Phonon induced line broadening and population of the dark exciton in a deeply trapped localized emitter in monolayer WSe₂.

We study trapped single excitons in a monolayer semiconductor with respect to their temperature stability, spectral diffusion and decay dynamics. In a mechanically exfoliated WSe₂ sheet, we could identify discrete emission features with emission energies down to 1.516 eV which are spectrally isolated in a free spectral range up to 80 meV. The strong spectral isolation of our localized emitter allow us to identify strong signatures of phonon induced spectral broadening for elevated temperatures accompanied by temperature induced luminescence quenching. A direct correlation between the droop in intensity at higher temperatures with the phonon induced population of dark states in WSe₂ is established. While our experiment suggests that the applicability of monolayered quantum emitters as coherent single photon sources at elevated temperatures may be limited, the capability to operate them below the GaAs band-edge makes them highly interesting for GaAs-monolayer hybrid quantum photonic structures.