Ultrafast Charge-Transfer Dynamics in Twisted MoS/WSe Heterostructures.

Two-dimensional transition metal dichalcogenides offer a fascinating platform for creating van der Waals heterojunctions with exciting physical properties. Because of their typical type-II band alignment, photoexcited electrons and holes can separate interfacial charge transfer. Furthermore, the relative crystallographic alignment of the individual layers in these heterostructures represents an important degree of freedom. Based on both effects, various fascinating ideas for applications in optoelectronics and valleytronics have been suggested. Despite its utmost importance for the design and efficiency of potential devices, the nature and the dynamics of ultrafast charge transfer are not yet well understood. This is mainly because the charge transfer can be surprisingly fast, usually faster than the temporal resolution of previous experimental approaches. Here, we apply time- and polarization-resolved second-harmonic imaging microscopy to investigate the charge-transfer dynamics for three MoS/WSe heterostructures with different stacking angles at a previously unattainable time resolution of ≈10 fs. For 1.70 eV excitation energy, electron transfer from WSe to MoS is found to depend considerably on the stacking angle with the fastest transfer time observed to be as short as 12 fs. At 1.85 eV excitation energy, ultrafast hole transfer from MoS to hybridized states at the Γ-point and to the K-points of WSe has to be considered. Surprisingly, the corresponding decay dynamics show only a minor stacking-angle dependence indicating that radiative recombination of momentum-space indirect Γ-K excitons becomes the dominant decay route for all samples.