266
Langmuir 1999, 15, 266-273
Assembly of Mesoscale Particles over Large Areas and Its Application in Fabricating Tunable Optical Filters Sang Hyun Park and Younan Xia* Department of Chemistry, University of Washington, Seattle, Washington 98195-1700 Received June 4, 1998. In Final Form: September 2, 1998 This paper describes a method for the crystallization of mesoscale particles over areas as large as ∼1 cm2. We injected an aqueous dispersion of spherical particles into a cell formed by two glass substrates and a square frame of photoresist that had been patterned on the surface of one of the substrates. One side of the frame had channels on its surface that could retain the particles while letting the solvent flow through. External gas pressure and sonication drove the particles into a cubic-close-packed (ccp) assembly with the (111) face parallel to the surfaces of the glass substrates. The smallest particles that have been crystallized by this method are polystyrene beads of ∼60 nm in diameter. The procedure presented here offers a number of attractive features: (i) It is relatively fast. For example, polystyrene beads of 0.48 µm in diameter can be crystallized into a 25 layer assembly over an area of ∼1 cm2 in ∼48 h. (ii) It has a tight control over the surface morphology and the number of layers of the crystalline assemblies. (iii) It works for aqueous dispersions of a wide range of spherical particles regardless of their chemical compositions and/or surface properties (such as charge densities and chemical functional groups). We have also demonstrated fabrication of tunable optical filters from these crystalline assemblies of particles. Each crystalline assembly effectively rejects a narrow wavelength interval (as determined by the Bragg condition) in the spectral region ranging from ultraviolet to near-infrared. The tunability (in terms of wavelength) of the filter can be achieved by changing the angle between the incident light and the normal to the surface of the filter, or by using particles with different diameters.
Introduction Mesoscale (∼10 nm to ∼10 µm) particles are probably the most commonly encountered forms of materials. They are also ubiquitous in chemistry and biology: typical examples include dendrimers, quantum dots, gold colloids, silica colloids, and latex particles in chemistry and proteins, viruses, and cells in biology.1 Monodispersed spherical particles of a wide range of materials now can be readily prepared in large quantities using procedures based on controlled nucleation or emulsion polymerization.1b,2 The ability to assemble these particles into twodimensional (2-D) or three-dimensional (3-D) crystalline structures is directly useful in many areas. For example, 2-D crystalline lattices of polystyrene beads can be used as arrays of microlenses in imaging,3 as physical masks for evaporation or reactive ion etching to fabricate regular arrays of micro- or nanostructures,4 and as masters to cast elastomeric stamps for use in microcontact printing;5 3-D crystalline lattices of colloidal particles can be used as templates to generate porous membranes6 or as * To whom correspondence should be addressed. E-mail: xia@ chem.washington.edu. (1) (a) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219-226. (b) Microspheres Catalog; Polysciences; Inc.: Warrington, PA, 1998. (2) See, for example: Matijevic, E. Langmuir 1994, 10, 8-16. (3) Hayashi, S.; Kumamoto, Y.; Suzuki, T.; Hirai, T. J. Colloid Interface Sci. 1991, 144, 538-547. (4) (a) Roxlo, C. B.; Deckman, H. W.; Gland, J.; Cameron, S. D.; Chianelli, R. R. Science 1987, 235, 1629-1631. (b) Buncick, M. C.; Warmack, R. J.; Ferrell, T. L. J. Opt. Soc. Am. B 1987, 4, 927-933. (c) Lenzmann, F.; Li, K.; Kitai, A. H.; Sto¨ver, H. D. H. Chem. Mater. 1994, 6, 156-159. (d) Burmeister, F.; Scha¨fle, C.; Matthes, T.; Bo¨hmisch, M. Langmuir 1997, 13, 2983-2987. (e) Boneberg, J.; Burmeister, F.; Scha¨fle, C.; Leiderer, P.; Reim, D.; Fery, A.; Herminghaus, S. Langmuir 1997, 13, 7080-7084. (f) Padeste, C.; Kossek, S.; Lehmann, H. W.; Musil, C. R.; Gobrecht, J.; Tiefenaur, L. J. Electrochem. Soc. 1997, 143, 38903895. (5) Xia, Y.; Tien, J.; Qin, D.; Whitesides, G. M. Langmuir 1996, 12, 4033-4038.
precursors to produce high-strength ceramics;7 and 3-D crystalline assemblies of polymer beads can be used as diffractive elements in fabricating sensors8 or optical components such as gratings,9 filters,10 switches,11 and photonic bad gap crystals.12 Crystalline lattices of colloidal particles have also been used as model systems to study fundamental phenomena such as crystallization,13 phase transition,14 and fracture mechanics.15 Most of these applications depend on the availability of crystalline assemblies of mesoscale particles of various materials with large enough domain sizes that can be easily adopted for macroscopic and optical measurements. A number of methods have been demonstrated for the crystallization of mesoscale particles.16-22 The method based on solvent evaporation and capillary attractive force is very effective in forming 2-D assemblies of mesoscale particles and proteins; these assemblies are polycrystalline with domain sizes usually less than 10-2 mm2.16 Sedimentation provides a simple and efficient route to mul(6) (a) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lenhoff, A. M. Nature 1997, 389, 447-448. (b) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538-540. (7) (a) Sacks, M. D.; Tseng, T.-Y. J. Am. Ceram. Soc. 1984, 67, 526532. (b) Calvert, P. Nature 1985, 317, 201. (8) (a) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829-832. (b) Holtz, J. H.; Holtz, J. S. W.; Munro, C. H.; Asher, S. A. Anal. Chem. 1998, 70, 780-791. (9) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959-960. (10) (a) Flaugh, P. L.; O’Donnell, S. E.; Asher, S. A. Appl. Spectrosc. 1984, 38, 847-850. (b) Spry, R. J.; Kosan, D. J. Appl. Spectrosc. 1986, 40, 782-784. (11) Chang, S.-Y.; Liu, L.; Asher, S. A. J. Am. Chem. Soc. 1994, 116, 6739-6744. (12) (a) Tarhan, I. I.; Watson, G. H. Phys. Rev. Lett. 1996, 76, 315318. (b) Vos, W. L.; Megens, M.; van Tats, C. M.; Bo¨secke J. Phys. Condens. Mater. 1996, 8, 9503-9507. (c) Miguez, H.; Meseguer, F.; Lo´pez, C.; Blanco, A.; Moya, J. S.; Requena, J.; Mifsud, A.; Forne´s, V. Adv. Mater. 1998, 10, 480-483. (13) Murray, C. A.; Grier, D. G. Am. Sci. 1995, 83, 238-245. (14) Larsen, A. E.; Grier, D. G. Phys. Rev. Lett. 1996, 76, 3862-3865. (15) Skjeltorp, A. T.; Meakin, P. Nature 1988, 335, 424-426.
10.1021/la980658e CCC: $18.00 © 1999 American Chemical Society Published on Web 12/09/1998
Assembly of Mesoscale Particles over Large Areas
Langmuir, Vol. 15, No. 1, 1999 267
tilayer assemblies of mesoscale particles. Procedures based on this strategy are relatively slow: it takes weeks to months to completely sediment submicron particles. The approach based on sedimentation also has very little control over the morphology of the top surface and the number of layers of the crystalline assemblies.18 The methodology based on electrostatic interactions seems to be the most powerful and successful one for generating multilayer assemblies of mesoscale particles: 3-D crystalline assemblies as large as ∼1 cm3 have been generated.20 This method, however, has very strict requirements on the experimental conditions such as the density of charges on the surface of the particle, the concentration of particles, and the concentration of free electrolyte molecules in the dispersion medium. The particles in the crystalline assemblies formed by this method are usually separated by a certain distance due to the repulsive electrostatic interactions among the particles. Here we would like to describe a method that is capable of producing crystalline assemblies of mesoscale particles of a variety of materials over relatively large areas (∼1 cm2).23 This method is relatively fast, and has a tight control over the surface morphology and the number of layers of the crystalline assemblies. The crystalline assemblies formed by this method are three-dimensionally periodic structures; they can be used as tunable optical notch filters that reject narrow wavelength intervals (as determined by the Bragg condition) of light in the spectral region ranging from ultraviolet to near-infrared. Experimental Section Materials and Substrates. Monodispersed polystyrene (PS) and poly(methyl methacrylate) (PMMA) beads (aqueous dispersions, ∼60 nm to ∼3 µm in diameter) were obtained from Polysciences (Warrington, PA) or Seradyn (Indianapolis, IN). Monodispersed colloidal particles of silica (aqueous dispersions, ∼300 and ∼450 nm in diameter) were obtained from Nissan Chemical Industries (Tarrytown, NY). All dispersions were diluted to ∼0.05% (wt) with deionized water before use. Precleaned glass substrates (Micro Slides, #2947) were obtained from Corning Glass Works (Corning, NY). Ultraviolet-curable (16) The method based on solvent evaporation: (a) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183-3190. (b) Dushkin, C. D.; Nagayama, K.; Miwa, T.; Kralchevsky, P. A. Langmuir 1993, 9, 3695-3701. (c) Lazarov, G. S.; Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Nagayama, K. J. Chem. Soc., Faraday Trans. 1994, 90, 2077-2083. (d) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303-1311. (e) Rakers, S.; Chi, L. F.; Fuchs, H. Langmuir 1997, 13, 7121-7124. (17) The method based on Lagmuir film technique: (a) Fulda, K.-U.; Tieke, B. Adv. Mater. 1994, 6, 288-290. (b) Kondo, M.; Shinozaki, K.; Bergstro¨m, L.; Mizutani, N. Langmuir 1995, 11, 394-397. (18) The method based on sedimentation: (a) 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, 257-260. (b) Donselaar, L. N.; Philipse, A. P.; Suurmond, J. Langmuir 1997, 13, 6018-6025. (c) Miguez, H.; Meseguer, F.; Lo´pez, C.; Mifsud, A.; Moya, J. S.; Va`zquez, L. Langmuir 1997, 13, 6009-6011. (19) The method based on electrophoretic deposition: (a) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408-3413. (b) Trau, M.; Saville D. A.; Aksay, I. A. Science 1996, 272, 706-709. (c) Yeh, S.-R.; Seul, M.; Shraiman, B. I. Nature 1997, 386, 57-59. (d) Solomentsev, Y.; Bo¨hmer, M.; Anderson, J. L. Langmuir 1997, 13, 6058-6068. (20) The method based on electrostatic interaction: (a) Pieranski, P.; Strzelecki, L.; Pansu, B. Phys. Rev. Lett. 1983, 50, 900-903. (b) Ise, N. Angew. Chem., Int. Ed. Engl. 1986, 25, 323-334. (c) Van Winkle, D. H.; Murray, C. A. Phys. Rev. 1986, 34, 562-573. (d) Sunkara, H. B.; Jethmalani, J. M.; Ford, W. T. Chem. Mater. 1994, 6, 362-364. (e) Larsen, A. E.; Grier, D. G. Nature 1997, 385, 230-233. 26. (21) The method based on optical manipulation: Burns, M. M.; Fournier, J.-M.; Golovchenko, J. A. Science 1990, 249, 749-754. (22) Other methods: (a) Kim, E.; Xia, Y.; Whitesides, G. M. Adv. Mater. 1996, 8, 245-247. (b) Neser, S.; Bechinger, C.; Leiderer, P.; Palberg, T. Phys. Rev. Lett. 1997, 79, 2348-2351. (23) Park, S. H.; Qin, D.; Xia, Y. Adv. Mater. 1998, 10 (13), 10281032.
Figure 1. Schematic outline of the experimental procedure that was used to assemble monodispersed spherical particles into crystalline lattices. precursors (NOA 60, 71, 72, 73, 88) to polyurethane were obtained from Norland Products (New Brunswick, NJ). Fabrication of the Cell. The cell (Figure 1) used in the assembly of mesoscale particles was constructed from two glass substrates and a square frame of photoresist and tightened with binder clips.23 A small hole (∼3 mm in diameter) was generated in the top glass substrate by etching in an aqueous HF solution, and a glass tube (∼6 mm in diameter) was attached to this hole using an epoxy adhesive. The frame of photoresist was fabricated on the surface of the bottom glass substrate using a three-stage procedure: (1) A 12-µm-thick film of photoresist (Microposit 1075, Shipley, Marlborough, MA) was spin-coated onto the bottom substrate and exposed to UV light for ∼67 s through a mask with a test pattern of a square frame (∼2 × 2 cm2 in area, and the line was ∼50 µm in width) on its surface. This photoresist film was then exposed to UV light for an additional ∼5 s through another mask covered with a test pattern of parallel lines (100 µm in width and separated by 500 µm). The two masks were registered such that the parallel lines on the second mask only overlapped with one side of the square frame. After developing, a square frame of photoresist was formed on the bottom substrate; one side of the frame had an array of shallow trenches (h ≈ 0.3 µm for all experiments described in this paper) on its surface that subsequently served as channels for the flow of the solvent used to disperse the particles. Confined Assembly of Particles. Figure 1 outlines the schematic procedure that we have used to pack mesoscale particles into crystalline assemblies.23 After an aqueous dispersion of monodispersed particles was injected into the cell through the rubber tube with a syringe, a positive pressure of N2 was applied through the glass tube to force the water to flow through the channels. As a result, the particles were accumulated at the bottom of the cell and were assembled into a close-packed structure under continuous sonication. When the dispersion of particles almost disappeared from the glass tube, the cell was placed in an oven at ∼65 °C for ∼4 h to evaporate the remaining
268 Langmuir, Vol. 15, No. 1, 1999
Park and Xia
water. After the top substrate was carefully removed, a crystalline assembly of particles was left behind on the bottom substrate. The rate of packing of polymer beads increases as the pressure of nitrogen increases; we have not systematically studied the influence of nitrogen pressure (or packing rate) on the order of the assembly. Instrumentation. The photolithography was carried out on a 3-in. aligner (Quintel-2001, Moutain View, CA). For particles g200 nm, the SEM images were taken on an environmental scanning electron microscope (ESEM 2020, Electroscan, Wilmington, MA). The accelerating voltage was 30 kV, and the pressure of the chamber was ∼5 Torr. The assemblies of mesoscale particles were sputtered with thin layers of gold (∼50 nm thick) before taking SEM images. For particles
