Electrically Tunable and Dramatically Enhanced Valley-Polarized Emission of Monolayer WS 2 at Room Temperature with Plasmonic Archimedes Spiral Nanostructures.

Monolayer transition metal dichalcogenides (TMDs) have intrinsic valley degrees of freedom, making them appealing for exploiting valleytronic applications in information storage and processing. WS ₂ monolayer possesses two inequivalent valleys in the Brillouin zone, each valley coupling selectively with a circular polarization of light. The degree of valley polarization (DVP) under the excitation of circularly polarized light (CPL) is a parameter that determines the purity of valley polarized photoluminescence (PL) of monolayer WS ₂ . Here efficient tailoring of valley-polarized PL from monolayer WS ₂ at room temperature (RT) through surface plasmon-exciton interactions with plasmonic Archimedes spiral (PAS) nanostructures is reported. The DVP of WS ₂ at RT can be enhanced from <5% to 40% and 50% by using 2 turns (2T) and 4 turns (4T) of PAS, respectively. Further enhancement and control of excitonic valley polarization is demonstrated by electrostatically doping monolayer WS ₂ . For CPL on WS ₂ -2TPAS heterostructures, the 40% valley polarization is enhanced to 70% by modulating the carrier doping via a backgate, which may be attributed to the screening of momentum-dependent long-range electron-hole exchange interactions. The manifestation of electrically tunable valley-polarized emission from WS ₂ -PAS heterostructures presents a new strategy toward harnessing valley excitons for application in ultrathin valleytronic devices.
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