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Transparent and Light-Emitting Epoxy Super-Nanocomposites Containing ZnO-QDs/SiO2 Nanocomposite Particles as Encapsulating Materials for Solid-State Lighting Yuan-Qing Li,† Yang Yang,† Shao-Yun Fu,*,† Xiao-Yan Yi,‡ Liang-Chen Wang,‡ and Hong-Da Chen‡ Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China ReceiVed: June 9, 2008; ReVised Manuscript ReceiVed: September 25, 2008
In this article, the ZnO quantum dots-SiO2 (Z-S) nanocomposite particles were first synthesized. Transparent Z-S/epoxy super-nanocomposites were then prepared by introducing calcined Z-S nanocomposite particles with a proper ratio of ZnO to SiO2 into a transparent epoxy matrix in terms of the filler-matrix refractive index matching principle. It was shown that the epoxy super-nanocomposites displayed intense luminescence with broad emission spectra. Moreover, the epoxy super-nanocomposites showed the interesting afterglow phenomenon with a long phosphorescence lifetime that was not observed for ZnO-QDs/epoxy nanocomposites. Finally, the transparent and light-emitting Z-S/epoxy super-nanocomposites were successfully employed as encapsulating materials for synthesis of highly bright LED lamps. 1. Introduction In response to ever-increasing energy demands coupled with serious concern for global warming, there has been a great interest in new light sources that can save electrical energy consumption. Solid-state lighting emitting diodes (LEDs) are thus receiving great attention as energy-saving light sources and are being intensely explored in this connection.1-7 The phosphor converted LEDs are highly promising because they possess high luminescence efficiency with only a single light-emitting diode (LED) chip.3-7 Traditionally, the phosphors are mixed with a transparent epoxy resin and the resulting mixture was coated on the surface of the LED chip in ultraviolet LED lamps, then the whole device is packaged with the transparent epoxy resin.3-7 However, since the mixture of phosphors with epoxy encapsulant is close to the chip as shown in Figure 1a, the radiant energy travels through the epoxy region close to the chip frequently; the epoxy would then yellow severely.3,6,7 By placing the phosphor layer far from the chip, light would escape the device rather than being trapped where it would cause the excessive yellowing of epoxy encapsulant, and then the lifetime of LEDs could be significantly improved.6,7 However, this improvement in lowering epoxy degradation is limited since the distance between the phosphors and the chip is restricted by the current LED lamp structure. Therefore, to completely solve the above problem, it is necessary to design a novel LED lamp structure in which there is no need of using conventional phosphors as shown in Figure 1b. In this new LED lamp structure, the encapsulating materials are responsible for emitting intense luminescence.8 ZnO is a wide-band gap semiconductor and its large exciton binding energy (59 meV) gives rise to the high efficiency exciton emission at room temperature.9 ZnO quantum dots (ZnO-QDs) show a broad luminescence emission spectrum in the blue-yellow region and have come up as a potential material as sources for * To whom correspondence should be addressed. Phone/fax: +86-1082543752. E-mail:
[email protected]. † Technical Institute of Physics and Chemistry. ‡ Institute of Semiconductors.
various colors of light including white light.8,10-12 At present, chemical synthesis methods have been most commonly used for growing uniformly dispersed ZnO-QDs but the ZnO-QDs are relatively unstable and encounter the difficulties in dispersion and preservation.13 For example, Spanhel and Anderson14 prepared ZnO-QDs via a sol-gel synthetic process. ZnO-QDs colloid was produced in an ethanol solution but it was difficult to separate ZnO-QDs from the solution and the particles would continuously grow and agglomerate during storage, even if they were stored at 0 °C.14 To avoid these problems, Abdullah et al.12 reported synthesis of ZnO/SiO2 nanocomposites by combining a sol-gel process and a spray drying method. Also, preparation of ZnO-QDs/SiO2 (Z-S) nanocomposite films was reported by Peng et al.13 using the target-attached radio frequency sputtering approach. Moreover, Amekura et al.15 reported synthesis of ZnO nanoparticles/SiO2 composites via the ion implantation and thermal oxidation method. In the Z-S nanocomposites, the particle size of ZnO does not change with aging since the particles are trapped in and protected by the solid silica matrix. Moreover, it was found that the defect structure and transition mechanisms could be modified by the amount and distribution of ZnO-QDs in SiO2 matrix to yield distinct luminescence properties.13,16 Very recently, ZnO quantum dots (QDs)/SiO2 (Z-S) nanocomposite particles were synthesized by hydrolyzing tetraethoxysilane (TEOS) in the ZnO-quantum dots (QDs)-containing ethanol solution.17 The fluorescence and phosphorescence of the Z-S nanocomposite particles were investigated as a function of calcination temperature. The results showed that the structure, fluorescence, and phosphorescence of Z-S particles were critically dependent on the calcination conditions, and the significant enhancements in the fluorescence and phosphorescence of ZnO-QDs/SiO2 nanocomposites have been achieved by the calcination.17 A high transparency is the prerequisite for encapsulating materials for LED so that they can be successfully used in practical applications. Nonetheless, it is very hard to maintain the high transparency of polymers after introduction of inorganic nanoparticles due to light scattering by nanoparticles because of the refractive index (RI) mismatch between the nanoparticles
10.1021/jp8050609 CCC: $40.75 2008 American Chemical Society Published on Web 11/04/2008
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Figure 1. Schematic of (a) a standard 5 mm LED7 and (b) a new LED structure.
Figure 2. Flowchart for preparation of transparent Z-S/epoxy super-nanocomposites.
and the polymer matrix. This is probably why in most fundamental research work, transparent polymer nanocomposites have been made in films with very small (normally less than 50 µm) thicknesses.18-20 However, in LED technology, transparent composites as encapsulating materials should be developed in bulk form with an approximate thickness of 1 mm. Fortunately, according to our very recent work,21 the RI of the Z-S nanocomposite particles can be tuned by controlling the ZnO-to-SiO2 ratio so that the RI matching principle between inorganic fillers and transparent epoxy matrix can be met to achieve highly transparent epoxy super-nanocomposites. In this article, ZnO quantum dots-SiO2 nanocomposite particles synthesized by hydrolyzing tetraethoxysilane (TEOS) in the ZnO-quantum dots (QDs)-containing ethanol solution are first calcined at a properly high temperature. Then, transparent ZnO quantum dots-SiO2(Z-S)/epoxy super-nanocomposites are readily prepared in terms of the refractive index matching principle as described in our recent work.21 The as-prepared transparent epoxy super-nanocomposites show intense luminescence with broad emission spectra. The interesting afterglow phenomenon with a long phosphorescence lifetime, namely the luminescence that is caused by the absorption of radiations and continues for a noticeable time after the radiations are stopped, is observed for the epoxy super-nanocomposites. Finally, the transparent and light-emitting Z-S/epoxy super-nanocomposites are successfully employed as encapsulating materials for synthesis of highly bright LED lamps.
2. Experimental Section Figure 2 shows the detailed procedures for preparation of transparent Z-S/epoxy super-nanocomposites. First, ZnO-QDs colloid was prepared using the modified one8,22 of the Spanhel and Anderson method.14 ZnO-QDs/silica (Z-S) nanocomposite particles were then synthesized by hydrolyzing tetraethylorthosilicate (TEOS with a chemical structure of Si(OCH2CH3)4) under the catalysis of ammonia in the ZnO-QDs-containing ethanol solution, similar to the preparation of silica nanoparticles by Sto¨ber method.21,23,24 The calculated amounts of TEOS and ammonia were added to the freshly prepared ZnO-QDscontaining ethanol solution under stirring at room temperature and after 24 h, the Z-S nanocomposite particles were harvested by centrifugation. The resulting Z-S nanocomposite particles were calcined in air at 500 °C for 2 h since at this temperature the calcined Z-S particles showed the optimal luminescent properties.25 Finally, transparent Z-S/epoxy super-nanocomposites were prepared using the in situ polymerization method with the ultrasonic technique. Transparent epoxy (EP-400 A and B) used for LED packaging was purchased from Bao and Lin Optoelectronic Co. Ltd. of China. The as-prepared Z-S nanocomposite particles and ZnO quantum dots were dispersed in anhydride curing agent (EP-400 B) using the ultrasonic technique for 10 min, the resulting mixture was then mixed with bis-phenol A epoxy (EP-400 A) at the weight ratio of 1:1. The epoxy and curing agent were well stirred until a homogeneous
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mixture was obtained. The mixture was poured into a stainless steel mold and heated at 130 °C for 1 h and at 100 °C for 6 h in an oven. After this curing process, the samples with a thickness of 4 mm were obtained.25,26 The UV-LED (370 nm) lamp was also cured with the above prepared epoxy mixture using the same curing process in a glass tube with a diameter of 1 cm in our laboratory and also in Institute of Semiconductors of CAS. TEM measurement was performed with a Hitachi JEOL JEM-2010 TEM. The transmittance spectra of Z-S/epoxy supernanocomposites were scanned using a UV-vis spectrophotometer Lambda 900 and the transmittance at 700 nm could then be obtained as a function of sample thickness.21 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 µm2 scan area from the prepared samples. Fluorescence emission spectra, phosphorescence emission spectra and phosphorescence lifetime were all performed on a Hitachi F4500 Fluorescence Spectrometer at room temperature under the excitation light source of 370 nm. 3. Results and Discussion The as-prepared ZnO-QDs are wurtzite spherical nanocrystals with an average diameter of about 3 nm and the XRD pattern and the absorption spectra of the ZnO sol have been reported previously.8,25 Since the relative weight ratio of ZnO to SiO2 could influence the refractive index of the Z-S nanocomposite particles and would then determine the transmittance of epoxy super-nanocomposites, Z-S nanocomposite particles with various ZnO-to-SiO2 weight ratios were prepared.25 The effect of Z-S composition (i.e., ZnO-to-SiO2 weight ratio) on the transmittance of epoxy super-nanocomposites is shown in Figure 3a, in which the Z-S content is 5 wt %. It can be seen that the epoxy supernanocomposites filled with pure ZnO-QDs is almost totally opaque within the whole UV-visible range. As the silica content increases, the visible light transmittance of Z-S/epoxy supernanocomposites increases up to the optimal transmittance when the weight ratio of ZnO/silica is 37 wt % (see Figure 3a and b). Further increase in the weight ratio of ZnO to silica leads to a reduced transmittance of Z-S/epoxy composites (Figure 3a and b). The variation in the transmittance of the epoxy supernanocomposites with the ZnO to silica weight ratio can be explained in terms of the RI matching principle between inorganic particles and epoxy matrix.21,27 The RI of Z-S nanocomposite particles, nZ-S, can be calculated via the effective medium theory as follows28 2 2 2 nZ-S ) nZnO VZnO + nSilica VSilica
(1)
where nZnO and nSilica are the RI of ZnO and silica, respectively. nZnO ) 2.0 and nSilica ) 1.42-1.46.21,29-31 VZnO and VSilica are, respectively, the corresponding volume fractions of ZnO and silica. When the weight ratio of ZnO/silica is 37 wt %, VZnO and nSilica can be determined by21
VZnO )
VZnO ) VZnO + VSilica
37wt % ×
FSilica FZnO
FSilica 37wt % × + 100% FZnO
VSilica ) 1 - VZnO
(2)
(3)
where FZnO and FSilica are the densities of ZnO and silica. We have FZnO ) 5.7 g/cm3, FSilica ) 2.2 g/cm3. Hence, by using eqs 1-3, we obtain nZ-S ) 1.50-1.54, which is close to the RI
Figure 3. (a) Transmittance of 4 mm thick Z-S/epoxy supernanocomposites with various ZnO-QD to SiO2 weight ratios, (b) transmittance of 4 mm thick epoxy super-nanocomposites at 700 nm and (c) optimal transmittance at the ZnO/SiO2 ratio of 37 wt % versus sample thickness.
()1.54) of the transparent epoxy. This is the RI matching case between inorganic particles and transparent polymer matrix. Therefore, at this optimal ZnO-QD to SiO2 weight ratio, the light scattering due to the RI mismatch would be minimal and the corresponding epoxy super-nanocomposite would show the optimal transmittance.21 The optimal transmittance at 700 nm of epoxy super-nanocomposites was shown as a function of sample thickness in Figure 3c. It shows that the transmittance of the epoxy super-nanocomposite with a thickness of 1 mm which is the practical case is as high as 82.4%. However, for other Z-S contents, it can be easily estimated that there will be an obvious difference in the RI between the Z-S nanocomposite particles and the epoxy matrix, leading to a relatively low transmittance.21,27
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Figure 4. (a) UV-vis spectra of Z-S/epoxy nanocomposites with different filler contents and (b, c) TEM micrographs of Z-S/epoxy nanocomposites with 5 and 15 wt % Z-S particles, respectively.
Epoxy super-nanocomposites containing various amounts of Z-S composite particles with the optimal ZnO-QD to SiO2 weight ratio have also been prepared and characterized. The results show that the transmittance of Z-S/epoxy super-nanocomposites decreases with increasing the Z-S filler content as shown in Figure 4a. The Z-S/epoxy super-nanocomposites with 5 wt % Z-S filler content show a transmittance of about 60% at 700 nm. Figure 4b and c shows the transmission electron microscopy (TEM) micrographs of the Z-S/epoxy supernanocomposites. It can be seen that the Z-S composite particles are basically homogenously dispersed with some degree of aggregation in the epoxy matrix. Some holes in the TEM photographs for the sample containing the 15 wt % Z-S content are observed since the thin slices for the TEM observation are brittle due to the fact that the sample contains 15 wt % hard Z-S particles in the brittle epoxy matrix. From the TEM observation and the IR-matching principle for the transmittance, the dispersion of Z-S particles would have an effect specially for the high Z-S content (15 wt %) case on the transmittance of epoxy super-nanocomposites. Therefore, the epoxy supernanocomposites with a filler content up to 5 wt % will be chosen for studying luminescent properties. The 2D and 3D AFM images of the surface topography obtained for the pure epoxy and super-nanocomposite samples are shown in Figure 5. It can be seen from Figure 5a and b that the sample surface prepared from pure epoxy is quite smooth and thus the light scattering due to the effect of surface roughness would be small on the transmittance. So, a high transmittance of pure epoxy resin has been observed as shown in Figure 4a. On the other hand, the addition of inorganic nanoparticles to a polymer would lead to an increase in viscosity, resulting in a rough surface of the super-nanocomposite samples
as shown in Figure 5c-f. And the surface roughness obviously increases with increasing the Z-S particle content. The light scattering caused by the surface roughness would to some extent reduce the transmittance of the transparent epoxy matrix. This is one reason for the decrease of the transmittance of epoxy super-nanocomposites with increasing the Z-S particle content. Figure 6a shows the fluorescence spectra of epoxy matrix and Z-S/epoxy super-nanocomposites, where the excitation spectrum is 370 nm. It can be observed that the Z-S/epoxy supernanocomposites show a broad fluorescence emission spectrum in the range of 400-650 nm. A weak fluorescence emission spectrum peaked at ∼442 nm has also 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 fluorescence emission intensity of the Z-S/epoxy supernanocomposites with the 1 and 3 wt % filler content has been enhanced by the addition of Z-S nanocomposite particles. However, the fluorescence emission intensity was lowered by a further increase of Z-S content (5 wt %). The decrease in the transmittance of the Z-S/epoxy composite with the 5 wt % Z-S content compared to the 1 and 3 wt % Z-S content cases would lower the measured fluorescence emission intensity. The increase of fluorescence emission intensity due to the increase in the filler content is not enough to compensate the decrease in the transmittance, thus leading to the observed decrease in the fluorescence emission intensity. Panels b and c of Figure 6 show, respectively, the phosphorescence spectra and phosphorescence lifetime for pure epoxy and Z-S/epoxy super-nanocomposites with different contents of Z-S nanocomposite particles calcined at 500 °C. Figure 6b exhibits that no phosphorescence emission peak can be observed
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Figure 5. AFM images of the surface topography for the pure epoxy (a, b) and the epoxy nanocomposites containing 1 (c, d) and 5 wt % (e, f) nanoparticles. It can be seen that the surface roughness is very small for the case of pure epoxy but the surface roughness increases with increasing the Z-S nanocomposite particle content.
for pure epoxy matrix. After introduction of Z-S nanocomposite particles into the epoxy matrix, a broad phosphorescence emission spectrum peaked at around 500 nm appears and the emission intensity increases monotonically with the increase of Z-S content. Figure 6c shows that the phosphorescence lifetime (namely the half-intensity time) of Z-S/epoxy super-nanocomposites is about 0.27 s and the afterglow phenomena can be clearly observed by the naked eye. Based on the analysis to the XRD patterns of Z-S particles calcined at different temperatures, the Z-S nanocomposite particles have a similar XRD spectrum to that of the uncalcined sample when the calcination temperature was 500 °C while
Zn1.7SiO4 instead of Zn2SiO4 was formed when the calcination temperature was 700 °C.17 In view of the report that surface defects can act as traps for phosphorescent emission,33 it can thus be deduced that on the surface of Z-S nanocomposite particles calcined at 500 °C there should exist plenty of Zn vacancies, which might be the reason for the observation of long phosphorescence lifetime. Of course, further investigation on the direct evidence for the long phosphorescence lifetime is still needed in future work. Moreover, as the Z-S content increases, a slight shift of the peak position was observed in both the fluorescence (Figure 6a) and phosphorescence (Figure 6b) spectra. This was possibly
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J. Phys. Chem. C, Vol. 112, No. 47, 2008 18621 chromaticity coordinates of (0.212 and 0.194) (see Supporting Information, Figure SI-1). The interesting afterglow phenomenon for the Z-S nanocomposite particles can be clearly observed as shown in the Supporting Information Movie SI-1. This interesting afterglow phenomenon should also appear for the Z-S/epoxy supernanocomposites, and indeed, the LED encapsulated using the Z-S/epoxy super-nanocomposite containing 5 wt % Z-S particles shows the afterglow phenomenon (see the Supporting Information Movie SI-2). In addition, we observed in measuring phosphorescence spectra that the Z-S nanocomposites and the Z-S/epoxy super-nanocomposites show similar phosphorescence spectra after excitation for various delay times, so the chromaticity does not change with delay time after excitation. 4. Conclusions In summary, herein we reported bulk preparation of novel transparent and light-emitting ZnO-QDs-silica/epoxy supernanocomposites by an ultrasonic technique via dispersion of Z-S nanocomposite particles with a matching refractive index in the transparent epoxy matrix. The Z-S/epoxy super-nanocomposites show a broad fluorescence emission spectrum and a broad phosphorescence emission spectrum with a long phosphorescence lifetime. The as-prepared novel transparent and light-emitting Z-S/epoxy super-nanocomposites have been successfully employed as encapsulating materials for solid-state lighting with no need of using conventional phosphors. The epoxy super-nanocomposites are very promising as novel encapsulating materials in solid-state LED devices for general illuminations in offices, houses and traffics etc due to their advantages of easy encapsulating process, high transparency and intense luminescence with a long phosphorescence lifetime.
Figure 6. (a) Fluorescence emission spectra of epoxy matrix and Z-S/ epoxy super-nanocomposites with different filler contents, (b) phosphorescence spectra and (c) phosphorescence lifetime of epoxy matrix and Z-S/epoxy super-nanocomposites with different filler contents and the inset is the photographs of the UV-LED lamp (370 nm as the emitting source) encapsulated with the pure epoxy (left-hand side) and the 5 wt % Z-S/epoxy super-nanocomposite (right-hand side).
because the slight aggregation of the Z-S nanocomposite particles might take place as shown in Figure 4a to form relatively large nanoparticles. This corresponds to the case of introducing relatively large nanoparticles into the transparent epoxy matrix as the Z-S content increases, leading to the redshift of the peak positions. The LED lamps were fabricated by encapsulating LED chips (370 nm) with the pure epoxy and the Z-S/epoxy supernanocomposites. Photographs were taken using a digital Olympus Camera for the encapsulated LED lamps as shown in the inset of Figure 6c.The LED lamp encapsulated with the pure epoxy (left-hand side) emits weak light. This is because that the pure epoxy emits light with a weak emission peak at ∼442 nm.21,32 On the other hand, under the excitation of 370 nm source light of the LED chip, the LED lamp encapsulated using the epoxy super-nanocomposite (right-hand side) emits intense light-green-like white light. By stopping the excitation, the LED lamp displays an interesting afterglow phenomenon and lasts for about 4 s (see Figure 6c). The LED lamp encapsulated with the epoxy super-nanocomposite containing 5 wt % Z-S particles exhibited blue light very closing to white light region with
Acknowledgment. This work was financially supported by the Beijing Municipal Natural Science Foundation and the National Natural Science Foundation of China. Supporting Information Available: CIE chromaticity diagram of the LED lamp encapsulated using the Z-S/epoxy supernanocomposite, and movies for light emitting of the Z-S nanocomposite particles and the Z-S/epoxy super-nanocomposite containing 5 wt % Z-S. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Nag, A.; Sarma, D. D. J. Phys. Chem. C 2007, 111, 13641. (2) Song, H.; Lee, S. Nanotechnology 2007, 18, 255202. (3) Li, Y. Q.; Yang, Y.; Fu, S. Y. Compos. Sci. Technol. 2007, 67, 3465. (4) Li, Y. Q.; Fu, S. Y.; Mai, Y. W. Polymer 2006, 47, 2127. (5) Fu, S. Y.; Li, Y. Q.; Yang, G. ; Li, M. China Patent Application Number. 200510068028.x. (6) Narendran, H.; Gu, Y.; Freyssinier-Nova, J. P.; Zhu, Y. Phys. Status Solidi 2005, 202, R60. (7) Narendran, N.; Gu, Y.; Freyssinier-Nova, J. P.; Yu, H.; Deng, L. J. Cryst. Growth 2004, 268, 449. (8) Yang, Y.; Li, Y. Q.; Fu, S. Y.; Xiao, H. M. J. Phys. Chem. C 2008, 112, 10553. (9) Reynold, D. C.; Look, D. C.; Jogai, B.; Morkoc, H. Solid State Commun. 1997, 101, 643. (10) Monticone, S.; Tufeu, R.; Kanaev, A. V. J. Phys. Chem. B 1998, 102, 2854. (11) Hung, C. H.; Whang, W. T. J. Mater. Chem. 2005, 15, 267. (12) Abdullah, M.; Shibamoto, S.; Okuyama, K. Opt. Mater. 2004, 26, 95. (13) Peng, Y. Y.; Hsieh, T. E. Appl. Phys. Lett. 2006, 89, 211909. (14) Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826. (15) Amekura, H.; Sakuma, Y.; Kono, K.; Takeda, Y.; Kishimoto, N.; Buchal, C. H. Physica B 2006, 376-377, 760.
18622 J. Phys. Chem. C, Vol. 112, No. 47, 2008 (16) Peng, Y. Y.; Hsieh, T. E.; Hsu, C. H. Nanotechnology 2006, 17, 174. (17) Li, Y. Q.; Yang, Y.; Sun, C. Q.; Fu, S. Y. J. Phys. Chem. C 2008, 112, in press. (18) Douce, J.; Boilot, J.; Biteau, J.; Scodellaro, L.; Jimenez, A. Thin Solid Films 2004, 46, 114. (19) Deng, Y.; Gu, A.; Fang, Z. Polym. Int. 2004, 53, 85. (20) Rubio, E.; Almaral, J.; Ramı´rez-Bon, R.; Castan˜o, V.; Rodrı´guez, V. Opt. Mater. 2005, 27, 1266. (21) Li, Y. Q.; Fu, S. Y.; Yang, Y.; Mai, Y. W. Chem. Mater. 2008, 20, 2637. (22) Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 5566. (23) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (24) Costa, C.; Leite, C.; Galembeck, F. J. Phys. Chem. B 2003, 107, 4747. (25) Li, Y. Q. Ph.D. Dissertation, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing China, May, 2007.
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