  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.
