pubs.acs.org/Langmuir © 2010 American Chemical Society
Nanometer-Sized Domains in Langmuir-Blodgett Films for Patterning SiO2 Sho Kataoka,* Yasutaka Takeuchi, and Akira Endo* National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received March 11, 2010. Revised Manuscript Received March 31, 2010 In this study, we prepared Langmuir-Blodgett films with domains ranging from 20 to 100 nm in size by using perfluorinated fatty acids. The domain size of the obtained LB films is markedly smaller than the ordinary domain size of hydrocarbons and fluorocarbons on the micrometer scale. The domains were prepared by controlling their growth through the addition of 2-propanol to the subphase of Langmuir monolayers. Furthermore, the prepared domains in the LB films were used as templates for patterning SiO2 films. The obtained SiO2 films have completely negative structures compared with those of the domains in the LB films.
Introduction Surface patterning is an important technique in various research areas including microelectronics and microanalytical devices. Numerous attempts have been made to create nanometer-scale patterned structures.1-6 Recently, the LangmuirBlodgett (LB) technique has been employed as a practical method for surface patterning over large areas.7 Although the LB technique is known as an organic monolayer coating method, lateral heterogeneity (domain structure) is often created owing to the phase separation of components.8,9 Such domains often have distinct shapes, which can be exploited for lateral patterning.10-13 However, because the domain size of LB films is generally greater than a few micrometers, this technique cannot be directly employed for nanometer-scale patterning. Over the last two decades, the shape and organization of the domains in Langmuir monolayers and LB films have been widely investigated using microscopy techniques.8,9,14-16 McConnell suggested the principle of the equilibrium domain size as follows9 Req
e3 δ λ ¼ exp 2 4 μ
ð1Þ
*Corresponding authors. Tel: þ81-29-861-4587. Fax: þ81-29-861-4660. E-mail:
[email protected],
[email protected]. (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (2) Gates, B. D.; Xu, Q. B.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1171–1196. (3) Kim, S. O.; Solak, H. H.; Stoykovich, M. P.; Ferrier, N. J.; de Pablo, J. J.; Nealey, P. F. Nature 2003, 424, 411–414. (4) Kadota, S.; Aoki, K.; Nagano, S.; Seki, T. J. Am. Chem. Soc. 2005, 127, 8266–8267. (5) Oishi, Y.; Kato, T.; Narita, T.; Ariga, K.; Kunitake, T. Langmuir 2008, 24, 1682–1685. (6) Liao, W. S.; Yang, T. L.; Castellana, E. T.; Kataoka, S.; Cremer, P. S. Adv. Mater. 2006, 18, 2240–2243. (7) Chen, X. D.; Lenhert, S.; Hirtz, M.; Lu, N.; Fuchs, H.; Chi, L. F. Acc. Chem. Res. 2007, 40, 393–401. (8) Mohwald, H. Annu. Rev. Phys. Chem. 1990, 41, 441–476. (9) McConnell, H. M. Annu. Rev. Phys. Chem. 1991, 42, 171–195. (10) Duschl, C.; Liley, M.; Corradin, G.; Vogel, H. Biophys. J. 1994, 67, 1229– 1237. (11) Ge, S. R.; Takahara, A.; Kajiyama, T. Langmuir 1995, 11, 1341–1346. (12) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173–175. (13) Matsumoto, M.; Tanaka, K.; Azumi, R.; Kondo, Y.; Yoshino, N. Langmuir 2003, 19, 2802–2807. (14) Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Science 1993, 259, 213–216. (15) Keller, S. L.; McConnell, H. M. Phys. Rev. Lett. 1999, 82, 1602–1605. (16) Khattari, Z.; Fischer, T. M. J. Phys. Chem. B 2002, 106, 1677–1683.
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where δ is the distance between a domain and its surroundings, λ is the line tension, and μ is the difference in dipole density. Fluorinated surfactants have commonly been used to reduce the domain size because of the low polarizability of their carbonfluorine bonds, which decreases the λ value.17-19 For the same reason, ion species are also added to the subphase of Langmuir monolayers to increase the μ value. However, in either case, the domains are still larger than 100 nm. Only a few studies have reported the successful formation of nanometer-sized domains in LB films.20-24 These studies used partially fluorinated surfactants, which are specially designed to have unique functions.19 In this study, we use commercially available fully fluorinated surfactants and create nanometer-scale domains in LB films by controlling their domain growth. The obtained LB films are further employed as templates for fabricating SiO2 thin films with nanometer-scale patterns.
Experimental Section Materials. All chemicals were used as received. Ethanol, 2-propanol, hexane, xylene, and 30% hydrogen peroxide were purchased from Kishida Chemical Co. (Osaka, Japan); tetrahydrofuran (THF) and magnesium chloride hexahydrate was purchased from Junsei Chemical Co. (Tokyo, Japan). Four perfluorocarboxylic acids were used as spreading monolayers. Perfluorododecanoic acid (C12F), perfluorotetradecanoic acid (C14F), and perfluorohexadecanoic acid (C16F) were purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan); perfluorooctadecanoic acid (C18F) was purchased from Apollo Scientific Ltd. (Cheshire, U.K.). The perfluorocarboxylic acids were dissolved in a hexane/THF mixture (9:1 v/v) to prepare 1 mg/ mL stock solutions. Perhydropolysilazane (PHPS) was purchased from AZ Electronic Materials (Aqua Micah NN110-20, Shizuoka, (17) Barriet, D.; Lee, T. R. Curr. Opin. Colloid Interface Sci. 2003, 8, 236–242. (18) Krafft, M. P.; Goldmann, M. Curr. Opin. Colloid Interface Sci. 2003, 8, 243–250. (19) Krafft, M. P.; Riess, J. G. Chem. Rev. 2009, 109, 1714–1792. (20) Kato, T.; Kameyama, M.; Ehara, M.; Iimura, K. Langmuir 1998, 14, 1786– 1798. (21) Overney, R. M.; Meyer, E.; Frommer, J.; Guntherodt, H. J.; Fujihira, M.; Takano, H.; Gotoh, Y. Langmuir 1994, 10, 1281–1286. (22) Gamboa, A. L. S.; Filipe, E. J. M.; Brogueira, P. Nano Lett. 2002, 2, 1083– 1086. (23) Zhang, G. F.; Marie, P.; Maalourn, M.; Muller, P.; Benoit, N.; Krafft, M. P. J. Am. Chem. Soc. 2005, 127, 10412–10419. (24) Matsumoto, M.; Watanabe, S.; Tanaka, K.; Kimura, H.; Kasahara, M.; Shibata, H.; Azumi, R.; Sakai, H.; Abe, M.; Kondo, Y.; Yoshino, N. Adv. Mater. 2007, 19, 3668–3671.
Published on Web 04/06/2010
DOI: 10.1021/la100998h
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Letter Japan) and was used without further purification. Milli-Q water (18 MΩ cm) was used to prepare an aqueous solution (Millipore, Billerica, MA). Freshly cleaved mica sheets (SPI Supplies, West Chester, PA) were used as substrates. LB Film Deposition. LB films were prepared using a KSV Langmuir trough system (KSV Minitrough/2000, KSV Instruments, Helsinki, Finland). A mixture of 2-propanol and a 5 mM Mg2þ aqueous solution (mole fraction of 2-propanol, x = 0.0048) was used as a subphase. The temperature of the trough was controlled at 23 °C with a water circulation system (Julabo F12-ED, Seelbach, Germany). The desired aliquot of perfluorocarboxylic acid solution was spread on the subphase and was left for 15 min to allow the solvent to evaporate. The molecules spread on the subphase were compressed to the desired surface pressure at a barrier speed of 10 mm min-1. The coating surface pressure was 25 mN/m for C12F, 17 mN/m for C14F, 14 mN/m for C16F, and 9 mN/m for C18F. After the surface pressure was controlled, the Langmuir film was transferred onto a mica sheet, which had previously been immersed in the subphase, by withdrawing it at a speed of 2.0 mm min-1. (Surface pressure-area isotherms are provided in the Supporting Information.) AFM Observation. Atomic force microscope (AFM) images of the LB films were observed with a tapping-mode AFM equipped with Si-DF20 cantilevers (SPA-400, SII Nanotechnologies Inc., Japan). SiO2 Film Formation. The prepared LB film was kept in a closed vessel overnight. Subsequently, the LB films were immersed in a 0.4% PHPS xylene solution for 60 h. PHPS is generally used for protective SiO2 films because of its hardness.25,26 In this study, it is exploited for a filling material between domains. The films were washed three times with xylene to remove excess adsorbed PHPS and then immersed in a mixture of 30% hydrogen peroxide and ethanol (1:99 v/v) to convert the adsorbed PHPS to SiO2 and to remove the LB films. When necessary, the films were further washed with water and then exposed to oxygen plasma at a “mid” level for 10 min (PDC-32G plasma cleaner, Harrick Scientific, Ossining, NY).
Results and Discussion AFM measurements revealed that the prepared LB films contained domains with distinct shapes ranging in size from 20 to 100 nm (Figure 1). Domains with a mean size of 20 nm are loosely packed in the C12F film (Figure 1a). Clear polygonal domains with sizes of 33 and 58 nm are closely packed in the C14F and C16F films (Figure 1b,c). Hexagonal domains with a size of 96 nm are aligned in an orderly way in the C18F film (Figure 1d). This domain shape is clearly different from that in the C14F and C16F films (Figure 1b,c). It is known that hexagonal domains can be formed through a phase transition from small circular domains.9 Presumably, the phase transition occurred in the C18F film because its long carbon chain shifted the equilibrium of the domain shape. Meanwhile, the coating surface pressure did not affect the domain size and shape but only the number density up to a critical point in this system. This coincides with previous theoretical studies.9 As the number density of domains increases, one could expect that domains are ideally aligned in a hexagonal lattice structure because of their electrostatic repulsion. Although the polygonal domains in the C14F film or the C16F film were not aligned in a hexagonal packing arrangement (Figure 1b,c), the hexagonal domains in the C18F film formed a hexagonal lattice structure (Figure 1d). Importantly, the domain size is markedly smaller than that of ordinary circular domains made of hydrocarbon or perfluorocarbon surfactants spread on aqueous (25) Saito, R.; Kobayashi, S. I.; Hayashi, H.; Shimo, T. J. Appl. Polym. Sci. 2007, 104, 3388–3395. (26) Yamano, A.; Kozuka, H. J. Phys. Chem. B 2009, 113, 5769–5776.
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Figure 1. AFM micrographs of LB films on mica substrates: (a) C12F, (b) C14F, (c) C16F, and (d) C18F. The image size is 1 μm 1 μm.
solutions. Qaqish et al. undertook a series of studies of C14F monolayers mixed with stearic or arachidic acid on aqueous solutions that revealed organized domains larger than a few micrometers.27,28 Therefore, the addition of 2-propanol appears to be the key to forming nanometer-sized domains. We also performed experiments with various 2-propanol mole fractions (Supporting Information).29 Whereas polygonal domains existed even at a mole fraction of x e 0.0024, some domains were flocculated up to several hundred nanometers. However, domains had a fractal shape at a mole fraction of x g 0.0072. It is known that small circular domains are first created in Langmuir monolayers (i.e., nucleation) and then grow via phase transition and/or flocculation.9 Because a 2-propanol/water mixture forms a Gibbs monolayer, the adsorption of 2-propanol molecules at the interface varies considerably at low mole fractions (x < 0.025).30 Thus, the flocculation of domains is prevented only when enough 2-propanol molecules exist at the interface (x > 0.0024). However, when the surface tension is too low (x > 0.0072), small polygonal domains cannot be formed. In addition, when the perfluorinated acids were spread on a 2-butanol/water Gibbs monolayer (x = 0.004), similar polygonal domains were observed. When the acids mixed with 1-hexanol (molar ratio 1:1) were spread on a 5 mM Mg2þ aqueous solution, small polygonal domains were observed in the C14F film whereas flocculated domains were observed in the C16F film. These supplemental results support the fact that the addition of alcohols hindered the further growth of domains under certain conditions. The equilibrium domain size in Figure 1 gradually increases with the carbon chain length. To assist in its interpretation, the (27) Qaqish, S. E.; Paige, M. F. Langmuir 2007, 23, 2582–2587. (28) Qaqish, S. E.; Urquhart, S. G.; Lanke, U.; Brunet, S. M. K.; Paige, M. F. Langmuir 2009, 25, 7401–7409. (29) When ion species other than Mg2þ were added to the subphase (i.e., Ca2þ and La3þ), the LB films contained large domains. (30) Kataoka, S.; Cremer, P. S. J. Am. Chem. Soc. 2006, 128, 5516–5522.
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Figure 2. Logarithmic mean domain size (Dmean) plotted against the carbon chain length. The dashed line is a guide to the eye.
logarithmic domain size is plotted against the chain length in Figure 2. The logarithmic domain size increases linearly with the chain length. This is in good agreement with McConnell’s principle.9 From eq 1, the logarithmic equilibrium domain size is directly proportional to the line tension, and the line tension is expected to be proportional to the chain length.31,32 Importantly, the result suggests that the domain size in this system can be altered by changing the carbon chain length. The C16F and C18F LB films with ordered domain structures were further exploited as templates for patterning SiO2. AFM images of the obtained SiO2 films show that they have ordered netlike structures (Figure 3). Importantly, both SiO2 films (Figure 3a,b) have completely negative structures compared with those of the C16F and C18F domains (Figure 1c,d). The film made with C16F (Figure 3a) contains holes with a mean size of 43 nm, which is smaller than the original domain size (58 nm). The film is about 5 nm high, which exceeds the original domain height. This is because of the increased volume of SiO2 converted from adsorbed PHPS (-SiH2NH-)n. Indeed, the SiO2 film has the appearance of a string of beads owing to its increased volume. This SiO2 film is slightly rough because PHPS was unevenly adsorbed outside the domains. The film made with C18F (Figure 3b) shows a clear hexagonal pattern on the substrate. The mean hole size is 72 nm, which is also smaller than the original domain size. These results indicate that PHPS was selectively adsorbed outside the domains. Although liquid-expanded monolayers of perfluorinated surfactants may exist outside the domains,33 they seemingly had no effect on PHPS adsorption.34 (31) Iimura, K.; Shiraku, T.; Kato, T. Langmuir 2002, 18, 10183–10190. (32) Melzer, V.; Vollhardt, D.; Brezesinski, G.; Mohwald, H. Thin Solid Films 1998, 327, 857–860. (33) Chi, L. F.; Gleiche, M.; Fuchs, H. Langmuir 1998, 14, 875–879. (34) Hao, J. Y.; Lu, N.; Xu, H. B.; Wang, W. T.; Gao, L. G.; Chi, L. F. Chem. Mater. 2009, 21, 1802–1805.
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Letter
Figure 3. SiO2 patterned films made with nanostructured LB films as templates: (a) C16F and (b) C18F. The image size is 1 μm 1 μm.
It should be noted that LB films must remain attached to substrates as templates during the selective deposition of SiO2 precursors. In our system, PHPS dissolved in xylene did not damage the LB films ionically bound to the substrate. It is demonstrated that the LB films can serve as template materials. In this study, we employed nanometer-sized perfluorinated surfactant domains as a template for making patterned SiO2 thin films. In fact, other metal oxide films can also be made using the templates (Supporting Information). In recent reports, several attempts have been made to fabricate metal oxide films with patterned structures smaller than 100 nm by using block copolymers and surfactant micelles as templates.35-38 These films can be employed as catalyst supports, adsorbents, membranes, and sensors; however, the shape and size of these patterned films are limited. We confirmed that the present method using domains in LB films can be exploited for this purpose. Acknowledgment. This work was financially supported by an AIST internal grant (Research Institute for Innovation in Sustainable Chemistry). We thank Dr. Toshiyuki Takagi (AIST) for fruitful discussions. Supporting Information Available: Additional AFM images of LB and metal oxide films and surface pressurearea isotherms. This material is available free of charge via the Internet at http://pubs.acs.org. (35) Trau, M.; Yao, N.; Kim, E.; Xia, Y.; Whitesides, G. M.; Aksay, I. A. Nature 1997, 390, 674–676. (36) Sanchez, C.; Boissiere, C.; Grosso, D.; Laberty, C.; Nicole, L. Chem. Mater. 2008, 20, 682–737. (37) Yamauchi, Y.; Nagaura, T.; Ishikawa, A.; Chikyow, T.; Inoue, S. J. Am. Chem. Soc. 2008, 130, 10165–10170. (38) Daiguji, H.; Tatsumi, N.; Kataoka, S.; Endo, A. Langmuir 2009, 25, 11221– 11224.
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