<i>In Situ</i> Imaging of Nanorod Adsorption and Assembly at Liquid Surfaces.

Abstract

Adsorption, surface orientation, interfacial interactions, and two-dimensional packing of ligand-stabilized nanorods at liquid surfaces were visualized at the single-particle level by <i>in situ</i> scanning electron microscopy. The nearly sphero-cylindrical silica-coated gold nanorods, functionalized with poly(ethylene glycol) ligands, had aspect ratios of ∼2, ∼3, and ∼5 and formed Gibbs adsorption layers at the surface of a nonvolatile ionic liquid. Incomplete layers densified over time and eventually jammed at areal fractions exceeding 0.50. Initially attached to the surface by a single end, most nanorods then rotated into the surface plane slowly, with reorientation strongly hindered by contact line pinning. The nanorod interfacial binding energies were several hundred kT, making binding irreversible, and the bound particles experienced significant capillary and steric interactions. In-surface-plane nanorods predominantly arranged side by side, defining flexible stacks that persisted into the jammed state. These kinetically trapped stacks, in conjunction with any out-of-plane nanorods, precluded the spontaneous adoption of long-range surface order. The fraction of out-of-surface-plane nanorods was much reduced at larger ligand lengths, suggesting that long enough ligands screened surface roughness. Nanorod shape, aspect ratio, ligand molecular weight, and rate of adsorption all affected the two-dimensional order observed. Comparable nanorod layers made by spreading rather than adsorption showed markedly different packing behavior.