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Chem. Mater. 2005, 17, 5268-5274
Two-Dimensional Non-Close-Packing Arrays Derived from Self-Assembly of Biomineralized Hydrogel Spheres and Their Patterning Applications Gang Zhang,† Dayang Wang,*,† Zhong-Ze Gu,‡ Ju¨rgen Hartmann,† and Helmuth Mo¨hwald† Max Planck Institute of Colloids and Interfaces, D-14424, Potsdam, Germany, and Department of Biological Science and Medical Engineering, Southeast UniVersity, 210096 Nanjing, People’s Republic of China ReceiVed February 23, 2005. ReVised Manuscript ReceiVed July 12, 2005
Two-dimensional (2D) non-close-packing arrays were fabricated by dip-coating hydrogel spheres unloaded and loaded with CaCO3 formed via in situ biomineralization. The separation distance between the particles can be well tuned by the dip-coating speed and the concentration of the gel spheres. The resultant 2D non-close-packing arrays can be used as templates to direct the self-assembly of silica colloidal spheres for formation of binary colloidal crystals with different patterns. The resulting binary structures may be manipulated by the surface wettability. Besides, they can be utilized as masks to generate functional patterned substrates.
Introduction Self-assembly of colloidal spheres of diameters, ranging from nanometers to micrometers, has recently become a quite dynamic field, because one may expect numbers of promising fundamental and technical applications.1 A number of techniques have been successfully developed for organizing colloidal spheres.2 The solid colloidal self-assemblies obtained so far exhibit predominantly a two- or threedimensional (2D or 3D) hexagonal or square close-packing array due to both their thermodynamically lowest state and their most robust mechanical stability.1 The arrays have a characteristic reflection band in a certain wavelength range due to the Bragg diffraction of light, exhibiting so-called structural colors.3 In this sense, they have an analogy to natural opals or diurnal insects’ eyes.4 In eyes of nocturnal * Author to whom correspondence should be addressed: fax 49-331-5679202; e-mail
[email protected]. † Max Planck Institute of Colloids and Interfaces. ‡ Southeast University.
(1) For recent reviews, see (a) Arsenault, A.; Fournier-Bidoz, S.; Hatton, B.; Mg´uez, H.; Te´treault, N.; Vekris, E.; Wong, S.; Yang, S.; Kitaev, V.; Ozin, G. J. Mater. Chem. 2004, 14, 781. (b) Wang, D.; Mo¨hwald, H. J. Mater. Chem. 2004, 14, 459. (c) Lopez, C. AdV. Mater. 2003, 15, 1679. (d) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. AdV. Mater. 2000, 12, 693. (e) Grier, D. From Dynamics to Devices: Directed SelfAssembly of Colloidal Materials, a special issue in MRS Bull. 1998, 23. (2) (a) Mayoral, R.; Requena, J.; Moya, J.; Lopez, C.; Cintas, A.; Miguez, H.; Meseguer, F.; Vazquez, L.; Holgado, M.; Blanco, A. AdV. Mater. 1997, 9, 257. (b) van Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (c) Jiang, P.; Bertone, J.; Hwang, K.; Colvin, V. Chem. Mater. 1999, 11, 2132. (d) Vickreva, O.; Kalinina, O.; Kumacheva, E. AdV. Mater. 2000, 12, 110. (e) Park, S.; Qin, D.; Xia, Y. AdV. Mater. 1998, 10, 1028. (f) Ozin, G.; Yang, S. M. AdV. Funct. Mater. 2001, 11, 95. (g) Garcia-Santamaria, F.; Miyazaki, H.; Urquia, A.; Ibisate, M.; Belmonte, M.; Shinya, N.; Meseguer, F.; Lopez, C. AdV. Mater. 2002, 14, 1144. (h) Gu, Z.; Fujishima, A.; Sato, O, Chem. Mater. 2002, 14, 760. (i) Wang, D.; Mo¨hwald, H. AdV. Mater. 2004, 16, 244. (3) (a) Biswas, R.; Sigalas, M.; Subramania, G.; Ho, K. M. Phys. ReV. B 1998, 57, 3701. (b) Busch, K.; John, S. Phys. ReV. E 1998, 58, 3896. (c) Moroz, A.; Sommers, C. J. Phys: Condens. Matter 1999, 11, 997.
insects, in contrast, 2D non-close-packing arrays are encountered. For instance, the corneas of moth eyes are composed of hexagonal non-close-packed arrays of chitin gel protuberances, which efficiently suppress the reflectivity at the eye surface over broad angular and frequency ranges for the purpose of night camouflage.4 As compared to closepacking ones, therefore, non-close-packing arrays are expected to show promising different uses besides that of photonic crystals. Up to now, non-close-packed arrays of isolated dots can be constructed only by means of lithographic techniques.5 These techniques are expensive, timeconsuming, and difficult to apply for large area construction. On the other hand, 2D close-packed colloidal assemblies have been used as masks to create inverted patterns on substrates, termed as nanosphere lithography.6 In comparison with conventional lithographic patterning techniques,5 however, this methodology lacks flexibility to create patterns with tunable separation distances between isolated domains. In this sense, 2D non-close-packed colloidal assemblies should be ideal templates. Even if colloidal spheres are arranged in non-close-packed ordered arrays, the growth of 2D non-close-packing arrays directly via self-assembly of colloidal spheres remains little studied. Asher and co-workers7 have successfully trapped 3D non-close-packing arrays of colloidal spheres by gelating (4) (a) Land, M.; Nilsson, D. Animal Eyes; Oxford University Press: Oxford, U.K., 2001. (b) Vukusic, P.; Sambles, J. Nature 2003, 424, 852. (c) Bernhard, C. EndeaVor 1967, 26, 79. (5) For a review, see Arden, W. Curr. Opin. Solid State Mater. Sci. 2002, 6, 371. (6) See, for example, (a) Fischer, U.; Zingsheim, H. J. Vac. Sci. Technol. 1981, 19, 881. (b) Deckman, H.; Dunsmuir, J. J. Vac. Sci. Technol B 1983, 1, 1109. (c) Haynes, C.; van Duyne, R. J. Phys. Chem. B 2001, 105, 5599. (7) (a) Sharma, A.; Jana, T.; Kesavamoorthy, R.; Shi, L.; Virji, M.; Finegold, D.; Asher, S. J. Am. Chem. Soc. 2004, 126, 2971. (b) Reese, C.; Mikhonin, A.; Kamenjicki, M.; Tikhonov, A.; Asher, S. J. Am. Chem. Soc. 2004, 126, 1493.
10.1021/cm050414x CCC: $30.25 © 2005 American Chemical Society Published on Web 09/28/2005
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their aqueous media via photopolymerization. These polymerized 3D non-close-packing arrays, similar to close-packing ones, also exhibit a bright reflection color due to the diffraction of light by their 3D ordered structures. With assistance of lithographically patterned substrates, van Blaaderen and co-workers have recently created 3D solid nonclose-packed colloidal assemblies.8 In the present work, we plan to demonstrate a facile and efficient approach to fabricate 2D non-close-packing arrays on substrates by using hydrogel spheres unloaded or loaded with CaCO3 via in situ mineralization inside the gels. Selfassembly of hydrogel spheres has largely been explored.9 Since all studies are associated with the goal to pursue the pseudo photonic band gap, hydrogel spheres self-assemble usually in the presence of external forces, for example, centrifugal forces, besides gravity to drive them closely packed. Furthermore, Lyon and co-workers9c have demonstrated the potential to use 2D close-packing arrays of hydrogel spheres as micrometer-sized lenses for imaging, presenting a similarity to the cornea of diurnal insect eyes. Two-dimensional non-close-packing arrays are occasionally encountered in certain areas in thin films of hydrogel spheres based on a deliberate design of the structure of hydrogel spheres in particular.10 Nevertheless, the present report is the first dedicated work to grow 2D non-close-packing arrays on substrates based on self-assembly of hydrogel spheres. Our methodology allows manipulation of the interparticle distances in 2D non-close-packing arrays obtained in a controlled manner, which provides a cheap alternative to lithographic techniques. The structures of the resulting nonclose-packing arrays were transferred into other materials such as Au. Besides, they were used as templates to control self-assembly of other colloidal spheres. Experimental Section Materials. N-Isopropyl acrylamide (NIPAM), N,N-methylenebis(acrylamide) (MBA), 4-vinylpyridine (4-VP), potassium persulfate (K2S2O8), calcium chloride (CaCl2), ammonium carbonate [(NH4)2CO3] and (heptadecafluoro-1,1,2,2-tetrahydrodecyl) dimethyl chlorosilane (HFDC) were purchased from Sigma-Aldrich. Silica spheres (144 and 596 nm) were purchased from Microparticles GmbH, Germany. NIPAM was purified by recrystallization from a toluene/hexane mixture (1:3); 4-VP was purified by distillation under vacuum. Other commercial materials were used without further purification. Silicon wafers or glass slides were soaked in a 7:3 volumetric mixture of 98% H2SO4 and 30% H2O2 for 30 min under boiling and then rinsed with water for use as substrate. The water in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity higher than 18.2 MΩ‚cm. Synthesis of Hydrogel Spheres. Hydrogel particles of copolymer of NIPAM and 4-VP, termed as PNIPVP, were prepared by a (8) Hoogenboom, J.; Retif, C.; de Bres, E.; van de Boer, M.; van LangenSuurling, A.; Romijn, J.; van Blaaderen, A. Nano Lett. 2004, 4, 205. (9) (a) Debord, J.; Eustins, S.; Debord, S.; Lofye, M.; Lyon, A. AdV. Mater. 2002, 14, 658. (b) Debord, J.; Lyon, A. J. Phys. Chem. B 2000, 104, 6327. (c) Serpe, M.; Kim, J.; Lyon, A. AdV. Mater. 2004, 16, 184. (d) Ma, G. H.; Fukutomi, T. Macromolecules 1992, 25, 1870. (10) (a) Saunders, B. R.; Vincent, B. AdV. Colloid Interface Sci. 1999, 80, 1. (b) Peppas, N.; Huang, Y.; Torres-Lugo, M.; Ward, J.; Zhang, J. Annu. ReV. Biomed. Eng. 2000, 2, 9. (c) Hoffman, A. AdV. Drug DeliVery ReV. 2002, 43, 3.
Chem. Mater., Vol. 17, No. 21, 2005 5269 surfactant-free emulsion polymerization.11 NIPAM (250 mg), 10 µL of 4-VP, 20 mg of K2S2O8, and 25 mg of MBA were dissolved in 25 mL of water in a 50 mL vial. The polymerization was carried out for 4 h at 70 °C under nitrogen atmosphere. The hydrogels were then purified by four successive centrifugations at 10000g for 10 min (Beckman preparative ultracentrifuge), each followed by decantation and redispersion in water. The hydrodynamic diameter of PNIPVP spheres is 675 nm at pH 7 at room temperature, determined by dynamic light scattering (DLS). Mineralization of CaCO3 within PNIPVP Spheres. In situ mineralization of CaCO3 within gel spheres was conducted following our previous study.11 In brief, in glass vials, the hydrogel spheres were well dispersed in 10 mM CaCl2 aqueous solution. The concentration of PNIPVP is 1 wt %. These vials were put in a closed desiccator where solid (NH4)2CO3 was placed as the CO2 vapor source.12 All mineralization experiments were carried out at room temperature. After three repetitions of centrifugation at 5000g for 10 min and washing with water, CaCO3-PNIPVP composite spheres were redispersed in water. Self-Assembly of Colloidal Spheres. PNIPVP spheres and CaCO3-PNIPVP composite ones self-assembled on silicon wafers or glass slides via a dip-coating process.2h The resulting structures were visualized by scanning electronic microscopy (SEM) and atomic force microscopy (AFM). To obtain monolayers, the concentrations of the gel spheres were chosen in the range of 0.10.5 wt % and the withdrawing speeds in the range of 1-5 µm/s. As a control, monolayers of 596 nm silica spheres were constructed on silicon wafers by dip-coating a 1.0 wt % dispersion. The withdrawing speed was 5 µm/s. Binary Colloidal Assemblies. On the resulting monolayers of CaCO3-PNIPVP particles, 144 nm silica spheres were consecutively dip-coated. The concentrations of silica spheres were in the range of 0.5-2.0 wt % and the withdrawing speed was 5 µm/s. On the other hand, silicon wafers coated with monolayers of CaCO3-PNIPVP particles were immersed into 1.0 wt % toluene solution of HFDC for 10 min. After consecutive washing with toluene and with ethanol two times, HFDC molecules were coated on the silicon wafers. Note that the CaCO3-PNIPVP particles cannot be silanized. Thus, HFDC molecules were located only in the silicon wafer valleys between CaCO3-PNIPVP particles. Subsequently, 144 nm silica spheres were dip-coated on these HFDC-modified substrates. Pattern Transfer. The resultant silicon wafers coated with monolayers of CaCO3-PNIPVP particles were employed as masks for Au vapor deposition. Cr layers (5 nm thick) and Au layers (30 nm thick) were consecutively deposited on the substrates. By immersion in an aqueous solution of hydrochloric acid (3.7 wt %), followed by 5 min of sonication, the CaCO3-PNIPVP particles were removed from the substrates. Characterization. DLS measurements were performed by using a commercial laser light scattering spectrometer (Malvern Autosizer 3000) with a 5 mW He-Ne laser. SEM images were recorded by means of a Gemini LEO 1550 instrument operated at 3 kV. The specimens were sputter-coated with gold prior to SEM imaging. AFM imaging was performed by using the Nanoscope Dimension 3100 system operating in tapping mode. Contact angle measurements were implemented with a contact angle measuring system G10 apparatus (Kru¨ss, Germany) at ambient temperature. (11) Kuang, M.; Wang, D.; Gao, M.; Hartmann, J.; Mo¨hwald, H. Chem. Mater. 2005, 17, 65. (12) Co¨lfen, H.; Antonietti, M. Langmuir 1998, 14, 582.
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Scheme 1. Schematic Illustration of Self-Assembly Derived from Hard and Soft Spheres during Water Evaporationa
a
Due to their large shrinkage, one can predict the formation of non-close-packing arrays by use of soft spheres, such as hydrogel spheres.
Results and Discussion Two-Dimensional Non-Close-Packing Arrays of PNIPVP Spheres. Spherical particles have the ability to self-assemble into a hexagonal or cubic packing array under a certain external field, for example, gravity. Although these spheres are in physical contact in the dried state, they are actually separated from each other in the wet state due to, for instance, electrostatic repulsion.1 During water evaporation, the attractive interaction between spheres, for example, capillary force, drives spheres closer, reducing the distance between spheres. In the meantime, the spheres themselves shrink, depending on the volume fraction of water in the spheres. If the reduction of the distance between spheres is faster and larger than the shrinkage of the individual spheres, closepacking arrays are formed. As depicted in Scheme 1, this may be usually implemented by use of latex and silica spheres, referred to as hard spheres that have a smaller shrinkage. Figure 1a shows a typical scanning electron microscope (SEM) picture of 2D colloidal assemblies of 596 nm silica spheres formed via a dip-coating process, in which a hexagonal close-packing array is observed. The existence of cracks evidences the collapse of non-close-packed colloidal assemblies in the wet state and the shrinkage of spheres after drying.1b,c In contrast, if the reduction of the intersphere distance is much smaller than the shrinkage of individual spheres, the non-close-packing arrays formed in the wet state should be maintained, as shown in Scheme 1. This realization is prerequisite that the spheres must have a large shrinkage, referred as soft spheres here. In this point of view, hydrogel spheres should be the best candidates to realize 2D nonclose-packing arrays. In our work, the concentrations of all aqueous dispersions of PNIPVP spheres were optimized to form 2D colloidal assemblies. Figure 1b shows a typical SEM image of 2D colloidal assemblies formed by dip-coating a 0.1 wt % PNIPVP dispersion on silicon wafers at a withdrawing speed of 5 µm/s. Obviously, plan-convex shaped particles of 600 nm in diameter self-organize into a 2D hexagonal non-close-
packing array. The center-to-center distance of neighboring particles is about 1080 nm, corresponding to an interparticle gap of 480 nm. Their height is around 120 nm, determined by atomic force microscopy (AFM). Nagayama and coworkers13 have extensively explored the convective 2D selfassembly of colloidal spheres in the process of dip-coating. When the substrate is slowly withdrawn from the colloidal suspension, a wetting thin film is formed on the substrate. The capillary force, induced by the water evaporation, transports the spheres from the bulk suspension to the substrate. When the thickness of this wetting film decreases during water evaporation, the component of the capillary force, vertical to the substrate, presses the spheres on the substrate. In the meantime, in the lateral direction another component, an attractive force, drives the spheres to approach each other. As the ordered packing has the higher entropy than the random one due to the translational degrees of freedom,14 the spheres are prone to self-assemble into an ordered non-close-packing array in the thin wetting films on the substrate. Once the drying front, the interfacial line between water, air, and the substrate, passes across the spheres, they are removed from the wet colloidal reservoir and pressed on the surface upon drying, thus losing mobility. In our work, the hydrogel spheres themselves shrink dramatically during water evaporation, following the model depicted in Scheme 1, so they may be expected to remain separated, thus creating a non-close-packing array in the dry samples. In the dip-coating process, the concentration of spheres provides a measure to control the density of spheres accumulating within the wetting thin film of the colloidal suspension on the substrate.2h Figure 2 shows a typical AFM image of 2D colloidal assemblies of PNIPVP spheres by dipcoating a 0.25 wt % dispersion, where close-packing arrays (13) (a) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183. (b) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303. (c) Nagayama, K. Colloid Surf. A. 1996, 109, 363. (14) Haw, M.; Poon, W.; Pusey, P.; Hebraud, P.; Lequeux, F. Phys. ReV. E 1998, 58, 4673.
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Figure 2. AFM image of a dried 2D colloidal assembly obtained by dipcoating a 0.25 wt % aqueous dispersion of PNIPVP spheres of diameter 670 nm in water, pH 7, on silicon wafers. The withdrawing speed is 5 µm/s.
Figure 1. SEM images of 2D colloidal assemblies produced by dip-coating (a) a 1.0 wt % aqueous dispersion of 596 nm silica spheres and (b) a 0.1 wt % aqueous dispersion of PNIPVP spheres of diameter 670 nm in water, pH 7, on silicon wafers. The withdrawing speed is 5 µm/s. The scale bar is 3 µm.
are clearly observed. Note that the surfaces of the dried colloidal assemblies of PNIPVP spheres are too smooth for SEM imaging. This is similarly reported in the literature.9 It is due to the fact that, as suggested in the literature,9d when hydrogel spheres approach, their surfaces may interpenetrate, which may impede movement of the gel spheres, thus creating less ordered close-packing arrays. Two-Dimensional Non-Close-Packing Arrays of CaCO3PNIPVP Spheres. Herein we conducted an in situ mineralization of CaCO3 in the gel spheres by use of slow diffusion of CO2, leading to the formation of CaCO3 on and within the gel spheres, denoted as CaCO3-PNIPVP. As suggested in our previous report,11 as the CO2 vapor diffuses from the outer shell into the center of the hydrogel spheres, the CaCO3 is mainly accumulated in the outer shells of the spheres, efficiently reducing the surface penetration. Due to their low
loading with CaCO3, the composite spheres swell similarly to PNIPVP ones, with little variation of the hydrodynamic diameter being observed. Figure 3a reveals a typical SEM image of 2D colloidal assemblies produced by dip-coating aqueous dispersions of CaCO3-PNIPVP composite spheres (0.1 wt %) at a withdrawing speed of 5 µm/s, exhibiting a non-close-packing array of disklike particles of around 550 nm in diameter. The center-to-center distance between particles is about 1040 nm, corresponding to a 490 nm interparticle gap. The height of these particles is around 200 nm, determined by AFM (Figure 4), which is higher than that of the particles from pure hydrogel spheres. As shown in Figure 3b, if the withdrawing speed is reduced from 5 to 1 µm/s, the center-to-center distance between particles decreases to 730 nm, corresponding to a 180 nm gap between particles, much smaller than that found in Figure 3a. The diameter and height of the disklike particles remain little changed. In the process of dip-coating, the withdrawing speed allows one to control the drying front movement; the lower withdrawing speed may slow the drying front movement.2h,13 Accordingly, it allows the hydrogel spheres within the wetting thin film of the suspensions to move closer, driven by the lateral attractive capillary force, thus leading to the smaller intersphere gaps. Furthermore, it enables more hydrogel spheres to accumulate within the wetting thin films or transport into the drying front, thus increasing the density of spheres and in turn reducing the distance of the neighboring gel spheres. A project to monitor the formation of 2D non-close-packing arrays based on hydrogel spheres is underway in our lab. Figure 3c shows a typical SEM image of 2D colloidal assemblies produced by dip-coating a 0.5 wt % CaCO3PNIPVP aqueous dispersion at a withdrawing speed of 5 µm/ s. The diameter of the disklike particles remains similar to those observed in Figure 3a,b, 550 nm, while the center-tocenter distance becomes much smaller, 620 nm, corresponding to an interparticle gap of 70 nm. Meanwhile, small fibers connecting the particles are observed, witnessing the surface penetration of neighboring spheres.9d After the withdrawing
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Figure 3. SEM images of 2D non-close-packing arrays formed by dip-coating a 0.1 wt % aqueous dispersion of CaCO3-PNIPVP spheres of diameter 670 nm in water, pH 7, at a withdrawing speed of 5 µm/s (a) or 1 µm/s (b), and SEM images of 2D non-close-packing arrays formed by dip-coating a 0.5 wt % aqueous dispersion of CaCO3-PNIPVP spheres of diameter 670 nm in water, pH 7, at a withdrawing speed of 5 µm/s (c) or 1 µm/s (d). The scale bar is 1 µm.
Figure 4. Three-dimensional AFM image of non-close-packing arrays formed by dip-coating a 0.1 wt % aqueous dispersion of CaCO3-PNIPVP spheres at a withdrawing speed of 5 µm/s.
Figure 5. Low-magnification SEM image of 2D non-close-packing arrays formed by dip-coating a 0.1 wt % aqueous dispersion of CaCO3-PNIPVP spheres of diameter 670 nm in water, pH 7, at a withdrawing speed of 5 µm/s. The scale bar is 10 µm.
speed is reduced to 1 µm/s, 2D close-packing arrays are obtained, as shown in Figure 3d, which is due to the fusion of gel spheres. This structure is similar to those found in Figure 2. Of importance is that with our procedure one can reproducibly grow non-close-packing 2D colloidal assemblies, shown in Figures 1 and 3, over areas as large as the substrates used. Figure 5 shows that the non-close-packed 2D assemblies are achieved over areas larger than 100 µm. In our work, we have conducted our procedure on silicon wafers and glass slides. It is worth noting that the best results were achieved always on silicon wafers rather than on glass slides, which is likely due to the exceedingly smooth surface of silicon wafers. Binary Colloidal Assemblies from 2D Non-ClosePacking Arrays. In the present work, we employed the
resultant 2D non-close-packing arrays of CaCO3-PNIPVP particles, shown in Figure 3a, as templates for directing selfassembly of silica spheres. The water contact angle of the silicon wafers coated with non-close-packed CaCO3PNIPVP monolayers is around 44°, much higher than that of pure silicon wafers (less than 5°). This suggests that CaCO3-PNIPVP particles are less hydrophilic than silicon wafers. When silica spheres are dip-coated on the silicon wafers covered with non-close-packing arrays of CaCO3PNIPVP particles, one may therefore expect that the hydrophilicity difference between the composite particle and silicon wafer between particles should guide hydrophilic silica spheres to accumulate between CaCO3-PNIPVP particles. Figure 6a shows a typical SEM image of binary structures by dip-coating 144 nm silica spheres of a concentration of 1.0 wt %. Obviously, all silica spheres are located between
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Figure 7. SEM image of the binary structure formed by dip-coating a 1.0 wt % dispersion of 144 nm silica spheres on the silicon wafer coated with 2D non-close-packing arrays of CaCO3-PNIPVP particles, modified by HFDC. The scale bar is 1 µm.
Figure 8. SEM image of Au patterns fabricated by using 2D non-closepacking arrays of CaCO3-PNIPVP particles as masks for Au vapor deposition. The scale bar is 1 µm.
Figure 6. SEM images of the binary structures formed by dip-coating the dispersion of 144 nm silica spheres with concentration of 1.0 wt % (a) and 0.5 wt % (b) on the silicon wafer coated with 2D non-close-packing arrays of CaCO3-PNIPVP particles. The scale bar is 1 µm.
CaCO3-PNIPVP particles. When dilute dispersions of silica spheres (0.5 wt %) were used for dip-coating, we found that silica spheres do not accumulate between CaCO3-PNIPVP particles but aggregate surrounding the composite particles (Figure 6b). This suggests that the hydrophilicity difference between the composite particle and silicon wafer may generate an attractive interfacial capillary force to drive silica spheres to self-assemble at the interfaces. Similar interfacial attraction has recently been observed by using the walls of the holes within patterned photoresist layers to confine nanoparticles.15 In the present work, we made silicon wafers hydrophobic by modifying them with HFDC. As CaCO3 usually is inert (15) Bo¨ker, A.; Lin, Y.; Chiapperini, K.; Horowitz, R.; Thompson, M.; Carreon, V.; Xu, T.; Abetz, C.; Skaff, H.; Dinsmore, A.; Emrick, T.; Russell, T. Nat. Mater. 2004, 3, 302.
to silanes, the surface wettability of CaCO3-PNIPVP particles remains little changed after the HFDC modification, thus being more hydrophilic than the HFDC-modified silicon wafers between the particles. After dip-coating of 144 nm silica spheres on the HFDC-modified substrates, one may clearly see than all silica spheres are located exclusively on the CaCO3-PNIPVP particles (Figure 7). Pattern Transfer of 2D Non-Close-Packing Arrays. Similar to close-packing ones, the resultant 2D non-closepacking arrays of PNIPVP and CaCO3-PNIPVP particles may also provide masks to pattern other materials. In the present work, we use them as masks for Au vapor deposition. After depositing Au thin films, followed by removal of the gel particles by sonication, we created ordered arrays of holes within Au thin films on silicon wafers, whose packing structures are exactly inverted to the template structures used. Figure 8 presents a typical SEM image of Au films patterned by templating with 2D non-close-packing arrays of CaCO3PNIPVP particles. This patterning technique should provide a complement for the existing nanosphere lithography, where the distances between the circular cavities can be varied in a controlled manner.
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Conclusion In summary, we have successfully fabricated 2D nonclose-packing arrays, based on dip-coating of PNIPVP and CaCO3-loaded PNIPVP spheres. Our procedure enables one to reproducibly fabricate non-close-packed 2D colloidal assemblies over large areas. The interparticle separation in the resulting non-close-packing structures may be manipulated in a controlled fashion. The more rapid the withdrawing speed and the more dilute the colloidal dispersion, the larger the interparticle distance. The resultant 2D non-close-packing arrays haven been employed as templates to direct the selfassembly of silica colloidal spheres, creating binary colloidal crystals. The structures of the resulting binary assemblies, especially the location of the silica spheres, can be manipulated by the wetting properties of the substrates. Our preliminary results demonstrate that these binary colloidal assemblies allow one to achieve superhydrophobicity. On
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the other hand, by using them as masks for Au vapor deposition, the resulting 2D non-close-packing arrays also yield ordered porous Au films, from which one may anticipate an extension for patterning other materials. Overall, the 2D non-close-packing arrays derived from hydrogel spheres and those loaded with minerals should be of great interest for patterning technology. One may expect formation of patterns with feature dimensions less than 100 nm by using smaller hydrogel spheres. This extension is one of our ongoing research topics. Acknowledgment. We thank the Max Planck Society for financial support. Z.-Z.G. acknowledges DAAD for a research fellowship and NSFC for financial support (Projects 60228002 and 60121101). CM050414X
