Here, we show that deep trapped “dark” exciton states are responsible for the surprisingly long lifetime of band-edge photoluminescence in acid-treated single-layer ${\mathrm{MoS}}_2$. Temperature-dependent transient photoluminescence spectroscopy reveals an exponential tail of long-lived states extending hundreds of meV into the band gap. These subband states, which are characterized by a 4 $\mu \mathrm{s}$ radiative lifetime, quickly capture and store photogenerated excitons before subsequent thermalization up to the band edge where fast radiative recombination occurs. By intentionally saturating these trap states, we are able to measure the “true” 150 ps radiative lifetime of the band-edge exciton at 77 K, which extrapolates to $\sim 600$ ps at room temperature. These experiments reveal the dominant role of dark exciton states in acid-treated ${\mathrm{MoS}}_2$, and suggest that excitons spend $>95\%$ of their lifetime at room temperature in trap states below the band edge. We hypothesize that these states are associated with native structural defects, which are not introduced by the superacid treatment; rather, the superacid treatment dramatically reduces nonradiative recombination through these states, extending the exciton lifetime and increasing the likelihood of eventual radiative recombination.

• Received 17 July 2017
• Revised 29 August 2017

DOI:https://doi.org/10.1103/PhysRevB.96.121404