pubs.acs.org/Langmuir © 2009 American Chemical Society
Fabrication of Disposable Topographic Silicon Oxide from Sawtoothed Patterns: Control of Arrays of Gold Nanoparticles Heesook Cho, Hana Yoo, and Soojin Park* School of Energy Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Korea Received November 2, 2009. Revised Manuscript Received November 29, 2009 Disposable topographic silicon oxide patterns were fabricated from polymeric replicas of sawtoothed glass surfaces, spin-coating of poly(dimethylsiloxane) (PDMS) thin films, and thermal annealing at certain temperature and followed by oxygen plasma treatment of the thin PDMS layer. A simple imprinting process was used to fabricate the replicated PDMS and PS patterns from sawtoothed glass surfaces. Next, thin layers of PDMS films having different thicknesses were spin-coated onto the sawtoothed PS surfaces and annealed at 60 °C to be drawn the PDMS into the valley of the sawtoothed PS surfaces, followed by oxygen plasma treatment to fabricate topographic silicon oxide patterns. By control of the thickness of PDMS layers, silicon oxide patterns having various line widths were fabricated. The silicon oxide topographic patterns were used to direct the self-assembly of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer thin films via solvent annealing process. A highly ordered PS-b-P2VP micellar structure was used to let gold precursor complex with P2VP chains, and followed by oxygen plasma treatment. When the PS-b-P2VP thin films containing gold salts were exposed to oxygen plasma environments, gold salts were reduced to pure gold nanoparticles without changing high degree of lateral order, while polymers were completely degraded. As the width of trough and crest in topographic patterns increases, the number of gold arrays and size of gold nanoparticles are tuned. In the final step, the silicon oxide topographic patterns were selectively removed by wet etching process without changing the arrays of gold nanoparticles.
Introduction Combination of bottom-up self-assembling system and topdown lithographic process has been an efficient way to control the spatial locations of organic and/or inorganic nanostructured materials in a specific area on solid or flexible substrates.1-9 Highly ordered and oriented nanopatterned materials have been of great interest owing to their potential applications in optical, wire-grid polarizer, and high-density media.10-13 As a top-down process, a rich variety of lithographic methods, like photolithography, extreme ultraviolet (UV) interference lithography, electron-beam lithography, soft lithography, multibeam interference lithography, and scanning probe microscopy (SPM) based litho*Corresponding author. Telephone: (þ82)-52-217-2515. Fax: (þ82)-52217-2909 E-mail:
[email protected]. (1) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. Adv. Mater. 2006, 18, 2505. (2) Marrian, C. R. K.; Tennant, D. M. J. Vac. Sci. Technol., A 2003, 21, S207. (3) Segalman, R. A.; Hexemer, A.; Kramer, E. J. Macromolecules 2003, 36, 6831. (4) Hawker, C. J.; Russell, T. P. MRS Bull. 2005, 30, 952. (5) Li, B.; Lu, G.; Zhou, X.; Cao, X.; Boey, F.; Zhang, H. Langmuir 2009, 25, 10455. (6) Li, B.; Goh, C. F.; Zhou, X.; Lu, G.; Tantang, H.; Chen, Y.; Xue, C.; Boey, F. Y. C.; Zhang, H. Adv. Mater. 2008, 20, 4873. (7) Zhou, X.; Chen, Y.; Li, B.; Lu, G.; Boey, F. Y. C.; Ma, J.; Zhang, H. Small 2008, 4, 1324. (8) Zhang, H.; Amro, N. A.; Disawal, S.; Eby, R. Nano Lett. 2004, 4, 1649. (9) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (10) Pelletier, V.; Asakawa, K.; Wu, M.; Adamson, D. H.; Register, R. A.; Chaikin, P. M. Appl. Phys. Lett. 2006, 88, 211. (11) Vahala, K. J. Nature 2003, 424, 839. (12) Kim, J. H.; Moyer, P. J. Opt. Express 2006, 14, 6595. (13) Thurn-Albrecht, T.; Schotter, J.; K€astle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (14) Glass, R.; Arnold, M.; Bl€ummel, J.; K€uller, A.; M€oller, M.; Spatz, J. P. Adv. Funct. Mater. 2003, 13, 569. (15) Solak, H. H.; David, C.; Govrecht, J.; Golovkina, V.; Cerrina, F.; Kim, S. O.; Nealey, P. F. Microelectron. Eng. 2003, 56, 67.
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graphy, have been used to fabricate organic/inorganic patterns having dimension ranging from tens of nanometer to submicrometer.14-24 As another approach, self-assembly of block copolymers (BCPs) can be used as templates or scaffolds for fabricating nanostructured materials with dimensions of 10100 nm. The BCPs have been used as an attractive route for overcoming the limitations of conventional lithographic techniques, owing to the ability of self-assembling into ordered nanoscale patterns, the easy tuning of size and distance of microdomains, the ease processing on flat and patterned surfaces without introducing disruptive technologies.4,25 In the absence of pre-patterns which were made from the lithographic process, the BCP nanostructures lack long-range alignment and lateral order. To this end, top-down approaches, like replica molding, extreme UV, and photolithography, have been used to pattern surfaces on microscopic length scales so that the patterning can be used to guide the self-assembly of the nanoscopic organic/inorganic elements.1,3,15 Several top-down/bottom-up approaches demonstrated that topographic patterns, chemically patterned substrates, and corrugated surfaces can not only significantly enhance the lateral ordering of nanoscopic elements but also induce different morphologies depending on the differences of patterned pitch and natural period of BCPs or chemical properties of the patterned surfaces.1,3,15 Although the topographical and (16) Xia, Y.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153. (17) Word, M. J.; Adesida, I.; Berger, P. R. J. Vac. Sci. Technol., B 2003, 21, L12. (18) Campbell, M.; Sharp, D. N.; Harrison, M. T.; Denning, R. G.; Turberfield, A. J. Nature 2000, 404, 53. (19) Resnick, D.; Sreenivasan, S. V.; Wilson, C. G. Mater. Today 2005, 8, 34. (20) Cai, Y.; Ocko, B. M. J. Am. Chem. Soc. 2005, 127, 16287. (21) Liu, G. Y.; Xu, S.; Quan, Y. L. Acc. Chem. Res. 2000, 33, 457. (22) Ginger, D. S.; Zhang, H.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 30. (23) Huo, F.; Zheng, Z.; Zeng, G.; Giam, L. R.; Zhang, H.; Mirkin, C. A. Science 2008, 321, 1658. (24) Basnar, B.; Willner, I. Small 2009, 5, 28. (25) Park, S.; Kim, B.; Wang, J.-Y.; Russell, T. P. Adv. Mater. 2008, 20, 681.
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chemically patterned surfaces prepared from photolithography, extreme UV, SPM lithography, and laser interference techniques are well-established, these methods are expensive, do complicate fabrication procedures such as patterning, etching, and lift-off process, and/or are limited to the patterning of small areas. These topographic or chemically patterned surfaces permanently remain at the substrate surface; thus, they may limit any further applications such as their use in integrated circuits.26,27 Among them, replica molding, one of the soft lithographic techniques, is a simple and economic method for patterning small features on the polymer or metal substrates. In general, an elastomeric stamp, like poly(dimethylsiloxane) (PDMS), is used to produce patterns with feature sizes ranging from tens of nanometers to hundreds of micrometers in soft lithography. Also, subsequent pattern transfer to polymeric or metallic surfaces can be performed.16 Here, we demonstrate a fabrication of silicon oxide topographic patterns having different crest widths from sawtoothed glass patterns and the polymeric replica, and subsequent fabrication of highly ordered BCP spherical micellar nanostructures and arrays of gold nanoparticles with high degree of lateral order. In the end, topographic silicon oxide patterns were removed via wet chemical etching process. By controlling the thickness of PDMS layers, annealing temperature, and plasma exposure conditions, silicon oxide patterns having different crest and trough widths were easily obtained. As-prepared topographic silicon oxide patterns can direct the self-assembly of BCP thin films via solvent annealing process. Subsequently, highly ordered micellar thin films were used as a template for incorporation of gold precursor to the functional group of the BCP components, and followed by oxygen plasma exposure to make gold nanoparticles with removal of BCP copolymers. In the final step, silicon oxide topographic patterns were selectively dissolved in dilute HF solutions, while the arrays of gold nanoparticles made unchanged. The top-down soft-lithographic process was used to fabricate arrays of metal dots with high degree of lateral ordering, and later the topographic patterns were selectively removed, if necessary.
Experimental Section Materials. Sawtoothed glass patterns having pitch of 550 nm and amplitude of 100 nm were purchased from Thorlabs. After cleaning the sawtoothed patterns with nitrogen blowing, PDMS (Sylgard 184, Dow Corning) mixed with cross-linker was spincoated on the sawtoothed glass surface, annealed at 80 °C for 2 h, cooled to room temperature, and then, peeled off to make the replicated PDMS patterns. The replicated PDMS grating was pressed on the polystyrene (PS) thin films with thickness of 98 nm at 130 °C for 30 min to fabricate the replicated PS patterns. Next, a thin layer of PDMS from hexane solution was spin-coated on the replicated PS, annealed to 60 °C for 1 h, and followed by oxygen plasma treatment (SPI Plasma Prep II) for 40 min at 50 W to make topographic silicon oxide patterns. Poly(styrene)-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer was purchased from Polymer Source, and were used without further purification (MnPS = 125 kg/mol, MnP2VP = 58.5 kg/mol, Mw/Mn = 1.05). The PS-b-P2VP copolymer was dissolved in toluene at room temperature for 12 h to yield a 0.5 wt % polymer solution. The PS-b-P2VP thin films were spin-coated on various topographic silicon oxide patterns, and annealed in toluene vapor for 4 h to enhance the lateral order of spherical micellar arrays. The thin films were immersed in gold salt solution (HAuCl4, 0.3 wt % in ethanol) for 2 min to let gold precursor complex with P2VP (26) Lambooy, P.; Russell, T. P.; Kellogg, G. J.; Mayes, A. M.; Gallagher, P. D.; Satija, S. K. Phys. Rev. Lett. 1994, 72, 2899. (27) Jeong, S.-J.; Kim, J. E.; Moon, H.-S.; Kim, B. H.; Kim, S. M.; Kim, J. B.; Kim, S. O. Nano Lett. 2009, 9, 2300.
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chains, and followed by plasma exposure to fabricate gold nanoparticles with removal of BCP thin films. Topographic silicon oxide patterns were removed in dilute HF (5 wt % in water) solution without changing the arrays of gold nanoparticles with high degree of lateral order. Characterization. Surface morphologies of PS-b-P2VP thin films and gold nanoparticles on flat or patterned substrates were imaged by scanning force microscope (Digital Instruments, Nanoscope V) in the tapping mode. The measurements were performed using commercial Si cantilevers. FE-SEM was performed on a NanoSEM 230 (FEI) at an accelerating voltage of 5 kV without any metal coating.
Results and Discussion Scheme 1 shows schematic illustration for fabricating arrays of highly ordered BCP micellar nanostructures and gold nanoparticles by combining top-down soft-lithographic and bottom-up BCP self-assembling process. An elastomeric material, PDMS containing cross-linker, was used to replicate the sawtoothed glass patterns which produced by physically forming grooves on a reflective surface by using a diamond tool.28 The cross-linked PDMS grating was reproduced in full reliefs to extremely close tolerances (Scheme 1A). When the replicated PDMS grating was pressed on polystyrene (PS) films at temperature well above the glass transition temperature (Tg) of PS (100 °C), the pitch and depth of PS grating were identical to those of the original PDMS grating (Scheme 1B). Next, thin layer of PDMS films were spincoated onto the replicated PS patterns, annealed to be drawn the PDMS into the valley of PS grating at well below the Tg of PS and at well above the Tg of PDMS, and then performed on oxygen plasma treatment to fabricate topographic silicon oxide patterns (Scheme 1C). As-prepared silicon oxide topographic patterns were used to guide the arrays of PS-b-P2VP BCP spherical micelles via solvent annealing process (Scheme 1D). The spherical micellar thin films with high degree of lateral order were immersed in gold salt solution to let the gold precursor complex with P2VP chains (Scheme 1E). Subsequently, gold-loaded films were reduced to gold nanoparticles under oxygen plasma environments without changing the lateral order of gold nanoparticles (Scheme 1F). Finally, silicon oxide patterns were removed in dilute HF solution, while gold nanoparticles were left behind (Scheme 1G). The sawtoothed patterns on the glass surfaces can be used as masters to imprint the surface topographies into a polymer at temperatures well above the Tg of the polymer. Subsequent cooling or cross-linking of the polymer freezes in the surface topography. One of the easiest ways to accomplish this is to use PDMS (Sylgard 184) which can be cross-linked at certain temperature. PDMS was one material used to replicate the sawtoothed glass surfaces, where the pitch and the amplitude are 550 and 100 nm, respectively. Thin PDMS films with cross-linker were spin-coated onto the glass surfaces, annealed at 80 °C for 2 h, cooled to room temperature, and then, peeled off to make the replicated PDMS patterns. The PDMS replica having pitch of 550 nm was seen in optical microscopy due to the peak-to-peak distances in visible regime (Figure 1A). Next, the patterns of replicated PDMS were transferred to 98 nm thick PS films using imprinting process. When the PDMS patterns were in contact with the PS films, and annealed at 130 °C for 2 h under pressure of 0.1 MPa, pitch and depth of replicated PS patterns are identical to those of the original PDMS as shown in Figure 1B and crosssectional line scans of scanning force microscopic (SFM) images (28) Hunter, W. R. Spectrometric Techniques; Academic Press: New York, 1985; Volume IV,p 63.
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Figure 1. Optical microscopic image of sawtoothed PDMS replica (A), height mode SFM images of replicated PS sawtoothed pattern (B), and representative topographic silicon oxide patterns having line widths of 120 nm (C) and 210 nm (D) prepared from replicated PS seen in part B). In the insets, cross-sectional line scans of SFM images were seen. Scale bars are 1 μm. Scheme 1. Schematic Illustrations for Arrays of Highly Ordered Gold Nanoparticles Prepared from the Combination of Top-Down Pressing and Bottom-Up Self-Assembling Processesa
a Soft-lithography process was applied to PDMS and PS films from sawtoothed glass patterns (A, B). When thin layer of PDMS films were spin-coated and thermally annealed at 60 °C for 1 h, the PDMS was drawn into the valley of sawtoothed patterns by capillary action, and followed by oxygen plasma treatment to make topographic silicon oxide patterns (C). The self-assembly of PS-b-P2VP thin films can be guided with topographic patterns (D), subsequently, loading of gold precursor solution (E), oxygen plasma etching (F), and wet chemical etching processes were performed to fabricate highly ordered arrays of gold nanoparticles with removal of topographic patterns (G).
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Figure 2. SEM images of silicon oxide topographic patterns having different line widths. When (A) 0.1, (B) 0.2, (C) 0.3, and (D) 0.5 wt % PDMS solutions were spin-coated onto replicated PS patterns seen in Scheme 1B, thermally annealed, and followed by oxygen plasma exposure, amorphous silicon oxide patterns having line widths of 50, 120, 210, and 260 nm were prepared, respectively. In the inset of Figure 2D, the plot of crest width vs polymer concentration was shown. Scale bars are 500 nm.
in the inset. It should be noted that cross-linked PDMS patterns can be used at least 10 times to make the replicated PS patterns even at high temperature of 160 °C. The polymeric replica can be used as templates for fabrication of nanowires or scaffolds for incorporation of inorganic functional materials, if the polymeric patterns have functional groups.16 When a thin layer of PDMS, which has the properties of low Tg and low surface energy, was spin-coated on the replicated PS, annealed at 60 °C for 1 h, and followed by oxygen plasma treatment, amorphous silicon oxide topographic patterns were obtained as shown in Figure 1, parts C and D. Upon annealing at temperature well above the Tg of PDMS and well below the Tg of PS, a thin layer of PDMS was drawn into the valley of sawtoothed PS patterns by capillary action. When the films were exposed to oxygen plasma environments, the PDMS was transformed to amorphous silicon oxide while PS was completely degraded at the same conditions.25,29 When 0.2 and 0.3 wt % PDMS solutions in hexane was spincoated on replicated PS surfaces, thermally annealed, and followed by oxygen plasma exposure, topographic silicon oxide patterns having crest widths of 120 and 210 nm were fabricated, respectively, as shown in Figure 1, parts C and D. It should be noted that thickness control of PDMS layer enables us to tune the line widths of topographic silicon patterns. Cross-sectional line scans of SFM images seen in Figure 1, parts B and D, show that replicated PS patterns follow the same sawtoothed shape as that of original glass patterns, while silicon oxide patterns were seen as a trapezoidal structure due to the volume contraction which (29) Brinkmann, M.; Chan, V. Z.-H.; Thomas, E. L. Chem. Mater. 2001, 13, 967.
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would take place during the conversion of PDMS to silicon oxide under oxygen plasma exposure (inset of Figure 1, parts B and D). As the PDMS films of various thicknesses were spin-coated onto replicated PS patterns, topographic silicon oxide patterns having a rich variety of crest widths were fabricated. When the 0.1, 0.2, 0.3, and 0.5 wt % PDMS solutions were used to spin-coat on the PS patterns, topographic patterns having crest widths of 50, 120, 210, and 260 nm were obtained (Figure 2). In the inset of Figure 2D, the plot of crest width vs concentration of PDMS solutions was seen. As the polymer concentration increases, crest widths of silicon oxide were linearly increased to ∼200 nm, but they were not much changed at higher concentration over 0.4 wt %. When more concentrated PDMS solution over 0.5 wt % were used to fabricate silicon oxide patterns, welldefined topographic patterns were not obtained due to the overloading of PDMS to replicated PS surfaces. As-prepared topographic silicon oxide patterns can be used to direct the self-assembly of BCP thin films. PS-b-P2VP (MnPS = 125 kg/mol, MnP2VP = 58.5 kg/mol, Mw/Mn = 1.05) copolymer was used in this study. The PS-b-P2VP copolymer was dissolved in toluene at room temperature for 12 h to yield a 0.5 wt % polymer solution. Toluene is selective solvent for PS, but a nonsolvent for P2VP, where spherical micelles consisting of PS corona and P2VP core formed.14,30 The PS-b-P2VP thin films were exposed to toluene vapor to induce enough mobility of polymer and allow near-perfect hexagonal arrays to occur. When spin-coated PS-b-P2VP thin films were annealed in toluene vapor (30) Riess, G. Prog. Polym. Sci. 2003, 28, 1107.
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Figure 3. SFM (height modes, A and C; phase modes, B and D) images of highly ordered PS-b-P2VP micellar arrays annealed in toluene vapor on topographic silicon oxide patterns. When the PS-b-P2VP thin films were annealed in solvent vapor onto two different topographic patterns having line widths of 120 nm (A, B) and 210 nm (C, D), the BCP self-assembly was guided along with topographic patterns. In the insets, Fourier transforms show characteristics of long-range order as the evidence of six diffraction spots. Scale bars are 200 nm.
for 4 h at room temperature in a closed vessel, highly ordered spherical micellar structures were developed as shown in Figure 3. After annealing, a monolayer of a PS shell around a P2VP core formed within the troughs have an average center-to-center √ distance, p, of 61 nm and a spacing, d, of 53 nm (d = 3p/2) between the rows of spherical micelles in a hexagonally closepacked two-dimensional array. These results show that the topographic silicon oxide patterns are efficient in promoting long-range order of PS-b-P2VP micellar arrays. Depending on the line width of crest and crest-to-trough depth in the topographic patterns, surface morphologies of solvent annealed PS-bP2VP thin films were different. The wetting layer of ∼12 nm in the crest and highly ordered micellar arrays in the trough were seen in PS-b-P2VP films annealed onto silicon oxide patterns having crest width of 120 nm (Figure 3, parts A and B). As the crest width increases, micellar structures are covered over entire regions of topographic patterns with preserving the high degree of lateral order of PS-b-P2VP micelles as shown in Figure 3, parts C and D. Fourier transform of panels A and C show the characteristics of the long-range order in the inset of Figure 3, parts A and C. When the well-developed PS-b-P2VP films were immersed in gold salt solution for 2 min to let the gold precursor complex with P2VP chains, followed by oxygen plasma exposure for 30 min, highly ordered arrays of gold nanoparticles were obtained as shown in Figure 4. To investigate the effect of topographic patterns on the self-assembly of BCP thin films, the PS-b-P2VP films spin-coated onto flat silicon (Figure 4A) and topographic Langmuir 2010, 26(10), 7451–7457
surfaces (Figure 4B-D) were annealed in toluene vapor for 4 h. An average separation distances and an average sizes of gold nanopartices for all samples are the same except for sample developed on the pattern having wide crests seen in Figure 4D. A significant difference in lateral ordering of gold nanoparticles was shown between flat silicon surfaces and topographically patterned surfaces as seen Fourier transform patterns in the inset. As an important point, the number of rows in the gold nanoparticles prepared from spherical micellar arrays is determined by the trough width. The gold arrays with N rows form for a confinement width of trough W where (N - 0.5)d < W < (N þ 0.5)d in the systems seen in Figure 4. It should be noted that the periodicities of spherical micellar thin films is compressed or expanded to adjust within the trough, depending on the widths of troughs. These results are in agreement with previous results using the sphere or cylinder-forming block copolymers.31-33 Another interesting phenomena is to control the crest widths of silicon oxide topographic patterns which, in turn, leads to multiple pathways for tuning the spatial distributions of gold nanoparticles on crest and/or in the trough of the topographic patterns. Gold nanoparticles were located in the trough of the patterns having narrow crest widths, 50 and 120 nm (Figure 4B, 4C), while (31) Cheng, J. Y.; Mayes, A. M.; Ross, C. A. Nat. Mater. 2004, 3, 823. (32) Xiao, S.; Yang, X.; Edwards, E. W.; La, Y.-H.; Nealey, P. F. Nanotechnology 2005, 16, S324. (33) Park, S.; Kim, B.; Yavuzcetin, O.; Tuominen, M. T.; Russell, T. P. ACS Nano 2008, 2, 1363.
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Figure 4. SEM images of arrays of gold nanoparticles prepared from PS-b-P2VP micellar films on flat silicon wafer (A), and on topographic patterns having line widths of 50 nm (B), 120 nm (C), and 260 nm (D). When the PS-b-P2VP films were immersed in gold precursor solution for 2 min, and followed by oxygen plasma treatment, arrays of gold nanoparticles were obtained with complete degradation of polymers. In the insets, Fourier transforms indicate that topographic patterns can direct the PS-b-P2VP micellar arrays with high degree of lateral order.
Figure 5. SEM images of highly ordered arrays of gold nanoparticles obtained after removal of silicon oxide topographic patterns. (A) When the films with arrays of gold nanopaticles were immersed in dilute HF solution, silicon oxide topographic patterns were selectively dissolved, while the gold nanoparticles were left behind as seen in part A. (B) When gold films coated with platinum layer of ∼7 nm were immersed in HF solution, the areas having silicon oxide topography were preferentially degraded.
gold nanoparticles were seen in both crest and trough regions in the case of system having wide crest width of 260 nm (Figure 4D). When the PS-b-P2VP thin films were spin-coated on topographic patterns with wide crest widths, immediately after spin-coating, polymers on the crest were partially drawn into the troughs due to the capillary actions as seen in the previous results.33 After solvent 7456 DOI: 10.1021/la904160y
annealing, polymers on the crests further moved to the trough regions, and the residual polymers formed larger spherical micelles due to the dewetting, as seen in the Fourier transform of the inset (Figure 4D). That is reason why sizes of gold nanoparticles on crest and troughs seen in Figure 4D are different. As the line width of the topographic pattern decreases, small amounts of Langmuir 2010, 26(10), 7451–7457
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polymers are left on the crests, and after solvent annealing, larger gold nanoparticles formed in the very narrow regions, as seen in Figure 4C. In particular, no gold nanoparticles were seen in the topographic patterns having crest width of 50 nm due to the existence of only wetting layer. It should be noted that arrays of highly ordered gold nanoparticles can be obtained by topographic patterns, and also arrays of nanoparticles having different separation distances can be fabricated by controlling the crest widths of topographic patterns. Line widths of topographic silicon patterns were easily tuned by controlling the thicknesses of PDMS layer on the same PS sawtoothed patterns. Topographically patterned substrates have been used to direct the self-assembly of BCP thin films, from which can be prepared laterally ordered, device-oriented nanostructured materials. However, the topographic patterns which can guide the BCP microdomains permanently remain at the substrate surface, thus, may limit any further applications like integrated circuits.27 Topographic silicon oxide patterns, used in this study, can easily be removed from wet etching process by immersing the patterns in dilute HF solutions. Figure 5A shows the SEM image of arrays of gold nanoparticles obtained after removing silicon oxide topography by immersing it in 5 wt % HF solution for 2 min. Silicon oxide was preferentially dissolved in HF, while the gold nanoparticles left behind.34 For the visual aid, the dotted line which indicates the original crest width of 120 nm, was seen in Figure 5A. It should be noted that disposable topographic silicon oxide patterns can be used to direct the arrays of gold nanoparticles, and then removed the topography of prepatterned substrates by wet etching process without changing the lateral (34) Park, S.; Wang, J.-Y.; Kim, B.; Xu, J.; Russell, T. P. ACS Nano 2008, 2, 766.
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order of gold nanoparticles. Even though thin layer of platinum (∼7 nm) was coated on gold nanoparticles and topographic silicon oxide, the topographic patterns can be removed in HF solution as shown in Figure 5B.
Conclusions In summary, we demonstrated the fabrication of disposable topographic silicon oxide from sawtoothed glass patterns and their applications to preparation of BCP template and arrays of gold nanoparticles with high degree of lateral order. Silicon oxide patterns having various crest widths were fabricated via oxygen plasma treatment of thin layer of PDMS spin-coated on replicated PS patterns. Topographic patterns directed the self-assembly of BCP thin films via solvent annealing process. Subsequently, highly ordered BCP thin films were immersed in gold salt solution to let gold precursor complex with functional groups, and followed by plasma etching, to fabricate the arrays of highly ordered gold nanoparticles. Controlling the thickness of PDMS layers on replicated PS surfaces and subsequent oxygen plasma exposure enable us to tune the line widths of silicon oxide patterns. In the final step, topographic silicon oxide was selectively dissolved in dilute HF solution, while the nanoparticles left behind. Disposable topographic patterns are very useful for guiding the organic and/or inorganic nanostructured materials with high degree of lateral order, after that, the topography of patterns can be removed via wet etching process, if it is not necessary. Acknowledgment. This work was supported by WCU (R312008-000-20012-0) programs.
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