Tuning c-Axis Orientation of Calcium Phosphate Hybrid Thin Films

Feb 19, 2019 - The orientation of the c-axis in octacalcium phosphate (OCP) nanocrystals that were incorporated into hybrid thin films were successful...
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Tuning the c‑Axis Orientation of Calcium Phosphate Hybrid Thin Films Using Polymer Templates Rino Ichikawa, Satoshi Kajiyama,* Misato Iimura, and Takashi Kato* Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

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S Supporting Information *

ABSTRACT: The orientation of the c-axis in octacalcium phosphate (OCP) nanocrystals that were incorporated into hybrid thin films was successfully tuned using poly(vinyl alcohol) (PVA) thin-film templates of varying thicknesses. This approach was inspired by biomineralization. Thicker PVA templates enhanced the c-axis orientation of the OCP crystals perpendicular to the substrate. Using this approach with a 900 nm thick PVA template, OCP/PVA hybrid thin films (1.8 μm thick) with a c-axis orientation perpendicular to the substrate were formed. Hydroxyapatite (HAP) hybrid thin films that also exhibited a perpendicular c-axis orientation were obtained through the topotactic transformation of the OCP/ PVA hybrid thin films in aqueous solution. The thickness change of the polymer templates had a significant effect on the structure of the OCP nanocrystals in the hybrid thin films. The structural control of the OCP hybrid thin films that were formed through the biomineralization-inspired approach allowed the formation of HAP hybrid thin films with controlled structures.

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INTRODUCTION Controlling the orientation of organic/inorganic hybrid materials is important to ensure that they exhibit specific and desired properties. Macroscopically oriented structures within organic/inorganic hybrids can be found in biominerals, including sea shells, bones, and teeth.1−5 The crystallographic orientation of CaCO 3 or hydroxyapatite (HAP; Ca5(PO4)3OH) nanocrystals imparts specific mechanical properties to nacre, bone, or teeth.3−5 These macroscopically ordered structures are composed of nanocrystals that are formed under mild conditions, and as such, this process is a good example that can be used to develop hierarchically ordered functional materials through self-organization processes under mild conditions. A key process in biomineralization involves biomacromolecules interacting with inorganic ions to form the ordered structures.1−8 Ordered organic/inorganic hybrid structures have been produced through biomineralization-inspired approaches using acidic water-soluble organic molecules and/or water-insoluble polymer templates.2,9−23 Cooperative effects between soluble and insoluble organic polymers lead to the formation of hierarchically ordered structures, similar to biominerals. Although there are many examples of organic additives that have been designed for the development of ordered hybrid materials,24−27 controlling ordered structures on the nano- to macroscale has not been achieved yet. This is largely because the role of the organic components in hybrids is not well understood, especially the role of insoluble templates. © XXXX American Chemical Society

Therefore, understanding how templates affect the formation of ordered structures is critical to developing hybrid materials with specific structures. We aim to understand the template effects on the formation of hybrid thin films that are composed of poly(vinyl alcohol) (PVA) and calcium phosphate. Thin-film PVA has been used previously as a matrix for inorganic components.22,23 Recently, we developed octacalcium phosphate (OCP; Ca8H2(PO4)6· 5H2O) hybrid thin films with a PVA thin-film template that was prepared by spin-coating a PVA solution on a substrate with a subsequent annealing process.23 OCP spontaneously crystallized on the annealed PVA templates that were placed in an aqueous solution containing poly(acrylic acid) (PAA). PAA inhibited crystallization within the solution. These hybrid thin films contained assemblies of OCP spherulitic thin films with the c-axis oriented parallel to the substrate. HAP hybrid thin films with the c-axis orientation parallel to the substrates were also developed from the OCP hybrid thin films through topotactic transformation.23 Understanding the way in which PVA acts as a template and how it affects the resulting morphology of a hybrid thin film is important to ensure the control of highly structured hybrid thin films based on calcium phosphate. Received: December 30, 2018 Revised: February 16, 2019 Published: February 19, 2019 A

DOI: 10.1021/acs.langmuir.8b04318 Langmuir XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Effects of the Template Thickness on Calcium Phosphate Crystallization. Three poly(vinyl alcohol) (PVA) thin-film templates that had varying thicknesses were prepared using different numbers of spin-coating and annealing cycles, as shown schematically in Figure 1. These PVA

Calcium phosphate-based materials have been developed for use in a variety of fields, including biomedical applications.9,28−45 The thin-film formation or coating with calcium phosphate crystals has received significant attention because thin films can be used as scaffolds for the regeneration of bone or teeth.15,18,21,39−43 Additionally, the performance of HAPbased materials can be enhanced by controlling the orientation of the c-axis as these materials exhibit anisotropic properties because of their crystal structure.46−48 Therefore, controlling the hierarchical structure of calcium phosphate-based materials from the nano- to macroscale is highly desirable, especially for the development of high-performance biomedical materials. Herein, we describe the tuning of the orientation of calcium phosphate-based hybrid thin films by varying the thickness of PVA thin-film templates. The effects of the thickness of the PVA template on the crystallization of OCP were investigated. We also fabricated HAP hybrid thin films based on hierarchically ordered structures from the OCP hybrid thin films because of the biomineralization-inspired approach using PVA templates with different thicknesses.

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Article

EXPERIMENTAL SECTION

Materials. Poly(vinyl alcohol) (PVA, MW = (1.46−1.86) × 105, 87−89% hydrolyzed) and poly(acrylic acid) (PAA, MW = 1.8 × 103) were purchased from Sigma-Aldrich (St. Louis, MO). Dimethyl sulfoxide (DMSO) was obtained from Kanto Chemical Co., Inc. (Tokyo, Japan). Dipotassium hydrogen phosphate (K2HPO4) and calcium chloride (CaCl2) were purchased from Fujifilm Wako Pure Chemical Corporation Ltd. (Osaka, Japan). All chemicals were used as received without further purification. Preparation of Thin-Film Polymer Templates. Thin-film polymer templates were prepared by spin-coating a polymer solution with subsequent annealing. PVA was dissolved in DMSO at a concentration of 4.0 wt %. The DMSO/PVA solution was spin-coated onto glass substrates at 1500 rpm for 60 s and then annealed at 200 °C. The thickness of the PVA template was controlled by changing the number of spin-coating and subsequent annealing cycles. The thinnest PVA templates were prepared by spin-coating once and then annealing at 200 °C for 30 min. Thicker PVA templates were prepared by repeating the spin-coating and annealing processes for three and five cycles, respectively. Spin-coated PVA is annealed for 30 min in the last cycle and otherwise annealed for 10 min. Preparation of Calcium Phosphate Hybrid Films. Calcium phosphate hybrid thin films were obtained by soaking the PVA templates in amorphous calcium phosphate solutions that were prepared according to a reported procedure.23 Briefly, 2.5 mL of an aqueous CaCl2 solution (40 mM) that also contained PAA (1.4 × 10−1 wt %) was added into an equal volume of an aqueous K2HPO4 solution (40 mM), which resulted in colloidal solutions. The PVA templates were immersed in the colloidal solution for 7 days. The temperature was maintained at 25 °C using an incubator. After 7 days, the thin films were washed with purified water and dried at ambient temperature. The hydroxyapatite (HAP) hybrid thin films were prepared by soaking the thin films in hot water at 80 °C for 30 min. Characterization. The thin-film morphology was examined using scanning electron microscopy (SEM) (Hitachi High Technologies, Tokyo, Japan, S-4700, operated at 3.0 kV). Platinum coating, used as a conductive treatment, was achieved using ion-sputter deposition (Hitachi High Technologies, Tokyo, Japan, E-1030 ion sputterer). Xray diffraction (XRD) patterns were recorded with a SmartLab X-ray diffractometer (Rigaku, Tokyo, Japan) using a paralleling beam method with Cu Kα radiation (λ = 0.154 nm). XRD measurements of the thin-film hybrids were recorded in two different geometries (i.e., out-of-plane with 2θ scanning and in-plane with 2θχ scanning). Thermogravimetric (TG) analyses were performed with TG-8120 (Rigaku, Tokyo, Japan) under a flow of N2 with a heating rate of 10 °C/min up to 800 °C.

Figure 1. Schematic illustration showing the fabrication of the hybrid thin films based using calcium phosphate and PVA thin-film templates with different thicknesses. This process allowed control over the structures that formed within the films.

templates were used as a matrix for the biomineralizationinspired crystallization of calcium phosphate using an amorphous colloidal precursor stabilized by poly(acrylic acid) (PAA). Scanning electron microscopy (SEM) was used to determine the thickness of the PVA templates, which were 200 nm (PVA200), 700 nm (PVA700), and 900 nm (PVA900) for one, three, and five cycles, respectively, as shown in Figure 2 (left panels). Hybrid thin films that consisted of octacalcium phosphate (OCP) and the PVA templates (OCP/PVA200, OCP/ PVA700, and OCP/PVA900) were obtained using the biomineralization-inspired crystallization described in the literature.21,23 The successful formation of the hybrid thin films was confirmed with polarizing optical microscopy (POM), SEM, and X-ray diffraction (XRD) measurements. Macroscopic observation with POM and SEM suggested that spherulitic thin films were formed in the PVA templates (Figures S1 and S2). After crystallization, the thicknesses of the films increased from 200, 700, and 900 nm to 600 nm (OCP/ PVA200), 1200 nm (OCP/PVA700), and 1800 nm (OCP/ PVA900), as shown in Figure 2 (right panels). Laminatedstructured templates frequently lead to the formation of layered structures.22,49,50 However, the hybrid structures in the present study consisted of a homogeneous mixture of the OCP nanocrystals and the PVA thin films, as evidenced from the cross-sectional SEM images (Figure 2, right panels). The OCP crystals formed oriented structures within in the PVA templates, which were examined using XRD in two B

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reflection was only present in the in-plane XRD pattern from the OCP/PVA200 film, which suggested that the c-axis was oriented parallel to the substrates (Figure 3a). This oriented structure was similar to those in our previous work using PVA thin films annealed at 200 °C for 60 min as a polymer template.23 Interestingly, the varying thickness of the PVA template induced different OCP structures within the hybrid thin films. The 002 reflection was present in the out-of-plane XRD patterns from OCP/PVA700 and OCP/PVA900 (Figure 3b,c). The intensities of these peaks in the in-plane and out-of-plane XRD patterns from OCP/PVA700 were similar (Figure 3b), whereas they were significantly more intense in the out-ofplane XRD pattern from OCP/PVA900 (Figure 3c). These results suggest that increasing the thickness of the PVA template enhanced the orientation of the c-axis perpendicular to the substrate. Therefore, using the PVA900 template, OCP hybrid thin films with the c-axis oriented perpendicular to the substrate were successfully developed. Preparation of HAP Hybrid Thin Films with Different Crystallographic Orientations. Hydroxyapatite (HAP) hybrid thin films were obtained from the OCP hybrid thin films through an aqueous solution process reported previously.21,23 The successful transformation of the OCP- to HAP-based hybrid thin films within 30 min was confirmed using XRD. Cross-sectional SEM images of the HAP hybrid thin films that were obtained from OCP/PVA200, OCP/ PVA700, and OCP/PVA900 are shown in Figure 4. Using

Figure 2. Cross-sectional SEM images of the three kinds of templates, (a) PVA200, (b) PVA700, and (c) PVA900 (left panels), and the OCP hybrid thin films (right panels) that used the corresponding PVA templates (scale bars = 500 nm).

different geometries (i.e., out-of-plane and in-plane). Peaks that were characteristic of OCP were observed in the XRD patterns of hybrid thin films (Figure 3). However, all XRD

Figure 4. (a−c) Cross-sectional SEM images of the (a) HAP/ PVA200, (b) HAP/PVA700, and (c) HAP/PVA900 hybrid thin films obtained from the OCP/PVA200, OCP/PVA700, and OCP/PVA900 precursors, respectively (scale bar = 500 nm). (d) Correlation between the thicknesses of hybrid thin films and the PVA thin-film templates.

Figure 3. Out-of-plane and in-plane XRD patterns from the OCP hybrid thin films using the (a) PVA200 (b) PVA700, and (c) PVA900 templates.

SEM, the thicknesses of HAP hybrid thin films (HAP/ PVA200, HAP/PVA700, and HAP/PVA900) were estimated to be 400, 900, and 1600 nm, respectively (Figure 4a−c). The thickness of HAP hybrid thin films decreased as compared with that of OCP hybrid thin films (Figures 2 and 4). This was likely caused by the dissolution of PVA and calcium phosphate crystals into the water as well as condensation of hybrid thin films during the transformation process. The surface morphology changed following the transformation to the

patterns from the OCP hybrid thin films did not contain the 100 reflection, which is the most characteristic peak (Figure 3). This might have been caused by incomplete formation of the OCP crystals51 because of interactions with PAA and PVA. Notably, the intensities of the peaks from the out-of-plane and in-plane XRD measurements were different (Figure 3). This difference was caused by the specific crystallographic orientation within the OCP hybrid thin films. The 002 C

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Figure 6. TEM images of the HAP/PVA200, (b) HAP/PVA700, and (c) HAP/PVA900 hybrid thin films. The right panels show magnified images from the black boxes in the left images. The insets in the left panels show the corresponding SAED patterns (scale bars = 50 nm (left panels) and 10 nm (right panels)). Figure 5. Out-of-plane and in-plane XRD patterns from the (a) HAP/ PVA200, (b) HAP/PVA700, and (c) HAP/PVA900 hybrid thin films obtained from the corresponding structured OCP hybrid thin films.

OCP crystals within the hybrid thin films were topotactically transformed into the HAP crystals through the aqueous solution process for 30 min. OCP crystals are used as a precursor for HAP crystals because both have similar crystal structures.52−54 However, the transformation can take several hours in aqueous solution at temperatures below 100 °C.55−57 The rate of transformation in the present study was significantly faster, even at lower temperatures, i.e., 80 °C. The nanocrystalline features of the OCP crystals formed in the hybrid thin films enabled the rapid, topotactic transformation to occur in the aqueous solution process.21,23,51 The inorganic amounts within the OCP and HAP hybrid thin films were estimated using thermogravimetric (TG) measurements under a flow of N2. Weight losses that were attributed to the removal of water molecules within the PVA templates and the decomposition of PVA were clearly observed in TG curves (Figure 7). During heating, OCP loses water molecules up to 200 °C,58 which results in a nonstoichiometric transformation to β-TCP and apatite crystals. The HAP crystals were stable up to 800 °C under the N2 atmosphere. Taking the thermal properties of OCP and HAP crystals into account, the proportion of inorganic components within the OCP and HAP hybrid thin films was calculated, as shown in Table 1. The amount of OCP decreased from 58.5 to 43.5 wt % as the thickness of the PVA templates was increased. This was presumably caused by a decreased rate of crystallization as the thickness of the PVA thin-film templates increased. The diffusion of ions into the thicker templates was more difficult. Additionally, the thicker PVA templates were subjected to longer annealing times, which would have increased the degree of cross-linking between the PVA chains. Thereby, the crystallization rate within the thicker PVA template became

different between the in-plane and out-of-plane XRD patterns. This suggested that the HAP hybrid thin films also had specific crystallographic orientations, and these structures were different for the different films (HAP/PVA200, HAP/PVA700, and HAP/PVA900). The intensity of the 002 reflection peak from the HAP in out-of-plane XRD patterns increased as the thickness of the thin films increased. This trend was similar to that of the OCP hybrid thin films, suggesting that the oriented structures within the hybrid thin films were maintained after the transformation using the aqueous solution process. Therefore, control of the structure and orientation of the OCP hybrid thin films by tuning the thickness of the PVA templates was an effective method to develop HAP hybrid thin films with controlled structures. Nanostructures of the OCP and HAP hybrid thin films were examined using transmission electron microscopy (TEM). Both OCP/PVA and HAP/PVA hybrid thin films contained assembled structures from nanorod crystals (Figures 6 and S4). Selected-area electron diffraction (SAED) patterns from the OCP hybrid thin films contained arched spots from the 002 reflections (Figure S4), which suggested that the nanocrystalline assemblies within the OCP/PVA hybrid thin films had caxis orientation. The HAP/PVA hybrid thin films were also composed of nanocrystals that had a rodlike morphology (width 5−10 nm, length 20−50 nm), as determined using TEM (Figure 6). The corresponding SAED patterns showed arched 002 reflections, which suggested that the HAP nanorods were oriented and elongated along the c-axis direction (Figure 6). These observations suggested that the D

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Figure 7. Thermogravimetric curves from the (a) OCP/PVA and (b) HAP/PVA hybrid thin films (heating rate = 10 °C/min up to 800 °C under a flow of N2).

Figure 8. SEM images at the initial stages of the formation of hybrid thin films using the (a) PVA200, (b) PVA700, and (c) PVA900 templates (images were obtained after 6 h crystallization time). Dotted boxes in the left panels indicate the regions used for obtaining the magnified SEM images shown in the right panels (scale bars = 1 μm (left panels) and 200 nm (right panels)).

Table 1. Composition of Hybrid Thin Films template

hybrid thin film

adsorbed water (wt %)

PVA200

OCP/PVA200 HAP/PVA200 OCP/PVA700 HAP/PVA700 OCP/PVA900 HAP/PVA900

11.7 10.5 5.9 5.0 12.6 6.1

PVA700 PVA900

organic components (wt %)

inorganic components (wt %)

29.8 38.0 43.3 39.5 43.9 45.9

58.5 51.5 50.8 55.5 43.5 48.0

of crystallization showed that PVA domains remained above and below the OCP thin films (Figures 8c and S5c). During the initial stages of crystallization after 6 and 24 h on OCP/ PVA700, PVA domains were observed above the spherulitic OCP thin films (Figures 8b and S5b). The thickness of these PVA domains (ca. 200 nm) provided enough space for further crystallization. In contrast, cross-sectional SEM images of OCP/PVA200 indicated that there were not enough PVA domains for further crystallization above the OCP spherulitic thin films (Figures 8a and S5a). By analyzing the cross-sectional SEM images of the films during the initial stages of crystallization, we have proposed a mechanism for the formation of the OCP/PVA hybrid thin films that had different crystallographic orientations, as schematically shown in Figure 9. Initially, nucleation occurred inside the PVA matrix. Crystal growth perpendicular to the substrates was inhibited by the PAA, which caused the growth of spherulitic thin films parallel to the substrates. This continued until neighboring spherulitic thin-film domains of the OCP crystals reached one another. Taking account of nanorod formation with an elongated c-axis (Figure S4), presumably, the c-axis of OCP crystals is consistent with the growing direction. XRD measurements suggested that hybrid thin films at the initial stage after 24 h crystallization exhibited a dominant c-axis orientation parallel to the substrates (Figure S6). The crystal growth in the PVA200 template was completed by the spherulitic thin-film formation and thus this hybrid thin film exhibited c-axis orientation parallel to the substrates (Figures 3a and 9a). PVA domains remained in PVA700 and PVA900 after the completion of the spherulitic thin-film growth parallel to the substrates, in which OCP could crystallize. The remaining PVA domains may act as templates for the formation of OCP nanorods with growth perpendicular to the substrate. Since the area of the remaining PVA domains increased as the thickness of the PVA templates was increased,

slow, resulting in the decrease of the amount of OCP crystals in the hybrid thin films. The TG results also showed that the proportion of inorganic material in the HAP/PVA hybrid thin films changed from the original OCP/PVA hybrid thin films. During the stoichiometric transformation from OCP into HAP, water molecules and PO43− ions are released from the OCP crystal structure.59 In addition, the transformation was performed in water at 80 °C, which would have partially dissolved the inorganic nanocrystals and the PVA templates. The HAP/PVA200, HAP/PVA700, and HAP/PVA900 hybrid thin films contained similar proportions of HAP nanocrystals (∼50 wt %, Table 1). These results showed that increasing the thickness of the PVA template did not significantly affect the proportion of inorganic material in the HAP/PVA hybrid thin films. Formation Process of the OCP Hybrid Thin Films in PVA Templates of Varying Thicknesses. Understanding what occurs during the initial stages of the hybrid thin-film formation is important to elucidate what effect the thickness of the PVA template has on the film structure. Cross-sectional SEM images of the OCP hybrid thin films after 6 h of crystallization are shown in Figure 8. Interestingly, crystallization occurred in the middle of the PVA900 template, whereas nucleation occurred at the interface of the PVA templates and glass substrates PVA200 and PVA700 (Figure 8). Spherulitic thin films that grew parallel to the substrates formed initially (up to 24 h of crystallization, Figure S5). Cross-sectional SEM images of OCP/PVA900 after 6 and 24 h E

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orientation within the hybrid thin films formed in the thickest thin-film template, PVA900. Therefore, increasing the thickness of polymer thin-film templates leads to drastic changes of hybrid structures accompanying the change of crystallization conditions in the template. The effects of template thickness on the structure of organic/inorganic hybrids that were observed in this work should allow the design of novel, highly ordered hybrid materials.

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CONCLUSIONS

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ASSOCIATED CONTENT

We have shown that the thickness of PVA thin-film templates affects the structure of PVA/HAP hybrid films that were formed using a biomineralization-inspired crystallization from amorphous precursors. This understanding should lead to improved template designs for organic/inorganic hybrid thin films that have highly ordered structures across a range of length scales. The OCP nanocrystals in the hybrid thin films exhibited a c-axis orientation perpendicular to the substrate, which was enhanced as the thickness of the PVA thin-film templates was increased. This control over the macroscopic caxis orientation was transferred to HAP hybrid thin films that were formed from the OCP hybrid thin films through topotactic transformation. Therefore, this biomineralizationinspired approach using polymer templates shows great promise for the development of ordered, HAP-based materials under mild conditions.

Figure 9. Schematic illustration showing the effects of the PVA template thickness on the oriented structures in the OCP hybrid thin films (blue arrows indicate the crystal growth direction of OCP nanorods, which corresponds to the c-axis).

the degree of perpendicular c-axis orientation was enhanced with increasing PVA template thickness (Figure 3). The SEM observations of the thin-film formation indicated that the OCP hybrid thin films that exhibited perpendicular caxis orientation were formed in two steps: (1) thin-film growth parallel to the substrate and (2) perpendicular overgrowth from the thin films toward the substrates. We previously reported a similar two-step crystal growth mechanism of formation for CaCO3-based hybrid thin films, in which the overgrowth of perpendicular aligned crystals occurred from spherulitic CaCO3 thin films that grew parallel to the direction of thin-film templates.60,61 Unlike the previous cases showing the first step of thin-film formation of CaCO3 on the surface of the PVA template, in the present OCP hybrid thin-film formation, both steps of spherulitic thin-film formation and overgrowth from the thin films occurred inside PVA hydrogel templates. In this situation, the PVA templates and confined surroundings significantly affect the OCP crystallization in both steps, leading to the development of well-ordered hierarchical structures. It is known that nanoscale confined surroundings significantly affect crystallization conditions including supersaturation and ion and mass diffusion behaviors.62 The confinement effects may have changed as the thickness of the templates was increased from 200 to 900 nm. The observations at the initial crystallization stage in PVA templates with three different thicknesses disclosed great potential for the effects of thickness change of the thin-film organic templates on ordered hybrid structures, although further observations are necessary for detailed elucidation of the thickness effects. It is noteworthy that the present case showed a high packing density of inorganic crystals at the overgrowth part, compared with hybrid thin films in our previous studies, which were formed through a two-step growth mechanism.60,61 The highly packed structures suggested that the overgrowth part passed through competition growth pathways, which play key roles in the formation of oriented structures in biomineral63 or biomimetic composites.13,21 It has been reported that crystalline assemblies formed through the competition growth pathways exhibited higher orientation as the distance of crystal growth increased.13,21,63 These effects may enhance the c-axis

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b04318. POM images for OCP/PVA and HAP/PVA hybrid thin films, SEM images for surface morphologies of OCP/ PVA and HAP/PVA hybrid thin film, TEM images and corresponding SAED patterns of OCP/PVA hybrid thin films, and cross-sectional SEM images for OCP/PVA hybrid thin films grown after 24 h of crystallization for the observation of crystallization at the initial stage (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.K.). *E-mail: [email protected] (T.K.). ORCID

Satoshi Kajiyama: 0000-0002-2200-7524 Takashi Kato: 0000-0002-0571-0883 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partially supported by JSPS KAKENHI Grant No. JP15H02179 and CREST, JST (JPMJCR15Q3). The authors would like to thank Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by “Nanotechnology Platform” of the MEXT, Japan, for TEM observation. S.K. is financially supported by the Murata Science Foundation. F

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