  We realize and investigate ionic liquid gated field-effect transistors (FETs)
on large-area MoS2 monolayers grown by chemical vapor deposition (CVD). Under
electron accumulation, the performance of these devices is comparable to that
of FETs based on exfoliated flakes. FETs on CVD-grown material, however,
exhibit clear ambipolar transport, which for MoS2 monolayers had not been
reported previously. We exploit this property to estimate the bandgap {\Delta}
of monolayer MoS2 directly from the device transfer curves and find {\Delta}
$\approx$ 2.4-2.7 eV. In the ambipolar injection regime, we observe
electroluminescence due to exciton recombination in MoS2, originating from the
region close to the hole-injecting contact. Both the observed transport
properties and the behavior of the electroluminescence can be consistently
understood as due to the presence of defect states at an energy of 250-300 meV
above the top of the valence band, acting as deep traps for holes. Our results
are of technological relevance, as they show that devices with useful
optoelectronic functionality can be realized on large-area MoS2 monolayers
produced by controllable and scalable techniques.
