Browsing by Author "Wu, Maw-Kuen"

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Electronic properties of free-standing TiO2 nanotube arrays fabricated by electrochemical anodization
(2015-01-01)
Chen, Chi Liang
;
Dong, Chung-Li
;
Chen, Chia-Hao
;
Wu, Jen-Wei
;
Lu, Ying-Rui
Nanotubular TiO2 has attracted considerable attention owing to its unique functional properties, including high surface area and vectorial charge transport along the nanotube, making it a good photocatalytic material. Anodic TiO2-nanotube (TiNT) arrays on a Ti foil substrate were prepared by electrochemical anodic oxidation and SEM/HRTEM/XRD analyses have suggested that the walls of TiO2 tubes are formed from stacked [101] planes (anatase). Both HRTEM and XRD indicate an interplanar spacing of d(101) = 0.36 nm in the wall structure. Despite the large amount of work done on nanotube synthesis, a thorough investigation of the electronic and atomic structures of free-standing TiNT arrays has not yet been carried out. X-ray absorption spectroscopy (XAS), resonant inelastic X-ray scattering (RIXS) and scanning photoelectron microscopy (SPEM) are employed herein to examine the electronic and atomic structures at the top and bottom of TiNT arrays. These analyses demonstrate the presence of mixed valence states of the Ti ions (Ti3+ and Ti4+) and a structural distortion at the bottom cap region of the TiNT. Additionally, the results obtained herein suggest the formation of a defective anatase phase at the bottom cap barrier layer between the Ti foil substrate and TiNT during the growth of electrochemically anodized nanotubes.
Magnetic fluctuations in FeSe(1-delta) and Cu-doped FeSe(1-delta): (77)Se NMR experiments
(2010-04-01)
Young, Ben-Li
;
Wu, Jack
;
Huang, Tzu-Wen
;
Yeh, Kuo-Wei
;
Wu, Maw-Kuen
The roles of Se deficiency and Fe atoms, for the superconductivity of FeSe, have been, respectively, investigated in FeSe(1-delta) and (Fe(1-x)Cu(x))Se(1-delta), by our NMR experiments. The data, for nuclear spin-lattice relaxation rate (1/T(1)), show that the spin fluctuations are weakened at a larger delta, and are correlated with the superconductivity in FeSe. The superconducting volume fraction, estimated by our ac susceptibility experiments, is found to vary inversely with delta. Our findings suggest that the Se-deficient FeSe has an inhomogeneous phase, where the superconductivity is associated with the regions having few or no Se vacancies. As for (Fe(1-x)Cu(x))Se(1-delta), T(c) is rapidly suppressed by Cu doping and vanishes around x=0.03. The (77)Se and the (63)Cu NMR linewidths suggest that a local moment is induced at the Fe sites and not at the Cu sites. However, 1/T(1) shows no obvious change with the Cu doping. We suspect that other effects, such as disorder or change in the density of states, have more influence on T(c) suppression since a metal-insulator transition, induced by Cu substitution, occurs in the resistivity measurements.
Magnetic fluctuations in FeSe1-delta and Cu-doped FeSe1-delta: Se-77 NMR experiments
(2010-04-01)
Young, Ben-Li
;
Wu, Jack
;
Huang, Tzu-Wen
;
Yeh, Kuo-Wei
;
Wu, Maw-Kuen
The roles of Se deficiency and Fe atoms, for the superconductivity of FeSe, have been, respectively, investigated in FeSe1-delta and (Fe1-xCux)Se1-delta, by our NMR experiments. The data, for nuclear spin-lattice relaxation rate (1/T-1), show that the spin fluctuations are weakened at a larger delta, and are correlated with the superconductivity in FeSe. The superconducting volume fraction, estimated by our ac susceptibility experiments, is found to vary inversely with delta. Our findings suggest that the Se-deficient FeSe has an inhomogeneous phase, where the superconductivity is associated with the regions having few or no Se vacancies. As for (Fe1-xCux)Se1-delta, T-c is rapidly suppressed by Cu doping and vanishes around x=0.03. The Se-77 and the Cu-63 NMR linewidths suggest that a local moment is induced at the Fe sites and not at the Cu sites. However, 1/T-1 shows no obvious change with the Cu doping. We suspect that other effects, such as disorder or change in the density of states, have more influence on T-c suppression since a metal-insulator transition, induced by Cu substitution, occurs in the resistivity measurements.