Mesostructure of the mesoporous silica films coated on ITO substrates

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nanostructured film, Raman scattering spectra were measured using Rhodamine B as a probe molecule. A Rhodamine B aqueous solution (1.0 μM) was drop-casted on nanostructured Au films in order to locate this probe on the substrate. The composite film before silica removal and a bare ITO substrate on which Au was electrodeposited under the same deposition conditions were also measured as references.

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Figure 2.2 (a) Cross-sectional HR-SEM image of the template film (spin coated at 4000 rpm), (b), (c) FFT images in the areas shown as (1) and (2) in (a), respectively.

vertical direction to the substrate by around 30 % of an ideal 2D hexagonal structure on the basis of the GI-SAXS data.⁵⁷ The cross-sectional HR-SEM images clearly indicate the shrinkage of 2D hexagonal mesostructures (Figure 2.2 and Figure 2.3) and the lying

Figure 2.3 High magnification images of Figure 2.2. (a) hexagonal pattern and (b) stripe pattern

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Figure 2.4 SEM image of the monodomain structure with hexagonal symmetry of the template film from the substrate surface to the film surface (spin coated at 4000 rpm).

mesochannels parallel to the substrate. However, the spinning rate significantly affected the mesochannel orientation at the interface between the film and the ITO substrate.

When the spinning rate was 4000 rpm, all the mesochannels were perfectly aligned parallel to the substrate over the whole thickness of the mesoporous silica films (Figure 2.2 and Figure 2.4). In contrast, with the decrease of the rate down to 3000 rpm, vertically aligned mesochannels were partially formed near the substrate surface, as was confirmed by cross-sectional HR-SEM images (Figure 2.5). As shown in Figure 2.5, the domain of vertically oriented mesochannels prevails over 500 nm in lateral direction and 150 nm along the perpendicular direction. A schematic view of the mesostructure of the film is shown in Figure 2.6. The domains mentioned above are frequently observed

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Figure 2.5 Direct observation of vertical aligned domain by HR-SEM (a) low magnification image and (b) high magnification image.

broad. Because the size of circle formed by slicing a sphere depends on the position of the slicing, the size of the vertical domain observed from the HR-SEM images may not reflect the largest size. For example, the cross-sectional images of planes (A) and (B) in Figure 2.6 should provide two distinctive images due to different domain sizes. The mesochannels in the domains with vertical orientation tend to fold as they are apart from the substrate and to smoothly merge into mesochannels lying parallel to the substrate.

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Figure 2.6 Schematic image of the mesostructure near the domain of vertical aligned mesopores. Light gray: mesopores, dark gray: domain of vertical aligned mesopores,

light blue: region of parallel aligned mesopores, and deep blue: substrate.

Both the slow rate of the mesostructural formation and the surface roughness of ITO substrates may cause the formation of the vertical orientation at the interface.

Tolbert et al. reported that the slow formation of mesostructures allows micelles to be aligned vertically on patterned substrate in mesoscale.⁵⁸ The slower rotating speed at spin coating forms a thicker liquid film of precursor solution, resulting in the slow formation of the mesostructures. Of course, only the slow formation of mesostructure does not change the alignment of micelles which must be achieved by a certain driving force. One of the possible forces to align micelles is the roughness of the substrate surface. The effect of rough surfaces on the alignment of mesochannels has already been revealed in a previous report which describes the vertical alignment of mesochannels of mesoporous silica on conical hole arrays of AAO.⁵⁹ The surface roughness of ITO substrates may stabilize the vertically aligned micelles, as found for the rough surface of AAO. Another hypothesis is the flow induced alignment by the Bénard-Marangoni convection. Because volatile moieties (like ethanol, HCl, and water) in the precursor solution evaporate at the early stage of spin coating, the convection

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cells are developed by the gradients of heat and concentration in the liquid fluid film. It is known that the Bénard-Marangoni convection occurs through sol-gel processes and affects the surface texture of formed films.⁶⁰ The vertical flow of the convection may align the micelles vertically, which is similar to the flow induced alignment methods.^61-65 The surface roughness of ITO substrates disturbs the convectional flow near the substrate surface.⁶⁶ Turbulent flows would generate near the surface and should affect the alignment of micelles. Because the vertical flow in convection cells are spatially distributed rod-like micelles should be vertically aligned in a partial domain, but not in the whole film. These two factors can induce the vertical alignment of mesochannels.

During spin coating, droplets are distributed from the rotation center to the substrate edges, inducing shear flows. This flow should force micelles to align along the parallel direction to the substrate. On the other hand, evaporation of volatiles during the spin coating induces a vertical compressive stress from the film surface to substrate surface. The stress also aligns rod-like micelles along the parallel direction.

Consequently, rod-like micelles are generally aligned along the parallel direction by these two factors. In this study, vertical aligned mesochannels were observed only in limited and partial regions. In the other areas, such as near the film surface, mesochannels are oriented along the parallel direction. Therefore, I can summarize that the shear flow mainly affect the alignment of micelles over the whole film and that vertical flow of the convection partially overcomes the shear flow and align the micelles vertically on the rough surface.

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