Stone-Wales defects in hexagonal boron nitride as ultraviolet emitters

Many quantum emitters have been measured close or near the grain boundaries of the two-dimensional hexagonal boron nitride where various Stone-Wales defects appear. We show by means of first principles density functional theory calculations that the pentagon-heptagon Stone-Wales defect is an ultraviolet emitter and its optical properties closely follow the characteristics of a 4.08-eV quantum emitter, often observed in polycrystalline hexagonal boron nitride. We also show that the square-octagon Stone-Wales line defects are optically active in the ultraviolet region with varying gaps depending on their density in hexagonal boron nitride. Our results may introduce a paradigm shift in the identification of fluorescent centres in this material.