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Anisotropic and reversible deformation of mesopores and mesostructures in silica-based films under mechanical stimuli toward adaptive optical components Kenji Okada, Genki Asakura, Tatsuya Yamamoto, Yasuaki Tokudome, and Masahide Takahashi ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00269 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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ACS Applied Nano Materials

Anisotropic and reversible deformation of mesopores and mesostructures in silica-based films under mechanical

stimuli

toward

adaptive

optical

components Kenji Okada, Genki Asakura, Tatsuya Yamamoto, Yasuaki Tokudome, and Masahide Takahashi* Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka, 599-8531, Japan KEYWORDS:

mesoporous silica, mesostructured hybrid, deformation, optical property,

anisotropic refractivity

ABSTRACT: Deformation of mesostructures in surfactant/silica hybrid films and mesoporous silica films prepared on bulky elastomeric substrates are investigated by compressing the substrates along an in-plane direction of the films. The mesostructure in the surfactant/silica hybrid film consists of organized micelles and silica matrix. The micelles are removed in the mesoporous silica films to leave mesopores. A grazing incidence small-angle X-ray scattering (GI-SAXS) investigation reveals that the mesostructures with C2mm symmetry anisotropically change under compressive strain of 6 % in both films; shrinkage of lattice parameters along the direction of

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compression and extension in normal direction. Deformation is reversible below the strain as high as 6 %. Anisotropic deformation of mesostructures results in an anisotropic change of refractive indices in normal and parallel to the direction of the compression. Compression of the surfactant/silica hybrid film induces a decrease of refractive index parallel to surface and an increase normal to surface in the order of 10-2. While, an opposite result is observed for the mesoporous silica film. The present finding gives us fundamental insights to develop adaptive optical components for sensing, attenuation, switching, modulation, shutters, filters and others in soft materials-based devices.

1. INTRODUCTION Mesoporous silica and mesostructured organics-silica hybrid films have attracted interests as low refractive index, ultralow-k dielectric and thermal-insulating materials in semiconductor industry1-6. An evaporation-induced self-assembly process using soft templating agents such as surfactants is well-used to obtain surfactant/silica hybrid films with well-defined mesostructures, where micelles of the soft templating agents are embedded in silica matrix7-10. Silica films with ordered mesopores are obtained after a removal of the templating agents from the hybrid films by thermal decomposition or solvent extraction11-15. The mesostructures (porosity as well as pore shape, size, and spatial arrangement) in the mesostructured hybrid and mesoporous silica films can be tuned by adjusting the synthesis conditions and the types of templating agents16. The control of mesostructures is important as it is correlated with optical and thermal properties of silica-based mesoporous and hybrid organics-silica mesostructured films. Both computational and experimental studies have revealed that refractive indices and dielectric


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constants of mesoporous silica films changed by their porosity as well as pore shape, size, and spatial arrangement17. Hutchinson et al. have demonstrated that mesoporous silica films with cubic and hexagonal-type mesostructures showed different reflectance and these results were consistent with computational simulations18. Similar investigation has been conducted on thermal conductivity and thermal-insulation properties of silica-based mesostructured films19,20. The computational studies also indicated that anisotropic refractive index and absorption index in inplane direction to the film surface were expected on silica-based films with anisotropic mesostructures21. Indeed, mesoporous silica films with a birefringence has been achieved by a control of pore alignment22. Although the impacts of mesostructures on optical and thermal properties have been investigated so far, these experiments were conducted on only static films. In some reports, with the aim of an application for drug delivery systems, openable and closable mesopores has been demonstrated on mesoporous silica nanoparticles modified with stimuliresponsible polymers so far23-25. Only dimensions of the stimuli-responsible polymers in the pores isotropically change. On the other hand, the silica-based moiety, that works as robust host matrix, remain unchanged. If we could induce stimuli-responsible changes of configuration, size, and shape of host silica matrix by applying anisotropic and reversible external stimuli such as a compressive stress, this material would be expected as adaptive optical components for sensing, attenuation, switching, modulation, shutters, filters and others in soft materials-based devices such as wearable biological sensors, local optical networking system, and so on. Herein, reversible and anisotropic deformation of mesostructures of silica-based films are reported for the first time by mechanical compression of mesostructured surfactant/silica hybrid films and mesoporous silica films formed on elastomeric substrates (Polydimethylsiloxane, PDMS). Anisotropic deformation of the mesostructures resulted in anisotropic changes of


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refractive index. This report helps to understand how the mesostructures and optical properties of silica-based films change by mechanical compression, which is important when the mesoporous silica or mesostructured surfactant/silica hybrid films will be used as low refractive index materials in flexible devises.

2. EXPERIMENTAL Preparation of surfactant/silica hybrid and mesoporous silica films. Both the surfactant/silica hybrid and mesoporous silica films were prepared from a same solution.26 Pluronic F127 (OH(CH2-CH2O)106(CHCH3CH2O)70 (CH2CH2O)106H) was purchased from Sigma-Aldrich Co., Ltd., tetraetylorthosilicate (TEOS), methyltriethoxysilane (MTES) and ethanol (EtOH) were purchased from Wako Pure Chemical Co., Ltd., and used as received. The precursor solution was prepared as follows. 0.75 g of Pluronic F127 was completely dissolved in a mixture containing 3.5 mL of EtOH, 0.75 mL of H2O and 0.20 mL of 1 M HCl in vial container. The solution was subsequently stirred for 10 min at room temperature and 15 min in the ice bath. Then, 1.45 mL of TEOS and 0.65 mL of MTES were added to the solution and stirred for 20 min in the ice bath. The final molar ratio was TEOS : MTES : Pluronic F127 : EtOH : HCl : H2O = 1.0 : 0.50 : 9.0×10-3 : 9.2 : 6.4 : 0.030. The precursor solution was aged for 24 hours at 60 °C. After aging, the precursor solution was allowed to cool down at room temperature for 15 min. The surfactant/silica hybrid film was prepared by spin-coating of above solution on an elastomeric substrate (Polydimethylsiloxane, PDMS) with a size of 17.5 mm (width) × 16 mm (length) × 14 mm (height). The PDMS substrate was prepared as follows. Silpot 184 and Silpot


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cat. 184 (Dow Corning Toray Co., Ltd., Japan) were mixed (Silpot 184 : Silpot cat. 184 = 10 : 1) and stirred for 3 min. After degassing for 90 min, the mixture was poured into a container with a size of 17.5 mm (width) × 16 mm (length) × 14 mm (height), and solidified on a hot plate (200 °C). Then, PDMS substrate was aged for 72 hours at 60 °C. Before coating of the solution for surfactant/silica hybrid film, the PDMS substrate was UV/O3 treated for 5 min in order to make the surface of PDMS hydrophilic. Then, the surfactant/silica hybrid solution was spin-coated on the hydrophilized PDMS substrate at 1000 rpm for 30 seconds. After coating, the surfactant/silica hybrid film was aged for 48 hours at 25 °C, 40 % RH. The mesoporous silica film on a PDMS substrate was prepare by a transfer method reported by Kozuka et al. with minor modification.27 At first, a polyvinylpyrrolidone (PVP) film was prepared on a Si substrate by spin-coating of an ethanolic solution (10 mL) containing 1.1g of PVP K90 (Wako Pure Chemical Co., Ltd., and used as received) at 8000 rpm for 30 seconds. The PVP film on a Si substrate was calcined at 450 °C for 45 min. Then, the solution for the surfactant/silica hybrid was spin-coated on the calcined PVP film at 1000 rpm for 30 seconds. After coating, the surfactant/silica hybrid film was aged for 48 hours at 25 °C, 40 % RH. A mesoporous silica film on the calcined PVP film was obtained by a calcination of the surfactant/silica hybrid film at 300 °C for 2 hours in order to remove the surfactant (Pluronic F127). Then, a PDMS substrate with a size of 17.5 mm (width) × 16 mm (length) × 14 mm (height) was solidified on the mesoporous silica film using a container on a hot plate (200 °C). The PDMS substrate was aged for 72 hours at 60 °C. After these processes, the mesoporous silica film was transferred onto the solidified PDMS substrate. Characterization.


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The compression of the films on the PDMS substrates was conducted by compressing the PDMS substrates using two metal plates. The mesostructures of mesoporous silica film were observed by a field emission SEM (S-4800, Hitachi Co., Ltd., Japan). The mesostructures were investigated by X-ray diffraction (XRD) experiments (SmartLab with and without HyPix-3000 system, Rigaku Co., Ltd., Japan, and Nano-Viewer, Rigaku Co., Ltd., Japan). The refractive indices of the films were measured using a prism coupler (Model 2010, Metricon, USA). Linearly polarized light at 632.8 nm by a He–Ne laser was used. The in-plane and out-of-plane refractive indices (nTE and nTM) of the film was measured by changing the polarization angle of the laser; parallel (TE mode) and perpendicular (TM mode) to the film, respectively. The spot size was 1.0 mm in a diameter.

3. RESULTS AND DISCUSSION The concept of the present study is illustrated in Figure 1. Mesostructured surfactant/silica hybrid films and mesoporous silica films with 2-3 μm in thickness were used for the investigation of reversible and anisotropic deformation of mesostructures. The surfactant/silica hybrid film with well-ordered mesostructures was deposited on bulk PDMS substrate by a spin-coating of a conventional precursor solution26. The mesoporous silica film was also prepared on the PDMS substrate from the same coating solution through a film-transfer-process using sacrificial polymer layer;27 the surfactant/silica hybrid film was first prepared on polyvinylpyrrolidone (PVP) sacrificial layer on a Si substrate, which was calcined at 300 °C in order to remove the surfactant. Subsequently, the mesoporous silica film on PVP layer was transferred onto a PDMS substrate. Both surfactant/silica hybrid and mesoporous silica films used in this study exhibited planar


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rectangular 2D mesostructures with C2mm symmetry, that was confirmed by grazing incidence small-angle X-ray scattering (GI-SAXS) measurements (Figure 1a and Figure S1)28,29. The mesostructure in the surfactant/silica hybrid film consists of organized micelle and silica matrix, and that in the mesoporous silica film consists of pore and silica matrix. The pores with ~7 nm in a diameter were observed in the mesoporous silica films by scanning electron microscopy (SEM) as shown in Figure 1b. A direct observation of the mesostructure in the surfactant/silica hybrid

Figure 1. (a) A schematic illustration of a mesostructured surfactant/silica hybrid film or a mesoporous silica film that has planar rectangular 2D mesostructures with C2mm symmetry. (b) A SEM image of the mesoporous silica film showing C2mm mesophase and ~7 nm pores in a diameter. (c) A concept of present study: anisotropic and reversible deformation of mesostructures of the mesostructured silica-based films by compression with accompanying deformation of the elastomeric substrates (PDMS).


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films by SEM was difficult because of a small electron density difference between micelle and wall materials. Both mesostructured surfactant/silica hybrid films and mesoporous silica films formed on elastomeric PDMS were compressed in in-plane direction to the film surface (Figure 1c). Hereafter, the axes parallel and normal to the direction of the compression is defined as x-axis and y-axis, respectively. Deformation of mesostructures by compression was investigated by insitu GI-SAXS measurement. The experimental set-up of GI-SAXS investigation upon compression of the films is illustrated in Figure 2a. X-ray was irradiated to the films perpendicularly to x-axis. Deformation of mesostructures in x-axis and y-axis was investigated. Both surfactant/silica hybrid and mesoporous silica films have mesostructures with polycrystalline domains (random orientation) in in-plane direction to the film as seen in Figure 1a. By investigating the shifts of the spots in the diffraction pattern, deformation of the organized surfactants and pores parallel to X-ray beam was investigated by GI-SAXS30 (Figure 2b). It should be noted that the both mesostructured surfactant/silica hybrid films and mesoporous silica films adhered to the surface of PDMS, as these films were not detached from the PDMS during the deformation investigations.


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Figure 2. (a) Schematic illustrations showing the experimental set-up of GI-SAXS investigation. Compression to the films was applied from vertical direction to the X-ray beam. (b) Mesostructures with polycrystalline domains (random orientation) in in-plane direction to the films. In the present study, deformation of the organized surfactants and pores parallel to X-ray beam (highlighted part) was investigated by GI-SAXS.


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Figure 3. GI-SAXS images (upper) and corresponding mesostructures (below) of the surfactant/silica hybrid film without (ε = 0 %) and with compression (ε = 6 %) (compressive strain, ε (%): dL/L0×100, see Figure S2). Figure 3 shows GI-SAXS images of the surfactant/silica hybrid film without (ε = 0 %) and with compression (ε = 6 %) (compressive strain, ε (%): dL/L0×100, see Figure S2). The illustrations under the images indicate the corresponding mesostructures obtained from the GISAXS measurements. While the space group of mesophase remained as C2mm after compression of the film, 6% shrinkage of a value from 14.14 nm to 13.22 nm in x-axis and 4.4% extension of b value from 21.70 nm to 22.14 nm in y-axis were observed. Relative changes of a and b value upon compression were further investigated by varying compressive strain. Relative change of a linearly decreased with compressive strain, while that of b linearly increased (Figure 4a). This result indicates that the mesostructures are anisotropically changed by compression. In addition,


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deformation was reversible because a and b values before compression was consistent with those after stress relaxation (Figure 4b). Similar changes of the mesostructure with compression were observed on the mesoporous silica film (Figure S3). It has been reported that silica-based mesoporous materials have flexibility as their mesostructures change through swelling by water adsorption31-35. The results presented here are the first observation to demonstrate that the anisotropic and reversible changes of mesostructures of surfactant/silica hybrid films and mesoporous silica films can be controlled by mechanical compression.

Figure 4. (a) The relative change of a and b values of mesostructures of the surfactant/silica hybrid film with varying compressive strain from 0 % to 6 %. (b) XRD patterns ((02) plane) of the surfactant/silica hybrid film with compressive strain at 0, 2, 4, 6 %, and 0 % (after stress relaxation from 6 % of compressive strain). Further compression of the surfactant/silica hybrid film over 6 % of compressive strain did not allow for obvious deformation of the mesostructures (Figure 5a). When the compressive strain exceeding 6 % is applied, the formation of wrinkle structures (6 % < ε < 8 %) and cracks (8 % ≤ ε) on the film in μm-scale was observed (Figure 5b). The compressive stress was relaxed by forming the wrinkles or cracks instead of deformation of the mesostructures. The formation of wrinkle structures with uniform periodicity and amplitude indicates that the component and


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structure in the surfactant/silica hybrid film are homogeneous over entire surface of the substrates.36 The estimation of mechanical parameters of the films (or multilayered system) of interest is theoretically possible as reported in the litaruture37, which will be worth investigating in the future. The formation of wrinkles over 6 % of compressive strain was not observed on the mesoporous silica film. Detachment of the mesoporous silica film from PDMS was partially observed at 10 % of compressive strain. This is presumably because elastic module of mesoporous silica film is too high to form wrinkles compared to the surfactant/silica hybrid film 38-40. From these results, we concluded that deformation of mesostructures of both surfactant/silica hybrid and mesoporous silica films is reversible within 6 % of compressive strain.


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Figure 5. (a) The relative change of b value in the surfactant/silica hybrid film with varying compressive strain from 0 % to 16 %. (b) Optical microscope images of the surfactant/silica hybrid film and corresponding schematic illustrations of mesostructures (nm-scale) in the films and surface macrostructures (μm-scale) at different compressive strains. The optical microscope images were taken at the compressive strain of 0, 6 and 8 %. Deformation of mesostructures of silica-based films allows for the change of optical properties as the mesostructure is correlated with optical properties17. Form the above results, the nanometer-scale mesostructures of surfactant/silica hybrid and mesoporous silica films could be controlled by compressing these films until the compressive strain as high as 6%. Deformation of mesostructure is of anisotropic along in-plane and out-of-plane directions to the film surface (x-


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axis and y-axis, respectively); shrinkage in x-axis and extension in y-axis. Thus, it is expected to control the out-of-plane and in-plane optical properties of these films by compression-induced deformation of mesostructures. Herein, we investigated changes of the refractive indices of both surfactant/silica hybrid and mesoporous silica films in x-axis and y-axis (Figure 6). The refractive indices of the films were investigated using a prism coupler with linearly polarized light at 632.8 nm. The refractive indices in x-axis (nTE) and y-axis (nTM) of the film was measured by changing the polarization angle of the light; parallel (TE mode) and perpendicular (TM mode) to the film surface, respectively. This method is commonly used to investigate the refractive index anisotropy of films.41-43 A proportion of refractive index in y-axis to refractive index in x-axis (nTM/nTE) was investigated by measuring nTM and nTE of both surfactant/silica hybrid and mesoporous silica films at compressive strain with 0, 2, 4 and 6 %. As shown in Figure 6, nTM/nTE of the surfactant/silica hybrid and mesoporous silica films exhibited opposite behaviors with compression; nTM/nTE increases in the surfactant/silica hybrid film, while nTM/nTE decreases in the mesoporous silica film. The both surfactant/silica hybrid and mesoporous silica films had C2mm-type mesostructures that is slightly shrunk in y-axis compared to isotropic mesostructures with hexagonal P6mm symmetry. In the case of the mesoporous silica film, the mesostructures are composed of air pore and silica matrix. A refractive index of air pore is lower than that of silica matrix. It has been reported so far that elliptical pores whose shorter length is directing to y-axis are formed due to a shrinkage of mesostructures in y-axis upon evaporation.26 With compression, the pores are squashed up in xaxis, resulting in an elliptical shape with longer length in y-axis. Consequently, a proportion of silica matrix to pores in y-axis becomes small and nTM/nTE decreased with compression. The opposite results were observed on the surfactant/silica hybrid film. This can be explained by an opposite reason of the mesoporous silica film. The mesostructures of the surfactant/silica hybrid


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film are composed of organized surfactant embedded in silica matrix. The refractive index of surfactant was slightly higher than silica matrix.44 Thus, an increase of nTM/nTE with compression was observed by the compression-induced deformation of mesostructures in the surfactant/silica hybrid film. In both cases, refractive index difference between TE and TM mode was exceeding 10-2 order. As mentioned above, the both films used in the present study have mesostructures with polycrystalline domains (random orientation) in in-plane direction to the film. If perfectly-oriented films with single-crystalline domains are used, much higher refractive index anisotropy would be expected because all the organized surfactants and pores are responsible to applied compression. This result also revealed how deformation of mesostructure in mesostructured silica-based films impacts on their optical properties. It should be also noted that the surfactant/silica hybrid film exhibited reversibility of refractive index anisotropy as similar to the reversible deformation of the mesostructure. Although refractive index anisotropy has been reported in the films with anisotropic micropores so far, refractive index anisotropy is investigated only in in-plane direction of the films and the optical responsibility does not show reversibility in the most cases.45 These silica-based film with mesostructures are expected to be used as advanced low refractive index materials on flexible devices. Thus, this report will help to understand correlation between deformation of films and anisotropic and reversible changes of refractive index of films.


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Figure 6. A proportion of refractive index in y-axis to refractive index in x-axis (nTM/nTE) as a function of compressive strain. nTE and nTM are refractive indices of the films against TE and TM polarization, respectively (TE mode: polarization angle of the light is parallel to the film surface, TM mode: polarization angle of the laser is perpendicular to the film surface.). Schematic illustrations showing the changes of mesostructures and nTM/nTE with compression for the mesoporous silica and mesostructured surfactant/silica hybrid films.

4. CONCLUSION Deformation of mesostructures in surfactant/silica hybrid and mesoporous silica films was investigated by mechanical compression of these films formed on elastomeric substrates. Mesostructures in both films anisotropically changed as the mesostructures extended in out-ofplane direction to the film surface and shrunk in in-plane direction. Deformation of mesostructures


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was reversible when the compressive strain was below 6%. Refractive index anisotropy was induced by anisotropic deformation of mesostructures. In the surfactant/silica hybrid film, a decrease of in-plane refractive index and an increase of out-of-plane refractive index were observed with compression. While, a decrease of out-of-plane refractive index and an increase of in-plane refractive index were observed with compression in the mesoporous silica film. Further detailed experiments for the films with different mesostructures will reveal how deformation of mesostructure of mesostructured silica-based films impacts on their optical properties. These mesostructured films would be expected as adaptive optical components for sensing, attenuation, switching, modulation, shutters, filters and others in soft materials-based devices such as wearable biological sensors, local optical networking system, and so on.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. GI-SAXS images, simulated reciprocal lattices and real lattices of the mesostructured surfactant/silica hybrid film and mesoporous silica film. Definition of compressive strain, ε (%). XRD patterns of the mesoporous silica film with compressive strain at 0, 5, 10%, and 0% (after stress relaxation from 10% of compressive strain). AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Phone: +81-72-254- 9309 ACKNOWLEDGMENT


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This work was partly supported by a Grant-in-Aid for Scientific Research (B) (26288108) from the Ministry of Education, Culture Sports, Science and Technology of Japan. We acknowledge Prof. Hironori Kaji and Dr. Tatsuya Fukushima (Kyoto University) for their support in the GISAXS measurements. REFERENCES (1) Falcaro, P.; Grosso, D.; Amenitsch, H.; Innocenzi, P. Silica Orthorhombic Mesostructured Films with Low Refractive Index and High Thermal Stability. J. Phys. Chem. B 2004, 108, 1094210948. (2) Soler-Illia, G. J. A. A.; Innocenzi, P. Mesoporous Hybrid Thin Films: The Physics and Chemistry Beneath. Chem. Eur. J. 2006, 12, 4478-4494. (3) Baskaran, S.; Liu, J.; Domansky, K.; Kohler, N.; Li, X.; Coyle, C.; Fryxell, G. E.; Thevuthasan, S.; Williford, R. E. Low Dielectric Constant Mesoporous Silica Films Through Molecularly Templated Synthesis. Adv. Mater. 2000, 12, 291-294. (4) Balkenende, A. R.; de Theije, F. K.; Kriege, J. C. K. Controlling Dielectric and Optical Properties of Ordered Mesoporous Organosilicate Films. Adv. Mater. 2003, 15, 139-143. (5) Miller, R. D. In Search of Low-k Dielectrics. Science 1999, 286, 421. (6) Theije, F. K.; Balkenende, A. R.; Verheijen, M. A.; Baklanov, M. R.; Mogilnikov, K. P.; Furukawa, Y. Structural Characterization of Mesoporous Organosilica Films for Ultralow-k Dielectrics. J. Phys. Chem. B 2003, 107, 4280–4289.


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