Visualizing Nanoparticle Packing at Liquid Interfaces

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NON-TECHNICAL ABSTRACTNanoparticles, objects not much larger than molecules, assemble in two- and three-dimensions into materials with unique and technologically important properties. Until recently, this assembly could not be directly seen because of the very small size of nanoparticles. For this proposal, a new imaging method has been developed that can resolve the structure and motions of individual nanoparticles assembled on a liquid interface. The method combines the high resolution of electron microscopy, about 100-1000 times greater than optical microscopy, with the nonvolatility of ionic liquids, essentially liquid salts, to probe the two-dimensional assembly of nanoparticles as a function of nanoparticle size, geometry, bulk chemistry, surface chemistry, and degree of liquid wetting. Unlike larger particles, nanoparticles can interact with each other over distances as large as the particles themselves, a feature that facilitates assembly into well-ordered structures. Also unlike larger particles, these interactions are weak, allowing the particles to "jiggle around" so as to find the best arrangement. In many experiments, different sorts of nanoparticles are mixed, increasing the diversity of the ordered structures, or in other cases, creating a frozen disordered "jammed" state. Determining conditions under which nanoparticle particles organize, understanding how these changes affect organization, and uncovering ways to make completely disordered systems are the key project objectives. This research is performed by undergraduate, graduate, and post-doctoral fellows, and the outcomes are often striking images or movies easily understandable by everyone. In the long term, the tools and techniques emerging from this research should apply easily to other technologically important materials such as gels, emulsions, liquid crystals, and suspensions, all of which display bulk properties determined by nanoscale features and events.TECHNICAL ABSTRACTDespite a need to impose vacuum, scanning electron microscopy can be performed on open liquid specimens when the components are wetted by, or dispersed in, a nonvolatile ionic liquid. At an imaging resolution of 3-5 nm, several frames per second can be acquired over an unlimited time. Preliminary experiments established the feasibility of scanning-electron-microscope, single-particle tracking even for dense nanoparticle packings and non-spherical particle shapes. The method functions much like optical video microscopy but at 10-100X greater magnification. Two-dimensional nanoparticle ordering on ionic liquid surfaces is now pursued comprehensively and quantitatively, examining effects of nanoparticle size, geometry, bulk chemistry, surface chemistry (i.e., polymer ligand type), and degree of liquid wetting. In addition, different types of nanoparticles are mixed. Unlike larger colloidal particles, nanoparticle interactions are typically weak, allowing equilibrium assembly under Brownian motion alone. The interaction potential will be evaluated for all listed parameters, with greatest attention on ligand type, which controls the interaction length scale, reaching or exceeding the particle size. Ligands also affect the contact angle, which dictates how deeply the interface-trapped particle protrudes into the underlying liquid, and, in turn, this depth influences particle dynamics. A new transmission-electron-microscopy approach to nanoparticle contact angle is being assessed, and by the new scanning-electron-microscopy tracking method, the associated interfacial nanoparticle dynamics are being quantified. Both ordered and disordered dense nanoparticle packings are being studied, with the latter facilitated by mixing particles of different size/shape. To control nanoparticle areal density, a unique device for control of interfacial area is placed inside the microscope, with areal adjustments made in situ as image sequences are collected. Assembly into dense structures is analyzed by calculating translational and orientational parameters. Lastly, an interesting and unexplained coupling between the electron beam of the microscope and solid metallic or metal-coated nanoparticles is being pursued to create precise nanoparticle patterns on liquid surfaces.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.