Investigation of photoluminescence and novel thermoluminescence dosimetric properties of NaGdF4: Tb3+ phosphors

The hexagonal phase β-NaGdF4: x % Tb3+ (x=0 to 5%) phosphor crystals with uniform morphology and considerable monodispersity were synthesized via tri-sodium assisted hydrothermal route. The photoluminescence studies showed the strong emission lines corresponding to 5D4 → 7FJ (J = 6, 5, 4, 3) transitions with prominent 5D4 → 7F5 green emission transition at 540 nm. The as-prepared samples were annealed at different temperatures (200-800 ᵒC), and the effect of thermal treatment on their structural, morphological, surface, photoluminescence and thermoluminescence properties were studied. A comparative study of pristine and annealed samples performed revealed phase alteration to mix (i.e. cubic and hexagonal) phase along with morphology conversion at higher annealing temperature. The photoluminescence emission intensity enhanced remarkably (~ 9 times) at 700 ᵒC owing to reduction in non-radiative processes from surface and volatilization of luminescence quenching (–OH) groups, which was correlated with surface FTIR studies. The systematic study on optimum concentrations of impurities and thermal treatment were studied to achieve high thermoluminescence (TL) efficiency. The best TL dosimetric glow curve was obtained for NaGdF4: 3 % Tb3+ annealed at 700 ᵒC, peaked at 230 ᵒC signifying deeper traps. NaGdF4: 3 % Tb3+ phosphor showed linear response over a very wide range of gamma dose from 100 Gy to 20 kGy revealing the NaGdF4: 3 % Tb3+ phosphor were quite well suited for high gamma dose measurements and their respective applications. Fading and reproducibility studies showed low fading and excellent reproducibility characteristics. Tm-Tstop technique in combination with the Initial Rise Method (IRM) was used to identify the number of overlapped TL glow peaks and trap level analysis. The kinetic parameters as activation energy (E), frequency factor (s), and order of kinetics (b) were estimated by Glow Curve Deconvolution (GCD) method. The Figure of Merit (FOM) value is found to be 2.2 %. The trapping parameters resulting from experimental Tm - Tstop study and theoretical GCD matches very well and is in good agreement with each other. Thus, along with high luminescence efficiency, the newly explored strong thermoluminescent properties of NaGdF4: Tb3+ make them a potential phosphor material and promise as research frontier to provide new avenue into other applications in radiation dosimetry such as agriculture/food sciences, archeological sciences, space dosimetry etc. Keywords: Phosphors, NaGdF4: Tb3+, photoluminescence, thermoluminescence, trapping parameters