Monitoring Self-Assembly and Ligand Exchange of PbS Nanocrystal Superlattices at the Liquid/Air Interface in Real Time.

We investigate in situ the structural changes during self-assembly of PbS nanocrystals from colloidal solution into superlattices, solvent evaporation, and ligand exchange at the acetonitrile/air interface by grazing incidence small-angle X-ray scattering (GISAXS). We simulate and fit the diffraction peaks under the distorted wave Born approximation (DWBA) to determine the lattice parameters. We observe a continuous isotropic contraction of the superlattice in two different assembly steps, preserving the body-centered cubic lattice with an overall decrease in the lattice constants of 11%. We argue that the first contraction period is due to a combination of solvent evaporation/annealing and capillary forces acting on the superlattice, whereas the second period is dominated by the effect of replacing oleic acid on the nanocrystal surface with the short and rigid cross-linker tetrathiafulvalene dicarboxylate. This work provides guidelines for optimized ligand exchange conditions and highlights the structural particularities arising from assembling NCs on liquid surfaces.