Gen Kamita

Unidirectional Alignment of Lamellar Bilayer in Hydrogel: One-Dimensional Swelling, Anisotropic Modulus, and Stress/Strain Tunable Structural Color

[∗] Dr. T. Kurokawa , Prof. J. P. Gong Faculty of Advanced Life Science Graduate School of Science Hokkaido University Sapporo, 060–0810 (Japan) E-mail: gong@sci.hokudai.ac.jp Dr. T. Kurokawa Creative Research Initiative Sousei Hokkaido University Sapporo, 001–0021 (Japan) M. A. Haque , G. Kamita Division of Biological Sciences Graduate School of Science Hokkaido University Sapporo, 060–0810 (Japan) Prof. K. Tsujii , Nanotechnology Research Center Research Institute for Electronic Science Hokkaido University (Retired) Sapporo, 001–0021 (Japan)

Digital Color in Cellulose Nanocrystal Films

Cellulose nanocrystals (CNCs) form chiral nematic phases in aqueous suspensions that can be preserved upon evaporation of water. The resulting films show an intense directional coloration determined by their microstructure. Here, microreflection experiments correlated with analysis of the helicoidal nanostructure of the films reveal that the iridescent colors and the ordering of the individual nematic layers are strongly dependent on the polydispersity of the size distribution of the CNCs. We show how this affects the self-assembly process, and hence multidomain color formation in such bioinspired structural films.

Controlled, bio-inspired self-assembly of cellulose-based chiral reflectors

Layered transparent photonic stacks are known to give rise to highly brilliant color in a variety of living organisms.[1] The biomimetic replication of these structures not only offers a wide range of applications, but can also be used as a tool to gain understanding of the biological processes responsible for the self-assembly of these structures in nature. Recent studies showed that cellulose microfibrils form helicoidal stacks in the plant cell wall, which selectively reflect circularly-polarised light of a specific wavelength.[2]–[5] Such structures are responsible for the bright colors in fruits[2] and leaves[3] of very different species of plants.[4,5] Similar photonic structures can be artificially produced using the same constituent material, cellulose nano-crystals (CNCs).[6,7] Slow evaporation of a CNC suspension gives rise to their spontaneous assembly into a chiral nematic liquid crystalline phase that can be preserved in the dry state.[8,9] The self-assembly process is strongly dependent on the properties of the nanoscale building blocks and on the macroscopic parameters that characterise the assembly.[10]–[12] Many factors influence the optical and mechanical properties of the obtained film, including temperature and pressure[13,15,16] the substrate,[14] and the surface chemistry of the CNCs.[17,18] Nevertheless the self-assembly process is robust and can be coupled with a range of chemical processes.[19]–[21]

Hierarchical Self-Assembly of Cellulose Nanocrystals in a Confined Geometry

Complex hierarchical architectures are ubiquitous in nature. By designing and controlling the interaction between elementary building blocks, nature is able to optimize a large variety of materials with multiple functionalities. Such control is, however, extremely challenging in man-made materials, due to the difficulties in controlling their interaction at different length scales simultaneously. Here, hierarchical cholesteric architectures are obtained by the self-assembly of cellulose nanocrystals within shrinking, micron-sized aqueous droplets. This confined, spherical geometry drastically affects the colloidal self-assembly process, resulting in concentric ordering within the droplet, as confirmed by simulation. This provides a quantitative tool to study the interactions of cellulose nanocrystals beyond what has been achieved in a planar geometry. Our developed methodology allows us to fabricate truly hierarchical solid-state architectures from the nanometer to the macroscopic scale using a renewable and sustainable biopolymer.

Layer-by-Layer Formation of Block-Copolymer-Derived TiO2 for Solid-State Dye-Sensitized Solar Cells

Morphology control on the 10 nm length scale in mesoporous TiO(2) films is crucial for the manufacture of high-performance dye-sensitized solar cells. While the combination of block-copolymer self-assembly with sol-gel chemistry yields good results for very thin films, the shrinkage during the film manufacture typically prevents the build-up of sufficiently thick layers to enable optimum solar cell operation. Here, a study on the temporal evolution of block-copolymer-directed mesoporous TiO(2) films during annealing and calcination is presented. The in-situ investigation of the shrinkage process enables the establishment of a simple and fast protocol for the fabrication of thicker films. When used as photoanodes in solid-state dye-sensitized solar cells, the mesoporous networks exhibit significantly enhanced transport and collection rates compared to the state-of-the-art nanoparticle-based devices. As a consequence of the increased film thickness, power conversion efficiencies above 4% are reached.

Controlling the Photonic Properties of Cholesteric Cellulose Nanocrystal Films with Magnets

The self-assembly of cellulose nanocrystals is a powerful method for the fabrication of biosourced photonic films with a chiral optical response. While various techniques have been exploited to tune the optical properties of such systems, the presence of external fields has yet to be reported to significantly modify their optical properties. In this work, by using small commercial magnets (≈ 0.5-1.2 T) the orientation of the cholesteric domains is enabled to tune in suspension as they assemble into films. A detailed analysis of these films shows an unprecedented control of their angular response. This simple and yet powerful technique unlocks new possibilities in designing the visual appearance of such iridescent films, ranging from metallic to pixelated or matt textures, paving the way for the development of truly sustainable photonic pigments in coatings, cosmetics, and security labeling.

Multilayer mirrored bubbles with spatially-chirped and elastically-tuneable optical bandgaps

We demonstrate the multifolding Origami manufacture of elastically-deformable Distributed Bragg Reflector (DBR) membranes that reversibly color-tune across the full visible spectrum without compromising their peak reflectance. Multilayer films composed of alternating transparent rubbers are fixed over a 300 μm wide pinhole and deformed by pressure into a concave shape. Pressure-induced color tuning from the near-IR to the blue arises from both changes in thickness of the constituent layers and from tilting of the curved DBR surfaces. The layer thickness and color distribution upon deformation, the band-gap variation and the repeatability of cyclic color tuning, are mapped through micro-spectroscopy. Such spatially-dependent thinning of the film under elastic deformation produces spatial chirps in the color, and are shown to allow reconstruction of complex 3D strain distributions.

Research data supporting "Controlling the Photonic Properties of Cholesteric Cellulose Nanocrystal Films with Magnets"

The data are organized and grouped in dedicated .zip files for each Figure they contribute to. All figures (ToC, Figures 1-4 and SI_1-17) are present in high resolution in each sub-folder. Software for file extensions: .mi (Gwyddion), .mat, .m and .fig (MATLAB), .blend (Blender). The following Info is also available in README_OpenData.pdf: *** ToC. Original photographs (.jpg) Figure 1. Original photographs (.jpg) Figure 2. Original graphics 2A-F (.png, .fig) spectrometer settings (.mat) Original photographs (Figure 2K) Figure 3. Original microscopy photographs (Figures 3A-L) (.png) and scale bar (.png) Bertrand lens and k-space calibration with grating (.png, .xlsx, xls) Figure 4. Original SEM photographs (.tif) Figure S1. AFM (.tiff, .mi) Figure S2. Titration file (.xlsx, xls, .png) Figure S3. Original photographs (.jpg) Sample preparation and Phase diagram (.xlsx, xls) Figure S4. Original photographs (.jgp) evaporation rate (.xlsx, xls) Figure S5. Original photographs (.jgp) Figure S6. Original photographs (.jgp) Figure S7. Original photographs (.jgp) Figure S8. Original photographs and schematics (.png) Figure S9. Original photographs and schematics (.jpg, .png) Figure S10. Original SEM images (.tif) Figure S11. Original SEM images (.tif) Figure S12. Original SEM images (.tif) Figure S13. Original SEM images (.tif) Figure S14. Experimental magnetic field mapped of the tilted field geometry (.xlsx, .xls) Visualization of the magnetic field for tilted field geometries (.m, .png, .fig) 3D schematic of the two magnets with the iron plate (.png, .blend) Visualization of the magnetic field for simple geometries (.pdf, .png) Figure S15. Computing of the magnetic field for the tilted field geometry (magnetic_field_calculated_mapping.xlsx, .xls) Computed magnetic field for the tilted field geometry (TwoMagVertAndTwoMagUpsideDown.xlsx, .xls) Script in MATLAB to create the figures (.m) Original graphic files (.fig, .png) Figure S16. Original figures (.fig, .png) Script in MATLAB illustrating the formula used to create the fits (.m) fit datapoints (.txt) Figure S17. Original photography (.jpg)