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Gelation of Colloidal Crystals without Degradation in Their Transmission Quality and Chemical Tuning Akiko Toyotama,† Toshimitsu Kanai,† Tsutomu Sawada,*,† Junpei Yamanaka,‡ Kensaku Ito,§ and Kenji Kitamura† National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, Graduate School of Pharmaceutical Sciences, Nagoya City University, 3-1 Tanabe-dori, Mizuho, Nagoya, Aichi 467-8603, Japan, and Faculty of Engineering, Toyama University, 3190 Gofuku, Toyama 930-8555, Japan Received April 15, 2005 A single-domain colloidal crystal with high transmission quality, prepared by a shear-induced process, was fixed as a hydrogel film by photopolymerization. Upon gelation, the original optical quality was almost perfectly preserved. By replacing the solvent, the gelled crystal could be converted to smaller lattice constant crystals without significant degradation in its transmission characteristics. The conversion results in a stop-band wavelength coverage across the entire visible light range.
Introduction 1
Colloidal crystals, which comprise three-dimensional periodic arrays of submicron particles, have recently attracted considerable attention because of their novel photonic applications,2 such as their use in the form of photonic crystals.3 If a colloidal crystal is fixed in a polymer gel,4 it could form a widely tunable photonic material.5 Charge-stabilized colloidal crystals have been tailored in large single-domain films with excellent optical characteristics of almost perfect opacity and high transparency at stop-band and pass-band wavelengths, respectively.6 However, gelled crystals possessing such transmission qualities have thus far not been reported. Whether the original transmission quality is maintained upon gelation is a crucial question in terms of transmission applications such as optical devices, although it might be relatively insignificant for applications based on reflection characteristics, such as chemical sensors. In this study, we demonstrate the possibility of fixing colloidal crystals of high quality optical transmission * Email correspondence to:
[email protected]. † National Institute for Materials Science. ‡ Nagoya City University. § Toyama University. (1) (a) Pieranski, P. Contemp. Phys. 1983, 24, 25. (b) Arora, K. A., Tata, B. V. R. Eds. Ordering and Phase Transitions in Charged Colloids; VCH Publishers: New York, 1996. (c) Gast, A. P.; Russel, W. B. Phys. Today 1998, 51, 24. (d) Van Blaaderen, A. MRS Bull. 2004, 29, 85. (e) Okubo, T. Naturwissenschaften 1992, 79, 317. (2) (a) Xia, Y. Ed. Adv. Mater. 2001, 13, 369. (b) Colvin, V. L. MRS Bull. 2001, 26, 637. (c) Meseguer, F.; Miguez, H. IEICE Trans. Electron. 2004, E87-C, 274. (d) Lopez, C. Adv. Mater. 2003, 15, 1679. (3) (a) Ohtaka, K. Phys. Rev. B 1979, 19, 5057. (b) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (c) John, S. Phys. Rev. Lett. 1987, 58, 2486. (d) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals; Princeton University Press: Princeton, NJ, 1995. (e) Sakoda, K. Optical Properties of Photonic Crystals; Springer-Verlag: Berlin, 2001. (4) (a) Kamenetzky, E. A.; Magliocco, L. G.; Panzer, H. P. Science 1994, 263, 207. (b) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959. (c) Takeoka, Y.; Watanabe, M. Adv. Mater. 2003, 15, 199. (d) Fudouzi, H.; Xia, Y. Adv. Mater. 2003, 15, 892. (e) Asher, S. A.; Holtz, J.; Weissman, J. MRS Bull. 1998, 23, 44. (f) Yamanaka, J.; Murai, M.; Iwayama, Y.; Yonese, M.; Ito, K.; Sawada, T. J. Am. Chem. Soc. 2004, 126, 7156. (5) (a) Foulger, S. H.; Jiang, P.; Ying, Y.; Lattam, A. C.; Smith, D. W., Jr.; Ballato, J. Adv. Mater. 2001, 13, 1898. (b) Iwayama, Y.; Yamanaka, J.; Takiguchi, Y.; Takasaka, M.; Ito, K.; Shinohara, T.; Sawada, T.; Yonese, M. Langmuir 2003, 19, 977. (6) (a) Kanai, T.; Sawada, T.; Toyotama, A.; Kitamura, K. Adv. Funct. Mater. 2005, 15, 25. (b) Sawada, T.; Suzuki, Y.; Toyotama, A.; Iyi, N. Jpn. J. Appl. Phys., Part 2 2001, 40, L1226.
characteristics in hydrogel without degradation. We also show that smaller lattice constant crystals can be derived from a single original gelled crystal by changing the composition of the solvent without affecting the spectral quality. This greatly simplifies the fabrication process. Experimental Section Our colloidal system was an aqueous dispersion of charged polystyrene particles (Duke Scientific Corp.; particle diameter: 198 nm; standard deviation: 3%) that was used after purification by dialysis, ultrafiltration, and the ion exchange method.7 The multistep fabrication process of gelled colloidal crystals is as follows: (1) preparation of the crystal-phase suspension including the gel reagents N-methylolacrylamide as the monomer (0.79 M), N,N′-methylenebisacrylamide as the cross-linker (10 mM), and camphorquinone as the photopolymerization initiator; (2) removal of dissolved oxygen from the sample solution by Ar bubbling to prevent the deactivation of polymerization-initiating radicals; (3) formation of a single-domain colloidal crystal in a straight, flat capillary cell (capillary space dimension: 0.1 × 9 × 60 mm) by processing with a strong shear flow of a short duration, wherein the starting suspension, which is in a polycrystalline state, is rearranged into a single domain if the driving pressure of the flow exceeds a certain critical value;6 (4) photoinduced polymerization by uniform irradiation from two sides of the cell by using blue light-emitting diode (LED) arrays (MBARB-5015, Moritex Corp., Tokyo, Japan). The crystalline phase of the ordered particle arrays in the dispersion had a cubic close-packed structure of spheres (i.e., the crystalline lattice is face-centered cubic) with a lattice constant of ∼540 nm (particle volume fraction: ∼10%). To inspect the three-dimensional crystalline order inside the samples, transmission spectra with air as the reference and transmission laser diffraction patterns, that is, Kossel patterns,8 in the transmission mode,9 were obtained with the incident light perpendicular to the gel surface. It should be noted that transmission measurements are more sensitive to disorder in the samples than are reflection measurements, although the latter have predominantly been used for characterizing colloidal crystals. (7) Toyotama, A.; Sawada, T.; Yamanaka, J.; Kitamura, K. Langmuir 2003, 19, 3236. (8) (a) Yoshiyama, T.; Sogami, I. In Ordering & Phase Transitions in Charged Colloids; Arora, A., Tata, B., Eds.; VCH Publishers: New York, 1996; p 41. (b) Clark, N.; Hurd, A.; Ackerson, B. Nature 1979, 281, 57. (c) Yoshiyama, T.; Sogami, I.; Ise, N. Phys. Rev. Lett. 1984, 53, 2153. (d) Rundquist, P. A.; Photinos, P.; Jagannathan, S.; Asher, S. A. J. Chem. Phys. 1989, 91, 4932. (9) Kanai, T.; Sawada, T.; Maki, I.; Kitamura, K. Jpn. J. Appl. Phys., Part 2 2003, 42, L655.
10.1021/la051018w CCC: $30.25 © 2005 American Chemical Society Published on Web 09/21/2005
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Figure 1. (A) Transmittance spectra before (thin solid line) and after (all other lines) the gelation (the samples were stored in the capillary cell). The thick solid line represents a successful case, whereas the two dashed lines represent unsuccessful cases. The dark level of the transmittance increases by a few percentages because of the characteristics of the charge-coupleddevice spectrometer. (B) A circular fragment of the gelled colloidal crystal film (diameter: 8 mm; thickness: ∼0.1 mm) placed in water. (C) The transmission Kossel pattern of a gelled crystal placed in water at a laser wavelength of 633 nm.
Figure 2. Spatial distribution of the central wavelength of the (111) dip along two arbitrary diametrical lines on the surface of the disk-shaped gel. The solid and dashed lines indicate the average values for the sets of data shown by the closed and open circles, respectively. Standard deviations for the respective sets of data are 5.8 and 4.7 nm.
Results and Discussions As a result of the fabrication process, we obtained a centimeter-sized single-domain gelled crystalline film in the capillary cell with a thickness of 0.1 mm that included over 300 particle layers in the direction of thickness. Figure 1A shows the transmission spectra for the colloidal crystal before and after gelation. The spectrum prior to gelation (thin solid line) shows a deep dip due to a stop band caused by Bragg diffraction (Bragg angle: 90°) from the (111) family of planes.9 It also shows a high transmittance at the pass-band wavelength. Both the stop-band and the pass-band characteristics are indicators of good crystallinity; however, the latter, in particular, indicates the absence of polygrains that are responsible for reducing the transmittance due to Bragg diffraction by irregularly oriented lattice planes.6 We observed that this quality of the transmittance spectrum can be preserved after the gelation process, as indicated in Figure 1A (thick solid line). However, when the polymerization condition is not set appropriately (insufficient Ar bubbling and/or low irradiation power), it results in the degradation of the transmission characteristics such as the deformation of the dip shape and a reduction in transmittance in the pass band (indicated by dashed lines in Figure 1A). This phenomenon suggests a gelation mechanism that is currently being extensively investigated. The most important fact to be noted is that it is possible to fix colloidal crystals in a gel network while maintaining almost the same transmission quality as that before gelation. Because the gels used in this study were self-standing, they could be removed from the fabrication cell and cut into circular fragments for the purpose of additional characterization, as shown in Figure 1B. The gel was transparent for wavelengths greater than approximately 500 nm, except for the (111) dip, and had a yellowish transmission color because of a lack of transmittance of the wavelength component for blue. Figure 1C shows the transmission Kossel pattern in which the outer concentric ring and hexagonal spoke lines are caused by Bragg reflections of the (111) and {002} family of planes, respectively.9 The existence of the spoke lines in the pattern provides strong evidence for the three-dimensional crystalline order of the particle arrays inside the sample because the {002} planes cannot be formed in the random stacking structure of the close-packed particle layers. One of the important characteristics of a large photonic crystal would be the uniformity of the stop-band wavelength. Figure 2 shows a typical example of the spatial distribution
Figure 3. (A-C) Change in the Bragg reflection color of the gelled crystal at a fixed Bragg angle (∼45°) for different EtOH concentrations: (A) 20%, (B) 65%, and (C) 80%. A Teflon ring (bore diameter: 12.3 mm) was used as a scale. (D) The transmission Kossel pattern of the gelled colloidal crystal with 65% EtOH concentration at a laser wavelength of 543.5 nm.
of the central wavelength of the (111) dip along two arbitrary diametrical lines on the surface of the diskshaped gel, where the measurement spot size is ∼100 µm2 (measured using ImSpector, Kawasaki Steel Technoresearch Corp., Tokyo, Japan). As demonstrated in Figure 2, the spatial fluctuation of the dip wavelength in the gel in terms of the standard deviation in these samples was approximately 5 nm. The swelling volume of the hydrogel is dependent on the solvent type and its composition.10 Accordingly, the gelled colloidal crystals could be chemically tuned to crystals with various lattice constants by replacing the solvent. We used the water/ethanol (EtOH) system for this purpose. Solvent exchange can be accomplished by merely soaking the gel in a solvent with a different composition for a few minutes. The main focus of our study is to determine whether the gel can change its size without significant degradation. Figure 3A-C shows photographs of the same gel that is depicted in Figure 1B at various EtOH concentrations (expressed in vol %). The photos were taken at a fixed Bragg angle (∼45°) for the (111) family of planes parallel to the gel surface. The gelled colloidal crystals shrank with an increase in EtOH concentration, whereas the iridescent color remained almost uniform. The blue shift of the reflection color indicates a reduction in the lattice constant. Figure 4A shows the transmittance spectra for various EtOH concentrations, and Figure 4B shows the dip wavelength as a function of the EtOH concentration. In this case, because of free swelling in the solvent, the dip wavelength at 0% EtOH is greater than that for the gel in the fabrication cell (Figure 1A). As expected in isotropic shrinkage, the dip wavelength was approximately proportional to the gel diameter (not shown). The smallest value of the dip wavelength at which the particles are in contact with each other is theoretically estimated to be (10) (a) Flory, P. J. Principle of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (b) Tanaka, T.; Fillmore, D. J. J. Chem. Phys. 1979, 70, 1214. (c) Tanaka, T.; Sato, E.; Hirokawa, Y.; Hirotsu, S.; Peetermans, J. Phys. Rev. Lett. 1985, 55, 2455.
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Figure 4. (A) Transmittance spectra for various EtOH concentrations. The gelled colloidal crystals shrink with an increase in the EtOH concentration, and a blue shift is observed in its spectrum. (B) The dip wavelength in the transmittance spectra shown in A as a function of the EtOH concentration.
∼495 nm. The spectral profiles simply shift to a shorter wavelength. This indicates that the degradation in crystallinity throughout the shrinking process was insignificant. The transmission Kossel patterns for the shrunk gels also clearly indicate the good quality of the crystalline order, as shown in Figure 3D for a 65% EtOH concentration. The transmittance of the gel decreased remarkably for an EtOH concentration above 90%, and the spectrum quality was not sustained. However, the condition of the gel was recovered with a reduction in the EtOH concentration. This shrinking-swelling process resulting from the solvent substitution was reversible and repeatable.
Letters
As shown in Figure 4B, the dip wavelength can be reduced to almost half its initial value. Therefore, the entire visible light range can be covered by designing the original gelled crystal to have a dip wavelength for red. The use of water and EtOH as solvents in the construction of durable devices might be disadvantageous in practice because of their high volatility. As alternatives with low vapor pressures, we found that mixtures of various polyols with varying affinities to hydrogel are effective in regulating the amount of swelling in the gel. These details will be reported in a separate paper. Conclusion In conclusion, we have shown that charge-stabilized colloidal crystals can be successfully fixed in a polymer gel while preserving their high optical transmission quality. By replacing the solvent, the gelled crystal could be converted into smaller lattice constant crystals without significant degradation of its transmission characteristics. The conversion results in a stop-band wavelength coverage across the entire visible light range. These results will pave the way for transmission applications of gelled colloidal crystals. LA051018W
