Permittivity model selection based on size and quantum-size effects in gold films

The article explores optical properties of nanostructures containing spherical gold nanoparticles of various radii. We explore the particle radius as a criterion to select a permittivity model aimed at describing optical absorption spectra of gold granules. The experiments showed splitting of the absorption band of granular gold films to form a second absorption peak. The first peak is associated with the phenomenon of plasmon resonance, while the second one reflects quantum hybridization of energy levels in gold. Quantum effects were shown to prevail over size effects at a granule diameter of about 5-6 nm. The Mie theory gives a rigorous solution for the scattered electromagnetic field on a sphere taking into account optical properties of the latter, however, it does not specify criteria of selecting a model to calculate dielectric permittivity. Both calculations and experiments confirmed the limiting diameter of gold nanoparticles where the Hampe-Shklyarevsky model is applied. Meanwhile, this model was still unable to predict splitting of the plasma absorption band. The data presented in the article can be used for a predetermined local field enhancement in composite media consisting of a biolayer and metal nanoparticles.