Nat. Commun. 7, 12174 (2016)

A metal–semiconductor junction can be used to detect low-energy photons, as the charge carriers photoexcited in the metal are injected into the semiconductor creating a photocurrent. However, the Schottky barrier at the interface sets a sharp lower limit for the photon energy required to trigger the process. Now, Frank Koppens and colleagues at the Institut de Ciencies Fotoniques in Spain and the National Institute for Materials Science in Japan demonstrate that a different mechanism is at work in vertical graphene–WSe2–graphene heterostructures, ideally allowing them to harvest photons with energies smaller than the Schottky barrier.

The researchers exploit the strong electron–electron and weak electron–phonon interactions in graphene. These properties induce a fast thermalization of the photoexcited hot charge carriers and increase the effective temperature of the electronic bath. This causes the electrons to be injected from graphene into WSe2 via the thermionic effect, generating a photocurrent that is then collected by a second graphene electrode.

The researchers can also tune the Schottky barrier using both a bias and a gating voltage, effectively controlling the intensity of the photocurrent. The thermionic effect is verified by the observation of an exponential dependence of the photocurrent on the Schottky barrier height and by a superlinear dependence of the photocurrent on the incoming laser power.