Visualizing Ultrafast Defect-Controlled Interlayer Electron-Phonon Coupling in Van der Waals Heterostructures.

Engineering ultrafast interlayer coupling provides access to new quantum phenomena and novel device functionalities in atomically thin van der Waals heterostructures. However, due to all the atoms of a monolayer material being exposed at the interfaces, the interlayer coupling is extremely susceptible to defects, resulting in high energy dissipation through heat and low device performance. The study of how defects affect the interlayer coupling at ultrafast and atomic scales remains a challenge. Here, using femtosecond transient absorption microscopy, a new defect-induced ultrafast interlayer electron-phonon coupling pathway is identified in a WS /graphene heterostructure, involving a three-body collision between electrons in WS and both acoustic phonons and defects in graphene. This interaction manifests as the reduced defect-related Raman resonant activity and the accelerated electron-phonon scattering time from 7.1 to 2.4 ps. Furthermore, the ultrafast interlayer coupling process is directly imaged. These insights will advance the fundamental knowledge of heat dissipation in nanoscale devices, and enable new ways to dynamically manipulate electrons and phonons via defects in van der Waals heterostructures.