Rapid Energy Sharing at Atomically Thin van der Waals Interfaces During Charge Transfer

Charge and energy transfer processes at the junction of atomically thin, two dimensional (2D) materials are an area of burgeoning interest because van der Waals crystals allow for the creation of arbitrary, atomically precise heterostructures simply by stacking disparate monolayers without the constraints of covalent bonding or epitaxy. At a type II heterojunction between two 2D semiconductors, ultrafast charge transfer has been previously determined to occur on the order of 10’s of femtoseconds after photoexcitation. However, the coupling between the lattice degrees of freedom of the photoexcited monolayers remains less understood. We use ultrafast electron diffraction to directly visualize lattice dynamics in the individual monolayers at the van der Waals heterojunction of WSe2 and WS2. We are able to track the transfer of energy from one layer to another by following the change in intensity of the Bragg peaks after photoexcitation. With the aid of first principles calculations, we uncover the role of layer-hybridized electronic states in enabling ultrafast charge and energy transport across atomic junctions through bidirectional phonon emission. This work is a result of fruitful collaborations with colleagues at various institutions including SLAC National Laboratory, Stanford, Berkeley Lab, UC Berkeley and NIMS Tsukuba.