Jung-Keun Yoo

Scalable fabrication of silicon nanotubes and their application to energy storage.

The facile synthesis of silicon nanotubes using a surface sol-gel reaction on pyridine nanowire templates is reported and their performance for energy storage is investigated. Organic-inorganic hybrid pyridine/silica core-shell nanowires prepared using surface sol-gel reaction were converted to silica nanotubes by pyrolysis in air; this was followed by the reduction to silicon nanotubes via magnesiothermic reaction. The electrochemical activity of the obtained silicon nanotubes showed excellent cycle stability, suggesting that the hollow one-dimensional structure would be a good candidate for a high-capacity anode for a lithium ion battery.

Unexpected discovery of low-cost maricite NaFePO4 as a high-performance electrode for Na-ion batteries

Battery chemistry based on earth-abundant elements has great potential for the development of cost-effective, large-scale energy storage systems. Herein, we report, for the first time, that maricite NaFePO4 can function as an excellent cathode material for Na ion batteries, an unexpected result since it has been regarded as an electrochemically inactive electrode for rechargeable batteries. Our investigation of the Na re-(de)intercalation mechanism reveals that all Na ions can be deintercalated from the nano-sized maricite NaFePO4 with simultaneous transformation into amorphous FePO4. Our quantum mechanics calculations show that the underlying reason for the remarkable electrochemical activity of NaFePO4 is the significantly enhanced Na mobility in the transformed phase, which is ∼one fourth of the hopping activation barrier. Maricite NaFePO4, fully sodiated amorphous FePO4, delivered a capacity of 142 mA h g−1 (92% of the theoretical value) at the first cycle, and showed outstanding cyclability with a negligible capacity fade after 200 cycles (95% retention of the initial cycle).

Thermal stability of Fe–Mn binary olivine cathodes for Li rechargeable batteries

The phase stability of binary Fe and Mn olivine materials is extensively studied with temperature-controlled in situ X-ray diffraction (XRD) for various Fe/Mn ratios and state of charges (SOCs). We identify that the thermal behavior of partially charged olivine materials is sensitively affected by the Fe/Mn ratio in the crystal. While Fe-rich binary olivine materials readily formed a solid solution phase of Li1−yFe1−xMnxPO4 near room temperature or with only slight heating, the Mn-rich binary olivine retained its two-phase characteristic up to ca. 250 °C before decomposition into non-olivine phases. The thermal stability and decomposition mechanism of fully delithiated olivine materials are more sensitively affected by the Fe/Mn ratio in the crystal. The decomposition temperature varies from 200 °C to 500 °C among the different Fe/Mn ratios. It is generally observed that the Mn-rich binary olivine materials are inferior to the Fe-rich ones with respect to the thermal stability in the delithiated state.

Tailoring of the PbS/metal interface in colloidal quantum dot solar cells for improvements of performance and air stability

Despite the outstanding advantages of a simple structure and cost-effectiveness of solution-based fabrication, Schottky junction quantum dot solar cells (QDSCs) often demonstrate low open-circuit voltage and power conversion efficiency (PCE) due to insufficient band bending at the QD/metal Schottky junction. Generally, this undesirable result stems from the presence of many defects at the QD/metal interface and the consequent Fermi-level pinning effect. Here, we show how the simple oxidation of PbS QDs at the PbS/metal interface can greatly improve the open-circuit voltage, fill factor, and PCE of Schottky junction QDSCs. On the basis of systematic analysis results using current–voltage characterization, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and light-soaking tests, we reveal that this enhancement originates from reduced interface states at the PbS/metal Schottky junction. Moreover, a significant enhancement of stability of the device is confirmed by the maintenance of >55% of its initial PCE even after 500 hours exposure in air without additional passivation.

Porous silicon nanowires for lithium rechargeable batteries

Porous silicon nanowire is fabricated by a simple electrospinning process combined with a magnesium reduction; this material is investigated for use as an anode material for lithium rechargeable batteries. We find that the porous silicon nanowire electrode from the simple and scalable method can deliver a high reversible capacity with an excellent cycle stability. The enhanced performance in terms of cycling stability is attributed to the facile accommodation of the volume change by the pores in the interconnect and the increased electronic conductivity due to a multi-level carbon coating during the fabrication process.

Self-Extinguishing Lithium Ion Batteries Based on Internally Embedded Fire-Extinguishing Microcapsules with Temperature-Responsiveness

User safety is one of the most critical issues for the successful implementation of lithium ion batteries (LIBs) in electric vehicles and their further expansion in large-scale energy storage systems. Herein, we propose a novel approach to realize self-extinguishing capability of LIBs for effective safety improvement by integrating temperature-responsive microcapsules containing a fire-extinguishing agent. The microcapsules are designed to release an extinguisher agent upon increased internal temperature of an LIB, resulting in rapid heat absorption through an in situ endothermic reaction and suppression of further temperature rise and undesirable thermal runaway. In a standard nail penetration test, the temperature rise is reduced by 74% without compromising electrochemical performances. It is anticipated that on the strengths of excellent scalability, simplicity, and cost-effectiveness, this novel strategy can be extensively applied to various high energy-density devices to ensure human safety.

Host-Guest Self-assembly in Block Copolymer Blends

Ultrafine, uniform nanostructures with excellent functionalities can be formed by self-assembly of block copolymer (BCP) thin films. However, extension of their geometric variability is not straightforward due to their limited thin film morphologies. Here, we report that unusual and spontaneous positioning between host and guest BCP microdomains, even in the absence of H-bond linkages, can create hybridized morphologies that cannot be formed from a neat BCP. Our self-consistent field theory (SCFT) simulation results theoretically support that the precise registration of a spherical BCP microdomain (guest, B-b-C) at the center of a perforated lamellar BCP nanostructure (host, A-b-B) can energetically stabilize the blended morphology. As an exemplary application of the hybrid nanotemplate, a nanoring-type Ge2Sb2Te5 (GST) phase-change memory device with an extremely low switching current is demonstrated. These results suggest the possibility of a new pathway to construct more diverse and complex nanostructures using controlled blending of various BCPs.

Magnetic resonant wireless power transfer for propulsion of implantable micro-robot

Recently, various types of mobile micro-robots have been proposed for medical and industrial applications. Especially in medical applications, a motor system for propulsion cannot easily be used in a micro-robot due to their small size. Therefore, micro-robots are usually actuated by controlling the magnitude and direction of an external magnetic field. However, for micro-robots, these methods in general are only applicable for moving and drilling operations, but not for the undertaking of various missions. In this paper, we propose a new micro-robot concept, which uses wireless power transfer to deliver the propulsion force and electric power simultaneously. The mechanism of Lorentz force generation and the coil design methodologies are explained, and validation of the proposed propulsion system for a micro-robot is discussed thorough a simulation and with actual measurements with up-scaled test vehicles.

Precise nonselective chemically assisted ion-beam etching of AlGaAs/GaAs Bragg reflectors byin situ laser reflectometry

Precision etch-depth control is realized by a chemically assisted ion-beam etching system incorporated within situ laser reflectometry. By counting the number of interference fringes, etch-depth control better than a quarter-wave thickness is easily obtained. Optimized etching conditions for highly anisotropic etching of bulk GaAs and AlGaAs/GaAs distributed Bragg reflectors are obtained. With the ability to etch-stop just below the active region by thein situ monitoring, InGaAs vertical-cavity surface-emitting lasers with CW threshold current density as low as 380 A cm-2 with output power >11 mW are fabricated. Spatial uniformity is 5% over a 1-cm2 sample, which corresponds to one pair over 20 pairs of quarter-wave stacks of AlGaAs/GaAs distributed Bragg reflectors.

Current redistribution of a current carrying superconducting tape in a perpendicular magnetic field

The current redistribution of a high Tc superconducting tape with increasing transport current in a magnetic field (Ha) was visualized. The external magnetic field was applied normal to the surface of the sample after zero-field cooling, and the transport current (Ia) was increased stepwise from 0 to 90% of the values of the critical current (Ic(Ha)) at Ha. The field distribution near the sample surface across the tape width (2w) was measured using the scanning Hall probe method. Applying an inversion to the measured field distribution, we obtained the underlying current distribution. The initial fieldlike distribution was gradually changed into a currentlike distribution with an increase of the transport current. The transition occurred when Ia and Ha approached the line f = c in the f–c plane where f = Ia/Ic and c = tanh(Ha/Hc). Hc is defined as Ic/(2πw).