Browsing by Author "Yang, S. H."

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Investigation of defects in organic semiconductors by charge based Deep Level Transient Spectroscopy (Q-DLTS)
(2009)
Nguyen, T. P.
;
Renaud, C.
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Le Rendu, P.
;
Yang, S. H.
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應用化學系
We report the results of measurements of traps in light emitting devices using a new derivative of poly(phenylene vinylene) (PPV) as an active material by the charge based Deep Level Transient Spectroscopy (Q-DLTS) technique. Diodes of structure Indium Tin Oxide (ITO)/PEDOT:PSS/poly(2-ethylhexyl)surfanyl-5- methoxy phenylene vinylene (MEH-S-PPV)/M with M = Al and M = Ca/Al were investigated by measurements of current-voltage-luminance characteristics. From analysis of these characteristics, evidence of charge trapping in devices was demonstrated. The trap parameters were then determined from Q-DLTS measurements, which were carried out on the samples as a function of the charging time, the applied voltage and the temperature. Five trap levels of activation energy in the range [0.-0.6 eV] and of density of order of 10(17) cm(-3) were identified in diodes with Ca/Al cathode. Electron (one level) and hole (four levels) traps were then clearly distinguished by performing measurements in hole-only devices. Trapping processes are discussed and tentatively proposed to performance of the light emitting diodes studied. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Investigation of the optical properties of polyfluorene/ZnO nanocomposites
(2011-04-01)
Zou, J. P.
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Le Rendu, P.
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Musa, I.
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Yang, S. H.
;
Dan, Y.
Polyfluorene (PF) and its derivatives are very promising candidates for organic light emitting diodes (OLEDs) in lighting applications because of their high photoluminescence and electroluminescence efficiencies. Recent investigations of potential materials for OLEDs have shown that introducing n-type inorganic nanoparticles into conjugated polymers is efficient to produce stable and high performance devices. In this study, composite thin films made by incorporation of zinc oxide (ZnO) nanoparticles into a PF derivative have been prepared and their optical properties have been investigated. The prepared thin films were stored in different media (in air, in vacuum, in the dark or exposed to light) in order to study environmental influences on the material stability. Analysis of spectral data obtained from infrared (IR), Raman, UV-vis, and photoluminescence (PL) measurements shows a large enhancement in luminescence for polymer nanocomposites while using high nanoparticle concentrations (within a limit of 10% ZnO). Time-resolved PL performed on those nanocomposite films corroborated the above result: it indicated that the light-emission enhancement can be explained by efficient energy transfer from nanoparticles to the polymer chains and increase of the chain separation distance. In addition, the nanocomposites were found to be more stable than pristine polymer films whatever the storage conditions were used. It was confirmed by IR analysis that incorporation of nanoparticles into polymers prohibited the formation of fluorenonyl groups in PF chains, which was identified as the main cause of the degradation of the polymer under photo-oxidation. (C) 2011 Elsevier B.V. All rights reserved.
Material characterization of laminated composite materials using a three-point-bending technique
(2009-05-01)
Kam, T. Y.
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Chen, C. M.
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Yang, S. H.
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機械工程學系
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Department of Mechanical Engineering
A three-point-bending technique is presented for identifying the elastic constants of laminated composite materials. In the proposed technique, three strains in the axial, lateral, and 45 directions on the bottom surface at the mid-span of a symmetric angle-ply beam subjected to three-point-bending testing are measured for elastic constants identification. The narrow beam theory together with the trial elastic constants is used to predict the theoretical strains of the beam. The theoretical and experimental strains of the beams are then used in a stochastic optimization method to identify the elastic constants of the beam. The accuracy and applications of the proposed technique are demonstrated by means of a number of examples on the elastic constants identification of graphite/epoxy (Gr/ep) or glass/epoxy (Gl/ep) laminated composite materials. The effects of specimen aspect ratio and thickness on the accuracy of the proposed method are investigated. (C) 2008 Elsevier Ltd. All rights reserved.
NMR evidence for the partially gapped state in CeOs(2)Al(10)
(2010-11-22)
Lue, C. S.
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Yang, S. H.
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Su, T. H.
;
Young, Ben-Li
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電子物理學系
We report the results of a (27)Al nuclear magnetic resonance (NMR) study of CeOs(2)Al(10) at temperatures between 4 and 300 K. This material has been of current interest due to indications of hybridization gap behavior below the transition temperature T(o)similar or equal to 29 K. Five (27)Al NMR resonance lines that are associated with five nonequivalent crystallographic aluminum sites have been resolved. For each individual aluminum site, the low-temperature NMR Knight shift goes over a thermally activated response. The temperature-dependent spin-lattice-relaxation rate exhibits a rapid drop below T(o), indicative of the formation of an energy gap in this material. We interpret the Knight shift and the relaxation-rate data in light of the presence of a pseudogap with residual electronic density of states at the Fermi level. Moreover, the magnitude of the pseudogap of 120 K is extracted from NMR results, in agreement with the value obtained from the inelastic neutron-scattering experiment.
NMR evidence for the partially gapped state in CeOs2Al10
(2010-11-22)
Lue, C. S.
;
Yang, S. H.
;
Su, T. H.
;
Young, Ben-Li
;
電子物理學系
We report the results of a Al-27 nuclear magnetic resonance (NMR) study of CeOs2Al10 at temperatures between 4 and 300 K. This material has been of current interest due to indications of hybridization gap behavior below the transition temperature T-o similar or equal to 29 K. Five Al-27 NMR resonance lines that are associated with five nonequivalent crystallographic aluminum sites have been resolved. For each individual aluminum site, the low-temperature NMR Knight shift goes over a thermally activated response. The temperature-dependent spin-lattice-relaxation rate exhibits a rapid drop below T-o, indicative of the formation of an energy gap in this material. We interpret the Knight shift and the relaxation-rate data in light of the presence of a pseudogap with residual electronic density of states at the Fermi level. Moreover, the magnitude of the pseudogap of 120 K is extracted from NMR results, in agreement with the value obtained from the inelastic neutron-scattering experiment.