Localized Gradual Photomediated Brightness and Lifetime Increase of Superacid Treated Monolayer MoS_2

Monolayer semiconducting transition metal dichalcogenides (S-TMDs) have been extensively studied as materials for next-generation optoelectronic devices due to their direct band gap and high exciton binding energy at room temperature. Under a superacid treatment of bis(trifluoromethane)sulfonimide (TFSI), sulfur-based TMDs such as MoS_2 can emit strong photoluminescence (PL) with photoluminescence quantum yield (PLQY) approaching unity. However, the magnitude of PL enhancement varies by more than two orders of magnitude in published reports. A major culprit behind the discrepancy is sulfur-based TMD's sensitivity to above band-gap photostimulation. Here, we present a detailed study of how TFSI-treated MoS_2 reacts to photostimulation with increasing PL occurring hours after constant or pulsed laser exposure. The PL of TFSI-treated MoS_2 is enhanced up to 74 times its initial intensity after 5 hours of continuous exposure to 532nm laser light. Photostimulation also enhances the PL of untreated MoS_2 but with a much smaller enhancement. Caution should be taken when probing MoS_2 PL spectra as above-bandgap light can alter the resulting intensity and peak wavelength of the emission over time. The presence of air is verified to play a key role in the photostimulated enhancement effect. Additionally, the rise of PL intensity is mirrored by an increase in measured carrier lifetime of up to  400ps consistent with the suppression of non-radiative pathways. This work demonstrates why variations in PL intensity are observed across samples and provides an understanding of the changes in carrier lifetimes to better engineer next-generation optoelectronic devices.