Fabrication of Spherical Colloidal Crystals Using Electrospray

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Fabrication of Spherical Colloidal Crystals Using Electrospray Seung-Hwan Hong, Jun Hyuk Moon, Jong-Min Lim, Shin-Hyun Kim, and Seung-Man Yang* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 305-701 Korea Received May 12, 2005. In Final Form: August 1, 2005 We demonstrated the use of electrohydrodynamic atomization to prepare uniform-sized emulsion droplets in which equal spheres of silica or polystyrene were dispersed. The size of the emulsion droplets was easily controlled by the electric field strength and the flow rate, independently of the diameter of the nozzles. During the evaporation of solvent in the droplets, spherical colloidal crystals were formed by self-assembly of the monodisperse colloidal spheres. The diameter of the spherical colloidal crystals was in the range of 10-40 µm. Depending on the stability of colloidal particles, the morphology of the self-assembled structure was varied. In particular, silica spheres in ethanol droplets were self-assembled into compactly packed silica colloidal crystals in spherical shapes, whereas polystyrene latex spheres in toluene droplets selfassembled into spherical colloidal crystal shells with hollow cores. The silica colloidal assemblies reflected diffraction colors according to the three-dimensionally ordered arrangement of silica spheres.

Introduction Self-assembled colloidal crystals have been of special interest due to their unique characteristics such as photonic band gaps, diffraction patterns, high catalytic throughput, and high packing density. Packed colloidal crystals with three-dimensional, hierarchical pore structures and high surface-to-volume ratios can be applied to catalytic and absorbent supports,1 separation and purification membranes,2 and biomaterials.3 Furthermore, the photonic band gaps of colloidal crystals are of practical significance for optical filters and switches,4-5 chemical sensors,6 and photonic inks.7 A plethora of self-assembly methods have been developed by utilizing external forces including electric fields,8,9 flow fields, capillary forces,10-12 temperature gradient,13 and oscillating stages.14 These approaches have been demonstrated to yield high-quality colloidal crystals containing few defects over large areas. The critical step toward functional colloidal crystals is * To whom correspondence should be addressed. Tel: 82-42869-3922. Fax: 82-42-869-3910. E-mail: [email protected]. (1) Al-Daous, M. A.; Stein, A. Chem. Mater. 2003, 15, 2638. (2) Takeoka, Y.; Watanabe, M. Langmuir 2003, 19, 9554. (3) Kotov, N. A.; Liu, Y. F.; Wang, S. P.; Cumming, C.; Eghtedari, M.; Vargas, G.; Motamedi, M. Nichols, J.; Cortiella, J. Langmuir 2004, 20, 7887. (4) Oswald, J. A.; Wu, B. I.; Mclntosh, K. A.; Mahoney, L. J.; Verghese, S. Appl. Phys. Lett. 2000, 77, 2098. (5) Pan, G.; Kesavamoorthy, R.; Asher, S. A. Phys. Rev. Lett. 1997, 78, 3860. (6) Iwayama, Y.; Yamanaka, J.; Takiguchi, Y.; Takasaka, M.; Ito, K.; Shinohara, T.; Sawada, T.; Yonese, M. Langmuir 2003, 19, 977. (7) Arsenault, A. C.; Miguez, H.; Kitaev, V.; Ozin, G. A.; Manners, I. Adv. Mater. 2003, 15, 503 (8) Holgado, M.; Garcia-Santamaria, F.; Blanco, A.; Ibisate, M.; Cintas, A.; Miguez, H.; Serna, C. J.; Molpeceres, C.; Requena, J.; Mifsud, A.; Meseguer, F.; Lo´pez, C. Langmuir 1999, 15, 4701. (9) Wen, W. J.; Qang, N.; Ma, H. R.; Lin, Z. F.; Tam, W. Y.; Chan, C. T.; Sheng, P. Phys. Rev. Lett. 1999, 82, 4248. (10) Danov, K. D.; Pouligny, B.; Kralchevsky, P. A. Langmuir 2001, 17, 6599. (11) Cong, H.; Cao, W. X. Langmuir 2003, 19, 8177. (12) Vlasov, Y. A.; Bo, X. Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414 289. (13) Wong. S.; Kitaev. V.; Ozin. G. A. J. Am. Chem. Soc. 2003, 125, 15589. (14) O. Vickreva; O. Kalinina; E. Kumacheva Adv. Mater. 2000, 12, 110

designing colloidal crystals in regular shapes. Colloidal crystals in regular shapes have been fabricated by confining the colloidal dispersions in various geometries such as spherical droplets15 and capillaries.16 Among the various confining geometries, dispersion droplets can be readily produced in both liquid and gaseous media and have been used for the fabrication of spherical colloidal crystals.15,17 In addition, colloidal crystallization on the interface of the droplet has been studied for colloidal crystals of spherical shells with a hollow core.18 An essential step toward the fabrication of spherical colloidal crystals of uniform size and structure is to devise a method that can produce uniform-sized emulsion droplets. Recently, our group has developed various approaches to produce monodisperse emulsion droplets including soft-microfluidic devices,15 micropipet injection,19 and consecutive double templating.20 However, these are all microscale processes and, as such, are limited in terms of practical application due to their low yield. Moon et al.21 reported for the first time a simple and large-scale production of spherical colloidal crystals via electrospraying. The electrospraying technique atomizes a droplet through a capillary needle, which is applied to an electric field. The electrospray has unique advantages over other atomization methods noted above; namely, capability to produce droplets in a wide range of sizes from a few nanometers up to hundreds of micrometers; emulsion stability due to high net charge on the surface of the generated droplets; and finally size-controllability by (15) (a) Yi, G.-R.; Thorsen, T.; Manoharan, V. N.; Hwang, M.-J.; Jeon, S.-J.; Pine, D. J.; Quake, S. R.; Yang, S.-M. Adv. Mater. 2003, 15, 1300. (b) Yi, G.-R.; Jeon, S.-J.; Thorsen, T.; Manoharan, V. N.; Pine, D. J.; Quake, S. R.; Yang, S.-M. Synth. Met. 2003, 139, 803. (16) Park, S. H.; Xia, Y. Adv. Mater. 1998, 10, 1045 (17) Xia, B.; Lenggoro, I. W.; Okuyama, K. Adv. Mater. 2001, 13, 1579 (18) Tsapis, N.; Bennett, D.; Jackson, B.; Weitz, D. A.; Edwards, D. A. Proc. Natl. Acad. Sci. 2002, 99, 12001. (19) Yi, G.-R.; Manoharan, V. N.; Klein, S.; Brzezinska, K. R.; Pine, D. J.; Lange, F. F.; Yang, S.-M. Adv. Mater. 2002, 14, 1137. (20) Yi, G.-R.; Moon, J. H.; Manoharan, V. N.; Pine, D. J.; Yang, S.-M. J. Am. Chem. Soc. 2002, 124, 13354. (21) Moon, J. H.; Yi, G.-R.; Yang, S.-M.; Pine, D. J.; Park, S. B. Adv. Mater. 2004, 16, 605.

10.1021/la051266s CCC: $30.25 © 2005 American Chemical Society Published on Web 10/06/2005

Fabrication of Spherical Colloidal Crystals

varying either the liquid flow rate or the electric field strength. In a previous study,21 we generated emulsion droplets in the field-enhanced dripping mode. The produced spherical colloidal crystals were highly monodisperse in size. However, the lower bound of the droplet size was limited to the range of 100-130 µm according to the size of the capillary needle. In this study, the frequency of the AC field was maintained sufficiently high so that the frequency of droplet generation could not match the electric field oscillations. Under these conditions, droplets were produced by the so-called cone-jet mode of atomization, which has been observed in a DC field. Obviously, a large amount of tiny droplets can be produced in the cone-jet mode regardless of the diameter of the nozzles or the geometry of the electrode.22-26 Therefore, this method can produce emulsion droplets that contain colloidal spheres without clogging the capillary needle even when the droplet size is comparable to the colloidal sphere size. For example, the present electrospray can be applied to mass production of colloidal clusters which contain several numbers of particles. The colloidal clusters or colloidal molecules have a great potential as light scatterers, diffusers, and novel building blocks.27,28 In the present study, we used two kinds of colloids, polystyrene (PS) in toluene and silica in ethanol, and obtained spherical colloidal assemblies. These spherical colloidal crystals can be used as supporting particles for membranes, catalysts, and drug delivery. Moreover, these spherical colloidal crystals can be used as interference pigment particles in paint and microdisplays because they reflect colors at specific wavelengths of photonic band gaps.

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Figure 1. SEM images of (A) silica colloidal particles of 252 nm in diameter; (B) polystyrene latex spheres of 265 nm in diameter. Scale bars are 1 µm.

Experimental Section Preparation of Silica Colloidal Dispersion in Ethanol. Monodisperse silica colloids were prepared following the Sto¨berFink-Bohn sol-gel synthesis.29 In this method, 144 mL of ethanol (Merck), 18 mL of distilled water, and 7 mL of ammonia solution (28%, Junsei Chemical) were mixed in a 500 mL one-neck flask. The mixture was stirred vigorously using a magnetic spin bar for homogenization. Then, a mixture containing 14 mL of tetraethyl orthosilicate (TEOS: 99.9% Si(OC2H5)4, Aldrich) dissolved in 56 mL of ethanol was added. The reaction proceeded at room temperature under continuous stirring for 2 h. The synthesized silica dispersions were purified by repeated washings and subsequently redispersed in ethanol. The particle size was measured from an SEM image (Philips, XL-30) and by a dynamic light scattering (DLS) system. As shown in Figure 1A, the particle size was 260 ( 1.3 nm in diameter. Preparation of Polystyrene Colloidal Dispersion in Toluene. Monodisperse cross-linked PS beads were prepared by emulsion copolymerization30 of styrene and divinylbenzene in the absence of an emulsifier. Distilled water (400 mL), 5 wt % of styrene (Kanto), and 10 wt % of divynylbenzene (DVB, Aldrich) were added to a reactor flask in a thermostatic water bath at 80 °C. Then, 0.0004 mol of potassium persulfate (KPS: K2S2O8, Kanto) was added dropwise. Polymerization proceeded (22) Rosell-Llompart, J.; Fernandez de La Mora, J. J. Aerosol Sci. 1994, 25, 1093. (23) Ferna´ndez de la Mora, J.; Navascue´s, J.; Ferna´ndez, F.; RosellLlompart, J. J. Aerosol Sci. 1990, 21, S673. (24) Ferna´ndez de la Mora, J.; Gomez, A. J. Aerosol Sci. 1993, 24, 691. (25) Tang, K. Q.; Gomez, A. J. Aerosol Sci. 1994, 25, 1237. (26) Tang, K. Q.; Gomez, A. J. Colloid Interface Sci. 1995, 175, 326. (27) Manoharan, V. N.; Elsesser, M. T.; Pine, D. J. Science 2003, 301, 483. (28) Yi, G.-R.; Manoharan, V. N.; Michel, E.; Elsesser, M. T.; Yang, S.-M.; Pine, D. J. Adv. Mater. 2004, 16, 1204. (29) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (30) Zou, D.; Derlich, V.; Gandhi, K.; Park, M.; Sun, L.; Kriz, D.; Lee, Y. D.; Kim, G.; Aklonis, J. J.; Salovey, R. J. Polym. Sci. Part A: Polym. Chem. 1990, 28, 1901.

Figure 2. Schematic of electrohydrodynamic atomization for the generation of emulsion droplets and the formation of spherical colloidal crystals. for 10 h. After polymerization, the polystyrene dispersion was filtered to remove undesirable coagulated substances and centrifuged to separate the unreacted monomer. The precipitate was redispersed in toluene and sonicated adequately for effective dispersion. An SEM image of the prepared PS latex spheres of 265 ( 1.8 nm is shown in Figure 1B. Meanwhile, the DLS data showed a bimodal size distribution of the PS spheres with diameter at 420 nm and 1.0-1.2 µm in toluene, indicating that the PS spheres were swollen in toluene solvent and formed some aggregates. Fabrication of Dispersion Droplets by Electrospray. The experimental setup is illustrated in Figure 2. The apparatus consists of atomizing, imaging, and sampling parts. The atomizing part is composed of a stainless capillary needle (metal hub needle, Hamilton) connected to a disposable 1 mL syringe and a ring electrode with a circular hole of 1 cm in diameter through which the atomized jet or spray passes. Both are connected to a high

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voltage power amplifier (20/20B, Trek Inc.) and an arbitrary function generator (DS345, Stanford Research Systems) to provide AC voltage. The inner and outer diameters of the needle are 0.11 mm and 0.24 mm, respectively. The flow rate of the colloidal dispersion was controlled by a syringe pump (KD Scientific, 12916-100). The imaging part is composed of an illuminator (fiber optic illuminator with a halogen lamp) and a CCD camera connected to a computer. They were focused to the tip of the needle in order to observe the meniscus and shapes of the jet. In the experimental runs, the dispersion was injected into a capillary needle, and the flow rate was kept in the range of 0.2-1.2 mL/h. For a stable generation of droplets in the conejet mode, we controlled the field intensity and the frequency. An AC field intensity in the range of 1.2-1.7 kV/mm was applied to the needle. The frequency range of the cone-jet mode was 1000-3000 Hz for the silica/ethanol dispersion and 300-500 Hz for the PS/toluene dispersion. The microscope images of Figure 3, panels a-c, show that the mode of atomization changed in the order of a dripping mode, a cone-jet mode, and an unstable multijet mode as the AC field strength was increased. In the cone-jet spray, the pendant drop became thinner in the direction of the electric field and a fairly slim jet was ejected from the liquid cone. The liquid jet subsequently breaks up into droplets by the so-called Rayleigh instability. The generated droplets have a bimodal size distribution as a result of the formation of satellite droplets during recoil of the surface and irregular breakup.31 The colloidal spheres of silica or PS in the electrically atomized emulsion were self-assembled into spherical colloidal crystals during evaporation of the solvent. The colloidal crystallization of spherical colloids is a self-organization induced by the evaporation of solvent, as illustrated schematically in Figure 2. Sampling of the spherical colloidal crystals was achieved by using a Petri dish placed underneath a three-feet-long acrylic tube. To measure the sizes of spherical colloidal crystals with an optical microscope, silicone oil (FL-100, ShinEtsu) and water containing 0.5 wt % of nonionic surfactant TWEEN were used for collecting the silica/ethanol and PS/toluene droplets, respectively. The samples were dried in a desiccator and placed in a drying oven of 70 °C for 1 or 2 days to ensure completely dried structures. The spherical colloidal crystals were observed using an optical microscope (Nikon Eclipse TE2000-U). The mean size and its standard deviation were calculated by the microscopic image analyses of approximately 150 spherical colloidal crystals. The morphology of the spherical colloidal crystals was characterized by SEM. A UV-vis spectrophotometer (Varian) was used for optical property of the crystals.

Yang et al.

Figure 3. Modes of electrospray with ethanol based silica dispersion. (a) dripping mode; (b) cone-jet mode; (c) multi-jet mode.

Fabrication of Spherical Silica Colloidal Crystals. The ethanol-based colloidal dispersion of silica spheres at 9.5 wt % was atomized by an AC electric field. Silica spheres inside the dispersion droplets self-organized into spherical colloidal crystals during the evaporation of ethanol. The capillary force provides a compressive force that leads to an arrangement of silica particles in spherical symmetry. van der Waals forces subsequently cement the silica particles. The entire process was completed in only several seconds. The size of the dispersion droplets was controlled by the AC field intensity and the flow rate of dispersion. Because the dispersion droplet shrinks to form a spherical colloidal crystal, the particle concentration also determines the final size of the colloidal crystal. As the field intensity increases, the dispersion protruding from the capillary needle becomes cusped with a conical shape and the liquid jet from the cone tip breaks into a stream of smaller charged droplets. The electrohydrodynamic atomization or emulsification is governed by two competing forces; namely, deforming electrical tangential stresses and

restoring surface tension. This can be characterized by a dimensionless Bond number Be ) DE2/σ, in which , σ, E, and D are the dielectric constant of air, surface tension of the liquid, electric field strength, and inner diameter of the needle tip. In Figure 4a, the average diameter of the spherical colloidal crystals is plotted against the square of the electric field strength (E2), which is proportional to Bond number. The liquid flow rate and the frequency were fixed at 0.5 mL/h and 3000 Hz, respectively. The average size was decreased and inversely proportional to the dimensionless Bond number.31 The size distribution was caused by the formation of satellite droplets during electrospray but the nonuniformity was reduced with an increase in the field strength. The standard deviation of the size of spherical colloidal crystals was decreased from 13 to 5 µm as the electric field was increased from 1.4 to 1.7 kV/mm. The effect of flow rate on the size of the spherical colloidal crystals is shown in Figure 4b. Dispersion droplets were produced at a fixed field strength of E ) 1.2 kV/mm and an AC frequency of 3000 Hz. An approximate scaling law considering Rayleigh instability shows that the diameter (d) of the breakup droplet is linearly proportional to the linear velocity of the liquid jet for a conducting liquid, i.e., d ∝ Q1/3 in which Q is the volume flow rate.32 As expected, Figure 4b shows that the

(31) Notz, P. K.; Basaran, O. A. J. Colloid Interface Sci. 1995, 213, 218.

(32) Hartman, R. P. A.; Brunner, D. J.; Camelot, D. M. A.; Marijnissen, J. C. M.; Scarlett, B. J. Aerosol Sci. 1999, 30, 823.

Results and Discussion

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Figure 4. Diameter of silica colloidal crystals as a function of (a) the electric field strength E and (b) flow rate Q.

average diameter of the spherical colloidal crystals increases linearly with the linear velocity for a given field strength and frequency. Here, the standard deviation of drop size was around 9 µm for all data points. Figure 5a shows an SEM image of spherical silica colloidal crystals. The diameter of spherical silica colloidal crystals was in the range of 10-40 µm. Figure 5b shows the SEM image of the spherical colloidal crystal surface. As shown in the inset of Figure 5b, the silica particles are arranged hexagonally within approximately five layers from the surface. The disordered structure in the core region was induced by the fast evaporation rate and packing in curved confined geometry. Figure 5c shows a collection of spherical silica colloidal crystals prepared in a large quantity. The colloidal crystals reflected green color, which is due to Bragg diffraction by the ordered layers near the surface in the spherical colloidal crystals. The yield of the spherical silica colloidal crystals in this experiment was estimated at approximately 74%, and the waste was due to deposition into the ring-type electrode and the inner wall of the acrylic tube. The production rate was calculated at about 34.0 mg/h. It can be expected that a scale-up for practical purposes is simple and can be achieved by mounting a number of capillaries as several reports have noted.33-34 Fabrication of Spherical PS Colloidal Crystals. Also, we used a polymeric colloidal dispersion for the electrospray. The toluene-based dispersion contained cross-linked PS latex spheres of 265 nm in diameter by 5.4 wt %. As in the previous case, the effects of the electric (33) Rulison A. J.; Flagan, R. C. Rev. Sci. Instrum. 1993, 64, 683. (34) Almekinders, J. C.; Jones, C. J. Aerosol Sci. 1999, 30, 969.

Figure 5. SEM images of (a) spherical silica colloidal crystals and (b) fractured spherical colloidal crystals. Inset shows a high-magnification SEM image of the surface. (c) A large collection of silica colloidal crystals.

field strength and flow rate on the size of the spherical colloidal crystals were examined. In Figure 6a, the average diameter of spherical PS colloidal crystals is plotted versus the square of the electric field strength for a fixed flow rate of 0.5 mL/h and field frequency of 360 Hz. Under this condition, the standard deviation of the size was less than 10.0 µm for all data points. As expected, the average size of the spherical colloidal crystals decreased as the square of the electric field strength was increased for a given liquid flow rate. Meanwhile, the diameter of the spherical PS colloidal crystals was proportional to the linear flow rate for a given electric field strength of E ) 1.4 kV/mm and field frequency of 360 Hz, as shown in Figure 6b. It is noteworthy that the threshold field strength for the electrospray of the PS/toluene dispersion was higher than that for silica/ethanol dispersion. This is because toluene has a smaller surface tension than ethanol, and consequently, a higher electric field is required in order to obtain the same size of dispersion droplets, as obvious from considering the Bond number.

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Figure 6. Diameter of polystyrene colloidal crystals as a function of (a) the electric field strength E and (b) flow rate Q. The standard deviation of the size was less than 10.0 µm for all data points.

Figure 7a,b show the spherical PS colloidal crystals. The produced spherical colloidal crystals possessed hollow cavities inside. The PS spheres were not well ordered compared with the spherical silica crystals. The shell of a hollow spherical colloidal crystal consisted of two layers of PS spheres. In our experiments, the spherical colloidal crystals possessed hollow core structures only in the case of the PS/toluene dispersion. It has been demonstrated that the hollow core structure can be formed when the evaporation rate of solvent is faster than the diffusion rate of the colloidal particles inside, and at the same time, the particles tend to straddle on the interface of the droplet in an emulsion system.18 It was also observed that colloidal spheres of the cross-linked PS were initially positioned randomly on the toluene droplets rather than dispersed inside of the droplets. In our case, the evaporation rate was very fast and the consolidation of colloidal spheres was completed within several seconds due to high vapor pressures in both ethanol and toluene. Moreover, PS particles in toluene were less stable than silica in ethanol. As we mentioned earlier, the size of silica particles in ethanol was highly monodisperse (0.5%) but PS particles shows bimodal size distribution (420 nm and 1.0-1.2 µm). Therefore, PS spheres might easily form aggregates and could be supersaturated near the droplet interface during fast drying. Then, the colloidal aggregates form a shell on the drop surface and further evaporation induces a convective flow that pulls the PS particles toward the preformed shell. Eventually, spherical PS colloidal crystals are assembled with hollow cores. Meanwhile, the swollen PS particles were shrunk during the evaporation of

Figure 7. (a) SEM images of polystyrene spherical colloidal crystals and (b, c) images of surface and the fractured spherical colloidal crystals, respectively. The arrow in (c) indicates the pinholes formed by shrinkage of PS particles during the evaporation of toluene.

toluene, which forms many pinholes on the surface as shown in Figure 7c. Optical Measurement of Spherical Silica Colloidal Crystals. To examine the optical properties of spherical colloidal crystals, spherical silica colloidal crystals were dispersed in deionized water. The inset image of Figure 8 captured by a digital camera shows spherical silica colloidal crystals dispersed in water. Because the spherical surface of the colloidal crystal is the (111) plane of the fcc lattice, the spherical colloidal crystals reflected the same color in all directions. Figure 8 is the corresponding optical transmission spectrum of the dispersed spherical silica colloidal crystals; a pronounced attenuation peak appears at approximately 578 nm. The stop band (or the reflection peak location) was consistent with the estimate from Bragg’s law35 for the refractive indices (1.45 for silica and 1.33 for water) and the diameter of the silica beads. However, the angle between the incident beam and the surface is not the same over the nonplanar, spherical surface of colloidal crystal, resulting in a rather broad reflection band.21 (35) Tarhan, I. I.; Watson, G. H. Phys. Rev. B 1996, 54, 7593.

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PS latexes in toluene. Under a cone-jet mode, the droplet size could be controlled by the strength of the electric field and the flow rate in a wide range of size, independently of the diameter of the nozzles. In this work, spherical colloidal crystals were in the range of 10-40 µm in diameter. The spherical silica colloidal crystals displayed a hexagonally packed structure of fcc lattice within a few layers from the surface. This ordered structure reflected the diffraction color in all directions. On the other hand, the relatively unstable colloidal dispersion of PS/toluene formed a consolidated structure on the droplet interface in the initial stage of the evaporation-induced crystallization and consequently spherical PS colloidal crystals possessed hollow cores. Our spherical colloidal crystals produced by electrospray are of practical significance in chromatography, drug delivery, and interference pigment particles for paint and display. Figure 8. Transmission spectrum of silica photonic balls composed of silica spheres of 252 nm in diameter. Inset is a digital camera image of silica colloidal crystals dispersed in water.

Conclusions We demonstrated the fabrication of spherical colloidal crystals by generating a cone jet mode of electrospray of the colloidal dispersions of silica spheres in ethanol and

Acknowledgment. This work was supported by the National R&D Project of Nano Science and Technology of Korea. We also acknowledge partial supports from the BK21 Program, the CUPS-ERC, and the Center for Nanoscale Mechatronics & Manufacturing of the 21st Century Frontier Research Program (M102KN01000205K1401-00214). LA051266S