Ultrafast\nCharge-Transfer Dynamics in Twisted MoS₂/WSe₂ Heterostructures

Abstract

Two-dimensional transition\nmetal dichalcogenides offer a fascinating\nplatform for creating van der Waals heterojunctions with exciting\nphysical properties. Because of their typical type-II band alignment,\nphotoexcited electrons and holes can separate <i>via</i> interfacial charge transfer. Furthermore, the relative crystallographic\nalignment of the individual layers in these heterostructures represents\nan important degree of freedom. Based on both effects, various fascinating\nideas for applications in optoelectronics and valleytronics have been\nsuggested. Despite its utmost importance for the design and efficiency\nof potential devices, the nature and the dynamics of ultrafast charge\ntransfer are not yet well understood. This is mainly because the charge\ntransfer can be surprisingly fast, usually faster than the temporal\nresolution of previous experimental approaches. Here, we apply time-\nand polarization-resolved second-harmonic imaging microscopy to investigate\nthe charge-transfer dynamics for three MoS₂/WSe₂ heterostructures with different stacking angles at a previously\nunattainable time resolution of ≈10 fs. For 1.70 eV excitation\nenergy, electron transfer from WSe₂ to MoS₂ is\nfound to depend considerably on the stacking angle with the fastest\ntransfer time observed to be as short as 12 fs. At 1.85 eV excitation\nenergy, ultrafast hole transfer from MoS₂ to hybridized\nstates at the Γ-point and to the K-points of WSe₂ has to be considered. Surprisingly, the corresponding decay dynamics\nshow only a minor stacking-angle dependence indicating that radiative\nrecombination of momentum-space indirect Γ-K excitons becomes\nthe dominant decay route for all samples.