Visible Emission on Nanostructured CeO₂ Thin Films Obtained by Spray Pyrolysis

Abstract

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 Ce 3þ in the films, even in heat-treated films (%25%). The Ce 3þ 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.

Strong texture tuning along different crystalline directions in glass-supported CeO2 thin films by ultrasonic spray pyrolysis

“…In conclusion, in the explored parameter ranges, higher temperatures and fluxes and lower precursor concentration favour faster reaction kinetics, which trigger texture along the (200) direction, while lower temperature and flux, as well as higher precursor concentration, favour slower reaction kinetics, promoting texture along the (111) direction. This result overcomes preceding studies regarding texture control of CeO 2 thin films on glass substrates by USD 13 , but it is consistent with ceria grown on silicon substrates 16 . This is an interesting result that suggests that texture is more influenced by the control of grown conditions than by the nature of the substrate.…”

“…Thus, the crystallite size determination via the Scherrer formula should be read as a minimum and not necessarily as a value well adhered to the real size. When the crystallite sizes are compared with films grown on crystalline (silicon) substrates 16 , similar grain size values are obtained for the (111) direction, but higher values are obtained for the other directions. This could be related to the ceria tendency to grow in a columnar structure.…”

“…However, it is very important to maintain a quasi-laminar flow, as turbulence can produce a coarsening of drops, which decreases the homogeneity of the deposition process. On the other hand, the director gas enables an increase in the kinetic energy of droplets arriving on the heated substrate but in the same way reduces the time required for droplets to approach the substrate 16 . Films were deposited onto glass slices, which were ultrasonically cleaned in different steps with trichloroethylene (C 2 HCl 3 ), acetone (C 3 H 6 O) and isopropanol (C 3 H 8 O).…”

“…Several physical and chemical processes have been employed to prepare CeO 2 thin films, including flash evaporation 7 , electron-beam evaporation 8 , sputtering 9 , laser ablation 10 , spin coating 11 and spray pyrolysis 12 – 16 . Among these techniques, spray pyrolysis is very attractive for its low cost and effectiveness in the deposition of high-quality thin oxide films 17 , 18 .…”

“…As ultrasonic spray pyrolysis is highly dependent on many deposition parameters, more studies are required to fill some of the gaps that remain in previous work. On the other hand, García Sánchez et al evaluated the influence of flow rates and substrate temperature on the structural, chemical and optical properties of CeO 2 thin films prepared by USP using a system with two flow rates 16 . They found that flow rate variations modified the substrate temperature, crystalline orientation and Ce 3+ /Ce 4+ ratio in the material.…”