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 (S_sa), 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 (S_cp). 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.