Unraveling Exciton Dynamics in van der Waals Heterostructures from First Principles

Van der Waals (vdW) heterostructures composed of vertically stacked two-dimensional semiconducting layers (such as transition metal dichalcogenides, TMDs) provide a fascinating playground to explore both fundamental physics and novel applications. Due to reduced dielectric screening and quantum confinement effect, TMD vdW heterostructures feature a prominent excitonic effect with large exciton binding energies, which play a crucial role in the dynamics of interfacial charge and energy transfer. Based on the single-particle picture, previous first-principles studies of TMD heterostructures failed to capture the excitonic effect, thus could not provide a comprehensive description of the underlying exciton dynamics. In this work, we present a first-principles framework that combines linear-response time-dependent density functional theory with non-adiabatic molecular dynamics which can capture the excitonic effect and exciton-phonon interactions in TMD heterostructures. Our first-principles method enables us to uncover the dynamic competition between energy and charge transfer in TMD bilayers, to unravel their mechanisms (Dexter and FÖrster) and relative efficiencies, and to explore the means to control their competition. The excitonic effect is found to drive the ultrafast energy and charge transfer in TMD heterostructures. Our work provides a basis to understand relevant phenomena in vdW heterostructures and paves the way for rational design of novel vdW heterostructures for optoelectronic and photovoltaic applications.