Exciton Dynamics at Reconstructed Edges in Monolayer Black Phosphorus

Atomically sharp edges of monolayer van der Waals materials form abrupt junctions that provide a novel platform to study symmetry-broken physical phenomenon and modification of the local electronic structure. Here, we report strong modification of the excitonic emission spectrum at the edges of monolayer black phosphorus (BP) samples. We find that while the quasi-1D exciton in the interior of monolayer BP exhibits a single Lorentzian-like photoluminescence (~40meV linewidth) at low temperatures (~5K), we observe additional fine-structure emission lines with very sharp linewidths of approximately 1-5 meV at the edges of monolayer BP flakes. We interpret the origin of such emission to be atomic reconstruction of the lattice at the edges of BP which modifies the quantum confinement. We observe the photoluminescence to be highly linearly polarized, with significant temperature and pump fluence dependence. Furthermore, we find that the fine-structure emission lines are highly tunable with electrostatic doping and can be turned on and off, on demand. 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.