Temperature Dependence of Photoluminescence Lifetimes of WSe₂ Monolayers

In photoluminescence (PL) spectra of atomically-thin layers of tungsten-based transition metal dichalcogenides (TMDs), two different types of components are typically observed: short-lived PL lines corresponding to free excitonic states, and lower-energy long-lived PL peaks related to so-called localized states. The nature of the latter features is, however, still not clear. Those lines exhibit a strong decrease of intensity with increasing temperature, which is most probably related to the opening of non-radiative recombination channels, either from relaxed excitonic states (from which the PL originates) or from excited states (acting as intermediate states during the non-resonant excitation process). In this work we analyze the properties of the WSe₂ PL peaks related to the localized states as a function of temperature with the use of time-resolved spectroscopy. We determine that the intensity of those peaks is inversely proportional to their lifetime, thus the decrease of PL intensity can be explained by the opening of recombination channels from relaxed localized states. This may shed some light on the nature of those states, which is important for future optoelectronics and valleytronics applications of TMDs.