Probing of free and localized excitons and trions in atomically thin WSe, WS, MoSe and MoS in photoluminescence and reflectivity experiments.

We report on detailed temperature dependent (T = 7-295 K) optical spectroscopy studies of WSe, WS, MoSe and MoS monolayers exfoliated onto the same SiO/Si substrate. In the high energy region of absorption type (reflectivity contrast-RC) and emission (photo-luminescence-PL) spectra of all the monolayers resonances related to the neutral and charged excitons (X and T) are detected in the entire measured temperature range. The optical amplitudes of excitons and trions strongly depend on the temperature and two dimensional carrier gas (2DCG) concentration. In the low energy PL spectra of WSe and WS we detect a group of lines (L) which dominates the spectra at low temperatures but rapidly quenches with the increase in the temperature. Interestingly, in the same energy range of the RC spectra recorded for WS, we observe an additional line (L ), which behaves in the same way as the L lines in the PL spectra. The optical amplitude of L and T resonances in the RC spectra strongly increases with the growth of the 2DCG concentration. On the base of these observations we identify the L resonance in the RC spectra as arising from the fine structure of the trion. We also propose that the line interpreted previously in PL spectra of WSe and WS as related to the biexciton emission is a superposition of the biexciton, trion and localized exciton emission. We find that with the temperature increase from 7-295 K the total PL intensity decreases moderately in WSe and WS, strongly in MoS and dramatically in MoSe.