Photoluminescence of ZnO:Eu³⁺ and ZnO:Tb³⁺ coatings formed by plasma electrolytic oxidation of pure zinc substrate

Abstract

Eu3+ and Tb3+ doped ZnO optical coatings were formed by the Plasma Electrolytic Oxidation (PEO) method from the pure Zn sheet, with various dopant concentrations. The morphologies show no significant change with the introduced dopant or their concentrations, with typical appearance of PEO formed coatings. All elements are uniformly distributed, and concentration of incorporated dopants is related to the concentrations of the Eu2O3 or Tb4O7 particles added to the basic electrolyte. Wurtzite structure of ZnO was identified in all the samples, with no change induced by the dopants. Under 260 nm excitation ZnO emits broad-band in the visible region attributed to oxygen interstitial defects and oxygen vacancies. Photoluminescence (PL) of ZnO:X3+ (X = Eu, Tb) is a sum of PL from ZnO and 4f–4f dopant emissions. Eu3+ and Tb3+ are most effectively excited in charge-transfer band and 4f5d levels, respectively. By increasing concentrations of the dopants the broad PL decreases in favor of the 4f–4f dopant transitions, an indication of the energy transfer. ZnO:Eu3+ spectra are dominated by the hypersensitive 5D0→7F2 transition. Eu3+ ions are incorporated in a highly asymmetric environment. PL emission at 541 nm is the most intense in ZnO:Tb3+. Judd-Ofelt intensity parameters rise with increasing Eu3+ concentration, up to Ω2 = 7.8·10−42 cm2 and Ω4 = 4.4·10−42 cm2. Radiative lifetime of 5D0 level drops to 1.12 ms. Chromaticity of PL varies with concentration only when the samples are irradiated by UV light, from white to orange-red or green for Eu3+ or Tb3+ doped samples, respectively. Eu3+ under 394 nm and 464 nm irradiation gives 99.5% color-purity.