LuPO4:Yb phosphor with concerted UV and IR thermoluminescent emissions by quantum cutting at high temperatures

Justyna Zeler*ab, Eugeniusz Zychb and Joanna Jedońb
aDepartment of Physics, CICECO-Aveiro Institute of Materials, University of Aveiro, Campus de Santiago, Aveiro, 3810-193, Portugal
bFaculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie Street, 50-383 Wroclaw, Poland. E-mail: justyna.zeler@chem.uni.wroc.pl

Received 4th June 2019 , Accepted 10th July 2019

First published on 10th July 2019


Thermoluminescence of LuPO4:0.1%Yb3+ sintered ceramics was investigated and simultaneous infrared 2F5/22F7/2 and UV-blue (YbCT3+)* → O2− charge transfer emissions of the Yb3+ impurity were observed around 150 °C (423 K) for the first time. Both photons were generated by one excited Yb3+*. LuPO4:Yb3+ was thus proved to be the first system showing the quantum cutting effect in thermoluminescence. Low concentration of the dopant was proved crucial to observe an intense CT emission at so high temperatures. These data revise deeply those reported previously on the thermal quenching of Yb3+ charge transfer luminescence in orthophosphates. In was formerly claimed that CT luminescence of Yb3+ in LuPO4 and similar hosts is quenched below 300 K. Similarly, the thermoluminescent emission of LuPO4:Yb3+ above room temperature was previously reported to appear only in the IR part of the spectrum around 980 nm. Our results fundamentally change this picture and prove that CT luminescence of Yb3+ in orthophosphates appears to be significant even above 150 °C (423 K). We demonstrate the great significance of the activator concentration in its CT luminescence thermal quenching. The Yb3+ impurity ion was found to act both as an electron trap and as a recombination center. Our data open the possibility to generate intense CT luminescence of Yb3+ in orthophosphates at room temperature and above which may make such phosphors rational for applications previously considered unattainable for them.


Introduction

Orthophosphates have been subjected to extensive research mainly as photoluminescent materials,1–6 high-speed scintillators when doped with Ce3+ (ref. 7–10) and, more recently, as energy storage phosphors.11–14 By thermoluminescence (TL) techniques, the latter were shown to trap free electrons and holes generated upon the impact of ionizing radiation and the trap depths could be effectively shaped by suitable choice of co-doping ions. In that research Ce3+ was often used to serve as a deep h-trap (∼4.0 eV) and other Ln3+ ions as shallower e-traps.11–13,15 Consequently, upon heating trapped electrons were released first and they recombined with holes at the Ce site giving rise to Ce3+ luminescence.16,17

The band gap energy of the LuPO4 host is about 9.2 eV.17,18 The recently developed semi-empirical Dorenbos model13 proved that excited states of practically all Ln3+ ions, even their 5d levels, are located below the bottom of this host conduction band (CB). This is essential and basically sufficient to let them produce luminescent photons.13,19 What is more, also ground states of most Ln2+ ions fall below the bottom of the CB which makes Ln3+ ions likely e-traps.20,21 In a classic series of papers, Dorenbos and co-workers proved that all that may be exploited to manage energy storage and persistent luminescence properties of such doubly activated compositions.11,13,15,22–24

More recently, it was shown that also singly doped orthophosphates, mainly LuPO4 and YPO4, may offer efficient energy storage. It was first shown for LuPO4:Eu sintered ceramics25–27 and recently for LuPO4 and YPO4 powders activated singularly with Sm3+, Eu3+ or Yb3+.28 These dopants – in agreement with the Dorenbos model13,29 and in accordance with their chemical properties – were postulated to serve as e-traps getting converted into 2+ states. Holes were inferred to get trapped at O2− ligands near Lu vacancies image file: c9cp03169c-t1.tif whose negative net charge might not only attract these carriers but also stabilize them after immobilization in the proximity of image file: c9cp03169c-t2.tif. Recently, by means of photoluminescence techniques, it was directly shown that X-rays indeed converted Eu3+ into Eu2+ and the latter was stable up to room temperature and could produce its own blue band emission in the range of 20–300 K.30

It was reported that heating of Eu- or Yb-doped LuPO4 previously exposed to X-rays produced orange luminescence by Eu3+ (ref. 31) or infrared emission by Yb3+.28,32–35 TL emission of the former ion was due to the 5D07FJ relaxation, while Yb3+ generated its ∼970–980 nm luminescence by means of the 2F5/22F7/2 intraconfigurational transition. This IR luminescence of Yb3+ might be of interest for bio-imaging as it fits the 1st bio-window (∼700–1000 nm) where absorption (as well as scattering) of radiation is minimal. Furthermore, emission in this range is also attractive for advanced military appliances as well as anti-counterfeiting solutions.36–38 This makes the research additionally attractive and important.

In this paper we present new data on TL of LuPO4:Yb polycrystalline sintered ceramics and we demonstrate that its TL emission spectrum is more complex and intriguing than reported by Lyu and Dorenbos in ref. 28. Namely, apart from the IR emission around 970–980 nm due to the 2F5/22F7/2 transition already reported by Lyu, the TL spectrum of LuPO4:Yb3+ may contain much more intense thermoluminescent emission due to the Yb3+ → O2− charge transfer (CT) transition appearing in the UV and blue part of spectrum. This TL covers the 250–550 nm range of wavelength with two maxima at 300 nm and 425–430 nm, exactly as expected for Yb3+ CT luminescence.37 The details of the LuPO4:Yb3+ TL are presented, discussed and compared to previous literature claiming that the CT luminescence of Yb3+ in orthophosphates is thermally quenched practically below 300 K. The CT emission around 300 nm is energetic enough to be of interest for destroying cancer cells,39 which makes its presence above room temperature additionally attractive. The presence of both CT and 2F5/22F7/2 emissions in TL makes the LuPO4:Yb3+ ceramics the first phosphor showing the quantum cutting effect in the thermoluminescence process.

Experimental

The LuPO4:x%Yb3+ (where x = 0.1, 1, 3) sintered ceramics were fabricated from nanopowders precipitated from a water solution. Lu(NO3)3·5H2O and Yb(NO3)3·5H2O were dissolved in water at 70–90 °C (343–363 K) and then (NH4)2HPO4 was added to precipitate the LuPO4:Yb3+ powders. These were separated by vacuum filtration, washed several times with the water–ethanol mixture and dried at 80 °C (353 K) in air and finally heated at 900 °C (1173 K) for 5 h. These powders were uniaxially cold-pressed upon the load of 5 tons into pellets 8 mm in diameter and were then sintered for 5 h at 1700 °C (1973 K) in air. Before measurements the pellets were mirror-polished.

The crystallographic purity of the sintered ceramics was tested by means of the powder X-ray diffraction (XRD) technique using a D8 Advance diffractometer from Bruker with Cu Kα1 radiation of 1.54060 Å wavelength. Scanning electron microscopy (SEM) studies were carried out on Hitachi S-3400N equipped with an energy dispersive X-ray spectroscopy (EDX) EDAX analyzer. The latter was used to evaluate the real content of Yb3+ in the sintered ceramics. All TL experiments were performed using a Lexsyg Research Fully Automated TL/OSL Reader from Freiberg Instruments GmbH. It used a Varian VF-50J RTG X-ray lamp with W-anode which operated under 15 kV and 0.1 mA for TL glove curves and 45 kV and 0.85 mA for TL emission spectra. The TL glow curves were recorded using a 9235QB-type photomultiplier from ET Enterprises in the range of 30–500 °C (303–773 K) and using the combination of HC340/26 and BP 365/50 filters. TL emission spectra were recorded using an OceanOptics HR2000 CG CCD camera in the 200–1100 nm range of wavelength and in the temperature range of 30–500 °C (303–773 K). All experiments were controlled by means of LexStudio 2 software and the resultant data were processed under LexEva 2 analytical software dedicated to the Fully Automated TL/OSL Reader and supplied by the manufacturer.

Results

The measured XRD diffraction patterns were compared to the simulated one expected for LuPO4 – pdf file #2025.40 These results are not shown here as very similar data were presented previously for Eu-doped LuPO4 pellets prepared in the same way.26,27 No measureable changes in the XRD peak positions were noted even for the 3% sample. This is understandable, as Yb3+ and Lu3+ have almost identical radii (1.125 Å and 1.117 Å, respectively). Unfortunately, quantitative analysis with EDX spectroscopy for real Yb3+ content was not possible due to a significant overlapping of all relevant EDX peaks of Yb3+ and Lu. Since the latter was present in a great majority, Yb3+ concentration could not be reliably extracted. Nevertheless, all the experience we have with the precipitation method we used to get raw powders makes it clear that real Yb3+ concentrations in the phosphors have to be noticeably lower than the nominal one.

Fig. 1-A01 (0.1%), A1 (1%), and A3 (3%) present maps of temperature- and wavelength-resolved thermoluminescence of LuPO4:Yb3+ ceramics after their exposure to X-rays. For the 0.1% (Fig. 1 – A01, B01 and C01) and 1% (Fig. 1 – A1, B1 and C1) samples the 2D plots prove that around 150 °C a strong thermoluminescence appears simultaneously in two spectral regions. A fraction of the TL emission comes around 975–980 nm (∼10[thin space (1/6-em)]200 cm−1) which is obviously due to the 2F5/22F7/2 intraconfigurational transition of Yb3+. The other, broad-band thermoluminescence covers the 250–550 nm range of wavelength and is composed of two components peaking at 295–300 nm and 425–430 nm. The latter band has a dip around 450 nm which is due to the CCD detector characteristics. Such characteristics of the UV-blue TL emission tell us to assign it to the (YbCT3+)* → O2− CT transition which terminates leaving the Yb3+ ion either at its 2F7/2 (when a UV band is generated) or 2F5/2 (blue band) state.41 The above presented characteristics of the Yb3+ CT thermoluminescence spectrum agree very well with the previously reported low-temperature photoluminescence data of LuPO4:Yb3+.42


image file: c9cp03169c-f1.tif
Fig. 1 2D temperature- and wavelength-resolved thermoluminescence maps of sintered LuPO4:x%Yb3+, where x = 0.1 (A01, B01 and C01), 1 (A1, B1 and C1) and 3 (A3, B3 and C3). Note the break of the Y-axis in B3.

Both the UV-blue and the IR thermoluminescent emissions have very similar glow curves; see the B01 and B1 windows for the 0.1% and 1% activator concentration in Fig. 1, respectively. They peak around 140 °C (413 K, UV emission) and 150 °C (423 K, IR). The small shift of the glow curve maxima is assigned to some thermal quenching of the CT luminescence.42 This reduces the intensity of the higher-temperature part of the CT emission and consequently shifts its TL glow curve peak to lower temperature. To the best of our knowledge, such a two-photon emission is the first observation of quantum cutting in the thermoluminescence process. For higher concentration of Yb3+ ions (3%, Fig. 1B3) the CT thermoluminescent emission is almost totally quenched while the IR TL could be easily recorded. Four lines in the orange-red part of the TL spectrum (window B01, B1 and B3 of Fig. 1) result from uncontrolled Eu3+ impurity, which was previously shown to be very active in thermoluminescence in this host.26

However, the presence of significant CT thermoluminescent emission of Yb3+ in LuPO4 around 150 °C (423 K) is in contradiction with the literature data. van Pieterson et al. reported42 that in photoluminescence the CT emission of Yb3+ in this phosphate is 50% quenched already at 250 K and quickly drops down upon further heating. Thus, based on these data, around 150 °C (423 K) when the TL reaches its maximum this emission should not be seen at all. Clearly, it is not the case of our LuPO4:0.1%Yb3+ system. What is more, in the recently published TL data on LuPO4:Yb3+ the presence of this emission was also not mentioned at all.28 In our case the CT thermoluminescence was nearly absent only for the most concentrated (3%) sample. We shall return to this problem in Discussion. Yet, the great effect of Yb3+ concentration is already obvious.

Fig. 2a presents TL glow curves taken after different doses of X-rays. With increasing dose the TL intensity increases, but the shape of the curve as well as the peak position practically does not change. The main peak appears at 150 °C (423 K). On its high-temperature side two components of much lower intensities come into view at 210 °C (483 K) and 270 °C (543 K) (see the inset in Fig. 2a). Also around 70 °C (343 K) a low intensity shoulder is visible. The position of each of the four components is independent of the dose. The dose dependence of the TL glow curve proves that the LuPO4:Yb3+ TL is a I-order kinetics process.43–45


image file: c9cp03169c-f2.tif
Fig. 2 Dependence of the LuPO4:0.1%Yb ceramics glove curve on the X-ray dose (a) and comparison of the Yb3+- and Eu3+-activated LuPO4 TL glow curves (b) illustrating their great similarity.

It is instructive to compare the glow curves of Eu3+- and Yb3+-activated LuPO4. Such a comparison is presented in Fig. 2b. Only a small shift of the main peak is observed and the more profound component around 280 °C (553 K) in the LuPO4:Eu3+ is due to much less significant temperature quenching of its luminescence.27 Also the mentioned shift of the TL glow curve peaks can be easily explained by the same effect – thermal quenching of the CT luminescence of Yb reduces the high-temperature intensity of TL emission causing what is indeed seen. Unfortunately, since the photo-excitation of the CT luminescence is around 190–200 nm we cannot measure the decay times of the CT luminescence. This precludes more quantitative analysis.

To learn more on the trap structure the so-called TmaxTstop (partial thermal cleaning) experiments were performed.46–49 The results are presented in Fig. 3a. It is immediately seen that not only the main TL peak but also the less intense ones at higher temperatures (250–300 °C/523–573 K) result from a series of traps of similar energies giving the so-called continuous distribution of trap energies.46–49 This conclusion comes directly from the observed linear growth of Tmax with increasing Tstop (see Fig. 3b). On the other hand, as seen in Fig. 3b, the low-intensity TL peak around 220 °C (493 K) appears to result from a single well-defined trap. For Tstop spanning the 140–200 °C (413–473 K) range of temperature, two parts (∼140–170 °C/413–443 K and ∼180–210 °C/453–483 K) of slightly different continuous distributions could be distinguished.


image file: c9cp03169c-f3.tif
Fig. 3 LuPO4:0.1%Yb3+ glow curves of consecutive TL measurements recorded with increasing Tstop (a). (b) TmaxTstop dependence derived from the data presented in (a). (c) Glow curves taken after irradiation of the material at the indicated temperature.

These data are in accordance with the results reported for the singly doped LuPO4:Eu3+.27 This is not surprising as in orthophosphates, according to the Dorenbos model,13,19,50,51 both Eu3+ and Yb3+ ions should serve as electron-traps. Then, TL emission of these dopants has to result from recombination of a hole after its liberation from the same type of trap(s) generated upon the high-temperature sintering. In LuPO4:Eu3+ it was postulated to be connected with Lu vacancy facilitating the hole localization at nearby oxygen ligands.26,30

The continuous distribution of trap energies is further confirmed by the changes in TL glow curves as the irradiation is performed at different temperatures in the range from room temperature up to 190 °C (463 K). The results are presented in Fig. 3c. Also in this case the glow curve peak shifts systematically towards higher temperatures with increasing temperature of irradiation as expected for continuous distribution of trap energies. Hence, it is clear that in LuPO4:Yb3+ the TL peaks result from liberation of holes from traps of similar but not equal depths. We may speculate that since holes are heavy charge carriers their migration along the valence band to the sites of trapped electrons may be hampered over larger distances. This could effectively be observed as the more distant holes were located at deeper traps. Thus, we do not exclude that it is the systematically increasing distance of the hole trap from the image file: c9cp03169c-t3.tif – the Yb3+ impurity ion which trapped an electron – which causes the continuous shift of the TL peak towards higher temperatures. Much deeper research, well beyond the scope of this paper, would be needed to reliably explain this problem.

The effect of the heating rate on the position and shape of the LuPO4:Yb3+ glow curve is presented in Fig. 4a. As the heating rate increases from 1 K s−1 to 10 K s−1 the glow curve shifts towards higher temperatures as its peak does. Calculated integrated TL luminescence intensities (areas under the glow curves) decrease by about 30% between the 1 K s−1 and 10 K s−1 heating rates. The decrease is presumably caused by the thermal quenching of the Yb3+ CT emission. This again confirms that thermal quenching of the CT luminescence of Yb3+ in the LuPO4 host is not that critical if the dopant concentration is low.


image file: c9cp03169c-f4.tif
Fig. 4 The heating rate dependence of the TL glow curve measured after the same X-ray dose (a) and the fading effect during the first 1080 min after irradiation of LuPO4:0.1%Yb3+ (b). β (Y-axis in (a)) stands for the heating rate.

Thermoluminescence of LuPO4:Yb3+ appears at moderate temperatures at least if the main TL peak is considered. This raises the question whether the energy is stored permanently or the carriers (holes at first) tend to leak out of their traps in time. The results of fading measurements for the 0.1% ceramics are presented in Fig. 4b. Surprisingly, within the first few hours after irradiation the peak intensity increases by about 9%. Repeated measurements gave always the same result proving that it is real. A closer examination reveals that this occurs at the expense of a low-intensity component appearing around 80–90 °C (353–363 K; see the inset of Fig. 4b). Clearly, the carriers from the trap(s) giving TL around 80–90 °C (353–363 K) fall continuously down to those giving TL around 155 °C (428 K). Thus, some redistribution of the trapped charge carriers between traps is observed. Altogether, during the first 10 hours, the total integrated intensity is practically stable within ±5%, which is within the experimental accuracy.

Discussion

The thermoluminescent glow curves of LuPO4:0.1%Yb3+ ceramics were found to be almost identical to those reported for Eu3+-activated LuPO427 (see Fig. 2b). This is in agreement with the Dorenbos model which predicts such a similarity assigning electron trapping properties to each of the two dopants.11,17,29 In the case of Yb3+ this process can be described by eqn (1). The profound similarity between TL glow curves of Yb3+- and Eu3+-doped LuPO4 leaves no doubt that hole-trapping in both compositions occurs at the same type of traps. EPR spectroscopy of LuPO4:Eu after its exposure to X-rays27 showed that it occurs at oxygen, presumably located in the vicinity of Lu vacancy, as illustrated with eqn (2).
 
image file: c9cp03169c-t4.tif(1)
 
image file: c9cp03169c-t5.tif(2)
Since the hole trap is shallower than the electron trap16,27 upon heating it is the trapped hole which is released and diffuses to the image file: c9cp03169c-t6.tif center ([triple bond, length as m-dash]Yb2+) where the trapped electron resides. Consequently, an excited (YbCT3+)* charge-transfer-like state appears which may relax sending off a photon of Yb3+ charge-transfer luminescence. Since the TL peaks practically do not change their position with Yb3+ concentration one may assume that the trap structure does not change within the investigated range of Yb3+ concentrations.

However, as we noted discussing the TL data presented in Fig. 1, appearance of the (YbCT3+)* → O2− charge transfer luminescence above room temperature is puzzling, especially that we see it up to ∼200 °C (473 K). van Pieterson et al.42 reported that this emission practically disappeared in their LuPO4:Yb sample around room temperature. What is more Lyu and Dorenbos in their very recent paper on thermoluminescence of LuPO4:Yb recorded only the 2F5/22F7/2 IR emission around 980 nm seeing nothing in the UV-blue part of the spectrum.28 Clearly, our results significantly diverge from those reported previously. On the other hand, Nakazawa, who was probably the first reporting the charge transfer luminescence in LuPO4:Yb3+,41 showed good quality CT spectra at both 80 K and 300 K. However, he did not present any data on the thermal quenching of this emission.

Nakazawa did not give any information on the Yb3+ concentration in the LuPO4:Yb3+ he investigated. And our data prove that this is a crucial parameter defining the susceptibility of the charge transfer luminescence of Yb3+ in LuPO4:Yb3+ for thermal quenching. van Pieterson et al. investigated the LuPO4:Yb3+ phosphor in which the real Yb3+ content was as high as 3%.42 Lyu and Dorenbos for their thermoluminescence experiments used a 0.5% sample.28 In our samples the Yb3+ nominal concentrations were 0.1%, 1% and 3%, but the real ones were certainly significantly lower due to our synthesis procedure. And only the first two – least concentrated – samples presented charge transfer TL emission of significant intensities. Furthermore, in the case of the 0.1% material the relative contribution from CT compared to the IR emission was clearly higher than in the 1% ceramics. We thus see that it is the low Yb concentration, preferentially around 0.1% or maybe even less, which is crucial for having efficient charge transfer emission of Yb3+ in LuPO4:Yb3+ at moderate temperatures of about 100–200 °C (373–473 K). In part the more significant T-quenching reported in the literature for PL emissions may also arise from significant local heating of the emitting Yb3+ as between its PL excitation and emission as much as ∼2 eV of energy is dissipated in extremely short time. This may notably raise the temperature of the emitting center critically reducing the CT luminescence intensity.

The above conclusion encourages one to revisit the UV-blue charge-transfer photoluminescence of LuPO4:Yb3+ and test its temperature quenching dependence on Yb3+ concentration. The data presented here tell that the dependence may be profound. Such research, well beyond the scope of this paper, is not easy, however, as the CT excitation is within deep UV, around 190–200 nm, and synchrotron (or deuterium lamp) radiation is needed for that.

Obviously, the concentration of the emitting entities affects the sensitivity for luminescence thermal quenching in many phosphors. In the case of charge transfer emissions the dependence is expected to be especially significant as the charge transfer state is spatially spread/diffused which makes it especially susceptible for interaction even with relatively distant features. Thus, even at moderate concentrations of the activator the Yb3+–Yb3+ interaction may easily allow for energy migration to defects serving as luminescence killing centers or the communicating Yb3+ ions may directly allow for energy dissipation by means of non-radiative transitions.

Conclusions

In this work the thermoluminescence properties of X-rayed LuPO4:Yb3+ sintered materials were investigated using a number of experimental techniques. Significant thermoluminescence was observed mainly around 150 °C (423 K). The TL spectrum changed with Yb3+ concentration although the glow curve shapes were practically not affected. For low dopant contents both IR emission due to the Yb3+ 2F5/22F7/2 transition and the (YbCT3+)* → O2− charge transfer luminescence of the activator were observed. As the Yb concentration increased the latter TL luminescence lessened much faster than the former and the sample with a Yb3+ concentration of 3% (nominally) showed TL emission mainly in IR only. To the best of our knowledge, the diluted LuPO4:Yb3+ is the first phosphor presenting quantum cutting in thermoluminescence – one image file: c9cp03169c-t7.tif center captures the thermally released hole and finally relaxes sending off up to two photons – one in the blue and one in the IR part of the spectrum. Precisely speaking, this statement applies to the bluish part of the CT TL emission component around 400 nm, as only then Yb3+ attains the excited 2F5/2 level and can additionally send off an IR photon.

Our results demonstrate that a deeper and more careful and critical investigation on the thermal quenching of the (YbCT3+)* → O2− charge transfer photoluminescence is needed and the results may be useful from both fundamental and practical research points of view. The previously published data collected mostly on samples with quite high Yb3+ concentrations are certainly not representative of the more diluted systems. This is not an academic problem only, as our data prove that the (YbCT3+)* → O2− charge transfer luminescence in orthophosphates may offer significant intensity well above room temperature. Consequently, it may be of practical interest for important areas of applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was partially supported by the Polish National Agency for Academic Exchange NAWA under the project #PPN/BEK/2018/1/00333/DEC/1 and by the Polish National Science Centre (NCN) under the grant number UMO-2015/19/N/ST5/00784.

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