Facile One-Step Fabrication of an Undercut Structure by Solution

May 7, 2010 - This paper reports a facile one-step method for making undercut structures by dewetting of poly(methyl methacrylate) (PMMA) chloroform ...
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J. Phys. Chem. C 2010, 114, 9845–9849

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Facile One-Step Fabrication of an Undercut Structure by Solution Dewetting on a Water/ Ice Mold Longjian Xue, Rubo Xing,* and Yanchun Han State Key Lab of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China ReceiVed: February 3, 2010; ReVised Manuscript ReceiVed: April 13, 2010

This paper reports a facile one-step method for making undercut structures by dewetting of poly(methyl methacrylate) (PMMA) chloroform solution on an ice/water mold. The ice mold is prepared by filling the microchannels with water (micromolding in capillaries (MIMIC) method) and then freezing. The curving surface of water mold stripes and the trapezoidal structure of ice mold stripes work as complementary structures devoted to the formation of undercut structures. The undercut structures with angles of 60° and 40° are obtained, which is determined by the states of ice or water mold. Meanwhile, if the solution concentration is increased, patterned polymer films with different configurations of undercut structures or microchannels would be gained. Introduction Organic light emitting diodes (OLEDs) trigger great interest in the area of flat panel displays (FPDs) because of their competitive performances and the ease and low cost of deposition.1-3 In passive-matrix OLED displays, the individual pixels are defined by the overlap of indium tin oxide (ITO) column strips (anodes) and metal row strips (cathodes). The anode strips can be easily formed by using conventional photolithography and wet etching processes before the organic layers are deposited. However, the cathode strips cannot be fabricated in the same way because the developing process would damage the underlying organic compounds. Because of this, a separator is normally used to form the cathode patterns during the vacuum evaporation of the cathode metal.4,5 Such a separator usually consists of a series of pillars with undercut structures. The separators with undercut structures are usually fabricated by photolithography6-9 and soft lithography.10-13 Photolithography and some modified photolithography usually generate an undercut structure in the photoresist.14,15 During the manufacturing process, the positive photoresist is exposed by UV light at the uncovered regions and the edge of the photomask covered regions because of the diffraction of UV light. After the removal of exposed photoresist, undercut structures formed. Lee et al.,7 however, created undercut structures by the diffusion of UV light instead of by diffraction, where an optical diffuser and randomized light are employed to form a patterned photoresist with circular or elliptical cross sections. Photolithography has good stability and repeatability; however, the multilayer resist configuration or some special photolithography process is needed which increases the manufacturing complexity and cost. Compared with the photolithography methods, soft lithography is simple, highly efficient, and low cost.16,17 An elastomeric polydimenthylsiloxane (PDMS) mold with reversely trapezoidal trenches is used to fabricate the undercut structure. The PDMS mold is placed on a substrate to form the channels with a * To whom correspondence should be addressed. Tel: +86-43185262267. Fax: +86-431-85262126. E-mail: [email protected].

reversely trapezoidal cross-section. Polymer solutions or precoated polymer layers are used for molding in the case of micromolding in capillaries (MIMIC) or soft-molding, respectively. After the removal of the PMDS mold, undercut structures are left on the substrate.10 Kim et al.13 also used the transfer printing method to fabricate the undercut structure. The prepolymer in the reversely trapezoidal trenches of the PDMS mold was cured with UV light. Then the PDMS mold was brought into contact with a substrate. By hot pressing, the reversely trapezoidal structures were transferred to the substrate. The base of this technique lies in the difference in adhesive works between the two interfaces. According to this mechanism, some terraced structures, in which the length dimension of the upper part is smaller than the lower part, can be transferred inversely onto another substrate without using the PDMS mold.12 These soft lithography methods are convenient to fabricate the undercut structures; however, the PDMS mold with reversely trapezoidal trenches should be fabricated first which is usually transferred from a preexisting reversely trapezoidal structure. That is to say, the fabricated undercut structure is transferred from another preexisting undercut structure. Besides, the selection of organic solvent should be carried out carefully since the organic solvent may lead to swelling of the PDMS mold which results in poor patterning quality or even makes the polymer solution unpatternable.18 Herein, we report a simple method based on dewetting of polymer solution on an ice/water mold to fabricate the undercut structure. Mostly, dewetting is an undesirable process for the preparation of homogeneous polymer films, because in this process the film often ruptures and forms disordered droplets on the solid surface.19,20 However, dewetting can also provide routes to fabricate patterns which cannot or can hardly be achieved by other methods.21-26 In this paper, ice and water strips fabricated by MIMIC were used as molds, on which the polymer solution dewetted. After the evaporation of organic solvent and water, undercut structures were generated spontaneously. Poly(methyl methacrylate) (PMMA) was used as the patterned material in our experiment, but this method could be extended to other insulating materials.

10.1021/jp1010567  2010 American Chemical Society Published on Web 05/07/2010

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Experimental Section Materials. Poly(methyl methacrylate) (PMMA, Mw ) 350 000 g/mol) was obtained from Acros Organics. Polystyrene used here was an industrial product. The solvent, chloroform (CHCl3) was procured from Beijing Chemical, China. Polydimethylsiloxane (PDMS) elastomer kits (Sylgard 184) were purchased from Dow Corning. ITO glass (glass slide with indium-tinoxide films) was purchased from Chinese South Glass Holding (CSC) Co., Ltd. ITO glass was used after the treatment of oxygen plasma. Sometimes, silicon wafer was also used for the convenience of characterization. Silicon wafer was boiled in a Piranha solution (7/3 (V/V) of 98% H2SO4/30% H2O2) for 30 min, then washed with deionized water, and dried with a nitrogen flow before use. Characterization. The structures of the molds and the fabricated undercut structures were investigated by field emission scanning electron microscopy (FESEM, Philips XL-30ESEM-FEG instrument). The samples for FESEM were coated with a 20-30 Å layer of Au to make them conductive. The optical photographs were imaged on a Leica optical microscopy (Leica Microsystems Ltd., Germany) in reflection mode with a CCD camera attachment. The freezing and thawing were also carried out on a Linkman hot stage (THMSE600) and were imaged simultaneously. The contact angles (CA) were measured on a KRu¨SS DSA10MK2 (KRu¨SS GmbH, Germany) drop shape analysis (DSA) system at an ambient temperature. The probe fluid was deionized water, and the droplet volume was 3 µL. The average CA value was obtained by measuring the same sample at five different positions. Results and Discussion Formation of Undercut Structure. An overall schematic of the fabrication process is depicted in Figure 1. PDMS molds with a stripe pattern were prepared by casting PDMS prepolymer on a photoresist master patterned by photolithography and curing at 65 °C for 4 h (Figure 1a). The strips of the photoresist master show trapezoidal cross-sections with bottom widths of 50 µm (spaced by 50 µm) and bottom angles of 60° (Figure 2a). The trapezoidal cross-section stripes of the photoresist master are the result of photolithography, which is usually neglected in most of the experiments.27-29 Only recently, our group used the nature of the trapezoidal cross-section to fabricate the undercut structure.17 The relief structure on the PDMS mold is a negative replica of the structure in the photoresist master as shown in Figure 2b. The width (W) and the height (H) of the stripe are 50 µm (spaced by 50 µm) and about 3.5 µm, respectively. The PDMS mold peeled from the photoresist master (Figure 1b) was then placed in contact with the ITO substrate forming the microchannels (Figure 1c). When a drop of water was placed at the entryway of microchannels, the water spontaneously filled into the microchannels driven by capillary forces (Figure 1d). Cooled below 0 °C (mostly at -10 °C) in a refrigerator, the water froze. After that, the PDMS mold was peeled off leaving ice strips on the substrate (Figure 1e). Subsequently, the icepatterned substrate was quickly dipped into PMMA/CHCl3 solutions with different concentrations (0.5, 0.8, 1, and 5 wt %) and withdrew from the solutions at a speed of 1- 3 cm/s (Figure 1f). Using method 1, the withdrawn sample was transferred to ambient atmosphere directly (Figure 1g1). After the complete evaporation of CHCl3 and water, the PMMA stripes with undercut structures were generated on the substrate (Figure 1h). Using method 2, the sample was kept at -10 °C in the refrigerator for about 20 min so that the solvent CHCl3

Figure 1. Fabrication procedure of PMMA separators with undercut structures supported on ITO glass: (a) PDMS prepolymer is casted on the patterned photoresist master and cured at 65 °C for 4 h. (b) PDMS mold with strip pattern is released from the photoresist master. (c) A PDMS mold is placed on an ITO glass forming the microchannels. (d) Water MIMIC into the microchannels and cools down at -10 °C for 20 min. (e) Remove the PDMS mold leaving the ice strips on the substrate. (f) Dip the ice mold into the PMMA/CHCl3 solution and withdraw. (g1) Method 1. Withdrawn sample is transferred to ambient atmosphere. (g2) Method 2. Withdrawn sample is kept -10 °C for 20 min until the complete evaporation of CHCl3. (h) CHCl3 and/or water evaporate leaving the PMMA undercut structures on the substrate.

Figure 2. ESEM images of cross sections of (a) photoresist master and (b) PDMS mold with patterned stripes fabricated from photoresist master in (a). The insets show the enlarged cross sections. The white line in the insets outlines the angles of the trapezoid structures.

would evaporate completely (Figure 1g2). After moving to the ambient atmosphere, the ice mold melted and evaporated

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Figure 4. (a) Optical photograph of patterned stripes with undercut structures made from 0.8 wt % PMMA/CHCl3 solution by method 2. (b) ESEM image of cross section of the undercut structure. (c) ESEM image of cross section of film with microchannels made from 5 wt % PMMA/CHCl3 solution by method 2. (d) The enlarged image of a single microchannel.

Figure 3. ESEM images of cross sections of patterned stripes with undercut structures made from (a, b) 0.8, (c, d) 0.5, and (e, f) 1 wt % PMMA CHCl3 solution and (g, h) microchannels made from 5 wt % PMMA CHCl3 solution using method 1. The inset in (a) shows an optical photograph of patterned stripes with an undercut structure made from 0.8 wt % PMMA CHCl3 solution. The white line in (b) outlines the angle of the undercut structure to be about 40°.

completely leaving behind the undercut structures on the substrate (Figure 1h). Undercut structures formed after the complete evaporation of CHCl3 and water. Typical images of PMMA undercut structures made from CHCl3 solution with different concentrations by method 1 are shown in Figure 3. An optical overview of PMMA undercut structures made from 0.8 wt % solution is shown in the inset of Figure 3a. The darker areas are the patterned PMMA stripes. Both sides of the stripe have different thicknesses from the center part since the light and shadow characteristics in the optical micrograph represent the height difference. From the center to the edge of the stripe, the brightness went dark first and then lighter, suggesting the presence of special structures on the edges of stripe. FESEM images of cross sections of the stripes are shown in Figure 3a,b indicating the presence of the undercut structure, which looks like the cornice of an ancient Chinese building. The height of the undercut structure is about 2 µm, and the angle is about 40° (Figure 3b). The thickness of the central part of the stripe pattern is about 150 nm, which is much smaller than the thickness near the root of the undercut structure that is about 800 nm. This large thickness at the root is helpful for the stability of undercut structures. By extending the concentration of polymer solution to a smaller concentration of 0.5 wt % and to a higher concentration of 1 wt %, the dimensional size of the stripe pattern changed. With the 0.5 wt % concentration, the thicknesses of the center part (90 nm) and the overhang part are much thinner (Figure

3c,d), and the tip of the overhang structure is not strictly linear but showed mild irregularity. The large concentration of 1 wt % (Figure 3e,f) caused a much larger thickness of about 500 nm at the center part of stripe pattern. Though the concentration of solutions changed, the angles of the undercut structures remained the same within error limit. When the concentration is as high as 5 wt %, microchannels instead of undercut structures formed (Figure 3g,h). These microchannels are composed of arched roofs instead of other shapes. Changing the experiment detail slightly so that the sample withdrawn from the solution was kept at -10 °C for about 20 min and then moved to ambient atmosphere (method 2) changed the characteristics of the resulting structure. The overview of resulting film made from 0.8 wt % solution by method 2 (Figure 4a) showed the same characteristics as that prepared by method 1. The cross section of the strip pattern also showed the presence of undercut structures of which the dimensional size (Figure 4b), however, changed. The height changed to about 1 µm and the thickness of the root part shifted to about 650 nm. Both are slightly smaller than that made by method 1. The angle of the undercut structure, however, shifted to a larger value of about 60° (Figure 4b). Meanwhile, the sample prepared from 5 wt % solution showed the microchannels with trapezoidal cross sections whose bottom angles were also about 60° (Figure 4c, d). Dewetting of Chloroform Solution Forms the Undercut Structure. The mechanism for the formation of undercut structure was proposed to be the dewetting of chloroform solution on the mold. When the substrate with ice stripes was withdrawn from the solution, a thin film of chloroform solution was initially deposited on the ice-mold-patterned substrate as depicted in Figure 1f. The thickness of the solution film reduced further because of the evaporation of solvent. According to the dewetting theory, the rupture of a thin liquid film is initiated only when the film thickness is smaller than a certain value, namely the capillary length λcap of the liquid. The capillary length is given by the equation λcap ) (γlv/Fg)1/2, where γlv is the liquid/vapor surface tension and F the density of the liquid.30,31 Smaller than this value, molecular forces dominate over gravitational forces and the film may dewet.32,33 The rupture, which may be initiated by the heterogeneous nucleation

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Figure 6. Outlines of cross sections of ice mold (dash line) and water mold (solid line) with same area. θ and R denote the angle at the three phase line of water mold and the bottom angle of ice mold, respectively.

Figure 5. (a) Water froze in the microchannels composed of PDMS mold and silicon wafer. White areas are ice. (b) The pattern of water columns on the silicon wafer after the peeling of PDMS mold at room temperature. The dark areas are water. The scale bar is 100 µm.

mechanism,25 would first take place on the mold where the thickness of liquid film is smaller and reaches the critical value first. The dewetted liquid flew away exposing the underlying mold. By method 1, chloroform solution dewetted on the water mold. Withdrawn from solution and transferred to ambient atmosphere, the melting of ice happened before the completed solidification of PMMA. In order to confirm this, a reference experiment was carried out. PDMS microchannels containing water were placed on a Linkman hot stage and cooled to -10 °C (Figure 5a). After the complete freezing of water, the sample was moved to ambient conditions. The ice strips melted within 2-3 s after the PDMS mold was peeled (Figure 5b). The water strips gained curved surfaces because of the surface tension.29 Meanwhile, the complete evaporation of chloroform would take more than 10 s. Thus, chloroform solution dewetted on the water mold in method 1 (Figure 1g1). When the sample was withdrawn from solution and kept at -10 °C (method 2) the ice mold would keep the original trapezoidal shape. The dewetting of chloroform solution then happened on the solid ice mold (Figure 1g2). Because of the persistence of water/ice mold during the whole evaporation process of chloroform, the curved surface of water mold and the trapezoidal ice mold worked as the complementary structure. After the removal of the water/ice mold by moving to ambient conditions, the deposited PMMA strips gained the complementary angles of the molds and formed the undercut structures. The dewetted solution flowed to the blank areas between the water/ice mold stripes. At the early stage of dewetting, the holes were still separated from each other that the contact line receded in all directions on the water/ice mold stripes. The receding direction along the long-axis of the mold stripe caused a thickness fluctuation in the chloroform solution resulting in an uneven surface of overhang structures, especially in the dilute

solution of 0.5 wt % (Figure 3d). After the impact and coalescence of holes, a long crack formed that the contact line receded normally to the long-axis of water/ice mold stripes. Because of the fast evaporation of chloroform, the dewetted solution accumulated near the edge of the water/ice stripes leaving behind a much thicker film than that of the center part of the PMMA stripes. Structure Tailoring by the State of Mold and the Solution Concentration. The state of the mold (liquid water or solid ice) determined the angle of the undercut structure. In the atmosphere of -10 °C, the ice mold worked as the complementary structure so that the bottom angle of the ice mold determined the angle of the undercut structure, which is about 60°. The viscosity of the chloroform solution was much higher at such a low temperature, which caused the flow of solution to be much slower. The dewetted solution from the ice mold then accumulated near the edge of the ice mold resulting in a much larger thickness of the root part of the undercut structure. The slower evaporation of chloroform (comparing with that at room temperature) permitted a longer time for the solution to dewet from the center of the ice stripe before the complete evaporation of solvent, which formed a smaller height (and thus the width) of the undercut structure. Moved to room temperature, the trapezoidal ice mold melted and turned into the curved water mold. Neglecting the shrinkage caused by melting, the contact angle θ of the water strip was calculated to be 39.6° (Figure 6), which agrees quite well with the experimental result. That is to say, parallel water stripes with contact angles of about 40° formed on the hydrophilic ITO surface. It is interesting to note that these channels stayed stable on the ITO surface as shown in Figure 5b. During the MIMIC process, capillary forces pull the water into the microchannels. Therefore, the most important concept of the process is to wet the walls of the PDMS mold and the substrate by water lowering the total free energy ∆G.34 The change in interfacial energies for water moving into a microchannel is given by eq 1 ∆G ) APDMS(γPDMS,W - γPDMS,V) + AITO(γITO,W - γITO,V) ) - APDMSγW,V cos θPDMS,W - AITOγW,V cos θITO,W

(1) The terms γPDMS,W, γPDMS,V, γITO,W, γITO,W, and γW,V, are PDMS-water, PDMS-vapor, ITO-water, ITO-vapor, and water-vapor interfacial free energies, respectively. APDMS and AITO are the areas of PDMS wall and ITO substrate in the microchannels, respectively. θPDMS,W and θITO,W are the contact angles of water on the PDMS and ITO surfaces, respectively. The contact angle of water on PDMS is about 90° that the ∆G can then be calculated approximately by eq 2

∆G ≈ -AITOγW,V cos θITO,W

(2)

Fabrication of Undercut Structure It suggests that the wetting of the ITO surface with water contributes mainly to the MIMIC process. In order to allow water to spontaneously fill into the microchannels composed of PDMS and ITO, the ITO surface was treated with oxygen plasma. The sessile contact angle of water on the oxygenplasma-treated ITO glass θITO,W is about 3°. Why can the water strips with contact angles of about 40° stay stable on such a hydrophilic ITO surface? It has been reported that, when the PDMS mold is placed on a substrate, low molecular weight oligomers from the PDMS mold transfer to the hydrophilic surface producing hydrophobic surfaces.35 The PDMS with the stripe pattern used here then produced hydrophobic stripes on the ITO surface. The boundaries of these hydrophobic regions pinned to the contact lines of water columns and the contact angle of about 40°, which is much larger than θITO,W, can be preserved.36 In order to conform this, a PDMS mold was immersed in acetone for 30 min to wash away the low molecular weight oligomers and then dried at 110 °C for 20 min. The treated PDMS mold was then used to fabricate the undercut structure. After the sample was withdrawn from the chloroform solution and transferred to ambient conditions, the ice mold melted and spread onto the whole substrate quickly. Without the pinning sites, the Laplace pressure in the water stripe, which is given by γ/R (γ being the surface tension and R the radius of curvature), leveled the curved surface and caused the contact angle to reach the equilibrium contact angle of about 3° reducing the system energy. The liquid film was floated, and a continuous PMMA film was generated. This phenomenon is even more obvious when a hydrophobic material, polystyrene, is used since polystyrene cannot wet the hydrophilic surface.37 The solution concentration determined the thickness of patterned PMMA films. When the concentration of solution was small (0.5, 0.8, and 1 wt %), a thin layer of PMMA solution deposited on the water/ice mold, and the thickness reduced further upon the evaporation of solvent. When the thickness reached the critical value, λcap, rupture happened in the film. Most of the dewetted solution collected at the boundary between the blank areas and the water/ice molds, which contributed little to the thickness of the central parts of PMMA stripes. Increasing the solution concentration, the thickness of resulting structures increased. When the concentration is large (e.g., 5 wt %), the film thickness can not reach λcap after solvent evaporation. Rupture would not happen in the film.32,33 A continuous PMMA film was deposited on the ice/water molds. After the removal of water, microchannels with curved or trapezoidal crosssections emerged. Conclusions A novel technique has been presented which enables one to fabricate undercut structures in one simple step. Chloroform solutions of polymers dewet on the water/ice mold and form the undercut structures. The states of the ice mold or water column determine the angle of the undercut structures; meanwhile, the concentration of polymer solution determines the thickness of the fabricated structure. The most significant advantage of this method is that the complementary material of the undercut structure is water which can be removed easily without any post-treatment. Our method could be extended to fabricate the undercut structures composed of other insulating polymer materials, and the only requirement seems to be that the materials used must be able to dissolve in a waterimmiscible, volatile organic solvent.

J. Phys. Chem. C, Vol. 114, No. 21, 2010 9849 Acknowledgment. This work was subsidized by the National Natural Science Foundation of China (20621401) and the Ministry of Science and Technology of China (2009CB930603 and 2009CB623604). References and Notes (1) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, Q. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (3) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughs, J. H.; Marks, R. N.; Tallini, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 14. (4) Kihima, Y.; Asai, N.; Kishii, N.; Tamura, S. IEEE Trans. Electron. DeVices 1997, 44, 1222. (5) Kim, S. W.; Hwang, B. H.; Lee, J. H.; Kang, J. I.; Min, K. W.; Kim, W. Y. Curr. Appl.Phys. 2002, 2, 335. (6) Py, C.; D’Iorio, M.; Tao, Y.; Stapledon, J.; Marshall, P. Synth. Met. 2000, 113, 155. (7) Lee, H. S.; Yoon, J. B. J. Micromech. Microeng. 2005, 15, 2136. (8) Huang, Z. H.; Qi, G. J.; Zeng, X. T.; Su, W. M. Thin Sol. Films 2006, 503, 246. (9) Xing, R.; Ye, T.; Ding, Y.; Ma, D.; Han, Y. Org. Electron. 2009, 10, 313. (10) Rhee, J.; Park, J.; Kwon, S.; Yoon, H.; Lee, H. H. AdV. Mater. 2003, 15, 1075. (11) Kang, H.; Park, J.; Lee, H. H. AdV. Mater. 2006, 18, 1603. (12) Seo, S.; Kim, J. H.; Kim, T.; Lee, H. H. Appl. Phys. Lett. 2006, 88, 023118. (13) Kim, M. J.; Song, S.; Kwon, S. J.; Lee, H. H. J. Phys. Chem. C 2007, 111, 1140. (14) Xing, R.; Xuan, Y.; Wang, Z.; Ma, D.; Han, Y. Curr. Appl. Phys. 2009, 9, 760. (15) Sze, S. M. Semiconductor DeVices: Physics and Technology, 2nd ed.; Wiley: New York, 2001; p 400. (16) Wang, Z.; Yuan, J.; Zhang, J.; Xing, R.; Yan, D.; Han, Y. AdV. Mater. 2003, 15, 1009. (17) Xing, R.; Xuan, Y.; Ma, D.; Han, Y. J. Vac. Sci. Technol. B 2008, 26, 1. (18) Kim, E.; Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1996, 118, 5722. (19) Reiter, G. Langmuir 1993, 9, 1344. (20) Herminghaus, S.; Jacobs, K.; Mecke, K.; Bischof, J.; Fery, A.; IbnElhaj, M.; Schlagowski, S. Science 1998, 282, 916. (21) Luo, C.; Xing, R.; Han, Y. Surf. Sci. 2004, 552, 139. (22) van Hameren, R.; Scho¨n, P.; van Buul, A. M.; Hoogboom, J.; Lazarenko, S. V.; Gerritsen, J. W.; Engelkamp, H.; Christianen, P. C. M.; Heus, H. A.; Maan, J. C.; Rasing, T.; Speller, S.; Rowan, A. E.; Elemans, J. A. A. W.; Nolte, R. J. M. Science 2006, 314, 1433. (23) Higgins, A. M.; Jones, R. A. L. Nature 2000, 404, 476. (24) Kim, E.; Whitesides, G. M.; Lee, L. K.; Smith, S. P.; Perentiss, M. AdV. Mater. 1996, 8, 139. (25) Lu, G.; Li, W.; Yao, J.; Zhang, G.; Yang, B.; Shen, J. AdV. Mater. 2002, 14, 1049. (26) Xue, L.; Gao, X.; Zhao, K.; Liu, J.; Yu, X.; Han, Y. Nanotechnology 2010, 21, 145303. (27) Wang, Z.; Xing, R.; Zhang, J.; Yuan, J.; Yu, X.; Han, Y. Appl. Phys. Lett. 2004, 85, 831. (28) Yu, X.; Xing, R.; Luan, S.; Wang, Z.; Han, Y. Appl. Surf. Sci. 2006, 252, 8544. (29) Suh, K. Y.; Lee, H. H. AdV. Funct. Mater. 2002, 12, 405. (30) Brochard-Wyart, F.; Daillant, J. Can. J. Phys. 1990, 68, 1084. (31) Seemann, R.; Herminghaus, S.; Neto, C.; Schlagowski, S.; Podzimek, D.; Konrad, R.; Mantz, H.; Jacobs, K. J. Phys.: Condens. Matter 2005, 17, S267. (32) de Gennes, P. G. ReV. Mod. Phys. 1985, 57, 827. (33) Brochard-Wyart, F.; Redon, C.; Rondelez, F. C. R. Acad. Sci., Ser 2 1988, 306, 1143. (34) Kim, E.; Xia, Y.; Whitesides, G. M. Nature 1995, 376, 581. (35) Briseno, A. L.; Roberts, M.; Ling, M.; Moon, H.; Nemanick, E. J.; Bao, Z. J. Am. Chem. Soc. 2006, 128, 3880. (36) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46. (37) Xue, L.; Cheng, Z.; Fu, J.; Han, Y. J. Chem. Phys. 2008, 129, 054905.

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