Modeling Luminescent Properties of HfO2:Eu Powders with Li, Ta, Nb

Article pubs.acs.org/JPCC

Modeling Luminescent Properties of HfO2:Eu Powders with Li, Ta, Nb, and V Codopants Aneta Wiatrowska† and Eugeniusz Zych*,†,‡ †

Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie Street, 50-383 Wroclaw, Poland Wroclaw Research Centre EIT+, 147/149 Stablowicka Street, 54-066 Wroclaw, Poland

‡

ABSTRACT: Monoclinic HfO2:Eu,M (M = Li, Ta, Nb, V) powders were prepared with the Pechini method. The M-dopants were added for charge compensation of Eu3+ positioned in Hf(IV) site. This was hoped to enhance the host-to-activator energy transfer and consequently the overall efficiency of X-ray excited luminescence. Only in the case of codoping with Nb, the final phosphor did not contained any foreign phases indicating that (Eu3+,Nb(V)) pairs easily dissolved in the host. In the HfO2:Eu,V composition, the phase purity was unsatisfying for higher concentrations. However, in the case of HfO2:Eu,Ta and HfO2:Eu,Li a foreign phase/phases could be easily detected even for quite low concentrations. The addition of Ta, Nb, or V allowed to create additional absorption/excitation bands located in UV around 225− 250 nm, 240−300 nm, and 240−320 nm, respectively. However, only in the presence of Nb the red Eu3+ photoluminescence could be efficiently excited and strongly enhanced upon stimulation into this extrinsic excitation structure compared to other investigated compositions as well as to singly doped HfO2:Eu reported previously. While XRDs showed that HfO2:Eu,Li contained the highest fraction of a foreign phase, this composition was found to produce the most efficient red emission upon irradiation with X-rays. It reached about 30% of the commercial Gd2O2S:Eu powder performance. This was achieved despite that a significant portion (roughly additional 30−35%) of the HfO2:Eu,Li radioluminescence was contained in a broad band covering the 400−600 nm range of wavelengths. Unfortunately, all the other codopants, Ta, Nb, and V, only suppressed the X-ray excited emission yield. Especially surprising was that the phase pure HfO2Eu,Nb powders, that presented the most efficient, truly bright photoluminescence, performed the least upon X-rays.

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INTRODUCTION With one of the highest densities among all compounds (∼9.7 g/cm3), high effective atomic number (Zeff = 67.2), and transparency over the whole visible and almost all UV part of electromagnetic spectru,m monoclinic hafnia, HfO2, is an extremely attractive host for modern X-ray phosphors and scintillators.1,2 It ensures an efficient absorption of X-rays and gamma particles and warrants a high photofraction (an appreciated property in many applications), an ability to absorb a whole energy of the incoming ionizing particle in a single position rather instead of multiple distant locations (due to Compton scattering).3,4 The literature on hafnia-based X-ray phosphors and scintillators is still rather scarce. Yet, the published results on spectroscopic properties of Eu-activated monoclinic HfO2, including excitation with synchrotron radiation over its energy gap, showed that the host was able to transfer the acquired energy to Eu3+ ions. Hence, the activator could get excited through host simulation, and the characteristic red luminescence of Eu3+ could be then produced.5−10 The early reports on the performance of HfO2:Eu under stimulation with X-rays were also quite optimistic though no quantitative results were showed concerning the light yield.6,7 Our recent measurements showed that monoclinc HfO2:Eu produced XEL whose © 2012 American Chemical Society

intensity was only about 20% of the performance of commercial Gd2O2S:Eu (GOS) phosphor.5 Exploring the feasibility of enhancing the XEL efficacy of HfO2:Eu, we selected a few codopants and examined their influence on the light yield and more generally on spectroscopic properties of this phosphor. As the second intentionally introduced impurity, Li+, Ta(V), Nb(V), and V(V), were chosen since they could serve as charge-compensating constituents of the system. The main dopant, Eu3+, generates a negatively charged site when it replaces Hf(IV). Hence, a defect is needed to ensure electrostatic neutrality of the system. We suspected that the uncontrolled defects in the singly activated HfO2:Eu might be mainly responsible for inefficient energy transfer from the excited host to the Eu3+ dopant and thus precluded its efficient scintillation. Li+ as the codopant was hoped to be able to locate in an interstitial position nearby Eu3+ and thus ensure electrostatic balance in the host. In contrast, Ta(V), Nb(V), and V(V) (hereafter denoted M(V)) were expected to replace Hf(IV), in close proximity of Eu3+ ion. Such a pair defect (M(V)−Eu3+) would need no extra charge compensation, hence no extra, hard to control defect(s) should be created Received: December 2, 2011 Revised: February 10, 2012 Published: February 21, 2012 6409

dx.doi.org/10.1021/jp211598m | J. Phys. Chem. C 2012, 116, 6409−6419

The Journal of Physical Chemistry C

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Figure 1. Variation of the X-ray powder diffraction patterns of (a) undoped HfO2 and HfO2:Eu,Li with different content of the dopants heattreatment at 1500 °C, (b) HfO2:5%Eu,5%Li annealed at different temperatures for 5 h, and (c) coactivated HfO2:5%Eu,5%M prepared at 1500 °C. Simulated XRD patterns of cubic, monoclinic, and tetragonal HfO2 and cubic Eu2O3 according to ICSD#53033, ICSD#27313, ICSD#173966, and ICSD#27997, respectively (d).

contrary to singly doped material. Furthermore, M(V) ought to introduce additional O2− → M(V) charge transfer energy states positioned below the band gap of the host. We believed that such a state might be able to mediate (e.g., facilitate) the transfer of energy from the excited monoclinic hafnia host to the Eu3+ luminescent center. This article reports on the findings of the research.

were recorded with the use of a Hitachi S-3400N equipped with an energy dispersive X-ray spectroscopy (EDX) EDAX analyzer. Photoluminescence (PL), photoluminescence excitation (PLE) spectra, and decay kinetics were recorded at room temperature with an FSL 920 Spectrofluorometer from Edinburg Instruments using a 450 W xenon lamp (PL and PLE) and a nitrogen flush lamp (decay kinetics) as an excitation sources. Emission and excitation spectra were also recorded at room temperature (RT) and 10 K using synchrotron radiation at the Superlumi station of Hasylab (Desy, Hamburg, Germany). The excitation spectra were corrected for the incident radiation intensity using sodium salicylate. Emission spectra were corrected for the recording system characteristics. X-ray excited luminescence spectra (XEL) were recorded at room temperature with Ocean Optics HR2000 CG spectrometer, whose resolution was about 1.2 nm. Spectra Suite dedicated software was used for that. White X-rays of a Cu lamp of DRON-1 diffractometer were used for excitation. For the calculations of efficiencies of the XEL of the various materials, Gd2O2S:Eu (GOS) commercial powder (UKL63/FR1) kindly donated by Gerry Sorce from Phosphor Technology Ltd. was used as a benchmark phosphor.

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EXPERIMENTAL SECTION Synthesis. Monoclinic powders of HfO2:Eu,M (M = Li+, Ta(V), Nb(V), V(V)) were prepared by the Pechini-based sol−gel process.11 The concentrations of Eu3+ were 0.5, 3, and 5 mol %. Concentrations of codopants were always the same as the activator. Starting materials were HfCl4 (Stanford Materials, 99.9%), 2 M C6H8O7 (prepared from citric acid monohydrate pure from POCH S.A.), C2H4(OH)2 (ethylene glycol, Chempur), Eu(NO3)3·6H2O (Aldrich, 99.99%), and NH4VO3 (Acros Organics, 99.5%), NbCl5 (ABCR GmbH&Co.KG, 99.99%), TaCl5 (ABCR GmbH&Co.KG, 99.99%), or LiCl (Lancaster, 99%). Stoichiometric amounts of HfCl4, Eu(NO3)3·6H2O, and LiCl, NH4VO3, TaCl5, or NbCl5 were dissolved in a 2 M water solution of citric acid. Next, this mixture was combined with ethylene glycol to achieve the molar ratio of metals/citric acid/glycol being 1:1:1, and the whole mixture was stirred for a few hours at 80 °C for condensation. Then, the temperature was slowly raised up to 600 °C, which led first to polymerization of the organics and finally the resin burned off leaving white powders. Such raw materials were air-heated at various temperatures in the range of 600−1750 °C for 5 h. Measurements. The powder X-ray diffraction (XRD) analyses were performed with D8 Advance X-ray Diffractometer from Bruker in the range of 2θ = 10−120° with the step of 2θ = 0.00857°; the counting time was 0.2 s. Nickel-filtered Cu Kα1 radiation (λ = 1.540596 Å) was used. Transmission electron microscopy (TEM) images were recorded with a FEI Tecnai G2 20 X-TWIN Transmission Electron Microscope working under the voltage of 200 kV. The samples were prepared by dispersing a small amount of a powder in methyl alcohol, and a droplet of the suspension was transferred onto a copper grid plate. Scanning electron microscopy (SEM) images

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RESULTS Structural Analysis. Figure 1 shows XRDs for HfO2:Eu,Li with different content of the dopants prepared at 1500 °C (Figure 1a), HfO2:5%Eu,5%Li prepared at different temperatures (Figure 1b), HfO2:5%Eu,5%M (M = Li, Ta, Nb, V) prepared at 1500 °C (Figure 1c), and a set of simulated patterns for cubic, monoclinic, and tetragonal HfO2 as well as cubic Eu2O3 (Figure 1d) for comparison. Not all of the synthesized powders were found to be single phase materials. Nevertheless, monoclinic hafnia, space group P21/c, always dominated the compositions. In the case of HfO2:Eu,Li, (analogously to HfO2:Eu),5 some foreign phase represented by diffraction line around 2θ = 30° appeared for 3% and 5% specimens, and this effect was slightly more profound for materials prepared at higher temperatures. At lower contents of the dopants only monoclinic phase was seen by means of XRDs. One can, however, suppose that even then a phase 6410



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Morphology Evolution with Preparation Temperature and Codopants. The morphology variations of HfO2:Eu with the synthesis temperature and type of codopant (Li, Ta, Nb, and V) were revealed by TEM and SEM images and are presented in Figures 2 and 3, respectively. For TEM images, the

separation may take place but is beyond this technique’s detection limit. The extra lines in concentrated systems were probably related to cubic hafnia (though tetragonal cannot be excluded), and this is quite typical behavior of HfO2 upon doping with triply ionized ions at higher concentrations.5,12 Clearly, the relatively large Eu3+ (Table 1), either by itself or in Table 1. Ionic Radii of Hf(IV), Eu3+, Li+, Ta(V), Nb(V), and V(V) Ions for Coordination Number CN = 613 metal

ionic radius (Å)

Hf(IV) Eu3+ Li+ Ta(V) Nb(V) V(V)

0.71 0.974 0.76 0.64 0.64 0.54

the presence of Li+ (which is not at all small compared to Hf(IV)) is not able to replace Hf(IV) without rather profound rearrangement of the constituting atoms around it and consequently without modification of the host structure partly. When V(V) or Nb(V) were used as codopants, virtually no foreign phase was seen; although in the case of the former, a detailed analysis revealed a trace of cubic HfV2O7, probably. An analogous observation was reported for HfO2:V.14 The addition of Ta(V) as the codopant ended up with some admixture of (presumably) cubic hafnia. Yet, from relative intensities of the diffraction lines, it could be concluded that the content of the foreign phase was considerably lower than in the case of HfO2:Eu5 or HfO2:Eu,Li powders. Hence, as we supposed the two aliovalent impurities, M(V) and Eu3+ applied simultaneously, indeed facilitated formation of products of better crystallographic purity. Besides the electrostatic balancing of the host, also sizes of the codopants favor their incorporation into HfO2. While Eu3+ is larger and all M(V) are smaller than Hf(IV) of which they replace, see Table 1, together they are easier to be tolerated by the host. Altogether, the XRDs revealed a very encouraging change in the materials when codoped with Nb and, to a lesser degree, also with Ta or V. These codopants hindered a separation of a foreign phase in the formed powders. This should facilitate a uniform distribution of the activator ions in the prepared phosphors, reduced the variety of defects and thus allow for a much better control over their properties.

Figure 3. SEM images of HfO2:Eu,V (a), HfO2:Eu,Nb (b), HfO2:Eu,Ta (c), and HfO2:Eu,Li (d) powders prepared at 1500 °C in air. The dopant concentrations were 0.5%.

materials were prepared at 600 °C (Figures 2a−d) and 1500 °C (Figures 2e−h). TEMs of powders prepared at 600 °C disclosed quite significant agglomeration of the crystallites in all cases but to the greatest extent in the Ta and Li codoped compositions. The Eu,V codoped material showed the lowermost (though still well seen) agglomeration, however. The sizes of crystallites were about 15 nm. After a heattreatment at 1500 °C, the crystallites grew significantly but to a degree depending strongly on the codopant. Interestingly, in the presence of the V-impurity (Figure 2e), the high temperature treatment gave powder without agglomeration, and the crystallites were quite uniform in size (∼1−2 μm) and shape. This was confirmed by SEM images (Figure 3a). Agglomeration was also rather minor when Li or Ta codopants were used. Yet, in these cases, the sizes of crystallites were smaller, 0.5−1 μm, and their shapes clearly less regular (Figures 2g,h and 3c,d). Nb-impurity led to crystallites with pretty much

Figure 2. TEM images of HfO2:Eu powders prepared at 600 °C (a−d) and 1500 °C (e−h) and codoped with V(V) (a,e), Nb(V) (b,f), Ta(V) (c,g), or Li+ (d,h). The dopant concentrations were 0.5%. 6411



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Figure 4. Excitation spectra of 480 and 614 nm emissions for HfO2:0.5%Eu,0.5%Li powders prepared at 1500 °C recorded at 300 K (a) and 10 K (b); emission spectra of HfO2:0.5%Eu,0.5%Li taken upon different excitation wavelengths recorded at 300 K (c) and 10 K (d). Panels a and d, terminal levels of the excitation transitions from the 7F0 and emission from the 5D0 state, respectively, are given.

irregular shapes and sizes varying in the range of 0.5−5 μm. This codopant also caused the most significant agglomeration (Figures 2f and 3b). EDX mapping of the constituent elements did not expose any irregularities in their distribution within the grains of the various powders (the data are not presented here). Also, elemental analysis with the EDX technique confirmed uniform distribution of the dopants. Photoluminescence of HfO2:0.5%Eu,0.5%Li. Figure 4a,b presents excitation spectra of the 480 and 614 nm emissions recorded at 300 and 10 K, respectively, for HfO2:0.5%Eu,0.5%Li powders prepared at 1500 °C in air. Luminescence spectra of this composition also recorded at 300 and 10 K are presented in Figure 4c,d. At both temperatures, the excitation spectra of the 480 nm emission consist of a broad high intensity band with the maximum at 280 nm and a few components at wavelengths shorter than about 230 nm. The later transitions fall within energies associated with valence-toconduction bands transitions of the host material.15 At 10 K, a characteristic doublet appears around 200−220 nm, hence in the vicinity of the band gap energy. This may be assigned to formation of free exciton (FE) (∼213 nm) and free carriers (below ∼208 nm). These observations imply that the broadband luminescence peaking around 480 nm may be connected with a defect present in the material with its own absorption around 280 nm and able to intercept the energy of free exciton and free carriers as well. It cannot be excluded, however, that the broad-band emission around 480 nm results from Tiimpurity, usually present in Hf-compounds.16 Excitation spectra of the 614 nm Eu3+ luminescence consist of narrow lines located at longer wavelengths and characteristic for f → f transitions of Eu3+ ion and a broad band located in UV and peaking at 280 nm, which might be assigned to O2− → Eu3+ charge transfer state. The latter coincides with the position of the UV excitation band of the 480 nm luminescence. At first, this might be seen as an indication of energy transfer from the

defect/Ti to Eu3+. Yet, experiments with powders containing higher concentrations of the dopants, among them decay kinetics measurements presented below, seem to deny it. Hence, positions of both bands seem to coincide only. This, in turn, means that both emitting centers in fact compete for the excitation energy, and therefore, the CT band in the excitation spectra of Eu3+ luminescence has unexpectedly low intensity compared to f → f transitions. In the 10 K excitation spectrum of the Eu3+ luminescence (Figure 4b), there is again a doublet seen around 210−215 nm, as in the case of the excitation of the 480 nm emission, already discussed. Hence, the energy of free excitons as well as free carriers is able to diffuse to the activator and excite it. However, at energies significantly exceeding the band gap, when the carriers become more mobile, the Eu luminescence is not effectively excited, especially at 10 K. At 300 K, the situation looks better. At energies greater than the band gap in the excitation spectrum, a reliable signal is seen. Yet, the real efficacy of the host-to-activator energy transfer will be verified by measurements of XEL efficiencies. These data will be presented at later stages of the article. Excitation into the broad structures at energies lower than the band gap (roughly in the range of 230−330 nm) produces mostly a broad intense luminescence covering the region of about 400−600 nm and peaking at 480 nm (Figure 4c,d). At 10 K, some red shift of the broad emission is observed. It is noteworthy that, upon stimulation into 250−300 nm bands, the luminescence appears white, as the superposition of the defect/ Ti and Eu3+ emissions cover the whole visible part of electromagnetic spectrum. The broad band luminescence was also observed in undoped hafnia.2,5 Hence, it should not be linked to the Eu activator or Li codopant. This luminescence was also observed in monoclinic HfO2:Eu/Ce and was considered to result from Ce3+ and/or Eu2+ ions.7 Since undoped hafnia gives analogous emission, this interpretation does not seem to be accurate, however. 6412



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Figure 5. Excitation spectra of 480 and 614 nm emissions of HfO2:0.5%Eu,0.5%Ta powders prepared at 1500 °C recorded at 300 K (a) and 10 K (b); emission spectra of HfO2:0.5%Eu,0.5%Ta taken upon different excitation wavelengths recorded at 300 K (c) and 10 K (d).

Figure 6. Excitation spectra of 614 nm emission for HfO2:0.5%Eu,0.5%Nb powder prepared at 1500 °C recorded at 300 K (a) and 10 K (b); emission spectra of HfO2:0.5%Eu,0.5%Nb taken upon different excitation wavelengths recorded at 300 K (c) and 10 K (d).

Eu3+ emission intensity appears quite insignificant upon excitation in the 230−330 nm range of wavelengths. However, stimulation of the material into its host fundamental absorption, at wavelengths shorter than ∼213 nm, produces mostly Eu3+ luminescence connected with the 5D0−7F0,1,2,3,4 transitions, and the broad emission intensity occurs much lower. Not much difference between 10 and 300 K emission spectra are seen. Yet, the broad luminescent structure is still

clearly present comprising at least about 25% of the total efficiency. All (Li,Eu) compositions exhibited pretty strong afterglow whose intensity was the most profound when the dopant contents were 0.5% and powders were heat-treated at 1500 °C. When the concentrations were higher or preparation temperature lower, the effect was less powerful and shorter in time. Nevertheless, it could not be totally diminished for any of the 6413



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Figure 7. Excitation spectra of 480 and 614 nm emissions for HfO2:0.5%Eu,0.5%V powders prepared at 1500 °C recorded at 300 K (a) and 10 K (b); emission spectra of HfO2:0.5%Eu,0.5%V taken upon different excitation wavelengths recorded at 300 K (c) and 10 K (d).

However, (Ta,Eu) codoped powders, like the (Li,Eu) ones, were noted to exhibit a significant afterglow, lasting even one hour after stopping the irradiation, as seen by the eye. While the afterglow got reduced at higher dopant concentrations, it was progressively stronger with increasing preparation temperature. Photoluminescence of HfO2:0.5%Eu,0.5%Nb. The excitation spectra of 614 nm of HfO2:0.5%Eu,0.5%Nb powder recorded at 300 and 10 K are presented in Figure 6a,b, respectively. In Figure 6c,d, emission spectra upon different excitation wavelengths at 300 and 10 K are given. The first great difference between (Nb,Eu) codoped powders compared to those already discussed, (Ta,Eu) and (Li,Eu) codoped ones, is that in the presence of Nb, the broad band emission around 480 nm was apparently absent at 300 K. Only at 10 K its residue could yet be seen. Compared to the (Ta,Eu) and (Li,Eu) codoped materials, now a yet broader and stronger excitation band appears in the range of 230−330 nm. This structure is so wide and intense that it extends toward and partially overlaps with the lower energy part of the host lattice excitation. Evidently, the efficient broad excitation UV band of the red Eu3+ luminescence has to be connected with the presence of Nb and its O2− → Nb(V) CT state. It has to be concluded from excitation spectrum of Eu3+ luminescence that incorporation of Nb is beneficial for the efficacy of the UV excited photoluminescence of HfO2:Eu. Indeed, while no quantitative measurements of the UV excited emission efficiencies were performed, the luminescence of the (Eu,Nb) codoped specimens was definitely the most significant among all the investigated compositions. Both at 10 and 300 K, the red emission of Eu3+ could be powerfully excited over about the 200−330 nm range of wavelengths. Hence, it can be concluded that the energy from the O2− → Nb(V) CT state is successfully transferred into the Eu3+ emitting center. This was one of the necessary, though not sufficient conditions for enhancing the efficiency of X-ray stimulated luminescence, as mentioned in

HfO2:Eu,Li powders. Also, undoped monoclinic HfO2 was reported to produce significant afterglow.5 Photoluminescence of HfO2:0.5%Eu,0.5%Ta. Figure 5a,b shows excitation spectra of 614 and 480 nm emissions of HfO2:0.5%Eu,0.5%Ta phosphor recorded at 300 and 10 K, respectively. Emission spectra at 10 and 300 K are presented in Figure 5c,d. Compared to the excitation spectra of the (Eu,Li) codoped material as well as Eu singly activated powders,5 one noticeable change is evidently seen. Namely, a strong broad band around 225−270 nm, hence in the nearest vicinity of the band gap, is present in excitation spectra of both the 614 and 480 nm emissions. Appearance of this structure is obviously a direct result of the Ta addition. Thus, it may be interpreted as resulting from the O2− → Ta(V) charge transfer transition.17 At shorter wavelengths, the FE and host lattice (HL) related structures are again well seen even at 300 K, although the partial superposition with the O2− → Ta(V) charge transfer band makes them appear less resolved. Independently on the excitation wavelength, the emission spectra are dominated by Eu3+ luminescence (Figure 5c,d), which is also different from the (Li,Eu) composition. This is another effect of Ta addition. Irradiation into the O2− → Ta(V) charge transfer state at 10 K led to Eu3+ luminescence solely. Both at 10 and 300 K, the VUV excitation of the host lattice (180 nm) generated exclusively Eu luminescence (Figure 5c,d). The lack of the broad emission upon VUV stimulation may be considered another positive effect of codoping with Ta. The broad band luminescence appeared only upon stimulation around 280 nm, but even then, its intensity related to Eu emission was by about 2 orders of magnitude lower than in the case of (Li,Eu) codoped powder. Increased annealing temperature caused some increase of this emission compared to the red luminescence of Eu3+. In the case of more concentrated (Ta,Eu) systems, the 480 nm emission disappeared completely leaving virtually only the f → f emission of Eu3+ ions independently on the stimulation energy. 6414



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Figure 8. RT luminescence decay traces of HfO2:0.5%Eu3+ powders codoped with 0.5%Li+, 0.5%Ta(V), 0.5%Nb(V), or 0.5%V(V) prepared at 1500 °C in air. Eu3+ luminescence was monitored at 614 nm (EX = 255 nm) (a) and the host-related emission at 480 nm (EX = 300 nm) (b).

and 10 K excitation of the host lattice, indeed, produced Eu3+ luminescence indicating that the activator can be fed by energy deposited in the host. In the excitation spectra of the 614 nm, both at 300 and 10 K, around 200−220 nm, the characteristic two components were again observed indicating that both FEs and free carriers can pass their energy to the activator. From the excitation spectra, it is not easy to extract the component(s) related to V. This is in contrast to the (Ta,Eu) and (Nb,Eu) codoped compositions. One may suppose that the structure around 300−330 nm with peak at 305−310 nm is due to the O2− → V(V) CT transition.18 This is reasonable, as the O2− → Nb(V) (see Figure 6a,b) was found to locate at slightly higher energies, as expected. An excitation spectrum of the 480 nm luminescence could be recorded only at 300 K and consisted of components located in UV below the band gap energies (220− 330 nm roughly) and some features at yet shorter wavelengths connected with transitions within the host material. At 300 K, the luminescence spectra (Figure 7c) excited below the band gap energy consist of both the broad band emission peaking around 480 nm and the characteristic red luminescence of Eu3+. Stimulation with energies sufficient to excite the hafnia host, hence with wavelengths shorter than 220 nm, led to the red emission of Eu3+, exclusively. It was intriguing that the broad band luminescence was totally absent at 10 K but appeared at 300 K, which behavior was perfectly contradictory to the (Nb,Eu) coactivated powder, see Figure 6. From Figure 7c,d, it is clear that the Eu3+ luminescence changes quite significantly with excitation wavelength. This indicates that the dopant experiences different symmetries of its environment. This is seen both at 300 K and even more profoundly at 10 K. This differs in the (V,Eu) doped composition from those containing Ta or Nb codopants. It may indicate that V and Eu do not locate in monoclinic hafnia so easily and that they perturb the host local structure. In fact, it accords with XRDs, which indicated some separation of HfV2O7 phase as already discussed. Definitely, the two dopants, V and Eu, are not capable of cooperating to produce efficient photoluminescence in monoclinic hafnia. Decay Kinetics. Figure 8 presents decay traces of the 614 nm (a) and 480 nm (b) emissions of hafnia powders activated with Eu3+ and codoped with Li+, Ta(V), Nb(V), or V(V) prepared at 1500 °C. Results of undoped and singly Eu-activated monoclinic HfO2 are enclosed for comparison. Table 2 presents time constants derived from the experimental traces for 0.5% and 5% concentrations in materials fabricated at 1000, 1300, and 1500 °C. Data for 480 and 614 nm emission are given.

the introduction. However, upon stimulation at higher energies (shorter wavelengths, below about 200 nm), the Eu3+ luminescence is not so powerful, though a noticeable excitation band appears around 70 nm, hence at the most energetic region accessible at Superlumi station. Altogether, this observation may indicate that transfer of energy from the excited host to Eu3+ via the O2− → Nb(V) CT state is not as vital as we wished. In the long-wavelength region of excitation, the narrow structures due to f → f transitions within Eu3+ are well seen, as expected. At higher concentrations of the Eu and Nb dopants (not presented here), the efficacy of UV-excited Eu luminescence appeared stronger, at least up to 5% of their contents. It is striking that both at RT and 10 K, a well distinguished structure around 212 nm, already assigned to free exciton, is seen in the excitation spectra of Eu luminescence. At 10 K, stimulation around 280−290 nm produced mostly the Eu emission, but the broad band defect/Ti luminescence around 480 nm could also be recorded (Figure 6d). Note that the 10 K excitation spectra of the two emissions differ to some extent in the UV part and that this disparity proves that in the UV region, both emissions (480 and 614 nm) get excited through overlapping but have rather distinct features. Thus, Nb addition induced at least two positive effects: enhanced the red photoluminescence efficiency of HfO2:Eu upon UV stimulation and also totally suppressed (at 300 K) the broad band host-related luminescence. Its effect on the XEL efficiency will be seen later. In contrast to the (Li,Eu) and (Ta,Eu) compositions, virtually no afterglow was observed from HfO2:Eu,Nb. Photoluminescence of HfO2:0.5%Eu,0.5%V. In the case of (V,Eu) codoped powders, an immediate observation was a very low emission intensity compared to all other investigated compositions. Independently, on the dopants content and preparation conditions, the luminescence was disappointingly weak, though it could be easily seen by eye and measured. That was the first sign that V addition did not function as it was supposed to in HfO2:Eu,V. Figure 7a shows RT excitation spectra of 480 and 614 nm emissions of HfO2:0.5%Eu,0.5%V powder, and Figure 7b presents a 10 K excitation spectrum of 614 nm luminescence. In the excitation spectra of the 614 nm luminescence, despite the narrow f → f transitions at longer wavelengths, a slightly structured broad band spreading from about 330 nm down to about 220 nm, hence to the band gap energies, is seen. Its intensity is strikingly low. At yet shorter wavelengths, features related to excitation of the host lattice were observed. Again, their intensities were also rather low. Nevertheless, both at 300 6415



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have its source in the structural perturbations already revealed above. With an increase of annealing temperature, thus enhanced crystallinity, the decay time of the 5D0 → 7F2 Eu3+ luminescence at 614 nm got shorter indicating increasing rate (probability) of the transitions. In most cases, the broad band emission with a maximum around 480 nm is characterized by the decay time of about 10 μs. This value remained independent of the dopant concentrations. Hence, it can be concluded that Eu3+ ions do not drain the energy from the already excited center emitting around 480 nm, whether it is a defect or Ti impurity as we stated above. In two cases, HfO2:5%Eu,5%Li and HfO2:0.5% Eu,0.5%Ta, the broad luminescence decayed with τ = 16−18 μs. This disparity remained unclear, unfortunately. X-ray Excited Luminescence. Figure 9 presents emission spectra under X-ray irradiation recorded for HfO2:0.5% Eu codoped with 0.5%Li (a), 0.5%Ta (b), 0.5%Nb (c), or 0.5%V (d) prepared at 1500 °C. Most of the compositions produced luminescence dominated by the 5D0 → 7FJ (J = 0−4) transitions of Eu3+ ions with the main component around 614 nm resulting from the hypersensitive 5D0 → 7F2 transition. Yet, the (Eu,Li) (Figure 9a) codoped specimen generated both the red emission of the activator and the broad band luminescence with a maximum around 480 nm already seen in PL spectra (Figure 4c). What is more, the integrated intensity of the latter was roughly as significant as the red luminescence of Eu3+. Some differences between the X-ray excited Eu3+ emissions from the codoped powders are easily seen. The (Eu,Li) (Figure 9a) and (Eu,Ta) (Figure 9b) compositions produced low intensity emission lines located within 525−580 nm roughly, which should be assigned to the relaxation of the 5D1 state. (Eu,Nb) (Figure 9c) and (Eu,V) (Figure 9d) codoped powders did not show luminescence from the 5D1 level. This can be understood as Nb and V introduced additional relatively low lying O2− → Nb(V) and O2− → V(V) CT states, which might

Table 2. Average Decay Time of 480 nm (EX = 300 nm) and 614 nm (EX = 255 nm) Emissions of HfO2:Eu,M (M = Li, Ta, Nb, V) Powders Prepared at Different Temperatures; Dopant Concentrations Were 0.5% or 5% annealing temp emission peaks lifetime HfO2:0.5%Eu HfO2:5%Eu HfO2:0.5% Eu,0.5%Li HfO2:5% Eu,5%Li HfO2:0.5% Eu,0.5%Ta HfO2:5% Eu,5%Ta HfO2:0.5% Eu,0.5%Nb HfO2:5% Eu,5%Nb HfO2:0.5% Eu,0.5%V HfO2:5% Eu,5%V

1000 °C

1300 °C

1500 °C

480 nm 614 nm 480 nm 614 nm 480 nm 614 nm (μs)

(ms)

(μs)

(ms)

(μs)

(ms)

12.1

1.17 1.11 1.19

12.3 11.3 11.2

0.75 0.83 0.72

9.8 9.3 10.5

0.74 0.65 0.76

0.92

18.5

0.77

9.6

0.67

1.52

9.3

1.22

16.0

1.09

12.7

17.9

1.13

1.09

0.99

1.31

1.15

1.02

1.15

1.13

0.96

0.92

0.74

0.52

0.81

7.5

0.51 0.62

Decay times are dependent on the codopant and annealing temperature, see Table 2. Taking into account the materials made at 1500 °C (presented in Figure 8), the longest decay time of Eu luminescence, τ = 1 ± 0.1 ms, was observed for Ta and Nb codoped powders. In the (Li,Eu) codoped compositions, the emission decayed with τ = 0.7 ms, which that value was also reported for singly doped HfO2:Eu.5 It is striking that V codoping decreased the decay time of Eu3+ luminescence to 0.5 ms. This low value accords with a low intensity of the luminescence of this composition. While detailed understanding of such behavior is not clear, it may well

Figure 9. XEL spectra of HfO2:0.5%Eu3+ powders codoped with 0.5%Li+ (a), 0.5%Ta(V) (b), 0.5%Nb(V) (c), or 0.5%V(V) (d) prepared at 1500 °C in air. Relative efficiencies of the red Eu3+ XEL against the commercial Eu-doped GOS powder are also given in the figure. 6416



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thus preclude radiative relaxation of the 5D1 level of Eu3+. The HfO2:0.5%Eu,0.5%Nb powder luminescence is generally less complex in terms of the number of observed lines, which may indicate higher ordering of the activator surrounding. Also, analysis of PL spectra led to an analogous conclusion. Figure 10 presents XEL efficiencies for materials prepared at 1000 °C, 1300 °C, 1500 °C, and 1750 °C and containing 0.5%

significant enhancement of PL was observed upon 254 nm irradiation too. Among the M(V) elements, only in the presence of Nb did XRDs not reveal any structural problems; new phases were not seen up to the concentration of 5%. PL spectroscopy also confirmed that the HfO2:Eu,Nb composition did not have any difficulties in incorporation of these two dopants simultaneously. What is more, emission and excitation spectra of different luminescent lines (not presented in this article) did not indicate existence of different Eu sites. Finally, the strong broad excitation band located in UV and connected with Nb proved to be an efficient stimulation path to produce very strong Eu3+ photoluminescence. This new Nb-related excitation component was not only broader but also more prominent in intensity than the O2− → Eu3+ CT band. Consequently, photoluminescence of HfO2:Eu,Nb powders was very strong upon stimulation in the 240−320 nm region. Clearly, (Nb,Eu) codoping was very beneficial for PL of HfO2. Unfortunately, (Eu,V) codopants did not dissolve in monoclinic hafnia so nicely, and some phase separation was observed. The observed luminescence varied with concentration but was of low intensity, which at least partially may be connected with the structural perturbations. That was disappointing, as V was expected to extend the broad UV excitation band even further toward longer wavelengths compared to the (Eu,Nb) compositions. While some indication of such effect was observed, due to a reduced incorporation of V into the monoclinic hafnia host, the overall result was deleterious both for PL and XEL efficiencies. A separate issue was the Li+ ion as a codopant. It was only expected to compensate the lower charge of Eu3+ positioned in the Hf(IV) site. Unfortunately, it turned out that these two ions, Eu3+ and Li+, were not able to form a uniform solid solution as proved by XRDs (Figure 1a). As a matter of fact, this effect cannot surprise since both Eu3+ and Li+ are larger than Hf(IV). It is a pity, as the HfO2:Eu,Li powders showed the most efficient radioluminescence among the investigated powders. This indicated that Li addition was indeed beneficial for XEL efficacy. Unfortunately, above 0.5%, XRDs showed a phase separation, which precluded a further enhancement of XEL efficacy. Qualitatively, Li did not influenced in any significant manner the photoluminescence compared to singly doped HfO2:Eu.5 This was not surprising as the codopant did not introduce any specific electronic level into the system. The main goal of introducing the various codopants was to enhance XEL efficacy, as we explained in the introduction. In fact, only in (Eu,Li) compositions some improvement of radioluminescence was observed compared to singly activated HfO2:Eu. It seems that even more light could be yielded by HfO2:Eu,Li, if uniform solid solutions with higher concentrations of both coactivators could be achieved. Since all the M(V) codopants, contrary to the expectations, lessened the XEL efficiency, even Nb, which did not separate in a foreign phase, it is worth to attempt a deeper understanding of the possible reasons of such behavior. Scintillation is basically a three-step process,3,19,20 which starts with conversion of the incoming particle energy into free carriers: electrons in conduction band and holes in valence band. Then, these carriers defuse toward the emitting center and excite it passing their energy to it. For that, the carriers may be sequentially intercepted by the luminescent ion or may first lower their energy forming exciton, which entity then moves toward the emitter to transmit the energy to it. Finally, the excited center

Figure 10. Dependence of the efficiency of XEL on different annealing temperature for HfO2:Eu powders codoped with V, Nb, Ta, or Li. Concentrations of dopants were 0.5 and 5%, and efficiency of XEL calculated in the range from 570 to 730 nm is given against the commercial Gd2O2S:Eu (GOS).

or 5% of the dopants. The highest light yield of the XEL was found for the (Eu,Li) powder. It reached about 30% of the commercial GOS, and this value was achieved taking into account only the red emission of Eu3+, hence disregarding the broad band luminescence around 480 nm (see Figure 9). Efficiencies of the XEL for (Eu,Ta), (Eu,Nb), and (Eu,V) were 17%, 3%, and 12% of GOS, respectively. For concentrations of the activators above 0.5%, the efficiencies were usually even lower (the only exception was V for which XEL effectiveness was pretty stable within the 0.5−5% range). These results are quite disappointing. Especially the efficacy of the (Eu,Nb) materials, whose UV-excited PL was indeed outstanding, is strikingly low.

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DISCUSSION The idea was to enrich the HfO2:Eu composition by the addition of charge compensating constituents and simultaneously, in the case of the M(V) codopants, introduce electronic states that might possibly guide the energy from the host to the activator. Obviously, the charge compensating codopants were hoped to build into the host forming uniform solid solutions as only then they could mediate the transfer of energy from the host to the emitter. As we saw already with XRDs, it was not always like that. V, and especially Ta, codopants clearly had tendency to separate, at least partially, forming a new crystallographic phase/phases. In such circumstances, it cannot surprise that XEL efficacy of HfO2:Eu,Ta did not get enhanced compared to singly doped HfO2:Eu. The O2− → Ta(V) CT band was found in the highest energy portion of the UV excitation region, and therefore, no 6417



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A different case is the (Eu,Li) codoped powder. Despite that monoclinic hafnia incorporated these dopants with difficulties (see XRDs, Figure 1), the light yield due to Eu3+ (disregarding the broad band luminescence) under excitation with X-rays was about 30−35% of GOS. This was noticeably more than from any other composition presently investigated (see Figure 10). Moreover, in the broad band around 480 nm (see Figure 9), a roughly analogous number of photons was contained. Hence, if one could find a way to transfer also this energy further to Eu3+ to let the dopant to produce its red luminescence, the total red XEL efficiency could roughly double reaching an appreciated value of about 60% of GOS. All of that without technological optimization. Unfortunately, at present, no pathway was found to transfer the bluish host-related broad band emission into the red one from Eu3+. Higher Eu concentrations indeed diminished the intensity of the broad band, but the red Eu3+ luminescence also weakened, unfortunately.

relaxes sending off a photon of light, which closes the cycle. The total productivity, the light yield, is a product of partial efficiencies of the three steps.3,19,20 The first phase can be assumed to be equally effective for all the compositions we researched as it is mostly dependent on the host band gap energy. In the case of the (Eu,Nb) powders, we saw that their XEL efficiency was the lowest, while PL efficacy was the highest, and the decay time was among the longest. Hence, the codopants did not drain the energy from the emitting states to any appreciated level. Then, it remains the particularly inefficient transfer of energy from the already excited host to the luminescent ions (Eu3+), the second step of the whole scintillation process, being responsible for the extremely low total efficacy of the XEL in (Eu,Nb) powders. It was mentioned in the introduction that the oxygen-to-codopant (Ta, Nb, and V) CT states were supposed to be able to enhance the host-toactivator energy transfer. The results of the (Eu,Nb) system shows that in the monoclinic HfO2, this channel of energy transfer to the activator (Eu) is not valid. Though (Eu,Ta) and (Eu,V) compositions performed slightly better upon X-rays, the above conclusion also remains valid for them. Eu3+ is generally an efficient emitter as its luminescence from the 5D0 level does not suffer from the classic concentration quenching;21 only because of energy migration, which occurs at higher concentrations, the quenching may take place when the energy reaches ions with a luminescence killing defect located nearby. Indeed, in the various materials investigated here, especially prepared at higher temperatures (see Table 2), there was no concentration quenching seen in PL at least up to 5%; the decay times were basically constant. However, efficiencies of XEL emissions were the highest when the content of both codopants was only 0.5% (see Figure 10). The reason of this disparity remains vague. The aliovalent impurities (dopant and codopant) replacing Hf(IV) not only distort their surrounding local structures but also create sites with negative (Eu3+) and positive (M(V)) electric charges. Both effects may easily hamper migration of both the electron and hole to the Eu3+ activator awaiting for the energy they carry. Eu3+ is said to prefer capturing an electron from the conduction band first and then a hole from the valence band to get excited.22 In hafnia, however, the first step, taking an electron, may appear difficult as Eu3+ occupying the Hf(IV) site has a negative effective charge. However, catching a hole is even more problematic due to a strong chemical instability of Eu4+ oxidation state. Altogether, this would explain the inefficient transfer of energy from the excited host directly to the Eu3+. Alternatively, the Ta(V), Nb(V), and V(V) codopants occupying the Hf(IV) position forms sites with effective positive charge. Hence, they would be more eager to intercept an electron becoming temporarily +4 charged. Let us assume it is possible. Then, whrn capturing a hole, they might turn into the excited (CT) state regaining the original charge. If things happened like that, at least in the case of Nb, the excited O2− → Nb(V) CT state would be able to pass its energy to Eu3+ as was proved through PL experiments, when the direct excitation into the O2− → Nb(V) CT state led to efficient luminescence. For some reasons, however, none of the codopants, Ta, Nb, and V, was found capable of mediating such a process. Consequently, in the presence of the Ta, Nb, or V codopants, a more efficient route of feeding Eu3+ with energy did not appear, and XEL emissions got even less potent.

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CONCLUSIONS Monoclinic HfO2:Eu powders additionally codoped with Li+, Ta(V), Nb(V), or V(V) were prepared with the classic Pechini method. Structural, morphological, and luminescent properties of these materials were investigated as a function of the dopant concentrations and of the temperature of preparation. Structural analysis proved that only in the case of (Eu,Nb) codoping the activators dissolved in the monoclinic hafnia host forming a uniform solid solution. In the case of (Eu,Li), (Eu,Ta), and (Eu,V) pairs as codopants, XRDs revealed formation of foreign phases. In the HfO2:Eu,Nb powders, an intense broad band excitation structure in the range of about 240−320 nm was found and proved to be at least partially connected with the presence of Nb. Excitation into this band led to a powerful red luminescence of the Eu3+ ion. Contrary to the efficient photoluminescence, X-ray excited emission of HfO2:Eu,Nb was found to be the least intense among all investigated compositions. Also in the case of (Eu,Ta,) and (Eu,V) powders, the radioluminescence was lower than from the singly doped HfO2:Eu. Hence, the pentavalent charge compensating codopants diminished the HfO2:Eu emission upon stimulation with X-rays. This was assigned to the anticipated separation of free carriers, electrons, and holes diffusing through the material toward the Eu activator due to contrast in the net charges of Eu3+ in the Hf position (negative) and M(V) in the Hf site (positive). An opposite effect was seen in the (Eu,Li) codoped powders. These produced noticeably enhanced luminescence upon X-rays compared to other coactivated powders as well as the singly doped HfO2:Eu. Unfortunately, the reasonably efficient red XEL was associated by a broad band host-related emission, which appeared even at higher (Eu,Li) contents.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +48713757248. Fax: +48713282348. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are indebted to Michalina Bańczyk for her assistance in sample preparation. This project was supported by the European Community under the grant number II-20090289 6418



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EC for experiments at Hasylab−DESY in Hamburg. A.W. acknowledges the support of the European Union under the European Social Found “Development of the potential and educational offer of the University of Wrocław: the chance to enhance the competitiveness of the University”.

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REFERENCES

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Modeling Luminescent Properties of HfO2:Eu Powders with Li, Ta, Nb

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