Quenching of the band gap of two-dimensional semiconductors with a perpendicular electric field

The electronic band structure of atomically thin semiconductors can be tuned by the application of a perpendicular electric field. The principle was demonstrated experimentally shortly after the discovery of graphene by opening a finite band gap in graphene bilayers that naturally are zero-gap semiconductors. So far, however, the same principle could not be employed to control a broader class of materials, because the required electric fields are beyond reach in current devices. To overcome this limitation, we have realized double ionic gated-transistors that enable the application of very large electric fields, due to the formation of electric double layers. Using these devices, we show that the band gap of few-layer semiconducting transition metal dichalcogenides, from bilayer to heptalayer WSe2, can be continuously suppressed from 1.6 eV to zero. Our results illustrate an unprecedented level of control of the band structure of 2D semiconductors.