Effect of Filler Concentration and Film Thickness on Structure and Optical Properties of Poly(p-Xylylene)−Cadmium Sulphide Nanocomposites

Abstract

The effect of filler concentration (C ≈ 0–100 vol %) and film thickness (d ≈ 0.02, 0.2, 0.5, and 1.0 μm) on the optical absorption spectra and surface morphology of thin nanocomposite films based on poly(p-xylylene) and cadmium sulphide (PPX–CdS) has been studied. The PPX–CdS films are prepared by low-temperature vapor deposition polimerization on quartz and silicon substrates. A nonmonotonic dependence of the absorption spectrum red shift on the filler concentration is revealed. The red shift of the spectrum reaches a maximum at some critical concentration of the filler C₀. It is observed that the value of the critical concentration C₀ increases with the film thickness and equals C₀ ≈ 11, 30, and 50 vol % for d ≈ 0.02, 0.5, and 1 μm, correspondingly. The average size of the nanoparticles is evaluated by an analysis of the absorption spectra (via estimating the exciton-peak wavelength). It is found that the shift in the absorption spectra is determined by a variation in the nanoparticle size. Atomic force microscopy reveals the effect of the filler content on the surface morphology of the polymer matrix in nanocomposite films and their surface roughness. The size distribution of the polymeric grains is evaluated. The most significant changes in the absorption spectra and surface morpology of the composites with a variation in the filler content are observed in the filler concentration range from 0 to C₀. The absorption spectra of the composites with the filler concentration above C₀ are similar, which can be attributed to the almost equal nanoparticle size in these nanocomposites. The correlation between changes in the matrix morphology and filler optical properties is established.