Investigation for the upconversion luminescence and temperature sensing mechanism based on BiPO4: Yb3+, RE3+ (RE3+ = Ho3+, Er3+ and Tm3+)

Journal of Alloys and Compounds 772 (2019) 371e380

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Investigation for the upconversion luminescence and temperature sensing mechanism based on BiPO4: Yb3þ, RE3þ (RE3þ ¼ Ho3þ, Er3þ and Tm3þ) Nan Wang a, Zuoling Fu a, *, Yanling Wei b, Tianqi Sheng a a Coherent Light and Atomic and Molecular Spectroscopy Laboratory, Key Laboratory of Physics and Technology for Advanced Batteries, College of Physics, Jilin University, Changchun, 130012, China b School of Media Mathematics &Physics, Jilin Engineering Normal University, Changchun, 130012, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 May 2018 Received in revised form 4 September 2018 Accepted 8 September 2018 Available online 8 September 2018

Rare earth ions doped phosphors have been commonly applied in temperature sensing field. However, it is urgent to specify temperature sensing mechanism owing to the troubles in selecting the suitable phosphors and ascertaining the temperature measurement range. In this work, the Yb3þ/Ho3þ, Yb3þ/Er3þ and Yb3þ/Tm3þ co-doped BiPO4 phosphors are prepared by a simple co-precipitation synthesis with further calcination. Under 980 nm excitation, the up-conversion emissions from the thermally couple levels (TCLs) of RE3þ (RE3þ ¼ Ho3þ, Er3þ and Tm3þ) are analyzed as a function of temperature from 313 K to 573 K. Using the fluorescence intensity ratio (FIR) technique, the temperature sensing mechanism based on BiPO4 matrix is investigated. In order to obtain the deeper comprehension of the temperature sensing mechanism, we explore both the relationship between the absolute sensitivity SA and temperature T and the relationship between the SA and energy gap DE in detail. The research results indicate that they would have an important guiding significance for the design and selection of phosphors with high temperature sensitivity in the future. © 2018 Elsevier B.V. All rights reserved.

Keywords: Co-precipitation synthesis Temperature sensing mechanism Up-conversion luminescence properties

1. Introduction The demand of precision for temperature measurement has put on the agenda in a broad spectrum of fields, including industrial manufacture and scientific research. Aside from the conventional contact temperature measurements, for examples, the thermistors, thermocouples, and electrical resistance, which all require electrical wiring [1,2], the weaker electromagnetic noise, and noncorrosive environment, the emerging non-contact thermometry methods are arousing more attention such as Raman spectroscopy, IR thermography, thermo-reflectance, luminescence, and so on [3,4]. Among all of the techniques, the thermometry based on the luminescence properties has got a lot of attention for its outstanding resolution, stability, accuracy, and repeatability [5e7]. There are six temperature-dependent luminescence parameters, such as intensity, spectral position, lifetime, bandwidth, polarization, band-shape [8]. The method based on the FIR of two emissions

* Corresponding author. E-mail address: [email protected] (Z. Fu). https://doi.org/10.1016/j.jallcom.2018.09.070 0925-8388/© 2018 Elsevier B.V. All rights reserved.

of a couple of thermally coupled levels from trivalent rare-earth ions is simple and efficient [9,10e13]. The FIR technique can achieve the improvement of sensitivity and the reduction of dependence for measurement conditions [5,14e18]. And the rare earth ions are abundant in energy levels from the intrinsic 4f transitions [19,20]. So the rare earth ions can produce the emissions in the range of from ultraviolet to infrared, which can release the desired emission bands [21]. Particularly, some rare-earth ions possess a couple of thermally coupled energy levels with the energy gap (DЕ) from 200 to 2000 cm1, and the intensity ratio of two emissions from the TCL will have a variation with the increasing temperature [22]. Herein, we select BiPO4 as the host matrix for its relative excellent chemical and thermal stabilities, especially for the nontoxicity and the radius of Bi3þ ion being comparable to the rareearth ions [23e25]. In addition, rare earth ions doped BiPO4 phosphors also have the potential applications in photonic, luminescence, and temperature sensor [22,26,27]. As far as we know, there are several reports in which rare earth ions including Eu3þ, Tb3þ, and Dy3þ can be doped into BiPO4 for down-conversion luminescence and Yb3þ, Ho3þ, Er3þ and Tm3þ for up-conversion

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luminescence [28,29]. However, little attention has been devoted to the investigation of temperature sensing mechanism based on Yb3þ, Ho3þ, Er3þ and Tm3þ doped BiPO4. Here, we undertake this task to investigate the temperature sensing mechanism based on the Yb3þ, Ho3þ, Er3þ and Tm3þ doped BiPO4 phosphors by analyzing the relationships between the absolute sensitivity and energy gap, and between the absolute sensitivity and temperature.

2. Experimental section 2.1. Materials Bismuth nitrate pentahydrate (Bi(NO3)3$5H2O, 99.99%), ammonium dihydrogen phosphate (NH4H2PO4, 99.99%), ytterbium nitrate pentahydrate (Yb(NO3)3$5H2O, 99.99%), erbium nitrate pentahydrate (Er(NO3)3$5H2O, 99.99%), thulium nitrate hexahydrate (Tm(NO3)3$6H2O, 99.99%) and holmium nitrate hexahydrate (Ho(NO3)3$6H2O, 99.99%) were used as the raw materials. Distilled water was used throughout. All of the chemical materials were used as received without further purification.

2.2. Sample preparation Bismuth phosphate powders doped with Yb3þ/Tm3þ, Yb3þ/Ho3þ and Yb3þ/Er3þ were obtained by co-precipitation method using water as solvent. In a typical synthetic procedure, 5 mL deionized water solvent was added into the beaker containing 2 mmol Bi(NO3)3$5H2O and other lanthanide nitrates with appropriate stoichiometric ratios for obtaining aqueous solution. Then above solution was vigorously stirred on the magnetic stirrer about 1 h for speeding up the dissolving. 2 mmol NH4H2PO4 was dissolved by using 65 mL distilled water medium under constantly stirring for 30 min. And this solution was then added dropwise into the former solution. Then a white homogeneous precipitate was acquired. After being stirred for 12 h at the room temperature, the white suspension was filtered, and washed with alcohol and water for several times. Finally, the resulting precipitate was dried at 60  C for 6 h. The product was calcined in air atmosphere at 600  C for 2 h.

3. Results and discussion 3.1. Structural and morphological analysis Fig. 1(a) displays the XRD diffraction pattern of BiPO4: 15% Yb3þ, 0.1% Ho3þ sample prepared at room temperature (RT). Fig. 1(b), (c) and (d) show the XRD patterns of BiPO4: 15% Yb3þ, 0.1% Ho3þ, BiPO4: 20% Yb3þ, 0.1% Tm3þ and BiPO4: 30% Yb3þ, 0.05% Er3þ phosphors sintered at 600  C for 2 h, respectively. It is noted that the XRD pattern in Fig. 1(a) is in agreement with the profile of standard hexagonal phase BiPO4 (JCPDS: 15-0766). No new other diffraction peaks are detected in the XRD pattern, signifying that these RE3þ ions have been appropriately introduced in the BiPO4 host lattice. However, after calcination at 600  C, the mixture crystalline phases of hexagonal phase (HP), high-temperature monoclinic phase (HTMP) and low-temperature monoclinic phase (LTMP) are observed in Fig. 1(b), (c) and (d). Our results are agreement exactly with the observation by Zhang et al. [30]. The typical morphologies of the resulting samples for the BiPO4: 0.1% Ho3þ, 15% Yb3þ phosphors are investigated by FE-SEM and TEM in Fig. 2. The resulting rice-shaped and micron-sized particles are composed of a number of short rod-like structures before calcination from Fig. 2(a). After calcination at 600  C for 2 h, the shape remains similar to rice shape as shown in Fig. 2(b). From TEM of Fig. 2(c), we can clearly notice that the length and width of the short rod-like structures are in the range of 70e400 nm and 30e200 nm, respectively. 3.2. Luminescence properties Fig. 3 depicts the up-conversion fluorescence spectra of Yb3þ/ Ho , Yb3þ/Tm3þ, and Yb3þ/Er3þ co-doped BiPO4 samples with calcination at 600  C for 2 h. Fig. 3(a) shows the up-conversion spectra of the BiPO4: x Ho3þ, 5% Yb3þ (x ¼ 0.05%, 0.1%, 0.3%, 0.5%, 1%); similarly, Fig. 3(b) shows the up-conversion spectra of the BiPO4: 0.1% Ho3þ, yYb3þ (y ¼ 5%, 10%, 15%, 20%, 25%). A strong red emission splitting two parts at 652 and 657 nm can be seen, which are ascribed to 5F5(1) / 5I8 and 5F5(2) / 5I8 transitions of Ho3þ ions, 3þ

2.3. Characterizations The phase structure of all the samples was characterized by Rigaku-Dmax 2500 diffractometer with Cu Ka radiation (l ¼ 0.15405 nm). The scanning rate of 15 per minute was employed to keep a record of patterns in the 2q range from 10 to 60 . A field emission scanning electron microscope (FE-SEM, XL30, Philips) was applied to character the morphologies and sizes of the samples. By using the JEOL-2100F microscope with a field emission gun operated at an accelerating voltage of 200 kV, the transmission electron microscopy (TEM) pattern was obtained. The photoluminescence was measured by an Andor Shamrock SR-750 fluorescence spectrometer. The signals of the samples were collected by means of a CCD detector combined with a monochromator from 400 to 750 nm. A diode laser (980 nm) was considered as the pump source. Andor SR-500i spectrometer (Andor Technology Co, Belfast, UK) was used for recording the luminescence spectra of the samples under the excitation of 980 nm. The Yb3þ/Tm3þ, Yb3þ/Er3þ and Yb3þ/Ho3þ co-doped BiPO4 samples were placed in an iron sample cell and the temperature of samples was increased in the range of the 313e573 K heated by resistive wire element.

Fig. 1. X-Ray powder diffraction patterns of the (a) BiPO4: 15% Yb3þ, 0.1% Ho3þ obtained at room temperature, (b) BiPO4: 15% Yb3þ, 0.1% Ho3þ, (c) BiPO4: 20% Yb3þ, 0.1% Tm3þ and (d) BiPO4: 30% Yb3þ, 0.05% Er3þ sintered 600  C for 2 h. Symbols * and ) represent the HP and LTMP phases, respectively.

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Fig. 2. FE-SEM images for the BiPO4: 0.1% Ho3þ, 15% Yb3þ sample (a) obtained at room temperature and (b) calcinated at 600  C for 2 h. The TEM image of BiPO4: 0.1% Ho3þ, 15% Yb3þ sample (c) obtained at 600  C for 2 h.

respectively. At the same time, the weaker blue and green emissions at 475 nm and 544 nm are assigned to 5F3 / 5I8 and 5F4, 5S2 / 5I8 of Ho3þ ions, respectively. With the decrease in the contents of Ho3þ ions from 1% to 0.05%, the luminescence intensity grows gradually before coming up to the value at x ¼ 0.1%. When the content of Ho3þ ions continues to decrease, the luminescence intensity is significantly reduced. Similarly, the optimal Yb3þ ions concentration with a constant Ho3þ ions content (x ¼ 0.1%) is determined to be 15%. And the dependence of integrated intensity on Yb3þ or Ho3þ ions contents is clearly displayed in the insets of Fig. 3(a) and (b), respectively. For this phenomenon, it can be attributed to the concentration quenching effect. Fig. 3(c) and (d) show the up-conversion emission spectra of BiPO4: x Tm3þ, 5% Yb3þ (x ¼ 0.05%, 0.1%, 0.3%, 1%) and BiPO4: 0.1% Tm3þ, y Yb3þ (y ¼ 5%, 10%, 20%, 25%, 30%) phosphors, respectively. The blue and red emission peaks centered at 475 nm and 648 nm correspond to the 1G4 / 3H6 and 1G4 / 3F4 of Tm3þ ions in the visible region, respectively. It is merit noting that the red emission is split into two parts from the 1G4 multiplet. And a strong nearinfrared (NIR) [31] emission at 804 nm is assigned to the intrinsic transition from 3H4 / 3H6 of Tm3þ ions. The supreme Yb3þ and Tm3þ ions concentrations with highest emission intensity are corresponding to be 20% and 0.1% through fixing the concentrations of activator of Tm3þ ions and the fixed concentration of sensitizer of Yb3þ ions, as shown in Fig. 3(c) and (d). The insets of Fig. 3(c) and (d) show the emission intensity variation with the contents of Tm3þ and Yb3þ ions, respectively. For up-conversion emission spectrum of Yb3þ/Er3þ co-doped BiPO4 phosphors, it is consisted of two green emission peaks from 512 nm to 573 nm centered at 525 and 551 nm, corresponding to the 2H11/2 / 4I15/2 and 4S3/2 / 4I15/2 transitions of Er3þ ions. Simultaneously, a stronger red emission

peak can be observed, which is assigned to 4F9/2 / 4I15/2 transition. And the BiPO4: 0.05% Er3þ, 30% Yb3þ phosphor is selected as the optimal luminescence material as shown in Fig. 3(e). To make it clear that the up-conversion process of Yb3þ/Ho3þ, 3þ Yb /Tm3þ and Yb3þ/Er3þ co-doped BiPO4 samples, the relationship between up-conversion emission intensity (IUC) and pump power (Ppump) obeys the mathematic equation:

IUC f Ppump


n

(1)

where “IUC”, “Ppump” and “n” represent the fluorescence intensity, the pump laser power and the number of pump photons required [32], respectively. As shown in Fig. 4, there is a perfect linear relationship between the logarithmic emission intensity and the logarithmic power. For BiPO4: 0.1% Ho3þ, 15% Yb3þ sample, the n values for blue, green and red emission bands are 2.34, 1.88 and 1.69 in Fig. 4(a), which indicates that two photons are involved for the appearance of these up-conversion emission bands. And the n values are determined to be 2.32 and 2.35 for blue and red emissions of Tm3þ ions from BiPO4: 0.1% Tm3þ, 20% Yb3þ phosphor in Fig. 4(b), indicating the three-photon up-conversion process [26]. A NIR emission centered at 804 nm of Tm3þ ions is assigned to be the two-photon process by the n value of 1.56. For the BiPO4: 0.05% Er3þ, 30% Yb3þ, the n value is 1.97 for green emission band centered at 525 nm, n value is 1.81 for green emission peak centered at 551 nm and n value is 1.54 for red up-conversion emission band centered at 660 nm in Fig. 4(c). All the emission bands can be attributed to be a process of two photons. As mentioned above, we show a detailed energy level diagram for Yb3þ, Ho3þ, Tm3þ and Er3þ ions in order to shed more light on the possible up-conversion emission mechanisms in Fig. 5. The observed blue, green and red emission bands from Ho3þ ions in

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part of populations from the 5I6 state are promoted to the 5S2 and F4 levels by means of excited-state absorption, and the rest decays nonradiatively to the 5I7 level. A second Yb3þ ion transfers its energy to Ho3þ ions in the 5I7 level and excites them from the 5I7 to 5F5 level, then to the ground 5I8 state. So a red emission peak centered at 654 nm can be observed. Finally, Ho3þ ions in 5S2 and 5F4 levels emit the green emission band at 544 nm following relaxation to the ground-state. Additionally, the blue emission band at 475 nm originates from the 5F3 / 5I8 transition, where the 5F3 level is populated by the cross-relaxation (CR) (5I7, 5F5 / 5I8, 5F3) energy transfer process. Notably, the fluorescence intensity of the red emission band is the strongest among all the transitions on account of the 5F5 state has a longer lifetime than the 5S2, 5F4 states. So the population of 5F5 state rapidly grows due to more possibilities to realize the transition of 5F5 / 5I8. Another reason is that the number of pump photons needed by red emission band is 1.69, which is the smallest in all the emission bands [35]. For BiPO4: 0.05% Er3þ, 30% Yb3þ phosphor, the Er3þ ions in the 4 I15/2 ground state absorb the energy from neighboring Yb3þ ions, and then promote to the 4I11/2 excited state, after which they decay nonradiatively to the 4I13/2 level. The Er3þ ions in the 4I13/2 level are excited to the 4F9/2 level via absorbing another photon provided by Yb3þ ions, generating an intense red emission band. The remaining ions will be further populated to the 4F7/2 state. The excited Er3þ ions will subsequently decay nonradiatively to the 2H11/2, 4S3/2 levels from 4F7/2 state. The two green emissions centered at 525 and 551 nm can be produced by radiatively decaying to the 4I15/2 ground state. In the case of BiPO4: 0.1% Tm3þ, 20% Yb3þ phosphor, two sequential energy transfer processes are required to distribute the 3 F2, 3 levels, which produce the 700 nm emission band. After that the Tm3þ ions in the 3F2, 3 levels can relax nonradiatively to 3H4 level and produce a NIR emission band centered at 804 nm. The 1G4 level can be populated through a three-photon process. Namely, three successive energy transfer processes are needed, with the Tm3þ ions at the level of 1G4 generating the blue and red emissions. In fact, the producing of blue and red emission bands respectively centered at 475, 648 nm is a three-photon process. Nevertheless, the pump photon numbers of n are 2.32 and 2.35 for blue and red bands, which are deviating from the ideal value of 3. Cooperative up-conversion energy process can shed more light on this scenario [26]. That is to say, the Tm3þ ions at the ground state absorb energy from two Yb3þ ions, after which directly are excited to the 1G4 level, without staying an intermediate level of Tm3þ ions. 5

3.3. Temperature sensing mechanism

Fig. 3. Up-conversion spectra of (a) BiPO4: x Ho3þ, 5% Yb3þ (x ¼ 0.05%, 0.1%, 0.3%, 0.5%, 1%), (b) BiPO4: 0.1% Ho3þ, y Yb3þ (y ¼ 5%, 10%, 15%, 20%, 25%), (c) BiPO4: x Tm3þ, 5% Yb3þ (x ¼ 0.05%, 0.1%, 0.3%, 1%), (d) BiPO4: 0.1% Tm3þ, y Yb3þ (y ¼ 5%, 10%, 20%, 25%, 30%) and (e) BiPO4: 0.05% Er3þ, 30% Yb3þ sample obtained under the excitation of 980 nm laser, and the emission intensity variation with the corresponding RE3þ contents (insets).

BiPO4: 0.1% Ho3þ, 15% Yb3þ phosphor may take place by the following process: under the excitation of 980 nm laser source, the excited Yb3þ ions transfer their energy to adjacent Ho3þ ions and excite them from ground 5I8 state to the excited 5I6 state [33,34]. A

3.3.1. Temperature sensing properties Fig. 6(a) presents the bar graph of two normalized upconversion fluorescence emission bands centered at 652 nm and 657 nm for Yb3þ/Ho3þ co-doped BiPO4 in the temperature range from 313 K to 573 K. The integrated intensity ratio of the two red up-conversion emissions peaks varies slightly with a rise of temperature. At 313 K, the intensity of emission band centered at 657 nm is nearly equal to the emission band at 652 nm, whereas the latter is gradually higher than the former with the increasing temperature. For the Yb3þ/Er3þ co-doped BiPO4 phosphor, the 2H11/ 4 2 and S3/2 are the thermally coupled energy levels and the emission intensity changes of 2H11/2 / 4I15/2 and 4S3/2 / 4I15/2 transitions with the increasing temperature are shown in Fig. 6(b). The integrated intensity of 2H11/2 / 4I15/2 transition centered at 525 nm gradually increases from 313 K to 573 K, whereas the emission intensity of 4S3/2 / 4I15/2 transition centered at 551 nm decreases. For the Yb3þ/Tm3þ co-doped BiPO4 phosphor, there are two couple of adjacent TCLs, 3F2,3 and 3H4, as well as 3H4(1) and 3H4(2). The

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Fig. 4. Logarithmic dependence pump power versus visible and NIR emission intensity for (a) BiPO4: 15% Yb3þ, 0.1% Ho3þ, (b) BiPO4: 20% Yb3þ, 0.1% Tm3þ and (c) BiPO4: 30% Yb3þ, 0.05% Er3þ.

Fig. 5. The energy level diagrams of the Yb3þ, Ho3þ, Tm3þ and Er3þ dopant ions and proposed upconversion mechanisms following the excitation of 980 nm laser diode.

corresponding emission spectra presenting the variation of emission intensity with temperature are also shown as Fig. 6(c). The intensity of emission band centered at 700 nm assigning to 3F2, 3 3þ ions is hardly observed at low temperature of 3 / H6 of Tm

318 K, after that, with the temperature elevating, the intensity is gradually growing. However, the intensity of emission peak centered at 804 nm attributing to 3H4 / 3H6 of Tm3þ ions is rapidly declining. For this phenomenon, it can be interpreted following the

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Fig. 6. The luminescence intensity variation with temperature changing in the temperature range of 313e573 K upon 980 nm laser diode: (a) BiPO4: 15% Yb3þ, 0.1% Ho3þ (b) BiPO4: 30% Yb3þ, 0.05% Er3þ (c) BiPO4: 20% Yb3þ, 0.1% Tm3þ.

reason: there is a thermal population process between the 3F2, 3 and 3 H4 levels [36], which makes the population of 3F2, 3 level rapidly increased. That is why the intensity of emission band at 700 nm increases and the intensity of emission band centered at 804 nm decreases. At the same time, the intensity ratio between the emission band at 794 nm corresponding to 3H4(1) / 3H6 transition and the emission band at 804 nm attributing to 3H4(2) / 3H6 transition has a weak variation. The FIR technique consists of measuring the thermal dependence of the intensity of fluorescence originating from two thermally coupled levels and calculates their ratio. The ratio of emission intensities can be modified as:

SA ¼

  dFIR DЕ DЕ DЕ ¼ ¼ В exp  FIR dΤ ΚВΤ ΚВΤ 2 ΚВΤ 2

(3)

SR ¼

1 dFIR DЕ ¼ FIR dΤ ΚВΤ 2

(4)

In order to be effective to design and select temperature sensing materials, we focus on the relationship between SA and T, as well as the relationship between SA and DЕ. 3.3.2. The relationship between SA and T We calculate the derivation of T by SA in order to understand clearly the relationship between SA and T:

FIR ¼

    N2 I2j g2 s2j u2j DЕ DЕ ¼ В exp ¼ ¼ exp  ΚВΤ ΚВΤ N1 I1j g1 s1j u1j

(2)

The N, I, g, s and u represent the populations of the corresponding energy level, the fluorescence intensity, the degeneracy of levels, the emission cross-section, and the absorption rates, respectively. DЕ is the energy gap between the two energy levels considered. KB is Boltzmann's constant, T is absolute temperature and B is the fitting constant relying on experimental system and spectroscopic parameters. The rate of FIR varied with the temperature is called optical sensor sensitivity, which shows the significant possibility for practical applications. The sensor sensitivity contains the absolute temperature sensitivity (SA) and the relative temperature sensitivity (SR), which can be expressed by the formulas:

dSA ¼ dΤ



   ВDЕ DЕ 2 exp  ΚВΤ ΚВΤ ΚВΤ 3

DЕ

(5)

From Eq. (5), we get dSA/dT ¼ 0, when (DE/KBT)-2 ¼ 0, and that is T ¼ DЕ/2KB. It implies that when the temperature sensor material (DЕ) is given, the temperature with respect to the maximum SA can be ascertained. We will investigate the relationship between SA and T in detail based on the Yb3þ/Ho3þ, Yb3þ/Er3þ and Yb3þ/Tm3þ codoped BiPO4 phosphors. Fig. 7 shows the monolog plots of the emission intensity ratio FIR versus 1/T and the FIR relative to absolute temperature T for Yb3þ/Ho3þ, Yb3þ/Er3þ and Yb3þ/Tm3þ co-doped BiPO4 phosphors, respectively. When the DЕ ¼ 46.06 cm1, B ¼ 1.61 from Fig. 7(a1) and (a2), the temperature is T ¼ 33.14 K, which can be confirmed by Fig. 8 (a). In Fig. 8 (a), the black curve obtained by Eq. (3) describes

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Fig. 7. The monolog plots of the emission intensity ratio LnFIR versus 1/T and the plots of FIR relative to absolute temperature: (a1) (a2) BiPO4: 15% Yb3þ, 0.1% Ho3þ; (b1) (b2) BiPO4: 30% Yb3þ, 0.05% Er3þ; (c1) (c2) BiPO4: 20% Yb3þ, 0.1% Tm3þ using FIR(I700/I804); (d1) (d2) BiPO4: 20% Yb3þ, 0.1% Tm3þ using FIR(I794/I804).

the relationship between SA theoretical value and temperature T, and the solid balls on the curve represent SA experimental value. It represents the fitting results of SA theoretical value (the black curve) and the SA experimental value (the solid balls on the curve) Due to the limitation of laboratory condition, the testing temperature covers the range of 313e573 K. Consequently, we can deduce that variation trend of SA experimental value with T is monotonically decreasing in the temperature of 313e573 K, which is verified by the solid balls within the dash frame of the curve in Fig. 8 (a). Naturally, the maximum of SA experimental value is 0.00079 K1 at 333 K. According to the SA theoretical value, Yb3þ/ Ho3þ co-doped BiPO4 phosphor is more suitable for low temperature zone. For the Yb3þ/Er3þ co-doped BiPO4 phosphor, similarly, when the DЕ ¼ 754.73 cm1, B ¼ 8.23 can be calculated by Fig. 7 (b1) and (b2), the temperature is T ¼ 542.96 K. We can deduce that the variation trend of SA experimental value with T is monotonically increasing in the temperature of 313e573 K. The conclusion is verified by Fig. 8 (b). The maximum of SA experimental value is 0.00413 K1 at 573 K. Therefore, for Yb3þ/Er3þ co-doped BiPO4 phosphor, it is more

suitable for high temperature zone. There are two TCLs in the Yb3þ/Tm3þ co-doped BiPO4 phosphor. Consequently, in order to distinguish them, the Yb3þ/Tm3þ codoped BiPO4 phosphor using FIR (I700/I804) is defined as Yb3þ/ Tm3þ co-doped BiPO4 phosphor, and the Yb3þ/Tm3þ co-doped 1 BiPO4 phosphor using FIR (I794/I804) is defined as Yb3þ/Tm3þ 2 codoped BiPO4 phosphor. For the Yb3þ/Tm3þ 1 co-doped BiPO4 phosphor, the DЕ ¼ 1457.29 cm1, B ¼ 1.40 can be calculated and shown in Fig. 7 (c1) and (c2). The T is 1048.39 K in the same method. For the 1 Yb3þ/Tm3þ 2 co-doped BiPO4 phosphor, when the DE ¼ 92.57 cm , B ¼ 0.93 by Fig. 7 (d1) and (d2), the temperature is T ¼ 66.60 K. Fig. 8 (c) and (d) certify the variation trend, respectively. In Fig. 8 (c) and (d), using Eq. (3), the fitting curves showing the relationship between SA theoretical value and T temperature of Yb3þ/Tm3þ 1 codoped BiPO4 phosphor and Yb3þ/Tm3þ 2 co-doped BiPO4 phosphor can be obtained, respectively. Meanwhile, the maximum of SA experimental value is 0.00022 K1 at 548 K for the former, and the maximum of SA experimental value is 0.00083 K1 at 313 K for the latter. From the above discussion, we know that Yb3þ/Ho3þ co-doped

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Fig. 8. The fitting curves of the relationship between SA and T: (a) BiPO4: 15% Yb3þ, 0.1% Ho3þ; (b) BiPO4: 30% Yb3þ, 0.05% Er3þ; (c) BiPO4: 20% Yb3þ, 0.1% Tm3þ 1 using FIR(I700/I804); (d) BiPO4: 20% Yb3þ, 0.1% Tm3þ 2 using FIR(I794/I804). The solid balls on the curve represent the experimental values, and the insets are the large version of pictures within the dashed frame.

BiPO4 and Yb3þ/Tm3þ 2 co-doped BiPO4 phosphors are more suitable for low temperature zone. In the same way, the Yb3þ/Er3þ co-doped BiPO4 and Yb3þ/Tm3þ 1 co-doped BiPO4 phosphors are more suitable for high temperature zone. So we can ascertain the suitable range of the temperature trial for the given materials. 3.3.3. The relationship between SA and DЕ As we had previously reported [37], we have calculated the derivation of DЕ by SA as follows:

dSA ¼ dDЕ

 1

DЕ ΚВΤ



  В DЕ exp  2 ΚВΤ Τ

(6)

From Eq. (6), we know when the dSA/dDE ¼ 0, the DE ¼ KBT. It means when the measure temperature range (T) is given, we should select the better materials satisfying the condition of DЕ ¼ KBT. For the low temperature zone, we select the TL ¼ 300 K, so the KBT ¼ 208.50 cm1. The fitting curve of SA changes with DЕ is shown in Fig. 9 (a). According to the above discussion, Yb3þ/Ho3þ co-doped BiPO4 and Yb3þ/Tm3þ co-doped BiPO4 phosphors are 2 more suitable for low temperature zone. The green point and yellow point on the black curve represent the Yb3þ/Tm3þ 2 co-doped BiPO4 phosphor and Yb3þ/Ho3þ co-doped BiPO4 phosphor, respectively. We know that the DE value for Yb3þ/Tm3þ 2 co-doped BiPO4 phosphor is closer to the value of 208.50 cm1 than that of the Yb3þ/Ho3þ co-doped BiPO4 phosphor. Consequently, we can

conclude that Yb3þ/Tm3þ 2 co-doped BiPO4 phosphor is more suitable for the temperature 300 K at the low temperature zone. For the high temperature zone, we select the TH ¼ 560 K, and the KBT ¼ 389.20 cm1. According to the above discussion, the Yb3þ/ Er3þ co-doped BiPO4 and Yb3þ/Tm3þ 1 co-doped BiPO4 phosphors are more suitable for high temperature zone. From Fig. 9 (b), the blue and red points on the curve express the DЕ values of Yb3þ/Er3þ and Yb3þ/Tm3þ 1 co-doped BiPO4 phosphors, respectively. Similarly, the DЕ for Yb3þ/Er3þ co-doped BiPO4 phosphor is closer to the value of 389.20 cm1 than that of Yb3þ/Tm3þ 1 co-doped BiPO4 phosphor. Naturally, we can conclude that Yb3þ/Er3þ co-doped BiPO4 phosphor is more suitable for the temperature range of 560 K at the high temperature zone. Relevant information is summarized in Table 1.

3.3.4. The relationship between SR and T The relative sensitivities SR for the Yb3þ/Er3þ, Yb3þ/Ho3þ, Yb3þ/ 3þ 3þ Tm3þ 1 and Yb /Tm2 in Fig. 10 show a monotone decreasing trend with the raising of temperature according to Eq. (4). Moreover, the values of SR for the Yb3þ/Er3þ and Yb3þ/Tm3þ 1 co-doped BiPO4 phosphors are larger than that of the Yb3þ/Ho3þ and Yb3þ/Tm3þ 2 codoped BiPO4 phosphors. It means that the Yb3þ/Er3þ co-doped BiPO4 and Yb3þ/Tm3þ 1 co-doped BiPO4 phosphors could be good candidate materials for optical temperature sensors.

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379

4. Conclusions In summary, under 980 nm excitation, up-conversion luminescence and the temperature dependence of up-conversion emission spectra from the thermal couple levels of rare earth ions based on the Yb3þ/Ho3þ, Yb3þ/Er3þ and Yb3þ/Tm3þ co-doped BiPO4 phosphors are studied for investigating temperature sensing mechanism. We explore the relationships among the absolute sensitivity (SA), relative sensitivity (SR) and temperature (T). When DE of the temperature sensing material is given, we can ascertain the optimal temperature ranging for reaching the maximum of absolute sensitivity by the T ¼ DE/2KB. In parallel, when the measurement temperature is given, we can select the excellent temperature sensing phosphors by the DE ¼ KBT. It is of great significance for the design and selection of temperature sensing materials. Acknowledgment This work was supported by the Science and Technology Development Planning Project of Jilin Province (No.20160101294JC), partially sponsored by Special funds for provincial industrial innovation in Jilin Province (No. 2018C043-4) and by the Education Department of Jilin Province (No.2016112). References

Fig. 9. The fitting curves of the relationship between SA and DE: (a) in the low temperature zone (b) in the high temperature zone. Table 1 The maximum sensitivity values for BiPO4: RE3þ by the FIR technique are shown, and the corresponding transitions from the TCL, emission wavelength and the energy gap (DE) are included.

DE (cm1) SA (K1) Tm(K)

Rare earth ions lem (nm) Transitions 3þ

3þ

Ho , Yb Er3þ, Yb3þ 3þ Tm3þ 1 , Yb 3þ Tm3þ 2 , Yb

652, 525, 700, 794,

657 551 804 804

5

5

5

F5(1), F5(2)/ I8 H11/2, 4S3/2 / 4I15/2 3 F2,3, 3H4/3H6 3 H4(1), 3H4(2)/3H6 2

46.06 754.73 1457.29 92.57

0.00079 0.00413 0.00022 0.00083

333 573 548 313

Fig. 10. The relative sensor sensitivity as a function of the temperature for Yb3þ/Er3þ co-doped BiPO4, Yb3þ/Ho3þ co-doped BiPO4 and Yb3þ/Tm3þ co-doped BiPO4.

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