Slow Cooling of High-Energy C Excitons Is Limited by Intervalley-Transfer in Monolayer MoS2

High-energy C exciton in 2D transition metal dichalcogenides with strong photon absorption can be utilized prospectively in light-harvesting and opto-electric devices. Here, a detailed study on C exciton dynamics in monolayer MoS2 is presented by femtosecond transient absorption (TA) spectroscopy. In the experiment, the C exciton with unique parallel band structure exhibits a slow process within tens of picoseconds (approximate to 10-50 ps) as well as a fast process within several picoseconds (approximate to 1-6 ps) instead of previous reported ultrafast cooling (<500 fs) process. From TA spectroscopy, the experimental results confirmed that the relatively slow cooling of C exciton is mainly limited by the rates of intervalley transfer rather than Pauli blocking effect from band-edge excitons. The timescale of intervalley transfer is longer than that of intraband relaxation of C exciton, therefore, leading to the slow cooling of C exciton. Moreover, intervalley transfer time are estimated with two processes of 5.1 +/- 0.6 ps and 69.5 +/- 8 ps when the pump fluence is approximate to 127 mu J cm(-2). This work provides further understanding of the ultrafast dynamics of C exciton in monolayer MoS2 and opens new opportunities for opto-electric related applications.