Exciton dynamics in atomically thin MoS2: Interexcitonic interaction and broadening kinetics

Abstract

We report ultrafast pump-probe spectroscopy examining exciton dynamics in atomically thin MoS 2 . Spectrally-and temporally-resolved measurements are performed to investigate the interaction dynamics of two important direct-gap excitons (A and B) and their associated broadening kinetics. The two excitons show strongly correlated inter-excitonic dynamic, in which the transient blue-shifted excitonic absorption originates from the internal A-B excitonic interaction. The observed complex spectral response is determined by the exciton collision-induced linewidth broadening; the broadening of the B exciton linewidth in turn lowers the peak spectral amplitude of the A exciton. Resonant excitation at the B exciton energy reveals that inter-excitonic scattering plays a more important role in determining the broadening kinetics than free-carrier scattering.

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] .…”

“…The non-equilibrium electrons rapidly scatter towards the conduction band minimum at the K point [14][15][16] , leading to a positive signal for probe photon energies above the bandgap ( ω ≥ 1.9 eV) through state filling (Fig. 2).…”

“…Physically, the pump pulse initially photoexcites carriers in the MoS 2 monolayer to higher energy states (primarily the C exciton states near the Γ point 14,16,29,30 ). The initial photoexcited carrier density is comparable to the Mott density of ~n~10 14 cm…”

“…UOM experiments were performed on individual MoS 2 flakes with different thicknesses as a function of the probe photon energy, with the pump fluence initially fixed at 320 μ J/cm 2 (producing an initial photoexcited carrier density of n~10 14 cm −2 , calculated using the absorbance in ref. 29).…”

Femtosecond spectroscopy on MoS2flakes from liquid exfoliation: surfactant independent exciton dynamics

“…In a more comprehensive study with different excitation fluences, it was found that the more the excited states the more the spectra are shifted to the red. 28 This has been explained as Stark effect due to photoexcited charges, 16,29 inter-excitonic interaction, 30 or band gap renormalization. 31 Additionally, at short delays, the A − and C − photoinduced absorption peaks appear artificially redshifted due to the strong overlap with the A 0 and C 0 bleaching peaks, respectively.…”