Influence of Surface Coverage on the Optical Properties of SiO₂@C Core–Shell Nanoparticle Films by Spectroscopic Ellipsometry

This study investigates the influence of Mie scattering and image charge effects on the optical properties of SiO2@C core–shell nanoparticle (NP) films with varying surface coverage ratios on silicon substrate. The NP films were prepared using a drop-coating self-assembly method and systematically characterized by TEM, SEM, and ellipsometry. As the number of drops increased from 1 to 6, the surface coverage ratio rose from 7.5 ± 2.5% to 34.8 ± 10.2%, transitioning the NP distribution from isolated particles to a continuous film. Ellipsometric data were analyzed using both a layered model and a layered-Effective Medium Approximation (EMA) model. At low coverage levels (1–3 drops, <20%), the derived NP volume ratio closely matched the specific surface area ratio (Ssa), indicating weak interparticle interactions. At higher coverage (4–6 drops, >20%), stronger NP interactions led the volume ratio to approach the surface coverage projective ratio (Scp). Analysis of the dielectric function revealed that at low coverage, image charge effects from the Si substrate induced magnetic dipole resonances in the NPs via Mie scattering, resulting in a negative imaginary component of the dielectric function. At high coverage, the formation of a continuous NP film attenuated substrate effects, and the dielectric properties gradually approached those of intrinsic SiO2. This work reveals the synergistic influence of image charge and Mie scattering mechanisms at varying coverage levels and demonstrates an optical phase transition in the films, providing a foundation for the development of tunable nanoparticle-based optical coatings.