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Langmuir 2002, 18, 7722-7727
Colloidal Crystals Made of Polystyrene Spheroids: Fabrication and Structural/Optical Characterization Yu Lu,† Yadong Yin,† Zhi-Yuan Li,‡ and Younan Xia*,‡ Department of Materials Science & Engineering and Department of Chemistry, University of Washington, Seattle, Washington 98195-1700 Received May 14, 2002. In Final Form: July 16, 2002 This paper describes the fabrication and characterization of colloidal crystals constructed from spheroidal building blocks with well-controlled major-to-minor ratios. Such a crystalline lattice was fabricated by infiltrating an opaline lattice of monodispersed polystyrene spheres with an elastomer precursor such as poly(dimethylsiloxane) (or PDMS), followed by thermal curing and stretching of the composite film at a temperature higher than the glass transition temperature of polystyrene. In this process, the polystyrene spheres were transformed into spheroids through viscoelastic deformation, while the long-range order of this three-dimensional lattice was essentially preserved. Because of the low contrast in the refractive index, the colloidal crystal fabricated in the present work exhibited a stop band (rather than a complete band gap) in the optical regime. The position of this stop band was determined by the diameter of the polystyrene spheres and the elongation ratio of the elastomeric composite. When the crystalline lattice was made of 240-nm polystyrene spheres, the stop band shifted from 594 to 522 nm, as the PDMS film was stretched by an elongation of 130%, and further down to 470 nm, as the incident angle was changed from 0 to 45°. These spectroscopic measurements were in good agreement with the partial band structures calculated using the plane-wave-expansion-method (PWEM). In addition to their use as a model system to investigate the dependence of photonic band structures on the shape (or symmetry) of lattice points, the nonspherical system described here also provides a potentially useful approach to fine-tuning the optical properties of colloidal crystals.
Introduction Photonic crystals are spatially periodic structures constructed from dielectric materials with different refractive indices. They have been a subject of extensive research ever since the concept was first proposed by Yablonovich and John in 1987.1 Under appropriate conditions (e.g., with a suitable lattice structure and a sufficiently high contrast between different dielectric regions), a crystal may exhibit a forbidden gap in its photonic band structure within which no optical modes (irrespective of their polarization directions) will be allowed to exist.2 Such band gaps provide a powerful means to confine and control lightsfor example, to manipulate a spontaneous emission process, to localize photons to a specified area, and to guide the propagation of light (at certain restricted frequencies) along a prespecified direction (even with sharp bends) and at low lost.3 All of these abilities to manipulate photons are technologically important because they can be potentially used, for instance, to greatly improve the performance of various kinds of quantum electronic devices such as semiconductor lasers.4 Self-assembly with colloidal particles as the building blocks represents an effective route to three-dimensionally * To whom correspondence should be addressed. E-mail: xia@ chem.washington.edu. † Department of Materials Science & Engineering. ‡ Department of Chemistry. (1) (a) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (b) John, S. Phys. Rev. Lett. 1987, 58, 2486. (2) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals; Princeton University Press: Princeton, NJ, 1995. (3) (a) Soukoulis, C. M. Photonic Band Gap Materials; Kluwer: Boston, 1996. (b) Scherer, A.; Doll, T.; Yablonovitch, E.; Everitt, H. O.; Higgins, J. A. A special issue in J. Lightwave Technol. 1999, 17, 1928. (c) Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. Nature 1997, 386, 143. (4) Painter, O.; Lee, R. K.; Scherer, A.; Yariv, A.; O’Brien, J. D.; Dapkus, P. D.; Kim, I. Science 1999, 284, 1819.
periodic structures characterized by long-range ordering.5-7 Most work in this area has been concentrated on facecentered-cubic (fcc) lattices of spherical colloids (polymer latexes and silica beads) that could be conveniently synthesized as monodispersed samples. As limited by the spherical symmetry of their lattice points, the photonic band structures of these crystalline lattices are often marked by degeneracy at the W- or U-point. It is generally accepted that spherical colloids are not suitable for the formation of complete band gaps, unless they can be assembled into the diamond lattice (which has only been achieved on the atomic scale)8 or inverted opaline structures with high enough contracts (>2.8) in refractive index are fabricated.7 Computational studies by several groups suggested that such a symmetry-induced degeneracy at the W- and U-points could be removed by constructing the (5) See, for example: (a) A special issue on photonic crystals in Adv. Mater. 2001, 13, 369. (b) Densmore, A. D.; Crocker, J. C.; Yodh, A. Curr. Opin. Colloid Interface Sci. 1998, 3, 5. (c) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693. (6) Opals: (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. (b) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829. (c) van Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (d) Gates, B.; Xia, Y. Adv. Mater. 2000, 12, 1329. (e) Jiang, P.; Ostojic, G. N.; Narat, R.; Mittleman, D. M.; Colvin, V. L. Adv. Mater. 2001, 13, 389. (f) Chen, K. M.; Jiang, X.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 26, 7825. (7) Inverse opals: (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) Vlasov, Y. A.; Yao, N.; Norris, D. J. Adv. Mater. 1999, 11, 165. (h) Braun, P. V.; Wiltzius, P. Nature 1999, 402, 603. (i) Jiang, P.; Cizeron, J.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 7957. (8) (a) Sozuer, H. S.; Haus, J. W.; Inguva, R. Phys. Rev. B 1992, 45, 13962. (b) Haus, J. W. J. Mod. Opt. 1994, 41, 195.
10.1021/la025946w CCC: $22.00 © 2002 American Chemical Society Published on Web 08/15/2002
Colloidal Crystals Made of Polystyrene Spheroids
crystalline lattices with nonspherical colloids.9 For example, computational studies based on the plane-waveexpansion-method (PWEM) have demonstrated that an fcc lattice made of dumbbell-shaped particles could exhibit a complete gap between the second and third bands once the refractive index contrast had reached a value of ∼2.4.10 A seemingly straightforward approach to colloidal crystals with nonspherical lattice points might involve the following two steps: (i) the synthesis of nonspherical colloids characterized by monodispersed sizes and shapes and (ii) the self-assembly of these nonspherical building blocks into 3D crystals with both spatial and orientational orders. Although a number of methods have been successfully demonstrated for generating nonspherical colloids with well-controlled sizes and shapes and in relatively large quantities,11-13 it still remains a great challenge to organize these nonspherical building blocks into 3D crystalline lattices over large areas that can be further characterized using spectroscopic methods. Here we overcame this difficulty by developing a procedure that generated 3D colloidal crystals with spheroidal building blocks by mechanically stretching an opaline lattice of polystyrene spheres embedded in the elastomeric matrix made of poly(dimethylsiloxane) (or PDMS). This method not only provides a good model system to investigate the dependence of photonic band structures on the shape (or symmetry) of lattice points, but it also offers a potentially useful approach to tune the properties of colloidal crystals self-assembled from spherical particles. Experimental Section Materials. Aqueous dispersions of monodispersed polystyrene (PS) spheres were obtained from Duke Scientific (Palo Alto, CA) and Polyscience (Warrington, PA). The liquid prepolymer of PDMS (Sylgard 184) was purchased from Dow Corning (Midland, MI). Poly(vinyl alcohol) (PVA) (FW ) 50 000-85 000) was obtained from Aldrich (Milwaukee, WI). All materials were used as received without further dilution or other modification. Stretching of PVA Thin Films Containing PS Spheres. In a typical procedure,14 one drop of PS spheres was added to ∼5 g of PVA aqueous solution (6%, wt), cast into thin films of ∼0.1 mm in thickness, and then cut into stripes of ∼2 cm in width. One end of each stripe was hung from the ceiling of a box oven (Model 1320, VWR, West Chester, PA) using a small binder clip (#72020, ACCO, Lincolnshire, IL), and the other end was uniaxially stretched by applying a known amount of weight through another small binder clip. The temperature of the oven was controlled around 200 °C. After the stretched film had been cooled to room temperature (while the strain was still applied), it was cut into small pieces (
