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

Monolayer transition metal dichalcogenides bear great potential for photodetection and light harvesting due to high absorption coefficients. However, these applications require dissociation of strongly bound photogenerated excitons. The dissociation can be achieved by vertically stacking different monolayers to realize band alignment that favors interlayer charge transfer. In such heterostructures, the reported recombination times vary strongly, and the charge separation and recombination mechanisms remain elusive. We use two color pump-probe microscopy to demonstrate that the charge separation in a MoSe2/WSe2 heterostructure is ultrafast (similar to 200 fs) and virtually temperature independent, whereas the recombination accelerates strongly with temperature. Ab initio quantum dynamics simulations rationalize the experiments, indicating that the charge separation is temperature-independent because it is barrierless, involves dense acceptor states, and is promoted by higher-frequency out-of-plane vibrations. The strong temperature dependence of the recombination, on the other hand, arises from a transient indirect-to-direct bandgap modulation by low-frequency shear and layer breathing motions.

Automated summary

Experimental and quantum dynamics simulations indicate that charge separation in MoSe2/WSe2 heterostructures is ultrafast and temperature-independent, while recombination accelerates significantly with temperature. Charge separation is temperature-independent due to its barrierless nature, involvement of dense acceptor states, and promotion by higher-frequency vibrations, while the strong temperature dependence of recombination arises from modulation of indirect-to-direct bandgap by low-frequency motions.