Browsing by Author "Pao, Chih-Wen"

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Atomistic nucleation sites of Pt nanoparticles on N-doped carbon nanotubes
(2013)
Sun, Chia-Liang
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Pao, Chih-Wen
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Tsai, Huang-Ming
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Chiou, Jau-Wern
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Ray, Sekhar C.
The atomistic nucleation sites of Pt nanoparticles (Pt NPs) on N-doped carbon nanotubes (N-CNTs) were investigated using C and N K-edge and Pt L-3-edge X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure (EXAFS) spectroscopy. Transmission electron microscopy and XANES/EXAFS results revealed that the self-organized Pt NPs on N-CNTs are uniformly distributed because of the relatively high binding energies of the adsorbed Pt atoms at the imperfect sites. During the atomistic nucleation process of Pt NPs on N-CNTs, stable Pt-C and Pt-N bonds are presumably formed, and charge transfer occurs at the surface/interface of the N-CNTs. The findings in this study were consistent with density functional theory calculations performed using cluster models for the undoped, substitutional-N-doped and pyridine-like-N-doped CNTs.
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
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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.
Defect induced ferromagnetic ordering in epitaxial Zn0.95Mn0.05O films on sapphire (0001)
(2019-12-04)
Liu, Wei-Rein
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Pao, Chih-Wen
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Yu, Doris Weijyun
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Chin, Yi-Ying
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Wu, Wen-Bin
We report the influence of Mn dopant on magnetic properties of Zn0.95Mn0.05O (ZMO)/Al2O3(0001) hetero-epitaxial systems grown by using pulsed-laser deposition. The room temperature (RT) intrinsic ferromagnetic (FM) ordering verified by superconducting quantum interference device magnetometer and x-ray magnetic circular dichroism spectrum of Mn L-2(,3) edges is ascribed to the substitutional Mn atoms in the Zn site of ZnO. Mn in ZMO has a tetrahedral local symmetry instead of the octahedral symmetry of MnO, after verifying the absence of the Mn-related impurities or clusters in ZMO epitaxial film by Mn K-edge and Zn K-edge x-ray absorption spectroscopy spectrum, as well as the analysis of long-range structural ordering on Renninger scan of forbidden (0005) reflection in x-ray diffraction, transmission electron microscopy and Raman spectrum. Comparison of x-ray absorption spectra of ZMO with those of ZnO epilayers at O K-, Zn K-, and L-3-edges indicates that the substitution of the Zn site with Mn enhances the charge-transfer (CT) transition and the presence of Zn vacancies (V-Zn) also dominate the photoluminescence (PL) spectrum, implying that the formation of numerous V-Zn defects plays an important role in activating FM interactions. The strong CT effect and the existence of high-density V-Zn suggest that the intrinsic RT FM ordering of insulating ZMO is a result of the formation of the bound magnetic polarons (BMPs) that interact with each other via intermediate magnetic impurities.
Effects of oxygen partial pressure on structural and gasochromic properties of sputtered VOx thin films
(2013-10-01)
Jang, Wei-Luen
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Lu, Yang-Ming
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Lu, Ying-Rui
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Chen, Chi-Liang
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Dong, Chung-Li
VOx films were deposited by radio-frequency reactive magnetron sputtering from a vanadium target in an Ar-O-2 gas mixture and pure O-2. For the films deposited in the gas mixture, the Ar flow rate was controlled at 20 sccm and the oxygen flow rate was controlled at 1, 3, and 5 sccm, respectively. A thin (similar to 5 nm) Pt layer was deposited on the VOx thin films as a hydrogen catalyst. The long-range structural order, short-range atom arrangement, and gasochromic properties of the deposited films were studied. The grazing incidence X-ray diffraction (GIXRD) results indicate that the deposited films are amorphous. Lamellar structures were found at oxygen flow rates of 3 sccm and above. The X-ray absorption spectroscopy (XAS) results show that the short-range atom arrangement of the lamellar VOx thin films is similar to that of crystal V2O5. The GIXRD and XAS results show that the film obtained with the gas mixture and at an oxygen flow rate of 1 sccm did not significantly change after exposure to hydrogen, whereas the other films exhibited decreased interlayer distance, oxidation state, and crystallinity. The color of the films changed from light or deep yellow to gray. The results suggest that the gasochromic properties of the VOx thin films are related to the V2O5-like atom arrangement and the interlayer distance of the lamellar structure. The films deposited with an oxygen flow rate of 3 sccm and above can be applied to H-2 gas sensors. (C) 2013 Elsevier B. V. All rights reserved.
Interface Engineering Strategy Utilizing Electrochemical ALD of Cu-Zn for Enabling Metallization of Sub-10 nm Semiconductor Device Nodes
(2019-08-28)
Joi, Aniruddha
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Venkatraman, Kailash
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Tso, Kuang-Chih
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Dictus, Dries
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Dordi, Yezdi
A novel interface engineering approach, utilizing electrochemical atomic layer deposition (e-ALD) of Cu(Zn) on a Ru liner, is presented for enabling the metallization of sub-10 nm interconnects in future semiconductor devices. Upon thermal treatment, Zn present in the e-ALD Cu layer at similar to 0.8 at.% is shown to diffuse through the Ru liner and react with the SiO2 to form a Zn-silicate layer at the Ru-SiO2 interface. This 'self-forming' interfacial layer provides adhesion enhancement to the Ru-SiO2 interface and serves as a diffusion barrier retarding Cu diffusion into the SiO2 layer while enabling void-free gap-filling of high aspect ratio trench structures. Absorption Near-Edge Spectroscopy and Extended X-ray Absorption Fine Structure analyses confirm that the self-formed barrier layer is composed primarily of Zn2SiO4. The interface engineering approach utilizing Cu(Zn) presented herein offers several potential advantages over traditional Mn-based self-forming barrier approaches, i.e., scalability to narrower dimensions and minimal impact to interconnect resistance. (C) The Author(s) 2019. Published by ECS.
Selective catalytic oxidation of benzene to phenol by a vanadium oxide nanorod (V-nr) catalyst in CH3CN using H2O2(aq) and pyrazine-2-carboxylic acid (PCA)
(2019-12-07)
Wanna, Wondemagegn Hailemichael
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Janmanchi, Damodar
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Thiyagarajan, Natarajan
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Ramu, Ravirala
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Tsai, Yi-Fang
Vanadium based catalysts are known to be highly efficient for oxidation reactions using O-2 and/or H2O2. In this study, we developed a vanadium catalyst that can be prepared by a reaction of VCl3 in acetonitrile with the addition of 35% hydrogen peroxide (H2O2). The catalyst was further characterized by a variety of methods including V-51 NMR, XRD, XPS, XAS, EA, ICP-AES, BET, SEM and TEM. Based on the results obtained, the catalyst was identified as a vanadium oxide material (V-nr) with the morphology and shape of a nanorod with dimensions of similar to 40 nm x 450 nm (width x length). The porosity of a material can be essential for small molecule activation on a heterogeneous surface. This material exhibits exceptional catalytic performance for the selective oxidation of benzene to phenol (PhOH) at room temperature using H2O2 as an oxidant in acetonitrile. The addition of pyrazine-2-carboxylic acid (PCA) as a co-catalyst can efficiently enhance the catalytic activity with a percentage selectivity of 87-99% phenol, producing p-benzoquinone (p-BQ) as a minor product. With the deployment of (H2O2)-O-18, we observed the product phenol to be highly enriched by O-18, indicating that the hydroperoxovanadium intermediate is essential for its catalytic conversion. Together with its high NIH shift ratio (80% deuterium remained) observed with the mechanistic probe of 4-[H-2(0,1)] toluene, the results suggested that the catalytic oxidation reaction mediated by Vnr may involve metal based oxygenated intermediates for its highly selective catalytic oxidation of benzene to phenol.