Langmuir 2002, 18, 3319-3323
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Ordered Arrays of Large Latex Particles Organized by Vertical Deposition Leonid M. Goldenberg,† Ju¨rgen Wagner,† Joachim Stumpe,*,† Bernd-R. Paulke, and Eckhard Go¨rnitz Fraunhofer Institute for Applied Polymer Research, Geiselbergstr. 69, 14476 Golm, Germany Received November 6, 2001. In Final Form: January 17, 2002 A simple preparation method of ordered multilayer films of submicrometer- and micrometer-sized monodisperse latex particles in the broad range of 0.2-2.5 µm is reported. The films were prepared from aqueous suspensions by vertical deposition at elevated temperatures. Both hydrophobic polystyrene and hydrophilic core-shell particles allowed good array formation. Three-dimensional lattices were proved by transmission and reflectance vis-NIR-IR spectroscopy. Well-developed Bragg peaks up to 5000 nm have been registered. An observation of Fabry-Perot resonance signals in the spectra also confirms the good array quality. Optical properties of the gratings were investigated by laser diffraction and by diffraction using an optical microscope equipped with a Bertrand lens.
Introduction Recently, a great deal of attention has been devoted to the fabrication of photonic crystals based on ordered arrays of colloidal particles.1 Among the recent developments in the preparation techniques, one finds exceptional efforts by the flow cell method2 and the vertical deposition technique.3 The first one based on the crystallization through physical confinement and hydrodynamic flow was suggested as a universal technique suitable for all types of materials and particle sizes; however, so far experimental results have been presented only for a particle size up to 0.5 or 1 µm for inversed opal. The method can also not be considered as simple. So far, the preparation of the cell requires well-developed photolithographic facilities and cannot be performed in a usual chemical laboratory. On the contrary, ordered array formation by vertical deposition is really a simple method, which was originally suggested for silica particles with a size up to 0.7 µm. Ethanol suspensions were used,3a,b and the method relies on the balance between ethanol evaporation (at room temperature) and particle sedimentation. Consequently, for a certain particle size the sedimentation is faster than ethanol evaporation and in this case the self-organization of the particles does not take place. Recently, Norris and Vlasov3b,c reported on an array of up to 1 µm silica particles formed by this technique. Surely, the rate of evaporation can be controlled by solvent and temperature, and the rate of sedimentation, by solvent and material density. More recently, they also succeeded in on-chip natural assembly of silica spheres directly on a Si wafer,3c obtaining * To whom correspondence should be addressed. E-mail:
[email protected]. † Also affiliated with the Institute of Thin Film Technology and Microsensorics, Teltow, Germany. (1) Xia, Y. N.; Gates, B.; Yin, Y. D.; Lu, Y. Adv. Mater. 2000, 12, 693. (2) (a) Park, S. H.; Xia, Y. N. Langmuir 1999, 15, 266. (b) Park, S. H.; Gates, B.; Xia, Y. N. Adv. Mater. 1999, 11, 693, 462. (c) Park, S. H.; Xia, Y. N. Chem. Mater. 1998, 10, 1745. (d) Park, S. H.; Qin, D.; Xia, Y. Adv. Mater. 1998, 10, 1028. (e) Yin, Y. D.; Gates, B.; Xia, Y. N. Adv. Mater. 2000, 12, 1426. (3) (a) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (b) Norris, D. J.; Vlasov, Y. A. NATO ASI Series C; Soukoulis, C. H., Ed.; Kluwer: Norwell, MA, Vol. 563, pp 229-238. (c) Vlasov, Y. A.; Bo, X.-Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289. (d) Ye, Y. H.; LeBlanc, F.; Hache, A.; Truong, V. V. Appl. Phys. Lett. 2001, 78, 52.
large single-crystalline domains (1 mm to 1 cm) using a temperature gradient in the vessel during the particle deposition. In another short publication3d is reported selfassembling of colloidal crystals from an aqueous suspension at a temperature around 45-65 °C. Polystyrene (PS) particles of 310 nm diameter were used, and colloidal crystals with a well-developed stop-band in the transmission spectra were obtained. Thus, the original technology3a,b covers the particle size of around 700 nm which allows one to obtain an important band gap at about 1.5 µm used for optical communications.4 An aqueous method for latexes has so far been applied only to small particles.3c In general, vertical deposition techniques have not yet been extended to the micrometer particle range. We were driven by the demands of multidimensional optoelectronics5a which requires mainly optical gratings with a periodic distance larger than 1 µm. To achieve the whole range of intelligent functions of human vision, threedimensional (3D) optical gratings with red-green-blue trichromatism are needed.5 Larger particles are also suitable for investigation by optical microscopy. Particles with a size of more than 1 µm allow one to shift the stopband further in the NIR region and even in the IR region. Such a rate of grating constant is also accessible by lithographic and holographic techniques. However, these techniques are generally suited to produce two-dimensional (2D) structures. Recently, a 3D grating with a periodic distance of 0.6 µm was produced by holographic lithography;6 however, this requires a sophisticated experimental setup. We show in this communication that the vertical deposition technique can be applied to latex particles of at least 2.5 µm diameter. In this way, both the usual hydrophobic PS particles and hydrophilic core-shell particles were assembled. The arrays manufactured were characterized by transmission and reflectance spectroscopy and optical microscopy and by their diffraction properties. (4) (a) Noda, S.; Tomoda, K.; Yamamoto, N.; Chutinan, A. Science 2000, 289, 604. (b) Blanco, A.; Chomski, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; van Driel, H. M. Nature 2000, 405, 437. (5) (a) Lauinger, N. Proc. SPIE-Int. Soc. Opt. Eng. 1999, 3837, 40. (b) Lauinger, N. Proc. SPIE-Int. Soc. Opt. Eng. 2000, 4197, 27. (c) Lauinger, N.; Pinnow, M.; Goernitz, E. J. Biol. Phys. 1997, 23, 73. (6) Campbell, M.; Sharp, D. N.; Harrison, M. T.; Denning, R. G.; Turberfield, A. J. Nature 2000, 404, 53.
10.1021/la015659c CCC: $22.00 © 2002 American Chemical Society Published on Web 03/09/2002
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Langmuir, Vol. 18, No. 8, 2002
Goldenberg et al.
Figure 1. Photographs of array films obtained from 0.32 µm PS (left) and 0.94 µm core-shell (right) particles at 56 °C and concentrations of 1.2 and 0.8 wt %, respectively (a); optical microscopic pictures at low magnification (objective 20×) of the arrays of the same particles (b).
Experimental Section Hydrophobic PS latex particles with a narrow size distribution and a diameter of 0.20 µm were prepared by classical emulsion polymerization.7 Hydrophobic, monodisperse PS colloids with diameters of 0.32, 0.54, 0.74, and 1.41 µm were synthesized by an emulsifier-free, aqueous radical polymerization.8 These colloids were equipped with a hydrophilic shell by a procedure described elsewhere9 to yield particles of 0.70, 0.94, 1.18, 1.37, and 1.41 µm diameter. Large, uncharged PS sterically stabilized tentacle particles with a diameter of 2.5 µm were prepared by nonaqueous dispersion polymerization.10 All dispersions were diluted with deionized water to a concentration of 0.3-1.2 wt % before use. The glass substrates were cleaned by a 1:1:5 mixture of ammonia, hydrogen peroxide, and water for 30 min at 70 °C and carefully rinsed. In a representative procedure, a glass slide (12 mm × 25 mm; thickness, 1 mm) was placed approximately vertical in a cylindric plastic vial (inner diameter, ca. 13 mm; volume, 2.5 mL). Then the vial was filled with approximately 1.5 mL of particle suspension and taken in a oven with controlled temperature. Depending on the particle size, the deposition was performed at 60 °C (for particles of
