The preparation and characterization of nanoparticle Ag–SiO2 composite films with super low refractive index

•The nanoparticle Ag–SiO2 composites were formed by annealing process of coatings.•The Ag nanoparticles are uniformly embedded into the SiO2 particles.•The optical constants of Ag–SiO2 composites were determined by using ellipsometry.•The refractive index of the Ag–SiO2 film is lower than 1.

The spin coating technique is used to prepare the porous Ag–SiO2 composite films, and the subsequent annealing process makes the nanoparticle Ag–SiO2 composite films formed. As the increase of the annealing temperature from 300 °C to 500 °C, the size of Ag–SiO2 composite nanoparticles gradually decreases. The SEM, TEM and XRD measurements of Ag–SiO2 composite films indicate that the single crystal metal nanoparticles are uniformly embedded into the SiO2 particles, forming Ag–SiO2 composite nanoparticles. The SPR absorption spectra of the Ag–SiO2 composite films show the content of metal Ag increases with the increase of annealing temperature, but when the annealing temperature increases to 500 °C, the SPR absorption is not observed on the absorption spectra although the total absorption of Ag–SiO2 composite films is enhanced. Furthermore, the optical constants of Ag–SiO2 composite films have been determined based on the measurement of ellipsometric parameters. It is more important the refractive index lower than 1 is obtained after annealing process of Ag–SiO2 composite films under 300 and 400 °C, but the extinction coefficient is low, which makes the film have low absorption.