Nano Lett. http://doi.org/t54 (2014)

Two-dimensional materials have been heralded as core components of future electronic devices. Graphene, hexagonal boron nitride and transition-metal dichalcogenides: each material has a different electronic band structure, and can be layered into an endless variety of stacks. It is anticipated that 'designer heterostructures' with specific physical properties can be engineered.

Marco Furchi and colleagues have now made and studied a device consisting of two transition metal dichalcogenides — molybdenum disulphide and tungsten diselenide — held together by van der Waals forces, on a substrate of silicon oxide. The authors found that their stacked sample acts not only as a diode, but also as a photovoltaic cell.

The observed behaviour is a consequence of the interplay between the band structures of the two layers. The diode function involves, for forward biasing, electrons or holes hopping from one layer into the other. As for the photovoltaic effect, when the device is irradiated with photons, electron–hole pairs form in both the MoS2 and the WSe2 layer. The ensuing carrier relaxation results in a charge transfer across the two layers — and a photocurrent.