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 WSe 2 and WS 2 monolayers, the dark exciton emission is observed at ∼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 MoSe 2 monolayer. Our experiments also show that the MoS 2 monolayer has a dark exciton ground state with a dark-bright exciton splitting energy of ∼100 meV.