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. 2014 Oct 28;8(10):10931-40.
doi: 10.1021/nn504760x. Epub 2014 Oct 2.

Ultrafast carrier thermalization and cooling dynamics in few-layer MoS2

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Ultrafast carrier thermalization and cooling dynamics in few-layer MoS2

Zhaogang Nie et al. ACS Nano. .

Abstract

Femtosecond optical pump-probe spectroscopy with 10 fs visible pulses is employed to elucidate the ultrafast carrier dynamics of few-layer MoS2. A nonthermal carrier distribution is observed immediately following the photoexcitation of the A and B excitonic transitions by the ultrashort, broadband laser pulse. Carrier thermalization occurs within 20 fs and proceeds via both carrier-carrier and carrier-phonon scattering, as evidenced by the observed dependence of the thermalization time on the carrier density and the sample temperature. The n(-0.37 ± 0.03) scaling of the thermalization time with carrier density suggests that equilibration of the nonthermal carrier distribution occurs via non-Markovian quantum kinetics. Subsequent cooling of the hot Fermi-Dirac carrier distribution occurs on the ∼ 0.6 ps time scale via carrier-phonon scattering. Temperature- and fluence-dependence studies reveal the involvement of hot phonons in the carrier cooling process. Nonadiabatic ab initio molecular dynamics simulations, which predict carrier-carrier and carrier-phonon scattering time scales of 40 fs and 0.5 ps, respectively, lend support to the assignment of the observed carrier dynamics.

Keywords: MoS2; carrier−carrier scattering; carrier−phonon scattering; nonthermal; quantum kinetics; ultrafast dynamics.

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