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Use of cross-relaxation for temperature sensing via a fluorescence intensity ratio
Accepted Manuscript Title: Use of Cross-Relaxation for Temperature Sensing via a Fluorescence Intensity Ratio Author: Weiwei Zhang Stephen F. Collins Greg W. Baxter Fotios Sidiroglou Changkui Duan Min Yin PII: DOI: Reference:
S0924-4247(15)30001-7 http://dx.doi.org/doi:10.1016/j.sna.2015.05.005 SNA 9177
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
24-10-2014 7-5-2015 8-5-2015
Please cite this article as: Weiwei Zhang, Stephen F.Collins, Greg W.Baxter, Fotios Sidiroglou, Changkui Duan, Min Yin, Use of Cross-Relaxation for Temperature Sensing via a Fluorescence Intensity Ratio, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2015.05.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Use of Cross-Relaxation for Temperature Sensing via a Fluorescence Intensity Ratio 1,2,* Weiwei Zhang , Stephen F. Collins2, Greg W. Baxter2, Fotios Sidiroglou2, Changkui Duan3, Min Yin3 1. Jiangxi Engineering Laboratory for Optoelectronics Testing Technology, Nanchang Hangkong University, Nanchang 330063, China 2. Optical Technology Research Laboratory, Victoria University, P.O. Box 14428, Melbourne City, Victoria 8001, Australia 3. Department of Physics, University of Science and Technology of China, Hefei 230026, China
*
Corresponding author. Tel.: +86 15170463677; fax: +86 791 83953461. E-mail address: [email protected] (Weiwei
Zhang).
Highlights
Cross-relaxation between the 5D0-7F1 and 7F1-5D0 transitions of Eu3+ at D3 and C3v sites respectively dominates the temperature dependent fluorescence of the hexagonal YBO3:Eu phosphor. Rate of energy transfer between donor and acceptor ions is temperature dependent, making a Fluorescence Intensity Ratio (FIR) of donor over acceptor emission indicator of temperature variation. A rough estimate using an expression 3/2kBT = ΔE gives an effective value of the low temperature limit of sensing range for FIR technique.
Abstract
A scheme that provides an alternative mechanism for temperature sensing via a fluorescence intensity ratio (FIR) is proposed. The rate of energy transfer between rare-earth ions in a crystal or glass host is temperature dependent, and as such exhibits a changing intensity ratio in the emission spectrum. Here a phosphor YBO3:Eu3+ was used to illustrate the proposed sensing mechanism. The temperature dependence of the observed fluorescence is similar to the phenomenon found for the conventional FIR technique. The potential benefit of the proposed scheme is access to numerous additional varieties of active materials for FIR temperature sensing. Keywords: Temperature; Fluorescence intensity ratio; Energy transfer.
1. Introduction The fluorescence intensity ratio (FIR) technique is known as an absolute temperature sensing method. Various rare-earth ions such as Eu3+, Nd3+, Er3+ and Sm3+ can be used in the technique [1]. The ions are generally doped into a glass or crystalline solid to form the active material. The underlying science has been articulated so that the performance of the standard FIR technique can be readily predicted [1, 2]. Most recent explorations of the FIR technique focus on the development of novel materials that display familiar characteristics, whilst reporting slight improvements in temperature sensitivity. Various thermal equilibrium systems, in addition to the FIR technique, may be utilized for absolute temperature sensing with the simple requirement that an observable characteristic is temperature dependent. When considering any thermometer, either optical or electronic, it is the thermal equilibrium distribution at the atomic or electron level that dominates temperature dependent changes. Considering the basic physics principles of such distributions, absolute temperature, T, always appears with the Boltzmann’s constant, kB, to produce energy, kBT. A wellknown example comes from a semiconductor thermometer whose current-voltage relation (namely the Shockley equation) includes Boltzmann’s distribution of carriers. Alternatively, new techniques of temperature sensing may be developed after investigating the broad array of thermal distributions of atoms or electrons. In this article, it is shown that a phosphor with emission lines from two interactive ions can be exploited successfully as a FIR material, through efficient cross relaxation between the doped rare-earth ions. The phosphor YBO3:Eu3+ is used to illustrate the sensing mechanism.
2. Experimental The phosphor YBO3:Eu3+ was prepared through a sol-gel process. An aqueous solution of rare-earth nitrates was mixed with tributyl borate, then stirred after adding ethanol until a clear sol was achieved. The gel formed overnight in a water bath held at 85C, and was then baked to form a bulk solid. This was then ground; the final product was sintered at 900C for two hours to insure crystallization. The phosphor’s luminescence was analyzed with a SPEX-1403 spectrometer under the excitation of a high pressure Hg lamp (output wavelength of 365 nm). In order to investigate the effect of temperature change on the fluorescence, the phosphor was compressed into a pellet and then glued on a brass heat sink in a temperature controlled vacuum chamber. The temperature range investigated was below room temperature.
3. Methodology of the schemes A. Conventional FIR Technique The conventional FIR technique employs active materials with two thermal coupled excitation energy levels [1]. The principle can be illustrated using a simplified diagram of the energy levels (Fig. 1). Spontaneous transition takes place from each of levels 2 & 1 to the ground state, level 0. The emission intensity Iij (i=2,1, j=0) relates to the spontaneous transition probability Aij , the frequency of the emitted light vij , and the population Ni of state i (i=2,1):
I ij Aij ij N i
Here non-radiative transition (phonon assisted relaxation) to the ground state is regarded as not important and its influence on the emission intensity has been ignored. As levels 2 & 1 are close to each other, they are thermally coupled within a certain temperature range. At thermal equilibrium, the population ratio N2/N1 follows a Boltzmann’s distribution. The intensity ratio R of transitions of state 2 to state 0 on state 1 to state 0 is then:
R C1 exp(
E21 ) k BT
where C1 is a constant for a certain active material and a certain measuring system, T is the temperature in Kelvin, and E21 is the energy gap between level 2 and level 1. For a solid phosphor, the transition energies E20 & E10 (observable from emission spectrum as hv20 and hv10) are not obviously dependent on temperature, meaning that E21 is constant. If a logarithmic operation is put on Eq. (2), a linear dependence of lnR on 1/T is evident. Furthermore, by dividing the determined slope of a plot of lnR against 1/T by E21, a value of the Boltzmann’s constant kB can be experimentally derived. When the experimental values were compared to the CODATA recommended constant [3], and the accuracy of the temperature sensing method can be judged via this comparison [4]. the discrepancy between the Boltzmann’s constant derived using the FIR technique in comparison to the recommended value can be less than 0.08% [411]. The sensing function of the FIR theory as represented eq. (2) can be seen to be highly accurate.
B. Alternative Scheme for the FIR technique An alternate scheme utilizing the FIR technique is proposed by considering temperature dependent transition processes other than that illustrated in Fig.1. Cross relaxation is a process where an excited ion (donor, D) non-radiatively transfers part of its energy to an ion (acceptor, A) in the neighborhood (Fig.2); the acceptor has a similar transition energy and therefore finds itself in a different energy state. The process normally requires a high degree of resonance between the two participating transitions, while the D and A can be the either same or different ions. The energy transfer process may be assisted by absorbing or emitting phonons, and will be efficient if the energy mismatch of the involved transitions is within one vibrational
quantum [12]. The probability of a non-resonant cross relaxation process with phonon annihilation is proportional to both the product of the oscillator strengths of the involved donor and acceptor transitions, and the phonon density. As the population of a phonon having a mode of frequency, v, at a temperature, T, is described by the BoseEinstein distribution n = 1/[exp(hv/kBT)–1], the phonon assisted cross relaxation process contributes to increase the probability of energy exchange at higher temperature. At low temperature, only energy transfers where a phonon is created are important. At higher temperature, both emission and absorption of phonons play a role. The energy transfer process changes the relative number of excited D and A ions at a constant excitation power. As both D and A ions radiate, the ratio of the fluorescence intensity (or FIR) is an indicator of the distribution ratio of excitation energy by D/A ions, and consequently is also an indicator of temperature. The D and A ions are thermally coupled by the phonon assisted cross relaxation process, and so the fluorescence intensity ratio of emissions from D and A is linked to temperature T, with T appearing in the expression of phonon density. This phenomenon is explored in this paper, following the presentation of the experimental results. 4. Results and discussions An X-ray diffraction pattern (Fig.3) of the phosphor used in the current work indicates a hexagonal calcite structure in accordance with JCPDS (Joint Committee on Powder Diffraction Standards) Files No. 13-483 and No.16-0277. However, there exists some ambiguity about the exact crystal structure of rare-earth borates [13-15]. A similar result suggested that in the matrix material, the doped rare-earth ions take two different types of lattice sites with C3 and C3/C3v symmetry respectively [13]. Later work on the popular matrix material YBO3 for VUV phosphors mostly agrees with such a lattice site symmetry analysis [16]. However, according to the observed emission lines which originate from the 5D0-7FJ (J=0,1,2) transitions of Eu3+ ions in the visible (bottom figure of Fig.4), similar lattice symmetries such as C3v and D3 could better explain the number and position of emission lines of Eu3+ in this material. Simulation of the position of the energy levels was found to be consistent with the experimental values, as shown in Table 1.
In the experiment temperature range below room temperature, no obvious thermal quenching occurs. The total emission intensity increased when the sample was heated, most likely benefiting from an improving absorption of the excitation UV light. Phonon-assisted absorption always plays the major role in such a luminescence process. Spectra recorded at various temperatures were normalized using the peak positioned at 16927 cm-1 (peak D in the top figure of Fig.4, a 5D0-7F1 line from the D3 site), the strong 5D0-7F1 emission (in the range 16800~17000 cm-1) of Y0.95BO3:Eu0.05 was studied by monitoring the relative intensity of the various peaks; specific peaks in this region were found to decrease in relative intensity with increasing temperature (the top
figure of Fig.4). The intensity ratio was calculated directly from the emission spectra. With such a simple operation, a monotonic relationship between intensity ratio with temperature has been derived. In the same way as the standard FIR technique, it is this monotonic dependence that can be used as a sensing function. By applying a mathematical best fit to the FIR data a portion of the plotted curve is seen to be exponential. Therefore a plot of lnR versus 1/T (Fig.5) was made to make clear any departure from the expected behavior; such a plot transforms a purely exponential curve into a straight line. The experimental linear relationship near room temperature (0.3 on the abscissa) is similar to the conventional FIR technique when it is fitted using Eq. (2) (the dashed lines in Fig.5). For fitting line C, the correlation coefficient is 0.996, indicating a near perfect linear fit. The other two fitted lines A and B give correlation coefficients 0.98 and 0.94 respectively, even though the uncertainty is much higher. After determining the slopes and using Boltzmann’s constant kB, the energy differences corresponding to line A, B and C are estimated as 37 cm-1, 28 cm-1 and 30 cm-1 respectively. These energy differences are sufficiently small to imply a thermal equilibrium Boltzmann’s distribution of the doping ions with different energy states, particularly at room temperature. Previous work with the phosphor has distinguished emission lines from the two lattice sites. Given the positions of energy levels (Table 1) the obtained gap values in this work are near the energy differences between D3 and C3v ions, while cross relaxation between the two sites are also possible. As shown in the table, the energy gap between the two 5D0 levels is 25 cm-1.
A relationship between cross relaxation and doping concentration is also evident. Besides temperature, the distance between interactive ions is also a factor that affects the energy transfer rate. The shorter the distance, the faster the rate will be. For electric-dipole interaction at a normally low doping concentration, the energy transfer probability between two neighbored ions scales with the ion-ion separation r as r-6 [17]. In Fig.6, a decreasing relative intensity of the 5D0-7F1 emission at a C3v site supports the existence of a more efficient energy transfer from an ion at a C3v site to that at a D3 site by absorbing phonons, which can be a result of a growing interaction between ions with shrinking distance. Increasing the temperature has effectively the same effect by producing more phonons to assist the energy transfer. By exploring concentration effects it is suggested an optimum doping concentration of Eu3+/Y3+ as 27 mol%, while at this doping level the concentration quenching is not so important whilst the phosphor gives the most intense fluorescence signals for temperature sensing. It is proposed that cross relaxation is occurring between 5D0-7F1(1,2) transitions at a D3 site and 7F1(1,2)-5D0 transitions at a C3v site. The transition between 5D0 and 7F0 levels of Eu3+ is almost forbidden, making it not possible to exchange energy between C3v and D3 sites via any transfer process involving the 7F0 level even though the 5D0 levels of ions in the two sites are only 25 cm-1 apart. However, as the 7F1 multiplets are only about 300 cm-1 above the ground level 7F0, they will be thermally populated strongly at room temperature. Furthermore, the oscillator strength of the 5D0-7F1
transitions is high as indicated by Fig.4. The last factor that contributes to a high probability of energy transfer is the small value of energy mismatch; the largest mismatch between the 5D0-7F1(1) and 7F1(2)-5D0 gaps at D3 and C3v sites respectively is only 89 cm-1, compared to the phonon energy as several hundred wavenumbers [18]. All these factors lead to a highly effective energy exchange between dopant ions; little change of relative intensity with temperature in the emission spectra would be observed if the single peak at ~16927 cm-1 is ignored. The analysis above provides an explanation of the observed slope variation. At low temperature, below 100 K, the 7F1 levels of Eu3+ is lightly populated, leading to the result that the population of 5D0 electrons seldom benefit from a 7F1-5D0 transition, thereby producing a gentle slope at low temperature as evident in Fig.5. When the population of 7F1 electrons increases exponentially under thermal excitation, the mentioned process of energy transfer becomes effective and increases the slope of the lnR~1/T curve. The temperature limit of the portion with a steep slope can be appreciated from Fig.5 as higher than 150 K. A rough estimate using the expression of thermal energy of a particle with three degrees-of-freedom, 3/2kBT = ΔE, gives an effective value of the low temperature limit of sensing range, where ΔE is the lowest energy needed to populate the 7F1 levels of Eu3+. In conventional FIR systems, this energy may appear as the gap between two upper excited levels (E21 in Eq. (2)). Under a temperature much lower than the given value, interactions between fluorescent centers (doped ions) are weak, making the statistical distribution of them away from the thermal equilibrium laws, as shown by the slopes change of any curves in Fig.5. As for the high temperature limit of the measuring range, it must be a negotiation with effects such as thermal quenching of luminescence and black-body irradiation. We didn’t reach it due to limitation of Such a limit was not observed for experimental conditions in this work. The discussion given above may also assist to some degree to clarifying the site symmetries of Eu3+ in hexagonal YBO3. All rare-earth ions must be in similar sites given that the energy levels’ splitting and position are measured to be close to each other. This suggests a strong overlap of transition peaks, allowing for a strong resonance and a high rate of energy transfer. It is difficult to identify the emission lines in the fluorescent spectra experimentally because of the overlap of multiple peaks; a direct result of the rare-earth ions being located in very similar sites. Fortunately the single peak at ~16927 cm-1 (5D0-7F1(1) transition of Eu3+ at a D3 site) in the emission spectra (Fig.4) is well resolved from the others. The corresponding energy differences with other transitions are not exactly resonant but still very small in the limit of one vibrational quantum (mismatch values as 49 cm-1, 69 cm-1, 89 cm-1 compared to phonon energy as several hundred wavenumbers [18]). It is the isolated peak position that has determined its temperature related characteristics to be different to the other peaks the 5D0-7F1(1) transition peak to be the reference peak for any ratiometric operations. In this work the considered energy transfer takes place between two Eu3+ ions with different site symmetries in the same host material. There are various solids that offer more than one site symmetry for the dopant ions, e.g. oxyfluoride glasses [19]. At the
same time some rare-earth ions can be co-doped into a matrix, in which they transfer energy as well, e.g. the well-known up-conversion phosphors [20, 21]. Hence the alternative new FIR mechanism proposed in this work may employ not only specific active ions like Eu3+, Sm3+, Er3+, Nd3+ etc. [1], but also multiple ion-pairs and various host solids. 5. Conclusions In conclusion, this article has demonstrated that additional fluorescent materials are available for temperature sensing using phenomenon similar to the FIR technique. Radiative emissions of the functional phosphors involve two or more sets of energy levels, and thermally dependent energy transfer takes place between the different sets of levels. The scheme has been explained and experimentally demonstrated with the temperature dependent fluorescence of the phosphor YBO3:Eu3+. The scheme demonstrates a much wider range of active materials in the application of FIR technique for temperature sensing.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 61167007), the Foundation of Aeronautic Science of China (Grant No. 2012ZD56007), and the China Scholarship Council (File No. 201308360026).
References [1] S. A. Wade, S. F. Collins, and G. W. Baxter, “Fluorescence intensity ratio technique for optical fiber point temperature sensing,” J. Appl. Phys. 94 (2003) 4743 - 4756. S. F. Collins, G. W Baxter, S. A. Wade, et al., “Comparison of fluorescence-based temperature sensor schemes: Theoretical analysis and experimental validation,” J. Appl. Phys. 84 (1998) 4649 - 4654. P. J. Mohr, B. N. Taylor, and D. B. Newell, “CODATA recommended values of the fundamental physical constants: 2010,” Rev. Mod. Phys. 84 (2012) 1527 - 1605, http://www.codata.org/. W. Zhang, Y. Gao, and X. He, “Boltzmann constant determined by fluorescent spectroscopy for verifying thermometers,” Front. Optoelectron. 7 (2014) 64 - 68. S. K. Singh, K. Kumar, and S. B. Rai, “Diode laser pumped Gd2O3:Er3+/Yb3+ phosphor as optical nano-heater,” Appl. Phys. B 100 (2010) 443 - 446.
L. Feng, B. Lai, J. Wang, et al., “Spectroscopic properties of Er3+ in a oxyfluoride glass and upconversion and temperature sensor behaviour of Er3+/Yb3+-codoped oxyfluoride glass,” J. Lumin. 130 (2010) 2418 - 2423. P. Haro-González, I. R. Martín, L. L. Martín, et al., “Characterization of Er3+ and Nd3+ doped Strontium Barium Niobate glass ceramic as temperature sensors,” Opt. Mater. 33 (2011) 742 - 745. Z. P. Cai, A. Chardon, H. Xu, et al., “Laser characteristics at 1535 nm and thermal effects of an Er:Yb phosphate glass microchip pumped by Ti:sapphire laser,” Opt. Commun. 203 (2002) 301 - 313. B. Y. Lai, L. Feng, J. Wang, et al., “Optical transition and upconversion luminescence in Er3+ doped and Er3+–Yb3+ co-doped fluorophosphate glasses,” Opt. Mater. 32 (2010) 1154 - 1160. Z. P. Cai and H. Y. Xu, “Point temperature sensor based on green upconversion emission in an Er:ZBLALiP microsphere,” Sens. Actuators A 108 (2003) 187 192. S. Zhou, C. Li, Z. Liu, et al., “Thermal effect on up-conversion in Er3+/Yb3+ co-doped silicate glass,” Opt. Mater. 30 (2007) 513 - 516. T. Miyakawa and D. L. Dexter, “Phonon sidebands, multiphonon relaxation of excited states, and phonon-assisted energy transfer between ions in solids,” Phys. Rev. B 1 (1970) 2961 - 2969. G. Chadeyron，M. El-Ghozzi, R. Mahiou, et al, “Revised Structure of the Orthoborate YBO3,” J. Solid State Chem. 128 (1997) 261 - 266. W. F. Bradley，D. L. Graf, and R. S. Roth, “The vaterite-type ABO3 rare-earth borates,” Acta Crystallogr. 20 (1966) 283 - 287. J. Holsa, “Luminescence of Eu3+ ion as a structural probe in high temperature phase transformations in lutetium orthoborates,” Inorg. Chem. Acta. 139 (1987) 257 259. S. Chawla, Ravishanker, A.F. Khan, et al., “Enhanced luminescence and degradation resistance in Tb modified Yttrium Borate core–nano silica shell phosphor under UV and VUV excitation,” Appl. Surf. Sci. 257 (2011) 7167 – 7171. D. L. Dexter, “A theory of sensitized luminescence in solids,” J. Chem. Phys. 21 (1953) 836 - 850. J. H. Denning and S. D. Ross, “The vibrational spectra and structures of some rareearth borates,” Spectrochim. Acta. A 28 (1972) 1775 - 1785. G. Lakshminarayana, E. M. Weis, A. C. Lira, et al., “Cross relaxation in rare-earthdoped oxyfluoride glasses ,” J. Lumin. 139 (2013) 132 - 142. A. Pandey, and V. K. Rai, “Optical thermometry using FIR of two close lying levels of different ions in Y 2O3:Ho3+–Tm3+–Yb3+ phosphor,” Appl. Phys. B 113 (2013) 221 225. A. Rapaport, J. Milliez, M. Bass, et al., “Numerical model of the temperature dependence of the up-conversion efficiency of fluoride crystals codoped with ytterbium and thulium,” J. Appl. Phys. 97 (2005) 123527.
Zhang Weiwei received his PhD in Condensed Matter Physics from University of Science and Technology of China in 2001. He is an associate professor at Nanchang Hangkong University, China, where, he is conducting research in fluorescent sensing, fiber optic sensors, and other optic measuring methods. Stephen Collins received his PhD in physics from University of Melbourne, Australia in 1985. He is a professor at Victoria University, Melbourne, Australia, where, for over 25 years, he has conducted research in optical fiber sensors especially those based on fluorescence or Bragg grating technology, and optoelectronic imaging. Currently he is President of the Australian Optical Society. Greg Baxter received his PhD in physics from University of Melbourne, Australia in 1990. He is a professor at Victoria University, Melbourne, Australia, where, he has conducted research in optoelectronic imaging, rare-earth doped optical fiber devices and optical fiber sensors. Fotios Sidiroglou graduated with a Ph.D. in Physics from The University of Melbourne (Australia) in 2007. He is currently a Research Fellow at Victoria University, where he is looking into the fabrication and characterization of rare-earth doped optical fibers. His other research interests include optical fiber sensing based on Bragg gratings. He has co-authored a number of journal and conference publications. Duan Changkui received his PhD in Condensed Matter Physics from University of Science and Technology of China in 1998. He is currently a professor at University of Science and Technology of China. He has conducted research in physics of luminescence for nearly 20 years. Yin Min received his PhD in physics from University of Science and Technology of China in 1995. He is a professor at University of Science and Technology of China since 2000. He has conducted research in luminescence, especially optical spectroscopy of rare earth ions, and luminescent materials for temperature sensing.
Fig. 1. Schematic of de-excitation transitions from upper energy levels for the conventional FIR technique. Fig 2. Schematic showing the relevant transitions and energy transfer processes between ions for the mechanism proposed. Fig. 3. X-ray diffraction pattern of the sol-gel prepared phosphor YBO3:Eu3+, in accordance with JCPDS No. 13-483 & No.16-0277. Figure 4. In-situ emission of YBO3:Eu3+ at various temperatures under excitation at 365 nm, spaced to avoid overlap. Top: The 5D0-7F1 emission peaks normalized to peak D, with curves at two extreme temperatures emphasized for a better observation.
Fig. 5. Temperature dependence of the YBO3 :Eu3+ fluorescence intensity ratio (R=I2/I1), where I2 and I1 are heights of an observed peak (peak A, B, or C in Fig. 4) and peak D respectively in Fig. 4. Fig. 6. 5D0-7F1 emission of the Y1-xBO3:Eux samples with different doping concentration under excitation at 365 nm at room temperature (normalized to peak D). The varying fluorescence intensity ratio (R=I2/I1) could be a result of cross relaxation.
Table 1. Energy levels (partial) of YBO3:Eu3+ level Experimental / Simulated / Assigned lattice cm-1 cm-1 symmetry 5 D0 17224 17218.1 D3 17199 17198.9 C3v 7 F1 (1) 297 311.6 D3 (2) 346 368.6 D3 (1) 341 341.2 C3v (2) 361 359.9 C3v 7 F0 0 0.2 D3 0 0.3 C3v
Figure 1
Figure 2
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Figure 3
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S0924-4247(15)30001-7 http://dx.doi.org/doi:10.1016/j.sna.2015.05.005 SNA 9177
To appear in:
Sensors and Actuators A
Received date: Revised date: Accepted date:
24-10-2014 7-5-2015 8-5-2015
Please cite this article as: Weiwei Zhang, Stephen F.Collins, Greg W.Baxter, Fotios Sidiroglou, Changkui Duan, Min Yin, Use of Cross-Relaxation for Temperature Sensing via a Fluorescence Intensity Ratio, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2015.05.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Use of Cross-Relaxation for Temperature Sensing via a Fluorescence Intensity Ratio 1,2,* Weiwei Zhang , Stephen F. Collins2, Greg W. Baxter2, Fotios Sidiroglou2, Changkui Duan3, Min Yin3 1. Jiangxi Engineering Laboratory for Optoelectronics Testing Technology, Nanchang Hangkong University, Nanchang 330063, China 2. Optical Technology Research Laboratory, Victoria University, P.O. Box 14428, Melbourne City, Victoria 8001, Australia 3. Department of Physics, University of Science and Technology of China, Hefei 230026, China
*
Corresponding author. Tel.: +86 15170463677; fax: +86 791 83953461. E-mail address: [email protected] (Weiwei
Zhang).
Highlights
Cross-relaxation between the 5D0-7F1 and 7F1-5D0 transitions of Eu3+ at D3 and C3v sites respectively dominates the temperature dependent fluorescence of the hexagonal YBO3:Eu phosphor. Rate of energy transfer between donor and acceptor ions is temperature dependent, making a Fluorescence Intensity Ratio (FIR) of donor over acceptor emission indicator of temperature variation. A rough estimate using an expression 3/2kBT = ΔE gives an effective value of the low temperature limit of sensing range for FIR technique.
Abstract
A scheme that provides an alternative mechanism for temperature sensing via a fluorescence intensity ratio (FIR) is proposed. The rate of energy transfer between rare-earth ions in a crystal or glass host is temperature dependent, and as such exhibits a changing intensity ratio in the emission spectrum. Here a phosphor YBO3:Eu3+ was used to illustrate the proposed sensing mechanism. The temperature dependence of the observed fluorescence is similar to the phenomenon found for the conventional FIR technique. The potential benefit of the proposed scheme is access to numerous additional varieties of active materials for FIR temperature sensing. Keywords: Temperature; Fluorescence intensity ratio; Energy transfer.
1. Introduction The fluorescence intensity ratio (FIR) technique is known as an absolute temperature sensing method. Various rare-earth ions such as Eu3+, Nd3+, Er3+ and Sm3+ can be used in the technique [1]. The ions are generally doped into a glass or crystalline solid to form the active material. The underlying science has been articulated so that the performance of the standard FIR technique can be readily predicted [1, 2]. Most recent explorations of the FIR technique focus on the development of novel materials that display familiar characteristics, whilst reporting slight improvements in temperature sensitivity. Various thermal equilibrium systems, in addition to the FIR technique, may be utilized for absolute temperature sensing with the simple requirement that an observable characteristic is temperature dependent. When considering any thermometer, either optical or electronic, it is the thermal equilibrium distribution at the atomic or electron level that dominates temperature dependent changes. Considering the basic physics principles of such distributions, absolute temperature, T, always appears with the Boltzmann’s constant, kB, to produce energy, kBT. A wellknown example comes from a semiconductor thermometer whose current-voltage relation (namely the Shockley equation) includes Boltzmann’s distribution of carriers. Alternatively, new techniques of temperature sensing may be developed after investigating the broad array of thermal distributions of atoms or electrons. In this article, it is shown that a phosphor with emission lines from two interactive ions can be exploited successfully as a FIR material, through efficient cross relaxation between the doped rare-earth ions. The phosphor YBO3:Eu3+ is used to illustrate the sensing mechanism.
2. Experimental The phosphor YBO3:Eu3+ was prepared through a sol-gel process. An aqueous solution of rare-earth nitrates was mixed with tributyl borate, then stirred after adding ethanol until a clear sol was achieved. The gel formed overnight in a water bath held at 85C, and was then baked to form a bulk solid. This was then ground; the final product was sintered at 900C for two hours to insure crystallization. The phosphor’s luminescence was analyzed with a SPEX-1403 spectrometer under the excitation of a high pressure Hg lamp (output wavelength of 365 nm). In order to investigate the effect of temperature change on the fluorescence, the phosphor was compressed into a pellet and then glued on a brass heat sink in a temperature controlled vacuum chamber. The temperature range investigated was below room temperature.
3. Methodology of the schemes A. Conventional FIR Technique The conventional FIR technique employs active materials with two thermal coupled excitation energy levels [1]. The principle can be illustrated using a simplified diagram of the energy levels (Fig. 1). Spontaneous transition takes place from each of levels 2 & 1 to the ground state, level 0. The emission intensity Iij (i=2,1, j=0) relates to the spontaneous transition probability Aij , the frequency of the emitted light vij , and the population Ni of state i (i=2,1):
I ij Aij ij N i
Here non-radiative transition (phonon assisted relaxation) to the ground state is regarded as not important and its influence on the emission intensity has been ignored. As levels 2 & 1 are close to each other, they are thermally coupled within a certain temperature range. At thermal equilibrium, the population ratio N2/N1 follows a Boltzmann’s distribution. The intensity ratio R of transitions of state 2 to state 0 on state 1 to state 0 is then:
R C1 exp(
E21 ) k BT
where C1 is a constant for a certain active material and a certain measuring system, T is the temperature in Kelvin, and E21 is the energy gap between level 2 and level 1. For a solid phosphor, the transition energies E20 & E10 (observable from emission spectrum as hv20 and hv10) are not obviously dependent on temperature, meaning that E21 is constant. If a logarithmic operation is put on Eq. (2), a linear dependence of lnR on 1/T is evident. Furthermore, by dividing the determined slope of a plot of lnR against 1/T by E21, a value of the Boltzmann’s constant kB can be experimentally derived. When the experimental values were compared to the CODATA recommended constant [3], and the accuracy of the temperature sensing method can be judged via this comparison [4]. the discrepancy between the Boltzmann’s constant derived using the FIR technique in comparison to the recommended value can be less than 0.08% [411]. The sensing function of the FIR theory as represented eq. (2) can be seen to be highly accurate.
B. Alternative Scheme for the FIR technique An alternate scheme utilizing the FIR technique is proposed by considering temperature dependent transition processes other than that illustrated in Fig.1. Cross relaxation is a process where an excited ion (donor, D) non-radiatively transfers part of its energy to an ion (acceptor, A) in the neighborhood (Fig.2); the acceptor has a similar transition energy and therefore finds itself in a different energy state. The process normally requires a high degree of resonance between the two participating transitions, while the D and A can be the either same or different ions. The energy transfer process may be assisted by absorbing or emitting phonons, and will be efficient if the energy mismatch of the involved transitions is within one vibrational
quantum [12]. The probability of a non-resonant cross relaxation process with phonon annihilation is proportional to both the product of the oscillator strengths of the involved donor and acceptor transitions, and the phonon density. As the population of a phonon having a mode of frequency, v, at a temperature, T, is described by the BoseEinstein distribution n = 1/[exp(hv/kBT)–1], the phonon assisted cross relaxation process contributes to increase the probability of energy exchange at higher temperature. At low temperature, only energy transfers where a phonon is created are important. At higher temperature, both emission and absorption of phonons play a role. The energy transfer process changes the relative number of excited D and A ions at a constant excitation power. As both D and A ions radiate, the ratio of the fluorescence intensity (or FIR) is an indicator of the distribution ratio of excitation energy by D/A ions, and consequently is also an indicator of temperature. The D and A ions are thermally coupled by the phonon assisted cross relaxation process, and so the fluorescence intensity ratio of emissions from D and A is linked to temperature T, with T appearing in the expression of phonon density. This phenomenon is explored in this paper, following the presentation of the experimental results. 4. Results and discussions An X-ray diffraction pattern (Fig.3) of the phosphor used in the current work indicates a hexagonal calcite structure in accordance with JCPDS (Joint Committee on Powder Diffraction Standards) Files No. 13-483 and No.16-0277. However, there exists some ambiguity about the exact crystal structure of rare-earth borates [13-15]. A similar result suggested that in the matrix material, the doped rare-earth ions take two different types of lattice sites with C3 and C3/C3v symmetry respectively [13]. Later work on the popular matrix material YBO3 for VUV phosphors mostly agrees with such a lattice site symmetry analysis [16]. However, according to the observed emission lines which originate from the 5D0-7FJ (J=0,1,2) transitions of Eu3+ ions in the visible (bottom figure of Fig.4), similar lattice symmetries such as C3v and D3 could better explain the number and position of emission lines of Eu3+ in this material. Simulation of the position of the energy levels was found to be consistent with the experimental values, as shown in Table 1.
In the experiment temperature range below room temperature, no obvious thermal quenching occurs. The total emission intensity increased when the sample was heated, most likely benefiting from an improving absorption of the excitation UV light. Phonon-assisted absorption always plays the major role in such a luminescence process. Spectra recorded at various temperatures were normalized using the peak positioned at 16927 cm-1 (peak D in the top figure of Fig.4, a 5D0-7F1 line from the D3 site), the strong 5D0-7F1 emission (in the range 16800~17000 cm-1) of Y0.95BO3:Eu0.05 was studied by monitoring the relative intensity of the various peaks; specific peaks in this region were found to decrease in relative intensity with increasing temperature (the top
figure of Fig.4). The intensity ratio was calculated directly from the emission spectra. With such a simple operation, a monotonic relationship between intensity ratio with temperature has been derived. In the same way as the standard FIR technique, it is this monotonic dependence that can be used as a sensing function. By applying a mathematical best fit to the FIR data a portion of the plotted curve is seen to be exponential. Therefore a plot of lnR versus 1/T (Fig.5) was made to make clear any departure from the expected behavior; such a plot transforms a purely exponential curve into a straight line. The experimental linear relationship near room temperature (0.3 on the abscissa) is similar to the conventional FIR technique when it is fitted using Eq. (2) (the dashed lines in Fig.5). For fitting line C, the correlation coefficient is 0.996, indicating a near perfect linear fit. The other two fitted lines A and B give correlation coefficients 0.98 and 0.94 respectively, even though the uncertainty is much higher. After determining the slopes and using Boltzmann’s constant kB, the energy differences corresponding to line A, B and C are estimated as 37 cm-1, 28 cm-1 and 30 cm-1 respectively. These energy differences are sufficiently small to imply a thermal equilibrium Boltzmann’s distribution of the doping ions with different energy states, particularly at room temperature. Previous work with the phosphor has distinguished emission lines from the two lattice sites. Given the positions of energy levels (Table 1) the obtained gap values in this work are near the energy differences between D3 and C3v ions, while cross relaxation between the two sites are also possible. As shown in the table, the energy gap between the two 5D0 levels is 25 cm-1.
A relationship between cross relaxation and doping concentration is also evident. Besides temperature, the distance between interactive ions is also a factor that affects the energy transfer rate. The shorter the distance, the faster the rate will be. For electric-dipole interaction at a normally low doping concentration, the energy transfer probability between two neighbored ions scales with the ion-ion separation r as r-6 [17]. In Fig.6, a decreasing relative intensity of the 5D0-7F1 emission at a C3v site supports the existence of a more efficient energy transfer from an ion at a C3v site to that at a D3 site by absorbing phonons, which can be a result of a growing interaction between ions with shrinking distance. Increasing the temperature has effectively the same effect by producing more phonons to assist the energy transfer. By exploring concentration effects it is suggested an optimum doping concentration of Eu3+/Y3+ as 27 mol%, while at this doping level the concentration quenching is not so important whilst the phosphor gives the most intense fluorescence signals for temperature sensing. It is proposed that cross relaxation is occurring between 5D0-7F1(1,2) transitions at a D3 site and 7F1(1,2)-5D0 transitions at a C3v site. The transition between 5D0 and 7F0 levels of Eu3+ is almost forbidden, making it not possible to exchange energy between C3v and D3 sites via any transfer process involving the 7F0 level even though the 5D0 levels of ions in the two sites are only 25 cm-1 apart. However, as the 7F1 multiplets are only about 300 cm-1 above the ground level 7F0, they will be thermally populated strongly at room temperature. Furthermore, the oscillator strength of the 5D0-7F1
transitions is high as indicated by Fig.4. The last factor that contributes to a high probability of energy transfer is the small value of energy mismatch; the largest mismatch between the 5D0-7F1(1) and 7F1(2)-5D0 gaps at D3 and C3v sites respectively is only 89 cm-1, compared to the phonon energy as several hundred wavenumbers [18]. All these factors lead to a highly effective energy exchange between dopant ions; little change of relative intensity with temperature in the emission spectra would be observed if the single peak at ~16927 cm-1 is ignored. The analysis above provides an explanation of the observed slope variation. At low temperature, below 100 K, the 7F1 levels of Eu3+ is lightly populated, leading to the result that the population of 5D0 electrons seldom benefit from a 7F1-5D0 transition, thereby producing a gentle slope at low temperature as evident in Fig.5. When the population of 7F1 electrons increases exponentially under thermal excitation, the mentioned process of energy transfer becomes effective and increases the slope of the lnR~1/T curve. The temperature limit of the portion with a steep slope can be appreciated from Fig.5 as higher than 150 K. A rough estimate using the expression of thermal energy of a particle with three degrees-of-freedom, 3/2kBT = ΔE, gives an effective value of the low temperature limit of sensing range, where ΔE is the lowest energy needed to populate the 7F1 levels of Eu3+. In conventional FIR systems, this energy may appear as the gap between two upper excited levels (E21 in Eq. (2)). Under a temperature much lower than the given value, interactions between fluorescent centers (doped ions) are weak, making the statistical distribution of them away from the thermal equilibrium laws, as shown by the slopes change of any curves in Fig.5. As for the high temperature limit of the measuring range, it must be a negotiation with effects such as thermal quenching of luminescence and black-body irradiation. We didn’t reach it due to limitation of Such a limit was not observed for experimental conditions in this work. The discussion given above may also assist to some degree to clarifying the site symmetries of Eu3+ in hexagonal YBO3. All rare-earth ions must be in similar sites given that the energy levels’ splitting and position are measured to be close to each other. This suggests a strong overlap of transition peaks, allowing for a strong resonance and a high rate of energy transfer. It is difficult to identify the emission lines in the fluorescent spectra experimentally because of the overlap of multiple peaks; a direct result of the rare-earth ions being located in very similar sites. Fortunately the single peak at ~16927 cm-1 (5D0-7F1(1) transition of Eu3+ at a D3 site) in the emission spectra (Fig.4) is well resolved from the others. The corresponding energy differences with other transitions are not exactly resonant but still very small in the limit of one vibrational quantum (mismatch values as 49 cm-1, 69 cm-1, 89 cm-1 compared to phonon energy as several hundred wavenumbers [18]). It is the isolated peak position that has determined its temperature related characteristics to be different to the other peaks the 5D0-7F1(1) transition peak to be the reference peak for any ratiometric operations. In this work the considered energy transfer takes place between two Eu3+ ions with different site symmetries in the same host material. There are various solids that offer more than one site symmetry for the dopant ions, e.g. oxyfluoride glasses [19]. At the
same time some rare-earth ions can be co-doped into a matrix, in which they transfer energy as well, e.g. the well-known up-conversion phosphors [20, 21]. Hence the alternative new FIR mechanism proposed in this work may employ not only specific active ions like Eu3+, Sm3+, Er3+, Nd3+ etc. [1], but also multiple ion-pairs and various host solids. 5. Conclusions In conclusion, this article has demonstrated that additional fluorescent materials are available for temperature sensing using phenomenon similar to the FIR technique. Radiative emissions of the functional phosphors involve two or more sets of energy levels, and thermally dependent energy transfer takes place between the different sets of levels. The scheme has been explained and experimentally demonstrated with the temperature dependent fluorescence of the phosphor YBO3:Eu3+. The scheme demonstrates a much wider range of active materials in the application of FIR technique for temperature sensing.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 61167007), the Foundation of Aeronautic Science of China (Grant No. 2012ZD56007), and the China Scholarship Council (File No. 201308360026).
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Zhang Weiwei received his PhD in Condensed Matter Physics from University of Science and Technology of China in 2001. He is an associate professor at Nanchang Hangkong University, China, where, he is conducting research in fluorescent sensing, fiber optic sensors, and other optic measuring methods. Stephen Collins received his PhD in physics from University of Melbourne, Australia in 1985. He is a professor at Victoria University, Melbourne, Australia, where, for over 25 years, he has conducted research in optical fiber sensors especially those based on fluorescence or Bragg grating technology, and optoelectronic imaging. Currently he is President of the Australian Optical Society. Greg Baxter received his PhD in physics from University of Melbourne, Australia in 1990. He is a professor at Victoria University, Melbourne, Australia, where, he has conducted research in optoelectronic imaging, rare-earth doped optical fiber devices and optical fiber sensors. Fotios Sidiroglou graduated with a Ph.D. in Physics from The University of Melbourne (Australia) in 2007. He is currently a Research Fellow at Victoria University, where he is looking into the fabrication and characterization of rare-earth doped optical fibers. His other research interests include optical fiber sensing based on Bragg gratings. He has co-authored a number of journal and conference publications. Duan Changkui received his PhD in Condensed Matter Physics from University of Science and Technology of China in 1998. He is currently a professor at University of Science and Technology of China. He has conducted research in physics of luminescence for nearly 20 years. Yin Min received his PhD in physics from University of Science and Technology of China in 1995. He is a professor at University of Science and Technology of China since 2000. He has conducted research in luminescence, especially optical spectroscopy of rare earth ions, and luminescent materials for temperature sensing.
Fig. 1. Schematic of de-excitation transitions from upper energy levels for the conventional FIR technique. Fig 2. Schematic showing the relevant transitions and energy transfer processes between ions for the mechanism proposed. Fig. 3. X-ray diffraction pattern of the sol-gel prepared phosphor YBO3:Eu3+, in accordance with JCPDS No. 13-483 & No.16-0277. Figure 4. In-situ emission of YBO3:Eu3+ at various temperatures under excitation at 365 nm, spaced to avoid overlap. Top: The 5D0-7F1 emission peaks normalized to peak D, with curves at two extreme temperatures emphasized for a better observation.
Fig. 5. Temperature dependence of the YBO3 :Eu3+ fluorescence intensity ratio (R=I2/I1), where I2 and I1 are heights of an observed peak (peak A, B, or C in Fig. 4) and peak D respectively in Fig. 4. Fig. 6. 5D0-7F1 emission of the Y1-xBO3:Eux samples with different doping concentration under excitation at 365 nm at room temperature (normalized to peak D). The varying fluorescence intensity ratio (R=I2/I1) could be a result of cross relaxation.
Table 1. Energy levels (partial) of YBO3:Eu3+ level Experimental / Simulated / Assigned lattice cm-1 cm-1 symmetry 5 D0 17224 17218.1 D3 17199 17198.9 C3v 7 F1 (1) 297 311.6 D3 (2) 346 368.6 D3 (1) 341 341.2 C3v (2) 361 359.9 C3v 7 F0 0 0.2 D3 0 0.3 C3v
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