Structure Selection in Ionic Colloidal Crystals via Independent Charge Tuning

Colloidal building blocks can assemble into many different crystal structures, yet selecting a specific lattice without redesigning the particles remains a central challenge in programmable self-assembly. Here we demonstrate that independent tuning of colloidal surface charge enables deterministic structure selection using a fixed pair of oppositely charged colloids. Micromolar additions of simple ionic surfactants independently adjust the surface charge of each colloidal species without significantly altering solution ionic strength, allowing the same building blocks to assemble into Th3P4-, CsCl-, Cu3Au-, and Li3Fe-like crystals, the last of which represents a previously unreported colloidal crystal structure. Experiments and molecular dynamics simulations reveal that structure selection is governed by the competition between Coulombic attraction and lattice-specific like-charge repulsion. Experiments across different particle-size ratios further demonstrate the generality of this approach, with additional surfactant-induced transitions between Th3P4-, AlB2-, and Cr3Si-like structures. These findings establish charge asymmetry as a general design parameter for ionic colloidal assembly.