Density-controlled electrochemical synthesis of ZnO nanowire arrays using nanotextured cathode.

Zinc oxide (ZnO) nanowires fabricated via wet chemical synthesis on flexible polymer substrates are inherently unstable against mechanical bending stress because of their high density and weak adhesion to the substrate. We introduce a novel method for controlling the density of such ZnO nanowire arrays using a three-dimensional corrugated metal substrate. These metal substrates, featuring extruded and recessed patterns fabricated via nanoimprint lithography, were employed as cathodes during the electrochemical deposition of ZnO nanowire arrays. The ZnO nanowire arrays synthesized on the patterned metal thin film exhibited smaller diameters and lower densities compared to those on non-patterned metal films. This reduction in density can be attributed to aligned nucleation and limited growth on the patterned metal surface. Crucially, ZnO nanowires synthesized on patterned metal substrates displayed remarkable mechanical robustness against external forces, a direct consequence of their reduced density. In contrast, nanowires synthesized on non-patterned metal substrates were broken under mechanical bending. Detailed morphological analyses performed after mechanical bending tests confirm that ZnO nanowires synthesized on nanoimprinted metal electrodes exhibited enhanced mechanical characteristics compared to those on non-patterned metal electrodes. These findings clearly demonstrate the promise of utilizing density-controlled ZnO nanowires in piezoelectric devices.

Scalable, solvent-free transparent film-based air filter with high particulate matter 2.5 filtration efficiency.

Over the course of the COVID-19 pandemic, people have realized the importance of wearing a mask. However, conventional nanofiber-based face masks impede communication between people because of their opacity. Moreover, it remains challenging to achieve both high filtration performance and transparency through fibrous mask filters without using harmful solvents. Herein, scalable transparent film-based filters with high transparency and collection efficiency are fabricated in a facile manner by means of corona discharging and punch stamping. Both methods improve the surface potential of the film while the punch stamping procedure generates micropores in the film, which enhances the electrostatic force between the film and particulate matter (PM), thereby improving the collection efficiency of the film. Moreover, the suggested fabrication method involves no nanofibers and harmful solvents, which mitigates the generation of microplastics and potential risks for the human body. The film-based filter provides a high PM2.5 collection efficiency of 99.9 % while maintaining a transparency of 52 % at the wavelength of 550 nm. This enables people to distinguish the facial expressions of a person wearing a mask composed of the proposed film-based filter. Moreover, the results of durability experiments indicate that the developed film-based filter is anti-fouling, liquid-resistant, microplastic-free and foldability.

High-performance bag filter with a super-hydrophobic microporous polytetrafluoroethylene layer fabricated by air-assisted electrospraying.

Reducing PM emissions from industrial sites has become increasingly important as the adverse health effects of particulate matter have been demonstrated by multiple epidemiological and toxicological studies. High-performance bag filters are often used for this purpose. We fabricated polytetrafluoroethylene (PTFE) nanoparticle (NP)-coated high-efficiency bag filters using air-assisted electrospraying (AAES) technology. AAES functionalized with a combination of airflow drag force and an applied electric field facilitates high-throughput without requiring additional purification or preparation process of a PTFE emulsion. PTFE NPs form a unique three-dimensional microporous structure on a foam-filter medium, enhancing mechanical filtration performance (diffusion and interception). Moreover, the surface hydrophobicity was significantly improved as the PTFE NPs covered the bag filter surface. These factors highlight the feasibility of large-scale implementation of PTFE NP-coated bag filters for reducing PM emissions from industrial sources.

Sensitive Wearable Temperature Sensor with Seamless Monolithic Integration.

Accurate temperature field measurement provides critical information in many scientific problems. Herein, a new paradigm for highly sensitive, flexible, negative temperature coefficient (NTC) thermistor-based artificial skin is reported, with the highest temperature sensing ability reported to date among previously reported NTC thermistors. This artificial skin is achieved through the development of a novel monolithic laser-induced reductive sintering scheme and unique monolithic structures. The unique seamless monolithic structure simultaneously integrates two different components (a metal electrode and metal oxide sensing channel) from the same material at ambient pressure, which cannot be achieved by conventional heterogeneous integration through multiple, complex steps of photolithography or vacuum deposition. In addition to superior performance, electronic skin with high temperature sensitivity can be fabricated on heat-sensitive polymer substrates due to the low-temperature requirements of the process. As a proof of concept, temperature-sensitive artificial skin is tested with conformally attachable physiological temperature sensor arrays in the measurement of the temperatures of exhaled breath for the early detection of pathogenic progression in the respiratory system. The proposed highly sensitive flexible temperature sensor and monolithic selective laser reductive sintering are expected to greatly contribute to the development of essential components in various emerging research fields, including soft robotics and healthcare systems.

Self-assembled stretchable photonic crystal for a tunable color filter.

In this Letter, we report the development of a continuously tunable color filter based on a self-assembled isotropically stretchable microbead monolayer. Spreading equidistantly upon the application of lateral strain, the isotropically stretchable monolayer serves as a dynamic diffraction grating whose diffraction angle can be mechanically modulated. Combined with a simple spatial filtering scheme, the spectra of the filtered light are solely controlled by external strain (up to 32% radial strain) to cover a broad visible spectrum. Through a finite-difference time-domain far-field diffraction simulation, we validate the working principle of the proposed color filter. The proposed continuously tunable color filter is expected to open original applications in next-generation display field.

Highly Controlled Nanoporous Ag Electrode by Vaporization Control of 2-Ethoxyethanol for a Flexible Supercapacitor Application.

Controlling the surface morphology of the electrode on the nanoscale has been studied extensively because the surface morphology of a material directly leads to the functionalization in various fields of studies. In this study, we designed a simple and cost-effective method to fine-tune the surface morphology and create controlled nanopores on the silver electrode by utilizing 2-ethoxyethanol and two successive heat treatments. High electrical conductivity and mechanical robustness of nanoporous silver corroborate its prospect to be employed in various applications requiring a certain degree of flexibility. As a proof-of-concept, a high-performance supercapacitor was fabricated by electrodepositing MnO2. This method is expected to be useful in various electronic applications as well as energy storage devices.

Low-Temperature Oxidation-Free Selective Laser Sintering of Cu Nanoparticle Paste on a Polymer Substrate for the Flexible Touch Panel Applications.

Copper nanomaterials suffer from severe oxidation problem despite the huge cost effectiveness. The effect of two different processes for conventional tube furnace heating and selective laser sintering on copper nanoparticle paste is compared in the aspects of chemical, electrical and surface morphology. The thermal behavior of the copper thin films by furnace and laser is compared by SEM, XRD, FT-IR, and XPS analysis. The selective laser sintering process ensures low annealing temperature, fast processing speed with remarkable oxidation suppression even in air environment while conventional tube furnace heating experiences moderate oxidation even in Ar environment. Moreover, the laser-sintered copper nanoparticle thin film shows good electrical property and reduced oxidation than conventional thermal heating process. Consequently, the proposed selective laser sintering process can be compatible with plastic substrate for copper based flexible electronics applications.

Nanotextured Polymer Substrate for Flexible and Mechanically Robust Metal Electrodes by Nanoimprint Lithography.

Metal thin film electrodes on flexible polymer substrates are inherently unstable against humidity and mechanical stresses because of their poor adhesion properties. We introduce a novel approach for improving the adhesion characteristics of metal-polymer interface based on the nanostructuring of the polymer substrate by using nanoimprint lithography. The adhesion characteristics of metal-polymer interface were measured by accelerated test, cyclic bending test and double cantilever beam (DCB) test. The interface of Au/Ti dual layer thin film and nanoimprinted PMMA substrate shows over 2.03 and 1.95 times higher adhesion energy (G(c)) than that of Au/Ti dual layer thin film and plane PMMA substrate in air and wet environments, respectively. The adhesion energy between metal thin film and polymer substrate was dramatically improved by the increased surface roughness and mechanical interlocking effect of numerous nanoscale anchors at the edges of nanoimprinted surface, which was verified by both experiment and numerical analysis.

Ag@Ni core-shell nanowire network for robust transparent electrodes against oxidation and sulfurization.

Silver nanowire (Ag NW) based transparent electrodes are inherently unstable to moist and chemically reactive environment. A remarkable stability improvement of the Ag NW network film against oxidizing and sulfurizing environment by local electrodeposition of Ni along Ag NWs is reported. The optical transmittance and electrical resistance of the Ni deposited Ag NW network film can be easily controlled by adjusting the morphology and thickness of the Ni shell layer. The electrical conductivity of the Ag NW network film is increased by the Ni coating via welding between Ag NWs as well as additional conductive area for the electron transport by electrodeposited Ni layer. Moreover, the chemical resistance of Ag NWs against oxidation and sulfurization can be dramatically enhanced by the Ni shell layer electrodeposited along the Ag NWs, which provides the physical barrier against chemical reaction and diffusion as well as the cathodic protection from galvanic corrosion.

Strong localized surface plasmon resonance effects of Ag/TiO2 core-shell nanowire arrays in UV and visible light for photocatalytic activity.

Periodic arrays of silver/titanium dioxide (Ag/TiO2) open core-shell nanowires have been investigated as enhanced plasmonic photocatalytic structures. Sequential top-down nanofabrication processes based on nanoimprinting, oblique angle evaporation, and selective electrodeposition were employed for the fabrication of various TiO2-shelled Ag nanowire arrays. Numerical simulation proves that the periodic array of Ag/TiO2 core-shell nanowire structures enables strong localized surface plasmon resonance (LSPR), which improves the electron generation and photocatalytic activities of the TiO2 shell. Enhanced photocatalytic performance was confirmed by the decomposition of methylene blue solution. Furthermore, the film composed of a Ag/TiO2 core-shell nanowire array shows photocatalytic reproducibility in the UV and visible light regions and mechanical robustness due to the periodic grating-like metal nanostructures. Our method allows reliable, controllable, and facile fabrication of large-scale plasmonic photocatalytic nanostructured films on various substrates such as glass, polymer, etc.