Stretched Polymer Nanohairs by Nanodrawing

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1 Letter Stretched Polymer Nanohairs by Nanodrawing Hoon Eui Jeong, Sung Hoon Lee, Pilnam Kim, and Kahp Y. Suh Subscriber access provided by University of Texas Libraries Nano Lett., 2006, 6 (7), DOI: /nl061045m Publication Date (Web): 27 June 2006 Downloaded from on March 7, 2009 More About This Article Additional resources and features associated with this article are available within the HTML version: Supporting Information Links to the 3 articles that cite this article, as of the time of this article download Access to high resolution figures Links to articles and content related to this article Copyright permission to reproduce figures and/or text from this article Nano Letters is published by the American Chemical Society Sixteenth Street N.W., Washington, DC 20036

2 Stretched Polymer Nanohairs by Nanodrawing NANO LETTERS 2006 Vol. 6, No Hoon Eui Jeong, Sung Hoon Lee, Pilnam Kim, and Kahp Y. Suh* School of Mechanical and Aerospace Engineering and Institute of AdVanced Machinery and Desgin, Seoul National UniVersity, Seoul, , Korea Received May 9, 2006; Revised Manuscript Received June 13, 2006 ABSTRACT A simple, yet innovative, method is presented for fabricating high-aspect-ratio polymer nanohairs (aspect ratio >20) on a solid substrate by sequential application of molding and drawing of a thin polymer film. The polymer film was prepared by spin coating on a rigid or flexible substrate, and the temperature was raised above the polymer s glass transition while in conformal contact with a poly(urethane acrylate) mold having nanocavities. Consequently, capillary forces induced deformation of the polymer melt into the void spaces of the mold and the filled nanostructure was further elongated upon removal of the mold due to tailored adhesive force at the mold/polymer interface. The optimum value of the work of adhesion at the mold/polymer interface ranged from 0.9 to 1.1 times that at the substrate/polymer interface. Thin polymer films are useful for various micro/nanolithography 1-4 and directed self-organization processes. 5-8 Lithographic methods utilize (i) chemical modification of a thin photoresist by radiation (photolithography) or (ii) physical deformation of a thermoplastic or ultraviolet (UV)- curable resin by a high mechanical pressure or capillary force (imprint or soft lithography). 9 These methods are primarily focused on precise transfer of a master pattern onto a thin polymer resist. Alternatively, some physical forces such as electric field 10,11 and temperature gradient 12 can drive the surface of a thin film into a regularly ordered structure using a featureless or an engraved master. As opposed to traditional lithographic methods, this approach can provide control over the spatial arrangement that is different from that of the master pattern. For example, a recent report 13 demonstrated that use of a bilayer consisting of two different polymers leads to a hierarchical structure with two independent length scales with the application of an electric field. These noncontact-based self-organization processes offer a new strategy for large area, sub-100-nm lithography and, in particular, would provide a simple route to mimicking nature s functional, complex surfaces such as lotus leaves, 14 water strider legs, 15 or biological adhesives. 16 In contact-based lithographies, efforts have been made to meet stringent pattern fidelity and exact replication that are needed for electronic or optical devices. Typically, materials with low surface tension have been used as a mold due to their easy demolding characteristic. For example, a number of poly(dimethylsiloxane) (PDMS, surface tension 20 mj/ * Corresponding author: phone, ; fax, ; , sky4u@snu.ac.kr. m 2 ) molds and a Teflon mold (surface tension 14 mj/ m 2 ) 21 have been used in a broad range of imprint or soft lithographic methods. Also, the surface of a mold has been treated with a fluorine-containing self-assembled monolayer (fluorination). 22,23 By contrast, use of a mold with high surface tension results in the delamination of a thin layer by a substantial difference in adhesive force between the mold/ film interface (W 12 ) and the film/substrate interface (W 23 ). On the basis of this concept, a number of detachment-based lithographies have been introduced using different mold and film materials Despite these efforts, little has been done to create a polymer structure that is different from that of the mold in contact-based lithographies. Here we present a novel, capillarity-driven rigiflex lithography aided by modulated interfacial tensions for unveiling high-aspect-ratio (AR) polymeric nanostructures. When the cavities of a mold are filled with a polymer having a relatively high adhesion (not too high) to the mold, the polymer can be elongated to form very long, stretched polymer nanohairs upon removal of the mold, which can be viewed as an intermediate process between simple replication (imprint or soft lithography) and detachment. This lithography offers a new insight into current contact-based lithographies and also a highly efficient method for fabricating polymeric high AR structures. Furthermore, surfaces containing these nanostructures are intrinsically adhesive and superhydrophobic, which can be used in various applications that aim to mimic nature s functional surfaces. Figure 1 shows a schematic illustration of the rigiflex lithography for nanodrawing of a thin polymer resist. In this process, the material that makes up the mold is important because it allows for control over the nanodrawing process /nl061045m CCC: $33.50 Published on Web 06/27/ American Chemical Society

3 Figure 1. A schematic illustration of the experimental procedure. First, the polymer nanostructures are fabricated by capillarity-driven molding and the filled nanostructures are further elongated due to tailored adhesive force at the mold/polymer interface. Here, a UV-curable mold based on poly(urethane acrylate) (PUA) was used, which was recently introduced for lithography of features smaller than 100 nm. 27 As previously demonstrated, PDMS is difficult to be used to fabricate high AR molds due to its sub-100-nm patterning resolution (elastic modulus 1.8 MPa). 28 In contrast, PUA molds are sufficiently rigid for fine patterning, yet they are also flexible such that they can make intimate contact with the substrate over a large area, combining major advantages of imprint and soft lithographies (so-called rigiflex lithography ). 29 A negative PUA mold replicated from the nanopatterned Si master shown in Figure 1 was not only sufficiently hard (Young s modulus of 40 MPa) but also flexible ( 50 µm thickness). These two features enable PUA molds to make intimate contact to smooth substrates with low pressure ( 10 3 Pa). After the PUA mold was prepared, the mold was placed on a polymer layer that was spin-coated onto a silicon substrate or flexible substrates such as poly(ethylene terephthalate) (PET) film ( 50 µm thickness) or commercial 3M tape ( 35 µm thickness). While heating in a vacuum oven for >1 h above T g (typically at 150 C), the polymer filled into the mold s cavities by capillary action. 4 The manual removal at a constant speed of 10 mm/s induced stretching of the preformed nanostructures, resulting in high AR, elongated nanohairs (AR > 20) over a large area. Figure 2a-d shows scanning electron microscopy (SEM) images of the PMMA and PS nanohairs on PET film (panels a, b, and c) or 3M tape (panel d). No appreciable differences were found between these two substrates since the flexibility of the PUA mold aids to make a conformal contact. Although not shown (see the supplemental figure parts a and b in Supporting Information), the nanohairs formed on a silicon substrate were similar in their structure. As shown in the figure, the height of nanohairs ranged from 1.5 to 2 µm(>5 in their structure compared to the original master) and the Figure 2. (a) A large-area SEM image of the stretched PMMA nanohairs on PET film. (b) The magnified view of (a). (c, d) Largearea SEM images of PS nanohairs on PET film and PMMA nanohairs on 3M tape, respectively. Simply replicated PMMA nanostructures without elongation are also shown (e). (f) A photograph demonstrating the large-area fabrication of nanohairs with minimal defects ( 2 2cm 2 ). (g) An SEM image of stretched nanohairs with a different mold. (h) An original replica without elongation (diameter of 700 nm, height of 900 nm). diameter was 80 nm (>50% decrease), rendering an AR larger than 20. Interestingly, each nanohair bulged at the tip with the thickness 1.5 times larger than that of the stalk, which appears to result from the demolding at the tip. Also, the nanohairs slightly leaned to one side due to manual removal in that direction. Figure 2e shows simply replicated nanostructures without elongation (height of 350 nm, diameter of 200 nm at bottom, 100 nm at top, pitch of 530 nm), corresponding to the shape of the original silicon master. A photograph shown in Figure 2f demonstrates the capability of large-area fabrication with minimum defects (< 2 2 cm 2 ). In addition, other mold geometries were also tested to obtain stretched nanopillars as shown in Figure 2g,h (diameter of 700 nm, height of 900 nm). In this case, the height was increased by 2.5 without the formation of the spherical tip. Along with cost-effectiveness and scalability of the proposed technique, the nanodrawing method presented here is highly efficient for fabricating high AR, hairy nanostructures. In particular, these structures are difficult to fabricate by other methods such as photolithography and electron- Nano Lett., Vol. 6, No. 7,

4 beam lithography because nanohair structures are prone to collapse during developing. Traditional microelectromechanical (MEMS) technologies such as deep reactive ion etching (DRIE) and LIGA are also difficult to use due to their limitations for sub-100-nm patterning. 30 As high AR structures are useful in sensor arrays, 31 high capacitance in DRAM, 32 field emitters, 33 and biological studies, 34 the approach presented here could be potentially useful for a diverse series of applications. Two processes are decisive for the formation of stretched nanohairs: capillarity and adhesive force. For the former, the experiment needs to be performed in a low ambient pressure to facilitate the capillary rise. For a less permeable PUA mold, a simple kinetic analysis revealed that the capillary rise increases linearly with time. 35 In addition, the contribution of viscosity and film thickness disappears such that the kinetics is solely governed by the permeation kinetics and capillary force. In a typical experiment, this capillary rise is extremely slow or sometimes stops in the middle of the rise. Thus, the vacuum condition alleviates this problem, allowing for complete capillary rise in a reasonable heating time (>1 h). For the latter, the adhesive force at the mold/ polymer interface needs to be modulated. Simply, the operability of the stretched nanohairs can be given by r ) W 23 A 23 W 12 A 12 = 1 (1) where W 12(23) and A 12(23) are the work of adhesion and surface area at the polymer/substrate (mold/polymer) interface, respectively. The physical interpretation of r is the adhesion energy ratio at the interface, which should be close to unity for nanodrawing of the preformed nanostructures. Otherwise, a simple replication or detachment occurs as mentioned before. The work of adhesion can be calculated by the geometric mean method, yielding 36 W 12 ) 4( γ d d 1 γ 2 γ d 1 + γ + γ p p 1 γ 2 (2) d 2 γ 1 p + γ 2 p) where the superscripts d and p are for the dispersion and polar components of the surface tension γ, respectively. W 23 can be defined in a similar way. With probing liquids such as deionized (DI) water and diiodomethane, the contact angles on various substrates were measured to obtain surface tension values that are needed to calculate the work of adhesion. To convert the measured contact angles into surface tensions, the following relation was used 36 γ l (1 + cos θ) ) 2(γ l γ s ) 1/2 + 2(γ l γ s ) 1/2 (3) where γ l and γ s mean the surface tension of liquid (DI water or diiodomethane) and solid surface to be measured, respectively. The results are summarized in Tables 1 and 2. Here, we used five PUA molds having different surface tensions, two polymers (PS, PMMA), and two substrates (PET, 3M Table 1. Contact Angles of Water (θ p ) and Diiodomethane (θ d ) on Various Surfaces in Degree and the Calculated Surface Tension Values in mj/m 2 by the Harmonic Mean Method θ d θ p γ d γ p γ total mold polymer PMMA PS substrate PET tape Table 2. Interfacial Energies at the Mold/Polymer Interface and at the Polymer/Substrate Interface for Various Combinations interfacial energy (mj/m 2 ) mold substrate polymer PET tape PMMA PS tape). In addition to the adhesion energy ratio, we further tested the effect of the drawing temperature (six temperatures tested in the range of -15 to 150 C). The drawing speed was maintained approximately at 10 mm/s throughout the experiment. According to an scanning electron microscopy (SEM) image of the nanopillars shown in Figure 2e, the initial interfacial area ratio (A 23 /A 12 ) was measured to be Shown in Figure 3a are the experimental data obtained for various values of r and drawing temperatures. These experimental findings can be classified into four categories: molding, stretching, delamination, and fracture. Usually, delamination and fracture of the preformed nanostructures occurred at the same time (total or partial delamination), leading to three regimes as shown in Figure 3a. A number of notable features are derived from the figure. First, the value of r was indeed close to unity for the fabrication of stretched nanohairs. Second, the drawing temperature was also important. Even with an optimum value of r, delamination or fracture at the middle or base of pillar took place below 90 C, which is primarily attributed to an extremely high mechanical modulus and viscosity below T g. In particular, three major phenomena (delamination, fracture, and elongation) were observed concurrently on the same sample in the temperature range of C, whereas the fracture was dominant for the temperature below 10 C. Hence, the drawing needs to be carried out above 90 C for uniform elongation of the nanostructures. The presence of the optimum temperature range indicates that the effects of temperature should be incorporated into the theory to accurately predict the nanodrawing process. A further study would be required to address this issue in terms of polymer viscosity and mechanical strength. To elaborate on the effect of geometry, we further tested other mold geometries with five different surface tensions 1510 Nano Lett., Vol. 6, No. 7, 2006

5 particular, the spherical tip of the nanohairs resembles a gecko s spatulae, thus further enhancing the contact area. Recently, many researchers have attempted to fabricate a dry adhesive by mimicking a gecko s foot hairs due to its wide applications to home or industrial appliance and biomimetic robots The gecko s outstanding adherent properties arise from van der Waals forces generated by nanohairy structures on its feet. 40, 41 To fabricate a flexible tape, a PET film that was 50 µm in thickness was attached to the sticky surface of a 3M tape. The subsequent procedure was the same after spin coating of a PMMA solution onto the nonsticky tape surface. Figure 4a shows an example of the flexible tape prepared by the nanodrawing method. To assess the adhesion capability of a single nanohair, we performed a force-distance loading test by atomic force microscopy (AFM) as demonstrated in Figure 4b,c. The blue solid line represents a force-distance curve when the tip approaches the top of a single nanohair while the red dashed line represents a curve when the tip retreats from the top. A hysteresis generally occurs as a result of the van der Waals interactions between the tip and the nanohair, leading to a peak height that is a measure of the adhesion force of a single nanohair. In theory, the adhesion force (F ad ) can be predicted by the Johnson-Kendall- Roberts (JKR) equation 42 F ad ) 3 2 πrw 12 (4) Figure 3. (a) A phase diagram for a wide range of drawing temperatures and values of r: molding, delamination and fracture, and stretching. The boxed area indicates the optimum condition for nanodrawing. (b) Test of other mold geometries showing that the optimum window for the work of adhesion at the mold/polymer interface ranged from 0.9 to 1.1 times that at the substrate/polymer interface. Below or above this range, a simple molding or detachment occurred, corresponding to earlier findings. each (three nanopillar molds with diameter of nm and six nanoline molds with width of nm). The description of these molds is summarized in the table in Supporting Information. During this experiment, the drawing temperature and drawing rate was maintained at 130 C and at 10 mm/s, respectively. As shown in Figure 3b, the optimum value of the work of adhesion at the mold/polymer interface ranged from 0.9 to 1.1 times that at the substrate/ polymer interface. Below or above this range, a simple molding or detachment occurred, corresponding to earlier findings. The formation of high AR nanostructures of various shapes demonstrates that this technique can be used irrespective of the shape of the mold. The nanodrawing method is potentially useful for applications that require high AR polymer nanostructures. To demonstrate such applicability, a flexible tape was prepared by fabricating polymer nanohairs on a commercial 3M tape (one side of wet adhesive and the other side of dry adhesive). This experiment was motivated by the fact that the stretched nanostructures are reminiscent of a gecko s foot hairs. In with R being the radius of a contacting hemisphere (the pillar top is assumed to be a hemisphere) and W 12 being the work of adhesion at the interface. In our experiment, the measured adhesion force was nn, in good agreement with the theoretical prediction of 20 nn (R ) 45 nm, W 12 ) 100 mj/m 2 ) and reported values. 43 Another notable feature is the hydrophobicity of the surface of stretched nanohairs as can be seen from the contact angle of a water droplet in Figure 4d ( 144 ). This is because air pockets trapped in the spaces between nanohairs resist the depression of a droplet into the structures. 44 The adhesive properties as well as superhydrophobicity of the flexible tape presented here provide a simple, versatile route to an artificial dry adhesive. Experimental Section. Fabrication of the Poly(urethane acrylate) (PUA) Mold. The PUA mold was composed of a functionalized prepolymer with acrylate groups for crosslinking, a monomeric modulator, a photoinitiator, and a radiation-curable releasing agent (commercial acrylate organomodified polysiloxane (Rad 2200N), TEGO Chemical Service) for surface activity. 27 The prepolymers contained both cycloaliphatic and long, linear chains. The former provides the rigidity, while the latter provides the flexibility. The liquid mixture was drop-dispensed onto a silicon master that had been prepared by photolithography or electron beam lithography, and a PET film with 50 µm thickness was gently placed on the liquid mixture followed by UV (λ ) nm) exposure for a few tens of seconds. After the UV curing, the mold was peel off from the master. Four PUA molds with different surface tensions (30.8, 32.8, 37.7, and Nano Lett., Vol. 6, No. 7,

6 Figure 4. (a) A photograph of the flexible tape consisting of a dry adhesive (PMMA nanohairy structures) at one side and a wet adhesive at the other sticky side of a 3M tape. (b) A conceptual sequence of the force-distance measurement using a tipless AFM cantilever on PMMA nanohairs fabricated on 3M tapes. (c) The measured data for PMMA and PS nanohairs. Some nonstretched nanopillars are also shown from (b). (d) A digital camera image of a water droplet sitting on the stretched PMMA nanohairs. The contact angle was measured to be 144, indicating that the surface is hydrophobic mj/m 2 ) were prepared by controlling the amount of the releasing agent (0.3, 0.2, 0.1, and 0.05 wt %, respectively). An additional PUA mold coated with Teflon (12.3 mj/m 2 ) was also prepared as a mold with the lowest surface tension. The resulting PUA molds had recessed dense nanopillars or nanolines with various feature sizes. Rigiflex Lithography for Fabricating Stretched Nanohairs. The rigiflex lithography consisted of two sequential steps: molding and drawing. The polymer solution was either poly(styrene) (PS, M w ) , T g ) 100 C) or poly- (methyl methacrylate) (PMMA, M w ) , T g ) 105 C) dissolved in toluene (10 wt %). For molding, cleaned silicon (100) or flexible substrates (50 µm PET film or 35 µm 3M tape) were spin-coated at 3000 rpm for 40 s with a 10 wt % PS or PMMA solution in toluene to the thickness of µm. The samples were then baked in a vacuum oven at 100 C for2htoremove any residual solvent in the film. A PUA mold was placed on the polymer surface under a slight pressure ( 10 3 Pa) to make conformal contact and the temperature was raised above T g (typically at 150 C) for various heating times (>1 h). For a uniform pressure distribution, the mold-substrate assembly was sandwiched between two thin PDMS blocks. For drawing, the temperature was first adjusted to the target drawing temperature in the range of -15 to 150 C. Then, the mold was removed in a vertical direction at a constant speed of 10 mm/s. Adhesion Test by Atomic Force Microscope (AFM). Pull-off force measurements for the adhesion of a single nanohair were performed using an AFM (XE-150, PSIA Inc., Santa Clara, CA) in the force-distance mode with a flat tip. The tipless silicon probe with the spring constant of 1.75 N/m was made from NSC 36 probe (Mikromasch, Japan) by cutting the tip with laser machining. The tip approached and retracted at a constant speed of 0.5 µm/s. Acknowledgment. This work was supported by the Micro Thermal System Research Center of Seoul National University and in part by the Ministry of Science and Technology through the Nanoscopia Center of Excellence at the same university. The authors are grateful to Professor Ali Khademhosseini for valuable comments to this manuscript and the Minuta Tech. for providing PUA materials with different surface tensions. Supporting Information Available: A table showing geometrical parameters and surface tensions of nine PUA molds used to construct Figure 3b and a figure showing SEM images. This material is available free of charge via the Internet at References (1) Xia, Y. N.; Whitesides, G. M. Annu. ReV. Mater. Sci. 1998, 28, (2) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272 (5258), (3) Hawker, C. J.; Russell, T. P. MRS Bull. 2005, 30 (12), (4) Suh, K. Y.; Kim, Y. S.; Lee, H. H. AdV. Mater. 2001, 13 (18), (5) Higgins, A. M.; Jones, R. A. L. Nature 2000, 404 (6777), (6) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403 (6766), (7) Bowden, N.; Brittain, S.; Evans, A. G.; Hutchinson, J. W.; Whitesides, G. M. Nature 1998, 393 (6681), (8) Kim, S. O.; Solak, H. H.; Stoykovich, M. P.; Ferrier, N. J.; de Pablo, J. J.; Nealey, P. F. Nature 2003, 424 (6947), (9) Gates, B. D.; Xu, Q. B.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. ReV. 2005, 105 (4), (10) Schaffer, E.; Thurn-Albrecht, T.; Russell, T. P.; Steiner, U. Nature 2000, 403 (6772), Nano Lett., Vol. 6, No. 7, 2006

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