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Control of the Optical Properties of Liquid Crystal-Infiltrated Inverse Opal Structures Using Photo Irradiation and/or an Electric Field Shoichi Kubo,† Zhong-Ze Gu,‡ Kazuyuki Takahashi,§ Akira Fujishima,| Hiroshi Segawa,†,⊥ and Osamu Sato*,|,# Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan, National Laboratory of Molecular and Biomolecular Electronics, Southeast UniVersity, Nanjing 210096, China, Institute for Molecular Science, 38 Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan, Kanagawa Academy of Science and Technology, KSP Bldg, East 412, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi, Kanagawa 213-0012, Japan, and Department of Chemistry, School of Arts and Sciences, The UniVersity of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan ReceiVed February 2, 2005. ReVised Manuscript ReceiVed March 10, 2005
Tunable photonic band gap crystals were prepared by the infiltration of photoresponsive liquid crystals into inverse opal structure films. Tuning of their optical properties could be realized by means of photoinduced phase transition of liquid crystals. The photoinduced phase transition behavior could be evaluated by measuring the change in their optical properties under light irradiation, and it was clearly observed that the behavior varied with temperature and light intensity. The materials could store and display images that were created by the irradiation with UV light through a photomask. A great advantage of this technique is that the films themselves can display an image without the use of polarizers or other assistant materials, and they can also render a color display that can be selected by varying the lattice distance of the inverse opal structure. In addition, we achieved the control of their optical properties by the application of an electric field. Because the state induced by the electric field is different from that engendered by a photoinduced phase transition, it is now possible to switch among three states by a combination of these two techniques. The materials that we have developed have possibilities for practical applications in optical devices.
Introduction Recently, photonic crystals (PCs) composed of spatially ordered dielectrics with lattice parameters comparable to the wavelength of visible light have received much attention due to their unique properties in controlling the propagation of light.1-28 Even those crystals that do not exhibit a complete * To whom correspondence should be addressed. E-mail: sato@ cm.kyushu-u.ac.jp. † School of Engineering, The University of Tokyo. ‡ Southeast University. § Institute for Molecular Science. | Kanagawa Academy of Science and Technology. ⊥ School of Arts and Sciences, The University of Tokyo. # Present address: Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga-koen, Kasuga 816-8508, Japan.
(1) Yablonovitch, E. Phys. ReV. Lett. 1987, 58, 2059. (2) John, S. Phys. ReV. Lett. 1987, 58, 2486. (3) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals: Molding the Flow of Light; Princeton University Press: Princeton, NJ, 1995. (4) Kim, B. G.; Parikh, K. S.; Ussery, G.; Zakhidov, A.; Baughman, R. H.; Yablonobitch, E.; Dunn, B. S. Appl. Phys. Lett. 2002, 81, 4440. (5) Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. Nature 1997, 386, 143. (6) Tarhan, I˙ . I˙ .; Watson, G. H. Phys. ReV. Lett. 1996, 76, 315. (7) Park, S. H.; Xia, Y. Langmuir 1999, 15, 266. (8) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. AdV. Mater. 2000, 12, 693. (9) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (10) Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 11630. (11) Mı´guez, H.; Meseguer, F.; Lo´pez, C.; Mifsud, A.; Moya, J. S.; Va´zquez, L. Langmuir 1997, 13, 6009. (12) Mı´guez, H.; Meseguer, F.; Lo´pez, C.; Lo´pez-Tejeira, F.; Sa´nchezDehesa, J. AdV. Mater. 2001, 13, 393.
photonic band gap can still show interesting optical properties, such as an optical stop band that can also be observed as a structural color. It is important to be able to control the photonic band structure through external stimuli for many practical applications.29-56 The photonic band structure (13) Bertone, J. F.; Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Colvin, V. L. Phys. ReV. Lett. 1999, 83, 300. (14) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lenhoff, A. M. Nature 1997, 389, 447. (15) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lenhoff, A. M.; Kaler, E. W. J. Am. Chem. Soc. 2000, 122, 9554. (16) Velev, O. D.; Kaler, E. W. AdV. Mater. 2000, 12, 531. (17) Blanco, A.; Chomski, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; Driel, H. M. v. Nature 2000, 405, 437. (18) Garcı´a-Santamarı´a, F.; Ibisate, M.; Rodrı´guez, I.; Meseguer, F.; Lo´pez, C. AdV. Mater. 2003, 15, 788. (19) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706. (20) van Blaaderen, A. Science 1998, 282, 887. (21) Reynolds, A. L.; Cassagne, D.; Jouanin, C.; Arnold, J. M. Synth. Met. 2001, 116, 453. (22) Romanov, S. G.; Maka, T.; Torres, C. M. S.; Mu¨ller, M.; Zentel, R. Synth. Met. 2001, 116, 475. (23) Yoshino, K.; Lee, S. B.; Tatsuhara, S.; Kawagishi, Y.; Ozaki, M.; Zakhidov, A. A. Appl. Phys. Lett. 1998, 73, 3506. (24) Vlasov, Y. A.; Astratov, V. N.; Karimov, O. Z.; Kaplyanskii, A. A.; Bogomolov, V. N.; Prokofiev, A. V. Phys. ReV. B 1997, 55, 13357. (25) Marlow, F.; Dong, W. ChemPhysChem 2003, 4, 549. (26) Gu, Z.-Z.; Fujishima, A.; Sato, O. Chem. Mater. 2002, 14, 760. (27) Gu, Z.-Z.; Hayami, S.; Kubo, S.; Meng, Q.-B.; Einaga, Y.; Tryk, D. A.; Fujishima, A.; Sato, O. J. Am. Chem. Soc. 2001, 123, 175. (28) Gu, Z.-Z.; Kubo, S.; Fujishima, A.; Sato, O. Appl. Phys. A 2002, 74, 127. (29) Yoshino, K.; Kawagishi, Y.; Ozaki, M.; Kose, A. Jpn. J. Appl. Phys. 1999, 38, L786. (30) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829.
10.1021/cm050249l CCC: $30.25 © 2005 American Chemical Society Published on Web 04/08/2005
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mainly depends on the lattice constants and the refractive indices of the dielectrics. Therefore, the approaches that are available for realizing the tuning of photonic band structures can be roughly divided into two categories: control of the lattice constants or control of the refractive indices. A variety of practical schemes have been reported that are based on the former concept. Some examples include tunable PCs that can be controlled by the application of an external mechanical force,29 the volume phase transition of hydrogels induced by changes in the environment,30-32 and the photoinduced phase transition of colloidal crystals in solution.33 Large shifts in the optical stop band have been demonstrated in these systems by controlling the lattice constants. However, these techniques have the disadvantage that structural changes of this type occur on the order of micrometer dimensions, and such changes may not be desirable in some cases. Techniques using nanogels have been proposed to resolve these problems.34 Alternative methods based on the concept of controlling the refractive indices of the materials by using dyes have also been studied.35,36 In these systems, an appropriate photochromic dye should be chosen, depending on the position of the stop band. Liquid crystals (LCs) are also candidate materials for the components of tunable PCs. LCs possess optical anisotropy; their refractive indices can be changed by controlling their orientation or temperature. The phase transition between the LC phase and the isotropic phase (31) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959. (32) Lee, K.; Asher, S. A. J. Am. Chem. Soc. 2000, 122, 9534. (33) Gu, Z.-Z.; Fujishima, A.; Sato, O. J. Am. Chem. Soc. 2000, 122, 12387. (34) Reese, C. E.; Mikhonin, A. V.; Kamenjicki, M.; Tikhonov, A.; Asher, S. A. J. Am. Chem. Soc. 2004, 126, 1493. (35) Gu, Z.-Z.; Hayami, S.; Meng, Q.-B.; Iyoda, T.; Fujishima, A.; Sato, O. J. Am. Chem. Soc. 2000, 122, 10730. (36) Gu, Z.-Z.; Iyoda, T.; Fujishima, A.; Sato, O. AdV. Mater. 2001, 13, 1295. (37) Zhou, J.; Sun, C. Q.; Pita, K.; Lam, Y. L.; Zhou, Y.; Ng, S. L.; Kam, C. H.; Li, L. T.; Gui, Z. L. Appl. Phys. Lett. 2001, 78, 661. (38) Li, B.; Zhou, J.; Li, L.; Wang, X. J.; Liu, X. H.; Zi, J. Appl. Phys. Lett. 2003, 83, 4704. (39) Xu, C.; Hu, X.; Li, Y.; Fu, X. L.; Zi, J. Phys. ReV. B 2003, 68, 193201. (40) Busch, K.; John, S. Phys. ReV. Lett. 1999, 83, 967. (41) Yoshino, K.; Satoh, S.; Shimoda, Y.; Kawagishi, Y.; Nakayama, K.; Ozaki, M. Jpn. J. Appl. Phys. 1999, 38, L961. (42) Kang, D.; Maclennan, J. E.; Clark, N. A.; Zakhidov, A. A.; Baughman, R. H. Phys. ReV. Lett. 2001, 86, 4052. (43) Johri, G. K.; Tiwari, A.; Johri, M.; Yoshino, K. Jpn. J. Appl. Phys. 2001, 40, 4565. (44) Shimoda, Y.; Ozaki, M.; Yoshino, K. Appl. Phys. Lett. 2001, 79, 3627. (45) Meng, Q.-B.; Fu, C.-H.; Hayami, S.; Gu, Z.-Z.; Sato, O.; Fujishima, A. J. Appl. Phys. 2001, 89, 5794. (46) Ozaki, M.; Shimoda, Y.; Kasano, M.; Yoshino, K. AdV. Mater. 2002, 14, 514. (47) Takeda, H.; Yoshino, K. J. Appl. Phys. 2002, 92, 5658. (48) Mertens, G.; Ro¨der, T.; Schweins, R.; Huber, K.; Kitzerow, H.-S. Appl. Phys. Lett. 2002, 80, 1885. (49) Mach, P.; Wiltzius, P.; Megens, M.; Weitz, D. A.; Lin, K.-h.; Lubensky, T. C.; Yodh, A. G. Phys. ReV. E 2002, 65, 031720. (50) Mach, P.; Wiltzius, P.; Megens, M.; Weitz, D. A.; Lin, K.-h.; Lubensky, T. C.; Yodh, A. G. Europhys. Lett. 2002, 58, 679. (51) Gottardo, S.; Wiersmaa, D. S.; Vos, W. L. Physica B 2003, 338, 143. (52) Ozaki, M.; Kasano, M.; Kitasho, T.; Ganzke, D.; Haase, W.; Yoshino, K. AdV. Mater. 2003, 15, 974. (53) Kasano, M.; Ozaki, M.; Yoshino, K.; Ganzke, D.; Haase, W. Appl. Phys. Lett. 2003, 82, 4026. (54) Maune, B.; Loncar, M.; Witzens, J.; Hochberg, M.; Baehr-Jones, T.; Psaltis, D.; Scherer, A.; Qiu, Y. M. Appl. Phys. Lett. 2004, 85, 360. (55) Kubo, S.; Gu, Z.-Z.; Takahashi, K.; Ohko, Y.; Sato, O.; Fujishima, A. J. Am. Chem. Soc. 2002, 124, 10950. (56) Kubo, S.; Gu, Z.-Z.; Takahashi, K.; Fujishima, A.; Segawa, H.; Sato, O. J. Am. Chem. Soc. 2004, 126, 8314.
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or between different kinds of phase can also induce changes in their optical properties. There have been reports of systems that use LCs to tune photonic band gap structures, and some of them have exhibited large changes in their optical properties.40-56 We have previously studied inverse opal films infiltrated with LCs as potential tunable photonic band gap crystals, and we reported the orientation of LCs in the voids in inverse opal structures by evaluating the effective refractive indices of the LCs.55,56 Large changes in the reflection spectra have been observed in such systems, caused by thermal or isothermal photoinduced phase transitions. The photoinduced phase transition of LCs containing azobenzene derivatives is a useful phenomenon for switching their optical properties. Some systems have been reported that control the photonic band structure by using photoinduced phase transitions.57,58 In this paper, we will report the photo switching of the optical properties of LC-infiltrated inverse opal structures by using photoinduced phase transitions, and we will give detailed information about photoinduced phase transitions in the inverse opal structure. We will also describe how to use such materials for the storage and display of images by means of photoswitching of their optical properties. Furthermore, we will also report the switching of their optical properties by use of an electric field, which makes it possible to control three different states by a combination of light irradiation and electric field. Experimental Section Materials. Monodispersed polystyrene spheres were purchased from Duke Scientific Corp. Monodispersed SiO2 spheres and an alcoholic colloidal solution of SiO2 nanoparticles were purchased from Catalysis & Chemical Industries Co., Ltd. (Japan). The liquid photopolymer NOA 60 was purchased from Norland Products Inc. The nematic liquid crystal 4-pentyl-4′-cyanobiphenyl (5CB) was purchased from Merck. All of the purchased reagents were used without further purification. 4-Butyl-4′-methoxyazobenzene (AzoLC) was synthesized according to the literature.59 Briefly, this involved synthesis via a diazo-coupling reaction between 4-butylaniline and phenol, followed by alkylation with methyl iodide. Fabrication of Inverse Opal Films. SiO2 inverse opal films were fabricated by a dipping method previously developed by us as follows.28 Polystyrene (PS) opal films, which were used as templates for the inverse opal structure, were fabricated by a vertical deposition method.9 Glass substrates were fixed vertically into a suspension containing 0.5 vol % of monodispersed PS spheres and were maintained at constant temperature and humidity of 50 °C and 30%, respectively. The PS opal films were then sintered at 80 °C for 30 min to enhance the connection between each of the spheres or between the spheres and the glass substrate. They were then immersed in an alcoholic colloidal solution of SiO2 nanoparticles with a diameter of 6 nm and lifted with a constant speed of 8 µm/s. During this procedure, SiO2 nanoparticles were infiltrated into the voids in the PS opal films, which became completely filled due to capillary forces and convection fluxes driven by evaporation. Finally, the samples were calcined at 500 °C for 1 h to remove the (57) Urbas, A.; Klosterman, J.; Tondiglia, V.; Natarajan, L.; Sutherland, R.; Tsutsumi, O.; Ikeda, T.; Bunning, T. AdV. Mater. 2004, 16, 1453. (58) Urbas, A.; Tondiglia, V.; Natarajan, L.; Sutherland, R.; Yu H.; Li, J.-H.; Bunning, T. J. Am. Chem. Soc. 2004, 126, 13580. (59) Zienkiewicz, J.; Galewski, Z. Liq. Cryst. 1997, 23, 9.
2300 Chem. Mater., Vol. 17, No. 9, 2005 PS spheres and to solidify the nanoparticles, thereby forming the inverse opal structure. Polymer inverse opal films were fabricated as follows. SiO2 opal films were fabricated by a vertical deposition method. Glass substrates were fixed vertically into an alcoholic suspension containing 1 vol % of monodispersed SiO2 spheres and were maintained at a constant temperature and humidity of 25 °C and 40%, respectively. Liquid photopolymer was dropped on the SiO2 opal films to fill the voids and the films were covered with another piece of glass substrate. After polymerization by exposure to UV light (λ ) 365 nm) for 2 h, the films were soaked in a 10% hydrofluoric acid solution for 3 days to remove the SiO2 spheres. The resulting films were rinsed with water and dried under reduced pressure. The structures of the inverse opal films were observed by scanning electron microscopy (SEM), which was carried out using a HITACHI model S-4500 SEM. Infiltration of Liquid Crystals into Inverse Opal Films. The prepared inverse opal films were fixed between glass substrates. LCs were then infiltrated into the voids in the films by using capillary forces at atmospheric pressure. Note that the LCs were heated above their phase transition temperature to convert them into the isotropic phase, thereby enabling them to be introduced into the voids. Photoinduced Changes in Optical Properties. The samples were irradiated with UV and visible light ito produce photoinduced changes in their optical properties. Light from an Hg-lamp was passed through an interference filter for use as the UV light source (λ ) 365 nm), while light from a Xe lamp was also passed through an interference filter for use as the visible light source (λ ) 450 nm). To study the application of the materials for display devices, the samples were irradiated with UV light through a patterned photomask. A single pulse from a YAG laser (355 nm, 6 ns fwhm) was used as the UV light source for time-resolved measurements, as detailed in the following description. Change in Optical Properties with Electric Field. LC infiltrated inverse opal films were fixed between a pair of indium tin oxide (ITO) coated glass substrates to study changes in their optical properties with changing electric field. The electric field was supplied by a function generator connected to an amplifier. Measurements of the Optical Properties. The optical properties of the LC-infiltrated inverse opal films were evaluated using the reflection or transmission spectra from vertically incident white light. The spectra were measured by using a multichannel photodetector with an Ocean Optics S2000 spectrometer. Time-resolved measurements were also performed for detailed analysis. The samples were irradiated with a single pump pulse from a YAG laser (355 nm), and the intensity of the probe light (He-Ne laser; 633 nm) that was transmitted through the sample was measured as function of time using a photodiode and a photomultiplier and recorded using an oscilloscope. The intensities of the pump and probe light were 112 µJ/pulse and 50 µW, respectively. Temperature-controlled measurements were carried out using an HCS402 Microscope Hot and Cold Stage with an STC2000 Controller (INSTEC).
Results and Discussion 1. Observation of the Inverse Opal Structure. Figure 1 shows SEM images of the opal structure composed of PS spheres with a diameter of 260 nm and the SiO2 inverse opal structure films. It can be observed that the spheres in the opal structure film form a closely packed arrangement, which agrees with past reports.11,26 The image of the inverse opal
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Figure 1. SEM images of polystyrene opal film (a) and SiO2 inverse opal film (b) prepared from polystyrene spheres with a diameter of 260 nm.
Figure 2. Reflection spectra of an inverse opal film with a diameter of 260 nm (black line), after infiltration with the liquid crystal mixture (brown line), and after subsequent irradiation with UV light (red line). The inset figures illustrate the orientation of the liquid crystals at each state.
Figure 3. Structures of 5CB and AzoLC.
film confirms that the opal structure was successfully inverted. The film contains hexagonal “air spheres,” which are derived from the FCC opal structure. Additionally, the underlying layer can be seen beneath the surface layer. This is because the air spheres are not isolated, but rather are connected to each other through holes that are derived from the contact points of the opal spheres. The structures of the SiO2 opal film and the polymer inverse opal film could also be observed. Their SEM images are shown in the Supporting Information. 2. Tuning of the Optical Properties of Inverse Opal Films with Liquid Crystals. As reported previously, the optical properties of inverse opal films can be controlled by combining them with LCs.55,56 Figure 2 shows a typical result of the change in reflection spectra of a photoresponsive LCinfiltrated SiO2 inverse opal structure with a diameter of 260 nm. The LC used in this experiment was a mixture of 5CB and AzoLC, with a volume ratio of 97:3. The structures of the 5CB and AzoLC are shown in Figure 3. This LC mixture will be abbreviated as PLC in this paper. The black line shows the reflection spectrum before infiltration of the PLC. The sample exhibited a peak at 410 nm. The optical properties of colloidal crystals have been well studied,6 and
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the peak position due to the optical stop band can be calculated approximately by using the equation for Bragg diffraction under normal incidence:7 λ ) 2x2/3 d(n2silica f + n2void (1 - f))1/2 where λ is the peak position, d is the diameter of the spherical voids, nsilica and nvoid are the refractive indices of the SiO2 and the medium in the voids of the inverse opal films, respectively, and f is the volume fraction of SiO2. The position of the observed peak agrees with that calculated by this equation. After the infiltration of the PLC into the voids in the inverse opal film, the spectrum changed to that indicated by the brown line. The peak at 410 nm disappeared and two broad peaks appeared at 600 and 630 nm. Irradiation with the UV light induced the large change in the reflection spectrum that is indicated by the red line, exhibiting a distinct peak at 610 nm. This peak decreased rapidly by subsequent irradiation with visible light, and the reflection spectrum completely reverted to the state that it exhibited before irradiation with UV light. These changes were triggered by the photoinduced phase transition of the PLC. When the PLCs are in the nematic phase, the refractive indices of the LCs in each of the voids in the inverse opal film are random, since the overall orientation of the LC molecules in the nematic phase is random. Such a random structure does not satisfy the conditions for Bragg diffraction. Therefore, most of the incident light is scattered. The two weak peaks could be attributed to diffractions from domains existing at different sites within the film. When the film is irradiated with UV light, a nematic (N)-isotropic (I) phase transition is induced in the PLCs, triggered by the trans-cis photoisomerization of the AzoLC.60,61 After the phase transition into the isotropic phase, the optical anisotropy of the LC disappears and a reflection peak due to Bragg diffraction can be observed because of the periodic refractive indices of the SiO2 and the LC. Subsequent irradiation with visible light brings about the cis-trans photoisomerization of the AzoLC, leading to the I-N phase transition, and the initial optical properties are restored. The detailed mechanism has been already described in our previous paper.56 3. Dependence of Photoinduced Phase Transition on Temperature and Light Intensity. The PLC-infiltrated inverse opal films also show changes in their transmission spectra (Figure 4). The transmittance is very low in the initial state, and does not exhibit any peaks because of lightscattering. After the UV light irradiation, the transmittance increases across the whole wavelength range and a peak due to the optical stop band appears. Subsequent irradiation with visible light caused the spectrum to revert to its initial state. Because the changes in the optical properties of PLCinfiltrated inverse opal films are derived from photoinduced phase transitions, the phase states can be estimated by measuring the optical properties. When the transmission is low, the PLC should be in the nematic phase, and when the transmission is high, the PLC should be in the isotropic (60) Tazuke, S.; Kurihara, S.; Ikeda, T. Chem. Lett. 1987, 911. (61) Sung, J. H.; Hirano, S.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T. Chem. Mater. 2002, 14, 385.
Figure 4. Changes in the transmission spectra of PLC-infiltrated inverse opal films: initial state (black), after UV light irradiation (blue), and after subsequent irradiation with visible light (green). The dotted red line indicates the wavelength used to evaluate the photoinduced phase transition of the PLC.
phase. The photoinduced phase transition behavior was evaluated on the basis of these phenomena. The transmittance at 633 nm, which is outside of the optical stop band (as shown in Figure 4), was measured to track the phase transition behavior. It should be noted that, the inverse opal films contain cracks of around 10 µm long and 1-2 µm wide that appear every 5-10 µm on average. Although these cracks filled with PLC would cause the scattering of light and reduce the transmittance on the whole, they would not have a serious effect on optical properties. To begin with, it was determined that the thermal phase transition temperature was 36.5 °C by the measurements of the transmittance at 633 nm with increasing temperature. This should agree with the thermal phase transition temperature without inverse opal structure as reported previously.56 3.1. N-I Phase Transition BehaVior under UV Light Irradiation (Forward Path). The transmission spectra of the films were measured during continuous UV light irradiation, and the dependence on the light intensity and temperature was studied. The left part of Figure 5 illustrates the changes in transmittance at 633 nm during the UV light irradiation under several different conditions. When the intensity of the UV light was 0.5 mW/cm2, no distinct change in transmittance was observed at any temperature. When the intensity was greater than 1 mW/cm2, the transmittance gradually increased, exhibited a steep rise, and finally saturated. This gradual increase is attributed to an increasing perturbation of the nematic phase alignment induced by the cis isomers, with a concomitant decrease in the optical anisotropy. Since the light scattering exhibited by PLCinfiltrated inverse opal films is caused by the randomness of refractive indices of PLCs in each spherical void in the films, the decrease in optical anisotropy brings about a reduction in the scattering of light. The abrupt change is due to a photoinduced phase transition. The disappearance of the optical anisotropy of the PLCs leads to a structure that contains the ordered refractive indices of PLC and SiO2. Such a structure satisfies the condition required for Bragg diffraction; light of a certain wavelength corresponding to the photonic band gap is reflected, while light of other wave-
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Figure 5. (Left) Change in transmittance at 633 nm during the irradiation with UV light at 30 °C (a) and 25 °C (b). The intensity of the UV light was 0.5 (black), 1.0 (green), 4.0 (blue), and 7.0 mW/cm2 (red). The changes observed at 35 and 34 °C were similar to that observed at 30 °C, and were completed within shorter time. Inset of (a): enlargements of the period in which the phase transition was completed. (Right) Change in transmittance at 633 nm during the irradiation with visible light at 30 °C (a′) and 25 °C (b′). The intensity of the visible light was 0.5 (black), 1.0 (green), 2.0 (blue), and 3.0 mW/cm2 (red). No switching behavior was observed at 35 and 34 °C.
lengths penetrates the film. The irregular rise and fall of the transmittance before saturation that was observed under some conditions could be mainly due to interference, attended by a large change in refractive indices during the phase transition of the PLC between the inverse opal structure and the glass that was used to cover the sample film. Switching of the optical properties did not proceed to completion under all conditions at 25 °C in this experiment, although the photoinduced phase transition seemed to be almost completed, since a peak in the transmittance spectra due to the optical stop band could be observed. 3.2. I-N Phase Transition BehaVior under Visible Light Irradiation (ReVerse Path). The reverse change induced by irradiation with visible light was also studied. Prior to the measurements, the film was irradiated with UV light to transform PLC into isotropic phase. The right part of Figure 5 shows the change in transmittance at 633 nm during the continuous visible light irradiation. Note that the initial intensity at 25 °C was different from that at other temperatures because the N-I phase transition did not proceed to the completion as described in the previous subsection. At 34 and 35 °C, no switching behavior could be observed at any intensity of visible light. On the other hand, at 30 °C, the transmittance remained for some time after the start of the visible light irradiation and exhibited a change due to the photoinduced phase transition from the
isotropic phase to the nematic phase. It is worth noting that the transmittance showed a strange behavior during the phase transition. Just after the decrease, it increased again and then began to decrease gradually. This can be explained as follows. By the irradiation with visible light, the cis isomers of the AzoLC transform to trans isomers, but the transmittance does not change because the PLC is still in the isotropic phase if the amount of the cis isomer is sufficient to disorganize the orientation of the PLC molecules. When the amount of cis isomer decreases and reaches the limit between the nematic phase and the isotropic phase, the reverse phase transition occurs and the transmittance decreases. At that time, there should be a temporary biphasic morphology consisting of nematic domains and isotropic domains, and the light is scattered strongly by the interface of such domains. After that, the PLCs are completely transformed into the nematic phase and the strong light scattering disappears. Then the transmittance exhibits a gradual decrease due to the increase of the optical anisotropy of the PLCs. At 25 °C, the transmittance decreased immediately and monotonically with the visible light irradiation without any strange behavior, unlike the case of 30 °C. This is because the nematic phase is much more stable than the isotropic phase. The I-N phase transition is so fast that the strange behavior cannot be observed although it may actually still occur.
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Figure 6. Dependence of the response time on the light intensity: N-I phase transition (a), and I-N phase transition (b).
3.3. Dependence of the Response Time for Phase Transition on the Irradiation Conditions. These results show the tendency of the photoinduced phase transition behavior. To summarize the dependence on the conditions, the relationship between the response time and the conditions was considered. The response time for the N-I phase transition (τN-I) was defined as the time necessary to rise to 90% of the gap between the initial state and the final state. Similarly, the response time for the I-N phase transition (τI-N) was defined as the time necessary to decrease by 90% of the gap between the initial state and the final state. Figure 6 shows the dependence of the response time on the light intensity. Stronger light induced the phase transition with a shorter response time in both directions. In terms of the temperature, higher temperatures could bring about a more rapid N-I phase transition, but caused a slower I-N phase transition. These results reflect the stability of the nematic phase alignment. The increase in temperature makes the nematic phase less stable because of the increase in the fluctuation of PLC molecules. That is, the smaller amount of cis isomers, which can be generated by the irradiation with UV light for shorter time, can disorganize the nematic phase and induce the N-I phase transition. Therefore, the higher the temperature is, the shorter τN-I is. On the other hand, it is difficult to induce the I-N phase transition, in which the ordered alignment is recovered, when the fluctuation of PLC molecules is large. That’s why the temperature dependence of τI-N is contrary to that of τN-I. In particular, the reverse phase transition could not be induced at 34 or 35 °C. This is because the concentration of cis isomer in the photostationary state under visible light irradiation is sufficient to disorganize the alignment of PLC molecules since the fluctuation is so large. These results are consistent with past reports.62 After considering these factors, the most suitable condition should occur between 25 and 30 °C. The application of this material on the basis of the change in the optical properties will be described in Section 5. 4. Time-Resolved Measurements of the Change in Optical Properties. The changes in the optical properties of PLC-infiltrated inverse opal films were studied for several different conditions as described above, and this should make (62) Lee, H.-K.; Kanazawa, A.; Shiono, T.; Ikeda, T. Chem. Mater. 1998, 10, 1402-1047.
it possible to find the optimal conditions for practical applications. However, it is also important to investigate how the photoinduced phase transition progresses in the voids in the inverse opal film in detail. For this reason, time-resolved measurements were carried out in order to investigate the changes in the optical properties of LC-infiltrated inverse opal films, and a noteworthy behavior was observed. The intensity of the probe light transmitted through the sample film was monitored with a photodiode, and the change in intensity with pulsed UV irradiation was monitored to follow the change in the state of the PLC and the progress of the phase transition behavior. The increase in the intensity of the probe light can be taken as the progress of the photoinduced phase transition leading to the change in the optical properties, because the transmittance of this sample increases for all wavelengths after UV light irradiation. Figure 7 shows the transient changes in the transmission intensity of the 633-nm laser through a PLC-infiltrated inverse opal film with a diameter of 260 nm. The spectra in each figure show the changes in intensity induced by the successive pulses (6 ns) of UV irradiation at 10-s intervals. At 35 °C, the intensity was increased by irradiation with the pulsed UV laser light, reaching a maximum within 2 ms and then remaining at the maximum. The transient changes were different at different temperatures. The lower the temperature, the more irradiation was required. The samples had to be irradiated for five, fifteen, or more than fifty times to attain maximum transmission intensity at 34, 30, and 25 °C, respectively. Such a dependence on temperature indicates that the PLC has greater capability for photoinduced phase transition at temperatures nearer to the thermal phase transition point. The molecules of PLC fluctuate furiously when the temperature is a little lower than the phase transition point, even though they are in the nematic phase, and can easily be changed into the isotropic phase by trans-cis photoisomerization of a few AzoLC molecules. At low temperature, the nematic phase is much more stable than the isotropic phase, and it is difficult for it to change into the isotropic phase by the adiition of a low concentration of the cis isomer. Prolonged irradiation promoted sufficient isomerization and induced the phase transition to the isotropic phase. The intermediate intensity states between the initial and final states are caused by perturbations in the alignment.
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Figure 7. Transient changes in the transmittance of PLC-infiltrated inverse opal film by successive pulses (6 ns) of UV irradiation at 10-s intervals at 35 °C (a), 34 °C (b), 30 °C (c), and 25 °C (d).
With the increase in concentration of cis isomers caused by repeated pulsed UV irradiation, the fluctuations are increased and the PLC becomes closer to the isotropic phase. Therefore, the optical anisotropy decreases and the light scattering of the PLC-infiltrated inverse opal structure is reduced. That brings about an increase in transmittance throughout the film. A much more noticeable phenomenon is that the intensity decreased after the first irradiation pulse, rather than the increase due to the phase transition at low temperature. The decrease in intensity was even observed after several irradiation events, although it increased initially. Similar phenomena were also observed at higher temperature. After the first irradiation pulse at 34 °C, for example, the intensity increased and reached a maximum within 1 ms. Just after that, it began to decrease and reached a level that was lower than the initial state. To study these phenomena, the same measurements were carried out over a prolonged time-scale. The results are shown in Figure 8. It could be observed that the intensity reached a much lower level than the initial state even at high temperatures, at which the intensity seemed to attain a maximum when measured on a short time scale. The intensity initially increased due to pulsed UV irradiation, as shown by the annotation [A] in Figure 8a, but immediately decreased (annotation [B] in Figure 8a) and recovered gradually. This took more than 1 s. Similar behavior was also observed at other temperatures. These results are quite different from those of conventional liquid crystals systems. Tsutsumi et al. have reported a
photoinduced phase transition from the nematic phase to the isotropic phase and the thermal recovery of the nematic phase.63 They performed time-resolved measurements in which homogeneously aligned LC films were set between two crossed polarizers and the transmittance of a probe light (633 nm) through them was measured. In such a system, the transmittance is high when LCs are in the nematic phase. When the phase transition from the nematic phase to the isotropic phase progresses, the transmittance decreases and reaches zero with the disappearance of optical anisotropy. According to their report, the intensity decreased immediately after the pulsed irradiation on the order of milliseconds, and recovered within the order of seconds. There are basically two states that can be observed: the high transmittance state and the low transmittance state. The change in transmittance in this report is quite different. There are three states: a low transmittance state, a high transmittance state, and a “lower” transmittance state. The former two states can be attributed to the nematic phase and the isotropic phase of the PLC, respectively. The distinctive state is the final one in the list. Such a change in the transmission intensity can be explained with the mechanism shown in Figure 9. In this figure, the red cylinder and the purple cylinder represent the routes of the probe and the pulse laser, respectively, while the light blue and orange parts represent the regions where the PLC (63) Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T.; Park, L.-S. Phys. Chem. Chem. Phys. 1999, 1, 4219.
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Figure 8. Long time-scale transient changes in the transmittance of PLC-infiltrated inverse opal film by successive pulses (6 ns) of UV irradiation at 10-s intervals at 35 °C (a), 34 °C (b), 30 °C (c), and 25 °C (d).
Figure 9. Model for the change in LC phase in the inverse opal structure. The transient spectrum after a single irradiation pulse at 35 °C is shown as a typical result.
is in the nematic phase and the isotropic phase, respectively. According to the past report, the cis isomers generated by irradiation with UV light can diffuse around on a millisecond order.64,65 By irradiating with the UV pulses (A), cis isomers (64) Shishido, A.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T.; Tamai, N. J. Am. Chem. Soc. 1997, 119, 7791. (65) Shishido, A.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T.; Tamai, N. J. Phys. Chem. B 1997, 101, 2806.
are generated and induce the isotropic phase around themselves (B). The isotropic part reduces light scattering, and the transmission intensity increases. After that, the cis isomers diffuse around their initial position and the isotropic region can be divided into some localized isotropic regions (C). In such a state, both nematic regions and localized isotropic regions are in the route of the probe light. Therefore, light should be strongly scattered by both the nematic region and the interface between the nematic and the isotropic regions. The subsequent diffusion of the cis isomers causes the localized isotropic regions to merge with the surrounding nematic regions and the light scattering decreases (D). At low temperature, the final state should be nearly the same as the initial nematic state because the amount of cis isomers generated by the pulsed irradiation is not sufficient to induce phase transition over the whole area. However, the fluctuation of the PLC will be increased by repeated irradiation with the UV pulses, which can be observed as an increase in the intensity in the initial state. At high temperature, the final state can be close to the isotropic phase, because a small amount of cis isomer can induce the phase transition into the isotropic phase. These results also suggest that the phase state of the LCs can be determined with inverse opal structure by measuring the transmission intensity. This could be a useful application. 5. Application to Storage and Display. As described above, PLC-infiltrated inverse opal films reflect at a specific
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Figure 10. Photographs of PLC-infiltrated inverse opal films with diameters of 200, 220, 240, 260, and 300 nm, respectively.
wavelength due to the presence of an optical stop band after UV light irradiation, and this state can remain if the films are irradiated with UV light for a reasonable time. The reflection peak disappears after subsequent irradiation with visible light. Since this change can be induced reversibly, PLC-infiltrated inverse opal films have a potential for application to storage and display devices. Furthermore, it might also be possible to realize multicolor displays because the wavelength of the reflection peak depends on the diameter of the inverse opal structure. Inverse opal films with five different diameters were prepared, and PLC was infiltrated into the voids in the films. Figure 10 shows photographs of the PLC-infiltrated inverse opal films after UV light irradiation from an Hg lamp. The reflection spectra of these films before and after PLC infiltration and following irradiation with UV light are shown in the Supporting Information. On the basis of the results concerning the response time to UV and visible light irradiation described in Section 3.3, the conditions for light irradiation were selected as 7 mW/ cm2 for 1 min and 3 mW/cm2 for 5 min at 27 °C for writing and erasing, respectively. The change in transmittance at 633 nm upon the light irradiation under this condition is shown in the Supporting Information. After the samples were irradiated with visible light to completely convert the PLC to the nematic phase, they were irradiated with UV light through a patterned photomask. Figure 11 shows photographs of the PLC-infiltrated inverse opal films after patterning. In Figure 11a, four kinds of samples (except for that with a diameter of 300 nm) were used, and all kinds of samples were used in Figure 11b. All of the letter patterns stored in the films could be clearly observed, and colors could also be displayed. These patterns could be erased with visible light and the samples could be reused for further patterning. The stored patterns also disappeared within several hours in the dark because of the cis-trans thermal isomerization of AzoLC leading to I-N phase transition. Patterning with a finer detailed photomask was also performed to examine the ability of the material to store more information. The sample was irradiated with UV light through photomasks including a test pattern. The film with a diameter of 260 nm was used here. The photomasks consisted of pairs of lines and spaces with several different widths. Figure 12a shows an optical micrograph of a mask with a width of 500 µm as a typical example. Figure 12b and c show optical micrographs of the sample after UV light irradiation through photomasks with widths of 500 µm and 125 µm, respectively. To prevent the light used within the microscope from inducing phase transitions in the PLC, a colored glass filter that cut off light at wavelengths shorter than 500 nm was used for the observation, which is why the images appear yellowish. The bright parts are the regions
Figure 11. Photographs of stored patterns in PLC-infiltrated inverse opal films with diameters of 200, 220, 240, and 260 nm (a), and those with diameters of 200, 220, 240, 260, and 300 nm (b).
Figure 12. Optical micrographs of the pattern stored in the PLC-infiltrated inverse opal film. Photomask with pairs of 500-µm-width lines and spaces (a), PLC-infiltrated inverse opal film patterned with a 500-µm-width photomask (b), and that patterned with a 125-µm-width photomask (c).
that were irradiated with UV lamp, and the dark parts are the areas that were not irradiated. The mesh-like patterns observed in the bright parts are derived from cracks in the inverse opal films that were formed during the fabrication procedure of the inverse opal structure. The patterning limit was reached at a width of about 50 µm. The resolution in this system is limited by the diffusion of the cis isomer, which makes the boundary unclear or induces phase transition into the isotropic phase in the regions surrounding those that were not irradiated with the UV light. It may be improved by using polymer LCs instead of low-molecularweight LCs to avoid the diffusion of cis isomer. The polymer LCs would also give the stability of the stored pattern because they can keep the isotropic orientation of main chain below Tg even after the thermal cis-trans isomerization. Pure AzoLC-infiltrated inverse opal films were also prepared that were not mixed with 5CB, and their optical properties and their applications to display devices were studied. In this case, a similar change in optical properties could be induced by irradiation with UV light. However, subsequent irradiation with visible light could not regenerate the initial state, which could only be recovered by a thermal process. This could be because the amount of cisisomer in a photostationary state under visible light irradiation is larger than that required for the phase transition into the isotropic phase. The interaction between cis-AzoLC and the surface of SiO2 inverse opal could also influence the phase transition
Control of Optical Properties of InVerse Opal
phenomena.66 Patterning could be realized using UV light irradiation through photomasks, as was the case for PLCinfiltrated inverse opal films. The inability to recover the original state using visible light led to a long stored time. The recorded pattern remained for about half a day, as against several hours in the case of the samples in which PLC was used. Note that the photographs and micrographs in Figures 11 and 12 were taken without polarizers. The reason the patterns could be stored and displayed with specific colors are that the light with a specific wavelength was reflected due to optical stop band, while other wavelengths penetrated in the irradiated regions and almost all of the incident light was scattered in the unirradiated regions. This could become a most important advantage for these materials compared with other liquid crystals systems. Although the polymer dispersed liquid crystals (PDLC) systems may also realize the capability to form displays without the use of polarizers, they can only display dark and bright states, without any colors. 6. Tuning of the Optical Properties by a Combination of Photo Irradiation and an Electric Field. We have already described ways of controlling the optical properties of LC-infiltrated inverse opal films by using photoinduced phase transition of the PLCs, and large changes in their optical properties have been achieved using those methods. If control of the optical properties could also be realized by the application of an electric field, then three different states could be switched by a combination of light irradiation and the electric field. To begin with, control of the optical properties by the application of electric field alone was studied. A polymer inverse opal film with a diameter of 300 nm was inserted between a pair of indium tin oxide (ITO) coated glass substrates, and PLC was infiltrated into the film. Insulating films with a thickness of 25 µm were also placed between the ITO coated glasses to avoid a short circuit. Changes in optical properties were measured during the application of an electric field (sine wave, 1 kHz) between ITO glass substrates. Figure 13 shows the changes in the transmission and reflection spectra of a PLC-infiltrated polymer inverse opal film during the application of an electric field. The transmittance was low on the whole because of the light scattering in the initial state. The transmittance increased with increasing voltage, and a peak appeared at around 604 nm (Figure 13a). Reorientation of the PLC molecules occurs when an electric field is applied. However, the orientation is not ordered sufficiently to exhibit an optical stop band initially, and only an increase in transmittance can be observed. The reorientation of the PLC molecules between the glass substrates and the inverse opal film (that is, the outside of the film) could also contribute to this increase in transmittance. When the voltage becomes sufficiently high, the PLC molecules are well aligned and the refractive indices become uniform. Such a state satisfies the condition required for Bragg diffraction, and a peak due to an optical stop band can be observed. Similar behavior could also be observed (66) Komitov, L.; Ruslim, C.; Matsuzawa, Y.; Ichimura, K. Liq. Cryst. 2000, 27, 1011.
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Figure 13. Changes in the transmittance spectra (a) and the reflection spectra (b) under the application of an electric field. The variation of the peak position with electric field (c).
in the reflection spectra (Figure 13b). A weak peak was observed in the initial state and at low voltage. This is because diffractions from a limited number of regions can be detected in the reflection measurements, although no such diffractions can be detected in the transmission measurements due to light scattering. When an electric field was applied, the peak position did not change until 6 V/µm. After that, it changed abruptly and showed a gradual decrease from 8 V/µm. This result indicates that the PLC molecules began to align parallel to the electric field at 6 V/µm. However, the orientation of the PLC was not sufficiently ordered to create periodic refractive indices. When the electric field
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Figure 14. Transmission spectra (a) and reflection spectra (b) of PLC-infiltrated polymer inverse opal film: initial state (black); during the application of a 16 V/µm electric field (green); after subsequent UV light irradiation under a 16 V/µm field (purple). (c) Schematic illustration of the switching of the optical properties with light irradiation and the application of an electric field.
increased above 8 V/µm, the peak increased rapidly and the wavelength of the peak position shifted slightly because the orientation of PLC adopted a well-ordered state. When the electric field decreased, the spectra did not return completely to the initial state. The peak position after the removal of the electric field was at shorter wavelength than that in the initial state (Figure 13c). This could be because the orientation of the PLC molecules could not return to the initial alignment after the molecules have once been aligned parallel to the electric field.47 After the second cycle, the change in the peak position could be repeated reversibly and completely. After it was confirmed that the optical properties of the PLC-infiltrated inverse opal film could be controlled by the application of an electric field, the combination of photo irradiation with an electric field was studied. Figure 14 shows the transmission and reflection spectra in the initial state, after the application of a 16 V/µm electric field, and after subsequent irradiation with UV light under a 16 V/µm field. The peak due to the optical stop band, which appeared after the application of the electric field, shifted from 604 to 612 nm when the film was irradiated with UV light, and reverted to 604 nm after the irradiation with visible light. This method has the advantage that both the initial state and the final state can be controlled. When phase transition of the LCs or reorientation with an electric field is used in isolation, the initial state cannot be controlled. Therefore, the orientation
of the LCs in inverse opal films would be random and strong light scattering should occur. In such a case, only switching between the scattering state and the colored state (that is, OFF state and ON state) can be achieved. By combining these two techniques, switching among the three states can be achieved as shown in Figure 14c: the initial state in which the light is scattered because of the random orientation of the PLCs in the voids of inverse opal structure; the state induced by the application of an electric field in which the light of a specific wavelength is reflected; and the other state induced by the irradiation with UV light in which light of another specific wavelength is reflected. These three states can be switched reversibly. Conclusions PLC-infiltrated inverse opal films were fabricated, and the phase transition behavior of the PLC in the films was studied by monitoring changes in transmittance during light irradiation. The results clearly showed a dependence of the response time on conditions such as temperature and irradiation intensity. The phase transition behavior was also studied using the time-resolved measurements, and the behavior that was quite different behavior from conventional LC-system was observed. This could be explained by the progressive mechanism of the photoinduced phase transitions. These results also suggested that the inverse opal structure could provide information about the detailed mechanism of the
Control of Optical Properties of InVerse Opal
changes in the LC phase. Furthermore, we also performed tests in which we succeeded in storing patterns such as letters and stripes by irradiation with UV light through photomasks, and were able to display those patterns that showed specific colors that depended on the diameter of the inverse opal structure. Reversible recording and erasing of patterns was realized by irradiating alternately with UV and visible light under suitable conditions that had been determined by the study of phase transition behavior. The developed materials can display the recorded patterns by themselves, without the use of polarizers. Ways of controlling the optical properties by using an electric field were also studied, and an alternative control method was developed that uses a combination of phase transitions and an electric field. Such materials have the possibility of applications in various optical devices.
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Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. This work was also supported by JSPS Research Fellowships for Young Scientists. Supporting Information Available: SEM images of SiO2 opal film and polymer inverse opal film, reflection spectra of PLCinfiltrated inverse opal films with diameters of 200, 220, 240, and 300 nm before and after infiltration of the PLC, and change in transmittance of PLC infiltrated inverse opal film under the condition used for the experiment of patterning (pdf). This material is available free of charge via the Internet at http://pubs.acs.org. CM050249L
