Electrostatic Superlattices beyond 1:1 Stoichiometry
Exotic nanoparticle superstructures can be accessed by harnessing nanoparticle softness and charge regulation, features often viewed as obstacles to structural control. Here, we show that regulated charge mismatch in polymer-grafted nanoparticles enables the assembly of high-stoichiometry cubic superlattices. By co-tuning grafting density, particle size, and bulk composition, we realize ionic-lattice analogues such as CaF2 and Th3P4, as well as single-component A3 and A7 superlattices without atomic counterparts. The A3 lattice has recently been identified theoretically as a photonic band-gap lattice. These phases emerge from a 1:1 "parent" lattice when local charge neutrality cannot be satisfied, driving either progressive interstitial filling or reorganization into a larger basis. For instance, the systematic occupation of ZnS tetrahedral sites yields CaF2, while ligand-swapping symmetry breaking converts CsCl into Th3P4. Upon heating, the assemblies exhibit reversible lattice contraction and pronounced negative thermal expansion. Furthermore, the energetic penalty for defects increases with nanoparticle size, facilitating the scalable production of high-quality, open superlattices for photonic applications.