Spectrally narrow neutral and charged exciton emission in MoS2 monolayers : optical doping and superacid treatment

In MoSe2 and WSe2 Monolayers (MLs) photoluminescence (PL) at low temperatures normally exhibits narrow neutral exciton (X) and trion (T) lines (FWHM 10 meV). In contrast, in MoS2 MLs only a single broad peak is observed (FWHM 50 meV) which has been attributed to a possible mixture of X and T emission. This peak is accompanied by a low energy emission attributed to defect-related transitions. Here we present results on MoS2 MLs treated by an inorganic superacid (TFSI) that has been previously shown to passivate defects. In these treated MLs, well identified X and T peaks dominate the optical spectrum at T=4K. Due to the clear X-T separation, we were able to initialize neutral exciton valley coherence (superposition of valley states) with linearly polarized excitation. In addition, we show that at very low laser excitation powers, the X and T peaks can be well identified in MLs even in the absence of any acid treatment, in contrast to what is usually found in the literature. A detailed investigation of the PL as a function of excitation power reveals a non-reversible change in the PL spectrum caused by laser exposure, at power densities that so far have been considered to be non-destructive.