(138a) Ultrafast Thermal Transport at Photoexcited 2D Van Der Waals Interfaces (invited)

Two-dimensional (2D), van der Waals crystals allow the creation of arbitrary, atomically precise heterostructures simply by stacking disparate monolayers without the constraints of covalent bonding or epitaxy. Charge and energy transfer processes at these novel junctions is an area of burgeoning interest both from the fundamental and application points of view. 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 of the van der Waals heterojunction. 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. We discover that under photoexcitation, thermal transport occurs at a rate orders of magnitude faster than what is explained by vibrational coupling alone, which we quantify by launching phonons across the junction through sub-bandgap excitation. With the aid of first principles calculations, we are able to shed light on the role of lattice dynamics during ultrafast electronic processes at 2D van der Waals heterojunctions.