Evaporation-Induced Phase Transitions in Free-Standing Plasmonic Nanoparticle Assemblies

Evaporation-induced self-assembly transforms dilute nanoparticle suspensions into ordered plasmonic superlattices, yet the microscopic mechanisms remain unclear for anisotropic particles in water. Here, we study the drying kinetics of silver nanorod (AgNR) dispersions in cetyltrimethylammonium chloride (CTAC) using time-resolved levitated small-angle X-ray scattering (SAXS), complemented by microbeam SAXS and focused ion beam-scanning electron microscopy on dried samples. The key parameter governing superlattice formation is the initial surfactant concentration, outweighing nanoparticle concentration and shape effects. AgNR ordering is synchronized with surfactant organization: CTAC micelles induce depletion attractions that drive nucleation and growth, followed by structural arrest upon CTAC gelation. These findings are directly relevant for improving the design of plasmonic metamaterial and nanoparticle (NP) self-assemblies in a broad sense. Moreover, the dual role played by CTAC micelles (promotion of NPs ordering followed by structural arrest) likely represents a mechanism applicable to other systems where depletants undergo gelation during drying.