In this paper the luminescence of the scheelite-based CaGd2(1-x)Eu2x(WO4)4 solid solutions is investigated as a function of the Eu content and temperature. All phosphors show intense red luminescence due to the 5D0 – 7F2 transition in Eu3+, along with other transitions from the 5D1 and 5D0 excited states. For high Eu3+ concentrations the intensity ratio of the emission originating from the 5D1 and 5D0 levels has a non-conventional temperature dependence, which could be explained by a phonon-assisted cross-relaxation process. It is demonstrated that this intensity ratio can be used as a measure of temperature with high spatial resolution, allowing the use of these scheelites as thermographic phosphor. The main disadvantage of many thermographic phosphors, a decreasing signal for increasing temperature, is absent.
© 2014 Optical Society of America
Scheelites are A1-□BO4 compounds (A = alkali, alkaline-earth or rare-earth element, B = Mo, W and some other tetrahedrally coordinated cations; □ denotes a cation vacancy) in which broad possibilities of isovalent and heterovalent cation substitutions result in a range of compounds with interesting optical properties, good stability and simple preparation [1–3]. Upon heterovalent substitution at the A sublattice, the charge balance can be maintained by the introduction of cation vacancies, giving rise to compositions characterized by a (A + A’):(BO4 + B’O4) ratio different from 1:1. This paper describes the temperature dependent luminescence properties of the CaGd2(1-x)Eu2x□(WO4)4 (x = 0 to 1, □ = vacancy) solid solutions and discusses their potential application as a thermographic phosphor.
The technological applications of luminescent materials are very diverse, going from lighting to display and imaging applications [4–10]. In some cases, the luminescence has a very specific temperature dependence, so that it can be used as temperature sensor [11–16]. This was first mentioned by Neubert in 1937 and the first application dates back to the 1950s, when a phosphor was painted on the wing surfaces of a wind-tunnel model in order to probe the temperature of different parts [17–19]. In principle, many different response modes are possible, but the three most important ones are the relative intensities of emission peaks, changes in decay time and shifts in peak position [20–22]. For the presently investigated scheelites, the focus will be on the fluorescence intensity ratio response mode. Kusama et al. were among the first to discuss this technique for Y2O2S:Eu3+, but the intensity ratios of the Eu3+ transitions were insufficiently dependent on the temperature to allow its use as thermographic phosphor . Until now, most research focused on Dy3+ or Sm3+ as more suitable thermographic rare earth activators, using the temperature dependence of the intensity ratio of different emission lines [24–28]. However, in this paper it will be shown that in the studied scheelite materials, the varying intensity ratio of the emission of the 5D0 and 5D1 energy level of Eu3+ is sensitive enough for use as temperature sensor. The mechanism behind the temperature dependence is explained by a phonon-assisted cross-relaxation process, and not by thermalisation like in most cases . To demonstrate the feasibility and prospects of the concept, a patterned resistive heater was coated with CaEu2(WO4)4 and illuminated with monochromatic light at 465 nm. The temperature map obtained by using the 5D0 to 5D1 intensity ratio showed high spatial resolution, in addition to an excellent correspondence with the temperature map recorded by more common infrared thermography.
2. Experimental section
Synthesis of the CaGd2(1-x)Eu2x(WO4)4 solid solutions: a solid state reaction of the tungstates CaEu2(WO4)4 and CaGd2(WO4)4 was performed. The CaGd2(WO4)4 and CaEu2(WO4)4 were prepared by heating stoichiometric amounts of CaCO3, WO3, Eu2O3 and Gd2O3 at 823 K for 10 h followed by annealing at 1203 K for 96 h in air. Appropriate amounts of CaEu2(WO4)4 and CaGd2(WO4)4 were mixed to obtain CaGd2(1-x)Eu2x□(WO4)4 and subsequently annealed at 1203 K.
Structure Analysis: The phase purity of the obtained CaGd2(1-x)Eu2x(WO4)4 (x = 0, 0.01, 0.05, 0.1, 0.25, 0.5, 1) solid solutions was checked with X-ray powder diffraction patterns collected on a Thermo ARL X’TRA powder diffractometer (CuKα radiation, λ = 1.5418 Å, Bragg–Brentano geometry, Peltier-cooled CCD detector).
Spectral characterization: Luminescence emission and excitation spectra were obtained with a FS920 spectrometer (Edinburgh Instruments), using a 450W xenon light source, double excitation monochromator and a R928P photomultiplier. All emission and excitation spectra were corrected for the sensitivity of the spectrometer and the intensity of the excitation. Decay times were collected using a pulsed dye laser (λexc = 385 nm) based on optical pumping by a nitrogen laser (λexc = 337 nm, pulse duration < 1ns, 1Hz repetition frequency) in combination with an intensified CCD detector (Andor Instruments) coupled to a 0.5 m monochromator. All measurements were taken at room temperature unless mentioned otherwise, and the temperature dependent measurements were performed using an Oxford Optistat CF cryostat.
Thermometry: The resistive heater was made by depositing a thin Al layer by electron beam evaporation onto a AF45 glass substrate (Präzisions Glas & Optik, 5x5 cm2), through a mask. Some irregularities were introduced in the heater to obtain temperature gradients upon application of a voltage over the outer contacts. Finally, the patterned heater was covered with a thin film of Al2O3 (100nm) by e-beam evaporation as a diffusion barrier. To deposit the thermographic phosphor, 0.06 g of CaEu2(WO4)4 was mixed with 0.18 g of Silres® (Wacker Chemie), painted on the patterned resistive heater and cured for two hours at 200°C. The material was excited with a Melles Griot argon ion laser with an excitation wavelength of 465 nm. The laser spot was expanded to approximately 3 cm in diameter. Photos were taken with a Nikon D3200 digital camera combined with a 535FS10-M52 green filter (Andover Corporation, transmission peak at 537 nm, 10 nm FWHM) or a red interference filter (peak wavelength of 625 nm) for selecting the green and red emission respectively. To obtain the reference temperature plot a FLIR A35sc (48°) infrared camera was used (320 x 256 resolution, temperature range from −40 °C to 550 °C with an accuracy of ± 5 °C).
3. Results and discussion
3.1 Structural characterization
According to the X-ray powder diffraction, the CaGd2(1-x)Eu2x(WO4)4 micron-sized powders are single phase and possess a body-centered monoclinically distorted scheelite-type structure. Upon the Gd/Eu substitution, the lattice parameters vary from a = 5.2313(2)Å, b = 5.2436(2)Å, c = 11.4382(4)Å, γ = 90.600(2)o for x = 0 to a = 5.2365(2)Å, b = 5.2629(2)Å, c = 11.4547(4)Å, γ = 91.152(2)o for x = 1. Besides the reflections from this basic monoclinic unit cell, satellites are visible on the X-ray powder diffraction patterns. These satellites originate from incommensurately modulated ordering of the A cations and vacancies. The detailed analysis of this modulation is out of scope of this article and is discussed elsewhere .
3.2 Luminescence emission and excitation spectra
In Fig. 1(a) the photoluminescence excitation spectrum is shown for CaGd1.8Eu0.2(WO4)4 upon monitoring the red emission at 612 nm, originating from the 4f6(5D0) – 4f6(7F2) transition within Eu3+. The excitation spectrum consists of the intraconfigurational 4f6-4f6 transitions of Eu3+ in the 300 to 500 nm region and a broad band in the region from 250 to 350 nm. The most intense Eu3+ 4f-4f excitation peak is due to the 7F0 - 5L6 (395 nm) transition. The small peak at 313 nm corresponds to the 8S7/2 - 6P7/2 transition within the 4f7 configuration of Gd3+. As the 6P7/2 level is the lowest excited level of Gd3+, no luminescence is expected from Gd3+ due to re-absorption and energy transfer to Eu3+. The presence of the 313 nm peak in the excitation spectrum of the Eu3+ emission points to energy transfer between the Gd3+ and Eu3+ ions. Apart from that, Gd3+ has only limited effect on the optical characteristics, which is common in Eu3+-Gd3+ co-doped samples [30, 31]. The excitation spectrum is representative for all studied samples, with composition CaGd2(1-x)Eu2x(WO4)4 (x = 0.01, 0.05, 0.1, 0.25, 0.5, 1). The broader band peaking at 285 nm is ascribed to a combination of a charge transfer (CT) transition from the 2p orbital of oxygen to the 3d orbital of tungsten in the (WO4)2- group and an O2- - Eu3+ charge transfer band [32–34]. A typical emission spectrum is shown in Fig. 1(b). The most intense emission peak in the region from 600 nm to 620 nm is due to the forced electric dipole transition 5D0 - 7F2. A magnetic dipole emission is found at 590 nm, originating from the 5D0 - 7F1 transition . The emission wavelengths of these 4f-4f transitions are only moderately influenced by the environment of the lanthanide ions since the partially filled 4f shell is well shielded by the filled 5s and 5p orbitals [36, 37]. No additional peaks or shifts in peak positions are observed when varying the europium concentration, only the relative intensities change. For the remainder of this work we will describe the emission originating from the 5DJ level to the 7FJ’ ground state multiplet as “5DJ emission”.
In Fig. 2 the dependence of the emission intensity of the 5D0 – 7F2 transition on the concentration of Eu3+ in CaGd2(1-x)Eu2x(WO4)4 is illustrated, which can be regarded as a measure for the external quantum efficiency. As can be seen, the intensity reaches a maximum for x = 0.5 and levels off for higher concentrations. Note that the concentration quenching is fairly limited, given the high emission intensity for the fully doped CaEu2(WO4)4.
3.3. Decay measurements
The luminescence lifetime of the emission from the different 5DJ levels was found to be independent of the final level 7FJ’. At 75 K all samples show a similar mono-exponential decay for the 5D0 emission (λexc = 385 nm), with a decay constant of 550 µs (Fig. 3), which is in line with previously reported decay times for similar materials [38–40]. For low dopant concentration, the decay profile shows almost no temperature dependence in the 75 K to 475 K range. For the highly doped samples, the temperature dependence is much more pronounced. The sample with full Eu substitution (i.e. CaEu2(WO4)4) has a mono-exponential decay profile, but the decay time decreases gradually from 470 µs at low temperature to 230 µs at 475 K.
For the intermediate case of CaGdEu(WO4)4 the decay starts deviating from a mono-exponential profile upon increasing temperature, and an accurate fit can be obtained by combining the low temperature decay component (τ1 = 550 µs) on the one hand and the changing decay component (τ2 = 550 µs to 230 µs) of CaEu2(WO4)4 at all temperatures on the other hand, in the following way:Table 1).
Taking into account the limited size difference of Gd3+(r = 1.053 Å) and Eu3+(r = 1.066 Å), we expect both cations to be uniformly distributed in the crystal. For low Eu concentrations this will result in a relatively large average distance between different Eu ions, leading to a low probability of Eu – Eu energy transfer.
As the Gd3+ ion is optically silent in the visible part of the spectrum, we can consider the Eu3+ ion as an isolated center in the Gd3+ dominated host for these low concentrations. For high Eu concentration, energy transfer is likely to occur between the different europium ions and this is obviously the case for the fully substituted CaEu2(WO4)4. Interestingly, we can consistently fit the thermal behavior of the decay for different dopant concentrations with only two types of Eu ions (i.e. isolated ones and those showing energy transfer), both characterized by their own specific thermal behavior.
3.4 Thermal quenching
The luminescence spectrum was monitored as a function of temperature in the range from 75 K to 475 K to evaluate the thermal quenching behavior. For the samples with low Eu concentration, the thermal quenching profile of the emission from the 5DJ levels is quasi-identical for all transitions. The integrated luminescence intensities upon excitation at 465 nm are shown in Fig. 4(a) for CaGd1.8Eu0.2(WO4)4, with each intensity normalized to the value at 75 K. At a temperature of 300 K, the emission has dropped to 25% of the initial intensity at liquid nitrogen temperature, suggesting that at room temperature a significant fraction of the emission is already quenched. This is somewhat in contrast to the luminescence decay behavior (§3.3), where hardly any decrease of the decay constants was observed upon increasing the temperature. In principle, thermal quenching is accompanied by a faster luminescence decay, due to an increase in non-radiative depopulation of the excited states. This discrepancy could not be explained yet, but might be related to the temperature dependent absorption strength for the excitation light.
Similar to the decay profiles, the temperature dependence of the luminescence is completely different for higher Eu concentrations. The 5D0 emission has similar quenching behavior compared to the samples with low Eu concentration, but for the 5D1 emission a strong absolute increase in intensity is observed for temperatures above room temperature. This is illustrated in Fig. 4(b) for CaEu2(WO4)4. For higher Eu concentration the emission output and the decay profiles show a marked temperature dependence and it is therefore expected that energy transfer mechanisms play a role.
In Fig. 5, the decay of the 5D1 emission of CaEu2(WO4)4 is depicted and apparently it consists of both a temperature independent component of the order of a microsecond and a temperature dependent slow component. The luminescence decay for Eu3+ is commonly much faster for the 5D1 emission than for the 5D0 emission, due to non-radiative depopulation of the higher excited state, e.g. by phonon relaxation or cross-relaxation processes. For higher dopant concentration – and thus for higher probability of energy transfer – the emission from 5D1 is often even not visible at all. Here we observe a fast component in the decay for 5D1 emission with a decay constant of 13 µs for CaGd1.8Eu0.2(WO4)4 and 1.8 µs for CaEu2(WO4)4, which is indeed considerably faster than the 5D0 emission. In addition, an unexpectedly slow component with a decay constant of hundreds of µs is observed as well. By comparing this slow component of the 5D1 emission with the decay of the 5D0 emission at the respective temperatures, a clear correspondence is observed.
The fraction of the slow component (defined by Eq. (2)) in the decay of the 5D1 emission varies from f2 = 0.3 at 75 K to f2 = 0.8 at 475 K. This points to an increased feeding of the 5D1 level from the 5D0 level upon increasing temperature for CaEu2(WO4)4, by an energy transfer process.
The same decay analysis was performed for CaGd1.8Eu0.2(WO4)4 (not shown). Although some transfer from 5D0 to 5D1 indeed occurs for these samples, the contribution is too low to result in a considerable increase of the 5D1 emission beyond the normal decay, which can also be observed in Fig. 4(a).
Both the thermal quenching and the decay profiles point at a concentration dependent cross-relaxation process, rather than a simple thermalisation process where the energy difference to populate the 5D1 level from the 5D0 level is fully covered by phonons. As will be substantiated in §3.5, the increase in the 5D1 emission can be explained by a thermally assisted cross-relaxation process with the involvement of two europium ions initially in the excited 5D0 and the 7F2 state:Fig. 6, favors the 5D1 emission of ion 2 (green arrow) at the expense of the 5D0 emission (red arrow), due to the transfer of the 7F2 – 7F0 energy difference from ion 1 (green dotted arrow). As this is not a resonant process, phonons are still needed to overcome the energy difference between 7F2 – 7F0 (1000 cm−1 +/− 100 cm−1) and 5D0 – 5D1 (1885 cm−1 +/− 100 cm−1). The energy differences between the 2S + 1LJ manifolds were derived from the emission spectrum. For higher Eu concentrations, this cross-relaxation process is more likely to occur due to a reduced average distance between the Eu ions. Note that a similar temperature dependence was observed for the emission of Eu3+ in GdVO4 by Nikolić et al, where the thermal quenching was explained by thermalisation from the lowest to the highest excited level, leading to a thermal barrier of about 1500cm−1 . However this thermalisation cannot account for the observed concentration dependence in our case.
The involvement of the phonons in Eq. (3) explains for the observed temperature dependence. This thermally assisted cross-relaxation should indeed result in a slow component in the decay of the 5D1 emission, comparable with this for the 5D0 emission, as the energy transfer can only occur as long as the 5D0 level remains populated. Note that the same temperature dependency for the decay constant of the 5D0 emission (Table 1) and the slow component in the 5D1 emission (Fig. 5) is found.
3.5 Use of CaEu2(WO4)4 as thermographic phosphor
Because of the strongly changing emission characteristics upon increasing temperature for CaEu2(WO4)4, this material is suitable as thermographic phosphor. Figure 7(a) shows the ratio of the 5D1 to 5D0 emission intensity as a function of temperature. The intensities are obtained by integrating over the wavelength ranges 535 to 545 nm (5D1-7F1) and 585 to 600 nm (5D0-7F1) respectively, upon steady state excitation at 395 nm. However, one is not limited to this excitation wavelength and specific 5DJ-7FJ’ transitions, as the cross-relaxation mechanism only depends on the emitting 5DJ levels.
Due to the non-resonant character of the cross-relaxation process, the 5D0 and 5D1 energy levels need to be thermally coupled for the process to occur. In case of thermal coupling, it can be shown that the relative population of both levels can be described by a Boltzmann distribution:42–44]. The uncertainty of the experimental data in Fig. 7(a) is on the order of a few degrees Celsius. Measurements were found to be reproducible upon consecutive heating cycles and did not lead to degradation of the material. The Arrhenius plot of the ratio R is given in the inset of Fig. 7(a), and the fit was obtained by varying both B and ΔE. The fitted value of 1068 cm−1 for ΔE should be compared with the energy of the phonons needed for the process. This energy is determined by the mismatch between the 7F2 – 7F0 (1000 cm−1 +/− 100 cm−1) and 5D0 – 5D1 (1885 cm−1 +/− 100 cm−1) energy difference. The variation on each energy difference arises from the multiplet splitting of 7F2 and 5D1. This value agrees reasonably well with the fitted value of 1068 cm−1, so the use of the Boltzmann equation is validated.
The one to one correspondence between the data and the fit makes CaEu2(WO4)4 applicable as thermographic phosphor from 300 K to at least 500 K. For a quantitative comparison of thermographic phosphors, the relative sensitivity Srel is the most appropriate parameter and is defined as follows [45, 46]:Fig. 7(b). The sensitivity varies from 0.014 K−1 at 300 K to 0.0047 K−1 at 475 K. These values are slightly lower than the relative sensitivities for other Eu3+ doped thermographic materials. For example Eu3+ doped (Y0.75Gd0.25)2O3 has a maximum relative sensitivity of 0.025K−1 . On the other hand, the scheelite thermographic phosphor has a much larger temperature operation range compared to most self-referencing materials . Compared to the performance of Dy3+, Sm3+ and Nd3+ doped materials or up-conversion nano-particles, the obtained maximum sensitivity is similar or even higher [27, 47–53].
To prove its usefulness in thermal imaging, CaEu2(WO4)4 phosphor powder was mixed in a 1:3 weight ratio with SILRES® binder and coated onto a patterned, purposely made resistive heater (Fig. 8) to obtain an inhomogeneous temperature distribution. The device was heated to a maximum local temperature of about 150°C. Then, the coated heater was illuminated with the 465 nm emission of an Ar laser to excite Eu3+ via the 7F0-5D2 transition. In order to map the temperature gradients of the heated surface, pictures were taken with a Nikon D3200 digital reflex camera combined with a green and a red narrow band filter, collecting the relevant fractions of the green 5D1 and the red 5D0 emission separately. Images were read out in the NEF data format, and divided pointwise. Using Eq. (4), a thermal map was obtained (Fig. 8, middle). As a reference, images were made with a thermal imaginginfrared camera (Fig. 8, right). For both types of measurements, a horizontal and a vertical temperature profile was extracted from the corresponding thermal maps (Fig. 9) at the positions indicated on the photograph of the coated heater (Fig. 8, left). It can be seen that the thermographic scheelite phosphor is capable of reproducing the temperature gradients in a very accurate way, which proves the applicability of the material in the postulated temperature range.
A major advantage specific for this response mode is that there is no need for an absolute intensity measurement, eliminating errors introduced by fluctuations in excitation intensity and light collection efficiency . Together with the obtained high spatial resolution and the non-destructiveness of the method, this makes the material a very promising candidate for thermometry. Indeed, after use, the phosphor coating can be easily removed using an appropriate solvent such as acetone. In addition, the involved emission peaks are very well separated, making the detection relatively easy. The main drawback for the material is the low 5D1 emission intensity compared to the 5D0 emission intensity. Nevertheless, the cross-relaxation process can still be optimized, for example by minimizing the defects in the host material. This reduces the non-radiative decay paths from 5D0 to 7FJ, which can then increase the probability of populating the 5D1 level. In addition, other host materials could be chosen with a different distance between the europium ions or with phonon energies having an improved match with the energy needed for the cross-relaxation process.
In this work, we investigated the temperature dependence of both the luminescence and the decay pathways of CaGd2(1-x)Eu2x(WO4)4 scheelites in a systematic way. All phosphors emit intense red light dominated by the 5D0 – 7F2 transition at 612 nm, and have similar excitation and emission spectra. The temperature dependence of the 5DJ emission intensities was however found to be strongly dependent on the europium concentration. For high Eu concentration strong cross-relaxation occurred upon increasing temperature due to energy transfer between nearby Eu ions. This thermally assisted cross-relaxation manifests itself in an increasing emission from the 5D1 level, characterized by the same decay time as the 5D0 emission, upon increasing temperature. Because of this specific temperature dependence, CaEu2(WO4)4 was proven to be suitable for intensity ratio based temperature measurements. The ratio of the 5D1 to 5D0 emission intensity follows a Boltzmann distribution equation. Consequently, an accurate estimate of the temperature can be made in the range from 300 K to at least 500 K by measuring the relative emission intensity of both peaks. This temperature range is large compared to other ratiometric systems and it was experimentally shown that the phosphor was able to reproduce temperature gradients with high accuracy and spatial resolution, confirming the applicability of the material for thermometry and thermography. A disadvantage of the present phosphor is the relatively low integrated emission intensity from the 5D1 emission compared to the dominating 5D0 emission, which lengthens the acquisition time for the thermal imaging. Future work will therefore focus on the optimization of the cross-relaxation process and the total emission intensity of the 5D1 state, by optimizing the host composition and reduction of the non-radiative decay paths.
This research was supported by FWO (projects G039211N, G006410), Research Foundation - Flanders. V.M. is grateful for financial support of the Russian Foundation for Basic Research (Grants 08-03-00593, 11-03-01164, and 12-03-00124). We gratefully thank Mathias Helsen and Jonas Botterman for assisting in the measurements.
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