We have combined spatially-resolved steady-state micro-photoluminescence (µPL) with timeresolved photoluminescence (TRPL) to investigate the exciton diffusion in a WSe2 monolayer encapsulated with hexagonal boron nitride (hBN). At 300 K, we extract an exciton diffusion length LX = 0.36 ± 0.02 µm and an exciton diffusion coefficient of DX = 14.5 ± 2 cm 2 /s. This represents a nearly 10-fold increase in the effective mobility of excitons with respect to several previously reported values on nonencapsulated samples. At cryogenic temperatures, the high optical quality of these samples has allowed us to discriminate the diffusion of the different exciton species : bright and dark neutral excitons, as well as charged excitons. The longer lifetime of dark neutral excitons yields a larger diffusion length of L X D = 1.5 ± 0.02 µm.Introduction.-Two-dimensional crystals of transition metal dichalcogenides (TMDC) such as MX 2 (M=Mo, W; X=S, Se, Te) are promising atomically flat semiconductors for applications in nanoelectronics and optoelectronics [1][2][3][4][5]. For example, solar cells [6],photodetectors [7] and laser prototypes [8] based on mono or few-layer MoS 2 have been recently demonstrated. In addition to their potential for unconventional, atomically thin and flexible optoelectronics, the interplay between inversion symmetry breaking and strong spin-orbit coupling in monolayers (MLs) also yields unique spin/valley properties which are expected to provide additional functionalities in future devices [9][10][11][12][13][14][15].
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