Jong-Woong Kim

Crack-induced Ag nanowire networks for transparent, stretchable, and highly sensitive strain sensors

Crack-based strain sensor systems have been known for its high sensitivity, but suffer from the small fracture strain of the thin metal films employed in the sensor which results in its negligible stretchability. Herein, we fabricated a transparent (>90% at 550u2009nm wavelength), stretchable (up to 100%), and sensitive (gauge factor (GF) of 30 at 100% strain) strain gauge by depositing an encapsulated crack-induced Ag nanowire (AgNW) network on a hydroxylated poly(dimethylsiloxane) (PDMS) film. Stretching the encapsulated AgNWs/PDMS resulted in the formation of a percolation network of nanowire ligaments with abundant percolation paths. The encapsulating polymer was designed to adhere strongly to both the AgNW and PDMS. The improved adhesion ensured the resistance of the crack-induced network of AgNWs varied reversibly, stably, and sensitively when stretched and released, at strains of up to 100%. The developed sensor successfully detected human motions when applied to the skin.

Photo-induced fabrication of Ag nanowire circuitry for invisible, ultrathin, conformable pressure sensors

We demonstrate ultrathin (thickness 88% at 550 nm), and conformable pressure-induced bending sensors with unprecedented flexibility and stretchability, produced by developing a photo-induced pattern of silver nanowires (AgNWs) on a 1.4 μm-thick polyethylene terephthalate (PET) sheet. This patterning approach does not require any additional materials to form a patterned barrier for AgNW etching or to enhance the adhesion between the AgNWs and PET. Simple irradiation using pulsed light on a masked AgNWs/1.4 μm-thick PET assembly followed by sonication formed a finely patterned AgNW network well-adhered to the underlying PET without impacting the optical transparency of the ultrathin PET. A pressure-induced bending-sensitive capacitive sensor fabricated by this approach was extremely flexible and stretch-compatible up to 100% uniaxial strain. This sensor, based on a simple tandem compound pattern, was reproducible, durable, and 90% more sensitive than an elastomeric pressure sensor made using the same sensor design. The functionality of the developed sensor system was successfully demonstrated in a sensitive acupressure sensor mounted on a gloved fingertip, in which the capacitance coincidently varied with the force applied to the fingertip.

Photo-induced healing of stretchable transparent electrodes based on thermoplastic polyurethane with embedded metallic nanowires

The incorporation of self-healing functionalities into stretchable thermoset systems can be associated with challenges such as large accumulation of inelastic strain as a result of repeated stretching or dramatic changes in modulus during repeated damage–self-healing cycles. Here, we successfully fabricate a stretchable and transparent electrode with photo-irradiation mediated self-healing capacity. This electrode was realised using Ag nanowires (AgNWs) and thermoplastic polyurethane (TPU) without employing any of the established dynamic self-healing chemistries. First, the AgNWs deposited on a cured TPU film were irradiated with intense-pulsed-light (IPL) to induce plasmonic heating, resulting in a large enhancement in conductivity and mechanically stable, stretchable transparent electrodes. Subsequent rounds of IPL irradiation were employed to repair the artificial cracks formed on the surface of the AgNW/TPU electrodes and the micro-cracks induced by repeated rounds of stretch-and-release testing. The surface analysis confirmed that both types of defects were successfully repaired by the IPL treatment owing to the enhanced flowability and thermal expansion of TPU during IPL irradiation. Multiple scratching with a cutting knife and healing demonstration revealed that the cracks formed at the same locations were healed repeatedly up to five times.

1.4 µm-Thick Transparent Radio Frequency Transmission Lines Based on Instant Fusion of Polyethylene Terephthalate Through Surface of Ag Nanowires

Though a percolated network of silver nanowires (AgNWs) has been considered the most promising flexible transparent electrode because of it high conductivity, high transmittance, and excellent flexibility, fabrication of AgNW-based transmission lines designed to conduct high frequency signals has been scarcely reported. The fabrication and performance of extremely thin (1.4xa0µm thick) and low lossy (smaller than −u200917xa0dB for reflection coefficient corresponding to 2.5xa0GHz) transmission lines with unprecedented transparency (higher than 90% for the entire visible light spectrum) are demonstrated in this study. AgNWs deposited onto a 1.4xa0µm-thick polyethylene terephthalate (PET) sheet were irradiated by intense-pulsed-light to selectively raise their temperature. The intensive photon energy delivered to the AgNWs simultaneously caused the active diffusion of Ag atoms and plasmonic welding, resulting in large drops in resistivity without drastic changes in their physical shape or the optical transmittance of the films. Furthermore, absorption of heat also thermally activated the underlying polymer and causing it to react with the surface of the AgNWs—this results in enhanced adhesion between the AgNWs and the PET. Measurements and simulation of specially designed coplanar waveguide circuits revealed that the fabricated electrode could simultaneously provide excellent transmission characteristics and mechanical stability and transparency.Graphical Abstract

Growth and Evaluation of GaN Grown on Patterned Sapphire Substrates

We report the microstructure and optical properties of gallium nitride (GaN) epilayers grown on lens shape patterned sapphire substrate (PSS) using metalorganic chemical vapor deposition (MOCVD) for various growth times. A lens shaped pattern was used to reduce the threading dislocation density and to improve optical emission efficiency. A scanning electron microscope (SEM) image shows flat and smooth surface of GaN grown on PSS at 80 min which could be achieved by lateral growth from the trench region. From the DCXRD spectra, full width at half maximun (FWHM) value was decreased with increasing growth time. FWHM of the sample grown at 80 min was 473.5 arc sec. This indicates there is an improvement in crystalline quality of the GaN grown on PSS as the growth time increases. From photoluminescence (PL) spectra, an increase in band edge emission intensity and a decrease in defect related yellow luminescence was observed for GaN on PSS as the growth time increased. From the PL spectra, FWHM was 82.2 meV at peak position 363.9 nm for the sample grown for 80 min. It is clearly seen that the threading dislocations can be reduced by lateral growth improving the light emission efficiency by internal light reflection on the lens surface for GaN grown on PSS.

Optical and Surface Morphological Study of GaN Nanocolumn Grown on Gallium Coated Si by Molecular Beam Epitaxy

Vertically aligned GaN nanocolumn arrays were grown by molecular beam epitaxy on Gallium coated silicon substrate. The dense packing of the NCs gives them the appearance of a continuous film in surface view, but cross-sectional analysis shows them to be isolated nanostructures. The GaN nanocolumns have uniform diameters of 85 nm, lengths up to 720 nm and possess a pyramid like tip. Photoluminescence measurements of NCs show excitonic emission with a dominant, narrow peak centered at 363 nm and FWHM of 68 meV. From the Raman spectrum, peaks at 566.9 and 730 cm-1 are assigned to the E2 and A1(LO) GaN phonons modes which clearly indicates that the grown nanocolumns are highly crystalline. The grown nanocolumns are highly oriented and perpendicular to the growth surface.