  Bottom-up assembly of nanocrystals (NCs) into ordered arrays, or
superlattices (SLs), is a promising route to design materials with new
functionalities, but the degree of control over assembly into functional
structures remains challenging. Using electrostatics, rather than density, to
tune the interactions between semiconductor NCs, we watch self-assembly
proceeding through a metastable liquid phase. We systematically investigate the
phase behavior as a function of quench conditions in situ and in real time
using small angle X-ray scattering (SAXS). Through quantitative fitting to
colloid, liquid, and SL models, we extract the time evolution of each phase and
the system phase diagram, which we find to be consistent with short-range
attractive interactions. Using the phase diagram's predictive power, we
establish control of the self-assembly rate over three orders of magnitude, and
identify one- and two-step self-assembly regimes, with only the latter
implicating the metastable liquid as an intermediate. Importantly, the presence
of the metastable liquid increases SL formation rates relative to the
equivalent one-step pathway, and SL order counterintuitively increases with the
rate, revealing a highly desirable and generalizable kinetic strategy to
promote and enhance ordered assembly.
