Temperature-dependent luminescence of a phosphor mixture of Li2TiO3: Mn4+ and Y2O3: Dy3+ for dual-mode optical thermometry

Journal of Alloys and Compounds 821 (2020) 153467

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Temperature-dependent luminescence of a phosphor mixture of Li2TiO3: Mn4þ and Y2O3: Dy3þ for dual-mode optical thermometry Chunyan Xie a, Peng Wang a, Yan Lin a, Xiantao Wei b, Min Yin a, Yonghu Chen a, * a

Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province, 230026, PR China b Physics Experiment Teaching Center, School of Physical Sciences, University of Science and Technology of China, Hefei, 230026, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 September 2019 Received in revised form 17 December 2019 Accepted 19 December 2019 Available online 20 December 2019

Mixture of Mn4þ-doped phosphor (Li2TiO3:Mn4þ) and Dy3þ-doped phosphor (Y2O3: Dy3þ) were prepared and investigated for the application of optical thermometry. X-ray powder diffraction and luminescence spectroscopy measurements were performed on all samples to analysis their optical properties. In particular, temperature-dependent luminescence and fluorescence lifetime of the mixture sample were measured in the temperature range of 273e373 K. Further analysis showed that the mixture sample used for temperature sensing has an excellent relative sensitivity with maximum value of 4.34% K1 at 288 K based on fluorescence intensity ratio (FIR) and 6.67% K1 at 339 K based on fluorescence lifetime, respectively. All these investigations suggest that the mixture phosphor is very promising in dual-mode high-sensitivity optical thermometry. © 2019 Elsevier B.V. All rights reserved.

Keywords: Phosphor mixture Fluorescence intensity ratio Lifetime Optical thermometry

1. Introduction Recently, non-contact optical thermometer based on the fluorescence intensity ratio (FIR) of rare earth (RE) elicits a tremendous fascination on many researchers because of its superior advantages such as immunity to fluorescent intensity losses, being independent of external interferences and insensitive to fluctuation of excitation power [1e14]. As a special example, the competition of electron population in the two thermally coupled states (TCS) of RE at different temperatures will lead to the variation of FIR between these two states. But, undesirably, the relative sensitivity (Sr) of thermometer based on the two TCS is proportional to, and thus limited by, the energy gap between them. The small energy gap will result in an unfavorable measurement sensitivity and annoying overlap between the two emission bands from the TCS. Nor the energy gap can be too large either, for then the two TCS cannot be thermally coupled [2,10,15,16]. Accordingly, a novel optical thermometer based on FIR with high sensitivity is still wanted. For this purpose, a multitude of researchers have examined the dualactivator luminescence for optical thermometry [5e7,9,14]. The mechanism of FIR based on dual-activator is owing to the

* Corresponding author. E-mail address: [email protected] (Y. Chen). https://doi.org/10.1016/j.jallcom.2019.153467 0925-8388/© 2019 Elsevier B.V. All rights reserved.

remarkable temperature quenching phenomena where the emission intensity decreases with the temperature rising. Particularly, the emission of transition metal ions usually experiences a rapid decline with temperature increase in the range of 0e100  C because of the environment sensitive nature of their 3dn emissions. While the trivalent lanthanide ions emissions due to shielded intraconfigurational 4f-4f transitions just have a negligible small change in this temperature range [17]. Therefore, the FIR between the transition metal ions and the trivalent lanthanide ions is expected to change dramatically with temperature rising, which will lead to more excellent temperature sensitivity. In this work, we intend to enrich this approach by preparing the phosphor mixture of Li2TiO3: 0.15% Mn4þ and Y2O3: 1% Dy3þ and improve the sensitivity. The emissions of Mn4þ and Dy3þ ions do not overlap completely, which is a paramount reason to choose the relevant activators (Mn4þ and Dy3þ ions in this paper). And secondly, the emissions of Mn4þ just have one band; Dy3þ have there peaks when excited by 355 nm, and the strongest is located at 574 nm which is used to defined the FIR. So the utilization ratio of the emission spectra of the sample is high, which is beneficial for improving the signal to noise ratio.Y2O3 phosphor exhibits pleasurable thermal stability at relatively low temperatures. Furthermore, it has other agreeable properties such as large bandgap, broad transparency and high refractive index. Last but not least, the ion radius of Dy3þ (0.912 Å) is close to the radius of Y3þ (0.900 Å).

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Based on the above properties, we choose Y2O3 as the host for Dy3þ. Y2O3: Dy3þ phosphor was obtained by the low temperature solution combustion method [13]. As for the reason for choosing Li2TiO3 as the host for Mn4þ, the main consideration is the available octahedral sites ([TiO6]) which is crucial for the red emission of Mn4þ ions. And Li2TiO3: Mn4þ phosphor was synthesized via solid-state reactions.

2. Experimental detail Single-doped Li2TiO3: 0.15% Mn4þ were synthesized by high temperature solid-state reaction method. The high purity Li2CO3 (3 N), TiO2 (CP) and MnCO3 (3 N) were weighed according to the stoichiometric ratio of samples. Mixtures of the raw powders were ground with appropriate ethanol in an agate mortar for 1 h to ensure homogeneity. And then the obtained mixtures were sintered at 1200  C for 4 h. The sample powders were obtained after grinding again. Y2O3: 1% Dy3þ phosphor was obtained by the low temperature solution combustion method. Firstly, RE(NO3)3 standard solutions (Y/Dy ¼ 99:1) was prepared by dissolving the raw material Y2O3 (4 N) and Dy2O3 (4 N) in hot dilute nitric acid. And then it was added into 24 mL aqueous solution containing 0.564 g glycine. Subsequently, the pH value of the mixture was adjusted to 7 using dilute ammonia. The solution was stirred to be transparent and then put into a muffle furnace preheated to 600  C for 10 min. After that, the combustion process finished and fluffy powder was obtained. Finally, the fluffy powder was collected and sintered at 1000  C for 2 h. We mix the different mole proportions of Li2TiO3: Mn4þ and Y2O3: Dy3þ, and then grind. Three final samples were obtained. The mole proportion of Mn4þ and Dy3þ in each sample is 1:1 (SAM1), 2:1 (SAM2) and 1:2 (SAM3), respectively. The crystal phases of the obtained samples were analyzed by using an X-ray powder diffractometer (Rigaku-TTR-III), equipped with nickel filtered Cu Ka radiation (l ¼ 0.15418 nm). Both the photoluminescence emission (PL) and excitation (PLE) spectra of each sample were recorded at room temperature by a HITACHI 850 fluorescence spectrometer, which utilized a 150W Xe lamp as its excitation source. The temperature-dependent emission spectra were obtained by a charge coupled device (CCD) (Andor DU401BVF), the decay curves were recorded by a Tektronix TDS2024 digital storage oscilloscope, and the tunable laser system (Model Opolette 355 LD OPO system) was used as an excitation light source, whose laser line width is 4e7 cm1 and pulse duration is 7 ns. The temperature was controlled by a temperature controller (Lake Shore Model 335) equipped with a type-K thermocouple and a heating tube.

3. Results and discussion 3.1. Structure and photoluminescence properties The XRD patterns of Li2TiO3: Mn4þ and Y2O3:Dy3þ power and the corresponding standard cards are shown in Fig. 1. The XRD patterns of Y2O3:Dy3þ and Li2TiO3:Mn4þ phosphor is consistent with the JCPDS cards No. 5e574 and No. 33e831, respectively, which means Y2O3:Dy3þ and Li2TiO3:Mn4þ phosphor are obtained successfully. Fig. 2(a) and Fig. 2(b) presents the PL and PLE spectra of Mn4þ single-doped Li2TiO3 and Dy3þ single-doped Y2O3 samples at room temperature, respectively. The most effective excitation light for both Mn4þ ions and Dy3þ ions are all situated around 350 nm as revealed in Fig. 2. The PL spectrum of Li2TiO3: Mn4þ under the excitation of 355 nm contains two emission peaks centered at 681 nm and 696 nm, which are corresponding to the transitions of 2 E/4A2. The PLE spectrum is obtained when monitoring 681 nm. Three wide excitation bands are obtained, which are corresponding to O2--Mn4þ (331 nm) charge transfer and Mn4þ: 4A2/4T1 (350 nm), and 4A2/4T2 (494 nm) transitions [18e21]. The PL spectrum of Y2O3:Dy3þ under the excitation of 355 nm shows two dominant emission peaks at around 486 nm and 574 nm, which is corresponding to 4F9/2 / 6H15/2 and 4F9/2 / 6H13/2 transitions of Dy3þ, respectively. The PLE spectrum monitored at 574 nm emission exhibits several characteristic strong lines in the range of 300e500 nm corresponding to the f-f transitions of Dy3þ, for instance, the peak centered at 350 nm is owing to 6H15/2 / 4I13/2 [22e24]. The energy levels and the luminescence mechanisms for both Li2TiO3:Mn4þ and Y2O3:Dy3þ are demonstrated in Fig. 3. Under ultraviolet 355 nm excitation, Mn4þ ions are excited from 4A2 to 2T2 level and then relaxed to the 2E level through non-radiation process. Eventually, these ions return to 4A2 ground state by producing a red emission at 681 nm. At the same time, Dy3þ ions are excited from the ground state to 4I13/2 level firstly and then relaxed to 4F9/2 level via non-radiation process. Subsequently, those Dy3þ ions at 4 F9/2 level return to the 6H15/2, 6H13/2 and 6H11/2 level by emitting light at 486 nm, 574 nm and 670 nm, respectively. 3.2. Temperature dependent luminescence To study the temperature sensitivity of SAM1 as a temperature sensor, the temperature-dependent PL spectra were measured at the temperature range of 273e373 K, which are plotted in Fig. 4(a). With increasing temperature from 273 to 373 K, both Mn4þ and Dy3þ emissions are gradually decreasing due to the thermal

Fig. 1. XRD patterns of the powder samples and the corresponding JCPDS cards (a) Li2TiO3: Mn4þ; (b) Y2O3:Dy3þ.

C. Xie et al. / Journal of Alloys and Compounds 821 (2020) 153467

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have been normalized at 574 nm to better display the relative change of the Dy3þ and the Mn4þ emission intensities. As shown in Fig. 4(b), the red light from Mn4þ decreases dramatically with the rise of temperature when the green light from Dy3þ is normalized. The same phenomenon also occurred in the SAM2 and SAM3, as shown in Fig. S1 and Fig. S5 of the supporting information. To further investigate the suitability of SAM1 for temperature sensing, it is necessary to study the relationship between FIR and temperature. The temperature-dependent FIR could be approximated by the following equation [2,4,6,7,12,14]:

FIR ¼

Fig. 2. The PL and PLE spectra at room temperature of (a) Li2TiO3: Mn4þ; (b) Y2O3: Dy3þ.

  IDy3þ DE þB ¼ Aexp  kB T IMn4þ

The temperature-dependent integral FIR between the green (568e586 nm) and the red (670e720 nm) are plotted in Fig. 5(a) to quantitatively analysis the temperature sensing sensitivity of SAM1. The olive green dots are experimental data, and the orange curve is the exponential fitting curve describing the relation between FIR and temperature, which demonstrates a tendency that the FIR decreases rapidly with the rise of 1/T. The result suggests that SAM1 could be used for temperature sensing. The sensitivity is another crucial parameter to characterize the performance of a sensor. The absolute sensitivity Sa and the relative sensitivity Sr can be defined respectively as [10,11,13,15,25,26]:

dFIR j dT

(2)

1 dFIR j j FIR dT

(3)

SaFIR ¼ j

SrFIR ¼

Fig. 3. The energy levels and the luminescence mechanisms of Mn4þ and Dy3þ ions.

quenching. However, the intensity of Mn4þ emission is observed to decrease much more rapidly compared with the insignificant change of Dy3þ emissions. The temperature-dependent PL spectra

(1)

The theoretical fitting of the relative sensitivity Sr-FIR and the absolute sensitivity Sa-FIR of SAM1 based on FIR in the temperature range from 273 K to 373 K are presented in Fig. 5(b). The Sr-FIR increases with the rise of temperature until reaches a maximum value of about 4.34% K1 at 288 K, then it gradually decreases after crossing 288 K. The value of Sa-FIR increases monotonously in this temperature range and the maximum sensitivity is about 2.63% K1 at 373 K. The temperature-dependent FIR of SAM2 and SAM3 have the same phenomena as SAM1, which are plotted in Fig. S2 and Fig. S6, respectively. For all the samples, the value of FIR decreases with the rise of 1/T. The value of both Sa and Sr increases at first, but then Sa continues to increase, while Sr decreases after reaching a maximum value as temperature rising in the range of 273 Ke373 K. From the above, the value of relative sensitivity Sr is different when the ratio of Li2TiO3: Mn4þ and Y2O3: Dy3þ is varied in the mixture. So, if this approach will be developed in practical

Fig. 4. (a) Temperature-dependent PL spectra of SAM1 excited by 355 nm. (b) Temperature-dependent PL spectra normalized at 574 nm of SAM1.

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C. Xie et al. / Journal of Alloys and Compounds 821 (2020) 153467

Fig. 5. (a) Temperature dependence of experimental FIR and the fitted curve, (b) the temperature-dependent Sr and Sa of SAM1.

applications, the proportion of Li2TiO3: Mn4þ and Y2O3: Dy3þ should be optimized by more accurate measurements. To further explore the temperature sensing potential of the mixture, we examine the lifetime of Mn4þ and Dy3þ emissions in the range of 273e373 K, as demonstrated in Fig. 6. The decay curves of Mn4þ monitored at 681 nm are shown in Fig. 6(a). The lifetime of Mn4þ decreases rapidly when temperature rises to 373 K from 273 K. Contrastingly, the lifetime of Dy3þ just manifests a slight decrease in the range of temperature as displayed in Fig. 6(b). The experimental data reveals a fact that the emission of Dy3þ has no significant thermal quenching compared with Mn4þ. And the result is consistent with the temperature-dependent PL spectra. The emission of Mn4þ decreases to nearly only a quarter of its initial intensity when temperature rises to 313 K from 273 K, while the emission intensity of Dy3þ has only negligible changes as shown in Fig. 4(a). Considering the significant thermal quenching of Mn4þ, the temperature sensing scheme based on lifetime can be realized. As mentioned above, the emission lifetime of Mn4þ decreases with temperature as shown in Fig. 6(a). The decay curves can be fitted with an exponential equation [4,27]:

  t þB IðtÞ ¼ Aexp 

t

(4)

Where I(t) is the luminescence intensity at time t, A and B are constants, t is the lifetime. The temperature-dependent lifetime fitted in the temperature range from 273 to 373 K are plotted in Fig. 7(a). The correlation between lifetime and temperature can be fitted by the following formula [26e28]:

1

t

¼

1

t0



  DE 1 þ Cexp kB T

(5)

Here t and t0 are the lifetime at temperature T and 0 K, C is constant, DE is the thermal-quenching activation energy and kB is Boltzmann constant, respectively. The experimental data are wellfitted by Eq. (5) as shown in Fig. 7(a). And the thermal-quenching activation energy of Mn4þ is calculated to be 3674.49 cm1. Similar to the scheme based on FIR, the absolute sensitivity Salifetime and the relative sensitivity Sr-lifetime based on the emission lifetime of Mn4þ can be calculated respectively by the following equations [5,30]:

Salifetime ¼ j

dt j dT

(6)

Srlifetime ¼

1 dt j j t dT

(7)

The absolute sensitivity Sa-lifetime and the relative sensitivity Srbased on the emission lifetime of Mn4þ are presented in Fig. 7(b). Both Sa-lifetime and Sr-lifetime have the same trend: the value of Sa-lifetime and Sr-lifetime increases with the increase of temperature until meets a maximal sensitivity, and then it will gradually decrease after crossing the maximal point. As a result, the maximal Sa-lifetime and Srlifetime values of SAM1 are 0.22% K1 at 308K and 6.67% K1 at 339 K, respectively. Compared the Sr values based on lifetime and FIR respectively, it is revealed that the value of Sr based on the lifetime is larger than the value based on FIR. It is owing to the slight change of Dy3þ emission in the scheme based on FIR. FIR is defined as the emission lifetime

Fig. 6. Temperature-dependent emission decay curves of (a) 2E of Mn4þ (lem ¼ 681 nm, lex ¼ 355 nm) and 4F9/2 of Dy3þ (lem ¼ 574 nm, lex ¼ 355 nm) in the range of 273e373 K of SAM1.

C. Xie et al. / Journal of Alloys and Compounds 821 (2020) 153467

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Fig. 7. (a) The temperature-dependent lifetime of 2E of Mn4þ (lem ¼ 681 nm, lex ¼ 355 nm) in the temperature range of 273 Ke373 K and fitted curve. (b) The relative sensitivity Sr and absolute sensitivity Sa based on the emission lifetime of Mn4þ.

Declaration of competing interest

Table 1 Several typical temperature sensors and their relative sensitivity. Materials

Temperature range (K) Sr

BaLaMgNbO6

230e470

K3Y(PO4)2 NaY(WO4)2 Lu2MoO6 Ca2Gd8(SiO4)6O2 Ba3Y4O9 YF3/Ga2O3

293e553 298e573 298e473 293e553 298e573 RT- 573

Li2TiO3/Y2O3

273e373

Max

(% K1) Mode

Ref.

1.82 2.43 1.31 1.21 1.12 1.10 1.34 0.8

FIR Lifetime FIR FIR Lifetime FIR FIR FIR

4.6 6.67

FIR This work Lifetime This work

[5] [12] [25] [28] [29] [31] [32]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC) (11574298, 61635012, 11974338) and the National Key Research and Development Program of China (2016YFB0701001). Appendix A. Supplementary data

intensity ratio of Dy3þ and Mn4þ. Even the emission intensity has a slight change with temperature rising, it will weaken the actual change of FIR as temperature elevating. Nevertheless, the values of Sr based on both lifetime and FIR are superior to recent results as shown in Table 1.

4. Conclusions In conclusion, the different proportion mixtures of Li2TiO3: Mn4þ and Y2O3: Dy3þ are successfully obtained and investigated for the temperature sensing application. The temperature-dependent PL spectra and lifetime of Dy3þ and Mn4þ were carefully measured and scrutinized. All sample show a promising temperature sensing performance in the temperature range of 273 Ke373 K due to the different thermal behaviors of Mn4þ and Dy3þ: the maximum value of FIR-based relative sensitivity Sr of SAM1, SAM2 and SAM3 is 4.34% K1, 4.6% K1 and 3.14% K1, respectively. What’s more, the fluorescence lifetime of Mn4þ is also employed for temperature sensing and the maximum relative sensitivity of SAM1 is 6.67% K1. The thermal-quenching activation energy of Mn4þ was calculated to be 2903 cm1 and 3674 cm1 based on FIR and lifetime, respectively. All these investigations demonstrate that the mixture of Li2TiO3: Mn4þ and Y2O3: Dy3þ could be a promising candidate for high sensitivity optical temperature sensors.

Author contributions section Yonghu Chen and Chuanyan Xie complete experiments and writing mostly; Peng Wang and Yan Lin participate in many discussions; Xiantao Wei sets up an experimental platform; And Min Yin revises the article.

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