Browsing by Author "Lai, Chun-Yen"

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Correlation among photoluminescence and the electronic and atomic structures of Sr2SiO4:xEu(3+) phosphors: X-ray absorption and emission studies
(2020-07-29)
Zheng, Shi-Yan
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Chiou, Jau-Wern
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Li, Yueh-Han
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Yang, Cheng-Fu
;
Ray, Sekhar Chandra
A series of Eu3+-activated strontium silicate phosphors, Sr2SiO4:xEu(3+) (SSO:xEu(3+), x=1.0, 2.0 and 5.0%), were synthesized by a sol-gel method, and their crystalline structures, photoluminescence (PL) behaviors, electronic/atomic structures and bandgap properties were studied. The correlation among these characteristics was further established. X-ray powder diffraction analysis revealed the formation of mixed orthorhombic alpha'-SSO and monoclinic beta-SSO phases of the SSO:xEu(3+) phosphors. When SSO:xEu(3+) phosphors are excited under ultraviolet (UV) light (lambda=250 nm,similar to 4.96 eV), they emit yellow (similar to 590 nm), orange (similar to 613 nm) and red (similar to 652 and 703 nm) PL bands. These PL emissions typically correspond to 4f-4f electronic transitions that involve the multiple excited D-5(0)-> F-7(J) levels (J=1, 2, 3 and 4) of Eu3+ activators in the host matrix. This mechanism of PL in the SSO:xEu(3+) phosphors is strongly related to the local electronic/atomic structures of the Eu3+-O2- associations and the bandgap of the host lattice, as verified by Sr K-edge and Eu L-3-edge X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure, O K-edge XANES and K-alpha X-ray emission spectroscopy. In the synthesis of SSO:xEu(3+) phosphors, interstitial Eu2O3-like structures are observed in the host matrix that act as donors, providing electrons that are nonradiatively transferred from the Eu 5d and/or O 2p-Eu 4f/5d states (mostly the O 2p-Eu 5d states) to the D-5(0) levels, facilitating the recombination of electrons that have transitioned from the D-5(0) level to the F-7(J) level in the bandgap. This mechanism is primarily responsible for the enhancement of PL emissions in the SSO:xEu(3+) phosphors. This PL-related behavior indicates that SSO:xEu(3+) phosphors are good light-conversion phosphor candidates for use in near-UV chips and can be very effective in UV-based light-emitting diodes.
Intensify the application of ZnO-based nanodevices in humid environment: O-2/H-2 plasma suppressed the spontaneous reaction of amorphous ZnO nanowires
(2014-06-02)
Lai, Chun-Yen
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Chien, Tzu-Chiao
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Lin, Ting-Yi
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Ke, Teng
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Hsu, Shih-Han
In this work, we have demonstrated that amorphous ZnO nanobranches (a-ZnO NBs) could spontaneously react from the crystalline ZnO NWs (c-ZnO NWs) at specific humid environment. The spontaneous reaction mechanism and result can be analyzed by humidity controlling and optical microscope (OM)/scanning electron microscope (SEM)/Kelvin probe force microscopy (KPFM)/transmission electron microscopy (TEM) system. We can make the c-ZnO NWs spontaneous reaction happen at different humid environments and suppress the a-ZnO NBs spontaneous reaction by oxygen/hydrogen plasma surface passivation. The hydrogen plasma surface treatment also can improve the UV sensing sensitivity more than twofold. This work provides the mechanism and methods of the a-ZnO NBs spontaneous growth and offers the passivation treatment for strengthening and enhancing ZnO-based nanodevice application in humid environment and UV light detection, respectively.
Surface defect engineering: gigantic enhancement in the optical and gas detection ability of metal oxide sensor
(2016-01-01)
Wang, Wen-Chieh
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Lai, Chun-Yen
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Lin, Yu-Ting
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Yua, Tzu-Hsuan
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Chen, Zong-Yi
In this work, the detection ability of nanosensors can be improved extraordinarily via surface defect engineering. A kinked SnO2-X/SnO2 nanostructure was fabricated by tuning the oxygen flow, and this kinked SnO2-X/SnO2 nanostructure was used to study the mechanism of surface defect (oxygen vacancy, V-O) effects via electric measurements. For UV light sensing, the response of the SnO2-X NW device is always better than the SnO2 NW device, and is two orders higher under pure O-2 surrounding conditions. The detection mechanism can be clarified by changing the detection environment (oxygen concentration) and the UV light detection sensitivity can be improved by increasing the surface V-O density. Furthermore, the SnO2-X NW device is very sensitive to its surrounding environment due to the high surface V-O density. Hence, CO/O-2 alternate-detection was used to verify our hypothesis; the results show that the SnO2-X NW device presents great detection abilities, compared with the SnO2 NW device. The sensitivity of the SnO2-X NW device is two orders higher and the reset/response time is faster, compared with the SnO2 NW device. To verify this hypothesis, the polycrystalline structure was fabricated to prove that the detection ability of metal oxide nanosensors can be improved gigantically by increasing surface defect amounts.
Synthesizing highly conductive cobalt sulfide hydrangea macrophylla using long carbon-chain sulfur source for supercapacitors
(2015-01-01)
Chiu, Jun-Ming
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Lin, Lu-Yin
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Yeh, Ping-Hung
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Lai, Chun-Yen
;
Teng, Ke
Cobalt sulfide has attracted much attention as the electroactive material for the supercapacitor (SC) because of its high capacitance and cost-effective features. A novel three-dimensional cobalt sulfide hydrangea macrophylla nanostructure with a thin carbon layer growing on the surface has been successfully synthesized by using a simple one-pot method with the addition of 1-dodecanethiol as the sulfur source connected with long carbon chains. The cobalt sulfide hydrangea macrophylla architecture is composed of several two-dimensional (2D) nanopetals intertwined together and thin carbon layers surrounding the nanopetals. A high specific capacitance (CF) of 324.17 F g(-1) is obtained for the cobalt sulfide hydrangea macrophylla-based SC electrode measured by cyclic voltammetry (CV) at a scan rate of 10 mV s(-1), due to the large surface area for charge accumulation and faradic reactions, as well as the high conductivity for charge transportation respectively benefitting from the 2D nanopetals and the carbon layers. The high CF value achieved in this study opens a window to morphology design for realizing highly effective pseudocapacitors.
Thermoelectric property and x-ray absorption near edge structure studies on Si-doped CaMnO3-delta
(2016-02-15)
Liu, Chia-Jyi
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Bhaskar, Ankam
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Yuan, J. J.
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Yang, Zong-Ren
;
Chen, Shih-Show
We report electrical resistivity, thermopower and Ca K-, Mn L-2,L-3- and O K-edges x-ray absorption near edge structure (XANES) studies on CaMn1-xSixO3-delta with x=0, 0.02, and 0.05. The transport properties measurements indicate that the sample of x=0.02 has the lowest electrical resistivity compared to the samples of x=0 and 0.05. Meanwhile, the temperature dependence of thermopower behaves significantly different for samples of x=0.02 and 0.05. These unusual phenomena are elucidated based on the charge transfer equilibrium principle together with the XANES results, which show that the Si substitution for Mn appears to generate an electron doping effect and lattice distortion. Among the samples, CaMn0.98Si0.02O3-delta exhibits the highest figure of merit of 0.0128 at 300 K. This value represents an improvement of about 52% compared to pristine CaMnO3-delta. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.