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황차환,이진호,송미연,박종필,심일운 대한화학회 2011 Bulletin of the Korean Chemical Society Vol.32 No.7
Colloidal cadmium telluride (CdTe) quantum dots (QDs) and their nanoparticles have been synthesized by one pot sonochemical reactions under multibubble sonoluminescence (MBSL) conditions, which are quite mild and facile compared to other typical high temperature solution-based methods. For a typical reaction, CdCl2 and tellurium powder with hexadecylamine and trioctylphosphine/trioctylphosphineoxide (TOP/TOPO) as a dispersant were sonicated in toluene solvent at 20 KHz and a power of 220W for 5-40 min at 60 ^oC. The sizes of CdTe particles, in a very wide size range from 2 nm-30 μm, were controllable by varying the sonicating and thermal heating conditions. The prepared CdTe QDs show different colors from pale yellow to dark brown and corresponding photoluminescence properties due mainly to the quantum confinement effect. The CdTe nanoparticles of about 20 nm in average were found to have band gap of 1.53 eV, which is the most optimally matched band gap to solar spectrum.
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Photoinduced Birefringence in an Azo Polymer Film
황의중,오차환,G. J. Lee,J. . Kim,P. . Kim,. H. ong,. Y. You,Y. K. Han,이영백 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.46 No.2
Photoinduced anisotropy and birefringence in an azobenzene polymer film were investigated by pumping beams with various wavelengths. Azobenzene-type polymer whose and n transition bands are far enough from each other to be distinguished was used in this experiment. The vertical and the parallel polarization components of the absorbance and also the birefringence on the polymer film were measured simultaneously during pumping with a linearly polarized beam at various wavelengths. We observed abnormal behavior of the birefringence for the pumping beam at specific wavelength. For explaining the photoinduced birefringence change with the illumination time, rate equations for angular distribution of trans fraction in the azobenzene polymer were introduced. By comparing the experimental results with the theoretical simulations by using the rate equations, it was found that the abnormal behavior of the birefringence is due to the incomplete cycle of the photoisomerization process.
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이세영,오차환,김필수,황의중 漢陽大學校 自然科學硏究所 1997 自然科學論文集 Vol.16 No.-
외부 광궤환 반도체 레이저의 출력 특성을 비율방정식의 수치해를 이용하여 분석하였다. 출력의 세기와 위상과 전자밀도를 시간에 대한 함수로 알아보았다. 또한 bifurcation diagram과 phase diagram을 통하여 궤환비율 k에 따른 레이저의 동작 특성을 조사하였다. 궤환광이 없을때는 이완발진이 감쇄한 후 안정한 출력특성이 나타나지만 되먹임 비율이 증가하면서 안정한 해의 갯수가 증가하다가 ??????이상의 매우 강한 광궤환에서는 출력특성이 극도로 불규칙해지고 무수히 많은 해를 갖는 혼돈현상이 나타남을 알 수 있었다. The output characteristics of a laser diode with external optical feedback were analyzed by numerical solutions of rate equations. The output power, phase and carrier density were plotted as a function of time. Dynamics of external cavity laser were investigated by the bifurcation diagram and the phase diagram with respect to the feedback rate k. If there was no feedback, relaxation oscillation reduced and laser showed stable output. For increasing feed back level, the number of stable state was increased. In very strong feedback rate above ??????, laser output showed extremely unstable and chaotic behavior.
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안경한,원종국,강여경,황차환,정인,김명길 한국물리학회 2019 Current Applied Physics Vol.19 No.2
The pursuit for a high-performance thermoelectric n-type bismuth telluride-based material is significant because n-type materials are inferior to their corresponding p-type materials in highly efficient thermoelectric modules. Herein, to improve the thermoelectric performance of an n-type Bi2Te3, we prepared Bi2Te3 nano-plates with a homogeneous sub-micron size distribution and thickness range of about a few tens of nanometers. This was achieved using a typical nano-chemical synthetic method, and the prepared materials were then spark plasma sintered to fabricate n-type nano-bulk Bi2Te3 samples. We observed a significant enhancement of the anisotropic electrical transport properties for the nano-bulk sample with a higher power factor along the in-plane direction (24.3 μW cm−1 K−2 at 300 K) than that along the out-of-plane direction (8.1 μW cm−1 K−2 at 300 K). However, thermal transport properties were insensitive along the measured direction for the nano-bulk sample. We used a dimensionless figure of merit ZT to calculate the thermoelectric performance. The results showed that the maximum ZT value of 0.69 was achieved along the in-plane direction at 440 K for the nano-bulk n-type Bi2Te3 sample, which was however smaller than that of the previously reported n-type samples (ZT of 1.1). We believe that a further enhancement of the ZT value in the fabricated nano-bulk sample could be accomplished by effectively removing the surface organic ligand of the Bi2Te3 nano-plate particles and optimizing the spark plasma sintering conditions, maintaining the nano-plate morphology intact.
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