Nanocrystals (NCs) act as building blocks and self-assemble into long-range periodic superlattice structures. Such assemblies not only maintain the size-dependent properties of individual nanocrystals, but also can give rise to enhanced or novel properties from near-field coupling. Control of this material-by-design approach requires a better understanding of nucleation and growth mechanism. One approach is to dry a pure solution and assemble supercrystals under equilibrium conditions. However, due to the weak interactions between NCs through ligands, it is often difficult to achieve large domain sizes, even in very slow processes, lasting from days to weeks.
Here, we use Fe2O3 spherical NCs as a model system to demonstrate an assembly approach based on the interdiffusion of NC solutions and anti-solvents. The anti-solvents are miscible to the solution but poor solvents for NCs (e. g., polar solvents for NCs coated with non-polar ligands). The diffusion at the interface creates a non-equilibrium condition to accelerate nucleation and allows growth control through the solvent/solution properties, such as polarity, miscibility, viscosity, solubility of solutes, etc. This approach not only accelerates the growth of superlattice into large grains, which could take weeks without anti-solvents, down to hours, but also facilitates exploration of new polymorphs. The solution processes are in situ characterized by SAXS/WAXS (Small and Wide Angle X-ray Scattering), to probe the periodic structures from atomic (NCs structure) to ~100nm (superlattice) length scales on the CMS beamline (11-BM) at NSLS II.