Interfacial Charge Transfer Circumventing Momentum Mismatch at 2D van der Waals Heterojunctions

TL;DR

This study demonstrates ultrafast interfacial charge transfer in 2D heterojunctions that bypasses momentum mismatch through excess energy and defects, advancing understanding of charge dynamics in layered materials.

Contribution

It reveals that charge transfer and recombination at 2D heterojunctions can circumvent momentum mismatch, independent of angular alignment, using femtosecond spectroscopy.

Findings

Ultrafast (<40 fs) electron transfer occurs regardless of angular alignment.

Charge recombination rates vary widely and are defect-mediated.

Momentum mismatch is bypassed via excess energy and defects.

Abstract

Interfacial charge separation and recombination at heterojunctions of monolayer transition metal dichalcogenides (TMDCs) are of interest to two dimensional optoelectronic technologies. These processes can involve large changes in parallel momentum vector due to the confinement of electrons and holes to the K-valleys in each layer. Since these high-momentum valleys are usually not aligned across the interface of two TMDC monolayers, how parallel momentum is conserved in the charge separation or recombination process becomes a key question. Here we probe this question using the model system of a type-II heterojunction formed by MoS2 and WSe2 monolayers and the experimental technical of femtosecond pump-probe spectroscopy. Upon photo-excitation specifically of WSe2 at the heterojunction, we observe ultrafast (<40 fs) electron transfer from WSe2 to MoS2, independent of the angular alignment and, thus, momentum mismatch between the two TMDCs. The resulting interlayer charge transfer exciton decays via nonradiatively recombination, with rates varying by up to three-orders of magnitude from sample to sample, but with no correlation with inter-layer angular alignment. We suggest that the initial interfacial charge separation and the subsequent interfacial charge recombination processes circumvent momentum mismatch via excess electronic energy and via defect-mediated recombination, respectively.