  Finding alternative optoelectronic mechanisms that overcome the limitations
of conventional semiconductor devices is paramount for detecting and harvesting
low-energy photons. A highly promising approach is to drive a current from the
thermal energy added to the free-electron bath as a result of light absorption.
Successful implementation of this strategy requires a broadband absorber where
carriers interact among themselves more strongly than with phonons, as well as
energy-selective contacts to extract the excess electronic heat. Here we show
that graphene-WSe2-graphene heterostructure devices offer this possibility
through the photo-thermionic effect: the absorbed photon energy in graphene is
efficiently transferred to the electron bath, leading to a thermalized hot
carrier distribution. Carriers with energy higher than the Schottky barrier
between graphene and WSe2 can be emitted over the barrier, thus creating
photocurrent. We experimentally demonstrate that the photo-thermionic effect
enables detection of sub-bandgap photons, while being size-scalable,
electrically tunable, broadband and ultrafast.
