Promising lanthanide-NaYbF4:Er3+@NaYbF4:Tm3+micro-phosphors for highly efficient upconversion luminescence and temperature sensing

In recent years, the fluorescence intensity ratio (FIR)-based technique for optical non-contact temperature measurement is of great interest to researchers due to its promising wide range of applications in high-voltage electricity fields, fires, corrosive environments, biological tissues, as well as other fields. Unfortunately, the FIR techniques based on thermally coupled energy levels (TCLs) often have the problem of low sensitivity as a result of the narrow energy gap. In contrast to FIR techniques with TCLs, the FIR technique with non-thermal coupled energy levels (NTCLs) is no longer restricted by the energy gap and is expected to achieve high signal resolution. In this study, we attempted to acquire a thermometric material that possesses high sensitivity as well as high signal resolution with the help of NTCLs. We successfully prepared NaYbF 4 :E r3+ @NaYbF 4 :Tm 3+ coreshell microcrystals by solvothermal technique using NaYbF 4 as matrix and Er 3+ and Tm 3+ as luminescent centers and made necessary investigations on their crystal structures, microscopic morphologies, intrinsic mechanisms, and thermometric properties. The sample phosphors in a 980 nm laser show bright upconversion (UC) luminescence with emission peaks corresponding to the characteristic energy level jumps of Er 3+ as well as Tm 3+ , respectively. The temperature sensing characteristic parameters have been investigated by FIR techniques with TCLs ( 2 H 11/2 -> 4 I 15/2 (Er 3+ )/ 4 S 3/2 -> 4 I 15/2 (Er 3+ )) & NTCLs ( 3 H 4 -> 3 H 6 (Tm 3+ )/ 1 G 4 -> 3 H 6 (Tm 3+ ), 3 H 4 -> 3 H 6 (Tm 3+ )/ 2 H 11/2 -> 4 I 15/2 (Er 3+ ), 3 H 4 -> 3 H 6 (Tm 3+ )/ 4 S 3/2 -> 4 I 15/2 (Er 3+ ) and 3 H 4 -> 3 H 6 (Tm 3+ )/ 4 F 9/2 -> 4 I 15/2 (Er 3+ )), respectively. It is shown that the maximum absolute & relative sensitivities are 0.5677 K -1 (495 K) as well as 1.20 % K -1 (445 K) respectively with good thermal stability. In addition, the NTCLs have good separation of the emission bands, which enables the material to have excellent signal discrimination in temperature detection. These results indicate that the material is expected to be a potential candidate for optical thermometers.