Visualizing Ultrafast Energy and Charge Transport in Semiconducting van der Waals Heterostructures

I will discuss two stories from our joint experimental and theoretical work focusing on the prototypical 2D semiconductor interface of monolayer WS₂ and WS₂. In part one, we use ultrafast electron diffraction to visualize lattice dynamics during the charge transfer process at the type-II heterojunction. Following the excitation of lower-gap WSe₂, we measure the concurrent<br/>heating of both layers on a picosecond timescale. This observation cannot be explained by phonon transport across the interface. In conjunction with ab initio theory, we uncover the critical role of layer-hybridized electronic states in enabling ultrafast energy and charge transport across such atomic junctions. In part two, we use optical spectroscopy and microscopy to visualize diffusion of interlayer excitons over a wide range of temperatures. While the measured exciton diffusivity decreases with temperature, it surprisingly plateaus below 90 K. Our observations cannot be explained by classical models like hopping in the moiré potential. Ab initio theory and molecular dynamics simulations suggest that low energy phonons arising from the mismatched lattices of a moiré heterostructures play a key role in understanding this anomalous behavior of exciton diffusion. This indicates that the moiré potential landscape is dynamic even at low temperatures.