Browsing by Author "Chang, Kao-Der"

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A hybrid simulated method for analyzing the optical efficiency of a head-mounted display with a quasi-crystal OLED panel
(2014-03-10)
Chang, Kao-Der
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Li, Chang-Yi
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Pan, Jui-Wen
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Cheng, Kuei-Yuan
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光電系統研究所
Organic light emitting diodes (OLEDs) with a quasi-crystal (QC) structure are analyzed and applied in a head-mounted display (HMD) system in this study. We adopt a hybrid simulated method to evaluate the light extraction efficiency (LEE) and far-field pattern in the air, and study the relationship between them. The simulation results show that OLEDs implanted with the QC structure can provide a collimated far-field pattern to increase the brightness. Using this 10-fold QC arrangement the maxima LEE of the OLEDs can be increased by 1.20 times. Compared with conventional OLEDs, the viewing angle of the OLED panel decreases from 120 degrees to 26 degrees with an improvement in the optical efficiency of the HMD system by 2.66 times. Moreover, the normalized on-axis intensity in the pupil of the eyepiece can be enlarged up to 3.95 times which suggests that the OLED panel can save 74.68% energy while achieving the same onaxis intensity as conventional OLEDs. (c) 2014 Optical Society of America
Ag-Nanoparticle-Decorated SiO2 Nanospheres Exhibiting Remarkable Plasmon-Mediated Photocatalytic Properties
(2012-09-06)
Chen, Kuang-Hsiu
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Pu, Ying-Chih
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Chang, Kao-Der
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Liang, Yi-Fan
;
Liu, Chia-Ming
We demonstrated for the first time that Ag-nanoparticle-decorated SiO2 nanospheres (NSs) may display noticeable photocatalytic activities upon surface plasmon resonance (SPR) excitation. The samples were prepared by reacting SiO2 NSs with AgNO3 in the seed-mediated growth process, from which the Ag particle size and decoration density can be readily controlled. The dependence of the SPR-mediated photocatalytic performance of Ag-decorated SiO2 NSs on the Ag morphology was investigated and presented. The as-prepared Ag-decorated SiO2 NSs showed a significantly red shifted and relatively broad SPR absorption when compared with the individually dispersed Ag nanoparticles. Owing to the considerably broad SPR absorption that spanned from the visible to the near-infrared region, Ag-decorated SiO2 NSs surpassed N-doped P-25 TiO2 powder and individually dispersed Ag nanoparticles in photocatalytic activity, demonstrating their potential as an active photocatalyst in nearly all the current photocatalysis applications. Furthermore, the result of performance evaluation under natural sunlight shows that the present Ag-decorated SiO2 NSs can be used as highly efficient photocatalysts that may practically harvest energy from sunlight. The current study provides a new paradigm for designing plasmonic metal nanostructures that can effectively absorb the entire solar spectrum and beyond for solar fuel generation.
Au Nanostructure-Decorated TiO2 Nanowires Exhibiting Photoactivity Across Entire UV-visible Region for Photoelectrochemical Water Splitting
(2013-08-01)
Pu, Ying-Chih
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Wang, Gongming
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Chang, Kao-Der
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Ling, Yichuan
;
Lin, Yin-Kai
Here we demonstrate that the photoactivity of Au-decorated TiO2 electrodes for photoelectrochemical water oxidation can be effectively enhanced in the entire UV-visible region from 300 to 800 nm by manipulating the shape of the decorated Au nanostructures. The samples were prepared by carefully depositing Au nanoparticles (NPs), Au nanorods (NRs), and a mixture of Au NPs and NRs on the surface of TiO2 nanowire arrays. As compared with bare TiO2, Au NP-decorated TiO2 nanowire electrodes exhibited significantly enhanced photoactivity in both the UV and visible regions. For Au NR-decorated TiO2 electrodes, the photoactivity enhancement was, however, observed in the visible region only, with the largest photocurrent generation achieved at 710 nm. Significantly, TiO2 nanowires deposited with a mixture of Au NPs and NRs showed enhanced photoactivity in the entire UV visible region. Monochromatic incident photon-to-electron conversion efficiency measurements indicated that excitation of surface plasmon resonance of Au is responsible for the enhanced photoactivity of Au nanostructure-decorated TiO2 nanowires. Photovoltage experiment showed that the enhanced photoactivity of Au NP-decorated TiO2 in the UV region was attributable to the effective surface passivation of Au NPs. Furthermore, 3D finite-difference time domain simulation was performed to investigate the electrical field amplification at the interface between Au nanostructures and TiO2 upon SPR excitation. The results suggested that the enhanced photoactivity of Au NP-decorated TiO2 in the UV region was partially due to the increased optical absorption of TiO2 associated with SPR electrical field amplification. The current study could provide a new paradigm for designing plasmonic metal/semiconductor composite systems to effectively harvest the entire UV visible light for solar fuel production.
Heteroepitaxial approach to explore charge dynamics across Au/BiVO4 interface for photoactivity enhancement
(2015-07-01)
Chien Nguyen Van
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Chang, Wei Sea
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Chen, Jhih-Wei
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Tsai, Kai-An
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Tzeng, Wen-Yen
Heterostructure provides a powerful route in manipulating electrical transport, optical response, electrolytic water splitting and water treatment of complex oxides. As a model for noble metal/ complex oxide heterostructures, we have successfully prepared Au/BiVO4 (BVO) heterostructures in which the Au nanoparticles (NPs) with various sizes and densities were uniformly deposited on the {001} facets of epitaxial BVO thin films. The heterostructures exhibit significantly enhanced photoactivities in both dye degradation and electrolytic water splitting. By employing X-ray photoelectron spectroscopy, the energy band alignment of Au/BVO heterojunction suggests a charge separation at their interfaces, that can manipulate the photoexcited electron-hole pairs and photocatalytic efficiency of the heterostructures. Photogenerated carrier injection, which mainly affects the photoactivity of photocatalysis, was detected across Au/BVO interfaces by ultrafast dynamics spectroscopy. This study delivers a general approach to probe and understand the photochemistry of noble metal-complex oxide heterostructures for photoconversion applications. (C) 2015 Elsevier Ltd. All rights reserved.
Light extraction efficiency analysis of GaN-based light-emitting diodes with nanopatterned sapphire substrates
(2013-03-01)
Pan, Jui-Wen
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Tsai, Pei-Jung
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Chang, Kao-Der
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Chang, Yung-Yuan
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光電系統研究所
In this paper, we propose a method to analyze the light extraction efficiency (LEE) enhancement of a nanopatterned sapphire substrates (NPSS) light-emitting diode (LED) by comparing wave optics software with ray optics software. Finite-difference time-domain (FDTD) simulations represent the wave optics software and Light Tools (LTs) simulations represent the ray optics software. First, we find the trends of and an optimal solution for the LEE enhancement when the 2D-FDTD simulations are used to save on simulation time and computational memory. The rigorous coupled-wave analysis method is utilized to explain the trend we get from the 2D-FDTD algorithm. The optimal solution is then applied in 3D-FDTD and LTs simulations. The results are similar and the difference in LEE enhancement between the two simulations does not exceed 8.5% in the small LED chip area. More than 104 times computational memory is saved during the LTs simulation in comparison to the 3D-FDTD simulation. Moreover, LEE enhancement from the side of the LED can be obtained in the LTs simulation. An actual-size NPSS LED is simulated using the LTs. The results show a more than 307% improvement in the total LEE enhancement of the NPSS LED with the optimal solution compared to the conventional LED. (C) 2013 Optical Society of America
Microprism for a capsule endoscope with wafer level optics
(2019-08-01)
Chen, Luan-Ying
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Pan, Jui-Wen
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Sheu, Ming-Jen
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Chang, Kao-Der
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光電系統研究所
Capsule endoscopes are pill-sized devices that physicians can utilize for diagnosis of a patient's small intestine. For diagnostic requirements, excellent image quality, large field of view (FOV) and compact size are crucial. In this paper, a multi-camera capsule endoscope (MCCE) composed of two wafer level optic modules and microprism elements which can offer both a large FOV and clear image quality within limited space conditions is described. The properties of the microprism needed to ensure the necessary image quality are further discussed, including working efficiency, the ability to increase the FOV, and the inhibition of chromatic aberration. Finally, the functioning of the MCCE is verified through an optical simulation and preliminary experiments. The results show that the max half FOV of the MCCE could be increased from 28 degrees to 37 degrees, and the relative illumination could be kept at 80% without a dark corner effect.
Plasmon-mediated charge dynamics and photoactivity enhancement for Au-decorated ZnO nanocrystals
(2018-03-14)
Chiu, Yi-Hsuan
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Chang, Kao-Der
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Hsu, Yung-Jung
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材料科學與工程學系
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Department of Materials Science and Engineering
With the capability of localizing optical energy via surface plasmon resonance (SPR), plasmonic Au nanostructures hold great promise for enhancing the solar water splitting of semiconductor photocatalysts. While the content of Au plays a critical role in mediating interfacial charge transfer, its quantitative influence on the efficiency of plasmon-assisted water splitting is still not fully understood. This work aimed to explore the correlations among plasmonic metal content, SPR-mediated charge transfer and electromagnetic response, and the resultant photoactivity enhancement toward photoelectrochemical (PEC) water splitting. The PEC system was constructed by employing Au particle-decorated ZnO nanocrystals (ZnO-Au) as the plasmonic photoanode. Time-resolved photoluminescence spectroscopy and finite-difference time-domain simulations were utilized to evaluate the optimal Au content which attained effective charge separation and imposed a significant SPR effect for achieving the largest photoactivity enhancement. The charge transfer at the photoanode/electrolyte interface and its dependence on the Au content were examined with electrochemical impedance analysis, which manifested the effectiveness of the optimal Au content in facilitating the hole transfer kinetics. The present study reports a technical advance in the realization of the quantitative effect of Au for designing sophisticated plasmonic PEC systems that enabled efficient solar-to-fuel energy conversion.
Surface Passivation of TiO2 Nanowires Using a Facile Precursor-Treatment Approach for Photoelectrochemical Water Oxidation
(2014-07-10)
Pu, Ying-Chih
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Ling, Yichuan
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Chang, Kao-Der
;
Liu, Chia-Ming
;
Zhang, Jin Z.
We developed a facile precursor-treatment approach for effective surface passivation of rutile TiO2 nanowire photoanode to improve its performance in photoelectrochernical (PEC) water oxidation. The approach was demonstrated by treating rutile TiO2 nanowires with titanium precursor solutions (TiCl4, Ti(OBu)(4), or Ti(OiP)(4)) followed by a postannealing process, which resulted in the additional deposition of anatase TiO2 layer on the nanowire surface. Compared to pristine TiO2, all the precursor-treated TiO2 nanowire electrodes exhibited a significantly enhanced photocurrent density under white light illumination. Among the three precursor-treated samples, Ti(OBu)(4)-treated TiO2 nanowires achieved the largest enhancement of photocurrent generation, which is approximately a 3-fold increase over pristine TiO2. Monochromatic incident photon-to-electron conversion efficiency (IPCE) measurements showed that the improvement of PEC performance was dominated by the enhanced photoactivity of TiO2 in the UV region. The photovoltage and electrochemical impedance spectroscopy (EIS) measurements showed that the enhanced photoactivity can be attributed to the improved charge transfer as a result of effective surface state passivation. This work demonstrates a facile, low-cost, and efficient method for preparing highly photoactive TiO2 nanowire electrodes for PEC water oxidation. This approach could also potentially be used for other photoconversion applications, such as TiO2 based dye-sensitized solar cells, as well as photocatalytic systems.
Surface Passivation of TiO2 Nanowires Using a Facile Precursor-Treatment Approach for Photoelectrochemical Water Oxidation
(2014-07-10)
Pu, Ying-Chih
;
Ling, Yichuan
;
Chang, Kao-Der
;
Liu, Chia-Ming
;
Zhang, Jin Z.
We developed a facile precursor-treatment approach for effective surface passivation of rutile TiO2 nanowire photoanode to improve its performance in photoelectrochernical (PEC) water oxidation. The approach was demonstrated by treating rutile TiO2 nanowires with titanium precursor solutions (TiCl4, Ti(OBu)(4), or Ti(OiP)(4)) followed by a postannealing process, which resulted in the additional deposition of anatase TiO2 layer on the nanowire surface. Compared to pristine TiO2, all the precursor-treated TiO2 nanowire electrodes exhibited a significantly enhanced photocurrent density under white light illumination. Among the three precursor-treated samples, Ti(OBu)(4)-treated TiO2 nanowires achieved the largest enhancement of photocurrent generation, which is approximately a 3-fold increase over pristine TiO2. Monochromatic incident photon-to-electron conversion efficiency (IPCE) measurements showed that the improvement of PEC performance was dominated by the enhanced photoactivity of TiO2 in the UV region. The photovoltage and electrochemical impedance spectroscopy (EIS) measurements showed that the enhanced photoactivity can be attributed to the improved charge transfer as a result of effective surface state passivation. This work demonstrates a facile, low-cost, and efficient method for preparing highly photoactive TiO2 nanowire electrodes for PEC water oxidation. This approach could also potentially be used for other photoconversion applications, such as TiO2 based dye-sensitized solar cells, as well as photocatalytic systems.
Yolk-shell nanostructures as an emerging photocatalyst paradigm for solar hydrogen generation
(2019-08-01)
Chiu, Yi-Hsuan
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Naghadeh, Sara Bonabi
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Lindley, Sarah A.
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Lai, Ting-Hsuan
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Kuo, Ming-Yu
With porous shells and mobile cores, yolk-shell nanostructures provide great structural advantage for mass transport-related applications such as photocatalysis. In this work, Au-Cu7S4 yolk-shell nanostructures are synthesized from Au-Cu2O core-shell templates. The Cu7S4 shell is then converted to CdS through a cation exchange process to produce Au-CdS yolk-shell photocatalysts for hydrogen generation. Ultrafast transient absorption and finite-difference time-domain simulation are used to investigate electronic interaction between Au nanoparticle core and the surrounding CdS shell. Additionally, a new method is presented to simulate chemical transport and quantitatively compare diffusion kinetics by monitoring mass transport through the porous CdS shell with dye molecules as optical probes. The highest hydrogen generation rate of 3390 mu mol g(-1) h(-1), corresponding to an adequate apparent quantum yield of 4.22% at 420 nm, is achieved for Au-CdS with the largest void size. The enhancement in photocatalytic performance with increase in void size is mostly attributed to improved mass transport kinetics, with additional gains from more efficient charge transfer and stronger surface plasmon resonance-mediated near-field effects. This comprehensive study demonstrates that void size is a critical structural parameter in optimizing the performance of yolk-shell nanostructures for photocatalysis or other mass-transport related applications.