Tunable localized surface plasmon resonance absorption in silver-based nanoplasmonics

Nanoplasmonic surfaces built up from silver nanoparticles (NPs) and core–shell Ag@Ag₂O NPs with highly tunable optical properties are synthesized along two different reaction channels. Their structural properties are studied in detail and new in-depth physical insights into their tunable localized surface plasmon resonant absorption (LSPR) are provided. Classical electrodynamics model with size-modified complex dielectric constant data for Ag predicts narrow symmetrical LSPR band in monodisperse population of small (5–20 nm) Ag NPs with a red shift of the band maximum position (λ_m) of 5 nm upon particle size increase from 5 to 20 nm. Compared to our experimental data, the predicted λ_m is too low by 54 nm (356 vs. 410 nm). In line with our experiments, the LSPR band undergoes inhomogeneous broadening when NPs size dispersion is included, accompanied with a shift of λ_m from 356 to 371 nm (still 40 nm below the experimental value). Excellent agreement with our experiments (λ_m shift to 410 nm and inhomogeneous broadening with high-wavelength side asymmetry) is achieved applying the effective medium approach with experimentally determined size dispersion, which accounts for the influence of the glass substrate and interparticle coupling in the close-packed Ag NPs on the LSPR absorption. When the synthesis is carried out along sonochemically induced reaction channel, Ag₂O shell is formed around the Ag core, leading to the most prominent LSPR band shift (~ 90 nm), which we attribute to a change in effective refraction index from 1.5 to 2.2.