Confined Excitons at Monolayer Black Phosphorus Edges

Atomically sharp or abrupt junctions provide a novel platform to study symmetry-broken physical phenomenon and modification of the local electronic structure at the interface – often resulting in localized or trapped state formation. We report emergence of quantum confined excitons in the emission spectrum in the edges of monolayer black phosphorus (BP). While the quasi-1D exciton in the interior of monolayer BP exhibits a single Lorentzian-like photoluminescence (~40 meV linewidth) at low temperatures (~5K), we observe additional red-shifted spectrally narrow (~1-5 meV) emission lines at the edges of monolayer BP flakes. We interpret the origin of such localized features to be due to a confining potential located at the physical edges of BP – supported by first principles GW-BSE calculations of BP nanoribbons. We observe the photoluminescence to be highly linearly polarized, with significant temperature and pump fluence dependence. Furthermore, we find that the emission is highly tunable with electrostatic doping and can be turned on and off, on demand – due to an interplay of doping and a linear perturbative Stark-effect. Our results help facilitate the understanding of fundamental optical excitations in monolayer BP under local structural modification and may enable further engineering of photoluminescence in BP nanostructures such as quantum dots or nanowires for bright, polarized, and reconfigurable light emitters.