Intrinsic optical properties of Ag-doped poly-(vinyl alcohol) nanocomposites: an analysis of the film thickness effect on the plasmonic resonance parameters

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The optical properties of silver nanoparticles originating from the collective oscillations of their conduction electrons (LSPR) play a major role in the development of nanotechnology-based optical devices, from optical sensors to bactericide materials via non-linear optical devices. In this study, nanocomposites polymer films embedding silver nanoparticles were prepared by thermal annealing of poly-(vinyl alcohol) films doped by silver nitrate using a “one-pot synthesis” method. In this upscalable solution-based process, the nanoparticles are directly grown in the polymer matrix, yielding a plasmonic nanocomposite with a high density of nano-objects. Low (2.5% w:w) and high (25% w:w) doping concentration of silver were considered as well as their effect on the optical properties of thin (30 nm) and thick (300 nm and more) films. The topography and the optical properties (refractive index n and extinction coefficient k) of such films were studied by atomic force microscopy and spectroscopic ellipsometry. Unexpectedly, for a given doping level, the parameters of the surface plasmon resonance (amplitude, position and width) were shown to be thickness-dependent. A multivariate statistical analysis technique was used to identify the differences in the optical behavior of thick and thin films.