The properties of Al-doped SnO_x films deposited via reactive co-sputtering were examined in terms of their potential applications for the fabrication of transparent and flexible electronic devices. Al 2.2-atom %-doped SnO_x thin-film transistors (TFTs) exhibit improved semiconductor characteristics compared to non-doped films, with a lower sub-threshold swing of ~0.68 Vdec⁻¹, increased on/off current ratio of ~8 × 10⁷, threshold voltage (V_th) near 0 V, and markedly reduced (by 81%) V_th instability in air, attributable to the decrease in oxygen vacancy defects induced by the strong oxidizing potential of Al. Al-doped SnO_x films maintain amorphous crystallinity, an optical transmittance of ~97%, and an adhesive strength (to a plastic substrate) of over 0.7 kgf/mm; such films are thus promising semiconductor candidates for fabrication of transparent flexible TFTs.

Since the ancient Egyptians had used sand as filter media for water purification, its principle has been inherited through generations and it is still being used now in industries. The sand filter consists of sand literally, and the voids within the sand bed are the pores for filtration. Here we present a filtration principle using nanoparticles, so that the voids between the nanoparticles can be considered as effective pores in nanoscale dimension. Anodic aluminum oxide (AAO) membrane has been used as the working template, and the nanoparticles have been injected and embedded within the pores of the AAO template. Nanoparticles with multiple sizes have been used in order to obtain smaller voids. Moreover, the nanoparticles have been functionalized, or electrically charged, with arginine/phenylalanine (RF) peptide group. In this way, filtration performance for charged particles or molecules, such as methylene blue, has been enhanced. Consequently, this study is expected to provide a new principle for fabrication of nano voids, or nano pores, and for filtration in nanoscale dimension.

The visual response is one of the most intuitive principles of sensors. Therefore, emission and change of the colors are widely studied for development of chemical, thermal and mechanical sensors. And it is still a challenging issue to fabricate them with a simple working mechanism, high sensitivity, good reliability, and a cost-effective fabrication process. In this study, we propose a mechanical strain sensor, which has 2D photonic crystal structures in nanoscale on stretchable polydimethylsiloxane (PDMS) substrate. Due to the periodic nanostructures, the surface of the sensor produces structural colors. And when it is stretched, the periodicity of the nanostructures changes, which results in the shift of the colors. Multiple nanostructures with different periodicities are integrated on the sensor in order to extend the working range up to 150% with high sensitivity. In addition, reusable and robust molds, which are fabricated by self-assembly of nanoparticles, are used for multiple replications of sensor substrates. Thus, the fabrication process of this study is believed to be potential for possible industrial manufacturing. This study is expected to contribute to strain sensors in the future for the applications of health care, infrastructure monitoring, soft robotics, and wearable devices.

Labels with covert-overt structural color based on photonic crystals (PCs) have drawn significant attention due to their unique color emission, offering promising solutions for anti-counterfeiting applications.

A lot of sensors using structural color are based on periodic nanostructures, or photonic crystals. So the nanostructures need to be fabricated with high reproducibility so that those sensors can be suitable for practical and commercial applications. Furthermore, achieving the reproducible fabrication is more challenging for hydrogel-based devices with structural color. In this study, we propose a novel molding approach to fabricate photonic crystal hydrogels with high reproducibility. A silicon wafer with a monolayer of self-assembled nanoparticles is used as a mold to transfer nanostructures onto the hydrogel surface. Since the molding technique is sensitive to the mechanical properties of the hydrogel, we optimized these properties by adjusting the monomer-to-crosslinker ratio. The ratio of 50:1 was identified as the optimal composition for the molding method to ensure both mechanical stability and chemical responsiveness. In order to demonstrate reproducibility, the molding processes were performed for over 50 cycles, resulting in hydrogel exhibiting structural colors with optical and mechanical integrity. Additionally, hydrogels showed reversible color changes in response to various solvents. Volume change of the hydrogel caused variation of periodicity of photonic crystal, which led to red-shifted colors upon swelling and blue-shifted colors upon contraction. This study shows that photonic crystal hydrogels can be fabricated with enhanced reproducibility by molding method. And it also shows that they can be applied to structural color-based sensors. The principle of this study can be extended to biosensing and environmental monitoring applications by incorporating selective molecules such as antibodies.