Understanding Variations in Circularly Polarized Photoluminescence in Monolayer Transition Metal Dichalcogenides

Abstract

Monolayer transition metal dichalcogenides are promising materials for valleytronic operations. They exhibit two inequivalent valleys in the Brillouin zone, and the valley populations can be directly controlled and determined using circularly polarized optical excitation and emission. The photoluminescence polarization reflects the ratio of the two valley populations. A wide range of values for the degree of circularly polarized emission, Pcirc, has been reported for monolayer WS2, although the reasons for the disparity are unclear. Here we optically populate one valley, and measure Pcirc to explore the valley population dynamics at room temperature in a large number of monolayer WS2 samples synthesized via chemical vapor deposition. Under resonant excitation, Pcirc ranges from 2% to 32%, and we observe a pronounced inverse relationship between photoluminescence (PL) intensity and Pcirc. High quality samples exhibiting strong PL and long exciton relaxation time exhibit a low degree of valley polarization, and vice versa. This behavior is also demonstrated in monolayer WSe2 samples and transferred WS2, indicating that this correlation may be more generally observed and account for the wide variations reported for Pcirc. Time resolved PL provides insight into the role of radiative and non-radiative contributions to the observed polarization. Short non-radiative lifetimes result in a higher measured polarization by limiting opportunity for depolarizing scattering events.

References in corpus (8)

• Valley polarization in MoS2 monolayers by optical pumping
• k.p theory for two-dimensional transition metal dichalcogenide semiconductors
• Robust optical emission polarization in MoS2 monolayers through selective valley excitation
• Vapor-Solid Growth of High Optical Quality MoS2 Monolayers With Near-Unity Valley Polarization
• Exciton Valley Dynamics probed by Kerr Rotation in WSe2 Monolayers
• The Effect of Preparation Conditions on Raman and Photoluminescence of Monolayer WS2
• Temperature Dependent Valley Relaxation Dynamics in Single Layer WS2 Measured Using Ultrafast Spectroscopy
• Spatially Resolved Electronic Properties of Single-Layer WS${}_2$ on Transition Metal Oxides

Cited by in corpus (5)

• Effects of CVD Growth Parameters on Global and Local Optical Properties of MoS${}_2$ Monolayers
• Effectuating tunable valley selection via multi-terminal monolayer graphene devices
• Circular dichroism induction in WS2 by a chiral plasmonic metasurface
• Laser-patterned submicron Bi2Se3-WS2 pixels with tunable circular polarization at room temperature
• Symmetric Domain Segmentation in WS2 Flakes: Correlating spatially resolved photoluminescence, conductance with valley polarization