Surface Recombination Limited Lifetimes of Photoexcited Carriers in Few-Layer Transition Metal Dichalcogenide MoS₂

Abstract

We present results on photoexcited carrier lifetimes in few-layer transition metal dichalcogenide MoS 2 using nondegenerate ultrafast optical pump-probe technique. Our results show a sharp increase of the carrier lifetimes with the number of layers in the sample. Carrier lifetimes increase from few tens of picoseconds in monolayer samples to more than a nanosecond in 10-layer samples. The inverse carrier lifetime was found to scale according to the probability of the carriers being present at the surface layers, as given by the carrier wavefunction in few layer samples, which can be treated as quantum wells. The carrier lifetimes were found to be largely independent of the temperature and the inverse carrier lifetimes scaled linearly with the photoexcited carrier density. These observations are consistent with defect-

Accelerated carrier recombination by grain boundary/edge defects in MBE grown transition metal dichalcogenides

“…The MBE sample has a thickness of 5 nm (7 atomic layers). It has been reported that multilayer MoS 2 samples with a thickness larger than 6 atomic layers have carrier lifetimes close to that of the bulk MoS 2 sample, 21 which suggests that the difference in structural defects but not the thickness should be responsible for the observed lifetime difference here. Generally, chalcogen vacancy is the most abundant defect species in TMD materials.…”

Experimental evidence of exciton capture by mid-gap defects in CVD grown monolayer MoSe2

“…When the probe photon energy is resonant with or smaller than the exciton state, the pump-induced absorption change of the probe (change in the imaginary part) dominates the optical response. 17,18,27 Using the reflectivity formula for the TMDs/SiO 2 /Si structure (see Eq. (1) in supplementary information), the change of reflectivity ΔR MoSe2 =R 0 due to small perturbations of the imaginary part of refractive index Δκ=κ ð Þ MoSe2 has been calculated (Inset of Fig.…”

“…Recently, a number of ultrafast experiments [17][18][19][20][21][22][23] and a theoretical study 24 have been done to investigate the exciton/carrier dynamics in two-dimensional TMDs, suggesting significant influences of defects on the exciton/carrier recombination dynamics. In these previous experiments, [17][18][19][20][21][22][23][24] exciton/carrier dynamics were detected by resonant probe or terahertz probe, and the interactions between exciton/carrier and defects were indirectly inferred from exciton/carrier population decay. One general observation of the previous studies is that the relaxation of photoexcited carriers/excitons shows a fast, few picoseconds decay followed by a slow several tens of picoseconds component.…”

Dark excitons and the elusive valley polarization in transition metal dichalcogenides

“…In writing the rate equations we have assumed identical recombination times for the dark and bright excitons which is reasonable in TMDs due to the strong non radiative recombination [49,50] [47,48,51] and τ r is in the range of picoseconds to hundreds of ps [23,24,49,50,[52][53][54][55][56]) and the dark exciton is the ground state (in WX 2 TMDs) it provides an important reservoir for valley polarization which tries to maintain the bright exciton population in the same valley (i.e. tries to maintain a Boltzmann distribution).…”