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Photoluminescence properties of perspective bolometric crystals Na2Mo2O7 and Na2W2O7 grown by low-thermal-gradient Czochralski technique
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Photoluminescence properties of perspective bolometric crystals Na2Mo2O7 and Na2W2O7 grown by low-thermal-gradient Czochralski technique Alexey A. Ryadun *, Mariana I. Rakhmanova, Veronika D. Grigorieva Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Science, 630090, Novosibirsk, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: Low-thermal-gradient Czochralski technique Na2Mo2O7 Na2W2O7 Luminescence lifetime Double beta-decay Bolometers Sensor sensitivity
Alkaline molybdate and tungstate crystals are promising bolometric materials for the search for rare events, especially for the search for neutrinoless double beta decay. Fluorescent thermometry for remote measurement of the temperature needs materials that demonstrate a significant change in the decay time of luminescence with a change in temperature. This paper presents the results of temperature-induced changes investigation in the photoluminescence of undoped tungstates and molybdates of the general formula Na2M2O7 (M ¼ Mo, W) in the temperature range of 77–300 K. Analyzing changes in the emission and excitation spectra, as well as in the decay kinetics allowed assessing the prospects of Na2Mo2O7 and Na2W2O7 crystals as materials for luminescence lifetime thermometry. The emission color chromaticity coordinates of Na2Mo2O7 and Na2W2O7 crystals were calculated and shown on Commission Internationale de L’Eclairage (CIE 1931). Correlated color temperature and color purity for both investigated crystals were estimated. For Na2Mo2O7 correlated color temperature is equal to 1672 K and color purity is 95%, for Na2Mo2O7 correlated color temperature is equal to 6460 K and color purity is 27%. The CIE investigation results show that Na2Mo2O7 expose strong red emission with color purity close to color purity of the monochromatic light sources.
1. Introduction Exact measurement of temperature values is necessary for under standing different physical and chemical processes for conducting ex periments and for control of technological processes under given conditions. The functional properties of materials used under extreme conditions (elevated temperature, pressure, aggressive chemical envi ronment and etc.) are highly temperature dependent. Sometimes it is impossible to measure the temperature directly; therefore, a non-contact measurement is required. Methods for solving the problem of contactless determination of temperature are described in Refs. [1,2]. For temperature remote determination described methods use changes in the optical properties of materials (emission, absorption, etc.). Widely used IR thermometry is not suitable for changing temperatures below the boiling point of liquid air, because a low value of the radiation intensity does not allow achieving a high-quality signal [3]. The field of luminescence thermometry is growing intensively showing significant breakthroughs in sensing, imaging, diagnostics, and therapy, among other areas [4]. This interest has been mostly
encouraged because many of the presenttechnological demands in areas such as microelectronics and nanoelectronics, photonics, nanomedicine, and microfluidics and nanofluidics have reached a point such that the use of traditional contact thermal probes (liquid-filled and bimetallic thermometers, thermocouples, pyrometers, and thermistors) is not capable to make reliable measurements when spatial resolution enters to the sub-micrometer range. This limitation of conventional thermometers for small systems has spurred the development of luminescent micro thermometers and nanothermometers, a research topic leaving its in flationary epoch accounting nowadays for more than 2.5% of the total publications in luminescence materials. Moreover, and from an indus trial point of view, this predicted miniaturization is expected to bring to the market new nanoscale thermal probes. Despite the recently devel oped luminescent nanothermometers are radically more sophisticated encompassing complex synthesis procedures, the fundamental problems and the applications that are being addressed are analogous to those reported in the field’s infancy: the understanding of heat transfer and energy transfer mechanisms, the optimization of temperature readout, and the developing of efficient and cost-effective sensors for front-edge medical and engineering tools. Among the distinct emitting centers used
* Corresponding author. E-mail address: [email protected] (A.A. Ryadun). https://doi.org/10.1016/j.optmat.2019.109537 Received 17 September 2019; Received in revised form 11 November 2019; Accepted 12 November 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Alexey A. Ryadun, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109537
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bio-imaging. The obtained results show that the ZnGa2O4:Cr3þ system has a high potential for the ratiometric thermometry applications from around ambient temperature to high temperatures. The relative sensi tivity of ZnGa2O4:Cr3þ for thermometry is estimated to be 2.8%K 1 at 310 K, which place this material at the top among the ratiometric fluorescent thermometers using Boltzmann distribution phenomenon for biological application. Thermometry of the luminescence lifetime is based on the temper ature dependence of the luminescence decay time constant [8]. This method can be used to measure the temperature of objects, when many other methods are not suitable. For example, high-temperature and chemically aggressive environments, MRI and other processes [9,10], Almost all applications of luminescent thermometry are related to the measurement of temperatures near ambient temperature. Rarely, this method was used to measure temperatures below 200 K, despite its advantages. The absence of direct contact between the sensor and the objects eliminates heat leakage and leads to more accurate measure ments. In Refs. [11–13], the authors gave examples of using thermom etry of the luminescence lifetime at low temperatures, in which La2O2S–Eu, MgFGeO6:Mn, CaF2-Yb and SrF2-Yb were used as the mea surement material. The main physical feature that imposes a restriction on the use of the method of luminescent thermometry is associated with the determination of temperature-dependent mechanisms that affect the change in the constant lifetime. Because the known mechanism of thermally activated non-radiative decay of excited states, which causes changes in the decay time constant, is limited by the temperature, below which it is poorly pronounced. Crystals of congruently melting molybdates and tungstates have significant scientific and practical importance as laser, acousto-optic, scintillating and bolometric materials [14,15]. At present time the scintillating properties of molybdate crystals are of particular interest for the detection of rare events at cryogenic temperatures since 100Mo isotope is one of the most promising sources of neutrinoless double beta-decay (0ν2β-decay) [16–18]. In present, ZnMoO4, and CaMoO4 single crystals are being applied as cryogenic scintillation bolometers [19,20]. However, despite the scientific and practical importance and long history of such studies, many complex problems of recording rare events have not been solved. Bolometric requirements in such experi ments include high molybdenum concentration in the volume, high energy resolution, ultra-low radiation background, large size of single crystals and adequate cost. That makes molybdates of light alkaline metals, such as Na, perspective bolometric materials for rare events physics, especially for neutrino study. Low-thermal-gradient (below 1� /cm) Czochralski technique (LTG Cz) was developed for growing large-sized oxide crystals of highest optical quality from melt. It procures such advantages as: suppression of decomposition and volatilization of melt components that prevents the formation of local melt nonstoichiometries and the loss of initial mate rials (especially important when working with such materials as 100Mo); and the decrease of thermoelastic stress in crystals that prevents cracking and increases maximal possible size of the crystals [21]. In this work we present the characterization results for two iso structural pure crystals of childrenite-eosphorite mineral group (sp. gr. Cmca) perspective in luminescence lifetime thermometry: sodium ditungstate Na2W2O7 (a ¼ 7216 Å, b ¼ 11,899 Å, c ¼ 14,716 Å) and so dium dimolybdate Na2Mo2O7 (a ¼ 7.164(6) Å, b ¼ 11.837(4) Å, c ¼ 14.713(2) Å). Both Na2W2O7 and Na2Mo2O7 compounds melt congruently at 738 � 5 and 632 � 5 � C, correspondingly. In this work, the excitation and emission spectra of the luminescence, as well as the decay curves in the temperature range 77–300 K were measured.
in luminescence thermometry, including proteins nucleic acids and other biomolecules, thermosresponsive polymers, organic dyes, and QDs, many Ln3þ-based luminescent thermometers have been reported, essentially in the last decade. Systems comprise molecular complexes, MOFs, polymers, and organic–inorganic hybrids, multifunctional heater-thermometer nanoplatforms, and etc. This review describes the use of these Ln3þ-based phosphors as luminescent ratiometric ther mometers in diverse applications, with focus on what the authors believe that will be the emergent new research areas in this fascinating research field: the use of luminescence thermometry for thermal imag ing, early tumor detection, and as a tool for unveiling properties of the thermometers themselves or of their local surroundings. Finally, to become a consolidated subject and not a temporary fashion, the research on luminescent thermometry must settle down as a strong node of fertile interactions among disparate communities, such as chemists, physicists, engineers, theoreticians, biologists, and physicians. The cross-fertilization of ideas and experiences at these interfaces will certainly induce important and exciting breakthroughs in future years. In Ref. [5] it has been shown that many luminescent systems (including polymers, organic dyes, rare earth doped crystals, rare earth doped nanocrystals, semiconductor nanocrystals, rare earth doped complexes, phosphorous and quantum dots) can be used as basic light-emitting materials for nanothermometry. It is not possible to highlight one material over the rest since the most suitable one would depend on the actual system to be thermally imaged. The continuous development of new microscopy techniques coupled with novel and cutting edge synthesis processes (allowing for the rational design of novel luminescent materials) will ensure the speedy development of luminescence nanothermometry, although working principles will be probably the same. Biological applications are by far the most chal lenging of all possible applications described in this review, due to the complex nature of the biological milieu. To overcome some of the hur dles associated with working in a biological environment, employing luminescent nanothermometers optically excited in the near-infrared (NIR) region is particularly promising. In combination with multi photon microscopy, this would allow for high-resolution, three-dimen sional thermal imaging of living specimens. Although a great deal of effort has been invested in the development of NIR excited luminescent nanothermometers, their real-world application in three-dimensional thermal imaging of bio-systems is far from being a reality. For the most part, this is because these nanothermometers typically emit in the visible range, where tissue transparency is low. In our opinion the forthcoming advances in the field of luminescence nanothermometry will be propelled by the development of luminescent nano thermometers, which operate within the biological window (700–900 nm). That is, both their excitation and emission wavelengths lie within this optimal window. The most groundbreaking results in the field of luminescent nanothermometry will be obtained by using ratio metric thermal sensors. Authors [6] demonstrated the potentiality of Cr3þ singly activated phosphors as ideal ratiometric luminescent thermometers not only for biological but for wider applications. Detailed spectroscopic investiga tion on the thermal behaviour of Cr3þ-doped Bi2Ga4O9 system showing the importance of an insightful analysis in a wide temperature range is reported. In addition, by considering the importance in real applications of the spectral change speed with temperature, a theoretical insight into the absolute sensitivity of singly activated systems characterized by couples of excited states obeying to the Boltzmann law is reported. Bi2Ga4O9:Cr3þ shows a remarkable absolute sensitivity of 0.28 � 0.02 K 1 at 300 K. In study [7], authors investigated the temperature dependence of photoluminescence spectrum of ZnGa2O4:Cr3þ and the possibility of using this material for thermometry. ZnGa2O4:Cr3þ is a well-known deep red (persistent) phosphor with R-lines due to the 2E→4T2 spin-forbidden transition of Cr3þ ions between 690 and 750 nm. Recently, this system has attracted a great deal of interest for in-vivo
2. Experimental 2.1. Crystal growth In our experiments deeply purified MoO3 and WO3 and commercial 2
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high purity Na2CO3 (15-2 TS 609 589–77, 99.99%) were used as initial compounds. Solid-state syntheses of pure Na2W2O7 and Na2Mo2O7 from the powders were carried out in the same 130 � 70 mm platinum cru cible, from which the crystals were grown, according to the reactions: 2WO3 þ Na2CO3 ¼ Na2W2O7 þ CO2 ↑ 2MoO3 þ Na2CO3 ¼ Na2Mo2O7 þ CO2 ↑ The compounds were mixed, heated up to 450 � C at the rate of 30� /h and kept at this temperature for 5 h. After that, temperature was elevated up to 20 � C above the melting point (758 � C for Na2W2O7, 652 � C for Na2Mo2O7) at the rate of 70 � C/h and kept at this temperature over 3 h for homogenization of the melt. Crystals were grown from the synthesized compounds by the lowthermal-gradient Czochralski technique (LTG Cz) in air atmosphere. The crucible covered with a platinum lid furnished with a pipe socket, through which the pull rode with an oriented seed was introduced into the crucible, was placed in a three-zone resistance furnace with low thermal conductivity top and bottom thermal insulation. Crystals stayed inside the crucible during the entire growth process. Unlike conven tional Czochralski technique, visual control is impossible in LTG Cz technique, thus, the growth processes were analyzed and controlled via weight signal channel at all stages of the process including the seeding. Temperature distribution between heating zones was preset in a way to ensure optimal growth conditions and flat crystallization front.
Fig. 1. The luminescence excitation and emission spectra of Na2W2O7 crystal (λem ¼ 540 nm; λex ¼ 310 nm; T ¼ 77 K).
2.2. Experimental techniques Excitation and emission photoluminescence spectra of 3x3x5 mm3 unpolished Na2W2O7 and Na2Mo2O7 crystal samples were recorded with a Horiba Jobin Yvon Fluorolog 3 photoluminescence spectrometer equipped with 450 W ozone-free Xe-lamp, cooled PC177CE-010 photon detection module with a PMT R2658 and double grating excitation and emission monochromators. Excitation and emission spectra were cor rected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves. Tem perature dependences of emission and excitation spectra and decay time curves were recorded with Optistat DN in temperature range 77–300 K.
Fig. 2. The luminescence excitation and emission spectra of Na2Mo2O7 crystal (λem ¼ 650 nm; λex ¼ 350 nm; T ¼ 77 K).
metastable levels. The PL temperature dependence is well described by Mott model with two recombination channels, radiative and non-radiative, from the relaxed excited state [29]. The emission color chromaticity coordinates of Na2W2O7 and Na2Mo2O7 crystals are calculated from emission spectra and shown on Commission Internationale de L’Eclairage (CIE1931) (Fig. 4). CIE chromaticity coordinates of Na2Mo2O7 with an excitation wavelength of 350 nm are (x ¼ 0.583, y ¼ 0.411) and of Na2W2O7 with an excitation wavelength of 310 nm are (x ¼ 0.300, y ¼ 0.434). It can be assumed that Na2Mo2O7 crystals are promising as red phosphorus for a white LED and Na2W2O7 as green. But it is necessary to calculate correlated color temperature and color purity. The correlated color temperature is calculated from McCamy’s formula [30]:
3. Results and discussion 3.1. Temperature dependence of Na2Mo2O7 and Na2W2O7 crystals photoluminescence properties Na2Mo2O7 and Na2W2O7 crystals were chosen as objects for this work, first of all due to the previously obtained results of their lumi nescence and scintillation properties investigations. Decay time constant for tungstate and molybdate crystals varies significantly with tempera ture [22–27]. Application of materials to contactless thermometry is based on intense luminescence response over the temperature range of interest. Thus, knowledge of temperature dependences of emission and excitation spectra is necessary. It was found that the luminescent properties of the materials under study exhibit significant temperature dependence. Na2W2O7 crystals exhibit a green glow (Fig. 1), and Na2Mo2O7 crystals a red glow (Fig. 2). Emission and excitation spectra temperature de pendences of Na2Mo2O7 and Na2W2O7 crystals are shown in Fig. 3. The emission of molybdates and tungstates is usually ascribed to the radiative annihilation of the excitons which are self-trapped at oxyanion groups. In Na2M2O7 there are two types of oxyanions - MO6 and MO4 (M ¼ Mo, W) complexes. The emission of Na2Mo2O7 and Na2W2O7 is also attributed to the excitons self-trapped at oxyanionic complexes [28] and the obtained results on decay kinetics allows to make suppositions about its localization site. The temperature behavior of the decay ki netics of this emission center is typical of a triplet emission originating from a triplet state split by spin-orbit interaction in emitting and
CCT ¼ 4449n3þ3525n2-6823.3nþ5520.33
(1)
Here n ¼ (x-xe)/(y-ye), (x; y) – chromaticity coordinates of light source, (xe ¼ 0.3320, ye ¼ 0.1858) is the coordinates of "epicenter” CCT, quite close to the isotherm intersection point. CCT is equal 1672 K for Na2Mo2O7 and 6460 K for Na2W2O7. The formula for color purity calculation is presented in Ref. [31]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx xi Þ2 þ ðy yi Þ2 CP ¼ *100% (2) ðxd xi Þ2 þ ðyd yi Þ2 Here (x; y) - chromaticity coordinates of light source; (xi; yi) - chroma ticity coordinates of the achromatic standard D65; (xd; yd) – chroma ticity coordinates of the dominant wavelength. Color purity for Na2Mo2O7 is 95% and for Na2W2O7 is 27%. This 3
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Fig. 3. Temperature dependences of luminescence excitation (left) and emission (right) spectra of Na2M2O7 (M ¼ Mo, W).
time for both crystals is about 70 μs, while at 300 K decay time is of the order of units of μs. In Ref. [33] the decay time for Na2W2O7 crystals at 100 K is about 55 μs, that quite comparable with our experiment results. The decay curves of the investigated crystals are described by a single-exponential function (Fig. 5). Fig. 6 shows the temperature dependence of the luminescence decay time constant in Na2Mo2O7 and Na2W2O7 crystals for a temperature range of 77–300 K, measured upon excitation by photons with wave lengths of 350 nm and 310 nm, respectively. Such excitation energies ensure that basically the intrinsic emission is excited. For both crystals, the luminescence decay constant increases with decreasing temperature. The observed features of the temperature dependence of the decay time in the crystals under study can be explained in the framework of the model that analyses the dynamics of radiative and non-radiative tran sitions between the excited and ground states of the emission center. According to this model, the shortening of the decay time with increasing temperature is due to the thermally stimulated re-distribution of particles between these states. An analytic expression for the tem perature dependence of luminescence decay time constant that takes into account thermal quenching at elevated temperatures has been derived in Ref. [34]. � � 1 k1 þ k2 expð D=kTÞ ΔE þ Kexp (3) ¼ 1 þ expð D=kTÞ kT τ
Fig. 4. CIE 1931 chromaticity diagram of Na2W2O7 with an excitation wave length of 310 nm and Na2Mo2O7 with an excitation wavelength of 350 nm.
calculation demonstrates that Na2Mo2O7 crystals may be prospective in different optical applications due to its excellent color purity and suit able correlated color temperature for red emission. For example, correlated color temperature and color purity of Na2Mo2O7 crystals make this material perspective in warm-white light-emitting diodes as red phosphorus [32]. Besides, red emission of Na2Mo2O7 makes the applications of this material as a red lighting source possible. But luminescence intensity is too weak to speak about practical applications right now. Further investigation of temperature dependences of decay kinetics will be discussed. The luminescence decay constant of Na2Mo2O7 and Na2W2O7 crystals strongly depends on temperature. At 77 K the decay
Here ΔE is the activation energy for the non-radiative transitions, k1 and k2 are rates, characteristic of radiative transitions from the excited levels 1 and 2 to the ground state, and D is the energy gap between these levels. The results of the fitting experimental results using Eq. (3) displayed in Fig. 6 as solid lines show good agreement between the theoretical model and the measured data. The fit was done using Origin software. For practical application, the sensor must have a good sensitivity, so a large derivative Δτ/ΔT is desirable, that should exhibit a sufficiently large value over the entire temperature range of interest. The other 4
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Fig. 5. Luminescence decay curves for a set of temperatures of Na2W2O7 (a) and Na2Mo2O7 (b) measured at excitation with 310 nm and 350 nm respectively.
Fig. 6. Temperature dependence of decay time constant measured for excita tion with 350 nm in Na2Mo2O7 (a) and 310 nm in Na2W2O7 (b). The solid lines show the fitting of experimental data using Eq. (3).
Fig. 7. The temperature dependence of the relative sensitivity |(Δτ/ΔT)τ 1| in Na2Mo2O7 (a) and Na2W2O7 (b) crystals. The curves are calculated by differ entiating the fitted τ ¼ f(T), displayed in Fig. 6. The horizontal at 0.002 and 0/ 01 K 1 separate the regions of low, modest and high relative sensitivity.
aspect of what makes a useful temperature-sensing material is the rate of photons emitted per time, which is proportional to 1/τ. It is necessary to normalise the derivative to the value of the decay time constant to ac count for this effect on probe sensitivity [35]. The modulus of the relative sensitivity α ¼ |(Δτ/ΔT)τ 1| provides a useful parameter for the characterization of the temperature sensor. Relative sensitivities of sensors for luminescence lifetime thermom etry vary between 10 3 and 10 1 K 1 [36]. For example sensitivity of some phosphors Y2O2S–Eu, Mg4FGeO6-Mn, and Y3Al5O12–Ce commonly used for luminescence lifetime thermometry is estimated to be 5 � 10 2, 7 � 10 3 and 8 � 10 3 K 1, respectively. To compare the sensitivity of the investigated crystals, we used 0.002 K-1 and 0.01 K-1 for the value α as a reference to differ the sensitivity of the temperature sensor. There are two characteristic ranges in τ ¼ ƒ (T) curves - high sensitivity at high and medium temperatures and low sensitivity at low temperatures (Fig. 7). The preferable material for luminescence lifetime thermometry is the one that exhibits good sensitivity over a wider temperature range. For Na2Mo2O7 good relative sensitivity is observed over the 160–300 K range when luminescence thermal quenching occurs and for Na2W2O7 good relative sensitivity is observed over the 200–300 K range. The α ¼ | (Δτ/ΔT)τ 1| plot exhibits a region with low relative sensitivity between 77 and 160 K for Na2Mo2O7 and between 77 and 200 K for Na2W2O7.
The temperature range spanning more than hundred K in which the responsivity is somewhat deficient, is clearly a disadvantage of these materials. The relative sensitivity of Na2Mo2O7 and Na2W2O7 up to 160–200 K is too weak to speak about it. It is necessary to indicate the value of relative sensitivity at least at 300 K, because most of the research focuses on biological topics. At 300 K, the relative sensitivity of Na2Mo2O7 is equal 0.65 %K 1 to and of Na2W2O7 – 0.21 %K 1. To estimate thermal resolution it is appropriative to consider the temperature range in which there is a significant decay time changes. So, as mentioned earlier, for Na2W2O7 crystal such range is 200–300 K, for Na2Mo2O7 – 160–300 K. Decay time accuracy is important for good thermal resolution. In our case, the magnitude order of measurement error is equal to microseconds. Thermal resolution for Na2W2O7 crystal is about 1.4 K/μs, for Na2Mo2O7 – 2 K/μs. Such values are not enough for the modern technology requirements. However, certainly such resolu tion are suitable for some measurements. 4. Conclusion Optimal conditions for obtaining high-quality Na2W2O7 crystals 5
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were 2.0–2.5 mm/h growth rate and 3 rpm rotation rate, for obtaining Na2Mo2O7 crystals were 0.8–1 mm/h growth rate and 10 rpm rotation rate, ensuring layered growth mechanism and faceted crystallization front. The size of obtained crystals was 60(L)x30(D)mm3 for Na2W2O7, 90(L)x44(D)mm3 for Na2Mo2O7. Crystal quality was strongly influenced by growth directions and distinctive facet families. Though growth be haviors of Na2W2O7 and Na2Mo2O7 were similar, Na2Mo2O7 showed stronger tendency to faceting and cracking along cleavage planes. For Na2Mo2O7 crystals optimal conditions were 0.8–1 mm/h growth rate and 10 rpm rotation rate, ensuring layered growth mechanism and faceted crystallization front. Na2Mo2O7 crystal quality was determined by growth direction and distinctive facet families. Luminescence lifetime thermometry for remote temperature moni toring requires materials that exhibit a suitably large change of the luminescence kinetics at preassigned temperature range. We carried out studies of the photoluminescent properties of Na2Mo2O7 and Na2W2O7 crystals in the temperature range 77–300 K to assess their prospects as materials for non-contact thermometry of the luminescence lifetime. It has been established that the excitation and emission spectra of Na2Mo2O7 and Na2W2O7 crystals exhibit strong temperature depen dence. It was shown that the decay time constant increases by an order of magnitude when cooled to 77 K. The CIE investigation results show that Na2Mo2O7 expose strong red emission with color purity close to color purity of the monochromatic light sources, but have too weak luminescence intensity to speak about practical applications.
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Mikhailik, H. Kraus, Studies of scintillation properties of CaMoO4 at millikelvin temperatures, Appl. Phys. Lett. 106 241904 (2015) https:// doi.org/10.1063/1.4922875. [26] V.B. Mikhailik, H. Kraus, Performance of scintillation materials at cryogenic temperatures, J. Phys. Stud. 14 (2010) 4201–4206, https://doi.org/10.1002/ pssb.200945500. [27] F.A. Danevich, D.M. Chernyak, A.M. Dubovik, B.V. Grinyov, S. Henry, H. Kraus, V. M. Kudovbenko, V.B. Mikhailik, L.L. Nagornaya, R.B. Podviyanuk, O.G. Polischuk, I.A. Tupitsyna, YuYa Vostretsov, MgWO4–A new crystal scintillator, Nucl. Instrum. Methods 608 (2009) 107–115, https://doi.org/10.1016/j.nima.2009.06.040. [28] Y.A. Borovlev, N.V. Ivannikova, V.N. Shlegel, Y.V. Vasiliev, V.A. Gusev Progress in growth of large sized BGO crystals by the low-thermal-gradient Czochralski technique J, Cryst. Growth 229 (1–4) (2001) 305–311, https://doi.org/10.1016/ S0022-0248(01), 01162–01169. [29] E.N. Galashov, T.M. Denisova, I.M. Ivanov, E.P. Makarov, V.N. Mamontov, V. N. Shlegel, Y.G. Stenin, Y.V. Vasiliev, V.N. Zhdankov Growing of 106CdWo4, ZnWo4, and ZnMoO4 scintillation crystals for rare events search by low thermal gradient Czochralski technique Funct, Materials 17 (4) (2010) 504–508. [30] J. Schanda, M. Danyi, Correlated color-temperature calculations in the CIE 1976 chromaticity diagram, Color Res. Appl. 2 (1977) 161–163, https://doi.org/ 10.1002/col.5080020403. [31] E.F. Schubert, Light-Emitting Diodes, second ed., Cambridge University Press, New York, 2006. [32] Z. Yahiaoui, M.A. Hassairi, M. Dammak, Synthesis, Optical Spectroscopy, Of YPO4: Eu3þ orange-red phosphors, J. Electron. Mater. 46 (2017) 4765–4773, https://doi. org/10.1007/s11664-017-5401-4. [33] I.R. Pandey, H.J. Kim, H.S. Lee, Y.D. Kim, M.H. Lee, V.D. Grigorieva, V.N. Shlegel, The Na2W2O7 crystal: a crystal scintillator for dark matter search experiment, Eur. Phys. J. C 78 (2018) 973, https://doi.org/10.1140/epjc/s10052-018-6462-0.
Contribution V.D. Grigorieva – grew crystals by the low-thermal-gradient Czo chralski technique, contributed to sample preparation. M.I. Rakhmanova and A.A. Ryadun carried out the experiment, performed the analytic calculations and performed the numerical simulations. All authors discussed the results and contributed to the final manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The reported study was funded by RFBR according to the research project N� 18-32-00024. References [1] P.R.N. Childs, J.R. Greenwood, C.A. Long, Review of temperature measurement, Rev. Sci. Instrum. 71 (2000) 2959–2978, https://doi.org/10.1063/1.1305516. [2] M.M. Kim, A. Giry, M. Mastiani, G.O. Rodrigues, A. Reis, P. Mandin, Microscale thermometry: a review, Microelectron. Eng. 148 (2015) 129–142, https://doi.org/ 10.1016/j.mee.2015.11.002. [3] E.H. Snell, H.D. Bellamy, G. Rosenbaum, M.J. van der Woerd, Non-invasive measurement of X-ray beam heating on a surrogate crystal sample, J. Synchrotron Radiat. 14 (2007) 109–115, https://doi.org/10.1107/S090904950604605X. [4] C.D.S. Brites, S. Balabhadra, L.D. Carlos Lanthanide-Based, Thermometers: at the cutting-edge of luminescence thermometry, Advanced Optical Materials (2018) 1801239, https://doi.org/10.1002/adom.201801239. [5] D. Jaque, F. Vetrone, Luminescence nanothermometry, Nanoscale 4 (15) (2010), https://doi.org/10.1039/c2nr30764b, 4301. [6] M. Back, J. Ueda, M.G. Brik, T. Lesniewski, M. Grinberg, S. Tanabe, Revisiting Cr3 þ-doped Bi2Ga4O9 spectroscopy: crystal field effect and optical thermometric behavior of NIR emitting singly activated phosphors, ACS Appl. Mater. Interfaces (2018), https://doi.org/10.1021/acsami.8b15607. [7] J. Ueda, M. Back, M.G. Brik, Y. Zhuang, M. Grinberg, S. Tanabe, Ratiometric optical thermometry using deep red luminescence from 4T2 and 2E states of Cr3þ in ZnGa2O4 host, Opt. Mater. 85 (2018) 510–516, https://doi.org/10.1016/j. optmat.2018.09.013.
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[34] V.B. Mikhailik, H. Kraus, S. Henry, A.J.B. Tolhurst, Scintillation studies of CaWO4 in the millikelvin temperature range, Phys. Rev. B 75 (2007) 184308, https://doi. org/10.1103/PhysRevB.75.184308. [35] P. Haro-Gonz� alez, L. Martínez-Maestro, I.R. Martín, D. Jaque García-Sol� e, High sensitivity fluorescence lifetime thermal sensing based on CdTe quantum dots, Small 8 (17) (2012) 2652–2658, https://doi.org/10.1002/smll.201102736.
[36] K.E. Homeyer, S. Pailhe‘s, R. Debord, V. Jary, C. Dujardin, G. Ledoux, Diamond contact-less micrometric temperature sensors, Appl. Phys. Lett. 106 (2015) 243502, https://doi.org/10.1063/1.4921177.
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Photoluminescence properties of perspective bolometric crystals Na2Mo2O7 and Na2W2O7 grown by low-thermal-gradient Czochralski technique Alexey A. Ryadun *, Mariana I. Rakhmanova, Veronika D. Grigorieva Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Science, 630090, Novosibirsk, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords: Low-thermal-gradient Czochralski technique Na2Mo2O7 Na2W2O7 Luminescence lifetime Double beta-decay Bolometers Sensor sensitivity
Alkaline molybdate and tungstate crystals are promising bolometric materials for the search for rare events, especially for the search for neutrinoless double beta decay. Fluorescent thermometry for remote measurement of the temperature needs materials that demonstrate a significant change in the decay time of luminescence with a change in temperature. This paper presents the results of temperature-induced changes investigation in the photoluminescence of undoped tungstates and molybdates of the general formula Na2M2O7 (M ¼ Mo, W) in the temperature range of 77–300 K. Analyzing changes in the emission and excitation spectra, as well as in the decay kinetics allowed assessing the prospects of Na2Mo2O7 and Na2W2O7 crystals as materials for luminescence lifetime thermometry. The emission color chromaticity coordinates of Na2Mo2O7 and Na2W2O7 crystals were calculated and shown on Commission Internationale de L’Eclairage (CIE 1931). Correlated color temperature and color purity for both investigated crystals were estimated. For Na2Mo2O7 correlated color temperature is equal to 1672 K and color purity is 95%, for Na2Mo2O7 correlated color temperature is equal to 6460 K and color purity is 27%. The CIE investigation results show that Na2Mo2O7 expose strong red emission with color purity close to color purity of the monochromatic light sources.
1. Introduction Exact measurement of temperature values is necessary for under standing different physical and chemical processes for conducting ex periments and for control of technological processes under given conditions. The functional properties of materials used under extreme conditions (elevated temperature, pressure, aggressive chemical envi ronment and etc.) are highly temperature dependent. Sometimes it is impossible to measure the temperature directly; therefore, a non-contact measurement is required. Methods for solving the problem of contactless determination of temperature are described in Refs. [1,2]. For temperature remote determination described methods use changes in the optical properties of materials (emission, absorption, etc.). Widely used IR thermometry is not suitable for changing temperatures below the boiling point of liquid air, because a low value of the radiation intensity does not allow achieving a high-quality signal [3]. The field of luminescence thermometry is growing intensively showing significant breakthroughs in sensing, imaging, diagnostics, and therapy, among other areas [4]. This interest has been mostly
encouraged because many of the presenttechnological demands in areas such as microelectronics and nanoelectronics, photonics, nanomedicine, and microfluidics and nanofluidics have reached a point such that the use of traditional contact thermal probes (liquid-filled and bimetallic thermometers, thermocouples, pyrometers, and thermistors) is not capable to make reliable measurements when spatial resolution enters to the sub-micrometer range. This limitation of conventional thermometers for small systems has spurred the development of luminescent micro thermometers and nanothermometers, a research topic leaving its in flationary epoch accounting nowadays for more than 2.5% of the total publications in luminescence materials. Moreover, and from an indus trial point of view, this predicted miniaturization is expected to bring to the market new nanoscale thermal probes. Despite the recently devel oped luminescent nanothermometers are radically more sophisticated encompassing complex synthesis procedures, the fundamental problems and the applications that are being addressed are analogous to those reported in the field’s infancy: the understanding of heat transfer and energy transfer mechanisms, the optimization of temperature readout, and the developing of efficient and cost-effective sensors for front-edge medical and engineering tools. Among the distinct emitting centers used
* Corresponding author. E-mail address: [email protected] (A.A. Ryadun). https://doi.org/10.1016/j.optmat.2019.109537 Received 17 September 2019; Received in revised form 11 November 2019; Accepted 12 November 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Alexey A. Ryadun, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109537
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bio-imaging. The obtained results show that the ZnGa2O4:Cr3þ system has a high potential for the ratiometric thermometry applications from around ambient temperature to high temperatures. The relative sensi tivity of ZnGa2O4:Cr3þ for thermometry is estimated to be 2.8%K 1 at 310 K, which place this material at the top among the ratiometric fluorescent thermometers using Boltzmann distribution phenomenon for biological application. Thermometry of the luminescence lifetime is based on the temper ature dependence of the luminescence decay time constant [8]. This method can be used to measure the temperature of objects, when many other methods are not suitable. For example, high-temperature and chemically aggressive environments, MRI and other processes [9,10], Almost all applications of luminescent thermometry are related to the measurement of temperatures near ambient temperature. Rarely, this method was used to measure temperatures below 200 K, despite its advantages. The absence of direct contact between the sensor and the objects eliminates heat leakage and leads to more accurate measure ments. In Refs. [11–13], the authors gave examples of using thermom etry of the luminescence lifetime at low temperatures, in which La2O2S–Eu, MgFGeO6:Mn, CaF2-Yb and SrF2-Yb were used as the mea surement material. The main physical feature that imposes a restriction on the use of the method of luminescent thermometry is associated with the determination of temperature-dependent mechanisms that affect the change in the constant lifetime. Because the known mechanism of thermally activated non-radiative decay of excited states, which causes changes in the decay time constant, is limited by the temperature, below which it is poorly pronounced. Crystals of congruently melting molybdates and tungstates have significant scientific and practical importance as laser, acousto-optic, scintillating and bolometric materials [14,15]. At present time the scintillating properties of molybdate crystals are of particular interest for the detection of rare events at cryogenic temperatures since 100Mo isotope is one of the most promising sources of neutrinoless double beta-decay (0ν2β-decay) [16–18]. In present, ZnMoO4, and CaMoO4 single crystals are being applied as cryogenic scintillation bolometers [19,20]. However, despite the scientific and practical importance and long history of such studies, many complex problems of recording rare events have not been solved. Bolometric requirements in such experi ments include high molybdenum concentration in the volume, high energy resolution, ultra-low radiation background, large size of single crystals and adequate cost. That makes molybdates of light alkaline metals, such as Na, perspective bolometric materials for rare events physics, especially for neutrino study. Low-thermal-gradient (below 1� /cm) Czochralski technique (LTG Cz) was developed for growing large-sized oxide crystals of highest optical quality from melt. It procures such advantages as: suppression of decomposition and volatilization of melt components that prevents the formation of local melt nonstoichiometries and the loss of initial mate rials (especially important when working with such materials as 100Mo); and the decrease of thermoelastic stress in crystals that prevents cracking and increases maximal possible size of the crystals [21]. In this work we present the characterization results for two iso structural pure crystals of childrenite-eosphorite mineral group (sp. gr. Cmca) perspective in luminescence lifetime thermometry: sodium ditungstate Na2W2O7 (a ¼ 7216 Å, b ¼ 11,899 Å, c ¼ 14,716 Å) and so dium dimolybdate Na2Mo2O7 (a ¼ 7.164(6) Å, b ¼ 11.837(4) Å, c ¼ 14.713(2) Å). Both Na2W2O7 and Na2Mo2O7 compounds melt congruently at 738 � 5 and 632 � 5 � C, correspondingly. In this work, the excitation and emission spectra of the luminescence, as well as the decay curves in the temperature range 77–300 K were measured.
in luminescence thermometry, including proteins nucleic acids and other biomolecules, thermosresponsive polymers, organic dyes, and QDs, many Ln3þ-based luminescent thermometers have been reported, essentially in the last decade. Systems comprise molecular complexes, MOFs, polymers, and organic–inorganic hybrids, multifunctional heater-thermometer nanoplatforms, and etc. This review describes the use of these Ln3þ-based phosphors as luminescent ratiometric ther mometers in diverse applications, with focus on what the authors believe that will be the emergent new research areas in this fascinating research field: the use of luminescence thermometry for thermal imag ing, early tumor detection, and as a tool for unveiling properties of the thermometers themselves or of their local surroundings. Finally, to become a consolidated subject and not a temporary fashion, the research on luminescent thermometry must settle down as a strong node of fertile interactions among disparate communities, such as chemists, physicists, engineers, theoreticians, biologists, and physicians. The cross-fertilization of ideas and experiences at these interfaces will certainly induce important and exciting breakthroughs in future years. In Ref. [5] it has been shown that many luminescent systems (including polymers, organic dyes, rare earth doped crystals, rare earth doped nanocrystals, semiconductor nanocrystals, rare earth doped complexes, phosphorous and quantum dots) can be used as basic light-emitting materials for nanothermometry. It is not possible to highlight one material over the rest since the most suitable one would depend on the actual system to be thermally imaged. The continuous development of new microscopy techniques coupled with novel and cutting edge synthesis processes (allowing for the rational design of novel luminescent materials) will ensure the speedy development of luminescence nanothermometry, although working principles will be probably the same. Biological applications are by far the most chal lenging of all possible applications described in this review, due to the complex nature of the biological milieu. To overcome some of the hur dles associated with working in a biological environment, employing luminescent nanothermometers optically excited in the near-infrared (NIR) region is particularly promising. In combination with multi photon microscopy, this would allow for high-resolution, three-dimen sional thermal imaging of living specimens. Although a great deal of effort has been invested in the development of NIR excited luminescent nanothermometers, their real-world application in three-dimensional thermal imaging of bio-systems is far from being a reality. For the most part, this is because these nanothermometers typically emit in the visible range, where tissue transparency is low. In our opinion the forthcoming advances in the field of luminescence nanothermometry will be propelled by the development of luminescent nano thermometers, which operate within the biological window (700–900 nm). That is, both their excitation and emission wavelengths lie within this optimal window. The most groundbreaking results in the field of luminescent nanothermometry will be obtained by using ratio metric thermal sensors. Authors [6] demonstrated the potentiality of Cr3þ singly activated phosphors as ideal ratiometric luminescent thermometers not only for biological but for wider applications. Detailed spectroscopic investiga tion on the thermal behaviour of Cr3þ-doped Bi2Ga4O9 system showing the importance of an insightful analysis in a wide temperature range is reported. In addition, by considering the importance in real applications of the spectral change speed with temperature, a theoretical insight into the absolute sensitivity of singly activated systems characterized by couples of excited states obeying to the Boltzmann law is reported. Bi2Ga4O9:Cr3þ shows a remarkable absolute sensitivity of 0.28 � 0.02 K 1 at 300 K. In study [7], authors investigated the temperature dependence of photoluminescence spectrum of ZnGa2O4:Cr3þ and the possibility of using this material for thermometry. ZnGa2O4:Cr3þ is a well-known deep red (persistent) phosphor with R-lines due to the 2E→4T2 spin-forbidden transition of Cr3þ ions between 690 and 750 nm. Recently, this system has attracted a great deal of interest for in-vivo
2. Experimental 2.1. Crystal growth In our experiments deeply purified MoO3 and WO3 and commercial 2
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high purity Na2CO3 (15-2 TS 609 589–77, 99.99%) were used as initial compounds. Solid-state syntheses of pure Na2W2O7 and Na2Mo2O7 from the powders were carried out in the same 130 � 70 mm platinum cru cible, from which the crystals were grown, according to the reactions: 2WO3 þ Na2CO3 ¼ Na2W2O7 þ CO2 ↑ 2MoO3 þ Na2CO3 ¼ Na2Mo2O7 þ CO2 ↑ The compounds were mixed, heated up to 450 � C at the rate of 30� /h and kept at this temperature for 5 h. After that, temperature was elevated up to 20 � C above the melting point (758 � C for Na2W2O7, 652 � C for Na2Mo2O7) at the rate of 70 � C/h and kept at this temperature over 3 h for homogenization of the melt. Crystals were grown from the synthesized compounds by the lowthermal-gradient Czochralski technique (LTG Cz) in air atmosphere. The crucible covered with a platinum lid furnished with a pipe socket, through which the pull rode with an oriented seed was introduced into the crucible, was placed in a three-zone resistance furnace with low thermal conductivity top and bottom thermal insulation. Crystals stayed inside the crucible during the entire growth process. Unlike conven tional Czochralski technique, visual control is impossible in LTG Cz technique, thus, the growth processes were analyzed and controlled via weight signal channel at all stages of the process including the seeding. Temperature distribution between heating zones was preset in a way to ensure optimal growth conditions and flat crystallization front.
Fig. 1. The luminescence excitation and emission spectra of Na2W2O7 crystal (λem ¼ 540 nm; λex ¼ 310 nm; T ¼ 77 K).
2.2. Experimental techniques Excitation and emission photoluminescence spectra of 3x3x5 mm3 unpolished Na2W2O7 and Na2Mo2O7 crystal samples were recorded with a Horiba Jobin Yvon Fluorolog 3 photoluminescence spectrometer equipped with 450 W ozone-free Xe-lamp, cooled PC177CE-010 photon detection module with a PMT R2658 and double grating excitation and emission monochromators. Excitation and emission spectra were cor rected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves. Tem perature dependences of emission and excitation spectra and decay time curves were recorded with Optistat DN in temperature range 77–300 K.
Fig. 2. The luminescence excitation and emission spectra of Na2Mo2O7 crystal (λem ¼ 650 nm; λex ¼ 350 nm; T ¼ 77 K).
metastable levels. The PL temperature dependence is well described by Mott model with two recombination channels, radiative and non-radiative, from the relaxed excited state [29]. The emission color chromaticity coordinates of Na2W2O7 and Na2Mo2O7 crystals are calculated from emission spectra and shown on Commission Internationale de L’Eclairage (CIE1931) (Fig. 4). CIE chromaticity coordinates of Na2Mo2O7 with an excitation wavelength of 350 nm are (x ¼ 0.583, y ¼ 0.411) and of Na2W2O7 with an excitation wavelength of 310 nm are (x ¼ 0.300, y ¼ 0.434). It can be assumed that Na2Mo2O7 crystals are promising as red phosphorus for a white LED and Na2W2O7 as green. But it is necessary to calculate correlated color temperature and color purity. The correlated color temperature is calculated from McCamy’s formula [30]:
3. Results and discussion 3.1. Temperature dependence of Na2Mo2O7 and Na2W2O7 crystals photoluminescence properties Na2Mo2O7 and Na2W2O7 crystals were chosen as objects for this work, first of all due to the previously obtained results of their lumi nescence and scintillation properties investigations. Decay time constant for tungstate and molybdate crystals varies significantly with tempera ture [22–27]. Application of materials to contactless thermometry is based on intense luminescence response over the temperature range of interest. Thus, knowledge of temperature dependences of emission and excitation spectra is necessary. It was found that the luminescent properties of the materials under study exhibit significant temperature dependence. Na2W2O7 crystals exhibit a green glow (Fig. 1), and Na2Mo2O7 crystals a red glow (Fig. 2). Emission and excitation spectra temperature de pendences of Na2Mo2O7 and Na2W2O7 crystals are shown in Fig. 3. The emission of molybdates and tungstates is usually ascribed to the radiative annihilation of the excitons which are self-trapped at oxyanion groups. In Na2M2O7 there are two types of oxyanions - MO6 and MO4 (M ¼ Mo, W) complexes. The emission of Na2Mo2O7 and Na2W2O7 is also attributed to the excitons self-trapped at oxyanionic complexes [28] and the obtained results on decay kinetics allows to make suppositions about its localization site. The temperature behavior of the decay ki netics of this emission center is typical of a triplet emission originating from a triplet state split by spin-orbit interaction in emitting and
CCT ¼ 4449n3þ3525n2-6823.3nþ5520.33
(1)
Here n ¼ (x-xe)/(y-ye), (x; y) – chromaticity coordinates of light source, (xe ¼ 0.3320, ye ¼ 0.1858) is the coordinates of "epicenter” CCT, quite close to the isotherm intersection point. CCT is equal 1672 K for Na2Mo2O7 and 6460 K for Na2W2O7. The formula for color purity calculation is presented in Ref. [31]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx xi Þ2 þ ðy yi Þ2 CP ¼ *100% (2) ðxd xi Þ2 þ ðyd yi Þ2 Here (x; y) - chromaticity coordinates of light source; (xi; yi) - chroma ticity coordinates of the achromatic standard D65; (xd; yd) – chroma ticity coordinates of the dominant wavelength. Color purity for Na2Mo2O7 is 95% and for Na2W2O7 is 27%. This 3
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Fig. 3. Temperature dependences of luminescence excitation (left) and emission (right) spectra of Na2M2O7 (M ¼ Mo, W).
time for both crystals is about 70 μs, while at 300 K decay time is of the order of units of μs. In Ref. [33] the decay time for Na2W2O7 crystals at 100 K is about 55 μs, that quite comparable with our experiment results. The decay curves of the investigated crystals are described by a single-exponential function (Fig. 5). Fig. 6 shows the temperature dependence of the luminescence decay time constant in Na2Mo2O7 and Na2W2O7 crystals for a temperature range of 77–300 K, measured upon excitation by photons with wave lengths of 350 nm and 310 nm, respectively. Such excitation energies ensure that basically the intrinsic emission is excited. For both crystals, the luminescence decay constant increases with decreasing temperature. The observed features of the temperature dependence of the decay time in the crystals under study can be explained in the framework of the model that analyses the dynamics of radiative and non-radiative tran sitions between the excited and ground states of the emission center. According to this model, the shortening of the decay time with increasing temperature is due to the thermally stimulated re-distribution of particles between these states. An analytic expression for the tem perature dependence of luminescence decay time constant that takes into account thermal quenching at elevated temperatures has been derived in Ref. [34]. � � 1 k1 þ k2 expð D=kTÞ ΔE þ Kexp (3) ¼ 1 þ expð D=kTÞ kT τ
Fig. 4. CIE 1931 chromaticity diagram of Na2W2O7 with an excitation wave length of 310 nm and Na2Mo2O7 with an excitation wavelength of 350 nm.
calculation demonstrates that Na2Mo2O7 crystals may be prospective in different optical applications due to its excellent color purity and suit able correlated color temperature for red emission. For example, correlated color temperature and color purity of Na2Mo2O7 crystals make this material perspective in warm-white light-emitting diodes as red phosphorus [32]. Besides, red emission of Na2Mo2O7 makes the applications of this material as a red lighting source possible. But luminescence intensity is too weak to speak about practical applications right now. Further investigation of temperature dependences of decay kinetics will be discussed. The luminescence decay constant of Na2Mo2O7 and Na2W2O7 crystals strongly depends on temperature. At 77 K the decay
Here ΔE is the activation energy for the non-radiative transitions, k1 and k2 are rates, characteristic of radiative transitions from the excited levels 1 and 2 to the ground state, and D is the energy gap between these levels. The results of the fitting experimental results using Eq. (3) displayed in Fig. 6 as solid lines show good agreement between the theoretical model and the measured data. The fit was done using Origin software. For practical application, the sensor must have a good sensitivity, so a large derivative Δτ/ΔT is desirable, that should exhibit a sufficiently large value over the entire temperature range of interest. The other 4
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Fig. 5. Luminescence decay curves for a set of temperatures of Na2W2O7 (a) and Na2Mo2O7 (b) measured at excitation with 310 nm and 350 nm respectively.
Fig. 6. Temperature dependence of decay time constant measured for excita tion with 350 nm in Na2Mo2O7 (a) and 310 nm in Na2W2O7 (b). The solid lines show the fitting of experimental data using Eq. (3).
Fig. 7. The temperature dependence of the relative sensitivity |(Δτ/ΔT)τ 1| in Na2Mo2O7 (a) and Na2W2O7 (b) crystals. The curves are calculated by differ entiating the fitted τ ¼ f(T), displayed in Fig. 6. The horizontal at 0.002 and 0/ 01 K 1 separate the regions of low, modest and high relative sensitivity.
aspect of what makes a useful temperature-sensing material is the rate of photons emitted per time, which is proportional to 1/τ. It is necessary to normalise the derivative to the value of the decay time constant to ac count for this effect on probe sensitivity [35]. The modulus of the relative sensitivity α ¼ |(Δτ/ΔT)τ 1| provides a useful parameter for the characterization of the temperature sensor. Relative sensitivities of sensors for luminescence lifetime thermom etry vary between 10 3 and 10 1 K 1 [36]. For example sensitivity of some phosphors Y2O2S–Eu, Mg4FGeO6-Mn, and Y3Al5O12–Ce commonly used for luminescence lifetime thermometry is estimated to be 5 � 10 2, 7 � 10 3 and 8 � 10 3 K 1, respectively. To compare the sensitivity of the investigated crystals, we used 0.002 K-1 and 0.01 K-1 for the value α as a reference to differ the sensitivity of the temperature sensor. There are two characteristic ranges in τ ¼ ƒ (T) curves - high sensitivity at high and medium temperatures and low sensitivity at low temperatures (Fig. 7). The preferable material for luminescence lifetime thermometry is the one that exhibits good sensitivity over a wider temperature range. For Na2Mo2O7 good relative sensitivity is observed over the 160–300 K range when luminescence thermal quenching occurs and for Na2W2O7 good relative sensitivity is observed over the 200–300 K range. The α ¼ | (Δτ/ΔT)τ 1| plot exhibits a region with low relative sensitivity between 77 and 160 K for Na2Mo2O7 and between 77 and 200 K for Na2W2O7.
The temperature range spanning more than hundred K in which the responsivity is somewhat deficient, is clearly a disadvantage of these materials. The relative sensitivity of Na2Mo2O7 and Na2W2O7 up to 160–200 K is too weak to speak about it. It is necessary to indicate the value of relative sensitivity at least at 300 K, because most of the research focuses on biological topics. At 300 K, the relative sensitivity of Na2Mo2O7 is equal 0.65 %K 1 to and of Na2W2O7 – 0.21 %K 1. To estimate thermal resolution it is appropriative to consider the temperature range in which there is a significant decay time changes. So, as mentioned earlier, for Na2W2O7 crystal such range is 200–300 K, for Na2Mo2O7 – 160–300 K. Decay time accuracy is important for good thermal resolution. In our case, the magnitude order of measurement error is equal to microseconds. Thermal resolution for Na2W2O7 crystal is about 1.4 K/μs, for Na2Mo2O7 – 2 K/μs. Such values are not enough for the modern technology requirements. However, certainly such resolu tion are suitable for some measurements. 4. Conclusion Optimal conditions for obtaining high-quality Na2W2O7 crystals 5
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were 2.0–2.5 mm/h growth rate and 3 rpm rotation rate, for obtaining Na2Mo2O7 crystals were 0.8–1 mm/h growth rate and 10 rpm rotation rate, ensuring layered growth mechanism and faceted crystallization front. The size of obtained crystals was 60(L)x30(D)mm3 for Na2W2O7, 90(L)x44(D)mm3 for Na2Mo2O7. Crystal quality was strongly influenced by growth directions and distinctive facet families. Though growth be haviors of Na2W2O7 and Na2Mo2O7 were similar, Na2Mo2O7 showed stronger tendency to faceting and cracking along cleavage planes. For Na2Mo2O7 crystals optimal conditions were 0.8–1 mm/h growth rate and 10 rpm rotation rate, ensuring layered growth mechanism and faceted crystallization front. Na2Mo2O7 crystal quality was determined by growth direction and distinctive facet families. Luminescence lifetime thermometry for remote temperature moni toring requires materials that exhibit a suitably large change of the luminescence kinetics at preassigned temperature range. We carried out studies of the photoluminescent properties of Na2Mo2O7 and Na2W2O7 crystals in the temperature range 77–300 K to assess their prospects as materials for non-contact thermometry of the luminescence lifetime. It has been established that the excitation and emission spectra of Na2Mo2O7 and Na2W2O7 crystals exhibit strong temperature depen dence. It was shown that the decay time constant increases by an order of magnitude when cooled to 77 K. The CIE investigation results show that Na2Mo2O7 expose strong red emission with color purity close to color purity of the monochromatic light sources, but have too weak luminescence intensity to speak about practical applications.
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Contribution V.D. Grigorieva – grew crystals by the low-thermal-gradient Czo chralski technique, contributed to sample preparation. M.I. Rakhmanova and A.A. Ryadun carried out the experiment, performed the analytic calculations and performed the numerical simulations. All authors discussed the results and contributed to the final manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The reported study was funded by RFBR according to the research project N� 18-32-00024. References [1] P.R.N. Childs, J.R. Greenwood, C.A. Long, Review of temperature measurement, Rev. Sci. Instrum. 71 (2000) 2959–2978, https://doi.org/10.1063/1.1305516. [2] M.M. Kim, A. Giry, M. Mastiani, G.O. Rodrigues, A. Reis, P. Mandin, Microscale thermometry: a review, Microelectron. Eng. 148 (2015) 129–142, https://doi.org/ 10.1016/j.mee.2015.11.002. [3] E.H. Snell, H.D. Bellamy, G. Rosenbaum, M.J. van der Woerd, Non-invasive measurement of X-ray beam heating on a surrogate crystal sample, J. Synchrotron Radiat. 14 (2007) 109–115, https://doi.org/10.1107/S090904950604605X. [4] C.D.S. Brites, S. Balabhadra, L.D. Carlos Lanthanide-Based, Thermometers: at the cutting-edge of luminescence thermometry, Advanced Optical Materials (2018) 1801239, https://doi.org/10.1002/adom.201801239. [5] D. Jaque, F. Vetrone, Luminescence nanothermometry, Nanoscale 4 (15) (2010), https://doi.org/10.1039/c2nr30764b, 4301. [6] M. Back, J. Ueda, M.G. Brik, T. Lesniewski, M. Grinberg, S. Tanabe, Revisiting Cr3 þ-doped Bi2Ga4O9 spectroscopy: crystal field effect and optical thermometric behavior of NIR emitting singly activated phosphors, ACS Appl. Mater. Interfaces (2018), https://doi.org/10.1021/acsami.8b15607. [7] J. Ueda, M. Back, M.G. Brik, Y. Zhuang, M. Grinberg, S. Tanabe, Ratiometric optical thermometry using deep red luminescence from 4T2 and 2E states of Cr3þ in ZnGa2O4 host, Opt. Mater. 85 (2018) 510–516, https://doi.org/10.1016/j. optmat.2018.09.013.
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