Bidirectional Phonon Emission in Two-Dimensional Heterostructures triggered by Ultrafast Charge Transfer

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 describe how we use ultrafast electron diffraction to uncover the role of layer-hybridized electronic states as a powerful route to control ultrafast energy transport across atomic junctions [1]. We measure the simultaneous heating of both WSe2 and WS2 in a WSe2/WS2 heterobilayer on a picosecond timescale after selective excitation of the WSe2 monolayer. This observation cannot be explained purely by phonon transport across the interface. Through first-principles calculations, we identify electronic states hybridized across the heterostructure that allow phonon-assisted interlayer transfer of photoexcited electrons, which leads to bidirectional phonon emission and simulatneous heating of both the layers.<br/><br/>[1] Sood, A., Haber, J.B. et al. Bidirectional phonon emission in two-dimensional heterostructures triggered by ultrafast charge transfer. Nat. Nanotechnol. 18, 29–35 (2023)