  Hexagonal boron nitride (hBN) is attracting a lot of attention in the last
years, thanks to its many remarkable properties. These include the presence of
single-photon emitters with superior optical properties, which make it an ideal
candidate for a plethora of photonic technologies. However, despite the large
number of experimental results and theoretical calculations, the structure of
the defects responsible for the observed emission is still under debate. In
this work, we visualize individual atomic-scale defects in hBN with atomic
force microscopy under ambient conditions and observe multiple narrow emission
lines from optically stable emitters. This direct observation of the structure
of the defects combined with density functional theory calculations of their
band structures and electronic properties allows us to associate the existence
of several single-photon transitions to the observed defects. Our work sheds
light on the origin of single-photon emission in hBN that is important for the
understanding and tunability of high-quality emitters in optoelectronics and
quantum technologies.
