Shape and Feature Size Control of Colloidal Crystal-Based Patterns

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Shape and Feature Size Control of Colloidal Crystal-Based Patterns Using Stretched Polydimethylsiloxane Replica Molds Hong Kyoon Choi,† Sang Hyuk Im,‡ and O Ok Park*,† †

Department of Chemical & Biomolecular Engineering (BK 21 Graduate Program), Korea Advanced Institute of Science and Technology(KAIST), 335 Gwahangno, Yuseong-gu, Daejeon, 305-701 Republic of Korea, and ‡ KRICT-EPFL Global Research Laboratory, Advanced Materials Division, Korea Research Institute of Chemical Technology, 19 Sinseongno, Yuseong, Daejeon, 305-600, Republic of Korea Received August 11, 2009. Revised Manuscript Received September 3, 2009

In this work, we fabricated various patterns using colloidal crystals as master molds via the soft lithography method. Even though colloidal crystals consist of spherical colloidal particles, nonspherical shaped patterns such as rectangular or elongated hexagonal shaped patterns can be fabricated using a stretched polydimethylsiloxane (PDMS) replica mold. The pattern shape and feature size can be easily controlled by changing the stretching axis and ratio of the PDMS replica mold. The deformations of the PDMS mold were simulated using the finite element method, and they are consistent with experimental results. The elongated patterns were used as templates to offer new types of colloidal crystal superlattice structures. A proposed pattern-control method will significantly expand the usefulness and diversity of micro/ nanopatterning technology.

Introduction Micro/nanopatterning is an essential technique for applications in micro- and optoelectronic devices,1-5 fabrication of chemical and biological sensors,6,7 and microfluidics.8 Many patterning methods have been developed over the past few decades, including photolithography, electron beam lithography, X-ray lithography, and soft lithography.9-12 Among these many approaches, soft lithography is one of the most efficient methods because of its simplicity, large area patterning, and low-cost process.12-14 In soft lithography, an elastomeric stamp or mold is used, which is typically prepared by curing polydimethylsiloxane (PDMS) prepolymer on a patterned master. Many research groups have taken advantage of the elastic properties of molds to fabricate various patterns on curved or flexible substrates.15,16 However, only a few reports are available showing that diverse sizes and shapes of the patterned features can be obtained by deforming the elastomeric mold itself. For instance, Xia et al. reduced the size of printed pattern features by deforming elasto*Corresponding author. E-mail: [email protected].

(1) Haynes, C. L.; Duyne, R. P. V. J. Phys. Chem. B 2001, 105, 5599. (2) Veinot, J. G.; Yan, C. H.; Smith, S. M.; Cui, J.; Huang, Q.; Marks, T. J. Nano Lett. 2002, 2, 333. (3) Behl, M.; Seekamp, J.; Zankovych, S.; Torres, C. M. S.; Zentel, R.; Ahopelto, J. Adv. Mater. 2002, 14, 588. (4) McAlpine, M. C.; Friedman, R. S.; Lieber, C. M. Nano Lett. 2003, 3, 443. (5) Lee, T.-W.; Zaumseil, J.; Bao, Z.; Hsu, J. W. P.; Rogers, J. A. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 429. (6) Shi, H. Q.; Tsai, W. B.; Garrison, M. D.; Ferrari, S. B.; Ratner, D. Nature 1999, 398, 593. (7) Cui, Y.; Wei, Q. Q.; Park, H. K.; Leiber, C. M. Science 2001, 293, 1289. (8) Manz, A.; Harrison, D. J.; Verpoorte, E. M. J.; Fettinger, J. C.; Paulus, A.; L€udi, H.; Widmer, H. M. J. Chromatogr. 1992, 593, 253. (9) Moreau, W. M. Semiconductor Lithography: Principles and Materials; Plenum: New York, 1988. (10) Broers, A. N.; Molzen, W.; Cuomo, J.; Wittels, N. Appl. Phys. Lett. 1976, 29, 596. (11) Dunn, P. N. Solid State Technol. 1994, 49. (12) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (13) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823. (14) Lee, T.-W.; Mitrofanov, O.; Hsu, J. W. P. Adv. Funct. Mater. 2005, 15, 1683. (15) Choi, W. M.; Park, O O. Nanotechnology 2004, 15, 1767. (16) Lee, K. J.; Fosser, K. A.; Nuzzo, R. G. Adv. Mater. 2005, 15, 557.

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meric PDMS stamps in a microcontact printing (μCP) process.17 In this paper, we demonstrate that it is possible to fabricate various patterns, which not only have different feature sizes but also different shapes, from colloidal crystal master patterns by stretching the PDMS replica mold. Colloidal crystals, which are self-assembled ordered structures formed from monodisperse colloidal particles, are promising materials for micro/nanopatterning because of their regular periodicity and small feature size, as well as their low-cost process compared to other micro/nanopatterning processes such as photolithographic techniques, electron-beam, and X-ray lithography.9-11 In micro/nanopatterning applications, colloidal crystals can be used as masks for lithography or templates for other nanostructures, such as nanorings or nanopillars, and they can also be used as masters in soft lithography to produce hexagonally arrayed nanostructures.18-22 Recently, we reported the successful fabrication of large-area and hexagonally well-ordered lens patterns from three-dimensional (3D) colloidal crystal masters using soft lithography. In addition, we obtained various shapes of the micro/nanopatterns by manipulating the surface of the colloidal crystal master.22 Despite the many advantages of colloidal crystals as master molds, only hemisphere-based shapes of the resulting patterns can be obtained, which limits the widespread use of colloidal crystal micro/nanopatterning. In this study, however, we developed a method for producing rectangular or hexagonal patterns from spherical colloidal crystal masters. An additional noteworthy aspect of the proposed method is that several patterns with various sizes and shapes can be produced from a single PDMS replica mold. (17) Xia, Y.; Whitesides, G. M. Langmuir 1997, 13, 2059. (18) Sun, Z.; Li, Y.; Zhang, J.; Li, Y.; Zhao, Z.; Zhang, K.; Zhang, G.; Guo, J.; Yang, B. Adv. Funct. Mater. 2008, 18, 4036. (19) Li, Y.; Cai, W.; Duan, G. Chem. Mater. 2008, 20, 615. (20) Heo, C. J.; Kim, S. H.; Jang, S. G.; Lee, S. Y.; Yang, S. M. Adv. Mater. 2009, 21, 1726. (21) Kim, M. H.; Choi, J.-Y.; Choi, H. K.; Yoon, S.-M.; Park, O O.; Yi, D. K.; Choi, S. J.; Shin, H.-J. Adv. Mater. 2008, 20, 457. (22) Choi, H. K.; Kim, M. H.; Im, S. H.; Park, O O. Adv. Funct. Mater. 2009, 19, 1594.

Published on Web 09/16/2009

DOI: 10.1021/la902982y

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Experimental Section Preparation of Polystyrene Colloidal Particles. Polystyrene (PS) particles with diameters of 270 and 470 nm were prepared by emulsifier-free emulsion polymerization.23,24 Deionized water (450 g) was poured into a reactor kept at a temperature of 70 °C upon stirring (350 rpm). Then, sodium styrene sulfonate (0.05 and 0.30 g) and sodium hydrogen carbonate (0.25 g) were added to the water as emulsifier and buffer, respectively. After 10 min, a styrene monomer (50 g) was added to the solution, and after an additional hour, potassium persulfate (0.25 g) was included as initiator. Finally, polymerization was carried out under nitrogen atmosphere for 18 h. Preparation of the 3D Colloidal Crystal Master. To fabricate the 3D colloidal crystals, PS colloidal particles were dispersed in deionized water (0.1 wt %), and a small amount of polyvinylpyrrolidone (Aldrich, Mw = 55000) was added to this suspension. Then, glass substrates were vertically dipped into the PS colloidal suspension (40 mL) and placed in a furnace of 70 °C.22 Preparation of the PDMS Replica Mold. The colloidal crystal master was passivated with chlorotrimethylsilane (Aldrich) by means of vapor deposition in a vacuum chamber in order to minimize the interaction force between the surface of the colloidal crystals and the cured PDMS mold. The PDMS base polymer and the cross-linker (Sylgard 184, Dow corning) were mixed at a weight ratio of 15:1, which includes a lower percentage of cross-linker than the standard weight ratio of 10:1. A lower amount of cross-linker leads to a higher elasticity, which is helpful in the stretching process. The mixed PDMS liquid prepolymer was poured onto the prepared colloidal crystal master with a thickness of about 2 mm. Then, the cast prepolymer was cured in an oven (at 60 °C) for 3 h. When the curing reaction was complete, the PDMS elastomer was carefully peeled off from the master. Pattern Transfer from the PDMS Replica Mold. A UVcurable acrylate solution was prepared containing 23.5 wt % dipentaerythritol hexaacrylate (DPHA) (SK CYTEC), 23.5 wt % tetraacrylate (PETIA) (SK CYTEC), 1 wt % igarcure 184 (Ciba), and 1 wt % darocure 1173 (Ciba) in methyl ethyl ketone (MEK) (Aldrich) and isopropyl alcohol (IPA) (Aldrich). This acrylate solution was spin-coated for 30 s (at 3000 rpm) onto a polyethylene terephthalate (PET) film (Skyrol, SK Co. Ltd.) which had been treated for 4 min with O2 plasma. Both ends of the thin PDMS replica mold were fixed with clamps corresponding to the stretching axis. The PDMS mold was stretched, and the strain was maintained by fixing both clamps. A spin-coated PET film was placed onto the stretched PDMS mold, and the acrylate monomer was polymerized by exposure to UV light (365-436 nm, 15 mW/ cm2, 4 min). Preparation of Superlattice Colloidal Structures. To fabricate colloidal crystals with a superlattice, replicated patterns from colloidal crystal particles with a diameter of 470 nm (and the corresponding stretched patterns) were used as the bottom layer. The confined convective assembly method was applied to deposit a second colloidal crystal layer containing particles with a diameter of 270 nm.25 The concentration of the 270 nm colloidal suspension was 0.27 wt %, and the lift-up rate was 2.5 cm/min.

Results and Discussion A schematic illustration of the experiment is shown in Figure 1. First, high-quality 3D colloidal crystals were fabricated on a slide glass via the vertical deposition method. Then, the PDMS prepolymer was poured onto the prepared 3D colloidal crystals. After curing the PDMS in an oven at 60 °C for 2 h, we carefully peeled off the PDMS mold. The detached PDMS mold was then (23) Yi, G. R.; Moon, J. H.; Yang, S.-M. Chem. Mater. 2001, 13, 2613. (24) Kim, J. H.; Chainey, M.; El-Aasser, M. S.; Vanderhoff, J. W. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 171. (25) Kim, M. H.; Im, S. H.; Park, O O. Adv. Funct. Mater. 2005, 15, 1329.

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stretched along the desired axis to deform the pattern shape. In this step, the direction of stretching is very important, as will be discussed below. The stretched PDMS mold was then placed on an acrylate-monomer-coated film and polymerized by exposing it to UV light for 3 min while keeping the PDMS mold stretched. Finally, after detaching the PDMS mold from the substrate, we obtained the pattern. If we release the strain applied to the PDMS mold, it returns to its original shape because of its elastic properties. Thus, the same mold can be reused many times. If we fabricate the pattern following the process described as route ‘c’ in Figure 1, we obtain a hemispherical lens pattern with a shape that is identical to that of the colloidal crystal surface. For routes ‘a’ and ‘b’, by contrast, different patterns are obtained depending on the stretching axis and degree of elongation. Mostly, the surface of 3D colloidal crystals are arranged in a close packed hexagonal array, which corresponds to the (111) plane of the face-centered cubic (fcc) structure. This hexagonal array has an orientational order that repeats every 60 degrees and, for convenience, we define two orientations, ‘A’ and ‘B’, as described in Figure 2c. If we fabricate the 3D colloidal crystals using the vertical deposition method, the orientational order of the fabricated colloidal crystals is always the same (the watersubstrate contact line (horizontal line) adopts the ‘B’ orientation). Therefore, we can recognize the orientation of the fabricated colloidal crystals without using a microscope. If we apply a stretching force on the colloidal particles (or colloidal crystals) to deform them, they adopt a spheroid shape from the spherical shape, as reported by Xia et al.26 Interestingly, if we stretch the PDMS replica mold, whose shape is the reciprocal of the shape of the surface pattern of the colloidal crystals, the particles do not deform to a spheroid shape, but they can be deformed into various shapes depending on the stretching axis. This phenomenon is attributed to the stretching force distribution on the surface of the PDMS replica mold [shown as arrow lines in Figure 2a,b]. Let us consider the center hole in Figure 2a: If the stretching force is applied along the ‘A’ axis, the zigzag arrow lines shown in Figure 2a tend to be straight, and the hole elongates having a straight side wall. On the other hand, if we apply the stretching force along the ‘B’ axis to the center hole in Figure 2b, a large portion of this force is applied on the lateral edges, as shown by the thick arrow lines, and the edges become sharp. Deformation of the PDMS mold can be simulated using the finite element method. For simplicity, we assume that the PDMS mold is a hexagonally arrayed two-dimensional (2D) mesh (this is similar to the outer surface of the PDMS mold). The simulation results show how pattern shape changes with varying the stretching direction. Figure 2e shows the original PDMS mold surface without stretching, and Figure 2d,f shows the simulation results for the PDMS mold stretched along the ‘A’ and ‘B’ axes, respectively. According to the simulation results, the circular holes in the PDMS mold deform into rectangular and elongated hexagonal shapes when elongated along the ‘A’ and ‘B’ axes, respectively. The experimental results are in good agreement with the simulations. Figure 3c shows a replicated pattern of a colloidal crystal master (with a particle diameter of 470 nm) without stretching of the PDMS mold. This pattern shape is almost identical to that of the surface pattern of the colloidal crystal master. Figure 3d-i shows the changes in the pattern shapes observed upon stretching the PDMS mold. Figure 3d,f,h shows the resulting patterns obtained when the PDMS mold was stretched along the ‘A’ axis with elongations of 125%, 150%, (26) Lu, Y.; Yin, Y.; Li, Z.-Y.; Xia, Y. Langmuir 2002, 18, 7722.

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Figure 1. Schematic illustration of the preparation of a PDMS replica mold and the pattern replication onto a polymer film. Route ‘c’ includes patterning using an unstretched PDMS mold, whereas routes ‘a’ and ’b’ correspond to pattering using stretched PDMS molds.

Figure 2. (a,b) Stretching-force distribution on the surface of a PDMS replica mold depending on the stretching direction. (c) Two orientational orders (‘A’,’B’) of the hexagonally arrayed colloidal crystals. (d,e,f) Simulation results showing the deformation of a 2D simplified PDMS replica mold under a stretching force.

and 200%, respectively. Upon increasing the elongation ratio, the shape of the replicated patterns gradually changes from circular to rectangular, becoming longer along the stretching axis while becoming shorter along the axis perpendicular to the stretching. The aspect ratio [l/w; indicated in Figure 3c] of the patterns increases from 1 (for the unstretched pattern) to 1.44, 2.05, and 3.34 for the patterns elongated by 125%, 150%, and 200%, respectively. Similarly, the pattern shape changes from circular to elongated hexagonal when the PDMS mold is stretched along the ‘B’ axis. Figure 3e,g,i shows the patterns obtained when the PDMS mold is stretched along the ‘B’ axis with elongations of 125%, 150%, and 200%; the aspect ratios are 0.67, 0.45, and 0.34, respectively. A comparison of the fast Fourier transform (FFT) images of the three different patterns (i.e., the original shape and the shape of the patterns elongated along the ‘A’ and ‘B’ axes) is shown in Figure S1 of the Supporting Information. The pattern-lattice changes induced by stretching are confirmed by spot-spacing changes in the FFT images. Stretching of the PDMS replica mold leads to pattern-height changes. The AFM image shown in Figure S2 of the Supporting Information reveals a height reduction of about 20% for the pattern obtained from a 200% elongated Langmuir 2009, 25(20), 12011–12014

Figure 3. (a,b) Stretching directions of the PDMS molds used in panels d, f, and h and in panels e, g, and i, respectively. (c) Scanning electron microscopy (SEM) image of a colloidal crystal replicated pattern obtained using an unstretched PDMS mold. SEM images of colloidal crystal replicated patterns obtained using PDMS molds stretched along the ‘A’ axis with elongations of 125 (d), 150 (f), and 200% (h), and along the ‘B’ axis with elongations of 125 (e), 150 (g), and 200% (i). The scale bars in d-i are 2 μm, and those in c and the insets of d-i are 1 μm. DOI: 10.1021/la902982y

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Figure 4. (c) SEM image showing the original shape of the colloidal crystal replicated pattern. (a,b) SEM images of deformed patterns obtained using PDMS replica molds stretched along an intermediate direction between the ‘A’ and ‘B’ axes and using a biaxially stretched PDMS mold, respectively. (d,e) SEM images of colloidal crystal replica patterns obtained using PDMS molds compressed along the ‘A’ and ‘B’ axes, respectively. The insets show the stretching or compressing axis in each case. Panels a-e are all in the same scale, and the scale bar represents 2 μm.

PDMS mold compared to the pattern obtained from an unstretched PDMS mold. The PDMS replica mold can be stretched not only along the ‘A’ and ‘B’ axes, but also in any other directions. Figure 4a shows the pattern obtained from a PDMS replica mold stretched along an intermediate direction between the ‘A’ and ‘B’ axes. The pattern shape also shows an intermediate shape between a rectangular and elongated hexagonal shape. Biaxially stretched PDMS replica molds can also be used to produce patterns with bigger feature sizes. As can be seen in Figure 4b, the patterns obtained from biaxially stretched PDMS molds show bigger feature sizes and sharper edges compared to those obtained from unstretched PDMS molds. We can obtain deformed patterns not only by stretching the PDMS molds but also by compressing them. Figure 4d,e shows the deformed patterns obtained using PDMS molds compressed along the ‘A’ and ‘B’ axes, respectively. The pattern shapes are similar to those of the patterns obtained from stretched PDMS molds, whereas the pattern feature sizes are reduced. However, it is difficult to avoid bending or humping of the PDMS mold during the compression process, and hence it is difficult to obtain a large-area pattern from a compressed PDMS mold. We used stretched patterns as template layers for producing colloidal crystals with 2D superlattices by additionally depositing a layer of small particles onto the colloidal crystal replicated patterns. The fabrication process is similar to that involving binary colloidal crystals, except for the fact that, in this case, we used colloidal crystal replicated patterns as the first layer instead of colloidal crystals. In a previous study, we introduced various binary colloidal crystal structures by changing the particle-size ratio of the layers and the concentration of the second-layered particles.27 However, in this system, we can additionally change the aspect ratio of the pattern on the first layer so as to produce a new type of superlattice structure. Figure 5a shows a typical (27) Kim, M. H.; Im, S. H.; Park, O O. Adv. Mater. 2005, 17, 2501.

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Figure 5. Schematic illustrations (a,b) and SEM images (c,d) of colloidal crystal superlattice structures. The bottom layers are: (a,c) a colloidal crystal replicated pattern obtained using an unstretched PDMS mold, and (b,d) a colloidal crystal replicated pattern obtained using a PDMS mold stretched along the ‘B’ axis. The scale bars correspond to 1 μm.

binary colloidal crystal structure in which the small particles have a graphite configuration. Figure 5c shows a fabricated sample consisting of a bottom layer composed of a colloidal crystal (particle size, 470 nm) replicated pattern and 270 nm particles located in the interstices of that bottom layer. On the other hand, if we use a stretched replicated pattern as the bottom layer, we can obtain a new type of superlattice structure, as shown in Figure 5b, d. The bottom layer of the sample in Figure 5d shows a replicated pattern obtained from a PDMS mold elongated to 150% along the ‘B’ direction (shown in Figure3g); then, 270 nm particles are deposited onto the prepared pattern. Since the interstitial distances of the bottom layer become longer along the stretching axis, the small particles form a zigzag line structure. Moreover, if we change the aspect ratio or the stretching axis of the bottom-layer pattern, we can further obtain various superlattice structures.

Conclusions Although colloidal crystals are attractive starting structures for micro/nanopatterning, until now they have only been able to offer circle- or sphere-shaped patterns. In this study, we have overcome this limitation by using stretched PDMS replica molds. Rectangular and elongated hexagon-shaped patterns with various aspect ratios have been successfully fabricated. Pattern shape and feature size can be easily controlled by changing the stretching axis and ratio of the PDMS mold. The simulated deformation of the PDMS replica mold is consistent with the experimental results. An additional noteworthy aspect of the proposed method is that several patterns introduced in this paper were produced from a single PDMS replica mold. The proposed pattern-control method can be applied not only to spherical shaped colloidal crystal master patterns but also to any other shaped master patterns such as line patterns or square patterns, which can be produced via photolithograpy or electron beam lithography. Therefore, we believe that this pattern-control method will significantly expand the usefulness and diversity of micro/nanopatterning technology. Acknowledgment. This work was supported by the ERC program of the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Ministry of Education, Science and Technology (MEST) (No. R11-2007-045-01002-0(2009)). Supporting Information Available: Supplementary figures are provided. This material is available free of charge via the Internet at http://pubs.acs.org. Langmuir 2009, 25(20), 12011–12014