In Situ Coherent X-ray Scattering Reveals Polycrystalline Structure and Discrete Annealing Events in Strongly-Coupled Nanocrystal Superlattices

Solution-phase bottom up self-assembly of nanocrystals into superstructuressuch as ordered superlattices is an attractive strategy to generate functionalmaterials of increasing complexity, including very recent advances thatincorporate strong interparticle electronic coupling. While the self-assemblykinetics in these systems have been elucidated and related to the productcharacteristics, the weak interparticle bonding interactions suggest thesuperstructures formed could continue to order within the solution long afterthe primary nucleation and growth have occurred, even though the mechanism ofannealing remains to be elucidated. Here, we use a combination of Braggcoherent diffractive imaging and X-ray photon correlation spectroscopy tocreate real-space maps of supercrystalline order along with a real-time view ofthe strain fluctuations in aging strongly coupled nanocrystal superlatticeswhile they remain suspended and immobilized in solution. By combining theresults, we deduce that the self-assembled superstructures are polycrystalline,initially comprising multiple nucleation sites, and that shear avalanches atgrain boundaries continue to increase crystallinity long after growth hassubstantially slowed. This multimodal approach should be generalizable tocharacterize a breadth of materials in situ in their native chemicalenvironments, thus extending the reach of high-resolution coherent X-raycharacterization to the benefit of a much wider range of physical systems.