V_NC_B defect as source of single photon emission from hexagonal boron nitride

Single photon emitters in 2D hexagonal boron nitride (hBN) have attracted a considerable attention because of their highly intense, stable, and strain-tunable emission. However, the precise source of this emission, in particular the detailed atomistic structure of the involved crystal defect, remains unknown. In this work, we present first-principles calculations of the vibrationally resolved optical fingerprint of the spin-triplet (2) ³B₁ to (1) ³B₁ transition of the V_NC_B point defect in hBN. Based on the excellent agreement with experiments for key spectroscopic quantities such as the emission frequency and polarization, the photoluminescence (PL) line shape, Huang–Rhys factor, Debye–Waller factor, and re-organization energy, we conclusively assign the observed single photon emission at ~2 eV to the V_NC_B defect. Our work thereby resolves a long-standing debate about the exact chemical nature of the source of single photon emission from hBN and establishes the microscopic understanding necessary for controlling and deploying such photons for quantum technological applications.