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Optical properties of Pb2MoO5 and Pb2WO5 single crystals as materials for practical applications
Optik - International Journal for Light and Electron Optics 226 (2021) 165912
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Original research article
Optical properties of Pb2MoO5 and Pb2WO5 single crystals as materials for practical applications Ryadun Alexey *, Rakhmanova Mariana, Grigorieva Veronika 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 Pb2MoO5 crystals Pb2WO5 crystals Luminescence lifetime Color chromaticity coordinates Non-contact thermometry
The low-gradient Czochralski method was used to obtain lanarkite type Pb2MoO5 and Pb2WO5 crystals. Photoluminescence properties under UV excitation were studied, such as temperature dependences of decay time and excitation and emission spectra. The Pb2MoO5 crystals demon strate intensive emission spectra in green range under 350 nm excitation and yellow-red range under 325 nm excitation. Pb2WO5 crystals exhibit green-blue luminescence under 400 nm exci tation and green-yellow luminescence under 340 nm excitation. Photoluminescence spectra for investigated crystals show a short wavelength shoulder in violet spectral region near 400− 420 nm. These violet component of photoluminescence for lead crystals are attribute to the 2transitions triplet states 1T1,2 of MoO2− 4 and WO4 groups, respectively. The band gap of Pb2MoO5 and Pb2WO5 crystals were estimated from UV absorption spectrum and are equal to about 3.45 eV. Correlated color temperatures and color purity for both crystals at different excitations were calculated and the conclusion was made about the possibility of using such crystals in warmwhite light emitted diodes and other solid-state lighting applications. Color chromaticity co ordinates are shown on CIE1931. Temperature dependence of decay time constant allowed us to calculate sensitivity for non-contact thermometry applications.
1. Introduction Molybdate and tungstate crystals are perspective materials in different practical application. Such materials are promising as a scintillation detector [1–6]. Special attention such materials attract due to opportunity to be used as detectors for rare event in vestigations [7,8]. For example, PbMoO4 is promising material to search for the neutrinoless double β decay of 100Mo isotope [7,9,10] and PbWO4 is promising as shield (anti-coincidence detector or active light guide) to suppress radioactive background to search for double β decay of 116Cd [11]. Nowadays various molybdate and tungstate crystals are investigated as potential scintillation materials for cryogenic bolometers [12,13]. The luminescence properties of Pb2MoO5 and Pb2WO5 crystals were investigated earlier. Recently, the field of luminescence thermometry is growing intensively showing significant breakthroughs in sensing, imaging, diagnostics, and therapy, among other areas [14]. 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 microthermometers and * Corresponding author. E-mail address: [email protected] (R. Alexey). https://doi.org/10.1016/j.ijleo.2020.165912 Received 1 August 2020; Accepted 27 October 2020 Available online 2 November 2020 0030-4026/© 2020 Elsevier GmbH. All rights reserved.
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nanothermometers, a research topic leaving accounting nowadays for more than 2.5 % of the total publications in luminescence materials. Among the distinct emitting centers used in luminescence thermometry are polymers, organic dyes, rare earth doped crystals, rare earth doped nanocrystals, semiconductor nanocrystals, rare earth doped complexes, phosphorous and quantum dots and other luminescent systems. Oxide crystals are promising materials for non-contact thermometry too. Detailed spectroscopic investigation on the thermal behavior of Cr3+-doped Bi2Ga4O9 system showing the importance of an insightful analysis in a wide temperature range for application in ratiometric luminescent thermometers not only for biological but for wider is reported in [15]. Bi2Ga4O9:Cr3+ shows a remarkable absolute sensitivity of 0.28 ± 0.02 K− 1 at 300 K. The possibility of ZnGa2O4:Cr3+ using for thermometry was investigated on base of the temperature dependence of photo luminescence spectra [16]. ZnGa2O4:Cr3+ is a well-known deep red phosphor with R-lines due to the 2E→4T2 spin-forbidden transition of Cr3+ ions between 690 and 750 nm. ZnGa2O4:Cr3+ system demonstrated opportunity for the ratiometric thermometry applications from around ambient to high temperatures. The relative sensitivity of ZnGa2O4:Cr3+ for thermometry is estimated to be 2.8 %K− 1 at 310 K, which is rather good for the ratiometric fluorescent thermometers using Boltzmann distribution phenomenon for biological application. Nowadays lead molybdatePb2MoO5and tungstatePb2WO5crystals are promising in various practical applications. And it is inter esting to investigate such crystals as materials for luminescence lifetime thermometry. Scheelite type PbMoO4 and PbWO4 crystals similar in composition to Pb2MoO5and Pb2WO5 were studied from manifold points of view. It is shown that lead containing crystals are perspective as acousto-optic [17], scintillation [18,19] and hosts materials for luminescent rare earthions [20]. But properties of lanarkite type Pb2MoO5and Pb2WO5crystals are poorly investigated. Structure data of Pb2MoO5 crystal appeared in 1951 [21], it was shown that the crystal has a monoclinic cell and crystal space group is C2/m. Refractive parameters measured in [22] with λ =589.3 nm was nх = 2.183, ny = 2.197 and nz = 2.319. The crystals studied in these works had small sizes and a yellowish-greenish color. Some results about optic properties and band structure of Pb2MoO5 crystals are presented in [23,24]. But crystal purity and quality of samples causes somewhat contradictory data. Besides, photoluminescence properties of Pb2MoO5 crystal was compared with PbMoO4 taking into consideration electronic structure calcu lation [25]. The optical properties of Pb2MoO5 and Pb2WO5 crystals were studied for acousto-optic applications [26–29]. Intrinsic lumines cence of Pb2MoO5 was studied under Xe-lamp excitation [30]. EPR investigations of Pb2MoO5 single crystals doped by copper ions presented in [31]. Low-thermal-gradient Czochralski technique was developed for growing large-sized oxide crystals of highest optical quality from melt. This method providefollowing advantages: suppression of decomposition and volatilization of melt components that prevents the formation of melt nonstoichiometry and the loss of initial materials; the decrease of thermoelastic stress in crystals, preventing cracking, thus, and increasing maximal possible size [32]. The purpose of present work is investigation of photoluminescence properties for Pb2MoO5 and Pb2WO5 single crystals grown by low-thermal-gradient Czochralski technique as materials for non-contact thermometry and lighting source. Measurements of lumi nescence excitation and emission spectra, as well as decay curves as a function of temperature over the 77–300 K temperature range were carried out. Color purity, correlated color temperature and decay time response to temperature change were calculated. 2. Experimental 2.1. Crystal grown and experimental techniques Pb2MoO5 and Pb2WO5 single crystals were grown by the Czochralski method under conditions of low temperature gradients (ΔT/ Δx<1 K/cm). The drawing speed was about 2 mm per day; the rotation speed was 30–40 revolutions per minute. The growing process was carried out from a platinum crucible with a diameter of 70 mm and a height of 120 mm on a machine with weight control of the process (laboratory unit HX-620 H). Detail description of the low thermal gradient Czochralski technique can be found elsewhere in references [33]. From the grown ingots, samples with dimension of 3 × 3 × 3 mm were cut. Corrected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves excitation and emission photo luminescence spectra of Pb2MoO5 and Pb2WO5 crystal samples were recorded with a Horiba Jobin Yvon Fluorolog 3 equipped with ozone-free Xe-lamp 450 W powder, cooled photon detector R928/1860 PFR technologies with refrigerated chamber PC177CE-010 and double grating monochromators. Temperature dependences of photoluminescence spectra and decay time curves were recorded with Optistat DN in temperature range 77− 300 K. 3. Results and discussion 3.1. Absorption spectra of Pb2MoO5 and Pb2WO5 crystals The band gap Eg value is corresponding to electronic transitions from valence to conduction band. Eg is fundamental parameter for scintillation materials. Estimated value Eg for Pb2WO5 and Pb2MoO5 crystals are close to each other and equal to about 3.45 eV. Estimated value of Eg for Pb2MoO5 using Urbah equation in [36] is about 4.16 eV. According to band structure calculations Eg for Pb2MoO5 is 2.41 eV [34] and 2.64 eV [35]. It is known that the scatter of the calculated values is associated with the features of the assessment methods used. In [36] temperature dependence of absorption spectra was investigated using Urbah equation and we can suggest such approach more accurate (Fig. 1). 2
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Fig. 1. UV/Vis absorption spectra of Pb2WO5 (red) and Pb2MoO5 (black) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 2. The luminescence excitation and emission spectra of Pb2MoO5 crystal sample at 77 K.
3.2. Photoluminescence spectra of Pb2MoO5 and Pb2WO5 crystals in 77− 300 K temperature range Temperature dependences of emission and excitation spectra is of significant relevance for possible practical applications. Pb2MoO5 crystals exhibit green luminescence under 350 nm excitation and yellow-red luminescence under 325 nm excitation (Figs. 2 and 5). Pb2WO5 crystals exhibit green-blue luminescence under 400 nm excitation and green-yellow luminescence under 340 nm excitation (Figs. 3 and 5). It should be noted that in earlier studies crystal samples of Pb2MoO5 demonstrate a photoluminescence emission peak in greenorange range near 530 nm [30]. From other investigations, we can see a peak of Pb2MoO5 photoluminescence close to 600 nm [23, 25]. In our investigation Pb2MoO5 crystals demonstrate the broad emission band with maximum at 580 nm under 325 nm excitation and broad emission band with maximum at 540 nm under 350 nm excitation. Emission band with maximum near 540 nm is very close to the green emission band of PbMoO4. Considered to be that this green emission of lead molybdates is generated by transitions from triplet states 3T1,2 to the ground state 1A1 of MoO2− 4 groups. So, we can suppose that our Pb2MoO5 samples contain some amount of PbMoO4 phase. Emission band with maximum at 580 nm are similar to literature data [23,25]. But in one case this emission band is explained by the same MoO2− 4 groups. It is claimed that the presence of broad emission band, which excited from the edge of ab sorption region is common for molybdates and this band is ascribed to the emission of self-trapped excitons, which located at MoO2− 4 groups. The position of this emission band is depends on molybdate crystal structure. So without reference to crystal structure of molybdates, there is MoO2− 4 group which responsible for the self-trapped excitons. Emission of Pb2MoO5 is related to such self-trapped excitons [23]. Difference in emission and excitation spectra of PbMoO4and Pb2MoO5 crystals gives reason to another presume that the photoluminescence emission of this crystals is due to different centers. As was mentioned in [31] chains defects in the linear configuration of Pb–O–Mo bonds appear. In addition, states of the oxygen atoms belong to Pb-O chains can be involved into lumi nescence processes. Unfortunately, observed data in our research is not enough to exclude one of these models. This moment requires further investigations. Pb2WO5 crystals demonstrate the broad emission band with maximum at 560 nm with the excitation at 340 nm and broad emission band with maximum at 480 nm with the excitation at 400 nm. Emission band with maximum near 480 nm is very close to the blue emission band of PbWO4. Possible, that Pb2WO5 samples contain some amount of PbWO4 phase, as in case of Pb2MoO5 samples. The emission band may be explained by self-trapped excitons of WO2− 4 groups, but by analogy with a Pb2MoO5 crystal, there may be another emission mechanism. Photoluminescence spectra for both crystals show a short wavelength shoulder near 400− 420 nm. Such band in violet range was 3
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Fig. 3. The luminescence excitation and emission spectra of Pb2WO5 crystal sample at 77 K.
observed earlier for some samples of PbMoO4. Violet component of photoluminescence for lead molybdate crystals are usually attribute to transitions from singlet states 1T1,2 to 1A1 of MoO2− 4 groups. Similar situation in the case of Pb2WO5 crystals, but instead of 2MoO2− 4 groups WO4 groups there are in such crystals. As it was presumed in [25,37,38] this violet emission band are related to the surface atoms of the crystal. It was explained by existence or absence of some point defects in samples and it depends on seed orientation. The temperature changes in the emission and excitation spectra of Pb2MoO5 and Pb2WO5 crystals shown in Fig. 4. The 4
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Fig. 4. Luminescence excitation (left) and emission (right) spectra of Pb2MoO5and Pb2WO5 for a range of temperatures 77–300 K.
Fig. 5. CIE 1931 chromaticity diagram of Pb2WO5 with an excitation wavelength of 340 nm (1) and 400 nm (2) and Pb2MoO5 with an excitation wavelength of 325 nm (3) and 350 nm (4).
photoluminescence properties of investigated crystals were found to exhibit strong temperature dependence. The emission color chromaticity coordinates at 77 K of Pb2WO5 crystals with an excitation wavelength of 340 nm and 400 nm and Pb2MoO5 crystals with an excitation wavelength of 325 nm and 350 nm are calculated from emission spectra and shown on Com mission Internationale de L’Eclairage (CIE1931) (Fig. 5). CIE chromaticity coordinates of Pb2WO5 with an excitation wavelength of 340 nm are (x = 0.388, y = 0.420) and with excitation wavelength of 400 nm are (x = 0.241, y = 0.347). Moreover, CIE chromaticity coordinates of Pb2MoO5 with an excitation wavelength of 325 nm are (x = 0.448, y = 0.464) and with an excitation wavelength of 350 nm are (x = 0.372, y = 0.455). It is necessary to calculate correlated color temperature and color purity to make a conclusion about practical perspective of investigated crystals in different optical applications. The formula for color purity calculation presented in [39]. √̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅ (x − xi )2 + (y − yi )2 CP = ∗100% (1) (xd − xi )2 + (yd − yi )2 Here k1 and k2 are rates, characteristic of radiative transitions from the excited levels 1 and 2 to the ground state, D is the energy gap between these levels and ΔE is the activation energy for the non-radiative transitions. Color purity for Pb2WO5 at 340 nm excitation is 45 % and at 400 nm excitation is 30 % and for Pb2MoO5 at 325 nm excitation is 74 % and at 350 nm excitation is 51 %. The closer to 100 % of the color purity of the monochromatic light sources, the better sample is obtained. In our case Pb2MoO5 crystal has the best color purity at 325 nm excitation. But it is not enough to tell about promising practical applications. It is necessary to calculate the correlated color temperature from McCamy’s formula [40] (2)
CCT=-449n3+3525n2-6823.3n+5520.33
Here n = (x-xe)/(y-ye), (x; y) – chromaticity coordinates of light source, (xe = 0.3320, ye = 0.1858) is the coordinates of "epicenter" CCT, quite close to the isotherm intersection point. Correlated color temperature is equal 4085 K at 340 nm excitation and 10,576 K at 400 nm excitation for Pb2WO5 and 3255 K at 325 nm excitation and 4583 K at 350 nm excitation for Pb2MoO5. So, correlated color temperature and color purity for Pb2MoO5 crystals are promising to use in solid state lighting applications. 3.3. Photoluminescence decay time constants in Pb2MoO5 and Pb2WO5:Cu2+ crystals in 77− 300 K temperature range The photoluminescence emission of Pb2MoO5 and Pb2WO5 crystals is characterized by microsecond decay times (Fig. 6). The luminescence decay times for both crystals under different excitations were fitted by a single exponential decay time function. The temperature dependence of decay time constant is shown in Fig. 7. The decay time constant temperature dependences for Pb2MoO5 5
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Fig. 6. Luminescence decay curves of (a) Pb2WO5 crystals with 340 nm excitation and (b) Pb2MoO5 with 325 nm excitation at 77 K.
Fig. 7. Decay time constant as function of temperature measured for excitation with 350 nm in Pb2MoO5 (a) and for excitation with 340 nm in Pb2WO5 (b). The solid lines show the experimental data fitting using Eq. (3).
Fig. 8. The temperature dependence of the specific responsivity |(Δτ/ΔT)τ− 1in Pb2MoO5 for excitation with 350 nm (a) and in Pb2WO5for exci tation with 340 nm (b). The curves are calculated by differentiating the fitted τ=f(T), displayed in Fig.7. The horizontal at 0.002 and 0.01 K− 1 separate the regions of low, modest and high specific sensitivity.
and Pb2WO5 crystals show similar trends with decreasing temperature. The decay time constant increase with decreasing temperature and found to be about 20 μs for both Pb2MoO5 and Pb2WO5 crystals at 77 K. The observed temperature dependence of the decay time in the crystals is explained in the framework of the model that analyses the dynamics of radiative and non-radiative transitions. The shortening of the decay time with increasing temperature is due to the thermally stimulated re-distribution of particles between the excited and ground states of the emission center and an analytic expression for the luminescence decay time constant τ as a function of temperature T that takes into account thermal quenching at 6
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elevated temperatures has been derived in [41]. ( ) 1 k1 + k2 exp(− D/kT) − ΔE + Kexp = 1 + exp(− D/kT) kT τ
(3)
The results of the fitting experimental results using Eq. (3) displayed in Fig. 7 as solid lines show good agreement between the theoretical model and the measured data. The fit was done using Origin software. Now we can characterize the materials under study from the point of view of temperature sensors. Using the approach described in the article [42] we can calculate the modulus of the specific sensitivity α = |(Δτ/ΔT)τ− 1| which provides a useful parameter for the characterization of the temperature sensor (Fig. 8). Obviously that the preferable material for luminescence lifetime thermometry is the one that exhibits good specific sensitivity over a wider temperature range.For Pb2MoO5 with 350 nm excitation. Good specific sensitivity is observed over the 80–300 K range when luminescence thermal quenching occurs and for Pb2WO5 with 340 nm excitation good specific sensitivity is observed over the 110–300 K range. The temperature range spanning more than hundred K in which the responsively is good, is clearly an advantage of these materials. Obtained results suggest that the crystals under study are promising as materials for contactless thermometry. 4. Conclusions High-quality Pb2MoO5 and Pb2WO5crystals were grown by the low-gradient Czochralski method. It has been established that the excitation and emission spectra of photoluminescence of Pb2MoO5 and Pb2WO5 crystals exhibit strong temperature dependence. The luminescence decay kinetics in Pb2MoO5 and Pb2WO5 crystals was studied in the temperature range 77–300 K and it was shown that the decay time constant increases by an order of magnitude when cooled to 77 K. Application of investigated crystals for the luminescence lifetime thermometry requires materials that have a significant change in the luminescence decay time constant for a given temperature range. We carried out studies of the luminescent properties of Pb2MoO5 and Pb2WO5 crystals in the temperature range 77–300 K to assess their prospects as materials for non-contact thermometry, because this crystals show good specific sensitivity in a large temperature range. The colorimetric parameters investigation results show that Pb2MoO5 and Pb2WO5 are not so good for solid-state lighting appli cations, in particularin light-emitting diodes. Only Pb2MoO5crystal at excitation with 325 nm demonstrate rather good color purity in 74 % and color temperature close to warm white lighting sources. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of Competing Interest The authors report no declarations of interest. References [1] P. Belli, A. Incicchitti, F. Cappella, Inorganic scintillators in direct dark matter investigation, Int. J. Mod. Phys. 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Ivannikova, EPR of copper ions in Pb2MoO5 crystals, J. Struct. Chem. 57 (4) (2016) 665, https://doi.org/ 10.1134/S0022476616040053. [32] E.N. Galashov, T.M. Denisova, I.M. Ivanov, E.P. Makarov, V.N. Mamontov, V.N. Shlegel, Yu.G. Stenin, Ya.V. Vasiliev, V.N. Zhdankov, Growing of (CdWO4)Cd106, ZnWO4, and ZnMoO4 scintillation crystals for rare events search by low thermal gradient Czochralski technique, Funct. Mater. 17 (2010) 504. [33] V.D. Grigorieva, V.N. Shlegel, N.V. Ivannikova, T.B. Bekker, A.P. Yelisseyev, A.B. Kuznetsov, J. Crystal Growth 507 (2019) 31, https://doi.org/10.1016/j. jcrysgro.2018.10.058. [34] O.Y. Khyzhun, V.L. Bekenev, V.V. Atuchin, L.D. Pokrovsky, V.N. Shlegel, N.V. Ivannikova, The electronic structure of Pb2MoO5: first-principles DFT calculations and X-ray spectroscopy measurements, Mater. Des. 105 (2016) 315, https://doi.org/10.1016/j.matdes.2016.04.095. [35] S. Nedilko, V. Chornii, Yu. Hizhnyi, M. Trubitsyn, I. Volnyanskaya, Luminescence spectroscopy and electronic structure of the PbMoO4 and Pb2MoO5 single crystals, Opt. Mater. 36 (2014) 1754, https://doi.org/10.1016/j.optmat.2014.03.019. [36] F.D. Fedyunin, D.A. Spassky, Urbach rule and estimate of the band gap in molybdates, Phys. Solid State 62 (8) (2020) 1179, https://doi.org/10.21883/ FTT.2020.08.49598.052. [37] T. Kajitani, M. Itoh, Phys. Status Solidi C 8 (2011) 108, https://doi.org/10.1002/pssc.201000665. [38] M. Tyagi, Sangeeta D.G. Desai, S.C. Sabharwal, J. Lumin. 128 (2008) 22, https://doi.org/10.1016/jlumin.2007.05.005. [39] J. Schanda, M. Danyi, Correlated color-temperature calculations in the CIE 1976 chromaticity diagram, Color Res. Appl. 2 (1977) 161, https://doi.org/10.1002/ col.5080020403/pdf. [40] E.F. Schubert, Light-Emitting Diodes, 2nd edition, Cambridge University Press, New York, 2006, https://doi.org/10.1017/CBO9780511790546. [41] V.B. Mikhailik, H. Kraus, S. Henry, A.J.B. Tolhurst, Phys. Rev. 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Original research article
Optical properties of Pb2MoO5 and Pb2WO5 single crystals as materials for practical applications Ryadun Alexey *, Rakhmanova Mariana, Grigorieva Veronika 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 Pb2MoO5 crystals Pb2WO5 crystals Luminescence lifetime Color chromaticity coordinates Non-contact thermometry
The low-gradient Czochralski method was used to obtain lanarkite type Pb2MoO5 and Pb2WO5 crystals. Photoluminescence properties under UV excitation were studied, such as temperature dependences of decay time and excitation and emission spectra. The Pb2MoO5 crystals demon strate intensive emission spectra in green range under 350 nm excitation and yellow-red range under 325 nm excitation. Pb2WO5 crystals exhibit green-blue luminescence under 400 nm exci tation and green-yellow luminescence under 340 nm excitation. Photoluminescence spectra for investigated crystals show a short wavelength shoulder in violet spectral region near 400− 420 nm. These violet component of photoluminescence for lead crystals are attribute to the 2transitions triplet states 1T1,2 of MoO2− 4 and WO4 groups, respectively. The band gap of Pb2MoO5 and Pb2WO5 crystals were estimated from UV absorption spectrum and are equal to about 3.45 eV. Correlated color temperatures and color purity for both crystals at different excitations were calculated and the conclusion was made about the possibility of using such crystals in warmwhite light emitted diodes and other solid-state lighting applications. Color chromaticity co ordinates are shown on CIE1931. Temperature dependence of decay time constant allowed us to calculate sensitivity for non-contact thermometry applications.
1. Introduction Molybdate and tungstate crystals are perspective materials in different practical application. Such materials are promising as a scintillation detector [1–6]. Special attention such materials attract due to opportunity to be used as detectors for rare event in vestigations [7,8]. For example, PbMoO4 is promising material to search for the neutrinoless double β decay of 100Mo isotope [7,9,10] and PbWO4 is promising as shield (anti-coincidence detector or active light guide) to suppress radioactive background to search for double β decay of 116Cd [11]. Nowadays various molybdate and tungstate crystals are investigated as potential scintillation materials for cryogenic bolometers [12,13]. The luminescence properties of Pb2MoO5 and Pb2WO5 crystals were investigated earlier. Recently, the field of luminescence thermometry is growing intensively showing significant breakthroughs in sensing, imaging, diagnostics, and therapy, among other areas [14]. 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 microthermometers and * Corresponding author. E-mail address: [email protected] (R. Alexey). https://doi.org/10.1016/j.ijleo.2020.165912 Received 1 August 2020; Accepted 27 October 2020 Available online 2 November 2020 0030-4026/© 2020 Elsevier GmbH. All rights reserved.
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nanothermometers, a research topic leaving accounting nowadays for more than 2.5 % of the total publications in luminescence materials. Among the distinct emitting centers used in luminescence thermometry are polymers, organic dyes, rare earth doped crystals, rare earth doped nanocrystals, semiconductor nanocrystals, rare earth doped complexes, phosphorous and quantum dots and other luminescent systems. Oxide crystals are promising materials for non-contact thermometry too. Detailed spectroscopic investigation on the thermal behavior of Cr3+-doped Bi2Ga4O9 system showing the importance of an insightful analysis in a wide temperature range for application in ratiometric luminescent thermometers not only for biological but for wider is reported in [15]. Bi2Ga4O9:Cr3+ shows a remarkable absolute sensitivity of 0.28 ± 0.02 K− 1 at 300 K. The possibility of ZnGa2O4:Cr3+ using for thermometry was investigated on base of the temperature dependence of photo luminescence spectra [16]. ZnGa2O4:Cr3+ is a well-known deep red phosphor with R-lines due to the 2E→4T2 spin-forbidden transition of Cr3+ ions between 690 and 750 nm. ZnGa2O4:Cr3+ system demonstrated opportunity for the ratiometric thermometry applications from around ambient to high temperatures. The relative sensitivity of ZnGa2O4:Cr3+ for thermometry is estimated to be 2.8 %K− 1 at 310 K, which is rather good for the ratiometric fluorescent thermometers using Boltzmann distribution phenomenon for biological application. Nowadays lead molybdatePb2MoO5and tungstatePb2WO5crystals are promising in various practical applications. And it is inter esting to investigate such crystals as materials for luminescence lifetime thermometry. Scheelite type PbMoO4 and PbWO4 crystals similar in composition to Pb2MoO5and Pb2WO5 were studied from manifold points of view. It is shown that lead containing crystals are perspective as acousto-optic [17], scintillation [18,19] and hosts materials for luminescent rare earthions [20]. But properties of lanarkite type Pb2MoO5and Pb2WO5crystals are poorly investigated. Structure data of Pb2MoO5 crystal appeared in 1951 [21], it was shown that the crystal has a monoclinic cell and crystal space group is C2/m. Refractive parameters measured in [22] with λ =589.3 nm was nх = 2.183, ny = 2.197 and nz = 2.319. The crystals studied in these works had small sizes and a yellowish-greenish color. Some results about optic properties and band structure of Pb2MoO5 crystals are presented in [23,24]. But crystal purity and quality of samples causes somewhat contradictory data. Besides, photoluminescence properties of Pb2MoO5 crystal was compared with PbMoO4 taking into consideration electronic structure calcu lation [25]. The optical properties of Pb2MoO5 and Pb2WO5 crystals were studied for acousto-optic applications [26–29]. Intrinsic lumines cence of Pb2MoO5 was studied under Xe-lamp excitation [30]. EPR investigations of Pb2MoO5 single crystals doped by copper ions presented in [31]. Low-thermal-gradient Czochralski technique was developed for growing large-sized oxide crystals of highest optical quality from melt. This method providefollowing advantages: suppression of decomposition and volatilization of melt components that prevents the formation of melt nonstoichiometry and the loss of initial materials; the decrease of thermoelastic stress in crystals, preventing cracking, thus, and increasing maximal possible size [32]. The purpose of present work is investigation of photoluminescence properties for Pb2MoO5 and Pb2WO5 single crystals grown by low-thermal-gradient Czochralski technique as materials for non-contact thermometry and lighting source. Measurements of lumi nescence excitation and emission spectra, as well as decay curves as a function of temperature over the 77–300 K temperature range were carried out. Color purity, correlated color temperature and decay time response to temperature change were calculated. 2. Experimental 2.1. Crystal grown and experimental techniques Pb2MoO5 and Pb2WO5 single crystals were grown by the Czochralski method under conditions of low temperature gradients (ΔT/ Δx<1 K/cm). The drawing speed was about 2 mm per day; the rotation speed was 30–40 revolutions per minute. The growing process was carried out from a platinum crucible with a diameter of 70 mm and a height of 120 mm on a machine with weight control of the process (laboratory unit HX-620 H). Detail description of the low thermal gradient Czochralski technique can be found elsewhere in references [33]. From the grown ingots, samples with dimension of 3 × 3 × 3 mm were cut. Corrected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves excitation and emission photo luminescence spectra of Pb2MoO5 and Pb2WO5 crystal samples were recorded with a Horiba Jobin Yvon Fluorolog 3 equipped with ozone-free Xe-lamp 450 W powder, cooled photon detector R928/1860 PFR technologies with refrigerated chamber PC177CE-010 and double grating monochromators. Temperature dependences of photoluminescence spectra and decay time curves were recorded with Optistat DN in temperature range 77− 300 K. 3. Results and discussion 3.1. Absorption spectra of Pb2MoO5 and Pb2WO5 crystals The band gap Eg value is corresponding to electronic transitions from valence to conduction band. Eg is fundamental parameter for scintillation materials. Estimated value Eg for Pb2WO5 and Pb2MoO5 crystals are close to each other and equal to about 3.45 eV. Estimated value of Eg for Pb2MoO5 using Urbah equation in [36] is about 4.16 eV. According to band structure calculations Eg for Pb2MoO5 is 2.41 eV [34] and 2.64 eV [35]. It is known that the scatter of the calculated values is associated with the features of the assessment methods used. In [36] temperature dependence of absorption spectra was investigated using Urbah equation and we can suggest such approach more accurate (Fig. 1). 2
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Fig. 1. UV/Vis absorption spectra of Pb2WO5 (red) and Pb2MoO5 (black) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 2. The luminescence excitation and emission spectra of Pb2MoO5 crystal sample at 77 K.
3.2. Photoluminescence spectra of Pb2MoO5 and Pb2WO5 crystals in 77− 300 K temperature range Temperature dependences of emission and excitation spectra is of significant relevance for possible practical applications. Pb2MoO5 crystals exhibit green luminescence under 350 nm excitation and yellow-red luminescence under 325 nm excitation (Figs. 2 and 5). Pb2WO5 crystals exhibit green-blue luminescence under 400 nm excitation and green-yellow luminescence under 340 nm excitation (Figs. 3 and 5). It should be noted that in earlier studies crystal samples of Pb2MoO5 demonstrate a photoluminescence emission peak in greenorange range near 530 nm [30]. From other investigations, we can see a peak of Pb2MoO5 photoluminescence close to 600 nm [23, 25]. In our investigation Pb2MoO5 crystals demonstrate the broad emission band with maximum at 580 nm under 325 nm excitation and broad emission band with maximum at 540 nm under 350 nm excitation. Emission band with maximum near 540 nm is very close to the green emission band of PbMoO4. Considered to be that this green emission of lead molybdates is generated by transitions from triplet states 3T1,2 to the ground state 1A1 of MoO2− 4 groups. So, we can suppose that our Pb2MoO5 samples contain some amount of PbMoO4 phase. Emission band with maximum at 580 nm are similar to literature data [23,25]. But in one case this emission band is explained by the same MoO2− 4 groups. It is claimed that the presence of broad emission band, which excited from the edge of ab sorption region is common for molybdates and this band is ascribed to the emission of self-trapped excitons, which located at MoO2− 4 groups. The position of this emission band is depends on molybdate crystal structure. So without reference to crystal structure of molybdates, there is MoO2− 4 group which responsible for the self-trapped excitons. Emission of Pb2MoO5 is related to such self-trapped excitons [23]. Difference in emission and excitation spectra of PbMoO4and Pb2MoO5 crystals gives reason to another presume that the photoluminescence emission of this crystals is due to different centers. As was mentioned in [31] chains defects in the linear configuration of Pb–O–Mo bonds appear. In addition, states of the oxygen atoms belong to Pb-O chains can be involved into lumi nescence processes. Unfortunately, observed data in our research is not enough to exclude one of these models. This moment requires further investigations. Pb2WO5 crystals demonstrate the broad emission band with maximum at 560 nm with the excitation at 340 nm and broad emission band with maximum at 480 nm with the excitation at 400 nm. Emission band with maximum near 480 nm is very close to the blue emission band of PbWO4. Possible, that Pb2WO5 samples contain some amount of PbWO4 phase, as in case of Pb2MoO5 samples. The emission band may be explained by self-trapped excitons of WO2− 4 groups, but by analogy with a Pb2MoO5 crystal, there may be another emission mechanism. Photoluminescence spectra for both crystals show a short wavelength shoulder near 400− 420 nm. Such band in violet range was 3
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Fig. 3. The luminescence excitation and emission spectra of Pb2WO5 crystal sample at 77 K.
observed earlier for some samples of PbMoO4. Violet component of photoluminescence for lead molybdate crystals are usually attribute to transitions from singlet states 1T1,2 to 1A1 of MoO2− 4 groups. Similar situation in the case of Pb2WO5 crystals, but instead of 2MoO2− 4 groups WO4 groups there are in such crystals. As it was presumed in [25,37,38] this violet emission band are related to the surface atoms of the crystal. It was explained by existence or absence of some point defects in samples and it depends on seed orientation. The temperature changes in the emission and excitation spectra of Pb2MoO5 and Pb2WO5 crystals shown in Fig. 4. The 4
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Fig. 4. Luminescence excitation (left) and emission (right) spectra of Pb2MoO5and Pb2WO5 for a range of temperatures 77–300 K.
Fig. 5. CIE 1931 chromaticity diagram of Pb2WO5 with an excitation wavelength of 340 nm (1) and 400 nm (2) and Pb2MoO5 with an excitation wavelength of 325 nm (3) and 350 nm (4).
photoluminescence properties of investigated crystals were found to exhibit strong temperature dependence. The emission color chromaticity coordinates at 77 K of Pb2WO5 crystals with an excitation wavelength of 340 nm and 400 nm and Pb2MoO5 crystals with an excitation wavelength of 325 nm and 350 nm are calculated from emission spectra and shown on Com mission Internationale de L’Eclairage (CIE1931) (Fig. 5). CIE chromaticity coordinates of Pb2WO5 with an excitation wavelength of 340 nm are (x = 0.388, y = 0.420) and with excitation wavelength of 400 nm are (x = 0.241, y = 0.347). Moreover, CIE chromaticity coordinates of Pb2MoO5 with an excitation wavelength of 325 nm are (x = 0.448, y = 0.464) and with an excitation wavelength of 350 nm are (x = 0.372, y = 0.455). It is necessary to calculate correlated color temperature and color purity to make a conclusion about practical perspective of investigated crystals in different optical applications. The formula for color purity calculation presented in [39]. √̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅ (x − xi )2 + (y − yi )2 CP = ∗100% (1) (xd − xi )2 + (yd − yi )2 Here k1 and k2 are rates, characteristic of radiative transitions from the excited levels 1 and 2 to the ground state, D is the energy gap between these levels and ΔE is the activation energy for the non-radiative transitions. Color purity for Pb2WO5 at 340 nm excitation is 45 % and at 400 nm excitation is 30 % and for Pb2MoO5 at 325 nm excitation is 74 % and at 350 nm excitation is 51 %. The closer to 100 % of the color purity of the monochromatic light sources, the better sample is obtained. In our case Pb2MoO5 crystal has the best color purity at 325 nm excitation. But it is not enough to tell about promising practical applications. It is necessary to calculate the correlated color temperature from McCamy’s formula [40] (2)
CCT=-449n3+3525n2-6823.3n+5520.33
Here n = (x-xe)/(y-ye), (x; y) – chromaticity coordinates of light source, (xe = 0.3320, ye = 0.1858) is the coordinates of "epicenter" CCT, quite close to the isotherm intersection point. Correlated color temperature is equal 4085 K at 340 nm excitation and 10,576 K at 400 nm excitation for Pb2WO5 and 3255 K at 325 nm excitation and 4583 K at 350 nm excitation for Pb2MoO5. So, correlated color temperature and color purity for Pb2MoO5 crystals are promising to use in solid state lighting applications. 3.3. Photoluminescence decay time constants in Pb2MoO5 and Pb2WO5:Cu2+ crystals in 77− 300 K temperature range The photoluminescence emission of Pb2MoO5 and Pb2WO5 crystals is characterized by microsecond decay times (Fig. 6). The luminescence decay times for both crystals under different excitations were fitted by a single exponential decay time function. The temperature dependence of decay time constant is shown in Fig. 7. The decay time constant temperature dependences for Pb2MoO5 5
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Fig. 6. Luminescence decay curves of (a) Pb2WO5 crystals with 340 nm excitation and (b) Pb2MoO5 with 325 nm excitation at 77 K.
Fig. 7. Decay time constant as function of temperature measured for excitation with 350 nm in Pb2MoO5 (a) and for excitation with 340 nm in Pb2WO5 (b). The solid lines show the experimental data fitting using Eq. (3).
Fig. 8. The temperature dependence of the specific responsivity |(Δτ/ΔT)τ− 1in Pb2MoO5 for excitation with 350 nm (a) and in Pb2WO5for exci tation with 340 nm (b). The curves are calculated by differentiating the fitted τ=f(T), displayed in Fig.7. The horizontal at 0.002 and 0.01 K− 1 separate the regions of low, modest and high specific sensitivity.
and Pb2WO5 crystals show similar trends with decreasing temperature. The decay time constant increase with decreasing temperature and found to be about 20 μs for both Pb2MoO5 and Pb2WO5 crystals at 77 K. The observed temperature dependence of the decay time in the crystals is explained in the framework of the model that analyses the dynamics of radiative and non-radiative transitions. The shortening of the decay time with increasing temperature is due to the thermally stimulated re-distribution of particles between the excited and ground states of the emission center and an analytic expression for the luminescence decay time constant τ as a function of temperature T that takes into account thermal quenching at 6
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elevated temperatures has been derived in [41]. ( ) 1 k1 + k2 exp(− D/kT) − ΔE + Kexp = 1 + exp(− D/kT) kT τ
(3)
The results of the fitting experimental results using Eq. (3) displayed in Fig. 7 as solid lines show good agreement between the theoretical model and the measured data. The fit was done using Origin software. Now we can characterize the materials under study from the point of view of temperature sensors. Using the approach described in the article [42] we can calculate the modulus of the specific sensitivity α = |(Δτ/ΔT)τ− 1| which provides a useful parameter for the characterization of the temperature sensor (Fig. 8). Obviously that the preferable material for luminescence lifetime thermometry is the one that exhibits good specific sensitivity over a wider temperature range.For Pb2MoO5 with 350 nm excitation. Good specific sensitivity is observed over the 80–300 K range when luminescence thermal quenching occurs and for Pb2WO5 with 340 nm excitation good specific sensitivity is observed over the 110–300 K range. The temperature range spanning more than hundred K in which the responsively is good, is clearly an advantage of these materials. Obtained results suggest that the crystals under study are promising as materials for contactless thermometry. 4. Conclusions High-quality Pb2MoO5 and Pb2WO5crystals were grown by the low-gradient Czochralski method. It has been established that the excitation and emission spectra of photoluminescence of Pb2MoO5 and Pb2WO5 crystals exhibit strong temperature dependence. The luminescence decay kinetics in Pb2MoO5 and Pb2WO5 crystals was studied in the temperature range 77–300 K and it was shown that the decay time constant increases by an order of magnitude when cooled to 77 K. Application of investigated crystals for the luminescence lifetime thermometry requires materials that have a significant change in the luminescence decay time constant for a given temperature range. We carried out studies of the luminescent properties of Pb2MoO5 and Pb2WO5 crystals in the temperature range 77–300 K to assess their prospects as materials for non-contact thermometry, because this crystals show good specific sensitivity in a large temperature range. The colorimetric parameters investigation results show that Pb2MoO5 and Pb2WO5 are not so good for solid-state lighting appli cations, in particularin light-emitting diodes. Only Pb2MoO5crystal at excitation with 325 nm demonstrate rather good color purity in 74 % and color temperature close to warm white lighting sources. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of Competing Interest The authors report no declarations of interest. References [1] P. Belli, A. Incicchitti, F. Cappella, Inorganic scintillators in direct dark matter investigation, Int. J. Mod. Phys. 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