Elsevier

Journal of Luminescence

Volume 207, March 2019, Pages 41-47
Journal of Luminescence

Temperature-dependent upconversion luminescence and spectra characteristic of Er3+/Yb3+ co-doped fluorotellurite glasses

https://doi.org/10.1016/j.jlumin.2018.10.028Get rights and content

Abstract

Er3+/Yb3+ co-doped fluorotellurite glasses were prepared. Intense green and red upconversion (UC) emissions corresponding to the transitions of 2H11/2, 4S3/24I15/2 and 4F9/24I15/2 were observed under 980 nm laser excitation, benefiting from the advantages of low phonon energy and good stability of fluorotellurite glasses. Temperature-dependent UC luminescence was carried on glasses with different doping concentration of Er3+ in the range of 298–568 K. Fluorescence intensity ratios (FIR) and absolute sensitivities (Sa) were calculated, and the maximal sensitivity value of 54.09 × 10−4 K−1 was obtained at 531 K in the glass with the lowest Er3+ concentration at 0.1 mol%. This study indicates that Er3+/Yb3+ co-doped fluorotellurite glasses can be promising materials applied to non-touching temperature sensors.

Introduction

Temperature, as a fundamental physical parameter, has been implemented in scientific research, industrial manufacture and biomedicine fields owing to its advantages in detecting objects of different scales and long distance [1], [2], [3], [4], [5]. Traditional contact temperature sensor cannot detect sub-micron scaled and contactless objects, such as in the circumstances with high voltage, high temperature, non-oxygen or corrosivity [1], [4]. However, non-contact temperature sensors overcome these disadvantages mentioned above to achieve high spatial resolution temperature detecting. Recent attention has been paid on the rare-earth (RE) doped upconversion (UC) luminescence in non-contact temperature sensing. Fluorescence intensity, effective bandwidth, peak wavelength, spectral shift, fluorescence intensity ratio (FIR) and fluorescence lifetime can be fundamental parameters to detect temperature [6], [7], [8].
Temperature sensors based on FIR of RE ions doped luminescent materials, utilizing the physical mechanism of thermal coupled energy levels (TCLs) of RE ions, are the most widely researched owing to their advantages of high precision measurements and being immune to the fluctuation noises of excitation light and external environmental disturbances [2], [3]. The upper levels and lower levels of TCLs can be depopulated by changing the temperature around samples [9], [10]. The energy gap of TCLs must be in the range of 200 cm−1–2000 cm−1 to ensure the electrons to be efficiently populated from lower levels to upper levels and there is no overlapping in emissions [11]. Generally, energy gap of Er3+:2H11/2, 4S3/2 (ΔE800cm1) can meet the requirements of this condition. In addition, Er3+ ions have broad emission bands locating from ultraviolet to infrared [12]. Er3+/Yb3+ co-doped systems can realize efficient upconversion emissions in visible region pumped by 980 nm laser, in which Yb3+ ions are used as sensitizers. On the other hand, hosts must have high RE ions solubility and low phonon energy to meet the requirements of efficient UC luminescence and high temperature sensitivity [13]. Therefore, seeking suitable materials is of great importance.
As host materials for FIR based temperature sensing, much attention has been paid on glasses. It is not only because glasses have the advantage of high RE solubility, but also because they are ease of fabrication [14], [15]. Pisarski et al. investigated optical temperature properties of Er3+/Yb3+ doubly doped lead-free fluorogermanate glasses and lead silicate glasses, which are promising for temperature sensing [15], [16]. Manzani et al. reported Er3+/Yb3+co-doped tellurite glass with high temperature sensitivity of 0.89 × 10−4 K−1 at 473 K [17]. Fluoride glasses have been intensively investigated for their low phonon energy (300–500 cm−1) [18], but fluoride is unstable and volatile. Tellurite glasses make up for the above shortcomings with their good chemical durability, thermal stability and high mechanical strength. Besides, tellurite glasses possess a wide transmission window (0.4–6 µm), high linear and nonlinear refractive induce, and low melting temperature [18], [19]. Herein, Er3+/Yb3+ co-doped fluorotellurite glasses with composition of TeO2-AlF3-NaF-BaF2-LaF3-ErF3-YbF3 were successfully fabricated by melt-quenching method. The absorption spectra, fluorescence decay lifetime and UC spectra under 980 nm excitation have been investigated. Judd-Ofelt (J-O) theory, FIR of the Er3+: 2H11/24I15/2,4S3/24I15/2, and temperature sensitivity from room temperature to 568 K have been calculated systematically. The obtained results show that the fabricated glasses are promising materials for temperature sensors.

Section snippets

Experimental

Er3+/Yb3+ co-doped fluorotellurite glasses were prepared by conventional melt-quenching method with composition of 50TeO2–10AlF3–5NaF-30BaF2–5LaF3-xErF3–4YbF3(x = 0.1, 0.5, 1, 3, 5) which were named as TAL:xEr4Yb. The high pure TeO2 (99.99%), AlF3 (99.99%), NaF (99.99%), BaF2 (99.99%), LaF3 (99.9%), ErF3 (99.9%), and YbF3 (99.99%) were used as raw materials. The raw materials were homogeneously mixed in a corundum crucible and melted at 1073 K for 1 h in an electric furnace under atmospheric

Absorption spectra and Judd-Ofelt analysis

Fig. 1 shows the absorption spectra of TAL: xEr4Yb glasses recorded in the range of 350–1700 nm. There are ten absorption bands locating at 378, 406, 452, 488, 521, 542, 654, 800, 978, 1530 nm, which are corresponding to the transitions form 4I15/2 ground state of Er3+ to 4G11/2, 2H9/2, 4F3/2 +4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2 excited states, respectively. TAL: xEr4Yb glasses exhibit an intensive absorption band at 978 nm, which can be ascribed to the overlapping

Conclusions

In summary, Er3+/Yb3+ co-doped fluorotellurite glasses were prepared by melt-quenching method. UC luminescence and temperature sensing properties based on FIR were investigated. Absorption spectra and J-O theory showed excellent spectra characteristics of TAL glasses. Moreover, under 980 nm laser excitation, the glasses exhibited intense UC emissions at 524, 548, 660 nm corresponding to the 2H11/2, 4S3/2, 4F9/24I15/2 transitions, expectively. The UC luminescence under different pump power

Acknowledgements

This work was supported by the International S&T Cooperation Program of China (No. 2014DFA52000), National Natural Science Foundation of China (No. 11574021, 11574017, 51372008), Special Foundation of Beijing Municipal Science & Technology Commission (Grant No. Z161100000216149), and the Fundamental Research Funds for the Central Universities (YWF-18-BJ-J-18).

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