Enhancing nanocrystal superlattice self-assembly near a metastable liquid binodal*

Self-assembly of colloidal nanocrystals (NCs) into superlattices (SLs) is an appealing strategy to design hierarchically organized materials with new functionalities. Mechanistic studies are needed to uncover SL self-assembly design principles, but they are difficult to perform due to the fast time- and short length scales of NC systems. To address this challenge, we developed an apparatus to directly measure in situ and in real time solutions of electrostatically stabilized NCs as they are quenched to form SLs using small angle X-ray scattering. By developing a quantitative model, we fit the time-dependent scattering patterns to obtain the phase diagram of the system and the kinetics as a function of varying quench conditions. The extracted phase diagram is consistent with particles whose interactions are short in range relative to their size. As a result, we can direct the self-assembly through one-step or two-step pathways in which SLs form either directly from the colloidal phase or from a metastable liquid intermediate whose fluctuations we quantified using X-ray photon correlation spectroscopy. By combining this in situ methodology with quantitative analysis tools and simulation, we propose design principles and kinetic strategies to optimize nanoscale self-assembly.