title: Ultrafast carrier thermalization and cooling dynamics in few-layer MoS2.
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.
authors.affiliation: Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University , Singapore 637371, Singapore.
authors.forename: Zhaogang
authors.initials: Z
authors.lastname: Nie
authors.forename: Run
authors.initials: R
authors.lastname: Long
authors.forename: Linfeng
authors.initials: L
authors.lastname: Sun
authors.forename: Chung-Che
authors.initials: CC
authors.lastname: Huang
authors.forename: Jun
authors.initials: J
authors.lastname: Zhang
authors.forename: Qihua
authors.initials: Q
authors.lastname: Xiong
authors.forename: Daniel W
authors.initials: DW
authors.lastname: Hewak
authors.forename: Zexiang
authors.initials: Z
authors.lastname: Shen
authors.forename: Oleg V
authors.initials: OV
authors.lastname: Prezhdo
authors.forename: Zhi-Heng
authors.initials: ZH
authors.lastname: Loh
keywords: MoS2; carrier−carrier scattering; carrier−phonon scattering; nonthermal; quantum kinetics; ultrafast dynamics