Effect of Phonons on Valley Depolarization in Monolayer WSe2

In this paper, temperature dependence of the excitonic bands in a mechanically exfoliated tungsten diselenide (WSe2) monolayer is studied using photoluminescence and circular dichroic photoluminescence (PL) in the temperature range between 8 and 300K. The peak energies associated with the neutral exciton (A), charged exciton (trion) and localized excitons are extracted from the PL spectra revealing a trion binding energy of around 30meV. The circular dichroic PL measured at 8K shows about 45% valley polarisation that sharply reduces with increasing temperature to 5% at 300K with photoexcitation energy of 1.96eV. A detailed analysis of the emission line-width suggests that the rapid decrease of valley polarisation with the increase of temperature is caused by the strong exciton-phonon interactions which efficiently scatter the excitons into different excitonic states that are easily accessible due to the supply of excess photoexcitation energy. The emission line-width broadening with the increase of temperature indicate residual exciton dephasing lifetime <100fs, that correlates with the observed rapid valley depolarisation. [GRAPHICS] .