Fabrication of Uniaxially Aligned Silica Nanogrooves with Sub-5 nm

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Fabrication of Uniaxially Aligned Silica Nanogrooves with Sub-5 nm Periodicity on Centimeter-Scale Si Substrate Using Poly(dimethylsiloxane) Stamp Keiya Hirota, Shintaro Hara, Hiroaki Wada, Atsushi Shimojima, and Kazuyuki Kuroda ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07714 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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Fabrication of Uniaxially Aligned Silica Nanogrooves with Sub-5 nm Periodicity on Centimeter-Scale Si Substrate Using Poly(dimethylsiloxane) Stamp Keiya Hirota,† Shintaro Hara,‡ Hiroaki Wada,† Atsushi Shimojima,† and Kazuyuki Kuroda*,†,§

† Department

of Applied Chemistry, Faculty of Science and Engineering, Waseda

University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan

‡Department

of Advanced Science and Engineering, 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

ACS Paragon Plus Environment

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ABSTRACT: The large-area fabrication of aligned nanopatterns with sub-5 nm feature size is crucial for developing nanodevices. Highly ordered nanostructures fabricated through molecular self-assembly exhibit substantial potential for sub-5 nm patterning techniques. Previously, we had reported the fabrication of silica nanogrooves with sub-5 nm periodicity on a Si substrate by using the outermost surfaces of cylindrical surfactant micelles as a template. However, uniaxial alignment of nanogrooves on the entire substrate surface has not yet been achieved. In this study, uniaxially aligned silica nanogrooves were prepared on the entire surface of a Si substrate (2 × 2 cm) by utilizing a poly(dimethylsiloxane) (PDMS) stamp with a striped pattern. The PDMS stamp was placed on the surface of a surfactant thin film precoated on the substrate, although the stamp was not in direct contact with the substrate as in the case of the soft nanoimprint technique. The substrate was then exposed to water vapor, during which cylindrical micelles were aligned in the direction of the guide. Subsequently, by exposing the substrate to an NH3–water vapor mixture, the outermost surfaces of the aligned micelles facing the substrate were replicated with soluble silicate species. The formation of uniaxially aligned nanogrooves on the entire surface of the centimeter-


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scale substrate was verified by scanning electron microscopy observations and grazingincidence small-angle X-ray scattering analysis. Thus, herein we provide a simple largearea fabrication method for uniaxially aligned nanopattern with ultra-fine pitch.

KEYWORDS: liquid crystals, nanopatterning, nanoimprint, directed self-assembly, alignment control

The fabrication of nanopatterns with single nanometer-scale feature size in a large area is essential for developing electronic devices. The control of sub-5 nm nanopatterns is in demand in the fields of the chemistries of nanomaterials and interface.1–4 Nanopatterns with sub-5 nm feature size afford surface functions such as strong electric-field enhancement5,6 and quantum confinement.7 The centimeter-scale alignment of sub-5 nm patterns is necessary for the practical application of these functions and for their use as anisotropic surfaces to guide the alignment of soft materials like organic semiconductors8–10 and liquid crystals.8,9,11 In general, conventional photolithographic methods exhibit a resolution limit,12 and the fabrication of


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sub-10 nm patterns is challenging. Extreme-UV lithography13 and electron beam lithography14 have been developed to realize sub-5 nm patterns; however, these methods require excessively expensive processes. Therefore, a straightforward fabrication method of uniaxially aligned nanopatterns with sub-5 nm pitch is in demand. A bottom-up approach using directed self-assembly of block copolymers (BCPs)15 is expected for the next-generation nanolithography. Ordered nanostructures can be used as nanoscale templates or masks for pattern transfer to substrates through selective etching processes of a component of BCPs and replication processes.16–20 The reduction of the periodicity requires the preparation of remarkably well-designed BCPs with high-immiscibility of each segment and low-molecular weight; this hinders the realization of nanopatterns with sub-5 nm periodicity.21 In recent progress, the fabrication of thin films with sub-5 nm periodic nanostructure was demonstrated by the self-assembly of small liquid crystalline molecules.22–26 Highly ordered nanostructures with centimeter-scale alignment are induced by external fields,22,27,28 surface modification of substrates,29,30 confinement in topographically patterns,24,31 and photoalignment.32,33 In particular, patterned substrates prepared by top-down


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lithography are mainly used for guiding the self-assembly. This top-down control of the bottom-up self-assembly is effective for desired shaping or long-range alignment of nanostructures. However, in order to use these nanostructures as sub-5 nm scale templates or functional substrates, convex–concave nanopatterns with uniaxial alignment must be prepared by the selective etching of a component. At sub-5 nm scales, high-resolution selective etching is significantly challenging even when the organic/inorganic hybrid LC molecules are used as the building blocks;23,24 hence, the realization of nanopatterns with sub-5 nm periodicity by using bottom-up self-assembly is challenging. Recently, we have reported a straightforward method for fabricating silica nanogrooves with sub-5 nm periodicity on a Si substrate, using lyotropic liquid crystals (LLCs) comprising cylindrical micelles as a template.34 However, centimeter-scale uniaxial alignment of nanogrooves on the entire surface of a substrate has not been achieved. Conventional top-down control of nanogrooves required time-consuming lithographic processes to prepare a guide pattern, and the alignment control is limited to


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the inside of the guide pattern (Figure 1a). Therefore, a simple method for alignment control on the entire surface of a substrate is necessary. To realize uniaxially aligned nanogrooves on an entire surface, the alignment control from a top layer of the LLCs by using a poly(dimethylsiloxane) (PDMS) stamp with a striped guide pattern is considered a suitable method. This “stamping method” is potentially beneficial for achieving long-range alignment without using modified substrates or complex instruments. Alignment control of cylindrical micelles guided by confinement in the striped pattern of the PDMS stamp has been previously reported.35,36 However, in the reported method, the convex surfaces of the stamp were in contact with the surface of the substrate, which resulted in partial alignment control of the micelles (Figure 1b). In order to control the alignment of micelles on the entire surface of a substrate, the stamp must not be in contact with the substrate, and several layers of micelles are required between the convex surfaces of the stamp and the surface of the substrate (Figure 1c). Here, we report the fabrication of uniaxially aligned nanogrooves on the entire surface of the centimeter-scale substrate, guided by a PDMS stamp with a striped pattern


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(shown in Scheme 1). In order to realize aligned nanogrooves on the entire surface of the substrate, we employed a modified stamping method. The PDMS stamp was placed on a surface of surfactant thin film, as found for the case of the soft nanoimprint technique, in which the stamp was not in contact with the substrate. The PDMS stamp with a sub-micrometer striped guide pattern was placed on a cetyltrimethylammonium chloride (CTAC: C16TAC) thin film formed on a Si substrate (2 × 2 cm) by spin-coating (Scheme 1a–c). When the substrate was exposed to water vapor, the hygroscopic CTAC molecules absorbed water vapor; this permitted the phase transition of the CTAC thin film from a lamellar phase to a 2D-hexagonal phase (Scheme 1d). The micelles confined in the cavities of the PDMS stamp were aligned in the lengthwise direction of the guide pattern. The alignment of the micelles propagates throughout the entire CTAC thin film and reaches the substrate-CTAC interface. Subsequently, by exposing the substrate to an NH3–water vapor mixture, the outermost surfaces of the aligned micelles facing the substrate were templated with soluble silicate species, generated from the Si substrate under basic conditions (Scheme 1e). After peeling off the PDMS stamp and


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removing the CTAC thin film by washing, the presence of uniaxially aligned nanogrooves on the entire surface of the Si substrate was verified (Scheme 1f).

Figure 1. Schematic illustrations of alignment control of micelles by using (a) top-down control, (b) a conventional stamping method, and (c) a modified stamping method.


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Scheme 1. Graphical representation of fabrication of uniaxially aligned nanogrooves on entire surface of Si substrate.

RESULTS and DISCUSSIONS Preparation of a PDMS stamp and a CTAC thin film and the placement of the stamp on the CTAC thin film. The PDMS stamp was prepared by replicating a patterned master substrate fabricated by lithographic printing (Scheme 1b). The transparent stamp (2 × 2 cm) with structural color arising from the striped pattern was obtained, and the thickness was 1–2 mm (Figure S1a). The feature size of the striped pattern was ~350 nm width, ~200 nm depth, and ~150 nm convex width, as shown in the atomic force microscopy (AFM) image and section analysis image (Figure S1b, c); the schematic illustrations of the PDMS stamp are shown in Figure 2a. The surface wetting behavior of the wall of the guide pattern strongly influences the alignment of micelles.36,37 In order to obtain the aligned micelles in the direction of the guide, modification of the surface properties for preferential contact with the hydrophilic


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surfaces of the micelle is required. Because PDMS is intrinsically hydrophobic, the hydrophilic PDMS stamp was prepared by exposure to oxygen plasma and subsequent immersion in a mixture of hydrochloric acid and tetraethoxysilane (TEOS).36 The hydrophilic PDMS stamp was simply placed on the surface of a CTAC thin film without external pressure (Scheme 1c). The initial thickness of the CTAC thin film hi was controlled at ~150 nm because several layers of CTAC micelles must remain between the surface of the substrate and the convex area of the PDMS stamp to obtain uniaxially aligned nanogrooves on the entire surface of the substrate. The final thickness hf between the surface of the substrate and the convex area of the stamp can be roughly estimated from hi. By scanning electron microscopy (SEM) observation, hi can be measured. According to the simple calculation, the contact with the surface of the substrate is circumvented when hi is over 70 nm. Assuming that the cavities of the PDMS stamp is filled with CTAC in the case of ~150 nm (hi), ~80 nm thickness of the CTAC layer (hf) should remain between the surface of the substrate and the convex area of the stamp (Figure 2b). The details on the estimation of relationship between hi and hf is shown in Supporting Information (Figure S2). Moreover, the cases of the


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various thicknesses of the CTAC thin film were investigated, and the details are discussed later. In the fabrication procedure of the nanogrooves, the CTAC thin film must be exposed to an NH3–water vapor mixture. In our previous report, it was revealed that the treatment with an NH3–water vapor mixture at 60 °C can cause the phase transition of a CTAC thin film from a lamellar phase to a 2D-hexagonal phase, and it generates soluble silicate species from a Si substrate.34 In the procedure reported here, pre-treatment with water–vapor prior to the NH3–water vapor treatment was introduced. By the consecutive progress of the phase transition and generation of soluble silicate species, the micelles that attained the equilibrium state could function as a template for the nanogrooves. Importantly, the high vapor permeability of PDMS38 enables the supply of the NH3–water vapor mixture to the CTAC thin film through the PDMS stamp. After the NH3–vapor treatment, the PDMS stamp could be conveniently peeled off from the substrate.


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Figure 2. Schematic illustration of contact between PDMS stamp and CTAC thin film. (a) The initial thickness of the CTAC thin film hi and the feature size of the striped pattern of the PDMS stamp are described. (b) When the value of hi was ~150 nm, the final thickness hf after stamping between the convex surface of stamp and the substrate should be ~80 nm.

Fabrication of uniaxially aligned nanogrooves using PDMS stamp. The scanning electron microscopy (SEM) images of the surface of the Si substrate after peeling off the PDMS stamp and washing the CTAC with water–ethanol (Scheme 1f) are shown in Figure 3. The high magnification cross-sectional SEM image from the direction of the guide pattern (Figure 3a) and the high-magnification top-view SEM image (Figure 3b) clearly display the nanogroove structure with sub-5 nm periodicity. The interface of nanogrooves–substrate and the elemental mappings were previously reported,34 and the formation of silica layer with 2~4 nm thickness on the Si substrate have been verified. The formation of nanogrooves indicates that the NH3–water vapor mixture


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permeated the PDMS stamp and penetrated the CTAC thin film. The low-magnification SEM image of the surface with a large area of 600 × 400 nm in size (Figure 3c) and the fast Fourier transform (FFT) pattern of this SEM image (inset of Figure 3c) display the formation of uniaxially aligned nanogrooves along the lengthwise direction of the guide pattern of the PDMS stamp; moreover, the periodicity of the nanogrooves is 4.7–4.8 nm. The continuous formation of the aligned nanogrooves on the substrate indicates that several layers of cylindrical micelles remained between the PDMS stamp and the substrate, and the alignment of the micelles completely reaches the substrate–CTAC interface.

Figure 3. SEM images of aligned nanogrooves. (a) High-magnification cross-sectional


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image from the direction parallel to the guide pattern and (b) high-magnification top-view image (insets: schematic illustrations of the nanogrooves). (c) Low-magnification topview SEM image (inset: FFT image). The blue arrow indicates the lengthwise direction of the guide pattern of the PDMS stamp.

To investigate the alignment of the nanogrooves on the entire surface of the substrate, the grazing incidence small-angle X-ray scattering (GI-SAXS) patterns (Figure 4) were recorded with the incident X-rays parallel and perpendicular, respectively, to the direction of the guide pattern of the PDMS stamp. When the incident direction of the Xray is parallel to the guide pattern, as shown in Figure 4a, a diffraction spot at 2θf = 1.87° corresponding to a d spacing of 4.7 nm was observed. This d spacing value is consistent with the spacing between the CTAC cylindrical micelles in the in-plane direction34 and is attributed to the nanogrooves. Meanwhile, when the incident direction of the X-ray was perpendicular to the guide pattern, as shown in Figure 4b, no spots were observed. Similar GI-SAXS patterns were observed at all the other points on the


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sample. These results strongly indicate that the nanogrooves are uniaxially aligned along the guide pattern on the entire surface of the substrate.

Figure 4. GI-SAXS patterns of substrate with nanogrooves. The projected directions of the incident X-rays on the substrate are (a) parallel and (b) perpendicular to the guide pattern of the PDMS stamp, as indicated by the red arrows. The blue arrows indicate the lengthwise direction of the guide pattern of the PDMS stamp.

The centimeter-scale uniaxial alignment of the nanogrooves was further investigated by SEM observation. As shown in Figure 5a, three samples were cut out from the 2 × 2 cm substrate with the nanogrooves, for SEM observation. Thirty-six areas on each SEM sample, whose configuration is shown in Figure 5b, were observed by SEM; totally 108


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FFT images were obtained from the SEM images (Figures 5c,d). As a result, the SEM images of the 108 areas revealed mostly identical FFT images. The distribution of the orientation angle obtained from each of the FFT images was analyzed to demonstrate the large-area alignment of the nanogrooves. The orientation angle is defined as the angle between a line perpendicular to the reference direction and a line which connects two points in an FFT image (Figure 5d). As shown in Figure 5e, the orientation histogram indicates the formation of centimeter-scale uniaxially aligned nanogrooves with a large domain on the entire surface of the substrate. The standard deviation of the orientation angle is 4.49° for the SEM observations, which is likely to have been caused by measurement errors such as the uncertainties in the cut-off angles of the sample plate and its placement on an SEM holder.


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Figure 5. Analysis of large-area alignment of nanogrooves. (a) Photograph of Si substrate with nanogrooves. Three SEM samples A–C were cut from the 2 × 2 cm substrate. The blue arrow indicates the lengthwise direction of the guide pattern of the PDMS stamp. (b) Configuration of 36 areas for SEM observation. (c) The example of SEM image and (d) FFT image of the SEM image (c). (e) Orientation histogram based on the orientation angles of the nanogrooves.


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In rare cases, the nanogrooves were not aligned in the direction of the guide pattern and rather largely curved (Figure S3) depending on the observation point. One of the reasons of the disordered alignment is defects such as dust and scratches on the surfaces of the CTAC thin film and the PDMS stamp. According to our previous report, the orientation of cylindrical micelles is disrupted by objects on a substrate;39 thus, dust and scratches are likely to cause the disorientation of micelles. Moreover, the dust on the surfaces also prohibits close contact between the CTAC thin film and the stamp, and the stamp does not act as a guide of micelles. These defects are highly likely to be improved by operating under cleaner conditions. In our previous report,34 well-aligned nanogrooves were verified when the nanogrooves were fabricated on the bottom faces of trenches whose width is 5 µm. Compared with the histogram based on the orientation angles of nanogrooves formed in the trench, much better alignment of nanogrooves was confirmed in this study, indicating that a guide with narrower width should lead to higher quality of nanogrooves. In previous reports about the top-down control of surfactant micelles, the feature size of the guide pattern affects the orientation of the micelles.40 When the width of the guide pattern is


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excessively wide (> 1 µm), the alignment regulating force is not applied, and random orientation of micelles is verified. Meanwhile, an excessively narrow width (< 100 nm) of the guide pattern results in a vertical orientation of the micelle. Furthermore, according to this reference,40 an appropriate width that results in uniaxial alignment of micelles is approximately 300–500 nm. In this study, the width of the guide pattern of the PDMS stamp is ~350 nm, which is considered an appropriate width for the alignment of micelles. In addition, when nanogrooves were fabricated on the patterned master mold consisting of silicon used as a template for the PDMS stamp, nanogrooves that were highly aligned in the long axis direction of the stripe pattern were observed inside large grooves (Figure S4). This experimentally verifies the suitability of the feature size of the PDMS stamp for the uniaxial alignment of micelles. Based on these results, the fabrication of uniaxially aligned silica nanogrooves on the entire surface of the substrate was verified by placing a PDMS stamp with a striped pattern. Because the walls of the striped pattern of the PDMS stamp and the outer surfaces of the CTAC micelles are hydrophilic, the micelles are aligned parallel to the lengthwise direction to preferentially contact with the wall. This “stamping method”


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provides significant benefits compared with other methods of alignment control. This method consists of simple steps without the use of complex instruments and permits convenient scale-up. Notably, this method is inexpensive because the stamp can be used repeatedly. Moreover, the guide pattern is not limited to a striped pattern, and the desired shaped patterns can be applied as a guide. According to previous reports, surfactant micelles can bend along the wall of the curved patterns and the surface of an object.39–41 This indicates that a PDMS stamp with guide patterns of other shapes such as curved lines and hole array results in variations in the orientation of the nanogrooves. Because of these advantages, it is important to apply this stamping method to the orientation control of other materials such as BCPs. Hierarchical structures of BCP films42,43 can be simply prepared on a substrate by stamping. Directed self-assembly of BCP by adopting a hard-mold nanoimprint method was reported;44 however, the uniaxial alignment of BCP using a soft stamp presents certain challenges45 and requires the optimization of experimental conditions.


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Verification of uniaxial orientation of micelles in CTAC films during the water vapor treatment.

The result of the formation of the uniaxially aligned silica nanogrooves

indicates that the micelles of the whole CTAC thin film should be uniaxially aligned in the direction of the guide as the convergence toward the equilibrium state during the water vapor treatment (Scheme 1e). However, because the nanogrooves were formed by replicating only the outermost surface of the cylindrical micelle in contact with the Si substrate, it has not yet been concluded that the PDMS stamp uniaxially guided the cylindrical micelles of the whole thin film during the water vapor treatment. In order to verify this, the whole structure of the LLC comprising CTAC cylindrical micelles was solidified with silicates. TEOS vapor and HCl vapor, rather than NH3–water vapor mixture, were supplied to the CTAC thin film covered with the PDMS stamp. Being different from the main procedure for forming nanogrooves, here, TEOS vapor was introduced to the CTAC film to “fix” the internal mesostructure comprising cylindrical micelles (Figure 6a). The supply of TEOS vapor to the LLC thin film permits the replication of all the surface of the micelles, and after calcination, mesoporous silica thin film can be prepared by templating the LLC structure.46 The orientation of the solidified


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micelles of the whole thin film was analyzed. The GI-SAXS patterns of the mesoporous silica thin film are shown in Figures 6b,c, and incident X-rays were projected from the two orthogonal directions, as shown in Figure 4. When the incident direction of the X-ray is parallel to the guide pattern, as shown in Figure 6b, diffraction spots corresponding to the (10) and (0l) planes of 2D-hexagonal phase are observed. Meanwhile, when the incident direction of the X-ray is perpendicular to the guide pattern, as shown in Figure 6c, a spot corresponding to the (01) plane of 2D-hexagonal phase is observed. The cross-sectional SEM image of the sample from the direction parallel to the guide pattern (Figure S5a) displays the thin film structure with sub-micrometer scale roughness; this indicates the solidification of the CTAC thin film with silicate and transcription of the guide pattern of the PDMS stamp to the CTAC thin film. The cross-sectional scanning transmission electron microscopy (STEM) image (Figure S5b) reveals that the mesochannels were aligned to the direction of the guide pattern. These results reveal that the fixed internal mesostructure during the process can be assigned to uniaxially aligned 2D-hexagonal structure; that is, the cylindrical micelles of the whole thin film were uniaxially aligned by the PDMS stamp.


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Moreover, these results also indicate the formation of mesoporous silica thin film with uniaxially aligned cylindrical mesochannels guided by the PDMS stamp. In conventional methods of alignment control of mesochannels using a guide pattern in the selfassembly process, several factors influence the mesostructures, such as the composition of a precursor solution, shear force, surface tension, relative humidity, and interactions of the liquid/wall interface; this is because mesostructures are generated through the cooperative self-assembly of silica sources and surfactants during evaporation of solvents.36,37 These factors are likely to cause an orientation of the mesochannels that is perpendicular to the guide pattern;36,37,40 therefore, a detailed consideration of the processing conditions is required. Meanwhile, the method reported here employs synthesis using vapor. TEOS vapor as a silica source was externally supplied through the PDMS stamp after the formation of the aligned 2D-hexagonal structure attained the equilibrium state by the treatment with water vapor. This indicates that this method will enable the fabrication of highly oriented mesoporous silica thin films.


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Figure 6. (a) Graphical representation of treatment with TEOS vapor to replicate all the surface of the micelles. (b, c) GI-SAXS patterns of mesoporous silica thin film. The projected directions of the incident X-rays on the substrate are (b) parallel and (c) perpendicular to the guide pattern of the PDMS stamp. Schematic illustrations adjacent to the GI-SAXS patterns display the indicated structures.

Influence of initial thicknesses of CTAC thin films. To investigate the influence of the initial thicknesses of CTAC thin films, CTAC films with varied thicknesses were


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prepared. The initial thicknesses of the CTAC thin films were regulated by the conditions of spin-coating. In the case of the CTAC film of thickness 30 nm, randomly oriented nanogrooves were observed on the entire surface (Figure 7a). When the initial thicknesses of the CTAC films were over 60 nm, uniaxially aligned nanogrooves were observed, similar to the case of 150 nm (Figure 7b). This difference should arise from the filling fraction of CTAC in the cavities of the stamp, as shown in Figure 7c,d. Because CTAC was not sufficiently confined in the cavities in the former case (Figure 7c), the cylindrical micelles were not aligned and were randomly oriented. Remarkably, in the case of the 900 nm thick CTAC film, uniaxially aligned nanogrooves were also observed (Figure S6); this indicates that the alignment of the CTAC micelles regulated by the confinement of the PDMS stamp propagates throughout the CTAC thin film over a sub-micrometer scale from the top. These results demonstrate that the patterned surface of the PDMS stamps can promote the alignment of micelles in a wide range in both the horizontal and vertical directions.


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Figure 7. (a, b) Top-view SEM images of surfaces of nanogrooves. (c, d) Schematic illustrations of cross-sectional view of the structures of CTAC thin film after placement of PDMS stamp. The initial thicknesses of the CTAC films are (a, c) 30 nm and (b, d) 60 nm.

Variation in chain lengths of surfactants used. The periodicity of the nanogrooves can be regulated by the alkyl chain length of the surfactant.34 As shown in Figure 8, the GISAXS patterns indicate that the formation of aligned nanogrooves was also achieved for the cases of the other surfactants. By using surfactants with shorter or longer alkyl chain (dodecyltrimethylammonium

chloride:

C12TAC

and

octadecyltrimethylammonium

chloride: C18TAC), the periodicities of the nanogrooves were 3.7 nm34 and 5.3 nm,


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respectively. These values are completely consistent with those obtained by the SEM images and FFT images of these samples (Figure S7). However, in the case of noctyltrimethylammonium chloride (C8TAC), the formation of nanogrooves was not verified, which is likely to be because micelles were not formed owing to the low hydrophobicity of the chain.47 When behenyltrimethylammonium chloride (C22TAC) was used under the identical conditions with others, nanogrooves were not found. At present, the periodicity of nanogrooves can be regulated in the range from 3.7 to 5.3 nm.


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Figure 8. (a, b) GI-SAXS patterns of sample in the cases using (a) C12TAC and (b) C18TAC. The projected directions of the incident X-rays on the substrate are parallel (left) and perpendicular (right) to the guide pattern of the PDMS stamp.

Further comments on uniaxially aligned silica nanogrooves. On the basis of all the results reported here, we have demonstrated the fabrication of uniaxially aligned silica nanogrooves on the entire surfaces of Si substrates. Uniaxially aligned silica nanogrooves exhibit substantial potentials in a variety of fields. In particular, this process does not require selective etching to remove organic segments like BCPs, and it is possible to use convex–concave nanopatterns. For example, the nanogrooves are expected to be used as a hard template for the large-area fabrication of uniaxially aligned metal nanopattern with sub-5 nm scale. Nanopatterned silica surfaces are known to function as a template for metal nanopatterns;48–50 therefore, silica nanogrooves are potentially beneficial for preparing high-density sub-5 nm metal nanogaps for strong field enhancements.5,6 In addition, uniaxially aligned nanogrooves


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can provide surface functions such as orientation control of molecular self-assembly8–11 and enhanced hydrophobicity4 because the surface properties of silica are conveniently well-designed by surface modification.51–53 Therefore, the sub-5 nm silica nanogrooves with uniaxial alignment on centimeter-scale substrates will provide benefits for chemistries of nanomaterials and nano-scale interfaces. However, the aligned gratings have a very small aspect ratio and the slope profile is curved (Figure 3a). The nanogrooves can only form on Si substrates at present. Moreover, the nanogrooves revealed in this study are yet to be completely straight, which is likely to make it insufficient for manufacturing nanodevices. The dynamic deformation of micelles is likely to cause defects in the nanogrooves.54 Further studies for minimizing the dynamic deformation of micelles and for quantitatively estimating the factors affecting the orientation of cylindrical micelles are required to improve the straightness of nanogrooves.

CONCLUSION


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The uniaxial alignment of cylindrical micelles on the entire surface of Si substrate was achieved using a PDMS stamp. The phase transition of a lamellar phase of CTAC thin film to a 2D-hexagonal phase and generation of soluble silicate species from a Si substrate occurred even after covering the CTAC thin film with a PDMS stamp; this is because the high vapor permeability of PDMS enables the supply of water vapor and an NH3–water vapor mixture to the CTAC thin film. When the initial thicknesses of CTAC films were over 60 nm, the cylindrical micelles were confined in the cavities of the PDMS stamp and aligned to the direction of the guide. Consequently, uniaxially aligned silica nanogrooves formed on the entire surface of the substrate by the replication the outermost surface of the aligned micelles. This method will be beneficial for the largearea fabrication of uniaxially aligned nanopatterns with ultra-fine pitch without complex steps and etching processes.

EXPERIMENTAL SECTION


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Materials. Cetyltrimethylammonium chloride (CTAC: C16TAC), aqueous ammonia (28 wt %), hydrochloric acid (1 mol/L), tetraethoxysilane (TEOS) (from Wako Pure Chemical Industries, Ltd.), n-octyltrimethylammonium chloride (C8TAC), dodecyltrimethylammonium chloride (C12TAC), octadecyltrimethylammonium chloride (C18TAC) (from Tokyo Chemical Industry Co., Ltd), and CATINAL DC-80, consisting of 80 wt % behenyltrimethylammonium chloride (C22TAC) and 20 wt % isopropyl alcohol, (from Toho Chemical Industry Co., Ltd) were used as received. Si(100) substrates were purchased from Silicon Technology Co., Ltd. Semicoclean 23, an alkaline aqueous solution of tetramethylammonium hydroxide (< 1 wt %) and nondisclosed reagents, was purchased from Furuuchi Chemical Corporation and was used to clean the substrate of the surfaces of the Si substrates. Sylgard 184 silicone elastomer kit (containing two parts, i.e., PDMS base polymer and PDMS curing agent) were purchased from Dow Corning Co., Ltd.

Preparation of CTAC Thin Films. The methods of preparation of CTAC thin film of thickness 150 nm were described in our previous reports.34,39 CTAC was dissolved in a


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mixture of ethanol and deionized water. In the main procedure reported here, the molar ratio of the solution was CTAC:ethanol:deionized water = 4:340:706. A Si(100) substrate (2 × 2 cm) was washed with Semicoclean 23 in an ultrasonic bath for 15 min and then with deionized water. After the substrate was dried, it was coated with a CTAC solution by spin-coating at 2000 rpm for 30 s in air atmosphere of 60% RH at 25 °C. CTAC thin films of varied thicknesses were prepared for comparison. The molar ratios of CTAC and the conditions of spin-coating are summarized in Table S1, and CTAC thin films of thicknesses 30, 60, and 900 nm were prepared.

Preparation of PDMS Stamps. PDMS base polymer and curing agent were mixed at a 10:1 mass ratio. The mixture was casted on a patterned master Si substrate prepared by lithography (2 × 2 cm); then, the PDMS was cured at 55 °C for a day in a vacuum. The PDMS stamp was carefully peeled off from the master substrate. To modify the PDMS stamp hydrophilic, the stamp was exposed to oxygen plasma for 120 s and immersed in an acidic aqueous solution (96 mM) of TEOS (3 mM), as previously


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reported.36 After drying, the hydrophilic PDMS stamp was simply placed on the surface of the CTAC thin film without external pressure.

Preparation of Silica Nanogrooves. The nanogrooves were prepared by a method described in our previous reports with a modification. The substrate, on which CTAC had been coated and the PDMS stamp placed, was directly attached to a plate, and the substrate on the plate was suspended inside a 500 mL separable flask. The flask was placed in an oven at a preset temperature of 60 °C. The flask with the substrate was preheated for 1 h. Deionized water (1.5 mL) was added to the flask without immersing the substrate in the water. The substrate was exposed for 3 h to the water vapor at 60 °C. Subsequently, aqueous ammonia (28 wt %, 1.5 mL) was added to the flask without immersing the substrate in the solution, and the substrate was exposed for 24 h to the NH3−water vapor mixture at 60 °C. After the reaction, the PDMS stamp was peeled off from the substrate, and the substrate was washed with deionized water and ethanol to remove the surfactants.


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Verification of Uniaxial Orientation of Micelles in CTAC Films by Vapor Synthesis of Mesoporous Silica Thin Film. The CTAC thin film with the PDMS stamp placed in Teflon vessel. A 3 mL glass vial filled with deionized water was placed in the vessel. The vessel was placed in an oven at 60 °C for 3 h. This step is same as the water vapor treatment of the main procedure to form nanogrooves. Subsequently, two 3 mL glass vials filled with TEOS and hydrochloric acid (1 M) were also placed in the vessel and heated at 60 °C for 48 h. After vapor treatment, the PDMS stamp was carefully peeled off, and the substrate was calcined at 400 °C in air for 6 h with a heating rate of 1 °C / min.

Characterization. An AFM image and a section profile of the PDMS stamp were recorded on a Nanoscope III (Digital Instruments Inc.) instrument using a tapping mode. Si probes (NCHV-10 V) were purchased from Bruker Nano Inc. Surface and crosssectional SEM images of the samples were recorded on a Hitachi S-5500 at accelerating voltages of 10−25 kV. Focused ion beam (FIB) processing was used for the cross-sectional observation of the mesoporous silica thin film. After the deposition of


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a carbon protective layer on the film, a sample was cut from the substrate by a gallium focused ion beam using a JEOL JIB-4000 instrument. A cross-sectional high angle annular dark-field STEM image of the film was recorded on a JEOL JEM-2100F microscope at an accelerating voltage of 200 kV. GI-SAXS patterns were recorded in reflection mode with a Rigaku NANO-Viewer with a Pilatus 2D X-ray detector (Dectris) using Cu Kα radiation (40 kV, 30 mA). The incident angle of the X-ray was 0.2°, and the sample−detector distance was 430 mm.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications

website. Details of the PDMS stamp, AFM image, SEM images, STEM image, and experimental condition (PDF)

The authors declare no competing financial interest.


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Page 36 of 47

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (K.K.)

ACKNOWLEDGMENT The authors thank Mr. M. Itoh (Waseda University) for aiding the AFM observation as well as Ms. M. Nakajima and Mr. S. Enomoto (Waseda University) for aiding the FIB processing and STEM observation.

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Fabrication of Uniaxially Aligned Silica Nanogrooves with Sub-5 nm

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