Characterization and optical properties of Au/SiO2 composite thin films

Au/SiO2 composite thin films were prepared by a multitarget sputtering method. The multilayer structure where the Au-particle layer was sandwiched between SiO2 layer was clearly observed in the as-deposited film by the cross-sectional TEM. The diameter of the Au particles in the film was estimated to be 2-3 nm and it began to increase when the film was heat-treated above 700 degrees C. The diameter was proportional to the cube root of the heat-treatment time, suggesting that the Au particles grew in the supersaturated solid solution. In the optical absorption spectra of the films, the absorption peak due to the surface-plasmon resonance of Au particles was observed at the wavelength of 528-544 nm. With increasing mean diameter of Au particles, the peak intensity became large and the full-width-at-half-maximum of the absorption band was decreased from 130 nm to 80 nm.