Spectro-ellipsometric studies of Au/SiO2 nanocomposite films

Au/SiO2 nanocomposite films in which Au nanoparticles were embedded in SiO2 matrix were fabricated via alternating sputtering of SiO2 and Au layers. In particular, we kept the nominal thickness of the Au layers below the threshold for the continuous layer formation whereas that of SiO2 layers was sufficient to form continuous layers. Transmission electron microscopy (TEM) images revealed that the Au nanoparticles were spherical in shape and that their average size was linearly proportional to the nominal thickness of Au layers. To account for the absorption variation of the composite films, including the absorption peak around 530 nm due to the surface plasmon resonance (SPR), we used Maxwell-Garnett effective medium theory (EMT) together with the modified Drude model. As it turned out, the combination of the modified Maxwell-Garnett effective medium theory and the aforementioned modification of the Drude model, which took the size of Au nanoparticles into consideration, were sufficient to model the variable-angle spectroscopic ellipsometry spectra. The values of the fitting parameters, such as film thickness, volume fraction of Au, and the diameter of Au nanoparticles, extracted from the modeling procedure were in good agreement with the results of TEM and spectrophotometer measurements.