Paper 2D+MI+NS+SS+TF-FrM3
Understanding Variations in Circularly Polarized Photoluminescence in Monolayer Transition Metal Dichalcogenides

Friday, November 3, 2017, 9:00 am, Room 15

The unique electronic band structure in monolayer transition metal dichalcogenides (TMDs) provides the ability to selectively populate a desired conduction band valley by exciting with circularly polarized light. The subsequent valley population can be interrogated by measuring helicity-resolved photoluminescence (PL). A high degree of circular polarization has been theoretically predicted for resonant excitation of TMDs, yet rarely observed experimentally. In fact, a wide range of values for the degree of circularly polarized emission (P_circ), has been reported for monolayer TMDs, although the reasons for the disparity are unclear. Here we investigate the room-temperature P_circ in several TMD monolayers synthesized via chemical vapor deposition. The samples include as-grown WS₂, as-grown WSe₂, and WS₂ monolayers that have been transferred to a fresh substrate. In each system, a wide range of P_circ and PL intensity values are observed. There is a pronounced inverse correlation between P_circ and PL intensity: samples that demonstrate weak PL emission and short exciton relaxation time exhibit a high degree of valley polarization. We attribute these effects to sample-dependent variations in the exciton radiative and non-radiative lifetime components. 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.