Yong-keon Ahn

Tunable terahertz generation using femtosecond pulse shaping

Femtosecond pulse shaping and photomixing methods are combined to make a tunable terahertz source. Grating-pair pulse shaper selects two main frequency components to produce pulses that are modulated with a period which is inversely proportional to the frequency separation. The shaped pulse is then photomixed in semi-insulating GaAs under external bias. Terahertz (THz) emission frequency follows the modulation frequency of the excitation pulses, from 0.5 to 3 THz. Contrary to cw photomixing, this method can easily be combined with electro-optic and photoconductive sampling detection methods.

All solid state flexible supercapacitors operating at 4 V with a cross-linked polymer–ionic liquid electrolyte

4 V-operated all solid symmetrical supercapacitors that employ mixtures of various weight compositions with cross-linked poly-4-vinylphenol (c-P4VPh) and 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) electrolytes have been demonstrated and characterized. The values at 1:3, 3.5, 4 and 4.5 (c-P4VPh:EMITFSI) offer free-standing membranes with high ionic conductivity. In the case of 1:3.5, the best specific capacitance (172.44 F g−1 in a single-electrode) and energy density (72.23 W h kg−1) were obtained at symmetrical cells based on porous carbon electrodes. Every prepared SC was reliable over 1000 cycles in the range of 0–4 V. They also have excellent flexibility and maintain capacitance after completing the bending test a thousand times.

Self-reducible copper ion complex ink for air sinterable conductive electrodes

Copper (Cu) based conductive inks have been widely studied with the objective of achieving highly conductive and low-cost electrodes for various electrical devices. However, the unstable oxidation properties of Cu inks make them difficult to be applied for a wide range of practical applications. The oxidation properties induce high resistivity in the electrode fabrication, and storage problem of ink. Herein, we introduce a novel self-reducible Cu ion complex ink (Cu-ink), composed by formate, alkanolamine groups and poly alcohols, for the air sinterable, low-cost, environmentally friendly fabrication of Cu conductive electrodes. The air sinterable properties of this novel Cu-ink are induced by the self-reducing activity of the Cu-ink ligand decomposition and the reduction-assistance properties of the polyol solvents. In particular, among various polyol solvents, glycerol was found to be the most suitable reduction assistant-material because of its relatively abundant hydroxyl groups, good evaporation properties, and environmentally friendly solvents. Through investigation of the Cu-ink sintering temperature and glycerol contents, we obtained the Cu electrode films with a low resistivity of 17 μΩ cm at 350 °C under air sintering conditions. Furthermore, various practical characteristics such as excellent storage stability (of up to 4 weeks), enhanced adhesion properties, and flexible retention characteristics for up to 2000 bending times (R/R0 < 1.2, bending radius 20 mm) were also demonstrated for Cu electrode films.

Enhanced electrochemical capabilities of lithium ion batteries by structurally ideal AAO separator

In this study, a novel inorganic separator, porous anodic aluminum oxide (AAO), is introduced for a rechargeable lithium ion battery system. The highly ordered AAO gives rise to an ideal structure for battery separators with appropriate porosity (67.4 %), extremely low tortuosity, and thermal durability. The prepared AAO separator has average pore sizes of 75 nm and thickness of 54 μm, which leads to enhanced ionic conductivity (2.196 mS cm−1), discharging capacity at high current rates (20.13 mA h g−1 at 10 C), and capacity retention (82.9%). Moreover, a computer simulation (COMSOL) model shows that the ideal AAO separator structure induces stable lithium ion battery operation in wide ranges of current rate, due to effective suppression of Li dendrite formation. The AAO separator has a strong potential in massive energy storage systems and electric vehicles.

Copper nanowire/multi-walled carbon nanotube composites as all-nanowire flexible electrode for fast-charging/discharging lithium-ion battery

A novel lightweight three-dimensional (3D) composite anode for a fast-charging/discharging Li-ion battery (LIB) was fabricated entirely using one-dimensional (1D) nanomaterials, i.e., Cu nanowires (CuNWs) and multi-walled C nanotubes (MWCNTs). Because of the excellent electrical conductivity, high-aspect ratio structures, and large surface areas of these nanomaterials, the CuNW-MWCNT composite (CNMC) with 3D structure provides significant advantages regarding the transport pathways for both electrons and ions. As an advanced binder-free anode, a CuNW-MWCNT composite film with a controllable thickness (∼600 μm) exhibited a considerably low sheet resistance, and internal cell resistance. Furthermore, the random CuNW network with 3D structure acting as a rigid framework not only prevented MWCNT shrinkage and expansion due to aggregation and swelling but also minimized the effect of the volume change during the charge/discharge process. Both a half cell and a full cell of LIBs with the CNMC anode exhibited high specific capacities and Coulombic efficiencies, even at a high current. More importantly, we for the first time overcame the limitation of MWCNTs as anode materials for fast-charging/discharging LIBs (both half cells and full cells) by employing CuNWs, and the resulting anode can be applied to flexible LIBs. This innovative anode structure can lead to the development of ultrafast chargeable LIBs for electric vehicles.

Synthesis of Copper Oxide/Graphite Composite for High‐Performance Rechargeable Battery Anode

A novel copper oxide/graphite composite (GCuO) anode with high capacity and long cycle stability is proposed. A simple, one-step synthesis method is used to prepare the GCuO, through heat treatment of the Cu ion complex and pristine graphite. The gases generated during thermal decomposition of the Cu ion complex (H2 and CO2 ) induce interlayer expansion of the graphite planes, which assists effective ion intercalation. Copper oxide is formed simultaneously as a high-capacity anode material through thermal reduction of the Cu ion complex. Material analyses reveal the formation of Cu oxide nanoparticles and the expansion of the gaps between the graphite layers from 0.34 to 0.40 nm, which is enough to alleviate layer stress for reversible ion intercalation for Li or Na batteries. The GCuO cell exhibits excellent Li-ion battery half-cell performance, with a capacity of 532 mAh g-1 at 0.2 C (C-rate) and capacity retention of 83 % after 250 cycles. Moreover, the LiFePO4 /GCuO full cell is fabricated to verify the high performance of GCuO in practical applications. This cell has a capacity of 70 mAh g-1 and a coulombic efficiency of 99 %. The GCuO composite is therefore a promising candidate for use as an anode material in advanced Li- or Na-ion batteries.

Conduction mechanism change with transport oxide layer thickness in oxide hetero-interface diode

An effective and facile strategy is proposed to demonstrate an engineered oxide hetero-interface of a thin film diode with a high current density and low operating voltage. The electrical characteristics of an oxide hetero-interface thin film diode are governed by two theoretical models: the space charge-limited current model and the Fowler-Nordheim (F-N) tunneling model. Interestingly, the dominant mechanism strongly depends on the insulator thickness, and the mechanism change occurs at a critical thickness. This paper shows that conduction mechanisms of oxide hetero-interface thin film diodes depend on thicknesses of transport oxide layers and that current densities of these can be exponentially increased through quantum tunneling in the diodes with the thicknesses less than 10 nm. These oxide hetero-interface diodes have great potential for low-powered transparent nanoscale applications.