  Point defects significantly influence the optical and electrical properties
of solid-state materials due to their interactions with charge carriers, which
reduce the band-to-band optical transition energy. There has been a demand for
developing direct optical imaging methods that would allow in-situ
characterization of individual defects with nanometer resolution. Here, we
demonstrate the localization and quantitative counting of individual optically
active defects in monolayer hexagonal boron nitride using single molecule
localization microscopy. By exploiting the blinking behavior of defect emitters
to temporally isolate multiple emitters within one diffraction limited region,
we could resolve two defect emitters with a point-to-point distance down to ten
nanometers. The results and conclusion presented in this work add unprecedented
dimensions towards future applications of defects in quantum information
processing and biological imaging.
