(Invited) Influence of Nd⁺ Doping and Thermal Annealing on Luminescent Properties and Thermal Sensing of Na₂Ti₆O₁₃ Nanocrystals.

This study investigates the effects of Nd⁺ doping and thermal annealing (at 250, 500, 650, and 800 °C) on the structural and luminescent properties of Nd⁺-doped Na₂Ti₆O₁₃ nanocrystals (NCs), with a focus on their potential for thermal sensing applications. The optimal doping concentration was found to be 0.5 wt% Nd⁺, where luminescence intensity decreases with higher concentrations due to concentration quenching. Thermal annealing significantly enhances both the crystallinity and luminescence intensity of the NCs, with the most notable improvements observed up to 500 °C. However, heating beyond 650 °C induces a phase transition from Na₂Ti₆O₁₃ to TiO₂, which impacts the NCs' structural and luminescent properties. Thermal sensing performance was evaluated using the fluorescence intensity ratio (FIR) between emissions at 1060 nm and 1340 nm across a temperature range of 300-343K, revealing the highest relative thermal sensitivity (S) of 3.28% K⁻¹ in the sample annealed at 250 °C. For applications requiring high emission intensity, the 0.5 wt% Nd⁺-doped Na₂Ti₆O₁₃ NCs annealed at 800 °C exhibited the highest figure of merit, combining high luminescence intensity at 1060 nm with excellent S, making them ideal for nanothermometry. Notably, the excitation (808 nm) and emission wavelengths (900, 1060, and 1340 nm) fall within the biological tissue windows, suggesting significant potential for biological nanothermometry applications. This study underscores the critical role of optimizing both doping concentration and thermal annealing conditions to enhance the properties of NCs, offering new insights into their use for advanced thermal sensing applications in biological and medical fields.