Eun Mi Kim

Flexible giant magnetoresistance (GMR) spin-valve sensors are attractive for touchless human-machine interfaces and robotic sensing, but their performance is often compromised by strain-induced magnetic anisotropy changes, known as the inverse-magnetostriction effect, and by fatigue of the metallic electrodes under repeated bending. In this work, flexible GMR spin-valve sensors were prepared with a low-magnetostriction Permalloy free layer and silver ink-printed electrodes to address both limitations. The Ni84.5Fe15.5 free-layer composition showed minimal variation in magnetic anisotropy after repeated bending, confirming reliable sensor performance under mechanical deformation. The silver ink-printed electrodes, fabricated by direct-write printing, also showed a small resistance variation ratio (ΔR/R0) of only about 0.003 under various curvatures and remained stable after 2,000 repetitive bending cycles. Comparative bending tests of a 4 mm × 2 mm strip GMR sensor showed that the coercivity of the Ni84.5Fe15.5 free layer varied by only about 1.2 Gauss up to a curvature of 0.638 mm⁻¹, while a Co free-layer control device showed a coercivity increase of over 6 Gauss under the same conditions. The 9 × 4 array of 300 μm GMR sensing units preserved this strain-insensitive response and showed excellent reversibility over repeated bending cycles between flat and curved states. Finally, the sensor array was mounted on a finger to demonstrate touchless communication, encoding and transmitting a Morse-code message through magnetic-field modulation from a palm-mounted permanent magnet. These results establish that compositionally optimized, low-magnetostriction GMR spin valves combined with printed flexible electrodes provide a reliable sensing platform for wearable and robotic touchless interfaces.

Smart windows capable of dynamically controlling light transmission are gaining increasing attention as a means of reducing building energy consumption while improving indoor comfort. However, indium tin oxide (ITO), the most widely used transparent electrode in such devices, suffers from poor mechanical flexibility due to its intrinsic brittleness, limiting its use in curved or flexible window applications. Here, we demonstrate a flexible ITO/DPP hybrid transparent electrode, fabricated by depositing a dimethyl sulfoxide (DMSO)-doped PEDOT:PSS (DPP) layer on ITO, that achieves markedly improved bending durability while maintaining electro-optical performance comparable to bare ITO. The hybrid electrode with an optimized DPP thickness of 120 nm exhibited a sheet resistance of 23.3 Ω/sq., about 20% lower than that of ITO (29 Ω/sq.), while maintaining comparable visible-range transmittance. A polymer-dispersed liquid crystal (PDLC) device fabricated with this electrode showed only a slight decline in driving voltage and transmittance relative to ITO. Notably, its resistance change under repeated bending at a curvature radius of 30 mm was only 25.1%, less than half that of ITO (51.6%). These results suggest that the ITO/DPP hybrid electrode is a promising candidate for transparent electrodes in curved and flexible smart windows and other flexible optoelectronic devices.

We report the use of four-layer graphene (4LG) as a highly reliable transparent conductive electrode (TCE) for polymer-dispersed liquid crystal (PDLC)-based smart window devices. The adhesion between 4LG and the substrate was successfully improved through a water-induced interface-cleaning (WIIC) process. We compared the performance of a device with a WIIC-processed 4LG electrode with that of devices with a conventional indium tin oxide (ITO) electrode and a 4LG electrode without a WIIC. With the application of the WIIC process, the PDLC smart window with a 4LG electrode exhibited reduced turn-on voltage and haze compared to 4LG without the WIIC process and characteristics comparable to those of the ITO electrode. The WIIC-processed 4LG electrode demonstrated enhanced electrical properties and better optical performance, leading to improved device efficiency and reliability. Furthermore, our study revealed that the WIIC process not only improved the adhesion between 4LG and the substrate but also enhanced the compatibility and interfacial interactions, resulting in the superior performance of the smart window device. These findings suggest that 4LG with WIIC holds great promise as a transparent conductive electrode for flexible smart windows, offering a cost-effective and efficient alternative to conventional ITO electrodes.

We report the use of four-layer graphene (4LG) as a highly reliable transparent conductive electrode (TCE) for polymer-dispersed liquid crystal (PDLC)-based smart window devices. The adhesion between 4LG and the substrate was successfully improved through a water-induced interface-cleaning (WIIC) process. We compared the performance of a device with a WIIC-processed 4LG electrode with that of devices with a conventional Indium tin oxide (ITO) electrode and a 4LG electrode without a WIIC. With the application of the WIIC process, the PDLC smart window with a 4LG electrode exhibited reduced turn-on voltage and haze compared to 4LG without the WIIC process and characteristics comparable to those of the ITO electrode. Based on our observations, we conclude that 4LG with WIIC is a promising TCE for flexible smart windows.

In this study, a TiO₂–alumina photocatalyst was manufactured by coating a surface-treated alumina substrate with TiO₂ sol using the sol–gel method, and the photolysis and conversion of acetaldehyde in the gas phase were evaluated. The effects of acetaldehyde flow rate (i.e., retention time), ultraviolet wavelength, moisture, and catalyst heat-treatment temperature on the conversion of acetaldehyde were investigated. The experiments confirmed that a decrease in flow rate (i.e., increase in retention time), increase in moisture level, and decrease in the ultraviolet wavelength of the light source increased the conversion rate of the gaseous acetaldehyde. Among the three heat-treatment temperatures (450, 650, and 850 °C) used in the catalyst manufacturing process, the catalyst treated at 650 °C had the highest acetaldehyde conversion rate. As a result of its increased acetaldehyde decomposition and photoefficiency, the newly manufactured TiO₂–alumina photocatalyst is expected to be used alongside a photoreactor as an air-purifying filter. Furthermore, the photocatalyst surface treatment demonstrated herein can be adopted to fabricate various environmentally friendly materials in the future.

We introduce a low temperature process for coating InGaN/GaN light-emitting diodes (LEDs) with h-BN as a passivation layer. The effect of h-BN on device performance and reliability is investigated. At −5 V, the leakage current of the h-BN passivated LED was −1.15 × 10⁻⁹ A, which was one order lower than the reference LED’s leakage current of −1.09 × 10⁻⁸ A. The h-BN layer minimizes the leakage current characteristics and operating temperature by acting as a passivation and heat dispersion layer. With a reduced working temperature of 33 from 45 °C, the LED lifetime was extended 2.5 times following h-BN passivation. According to our findings, h-BN passivation significantly improves LED reliability.

The demand for high-performance two-dimensional gas barrier materials is increasing owing to their potential for application in optoelectronic devices. These materials can help the devices maintain their properties over a long period. Therefore, in this study, we investigated the gas barrier performance of hexagonal boron nitride (h-BN) monolayers grown on copper foils via electrochemical polishing (ECP). The ECP treatment helped reduce the surface roughness of the copper foils. As a result, the nucleation density was reduced and highly crystalline h-BN monolayers were produced. The gas barrier performance of h-BN monolayers on copper foils with ECP was comparable to that of graphene. Our finding demonstrates the potential of monolayer h-BN as a high-performance and economical gas barrier material for organic-based optoelectronic devices.