Yb3+ phosphors based on fluorescence intensity ratios

Journal Pre-proof Optical Temperature Sensing Behavior for KLa(MoO4 )2 :Ho3+ /Yb3+ Phosphors Based on Fluorescence Intensity Ratios Yuhong Zhang, Tingting Wang, Hang Liu, Liu Dan, Yunhe Liu, Zuoling Fu

PII:

S0030-4026(19)31999-0

DOI:

https://doi.org/10.1016/j.ijleo.2019.164100

Reference:

IJLEO 164100

To appear in:

Optik

Received Date:

18 November 2019

Accepted Date:

18 December 2019

Please cite this article as: Zhang Y, Wang T, Liu H, Liu D, Liu Y, Fu Z, Optical Temperature Sensing Behavior for KLa(MoO4 )2 :Ho3+ /Yb3+ Phosphors Based on Fluorescence Intensity Ratios, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.164100

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Optical Temperature Sensing Behavior for KLa(MoO4)2:Ho3+/Yb3+ Phosphors Based on Fluorescence Intensity Ratios Yuhong Zhanga*, Tingting Wanga, Hang Liua*, Dan Liua, Yunhe Liua, Zuoling Fub a.School of electrical and computer engineering, Jilin jianzhu university, Changchun 130118,China b.College of Physics, Jilin University; and Key Lab of Coherent Light, Atomic and Molecular Spectroscopy, Ministry of Education, Changchun 130012, China

E-mail: [email protected] [email protected]

Abstract: KLa(MoO4)2:Yb3+/Ho3+ phosphors were synthesized using hydrothermal method. The phosphors showed intense green (~546 nm) and weak red emissions

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(~651 nm, ~756 nm) by 980 nm excitation. Temperature-dependent fluorescence

intensity ratios (FIRs) are analyzed. The two spark green(5F4 /5S2, I540 nm/I550 nm) and red-green (I641

nm/I540 nm)

FIRs were studied at

313 K-503 K. The maximum

sensitivity was obtained at 503 K, which is 3.56% K-1. The results imply that the superior

to

most

of

the

reported

Ho3+

doped

materials.

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sensitivity is

KLa(MoO4)2:Yb3+/Ho3+ phosphors could potentially be used to temperature sensor.

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Keywords: upconversion luminescence, fluorescence intensity ratio, sensitivity,

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optical thermometry

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1.Introduction The measurement of temperature is important in many fields. In particular the non-contact temperature measurement technique has enhanced much interest of the researcher, due to it is a noninvasive measuring method. It can work in hash environments, oil refineries, and strong electromagnetic fields[1-4]. Therefore, the doped rare earth (RE) ions materials can be applied in the non-contact temperature sensor, which are based on FIR. In general, the FIR varies with temperature, the temperature can be obtained by calculating FIR from two thermally coupled levels (TCELs). And the FIR technology has the unique merits of high thermal and fast response [5-9]. Er3+, Tm3+, Dy3+, Eu3+, Nd3+, and Pr3+ have been studied for FIR-based temperature sensors[10-14]. These RE ions all have one and more TCELs. The population distribution of two coupled levels comply the well known Boltzmann distribution law. The temperature sensitivity relates to the energy difference(ΔE of 200 cm-1 ＜ΔE＜2000 cm-1). Then a large ΔE will contribute to obtain the high sensitivity. But the energy gap was limited. Because of this restriction, it needs to

explore new thermometry strategy for gaining high temperature sensitivity at present. Ho3+ has been verified as a promising candidate. On one hand, the Yb3+-Ho3+ codoped temperature sensing phosphors have been studied according to FIR technique based on a modified Boltzmann distribution formula of TCELs[15-17]. Besides, the FIR of the non-TCELs of Ho3+ are also contribute to the development of the Yb3+-Ho3+ codoped temperature sensing phosphor [18,19]. To obtained high quality temperature sensing phosphor, the selection of the matrix is another important factor besides the luminescent ions. The phosphor materials need to have low phonon frequency for high UC luminous efficiency. The alkali metal molybdate ALn(MoO4)2 with scheelite-like structure are regarded as a type of excellent matrix materials. They have good thermal stability and chemical

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properties[20,21]. They can act as light emitting center and yield effective energy transfer from host to luminescent centers. Higher luminous efficiency of the doped rare earth ions can be obtained in such materials. To the best of our knowledge, the

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2. Experimental The KLa1-x(MoO4)2:

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UC luminescence and temperature-dependence capabilities of KLa(MoO4)2: Ho3+ systems were rarely reported. KLa(MoO4)2:x%Ho3+/y%Yb3+ phosphors were prepared using a hydrothermal method. Their UC luminescence properties were presented and the temperature dependence of FIR was investigated. The green FIR (I540/I550) and red-green FIR (I641/I540) were studied for analyzing the temperature sensing capabilities from 313 K to 503 K. 5%Yb3+/x%Ho3+(1≤x≤3),

KLa1-y(MoO4)2:

y%Yb3+/

2%Ho3+(5≤y≤20) phosphors were prepared using hydrothermal method. First the

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solution A was obtained by mixing the solutions of La(NO3)3, Ho(NO3)3 and Yb(NO3)3. And the solution B comes from the (NH4)6Mo7O24·4H2O dissolved in distilled water. The two prepared solutions (A and B) were mixed and continuously

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stirred at room temperature until the solution was a milky white suspension. Then the

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KOH was added for adjusting the PH=7. Then the mixture was maintained at 180 ℃ for 12 h in a Teflon bottle. Finally, the samples were washed and dried at 60 ℃ for 8 h. The uniform distributing phosphors were obtained. The basic crystal structure of KLa(MoO4)2: Ho3+/Yb3+ samples was confirmed by X with CuKα radiation (λ=0.15406 nm) in the10–80° range (Rigaku D/Max-2500). The morphology of the samples was inspected by FE-SEM (JEOL JEM-6700F). The photoluminescence spectra were monitored using a Zolix Omni-λ500 spectrograph. The pumping is 980 nm laser. The temperature characteristics were measured from

313 to 503 K. 3. Results and discussion 3.1 XRD and SEM study For analyzing the phase purity of the phosphors, the XRD patterns of KLa(MoO4)2: 5%Yb3+ with different Ho3+ concentration are presented in Fig.1(a). It can be found that no any impurity or secondary phase was observed, which indicates the phosphor samples are purely fragrant and the RE ions have been well incorporated into the host lattice. For analyzing the material morphology, the SEM image of 5%Yb3+/2%Ho3+ codoped KLa(MoO4)2 sample was measured(Fig.1(b)). It is clearly

smooth, revealing high crystallinity of the sample. 3.2 UC luminescence properties

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seen that the particles are discrete with the size about ~2 um. The particle planes are

Fig.2 illustrates the emission spectra of the KLa(MoO4)2: x%Yb3+/y%Ho3+ phosphors by 980 nm excitation. The emission spectra are composed of intense green

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emission(~545 nm, (5F4,5S2)→5I8), and relatively weak red emission centered at 651

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nm(5F5→5I8) and 755 nm((5F4,5S2) →5I7). As shown in Fig.2a, the intensity of UC emissions reaches the maximum under the concentration of Ho3+ was fixed at 2 mol%.

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The concentration quenching effect would occur when the fixed Ho3+ exceeded 2%. Fig.2b illustrates the emission spectra of KLa(MoO4)2: Yb3+/Ho3+ with different Yb3+ concentration. The UC emission first intensity enhanced gradually, than

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decreased with enhancing Yb3+ concentration. The KLa(MoO4)2: 15%Yb3+/2%Ho3+ phosphor has the emission maximum value. Because of Yb3+ ion with a large absorption cross section at 980 nm excitation, the UC emission intensity of the Ho3+

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ions can be enhanced by the energy transfer (ET) process from Yb3+ to Ho3+ ion. The ET process is efficient at the Yb3+ concentration around 5% ~15%. But the distance

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between Yb3+-Yb3+ ions became shorter when the Yb3+ doping concentration reached 20%, then resulted in stronger interactions. So the phenomenon of concentrationdependent quenching generated. Based on the spectra revealed in Fig.2, the best doping concentrations of 2% Ho3+ and 15% Yb3+ are obtained. To better understand the UC process of KLa(MoO4)2: Ho3+/Yb3+ phosphors, the number of photons for populating the exciting state can be calculated by the following relation:

.

The n indicates the photon numbers. The n value can be obtained the slope of the

straight line of ln(I) vs ln(P). As Fig.3 shown, the calculated slopes are about 1.83, 1.32 and 2.21 for 545 nm (5F4/5S2 → 5I8) and 651 nm, 755 nm (5F5 → 5I8 and 5F4/5S2 → 5I7). The three n values were almost near 2. So the three emission processes all belonged to two-photon UC mechanism. According to the above results, the schematic energy level diagram is presented in Fig.4. First the transition from 2F7/2 to excited state 2F5/2 of Yb3+ ion based on ground state adsorption (GSA), and then the excited level 5I6 of Ho3+ ions was populated through the energy transfer (ET) processes from Yb3+ to Ho3+[As shown, ET1: 5I8(Ho3+)+2F5/2(Yb3+) →5I6(Ho3+)+2F7/2(Yb3+)]. And the population 5I6 state of Ho3+ ions also can be came from 5I8 to 5I6 by GSA. The excited ions in the 5I6 state are 5

F4/5S2 by coupling state[ET2:

5

I6(Ho3+)+2F5/2(Yb3+) →

5

F4/5S2

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excited to the

(Ho3+)+2F7/2(Yb3+)] or excited state absorption. And Ho3+ ions excited to 5I5 might be further excited to the

5

F2/5K8 state by ET3[ET3:

5

I5(Ho3+)+2F5/2(Yb3+) →

F2/5K8(Ho3+)+2F7/2(Yb3+)]. Thus, the populated 5F4/5S2 level relaxes radiation to the

5

I8 and 5I7 states, leading to emit the emission(549 nm, 756 nm). The populating the

5

F5 state of Ho3+ ions come from two processes. One process is a nonradiative(NR)

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5

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process (5F4, 5S2→5F5), the another process is the intermediary level 5I6 of Ho3+ relaxing to 5I7 and then populating the 5F5 state through ESA. The transition from 5F5

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to 5I8 of Ho3+ ions leads to emit the red emission (667 nm). 3.3 Optical temperature sensing behavior

For study the temperature sensing characteristics, the UC luminescence spectra

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of the KLa(MoO4)2: 15%Yb3+/2%Ho3+ sample were monitored in the temperature range from 313 K to 503 K at a 980 nm excitation. Fig.5 indicates the spectra from 520 nm to 570 nm at 333 K-463 K. It was observed that the green emission band

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decreased due to thermal quenching effect with increasing the sample temperature.

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The emission intensity at ~540 nm (5F4→5I8) is less comparison to that at ~550 nm (5S2→5I8) at 333 K, while the reverse trend is seen at higher temperature (463 K). Accordingly the intensity ratio of ∼540 nm and ∼550 nm was estimated for analyzing thermometry behavior. As shown in Fig.6(a), the FIR of ∼540 nm and ∼550 nm obviously changes. According to the literature, the ratio of ∼540 nm and ∼550 nm followed a Boltzmann type distribution. And the equation between them is[22] :

(1) Where I540 and I550 are the intensities of radiation from ∼540 nm and ∼550 nm, C is constant, k is the Boltzmann constant, △E is the energy gap between the 5F4 and 5

S2 levels, and T is the absolute temperature.

The variation of FIR with absolute temperature is shown in Fig.6(a). The experimental data can be well fitted with the following equation: R=19.2exp(527.2/T). From the value of ∆E/k, it can be calculated that the energy gap between 5

F4 and 5S2 is 373.19 cm-1. This value is close to the theoretical value 333 cm-1

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obtained from the up-conversion spectrum, indicating that our fitting operation is reliable. To study temperature sensing, the sensitivity(S) is an important parameter.

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According to the definition of sensitivity [22]:

(2)

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Fig.6(b) shows the calculated sensitivity of the KLa(MoO4)2: Yb3+/Ho3+ phosphor sensitivity curve calculated by equation (2). As can be seen that the sensitivity of the sample gradually decreased with increasing temperature. The maximum sensitivity

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value is 0.00556K-1 at 313 K.

As shown in Fig.7, With the increase of temperature, both the UC emission intensity of the red (5F5-5I8) and green (5F4/5S2-5I8) transitions decreased gradually.

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But the green intensities decreased much faster than the red UC emission intensity. However the energy gap between 5F4/5S2 and 5F5 states is about ~3000 cm-1, so the 5

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FIR technique is valid. Fortunately, As can be seen from Fig.4, the population of the F5 level comes from three pathways, but the two process related to the nonradiative

process. The one part of 5F5 level comes from the 5I6 level nonradiative relaxation to

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the 5I7 level in absorbing one photon energy. The another part of it comes from the nonradiative process of 5F4/5S2→5F5. Since the nonradiative relaxation is generated by the main lattice vibration, the nonradiative relaxation rate can enhance with the increasing temperature. This phenomenon can also be explained by the formula of the non-radiative relaxation rate of the excited state: (3)

(4) Apparently, the red and the green emissions can be applied for FIR temperature sensing due to their different thermal response. Fig.8(a) shows the plot of Ired/Igreen thermal evolution. The FIR values continuously increase with temperature increasing. The solid line can fit the extermination data by the following formula: (5)

For optical thermometry, the absolute sensor sensitivity is important. S also can

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be expressed as follows: S=dFIR/dT The sensitivity of red and green light intensity is shown in Fig.8(b). As can be seen that the sensitivity reach the maximal value of 3.56%K-1 at 503 K. For comparison, the other Ho3+ doped thermometric materials are listed in Table 1. Overall, the KLa(MoO4)2: Yb3+/Ho3+ showed higher sensitivities. The above results

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imply that the KLa(MoO4)2: Yb3+/Ho3+ could be a versatile material for optical thermometry.

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4. Conclusion In conclusion, KLa(MoO4)2: Yb3+/Ho3+ phosphors have been synthesized by a hydrothermal method. Under 980 nm excitation, the phosphors showed a strong green

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band peaking at 546 nm and two weak red emission at 651 nm and 756 nm. The best optimal luminescence is obtained in KLa(MoO4)2: 15%Yb3+, 2%Ho3+. The possible UC mechanism was discussed, which can be attributed to the dependence of emission intensities on the pumping power, and the green (5F4/5S2 → 5I8) and red (5F5 → 5I8 and 5 F4/5S2 → 5I7) emissions were related to two-photon processes. Furthermore, the FIR(I540 /I550) originated from the TCEL (5F4 and 5S2) is applied for temperature range from 313 K to 503 K. The FIR (Ired /Igreen) originated from non-TCEL (5F4/5S2 and 5F5) also is discussed. The highest temperature sensitivity is 3.56%K-1 at 503 K, which is superior to most Ho3+ ions doped materials. Hence, the KLa(MoO4)2: Yb3+/Ho3+ could be applied for temperature sensor.

Declaration of Interest Statement The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgments This work was supported by Natural Science Foundation of China (Grant No:61705077); Project of Jilin Provincial Science and Technology Department (No:20190303064SF); Project of Jilin Province Development and Reform Commission (2019C048-4); Science Foundation of Jilin

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Province Education Department (JJKH20190853KJ).

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Fig.1(a) The X-ray diffraction pattern of KLa(MoO4)2: 5%Yb3+/x%Ho3+ Fig.1(b) The SEM image of KLa(MoO4)2: 5%Yb3+/2%Ho3+ Fig.2 The UC emission spectra of the KLa(MoO4)2: x%Yb3+/y%Ho3+ phosphors under 980 nm excitation

(a) Spectra of the KLa(MoO4)2: 5%Yb3+/x%Ho3+

(b) Spectra of the KLa(MoO4)2:

x%Yb3+/2%Ho3+ Fig.3 The UC intensity of red and green on the pump power dependence Fig.4 The proposed UC upconversion mechanism of KLa(MoO4)2: Yb3+/Ho3+ Fig.5 Temperature dependence on UC spectra of KLa(MoO4)2: 15%Yb3+/2%Ho3+ around 500-600 nm Fig.6 (a) Single exponential fitting of FIR of I540/I550 at 313 K-503 K

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(b) The measured sensitivities for the I540/I550 ratio

Fig.7 Temperature dependence on UC spectra of KLa(MoO4)2: 15%Yb3+/2%Ho3+ around 500-700 nm

Fig.8(a) Single exponential fitting of FIR of Iren/Igreen at 313 K-503 K

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(b) The measured sensitivities for the Ired/Igreen ratio

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Table 1 Summarized performance of Ho3+ doped materials showing temperature sensing parameters Temperature range [K]

Excitation wavelength (nm)

Maximum sensitivity[K-1]

Reference

ZnWO4: Ho3+/Yb3+

83-503

980

0.0064

[19]

Ho3+ (glass ceramic)

303-643

980

0.0102

[23]

β-NaLuF4: Yb3+/ Ho3+

390-780

980

0.0083

[24]

NaYb(MoO4)2: Yb3+/ Ho3+

323-573

980

0.00035

[25]

Ba2Gd2Si4O13:Yb3+ /Ho3+

293-553

980

0.0016

[26]

(Y0.88La0.09Zr0.03)2O3: Ho3+/ Yb3+

293-563

980

0.007128

[18]

CaMoO4:Ho3+/Yb3+/Mg2+

303-543

980

0.0066

[15]

Y2O3:Ho3+/Yb3+/Zn2+

299-673

980

0.00302

[27]

KLa(MoO4)2: Ho3+/Yb3+

293-473

980

0.0356

This work

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Sample