• Journal of the Korean Magnetics Society
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• v.5 no.5
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• pp.589-596
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• 1995

In the present paper some of the magnetic properties of the nanocrystalline Fe-based magnets produced by an appropriate annealing of their metallic glass precursors are reviewed. These properties are discussed on the grounds of their characteristics measured at the elevated temperatures. It is shown that the effective magnetostriction these magnets display, results from the competition among two contributions of the opposite sign originating from the individual magnetic phases, crystalline phase and the residual glassy matrix in which the nanocrystallites are embedded. It is also shown that at certain conditions the magnets considered expose superparamagnetic behavior and that their isothermal magnetization characteristics can successfully be used to calculate the distribution of the particle volumes. Application of the recently invented new genetic algorithm method, a powerful tool to calculate these distributions is, finally, presented.

• Bulletin of the Korean Chemical Society
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• v.31 no.5
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• pp.1123-1127
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• 2010

We report a simple and quick route for the preparation of high-quality, nearly monodisperse $Bi_2Te_3$, $Sb_2Te_3$, and $Bi_xSb_{2-x}-Te_3$ nanocrystallites. The reactions of bismuth acetate or antimony acetate with Te in oleic acid result in pure phase of $Bi_2Te_3$ or $Sb_2Te_3$ nanoparticles, respectively. Also, ternary $Bi_xSb_{2-x}Te_3$ nanoparticles were successfully synthesized using the same method. The size and morphology of the nanoparticles were controlled by varying the stabilizing agents. The as-prepared nanoparticles are characterized by X-ray diffraction, scanning electron microscope, and high-resolution transmission electron microscope using an energy dispersive spectroscopy.

• Transactions on Electrical and Electronic Materials
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• v.15 no.2
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• pp.73-76
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• 2014

$Pt/Al_2O_3/La_2Si_5O_x/SiO_2/Si$ charge trap memory capacitors were prepared, in which the $La_2Si_5O_x$ film was used as the charge trapping layer, and the effects of post annealing atmospheres ($NH_3$ and $N_2$) on their memory characteristics were investigated. $La_2O_3$ nanocrystallites, as the storage nodes, precipitated from the amorphous $La_2Si_5O_x$ film during rapid thermal annealing. The $NH_3$ annealed memory capacitor showed higher charge storage performances than either the capacitor without annealing or the capacitor annealed in $N_2$. The memory characteristics were enhanced because more nitrogen was incorporated at the $La_2Si_5O_x/SiO_2$ interface and interfacial reaction was suppressed after the $NH_3$ annealing treatment.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.133-133
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• 2013

For both oxygen reduction (ORR) and hydrogen oxidation reactions (HOR) of proton electrolyte membrane fuel cells (PEMFCs), alloying Pt with another transition metal usually results in a higher activity relative to pure Pt, mainly due to electronic modification of Pt and bifunctional behaviour of alloy surface for ORR and HOR, respectively. However, activity and stability are closely related to the preparation of alloy nanoparticles. Preparation conditions of alloy nanoparticles have strong influence on surface composition, oxidation state, nanoparticle size, shape, and contamination, which result from a large difference in redox priority of metal precursors, intrinsic properties of metals, increasedreactivity of nanocrystallites, and interactions with constituents for the synthesis such as solvent, stabilizer, and reducing agent, etc. Carbon-supported Pt-Ni alloy nanoparticles were prepared by the borohydride reduction method in anhydrous solvent. Pt-Ru alloy nanoparticles supported on carbon black were also prepared by the similar synthetic method to that of Pt-Ni. Since electrocatalytic reactions are strongly dependent on the surface structure of metal catalysts, the atom-leveled design of the surface structure plays a significant role in a high catalytic activity and the utilization of electrocatalysts. Therefore, surface-modified electrocatalysts have attracted much attention due to their unique structure and new electronic and electrocatalytic properties. The carbon-supported Au and Pd nanoparticles were adapted as the substrate and the successive reduction process was used for depositing Pt and PtM (M=Ru, Pd, and Rh) bimetallic elements on the surface of Au and Pd nanoparticles. Distinct features of the overlayers for electrocatalytic activities including methanol oxidation, formic acid oxidation, and oxygen reduction were investigated.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2003.10a
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• pp.54-54
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• 2003

We have developed a Cu-based bulk amorphous composite reinforced with a micron-sized crystalline phase, the (Cu60Zr30Ti10)95Ta5 amorphous matrix composite. The composite demonstrates the ultimate strength of 2332 MPa with a dramatically enhanced fracture strain of 15.3 %. Macroscopic observation of the fractured (Cu60Zr30Ti10)95Ta5 amorphous matrix composite showed the development of multiple shear bands along with numerous branching and deflection of shear bands. Microscopic observation on the amorphous matrix of the composite showed that cracks propagate through the residual amorphous matrix located between nanocrystallites, which had formed during deformation. Simulations based on finite element method were conducted to understand the formation mechanisms of multiple shear bands, the initiation site of shear bands, and interaction of shear bands with crystalline particles. Other microscopic fracture mechanism responsible for the enhanced plasticity was discussed.

• Journal of the Korean Chemical Society
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• v.43 no.1
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• pp.1-7
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• 1999

Metal colloidal particles have been prepared by a photo-chemical process in an aqueous solution containing semiconductor nanocrystallites. Metal colloidal particles produced in CdS and AgBr exhibit different absorption spectra. Au particles produced in solution with CdS show typical Au plasmon resonance absorption spectra. On the other hand Ag particles in solution with AgBr shows surface plasmon resonance absorption spectra which are red-shifted, as compared to that of a dispersion of homogeneous Ag colloidal particles in the same host. The extent of red-shift depends on the UV illumination time. This phenomenon is interpreted within the context of effective medium theory for small volume fractions. From the theory, a metal coated particle predicts Ag plasmon resonance, red shifted with respect to 400 nm that would be associated with a silver particle in solution. The absorption peak position is very sensitive to the coating thickness.

• Korean Journal of Materials Research
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• v.28 no.2
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• pp.124-128
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• 2018

N-type porous silicon (PS) layers and thermally oxidized PS layers have been characterized by various measuring techniques such as photoluminescence (PL), Raman spectroscopy, IR, HRSEM and transmittance measurements. The top surface of PS layer shows a stronger photoluminescence peak than its bottom part, and this is ascribed to the difference in number of fine silicon particles of 2~3 nm in diameter. Observed characteristics of PL spectra are explained in terms of microstructures in the n-type PS layers. Common features for both p-type and n-type PS layers are as follows: the parts which can emit visible photoluminescence are not amorphous, but crystalline, and such parts are composed of nanocrystallites of several nm's whose orientations are slightly different from Si substrate, and such fine silicon particles absorb much hydrogen atoms near the surfaces. Light emission is strongly dependent on such fine silicon particles. Photoluminescence is due to charge carrier confinement in such three dimensional structure (sponge-like structure). Characteristics of visible light emission from n-type PS can be explained in terms of modification of band structure accompanied by bandgap widening and localized levels in bandstructure. It is also shown that hydrogen and oxygen atoms existing on residual silicon parts play an important role on emission stability.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.37-37
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• 2008

Hydrogenated nanocrystalline Si (nc-Si:H) thin films were prepared by plasma enhanced chemical vapor deposition (PECVD). The films were deposited with a radio frequency power of 100 W, while substrates were exposed to direct current (DC) biases in the range from 0 to -400 V. The effects of the DC bias on the formation of nanoscale Si crystallites in the films and on their optical characteristics were investigated. The size of the Si crystallites in the films ranges from ~ 1.9 to ~ 4.1 nm. The relative fraction of the crystallites in the films reached up ~ 56.5 % when the DC bias of -400 V was applied. Based on the variation in the structural, chemical, and optical features of the films with DC bias voltages, a model for the formation of nanostructures of the nc-Si:H films prepared by PECVD was suggested. This model can be utilized to understand the evolution in the size and relative fraction of the nanocrystallites as well as the amorphous matrix in the nc-Si:H films.

• Journal of the Korean Chemical Society
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• v.63 no.2
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• pp.94-101
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• 2019

Nb, Mo-doped and Nb/Mo-codoped $VO_2(M)$ nanocrystallites with various doping levels were synthesized for the first time by a hydrothermal and post thermal transformation method. The reversible phase transition characteristics of those doped $VO_2(M)$ was comparatively investigated. Nb-doping of $VO_2(M)$ by this method resulted in a very efficient lowering of the transition temperature ($T_c$) with a rate of $-16.7^{\circ}C/at.%$ that is comparable to W-doping, while Mo-doping did not give a serious reduction of $T_c$ with only a rate of $-5.1^{\circ}C/at.%$. Nb/Mo-codoping gave a similar result to Nb-doping without a noticeable difference. The thin films of Nb-doped and Nb/Mo-codoped $VO_2(M)$ with a thickness of ca. 120 nm were prepared by a wet-coating of the nanoparticle-dispersed solutions. Those films showed a good thermochromic modulation of near infrared radiation with 30-35% for Nb-doped $VO_2(M)$ and 37-40% for Nb/Mo-codoped ones. Nb/Mo-codoped $VO_2(M)$ film showed slightly enhanced thermochromic performance compared with Nb-doped $VO_2(M)$ film.

• Journal of the Korean Magnetics Society
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• v.11 no.6
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• pp.262-266
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• 2001

The magnetic switching volume is known as an important parameter to understand the magnetization reversal process, thermal stability of the written information and media noise. This parameter is influenced significantly by the microstructure of the magnetic layer as well as underlayer. Therefore, we fabricated CoSm/Cr thin films with varying magnetic layer thickness under constant sputtering by using a dc magnetic sputtering machine. The magnetic layer thickness effect on the magnetic switching volume have been studied by the means of magnetic viscosity and dc demagnetization remanence curve mesurements. From these measurements, we found that the switching volumes increased with increasing the magnetic layer thickness, whereas the coercivity showed different behavior. These may be a result of the increased intergranular coupling and the larger volume fraction of the magnetic layer.