Scattering vs Interference in Interferometric Scattering Spectroscopy of Plasmonic Nanoparticles

Interferometric scattering (iSCAT) is a powerful tool to study single plasmonic nanoparticles (NPs), particularly when the particles become too small to be observed by their scattering signal alone. This sensitivity to NP size makes the technique a promising tool to monitor dynamic morphological changes in NPs in electrochemical or other reactive environments. However, because the signal measured in iSCAT consists of both the NP scattering and its interference with a reflected reference field, the role of the substrate and local environment can have an outsize influence, leading to significant differences between iSCAT and dark-field scattering spectra, even for large particles where scattering is expected to dominate. In this work, we show that the iSCAT contrast spectra of gold NPs can be tuned between scattering- or interference-dominated regimes by changing the refractive index of the embedding medium, the reflectivity of the substrate-medium interface, and the size of the NP. We compare the iSCAT spectra to dark-field scattering spectra to show how the interference contribution can shift spectral features away from the plasmon resonance and use a dipole oscillator model to explain the observed spectral lineshapes. Lastly, we demonstrate the need to measure the iSCAT signal at multiple illumination wavelengths during electrodissolution experiments to extract kinetic parameters that are representative of the NP's morphological changes.