Geometric Effects of Plasmonic Nanoscale Heterostructures on Infrared Activity

Electron probes can resolve bright and dark optical modes at subwavelength scales to distinguish localized effects, e.g., those of composition and geometry, via energy loss measurements. In this work, electron energy loss spectra of a metal nanospheroid (NS) near a van der Waals material were simulated to show effects of NS shape and structure on plasmon and exciton energies. Hollowing or elongating the NS intensified and shifted its plasmon bright and dark mode energy losses. Simultaneous hollowing and elongation intensified and redshifted bifurcated bright modes more than adding effects of separate alterations, whereas the dark mode intensified additively and redshifted subadditively. Proximity to a transition metal dichalcogenide (TMD) nanodisk differentiated redshifting of bright modes (more) and dark (less) modes and fractured the modes across multiple spectral features. Some bright and dark mode energies were pinned at TMD exciton energies. Measured optical spectra exhibiting such effects corresponded to simulation. Only simultaneous hollowing and elongation above a TMD nanodisk redshifted primary components of each bright and dark mode entirely into the near-infrared (NIR) biological water window. Simulating energy electron loss spectra identifies nanoheterostructure geometry and composition that enhances bright- and dark-mode activity at biologically transparent NIR energies to potentiate bio/catalytic activity.