Dual-Mode Photoluminescence of Er3+ -Doped Ta2O5 Coatings Formed by Plasma Electrolytic Oxidation of Tantalum

Orthorhombic Ta2O5: Er3+ coatings were obtained by plasma electrolytic oxidation (PEO) of tantalum in an alkaline electrolyte with the addition of Er2O3 particles in various concentrations. Down-conversion photoluminescence (PL) emission spectra is featured by sharp bands attributed to 4f-4f transitions of Er3+. The most intense green PL emission band centered at 544 nm is attributed to the S-4(3/2) -> I-4(15/2) emission of Er3+. Population paths of the Er3+ down-conversion PL happen under resonant excitation conditions of the Er3+ from ground state I-4(15/2) to higher levels, and via a broad violet charge transfer excitation band. Both excitation energy and Er content have an impact on the PL emission response in Ta2O5: Er3+ coatings. The I-4(15/2). (4)G(11/2) transition is the dominant excitation pathway for the PL emission of Er3+. Up- conversion PL spectra created after 980 nm irradiation of Ta2O5: Er3+ coatings have characteristic 4f-4f transitions of Er3+. The intensity of the green S-4(3/2). I-4(15/2) is much higher compared to the red F-4(9/2) -> I-4(15/2) emission of Er3+. Co-doping Ta2O5: Er3+ coatings with sensitizer Yb3+ ions increased the up-conversion emission PL intensity of Er3+ thanks to the efficient energy transfer from Yb3+ to Er3+. The red up-conversion PL intensity of F-4(9/2) -> I-4(15/2) transition increases more rapidly with respect to green emission S-4(3/2) -> I-4(15/2). A phonon-assisted energy back transfer process from Er3+ ions (S-4(3/2) -> I-4(13/2)) to the neighboring Yb3+ ions (F-2(7/2) -> F-2(5/2)) leads to enhanced red emission F-4(9/2) -> I-4(15) and decreased green emission S-4(3/2) -> I-4(15/2).