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Self-Controlled Growth of Silica Thin Films for Nanopatterning Gyoujin Cho,*,† Jongkwan Jang,† Sunggi Jung,† Il-Shik Moon,† Jae-Suk Lee,‡ Young-Sun Cho,‡ Bing M. Fung,§ Wei-Li Yuan,| and Edgar A. O’Rear| Department of Chemical Engineering and Nanotechnology Center, Sunchon National University, 315 Maegok Sunchon, Chonnam 540-742 Korea; Department of Material Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), Kwangju 506-712 Korea; Department of Chemistry and Biochemistry, The University of Oklahoma, Norman, Oklahoma 73019; and Department of Chemical Engineering and Materials Science, The University of Oklahoma, Norman, Oklahoma 73019 Received November 20, 2001. In Final Form: March 3, 2002 We report a new method to control both the nucleation and growth of SiO2 to form nanopatterns width in an oil phase, using poly(styrene-b-4-vinylpyridine) (PS-b-4PV) as a template. The resulting SiO2 can be selectively grown on protonated 4VP blocks up to a height of 8 nm by 60 nm.
In this letter, we would like to report the results of using an interfacial phenomenon to control both the nucleation and the growth of nanopatterns of SiO2. The preparation of inorganic materials with 3-dimensional and nanosized periodic patterns is one of the central techniques in designing miniature devices.1 A simple periodic pattern at micrometer-size is often prepared using conventional photolithography.2 On the other hand, complex materials with periodic patterns having sizes below 0.1 µm cannot be simply constructed using conventional photolithography. Recently, an alternative approach to attain the complex nanostructures has been developed by using microphase-separated block copolymer as a template.3,4 The block copolymer can self-assemble to produce periodic nanodomains, such as lamellar, hexagonal, and sphere phases.5 These self-assembled structures of 10-100 nm in length scale are ideally suited to control the nucleation, growth, and morphology of inorganic materials6 and are used for nanoscale patterning if the orientations of the self-assembled structures can be controlled normal to the surface.3,4 To do this, regioselective nucleation is regulated by the interfacial phenomenon †
Sunchon National University. Kwangju Institute of Science and Technology (K-JIST). Department of Chemistry and Biochemistry, The University of Oklahoma. | Department of Chemical Engineering and Materials Science, The University of Oklahoma. ‡ §
(1) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. J. Vac. Sci. Technol., B 1996, 14, 4129. (2) Moreau, M. Semiconductor Lithography: Principles and Materials, Plersume: New York, 1988. (3) Meiners, J. C.; Ritzi, E. A.; Mlynek, J.; Krausch, G. J. Appl. Phys. 1996, 80, 2224. Lee, T.; Yao, N.; Aksay, I. Langmuir 1997, 13, 3866. Tsutsumi, K.; Funaki, Y.; Hirokawa, Y.; Hashimoto, T. Langmuir 1999, 15, 5200. Zehner, R. W.; Sita, L. R. Langmuir 1999, 15, 6139. Rosa, C. D.; Park, C.; Thomas, E.; Lotz, B. Nature (London) 2000, 405, 433. (4) Liu. G.; Ding, J.; Guo, A.; Herfort, M.; Bazett-Jones, D. Macromolecules 1997, 30, 1851. Liu, G.; Ding, J.; Hashimoto, T.; Kimishima, K.; Winnik, F. M.; Nigam, S. Chem. Mater. 1999, 11, 2233. ThurnAlbrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell, T. Adv. Mater. 2000, 12, 787. (5) Abetz, V.; Goldacker, T. Macromol. Rapid Commun. 2000, 21, 16 and references therein. (6) Swift, D. M.; Wheeler, A. P. J. Phycol. 1992, 28, 202. Wolber, P. K.; Warren, G. J. Trends Biochem. Sci. 1989, 14, 179. Mann, S.; Archibald, D. D.; Didymus, J. M.; Douglas, T.; Heywood, B. R.; Meldrum, F. C.; Reeves, N. J. Science 1993, 261, 1286.
between the reaction medium and different nanodomains of the block copolymer.6 Usually, heterogeneous nucleation is favored onto the nanodomains which have a lower interfacial energy. In fact, the interfacial phenomenon is the central principle in the biomineralization for the control of particle size, structure, morphology, aggregation, and crystalographic orientation of the inorganic phase.7,8 Although the use of block copolymers as templates was successful for two-dimensional nanopatterning, we found that the influence of the structured interface on the surface morphology of the block copolymer becomes weaker as the structure builds up, so that the boundaries between patterns become gradually blurred as the thickness increased. To over come this problem, we have developed a new method to promote the nucleation and growth of SiO2 regioselectivly (Scheme 1). While we were preparing the manuscript, Russell and co-workers have reported a route to nanoscopic SiO2 posts using thin films of poly(styrene-b-methyl methacrylate) (PS-b-PMMA) as a template.9 In their method, PMMA cylinders in a PS matrix orienting normal to the surface was prepared using anchored random copolymers of styrene and methyl methacrylate. The oriented PMMA cylinders were then removed to generate nanopores after ultraviolet (UV) exposure and acetic acid rinsing. Finally, SiO2 posts were grown through the hydrolysis of SiCl4 in the nanopores. The method outlined here uses a different approach, namely utilizing an interface phenomenon of a block copolymer for the SiO2 nanopatternings. To do this, we prepared poly(styrene-b-4-vinylpyridine) (PS-b4PV) with Mw of 58 000 mol/g and PD of 1.1 using anionic polymerization.10 The ratio of PS/4PV was 0.95 calculated (7) Mann, S.; Webb, J.; Williams, R. J. P. Biomineralization: Chemical and Biochemical prospectives, VCH Verlagsgesellschaft: Weinheim, Germany, 1989. Lowenstam, H. A.; Weiner, S. On Biomineralization, Oxford University Press: Oxford, England, 1989. (8) Titiloye, J. O.; Parker, S. C.; Osguthorpe, D. J.; Mann, S. J. Chem. Soc., Chem. Commun. 1991, 1494. Heywood, B. R.; Mann, S. Langmuir 1992, 8, 1492. Weissbuch, I.; Addadi, L.; Lahav, M.; Leiserowitz, L. Science 1991, 253, 637. Addadi, L.; Berkovitch-Yellin, Z.; Weissbuch, I.; Mil, J. V.; Shimon, L. J. W.; Leiserowitz, L. Angew. Chem., Int. Ed. Engl. 1985, 24, 466. (9) Kim, H. C.; Jia, X.; Stafford, C. M.; Kim, D. H.; McCarthy, T. J.; Tuominen, M.; Hawker, C. J.; Russell, T. P. Adv. Mater. 2001, 13, 795. (10) Thurmond, K. B.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1997, 119, 6656.
10.1021/la015688n CCC: $22.00 © 2002 American Chemical Society Published on Web 04/02/2002
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Figure 1. TEM image of a PS-b-4PV film after staining (A) and AFM images of a PS-b-4PV film (B) before and (C) after acid treatment. Scheme 1. Descriptive Illustrations for the Growth of SiO2 on the Films of PS-b-4PV under Aqueous and Oil Phases
by integrating the 1H NMR peaks of PS and 4PV. The main reason for using PS-b-4PV is that the pyridine ring can be easily protonated, so that there is a large difference in the surface energy between PS and 4PV blocks. Furthermore, the protonated 4PV can act as an acid catalyst site for the hydrolysis of tetraethoxysilane (TEOS),11 so that SiO2 can nucleate onto only the 4-PV block and grow from the surface. After the regioselective nucleation, the direction of SiO2 growth can be controlled by using an oil phase instead of an aqueous medium. To regioselectively nucleate SiO2 for the nanopatternings, a heterogeneous surface exhibiting an ordered array of nanoscopic areas of different chemical compositions (PV and 4PV) should be formed on the surface. Therefore, a prerequisite for the nanopatterning process using block copolymer as a template is control over the orientation of the nanodomains. The orientations of nanodomains normal to the surface are often achieved by generating a neutral surface using random copolymers anchored to the substrate12 or casting the films with a thickness less than Lo (equilibrium period)12 or using a substrate with a low surface energy such as self-assembled hydrocarbon monolayer.13 In this work, a graphite plate was used as a substrate to attain the low surface energy. (11) Advanced contact angles for polystyrene and poly(4-vinylpyridine) films were 90 and 60° respectively. After acid treatement of poly(4-vinylpyridine) film, the advanced contact angle was 30° because of the formation of pyridinium salt. (12) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. J. Science 1997, 275, 1458. Huang, E.; Rockford, L.; Russell, T. P.; Hawker, C. J.; Mays, J. Nature 1998, 395, 757. (13) Heier, J.; Kramer, E. J.; Walheim, S.; Krausch, G. Macromolecules 1997, 30, 6610.
To examine the process of controlled nanopatterning, PS-b-4PV films were cast on either carbon-coated nickel TEM specimen grids or graphite plates (1 × 1 cm2) in the following way. First, the block copolymer was dissolved in methylethyl ketone (1 mg/mL). Then, the substrate was dipped into this solution for seconds. Finally, the solvent was evaporated under ambient condition, and the dried grid or plate was annealed under nitrogen at 150 °C for 3 h. The morphology of microphase-separated PSb-4PV on the grid was investigated using TEM. Figure 1A shows a plain TEM image of the resulting thin film after staining with iodine. The nanodomains of 4PV in the PSb-4PV appear as lamellar nanodomains with 30 nm interspacing.14 The thickness of a PS-b-4PV film on the surface of graphite was determined by using the R-step and contact AFM (Park Science AFM CP using silicon nitride tip with a spring constant of 0.05 N/m), and the value was found to be 20 nm. Figure 1B shows that the AFM image of the nanodomains is very similar to the TEM image, except that the surface nanodomains in the AFM image (with 50 nm interspacing) were slightly enlarged, probably due to the AFM tip artifact. Another possible reason for the difference in the domain sizes is the different properties of the substrate surfaces. Since the surface of 4PV has proton acceptor sites, they can be protonated by simply treating with 1 N HCl for 20 s. The acid-treated PS-b-4PV film was rinsed thoroughly with deionized water and dried at room temperature. As seen in the AFM image (Figure 1C), the nanodomains on (14) Heier, J.; Kramer, I. J.; Walheim, S.; Krausch, G. Macromolecules 1997, 30, 6610. Li, Z.; Zhao, W.; Liu, Y.; Rafailovich, M. H.; Sokolov, J. J. Am. Chem. Soc. 1996, 118, 10892.
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Figure 2. TEM images of SiO2 on the film of PS-b-4PV without staining after (A) 10, (B) 20, and (C) 300 min of immersion into aqueous solution for SiO2 growth.
Figure 3. TEM images of SiO2 on PS-b-4PV without staining after (A) 60 and (B) 300 min of immersion into an oil phase.
the film were swollen to 60 nm interspacings after the acid treatment. From the cross-sectional analysis of AFM images of Figure 1, parts B and C, the mean height of the protruded units (seemingly 4PV domains) was 1.93 nm before acid treatment and changed to about 2.47 nm after acid treatment, indicating that protonation of the pyridine units increases the protrusion by about 0.54 nm. A detailed investigation of the change of the surface nanodomains by acid treatment15 is beyond the scope of this letter, and the full characterization will be published separately. To deposit SiO2 posts onto the acid-treated PS-b-4PV films, a sample was immersed into either an aqueous or an oil phase containing 0.01 M TEOS. The immersion time ranged from 10 to 300 min, and the temperature was kept at 25 °C. After taking out the sample from the aqueous solution, it was rinsed with stirring deionized water for 5 min. For samples immersed in the silicone oil solution, n-hexane was used for rinsing. All of the SiO2-fabricated PS-b-4PV samples were investigated using TEM without staining. Parts A-c of Figure 2 show the TEM images of SiO2 nanopatterning grown in an aqueous phase with 10, 20, and 300 min growth times, respectively. It can be seen that SiO2 starts to nucleate following the nanodomains of 4PV after 10 min (Figure 2A), nucleates onto most of the 4PV nanodomains after 20 min (Figure 2B), and completely nucleates onto all of the 4PV nanodomains after 300 min (Figure 2C). The reason for the fast regioselective nucleation only onto the protonated hydrophilic 4PV block is that the interfacial energy between water and the nanodomains of protonated 4PV is lower than that for the PS nanodomains.16 However, because the heterogeneous nucleation of SiO2 is very fast in the aqueous phase, there is no favored growth direction to decrease the interfacial (15) Senshu, K.; Kobayashi, M.; Ikawa, N.; Yamashita, S.; Hirao, A.; Nakahama, S. Langmuir 1999, 15, 1763. (16) Rockford, L.; Liu, Y.; Mansky, P.; Russell, T. P. Phys. Rev. Lett. 1999, 82, 2602.
energy. Furthermore, at longer growth time, SiO2 can nucleate on the PS nanodomains because nucleation with less selectivity can occur for longer growth time. As a result, the patterning becomes blurred after 60 min of growth time and completely lost after 300 min (Figure 2C). In contrast, the initiation of regioselective nucleation in an oil phase required a much longer time (60 min; Figure 3A), but clear nanopatterns as sharp as the PS-b-PV template were preserved on all samples for growth times up to 300 min. For further characterization, SiO2 nanopatterns were also grown onto a graphite plate under the same conditions for AFM studies with contact mode. The AFM images (Figure 4, parts A-D) show that SiO2 nanopatterns were homogeneously grown, and the SiO2 nanopatterns are similar to those observed from TEM (Figure 3B). On the basis of the cross-sectional analysis of the AFM image for samples prepared with 300 min of growth (Figure 4D), the height of the SiO2 posts is estimated to be about 5-8 nm with 60 nm width. The remarkable difference in the nanopatterning growth of SiO2 in the two phases can be explained by the following consideration. Because protonated pyridine rings are hydrophilic, each nanodomain of 4PV may be covered by a very thin layer of water (Scheme 1), which does not mix with the surrounding medium in an oil phase. It may be compared to the liquid flux in both vapor-liquid-solid (VLS)17 and solution-liquid-solid (SLS)18 methods of crystal growth. The thin strips of water film can act as acid catalyst to let SiO2 nucleate regioselectively on the surface of 4PV block. After the nucleation, SiO2 can further grow only along the vertical direction because the thin water strips do not mix with oil and stay on top of the SiO2 columns. During the growth, more TEOS from the oil phase is slowly fed into the thin water strips and hydrolyze into SiO2, following only the patterns of the 4PV block in the (17) Morales, A. M.; Lieber, C. M. Science, 1998, 279, 208. (18) Trenfler, T. J.; Hickman, K. M.; Goel, S. C.; Viano, A. M.; Gibbons, P. C.; Buhro, W. E. Science 1995, 270, 1791.
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Figure 4. AFM images of SiO2 nanopatterns grown in an oil phase: (A) 10 × 10, (B) 5 × 5, and (C) 2 × 2 µm2. (D) Cross-sectional analysis of image C.
template. In other words, after the slow regioselective nucleation, the nucleated SiO2 posts can self-control to find the minimum interfacial energy and grow at a direction normal to the surface. In summary, we have demonstrated a new method to copy the nanostructures of the microphase-separated copolymer thin films to form silica thin films up to a height of 8 nm with 60 nm width. Detailed studies for morpho-
logical changes and growing mechanism relating to changes in interfacial tensions of the medium are being carried out. Acknowledgment. This work was supported by KOSEF (R03-2001-00024 to G.C.) and Ministry of Education (BK21 to J.-S.L.), for which we are grateful. LA015688N
