Tunable photoluminescence properties and energy transfer in oxyapatite-based Ca2Tb8(SiO4)6:Eu3+ phosphors for UV-LEDs

New phosphors Ca2Tb8-xEux(SiO4)6O2 with color-tunability were synthesized in this work. The Rietveld refinement indicated that the prepared phosphors were crystallized to a hexagonal structure with space group P63/m (no. 176). When Eu3+ ions were introduced into Ca2Tb8(SiO4)6O2, characteristic red-orange emissions of Eu3+ ions and the green emission of Tb3+ ions were obtained. Eu3+-doped phosphors emitted with enhanced intensity and had broader excitation spectra than the undoped Ca2Tb8(SiO4)6O2 phosphors. As Eu3+ content increased, a decline in Tb3+ emission was observed owing to resonant energy transfer from the 5D3 level of Tb3+ ions to the 5D0 level of Eu3+ ions. Moreover, the transfer of energy from Tb3+ ions to Eu3+ ions proceeded via a dipole-dipole interaction, and the critical distance for this energy transfer was calculated to be 7.2 Å. The efficiency of energy transfer in Ca2Tb8−xEux(SiO4)6O2 increased from 72% to 87% with an increase in the Eu3+ concentration, x, from 0.02 to 0.08. The emitted color of Ca2Tb8−xEux(SiO4)6O2 was tuned from green to red via controlling the concentration of Eu3+ ions. Meanwhile, the optimum phosphor system Ca2Tb7.94Eu0.6(SiO4)6O2 exhibited adequate thermal stability even at 200 °C and the corresponding activation energy was calculated to be 0.027 eV. These results suggest that Ca2Tb8(SiO4)6O2:Eu3+ may be potential phosphors for the application in UV LEDs.