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

Abstract

The hexagonal phase β-NaGdF₄:x% Tb³⁺ (x = 0 to 5%) phosphor crystals with uniform morphology and considerable monodispersity were synthesized via a tri-sodium citrate-assisted hydrothermal route. The photoluminescence studies showed the strong emission lines corresponding to ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transitions with a prominent ⁵D₄ → ⁷F₅ green emission transition at 540 nm. The as-prepared samples were annealed at different temperatures (200–800 °C), 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 revealed phase alteration to the mixed (i.e. cubic and hexagonal) phase along with morphology transformation at higher annealing temperatures. The photoluminescence emission intensity was remarkably enhanced (∼9 times) at 700 °C owing to the reduction in non-radiative processes from the surface and the volatilization of luminescence quenching (–OH) groups, which were corroborated by surface FTIR studies. The systematic study of the 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 °C, which peaked at 230 °C, signifying deeper traps. The NaGdF₄:3% Tb³⁺ phosphor showed a linear response over a very wide range of gamma doses from 100 Gy to 20 kGy, revealing that the NaGdF₄:Tb³⁺ phosphor was quite well suited for high gamma dose measurements and their respective applications. Fading and reproducibility studies showed low fading and excellent reproducibility characteristics. The T_m–T_stop 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, such as activation energy (E), frequency factor (s), and order of kinetics (b), were estimated by the Glow Curve Deconvolution (GCD) method. The Figure of Merit (FOM) value was found to be 2.2%. The trapping parameters resulting from the experimental T_m–T_stop study and theoretical GCD match closely and are in good agreement with each other. Thus, along with high luminescence efficiency, the newly explored strong thermoluminescence properties of NaGdF₄:Tb³⁺ make them potential phosphor materials and could provide new avenues into other applications in radiation dosimetry such as agriculture/food sciences, archeological sciences, space dosimetry, etc.