Curvature-Enhanced Localised Emission from Dark States in Wrinkled Monolayer WSe₂ at Room Temperature

Localised emission from defect states in monolayer transition metal dichalcogenides is of great interest for optoelectronic and quantum device applications. Recent progress towards high temperature localised emission relies on the application of strain to induce highly confined excitonic states. In this study we consider freestanding wrinkles of monolayer tungsten diselenide (WSe₂), which exhibit room temperature emission from states up to 150 meV below the main A-exciton emission of flat material. Using tip-enhanced optical spectroscopy we are able to probe this emission with nanoscale resolution revealing that it arises from the wrinkle apex and has an out-of-plane transition dipole. The Raman scattering and photoluminescence spectra show no evidence of substantial in-plane strain associated with the localised emission, this is supported by strain calculations based on simple materials engineering considerations. Our results cannot be accommodated within the dominant paradigm in published literature based on strain-localised emission and/or exciton funnelling. Rather, we propose that curvature, instead of in-plane strain, of the 1L WSe₂ offers a more appropriate means of understanding these results.<br/><br/>Whilst it has been shown elsewhere that in-plane tensile strain results in narrowing of the WSe₂ bandgap and is associated with highly localised emission, the distinction between in-plane strain and bending strain has not been adequately explored. The case of free-standing wrinkles is an instructive example since such buckling delaminations form under compressive strain, where the out-of-plane deflection acts to minimise the in-plane strain. This is supported by a simple evaluation of the local strain based on nanoscale surface topography measurements. Our tip-enhanced spectroscopy instrument uses side-illumination to probe the sample using out-of-plane optical polarisation so to explain our observations we propose the existence of a manifold of spin-forbidden excitonic states that we are able to access as a result of local symmetry breaking due to geometric curvature.<br/><br/>The results of this study suggest curvature engineering of 1L WSe₂ as an effective route towards localised emission at high temperature, and an alternative strategy to the pursuit of increasing in-plane stretching strain to confine excitons. The spectral features we report at room temperature show energetic confinement comparable with reports of single photon emission in 1L WSe₂ so photon antibunching experiments are an interesting area for future study. It is also significant that the emission we report here originates from optical coupling to spin-forbidden dark states, which are typically long-lived and therefore of potential interest for quantum information technologies. The out-of-plane transition dipoles for this emission are also relevant for many proposed applications of TMDs for plasmonic circuits and on-chip photonic devices, which require the coupling of light into ‘horizontal’ waveguides. This is in contrast to the typically-observed free bright excitons in TMDs, which have in-plane dipoles that efficiently couple light in/out of thin film devices structures in a ‘vertical’ direction. Furthermore, the localised emission we observe exhibits semi-continuous spatial variation in the spectral positions of the peaks so we speculate that it may be possible to tune the emission through the local curvature of the 1L WSe₂.