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A- and B-exciton photoluminescence intensity ratio as a measure of sample quality for transition metal dichalcogenide monolayers
摘要： 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.
关键词： transition metal dichalcogenides,valley polarization,A-exciton,sample quality,B-exciton,monolayers,photoluminescence
更新于2025-09-09 09:28:46
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Assessing UO2 sample quality with μ-Raman spectroscopy
摘要： m-Raman spectroscopy of UO2 has demonstrated its value as a tool in nuclear forensics and for analyzing nuclear fuels, but its ability to evaluate UO2 sample quality has been relatively unexplored. To investigate this potential, a three-pronged study that uses Raman spectroscopy to analyze 1) three distinct qualities of UO2 single crystals with two different excitation lasers, 2) a high quality single crystal UO2 sample that is progressively damaged with a laser, and 3) a single crystal of UO2 grown on top of a ThO2 substrate was undertaken. The results demonstrate that the peak height ratio between the second order longitudinal optical phonon signal near 1145 cm?1 and the T2g peak located near 450 cm?1 is directly correlated with crystal quality when a 532 nm excitation source is used. Furthermore, this observed pattern relies upon the two most intense peaks that are present in the Raman spectra, making it more robust to lower signal-to-noise ratios than similar trends present in the peak height ratio between the first order longitudinal optical phonon signal near 565 cm?1 and the T2g peak. Both peak height ratios are more sensitive indicators of the crystal quality than the width ratios of the same signals or the associated wavenumber shift in the T2g peak location. The correlation between the 2LO and T2g peak heights and the overall crystal quality offers the potential to directly assess and compare the quality of different samples of single crystal UO2.
关键词： Uranium dioxide,Sample quality,m-Raman spectroscopy,UO2,Single crystal
更新于2025-09-09 09:28:46