  Self-assembly of colloidal nanocrystals (NCs) into superlattices (SLs) is an
appealing strategy to design hierarchically organized materials with new
functionalities. Mechanistic studies are still needed to uncover the design
principles for SL self-assembly, but such studies have been 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 the evolving
phases \textit{in situ} and in real time of an electrostatically stabilized Au
NC solution before, during, and after it is quenched to form SLs using small
angle X-ray scattering (SAXS). By developing a quantitative model, we fit the
time-dependent scattering patterns to obtain the phase diagram of the system
and the kinetics of the colloidal and SL phases as a function of varying quench
conditions. The extracted phase diagram is consistent with particles whose
interactions are short in range relative to their diameter. We find the degree
of SL order is primarily determined by fast (sub-second) initial nucleation and
growth kinetics, while coarsening at later times depends non-monotonically on
the driving force for self-assembly. We validate these results by direct
comparison with simulations and use them to suggest dynamic design principles
to optimize crystallinity within a finite time window. The combination of this
measurement methodology, quantitative analysis, and simulation should be
generalizable to elucidate and better control the microscopic self-assembly
pathways of a wide range of bottom-up assembled systems and architectures.
