VISUALIZING PHASE TRANSFORMATIONS & LIGHT-MATTER INTERACTIONS WITHIN INDIVIDUAL NANOPARTICLES

In Pixar's Inside Out, Joy proclaims, "Do you ever look at someone and wonder, what is going on inside?" Dr. Dionne's group asks the same question about nanomaterials whose function plays a critical role in energy, biology, and information-relevant processes. In this presentation, Jen will describe new techniques that enable visualization of nanoparticle phase transitions in reactive environments as well as light-matter interactions with near-atomic-scale resolution. First, hydrogen absorption and desorption in individual palladium nanocrystals is directly monitored. The approach is based on in-situ electron energy-loss spectroscopy in an environmental transmission electron microscope. Probing hydrogen-induced shifts of the palladium plasmon resonance leads to the discoverey that hydrogen loading and unloading isotherms are characterized by abrupt phase transitions and macroscopic hysteresis gaps. These results suggest that alpha and beta phases do not coexist in single-crystalline nanoparticles, in striking contrast with ensemble measurements of Pd nanoparticles. Second, a novel tomographic technique-cathodoluminescence spectroscopic tomography-is introduced 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. Tomograms allow the location of regions of efficient cathodoluminescence across visible and near-infrared wavelengths, with contributions from material luminescence and radiative decay of electromagnetic eigenmodes. The experimental signal can further be correlated with the radiative local density of optical states in particular regions of the reconstruction. The results provide a general framework for visualizing chemical reactions and light-matter interactions with nanometer-scale resolution and in three-dimensions.