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Layer-dependent signatures for exciton dynamics in monolayer and multilayer WSe2 revealed by fluorescence lifetime imaging measurement

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Two-dimensional (2D) transition-metal dichalcogenide (TMD) materials have aroused noticeable interest due to their distinguished electronic and optical properties. However, little is known about their complex exciton properties together with the exciton dynamics process which have been expected to influence the performance of optoelectronic devices. The process of fluorescence can well reveal the process of exciton transition after excitation. In this work, the room-temperature layer-dependent exciton dynamics properties in layered WSe2 are investigated by the fluorescence lifetime imaging microscopy (FLIM) for the first time. This paper focuses on two mainly kinds of excitons including the direct transition neutral excitons and trions. Compared with the lifetime of neutral excitons (< 0.3 ns within four-layer), trions possess a longer lifetime (~ 6.6 ns within four-layer) which increases with the number of layers. We attribute the longer-lived lifetime to the increasing number of trions as well as the varieties of trion configurations in thicker WSe2. Besides, the whole average lifetime increases over 10% when WSe2 flakes added up from monolayer to four-layer. This paper provides a novel tuneable layer-dependent method to control the exciton dynamics process and finds a relatively longer transition lifetime of trions at room temperature, enabling to investigate in the charge transport in TMD-based optoelectronics devices in the future.

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This work is supported by the National Natural Science Foundation of China (Nos. 51527901, 51575298, 51705285, and 11890672). And we are grateful to Tsinghua-Nikon Imaging Core Facility for providing technical support and to Yanli Zhang for assistance with confocal microscopy and image processing.

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Correspondence to Dameng Liu.

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Liu, Y., Li, H., Qiu, C. et al. Layer-dependent signatures for exciton dynamics in monolayer and multilayer WSe2 revealed by fluorescence lifetime imaging measurement. Nano Res. (2020). https://doi.org/10.1007/s12274-020-2670-7

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  • two-dimensional (2D) WSe2
  • exciton dynamics
  • fluorescence lifetime
  • fluorescence lifetime imaging microscopy (FLIM)
  • density functional theory (DFT)