• 한국진공학회:학술대회논문집
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• 한국진공학회 2012년도 제43회 하계 정기 학술대회 초록집
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• pp.391-392
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• 2012

Graphene is a perfectly two-dimensional (2D) atomic crystal which consists of sp2 bonded carbon atoms like a honeycomb lattice. With its unique structure, graphene provides outstanding electrical, mechanical, and optical properties, thus enabling wide variety of applications including a strong potential to extend the technology beyond the conventional Si based electronic materials. Currently, the widespread application for electrostatically switchable devices is limited by its characteristic of zero-energy gap and complex process in its synthesis. Several groups have investigated nanoribbon, strained, or nanomeshed graphenes to induce a band gap. Among various techniques to synthesize graphene, chemical vapor deposition (CVD) is suited to make relatively large scale growth of graphene layers. Direct growth of graphene on hexagonal boron nitride (h-BN) using CVD has gained much attention as the atomically smooth surface, relatively small lattice mismatch (~1.7％) of h-BN provides good quality graphene with high mobility. In addition, induced band gap of graphene on h-BN has been demonstrated to a meaningful value about ~0.5 eV.[1] In this paper, we report the synthesis of grpahene / h-BN bilayer in a chemical vapor deposition (CVD) process by controlling the gas flux ratio and deposition rate with temperature. The h-BN (99.99％) substrate, pure Ar as carrier gas, and $CH_4$ are used to grow graphene. The number of graphene layer grown on the h-BN tends to be proportional to growth time and $CH_4$ gas flow rate. Epitaxially grown graphene on h-BN are characterized by scanning electron microscopy, atomic force microscopy, and Raman spectroscopy.

• Current Photovoltaic Research
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• 제4권1호
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• pp.25-29
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• 2016

$Eu^{3+}$-activated $MgMoO_4$ phosphor thin films were grown at $400^{\circ}C$ on quartz substrates by radio-frequency magnetron sputter deposition from a 15 mol% Eu-doped $MgMoO_4$ target. After the deposition, the phosphor thin films were annealed at several temperatures for 30 min in air. The influence of thermal annealing temperature on the structural and optical properties of $MgMoO_4:Eu^{3+}$ phosphor thin films was investigated by using X-ray diffraction (XRD), photoluminescence (PL), and ultraviolet-visible spectrophotometry. The transmittance, optical band gap, and intensities of the luminescence and excitation spectra of the thin films were found to depend on the thermal annealing temperature. The XRD patterns indicated that all the thin films had a monoclinic structure with a main (220) diffraction peak. The highest average transmittance of 91.3% in the wavelength range of 320~1100 nm was obtained for the phosphor thin film annealed at $800^{\circ}C$. At this annealing temperature the optical band gap energy was estimated as 4.83 eV. The emission and excitation spectra exhibited that the $MgMoO_4:Eu^{3+}$ phosphor thin films could be effectively excited by near ultraviolet (281 nm) light, and emitted the dominant 614 nm red light. The results show that increasing RTA temperature can enhance $Eu^{3+}$ emission and excitation intensity.

• JSTS:Journal of Semiconductor Technology and Science
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• 제17권1호
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• pp.141-146
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• 2017

In this letter, we propose the use of tunneling field effect transistors (TFET) as a biosensor that detects bio-molecules on the gate oxide. In TFET sensors, the charges of target molecules accumulated at the surface of the gate oxide bend the energy band of p-i-n structure and thus tunneling current varies with the band bending. Sensing parameters of TFET sensors such as threshold voltage ($V_t$) shift and on-current ($I_D$) change are extracted as a function of the charge variation. As a result, it is found that the performances of TFET sensors can surpass those of conventional FET (cFET) based sensors in terms of sensitivity. Furthermore, it is verified that the simultaneous sensing of two different target molecules in a TFET sensor can be performed by using the ambipolar behavior of TFET sensors. Consequently, it is revealed that two different molecules can be sensed simultaneously in a read-out circuit since the multi-sensing is carried out at equivalent current level by the ambipolar behavior.

• 천문학회보
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• 제39권2호
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• pp.98-98
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• 2014

Due to the simple vertical structure of magnetic field, pores can be exploited to study the transport of mechanical energy by waves along the magnetic field to the chromosphere and corona. For a better understanding of physics of pores, we have investigated chromospheric traveling features running across two merged pores from their centers at the speed about 55 km s-1, in the active region AR 11828. The pores were observed on 2013 August 24 by using high time, spatial, and spectral resolution data from the Fast Imaging Solar Spectrograph (FISS) of the 1.6 meter New Solar Telescope (NST). We infer a LOS velocity by applying the bisector method to the Ca II $8542{\AA}$ band and $H{\alpha}$ band, and investigate intensity and the line-of-sight velocity changes at different wavelengths and different positions at the pores. We find that they have 3 minutes oscillations, and the intensity oscillation from the line center is preceded by that from the core ($-0.3{\AA}$) of the bands. There is no phase difference between the intensity and the LOS velocity oscillations at a given wavelength. The amplitude of LOS velocity from near the core spectra is greater than that from the far core spectra. These results support the interpretation of the observed wave as a slow magnetoacoustic wave propagating along the magnetic field lines in the pores. The apparent horizontal motion and a sudden decrease of its speed beyond the pores can be explained by the projection effect caused by inclination of the magnetic field with a canopy.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2014년도 제46회 동계 정기학술대회 초록집
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• pp.356.1-356.1
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• 2014

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 reduction-sulfurization 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 mono-, 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.

• 대기
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• 제18권4호
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• pp.441-458
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• 2008

A TP/WVP-3000A, ground-based microwave radiometer, that was first introduced to South Korea has been operated since August 22, 2007 at the National Center for Intensive Observation of Severe Weathers (NCIO). Using the dual-band, the radiometer provides temperature and humidity soundings from the surface up to 10 km height with the high-temporal resolution of a few minutes. In this study, the performance of the radiometer on the predictability of the high impact weathers was evaluated and various practical applications were investigated. To verify the retrieved profile data from the radiometer, temperature and relative humidity soundings are compared with those from the rawinsonde launched at the NCIO and Gwangju station. The root mean squared errors for temperature and relative humidity soundings were smaller under rainy weather conditions. The correlation coefficient between PWVs (Precipitable Water Vapors) obtained from the radiometer and Global Positioning System satellite at Mokpo station is 0.92 on average. In order to investigate the structure and characteristics of precipitation, stability indexes related to rainfall such as the Convective Available Potential Energy (CAPE), K-index, and Storm RElative Helicity (SREH) were calculated using windprofiler at the NCIO from 14 to 16 September, 2007. CAPE and K-index tended to be large when the thermodynamic unstability was strong. On the other hand, SREH index was dominantly large when the dynamic unstability was strong due to the passage of the typhoon 'Nari'.

• 한국재료학회지
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• 제22권5호
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• pp.215-219
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• 2012

Red phosphors $Ca_{1-1.5x}WO_4:{Eu_x}^{3+}$ were synthesized with different concentrations of $Eu^{3+}$ ions by using a solid-state reaction method. The crystal structure of the red phosphors was found to be a tetragonal system. X-ray diffraction (XRD) results showed the (112) main diffraction peak centered at $2{\theta}=28.71^{\circ}$, and the size of crystalline particles exhibited an overall decreasing tendency according to the concentration of $Eu^{3+}$ ions. The excitation spectra of all the phosphors were composed of a broad band centered at 275 nm in the range of 230-310 nm due to $O^{2-}{\rightarrow}W^{6+}$ and a narrow band having a peak at 307 nm caused by $O^{2-}{\rightarrow}Eu^{3+}$. Also, the excitation spectrum presents several strong lines in the range of 305-420 nm, which are assigned to the 4f-4f transitions of the $Eu^{3+}$ ion. In the case of the emission spectrum, all the phosphor powders, irrespective of $Eu^{3+}$ ion concentration, indicated an orange emission peak at 594 nm and a strong red emission spectrum centered at 615 nm, with two weak lines at 648 and 700 nm. The highest red emission intensity occurred at x = 0.10 mol of Eu3+ ion concentration with an asymmetry ratio of 12.5. Especially, the presence of $Eu^{3+}$ in the $Ca_{1-1.5x}WO_4:{Eu_x}^{3+}$ shows very effective use of excitation energy in the range of 305-420 nm, and finally yields a strong emission of red light.

• 한국지반환경공학회 논문집
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• 제10권1호
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• pp.35-41
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• 2009

구조물과 지반상호간의 동적 특성 및 지진원 등을 보다 신뢰성 있게 평가하기 위해 지반의 증폭특성을 반드시 고려하여야 한다. 지반증폭 특성을 분석하는 방법은 여러 가지가 제시되어 있으나 본 연구에서는 Nakamura(1989)에 의해 제시된 방법을 적용하였다. 이 방법은 얕은 지반의 상시미동의 표면파의 특성을 이해하기 위해 제시되어 한계점이 존재하나 근래에 와서 전단파 에너지 등에 적용범위가 넓어지면서 간단한 지반의 동적인 증폭특성 연구에 많이 이용되고 있다. Nakamura(1989)가 제시한 방법을 이용하여 본진 및 11개 후진을 포함한 후쿠오카 지진으로부터 최근 한반도 남부에 설치된 지진관측계에 관측된 지진자료를 이용하여 각각 지진관측소 지반의 동적인 증폭특성을 분석하였다. 결과에 의하면 지진관측소마다 고유주파수, 고주파수 및 저주파수 대역에서 서로 다른 증폭특성을 보여주었다. 특히 각각 관측소 부지의 고유주파수는 각 관측소에서 관측한 지진자료의 질을 좌우하므로 정확한 분석이 필요하다. 또한 본 연구결과를 이용하여 국내 지반의 분류 연구에 많은 정보를 줄 수 있으며 또한 관측 지반진동으로부터 지반고유의 증폭특성을 제거하면 지진원 변수를 보다 신뢰성 있게 도출할 수 있다.

• Bulletin of the Korean Chemical Society
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• 제30권12호
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• pp.3021-3024
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• 2009

$Zn_2Ti_{1-x}Fe_xO_4\;(0\;{\leq}\;x\;{\leq}\;0.7)$ photocatalysts were synthesized by polymerized complex (PC) method and investigated for its physico-chemical as well as optical properties. $Zn_2Ti_{1-x}Fe_xO_4$ can absorb not only UV light but also visible light region due to doping of Fe in the Ti site of $Zn_2TiO_4$ lattice because of the band transition from Fe 3d to the Fe 3d + Ti3d hybrid orbital. The photocatalytic activity of Fe doped $Zn_2TiO_4$ samples for hydrogen production under UV light irradiation decreased with an increase in Fe concentration in $Zn_2TiO_4$. Consequently, there exists an optimized concentration of iron for improved photocatalytic activity under visible light (${\lambda}{\leq}$420 nm)

• 한국세라믹학회지
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• 제40권11호
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• pp.1113-1119
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• 2003

저온 박막 공정을 위해 비등점이 낮은 용매인 isopropanol을 사용하였고, 용질로 zinc acetate의 몰 농도를 0.3∼1.3 mol/l까지 변화시켜 sol을 합성하였다. Zn 농도 변화에 따른 ZnO 박막의 구조 및 광학, 전기적 특성을 분석하였다. XRD 측정에서 Zn의 농도가 0.7 mol/l 일 때 c-축으로 결정 배향성이 뚜렷하였다. SEM으로 박막의 표면 morphology를 관찰한 결과 0.7 mol/l 에서 균일한 표면층을 갖는 나노구조를 이루고 있었다. UV-vis. 측정을 통한 ZnO 박막의 광투과도는 Zn의 농도가 0.7 mol/l 이하에서 87%였으나, 1.0 mol/l 이상의 농도에서는 급격히 감소하였다. 이때 광 밴드갭 에너지는 3.07∼3.22 eV의 값을 나타내며, 벌크 ZnO의 특성과 유사하였다. 박막의 전기 비저항 값은 150 $\Omega$-cm로 Zn의 농도변화에 따라 큰 변화를 보이지 않았으며, I-V 특성분석에서 전형적인 ohmic contact 특성을 보였다.