We present a detailed investigation of the exciton and trion dynamics in naturally doped MoSe2 and WSe2 single atomic layers as a function of temperature in the range 10-300K under above band-gap laser excitation. By combining time-integrated and time-resolved photoluminescence (PL) spectroscopy we show the importance of exciton and trion localization in both materials at low temperatures. We also reveal the transition to delocalized exciton complexes at higher temperatures where the exciton and trion thermal energy exceeds the typical localization energy. This is accompanied with strong changes in PL including suppression of the trion PL and decrease of the trion PL life-time, as well as significant changes for neutral excitons in the temperature dependence of the PL intensity and appearance of a pronounced slow PL decay component. In MoSe2 and WSe2 studied here, the temperatures where such strong changes occur are observed at around 100 and 200 K, respectively, in agreement with their inhomogeneous PL linewidth of 8 and 20 meV at T~10K. The observed behavior is a result of a complex interplay between influences of the specific energy ordering of bright and dark excitons in MoSe2 and WSe2, sample doping, trion and exciton localization and various temperature-dependent non-radiative processes.
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