Youn-Kyoung Baek

Cylindrical posts of Ag/SiO2/Au multi-segment layer patterns for highly efficient surface enhanced Raman scattering

We fabricated a regular array of Ag/SiO₂/Au multi-segment cylindrical nanopatterns to create a highly efficient surface enhanced Raman scattering (SERS) active substrate using an advanced soft-nanoimprint lithographic technique. The SERS spectra results for Rhodamine 6G (R6G) molecules on the Ag/SiO₂/Au multi-segment nanopatterns show that the highly ordered patterns and interlayer thickness are responsible for enhancing the sensitivity and reproducibility, respectively, The multi-segment nanopattern with a silica interlayer generates significant SERS enhancement (~EF = 1.2 x 10⁶) as compared to that of the bimetallic (Ag/Au) nanopatterns without a dielectric gap (~EF = 1.0 x 10⁴). Further precise control of the interlayer distances between the two metals plays an essential role in enhancing SERS performance for detecting low concentrations of analytes such as fluorescent (Rhodamine 6G) and DNA molecules. Therefore, the highly ordered multi-segment patterns provide great sensitivity and reproducibility of SERS based detection, resulting in a high performance of the SERS substrate.

Effect of the dehydrogenation speed and Nd content on the microstructure and magnetic properties of HDDR processed Nd-Fe-B magnets

The effect of Nd content and dehydrogenation speed on the microstructure and magnetic properties of hydrogenation-disproportionation-desorption-recombination (HDDR) processed Nd-Fe-B magnetic powders was studied. The NdxB6.4Ga0.3Nb0.2Febal (x=12.5–13.5, at.%) mold casting alloys were subjected to HDDR process after homogenization heat treatment. During desorption-recombination stage, dehydrogenation speed and time were systematically changed to control the speed of the desorption-recombination reaction. The higher Nd content resulted in better magnetic properties of the HDDR powder, and this was attributed to the thicker and more uniform Nd-rich phase at grain boundaries. It was also confirmed that the slow dehydrogenation speed could maximize the effect of high Nd content on the magnetic properties of HDDR powder. At the optimized dehydrogenation speed, the coercivity and remanence was 15.3 kOe and 13.0 kG, respectively, at 12.9 at.% Nd content, which resulted in a (BH)max of 36.8 MGOe.

Patterned nano-sized gold dots within FET channel: from fabrication to alignment of single walled carbon nanotube networks†

We have introduced patterned gold dots between the electrodes of single walled carbon nanotube (SWNT) network films, to effectively guide the path of randomly assembled nanotube networks. Direct visualization results show well-aligned SWNT networks across the gold dots, generating improved SWNT–FET performance with a high on/off ratio of 104 and 85 cm2 V−1s−1 mobility, a remarkable enhancement compared to SWNT–FET in the absence of patterned gold dots. Unlike previously reported alignment techniques used, the proposed method affords the selective alignment of SWNTs within electrodes at room temperature without any direct growth on the device surface, and without the need for further alignment procedures. Thus, this gold-dot pattern technique within FET channel has proven to be truly simple, reliable and cost-effective. We discuss the effect of the voltage, channel size and feature dimensions of the gold nanopatterns in field effect transistor (FET) structures.

Tin nanoparticle thin film electrodes fabricated by the vacuum filtration method for enhanced battery performance

A novel method for fabricating tin nanoparticle thin film electrodes that show good performance in lithium ion batteries during cycling is reported. The vacuum filtration method has the advantage of affording a high degree of dispersion of the electrode components, thereby providing good electrical contacts between the tin nanoparticles and the conductive carbon or current collector. The reversible capacity and initial Coulombic efficiency are 726 mA h g(-1) and 85.3%, respectively, with this thin film electrode. Cycle life performance tests under real battery conditions show that the battery capacity and reaction peaks remained stable for up to 50 cycles. SEM shows that the uniform morphology of the vacuum filtered film was maintained throughout the cycle life test. This novel vacuum filtration method for providing nanoparticle-based film electrodes has further potential applications for use in various devices such as high power, thin film batteries, supercapacitors and organic-inorganic hybrid photovoltaic cells.

The Influence of Dehydrogenation Speed on the Microstructure and Magnetic Properties of Nd-Fe-B Magnets Prepared by HDDR Process

The influence regarding the dehydrogenation speed, at the desorption-recombination state during the hydrogenation-disproportionation-desorption-recombination (HDDR) process, on the microstructure and magnetic properties of Nd-Fe-B magnetic powders has been studied. Strip cast Nd-Fe-B-based alloys were subjected to the HDDR process after the homogenization heat treatment. During the desorption-recombination stage, both the pumping speed and time of hydrogen were systematically changed in order to control the speed of the desorption-recombination reaction. The magnetic properties of HDDR powders were improved as the pumping speed of hydrogen at the desorption-recombination stage was decreased. The lower pumping speed resulted in a smaller grain size and higher DoA. The coercivity and the remanence of the 200-300 μm sized HDDR powder increased from 12.7 to 14.6 kOe and from 8.9 to 10.0 kG, respectively. In addition, the remanence was further increased to 11.8 kG by milling the powders down to about 25-90 μm, resulting in (BH) max of 28.8 MGOe.

Anisotropic Consolidation Behavior of Isotropic Nd–Fe–B HDDR Powders During Hot-Deformation

In this paper, the anisotropic consolidation behavior of isotropic Nd–Fe–B HDDR powders has been studied as a function of the strain, strain rate, and processing temperature. The compacts, produced by hot-pressing at 700 °C under 400 MPa in vacuum, were subjected to die-upsetting at 700 °C and 800 °C with different deformation conditions of strain rate 0.01–0.001 s⁻¹ and strain 0.5–1.5. After die-upset, the coercivity and remanence decreased and increased with increasing strain, respectively, with the change of grain morphology from the sphere to platelet. And the high deformation temperature and slow deformation rate reduce the coercivity further. The coercivity of die-upset magnet, produced at 700 °C with a strain of 1.4 and a strain rate of 0.001 s⁻¹, was largely increased about 2 kOe after post-annealing at 800 °C. The maximum coercivity, remanence, and (BH)_max were 10.4 kOe, 12.4 kG, and 35.6 MGOe, respectively.

Synthesis of KIT-1 Mesoporous Silicates Showing Two Different Macrosporous Strucrtues; Inverse-opal or Hollow Structures

We report a facile method for preparing KIT-1 mesoporous silicates with two different macroporous structures by dual templating. As a template for macropores, polystyrene (PS) beads are assembled into uniform three dimensional arrays by ice templating, i.e., by growing ice crystals during the freezing process of the particle suspension. Then, the polymeric templates are directly introduced into the precursor-gel solution with cationic surfactants for templating the mesopores, which is followed by hydrothermal crystallization and calcination. Later, by burning out the PS beads and the surfactants, KIT-1 mesoporous silicates with macropores are produced in a powder form. The macroporous structures of the silicates can be controlled by changing the amount of EDTANa4 salt under the same templating conditions using the PS beads and inverse-opal or hollow structures can be obtained. This strategy to prepare mesoporous powders with controllable macrostructures is potentially useful for various applications especially those dealing with bulky molecules such as, catalysis, separation, drug carriers and environmental adsorbents.

Synthesis of Silica-Core Gold-Satellite Nanoparticles and Their Surface-enhanced Raman Scattering Based Sensing Application

【In this study, we synthesize silica-core gold-satellite nanoparticles (SGNPs) for the surface-enhanced Raman scattering (SERS) based sensing applications. They consist of gold satellite nanoparticles (AuNPs) fixed on the silica core nanoparticles, which sizes of AuNPs can be tunned by varying the amount of reactants (growth solution and reducing agent). Their surface plasmon resonance (SPR) properties were characterized by using UV-vis spectroscopy, showing that the growth of AuNPs on silica cores leads to the light absorption in the longer wavelength region. Furthermore, the size increase of AuNPs exhibited the dramatic change in SERS activity due to the formation of hot spots. The optimized SGNPs showing enhancement factor

Synthesis of Highly Dispersible Metal Nanoparticles in P3HT:PCBM Layers and Their Effects on the Performance of Polymer Solar Cells

{\sim}3.8{\times}10^6

The Effect of Mn Addition on Nitrogenation Behavior and Magnetic Properties of Sm-Fe Alloy Powder Produced by Reduction-diffusion Method

exhibited a detection limit of rhodamine 6G (R6G) as low as