  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.
