Multi-channel lifetime thermometry and security films based on Ln (Ln = Sm³⁺, Tb³⁺, and Sm³⁺/Tb³⁺) doped NaKLaNbO₅ phosphors

In this report, we synthesized a series of Sm³⁺ and Tb³⁺ single-/co-doped NaKLaNbO₅ phosphors using a solid-state reaction technique and studied their crystal structures, elemental compositions, morphologies, and photoluminescence properties (e.g., excitation/emission spectra, concentration-quenching mechanisms, thermal stabilities, chromaticity coordinates, and luminescence dynamics). The optimal doping concentrations of the NaKLaNbO₅:Sm³⁺ and NaKLaNbO₅:Tb³⁺ phosphors were determined to be 0.05 and 0.3 mol, respectively. Additionally, the concentration quenching mechanisms of both the NaKLaNbO₅:Sm³⁺ and NaKLaNbO₅:Tb³⁺ phosphors were verified as electric dipole–dipole interactions. Subsequently, a typical NaKLaNbO₅:0.05Sm³⁺/0.3Tb³⁺ phosphor was prepared and used for multi-channel luminescence lifetime thermometry based on different excitation wavelengths and luminescence centers. Notably, the relative sensitivity values of the NaKLaNbO₅:0.05Sm³⁺/0.3Tb³⁺ phosphor were calculated to be 0.106 % K⁻¹@463 K (λ_ex = 250 nm, λ_em = 550 nm), 0.074 % K⁻¹@463 K (λ_ex = 250 nm, λ_em = 665 nm), 0.072 % K⁻¹@463 K (λ_ex = 284 nm, λ_em = 550 nm), and 0.091 % K⁻¹@303 K (λ_ex = 284 nm, λ_em = 665 nm), respectively, suggesting that the excitation wavelength and luminescence center affect lifetime sensitivity. Furthermore, the NaKLaNbO₅:0.3Tb³⁺ powder-based films were tested for security applications.