Substrate and coverage engineered visible-range Fano resonance in SiO₂@C core-shell nanoparticles

Substrate-mediated Fano-like resonance in dielectric core-shell nanoparticles (NPs) offers a promising pathway for tailoring light-matter interactions, yet the role of surface coverage-dependent aggregation remains underexplored. Here, we systematically investigate the optical responses of SiO₂@C core-shell NP films on metallic (Cu) and non-metallic (Si) substrates using spectroscopic ellipsometry combined with critical-point analysis. On Cu substrates, increasing NP coverage (6 ± 2% to 27 ± 4%) drives a transition from isolated clusters to interconnected networks, inducing visible-range (300∼500 nm) Fano-like resonance through Mie-plasmon coupling. In contrast, Si substrates promote large-area sheet-like films (8 ± 3% to 35 ± 10%) that suppress resonance due to diminished scattering heterogeneity and dominant substrate electronic transitions. The COMSOL simulations confirm that larger clusters amplify magnetic quadrupole contributions, aligning with experimental spectral evolution. Additional validation on fused quartz substrates highlights asymmetric Fano signatures at high coverage, emphasizing substrate-driven modulation. These findings advance the design of dielectric core-shell NPs for tunable light-harvesting and nanophotonic applications, providing mechanistic insights into substrate-mediated resonance engineering.