Analogy of different optical temperature sensing techniques in LaNbO4:Er3+/Yb3+ phosphor

Abstract

Three of the possible approaches to optical temperature sensing of a thermographic phosphor were formally compared in upconversion (UC) of Er3+ and Yb3+ doped lanthanum orthoniobate (LNO) phosphor. The three approaches used in the study were namely fluorescence intensity ratio (FIR) of thermally coupled levels (TCL), Valley to Peak ratio (VPR) and the ratio of non-thermally coupled levels (NTCL), respectively. The TCLs in the study were the ratio of intensities of 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 UC transitions of Er3+ ion. VPR method was verified within 2H9/2 → 4I15/2 transition of Er3+ UC. The other ratios of transitions which are not fit in TCL were investigated through NTCL technique. Ratios of several sets of transitions namely 4S3/2 → 4I15/2/2H9/2 → 4I15/2, 4S3/2 → 4I15/2/2I9/2 → 4I15/2, 4S3/2 → 4I15/2/4F9/2 → 4I15/2, 4F7/2 → 4I15/2/2H11/2 → 4I15/2, 2H11/2 → 4I15/2/4I9/2 → 4I15/2 and 2H11/2 → 4I15/2/2G11/2 were considered for NTCL technique. Moreover, the effect of temperature in the emission color was analyzed and a change from yellow (0.3310; 0.5990) to green (0.2590; 0.6740) was observed when the temperature rise has been increased from 14 to 300 K. Results obtained indicate that the LNO:Er3+/Yb3+ phosphor could be employed in optical thermometry and this phosphor would be a candidate with high potential as sensor operating from low to room temperature using TCL, VPR and/or NTCL methods. The most prominent sensing technique in the case of LNO:Er/Yb material among the three techniques is found to be NTCL with the high relative sensitivity of 1.19% K−1 at 300 K.