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Growth of Large Crystals of Monodispersed Spherical Colloids in Fluidic Cells Fabricated Using Non-photolithographic Methods Yu Lu,† Yadong Yin,† Byron Gates,‡ and Younan Xia* Department of Materials Science & Engineering and Department of Chemistry, University of Washington, Seattle, Washington 98195-1700 Received March 20, 2001. In Final Form: May 9, 2001 This paper describes a convenient approach to the fabrication of fluidic cells to be used for crystallizing spherical colloids into three-dimensionally periodic lattices over large areas. The major component of the fluidic cell was a rectangular gasket sandwiched between two glass substrates. Here we demonstrate that these gaskets could be simply cut out of commercial Mylar films. Three non-photolithographic methods were also demonstrated to create shadow channels between the Mylar film and two glass substrates: (i) by wiping (along one single direction) both sides of the Mylar film with a piece of soft paper (Kimwipes EX-L); (ii) by coating both surfaces of the Mylar film with polymer beads whose size was smaller than those to be packed in the cell; and (iii) by patterning the surface of the bottom glass substrate with an array of gold channels using a combination of microcontact printing and selective etching. When an aqueous dispersion of monodispersed spherical colloids was injected into this packing cell, a crystalline lattice nucleated and grew from the edge(s) of the cell as a result of solvent depletion through the channels between the Mylar film and the glass substrates. The capability and feasibility of this new approach have been demonstrated by the fabrication of uniform opaline lattices of polystyrene beads and silica colloids over areas as large as several square centimeters. Because Mylar films with thicknesses in the range 20-100 µm are commercially available in large quantities and at reasonably low costs, the present approach offers a flexible tool to those who want to explore the use of large crystals of spherical colloids but have no access to clean room facilities.
Introduction Crystalline arrays of monodispersed spherical colloids have been demonstrated or proposed for use in a wide variety of areas. Interesting examples include their use as precursors to produce high-strength ceramics;1 as model systems to investigate crystallization and phase transitions;2 as templates to generate porous materials with highly ordered structures;3 as diffractive elements to fabricate optical switches and smart sensors;4 and as prototypes of photonic band gap materials.5 These crystalline lattices are particularly useful in diffractive optics because their properties can be readily changed by * To whom correspondence should be addressed. E-mail: xia@ chem.washington.edu. † Department of Materials Science & Engineering. ‡ Department of Chemistry. (1) See, for example: (a) Sacks, M. D.; Tseng, T.-Y. J. Am. Ceram. Soc. 1984, 67, 526. (b) Calvert, P. Nature 1985, 317, 201. (2) (a) Murray, C. MRS Bull. 1998, 23 (10), 33. (b) Murray, C. A.; Grier, D. G. Am. Sci. 1995, 83, 238. (3) (a) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lenhoff, A. M. Nature 1997, 389, 447. (b) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (c) Park, S. H.; Xia, Y. Chem. Mater. 1998, 10, 1745. (d) Yang, P.; Deng, T.; Zhao, D.; Feng, P.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1998, 282, 2244. (e) Wijnhoven, J. E. G. J.; Vos, W. L. Science 1998, 281, 802. (f) Zakhidov, A. A.; Baughman, R. H.; Iqbal, Z.; Cui, C.; Khayrullin, I.; Dantas, S. O.; Marti, J.; Ralchenko, V. G. Science 1998, 282, 897. (g) Johnson, S. A.; Ollivier, P. J.; Mallouk, T. E. Science 1999, 283, 963. (h) Vlasov, Y. A.; Yao, N.; Norris, D. J. Adv. Mater. 1999, 11, 165. (i) Subramanian, G.; Manoharan, V. N.; Thorne, J. D.; Pine, D. J. Adv. Mater. 1999, 11, 1261. (j) Jiang, P.; Cizeron, J.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 7957. (k) Braun, P. V.; Wiltzius, P. Nature 1999, 402, 603. (l) Blanco, A.; Chomski, E.; Grabtchak, M. I.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Migues, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; van Driel, H. M. Nature 2000, 405, 437. (4) (a) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829. (b) Chang, S.-Y.; Liu, L.; Asher, S. A. J. Am. Chem. Soc. 1994, 116, 6739. (c) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959.
controlling a number of parameterssfor example, the size of the colloids, the refractive index contrast between the colloids and the surrounding medium, and the pitch of the three-dimensionally periodic lattices. The success of all these applications strongly depends on the ability to assemble monodispersed spherical colloids into crystalline lattices with a well-defined structure, a tightly controlled thickness, and sufficiently large domain sizes. A large number of different approaches have been demonstrated for crystallizing spherical colloids into threedimensionally periodic lattices.6 Several of them were, in particular, useful in generating relatively large crystalline lattices with well-controlled structures. For example, the method based on repulsive electrostatic interactions was able to organize highly charged colloids into body-centercubic (bcc) or face-center-cubic (fcc) crystals of up to several hundred layers in thickness.7 The approach based on attractive capillary forces (due to solvent evaporation) could be employed in a layer-by-layer fashion to generate opaline lattices of colloids with thicknesses from a monolayer up to 50 layers.8 Our group also demonstrated a versatile method based on hydrodynamic flow and physical confinement that allowed for the assembly of (5) (a) Tarhan, I. I.; Watson, G. H. Phys. Rev. Lett. 1996, 76, 315. (b) Mayoral, R.; Requena, J.; Moya, J. S.; Lo´pez, C.; Cintas, A.; Miguez, H.; Meseguer, F.; Va´zquez, L.; Holgado, M.; Blanco, A. Adv. Mater. 1997, 9, 257. (c) Donselaar, L. N.; Philipse, A. P.; Suurmond, J. Langmuir 1997, 13, 6018. (d) Miguez, H.; Meseguer, F.; Lo´pez, C.; Mifsud, A.; Moya, J. S.; Va´zquez, L. Langmuir 1997, 13, 6009. (e) van Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (f) Chen, K. M.; Jiang, X.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 26, 7825. (6) Recent reviews: (a) Densmore, A. D.; Crocker, J. C.; Xia, Y. Curr. Opin. Colloid Interface Sci. 1998, 3, 5. (b) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693. (7) (a) Flaugh, P. L.; O’Donnell, S. E.; Asher, S. A. Appl. Spectrosc. 1984, 38, 847. (b) Ise, N. Angew. Chem., Int. Ed. Engl. 1986, 25, 323. (c) Sunkara, H. B.; Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1994, 6, 362.
10.1021/la010419i CCC: $20.00 © 2001 American Chemical Society Published on Web 08/16/2001
Large Crystals of Monodispersed Spherical Colloids
spherical colloids into crystalline lattices over large areas and with layer thicknesses ranging from one up to several hundred monolayers.9 In this method, a packing cell was fabricated by sandwiching a square-shaped frame of a photoresist pattern between two glass substrates. An array of trenches was also generated on the surface of the photoresist pattern that could serve as channels for the flow of solvent. When an aqueous dispersion of spherical colloids was injected into this cell, the colloids were retained, concentrated, and subsequently transformed into an opaline array within the confinement of the cell under continuous sonication. The crystal formed in this way had its (111) planes parallel to the surfaces of both glass substrates. The total number of (111) planes was determined by the thickness of the photoresist film relative to the diameter of the spherical colloids, and these two parameters could be changed in a controllable fashion. Although these three methods have been effective in generating well-defined crystals of colloids with large domain sizes, they all need to be improved before they will become methods of choice for large-scale or low-cost fabrication tasks. The first approach, for example, requires strong repulsive interactions between colloids, which can only be achieved by increasing the density of charges on the colloidal surface and decreasing the concentrations of electrolytes (through ion-exchange) in the dispersion medium. The second method may become problematic (mainly due to the time-consuming aspect) when crystals more than hundreds of layers are needed. The third method, on the other hand, requires the extensive use of clean room facilities that might not be accessible to chemists and material scientists. It has also been difficult to fabricate 3D crystalline lattices of colloidal particles with thicknesses >25 µm because of the difficulties involved in patterning thick photoresist films. Here we wish to report an approach that was remarkable for its simplicity in fabricating the fluidic cells; for its speed in generating crystalline lattices of spherical colloids over large areas; and for its effectiveness in controlling the structure and thickness of the 3D opaline lattice. This approach represents a modification to the previous method based on hydrodynamic flow, and the most significant improvement is the substitution of the photoresist pattern with a gasket structure cut from a piece of Mylar thin film. All the steps involved in this new approach could be carried out under ambient laboratory conditions without using clean room facilities. More importantly, this new approach also provides a control over the number of (111) planes of the 3D crystalline lattice by varying the ratio between the thickness of the Mylar film and the size of the colloidal particles. The largest crystal we have generated using this approach was approximately 20 µm × 6 cm2 in size. Experimental Section Materials and Substrates. Monodispersed polystyrene (PS) beads (∼240 nm in diameter, 1 wt %) and silica colloids (∼1.0 µm in diameter, 1 wt %) were purchased from Duke Scientific (Palo Alto, CA). Carboxylate-terminated PS beads (∼226 nm in diameter, 10 wt %, PA ) 5) were obtained from Seradyn (8) (a) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. (b) Dushkin, C. D.; Nagayama, K.; Miwa, T.; Kralchevsky, P. A. Langmuir 1993, 9, 3695. (c) Lazarov, G. S.; Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Nagayama, K. J. Chem. Soc., Faraday Trans. 1994, 90, 2077. (d) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303. (e) Jinag, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (9) (a) Park, S. H.; Qin, D.; Xia, Y. Adv. Mater. 1998, 10, 1028. (b) Park, S.; Xia, Y. Langmuir 1999, 15, 266. (c) Mayers, B. T.; Gates, B.; Xia, Y. Adv. Mater. 2000, 12, 1629.
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Figure 1. (A) Schematic illustration of the fluidic cell that was used to crystallize spherical colloids into 3D opaline lattices. (B-D) Three different approaches that have been demonstrated for generating small channels that could let the solvent flow while retaining the spherical colloids to the cell: (B) by wiping the Mylar film with a piece of soft paper to generate shadow scratches; (C) by decorating the surface of the Mylar film with some PS beads with sizes less than (or equal to) those of the spherical colloids to be crystallized in the cell; and (D) by covering the surface of the bottom substrate with channel structures patterned in a thin film of gold. (Indianapolis, IN). Silica colloids (∼450 nm in diameter, 5 wt %) were purchased from Polyscience (Warrington, PA). Silica colloids (∼300 nm in diameter, 40 wt %) were supplied (as a free gift) by Nissan Chemicals (New York, NY). All colloidal dispersions were used as-received without further dilution or other modification. Precleaned glass slides (Micro slides, #2947) were obtained from Corning Glass (Corning, NY). Thin Mylar films with thickness in the range 20-100 µm were obtained from Fralock (Canoga, CA). Fabrication of Packing Cells with Mylar Film as the Spacer. The major component of the packing cell is a gaskettype structure sandwiched between two flat glass substrates (Figure 1A). In our previous work,9 this structure was patterned in a thin film of photoresist using a two-step photolithographic method. Here we have replaced the photoresist film with an ordinary polymer film, in an effort to avoid the use of clean room facilities. Any polymer film that meets the following two requirements should be suitable for this purpose: (i) the surface of the film must be smooth enough to provide packing cells that can retain colloidal particles as small as ∼200 nm in size; and (ii) the surface of the polymer film should be soft enough to ensure the formation of conformal contact between rigid glass substrates and the polymer film. We have tested a variety of thin films of organic polymers and found that the Mylar film manufactured by Fralock worked best for this purpose. The gasket-type structure (rectangular or square-shaped frame) could be directly cut out of the Mylar film with a razor blade. Figure 2A shows an AFM image of the surface of a Mylar film that had been washed with deionized water. Note that the surface of this film was remarkably smooth over relatively large areas, and the main features one could find were some very small bumps with heights less than ∼20 nm. When this film was directly used to fabricate the packing cell, no 3D crystalline lattice of colloids formed in the cell because the colloidal dispersion could not be concentrated (via the removal of solvent) to a concentration high enough for the disorder-order phase transition.10 When the Mylar film was used as-received (without cleaning), the dust particles on their surfaces provided some additional spaces between the glass substrates and the Mylar film, and the solvent could flow or evaporate through these void spaces. In this case, the colloids could be accumulated or concentrated at the edge(s) of the packing cell as a result of solvent depletion, and eventually be crystallized into a highly ordered 3D lattice. Although this (10) Gast, A. P.; Russel, W. B. Phys. Today 1998, December, 24.
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Figure 2. (A) AFM image of the surface of a pristine Mylar film. (B) AFM image of a Mylar film whose surface had been wiped with a piece of soft paper, showing a typical scratch created by this process. The inset shows a cross-sectional profile of this scratch. (C) SEM image of a Mylar film whose surface had been coated with some 255-nm PS beads. (D) Optical micrograph of a 2D array of channels that were patterned in a thin film of gold (∼300 nm thick, sputtered on the surface of the bottom glass substrate) using a combination of µCP and selective wet etching. The inset shows the profile of these gold channels measured using an Alpha-Step profilometer. procedure was simple and straightforward to follow, the yield of this process was too low (
