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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 5480 Received 3rd December 2013, Accepted 26th December 2013
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Highly luminescent and transparent ZnO quantum dots–epoxy composite used for white light emitting diodes† Jia Jia Huang, Yi Bin Ye, Zhou Qiao Lei, Xiao Ji Ye, Min Zhi Rong* and Ming Qiu Zhang*
DOI: 10.1039/c3cp55098b www.rsc.org/pccp
By using epoxy silane as a modifier and stabilizer, ZnO quantum dots (QDs) were synthesized by a one-step precipitation approach. The ZnO QDs acquired showed satisfactory redispersibility and exhibited strong and stable photoluminescence in both solution and dry states. When the ZnO QDs content was as high as 8 wt%, the ZnO QDs–epoxy nanocomposite was still highly transparent and luminescent. Accordingly, the ZnO QDs can be used as a luminescent material, and a cool white light emitting diode (WLED) lamp was made by encapsulating a UV chip with the nanocomposite, without traditional tricolor rare earth phosphors. Additionally, the high loading nanocomposite possessed high ultraviolet resistance, which would help to improve the lifetime of the WLED.
White light emitting diodes (WLEDs) have attracted increasing interest because of their applications as new illumination sources.1 Among the strategies to create white light, the use of ultraviolet (UV)/near ultraviolet (NUV) chips in cooperation with tricolor phosphors is growing rapidly, owing to their high color rendering index, excellent light quality and low re-absorption characteristics.2–4 More importantly, they are believed to be the main route to evade existing patent barriers. Nevertheless, rare earth phosphors still serve as the luminescent material in this case, which inevitably brings about the drawbacks of limited resources and cost ineffectiveness. To the best of our knowledge, no unclassified information about WLEDs fabricated from UV chips and metallic oxide luminescent materials is available in the literature.5 ZnO has been found to be an emerging semiconductor material with wide band gap of 3.37 eV and high exciton Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China. E-mail: [email protected], [email protected]; Fax: +86-20-84114008; Tel: +86-20-84114008 † Electronic supplementary information (ESI) available: materials and methods, PL properties, UV-vis absorption spectra, XRD patterns, TEM photos, Fourier transformation of HRTEM image, photographs, thickness, drift corrected spectrum image scanning, and temperature dependence of loss factor. See DOI: 10.1039/c3cp55098b
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binding energy (60 meV). Compared with conventional semiconductor quantum dots (QDs) such as CdTe QDs, ZnO QDs are characterized by their low cost and non-toxicity.6 ZnO QDs can be used not only as an excellent ultraviolet absorbent to increase the UV resistance of polymers,7–10 but also as novel fluorescent powders in LEDs owing to their strong and stable visible emission.11–14 ZnO QDs are generally prepared by sol–gel or precipitation approaches. With the use of suitable ligands or polymeric encapsulation,15–23 monodispersed ZnO QDs with strong and tunable emission can be obtained. However, the poor separation–redispersion characteristics of ZnO QDs and their non-compatibility with polymers limit their application as luminescent materials for WLEDs.5,8 Most of the ZnO QDs–epoxy composites reported so far only exhibit improved UV resistance, but fail to emit fluorescence because purified ZnO QDs are difficult to redisperse in solvents like acetone, which is a requirement for obtaining a homogeneous mixture of ZnO QDs and epoxy. In this work we report the synthesis of ZnO QDs by a one-step precipitation method in the presence of g-(2,3-epoxypropoxy)propytrimethoxysilane (KH560) as a surface modifier. The resultant QDs have an enhanced quantum yield (QY) and redispersibility, exhibiting strong and stable photoluminescence in both solution and dry states. In addition, when the as-produced ZnO QDs (ZnOKH560) are compounded with epoxy, a uniformly dispersed ZnO QDs–epoxy nanocomposite with high loading is produced because the epoxy groups of silane KH560 help to greatly improve the interfacial compatibility. This sets the stage for ZnO QDs to be used as a luminescent material in an epoxy matrix. To verify this idea, a UV LED chip (370 nm excitation) was simply encapsulated in the as-prepared ZnO QDs–epoxy nanocomposite without any traditional phosphors. The LED lamp indeed emits a cool white light with CIE coordinates of (0.305, 0.362) under an operating current of 10 mA (see inset of Fig. 1). A narrow peak at 408 nm and a broad peak at about 550 nm appear in the emission spectrum of the LED lamp (Fig. 1), which are attributed to the emissions of the epoxy and ZnO QDs, respectively. As the ZnO QDs are uniformly distributed in
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Fig. 1 Emission spectrum of the LED lamp (370 nm) encapsulated by the ZnO QDs–epoxy nanocomposite. Content of ZnO QDs: 5 wt%. Inset: the LED lamp in operation (left); CIE 1931 chromaticity coordinates calculated from the emission spectra (right).
the epoxy and are relatively far away from the LED chip, the lifetime of the lamp would be increased accordingly. The result demonstrates that the ZnO QDs can replace traditional rare earth phosphors to produce white light LEDs, or more precisely, the ZnO QDs–epoxy nanocomposite can replace the conventional compounds of silicon resin and rare earth phosphors to be coupled with UV/NUV chips for producing white light LEDs. The WLED shown in Fig. 1 reveals the potential of the ZnO QDs as a candidate for luminescent materials. The key issue lies in fabrication of a highly luminescent and transparent epoxy nanocomposite containing a high loading of ZnO QDs. We modified the surfaces of ZnO QDs with silane KH560, which provides the quantum dots with a protective layer. Consequently, the treated ZnO QDs are no longer affected by external interference, from factors such as solvents, moisture in the air and aggregation. Fluorescence quenching is thus minimized. Fig. 2 gives the results of a spectroscopic study of ZnOKH560 in comparison with that of ZnObare, which was synthesized
Fig. 2 (a) XPS survey scan spectrum of ZnOKH560, (b) experimental and fit curves for O 1s XPS spectrum of ZnOKH560, (c) FTIR spectra of ZnObare and ZnOKH560, and (d) thermogravimetric curves of ZnObare and ZnOKH560.
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without the silane. The X-ray photoelectron spectroscopy (XPS) spectrum in Fig. 2a shows that C, O, Zn, and Si appear on the surface of ZnOKH560. Moreover, deconvolution of the O 1s spectrum (Fig. 2b) reveals the presence of O2 (532.30 eV) ions in the oxygen-deficient regions within the matrix of ZnO,24 and C–O–C (533.45 eV). According to the Fourier transform infrared (FTIR) spectra (Fig. 2c), additional peaks of nC–H, nC–O–C and nSi–O–Si are observed in the spectrum of ZnOKH560. These prove that the silane KH560 has been bonded to ZnO with a grafting percentage of about 16 wt%, as determined from the thermal decomposition curve in Fig. 2d. The silane surface modifier has as effect on the formation, structure and photoluminescence of ZnO QDs.22 It efficiently suppresses the aggregation and excessive growth of the ZnO QDs through hydrolysis and condensation reactions with the hydroxyl groups on the surface of the ZnO QDs. As a result, the UV-vis absorption peak of ZnOKH560 has a blue shift of 20 nm compared to that of ZnObare (ESI,† Fig. S2), meaning that the diameter of ZnOKH560 has decreased from 4.21 to 2.89 nm.15 On the other hand, the ZnO QDs prepared through the traditional sol–gel and precipitation routes are difficult to redispersed in solution after drying, but the dried ZnOKH560 can be easily broken up in polar solvents such as ethanol, acetone, dimethyl formamide and dimethylsulfoxide. The highest concentration of the suspension of ZnOKH560 in acetone was 100 mg ml 1, which remains stable even after six months. X-ray diffraction (XRD) patterns of the ZnO QDs located at 2y = 31.84, 34.52, 36.24, 47.56, 56.60, 62.76, and 67.881 (ESI,† Fig. S3) can be perfectly indexed to a hexagonal wurtzite structure of ZnO (JCPDS: 36-1451).18 Nevertheless, the crystallinity of ZnOKH560 is much poorer than that of ZnObare due to the hindering effect of KH560, which leads to more defects on the surface of the former sample.25 Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images (ESI,† Fig. S4 and Fig. 3) show that the ZnOKH560 particles are uniform and well dispersed, which is in stark contrast to the aggregated ZnObare. Direct measurement of Fig. 3 indicates that the average diameters of ZnOKH560 and ZnObare are 3.0 0.5 and 4.5 0.5 nm, respectively, which is in agreement with the above estimation based on the UV-vis absorption spectra (ESI,† Fig. S2). Moreover, Fourier transformation of Fig. 3a gives interplanar spacings of 0.279, 0.258, 0.243, 0.187,
Fig. 3 HRTEM images of (a) ZnObare and (b) ZnOKH560. The insets show enlarged views of the selected regions.
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Fig. 4 PL spectra of ZnObare and ZnOKH560 in acetone (3 mg ml 1). The inset photographs of ZnObare (left) and ZnOKH560 (right) in acetone (3 mg ml 1) were taken under 365 nm irradiation.
0.164, and 0.145 nm, which correspond to the (100), (002), (101), (102), (110) and (103) planes of wurtzite ZnO (ESI,† Fig. S5) in agreement with the XRD measurements. The visible photoluminescence (PL) of ZnO QDs is mainly dependent on their size and surface defects.16,24–27 The former influences the position of the emission peak, while the latter controls the emission intensity. In accordance with the above discussion of the function of silane KH560 during the growth of the ZnO QDs, the PL spectra (Fig. 4) show that the excitation and emission peaks of ZnOKH560 in acetone have an obvious blue shift due to the smaller size of the QDs. The PL peak intensities of ZnOKH560, which possesses more surface defects, are higher than those of ZnObare. The absolute quantum yield of ZnOKH560 in acetone is 38%, which is thrice the value of ZnObare (12%). Additionally, the photoluminescence of dried ZnOKH560 is quite strong, while no light was given out by dried ZnObare (ESI,† Fig. S6, Table S1). The subsequent durability tests further demonstrated that the emission intensity of ZnOKH560 in either acetone or dry state remained unchanged after six months of storage under ambient conditions. Having been compounded with epoxy, the ZnO QDs are still highly luminescent due to their good distribution. Fig. 5 shows that the emission intensity of the ZnO QDs–epoxy nanocomposites increases with the increasing content of ZnO QDs. The highest absolute QY of the nanocomposite is 27% (ESI,† Table S1), but the QY value shows the opposite trend with the increasing content of ZnO QDs due to the concentration quenching effect. On the other hand, the transparency of epoxy remained almost unchanged after the incorporation of the ZnO QDs, because the particles were evenly dispersed. Fig. 6 shows the transmittance of the ZnO QDs–epoxy nanocomposite. Only marginal decrease was detected with the addition of ZnOKH560 from 0 to 8 wt% as expected. The transmittance of the nanocomposite containing 8 wt% ZnOKH560 at 800 nm was as high as 84%, demonstrating that the nanocomposite is basically as transparent as the unfilled epoxy. Fig. 6 also shows that the cut-off wavelength of the epoxy based nanocomposite increases from 300 to 350 nm due to the
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Fig. 5 PL spectra of ZnO QDs–epoxy nanocomposites with different ZnOKH560 contents.
Fig. 6 UV-vis transmittance of ZnO QDs–epoxy nanocomposites (B4.5 mm thick, as shown in ESI,† Fig. S7) with different ZnOKH560 contents. The inset photographs of the nanocomposites with different filler contents were taken under room light (top) and 365 nm irradiation (bottom), respectively.
excellent ultraviolet absorption capability of ZnO QDs. This means that the UV resistance of the epoxy was greatly improved. Meanwhile, the as-prepared ZnO QDs–epoxy nanocomposite maintained strong and stable PL properties (see inset of Fig. 6). At present, WLEDs based on blue chips and yellow phosphors are mainstream products. As the most important packaging material of LEDs, epoxy resin exerts a great influence on the lifetime of the entire device.28 Degradation and yellowing of epoxy, mainly induced by UV irradiation and light scattering from the phosphors, would significantly deteriorate its transparency and the luminous effect of the lamp as well. In this context, improving the UV resistance of epoxy is very important for producing durable WLED lamps.28,29 Since the above experiments indicate that the presence of ZnO QDs does not lower the transmittance of the epoxy and increases its UV absorbance, the longevity of WLEDs with blue chips would be extended accordingly. To directly observe the dispersion of ZnO QDs in the consolidated epoxy, scanning transmission electron microscopic (STEM) images of the nanocomposites were taken (Fig. 7).
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material for WLEDs based on a UV chip without the need for traditional tricolor phosphors. Since rare earth phosphors can be replaced in this way, production and application of WLEDs would be greatly promoted accordingly. In addition, the nanocomposite has with excellent UV resistance and high transmittance even in the case of 8 wt% ZnO QDs, which should benefit the longer life of WLEDs based on blue chips and yellow phosphors.
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Acknowledgements Fig. 7 STEM images of ultrathin sections (40 nm thick) of ZnO QDs– epoxy nanocomposites containing (a) 1 wt% and (b) 8 wt% ZnOKH560.
We can see that the ZnO QDs are uniformly dispersed in the matrix without obvious aggregation, even when the content of ZnO QDs reached up to 8 wt%. The homogeneous distribution of Zn atoms in the nanocomposite exhibited by the drift corrected spectrum image scanning shows the above analysis from another angle (ESI,† Fig. S8). As mentioned above, the silane KH560 modified ZnO QDs have good compatibility with the epoxy, due to the epoxy groups of KH560 that were attached to the QDs surface. When ZnO QDs were mixed with epoxy to make the composite material, the probability of unwanted QDs aggregation was reduced. Furthermore, the refractive index of silane KH560 (1.42) is much lower than those of ZnO (1.90) and epoxy (1.54). The intimate coverage of the silane on the ZnO QDs decreases the refractive index of the latter remarkably,29 and the resulting ZnOKH560 possesses a refractive index close to that of epoxy. During the curing process, crosslinking of the epoxy groups on the surface of modified ZnO QDs with the matrix epoxy prevents phase separation and eventually reduces the difference in refractive index between the ZnO QDs and the epoxy matrix. The strengthened interfacial adhesion between ZnOKH56 and the matrix epoxy is evidenced by the higher Tg of the nanocomposites than that of the matrix (ESI,† Fig. S9), because motion of the chain segments is restricted by the quantum dots. These ensure the high transparency of the epoxy nanocomposite. On the whole, although ZnO QDs have demonstrated their ability to act as a luminescent material in WLEDs excited by a UV chip, their size, color rendering index, luminous efficiency and consistency of light color must be further optimized. Moreover, measurement of lifetime and related investigations of WLEDs based on a blue chip with the ZnO QDs–epoxy composite as packaging material are on-going in our lab.
Conclusions Epoxy silane functionalized ZnO QDs with ultrastable and strong luminescence in both solution and dry states have been successfully prepared by a one-step precipitation method. Owing to the improved interfacial interaction between silane modified ZnO QDs and the epoxy resin, the quantum dots are well dispersed in epoxy and the resultant nanocomposite shows strong visible emission. Therefore, it can serve as a fluorescent
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The authors are grateful for the support of the Science and Technology Program of Guangdong Province (grant: 2010A011300004), the Project of Key Technological Breakthrough for Emerging Industries of Strategic Importance (grants: 2011A091102001 and 2011A091102003), and the Natural Science Foundation of China (grant: U1201243).
Notes and references 1 E. F. Schubert and J. K. Kim, Science, 2005, 308, 1274–1278. 2 X. B. Wang, X. S. Yan, W. W. Li and K. Sun, Adv. Mater., 2012, 24, 2742–2747. 3 L. Chen, C. C. Lin, C. W. Yeh and R. S. Liu, Materials, 2010, 3, 2172–2195. 4 M. M. Shang, G. G. Li, X. J. Kang, D. M. Yang and J. Lin, J. Electrochem. Soc., 2011, 158, H565–H571. 5 D. I. Son, B. W. Kwon, D. H. Park, W. S. Seo, Y. Yi, B. Angadi, C. L. Lee and W. K. Choi, Nat. Nanotechnol., 2012, 7, 465–471. 6 D. Zhou, M. Lin, X. Liu, J. Li, Z. L. Chen, D. Yao, H. Z. Sun, H. Zhang and B. Yang, ACS Nano, 2013, 7, 2273–2283. 7 S. Li, M. S. Toprak, Y. S. Jo, J. Dobson, D. K. Kim and M. Muhammed, Adv. Mater., 2007, 19, 4347–4352. 8 Y. Zhang, X. Wang, Y. X. Liu, S. Y. Song and D. P. Liu, J. Mater. Chem., 2012, 22, 11971–11977. 9 Y. Yang, W. N. Li, Y. S. Luo, H. M. Xiao, S. Y. Fu and Y. W. Mai, Polymer, 2010, 51, 2755–2762. 10 Y. Tu, L. Zhou, Y. Z. Jin, C. Gao, Z. Z. Ye, Y. F. Yang and Q. L. Wang, J. Mater. Chem., 2010, 20, 1594–1599. 11 M. J. Chen, J. R. Yang and M. Shiojiri, Semicond. Sci. Technol., 2012, 27, 074005. 12 H. V. Demir, S. Nizamoglu, T. Erdem, E. Mutlugun, ¨ller, Nano Today, 2011, 6, 632–647. N. Gaponik and A. Eychmu 13 Q. Qiao, B. H. Li, C. X. Shan, J. S. Liu, J. Yu, X. H. Xie, Z. Z. Zhang, T. B. Ji, Y. Jia and D. Z. Shen, Mater. Lett., 2012, 74, 104–106. 14 F. Wang, D. Zhao, Z. Guo, L. Liu, Z. Zhang and D. Shen, Nanoscale, 2013, 5, 2864–2869. 15 H. M. Xiong, D. G. Shchukin, H. Mohwald, Y. Xu and Y. Y. Xia, Angew. Chem., Int. Ed., 2009, 48, 2727–2731. 16 Y. S. Fu, X. W. Du, S. A. Kulinich, J. S. Qiu, W. J. Qin, R. Li, J. Sun and J. Liu, J. Am. Chem. Soc., 2007, 129, 16029–16033. 17 H. M. Xiong, Y. Xu, Q. G. Ren and Y. Y. Xia, J. Am. Chem. Soc., 2008, 130, 7522–7523. 18 Y. Zhang, H. Wang, H. Jiang and X. Wang, Nanoscale, 2012, 4, 3530–3535.
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25 L. Y. Zhang, L. W. Yin, C. X. Wang, N. Lun, Y. X. Qi and D. Xiang, J. Phys. Chem. C, 2010, 114, 9651–9658. 26 H. B. Zeng, G. T. Duan, Y. Li, S. K. Yang, X. X. Xu and W. P. Cai, Adv. Funct. Mater., 2010, 20, 561–572. 27 A. Asok, M. N. Gandhi and A. R. Kulkarni, Nanoscale, 2012, 4, 4943–4946. 28 Y. Q. Li, Y. Yang, S. Y. Fu and H. M. Xiao, J. Phys. Chem. C, 2008, 112, 10553–10558. 29 Y. Yang, Y. Q. Li, S. Y. Fu, X. Y. Yi, L. C. Wang and H. D. Chen, J. Phys. Chem. C, 2008, 112, 18616–18622.
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19 Z. Y. Zhang, Y. D. Xu, Y. Y. Ma, L. L. Qiu, Y. Wang, J. L. Kong and H. M. Xiong, Angew. Chem., Int. Ed., 2013, 52, 4127–4131. 20 H. M. Xiong, Z. D. Wang, D. P. Liu, J. S. Chen, Y. G. Wang and Y. Y. Xia, Adv. Funct. Mater., 2005, 15, 1751–1756. 21 H. M. Xiong, J. Mater. Chem., 2010, 20, 4251–4262. 22 H. Q. Shi, W. N. Li, L. W. Sun, Y. Liu, H. M. Xiao and S. Y. Fu, Chem. Commun., 2011, 47, 11921–11923. 23 H. M. Xiong, Adv. Mater., 2013, 25, 5329. 24 X. Xu, C. Xu, J. Dai, J. Hu, F. Li and S. Zhang, J. Phys. Chem. C, 2012, 116, 8813–8818.
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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 5480 Received 3rd December 2013, Accepted 26th December 2013
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Highly luminescent and transparent ZnO quantum dots–epoxy composite used for white light emitting diodes† Jia Jia Huang, Yi Bin Ye, Zhou Qiao Lei, Xiao Ji Ye, Min Zhi Rong* and Ming Qiu Zhang*
DOI: 10.1039/c3cp55098b www.rsc.org/pccp
By using epoxy silane as a modifier and stabilizer, ZnO quantum dots (QDs) were synthesized by a one-step precipitation approach. The ZnO QDs acquired showed satisfactory redispersibility and exhibited strong and stable photoluminescence in both solution and dry states. When the ZnO QDs content was as high as 8 wt%, the ZnO QDs–epoxy nanocomposite was still highly transparent and luminescent. Accordingly, the ZnO QDs can be used as a luminescent material, and a cool white light emitting diode (WLED) lamp was made by encapsulating a UV chip with the nanocomposite, without traditional tricolor rare earth phosphors. Additionally, the high loading nanocomposite possessed high ultraviolet resistance, which would help to improve the lifetime of the WLED.
White light emitting diodes (WLEDs) have attracted increasing interest because of their applications as new illumination sources.1 Among the strategies to create white light, the use of ultraviolet (UV)/near ultraviolet (NUV) chips in cooperation with tricolor phosphors is growing rapidly, owing to their high color rendering index, excellent light quality and low re-absorption characteristics.2–4 More importantly, they are believed to be the main route to evade existing patent barriers. Nevertheless, rare earth phosphors still serve as the luminescent material in this case, which inevitably brings about the drawbacks of limited resources and cost ineffectiveness. To the best of our knowledge, no unclassified information about WLEDs fabricated from UV chips and metallic oxide luminescent materials is available in the literature.5 ZnO has been found to be an emerging semiconductor material with wide band gap of 3.37 eV and high exciton Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China. E-mail: [email protected], [email protected]; Fax: +86-20-84114008; Tel: +86-20-84114008 † Electronic supplementary information (ESI) available: materials and methods, PL properties, UV-vis absorption spectra, XRD patterns, TEM photos, Fourier transformation of HRTEM image, photographs, thickness, drift corrected spectrum image scanning, and temperature dependence of loss factor. See DOI: 10.1039/c3cp55098b
5480 | Phys. Chem. Chem. Phys., 2014, 16, 5480--5484
binding energy (60 meV). Compared with conventional semiconductor quantum dots (QDs) such as CdTe QDs, ZnO QDs are characterized by their low cost and non-toxicity.6 ZnO QDs can be used not only as an excellent ultraviolet absorbent to increase the UV resistance of polymers,7–10 but also as novel fluorescent powders in LEDs owing to their strong and stable visible emission.11–14 ZnO QDs are generally prepared by sol–gel or precipitation approaches. With the use of suitable ligands or polymeric encapsulation,15–23 monodispersed ZnO QDs with strong and tunable emission can be obtained. However, the poor separation–redispersion characteristics of ZnO QDs and their non-compatibility with polymers limit their application as luminescent materials for WLEDs.5,8 Most of the ZnO QDs–epoxy composites reported so far only exhibit improved UV resistance, but fail to emit fluorescence because purified ZnO QDs are difficult to redisperse in solvents like acetone, which is a requirement for obtaining a homogeneous mixture of ZnO QDs and epoxy. In this work we report the synthesis of ZnO QDs by a one-step precipitation method in the presence of g-(2,3-epoxypropoxy)propytrimethoxysilane (KH560) as a surface modifier. The resultant QDs have an enhanced quantum yield (QY) and redispersibility, exhibiting strong and stable photoluminescence in both solution and dry states. In addition, when the as-produced ZnO QDs (ZnOKH560) are compounded with epoxy, a uniformly dispersed ZnO QDs–epoxy nanocomposite with high loading is produced because the epoxy groups of silane KH560 help to greatly improve the interfacial compatibility. This sets the stage for ZnO QDs to be used as a luminescent material in an epoxy matrix. To verify this idea, a UV LED chip (370 nm excitation) was simply encapsulated in the as-prepared ZnO QDs–epoxy nanocomposite without any traditional phosphors. The LED lamp indeed emits a cool white light with CIE coordinates of (0.305, 0.362) under an operating current of 10 mA (see inset of Fig. 1). A narrow peak at 408 nm and a broad peak at about 550 nm appear in the emission spectrum of the LED lamp (Fig. 1), which are attributed to the emissions of the epoxy and ZnO QDs, respectively. As the ZnO QDs are uniformly distributed in
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Fig. 1 Emission spectrum of the LED lamp (370 nm) encapsulated by the ZnO QDs–epoxy nanocomposite. Content of ZnO QDs: 5 wt%. Inset: the LED lamp in operation (left); CIE 1931 chromaticity coordinates calculated from the emission spectra (right).
the epoxy and are relatively far away from the LED chip, the lifetime of the lamp would be increased accordingly. The result demonstrates that the ZnO QDs can replace traditional rare earth phosphors to produce white light LEDs, or more precisely, the ZnO QDs–epoxy nanocomposite can replace the conventional compounds of silicon resin and rare earth phosphors to be coupled with UV/NUV chips for producing white light LEDs. The WLED shown in Fig. 1 reveals the potential of the ZnO QDs as a candidate for luminescent materials. The key issue lies in fabrication of a highly luminescent and transparent epoxy nanocomposite containing a high loading of ZnO QDs. We modified the surfaces of ZnO QDs with silane KH560, which provides the quantum dots with a protective layer. Consequently, the treated ZnO QDs are no longer affected by external interference, from factors such as solvents, moisture in the air and aggregation. Fluorescence quenching is thus minimized. Fig. 2 gives the results of a spectroscopic study of ZnOKH560 in comparison with that of ZnObare, which was synthesized
Fig. 2 (a) XPS survey scan spectrum of ZnOKH560, (b) experimental and fit curves for O 1s XPS spectrum of ZnOKH560, (c) FTIR spectra of ZnObare and ZnOKH560, and (d) thermogravimetric curves of ZnObare and ZnOKH560.
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without the silane. The X-ray photoelectron spectroscopy (XPS) spectrum in Fig. 2a shows that C, O, Zn, and Si appear on the surface of ZnOKH560. Moreover, deconvolution of the O 1s spectrum (Fig. 2b) reveals the presence of O2 (532.30 eV) ions in the oxygen-deficient regions within the matrix of ZnO,24 and C–O–C (533.45 eV). According to the Fourier transform infrared (FTIR) spectra (Fig. 2c), additional peaks of nC–H, nC–O–C and nSi–O–Si are observed in the spectrum of ZnOKH560. These prove that the silane KH560 has been bonded to ZnO with a grafting percentage of about 16 wt%, as determined from the thermal decomposition curve in Fig. 2d. The silane surface modifier has as effect on the formation, structure and photoluminescence of ZnO QDs.22 It efficiently suppresses the aggregation and excessive growth of the ZnO QDs through hydrolysis and condensation reactions with the hydroxyl groups on the surface of the ZnO QDs. As a result, the UV-vis absorption peak of ZnOKH560 has a blue shift of 20 nm compared to that of ZnObare (ESI,† Fig. S2), meaning that the diameter of ZnOKH560 has decreased from 4.21 to 2.89 nm.15 On the other hand, the ZnO QDs prepared through the traditional sol–gel and precipitation routes are difficult to redispersed in solution after drying, but the dried ZnOKH560 can be easily broken up in polar solvents such as ethanol, acetone, dimethyl formamide and dimethylsulfoxide. The highest concentration of the suspension of ZnOKH560 in acetone was 100 mg ml 1, which remains stable even after six months. X-ray diffraction (XRD) patterns of the ZnO QDs located at 2y = 31.84, 34.52, 36.24, 47.56, 56.60, 62.76, and 67.881 (ESI,† Fig. S3) can be perfectly indexed to a hexagonal wurtzite structure of ZnO (JCPDS: 36-1451).18 Nevertheless, the crystallinity of ZnOKH560 is much poorer than that of ZnObare due to the hindering effect of KH560, which leads to more defects on the surface of the former sample.25 Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images (ESI,† Fig. S4 and Fig. 3) show that the ZnOKH560 particles are uniform and well dispersed, which is in stark contrast to the aggregated ZnObare. Direct measurement of Fig. 3 indicates that the average diameters of ZnOKH560 and ZnObare are 3.0 0.5 and 4.5 0.5 nm, respectively, which is in agreement with the above estimation based on the UV-vis absorption spectra (ESI,† Fig. S2). Moreover, Fourier transformation of Fig. 3a gives interplanar spacings of 0.279, 0.258, 0.243, 0.187,
Fig. 3 HRTEM images of (a) ZnObare and (b) ZnOKH560. The insets show enlarged views of the selected regions.
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Fig. 4 PL spectra of ZnObare and ZnOKH560 in acetone (3 mg ml 1). The inset photographs of ZnObare (left) and ZnOKH560 (right) in acetone (3 mg ml 1) were taken under 365 nm irradiation.
0.164, and 0.145 nm, which correspond to the (100), (002), (101), (102), (110) and (103) planes of wurtzite ZnO (ESI,† Fig. S5) in agreement with the XRD measurements. The visible photoluminescence (PL) of ZnO QDs is mainly dependent on their size and surface defects.16,24–27 The former influences the position of the emission peak, while the latter controls the emission intensity. In accordance with the above discussion of the function of silane KH560 during the growth of the ZnO QDs, the PL spectra (Fig. 4) show that the excitation and emission peaks of ZnOKH560 in acetone have an obvious blue shift due to the smaller size of the QDs. The PL peak intensities of ZnOKH560, which possesses more surface defects, are higher than those of ZnObare. The absolute quantum yield of ZnOKH560 in acetone is 38%, which is thrice the value of ZnObare (12%). Additionally, the photoluminescence of dried ZnOKH560 is quite strong, while no light was given out by dried ZnObare (ESI,† Fig. S6, Table S1). The subsequent durability tests further demonstrated that the emission intensity of ZnOKH560 in either acetone or dry state remained unchanged after six months of storage under ambient conditions. Having been compounded with epoxy, the ZnO QDs are still highly luminescent due to their good distribution. Fig. 5 shows that the emission intensity of the ZnO QDs–epoxy nanocomposites increases with the increasing content of ZnO QDs. The highest absolute QY of the nanocomposite is 27% (ESI,† Table S1), but the QY value shows the opposite trend with the increasing content of ZnO QDs due to the concentration quenching effect. On the other hand, the transparency of epoxy remained almost unchanged after the incorporation of the ZnO QDs, because the particles were evenly dispersed. Fig. 6 shows the transmittance of the ZnO QDs–epoxy nanocomposite. Only marginal decrease was detected with the addition of ZnOKH560 from 0 to 8 wt% as expected. The transmittance of the nanocomposite containing 8 wt% ZnOKH560 at 800 nm was as high as 84%, demonstrating that the nanocomposite is basically as transparent as the unfilled epoxy. Fig. 6 also shows that the cut-off wavelength of the epoxy based nanocomposite increases from 300 to 350 nm due to the
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Fig. 5 PL spectra of ZnO QDs–epoxy nanocomposites with different ZnOKH560 contents.
Fig. 6 UV-vis transmittance of ZnO QDs–epoxy nanocomposites (B4.5 mm thick, as shown in ESI,† Fig. S7) with different ZnOKH560 contents. The inset photographs of the nanocomposites with different filler contents were taken under room light (top) and 365 nm irradiation (bottom), respectively.
excellent ultraviolet absorption capability of ZnO QDs. This means that the UV resistance of the epoxy was greatly improved. Meanwhile, the as-prepared ZnO QDs–epoxy nanocomposite maintained strong and stable PL properties (see inset of Fig. 6). At present, WLEDs based on blue chips and yellow phosphors are mainstream products. As the most important packaging material of LEDs, epoxy resin exerts a great influence on the lifetime of the entire device.28 Degradation and yellowing of epoxy, mainly induced by UV irradiation and light scattering from the phosphors, would significantly deteriorate its transparency and the luminous effect of the lamp as well. In this context, improving the UV resistance of epoxy is very important for producing durable WLED lamps.28,29 Since the above experiments indicate that the presence of ZnO QDs does not lower the transmittance of the epoxy and increases its UV absorbance, the longevity of WLEDs with blue chips would be extended accordingly. To directly observe the dispersion of ZnO QDs in the consolidated epoxy, scanning transmission electron microscopic (STEM) images of the nanocomposites were taken (Fig. 7).
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material for WLEDs based on a UV chip without the need for traditional tricolor phosphors. Since rare earth phosphors can be replaced in this way, production and application of WLEDs would be greatly promoted accordingly. In addition, the nanocomposite has with excellent UV resistance and high transmittance even in the case of 8 wt% ZnO QDs, which should benefit the longer life of WLEDs based on blue chips and yellow phosphors.
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Acknowledgements Fig. 7 STEM images of ultrathin sections (40 nm thick) of ZnO QDs– epoxy nanocomposites containing (a) 1 wt% and (b) 8 wt% ZnOKH560.
We can see that the ZnO QDs are uniformly dispersed in the matrix without obvious aggregation, even when the content of ZnO QDs reached up to 8 wt%. The homogeneous distribution of Zn atoms in the nanocomposite exhibited by the drift corrected spectrum image scanning shows the above analysis from another angle (ESI,† Fig. S8). As mentioned above, the silane KH560 modified ZnO QDs have good compatibility with the epoxy, due to the epoxy groups of KH560 that were attached to the QDs surface. When ZnO QDs were mixed with epoxy to make the composite material, the probability of unwanted QDs aggregation was reduced. Furthermore, the refractive index of silane KH560 (1.42) is much lower than those of ZnO (1.90) and epoxy (1.54). The intimate coverage of the silane on the ZnO QDs decreases the refractive index of the latter remarkably,29 and the resulting ZnOKH560 possesses a refractive index close to that of epoxy. During the curing process, crosslinking of the epoxy groups on the surface of modified ZnO QDs with the matrix epoxy prevents phase separation and eventually reduces the difference in refractive index between the ZnO QDs and the epoxy matrix. The strengthened interfacial adhesion between ZnOKH56 and the matrix epoxy is evidenced by the higher Tg of the nanocomposites than that of the matrix (ESI,† Fig. S9), because motion of the chain segments is restricted by the quantum dots. These ensure the high transparency of the epoxy nanocomposite. On the whole, although ZnO QDs have demonstrated their ability to act as a luminescent material in WLEDs excited by a UV chip, their size, color rendering index, luminous efficiency and consistency of light color must be further optimized. Moreover, measurement of lifetime and related investigations of WLEDs based on a blue chip with the ZnO QDs–epoxy composite as packaging material are on-going in our lab.
Conclusions Epoxy silane functionalized ZnO QDs with ultrastable and strong luminescence in both solution and dry states have been successfully prepared by a one-step precipitation method. Owing to the improved interfacial interaction between silane modified ZnO QDs and the epoxy resin, the quantum dots are well dispersed in epoxy and the resultant nanocomposite shows strong visible emission. Therefore, it can serve as a fluorescent
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The authors are grateful for the support of the Science and Technology Program of Guangdong Province (grant: 2010A011300004), the Project of Key Technological Breakthrough for Emerging Industries of Strategic Importance (grants: 2011A091102001 and 2011A091102003), and the Natural Science Foundation of China (grant: U1201243).
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