EPSD Colloquium

Bottom-up assembly of functional materials is a multiscale phenomenon and one of the grand challenges of materials science. The materials thus formed are hierarchical – that is, they exhibit a hierarchy of bonding interactions that can be especially challenging to control to achieve long-range ordered, functional structures. Specifically, when building electronic or optoelectronic materials out of colloidal semiconductor nanocrystals, solution-phase electrostatics offers a new strategy to more precisely control emergent material properties and the pathways to achieve them by leveraging explicit short-range attractive interactions. I will first show, via in situ small-angle X-ray scattering, how this approach reveals a new kinetic design principle that invokes a metastable high-density fluid intermediate that unexpectedly simultaneously boosts both crystallinity and crystallization rate as the driving force is increased. Second, by leveraging the power of MHz-rate coherent X-ray scattering, I will explain how equilibrium crystalline order emerges from strikingly non-equilibrium intermediates: jammed prenucleation clusters within dynamically heterogeneous nanocrystal fluids. Third, I will describe our current understanding of how electrolyte molecules dynamically reorganize around the nanocrystals to facilitate the spectrum of high-density fluid interactions observed, thanks to wide-angle scattering pair distribution function (PDF) analysis. Finally, I will show how the phase behavior of electrostatically controlled nanocrystals is actively tuned by photoexcitation and share a vision of autonomously guided solution-phase materials formation via closed-loop adaptive feedback to external stimuli that leverages real-time in situ observation.