Tuning carrier concentration in a superacid treated MoS$_2$ monolayer

TL;DR

This study demonstrates that superacid treatment of MoS₂ monolayers effectively passivates defects and reduces carrier density, leading to significant changes in optical and Raman properties, including exciton and trion behavior.

Contribution

It provides a detailed analysis of how superacid treatment modulates carrier concentration and optical responses in MoS₂ monolayers, revealing the role of excitonic charge states in resonance Raman scattering.

Findings

Progressive quenching of trion emission with treatment

Redshift of neutral exciton emission after treatment

Enhanced Raman scattering near neutral exciton resonance

Abstract

The effect of bis(trifluoromethane) sulfonimide (TFSI, superacid) treatment on the optical properties of MoS$_2$ monolayers is investigated by means of photoluminescence, reflectance contrast and Raman scattering spectroscopy employed in a broad temperature range. It is shown that when applied multiple times, the treatment results in progressive quenching of the trion emission/absorption and in the redshift of the neutral exciton emission/absorption associated with both the A and B excitonic resonances. Based on this evolution, a trion complex related to the B exciton in monolayer MoS$_2$ is unambiguously identified. A defect-related emission observed at low temperatures also disappears from the spectrum as a result of the treatment. Our observations are attributed to effective passivation of defects on the MoS$_{2}$ monolayer surface. The passivation reduces the carrier density, which in turn affects the out-of-plane electric field in the sample. The observed tuning of the carrier concentration strongly influences also the Raman scattering in the MoS$_2$ monolayer. An enhancement of Raman scattering at resonant excitation in the vicinity of the A neutral exciton is clearly seen for both the out-of-plane A$_1^{'}$ and in-plane E$^{'}$ modes. On the contrary, when the excitation is in resonance with a corresponding trion, the Raman scattering features become hardly visible. These results confirm the role of the excitonic charge state plays in the resonance effect of the excitation energy on the Raman scattering in transition metal dichalcogenides.