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 both WSe$_2$ and WS$_2$ monolayers, the dark exciton emission is observed at $\sim$50 meV below the bright exciton peak and displays a characteristic doublet structure which intensity is growing 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 $\sim$100 meV. 
M. R. Molas, C. Faugeras, A. O. Slobodeniuk, K. Nogajewski, M. Bartos,   D. M. Basko, M. Potemski
cond-mat.mes-hall