Ultra-low power threshold for laser induced changes in optical properties of 2D Molybdenum dichalcogenides

TL;DR

This study reveals that 2D molybdenum dichalcogenides undergo irreversible optical property changes even at low laser excitation powers, with effects varying based on substrate and material type, challenging assumptions of non-intrusiveness.

Contribution

It demonstrates that low-power laser excitation causes irreversible doping and optical changes in 2D MoSe2 and MoS2, highlighting substrate-dependent effects and the need for careful laser use in experiments.

Findings

Laser treatment increases trion emission in MoSe2.

Laser exposure can cause irreversible PL peak shifts in MoS2.

Substrate influences laser-induced doping efficiency.

Abstract

The optical response of traditional semiconductors depends on the laser excitation power used in experiments. For two-dimensional (2D) semiconductors, laser excitation effects are anticipated to be vastly different due to complexity added by their ultimate thinness, high surface to volume ratio, and laser-membrane interaction effects. We show in this article that under laser excitation the optical properties of 2D materials undergo irreversible changes. Most surprisingly these effects take place even at low steady state excitation, which is commonly thought to be non-intrusive. In low temperature photoluminescence (PL) we show for monolayer (ML) MoSe2 samples grown by different techniques that laser treatment increases significantly the trion (i.e. charged exciton) contribution to the emission compared to the neutral exciton emission. Comparison between samples exfoliated onto different substrates shows that laser induced doping is more efficient for ML MoSe2 on SiO2/Si compared to h-BN and gold. For ML MoS2 we show that exposure to laser radiation with an average power in the $\mu$W/$\mu$m$^2$ range does not just increase the trion-to-exciton PL emission ratio, but may result in the irreversible disappearance of the neutral exciton PL emission and a shift of the main PL peak to lower energy.