Fractal characteristics of TiO2-Ag nanocomposite films deposited by a grid-assisted co-sputtering method

TiO2-Ag composite films are deposited on glass substrates by a novel grid-assisted co-sputtering method, aiming at preparing films with different silver contents. The films are characterized by energy-dispersive X-ray spectroscopy, field emission scanning electron microscopy, atomic force microscopy (AFM), ultraviolet-visible spectroscopy, and photoluminescence spectroscopy. Furthermore, the parameters of saturation roughness, fractal spectra, and permutation entropy are utilized to analyze the AFM images; the permutation entropy is employed in a 2D matrix to measure the complexity. The atomic ratios of silver to titanium range from 0.09 to 7.80, and the observed optical band gaps are in the range of 3.17 to 3.26 eV. Photoluminescence spectroscopy reveals the photoinduced-electron-trapping effect of Ag particles in the structure and surface of the films governs the emission intensities. The AFM images show the roughness of the films increases by increasing the Ag content, and it can be seen that an increase in the TiO2 content of the films results in an increase in fluctuation per area, hence increasing the permutation entropy. Therefore, it is proposed that the Ag content of the films could be predicted by surface roughness and permutation entropy measurements; the fractality of the data allows a more precise determination of silver content.