![]() The Raman spectra of a representative MoS 2 sample display the in-plane E 1 2g mode at 384 cm −1 and out-of-plane A 1g mode at 404 cm −1, consistent with monolayer MoS 2. Raman, PL, and reflectance measurements are acquired using a commercial system equipped with a 50× objective so that the area analyzed is approximately a 2 µm diameter circle. The as-grown TMDs are characterized at room temperature under ambient conditions. This additional relaxation pathway shortens the effective lifetime for B-excitons, subsequently reducing the opportunity for intervalley scattering and enhancing the degree of valley polarization. The high polarization is a consequence of the shorter B-exciton lifetime resulting from rapid relaxation of excitons from the higher energy spin-orbit split state (B-exciton) to the ground state (A-exciton) of the valence band. By comparing the degree of valley polarization from A- and B-excitons for a given sample, we find a notably higher valley polarization in the B-exciton. This relationship between PL profile and exciton dynamics provides a facile method to assess sample quality: a low B/A ratio indicates low defect density and high sample quality, whereas a large B/A ratio signals a high defect density and poor-quality material. We show that these observed variations arise from differences in the non-radiative recombination associated with the defect density in a given sample. We find that both A- and B-emission intensities can vary widely from sample-to-sample, consistent with other reports, leading to a variety of emission profiles as well as B/A intensity ratios. Here we address the discrepancies in interpretation of these PL emission features by analyzing a large number of different monolayer TMDs (MoS 2, MoSe 2, WS 2, and WSe 2) to better understand the conditions responsible for various emission characteristics and valley polarizations. Our work clarifies disparities reported in the literature relating to the emission profile and provides a straightforward means to assess sample quality. 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. We observe a notably higher valley polarization in the B-exciton relative to the A-exciton. Emission from TMD monolayers is governed by unique optical selection rules which make them promising materials for valleytronic operations. We also performed polarization-resolved PL measurements. Therefore, the relative intensities of the A- and B-emission features can be used to qualitatively assess the non-radiative recombination and a low B/A ratio is indicative of low defect density and high sample quality. We determine that PL variations arise from differences in the non-radiative recombination associated with defect densities. In this work, we analyze the room temperature PL from MoS 2, MoSe 2, WS 2, and WSe 2 monolayers and identify the underlying cause of observed variations in emission profile. The intensity ratio of these two features varies widely, and several contradictory interpretations have been reported. ![]() ![]() The photoluminescence (PL) in monolayer transition metal dichalcogenides (TMDs) is dominated by the 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).
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