Yb3+ co-doped fluorotellurite glasses

Journal of Luminescence 207 (2019) 41–47

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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

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

T

Xin Huang, Jiaming Liu, Heng Pan, Chengcai Tian, Hao Zhang, Xiaojuan Chen, Anping Huang, ⁎ Zhisong Xiao Key Laboratory of Micro-nano Measurement, Manipulation and Physics (Ministry of Education), School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Fluorotellurite glass Er3+/Yb3+ co-doped Upconversion Temperature sensing

Er3+/Yb3+ co-doped fluorotellurite glasses were prepared. Intense green and red upconversion (UC) emissions corresponding to the transitions of 2H11/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 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.

1. Introduction

upper levels and there is no overlapping in emissions [11]. Generally, energy gap of Er3+:2H11/2, 4S3/2 ( ΔE ≈ 800cm−1) can meet the requirements of this condition. In addition, Er3+ ions have broad emission bands locating from ultraviolet to infrared [12]. Er3+/Yb3+ codoped 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

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 [1–5]. 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 [6–8]. 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

⁎

Corresponding author. E-mail address: [email protected] (Z. Xiao).

https://doi.org/10.1016/j.jlumin.2018.10.028 Received 6 June 2018; Received in revised form 1 September 2018; Accepted 8 October 2018 Available online 14 October 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.

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melting temperature [18,19]. Herein, Er3+/Yb3+ co-doped fluorotellurite glasses with composition of TeO2-AlF3-NaF-BaF2-LaF3-ErF3YbF3 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+: 2H11/2→4I15/2,4S3/2→4I15/2, 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.

comparing with the tellurite glasses previously [18], which is due to the shift of the absorption onset of the glass to lower wavelength. According to the obtained data from absorption spectra and JuddOfelt (J-O) theory, the experimental oscillator strength ( fexp ) can be expressed by the following formula [20]:

fexp =

me c 2 πe 2λ 2N

2

∫ α (λ) dλ = πem2λe c2N

×

1 0.43L

∫ OD (λ) dλ

(1)

where L is the thickness of the sample, N is the concentration of RE ions, λ ̅ is the average wavelength of the linear factor, c is the light velocity, m is the electron mass, e is the electron charge, α(λ) and OD(λ) are the absorption coefficient and optical density, respectively. The expression for the theoretical oscillator strength is [20]:

2. 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 pressure. After that, the molten glass was poured into a preheated stainless-steel mold and annealed at 623 K for 10 h to release internal stress. Finally, the obtained glasses were cut and polished into a size of 15 × 10 × 3 mm3. Optical absorption spectra at room temperature was recorded on a UV-VIS-NIR spectrophotometer (Cary 5000, Agilent). The fluorescence decay lifetime was measured by a spectrometer (FLS980, Edinburgh). Visible UC spectra was performed on a spectrometer (SR500I-B1, ANDOR).

fcal = fed + fmd =

Sed = Smd =

∑λ,2,4,6 Ωλ

2 2 8π 2mc ⎛ (n + 2) S + χ S ⎞⎟ ⎜ ed md md 3h (2J + 1) λ ⎝ 9n ⎠

ΨJ ‖U (λ) ‖Ψ ′J ′

h2 ΨJ ‖L + 2S‖Ψ ′J ′ 16π 2m2c 2

(2)

2

(3) 2

(4)

h is the Plank constant, n is the refractive index, J and J′ are the total angular momentum quantum numbers of energy levels, Ωλ (λ = 2,4,6) is defined as J-O parameter. ΨJ ‖U (λ) ‖Ψ ′J ′ and ΨJ ‖L + 2S‖Ψ ′J ′ correspond to the reduced matrix elements of electronic-dipole transition and magnetic-dipole transition respectively [20]. Sed and Smd are line intensities of electric dipole and magnetic dipole transitions. By assuming fcal = fexp , six intense absorption bands, from 4I15/2 to 4F7/2, 2 H11/2, 4S3/2, 4F9/2, 4I9/2, 4I13/2, were taken into calculation to determine Ωλ (λ = 2, 4,6) by a least-square fitting approach [21], shown in Table 1. The intense absorption band at 980 nm was excluded because of the overlapping absorption band of Yb3+ at 2F5/2 level and Er3+ at 4I11/2, which will cause a large calculation error [20]. The rootmean-square deviation between the experimental and calculated oscillator strength is given by:

3. Results and discussions 3.1. 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 4 G11/2, 2H9/2, 4F3/2 +4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4 I13/2 excited states, respectively. TAL: xEr4Yb glasses exhibit an intensive absorption band at 978 nm, which can be ascribed to the overlapping transitions 4I15/2→4I11/2 of Er3+ and 2F7/2→2F5/2 of Yb3+ [18]. It can be induced that there exists energy transfer between Er3+ and Yb3+. We also observed an additional absorption band at 378 nm

Δrms =

∑ (fexp − fcal )2 q−p

(5)

where q is the number of absorption bands involved in fitting, and p = 3 is the number of the parameters determined. J-O parameters of TAL:0.1Er4Yb and TAL:1Er4Yb glasses were obtained to be Ω2 = 15.17 × 10–20 cm2, Ω4 = 7.61 × 10–20 cm2, Ω6 = 1.02 × 10–20 cm2 and Ω2 = 22.02 × 10–20 cm2, Ω4 = 5.76 × 10–20 cm2, Ω6 = 3.62 × 10–20 cm2, respectively. Ω2 reflects the environmental symmetry and the covalency between RE ions and surrounding anions. The value of Ω2 is large for the prepared glasses, indicating the strong covalent bonds of Er3+ and its high asymmetric potential in these glasses [21]. χ = Ω4 / Ω6 is defined as the spectroscopic factor, which is used to predict the stimulated emission in a laser active medium [22]. The larger value of χ presents, the stronger intensity of the laser transition [23,24]. For TAL:xEr4Yb glasses (x = 0.1,1), χ were calculated to be 7.46 and 3.82, respectively. It is shown that the TAL glasses exhibit high qualities of spectroscopic characteristics. As it is shown in Table 2, the spontaneous radiation rates A(J → J ′) , fluorescence branch ration β(J → J ′) and radiation lifetime τrad can be obtained as well by the following formulas:

AJ → J ′ = Aed + Amd =

β(J → J ′) =

τrad =

Fig. 1. Absorption spectra of TAL:xEr4Yb glasses from 350 nm to 1700 nm. 42

2 2 64π 4e 2 ⎡ n (n + 2) S + n3S ⎞⎟ ed md 3⎢ 9 3h (2J + 1) λ ⎣ ⎠

(6)

A(J → J ′) ∑J ′ A(J → J ′)

1 ∑J ′ A (J , J ′)

(7)

(8)

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Table 1 Experimental and calculated oscillator strengths for TAL: xEr4Yb (x = 0.1,1) glasses. λ (nm)

Transitions

TAL:0.1Er4Yb

TAL:1Er4Yb

−6

fexp (× 10 I15/12→4F7/2 I15/12→2H11/2 4 I15/12→4S3/2 4 I15/12→4F9/2 4 I15/12→4I9/2 4 I15/12→4I13/2 4

488 522 544 652 798 1532

4

−6

)

fcal (× 10

4.252 37.443 0.584 8.147 0.976 5.378 Δrms= 2.537 × 10−6

6.049 37.466 0.518 7.205 1.583 1.798

fexp (× 10−6)

fcal (× 10−6)

5.589 35.913 1.273 7.534 0.785 5.138 Δrms= 1.018 × 10−6

6.587 35.924 1.479 7.134 1.244 4.310

transitions from 4F9/2 to 4I15/2 (660 nm) are approximately contributed by two-photon involved process. The slopes are smaller than 2 because of the saturation effect which reduces the power dependence of emissions [28,29]. When glasses are pumped at lower power, as it is shown in the region before black line of Fig. 2(d), the values of the slopes are smaller than the high pump power region owing to the lower enhancement of UC emission rate [26]. The values of n vary at 524, 548, 660 nm emissions, which means the red-to-green ratios change as well with pump power. This phenomenon is mainly ascribed to the varied pump power induced temperature changes, leading to the populations change in energy levels [7]. The fluorescence decay curves of 2H11/2, 4 S3/2 and 4F9/2→ 4I15/2 (Er3+:524, 548, 660 nm) transitions excited at 980 nm are shown in Fig. 3. All the decay lifetimes were calculated shown in Table 3. With the increasing concentration of Er3+, the lifetimes should be increased if the Er3+ could be pumped effectively by 980 nm laser. However, the lifetimes decreased with the increasing concentration of Er3+. It can be deduced that the energy transfer from Yb3+ to Er3+ is dominant at 980 nm excitation rather than the Er3+→ Yb3+ back-transfer [30,31]. Fig. 4 shows the possible energy level diagrams of Er3+/Yb3+ co-doped TAL glasses, in which the UC energy transfer mechanisms are proposed. Under 980 nm excitation, electrons at Er3+: 4I15/2 and Yb3+: 2F7/2 are both excited to excited states:4I11/ 3+ ), 2F5/2(Yb3+) respectively by ground state absorption (GSA). 2(Er Since Yb3+ has a broader absorption cross-section at 980 nm than Er3+, the ETU processes from Yb3+ to Er3+ are highly efficient [32,33]. Yb3+

3.2. UC emission and decay lifetime The UC emission spectra recorded from 450 nm to 750 nm of TAL:xEr4Yb glasses pumped by 980 nm laser is shown in Fig. 2(a). Three obvious emission bands at 524 nm, 548 nm and 660 nm are observed in the spectra, which are assigned to the transitions from excited states 2H11/2, 4S3/2, 4F9/2 to ground state 4I15/2, respectively. According to Fig. 2(b), it is obviously shown that with the increase of Er3+ concentration, green and red emission intensities are both increased until the molar concentration of Er3+ reaches maximum at 1 mol%. This phenomenon can be ascribed to the concentration quenching of Er3+ ions. The distances between Er3+ ions get closer with the increase of Er3+ concentration, which will increase the probabilities of multiphonon relaxation and lead to luminescence quenching eventually [25]. In order to further confirm the energy transfer mechanisms between Er3+ and Yb3+, the UC emission spectra under different pump power are carried under 980 nm excitation, as it is shown in Fig. 2(c). The emission intensity I and pump power P follow the following relation [26,27]: (9)

I ∝ Pn

)

where n is the pump photon required to populate the emitting states. The slopes of Fig. 2(d) represent the numbers of how many photons to be responsible for the emitting states. The values of n for the green and red emissions are fitted to be 1.75,1.40,1,30 respectively. This suggests that emissions arising from 2H11/2, 4S3/2 to 4I15/2 (524 nm, 548 nm) and

Table 2 Spontaneous radiation rates, radiative lifetimes, and fluorescence branch rations for the energy levels of Er3+ in TAL:xEr4Yb (x = 0.1,1) glasses. Transitions

4 F7/2→4F9/2 →4I9/2 →4I11/2 →4I13/2 →4I15/2 2 H11/2→4F9/2 →4I9/2 →4I11/2 →4I13/2 →4I15/2 4 S3/2→4I9/2 →4I11/2 →4I13/2 →4I15/2 4 F9/2→4I9/2 →4I11/2 →4I13/2 →4I15/2 4 I9/2→4I11/2 →4I13/2 →4I15/2 4 I11/2→4I13/2 →4I15/2 4 I13/2→4I15/2

λ (nm)

1948 1264 974 716 488 2616 1516 1117 790 522 1717 1222 841 544 3602 1948 1132 652 4299 1659 798 2673 976 1532

TAL:0.1Er4Yb

TAL:1Er4Yb

A (s−1)

β%

τrad (ms)

A (s−1)

β%

τrad (ms)

18.144 278.904 706.097 2201.349 26,782.960 73.553 280.547 237.179 457.495 64,204.840 120.566 65.059 938.041 4443.598 12.045 134.737 218.153 5564.055 2.103 87.361 607.773 40.683 338.615 203.423

0.0605 0.930 2.355 7.341 89.314 0.113 0.430 0.363 0.701 98.393 2.166 1.169 16.849 79.817 0.203 2.273 3.679 93.845 0.302 12.530 87.169 10.726 89.274 100

0.033

20.485 362.037 711.014 1710.449 10,225.250 87.890 338.096 247.394 447.921 31,539.260 159.065 95.305 1311.054 3552.034 14.620 197.798 226.318 4559.754 2.659 136.887 513.853 57.168 449.975 294.562

0.157 2.779 5.457 13.128 78.479 0.269 1.035 0.757 1.371 96.567 3.108 1.862 25.619 69.410 0.292 3.957 4.528 91.223 0.407 20.950 78.643 11.273 88.727 100

0.076

0.015

0.180

0.169

1.434

2.636 4.916

43

0.031

0.195

0.200

1.530

1.972 3.395

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Fig. 2. (a) UC emission spectra of TAL:xEr4Yb glasses ranging from 450 nm to 750 nm pumped by 980 nm laser; (b) Emission intensities with the increase of Er3+ concentration locating at 524, 548, 660 nm; (c) The UC emission spectra of the TAL:1Er4Yb glass under adjustable power excitation of 980 nm; (d) In–In plots of the UEC emission intensity vs. the excitation power for the TAL:1Er4Yb glass.

ions transfer their energy to Er3+ via ETU:2F5/2(Yb3+) + 4I15/2(Er3+) → 2F7/2(Yb3+) + 4I11/2(Er3+). Then electrons at 4I11/2(Er3+) are promoted up to 4F7/2 energy level by ETU and excited state absorption (ESA): 2F5/2(Yb3+) + 4I11/2(Er3+) → 2F7/2(Yb3+) + 4F7/2(Er3+). Afterwards, Er3+ ions at 4F7/2 are quickly relaxed to 2H11/2 ,4S3/2 and 4 F9/2 states through multi-photon relaxation. It is worth noting that redto-green ratios differ dramatically with different Er3+ concentration, and the red emission is more pronounced in the high Er3+ doped sample, which is mainly due to the decreased distances and stronger interactions between Er3+ ions. The populations of Er3+:4F9/2 become

dominant through cross-relaxation (CR) process of Er3+: 4F7/2 + 4I11/2 → 4F9/2 +4F9/2 [30,31,34,35]. 3.3. Temperature-dependent UC emission and temperature sensing behaviour In order to explore the possible applications of Er3+/Yb3+ co-doped TAL glasses in temperature sensing, temperature-dependent UC emission spectra was measured in the range of 298K–568K excited by 980 nm laser, as depicted in Fig. 5. It can be observed that the two

Fig. 3. Fluorescence decay curves of TAL:xEr4Yb glasses at (a) 524 nm: 2H11/2→4I15/2 (b) 548 nm: 4S3/2→4I15/2, (c) 660 nm:4F9/2→4I15/2. 44

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Table 3 Fluorescence decay lifetimes of TAL:xEr4Yb (x = 0.1, 0.5,1,3,5) glasses at 524 nm, 548 nm, 660 nm. Sample

TAL: TAL: TAL: TAL: TAL:

xEr3+4Yb3+ xEr3+4Yb3+ xEr3+4Yb3+ xEr3+4Yb3+ xEr3+4Yb3+

x

Lifetime at 524 nm (µs)

Lifetime at 548 nm (µs)

Lifetime at 660 nm (µs)

0.1 0.5 1 3 5

205.845 168.989 132.560 86.898 74.770

208.148 207.910 137.589 90.250 76.179

295.728 234.298 195.028 145.555 141.820

Fig. 6. Fluorescence intensity ratio (FIR) as a function of inverse temperature of TAL:xEr4Yb glasses (x = 0.1,0.5,1,3,5).

constant. Absolute temperature, Boltzmann constant and energy gap between the two energy levels are symbolized by T, kB, and ΔE. On basis of Eq. (10), FIR as a function of inverse absolute temperature was fitted shown in Fig. 6. According to the fitting data, the energy gap ΔE and the pre-exponential constant B were obtained for each sample listed in Table 4, which are vital parameters for calculating the absolute sensor senstivity by the following equation [38,39]:

Fig. 4. Mechanism for UC and energy transfer processes of Er3+/Yb3+ codoped TAL glasses under 980 nm excitation.

Sa =

emission bands locating at 524 nm and 548 nm show dependence on the temperature due to the TCLs of Er3+: 2H11/2, 4S3/2. When the temperature increases, populations at 2H11/2 state increase while populations at 4S3/2 state decrease. This phenomenon matches well with Boltzmann's law. According to Boltzmann's law, the FIR between TCLs can be expressed by the following equation [36,37]:

FIR =

ωH AH gH I524 −ΔE ⎞ = = B exp ⎛ I548 ωS AS gS ⎝ kB T ⎠ ⎜

d (FIR) ΔE ⎞ ΔE −ΔE ⎞ = FIR ⎛ =B exp ⎛ 2 kB T 2 dT ⎝ kB T ⎠ ⎝ kB T ⎠ ⎜

⎟

⎜

⎟

(11)

The calculated sensor sensitivity curve as a function of temperature is shown in Fig. 7. The absolute sensor sensitivity reaches its maximal value to 54.09 × 10−4 K−1 at 531 K for TAL:0.1Er4Yb glass. Comparing with other Er3+/Yb3+ co-doped materials, this value is higher than lead-free fluorogermanate glasses (42.5 × 10−4 K−1) [16], Sr2Bi4Ti5O18 ceramics (42.0 × 10−4 K−1) [40], and β-NaYF4 nanocrystals (48.4 × 10−4 K−1) [41]. In general, the maximal absolute sensitivities decrease with the increase of Er3+ concentration. The reabsorption mechanism, which will cause the radiative transfer processes, can be avoided if the sample is doped at a very low concentration. Hence, the sample with a low Er3+ concentration is more likely to present a larger absolute sensitivity [42]. Moreover, in order to

⎟

(10)

where I524 and I548 are the integrated UC intensities corresponding to the 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions, g, ω, and A represent degeneracy, angular frequency of fluorescent transition from the 2H11/2 or 4S3/2 excited state to the 4I15/2 ground state, and spontaneous radiation rate of the corresponding transition, respectively. B is the

Fig. 5. (a) Temperature-dependent UC emission spectra (b) Intensities at 524, 548 nm recorded from 298 K to 568 K excited by 980 nm laser of TAL:0.1Er4Yb glass. 45

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be expressed by the following equation through J-O parameters [10,13,42]:

Table 4 Values for the pre-exponential constant B, energy gap ΔE between 2H11/2 and 4 S3/2, and the maximum sensitivity as a function of Er3+ concentration. Er3+ concentration (mol%)

B

ΔE (cm−1)

Maximum sensitivity Smax (× 10−4 K−1)

Tmax (K)

0.1 0.5 1 3 5

10.49 4.95 4.48 4.18 4.26

724.6 620.4 565.5 557.8 676.9

54.09 29.76 29.62 27.98 23.53

531 449 408 403 480

2

B=

∑λ = 2,4,6 Ωλ 4I15/2‖U (λ) ‖2H11/2 ω2 4 0.1758Ω2 + 0.4138Ω4 × = 2 4 ω1 0.2225Ω6 ∑λ = 2,4,6 Ωλ 4I15/2‖U (λ) ‖4S3/2 +

0.0927 0.2225

(13) 4

Since the energy level at S3/2 has only one nonzero reduced matrix elements Γ6, 2H11/2 has no nonzero reduced matrix element, but larger Γ2 which is more sensitive to the environment, it is considered that transition from 2H11/2 to 4I15/2 is more hypersensitive than transition 4 S3/2→4I15/2 [43]. Ω2 is called short-range parameter which is strongly associated with the covalent chemical bonding or the local structural distortion of RE. Ω4 and Ω6 are long-range parameters linked with bulk natures of hosts, like rigidity and basicity. Ω2 is more sensitive to the environment of lanthanide ions [44]. According to Eq. (11) and Eq. (13), coefficient B is mainly determined by Ω2, which means that the Ω2 exerts great influence on Sa. According to the J-O parameters calculated by J-O theory listed in Table 1, Er3+/Yb3+ co-doped TAL glasses are considered to present high absolute sensitivities in theory. 4. 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, 4 F9/2→4I15/2 transitions, expectively. The UC luminescence under different pump power explained the two-phonon absorption process. Decay lifetime measurements showed that with the increase of Er3+ concentration, lifetimes of Er3+: 524, 548, 660 nm decrease, which confirmed the efficient energy transfer from Yb3+ to Er3+. Finally, temperature dependent UC emissions were recorded and analyzed based on the thermal coupled levels of Er3+: 2H11/2, 4S3/2. The FIR of Er3+/Yb3+ co-doped fluorotellurite glasses was monitored to estimate the temperature sensing abilities. The absolute sensitivities of glasses with increased Er3+ concentration were further calculated. The highest value of absolute sensitivity was found to be 54.09 × 10−4 K−1 at 531 K for the sample with the lowest Er3+ concentration at 0.1 mol%. The obtained results provided Er3+/Yb3+ co-doped fluorotellurite glasses potential applications in non-touching temperature sensors.

Fig. 7. Absolute temperature sensitivities S for TAL:xEr4Yb glasses ranging 298–568 K.(x = 0.1,0.5,1,3,5).

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).

Fig. 8. Relative temperature sensitivities SR for TAL:xEr4Yb glasses ranging 298–568 K.(x = 0.1,0.5,1,3,5).

compare the sensitivities of different materials, relative sensitivities were derived by the following calculation [38]:

SR =

ΔE kB T 2

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As it is shown in Fig. 8, all the relative sensitivities remain the same tendency. They decrease with the increase of absolute temperature. They are supposed to be exactly the same since they are only determined by the ΔE between 2H11/2 and 4S3/2, while the discrepancies may arise from the calculations of experimental ratios and their derivations [42]. The energy gap between TCLs differs little in different materials due to the protection of outer electron shells (5s25p6) to inner 4f electrons [13]. While coefficient B is quite different in different hosts, which can 46

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