Tailoring Y2Ge2O7: Tm3+ nanophosphors for dual-mode photonic functionality: for high-sensitivity near-infrared thermometry and blue light emission

Abstract

Nanocrystalline Y2Ge2O7 powders doped with 0.2 to 4 mol% Tm3+ were synthesized by a sol–gel route and heat-treated at 1100 °C for 1 h. XRD confirmed the formation of a single-phase tetragonal structure with successful Tm3+ substituting Y3+ sites. Diffuse reflectance spectra showed high transparency in the visible region and an optical bandgap of ∼6 eV, unaffected by dopant concentration. Under UV excitation, the powders exhibited intense deep-blue emission at 455 nm (1D2 → 3F4), with optimal intensity and an internal quantum yield of ∼11.8% at 0.5 mol% Tm3+. All samples displayed colour purity >98% and a CCT of ∼1770 K. Dexter model analysis indicated that dipole-dipole interactions govern concentration dependence. The 1D2 excited state exhibits monoexponential emission intensity decay, with the lifetime decreasing from 22 μs to 8 μs when the dopant increases from 0.2 to 4 mol% Tm3+. Temperature-dependent blue emission yielded an activation energy of 0.193 eV for 0.5 mol. % Tm3+ sample using the Arrhenius approach. Conversely, the broadband NIR emission around 1480 nm (3H4 → 3F4 transition) exhibits negative thermal suppression, with up to 30% enhancement for 1 mol% Tm3+. The luminescence intensity ratio (LIR)-based thermometry using the ratio between infrared emissions bands at 1375 nm and 1518 nm arising from Stark sublevels of the 3H4 → 3F4 transition, which achieved a relative thermal sensitivity (SR) = 0.7 % K−1 at room temperature for the 0.5 mol% Tm3+. The accuracy of measurements is supported by thermal resolution of 1.83 K at 293 K, improving to 0.4 K at 317.5 K and remaining below 2 K throughout the physiological range for the 0.5 mol% Tm3+ composition. These results indicate that Tm3+-doped Y2Ge2O7 is a promising bifunctional photonic material, combining efficient deep-blue emission with Boltzmann-type NIR thermometry performance in the third biological window.