  We realized ambipolar Field-Effect Transistors by coupling exfoliated thin
flakes of tungsten disulphide (WS2) with an ionic liquid-dielectric. The
devices show ideal electrical characteristics, including very steep
sub-threshold slopes for both electrons and holes and extremely low OFF-state
currents. Thanks to these ideal characteristics, we determine with high
precision the size of the band-gap of WS2 directly from the gate-voltage
dependence of the source-drain current. Our results demonstrate how a careful
use of ionic liquid dielectrics offers a powerful strategy to study
quantitatively the electronic properties of nano-scale materials.
