Temperature-Controlled Reversible Formation and Phase Transformation of 3D Nanocrystal Superlattices Through In Situ Small-Angle X-ray Scattering

For decades, the spontaneous organization of nanocrystalsintosuperlattices has captivated the scientific community. However, achievingdirect control over the formation of the superlattice and its phasetransformations has proven to be a grand challenge, often resultingin the generation of multiple symmetries under the same experimentalconditions. Here, we achieve direct control over the formation ofthe superlattice and its phase transformations by modulating the thermalenergy of a nanocrystal dispersion without relying on solvent evaporation.We follow the temperature-dependent dynamics of the self-assemblyprocess using synchrotron-based small-angle X-ray scattering. Whencooled below -24.5 degrees C, lead sulfide nanocrystals formmicrometer-sized three-dimensional phase-pure body-centered cubicsuperlattices. When cooled below -35.1 degrees C, these superlatticesundergo a collective diffusionless phase transformation that yieldsdenser body-centered tetragonal phases. These structural changes canbe reversed by increasing the temperature of the dispersion and maylead to the direct modulation of the optical properties of these artificialsolids.