The temperature influence on the brightening of neutral and charged dark excitons in WSe${}_2$ monolayer

The optically dark states play an important role in the electronic and optical properties of monolayers (MLs) of semiconducting transition metal dichalcogenides. The effect of temperature on the in-plane-field activation of the neutral and charged dark excitons is investigated in a WSe${}_2$ ML encapsulated in hexagonal BN flakes. The brightening rates of the neutral dark (X${}^D$) and grey (X${}^G$) excitons and the negative dark trion (T${}^D$) differ substantially at a particular temperature. More importantly, they vanish considerably by about 3 – 4 orders of magnitude with the temperature increased from 4.2 K to 100 K. The quenching of the dark-related emissions is accompanied by the two-order-of-magnitude increase in the emissions of their neutral bright counterparts, $i\mathrm{.}e\mathrm{.}$ neutral bright exciton (X${}^B$) and spin-singlet (T${}^S$) and spin-triplet (T${}^T$) negative trions, due to the thermal activations of dark states. Furthermore, the energy splittings between the dark X${}^D$ and T${}^D$ complexes and the corresponding bright X${}^B$, T${}^S$, and T${}^T$ ones vary with temperature rises from 4.2 K to 100 K. This can be explained in terms of the different exciton-phonon couplings for the bright and dark excitons stemming from their distinct symmetry properties.