Are rare earth phosphates suitable as hosts for upconversion luminescence? Studies on nanocrystalline REPO4 (RE=Y, La, Gd, Lu) doped with Yb3+ and Eu3+, Tb3+, Ho3+, Er3+ or Tm3+ ions

Rare earth phosphate nanomaterials doped with lanthanide ions (Yb3+, Ho3+, Er3+, Tm3+, Tb3+ and Eu3+) showing upconversion (UC) luminescence may play an important role in photonics, optics, electronics and in biomedical diagnostics. Year by year, the significance of rare earth compounds increases. Here, we present structural, morphological and spectroscopic investigations on REPO4 nanoparticles, which were successfully synthesized by the co-precipitation method followed by annealing at 1000°C. Obtained tetragonal (YPO4, LuPO4) and monoclinic (LaPO4, GdPO4) nanocrystals were single phase and homogeneous, with dimensions not exceeding 75 nm. These samples exhibited strong luminescence when excited in NIR range (975-978 nm) and the UC mechanisms were proposed. After initial photon absorption by Yb3+ ions, the energy transfer (ET) to emitting ions (Ho3+, Er3+, Tm3+, Tb3+, Eu3+) occurred. Luminescence rise and decay times were short with the exception for Tb3+ and Eu3+ ions, which showed up to 5 ms long luminescence. Dependencies of integral luminescence intensity on laser energy indicated that, most of the transitions were two-photon what is typical value for this kind of ions. However, some of transitions were three- and even four-photon (especially in Tm3+ ions) and some of slope values were lower than two, what was caused by the saturation and local thermal effects.