  Diamond nanocrystals that host room temperature narrowband single photon
emitters are highly sought after for applications in nanophotonics and
bio-imaging. However, current understanding of the origin of these emitters is
extremely limited. In this work we demonstrate that the narrowband emitters are
point defects localized at extended morphological defects in individual
nanodiamonds. In particular, we show that nanocrystals with defects such as
twin boundaries and secondary nucleation sites exhibit narrowband emission that
is absent from pristine individual nanocrystals grown under the same
conditions. Critically, we prove that the narrowband emission lines vanish when
extended defects are removed deterministically using highly localized electron
beam induced etching. Our results enhance the current understanding of single
photon emitters in diamond, and are directly relevant to fabrication of novel
quantum optics devices and sensors.
