Particle Shape Effects in Vibrational Electron Energy-Loss Spectroscopy

Vibrational spectra in the scanning transmission electron microscope are strongly influenced by the shape of the sample being studied. We explore these shape effects using the local phenomenological model of Born and Huang. We quantize this model and characterize the general properties of its harmonic modes to simulate electron energy-loss spectra from several regular geometries (foil, cylinder, sphere, prolate and oblate spheroid). We then extend these methods to the vibrational modes and surface plasmons of irregularly shaped particles. Finally, we show how vibrational modes of a surface molecule coupled to the surface plasmons of a metallic nanoparticle can yield Fano-type spectral profiles.