Connected Open Structures from Close-Packed Colloidal Crystals by

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Connected Open Structures from Close-Packed Colloidal Crystals by Hyperthermal Neutral Beam Etching Young-Sang Cho,† Gi-Ra Yi,‡ Jun Hyuk Moon,† Dae-Chul Kim,§ Bong-Ju Lee,§ and Seung-Man Yang*,† Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Korea, Corporate R&D Center, LG Chem Research Park, Korea, and Korea Basic Science Institute, Korea Received June 13, 2005. In Final Form: August 1, 2005 We report the fabrication of connected open structures from close-packed colloidal crystals by hyperthermal neutral beam etching. Colloidal crystal films of polystyrene microspheres were prepared by a vertical deposition method. Exposure of the colloidal crystal films to hyperthermal neutral beam made isolated microspheres in the face-centered cubic lattice, each of which was connected with its twelve nearest neighbors through very thin cylinders. Due to the charge neutrality of impinging gas molecules of the hyperthermal neutral beam, the spherical shape of polymer microspheres was almost maintained during the etching process. The Bragg reflection peaks were modulated by the etched volume of colloidal crystals. Finally, the inverse structures of such open structures were replicated by a simple room-temperature chemical vapor deposition and subsequently burning out polymer template spheres.

Introduction The dense packing of monodisperse colloidal particles is important for a number of applications including sensors, membranes, catalytic supports, and photonic crystals.1 Over the years, different methods have been developed for controlling the super-structures of a large number of all-identical microspheres, thereby enabling the fabrication of artificial opals, and complex surface patterns.2,3 Colloidal photonic crystals self-assembled by using microspheres as building blocks have a packing structure of face centered cubic (fcc) symmetries and do not exhibit a full photonic band gap but a few pseudogaps. Moreover, the pseudogap widths are usually so narrow that a few defects may erase the band gaps. Motivated by recent interest in colloidal crystals with robust photonic band gaps, many research groups have tried either to develop synthetic routes to nonconventional colloidal crystals by using colloidal clusters of microspheres as building blocks or to modify close-packed structures for controlled photon dispersion.4-9 Among them, a few groups * Corresponding author. E-mail: [email protected]. † Korea Advanced Institute of Science and Technology. ‡ LG Chem Research Park. § Korea Basic Science Institute. (1) Mirkin, C. A.; Letsinger, R. L.; Music, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (2) (a) Wong, S.; Kitaev, V.; Ozin, G. A. J. Am. Chem. Soc. 2003, 125, 15589. (b) Reculusa, S.; Ravaine, S. Chem. Mater. 2003, 15, 598. (c) Gong, T.; Wu, D. T.; Marr, D. W. M. Langmuir 2003, 19, 5967. (d) Choi, D.-G.; Kim, S.; Lee, E.; Yang, S.-M. J. Am. Chem. Soc. 2005, 127, 1636. (3) Ozin, G. A.; Yang, S. M. Adv. Funct. Mater. 2001, 11, 95 (4) (a) Xu, X.; Friedman, G.; Humfeld, K. D.; Majetich, S. A.; Asher, S. A. Adv. Mater. 2001, 13, 1681. (b) Xu, X.; Friedman, G.; Humfeld, K. D.; Majetich, S. A.; Asher, S. A. Chem. Mater. 2002, 14, 1249. (c) Xu, X.; Majetitich, S. A.; Asher, S. A. J. Am. Chem. Soc. 2002, 124, 13864. (5) (a) Kubo, S.; Gu, Z.-Ze.; Takahashi, K.; Fujishima, A.; Segawa, H.; Sato, O. J. Am. Chem. Soc. 2004, 126, 8314. (b) Blanford, C. F.; Schroden, R. C.; Al-Daous, M.; Stein, A. Adv. Mater. 2001, 13, 26. (6) (a) Hu, Z.; Lu, X.; Gao, J. Adv. Mater. 2001, 13, 1708. (b) Debord, J. D.; Eustis, S.; Debord, S. B.; Lofye, M. T.; Lyon, L. A. Adv. Mater. 2002, 14, 658. (7) (a) Gu, Z.-Z.; Kubo, S.; Fujishima, A.; Sato, O. Appl. Phys. A 2002, 74, 127. (b) Kuai, S.; Badilescu, S.; Bader, G.; Bruning, R.; Hu, X.; Truong, V.-V. Adv. Mater. 2003, 15, 73. (8) Gates, B.; Park, S. H.; Xia, Y. Adv. Mater. 2000, 12, 653. (9) Fudouzi, H.; Xia, Y. Adv. Mater. 2003, 15, 892.

have developed novel structures of colloidal microspheres by using nonisotropic colloidal clusters (or molecules) as building blocks.10 Recently, clever schemes for the preparation of such colloidal molecules have been proposed and demonstrated, and it is under way to self-organize such colloidal molecules in inexpensive way. As alternative and practical means, tuning of the photonic band gaps by modifying the structures of opaline colloidal crystals of fcc packing symmetry has been extensively studied by various groups. The factors affecting photonic band gaps are the refractive index mismatch, lattice constant and shape of lattice “atoms” of colloidal crystals. In this context, band gap tuning has been attempted by a number of strategies including swelling the PDMS matrix of a polystyrene-PDMS composite colloidal crystal, converting the shape of lattice “atoms” from spheres to ellipsoids, changing interparticle spacing between magnetically controllable building block particles, infiltrating liquid crystal or solvents to opals or inverse opals, and using hydrogel opals.4-6,9 In particular, Dong et al. developed the so-called “skeleton structure”, by infiltrating partially hydrophobic ceramic precursors into opals of polymeric microspheres, which were used as sacrificial templates. These structures may produce unusual optical properties; for example, the skeleton structure exhibits dual photonic band gaps.11 However, these indirect modification methods have some difficulties in the precise control of the lattice structures due to the limited number of manipulation parameters. One of the promising direct routes to the fabrication of novel photonic band gap materials from colloidal opaline structures is a selective etching of the close-packed (10) (a) Lu, Y.; Yin, Y.; Li, Z.-Y.; Xia, Y. Langmuir 2002, 18, 7722. (b) Sumioka, K.; Kayashima, H.; Tsutsui, T. Adv. Mater. 2002, 14, 1284. (c) Velikov, K. P.; Dillen, T. v.; Polman, A.; Blaaderen, A. v. Appl. Phy. Lett., 2002, 81, 838. (d) Snoeks, E.; Blaaderen, A. v.; Dillen, T. v.; Kats, C. M. v.; Velikov, K.; Brongersma, M. L.; Polman, A. Nucl. Instrum. Methods B 2001, 178, 62. (e) Ji, L.; Rong, J.; Yang, Z. Chem. Comm. 2003, 1080. (11) (a) Dong, W.; Bongard, H.; Tesche, B.; Marlow, F. Adv. Mater. 2002, 14(20), 1457. (b) Dong, W.; Bongard, H. J.; Marlow, F. Chem. Mater. 2003, 15, 568. (c) Marlow, F.; Dong, W. ChemPhysChem 2003, 4, 549.

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spherical lattice atoms of fcc symmetry. Up to now, two different etching techniques have been developed for the production of nonclose-packed colloidal crystal films; namely, reactive ion or plasma etching of polystyrene colloidal crystals and wet etching of silica colloidal crystal with hydrofluoric acid (HF) solution.12,13 Recently, Fenollosa and his colleagues have demonstrated that nonclosepacked artificial opals could be produced by a liquid-phase partial etching of sintered silica colloidal crystals with HF.12 Their interconnected nonclose-packed structures exhibited a shift of stop band position as well as an intensified reflectance peak compared to the original colloidal crystal. However, the wet etching process has some drawbacks for the limited number of control parameters and the use of the harmful HF solution. Meanwhile, a number of research groups have used conventional reactive ion etching (RIE) to convert the closepacked polystyrene spheres into nonclose-packed oblate particles.13 However, this RIE method has some limitations in that isotropic etching of building block particles is impossible and the reactive ion beam causes random rugged structures on the particle surfaces. To overcome the problems in RIE, Freymann et al. used a dry plasma etching to modify a woodpile structure fabricated by direct laser writing and studied the photonic bad gap of partially etched polymeric photonic crystals.14 More recently, dry O2 plasma etching has been performed by Ozin and his colleagues for the fabrication of nonclose-packed artificial opals.15 They have observed enhanced coupling to slow photon modes from their graded colloidal crystals. However, the O2 plasma etching produced a nonuniform, graded structure due to the weak etching efficiency and charged ionic species of plasma. Consequently, a more powerful and practical dry etching technique is needed for the connected open structures, and hyperthermal neutral beam (henceforth, referred to as HNB) etching is a promising alternative for this purpose. In this article, we demonstrated a practical and facile method to produce polymeric colloidal crystal films of connected nonclose-packed microspheres in fcc lattice by partial etching with HNB. Also, we produced the inverse structures of such connected open structures by a simple room-temperature chemical vapor deposition (CVD) and subsequently removing the polymeric template by thermal decomposition. HNB-assisted dry etching techniques have advantages over liquid-phase etching for fabricating complex nanostructures by controlling the operating parameters such as RF power to generate plasma, plasma gas flow rate, composition of etching gas species, and etching time.16-20 HNB has been widely adopted for the fabrication of deep submicron integrated circuits and the surface modification of photoresist or polymeric films without serious problems such as radiation damage, charging and notching. (12) Fenollosa, R.; Meseguer, F. Adv. Mater. 2003, 15, 1282. (13) (a) Fujimura, T.; Tamura, T.; Itoh, T. Appl. Phys. Lett. 2001, 78 (11), 1478. (b) Choi, D.-G.; Yu, H. K.; Jang, S. G.; Yang, S.-M. J. Am. Chem. Soc. 2004, 126, 7019. (14) Freymann, G. v.; Chan, T. Y. M.; John, S.; Kitaev, V.; Ozin, G. A.; Deubel, M.; Wegener, M. Photonics Nanostruct. 2004, 2, 191. (15) Freyman, G. v.; John, S.; Kitaev, V.; Ozin, G. A. Adv. Mater. 2005, 17, 1273. (16) Lee, D. H.; Bae, J. W.; Park, S. D.; Yeom, G. Y., Thin Solid Films 2001, 647, 398. (17) Mizutani, T.; Yunogami, T. Jpn. J. Appl. Phys. 1990, 29, 2220. (18) Gottscho, R. A.; Jurgensen, C. W.; Vitkavage, D. J. J. Vac. Sci. Technol. B 1992, 10, 2133. (19) Giapis, K. P.; Moore, T. A.; Minton, T. K. J. Vac. Sci. Technol. A 1995, 13, 959. (20) Tagawa, M.; Ohki, Y.; Yokota, K.; Ohmae, N. J. Adhesion Sci. Technol. 2004, 18, 243.

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Experimental Section Materials. Styrene (99%), potassium persulfate (initiator, 98%), and sodium hydrogen carbonate (buffer, 99%) were purchased from Kanto Chemicals. The comonomer, sodium styrene sulfonate, was obtained from Aldrich. Milli-Q water (18.2 MΩcm-1) was used as the reaction medium for the polymerization. Reagent grade methanol, acetone, and 2-propanol were obtained from Merck. Instrumentation. Fabrication of colloidal crystals by the dipcoating method was performed by using a stage controller (Sigma Koki co., ltd, MARK202). Before dip-coating, the glass or silicon wafer substrate was treated with oxygen plasma by using an O2 plasma cleaner/sterilizer (HARRICK). The annealing of the colloidal crystal film was performed by heating inside a drying oven (Daihan). A field emission scanning electron microscope (FE-SEM, Philips, XL305FEG) was used to observe the connected open structure of colloidal crystals after the HNB etching of the sample. The reflectance spectra of dry-etched samples were measured by using a reflectance measurement system composed of a xenon lamp (Hamamatsu, C2577) and a monochromator (Dongwoo Optron, PM201). A muffle furnace (Isuzu, PK9712150030-9) was used to burn out organic cores of polystyrene to create silica inverse opal. Synthesis of Aqueous Monodisperse PS Latex. Monodisperse PS microspheres with mean diameters of 280 nm were prepared by an emulsifier-free emulsion polymerization method using styrene as the monomer, potassium persulfate as the initiator, and sodium hydrogen carbonate as the buffer.21 The size of the PS particles could be controlled by varying the amount of the comonomer, sodium styrene sulfonate. During polymerization, the reaction temperature was maintained at 70 °C, and an impeller was used for vigorous agitation of the reactants at 300 rpm. After completion of reaction, unreacted precursors were removed through solvent exchange twice by centrifugation and redispersion of particles in distilled water by ultrasonication for 30 min. Preparation of a Colloidal Crystal Film on the Substrate. Colloidal crystal films of PS microspheres were prepared on a glass substrate by a modified vertical deposition method.22 Before the deposition of PS particles, slide glasses or silicon wafers were washed by using methanol, acetone, and 2-propanol to remove organic contaminants on the surface. Then, oxygen plasma was treated for one minute to increase the hydrophilicity of the substrates. During the dip-coating procedure, the withdrawing speed of the glass substrate was varied to control the thickness of the colloidal crystal film by using a stage controller. The annealing of the colloidal crystal film was performed by heating inside a drying oven at 100 °C during different time intervals. Creation of Connected Open Structures by Dry Etching. Isotropic dry etching of PS colloidal crystals was performed to 2 × 2 mm2 colloidal crystal film with HNB to prepare nonclosepacked polystyrene colloidal crystals. The plasma source was a planar type of inductively coupled plasma source, and the ions produced in the plasma were accelerated from the sheath boundary onto the reflector plate and changed into neutrals through surface reflection neutralization.23 The plasma reactor was operated with 800 W of RF power. The etching time or input voltage of the plasma reactor was changed to obtain proper experimental conditions from 1 to 5 min and -10 to -70 V, respectively. Structure Inversion of Connected Open Structures. Inversion of the connected open structure was accomplished by CVD of silicon tetrachloride into interstitial voids between particles.24 A CVD reactor was used to feed the bubbled gas of silicon tetrachloride and water to the sample alternatively. After short exposure to the precursor gases, the organic template was removed by burning out at 500 °C for 5 h. Sample Characterization. Samples for imaging with FESEM were prepared by coating with gold to avoid possible (21) Yi, G.-R.; Moon, J. H.; Yang, S.-M. Chem. Mater. 2001, 13, 2613. (22) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (23) Yoo, S. J.; Yang, H. L.; Jung, M.; Lho, T.; Kim, D. C.; Lee, B. J.; Kim, J. S.; Kim, G. H. Fusion Sci. Technol. 2003, 43, 286. (24) Miguez, H.; Tetreault, N.; Hatton, B.; Yang, S. M.; Perovic, D.; Ozin, G. A. Chem. Commun. 2002, 2736.

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Figure 1. Schematic of the fabrication of nonclose-packed artificial opals with interconnecting networks: (a) PS colloidal crystal film, (b) sintered colloidal crystal film, and (c) nonclose-packed artificial opal

Figure 2. Surface SEM images of nonclose-packed PS opals after severe sintering treatment. The original PS spheres were 280 nm in diameter, sintered at 100 °C for 12 h and oxygen HNB treated for 1, 3, and 5 min (scale bar indicates 500 nm): (a) original colloidal crystal, (b) after sintering, (c) after etching for 1 min, (d) after etching for 3 min, and (e) after etching for 5 min charging. The reflectance spectra of HNB-etched samples were obtained from reflectance measurement system. The incident light was unpolarized and all measurements were made at normal incident angle with respect to the surface of colloidal crystal film.

Results and Discussion The procedure for the fabrication of connected open structures is described schematically in Figure 1. First, the colloidal crystal film of 280-nm polystyrene (PS) microspheres on substrates was prepared by a modified vertical deposition method developed previously22 and was annealed at 100 °C for 12 h to make the PS spheres sintered and necked. The sintered colloidal crystals were then exposed to HNB of oxygen gas for the partial etching of PS microspheres. In doing this, the energetic neutral oxygen gas collides with PS microspheres and reacts with the hydrophobic surfaces to generate gaseous H2O, CO, or CO2 leaving behind the partially etched particles, each

connected with twelve nearest neighbors through very thin cylinders.25 The etching chamber was operated at 80 mTorr of oxygen pressure for given etching time intervals. Figure 2, panels a and b, shows the SEM images of the PS colloidal crystals before and after sintering, respectively. Indeed, after sintering the shape of PS particles became hexagonal and in close contact with each other. Such a morphological change is because of the viscoelastic deformation of PS entailing the space-filling shape of colloidal crystals.8,26 Then, the sintered colloidal crystals in Figure 2b were exposed to HNB in three different time intervals of 1, 3, and 5 min and their SEM images are shown in Figure 2, panels c-e, respectively. As shown in (25) Manos, D. M.; Flamn, D. L. Plasma Etching: An Introduction; Academic Press: New York, 1989. (26) (a) Mazur, S.; Beckerbauer, R.; Buckholz, J. Langmuir 1997, 13, 4287. (b) Pan, G.; Kesavamoorthy, R.; Asher, S. A. J. Am. Chem. Soc. 1998, 120, 6525. (c) Rundquist, P. A.; Jagannathan, S.; Kesavamoorthy, R.; Brnaric, C.; Xu, S.; Asher, S. A. J. Chem., Phys. 1991, 94, 711.

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Figure 3. (a-c) Cross-sectional SEM images of nonclosepacked PS opals at various viewing angles. The original PS spheres were 280 nm in diameter, sintered at 100 °C for 12 h and oxygen HNB treated for 3 min. Scale bars are 1 µm for (a) and 500 nm for both (b) and (c). (d) Variation of the size of HNB-etched PS particles across the 40-layer colloidal crystal film. The PS size was normalized with respect to the initial value of 280 nm. Also included in (d) for comparison is the normalized size variation of O2 plasma-etched PS particles in the 17-layer film produced by Freyman et al.15 The initial size was 850 nm in the O2 plasma etching.

Figure 2c, a slight etching of PS microspheres was achieved and short networks between particles were created as the diameter of the PS spheres was reduced from 280 to 240 nm. These interconnecting cylinders disappeared after a more prolonged etching as shown in Figure 2d. The diameter of particles in the top layer was reduced to 180 nm, whereas the particle size in the second layer was decreased to 227 nm with an increase in the etching time. As shown in these images, nearly spherical microspheres form nonclose-packed hexagonal arrays due to the partial etching of PS particles. Moreover, the PS microspheres of the top layer completely disappeared, and the sphericity of particles in the next layer was reduced due to the overetching for 5 min, as noted from Figure 2e. In addition, we also observed the morphology of PS colloidal crystals after a more prolonged etching for 30 min, although the result is not reproduced here. The prolonged HNB etching melted the colloidal crystal films slowly because of the exhausting ceaseless bombardment of highly energetic oxygen gas resulting in the deterioration of the sample. A nonclose-packed structure of isolated microspheres with thin connecting networks can be confirmed from the SEM images of various viewing angles in panels a-c of Figure 3, which were taken after HNB etching for 3 min. Our HNB etching shows three important features of colloidal assemblies. First, the size of partially etched PS particles was quite uniform except for the top layer, which

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was bombarded most severely and sacrificed by HNB. Second, the spherical morphology of particles was maintained, indicating that the etching was nearly isotropic. The isotropic partial etching of the microspheres is because the impinging gas of HNB onto the colloidal crystal in the chamber is charge-neutral so that the oxygen gas molecules or radicals collide uniformly with the hydrophobic PS surface. This is an advantageous feature of the present HNB dry etching; conventional reactive ion etching produced the anisotropic morphology of PS microspheres.13 Third, the particle surface of our open structure is much smoother than the conventionally etched one, which means that the damaged rugged surface by anisotropic etching can be prevented by oxygen HNB. The degree of HNB etching along the depth of the film can be seen from a full cross-sectional SEM image of 40layered PS microspheres in Figure 3a. We measured the size variation of the HNB etched PS particles across the 40-layer colloidal crystal film, and the result is shown in Figure 3d. Also included here for comparison is the size change of O2-plasma-etched PS particles across a 17-layer colloidal crystal film produced by Freyman et al.15 In this plot, the sizes of etched particles were normalized with respect to the initial sizes. From Figure 3d, we can see typical features of the present HNB etching in contrast to the preceding plasma etching. In particular, in the O2plasma etching, PS particles were etched mostly in the first five layers, beyond which the PS particles were not etched appreciably and the size was reduced only by 5% or less. Therefore, the plasma etching forms the size gradient of PS particles only in the first few layers. However, the present HNB etching proceeds throughout the colloidal crystal film and forms a moderate size gradient in the entire colloidal crystal because the hyperthermal neural beam can penetrate well into the colloidal crystal film. As noted, the particle diameter of the 40th layer on the substrate was about 260 nm, which was still smaller than the original 280 nm before HNB etching. Therefore, the size of HNB-etched PS particles was reduced by 9% or more throughout the 40-layer colloidal film. This implies that the HNB of oxygen reached the bottom of the film and etched effectively a 40-layered PS colloidal crystal film. Indeed, the top layer was etched severely because of the etching-mask function of the toplayer PS spheres during the HNB process. However, the size of partially etched PS particles was almost uniform from the 2nd to 10th layers, and the diameter of the etched PS microspheres was 225 nm on average. As we shall see shortly, the reflectance spectrum became broader when the HNB etching was applied to the colloidal crystal film due to the graded size distribution. It can be also noted from Figure 3d that the etching power of the O2-plasma is so weak that it requires a long time for the desired reduction of particle size. As expected, the HNB etching is powerful, and only a short exposure to HNB is enough for the substantial reduction of particle size even for the 40th layer of the colloidal crystal film. Recently, Fenollosa and his colleagues reported a novel wet etching technique for nonclose-packed colloidal crystals with hydrofluoric acid, which is especially good for silica microspheres.12 In contrast to their procedure, our HNB etching works with dry oxygen gas which is a very effective etchant for polymeric latex spheres. Moreover, besides the duration time of etching, we have other controlling parameters such as applied voltage for the beam formation of neutral gas and the composition of etchant gases, e.g., SF6 or CF4 for partial etching of inorganic microspheres. For example, we can create a nonclose-packed connected structure of silica particles by

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Figure 4. Reflectance spectra of sintered colloidal crystal and nonclose-packed opals prepared by HNB etching. The original PS spheres were 280 nm in diameter, sintered at 100 °C for 12 h and oxygen HNB treated for 1-5 min.

a similar HNB etching using SF6 or CF4 as an etchant gas instead of O2. However, it is noteworthy that the annealing or sintering should be conducted at ca 1000 °C to create the connected thin networks between silica microspheres. Thus, the present HNB etching can provide a versatile way for the fabrication of the connected open structures for either silica or polymeric latexes. To investigate the effect of HNB etching on the stop band position of the colloidal crystal, we measured the reflectance spectra of the annealed PS colloidal crystal and its connected open structures of Figure 2, panels b-e, prepared by the HNB treatment. As shown in Figure 4, the maximum reflectance peaks due to Bragg diffraction were blue-shifted with the etching time since the volume fraction of building blocks was decreased during etching. It is noteworthy that the lattice spacing of the fcc structure was maintained due to the presence of robust interconnecting cylindrical networks between microspheres.12 As noted from Figure 4, all samples exhibited the FabryPerot resonances as minor peaks at wavelengths longer than 650 nm, indicating a good colloidal crystallinity and optical quality. The intensity of the maximum reflectance peak was reduced with the etching time, which was consistent with the evolution of reflectance spectra of polymeric woodpile structures etched by oxygen plasma by Wegener and his colleagues.14 Consequently, the reduction of volume fraction of polystyrene microspheres with the etching time was responsible for the reduction of peak heights in reflectance spectra.14 Moreover, the broadening of the reflectance peak for the sample with HNB-treatment for 5 min was observed, which was caused by the size variation of polystyrene microspheres across the connected open structure film.14 This result provides a possible route to fine-tuning of stop bands (both positions and widths) of colloidal photonic crystals by changing the size of building blocks during the HNB etching procedure. The structure of the nonclose-packed network depended also on the condition of annealing prior to HNB etching. For colloidal crystals annealed at 70 °C which was below the glass transition temperature, the interconnected networks between polystyrene microspheres were not produced by the same etching procedure since the sintering between particles was not sufficient. In this work, therefore, the annealing temperature was fixed at 100 °C near the glass transition temperature. To control the diameter of connecting networks of our nonclose-packed artificial opals, we have changed the annealing time prior

Figure 5. Surface SEM images of connected open structures prepared by HNB etching for 1 min (a) annealed sample for 6 h (b) annealed sample for 12 h (c) annealed sample for 18 h. The original PS sphere was 280 nm in diameter, annealed at 100 °C. Scale bar is 250 nm.

to 1 min HNB etching. As noted from Figure 5a, the diameter of connecting networks between microspheres was about 40 nm after 1 min HNB etching for the sample annealed at 100 °C for 6 h. These thin connecting networks were 90 nm thick by increasing the annealing time to 12 h, as shown in Figure 5b. Since the more prolonged annealing of a colloidal crystal results in a stronger and closer contact between PS microspheres during viscoelastic deformation, the bombardment of oxygen HNB will generate the thicker connecting networks between nonclose-packed microspheres. It can be seen from Figure 5c that the connecting networks were not produced for the sample annealed for 18 h, since very strong contact between microspheres may withstand the oxygen neutral beam during the etching step. Finally, we used our connected open structures as templates to produce high refractive index materials with ordered arrays of air cavities and interconnecting channels by the colloidal templating method.27 The remarkable feature of this inverted structure is that the volume fraction of air cavities can be adjusted to be smaller than 74% and fine-tuning of stop bandwidth is possible by (27) (a) Moon, J. H.; Kim, S.; Yi, G.-R.; Yang, S.-M. Langmuir 2004, 20, 2033. (b) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111.

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Figure 6. (a) Schematic of the fabrication of silica inverse opal obtained from connected open structured template. (b and c) SEM images of inverted structures. The original PS spheres was 280 nm in diameter, sintered at 100 °C for 12 h and oxygen HNB treated for 3 min. Scale bars in (b) and (c) are 1 µm and 200 nm, respectively.

changing the size of air cavities.28 Nevertheless, only one precedent report deals with the realization of this inverted structure by wet etching of silica particles in a polymeric matrix.29 In the present study, we fabricated this novel macroporous structure by the chemical vapor deposition (CVD) method which was originally developed by Ozin and co-workers.24 Though other sol-gel precursors for higher refractive index materials such as titanium tetrachloride could be also used, a less reactive silicon-based precursor was used for our experiments due to the ease of handling. First, the connected open structured template was placed into a CVD reaction chamber. Then, silicon tetrachloride and water vapor bubbled with nitrogen gas were fed to the chamber alternatively to induce hydrolysis and condensation reaction of the precursor inside the interstitial voids of the template. After 20 min of sol-gel reaction, the sample was heated at 500 °C for 5 h to burn out the PS template leaving behind the silica inverse structure. The inversion procedures are depicted schematically in Figure 6a, and panels b and c of Figure 6 contain the SEM images of the surfaces of silica inverse opal obtained from the connected open structured template in Figure 3. As noted from the SEM images, relatively thick shells were formed between hexagonally ordered air cavities, each with six interconnecting side channels. This implies a successful replication of open packing of isolated microspheres with connecting networks. The diameter of macropores is about 160 nm, indicating that the open connected structure shown in Figure 3 was inverted with about 11% of shrinkage during calcination. (28) Doosje, M.; Hoenders, B. J.; Knoester, J. J. Opt. Soc. Am. B 2000, 17, 600. (29) Jiang, P.; McFarland, M. J. J. Am. Chem. Soc. 2004, 126, 1377813786.

Conclusions In this work, we developed a novel strategy for the fabrication of open structures of connected nonclosepacked microspheres in a fcc lattice using the HNB etching technique. The annealing of PS colloidal crystals allowed the formation of cylindrical networks between neighboring particles, after the isotropic etching of PS particles. These connected open structures revealed different optical properties compared to the original colloidal crystals, resulting in the blue-shift of reflectance spectra due to the reduction of the particle volume fraction. The diameters of connecting networks between isolated microspheres were also controlled by changing the annealing time before HNB treatment. Moreover, the inverse structures of such open structures were replicated by CVD, which has diverse applications for the preparation of newtypes of inverse opals with higher filling fraction, separation medium for fractionation of particles with polydisperse size distribution, and microreactors with interconnecting channels. Acknowledgment. We express our gratitude for the financial support for this research by the National R&D Project of Nano Science and Technology of Korea, and partial support from the BK21 Program, the CUPS-ERC and the Center for Nanoscale Mechatronics & Manufacturing of the 21st Century Frontier Research Program (M102KN010002-05K1401-00214). The Korea Basic Science Institute is also acknowledged for allowing us to use their scanning electron microscope. LA051558T