Jeng-Lung Chen

Local geometric and electronic structures of gasochromic VOx films

VOx films were deposited by radio-frequency reactive magnetron sputtering from a vanadium target at room temperature. Local atomic and electronic structures of the films were then modified by thermal annealing. The oxidation state and structural and gasochromic properties of the films were elucidated by X-ray absorption spectroscopy. Analytical results indicate that the as-deposited VOx films were amorphous with mixed V(4+) and V(5+) valences. The amorphous VOx had a disordered and expanded lamellar structure resembling that of polymer-intercalated V2O5 gels. VOx films were crystallized into orthorhombic V2O5 at 300 °C, and the lamellar structure was eliminated at 400 °C. Additionally, the gasochromic reaction reduced the vanadium valence via intervalence transitions between V(5+) and V(3+). Moreover, removing the lamellar structure reduced the gasochromic rate, and the gasochromic reaction transformed the V2O5 crystalline phase irreversibly into an H1.43V2O5 phase. Based on the results of this study, amorphous VOx with a lamellar structure is recommended for use in H2 gas sensors.

Mechanism of light emission and electronic properties of a Eu3+-doped Bi2SrTa2O9 system determined by coupled X-ray absorption and emission spectroscopy

The origin of light emission from newly discovered orange-red UV light emitting diodes, and their electronic properties are critical issues yet to be understood. In this study, X-ray absorption spectroscopy (XAS) and emission spectroscopy (XES) are utilized to examine the electronic structure of the Eu3+-doped Bi2SrTa2O9 system. While no significant change in the electronic structure is observed around the Bi and Ta sites, variation around the Eu and Sr atoms is observed, along with even more significant changes in the O 2p states in the conduction band. Upon UV irradiation, Eu-induced states within the conduction band are observed and found to shift to the conduction band minimum upon substitution of Sr with Eu. This phenomenon is the result of the creation by Eu of an excitable state and the fact that Eu is more electronegative than Sr, such that the substitution lowers the Eu 4f5d–O 2p hybridized states. Consequently, the substitution reduces the energy of electron recombination between the valence and conduction bands, which is consistent with the red shift in the photoluminescence spectra. The presence of the newly formed hole states distributed over the O 2p states in the conduction band is strongly correlated with the emission intensity. The results and analyses demonstrate that Eu can be introduced to tailor the Eu–O hybridized states within the conduction band and change the route of recombination, suggesting that Eu is critically involved in light emission in these UV-induced orange-red emitting LED materials.

Enhancement of thermoelectric figure of merit in β-Zn4Sb3 by indium doping control

We demonstrate the control of phase composition in Bridgman-grown β-Zn4Sb3 crystals by indium doping, an effective way to overcome the difficulty of growing very pure β-Zn4Sb3 thermoelectric material. The crystal structures are characterized by Rietveld refinement with synchrotron X-ray diffraction data. The results show an anisotropic lattice expansion in In-doped β-Zn4Sb3 wherein the zinc atoms are partially substituted by indium ones at 36f site of R-3c symmetry. Through the elimination of ZnSb phase, all the three individual thermoelectric properties are simultaneously improved, i.e., increasing electrical conductivity and Seebeck coefficient while reducing thermal conductivity. Under an optimal In concentration (x = 0.05), pure phase β-Zn4Sb3 crystal can be obtained, which possesses a high figure of merit (ZT) of 1.4 at 700 K.

Atomic and electronic aspects of the coloration mechanism of gasochromic Pt/Mo-modified V2O5 smart films: an in situ X-ray spectroscopic study

In this work, gasochromic pristine and Mo-modified V2O5 thin films were prepared by the sol-gel spin coating method. Both films exhibit excellent gasochromic coloration. Synchrotron grazing incidence X-ray diffraction reveals that the Mo-modified V2O5 thin film is more amorphous than the pristine V2O5 thin film. X-ray absorption spectroscopy (XAS) was utilized to elucidate the modifications of the local electronic and atomic structures that are caused by Mo. In situ soft-XAS and in situ hard-XAS were performed to monitor the effect of the adsorption of dihydrogen on the charge state of vanadium and local atomic rearrangement in the gasochromic thin films. The gasochromic V2O5 film has a significantly pyramid-like oxygen-coordinated environment. However, the Mo-modified film exhibits mixed pyramid- and octahedral-like structures. Analytic results indicate that upon gasochromic coloration, adsorption of hydrogen adds electrons to the V 3d t2g orbital, lowering the charge state of vanadium. The films undergo structural modification before the valence is changed. The Mo-modified V2O5 film exhibits faster coloration because the apical V-O bond differs from that in the pristine V2O5 film. This in situ XAS allows real-time monitoring of changes in the element-specific local atomic structure during the gasochromic reaction and enables the elucidation of the gasochromic mechanism.

Electronic and atomic structures of gasochromic V2O5 films

A gasochromic Pt/V2O5 film was fabricated by the sol-gel process and it exhibited excellent color change performance from yellow to gray/black upon exposure to hydrogen gas under ambient conditions. X-ray absorption spectroscopy was employed to study the unoccupied electronic states in detail and to explore the gasochromic effect and its coloration mechanism. Upon injection of hydrogen, gashochromism was revealed to correlate strongly with the change of both electronic and atomic structures. Since the hydrogen supplied an extra electron, the hybridization between vanadium 3d and oxygen 2p was modified, reducing the valence state of vanadium from 5+ to 4.1+. Linear polarized synchrotron X-rays were utilized to investigate the change in the structural symmetry and the orbital orientation. Along with the electronic structure, the geometric modulation of the VO6 octahedron is crucial to the gasochromic mechanism, as also revealed in this study. Copyright c EPLA, 2013

Electronic and atomic structures of the Sr 3 Ir 4 Sn 13 single crystal: A possible charge density wave material

X-ray scattering (XRS), x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopic techniques were used to study the electronic and atomic structures of the high-quality Sr3Ir4Sn13 (SIS) single crystal below and above the transition temperature (T* ≈ 147 K). The evolution of a series of modulated satellite peaks below the transition temperature in the XRS experiment indicated the formation of a possible charge density wave (CDW) in the (110) plane. The EXAFS phase derivative analysis supports the CDW-like formation by revealing different bond distances [Sn1(2)-Sn2] below and above T* in the (110) plane. XANES spectra at the Ir L3-edge and Sn K-edge demonstrated an increase (decrease) in the unoccupied (occupied) density of Ir 5d-derived states and a nearly constant density of Sn 5p-derived states at temperatures T < T* in the (110) plane. These observations clearly suggest that the Ir 5d-derived states are closely related to the anomalous resistivity transition. Accordingly, a close relationship exists between local electronic and atomic structures and the CDW-like phase in the SIS single crystal.

Anisotropy in the thermal hysteresis of resistivity and charge density wave nature of single crystal SrFeO 3-δ : X-ray absorption and photoemission studies

The local electronic and atomic structures of the high-quality single crystal of SrFeO3-δ (δ~0.19) were studied using temperature-dependent x-ray absorption and valence-band photoemission spectroscopy (VB-PES) to investigate the origin of anisotropic resistivity in the ab-plane and along the c-axis close to the region of thermal hysteresis (near temperature for susceptibility maximum, Tm~78 K). All experiments herein were conducted during warming and cooling processes. The Fe L3,2-edge X-ray linear dichroism results show that during cooling from room temperature to below the transition temperature, the unoccupied Fe 3d eg states remain in persistently out-of-plane 3d3z2-r2 orbitals. In contrast, in the warming process below the transition temperature, they change from 3d3z2-r2 to in-plane 3dx2-y2 orbitals. The nearest-neighbor (NN) Fe-O bond lengths also exhibit anisotropic behavior in the ab-plane and along the c-axis below Tm. The anisotropic NN Fe-O bond lengths and Debye-Waller factors stabilize the in-plane Fe 3dx2-y2 and out-of-plane 3d3z2-r2 orbitals during warming and cooling, respectively. Additionally, a VB-PES study further confirms that a relative band gap opens at low temperature in both the ab-plane and along the c-axis, providing the clear evidence of the charge-density-wave nature of SrFeO3-δ (δ~0.19) single crystal.

Effect of local atomic and electronic structures on thermoelectric properties of chemically substituted CoSi

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.