LETTER pubs.acs.org/Langmuir
Simple Fabrication of Asymmetric High-Aspect-Ratio Polymer Nanopillars by Reusable AAO Templates Moon Kee Choi,† Hyunsik Yoon,† Kyunghee Lee, and Kyusoon Shin* School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea
bS Supporting Information ABSTRACT: We present a simple method of utilizing anodized aluminum oxide (AAO) as a reproducible template for fabricating high-aspect-ratio uniformly bent polymeric nanopillars that can be used as a physical adhesive. It is shown how to achieve straight high-aspect-ratio nanopillars with concepts of the work of adhesion and lateral collapse between polymer pillars without serious damage to the master template. With the support of manufacturing polymeric nanopillars from the reusable AAO, a simple route to asymmetric dry adhesive nanopillars bent by residual stresses was demonstrated.
’ INTRODUCTION Making 1D nanoscopic structures especially with asymmetry, inspired by Mother Nature, is still a great challenge to overcome, although 1D nanomaterials such as asymmetric high-aspect-ratio polymer nanopillars have received considerable attention because of their wide potential applications including Gecko-mimicking dry adhesives,1 microfluidics,2,3 water delivery,4 unidirectional wetting,5,6 nanotemplates,7-10 piezoelectric nanogenerators,11,12 and micromechanical sensors.13,14 Nanopillars are generally fabricated by photolithography,15 e-beam lithography,16 and soft lithography.17 Unfortunately, each of these techniques has their inherent bottlenecks for productivity. Taking e-beam lithography as an example, frequently used for nanoscale structure manufacturing, it requires high cost equipments and suffers low throughput.18 In addition, it is difficult to fabricate a master template that possesses high-aspectratio nanoscopic structures as well. As a solution to this problem, exploiting anodized aluminum oxide (AAO) as a template for highaspect-ratio nanopillars was proposed.19-37 Owing to the inherent self-assembly mechanism of nanopore formation, the anodized alumina has high-aspect-ratio nanoscopic pores that are uniform and hexagonally packed highly ordered. Moreover, the formation process of AAO pores is well developed, and the diameter and length of nanopores is easy to control.38 Therefore, apart from endowing asymmetry, employing AAO as a template in the generation of 1D structure must be advantageous to the economic production of nanopillars. The injection molding technique is one of the most popular methods of obtaining high-aspect-ratio nanopillars from AAO.39-41 This method is based on the wetting of polymeric materials on the AAO wall as well as high-temperature conditions in order to introduce the viscous polymeric materials into the r 2011 American Chemical Society
nanopores. However, this method requires the dissolution of the master template with strong acid after the infiltration of polymeric materials into the nanopores in order to obtain the nanopillars out of the high-aspect-ratio nanopores. The removal of the master template makes the recycling of the master mold unavailable in this method and the whole manufacturing process very inefficient. In addition to the template-removal issue, the clumping of the high-aspect-ratio nanopillars is also important to prevent, but it is very hard to avoid because of the van der Waals interaction among the densely packed nanopillars. Because the separation of nanopillars is critically important to the performance of dry adhesion and device applications, methods to prevent the lateral collapse of the nanopillars such as freezing or supercritical drying steps are necessarily applied in certain cases.42 Thus, it is necessary to resolve a couple of issues in the manufacturing of high-aspect-ratio nanopillars. As a solution to the reuse of the template, UV-curable polymers for mold casting might be applied.31,43,44 In the manufacturing of high-aspectratio nanopillars, however, it is still difficult to release the nanopillars from the mold because the difference in the surface energy of AAO and the polymer is small, which limits the aspect ratio of the fabricated nanopillars to less than ∼5 and lowers their uniformity.43,45 In connection with these issues, herein we demonstrate a simple route to asymmetric polymer nanorod arrays from a repeatedly usable AAO master mold on the basis of a UV-assisted nanomolding technique. The conversion from straight nanopillars Received: December 5, 2010 Revised: January 11, 2011 Published: January 19, 2011 2132
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to uniformly bent Janus nanopillars was available by the angled metal-evaporation method that was proposed quite recently.46,47 High-aspect-ratio polymeric nanopillars were obtained on a large scale without lateral collapse and the sticking problem between the mold and the nanopillars, and asymmetric uniform bending was imparted to the nanopillars by the evaporation of metals on the nanopillars with tilt. We diversify the size of polymeric nanopillars from AAO nanotemplates by varying the interpore distance, aspect ratio, and diameter. The effects of these three factors on the fabrication process are studied and discussed in detail here.
’ EXPERIMENTAL SECTION Fabrication of the AAO Mold. AAO templates were fabricated via the two-step anodization of aluminum sheets (99.999%, Goodfellow).48-50 Aluminum sheets were treated with 4:1 v/v ethanol/HClO4 using an electropolishing technique. Then, the sheets were anodized at 190 V in 0.1 M phosphoric acid electrolyte while the temperature was fixed at 0 °C. After the first anodization, the aluminum oxide layer was dissolved in chromic acid at 75 °C and washed with distilled water and ethanol. The second anodization process was carried out under the same condition as the first one. The pore length was proportional to the anodization time. By using two-step anodization, hexagonally packed uniform nanopores were obtained. Pore diameters were varied from 120 to 190 nm by controlling the pore-widening time in 0.1 M phosphoric acid. With this process, we fabricated nanopores with uniform inner diameters with an interpore distance of 500 nm. To obtain an inner diameter of 45 nm and an interpore distance of 105 nm, the voltage and temperature were set as 40 V and 15 °C, respectively, and 0.3 M oxalic acid was used as the electrolyte. Surface Modification of the AAO. First, the AAO mold was treated with oxygen plasma. The instrument was operated at 50 W with the gas flow rate of 10 sccm and a pressure of 100 mTorr (PVTronics Etch-100). Then, the mold was immersed in a 0.5 wt % aqueous solution of 3-(aminopropyl triethoxysilane) (APTES, Aldrich) for 10 min. Unreacted APTES was washed with distilled water. The mold was treated with monoglycidyl ether-terminated poly(dimethylsiloxane) (PDMS, Mn = 5000, Aldrich) by holding at 80 °C for 4 h. After heat treatment, the mold was immersed in isopropyl alcohol and sonicated for 1 min to remove unreacted PDMS.51 The thickness of PDMS was less than 10 nm, which could be confirmed by comparing the pillar diameters and the pore sizes. This could be another strong point against the silane treatment changing the pore size to about 40 nm.31 Fabrication of Polymer Nanopillars. Polyurethane acrylate (PUA, 301RM and 311RM, Minuta Tech.) was drop cast onto an AAO mold, and a polyethylene terephthalate (PET) film with 50 μm thickness was gently placed on the PUA layer as a substrate. The UVcurable PUA consisted of a functionalized prepolymer with an acrylate group for cross-linking, a photoinitiator, and some additives. It had a good elongation property and high mechanical stiffness after crosslinking.52,53 The polymer layers on the AAO mold were irradiated with UV light (λ = 250 to 400 nm) for a few seconds at a dose of 900 mJ/cm2. The PET film played the roles of a stiff backplane and a blocking layer against the permeation of oxygen gas that could interfere with UV curing. After UV curing, the PUA replica was detached from the AAO mold. The film supported by a stiff PET film was carefully peeled off of the AAO template at a constant speed of ∼3 mm/s while maintaining the angle between the AAO surface and the film at less than 20°. To eliminate unreacted precursors, further UV exposure was performed for 3 h. The polymer nanopillars obtained were investigated with a JSM6701F scanning electron microscope (SEM, JEOL). Fabrication of Asymmetrically Bent Nanopillars. One lateral side of the polymer nanopillars was coated with gold that was tens of
Figure 1. Schematic illustration of the procedure for the fabrication of highly packed, uniformly bent polymeric Janus nanopillar arrays prepared from reusable AAO templates. nanometers thick by oblique thermal evaporation. The deposition rate was 1 Å/s under high vacuum (10-6 Torr). The inclined holder at an angle of 45° was used for selective Au coating on only one side of the nanopillars to ensure their Janus structure. Janus nanopillars were bent toward the Au-deposited side, and their curvature was controlled by the thickness of the Au layer. Details of the experimental setup and the mechanism for the fabrication of bent Janus nanopillars can be found elsewhere.46,47 Measurement. To measure the adhesion properties of the nanopillars, the macroscopic shear adhesion strength was evaluated by a hanging test in which a flexible dry adhesion patch (1 cm 2 cm) was attached to a Si surface under a preload of 0.1 N/cm2. During the shear adhesion test, no external normal load was applied. The generated patterns were examined by scanning electron microscopy (SEM, JSM6701F) after platinum sputtering (SCD 005) onto the polymeric nanopillars with a thickness of less than 3 nm.
’ RESULTS AND DISCUSSION Uniform asymmetric high-aspect-ratio polymeric nanopillars, replicated from a reusable template, were fabricated by utilizing the surface-modified nanoporous alumina and UV-curable polymer. Figure 1 shows a schematic illustration of the fabrication procedure used in this study. In the first step, we prepared an AAO mold of well-aligned nanopores by a conventional two-step anodization process. The surface of the AAO template was treated with oxygen plasma to enhance its reactivity towards silanization. Hydroxyl groups on the surface of the AAO template were substituted with amino groups by a silanization reaction with APTES. Amino groups made strong covalent bonds with the epoxy group of monoglycidyl ether-terminated PDMS, which led to the formation of a PDMS layer on the surface of the AAO template. Note that the water contact angle on the AAO template (interpore distance, 110 nm; range of pore diameter, 30-60 nm) before the PDMS coating was 77° and changed uniformly to 114° after the coating. PUA prepolymer liquid was drop cast onto the surface-treated AAO template with a PET substrate as a backplane. After the UV-curing procedure, cross-linked PUA nanopillars on the PET backplane were detached from the AAO mold. After manufacturing the high-density nanorod arrays, asymmetric uniformly bent nanorods were formed by metal evaporation. In our methods, the diameter and height of the 2133
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Langmuir nanopillars were controlled by adjusting the etching time of the AAO mold. The diameters of the nanopillars d were in the range of 45 to 200 nm and their heights h were in the range of 40 nm to 2 μm. The center-to-center distance of the nanopillars s was controlled by varying the voltage and the type of anodizing electrolytes during AAO etching and in the range of 105 to 500 nm (the lowest panel in Figure 1).54 The nanopillars made by the simple “pulling” method were easily converted to the bent structure by following a process (Janus nanopillars) reported previously, as shown in Figure 2a,b.47 During gold deposition, the nanopillars were bent toward the metal face because of a residual tensile stress, and the bending direction was changed in the opposite direction when aluminum was deposited. These nanopillars that are asymmetrically bent in one direction have anisotropic shear adhesion behavior and
Figure 2. Uniformly bent Janus nanopillars and the shear adhesion force of a substrate that has the bent Janus nanopillars: (a) Schematic illustration of the fabrication of bent Janus nanopillars and (b) SEM image of bent nanopillars prepared by 20-nm-thick Au evaporation. The scale bar of this image is 500 nm. (c) Macroscopic shear adhesion force of low-modulus PUA layers with nanopillar arrays on a PET substrate: a flat film without nanostructure (flat), straight nanopillars (straight), and bent nanopillars (L- and R-bent).
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directional wetting characteristics.1,5,6 These properties could be useful for some applications, such as climbing robots, microfluidics, and so forth. In Figure 2c, the adhesion characteristics of the flat film, the straight nanopillars, and bent Janus nanopillars are compared. To measure the adhesion properties of the nanopillars, the macroscopic shear adhesion strength was evaluated by a hanging test in which a flexible dry adhesion patch (1 cm 2 cm) was attached against a Si surface under a preload of 0.1 N/cm2. During the shear adhesion test, no external normal load was applied. The dimensions of the bent Janus nanopillars were the same as those of the straight ones before the metal deposition (150 nm in diameter and 1.5 μm in height). The shear adhesion force was measured to be 4.12 N/cm2 for the straight nanopillars. However, the shear adhesion forces of the bent Janus nanopillars were 6.45 and 1.20 N/cm2, respectively, for the two opposite directions that are parallel and antiparallel to the curved direction of the nanopillars. This result shows that the nanopillars of anisotropic adhesion characteristics can be fabricated by a simple template method. It is noted that the values of the adhesion force obtained by bent nanopillars are lower than previous results.47 This is probably because of the microroughness of the AAO template interrupting the contact between the nanopillars and the smooth substrates (Supporting Information S1). One of the essential features of the current method that contributed to the formation of uniform, high-aspect-ratio nanopillars with reusable molds is the surface treatment of the AAO mold to form a PDMS layer on its surface. The PDMS treatment lowers the surface energy inside the pores of AAO and reduces the adhesion force between the polymer and the AAO mold during the mold-casting process. As a result, polymer nanopillars with a relatively high aspect ratio were easily released from the mold without damage. In this way, the nanopillars were obtained from the mold within only a few seconds. Moreover, the PDMS layer on the surface of the AAO mold was so rigid that significant damage to the nanopillars was not easily observed during the repetitive use of the same mold more than 100 times. The low-cost manufacturing of the AAO template owing to selfassembly and the high throughput and the excellent durability of the mold for the recycling of the current synthesis method are very advantageous for high productivity and low cost.
Figure 3. AAO templates and polymer nanopillar arrays with a range of aspect ratios. The upper images are cross-sectional SEM images of the AAO templates (scale bar, 100 nm), and the lower images are 45°-tilted views of polymer nanopillars (scale bar, 500 nm). The center-to-center distance and diameter of the nanopillars were kept at 110 and 45 nm, and the lengths were varied. 2134
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Figure 4. SEM images of the top and side views of the polymer nanopillars of different diameters and heights with a fixed center-to-center distance (∼500 nm). Images on the left and right sides of each panel are top and cross-sectional views of the nanopillars, respectively. The regime in the gray background indicates the optimal dimensional conditions for the manufacture of straight, uniform nanopillar arrays.
Straight nanopillars with a higher aspect ratio are favorable for applications such as dry adhesive and superhydrophobic surfaces. However, high-aspect-ratio polymeric nanopillars tend to stick to the interior of the AAO template. With the PDMS coating in our method, it was possible to obtain polymer nanopillars with a higher aspect ratio from the mold because of the reduction in the adhesive force between the polymeric rods and the surface of the AAO template wall. The nanopillars on the substrate can be pulled off of the mold when the work of adhesion between the nanopillars and the substrate is larger than that between the nanopillars and the mold. This relationship is written as follows WPS APS > WPM APM
ð1Þ
where WPS and WPM are the work of adhesion between the polymer nanopillars and the substrate and between the polymer nanopillars and the mold, respectively. The areas of the corresponding interfaces, APS and APM, can be calculated from the values of d, h, and s of the nanopillars. Then, the limit of the aspect ratio h/d for the release of the nanopillars from the mold is given as pffiffiffi 2 3 s WPS AR < -1 ð2Þ 2π d WPM As shown in this equation, the smaller WPM, realized by the PDMS coating in our method, leads to higher-aspect-ratio nanopillars. In Figure 3, SEM images of the polymer nanopillars with high aspect ratios are shown. The nanopillars, having diameters of 45 nm, were easily released from the mold until the aspect ratio was increased to 15. However, with an aspect
ratio as large as 19, it was very hard to detach the nanopillars from the mold without damage. According to eq 2, the ratio of the work of adhesion WPS/WPM for the upper limit of the aspect ratio is in the range of 8.5-12.0 when d = 45 nm and s = 105 nm. The nanopillars have an attractive force between pillars adjacent to each other to induce lateral collapse. This effect becomes more significant because the nanopillars have higheraspect-ratios, as shown in Figures 3 and 4.55-57 Because laterally collapsed nanopillars are undesirable for their applications in some cases, the fabrication of straight, high-aspect-ratio nanopillars is very important. The maximum aspect ratio of the nanopillars that can resist the attractive force for tip-to-tip lateral collapse is determined by the distance between the nanopillars s-d, the surface energy γsv, Young’s modulus E, and Poisson’s ratio ν as described in the following equation:58 "
AR max
63 π4 ðs - dÞ6 ¼ 0:25 ð1 - ν2 Þd2
#1=12
E γsv
!1=3 ð3Þ
In Figure 4, SEM images of the various nanopillars are shown in the order of their diameters and aspect ratios. These nanopillars are replicated by the depth-controlled AAO mold (Supporting Information S2). As indicated by the gray box, there is a boundary between the straight and the laterally collapsed nanopillars. In Figure 5, the phases of the nanopillars of various diameters and aspect ratios are shown for the polymers of two different Young’s moduli. In the Figure, the phase boundaries between the straight and the laterally collapsed nanopillars are in reasonable agreement with the boundary curves calculated with 2135
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’ ASSOCIATED CONTENT
bS
Supporting Information. SEM images of the AAO surface showing microscale roughness after the etching process, followed by the first anodization, and of the AAO molds used to obtain polymer nanopillars with different diameters and heights. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Author Contributions †
Figure 5. Diagrams representing the status of the nanopillar arrays as a function of the diameter and aspect ratio (AR), fabricated using UVcurable polymer having (a) softer and (b) harder Young’s moduli. The straight nanopillars and the laterally collapsed nanopillars are indicated as open circles and triangles, respectively. The boundary curves (solid lines) were calculated using eq 3.
eq 3. For the calculations, the value of E/γsv was used as the adjusting parameter. We could get the values of surface tension from refs 59 and 60. The fitted values of E were 451 MPa for the high-modulus polymer (311RM) and 374 MPa for the lowmodulus one (301RM), respectively. The former one is in agreement with the value reported by Yoo et al., but the latter one is much larger.53 We believe that this simple but quantitative discussion, eqs 2 and 3, would be applicable to the micropillars in principle. In addition to the center-to-center distance and aspect ratio of the nanopillars, the diameter can also affect the structural stability of the nanopillar arrays. A larger pillar diameter in general improves the stability of the pillars and leads to a decrease in the degree of lateral collapse. In our system, however, the observed stability of the pillars was almost the same or rather poorer, although the diameter of the nanorods was enlarged by a factor of five-thirds. This is probably because the interpillar separation distance is decreased with the increase in pillar diameter under the condition of a fixed center-to-center distance. This effect offsets the stabilizing effect from the enlarged diameter; therefore, the experimental maximum aspect ratio does not show significant differences upon the variation of diameter.
’ CONCLUSIONS A simple approach to repeatedly fabricating uniformly bent high-aspect-ratio polymer nanopillars has been demonstrated on the basis of a surface-treated reusable AAO mold. A dry adhesive property with the fabricated high-aspect-ratio pillars was also shown. By introducing the PDMS coating onto the surface of the mold, it was available to replicate nanopillars from the AAO mold without serious damage to the mold more than 100 times. We presented the criteria to achieve optimized straight, high-aspectratio nanopillars with the concepts of adhesion and lateral collapse. We believe that the concept presented here could be exploited for fabricating asymmetric high-aspect-ratio nanopillars and is useful and applicable to various fields because of the self-assembly-based methodology.
These authors contributed equally to this work.
’ ACKNOWLEDGMENT We appreciate the support from the Construction Technology Innovation Program funded by the Korea Ministry of Land, Transportation and Maritime Affairs (MLTM, 09CCTIB050566-02-000000) and the Eco-Technopia 21 project funded by the Korea Ministry of Environment (102-081-067). ’ REFERENCES (1) Jeong, H. E.; Suh, K. Y. Nano Today 2009, 4, 335–346. (2) Chan-Park, M. B.; Zhang, J.; Yan, Y. H.; Yue, C. Y. Sens. Actuators B 2004, 101, 175–182. (3) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Yang, S. Langmuir 2004, 20, 3824–3827. (4) Cho, W. K.; Choi, I. S. Adv. Funct. Mater. 2008, 18, 1089–1096. (5) Chu, K. H.; Xiao, R.; Wang, E. N. Nat. Mater. 2010, 9, 413–417. (6) Kim, T. I.; Suh, K. Y. Soft Matter 2009, 5, 4131–4135. (7) Chandra, D.; Yang, S. Acc. Chem. Res. 2010, 43, 1080–1091. (8) Byun, J.; Lee, J. I.; Kwon, S.; Jeon, G.; Kim, J. K. Adv. Mater. 2010, 22, 2028–2032. (9) Haberkorn, N.; Weber, S. A. L.; Berger, R.; Theato, P. ACS Appl. Mater. Interfaces 2010, 2, 1573–1580. (10) Haberkorn, N.; Lechmann, M. C.; Sohn, B. H.; Char, K.; Gutmann, J. S.; Theato, P. Macromol. Rapid Commun. 2009, 30, 1146–1166. (11) Wang, Z. L.; Song, J. Science 2006, 312, 242. (12) Lu, M. P.; Song, J.; Lu, M. Y.; Chen, M. T.; Gao, Y.; Chen, L. J.; Wang, Z. L. Nano Lett. 2009, 9, 1223. (13) Evans, B. A.; Shields, A. R.; Carroll, R. L.; Washburn, S.; Falvo, M. R.; Superfine, R. Nano Lett. 2007, 7, 1428–1434. (14) Ghatnekar-Nilsson, S.; Karlsson, I.; Kvennefors, A.; Luo, G.; Zela, V.; Arlelid, M.; Parker, T.; Montelius, L.; Litwin, A. Nanotechnology 2009, 20, 175502. (15) Shirtcliffe, N. J.; Aqil, S.; Evans, C.; McHale, G.; Newton, M. I.; Perry, C. C.; Roach, P. J. Micromech. Microeng. 2004, 14, 1384–1389. (16) Valamontes, E.; Chatzichristidi, M.; Tsikrikas, N.; Goustouridis, D.; Raptis, I.; Potiriadis, C.; van Kan, J. A.; Watt, F. Jpn. J. Appl. Phys. 2008, 47, 8600–8605. (17) del Campo, A.; Arzt, E. Chem. Rev. 2008, 108, 911–945. (18) Lim, K.; Wi, J. S.; Nam, S. W.; Park, S. Y.; Lee, J. J.; Kim, K. B. Nanotechnology 2009, 20, 495303. (19) Chik, H.; Xu, J. M. Mater. Sci. Eng., R 2004, 43, 103–138. (20) Lee, J.; Kim, J.; Hyeon, T. Adv. Mater. 2006, 18, 2073–2094. (21) Miao, Z.; Xu, D. S.; Ouyang, J. H.; Guo, G. L.; Zhao, X. S.; Tang, Y. Q. Nano Lett. 2002, 2, 717–720. (22) Chik, H.; Liang, J.; Cloutier, S. G.; Kouklin, N.; Xu, J. M. Appl. Phys. Lett. 2004, 84, 3376–3378. (23) Wang, J. G.; Tian, M. L.; Kumar, N.; Mallouk, T. E. Nano Lett. 2005, 5, 1247–1253. 2136
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