Alkali Silicates Codoped with NIR-Emitting RE (Nd³⁺ and Yb³⁺) Ions for Thermometry Applications

In the present study, the synthesis of BaSrSiO₄ co-doped Yb³⁺ and Nd³⁺ nanophosphors (NPs) was successfully achieved through the conventional sol-gel method, as confirmed by X-ray diffraction and SEM analysis, verifying the formation of pure NPs. The FTIR and Raman spectra analysis confirm the formation of silicates, as different modes and vibrations of Si-O and Si-O-Si were seen at 800-1000 cm^-1. The energy transfer (ET) mechanism between Nd³⁺ and Yb³⁺ ions was seen as the emission spectra showed a rise in intensity of one over another. PLE emission spectra showed transitions at ²F_7/2-²F_5/2 for Yb³⁺ and from ⁴F_3/2 to (⁴I_9/2, ⁴I_11/2, and ⁴I1_3/2) for Nd³⁺ when excited at 785 nm. All the samples record low activation energy, which shows that the rate of reaction will be higher in all the samples, and it will be highest for 1 mol% Nd³⁺ and 1 mol% Yb³⁺. An increasing value of τ was seen with increasing Yb³⁺ concentration, which confirms the increase in the population of trap centers. The positron annihilation lifetime (PAL) curve showed that 1 mol% Yb³⁺ and 2 mol Nd³⁺ have single vacancies or shallower positron traps, whereas 3 mol% Yb³⁺ and 2 mol% Nd³⁺ have larger defects like surface oxygen vacancy clusters. The other two samples have balance vacancies, which makes them best for thermometry applications. The fluorescence intensity ratio (FIR) was calculated to get sensitivity for thermometry application. 2.13% K^-1 sensitivity achieved at 303-333 K temperature.