pubs.acs.org/Langmuir © 2009 American Chemical Society
Formation of Optically Anisotropic Films from Spherical Colloidal Particles Susumu Inasawa* and Yukio Yamaguchi Department of Chemical System Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan Received May 8, 2009. Revised Manuscript Received June 24, 2009 Colloidal silica films, formed by the drop evaporation method, showed birefringent spherulite optical properties. They displayed a Maltese cross pattern under crossed polarizers, and interference colors, such as blue and orange-red, under crossed polarizers with a compensator. The difference in refractive index was estimated to be 9 10-4 from the interference colors. Scanning electron microscopy (SEM) results revealed anisotropic structures in the colloidal films. Particles formed radially ordered hexagonal arrays. The drop evaporation method used in this report, which dries from the edge to the center, resulted in a radially ordered colloidal film. When a colloidal silica film was prepared using a unidirectional drying method, particles were packed in an ordered structure corresponding to the drying direction and the resulting film showed different birefringent optical properties. Our results show that a variety of birefringent films can be obtained from spherical colloidal dispersions through control of the drying method.
1. Introduction Colloidal thin films that have a periodic structure in three dimensions are called colloidal crystals. Because these colloidal crystals have a periodic distribution to their refractive index, photonic bandgaps are formed in which light of a specific wavelength range cannot propagate.1 Colloidal crystals are currently being studied because of these interesting properties.2-4 Because the simplest technique to form a colloidal film is to coat a substrate with colloids, many preparation methods, such as spin coating,5 Langmuir-Blodgett (LB)-based methods,6 thermally induced unidirectional crystallization,7 vertical deposition,8,9 and convective assembly,10 have been proposed. In most cases, coating involves the evaporation of a solvent, leaving the colloidal particles in an ordered structure that is formed during this drying process. Although the evaporation of solvents is a ubiquitous phenomenon, the physical picture of the drying process of colloidal suspensions is not fully understood. This is because many physical phenomena, such as the evaporation of the solvent,11 Marangoni flow,12 capillary flow inside the droplet,13,14 the trapping of particles at the air/solvent interface,15 and the consolidation of particles,16,17 occur at the same time. At the last *Author to whom correspondence should be addressed. E-mail: inasawa@ chemsys.t.u-tokyo.ac.jp.
(1) Vlasov, Y. A.; Bo, Z. S.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289. (2) Busch, K.; John, S. Phys. Rev. E 1998, 58, 3896. (3) Subramania, G.; Biswas, R.; Constant, K.; Sigalas, M. M.; Ho, K. M. Phys. Rev. B 2001, 63, 235111. (4) Nagy, N.; Deak, A.; Horvolgyi, Z.; Fried, M.; Agod, A.; Barsony, I. Langmuir 2006, 22, 8416. (5) Jiang, P.; McFarland, M. J. J. Am. Chem. Soc. 2004, 126, 13788. (6) Marquez, M.; Grady, B.; P. Langmuir 2004, 20, 10998. (7) Toyotama, A.; Yamanaka, J.; Shinohara, M.; Onda, S.; Sawada, T.; Yonese, M.; Uchida, F. Langmuir 2009, 25, 589. (8) Zhang, T.; Tuo, X.; Yuan, J. Langmuir 2009, 25, 820. (9) Wang, L.; Zhao, X. S. J. Phys. Chem. C 2007, 111, 8538. (10) Prevo, B. G.; Velev, O. D. Langmuir 2004, 20, 2099. (11) Kuncicky, D. M.; Velev, O. D. Langmuir 2008, 24, 1371. (12) Hu, H.; Larson, R. G. J. Phys. Chem. B 2006, 110, 7090. (13) Tirumkudulu, M. S.; Russel, W. B. Langmuir 2004, 20, 2947. (14) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827. (15) Bigioni, T. P.; Lin, X. M.; Nguyen, T. T.; Corwin, E. I.; Witten, T. A.; Jaeger, H. M. Nat. Mater. 2006, 5, 265. (16) Juillerat, F.; Bowen, P.; Hofmann, H. Langmuir 2006, 22, 2249. (17) Sarkar, A.; Tirumkudulu M. S. Langmuir 2009, 25, 4945.
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stage of drying, the solvent between particles is replaced with air (air invasion)18,19 and cracks often form.20-23 These cracks propagate as the solvent continues to evaporate.22,23 While the preparation of a colloidal film by coating is uncomplicated, the processes involved in their formation are complex, and this has proven to be a fruitful area for physics and physical chemistry research. Obtained colloidal arrays generally are optically evaluated. Both transmission and reflectance measurements have been conducted to examine the photonic bandgaps of the crystals.1,3 This is because one of the main industrial target applications is use as an optical device that is based on the photonic bandgap. Light diffraction from colloidal arrays can also be used to detect chemical species by observing the diffraction pattern.24 Thus, colloidal crystals have the potential for practical applications in a variety of fields. Although extensive studies have been conducted on colloidal crystals, most have focused on the optical responses, such as absorption/transmission and reflection by the crystals. However, little is known about the refractive indices of these films. The refractive index is the one of the dominant factors in determining the optical properties of materials. A few papers have briefly reported effective refractive indices of colloidal films;3,4 however, few papers have reported anisotropy in these refractive indices. Because the photonic bandgaps and diffraction wavelengths are strongly dependent on the structure of the colloidal crystals, the overall refractive indices of colloidal films are also thought to be dependent on the structures of the colloidal films. Optically anisotropic properties of colloidal films are seen when colloidal particles with anisotropic shapes align in a specific direction. The birefringence of colloidal films that consist of nonspherical particles, such as single-walled carbon nanotubes (SWCNTs)25 (18) Xu, L.; Davies, S.; Schofield, A. B.; Weitz, A. D. Phys. Rev. Lett. 2008, 101, 094502. (19) Shaw, T. M. Phys. Rev. Lett. 1987, 59, 1671. (20) Singh, K. B.; Tirumkudule, S. Phys. Rev. Lett. 2007, 98, 218302. (21) Tirumkudule, M. S. Langmuir 2005, 21, 4938. (22) Dufresne, E. R.; Stark, D. J.; Greenblatt, N. A.; Cheng, J. X.; Hutchinson, J. W.; Mahadevan, L.; Weitz, D. A. Langmuir 2006, 22, 7144. (23) Gauthier, G.; Lazarus, V.; Pauchard, L. Langmuir 2007, 23, 4715. (24) Asher, S. A.; Kimble, K. W.; Walker, J. P. Chem. Mater. 2008, 20, 7501. (25) Duggal, R.; Hussain, F.; Pasquali, M. Adv. Mater. 2006, 18, 29.
Published on Web 07/09/2009
DOI: 10.1021/la901642b
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and rod-shaped cadmium selenide (CdSe) nanoparticles,26 have all been reported. The radial structure of these films is called a spherulite structure, and this is the origin of the observed birefringence.25 Spherulites are frequently formed in the crystallization of polymers27-32 and liquid crystals.33 In these cases, the formation of chainlike groupings of molecules is responsible for the spherulite structure. Although one clearly can form birefringent thin films from anisotropic particles/molecules such as the examples previously mentioned, little work has been done to investigate if similar birefringent materials can be made from optically isotropic spherical particles. This leads to the motivation for this study: to form an oriented, ordered film from spherical colloids and evaluate the birefringence of the colloidal film. The films obtained from a colloidal suspension have been characterized by optical microscopy with a set of polarized filters with and without a compensator. The difference in the refractive indices of the films has been evaluated quantitatively from the optical characterizations. The surface structures of films have been analyzed by electron microscopy.
2. Experimental Section Colloidal silica dispersed in water (KE-W10, Nihon Shokubai) was used as supplied. The average diameter of silica colloids was ca. 120 nm. The colloidal films were obtained by a drop drying method. An aliquot of 2 μL of the colloidal suspension (11% by volume) was deposited dropwise on a cover glass and dried under ambient conditions. During drying, the contact line of the solvent moved from the edge to the center. When the drying was complete, the colloidal film was observed using an optical microscope (SMZ800, Nikon) equipped with polarized filters (SPF-50C-32, Sigma Koki) and a compensator (HI-RETAX-1λ, Luceo). The sample was irradiated from underneath the sample stage. Observed images were recorded with a digital camera (D1x, Nikon). When the film was being observed under polarized light, a sample colloidal film was placed between the polarizer and the analyzer filters and the two filters were set to crossed polarization. To observe the degree to which the films retarded the phase of the transmitted light, a compensator, which introduces a retardation in phase with the transmitted light, was inserted between the film and the analyzer. The slow axis of the compensator was oriented 45° to the polarization directions.34 Compensators cause a larger retardation of the phase, resulting in interference colors. These colors were analyzed with a spectrophotometer (PMA-11, Hamamatsu) through one ocular lens of the microscope. Two types of latex microspheres (5014B and 5100B, 10 vol %, Duke Scientific) with different sizes (140 nm and 1 μm in diameter, respectively), were also used. Furthermore, colloidal silica with different sizes (50 nm in diameter, 10 vol %, Snowtex OL, Nissan Chemical) was used. These colloidal particles were dispersed in water and used as received. For unidirectional drying experiments, a small glass chip was constructed. Small square glass capillary tubes (ST8505, Vitrocom, 100 μm 100 μm) were used as spacers. A pair of glass tubes with a length of ca. 6 mm was put on a cut cover glass (26) Talapin, D. V.; Shevchenko, E. V.; Murray, C. B.; Kornowski, A.; F€orster, S.; Weller, H. J. Am. Chem. Soc. 2004, 126, 12984. (27) Tuteja, B.; Moniruzzaman, M.; Sundararajan, P. R. Langmuir 2007, 23, 4709. (28) Yun, J. H.; Kuboyama, K.; Ougizawa, T. Polymer 2006, 47, 1715. (29) Yun, J. H.; Kuboyama, K.; Chiba, T.; Ougisazawa, T. Polymer 2006, 47, 4831. (30) Yoshioka, T.; Fujimura, T.; Manabe, N.; Yokota, Y.; Tsuji, M. Polymer 2007, 48, 5780. (31) Nishida, R.; Takahashi, M. Polym. J. 2008, 40, 148. (32) Terech, P.; Sangeetha, N. M.; Maitra, U. J. Phys. Chem. B 2006, 110, 15224. (33) Fernandez-Nieves, A.; Link, D. R.; Marquez, M.; Weitz, D. A. Phys. Rev. Lett. 2007, 98, 087801. (34) Scharf, T. Polarized Light in Liquid Crystals and Polymers; John Wiley and Sons: Hoboken, NJ, 2007.
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Figure 1. Schematic illustration of the handmade glass chip used for the unidirectional drying experiment: (a) a colloidal suspension (ca. 2 μL) was introduced into the narrow space in the chip; (b) after evaporation of the solvent water, a colloidal film was formed at the narrow end; and (c) the top cover glass was removed prior to scanning electron microscopy (SEM) observation of the film. (4 mm 8 mm). The distance between the two capillary tubes was not uniform, roughly 1.5 mm at one end, increasing to 3 mm at the other, to ensure that the colloidal particles accumulated at the narrow end of the chip during drying. The spacers were fixed to the glass substrate with optical glue (NOA 61, Norland). After curing under ultraviolet (UV) irradiation, another cut cover glass was fixed to the top of the spacers to create a narrow space with two open ends. A small amount of grease was used to fix the top cover glass, which enabled it to be removed after the film had formed. This allowed direct observation of the surface structure of the films. A schematic illustration of the handmade glass chip for unidirectional drying is shown in Figure 1. The surface structure of the colloidal films were observed via scanning electron microscopy (SEM) (Model S-900, Hitachi) with an acceleration voltage of 6 kV. The same optical glue was used to support the silica films for these observations, because the colloidal films did not adhere well to a glass substrate and were easily moved. A small amount of optical glue was inserted between the film and the substrate and cured with UV light. Prior to observation, the sample was inspected to confirm that the glue support had not affected the shapes and structures of colloidal films.
3. Results Optical microscope images of a typical colloidal film are shown in Figure 2, as formed using a simple drop drying method. Cracks can clearly be seen in the natural light image (see Figure 2a). As shown in Figure 2b, the film exhibits birefringence with a Maltese cross pattern being observed under crossed polarizing filters. When the phase retardation caused by the birefringence of the film was investigated with a compensator, bright blue and orange-red colors were observed (see Figure 2c). The Maltese cross pattern is one of the typical birefringent properties of a spherulite.26,28,32,33 Interference colors were not observed under crossed polarizers, indicating that the difference in refractive indices was not large and the retardation in phase caused by the colloidal film was small.29 This small retardation can be visualized as a color difference by inserting a compensator.34 In the interference color image shown in Figure 2c, the first and third quadrants are blue, while the other two are orange-red. This means that the birefringent film is optically positive.34 The spatially resolved spectra of the observed interference colors are shown in Figure 2d. Each region exhibits a minimum in the light intensity at 534 nm, 611 nm, and 465 nm for the background purple, bright blue, and orange-red regions, respectively. The effect of the sizes and materials of the colloidal particles on the birefringence of the formed colloidal films were studied. Silica Langmuir 2009, 25(18), 11197–11201
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Figure 2. Optical microscopy images of the colloidal film under (a) normal light, (b) crossed polarizers, and (c) crossed polarizers with a compensator. The direction of the Maltese cross shown in panel b corresponds to the direction of the crossed polarizers. Visible light spectra of the observed interference colors are shown in panel d; all spectra are normalized to the maximum intensity. The black, blue, and red solid lines correspond to the spectra of the background purple, bright blue, and orange-red regions respectively shown in panel c. The inset in panel d is an enlargement of the orange-red spectrum and is shown to clarify the low-intensity region. The diameter of the film was ∼2 mm.
particles with a diameter of 50 nm and latex particles with diameters of 140 nm and 1 μm were used. The different film patterns after complete evaporation are shown in Figure 3. The mechanism of the formation of these patterns is another interesting topic, which has been extensively studied by many groups.11,14,35,36 For both the smaller particles of silica (50 nm) and latex (140 nm), the dried films clearly exhibited the optical properties of spherulites (see Figures 3b, 3c, 3e, and 3f). However, the colloidal film of the 1 μm latex particles did not show any birefringence. One possible reason is the scattering effect of the larger latex particles. The film in Figure 3g is darker than those of the smaller particles. This suggests that most of the light is not transmitted through the film, because of scattering. Detailed study into the effect of particle sizes on birefringence in colloidal films is an important subject; however, it is beyond the scope of this paper. After complete evaporation of the solvent, the colloidal films became filled with air. To understand the effect of the surrounding media on the birefringence of colloidal films, a drop of water was added to a dried film. The original colloidal film (Figure 4a) shows birefringence in Figure 4d. However, the film that has been refilled with water (Figure 4b) does not show any birefringence (see Figure 4e). After the water had evaporated once again, (Figure 4c), additional cracks had formed and the birefringence had reappeared (see Figure 4f). The refractive indices of water, silica, and air are ∼1.33, ∼1.46, and ∼1.00, respectively.37 Despite the difference in the refractive index of water and silica being a nontrivial 0.13, no birefringence can be observed. These results indicate that the difference in refractive index between particles and the surrounding media must be larger than 0.13 to observe birefringence. The surface structures of the colloidal film were observed by SEM. Figure 5 shows both optical and SEM images. The colloidal particles are closely packed on the surface of the film. The Fourier (35) Deegan, R. D. Phys. Rev. E 2000, 61, 475. (36) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. Rev. E 2000, 62, 756. (37) Handbook of Chemistry and Physics, 83rd Edition: CRC Press: Boca Raton, FL, 2002.
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Figure 3. Optical microscopy images of the different colloidal films. (a-c) 50-nm silica colloidal particles, (d-f) 140-nm latex particles, and (g-i) 1-μm latex particles. The images were obtained under normal light (panels a, d, and g), crossed polarized filters (panels b, e, and h), and crossed polarized filters with the compensator (panels c, f, and i). The scale bar represents 1 mm.
Figure 4. Effect of rehydration on optical properties of a colloidal film: (a) the original colloidal film, (b) after the addition of water, and (c) after the evaporation of water. Panels d-f correspond to images observed under crossed polarizers with a compensator.
transform of the SEM image is shown in the inset of Figure 5b. It shows a hexagonal pattern, indicating that particles are packed in an ordered structure.
4. Discussion 4.1. Origin and Evaluation of Birefringence in Colloidal Films. Generally, the spherulites of polymers and nonspherical particles have a radial structure.25,28-30,33 In our case, spherical particles are closely packed and formed an ordered structure, as seen in Figure 5. Because our particles are optically isotropic spherical particles, the packed structure of particles must be anisotropic to exhibit birefringence in a colloidal film. The direction of the smallest hexagonal shape in the Fourier-transformed images was chosen for analysis because it represents the largest ordered structures in the SEM image. The direction of the hexagonal shape was defined as follows. The angle of the observed point on the colloidal film was defined as θ, as depicted in Figure 5a. The hexagonal shape in a Fourier-transformed image has three pairs of sides, and each pair is oriented in a different DOI: 10.1021/la901642b
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Figure 2d, the relative light intensity, T, as transmitted by uniaxial anisotropic materials, is described as πΔnd ð1Þ TðλÞ ¼ sin2 λ where λ is the wavelength of light, Δn the difference in refractive indices between nr and nθ, and d the thickness of the material.34 Because a compensator has been used, eq 1 should be modified to consider the optical path difference introduced by the compensator (Δ1), as34 πðΔnd ( Δ1 Þ ð2Þ TðλÞ ¼ sin2 λ
Figure 5. (a) Optical and (b) SEM images of the colloidal silica film. The SEM image shown in panel b was taken at the white dot shown in panel a. The white dashed lines in panel a represent the center position of the colloidal film. The inset in panel b is the Fourier transformed image of panel b. The scale bar in panel b represents 1 μm. A scheme illustrating “the direction of the hexagonal shape” in the Fourier transformed image is shown in panel c.
Figure 6. Relationship between θ and θSEM. The solid line represents θ = θSEM.
direction. Among these three angles, the angle that was closest to θ was selected and defined the direction of the hexagonal shape, θSEM, as shown in Figure 5c. The values of θ and θSEM were between 0° and 180°. The relationship between θ and θSEM is shown in Figure 6. The value of θSEM is almost the same as θ, indicating that the hexagonal arrays of colloidal particles are oriented toward the center of the film. Therefore, we conclude that the origin of the spherulite’s optical properties in the colloidal films is the radial structure of the particles. Colloidal crystals with well-aligned radial structures are obtained by spin coating, and these crystals show diffraction patterns with 6-fold symmetry.5 Compared to these well-aligned colloidal crystals, our films are less circularly symmetric, as shown in Figures 5b and 6, and we do not observe any clear diffraction. This implies that the formation of a “highlyordered alignment” of colloidal particles is not required to realize birefringent films, whereas almost-complete alignments of colloidal particles are necessary to obtain photonic crystals. The difference in refractive indices in the silica colloidal film was also evaluated. Because the colloidal films have formed radial structures, the refractive index in the radial direction (nr) will be different from that in the circumferential direction (nθ). To analyze the spectra of the interference colors, as shown in 11200 DOI: 10.1021/la901642b
A compensator introduces interference colors by adding or subtracting Δ1 from the phase of the transmitted light. The orientation of the optically slow axis of the compensator relative to that of the birefringent materials determines whether Δ1 is added or subtracted.34 This corresponds to the plus or minus sign before Δ1 in eq 2, with the addition region being bright blue and the minus region being orange-red in Figure 2. From eq 2, when π(Δnd ( Δ1)/λ is equal to mπ, and m is an integer, the intensity of the transmitted light is zero. The compensator used here prohibits the transmission of light at ∼530 nm and the complementary color, purple, is observed in the background in Figure 2c. The spectrum of this region is shown as the black line in Figure 2d, and the transmitted light intensity is almost zero at a wavelength of 534 nm. This corresponds to the optical path difference of the compensator, Δ1. Because no interference colors were observed under crossed polarized filters in Figure 2b, the magnitude of the retardation caused by the film (|Δnd|) must be
