Single Particle Imaging of the Assembly of Elongated Nanoparticles at a Liquid Interface

Despite conceptual and technological importance, the attachment and subsequent ordering of anisotropic particles at liquid interfaces remains poorly understood. Here, in situ scanning electron microscopy provided insights into the near two-dimensional interfacial assembly of model PEG-grafted anisotropic nanoparticles (NPs) of both ellipsoidal and sphero-cylindrical shape; aspects ratios varied from 2 to 7.  Due to PEG's lower surface energy, bulk NPs segregated to the vacuum-liquid interface, jamming at high interfacial areal density after hours or days. The positions and orientations of individual NPs were unambiguously visualized throughout, enabling real-time characterization of nematic and smectic order parameters along with orientational correlation functions. The NPs formed long, persistent side-by-side stacks, as presumably driven by capillary forces. PEG length, as well as solvent surface tension, altered NP interactions, with stronger attractions kinetically trapping co-existing regions of orientational order and disorder. Ellipsoids and rods behaved similarly.