Circularly Polarized Luminescence from Pure and Eu-Doped Trigonal TbPO₄·nH₂O Nanocrystals

In this contribution, we describe the preparation, by means of a precipitation reaction from aqueous solution at 40 °C, and the structural characterization of nanocrystalline powders of trigonal Tb_1-EuPO₄·nH₂O (with x = 0, 0.005, 0.01, 0.05, and 0.1; n tentatively assigned as 0.67) which crystallize in the two possible P3₁21 or P3₂21 enantiomorphic space groups. While the volume of the crystal lattice is not significantly affected by the Tb³⁺/Eu³⁺ substitution, the average crystallite size seems to depend on the Eu³⁺ dopant concentration and ranges from 13 to 30 nm. The desired handedness of the crystals has been induced by using, during the synthesis, one of the two possible enantiomers of tartaric acid (l or d). The analysis of the luminescence excitation and emission spectra, together with the decay kinetics of the ⁵D₄ Tb³⁺ excited state, suggests the presence of a very efficient Tb³⁺ → Eu³⁺ energy transfer process in the Eu³⁺-doped orthophosphates. Upon excitation of Tb³⁺ ions at 368 nm, the enantiomorphic powders grown with l- or d-tartaric acid (i.e., l-TbPO₄·0.67H₂O/d-TbPO₄·0.67H₂O, l-Tb_0.995Eu_0.005PO₄·0.67H₂O/d-Tb_0.995Eu_0.005PO₄·0.67H₂O, and l-Tb_0.9Eu_0.1PO₄·0.67H₂O/d-Tb_0.9Eu_0.1PO₄·0.67H₂O) exhibited mirror circularly polarized luminescence signals in the visible spectral region (in the green and/or in the red).