Temperature optical sensing and color tuning in Er3+/Tm3+/Yb3+ tri-doped NaNbO3 nanoparticles under 1064 nm anti-Stokes Excitation

We report on the temperature-dependent upconversion photoluminescence of NaNbO₃ nanoparticles triply doped with Er³⁺, Tm³⁺, and Yb³⁺ ions under 1064 nm anti-Stokes excitation, a wavelength within the second biological window. Phonon-assisted absorption by Yb³⁺ ions enables efficient red, green, and blue emissions through sequential energy transfer to Er³⁺ and Tm³⁺. Contrary to conventional quenching, the emissions increase with temperature over 313–447 K. The fluorescence intensity ratio between the 554 nm (Er³⁺) and 483 nm (Tm³⁺) bands provided the best thermometric performance, yielding a maximum relative sensitivity of 0.52 %·K⁻¹ and sub-kelvin resolution (≈0.8–0.9 K). The thermometer demonstrated excellent reproducibility after multiple heating–cooling cycles, stability under continuous excitation, and independence from excitation power. TEM analysis confirmed ∼72 nm cubic particles with narrow size dispersion. Additionally, CIE chromaticity analysis revealed marked temperature-induced color shifts, confirming the multifunctionality of these nanoparticles for luminescent nanothermometry and thermally responsive photonic applications.