Photoluminescence studies of ZrO2:Tm3+/Yb3+ coatings formed by plasma electrolytic oxidation

Zirconia coatings doped with various contents of Tm3+ and Yb3+ were formed by the plasma electrolytic oxidation process of zirconium foils for 15 min in alkaline electrolyte containing Tm2O3 and Yb2O3 powders in different concentrations. The coatings were composed of monoclinic and tetragonal ZrO2 phases, but Tm and Yb incorporated into ZrO2 coatings induces phase transformation from monoclinic phase to tetragonal phase. The excitation photoluminescence (PL) spectra of ZrO2:Tm3+/Yb3+ coatings were featured by a broad band at around 280 nm related to zirconia host, and a peak at around 359 nm corresponding to the H-3(6)-> D-1(2) transition of Tm3+. The coatings were excited by the 280 nm and 359 nm incident light, resulting in a strong zirconia PL band, and H-3(4)-> H-3(6) down-conversion luminescence of Tm3+, respectively. With 980 nm excitation and subsequent absorption by Yb3+, an up-conversion PL was observed. Emission at 799 nm is attributed to the H-3(4) -> H-3(6) transition of Tm3+. PL spectra of samples doped with 8 different combinations of Tm3+ and Yb3+, allow the analysis of the responsible mechanisms and their presentation in the Jablonski diagram. CIE chromaticity coordinates were calculated, revealing dependence of the PL color upon the Tm3+/Yb3+ doping concentration and excitation light wavelength. By 280 nm and 359 nm excitation, the samples emit cyan and purple light, respectively.