Article Cite This: Langmuir XXXX, XXX, XXX−XXX
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Formation of Concentric Silica Nanogrooves Guided by the Curved Surface of Silica Particles Shintaro Hara,† Keiya Hirota,‡ Yuka Tabe,§,∥ Hiroaki Wada,‡ Atsushi Shimojima,‡ and Kazuyuki Kuroda*,‡,∥ †
Department of Advanced Science and Engineering, Faculty of Science and Engineering, ‡Department of Applied Chemistry, Faculty of Science and Engineering, and §Department of Applied Physics, Faculty of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan ∥ Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan S Supporting Information *
ABSTRACT: The flexible control of nanopatterns by a bottom-up process at the nanometer scale is essential for nanofabrication with a finer pitch. We have previously reported that for the fabrication of linear nanopatterns with sub-5 nm periodicity on Si substrates the outermost surfaces of assembled micelles facing the substrates can be replicated with soluble silicate species generated from the Si substrates under basic conditions. In this study, concentrically arranged nanogrooves with a sub-5 nm periodicity were prepared on Si substrates by replicating the outermost surfaces of bent micelles guided by silica particles. The Si substrates, where silica particles and surfactants films were deposited, were exposed to an NH3−water vapor mixture. During the vapor treatment, cylindrical micelles became arranged in concentric patterns centered on the silica particles, and their outermost surfaces facing the substrates were replicated by soluble silicate species on the Si substrates. The thinness of the surfactant film on the substrate is crucial for the formation of concentric silica nanogrooves because the out-of-plane orientations of the micelles are suppressed at the interface. Surprisingly, the domains of the concentric silica nanogrooves spread to much larger areas than the maximum cross-sectional areas of the particles, and the size of the domains increased linearly with the radii of the particles. The extension of concentric nanogrooves is discussed on the basis of the orientational elastic energies of the micelles around one silica particle. This study of the formation of bent nanogrooves guided by the outlines of readily deposited nanoscale objects provides a new nanostructure-guiding process.
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INTRODUCTION The development of nanopatterning techniques is crucial for the further miniaturization of electronic devices. The fabrication of nanopatterns with single nanometer-scale periodicity is a hot topic in nanoscale interface science.1,2 Nanopatterning defined by molecular self-assembly is one of the most promising alternatives to conventional lithographical methods because of the low capital cost and potential for creating sub-5 nm features.3 Techniques for flexibly controlling the sub-5 nm nanopatterns defined by self-assembled molecules, including bent and curved patterns, will be required because analogous studies are developing at a larger scale for the fabrication of nanodevices.4−10 In addition, concentric nanopatterns with sub-5 nm periodicity for use as templates for nanostructured noble metals have the potential to dramatically increase surface-enhanced Raman scattering.11,12 The concentric nanopatterns are reported to be used as a template for nanostructured metals applicable to a microzone plate in soft Xray microscopy.13,14 Self-assembled block copolymers are known to bend along the curved surfaces of spheres,15,16 and the surrounding nanostructures are expected to be elastically deformed to © XXXX American Chemical Society
form concentric structures in the same way that the orientation directions of liquid crystals are distorted by spheres.17 However, the formation of continuously and concentrically curved nanopatterns of self-assembled block copolymers or micelletemplated mesostructures on substrates centered on spheres has never been achieved. A possible reason is the random orientation of nanostructured cylinders composed of selfassembled micelles or block copolymers on spheres (Figure 1a), which is partly supported by the formation of multiple liquid-crystal domains on spheres.18 The unidirectional orientation on spheres is required for using spheres as a guide (Figure 1b). Certainly, self-assembled block copolymers and micelles are bent along the walls of objects with confined spaces, including flat substrates with circular trenches,6−8 porous anodic aluminum oxide with cylindrical spaces,19−21 and three-dimensionally ordered macroporous materials (3DOM),22 and such copolymers and micelles are concentrically arranged in the confined spaces. These methods, however, Received: October 31, 2017 Revised: December 20, 2017 Published: December 22, 2017 A
DOI: 10.1021/acs.langmuir.7b03777 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
outermost surfaces of the micelles facing the substrate were replicated with soluble silicate species generated from the Si substrates under basic conditions. When the LLC layers were sufficiently thin, the horizontal orientation of micelles was promoted, and micelles were concentrically arranged (Scheme 1d), reflecting the shape of the inner micelles arranged unidirectionally on spheres (Figure 1b). The substrate was washed to remove the surfactants (Scheme 1e), and the silica particles were removed without the destruction of the nanogrooves by removing the silica particles with an adhesive tape. The relationship between the domain sizes of concentric nanogrooves and particle radii was also investigated from experimental and theoretical perspectives.
Figure 1. Orientation directions of nanostructured cylinders composed of self-assembled micelles or block copolymers on spheres. (a) Random orientation and (b) unidirectional orientation.
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require elaborate processes to fabricate the confined spaces and cannot be applied to flat surfaces; in contrast, for flat surfaces, nanoparticles can be readily deposited and removed, which ensures the feasibility of the process and the flatness of the substrate. The preparation of mesoporous thin films on substrates with monolayer of spheres was reported, but the concentric arrangement of mesochannels centered on spheres was not confirmed although some mesochannels were aligned along to the surfaces of spheres.23 Recently, we have reported a simple method for the fabrication of nanogrooves with sub-5 nm periodicity on a Si substrate using lyotropic liquid crystals (LLCs) consisting of cylindrical micelles as a nanoimprint mold.24 The interspaces between the Si substrate and LLCs were infilled with silicate species generated from the Si substrate by treatment with an NH3−water vapor mixture. The alignment of the nanogrooves was realized by confining the lyotropic liquid crystals in the micrometer-scale grooves. However, the report has focused on the formation of linear nanogrooves. Here, we report the formation of concentric nanogrooves on Si substrates centered on silica particles, as schematically shown in Scheme 1. After the deposition of the silica particles on a Si substrate, a cetyltrimethylammonium chloride (CTAC) film was deposited by spin-coating (Scheme 1a−c). When the substrate was exposed to an NH3−water vapor mixture, the hygroscopic CTAC molecules, being initially present as a lamellar phase, absorbed water, and this phase was converted to a 2D-hexagonal phase of lyotropic liquid crystals (LLC) (Scheme 1d). The micelles in contact with the silica particles were bent along the curved surfaces of the silica particles (Scheme 1d). After the formation of concentric micelles, the
EXPERIMENTAL METHODS
Materials. Cetyltrimethylammonium chloride (CTAC), cetyltrimethylammonium bromide (CTAB), lauryltrimethylammonium chloride, aqueous ammonia, NaOH, tetraethoxysilane (TEOS) (all from Wako Pure Chemical Industries, Ltd.), ethanol (Kanto Chemical Co.), and fumed silica (Aldrich) were used as received. Si(100) substrates were purchased from Silicon Technology Co., Ltd. Semicoclean 23 was used to clean the substrate and improve the wettability of the surfaces of the Si substrates and was purchased from Furuuchi Chemical Corporation. Semicoclean 23 is an alkaline aqueous solution of tetramethylammonium hydroxide ( x is 22 nm. (34) Taratuta, V. G.; Lonberg, F.; Meyer, R. B. Anisotropic mechanical properties of a polymer nematic liquid crystal. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 37, 1831−1834. (35) Lee, S.-D.; Meyer, R. B. Crossover Behavior of the Elastic Coefficients and Viscosities of a Polymer Nematic Liquid Crystal. Phys. Rev. Lett. 1988, 61, 2217−2220. (36) Hakemi, H.; Roggero, A. Anisotropy of turbidity and elastic constants of liquid-crystalline polypeptide solutions. Polymer 1990, 31, 84−91. (37) Jung, C.; Kirstein, J.; Platschek, B.; Bein, T.; Budde, M.; Frank, I.; Müllen, K.; Michaelis, J.; Bräuchle, C. Diffusion of Oriented Single Molecules with Switchable Mobility in Networks of Long Unidimensional Nanochannels. J. Am. Chem. Soc. 2008, 130, 1638−1648. (38) Kruk, M.; Jaroniec, M.; Sakamoto, Y.; Terasaki, O.; Ryoo, R.; Ko, C. H. Determination of Pore Size and Pore Wall Structure of I
DOI: 10.1021/acs.langmuir.7b03777 Langmuir XXXX, XXX, XXX−XXX
