Enhanced vibrational electron energy-loss spectroscopy of adsorbate molecules

Plasmonic excitations in metallic nanoparticles are known to depend strongly on nanoparticle size and shape. Here we explore how such excitations can be harnessed to enhance the vibrational signals from molecules adsorbed on nanoparticle surfaces, as detected using electron energy-loss spectroscopy in the scanning transmission electron microscope. We use the Born-Huang formalism in the electrostatic approximation to develop a theoretical model for electron energy-loss from an adsorbed molecule (represented by a point dipole) on the surface of a dielectric nanoparticle. We find that the adsorbate contribution to the energy-loss spectrum is approximately proportional to the square of the electric field at the adsorption site, and hence we find that the enhancement of the molecule's vibrational signal is greatest for molecules adsorbed on small, sharp nanoparticles. When the molecular frequency is near one of the nanoparticle frequencies, we generally find an asymmetric Fano-type spectrum line shape whose asymmetry is attributable to multimodal contributions. Our calculations for a molecule adsorbed on the tip of a prolate spheroidal silver nanoparticle predict that signal enhancements of several hundred times should be readily achievable, and up to several thousand times if the nanoparticle's plasmonic mode is "tuned" to the molecular frequency. Such enhancement effects potentially make vibrational STEM-EELS a powerful tool for the characterization of surface-functionalized nanoparticles and nanomaterials used for chemical sensing.