Structure-property relationships for Eu doped TiO2 thin films grown by a laser assisted technique from colloidal sols

Abstract

Photoactive europium doped titanium dioxide (Eu:TiO₂) thin films were grown by a matrix assisted pulsed laser evaporation technique. TiO₂ and Eu cation-adsorbed TiO₂ nanoparticles (NPs) sols were used as starting materials. The sols were synthesized by a colloidal sol–gel route. In order to obtain solid targets, the sols were cooled down until solidification in liquid nitrogen. The irradiation of the solid targets was performed in a controlled oxygen atmosphere using an UV KrF* (λ = 248 nm, τ_FWHM ≤ 25 ns, ν = 10 Hz) excimer laser source. The NPs were transferred and deposited onto solid substrates placed in the front of the frozen targets, forming continuous thin films. The as-grown undoped and Eu:TiO₂ thin films were submitted to post-deposition thermal treatments. The surface morphology, crystalline structure, and chemical composition of the thin films were characterized by field emission scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Functional optical properties of the films were investigated by UV-VIS-NIR spectroscopy, spectroscopic ellipsometry, and photoluminescence as a function of dopant concentration in the TiO₂ host and annealing temperature. A direct correlation was established between the processing conditions, films structure, chemical composition and oxidation states studied both at the top surface and in-depth as well as their functional, optical properties. Eu doping induces a blue-shift of the absorption edge with respect to the as-grown samples. Under visible light excitation (λ = 488 nm, emitted by an Ar⁺ laser source), the Eu:TiO₂ samples show characteristic photoluminescence corresponding to the ⁵D₀ → ⁷F_i (i = 0, 1, 2, 3, 4) transitions of Eu⁺ ions in the visible spectral range. The samples treated at the highest annealing temperature are characterized by strong photoluminescence emission, high transmittance in the VIS-NIR spectral regions, and high refractive index.