Langmuir 2007, 23, 4549-4553
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Rigiflex, Spontaneously Wettable Polymeric Mold for Forming Reversibly Bonded Nanocapillaries Pilnam Kim and Kahp Y. Suh* School of Mechanical and Aerospace Engineering, Seoul National UniVersity, Seoul 151-742, Korea ReceiVed NoVember 21, 2006 We present a novel ultraviolet (UV)-curable mold that enables the formation of reversibly bonded nanocapillaries (500-50 nm) on a gold or silicon substrate. A sheet-type (∼50 µm) polyethylene diacrylate (PEG-DA) mold was used for its rigiflex nature; it provides rigidity high enough for maintaining nanostructures (elastic modulus >70 MPa) and also flexibility good enough for intimate contact over a large area aided by weak electrostatic forces (zeta potential ≈ -113.55 mW). The electrostatic charge is generated on a rigiflex PEG-DA mold upon peeling from an original engraved silicon master by mechanical friction, thereby assisting the formation of spontaneous contact with the gold or silicon substrate.
Introduction Recently, extensive efforts have been made to create nanofluidic channels (nanochannels) for studying transport phenomena, identification, and separation of small biological species (e.g., DNA analysis).1-5 Nanochannel fabrication techniques include conventional micromachining,6-8 nanoimprint lithography followed by thermal bonding,9 and a simple bonding process utilizing fusion or anodic bonding,10-12 leading to irreversibly bonded nanochannels. While compromising the robustness of the fabricated channel, reversibly bonded nanocapillaries (RBNs) could find uses in residual-layer free nanofabrication and guided growth/chemical reaction into nanocapillaries. A decade ago, reversibly bonded microcapillaries were first demonstrated by Whitesides and co-workers in a technique called micromolding in capillaries (MIMIC).13,14 A potential limitation of this technique is that the sub-100-nm resolution is nearly impossible to obtain because of the low elastic modulus (∼3.2 MPa) of PDMS. Several researchers have reported that PDMS gives rise to deformation, bucking, or collapse of relief features when peeled from the original master.15,16 To overcome these mechanical shortcomings of PDMS, a number of alternative molds have been introduced with enhanced mechanical properties.17-23 These molds, however, are difficult * Corresponding author. E-mail:
[email protected]. Phone: +82-2-8809103. Fax: +82-2-880-1725. (1) Craighead, H. G. Nature 2006, 442, 387. (2) Han, J.; Turner, S. W.; Craighead, H. G. Phys. ReV. Lett. 1999, 83, 1688. (3) Rzayev, J.; Hillmyer, M. A. J. Am. Chem. Soc. 2005, 127, 13373. (4) Karnik, R.; Castelino, K.; Fan, R.; Yang, P.; Majumdar, A. Nano Lett. 2005, 5, 1638. (5) Mijatovic, D.; Eijkel, J. C. T.; van den Berg, A. Lab Chip 2005, 5, 492. (6) Tas, N. R.; Berenschot, J. W.; Mela, P.; Jansen, H. V.; Elwenspoek, M.; van den Berg, A. Nano Lett. 2002, 2, 1031. (7) Lee, C.; Yang, E. H.; Myung, N. V.; George, T. Nano Lett. 2003, 3, 1339. (8) Eijkel, J. C. T.; Bomer, J.; Tas, N. R.; van den Berg, A. Lab Chip 2004, 4, 161. (9) Guo, L. J.; Cheng, X.; Chou, C. F. Nano Lett. 2004, 4, 69. (10) Berthold, A.; Nicola, L.; Sarro, P. M.; Vellekoop, M. J. Sens. Actuators, A 2000, 82, 224-228. (11) Iglesias, C.; Seyama, M.; Horiuchi, T.; Miura, T.; Haga, T.; Iwasaki, Y. Electrochemistry 2006, 74, 169. (12) Cheng, G. J.; Pirzada, D.; Dutta, P. J. Microlithogr. Microfabr. 2005, 4, Art. No.013009. (13) Kim, E.; Xia, Y. N.; Whitesides, G. M. Nature 1995, 376, 581. (14) Kim, E.; Whitesides, G. M. J. Phys. Chem. B 1997, 101, 855. (15) Hui, C. Y.; Jagota, A.; Lin, Y. Y.; Kramer, E. J. Langmuir 2002, 18, 1394. (16) Bietsch, A.; Michel, B. J. Appl. Phys. 2000, 88, 4310. (17) Schmid, H.; Michel, B. Macromolecules 2000, 33, 3042.
to apply when forming RBNs because of poor resolution or a weak ability to make conformal contact. To achieve RBNs, the mold needs to be rigiflex; the mold should provide rigidity high enough for fine patterning and also flexibility good enough for intimate contact over a large area, combining the major advantages of imprint and soft lithography.24 We found that a sheet-type (∼50 µm), UV-curable polyethylene diacrylate (PEG-DA) mold meets these two requirements through its relatively high mechanical hardness (elastic modulus >70 MPa), good flexibility, and weak negative charges when detached from the silicon master. It is noted that the adequate mechanical property of PEG-DA prevents the collapse of the engraved nanostructures and that the film mold promotes flexibility and wettability. Furthermore, the surface charge generated on the film mold induces electrostatic attraction when the mold is brought into contact with a gold or silicon substrate, allowing for spontaneous conformal contact without any external stimuli such as a temperature rise or the application of mechanical pressure. Experimental Methods Fabrication of the PEG-DA Mold with Nanocapillaries. A small amount (∼0.1-0.5 mL) of a UV-curable PEG polymer such as PEG diacrylate (PEG-DA, Mw ) 575, Aldrich) was dropdispensed onto silicon master, and a supporting poly(ethylene terephthalate) (PET) film was carefully placed on top of the surface to make conformal contact. The PET film used in this study was surface modified with urethane groups to increase adhesion to the acrylate-containing monomer (Minuta Tech., Korea). The silicon masters had been prepared by photolithography or electron-beam lithography depending on the feature size. The geometrical parameters of five nanolines and one nanopillar mold used in the experiments are summarized in Table 1. To cure, the sample was exposed to UV (250-356 nm) for 3 s at an intensity of 100 mW/cm2 after adding 1 wt % of the UV initiator (2,2-dimethoxy-2-phenylacetophenone, Aldrich) with respect to the amount of polymer. After UV curing, the fabricated PEG-DA mold was peeled off of the master using a sharp tweezer. (18) Odom, T. W.; Love, J. C.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Langmuir 2002, 18, 5314. (19) Choi, K. M.; Rogers, J. A. J. Am. Chem. Soc. 2003, 125, 4060. (20) Csucs, G.; Kunzler, T.; Feldman, K.; Robin, F.; Spencer, N. D. Langmuir 2003, 19, 6104. (21) Suh, K. Y.; Langer, R.; Lahann, J. Appl. Phys. Lett. 2003, 83, 4250. (22) Khang, D. Y.; Lee, H. H. Langmuir 2004, 20, 2445. (23) Choi, S. J.; Yoo, P. J.; Baek, S. J.; Kim, T. W.; Lee, H. H. J. Am. Chem. Soc. 2004, 126, 7744. (24) Suh, D.; Choi, S. J.; Lee, H. H. AdV. Mater. 2005, 17, 1554.
10.1021/la0633942 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/08/2007
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Table 1. Geometrical Parameters of Various Nanocapillaries Used in the Experiments NC-500L NC-300L NC-200L NC-70L NC-50L NC-50P
shape
geometry
nanolines nanolines nanolines nanolines nanolines nanopillars
500 nm width, 1000 nm height 300 nm width, 1000 nm height 200 nm width, 200 nm height 70 nm width, 200 nm height 50 nm width, 150 nm height 50 nm diameter, 300 nm height
Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) Measurements. ATR-FTIR spectra were recorded on a Perkin-Elmer Spectrum 2000 Fourier transform infrared (FTIR) spectrometer in the mid-infrared range from 4000 to 600 cm-1 in absorbance mode for resin characterization during the curing process. The spectra were obtained using attenuated total reflectance (ATR). The temperature and relative humidity measured were 25°C and 70%, respectively. A sample was placed in contact with an internal reflection element (i.e., trapezoidal ZnSe crystal), and an IR beam was passed through the crystal undergoing total internal reflection at the sample/crystal interface. Nanoindentation. The elastic modulus and hardness were measured using a nanoindenter (Nano Indenter XP, MTS) with a low-force to high-force range and also including a CSM (continuous stiffness measurement) mode. The target depth was 300 nm, and the typical mean load on the bone was 0.2-0.4 mN. The presented values were measured over at least 10 points per single sample. Nanofabrication Using RBNs. The RBNs were used as channels for flowing a UV-curable liquid prepolymer (NOA 71, Norland). For this experiment, both ends of the mold were cut such that NOA 71 spontaneously filled the capillaries. For a liquid prepolymer, the filled prepolymer was exposed to UV for 30 min, and the mold was removed using a sharp tweezer, leaving behind a polymer replica without a residual layer. To ease the release of the cured nanostructure, the supporting substrate (Au or Si) was treated with oxygen plasma (40 s, 60 W, PDC-32G, Harrick Scientific Inc.) to render relatively high surface tension (∼>70 mJ/m2), and the as-formed capillaries were exposed to UV extensively (>3 h) for inertness of the mold surface prior to flow of a prepolymer. Contact Angle Measurements. Static contact angles were measured with a Rame´-Hart goniometer (Mountain Lakes) equipped with a video camera. Reported values represent the averages of at least six independent measurements. Scanning Electron Microscopy (SEM). SEM images were taken using high-resolution SEM (S4800, Hitachi, Japan) at an acceleration voltage higher than 5 kV. Samples were coated with a 10 nm Au layer prior to analysis to prevent charging.
Results and Discussion Figure 1 shows a schematic diagram of the experimental procedure for forming RBNs and its application to fabricating polymeric nanostructures in a residual-layer-free fashion. During the fabrication of the PEG-DA channel mold, care was taken to maintain the surface of the mold as sufficiently rigid but incompletely cured (curing time ≈ 3 s). The elastic modulus after this initial curing was measured to be ∼70 MPa, which is far below an upper bound on the elastic modulus for a material (∼100 MPa) to appear sticky.25 Also the use of a supporting polyethylene terephthalate (PET) film to peel off the replicated PEG-DA mold was essential because it helps to release the mold from the silicon master and prevent swelling of the PEG layer when in contact with a biological fluid (Figure 1a). The PET film used in this experiment was surface modified with urethane groups to increase adhesion to the acrylate-containing PEG-DA mold. Another important characteristic of reversible contact is frictioninduced negative charges that can be generated on the surface (25) Dahlquist, C. A. Adhesion Fundamentals and Practice; McLaren and Sons: London, 1966.
Figure 1. Schematic diagram of the experimental procedure. (a) PET support of 50 µm thickness that was used to peel off the cured PEG-DA film (∼3 µm thickness) from the silicon master. (b) PEGDA surface showing negative charges when removed from the master, which induced electrostatic attraction when the mold was brought into contact with the gold or silicon substrate. (c) RBNs used for fabricating nanostructures by flowing a UV-curable prepolymer (NOA 71) via capillary action and subsequent UV exposure.
when detached from the silicon master. Because the PEG-DA mold contains oxygen-abundant acrylate groups, it is prone to exhibiting negative charges (zeta potential was -113.55 mW as measured by an electrophoretic light-scattering spectrophotometer with a 10 mM NaCl solution). This negatively charged surface induces initial spontaneous contact with a gold (100 nm of Au on 50 nm of Ti-coated Si) or p-type silicon (100) substrate as shown in the optical image in Figure 1b. Because the generation of the charged surface arises from mechanical friction upon peeling from the silicon master, the PEG-DA mold could not be reused without additional treatment to create electrostatic forces. In the experiment, the mold was used once and discarded because the mold is cheap. The presence of negative charge was supported by the fact that the mold did not wet the negatively charged glass substrate as a result of charge repulsion (not shown). After the initial contact, the mold was exposed to UV for 3 h to render the surface of the mold mechanically rigid and chemically inert. This step turned out to be important for preventing cohesion mechanical failure from the relatively soft PEG-DA mold and for enabling subsequent nanofabrication with a UV-curable liquid prepolymer such as NOA 71 (Figure 1c). For nanofabrication, both ends of the mold were cut such that a liquid prepolymer spontaneously filled the nanocapillaries via capillary action.13,14 To ensure easy release of the cured polymeric nanostructures, the mold surface needs to be inert or contain a minimum number of active UV-curable groups. For this purpose, the mold was cured at 100 mJ/cm2 for ∼3 h after initial contact such that most of the acrylate groups on the surface would be destroyed.26,27 Further UV exposure lead to the stabilization of the surface with chain reorganization and crosslinking of inner acrylate groups with enhanced structural integrity or rigidity.27 To monitor these changes, we performed ATR-FTIR spectroscopy and nanoindentation measurements (Figure 2a). The FTIR spectra show that the number of acrylate groups decreased with curing time as seen from the substantial decrease of three IR peaks in Figure 2a (812 cm-1, CdCH; 1415 cm-1, CHdCH2; and 1613 cm-1, (26) Lin, H. Q.; Kai, T.; Freeman, B. D.; Kalakkunnath, S.; Kalika, D. S. Macromolecules 2005, 38, 8381. (27) Witte, R. P.; Blake, A. J.; Palmer, C.; Kao, W. J. J. Biomed. Mater. Res., Part A 2004, 71, 508.
Mold for Forming ReVersibly Bonded Nanocapillaries
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Figure 2. (a) ATR-FTIR spectra of the PEG-DA mold with curing time. Three peaks for the acrylate groups at 812, 1410, and 1613 cm-1 almost disappeared after exposure for 3 h. The peak intensities were normalized with the CdO group at 1740 cm-1 because this peak was unchanged after crosslinking. (b) Measurement of the elastic modulus and hardness with curing time.
CdC),28,29 suggesting that the crosslinking of PEG-DA is nearly complete after 3 h of exposure. Also, the elastic modulus or hardness was monotonically increased with curing time (Figure 2b), which in turn leads to an increase in cohesive strength and thus assists in the clean release of the mold without cohesion failure that has been frequently found in elastomeric molds such as polydimethylsiloxane (PDMS).30,31 It is intriguing that good contact was achieved for the time span of UV exposure of more than 3 h (elastic modulus ≈ 125 MPa). If a material is highly deformable, then good contact may be easily made despite the roughness through surface interactions on the molecular scale, with no need for applied pressure. For the highly cured PEG-DA mold, however, it may be relatively stiff, and thus it is prone to detachment (slightly above the upper limit of the elastic modulus for stickiness, ∼100 MPa).25 It seems that the electrostatic attraction gives rise to good contact even for a relatively high elastic modulus with aid of the flat surface topography (rms < ∼2 nm). The stabilization of the mold surface can also be measured by the work of adhesion. Simply, the condition for the operation of clean release is that the work of adhesion at the mold (1)/polymer (2) interface should be smaller than that at the polymer (2)/ substrate interface (3), yielding
W12 < W23
(1)
where W12 is the work of adhesion at the mold/polymer interface and W23 is the work of adhesion at the polymer/substrate (Au or Si) interface. The work of adhesion can be calculated for each interface on the basis of contact angle measurements based on the harmonic mean method.32
W12 ) 4
(
γ1dγ2d
γ1d + γ2
+ d
)
γ1pγ2p γ1p + γ2p
(2)
where the superscripts d and p are for the dispersion and polar components of the surface tension γ, respectively. With probing liquids of water and diiodomethane, the contact angles were (28) Decker, C.; Moussa, K. J. Appl. Polym. Sci. 1987, 34, 603. (29) Scorates, G. Infrared Characteristic Group Frequencies, 2nd ed.; Wiley: New York, 1994. (30) Childs, W. R.; Nuzzo, R. G. J. Am. Chem. Soc. 2002, 124, 13583. (31) Ahn, H.; Lee, K. J.; Shim, A.; Rogers, J. A.; Nuzzo, R. G. Nano Lett. 2005, 5, 2533. (32) Wu, S. Polymer Interface and Adhesion; Marcel Dekker: New York, 1982.
Table 2. Measured Contact Angles of Water (θp) and Diiodomethane (θd) on Various Substrates and Surface Tensions Calculated by the Harmonic Mean Method contact angles (deg) surface tension (mJ/m2) substrate prepolymer PEG-DA moldb
Au Au*a NOA A B C D
θp
θd
γp
γd
γtotal
81.28 40.88 77.24 43.44 45.39 58.89 60.26
29.48 22.09 25.15 18.05 24 30.69 38.66
38.44 26.01 38.53 27.89 26.62 25.06 25.01
4.63 52.55 6.84 47.34 46.65 32.67 31.11
43.07 78.56 45.37 75.23 73.27 57.73 56.12
a Au substrate with oxygen plasma treatment. b PEG-DA film with different curing times: 3 s (A), 30 min (B), 3 h (C), and 10 h (D).
Table 3. Calculated Work of Adhesion at Each Interface work of adhesion (mJ/m2) prepolymer NOA prepolymer NOA substrate Au Au*a a
86.30 88.04
PEG-DA mold A
B
C
D
88.61 86.83 83.35 83.08 81.53 79.77 76.91 76.73 153.45 151.47 131.63 129.16
Au substrate with oxygen plasma treatment.
measured on various substrates (PEG-DA, Au, Au* (plasmacleaned Au), and NOA), which provide surface tension values that are needed to calculate the work of adhesion in eq 2. Details on the measured data and calculated work of adhesion values are summarized in Tables 2 and 3. A number of notable findings are derived from the measurements. First, the gold surface treated with oxygen plasma showed a substantial increase in surface tension (43.07 to 78.56 mJ/m2) as a result of the removal of contaminant particles and partial oxidation. As a result, the substrate denoted as Au* (with oxygen plasma treatment) showed better performance in releasing the mold. Second, the surface tension of the PEG-DA mold decreased with prolonged exposure time of plasma treatment (from A to D) because of surface stabilization by chain reorganization. The calculated work of adhesion values are 88.04 mJ/m2 for the NOA/ Au* interface and 88.61, 86.83, 83.35, and 83.08 mJ/m2 for the NOA/PEG-DA interface with increasing exposure time (3 s, 30 min, 3 h, and 10 h in order). Thus, a higher UV exposure time to the PEG-DA mold could ensure a clean release. Also, the minimum exposure time for clean release may be 30 min for the conditions used in the experiments. A plot in Figure 3 summaries
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Figure 3. Plot showing two distinct regions of sticking and nonsticking of the cured nanostructures using NOA 71 with different UV exposure times: (A) 3 s, (B) 30 min, (C) 3 h, and (D) 10 h. For successful pattern generation, the work of adhesion at the mold/ polymer interface needs to be smaller than that at the polymer/ substrate interface (W12 < W23).
these observations, confirming that the work of adhesion at the mold/polymer interface needs to smaller than that at the polymer/ substrate interface for the cured nanostructures to be nonsticking (W12 < W23). In addition, the average flow rate was measured using two RBNs (NC-50L and NC-500L shown in Table 1). For Newtonian flow in a rectangular duct, the volumetric flow rate (Q) can be given by simple Poiseuille formulation33
Q)
3
WH ∆P F 12η z P
(3)
where W and H are the width and height of the nanocapillaries, respectively, η is the viscosity of the liquid, ∆P is the pressure drop, z is the filling length, and FP is the shape factor that is given by an infinite series as a function of the aspect ratio W/H (FP was 0.087 for NC-50L, H/W ) 3 and 0.171 for NC-500L, and H/W ) 2). Here, ∆P can be replaced by the Laplace pressure (γLV cos θ/ ), where γ R LV and θ are the surface tension and equilibrium contact angle of a liquid on a solid substrate, respectively, and R is the hydraulic radius (WH/2(W + H)).6 Then, the filling length z for the elapsed time T is given by
z)
x
γLV cos θ(W + H)HFP T 3ηW
(4)
Using NOA 71 as a prepolymer (γLV ≈ 39 mJ/m2, η ) 200 cP),16 ∼0.6 mm for NC-50L and ∼2.8 mm for NC-500L were measured for an elapsed time of 6 min. These values qualitatively agree with the theoretical values given by eq 4 (0.96 mm for NC-50L and 3.01 mm for NC-500L). For residual-layer-free nanofabrication, various nanocapillaries were used: five nanochannels with widths ranging from 500 to 50 nm (NC-500L, 300L, 200L, 70L, and 50L) and one nanopillar with a diameter of 50 nm (NC-50P). As shown in Figure 4, these nanocapillaries can act as a guiding channel for the fabrication of polymeric nanostructures on a gold substrate by allowing UV-curable NOA 71 to flow inside nanocapillaries. In panel a, the progressing end of rectangular nanostructures (NC-500L, aspect ratio ) 2, equal spacing) is shown, along with a magnified (33) Middleman, S. Fundamentals of Polymer Processing; McGraw-Hill: New York, 1977.
Figure 4. SEM images of fabricated nanostructures on a gold substrate using RBNs for (a) 500 nm lines with an apparent contact angle of 55°, (b) 300 nm lines with one detached, freestanding line, (c) 50 nm lines, (d) the interface between the nanoline mold (70 nm) and gold substrate, (e) 50 nm wells with the mold with pillar geometry, (f) large-area fabrication for nanowells, (g) interface between the nanopillar mold (50 nm) and gold substrate, and (h) freestanding 300 nm nanofibers delaminated from the substrate.
view in the inset. A meniscus was seen at the three-phase contact line, exhibiting a contact angle of ∼55°. Also, an image of rectangular nanostructures (NC-300L, aspect ratio ) 3.3, equal spacing) with one detached, freestanding line (panel b) indicates that the nanostructures are distinctly separated with a high aspect ratio. (See the side view of the nanostructure in the inset.) In panel c, 50 nm nanostructures were shown over a large area (NC-50L, aspect ratio ) 3, 150 nm spacing). Shown in panel d is a cross-sectional image taken at the interface between the nanoline mold (NC-70L, aspect ratio ) 2.9, equal spacing) and the gold substrate, showing a reversible seal without an appreciable gap. Interestingly, nanowells of ∼50 nm diameter could also be formed using the same approach with NC-50P (aspect ratio ) 6450 nm spacing) (panels e and f) in spite of the limited contact area that the pillar geometry can offer. The image in panel e along with the inset image indicates that the substrate surface was exposed. Furthermore, large-area fabrication was possible as seen from the image in panel f. Shown in panel g is a cross-sectional image taken at the interface between the nanopillar mold and gold substrate. Although the contact was slightly distorted as a result of the limited contact area (pillars inclined to one side), the contact interface was remarkably stable against appreciable collapse or deformation. Also, freestanding nanofibers were frequently observed as a result of delamination from the surface during sample cutting (panel h). Here, a few comments follow regarding potential limitations of allowing a viscous prepolymer to flow into the RBNs. First,
Mold for Forming ReVersibly Bonded Nanocapillaries
the length of capillary movement was restricted such that the polymer stopped flowing at a distance of less than ∼1 mm from the inlet. (The total length is ∼1 cm for channels and ∼2 cm for pillars.) To overcome this limitation, an application of negative pressure (suction) or the use of a hydrophilic polymer would be necessary. Second, partial elevation of the mold was observed when the surface roughness was relatively large (>10 nm), which is attributed to the high elastic modulus of the PEG-DA mold. Hence, the use of the RBNs needs to be limited to a flat substrate.
Conclusions In this article, we have presented a rigiflex, spontaneously wettable PEG-DA mold for forming RBNs and fabricating polymeric nanostructures in a residual-layer-free fashion. It turns out that the sheet-type PEG-DA mold supported on a PET film provided adequate mechanical properties as well as flexibility, allowing for conformal, spontaneous contact over a large area. Furthermore, the weak negative charge generated on the PEGDA surface induced an initial reversible seal on the gold or silicon substrate without applying any external force or modification.
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The nanocapillaries were used to create well-defined polymeric nanostructures without a residual layer using a UV-curable liquid prepolymer. These unique features of the PEG-DA mold should be useful for various nanopatterning applications. If properly controlled, the RBNs could be used to fabricate 1D or 2D arrays of various nanoelements such as carbon nanotubes, nanowires, and nanoparticles in a conventional laboratory setup, which is currently under investigation in our laboratory. Acknowledgment. This work was supported by the Ministry of Science and Technology through the Nanoscopia Center of Excellence and the Micro Thermal System Research Center at Seoul National University. Supporting Information Available: EOS plot and diagram of the zeta potential of a PEG-DA film mold measured by an ELS (electrophoretic light scattering) spectrophotometer and some examples of cohesion failure in the absence of additional UV exposure. This material is available free of charge via the Internet at http://pubs.acs.org. LA0633942
