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 WSe
2
, 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.