  We investigate electrical gating of photoluminescence and optical absorption
in monolayer molybdenum disulfide (MoS$_2$) configured in field effect
transistor geometry. We observe an hundredfold increase in photoluminescence
intensity and an increase in absorption at ~660 nm in these devices when an
external gate voltage is decreased from +50 V to -50 V, while the
photoluminescence wavelength remains nearly constant. In contrast, in bilayer
MoS$_2$ devices we observe almost no changes in photoluminescence with gate
voltage. We propose that the differing responses of the monolayer and bilayer
devices are related to the interaction of the excitons in MoS$_2$ with charge
carriers.
