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.

Nonequilibrium carrier dynamics in transition metal dichalcogenide semiconductors

“…This process is determined by the slower dynamics of the hole relaxation. The same tendency was found in [7] for few-layer MoS 2 . The origin for carrier relaxation from high-energy states being faster than intervalley scattering is the better availability of final states for the assisting scattering process.…”

Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

“…By tuning the probe photon energy through the MoS 2 band gap (both indirect and direct), our UOM measurements show that conduction and valence band states are rapidly populated on a sub-picosecond (ps) time scale in a MoS 2 monolayer after photoexcitation at 3.1 eV, consistent with previous work [14][15][16][17][18] . Pump fluence-dependent measurements reveal that subsequent carrier relaxation in our samples is primarily due to surface-related defects and trap states, not the Auger processes observed in previous measurements on MoS 2 and other semiconductor nanosystems 12,[18][19][20][21][22][23] .…”

“…Our curve fits indicate that these carriers relax within ~30 ps. We also examined the dependence of the observed carrier dynamics on pump fluence for the single MoS 2 monolayer flake, in a regime (F~95-500 μ J/cm 2 ) that has not been extensively studied 17,18,34,35 . Figure 3(a) shows the normalized transmission changes for various pump fluences at a probe photon energy of ω = 1.91 eV, revealing that the initial decay time τ increases as a function of pump fluence.…”

Libra: An open-Source “methodology discovery” library for quantum and classical dynamics simulations

“…In the conclusion, I emphasize that the present approach is distinct to CPA, previously used in various studies . In CPA, only a single ground state trajectory is utilized and many stochastic realizations of the surface hopping algorithm are computed for the single nuclear trajectory.…”