Targeted dielectric coating of silver nanoparticles with silica to manipulate optical properties for metasurface applications

An epsilon-negative metamaterial (ENM) containing core@shell nanoparticles (NPs) was designed, where silver (Ag) NPs served as core and silica (SiO₂) was used as spacer shell. AgNPs were synthesized in large scale, using microwave-assisted polyol method, in three average particle sizes, as 30, 54, and 61 nm, with a narrow particle size distribution. Optical absorption of Ag NPs was investigated using UV–Vis spectroscopy. Their optical behavior was also theoretically predicted for different thicknesses of the SiO₂ shell immersed in media of different refractive indices using the Clausius - Mossotti equation. Based on the results, optimal outputs were obtained with a SiO₂ shell of 10 nm in thickness encompassing 54 nm Ag NPs based on the analytical model and numerical simulations here developed for core-shell structures. Then 10 nm SiO₂ shell was grown on 54 nm Ag NPs by sol-gel synthesis. The NPs were then characterized by UV–Vis, TEM, SEM, EDX, DLS, and zeta potential analyses. The synthesized core-shell NPs can be used to establish epsilon-negative properties in polymer layers within visible range of wavelengths.