In Situ X-Ray Scattering of Atypical Nucleation and Kinetics of Strongly Coupled Nanocrystal Superlattice Self-Assembly

Self-assembly of colloidal nanocrystals (NCs) into long-range ordered arrays represents an appealing strategy toward the bottom-up design of hierarchically organized functional and multifunctional materials. These superlattices (SLs) are typically assembled from NCs with long, insulating organic ligands that inhibit strong electronic coupling. Recently, our team has developed a novel method to self-assemble metallic and high dielectric semiconductor NCs with short, multivalent, inorganic chalcogenometallate (ChM) ligands into strongly electronically coupled ordered SLs in ChM electrolytic environments. We are gaining a deeper understanding of the nucleation, kinetics, and phase diagram associated with this self-assembly process as a function of the system and environment composition, in order to better control assembly of these high dielectric semiconductor NCs and extend this method of assembly to semiconductor NCs with more typical, lower dielectric constants. To do so, we employ in situ small angle X-ray scattering (SAXS) to non-perturbatively probe the structure of PbS NC colloid, liquid, and SL phases as a function of time during self-assembly with millisecond time resolution. From fitting in situ SAXS patterns, we calculate the densities of different phases and thereby quantitatively map experimental phase diagrams for the first time. By identifying and exploring different regions of the phase diagram, we have also observed both classical one-step SL nucleation and non-classical two-step SL nucleation, where a liquid of NCs nucleates from colloidal NCs first, and crystallization into a SL occurs from within the liquid phase. This more complex nucleation process has been observed in protein crystallization and predicted for inorganic NC systems, but had not yet been shown experimentally. We have further explored the non-classical nature of this system by quantitatively comparing rates of assembly to classical nucleation theory, as well as by studying NC diffusion and cluster density fluctuations that lead to nucleation, in order to better control the nucleation process and final ordered configurations. The kinetic and dynamic lessons from these novel in situ experiments will help us push forward the goal of assembling strongly coupled SL materials from semiconducting NCs.