Upconversion luminescence and favorable temperature sensing performance of eulytite-type Sr3Y(PO4)3:Yb3+/Ln3+ phosphors (Ln=Ho, Er, Tm)

Weigang Liu,Xuejiao Wang,Qi Zhu,Xiaodong Li,Xudong Sun,Ji-Guang Li

doi:10.1080/14686996.2019.1659090

Abstract

ABSTRACT Phase-pure eulytite-type Sr3Y0.88(PO4)3:0.10Yb3+,0.02Ln3+ upconversion (UC) phosphors (Ln = Ho, Er, Tm) were synthesized via gel-combustion and subsequent calcination at 1250°C. Their UC luminescence, temperature-dependent fluorescence intensity ratio of thermally and/or non-thermally coupled energy levels, and performance of optical temperature sensing were systematically investigated. The phosphors typically exhibit green, orange-red and blue luminescence under 978 nm NIR laser excitation for Ln = Er, Ho and Tm, respectively, which were discussed from two- and three-photon processes. The 524 nm green (Er3+), 657 nm red (Ho3+) and 476 nm blue (Tm3+) main emissions were analyzed to have average decay times of ~52 ± 2, 260.6 ± 0.7 and 117 ± 1 μs, respectively. It was shown that (1) the Er3+ doped phosphor has a better overall performance of temperature sensing with thermally coupled 2H11/2 and 4S3/2 energy levels, whose maximum absolute (S A) and relative (SR ) sensitivities are ~5.07 × 10−3 K−1 at 523 K and ~1.16% at 298 K, respectively; (2) the Ho3+ doped phosphor shows maximum S A and SR values of ~0.019 K−1 (298–573 K) and 0.42% at 573 K for the non-thermally coupled energy pairs of 5F5/(5F4,5S2) and 5I4/5F5, respectively; (3) the Tm3+ doped phosphor has a maximum S A of ~12.74 × 10−3 K−1 at 573 K for the non-thermally coupled 3F2,3/1G4 energy levels and a maximum SR of ~1.74% K−1 at 298 K for the thermally coupled 3F2,3/3H4 levels. Advantages of the current phosphors in optical temperature sensing were also revealed by comparing with other typical UC phosphors.

Highlights

Upconversion (UC) luminescence is a process of converting low-energy light, usually near-infrared (NIR) or infrared, into high-energy light through multiple absorption and/or energy transfer [1,2,3]
Since the energy separation ΔE of such levels is usually restricted to 200–2000 cm−1 to avoid strong overlapping of two emission bands [16,17,18,19], the sensitivity of temperature sensing, which is proportional to ΔE, can hardly be further improved to a higher level according to the Boltzmann distribution
(1360°C) needed for the synthesis of Ba3La(PO4)3: Yb3+/Ln3+ (Ln = Er, Tm) via solid reaction [14], which may originate from the better cation homogeneity of sol-gel processing

Summary

Introduction

Upconversion (UC) luminescence is a process of converting low-energy light, usually near-infrared (NIR) or infrared, into high-energy light (ultraviolet or visible) through multiple absorption and/or energy transfer [1,2,3]. Since the energy separation ΔE of such levels is usually restricted to 200–2000 cm−1 to avoid strong overlapping of two emission bands [16,17,18,19], the sensitivity of temperature sensing, which is proportional to ΔE, can hardly be further improved to a higher level according to the Boltzmann distribution For this reason, the use of non-thermally coupled energy levels is being considered as an effective complement to a better sensitivity of FIR. We originally synthesized in this work a series of eulytite-type Sr3Y(PO4)3:Yb3+/Ln3+ UC phosphors (Ln = Ho, Er, Tm) via gel-combustion, followed by a thorough investigation of their UC luminescence and performance of optical temperature sensing with thermally coupled and/or nonthermally coupled energy levels. We report the synthesis, characterization and optical properties of Sr3Y(PO4)3:Yb3+/Ln3+ UC phosphors

Materials and synthesis

Characterization

Phase analysis and morphology

Conclusions