Multifunctional Silicone Nanocomposites for ...

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Tao, Peng; Rensselaer Polytechnic Institute, Department of Materials. Science ... bimodal polymer brush design enables homogenous dispersion of the surface ...
Multifunctional Silicone Nanocomposites for Advanced LED Encapsulation

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2013 MRS Spring Meeting Draft Symposium M n/a Li, Ying; Rensselaer Polytechnic Institute, Department of Materials Science and Engineering Tao, Peng; Rensselaer Polytechnic Institute, Department of Materials Science and Engineering Siegel, Richard; Rensselaer Polytechnic Institute, Department of Materials Science and Engineering Schadler, Linda; Rensselaer Polytechnic Institute, Department of Materials Science and Engineering optical properties, photoemission, composite

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Multifunctional silicone nanocomposites for advanced LED encapsulation Ying Li, Peng Tao, Richard W. Siegel, and Linda S. Schadler Department of Materials Science and Engineering and Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, NY 12180, U. S. A. ABSTRACT The addition of high refractive index (RI) inorganic nanoparticles (NPs) to LED encapsulation materials can lead to higher light extraction efficiency. In addition, the NPs can be carriers for additional functionality such as color conversion. Using a simple “grafting-to” approach, bimodal polydimethylsiloxane (PDMS) brushes were grafted onto high-RI ZrO2 NPs. Subsequently, an organic phosphor, 6-[fluorescein-5(6)-carboxamido]hexanoic acid (FCHA), was attached onto the PDMS-grafted ZrO2 NPs via a facile ligand exchange process. The bimodal polymer brush design enables homogenous dispersion of the surface functionalized NPs within the silicone matrix. The functionalized NPs with ~53 wt% ZrO2 core have a ~0.08 higher RI than neat silicone, and the NP-filled silicone nanocomposites exhibit a transparency of ~ 90% in the 550-800 nm wavelength range. In addition, the nanocomposites could be excited at a wavelength around 455 nm by a blue LED and undergo secondary yellow emission at around 571 nm. It is expected that the prepared nanocomposites can be used as high-efficiency, nonscattering, color-tuned materials for advanced LED encapsulation. INTRODUCTION Compared to epoxy-based LED encapsulants, which tend to yellow over time with exposure to high operating temperatures and/or absorption of UV-blue light, silicone resins have higher photochemical and thermal stability, high transparency in the UV-visible region, low water permeability, and tunable hardness.[1] Silicone encapsulants would open up exciting new luminaire designs and allow for penetration of phosphor-converted LED lamps into the solidstate-lighting market if they had a higher RI and better color-conversion properties.[2, 3] Increasing the RI increases the angle of the light-escape cone, thereby enhancing the light extraction efficiency.[4] Adding non-scattering color conversion properties to the encapsulant opens up new luminaire geometries. Currently, the most commonly used phosphors are composed of an inorganic host substance, such as yttrium aluminum garnet (YAG), doped with rare-earth elements.[5] With increasing concerns over the resource depletion of rare-earth elements, organic fluorescent materials have attracted attention because of their low cost, ease of fabrication, color tuning via modifying π–π* transitions through molecular/structure design, solubility in organic solvents enabling molecular-level doping into polymers, and generally good compatibility with polymer matrices.[3] In this work, we have demonstrated the preparation of multifunctional silicone nanocomposites with combined high RI and color conversion functionality by uniformly dispersing organic-phosphor functionalized high-RI ZrO2 NPs within a silicone matrix. In order to achieve higher NP loading, and thus higher RI of the nanocomposite, while minimizing transparency lose due to Rayleigh scattering as well as fluorescent quenching, homogenous dispersion of the NPs is critical. Conventional attempts to use monomodal (single population) grafted polymer brushes to control nanoparticle dispersion are challenged by the need for high

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graft density to shield particle core-core attractions and the need for low graft density to facilitate the penetration of matrix chains into the brush to suppress dewetting.[6, 7] In the present study silicone compatible PDMS brushes with two significantly different molecular weights were grafted to the surface of ZrO2 NP cores to compatibilize the nanoparticle, both enthalpically and entropically. The yellow-emitting FCHA dye was subsequently attached onto the grafted NPs via a facile ligand exchange process to introduce color-conversion functionality. The resulting silicone nanocomposites with tunable interfacial properties and optical functionalities enable new opportunities for advanced LED packaging. EXPERIMENT Spherical ZrO2 NPs with a radius of ~1.9 nm were synthesized using a non-aqueous, surfactant free synthetic approach.[8] In a typical synthesis, 2.22 g of zirconium isopropoxide isopropanol complex (98%, Alfa Aesar) was dissolved in 30 mL of benzyl alcohol (99%, Sigma Aldrich), and the mixture was transferred to a 45 mL stainless steel pressure vessel (Parr), which was heated to 240 °C. After 4 days, a white turbid suspension was retrieved from the cooled vessel and wet ZrO2 NPs were isolated by centrifugation at 10,000 rpm for 10 min and redispersed in chloroform. Silicone compatible PDMS brushes were synthesized through direct modification of commercial hydroxyl-terminated PDMS (Gelest) into a phosphate-terminated PDMS based on a method reported in the literature.[9] The phosphate head group replaces the weakly bonded capping ligands on the as-synthesized NP surfaces. The phosphate-terminated PDMS was added to the transparent NP chloroform solution and refluxed under stirring for 24 h to complete the “grafting-to” process. The grafted NPs were washed using methanol and redispersed in chloroform. FCHA (Sigma-Aldrich) was used to introduce color-conversion capability into the nanocomposite system. FCHA was completely dissolved in mixtures of chloroform and dimethylformamide (DMF), and then added into the grafted NP chloroform solution in a bath sonicator. The reaction mixture was sonicated for 30 min to facilitate the attachment of organic phosphor molecules via the carboxylic acid anchoring group. The phosphor-functionalized NPs were then washed and re-dispersed in chloroform. The functionalized NP chloroform solution was subsequently mixed with a silicone resin (Gelest). The mixture was homogeneously stirred and then put in a vacuum oven overnight for solvent removal at room temperature. After complete solvent removal, the nanocomposite was cured at 120 °C. In a typical TGA analysis, the NP sample was heated from 30 °C to 800 °C under a N2 flow at a heating rate of 10 °C/min. The refractive index of the nanocomposites was measured using variable angle spectroscopic ellipsometry (VASE, J.A Woollam Co.) on a spin-coated sample with a thickness in the range of 50 to 100 nm on a Si wafer. The measured results were fitted with the Cauchy model. The nanocomposite samples applied on glass slides (~1 mm thick) were used to measure optical transparency (UV-vis, Perkin–Elmer Lambda 950) and luminescence spectra (F-4500, Hitachi). RESULTS AND DISCUSSION The morphology and nanocrystal structure of the as-synthesized ZrO2 NPs are shown in Figure 1. The TEM characterization shows homogeneously distributed, near monodisperse ZrO2 NPs with an average radius of ~1.9 nm. All the peaks in the XRD pattern can be assigned to the

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ZrO2 cubic phase (JCPDS, 27-997). The small-size of the ZrO2 NPs is desirable for reducing transparency loss due to Rayleigh scattering. Also, the relatively large band gap of ZrO2 (~5 to 7 eV) compared to other metal oxide nanofillers, such as TiO2 (~3.5 eV), leads to less photocatalytic activity and thus better polymer stability.[10, 11]

Figure 1. (a) TEM image of as-synthesized ZrO2 NPs, and (b) their XRD pattern. The bimodal PDMS brush grafted NPs were then prepared using a two-step “grafting-to” approach. The phosphate-terminated PDMS brushes robustly anchor onto ZrO2 NPs through the strong binding of organo-phosphate with metal oxides. After attaching the long brush in the first step, the short brush filled in the remaining space on the particle surface at a higher graft density. As illustrated in Figure 2a, the FCHA phosphor molecule was then attached to the grafted NPs via the coupling of the carboxylic acid head group with the NP surfaces. As shown in Figure 2b, the transparent yellow-orange chloroform solution of the functionalized NPs is preliminary evidence of the successful attachment of FCHA, since the pure FCHA phosphor cannot be dissolved in chloroform as free molecules. The change in weight percentage of the functionalized NPs shown in the normalized TGA curves further confirms the existence of the ligand exchange process during functionalization, and the increased weight loss during the TGA analysis indicates the successful attachment of FCHA. b 100 90 80

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Figure 2. (a) Schematic illustration of a functionalized NP with bimodal PDMS polymer brushes and FCHA molecules attached. Gray polymer chains represent long brushes (Mw=36 kg/mol) and lighter gray chains represent short brushes (Mw=10 kg/mol). (b) Normalized TGA curves for PDMS grafted ZrO2 NPs before and after FCHA functionalization.

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Figure 3 shows an increase of RI (~0.08) over neat silicone for silicone containing functionalized NPs with ~53 wt% ZrO2 core. The solution absorption and photoluminescence (PL) emission spectra of the FCHA-functionalized NPs are nearly the same as those of the free FCHA, except for the strong absorption of UV light attributable to the ZrO2 core. Since the optical properties of the organic phosphors are very sensitive to their dispersion state, the unaltered PL spectrum indicates a homogeneous distribution of the organic phosphor molecules on the NP surface and effective shielding provided by the matrix-compatible PDMS brushes. White light is usually produced by mixing the blue emission (~460 nm) from InGaN and the yellow luminescence converted from the phosphors (~560 nm). Therefore, the PL behavior of the functionalized NP solution suggests that such high-RI, FCHA-functionalized NPs would be promising candidates for white light conversion in LEDs.

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Figure 3. (a) Comparison of RI dispersion of neat silicone and silicone with functionalized ZrO2 NPs. (b) Solution PL emission and UV–Vis absorption spectra of functionalized NPs. With the bimodal PDMS grafted chain design, the organic phosphor functionalized NPs were dispersed into a silicone matrix at 30 wt% loading. As shown in Figure 4a, the nanocomposite exhibits a transparency of ~ 90% in the 550-800 nm wavelength range, indicating homogenous dispersion of functionalized NPs attributable to the bimodal surface modification. Below 550 nm, the organic phosphors were excited and the incident light was absorbed. The short brushes are grafted at relatively high graft densities and enthalpically screen the NP corecore attraction, which is especially critical for inorganic NPs dispersed in organic matrices considering their large surface energy mismatch. The sparsely grafted long brush suppresses entropic dewetting of high-molecular-weight commercial silicone matrices.[6, 7] The prepared transparent silicone nanocomposite can minimize the scattering loss of the converted light. This is superior to the state-of-the-art silicone filled with micron-sized inorganic phosphor particles. Another advantage of the bimodal grafted chain design is the easy tuning of the inter-molecular distance of organic phosphor molecules on the particles. It can be expected that the light emission luminosity and operating lifetime can be tailored by optimizing the bimodal PDMS brush design. The maximum PL intensity of the nanocomposite was observed at 571 nm, which is yellow in color. The red-shifted PL spectrum of the nanocomposite (~ 30 nm), compared to the functionalized NPs in chloroform (Figure 3b), is probably attributable to the stronger interactions between fluorescein units in the solid state.[2] However, the strong absorption at 400-550 nm

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wavelength suggests that the phosphor concentration in the current nanocomposites might be too high for blue LED encapsulation, since they completely absorbed the blue emission from the LED chip. For future white LED applications, either the loading of functionalized NPs or the number of phosphor molecules on each NP should be reduced, which could be achieved by increasing the graft density of the PDMS brushes. 100

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Figure 4. (a) UV-vis transmittance spectrum (actual sample shown in inset) and (b) PL emission spectrum of functionalized silicone nanocomposite with 30 wt% loading of organic phosphor functionalized ZrO2 NPs. CONCLUSIONS High RI ZrO2 NPs were synthesized, surfaced modified with bimodal PDMS polymer brushes, and then functionalized with organic phosphor molecules. The functionalized NPs achieved homogenous dispersion within the silicone matrix. The high-RI and color-converting functionalities are introduced in the silicone nanocomposite without sacrificing the good transparency and flexibility of the silicone resin. The bimodal surface ligand design provides an effective tool for compatibilizing the inorganic nanofiller with the organic matrix. The multifunctional silicone nanocomposite exhibits desired fluorescent properties for colorconverting in LED encapsulation. By varying the graft density of the bimodal PDMS brushes and functionalized NP loading, the PL property of the multifunctional silicone nanocomposite can be further optimized for white LED applications. ACKNOWLEDGMENTS This work was supported by the Engineering Research Centers Program of the National Science Foundation under Cooperative Agreement EEC-0812056 and by NYSTAR under contracts C080145 and C090145 and by the Nanoscale Science and Engineering Initiative of the National Science Foundation under NSF award number DMR-0642573.

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