Browsing by Author "Cheng, Hsin-Ming"

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Band gap engineering and stimulated emission of ZnMgO nanowires
(2006-07-03)
Hsu, Hsu-Cheng
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Wu, Chun-Yi
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Cheng, Hsin-Ming
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Hsieh, Wen-Feng
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光電工程學系
We report a simple method for fabricating heterostructured ZnMgO nanowires by annealing the preformed ZnO/MgO core-shell structure. Photoluminescence from the alloy nanowires shows strong near-band-edge (NBE) emission, reflecting good material quality. A blueshift of the NBE emission at room temperature after the annealing treatment is attributed to the diffusion of Mg from the shell into the core ZnO of the nanowires to form a ternary ZnMgO alloy. Band gap engineering and stimulated emissions of ZnMgO nanowires with different Mg doping concentrations are also demonstrated. (c) 2006 American Institute of Physics.
Bandgap engineering and spatial confinement of optical phonon in ZnO quantum dots
(2006-01-01)
Hsieh, Wen-Fen
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Lin, Kuo-Feng
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Cheng, Hsin-Ming
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Hsu, Hsu-Cheng
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光電工程學系
Upon decreasing size of ZnO quantum dots, the blueshift in bandgap reveals the quantum confinement effect and the diminishing ratio of the second- to first-order resonant Raman intensity reveals reducing the electron-phonon Frohlich interaction.
Efficient electron transport in tetrapod-like ZnO metal-free dye-sensitized solar cells
(2009)
Chiu, Wei-Hao
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Lee, Chia-Hua
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Cheng, Hsin-Ming
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Lin, Hsiu-Fen
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Liao, Shih-Chieh
The tetrapod-like ZnO nanopowders are employed to construct an efficient electron transport network as the photoanode of the dye-sensitized solar cells (DSCs). Due to the high extinction coefficient of the metal-free D149 dye offering a high photocurrent through the ZnO network, the best performance of DSCs based on a 42 mu m terapod-like ZnO film showed a high energy conversion efficiency of 4.9% with a high short-circuit photocurrent density of 12.3 mA cm(-2), an open-circuit photovoltage of 0.6 V, and a fill factor of 0.65 under AM 1.5 irradiation. Highly efficient electron transport may also be ascribed by a long effective electron diffusion length of 46 mu m determined from the electrochemical impedance spectroscopy which is consistent with thickness dependent J(SC) measurements.
Electron transfer properties of organic dye-sensitized solar cells based on indoline sensitizers with ZnO nanoparticles
(2010-12-03)
Cheng, Hsin-Ming
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Hsieh, Wen-Feng
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光電工程學系
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Department of Photonics
Two indoline dyes, coded D149 and D205, were used as the sensitizers of ZnO dye-sensitized solar cells (DSCs) with optimal energy conversion efficiencies of more than 5%, under AM 1.5 full sunlight illumination (100 mW cm(-2)). Higher interfacial charge transfer rate and retardant fluorescence decay confirmed from transient fluorescence illustrated that D205-sensitized ZnO DSCs could possess better electron transport than D149-sensitized ZnO DSCs. The enhancement of V(oc) and J(sc) for D205-sensitized ZnO DSCs was ascribed to the effective suppression of electron recombination by extending the alkyl chain on the terminal rhodanine moiety from ethyl to octyl. The evidence of enhanced electron diffusion coefficient was further shown by electrochemical impedance spectroscopy (EIS).
Formation of Branched ZnO Nanowires from Solvothermal Method and Dye-Sensitized Solar Cells Applications
(2008-10-01)
Cheng, Hsin-Ming
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Chiu, Wei-Hao
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Lee, Chia-Hua
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Tsai, Song-Yeu
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Hsieh, Wen-Feng
Branched ZnO nanowires have been fabricated on conductive glass substrates via a solvothermal method for dye-sensitized solar cells (DSCs). The 1D branched nanostructures can afford a direct conduction pathway instead of interparticle hops while using nanoparticles. Furthermore, the short-circuit current density and the energy conversion efficiency of the branched ZnO nanowire DSCs are 4.27 mA/cm(2) and 1.51%,which are twice as high as the bare ZnO nanowire ones. The improvement was a consequence of the enlargement of the internal surface area within the photoelectrode and allowed us to achieve higher dye adsorption to significantly enhance the performance of the DSCs.
High-efficiency metal-free organic-dye-sensitized solar cells with hierarchical ZnO photoelectrode
(2010)
Cheng, Hsin-Ming
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Hsieh, Wen-Feng
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光電工程學系
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Department of Photonics
Self-assembled ZnO secondary nanoparticles have been fabricated as an effective photoelectrode for dye-sensitized solar cells (DSCs). The hierarchical architecture, which manifested the significant light-scattering, can provide more photon harvesting. In addition, dye-molecule adsorption was sufficient due to enough internal surface area provided by the primary single nanocrystallites. Two indoline dyes, coded D149 and D205, were used as the sensitizers of ZnO DSCs with the optimal energy conversion efficiencies of 4.95% and 5.34%, respectively, under AM 1.5 full sunlight illumination (100 mW cm(-2)). The enhancement of the open-circuit photovoltage (V(oc)) and the short-circuit photocurrent density (J(sc)) for D205-sensitized ZnO DSCs was ascribed to the effective suppression of electron recombination by extending the alkyl chain on the terminal rhodanine moiety from ethyl to octyl. Further evidence is obtained from the electrochemical impedance spectroscopy (EIS) which exhibits a longer electron lifetime for D205-sensitized ZnO DSC in comparison with the D149-sensitized one.
High-power passively continuous-wave and Q-switching mode-locked Nd:LuVO4 laser by few-layer graphene-oxide films
(2016-12-01)
Chen, Hou-Ren
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Tsai, Chih-Ya
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Cheng, Hsin-Ming
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Lin, Kuei-Huei
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Chen, Chyong-Hua
Using a series of different concentration polymer-free graphene oxide (GO) films as saturable absorbers (SAs), we demonstrate high-power passively continuous-wave modelocking (CW-ML) and Q-switching mode-locking (QS-ML) for a Nd: LuVO4 laser at 1 mu m. CW-ML pulses of 6.62 ps with maximum output power of 3.89 W have been achieved to give the highest pulse peak power of 4.85 kW and pulse energy of 32.14 nJ under 15-W pumping by using SAs of 84.4% transmittance. Using SAs with a lower transmittance having a larger modulation depth of 75.9% and 63.4%, we obtain both the CW-ML pulses of 4.85-ps and QSML pulses with 2.73 W and 1.92 W output powers, respectively. (C) 2016 Optical Society of America
High-quality and Large-size Topological Insulator Bi2Te3-Gold Saturable Absorber Mirror for Mode-Locking Fiber Laser
(2016-12-05)
Chen, Hou-Ren
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Tsai, Chih-Ya
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Cheng, Hsin-Ming
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Lin, Kuei-Huei
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Yen, Po-Hsiu
A novel high-quality, large-size, reflection-type topological insulator Bi2Te3-Gold (BG) film-based nonlinear optical modulator has been successfully fabricated as a two-dimensional saturable absorber mirror (SAM) by pulsed laser deposition (PLD). This BG-SAM possesses saturation fluence of 108.3 mu J/cm(2), modulation depth (Delta R) of 6.5%, non-saturable loss of 38.4%, high damage threshold above 1.354 mJ/cm(2) and excellent uniformity providing for the generation of passive mode-locked (ML) pulses for erbium-doped fiber lasers (EDFLs) on a large sample area. Under 124 mW 976 nm pumping, We obtained 452-fs continuous-wave ML pulses with pulse energy of 91 pJ and full width at halfmaximum (FWHM) of 6.72-nm from this EDFL. The results clearly evidence that the PLD is an efficient method for fabricating BG-SAM that is suitable for a compact ultrafast ML fiber laser system.
Highly efficient solar-heat shield based on the bipolaron-assisted PEDOT:PSS thin film
(2020-08-01)
Thakur, Diksha
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Wu, Jia-Ren
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Chandel, Anjali
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Liu, Shih-Yao
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Lin, Pei-Chen
We report on an efficient solar-heat shield based on the highly transparent PEDOT:PSS thin film, which significantly reduces the temperature from 38.2 degrees C to 33.5 degrees C. To understand the superior solar-heat shielding ability, the modified PEDOT:PSS thin films were investigated by using the transmittance spectra, Raman scattering spectra, atomic-force microscopic (AFM) images and broadband refractive index spectra. The experimental results show that the molecular structure of the PEDOT chains can be manipulated by varying the Brownian motion of PEDOT polymers in the PEDOT:PSS solutions and the spin rate. The lower Brownian motion of PEDOT polymers and the higher spin rate both can change the molecular structure of the PEDOT chains from bended structures (de-doping states) to linear structures (doping states), which simultaneously increase the near-infrared absorption strength and the visible transparency due to the formation of bipolarons.
Passive mode locking of ytterbium- and erbium-doped all-fiber lasers using graphene oxide saturable absorbers
(2014-06-02)
Chen, Hou-Ren
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Tsai, Chih-Ya
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Cheng, Hsin-Ming
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Lin, Kuei-Huei
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Hsieh, Wen-Feng
Broadband graphene oxide/PVA films were used as saturable absorbers (SAs) for mode locking erbium-doped fiber laser (EDFL) and ytterbium-doped fiber laser (YDFL) at 1.06 mu m and 1.55 mu m. They provide modulation depths of 3.15% and 6.2% for EDFL and YDFL, respectively. Stable self-starting mode-locked pulses are obtained for both lasers, confirming that the graphene oxide is cost-effective. We have generated mode-locked pulses with spectral width, repetition rate, and pulse duration of 0.75 nm, 9.5 MHz, and 2.7 ps. This is the shortest pulse duration directly obtained from an all-normal-dispersion YDFL with graphene-oxide saturable absorber. (C) 2014 Optical Society of America
Saturation and beating of acoustic phonon oscillations excited near the exciton resonance of strained polar ZnO/Zn0.8Mg0.2O multiple quantum wells
(2018-01-01)
Liu, Wei-Rein
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Lin, Ja-Hon
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Chen, Jyun-Sian
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Cheng, Hsin-Ming
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Li, Sheng-Jie
Saturation and beating of coherent acoustic phonon (CAP) oscillations were observed and attributed to the screening of a built-in electric field with increasing pump power using degenerate pump-probe measurements near the exciton resonance of polar ZnO/Zn0.8Mg0.2O multiple quantum wells (MQWs). After purifying the CAP signals by using an empirical mode decomposition, we found not only that the CAP amplitude follows the trend of the band gap renormalization (BGR) and shows saturation at high pump power, but also that the CAP oscillation period coincides with that of the MQWs, consistent with the XRD and TEM results. An additional low-frequency oscillation modifying the CAP signal is revealed due to the negative change in refractive index caused by BGR as the pump power increases.
Structural and optical properties of ZnMgO nanostructures formed by Mg in-diffused ZnO nanowires
(2007-04-01)
Pan, Ching-Ju
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Hsu, Hsu-Cheng
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Cheng, Hsin-Ming
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Wu, Chun-Yi
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Hsieh, Wen-Feng
ZnMgO nanostructures with wurtzite phase were prepared by thermal diffusion of Mg into the ZnO nanowires. As ZnO light-emitting devices have been operated by using ZnMgO layers as energy barrier layers to confine the carriers, it is essential to realize the characterization of ZnMgO particularly. In this work, the Mg content in Zn1-xMgxO alloy determined by X-ray diffraction (XRD) and photoluminescence (PL) shows a good coincidence. The variation of lattice constant and the blueshift of near-band-edge emission indicate that Zn2+ ions are successfully substituted by Mg2+ ions in the ZnO lattice. In Raman-scattering studies, the change of E-2(high) phonon line shape in ZnO:Mg nanostructures reveals the microscopic substitutional disorder. In addition to the host phonons of ZnO, two additional bands around 383 and 510cm(-1) are presumably attributed to the Mg-related vibrational modes. (C) 2007 Elsevier Inc. All rights reserved.