Charge-transfer-induced ultrafast valley depolarization in WS2/GaSe van der Waals heterostructures

Abstract

The interfacial processes of two-dimensional van der Waals heterostructures (vdWHs) are of critical importance to their ultrafast carrier transfer dynamics and device performances as optoelectronic and valleytronic devices. Herein, the intricate interfacial dynamics and valley polarization behavior in WS2/GaSe vdWHs have been studied. Charge transfer (CT) is suggested to be the major interfacial process governing the photocarrier dynamics in this WS2/GaSe heterostructure (HS) by the control experiments, in which an 11-nm h-BN spacer between WS2 and GaSe effectively confirms the pronounced effect of the CT process on the dynamics of the HS. Considering spectral overlap between WS2 and GaSe excitons and multilayer interference effects, contribution from non-radiative energy transfer cannot be fully excluded. Furthermore, valley-resolved measurements reveal that the CT process drastically shortens the valley polarization lifetime to 151 fs in WS2/GaSe compared to 362 fs in monolayer WS2. This acceleration of valley depolarization may be interpreted by spin-degenerate valence bands in GaSe arising from its weak spin–orbit coupling. Acting as interlayer intermediate states, this valence band could facilitate an indirect carrier scattering pathway of “WS2 K valley → GaSe valence band → WS2 K′ valley.” This valley lifetime partially recovers when CT is blocked by h-BN insertion, directly correlating accelerated valley depolarization with interfacial CT. This work gains deep insights into the relationship between the charge separation and valley polarization dynamics in vdWHs, which is crucial for designing future valleytronic and optoelectronic devices.