Well separated trion and neutral excitons on superacid treated MoS2 monolayers

TL;DR

This study demonstrates that superacid treatment of MoS2 monolayers significantly enhances photoluminescence and allows clear spectral separation of neutral and charged excitons, enabling detailed analysis of their dynamics and valley properties.

Contribution

The paper introduces a superacid treatment method that improves PL intensity and spectral resolution of excitons in MoS2 monolayers, revealing distinct exciton features and valley phenomena.

Findings

PL intensity increases by up to 60 times at room temperature

Spectral separation of neutral and charged excitons achieved

Valley polarization observed for both excitons, valley coherence only for neutral

Abstract

Developments in optoelectronics and spin-optronics based on transition metal dichalcogenide monolayers (MLs) need materials with efficient optical emission and well-defined transition energies. In as-exfoliated MoS2 MLs the photoluminescence (PL) spectra even at low temperature consists typically of broad, overlapping contributions from neutral, charged excitons (trions) and localized states. Here we show that in superacid treated MoS2 MLs the PL intensity increases by up to 60 times at room temperature. The neutral and charged exciton transitions are spectrally well separated in PL and reflectivity at T=4 K, with linewidth for the neutral exciton of 15 meV, but with similar intensities compared to the ones in as-exfoliated MLs at the same temperature. Time resolved experiments uncover picoseconds recombination dynamics analyzed separately for charged and neutral exciton emission. Using the chiral interband selection rules, we demonstrate optically induced valley polarization for both complexes and valley coherence for only the neutral exciton.