J. F. Lee

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

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Publications (116)227.61 Total impact

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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. · 2.39 Impact Factor
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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 01/2014; 181:50–53. · 1.53 Impact Factor
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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 01/2014; 104(8):082104-082104-5. · 3.79 Impact Factor
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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 01/2014; 252:292–297. · 4.68 Impact Factor
  • Chemistry - An Asian Journal 01/2014; · 4.57 Impact Factor
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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; 48:2618-2623. · 2.16 Impact Factor
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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. · 2.39 Impact Factor
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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; · 2.39 Impact Factor
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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. · 2.56 Impact Factor
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    Thin Solid Films 01/2013; · 1.60 Impact Factor
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    ABSTRACT: Defect engineering techniques have been employed to generate and remove oxygen vacancy defects in nanoparticles of Y2O3:Mn diluted magnetic oxide (DMO). These samples were prepared by thermal decomposition method followed by a series of thermal annealing in oxygen and forming gas. The x-ray absorption analysis reveals that O vacancies surrounding Mn and Y atoms were appreciably increased by forming-gas-annealing and decreased by oxygen-annealing, accompanied by enhanced and reduced saturation magnetization as demonstrated by magnetic measurements, respectively. Our results demonstrate strong correlation between magnetism and O vacancies and therefore strongly support the bound magnetic polaron model for these high-k DMOs.
    Applied Physics Letters 07/2012; 101(2). · 3.79 Impact Factor
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    ABSTRACT: Co-based catalysts of the reaction by which hydrogen was obtained from NaBH4 solution were prepared by chemical reduction in a liquid phase. X-ray diffraction and scanning electron microscopy analyses showed that the as-prepared Fe@Co catalyst was ultrafine and amorphous. The calculated Arrhenius activation energy of the Fe@Co catalyst was 35.62(1) kJ mol−1 while that of the Co catalyst was 38.81(2) kJ mol−1, demonstrating that Fe@Co nanoparticles reduce the activation energy of the reaction more than does a Co nanocatalyst. X-ray absorption spectroscopy (XAS) clearly reveals the valences of Fe and Co. The Fe valence of Fe@Co is smallest among three catalysts because of the Co shell. The molar ration of Fe to Co is 1: 2 as determined by using XPS analysis, indicating that the novel catalyst reduces costs. The generation of hydrogen is schematically elucidated.
    International Journal of Hydrogen Energy 02/2012; 37(4):3338–3343. · 3.55 Impact Factor
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    ABSTRACT: Local electronic structures of ruthenocuprate RuSr2EuCu2O8 (RuEu-1212) were investigated by using x-ray absorption near-edge structure (XANES) and valence-band photoemission (VB-PES) measurements at room temperature, 80 K, and 25 K. The XANES results indicate that when RuEu-1212 is below Curie temperature, TM, electrons are transferred not only from Cu 3d states, but also from Ru 4 d states, to O 2p orbitals. Additionally, the partial spectral weight distributions derived from VB-PES measurements provide evidence of a weak overlapping between Ru 4 d states and strongly Cu 3d−O 2p hybridized states below the Fermi level, which results in a weak coupling between the CuO2 and RuO2 layers. The analysis of extended x-ray absorption fine structure shows that the appearance of weak ferromagnetism and superconductivity is accompanied by a significant decrease of dynamic local lattice distortions of high-shell Cu-Sr and Ru-Sr bonding in RuEu-1212.
    Physical review. B, Condensed matter 01/2012; 85(1).
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    ABSTRACT: NiAl2O4,NiFe2O4, and their solid solution Ni(FezAl1−z)2O4 have been studied using high temperature oxide melt solution calorimetry in molten 2PbO·B2O3 at 973 K. X-ray absorption near edge structure (XANES) measurements and Mössbauer spectroscopy investigation of the cation distribution showed that the Ni fraction in octahedral sites increases with increasing iron content. Despite the zero heat of mixing, the solid solution is not thermodynamically ideal in the sense of Raoult's law. The entropies of mixing are similar to those for a solid solution of two inverse spinels and the activities are approximated as the square of the mole fractions. The stability of the solid solutions relative to oxide components (NiO,Al2O3,Fe2O3) increases with increasing iron content. The solid solution is a suitable waste form for nickel from industrial processing with higher iron content potentially beneficial to its stability.
    Journal of the American Ceramic Society 01/2012; 95:423. · 2.11 Impact Factor
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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 01/2012; 86:241103. · 3.77 Impact Factor
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    Applied Physics Letters 11/2011; 99(22). · 3.79 Impact Factor
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    ABSTRACT: In the present work we have reported the results of investigations on local structures of e-beam evaporated (Gd2O3-SiO2) composite thin films by synchrotron based EXAFS measurements. The evolution of local structure in the case of the Gd2O3-SiO2 system is found to be different from the HfO2-SiO2 system reported by us earlier. The EXAFS analysis has shown that the Gd-O bond length decreases monotonically as SiO2 content in the films increases. Also the amplitudes of the peaks in the FT-EXAFS spectra of the samples, which depend jointly on the coordination numbers as well as the Debye-Waller factors (measure of disorder) are found to decrease monotonically with increase in SiO2 contents in the Gd2O3 matrix. Atomic force microscopy (AFM) measurements of the samples also show continuous evolution of amorphous-like denser microstructure with increase in SiO2 content in the films. Hence incorporation of SiO2 in the Gd2O3 matrix, results in a continuous change in oxygen coordination yielding a change in the Gd-O bond length and also results in a continuous increase in amorphousness and a smoother morphology of the samples and, unlike the HfO2-SiO2 system, does not show any maximum for a particular SiO2 concentration.
    Journal of Applied Physics 09/2011; 110:063527. · 2.21 Impact Factor
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    ABSTRACT: Critical issues concerning emerging Fe-based superconductors include the degree of electron correlation and the origin of the superconductivity. X-Ray absorption spectra (XAS) and resonant inelastic X-ray scattering spectra (RIXS) of FeSe(1-x)Te(x) (x = 0-1) single crystals were obtained to study their electronic properties that relate to electron correlation and superconductivity. The linewidth of Fe L(2,3)-edges XAS of FeSe(1-x)Te(x) is narrower than that of Fe-pnictides, revealing the difference between their hybridization effects and localization character and those of other Fe-pnictides. While no significant differences exist between the Fe L-edge XAS and RIXS of FeSe(1-x)Te(x) and those of Fe-pnictides, Se K-edge and Te K-edge XAS exhibit substantial edge shift, suggesting that the superconductivity in an Fe-Se superconductor is strongly associated with the ligand states. A comparison of the Se K-edge and Te K-edge spectra reveals that the charge transfer may occur between Se and Te. Given the Coulomb interaction and the bandwidth, the spectral results indicate that FeSe(1-x)Te(x) is unlikely to be a weakly correlated system unlike the Fe-pnictides of the "1111" and "122" families. The spectral results further demonstrate that superconductivity in this class of Fe-based compounds is strongly associated with the ligand 4p hole state.
    Physical Chemistry Chemical Physics 09/2011; 13(34):15666-72. · 3.83 Impact Factor
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    ABSTRACT: The Sn-doped Ge semiconductor thin films have shown direct bandgap and many other interesting physical properties with great potential for technological applications. To understand the underlying mechanism for the unique properties of these materials, information on the locations of Sn atoms in the matrix and the effects of Sn doping on the crystal structure of Ge host is an important prerequisite. Samples of Sn-doped Ge thin films of thickness around 300å and Sn concentration 4 at.% to 28 at.% have been prepared by molecular beam epitaxy (MBE) method. Long-range-order and short-range-order structures in these films have been probed by using x-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS) techniques, respectively. Our x-ray results demonstrate that Sn impurity atoms are located on the substitutional sites in the Ge films with Sn concentration up to 20 at.%. Variation of lattice constant as a result of Sn doping in the Ge host will also be presented.
    03/2011;