Er³⁺/Yb³⁺ co-doped La₂O₃–TiO₂–Gd₂O₃ glasses with upconversion luminescence properties prepared by aerodynamic container-less levitation

Container-less levitation technology, originally developed for simulating microgravity conditions in space, has become a crucial method for preparing advanced materials free from contamination and heterogeneous nucleation caused by containers. This technique is particularly valuable for synthesizing heavy-metal oxide glasses, which are challenging to produce using conventional melting methods due to their poor glass-forming ability and the need for rapid cooling. This study used aerodynamic levitation to fabricate novel Er³⁺/Yb³⁺ co-doped La₂O₃–TiO₂–Gd₂O₃ (LTG) glasses and systematically investigated how different Gd₂O₃ concentrations affected the thermal stability and upconversion luminescence properties of the glasses. The goal was to develop high-performance upconversion materials for potential applications in solid-state lasers, optical temperature sensors, and biological labeling systems.

Glasses with the nominal composition (La_0.78−xGd_xEr_0.04Yb_0.18) Ti_2.25O₆ (where x = 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, or 0.35%) were prepared using an aerodynamic levitation furnace. High-purity oxide powders were thoroughly mixed, pressed into rods, and introduced into the levitator. The samples were levitated and melted using a CO₂ laser in an oxygen atmosphere, followed by rapid cooling to form spherical glass beads with a diameter of ~3 mm. The amorphous nature of the obtained glasses was confirmed through X-ray diffraction (XRD). Thermal properties, including the glass transition temperature (T_g), onset crystallization temperature (T_c), and crystallization peak temperature (T_p), were determined through simultaneous thermal analysis (STA) at a heating rate of 10 K/min. Upconversion luminescence spectra were recorded under 980 nm laser excitation using a fluorescence spectrophotometer equipped with a photomultiplier tube.

XRD patterns confirmed the amorphous structure of all prepared LTG samples. DTA results indicated that T_g, T_c, and T_p decreased with increasing Gd₂O₃ concentration. The thermal stability parameter (ΔT = T_c − T_g), which reflects the glass-forming ability, also decreased from 59.4 ℃ to 42.6 ℃ as Gd₂O₃ concentration increased from 0.10% to 0.35%, suggesting a reduction in the thermal stability and glass-forming tendency with increasing Gd₂O₃ concentration. Under 980 nm laser excitation, intense green and red upconversion emissions were observed. The emission bands centered at 535, 553, and 672 nm corresponded to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, respectively. The green emission (553 nm) was notably more intense than the red emission across all compositions. The integrated upconversion luminescence intensity initially increased with increasing Gd₂O₃ concentration and peaked at x = 0.15% before decreasing. Furthermore, the ratio of green-to-red emission intensity exhibited a nonmonotonic trend: it first increased and then decreased, indicating that high Gd₂O₃ concentrations favored red emissions over green emissions. The upconversion mechanism involved ground-state absorption and energy transfer processes from Yb³⁺ to Er³⁺, followed by excited-state absorption, leading to the filling of high-energy levels and subsequent radiative transitions.

This study successfully demonstrated the effectiveness of aerodynamic levitation for fabricating Er³⁺/Yb³⁺ co-doped LTG heavy-metal oxide glasses, which are difficult to produce by conventional methods. Gd₂O₃ significantly affected both the thermal and luminescent properties of the glasses. Although increasing Gd₂O₃ concentration reduced thermal stability, it enabled the upconversion emission intensity and color ratio to be adjusted, with optimal luminescence performance achieved at a Gd₂O₃ concentration of x = 0.15%. The strong green emissions underscore the potential of these LTG glasses for various photonic applications. The combination of container-less processing and strategic compositional design provides a robust approach for developing new functional glass materials with tailored properties.