Tae-Yeon Hwang

Morphology control of ordered Si nanowire arrays by nanosphere lithography and metal-assisted chemical etching

Metal-assisted chemical etching (MACE) using a nanosphere lithography (NSL) technique is regarded as a general fabrication method for silicon nanowire (SINW) arrays. However, morphology control of SiNWs using this method has not been reported. In this study, silicon nanowire (SINW) and silicon nanocone (SINC) arrays were fabricated by MACE using a NSL. Depending on the concentration of etchants in the etching solution, the morphology of the wires and etching rate were systemically changed. At high concentrations, the wires were etched cylindrically and at low concentrations, tapered (cone-like) wires were obtained. To quantify the degree of tapering, the volume ratio of the etched part was calculated from their morphology. The degree of tapering increased as the concentrations of etchants decreased. We suggest that the mechanism of the formation of nanocone is related to the degree of hole diffusion under the metal layer, which was supported by field emission scanning electron microscope (FE-SEM) images.

Synthesis of Samarium-Cobalt Sub-micron Fibers and Their Excellent Hard Magnetic Properties

High-throughput synthesis of Samarium-Cobalt sub-micron fibers with controlled composition and dimension was demonstrated by combining electrospinning and reduction-diffusion processes. The composition of fibers was readily varied (8 < Sm < 20 at.%) by adjusting precursor composition whereas the diameter of fibers was precisely controlled by varying electrospinning parameters (e.g., applied voltage, solution feed rate, temperature, and humidity) to reach single-domain size. X-ray diffraction patterns confirmed that single phase Sm2Co17 fibers were synthesized when the metal precursor ratio (Sm3+/(Sm3++Co2+)) was precisely controlled at 10.6%, whereas mixed phases (i.e., Co-Sm2Co17 or Sm2Co17-Sm2Co7) were observed when the ratio is deviated from the stoichiometric. Magnetic saturation (Ms) of the synthesized fibers monotonically decreased with an increased in Sm content. In contrast, coercivity (Hci) monotonically increased with an increase in Sm content.

A noble gas sensor platform: linear dense assemblies of single-walled carbon nanotubes (LACNTs) in a multi-layered ceramic/metal electrode system (MLES)

Monodispersed Pt nanocatalyst-doped and undoped assemblies of single-walled carbon nanotubes (SWCNTs) were aligned with high anisotropy on a multi-layered ceramic/metal electrode system (MLES), which was used as a cheap and cost-effective electrode system, by dielectrophoretic orientation to form linear dense assemblies of SWCNTs (LACNTs). It is envisaged that they can be used as a potentially inexpensive platform for multi-gas sensing nano-devices. To form homogeneously dispersed Pt nanocatalysts (NCs), an aqueous chloroplatinic acid solution (H2PtCl6) was deposited on the surface of acid-treated SWCNTs. This was followed by alignment under the application of alternative currents ranging from 5 kHz to 50 MHz to create electrically conducting LACNT bundles on the MLES platform and reduction under hydrogen plasma. The analysis showed that the Pt nanocatalysts (2 nmav, deviation: ±1 nm) are monodispersed on the SWCNTs, which contributed to a noticeable enhancement in the gas sensing properties towards H2, NO2, H2S and NH3 gases. In addition, the principal component analysis results showed fairly good discrimination of the gases by employing a sensor array composed of Pt/LACNTs and SWCNTs, and the hydrogen sensing performance was high in terms of sensitivity compared to other literature studies. This gas sensing device based on an MLES may pave the way for extended applications of new multi-gas sensing devices as a low-cost and life-long gas sensing platform.

Near theoretical ultra-high magnetic performance of rare-earth nanomagnets via the synergetic combination of calcium-reduction and chemoselective dissolution

Rare earth permanent magnets with superior magnetic performance have been generally synthesized through many chemical methods incorporating calcium thermal reduction. However, a large challenge still exists with regard to the removal of remaining reductants, byproducts, and trace impurities generated during the purifying process, which serve as inhibiting intermediates, inducing productivity and purity losses, and a reduction in magnetic properties. Nevertheless, the importance of a post-calciothermic reduction process has never been seriously investigated. Here, we introduce a novel approach for the synthesis of a highly pure samarium-cobalt (Sm-Co) rare earth nanomagnet with near theoretical ultra-high magnetic performance via consecutive calcium-assisted reduction and chemoselective dissolution. The chemoselective dissolution effect of various solution mixtures was evaluated by the purity, surface microstructure, and magnetic characteristics of the Sm-Co. As a result, NH4Cl/methanol solution mixture was only capable of selectively rinsing out impurities without damaging Sm-Co. Furthermore, treatment with NH4Cl led to substantially improved magnetic properties over 95.5% of the Ms for bulk Sm-Co. The mechanisms with regard to the enhanced phase-purity and magnetic performance were fully elucidated based on analytical results and statistical thermodynamics parameters. We further demonstrated the potential application of chemoselective dissolution to other intermetallic magnets.

Near-infrared absorbance properties of Cu2−xS/SiO2 nanoparticles and their PDMS-based composites

We synthesized different Cu2−xS nanoparticles (CuS and Cu1.8S NPs) with localized surface plasmon resonance (LSPR) absorbance in the near infrared (NIR) region and applied a silica layer on the surface of Cu2−xS nanoparticles (NPs) to achieve high photo-stability and dispersion stability. These plasmonic Cu2−xS NPs have been much utilized as thermal shielding materials of energy-saving windows, due to shielding the ultraviolet (UV) and NIR regions, and transmitting the visible (Vis) region. However, this application is limited because Cu2−xS NPs show photocatalyst properties that lead to the decomposition of organic matter, and dispersing these NPs into polymer films is difficult. Herein, silica coating successfully suppressed the generation of hydroxyl radicals on the surface of CuS NPs, which caused the photo-degradation of a polydimethylsiloxane (PDMS) polymer. Furthermore, a silica layer with the structure similar to that of PDMS provided the dispersion stability of CuS NPs. The achievement of good photo-stability and dispersion stability in the Cu2−xS NPs was demonstrated by the thermal shielding effect using a simulated experiment. The temperature change of a box shielded with a CuS/SiO2–PDMS film (ΔT = 6.8 °C) was smaller than that when the box was shielded with common glass (ΔT = 12.7 °C) or a CuS–PDMS film (ΔT = 9.2 °C). This research introduces a new and reliable thermal shielding material.