Crack-Free Structural Color Materials Prepared without Disrupting the Particle Arrangement by Controlling the Internal Stress Relaxation and Interactions of the Melanin Particles.

In fabricating structural color materials with assembled colloidal particles, there is a trade-off between the internal stresses acting on the particles and the interactions between the particles during solvent volatilization. It is crucial to fabricate crack-free materials that maintain the periodic arrangements of the particles by understanding the mechanism for crack initiation. Here, we focused on the composition and additives of melanin particle dispersions to obtain crack-free structural color materials without disturbing the particle arrangements. The use of a water/ethanol mixture as a dispersant effectively reduced the internal stresses of the particles during solvent evaporation. Furthermore, the addition of low-molecular-weight, low-volatility ionic liquids ensured that the arrangement and interactions of the particles were maintained after solvent volatilization. Optimization of the composition and additives of the dispersion made it possible to achieve crack-free melanin-based structural color materials while maintaining vivid, angular-dependent color tones.

Rapid Growth of Colloidal Crystal Films from the Concentrated Aqueous Ethanol Suspension.

Developing a rapid fabrication of colloidal crystal film is one of the technical issues to apply to wide and various fields. We have been investigating a drying process of colloidal aqueous ethanol (EtOH) suspension formed by electrophoretic deposition (EPD). Here, the detailed formation mechanism of the colloidal crystal films with the closest packing structure was investigated by optical microscope and spectroscopy. The growth mechanism from the colloidal suspension to the colloidal crystal film was found to consist of four stages. In the first stage, concentrated colloidal suspension changed to order structure, i.e., nonclosely packed colloidal crystal by Alder phase transition. After this crystallization, we observed Bragg's diffraction peak and structural color. In the second stage, the diffraction peak shifts toward the shorter-wavelength direction (blue shift) due to the reduction of the interparticle distance of the nonclosely packed colloidal crystal. Finally, this peak shift continued until the closely packed colloidal crystal film was formed. In the third stage, the diffraction peak kept almost a similar wavelength due to the liquid film of aqueous EtOH covering on the colloidal crystal film. In the fourth stage, the colloidal crystal film changed from wet to dry condition. The structural color changes from green to blue by the evaporation of the solvent from the interspace of the colloidal crystal film. This color change is explained by the change in the refractive index of the interparticle medium from solvent to air. One of the key findings in our process is a rapid crystal growth using concentrated colloid aqueous EtOH suspension. Drying the concentrated suspension formed a closely packed colloidal crystal film within 55 s. This process has the potential for high-speed deposition of the colloidal crystalline thin films.

Formation of Uniform and High-Coverage Monolayer Colloidal Films of Midnanometer-Sized Gold Particles over the Entire Surfaces of 1.5-in. Substrates.

We report a simple and facile method for fabricating monolayer colloidal films of alkanethiol-capped gold nanoparticles (AuNPs) on glass substrates. The new method consists of two sequential sonication processes. The first sonication is performed to obtain a well-dispersed state of alkanethiol-capped AuNPs in hexane/acetone in the presence of a substrate. After additional static immersion in the colloidal solution for 5 min, the substrate is subjected to sonication in hexane. By using this method, we succeeded in forming uniform and stable assemblies of midnanometer-sized AuNPs (14, 34, and 67 nm in diameter) over the entire surface of 10-mm square glass substrates in a short processing time of less than 10 min. It was also demonstrated that this method can be applied to a 1.5-in. octagonal glass substrate. The mechanism of monolayer colloidal film formation was discussed based on scanning electron microscopy observations at each preparation step. We found that the second sonication was the key process for uniform and high-surface-coverage colloidal film formation of midnanometer-sized AuNPs. The second sonication promotes the migration of AuNPs on top of the monolayer in contact with the substrate surface, decreasing both the multilayer region and the bare surface area. Eventually, a nearly perfect monolayer colloidal film is formed.

Sub-micrometer soft lithography of a bulk chalcogenide glass.

We demonstrate, for the first time, time- and cost-effective replication of sub-micrometer features from a soft PDMS mold onto a bulk chalcogenide glass over a large surface area. A periodic array of sub-micrometer lines (diffraction grating) with period 625 nm, amplitude 45 nm and surface roughness 3 nm was imprinted onto the surface of the chalcogenide AsSe(2) bulk glass at temperature 225°C, i.e. 5°C below the softening point of the glass. Sub-micrometer soft lithography into chalcogenide bulk glasses shows good reliability, reproducibility and promise for feasible fabrication of various dispersive optical elements, anti-reflection surfaces, 2D photonic structures and nano-structured surfaces for enhanced photonic properties and chemical sensing.

Inverse opal photonic crystal of chalcogenide glass by solution processing.

Chalcogenide opal and inverse opal photonic crystals were successfully fabricated by low-cost and low-temperature solution-based process, which is well developed in polymer films processing. Highly ordered silica colloidal crystal films were successfully infilled with nano-colloidal solution of the high refractive index As(30)S(70) chalcogenide glass by using spin-coating method. The silica/As-S opal film was etched in HF acid to dissolve the silica opal template and fabricate the inverse opal As-S photonic crystal. Both, the infilled silica/As-S opal film (Δn ~ 0.84 near λ=770 nm) and the inverse opal As-S photonic structure (Δn ~ 1.26 near λ=660 nm) had significantly enhanced reflectivity values and wider photonic bandgaps in comparison with the silica opal film template (Δn ~ 0.434 near λ=600 nm). The key aspects of opal film preparation by spin-coating of nano-colloidal chalcogenide glass solution are discussed. The solution fabricated "inorganic polymer" opal and the inverse opal structures exceed photonic properties of silica or any organic polymer opal film. The fabricated photonic structures are proposed for designing novel flexible colloidal crystal laser devices, photonic waveguides and chemical sensors.

Tunable structural color in organisms and photonic materials for design of bioinspired materials.

In this paper, the key topics of tunable structural color in biology and material science are overviewed. Color in biology is considered for selected groups of tropical fish, octopus, squid and beetle. It is caused by nanoplates in iridophores and varies with their spacing, tilting angle and refractive index. These examples may provide valuable hints for the bioinspired design of photonic materials. 1D multilayer films and 3D colloidal crystals with tunable structural color are overviewed from the viewpoint of advanced materials. The tunability of structural color by swelling and strain is demonstrated on an example of opal composites.

Photonic rubber sheets with tunable color by elastic deformation.

This article describes an elastic silicone sheet with reversible tuning structural color. The sheet has a thin layer of cubic close-packed, ccp, colloidal particles embedded in poly(dimethylsiloxane), PDMS, elastomer. The array of ccp (111) planes diffracts light of selective wavelengths according to Bragg's law. This is responsible for the structural color of the PDMS sheet. Because the sheet was stretched in the horizontal direction, it was reduced in size in the vertical direction. As a result, the lattice distance of ccp (111) planes decreased, and the reflected wavelength of light shifted to shorter wavelengths. For example, the peak of reflection was tuned from 589 to 563 nm as a function of sheet elongation. The peak position decreased linearly with deformation when the deformation was within 20% of its elongation. Accordingly, the color of the PDMS sheet changed from red to green. When the mechanical strain on the PDMS sheet was released, the peak returned to its original position, and the color of the PDMS sheet also changed back to red. Tuning the color of the PDMS sheet is a reversible and repeatable process. The novel PDMS sheet has the potential to be applied to mechanical strain sensing.

Fabricating high-quality opal films with uniform structure over a large area.

Films of opal, a colloidal crystalline lattice with closely packed structure, are anticipated to become a fundamental material in photonic crystal engineering. One of the technological issues is forming the opal film with a flat and uniform surface over a large area. This article describes a new and simple method for forming an opal film without special equipment. The opal film is formed by drying a colloidal suspension covered on a hydrophilic solid substrate. In the conventional method, a ring-shaped opal usually forms at the edge (contact line) of the suspension on the substrate. The new method improved the process of drying the colloidal suspension free from the ring formation. The driving force of this ring formation is based on capillary flow in the suspension from inside to outside because of the high evaporation rate at the contact line. To prevent capillary flow, the contact line of the suspension was covered with hydrophobic silicone liquid. As a result, ring formation was depressed and flat opal films with uniform structure were formed. The structure comprised cubic closely packed (111) planes, and the opal films were grown to grain sizes larger than 200 microm. In addition, the coating area of the opal film was greater than 75 cm2 using a 4-in. silicone wafer. This new method should be useful for coating high-quality opal film over large areas on solid substrates.