Electrostatically Driven Superstructures Assembly of Charged Gold Nanoparticles

We demonstrate the formation of CsCl-like superstructures by controlling the surface charges of grafted nanoparticles (NPs), showing that these structures remain robust across varying charge densities and temperatures. Gold nanoparticles (AuNPs) are grafted with water-soluble poly(ethylene glycol) (PEG) terminated with charged end groups. PEG with terminal -NH₂ groups create positively charged surfaces, while PEG with terminal -COOH groups provides negatively charged surfaces. We explore the conditions for assembled structures to form by using synchrotron X-ray diffraction and tuning surface charge density by pH adjustments. We show that ionic-like superstructures emerge with pseudo-stoichiometries reflecting the particles' charge density and the binary mixing ratios. In two dimensions, we observe checkerboard-like patterns, while in bulk, CsCl-like structures form, though defects arise from the particle ratio of a binary mixture. We also find that perovskite-like superstructures are formed and assembled under specific pH and binary mixing ratio conditions into body-centered cubic superstructures.