  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, which 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. Using these devices, we show that the band gap of few-layer
semiconducting transition metal dichalcogenides can be continuously suppressed
from 1.5 eV to zero. Our results illustrate an unprecedented level of control
of the band structures of 2D semiconductors, which is important for future
research and applications.
