pubs.acs.org/Langmuir © 2009 American Chemical Society
Cross-Linked Random Copolymer Mats As Ultrathin Nonpreferential Layers for Block Copolymer Self-Assembly Eungnak Han and Padma Gopalan* Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706 Received July 9, 2009. Revised Manuscript Received September 14, 2009 We report the effect of increasing amounts of glycidyl methacrylate (GMA) in random copolymers of P(S-r-MMAr-GMA) on the formation of nonpreferential mats for the assembly of P(S-b-MMA) block copolymers. Increasing the GMA concentration in the random copolymer from 1 (PG1) to 4 (PG4) mole % increased the cross-linking efficiency and reduced the effective minimum thickness of the cross-linked mat for perpendicular alignment of P(S-b-MMA) from ∼6 nm to ∼2 nm. The compositional window (so-called perpendicular window) of PG4 was defined for both symmetric and asymmetric P(S-b-MMA). Compared to PG1, incorporation of higher amount of polar comonomer (GMA) in PG4 shifted the perpendicular window toward higher styrene fraction as a result of the increased polarity. The defined perpendicular window for P(S-r-MMA-r-GMA) is equally applicable for random copolymers prepared by both controlled living and classical free-radical polymerizations.
Introduction Block copolymer (BCP) lithography has emerged as a powerful and cost-effective tool for creating dense periodic nanostructures.1-4 The use of BCP materials in thin-films as templates involves controlling the size, shape, and the orientation of the domains. Perpendicularly oriented domains in BCP thin-films have received attention as etch masks or nanotemplates as they offer high aspect ratio and direct connectivity to the underlying substrate.5-7 In general, the shape and size of the assembled domains in BCP thin-films is controlled by intrinsic characteristics of BCP such as molecular weight and volume fraction. The orientation of the microdomains in thin films is mainly controlled by the interaction of each block with the substrate and the free surface through wetting energetics and polymer confinement effects resulting from the film thickness. Typically, preferential wetting of the substrate by one of the blocks due to lower interfacial energy results in parallel orientation of the domains. To induce perpendicular domain *Author to whom correspondence should be addressed. E-mail: pgopalan@ wisc.edu. (1) Lazzari, M.; Lopez-Quintela, M. A. Adv. Mater. 2003, 15(19), 1583–1594. (2) Hamley, I. W. Nanotechnology 2003, 14(10), R39–R54. (3) Hawker, C. J.; Russell, T. P. MRS Bull. 2005, 30, 952–966. (4) Segalman, R. A. Mater. Sci. Eng., R: Rep. 2005, 48(6), 191–226. (5) Shin, K.; Leach, K. A.; Goldbach, J. T.; Kim, D. H.; Jho, J. Y.; Tuominen, M.; Hawker, C. J.; Russell, T. P. Nano Lett. 2002, 2(9), 933–936. (6) Xiao, S. G.; Yang, X. M.; Edwards, E. W.; La, Y. H.; Nealey, P. F. Nanotechnology 2005, 16(7), S324–S329. (7) Park, S. M.; Stoykovich, M. P.; Ruiz, R.; Zhang, Y.; Black, C. T.; Nealey, P. E. Adv. Mater. 2007, 19(4), 607–611. (8) Kim, S. H.; Misner, M. J.; Xu, T.; Kimura, M.; Russell, T. P. Adv. Mater. 2004, 16(3), 226–231. (9) Thurn-Albrecht, T.; Schotter, J.; Kastle, C. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290(5499), 2126–2129. (10) Kim, S. O.; Solak, H. H.; Stoykovich, M. P.; Ferrier, N. J.; de Pablo, J. J.; Nealey, P. F. Nature 2003, 424(6947), 411–414. (11) Stoykovich, M. P.; Muller, M.; Kim, S. O.; Solak, H. H.; Edwards, E. W.; de Pablo, J. J.; Nealey, P. F. Science 2005, 308(5727), 1442–1446. (12) Peters, R. D.; Yang, X. M.; Kim, T. K.; Sohn, B. H.; Nealey, P. F. Langmuir 2000, 16(10), 4625–4631. (13) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275(5305), 1458–1460.
Langmuir 2010, 26(2), 1311–1315
orientation, several methods such as solvent annealing,8 e-field alignment,9 chemical patterns,10,11 substrate modification,12-18 and rough substrates19,20 have been used. The formation of a nonpreferential surface for P(A-b-B) diblock copolymer using P(A-r-B) copolymers was demonstrated in a seminal work by Mansky et al.13,21 The interfacial interactions of the polystyrene (PS) and poly(methyl methacrylate) (PMMA) blocks with the substrate can be balanced by end grafting a random copolymer of P(S-r-MMA) (MMA = methyl methacrylate) having a 0.58 styrene (St) fraction to the substrate. End-grafting is achieved through the dehydration reaction between the terminal hydroxy group in the P(S-r-MMA) and the native oxide layer on silicon wafer. The kinetically stable brush layer prevents diffusion of random copolymers into the overlying BCP film during spin coating or the thermal annealing process. As an alternate method, the feasibility of using a third comonomer (C) in the random copolymer P(A-r-B-r-C) for nonpreferential layer formation for a P(A-b-B) diblock copolymer was demonstrated by us and others.14-17 For example, Ryu et al. reported the random copolymer containing thermally crosslinkable benzocyclobutene (BCB) monomer to obtain an insoluble cross-linked thin-film.14 We have focused our efforts on a simple strategy, where instead of end-grafting we used the sidechain grafting of the random copolymer to the substrate by incorporating hydroxyethyl methacrylate (HEMA) as a third (14) Ryu, D. Y.; Shin, K.; Drockenmuller, E.; Hawker, C. J.; Russell, T. P. Science 2005, 308(5719), 236–239. (15) In, I.; La, Y. H.; Park, S. M.; Nealey, P. F.; Gopalan, P. Langmuir 2006, 22 (18), 7855–7860. (16) Bang, J.; Bae, J.; Lowenhielm, P.; Spiessberger, C.; Given-Beck, S. A.; Russell, T. P.; Hawker, C. J. Adv. Mater. 2007, 19(24), 4552–4557. (17) Han, E.; In, I.; Park, S. M.; La, Y. H.; Wang, Y.; Nealey, P. F.; Gopalan, P. Adv. Mater. 2007, 19(24), 4448–4452. (18) Ji, S. X.; Liu, G. L.; Zheng, F.; Craig, G. S. W.; Himpsel, F. J.; Nealey, P. F. Adv. Mater. 2008, 20(16), 3054–3060. (19) Sivaniah, E.; Hayashi, Y.; Matsubara, S.; Kiyono, S.; Hashimoto, T.; Kukunaga, K.; Kramer, E. J.; Mates, T. Macromolecules 2005, 38(5), 1837–1849. (20) Sivaniah, E.; Hayashi, Y.; Iino, M.; Hashimoto, T.; Fukunaga, K. Macromolecules 2003, 36(16), 5894–5896. (21) Huang, E.; Rockford, L.; Russell, T. P.; Hawker, C. J. Nature 1998, 395 (6704), 757–758.
Published on Web 09/30/2009
DOI: 10.1021/la902483m
1311
Article
Han and Gopalan
comonomer. A random copolymer layer of P(S-r-MMAr-HEMA) having a 0.02 fraction of HEMA and a 0.58 fraction of St showed “nonpreferential” wetting behavior toward symmetric P(S-b-MMA) along with fast binding kinetics to the substrate.15 We further demonstrated that photocross-linkable random copolymers can be made by functionalizing the hydroxyl group of HEMA with acryloyl groups.17 Alternatively, we reported a thermally and photochemically cross-linkable random copolymer by copolymerization of glycidyl methacrylate (GMA) with St and MMA.17 In all the above reports, the St mole fraction was fixed to 0.58, and the third monomer fraction (C) in the P(A-r-B-r-C) copolymers was fixed to 0.02 or below to preserve the neutrality of the random copolymer layer. Here we report the formation of an ultrathin nonpreferential layer by increasing the third polar monomer (C) concentration in the random copolymer from 1 to 4 mol %. The increase in the concentration of the crosslinkable groups leads to denser random-copolymer mats, consequently lowering the effective minimum thickness to ∼2 nm and eliminates the need for an additional washing step to remove the un-cross-linked copolymer. We further examined the effect of changing the amount of St (FSt) from 0.50 to 0.75 and increasing the concentration of the C monomer from 1 to 4 mol % on the nonpreferentiality of the random copolymer toward the BCP. Increase in the third polar monomer concentration shifts the perpendicular window to higher St fraction in the copolymer to compensate for the increased polarity.
the cross-linked mat. The substrates were sonicated in toluene for 1 min and rinsed with fresh toluene repeatedly to remove any un-cross-linked random copolymers. On these random copolymer modified wafers, a solution of lamellae-forming L5252 in toluene (1.5% w/w), PMMA-cylinder-forming PMMA-C5020 in toluene (1% w/w), or PS-cylinder-forming PS-C2050 in cyclopentanone (1% w/w) was spin-coated at 4000 rpm to produce films with thicknesses of 45, 32, and 22 nm, respectively. All BCP films were annealed at 190 °C for 72 h under vacuum to attain the equilibrium morphology prior to imaging. Characterization. For each synthesized random copolymer, a 1 H NMR spectrum was recorded in CDCl3 with a Bruker ACþ 300 (300-MHz) spectrometer. GPC was performed with a Viscotek GPCmax VE-2001 chromatograph using two columns (VARIAN 5M-POLY-008-27 and VARIAN 5M-POLY-008-32). THF was used as an eluent with a flow rate of 1 mL/min at 30 °C. Monodispersed PS standards were used for calibration. The film thicknesses of the random copolymer and BCP layers were measured by ellipsometry (Rudolph Research Auto EL). Top-down scanning electron microscope (SEM) images of the BCP microdomains were acquired with a LEO-1550 VP field-emission instrument using an accelerating voltage of 1 kV. For PMMA-C5020, the PMMA domain was etched by UV exposure and acetic acid washing prior to imaging. The etch rate of the cross-linked mat was measured with oxygen plasma (Plasma Etch, Inc., PE-200) (RF power = 50 W, pressure = 25 mTorr). The surface topography of the cross-linked mat was examined using a Nanoscope III Multimode atomic force microscope (Digital Instruments) in tapping mode.
Experimental Section
Results and Disscusions
Materials. St, MMA, and GMA were purchased from Aldrich and distilled under reduced pressure. Three BCPs were purchased from Polymer Source, Inc. (Dorval, Quebec, Canada) and used without further purification: symmetric P(S-b-MMA) (PS 52 kg/mol, PMMA 52 kg/mol, polydispersity index (PDI), 1.09), asymmetric PMMA cylinder forming P(S-b-MMA) (PS 50.5 kg/mol, PMMA 20.9 kg/mol, PDI, 1.06), and asymmetric PS cylinder forming P(S-b-MMA) (PS 20.2 kg/mol, PMMA 50.5 kg/mol, PDI, 1.07). These three BCPs are referred to throughout as L5252, PMMA-C5020, and PS-C2050, respectively. Synthesis of P(S-r-MMA-r-GMA). In this paper, PG1 and PG4 refers to P(S-r-MMA-r-GMA) copolymers with 1 and 4% GMA, respectively, synthesized by nitroxide mediated polymerization (NMP) following a procedure reported earlier.22,23 PG’4 refers to P(S-r-MMA-r-GMA) copolymers with 4% GMA synthesized by classical free radical polymerization (FRP) using azobisisobutyronitrile (AIBN). The number after the dashed line represents the St fraction in the copolymer. For example, PG’4-67 is the copolymer containing 4% GMA and 67% St made by FRP. A typical procedure, for example, to synthesize PG’4-67 involved degassing a mixture of AIBN (0.13 g), St (6.72 g), MMA (2.40 g), GMA (0.52 g), and toluene (10 mL) by three freeze/thaw cycles and sealed under nitrogen. The polymerization mixture was reacted at 80 °C for 24 h, and the resulting viscous mixture was diluted with THF and precipitated into methanol. The precipitated solid was filtered and dried under reduced pressure to yield the random copolymer PG’4 as a white solid. In the case of NMP, bulk polymerization was carried out at 120 °C for 36 h. Surface Modification and BCP Deposition. As a general procedure, substrate modification was done by spin coating a random copolymer solution in toluene (0.3% w/w) onto silicon wafer, followed by annealing in vacuum at 160 °C for 3 h to yield
Copolymer Synthesis. We synthesized a series of random copolymers (Figure 1) where the St fraction was varied from 0.50 to 0.75 and the amount of GMA was increased from 1 to 4 mol % (Table 1). Classical FRP using AIBN as an initiator gave random copolymers with higher polydispersity compared to NMP. The PDI was e1.3 for polymers made by NMP and close to 1.8 for those made by FRP. The copolymer composition was determined by evaluation of the 1H NMR spectrum. Small deviations in the feed ratio and the actual St fraction (FSt) were observed for both FRP and NMP, probably as a result of variations in conversion. The temperature of polymerization especially by NMP should be controlled to be below 130 °C, as higher temperatures lead to cross-linking and subsequent gelation during polymerization. Cross-Linked Mat Formation. Three different copolymers, PG1-69 (Mw = 47 200), PG4-69 (Mw = 20 000), and PG4-70 (Mw = 46 700), were used to investigate the effect of cross-linker density and molecular weight on the formation of cross-linked mat. The expected higher cross-link density in PG4 compared to PG1 can be observed by measuring the remaining thickness of the cross-linked film. Solutions of PG1 and PG4 (0.4% w/w) in toluene were spin coated onto silicon wafers to yield ∼16 nm thick films. The coated substrates were heated in nitrogen atmosphere at 160 or 230 °C for predetermined times and quenched to room temperature. After sonication and repeated washing in toluene, the samples were dried under nitrogen, and the remaining thickness was measured by ellipsometry. As a result of the volume shrinkage during cross-linking and solvent drying, film thickness after cross-linking but prior to the washing step was used as the initial thickness instead of the thickness of the as-cast film. As shown in Figure 2, the remaining thicknesses for both PG1 and PG4 mats rapidly increases with increasing cross-linking time for the first 20 min, reaching saturation after 3 h at 160 °C and after 60 min at 230 °C. Compared to PG1, in PG4 quantitative cross-linking can be achieved, which makes the washing
(22) Han, E.; Stuen, K. O.; La, Y. H.; Nealey, P. F.; Gopalan, P. Macromolecules 2008, 41(23), 9090–9097. (23) Han, E.; Stuen, K. O.; Leolukman, M.; Liu, C. C.; Nealey, P. F.; Gopalan, P. Macromolecules 2009, 42(13), 4896–4901.
1312 DOI: 10.1021/la902483m
Langmuir 2010, 26(2), 1311–1315
Han and Gopalan
Article
Figure 1. Schematic representation of substrate modification with P(S-r-MMA-r-GMA) for perpendicular domain orientation of P(S-b-MMA). Table 1. Characteristics of Synthesized P(S-r-MMA-r-GMA) Copolymers polymer
polym methoda
fStb
FStb
Mw
PDI
PG1-69 NMP 0.70 0.69 47 200 1.3 PG4-50 NMP 0.50 0.50 50 700 1.3 PG4-56 NMP 0.55 0.56 51 400 1.3 PG4-61 NMP 0.60 0.61 41 800 1.2 PG4-65 NMP 0.65 0.65 50 500 1.3 PG4-69 NMP 0.70 0.69 20 000 1.2 PG4-70 NMP 0.72 0.70 46 700 1.2 PG4-75 NMP 0.78 0.75 44 400 1.2 PG’4-58 classical 0.60 0.58 38 200 1.7 PG’4-63 classical 0.65 0.63 38 600 1.7 PG’4-67 classical 0.70 0.67 36 400 1.8 a Method of polymerization of random copolymers (NMP = nitroxidemediated controlled radical polymerization, classical = classical free-radical polymerization using AIBN). b fSt is the mole fraction of St in the feed, and FSt is the mole fraction of St in the resulting random copolymer as measured by 1H NMR.
Figure 2. Remaining thickness of cross-linked mats of PG1 and PG4 as a function of cross-linking time at 160 °C and inset shows the result at 230 °C.
step unnecessary. Hence, the efficiency of cross-linking (defined as the percentage of initial thickness remaining) increases as the fraction of GMA units in the random copolymer increases. At the same time, the molecular weights of the copolymers are also important. Typically higher molecular weights lead to more physical cross-links, which can prevent the loss of polymer chains from the mat during the washing step. This is apparent by Langmuir 2010, 26(2), 1311–1315
comparing the remaining thickness between low and high molecular weight PG4 copolymers. As seen in Figure 2, a comparison of PG4 with a Mw = 47 200 and Mw = 20 000 shows that, even though the cross-linking efficiency after 3 h at 160 °C is similar, significant differences can be seen at shorter cross-linking times. The higher molecular weight PG4 mat has approximately 10% higher remaining thickness compared to the lower molecular weight PG4 mat for the same cross-linking time. The root-mean-square (rms) roughness of cross-linked PG1-69 and PG4-70 mats was measured to be 0.221 and 0.216 nm, respectively, using an atomic force microscope. These values are comparable to those of a covalently grafted brush system (∼0.19 nm)15 and significantly lower than the rough surfaces (∼15 nm) that were used by Sivaniah et al.19,20 to induce perpendicular orientation of symmetric P(S-b-MMA). Therefore, the perpendicular orientation of P(S-b-MMA) domains in this study solely results from the balanced interfacial interactions and not the surface roughness. Compositional Ranges of the PG4 Mats for Perpendicular Orientation of Domains in P(S-b-MMA) BCPs. The composition range of the random copolymers that induce perpendicular orientation of domains (denoted as perpendicular window) in P(S-b-MMA) depends on the relative volume fraction of each block of BCP and BCP film thickness.22,24 The three BCPs, L5252, PMMA-C5020, and PS-C2050, were spin coated and annealed over 10 nm thick PG4 modified substrates to define the perpendicular windows. Figure 3 shows the SEM images of self-assembled BCPs on PG4-modified substrates. Perpendicular orientation of lamellae, PMMA cylinders, and PS cylinders were successfully achieved on PG4 mats with 0.50 < FSt e 0.70, 0.61 e FSt e 0.75, and 0.61 < FSt e 0.65, respectively (FSt is the mole fraction of St in the random copolymer as measured by 1 H NMR). Perpendicular window of PMMA-C5020 is at higher FSt than others, and PS-C2050 showed a very narrow perpendicular window compared to L5252 or PMMA-C5020. We reported a phenomenological description for the observed perpendicular window for all three BCPs on PG1-modified substrates in an earlier publication.22 These discussions are equally valid for PG4 mats. The observed perpendicular window was unique for each BCP, as the orientation of BCP domains in thin films on these (24) Ham, S.; Shin, C.; Kim, E.; Ryu, D. Y.; Jeong, U.; Russell, T. P.; Hawker, C. J. Macromolecules 2008, 41(17), 6431–6437.
DOI: 10.1021/la902483m
1313
Article
Han and Gopalan
Figure 3. Top view SEM images of self-assembled BCP (L5252, 45 nm thick in the first row, PMMA-C5020, 32 nm thick in the second row, PS-C2050, 22 nm thick in the third row) films on PG4-modified surfaces containing different mole fractions of St in the random copolymer. Black scale bars represent 200 nm.
Figure 4. Top-view SEM images of self-assembled PMMA-C5020 on PG1-69 (first row) and PG4-70 (second row) modified substrates where the thickness of cross-linked mat (t) was varied from ∼5 nm to ∼2 nm. (scale bar = 200 nm).
copolymer layers is a result of free energy minimization, which is affected not only by the interfacial interactions between the mat and BCP, but also the BCP composition and the interaction of BCP with the free surface. The observed FSt ranges for PG4 are higher than those observed for PG1.22 These observations are consistent with our earlier reports where the incorporation of 1 mol % of HEMA or 1 mol % of GMA in the random copolymers P(S-r-MMAr-HEMA) and P(S-r-MMA-r-GMA) respectively shifted the perpendicular window toward higher FSt compared to endhydroxyl terminated P(S-r-MMA) to compensate for the increased polarity of random copolymer.22 Thus by simultaneously increasing the St fraction relative to the MMA fraction in the random copolymer, a higher amount of polar comonomer (GMA) can be incorporated without altering the orientation of the domains in the assembled BCP film. Effective Minimum Thickness of a Cross-Linked Mat. In addition to increasing the cross-linking efficiency, higher GMA content in the random copolymer also increases the cross-linking density leading to denser mats. These denser mats can more effectively screen the overlaying BCP film from the underlying substrate compared to less densely cross-linked mats. To find the effective minimum thickness of PG1 and PG4 mats, 32 nm thick films of PMMA-C5020 were deposited on predetermined thicknesses of cross-linked PG1-69 and PG4-70 mats. The assembled morphologies were examined by SEM (Figure 4). With PG1 1314 DOI: 10.1021/la902483m
Figure 5. Etched thickness of cross-linked mat of PG1-69 (etch rate ∼ 2.7 nm/10 s) and PG4-70 (etch rate ∼ 2.6 nm/10 s) under O2 plasma.
as the cross-linked copolymer, a minimum effective thickness of ∼6 nm was required. As the thickness of PG1 decreased below 6 nm, the number of parallel cylinders or defects increased. In contrast to the PG1 layer, even the 2.3 nm PG4 layer shows perpendicular cylinders in the overlying BCP film. This confirms our hypothesis that an ultrathin layer of PG4 mat can prevent the penetration of BCP and effectively screen the influence of the bare Langmuir 2010, 26(2), 1311–1315
Han and Gopalan
Article
respectively, are comparable as shown in Figure 5. Hence, the ability to work with ∼2 nm of a PG4 mat layer is certainly advantageous, as it minimizes the loss of PS template during the oxygen plasma etching step. As a comparison, PS homoplymer has an etch rate of 1.9 nm/10 s under the same etching conditions. Effectiveness of Classical FRP. While most of the reports in the literature on the synthesis of random copolymers for substrate modification have utilized controlled FRP, we demonstrated in an earlier work that HEMA containing copolymers synthesized by classical FRP with an AIBN initiator can be equally effective in inducing the perpendicular orientation of BCP domains.15 Hence, we synthesized three different compositions of PG’4 by FRP and examined the assembly of both symmetric and asymmetric BCPs as shown in Figure 6. Overall quality of assembled structures on PG’4 is comparable to its PG4 counterpart implying that the perpendicular window defined for PG4 is equally applicable for PG’4. Therefore as long as the compositions of PG’4 as determined by 1H NMR falls within the perpendicular window for a given composition of BCP and a given BCP film thickness, perpendicular orientation can be predictably achieved.
Conclusions
Figure 6. Top view SEM images of self-assembled (a) 45 nm thick L5252 on PG’4-58, (b) 32 nm thick PMMA-C5020 on PG’4-67, and (c) 22 nm thick PS-C2050 on PG’4-63. Black scale bars represent 200 nm.
substrate. Similar results were reported earlier by Ryu et al., where at least a 5.5 nm layer of random copolymer P(S-r-MMA-r-BCB) containing 2% BCB is needed to prevent the penetration of the BCP and to minimize the defects in the assembled structure.14,25 Typically the PMMA domain in the cylinder forming P(S-b-MMA) is removed by UV exposure followed by washing with acetic acid to generate a nanoporous PS film, which can be used as a nanotemplate. However, the underlying random copolymer layer cannot be effectively removed by the wet etching process alone and requires an additional oxygen plasma etching. Although the cross-link density increases as we move from PG1 to PG4, the measured etch rates of 2.7 nm/10 s and 2.6 nm/10 s, (25) Ryu, D. Y.; Wang, J. Y.; Lavery, K. A.; Drockenmuller, E.; Satija, S. K.; Hawker, C. J.; Russell, T. P. Macromolecules 2007, 40(12), 4296–4300.
Langmuir 2010, 26(2), 1311–1315
The influence of the bare substrate on the overlying BCP was effectively screened by modifying the substrate with an ultrathin cross-linked random copolymer mat PG4 having 4 mol % of GMA. The increased cross-link density in PG4 compared to PG1 reduces the effective minimum thickness of the copolymer layer to less than 3 nm. A perpendicular window of PG4 layer was defined for symmetric and the two asymmetric P(S-b-MMA) BCP films. Compared to PG1, incorporation of a higher amount of polar comonomer (GMA) in PG4 shifted the perpendicular window toward a higher St fraction as a result of the increased polarity. Perpendicular windows defined for PG4 are equally applicable for PG’4 copolymers made by classical FRP. The ability to work with ∼2 nm of PG4 or PG’4 mat is advantageous for nanotemplate fabrication, as it minimizes the loss of PS template during the oxygen plasma etching step. Acknowledgment. The authors would like to acknowledge support from staff and the use of equipment at the Center for Nanotechnology and the Synchrotron Radiation Center at the University of Wisconsin (National Science Foundation Grant No. DMR-0537588). E.H. would like to acknowledge Melvina Leolukman for the AFM measurements. This research was funded by the Global Research Corporation (Grant Nos. 2005OC-985 and 2008-OC-164) and the National Science Foundation Nanoscale Science and Engineering Center at the University of Wisconsin - Madison (Grant No. DMR0425880).
DOI: 10.1021/la902483m
1315
