Colloidal Photonic Crystals Containing Silver Nanoparticles with Tunable Structural Colors

Spray-Dried Photonic Balls with a Disordered/Ordered Hybrid Structure for Shear-Stress Indication

Optical microscale shear-stress indicator particles are of interest for the in situ recording of localized forces, e.g., during 3D printing or smart skins in robotic applications. Recently developed particle systems are based on optical responses enabled by integrated organic dyes. They thus suffer from potential chemical instability and cross-sensitivities toward humidity or temperature. These drawbacks can be circumvented using photonic balls as shear-stress indicator particles, which employ structural color as the element to record forces. Here, such photonic balls are prepared from silica and iron oxide nanoparticles via the scalable and fast spray-drying technique. Process parameters to create photonic balls with a disordered core and an ordered particle structure toward the exterior of the supraparticles are reported. This hybrid disordered-ordered structure is responsible for a color loss of the indicator particles during shear-stress application because of irreversible structural destruction. By adjusting the primary silica particle sizes, nearly all colors of the visible spectrum can be achieved and the sensitivity of the response to shear stress can be adjusted.

Co-Assembly of Colloids and Eumelanin Nanoparticles in Droplets for Structural Pigments with High Saturation

Colloidal crystals have been used to develop structural colors. However, incoherent scattering causes the colors to turn whitish, reducing the color saturation. To overcome the problem, light-absorbing additives have been incorporated. Although various additives have been used, most of them are not compatible with a direct co-assembly with common colloids in aqueous suspensions. Here, the authors suggest eumelanin nanoparticles as a new additive to enhance the color chroma. Eumelanin nanoparticles are synthesized to have diameters of several nanometers by oxidative polymerization of precursors in basic solutions. The nanoparticles carry negative charges and do not weaken the electrostatic repulsion among same-charged polystyrene particles when they are added to aqueous suspensions. To prove the effectiveness of eumelanin as a saturation enhancer, the authors produce photonic balls through direct co-assembly of polystyrene and eumelanin using water-in-oil emulsion droplets, while varying the weight ratio of eumelanin to polystyrene. The high crystallinity of colloidal crystals is preserved for the ratio up to at least 1/50 as the eumelanin does not perturb the crystallization. The eumelanin effectively suppresses incoherent scattering while maintaining the strength of structural resonance at an optimum ratio, improving color chroma without compromising brightness.

Colorful and facile in situ nanosilver coating on sisal/cotton interwoven fabrics mediated from European larch heartwood

This study reports on a novel coloration approach for sisal/cotton interwoven fabric via in situ synthesis of European larch (Larix decidua) heartwood-anchored sustainable nanosilver. The heartwood extracts functioned as the reducing and stabilizing agent in reaction systems. The deposited silver nanoparticles (AgNPs) over the fabric surfaces displayed brilliant coloration effects with improved fastness ratings and color strengths (K/S). The successful depositions of nanosilvers were quantified and increasing trends in K/S values with the increase in silver precursor loading were discovered. The concentrations of AgNPs deposited on fabric surfaces were found to be 16 mg/L, 323 mg/L, and 697 mg/L, which were measured through an iCP OES (atomic absorption spectroscopy) test. The K/S values obtained for different loadings of silver precursors (0.5, 1.5, and 2.5 mM (w/v)) are 2.74, 6.76, and 8.96. Morphological studies of the control and AgNP-treated fabrics also displayed a uniform and homogeneous distribution of AgNPs over the fabric surfaces. FTIR (Fourier transform infrared spectroscopy) studies of the sustainably developed materials further confirms the successful bonding between the fabrics and AgNPs. Furthermore, stability against temperature was also noticed as per TGA (thermogravimetric analysis) and DTG (derivative TG) analysis although there was a slight decline from the control sisal/cotton interwoven fabrics observed. Statistically, regression analysis and ANOVA tests were conducted to understand the significance of increased nanosilver loading on sisal/cotton interwoven fabrics. In summary, the perceived results demonstrated successful coloration and functionalization of sisal/cotton interwoven fabrics through green AgNPs, which could indicate a new milestone for industrial production units.

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.

Synthesis of High-Performance Photonic Crystal Film for SERS Applications via Drop-Coating Method

Silica nanospheres with a well-controlled particle size were prepared via a nucleation-to-growth synthesis process. A facile method is proposed for improving the self-assembly behavior of silica colloidal particles in droplet coatings by the simple controlling of the drying temperature. It is shown that a periodically arranged, opal-structured, photonic crystal film with a large area of approximately 4.0 cm2 can be prepared, even when the particle size is up to 840 nm. When the band gap of the silica photonic crystals falls in the visible-light region, the crystals exhibit distinct structural colors. Moreover, the wavelength of the reflected light increases with an increasing particle size of silica. When the photonic band gap overlaps the wavelength of the laser source, the overall Raman spectrum intensity is significantly enhanced. Accordingly, the proposed nucleation-to-growth process and drop-coating method provides a cheap and simple approach for the manufacture of uniform sized silica and surface-enhanced Raman scattering substrates, respectively.

Nanosphere Lithography for Structuring Polycrystalline Diamond Films

This paper deals with the structuring of polycrystalline diamond thin films using the technique of nanosphere lithography. The presented multistep approaches relied on a spin-coated self-assembled monolayer of polystyrene spheres, which served as a lithographic mask for the further custom nanofabrication steps. Various arrays of diamond nanostructures—close-packed and non-close-packed monolayers over substrates with various levels of surface roughness, noble metal films over nanosphere arrays, ordered arrays of holes, and unordered pores—were created using reactive ion etching, chemical vapour deposition, metallization, and/or lift-off processes. The size and shape of the lithographic mask was altered using oxygen plasma etching. The periodicity of the final structure was defined by the initial diameter of the spheres. The surface morphology of the samples was characterized using scanning electron microscopy. The advantages and limitations of the fabrication technique are discussed. Finally, the potential applications (e.g., photonics, plasmonics) of the obtained nanostructures are reviewed.

Optical Characteristics of ZnCuInS/ZnS (Core/Shell) Nanocrystal Flexible Films Under X-Ray Excitation

The aim of this article is to evaluate optical characteristics, such as the intrinsic conversion efficiency and the inherent light propagation efficiency of three polymethyl methacrylate (PMMA)/methyl methacrylate (MMA) composite ZnCuInS/ZnS (core/shell) nanocrystal flexible films. The concentrations of these were 100 mg/mL, 150 mg/mL, and 250 mg/mL, respectively. Composite films were prepared by homogeneously diluting dry powder quantum dot (QD) samples in toluene and subsequently mixing these with a PMMA/MMA polymer solution. The absolute luminescence efficiency (AE) of the films was measured using X-ray excitation. A theoretical model describing the optical photon propagation in scintillator materials was used to calculate the fraction of the generated optical photons passed through the different material layers. Finally, the intrinsic conversion efficiency was calculated by considering the QD quantum yield and the optical photon emission spectrum.