Defect healing and charge transfer mediated valley polarization in MoS${}_2$/MoSe${}_2$/MoS${}_2$ trilayer van der Waals heterostructures

Monolayer transition metal dichalcogenides (TMDC) grown by chemical vapor deposition (CVD) are plagued by a significantly lower optical quality compared to exfoliated TMDC. In this work we show that the optical quality of CVD-grown MoSe${}_2$ is completely recovered if the material is sandwiched in MoS${}_2$/MoSe${}_2$/MoS${}_2$ trilayer van der Waals heterostructures. We show by means of density-functional theory that this remarkable and unexpected result is due to defect healing: S atoms of the more reactive MoS${}_2$ layers are donated to heal Se vacancy defects in the middle MoSe${}_2$ layer. In addition, the trilayer structure exhibits a considerable charge-transfer mediated valley polarization of MoSe${}_2$ without the need for resonant excitation. Our fabrication approach, relying solely on simple flake transfer technique, paves the way for the scalable production of large-area TMDC materials with excellent optical quality.