Patterned Polymeric Domes with 3D and 2D Embedded Colloidal Crystals using Photocurable Emulsion Droplets

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1 Patterned Polymeric Domes with 3D and 2D Embedded Colloidal Crystals using Photocurable Emulsion Droplets By Shin-Hyun Kim, Se-Heon Kim, and Seung-Man Yang* Complex systems of colloidal particles and emulsion droplets or bubbles have been studied intensively due to non-conventional rheological behavior [1] and practical applications, ranging from cosmetics and drugs to flotation of mineral ores. Recent advances in microfluidics have enabled to make emulsion droplets or bubbles in a precisely controlled manner, and more elaborate drop manipulation techniques are still being developed, particularly for biological applications. [2,3] Along with the recent development of programmatic microfluidics, self organization of colloidal particles confined in emulsion droplets has attracted great attention. Depending on the surface properties, size, and concentration of the colloidal particles, the emulsion droplets can confine the colloids within the 3D space of the drop volume or the 2D planar space of the drop surface. Using the 3D confinement effect, spherical colloidal crystals have been prepared by evaporating the emulsion drop phase or by inducing repulsive forces between colloidal particles. [4 6] On the other hand, the interfacial 2D confinement can provide a visualized clue to the longstanding Thomson s problem by analyzing the arrays of charged colloids on the emulsion interface, which is related to the ground states of repulsive species on the spherical surface. [7] In addition, complex colloidal structures such as colloidal clusters, [8] colloidosomes, [9] raspberry- or golf-ball-like particles [10] were prepared by the method of emulsion templating. For practical applications, however, it is still required to make the colloidal crystals into well-organized patterns and, at the same time, to tune the lattice constant in a controlled manner. Here, we report a novel and controllable patterning technique for 3D or 2D colloidal arrays of polymeric domes using photocurable emulsion droplets as templates. In particular, the oil-in-water emulsion droplets, which tend to adhere selectively on the surface with a high interfacial affinity, are patterned in a single step using prepatterned substrates with different surface properties without complicate patterning devices such as motorized stages and dispensers. [5] We also demonstrate that the colloidal structures can be used as reflective color pigments or near-field-microlens arrays. [*] Prof. S.-M. Yang, Dr. S.-H. Kim, Dr. S.-H. Kim Department of Chemical and Biomolecular Engineering National Creative Research Initiative Center for Integrated Optofluidic Systems KAIST, Daejeon, (Korea) smyang@kaist.ac.kr DOI: /adma Our strategy for patterning polymeric domes with embedded 3D or 2D colloidal crystals is described in Scheme 1. First, we performed microcontact printing with octadecyltrichlorosilane (OTS) as a hydrophobic ink to prepare patterned glass substrates. The OTS molecules were transferred from a poly(dimethylsiloxane) (PDMS) stamp onto the clean glass surface, where they bind covalently. The soft-lithographically featured PDMS stamps had cylindrical posts, which were patterned into arrays or characters. Then, oil-in-water emulsion drops were poured over the prepatterned glass substrate. The oil phase was a photocurable resin of ethoxylated trimethylolpropane triacrylate (ETPTA). In water, the ETPTA drops form a relatively high contact angle of on the bare glass, while being wettable and likely to spread over the OTS-coated glass with a low contact angle of 88 to reduce the water/ots interface with the highest free energy. Therefore, the ETPTA drops tend to sit selectively on the patterned OTS dots. To prepare a pattern of hemispherical colloidal crystals or so-called photonic domes, we used ETPTA droplets containing concentrated silica particles. The droplets of all-equal size were prepared with a microfluidic device composed of glass capillaries, and silica particles confined in the droplet formed spontaneously a face-centered-cubic (fcc) structure due to the repulsive interparticle potential. [6a] Optical microscopy (OM) images of the patterned square arrays of the colloidal crystal domes with different intervals l d ¼ 500 and 350 mm are shown in Figure 1a c. In these cases, the ETPTA emulsion droplets were 250 mm in average diameter and composed of 170-nm silica particles at 33 vol%. Other types of dome patterns such as KAIST, IOFS have been produced (Supporting Information, Fig. S1). In Figure 1d, we display scanning electron microscopy (SEM) images of the colloidal crystal domes. The inset shows the hexagonal arrangement of colloidal silica particles on the surface of the dome, which corresponds to the (111) plane of the fcc lattice. Due to the periodic modulation of the refractive index in the colloidal crystals, they exhibit a stop band (the so-called L-gap), which is responsible for the reflected green color, as can be estimated from the Bragg s equation (Supporting Information, Equation (1)). Because the (111) plane of the fcc lattice was generated over the entire free interface, the photonic domes show isotropic reflection colors for normal incident light on the spherical surface of photonic domes, which is confirmed by the side view of the photonic dome shown in the inset of Figure 1b. [6b] On the other hand, the silica particles in the ETPTA matrix can be removed selectively using HF solution as wet etchant, leaving behind porous photonic domes. [6a,b] Figure 2a and its inset show a Adv. Mater. 2009, 21, ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3771

2 Scheme 1. Schematic of mounting and solidifying of droplets on a substrate prepatterned with a hydrophobic moiety. Figure 1. a, b) OM images of patterned photonic domes of a 260-mm diameter with a 500-mm interval at two different magnifications. They are composed of 170-nm silica particles (33 vol%) in an ETPTA matrix. The inset of b) is a side view of a single photonic dome, which shows the same reflection color. c) OM image of the pattern with a 350-mm interval. d) SEM image of the dome pattern; the inset shows the surface morphology of the dome. The scale bars in (a,d) and (b,c) are 500 and 200 mm, respectively. porous dome with arrays of air cavities after the removal of the silica colloids. OM images in Figure 2b and c show the porous photonic domes reflecting blue color, which is induced by a blue-shift of the L-gap owing to the decrease in the effective refractive index of the photonic structure. This is clearly seen from the reflection spectra in Figure 2d, measured before and after removal of the silica colloids. When the repulsive colloidal particles are likely to reside inside an oily emulsion phase, they usually form 3D crystals. However, when colloids are present in a continuous aqueous phase, they tend to anchor on the interface to form 2D crystals on the surface of the drop. To capture the structure of 2D crystals on the spherical interface, we used photocurable ETPTA drops anchored by 1-mm polystyrene (PS) particles with a carboxyl group. The polydisperse emulsion droplets were produced by shear-induced emulsification of the mixture of ETPTA and aqueous PS suspension. The PS particles in the continuous phase were anchored on the emulsion interface during emulsification with a contact angle of about [10] We did not need to add any emulsion stabilizer because the anchored PS particles stabilize the emulsion drops by reducing the total interfacial energy and prevent coalescence by forming the barrier. Following the same procedure for 3D crystals, the drops were mounted selectively onto the hydrophobic dots on glass substrates and solidified by UV exposure. Figure 3a is the SEM image of a dome array prepared on the prepattern of OTS-coated dots with a diameter of d d ¼ 50 mm and interval of l d ¼ 150 mm. A magnified SEM image of a single dome in Figure 3b shows an ordered array of the PS particles on the surface, where the ratio of the dome diameter (d d ) to the interparticle distance (a) is 69. Figure S2 shows the dome pattern over a large area. Patterns of smaller domes with different intervals were also prepared, as shown in Figure 3c, d (d d ¼ 30 mm, l d ¼ 75 mm) and 3e, f (d d ¼ 20 mm, l d ¼ 50 mm). The contact angle and size of the domes show a distribution due to the polydisperse ETPTA droplets. In addition, some OTS dots are unoccupied with any emulsion droplets, leaving behind vacancy defects in the dome array. The vacancy-defect density can be reduced by using an excess number of emulsion droplets with OTS dots of comparable sizes so that the effective contact probability between them is enhanced. This can be seen clearly from Figure 3 for various dot sizes. Indeed, the contact induces mounting only for emulsion droplets of similar sizes, when the patterns were prepared from the same droplet suspension as shown in Figure 3. We analyzed the 2D arrangement of colloidal particles on the curved surface using an SEM image of a dome with d d /a ¼ 33, as shown in Figure 4a. The particle locations were determined from the image and Delaunay triangulation was carried out. (We used Image Pro 6.0 and Matlab 7.0 for the particle-location data and Delaunay triangulation, respectively.) The result shows that defects occur as chains of repetitive five- and seven-fold defects (so-called scar) or as isolated pairs of five- and seven-fold defects, which coincides with the spherical crystallography for d d / a > 10. [7] Red and yellow dots represent five- and seven-fold defects, respectively; other vertices in the mesh exhibit six-fold symmetry except for the outermost vertices. The surface morphology of the 2D colloidal crystal can be transferred to other materials by double transfer molding. Hereby, the dome structure, decorated with 2D colloidal crystals, is used as a 3772 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21,

3 Figure 2. a) SEM image of a single porous dome. The inset shows the porous surface morphology. b,c) OM images of porous photonic domes reflecting blue color at different magnifications. d) Normalized reflectance spectra for composite and porous photonic domes. Scale bars in (a), (b), and (c) are 100, 500, and 200 mm, respectively. Figure 3. SEM images of the dome patterns with 2D crystals on the hemispherical surfaces. l d is 150 mm in (a,b), 75 mm in (c,d), and 50 mm in (e,f). d d /a is 69 in (b), 35 in (d) and 20 in (f). Scale bars in (a,c,e), (b), (d), and (f) are 500, 20, 10, and 5 mm, respectively. template to produce the negative mold of PDMS. In Figure S3, we displayed a schematic of the molding, together with SEM and OM images of the negative mold and the reproduced ETPTA dome with embossed surface. Microparticles can act as lenses for point light emitters, such as nanocrystal quantum dots or fluorescent molecules, as reported by Brody and Quake. [11] In particular, domes decorated with a number of colloids will behave like a microlens array or compound eyes and capture effectively the emission from such subwavelength-size sources and transmit the radiation from any position of the dome to its center owing to the spherical symmetry. To demonstrate this capturing performance, we carried out finite-difference timedomain (FDTD) simulations. [12] Here, the detailed surface morphology was constructed from Surface Evolver simulations as shown in Figure 4b (and Figure S4 for details). [1b,13,14] The particle-position data on the mounted droplet obtained from the Surface Evolver simulation were used to generate the FDTD simulation structure, as shown in Figure 4c. We have inserted an electric dipole source (emission wavelength of 700 nm) as a pointlike light emitter continuously oscillating in the x-direction. Such a dipole source is to be placed near the top-positioned particle, indicated by the red dot in Figure 4c. One of the imaginary surfaces, S1 (in Fig. 4c), completely encloses the whole structure with the dipole source in its center. The other surface, S2, positioned at the bottom of the dome, is designed to capture the downward propagating energy distribution (P 2 (x, y)). Thus, one can obtain complete 3D information on the emission profile (P 1 (u, f)) via Poynting energy distribution over S1, which will be represented using an equirectangular projection. Figure 4d,e show remarkably enhanced directional emission compared with a single dipole source positioned in air, as shown in Figure S5. About 53.6% of the total emitted energy can be collected within 308 from the bottom (over six times enhancement). Moreover, using a further-reduced collection solid angle down to 108, the enhancement factor can be up to 13 times more pronounced, for example 12.3% for the single dipole source on the compound-eye structure, while 0.9% for the single dipole source in air. In addition, the directional emission is effective for a light source on a rotated position to the side particles as shown in Figure S6. Although the positional deviation of the point source from the top of the PS particle results in a lower collection efficiency, as shown in Figure S7, the compound-eye structure might be effective for many point sources randomly distributed over the surface of the particles on the dome. Furthermore, the spontaneous emission rate of a point dipole source, which depends on the photon density of states, can be enhanced. [15] By comparing the superposed sum of the Poynting energy over S1 for the point dipole source placed on top of the Adv. Mater. 2009, 21, ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3773

4 hydrophobic moieties. Due to the repulsive interparticle potential, silica particles formed non-close-packed 3D colloidal crystals. Similarly, we patterned embossed domes with PS particle-anchored photocurable emulsion droplets and demonstrated their potential application as a miniaturized compound eye for pointlike light emitters in near-field by FDTD simulation. We believe that this focusing property will find unique, promising applications, such as the detection of biomolecules tagged with quantum dots or fluorescent dyes and efficient evanescent coupling with photonic crystal nanolasers. [18] Figure 4. a) SEM image of the PS particle-decorated dome. The mesh shows the result of Delaunay triangulation, where red and yellow dots correspond to five- and seven-fold symmetries, respectively. b) Surface Evolver-simulation results for a 312 particle-decorated mounted drop and c) reconstructed structure for FDTD simulation. d) Electric field intensity distributions (jej 2 )in logarithmic scale in the plane of y ¼ 0 and e) equirectangular projection of the Poynting energy distribution over S1 for a single dipole source on the top particle. The inset of (d) shows the downward-propagating energy distribution over S2. The rainbow scale bar in (e) can be applied independently. The scale bar in (a) is 10 mm. small particle on the dome and the source in air, we can get an enhancement ratio (also known as Purcell factor, F p ) of 4.08, which also explains a fraction of the downward propagating energy of 80% (4.08/(1 þ 4.08)). (It should be noted that F p 4.08 is not too small considering that normally an F p of 10 is obtained, even with photonic crystal nanocavities, and the record Purcell factor of 75 is only obtained through careful spatial/ spectral tunings.) We expect that the use of titania particles will be more advantageous for such near-field focusing due to a high density of states induced by a higher refractive index. [16,17] In conclusion, we prepared patterns of photonic domes of 3D colloidal crystals and embossed domes of 2D crystals via a novel single-step patterning process. Photonic domes with an isotropic bandgap property were produced using photocurable emulsion droplets of all-equal size on prepatterned glass substrates with Experimental Preparation of the Substrates: Patterned glass substrates with a hydrophobic moiety were prepared by contact printing. The PDMS stamps with square arrays or characters of cylindrical posts were fabricated by conventional soft lithography. First, OTS ink (6 mm, in toluene) was deposited onto the stamp and dried in a flow of nitrogen gas for a few seconds. Then, for 1 min at 70 8C, the stamp was brought into contact with the glass substrate, which was pretreated under oxygen plasma for 2 min. After release of the stamp, the glass was rinsed with acetone. Patterning of Photonic Domes: For the photonic domes, monodisperse ETPTA emulsion droplets, 250 mm in diameter, which contained 170-nm silica particles at 33 vol%, were prepared by a microfluidic device composed of coaxial glass capillaries [6a]. The emulsion droplets were introduced onto the OTS-patterned substrate with 200-mm dots, which were separated by 500 or 350 mm intervals. Gentle stirring of the substrate for 5 min led to the mounting of the droplets onto the OTS-inked dots, and subsequent UV irradiation for 10 s induced the photo-polymerization of the droplets. For porous photonic domes, the colloidal silica particles were removed from the composite domes. To achieve this, first, the interstices between the domes were filled with pure ETPTA of about 100 mm thickness, which was solidified subsequently under UV irradiation. Then, the ETPTA film with embedded domes was detached from the glass substrate and immersed in 5% HF solution for 12 h. Patterning of Embossed Domes: For embossed domes with 2D colloidal crystals on the surfaces, ETPTA emulsion droplets were prepared by shearing the mixture of ETPTA and an aqueous suspension of 1-mm PS particles with carboxyl groups (Interfacial Dynamics, ) at rpm for 30 s using a homogenizer (Heidolph, Silentcrusher M) [10]. The emulsion droplets were introduced onto the three different OTS-patterned substrates: 50 mm dots with a 150 mm interval, 30 mm dots with a 75 mm interval, and 20 mm dots with a 50 mm interval. With a similar method as used for the 3D crystal domes, the substrates were stirred gently and subsequently exposed to UV light. The surface morphology of the 2D colloidal crystal could be transferred to other materials by a double transfer molding technique (see Supporting Information for details). Characterization: The colors and morphologies of the patterned domes were observed using an OM (Nikon, L150) and a SEM (Philips, XL30), respectively. For reflectance spectra and contact-angle measurements, an 3774 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21,

5 optical spectrometer (Newport, OSM-100-UV-NIR) was used, attached to the OM and a drop shape analysis system (KRÜSS, DSA10), respectively. Acknowledgements This work was supported by a grant from the Creative Research Initiative Program of the Ministry of Education, Science and Technology for Complementary Hybridization of Optical and Fluidic Devices for Integrated Optofluidic Systems. Supporting Information is available online from Wiley InterScience or from the author. Received: April 13, 2009 Published online: August 3, 2009 [1] a) A. B. Subramaniam, M. Abkarian, L. Mahadevan, H. A. Stone, Nature 2005, 438, 930. b) M. Abrikan, A. B. Subramaniam, S.-H. Kim, R. Larsen, S.-M. Yang, H. A. Stone, Phys. Rev. Lett. 2007, 99, [2] a) H. Song, J. D. Tice, R. F. Ismagilov, Angew. Chem. Int. Ed. 2003, 42, 767. b) B. Zheng, J. D. Tice, L. S. Roach, R. F. Ismagilov, Angew. Chem. Int. Ed. 2004, 43, [3] J. Shim, G. Cristobal, D. R. Link, T. Thorsen, Y. Jia, K. Piattelli, S. Fraden, J. Am. Chem. Soc. 2007, 129, [4] a) O. D. Velev, A. M. Lenhoff, E. W. Kaler, Science 2000, 287, b) G.-R. Yi, V. N. Manoharan, S. Klein, K. R. Brzezinska, D. J. Pine, F. F. Lange, S.-M. Yang, Adv. Mater. 2002, 14, c) S.-H. Kim, S. Y. Lee, G.-R. Yi, D. J. Pine, S.-M. Yang, J. Am. Chem. Soc. 2006, 128, d) X. Zhao, Y. Cao, F. Ito, H.-H. Chen, J. Nagai, Y.-H. Zhao, Z.-Z. Gu, Angew. Chem. Int. Ed. 2006, 45, [5] Y. Masuda, T. Itoh, K. Koumoto, Adv. Mater. 2005, 17, 841. [6] a) S.-H. Kim, S.-J. Jeon, G.-R. Yi, C.-J. Heo, J. H. Choi, S.-M. Yang, Adv. Mater. 2008, 20, b) S.-H. Kim, J.-M. Lim, W. C. Jeong, D.-G. Choi, S.-M. Yang, Adv. Mater. 2008, 20, c) S.-H. Kim, S.-J. Jeon, W. C. Jeong, H. S. Park, S.-M. Yang, Adv. Mater. 2008, 20, d) S.-H. Kim, S.-J. Jeon, S.-M. Yang, J. Am. Chem. Soc. 2008, 130, [7] A. R. Bausch, M. J. Bowick, A. Cacciuto, A. D. Dinsmore, M. F. Hsu, D. R. Nelson, M. G. Nikolaides, A. Travesset, D. A. Weitz, Science 2003, 299, [8] V. N. Manoharan, M. T. Elsesser, D. J. Pine, Science 2003, 299, 483. [9] A. D. Dinsmore, M. F. Hsu, M. G. Nikolaides, M. Marquez, A. R. Bausch, D. A. Weitz, Science 2002, 298, [10] S.-H. Kim, C.-J. Heo, S. Y. Lee, G.-R. Yi, S.-M. Yang, Chem. Mater. 2007, 19, [11] J. P. Brody, S. R. Quake, Appl. Phys. Lett. 1999, 74, 144. [12] A. Taflove, S. C. Hagness, Computational Electrodynamics: The Finite- Difference Time-Domain Method, Artech House, Norwood, MA [13] K. A. Brakke, Exp. Math. 1992, 1, 141. [14] E. Lauga, M. P. Brenner, Phys. Rev. Lett. 2004, 93, [15] Y. Xu, J. Vuckovic, R. K. Lee, O. J. Painter, A. Scherer, A. Yariv, J. Opt. Soc. Am. B 1999, 16, 465. [16] X. Jiang, T. Herricks, Y. Xia, Adv. Mater. 2003, 15, [17] H. K. Yu, G.-R. Yi, J.-H. Kang, Y.-S. Cho, V. N. Manoharan, D. J. Pine, S.-M. Yang, Chem. Mater. 2008, 20, [18] H.-G. Park, S.-H. Kim, S.-H. Kwon, Y.-G. Ju, J.-K. Yang, J.-H. Baek, S.-B. Kim, Y.-H. Lee, Science 2004, 305, Adv. Mater. 2009, 21, ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3775