Observation of Room-Temperature Dark Exciton Emission in Nanopatch-Decorated Monolayer WSe2 on Metal Substrate

The presence of strong spin-orbit coupling in the valence band and weak spin-splitting in the conduction band result in the lowest energy exciton in WX2 (X = S, Se) being spin forbidden and optically dark. Because of their long lifetimes, dark excitons are highly attractive for quantum optics and optoelectronic applications. To date, studying dark excitonic emissions is limited to cryogenic temperatures or requires very complex experimental configurations to observe them at room temperature (RT). Here, the radiative decay of dark exciton related emission in WSe2 monolayers is studied using both conventional and tip-enhanced photoluminescence (TEPL) at RT. Monolayer WSe2 flakes are sandwiched between noble metal substrates and polydimethylsiloxane nanopatches providing a strong local electrostatic out-of-plane dipole moment with respect to the 2D plane resulting in the observation of dark excitonic emission at RT. The spatial distribution of this dark exciton related emission is studied by TEPL with a spatial resolution of <10 nm confirming the confinement of these excitons within the polydimethylsiloxane nanopatches. The tip-enhanced Raman scattering (TERS) investigation excludes any local strain induced effects and reveals a direct correlation between dark excitons and defects in WSe2. Finally, removal of the nanopatches led to the recovery of bright excitonic emission in WSe2.