VOLUME 16, NUMBER 19
SEPTEMBER 21, 2004
© Copyright 2004 by the American Chemical Society
Communications Ordered Honeycomb Structures on a Substrate by Coating with Ring-Shaped Particles of 3D Siloxanes Ippei Noda*,† and Akira Nakamura‡ Research and Development Division, Takemoto Oil and Fat Co., Ltd., 2-5 Minato-machi, Gamagori, Aichi 443-8611, Japan and OM Research, 7-2-1308 Minami-Ogimachi, Kita-ku, Osaka 530-0052, Japan Received April 30, 2004 Revised Manuscript Received July 26, 2004 Advanced techniques for nano- or microfabrication are now essential for progress in many areas of science and technology.1 We have already reported novel highly ordered meso- or macroporous monolayers formed by self-assembly of hollow hemispheres.2,3 For demanding applications as functional devices of microelectronics or optoelectronics, the ordered arrangement still needs to be improved. Significant improvements in reducing the number of defects in the self-assembled monolayers (SAMs) will be essential to support the widespread application of this technique to the manufacture of nanostructures. In the case of the hollow hemispheres, the quality of the ordered arrangement is affected by their affinity toward a substrate since the defects in the SAMs are generated by rolling of the hemispheres.2 For * Corresponding author. Tel: +81-533-68-2149. Fax: +81-564-547708. E-mail:
[email protected]. † Takemoto Oil and Fat Co., Ltd. ‡ OM Research. (1) (a) Madou, M. Fundamentals of Microfabrication; CRC Press: Boca Raton, FL, 1997. (b) Kovacs, G. T. A. Micromachined Transducers Sourcebook; McGraw-Hill: New York, 1998. (2) Noda, I.; Yamada, M. Chem. Mater. 2002, 14, 2348. (3) Noda, I.; Yamada, M. Adv. Mater. 2002, 14, 1236.
decreasing such defects further, we investigate whether hollow flat particles as a red-blood cell shape or ringshaped particles can be obtained. For topological reasons, trifunctional units of (CH3)SiO3/2 (T3) and (HO)SiO3/2 (Q3) exist at the surface of spherical particles, having the empirical formula (CH3)0.9SiO1.55, since the geometry of trifunctional units is fit to form circular shells, whereas tetrafunctional units of SiO4/2 (Q4) exist predominantly in the inner part of the particle based on XPS atomic composition data.4 Although difunctional units, (CH3)2SiO2/2 (D2) and (CH3)Si(OH)O2/2 (T2), having more structural freedom, can also exist in the surface of a spherical particle with a low degree of cross-linking, polymers formed by the difunctional units tend to become a linear polymer. To reduce the sphericalsurface area in the particle shape, we tried to control the degree of cross-linking in a hollow hemisphere. First, we investigated the replacement of a portion of the Q4, Q3, T3, and T2 units with D2 units. As a result, clamshells and ellipses were obtained as the intermediate topologies, and the shape became spherical with a further increase in D2 units since the spherical shape is most stable against external water pressure.2 It was assumed that the spherical shape was caused by reducing the proportion ∆ [)Q4/(D2 + T2 + T3 + Q3)] of the Q4 unit to D2, T2, T3, and Q3units. Second, we attempted to increase the difunctional unit without changing the ∆. To satisfy this requirement, we investigated a synthetic approach using a silicon monomer which generates RSi(OH)3 in place of methyltrimethoxysilane since the large group (R) on the Si atom interferes with condensation of the neighboring silanol. In a manner similar to the preparation of hemispheres,4,5 particles with an intermediate mor(4) Noda, I.; Kamoto, T.; Sasaki, Y.; Yamada, M. Chem. Mater. 1999, 11, 3693. (5) Noda, I.; Kamoto, T.; Yamada, M. Chem. Mater. 2000, 12, 1708.
10.1021/cm0403412 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/24/2004
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Communications Scheme 1. Schematic Representation of Morphologies Formed by Changing an Amount of T′2 Unit in a Particle
Figure 1.
29Si
CP/MAS NMR of ring-shaped particles.
phology are obtained by copolymerization of tetraethoxysilane [TEOS: Si(OC2H5)4], methyltrimethoxysilane [MTMS: CH3Si(OCH3)3], and [3-(methacryloxy)propyl]trimethoxysilane (MPTMS) as the RSi(OCH3)3.6 Based on 29Si CP/MAS NMR data, difunctional units in the particles increase as the amount of MPTMS increases. Figure 1 shows 29Si CP/MAS NMR of ring-shaped particles. The topologies of these particles are characterized by the particle size distributions7 and by the SEM images. First, we investigated how the shape of a hollow hemisphere with a thick wall covering the hollow center is changed by increasing the amount of difunctional units in the particle. Figure 2A-C shows that the size distributions become a single peak around the median size as the amount of MPTMS increases, and the single peak looks like a narrow distribution of spherical particles. Actually, this particle with the size distribution is identified as a disk-shaped particle by the SEM image (Figure 3A). The flatness of the particle shape increases as the depth of the hollow becomes shallow; therefore, the final shape leads to a hollow disk. Second, when the thickness of the wall covering the hollow portion becomes thin, the peak centered at 0.47 µm of the observed two peaks increases as the amount of MPTMS increases (Figure 2D-F). The size distribution in Figure 2F demonstrates that the median radius is much smaller than the average outside diameters estimated from the SEM images in Figure 3B. The SEM images show that the particle has a ring shape and that the average outside and inside diameters are 2.2 and 1.5 µm, respectively. Finally, the particle shape is also identified as a ring-shaped by cross-sectional SEM images of the particles before aging. Figure 2E is assumed to be the size distribution of particles with an intermediate shape between hemisphere and ring. These results support the topologies to be ring-shaped, truncated hollow hemispherical, and hollow disk(6) Typical experimental procedure based on a method described by our reports.4,5 A mixture of TEOS (79.2 g, 0.38 mol), MTMS (31.0 g, 0.23 mol), and MTS (84.8 g, 0.34 mol) was added slowly into pure water (831.6 mL) and 48% sodium hydroxide (0.65 g) over a period of 20 min, maintaining the two layers. The mixture was slowly stirred at 14 °C. After 50 min, 5% sodium dodecylbenzensulfonic acid (6.0 g) was added dropwise into the reaction mixture. After the mixture was aged for 10 h at 50 °C, separation of the resulting precipitates by centrifugation, washing with pure water (200 mL), and drying for 12 h at 100 °C afforded 45.8 g (46%) of silicone particles. (7) Topologies of these particles are characterized by the particle size distributions measured with a laser scattering particle distributor (HORIBA LA-920). The accuracy in the size distributions are improved by the removal of extra space between counters, compared with size distributions measured by LA-720.
shaped. Scheme 1 outlines the topologies observed by changing the amount of D unit in the particle. This ability to control the shapes of particles affords an opportunity to yield colloidal crystals with desirable optical characteristics from the viewpoint of applications. In particular, such colloidal crystals are applicable to a technique of holography commonly accomplished through embossing the surface of clear or reflective films.8,9 First, by use of SAMs fabricated by ring-shaped particles, the methodologies for generation of patterned structures on a plastic thin film were examined. One of the conventional methods based on lithographic technique uses the SAMs as a mold to imprint relief structures on a thermoplastic thin film that has been thermally softened.2,3 Figure 4A shows SEM images of a holographic patterned poly(ethylene terephthalate) (PET) film.10 The embossing technique has the advantage of simplicity and convenience. Once the stamp is available, multiple copies of the pattern can be rapidly produced in a fabrication process for PET films. Another type of embossed holographic films can be fabricated by antireflection-coating of the films with small holographic particles,8 which reduces the optical losses further.11,12 With use of the ring-shaped particles as the holographic particles, the ordered array of holes on a substrate attains special optical effects in different application media. Figure 4B shows SEM images of holographic films made by coating a PET film with these particles and thermoplastic resins. The thermoplastic resin is a binder that can settle particles on the PET film, which has a flat and inactive surface. Figure 4B shows an ordered array of holes fabricated by deposition of the ring-shaped particles, which were fixed on the PET film by a small amount of partially etherified poly(vinyl alcohol) (PVA). Angle-dependent optical effects of the film are derived from the ordered structures including that fabricated by hollow hemispheres, and these effects should be widely useful in industry to manufacture holographic diffraction gratings, antiscattering coatings, antiglare coatings, and antireflection coatings. Consequently, these procedures are possible approaches to the manufacturing of polymer-dispersed liquid-crystal (8) Pfaff, G.; Reynders, P. Chem. Rev. 1999, 99, 1963. (9) Walheim, S.; Scha¨ffer, E.; Mlynek, J.; Steiner, U. Science 1999, 283, 520. (10) The film is susceptible to wrinkling since the thermalrelaxation process is omitted. (11) Hattori, H. Adv. Mater. 2001, 13, 51. (12) Wang, C.; Zhang, C.; Lee, M. S.; Dalton, L. R.; Zhang, H.; Steier, W. H. Macromolecules 2001, 34, 2359.
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Figure 2. Size distribution of the SixOyMez particles. (A) Size distribution of particles with an empirical formula of (CH3)0.7SiO1.65 fabricated by TEOS and MTMS. (B) 40 mol % of MTMS replaced by MTS. (C) 60 mol % of MTMS replaced by MTS. (D) Size distribution of particles with (CH3)0.6SiO1.70. (E) 40 mol % of MTMS replaced by MTS. (F) 60 mol % of MTMS replaced by MTS.
Figure 3. (A) SEM images of hollow disk-shaped particles. (B) SEM images of ring-shaped particles after heating at 50 °C for 10 h. The bars are 1.0 and 10.0 µm, respectively.
(PDLC) films.13 Furthermore, these particles with a high porosity should form a porous layer on the flat surface of transparent plastic films, together with water-soluble binders. With use of the transparent films as a substrate, ink-jet printing has emerged as an attractive patterning technique for electrodes in light-emitting diodes (LEDs) and electroluminescence displays (ELDs) using light-emitting diodes.14 In addition, the ordered structure on a polymer film used as an insulator improves the peel strength of the electrode on a highdensity circuit board. We have demonstrated the concept of formation of patterned SAMs of ring-shaped particles by coating plastic thin films with the dispersed solution. This (13) Luther, B.; Springer, G. H.; Higgins, D. A. Chem. Mater. 2001, 13, 2281. (14) (a) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. Science 2000, 290, 2123. (b) Calvert, P. Chem. Mater. 2001, 13, 3299.
Figure 4. (A) SEM images of an ordered array of holes on a surface of PET film prepared by compression micromolding using SAMs of ring-shaped particles with an average outside diameter of 5.0 µm. (B) SEM images of surfaces of PET thin films coated with ring-shaped particles with an average outside diameter of 2.2 µm, together with a small amount of PVA. The bars are 10.0 µm, respectively.
method contributes to the development of formation methodology of SAMs, which should allow a greater level of control, and provides for the efficient manufacturing of nano- and microstructures. Acknowledgment. We thank Mr. Masahiko Yamada for helpful discussions. Supporting Information Available: 29Si CP/MAS spectra of 3D siloxanes, SEM images of ring-shaped particles and ordered arrays of holes on the bent surface, silicon distribution map in the SEM image, SEM images of truncated hollow hemispheres, tilt-angle transmission spectra, and SEM images of the surface of epoxy insulator (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM0403412
