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Controlling Exciton and Valley Dynamics in Two-Dimensional Heterostructures with Atomically Precise Interlayer Proximity.

2D materials and heterostructures with strong excitonic effect and spin/valley properties have emerged as an exciting platform for optoelectronic and spin/valleytronic applications. There, precisely control of exciton transformation process (including intralayer to interlayer exciton transition and recombination) and valley polarization process vis structural tuning is crucial but remains largely unexplored. Here using hexagonal boron nitride (BN) as an intermediate layer, we show the fine tuning of exciton and valley dynamics in 2D heterostructures with atomic precision. Both interfacial electron and hole transfer rates decrease exponentially with increasing BN thickness, which can be well described with quantum tunneling model. The increased spatial separation with BN intercalation weakens the electron-hole Coulomb interaction and significantly prolongs the interlayer exciton population and valley polarization lifetimes in van der Waals heterostructures. For example, WSe2/WS2 heterostructures with monolayer BN intercalation exhibit a hole valley polarization lifetime of ~ 60 ps at room temperature, which is about three folds and three orders of magnitude longer than that in WSe2/WS2 heterobilayer without BN and WSe2 monolayer, respectively. Considering a large family of layered materials, this study suggests a general approach to tailor and optimize exciton and valley properties in vdW heterostructures with atomic precision.