Browsing by Author "Lin, Yan-Cheng"

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Atomic origin of the spin-polarization of the Co2FeAl Heusler compound
(2016-02-25)
Liang, Jaw-Yeu
;
Lam, Tu-Ngoc
;
Lin, Yan-Cheng
;
Chang, Shu-Jui
;
Lin, Hong-Ji
Using synchrotron x-ray techniques, we studied the Co2FeAl spin-polarization state that generates the half-metallicity of the compound during an A(2) (low-spin) -> B-2 (high-spin) phase transition. Given the advantage of element specificity of x-ray techniques, we could fingerprint the structural and magnetic cross-reactions between Co and Fe within a complex Co2FeAl structure deposited on a MgO (0 0 1) substrate. X-ray diffraction and extended x-ray absorption fine structure investigations determined that the Co atoms preferably populate the (1/4,1/4,1/4) and (3/4,3/4,3/4) sites during the development of the B-2 phase. X-ray magnetic spectroscopy showed that although the two magnetic elements were ferromagnetically coupled, they interacted in a competing manner via a charge-transfer effect, which enhanced Co spin polarization at the expense of Fe spin polarization during the phase transition. This means that the spin-polarization of Co2FeAl was electronically dominated by Fe in A(2) whereas the charge transfer turned the dominance to Co upon B-2 formation. Helicity-dependent x-ray absorption spectra also revealed that only the minority state of Co/Fe was involved in the charge-transfer effect whereas the majority state was independent of it. Despite an overall increase of Co2FeAl magnetization, the charge-transfer effect created an undesired trade-off during the Co-Fe exchange interactions, because of the presence of twice as many X sites (Co) as Y sites (Fe) in the Heusler X(2)YZ formula. This suggests that the spin-polarization of Co2FeAl is unfortunately regulated by compromising the enhanced X (Co) sites and the suppressed Y (Fe) sites, irrespective of the development of the previously known high-spin-polarization phase of B-2. This finding provides a possible cause for the limited half-metallicity of Co2FeAl discovered recently. Electronic tuning between the X and Y sites is necessary to further increase the spin-polarization, and likely the half-metallicity as well, of the compound.
Carrier dynamics in dilute II-VI oxide highly mismatched alloys
(2014)
Lin, Yan-Cheng
;
Chou, Wu-Ching
;
Chyi, Jen-Inn
;
Tanaka, Tooru
;
電子物理學系
This study explores comprehensively the carrier dynamics in ZnSeO and ZnTeO using photoluminescence (PL) and time-resolved PL spectroscopy. As the O concentration increases, the PL emissions shift toward lower energies. Additionally, the PL lifetime increases with increasing O contents and the decay curves exhibit complex behavior. In the case of ZnSeO, the mechanism of carrier recombination undergoes a complicated change from trapped to free excitons with the increase in temperature. The incorporation of O in ZnTe generates a wide distribution of electron localization below the energy of the E- conduction subband, and these cause broad PL emission and serve as another intermediate band. Electrons in both the E+ and the E- conduction subbands favor rapid relaxation to low energy states. Moreover, temperature-independent long carrier lifetimes (> 130.0 ns) that are induced by localized electrons increase with O concentration.
Carrier generation and recombination dynamics in type-II ZnSeTe/ZnMnSe quantum structures
(2015-07-31)
Lin, Yan-Cheng
;
Chou, Wu-Ching
;
電子物理學系
;
Department of Electrophysics
Type-II band alignment structure is coveted in the design of photovoltaic devices, since it is beneficial for the transport of photogenerated carriers. Here we study the generation and recombination dynamics of carriers in a type-II quantum structure composed of ZnSe0.92Te0.08 highly mismatched alloys (HMAs) and Zn0.97Mn0.03Se. The photoinduced holes at the ZnSe0.92Te0.08 HMAs firstly undergo rapid relaxation to the isoelectronic centers above the valence band edge and subsequently recombine with the free electrons in the Zn0.97Mn0.03Se. The long carrier lifetimes over 120 ns induced by spatially indirect excitons that are bound to isoelectronic Te trapping states further increase with increasing temperature.
CdxHg(1x)Te Alloy Colloidal Quantum Dots: Tuning Optical Properties from the Visible to Near-Infrared by Ion Exchange
(2013-04-01)
Gupta, Shuchi
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Zhovtiuk, Olga
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Vaneski, Aleksandar
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Lin, Yan-Cheng
;
Chou, Wu-Ching
The energy gap between valence and conduction levels in colloidal semiconductor quantum dots can be tuned via the nanoparticle diameter when this is comparable to or less than the Bohr radius. In materials such as cadmium mercury telluride, which readily forms a single phase ternary alloy, this quantum confinement tuning can also be augmented by compositional tuning, which brings a further degree of freedom in the bandgap engineering. Here it is shown that compositional control of 2.3 nm diameter CdxHg(1x)Te nanocrystals by exchange of Hg2+ in place of Cd2+ ions can be used to tune their optical properties across a technologically useful range, from 500 nm to almost 1200 nm. Data on composition-dependent changes in the optical properties are provided, including bandgap, extinction coefficient, emission energy and spectral shape, Stokes shift, quantum efficiency, and radiative lifetimes as the exchange process occurs, which are highly relevant for those seeking to use these technologically important QD materials.
Gold nanoparticles: BSA (Bovine Serum Albumin) coating and X-ray irradiation produce variable-spectrum photoluminescence
(2015-01-15)
Lee, Kuo-Hao
;
Lai, Sheng-Feng
;
Lin, Yan-Cheng
;
Chou, Wu-Ching
;
Ong, Edwin B. L.
We show that by using different x-ray irradiation times of BSA-coated Au nanoparticles (NPs) we can change their ultraviolet-stimulated photoluminescence and shift the spectral weight over the visible spectral range. This is due to the interplay of two emission bands, one due to BSA and the other related to gold. The emission properties did not change with time over a period of several months. (C) 2014 Published by Elsevier B.V.
Heteroepitaxial approach to explore charge dynamics across Au/BiVO4 interface for photoactivity enhancement
(2015-07-01)
Chien Nguyen Van
;
Chang, Wei Sea
;
Chen, Jhih-Wei
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Tsai, Kai-An
;
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.
Photoluminescence and time-resolved carrier dynamics in thiol-capped CdTe nanocrystals under high pressure
(2013)
Lin, Yan-Cheng
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Chou, Wu-Ching
;
Susha, Andrei S.
;
Kershaw, Stephen V.
;
Rogach, Andrey L.
The application of static high pressure provides a method for precisely controlling and investigating many fundamental and unique properties of semiconductor nanocrystals (NCs). This study systematically investigates the high-pressure photoluminescence (PL) and time-resolved carrier dynamics of thiol-capped CdTe NCs of different sizes, at different concentrations, and in various stress environments. The zincblende-to-rocksalt phase transition in thiol-capped CdTe NCs is observed at a pressure far in excess of the bulk phase transition pressure. Additionally, the process of transformation depends strongly on NC size, and the phase transition pressure increases with NC size. These peculiar phenomena are attributed to the distinctive bonding of thiols to the NC surface. In a nonhydrostatic environment, considerable flattening of the PL energy of CdTe NC powder is observed above 3.0 GPa. Furthermore, asymmetric and double-peak PL emissions are obtained from a concentrated solution of CdTe NCs under hydrostatic pressure, implying the feasibility of pressure-induced interparticle coupling.
Recombination dynamics and carrier lifetimes in highly mismatched ZnTeO alloys
(2013-12-23)
Lin, Yan-Cheng
;
Tasi, Ming-Jui
;
Chou, Wu-Ching
;
Chang, Wen-Hao
;
Chen, Wei-Kuo
This study investigates the recombination dynamics in highly mismatched ZnTeO alloys using time-resolved photoluminescence (PL) spectroscopy. The large PL energy redshift with increasing O content and the disappearance of the ZnTe emission verify the O-induced conduction band anticrossing effect. The incorporation of O generates electron localization below the E_ conduction subband tail, which provide additional optical transitions and cause complex recombination mechanisms. Photoexcited free electrons in both the E+ and the E- conduction subbands favor rapid relaxation to low energy states. Additionally, temperature-independent long carrier lifetimes (>130.0 ns) that are induced by localized electrons increase with O concentration. (C) 2013 AIP Publishing LLC.
Superior photoelectrochemical activity of self-assembled NiWO4-WO3 heteroepitaxy
(2016-05)
Thi Hien Do
;
Chien Nguyen Van
;
Tsai, Kai-An
;
Le Thi Quynh
;
Chen, Jhih-Wei
Self-assembled nanocomposites with a high interface-to-volume ratio offer tremendous opportunities of tailoring functionalities by a proper combination of components. In this study, we report the first fabrication of self-assembled NiWO4-WO3 heteroepitaxy and its excellent performance on photoel-ectrochemical water splitting. The heteroepitaxy is evidenced by a combination of x-ray diffraction and transmission electron microscopy. The energy band diagram of NiWO4-WO3 heterostructure probed by x-ray photoelectron spectroscopy suggests an efficient charge separation at their interface by injecting photo -generated electrons from NiWO4 to WO3 and holes from WO3 to NiWO4. Therefore, a great improvement in the photoelectrochemical activity of self -assembled NiWO4-WO3 heteroepitaxy was achieved (1400 mu A cm(-2) at 0.7 V vs. Ag/AgCl). This work delivers a new category of composite materials for promising photoelectrode in photoelectrochemical cells. (C) 2016 Elsevier Ltd. All rights reserved.
Temperature-dependent decay dynamics in highly mismatched ZnSe1-xTex alloy
(2012-02-13)
Lin, Yan-Cheng
;
Jiang, Wei-Shi
;
Chou, Wu-Ching
;
Chen, Wei-Kuo
;
Chang, Wen-Hao
This study investigates the temperature-dependent decay dynamics in highly mismatched ZnSe0.950Te0.050 alloy using photoluminescence (PL) and time-resolved PL spectroscopy. The PL peak energy exhibits a V-shaped dependence on temperature (10-300 K), indicating strong carrier localization. Kohlrausch's stretched exponential law, in which the deduced stretching exponent beta is highly consistent with the V-shaped PL peak shift, closely corresponds to the complex decay curves over a wide temperature range. Additionally, the PL lifetime tau initially increases and then monotonically declines as the temperature increases. These findings agree excellently with the low electron-hole binding energy upon thermal ionization of weakly bound electrons. (C) 2012 American Institute of Physics. [doi:10.1063/1.3687187]
Water-Soluble CdTe Nanocrystals under High Pressure
(2015-01-01)
Lin, Yan-Cheng
;
電子物理學系
;
Department of Electrophysics
The application of static high pressure provides a method for precisely controlling and investigating many fundamental and unique properties of semiconductor nanocrystals (NCs). This study systematically investigates the high-pressure photoluminescence (PL) and time-resolved carrier dynamics of thiol-capped CdTe NCs of different sizes, at different concentrations, and in various stress environments. The zincblende-to-rocksalt phase transition in thiol-capped CdTe NCs is observed at a pressure far in excess of the bulk phase transition pressure. Additionally, the process of transformation depends strongly on NC size, and the phase transition pressure increases with NC size. These peculiar phenomena are attributed to the distinctive bonding of thiols to the NC surface. In a nonhydrostatic environment, considerable flattening of the PL energy of CdTe NCs powder is observed above 3.0 GPa. Furthermore, asymmetric and double-peak PL emissions are obtained from a concentrated solution of CdTe NCs under hydrostatic pressure, implying the feasibility of pressure-induced interparticle coupling.
WO3 mesocrystal-assisted photoelectrochemical activity of BiVO4
(2017-03-01)
Chien Nguyen Van
;
Thi Hien Do
;
Chen, Jhih-Wei
;
Tzeng, Wen-Yen
;
Tsai, Kai-An
Self- assembled nanocomposites have gained much attention over the past decade due to their intriguing properties and functionalities. In this work, we developed a self-assembled nanocomposite photoanode composed of an epitaxial BiVO4 matrix embedded with WO3 mesocrystals for photoelectrochemical (PEC) applications in the visible- light regime. The orientation of the crystal facet and interface provides a superior template to understand the intimate contact between the two constituent phases. We demonstrate that the interfacial coupling of the mesocrystal and matrix improves the separation of photoexcited carriers and the properties of charge transfer, resulting in a greatly enhanced PEC performance compared with their parent compounds. The current study demonstrates that the utilization of the interface-to-volume ratio to optimize charge interactions in the nanocomposite is essential for the advanced design of novel mesocrystal-embedded nanocomposite photoelectrodes.