Visualization of Assembly and Dynamics of Anisotropic Nanoparticles on Liquid Surfaces

The 2D phase behavior of assemblies of silica-coated gold nanorods (NRs), silica-coated hematite nano-ellipsoids (NEs), and their binary mixtures with silica nanospheres at a liquid surface was studied by in situ scanning electron microscopy (SEM). All the nanoparticles (NPs) were functionalized with polyethylene glycol (PEG) and then dispersed in ionic liquid (IL) 1-ethyl 3-methylimidazolium ethyl sulfate ([EMIM][EtSO4]). The NPs gradually segregated to the IL surface until the self-assembled monolayer reached a jammed state. The evolution of the assembly was directly visualized by SEM. The dense assemblies were characterized by the nematic and smectic order parameters (global and local) and orientational correlation functions. Partially adsorbed NPs with orientations out of the interfacial plane reduced the in-plane orientational order in the jammed monolayers. NP-NP interactions and the fraction of out of plane NPs were shape-dependent; NEs preferentially associated side-by-side to form flexible stacks while NRs did not. In the dense assemblies of binary mixtures of NRs and NEs with nanospheres, uniformly mixed and phase-separated morphologies were obtained controlled by the NP-NP interactions and NP adsorption kinetics.