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Time-integrated luminescence thermometry of Eu3+ and Dy3+ doped YVO4
Sensors and Actuators A 295 (2019) 450–455
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Time-integrated luminescence thermometry of Eu3+ and Dy3+ doped YVO4 b ´ c´ a , Stevan Stojadinovic´ a,∗ , Miroslav D. Dramicanin ´ Aleksandar Ciri a b
University of Belgrade, Faculty of Physics, Studentski trg 12–16, Belgrade 11000, Serbia University of Belgrade, Vinˇca Institute of Nuclear Sciences, P.O. Box 522, Belgrade 11001, Serbia
a r t i c l e
i n f o
Article history: Received 20 April 2019 Received in revised form 24 May 2019 Accepted 17 June 2019 Available online 18 June 2019 Keywords: YVO4 Eu3+ YVO4 Dy3+ Thermographic phosphor Line-broadening Line-shift Luminescence intensity ratio
a b s t r a c t Photoluminescence spectra of bulk YVO4 doped with 1 mol% of Eu3+ and Dy3+ were recorded on the range from room temperature to 733 K, in 20 K steps. The temperature sensing performances were estimated by the luminescence intensity ratio, line-broadening and line-shift methods. The luminescence intensity ratio was tested by the ratio of intensities of 5 D1 →7 F1 and 5 D0 →7 F2,4 transitions of Eu3+ , and 4 I15/2 →6 H15/2 and 4 F9/2 →6 H15/2 transitions of Dy3+ . The temperature dependent line-broadenings were measured on 5 D0 →7 F2 and 4 F9/2 →6 H13/2 of Eu3+ and Dy3+ , respectively. The line-shifts were investigated on 5 D0 →7 F1 and 4 F9/2 →6 H15/2 of Eu3+ and Dy3+ , respectively. The experimental data of all three methods is fitted and is in excellent agreement with the theory. The calculated thermometric figures of merit, absolute and relative sensitivities, show the sensor performances at the given temperature, and allow the selection of the best sensor material or transition to be chosen for the desired temperature range. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Temperature is the most frequently measured physical quantity next to the length and time, which is why the temperature sensors comprise the 80% of the total sensor market share and are used in every scientific field [1]. Temperature measurements can be done by invasive (e.g. thermocouples), non-invasive (optical pyrometers), or semi-invasively techniques. The latter are the methods that include alternating the measurand in a manner which allows the non-contact measurements [2]. Out of the materials for semi-invasive measurements, thermographic phosphors have the best overall properties and the widest applicability range [3]. Thermographic phosphors (TP) are materials with temperature sensitive photoluminescent (PL) properties. Consequently, the luminescence thermometry is the optical technique for temperature measurement via the temperature-dependent PL. Temperature sensors based on TP are useful in hostile or inaccessible environments, as they are non-intrusive, durable and highly accurate on a large temperature range [4], which is why they found their usage even in e.g. rocket engines or gas turbines [5]. Out of the TP materials, the lanthanides have attracted the most attention due the narrow emission lines ranging from the UV to NIR region [6].
∗ Corresponding author. ´ E-mail address: [email protected] (S. Stojadinovic). https://doi.org/10.1016/j.sna.2019.06.035 0924-4247/© 2019 Elsevier B.V. All rights reserved.
The choice of lanthanide for a given application primarily depends on the desired temperature range, while the selection of the host matrix depends mostly on the chemical and physical stability in the measured environment [7]. The temperature read-out schemes in rare-earth luminescence thermometry can be classified to the time-resolved and timeintegrated methods, depending on the temporal dependence of the measured quantity [3]. Time-integrated, or the steady-state methods use a single spectral band or two spectral bands. Out of the methods still employed, the former uses the temperature-induced changes in the transition line-shifts or line-broadening. The latter includes the most popular of the semi-invasive methods, the luminescence intensity ratio, out of which the specific class is when the two observed spectral bands are from the single-emission center and from the energetically close levels. Then, the emission intensity ratio is well-understood process that can be adequately modeled theoretically by the Boltzmann distribution. Majority of researches on TPs are concerned with temperature sensing by LIR of lifetime methods, and only a few reports exist that investigate sensing by line-broadening or line-shifts [7–9]. The TP quality can be evaluated by the thermometric figures of merit: absolute and relative sensitivities, temperature and spatial resolution. The absolute sensitivity is defined as a quotient of the change of indication and the change of temperature, while the relative sensitivity is the absolute sensitivity divided by the indication, and are given by [10]:
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Fig. 1. Temperature dependent photoluminescence spectra of 1 mol% doped: (a) YVO4 :Eu3+ and (b) YVO4 :Dy3+ .
∂Q −1 1 ∂Q , S %K = Q ∂T · 100% ∂T
S K −1 =
(1)
where Q is an indication, which can be luminescence intensity ratio, line-shift, or line-broadening from the time-integrated methods. The temperature and spatial resolutions can be estimated from the absolute sensitivity and the measurement uncertainty, : Tmin =
dx , xmin = Tmin S dT
(2)
The measurement uncertainty is an instrumental property. The spectrofluorometers usually have smaller uncertainty in measuring positions, than intensities. Lanthanide-doped oxides are known to be thermally resilient ceramics with excellent luminescent properties. Orthovanadates RYVO4 (R = Sc, Y, La, Gd or Lu) have a high melting point (2083 K) and good hardness (Mohs 4) [11]. Due to the linear coordination of the incorporated trivalent lanthanide with the V and O ions, orthovanadates are highly luminescent host materials [12]. Among lanthanides, in YVO4 , the Eu3+ ion gives the highest PL intensity, following by the Dy3+ [7]. Coincidently, those two ions are featured by the largest energy gaps between their thermalized levels, providing the largest relative sensitivities among lanthanides. However, the majority of thermometric researches are conducted on Nd3+ doped yttrium vanadate, due to its use for lasing [8]. On YVO4 :Eu3+ ,
the various thermometric reports are available: LIR from chargetransfer excitation band is given in Ref. [13], LIR from Stark levels is given in Ref. [14]. Ref [15]. reported on LIR of 5 D1,0 →7 F1 levels, not exploring the larger sensitivities the other emissions provide. Thus, there are no investigations of the LIR of 5 D1 →7 F1 and 5 D0 →7 F2,4 transitions, which give the most intense peaks in YVO4 :Eu3+ and provide higher sensitivities, as it will be presented later. YVO4 :Dy3+ performance as a TP phosphor is investigated only by the timeresolved methods [16]. This research is concerned with the gaps in the current research regarding the evaluation of thermometric potentials of lanthanide doped YVO4 , as one of the most significant PL host matrices. Completing the picture of this significant phosphor may provide the values significant for the creation of novel temperature sensors. Thus, YVO4 :Dy3+ and YVO:Eu3+ TPs are to be investigated by the LIR, line-broadening and line shift methods. For each method, the most sensitive transition must be chosen for investigation and theoretical fitting. In the case of Eu3+ , the preference of two competing lower-energy transitions 5 D0 →7 F2,4 should be established by the estimated sensitivities. The sensitivities on all the methods should be given on a measured temperature range, in order to allow for the selection of the best method for a given temperature.
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2. Experimental YVO4 :Eu3+ and YVO4 :Dy3+ samples with 1 mol% doping concentration were prepared by the high-temperature polymer complex solution method at high temperature, as described in Ref. [17]. YVO4 :Eu3+ and YVO4 :Dy3+ samples with 1 mol% doping concentration were prepared by the high-temperature polymer complex solution method at high temperature, as described in Ref. [17]. Mixture of YVO4 with Eu2 O3 or Dy2 O3 (Alfa Aesar, 99.9%) were dissolved in hot nitric acid and cooled to room temperature. Then, the polyethylene glycol, with average molecular weight of 200, was added to the solution. The resulting solution was stirred at 353 K for a few hours, and then heated at 1073 K for additional 2 h. Afterwards, the powder is cold pressed into pellets. In the final stage, the pellets were sintered at 1273 K in order to produce the high crystallinity of the samples. Horiba-Jobin Yvon Fluorolog-3 FL3-221 spectrofluorometer was employed for recording the PL spectra. The samples were irradiated by the 450 W xenon lamp, via the optical fiber bundle. The samples were heated by the custom-built hot-plate, whose construction is described in Ref. [18]. 3. Results 3.1. Photoluminescence spectra PL spectra of YVO4 :Eu3+ and YVO4 :Dy3+ , recorded from the room temperature in 20 ◦ C steps, are presented in Fig. 1a and b, respectively. The excitation of YVO4 :Eu3+ and YVO4 :Dy3+ was performed by pumping into 5 L6 and 6 P5/2 levels, respectively, by 396 nm and 367 nm irradiation, respectively. Subsequently, Eu3+ ion nonradiatively decays to 5 D0,1 levels, while Dy3+ decays to 4I 4 15/2 and F9/2 , with radiative 4f-4f emissions observed at typical positions for Eu3+ and Dy3+ ions. The observed emission peaks of YVO4 :Eu3+ are 5 D1 →7 F1 at 537 nm, and 5 D0 →7 F1,2,4 at 594 nm, 619 nm and 698 nm, respectively. Emission peaks of YVO4 :Dy3+ are 4 I15/2 →6 H15/2 at 454 nm, and 4 F9/2 →6 H15/2, 13/2, 11/2 at 483 nm, 573 nm, and 661 nm, respectively. The energy level difference between 5 D1 and 5 D0 levels of Eu3+ is equal to 1700 cm−1 , calculated by the difference between the barycenters of 5 D1,0 →7 F1 transitions. Similarly, the energy difference between the 4 I15/2 and 4 F9/2 levels was calculated to be 1158 cm−1 , by subtracting the barycenter of 4 I15/2 →6 H15/2 from the 4 F9/2 →6 H15/2 transition barycenter. The intensities of both Eu3+ and Dy3+ doped YVO4 unexpectedly reach their maxima at 653 K and 533 K, respectively, for the reasons beyond the scope of this article.
Fig. 2. (a) Luminescence intensity ratio of 1 mol% doped YVO4 :Eu3+ and YVO4 :Dy3+ and theoretical fits, (b) corresponding absolute and (c) relative sensitivities.
3.2. Luminescence intensity ratio If two excited energy levels are separated by less than 2000 cm−1 , they are said to be thermally coupled, meaning that the gap E is sufficiently small for electrons to make a leap solely by using the thermal energy. Then, the relative population of the higher (H) and lower (L) states is given by the Boltzmann distribution: NH /NL = gH /gL ·exp(-E/kT), where k =0.695 cm−1 K−1 is the Boltzmann constant, and g = 2J + 1 is the level degeneracy. The intensity of emission is given by [19]: I = hNA, where is the transition barycenter, h is the Planck constant, and A is the radiative transition probability. The ratio of the intensities of H to L emissions is termed the luminescence intensity ratio, and is given by [20]: LIR (T ) =
hH NH AH E IH = exp − IL hL NL AL kT
E
= Bexp −
kT
+A
(3)
where B is the temperature independent constant for a given material.
The absolute and relative sensitivities in LIR scheme are given by, respectively [6,21]:
∂LIR (T ) E = (LIR (T ) − A) ∂T kT 2 1 ∂LIR (T ) · 100% = LIR (T ) − A E · 100% SR (T ) = LIR (T ) kT 2 LIR(T ) ∂T S (T ) =
(4)
(5)
Since the energy level difference between 5 D1,0 levels is less than 2000 cm−1 , they are considered as thermally coupled, i.e. the population of 5 D1 to the expense of the 5 D0 level is probable via thermal energy, and accordingly rises with increasing temperature. LIRs with H transition 5 D1 →7 F1 and L transitions 5 D0 →7 F2,4 are given in Fig. 2a, together with their fits to Eq. (3). The estimated energy difference is in good agreement with the value obtained by the barycenter differences, as explained in Section 3.1. The quality of fit is very high, thus the relative populations of YVO4 :Eu3+
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Fig. 3. (a) Line-broadening of 1 mol% doped YVO4 :Eu3+ and YVO4 :Dy3+ and theoretical fits, (b) corresponding absolute and (c) relative sensitivities.
1 mol% can be adequately described by the Boltzmann distribution. Accordingly, the absolute and relative sensitivities are given in Fig. 2b and c, respectively, favoring the 5 D0 →7 F4 as L transition for temperature sensing by the LIR method. The maximum absolute sensitivities are predicted from the fits to be 0.0011 K−1 at 1319 K and 0.0003 K−1 at 1283 K for 5 D0 →7 F4 and 5 D0 →7 F2 , respectively. Analogously to Eu3+ , 4 I15/2 (H) of Dy3+ is thermally coupled to 4F 9/2 (L) level by the energy difference significantly smaller than it is the case with Eu3+ . Thus, the promotion to the H level needs less thermal energy. The E = (4 I15/2 ) – (4 F9/2 ) values obtained from the barycenter differences and by fitting to the LIR data in Fig. 2a are in good agreement. LIR is excellently fitted by the Eq. (3). The sensitivities of YVO4 :Dy3+ 1 mol% are presented in Fig. 2b and c, showing that the absolute sensitivity of Dy3+ doped sample is higher than that of Eu3+ . The maximum value of absolute sensitivity is equal to 0.001 K−1 at 900 K. 3.3. Line broadening The direct consequence of interaction of ions with phonons is the homogeneous broadening of the transition line widths, which is given by the McCumber-Sturge equation [22,23]:
Fig. 4. (a) Line-shifting of 1 mol% doped YVO4 :Eu3+ and YVO4 :Dy3+ and theoretical fits, (b) corresponding absolute and (c) relative sensitivities.
FWHM cm−1 = FWHM0 + ˛ ¯
T 7 TD
0
TD /T
x 6 e2 (ex
− 1)2
dx
(6)
¯ are the are effective Debye temperature and where TD and ˛ electron-phonon interaction coefficients, respectively. As it is demonstrated in Ref. [8], the Eq. (6) can be approximated with:
FWHM cm−1 ≈ FWHM0 + Aexp(R0 T )
(7)
In YVO4 doped with 1 mol% of Eu3+ and Dy3+ the experimentally obtained line widths of transitions 5 D0 →7 F2 and 4 F9/2 →6 H13/2 , respectively, are presented in Fig. 3a. Taking the points at the borders of the measured range, the FWHM changes by the rate of 0.064 cm−1 /K and 0.11 cm−1 /K for Eu3+ and Dy3+ , respectively. Line broadening of 5 D0 →7 F2 transition is adequately fitted with Eq. (7), while the broadening of the 4 F9/2 →6 H13/2 linearly increases with increasing temperature. From the obtained fits, the absolute and
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relative sensitivities are given in Fig. 3b and c, respectively. The relative sensitivity of Eu3+ is better, regarding the line-broadening in YVO4 , at temperatures above 350 K, while Dy3+ is more sensitive at the lower temperatures. 3.4. Line shift The second McCumber-Sturge equation describes the temperature-induced line-shift of rare-earth transitions [22]:
T 4
cm−1 = ˛
TD
0
TD /T
x3 dx ex − 1
(8)
Analogously to the line-broadening in Eq. (7), line-shift can also be approximated by the exponential equation:
cm−1 ≈ 0 + Aexp(R1 T )
(9)
Line broadenings of 5 D0 →7 F1 and 4 F9/2 →6 H15/2 transitions, recorded from the room temperature to 733 K and 653 K is presented in Fig. 4a, together with fits to Eq. (9). By taking the points at the borders of the measured range, the line-shift changes by the rate of 0.13 cm−1 /K and 0.066 cm−1 /K for Eu3+ and Dy3+ , respectively. The absolute and relative sensitivities on a given temperature range are given in Fig. 4a and b, respectively. The sensitivities are larger for Eu3+ doped sample at temperatures above ca. 350 K, the same as it is with line-broadening (see Section 3.3). 4. Conclusion Eu3+ and Dy3+ doped YVO4 samples were successfully synthetized by the high-temperature polymer-complex solution method. The recorded PL spectra shows emission peaks at typical positions for Eu3+ and Dy3+ materials. The intensities of the peaks unexpectedly rise at higher temperatures, which might be the grounds for another research. Our study was the first to demonstrate the potential of YVO4 :Eu3+ as the thermometric phosphor for temperature sensing by the LIR of 5 D1 →7 F1 and 5 D0 →7 F2,4 transitions, the first LIR investigation of YVO4 :Dy3+ , and the first investigation of lanthanide doped YVO4 by line-shift and line-broadening methods. By the LIR method, YVO4 :Eu3+ is most sensitive by using the 5 D0 →7 F4 transition. The line-broadening and line-shift sensitivities of various materials are scarcely reported in literature. However, while linebroadening gives relatively low values comparing to Ref. [8], the line-shifts sensitivities of both YVO4 :Eu3+ and YVO4 :Dy3+ are the largest. Both YVO4 :Eu3+ and YVO4 :Dy3+ are excellent thermographic phosphors with great potential for practical temperature sensors, due to both the high luminescence and quantum efficiencies, and large sensitivities. Each lanthanide dominates in its own temperature range, and for all of the three methods, the bounds are at ca. 350 K: Dy3+ is as expected better at lower temperatures than Eu3+ and vice versa.
[2] P.R.N. Childs, J.R. Greenwood, C.A. Long, Review of temperature measurement, Rev. Sci. Instrum. 71 (2000) 2959–2978, http://dx.doi.org/10.1063/1.1305516. [3] J. Brübach, C. Pflitsch, A. Dreizler, B. Atakan, On surface temperature measurements with thermographic phosphors: a review, Prog. Energy Combust. Sci. 39 (2013) 37–60, http://dx.doi.org/10.1016/j.pecs.2012.06.001. [4] B.W. Noel, W.D. Turley, S.W. Allison, Thermographic-Phosphor Temperature Measurements: Commercial and Defense-Related Applications, 1994 https:// www.osti.gov/servlets/purl/10159239. ´ D.J. Jovanovic, ´ M.D. Dramicanin, ´ Temperature dependence of [5] M.G. Nikolic, emission and lifetime in Eu3+ - and Dy3+ - doped GdVO4, Appl. Opt. 52 (2013) 1716, http://dx.doi.org/10.1364/AO.52.001716. ´ [6] M.D. Dramicanin, Sensing temperature via downshifting emissions of lanthanide-doped metal oxides and salts. A review, Methods Appl. Fluoresc. 4 (2016), 042001, http://dx.doi.org/10.1088/2050-6120/4/4/042001. [7] S.W. Allison, G.T. Gillies, Remote thermometry with thermographic phosphors: instrumentation and applications, Rev. Sci. Instrum. 68 (1997) 2615–2650, http://dx.doi.org/10.1063/1.1148174. [8] I.E. Kolesnikov, A.A. Kalinichev, M.A. Kurochkin, E.V. Golyeva, E.Y. Kolesnikov, A.V. Kurochkin, E. Lähderanta, M.D. Mikhailov, YVO4 :Nd3+ nanophosphors as NIR-to-NIR thermal sensors in wide temperature range, Sci. Rep. 7 (2017) 18002, http://dx.doi.org/10.1038/s41598-017-18295-w. [9] H. Kusama, O.J. Sovers, T. Yoshioka, Line shift method for phosphor temperature measurements, J. Appl. Phys. 15 (1976) 2349–2358, http://dx. doi.org/10.1143/JJAP.15.2349. ´ Luminescence Thermometry, Methods, Materials and [10] M.D. Dramicanin, Applications, Woodhead Publishing, 2018, http://dx.doi.org/10.1016/B978-008-102029-6.00006-3. [11] S. Erdei, Growth of oxygen deficiency-free YVO4 single crystal by top-seeded solution growth technique, J. Cryst. Growth 134 (1993) 1–13, http://dx.doi. org/10.1016/0022-0248(93)90002-E. [12] G. Blasse, On the Eu 3+ fluorescence of mixed metal oxides. IV. The photoluminescent efficiency of Eu3+ -activated oxides, J. Chem. Phys. 45 (1966) 2356–2360, http://dx.doi.org/10.1063/1.1727946. [13] S. Zhou, C. Duan, M. Yin, S. Zhang, C. Wang, High-sensitive optical temperature sensing based on 5 D1 emission of Eu3+ in YVO4 , J. Alloys. Compd. 784 (2019) 970–974, http://dx.doi.org/10.1016/j.jallcom.2019.01.143. [14] I.E. Kolesnikov, E.V. Golyeva, E. Lähderanta, A.V. Kurochkin, M.D. Mikhailov, Ratiometric thermal sensing based on Eu3+ -doped YVO4 nanoparticles, J. Nanopart. Res. 18 (2016) 354, http://dx.doi.org/10.1007/s11051-016-3675-8. [15] I.E. Kolesnikov, A.A. Kalinichev, M.A. Kurochkin, D.V. Mamonova, E.Y. Kolesnikov, E. Lähderanta, Ratiometric optical thermometry based on emission and excitation spectra of YVO4 :Eu3+ nanophosphors, J. Phys. Chem. C. 123 (2019) 5136–5143, http://dx.doi.org/10.1021/acs.jpcc.9b00284. [16] S.W. Allison, M.R. Cates, G.T. Gillies, Phosphor thermometry techniques for the realization of thermal standards, Tech. Pap. 422 (2002) 779–788 https:// www.isa.org/intech/phosphors/. ´ Zˇ . Antic, ´ M.G. Nikolic, ´ M. Mitric, ´ M.D. Dramicanin, ´ [17] R.M. Krsmanovic, Preparation of Y2 O3 :Eu3+ nanopowders via polymer complex solution method and luminescence properties of the sintered ceramics, Ceram. Int. 37 (2011) 525–531, http://dx.doi.org/10.1016/j.ceramint.2010.09.040. [18] A. Ciric, S. Stojadinovic, M.D. Dramicanin, Custom-built thermometry apparatus and luminescence intensity ratio thermometry of ZrO2 :Eu3+ and Nb2 O5 :Eu3+ , Meas. Sci. Technol. (2019), http://dx.doi.org/10.1088/1361-6501/ ab01b4. [19] B.F. dos Santos, M.V. dos Santos Rezende, P.J.R. Montes, R.M. Araujo, M.A.C. dos Santos, M.E.G. Valerio, Spectroscopy study of SrAl2 O4 :Eu3+ , J. Lumin. 132 (2012) 1015–1020, http://dx.doi.org/10.1016/j.jlumin.2011.09.008. [20] S.A. Wade, S.F. Collins, G.W. Baxter, Fluorescence intensity ratio technique for optical fiber point temperature sensing, J. Appl. Phys. 94 (2003) 4743, http:// dx.doi.org/10.1063/1.1606526. [21] S.F. Collins, G.W. Baxter, S.A. Wade, T. Sun, K.T.V. Grattan, Z.Y. Zhang, A.W. Palmer, Comparison of fluorescence-based temperature sensor schemes: theoretical analysis and experimental validation, J. Appl. Phys. 84 (1998) 4649–4654, http://dx.doi.org/10.1063/1.368705. [22] X. Chen, Y. Liu, D. Tu, Lanthanide-doped Luminescent Nanomaterials, Springer, Berlin Heidelberg, Berlin, Heidelberg, 2014, http://dx.doi.org/10. 1007/978-3-642-40364-4. [23] D.E. McCumber, M.D. Sturge, Linewidth and temperature shift of the R lines in Ruby, J. Appl. Phys. 34 (1963) 1682–1684, http://dx.doi.org/10.1063/1. 1702657.
Acknowledgements
Biographies
This work is supported by the Ministry of Education, Science, and Technological Development of the Republic of Serbia under Project No. 171035 and 45020.
´ c is now pursing his Ph.D. degree with the supervision of Prof. Aleksandar Ciri´ ´ at Faculty of Physics at University of Belgrade. His main Miroslav Dramicanin research interests focus on Judd-Ofelt theory and thermometry of europium doped materials.
References [1] X. Wang, O.S. Wolfbeis, R.J. Meier, Luminescent probes and sensors for temperature, Chem. Soc. Rev. 42 (2013) 7834, http://dx.doi.org/10.1039/ c3cs60102a.
Prof. Stevan Stojadinovi´c studied Faculty of Physics at University of Belgrade, Serbia, where he received a Ph.D in Applied Physics in 2004. He became a full professor in 2017 at the same University. He is currently Vice Dean for Science at Faculty of Physics. His main research topics include plasma electrolytic oxidation, galvanoluminescence, photocatalysis, and photoluminescence of rare earth ions doped materials.
´ iri´c et al. / Sensors and Actuators A 295 (2019) 450–455 A. C Dr. Miroslav Drami´canin is currently a research professor at Vinca Institute of Nuclear Science at the University of Belgrade. He also acts as a full professor at the Faculty of Physics and the head of the Laboratory for Radiation Physics and Chemistry where he studies luminescent materials. His current project involves
455
studying materials of reduced dimensions for efficient light harvesting and energy conversion. He has served as chairman and on the organizing committee of internal conferences for optical materials, and as editor of multiple international conference proceedings. He has authored more than 250 scientific papers.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Time-integrated luminescence thermometry of Eu3+ and Dy3+ doped YVO4 b ´ c´ a , Stevan Stojadinovic´ a,∗ , Miroslav D. Dramicanin ´ Aleksandar Ciri a b
University of Belgrade, Faculty of Physics, Studentski trg 12–16, Belgrade 11000, Serbia University of Belgrade, Vinˇca Institute of Nuclear Sciences, P.O. Box 522, Belgrade 11001, Serbia
a r t i c l e
i n f o
Article history: Received 20 April 2019 Received in revised form 24 May 2019 Accepted 17 June 2019 Available online 18 June 2019 Keywords: YVO4 Eu3+ YVO4 Dy3+ Thermographic phosphor Line-broadening Line-shift Luminescence intensity ratio
a b s t r a c t Photoluminescence spectra of bulk YVO4 doped with 1 mol% of Eu3+ and Dy3+ were recorded on the range from room temperature to 733 K, in 20 K steps. The temperature sensing performances were estimated by the luminescence intensity ratio, line-broadening and line-shift methods. The luminescence intensity ratio was tested by the ratio of intensities of 5 D1 →7 F1 and 5 D0 →7 F2,4 transitions of Eu3+ , and 4 I15/2 →6 H15/2 and 4 F9/2 →6 H15/2 transitions of Dy3+ . The temperature dependent line-broadenings were measured on 5 D0 →7 F2 and 4 F9/2 →6 H13/2 of Eu3+ and Dy3+ , respectively. The line-shifts were investigated on 5 D0 →7 F1 and 4 F9/2 →6 H15/2 of Eu3+ and Dy3+ , respectively. The experimental data of all three methods is fitted and is in excellent agreement with the theory. The calculated thermometric figures of merit, absolute and relative sensitivities, show the sensor performances at the given temperature, and allow the selection of the best sensor material or transition to be chosen for the desired temperature range. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Temperature is the most frequently measured physical quantity next to the length and time, which is why the temperature sensors comprise the 80% of the total sensor market share and are used in every scientific field [1]. Temperature measurements can be done by invasive (e.g. thermocouples), non-invasive (optical pyrometers), or semi-invasively techniques. The latter are the methods that include alternating the measurand in a manner which allows the non-contact measurements [2]. Out of the materials for semi-invasive measurements, thermographic phosphors have the best overall properties and the widest applicability range [3]. Thermographic phosphors (TP) are materials with temperature sensitive photoluminescent (PL) properties. Consequently, the luminescence thermometry is the optical technique for temperature measurement via the temperature-dependent PL. Temperature sensors based on TP are useful in hostile or inaccessible environments, as they are non-intrusive, durable and highly accurate on a large temperature range [4], which is why they found their usage even in e.g. rocket engines or gas turbines [5]. Out of the TP materials, the lanthanides have attracted the most attention due the narrow emission lines ranging from the UV to NIR region [6].
∗ Corresponding author. ´ E-mail address: [email protected] (S. Stojadinovic). https://doi.org/10.1016/j.sna.2019.06.035 0924-4247/© 2019 Elsevier B.V. All rights reserved.
The choice of lanthanide for a given application primarily depends on the desired temperature range, while the selection of the host matrix depends mostly on the chemical and physical stability in the measured environment [7]. The temperature read-out schemes in rare-earth luminescence thermometry can be classified to the time-resolved and timeintegrated methods, depending on the temporal dependence of the measured quantity [3]. Time-integrated, or the steady-state methods use a single spectral band or two spectral bands. Out of the methods still employed, the former uses the temperature-induced changes in the transition line-shifts or line-broadening. The latter includes the most popular of the semi-invasive methods, the luminescence intensity ratio, out of which the specific class is when the two observed spectral bands are from the single-emission center and from the energetically close levels. Then, the emission intensity ratio is well-understood process that can be adequately modeled theoretically by the Boltzmann distribution. Majority of researches on TPs are concerned with temperature sensing by LIR of lifetime methods, and only a few reports exist that investigate sensing by line-broadening or line-shifts [7–9]. The TP quality can be evaluated by the thermometric figures of merit: absolute and relative sensitivities, temperature and spatial resolution. The absolute sensitivity is defined as a quotient of the change of indication and the change of temperature, while the relative sensitivity is the absolute sensitivity divided by the indication, and are given by [10]:
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Fig. 1. Temperature dependent photoluminescence spectra of 1 mol% doped: (a) YVO4 :Eu3+ and (b) YVO4 :Dy3+ .
∂Q −1 1 ∂Q , S %K = Q ∂T · 100% ∂T
S K −1 =
(1)
where Q is an indication, which can be luminescence intensity ratio, line-shift, or line-broadening from the time-integrated methods. The temperature and spatial resolutions can be estimated from the absolute sensitivity and the measurement uncertainty, : Tmin =
dx , xmin = Tmin S dT
(2)
The measurement uncertainty is an instrumental property. The spectrofluorometers usually have smaller uncertainty in measuring positions, than intensities. Lanthanide-doped oxides are known to be thermally resilient ceramics with excellent luminescent properties. Orthovanadates RYVO4 (R = Sc, Y, La, Gd or Lu) have a high melting point (2083 K) and good hardness (Mohs 4) [11]. Due to the linear coordination of the incorporated trivalent lanthanide with the V and O ions, orthovanadates are highly luminescent host materials [12]. Among lanthanides, in YVO4 , the Eu3+ ion gives the highest PL intensity, following by the Dy3+ [7]. Coincidently, those two ions are featured by the largest energy gaps between their thermalized levels, providing the largest relative sensitivities among lanthanides. However, the majority of thermometric researches are conducted on Nd3+ doped yttrium vanadate, due to its use for lasing [8]. On YVO4 :Eu3+ ,
the various thermometric reports are available: LIR from chargetransfer excitation band is given in Ref. [13], LIR from Stark levels is given in Ref. [14]. Ref [15]. reported on LIR of 5 D1,0 →7 F1 levels, not exploring the larger sensitivities the other emissions provide. Thus, there are no investigations of the LIR of 5 D1 →7 F1 and 5 D0 →7 F2,4 transitions, which give the most intense peaks in YVO4 :Eu3+ and provide higher sensitivities, as it will be presented later. YVO4 :Dy3+ performance as a TP phosphor is investigated only by the timeresolved methods [16]. This research is concerned with the gaps in the current research regarding the evaluation of thermometric potentials of lanthanide doped YVO4 , as one of the most significant PL host matrices. Completing the picture of this significant phosphor may provide the values significant for the creation of novel temperature sensors. Thus, YVO4 :Dy3+ and YVO:Eu3+ TPs are to be investigated by the LIR, line-broadening and line shift methods. For each method, the most sensitive transition must be chosen for investigation and theoretical fitting. In the case of Eu3+ , the preference of two competing lower-energy transitions 5 D0 →7 F2,4 should be established by the estimated sensitivities. The sensitivities on all the methods should be given on a measured temperature range, in order to allow for the selection of the best method for a given temperature.
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2. Experimental YVO4 :Eu3+ and YVO4 :Dy3+ samples with 1 mol% doping concentration were prepared by the high-temperature polymer complex solution method at high temperature, as described in Ref. [17]. YVO4 :Eu3+ and YVO4 :Dy3+ samples with 1 mol% doping concentration were prepared by the high-temperature polymer complex solution method at high temperature, as described in Ref. [17]. Mixture of YVO4 with Eu2 O3 or Dy2 O3 (Alfa Aesar, 99.9%) were dissolved in hot nitric acid and cooled to room temperature. Then, the polyethylene glycol, with average molecular weight of 200, was added to the solution. The resulting solution was stirred at 353 K for a few hours, and then heated at 1073 K for additional 2 h. Afterwards, the powder is cold pressed into pellets. In the final stage, the pellets were sintered at 1273 K in order to produce the high crystallinity of the samples. Horiba-Jobin Yvon Fluorolog-3 FL3-221 spectrofluorometer was employed for recording the PL spectra. The samples were irradiated by the 450 W xenon lamp, via the optical fiber bundle. The samples were heated by the custom-built hot-plate, whose construction is described in Ref. [18]. 3. Results 3.1. Photoluminescence spectra PL spectra of YVO4 :Eu3+ and YVO4 :Dy3+ , recorded from the room temperature in 20 ◦ C steps, are presented in Fig. 1a and b, respectively. The excitation of YVO4 :Eu3+ and YVO4 :Dy3+ was performed by pumping into 5 L6 and 6 P5/2 levels, respectively, by 396 nm and 367 nm irradiation, respectively. Subsequently, Eu3+ ion nonradiatively decays to 5 D0,1 levels, while Dy3+ decays to 4I 4 15/2 and F9/2 , with radiative 4f-4f emissions observed at typical positions for Eu3+ and Dy3+ ions. The observed emission peaks of YVO4 :Eu3+ are 5 D1 →7 F1 at 537 nm, and 5 D0 →7 F1,2,4 at 594 nm, 619 nm and 698 nm, respectively. Emission peaks of YVO4 :Dy3+ are 4 I15/2 →6 H15/2 at 454 nm, and 4 F9/2 →6 H15/2, 13/2, 11/2 at 483 nm, 573 nm, and 661 nm, respectively. The energy level difference between 5 D1 and 5 D0 levels of Eu3+ is equal to 1700 cm−1 , calculated by the difference between the barycenters of 5 D1,0 →7 F1 transitions. Similarly, the energy difference between the 4 I15/2 and 4 F9/2 levels was calculated to be 1158 cm−1 , by subtracting the barycenter of 4 I15/2 →6 H15/2 from the 4 F9/2 →6 H15/2 transition barycenter. The intensities of both Eu3+ and Dy3+ doped YVO4 unexpectedly reach their maxima at 653 K and 533 K, respectively, for the reasons beyond the scope of this article.
Fig. 2. (a) Luminescence intensity ratio of 1 mol% doped YVO4 :Eu3+ and YVO4 :Dy3+ and theoretical fits, (b) corresponding absolute and (c) relative sensitivities.
3.2. Luminescence intensity ratio If two excited energy levels are separated by less than 2000 cm−1 , they are said to be thermally coupled, meaning that the gap E is sufficiently small for electrons to make a leap solely by using the thermal energy. Then, the relative population of the higher (H) and lower (L) states is given by the Boltzmann distribution: NH /NL = gH /gL ·exp(-E/kT), where k =0.695 cm−1 K−1 is the Boltzmann constant, and g = 2J + 1 is the level degeneracy. The intensity of emission is given by [19]: I = hNA, where is the transition barycenter, h is the Planck constant, and A is the radiative transition probability. The ratio of the intensities of H to L emissions is termed the luminescence intensity ratio, and is given by [20]: LIR (T ) =
hH NH AH E IH = exp − IL hL NL AL kT
E
= Bexp −
kT
+A
(3)
where B is the temperature independent constant for a given material.
The absolute and relative sensitivities in LIR scheme are given by, respectively [6,21]:
∂LIR (T ) E = (LIR (T ) − A) ∂T kT 2 1 ∂LIR (T ) · 100% = LIR (T ) − A E · 100% SR (T ) = LIR (T ) kT 2 LIR(T ) ∂T S (T ) =
(4)
(5)
Since the energy level difference between 5 D1,0 levels is less than 2000 cm−1 , they are considered as thermally coupled, i.e. the population of 5 D1 to the expense of the 5 D0 level is probable via thermal energy, and accordingly rises with increasing temperature. LIRs with H transition 5 D1 →7 F1 and L transitions 5 D0 →7 F2,4 are given in Fig. 2a, together with their fits to Eq. (3). The estimated energy difference is in good agreement with the value obtained by the barycenter differences, as explained in Section 3.1. The quality of fit is very high, thus the relative populations of YVO4 :Eu3+
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Fig. 3. (a) Line-broadening of 1 mol% doped YVO4 :Eu3+ and YVO4 :Dy3+ and theoretical fits, (b) corresponding absolute and (c) relative sensitivities.
1 mol% can be adequately described by the Boltzmann distribution. Accordingly, the absolute and relative sensitivities are given in Fig. 2b and c, respectively, favoring the 5 D0 →7 F4 as L transition for temperature sensing by the LIR method. The maximum absolute sensitivities are predicted from the fits to be 0.0011 K−1 at 1319 K and 0.0003 K−1 at 1283 K for 5 D0 →7 F4 and 5 D0 →7 F2 , respectively. Analogously to Eu3+ , 4 I15/2 (H) of Dy3+ is thermally coupled to 4F 9/2 (L) level by the energy difference significantly smaller than it is the case with Eu3+ . Thus, the promotion to the H level needs less thermal energy. The E = (4 I15/2 ) – (4 F9/2 ) values obtained from the barycenter differences and by fitting to the LIR data in Fig. 2a are in good agreement. LIR is excellently fitted by the Eq. (3). The sensitivities of YVO4 :Dy3+ 1 mol% are presented in Fig. 2b and c, showing that the absolute sensitivity of Dy3+ doped sample is higher than that of Eu3+ . The maximum value of absolute sensitivity is equal to 0.001 K−1 at 900 K. 3.3. Line broadening The direct consequence of interaction of ions with phonons is the homogeneous broadening of the transition line widths, which is given by the McCumber-Sturge equation [22,23]:
Fig. 4. (a) Line-shifting of 1 mol% doped YVO4 :Eu3+ and YVO4 :Dy3+ and theoretical fits, (b) corresponding absolute and (c) relative sensitivities.
FWHM cm−1 = FWHM0 + ˛ ¯
T 7 TD
0
TD /T
x 6 e2 (ex
− 1)2
dx
(6)
¯ are the are effective Debye temperature and where TD and ˛ electron-phonon interaction coefficients, respectively. As it is demonstrated in Ref. [8], the Eq. (6) can be approximated with:
FWHM cm−1 ≈ FWHM0 + Aexp(R0 T )
(7)
In YVO4 doped with 1 mol% of Eu3+ and Dy3+ the experimentally obtained line widths of transitions 5 D0 →7 F2 and 4 F9/2 →6 H13/2 , respectively, are presented in Fig. 3a. Taking the points at the borders of the measured range, the FWHM changes by the rate of 0.064 cm−1 /K and 0.11 cm−1 /K for Eu3+ and Dy3+ , respectively. Line broadening of 5 D0 →7 F2 transition is adequately fitted with Eq. (7), while the broadening of the 4 F9/2 →6 H13/2 linearly increases with increasing temperature. From the obtained fits, the absolute and
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relative sensitivities are given in Fig. 3b and c, respectively. The relative sensitivity of Eu3+ is better, regarding the line-broadening in YVO4 , at temperatures above 350 K, while Dy3+ is more sensitive at the lower temperatures. 3.4. Line shift The second McCumber-Sturge equation describes the temperature-induced line-shift of rare-earth transitions [22]:
T 4
cm−1 = ˛
TD
0
TD /T
x3 dx ex − 1
(8)
Analogously to the line-broadening in Eq. (7), line-shift can also be approximated by the exponential equation:
cm−1 ≈ 0 + Aexp(R1 T )
(9)
Line broadenings of 5 D0 →7 F1 and 4 F9/2 →6 H15/2 transitions, recorded from the room temperature to 733 K and 653 K is presented in Fig. 4a, together with fits to Eq. (9). By taking the points at the borders of the measured range, the line-shift changes by the rate of 0.13 cm−1 /K and 0.066 cm−1 /K for Eu3+ and Dy3+ , respectively. The absolute and relative sensitivities on a given temperature range are given in Fig. 4a and b, respectively. The sensitivities are larger for Eu3+ doped sample at temperatures above ca. 350 K, the same as it is with line-broadening (see Section 3.3). 4. Conclusion Eu3+ and Dy3+ doped YVO4 samples were successfully synthetized by the high-temperature polymer-complex solution method. The recorded PL spectra shows emission peaks at typical positions for Eu3+ and Dy3+ materials. The intensities of the peaks unexpectedly rise at higher temperatures, which might be the grounds for another research. Our study was the first to demonstrate the potential of YVO4 :Eu3+ as the thermometric phosphor for temperature sensing by the LIR of 5 D1 →7 F1 and 5 D0 →7 F2,4 transitions, the first LIR investigation of YVO4 :Dy3+ , and the first investigation of lanthanide doped YVO4 by line-shift and line-broadening methods. By the LIR method, YVO4 :Eu3+ is most sensitive by using the 5 D0 →7 F4 transition. The line-broadening and line-shift sensitivities of various materials are scarcely reported in literature. However, while linebroadening gives relatively low values comparing to Ref. [8], the line-shifts sensitivities of both YVO4 :Eu3+ and YVO4 :Dy3+ are the largest. Both YVO4 :Eu3+ and YVO4 :Dy3+ are excellent thermographic phosphors with great potential for practical temperature sensors, due to both the high luminescence and quantum efficiencies, and large sensitivities. Each lanthanide dominates in its own temperature range, and for all of the three methods, the bounds are at ca. 350 K: Dy3+ is as expected better at lower temperatures than Eu3+ and vice versa.
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Acknowledgements
Biographies
This work is supported by the Ministry of Education, Science, and Technological Development of the Republic of Serbia under Project No. 171035 and 45020.
´ c is now pursing his Ph.D. degree with the supervision of Prof. Aleksandar Ciri´ ´ at Faculty of Physics at University of Belgrade. His main Miroslav Dramicanin research interests focus on Judd-Ofelt theory and thermometry of europium doped materials.
References [1] X. Wang, O.S. Wolfbeis, R.J. Meier, Luminescent probes and sensors for temperature, Chem. Soc. Rev. 42 (2013) 7834, http://dx.doi.org/10.1039/ c3cs60102a.
Prof. Stevan Stojadinovi´c studied Faculty of Physics at University of Belgrade, Serbia, where he received a Ph.D in Applied Physics in 2004. He became a full professor in 2017 at the same University. He is currently Vice Dean for Science at Faculty of Physics. His main research topics include plasma electrolytic oxidation, galvanoluminescence, photocatalysis, and photoluminescence of rare earth ions doped materials.
´ iri´c et al. / Sensors and Actuators A 295 (2019) 450–455 A. C Dr. Miroslav Drami´canin is currently a research professor at Vinca Institute of Nuclear Science at the University of Belgrade. He also acts as a full professor at the Faculty of Physics and the head of the Laboratory for Radiation Physics and Chemistry where he studies luminescent materials. His current project involves
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studying materials of reduced dimensions for efficient light harvesting and energy conversion. He has served as chairman and on the organizing committee of internal conferences for optical materials, and as editor of multiple international conference proceedings. He has authored more than 250 scientific papers.