Transparent and Light-Emitting Epoxy Nanocomposites Containing

Jun 21, 2008 - Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China, Graduate School of Chinese Academy of...
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J. Phys. Chem. C 2008, 112, 10553–10558

10553

Transparent and Light-Emitting Epoxy Nanocomposites Containing ZnO Quantum Dots as Encapsulating Materials for Solid State Lighting Yang Yang,†,‡ Yuan-Qing Li,† Shao-Yun Fu,*,†,§ and Hong-Mei Xiao† Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China, Graduate School of Chinese Academy of Sciences, Beijing 100039, China, and International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China ReceiVed: March 11, 2008; ReVised Manuscript ReceiVed: April 22, 2008

Polymer nanocomposites are usually made by incorporating dried nanoparticles into polymer matrices. This would easily lead to aggregation of nanoparticles and then would readily bring about opaqueness for nanocomposites based on functionally transparent polymers. In this work, preparation of highly transparent ZnO quantum dots (ZnO-QDs)/epoxy nanocomposites that can emit intense luminescence was reported for the first time by uniformly dispersing ZnO quantum dots in a transparent epoxy resin via a direct dispersion method. The direct dispersion of ZnO quantum dots without drying in the epoxy matrix could effectively avoid aggregation of fine quantum dots and showed a good dispersity of ZnO-QDs in the epoxy matrix. Such materials showed a high transparency in the visible region and exited a broad emission spectrum peaked at 442 nm. On the other hand, in traditional solid state lighting emission diodes, semiconductor quantum dots are used as phosphors that are attached to the die of the lighting emission diode (LED) lamps for emitting luminescence. In this work, the as-prepared ZnO-QDs/epoxy nanocomposites were successfully employed as packaging materials for solid state lighting emission diodes in which conventional phosphors are not required while the nanocomposite encapsulating materials are responsible for emitting luminescence and thus the encapsulated LED lamps have an innovative structure. Consequently, the as-prepared ZnO-QDs/epoxy nanocomposites are promising for use as novel encapsulating materials in LED technology due to the much simplified encapsulating process. 1. Introduction Incandescent bulbs and halogen lamps have long been widely used for lighting in houses, offices, and traffic lights but they have disadvantages of relatively high energy consumption, low reliability, short lifetime, etc. Thus, solid state lighting emitting diodes (LEDs) have attracted great interests for general illumination because LEDs have advantages over the existing incandescent and halogen lamps in terms of power efficiency, reliability and long lifetime etc.1,2 In standard solid state lighting emitting diode (LED) technology, transparent epoxy resins are most frequently employed as packaging materials due to their high transparency, high-glass transition temperature, low-water absorption and standard process ability.3–9 In practical applications, the die of the LED is placed on a substrate, connected to an anode and a cathode, and then covered with a transparent epoxy resin. Phosphors are first mixed with epoxy resin and the mixture (inner layer) is placed at a position close to the die. Then, the whole LED lamp is encapsulated with a transparent epoxy resin (outer layer), as illustrated in Figure 1a.7 However, the severe yellowing of epoxy encapsulant would occur due to junction heat and short-wavelength emission.7,9 This would significantly reduce the lifetime of LEDs. Since the mixture of phosphors with epoxy encapsulant is close to the die, the radiant energy travels through the epoxy region close to the die frequently, the epoxy would then yellow severely. By placing * Corresponding author. Tel.: +86-10-82543752. Fax: +86-10-82543752. E-mail: [email protected]. † Technical Institute of Physics and Chemistry. ‡ Graduate School of Chinese Academy of Sciences. § International Centre for Materials Physics.

the phosphor layer far from the die, light would escape the device rather than being trapped where it will cause the excessive yellowing of epoxy encapsulant to improve the lifetime of LEDs.7,9 Therefore, it was suggested that the structure of LEDs could be designed with the phosphor layer far away from the die to lower the degradation of epoxy.7,9 But, this improvement in lowering epoxy degradation is limited since the distance between the phosphors and the die is restricted by the current LED structure. To avoid the above shortcoming, the phosphors must be far away from the die. This would be possible if the phosphors could be dispersed in the outer epoxy layer. However, the conventional phosphors or nanophosphor aggregates usually are of the micrometer or submicrometer sized grade with a broad particle size distribution, and hence, the mixture of the phosphors with the transparent epoxy resin would readily lead to the complete opaqueness due to light scattering caused by the inorganic particles with a different refractive index from that of the polymer matrix as pointed out in our recent work,10 that is not desired for the outer encapsulating layer for the LED lamp. On the other hand, polymer nanocomposites are usually made by incorporating dried nanoparticles into polymer matrices.3,4,10–12 This would easily lead to aggregation of nanoparticles and then would readily bring about opaqueness for nanocomposites based on functionally transparent polymers even at a low nanofiller content (less than 0.5 wt %).3,11 Inspired by the works of Narendran et al.,7,9 the present study is aimed to find a suitable solution for the above problem by preparing highly transparent epoxy nanocomposites that can emit light as encapsulating materials for the ultraviolet (UV) LED lamps with no need of using conventional phosphors as shown in Figure 1b. And the

10.1021/jp802111q CCC: $40.75  2008 American Chemical Society Published on Web 06/21/2008

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Yang et al.

Figure 1. Schematic of (a) a standard 5 mm LED7 and (b) a new LED structure in the present study.

Figure 3. FT-IR spectra of pure epoxy and ZnO-QDs/epoxy nanocomposites. The inset indicates the enlarged vibration band of ZnsO.

Figure 2. (a) X-ray diffraction pattern and (b) UV-vis absorption and PL emission spectra of detected at λ ) 370 nm of ZnO sol.

quantum dots with very fine sizes in the nanocomposites are uniformly dispersed in the transparent epoxy matrix via a direct dispersion method without drying quantum dots to maintain the high transmittance of epoxy resins and thus are naturally far away from the LED die. Zinc oxide (ZnO) is a notable inorganic material closely related with ultraviolet light. In our previous works,3–5 ZnO nanoparticles with a low filler content up to 0.15% were

employed for improving the UV shielding efficiency of encapsulating materials and hence the lifetime of the ultraviolet lightbased white light-emitting dioxides (UV-WLED) lamps. Li et al.12 recently reported the preparation of transparent UVshielding poly(methyl methacrylate) (PMMA) nanocomposites by incorporating ZnO quantum dots into the PMMA matrix, in which the ZnO-QDs content is very low up to 0.11 wt % to keep the high transparency of PMMA matrix. On the other hand, ZnO is also a versatile direct-band gap semiconductor with a wide band gap of 3.37 eV, and its large exciton binding energy (60 meV) makes the exciton state stable even at room temperature. Furthermore, it is an environmentally friendly material and shows a broad luminescence emission spectrum in the blueyellow region. Therefore, the synthesis of ZnO-QDs has been intensively studied.13–23 Koch et al.,13 Bahnemann et al.,14 and Haase et al.15 employed the colloidal method for preparing ZnO nanoparticles by hydrolyzing zinc salts in basic alcoholic solutions. Spanhel and Anderson systematically studied the synthesis of colloidal ZnO nanoparticles by using zinc acetate dehydrate (Zn(Ac)2 · 2H2O) and lithium hydroxide as the starting materials and ethanol as the solvent.16 Also, this method has been widely accepted for preparing ZnO nanoparticles.17–23 However, in their method, it was difficult to separate ZnO-QDs from ZnO-QDs colloid that was produced in an ethanol solution. Moreover, the particles would continuously grow and agglomerate during storage, even if they were stored at 0 °C. Therefore, the preparation of ZnO-QDs/poly(vinyl pyrrolidone) (PVP) was carried out by the introduction of PVP as

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J. Phys. Chem. C, Vol. 112, No. 28, 2008 10555

Figure 4. AFM images of phase and surface topography for the pure epoxy (a,b) and the epoxy nanocomposites (c,d) with the 4 wt % ZnO-QDs content.

Figure 5. UV-vis spectra of pure epoxy and ZnO-QDs/epoxy nanocomposites with a thickness of 4 mm.

capping molecules for the ZnO nanoparticles.24 It serves to stabilize the ZnO nanoparticles and passivate the surface to reduce the oxygen vacancy sites. PVP-capped ZnO nanoparticles are much more stable than noncapped ZnO nanoparticles. Also, ZnO-QDs/poly(ethylene glycol) electrolyte nanocomposites that emit blue to yellow luminescence were prepared by an in situ method.25,26 Moreover, polymer nanocomposites emitting blue to yellow luminescence, in which ZnO nanoparticles with a size from 2 to 8 nm and poly(ethylene glycol methyl ether) (PEGME)

molecules are connected by covalent bonds, were synthesized by a sol-gel method.27 In the above polymer nanocomposites, polymers are used either for modifying the surface of ZnO nanocrystals or in the gel-like state. However, ZnO-QDs have not been incorporated into solid-state polymer matrices like epoxy resins for solid state lighting emission diodes. The present study is aimed to use ZnO quantum dots/epoxy nanocomposites as encapsulating materials that can emit intense luminescence for LED devices. A high transmittance is required for the epoxy nanocomposites as encapsulating materials so that they can be successfully used in practical applications. Consequently, novel transparent Z-S/epoxy nanocomposites that can emit intense luminescence will be prepared in bulky form (4 mm thick) by homogeneously dispersing ZnO-QDs in a transparent epoxy matrix. In this article, the preparation of highly transparent ZnOQDs/epoxy nanocomposites which can emit intense luminescence was reported for the first time by homogeneously dispersing ZnO quantum dots without drying in a transparent epoxy resin via a direction dispersion method using an ultrasonic technique.28 The as-prepared epoxy nanocomposites after cured are in the solid state so that the ZnO-QDs would be fixed in the epoxy matrix and would not aggregate to become larger particles, which would take place in the gel-like polymer nanocomposites.25–27 Supporting Information Figure 1 presents the flowchart for the whole process of preparing highly transparent ZnO-QDs/epoxy nanocomposites. The luminescent

10556 J. Phys. Chem. C, Vol. 112, No. 28, 2008 properties of epoxy nanocomposites were well characterized and the as-prepared transparent epoxy nanocomposites were successfully employed as encapsulating materials for solid state lighting in the LED lamps with an innovative structure as shown in Figure 1b, in which traditional phosphors were not used for emitting light so that the encapsulating process has been much simplified. Experimental Section ZnO-QDs were first prepared using a synthetic procedure similar to that of Meulenkamp,17 namely the modified one to the Spanhel and Anderson method.16 A 2.20 g sample of Zn(Ac)2 · 2H2O was dissolved in 100 mL boiling absolute ethanol under vigorous stirring at atmospheric pressure. After cooling to room temperature, the solution was further cooled to 0 °C in an ice water bath. A 0.58 g sample of LiOH · H2O was dissolved in 50 mL absolute ethanol at room temperature in an ultrasonic bath and cooled to 0 °C. The hydroxidecontaining solution was then added to the zinc acetate solution under vigorous stirring to yield a clear and homogeneous solution, which is namely a transparent ZnO sol. Then 0.9 mL of oleic acid was added to the ZnO sol under vigorous stirring to precipitate ZnO nanocrystals. The supernatants were removed by centrifugation and then the ZnO precipitated nanocrystals were dispersed in anhydride curing agent (EP 400 B) using acetone as solvent in ultrasonic bath for 30 min until the mixture is transparent. During this process, the direct dispersion of ZnO quantum dots without drying was important to avoid aggregation, and acetone was used as solvent because it can be easily extracted as a nonpolar solvent with a low boiling point from epoxy. The resulting mixture was then mixed with epoxy resin (EP 400 A) and the final mixture was well stirred until a homogeneous mixture was obtained. Vacuity technique was used to remove the solvent, and the mixtures were poured into a steel mold with the thickness of 4 mm that were then cured in an oven for 1 h at 130 °C followed by 8 h at 100 °C. Transmittance and fluorescence of the pure epoxy resin and epoxy nanocomposites were detected using a UV-vis spectrometer (Model Lambda 900) and a fluorescent photometer (Hitachi F-4500). Dispersion of ZnO-QDs in the epoxy resin was characterized with a high resolution transmission electron microscopy (HRTEM) instrument (JEOL JEM-2010). An atomic force microscope (AFM; Nanoscope IIIa, Digital Instruments Co) was used to investigate the surface phase and topography of the pure epoxy and epoxy nanocomposites. AFM images were taken with 3 × 3 µm scan area from the prepared samples. UV LED lamps were encapsulated respectively with the pure transparent epoxy resin and the as-prepared transparent nanocomposites. The synthetic procedure of ZnO nanocrystals is similar to but has advantages in precipitation of ZnO-QDs over that of Meulenkamp17 and Hoyer and Wellerl.22 Meulenkamp17 used an organic “non-polar solvent” such as hexane and heptane in precipitation of ZnO nanocrystals. However, the required volume ratio of hexane or heptane to ZnO sol is 1:1 to 2:1, which would highly increase the separation cost. In this study, oleic acid is used in the precipitation process and only 0.6 mL of oleic acid can give a complete precipitation of 100 mL ZnO sol, showing the advantages of low cost and high efficiency. Moreover, Hoyer and Weller22 precipitated ZnO aggregates via addition of water, but water could cause growth of ZnO particles and hence would readily lead to large-sized ZnO particles in the product. Results and Discussion The XRD pattern of the as-prepared ZnO quantum dots is shown in Figure 2a. The diffraction pattern can be well matched

Yang et al. to the standard diffraction pattern of wurtzite ZnO,29 demonstrating the successful formation of wurtzite ZnO nanocrystals. Thus, the average size of quantum dots was obtained to be 3.1 nm ( 0.3 nm from 50 measurements of the sizes of ZnO nanoparticles from the HRTEM images as shown in Supporting Information Figure 2 using the software SemAfore 4.0. The enlarged inset at the left-top corner of Supporting Information Figure 2 shows the lattice fringes belonging to wurtzite structure of ZnO nanocrystals. UV-vis absorption and photoluminescence (PL) emission spectra of ZnO-QDs sol are presented in Figure 2b. The absorption peak at 325 nm in UV-vis absorption spectra shows an obvious blue shift compared to the bulk ZnO material (373 nm), which is attributed to the quantum confinement effect caused by the extremely small size.17 An expression can be used for description of the ZnO quantum dot size (D) as follows: 1240/λ1/2 ) 3.301 + 294.0/D2 - 1.09/D, where λ1/2 is the wavelength at which the absorption is 50% of that at the excitonic peak.17 According to this calculation, we obtain a size of 2.98 nm for the ZnO nanocrystals, which is in good agreement with the size obtained from the above HRTEM result. The emission peak at ∼ 509 nm of ZnO quantum dots was observed when the sample was excited at 370 nm, which may be attributed to the surface traps of oxygen interstitials that has been reported in bulk ZnO as well as in ZnO quantum dots.22 ZnO/epoxy nanocomposites containing 2 and 4 wt % ZnO quantum dots have been prepared and characterized. Nanocomposites containing a higher content of ZnO quantum dots showed a cracked appearance possibly because the high loading of ZnO hindered the curing reaction of epoxy resin. Figure 3 illustrates the FT-IR spectra of the pure epoxy and ZnO-QDs/epoxy nanocomposites. In contrast to pure epoxy, the nanocomposites have a broad absorption at around 3500 cm-1, which is assigned to the absorbed water and Zn-OH residues in the nanocomposites. The intensity of the vibration band belonging to Zn-O30 near 460 cm-1 is low as shown in Figure 3 because the content of ZnO-QDs is low (4 wt %) in the nanocomposite and after enlargement, the vibration band can be clearly seen in the inset on the right-bottom corner within Figure 3. The surface morphology of the samples was examined by atomic force microscope (AFM) (Figure 4). AFM images were taken with 3 * 3 µm scan area from the prepared samples. The full height scale is 100 nm in the 3D AFM images and it can be seen that the surface roughness is very small for the case of pure epoxy while the surface of nanocomposites containing ZnO quantum dots is somewhat rough. Moreover, Figure 4c shows that there is a good dispersion of ZnO quantum dots in the epoxy matrix, where white dots are referred to ZnO quantum dots. The distribution of ZnO quantum dots in the epoxy matrix was also examined by HRTEM (Supporting Information Figure 3). The lattice fringes belonging to ZnO nanocrystals can be clearly observed in the HRTEM image. Moreover, Supporting Information Figure 3 shows that the ZnO quantum dots are well dispersed in the epoxy matrix. Therefore, the high transmittance has been observed not only for the pure epoxy but also for the ZnO-QDs/epoxy nanocomposite samples with a thickness of 4 mm. The introduction of ZnO nanocrystals leads to only a slight decrease in the transmittance, mainly resulting from the surface roughness for the nanocomposites containing ZnO quantum dots as shown in Figure 4d. Therefore, the transmittance of the ZnOQDs/epoxy nanocomposite with the 4 wt % ZnO-QD content maintains a high transmittance of g80% at 700 nm with only a slight reduction (9%) compared to that of pure transparent epoxy matrix (Figure 5).

Transparent and Light-Emitting Epoxy Nanocomposites

J. Phys. Chem. C, Vol. 112, No. 28, 2008 10557 encapsulated with the pure epoxy emits weak light. This is because that the pure epoxy emits light with a weak emission peak at ∼442 nm. The LED lamp encapsulated with the ZnOQDs/epoxy nanocomposite emits white light and shows a much stronger intensity of luminescence compared to that encapsulated using the pure transparent epoxy resin. This is because the epoxy nanocomposite exhibits a broad photoluminescence emission spectrum and ZnO-QDs in the epoxy nanocomposite emit bright luminescence. The encapsulated LED lamps in our laboratory are much larger in size than commercial ones so that the light intensity becomes weak at the top of the LED lamps. 4. Conclusions

Figure 6. Fluorescence emission spectra of pure epoxy and ZnO-QDs/ epoxy nanocomposites excited at 370 nm. The inset shows the photographs of the UV-WLED lamps (370 nm) encapsulated with the pure transparent epoxy (left) and the ZnO-QDs/epoxy nanocomposite (right) containing the 4 wt % ZnO-QDs content.

The intensity loss of transmitted light in the nanocomposites due to light scattering can be estimated by10,31

[

I/I0 ) exp

( )]

-3Vpxr3 np -1 nm 4λ4

(1)

which is valid for spherical particles with a radius r and a refractive index np uniformly dispersed in a matrix with a refractive index nm; where I is the intensity of the light passing through the sample, I0 the intensity of the light that would pass the sample without scattering (the intensity of the incoming light for nonabsorbing materials), Vp the volume fraction of the particles, λ the light wavelength and x the optical path length. The refractive index of the ZnO is 1.9-2.0 and the refractive index of epoxy matrix is ∼1.54-1.55.10 Referring to the system discussed here, the radius r is around 3.0 nm which is much smaller than the light wavelength, the very low intensity loss (∼0.1%) of the incident light by scattering can be estimated. It can thus be concluded that the slight reduction in the transmittance of nanocomposites compared to pure epoxy resin is mainly caused by the surface roughness of the nanocomposite sample as shown in Figure 4d. The fluorescence spectra of epoxy and transparent ZnO-QDs/ epoxy nanocomposites under the excitation of 370 nm are shown in Figure 6. From Figure 6, a weak emission peak at ∼ 442 nm has been observed for pure epoxy, which is similar to the previous report that the emission peak was observed at 431 nm for methyltetrahydrophtalic anhydride cured epoxy resin under the excitation of 360 nm.32 The broad emission spectrum of ZnO-QDs/epoxy nanocomposites in the range of 400 nm to ca. 640 nm with a long tail was observed with an emission peak at about 442 nm, and the fluorescent intensity of nanocomposites increases with increasing the content of ZnO quantum dots. The strong emission of ZnO-QDs/epoxy nanocomposites should arise from both the ZnO nanocrystals and the organic epoxy resin. Moreover, the peak of the epoxy nanocomposites has been blueshifted from the 509 nm for the ZnO-QDs to the 442 nm for the pure epoxy resin, indicating that the epoxy matrix plays an important role in determining the peak location. The LED lamps were fabricated by encapsulating LED chips (370 nm) with the pure epoxy and the ZnO-QDs/epoxy nanocomposite containing the 4 wt % ZnO-QDs. Photographs were taken using a digital Olympus camera for the encapsulated LED lamps as shown in the inset of Figure 6. The LED lamp

In summary, highly transparent ZnO-QDs/epoxy nanocomposites that can emit intense luminescence have been successfully prepared by uniformly dispersing ZnO quantum dots in a transparent epoxy resin via a direct dispersion method. The direct dispersion of ZnO quantum dots without drying can effectively avoid aggregation and showed a good dispersity of ZnO-QDs in the epoxy matrix. Such materials showed a great transparency in the visible region and exited a broad emission spectrum peaked at 442 nm. In addition, the as-prepared ZnO/ epoxy nanocomposites were successfully used as packaging materials for solid state lighting emission diodes with no need of using conventional phosphors and the resulting LED lamp can emit intense white light. The encapsulating process of LED lamps has been much simplified since no conventional phosphors are required in the new LED lamps with an innovative structure as shown in Figure 1b. The highly transparent ZnOQDs/epoxy nanocomposites developed in the present study that are capable of emitting intense luminescence will be very promising for use as novel encapsulating materials in LED technology. Acknowledgment. We thank Prof. Xiao Hu and Prof. MeiXiang Wan for helpful discussions. The work was financially supported by the Chinese Academy of Sciences and the National Natural Science Foundation of China (Nos. 10672161 and 573090). Supporting Information Available: Experimental procedure and HRTEM images of ZnO-QDs and ZnO-QDs/epoxy nanocomposites are provided in detail. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Heeger, A. J. Solid State Commun. 1998, 107, 673–679. (2) Yam, F. K.; Hassan, Z. Microelectron. J. 2005, 36, 129–137. (3) Li, Y. Q.; Fu, S. Y.; Mai, Y. W. Polymer 2006, 47, 2127–2132. (4) Li, Y. Q.; Yang, Y.; Fu, S. Y. Compos. Sci. Technol. 2007, 67, 3465–3471. (5) Fu, S. Y.; Li, Y. Q.; Yang, G.; Li, M. China Patent Application Number 200510068028.x, 2005. (6) Huang, J. C.; Chu, Y. P.; Wei, M.; Deanin, R. D. AdV. Polym. Technol. 2004, 23, 298–306. (7) Narendran, N.; Gu., Y.; Freyssinier-Nova, J. P.; Yu, H.; Deng, L. J. Cryst. Growth. 2004, 268, 449–456. (8) Morita, Y. J. Appl. Polym. Sci. 2005, 97, 1395–1400. (9) Narendran, H.; Gu, Y.; Freyssinier-Nova, J. P.; Zhu, Y. Phys. Status Solidi 2005, 202, 60–62. (10) Li, Y. Q.; Fu, S. Y.; Yang, Y.; Mai, Y. W. Chem. Mater. 2008, 20, 2637–2643. (11) Ryszkowska, J.; Zawadzak, E. A.; Łojkowski, W.; Opalin´ska, A.; Kurzydłowski, K. J. Mater. Sci. Eng., C 2007, 27, 994–997. (12) Li, S. H.; Toprak, M. S.; Jo, Y. S.; Dobson, J.; Kim, D. K. AdV. Mater. 2007, 19, 4347–4352. (13) Koch, U.; Fojtic, A.; Weller, H.; Henglein, A. Chem. Phys. Lett. 1985, 122, 507–510.

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