  Monolayer hexagonal boron nitride (hBN) has recently become the focus of
intense research as a material to host quantum emitters. Although it is well
known that such emission is associated with point defects, so far no conclusive
correlation between the spectra and specific defects has been demonstrated.
Here, we prepare atomically clean suspended hBN samples and subject them to
low-energy ion irradiation. The samples are characterized before and after
irradiation via automated scanning transmission electron microscopy imaging to
assess the defect concentrations and distributions. We find an intrinsic defect
concentration of ca. 0.03/nm2 (with ca. 55% boron and 8% nitrogen single
vacancies, 20% double vacancies and 16% more complex vacancy structures). To be
able to differentiate between these and irradiation-induced defects, we create
a significantly higher (but still moderate) concentration of defects with the
ions (0.30/nm2), and now find ca. 55% boron and 12% nitrogen single vacancies,
14% double vacancies, and 18% more complex vacancy structures. The results
demonstrate that already the simplest irradiation provides selectivity for the
defect types, and open the way for future experiments to explore changing the
selectivity by modifying the irradiation parameters.
