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Stacking atomic monolayers of semiconducting transition metal dichalcogenides(TMDs) has emerged as an effective way to engineer their properties. Inprinciple, the staggered band alignment of TMD heterostructures should resultin the formation of inter-layer excitons with long lifetimes and robust valleypolarization. However, these features have been observed simultaneously only inMoSe$_2$/WSe$_2$ heterostructures. Here we report on the observation of longlived inter-layer exciton emission in a MoS$_2$/MoSe$_2$/MoS$_2$ trilayer vander Waals heterostructure. The inter-layer nature of the observed transition isconfirmed by photoluminescence spectroscopy, as well as by analyzing thetemporal, excitation power and temperature dependence of the inter-layeremission peak. The observed complex photoluminescence dynamics suggests thepresence of quasi-degenerate momentum-direct and momentum-indirect bandgaps. Weshow that circularly polarized optical pumping results in long lived valleypolarization of inter-layer exciton. Intriguingly, the inter-layer excitonphotoluminescence has helicity opposite to the excitation. Our results showthat through a careful choice of the TMDs forming the van der Waalsheterostructure it is possible to control the circular polarization of theinter-layer exciton emission.

Probing the inter-layer exciton physics in a MoS$_2$/MoSe$_2$/MoS$_2$ van der Waals heterostructure