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Magnetic anisotropy in symmetric magnetic colloidal quantum dots doped with few Mn2+ impurities
(2009-06-01)
Cheng, Shun-Jen
;
電子物理學系
;
Department of Electrophysics
We present numerical exact diagonalization studies of the magnetism in magnetic II-VI colloidal semiconductor quantum dots charged with controlled number of electrons coupled to few magnetic-ion (Mn2+) dopants. The interplay between various relevant spin interactions and the discrete nature of Mn2+ spin distribution is shown to be essential in the magnetism of few Mn2+-doped quantum dots. An interesting revealed feature is the existence of pronounced magnetic anisotropies in symmetric magnetic quantum dots, which are related to the orbital quenchings induced by electron-Mn spin interactions. The orbital quenching effects further lead to significant deviations of the magnetizations of randomly Mn-doped dots from the standard Brillouin-function descriptions.
Time-resolved photoluminescence of isoelectronic traps in ZnSe1-xTex semiconductor alloys
(2008-12-15)
Lin, Y. C.
;
Chou, W. C.
;
Fan, W. C.
;
Ku, J. T.
;
Ke, F. K.
Kohlrausch's stretched exponential law correlates well with the photoluminescence (PL) decay profiles of ZnSe1-xTex. As the Te concentration increases, the stretching exponent beta initially declines and then monotonically increases. This result can be understood using the hopping-transport and energy transfer model. The increase in the number of isoelectronic Te localized traps can reduce the PL decay rate and increase the linewidth, whereas the hybridization of the Te localized states with the valence-band edge states causes a reduction in both the lifetime and linewidth.
A comprehensive study of temperature-dependent reflectance and photoluminescence of Zn1-xMnxO thin films grown on c-Al2O3
(2009-03-01)
Lin, D. Y.
;
Lin, H. J.
;
Wu, J. S.
;
Chou, W. C.
;
Yang, C. S.
We present a systematic study of temperature-dependent reflectance (R) and photoluminescence (PL) measurements on ZnMnO films grown by plasma-assisted molecular beam epitaxy. For the first time, the three free-exciton transitions FXA (Gamma(7c)-Gamma(u)(7v)), FXB (Gamma(7c)-Gamma(9v)), and FXC (Gamma(7c)-Gamma(l)(7v)) and the longitudinal-optical phonon replicas of FXB and FXC of ZnMnO films have been clearly observed in the R spectra. The parameters describing the activation energy and the temperature dependence of the transition energy and broadening have been extracted by fitting the experimental R and PL spectra. The spectral data of ZnMnO films not only show the deterioration of crystalline quality with increasing Mn composition fraction but also indicate the Mn clustering caused by Mn atom segregation.
Carrier dynamics in isoelectronic ZnSe1-xOx semiconductors
(2010-07-26)
Lin, Y. C.
;
Chung, H. L.
;
Chou, W. C.
;
Chen, W. K.
;
Chang, W. H.
This study explores the effects of both Oxygen and temperature on the carrier dynamics of isoelectronic ZnSe1-xOx (x=0.027 and 0.053) semiconductors using photoluminescence (PL) and time-resolved PL spectroscopy. We find that the Kohlrausch law is highly consistent with the complex decay traces induced by isoelectronic O traps, and the mechanism of carrier recombination undergoes a complicated change from trapped to free excitons with the increase in temperature. Complex recombination processes are clarified using the relaxation model based on various decay channels. These findings are consistent with the initial fall in the stretching exponent beta followed by its monotonic increase with increasing temperature. (C) 2010 American Institute of Physics. [doi:10.1063/1.3473776]
Enhanced phosphorescence and electroluminescence in triplet emitters by doping gold into cadmium selenide/zinc sulfide nanoparticles
(2005-10-01)
Liu, HW
;
Laskar, IR
;
Huang, CP
;
Cheng, JA
;
Cheng, SS
Gold-cadmium selenide/zinc sulfide (Au-CdSe/ZnS) nanocomposites (NCs) were synthesized and characterized by transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, ultraviolet-visible (UV-visible) absorption and photoluminescence (PL) emission spectroscopy. The PL intensity in the Au-CdSe/ZnS NCs system was found to be much greater than that of CdSe/ZnS nanoparticles (NPs) alone, because of the surface-enhanced Raman scattering of An NPs. Adding Au-CdSe/ZnS NCs to the cyclometalated iridium(Ill) complex (Ir-complex) greatly enhanced the PL intensity of a triplet emitter. Three double-layered electroluminescence (EL) devices were fabricated where the emitting zone contains the definite mixture of Ir-complex and the NCs [molar concentration of Ir-complex/NCs = 1:0 (Blank, D-1), 1: 1 (D-2) and 1:3 (D-3)] and the device D-2 exhibited optimal EL performances. (c) 2005 Elsevier B.V. All rights reserved.
Fabrication and CO Sensing Properties of Mesostructured ZnO Gas Sensors
(2009)
Liu, Chueh-Yang
;
Chen, Chia-Fu
;
Leu, Jih-Perng
;
材料科學與工程學系
;
Department of Materials Science and Engineering
This study presents the synthesis of mesostructured ZnO using a template replication method and the fabrication of a carbon monoxide sensor using the synthesized ZnO using the dielectrophoresis process. The mesoporous carbon, CMK-3, was employed for the template and zinc nitrite was used as the precursor for synthesizing the ordered porous ZnO. The mesostructured ZnO was analyzed using X-ray diffraction (XRD) patterns, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and N(2) adsorption-desorption isotherms. The XRD patterns indicated that the ZnO exhibited a highly ordered structure after the template was removed. The morphology of the ZnO was randomly oriented and the structure was polycrystalline. The surface area, pore volume, and pore size of the porous ZnO were 61.3 m(2) g(-1), 0.31 cm(3) g(-1), and 5.3 nm, respectively. When CO gas was injected, the optimum sensitivities at 250 degrees C were 12.1, 14.6, 18.4, 26.0, and 60% when the CO concentrations were 10, 20, 30, 50, and 70 ppm, respectively.
Large vacuum Rabi splitting in ZnO-based hybrid microcavities observed at room temperature
(2009-02-09)
Chen, Jun-Rong
;
Lu, Tien-Chang
;
Wu, Yung-Chi
;
Lin, Shiang-Chi
;
Liu, Wei-Rein
Wide-band gap ZnO semiconductors are attractive materials for the investigation of microcavity exciton polaritons due to the large exciton binding energy and oscillator strength. We report the growth and characterization of bulk ZnO-based hybrid microcavity. The phenomenon of strong exciton-photon coupling at room temperature has been observed in the ZnO-based hybrid microcavity structure, which consists of 30 pair epitaxially grown AlN/AlGaN distributed Bragg reflector (DBR) on the bottom side of the 3/2 lambda thick ZnO cavity and 9 pair SiO(2)/HfO(2) DBR as the top mirror. The cavity quality factor is about 221. The experimental results show good agreement with theoretically calculated exciton-polariton dispersion curves based on transfer matrix method. From the theoretical and experimental exciton-polariton dispersion curves with two different cavity-exciton detuning values, the large vacuum Rabi splitting is estimated to be about 58 meV in the ZnO-based hybrid microcavity.
Magnetophotoluminescence properties of Co-doped ZnO nanorods
(2009-04-13)
Lin, C. Y.
;
Wang, W. H.
;
Lee, C. -S.
;
Sun, K. W.
;
Suen, Y. W.
We present the detailed experimental results of the magnetic and optical properties of cobalt doped ZnO nanorods, especially the temperature and magnetic field dependence of photoluminescence up to 14 T. The Raman measurements indicate that our Co-doped ZnO nanorods have the same lattice constant as crystalline bulk ZnO. Sharp luminescence peaks centered at around 670 nm were observed at low temperature and their intensity decreased with increasing magnetic field. The luminescence peaks were attributed to d-d transitions in the Ligand field from the doped Co ions. We also observed a diamagnetic shift at a temperature of 1.5 K when the magnetic field was scanned from 0 to 14 T. The exciton radius of the Co-doped ZnO nanorods was deduced from the magnetophotoluminescence results.
A low-temperature method for improving the performance of sputter-deposited ZnO thin-film transistors with supercritical fluid
(2009-04-20)
Chen, Min-Chen
;
Chang, Ting-Chang
;
Huang, Sheng-Yao
;
Chang, Kuan-Chang
;
Li, Hung-Wei
A low-temperature method, supercritical CO(2) (SCCO(2)) fluid technology, is employed to improve the device properties of ZnO TFT at 150 degrees C. In this work, the undoped ZnO films were deposited by sputter at room temperature and treated by SCCO(2) fluid which is mixed with 5 ml pure H(2)O. After SCCO(2) treatment, the on/off current ratios and threshold voltage of the device were improved significantly. From x-ray photoelectron spectroscopy analyses, the enhancements were attributed to the stronger Zn-O bonds, the hydrogen-related donors, and the reduction in dangling bonds at the grain boundary by OH passivation.
Blinking suppression of colloidal CdSe/ZnS quantum dots by coupling to silver nanoprisms
(2009-06-15)
Yuan, C. T.
;
Yu, Pyng
;
Tang, Jau
;
光電工程學系
;
Department of Photonics
Fluorescence blinking of quantum dots (QDs) is a common but annoying behavior for practical applications as fluorescent markers or single photon sources. We demonstrate here, by coupling colloidal CdSe/ZnS QDs to silver nanoprisms, fluorescence blinking behavior can be suppressed. For most of coupled QDs, two emissive states with different fluorescence intensities and lifetime were observed. We suggest that energy transfer could play an important role in suppressing blinking behavior.

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