Intense light-matter interactions and unique structural and electrical properties make Van der Waals heterostructures composed by Graphene (Gr) and monolayer transition metal dichalcogenides (TMD) promising building blocks for tunnelling transistors, flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics and QLEDs, bright and narrow-line emitters using minimal amounts of active absorber material. The performance of such devices is critically ruled by interlayer interactions which are still poorly understood in many respects. Specifically, two classes of coupling mechanisms have been proposed: charge transfer (CT) and energy transfer (ET), but their relative efficiency and the underlying physics is an open question. Here, building on a time resolved Raman scattering experiment, we determine the electronic temperature profile of Gr in response to TMD photo-excitation, tracking the picosecond dynamics of the G and 2D bands. Compelling evidence for a dominant role ET process accomplished within a characteristic time of ~ 3 ps is provided. Our results suggest the existence of an intermediate process between the observed picosecond ET and the generation of a net charge underlying the slower electric signals detected in optoelectronic applications.
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