UC Berkeley

In the fascinating family of two-dimensional (2D) materials, layers of atoms are bonded strongly into sheets that are held together by weak van der Waals bonds. Consequently, atomically-thin layers of various 2D materials can be isolated into monolayers and stacked together to form synthetic materials with emergent electronic and optical properties. The behavior of a 2D stack is determined by the behavior of electrons within each layer and interactions between the layers, which provides remarkable opportunities for creativity and control in electronic material design. In the past few years, a unique and exciting prospect for 2D stacks has emerged: the ability to tune the twist angle between atomic layers. With a small twist angle or lattice mismatch, the interference of two atomic lattices produces a large periodic pattern, called a moiré superlattice, where the alignment between atoms varies periodically across the system. Recently, it has become clear that moiré superlattices can dramatically modify the electronic properties of a 2D stack, resulting in novel states of matter, modified light-matter interactions, and more. In this dissertation, I focus on moiré superlattices formed from WSe2 and WS2 monolayers, which are both members of the semiconducting transition metal dichalcogenide (TMD) family known for their strong light-matter interactions and promise for optoelectronic applications. I begin with an introduction to semiconducting TMDs, which guides our understanding of TMD stacks and, in particular, moiré superlattices. I then describe several experimental studies highlighting how a simple interference pattern can produce fascinating electronic and optical behavior. First, I consider how a moiré superlattice modifies the excitonic properties of TMDs, where our combined experimental and theoretical investigations reveal exciton states unique to superlattices. From there, I describe how moiré superlattices can lead to novel correlated states of matter by shifting the balance between the kinetic energy and interaction energy of electrons. Using a new experimental approach, we reveal the presence of generalized Wigner crystal states in TMD moiré superlattices thanks to the enhanced influence of electron-electron interactions. Finally, I demonstrate an optical technique for measuring spin transport in TMD moiré superlattices that provides insight into how spin behave in correlated 2D materials.