Lanthanide-doped Sc2O3 for highly sensitive temperature sensor

Optical Materials 93 (2019) 39–43

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Lanthanide-doped Sc2O3 for highly sensitive temperature sensor a,1

Hongyuan Sha , Zizheng He Zhongmin Sua,b,∗∗∗

a,1

a,b,∗

, Chun Li

a

a

, Xinyu Wang , Qian Jiang , Fanming Zeng

T a,b,∗∗

,

a School of Materials Science and Engineering, Optoelectronic Functional Materials Engineering Research Center of the Ministry of Education, Changchun University of Science and Technology, Changchun, Jilin, 130022, China b Jilin Provincial Science and Technology Innovation Center of Optical Materials and Chemistry, Changchun, Jilin, 130022, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Scandium oxide Nanoparticle Lanthanide doping Temperature sensing Up-conversion

Luminescent temperature sensors operated at wide temperature interval show promising technological potential. Here, the holmium and ytterbium co-doping of cubic phase Sc2O3:x%Ho3+/8%Yb3+ (x = 1.0–3.0) nanocrystals are synthesized by a facile coprecipitation annealing method. The strongest green and red up-conversion emission was obtained from Sc2O3:2%Ho3+/8%Yb3+ with 980 nm excitation. The temperature sensitivity steadily increases to 0.1285 K−1 at ∼83 K during cooling. Accordingly, the ratio of green and red up-conversion emission increases from 0.966 to 3.469 when cooling from 483 K to 83 K. This is in accordance with the changing of cross-relaxation process at different temperatures. This work highlights the fascinating potential of lanthanide-loped Sc2O3 for highly sensitive wide temperature monitoring.

1. Introduction

[11]. However, these thermally coupled energy levels up-conversion emission intensity is far lower than the green or red up-conversion emission intensity. Thus, it is very interesting to explore the ratio of the green to the red emissions of Ho3+ for high temperature sensing. There have been few studies of temperature dependent up-conversion luminescence properties in Ho3+/Yb3+ co-doped systems, due to the serious thermal quenching effect at high temperature lead to low up-conversion emission intensity. It is very important to find a highly efficient up-conversion host material to study the temperature sensitivity in Ho3+/Yb3+ co-doped system. Besides, the non-contact thermometry technique usually needs the host materials to have highly chemical, thermodynamic and physical stability for wide temperature thermometry [12]. As a rare-earth oxide, Sc2O3 exhibits excellent stable thermodynamic properties and optical properties, such as high melting point (2340 °C) [13,14], high volume refractive index (nH = 2.0 at, λ = 300 nm) [15] and high bandgap (5.7 eV) [16], it is an excellent candidate for optical temperature sensing. More importantly, Sc2O3 has smaller lattice constant (0.979 nm) [17] and larger density of cationic sites (3.34 × 1022 cm−1) [18], which greatly increases the energy

Lanthanide ions doped up-conversion luminescent materials have been widely used in biological imaging [1], LED [2], optical temperature sensing [3] and other fields. These promising applications primarily stem from the photoluminescence phenomenon, namely, photon conversion processes involving the internal electronic transitions of the 4f shell [4]. Among lots of Ln3+ ions, Ho3+ doped system exhibits intrinsic visible green or red up-conversion emission [5,6]. Unfortunately, there exists low light absorption of Ho3+ ions under the near-infrared illumination, which hinders its extensive technological applications. The co-doping with Yb3+ ions is an efficient way to obtain intensive upconversion emission [7,8]. It is original from the large absorption cross section of Yb3+ and the energy resonance between Yb3+ and intermediate energy level of Ho3+. Besides, the Ho3+/Yb3+ co-doped systems have greatly potential to optical temperature sensing [9,10]. This temperature sensitivity evaluation is generally based on the luminescence intensity ratio of two thermally coupled energy levels (5F2,3/3K8 and 5G6/5F1) with appropriate bandgap and population distribution

∗ Corresponding author. School of Materials Science and Engineering, Optoelectronic Functional Materials Engineering Research Center of the Ministry of Education, Changchun University of Science and Technology, Changchun, Jilin, 130022, China. ∗∗ Corresponding author. School of Materials Science and Engineering, Optoelectronic Functional Materials Engineering Research Center of the Ministry of Education, Changchun University of Science and Technology, Changchun, Jilin, 130022, China. ∗∗∗ Corresponding author. School of Materials Science and Engineering, Optoelectronic Functional Materials Engineering Research Center of the Ministry of Education, Changchun University of Science and Technology, Changchun, Jilin, 130022, China. E-mail addresses: [email protected] (C. Li), [email protected] (F. Zeng), [email protected] (Z. Su). 1 These authors contributed equally to this work and should be considered co-first authors.

https://doi.org/10.1016/j.optmat.2019.04.067 Received 12 March 2019; Received in revised form 28 April 2019; Accepted 30 April 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.

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transfer probability among various Ln3+ ions and further enhances the up-conversion emission in Sc2O3 [19]. Some reports are available for the temperature sensing study of Ho3+/Yb3+ co-doped materials but not discuss by the relationship with cross-relaxation process and temperature [10–12], thus the study of temperature sensing properties for Sc2O3:Ho3+/Yb3+ is important and necessary. In this paper, excellent cubic phase Sc2O3:2%Ho3+/8%Yb3+ can be prepared with 60 min at 900 °C, its high temperature sensing properties were investigated. And it discusses energy transfer process, analyzed the effect on temperature. 2. Experimental 2.1. Synthesis of lanthanide-doped Sc2O3 nanoparticles High purity powders of Sc2O3 (99.999%), Yb2O3 (99.999%) and Ho2O3 (99.999%) were used as initial materials, and were accurately weighed according to the calculated results. After pouring the initial materials into a beaker, appropriate amount of concentrated nitric acid was added with a magnetic stirring and heating (60 °C) until the solution was clear. When the nitrate solution was cooled to room temperature, the solution was anti-dropped into NH4HCO3 solution using the burette. The amount of NH4HCO3 solution should be calculated based on the amount of nitrate solution, and the nitrite should be slowly dropped to ensure the simultaneous precipitation of Ln3+. The pH of the reacted solution was adjusted to about 7 and continued heating for a period. Then, the heating device was turn off and the reacted solution kept stirring for 12 h, and then aging for another 12 h. Next, the precipitates were collected by centrifugation and washed three times with water and alcohol, respectively, which were then dried at 60 °C in an oven. After that, the obtained the precursor powder was placed in gradient furnaces at a calcining temperature of 900 °C and then calcined from 1 h. After cooled, it is taken out and the samples were obtained.

Fig. 1. XRD patterns of Sc2O3:x%Ho3+/8%Yb3+ (x=1.0, 1.5, 2.0, 2.5, 3.0) nanoparticals.

2.2. Characterization

Fig. 2. SEM micrograph of Sc2O3:2%Ho3+/8%Yb3+ nanoparticals.

The precursor powder and samples’ phase was analyzed by the Rigaku D/Max-rA Turn target X-ray diffractometer (radiation source: Cu target Kα1 ray, target voltage: 40 kV, electric current: 20 mA, step length: 0.02°, scan range: 10°∼80°). The phase micro-morphology was observed by Hitachi S-4800 scanning electron microscope. The luminescence spectra of samples with different doped-concentrations were measured by Edinburgh FLS980 (pump source: 980 nm). Temperature of the samples, ranging from 83 K to 483 K, was controlled by a temperature-controlled stage. 3. Results and discussion 3.1. Phase and morphology of lanthanide-doped Sc2O3 nanoparticle XRD patterns of Sc2O3:x%Ho3+/8%Yb3+ (x = 1.0, 1.5, 2.0, 2.5, 3.0) nanoparticles are presented in Fig. 1. It can be seen from the figure that excellent cubic phase Sc2O3 (JCPDS No.74-1128) can be got at different doping concentrations. With the increase of doping concentrations of Ho3+, the intensity of peaks decreased, the full width at half maximum of peaks increase, and the peak position gradually shifts to small-angle, indicating that doped Ln3+ (Yb3+ = 0.868 Å, Ho3+ = 0.901 Å, Sc3+ = 0.745 Å) with larger radius occupy Sc3+ sites result the lattice bigger and crystallization degree smaller. All peaks exhibit no significant change with the increase of calcination time. The SEM micrograph of Sc2O3:2%Ho3+/8%Yb3+ nanoparticles shows that the size of the nanoparticles is nearly 35 nm in Fig. 2.

Fig. 3. Up-conversion emission spectra of Sc2O3:x%Ho3+/8%Yb3+ (x=1.0, 1.5, 2.0, 2.5, 3.0) nanoparticals, inset shows relationship between up-conversion emission intensity and pump power for Sc2O3:2%Ho3+/8%Yb3+ nanoparticals.

main emission bands can be clearly distinguished in the spectrum. The green band (530-560 nm) is derived from the 5F4/5S2→5I8 transition of Ho3+; the red band (630-680 nm) is derived from the 5F5→5I8 transition of Ho3+. Additionally, the up-conversion emission intensity increases with the increasing Ho3+ concentration, and the largest intensity of emission peak is obtained from Sc2O3:2%Ho3+/8%Yb3+ nanoparticles. This is because that with the increase in the

3.2. Up-conversion emission of lanthanide-doped Sc2O3 nanoparticle The emission spectra of Sc2O3:x%Ho3+/8%Yb3+ (x = 1.0, 1.5, 2.0, 2.5, 3.0) excited by 980 nm laser were shown in Fig. 3. Obviously, two 40

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dN1/dt = (I σYb/ hν980) N0 − W1 N1 N2 − W2 N1 N4 − W3 N1 N3 − R1 N1 = 0 (3)

dN2/dt = −(I σHo/ hν980) N2 − W1 N1 N2 − CR1 N2 + R3 N3 + R 4 N4 + R5 N5 + R6 N6 + CR1 N6 = 0

dN3/dt = −W3 N1 N3 + CR1 N2 + NR1 N4 − R3 N3 = 0

(4) (5)

dN4 /dt = (I σHo/ hν980) N2 + W1 N1 N2 − W2 N1 N4 − NR1 N4 − R 4 N4 = 0 (6)

Fig. 4. Energy level diagram of Sc2O3:2%Ho version luminescence mechanism.

3+

3+

/8%Yb

and possible up-con-

concentration of Ho3+, the concentration of luminescent centers in the matrix is increased and causes the enhancement of up-conversion emission intensity. With the further increase of the concentration of Ho3+, the up-conversion emission intensity decreases, which can be attributed to the concentration quenching. The inset shows the red and green up-conversion emission intensity (I) measured at different laser pump powers (P) at 980 nm laser diodes. For the up-conversion process:

lnI = n × lnP

dN5/dt = W3 N1 N3 + NR2 N6 − R5 N5 = 0

(7)

dN6/dt = W2 N1 N4 − CR1 N6 − NR2 N6 − R6 N6 = 0

(8)

N0 (t = 0) = N2 (t = 0) = 1

(9)

N1 (t = 0) = N3 (t = 0) = N4 (t = 0) = N5 (t = 0) = N6 (t = 0) = 0 (10) In these equations, I is the laser intensity and hν980 is the contained energy of a photon under 980 nm laser diode excitation, σYb is the Yb3+ absorption cross-section for transition 2F7/2 → 2F5/2 and σHo is the Ho3+ absorption cross-section for transition 5I8→5I6. The energy transfer rate parameters W1, W2 and W3 are correspond to the processes ETU1, ETU2 and ETU3 in Fig. 8, respectively. Ni (i = 0-6) are the population densities of the 2F7/2, 2F5/2, 5I8, 5I7, 5I6, 5F5 and 5F4/5S2 respectively. CR1 is the cross-relaxation energy transfer (CR) rates of process 5 I7+5F4/5S2→5I6+5F5 and 5I8+5F4/5S2→5I7+5I4, respectively. NR1 is the non-radiation rates of 5I6→5I7 and NR2 is the non-radiation rates of 5 F4/5S2→5F5. R1, R3, R4, R5 and R6 are radiation rates. By further simplifying equations (1)–(7), it can be got:

(1)

For the sample, n of the green and red emission is 1.84 and 2.09, respectively, they are closer to 2, indicating that both green and red upconversion emission belong to two-photon process [20–22]. Fig. 4 shows the Ho3+ and Yb3+ energy level structures. The green up-conversion emission mechanism is that the Yb3+ in the ground state absorbs one photon transition to the excited state of the 2F5/2 energy level, and the excited Yb3+ transfers energy to the Ho3+ in the ground state to make it transition to the 5I6 energy level with the redundant energy dissipated to the crystal lattice by phonons (up-conversion en2 ergy transfer (ETU): F5/2(Yb3+)+5I8(Ho3+)→2F7/2(Yb3+) +5I6(Ho3+)). The Ho3+ in the ground state can also be assigned to the 5 I6 energy level by absorbing one photon (ground state absorption (GSA): 5I8(Ho3+)+hν980→5I6(Ho3+)). In this energy transfer process, the energy resonance between Yb3+ (2F5/2 ∼ 2F7/2) and Ho3+ (5I8 ∼ 5 I6) section dominates, because of the large absorption cross section of Yb3+ and the energy resonance between Yb3+ and intermediate energy level of Ho3+. Then, the Ho3+ absorb energy from Yb3+ (ETU: 5 I6(Ho3+)+2F5/2(Yb3+)→5F4/5S2(Ho3+)+2F7/2(Yb3+)) or photons (excited state absorption (ESA): 5I6(Ho3+)+hν980→5F4/5S2(Ho3+)) in this energy level and upward transition to the 5F4/5S2, then Ho3+ ion radiative transitions to the ground state and radiates 530-560 nm green up-conversion emission. The red up-conversion emission mechanism has two paths: (1) the Ho3+ at the 5I6 level non-radiates to the 5I7 level (non-radiative transition (NR): 5I6(Yb3+)→5I7(Yb3+)), and transfers the excess energy to the crystal lattice, and then absorbs the energy from Yb3+ (ETU: 5 I7(Ho3+)+2F5/2(Yb3+)→5F5(Ho3+)+2F7/2(Yb3+)) or photons (ESA: 5 I7(Ho3+)+hν980→5F5(Ho3+)) to the 5F5 energy level, the particles populated at this energy level radiate to the ground state, emitting red up-conversion emission at 640-680 nm; (2) the Ho3+ ion at the 5F4/5S2 level non-radiates to the 5F5 level (NR: 5F4/5S2(Ho3+)→5F5(Ho3+)), so that radiative transitions to the ground state and emits 630-680 nm red up-conversion emission.

N1 =

IσYb N0/ hν980 ∝I∝P (N0 + W1 N2 + W2 N4 + W3 N3)

N5 =

W1 N1⋅W2 N1⋅N2 R3 R5 NR2 + W3 N1⋅N2 (W1 N1⋅R6 NR1 + R 4 R6 CR1) ∝ In R3 R 4 R5 R6

∝ Pn N6 =

W1 N1⋅W2 N1⋅N2 ∝ I n ∝ Pn (R 4 + NR1)(R6 + CR1 + NR2)

(11)

(12) (13)

I553 = R6 N6 ν553

(14)

I672 = R5 N5 ν672

(15)

I553 W1 N1⋅W2 N1⋅R3⋅R6 ν553 = I672 R 4 R6 + R 4 CR1 + W3 N1 CR1 ν672

(16)

3.3. Rate equation theoretical of lanthanide-doped Sc2O3 nanoparticle The rate equations used to discuss the energy transfers process based on the energy level structures diagram proposed in Fig. 4 and explained above are:

Fig. 5. Up-conversion emission spectra of Sc2O3:2%Ho3+/8%Yb3+ with the temperature range from 83 to 483 K, inset shows CIE chromaticity diagram with Sc2O3:2%Ho3+/8%Yb3+ at the temperature range from 83 to 483 K.

dN0/dt = −(IσYb/ hν980) N0 + W1 N1 N2 + W2 N1 N4 + W3 N1 N3 + R1 N1 = 0 (2) 41

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Theoretically, the emission intensity of green up-conversion should be stronger more than that of red up-conversion emission [23]. This due to the non-radiative transitions of NR1 and NR2 are necessary for obtaining the red emission, and those redundant energy that dissipated to the crystal lattice higher than the phonon energy of Sc2O3 [24], so those non-radiative transition not easy to occur. However, due to the large cation concentration and the spacing between cations is small, making Ho3+ produces efficient cross-relaxation energy (CR) transfer in the lattice field. Therefore, Ho3+ ion transitions from the 5F4/5S2 level downward to the 5I4 level, transferring energy to another Ho3+ ion at the 5I8 level, causing it to upward transition to the 5I7 level to achieve red emission (CR1). By the way, due to the predominant position of N6, the possibility of CR1 implementation is further enhanced. In addition, the lifetime of 5I6 of Ho3+ ion is lower than that of 5I7 in general [25,26]. Therefore, the Ho3+ ions in the 5I7 level to absorb energy and have a high probability of upward transition, which increased the value of N5 and I672. Those to a certain extent makes the probability of red upconversion emission slightly higher than expected. Therefore, green upconversion is slightly stronger than that red up-conversion emission.

Fig. 6. Intensity of ratio of 552 nm and 672 nm with the temperature range from 83 to 483 K, inset shows the fitting function of FIR-T.

3.4. Temperature sensing property lanthanide-doped Sc2O3 nanoparticle The Sc2O3:2%Ho3+/8%Yb3+ nanoparticles temperature sensing properties were investigated that the strongest up-conversion emission can be got at this sample. The up-conversion emissions intensity decreased with the temperature increased 83–483 K is shown in Fig. 5. With increasing the temperature, the position of the up-conversion emission peaks didn't have obviously change. This phenomenon could contribute to the thermal agitation. It can be observed that the upconversion emission intensity decreased gradually with temperature due to the increased non-radiative relaxation of electrons between Ln3+ energy levels. Especially, the green up-conversion emission intensity decreased quickly, and red up-conversion emission intensity showed a slight decrease, and lead to the FIR (I553/I671) decreased greatly. In addition, the emission light of samples gradually closer to the red-light of the CIE chromaticity diagram with this temperature range shown in inset, and the distance between coordinates are increased with temperature increased. Besides, the FIR can be defined by the following equation [27]: Fig. 7. Sensing sensitivity of Sc2O3:2%Ho3+/8%Yb3+ as a function of temperature.

FIR =

I553 ΔE ⎞ = Aexp ⎛− +B I672 ⎝ kB T ⎠ ⎜

⎟

(17)

Here, FIR is luminescence intensity ratio, A and B are preexponential constant, ΔE is energy difference between 4f levels of the lanthanide ion, kB is the Boltzmann constant and T is temperature. Fig. 6 shows the ratio of the 553 nm and 672 nm up-conversion emissions against the absolute temperature. Due to the possibilities of non-radiative transition of all levels becomes stronger as the temperature increases by the assistance of phonons of the matrix, the NR1, NR2 and N5 are increased, and red up-conversion emission intensity decreased slowly. Therefore, the FIR decreased from 3.469 to 0.966 with the temperature increasing from 83 K to 483 K. By fitting the experimental date with Eq. (17), the coefficients A, B, ΔE/kB and R2 are −4.562, 3.665, 255.102 K and 0.998 shown at inset of Fig. 6. Considering practical applications, it is very important to investigate the relative sensitivity Sr, which can be defined as [28]:

Sr =

Fig. 8. Thermal stability of Sc2O3:2%Ho cooling cycle between 83 and 483 K.

3+

/8%Yb

3+

1 dFIR ΔE = FIR dT kB T 2

(18)

Fig. 7 presents the Sr as a function of temperature. So, the Sr will have increased with decreased of the T. The sensitivity increased gradually with decreasing temperature and reaching a maximum of 0.1285 K−1 at 83 K, and a high Sr (0.0041 K-1) can be got at 323 K. This temperature sensitivity is much higher than Yb3+/Ho3+ co-doped Y2O3 (0.0038 K-1) and Yb3+/Er3+ co-doped Gd2O3 (0.0039 K-1) in high

during the heating and

42

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temperature [29,30], and this value higher than the Yb3+/Ho3+ codoped Y2O3(0.097 K-1) in low temperature [31]. Considering higher value of CR1 at Sc2O3, lower T and higher ΔE of Ho3+ in Sc2O3, that produce such a high Sr value. Fig. 8 illustrates the thermal stability of the Sc2O3:x%Ho3+/8% 3+ Yb during the heating and cooling cycle between 83 and 483 K. The FIR recorded at 83, 283, and 483 K were nearly unchanged with uncertainty be close to 1% in 6 cycles of heating and cooling processes. That show reproducibility of this material and method. Moreover, based on the relative sensitivity, the temperature resolution (δT) can be defined as it is [32]:

δT =

[2]

[3]

[4]

[5]

[6]

1 δFIR Sr FIR

(19) [7]

Where δFIR/FIR is the relative uncertainty of temperature determination (in this case 0.3%) [33]. Evidently, the larger value of Sr will cause the smaller value of δT. Therefore, the value of δT is 0.0233, 0.7317 and 2.7273 at 83, 283, and 483 K, respectively. Due to the high melting point (2340 °C), high thermal conductivity (17 W/mK) [16] and high luminescence intensity, the Sc2O3:2%Ho3+/8%Yb3+ is a promising candidate for optical temperature sensing. Last but not least, it produced error in temperature detection generally that higher laser pump power would disturb the surface temperature of the sample. So, lower laser power excitation and lower temperature is obviously enhanced the detection accuracy. Under low temperature conditions, the occurrence of NR is extremely difficult due to the low probability of thermal vibration. So, this numerical value is approximately 0, and R3 and R4 are approximately constant. Through simultaneous Eqs. (16) and (17) at low temperatures, the relationship equation between CR1 and temperature can also be obtained:

FIR ≈

W1 N1 W2 N2 R3 νgreen

[9] [10]

[11]

[12] [13]

[14]

[15] [16]

C ΔE ⎞ = = Aexp ⎛− +B CR1 k ⎝ BT ⎠ ⎜

W3 N1 R 4 CR1 νred

[8]

⎟

[17]

(20)

[18]

At high temperatures, as thermal vibrations increase, lattice energy increases, phonon energy increases, and the probability of NR is not negligible, and R3 and R4 also change with temperature, so FIR deviates from this formula. The CR increased with temperature increased because the enhancement of lattice thermal vibration, and the distance of the lanthanide ions will be close sometime.

[19]

[20]

[21]

4. Conclusion

[22] 3+

3+

This paper presents the Ho and Yb doping induced high temperature sensing performance in Sc2O3 and energy transfer process analysis especially cross-relaxation. Under the excitation of 980 nm laser, the Sc2O3:2%Ho3+/8%Yb3+ nanoparticle can get the highest luminous intensity, and the relative sensitivity temperature dependent reaches the maximum value 0.1285 K−1 at 83 K. And it can be used as a low temperature warning indicator in space. Therefore, the Sc2O3:2% Ho3+/8%Yb3+ nanoparticle was a good candidate for optical lowtemperature sensing as green up-conversion material.

[23] [24] [25] [26]

[27]

[28]

Acknowledgements

[29]

This work was supported by Department of Science and Technology of Jilin Province of China (No.20160414043GH), Chinese Academy of Weapons (No.6141B012822, No.6141B012823), Changchun University of Science and Technology technology Innovation Fund (XJJLG-201812), and the Open Project of State Key Laboratory of Super hard Materials (Jilin University) (No.201807). It is grateful for Y. Liang of Institute of Chemistry, Chinese Academy of Sciences.

[30] [31]

[32]

[33]

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