Model 2D Ordering: Structure and Dynamics of Nanoparticles and Their Mixtures at Liquid Interfaces

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Non-Technical Abstract:Nanoparticles, too small to see with even the best optical microscopes and just larger than individual molecules, can attach to a liquid surface and pack there at increasing density to create a two-dimensional layer only one particle thick. To keep the particles from aggregating, they typically are coated with a solvent-swollen polymer that prevents the particles from physically touching each other. The individual nanoparticles form layers that possess magnetic, conductive, optical or sieving properties. These nanoparticle assemblies are of increasing importance for emerging technologies, ranging from displays to storage media to flexible electronics. In this project, a new electron microscopy method enables the in situ, real-time imaging of nanoparticles as they rearrange and possibly crystallize or jam at a liquid surface. Among the phenomena visualized at single-particle resolution are mixing/separation of dissimilar nanoparticles, control of packing through variation of polymer chemistry, and mutual organization/orientation of rod-like nanoparticles. Videos document in time how the nanoparticles organize themselves. A further project goal is to build electron microscope devices that mechanically stress a liquid surface laden with nanoparticles, forcing them to re-organize, opening routes to more perfect packings or even the formation of surface patterns when two or more different types of nanoparticles are used. In developing and applying the new imaging method, a Ph.D. student and a post-doctoral student are gaining skills at the disciplinary intersection of polymers, nanoparticles, microscopy, and interfaces. The collected images and movies of nanoscale processes offer a visually appealing introduction to nanotechnology and soft matter research that is integrated in K12 and public outreach activities.Technical Abstract:Nanoparticles attached to liquid interfaces as Gibbs monolayers are key to numerous technologies, and more generally, these systems provide excellent models for probing two-dimensional (2D) assembly processes. However, nanoparticles are too small to resolve in an optical microscope, and little is known about their behavior at liquid interfaces. Different than larger particles, the flexible polymeric ligands that stabilize nanoparticles often approach or even exceed the particle size; how these ligands promote and stabilize nanoparticles on a liquid surface has scarcely been explored. A new in situ scanning electron microscopy (SEM) method can visualize, to the single particle level and in real-time, the microstructures and dynamics of densely packed interfacial nanoparticles, even those that are undergoing 2D crystallization and jamming. Here, this method addresses several outstanding problems in interfacial nanoparticle assembly, including the role of ligands in modulating interfacial nanoparticle interactions, the influence of nanoparticle shape on monolayer assembly, the phase behavior of nanoparticle interfacial mixtures, and the development of electromechanical devices for in-situ manipulation of nanoparticle-decorated liquid interfaces. While the SEM method’s foundational experiments relied on the unique properties of ionic liquids, using a variable pressure SEM the need for such special liquids can be lifted, vastly expanding the range of nanoparticles, liquids, and ligands accessible to imaging. With the insights gained in this study, possibilities for interfacial nanoparticle technologies are being greatly expanded, with microstructures, dynamics, and physical properties made predictable for the first time. Further, the improved understandings are leading to novel behaviors that will create new technologies in areas such as 3D printing, selective molecular sieving, and nanoscale surface patterning.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.