  We correlate spatially resolved fluorescence (-lifetime) measurements with
X-ray nanodiffraction to reveal surface defects in supercrystals of
self-assembled caesium lead halide perovskite nanocrystals and study their
effect on the fluorescence properties. Upon comparison with density functional
modelling, we show that a loss in structural coherence, an increasing atomic
misalignment between adjacent nanocrystals, and growing compressive strain near
the surface of the supercrystal are responsible for the observed fluorescence
blueshift and decreased fluorescence lifetimes. Such surface defect-related
optical properties extend the frequently assumed analogy between atoms and
nanocrystals as so-called quasi-atoms. Our results emphasize the importance of
minimizing strain during the self-assembly of perovskite nanocrystals into
supercrystals for lighting application such as superfluorescent emitters.
