J. F. Lee

National Synchrotron Radiation Research Center (NSRRC), Hsin-chu-hsien, Taiwan, Taiwan

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Publications (136)347.26 Total impact

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    ABSTRACT: We study the magnetic anisotropic properties of as-grown and annealed Pd/Co/Pd trilayer films based on their atomic and electronic structures using extended x-ray absorption fine structure (EXAFS), x-ray absorption near edge structure (XANES) spectroscopy and magnetic circular dichroism (XMCD) measurements. The annealed film exhibits interesting perpendicular magnetic anisotropy (PMA) whereas as-grown film exhibits in-plane anisotropy. Cross-sectional transmission electron microscopic analysis together with the Co K-edge EXAF results confirm the formation of ordered-alloy CoPd phase in the annealed film, whereas the as-grown film has hcp Co-like phase. Co L3,2-edge XMCD measurements reveal an enhanced ratio of Co 3d orbital to spin moments of the annealed film providing evidence of the observed PMA upon annealing.
    RSC Advances 02/2015; 5(25). DOI:10.1039/C4RA15683H,
  • Journal of The Electrochemical Society 01/2015; 162(5):A5106-A5114. DOI:10.1149/2.0171505jes
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    ABSTRACT: Short-range-order and long-range-order structures in Ge1 − xSnx /Ge thin films grown by molecular beam epitaxy (MBE) were investigated by using extended x-ray absorption fine structure (EXAFS) and x-ray diffraction (XRD) techniques, respectively. These materials are of great potential for constructing efficient optoelectronic devices. The EXAFS analysis demonstrates that Sn atoms occupy Ge sites in these thin-film samples with Sn concentration up to 20 at.%. The Ge-Sn bonds expected in the substitutional model were also observed in Raman spectra of these samples. The XRD results show that, in the out-of-plane direction, the lattice constants of the films are distinctly larger than that of the Ge substrates. However, such increased lattice parameters were not observed in the in-plane direction. Our x-ray and Raman results have clearly revealed substitutional incorporation of Sn with high concentration in dislocation-free MBE-grown Ge films of practical-device thickness.
    Semiconductor Science and Technology 09/2014; 29(11):115008. DOI:10.1088/0268-1242/29/11/115008
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    ABSTRACT: The changes of physical properties occurring at a first-order-like transition among a wide family of (Pr1−yLny)1−xCaxCoO3 compounds (Ln being a lanthanide) are controversially discussed as a transition of Co3+ ions from states of low spin (LS) to intermediate spin (IS), or from LS to a mixture of LS and high spin (HS). The debate also includes the nature of the spin state of Co4+, as well as the degree of valence shift between Pr3+ and Pr4+ which accompanies this transition. In the present paper, we investigated the evolution of spin and valence states of Co ions and the valence state of Pr ions in (Pr0.7Sm0.3)0.7Ca0.3CoO3, at high temperature up to 750 K and under pressure up to 36.5 GPa. For this purpose, the data of three spectroscopic techniques were combined: high-resolution Co K-edge partial-fluorescence-yield x-ray absorption spectra, Pr L2-edge x-ray absorption spectra, and Co Kβ x-ray emission spectra. The spectral weight transfer of the pre-edge peaks of the Co K-edge spectra reveals a continuous redistribution of 3d electrons between the t2g and eg levels of Co ions, reflecting a gradual increase of the average Co spin state upon heating. This is further confirmed by Co Kβ x-ray emission spectra, which show an increase of HS Co3+ population when increasing temperature from 300 to 750 K. Applying high pressure at 300 K, the Co Kβ x-ray emission spectra indicate a relatively sharp increase of LS Co3+ population below 4 GPa. Pr L2-edge x-ray absorption spectra of (Pr0.7Sm0.3)0.7Ca0.3CoO3 exhibit a valence-state transition from Pr3+ to Pr3.26+, occurring predominantly upon applying pressure in the range ∼4 to ∼12 GPa. Owing to the charge balance, increasing pressure in this regime induces complex valence and spin-state transitions within the Co ions. Moreover, at high enough pressure, it is found that the Co4+ can also undergo a spin-state transition (from IS to LS), in addition to that affecting the Co3+ (HS to LS).
    Physical Review B 07/2014; 90(3):035107. DOI:10.1103/PhysRevB.90.035107
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    ABSTRACT: The electronic and magnetic properties of tetravalent-ion-doped La0.85Zr0.15MnO3 (LZMO) thin films that were epitaxially grown on SrTiO3 (STO) and MgO substrates were studied using temperature-dependent x-ray diffraction (XRD), x-ray absorption near-edge structure, x-ray linear dichroism, and x-ray magnetic circular dichroism at the Mn L3,2- and K-edge. XRD studies reveal that the LZMO thin films have compressive and tensile strains (along the c-axis) on the STO and MgO substrates, respectively. As the temperature is reduced from room temperature to below magnetic transition temperature, the preferentially occupied Mn majority-spin eg orbital changes from the in-plane dx2-y2 to the out-of-plane d3z2-r2 orbital for LZMO/STO, and vice versa for LZMO/MgO. Experimental results suggest that the new hopping path that is mediated by the Mn2+ ions triggers a stronger d3z2-r2 orbital ordering of Mn3+ ions and enhances the ferromagnetic coupling between the Mn spin moments of t2g electrons in LZMO/STO, whereas the strong tensile strain stabilizes the dx2-y2 orbital by inducing lattice distortions of the MnO6 octahedra in LZMO/MgO.
    Journal of Applied Physics 06/2014; 115(23):233713-233713-9. DOI:10.1063/1.4884230
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    ABSTRACT: We report the effects of Ge partial substitution for Si on local atomic and electronic structures of thermoelectric materials in binary compound cobalt monosilicides (). Correlations between local atomic/electronic structure and thermoelectric properties are investigated by means of X-ray absorption spectroscopy. The spectroscopic results indicate that as Ge is partially substituted onto Si sites at , Co in CoSi1−xGex gains a certain amount of charge in its 3d orbitals. Contrarily, upon further replacing Si with Ge at , the Co 3d orbitals start to lose some of their charge. Notably, thermopower is strongly correlated with charge redistribution in the Co 3d orbital, and the observed charge transfer between Ge and Co is responsible for the variation of Co 3d occupancy number. In addition to Seebeck coefficient, which can be modified by tailoring the Co 3d states, local lattice disorder may also be beneficial in enhancing the thermoelectric properties. Extended X-ray absorption fine structure spectrum results further demonstrate that the lattice phonons can be enhanced by Ge doping, which results in the formation of the disordered Co-Co pair. Improvements in the thermoelectric properties are interpreted based on the variation of local atomic and electronic structure induced by lattice distortion through chemical substitution.
    EPL (Europhysics Letters) 05/2014; 106(3):37007. DOI:10.1209/0295-5075/106/37007
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    ABSTRACT: A higher capacity and better cyclability are apparent when magnesium is introduced into the structure of LiCoO2 (y = 0.15). XRD analysis of LiMgyCo1−yO2 (y = 0, 0.1, 0.15), synthesized at 800 °C using a microwave assisted method, shows that the material is in the R-3m space group and to have a slightly expanded unit cell that increases with greater magnesium doping. Structural analysis by X-ray absorption spectroscopy (XAS) at the Co K-edge, L-edge and O K-edge shows that the magnesium is located in the transition metal layer rather than in the lithium layer and the charge balance results from the formation of oxygen vacancies rather than Co4+, while cobalt remains in the 3+ oxidation state. Interestingly, oxygen is found to participate in the charge compensation. Both magnesium, in the transition metal layer, and the Co-defect structure are attributed to the contribution towards structural stabilization of LiCoO2, thereby resulting in its enhanced electrochemical performance.
    Journal of Power Sources 04/2014; 252:292–297. DOI:10.1016/j.jpowsour.2013.10.130
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    ABSTRACT: Ruthenium (Ru = 0%, 1% and 2%) doped nano-crystalline zinc oxide (ZnO) nanorods were synthesized by using well-known sol–gel technique. X-ray diffraction (XRD) results show that Ru (0%, 1% and 2%) doped ZnO nanorods crystallized in the wurtzite structure having space group C3v (P63mc). Williamson and Hall plot reveal that in the nanoscale dimensions, incorporation of Ru induced the tensile strain in ZnO host matrix. Photoluminescence (PL) and Raman studies of Ru doped ZnO nanorods show the formation of singly ionized oxygen vacancies which may account for the observed room temperature ferromagnetism (RTFM) in 2% Ru doped ZnO. X-ray absorption spectroscopy (XAS) reveals that Ru replace the Zn atoms in the host lattice and maintain the crystal symmetry with slightly lattice distortion.
    Journal of Alloys and Compounds 03/2014; Volume 588:705-709.
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    ABSTRACT: The electronic structure of Nd1−xYxMnO3 (x=0–0.5) is studied using x-ray absorption near-edge structure (XANES) spectroscopy at the Mn K-edge along with the DFT-based LSDA+U and real space cluster calculations. The main edge of the spectra does not show any variation with doping. The pre-edge shows two distinct features which appear well-separated with doping. The intensity of the pre-edge decreases with doping. The theoretical XANES were calculated using real space multiple scattering methods which reproduces the entire experimental spectra at the main edge as well as the pre-edge. Density functional theory calculations are used to obtain the Mn 4p, Mn 3d and O 2p density of states. For x=0, the site-projected density of states at 1.7 eV above Fermi energy shows a singular peak of unoccupied eg (spin-up) states which is hybridized Mn 4p and O 2p states. For x=0.5, this feature develops at a higher energy and is highly delocalized and overlaps with the 3d spin-down states which changes the pre-edge intensity. The Mn 4p DOS for both compositions, show considerable difference between the individual px, py and pz states. For x=0.5, there is a considerable change in the 4p orbital polarization suggesting changes in the Jahn–Teller effect with doping.
    Solid State Communications 03/2014; 181:50–53. DOI:10.1016/j.ssc.2013.11.015
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    ABSTRACT: We present the orbital structure of MnTiO3 with polarization dependent x-ray absorption and resonant x-ray emission spectra accompanied with electronic structure calculations. The results clearly indicate a strongly anisotropic O 2p-Mn 3d orbital hybridization whereas the Mn 3d hole state shows a highly delocalized characteristic ascribed to the 3d-4p mixing. The extended Mn 4p orbital could enhance the exchange interaction between Mn (3d)-O (2p)-Mn (3d) leading to an asymmetric charge distribution in Mn-O bonds. The delocalized characteristic of Mn 3d holes is indispensable to the mechanism of spin-dependent-metal-ligand hybridization to explain magnetically induced ferroelectricity.
    Applied Physics Letters 02/2014; 104(8):082104-082104-5. DOI:10.1063/1.4866462
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    ABSTRACT: The in-situ X-ray absorption spectroscopy of three tungsten oxide films was performed to study the electronic and atomic structures following repeated cycles of coloration and bleaching processes. The transparent tungsten oxide films become deep blue upon intercalation of Li+ ions in the WO6 octahedra when an external electrical bias was applied. These films reverted to transparent when a reverse external electrical bias was applied. W L3-edge X-ray absorption near-edge structure (XANES) measurements of the nanocrystalline and crystalline tungsten oxide films revealed that the intensity of the white-line feature decreases after coloration and recoverably increases after bleaching owing to the filling and unfilling of the W 5d-O 2p conduction band states. The second derivative of the W L3-edge XANES spectra indicated an increase in structural disordering following repeated cycles of coloration and bleaching. However, the extended X-ray absorption fine structure analysis showed that the nearest-neighbor W-O bond distances in the samples overall remain unchanged by coloration and bleaching. The nanocrystalline tungsten oxide film exhibited more effective recovery (97% after first cycle) of the electronic structures than the other two crystalline samples in terms of the filling and unfilling of the W 5d-O 2p conduction band states after repeated coloration and bleaching. These results show that the nanocrystalline tungsten oxide sample has superior electrochromic properties to the crystalline samples.
    RSC Advances 01/2014; 4(10):5036. DOI:10.1039/c3ra45421e
  • Chemistry - An Asian Journal 01/2014;
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    ABSTRACT: Ni doped ZnS (Zn1−xNixS) nanoparticles with Ni concentration x = 0.01, 0.03 and 0.05 were synthesized by the chemical co-precipitation technique. Powder X-ray diffraction (XRD) reveals that Ni incorporates into the ZnS crystal lattice with a cubic structure. Transmission electron microscopy (TEM) measurements show that the average size of these nanoparticles is in the range 2–3 nm. Micro-Raman and photololuminescence (PL) studies also confirm the incorporation of Ni in the lattice. X-ray absorption spectroscopy (XAS) reveals that Ni replace the Zn atoms in the host lattice without forming second phases. The ZnS nanoparticles with lower Ni concentration exhibit saturated hysteresis loop at room temperature indicating ferromagnetism where as samples having higher Ni concentrations did not show such saturation confirming the intrinsic nature of the ferromagnetism at smaller doping.
    Journal of Alloys and Compounds 12/2013; 554:357-362. DOI:10.1016/j.jallcom.2012.12.001
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    ABSTRACT: Co and N co-doped ZnO nanopowders with Co and N concentration 4, 6, and 8 mol% were synthesized by sol–gel method. Powder X-ray diffraction reveals that Co and N co-doped ZnO crystallize in wurtzite structure having space group C6v. Photo-luminescence studies show the reduction in band gap with increase in concentration of dopants. Micro Raman studies show the red shift for 1LO phonon peak with increase in doping, generally credited to the stress caused by lattice mismatch due to N doping in ZnO. X-ray absorption spectroscopy reveals that Co replaces the Zn atoms and N replace the O atoms in the host ZnO lattice. Magnetic studies show that Co and N codoped ZnO nanopowders exhibit ferromagnetic character at room temperature.
    Journal of Materials Science 12/2013; 48(6):2618-2623. DOI:10.1007/s10853-012-7055-1
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    ABSTRACT: Ruthenium (Ru = 0%, 1% and 2%) doped nano-crystalline zinc oxide (ZnO) nanorods were synthesized by using well-known sol gel technique. X-ray diffraction (XRD) results show that Ru (0%, 1% and 2%) doped ZnO nanorods crystallized in the wurtzite structure having space group C3v (P63mc). Williamson and Hall plot reveal that in the nanoscale dimensions, incorporation of Ru induced the tensile strain in ZnO host matrix. Photoluminescence (PL) and Raman studies of Ru doped ZnO nanorods show the formation of singly ionized oxygen vacancies which may account for the observed room temperature ferromagnetism (RTFM) in 2% Ru doped ZnO. X-ray absorption spectroscopy (XAS) reveals that Ru replace the Zn atoms in the host lattice and maintain the crystal symmetry with slightly lattice distortion.
    Journal of Alloys and Compounds 12/2013; DOI:10.1016/j.jallcom.2013.11.137
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    ABSTRACT: Ruthenium (Ru = 0%, 1% and 2%) doped nano-crystalline zinc oxide (ZnO) nanorods were synthesized by using well-known sol gel technique. X-ray diffraction (XRD) results show that Ru (0%, 1% and 2%) doped ZnO nanorods crystallized in the wurtzite structure having space group C3v (P63mc). Williamson and Hall plot reveal that in the nanoscale dimensions, incorporation of Ru induced the tensile strain in ZnO host matrix. Photoluminescence (PL) and Raman studies of Ru doped ZnO nanorods show the formation of singly ionized oxygen vacancies which may account for the observed room temperature ferromagnetism (RTFM) in 2% Ru doped ZnO. X-ray absorption spectroscopy (XAS) reveals that Ru replace the Zn atoms in the host lattice and maintain the crystal symmetry with slightly lattice distortion.
    Journal of Alloys and Compounds 11/2013;
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    ABSTRACT: 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.
    Thin Solid Films 10/2013; 544. DOI:10.1016/j.tsf.2013.02.083
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    ABSTRACT: In this study, X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray magnetic circular dichroism (XMCD) and element- and site-specific magnetic hysteresis (ESMH) are used to elucidate the effect of geometry (0-3- and 2-2-type) on the magnetic properties of CoFe2O4–PbTiO3 (CFO–PTO) multiferroic composites by comparison with those of the reference CFO and PTO powders. Magnetic Co ions in CFO have been confirmed to be located at both the tetrahedral (A)- and octahedral (B)-sites. CFO retains its mixed-spinel structure as verified by the EXAFS, XMCD and ESMH measurements. ESMH measurements further demonstrate that the magnetic moments of Co2+ and Fe3+/Fe2+ cations at both the A- and B-sites in the composites are smaller than those of the CFO powder. The reduction of the magnetic moments in the 2-2-type composite was larger than that in the 0-3-type composite. The reduction of the magnetic moments in the composites was attributable to the formation of anti-phase boundaries owing to the compressive strain in CFO, which is the largest strain in the 2-2-type composite. Based on the Ti L3,2-edge XMCD measurements of the CFO–PTO composites, no induced magnetic moment was observed at the Ti sites in the PTO matrix, excluding the possibility that the Ti ions in the PTO matrix affect the magnetic properties of these CFO–PTO composites.
    RSC Advances 05/2013; 3(21):7884-7893. DOI:10.1039/C3RA00104K
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    ABSTRACT: The correlation between the p-type hole conduction and the electronic structures of Cr-deficient CuCr1−xO2 (x=0−0.1) compounds was investigated using O K-, Cu, and Cr L3,2-edge x-ray absorption near-edge structure (XANES), scanning photoelectron microscopy, and x-ray emission spectroscopy measurements. XANES spectra reveal a gradual increase in the Cu valence from Cu1+ to Cu2+ with increasing Cr deficiency x, whereas, the valence of Cr remains constant as Cr3+. These results indicate that the p-type conductivity in the CuCr1−xO2 samples is enhanced by a Cu1+-O-Cu2+ rather than a Cr3+-Cr4+ or direct Cu1+-Cu2+ hole mechanism. Remarkable Cr-deficiency-induced changes in the densities of Cu 3d, Cu 3d-O 2p, and O 2p states at or near the valence-band maximum or the Fermi level were also observed. In addition, a crossover of conduction mechanism from thermally activated (TA) hopping to a combination of TA and Mott's three-dimensional variable range hopping occurs around 250 K.
    Physical Review B 12/2012; 86:241103. DOI:10.1103/PhysRevB.86.241103

Publication Stats

1k Citations
347.26 Total Impact Points

Institutions

  • 1997–2015
    • National Synchrotron Radiation Research Center (NSRRC)
      Hsin-chu-hsien, Taiwan, Taiwan
  • 2011
    • National Chiao Tung University
      • Department of Electrophysics
      Hsin-chu-hsien, Taiwan, Taiwan
  • 2002–2007
    • National Taiwan University of Science and Technology
      • • Department of Electronic and Computer Engineering
      • • Department of Chemical Engineering
      T’ai-pei, Taipei, Taiwan
  • 2005
    • Massachusetts Institute of Technology
      • Department of Materials Science and Engineering
      Cambridge, Massachusetts, United States
  • 2001
    • National Tsing Hua University
      • Department of Engineering and System Science
      Hsin-chu-hsien, Taiwan, Taiwan