Visualization of Nanorod Assembly and Dynamics on Liquid Surfaces*

The two-dimensional phase behavior of rod-like particles was studied by dispersing silica-coated gold nanorods on a liquid surface. Nanorods with aspect ratio ranging from 3 to 6 were synthesized, coated with silica, functionalized with polyethylene glycol (PEG), and then dispersed in ionic liquid 1-ethyl 3-methylimidazolium ethyl sulfate ([EMIM][EtSO4]). The dispersed rods, driven to the surface by the low surface energy of PEG, were observed directly on the surface of an ionic liquid droplet by scanning electron microscopy. Rod diffusion coefficients were calculated in dilute systems by tracking position and orientation of single nanorods. In concentrated systems, the nanorod packing was studied using an in-situ liquid cell to control volume and surface area of the droplet. The 2D packing was characterized by pair correlation function, translational order parameter, orientational order parameter, and Voronoi tessellation. Order parameters were calculated as a function of areal density and nanorod aspect ratio. An increase in local orientational order was observed at higher areal densities but a liquid crystalline phase was not observed due to low aspect ratio.