Understanding the formation mechanism of colloidal nanocrystals is of paramount importance in order to design new nanostructures and synthesize them in a predictive fashion. However, reliable data on the pathways leading from molecular precursors to nanocrystals are not available yet. We used synchrotron-based time-resolved in situ small and wide-angle X-ray scattering to experimentally monitor the formation of CdSe quantum dots synthesized in solution through the heating up of precursors in octadecene at 240 °C. Our experiment yields a complete movie of the structure of the solution from the self-assembly of the precursors to the formation of the quantum dots. We show that the initial cadmium precursor lamellar structure melts into small micelles at 100 °C and that the first CdSe nuclei appear at 218.7 °C. The size distributions and concentration in nanocrystals are measured in a quantitative fashion as a function of time. We show that a short nucleation burst lasting 30 s is followed by a slow decrease of nanoparticle concentration. The rate-limiting process of the quantum dot formation is found to be the thermal activation of selenium.

SignificanceColloidal suspensions of electrically charged nanometric sheets (nanosheets), like graphene oxide or clays, which are widely used in industry, form liquid-crystalline phases. These include the nematic phase, where all nanosheets are approximately parallel, and the lamellar phase, where they also form equidistant layers. When the particle diameter is large (100 nm to 1 µm), distinguishing these phases is quite challenging. Using newly available synchrotron small-angle X-ray scattering setups, we unambiguously identified both phases, in H3Sb3P2O14 nanosheet suspensions, by analyzing their X-ray scattering patterns. The lamellar domain size reaches 20 µm, which means each layer is made of ∼1,000 nanosheets. Because the lamellar phase was rarely predicted in suspensions of charged disks, these systems should be revisited by theory or simulations.