Design advances for the bottom-up assembly of highly ordered functional nanomaterials have generated a wide range of fundamental questions that must be answered to continue to advance material properties, such as strong electronic and mechanical coupling. Compared to the microscale realm of self-assembling soft matter, nanoscale building blocks are generally more challenging to manipulate with a high degree of tunability to achieve different desirable outcomes. This challenge is predicated on scale itself – it is difficult to specify the interparticle interactions of nanoscale building blocks in the solution phase due to the non-additivity of different attractive and repulsive contributions, especially because of the finite size of solvent molecules relative to the building blocks themselves. We nevertheless focus on colloidal nanocrystal advances that incorporate electrostatics to promote the formation of ordered superlattice structures from nanocrystals with high dielectric constants.¹ Through a multiscale suite of hard X-ray scattering experiments ranging from small- to wide-angle, incoherent to coherent, and storage ring to free electron laser, we characterize the phases, their fluctuations, and the dynamic interconversion between phases of this enigmatic system, non-invasively and in real-time, identifying the helpful role of a liquid-like intermediate phase that admits an unusually high degree of control over supercrystalline product yield, size, and order. We find that controlled, ordered assembly requires a balance of surface charge, screening, and van der Waals attraction that is facilitated by moderate to high dielectric ratios between the nanocrystals and their surroundings. We also find that laser absorption reversibly suppresses growth of the ordered superlattice phase and intend to leverage these finding to infer strategies to self-assemble more common low-dielectric nanocrystals into ordered structures by driving them far from equilibrium with optical excitation.

Time-permitting, I will also share a few vignettes on larger scale driven colloidal dynamics of monolayers of silica nanoparticles pinned to an ionic liquid-vacuum interface that are both driven and imaged with a scanning electron beam. We have followed phenomena such as coarsening and melting high density monolayers² and a percolation transition of sparse smaller particles in a size-bidisperse mixture rationalized via geometric non-additivity³, both of which rely on controllable particle charging by the electron beam. Recent work at lower particle density reveals additional ways to controllably drive the system with seemingly Marangoni-like flows using radical chemistry in the ionic liquid. Control of such effects can also be used to write and preserve patterns with the particles.

1.    Coropceanu, I. et al. Self-assembly of nanocrystals into strongly electronically coupled all-inorganic supercrystals. Science 375, 1422–1426 (2022).

2.    Bischak, C. G., Raybin, J. G., Kruppe, J. W. & Ginsberg, N. S. Charging-driven coarsening and melting of a colloidal nanoparticle monolayer at an ionic liquid–vacuum interface. Soft Matter 16, 9578–9589 (2020).

3.    Raybin, J. G., Wai, R. B. & Ginsberg, N. S. Nonadditive Interactions Unlock Small-Particle Mobility in Binary Colloidal Monolayers. ACS Nano 17, 8303–8314 (2023).