Phonon-Mediated Interlayer Charge Separation and Recombination\nin a MoSe₂/WSe₂ Heterostructure

Abstract

Monolayer\ntransition metal dichalcogenides bear great potential\nfor photodetection and light harvesting due to high absorption coefficients.\nHowever, these applications require dissociation of strongly bound\nphotogenerated excitons. The dissociation can be achieved by vertically\nstacking different monolayers to realize band alignment that favors\ninterlayer charge transfer. In such heterostructures, the reported\nrecombination times vary strongly, and the charge separation and recombination\nmechanisms remain elusive. We use two color pump–probe microscopy\nto demonstrate that the charge separation in a MoSe₂/WSe₂ heterostructure is ultrafast (∼200 fs) and virtually\ntemperature independent, whereas the recombination accelerates strongly\nwith temperature. <i>Ab initio</i> quantum dynamics simulations\nrationalize the experiments, indicating that the charge separation\nis temperature-independent because it is barrierless, involves dense\nacceptor states, and is promoted by higher-frequency out-of-plane\nvibrations. The strong temperature dependence of the recombination,\non the other hand, arises from a transient indirect-to-direct bandgap\nmodulation by low-frequency shear and layer breathing motions.