Microscopic and macroscopic investigation of localized surface plasmons on Ag nanoparticles embedded in porous TiO₂ glass

We have macroscopically and microscopically investigated the localized surface plasmons (LSPs) on Ag nanoparticles embedded in porous TiO₂ glass. We have prepared TiO₂ glass containing Ag nanoparticles (Ag/TiO₂) by the sol-gel process and changed the porosity of the TiO₂ glass by drying under a high vacuum condition. Using a spectrometer, we have macroscopically measured the absorption spectra (ABS) on all Ag nanoparticles and using near-field scanning optical microscopy (NSOM), we have microscopically measured the scattering spectra on a single Ag nanoparticle in the Ag/TiO₂ after each drying process. Fitting the spectra derived from the numerical calculation based on the Mie theory to that evaluated from the ABS and the NSOM measurements, we have evaluated the deviation of the full width at half maximum evaluated by the experimental measurements from that derived from the theoretical calculation (ΔFWHM) and the bound (Hashin-Shtrikman bounds) of the porosity of the TiO₂ glass by the use of the Hashin Shtrikman limit, similar to the literature [1]. It has been found that the ΔFWHM of the scattering spectra on the single Ag nanoparticle can be much smaller than one of the ABS on all Ag nanoparticles in the Ag/TiO₂. This result is due to that the ABS is influenced by the aggregation and the size distribution of all Ag nanoparticles. Furthermore, on extrapolating from the plot of ΔFWHM vs porosity, the ΔFWHM is reduced to zero when the porosity is about 15%.