  Solution-phase bottom up self-assembly of nanocrystals into superstructures
such as ordered superlattices is an attractive strategy to generate functional
materials of increasing complexity, including very recent advances that
incorporate strong interparticle electronic coupling. While the self-assembly
kinetics in these systems have been elucidated and related to the product
characteristics, the weak interparticle bonding interactions suggest the
superstructures formed could continue to order within the solution long after
the primary nucleation and growth have occurred, even though the mechanism of
annealing remains to be elucidated. Here, we use a combination of Bragg
coherent diffractive imaging and X-ray photon correlation spectroscopy to
create real-space maps of supercrystalline order along with a real-time view of
the strain fluctuations in aging strongly coupled nanocrystal superlattices
while they remain suspended and immobilized in solution. By combining the
results, we deduce that the self-assembled superstructures are polycrystalline,
initially comprising multiple nucleation sites, and that shear avalanches at
grain boundaries continue to increase crystallinity long after growth has
substantially slowed. This multimodal approach should be generalizable to
characterize a breadth of materials in situ in their native chemical
environments, thus extending the reach of high-resolution coherent X-ray
characterization to the benefit of a much wider range of physical systems.
