Abstract: Short-range-interacting particles can in principle crystallize via so-called non-classical pathways invoking a metastable liquid intermediate, yet non-equilibrium gelation often occurs before a metastable liquid can form. Using in situ X-ray scattering, we nevertheless watch electrostatically stabilized colloidal semiconducting nanocrystals self-assemble into long-range-ordered superlattices via this non-classical pathway and show how the pathway increases the rate of crystallization over that of direct crystallization from the colloidal phase. Furthermore, by mapping the phase behavior and kinetics as a function of nanocrystal density and electrostatically tuned driving force for assembly, we demonstrate a highly unusual degree of control of a nanoscale system. This control is exemplified by varying the self-assembly rate by over three orders of magnitude, along with predictive control of superlattice yield, size, and crystallinity. Most strikingly, we reveal that this non-classical pathway increases crystallinity of the superlattice simultaneously with the crystallization rate. To further elucidate the elusive nature of the short-range interactions at the nanoscale, we also study the microscopic fluctuations of colloidal suspensions and liquid droplets of the nanocrystals via free-electron laser MHz X-ray photon correlation spectroscopy (XPCS). We discover suppressed nanocrystal self-diffusion in the liquid state, which we attribute to the explicit attractive interactions that are not captured by typical charged particle hydrodynamic models. The combined results suggest design rules for the shape of interaction potentials not only to leverage liquid intermediates in crystallization processes but also to avoid gelation for better control of phase behaviors.
Energy transport in these and other materials is an important emergent property to also characterize at the nanoscale, especially since the solids created often still contain nanoscale heterogeneities. I will therefore also share recent advances in detecting, tracking, and discerning the spatiotemporal evolution of charge carriers, excitons, heat and ions as they interconvert and explore emerging materials’ structure and heterogeneity on multiple scales. I will share our development of sub-picosecond and single-digit nanometer sensitivity stroboscopic optical scattering microscopy (stroboSCAT) through a series of examples of increasing complexity, ranging from solution-processed semiconductors to transition metal oxide photoelectrodes for artificial photosynthesis to the most direct measurements to-date of exciton transport in natural photosynthesis.