In Pixar's Inside Out, Joy proclaims, "Do you ever look at someone and wonder what's going on inside?" My group asks the same question about nanomaterials whose function plays a critical role in energy, biology, and information-relevant processes. In this presentation, I will describe new techniques that enable visualization of nanoparticle phase transitions and light-matter interactions with nanometer-scale resolution. First, we explore nanomaterial phase transitions induced by solute intercalation, to understand and improve materials for energy storage applications. As a model system, we investigate hydrogen absorption and desorption in individual palladium nanocrystals. Our approach is based on in-situ electron energy-loss spectroscopy in an environmental transmission electron microscope. By probing hydrogen-induced shifts of the palladium plasmon resonance, we find that loading pressures are strongly size-dependent and that sub-30nm single-crystals do not exhibit phase coexistence. Then, we introduce a novel tomographic technique, cathodoluminescence spectroscopic tomography, to probe optical properties in three dimensions with nanometer-scale spatial and spectral resolution. Particular attention is given to reconstructing a 3D metamaterial resonator supporting broadband electric and magnetic resonances at optical frequencies. Our tomograms allow us to locate regions of efficient cathodoluminescence across visible and near-infrared wavelengths, with contributions from material luminescence and radiative decay of electromagnetic eigenmodes. This tomographic technique could be used to precisely locate radiative recombination centers in light-emitting diodes, to probe the nanoscale distribution of defect states in organic photovoltaics, and potentially to provide new label-free avenues for biological imaging. Taken together, our results provide a general framework for high-resolution visualization of chemical reactions and light-matter interactions, well below the diffraction limit and in three-dimensions.
Materials Science and Engineering / Stanford University, California