Single photon emission from point defects in hexagonal boron nitride nanosheets enabled via ambient annealing

Single photon emitters (SPEs) in two-dimensional van der Waals crystals are essential for developing quantum technologies due to their ready integration into photonic circuits and high photon extraction efficiency. Hexagonal boron nitride (h-BN) exhibits an ultra-wide bandgap that can host multiple defect states emitting stable single photons with high brightness at room-temperature. The fabrication and regulation of the defects that determine the spin and optoelectronic physics of h-BN are thus important. Herein, we demonstrate the composite defects modulation in h-BN nanosheets by thermal annealing treatment in air that can generate stable room-temperature SPEs with high photon purity and brightness. Strong and sharp zero-phonon lines appear at ∼386 nm (3.21 eV) and ∼573 nm (2.16 eV) after annealing. The ultraviolet light emission is induced by the formation of a boroxyl ring in h-BN commensurate with the optical transition of nitrogen vacancies, which is characterized by the spectral analysis combined with first-principle calculations. The thermal annealing suppresses the fluorescence background, leading to the population of anti-site nitrogen vacancy complex defects, achieving visible single photon emissions. The results of our work provide a practical post-synthesis process for engineering ensembles of emitters in h-BN for their future integration in quantum photonics.