Visible Emission on Nanostructured CeO₂ Thin Films Obtained by Spray Pyrolysis

Herein, the influence of the flow rate, substrate temperature, and thermal annealing at 700 °C on the optical and structural parameters of nanostructured ceria thin films prepared by ultrasonic spray pyrolysis is evaluated. The morphology, structure, and optical properties are studied by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL). The spray conditions are optimized for obtaining smooth, dense, and homogeneous nanocrystalline films with grain sizes smaller than 20 nm. XPS demonstrates a high percentage (≈40%) of Ce³⁺ in the films, even in heat-treated films (≈25%). The Ce³⁺ concentration does not depend on the growth temperature. Intense peaks at 575 and 618 nm are observed in PL measurements. These peaks are not reported in pure ceria thin films, they are dependent on the growth parameters (director and carrier gas flow rates and substrate temperature), and they are associated with levels produced by oxygen defects in the films. Variations in the flow rate modify the optimal temperature and the kinetic reaction, which also modifies the preferential orientation and the defect distribution in the films.