Engineering Interfacial Effects and Layered Behavior in 2D Materials

Thesis (Ph.D.)--University of Washington, 2020The rise of two-dimensional (2D) materials over the last one-and-a-half decades can be attributed to two separate reasons. The first is the discovery of new materials with unique properties that derive partially, if not entirely from their low dimensionality, as is the case for 2D Chern insulators. The second is the ability to combine 2D materials together arbitrarily through van der Waals stacking to form custom layered structures with engineered properties. But more than just offering a new knob for controlling materials, van der Waals heterostructures have produced to breakthrough discoveries in basic science, helping us create new electronic phases and explore topology in new ways. Central to producing these effects are the different types of interactions between layers, from the most trivial effects like dielectric screening to more complex interactions like interlayer charge transfer and exchange interactions. Consequently, learning how to control interlayer interactions and predict the physical outcomes they produce has become a key scientific challenge in the 2D materials community. In this dissertation, several approaches to engineering, controlling, and harnessing the power of interlayer and interfacial effects in various 2D material systems are explored. First, we introduce a new, powerful spectroscopic tool for understanding the effects of quantum confinement on excitons, optically excited and bound electron-hole pairs, in 2D semiconductors, and use it to explore the intricacies of the Hamiltonian of these 2D excitons. We then discuss the novel properties of heterobilayers of 2D semiconductors which host interlayer excitons in which the electron and hole reside in opposite layers, and present a simple approach to enhancing the beneficial properties of these interlayer excitons for studying their many-body physics by modifying the interlayer interaction with a tunneling barrier. Finally, we use various optical and spectroscopic probes to investigate a new type of behavior found in layered magnetic van der Waals materials, layered magnetism, which results from the fundamental anisotropy of the materials. We analyze a simple approach to engineering the interlayer magnetic coupling in such materials, and uncover a deep, underlying connection between the layered magnetic order and the optical excitations in a newly discovered 2D magnetic semiconductor