Yongcheol Jo

Multi-functional reactively-sputtered copper oxide electrodes for supercapacitor and electro-catalyst in direct methanol fuel cell applications.

This work reports on the concurrent electrochemical energy storage and conversion characteristics of granular copper oxide electrode films prepared using reactive radio-frequency magnetron sputtering at room temperature under different oxygen environments. The obtained films are characterized in terms of their structural, morphological, and compositional properties. X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscope studies reveal that granular, single-phase Cu2O and CuO can be obtained by controlling the oxygen flow rate. The electrochemical energy storage properties of the films are investigated by carrying out cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy tests. The electrochemical analysis reveals that the Cu2O and CuO electrodes have high specific capacitances of 215 and 272 F/g in 6 M KOH solution with a capacity retention of about 80% and 85% after 3000 cycles, respectively. Cyclic voltammetry and chronoamperometry are used to study the electrochemical energy conversion properties of the films via methanol electro-oxidation. The results show that the Cu2O and CuO electrodes are electro-catalytically active and highly stable.

Self-assembled two-dimensional copper oxide nanosheet bundles as an efficient oxygen evolution reaction (OER) electrocatalyst for water splitting applications

A high activity of a two-dimensional (2D) copper oxide (CuO) electrocatalyst for the oxygen evolution reaction (OER) is presented. The CuO electrode self-assembles on a stainless steel substrate via chemical bath deposition at 80 °C in a mixed solution of CuSO4 and NH4OH, followed by air annealing treatment, and shows a 2D nanosheet bundle-type morphology. The OER performance is studied in a 1 M KOH solution. The OER starts to occur at about 1.48 V versus the RHE (η = 250 mV) with a Tafel slope of 59 mV dec−1 in a 1 M KOH solution. The overpotential (η) of 350 mV at 10 mA cm−2 is among the lowest compared with other copper-based materials. The catalyst can deliver a stable current density of >10 mA cm−2 for more than 10 hours. This superior OER activity is due to its adequately exposed OER-favorable 2D morphology and the optimized electronic properties resulting from the thermal treatment.

Nickel titanate lithium-ion battery anodes with high reversible capacity and high-rate long-cycle life performance

We demonstrate the impressive performance of sparsely studied nickel titanate anode materials for Li-ion batteries (LIBs). The nickel titanate anode delivers a high reversible discharge capacity of 435 mA h g−1 at a current density of 35 mA g−1, high-rate performance and excellent cycling retention of 96% with a long-term cycling stability at 1500 mA g−1 over 300 cycles. The coulombic efficiency is obtained as high as 98%. This superior nickel titanate electrode material could be used as a safe, low-cost, long cycle life anode material for next-generation LIBs with a high power capability.

Resistance switching mode transformation in SrRuO3/Cr-doped SrZrO3/Pt frameworks via a thermally activated Ti out-diffusion process

This work reports on a mechanism for irreversible resistive switching (RS) transformation from bipolar to unipolar RS behavior in SrRuO3 (SRO)/Cr-doped SrZrO3 (SZO:Cr)/Pt capacitor structures prepared on a Ti/SiO2/Si substrate. Counter-clockwise bipolar RS memory current-voltage (I–V) characteristics are observed within the RS voltage window of −2.5 to +1.9 V, with good endurance and retention properties. As the bias voltage increases further beyond 4 V under a forward bias, a forming process occurs resulting in irreversible RS mode transformation from bipolar to unipolar mode. This switching mode transformation is a direct consequence of thermally activated Ti out-diffusion from a Ti adhesion layer. Transition metal Ti effectively out-diffuses through the loose Pt electrode layer at high substrate temperatures, leading to the unintended formation of a thin titanium oxide (TiOx where x < 2) layer between the Pt electrode and the SZO:Cr layer as well as additional Ti atoms in the SZO:Cr layer. Cross-sectional scanning electron microscopy, transmission electron microscopy and Auger electron spectroscopy depth-profile measurements provided apparent evidence of the Ti out-diffusion phenomenon. We propose that the out-diffusion-induced additional Ti atoms in the SZO:Cr layer contributes to the creation of the metallic filamentary channels.

Effect of Electronegativity on Bipolar Resistive Switching in a WO3-Based Asymmetric Capacitor Structure

This study investigates the transport and switching time of nonvolatile tungsten oxide based resistive-switching (RS) memory devices. These devices consist of a highly resistive tungsten oxide film sandwiched between metal electrodes, and their RS characteristics are bipolar in the counterclockwise direction. The switching voltage, retention, endurance, and switching time are strongly dependent on the type of electrodes used, and we also find quantitative and qualitative evidence that the electronegativity (χ) of the electrodes plays a key role in determining the RS properties and switching time. We also propose an RS model based on the role of the electronegativity at the interface.

Switching Power Universality in Unipolar Resistive Switching Memories.

We investigate the resistive switching power from unipolar resistive switching current-voltage characteristics in various binary metal oxide films sandwiched by different metal electrodes, and find a universal feature (the so-called universality) in the switching power among these devices. To experimentally derive the switching power universality, systematic measurements of the switching voltage and current are performed, and neither of these correlate with one another. As the switching resistance (R) increases, the switching power (P) decreases following a power law P ∝ R−β, regardless of the device configurations. The observed switching power universality is indicative of the existence of a commonly applicable switching mechanism. The origin of the power universality is discussed based on a metallic filament model and thermo-chemical reaction.

Reversible conductance switching characteristics in a polymer-In2O3 nanocrystals junction

A transparent polymer-based resistive switching device containing In2O3 nanocrystals (NCs) is fabricated, and its nonvolatile memory characteristics are evaluated. Very clear reversible counter-clockwise bipolar-type resistive switching phenomena are observed. Stable retention is demonstrated. An Analysis of the temperature dependence of the bistable resistance states reveals additional features, not reported in previous studies, that the observed resistance switching is due to oxygen ions drift-induced redox reactions at the polymer/In2O3 NCs interface. The RESET and SET switching times (τRESET and τSET), which are defined as pulse widths extrapolated by the steepest slopes in the transition region, are τRESET ∼ 550 nsec and τSET ∼ 900 nsec. The authors propose that microscopic potential modification occurring near the polymer/In2O3 NCs boundaries plays a key role in determining resistive switching properties.

Self‐Assembled Nanostructured CuCo2O4 for Electrochemical Energy Storage and the Oxygen Evolution Reaction via Morphology Engineering

CuCo2 O4 films with different morphologies of either mesoporous nanosheets, cubic, compact-granular, or agglomerated embossing structures are fabricated via a hydrothermal growth technique using various solvents, and their bifunctional activities, electrochemical energy storage and oxygen evolution reaction (OER) for water splitting catalysis in strong alkaline KOH media, are investigated. It is observed that the solvents play an important role in setting the surface morphology and size of the crystallites by controlling nucleation and growth rate. An optimized mesoporous CuCo2 O4 nanosheet electrode shows a high specific capacitance of 1658 F g-1 at 1 A g-1 with excellent restoring capability of ≈99% at 2 A g-1 and superior energy density of 132.64 Wh kg-1 at a power density of 0.72 kW kg-1 . The CuCo2 O4 electrode also exhibits excellent endurance performance with capacity retention of 90% and coulombic efficiency of ≈99% after 5000 charge/discharge cycles. The best OER activity is obtained from the CuCo2 O4 nanosheet sample with the lowest overpotential of ≈290 mV at 20 mA cm-2 and a Tafel slope of 117 mV dec-1 . The superior bifunctional electrochemical activity of the mesoporous CuCo2 O4 nanosheet is a result of electrochemically favorable 2D morphology, which leads to the formation of a very large electrochemically active surface area.

Temperature dependence of ballistic mobility in a metamorphic InGaAs/InAlAs high electron mobility transistor

We have investigated the temperature dependence of ballistic mobility in a 100 nm-long InGaAs/InAlAs metamorphic high-electron-mobility transistor designed for millimeter-wavelength RF applications. To extract the temperature dependence of quasi-ballistic mobility, our experiment involves measurements of the effective mobility in the low-bias linear region of the transistor and of the collision-dominated Hall mobility using a gated Hall bar of the same epitaxial structure. The data measured from the experiment are consistent with that of modeled ballistic mobility based on ballistic transport theory. These results advance the understanding of ballistic transport in various transistors with a nano-scale channel length that is comparable to the carriers mean free path in the channel.

Linearly polarized photoluminescence of InGaN quantum disks embedded in GaN nanorods

We have investigated the emission from InGaN/GaN quantum disks grown on the tip of GaN nanorods. The emission at 3.21 eV from the InGaN quantum disk doesn’t show a Stark shift, and it is linearly polarized when excited perpendicular to the growth direction. The degree of linear polarization is about 39.3% due to the anisotropy of the nanostructures. In order to characterize a single nanostructure, the quantum disks were dispersed on a SiO2 substrate patterned with a metal reference grid. By rotating the excitation polarization angle from parallel to perpendicular relative to the nanorods, the variation of overall PL for the 3.21 eV peak was recorded and it clearly showed the degree of linear polarization (DLP) of 51.5%.