Seonno Yoon

Three-Dimensional Hierarchically Mesoporous ZnCo2O4 Nanowires Grown on Graphene/Sponge Foam for High-Performance, Flexible, All-Solid-State Supercapacitors

To achieve high energy storage on three-dimensional (3D) structures at low cost, materials with high power and long cycle life characteristics have to be developed. We synthesized ZnCo2 O4 /reduced graphene oxide (rGO) binary composites in commercial sponges. ZnCo2 O4 nanosheets were grown on the surface of GO/sponge through a hydrothermal reaction. The resulting flexible, free-standing ZnCo2 O4 /rGO/sponge electrodes were used as the electrodes in a symmetric supercapacitor. ZnCo2 O4 /rGO/sponge electrodes have a large specific capacitance of 1116.6 F g-1 at a scan rate of 2 mV s-1 in aqueous electrolyte. The all-solid-state flexible supercapacitor is assembled based on ZnCo2 O4 /rGO/sponge electrodes, which show excellent electrochemical performances with a specific capacitance of 143 F g-1 at a current density of 1 A g-1 . The as-fabricated supercapacitor also exhibits excellent cycling stability (93.4 % capacitance retention after 5000 cycles) and exceptional mechanical flexibility. These results demonstrate the potential of ZnCo2 O4 /rGO/sponge as an electrode in flexible, high-performance supercapacitors.

Lateral photovoltaic effect in flexible free-standing reduced graphene oxide film for self-powered position-sensitive detection.

Lightweight, simple and flexible self-powered photodetectors are urgently required for the development and application of advanced optical systems for the future of wearable electronic technology. Here, using a low-temperature reduction process, we report a chemical approach for producing freestanding monolithic reduced graphene oxide papers with different gradients of the carbon/oxygen concentration ratio. We also demonstrate a novel type of freestanding monolithic reduced graphene oxide self-powered photodetector based on a symmetrical metal–semiconductor–metal structure. Upon illumination by a 633-nm continuous wave laser, the lateral photovoltage is observed to vary linfearly with the laser position between two electrodes on the reduced graphene oxide surface. This result may suggest that the lateral photovoltaic effect in the reduced graphene oxide film originates from the built-in electric field by the combination of both the photothermal electric effect and the gradient of the oxygen-to-carbon composition. These results represent substantial progress toward novel, chemically synthesized graphene-based photosensors and suggest one-step integration of graphene-based optoelectronics in the future.

Localized TiSi and TiN phases in Si/Ti/Al/Cu Ohmic contacts to AlGaN/GaN heterostructures

Microstructural changes in Si/Ti/Al/Cu (10/40/60/50 nm) Ohmic contacts to AlGaN/GaN heterostructure were investigated for complementary metal-oxide semiconductor compatible processes. Si/Ti/Al/Cu metallization exhibited a low specific contact resistance of 3.6 × 10−6 Ω-cm2 and contact resistance of 0.46 Ω-mm when a Si interfacial layer was used. Without a designated barrier metal, TiSix alloys that formed in the metallic region effectively suppressed Cu diffusion. The shallow TiN junction in AlGaN/GaN was attributed to TiSix in the metallic regions. Microstructural changes were detected by systematic physical characterization.

Three-Dimensional Flexible All-Organic Conductors for Multifunctional Wearable Applications

Wearable textile electrodes based on π-conjugated polymers are appealing alternatives to carbon fabrics, conductive yarns, or metal wires because of their design flexibility, low cost, flexibility, and high throughput. This provides the benefits of both electronics and textiles. Herein, a general and new method has been developed to produce tailorable, wearable energy devices that are based on three-dimensional (3D) poly(3,4-ethylenedioxythiophene) (PEDOT)-coated textile conductors. To obtain the desired electrode materials, both facile solution-dropping polymerization methods are used to fabricate a 3D flexible PEDOT conductor on a cotton textile (PEDOT/textile). PEDOT/textile shows a very low sheet resistance of 4.6-4.9 Ω·sq-1. Here, we employ the example of this 3D network-like structure and the excellent electrical conductivities under the large deformation of PEDOT/textiles to show that wearable and portable heaters have immense potential. A flexible textile heater with a large area (8 × 7.8 cm2) reached a saturation temperature of ∼83.9 °C when a bias of 7 V was applied for ∼70 s due to the good electrical conductivity of PEDOT. To demonstrate the performance of all-solid-state supercapacitors, nano-ascidian-like PEDOT (PEDOT-NA) arrays were prepared via a simple vapor-phase polymerization of 3,4-ethylenedioxythiophene on PEDOT/textile to increase both the surface area and the number of ion diffusion paths. The PEDOT-NA arrays on PEDOT/textile showed outstanding performance with an areal capacitance of 563.3 mF·cm-2 at 0.4 mA·cm-2 and extraordinary mechanical flexibility. The maximum volumetric power density and energy density of the nanostructured PEDOT on the textile were 1.75 W·cm-3 and 0.0812 Wh·cm-3, respectively. It is expected that the wearable nanostructured conducting polymers will have advantages when used as structures for smart textronics and energy conversion/storage.

Atomic-Layer Deposition of Single-Crystalline BeO Epitaxially Grown on GaN Substrates

We have grown a single-crystal beryllium oxide (BeO) thin film on a gallium nitride (GaN) substrate by atomic-layer deposition (ALD) for the first time. BeO has a higher thermal conductivity, bandgap energy, and dielectric constant than SiO2. As an electrical insulator, diamond is the only material on earth whose thermal conductivity exceeds that of BeO. Despite these advantages, there is no chemical-vapor-deposition technique for BeO-thin-film deposition, and thus, it is not used in nanoscale-semiconductor-device processing. In this study, the BeO thin films grown on a GaN substrate with a single crystal showed excellent interface and thermal stability. Transmission electron microscopy showed clear diffraction patterns, and the Raman shifts associated with soft phonon modes verified the high thermal conductivity. The X-ray scan confirmed the out-of-plane single-crystal growth direction and the in-plane, 6-fold, symmetrical wurtzite structure. Single-crystalline BeO was grown on GaN despite the large lattice mismatch, which suggested a model that accommodated the strain of hexagonal-on-hexagonal epitaxy with 5/6 and 6/7 domain matching. BeO has a good dielectric constant and good thermal conductivity, bandgap energy, and single-crystal characteristics, so it is suitable for the gate dielectric of power semiconductor devices. The capacitance-voltage (C-V) results of BeO on a GaN-metal-oxide semiconductor exhibited low frequency dispersion, hysteresis, and interface-defect density.