J. Phys. Chem. B 2002, 106, 3805-3812
3805
Spectroscopic Properties of Lu2O3/Eu3+ Nanocrystalline Powders and Sintered Ceramics E. Zych,*,† D. Hreniak,‡ and W. Strek‡ Faculty of Chemistry, UniVersity of Wrocław, 14 F. Joliot-Curie Street, 50-383 Wrocław, Poland, and Institute for Low Temperature and Structure Research, Polish Academy of Sciences, Wrocław, Poland ReceiVed: June 29, 2001; In Final Form: January 29, 2002
Europium-doped Lu2O3 nanocrystalline powders with Eu concentrations of 1-13 m/0 were prepared via a combustion route. Their morphology was determined with TEM measurements. It was found that the sizes of nanocrystallites were around 10-13 nm. These powders were sintered at 1700 °C into tablets whose grains were a few micrometers wide. Absorption and emission spectra of all materials were measured. Pronounced changes of UV absorption bands with increasing concentration were observed in the case of nanoparticulate powders. The Eu emission lifetimes of nanopowders demonstrated significant concentration quenching. No such effect was observed for the sintered materials up to 13 m/0 of Eu. A strong decrease of the emission rise time with rising Eu concentration was found for both types of the material at room temperature. At the temperature of liquid nitrogen, the longest rise time in the case of nanomaterials (∼110 µs) was found for a sample containing 5% of Eu, whereas for the sintered ceramics, the behavior was very similar to that at room temperature. Spectroscopic results are elucidated by a concept of strong aggregation of Eu3+ ions in nanocrystallites when the ion exceeds 3%. An aggregation of Eu3+ ions can be deduced from the nonexponential decay kinetics and leads to fast energy migration, resulting in the linear concentration dependence of the decay rate.
Introduction The last two decades witnessed a rapidly growing interest in lutetium-based compounds and materials. This interest reflected an increasing need for new materials for the detection of ionizing radiation, mainly gamma- and X-rays. The call for highperformance scintillators and X-ray phosphors came both from science, mainly high-energy physics, and from industry. In the latter case, for example, novel state-of-the-art diagnostic equipment in medicine could not be manufactured without developing more efficient scintillator materials. Lower doses and better quality images can be achieved only through the application of scintillators with higher light output (sometimes accompanied with a very short decay time) and with higher stopping power for ionizing radiation. The last requirement directed interest toward lutetium compounds because they are characterized by high absorption coefficients for any kind of high-energy ionizing radiation because of their high density. Even more, the high Z number of Lu results in a high value of photofraction for Lucontaining materials.1 This parameter determines if the ionizing particle energy is preferentially deposited at a single position in the material or, because of a Compton scattering, if there are a few spatially separated energy depositions. The last case is, obviously, not preferable. The value of photofraction can be calculated from a simple equation1
) σp/σp + σc where σp and σc are the cross sections for photoelectric absorption and Compton scattering, respectively. A higher value * Corresponding author. E-mail:
[email protected]. Internet address: http://www.chem.uni.wroc.pl/personal/zych.htm. Tel: +48 71 3757304. Fax: +48 71 3282348. † University of Wrocław. ‡ Polish Academy of Sciences.
of photofraction translates directly into better quality images. This research did indeed bring some interesting materials on the scene. Good examples are Ce-doped Lu2SiO5 and LuAlO3,2-7 which compete as materials of choice for postitron emission tomography (PET) cameras. PET is a wonderful although expensive tool in medical diagnosis. The principle of PET can be found in ref 8 or on the Internet. It has been recognized recently1 that the simple oxide Lu2O3 could serve as a convenient host lattice for some activators to form promising scintillators or X-ray phosphors. Indeed, lutetium oxide is one of the densest inorganic materials, with a band gap large enough to accommodate the energy levels of many luminescent activators. Moreover, its structural analogue (whose density is unfortunately too low to serve as a novel scintillator), Y2O3, is known to be a very efficient phosphor when doped with appropriate activators, for example, Eu3+ and Tb3+ ions. Thus, the high density and high Z-number of Lu make lutetium oxide a noteworthy host material for some novel applications, like digital radiography.9-11 Lutetium oxide is practically unaffected by moisture or any other component of the atmosphere, which makes the compound additionally attractive for practical applications. Furthermore, because it has an isotropic composition, its powders may be converted into transparent plates by the application of the proper densification technique, such as sintering, hot pressing, or hot isostatic pressing.12-16 Thus, transparent bodies of Lu2O3 may be fabricated without pulling crystals at temperatures significantly lower than that of its high melting point (2487 °C). The ability to make polycrystalline but transparent plates is important because for practical applications such a form is often more desired than powders. On the other hand, the susceptibility of powders to sinter into optically transparent bodies depends on many factors. Among the most important ones are the thermal history and the grain size of the starting powders. In general,
10.1021/jp012468+ CCC: $22.00 © 2002 American Chemical Society Published on Web 03/22/2002
3806 J. Phys. Chem. B, Vol. 106, No. 15, 2002 the finer particles sinter more easily and at lower temperatures. Furthermore, the lower the temperature of the powder prior to sintering, the more susceptible it is to sintering into high density and transparency. These facts directed the attention of researchers to nanostructures as starting compositions for sintering because such materials can be typically made at relatively low temperatures and, obviously, they are extremely fine. Some experimental results confirmed that nanocomositions may be very attractive for making highly consolidated transparent or translucent ceramic materials at relatively low temperatures.17-21 Nanoceramics are also very interesting for other reasons. Because of geometrical restrictions and extremely high surfaceto-volume ratios, some changes are observed in the spectroscopic properties of the materials when they are compared to their classic micronsized counterparts. This field of research is still rather novel field, and our understanding of the nanomaterials is very superficial. Even worse, among very reliable and solid articles,22-26 we can also encounter reports containing conclusions drawn without appropriate caution.27,28 For the most part, the authors point out some changes in the decay kinetics of the observed luminescence lines and variations, which are sometimes significant, in the branching ratio of the various emissions. Also, broadening of both emission and absorption lines was reported. While it seems now that for insulators the changes in spectroscopy between micrometer-sized to nanometer-sized materials is not so dramatic, it is still challenging to describe the variations and understand them. In this article, we report on the spectroscopic measurements of the Eu-doped Lu2O3 nanocrystalline powders and, where appropriate, compare the results to those found for micronmetersized polycrystalline sintered specimens fabricated from nanoparticulate powders. Materials and Experiments. Powder samples of Lu2O3, both undoped and doped with Eu3+ ions, were prepared via a combustion synthesis by utilizing the reaction between Lu(NO3)3‚5H2O (eventually with an appropriate admixture of Eu(NO3)3‚6H2O) and urea (NH2)2CO). The components were dissolved in a small amount of water. The solution was dried at 130-150 °C, and the solid residue was transferred into a furnace preheated to about 650 °C, where a vigorous reductionoxidation reaction quickly took place. The process stoichiometry may be given as follows:
2Lu(NO3)3 + 5(NH2)2CO f Lu2O3 + 8N2 + 10H2O + 5CO2 (1) In reality, the chemistry of the reaction is more complex, and nitrogen oxide is also liberated. The optimal furnace temperature was experimentally determined previously.29 The Lu and Eu nitrates were obtained by reacting Lu2O3 (99.995%) and Eu2O3 (99.99%) (Stanford) with analytical grade HNO3 and drying the solution in a vacuum desiccator over KOH and finally over P2O5. Nanopowders obtained via the combustion reactions were cold pressed under nine tons of load into tablets of 10-mm diameter. Afterward, the tablets were vacuum sintered at 1700 °C for 5 h. This procedure allowed us to obtain samples of the same chemical compositions as those of the relevant nanopowders but with significantly bigger crystallites. X-ray analyses were performed with a DRON-2 diffractometer using Co radiation filtered with Fe. The diffractograms were recorded with a step of 0.05 degrees for 2Θ ) 20-80. Scherrer’s relation was used to analyze the crystallite sizes.30 High-resolution transmission electron microscopy (HRTEM)
Zych et al. images and selected-area electron diffraction (SAED) patterns were taken with the Philips CM20 Super Twin microscope operating at 200 kV and providing 0.25-nm resolution. Specimens for TEM study were prepared by dispersing the fine powder, obtained by grinding the samples in a mortar with methanol addition followed by ultrasonic agitation, and depositing a droplet of the suspension on a copper microscope grid covered with porous carbon film. An SEM picture of a sintered ceramic specimen was taken with Joel JSM 5800LV. For that purpose the sample was broken, and the surface obtained in such a way was imaged. Reflection spectra of the powders were taken with a CaryVarian Cary 5 UV-Vis-NIR spectrophotometer and were automatically converted by the spectrophotometer software to be viewed as absorption. The same spectrophotometer was used to obtain absorption spectra of the sintered ceramics. Emission spectra were measured by means of a Jobin Yvon TRW 1000 spectrophotometer equipped with a Lambda Physics excimer laser as an excitation source and a Hamamatsu R928 photomultiplier. Emission kinetics were measured with a Tektronics 1000 TDS 380 oscilloscope using an excimer laser (308 nm) as the excitation source. The experimental results were fitted using a two- or three-exponent equation:
I/I0 ) A1 exp(-t/τ1) + A2 exp(-t/τ2) + A3 exp(-t/τ3)
(2)
For those the decays that exhibited a measurable rise of the signal, the appropriate rise time constant was determined, allowing for a negative value of A1.31 Excitation spectra were recorded with an SPF 500 spectrofluorimeter equipped with a 300 W Xe lamp with a sapphire window and an Al-coated parabolic reflector and were corrected for the excitation light intensity. The reference channel was equipped with a Hamamatsu R2777 photomultiplier, whereas the fluorescence channel used a Hamamatsu R928P PMT. The excitation monochromator had a 0.25 m focal length and an f/4 aperture. Results and Discussion The very exothermic reaction between urea and Lu(NO3)3 causes a significant, though short-lived, increase of the reacting mixture temperature, which allows the forming oxide to crystallize. On the other hand, the huge volume of escaping gases (more than 20 moles per one mole of the main product) does not let the crystallites grow into bigger particles. Indeed, X-ray analysis confirms that the created oxides are nanocrystalline materials, with an average crystallite size of about 1013 nm.32 The same may be concluded from the high-resolution transmission electron microscopy picture of Lu2O3/Eu3+, which is shown in Figure 1a. The aggregates of crystallites shown in Figure 1a and b produce a SAED pattern (inset to Figure 1b) with diffraction rings that are characteristic for interplanar distances of cubic Lu2O3.33-35 On the other hand, the hightemperature sintering allowed the formation of much larger crystallites, as shown in Figure 1c. The sintered material is made of tightly packed grains whose sizes are mostly on the order of a few micrometers. There are many voids between the grains, however, that make the air-sintered specimens only slightly translucent. Figure 2a and b show absorption spectra for samples containing varying amounts of europium. The undoped sample is characterized by a single strong band peaking around 212 nm resulting from the fundamental absorption of the host material. This value agrees with that found for single crystals
Properties of Lu2O3/Eu3+ Nanocrystalline Powders
Figure 1. HRTEM (a) and TEM/SAED (b) pictures of the as-made Lu2O3/1% Eu3+ nanocrystalline powder and an SEM (c) picture of the air-sintered Lu2O3/1% Eu3+ ceramic specimen showing the material’s microstructure.
by Yen and co-workers.36,37 This value gives a valence-toconduction band transition energy of about 47 170 cm-1. The spectra of Eu3+ doped materials contain additional features. The intense and broad band peaking around 270 nm may be reliably ascribed to the charge-transfer transitions from the O ion to the Eu ion.31 The intensity of the band clearly tracks the dopant concentration. We can easily note, however, that this band changes its position significantly with varying Eu3+ concentration, moving into the blue for higher activator content. This behavior is especially true for materials containing more than 3% of Eu3+. This effect might be associated with an appearance of Eu3+-Eu3+ pairs in the system because of the decrease in the average distance between Eu3+ ions; this effect could be further enhanced by the tendency of the Eu3+ ions to aggregation. In Figure 2b, for comparison with the above results, we present absorption spectra of a sintered ceramic sample of Lu2O3/Eu 10% and its nanopowder counterpart. For the ceramic specimen in the region above about 290 nm, we can see nicely resolved lines resulting from the transitions within the 4f6 configuration of the Eu3+ ion. In the 200-290 nm region, a broad-structured band with at least three components around 265, 240, and 215 nm can be noted. Obviously, the 215 nm component results from the already identified fundamental
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3807
Figure 2. Absorption spectra derived from reflection spectra of Lu2O3/ Eu nanocrystallites measured for different concentration of Eu3+ ions (a) and absorption spectra of Lu2O3/10% Eu nanopowder and bulk ceramic (b).
absorption of the Lu2O3 host itself. The other two strongly superimposed components (∼240 and 265 nm) must be related to the charge-transfer transitions. In fact, it should be expected that the CT band will be a superposition of two components because the Lu2O3 host offers two crystallographic sites for the dopant: C2 and centrosymmetric S6.38-40 For each of the sites, the Eu3+ CT band may, obviously, have a slightly different energy. Comparison of the absorption spectra of the sintered ceramic and the nanopowder (Figure 2b) further indicates that in the latter one the f-f lines are much less resolved and their intensities, when related to CT band, are much lower compared to those of the ceramic material. We shall see analogous variations shortly in excitation spectra. The 611-nm Eu emission-excitation spectra of Lu2O3/Eu3+ of varying dopant content are shown in Figure 3a (nanopowders) and b (sintered ceramics). It is easy to see that some characteristics of the spectra of nanopowders change very little within the whole range of the Eu concentration, despite the fact that it changes by more than an order of magnitude. Among such attributes are the positions and ratios of the intensities of the weak intraconfigurational 4f transitions located above 280 nm. All the spectra contain a broad structure of much higher intensity situated around 250 nm. This band coincides with a similar one seen in absorption spectra of the powders (Figure 2a), which we assigned to a charge-transfer transition. Its location is very typical of Eu3+ CT excitation in oxide hosts.9,31 We can easily note that for nanopowders (Figure 3a) the intensity of CT
3808 J. Phys. Chem. B, Vol. 106, No. 15, 2002
Figure 3. Excitation spectra of the 611-nm emission of Lu2O3/Eu nanocrystallites (a) and bulk ceramics (b) measured at room temperature for different concentrations of Eu3+ ions.
band, when related to the weaker f-f lines, changes with Eu content, which is especially true for samples where Eu concentration exceeds 5% for which the CT band strength systematically weakens and its shape becomes more and more distorted. Such behavior of the CT band leads us to conclude that if the stimulating energy is high enough to excite the system to transfer electrons from O2- ions to Eu3+ ions (CT transitions) then the probability that they will expel the gained energy in the form of visible radiation decreases for materials containing more than 5% of Eu. Therefore, if the character of the excitation is to some degree delocalized, as is true for the CT transitions, then the excited electron in the more concentrated system may dissipate its extra energy through a nonradiative process, with the probability rising with increasing Eu3+ concentration. This result may be understood by remembering that the CT states are widely spread. Thus, the energy migration between Eu3+ ions can occur even if the distance between them is relatively large.9,31 Such a long migration of an excited electron significantly increases the probability that it encounters a defect state, allowing it to get rid of its energy through a nonradiative process. The excitation spectra seem to give us such information. The band located at yet higher energies (below 220 nm), seen in all the spectra, is associated with the fundamental absorption of the host lattice itself. This band is observed for all samples, indicating that the energy deposited into the lattice is able to find its way to the Eu3+ ions, excite them, and cause the appearance of Eu3+ emission. Thus, we may conclude that there exists an efficient transfer of energy from the excited host to
Zych et al. Eu3+. Indeed, we recently found that stimulating the material with 210-nm photons produced an efficient Eu3+ emission. Also, stimulation of the material with X-ray radiation, which in principle excites the host, created a Eu3+ emission of remarkable intensity. These results will be published separately. In Figure 3b, we show an excitation spectra of the Eu emission in a sintered ceramic specimen containing varying concentration of the activator. We note a very significant difference between the excitation spectra of the nanopowders (Figure 3a) and those of the sintered specimens. First of all, comparing the ceramic materials to the nanoparticulate materials, we observe a great increase in the relative intensities of the f-f lines and the CT bands. It seems, both from absorption (Figure 2) and excitation (Figure 3) spectra, that in the small nanoparticles of the Lu2O3 host lattice the f-f and CT transition probabilities of the Eu ions are significantly altered in comparison to their large-grained counterparts. For the sintered specimens, we also see systematic changes in excitation spectra with Eu concentration. The CT band is clearly a superposition of two components whose relative intensities changes in favor of the longer constituent with rising Eu content. In the case of nanopowders (Figure 3a), the band was also slightly distorted, but only for higher dopant concentrations. Nevertheless, the components could not be clearly seen for any of the powder samples. Furthermore, for nanopowders, it is the shorter wavelength component of the CT band whose intensity seems to rise faster with increasing Eu concentration. This behavior is especially well seen in the absorption spectra, as presented in Figure 2a. It is also clear that both the f-f lines and CT bands for nanopowders are significantly broader compared to those of the sintered ceramics, which may indicate some random distortion of the site symmetry in the powders. Such a possibility cannot be rejected because the synthesis procedure leads to very rapid formation of the nanocrystallites, which is, obviously, not in favor of the creation of a perfect lattice structure. Thus, the noted broadening of the lines for nanopowders would be an inhomogeneous broadening. The emission spectra of Lu2O3/Eu3+ nanopowders were measured for different concentrations of the active ions and are shown in Figure 4. Analogous spectra for sintered ceramics are very similar, with practically the only difference being a slight narrowing of the observed lines. We can see the intense and quite well-resolved emission lines even at room temperature. We also note an extremely strong intensity of the hypersensitive 5D f 7F transition band around 611 nm. The cubic C-type 0 2 Lu2O3, an analogue of Y2O3, has two crystallographically different sites, one with C2 and one with S6 inversion symmetry.38-40 Because only a magnetic-dipole transition is allowed for inversion symmetry, for Eu3+ in the S6 (C3i) site we can expect only the 5D0-7F1 emission. The assignment of all observed emission lines is given in Table 1. Statistically, there are three times more C2 sites than S6 sites.9,38-40 We found that with an increasing concentration of Eu3+ ions the width of emission bands noticeable increased. This effect is especially true when the Eu3+ content increases above 3%, although the first indications of such behavior come into view when the Eu3+ concentration changes from 1to 3%. Such a broadening of the emission lines seems to indicate the appearance of some interaction between the emitting centers, which is not surprising because the higher dopant content must necessarily reduce the average distance between the activator ions. Measurements of the emission kinetics shed more light on this issue.
Properties of Lu2O3/Eu3+ Nanocrystalline Powders
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3809
Figure 4. Emission spectra of Lu2O3/Eu nanocrystallites measured at room temperature (a) and at 77 K (b) for different concentration of active ions.
TABLE 1: Energy of Lines Observed in the Excitation and Emission Spectra of Lu2O3/Eu Nanoparticulate Powders energy /cm-1
transition excitation
emission
7F f 5F 0 2
7F f 5H 0 J
7
F0f 5D4 7F f 5G 0 J 7F f 5L 0 6
7F f 5D 0 3 7F f 5D 0 2
7
F0f 5D1
7F f 5D 0,1 0 5D f 7F 0 0 5D f 7F 1 3 5D f 7F 0 1
5D f 7F 0 2 5D f 1 5 D0f 5D f 1 5D f 0 5D f 1
a
7F
4
7
F3
7F
5
7F
4
7F
6
300 K 34 423, 33 557, 33 289, 32 895, 32 510 31 378, 31 172, 30 912, 30 294, 30 130 27 450, 27 270 26 281, 26 117, 26 035, 25 753 25 381, 25 284, 25 094,24 907, 24 685 24 131, 23 958 21 487, 21 390, 21 160, 20 986, 20 829 18 947, 18 744, 18 577, 18 162 17 224,a 17 030, 16 722 17 244 17 042 17 224,a 17 077,a 17 018, 16 846, 16 684 16 647, 16 353, 16 308 15 936, 15 803 15 378, 15 177 15 058 14 531, 14 397 14 126, 14 082, 14 013
77 K
17 388,a 17 244 17 036 17 224,a 17 077,a 16 849, 16 684 16 647, 16 359, 16 311 15 931, 15 803 15 394, 15 179 15 053 14 537, 14 399 14 122, 14 081, 14 008
The second site (S6).
The emission decays were measured at room and at liquid nitrogen temperatures for all specimens. In the case of the sintered ceramics, the decays were single exponential within the whole region of investigated concentrations. For nanopowders, however, the situation is quite different, and the decay kinetics change significantly with the concentration of Eu3+ ions, as seen in Figure 5. The observed decay times significantly shortened with concentration. Moreover, only for the low
Figure 5. Effect of concentration on emission decay curves of Lu2O3/ Eu nanocrystallites measured at room temperature. The inset shows the concentration effect on the rise times of emission kinetics.
concentration of 1%, the emission decay curve was a perfect single-exponential function, whereas with the Eu concentration exceeding 3%, the decay curves became nonexponential. Considering the dependence of Eu3+ emission kinetics on the dopant concentration, we should point on two main characteristic features linked with a rise and a decay of the emission. First, the decays of all samples with Eu concentration lower than 10% exhibit a characteristic build-up of the signal, which delays the appearance of maximum emission intensity when it is related to the excitation pulse. As seen in Figure 6, at room temperature, the characteristic rise time decreases systematically with rising Eu3+ concentration. At liquid nitrogen temperature, for the 1% sample, the rise time is similar to that seen at room temperature. However, for all other concentrations, its low-temperature value is much higher, with the most profound difference seen for the 5% specimen. In principle, the appearance of the rise time indicates the presence of a slow relaxation processes feeding
3810 J. Phys. Chem. B, Vol. 106, No. 15, 2002
Zych et al. The integrated fluorescence intensity of donor fluorescence is proportional to the number of excited donors ND, described by31,41
ND(t) ) ND0 exp(-γRt) f(t)
(4)
where γR is the reciprocal of the lifetime of the excited state of the donor ion and
f(t) ≈
∏l [1 - x + x exp -(X0l + W0l)t] cosh(W0lt)
(5)
where Πl is the product over all lattice sites, X0l is the cross relaxation trapping rate between a donor at site 0 and an acceptor at site l, x is the concentration of acceptor ions, and W0l is the rate of energy migration (diffusion) between a donor at site 0 and a donor at site l. Equation 4 predicts in general the nonexponential decay of donor emission. Its exact solutions are possible only in the two limiting cases. In the first case, when donor-donor interaction is negligible, W0l ) 0, and donor-acceptor interaction is weak (Inokuti-Hirayama model41)
f(t) )
∏l [1 - x + x exp(-X0lt)]
(6)
The second case involves rapid donor-donor transfer and negligible cross relaxation, X0l ≈ 0:
f(t) ) exp(-x Figure 6. Concentration dependence of emission lifetimes and emission rise times of Eu3+ luminescence under 308-nm excitation in nanocrystalline (a) and bulk (b) Lu2O3/Eu. 5D
Eu3+
the emitting level. The 0 of isolated ions is populated directly via radiative and radiationless (multiphonon) transitions from higher-lying levels. In principle, their rise rates should be similarly independent of concentration. Hence, a drastic decrease of rise time for higher contents of Eu3+ ions must be associated with cooperative processes. It is known that the emission from higher excited 5D3, 5D2, and 5D1 states sometimes is observed at low concentrations of Eu3+ ions in low-energy phonon matrixes. In fact, we could observe the emission from 5D1 only for low-concentration samples at liquid nitrogen temperature. As seen in Figure 6, the rise times for low-concentration samples were comparable at 77 K and RT, whereas for higher dopant content, they became noticeable different, being much longer at low temperature. For samples containing 5% or more of Eu, the 77 K rise times were at least 10 times longer than those measured at room temperature. Because the emission rise time demonstrates the concentration dependence at room and liquid nitrogen temperatures, the mechanism of population of the 5D0 level should be associated with cooperative interactions, predominantly with the cross relaxation following the scheme 5
D1 f 5D0 T 7F0 f 7F2, 7F1 f 7F3
(3)
The second process is associated with a prior population of the state, and obviously the total cross relaxation is faster at room temperature, which well-elucidates the observed dependence. The concentration quenching of the emission from 5D0 in the system of Eu3+ ions is described by incoherent energy transfer involving donor-donor and donor-acceptor (via crossrelaxation) processes. 7F 1
∑l X0lt)
(7)
In this case, the decay time of donor emission is given by
τ-1 ) τ0-1 + x
∑l X0l
(8)
The decay rate τ-1 is a linear function of the acceptor ion concentration. An analysis of the energy-level diagrams of Eu3+ systems (see Table 1) suggests a lack of simple cross-relaxation processes for the 5D0 emission decay. Moreover, the energy migration associated with donor-donor interaction of the 5D0 state should be, in principle, weak. Therefore, for a system with negligible donor-donor interaction and weak cross-relaxation, a single-exponential decay should be observed. In fact, such a decay was observed for a small concentration of Eu3+ ions. With increasing concentration, the observed decay becomes nonexponential. A source of such behavior may be only an increase of the donor-acceptor interaction (cross-relaxation). Because there is no possibility of resonant cross-relaxation, we find that with increasing concentration there appear new trapping centers associated with the Eu3+ aggregates. Following Buijs, Meijerink, and Blasse40, efficient phononassisted energy transfer from Eu3+ at the S6 site to Eu3+ at the C2 site occurs. There are, in general, three times more C2 sites than there are S6 sites. Because the S6 center has inversion symmetry, the 5D0-7FJ transitions have strongly forbidden character, which is only slightly relaxed at the 7F1 terminal level. The measured emission decay rate τ-1 is given by
τ-1 ) τ0-1 + τQ-1
(9)
where τ0-1 is the decay rate of an isolated Eu3+ ion and τQ-1 is the quenching rate constant. The measured decay time of an isolated Eu3+ ion in Y2O3 was almost 7 times higher at the S6
Properties of Lu2O3/Eu3+ Nanocrystalline Powders
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3811 Conclusions
Figure 7. Concentration dependence of emission decay rate of Lu2O3/ Eu nanocrystallites.
TABLE 2: Emission Decay Constant Determined from the Deconvolution of Emission Decay Profiles of Lu2O3/Eu Nanoparticulate Powders (300 K) % Eu
τ1/ms
A1
τQ/µs
AQ
13 10 7 5 3 1
1.67 1.67 1.67 1.67 1.67 1.67
0.068 0.121 0.178 0.211 0.501
400 ( 5 450 ( 4 570 ( 4 680 ( 9 1060 ( 10
0.283 0.347 0.328 0.291 0.455
site than it was at the C2 site.40 Our very recent measurements showed that in Lu2O3/Eu the ratio is about 4.42 The Eu3+ ions may occur at the both neighboring sites, with a distance of about 3.5 Å. Because the energy of the 5D0 state is slightly higher than that for the S6 site, an extremely efficient phonon-assisted energy transfer from S6 to C2 at low temperature is observed. With increasing temperature, the opposite C2 to S6 energy transfer may occur, too. The distance between the two Eu3+ ions at the C2 and S6 sites (3.4 Å) is much shorter than that between two the Eu3+ ions at the S6 sites (5.2 Å) and is comparable to the distance between two the Eu3+ ions at the C2 sites (3.5 Å).33-35 The emission yield from Eu3+ at the C2 sites is much stronger than that at the S6 site, so this energy transfer may be considered to be an effective cross-relaxation process leading to the concentration quenching of emission. In Table 2, we have listed both of the decay constants obtained from deconvolution of the decay curves into two decay constants; one (τ1) is associated with isolated ions and one (τQ) is ascribed to the cross-relaxation rate. We have assumed that the decay time for the 1% Eu3+-doped Lu2O3 sample corresponds to the radiative lifetime of an isolated Eu3+ ion, which was determined to be 1670 µs. One can note that the decay rate τQ-1 quickly increases with concentration. In Figure 7, the dependence of the emission decay rate on the concentration of Eu3+ ions is plotted. We have found that this dependence is almost linear. The result suggests that concentration quenching in the Eu3+-doped Lu2O3 nanocrystallites corresponds to the limit of fast energy migration and weak cross-relaxation. In fact, a fast excitation-energy migration over the Eu3+ (C2) sublattice was found to be responsible for concentration quenching in Y2O3/Eu.40 A strong increase in energy migration with rising Eu content is due to closer contact of the donor ion with another ion at C2 sites connected via a superexchange interaction.
The results presented in this article demonstrate the optical behavior of Lu2O3/Eu nanocrystalline powders, obtained by a combustion route, as well as their micron-sized sintered counterparts. The morphology of the obtained nanocrystallites was investigated by means of HRTEM. The average size of strongly agglomerated nanocrystallites was found to be 10-13 nm. The absorption spectra of nanoparticulate powders demonstrated a significant blue shift of the CT bands appearing in the UV with increasing Eu content. A similar but less profound effect was seen in the excitation spectra of nanopowders. Exactly the opposite situation was found for the CT band of the sintered large-grained ceramics. The emission spectra were characterized by rather sharp, narrow lines, whose widths systematically increased for Eu concentrations higher than 3%. For ceramic specimens, the emission lines were even narrower. Strong concentration quenching of Eu emission in nanopowders was observed when the doping level exceeded 3%. Moreover, the decay profiles became nonexponential when the concentration of Eu3+ was higher than 3%. which pointed out the significant role of aggregation of Eu3+ ions in nanocrystalline seeds for higher concentrations of the activator. Such effects were completely absent in the case of the sintered large-grained ceramics, for which no concentration quenching was observed for dopant content up to 13%. From excitation spectra, we can also conclude that the host material is able to transfer acquired energy to the Eu3+ ions and excite them, ultimately causing the appearance of the characteristic red emission of Eu3+ both in nano- and coarsegrained materials, which indicates that the Lu2O3/Eu compositions are noteworthy for high-energy ionizing radiation detection. On the other hand, the appearance of rise time in the decays points to possible problems with the emission kinetics under ionizing radiation excitation. This effect, however, remains to be seen. Acknowledgment. We thank to Dr. L. Kepinski from the Institute for Low Temperature and Structure Research, Polish Academy of Sciences, for TEM measurements and Krystyna Haimann from the Institute of Material Sciences and Applied Mechanics, Technical University of Wroclaw, for taking the SEM picture. We greatly acknowledge funding of this project by the Polish Scientific Committee (KBN) under grant no. 3 T09B 031 16 and by NATO under grant no. PST.CLG.976212. References and Notes (1) Derenzo, S. E.; Moses, W. W.; Weber, M. J.; West, Z. C. Mater. Res. Soc. Symp. Proc. 1994, 348, 39. (2) Melcher, C. L.; Schweitzer, S. Nucl. Instrum. Methods Phys. Res., Sect. A 1992, 314, 212. (3) Melcher, C. L.; Schweitzer, S. IEEE Trans. Nucl. Sci. 1992, 39, 502. (4) Lempicki, A.; Randles, M. H.; Wisniewski, D.; Balcerzyk, M.; Brecher, C.; Wojtowicz, A. J. IEEE Trans. Nucl. Sci. 1995, 42, 280. Conference Record, IEEE Nuclear Science Symposium and Medical Imaging Conference, 1994; Trendler, R.C., Ed.; IEEE Inc.: Piscataway, NJ, 1995; p 307. (5) Lempicki, A.; Brecher, C.; Wisniewski, D.; Zych, E.; Wojtowicz, A. J. IEEE Trans. Nucl. Sci. 1996, 43, 1316. Conference Record, IEEE Nuclear Science Symposium and Medical Imaging Conference, 1995; Moonier, P. A., Ed.; IEEE Inc.: Piscataway, NJ, 1996. (6) Wojtowicz, A. J.; Berman, E.; Koepke, Cz.; Lempicki, A. In Cerium Compounds as Scintillators, Conference Record, IEEE Nuclear Science Symposium and Medical Imaging Conference, 1991; Baldwin, G. T., Kirsten, F. A., Eds.; IEEE Inc.: Piscataway, NJ, 1992; Vol. 1, pp 153157. (7) Lempicki, A.; Wojtowicz, A. J.; Brecher, C. In Wide-Gap Luminescent Materials: Theory and Applications; Rotman, S. R., Ed.; Kluwer Academic Publishers: Norwell, MA, 1996.
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