Theory of ultraviolet defect emitters in hexagonal boron nitride

In this paper, we summarize recent theoretical and experimental progress in determining the microscopic nature of the sharp ultraviolet emission in hexagonal boron nitride. Initially associated with intrinsic point defects or bound excitons, this UV feature is now widely attributed to a carbon dimer defect, as supported by hybrid density functional–many-body perturbation theory calculations. The carbon dimer model accurately reproduces the ZPL energy, emission lifetime, and phonon sideband characteristics. Recent advances in isotope engineering, pressure-dependent spectroscopy, and moiré superlattice control have allowed a highly accurate validation of this determination, including direct observation of isotope-induced shifts. These results also demonstrate a high sensitivity of the emitter to the stacking order and lattice symmetry, opening the way to tuning the emission properties and distinguishing BN polytypes. These findings confirm that the carbon dimer is the most consistent microscopic source of the 4.1-eV line and a robust probe of local structure in van der Waals materials.