A comparative study of spectral and temperature sensing properties of Er3+ mono-doped LnNbO4 (Ln = Lu, Y, Gd) phosphors under 980 and 1500 nm excitations

LnNbO₄:Er³⁺ (Ln = Lu, Y, Gd) phosphors were prepared through a conventional solid-state reaction method. The phase purity of the samples was characterized by X-ray diffraction (XRD). A comparative study of spectral and temperature sensing properties of LnNbO₄:Er³⁺ (Ln = Lu, Y, Gd) phosphors was presented. The excitation spectra of the samples show a strong band in the region of 200–350 nm coming from the NbO₄⁴⁻, thus indicating efficient energy transfer from the host to Er³⁺. From the dependence of upconversion (UC) luminescence spectra on laser diode working current, it was confirmed that 2- and 3-photon processes were respectively responsible for both the green and the red UC emissions under 980 and 1550 nm excitations. The temperature sensing properties for LnNbO₄:Er³⁺ were compared under individual excitation of 980 and 1550 nm. It was found that the temperature sensing was independent from the excitation wavelength, but LuNbO₄:Er³⁺ phosphors yielded the highest luminescence intensity and maximum temperature sensitivity.