Lattice Matching in the Epitaxial Formation of Mesostructured Silica

Dec 18, 2012 - Crystallographic orientation of mesostructured silica films on a substrate drastically changes when the substrate is modified with an a...
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Lattice Matching in the Epitaxial Formation of Mesostructured Silica Films Hirokatsu Miyata,*,† Shimon Kobori,‡ Wataru Kubo,† Masatoshi Watanabe,§ and Kazuyuki Kuroda*,‡,∥ †

Frontier Research Center, Canon Inc., 3-30-2 Shimomaruko, Ohta-ku, Tokyo 146-8501, Japan Department of Applied Chemistry, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan § Nanomaterials R&D Center, Canon Inc., 3-30-2 Shimomaruko, Ohta-ku, Tokyo 146-8501, 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: Crystallographic orientation of mesostructured silica films on a substrate drastically changes when the substrate is modified with an anisotropic surface. The ⟨01⟩ axis of a two-dimensional (2D) hexagonal structure of the film prepared on a polyimide surface using C22EO20 as a structuredirecting agent changes from perpendicular to parallel with respect to the substrate after a rubbing treatment of polyimide, which is accompanied by the simultaneous unidirectional alignment of the cylindrical pores in the plane of the film. The normal direction of the film is ⟨21⟩̅ , which has never been observed in the mesostructured silica films reported so far including those with controlled in-plane alignment of the mesochannels. The change of the orientation with respect to the substrate can be explained by the increased lateral distance between the adjacent surface micelles, which is caused by the elongation of the alkyl chains of the surfactant molecules induced by the adsorption onto the polymer surface with a molecular-level anisotropy. These results show that the total structural orientation of the mesostructured silica film is determined by the matching of the intrinsic lattice constant of the mesostructured silica with that of the surface micelle structure on a substrate.

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INTRODUCTION In epitaxial growth of a crystalline material, the crystallographic orientation of the material formed on a substrate is correlated with the orientation of the substrate.1,2 The epitaxial layer is grown in a way to minimize the interfacial energy between the two materials. The matching of the lattice constants of the two materials at the interface and the consequent lowering of the interfacial energy is required: otherwise, the large deformation energy does not allow the formation of a good epitaxial layer. Due to this requirement, the crystallographic orientation of an epitaxial layer is inevitably determined by the lattice constant of the substrates. In a broad sense, “epitaxy” can be applied to some soft materials for which crystallographic orientation can be defined.3,4 Orientation-controlled self-assembly of block copolymer films on a lithographically patterned substrate5−8 and formation of periodic mesostructured films with macroscopically controlled structures on well-designed substrates9−24 are included in the concept of epitaxy. The controlled orientation of soft materials is achieved as a result of minimization of the interfacial energy, which is substantially the same mechanism as the epitaxy of crystalline materials. However, in the epitaxy of soft materials, the interfacial energy should be considered acting over longer distances than those © 2012 American Chemical Society

for atomic and molecular level assemblies. Also, the increase of the energy by deformation is much smaller in soft materials. Therefore, the concept of lattice matching has been difficult to apply to the epitaxy of soft materials. However, as long as the origin of epitaxy is common in the minimization of the interfacial energy, the growth has to be influenced by the geometry of the surface on which epitaxial growth takes place. Here, we report a complete change of the crystallographic orientation of a mesostructured silica film with a 2D-hexagonal structure by changing the conditions of the substrate surface. In general, the orientation of mesostructured films with a 2Dhexagonal structure is ⟨01⟩ with respect to the substrate surface,25 which is determined by the boundary conditions at both the air/film and the substrate/film interfaces. This preferred orientation has been commonly observed for most of the substrates that allow the formation of mesostructured films thereon. Though the use of substrates with surface anisotropy, such as single crystals9−11 and those with anisotropic polymer coatings,12−14 can accomplish the inplane alignment of the mesochannels in a 2D-hexagonal Received: November 8, 2012 Revised: December 14, 2012 Published: December 18, 2012 761

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Brentano and in-plane geometries were performed with a Philips X’pert Pro X-ray diffractometer and a Rigaku ATX-G diffractometer equipped with a 4-axes goniometer using monochromated CuKα radiation. The 2D-XRD patterns were recorded in reflection mode using CuKα radiation and a Pilatus 2D X-ray detector (Dektris). Grazing incidence of 0.2° was commonly employed for the in-plane and the 2D-XRD measurements. The details of the in-plane XRD are shown in our previous papers.12 Cross-sectional images of transmission electron microscopy (TEM) were recorded on a HITACHI H-800 instrument at an accelerating voltage of 200 kV. The film sample with a size of 2 × 10 μm was cut out from the substrate by a Gallium focused ion beam (FIB) and transferred to a copper grid by a microsampling technique. It was further thinned by FIB to a thickness below 100 nm to allow observation of the cross section.

structure and may influence the degree of distortion from an ideal hexagonal structure, the orientation with respect to the substrate has never changed. This is partially ascribed to the flexibility of mesostructured materials, which allows relatively large mismatch between the intrinsic lattice constants of mesostructured materials and the periodicity of the regularly arranged surface micelles formed by adsorption of surfactant molecules on a surface. In other words, mesostructured materials are deformed in a way to match the lattice constants at the substrate surface. In this report, we show the drastic change of the crystallographic orientation of a mesostructured silica film prepared using a nonionic surfactant with a long alkyl chain, C22H45(OC2H4)20OH, on a polyimide (PI) coating on a substrate by inducing anisotropy through a rubbing treatment. The ⟨01⟩ orientation of the film on a bare PI coating changes into ⟨21̅⟩ when the PI has been subjected to a rubbing treatment, which accompanies the in-plane alignment of the mesochannels perpendicular to the rubbing direction (Figure 1). The observed change of the orientation of the film with

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RESULTS X-ray Diffraction. The 2D-XRD patterns of the film recorded under the geometry in which the rubbing direction is parallel and perpendicular to the X-rays are shown in Figure 2A

Figure 2. Two-dimensional XRD patterns of the mesostructured silica films. (A, B) Film on the rubbing-treated PI. (C) Film on a bare PI without a rubbing treatment. The incident X-rays are parallel (A) and perpendicular (B) to the rubbing direction.

and B, respectively. The structural anisotropy in the plane of the film is apparent from these two patterns. In Figure 2A, only one spot ascribed to the reflection on the film surface is observed and substantially no diffraction spots are recognized in this angle range, whereas well-defined diffraction spots are observed in Figure 2B. This observed anisotropy of the diffraction patterns proves anisotropic structure of the film. Here, the most important feature in the pattern of Figure 2B is the absence of a principal diffraction spot on the 2θ_in‑plane = 0° line (shown as a yellow dotted line), which shows that the basal planes of the mesostructured silica film are not oriented with respect to the substrate. This is a fundamental structural difference of this film from the conventional mesostructured silica films reported so far, and suggests an unusual crystallographic orientation of this film with respect to the substrate. Considering the features of the diffraction patterns in Figure 2, including the comparable intensity of the diffraction spots in Figure 2B, we assume a 2D hexagonal structure with a ⟨21⟩̅ orientation with respect to the substrate as the most probable structure of this film. The XRD patterns recorded under the Bragg−Brentano geometry are shown in Figure 3A. The anisotropy of the patterns is consistent with the 2D-XRD patterns in Figure 2. In each pattern in Figure 3A, a relatively weak diffraction peak is commonly observed at the same position of 2θ = 2.3°, which can consistently be assigned as (21)̅ of the assumed 2D hexagonal structure. The corresponding spots are not found in Figure 2 because the positions of the diffraction spots are out of the area of the 2D detector. In the in-plane XRD study, the evident in-plane diffraction peaks are observed when the projection of the incident X-rays is perpendicular to the rubbing direction, and substantially no peaks are found under the parallel geometry, as shown in Figure

Figure 1. Schematic illustration of the structural changes in the mesostructured silica film prepared using C22EO20 by a rubbing treatment: in-plane alignment of mesopores and the change of the crystallographic orientation with respect to the substrate concomitantly take place.

respect to the substrate can be explained by the increased spacing of the regular surface micelles, which cannot allow the conventional ⟨01⟩ orientation of the mesostructured silica by matching of the lattice constants at the interface through distortion. The results of the present paper suggest the universal preparation of the films of single-crystalline mesoporous materials on a substrate, which will open up the gate for new applications of mesostructured materials to optical devices in the X-ray region by controlling the direction of the characteristic strong diffraction caused by their periodic structure.

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EXPERIMENTAL SECTION

Preparation of Mesoporous Silica Films. The details of the preparation of the substrate coated with a rubbing-treated poly(hexamethylenepyromellitimide) film are shown in our previous papers.12,13 The substrate was dip-coated with a precursor solution prepared by mixing tetrapropoxysilane (TPOS), ethanol, C22H45(OC2H4)20OH (abbreviated as C22EO20, Wako Chemical Co.), hydrochloric acid, and water with the molar ratio of 1.0:22:0.06:0.004:4.0 at a coating speed of 3 mm/s. The dip-coating is performed in a chamber to prevent the disturbance by air current and to keep the ambient conditions constant. The mesostructured silica films were obtained after drying the film for 1 h in the ambient atmosphere. Characterization. The structure of the films was characterized by X-ray diffraction (XRD). The XRD measurements with the Bragg− 762

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Figure 3. XRD patterns of the mesostructured silica films. (A) θ−2θ scanning, (B) in-plane ϕ−2θ_in‑plane scanning profiles: the incident X-rays are perpendicular (blue line) and parallel (red line) to the rubbing direction. (C) In-plane ϕ-scanning profile.

3B. The in-plane rocking curve of the principal in-plane diffraction peak is shown in Figure 3C. This profile is just the same as those of the conventional mesostructured silica films with an aligned 2D hexagoanl structure,12−14 whose orientation with respect to the substrate is ⟨01⟩. However, this profile is consistent with the assumed structure, because the 2D hexagonal film with a ⟨21⟩̅ orientation gives the same inplane rocking curve when the alignment of the mesochannels are controlled in one direction in the plane of the film. We prepared the film on a substrate with the same PI coating without rubbing treatment using the same precursor solution and the same conditions. Interestingly, the structure of the formed film is the conventional 2D hexagonal structure with a ⟨01⟩ orientation with respect to the substrate, as confirmed from the 2D XRD pattern shown in Figure 2C. It should be noted that the intensity of the (10) and the (1̅1) spots are weaker than that of the (01) spot in Figure 2C because of the lack of in-plane alignment of the mesochannels. From these results, it is confirmed that the ⟨21̅⟩ orientation is achieved only when the PI on the substrate is given anisotropy by the rubbing treatment. Cross-Sectional TEM. The structure of the film is finally confirmed by cross-sectional TEM observation. Figure 4A and

B shows the images taken for the cross sections sliced parallel and perpendicular to the rubbing direction, respectively. Figure 4A clearly shows that the crystallographic orientation of this film with respect to the substrate is ⟨21̅⟩ of the 2D hexagonal structure. The alignment of the cylindrical mesochannels is confirmed by Figure 4B, which shows the stripes parallel to the substrate. For clear observation of the stripe pattern in Figure 4B, the specimen needs to be inclined with respect to the incident electron beam by α = 16.5° as schematically shown in Figure 4D, which leads to the different apparent film thickness in the two images. When the cross section sliced perpendicularly to the rubbing direction is observed in the normal direction to the cross section, the film looks featureless (Figure S1, Supporting Information) because of the overlapping of the mesochannels in the thickness direction of the specimen. In this film, one of the principal lattice planes of a 2D hexagonal structure, (01), is oriented in the direction perpendicular to the rubbing direction in the plane of the film. This causes the strong diffraction peak assigned to be (01) in the in-plane XRD pattern when the X-rays are perpendicular to the rubbing direction. Close observation of Figure 4A gives us the information at the mesostructured silica film/PI interface. The enlarged image of the interface in Figure 4A is shown in Figure 4C. As shown in the image, the formation of the hemicylindrical surface micelles can be observed. This is the first direct image of the structure of the interface, where the alignment of the mesochannels is determined, and it is consistent with the alignment mechanism of mesochannels in our previous reports.12

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DISCUSSION We have reported in-plane alignment control of mesochannels of mesostructured silica films using various substrates with surface structural anisotropy including those with a rubbingtreated PI coating. For all the films with a 2D-hexagonal structure in the previous study, the crystallographic orientation with respect to the substrate is ⟨01⟩.12 Though the use of alkylPEO nonionic surfactants with different alkyl length, decyl, dodecyl, and hexadecyl groups, allows the formation of aligned mesostructured silica films on the rubbing-treated PI, the crystallographic orientation with respect to the substrate has never changed.26 The observed drastic change of the orientation is definitely related to the long alkyl chain, C22H45, in the surfactant used in this study.

Figure 4. Cross-sectional TEM images of the mesostructured silica film. Cross section sliced (A), (C) parallel and (B) perpendicular to the rubbing direction. (B) Inset: enlarged image with improved contrast. (C) Enlarged image of (A). (D) Illustration explaining the directions of the TEM observations: the tilt angle α of the specimen for (B) is 16.5°. 763

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large distortion because d01 is √3 times larger than d21̅ for an ideal 2D hexagonal structure. Actually, the proposed model can be confirmed directly from the high-resolution cross-sectional TEM image of the substrate/film interface. In Figure 4C, the hemicylindrical surface micelles can be observed at the interface in every other column, which is exactly the same as the schematic illustration shown in Figure 6C. Because this reorientation requires a large change in the elastic energy, the thickness of the film that allows the total orientation change is as thin as 150 nm, as shown in Figure 4. Consequently, the range of optimum conditions for the preparation of the film with the total ⟨21̅⟩ orientation becomes narrow. The surfactant used in this study is C22EO20, whose alkyl chain is much longer than that of C16EO10 used for the preparation of aligned mesoporous silica films with ⟨01⟩ orientation in previous reports.12b This longer alkyl chain causes an increase in the distance between the adjacent surface micelles when the surfactants are adsorbed on the rubbingtreated PI substrates. This leads to the larger increase of the elastic energy in the C22EO20 system, because the elastic energy is proportional to the square of deformation.28 The elastic constant of the silicatropic liquid crystal made of C22EO20 could also be larger than that in the C16EO10 system because of the larger hydrophilic part, wherein the surfactant molecules and siliceous species interact to form an ordered micelle structure analogous to liquid crystals. The larger elastic constant also contributes to the larger increase of the elastic energy by the deformation. The larger increase of the elastic energy as explained above would be the reason why the use of C22EO20 provides such total change of the crystallographic orientation, which has never been observed using surfactants with shorter alkyl chains such as Brij 56. The results in this study suggest that the crystallographic orientation of mesostructured films can be controlled not only in the plane of the film but also perpendicular to it by carefully designing the interfacial chemistry. For the films with ⟨21̅⟩ orientation in this report, the strong XRD caused by the principal (01) lattice planes takes place in the plane of the film. Such a unique X-ray diffraction behavior will give opportunities for new applications to these films in the field of X-ray optics such as planar double-crystal monochromators. The orientation control by lattice matching at an interface could be extended to other soft materials such as block copolymers, which will expand possible applications using their beautiful structural regularity.

The observed drastic change of the crystallographic orientation with respect to the substrate by a rubbing treatment can be explained by the change of the distance of the adjacent surface micelles on the substrate and the consequent increase of the elastic energy in the silicatropic liquid crystal phase27 formed on the substrate by the evaporation-induced selfassembly (EISA) process. Unlike the microphase separation of block copolymers or layer-by-layer formation of polymer multilayers, the orientation control of mesostructured silica in the present study can be considered being based on a simple elastic model without referring to detailed interactions between the polymer chains of the surfactant. In the mesostructured silica film preparation by the EISA process, the surface micelle structure formed on a substrate is influenced by the nature of the substrate surface. The rubbingtreated PI has an anisotropic structure, in which the polymer chains are elongated and aligned in the rubbing direction. On this anisotropic polymer, the surfactant molecules are adsorbed parallel to the aligned PI chains through hydrophobic interactions, and consequently, the aligned surface micelle structure is formed. During this adsorption, the alkyl chains of the surfactant are elongated along the rubbing direction through a change of the conformation. This is likely due to the reduction of possible cis and gauche conformations in the alkyl chains by the strong interactions with the aligned alkyl chains of the polyimide. Because of this anisotropic adsorption and the consequent elongation of the alkyl chains of the surfactant, the horizontal distance between the surface micelles becomes longer compared to that formed on the same isotropic polyimide surface (ls → ls′), as schematically shown in Figure 5. The silicatropic liquid crystal phase formed on this surface

Figure 5. Schematic illustration of the aligned cylindrical surface micelles on the (A) bare and (B) rubbing-treated PI surfaces.

micelle structure needs to deform to match its lattice with the increased periodicity of the surface micelle structure (Figure 6A, B). However, when the required deformation is large, the retention of the original orientation becomes unfavorable because of the excessive increase of the elastic energy. Then, the total change of the orientation would eventually take place from ⟨01⟩ to ⟨21̅⟩ (Figure 6C), which does not need such a

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CONCLUSION The structural anisotropy of a substrate, which is provided by rubbing treatment of a polyimide film on a substrate, drastically changes the crystallographic orientation of a 2D hexagonal mesostructured silica film formed thereon. In addition to the uniaxial in-plane alignment of the mesochannels, the orientation with respect to the substrate changes from ⟨01⟩ to ⟨21⟩̅ . This novel orientation is achieved as a result of the lattice matching between the aligned surface micelle structure and the silicatropic liquid crystal, which are both formed by the EISA process. The structure at the substrate/film interface, where the total structure of the film is fixed, is directly observed by high-resolution TEM: the obtained image is exactly the same as the proposed model. These results show that the crystallographic orientation of soft materials with structural regularity can be universally controlled by interactions with

Figure 6. Illustration of the relationship between the crystallographic orientation of the mesostructured silica film and the distance of the adjacent surface micelles. (A) ⟨01⟩ orientation on an isotropic substrate, (B) largely distorted ⟨01⟩, and (C) ⟨21̅⟩ orientations on the rubbing-treated PI. 764

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substrates, which is useful for designing future optical and electronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

Cross-sectional TEM image of the mesostructured silica film sliced perpendicular to the substrate observed in the normal direction to the cross section. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.M.); kuroda@ waseda.jp (K.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. O. Albrecht for careful reviewing of the manuscript. The authors thank Professor S. H. Tolbert for useful discussion and suggestion. The authors thank Ms. S. Hayase for preparation of film samples.

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REFERENCES

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