Anomalously polarised emission from a MoS/WS heterostructure.

We report circularly polarised emission, with helicity opposite to the optical excitation, from a van der Waals heterostructure (HS) consisting of a monolayer MoS and three-layer WS. Selective excitation of the MoS layer confirms that this cross-polarized emission is due to the charge transfer from the WS layers to the MoS layer. We propose that the high levels of n-doping in the constituent layers due to sulphur vacancies and defects give rise to an enhanced transition rate of electrons from the valley of WS to the ' valley of MoS, which leads to the emission, counter polarized to the excitation. Simulations based on the rate equation model support this conclusion.