  The effect of bis(trifluoromethane) sulfonimide (TFSI, superacid) treatment
on the optical properties of MoS$_2$ monolayers is investigated by means of
photoluminescence, reflectance contrast and Raman scattering spectroscopy
employed in a broad temperature range. It is shown that when applied multiple
times, the treatment results in progressive quenching of the trion
emission/absorption and in the redshift of the neutral exciton
emission/absorption associated with both the A and B excitonic resonances.
Based on this evolution, a trion complex related to the B exciton in monolayer
MoS$_2$ is unambiguously identified. A defect-related emission observed at low
temperatures also disappears from the spectrum as a result of the treatment.
Our observations are attributed to effective passivation of defects on the
MoS$_{2}$ monolayer surface. The passivation reduces the carrier density, which
in turn affects the out-of-plane electric field in the sample. The observed
tuning of the carrier concentration strongly influences also the Raman
scattering in the MoS$_2$ monolayer. An enhancement of Raman scattering at
resonant excitation in the vicinity of the A neutral exciton is clearly seen
for both the out-of-plane A$_1^{'}$ and in-plane E$^{'}$ modes. On the
contrary, when the excitation is in resonance with a corresponding trion, the
Raman scattering features become hardly visible. These results confirm the role
of the excitonic charge state plays in the resonance effect of the excitation
energy on the Raman scattering in transition metal dichalcogenides.
