Nanoparticles at liquid-vacuum interfaces can be resolved to the single particle level by <i>in </i>situ, real-time scanning electron microscopy (SEM), providing crucial insights into their static and dynamic behaviors. Here, interactions and packing of nanorods and nanoellipsoids were explored for Gibbs nanoparticle monolayers of dilute to jammed packing densities. Individual SEM images and image sequences uncovered behaviors not seen before. (i) As an isolated, ligand-stabilized rod breaches a liquid interface, typically by one end, dewetting to align the rod in the interfacial plane can take many hours. This timescale is affected by the the ligand (and probably the surface roughness). When dewetting is slower than nanoparticle adsorption, the resulting jammed monolayers contain aligned and out-of-plane rods, enhancing 2D packing disorder. (ii) Beyond nonvolatile liquids (e.g., ionic liquids), variable pressure SEM can image nanorods at the surface of low volatility liquids such as glycerol, expanding the range of systems imageable. (iii) While most interfacial nanoparticles are stabilized with dense ligand layers, typically grafted PEG, several interfacial nanoparticle systems without ligands proved stable, a finding that should help isolate the ways that ligands control nanoparticle interactions. (iv) Packing in jammed monolayers is typically kinetically controlled. For most systems examined (PEGylated gold and hematite nanorods and nanoellipsoids of aspect ratio 3 to 7), attractive side-by-side interactions induce nanoparticle stacking (see image); the semiflexible stacks persist through jamming.

Imaging nanorod and nanoellipsoid packing at liquid surfaces to the single particle level

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