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 4S3/2 → 4I15/2 emission of Er3+. Population paths of the Er3+ down-conversion PL happen under resonant excitation conditions of the Er3+ from ground state 4I15/2 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 4I15/2 → 4G11/2 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 4S3/2 → 4I15/2 is much higher compared to the red 4F9/2 → 4I15/2 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 4F9/2 → 4I15/2 transition increases more rapidly with respect to green emission 4S3/2 → 4I15/2. A phonon-assisted energy back transfer process from Er3+ ions (4S3/2 → 4I13/2) to the neighboring Yb3+ ions (2F7/2 → 2F5/2) leads to enhanced red emission 4F9/2 → 4I15 and decreased green emission 4S3/2 → 4I15/2.