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Fabrication and optical properties of Alq3 doped PMMA microsphere arrays templated by ZnO inverse opal structure
Optical Materials 32 (2010) 1210–1215
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabrication and optical properties of Alq3 doped PMMA microsphere arrays templated by ZnO inverse opal structure Ming Fu *, Lier Deng, Ailun Zhao, Yongsheng Wang, Dawei He Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China
a r t i c l e
i n f o
Article history: Received 23 November 2009 Received in revised form 28 February 2010 Accepted 31 March 2010
Keywords: PMMA Microsphere arrays Inverse opal Double replicating template Photonic crystals Colloidal crystals
a b s t r a c t PMMA microsphere arrays are fabricated by a double replicating method with common used polystyrene colloidal crystal template. High quality ZnO inverse opals formed by electrodeposition play the key role between the PMMA microsphere arrays and polystyrene colloidal crystals. The electrodeposition method has advantage on fabricating IO structures with high solid fraction. After the subsequently in-situ polymerization of MMA in the voids of ZnO inverse opals, the ZnO is removed by hydrochloric acid solution. Microsphere arrays fabricated by PMMA or PMMA doped with Alq3 are prepared. Reflection stop bands are detected from the formed PMMA microsphere arrays. Solid fraction from 37% to 50% of the PMMA arrays can be formed by different in-situ polymerization modes of MMA. The photoluminescence of Alq3 in the PMMA spheres is partly suppressed at the wavelength of the photonic stop band induced by PMMA arrays. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Three-dimensionally (3D) well ordered structure fabricated by microsphere arrays have attracted much attention due to their wide range of applications in photonic crystals, catalysts, separation technique, etc. One current approach for fabricating the ordered arrays is the self-assembly method by the monodispersed colloids or latex [1–4]. Monodispersed microsphere fabricated by kinds of materials including silica [5], polystyrene [6], ZnO [7], ZnS [8], Y2O3 [9], TiO2 [10] have been successfully made and applied to the further self-assembly process. But the variety of the materials which can be fabricated with monodispersed sphere shape is still limited. An alternatively versatile method for fabricating microsphere arrays with desired materials is the double replicating template method, also note as nanoscale lost-wax method [11]. A two-step templating route [12–14] is employed for the preparation of the microsphere arrays by using silica or polystyrene colloidal crystal template. In addition, the two-step templating route has advantage on the fabrication of photonic crystals with special geometry, such as heterostructure photonic crystal fabricated by sphere arrays with different solid fractions. An inverse opal (IO) structure is firstly fabricated by silica or polystyrene colloidal crystals as the final template for the formation of microsphere arrays of other desired materials. Several kinds of IO structures such as polymer [11,13] or carbon * Corresponding author. Tel.: +86 1051688605. E-mail address: [email protected] (M. Fu). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.03.031
[12] IO were investigated for the final formation of solid microsphere arrays or hollow sphere arrays. Though the structure of the arrays depends on the filling process, the quality of the IO template, including the number of defects, the orderliness, the solid volume fraction, is the key factor to the shape of the formed sphere arrays. Among the many methods for the formation of the IO structure, electrodeposition method has advantage on filling the voids of colloidal crystals with high ratio and decreasing the introduction of extra defects. It is helpful to maintain the orderliness during the two-step templating process. Therefore, in this paper, the high quality ZnO IO structure was employed for fabrication of the microsphere arrays. PMMA microsphere arrays (opalline structures) doped with Alq3 were prepared via the in-situ thermo polymerization process. The photonic band gap effects on the spontaneous emission of Alq3 were also investigated.
2. Experimental The colloidal crystal templates were assembled by vertical deposition method onto the indium tin oxide (ITO) substrates. The suspension of the monodispersed polystyrene microspheres was diluted to about 1% vol using deionized water. The ITO substrates are vertically immersed into suspensions and placed in an incubator at 50 °C and 30% humidity. Monodispersed polystyrene microspheres with different diameters were used to synthesize the ZnO IO templates.
M. Fu et al. / Optical Materials 32 (2010) 1210–1215
The ZnO IO structures are fabricated by electrodeposition methods. A three-electrode system (CHI660A Electrochemical Workstation) was used for the electrochemical depositions. The colloidal crystal-covered ITO substrates were used as the electrodeposition working electrode, while the saturated calomel electrode works as a reference electrode. The ZnO was firstly deposited in zinc nitrate solution (0.1 M) at 70 °C for 30 min. Then the deposits were calcinated at 450 °C for 2 h to remove the colloidal crystal template and create the macroporous ZnO IO template. The PMMA microspheres were then prepared by in-situ thermal polymerization of the methyl methacrylate (MMA) in the ZnO template. MMA was firstly purified for wiping off the inhibitor. The ZnO IO template was placed at the bottom of the freezing bottom. The bottom was sealed for avoiding the volatilization of MMA. The volume of the MMA was controlled to be just less than the thickness of the IO template or the IO template was fully immersed in the MMA solution. The polymerization reaction was carried out at 80 °C for 72 h. After the PMMA were formed in the interstice of the template, the ZnO were removed by immerse in HF solution for 72 h. A film constructed with PMMA ordered microsphere arrays were formed. The micro-region reflection spectra and photoluminescence spectra of the structure can be characterized by consist of a Fiber Optic Spectrometer (Acton InSpectrum Spectrograph with Embedded CCD, InSpectrum 150) which is helped by a microscope (Leica DMR). The fiber of spectrometer is placed in position of the beam path after the detecting light goes through the object lens. The light is introduced by the aid of a focusing lens with the same magnification of eye lens. An objective lens of 20 in magnification was used via measuring the spectra. Therefore, the spectra of a
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detecting area about 150 150 square micrometers is recorded by the spectrometer. The micro-region photoluminescence spectra are testing by a mercury lamp which is helped by a group of UV fluorescence filters. The incident light and collection light are both vertical to the surface of the substrate. 3. Results and discussion The typical SEM images of ZnO IO structure are shown in Fig. 1a. The structure is fabricated by the colloidal crystal template (265 nm) via electrodepositing at 70 °C for 30 min. The surface of the porous structure has ordered pore periodicity. The pore arrays in the surface have 2D hexagonal symmetry which is consistent with the face cubic centered (FCC) arrangement of colloidal microsphere arrays. Three cavities can be seen in each hole which is induced by the removed colloidal microspheres in the next {1 1 1} plane. Long-range order of pore arrays in surface of the macroporous ZnO structure can also be confirmed by Fast-Fourier transform (FFT) of the SEM image as shown in Fig. 1a inset. Fig. 1b shows a typical cross-section SEM image of ZnO IOs. The IO structures formed by electrodeposition method have high infilling ratios with high solid volume fraction. Therefore, the infilling structure has less shrinkage during the remove process of the polystyrene microspheres. The diameter of the pore arrays of the ZnO IO is almost equal to the diameter of the polystyrene microspheres. The electrodeposited template is more suitable for the fabrication of the microsphere arrays with high quality compared with other double-replica template, such as IO structure fabricated by sol–gel method. It is helpful for the transformation with the exactly shape and size of the fabricated microsphere arrays from the original
Fig. 1. (a) The typical SEM image of ZnO inverse opal structure fabricated by electrodeposition; (b) the typical cross-section SEM image of ZnO inverse opal; and (c) SEM image of the ordered inverse opal structures in large area, no extra defects are introduced.
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colloidal crystal microspheres. Large area of the ordered IO structures is formed by the electrodeposition method as shown in Fig. 1c. The defects in the ZnO IO are the solid ZnO walls between original colloidal crystal grains as shown in the right edge of Fig. 1a. The walls are induced by fully filling ZnO into the cracks (grain boundary) of the polystyrene colloidal crystals. (When the colloidal crystal is dried, little shrinkage of the microsphere lead the cracks of the colloidal crystals.) From the surface view of SEM images in Fig. 1a and c, no cracks are introduced during the first templating process. Therefore, the ZnO IO structure by electrodeposition is one of the best templates for the fabrication of microsphere arrays. The PMMA microsphere is formed by in-situ thermal polymerization of MMA in the ZnO template. The quality of the microsphere arrays is partly decided by thermal polymerization process. The shape of the PMMA spheres can be affected by the filling thickness of the MMA in the ZnO IO template. In some cases, the IO template is fully immersed in MMA. Polymerization not only happens in the interstice of the IO template, thick PMMA film also forms on the surface of the template. All the interstice of the template is filled by immersing in the extra MMA monomer. In other cases, limited MMA is filled into the interstice of the inverse tem-
plate. The thickness of the fabricated PMMA microsphere arrays is less than the one of the IO template. Therefore, the whole PMMA film is made of porous structure. When limited MMA is filled into the interstice of the template, the microsphere arrays can be characterized from the surface view of the scanning electron microscopy. When enough MMA is provided during the polymerization process, the surfaces of the fabricated PMMA microsphere arrays are covered by solid PMMA films. The spheres arrays can be characterized from the bottom face when the porous PMMA films are peeled off by HF solution. Fig. 2a shows the cross-section image of PMMA infilled IO structure. The erose cross-section of the structure is due to the plasticity of polymer. Though the ordered pores of inverse opals can be seen in several areas of the cross-section image, all the pores are filled with materials. Fig. 2b shows the SEM image of PMMA microsphere arrays from the surface view. The surface spheres have typical 2D hexagonal ordered structure. The orderliness of the surface of the PMMA arrays is evaluated by the FFT of the SEM image, which is shown in Fig. 2b inset. The orderliness of the arrays falls down compared with the FFT image of the IO structure in the same scale. A few cracks can be seen in the surface of the PMMA microsphere arrays, which may be induced by the uneven polymeriza-
Fig. 2. (a) Cross-section SEM image of the PMMA infilled ZnO inverse opal; (b and c) typical SEM images of formed PMMA microsphere arrays fabricated by immerging in limited MMA before polymerization; (d) high magnification SEM image which shows the linked nanorods between PMMA microspheres; (e and f) SEM images of formed PMMA microsphere arrays fabricated by immerging in enough MMA before polymerization; (e) the cross-section of the PMMA microsphere arrays is exposed; and (f) the surface view of the PMMA arrays.
M. Fu et al. / Optical Materials 32 (2010) 1210–1215
tion of the PMMA. Fig. 2c gives a typical SEM image of microsphere arrays. The PMMA microspheres are assembled in unattached arrays. Though the diameter of the fabricated PMMA sphere is smaller than the one of the template polystyrene microspheres, the periodicity of the arrays is equal to the diameter of the templated polystyrene microspheres. There are uniform intervals between each microsphere in the arrays. The intervals are induced by the shrinkage when PMMA polymerize from monomer. Even shrinkage of the microsphere happens for each microsphere. The unattached microsphere arrays are linked and supported by the small nanorods between each others as shown in Fig. 2d. Some of the linked nanorods in Fig. 2d were burned by the energy of the electron beam of the microscopy, which were not recorded in the image. In the view from the screen of the microscopy in the first several seconds, there were linked nanorods between all the neighboring microspheres. The average diameter of the spheres is about 165 nm. The periodicity of the arrays maintains about 260 nm which is equal to the diameter of the templated polystyrene microspheres. When the IO structure is immersed in enough monomers of MMA which is thicker than IO template, the diameter of the formed PMMA microspheres is always much closer to the one of the templated polystyrene microspheres. Extra MMA continues being filled into the sphere voids of the IOs. The induced voids from the shrinkage during the thermo polymerization process are also filled by the extra MMA. The air voids in the IO template are about 74% vol fraction or more. The large fraction of air void made the unpolymerizated MMA monomer can be supplemented to the shrinkage space of the PMMA in the sphere voids. Therefore, the volume of the formed microspheres is larger than the ones formed in limited MMA. Though the structure is partly distorted when peeled off from the ITO slide, the contacted microsphere arrays without intervals and linked nanorods are revealed as shown in Fig. 2e and f. The cross-section of the PMMA microsphere arrays is exposed the image of Fig. 2e, which proves the orderliness of the PMMA arrays of the bottom layers. An important evidence of the orderliness of periodically layers of PMMA microsphere arrays is the photonic stop band. The micro-region reflection spectra of the IO template and PMMA filled IO is shown in Fig. 3. The photonic band gap along the normal direction of the surface for the ZnO IO template fabricated by colloidal crystal with 240 and 265 nm microsphere in diameter is at about 452 and 530 nm, respectively. The spectral positions of the stop bands for an inverse opal photonic crystal can be calculated by Bragg’s law:
k ¼ 2dneff
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Fig. 3. Micro-region reflection spectra of inverse opal template which fabricated by monodispersed polystyrene colloidal crystals with 240, 255, 265 nm microspheres in diameter; and micro-region reflection spectra of PMMA filled inverse opals fabricated by colloidal crystals with 240, 265 nm microspheres in diameter.
When the ZnO template is filled with PMMA, the air void is replaced by PMMA. Red shift of the band gap is induced by the increase of the effective refractive index of the whole ordered structure. Reflection peaks at 518 nm, 617 m, and 625 nm are found for the PMMA filled ZnO IO fabricated by colloidal crystal templates with 240, 255, and 265 nm microspheres in diameter, respectively. Brightly color with strong reflection peaks proves the high orderliness of the formed PMMA microsphere arrays along its normal directions. The effective refractive index of PMMA filled IO structure can be estimated as following equation:
N2eff ¼ n2ZnO fZnO þ n2air ð1 fZnO fPMMA Þ þ n2PMMA fPMMA :
ð3Þ
The solid fraction of the PMMA spheres in the ZnO IO structure is about 0.49 as estimated from Eq. (3). When the ZnO IO is removed after the polymerization, the reflection stop bands of the PMMA microsphere arrays have blue shifts compared with PMMA filled IOs. The filling ratios of the PMMA in the IO template and the diameter of the PMMA microspheres is not same for method with different immerging mode of MMA, the positions of the photonic band gap of the PMMA filled
ð1Þ
where neff is the effective refractive index of the structure, d is the interplanar h1 1 1i spacing. In the FCC lattice, the relationship between d and diameters of the periodicity of structure (i.e. the periodicity of spheres or of the inverse replicas) is: d = (2/3)0.5D. The wavelength (k) is the photonic band of the ordered structures. It can be measured by reflection spectra from the normal direction. The average diameters (D) of holes can be measured by SEM. Therefore, neff can be estimated from Bragg equation (neff = (3/8)0.5k/D). Furthermore, neff can be independently estimated using the following relation:
N2eff ¼ n2ZnO fZnO þ n2air ð1 fZnO Þ
ð2Þ
where fZnO is the filling factor of the ZnO, nZnO and nair are the refractive index of ZnO and air. The periodicity of the ZnO IO structure has little shrinkage as estimated from the SEM image of ZnO IOs. nZnO is considered to be 1.9 when calculating the volume fraction. The volume fraction of ZnO in the IO structure is calculated to be 0.214, which is close to the idea void fraction of colloidal crystals (0.26).
Fig. 4. Micro-region reflection spectra of PMMA filled inverse opals and PMMA sphere arrays fabricated by colloidal crystals with 265 nm microspheres in diameter by different in-situ polymerization modes of MMA.
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IO structure have 22 nm difference as shown in Fig. 4. The solid fraction of the PMMA arrays formed in limited MMA is about 37% as calculated by Eq. (3) with its reflection band gap. When the IO structure is immersed in enough monomers of MMA, the solid fraction of PMMA arrays is estimated to be 0.498. The change of the band gap position for pure PMMA microsphere arrays without ZnO template is more sensitive to the filling ratio of PMMA. When the PMMA microsphere arrays are infilled in the ZnO IO template, the change of photonic band is affected by the average effects of the ordered structure of IOs and PMMA. The band gap of the PMMA microsphere arrays for method with different immerging mode of MMA has about 36 nm difference as shown in Fig. 4. Fig. 5 gives the photonic reflection band for the colloidal crystal template with 265 nm polystyrene microspheres in diameter, the ZnO IO template fabricated by the forementioned colloidal crystal template, PMMA filled IO, and pure PMMA microsphere arrays fabricated by IO template. The photonic band gap of the polystyrene microsphere arrays is centered at 600 nm. The wavelength of the
Fig. 5. Micro-region reflection spectra of the colloidal crystal template with 265 nm polystyrene microsphere arrays in diameter, the ZnO IO template fabricated by the forementioned colloidal crystal template, PMMA filled IO, and pure PMMA microsphere arrays.
band gap at 530 nm for ZnO IO is much shorter than its replica template due to the low effective refractive index of the whole structure. The thermo polymerization of PMMA in the IO structure made the wavelength of the reflection band has 70 nm red shift. When the ZnO is removed, the wavelength of the band gap of PMMA arrays turns blue. The band gap of the formed PMMA microsphere arrays has about 95 nm blue shift compared with the ones of the templated polystyrene arrays, which is mainly induced by low solid content of the PMMA microspheres. Further, the lower refractive index of PMMA (1.49) compared with the one of polystyrene (1.55) can also brings the blue shifts of the band gap. Microsphere arrays made from Alq3 doped PMMA can also be fabricated by templated thermo polymerization. Tiny Alq3 is dissolved into the MMA before polymerization. The microstructure of the arrays is similar to the pure PMMA arrays. The photoluminescence of the Alq3 in PMMA arrays is partly suppressed in the case the photonic band gap is overlapped with the luminescence band. The modified spontaneous emission properties of Alq3 in both PMMA filled IOs and PMMA arrays are investigated by micro-region photoluminescence spectra (see Fig. 6). The intensity of luminescent band at about 520 nm is much higher than the one at about 750 nm for the PMMA filled ZnO IO fabricated by polystyrene colloidal crystals with 265 nm microspheres in diameter. The photonic stop band of the sample at 630 nm has no overlap with the emission band. As the electrodeposited ZnO in the IO structure has poor crystallization without treating in high temperature, no photoluminescence of the ZnO IOs is excitated by the mercury lamb in the fluorescence microscope. Therefore, the photoluminescence of PMMA filled IOs have no relationship with ZnO. The photoluminescence spectra of the PMMA filled ZnO IO fabricated by polystyrene colloidal crystals with 265 nm microspheres in diameter is much similar to the solid PMMA films doped with Alq3. Both the PMMA filled zinc oxide inverse opal (by 265 nm colloidal crystal) and solid PMMA films plays the roles as the reference when discussing about the photonic band gap effects on the photoluminescence. When the ZnO template is removed, the photonic band gap of the structure moves to the wavelength at 505 nm. It is overlapped with the luminescence band of Alq3 at 520 nm, which suppresses the luminescence at this wavelength. The intensity of the detected luminescence band at 520 nm of the PMMA arrays fabricated by colloidal crystals with 265 nm polystyrene spheres is much lower than its band at 750 nm. There
Fig. 6. Micro-region reflection spectra and micro-region photoluminescence spectra of PMMA filled inverse opals and PMMA sphere arrays fabricated by colloidal crystals with 265 nm microspheres in diameter, and MMA filled inverse opals by colloidal crystals with 240 nm microspheres.
M. Fu et al. / Optical Materials 32 (2010) 1210–1215
also are photonic stop band at about 535 nm for some IO structure filled with PMMA fabricated by colloidal crystal template with 240 nm microspheres. The overlaps between the luminescence band and photonic stop band also partly exhibited the luminescence at 520 nm, which intensity is lower than the one at about 750 nm.
4. Conclusions In summary, a high quality ZnO IO template which formed by polystyrene colloidal crystals is used for the fabrication of PMMA microsphere arrays. The electrodeposition method has advantage on fabricating IO structures with high solid fraction among the many methods. Different solid fractions of the PMMA arrays can be formed, which has potential applications on the fabrication of photonic crystals with special geometry. The double replicating method with the high quality ZnO IO gives a versatile method for fabricating microsphere arrays with varieties of materials. Acknowledgements This work was supported by National Science Foundation of China under Grants of 60825407, 60877025, and 50902008, Beijing
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Science and Technology Committee under Grant Z08000303220803, and Beijing Natural Science Fund Project under Grant 2092024. References [1] S. Wong, V. Kitaev, G.A. Ozin, J. Am. Chem. Soc. 125 (2003) 15589. [2] P. Jiang, J.F. Bertone, K.S. Hwang, V.L. Colvin, Chem. Mater. 11 (1999) 2132. [3] M. Holgado, F.G. Santamaria, A. Blanco, M. Ibisate, A. Cintas, H. Miguez, C.J. Serna, C. Molpeceres, J. Requena, A. Mifsud, F. Meseguer, C. Lopez, Langmuir 15 (1999) 4701. [4] Y. Xia, B. Gates, Y. Yin, Y. Lu, Adv. Mater. 12 (2000) 693. [5] W. Stöber, A. Fink, J. Colloid Interface Sci. 26 (1968) 62. [6] I. Piirma (Ed.), Emulsion Polymerization, Academic, New York, 1982. [7] H. Jiang, J. Hu, F. Gu, C. Li, J. Phys. Chem. C 112 (2008) 12138. [8] H.Z. Zhong, Z.X. Wei, M.F. Ye, Y. Yan, Y. Zhou, Y.Q. Ding, C.H. Yang, Y.F. Li, Langmuir 23 (2007) 9008. [9] J. Li, X. Li, X.G. Sun, I. Takamasa, J. Phys. Chem. C 112 (2008) 11707. [10] H. Yu, G. Yi, J. Kang, Y. Cho, V.N. Manoharan, D.J. Pine, S. Yang, Chem. Mater. 20 (2008) 2704. [11] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 443. [12] Z. Lei, J. Li, Y. Ke, Y. Zhang, H. Zhang, F. Lic, J. Xing, J. Mater. Chem. 11 (2001) 2930. [13] Lianbin Xu, Le Duc Tung, Leonard Spinu, Anvar A. Zakhidov, Ray H. Baughman, John B. Wiley, Adv. Mater. 15 (2003) 1562. [14] L. Xu, A. Zakhidov, R. Baughman, J. Wile, in: Materials Research Society Symposium Proceedings, EXS-2 (Nontraditional Approaches to Patterning), 2004, pp. 93–95.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabrication and optical properties of Alq3 doped PMMA microsphere arrays templated by ZnO inverse opal structure Ming Fu *, Lier Deng, Ailun Zhao, Yongsheng Wang, Dawei He Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China
a r t i c l e
i n f o
Article history: Received 23 November 2009 Received in revised form 28 February 2010 Accepted 31 March 2010
Keywords: PMMA Microsphere arrays Inverse opal Double replicating template Photonic crystals Colloidal crystals
a b s t r a c t PMMA microsphere arrays are fabricated by a double replicating method with common used polystyrene colloidal crystal template. High quality ZnO inverse opals formed by electrodeposition play the key role between the PMMA microsphere arrays and polystyrene colloidal crystals. The electrodeposition method has advantage on fabricating IO structures with high solid fraction. After the subsequently in-situ polymerization of MMA in the voids of ZnO inverse opals, the ZnO is removed by hydrochloric acid solution. Microsphere arrays fabricated by PMMA or PMMA doped with Alq3 are prepared. Reflection stop bands are detected from the formed PMMA microsphere arrays. Solid fraction from 37% to 50% of the PMMA arrays can be formed by different in-situ polymerization modes of MMA. The photoluminescence of Alq3 in the PMMA spheres is partly suppressed at the wavelength of the photonic stop band induced by PMMA arrays. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Three-dimensionally (3D) well ordered structure fabricated by microsphere arrays have attracted much attention due to their wide range of applications in photonic crystals, catalysts, separation technique, etc. One current approach for fabricating the ordered arrays is the self-assembly method by the monodispersed colloids or latex [1–4]. Monodispersed microsphere fabricated by kinds of materials including silica [5], polystyrene [6], ZnO [7], ZnS [8], Y2O3 [9], TiO2 [10] have been successfully made and applied to the further self-assembly process. But the variety of the materials which can be fabricated with monodispersed sphere shape is still limited. An alternatively versatile method for fabricating microsphere arrays with desired materials is the double replicating template method, also note as nanoscale lost-wax method [11]. A two-step templating route [12–14] is employed for the preparation of the microsphere arrays by using silica or polystyrene colloidal crystal template. In addition, the two-step templating route has advantage on the fabrication of photonic crystals with special geometry, such as heterostructure photonic crystal fabricated by sphere arrays with different solid fractions. An inverse opal (IO) structure is firstly fabricated by silica or polystyrene colloidal crystals as the final template for the formation of microsphere arrays of other desired materials. Several kinds of IO structures such as polymer [11,13] or carbon * Corresponding author. Tel.: +86 1051688605. E-mail address: [email protected] (M. Fu). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.03.031
[12] IO were investigated for the final formation of solid microsphere arrays or hollow sphere arrays. Though the structure of the arrays depends on the filling process, the quality of the IO template, including the number of defects, the orderliness, the solid volume fraction, is the key factor to the shape of the formed sphere arrays. Among the many methods for the formation of the IO structure, electrodeposition method has advantage on filling the voids of colloidal crystals with high ratio and decreasing the introduction of extra defects. It is helpful to maintain the orderliness during the two-step templating process. Therefore, in this paper, the high quality ZnO IO structure was employed for fabrication of the microsphere arrays. PMMA microsphere arrays (opalline structures) doped with Alq3 were prepared via the in-situ thermo polymerization process. The photonic band gap effects on the spontaneous emission of Alq3 were also investigated.
2. Experimental The colloidal crystal templates were assembled by vertical deposition method onto the indium tin oxide (ITO) substrates. The suspension of the monodispersed polystyrene microspheres was diluted to about 1% vol using deionized water. The ITO substrates are vertically immersed into suspensions and placed in an incubator at 50 °C and 30% humidity. Monodispersed polystyrene microspheres with different diameters were used to synthesize the ZnO IO templates.
M. Fu et al. / Optical Materials 32 (2010) 1210–1215
The ZnO IO structures are fabricated by electrodeposition methods. A three-electrode system (CHI660A Electrochemical Workstation) was used for the electrochemical depositions. The colloidal crystal-covered ITO substrates were used as the electrodeposition working electrode, while the saturated calomel electrode works as a reference electrode. The ZnO was firstly deposited in zinc nitrate solution (0.1 M) at 70 °C for 30 min. Then the deposits were calcinated at 450 °C for 2 h to remove the colloidal crystal template and create the macroporous ZnO IO template. The PMMA microspheres were then prepared by in-situ thermal polymerization of the methyl methacrylate (MMA) in the ZnO template. MMA was firstly purified for wiping off the inhibitor. The ZnO IO template was placed at the bottom of the freezing bottom. The bottom was sealed for avoiding the volatilization of MMA. The volume of the MMA was controlled to be just less than the thickness of the IO template or the IO template was fully immersed in the MMA solution. The polymerization reaction was carried out at 80 °C for 72 h. After the PMMA were formed in the interstice of the template, the ZnO were removed by immerse in HF solution for 72 h. A film constructed with PMMA ordered microsphere arrays were formed. The micro-region reflection spectra and photoluminescence spectra of the structure can be characterized by consist of a Fiber Optic Spectrometer (Acton InSpectrum Spectrograph with Embedded CCD, InSpectrum 150) which is helped by a microscope (Leica DMR). The fiber of spectrometer is placed in position of the beam path after the detecting light goes through the object lens. The light is introduced by the aid of a focusing lens with the same magnification of eye lens. An objective lens of 20 in magnification was used via measuring the spectra. Therefore, the spectra of a
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detecting area about 150 150 square micrometers is recorded by the spectrometer. The micro-region photoluminescence spectra are testing by a mercury lamp which is helped by a group of UV fluorescence filters. The incident light and collection light are both vertical to the surface of the substrate. 3. Results and discussion The typical SEM images of ZnO IO structure are shown in Fig. 1a. The structure is fabricated by the colloidal crystal template (265 nm) via electrodepositing at 70 °C for 30 min. The surface of the porous structure has ordered pore periodicity. The pore arrays in the surface have 2D hexagonal symmetry which is consistent with the face cubic centered (FCC) arrangement of colloidal microsphere arrays. Three cavities can be seen in each hole which is induced by the removed colloidal microspheres in the next {1 1 1} plane. Long-range order of pore arrays in surface of the macroporous ZnO structure can also be confirmed by Fast-Fourier transform (FFT) of the SEM image as shown in Fig. 1a inset. Fig. 1b shows a typical cross-section SEM image of ZnO IOs. The IO structures formed by electrodeposition method have high infilling ratios with high solid volume fraction. Therefore, the infilling structure has less shrinkage during the remove process of the polystyrene microspheres. The diameter of the pore arrays of the ZnO IO is almost equal to the diameter of the polystyrene microspheres. The electrodeposited template is more suitable for the fabrication of the microsphere arrays with high quality compared with other double-replica template, such as IO structure fabricated by sol–gel method. It is helpful for the transformation with the exactly shape and size of the fabricated microsphere arrays from the original
Fig. 1. (a) The typical SEM image of ZnO inverse opal structure fabricated by electrodeposition; (b) the typical cross-section SEM image of ZnO inverse opal; and (c) SEM image of the ordered inverse opal structures in large area, no extra defects are introduced.
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colloidal crystal microspheres. Large area of the ordered IO structures is formed by the electrodeposition method as shown in Fig. 1c. The defects in the ZnO IO are the solid ZnO walls between original colloidal crystal grains as shown in the right edge of Fig. 1a. The walls are induced by fully filling ZnO into the cracks (grain boundary) of the polystyrene colloidal crystals. (When the colloidal crystal is dried, little shrinkage of the microsphere lead the cracks of the colloidal crystals.) From the surface view of SEM images in Fig. 1a and c, no cracks are introduced during the first templating process. Therefore, the ZnO IO structure by electrodeposition is one of the best templates for the fabrication of microsphere arrays. The PMMA microsphere is formed by in-situ thermal polymerization of MMA in the ZnO template. The quality of the microsphere arrays is partly decided by thermal polymerization process. The shape of the PMMA spheres can be affected by the filling thickness of the MMA in the ZnO IO template. In some cases, the IO template is fully immersed in MMA. Polymerization not only happens in the interstice of the IO template, thick PMMA film also forms on the surface of the template. All the interstice of the template is filled by immersing in the extra MMA monomer. In other cases, limited MMA is filled into the interstice of the inverse tem-
plate. The thickness of the fabricated PMMA microsphere arrays is less than the one of the IO template. Therefore, the whole PMMA film is made of porous structure. When limited MMA is filled into the interstice of the template, the microsphere arrays can be characterized from the surface view of the scanning electron microscopy. When enough MMA is provided during the polymerization process, the surfaces of the fabricated PMMA microsphere arrays are covered by solid PMMA films. The spheres arrays can be characterized from the bottom face when the porous PMMA films are peeled off by HF solution. Fig. 2a shows the cross-section image of PMMA infilled IO structure. The erose cross-section of the structure is due to the plasticity of polymer. Though the ordered pores of inverse opals can be seen in several areas of the cross-section image, all the pores are filled with materials. Fig. 2b shows the SEM image of PMMA microsphere arrays from the surface view. The surface spheres have typical 2D hexagonal ordered structure. The orderliness of the surface of the PMMA arrays is evaluated by the FFT of the SEM image, which is shown in Fig. 2b inset. The orderliness of the arrays falls down compared with the FFT image of the IO structure in the same scale. A few cracks can be seen in the surface of the PMMA microsphere arrays, which may be induced by the uneven polymeriza-
Fig. 2. (a) Cross-section SEM image of the PMMA infilled ZnO inverse opal; (b and c) typical SEM images of formed PMMA microsphere arrays fabricated by immerging in limited MMA before polymerization; (d) high magnification SEM image which shows the linked nanorods between PMMA microspheres; (e and f) SEM images of formed PMMA microsphere arrays fabricated by immerging in enough MMA before polymerization; (e) the cross-section of the PMMA microsphere arrays is exposed; and (f) the surface view of the PMMA arrays.
M. Fu et al. / Optical Materials 32 (2010) 1210–1215
tion of the PMMA. Fig. 2c gives a typical SEM image of microsphere arrays. The PMMA microspheres are assembled in unattached arrays. Though the diameter of the fabricated PMMA sphere is smaller than the one of the template polystyrene microspheres, the periodicity of the arrays is equal to the diameter of the templated polystyrene microspheres. There are uniform intervals between each microsphere in the arrays. The intervals are induced by the shrinkage when PMMA polymerize from monomer. Even shrinkage of the microsphere happens for each microsphere. The unattached microsphere arrays are linked and supported by the small nanorods between each others as shown in Fig. 2d. Some of the linked nanorods in Fig. 2d were burned by the energy of the electron beam of the microscopy, which were not recorded in the image. In the view from the screen of the microscopy in the first several seconds, there were linked nanorods between all the neighboring microspheres. The average diameter of the spheres is about 165 nm. The periodicity of the arrays maintains about 260 nm which is equal to the diameter of the templated polystyrene microspheres. When the IO structure is immersed in enough monomers of MMA which is thicker than IO template, the diameter of the formed PMMA microspheres is always much closer to the one of the templated polystyrene microspheres. Extra MMA continues being filled into the sphere voids of the IOs. The induced voids from the shrinkage during the thermo polymerization process are also filled by the extra MMA. The air voids in the IO template are about 74% vol fraction or more. The large fraction of air void made the unpolymerizated MMA monomer can be supplemented to the shrinkage space of the PMMA in the sphere voids. Therefore, the volume of the formed microspheres is larger than the ones formed in limited MMA. Though the structure is partly distorted when peeled off from the ITO slide, the contacted microsphere arrays without intervals and linked nanorods are revealed as shown in Fig. 2e and f. The cross-section of the PMMA microsphere arrays is exposed the image of Fig. 2e, which proves the orderliness of the PMMA arrays of the bottom layers. An important evidence of the orderliness of periodically layers of PMMA microsphere arrays is the photonic stop band. The micro-region reflection spectra of the IO template and PMMA filled IO is shown in Fig. 3. The photonic band gap along the normal direction of the surface for the ZnO IO template fabricated by colloidal crystal with 240 and 265 nm microsphere in diameter is at about 452 and 530 nm, respectively. The spectral positions of the stop bands for an inverse opal photonic crystal can be calculated by Bragg’s law:
k ¼ 2dneff
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Fig. 3. Micro-region reflection spectra of inverse opal template which fabricated by monodispersed polystyrene colloidal crystals with 240, 255, 265 nm microspheres in diameter; and micro-region reflection spectra of PMMA filled inverse opals fabricated by colloidal crystals with 240, 265 nm microspheres in diameter.
When the ZnO template is filled with PMMA, the air void is replaced by PMMA. Red shift of the band gap is induced by the increase of the effective refractive index of the whole ordered structure. Reflection peaks at 518 nm, 617 m, and 625 nm are found for the PMMA filled ZnO IO fabricated by colloidal crystal templates with 240, 255, and 265 nm microspheres in diameter, respectively. Brightly color with strong reflection peaks proves the high orderliness of the formed PMMA microsphere arrays along its normal directions. The effective refractive index of PMMA filled IO structure can be estimated as following equation:
N2eff ¼ n2ZnO fZnO þ n2air ð1 fZnO fPMMA Þ þ n2PMMA fPMMA :
ð3Þ
The solid fraction of the PMMA spheres in the ZnO IO structure is about 0.49 as estimated from Eq. (3). When the ZnO IO is removed after the polymerization, the reflection stop bands of the PMMA microsphere arrays have blue shifts compared with PMMA filled IOs. The filling ratios of the PMMA in the IO template and the diameter of the PMMA microspheres is not same for method with different immerging mode of MMA, the positions of the photonic band gap of the PMMA filled
ð1Þ
where neff is the effective refractive index of the structure, d is the interplanar h1 1 1i spacing. In the FCC lattice, the relationship between d and diameters of the periodicity of structure (i.e. the periodicity of spheres or of the inverse replicas) is: d = (2/3)0.5D. The wavelength (k) is the photonic band of the ordered structures. It can be measured by reflection spectra from the normal direction. The average diameters (D) of holes can be measured by SEM. Therefore, neff can be estimated from Bragg equation (neff = (3/8)0.5k/D). Furthermore, neff can be independently estimated using the following relation:
N2eff ¼ n2ZnO fZnO þ n2air ð1 fZnO Þ
ð2Þ
where fZnO is the filling factor of the ZnO, nZnO and nair are the refractive index of ZnO and air. The periodicity of the ZnO IO structure has little shrinkage as estimated from the SEM image of ZnO IOs. nZnO is considered to be 1.9 when calculating the volume fraction. The volume fraction of ZnO in the IO structure is calculated to be 0.214, which is close to the idea void fraction of colloidal crystals (0.26).
Fig. 4. Micro-region reflection spectra of PMMA filled inverse opals and PMMA sphere arrays fabricated by colloidal crystals with 265 nm microspheres in diameter by different in-situ polymerization modes of MMA.
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IO structure have 22 nm difference as shown in Fig. 4. The solid fraction of the PMMA arrays formed in limited MMA is about 37% as calculated by Eq. (3) with its reflection band gap. When the IO structure is immersed in enough monomers of MMA, the solid fraction of PMMA arrays is estimated to be 0.498. The change of the band gap position for pure PMMA microsphere arrays without ZnO template is more sensitive to the filling ratio of PMMA. When the PMMA microsphere arrays are infilled in the ZnO IO template, the change of photonic band is affected by the average effects of the ordered structure of IOs and PMMA. The band gap of the PMMA microsphere arrays for method with different immerging mode of MMA has about 36 nm difference as shown in Fig. 4. Fig. 5 gives the photonic reflection band for the colloidal crystal template with 265 nm polystyrene microspheres in diameter, the ZnO IO template fabricated by the forementioned colloidal crystal template, PMMA filled IO, and pure PMMA microsphere arrays fabricated by IO template. The photonic band gap of the polystyrene microsphere arrays is centered at 600 nm. The wavelength of the
Fig. 5. Micro-region reflection spectra of the colloidal crystal template with 265 nm polystyrene microsphere arrays in diameter, the ZnO IO template fabricated by the forementioned colloidal crystal template, PMMA filled IO, and pure PMMA microsphere arrays.
band gap at 530 nm for ZnO IO is much shorter than its replica template due to the low effective refractive index of the whole structure. The thermo polymerization of PMMA in the IO structure made the wavelength of the reflection band has 70 nm red shift. When the ZnO is removed, the wavelength of the band gap of PMMA arrays turns blue. The band gap of the formed PMMA microsphere arrays has about 95 nm blue shift compared with the ones of the templated polystyrene arrays, which is mainly induced by low solid content of the PMMA microspheres. Further, the lower refractive index of PMMA (1.49) compared with the one of polystyrene (1.55) can also brings the blue shifts of the band gap. Microsphere arrays made from Alq3 doped PMMA can also be fabricated by templated thermo polymerization. Tiny Alq3 is dissolved into the MMA before polymerization. The microstructure of the arrays is similar to the pure PMMA arrays. The photoluminescence of the Alq3 in PMMA arrays is partly suppressed in the case the photonic band gap is overlapped with the luminescence band. The modified spontaneous emission properties of Alq3 in both PMMA filled IOs and PMMA arrays are investigated by micro-region photoluminescence spectra (see Fig. 6). The intensity of luminescent band at about 520 nm is much higher than the one at about 750 nm for the PMMA filled ZnO IO fabricated by polystyrene colloidal crystals with 265 nm microspheres in diameter. The photonic stop band of the sample at 630 nm has no overlap with the emission band. As the electrodeposited ZnO in the IO structure has poor crystallization without treating in high temperature, no photoluminescence of the ZnO IOs is excitated by the mercury lamb in the fluorescence microscope. Therefore, the photoluminescence of PMMA filled IOs have no relationship with ZnO. The photoluminescence spectra of the PMMA filled ZnO IO fabricated by polystyrene colloidal crystals with 265 nm microspheres in diameter is much similar to the solid PMMA films doped with Alq3. Both the PMMA filled zinc oxide inverse opal (by 265 nm colloidal crystal) and solid PMMA films plays the roles as the reference when discussing about the photonic band gap effects on the photoluminescence. When the ZnO template is removed, the photonic band gap of the structure moves to the wavelength at 505 nm. It is overlapped with the luminescence band of Alq3 at 520 nm, which suppresses the luminescence at this wavelength. The intensity of the detected luminescence band at 520 nm of the PMMA arrays fabricated by colloidal crystals with 265 nm polystyrene spheres is much lower than its band at 750 nm. There
Fig. 6. Micro-region reflection spectra and micro-region photoluminescence spectra of PMMA filled inverse opals and PMMA sphere arrays fabricated by colloidal crystals with 265 nm microspheres in diameter, and MMA filled inverse opals by colloidal crystals with 240 nm microspheres.
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also are photonic stop band at about 535 nm for some IO structure filled with PMMA fabricated by colloidal crystal template with 240 nm microspheres. The overlaps between the luminescence band and photonic stop band also partly exhibited the luminescence at 520 nm, which intensity is lower than the one at about 750 nm.
4. Conclusions In summary, a high quality ZnO IO template which formed by polystyrene colloidal crystals is used for the fabrication of PMMA microsphere arrays. The electrodeposition method has advantage on fabricating IO structures with high solid fraction among the many methods. Different solid fractions of the PMMA arrays can be formed, which has potential applications on the fabrication of photonic crystals with special geometry. The double replicating method with the high quality ZnO IO gives a versatile method for fabricating microsphere arrays with varieties of materials. Acknowledgements This work was supported by National Science Foundation of China under Grants of 60825407, 60877025, and 50902008, Beijing
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Science and Technology Committee under Grant Z08000303220803, and Beijing Natural Science Fund Project under Grant 2092024. References [1] S. Wong, V. Kitaev, G.A. Ozin, J. Am. Chem. Soc. 125 (2003) 15589. [2] P. Jiang, J.F. Bertone, K.S. Hwang, V.L. Colvin, Chem. Mater. 11 (1999) 2132. [3] M. Holgado, F.G. Santamaria, A. Blanco, M. Ibisate, A. Cintas, H. Miguez, C.J. Serna, C. Molpeceres, J. Requena, A. Mifsud, F. Meseguer, C. Lopez, Langmuir 15 (1999) 4701. [4] Y. Xia, B. Gates, Y. Yin, Y. Lu, Adv. Mater. 12 (2000) 693. [5] W. Stöber, A. Fink, J. Colloid Interface Sci. 26 (1968) 62. [6] I. Piirma (Ed.), Emulsion Polymerization, Academic, New York, 1982. [7] H. Jiang, J. Hu, F. Gu, C. Li, J. Phys. Chem. C 112 (2008) 12138. [8] H.Z. Zhong, Z.X. Wei, M.F. Ye, Y. Yan, Y. Zhou, Y.Q. Ding, C.H. Yang, Y.F. Li, Langmuir 23 (2007) 9008. [9] J. Li, X. Li, X.G. Sun, I. Takamasa, J. Phys. Chem. C 112 (2008) 11707. [10] H. Yu, G. Yi, J. Kang, Y. Cho, V.N. Manoharan, D.J. Pine, S. Yang, Chem. Mater. 20 (2008) 2704. [11] P. Jiang, J.F. Bertone, V.L. Colvin, Science 291 (2001) 443. [12] Z. Lei, J. Li, Y. Ke, Y. Zhang, H. Zhang, F. Lic, J. Xing, J. Mater. Chem. 11 (2001) 2930. [13] Lianbin Xu, Le Duc Tung, Leonard Spinu, Anvar A. Zakhidov, Ray H. Baughman, John B. Wiley, Adv. Mater. 15 (2003) 1562. [14] L. Xu, A. Zakhidov, R. Baughman, J. Wile, in: Materials Research Society Symposium Proceedings, EXS-2 (Nontraditional Approaches to Patterning), 2004, pp. 93–95.