Charge Trapping and Exciton Dynamics in Large-Area CVD Grown MoS2

There is keen interest in monolayer transition metal dichalcogenide films for a variety of optoelectronic applications due to their direct band gap and fast carrier dynamics. However, the mechanisms dominating their carrier dynamics are poorly understood. By combining time-resolved terahertz (THz) spectroscopy and transient absorption, we are able to shed light on the optoelectronic properties of large area CVD grown mono- and multilayer MoS2 films and determine the origins of the characteristic two-component excited state dynamics. The photoinduced conductivity shows that charge carriers, and not excitons, are responsible for the subpicosecond dynamics. Identical dynamics resulting from sub-optical gap excitation suggest that charge carriers are rapidly trapped by midgap states within 600 fs. This process complicates the excited state spectrum with rapid changes in line-width broadening in addition to a red-shift due to band gap renormalization and simple state-filling effects. These dynamics are insensitive to film thickness, temperature, or choice of substrate, which suggests that carrier trapping occurs at surface defects or grain boundaries. The slow dynamics are associated with exciton recombination and lengthen from 50 ps for monolayer films to 150 ps for multilayer films indicating that surface recombination dominates their lifetime. We see no signatures of trions in these MoS2 films. Our results imply that CVD grown films of MoS2 hold potential for high-speed optoelectronics and provide an explanation for the absence of trions in some CVD grown MoS2 films.