Auger Processes and Excited State Dynamics in WS2/Graphene Heterostructures: A First-Principles Perspective.

Abstract

van der Waals heterostructures comprised of graphene and transition metal dichalcogenides (TMDs) represent a fascinating platform to pursue both fundamental science and novel applications. However, microscopic mechanisms underlying their charge and energy transfer dynamics remain poorly understood and controversies abound in literature. In this work, by means of first-principles calculations, we conduct a comprehensive study on excited state dynamics in a representative WS2/graphene heterostructure and provide critical insights and potential resolutions to key controversies in literature. We show that direct interlayer excitations are too weak to yield efficient charge transfer. Instead, ultrafast charge transfer stems primarily from interlayer Auger-like processes driven by strong electron-hole interactions. Electron-phonon coupling, essential for hot carrier relaxation, cannot outcompete Auger processes owing to phonon bottleneck. Thus, interfacial charge transfer occurs well before charge carriers are thermalized. Transient charge-separated states with highly asymmetric dynamics are shown to result from a disparity in density of states of the heterostructure. The interplay (co-operation and competition) between Auger processes and electron-phonon scattering is elucidated.