Exciton transport is a fundamental process of energy conversion in semiconductors. The strong interaction of light with two-dimensional (2D) semiconductors provides the opportunity to optimize exciton transport for 2D optoelectronic devices. However efficient exciton transport in atomically thin 2D semiconductors is still challenging due to various scattering sources such as defects and traps. Here we show that photoinduced processes can achieve a giant enhancement in exciton diffusivity (from 1.5 to 22.5 cm2/s in monolayer MoS2 crystals)[1] and can be revealed by in situ optical spectroscopy monitoring. The mechanism of the enhancement is revealed: the scattering of excitons is screened by trapped charges generated by a photoinduced electron-hole plasma. This understanding of how to improve and control exciton transport in 2D semiconductors opens new avenues for the development of high-performance excitonic and photovoltaic devices. [1] Y. Yu et al., Sci. Adv., 2020, eabb4823.
Single layer transition metal dichalcogenides are 2D semiconducting systems with unique electronic band
structure. Two-valley energy bands along with strong spin-orbital coupling lead to valley-dependent carrier spin
polarization, which is the basis for recently proposed valleytronic applications. These systems also exhibit
unusually strong many body effects, such as strong exciton and trion binding, due to reduced dielectric
screening of Coulomb interactions. Not much is known about the impact of strong many particle correlations on
spin and valley polarization dynamics. Here we report direct measurements of ultrafast valley specific relaxation
dynamics in single layer MoS2 and WS2. We found that excitonic many body interactions significantly
contribute to the relaxation process. Biexciton formation reveals hole valley/spin relaxation time in MoS2. Our
results suggest that initial fast intervalley electron scattering and electron spin relaxation leads to loss of valley
polarization for holes through an electron-hole spin exchange mechanism in both MoS2 and WS2.
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