Structural imaging and electro-optical control of two-dimensional semiconductors

Abstract

Monolayer transition metal dichalcogenides (TMDs) have attracted great interest due to their unique optical and electronic properties. As gapped, semiconducting counterparts to graphene, they display particularly strong light-matter interactions through tightly bound electron-hole pairs (excitons). Atomically-thick layers of distinct materials can further be combined into layered van der Waals (vdW) heterostructures to engineer materials with far more diverse properties than their constituent parts. Beyond just their composition, the relative alignment and resulting atomic stacking order of the layers in a vdW heterostructure can give rise to exotic effects that are entirely absent at different lattice orientations. For instance, periodic variation of local atomic registry due to a small angle or lattice-constant mismatch, known as a moiré pattern, has given rise to superconductivity in twisted bilayer graphene and exotic exciton states in TMD heterobilayers. In this dissertation, I explore monolayer and bilayer TMD structures with properties that show promise for the realization of controlled quantum devices. We identity and examine an optically induced current perpendicular to the plane of a TMD monolayer encapsulated in hexagonal boron nitride (hBN). Contrasted with the conventional picture of hBN as an insulating dielectric, these results point to both limitations and opportunities for the control of charge in TMD devices. Next, I present a new scanning electron microscopy (SEM) technique that makes it possible to nondestructively image the local stacking order in complete vdW devices, and apply it to correlate the local moiré periodicity with optical properties. These results provide important new tools and insights for the study of moiré heterostructures and the engineering of controllable quantum systems in TMDs.