Transient absorption of transition metal dichalcogenide monolayers studied by a photodope-pump-probe technique

Abstract

We report three-pulse photodope-pump-probe measurements on photocarrier dynamics in semiconducting transition metal dichalcogenide monolayers of MoS 2 , WS 2 , MoSe 2 , and WSe 2 . The samples are fabricated by metal-organic chemical vapor deposition and mechanical exfoliation techniques and characterized by photoluminescence spectroscopy. In the time-resolved measurement, the samples are first photodoped by a prepulse, which injects background photocarriers of various densities. A pump pulse then injects photocarriers, whose dynamics is monitored by measuring a differential reflection of a time-delayed probe pulse. We found that the ultrafast decay component of the differential reflection signal, which has been widely reported before, shows minimal dependence on the background exciton density. This observation shows that a previously suggested carrier-trapping model cannot account for this component. The results thus further support an exciton-formation model that was previously proposed based on spectroscopic evidence.

Excitonic Dynamics in Janus MoSSe and WSSe Monolayers

“…Previously, we have shown that the fast decay process of the differential reflectance signal in TMD monolayers is due to the formation of excitons from pump-injected free electron−hole pairs. 32,33 The deduced exciton formation times have also been confirmed independently by another experimental technique 34 and are consistent with a theory. 35 Hence, the results suggest that the exciton formation time in MoSSe is faster than that in MoS 2 by about 30%.…”

Photocarrier Dynamics in MoTe2 Nanofilms with 2H and Distorted 1T Lattice Structures

“…Since the intercarrier scattering rate increases with their density, the decrease of the rise time with the pump fluence is reasonable. Meanwhile, excitons are also expected to form from the free electron–hole pairs on this time scale: Previous studies have revealed that in monolayer semiconductors, the exciton binding energies can reach several hundreds of millielectronvolts, , and electron–hole pairs can form excitons within 1 ps. − In the nanofilm studied here, we expect similarly large exciton binding energies. Hence, the electron–hole pairs can form hot excitons in this few-picoseconds time range.…”

Ultrafast Interlayer Charge Transfer between Bilayer PtSe2 and Monolayer WS2

“…The decay of the WS 2 signal is fit by a triple exponential function, Δ R / R 0 ( t ) = A 1 exp­(− t /τ 1 ) + A 2 exp­(− t /τ 2 ) + A 3 exp­(− t /τ 3 ), as shown by the cyan curve over the black squares in Figure b,c, with three decay constants (and their weights) of 0.24 ± 0.03 ps (83%), 2.4 ± 0.3 ps (9%), and 27 ± 2 ps (8%), respectively. The initial sub-picosecond fast decay has been previously assigned to the exciton formation process of the excited electron–hole pairs. − The 2.4 and 27 ps processes reflect the excitonic dynamics in the monolayer WS 2 , with the latter being the exciton recombination lifetime. These results agree well with previous studies .…”