Quantifying the creation of negatively charged boron vacancies in He-ion irradiated hexagonal boron nitride

Hexagonal boron nitride (hBN) hosts luminescent defects possessing spin qualities compatible with quantum sensing protocols at room temperature. Vacancies, in particular, are readily obtained via exposure to high-energy ion beams. While the defect creation mechanism via such irradiation is well understood, the occurrence rate of optically active negatively charged vacancies (VB-) is still an open question. In this work, we exploit focused helium ions to systematically create optically active vacancy defects in hBN flakes at varying densities. By comparing the density-dependent spin splitting measured by magnetic resonance to calculations based on a microscopic charge model, in which we introduce a correction term due to a constant background charge, we are able to quantify the number of VB- defects created by the ion irradiation. We find a lower bound for the fraction (0.2%) of all vacancies in the optically active, negatively charged state. Our results provide a protocol for measuring the creation efficiency of VB-, which is necessary for understanding and optimizing luminescent centers in hBN.