- Email: [email protected]
Correlation of the structure and the luminescence properties of Eu3+-doped Gd2O3 oxide between fiber single crystal and the nano-size powders
ARTICLE IN PRESS
Journal of Crystal Growth 265 (2004) 459–465
Correlation of the structure and the luminescence properties of Eu3+-doped Gd2O3 oxide between fiber single crystal and the nano-size powders C. Louisa, K. Lebboua,*, M.A. Flores-Gonzaleza, R. Bazzia, B. Hautefeuillea, B. Merciera, S. Rouxa, P. Perriatb, C. Olagnonb, O. Tillementa a
Physical Chemistry of Luminescent Materials, Claude Bernard/Lyon1 University, CNRS UMR 5620, Bat. A.Kastler, 10 rue Ampere, 69622 Villeurbanne Cedex, France b UMR 5510 CNRS INSA of Lyon, France Received 15 December 2003; accepted 8 February 2004 Communicated by M. Schieber
Abstract Gd2xEuxO3 powder materials doped with Eu3+ using different concentration from x ¼ 0 to 2 were prepared by a new sol-lyophilization synthesis and fiber single crystal (Gd2O3–Gd2O3/Eu3+) oxides were grown using laser-heated pedestal growth (LHPG) technique. X-ray powder diffraction performed at room temperature showed that the materials obtained by lyophilization technique are cubic, lies in the nanometer range and different from bulk materials. The materials are a continued solid solution and the lattice parameter a-axis increases with Eu3+ concentration. The fibers grown by LHPG technique were single crystal, transparent, red in color when Eu3+ concentration is high, and the structure is monoclinic for Eu3+o15%. The luminescence of Eu3+ from C2 crystallographic site was been detected. In the case of nano-particles oxide, an additional peak in emission spectral is obtained at l ¼ 610 nm. The influence of Eu3+, particles sizes and the structure on the luminescence spectral of Eu3+ ions was been described. r 2004 Elsevier B.V. All rights reserved. PACS: 78.67.n; 78.60.b; 61.66.Fn; 81.10.h Keywords: A1. Nanostructures; A1. Segregation; A1. Solid solutions; A2. Laser heated pedestal growth; B2. phosphors
1. Introduction Our research group is involved in crystal growth materials for optical application using different *Corresponding author. Tel.: +33-4-72-44-81-30; fax: +334-72-43-12-33. E-mail address: [email protected] (K. Lebbou).
crystal growth techniques and recently we extended our research topics to the design and synthesis of nanoparticles luminescent inorganic materials doped with RE3+ activators ions. Since long time europium-activated phosphors have been investigated as an important topic for a large variety of materials [1–4]. Gd2O3 doped with Eu3+ is important and it shows an optical behavior
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.02.005
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C. Louis et al. / Journal of Crystal Growth 265 (2004) 459–465
which depends on the particle size, the dopant concentration and the elaboration technique [5–7]. The trivalent europium ion fluorescence in crystalline host is red orange. It is composed from a narrow peak spectral between 460 and 720 nm with intense peaks located near 610 nm. The peaks correspond to the transition between the excited level 5D0, 5D1, 5D2 and the fundamental multiple 7 Fj (with j ¼ 0; 1; 2; 3; 4). All these levels are decomposed by Stark effect because of crystal field [8]. This decomposition is realized with different manners as a function of crystal field which means that symmetry is attached to the europium ion position in the compound. It is then clear that, the oriented single crystals materials are important to study the optical properties and the environmental ions effect in any materials. However, several problems could be observed during crystal growth process, especially the best starting percentage dopant concentration, which permits to obtain the nice crystal, defect free, transparent for the utilized wave length with segregation coefficient close to one [9]. In the case of Gd2O3 crystals, quite often these crystals are obtained from the melt, are monoclinic [10] and the europium segregation coefficient is less than one. The Gd2O3 crystal growth process is complex because of its high melting temperature (2673 K). It is difficult to insert Eu3+ in the monoclinic Gd2O3 crystals [11]. The Gd2O3 monoclinic structure is not supposed to generate a red intense light. On the contrary, the cubic phase permits intense luminescence to be achieved. Such phases can be obtained when preparing samples at temperatures lower than 1573 K. This has been done for a long time using classical solid-state reactions. However, only chemical processes allow to obtain samples with well-controlled composition, narrow particle size and free of the defects generally induced by comminution. That explains why we have developed a new sol-lyophilization technique which leads to submicronic particles size In this paper, we report the synthesis and the characterization of a variety of luminescent nano-particles Gd2O3 powders oxide doped with Eu3+ at different scales. The nanoscaled powders are obtained by the above-mentioned sol/lyophilization technique. The single crystal fibers are grown by laser-heated
pedestal growth (LHPG) technique. That allows a direct comparison between some nanoscaled and macroscopic compounds that is very rarely achieved in literature. In particular the preliminary results about the comparison of the optical properties at the two scales will be discussed.
2. Experimental procedure Single crystal fibers were grown using CO2 IR laser beam emitting at 10.06 mm. The maximum power was about 200 W, which permit to melt the majority of oxides. The source material was used in the form of Gd2O3 and Gd2O3–Eu3+ square cross-section rods from polycrystalline material that had been fabricated from pressed and sintered powders at 1573 K for a long time to be densified. After densification, the ceramic pellets were cut into thin rods with the dimension 1 1 2.5 mm3. The rod source and seed crystal were vertical, the pulling rate rod source and seed were independent and varied from 1.1 to 100 mm/h. At the first step, to initiate the growth condition, a polycrystalline Gd2O3 rod was used as seed. A single grain dominated the cross-section after only a short distance of the growth. In the case of Gd2O3 fiber crystal growth, the pulling rate was 30 mm/h for seed rod and 15 mm/h for the rod row material. The melting zone was maintained in equilibrium between the two rods because of superficial tension force. This technique is crucible free and is not required to have a furnace insulation (Fig. 1) and contamination problems are completely minimized. Finely divided europium-doped gadolinium oxide powders were prepared according to the sollyophilization process described elsewhere [7]. In summary, gadolinium nitrate (NO3)3Gd H2O and europium nitrate (NO3)3Eu H2O with certified purity of 99.9% from Aldrich were dissolved in acidified water and with a molar proportion of varying Eu3+/Gd3+ from 0 to 1. During the addition of a basic solution, precipitation was instantaneous and the hydroxide was formed. The so-obtained precipitates were washed under ultrasonication and each washing was followed by centrifugation. A sol was obtained, freeze-dried at 10 C, dried at 400 K and thermally annealed at
ARTICLE IN PRESS C. Louis et al. / Journal of Crystal Growth 265 (2004) 459–465
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estimate of the isotropic broadeningpdue ffiffiffiffiffi to the presence of microstrains: e ¼ ðp=1:8Þ U (e is the defect concentration in %) and IG is a p measure of ffiffiffiffiffiffi the isotropic size effect: T ¼ 180 Kl=p IG (T is the size in A; l is the wavelength in A and K is the Scherrer constant here equal to 43) [12]. The fwhm have been corrected from the instrumental corrections obtained from an annealed BaF2 reference. Scanning electron microscopy (SEM) was carried out using a Philips microscope operating at 20 kV. Fluorescence spectra was obtained using excitation with a Xe lamp and a J.Y. H10D monochromator (set at l ¼ 250 nm). Light was collected with an UV optical fiber coupled with a J.Y. TRIAX 320 and a CCD camera (1200 g/ 320 nm grating, resolution=0.65).
3. Results
Fig. 1. Schematic illustration of the LHPG technique principle.
different temperatures between 700 and 1300 K in order to obtain europium-doped gadolinium oxide. X-ray diffraction diagrams were carried out using a D5000 Siemens diffractometer with CuKa1 and Cu-Ka2 X-rays (l ¼ 0:15406 and 0:15444 nm). The diffraction pattern was scanned over the 2y range 3–50 in steps of 0.02 and a counting time of 8 s/step. The parameter of the crystalline sample has been refined taking into account the aberration arising from the specimen displacement, D2y ; given by the formula: D2y ¼ ð2s=RÞcos 2y where s is the is placement of the sample surface with respect to the axis of the goniometer and R is the radius of the goniometer circle. Similarly, the microstrains and the size of the coherent domains have been derived from a refinement of the full-width at half-maximum (fwhm), b; of the patterns fitted with Pseudo-Voigt functions according to the following relation: b2 ¼ U tan2 y þ IG=cos2 y: In the latter relation U is an
Using LHPG technique we have grown, under air atmosphere, undoped and Eu3+-doped Gd2O3 oxide fiber single crystal of 20–50 mm length with roughly cylindrical cross-section. The feeding rate was adjusted so that the fiber diameter was between 0.5 and 1 mm to minimize crack propagation, to keep melting zone interface plane and the quasi-equilibrium state during the crystal growth process. For pulling rate higher than 30 mm/h, the melting zone becomes unstable and seed disconnection is observed. Temperature stability is a critical factor as we also observed polycrystal formation when the temperature abruptly decreased because of an accidental power instability in the CO2 laser of LHPG machine. The obtained fibers are single crystal and free of impurities. For Eu3+ belonging to (0–15%), the fiber structure is monoclinic, in good agreement with JCPDS file no. 43-1015. The undoped Gd2O3 fibers are transparent, with uniform shape and free of macroscopic defects such as cracks, bubbles or inclusions (Fig. 2). The cracks and diameter variation were observed only for fibers with high Eu3+ concentrations (Eu3+>5%) and red color was observed because of Eu3+. For high Eu3+ concentration, the composition is not homogeneous and varied along the growth direction, the first part of the fiber contains high Eu3+
ARTICLE IN PRESS C. Louis et al. / Journal of Crystal Growth 265 (2004) 459–465
462
(222) 1300K
Intensity (arb.unit.)
(400)
Fig. 2. Gd2O3 fiber single crystal grown by LHPG technique (scale in mm).
(440)
(411) (332) (431)
(211)
1100K
700K
Emission spectra : λexc.= 254nm. 15
D0
7
18
21
24
27
30
33
36
39
42
45
48
2θ (deg)
F2
Fig. 4. Effect of temperature reaction on 2.5% molar Eu3+doped Gd2O3 cubic oxide formation.
5
7
F4
D0
5
5
580
7
D0
600
620
640
F3
660
680
700
720
wavelength (nm)
Fig. 3. Room temperature emission spectra of 15% molar Eu3+-doped Gd2O3 fiber single crystal (lexc ¼ 254 nm).
concentration and continuously decreases even with high pulling rate because of segregation problem. The effective distribution coefficient (Keff ) of Eu3+ ions was calculated on the basis of Pfann Eq. (1) [13] as follows: Cs =C0 ¼ keff ð1 gÞkeff 1 ;
ð1Þ 3+
D0-7Fj (j=0 to 4) transitions are detailed in the Fig. 3. For 0%oEu3+o15%, it was difficult to stabilize the cubic structure during crystal growth process because the high temperature monoclinic phase is the stable state, in addition the monoclinic–cubic transition can disturb the crystal qualities and it is difficult to insert high Eu3+ concentration in single crystal Gd2O3 monoclinic structure because of segregation problem, especially in incongruent composition (solid solution). This is why we have thought to find another way to insert high Eu3+ concentration in Gd2O3 oxide, to generate, keeping the cubic structure and to improve the luminescence properties. We extend our experiment to the utilization of a new lyophilization synthesis method. The initially obtained material is a slightly crystallized Eu3+doped hydroxide Gd(OH)3. The hydroxide geometry shape is of a homogeneous elongated plate [7]. The first Gd2O3 particle oxide appears after annealing at temperature 700 K (Fig. 4). For annealing temperature less than 600 K, the materials is Gd(OH)3 hydroxide with low crystallinity. The proportion of Gd2O3(Eu3+) cubic oxide (JCPDS file 43-1014) increases with increasing annealing temperature. The oxide quantity has been followed by the intensity evolution of the (2 2 2) peak of Gd2O3 cubic oxide. From 1100 K, the samples are Gd2O3 oxide single phase, belonging to the space group Ia3 (206) and are 5
D0
D0
5
F0
7
F1
15% Eu 2 O3 doped Gd2O3
7
Intensity (arb. unit.)
5
where C0 and Cs is the Eu concentration in the starting melts and in the crystal at the solidification fraction (g), respectively. The logarithm of Eq. (1) permits to calculate keff of Eu3+ concentration. In the case of Gd2O3 (Eu3+=5%), keff ¼ 1: For Eu3+ concentration>5%, keff o1: Fig. 3 shows the luminescence spectra for the excitation at 254 nm of Eu3+-doped Gd2O3 fiber single crystal (Eu3+=15%), and the spectral belongs to the luminescence of monoclinic Gd2O3 structure different from the cubic materials as already shown in our Ref. [7]. In the case of monoclinic Gd2O3 one, there are three C5 sites [14]. Only the
ARTICLE IN PRESS C. Louis et al. / Journal of Crystal Growth 265 (2004) 459–465
monoclinic phase-free. Fig. 5 shows the particle size evolution as a function of annealing temperature. The oxide particle sizes are less than 60 nm with high crystallinity. At low temperatures the particle sizes are less than 10 nm. It is clear that an annealing for a long time or at a temperature higher than 1300 K could be a way to increase particle sizes because of diffusion and germination phenomena which can be observed during the reaction. Fig. 6 shows the evolution of powder X-ray diffraction patterns as a function of Eu3+ con-
50
30
20
10
0 700
800
900
1000 1100 1200 Temperature (K)
1300
1400
Fig. 5. Particles size evolution as a function of temperature variation of 2.5% molar Eu3+-doped Gd2O3 cubic oxide.
10.87
0%
10.86
1% (400) (411) (332) (431)
(440) 2.5%
3+
(222)
X Eu
Intensity (arb.unit.)
Gd2O3
(211)
centration at the annealing temperature of 1300 K. The samples are monophased in the whole range of composition, from Eu3+=0 to 1. The Miller indices (h k l) can be assigned according to cubic symmetry in good agreement with JCPDS data card No. 43-1014. No presence of impurities or second phases belonging to another domain outside the solid-solution range in the Gd2O3–Eu2O3 equilibrium diagram [12] are detected. Fig. 7 shows the evolution of the lattice parameter as a function of Eu3+concentration. An important increase in aaxis of the Gd2O3 compound is observed with the increasing of the Eu3+ concentration. This can be explained on the basis of the ionic radii of Gd3+(0.0938 nm) smaller than Eu3+(0.0947 nm). The linear dependence observed clearly demonstrates the existence of a complete solid solution at the nanometer scale in the Gd2O3–Eu2O3 system. Emission spectra were performed for Gd2O3 as a function of Eu3+ concentration for the materials submitted to annealing temperature from 700 to 1300 K. In gadolinium oxide Eu3+ exhibits 4f6 5 D0-4f6 7Fj (j ¼ 026) emission lines. Fig. 8 shows the luminescence spectral for the excitation at 254 nm of the samples annealed at 1300 K. Only the 5D0-7Fj (j ¼ 022) transitions are detailed in Fig. 9. In the case of cubic Gd2O3 oxide, generally the spectrum is dominated by the well-known emission lines typical of Eu3+ in the C2 site without inversion center and the weak S6 site peaks belonging to the 5D0-7F1 transition. But
25%
50%
Lattice parameter (pm)
Size (nm)
40
463
10.85 10.84 10.83 10.82 10.81 10.80
Eu2O3
100%
16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
2θ (deg)
10.79 10.78 0
10
20
30
40
50
60
70
80
90
100
3+
Fig. 6. Evolution of X-ray powder diffraction at room temperature as a function of Eu substitution of powders annealing at 1300 K.
% molar Eu doped Gd2 O3
Fig. 7. Evolution of lattice parameter as a function of Eu3+ concentration of powders annealing at 1300 K.
ARTICLE IN PRESS C. Louis et al. / Journal of Crystal Growth 265 (2004) 459–465
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7
5
F2(C2)
D0
5%
1.6 1.2
F1(C2)
F1(S6)
7
5
7
D0
F0(C2)
5
0.4
D0
7
D0
0.8
10% 2.5%
1%
5
Intensity (arb.unit.)
2.0
25% 50%
0.0 585 590 595 600 605 610 615 620 625 630
λ (nm)
Fig. 8. Room temperature emission spectra as a function of Eu3+ concentration in Gd2O3 oxide at annealing temperature 1300 K (lexc: ¼ 254 nm).
7
5
F1(C2)
600
610
620
630
4. Conclusion
nm
700K
5
D0
700K
590
7
F1(S6)
7
D0
5
F0(C2)
7
D0
0.2
900K
0.0035
Intensity (a.u.)
1100K
0.6
5
Intensity (arb.unit.)
1300K
0.8
0.4
F2(C2)
D0
1.0
centration. The emission spectra change drastically in the case of the sample annealed at low temperature and high Eu3+ concentration (Fig. 9). After annealing temperature higher than 700 K the broad bands observed in the sample annealed at low temperature change to sharp emissions lines. The line positions are in good agreement with the level energies given in Ref. [16]. For Eu3+>5%, the increasing Eu3+ concentration and the decreasing temperature annealing reduces the luminescence intensity and gives a broad band that confirms the location of the Eu3+ ions in a disordered gadolinium oxide matrix for a low annealing temperature. Finally, in the case of single crystal fibers grown by LHPG technique, a monoclinic solid solution exist until Eu3+=15%, but crystal growth processes are complex because of the instability of the melting zone and segregation problem. For Eu3+>15% the study is on progress and will be initiated soon. In the lyophilized materials a complete solid solution with cubic structure, the best luminescent properties, is observed for Eu3+=5%.
0.0 585 590 595 600 605 610 615 620 625 630 635 640
λ (nm) Fig. 9. Room temperature emission spectra of 2.5% molar Eu3+-doped Gd2O3 annealed at a different temperature.
several additional emission lines are also observed. These ones are attributed to some surface effect [15]. The emission 5D0-7F2 is higher than 5D0-7F1 and the ratio of 5D0-7F2/5D0-7F1 is about 4. An additional peak is observed at about 610 nm, which is absent for high Eu3+ concentration, at low temperature annealing (To700 K) and in the monoclinic structure. This could be related to the size or surface effect; the research is under progress and will be soon published. For Eu3+o5%, the luminescence intensity increases with Eu3+ con-
We have used two different methods to prepare Eu3+-doped Gd2O3 luminescent materials. The first one is LHPG technique and the second one is a sol-lyophilization method. By LHPG technique we obtained a monoclinic single crystals fibers with low luminescent properties although the materials are single crystal. Also, related to segregation phenomena, the crystal qualities decrease when incorporating Eu3+ ions in the Gd3+ sites of Gd2O3 oxide. In the case of the sol-lyophilization method, the present investigation gives an overall picture of these so-synthesized compounds obtained after an optimized thermal annealing with a fuel process at starting reaction state and the annealing at selected temperature. Cubic single phase, with a well-distributed particle and the grain sizes in the range of 10–60 nm are obtained. The Eu3+ emission spectral depends strongly on the quantities of Eu3+ concentration and annealing temperature. For the annealing temperature of 1300 K, the Eu3+ composition corresponding to
ARTICLE IN PRESS C. Louis et al. / Journal of Crystal Growth 265 (2004) 459–465
the maximal luminescence intensity corresponds to europium contents higher than 5%. The samples obtained by the two techniques are clearly different, especially the luminescent properties and the structure of the obtained phases.
References [1] A. Bril, W.L. Wanmaker, J. Electrochem. Soc. 111 (1964) 1363. [2] B.M.J. Smets, Mater. Chem. Phys. 16 (1987) 283. [3] N.A. Diakides, Proc. Soc. Photo-Opt. Instrum. Eng. 42 (1973) 83. [4] H. Forest, G. Ban, J. Electrochem. Soc. 116 (1969) 474. [5] R.S. Roth, S.J. Scheneider, J. Res. Nat. Bur. Stand. 64A (1960) 309.
465
[6] A. Garcia-Murillo, C. Le Lueyer, C. Garapon, C. Dujardin, E. Bernstein, C. Pedrini, J. Mugnier, Opt. Mater. 19 (2002) 161. [7] C. Louis, R. Bazzi, M.A. Flores, W. Zheng, K. Lebbou, O. Tillement, B. Mercier, C. Dujardin, P. Perriat, J. Solid State Chem. 173 (2003) 335. [8] H.T. Hintzen, H.M. Van Noort, J. Phys. Chem. Solids 49 (1996) 783. [9] K. Lebbou et al., in preparation. [10] J. Dexpert-Ghys, M. Faucher, P. Caro, Phys. Rev. 23 (2) (1981) 607. [11] V.C. Costa, M.J. Lochhead, K.L. Bray, Chem. Matter 8 (1996) 783. [13] W.G. Pfann, Trans. Am. Inst. Min. Metall. Eng. 194 (1952) 747. [12] J.B. Langford, Aust. J. Phys. 41 (1988) 173. [14] B. Beaury, et al., J. Lumin. 40–41 (1998) 651. [15] D.B.M. Klaasen, et al., J. Lumin. 40–41 (1988) 651. [16] M. Buijs, A. Meijerink, G. Blasse, J. Lumin. 37 (1987) 9.
Journal of Crystal Growth 265 (2004) 459–465
Correlation of the structure and the luminescence properties of Eu3+-doped Gd2O3 oxide between fiber single crystal and the nano-size powders C. Louisa, K. Lebboua,*, M.A. Flores-Gonzaleza, R. Bazzia, B. Hautefeuillea, B. Merciera, S. Rouxa, P. Perriatb, C. Olagnonb, O. Tillementa a
Physical Chemistry of Luminescent Materials, Claude Bernard/Lyon1 University, CNRS UMR 5620, Bat. A.Kastler, 10 rue Ampere, 69622 Villeurbanne Cedex, France b UMR 5510 CNRS INSA of Lyon, France Received 15 December 2003; accepted 8 February 2004 Communicated by M. Schieber
Abstract Gd2xEuxO3 powder materials doped with Eu3+ using different concentration from x ¼ 0 to 2 were prepared by a new sol-lyophilization synthesis and fiber single crystal (Gd2O3–Gd2O3/Eu3+) oxides were grown using laser-heated pedestal growth (LHPG) technique. X-ray powder diffraction performed at room temperature showed that the materials obtained by lyophilization technique are cubic, lies in the nanometer range and different from bulk materials. The materials are a continued solid solution and the lattice parameter a-axis increases with Eu3+ concentration. The fibers grown by LHPG technique were single crystal, transparent, red in color when Eu3+ concentration is high, and the structure is monoclinic for Eu3+o15%. The luminescence of Eu3+ from C2 crystallographic site was been detected. In the case of nano-particles oxide, an additional peak in emission spectral is obtained at l ¼ 610 nm. The influence of Eu3+, particles sizes and the structure on the luminescence spectral of Eu3+ ions was been described. r 2004 Elsevier B.V. All rights reserved. PACS: 78.67.n; 78.60.b; 61.66.Fn; 81.10.h Keywords: A1. Nanostructures; A1. Segregation; A1. Solid solutions; A2. Laser heated pedestal growth; B2. phosphors
1. Introduction Our research group is involved in crystal growth materials for optical application using different *Corresponding author. Tel.: +33-4-72-44-81-30; fax: +334-72-43-12-33. E-mail address: [email protected] (K. Lebbou).
crystal growth techniques and recently we extended our research topics to the design and synthesis of nanoparticles luminescent inorganic materials doped with RE3+ activators ions. Since long time europium-activated phosphors have been investigated as an important topic for a large variety of materials [1–4]. Gd2O3 doped with Eu3+ is important and it shows an optical behavior
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.02.005
ARTICLE IN PRESS 460
C. Louis et al. / Journal of Crystal Growth 265 (2004) 459–465
which depends on the particle size, the dopant concentration and the elaboration technique [5–7]. The trivalent europium ion fluorescence in crystalline host is red orange. It is composed from a narrow peak spectral between 460 and 720 nm with intense peaks located near 610 nm. The peaks correspond to the transition between the excited level 5D0, 5D1, 5D2 and the fundamental multiple 7 Fj (with j ¼ 0; 1; 2; 3; 4). All these levels are decomposed by Stark effect because of crystal field [8]. This decomposition is realized with different manners as a function of crystal field which means that symmetry is attached to the europium ion position in the compound. It is then clear that, the oriented single crystals materials are important to study the optical properties and the environmental ions effect in any materials. However, several problems could be observed during crystal growth process, especially the best starting percentage dopant concentration, which permits to obtain the nice crystal, defect free, transparent for the utilized wave length with segregation coefficient close to one [9]. In the case of Gd2O3 crystals, quite often these crystals are obtained from the melt, are monoclinic [10] and the europium segregation coefficient is less than one. The Gd2O3 crystal growth process is complex because of its high melting temperature (2673 K). It is difficult to insert Eu3+ in the monoclinic Gd2O3 crystals [11]. The Gd2O3 monoclinic structure is not supposed to generate a red intense light. On the contrary, the cubic phase permits intense luminescence to be achieved. Such phases can be obtained when preparing samples at temperatures lower than 1573 K. This has been done for a long time using classical solid-state reactions. However, only chemical processes allow to obtain samples with well-controlled composition, narrow particle size and free of the defects generally induced by comminution. That explains why we have developed a new sol-lyophilization technique which leads to submicronic particles size In this paper, we report the synthesis and the characterization of a variety of luminescent nano-particles Gd2O3 powders oxide doped with Eu3+ at different scales. The nanoscaled powders are obtained by the above-mentioned sol/lyophilization technique. The single crystal fibers are grown by laser-heated
pedestal growth (LHPG) technique. That allows a direct comparison between some nanoscaled and macroscopic compounds that is very rarely achieved in literature. In particular the preliminary results about the comparison of the optical properties at the two scales will be discussed.
2. Experimental procedure Single crystal fibers were grown using CO2 IR laser beam emitting at 10.06 mm. The maximum power was about 200 W, which permit to melt the majority of oxides. The source material was used in the form of Gd2O3 and Gd2O3–Eu3+ square cross-section rods from polycrystalline material that had been fabricated from pressed and sintered powders at 1573 K for a long time to be densified. After densification, the ceramic pellets were cut into thin rods with the dimension 1 1 2.5 mm3. The rod source and seed crystal were vertical, the pulling rate rod source and seed were independent and varied from 1.1 to 100 mm/h. At the first step, to initiate the growth condition, a polycrystalline Gd2O3 rod was used as seed. A single grain dominated the cross-section after only a short distance of the growth. In the case of Gd2O3 fiber crystal growth, the pulling rate was 30 mm/h for seed rod and 15 mm/h for the rod row material. The melting zone was maintained in equilibrium between the two rods because of superficial tension force. This technique is crucible free and is not required to have a furnace insulation (Fig. 1) and contamination problems are completely minimized. Finely divided europium-doped gadolinium oxide powders were prepared according to the sollyophilization process described elsewhere [7]. In summary, gadolinium nitrate (NO3)3Gd H2O and europium nitrate (NO3)3Eu H2O with certified purity of 99.9% from Aldrich were dissolved in acidified water and with a molar proportion of varying Eu3+/Gd3+ from 0 to 1. During the addition of a basic solution, precipitation was instantaneous and the hydroxide was formed. The so-obtained precipitates were washed under ultrasonication and each washing was followed by centrifugation. A sol was obtained, freeze-dried at 10 C, dried at 400 K and thermally annealed at
ARTICLE IN PRESS C. Louis et al. / Journal of Crystal Growth 265 (2004) 459–465
461
estimate of the isotropic broadeningpdue ffiffiffiffiffi to the presence of microstrains: e ¼ ðp=1:8Þ U (e is the defect concentration in %) and IG is a p measure of ffiffiffiffiffiffi the isotropic size effect: T ¼ 180 Kl=p IG (T is the size in A; l is the wavelength in A and K is the Scherrer constant here equal to 43) [12]. The fwhm have been corrected from the instrumental corrections obtained from an annealed BaF2 reference. Scanning electron microscopy (SEM) was carried out using a Philips microscope operating at 20 kV. Fluorescence spectra was obtained using excitation with a Xe lamp and a J.Y. H10D monochromator (set at l ¼ 250 nm). Light was collected with an UV optical fiber coupled with a J.Y. TRIAX 320 and a CCD camera (1200 g/ 320 nm grating, resolution=0.65).
3. Results
Fig. 1. Schematic illustration of the LHPG technique principle.
different temperatures between 700 and 1300 K in order to obtain europium-doped gadolinium oxide. X-ray diffraction diagrams were carried out using a D5000 Siemens diffractometer with CuKa1 and Cu-Ka2 X-rays (l ¼ 0:15406 and 0:15444 nm). The diffraction pattern was scanned over the 2y range 3–50 in steps of 0.02 and a counting time of 8 s/step. The parameter of the crystalline sample has been refined taking into account the aberration arising from the specimen displacement, D2y ; given by the formula: D2y ¼ ð2s=RÞcos 2y where s is the is placement of the sample surface with respect to the axis of the goniometer and R is the radius of the goniometer circle. Similarly, the microstrains and the size of the coherent domains have been derived from a refinement of the full-width at half-maximum (fwhm), b; of the patterns fitted with Pseudo-Voigt functions according to the following relation: b2 ¼ U tan2 y þ IG=cos2 y: In the latter relation U is an
Using LHPG technique we have grown, under air atmosphere, undoped and Eu3+-doped Gd2O3 oxide fiber single crystal of 20–50 mm length with roughly cylindrical cross-section. The feeding rate was adjusted so that the fiber diameter was between 0.5 and 1 mm to minimize crack propagation, to keep melting zone interface plane and the quasi-equilibrium state during the crystal growth process. For pulling rate higher than 30 mm/h, the melting zone becomes unstable and seed disconnection is observed. Temperature stability is a critical factor as we also observed polycrystal formation when the temperature abruptly decreased because of an accidental power instability in the CO2 laser of LHPG machine. The obtained fibers are single crystal and free of impurities. For Eu3+ belonging to (0–15%), the fiber structure is monoclinic, in good agreement with JCPDS file no. 43-1015. The undoped Gd2O3 fibers are transparent, with uniform shape and free of macroscopic defects such as cracks, bubbles or inclusions (Fig. 2). The cracks and diameter variation were observed only for fibers with high Eu3+ concentrations (Eu3+>5%) and red color was observed because of Eu3+. For high Eu3+ concentration, the composition is not homogeneous and varied along the growth direction, the first part of the fiber contains high Eu3+
ARTICLE IN PRESS C. Louis et al. / Journal of Crystal Growth 265 (2004) 459–465
462
(222) 1300K
Intensity (arb.unit.)
(400)
Fig. 2. Gd2O3 fiber single crystal grown by LHPG technique (scale in mm).
(440)
(411) (332) (431)
(211)
1100K
700K
Emission spectra : λexc.= 254nm. 15
D0
7
18
21
24
27
30
33
36
39
42
45
48
2θ (deg)
F2
Fig. 4. Effect of temperature reaction on 2.5% molar Eu3+doped Gd2O3 cubic oxide formation.
5
7
F4
D0
5
5
580
7
D0
600
620
640
F3
660
680
700
720
wavelength (nm)
Fig. 3. Room temperature emission spectra of 15% molar Eu3+-doped Gd2O3 fiber single crystal (lexc ¼ 254 nm).
concentration and continuously decreases even with high pulling rate because of segregation problem. The effective distribution coefficient (Keff ) of Eu3+ ions was calculated on the basis of Pfann Eq. (1) [13] as follows: Cs =C0 ¼ keff ð1 gÞkeff 1 ;
ð1Þ 3+
D0-7Fj (j=0 to 4) transitions are detailed in the Fig. 3. For 0%oEu3+o15%, it was difficult to stabilize the cubic structure during crystal growth process because the high temperature monoclinic phase is the stable state, in addition the monoclinic–cubic transition can disturb the crystal qualities and it is difficult to insert high Eu3+ concentration in single crystal Gd2O3 monoclinic structure because of segregation problem, especially in incongruent composition (solid solution). This is why we have thought to find another way to insert high Eu3+ concentration in Gd2O3 oxide, to generate, keeping the cubic structure and to improve the luminescence properties. We extend our experiment to the utilization of a new lyophilization synthesis method. The initially obtained material is a slightly crystallized Eu3+doped hydroxide Gd(OH)3. The hydroxide geometry shape is of a homogeneous elongated plate [7]. The first Gd2O3 particle oxide appears after annealing at temperature 700 K (Fig. 4). For annealing temperature less than 600 K, the materials is Gd(OH)3 hydroxide with low crystallinity. The proportion of Gd2O3(Eu3+) cubic oxide (JCPDS file 43-1014) increases with increasing annealing temperature. The oxide quantity has been followed by the intensity evolution of the (2 2 2) peak of Gd2O3 cubic oxide. From 1100 K, the samples are Gd2O3 oxide single phase, belonging to the space group Ia3 (206) and are 5
D0
D0
5
F0
7
F1
15% Eu 2 O3 doped Gd2O3
7
Intensity (arb. unit.)
5
where C0 and Cs is the Eu concentration in the starting melts and in the crystal at the solidification fraction (g), respectively. The logarithm of Eq. (1) permits to calculate keff of Eu3+ concentration. In the case of Gd2O3 (Eu3+=5%), keff ¼ 1: For Eu3+ concentration>5%, keff o1: Fig. 3 shows the luminescence spectra for the excitation at 254 nm of Eu3+-doped Gd2O3 fiber single crystal (Eu3+=15%), and the spectral belongs to the luminescence of monoclinic Gd2O3 structure different from the cubic materials as already shown in our Ref. [7]. In the case of monoclinic Gd2O3 one, there are three C5 sites [14]. Only the
ARTICLE IN PRESS C. Louis et al. / Journal of Crystal Growth 265 (2004) 459–465
monoclinic phase-free. Fig. 5 shows the particle size evolution as a function of annealing temperature. The oxide particle sizes are less than 60 nm with high crystallinity. At low temperatures the particle sizes are less than 10 nm. It is clear that an annealing for a long time or at a temperature higher than 1300 K could be a way to increase particle sizes because of diffusion and germination phenomena which can be observed during the reaction. Fig. 6 shows the evolution of powder X-ray diffraction patterns as a function of Eu3+ con-
50
30
20
10
0 700
800
900
1000 1100 1200 Temperature (K)
1300
1400
Fig. 5. Particles size evolution as a function of temperature variation of 2.5% molar Eu3+-doped Gd2O3 cubic oxide.
10.87
0%
10.86
1% (400) (411) (332) (431)
(440) 2.5%
3+
(222)
X Eu
Intensity (arb.unit.)
Gd2O3
(211)
centration at the annealing temperature of 1300 K. The samples are monophased in the whole range of composition, from Eu3+=0 to 1. The Miller indices (h k l) can be assigned according to cubic symmetry in good agreement with JCPDS data card No. 43-1014. No presence of impurities or second phases belonging to another domain outside the solid-solution range in the Gd2O3–Eu2O3 equilibrium diagram [12] are detected. Fig. 7 shows the evolution of the lattice parameter as a function of Eu3+concentration. An important increase in aaxis of the Gd2O3 compound is observed with the increasing of the Eu3+ concentration. This can be explained on the basis of the ionic radii of Gd3+(0.0938 nm) smaller than Eu3+(0.0947 nm). The linear dependence observed clearly demonstrates the existence of a complete solid solution at the nanometer scale in the Gd2O3–Eu2O3 system. Emission spectra were performed for Gd2O3 as a function of Eu3+ concentration for the materials submitted to annealing temperature from 700 to 1300 K. In gadolinium oxide Eu3+ exhibits 4f6 5 D0-4f6 7Fj (j ¼ 026) emission lines. Fig. 8 shows the luminescence spectral for the excitation at 254 nm of the samples annealed at 1300 K. Only the 5D0-7Fj (j ¼ 022) transitions are detailed in Fig. 9. In the case of cubic Gd2O3 oxide, generally the spectrum is dominated by the well-known emission lines typical of Eu3+ in the C2 site without inversion center and the weak S6 site peaks belonging to the 5D0-7F1 transition. But
25%
50%
Lattice parameter (pm)
Size (nm)
40
463
10.85 10.84 10.83 10.82 10.81 10.80
Eu2O3
100%
16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50
2θ (deg)
10.79 10.78 0
10
20
30
40
50
60
70
80
90
100
3+
Fig. 6. Evolution of X-ray powder diffraction at room temperature as a function of Eu substitution of powders annealing at 1300 K.
% molar Eu doped Gd2 O3
Fig. 7. Evolution of lattice parameter as a function of Eu3+ concentration of powders annealing at 1300 K.
ARTICLE IN PRESS C. Louis et al. / Journal of Crystal Growth 265 (2004) 459–465
464
7
5
F2(C2)
D0
5%
1.6 1.2
F1(C2)
F1(S6)
7
5
7
D0
F0(C2)
5
0.4
D0
7
D0
0.8
10% 2.5%
1%
5
Intensity (arb.unit.)
2.0
25% 50%
0.0 585 590 595 600 605 610 615 620 625 630
λ (nm)
Fig. 8. Room temperature emission spectra as a function of Eu3+ concentration in Gd2O3 oxide at annealing temperature 1300 K (lexc: ¼ 254 nm).
7
5
F1(C2)
600
610
620
630
4. Conclusion
nm
700K
5
D0
700K
590
7
F1(S6)
7
D0
5
F0(C2)
7
D0
0.2
900K
0.0035
Intensity (a.u.)
1100K
0.6
5
Intensity (arb.unit.)
1300K
0.8
0.4
F2(C2)
D0
1.0
centration. The emission spectra change drastically in the case of the sample annealed at low temperature and high Eu3+ concentration (Fig. 9). After annealing temperature higher than 700 K the broad bands observed in the sample annealed at low temperature change to sharp emissions lines. The line positions are in good agreement with the level energies given in Ref. [16]. For Eu3+>5%, the increasing Eu3+ concentration and the decreasing temperature annealing reduces the luminescence intensity and gives a broad band that confirms the location of the Eu3+ ions in a disordered gadolinium oxide matrix for a low annealing temperature. Finally, in the case of single crystal fibers grown by LHPG technique, a monoclinic solid solution exist until Eu3+=15%, but crystal growth processes are complex because of the instability of the melting zone and segregation problem. For Eu3+>15% the study is on progress and will be initiated soon. In the lyophilized materials a complete solid solution with cubic structure, the best luminescent properties, is observed for Eu3+=5%.
0.0 585 590 595 600 605 610 615 620 625 630 635 640
λ (nm) Fig. 9. Room temperature emission spectra of 2.5% molar Eu3+-doped Gd2O3 annealed at a different temperature.
several additional emission lines are also observed. These ones are attributed to some surface effect [15]. The emission 5D0-7F2 is higher than 5D0-7F1 and the ratio of 5D0-7F2/5D0-7F1 is about 4. An additional peak is observed at about 610 nm, which is absent for high Eu3+ concentration, at low temperature annealing (To700 K) and in the monoclinic structure. This could be related to the size or surface effect; the research is under progress and will be soon published. For Eu3+o5%, the luminescence intensity increases with Eu3+ con-
We have used two different methods to prepare Eu3+-doped Gd2O3 luminescent materials. The first one is LHPG technique and the second one is a sol-lyophilization method. By LHPG technique we obtained a monoclinic single crystals fibers with low luminescent properties although the materials are single crystal. Also, related to segregation phenomena, the crystal qualities decrease when incorporating Eu3+ ions in the Gd3+ sites of Gd2O3 oxide. In the case of the sol-lyophilization method, the present investigation gives an overall picture of these so-synthesized compounds obtained after an optimized thermal annealing with a fuel process at starting reaction state and the annealing at selected temperature. Cubic single phase, with a well-distributed particle and the grain sizes in the range of 10–60 nm are obtained. The Eu3+ emission spectral depends strongly on the quantities of Eu3+ concentration and annealing temperature. For the annealing temperature of 1300 K, the Eu3+ composition corresponding to
ARTICLE IN PRESS C. Louis et al. / Journal of Crystal Growth 265 (2004) 459–465
the maximal luminescence intensity corresponds to europium contents higher than 5%. The samples obtained by the two techniques are clearly different, especially the luminescent properties and the structure of the obtained phases.
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[6] A. Garcia-Murillo, C. Le Lueyer, C. Garapon, C. Dujardin, E. Bernstein, C. Pedrini, J. Mugnier, Opt. Mater. 19 (2002) 161. [7] C. Louis, R. Bazzi, M.A. Flores, W. Zheng, K. Lebbou, O. Tillement, B. Mercier, C. Dujardin, P. Perriat, J. Solid State Chem. 173 (2003) 335. [8] H.T. Hintzen, H.M. Van Noort, J. Phys. Chem. Solids 49 (1996) 783. [9] K. Lebbou et al., in preparation. [10] J. Dexpert-Ghys, M. Faucher, P. Caro, Phys. Rev. 23 (2) (1981) 607. [11] V.C. Costa, M.J. Lochhead, K.L. Bray, Chem. Matter 8 (1996) 783. [13] W.G. Pfann, Trans. Am. Inst. Min. Metall. Eng. 194 (1952) 747. [12] J.B. Langford, Aust. J. Phys. 41 (1988) 173. [14] B. Beaury, et al., J. Lumin. 40–41 (1998) 651. [15] D.B.M. Klaasen, et al., J. Lumin. 40–41 (1988) 651. [16] M. Buijs, A. Meijerink, G. Blasse, J. Lumin. 37 (1987) 9.