Optical studies on sputter-deposited Ag-SiO₂ nanoparticle composites

Silicon dioxide with nano-size Ag (Au or Cu) particles embedded on its surface has unique optical properties due to the phenomenon known as surface plasmon resonance (SPR). SPR is the resonance of conduction band electrons which generate electromagnetic waves propagating along the dielectric/metal interface. In this study, Ag/SiO₂ films with different Ag layers (3/5/7/10 nm) were deposited on glass in sequence by magnetron sputtering. Various silver particle sizes were then created by the subsequent rapid thermal annealing (RTA). The RTA was carried out at elevated temperatures from 450 to 600 °C over different periods. All samples were examined by field-emission scanning electron microscope (FESEM) for their surface morphology. For optical properties, the films' transmittance and reflectance were measured with wavelengths ranging from 300 to 900 nm by a UV-VIS-NIR spectrophotometer. Experimental results show that for longer annealing time (15 min) and thicker deposited Ag film, larger Ag particles can be produced. These larger Ag particles on SiO₂ surface serve as the resonators of electrons to result in a broader absorption under the visible light irradiation. Following the experimental results, the Mie theory for a small particle in a dielectric medium can be used to estimate the spectrum of absorption. However, such estimate needs an equivalent dielectric constant of medium (SiO₂ + air) as input a priori. A newly proposed fitting scheme based on the Mie solution was tested. This scheme yielded a good fit to the experimental absorbance and simultaneously gave a reasonable estimate on the effective dielectric constant.