Upon solvent evaporation, monodisperse colloids undergo a phase change in which the isolated, disordered, noninteracting colloidal particles self-assemble into a highly ordered, close-packed superlattice. While the initial and final states can be readily characterized, little is known about the pathway and dynamics between these two disparate states. Using in situ grazing-incidence X-ray scattering, we tracked the self-assembly of lead sulfide nanocrystals in real-time as they progressed from colloid to superlattice. Following the first appearance of an ordered arrangement, the superlattice underwent uniaxial contraction and collective rotation as it approached its final body-centered cubic (BCC) formation. Importantly, the nanocrystals became crystallographically aligned early in the overall self-assembly process, showing that nanocrystal ordering occurs first and on a faster timescale than superlattice densification. These experiments demonstrate that this synchrotron technique is a viable method for imaging self-assembly in its native environment, with ample time resolution to extract kinetic rates and observe previously unknown intermediate configurations. Such a technique could be used for real-time control of directed self-assembly processes, and reveals new insight into the forces governing self-organization of soft materials.

Kinetics of nanocrystal superlattice self-assembly revealed by real-time in situ x-ray scattering

251st National Meeting (2016)

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