Painting Potential Landscapes on an Atomically Thin Canvas

Atomically thin van der Waals crystals like graphene and transition metal dichalcogenides allow for the creation of arbitrary, atomically precise heterostructures simply by stacking disparate monolayers without the constraints of covalent bonding or epitaxy. While these are commonly described as nanoscale LEGO blocks, many intriguing phenomena have been discovered in the recent past that go beyond this simple analogy. In this talk, I will discuss two stories from our joint experimental and theoretical work focusing on the rotationally-aligned 2D semiconductor heterostructure of monolayer WS2 and WSe2. In part one, we use electron microscopy and electron energy loss spectroscopy to directly visualize and correlate the nanoscale real space localization of excitonic states and structural reconstruction within the moiré unit cell, opening up the possibility for on-demand engineering of excitonic superlattices with nanometer precision. In part two, we use time- and spatially-resolved spectroscopies to investigate the diffusion of various interlayer exciton species as a function of temperature. We uncover the role of low energy moiré phonons or phasons in enabling exciton diffusion in the moiré potential even at low temperatures. These works are a result of fruitful collaborations with colleagues at various institutions including Berkeley Lab, UC Berkeley, Purdue University, Imperial College, Stanford Unviersity and NIMS Tsukuba.