Dark-state impact on the exciton recombination of WS2 monolayers as revealed by multi-timescale pump-probe spectroscopy

Takashi Kuroda; Yusuke Hoshi; Satoru Masubuchi; Mitsuhiro Okada; Ryo Kitaura; Kenji Watanabe; Takashi Taniguchi; Tomoki Machida

doi:10.1103/physrevb.102.195407

Article Dark-state impact on the exciton recombination of WS2 monolayers as revealed by multi-timescale pump-probe spectroscopy

Takashi Kuroda ; Yusuke Hoshi ; Satoru Masubuchi ; Mitsuhiro Okada ; Ryo Kitaura (National Institute for Materials Science) ; Kenji Watanabe ; Takashi Taniguchi ; Tomoki Machida

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Takashi Kuroda, Yusuke Hoshi, Satoru Masubuchi, Mitsuhiro Okada, Ryo Kitaura, Kenji Watanabe, Takashi Taniguchi, Tomoki Machida. Dark-state impact on the exciton recombination of WS2 monolayers as revealed by multi-timescale pump-probe spectroscopy. Physical Review B. 2020, 102 (19), 195407. https://doi.org/10.1103/physrevb.102.195407

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The luminescence yield of transition metal dichalcogenide monolayers frequently suffers from the formation of long-lived dark states, which include excitons with intervalley charge carriers, spin-forbidden transitions, and a large center-of-mass momentum located outside the light cone of dispersion relations. Efficient relaxation from bright exciton states to dark states suppresses the quantum yield of photon emission. In addition, the radiative recombination of excitons is heavily influenced by Auger-type exciton-exciton scattering, which yields another nonradiative relaxation channel at room temperature. Here, we show that Auger-type scattering is promoted not only between (bright) excitons but also between excitons and long-lived dark states. We studied the luminescence dynamics of monolayer WS2 capped with hexagonal BN over broad time ranges of picoseconds to milliseconds using carefully designed pump-and-probe techniques. We observed that luminescence quenching associated with Auger-type scattering occurs on 1-100 μs time scales, which thus correspond to the lifetimes of the relevant dark states. The broad distribution of the measured lifetimes implies the impact of various types of long-lived states on the exciton annihilation process.

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