Enhanced Raman and photoluminescence response in monolayer MoS$_2$ due to laser healing of defects

TL;DR

This study demonstrates that brief laser exposure in air can enhance photoluminescence and Raman signals in monolayer MoS$_2$ by healing defects, reducing non-radiative processes, and altering lattice dynamics.

Contribution

It introduces a laser healing technique that simultaneously boosts photoluminescence and Raman responses in monolayer MoS$_2$, revealing defect reduction as a key factor.

Findings

Photoluminescence from trions and excitons increases with laser exposure.

Raman intensities of first and second order modes increase 3-5 times.

Lattice modes show shifts indicating changes in electron concentration and lattice structure.

Abstract

Bound quasiparticles, negatively charged trions and neutral excitons, are associated with the direct optical transitions at the K-points of the Brillouin zone for monolayer MoS$_2$. The change in the carrier concentration, surrounding dielectric constant and defect concentration can modulate the photoluminescence and Raman spectra. Here we show that exposing the monolayer MoS$_2$ in air to a modest laser intensity for a brief period of time enhances simultaneously the photoluminescence (PL) intensity associated with both trions and excitons, together with $\sim$ 3 to 5 times increase of the Raman intensity of first and second order modes. The simultaneous increase of PL from trions and excitons cannot be understood based only on known-scenario of depletion of electron concentration in MoS$_2$ by adsorption of O$_2$ and H$_2$O molecules. This is explained by laser induced healing of defect states resulting in reduction of non-radiative Auger processes. This laser healing is corroborated by an observed increase of intensity of both the first order and second order 2LA(M) Raman modes by a factor of $\sim$ 3 to 5. The A$_{1g}$ mode hardens by $\sim$ 1.4 cm$^{-1}$ whereas the E$^1_{2g}$ mode softens by $\sim$ 1 cm$^{-1}$. The second order 2LA(M) Raman mode at $\sim$ 440 cm$^{-1}$ shows an increase in wavenumber by $\sim$ 8 cm$^{-1}$ with laser exposure. These changes are a combined effect of change in electron concentrations and oxygen-induced lattice displacements.