A- and B-Exciton Photoluminescence Intensity Ratio as a Measure of Sample Quality for Transition Metal Dichalcogenide Monolayers

TL;DR

This study demonstrates that the ratio of B- to A-exciton photoluminescence intensities in monolayer TMDs correlates with defect density and sample quality, providing a simple optical metric for assessing material quality.

Contribution

It introduces the B/A exciton intensity ratio as a qualitative measure of defect-related non-radiative recombination in TMD monolayers, linking optical properties to material quality.

Findings

Lower B/A ratio indicates fewer defects and higher quality.

Higher valley polarization observed in B-exciton due to shorter lifetime.

PL variations are primarily due to defect-related non-radiative processes.

Abstract

The photoluminescence (PL) in monolayer transition metal dichalcogenides (TMDs) is dominated by recombination of electrons in the conduction band with holes in the spin-orbit split valence bands, and there are two distinct emission features referred to as the A-peak (ground state exciton) and B-peak (higher spin-orbit split state). The intensity ratio of these two features varies widely and several contradictory interpretations have been reported. We analyze the room temperature PL from MoS2, MoSe2, WS2, and WSe2 monolayers and show that these variations arise from differences in the non-radiative recombination associated with defect densities. Hence, the relative intensities of the A- and B-emission features can be used to qualitatively asses the non-radiative recombination, and thus the quality of the sample. A low B/A ratio is indicative of low defect density and high sample quality. Emission from TMD monolayers is governed by unique optical selection rules which make them promising materials for valleytronic operations. We observe a notably higher valley polarization in the B-exciton relative to the A-exciton. The high polarization is a consequence of the shorter B-exciton lifetime resulting from rapid relaxation of excitons from the B-exciton to the A-exciton of the valence band.