Spectroscopic Properties of Persistent Luminescence Phosphors

Oct 28, 2009 - The TL glow curves―even if only qualitatively analyzed, as shall be done here―often deliver valuable information about systems exhi...
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J. Phys. Chem. C 2009, 113, 20493–20498

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Spectroscopic Properties of Persistent Luminescence Phosphors: Lu2O3:Tb3+,M2+ (M ) Ca, Sr, Ba) Joanna Trojan-Piegza,*,† Eugeniusz Zych,† Jorma Ho¨lsa¨,‡ and Janne Niittykoski‡ Faculty of Chemistry, Wrocław UniVersity, 14 F. Joliot-Curie Street, 50383 Wrocław, Poland, and Department of Chemistry, UniVersity of Turku, FI-20014 Turku, Finland ReceiVed: June 30, 2009; ReVised Manuscript ReceiVed: October 2, 2009

A new family of persistent luminescence phosphors, Lu2O3:Tb3+,M2+ (M ) Ca, Sr, Ba), was found and the materials’ most important spectroscopic properties related to the persistent luminescence phenomenon were reported and discussed. The persistent luminescence seen by the naked eye in the dark lasted for at least 10 h. The green persistent luminescence is exclusively due to emission from the Tb3+ ion. The thermoluminescence glow curves change with the codopant. For Ca2+ codoping, the main TL band is located around 100 °C. When Sr2+ is added, a similar TL band is accompanied with another structure around 240 °C, and for Ba2+ codoped material, the latter band becomes the most prominent feature in the TL glow curve. Analysis of the various findings leads to the conclusion that the persistent luminescence is governed by a temperature-assisted tunneling mechanism and the point defects get spatially linked during the materials processing into [Tb× LuVO••-2M'Lu] cluster entities. Introduction Persistent luminescence is an intriguing effect of sending off visible photons by a material for a long time after ceasing the excitation. For example, ZnS:Cu+ was found to emit light for tens of minutes after irradiation with sunlight.1-3 The Co2+ codoped version of the material emits for as long as about 10 h, which allows for its practical exploitation.4 Some other sulfides show a similar effect but with duration rarely exceeding 1 h.5,6 In the middle of the 1990s, it was discovered that SrAl2O4 (or CaAl2O4) doped with Eu2+ and codoped with Dy3+ (or Nd3+) shows very efficient persistent luminescence whose duration reached 20 h and even longer.7 The introduction of codopants, Dy3+ or Nd3+, greatly enhanced both the intensity and the duration of persistent luminescence from the aluminates.8-10 This discovery revived the research on the persistent luminescence phosphors and new projects were launched bringing new interesting materials on the scene, mostly various aluminates11-13 and silicates,14-21 with visible persistent luminescence exceeding 10 h.22 Tracking the detailed mechanism of persistent luminescence is often a complex and difficult work; it took some time to reach a common agreement on that subject for the aluminates.23-28 Yet, there is a general consensus that excited electrons and/or holes are entrapped at lattice defects (vacancies, color centers, dopant or codopant sites). From these sites, the charge carriers are freed thermally or by tunneling and further diffuse to the emitting centers at which they recombine radiatively.23,24 A thorough description of the various aspects of thermoluminescence is given in the classic book by McKeever.29 In a previous paper, energy storage in Lu2O3:Tb30 was reported together with its possible recovery by stimulation with infrared radiation or red light presumably leading to the green emission characteristic for the Tb3+ ion. Recently, a detailed * To whom correspondence should be addressed. E-mail: jtp@ eto.wchuwr.pl. † Wrocław University. ‡ University of Turku.

analysis of persistent luminescence and thermoluminescence of vacuum-sintered Lu2O3:Tb3+,Ca2+ ceramics was published, too.31 In the present article, it will be shown that codoping of the Tb3+ doped material with the divalent M2+ ions, Ca, Sr, or Ba, combined with sintering at the more reducing atmosphere of a N2-H2 mixture leads to phosphors showing yet more efficient persistent luminescence seen by the naked eye in the dark for 10-20 h, depending on the codopant. The Kro¨ger-Vink notation of defects32,33 was chosen to be used throughout the entire report. It seems to be a good standard allowing confusion to be avoided. Experimental Section The investigated materials, Lu2O3:Tb3+,M2+ (M ) Ca, Sr, Ba) were prepared in the form of ceramics sintered at 1700 °C for 5 h in the atmosphere of a N2-H2 mixture (9:1 in volume). The starting powders were prepared applying the combustion synthesis with glycine as the fuel as described earlier.34 In short, metal nitrates and glycine were dissolved in a small amount of water, which was evaporated at 80-120 °C afterward. The residuals left in a beaker were transferred into a furnace preheated up to 650 °C. A vigorous reaction occurred shortly and crystalline powder of Lu2O3:Tb,M was obtained with average sized crystallites in the range of 30-50 nm. Before sintering the powders were cold pressed under 9 tons load into pellets 10 mm in diameter. The concentration of the Tb3+ dopant was set at 0.05 mol % with respect to Lu and the content of the codopant M2+ (Ca, Sr, or Ba) was 1.5 mol %. To measure a real excitation spectrum of Tb luminescence (not influenced by the persistent emission) one sample with Tb content of 1 mol % was prepared by the same method. The purity of all reagents but BaCO3 (99%) was not lower than 99.99%. The 270 nm excited photoluminescence, 543 nm luminescence excitation, and persistent luminescence spectra (integration time was 5 s), as well as persistence decay kinetics, were recorded with a FLS 920 spectrometer from Edinburgh Instruments. All luminescent spectra were taken with about 1 nm

10.1021/jp906127k CCC: $40.75  2009 American Chemical Society Published on Web 10/28/2009

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Figure 2. Comparison of the 543 nm luminescence excitation spectrum of the Lu2O3:Tb 1% ceramics and the Earth-reaching sunlight spectrum.

Figure 1. Representative photoluminescence (PL) spectrum of the Lu2O3:Tb3+,Ca2+ and the persistent luminescence spectra of the Lu2O3: Tb3+,M2+ (M ) Ca, Sr, Ba) ceramics. The persistent luminescence was recorded 20 h after ceasing the irradiation of the materials with the UV radiation at 270 nm. The PL spectra of other materials do not differ from the one presented here. The spectra were normalized to the same height at 543 nm.

resolution and the excitation spectrum was taken with 0.25 nm resolution. Prior to the persistent luminescence measurements, the materials were irradiated with 270 nm UV radiation for 10 min, using the 450 W Xe lamp of the spectrometer. The persistent luminescence was measured starting 18 h after ceasing the irradiation. The decay traces were recorded omitting the first 3 min of the process, which allowed adjusting much better measurement parameters to keep good signal-to-noise ratio after a few hours of recording of the persistent emissions. The thermoluminescence (TL) glow curves were recorded with a Risø TL/OSL-DA-12 apparatus with a linear heating rate of 5 deg/min. About 40-80 mg samples of the materials were used for these experiments. The TL intensities were recalculated per mass unit (mg) afterward to compare the results in a quantitative manner. Yet, it seems that accuracy better than 20-25% should not be expected. Prior to the TL measurements, the materials were exposed to radiation from a combination of two UV lamps: Philips TL 20W/05 (maximum emission at 380 nm) and TL 20W/03 (420 nm). The sunlight spectrum was recorded with an Ocean Optics HR2000 CG spectrometer covering the range of 200-1100 nm. Results The codoped materials formed high-density pellets when sintered. The X-ray diffraction measurements, results of which are not presented here, confirmed that all materials crystallized in the cubic C-type structure, characteristic for Lu2O3.35 No unknown diffraction lines were detected, which assures that, down to the detection limit of this technique, the materials were crystallographically pure. The green part of the characteristic photoluminescence (PL) from the Lu2O3:Tb3+,Ca2+ material, which is perfectly representative for all other compositions, and persistent luminescence from all the codoped Lu2O3:Tb3+,M2+ (M ) Ca, Sr, Ba) ceramic materials are shown in Figure 1. All spectra were normalized to the same height of their peak intensity located at about 543

nm. Clearly, the persistent luminescence results from a radiative relaxation of the excited Tb3+ ion and there are observed only transitions from the 5D4 to the 7FJ states. Thus, the activator attracts the carriers, electrons, and/or holes, diffusing or tunneling from the trapping sites, hence overcoming the energy barrier between the dopant and the traps, and serves as the only center for their radiative recombination. This will be further treated in the Discussion section. It is worth noting that despite the very low Tb3+ concentration, there was no emission observed from the higher lying, 5D3 level of Tb3+ ion, not even in photoluminescence. Efficient multiphonon 5D3 f 5D4 relaxation is expected to be responsible for that as the alternative crossrelaxation route is not likely to be as effective due to a relatively high average separation of the Tb3+ ions. The persistent luminescence occurred only if the stimulating radiation was shorter than 330 nm, hence as is shown in Figure 2, upon direct excitation of the Tb3+ ion through its 4f f 5d absorption occurring in the range of 240-330 nm36 or by stimulation of the host material through its fundamental valenceto-conduction band absorption located at yet shorter wavelengths, below 240 nm.36-38 In this respect, the present materials differ from many other efficient persistent luminescence phosphors (MAl2O4:Eu2+,Dy3+ where M ) Ca, Sr, Ba; Ba2MgSi2O7: Eu2+,Tm3+; Sr2MgSi2O7:Eu2+, Dy3+) for which the long-lasting emission can be triggered with the Earth-reaching sunlight. As is clearly seen from Figure 2 such sunrays do not carry photons of energy high enough to overlap the f f d and/or HL absorption of Lu2O3:Tb. Consequently, no persistent emission is generated upon exposure of the Lu2O3:Tb3+,M2+ ceramics to daylight. This obviously raises the question about their possible usability. The typical, classic ones are obviously excluded.39,40 While this problem is not intended to be discussed here it can be noted that such characteristics could be beneficial considering their application as sensitive elements of fiber optic thermometers.41,42 Yen used to say that application of persistent phosphors is constrained mostly by the limited imagination of ourselves and this might be a good comment here. Yen and co-workers37,38,43 found that 5d levels of the Ce3+ ion are immersed in the conduction band of the Lu2O3 host, which precluded the luminescence of Lu2O3:Ce. This finding is very helpful in the analysis of processes occurring upon the f f d excitation of Tb3+ in lutetia. Namely, according to Dorenbos’ method of positioning of the 5d states of trivalent lanthanides against the valence and conduction bands of a host,44-47 the 5d levels of Tb3+ must also be localized within

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Figure 3. Picture of Lu2O3:Tb,Ba taken in the darkness with a digital camera 30 h after ceasing the material irradiation at 270 nm. The exposure time was 6 min, and the camera-sample distance was 30 cm.

the conduction band of the Lu2O3 host. Consequently, the 5d orbitals of Tb3+ are coupled with the lutetia host conduction band and the electron elevated to the 5d state of the dopant can easily escape it and become trapped afterward in a defect leaving a hole behind. This basically means that Tb3+ can become oxidized to Tb4+ by UV radiation shorter than about 330 nm. This paper will try to disclose, among other things, the possible trapping sites for the freed electrons and holes in Lu2O3:Tb,M, where they could be stored for hours at room temperature. The green persistent luminescence could be easily seen by the naked eye accustomed to darkness even 10-20 h after ceasing the irradiation of the specimen. The picture of the persistent luminescence of the Lu2O3:Tb3+,Ba2+ ceramic material (Figure 3), taken in complete darkness 30 h after ceasing the irradiation of the material, proves that even after such a long time the material still emits measurable green light. Taken into account the fact that the compositions of the materials were not fully optimized, the efficiency and duration of the persistent luminescence is impressive for all materials. The decay of the persistent luminescence was drawn in the log-log scale (Figure 4) instead of the more classical log(I) vs. time t dependence (not shown here). The latter could not give a reasonable linear fit for any of the materials investigated, not even by using a three-exponential term. This kind of behavior is rather typical for the persistent luminescence phosphors;their emission usually fades nonexponentially. However, the log(I) vs. log(t) curves for the Lu2O3:Tb3+,M2+ ceramics are very characteristic. For the Ca2+ codoped material, a linear dependence was found within practically the entire time range (18 h) investigated with the exception of the first hour after excitation. For the Sr2+ and Ba2+ containing materials, two regions of linearity were observed: one for the roughly first 2 h and the other one after that. For the Sr2+ codoped material, both parts have very similar slopes, while for the Ba2+ codoped specimen, the slopes of the two linear sections differ significantly from each other. These results will be further treated in the Discussion section. To elucidate further the processes responsible for the persistent luminescence of the Lu2O3:Tb3+,M2+ ceramics, their TL glow curves were recorded above room temperature, which is the primary application range. The TL glow curves;even if only qualitatively analyzed, as shall be done here;often deliver valuable information about systems exhibiting persistent luminescence. For the Ca2+ codoped material, the glow curve consists of an asymmetric band of significant intensity covering the temperature range roughly between 60 and 200 °C (Figure 5). The shape of this broadband suggests that it probably consists of three strongly overlapping components located around 100, 120, and 160 °C. Yet another, low-intensity TL band is visible around 240 °C.

Figure 4. Intensity vs. time logarithmic dependence of the persistent luminescence of the investigated ceramics of Lu2O3:Tb3+,M2+ (M ) Ca, Sr, Ba) recorded within 18 h after the irradiation of the materials with the UV radiation at 270 nm.

Figure 5. Comparison of the thermoluminescence glow curves obtained from the Lu2O3:Tb3+,M2+ (M ) Ca, Sr, Ba) ceramic materials sintered in a N2-H2 mixture after irradiation with UV radiation.

The codoping with Sr2+ instead of Ca2+ changes the TL glow curve significantly. Although there is still a band around 110 °C, its intensity is clearly lower compared to that of the Ca2+ codoped material. The component around 240 °C, with only insignificant intensity for the Ca2+ codoped specimen, is now the dominant one with intensity at least twice as high as that of the lower temperature band. For the Sr2+ codoped ceramic material yet another, high-temperature component around 380 °C appears, which feature also differentiates the Sr2+ codoped ceramics from the Ca2+ codoped counterpart. The trap responsible for this high-temperature TL band is certainly too deep to contribute to persistent luminescence at room temperature,

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however. Fortunately, the density of these traps is low enough not to jeopardize the efficiency of the persistent luminescence. The Ba2+ codoping further changes the observed TL glow curve. Although the low-temperature structured band with at least two components peaking around 90 and 140 °C is still observed, the majority of the TL intensity is comprised in the strongest band peaking around 240 °C. Also for this material, as for the Sr2+ containing one, a high-temperature weak TL component around 380 °C can be observed. Summarizing the findings in the TL glow curves, systematic variations can be noted when the codopant changes from the smallest Ca2+ through the intermediate Sr2+ to the largest of them, Ba2+. In the range of moderate temperatures, 20-300 °C, all glow curves consist of bands at quite similar temperatures. However, with increasing size of the codopant ion a progressively higher fraction of the total TL intensity is being comprised in the 240 °C component of the TL glow curve. Irrespective of the codopant, the TL glow curves prove that sets of traps of different depths and relative densities are present. While for the Ca2+ codoped material all the traps produce structured but basically only one TL band, for the Sr2+ or Ba2+ containing ceramics, the trap depths are such that the TL bands are significantly more spread. Yet, most of them overlap to some extent. Discussion The previous papers reported on the afterglow emission30 and thermoluminescence31 from Lu2O3 doped only with the Tb3+ ion. The singly activated material showed much shorter and less intense, ca. 3-5 times, persistent luminescence compared to any of the codoped ceramics reported on presently. The Tb3+ doped lutetia possesses a variety of traps giving rise to TL bands of comparable intensities located both in the range of low temperatures, below 300 °C, and at high temperatures, above 350 °C. This property made it neither a good persistent luminescent phosphor nor a good storage phosphor.29,48 Recently, it has been reported that Ca2+ codoping and vacuum sintering produced persistent luminescence ceramics characterized by virtually a single TL band around 100 °C.31 The intensity and duration of persistent luminescence from these ceramics were much less, roughly by a factor of 1.5-2, than from any of the materials described in the present report. This shows how important a role the strongly reducing atmosphere (N2-H2 mixture against vacuum) plays in obtaining efficient persistent luminescence from the Lu2O3:Tb3+,M2+ ceramics. The results presented in the current report clearly prove that codoping not only with Ca2+, but also with Sr2+ or Ba2+, simplified the traps structure leaving virtually only those giving rise to TL bands located below 300 °C. In other words, the divalent ions introduced as codopants to Lu2O3:Tb3+ increased the density of the shallow traps at the expense of the deeper ones converting the Tb3+ doped Lu2O3 into persistent instead of storage phosphor. The two conditions (Ca2+/Sr2+/Ba2+ codoping and a reducing atmosphere) to obtain efficient persistent luminescence from Lu2O3:Tb3+,M2+ are necessary but not sufficient, however. Another very important parameter is the temperature of the materials processing. This should not be lower than about 1700 °C. Materials prepared at 1550 °C showed much less profound persistent luminescence. The modification of the investigated materials into efficient persistent luminescence phosphors requires the following three conditions to be fulfilled simultaneously: (1) The materials have to be codoped with Ca2+, Sr2+, or Ba2+. (2) The processing of the materials has to be

Trojan-Piegza et al. carried out in a reducing atmosphere such as the N2-H2 mixture. (3) The ceramics should be processed at temperatures not lower than about 1700 °C. Adding the M2+ ions to the host lattice of Lu2O3 induces at ' , least two effects. First, the codopant forms a defect site, MLu with a negative net charge, and second this site requires a charge compensation. A few charge compensation schemes are possible. A straightforward one is a simultaneous creation of an oxygen vacancy, VO••, the net charge of which would be doubly positive and thus one such defect would be enough to counterbalance ' sites. It is important to the negative net charges of two MLu note that even processing in an oxidizing atmosphere is not able to convert any significant part of Tb3+ to Tb4+, since the air treated materials show regular green luminescence of significant intensity characteristic for Tb3+. From this analysis, it is reasonable to deduce that the dopant enters the host lattice as a × defect sites, and it is rather resistant Tb3+ ion, giving the TbLu • ). Hence, it seems that to permanent oxidation to Tb4+ (TbLu ' site by the positively balancing the negative charge of the MLu charged Tb•Lu (stable Tb4+) is of little probability and may occur only occasionally. Consequently, it can be anticipated that the most probable charge compensation of the positively charged ' sites occurs through the creation of the oxygen vacancy, MLu VO••, sites. The prerequisite to fabricate the materials in the strongly reducing environment of the N2-H2 mixture to obtain really efficient and long-lasting persistent luminescence, see the condition 2, makes it even more probable that V••O sites are indeed created. After all, the reducing atmosphere of synthesis can only be considered favorable to enhance their density, while it certainly does not facilitate formation of oxidized Tb4+, to say the least. Yet, temporal creation of Tb4+ after photoionization with UV photon seems to occur indeed. Having dealt with the assumption that there are two main types of point defects present in the ceramics containing the M2+ ions and processed in a reducing atmosphere, one should ' and VO••, are also raise the question whether these defects, MLu spatially linked or rather only statistically distributed in the host. In fact, this question should be made more general and the × sites should also be included in the location of the TbLu consideration. To reasonably analyze this topic, one should turn the attention to condition 3 of converting the materials into efficient persistent luminescence phosphors. A very high temperature of 1700 °C is necessary to attain this goal. Without fulfilling this condition the persistent luminescence is only weak in intensity and short in time, although efficient photoluminescence can be obtained from the ceramics processed at much lower temperatures, even at 1100-1200 °C. There must clearly occur some changes in the materials when the temperature is raised to 1700 °C, changes which are not capable of taking place at lower temperatures. Evidently, the higher temperatures enhance the mobility of various entities making up or just present in the host lattice. As the temperature gets higher, the lattice vibrations are boosted and diffusion of ions and defects become more and more easy and probable. Hence, at higher temperatures the system attains the possibility to reduce its overall energy by rearranging/repositioning some of its constituents. This especially refers to the diffusion of point defects, which could easily lead to their aggregation and clustering. As was mentioned before, three main point defects are expected to exist in the materials taking into account their × , VO••, and MLu ' . composition and processing parameters: TbLu ' defect sites is expected to Further, the population of the MLu roughly double the population of VO••, as the latter are supposed to compensate for the net charges of the former. Tb3+ and all

Persistent Luminescence Phosphors: Lu2O3:Tb3+,M2+ three M2+ ions are larger than Lu3+ for which they are thought to substitute in the lutetia host lattice.49 Consequently, these dopants must inevitably deform their neighborhood in the Lu2O3 lattice by pushing aside the surrounding atoms to some extent. In contrast, the V••O defect site is simply an empty space originally × occupied by an oxygen. Since TbLu and MLu ' need more space than formally offered by the Lu2O3 host and since the VO•• sites offer an empty space a combination of these three entities into one complex species would reduce the strain in the host and consequently would lower the overall lattice energy. In view × of this reasoning, it can be postulated that the [TbLu -VO••-2MLu ' ] defect clusters are created from the three point defects in the Lu2O3:Tb3+,M2+ codoped materials upon heating at 1700 °C in the reducing atmosphere. This conclusion based on purely sterical constraints gets support from electrostatic considerations, × as formation of the [TbLu -VO••-2MLu ' ] defect clusters also has to be driven by mutual electrostatic attraction of the charged point defects. Since the concentration of Tb3+ is much lower than ' ] that of M2+, also the formation of the less complex [VO••-2MLu clusters can be anticipated. However, as will become clearer × later on, the existence of these, TbLu -uncoupled, does not seem to be directly related to persistent luminescence. The postulated clustering of point defects means also that the carrier traps are spatially linked and not very distant. It is known that the tunneling efficiency decreases exponentially with increasing distance between the entities involved,29 and for longer distances tunneling becomes insignificant. The present observations may be taken as the first indications that tunneling can be a mechanism of persistent luminescence in the phosphors dealt with in the present report. When comparing the efficiency of persistent luminescence of materials sintered either in vacuum31 or in a N2-H2 mixture, it was undeniably found that the latter performed much more effectively, by a factor of 1.5-2 taking the initial intensity. The heating in the strongly reducing atmosphere of a N2-H2 mixture should virtually produce even more oxygen vacancies (V••O) when compared to the similar processing under vacuum. Consequently, the density of the VO•• defects appears to be the critical factor for the persistent luminescence phenomenon to appear in Lu2O3:Tb3+,M2+ ceramics. This, in turn, implies that these are the oxygen vacancies which trap electrons escaping from Tb3+ ions after excitation by UV photons (shorter than 330 nm) to 5d states, as was discussed earlier. This conclusion, when × combined with the postulated creation of the [TbLu -VO••-2MLu ' ] defect clusters, entails that the beneficial influence of the addition of M2+ is related to promoting both the clustering of point defects and the formation of the oxygen vacancy (VO••) sites serving as a reservoir for electrons escaping excited Tb3+ ions. One should not forget the modification of the trap energies by the M2+ codoping, either. After losing an electron, the Tb3+ ion becomes oxidized to Tb4+. It is not clear at present if the hole remains trapped by the dopant giving a positively charged TbLu ′ site, or if it goes further to valence band to become trapped in the MLu ' site × subsequently turning it into MLu . EPR spectra recorded before and after irradiation with UV photons did not give any definitive, clear answer concerning this problem; no clear signal could be observed. EPR spectroscopy should easily see free electrons, free holes, as well as Tb4+.50,51 Lack of an EPR signal may be taken as an indication that the trapped carriers are not that free in the Lu2O3:Tb3+,M2+ persistent phosphors and also that Tb4+ is not the final stage of hole trapping. Yet, lack of EPR signal characteristic for free electrons and/or holes supports supposition

J. Phys. Chem. C, Vol. 113, No. 47, 2009 20497 that both carriers are indeed trapped in close proximity, which × is to say, in the [TbLu -VO••-2MLu ' ] defect cluster. The analysis of the decay kinetics (Figure 4) can lead to interesting conclusions too. The kinetics of the persistent luminescence depends on the mechanism. Consequently, analysis of the decay processes may give some insight into the mechanism. In very general terms, persistent luminescence may involve two different routes as far as the trap bleaching is concerned. The charge carriers (electron or hole) may reach the recombination and luminescence center (Tb3+) either being thermally freed to the conduction or the valence bands, respectively, or by means of tunneling to the center from the trapping sites. The first of the two paths is obviously a temperature-dependent one as the release of the carriers to the appropriate bands needs their thermal excitation to overcome the potential barrier. In contrast, the tunneling mechanism is in principle a temperatureindependent process. It is, however, also possible that between a trapped carrier and its destination;the recombination (and luminescence) center;there exists a potential barrier to be overcome with thermal stimulation. In such a case, a temperature-assisted tunneling may occur.33 Persistent luminescence in each of the investigated materials is temperature dependent, as proved by the TL glow curves. The decay kinetics are characterized by the linear log(I) vs. log(t) dependence, which cannot be accidental especially for such long time ranges. Each linear part of the decay curve can be well reproduced with the following equation:52,53

I ) I0(1 + γt)-n

(1)

which basically means that the intensity of persistent luminescence follows the I ) f(t-n) relationship. For the Tb3+,Ba2+ codoped material two time regions with strongly different slopes (n equal to 1.5 during the first 2 h and 0.54 afterward) are visible. For the Tb3+,Ca2+ and Tb3+,Sr2+ codoped materials, the slopes are very close to unity within the whole measurement time. In literature, the n value close to unity was considered to be a proof that persistent luminescence is governed by the tunneling mechanism.54-56 Yet, it has recently been showed that such a mechanism may generate the n values in the range of 0.95-1.5.57 This is the case of materials reported here with the exception of the Tb3+,Ba2+ codoped material for which the persistent luminescence decays with the log-log slope giving n equal to 0.54 after the first 2 h. Such a low value of n was reported and discussed in only a few papers, mostly published in recent years.56-59 The literature data available do not give any definitive solution as to the reasons of n reaching such low values. However, especially in the very recent paper by Yamaga et al.,59 it is concluded that n much lower than 1 indicates that the trapped hole tunnels to the recombination center by hopping, which requires significant thermal stimulation to overcome the potential barrier between the carrier trap and the destination. This is in qualitative agreement with the TL glow curves (Figure 5), which shows that this is the Ba2+ codoping that produces the most significant TL band at relatively high temperatures, around 240 °C. When Ca2+ is used as the codopant, the main TL band drops to about 100 °C with a tail extending close to room temperature. On the other hand, the n value around 0.5 has also been interpreted to result from two sets of traps,57,58 yet with tunneling as the main mechanism still valid. Conclusions In this report, it was shown that the Tb3+,M2+ codoping (M ) Ca, Sr, Ba) of the Lu2O3 ceramics, combined with heat

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treatment at temperatures around 1700 °C in a reducing atmosphere, yields materials with surprisingly long-lasting and efficient persistent luminescence. The persistent luminescence could be observed even for 10-20 h after ceasing the irradiation of the materials with UV photons (λ e 330 nm). It is postulated that the individual trapping sites cluster at the high temperature •• of materials manufacture into the [Tb× Lu-VO-2M'Lu] entities, with the O-vacancy, VO••, serving as an electron trap. The decay kinetics suggested that (temperature-assisted) tunneling is the main mechanism of persistent luminescence in the Tb3+,M2+ codoped Lu2O3 ceramics. The existence of the postulated defect clusters then would be beneficial for both the efficiency and duration of persistent luminescence as tunneling effectiveness, whether temperature assisted or temperature independent, decreases exponentially with distance, and for larger separation between the trapped carrier and the potential recombination site it becomes inefficient. Acknowledgment. Financial support from the Minister of Science and Higher Education under grant N205 024 31/1207 is gratefully acknowledged by E.G. and J.T.-P. The research mobility agreements (112812/2006 and 209438/2006) between the Academy of Finland and the Polish Academy of Sciences are acknowledged by J.H. References and Notes (1) Garlick, G. F. J. Luminescent Materials; Oxford University Press: London, UK, 1949. (2) Shionoya, S. Luminescence of Inorganic Solids; Academic Press: London, UK, 1966. (3) Kinoshita, T.; Yamazaki, M.; Kawazoe, H.; Hosono, H. J. Appl. Phys. 1999, 86, 3729. (4) Uheda, K.; Maruyama, T.; Takizawa, H.; Endo, T. J. Alloys Compd. 1997, 262-263, 60. (5) Murayama, Y. Phosphor Handbook; CRC Press: Boca Raton, FL, 1999. (6) Jia, D.; Yen, W. M. J. Lumin. 2003, 101, 115. (7) Matsuzawa, T.; Aoki, Y.; Takeuchi, N.; Murayama, Y. J. Electrochem. Soc. 1996, 143, 2670. (8) Ho¨lsa¨, J.; Jungner, H.; Lastusaari, M.; Niittykoski, J. J. Alloys Compd. 2001, 323-324, 326. (9) Aitasalo, T.; Deren´, P.; Ho¨lsa¨, J.; Jungner, H.; Krupa, J.-C.; Lastusaari, M.; Legendziewicz, J.; Niittykoski, J.; Streˆk, W. J. Solid State Chem. 2003, 171, 114. (10) Jia, W.; Yuan, H.; Lu, L.; Liu, H.; Yen, W. M. J. Lumin. 1998, 76-77, 424. (11) Lin, Y.; Tang, Z.; Zhang, Z.; Nan, C. J. Eur. Ceram. Soc. 2003, 23, 175. (12) Zhao, C.; Chen, D. Mater. Lett. 2007, 61, 3673. (13) Chang, C.; Mao, D.; Shen, J.; Feng, C. J. Alloys Compd. 2003, 348, 224. (14) Liu, B.; Shi, C.; Yin, M.; Dong, L.; Xiao, Z. J. Alloys Compd. 2005, 387, 65. (15) Lin, Y.; Zhang, Z.; Tang, Z.; Wang, X.; Zhang, J.; Zheng, Z. J. Eur. Ceram. Soc. 2001, 21, 683. (16) Lin, Y.; Tang, Z.; Zhang, Z.; Zhang, J.; Chen, Q. Mater. Sci. Eng., B 2001, 86, 79. (17) Lin, Y.; Nan, C.-W.; Zhou, X.; Wu, J.; Wang, H.; Chen, D.; Xu, S. Mater. Chem. Phys. 2003, 82, 860. (18) Liu, B.; Shi, C.; Qi, Z. Appl. Phys. Lett. 2005, 86, 191111.

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