We studied the effective exciton radiative lifetimes of monolayer two-dimensional transition-metal dichalcogenides MX 2 (M = Mo, W; X = S, Se). The photoluminescence (PL) quantum yield and PL decay time were measured for monolayer MoS2, MoSe2, WS2, and WSe2 at room temperature. Effective exciton radiative lifetimes of >10 ns were determined from the PL quantum yield of 10-2 to 10-3 and the PL decay time of several hundred picoseconds. The results are explained on the basis of the long radiative lifetime of bright excitons at a low temperature limit, a finite exciton coherence area of several square nanometers, and the population of dark exciton states.
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Acknowledgments This work was supported by Asahi Glass Foundation, JSPS KAKENHI Grant Numbers JP22740195, JP25400324, JP15K13337, JP15H05408, JP16H00911, JP17K19055, JP25610074, JP26107522, JP16H00910, JP15K13500, and JP16H06331, and by JST CREST (JPMJCR16F3). The authors thank A. Wakamiya and Y. Murata for the use of experimental equipment.
© 2018 The Japan Society of Applied Physics.