• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2005.05a
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• pp.51-53
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• 2005

$Zn_{1-x}Cd_xO$ thin films were grown on (0001) sapphire substrates by pulsed laser deposition. The energy bandgap of $Zn_{1-x}Cd_xO$ films decreases withincreasing Cd content. An increase of Cd content also leads to the emission broadening and degraded crystallinity. The absorption edge and ultraviolet emission peak shift to lower energy from 3.357 eV to 3.295 eV and 3.338 eV to 3.157 eV, respectively, with increasing Cd content from 0.3% to 3%. The Stokes' shift between the absorption and emission indicates the increase of localization of exciton with Cd content.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.12 no.3
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• pp.138-143
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• 2002

In this study, ZnS: Cu nano-crystals are synthesized by solution synthesis technique (SST). The structural properties such as crystal structure and particle morphology, and the optical properties such as light absorption/transmittance, energy bandgap, and photoluminescence (PL) excitation/emission are investigated. In an attempt to realize the Cu-doping easiness, the synthesis temperature (~$80^{\circ}C$) is applied to the synthesis bath, and the thiourea is used as sulfur precursor, unlike other general chemical synthesis route. Both undoped ZnS and ZnS : Cu nano-crystals have the cubic crystal structure and have the spherical particle shape. The position of light absorption edge is ~305 nm, indicating the occurrence of quantum size effect. The PL emission intensity and line-width are maximum and minimum, respectively, for Cu-doping concentration 0.03M. In particular, the dependence of PL intensity and line-width on the Cu-doping concentration for ZnS : Cu nano-crystals synthesized by SST is reported for the first time in this study. Experimental results of the absorption edge and the PL excitation show that the main emission peak of ZnS : Cu nano-crystals (~510 nm) in this study is due to the radiative recombination center in the energy bandgap induced by Cu dopant.

• Progress in Superconductivity
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• v.14 no.2
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• pp.99-101
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• 2012

Fluorescent yield X-ray absorption fine structure (XAFS) spectroscopy is useful for analyzing local structure of specific elements in matrices. We developed an XAFS apparatus with a 100-pixel superconducting tunnel junction (STJ) detector array with a high sensitivity and a high resolution for light-element dopants in wide-gap semiconductors. An STJ detector has a pixel size of $100{\mu}m$ square, and an asymmetric layer structure of Nb(300 nm)-Al(70 nm)/AlOx/Al(70 nm)-Nb(50 nm). The 100-pixel STJ array has an effective area of $1mm^2$. The XAFS apparatus with the STJ array detector was installed in BL-11A of High Energy Accelerator Research Organization, Photon Factory (KEK PF). Fluorescent X-ray spectrum for boron nitride showed that the average energy resolution of the 100-pixels is 12 eV in full width half maximum for the N-K line, and The C-K and N-K lines are separated without peak tail overlap. We analyzed the N dopant atoms implanted into 4H-SiC substrates at a dose of 300 ppm in a 200 nm-thick surface layer. From a comparison between measured X-ray Absorption Near Edge Structure (XANES) spectra and ab initio FEFF calculations, it has been revealed that the N atoms substitute for the C site of the SiC lattice.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.315-315
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• 2013

We report the electronic structure modification in the swift heavy ion (SHI) irradiated N-doped ZnO thin films prepared by RF sputtering from ZnO target in different ratio of Ar/$N_2$ gas mixture using highly pure $N_2$ gas. The different N-ZnO thin lms were then irradiated with 120 MeV Ag ion beam with different doses ranging from $1{\times}10^{11}$ to $5{\times}10^{12}$ ions/$cm^2$ and characterized by XRD and near edge X-ray absorption ne structure (NEXAFS) at N and O K-edges. The NEXAFS measurements provide direct evidence of O 2p and Zn 3d orbital hybridization and also the bonding of N ions with Zn and O ions. The minimum value of resistivity of $790{\Omega}cm$, a Hall mobility of $22cm^2V^-1s^-1$ and the carrier concentration of $3.6{\times}10^{14}cm^{-3}$ were yielded at 75% $N_2$. X-ray diffraction (XRD) measurements revealed that N-doped ZnO films had the preferential orientation of (002) plane for all samples, while crystallinity start decreasing at 32.5% $N_2$. The average crystallite size varies from 5.7 to 8.2 nm for 75% and then decreases to 7.8 nm for 80% $Ar:N_2$ ratio.

• Bulletin of the Korean Chemical Society
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• v.33 no.11
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• pp.3761-3766
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• 2012

$Ag_2Se$-Graphene/$TiO_2$ composite was synthesized by a facile sonochemical method. The as-prepared products were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis, transmission electron microscopy (TEM) and UV-vis diffuse reflectance spectrophotometer. During the reaction, both of the reduction of graphene oxide and loading of $Ag_2Se$ and $TiO_2$ particles were achieved. The as-prepared $Ag_2Se$-Graphene/$TiO_2$ composites possessed great adsorptivity of dyes, extended light absorption range, and efficient charge separation properties simultaneously. Hence, in the photodegradation of rhodamine B (Rh.B), a significant enhancement in the reaction rate was observed with $Ag_2Se$-Graphene/$TiO_2$ composites, compared to the pure $TiO_2$. The high activity can be attributed to the synergetic effects of high charge mobility, and red shift in absorption edge of $Ag_2Se$-Graphene/$TiO_2$ composites.

• Journal of KSNVE
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• v.8 no.1
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• pp.146-156
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• 1998

To mitigate excessive noise from highways, and high speed rail road, it is often necessary to construct a noise barrier. Absorptive barroer attenuation solution is obtained for the problem of diffration of a plane wave sound source by a semi-infinite plane. A finite region in the vicinity of the edge has an highly absorbing boundary condition ; the remaining portion of the half plane is rigid. The problem which is solved is a mathematical model for a hard barrier with an absorbing edge. If the wavelength of the sound is much smaller than the length scale associated with the barrier, the diffraction process is governed to all intents and purpose by the solution to a standard problem of diffraction by a semi-infinite hard plane with an absorbent edge. It is concluded that the absorbing material that comprises the edge need only be of the order of a wavelength long to have approximately the same effect, on the sound attenuation in the shadow side of the barrier. Traffic noise is composed of thousands of sources with varying frequency content. To simplify noise predictions when barriers are present, an effective frequency of 550Hz may be used to represent all vehicles. The wavelength of sound at f=550Hz for traffic noise is about 2 feet. According to the above conclusion, an absorptive highway noise barrier is only needed to cover to cover approximately a 2 foot length of absorbing material. It would be more economical to cover only the region in the immediate vicinity of the edge with highly sound obsorbent material.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.9 no.2
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• pp.180-184
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• 1999

The single crystals of $Mg_{0.15}Zn_{0.85}Te$ and $Mg_{0.15}Zn_{0.85}Te:Co^{2+}$(0.001%) were grown by vertical Bridgman method. Optical absorption properties of this compound were studied. As a result of the optical absorption spectra of $Mg_{0.15}Zn_{0.85}Te$, absorption peaks were related to exciton and the exciton level redshifts with increasing temperature, and temperature coefficient given to the value of $-5.8{\times}10^{-4}\;eV/K$ for the temperature range above 100 K. in the $Mg_{0.15}Zn_{0.85}Te:Co^{2+}$(0.001%) single crystal, the intracenter transitions due to $Co^{2+}$ ions were detected for $A-band:^4A_2(^4F) {\to}^4T_1(^4F),\; B-band:\; ^4A_2(^4F){\to}^4T_1(^4P)$, and the charge transfer transition near the absorption edge was observed in the wavelength range of 500 to 800 nm. According to the crystal field theory and Lucovsky formula, the crystal field parameter, Racah parameter and charge transfer energy were determined.

• Journal of the Korean Ceramic Society
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• v.49 no.5
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• pp.469-474
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• 2012

The crystallinity and phase ratio of anatase to rutile in $TiO_2$ were estimated by x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS). Traditionally, the structural characterization of $TiO_2$ powders has been carried out by XRD techniques, which are comparatively easy in use and analysis. However, materials with amorphous phase, nano-sized or nano-structured crystallinities cannot be fully characterized by XRD because XRD analysis has a limit for abnormal contributions of the nano-crystal such as the surface contribution. From the comparison with the experimental and calculated Ti K-edge XAS spectra, we found the possibility of efficient estimation in the crystalinites and the phase ratio of anatase to rutile for nano-sized $TiO_2$ mixture.

• Bulletin of the Korean Chemical Society
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• v.35 no.9
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• pp.2660-2664
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• 2014

A simple method of depositing titanium dioxide ($TiO_2$) nanoparticles onto graphene oxide (GO) as a catalytic support was devised for photocatalytic degradation of methylene blue (MB). Thiol groups were utilized as linkers to secure the $TiO_2$ nanoparticles. The resultant GO-supported $TiO_2$ (GO-$TiO_2$) sample was characterized by transmission electron microscopy (TEM), near-edge X-ray absorption fine structure (NEXAFS), and X-ray photoelectron spectroscopy (XPS) measurements, revealing that the anatase $TiO_2$ nanoparticles had effectively anchored to the GO surface. In the photodegradation of MB, GO-$TiO_2$ exhibited remarkably enhanced photocatalytic efficiency compared with thiolated GO and pure $TiO_2$ nanoparticles. Moreover, after five-cycle photodegradation experiment, no obvious deactivation was observed. The overall results showed that thiolated GO provides a good support substrate and, thereby, enhances the photodegradation effectiveness of the composite photocatalyst.

• Journal of the Korean Institute of Telematics and Electronics A
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• v.32A no.11
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• pp.29-34
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• 1995

Mirror coating is applied to laser facets to improve properties of edge emitting laser diodes. In this experiment, InGaAsP/GaAs high power laser diodes were studied with respect to different degrees of anti-reflective coating. Sputterred $Al_{2}$O$_{3}$ was used as the coating material and the HR coating was kept constant at 90%. Threshold current density, differential quantum efficiency, emission wavelength and the operating current at 500mW were measured for a range of AR coating and compared with theoretically calculated values; that showed good agreements. Precise wavelength control is important for laser diodes for solid state pumping because of small absorption bandwidth. In addition, since these lasers operate under CW condition, a lowest possible operating current for a given power is desired in order to minimize the heat produced. From the results of this experiment, we were able to obtain a optimum range of AR coatings for minimum operating current. The wavelength can be varied up to 4nm within this range.