Journal of Colloid and Interface Science 465 (2016) 42–46
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Short Communication
Enhanced light extraction by heterostructure photonic crystals toward white-light-emission Heng Li a, Zhaohua Xu a,⇑, Bin Bao b, Yanlin Song b,⇑ a
Department of Material Technology, Jiangmen Polytechnic, Jiangmen, Guangdong 529090, PR China Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green Printing, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China b
g r a p h i c a l a b s t r a c t The simultaneous enhancement of red, green, and blue emission intensity by heterostructure photonic crystals with tri-stopbands is presented, which have potential for developing optical devices of high performance.
a r t i c l e
i n f o
Article history: Received 22 September 2015 Revised 20 November 2015 Accepted 20 November 2015 Available online 22 November 2015 Keywords: Photonic crystals Tri-stopbands RGB emission Heterostructure
a b s t r a c t In this work, we present a novel approach on the simultaneous enhancement of intensity of red, green, and blue (RGB) emission by heterostructure colloidal photonic crystals (PCs) with tri-stopbands. The intensity of RGB emission on heterostructure PCs with tri-stopbands overlapping emission wavelengths of RGB QDs can be up to about 8-fold enhancement in comparison to that on the control sample. Furthermore, CIE diagrams show the chromaticity parameters approaching that of white light. The method will be favorable for developing optical devices of high performance. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction White-light-emission systems based on phosphors with flexibly selected emission color and high efficiency have drawn a greatly ⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Xu), [email protected] (Y. Song). http://dx.doi.org/10.1016/j.jcis.2015.11.052 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
increased interest in the fields of optoelectronics [1,2]. Basically, white-light-emission systems can be achieved using the complementary color wheel rule composed of two-color system (blue and yellow) or, more commonly, a three-color system (blue, green, and red, that is RGB) [3]. In this regard, three-color system through the mixing of independently emitting quantum dots (QDs) could
H. Li et al. / Journal of Colloid and Interface Science 465 (2016) 42–46
be proposed to be the most promising materials in the optoelectronic fabrication due to high luminescence quantum yield, good photo-stability, size-tunable emission and cheap solution processability [4,5]. Combined with UV light, highly efficient RGB QDs have been utilized as light conversion phosphors for white light emission [6–9]. However, most of these optical films based on the QDs have small Stokes shift owing to a strong aggregation phenomena and low light-extraction efficiency resulted from the scattering problem [10,11]. Therefore, it is still challenging to develop the white light film for incorporating QDs that can keep their high luminescent performance. As we know, photonic crystals (PCs) as unique optical materials have received great attention due to its special light manipulation property [12–14]. Especially, optical gain in PC science is regarded optical amplification mediated by stimulated emission of photons [15–19]. Additionally, the PCs surface possesses large surface-tovolume ratios for the effective dispersion of phosphors, which can avoid self-quenching effect. For example, Cunningham et al. demonstrated 108-fold enhancement of photoluminescence on the PC slabs by a combination of high-intensity near fields with strong coherent scattering effects of guided resonance [18,19]. In our previous work, a 162-fold enhancement of luminescent signal based on heterostructure PCs with dual stopbands was also demonstrated [20]. Thus, PCs as the excellent optical substrate are a promising tool to manipulate and improve luminescent signal. Unfortunately, at that time our work did not involve the application of heterostructure PCs. Herein, a strategy of combining heterostructure PCs as the substrate and RGB QDs applied as the optical film is developed for white-light-emission. We show that an obvious enhancement of white-light intensity based on heterostructure PCs with tri-stopbands comparison to that on the control sample. This is achieved by engineering the structure of PCs to make tri-stopbands overlapping the emission wavelength of RGB QDs, respectively. Furthermore, RGB emission can be improved simultaneously at a single excitation wavelength, and CIE diagrams show the chromaticity parameters approaching that of white light. The means of white-light enhancement on heterostructure PCs with tri-stopbands can lead to strong light harvestings and provide an opportunity for developing the optical device with high performance. 2. Experimental 2.1. Materials Styrenes (St), methyl methacrylate (MMA), acrylic acid (AA) were purified by distillation under reduced pressure. The initiator of (NH4)2S2O4 (APS) was recrystallized three times. All reagents and materials were purchased from Aldrich unless otherwise noted. 2.2. Fabrication of heterostructure PC films Monodisperse latex spheres of Poly(St-MMA-AA) were synthesized via our previous method [21]. The resulting latex spheres were used directly without purification. The polydispersity of the latex spheres was about 0.5%, which was detected by ZetaPALS BI-90plus (Brookhaven Instrument). The heterostructure PCs with tri-stopbands are fabricated by the successive vertical depositions of three latex spheres with different diameters onto glass substrates at the constant temperature of 60 °C and humidity of 60%. The glass slides were first treated with a chromic–sulfuric acid solution to ensure clean surfaces. After the samples were dry, they were sintered at 85 °C for 30 min to increase the stability of the samples.
43
2.3. Fabrication of crosslinked heterostructure PC films Photo-crosslinkaged heterostructure PC films were fabricated by immersing acrylamide solution for illumination of ultraviolet light via the Ref. [22]. As a result, a crosslinked polymer network would form among latex spheres and solvent resistance of the PCs got improved. 2.4. Preparation of luminescent PC films The blue, green, and red QDs solution (10 mg/mL chloroform) (mass radio of R:G:B is = 7:6:3), were added into a 1 ml transparent PMMA/chloroform solution (1 g of PMMA mixed with 10 ml of chloroform solution). The RGB QDs-loaded PCs films were prepared by spin-coating the chloroform solution of RGB QDs and PMMA mixtures onto photo-crosslinkaged heterostructure PC films and glass substrates (as the control sample) at 1200 rpm for 20 s, respectively. 2.5. Characterization The SEM image was obtained with a field-emission SEM (JEOL JSM-4800, Japan), after sputtering the samples with a thin layer of gold. The UV–Vis absorbance spectrum was obtained by an ultraviolet–visible spectrophotometer (UV-2600, Japan). The photoluminescence spectrum was measured by a Hitachi F-4500 fluorescence spectrophotometer. The micro-reflectance spectrum observation of the PC films was carried out by combining a reflected microscope (Olympus MX40, Japan) and a fiber optic UV–Vis spectrometer (Ocean Optic HR 4000, USA). The illuminating light was focused onto the PC films through an objective lens and the reflected light was collected by the same lens and then transported to the spectrometer through the optic fiber. 3. Results and discussion Commercially available RGB QDs which can be adjusted to generate white light emitting are chose. UV–Visible absorption spectra of the trichromatic QDs in chloroform solutions used are presented in Fig. 1a. The trend of the absorption toward longer wavelength is observed. The photoluminescence (PL) spectra of the three samples exhibit band-to-band emission bands centered at 467 nm, 520 nm and 610 nm, respectively. Their respective PL spectra show large Stokes shifts, indicating that the emission can be dominated by defect-related mechanisms [23]. Moreover, it can be seen that their full width at half maximums are about 30 nm, indicating their narrow size distribution. Here, we selected PC films from latex spheres with diameter 197 nm, 210 nm and 265 nm, whose stopbands are at 473, 510 and 613 nm, spectrally corresponding to the emission wavelength of the trichromatic QDs, respectively. As shown in Fig. 1b, the three stopbands of PC films match clearly the emission peak of the individual QDs. The triple period structure of the PCs can offer tri-dielectric stopbands, which leads to a dramatic modification of light propagation and emission properties of the luminescent medium. Heterostructure PCs are fabricated by the successive depositions of three particles with different diameters onto a substrate [24–28]. Fig. 2a–c presents scan electron microscope (SEM) side views of heterostructure PC films composed different selfassembly concentrations of P(St-MMA-AA) spheres with 197down– 210middle–265up. This clearly shows the growth of one PC over another with good ordering. In general, the film thickness depends on such parameters as solution concentration, temperature, air humidity, and sphere size. Keeping all parameters constant except the solution concentration, we explored the effect upon the film
H. Li et al. / Journal of Colloid and Interface Science 465 (2016) 42–46
(b) 1.2
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0.8 0.6 0.4 0.2
473 nm 510 nm 613 nm Blue Green Red
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0.8 0.6 0.4 0.2 0.0
0.0 300
400
500
600
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400
500
600
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700
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Fig. 1. (a) UV–visible absorption spectra of the trichromatic QDs in chloroform solutions. (b) The reflection spectra for only PCs assembled from latex sphere with diameter 197 nm (black line), 210 nm (red line) and 265 nm (blue line), measured with a light incident along the normal surface of PCs [1 1 1] direction, and PL spectra of the trichromatic QDs (dotted line) in chloroform solutions under excitation wavelength of 365 nm.
Fig. 2. (a–c) SEM side views of heterostructure PC films made of different self-assembly concentrations of P(St-MMA-AA) spheres with 197down–210middle–265up (the number indicates the diameter (nm) of the sphere used). The self-assembly concentration: (a) 0.05 wt.%, (b) 0.1 wt.%, and (c) 0.2 wt.%. (d) The reflection spectra of heterostructure PC films with different self-assembly concentrations of P(St-MMA-AA) spheres with 197down–210middle–265up.
thickness. With increasing the self-assembly concentration, interfaces between the two layers from different diameters of particles are apparent, especially for 0.2 wt.% self-assembly concentration, indicating that a critical concentration is necessary to build up an outstanding close-packed film on a substrate. It is notable that PC films are very fragile and tends to fall off if continue to increase the self-assembly concentration. From Fig. 2c, the thickness of top layer from 265 nm particles is less that of middle layers from 210 nm particles and bottom layers from 197 nm particles at the same self-assembly concentration, indicating difficult depositions for high concentration. Using particles with different sizes, the layers can be discriminated by UV–visible spectroscopy. Fig. 2d exhibits that the PC heterostructure with three peaks having a one-to-one correspondence confirming the signature of the photonic
stopband. The center positions of the tri-stopbands of heterostructure PCs are at approximately 470, 510 and 610 nm, but it is found that the reflectivity intensity from three particles at the identical concentration for the same heterostructure PCs is not equal due to disorder at the interface resulted in increased bandwidth, reduced peak reflectance. At the beginning, the deposition of an increasing concentration induces the overall intensity increases shown in black line and red dashed line of Fig. 2d. Moreover, center positions of peak shifts toward long wavelengths. Clearly, the reflectivity of heterostructure PC films diminishes when the PCs are thicker, which is probably owing to more defects in thicker PCs [29]. The three-component QDs mixtures are employed to achieve equal RGB emission intensities, demonstrating a simple and effective
H. Li et al. / Journal of Colloid and Interface Science 465 (2016) 42–46
method of fine color rendering capability for illumination devices. Clearly, Fig. 3a exhibits the intensities in each of the RGB spectrum regions in a solution are almost equal, producing the bright white color. From analyzing the emission spectrum, the Commission International de I’Eclairage (CIE) chromaticity coordinates for the RGB QD emission yield values of x = 0.32 and y = 0.34 laid in the white light region. The RGB QDs mixed with a common thermoplastic poly(methyl methacrylate) (PMMA) in organic media were coated crosslinkaged heterostructure PC film. The crosslinkaged heterostructure PC film is characterized by SEM. As shown in Fig. 3c, there is the SEM image of heterostructure PC film before crosslinkage. The SEM image illustrates that the colloid spheres are in a face-centered cubic arrangement with a closepacked plane (1 1 1) oriented parallel to the substrate. The closepacked arrangement can extend over a large area, which provides
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a simple and inexpensive technique for practical application of optical devices. From Fig. 3d, it can be seen that the surface of the film seemed to be overcasted, which implies that part of polyacrylamide (PAAm) is infiltrated among the interstice of latex spheres. As a result, a crosslinked polymer network is formed. In addition, solvent resistance of the heterostructure PC film can be improved. In order to obtain the solid white-light film, RGB colors are mixed for creating white light. As shown in Fig. 4a, owing to the tricolor mixing with a suitable ratio, white light emission is satisfactorily achieved. The PL spectra of the solution and the solid film are nearly identical. This observation demonstrates that there is no significant interaction between the three QDs when they are doped in the matrix. Remarkably, a white-light emission film on the control sample with CIE coordinates of (0.30, 0.31) is obtained and is
Fig. 3. (a) PL spectrum of trichromatic QDs solution mixture under excitation wavelength of 365 nm. (b) The CIE coordinates of the emitting trichromatic QDs. (c and d) Typical SEM images of the heterostructure PC film before and after photo-crosslinkage.
Fig. 4. (a) PL spectra and (b) the CIE coordinates of a mixture of RGB QDs embedded in PMMA matrix on the control and on crosslinkaged heterostructure PC films with different self-assembly concentrations of P(St-MMA-AA) spheres with 197down–210middle–265up under excitation wavelength of 365 nm, respectively.
46
H. Li et al. / Journal of Colloid and Interface Science 465 (2016) 42–46
very close to the pure white light (0.33, 0.33). In contrast to the control sample from the QDs emission under a single excite source, the peak wavelength positions remain almost unchanged, but the emission intensity from the heterostructure PC film with 0.05% self-assembly concentration is significantly stronger than that from control sample, by factor of 8. The enhancement of emission can be attributed to its periodic arrangement, especially, the tristopbands of heterostructure PCs overlaps the emission wavelengths of RGB QDs, respectively. Here, PCs are acted as a Bragg mirror, which can produce a strong resonant mode for the emission propagation. Photons can couple to the overlapping local resonance mode and Bragg scatter out of the structure, thereby greatly reducing the amount of light trapped as guided modes. Moreover, high density of states near the tri-stopband enhances the coupling of spontaneously emitted photons [30,31]. With increasing the self-assembly concentration, we observe a continual increase of RGB emission. It can be seen that the enhancement ration of blue spectral component is relatively higher compared with that of other colors. The evolution of spectral feature is consistent with reflectance intensity of heterostructure PCs, indicating that more photons are harvested. Additionally, the color coordinates shift from (0.30, 0.31) to (0.27, 0.27) upon increasing the blue spectral component. However, further increase of the selfassembly concentration will not be very effective in further enhancing the RGB emission intensity since more defects can effect optical properties in thicker heterostructure PCs. 4. Conclusions We demonstrate over 8-fold enhancement of white-light intensity from three-dimensional heterostructure PCs with 0.05% self-assembly concentration. This is obtained by engineering the structure of PCs to make tri-stopbands overlapping emission wavelengths of RGB QDs, respectively. Moreover, CIE diagrams show the chromaticity parameters approaching that of white light. The approach will be of great significance for building a multilayer system of PCs with the structural engineering and complex optical properties, which provides the reasonable guideline for developing optical devices of high performance. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51302107) and the Natural Science Foundation of Guangdong Province, China (Grant No. 2015A030313804).
References [1] M. Shang, C. Li, J. Lin, Chem. Soc. Rev. 43 (2014) 1372. [2] B. Chen, J.C. Feng, J. Phys. Chem. C 119 (2015) 7865. [3] J. Kim, J. Noh, S. Jo, K.E. Park, W.H. Park, T.S. Lee, ACS Appl. Mater. Interfaces 5 (2013) 6038. [4] X.Q. Li, Y.W. Wu, D. Steel, D. Gammon, T.H. Stievater, D.S. Katzer, D. Park, C. Piermarocchi, L.J. Sham, Science 301 (2003) 809. [5] J.Y. Kim, O. Voznyy, D. Zhitomirsky, E.H. Sargent, Adv. Mater. 25 (2013) 4986. [6] T.E. Rosson, S.M. Claiborne, J.R. McBride, B.S. Stratton, S.J. Rosenthal, J. Am. Chem. Soc. 134 (2012) 8006. [7] R.Z. Liang, D.P. Yan, R. Tian, X.J. Yu, W.Y. Shi, C.Y. Li, M. Wei, D.G. Evans, X. Duan, Chem. Mater. 26 (2014) 2595. [8] S. Kim, S. HyukIm, S.-W. Kim, Nanoscale 5 (2013) 5205. [9] C. Sun, Y. Zhang, Y. Wang, W.Y. Liu, S. Kalytchuk, S.V. Kershaw, T.Q. Zhang, X.Y. Zhang, J. Zhao, W.W. Yu, A.L. Rogach, Appl. Phys. Lett. 104 (2014) 261106. [10] C. Dang, J. Lee, Y. Zhang, J. Han, C. Breen, J.S. Steckel, S. Coe-Sullivan, A. Nurmikko, Adv. Mater. 24 (2012) 5915. [11] B.X. Zhao, D. Zhang, K. Sun, X.B. Wang, R.H. Mao, W.W. Li, RSC Adv. 4 (2014) 45155. [12] H.J. Kim, S. Kim, H. Jeon, J. Ma, S.H. Choi, S. Lee, C. Ko, W. Park, Sens. Actuators, B 124 (2007) 147. [13] J. Hu, X.W. Zhao, Y.J. Zhao, J. Li, W.Y. Xu, Z.Y. Wen, M. Xu, Z.Z. Gu, J. Mater. Chem. 19 (2009) 5730. [14] F. Jin, L.T. Shi, M.L. Zheng, X.Z. Dong, S. Chen, Z.S. Zhao, X.M. Duan, J. Phys. Chem. C 117 (2013) 9463. [15] P. Lodahl, A.F. van Driel, I.S. Nikolaev, A. Irman, K. Overgaag, D.L. Vanmaekelbergh, W.L. Vos, Nature 430 (2004) 654. [16] A.C. Arsenault, T.J. Clark, G. Von Freymann, L. Cademartiri, R. Sapienza, J. Bertolotti, E. Vekris, S. Wong, V. Kitaev, I. Manners, R.Z. Wang, S. John, D. Wiersma, G.A. Ozin, Nat. Mater. 5 (2006) 179. [17] M.N. Shkunov, Z.V. Vardeny, M.C. DeLong, R.C. Polson, A.A. Zakhidov, R.H. Baughman, Adv. Funct. Mater. 12 (2002) 21. [18] N. Ganesh, W. Zhang, P.C. Mathias, E. Chow, Jant Soares, V. Malyarchuk, A.D. Smith, B.T. Cunningham, Nat. Nanotechnol. 2 (2007) 515. [19] P.C. Mathias, H.Y. Wu, B.T. Cunningham, Appl. Phys. Lett. 95 (2009) 021111. [20] H. Li, J.X. Wang, F. Liu, Y.L. Song, R.M. Wang, J. Colloid Interface Sci. 356 (2011) 63. [21] J.X. Wang, Y.Q. Wen, H.L. Ge, Z.W. Sun, Y.M. Zheng, Y.L. Song, L. Jiang, Macromol. Chem. Phys. 207 (2006) 596. [22] E.T. Tian, J.X. Wang, Y.M. Zheng, Y.L. Song, L. Jiang, D.B. Zhu, J. Mater. Chem. 18 (2008) 1116. [23] W. Liu, Y. Zhang, W. Zhai, Y. Wang, T. Zhang, P. Gu, H. Chu, H. Zhang, T. Cui, Y. Wang, J. Zhao, W.W. Yu, J. Phys. Chem. C 117 (2013) 19288. [24] S. Wong, V. Kitaev, G.A. Ozin, J. Am. Chem. Soc. 125 (2003) 15589. [25] S. Reculusa, P. Masse, S. Ravaine, J. Colloid Interface Sci. 279 (2004) 471. [26] P. Masse, S. Ravaine, Chem. Mater. 17 (2005) 4244. [27] M. Bardosova, M.E. Pemble, I.M. Povey, R.H. Tredgold, D.E. Whitehead, Appl. Phys. Lett. 89 (2006) 093116. [28] R.V. Nair, R. Vijaya, J. Appl. Phys. 102 (2007) 056102. [29] S. Reculusa, S. Ravaine, Chem. Mater. 15 (2003) 598. [30] S.H. Fan, P.R. Villeneuve, J.D. Joannopoulos, E.F. Schubert, Phys. Rev. Lett. 78 (1997) 3294. [31] M. Zelsmann, E. Picard, T. Charvolin, E. Hadji, M. Heitzmann, B. Dal’zotto, M.E. Nier, C. Seassal, P. Rojo-Romeo, X. Letartre, Appl. Phys. Lett. 83 (2003) 2542.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Short Communication
Enhanced light extraction by heterostructure photonic crystals toward white-light-emission Heng Li a, Zhaohua Xu a,⇑, Bin Bao b, Yanlin Song b,⇑ a
Department of Material Technology, Jiangmen Polytechnic, Jiangmen, Guangdong 529090, PR China Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green Printing, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China b
g r a p h i c a l a b s t r a c t The simultaneous enhancement of red, green, and blue emission intensity by heterostructure photonic crystals with tri-stopbands is presented, which have potential for developing optical devices of high performance.
a r t i c l e
i n f o
Article history: Received 22 September 2015 Revised 20 November 2015 Accepted 20 November 2015 Available online 22 November 2015 Keywords: Photonic crystals Tri-stopbands RGB emission Heterostructure
a b s t r a c t In this work, we present a novel approach on the simultaneous enhancement of intensity of red, green, and blue (RGB) emission by heterostructure colloidal photonic crystals (PCs) with tri-stopbands. The intensity of RGB emission on heterostructure PCs with tri-stopbands overlapping emission wavelengths of RGB QDs can be up to about 8-fold enhancement in comparison to that on the control sample. Furthermore, CIE diagrams show the chromaticity parameters approaching that of white light. The method will be favorable for developing optical devices of high performance. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction White-light-emission systems based on phosphors with flexibly selected emission color and high efficiency have drawn a greatly ⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Xu), [email protected] (Y. Song). http://dx.doi.org/10.1016/j.jcis.2015.11.052 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
increased interest in the fields of optoelectronics [1,2]. Basically, white-light-emission systems can be achieved using the complementary color wheel rule composed of two-color system (blue and yellow) or, more commonly, a three-color system (blue, green, and red, that is RGB) [3]. In this regard, three-color system through the mixing of independently emitting quantum dots (QDs) could
H. Li et al. / Journal of Colloid and Interface Science 465 (2016) 42–46
be proposed to be the most promising materials in the optoelectronic fabrication due to high luminescence quantum yield, good photo-stability, size-tunable emission and cheap solution processability [4,5]. Combined with UV light, highly efficient RGB QDs have been utilized as light conversion phosphors for white light emission [6–9]. However, most of these optical films based on the QDs have small Stokes shift owing to a strong aggregation phenomena and low light-extraction efficiency resulted from the scattering problem [10,11]. Therefore, it is still challenging to develop the white light film for incorporating QDs that can keep their high luminescent performance. As we know, photonic crystals (PCs) as unique optical materials have received great attention due to its special light manipulation property [12–14]. Especially, optical gain in PC science is regarded optical amplification mediated by stimulated emission of photons [15–19]. Additionally, the PCs surface possesses large surface-tovolume ratios for the effective dispersion of phosphors, which can avoid self-quenching effect. For example, Cunningham et al. demonstrated 108-fold enhancement of photoluminescence on the PC slabs by a combination of high-intensity near fields with strong coherent scattering effects of guided resonance [18,19]. In our previous work, a 162-fold enhancement of luminescent signal based on heterostructure PCs with dual stopbands was also demonstrated [20]. Thus, PCs as the excellent optical substrate are a promising tool to manipulate and improve luminescent signal. Unfortunately, at that time our work did not involve the application of heterostructure PCs. Herein, a strategy of combining heterostructure PCs as the substrate and RGB QDs applied as the optical film is developed for white-light-emission. We show that an obvious enhancement of white-light intensity based on heterostructure PCs with tri-stopbands comparison to that on the control sample. This is achieved by engineering the structure of PCs to make tri-stopbands overlapping the emission wavelength of RGB QDs, respectively. Furthermore, RGB emission can be improved simultaneously at a single excitation wavelength, and CIE diagrams show the chromaticity parameters approaching that of white light. The means of white-light enhancement on heterostructure PCs with tri-stopbands can lead to strong light harvestings and provide an opportunity for developing the optical device with high performance. 2. Experimental 2.1. Materials Styrenes (St), methyl methacrylate (MMA), acrylic acid (AA) were purified by distillation under reduced pressure. The initiator of (NH4)2S2O4 (APS) was recrystallized three times. All reagents and materials were purchased from Aldrich unless otherwise noted. 2.2. Fabrication of heterostructure PC films Monodisperse latex spheres of Poly(St-MMA-AA) were synthesized via our previous method [21]. The resulting latex spheres were used directly without purification. The polydispersity of the latex spheres was about 0.5%, which was detected by ZetaPALS BI-90plus (Brookhaven Instrument). The heterostructure PCs with tri-stopbands are fabricated by the successive vertical depositions of three latex spheres with different diameters onto glass substrates at the constant temperature of 60 °C and humidity of 60%. The glass slides were first treated with a chromic–sulfuric acid solution to ensure clean surfaces. After the samples were dry, they were sintered at 85 °C for 30 min to increase the stability of the samples.
43
2.3. Fabrication of crosslinked heterostructure PC films Photo-crosslinkaged heterostructure PC films were fabricated by immersing acrylamide solution for illumination of ultraviolet light via the Ref. [22]. As a result, a crosslinked polymer network would form among latex spheres and solvent resistance of the PCs got improved. 2.4. Preparation of luminescent PC films The blue, green, and red QDs solution (10 mg/mL chloroform) (mass radio of R:G:B is = 7:6:3), were added into a 1 ml transparent PMMA/chloroform solution (1 g of PMMA mixed with 10 ml of chloroform solution). The RGB QDs-loaded PCs films were prepared by spin-coating the chloroform solution of RGB QDs and PMMA mixtures onto photo-crosslinkaged heterostructure PC films and glass substrates (as the control sample) at 1200 rpm for 20 s, respectively. 2.5. Characterization The SEM image was obtained with a field-emission SEM (JEOL JSM-4800, Japan), after sputtering the samples with a thin layer of gold. The UV–Vis absorbance spectrum was obtained by an ultraviolet–visible spectrophotometer (UV-2600, Japan). The photoluminescence spectrum was measured by a Hitachi F-4500 fluorescence spectrophotometer. The micro-reflectance spectrum observation of the PC films was carried out by combining a reflected microscope (Olympus MX40, Japan) and a fiber optic UV–Vis spectrometer (Ocean Optic HR 4000, USA). The illuminating light was focused onto the PC films through an objective lens and the reflected light was collected by the same lens and then transported to the spectrometer through the optic fiber. 3. Results and discussion Commercially available RGB QDs which can be adjusted to generate white light emitting are chose. UV–Visible absorption spectra of the trichromatic QDs in chloroform solutions used are presented in Fig. 1a. The trend of the absorption toward longer wavelength is observed. The photoluminescence (PL) spectra of the three samples exhibit band-to-band emission bands centered at 467 nm, 520 nm and 610 nm, respectively. Their respective PL spectra show large Stokes shifts, indicating that the emission can be dominated by defect-related mechanisms [23]. Moreover, it can be seen that their full width at half maximums are about 30 nm, indicating their narrow size distribution. Here, we selected PC films from latex spheres with diameter 197 nm, 210 nm and 265 nm, whose stopbands are at 473, 510 and 613 nm, spectrally corresponding to the emission wavelength of the trichromatic QDs, respectively. As shown in Fig. 1b, the three stopbands of PC films match clearly the emission peak of the individual QDs. The triple period structure of the PCs can offer tri-dielectric stopbands, which leads to a dramatic modification of light propagation and emission properties of the luminescent medium. Heterostructure PCs are fabricated by the successive depositions of three particles with different diameters onto a substrate [24–28]. Fig. 2a–c presents scan electron microscope (SEM) side views of heterostructure PC films composed different selfassembly concentrations of P(St-MMA-AA) spheres with 197down– 210middle–265up. This clearly shows the growth of one PC over another with good ordering. In general, the film thickness depends on such parameters as solution concentration, temperature, air humidity, and sphere size. Keeping all parameters constant except the solution concentration, we explored the effect upon the film
H. Li et al. / Journal of Colloid and Interface Science 465 (2016) 42–46
(b) 1.2
1.0
B G R
0.8 0.6 0.4 0.2
473 nm 510 nm 613 nm Blue Green Red
1.0
Reflection (a.u.)
Absorbance (a.u.)
(a)
0.8 0.6 0.4 0.2 0.0
0.0 300
400
500
600
700
Wavelength (nm)
400
500
600
Photoluminescence (a.u.)
44
700
Wavelength (nm)
Fig. 1. (a) UV–visible absorption spectra of the trichromatic QDs in chloroform solutions. (b) The reflection spectra for only PCs assembled from latex sphere with diameter 197 nm (black line), 210 nm (red line) and 265 nm (blue line), measured with a light incident along the normal surface of PCs [1 1 1] direction, and PL spectra of the trichromatic QDs (dotted line) in chloroform solutions under excitation wavelength of 365 nm.
Fig. 2. (a–c) SEM side views of heterostructure PC films made of different self-assembly concentrations of P(St-MMA-AA) spheres with 197down–210middle–265up (the number indicates the diameter (nm) of the sphere used). The self-assembly concentration: (a) 0.05 wt.%, (b) 0.1 wt.%, and (c) 0.2 wt.%. (d) The reflection spectra of heterostructure PC films with different self-assembly concentrations of P(St-MMA-AA) spheres with 197down–210middle–265up.
thickness. With increasing the self-assembly concentration, interfaces between the two layers from different diameters of particles are apparent, especially for 0.2 wt.% self-assembly concentration, indicating that a critical concentration is necessary to build up an outstanding close-packed film on a substrate. It is notable that PC films are very fragile and tends to fall off if continue to increase the self-assembly concentration. From Fig. 2c, the thickness of top layer from 265 nm particles is less that of middle layers from 210 nm particles and bottom layers from 197 nm particles at the same self-assembly concentration, indicating difficult depositions for high concentration. Using particles with different sizes, the layers can be discriminated by UV–visible spectroscopy. Fig. 2d exhibits that the PC heterostructure with three peaks having a one-to-one correspondence confirming the signature of the photonic
stopband. The center positions of the tri-stopbands of heterostructure PCs are at approximately 470, 510 and 610 nm, but it is found that the reflectivity intensity from three particles at the identical concentration for the same heterostructure PCs is not equal due to disorder at the interface resulted in increased bandwidth, reduced peak reflectance. At the beginning, the deposition of an increasing concentration induces the overall intensity increases shown in black line and red dashed line of Fig. 2d. Moreover, center positions of peak shifts toward long wavelengths. Clearly, the reflectivity of heterostructure PC films diminishes when the PCs are thicker, which is probably owing to more defects in thicker PCs [29]. The three-component QDs mixtures are employed to achieve equal RGB emission intensities, demonstrating a simple and effective
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method of fine color rendering capability for illumination devices. Clearly, Fig. 3a exhibits the intensities in each of the RGB spectrum regions in a solution are almost equal, producing the bright white color. From analyzing the emission spectrum, the Commission International de I’Eclairage (CIE) chromaticity coordinates for the RGB QD emission yield values of x = 0.32 and y = 0.34 laid in the white light region. The RGB QDs mixed with a common thermoplastic poly(methyl methacrylate) (PMMA) in organic media were coated crosslinkaged heterostructure PC film. The crosslinkaged heterostructure PC film is characterized by SEM. As shown in Fig. 3c, there is the SEM image of heterostructure PC film before crosslinkage. The SEM image illustrates that the colloid spheres are in a face-centered cubic arrangement with a closepacked plane (1 1 1) oriented parallel to the substrate. The closepacked arrangement can extend over a large area, which provides
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a simple and inexpensive technique for practical application of optical devices. From Fig. 3d, it can be seen that the surface of the film seemed to be overcasted, which implies that part of polyacrylamide (PAAm) is infiltrated among the interstice of latex spheres. As a result, a crosslinked polymer network is formed. In addition, solvent resistance of the heterostructure PC film can be improved. In order to obtain the solid white-light film, RGB colors are mixed for creating white light. As shown in Fig. 4a, owing to the tricolor mixing with a suitable ratio, white light emission is satisfactorily achieved. The PL spectra of the solution and the solid film are nearly identical. This observation demonstrates that there is no significant interaction between the three QDs when they are doped in the matrix. Remarkably, a white-light emission film on the control sample with CIE coordinates of (0.30, 0.31) is obtained and is
Fig. 3. (a) PL spectrum of trichromatic QDs solution mixture under excitation wavelength of 365 nm. (b) The CIE coordinates of the emitting trichromatic QDs. (c and d) Typical SEM images of the heterostructure PC film before and after photo-crosslinkage.
Fig. 4. (a) PL spectra and (b) the CIE coordinates of a mixture of RGB QDs embedded in PMMA matrix on the control and on crosslinkaged heterostructure PC films with different self-assembly concentrations of P(St-MMA-AA) spheres with 197down–210middle–265up under excitation wavelength of 365 nm, respectively.
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very close to the pure white light (0.33, 0.33). In contrast to the control sample from the QDs emission under a single excite source, the peak wavelength positions remain almost unchanged, but the emission intensity from the heterostructure PC film with 0.05% self-assembly concentration is significantly stronger than that from control sample, by factor of 8. The enhancement of emission can be attributed to its periodic arrangement, especially, the tristopbands of heterostructure PCs overlaps the emission wavelengths of RGB QDs, respectively. Here, PCs are acted as a Bragg mirror, which can produce a strong resonant mode for the emission propagation. Photons can couple to the overlapping local resonance mode and Bragg scatter out of the structure, thereby greatly reducing the amount of light trapped as guided modes. Moreover, high density of states near the tri-stopband enhances the coupling of spontaneously emitted photons [30,31]. With increasing the self-assembly concentration, we observe a continual increase of RGB emission. It can be seen that the enhancement ration of blue spectral component is relatively higher compared with that of other colors. The evolution of spectral feature is consistent with reflectance intensity of heterostructure PCs, indicating that more photons are harvested. Additionally, the color coordinates shift from (0.30, 0.31) to (0.27, 0.27) upon increasing the blue spectral component. However, further increase of the selfassembly concentration will not be very effective in further enhancing the RGB emission intensity since more defects can effect optical properties in thicker heterostructure PCs. 4. Conclusions We demonstrate over 8-fold enhancement of white-light intensity from three-dimensional heterostructure PCs with 0.05% self-assembly concentration. This is obtained by engineering the structure of PCs to make tri-stopbands overlapping emission wavelengths of RGB QDs, respectively. Moreover, CIE diagrams show the chromaticity parameters approaching that of white light. The approach will be of great significance for building a multilayer system of PCs with the structural engineering and complex optical properties, which provides the reasonable guideline for developing optical devices of high performance. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51302107) and the Natural Science Foundation of Guangdong Province, China (Grant No. 2015A030313804).
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