Jinhyeong Kwon

Nonvacuum, Maskless Fabrication of a Flexible Metal Grid Transparent Conductor by Low-Temperature Selective Laser Sintering of Nanoparticle Ink

We introduce a facile approach to fabricate a metallic grid transparent conductor on a flexible substrate using selective laser sintering of metal nanoparticle ink. The metallic grid transparent conductors with high transmittance (>85%) and low sheet resistance (30 Ω/sq) are readily produced on glass and polymer substrates at large scale without any vacuum or high-temperature environment. Being a maskless direct writing method, the shape and the parameters of the grid can be easily changed by CAD data. The resultant metallic grid also showed a superior stability in terms of adhesion and bending. This transparent conductor is further applied to the touch screen panel, and it is confirmed that the final device operates firmly under continuous mechanical stress.

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 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.

Direct selective growth of ZnO nanowire arrays from inkjet-printed zinc acetate precursor on a heated substrate

Inkjet printing of functional materials has drawn tremendous interest as an alternative to the conventional photolithography-based microelectronics fabrication process development. We introduce direct selective nanowire array growth by inkjet printing of Zn acetate precursor ink patterning and subsequent hydrothermal ZnO local growth without nozzle clogging problem which frequently happens in nanoparticle inkjet printing. The proposed process can directly grow ZnO nanowires in any arbitrary patterned shape, and it is basically very fast, low cost, environmentally benign, and low temperature. Therefore, Zn acetate precursor inkjet printing-based direct nanowire local growth is expected to give extremely high flexibility in nanomaterial patterning for high-performance electronics fabrication especially at the development stage. As a proof of concept of the proposed method, ZnO nanowire network-based field effect transistors and ultraviolet photo-detectors were demonstrated by direct patterned grown ZnO nanowires as active layer.

Single nanowire resistive nano-heater for highly localized thermo-chemical reactions: localized hierarchical heterojunction nanowire growth.

A single nanowire resistive nano-heater (RNH) is fabricated, and it is demonstrated that the RNH can induce highly localized temperature fields, which can trigger highly localized thermo-chemical reactions to grow hierarchical nanowires directly at the desired specific spot such as ZnO nanowire branch growth on a single Ag nanowire.

Low-temperature rapid fabrication of ZnO nanowire UV sensor array by laser-induced local hydrothermal growth

We demonstrate ZnO nanowire based UV sensor by laser-induced hydrothermal growth of ZnO nanowire. By inducing a localized temperature rise using focused laser, ZnO nanowire array at ∼15 µm size consists of individual nanowires with ∼8 µm length and 200∼400 nm diameter is readily synthesized on gold electrode within 30 min at the desired position. The laser-induced growth process is consecutively applied on two different points to bridge the micron gap between the electrodes. The resultant photoconductive ZnO NW interconnections display 2∼3 orders increase in the current upon the UV exposure at a fixed voltage bias. It is also confirmed that the amount of photocurrent can be easily adjusted by changing the number of ZnO NW array junctions. The device exhibits clear response to the repeated UV illumination, suggesting that this process can be usefully applied for the facile fabrication of low-cost UV sensor array.