Er3+ codoped La2O2S phosphor

Er3+ codoped La2O2S phosphor

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Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint

Optical thermometry based on the upconversion fluorescence from Yb3 þ /Er3 þ codoped La2O2S phosphor Yanmin Yanga,n, Chao Mia, Fang Yua, Xianyuan Sua, Chongfeng Guob,n, Gang Lia, Jiao Zhanga, Linlin Liua, Yanzhou Liua, Xiaodong Lia a College of Physics Science and Technology, Hebei University, Baoding 071002, China National Key Laboratory of Photo electric Technology and Functional Materials (Culture Base) in Shaanxi Province, National Photoelectric Technology and Functional Materials & Application of Science and Technology International Cooperation Base, Institute of Photonics & Photon-Technology and Department of Physics, Northwest University, Xi'an 710069, China b

Received 16 February 2014; received in revised form 20 February 2014; accepted 20 February 2014

Abstract The fluorescence intensity ratio (FIR) of the two green emission bands centered at 524 and 547 nm in Yb3 þ /Er3 þ co-doped La2O2S phosphor was studied as a function of temperature in the range 300–573 K for optical temperature measurement. Upconversion phosphor La2O2S: Yb3 þ , Er3 þ show intense double-band green emission at low pump power, and the maximum sensor sensitivity derived from the FIR of the upconversion green emissions was approximately 0.008 K  1. This indicates that the phosphor has potential application in optical temperature sensors. Based on the Judd–Ofelt theory, an interpretation on how the factors influence the sensitivity of temperature sensing was proposed for the first time. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Optical sensing and sensor; Rare-earth-doped materials; Temperature; Upconversion

1. Introduction Yb3 þ /Er3 þ codoped upconversion (UC) fluorescence materials are of interest for optical temperature sensors, as was first discussed by Berthou and Jorgensen in ZBLA glass [1]. In this method, the sample's temperature is determined by measuring the signal ratio of two fluorescence lines associated with excited states of trivalent rare earth ions, such as the two energy levels 2H11/2 and 4S3/2 in Er3 þ ions, and the signal processing of a fluorescent sensor is simplified [2–7]. An important requirement for practical applications of the fluorescence intensity ratio (FIR) method is the selection of an appropriate material that must present a high efficient fluorescence signal and a good thermal stability to work in a high n Corresponding authors at: Hebei University, College of Physics Science and Technology, Wusi East Road 180, Baoding, Hebei 071002, China. Tel.: þ 86 159 3028 4830; fax: þ86 312 5079423. E-mail address: [email protected] (Y. Yang).

temperature area. Due to the efficient energy transition between Yb3 þ and Er3 þ , different kinds of materials with Yb3 þ /Er3 þ co-doped have been extensively studied [8–20], but ideal material for optical temperature sensing is still in short supply. It has been recognized that the matrix has a great influence on the fluorescence properties of the rare-earth doped material. On the basis of lattice relaxation theory, the lower the phonon energy of the matrix where rare earth ion is embedded in, the stronger the intensity of the UC emission . Up to now, NaYF4 is still recognized as the best UC phosphor host due to its low phonon energy. However, it is not ideal for the temperature sensor due to the lower sensitivity of an optical thermometer [8,9]; Er3 þ doped fluoride glasses exhibit a characteristic of higher sensitivity for temperature sensing, but its unstable chemical nature limits the practical application in high temperature measurement [10–12]. Although the oxides can work well in high temperature zone on account of the stable chemical properties, a lower fluorescence efficiency

http://dx.doi.org/10.1016/j.ceramint.2014.02.081 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Y. Yang, et al., Optical thermometry based on the upconversion fluorescence from Yb3 þ /Er3 þ codoped La2O2S phosphor, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.02.081

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partially limited the potential application in an FIR technique [13–17]. Thus, looking for a proper matrix material with high fluorescence efficiency and high stability for practical application is much meaningful. According to M. Pokhrel's research [21], Yb3 þ /Er3 þ codoped La2O2S phosphor showed the higher UC quantum yield than Yb3 þ /Er3 þ co-doped NaYF4 phosphor at lower excitation density; meanwhile, sulfur oxides have better chemical properties for practical applications than traditional fluoride materials, which makes Yb3 þ /Er3 þ co-doped La2O2S phosphor as a promising material in optical temperature sensing. However, no studies are available on the properties of temperature sensing for Yb3 þ /Er3 þ co-doped La2O2S phosphor. Otherwise, the higher sensitivity is required for temperature sensors in practical applications. Though many papers on the sensitivity of temperature sensing based on UC fluorescence have been reported, which kind of material has higher sensitivity or which factors affect the sensitivity of temperature sensing? no report has been given on these problems.

Fig. 1. XRD pattern of the La2O2S: 3% Er3 þ , 1% Yb3 þ sample.

Fig. 2. (a) The UC emission spectrum of La2O2S: Yb3 excitation power.

þ

In the present paper, we report the character of temperature sensing of Yb3 þ /Er3 þ co-doped La2O2S phosphor. The factors affecting the sensitivity of temperature sensing were given and discussed. The highest sensitivity is predicted according to the data obtained from literatures. 2. Experimental procedure La2O2S: 3% Yb3 þ , 1% Er3 þ phosphors were prepared by a high temperature solid state method. The starting materials include La2O3, Yb2O3, Er2O3 (Sigma-Aldrich, all 99.999%) and S (powder, 99.5%). The nominal composition of the sample was La1.92Yb0.06Er0.02O2S. First, La2O3, Yb2O3, Er2O3 and S were thoroughly mixed with the molar ratio of 1:4 in an agate mortar and transferred into the aluminum oxide crucible. The mixture was heated at 1200 1C for 6 h under CO reducing atmosphere. It was then cooled to room temperature spontaneously and crushed to obtain fine La2O2S: 3% Yb3 þ , 1% Er3 þ powder. An XRD pattern was recorded to confirm the structure and the purity of the phase using a Bruker D8 advance X-ray diffractometer (Bruker Optics, Ettlingen, Germany) with CuKα radiation. The excitation source in the experiment was a 971 nm controlled temperature CW semiconductor laser diode (LD) with Pmax ¼ 3 W. The diode was coupled to a fiber (the core diameter 200 mm, numerical aperture 0.22). The luminescence spectra were recorded by exciting the sample with 971 nm LD using an Andor SR-500i spectrometer (Andor Technology Co., Belfast, UK). The Yb3 þ /Er3 þ co-doped La2O2S sample was filled in an iron sample cell and the temperature of the sample was increased from 290 to 573 K heated by resistive wire elements. A copper-constant

, Er3 þ under 971 nm LD excitation. (b) The log–log plot of the green UC emission as a function of laser

Please cite this article as: Y. Yang, et al., Optical thermometry based on the upconversion fluorescence from Yb3 þ /Er3 þ codoped La2O2S phosphor, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.02.081

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thermocouple buried in the sample was used to monitor the sample's temperature with a measurement error of 7 1.5 K. 3. Results and discussion Fig. 1 shows the typical powder X-ray diffraction pattern of the Yb3 þ /Er3 þ co-doped La2O2S samples in contrast to the standard card (JCPDS 27-0263), which indicates that La2O2S: Er3 þ , Yb3 þ samples exhibit pure hexagonal in phase and belongs to space group P3m1. No additional peaks of other phases can be detected in the XRD patterns, revealing that Yb3 þ and Er3 þ ions have been entered into the host lattices. Fig. 2 shows the UC emission spectrum of La2O2S: Yb3 þ , 3þ Er in the wavelength range of 500–710 nm at room temperature and the log–log plot of the green UC emission as a function of laser excitation power, respectively. It is observed that the UC spectrum of sample includes double bands peaked at 524 and 547 nm in the green region, and one band centered at 667 nm in the red region, which are shown in Fig. 2(a). The former is due to the 2H11/2, 4S3/2-4I15/2 transitions, and the latter is attributed to 4F9/2-4I15/2 transitions of Er3 þ ions, respectively. The dependence of the green UC emission on the excitation power is shown in Fig. 2(b), which indicated that the green UC emission of La2O2S: Yb3 þ , Er3 þ was a two-photon process based on the relational expression Iup p Pnpump. This means that the UC emission intensity Iup is proportional to the nth power of the pump power Ppump and n is the number of photons used to populate the UC emission state [22,23]. A possible UC mechanism is proposed and shown in Fig. 3. Under the excitation of 971 nm LD, the Yb3 þ ions in the ground state absorb a 971 nm photon to provoke the 2F7/2-2F5/2 transition, then the metastable energy level 4I11/2 of Er3 þ ions is populated by a first Yb3 þ -Er3 þ energy transfer (ET) step: 2F5/2(Yb3 þ ) þ 4I15/2 (Er3 þ )-2F7/2(Yb3 þ ) þ 4I11/2(Er3 þ ), Er3 þ in 4I11/2 state is excited to 4F7/2 state by ET2: 2F5/2(Yb3 þ )þ 4I11/2(Er3 þ )2 F7/2(Yb3 þ ) þ 4F7/2(Er3 þ ) or the excited state absorption ESA: 4 I11/2 (Er3 þ ) þ one photon-4F7/2(Er3 þ ). Then populated to the lower levels 2H11/2(4S3/2) by a multiphonon relaxation, the radiative relaxation from the thermally coupled levels 2 H11/2(4S3/2) to the ground state 4I15/2 of Er3 þ results in the green emissions.

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Fig. 4(a) shows the green UC emission spectra of La2O2S: Yb3 þ , Er3 þ at different temperatures with the 9.3 W/cm2 excitation density. It can be observed that the fluorescence intensity ratio of two emissions from 2H11/2, 4S3/2-4I15/2 transitions increases with increasing temperature. The fluorescence intensity ratio of two emissions can be written as follows [2,24,25]: R¼

I 2 N 2 A2 hν2 ¼ I 1 N 1 A1 hν1

ð1Þ

where I2 and I1 are the integrated intensities of the transitions of H11/2-4I15/2 and 4S3/2-4I15/2, respectively. N2 and N1 are the populations at the 2H11/2 and 4S3/2 levels, and A2 and A1 are the total spontaneous-emission rates of the 2H11/2 and 4S3/2 levels, respectively. Due to the low energy gap between 2H11/2 and 4S3/2 levels, the 2H11/2 level can be populated from the 4S3/2 level by thermal excitation, resulting in the relative change of intensity between the two green emissions to ensure a quasi-thermal equilibrium [12,24]. With thermalization of populations at the 2 H11/2 and 4S3/2 levels, the fluorescence intensity ratio of green UC emissions from the 2H11/2, 4S3/2-4I15/2 transitions can be written as follows:   I 2 N 2 A2 hν2 g2  ΔE hν2 A2 R¼ ¼ ¼ exp ð2Þ k B T hν1 A1 I 1 N 1 A1 hν1 g1 2

where g2, g1 are the degeneracies 2Jþ 1 of the 2H11/2 and 4S3/2 levels, respectively. According to the Judd–Ofelt theory [26,27], Afi represents the radiative transition probability from the level f to level i, it could beobtained from the following equation: Af i ¼

64π 2 e2 ν3 ðn2 þ 2Þ2 ∑ Ωλ j〈4f N ðαCLÞJjjU ðλÞ jj4f N ðα0 C 0 L0 J 0 Þ〉j2 3hgf 9n λ¼2;4;6

ð3Þ 1

between the two where ν is the energy of transition in cm multiplets, U(λ) (λ=2, 4, and 6) are the unit tensor operators of rank λ and Ωλ is the J–O intensity parameters. The terms j〈4f N ðαCLÞJjjU ðλÞ jj4f N ðα0 C 0 L0 J 0 Þ〉j2 are the reduced matrix elements. The electric-dipole line strength C can be written as follows [26–28]: C ¼ gi j o f jμji4j2 ¼ ∑ j o f jμji; m 4 j2 m

¼ ∑ Ωλ j〈4f ðαCLÞJjjU ðλÞ jj4f N ðα0 C 0 L0 J 0 Þ〉j2 N

λ¼2;4;6

ð4Þ

Thus, the fluorescence intensity ratio of green UC emissions from the 2H11/2, 4S3/2-4I15/2 transitions can be written as follows:

   ΔE v42 C 2 R ¼ exp kB T v41 C 1      ΔE v42 Ω2 n0:7158 þ Ω4 n0:4138 ¼ exp þ 0:0927=0:2225 4 k T v1 0:2225nΩ6 B   ΔE ð5Þ ¼ Bexp kB T

Fig. 3. Simplified energy level diagram of Yb3 þ , Er3 þ and a possible mechanism in La2O2S: Yb3 þ , Er3 þ phosphor under 971 nm LD excitation.

Fig. 4(b) shows the fluorescence intensity (integrated area below the fluorescence curves) ratio of green UC emissions I524 and I547 as a function of inverse absolute temperature in the range of 290–573 K on a monolog scale. The experimental

Please cite this article as: Y. Yang, et al., Optical thermometry based on the upconversion fluorescence from Yb3 þ /Er3 þ codoped La2O2S phosphor, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.02.081

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Fig. 4. Temperature sensing based on La2O2S: Yb3 þ , Er3 þ . The excitation density was 9.3 W/cm. (a) UC spectra of the La2O2S: Yb3 þ , Er3 þ for green emissions at 294 K, 439 K and 562 K, (b) The monolog plot of R as a function of inverse absolute temperature. (c) R between the green emissions at temperatures ranging from 290 to 573 K. (d) The sensor sensitivity dR/dT as a function of the temperature.

data are fitted to a straight line with a slope of about 1143.3. The dependence of R (I524/I547) on the temperature in the range of 290–573 K is shown in Fig. 4(c). The value of coefficient B for the best fit curve on the experimental data is found to be 17.27. The sensor sensitivity of an optical thermometer has an important reference value in the practical applications. The sensor sensitivity can be defined as follows [6,11,29]:     dR ΔE ΔE  ΔE S¼ ¼R exp ¼ B ð6Þ dT kB T kB T 2 kB T 2 It can be seen from Fig. 4(d) that the sensitivity of La2O2S: Yb3 þ , Er3 þ reached the maximum of about 0.008 K  1 at the temperature of 562 K, which is higher among the Er3 þ -doped and Yb3 þ /Er3 þ co-doped hosts (as shown in Table 1). As shown in Table 1, it is observed that the sensitivity alters from 0.0012 to 0.014, which is decided by two variables B and ΔE/kB. In order to analyze the impact of the change of the two variables on the sensitivity, the curves of the sensor sensitivity versus ΔE/kB with fixed B values 32.45 are shown in Fig. 5(a). The curves of the sensor sensitivity versus B were also given as ΔE/kB is a constant value of 523.1, which are shown in Fig. 5(b). It can be seen that as ΔE/kB increases, the sensitivity drops faster in low temperature zone from 200 K to 600 K and the maximum value of the sensitivity shifts to high temperature direction. As shown in Fig. 5(b), the sensitivity rises significantly with the increase of the value of B, owing to the sensitivity is directly proportional to B, as shown in Eq. (6).

To account for this source of variability, the samples have to meet two requirements in order to obtain high temperature sensing sensitivity in low temperature zone: one is the lower energy gap ΔE between 2H11/2 and 4S3/2 levels; another is the higher value of B. As shown in Table 1, the lowest ΔE/kB is 523.1 and the highest value of B is 32.45, if these two conditions could be combined together, the highest sensitivity can go up to 0.03357 K  1. It is worth noting that the sensor sensitivity hardly changed with ΔE in higher temperature from 600 K to1000 K. So, the sensor sensitivity is directly proportional to B in the high temperature zone. What determines the value of B? From Eq. (5), B can be rewritten as follows:   ν4 Ω2 n0:7158þ Ω4 n0:4138 B ¼ 24 þ 0:0927 0:2225 0:2225nΩ6 ν1 

Ω2 n0:7158þ Ω4 n0:4138 þ 0:0927=0:2225 0:2225nΩ6

ð7Þ

In view of the fact that ν42 =ν41 approaches 1 in Eq. (7), B is determined only by Ωλ ¼ 2,4,6. The energy level of 4S3/2 has only one nonzero reduced matrix element Ω6 and the energy level of 4H11/2 has no nonzero reduced matrix elements, but has a bigger Ω2. Transitions characterized by large values of the Ω2 matrix element are in general rather sensitive to the environment of the lanthanide ion and have been called ‘hypersensitive’ by Jorgensen and Judd [28]. Otherwise, it is well known that Ω2 is more sensitive to the environments than others amongst J–O parameters. So B is mainly determined by

Please cite this article as: Y. Yang, et al., Optical thermometry based on the upconversion fluorescence from Yb3 þ /Er3 þ codoped La2O2S phosphor, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.02.081

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Table 1 The fluorescence intensity ratio parameters and values of the maximum sensitivity in different Er3+ doped hosts. Temperatures for the maximum sensitivity as well as the excitation wavelength are also included. Host

B

ΔE/kB (K)

SMAX (K−1)

T (K)

Excitation Wavelength (nm)

Ref.

fluorotellurite glass fluorotellurite glass fluorotellurite glass Gd2O3 nanowires Gd2O3 nanophosphor silicate glass silicate glass Yttrium silicate powders Yttrium silicate powders phosphate glass ceramics Ga2S3:La2O3 glass Al2O3 nanoparticles. NaYF4 nanoparticle NaYF4 Zbyban LiNbO3 YbAG YNbO4 BaTiO3 nanocrystals nanocrystals Yb2Ti2O7 nanophosphor Al2O3 La2O2S

1.54 – 11.2 16 3.19 7.92 3.65 – – 1.58 8.85 9.63 5.70 8.06 3 32.45 8.82 13.19 9.97 9.3 11.2 17.27

559 – 1108.9 1007 746.4 1289.09 592.6 817 1226 523.1 928.98 964.1 1028 1082.1 1300 1250 900 1016 1200 679.2 956 1143.3

0.0015 0.0054 0.0055 0.0085 0.0039 0.0031 0.0033 0.0056 0.0070 0.0016 0.0052 0.0051 0.0030 0.0040 0.0012 0.014 0.0048 0.0073 0.0045 0.0074 0.0051 0.0080

279 547 550 500 300 550 286 400 630 260 443 495 510 535 650 628 450 473 600 345 450 562

980 800 1480 978 976 970 978 975 975 (pulsed) 975 1060 978 920 980 975 980 976 976 980 976 976 971

[9] [10] [1]1 [12] [13] [14] [15] [18] [30] [5] [17] [16] [8] [7] [29] [19] [31] [32] [33] [28] [6] This work

Fig. 5. (a) The curves of sensor sensitivity versus ΔE/kB with fixed B value at 32.45. (b) The curves of the sensor sensitivity versus B with fixed ΔE/kB value at 523.1.

Ω2. Jorgensen investigated the three J–O intensity parameters for erbium (III) in 17 different environments. The obtained data showed that the parameter Ω2 was strongly affected by covalent chemical bonding [34]. In other words, the B value can be judged by analyzing the ligand covalency of the hosts [35–37]. In low temperature zone from 200 to 500 K, the energy gap ΔE between 2H11/2 and 4S3/2 levels of Er3 þ ions may not be neglected. As well known, the levels of trivalent rare earth ions are little affected by the crystalline field of the hosts due to the shield of the outermost electron 5s25p6. However, the energy gap ΔE between 2H11/2 and 4S3/2 levels of Er3 þ ions is very small, about 600 cm  1, the influence of the crystalline field of the hosts may not be neglected. In theory, the energy gap ΔE is

decided by the even-parity of the crystalline field of the hosts and is related to the symmetries of the crystal system. 4. Conclusions An optical temperature sensor based on the La2O2S: Yb3 þ , Er has been developed. Due to its high sensor sensitivity and intense green UC emission at lower excitation power densities, La2O2S: Yb3 þ , Er3 þ has potential applications in optical temperature sensors. Through analysis of the influencing factors of the sensor sensitivity, we can judge the sensor sensitivity of the samples by the ligand covalency of the hosts in the high temperature zone. In low temperature zone, the energy gap ΔE between 2H11/2 and 4S3/2 levels of Er3 þ ions is 3þ

Please cite this article as: Y. Yang, et al., Optical thermometry based on the upconversion fluorescence from Yb3 þ /Er3 þ codoped La2O2S phosphor, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.02.081

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Please cite this article as: Y. Yang, et al., Optical thermometry based on the upconversion fluorescence from Yb3 þ /Er3 þ codoped La2O2S phosphor, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.02.081