  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 quantum light emitting
devices (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 ~ 4 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.
