A- and B-exciton photoluminescence intensity ratio as a measure of sample quality for transition metal dichalcogenide monolayers

Kathleen M Mccreary,Saujan V Sivaram,Aubrey T Hanbicki,Berend T Jonker

doi:10.1063/1.5053699

Kathleen M Mccreary, Saujan V Sivaram + Show 2 more

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https://doi.org/10.1063/1.5053699

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Journal: APL Materials	Publication Date: Nov 1, 2018
Citations: 106	License type: cc-by

Affiliation: United States Naval Research Laboratory

Abstract

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). The intensity ratio of these two features varies widely, and several contradictory interpretations have been reported. In this work, we analyze the room temperature PL from MoS2, MoSe2, WS2, and WSe2 monolayers and identify the underlying cause of observed variations in emission profile. We determine that PL variations arise from differences in the non-radiative recombination associated with defect densities. 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 also performed polarization-resolved PL measurements. 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. Our work clarifies disparities reported in the literature relating to the emission profile and provides a straightforward means to assess sample quality.

Highlights

A- and B-exciton photoluminescence intensity ratio as a measure of sample quality for transition metal dichalcogenide monolayers
As a case in point: in monolayer MoS2, some studies report only a single emission feature associated with optical transitions between the highest valence band and the conduction band at the K-points, often referred to as the ground state A-exciton emission.[3,5]
Others observe a strong A-emission feature accompanied by a weaker second emission peak at 100-200 meV higher energy.[6,7,8]

Summary

Introduction

A- and B-exciton photoluminescence intensity ratio as a measure of sample quality for transition metal dichalcogenide monolayers. Careful inspection reveals a small PL emission peak identified as the B-exciton ∼190 meV above the A-exciton [Fig. 3(f)], consistent with the expected valence band splitting and previous observations.[24] Behavior in the selenium-based material is nearly identical to that observed in MoS2 monolayers, with all samples exhibiting a non-zero B-peak emission, a linear relationship between the A- and B-peak intensities with a non-zero intercept [Fig. 3(g)], and a variety of B/A ratios [Fig. 3(h)].

Results

Conclusion