Synthesis and optical characterization of Tm3+ doped CaAl4O7 for near-UV LED-based white light

A blue light emitting phosphor Ca1−xAl4O7:Tmx3+ was synthesized by the conventional Pechini sol-gel method. Synthesized polymeric resin was calcined at 800, 850, 900, 950 and 1000 °C for 2 h. The samples were analyzed with powder XRD, TG-DSC, UV-vis-NIR (diffuse reflectance spectra - DRS) and spectrofluorophotometer. Thermal degradation of the derived gel was observed up to 400 °C and the desired phase formation initiates from 800 °C. XRD pattern confirms the monoclinic phase of CaAl4O7:Tm3+ phosphors with a=12.860 Å, b=8.865 Å, c=5.396 Å, and β=107.06°. Diffuse reflectance and photoluminescence excitation spectral studies reveal the corresponding absorption and emission properties of trivalent state of thulium ion. The major emission peak at 460 nm corresponding to 1D2→3F4 of Tm3+ ions was observed. The decrease in emission intensity and luminescence life-times with increase of activator concentration was observed and it is mainly due to the phonon - assisted cross - relaxation processes between Tm3+ ions. The energy transfer mechanism between the nearby Tm3+ ions was analysed by fitting the non-exponential fitting curve by IH model and the results were discussed. The critical distance for the energy transfer of the phosphor is calculated to be 6 Å. Emission in the blue region is expected and the same can be evidenced in the CIE chromaticity diagram with high color purity (73%). Concentration quenching occurs with Tm3+ at 5 mol%. The obtained CaAl4O7:Tm3+ phosphors have potential applications in the area of near UV based white light emitting diodes.