Down-conversion photoluminescence of Ta2O5:Ho3+ and up-conversion photoluminescence of Ta2O5:Ho3+/Yb3+ coatings formed by plasma electrolytic oxidation of tantalum

This study demonstrated that Ta2O5 coatings formed by plasma electrolytic oxidation of tantalum substrate in an alkaline electrolyte containing Ho2O3 nanoparticles have the potential to be used as a host for Ho3+ ions. Down-conversion photoluminescence (PL) intensity of Ta2O5:Ho3+ is strongly dependent on the concentration of Ho3+ incorporated into Ta2O5 coatings, which depends on the duration of the PEO process and the concentration of Ho2O3 nanoparticles in the electrolyte. In the PL emission spectra, dominant green PL emission bands of Ho3+ centered at around 544 nm are assigned to (S2,F4)-S-5-F-5 & RARR; I-5(8) transitions. Also observed are red and near-infrared emission bands centered at around 658 nm and 758 nm, corresponding to the F-5(5) & RARR; I-5(8) and (S2,F4)-S-5-F-5 & RARR; I-5(7) transitions of Ho3+, respectively. The PL excitation spectra of Ho3+ doped Ta2O5 coatings show a broad band from 250 to 325 nm caused by electron transfer between completely filled 2p orbitals of O2- ions and 4f orbitals of Ho3+ ions and a series of sharp peaks between 325 and 500 nm attributed to direct excitation of Ho3+ ions from the I-5(8) ground state to higher levels of the 4f-manifold. Ta2O5:Ho3+ coatings do not exhibit detectable up-conversion PL under 980 nm excitation, but Ta2O5:Ho3+/Yb3+ coatings formed in an alkaline electrolyte containing both Ho2O3 and Yb2O3 nanoparticles do. Also discernible are three groups of up-conversion PL emission bands linked to the 4f-4f transition of Ho3+ ions. With increasing Yb3+ ion concentration in coatings, there is a progressive increase in the PL intensity of all up-conversion emission bands, indicating an efficient energy transfer from excited Yb3+ ions to Ho3+ ions. The potential mechanisms for up-conversion that could result in the population of different excited levels are discussed.