Probing 2D Phase Separation in Mixed Silica and Gold Nanoparticle Assemblies

While much attention has been directed toward interfaces decorated with functional, uniform NPs, less effort has focused on mixed NP assemblies that have the potential to prepare well-defined heterogeneous surfaces and 2D phase separated morphologies. We capitalize on nanoparticle surfactant (NPS) assembly at a liquid-liquid interface to increase NP binding energy and provide a platform to probe highly packed layers of mixed silica and gold NPSs by UV-Vis reflection spectroscopy. As predicted by the Mie theory, the plasmon excitation energy depends not only on the properties of individual gold NPs but also on the number of NPs in the ensemble, their relative location, and interparticle distances. Silica prevents significant ordering of the gold NPs into larger structures. By tuning the size, number, and adsorption rate of each NP species in the assembly, we can control the morphology of the jammed assembly using the rate of 2D phase separation. The maximum reflection intensity and corresponding wavelength reveal structural correlations of the gold NP clusters. This method can be used to understand and control the molecular and nanoscopic factors that govern the phase separation in two dimensions.