  Supersaturation is the fundamental parameter driving crystal formation, yet
its dynamics during the growth of colloidal nanocrystals (NCs) are poorly
understood. Experimental characterization of supersaturation in colloidal
syntheses has been difficult, limiting insight into the phenomena underlying NC
growth. Hence, despite significant interest in the topic, how many types of NCs
grow remain unclear. Here, we develop a framework to quantitatively
characterize supersaturation in situ throughout NC growth. Using this approach,
we investigate the seed-mediated synthesis of colloidal Au nanocubes, revealing
a triphasic sequence for the supersaturation dynamics: rapid monomer
consumption, sustained supersaturation, and then gradual monomer depletion.
These NCs undergo different shape evolutions in different phases of the
supersaturation dynamics. As shown with the Au nanocubes, we can use the
supersaturation profile to theoretically predict the growth profile of NCs. We
then apply these insights to rationally modulate shape evolutions, decreasing
the yield of impurity NCs. Our findings demonstrate that the supersaturation
dynamics of NC growth can be more complex than previously understood. While
this study focuses experimentally on Au NCs, our framework is facile and
applicable to a broad range of NCs undergoing classical growth. Thus, our
methodology facilitates deeper understanding of the phenomena governing
nanoscale crystal growth and provides insight towards the rational design of
NCs.
