Excitons are electronic excitations formed by an electron-hole pair. At low energies (below a few hundreds of meV), they control the optical properties of transition metal dichalcogenide monolayers. Knowledge of the fine structure of these excitons is therefore essential to understand the optoelectronic properties of these 2D materials. Here we have investigated the exciton fine structure of MoS₂ and MoSe₂ monolayers encapsulated in boron nitride by magneto-photoluminescence spectroscopy in magnetic fields up to 30 T. The experiments performed in transverse magnetic field reveal a brightening of the spin-forbidden dark excitons in MoS₂ monolayer: we find that the dark excitons appear at 14 meV below the bright ones (see Figure). Magnetic brightening experiments thus appear crucial to determine the band structure of monolayer semiconductors. These experiments are possible thanks to the high static magnetic fields accessible at LNCMI-G.

Publication - Measurement of the Spin-Forbidden Dark Excitons in MoS₂ and MoSe₂ monolayers
Robert, B. Han, P. Kapuscinski, A. Delhomme, C. Faugeras, T. Amand, M. R. Molas, M. Bartos, K. Watanabe, T. Taniguchi, B. Urbaszek, M. Potemski, X. Marie Nature Communications 11, 1037, (2020)

Figure -In-plane magnetic field B_// – Spin-forbidden dark exciton in MoS₂ monolayer encapsulated in hBN revealed by photoluminescence (PL). (a) Color map of the variation of the PL intensity as a function of B_//  (the PL intensity of the bright exciton, X_B, has been normalized at each field); (b) PL spectra for magnetic fields from 0 to 30 T showing the emergence of the brightened dark exciton, X_D, at low energy. (c) Ratio of the PL intensity of dark (X_D) and bright (X_B) excitons as a function of magnetic field. Inset: sketch of the excitonic fine structure. The arrows ↑ and ↓ represent the main spin contribution of conduction and valence electrons involved in the exciton states.