in vibrational frequencies in sufficiently thin samples. Along with Raman spectroscopy, photoluminescence spectroscopy is carried out with the aim of uncovering the mechanisms behind this CDW suppression, and indicates that charge transfer between the materials likely occurs; however, the exact nature of the interfacial interaction that leads to CDW suppression remains somewhat elusive. Chapter 3 explores the modification of rare-earth tritelluride materials RTe3, particularly HoTe3 by electrochemical intercalation. HoTe3 is synthesized, and characterization by Raman spectroscopy and electronic transport corroborate previous studies of RTe3 materials. Electrochemical studies are carried out, allowing the intercalation of HoTe3 by Li+ to be observed in-situ by optical microscopy, revealing a pattern of several distinct visual changes that occur in HoTe3 flakes upon intercalation. A concurrent optical microscopy and electronic transport measurement during intercalation suggests that these visual stages are accompanied by a transition to an increasingly insulating phase and/or possible sample degradation. Together, these chapters demonstrate two distinct methods to modify 2D CDW materials and the use of multiple experimental techniques to analyze the effects of these modifications. The studies presented here contribute to a broader understanding of tunable properties in 2D CDW materials.

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.