Breaking symmetry in nanoparticle superlattices via spontaneous valency

Nanoparticle superlattices enable programmable material structure through ligand-mediated interactions, but attractive ligand interactions typically limit assembly to densely packed lattices that maximize nearest-neighbour bonding. Intentionally programmed directional bonding and valency are therefore typically assumed to require particle-level anisotropy in shape or surface patterning. Here, using assemblies of flexible, electrostatically active isotropic nanoparticles, we show that valency need not be encoded by particle geometry, but can instead emerge spontaneously during assembly. When ligand coronas are sufficiently deformable, ligands reconfigure to align along interparticle interfaces, generating an effective valency. Using a combination of experiment and simulation, we show that increasing ligand deformability lowers lattice coordination numbers, and identify design considerations for assembling isotropic building blocks into lattices with lower coordination, packing density and symmetry than would be predicted from hard-sphere packing. The work presented here therefore establishes ligand flexibility as a design parameter for programming coordination in nanoparticle assembly.