Investigation of luminescence from LuAG: Mn4+ for physiological temperature sensing

Optical Materials 66 (2017) 447e452

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Optical Materials journal homepage: www.elsevier.com/locate/optmat

Investigation of luminescence from LuAG: Mn4þ for physiological temperature sensing Fei Li, Jiajia Cai, FengFeng Chi, Yonghu Chen*, Changkui Duan, Min Yin Department of Physics, University of Science and Technology of China, Hefei 230026, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 January 2017 Received in revised form 24 February 2017 Accepted 26 February 2017

Optical thermometry based on luminescent materials has garnered much attention due to its many advantages. But higher sensitivity is still expected in physiological temperature range which is of special significance in medicine and biology. For this purpose, quadrivalent manganese doped lutetium aluminum garnet, Lu3Al5O12: Mn4þ, or simply LuAG: Mn4þ, has been successfully synthesized by sol-gel method and its temperature dependent luminescence has been investigated in the present work. Compared to the common red emission phosphors Y3Al5O12: Mn4þ (YAG:Mn4þ) with same structure, LuAG:Mn4þ has a stronger crystal field strength and a higher thermal-quenching activation energy (DE) of 5732 cm1. Rapid thermal quenching of the Mn4þ luminescence occurred above room temperature around 90  C for our LuAG:Mn4þ sample. Temperature dependent decay curves of Mn4þ emission from LuAG:Mn4þ revealed that an extraordinary high sensitivity can be achieved from luminescence lifetime measurements covering physiological temperature range with a sensitivity of 3.75% K1 at 38  C. © 2017 Elsevier B.V. All rights reserved.

Keywords: Solegel processes Lifetime Thermal properties Optical thermometry

1. Introduction Nowadays, transition metal ions and rare earth ions doped phosphors of various kinds have witnessed an increasingly widespread popularity for their application in diverse fields such as LED [1,2], bio-cultivation [3,4], and temperature sensing [5e7]. Employed in temperature sensing, luminescence-based thermometers have the advantages of noninvasiveness, accuracy, fast response, high spatial resolution, and environmental applicability [8]. Several temperature-dependent fluorescent properties of phosphors, such as changes in transition energy [8], lifetime [9] and intensity [10], can be used as the main metrics in temperature measurements. To date, several luminescent materials have been explored and developed as optical thermometers. For instance, as the commonly used temperature materials, lanthanide doped upconversion luminescent materials are based on fluorescence intensity ratio technique [9e11], which can work in a wide temperature range with a relatively high sensitivity [12]. However, materials and performance for sensing in the physiological temperature range (298e318 K) of special interests of medicine and biology are still need to be explored and improved [8,12].

* Corresponding author. E-mail address: [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.optmat.2017.02.054 0925-3467/© 2017 Elsevier B.V. All rights reserved.

As well-known, transition metal doped luminescence materials exhibit a strong electronephonon coupling [13], which usually makes their emissions easily thermally quenched, as observed, for instances, in YAlO3:Mn4þ [14] and BaGe4O9:Mn4þ [15]. Therefore, temperature sensitive luminescence can be expected from these kinds of materials. Moreover, as cheap and easy-to-obtain raw materials [16,17], Mn4þ doped red phosphors have received increasing attention these years. In this work, we focus on the application prospect of Mn4þ doped phosphors in thermometers. As a novel red phosphor previously reported, YAG: Mn4þ [17] has a thermal-quenching activation energy as high as 2230 cm1 [13]. On the other hand, it is also found in RE2Sn2O7:Mn4þ(RE3þ ¼ Y3þ, Lu3þ or Gd3þ) that crystal field strength of Mn4þ enhanced with the decrease of RE3þ ionic radius and Mn4þ-O2 distance, as expected from the electrostatic point charge model [18]. Generally, thermalquenching activation energy (DE), as well as crystal field strength, could increase with metal-ligand distance decreased [19,20] and while larger thermal-quenching activation energy (DE) usually means higher temperature sensitivity [12,15]. Therefore we choose lutetium aluminum garnet (LuAG) as host for Mn4þ doping, in consideration of the smaller ionic radius of Lu3þ than Y3þ and shorter Mn4þ-O2 distance in LuAG:Mn4þ. A stronger crystal field strength and a higher thermal-quenching activation energy (DE) are expected in LuAG: Mn4þ than in YAG: Mn4þ, which could possibly leads to a better temperature sensing performance.

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Accordingly, temperature dependent luminescence is recorded to characterize LuAG: Mn4þ. A spectral blue-shift of Mn4þ emission is also observed, evidencing the stronger crystal field strength in LuAG host. A strong thermal quenching of the luminescence occur at above 90  C. However, a remarkable high temperature sensitivity is registered in physiological temperature range when using decay time as indicator of temperature, recommending it as a promising candidate for thermometry applications in medicine and biology. 2. Experiments The lutetium aluminum garnet doped with quadrivalent manganese, LuAG:Mn4þ was synthesized by sol-gel method [21,22]. As reported in YAG, Mn4þ ions can better enter the YAG crystal lattice when additional charge compensator is applied, e.g., Mg2þ [23]. Besides, large amount of additional Mg2þ ions can be used to improve the Mn4þ emissions in YAG:Mn4þ and an optimal formula Y3Al4.959Mn0.001Mg0.04O12 was found in the previous work [17]. Therefore, a similar formula Lu3Al4.959Mn0.001Mg0.04O12 was used for the present preparation of LuAG:Mn4þ in consideration of the structural similarity between YAG and LuAG. Lutetium oxide (Lu2O3, A. R.), aluminum nitrate (Al(NO3)3, A. R.), manganous carbonate MnCO3 (99.99%) and basic magnesium carbonate Mg2(OH)2CO3 were used as starting materials. Firstly, the stoichiometric amount of Lu2O3, MnCO3 and Mg2(OH)2CO3 were dissolved into hot diluted nitric acid. Then the required amounts of aluminum nitrate Al(NO3)3 and citric acid (C6H8O7.H2O) (citric acid: metal ion ¼ 2:1) were added and dissolved into the above solution. The mixture was stirred and heated in water bath of 80  C. After several hours of heating and stirring, yellow gel was formed. Subsequently, the gel was heated at 180  C for 4 h in drying oven to get white gel. The white gel were thoroughly ground in an agate mortar and then be annealed at 600  C for 4 h in a muffle furnace in air to remove the residual carbon. Finally, the obtained powders were calcinated for 4 h at 1000  C in air to yield the final product of LuAG:Mn4þ. The crystal structures of LuAG:Mn4þ was analyzed by a MXPAHF rotating anode X-ray diffractometer using Cu Ka radiation (l ¼ 0.15418 nm). The XRD profiles were collected in the range 10 < 2q < 70 . Photoluminescence excitation spectra, emission spectra and the decay curves at different temperatures were characterized on a HITACHI 850 fluorescence spectrometer with a 150 W Xe lamp as excitation source. The signal was analyzed using an EG&G 7265 DSP lock-in amplifier. The decay curves of the sample were measured at each temperature using a Tektronix TDS2024 digital storage oscilloscope. The temperature of the sample fixed on a copper post was controlled over the range of 34e110  C using a temperature controller (FOTEK MT48-V-E, Taipei) with a type-K thermocouple and a heating tube. 3. Results and discussion 3.1. Phase and crystal structure analysis Fig. 1 shows the XRD patterns of the obtained LuAG:Mn4þ sample in comparison with the standard data of LuAG(PDF#741368). The two XRD patterns match well and no redundant crystal phases are observable in our sample, indicating that the pure phase LuAG:Mn4þ has been obtained. The Al3þ in LuAG have two kinds of sites, one is octahedral point symmetry (coordination number CN ¼ 6) and the other is tetragonal point symmetry (CN ¼ 4); and the Lu3þ has only one site with D2 point symmetry (CN ¼ 8) [24]. Because of the similar revised effective ionic radii of Al3þ (r ¼ 0.535 Å, CN ¼ 6) and Mn4þ (r ¼ 0.530 Å, CN ¼ 6) and the stronger ligand-field stabilization energy of Mn4þ in the hexa-

Fig. 1. Powder XRD patterns of LuAG:Mn4þ sample (a) and the standard data of LuAG (PDF#74-1368).

coordination [17,25], Mn4þ are most likely to incorporate themselves into octahedral positions of Al3þ ions [23]. Obviously, the structure of LuAG crystal has almost no change when Al3þ is substituted by Mn4þ and/or Mg2þ.

3.2. Photoluminescence analysis The room temperature photoluminescence emission (PL) spectra and the excitation (PLE) spectra of LuAG:Mn4þ and YAG:Mn4þ (from Ref. [17]) phosphors are both presented in Fig. 2(b) and (c), respectively. Though the two samples have similar spectral shape for Mn4þ luminescence, an obvious blue shift is observed from YAG:Mn4þ to LuAG:Mn4þ in both the emission and the excitation spectra. When excited by 355 nm the PL spectrum of LuAG:Mn4þ contains two emission bands with the stronger band centered at 666 nm and another centered around 638 nm. The two bands can be attributed to 2E / 4A2 transitions of Mn4þ ions. However, there is only one octahedral site of Al3þ for Mn4þ substitution, excluding the possibility of luminescence from different sites. Actually, the two bands can be attributed to the phononassisted Stokes and the anti-Stokes 2E / 4A2 transitions. It is reported that due to the parity- and spin-forbidden nature of the 2E / 4A2 transition, the emission intensity of the zero-phonon line (ZPL) of 2Eg / 4A2 would be rather weak and the phonon-assisted vibronic transitions usually dominate the emission of Mn4þ [26], and the Stokes and anti-Stokes of 2E / 4A2 transition would be prominent. The PLE spectra of LuAG:Mn4 monitoring 666 nm emission consist of two strong excitation bands centered at 345 and 480 nm, respectively. The excitation band located in blue region is assigned to 4A2 / 4T2 transitions of Mn4þ. And the broad excitation band located in UV region (345 nm) can be assigned to 4A2 / 4T1 and 4A2 / 2T1 transitions, of which 4A2 / 4T1 transition plays a dominant role and 4A2 / 2T1 transition can be ignored. The above assignment of the excitation bands is supported by the fact that the 4 A2 / 4T1 and the 4A2 / 4T2 transitions are spin-allowed and the transition 4A2 / 2T1 is spin-forbidden according to the spin selection rule of DS ¼ 0. However, the spin-forbidden transition 4A2 / 2T1 is still be found in some cases of Mn-incorporated phosphors, such as CaAl12O19: Mn4þ, SrMgAl10O17: Mn4þ [27]. The splitting of the Mn4þ (3d3) energy levels in an octahedral

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Fig. 2. Tanabe-Sugano diagram of the d3 electron configuration in an octahedral crystal field (a). Photoluminescence excitation (PLE) spectra (b) and emission (PL) spectra (c) (excited by 355 nm) of LuAG:Mn4þ and YAG:Mn4þ [15]. PLE spectra of LuAG:Mn4þ and YAG:Mn4þ is monitoring at 666 nm and 672 nm, respectively.

crystal field can be described by the Tanabe-Sugano diagram as shown in Fig. 2(a). The emission and the excitation spectra of Mn4þ ions in different octahedron are similar, but the peak position may shift due to specific crystal field strength in the octahedron [27]. The effective ionic radii of Lu3þ ions (r ¼ 0.977 Å, CN ¼ 8) in LuAG is less than Y3þ (r ¼ 1.019 Å, CN ¼ 8) in YAG [25] and the unit cell volume of the LuAG (with lattice constant a ¼ 11.906 Å) would shrink in comparison with YAG (a ¼ 12.002 Å) [18]. Consequently, the Mn4þ-O2 (Al3þ-O2) distance in the MnO6 octahedron of LuAG: Mn4þ would be shorter than that in YAG: Mn4þ. As expected from electrostatic point charge model, the crystal field strength for Mn4þ will increases with the decreasing of Mn4þ-O2 distance [18]. Therefore, the energy of the 4T1, 4T2 and 2E state of Mn4þ shown in Fig. 2(a) increase, leading to the blue shift observed in the spectra.

emission intensity at high temperature, we choose emission decay time instead of luminescence intensity as indicator of temperature. The decay curves (monitoring 666 nm emission) of LuAG:Mn4þ in different temperature range from 34 to 110  C is shown in Fig. 4(a). The emission lifetime of LuAG:Mn4þ decreases when

3.3. Temperature dependence of the Mn4þ luminescence To further study the temperature dependence of the luminescence, temperature-dependent PL spectra of LuAG:Mn4þ (excited by 355 nm) from 34 to 100  C is shown in Fig. 3. The intensity of the Mn4þ emission is found to decrease quickly with an elevation in temperature, and a strong thermal quenching of Mn4þ luminescence of LuAG: Mn4þ was revealed. Unfortunately, the quenching temperature is too low that LuAG: Mn4þ is unsuitable for most of lighting application, though phosphors based in YAG host are widely used in this direction. However, the emission intensity is still fairly sufficient at temperature range from room temperature to 90  C for thermometry application. Moreover, to avoid the unfavorable influence of the low signal-to-noise ratio of the weak

Fig. 3. Temperature-dependent PL spectra of LuAG:Mn4þ (excited by 355 nm) from 34 to 100  C.

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Fig. 4. (a) The decay curves of LuAG:Mn4þ (monitoring 666 nm emission) in different temperature range from 34 to 110  C. (b) Schematic configuration coordinate diagram for illustration of the thermal quenching of Mn4þ in LuAG host.

temperature is raised. The thermal quenching of the luminescence can be explained using a configuration coordinate curves shown in Fig. 4(b) [15]. When excited by UV or blue light, the electrons of Mn4þ will be pumped from the ground state 4A2 to the excited states 2E, 4T1 and 4T2. Usually, the electrons will nonradiatively relaxed to the lowest excited state 2E, and followed by radiative transition to the ground state 4A2. However, due to strong electronephonon coupling [13], more and more electron in 2E state can be thermally excited to 4A2 state when the temperature is elevated, via the crossover point of the states 4A2 and 2E [15]. As a result, the emission intensity and luminescence lifetime of LuAG:Mn4þ decrease when temperature enhanced. To future explore the possible application of LuAG:Mn4þ in optical thermometry of physiological temperature, the decay curves of LuAG:Mn4þ at temperature range from 34 to 50  C is shown in detail in Fig. 5. An obvious shortening in decay time due to thermal quenching is observed. Besides, the emission intensity is fairly sufficient to distinguish for thermometry application. The

intensity and the photoluminescence lifetime of LuAG:Mn4þ can be used as the indicator of the corresponding temperature. For evaluation of the thermal-quenching activation energy (DE) of thermal quenching, we can fit the temperature-dependent lifetimes rather than emission intensity to a modified Arrhenius equation [15]:

tðTÞ ¼

t0

(1)

DE

1 þ CeKB T

or

1

t

¼

1

t0

DE

1 þ CeKB T



where t (T) is the emission decay lifetime at temperature T, t0 is the emission lifetime at 0 K, C is a rate constant for the process and DE is the activation energy of the thermal quenching. To intuitively shed light on the relationship between the lifetime of LuAG:Mn4þ (t) and the temperature (T), the plot of inverse absolute lifetime (1/t) on a single logarithmic scale as a function of temperature range from 307 K to 383 K were employed in Fig. 6, the data in the range from 307 to 323 K is plotted in detail in the inset. And the experiment dates can be fitted well by Eq. (2). Consequently, the obtained energy gap DE, and the constant t0 and C of Eq. (2) are 5732 cm1, 1.70 ms and 2.40  1011, respectively. Meanwhile, the thermalquenching activation energy (DETM) was calculated as only 2230 cm1 for the Mn4þ single-doped YAG sample [13]. The thermal quenching phenomenon of LuAG:Mn4þ is apparently stronger than YAG:Mn4þ. It could be accounted by the large electronphonon coupling effect caused by the larger crystal field strength of LuAG:Mn4þ than that of YAG:Mn4þ. The relative sensitivity (SR) of is defined as following equation:

dt DE ¼ t C eKB T DE SR ¼ tdT t0 KB T 2

Fig. 5. The decay curves of LuAG:Mn4þ in different temperature range from 34 to 50  C.

(2)

(3)

The calculated curve of relative temperature sensitivity (SR) as a function of absolute temperature (T) is plotted in Fig. 7. It can be seen that the relative sensitivity of LuAG:Mn4þ is very high during our experimental temperature range, and has a maximum value of

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4. Conclusion A novel transition metal doped phosphor LuAG:Mn4þ have been synthesized by sol-gel method. X-ray powder analysis confirmed the formation of single phase of LuAG. The luminescence spectra of LuAG:Mn4þ showed a blue shift compared to that of YAG:Mn4þ because of stronger crystal field strength in LuAG host. The photoluminescence lifetime of LuAG:Mn4þ can be employed for temperature measurements in the range from 34 to 50  C. The thermalquenching activation energy of LuAG:Mn4þ is 5732 cm1, higher than that of YAG:Mn4þ (2230 cm1). The relative sensitivity of LuAG:Mn4þ is extraordinary high in the physiological temperature range with a large sensitivity of 3.75% K1 at 38  C, suggesting its potential application for temperature sensing in this temperature range of special interests. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 11574298, 11374291, and 11274299). Fig. 6. Single logarithmic plot of inverse absolute lifetime of LuAG:Mn4þ (1/t as a function of temperature (T) range from 307 to 383 K. The inset is the fitting of the data in the range from 307 to 323 K.

6.35% at 72  C and a typical sensitivity of 3.75% K1 at 38  C. While the maximal relative temperature sensitivity for the Eu3þ/ Mn4þ:YAG are 4.81% K1 [13]. Besides, to compared the relative sensitivity (SR) of LuAG:Mn4þ phosphor and common thermometry material, the relative sensitivity of FIR based thermometry material with high energy gap DE, such as NaYF4:Er3þ,Yb3þ nanoparticles (DE ¼ 812 cm1) [11], is also plotted in Fig. 7. The relative sensitivity (SR) of optical thermometry material based on fluorescence intensity ratio (FIR) is equal to DE/ kBT2, where DE is the energy gap between the two thermally coupling emission levels involved [12]. The relative sensitivity of LuAG:Mn4þ is obvious higher than FIR based thermometry material NaYF4:Er3þ,Yb3þ nanoparticles, especially in the physiological temperature range. Impressively, it is expected that this LuAG:Mn4þ could be used in optical temperature sensing in physiological temperature range.

Fig. 7. The relative sensitivity (SR) of LuAG:Mn4þ and NaYF4:Er3þ,Yb3þ nanoparticles at various temperatures from 30 to 110  C. The SR of LuAG:Mn4þ is defined as Eq. (3); and the SR of NaYF4: Er3þ,Yb3þ nanoparticles is defined as 1171/T2 [11].

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