  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.
