Revealing the strong coupling of atomic motion to interlayer excitons through photocurrent imaging at room temperature.

In atomically thin van der Waals crystals photoexcited excitons are expected to be delocalized over many atomic sites, weakening the electron-phonon coupling strength. Surprisingly, we show that a heterostructure composed of stacked layers exhibits interlayer excitons that are strongly coupled to atomic vibrational motion. To overcome the small oscillator strength of interlayer excitons, we use photocurrent measurements for their high sensitivity and relatively large quantum efficiency. Using Multi-Parameter Dynamic Photoresponse Microscopy on heterostructures made of monolayer MoSe₂ and bilayer WSe₂ encapsulated with hBN, we isolate the signal from the interlayer exciton under precise charge neutrality conditions. We vary the voltage across the material interface while maintaining charge neutrality, thus tuning the potential energy landscape of the interlayer exciton. This results in photoconductance oscillations with a period of 30 meV, the energy of the dominant phonon mode. Interestingly, even as the interlayer exciton exhibits signatures of localization and strong coupling to atomic motion, the large interlayer photocurrent measured in the heterojunction means that the electrons and holes that make up the interlayer exciton are delocalized and move freely in the MoSe₂/WSe₂.