Real-Time X-ray Scattering Discovers Rich Phase Behavior in PbS Nanocrystal Superlattices during in Situ Assembly

During the self-organization of colloidal semiconductor nanoparticles by solvent evaporation, nanoparticle interactions are substantially determined by the organic ligands covering the inorganic core. However, the influence of the ligand grafting density on the assembly pathway is often not considered in experiments. Here, we carry out an in situ synchrotron small-angle X-ray scattering and X-ray cross-correlation analysis study of the real-time assembly of oleic acid-capped PbS nanocrystals at a low ligand coverage of 2.7 molecules/nm2. With high temporal and spatial resolution, we monitor the transitions from the colloidal suspension through the solvated superlattice states into the final dried superstructure. In a single in situ experiment, we observe a two-dimensional hexagonal, hexagonal close-packed, body-centered cubic, body-centered tetragonal (with different degrees of tetragonal distortion), and face-centered cubic superlattice phases. Our results are compared to the self-organization of PbS nanocrystals with a higher ligand coverage up to 4.5 molecules/nm2, revealing different assembly pathways. This highlights the importance of determining the ligand coverage in assembly experiments to approach a complete understanding of the assembly mechanism as well as to be able to predict and produce the targeted superstructures.