Purcell enhanced nano-spectroscopy of strain-localized dark exciton and defect states in WSe₂

With broken spin- and momentum- degeneracy, WSe₂ hosts a range of bright, dark, charged, and defect-localized excitons with a complex system of scattering and relaxation dynamics. Tip-enhanced photoluminescence (TEPL) has previously demonstrated up to 2000x enhancement of long-lived spin-forbidden dark excitons and defect-bound states localized to strain. Yet multiple conflicting models have been proposed to explain the interaction of dark and defect-localized excitons with strain and the room-temperature dynamics remain unclear due to overlapping PL peaks. Here we use a multiparameter approach to separate distinct exciton responses and their dependence on strain through power dependence, hot electron injection, and tip-induced Stark effects in Purcell-enhanced nano-spectroscopy (PENS). We find dark exciton emission correlated with additional low-energy peaks and localized to the edges of nanobubbles, while confirming the defect-localized nature of the low-energy peaks through their saturation and out-of-plane dipole moments of μ≳0.4 Debye. Through a comparison of existing strain models, we identify regions of high curvature and strain near the edges of nanobubbles as the cause of the localization. Our work is a vital step towards control of dark excitons as long-lived states for valley- and strain-tronics along with defect localized single photon sources for 2D material photonics.