The unique electronic band structure in monolayer transition metal dichalcogenides (TMDs) provides the ability to selectively populate a desired valley by exciting with circularly polarized light. The valley population is reflected through the circular polarization of photoluminescence (PL) and a high degree of circular polarization has been theoretically predicted for resonant excitation of TMDs such as MoS2, MoSe2, WS2 and WSe2, yet rarely observed experimentally. In fact, a wide range of values for the degree of circularly polarized emission, Pcirc, has been reported for monolayer TMDs, although the reasons for the disparity are unclear. Here we investigate the room-temperature Pcirc in TMD monolayers synthesized via chemical vapor deposition. In each material system, a wide range of Pcirc and PL intensity values are observed. However, there is a pronounced inverse correlation between Pcirc and PL intensity, which we attribute to sample-dependent variations in the exciton radiative and non-radiative lifetime components. Samples that demonstrate weak PL emission and short exciton relaxation time exhibit a high degree of valley polarization. The short exciton lifetime results in a higher measured polarization by limiting opportunity for depolarizing scattering events. These findings clarify the disparities among previously reported values and suggest a means to engineer valley polarization via controlled introduction of defects and non-radiative recombination sites.

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Kathleen McCreary, Aubrey Hanbicki, Marc Currie, Hsun-Jen Chuang, and Berend Jonker, "Understanding variations in circularly polarized photoluminescence in monolayer transition metal dichalcogenides (Conference Presentation)," Proc. SPIE 10732, Spintronics XI, 1073228 (Presented at SPIE Nanoscience + Engineering: August 22, 2018; Published: 18 September 2018); https://doi.org/10.1117/12.2320733.5836443473001.