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Large-Domain Colloidal Crystal Films Fabricated Using a Fluidic Cell Masahiko Ishii,* Hiroshi Nakamura, Hideyuki Nakano, Azusa Tsukigase, and Masashi Harada Toyota Central Research & Development Laboratories, Incorporated, Nagakute, Aichi 480-1192, Japan Received January 16, 2005. In Final Form: March 20, 2005 The growth of colloidal crystal films from a dispersion of monodispersed silica spheres using a simple cell with one opening was investigated. Colloidal crystal films with large domain sizes were successfully fabricated almost over the cell (∼10 cm2) without applying any external force at room temperature. During the drying process, three distinct conditions were observed, in each of which the films exhibited different optical properties. Films with high transmittance were formed in the first stage. Upon further solvent evaporation, the films entered a medium transmittance state via an extremely low transmittance state. Angle-resolved reflection spectroscopy, which was used to analyze the three conditions, revealed that close-packed arrays with water-filled spaces between were formed in the first stage. One-directional flow was generated in the cell because water evaporation occurred only at the opening. The flow caused the spheres to be arranged epitaxially, resulting in a large domain size.
Introduction One approach for fabricating three-dimensional photonic band gap (PBG) materials is to utilize monodispersed colloidal spheres. The colloidal spheres can be selfassembled into crystalline arrays, which are called synthetic opals or widely defined colloidal crystals: colloidal crystals are strictly defined as periodic arrays of colloidal spheres in suspensions. Such a crystalline array has a pseudo-PBG in the visible light region since it has a periodic structure on the order of the light wavelength.1 Thus, synthetic opals have been extensively studied in applications, such as chemical and biochemical sensors,2,3 optical switches,4,5 or photonic devices with a complete PBG.6 One of the challenges for the success of these applications is to create opaline films with minimal imperfection, a uniform thickness, and sufficiently large domain sizes. To meet this challenge, many approaches of colloidal crystal formation have been demonstrated. One of these is vertical deposition utilizing evaporation-induced selfassembly driven by capillary forces, which produces superior quality colloidal crystals with less defects and a non-polycrystalline structure.7,8 This approach, however, has two limitations: first, the long evaporation time required to fabricate crystals more than hundreds of layers thick and, second, the fact that deposition is limited to smaller colloidal spheres (less than ca. 500 nm in the case of silica spheres) because of the influence of sedimentation. Two approaches were proposed for preventing sedimenta* To whom correspondence should be addressed. E-mail: m-ishi@ mosk.tytlabs.co.jp. (1) Vlasov, Y. A.; Astratov, V. N.; Karimov, O. Z.; Kaplyanskii, A. A.; Bogomolov. V. N.; Prokofiev. A. V. Phys. Rev. B 1997, 55, R13357. (2) Holts, J. H.; Asher, S. A. Nature 1997, 389, 829. (3) Velev, O. D.; Kaler, E. W. Langmuir 1999, 15, 3693. (4) Pan, G.; Kesavamoorthy, R.; Asher, S. A. Phys. Rev. Lett. 1997, 78, 3860. (5) Mazurenko, D. A.; Kerst. R.; Dijkhunis, J. I. Phys. Rev. Lett. 2003, 91, 213903. (6) Te´treault, N.; Mı´guez, H.; Ozin, G. A. Adv. Mater. 2004, 16, 1471. (7) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (8) Gu, Z.-Z.; Fujishima, A.; Sato, O. Chem. Mater. 2002, 14, 760.
tion of the spheres: mechanical agitation9 and convection10,11 induced by the temperature gradient. These methods were successfully applied to the fabrication of colloidal crystals from dispersions of silica colloids, 1 µm in diameter. Despite these improvements, colloidal crystal films grown by vertical deposition exhibit cracks with a typical density of about one per hundred to several hundred spheres because drying of the solvation layers forming capillary necks between the spheres causes a decrease in the individual sphere size.10 Another method utilizing hydrodynamic flow and physical confinement has been demonstrated.12 In this method, an aqueous dispersion of colloidal spheres was injected into a packing cell composed of two glass substrates and a square-shaped thin spacer sandwiched between them. The spheres retained, concentrated, and subsequently transformed into a crystalline array within the cell by evaporation of the solvent through small channels formed on the spacer by mechanical scratch or thin film patterning. The crystal film formed had a uniform thickness determined by the thickness of the spacer and had a domain size as large as the internal dimensions of the cell. Continuous sonication was, however, necessary to obtain such a large crystal. Despite these successes,7-12 the colloidal crystal with crystallinity high enough for a template of a complete PBG material has not been realized. Understanding of the growth process of colloidal crystals is necessary for control and decrease of crystalline defects. We fabricated colloidal crystal films using a fluidic cell with a simple structure and investigated the growth process of the films. In this paper, we report analysis results of the growth process by using angle-resolved reflection spectroscopy as well as characterization of the films obtained. The analysis revealed that three conditions with different (9) Yang, S. M.; Miguez, H.; Ozin, G. A. Adv. Funct. Mater. 2002, 12, 425. (10) Vlasov, Y. A.; Bo, X.-Z.; Strum, J. C.; Norris, D. J. Nature 2001, 414, 289. (11) Wong, S.; Kitaev, V.; Ozin, G. A. J. Am. Chem. Soc. 2003, 125, 15589. (12) Lu, Y.; Yin, Y.; Gates, B.; Xia, Y. Langmuir 2001, 17, 6344.
10.1021/la050124v CCC: $30.25 © 2005 American Chemical Society Published on Web 05/12/2005
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Figure 2. Images of a sample in conditions A, B, and C. (A) condition A, (B) condition B, and (C) condition C.
Figure 1. Schematic drawing of the fluidic cell used to fabricate colloidal crystal films.
optical properties appeared in the growth process and a close-packed array of spheres with water-filled spaces between was formed in the first stage of the process. A mechanism for the large domain sizes was discussed on the basis of the analysis. Experimental Section Materials. An aqueous dispersion of monodispersed silica spheres (290 nm in diameter, 20 wt %) was purchased from Nippon Shokubai Co., Ltd. The dispersion was diluted with ultrapure water (Millipore) to a concentration of 5 wt % and was deionized with mixed-bed ion-exchange resin (Bio-Rad, AG501X8(D)). Fabrication of Fluidic Cells. The fluidic cell used in this study is composed of two flat glass substrates and two spacers sandwiched between them (Figure 1). Glass microscope slides (Matsunami, S-1111, 76 × 26 × 1 mm) were used as glass substrates and cleaned in a UV/ozone cleaner (Nippon Laser) for 20 min before assembling the cell. Double-stick tape (Nitto Denko Corporation, No. 5601, 10 µm in thickness) was used as spacer. The cell has two openings with the same thickness as the spacer. A fluid reservoir is connected with one of the openings (not shown in Figure 1). Growth of Colloidal Crystal Films. Upon injection into the reservoir, the dispersion of silica spheres penetrated into the space of the cell by capillary forces. The cell was held at a slope of about 15° to supply the dispersion into the cell from the reservoir until the crystal film formed across the inside surface of the cell. The colloidal crystal grew gradually from the vicinity of another opening and reached over several square centimeters after about 10 h. Once the crystal had reached the required size, the dispersion supply was stopped, and the sample dried due to solvent evaporation at room temperature. During this drying process, the crystal changed from a high light transmittance state (condition A) to a medium transmittance state (condition C) via a low transmittance state (condition B) over a long period of more than 150 h. Condition C is the final state of the colloidal crystal film fabricated in this study. Characterization. Scanning electron microscope (SEM) images were taken with a conventional SEM (Hitachi, S-3600N) at an acceleration voltage of 15 kV in order to evaluate the crystallinity of the samples. The SEM samples were prepared by careful removal of one of the glass substrates and coating of the film with thin layers of osmium. Optical properties of colloidal crystal films in conditions A, B, and C were evaluated by measuring their reflection and transmission spectra at normal incidence using a multichannel spectrometer (Soma Optics, Fastevert S-2650). Structural analysis of the conditions was performed by angle-resolved reflection spectroscopy. Angle-Resolved Reflection Spectroscopy. Angle-resolved reflection spectra were measured in order to determine the structural parameters of the crystalline arrays fabricated and to evaluate the crystal orientation in a plane of these arrays. For the structural parameter measurements, the incident angle θ between the beam and the normal to the sample surface was
Figure 3. Reflection (A) and transmission (B) spectra measured for the three conditions observed in the drying process. varied from 9° to 34° and the light scattered was collected in the Bragg configuration. The wavelength λpeak of each reflection peak was plotted against θ. The interplanar spacing d and the effective refractive index neff were determined by fitting the Bragg condition to the plotted data. The Bragg condition is given by
λpeak ) 2dxneff2 - sin2 θ
(1)
To increase measurement accuracy, the spectra were measured at many different angles and λpeak was determined by fitting each reflection peak to the Gaussian curve. To evaluate the crystal orientation, the spectra were measured from 44° to 64° in two perpendicular directions.
Experimental Results Three Conditions Observed in the Drying Process. Three conditions (conditions A, B, and C) with different optical properties were observed in the drying process. Figure 2 shows images of a sample in conditions A, B, and C. Figure 3 shows reflection and transmission spectra measured for these conditions. The reflection and transmission peaks shifted to shorter wavelength in going from condition A to condition C. In addition, transmittance at wavelengths other than the Bragg diffraction resonance changed dramatically with the condition: high, extremely low, and medium transmittance was observed for conditions A, B, and C, respectively.
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Figure 4. (A) Angle-resolved reflection spectra measured for the three conditions. (B) Plots of λpeak vs θ for the three conditions. Table 1. Interplanar Spacing d and Refractive Index neff Determined by Angle-Reflection Spectroscopy condition A condition B
d (nm)
neff
237 238
1.42 1.33
condition C
d (nm)
neff
238
1.31
To investigate the structural difference between these conditions, we measured angle-resolved reflection spectra. These spectra and the plots of reflection peak wavelength λpeak versus incident angle θ are shown in Figure 4. The solid lines indicated in Figure 4B are the curves obtained by fitting eq 1 to the plotted data. The fitted curves matched closely with the data. The interplanar spacing d and the effective refractive index neff determined by fitting each condition are shown in Table 1. Although d changed little, neff decreased in going from condition A to condition C. The average diameter of the spheres D was calculated to be 290-291 nm using D ) (3/2)1/2d. This calculated value matched the mean diameter of the spheres used in this study. These results confirmed that the spheres had already attained a close-packed arrangement in condition A. Since the change in neff seemed to be due to the medium around the spheres, neff was calculated for two media, namely water and air, using the following equation:
neff ) (1 - f)nm + f ns
(2)
where f is the volume fraction of the spheres (f ) 0.74). nm and ns are the refractive indices of the medium and the sphere, respectively. The value of nm is 1.33 for water and 1.0 for air. The value of ns is 1.46 which has been reported as the refractive index of silica spheres.14 The calculated values of neff are 1.42 and 1.33 for water and air, respectively, which are in good agreement with the measured neff for conditions A and B. Thus, we conclude that the two colloidal crystal films in conditions A and B (13) Romanov, S. G.; Maka, T.; Sotomayor Torres, C. M.; Mu¨ller, M.; Zentel, R.; Cassagne, D.; Manzanares-Martinez, J.; Jouanin, C. Phys. Rev. E 2001, 63, 056603. (14) McComb, D. W.; Treble, B. M.; Smith, C. J.; De La Rue, R. M.; Johnson, N. P. J. Mater. Chem. 2001, 11, 143.
Figure 5. Top-view SEM images of a colloidal crystal film fabricated using the cell: (A) a low-magnification image; (B) a high-magnification image showing a crack. Table 2. Structure Summary of Conditions A, B, and C
condition A condition B condition C
arrangement of spheres
medium
spheres
close-packed array close-packed array close-packed array
water air air
wet wet dry
differ in that the medium of the former is water, while that of the latter is air, though both consist of close-packed arrays of silica spheres. On the other hand, the colloidal crystal in condition C had an neff smaller than 1.33. Condition C presumably showed such a small neff because of the decrease in the volume ratio and/or the refractive index of the spheres. The volume ratio of the spheres will decrease as a result of the shrinking of the spheres which occurs when the water in the crystal evaporates.15 However, this effect is probably not the dominant factor because the interplanar spacing changed little in the present study. It is likely that the water evaporation reduces the refractive index of the silica spheres, since it has been reported that silica spheres prepared by the Stober-Fink-Bohn method contain adsorbed molecules of water, which are removed below 200 °C.16 The spheres in condition B presumably contain a large amount of adsorbed water, some of which will evaporate in going from condition B to condition C. Consequently, the refractive index of the spheres will decrease. The reason such a change by removal of the adsorbed water has not been reported previously is possibly that the refractive index was often measured with the Index Matching method,17 which involves measuring in a dispersion. The structures of conditions A, B, and C, as determined from the analysis using angle-resolved spectroscopy, are summarized in Table 2. (15) Fustin, C.-A.; Glasser, G.; Spiess, H. W. Jonas, U. Langmuir 2004, 20, 9114. (16) Garcı´a-Santamarı´a, F.; Mı´guez, H.; Ibisate, M.; Meseguer M.; Lo´pez, C. Langmuir 2002, 18, 1942. (17) van Blaaderen, A.; Vrij, A. J. Colloid Interface Sci. 1993, 156, 1.
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in diameter) have the same crystal orientation in the plane parallel to the substrate and that the 〈110〉 direction is parallel to the growth direction, that is, the long side of the cell. These results were observed throughout the sample except in the vicinity of the spacers. Thus, we concluded that the colloidal crystal film prepared here had a long-range ordering on the order of centimeters, even though it had some cracks and defects. The cracks were probably generated after the formation of crystalline arrays.12
Figure 6. Reflection spectra measured parallel (B) and perpendicular (C) to the long side of the cell at incident angle from 44° to 64°. (A) An illustration explaining the directions.
Crystal Structure of Colloidal Crystal Films. The colloidal crystal film fabricated was observed by SEM, as shown in Figure 5. Figure 5A shows a typical lowmagnification image, which indicates that the flat surface of the film extends to a large region and the density of the cracks observed as bright lines is very small. Figure 5B shows a high-magnification image of a section containing a crack. This image confirms that the bright lines seen in Figure 5A are cracks having widths ranging from a fraction of the sphere diameter to several times the sphere diameter. In addition, this image suggests that the (111) planes of the face-centered-cubic (fcc) lattice are parallel to the glass substrate, and that there are many point and line defects in the film. The main cause of these defects is polydispersity of the silica colloidal spheres used (5%). Some point defects and all isolated spheres observed in the uppermost layer were generated when the top substrate was separated from the sample. To evaluate the orientation of the lattices in the plane parallel to the substrate, reflection spectra were measured at several incident angles in two perpendicular directions. Since the diameter of the incident light beam is approximately 5 mm, the spectra provide averaged data over the area corresponding to the light flux. Figure 6 shows the reflection spectra measured in the directions parallel and perpendicular to the long side of the cell; the directions are illustrated in Figure 6A. The reflection peaks observed were due to the Bragg diffraction resonance. In the parallel direction, the reflection peak monotonically shifted to shorter wavelengths with increasing incident angle. On the other hand, in the perpendicular direction the reflection peak was split into double peaks at the incident angles of 48° to 56°. Romanov et al. linked this phenomenon to photonic band branching at the U point of the Brillouin zone.13 This means that both the diffractions from (111) and (200) plane were observed at these incident angles. In our results, the appearance of the double peaks depended on the measurement direction of the reflection spectra, which was confirmed to have rotational symmetries through 60°. This suggests that the sphere arrangements in the measured area (∼5 mm
Discussion The mechanism whereby the present colloidal crystal films with a large domain (∼10 cm2) were fabricated using a simple cell is discussed on the basis of the structural analysis of condition A. In condition A, which spread gradually from the opening, water was present in the spaces between the arrays of spheres. This suggests that water evaporated only in the vicinity of the opening as long as the crystal film maintained condition A. This generated a one-directional flow toward the opening. The water evaporation pockmarked the air-water interface with nanomenisci, which create capillary tension pulling water through the array of spheres, keeping the interface fixed at the first layer of spheres.18 Thus, the dispersion in the cell flowed in only one direction, namely toward the opening. The spheres were carried toward the opening by the flow and were gradually ordered from the opening across the inside of the cell. The slow evaporation at room temperature possibly contributed to the epitaxial growth. The formation of the colloidal crystal film through transformation from condition A to condition C is thought to help minimize crack generation. In the present method, a close-packed array with direct contact between spheres was formed during the first stage of crystalline formation, that is, while the spaces between the arrayed spheres were filled with water (condition A). After that, the dried crystalline array (condition C) was obtained by maintaining the close-packed structure through gradual evaporation of the water. Thus, the change in the size of the array, and hence crack generation, was kept to a minimum. The issue of crack generation is not discussed further because of the difficulty associated with such a quantitative discussion. The light transmittance at wavelengths other than the Bragg diffraction resonance depended strongly on condition (A, B, or C), as shown in Figure 3B. The high transmittance of condition A was due to the small difference between the refractive indices of the spheres and the medium (1.46 and 1.33). The transmittance in condition C was lower than that in condition A since the large index difference (1.43 and 1.0) caused strong light scattering, mainly due to the Mie scattering. Condition B showed extremely low transmittance, though it involved almost the same refractive index as condition C. The low transmittance in condition B was probably due to strong scattering because condition B showed almost the same reflection properties as condition C. As condition B is a transition state between conditions A and C, it may show an inhomogeneous structure consisting of blocks larger than the sphere diameter. If condition B has such nonuniformity, it will show a stronger scattering and, hence, a lower transmittance compared to those of condition C. This will be clarified by further analysis of the structure of condition B, as well as theoretical analysis of light scattering. (18) Dufresne, E. R.; Corwin, E. I.; Greenblatt, N. A.; Ashmore, J.; Wang, D. Y.; Dinsmore, A. D.; Cheng, J. X.; Xie, X. S.; Hutchinson, J. W.; Weitz, D. A. Phys. Rev. Lett. 2003, 91, 224501.
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of 10 to 100 µm in thickness. Figure 7B shows the intensities and full width at half-maximum (fwhm) of the peaks obtained by subtracting backgrounds. These results obviously indicate that the colloidal film prepared using the cell with the spacers of 100 µm in thickness had less crystallinity than the others. In the cell used in the present study, disarray probably increases with the thickness of the spacers because the one-direction flow has no effect on controlling the arrangement in the through-thickness direction. Thus, the upper limitation of the thickness is around 50 µm for the experimental condition studied. The limitation will change for the experimental condition such as diameters of spheres and concentrations of dispersions. Dispersions with a wide range of concentrations are available for the fabrication of colloidal crystal films using the present cell, though only a 5 wt % dispersion was used in this study. Lower concentrations will require a longer fabrication time and higher supply of dispersion. Higher concentrations seemed to induce lower crystallinity because the growth rate is expected to be higher. Further experiments to verify the influence of concentration are in progress.
Figure 7. (A) Reflection spectra measured for samples prepared using the cells with the spacers of 10 to 100 µm in thickness. (B) Intensities and full width at half-maximum (fwhm) of the reflection peaks. Open and closed circles indicate peak intensities and fwhm of the peaks, respectively.
Silica was the only species of spheres used in this study. We can also utilize polystyrene spheres which are commercially available as monodispersed spheres. Since polystyrene spheres would shrink more than silica spheres as a result of drying, a polystyrene crystal film fabricated using the cell would presumably have more cracks. The thickness of the colloidal crystal film prepared in this study is governed by the thickness of the spacers used in the cell. Although the thickness used in this study was only 10 µm, we can use to a wider range of thicknesses to generate capillary action: from slightly larger than the sphere diameter to the thickness at which capillary action disappears. However, too thick spacers will decrease the crystallinity. Figure 7A shows reflection spectra measured for the samples fabricated using the cells with the spacers
Conclusions We investigated the growth of colloidal crystal films from a dispersion of silica spheres utilizing a fluidic cell with one opening. Large-domain colloidal crystal films were fabricated in the cell through three conditions (conditions A, B, and C). The three conditions showed different optical properties: high, extremely low, and medium transmittances were observed in conditions A, B, and C, respectively. Their structure was revealed by an analysis using angle-resolved reflection spectroscopy. A close-packed array already formed in condition A, which was the first stage of crystal formation. Since solvent evaporation occurred only at the opening, a one-directional flow was generated in the cell. The flow caused the spheres to be arranged epitaxially, resulting in the large domain size. Acknowledgment. We thank M. Kato for her assistance with reflection spectra measurements and analysis. Supporting Information Available: Photographs showing the growth and time evolution of the state of the colloidal crystal film. This material is available free of charge via the Internet at http://pubs.acs.org. LA050124V
