Brightening of dark excitons in monolayers of semiconducting transition metal dichalcogenides

We present low-temperature magneto-photoluminescence experiments which demonstrate the brightening of dark excitons by an in-plane magnetic field B applied to monolayers of different semiconducting transition metal dichalcogenides. For WSe2 and WS2 monolayers, the dark exciton emission is observed at similar to 50 meV below the bright exciton peak and displays a characteristic doublet structure whose intensity grows with B-2, while no magnetic field induced emission peaks appear for MoSe2 monolayer. Our experiments also show that the MoS2 monolayer has a dark exciton ground state with a dark-bright exciton splitting energy of similar to 100 meV.