Modeling of optical absorption of silver prolate nanoparticles embedded in sol-gel glasses

Silver prolate nanoparticles were obtained in silica gels prepared by the sol-gel process. Heating them at 900degreesC for few minutes, the samples showed a yellow-orange color. A strong optical absorption with an asymmetric peak centred at 425 nut due to surface plasmon resonance of silver nanoparticles was observed. High-resolution transmission electron microscopy images showed silver prolate particles (average axial ratio AR = 0.76) randomly oriented with broad size distribution. The size changed from 9 to 3 mn and the prolate form changed to almost spherical (AR = 0.92) when the samples were heated longer time at 900degreesC. In these samples, the absorption peak was shifted from 425 up to 460 nm. After heat treatment, the absorption spectrum did not change any more in some months, indicating that the particles obtained through this method are stable at room temperature. The Gans theory was used to fit the experimental spectra. The fit was not good until we assumed in the calculations all the physical features come from the system such as the volume fraction, shape and size of the metallic particles, and refractive index of the silica matrix. It was necessary to consider also a refractive index that come from oxidation on the surface of the metallic particles. With these considerations the fit with the Gans theory was good enough, and the difference between the calculated and experimental spectra was very small, factor 20 better than when oxidation is ignored. So then, the oxidation from the metallic particles must be taken in account to explain the experimental absorption spectra. These results are discussed. (C) 2004 Elsevier B.V. All rights reserved.