Direct Nanoscopic Imaging of Mesophase Formation from Anisotropic Nanoparticles

We utilize the emergent nanoscopic imaging technique, liquid phase transmission electron microscopy, to suggest a paradigm shift from imaging the dynamics and transformations of micron-sized systems to nanoscale objects in liquids. Here highly anisotropic nanoparticles are used as a prototypical system and are observed to assemble into a non-trivial mesophase, a plastic crystal with positional order and orientational disorder which has been predicted to be impossible for this shape using hard colloidal models. With a combination of direct imaging and Monte Carlo simulation, we show that the nanoscale ``non-hard'' interactions render configurational entropy sufficient to randomize nanoparticle orientations and induce the crystallization into a 3D hierarchical plastic crystal. The long-standing hypothesized kinetic intermediates in nanoscale crystallization is also visualized and analyzed for the first time.