A non-equilibrium transient phase revealed by in situ GISAXS tracking of the solvent-assisted nanoparticle self-assembly

We report on a time-resolved study of the colloidal nanoparticle self-assembly into a high-quality nanoparticle crystal with the face-centered cubic crystallographic symmetry. In particular, the grazing-incidence small-angle X-ray scattering technique was employed to track kinetics of the solvent evaporation driven self-assembly on casting a drop of plasmonic Ag nanoparticles on a silicon substrate. The short-range (cumulative) disorder typical for paracrystal structures before the complete solvent evaporation at 300-350 s from the drop casting was found with the exception of the time window of 125-150 s where a highly regular transient phase with the long-range order was observed. This temporary improvement of the nanoparticle crystal perfection occurring shortly before the complete solvent evaporation is the main message of the paper. It is attributed to interaction between the surfactant shells of the neighboring nanoparticles getting into contact in the presence of solvent residua to the end of the solvent evaporation which results in a larger nanoparticle hydrodynamic diameter with a smaller dispersion and improvement of the crystallization. This process has direct impact on the quality of the resulting nanoparticle crystal and tailoring its properties.