Optical Materials 33 (2011) 1124–1127

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Optical Materials

journal homepage: www.elsevier .com/locate /optmat

Synthesis of YAG:Ce/TiO2 nanocomposite films

Amélie Revaux a,⇑, Géraldine Dantelle a, Dominique Decanini b, Anne-Marie Haghiri-Gosnet b,Thierry Gacoin a,⇑⇑, Jean-Pierre Boilot a

a Laboratoire de Physique de la Matière Condensée, Ecole Polytechnique – CNRS, 91128 Palaiseau, Franceb Laboratoire de Photonique et Nanostructures, CNRS, 91460 Marcoussis, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 31 May 2010Received in revised form 20 July 2010Accepted 31 July 2010Available online 15 September 2010

Keywords:YAG:Ce3+ nanoparticlesTiO2 sol–gel filmsPhotoluminescence quantum efficiencyGlycothermal methodSoft nano-imprint lithographySurface patterning

0925-3467/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.optmat.2010.07.022

⇑ Corresponding author.⇑⇑ Corresponding author.

E-mail addresses: amelie.revaux@polytechnique.ed@polytechnique.fr (T. Gacoin).

Our work is devoted to the development of YAG:Ce3+ phosphor nanoparticle-based converter layer forwhite LEDs. To avoid losses due to scattering effects, the strategy is to control separately the down-conversion and the extraction of light instead of using micron-sized luminescent particles acting simul-taneously as both converter and scatterer. YAG:Ce nanoparticles were synthesized by a glycothermalmethod in autoclave at low temperature (300 �C). Y3Al5O12 garnet phase with a crystallite size of25 nm was obtained, as verified by X-ray diffraction and electron microscopy. The quantum yield ofnanoparticles is 55%. The colloidal nanoparticles are finally incorporated into a sol–gel matrix of TiO2.The small difference in refractive index between particles and matrix and the nanosize of the particlescontribute to the transparency of the converter films. The surface of these layers can be periodically pat-terned by soft nano-imprint lithography. The diffraction due to the obtained photonic crystal at the sur-face may offer the opportunity to compensate the absence of scattering to extract the converted light.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Yttrium aluminium garnet (Y3Al5O12) doped with Ce3+ ions(YAG:Ce) is a well-known phosphor, commonly used as light con-verter in commercial white LEDs. Indeed, its capacity to efficientlyabsorb blue light and emit in the yellow range (thanks to the 4f–5dtransitions of Ce3+) allows generation of white light from blue In-GaN LED chips.

Traditionally, YAG:Ce particles, produced by solid state reactionat high temperature (P1400�), are incorporated into epoxy bulbsand deposited on LEDs as shown in Fig. 1a. Because of their synthe-sis route, these particles are in the micrometer size range, and thuspresent strong light scattering effects. This drastically affectsabsorption, dissipation and extraction properties of the converterlayer. This scattering can be beneficial because it contributes tothe extraction of light out from the converter layer. But it also in-duces losses such as the absorption of backscattered light into thep–n junction, thus decreasing the external yield of the device.

In this work, we investigate a strategy that aims at controllingseparately the down-conversion and the light extraction insteadof using micron-sized particles simultaneously acting as converterand scatterer. For this purpose, the strategy can consist in formingYAG:Ce thin films onto blue LEDs. The luminescent thin films

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should be transparent (i.e. non diffusive) and present a high quan-tum yield. Due to the absence of loss by scattering, these films arealso very good waveguides. Thus, for white LEDs applications, thelight has to be extracted from the top of the film. This could be con-trolled by photonic crystal diffraction at the surface of the film,thus allowing also some control of the light emission directionality(Fig. 1b) [1].

Few examples of synthesis of YAG:Ce thin films for light conver-sion are found in literature by sol–gel [2], rf magnetron sputtering[3], liquid phase epitaxy [4] or pulsed laser deposition [5]. Theadvantage of these techniques is that thin-film phosphors can beeasily integrated onto LEDs arrays. However, it has severaldrawbacks:

(1) These films need to be further annealed (above 900 �C) afterdeposition so that the YAG phase crystallizes, meaning thatthe substrate needs to be resistant to high temperatures.

(2) The obtained films are polycrystalline, presenting grainboundaries, cracks and rough surface that induce lightscattering.

To realize transparent down-conversion layer for white emit-ting devices, our strategy consists in synthesizing luminescentYAG:Ce particles with a nanometer size (limiting light scatteringin the converting layer), then incorporating these already crystal-lized particles into a matrix and finally depositing the compositeas a thin film onto a LED. The refractive index n of the surround-ing matrix should be relatively high in order to limit the total

Fig. 1. Schematic of light propagation in (a) standard LEDs and (b) transparent thin-film based LEDs with light extraction by surface patterning. (a) In standard LEDs,micron-sized powders acting as scatterer and emitter are represented by yellowcircles. Arrows materialize light scattering from particles. (b) Transparent converterfilm is represented in yellow on the blue LED. The yellow light is guided anddirectionally extracted from the layer by a periodic surface pattering.

A. Revaux et al. / Optical Materials 33 (2011) 1124–1127 1125

reflection of the excitation blue light at the film/GaN substrateinterface and thus allowing light extraction at the textured film/airinterface. n should also be comparable to the YAG refractive indexto avoid light losses due to n mismatch.

Recently, colloidal suspensions of YAG nanoparticles in ethanolwere synthesized through a glycothermal route at relatively lowtemperature (6300 �C) [6]. This technique does not require anyfurther thermal annealing for crystallization, leading to nanoparti-cles with an average size of 30 nm and dispersible in ethanol.When doped with cerium, these YAG: particles present a quantumyield between 21% and 56% depending on the experimental condi-tions [7,8].

In the first part of our work, we revisit this synthesis strategy toprepare colloidal solutions of YAG:Ce nanoparticles with controlledaverage size, size distribution and dispersion in ethanol. The sec-ond part concerns the incorporation of the luminescent YAG:Cenanoparticles into transparent TiO2 films deposited by spin-coat-ing. TiO2 is chosen because of its high refractive index, whichapproximately corresponds to the refractive index of the substrateand the particles. Moreover, our TiO2 films are elaborated bysol–gel chemistry, starting from inorganic precursors soluble inethanol. Considering that the previous YAG:Ce nanoparticles arewell-dispersed in ethanol, TiO2 films encapsulating YAG:Ce nano-particles can be prepared without requiring any solvent exchangeor surface functionalization, and the good dispersion of the parti-cles is preserved.

After deposition of the converter layer, the last step is the sur-face patterning of the film to control the extraction of light out ofthe converter layer and to compensate the absence of scattering.Nano-imprint lithography, developed by Chou et al. [9] is welladapted for our system. This technique, more accessible thanexpensive and time-consuming techniques like electron beamlithography or reactive ion etching, allows sub-100 nm resolutiononto large surface area. It is commonly used to pattern thermoplas-tics and UV curable resists but the sol–gel technique that we usealso allows nano-imprint lithography. Last part of our work showsthe successful application of this technique for the patterning ofthe surface of the composite layer, opening the way toward thecareful study of the best structure adapted for the light extractionin our systems.

2. Experimental

2.1. YAG:Ce nanoparticles synthesis

The synthesis of YAG nanoparticles doped with 1% cerium wasadapted from the one reported in [10]. Typically, 2.5 mmol of alu-minium isopropoxide, 7.425 mmol of yttrium acetate hydrate and0.075 mmol of cerium III acetate hydrate were mixed together in

20 mL of 1,4 butandiol (all products from Sigma Aldrich). Whenhomogeneous, the mixture was poured in a 71 mL non-stirredautoclave (Parr, Series 4740 High Pressure Vessel) with 33 mL ofsolvent used to rinse the previous container. The vessel was placedin a ceramic heater at 300 �C for 3 h. The inner pressure reached70 bars after 3 h. The synthesis was conducted with stirring byinserting a magnetic stirring bar in the vessel and placing a stirringplate underneath the heater.

After reaction, the obtained yellow solution was washed threetimes in ethanol by centrifugation at 11,000 rpm for 10 min, to re-trieve a stable colloidal solution in ethanol.

The size of the particles in suspension was investigated by elec-tron microscopy (Hitachi S4800 scanning electron microscope FEG-SEM and Philips CM30 transmission electron microscope TEM) anddynamic light scattering (DLS, Malvern Zetasizer). X-ray Diffraction(XRD, Philips X-Pert Cu Ka radiation) was performed on dry pow-der. The luminescent properties of solutions were measured with afluorimeter (Xe lamp, HITACHI F-4500FL). The absolute fluores-cence quantum yields of the powders were measured using anintegrating sphere coated with Lambertian reflector as describedby Greenham [11]. For this purpose, powders were mixed in epoxyresin and deposited on silica slide. These samples were inserted inthe sphere and excited by an Ag laser. The light coming out fromthe sphere was measured by a calibrated silicon photodiode.Absorption and emission could be successively measured usingproper filter between the sphere and the detector (UV and visiblebandpass filter, respectively).

2.2. Incorporation of nanoparticles in TiO2 and film deposition

The sol–gel method consists in depositing films from a liquidsolution containing TiO2 molecular precursors (called ‘‘sol”) thatcan be converted into nanocrystalline TiO2 after a thermal treat-ment. Titania sol was prepared by mixing 18 mL of Titanium (IV)butoxide with 9.8 mL of butanol during 10 min. Then, 27.3 mL ofacetic acid was added and the mixture was heated at 50 �C. After30 min, the solution was cooled to 0 �C with ice during 1 h.8.9 mL of DI water and 37.6 mL of ethanol were mixed togetherand added drop by drop in the cold solution. The resulting mixturewas heated at 50 �C for 1 h. The solution was stirred during allsteps of the synthesis. Finally, the sol was filtered with a 45 lmporous membrane. Colloidal solution of nanoparticles in ethanolwas added to titania sol. Then ethanol was evaporated from themixture to condensate the solution. The less ethanol is remaining,the thicker will be the film. The films were deposited onto siliconsubstrates by spin-coating (2000 rpm, 30 s). The silicon waferswere previously cleaned by 20 min immersions in a fresh piranhasolution (H2O2 (30%)/H2SO4 1:3 vol). This easy in-house elabora-tion technique gives access to various high refractive indices andfilm thicknesses depending on dilution of the sol and post-deposi-tion thermal annealing. It is also suitable for low-cost fabrication.

2.3. YAG:Ce doped TiO2 thin-film patterning and characterization

The architecture of a periodically patterned PDMS mold can betransferred to a TiO2 sol–gel film by soft nano-imprint lithography[12,13]. A TiO2 precursor solution was spin-coated onto the film.Before condensation of the gel, the anti-adhesive treated PDMSmold was deposited on it. The system was heated at 110 �C underpressure (20 PSI) during 5 min in a NXR2500 from Nanoneximprinter. During this step, the viscous sol fills the cavities of themold and becomes solid because of the polymerization of thetitania network induced by heat. After cooling, patterned filmwas separated from the stamp.

2500

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The layer thickness and refractive index of the film were mea-sured by spectroscopic ellipsometry (Jobin–Yvon Horiba). Filmswere also observed by scanning electron microscopy.

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Fig. 4. Excitation (with the emission wavelength at 550 nm) and emission spectra(with the excitation wavelength at 450 nm) of YAG:Ce nanoparticles suspension inethanol at room temperature.

3. Experimental results and discussion

3.1. Characterization of powders and colloidal solution

After 3 h of reaction, under stirring, a yellow suspension wasobtained. Fig. 2 presents the X-ray diffraction diagram of YAG:Cepowders obtained after washing, dispersion in ethanol and drying.Most of XRD peaks agree with those of Y3Al5O12, which presents acubic garnet structure. We can also detect an additional phase ofboehmite (AlO(OH)), as previously shown by Nyman et al. (layeredalumina phase) [8]. This additional phase is also observed on TEMimaging of powders as shown on Fig. 3a. The crystallite size calcu-lated from the main peak (4 0 0) width by Scherrer’s equation is25 nm. This is quite consistent with the size observed on HR-TEM imaging from the inset of Fig. 2.

The suspension in ethanol is very stable as controlled byDynamic light Scattering. The nanoparticles size in suspension is175 nm but decreases to 75 nm after only 1 min centrifugation at10,000 rpm which allows removing some residual aggregates.This size is consistent with TEM and SEM imaging shown onFig. 3b and c.

Fig. 2. XRD diagram of YAG:Ce powders. The inset presents a HR-TEM imaging of aYAG:Ce nanoparticles.

Fig. 3. (a) TEM imaging of a YAG:Ce nanoparticle surrounding by boehmite layered cryYAG:Ce suspension deposited on a silicon wafer.

Excitation and emission spectra of YAG:Ce colloid in ethanol isshown on Fig. 4. Two excitation peaks are observed at 340 and450 nm, respectively associated with the 4f?5d(2B1g) and4f?5d(2A1g) transitions [14]. Contrary to bulk YAG:Ce, the lowestenergy excitation band presents a shoulder around 400 nm. Thisis probably due to Ce3+ ions on the surface of the particles.The emission of Ce3+ peaks at 550 nm, corresponding to the5d(2A1g)?4f transition. Using a calibrated integrated sphere,the quantum yield (g) of the YAG:Ce nanoparticles was measuredto be 55%, which is similar to the values found by Nyman et al.[8]. For bulk YAG:Ce, g was found to be 85%. The difference canbe explained by surface effects, such as defects or adsorbed mole-cules that are known to quench the luminescence of nanoparticles.However, the difference of quantum yield between bulk and nano-particles remains small, probably because Ce3+ ions do not haveintermediate states that are very sensitive to non radiativerelaxations.

3.2. Characterization of TiO2/YAG:Ce films

Fig. 5a is a SEM image of a TiO2 film charged with YAG:Cenanoparticles with a low concentration (molar ratio nYAG/nTi = 0.4%). We approximate that all Ti atoms are in TiO2 networkand that the matrix is a mixture of 75% of dense TiO2 witha density of 3.8% and 25% of void as verified by ellipsometry.

stallites. (b) TEM imaging of YAG:Ce nanoparticles suspension. (c) SEM imaging of

Fig. 5. SEM imaging of TiO2 films charged with YAG:Ce nanoparticles. (a) Diluted: nYAG/nTi = 0.4%, VYAG=VTiO2 ¼ 2% and (b) concentrated: nYAG/nTi = 15%, VYAG=VTiO2 ¼ 68%

Fig. 6. SEM imaging of a region with both patterned and nonpatterned YAG:Ce/TiO2

film surface.

A. Revaux et al. / Optical Materials 33 (2011) 1124–1127 1127

This dilution thus corresponds to a volume fraction of YAG(VYAG=VTiO2 ) close to 2%. The particles appear well-dispersed inthe film, proving that they do not aggregate in the initial titaniasol and that they remain well-separated even during the filmdeposition and the solvent evaporation. These films are perfectlytransparent and show no light scattering. Fig. 5b shows an imageof a much more concentrated film, with a YAG volume fractionof 68% (nYAG/nTi = 15%).

TiO2 films doped with YAG:Ce nanoparticles were analyzed byspectroscopic ellipsometry after deposition and drying withoutfurther thermal annealing. A thickness of 110 and 105 nm anda refractive index of 1.76 and 1.74 at 550 nm for the charged(molar ratio nYAG/nTi = 15%, Fig. 5b) and diluted films (molar rationYAG/nTi = 0.4%, Fig. 5a) were respectively measured. Withoutthermal annealing, TiO2 is not completely condensed and canbe modelized by a mixture of TiO2 and void, thus leading to alower refractive index as compared to the crystallized matrix.As YAG presents a higher refractive index than non-crystallizedTiO2, the more particles are incorporated in the film, the higherthe refractive index of the film. It is possible to deposit thinnerfilms by diluting the sol with ethanol or to deposit thicker filmsby successive multilayer depositions.

Fig. 6 shows an example of one dimensional photonic crystal atthe surface of YAG:Ce/TiO2 nanocomposite film. One can observethat the structure is well-printed onto the film. No cracks can beseen, showing that the surface patterning does not damage the filmaspect. Such surface architecture can be used to force light outfrom the top of the film.

4. Conclusion

The goal of this work was to synthesize transparent YAG:Cenanoparticle-based converter layer for white LEDs, to furthercontrol the extraction of light outside of the film without depend-ing on the microstructure as for conventional micronic phosphors.A very stable suspension of luminescent YAG:Ce nanoparticles inethanol was obtained after synthesis. These particles were incorpo-rated into a titania sol without requiring any solvent exchange be-fore deposition by spin-coating. Thanks to the relative matching ofrefractive indexes between the TiO2 matrix and the YAG particles,and to the sub-wavelength size of these particles, the resultingfilms are transparent (in the sense of non scattering). When usedas light converters for white LEDs, these films could offer theopportunity to diminish the backscattered light absorption losses.We showed that sol–gel TiO2 films can be patterned by soft nano-imprint lithography. The diffraction due to the architecture of thefilm surface should compensate the absence of scattering for theextraction of the converted light out of the guiding layer and leadto high external yield devices.

Acknowledgments

We thank Dr. Alexander Mikhailovsky for his help on QuantumYield of luminescence measurements. Pr. Ram Seshadri andPr. Claude Weisbuch are also acknowledged for valuablediscussions.

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