Journal of Non-Crystalline Solids 355 (2009) 1333–1337
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Tb3+ luminescence enhancement of YAG:Tb3+ nanocrystals embedded in silica xerogel Mariusz Kubus a, H.-Jürgen Meyer b, Lorenz Kienle c, Andrzej M. Kłonkowski a,* a
´ sk, Sobieskiego 18, 80-952 Gdan ´ sk, Poland Faculty of Chemistry, University of Gdan Institut für Anorganische Chemie, Universität Tübingen, Ob dem Himmelreich 7, 72074 Tübingen, Germany c Max-Planck-Institut für Festköperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany b
a r t i c l e
i n f o
Article history: Available online 13 June 2009 PACS: 78.60.Sq 76.30.Kg 78.30.Ly 78.40.-q 78.55.-m Keywords: Composite materials Rare-earth ions Disordered solids Absorption and reflection spectra Photoluminescence
a b s t r a c t The luminescence behavior of composite materials consisting of nanocrystals of Y3xAl5O12:Tb (YAG:Tb3+) embedded into silica xerogel has been studied. Blue and green luminescence of the materials is due to a cross-relaxation effect in Tb3+ ions doped into a YAG lattice. The materials with YAG:Tb3+ nanocrystals immobilized in silica exhibit enhancement of Tb3+ luminescence in comparison with the macrocrystalline YAG:Tb3+ powder. The Tb3+ luminescence intensity of a composite material dried at room temperature can be improved when higher aliphatic alcohols are applied in a one-pot procedure during a sol–gel synthesis. On the other hand, the Tb3+ luminescence is quenched in the presence of Ag nanoparticles in the material. The composite material (YAG:Tb3+ in silica) exhibits thermal stability at higher temperatures and achieves the highest emission intensity after having been annealed at 700 °C. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction A luminescent material, also called a phosphor, is a solid which converts different types of energy into electromagnetic radiation over and above thermal radiation. The electromagnetic radiation emitted by a luminescent material is usually in the visible range, but can also be in other spectral regions, such as ultraviolet or infrared [1]. Luminescent materials are often in form of crystal grains or powders. The system consists of a host lattice (matrix) and a luminescent center, often called the activator. The introduction of lanthanide activated phosphors into fluorescent lamps during last two decades has drastically improved the light output and the color rendering. A modern fluorescent lamp contains the following lanthanide ions: Eu2+, Ce3+, Gd3+, Tb3+, Y3+ and Eu3+. It is the Tb3+ emitting ions from among them that are in focus of our interest. Tb3+ cation emits a lemon-yellow color that results from a combination of a strong green emission line attributed to 5D4 ? 7F5 transition and other lines in orange and red corresponding to 5 D4 ? 7F6 and 5D4 ? 7F7 transitions, respectively. Therefore, Tb3+ ions are used in green phosphors in fluorescent lamps and color * Corresponding author. Tel.: +48 58 345 04 43; fax: +48 58 523 54 00. E-mail address:
[email protected] (A.M. Kłonkowski). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.05.021
TV tubes. However, terbium(III) doped YAlO3 or GaTaO4 are also blue emitting phosphors owing to the 5D3 ? 7FJ transition. The Tb3+ emission color is controlled by a cross-relaxation process depending on the Tb3+ concentration [2,3]. Thus, terbium ‘blue’ and ‘green’ phosphors can be combined with trivalent europium red phosphors to provide the ‘trichromatic’ lighting technology which is by far the largest consumer of the world’s terbium supply. The yttrium aluminum garnet (YAG, Y3Al5O12) is a prominent crystalline material of the garnet group. The Tb(III) doped YAG (YAG:Tb) is a phosphor used in cathode ray tubes. It emits a yellow-green color owing to the dominating 5D4 ? 7F5 band at 543 nm. It is well known that chemical and physical properties of materials with strong chemical bonding depend on the particle size effects. It has been demonstrated that also other nanocrystals display widely interesting size-dependent optical, electronic and chemical properties, similarly to noble metal clusters [4–6]. These effects are observed in the scale regime where neither quantum nor classical physics hold [7]. We have incorporated YAG:Tb3+ (Y3xAl5O12:Tbx, where x = 0.05) nanocrystals into a silica xerogel matrix. The sol–gel process is a very attractive procedure for preparation of oxide matrices for optical materials [8,9]. In many applications the sol–gel derived material is used to provide a transparent chemically and mechanically stable matrix in the UV–vis range.
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The aim of this work was to prepare a composite phosphor from YAG:Tb3+ nanocrystals incorporated into a silica xerogel matrix. Due to nanometer size of the YAG:Tb3+ particles in the material intensity of the Tb3+ emission was enhanced in comparison to YAG:Tb3+ crystals. On the other hand, owing to the cross-relaxation effect, the composite material has shown that the dominant emission band shifts to a longer wavelength with the increasing Tb3+ concentration in the matrix. The presence of Ag nanostructures in the vicinity of immobilized YAG:Tb3+ nanocrystals causes Tb3+ luminescence quenching.
2. Experimental 2.1. Preparation Y3xAl5O12:Tb3+, where x = 0.05 (YAG:Tb3+), was synthesized by the sol–gel method proposed by Veith et al. [10]. YAG:Tb3+ crystals were powdered in an agate mortar and then pulverized in methanol to nanosized particles by an ultrasonic technique for 3 h. The preparation of Ag nanoparticles was based on AgNO3 reduction in the presence of trisodium citrate [11]. A 50 cm3 aqueous solution containing 1.25 105 mole AgNO3 and 6.8 105 mole sodium citrate was prepared. The solution was boiled for 20 min. The sodium citrate served only as a stabilizer, since it cannot reduce the silver salt at a room temperature. The obtained silver sol was diluted to an Ago concentration of 2.35 104 M. The YAG:Tb3+ nanoparticles were entrapped into silica xerogel by the sol–gel method in a one-pot procedure [8]. About 2.46 cm3 of TMOS was dissolved in 5.0 cm3 of MeOH and 1.0 cm3 of water was added. The mixture was stirred for 15 min and 0.2 cm3 of NH3(aq) was added dropwise as a catalyst. Then, the methanol sol consisting of YAG nanoparticles (possibly higher aliphatic alcohols or Ag nanostructures) was added. Next, the mixture was stirred for 15 min again. The final molar ratio of TMOS:H2O:MeOH was 1:4:8. The YAG, higher aliphatic alcohol and Ago concentrations were equal to 22 106, 20 104 and 5 109 mol/ g silica, respectively. After two days, a rigid wet gel with YAG nanoparticles was obtained. The samples were dried at 60 °C, then crushed and sieved (0.25–0.50 mm). Finally the materials were annealed at temperatures of up to 1000 °C.
Fig. 1. TEM micrograph of the material consisting of YAG:Tb3+ nanocrystals (YAG) immobilized in SiO2 xerogel.
2.2. Apparatus The YAG:Tb3+ crystals were pulverized to nanosized particles. The methanol slurry was prepared by ultrasound radiation for 3 h. The ultrasonic equipment for the sample disintegration was Sonorex Super RK 103H. The samples were annealed in an air atmosphere in a programmable oven equipped with an SM-946 temperature controller. The products were characterized by a transmission electron microscope (TEM), a high resolution transmission electron microscope (HR-TEM) and a selected-area electron diffraction (SAED). The images of nanoparticles were obtained with a Philips CM 30ST at an accelerating voltage of 300 kV. UV–vis absorption spectra were measured using a Perkin–Elmer Lambda 25 spectrophotometer equipped with a Labsphere accessory for powders. The photoluminescence excitation and emission spectra were recorded using a Perkin–Elmer LS 50B spectrofluorimeter with a reflection attachment. 3. Results The TEM micrograph in Fig. 1 shows YAG:Tb3+ nanocrystals entrapped into a silica xerogel by the sol–gel method. The shapes and dimensions of the YAG:Tb3+ particles are not uniform but their sizes are in the nanometer range.
Fig. 2. High resolution TEM image of the YAG:Tb3+ nanocrystal in silica. Inset: SAED image of the nanocrystal.
Fig. 2 shows a high resolution TEM micrograph of YAG:Tb3+ nanoparticle in silica, while the precession electron diffraction pattern in the inset proves a single crystalline structure. A reflectance spectrum of the composite material with nanocrystals of YAG:Tb3+ incorporated into a silica xerogel consists of two bands at 227 and 272 nm (Fig. 3). The excitation spectrum for kem = 453 nm emission related to the 5D4 ? 7F5 band of Tb3+ consists of one band peaked at 209 nm, a three-component band with a maximum at 227 nm, and a dominating band at 272 nm (Fig. 4). The excitation wavelength corresponding to the 272 nm dominating band position was applied to induce the most intense emission of the YAG:Tb3+ nanocrystals entrapped into silica and of the YAG:Tb3+ powder. The emission spectra displayed in Fig. 5 shows UV bands attributed to 5D3 ? 7FJ transitions (where: J = 6, 5 and
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Reflectance R / %
Emission intensity I / arb. units
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0
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0
1
2 3 4 Number of C atoms n
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Fig. 6. Emission intensity I of the band at 543 nm related to 5D4 ? 7F5 transition in Tb3+ vs. number of C atoms in n-aliphatic alcohol formula CnH2n + 1OH. Alcohol concentration 2.6 104 mol/g silica. kexc = 272 nm.
Fig. 3. Reflectance spectrum of YAG:Tb3+ entrapped in silica xerogel.
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Emission intensity I / arb.units
Emission intensity I/ arb.units
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272 nm
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200 180 160 5.2
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Fig. 4. Excitation spectrum of YAG:Tb3+ immobilized in silica xerogel. kem = 543 nm corresponds to kmax of the intensive Tb3+ emission band attributed to 5D4 ? 7F5 transition.
2.6
220
1.3 0
5 10 15 20 -4 -1 n -hexanol concentration c / 10 mol.g SiO2
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Fig. 7. Emission intensity I of the Tb3+ band at 543 nm of the composite vs. nhexanol concentration chex. Tb3+ concentration 5.5 106 mol/g silica. kexc = 272 nm.
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Emission intensity I/ arb. units
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J=6
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D4 - FJ 5
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J=6 4
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450 500 550 Wavelength / nm
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Fig. 5. Emission spectra of: (a) YAG:Tb3+ powder and (b) YAG:Tb3+ nanocrystals incorporated into silica xerogel. Excitation wavelength 272 nm. Tb3+ concentration 5.5 106 mol/g silica.
4) and four distinct bands in the visible range ascribed to 5D4 ? 7FJ transitions (J = 6, 5, 4 and 3). In spite of a much lower concentration of Tb3+ ions in the YAG:Tb3+ nanocrystals immobilized into the silica matrix than in the basic YAG:Tb3+ powder (5 mol% of Tb), the luminescence intensity of the composite is ca. three times higher than in the case of the crystal powder. Nanoparticles without capping exhibit a trend to agglomerate. We applied higher n-aliphatic alcohols as capping agents during the syntheses to avoid this disadvantage. The intensity of the Tb3+ band peaked at 543 nm has been used as a measure of the emission intensity. As displayed in Fig. 6, the emission intensity does not change monotonically with the increasing hydrocarbon chain length. The highest luminescence intensity among the applied alcohols is observed for n-hexanol. If the n-hexanol concentration is increased in the reaction mixture before gelation, the band’s maximum intensity peaked at 543 nm of the composite is attained for a concentration equal to 2.6 104 mol/g silica (Fig. 7).
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240 5.5
Emission intensity I / arb.units
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11
180 1.1
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140 120 100 80 0.11
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5
10
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20 -6
25
-1
Tb(III) concentration c / 10 mol.g SiO2 Fig. 8. Emission intensity I of the Tb3+ band at 543 nm vs. Tb3+ concentration. kexc = 272 nm. Fig. 10. TEM micrograph of Ag nanostructures in the material consisting of YAG:Tb3+ nanocrystals incorporated into silica xerogel.
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(b)
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(b) Emission intensity I / arb. units
Emission intensity I / a.u.
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Fig. 9. Tb3+ emission spectra of the material consisting of YAG:Tb3+ nanocrystals and n-hexanol (chex = 2.6 104 mol/g silica) immobilized into silica xerogel. Tb3+ concentration: (a) 1.1 107, (b) 5.5 106 and (c) 2.2 105 mol/g silica.
The highest enhancement of luminescence is demonstrated by the composite prepared with n-hexanol (chex = 2.6 104 mol/g silica), if the concentration of Tb3+ in it is equal to 5.5 106 mol/g silica. The plot in Fig. 8 shows the change of the 543 nm band intensity vs. the Tb3+ concentration. However, the Tb3+ emission spectra in Fig. 8 show intensity changes of the bands attributed to 5D3 ? 7FJ transitions (below 480 nm) in comparison to the 5D4 ? 7FJ transitions. The material with the lowest Tb3+ concentration (1.1 107 mol/g) exhibits decay of the band groups corresponding to 5D4 ? 7FJ. While, in Tb3+ concentration equals to 5.5 106 mol/g the 5D4 ? 7FJ band group attains its highest intensity. Ag particles were immobilized into a silica xerogel together with YAG:Tb3 nanocrystals by a one-pot procedure. It turned out that the particles were nanosized and not uniform in structure (Fig. 10). As shown above, the presence of the nanosized YAG:Tb3+ particles in the composite material causes improvement of the Tb3+ luminescence. On the contrary, the Tb3+ emission in the vicinity
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400 600 Temperature / °C
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Fig. 11. Changes of the emission intensity I of the Tb3+ band at 543 nm with increasing of annealing temperature for composites consisting of YAG:Tb3+ nanocrystals entrapped into a silica xerogel matrix in each case. (a) The YAG:Tb3+ nanocrystals only, (b) the nanocrystals treated with n-hexanol and (c) YAG:Tb3+ nanocrystals in the vicinity of Ag nanostructures. Concentrations: cTb = 5.5 106, chex = 2.6 104 and cAg = 5 109 mol/g silica. kexc = 272 nm.
of silver nanostructures is depressed in comparison to the composite (YAG:Tb3+ nanocrystals in silica). These effects are demonstrated in Fig. 11. However, plots of the emission intensity I vs. annealing temperature in the same figure exhibit non-monotonical changes. Thus, the two-component material [curve (a)] achieves maximum emission intensity after annealing at 700 °C, while the emission maxima fall at 600 and 400 °C for the three-component materials [with n-hexanol and Ag nanostructure components, curves (b) and (c), respectively]. 4. Discussion Both components, YAG:Tb3+ crystals and Ag clusters used in the prepared materials are of nanometer-size particles (see Figs. 1, 2 and 10). Owing to the nanoscaled YAG:Tb3+ component immobi-
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lized into a silica matrix, a luminescent material is obtained yielding a higher emission intensity than the crystalline YAG:Tb3+ powder (Fig. 5). It is very probable that this effect is due to the energy transfer from an excited defect state of an amorphous silica xerogel to a Tb3+ ion because a similar effect has been observed for an SiO2 xerogel doped with Tb3+ [12]. The reflectance and excitation spectra of the two-component materials (Figs. 3 and 4) consist of two distinct Tb3+ bands at 227 and 272 nm related to 4f8 ? 4f75d transition as well as f–f transition lines of low intensity at longer wavelengths (e.g. 350 nm). The Tb3+ emission spectra of the prepared luminescent materials could be divided into two groups upon excitation at the spin-allowed 4f8 ? 4f75d transition at 272 nm, (Fig. 5) – blue emission below 480 nm owing to 5D3 ? 7FJ (J = 6, 5 and 4) transitions and green emission above 543 nm due to 5D4 ? 7FJ (J = 6, 5, 4 and 3) transitions [13,14]. However, the observed emission color tuning from blue to green with the increasing Tb3+ concentration (Fig. 9) can be explained in terms of cross-relaxation [15,16]. The cross-relaxation as a one-step transfer of some of the energy from the excited donors to acceptors is established as the equilibrium between the 5D3 emission and the 5D4 emission (cf. spectra in Figs. 5 and 9) that can be expressed in Tb3+ by the following process [17]: 3þ
3þ
3þ
3þ
Tb ð5 D3 Þ þ Tb ð7 F6 Þ $ Tb ð5 D4 Þ þ Tb ð7 F0 Þ: 5
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the density spectrum of the electronic states of the Ag nanostructures. On the other hand, the energy transfer quenching is also very sensitive to electronic state density changes in the energy donors [11]. The results in our case (see Fig. 10) suggest that the donor Tb3+ ions, acting as the emission centers in the vicinity of Ag clusters, possess the appropriate conditions to depress the emission of a luminescent material. 5. Conclusions (1) Owing to the cross-relaxation effect for the Tb3+ doped YAG nanocrystals, the material can be treated as a blue and green phosphor. (2) Probably owing to the energy transfer: the excited defect state of a silica matrix ? Tb3+ ions the material consisting of YAG:Tb3+ nanocrystalites attains more intensive luminescence than the YAG:Tb3+ crystalline powder. (3) The agglomeration tendency of YAG:Tb3+ nanocrystals before gelation can be depressed by n-hexanol as a capping agent. (4) The presence of Ag nanostructures in the vicinity of YAG:Tb3+ nanocrystals causes the Tb3+ emission quenching due to the surface plasmon phenomenon.
ð1Þ
This emission process from higher D3 state is mainly present for a low Tb3+ concentration and this emission is quenched for higher concentrations of terbium ions by a cross-relaxation resulting from the emission enhancement for a lower 5D4 state [2]. The three-component materials prepared with YAG:Tb3+ nanocrystals, silica, and higher n-aliphatic alcohols as capping agents show probably the odd–even effect with an increasing number of C atoms in the alcohol molecules (Fig. 6). The best result in the sense of emission intensity is exhibited by the sample prepared with n-hexanol. However, it was only the sample which was dried at room temperature that emitted more intensely than the twocomponent samples with YAG:Tb3+ in silica. Annealing at higher temperatures reduces the Tb3+ emission of samples with n-hexanol due to evaporation or thermal degradation of the alcohol (compare with the two-component sample in Fig. 11). The two-component material achieves a maximum of the Tb3+ luminescence after annealing at 700 °C. In general, the presence of Ag nanostructures in the vicinity of Tb3+ ions causes depression of their emission activity with only one exception, viz. higher emission is observed for the sample annealed at 400 °C. The general quenching effect is caused by the surface plasmon that can act as an energy acceptor even at a distance of 1 nm between the Tb3+ emission center and the silver surface [18]. Metal nanoparticles have a continuum (more or less) of electronic states and exhibit energy transfer behavior as excited state quenchers [19]. The distance of the closest approach of terbium ions to an Ag nanostructure core will change with the particle size. Thus, the origin of the nanostructure size dependence on the quenching process lies also in the core-size related differences in
Acknowledgements This work is supported by the Ministry of Science and High Education (Grant DBN 3376/T02/2007/32). The authors would like to thank Mrs. V. Duppel for practical TEM work and Prof. Dr. Dr. h. c. mult. A. Simon for enabling the TEM experiments.
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