Langmuir 2007, 23, 3503-3505
3503
Tuning the Effective Width of the Optical Stop Band in Colloidal Photonic Crystals Toshimitsu Kanai,†,§ Tsutomu Sawada,*,† Akiko Toyotama,† Junpei Yamanaka,‡ and Kenji Kitamura† National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and Faculty of Pharmaceutical Sciences, Nagoya City UniVersity, 3-1 Tanabe, Mizuho, Nagoya, Aichi 467-8603, Japan ReceiVed July 20, 2006. In Final Form: December 20, 2006 The optical stop band in colloidal crystals is characterized by the central frequency and bandwidth. Although the former is known to be highly tunable by changing the lattice constant, the latter is basically determined by the refractive index contrast between the particles and the background medium that is intrinsic to the materials. In this study, we show that the effective bandwidth in gelled colloidal crystals can also be tuned by controlling the fabrication conditions. Single-domain gelled colloidal crystals were prepared by photopolymerization under various photoirradiation conditions. It was observed that the width of the stop band in the transmission or reflectance spectrum could be expanded by simply adjusting the irradiation time.
Introduction Charged colloidal crystals are 3D periodic arrays of submicrometer particles in a liquid medium; the arrays are spontaneously ordered by the electrostatic interaction between the particles.1 The resulting spatial periodicities of the dielectric constant range from ca. 102 to 103 nm, and they have a photonic band gap in the visible regime.2 Therefore, these materials have potential applications as optical rejection filters, switches, limiters, and sensors.3 Although the colloidal crystalline arrays in a liquid medium do not exhibit a high degree of stability against external disturbances such as mechanical vibrations, convection in the colloid, and chemical contamination, approaches to immobilize the ordered arrays in a hydrogel by the polymerization of a monomer in a dispersion medium have been examined.4 The optical stop band frequency of the gelled colloidal crystal is highly tunable; this frequency can be tuned by changing the lattice constant. This is achieved by appropriately selecting the particle volume concentration or diameter during fabrication and by applying mechanical stress5 or replacing the liquid medium6 * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: +81-29-851-3354. Fax: +81-29-8527449. † National Institute for Materials Science. ‡ Nagoya City University. § Current address: RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan. (1) (a) Pieranski, P. Contemp. Phys. 1983, 24, 25. (b) Ito, K.; Sumaru, K.; Ise, N. Phys. ReV. B 1992, 46, 3105. (c) Ise, N.; Smalley, M. V. Phys. ReV. B 1994, 50, 16722. (d) Ordering and Phase Transitions in Charged Colloids; Arora, A. K., Tata, B. V. R., Eds.; VCH: New York, 1996. (e) Gast, A. P.; Russel, W. B. Phys. Today 1998, 51, 24. (2) (a) Yablonovitch, E. Phys. ReV. Lett. 1987, 58, 2059. (b) John, S. Phys. ReV. Lett. 1987, 58, 2486. (c) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals; Princeton University Press: Princeton, NJ, 1995. (d) Sakoda, K. Optical Properties of Photonic Crystals; Springer-Verlag: Berlin, 2001. (e) Weitz, D. A.; Russel, W. B., Eds. MRS Bull. 2004, 29, 82. (f) Polman, A.; Wiltzius, P., Eds. MRS Bull. 2001, 26, 608. (g) Grier, D. G., Ed. MRS Bull. 1998, 23, 21. (3) (a) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959. (b) Kamenetzky, E. A.; Magliocco, L. G.; Panzer, H. P. Science 1994, 263, 207. (c) Velev, O. D.; Kaler, E. W. Langmuir 1999, 15, 3693. (4) (a) Sunkara, H. B.; Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1994, 6, 362. (b) Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1996, 8, 2138. (c) Foulger, S. H.; Jiang, P.; Ying, Y.; Lattam, A. C.; Smith, D. W.; Ballato, J. AdV. Mater. 2001, 13, 1898. (5) (a) Asher, S. A.; Holtz, J.; Liu, L.; Wu, Z. J. Am. Chem. Soc. 1994, 116, 4997. (b) Foulger, S. H.; Jiang, P.; Lattam, A.; Smith, D. W.; Ballato, J.; Dausch, D. E.; Grego, S.; Stoner, B. R. AdV. Mater. 2003, 15, 685. (c) Iwayama, Y.; Yamanaka, J.; Takiguchi, Y.; Takasaka, M.; Ito, K.; Shinohara, T.; Sawada, T.; Yonese, M. Langmuir 2003, 19, 977.
during use. However, the width of the optical stop band for a particular volume concentration is intrinsically determined by the dielectric contrast between the particles and the medium. Therefore, controlling the bandwidth, which is significant for device applications, is difficult. In an opal system, the bandwidth has been effectively broadened by introducing graded structures where the superposition of shifted stop bands effectively yields a wider stop band.7 The fabrication processes for such graded structures require multiple steps in addition to the steps for fabricating ordinary opals. In this letter, we show that for gelled colloidal crystals prepared by photopolymerization the effective width of the optical stop band can be broadened by simply adjusting the photoirradiation time. This process can be readily incorporated into the established fabrication method8,9 for the single-domain gelled colloidal crystals, where the effective stop band is spontaneously broadened while preserving excellent uniformity over the entire area of the gelled crystals. This broadening could be frozen at a desired value by additional irradiation. The colloidal system in this study is an aqueous dispersion of negatively charged polystyrene particles. Polystyrene latex (Duke Scientific Corp., Palo Alto, CA; particle diameter, 198 nm; standard deviation, 3%) was deionized by using a mixed-bed ion-exchange resin (Bio-Rad, AG501-X8, Hercules, CA) in vials until the suspension exhibited iridescence; this is indicative of crystallization. This suspension was mixed with aqueous solutions of N-methylol-acrylamide (1.2 M) as a monomer, N,N′-methylenebis-acrylamide (10 mM) as a crosslinker, and camphorquinone (0.4 mM) as a photoinduced polymerization initiator; camphorquinone has absorption in the wavelength region from ∼400 to 500 nm (with an absorption maximum at 468 nm).10 The (6) (a) Holtz, J.; Asher, S. A. Nature 1997, 389, 829. (b) Saito, H.; Takeoka, Y.; Watanabe, M. Chem. Commun. 2003, 17, 2126. (c) Fudouzi, H.; Xia, Y. AdV. Mater. 2003, 15, 892. (7) (a) Jiang, P.; Ostojic, G. N.; Narat, R.; Mittleman, D. M.; Colvin, V. L. AdV. Mater. 2001, 13, 389. (b) Park, J. H.; Choi, W. S.; Koo, H. Y.; Hong, J. C.; Kim, D. Y. Langmuir 2006, 22, 94. (8) (a) Sawada, T.; Suzuki, Y.; Toyotama, A.; Iyi, N. Jpn. J. Appl. Phys. 2001, 40, L1226. (b) Kanai, T.; Sawada, T.; Maki, I.; Kitamura, K. Jpn. J. Appl. Phys. 2003, 42, L655. (c) Kanai, T.; Sawada, T.; Kitamura, K. Langmuir 2003, 19, 1984. (d) Toyotama, A.; Kanai, T.; Sawada, T.; Yamanaka, J.; Ito, K.; Kitamura, K. Langmuir 2005, 21, 10268. (9) Kanai, T.; Sawada, T.; Toyotama, A.; Kitamura, K. AdV. Funct. Mater. 2005, 15, 25. (10) Emami, N.; So¨derholm, K.-J. M. J. Mater. Sci. Mater. Med. 2005, 16, 47.
10.1021/la0621160 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/21/2007
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Figure 1. Temporal changes in transmission spectra of colloidal crystals after (A) 3 and (B) 1 h of light irradiation. The dashed line indicates the spectrum before irradiation. The dark level of the transmittance increases by a few percentage points as a result of the characteristics of a CCD spectrometer. (C) Time courses of the (111) dip width for A (2) and B (b). The open circle (O) indicates the change in dip width for a multistep case; 1 h of initial irradiation followed by 7 h of retention in the dark, after which additional irradiation for 3 h was carried out. (D) Saturation value for the expanded (111) dip width as a function of light irradiation time.
reaction solution (particle volume concentration, 10%) was bubbled with Ar gas for 10 min to remove any dissolved O2 and CO2. It was then injected into a flat capillary cell (thickness, 0.1 mm; width, 9 mm; and length, 70 mm) made of fused quartz, after which it was processed with a momentary strong shear flow to obtain a centimeter-sized single domain in the capillary cell.8,9 The single-domain crystal thus obtained was polymerized by uniform photoirradiation with two sets of high-brightness blue LED arrays (peak wavelength of emission, 470 nm; 36 LEDs aligned in an area of 50 × 15 mm2; MBARB-5015, Moritex Corp., Tokyo, Japan) 20 mm away from both sides of the cell surface through light diffusers. The light intensity at the sample position was measured to be 21 mW/cm2. This enabled the fabrication of a quartz-packaged gel film. The transmission spectral measurements for the packaged colloidal crystals were performed using an imaging spectrophotometer (ImSpector, JFE Techno-Research Corp., Chiba, Japan) with incident light normal to the cell surface and air as the reference.11 The reflection spectra for the samples were measured using a multichannel spectrometer (Fastevert S-2600, Soma Optics, Tokyo, Japan). Although all of the single-domain colloidal crystals exhibited excellent spectral quality before gelation (for example, see dashed lines in Figure 1A,B), the spectra for gelled crystals significantly depended on the photoirradiation conditions. When the light intensity of the irradiation was considerably low (but strong enough to form a self-standing film), the spectral profile observed immediately after finishing the irradiation differed markedly from the original spectral profile; this difference indicates a large shift in the dip wavelength, a reduction in the transmittance at the passband wavelength, and the appearance of additional satellite dips (not shown). By increasing the light intensity, the shape of the spectrum was preserved against irradiation. However, even in this case, the dip expansion occurred during subsequent equilibration in the dark. The rate of this dip expansion and the settled value of the dip width depend on the irradiation time. Figure 1A,B shows temporal changes in the transmission spectra of the colloidal crystals during equilibration after light (11) Kanai, T.; Sawada, T.; Kitamura, K. Chem. Lett. 2005, 34, 904.
Letters
irradiation for the two representative cases. The deep dip at 840 nm is due to the stop band caused by Bragg diffraction from the (111) lattice planes of the face-centered-cubic structure parallel to the cell surface. In the case in which photopolymerization is induced by light irradiation for 3 h (Figure 1A), the initial spectrum appeared to be frozen at least within the experimental time range. However, in the case of the 1 h irradiation (Figure 1B), the (111) dip gradually expanded during equilibration in the dark after the irradiation was complete. It is noteworthy that in the latter case the spectrum profile except for the dip region is preserved (i.e., the high transmittance at the passband and the absence of a band edge shift at approximately 450 nm is observed; this gives the impression that the photonic band gap at 840 nm is expanded). Figure 1C shows the time courses of the (111) dip width in the equilibration process after light irradiation is completed for both of the cases shown in Figure 1A, B. Here, dip width is defined as full width at 40% transmittance. The dip width of the sample irradiated for 3 h remains constant whereas that of the sample irradiated for 1 h increases with equilibration time, after which it levels off. In every measurement, because the dip width appeared unchanged after 50 h of equilibration, we hereafter regard the value at 50 h as the saturation value for the dip width. Guaranteeing stability on a much longer time scale becomes a difficult task because of the effect of the gradual evaporation of water from the cell, although it was regarded as being negligible over a relatively shorter time scale such as the 50 h interval. Figure 1D shows the saturation values of the expanded dip width as a function of the light irradiation time after equilibration. For irradiation time exceeding 3 h, the initial dip width was frozen, whereas for an irradiation time of less than 3 h it was observed that a decrease in the irradiation time resulted in larger saturation values. On the basis of these observations, it can be concluded that the final dip width in gelled colloidal crystals can be controlled by the irradiation time. In our experiment, the smallest and largest values of the final dip width are 46 and 156 nm, respectively, showing a tunability that changes by a factor of approximately 3. Another interesting feature is that dip expansion can be suspended and frozen by additional light irradiation. The dashed line in Figure 1C represents an example where the sample was first irradiated for 1 h, retained in the dark for 7 h, and subsequently irradiated again for 3 h. The expansion of the width was frozen at 72 nm by the second irradiation; however, it would have increased to 144 nm without the additional irradiation. As observed from the comparison during the initial 7 h between the behavior of the open and closed circles in Figure 1C, the two different sets of experiments with the same irradiation condition show almost identical behavior; therefore, the phenomenon of dip expansion has good reproducibility. The reproducibility was also confirmed by repeating the same sets of experiments several times. Dip expansion is not a local phenomenon; it is observed over the entire area of a specimen. Figure 2 shows 2D images of the transmittance distribution of the same sample across a wide area at several representative wavelengths before and after the dip expansion. The absence of uniformity in the optical properties is detected as the contrast in brightness.9 These images indicate that the resultant samples exhibit excellent uniformity across a wide area; therefore, dip expansion is a global phenomenon. The dip expansion observed in this study can be explained if the gel possesses some inhomogeneity as follows. The lattice structure of the colloidal crystals captured in an inhomogeneous gel network would become inhomogeneous as a result of subsequent swelling equilibration. This implies that the interplanar spacing of the (111) lattice planes, which determines the dip
Letters
Figure 2. Images of the transmittance distribution with brightness at several representative wavelengths for the same sample (A) before and (B) after irradiation in the case of 1 h of initial irradiation followed by 7 h of retention, after which additional irradiation for 3 h was carried out. The spatial resolution is 100 µm.
wavelength, is not constant but fluctuates around a value for the homogeneous case. If the length scale of such an inhomogeneity is smaller than the spatial resolution of the spectral measurement (100 µm) in the lateral direction, then the spectrum of the specimen would be observed as the superposition of spectra with different dip wavelengths, resulting in an apparent dip expansion. We conceive two possible mechanisms for generating such an inhomogeneous gel. One is inhomogeneity along the thickness direction due to the attenuation of irradiated light. Because the transmittance of the sample at the absorption wavelength (∼468 nm) of the photoinitiator (camphorquinone) is not very high (∼20%), the polymerization reaction would be incomplete in the deeper interiors of the sample when the irradiation time is insufficient; in other words, inhomogeneity would be generated along the thickness direction. The lattice spacing of the colloidal crystals captured in such an inhomogeneous gel would become inhomogeneous along the thickness direction in the subsequent swelling equilibration process, thus resulting in dip expansion. By increasing the irradiation time, the polymerization reaction is promoted even in the deep interiors of the sample where the light intensity is relatively low, and the sample becomes more homogeneous. Ultimately, when the irradiation time is sufficiently long, complete polymerization should occur even in the deep interiors of the sample, and a homogeneous gel sample can be obtained. For such a sample, inhomogeneities in the lattice constant are not generated in the subsequent equilibration process, and hence the dip expansion does not occur. The other one relates to a polymerization mechanism. In general, numerous microgels are formed in the initial stage of the polymerization process; these microgels then grow into a global gel network.12 In this process, if the light irradiation is aborted and the polymerization reaction is stopped before the completion of the global gel (12) Flory, P. J. Principles of Polymer Chemistry; Cornel University Press: Ithaca, NY, 1953; Chapter 9.
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Figure 3. Reflection spectra of samples irradiated for 1 and 3 h.
network, then it would generate a microscopically inhomogeneous gel network like the coexistence of polymer-rich regions occupied by microgels with other polymer-poor regions. Such inhomogeneity will depend on the irradiation time. Because the inhomogeneity in the gel network reflects the inhomogeneity in the lattice constant of the colloidal crystals through the subsequent equilibration process as discussed above, it leads to dip expansion. Therefore, the dependence of dip expansion on the irradiation time can also be explained by this mechanism. The proposed mechanisms for dip expansion are speculations that are consistent with the present experimental observations. For further verification of the mechanisms, the most effective way would be to make microscopic observations of the lattice spacing of the gelled colloidal crystals and the density of the gel network during and after light irradiation, both of which seem to be challenging. These are left for future study. Although the dip expansion by these mechanisms is essentially different from the expansion of the photonic band gap in a perfect crystal, it has an equivalent effect on the apparent spectral property.7 In addition to the transmission spectrum, a reflection spectrum also looks as if the band gap expanded. Figure 3 shows the normal reflection spectra of samples irradiated for 1 and 3 h, where the expansion of the (111) reflection peak of the sample irradiated for 1 h was similar to the dip expansion in the transmission spectrum. In conclusion, we have observed that the width of the optical stop band in gelled colloidal crystals can be effectively expanded in a simple manner by controlling the light irradiation conditions of photoinduced polymerization. The dip width in the transmission spectrum increased to 3 times its original value while preserving the high macroscopic uniformity in the gelled crystal. The tunability of the width as well as the central frequency of the optical stop band will greatly increase the scope of the optical applications of colloidal crystals. LA0621160
