Large single crystal domains of WS2 are grown by chemical vapour deposition and their photoluminescent properties under lateral electric field are studied. We demonstrate that monolayer and bilayer WS2 have opposite responses to lateral electric fields, with lateral electric fields causing monolayer WS2 photoluminescence (PL) to substantially reduce, whilst it increases PL in bilayer WS2. Temperature dependent PL measurements are also undertaken and show distinctly different behaviour to the lateral electric field effects, ruling out heating as the cause of the PL changes. The PL variation in both mono- and bilayer WS2 is attributed to the transfer of photo-excited electrons from one conduction band valley to another, modifying the resultant recombination pathways. The effect of this physical process on luminescent properties can be only observed in 2D TMDs other than bulk semiconductors, due to the much larger exciton binding energy than the energy difference between two conduction band extrema.
