Young Duk Suh

Highly Stretchable and Transparent Metal Nanowire Heater for Wearable Electronics Applications

A highly stretchable and transparent electrical heater is demonstrated by constructing a partially embedded silver nanowire percolative network on an elastic substrate. The stretchable network heater is applied on human wrists under real-time strain, bending, and twisting, and has potential for lightweight, biocompatible, and versatile wearable applications.

Highly Sensitive and Stretchable Multidimensional Strain Sensor with Prestrained Anisotropic Metal Nanowire Percolation Networks.

To overcome the limitation of the conventional single axis-strain sensor, we demonstrate a multidimensional strain sensor composed of two layers of prestrained silver nanowire percolation network with decoupled and polarized electrical response in principal and perpendicular directional strain. The information on strain vector is successfully measured up to 35% maximum strain with large gauge factor (>20). The potential of the proposed sensor as a versatile wearable device has been further confirmed.

Highly Stretchable and Transparent Supercapacitor by Ag–Au Core–Shell Nanowire Network with High Electrochemical Stability

Stretchable and transparent electronics have steadily attracted huge attention in wearable devices. Although Ag nanowire is the one of the most promising candidates for transparent and stretchable electronics, its electrochemical instability has forbidden its application to the development of electrochemical energy devices such as supercapacitors. Here, we introduce a highly stretchable and transparent supercapacitor based on electrochemically stable Ag-Au core-shell nanowire percolation network electrode. We developed a simple solution process to synthesize the Ag-Au core-shell nanowire with excellent electrical conductivity as well as greatly enhanced chemical and electrochemical stabilities compared to pristine Ag nanowire. The proposed core-shell nanowire-based supercapacitor still possesses fine optical transmittance and outstanding mechanical stability up to 60% strain. The Ag-Au core-shell nanowire can be a strong candidate for future wearable electrochemical energy devices.

Ag/Au/Polypyrrole Core-shell Nanowire Network for Transparent, Stretchable and Flexible Supercapacitor in Wearable Energy Devices

Transparent and stretchable energy storage devices have attracted significant interest due to their potential to be applied to biocompatible and wearable electronics. Supercapacitors that use the reversible faradaic redox reaction of conducting polymer have a higher specific capacitance as compared with electrical double-layer capacitors. Typically, the conducting polymer electrode is fabricated through direct electropolymerization on the current collector. However, no research have been conducted on metal nanowires as current collectors for the direct electropolymerization, even though the metal nanowire network structure has proven to be superior as a transparent, flexible, and stretchable electrode platform because the conducting polymer’s redox potential for polymerization is higher than that of widely studied metal nanowires such as silver and copper. In this study, we demonstrated a highly transparent and stretchable supercapacitor by developing Ag/Au/Polypyrrole core-shell nanowire networks as electrode by coating the surface of Ag NWs with a thin layer of gold, which provide higher redox potential than the electropolymerizable monomer. The Ag/Au/Polypyrrole core-shell nanowire networks demonstrated superior mechanical stability under various mechanical bending and stretching. In addition, proposed supercapacitors showed fine optical transmittance together with fivefold improved areal capacitance compared to pristine Ag/Au core-shell nanowire mesh-based supercapacitors.

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.

All-solid-state flexible supercapacitors by fast laser annealing of printed metal nanoparticle layers

A flexible all-solid-state supercapacitor was demonstrated with a flexible Ag nanoparticle current collector which is prepared by a roll-to-roll (R2R) gravure printing process combined with a fast, low temperature laser annealing process. The laser annealing could yield good electrical conductivity of the printed Ag nanoparticle (NP) very rapidly without any noticeable polymer substrate damage and with outstanding adhesion to the underlying polymer substrate which is essential for the fabrication of stable energy devices. The laser annealed Ag NP films are subsequently sandwiched with a carbon slurry and a polymer layer as the active material and the electrolyte to assemble flexible all solid-state supercapacitors that can be bent up to 135° without any severe decrease of the electrochemical performance. By combining the proposed laser process with the existing R2R system, we expect that the printing process for flexible electronic devices could be greatly improved in terms of processing time and space.

Random nanocrack, assisted metal nanowire-bundled network fabrication for a highly flexible and transparent conductor

The most viable flexible and transparent conductor alternative to indium tin oxide (ITO) is metal mesh on plastic including metal micro-lines at regular spacing and metal nanowire percolation networks. Applications in flexible and transparent devices have been hampered by either moire pattern problems caused by regular patterning or low mechanical robustness of the nanowire network. In this study, we demonstrate a novel class of flexible transparent conductor based on metal nanowire micro-bundled networks at random patterns. Original random patterns are prepared from controlled random cracking of high-stress silicon nitride on the silicon substrate, and employed as repetitively usable master molds with independently controllable pattern density and linewidth. Silver nanowires are subsequently placed in the random crack channels through a facile solution process and transferred to the polymer substrate with UV curable epoxy resin. The resultant flexible and transparent conductor, spanning over wafer scale at high reproducibility, not only exhibits enhanced mechanical robustness upon repeated bending or scratching, which often occurs when used as touch-screen panel, but also is free from the moire pattern problem due to the random nature of nanowire bundle patterns. Further application of the resultant flexible transparent conductor as a touch-screen panel confirms easy large-scale fabrication of this robust and flexible transparent conductor.

Laser-Induced Hydrothermal Growth of Heterogeneous Metal-Oxide Nanowire on Flexible Substrate by Laser Absorption Layer Design

Recent development of laser-induced hydrothermal growth enabled direct digital growth of ZnO nanowire array at an arbitrary position even on 3D structures by creating a localized temperature field through a photothermal reaction in liquid environment. However, its spatial size was generally limited by the size of the focused laser spot and the thermal diffusion, and the target material has been limited to ZnO. In this paper, we demonstrated a next generation laser-induced hydrothermal growth method to grow nanowire on a selected area that is even smaller than the laser focus size by designing laser absorption layer. The control of laser-induced temperature field was achieved through adjusting the physical properties of the substrate (dimension and thermal conductivity), and it enabled a successful synthesis of smaller nanowire array without changing any complex optics. Through precise localized temperature control with laser, this approach could be extended to various nanowires including ZnO and TiO2 nanowires even on heat sensitive polymer substrate.

Nanowire reinforced nanoparticle nanocomposite for highly flexible transparent electrodes: borrowing ideas from macrocomposites in steel-wire reinforced concrete

Inspired by steel-wire reinforced concrete, which is a strong building material, a novel nanocomposite of a nanowire reinforced nanoparticle matrix film was developed for flexible and transparent electrode applications. The transparent electrode made in this study exhibits superior mechanical characteristics under tensile as well as compressive bending, in comparison to conventional sintered metal nanoparticle based metallic grids.

Control and Manipulation of Nano Cracks Mimicking Optical Wave

Generally, a fracture is considered as an uncontrollable thus useless phenomenon due to its highly random nature. The aim of this study is to investigate highly ordered cracks such as oscillatory cracks and to manipulate via elaborate control of mechanical properties of the cracking medium including thickness, geometry, and elastic mismatch. Specific thin film with micro-sized notches was fabricated on a silicon based substrate in order to controllably generate self-propagating cracks in large area. Interestingly, various nano-cracks behaved similar to optical wave including refraction, total internal reflection and evanescent wave. This novel phenomena of controlled cracking was used to fabricate sophisticated nano/micro patterns in large area which cannot be obtained even with conventional nanofabrication methods. We also have showed that the cracks are directly implementable into a nano/micro-channel application since the cracks naturally have a form of channel-like shape.