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Temperature-dependent luminescence of BaLaMgNbO6:Mn4+, Dy3+ phosphor for dual-mode optical thermometry
Optical Materials 95 (2019) 109199
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Temperature-dependent luminescence of BaLaMgNbO6:Mn4+, Dy3+ phosphor for dual-mode optical thermometry
T
Yan Lina, Lu Zhaoa, Bin Jianga, Jiashan Maoa, Fengfeng Chib, Peng Wanga, Chunyan Xiea, Xiantao Weic, Yonghu Chena,**, Min Yina,* 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 New Energy Technology Engineering Laboratory of Jiangsu Province & School of Science, Nanjing University of Posts and Telecommunications, Nanjing Province, 210023, PR China c Physics Experiment Teaching Center, School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui Province, 230026, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: BaLaMgNbO6 phosphor Fluorescence intensity ratio Lifetime Optical thermometry
We have co-doped Mn4+ and Dy3+ into a perovskite structure host BaLaMgNbO6 (BLMN) in order to develop a new scheme of dual-mode optical thermometry. All the samples including single-doped and co-doped samples were successfully synthesized via a high-temperature solid-state reaction method. The phase purity and luminescence properties of the phosphors were characterized by X-ray powder diffraction, photoluminescence excitation and emission spectra. In particular, temperature-dependent emission spectra as well as luminescence decay curves measurements were performed on the co-doped BLMN:Mn4+, Dy3+ sample to inspect its temperature-dependent luminescence properties. Under 355 nm excitation, the fluorescence intensity ratio (FIR) between Dy3+ emission at 580 nm (4F9/2 → 6H13/2) and Mn4+ emission at 698 nm (2Eg→4A2g) as well as the fluorescence lifetime of Mn4+ (2Eg→4A2g) were investigated in the temperature range from 230 K to 470 K, and the maximum relative sensitivities are 1.82% K−1 at 457 K and 2.43% K−1 at 437 K, respectively. It should be noted that the emission peak of Mn4+ whose luminescence can be employed by lifetime-mode temperature sensing is located in the optical window of biological tissue, which could be used in biological application. All these investigations suggest that the BLMN:Mn4+, Dy3+ phosphor is very promising in dual-mode high-sensitivity optical thermometry.
1. Introduction Recently, non-contact optical thermometers have attracted extensive attentions due to their higher spatial resolution and faster response than the conventional contact temperature sensors among other advantages which open their wide applications in industrial and scientific fields [1–3]. Among the existing schemes for the optical thermometers already realized, rare earth (RE) based materials have been widely explored because of their special temperature-dependent luminescent properties such as emission bandwidth, spectral shape, peak position, emission intensity, fluorescence intensity ratio (FIR) and fluorescence lifetime [4–14]. In particular, optical thermometry based on the FIR are generally considered to be the most promising by virtue of their independence on external interferences and immunity to spectral losses, as well as fluctuations of the excitation density [15–17].
*
Most optical thermometers of FIR approach are based on the emission of thermally coupled energy levels (TCELs), whose temperature detection sensitivity and corresponding temperature detection range are determined by the energy level structure of relevant TCELs [18–20]. The relative temperature sensitivity of FIR technique based on TCELs is proportional to the energy gap between two TCELs. However, the energy gap of TCELs is restricted to be in the range of 200–2000 cm−1 and TCELs will be thermally decoupled if their energy gap is beyond 2000 cm−1. Therefore, the relative temperature sensitivity in FIR technique based on TCELs has limitations imposed by the restriction of TCELs. Considering the disadvantage of TCELs mentioned above, new FIR-type TCELs-free scheme for temperature sensing is still in need. As a promising approach, the FIR technique based on dual-activators, transition metal and rare earth (TM/RE) ions, have been explored
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Chen), [email protected] (M. Yin).
**
https://doi.org/10.1016/j.optmat.2019.109199 Received 19 April 2019; Received in revised form 28 May 2019; Accepted 14 June 2019 0925-3467/ © 2019 Published by Elsevier B.V.
Optical Materials 95 (2019) 109199
Y. Lin, et al.
owing to their different thermal quenching channels for RE ions with 4fn electronic configurations having a weak electron-lattice interaction and TM ions with 3dn ones exhibiting a strong electron-phonon coupling, which may induce significant temperature-sensitive FIR to get a higher temperature sensitivity [21]. In order to realize efficient emissions of both TM and RE activators, the most challenging task is to find appropriate hosts. Because hosts are required to provide different sites for TM and RE activators. Furthermore, TM ions in those host are able to exhibit a strong electron-phonon coupling and the temperature-dependent luminescence properties. The reported hosts which are suitable for TM/RE dual-activators currently are quite scarce. Up to now, the reported hosts used for TM/RE co-doping only has M3Al5O12(M = Y, Lu, Gd) [22–26]. As an alternative, using a mixture (such as Y2O3:Ho3+/Mg2TiO4:Mn4+ [27]) avoids the difficulty of selecting appropriate hosts. However, the fatal shortcoming of the mixture is that the service life of the two materials is different, which may cause inconvenience in the future. Thus, searching appropriate hosts for TM/RE ions is still urgent. In recent years, Mn4+-activated red phosphors with perovskite structure have been extensively studied for their application in LEDs. One of them, BaLaMgNbO6 (BLMN) [28], is regarded as potential hosts for TM/RE ions co-doping by our research, which can also broaden the application of perovskite materials in the temperature sensing. Mn4+-doping BLMN produces bright luminescence from the transition of 2Eg→4A2g which dramatically decreases with the rising temperature [29]. In addition, the luminescence of Mn4+ is usually in red-infrared region which is in the optical window of biological tissue. Thus the temperature-dependent fluorescence lifetime of Mn4+ employed as lifetime-mode optical thermometry should also be investigated to explore its latent applications in bioscience. This means if we co-dope Mn4+ ions with one kind of RE ions into BLMN, we may obtain a kind of phosphor with dual-mode temperature sensing function. In the present work, the novel BLMN:Mn4+, Dy3+ phosphor with efficient dual-activators luminescence was designed and synthesized via a high-temperature solid-state reaction method. Under 355 nm excitation, the FIR between Dy3+ (4F9/2 → 6H13/2) and Mn4+ (2Eg→4A2g) as well as the fluorescence lifetime of Mn4+ (2Eg→4A2g) were investigated in the temperature range from 230K to 470K. All these investigations demonstrated that the BLMN:Mn4+, Dy3+ phosphor is a promising material for applications as dual-mode temperature sensor with high sensitivity.
Fig. 1. XRD patterns of (a) BLMN:0.02% Mn4+, (b) BLMN:3% Dy3+, (c) BLMN:0.02% Mn4+, 3% Dy3+ samples as well as (d) the standard cubic BLMN (ICSD-51598) diffraction data.
Ltd., MXP18AHF, Tokyo, Japan), using nickel-filtered Cu Kα radiation (λ = 0.15418 nm) with the accelerating voltage 40.0 kV and the tube current 100.0 mA, respectively. The XRD profiles were collected in the 2θ range from 10° to 70°. Both the photoluminescence emission (PL) and excitation (PLE) spectra of each sample were measured by a HITACHI 850 fluorescence spectrometer with a 150 W Xe lamp as the excitation source at room temperature. To study the temperature dependence of luminescence, the PL spectra at different temperatures were measured by using a charge coupled device (CCD) (Andor DU401A-BVF). An Opolette 355 LD laser, which is tunable in the range of 410–2200 nm with a spectral line-width of 4–7 cm−1 and pulse duration of 7 ns, was used as excitation source. And a temperature controller (OMRON E5CC-800) with a type-K thermocouple and a heating tube was used to control the temperature. 3. Results and discussion 3.1. Phase and crystal structure analysis
2. Experimental 4+
2.1. Synthesis of Mn
3+
and Dy
The crystal structure of Mn4+, Dy3+, Mn4+/Dy3+ doped BLMN phosphors were identified by the X-ray diffraction patterns, as shown in Fig. 1. All diffraction peaks of samples match well with the standard BLMN (ICSD-51598), which indicate that the phosphors were successfully synthesized via the high-temperature solid-state reaction method. BLMN was selected as the host because it has suitable sites for both Mn4+ and Dy3+ emitting centers. Fig. 2 presents the crystal structure of BLMN. The BLMN crystal belongs to double-perovskite oxide and ex3 m [28]. Because of the hibits a cubic system with space group Fm ‾ approximate ionic radii between Mn4+ (r = 0.53 Å, CN = 6) and Nb5+ (r = 0.64 Å, CN = 6), the Mn4+ activator is easily entered into the octahedral site of Nb5+. In addition, Dy3+ ions prefer to substitute for the La3+ ions due to the similar radii between Dy3+ (r = 1.08 Å, CN = 9) and La3+ (r = 1.32 Å, CN = 12). (CN is Coordination Number)
doped BLMN phosphors
The samples of 0.02%Mn4+, 3%Dy3+, 0.02%Mn4+/3%Dy3+ doped BLMN were successfully synthesized through a high-temperature solidstate reaction method. Typically, raw materials containing BaCO3 (A.R.), MgO (99.99%), La2O3 (99.99%), Nb2O5 (A.R.), Dy2O3 (99.95%) and MnCO3 (99.99%) were stoichiometrically weighed according to the required chemical formula, then mixed and ground for about 30 min in an agate mortar with a pestle after adding an appropriate amount of ethanol until it dried to achieve uniformity. The mixtures were transferred to ceramic crucibles in a muffle furnace for pre-calcination at 500 °C for 3 h with a heating rate of 5 °C/min. After that the mixtures underwent another 1 min of grinding and were then transferred to the furnace for anneal at 1500 °C for 6 h in the air. Finally, the samples were cooled to room temperature naturally in the furnace, and they were reground again to obtained final samples for further characterizations.
3.2. Photoluminescence analysis The room temperature photoluminescence excitation (PLE) and emission (PL) spectra of BLMN:Mn4+, BLMN:Dy3+ and BLMN:Mn4+,Dy3+ phosphors are all presented in Fig. 3(a)-(c), respectively. As presented in Fig. 3(a), when monitored at 698 nm, the PLE spectrum of the Mn4+ single-doped BLMN consists of two broad excitation bands in the region of 300–600 nm, which are attributed to
2.2. Characterization of samples The crystal structures and phase purity of the obtained samples were verified by an X-ray powder diffractometer (MAC Science Co., 2
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Fig. 2. The crystal structure of BLMN which belongs to double-perovskite oxide 3 m. and exhibits a cubic system with space group Fm ‾
Fig. 4. The schematic energy level diagram of Mn4+ and Dy3+ in BLMN. Mn4+ and Dy3+ can be simultaneously excited under 355 nm and the emission we concern are 4F9/2 → 6H13/2 (Dy3+) and 2Eg→4A2g (Mn4+).
nonradiative relaxation and the emission from 4F9/2 → 6H15/2, 4F9/2 → H13/2, 4F9/2 → 6H11/2 and 4F9/2 → 6H9/2+6F11/2 could be detected.
6
3.3. Temperature-dependent luminescence analysis To further study the temperature properties of the luminescence, normalized temperature-dependent PL spectra of BLMN:Mn4+, Dy3+ phosphor in the temperature range from 230K to 470K is provided in Fig. 5(a). The intensity of Mn4+ luminescence is found to decrease quickly with increase of temperature. As transition metal ion, Mn4+ ion with an outer 3d3 electronic configuration is very sensitive to the surrounding environment, thus leading to a stronger electron-phonon coupling and more drastic thermal quenching. Configurational coordinate diagram of Mn4+ emitting centers in the BLMN host is depicted in Fig. 5(b). O and A are the lowest point in the 2Eg parabola and the crossing point of 2Eg and 4A2g states, respectively. ΔE is the activation energy which is the energy separation between O and A. At the room temperature, most of the electrons of 2Eg state return to the ground state 4A2g directly, issuing the emission corresponding to 2 Eg→4A2g transition. With the increasing of temperature, the electrons of 2Eg state can return to the ground state 4A2g by thermally overcoming an activation energy barrier of ΔE . The Mn4+ luminescence is nonradiatively thermally quenched between the 2Eg excited state and the 4 A2g ground state due to the role of strong electron–phonon coupling. However, the luminescence of Dy3+ ion coming from the transition of
Fig. 3. The room temperature PLE and PL spectra of (a) BLMN:Mn4+, (b) BLMN:Dy3+, (c) BLMN:Mn4+, Dy3+. 4 A2g → 4T1g and 4A2g → 2T2g+4T2g transitions of Mn4+ ions, respectively [28]. Under 355 nm excitation, the BLMN: Mn4+ phosphor shows an intense deep red emission peaked at 698 nm, which can be attributed to 2Eg→4A2g transition of Mn4+ ions in the octahedral environment. Fig. 3(b) shows that the PLE spectrum of Dy3+ single-doped BLMN consists of several excitation peaks from 4f-4f transitions when monitored at 584 nm. And under 355 nm excitation, some characteristic Dy3+ emission peaks located at 488 nm, 580 nm, 667 nm and 770 nm can be observed, which are attributed to 4F9/2 → 6H15/2, 4F9/2 → 6H13/2, 4 F9/2 → 6H11/2 and 4F9/2 → 6H9/2+6F11/2 transition of Dy3+ ions, respectively [29]. The results above show the PLE spectral overlap between Mn4+ and Dy3+ ions, which make possible of simultaneous excitation of Mn4+ and Dy3+ under a certain excitation wavelength such as 355 nm in Fig. 3(c). In addition, when monitored at 698 nm, the PLE spectra of the Mn4+ and Dy3+ co-doped BLMN is only consistent with the PLE spectra of the Mn4+, which indicates that there is no evident energy transfer between Mn4+ and Dy3+ ions. The schematic energy level diagram of Mn4+ and Dy3+ ions is depicted in Fig. 4. Mn4+ and Dy3+ can be simultaneously excited under 355 nm as analyzed above. Mn4+ situated in the ground-state 4A2g could be excited to 4T1g, which was followed by a series of nonradiative relaxation and the emission from 2Eg→4A2g could be detected. Furthermore, under the same excitation, Dy3+ situated in the ground-state 6 H15/2 could be excited to 6P7/2, which was followed by a series of
Fig. 5. (a) The normalized temperature-dependent PL spectra of BLMN: Mn4+, Dy3+ phosphors in the temperature range from 230K to 470K, which excited by 355 nm laser. (b) Configurational coordinate diagrams of Mn4+ emitting centers in the BLMN host, showing the quenching mechanism for the Mn4+ activator. 3
Optical Materials 95 (2019) 109199
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SR FIR =
1 dFIR × 100% FIR dT
(3)
SA FIR =
dFIR × 100% dT
(4)
The theoretical fitting of relative sensitivity SR and absolute sensitivity SA of BLMN:Mn4+, Dy3+ in the temperature range from 230K to 470K are presented in Fig. 7(b). It is found that SA reaches a maximum value of about 2.61% K−1 at 470K. Moreover, the SR value increases gradually and the maximum sensitivity is about 1.82% K−1 at 457K. 3.3.2. Temperature-dependent lifetime In addition to FIR, temperature-dependent fluorescence lifetime can also be exploited to realize in temperature sensing. Fig. 8 shows the normalized decay curves of 2Eg→4A2g transition of Mn4+ at a series of temperatures, revealing that the emission lifetime of Mn4+ decreases with temperature rise. The decay curves can be well-fitted with a double exponential equation:
Fig. 6. The integrated fluorescence intensity of 2Eg→4A2g transition of Mn4+ and 4F9/2 → 6H13/2 transition of Dy3+ in the temperature range from 230 K to 470 K.
t t I (t ) = C1 exp ⎛− ⎞ + C2 exp ⎛− ⎞ ⎝ τ1 ⎠ ⎝ τ2 ⎠ ⎜
3.3.1. Temperature-dependent FIR In FIR measurement, we chose the integrated area of certain range within 568–595 nm of Dy3+ (4F9/2 → 6H13/2) and 675–735 nm of Mn4+ (2Eg→4A2g) respectively, rather than the whole range to effectively improve signal to noise ratio. To quantitatively display the temperature-dependent variation, the integrated fluorescence intensity from these two ranges were calculated and presented in Fig. 6. The integrated fluorescence intensity of 2Eg→4A2g transition of Mn4+ is found to decrease quickly with the increase of temperature, whereas the integrated fluorescence intensity of 4F9/2 → 6H13/2 transition of Dy3+ only exhibits an insignificant increase. Therefore, the luminescence intensity of Dy3+ activator can be approximated as a constant. For the Mn4+ activator, the decrease in the PL intensity with an increase temperature can be expressed by the following equation [22].
‾τ =
IDy IMn
= B + Cexp (−ΔE / kB T )
⎟
(5)
∫ I (t ) tdt C1 τ12 + C2 τ22 = C1 τ1 + C2 τ2 ∫ I (t ) dt
(6)
The average lifetime calculated in the temperature range from 230K to 470K and the fitting are presented in Fig. 9(a). For evaluation of the thermal-quenching activation energy ΔE of the thermal quenching, we can fit the temperature-dependent lifetimes to a modified Arrhenius equation [32]:
τ0
τ(T) =
−ΔE
1 + Ce kB T
(7)
or −ΔE 1 1 = ⎛1 + Ce kB T ⎞ τ τ0 ⎝ ⎠
(8)
Here, τ(T) and τ0 are the lifetime at temperature T and 0 K, C is constant, ΔE is the thermal-quenching activation energy and kB is Boltzmann constant, respectively. As shown in Fig. 9(a), the experiment data can be fitted well by Eq. (9). And the thermal-quenching activation energy of Mn4+ was calculated to be 3871 cm−1, which is not much different from the result calculated by FIR. Similarly, the relative sensitivity SR and absolute sensitivity SA based on the emission lifetime of Mn4+ are calculated respectively by the following equations:
(1)
Here, I (T ) is the luminescence intensities at temperature T , I0 is the initial emission intensity, A is constant, ΔE is the thermal-quenching activation energy and kB is Boltzmann constant, respectively. The expression for the temperature dependence of FIR between Dy3+ and Mn4+ can be obtained:
FIR =
⎜
Here, I is the luminescence intensity, C1 and C2 are constants, τ1 and τ2 are the lifetimes, respectively. The result shows that the decay process of Mn4+ consist of fast and slow decay components which may be due to the different doping concentration distribution and the resulting different local environments around Mn4+ in the host [30,31]. The average lifetime can be calculated as the following equation:
its shielded 4f electrons is not greatly affected by the lattice environment, due to the screening of the outer 5s and 5p shell electrons. Therefore, a highly sensitive temperature detection can be expected by using FIR method in the Mn4+ and Dy3+ co-doped sample.
I0 I (T ) = 1 + Aexp (−ΔE / kB T )
⎟
(2)
As shown in Fig. 7(a), the experiment dates can be fitted well by Eq. (2). The change in FIR (IDy / IMn ) with variation of temperature is remarkable and the thermal-quenching activation energy of Mn4+ was calculated to be 3323 cm−1. To gain more insight into the temperature sensitivity, it is of great significance to investigate the sensing sensitivity of BLMN:Mn4+, Dy3+ for temperature sensing. The relative sensitivity SR and absolute sensitivity SA are two key parameters to evaluate the performance of the temperature sensors. The SR and SA are defined as the relative and absolute change of the FIR in response to the variation of temperature respectively, which can be obtained by the following equation [5,6,8–10].
SR Lifetime =
1 d ‾τ × 100% ‾τ dT
SA Lifetime =
d ‾τ dT
(9)
(10)
The relative sensitivity SR and absolute sensitivity SA based on the average fluorescence lifetime of Mn4+ are calculated and presented in Fig. 9(b), respectively. The SA represents the amount of change in lifetime over a unit temperature and its value increases at first and then decreases, reaching the maximum sensitivity of 0.51 × 10−2 ms⋅K−1 at 378 K. Moreover, it can be seen that SR reaches a maximum value of 2.43% K−1 at 437 K. Several typical temperature sensors based on Mn4+ or Dy3+ and 4
Optical Materials 95 (2019) 109199
Y. Lin, et al.
Fig. 7. (a) The temperature dependence of FIR between 2Eg→4A2g and 4F9/2 → 6H13/2 and the polynomial fitting. (b) The theoretical fitting of relative sensitivity SR and absolute sensitivity SA based on the FIR between Mn4+ and Dy3+. Table 1 Several typical temperature sensors based on Mn4+ or Dy3+ and their relative sensitivity. Materials
Temperature range (K)
SR Max (% K−1)
Mode
Ref.
Gd2Ti2O7:Dy3+ Y2Mg2TiO6:Mn4+ Lu3Al5O12:Mn4+ Gd3Al5O12:Mn3+, Mn4+ Y3Al5O12:Dy3+, Mn4+ Y2O3:Ho3+/Mg2TiO4:Mn4+
293–443 10–513 303–383 120–570 293–393 298–373
BaLaMgNbO6:Dy3+, Mn4+
230–470
1.68 0.14 3.75 2.08 3.16 4.61 0.9 1.82
FIR FIR Lifetime Lifetime FIR FIR Lifetime FIR
2.43
Lifetime
[33] [34] [32] [25] [22] [27] [27] This work This work
Mg2TiO4:Mn4+ because the service life of the two materials is different for mixture which may cause inconvenience in application. All these results indicate that BLMN is the appropriate host for TM/RE and BLMN:Mn4+, Dy3+ phosphor can be a potentially excellent candidate for optical temperature sensor with high relative sensitivity, which is based on both FIR and lifetime temperature readout. In addition, the emission peak of Mn4+ is located in the optical window of biological tissue which provides a possibility for application in biological temperature detection.
Fig. 8. The normalized decay curves of 2Eg→4A2g transition of Mn4+ at the temperatures range from 230 K to 470 K when monitored at 698 nm under 355 nm excitation.
their relative sensitivity were displayed in Table 1. Compared with those, the sensitivity of our sample based on FIR and fluorescence lifetime technique are not the highest but they are pretty good. For FIR technique, the sensitivity of our sample is higher than the sensitivity of Gd2Ti2O7:Dy3+ [33] which is based on the emission of TCELs. Furthermore, our sample may more practical than Y2O3:Ho3+/
4. Conclusion In conclusion, the novel BLMN:Mn4+, Dy3+ phosphor with dual-
Fig. 9. (a) The average lifetime calculated in the temperature range from 230 K to 470 K and the polynomial fitting. (b) The theoretical fitting of relative sensitivity SR and absolute sensitivity SA based on the emission lifetime of Mn4+. 5
Optical Materials 95 (2019) 109199
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activators luminescence was successfully designed and synthesized via a high-temperature solid-state reaction method. The temperature-dependent luminescent properties of BLMN:Mn4+, Dy3+ sample for the potential application in dual-mode optical temperature sensor were studied. Under 355 nm excitation, the change of FIR (IDy / IMn ) with variation of temperature is dramatic from 230 K to 470 K, the relative sensitivity reached a maximum value of 1.82% K−1 at 457 K, and the thermal-quenching activation energy of Mn4+ was calculated as 3323 cm−1. In addition, the fluorescence lifetime of Mn4+ (2Eg→4A2g) can also be employed for temperature sensing and the relative sensitivity reaches a maximum value of 2.43% K−1 at 437 K. The thermalquenching activation energy of Mn4+ was calculated to be 3323 cm−1 and 3871 cm−1 based on FIR and lifetime, respectively. These two results are not much different. Moreover, the emission peak of Mn4+ is located in the optical window of biological tissue in the lifetime mode readout, which provides a possibility for application in biological temperature detection. All these investigations demonstrated that the BLMN:Mn4+, Dy3+ phosphor could be a promising candidate for highsensitivity optical temperature sensors.
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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 financially supported by the National Natural Science Foundation of China (NSFC) (11574298, 61635012) and the National Key Research and Development Program of China (2016YFB0701001). References [1] A.H. Khalid, K. Kontis, Thermographic phosphors for high temperature measurements: principles, current state of the art and recent applications, Sensors-Basel 8 (2008) 5673–5744. [2] E.J. McLaurin, V.A. Vlaskin, D.R. Gamelin, Water-soluble dual-emitting nanocrystals for ratiometric optical thermometry, J. Am. Chem. Soc. 133 (2011) 14978–14980. [3] V. Lojpur, M.G. Nikolic, D. Jovanovic, M. Medic, Z. Antic, M.D. Dramicanin, Luminescence thermometry with Zn2SiO4:Mn2+ powder, Appl. Phys. Lett. 103 (2013). [4] Y. Zhou, F. Qin, Y. Zheng, Z. Zhang, W. Cao, Fluorescence intensity ratio method for temperature sensing, Opt. Lett. 40 (2015) 4544–4547. [5] F.F. Chi, X.T. Wei, S.S. Zhou, Y.H. Chen, C.K. Duan, M. Yin, Inorg. Enhanced 5 D0→7F4 transition and optical thermometry of garnet type Ca3Ga2Ge3O12:Eu3+ phosphors, Chem. Front. 5 (2018) 1288–1293. [6] L. Zhao, J. Mao, B. Jiang, X. Wei, Y. Chen, M. Yin, Temperature-dependent persistent luminescence of SrAl2O4:Eu2+, Dy3+, Tb3+: a strategy of optical thermometry avoiding real-time excitation, Opt. Lett. 43 (2018) 3882–3884. [7] Y. Song, B.Q. Shao, Y. Feng, W. Lü, G.X. Liu, H.P. You, A novel strategy to enhance the luminescence performance of NaGdF4: ln3+ nanocrystals, Dalton Trans. 45 (2016) 9468. [8] Z. Cao, X. Wei, L. Zhao, Y. Chen, M. Yin, Investigation of SrB4O7:Sm2+ as a multimode temperature sensor with high sensitivity, ACS Appl. Mater. Inter. 8 (2016) 34546–34551. [9] L. Zhao, J. Cai, F. Hu, X. Li, Z. Cao, X. Wei, Y. Chen, M. Yin, C. Duan, Optical thermometry based on thermal population of low-lying levels of Eu3+ in Ca2.94Eu0.04Sc2Si3O12, RSC Adv. 7 (2017) 7198–7202.
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Temperature-dependent luminescence of BaLaMgNbO6:Mn4+, Dy3+ phosphor for dual-mode optical thermometry
T
Yan Lina, Lu Zhaoa, Bin Jianga, Jiashan Maoa, Fengfeng Chib, Peng Wanga, Chunyan Xiea, Xiantao Weic, Yonghu Chena,**, Min Yina,* 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 New Energy Technology Engineering Laboratory of Jiangsu Province & School of Science, Nanjing University of Posts and Telecommunications, Nanjing Province, 210023, PR China c Physics Experiment Teaching Center, School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui Province, 230026, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: BaLaMgNbO6 phosphor Fluorescence intensity ratio Lifetime Optical thermometry
We have co-doped Mn4+ and Dy3+ into a perovskite structure host BaLaMgNbO6 (BLMN) in order to develop a new scheme of dual-mode optical thermometry. All the samples including single-doped and co-doped samples were successfully synthesized via a high-temperature solid-state reaction method. The phase purity and luminescence properties of the phosphors were characterized by X-ray powder diffraction, photoluminescence excitation and emission spectra. In particular, temperature-dependent emission spectra as well as luminescence decay curves measurements were performed on the co-doped BLMN:Mn4+, Dy3+ sample to inspect its temperature-dependent luminescence properties. Under 355 nm excitation, the fluorescence intensity ratio (FIR) between Dy3+ emission at 580 nm (4F9/2 → 6H13/2) and Mn4+ emission at 698 nm (2Eg→4A2g) as well as the fluorescence lifetime of Mn4+ (2Eg→4A2g) were investigated in the temperature range from 230 K to 470 K, and the maximum relative sensitivities are 1.82% K−1 at 457 K and 2.43% K−1 at 437 K, respectively. It should be noted that the emission peak of Mn4+ whose luminescence can be employed by lifetime-mode temperature sensing is located in the optical window of biological tissue, which could be used in biological application. All these investigations suggest that the BLMN:Mn4+, Dy3+ phosphor is very promising in dual-mode high-sensitivity optical thermometry.
1. Introduction Recently, non-contact optical thermometers have attracted extensive attentions due to their higher spatial resolution and faster response than the conventional contact temperature sensors among other advantages which open their wide applications in industrial and scientific fields [1–3]. Among the existing schemes for the optical thermometers already realized, rare earth (RE) based materials have been widely explored because of their special temperature-dependent luminescent properties such as emission bandwidth, spectral shape, peak position, emission intensity, fluorescence intensity ratio (FIR) and fluorescence lifetime [4–14]. In particular, optical thermometry based on the FIR are generally considered to be the most promising by virtue of their independence on external interferences and immunity to spectral losses, as well as fluctuations of the excitation density [15–17].
*
Most optical thermometers of FIR approach are based on the emission of thermally coupled energy levels (TCELs), whose temperature detection sensitivity and corresponding temperature detection range are determined by the energy level structure of relevant TCELs [18–20]. The relative temperature sensitivity of FIR technique based on TCELs is proportional to the energy gap between two TCELs. However, the energy gap of TCELs is restricted to be in the range of 200–2000 cm−1 and TCELs will be thermally decoupled if their energy gap is beyond 2000 cm−1. Therefore, the relative temperature sensitivity in FIR technique based on TCELs has limitations imposed by the restriction of TCELs. Considering the disadvantage of TCELs mentioned above, new FIR-type TCELs-free scheme for temperature sensing is still in need. As a promising approach, the FIR technique based on dual-activators, transition metal and rare earth (TM/RE) ions, have been explored
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y. Chen), [email protected] (M. Yin).
**
https://doi.org/10.1016/j.optmat.2019.109199 Received 19 April 2019; Received in revised form 28 May 2019; Accepted 14 June 2019 0925-3467/ © 2019 Published by Elsevier B.V.
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owing to their different thermal quenching channels for RE ions with 4fn electronic configurations having a weak electron-lattice interaction and TM ions with 3dn ones exhibiting a strong electron-phonon coupling, which may induce significant temperature-sensitive FIR to get a higher temperature sensitivity [21]. In order to realize efficient emissions of both TM and RE activators, the most challenging task is to find appropriate hosts. Because hosts are required to provide different sites for TM and RE activators. Furthermore, TM ions in those host are able to exhibit a strong electron-phonon coupling and the temperature-dependent luminescence properties. The reported hosts which are suitable for TM/RE dual-activators currently are quite scarce. Up to now, the reported hosts used for TM/RE co-doping only has M3Al5O12(M = Y, Lu, Gd) [22–26]. As an alternative, using a mixture (such as Y2O3:Ho3+/Mg2TiO4:Mn4+ [27]) avoids the difficulty of selecting appropriate hosts. However, the fatal shortcoming of the mixture is that the service life of the two materials is different, which may cause inconvenience in the future. Thus, searching appropriate hosts for TM/RE ions is still urgent. In recent years, Mn4+-activated red phosphors with perovskite structure have been extensively studied for their application in LEDs. One of them, BaLaMgNbO6 (BLMN) [28], is regarded as potential hosts for TM/RE ions co-doping by our research, which can also broaden the application of perovskite materials in the temperature sensing. Mn4+-doping BLMN produces bright luminescence from the transition of 2Eg→4A2g which dramatically decreases with the rising temperature [29]. In addition, the luminescence of Mn4+ is usually in red-infrared region which is in the optical window of biological tissue. Thus the temperature-dependent fluorescence lifetime of Mn4+ employed as lifetime-mode optical thermometry should also be investigated to explore its latent applications in bioscience. This means if we co-dope Mn4+ ions with one kind of RE ions into BLMN, we may obtain a kind of phosphor with dual-mode temperature sensing function. In the present work, the novel BLMN:Mn4+, Dy3+ phosphor with efficient dual-activators luminescence was designed and synthesized via a high-temperature solid-state reaction method. Under 355 nm excitation, the FIR between Dy3+ (4F9/2 → 6H13/2) and Mn4+ (2Eg→4A2g) as well as the fluorescence lifetime of Mn4+ (2Eg→4A2g) were investigated in the temperature range from 230K to 470K. All these investigations demonstrated that the BLMN:Mn4+, Dy3+ phosphor is a promising material for applications as dual-mode temperature sensor with high sensitivity.
Fig. 1. XRD patterns of (a) BLMN:0.02% Mn4+, (b) BLMN:3% Dy3+, (c) BLMN:0.02% Mn4+, 3% Dy3+ samples as well as (d) the standard cubic BLMN (ICSD-51598) diffraction data.
Ltd., MXP18AHF, Tokyo, Japan), using nickel-filtered Cu Kα radiation (λ = 0.15418 nm) with the accelerating voltage 40.0 kV and the tube current 100.0 mA, respectively. The XRD profiles were collected in the 2θ range from 10° to 70°. Both the photoluminescence emission (PL) and excitation (PLE) spectra of each sample were measured by a HITACHI 850 fluorescence spectrometer with a 150 W Xe lamp as the excitation source at room temperature. To study the temperature dependence of luminescence, the PL spectra at different temperatures were measured by using a charge coupled device (CCD) (Andor DU401A-BVF). An Opolette 355 LD laser, which is tunable in the range of 410–2200 nm with a spectral line-width of 4–7 cm−1 and pulse duration of 7 ns, was used as excitation source. And a temperature controller (OMRON E5CC-800) with a type-K thermocouple and a heating tube was used to control the temperature. 3. Results and discussion 3.1. Phase and crystal structure analysis
2. Experimental 4+
2.1. Synthesis of Mn
3+
and Dy
The crystal structure of Mn4+, Dy3+, Mn4+/Dy3+ doped BLMN phosphors were identified by the X-ray diffraction patterns, as shown in Fig. 1. All diffraction peaks of samples match well with the standard BLMN (ICSD-51598), which indicate that the phosphors were successfully synthesized via the high-temperature solid-state reaction method. BLMN was selected as the host because it has suitable sites for both Mn4+ and Dy3+ emitting centers. Fig. 2 presents the crystal structure of BLMN. The BLMN crystal belongs to double-perovskite oxide and ex3 m [28]. Because of the hibits a cubic system with space group Fm ‾ approximate ionic radii between Mn4+ (r = 0.53 Å, CN = 6) and Nb5+ (r = 0.64 Å, CN = 6), the Mn4+ activator is easily entered into the octahedral site of Nb5+. In addition, Dy3+ ions prefer to substitute for the La3+ ions due to the similar radii between Dy3+ (r = 1.08 Å, CN = 9) and La3+ (r = 1.32 Å, CN = 12). (CN is Coordination Number)
doped BLMN phosphors
The samples of 0.02%Mn4+, 3%Dy3+, 0.02%Mn4+/3%Dy3+ doped BLMN were successfully synthesized through a high-temperature solidstate reaction method. Typically, raw materials containing BaCO3 (A.R.), MgO (99.99%), La2O3 (99.99%), Nb2O5 (A.R.), Dy2O3 (99.95%) and MnCO3 (99.99%) were stoichiometrically weighed according to the required chemical formula, then mixed and ground for about 30 min in an agate mortar with a pestle after adding an appropriate amount of ethanol until it dried to achieve uniformity. The mixtures were transferred to ceramic crucibles in a muffle furnace for pre-calcination at 500 °C for 3 h with a heating rate of 5 °C/min. After that the mixtures underwent another 1 min of grinding and were then transferred to the furnace for anneal at 1500 °C for 6 h in the air. Finally, the samples were cooled to room temperature naturally in the furnace, and they were reground again to obtained final samples for further characterizations.
3.2. Photoluminescence analysis The room temperature photoluminescence excitation (PLE) and emission (PL) spectra of BLMN:Mn4+, BLMN:Dy3+ and BLMN:Mn4+,Dy3+ phosphors are all presented in Fig. 3(a)-(c), respectively. As presented in Fig. 3(a), when monitored at 698 nm, the PLE spectrum of the Mn4+ single-doped BLMN consists of two broad excitation bands in the region of 300–600 nm, which are attributed to
2.2. Characterization of samples The crystal structures and phase purity of the obtained samples were verified by an X-ray powder diffractometer (MAC Science Co., 2
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Fig. 2. The crystal structure of BLMN which belongs to double-perovskite oxide 3 m. and exhibits a cubic system with space group Fm ‾
Fig. 4. The schematic energy level diagram of Mn4+ and Dy3+ in BLMN. Mn4+ and Dy3+ can be simultaneously excited under 355 nm and the emission we concern are 4F9/2 → 6H13/2 (Dy3+) and 2Eg→4A2g (Mn4+).
nonradiative relaxation and the emission from 4F9/2 → 6H15/2, 4F9/2 → H13/2, 4F9/2 → 6H11/2 and 4F9/2 → 6H9/2+6F11/2 could be detected.
6
3.3. Temperature-dependent luminescence analysis To further study the temperature properties of the luminescence, normalized temperature-dependent PL spectra of BLMN:Mn4+, Dy3+ phosphor in the temperature range from 230K to 470K is provided in Fig. 5(a). The intensity of Mn4+ luminescence is found to decrease quickly with increase of temperature. As transition metal ion, Mn4+ ion with an outer 3d3 electronic configuration is very sensitive to the surrounding environment, thus leading to a stronger electron-phonon coupling and more drastic thermal quenching. Configurational coordinate diagram of Mn4+ emitting centers in the BLMN host is depicted in Fig. 5(b). O and A are the lowest point in the 2Eg parabola and the crossing point of 2Eg and 4A2g states, respectively. ΔE is the activation energy which is the energy separation between O and A. At the room temperature, most of the electrons of 2Eg state return to the ground state 4A2g directly, issuing the emission corresponding to 2 Eg→4A2g transition. With the increasing of temperature, the electrons of 2Eg state can return to the ground state 4A2g by thermally overcoming an activation energy barrier of ΔE . The Mn4+ luminescence is nonradiatively thermally quenched between the 2Eg excited state and the 4 A2g ground state due to the role of strong electron–phonon coupling. However, the luminescence of Dy3+ ion coming from the transition of
Fig. 3. The room temperature PLE and PL spectra of (a) BLMN:Mn4+, (b) BLMN:Dy3+, (c) BLMN:Mn4+, Dy3+. 4 A2g → 4T1g and 4A2g → 2T2g+4T2g transitions of Mn4+ ions, respectively [28]. Under 355 nm excitation, the BLMN: Mn4+ phosphor shows an intense deep red emission peaked at 698 nm, which can be attributed to 2Eg→4A2g transition of Mn4+ ions in the octahedral environment. Fig. 3(b) shows that the PLE spectrum of Dy3+ single-doped BLMN consists of several excitation peaks from 4f-4f transitions when monitored at 584 nm. And under 355 nm excitation, some characteristic Dy3+ emission peaks located at 488 nm, 580 nm, 667 nm and 770 nm can be observed, which are attributed to 4F9/2 → 6H15/2, 4F9/2 → 6H13/2, 4 F9/2 → 6H11/2 and 4F9/2 → 6H9/2+6F11/2 transition of Dy3+ ions, respectively [29]. The results above show the PLE spectral overlap between Mn4+ and Dy3+ ions, which make possible of simultaneous excitation of Mn4+ and Dy3+ under a certain excitation wavelength such as 355 nm in Fig. 3(c). In addition, when monitored at 698 nm, the PLE spectra of the Mn4+ and Dy3+ co-doped BLMN is only consistent with the PLE spectra of the Mn4+, which indicates that there is no evident energy transfer between Mn4+ and Dy3+ ions. The schematic energy level diagram of Mn4+ and Dy3+ ions is depicted in Fig. 4. Mn4+ and Dy3+ can be simultaneously excited under 355 nm as analyzed above. Mn4+ situated in the ground-state 4A2g could be excited to 4T1g, which was followed by a series of nonradiative relaxation and the emission from 2Eg→4A2g could be detected. Furthermore, under the same excitation, Dy3+ situated in the ground-state 6 H15/2 could be excited to 6P7/2, which was followed by a series of
Fig. 5. (a) The normalized temperature-dependent PL spectra of BLMN: Mn4+, Dy3+ phosphors in the temperature range from 230K to 470K, which excited by 355 nm laser. (b) Configurational coordinate diagrams of Mn4+ emitting centers in the BLMN host, showing the quenching mechanism for the Mn4+ activator. 3
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SR FIR =
1 dFIR × 100% FIR dT
(3)
SA FIR =
dFIR × 100% dT
(4)
The theoretical fitting of relative sensitivity SR and absolute sensitivity SA of BLMN:Mn4+, Dy3+ in the temperature range from 230K to 470K are presented in Fig. 7(b). It is found that SA reaches a maximum value of about 2.61% K−1 at 470K. Moreover, the SR value increases gradually and the maximum sensitivity is about 1.82% K−1 at 457K. 3.3.2. Temperature-dependent lifetime In addition to FIR, temperature-dependent fluorescence lifetime can also be exploited to realize in temperature sensing. Fig. 8 shows the normalized decay curves of 2Eg→4A2g transition of Mn4+ at a series of temperatures, revealing that the emission lifetime of Mn4+ decreases with temperature rise. The decay curves can be well-fitted with a double exponential equation:
Fig. 6. The integrated fluorescence intensity of 2Eg→4A2g transition of Mn4+ and 4F9/2 → 6H13/2 transition of Dy3+ in the temperature range from 230 K to 470 K.
t t I (t ) = C1 exp ⎛− ⎞ + C2 exp ⎛− ⎞ ⎝ τ1 ⎠ ⎝ τ2 ⎠ ⎜
3.3.1. Temperature-dependent FIR In FIR measurement, we chose the integrated area of certain range within 568–595 nm of Dy3+ (4F9/2 → 6H13/2) and 675–735 nm of Mn4+ (2Eg→4A2g) respectively, rather than the whole range to effectively improve signal to noise ratio. To quantitatively display the temperature-dependent variation, the integrated fluorescence intensity from these two ranges were calculated and presented in Fig. 6. The integrated fluorescence intensity of 2Eg→4A2g transition of Mn4+ is found to decrease quickly with the increase of temperature, whereas the integrated fluorescence intensity of 4F9/2 → 6H13/2 transition of Dy3+ only exhibits an insignificant increase. Therefore, the luminescence intensity of Dy3+ activator can be approximated as a constant. For the Mn4+ activator, the decrease in the PL intensity with an increase temperature can be expressed by the following equation [22].
‾τ =
IDy IMn
= B + Cexp (−ΔE / kB T )
⎟
(5)
∫ I (t ) tdt C1 τ12 + C2 τ22 = C1 τ1 + C2 τ2 ∫ I (t ) dt
(6)
The average lifetime calculated in the temperature range from 230K to 470K and the fitting are presented in Fig. 9(a). For evaluation of the thermal-quenching activation energy ΔE of the thermal quenching, we can fit the temperature-dependent lifetimes to a modified Arrhenius equation [32]:
τ0
τ(T) =
−ΔE
1 + Ce kB T
(7)
or −ΔE 1 1 = ⎛1 + Ce kB T ⎞ τ τ0 ⎝ ⎠
(8)
Here, τ(T) and τ0 are the lifetime at temperature T and 0 K, C is constant, ΔE is the thermal-quenching activation energy and kB is Boltzmann constant, respectively. As shown in Fig. 9(a), the experiment data can be fitted well by Eq. (9). And the thermal-quenching activation energy of Mn4+ was calculated to be 3871 cm−1, which is not much different from the result calculated by FIR. Similarly, the relative sensitivity SR and absolute sensitivity SA based on the emission lifetime of Mn4+ are calculated respectively by the following equations:
(1)
Here, I (T ) is the luminescence intensities at temperature T , I0 is the initial emission intensity, A is constant, ΔE is the thermal-quenching activation energy and kB is Boltzmann constant, respectively. The expression for the temperature dependence of FIR between Dy3+ and Mn4+ can be obtained:
FIR =
⎜
Here, I is the luminescence intensity, C1 and C2 are constants, τ1 and τ2 are the lifetimes, respectively. The result shows that the decay process of Mn4+ consist of fast and slow decay components which may be due to the different doping concentration distribution and the resulting different local environments around Mn4+ in the host [30,31]. The average lifetime can be calculated as the following equation:
its shielded 4f electrons is not greatly affected by the lattice environment, due to the screening of the outer 5s and 5p shell electrons. Therefore, a highly sensitive temperature detection can be expected by using FIR method in the Mn4+ and Dy3+ co-doped sample.
I0 I (T ) = 1 + Aexp (−ΔE / kB T )
⎟
(2)
As shown in Fig. 7(a), the experiment dates can be fitted well by Eq. (2). The change in FIR (IDy / IMn ) with variation of temperature is remarkable and the thermal-quenching activation energy of Mn4+ was calculated to be 3323 cm−1. To gain more insight into the temperature sensitivity, it is of great significance to investigate the sensing sensitivity of BLMN:Mn4+, Dy3+ for temperature sensing. The relative sensitivity SR and absolute sensitivity SA are two key parameters to evaluate the performance of the temperature sensors. The SR and SA are defined as the relative and absolute change of the FIR in response to the variation of temperature respectively, which can be obtained by the following equation [5,6,8–10].
SR Lifetime =
1 d ‾τ × 100% ‾τ dT
SA Lifetime =
d ‾τ dT
(9)
(10)
The relative sensitivity SR and absolute sensitivity SA based on the average fluorescence lifetime of Mn4+ are calculated and presented in Fig. 9(b), respectively. The SA represents the amount of change in lifetime over a unit temperature and its value increases at first and then decreases, reaching the maximum sensitivity of 0.51 × 10−2 ms⋅K−1 at 378 K. Moreover, it can be seen that SR reaches a maximum value of 2.43% K−1 at 437 K. Several typical temperature sensors based on Mn4+ or Dy3+ and 4
Optical Materials 95 (2019) 109199
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Fig. 7. (a) The temperature dependence of FIR between 2Eg→4A2g and 4F9/2 → 6H13/2 and the polynomial fitting. (b) The theoretical fitting of relative sensitivity SR and absolute sensitivity SA based on the FIR between Mn4+ and Dy3+. Table 1 Several typical temperature sensors based on Mn4+ or Dy3+ and their relative sensitivity. Materials
Temperature range (K)
SR Max (% K−1)
Mode
Ref.
Gd2Ti2O7:Dy3+ Y2Mg2TiO6:Mn4+ Lu3Al5O12:Mn4+ Gd3Al5O12:Mn3+, Mn4+ Y3Al5O12:Dy3+, Mn4+ Y2O3:Ho3+/Mg2TiO4:Mn4+
293–443 10–513 303–383 120–570 293–393 298–373
BaLaMgNbO6:Dy3+, Mn4+
230–470
1.68 0.14 3.75 2.08 3.16 4.61 0.9 1.82
FIR FIR Lifetime Lifetime FIR FIR Lifetime FIR
2.43
Lifetime
[33] [34] [32] [25] [22] [27] [27] This work This work
Mg2TiO4:Mn4+ because the service life of the two materials is different for mixture which may cause inconvenience in application. All these results indicate that BLMN is the appropriate host for TM/RE and BLMN:Mn4+, Dy3+ phosphor can be a potentially excellent candidate for optical temperature sensor with high relative sensitivity, which is based on both FIR and lifetime temperature readout. In addition, the emission peak of Mn4+ is located in the optical window of biological tissue which provides a possibility for application in biological temperature detection.
Fig. 8. The normalized decay curves of 2Eg→4A2g transition of Mn4+ at the temperatures range from 230 K to 470 K when monitored at 698 nm under 355 nm excitation.
their relative sensitivity were displayed in Table 1. Compared with those, the sensitivity of our sample based on FIR and fluorescence lifetime technique are not the highest but they are pretty good. For FIR technique, the sensitivity of our sample is higher than the sensitivity of Gd2Ti2O7:Dy3+ [33] which is based on the emission of TCELs. Furthermore, our sample may more practical than Y2O3:Ho3+/
4. Conclusion In conclusion, the novel BLMN:Mn4+, Dy3+ phosphor with dual-
Fig. 9. (a) The average lifetime calculated in the temperature range from 230 K to 470 K and the polynomial fitting. (b) The theoretical fitting of relative sensitivity SR and absolute sensitivity SA based on the emission lifetime of Mn4+. 5
Optical Materials 95 (2019) 109199
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activators luminescence was successfully designed and synthesized via a high-temperature solid-state reaction method. The temperature-dependent luminescent properties of BLMN:Mn4+, Dy3+ sample for the potential application in dual-mode optical temperature sensor were studied. Under 355 nm excitation, the change of FIR (IDy / IMn ) with variation of temperature is dramatic from 230 K to 470 K, the relative sensitivity reached a maximum value of 1.82% K−1 at 457 K, and the thermal-quenching activation energy of Mn4+ was calculated as 3323 cm−1. In addition, the fluorescence lifetime of Mn4+ (2Eg→4A2g) can also be employed for temperature sensing and the relative sensitivity reaches a maximum value of 2.43% K−1 at 437 K. The thermalquenching activation energy of Mn4+ was calculated to be 3323 cm−1 and 3871 cm−1 based on FIR and lifetime, respectively. These two results are not much different. Moreover, the emission peak of Mn4+ is located in the optical window of biological tissue in the lifetime mode readout, which provides a possibility for application in biological temperature detection. All these investigations demonstrated that the BLMN:Mn4+, Dy3+ phosphor could be a promising candidate for highsensitivity optical temperature sensors.
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