• Korean Journal of Materials Research
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• v.26 no.7
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• pp.363-369
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• 2016

$NaCaLa_{1-x}(MoO_4)_3:Ho^{3+}/Yb^{3+}$ ternary molybdates with proper doping concentrations of $Ho^{3+}$ and $Yb^{3+}$ (x = $Ho^{3+}+Yb^{3+}$, $Ho^{3+}$ = 0.05 and $Yb^{3+}$ = 0.35, 0.40, 0.45 and 0.50) were successfully synthesized by microwave sol-gel method. Well-crystallized particles formed after heat-treatment at $900^{\circ}C$ for 16 h showed a fine and homogeneous morphology with particle sizes of $3-5{\mu}m$. Under excitation at 980 nm, the UC intensities of the doped samples exhibited strong yellow emissions based on the combination of strong emission bands at 520-nm and 630-nm emission bands in the green and red spectral regions, respectively. The optimal $Yb^{3+}:Ho{3+}$ ratios were obtained at 9:1 and 10:1, as indicated by the composition-dependent quenching effect of the $Ho^{3+}$ ions. The pump power dependence of the upconversion emission intensity and the Commission Internationale de L'Eclairage chromaticity coordinates of the phosphors were evaluated in detail.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.22 no.5
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• pp.241-246
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• 2012

$Yb^{3+}$, $Er^{3+}$ co-doped $SrMoO_4$ ($SrMoO_4$ : $Yb^{3+}/Er^{3+}$) specimens have been successfully synthesized via the complex citrate-gel method and their structural and optical properties were investigated in detail. Under 980 nm excitation, $SrMoO_4$ : $Yb^{3+}/Er^{3+}$ UC phosphors have been emitted strong green luminescence at 530 and 550 nm with weak red emission around 670 nm corresponding to the intra 4f transitions of $Er^{3+}$ ($^4F_{9/2}$, $^2H_{11/2}$, $^4S_{3/2}$) ${\rightarrow}$ $Er^{3+}$ ($^4I_{15/2}$). The optimal doping concentrations of $Er^{3+}$ and $Yb^{3+}$ ions were verified to 2/16 mol% and a possible upconversion mechanism depending on pump power dependence is studied in detail.

• Polymer(Korea)
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• v.29 no.4
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• pp.363-367
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• 2005

The effects of organic solvent on the charge transport behavior of poly (3,4-ethylenedioxythioph one)/p-toluene-sulfonate(PEDOT-OTs) are investigated. The use of different organic solvents during the oxidative chemical polymerization of 3,4-ethylenedioxythiophene (EDOT) with Iron(III) -tosylate can greatly vary the DC conductivity of PEDOT-OTs along with molecular structure and doping concentration. For example, PEDOT-OTs prepared from methanol shows the conductivity of 19.5 S/cm, which is an increase by a factor of $10^8$ compared to PEDOT-OTa prepared from acetone. From the X-ray diffraction (XRD) experiments, it was found that PEDOT-OTs with ketone is amorphous state, while PEDOT-OTs with alcoholic solvent shows the better defined crystalline structure in which the charge transport along and between the PEDOT chains are promoted. Chemical analysis employing X-ray photoelectron spectroscopy (XPS) revealed that the doping concentration of PEDOT-OTs with alcoholic solvent is much higher than that of PEDOT-OTs with ketones. It is proposed that the interactions between the organic solvent and doping anion can cause the variation in doping concentration and, therefore, result in the PEDOT-OTs of different conductivities and chain structures.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.1
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• pp.63-68
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• 2018

This paper presents an integrated photochemical reaction system (i.e., an artificial leaf) that uses earth-abundant catalysts for artificial photosynthesis with a carbon dioxide ($CO_2$) fixation process. The performance of the system was investigated in terms of the energy capture and conversion capabilities. A wireless configuration was achieved by directly doping cobalt oxide as an oxygen-evolving catalyst for water splitting reaction on the illuminated surface of photovoltaic (PV) cell, as well as molybdenum disulfide ($MoS_2$) as an efficient catalyst for $CO_2$ reduction on the back substrate surfaces of the PV cell. The system produces hydrogen and carbon monoxide (CO) as sustainable fuels (i.e., synthesis gas) at around 4.5% efficiency, which implies more than 75% catalytic efficiency at the cathode. The process of solar-driven $CO_2$ conversion and water-splitting reaction is contained in one system, which is one step closer to the successful realization of artificial photosynthesis.

• Journal of the Korean Ceramic Society
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• v.51 no.6
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• pp.549-553
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• 2014

$CoSb_3$-based skutterudite compounds are candidate materials for thermoelectric power generation in the mid-temperature range (600 - 900 K) because their thermoelectric properties can be enhanced by doping and filling. The joining property of thermoelectric module electrodes containing thermoelectric materials is of great importance because it can dominate the efficiency of the thermoelectric module. This study examined the properties of $CoSb_3$/Al/Ti/CuMo joined by the spark plasma sintering technique. Titanium thin foil was used to prevent the diffusion of copper into $CoSb_3$ and Aluminum thin foil was used to improve the adhesion between $CoSb_3$ and Ti. The insertion of an Aluminum interlayer between the Ti and $CoSb_3$ was effective for joining $CoSb_3$ to Ti by forming an intermediate layer at the Al-$CoSb_3$ boundary without any micro cracks. Specifically, the adhesion strength of the Ti/Al/$CoSb_3$ joining interface showed a remarkable improvement compared with our previous results, without deterioration of electrical property in the interface.

• Journal of the Korean Ceramic Society
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• v.53 no.3
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• pp.338-342
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• 2016

ZnO thin film co-doped with F and Al was prepared on a glass substrate via simple non-alkoxide sol-gel spin coating. For a fixed F concentration, the addition of Al co-dopant was shown to reduce the resistivity mainly due to an increase in electrical carrier density compared with ZnO doped with F only, especially after the second post-heat-treatment in a reducing environment. There was no effective positive contribution to the reduction in resistivity due to the mobility enhancement by the addition of Al co-dopant. Optical transmittance of the ZnO thin film co-doped with F and Al in the visible light domain was shown to be higher than that of the ZnO thin film doped with F only.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.280.2-280.2
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• 2016

The graphene, a single atomic sheet of graphite, has attracted tremendous interest owing to its novel properties including high intrinsic mobility, optical transparency and flexibility. However, for more diverse application of graphene devices, it is essential to tune its transport behavior by shifting Dirac Point (DP) of graphene. So, in the following context, we suggest a method to tune structural and electronic properties of graphene using atomic layer deposition. By atomic layer deposition of zinc oxide (ZnO) on graphene using 4-mercaptophenol as linker, we can fabricate n-doped graphene. Through ${\pi}-{\pi}$ stacking between chemically inert graphene and 4-mercaptophenol, conformal deposition of ZnO on graphene was enabled. The electron mobility of graphene TFT increased more than 3 times without considerably decreasing the hole mobility, compared to the pristine graphene. Also, it has high air stability. This ZnO doping method by atomic layer deposition can be applicable to large scale array of CVD graphene TFT.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2010.06a
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• pp.352-352
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• 2010

Silicon Carbide (SiC) power device possesses attractive features, such as high breakdown voltage, high-speed switching capability, and high temperature operation. In general, device design has a significant effect on the switching characteristics. In this paper, we report the effect of the P-base doping concentration ($N_{PBASE}$) on the transient characteristics of 4H-SiC DMOSFETs. By reducing $N_{PBASE}$, switching time also decreases, primarily due to the lowered channel resistance. It is found that improvement of switching speed in 4H-SiC DMOSFETs is essential to reduce the and channel resistance. Therefore, accurate modeling of the operating conditions are essential for the optimization of superior switching performance.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2010.06a
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• pp.355-355
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• 2010

We studied the dependence of the performance of schottky barrier metal-oxide-field effect transistors(SB MOSFETs) on the work function of source/drain metals. A strong impact of the various work functions and the light wavelengths on the transistor characteristics is found and explained using experimental data. We used an insulator of a high thickness (100nm) and back gate issues in SOI substrate, subthreshold swing was measured to 300~400[mV/dec] comparing with a ideal subthreshold swing of 60[mV/dec]. Excellent characteristics of Al/Si was demonstrated higher on/off current ratios of ${\sim}10^7$ than others. In addition, extensive photoresponse analysis has been performed using halogen and deuterium light sources(200<$\lambda$<2000nm).

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.30 no.7
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• pp.423-426
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• 2017

The designing approaches with consideration offabrication process technologies for high-frequency, high-powered, silicon-based static induction thyristors (SITH) are presented. The effects of doping concentration and thickness on the I-V characteristics and power performance of the devices are discussed. The dependence of SITH switching performances on material, geometric structure, and technological parameters isexamined by using two-dimensional simulations. Thick-epitaxy technology is found to be one of the most critical steps in realizing the proposed structure and switching times, $t_{off}$, of SITH, which may be reduced to below ${\sim}0.26{\mu}s$ for the proposed 1,700 V SITH devicesafter optimization.