• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.287.1-287.1
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• 2013

The Isolation of few-layered transition metal dichalcogenides has mainly been performed by mechanical and chemical exfoliation with very low yields. in particular, the two-dimensional layer of molybdenum disulfide (MoS2) has recently attracted much interest due to its direct-gap property and potential application in optoelectronics and energy harvesting. However, the synthetic approach to obtain high-quality and large-area MoS2 atomic thin layers is still rare. In this account, a controlled thermal reductionsulfurization method is used to synthesize large-MoOx thin films are first deposited on Si/SiO2 substrates, which are then sulfurized (under vacuum) at high temperatures. Samples with different thicknesses have been analyzed by Raman spectroscopy and TEM, and their photoluminescence properties have been evaluated. We demonstrated the presence of single-, bi-, and few-layered MoS2 on as-grown samples. It is well known that the electronic structure of these materials is very sensitive to the number of layer, ranging from indirect band gap semiconductor in the bulk phase to direct band gap semiconductor in monolayers. This synthetic approach is simple, scalable, and applicable to other transition metal dichalcogenides. Meanwhile, the obtained MoS2 films are transferable to arbitrary substrates, providing great opportunities to make layered composites by stacking various atomically thin layers.

• Journal of the Korean Vacuum Society
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• v.7 no.s1
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• pp.57-77
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• 1998

Heteroepitaxial $Y_2O_3$ films were grown on a Si(111) substrate by ionized cluster beam deposition(ICBD) in ultra high vacuum, and its qualities such as crystllitnity, film stress, and morphological characteristics were investigated using the various measurement methods. The crystallinity was investigated by x-ray diffraction (XRD) and reflection high energy electron diffraction (RHEED). Interface crystallinity was also examined by Rutherford backscattering spectroscopy(RBS) channeling, transmission electron microscopy(TEM). The stress of the films was measured by RBS channeling and XRD. Surface and interface morphological characteristics were investigated by atomic force microscopy (AFM) and x-ray scattering method. Comparing the interface with the surface characteristics, we can conclude that many defects at the interface region were generated by interface reaction between the yttrium metal and SiO2 layer and by ion beam characteristic such as shallow implantation, so that they influenced the film qualities. The film quality was dominantly depended on the characteristic temperature range. In the temperature range from $500^{\circ}C$ to $600^{\circ}C$, the crystallinity was mainly improved and the surface roughness was drastically decreased. On the other hand, in the temperature range from $600^{\circ}C$ to $700^{\circ}C$, the compressive stress and film density were dominantly increased, and the island size was more decreased. Also the surface morphological shape was transformed from elliptical shape to triangular. The film stress existed dominantly at the interface region due to the defects generation.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.26 no.6
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• pp.446-450
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• 2013

Growth mechanism of GS-MBE(Gas source-Molecular Beam Epitaxy) has been investigated. We observed that the growth rate of GaN films is changing from 520 nm/h to 440 nm/h by the variation of V/III ratio under nitrogen-rich growth condition. It was explained that the amount of hydrogen on the growth front varies by the ammonia flow, and gallium hydrides are generated on the surface by a reaction of hydrogen and gallium, resultantly the amount of gallium supplying is changing along with the $NH_3$ flow. Reflection high energy electron diffraction (RHEED) observation was used to confirm the N-rich condition. The crystal quality of GaN was estimated by photoluminescence (PL) and X-ray diffraction (XRD).

• Korean Journal of Materials Research
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• v.12 no.3
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• pp.190-194
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• 2002

InN/GaN multi-layers were grown by metalorganic chemical vapor deposition(MOCVD) in order to get the appropriate structure for an high power blue-green light emitting diode(LED), and effects of growth conditions (growth temperature, pressure, and $trimethylindium(TMIn)-NH_3-N_2\; flow\; rare)$ on the integrated photoluminescence (PL) intensity and PL peak energy in InN/GaN multi-layers were investigated. The optimized growth conditions with the highest integrated PL intensity for InN/GaN multi-layers were obtained: the growth temperature at $780^{\circ}C$, the growth pressure at 325 Torr, the TMIn flow rate with 150 $m\ell$/min, the $NH_3$flow rate with 3.2 ι/min, and $N_2$ flow rate with 2 ι/min.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.12-12
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• 2010

By using scanning tunneling microscopy/spectroscopy (STM/S), we can make images of various physical properties in nanometer-scale spatial resolutions. Here, I demonstrate imaging of two electron-correlated subjects; screening and superconductivity by STM/S. The electrostatic potential around a charge is described with the Coulomb potential. When the charge is located in a metal, the potential is modified because of the free electrons in the host. The potential modification, called screening, is one of the fundamental phenomena in the condensed matter physics. Using low-temperature STM we have developed a method to measure electrostatic potential in high spatial and energy resolutions, and observed the potential around external charges screened by two-dimensional surface electronic states. Characteristic potential decay and the Friedel oscillation were clearly observed around the charges [1]. Superconductivity of nano-size materials, whose dimensions are comparable with the coherence length, is quite different from their bulk. We investigated superconductivity of ultra-thin Pb islands by directly measuring the superconducting gaps using STM. The obtained tunneling spectra exhibit a variation of zero bias conductance (ZBC) with a magnetic field, and spatial mappings of ZBC revealed the vortex formation [2]. Size dependence of the vortex formation will be discussed at the presentation.

• Nuclear Engineering and Technology
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• v.56 no.2
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• pp.477-484
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• 2024

Experimental investigations on gamma - rays attenuation parameters and dielectric spectroscopic properties were done on a polymeric mixture with chemical composition (100-x) polyethylene + x basalt, where x = 0, 1, 3, 5, 10, and 20 wt%. Using the melting blending technique,six nanocomposite polymeric samples were prepared. The linear attenuation coefficient µ of each prepared set of samples was measured using a gamma-ray spectrometer including High Purity Germanium detector (HPGe) at energies 662.5, 1173.24, and 1332.51 keV. Based on the measured values of (µ) and sample density, the other effective shielding parameters were calculated. The values of µ showed an increase with increasing the dopant ratios from 0.0 up to 20.0 wt%. In addition, the µ values decreased with the photon's energy. The µ values were found 0.0847 up to 0.1175 cm^-1, 0.0571 up to 0.0855 cm^-1, and 0.0543 up to 0.075 cm^-1 at 662.5, 1173.24, and 1332.51 keV. for B0 up to B20, respectively. The ATR spectroscopy was done on the prepared samples, and a good evidence of adding the filler to the pure polyethylene (HDPE) was obtained. Besides, an enhancement in dielectric constant by insertion of basalt NPs also recorded and can be attributed to the large dielectric constant of basalt compared to pure HDPE.

• Journal of The Korean Astronomical Society
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• v.37 no.5
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• pp.605-609
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• 2004

We present a set of high-resolution 3D MHD simulations exploring the evolution of light, supersonic jets in cluster environments. We model sets of high- and low-Mach jets entering both uniform surroundings and King-type atmospheres and propagating distances more than 100 times the initial jet radius. Through complimentary analyses of synthetic observations and energy flow, we explore the detailed interactions between these jets and their environments. We find that jet cocoon morphology is strongly influenced by the structure of the ambient medium. Jets moving into uniform atmospheres have more pronounced backflow than their non-uniform counterparts, and this difference is clearly reflected by morphological differences in the synthetic observations. Additionally, synthetic observations illustrate differences in the appearances of terminal hotspots and the x-ray and radio correlations between the high- and low-Mach runs. Exploration of energy flow in these systems illustrates the general conversion of kinetic to thermal and magnetic energy in all of our simulations. Specifically, we examine conversion of energy type and the spatial transport of energy to the ambient medium. Determination of the evolution of the energy distribution in these objects will enhance our understanding of the role of AGN feedback in cluster environments.

• Journal of Welding and Joining
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• v.35 no.3
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• pp.62-67
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• 2017

The automotive industry is in constant pursuit of alternative materials and processes to address the ever changing needs of their customers and the environment. Applications of laser welding have increased steadily in recent years due to its benefits including high speed, high productivity, and high energy density of heat source. This paper investigated the relationship between laser welding parameters and penetration characteristics. The Welding power was fixed at 5kW and welding speeds were varied for the thickness of the workpiece material. Full penetration occurred in the energy per unit area of $21J/mm^2$. The hardness of the weld zone was different for each strength of the workpiece material, however the lowest hardness values were the same in the heat-affected zone.

• Journal of the Optical Society of Korea
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• v.7 no.3
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• pp.135-138
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• 2003

A chirped-pulse amplification femtosecond Ti:sapphire laser operating at 1 KHz has been developed. The laser system consisted of a long-cavity femtosecond oscillator, a four-pass grating pulse stretcher, two multi-pass amplifiers and a double-pass grating pulse compressor. Thermal lensing at the amplifiers was reduced by cooling Ti:sapphire crystals using Peltier coolers. Gain narrowing and residual phase errors were compensated for by the use of an acousto-optic pulse shaper. The final laser output had an energy per pulse of 2.0 mJ and a pulse duration of 19.5 fs, reaching 0.1 TW at 1 KHz.

• Proceedings of the Korean Magnestics Society Conference
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• 2007.05a
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• pp.306.1-306.1
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• 2007