Acta Chim. Slov. 2007, 55, 179–183
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Scientific paper
Properties of the (Y0.75Gd0.25)2O3:Eu3+
Scintillating Nanopowder
@eljka Andri},a Radenka Krsmanovi},a Miodrag Mitri},a Zoran Markovi},a Bruno Vianab and Miroslav D. Drami}anina*
a Institute of Nuclear Sciencies “Vin~a,” P.O. Box 522, 11000 Beograd, Serbia
b Laboratoire de Chimie de la Matière Condensée de Paris (ENSCP), CNRS UMR 7574 11, rue P et M Curie, 75231 Paris cedex 05, France
* Corresponding author: E-mail: dramican@vin.bg.ac.yu
Received: 09-07-2007
Abstract
Yttrium-gadolinium-oxide phosphors are regarded promising host for many important optical applications, for example in medical diagnostic X-ray systems and plasma displays. In this work, we investigated the procedure for (Y0.75Gd0.25)2O3:Eu3+ nanopowder synthesis using polymer complex solution (PCS) method based on polyethylene gly-col (PEG) as fuel. Detailed information, on both the structure and luminescence, are obtained using X-ray powder diffraction, electron microscopy and luminescence spectroscopy. The synthesized powder consists of nanoparticles of about 36 nm in size, and has excellent structural ordering in cubic bixbyte structure (space group Ia3). Unit cell parameter, ionic coordinates, crystal coherence size and microstrains are determined from Rietveld analysis. Luminescence emission measurements show characteristic transition of the trivalent europium ion incorporated into insulating host. Splitting of the Stark components of the 7F1 manifold is presented. 5D0, 5D1 and 5D2 decay time values are measured to obtain information on different kinetic processes occurring for these three emitting levels.
Keywords: Combustion synthesis, yttrium-gadolinium oxide, PEG, luminescence spectroscopy.
1. Introduction
The Ln2O3 rare-earth oxides in the lanthanide series, or sesquioxides, have been of particular interest to the research community over the last century. The rare-earth oxides, being the most widespread lanthanide compounds, are commonly utilized as a catalyst for the synthesis of many other 4f-materials. Phosphors based on Eu3+ doped rare-earth (RE) sesquioxides such as yttrium, lanthanum and gadolinium are important materials with numerous applications in different fields. In particular, Eu3+ doped (Y, Gd)2O3 is well-known luminescent material widely used to provide red light emission for modern optoelectronic devices.1–5
Nanoparticles have sparked intense interest expecting that this range of materials dimension will yield size-dependant properties. Both physical and chemical properties vary with size and the use of ultrafine particles clearly represents a fertile field of materials research. During the past decade, optical properties of nanosized materials have attracted considerable interest. Stronger luminescence
emission in nanocrystalline materials compared to bulk materials and modification of radiative lifetime have been reported.6 It is shown that doping uniformity and phase purity are easier to achieve with nanomaterials. Internal scattering in nanomaterial is reduced as the size of the phosphor particles is much smaller than the wavelength of the visible light. Moreover, in nanophosphors the dopant concentrations can be increased as the quenching effect due to the energy transfer between emission centers is greatly reduced.
Common methods for preparation of nanophosphors are sol-gel, precipitation, co-precipitation, emulsion, combustion, spray pyrolysis, etc. It is known that several synthetic procedures could facilitate formation of C-type stable solid solutions for all Y2O3–Gd2O3 compositions.7, 8, 9 We present here polymer complex solution method – a variation of combustion technique, as synthesis technique for nanocrystalline Y0.75Gd0.25O3: Eu3+ (3 at % of Eu). This method is based on the fact that the reaction of polymer complex solution is very fast and intense combustion reaction where polymer substance (in our case polyethyle-
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Acta Chim. Slov. 2007, 55, 179–183
ne glycol – PEG) is used as a fuel. In this process large amount of gaseous products and heat are generated resulting in very porous material. The characterization is done with XRD, SEM, TEM and luminescence spectrometry and obtained results presented in this paper.
2. Results and Discussion
XRD analysis of (Y0.75Gd0.25)2O3 sample confirms
that the powder crystallizes in cubic bixbyte structure
(space group Ia3), same structure of the starting materials
Y2O3 and Gd2O3. In this structural type cations occupy
two different crystallographic positions 8b (¼, ¼, ¼) with
– point symmetry 3 (S6, C3i) and 24d (x, 0, ¼) with point
symmetry 2 (C2). Both cations are octahedral coordinated.
The oxygen ion, in the general 48e position, is tetrahe-
Figure 1 The observed (crosses), calculated (solid line) and difference XRD patterns for (Y0.75Gd0.25)2O3:Eu3+ nanopowder.
drally surrounded.10 Elementary cell is body centered with 16 formulae units. Measured XRD pattern together with final Rietveld plot is given in Figure 1, and relevant data are reported in Table 1.
SEM observations revealed the nature of the combustion synthesis: at micron-level our sample has a porous, sponge-like morphology, with lot of pores and voids, as can be seen in one representative SEM image in Figure 2. The presence of pores and voids results from the large quantity of escaping gases during combustion reaction.
Figure 2 Typical SEM image of (Y0.75Gd0.25)2O3:Eu3+ powder.
Figure 3 TEM image of one region containing agglomerates of (Y0.75Gd0.25)2O3:Eu3+ nanoparticles. To make them more visible we delineated the contours of nanoparticles in the agglomerate indicated with an arrow.
Morphology and particle sizes are further checked with TEM (Figure 3). At nanometer-level phosphor particles have irregularly spherical or elliptical morphology and form agglomerates. The particle sizes are in the range of 25 to 50 nm. Probably the final, post-synthesis calcination is the reason for some inter-particle sintering (see arrowed agglomerate in Figure 3 and the formation of larger agglomerates.
Figure 4 shows the fluorescence decay curves of the 5D0 and 5D1 emitting levels obtained under excitation at 21598 cm–1 (463 nm) (?em(5D0) = 16345 cm–1 (611.8 nm); ?em(5D1) = 18762 cm–1 (533 nm)). In both cases, fluorescence decay profiles could be adjusted by a single-exponential function and the obtained lifetimes are given in the Table 2. The decay profile of 5D2 emitting level (excitation at 21598 cm–1 (463 nm), emission at 20000 cm–1 (500 nm)) has a complex profile and can be approximated with
Table 1 Selected parameters obtained after Rietveld analysis of XRD patterns for (Y0.75Gd0.25)2O3:Eu3+.
Cell parameter a [Å]
Y3+, Gd3+
x                          x                          y
Microstrain      Crystal coherence size ?a/a [%]                      [nm]
10.6784 (1)
-0.0313 (1)         0.3923 (5)
0.1523 (4)         0.3829 (6)         0.081 (8)
35.6 (4)
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Figure 4 Fluorescence decay profile for 5D0 (z = 1.56 ms) and 5D1 (z = 30 µs) levels.
Figure 5 Fluorescence decay profile for 5D2 level.
Figure 6 Luminescence spectra of (Y0.75Gd0.25)2O3:Eu3+ obtained with 200 µs delay time.
three exponential functions with decay times of 19.2, 5.4 and 0.4 µs. (Figure 5). Further experiments are in progress to describe processes responsible for behavior. Quite high values are obtained for 5D1 and 5D2 lifetimes. In this case, cross relaxation effects, which is the major process involved in the reduction of the trivalent europium 5D1,2 lifetimes, seems to be weak. This indicates homogeneous distribution of Eu3+ ions in nanoparticles.
Luminescence emission spectra of (Y0.75Gd0.25)2 O3:Eu3+ nanopowders are presented on Figures 6–8. Measurements are performed with different time delays after the laser pulse in order to observe the contributions of dif-
ferent emitting levels. Luminescence spectrum obtained with the 200 µs delay time is presented in Figure 6. Taking into account measured values of 5D1 and 5D2 lifetimes (30 µs and 19 µs, respectively), the observed features primarily corresponds to 5D0 emission. In this figure, five characteristic bands centered at around 17223, 16853, 16345, 15860 and 14250 cm–1 associated to 5D0 —> 7Fi (i = 0, 1, 2, 3 and 4) spin forbidden f-f transitions are observed. The 5D0 —> 7F1 transition is the parity-allowed magnetic dipole transition (AJ = 1) and its intensity does not vary with the host. On the contrary, the 5D0 —> 7F2 electric dipole transition (AJ = 2) is very sensitive to the local environment
Table 2 Selected parameters obtained after luminescence measurements for (Y0.75Gd0.25)2O3:Eu3+.
5D0 —> 7F0 C2 site [cm"1]	5D0 —> 7FX C2 site [cm"1]	5D0 —> 7FX S6 site [cm"1]	R	AE [cm"1]	T(5D0 —> 7F2) [ms]	x(5D10 —>	7F2)	T(5D20 —> 7F2) dis]
17223.3	17011.3 17169.2 16673.6	16853.8	0.0997	337.7	1.56	30		19.2, 5.4 and 0.4
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Figure 7 Luminescence spectra of (Y0.75Gd0.25)2O3:Eu3+ obtained with 30 µs delay time.
around Eu3+ ion, and its intensity depends on the symmetry of the crystal field around Eu3+ ion.
The symmetry ratio of the integrated intensity of the 5D0 ? 7F2 and 5D0 ? 7F1 transitions, i.e. R = I (5D0 ? 7F1) / I (5D0 ? 7F2), is considered to be the indicator of the symmetry of the coordination environment around the Eu3+ ion (see Table 2). Stark components of the 7F1 manifold are clearly visible and their values, together with values of maximum splitting ?E, are given in Table 2.
Luminescence spectra obtained with 30 µs and 30 ns time delay are presented in Figure 7 and Figure 8, respectively. In the former both the 5D2 and 5D1 levels are observed, while in the later the faster kinetics is favoured (5D2 level). From these measurements the 5Di ? 7Fj transitions are identified and labeled on Figures 6–8.
Figure 8 Luminescence spectra of (Y0.75Gd0.25)2O3:Eu3+ obtained with 30 ns delay time.
3. Conclusions
Nanocrystalline, pure phase of yttrium-gadolinium mixed oxide ((Y0.75Gd0.25)2O3) has been successfully synthesized by a complex polymer solution method, em-
ploying PEG as fuel. Well crystallized particles have size of about 36 nm. The characterization is done by XRD, SEM and TEM, and luminescence spectroscopy and the main conclusions are summarized as follows:
(i) Data obtained from Rietveld analysis of XRD patterns and luminescence measurements demonstrated that perfect mixing of yttrium oxide and gadolinium oxide has been accomplished for the studied oxide composition: (Y0.75Gd0.25)2O3:Eu3+. (ii) The TEM observations indicate that irregularly spherical nanocrystallites are the main constitute elements of µm-size, very porous regions observed at SEM. The average particle diameter determined from TEM images is about 36 nm that is consistent with the mean crystallite size determined from XRD data. (iii) When excited into 5D2 absorption band nanopowder exhibit strong red luminescence typical for trivalent europium ion. Different delay times after excitation provided luminescence spectra which favors luminescence from different 5DJ (J = 0, 1, 2) levels, and enable detection of all transition present in the visible spectral range. This is in contrast to bulk phosphor where 3 at % concentration of Eu were sufficient to quench higher level emission (i.e. 5DJ ? 7F, J > 0).11 Taking into account also quite high values obtained for 5D1 and 5D2 lifetime, of 30 µs and 19 µs, respectively, we may point to the existence of inefficient cross relaxation processes and homogeneous distribution of Eu3+ ions in nanoparticles. (iv) The 5D0 lifetime of 1.56 ms is much higher than those found in bulk Y2O3 and Gd2O3 (respectively 1 and 1.1 ms12) indicating the possibility of production of material with higher emission yield. The explanation for observed high value of 5D0 lifetime can be found in a refractive index variation in effective medium.6
4. Experimental
Nanocrystalline (Y0.75Gd0.25)2O3:Eu3+ (3 at % of Eu) was prepared by polymer complex solution (PCS) method. The initial material – Y2O3, Gd2O3 and Eu2O3 were dissolved in hot nitric acid. In obtained solutions of stoic-hiometric quantities of Y, Gd and Eu-nitrate PEG was added in 1:1 mass ratio to used oxides. After forming metal-PEG solutions and stirring at 80 °C, metal-PEG solid complex was formed. Complex is combusted at 800 °C in air and then calcinated at same temperature for 2h.
X-ray diffraction measurements are obtained with Philips PW 1050 instrument, using Ni filtered Cu Ka1,2 radiation. Diffraction data were recorded in 2? range from 10° to 120° counting for 15 s in 0.02° steps. Structure analysis was performed using KOLARIE computer software based on a Rietveld full profile refinement method.
Particle morphology and agglomeration state of the synthesized powders were observed with scanning electron microscopy (SEM – JEOL: JKSM-5300). Particle size of (Y0.75Gd0.25)2O3:Eu3+ nanoparticles were determined
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by transmission electron microscopy – TEM (Phillips CM100 instrument). SEM and TEM specimens were prepared with standard powder preparation techniques.
The emission spectra have been collected at room temperature after excitation into the 7F0 ? 5D2 absorption band. The excitation source was an Optical Parametric Oscillator (O.P.O.) pumped by the third harmonic of the Nd:YAG laser. The emission has been analyzed using HR250 monochromator (Jobin-Yvon) and then detected by an ICCD camera (Princeton Instrument). The emissions are recorded with the following delay time after the laser excitation: 30 ns, 30 ms and 200 ms, in order to limit contributions from different emitting levels. Lifetime measurements have been recorded under pumping by the O.P.O. at 463 nm at room temperature. 5D2, 5D1 and 5D0 decay time values provide information on different kinetic processes occurring for these three emitting levels.
5. Acknowledgements
This work was supported by the Ministry of Science of the Republic of Serbia (Project 142066).
6. References
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Povzetek
Itrij-gadolinijevi oksidi so obetavni materiali za {tevilne pomembne aplikacije v optiki, kot so na primer sistemi za Rentgensko medicinsko diagnostiko in plazemski ekrani. V tem prispevku je opisan postopek za sintezo nanoprahov (Y0.75Gd0.25)2O3:Eu3+ z uporabo metode kompleksne raztopine polimerov (PCS) na osnovi polietilenglikola (PEG) kot goriva. Podatke o strukturi in luminiscenci sintetiziranega materiala smo dobili z X-`arkovno difrakcijo prahov, elektronsko mikroskopijo in luminiscen~no spektroskopijo. Sintetizirani material sestavljajo nanodelci velikosti 30 nm s ku-bi~no biksbitno strukturo (prostorska grupa Ia3). Parametre osnovne celice, ionske kordinate, koheren~no dol`ino in mi-kronapetosti smo dolo~ili z Rietveldovo analizo. Meritve luminiscence so pokazale energijske prehode trivalentnih ionov evropija, zna~ilne za koordinacijo sosednjih atomov v sintetiziranem nanoprahu. Izmerili smo razpolovne dobe 5D0, 5D1 in 5D2 stanj in dobili podatke o razli~nih kineti~nih procesih za tri emisijske nivoje.
Andri} et al.:   Properties of the (Y0.75Gd0.25)2O3:Eu3+ Scintillating Nanopowder