Low Symmetry Nanoparticle Superlattices via Spontaneous Valency

In colloidal self-assembly, particle valency is controlled through the use of specific particle shapes or surface functionalization anisotropy. Although spheres are typically the easiest particle shape to produce, their inability to produce directional bonding limits their structures to densely packed, high symmetry unit cells. Here, we present an alternative route towards lower-symmetry lattice formation: isotropic, soft nanoparticles that undergo spontaneous symmetry breaking during the assembly process. This in-situ asymmetry generation, enabled by the recruitment of flexible electrostatic chain ends to the interfaces between nanoparticles, leads to an effective valency which determines final lattice structure. Combined with evidence from molecular dynamics simulations, we use a series of CsCl-, Th 3 P 4 -, and NaCl-type lattices assembled from identical but oppositely charged nanoparticles to elucidate the relationship between nanoparticle core size, polymer graft length, and resulting lattice coordination. We also report the first use of wide-angle x-ray scattering to confirm the recruitment and alignment of polymer chains at nanoparticle-nanoparticle interfaces, and assemble the first experimentally realized CaF 2 -and 𝛽-UH 3 -type nanoparticle superlattices. Together, this work demonstrates a potentially new paradigm in nanoparticle superlattice engineering, opening additional and more accessible routes towards low-symmetry and low-packing density structures.