Chung-Chieh Chang

Heterojunction of Zinc Blende/Wurtzite in Zn1-xCdxS Solid Solution for Efficient Solar Hydrogen Generation: X-ray Absorption/Diffraction Approaches.

In the past decade, inorganic semiconductors have been successfully demonstrated as light absorbers in efficient solar water splitting to generate chemical fuels. Pseudobinary semiconductors Zn1-xCdxS (0≤x≤1) have exhibited a superior photocatalytic reactivity of H2 production from splitting of water by artificial solar irradiation without any metal catalysts. However, most studies had revealed that the extremely high efficiency with an optimal content of Zn1-xCdxS solid solution was determined as a result of elevating the conduction band minimum (CBM) and the width of bandgap. In addition to corresponding band structure and bandgap, the local crystal structure should be taken into account as well to determine its photocatalytic performance. Herein, we demonstrated the correlations between the photocatalytic activity and structural properties that were first studied through synchrotron X-ray diffraction and X-ray absorption spectroscopy. The crystal structure transformed from zinc blende to coexisted phases of major zinc blende and minor wurtzite phases at a critical point. The heterojunction formed by coexistence of zinc blende and wurtzite phases in the Zn1-xCdxS solid solution can significantly improve the separation and migration of photoinduced electron-hole pairs. Besides, X-ray absorption spectra and UV-vis spectra revealed that the bandgap of the Zn0.45Cd0.55S sample extended into the region of visible light because of the incorporation of Cd element in the sample. These results provided a significant progress toward the realization of the photoelectrochemical mechanism in heterojunction between zinc blende and wurtzite phases, which can effectively separate the charge-carriers and further suppress their recombination to enhance the photocatalytic reactivity.

Magnetodielectric study in SiO2-coated Fe3O4 nanoparticle compacts

The dielectric properties of Fe3O4 magnetic nanoparticles with an insulating coating layer of SiO2 were investigated. At high temperatures, the changes in the dielectric constant and loss induced by the magnetic field are opposite in sign and strongly frequency-dependent, which originates from extrinsic magnetodielectric coupling-the Maxwell–Wagner effect combined with magnetoresistance. And the interface defects leads to the obvious hysteresis phenomena observed in the measurements. On the other hand, the strong coupling of dielectric and magnetic properties at low temperatures contradicts the Maxwell–Wagner model, suggesting the intrinsic magnetodielectric coupling. Our observations are consistent with the recent polarization switching measurements, which confirm the low-temperature multiferroic state existing in highly lossy Fe3O4. And the core/shell nanostructure may provide a new route to achieve applicable magnetoelectric materials with low loss.

CCT- and CRI-tuning of white light-emitting diodes using three-dimensional non-close-packed colloidal photonic crystals with photonic stop-bands

This study exhibited the correlated color temperature (CCT)- and color-rendering index (CRI)-tuning behavior of light emission from white light-emitting diodes (WLEDs) using three-dimensional non-close-packed (3D NCP) colloidal photonic crystals (CPhCs). The CCT of approximately 5300 K (characteristic of cold WLEDs) of white light propagated through the NCP CPhCs dropped to 3000 K (characteristic of warm WLEDs) because of the photonic stop-bands based on the photonic band structures of NCP CPhCs. This study successfully developed a novel technique that introduces lower-cost CCT- and CRI-tuning cold WLEDs with a CRI of over 90 that of warm WLEDs by using 3D NCP CPhCs.

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.

Disordered Fe vacancies and superconductivity in potassium-intercalated iron selenide (K2−xFe4+ySe5)

In the high-T c potassium-intercalated FeSe, there has been significant debate regarding what the exact parent compound is. Here we show that the Fe-vacancy ordered K2Fe4Se5 is the magnetic, Mott insulating parent compound of the superconducting state. Non-superconducting K2Fe4Se5 becomes a superconductor after high-temperature annealing, and the overall picture indicates that superconductivity in K2−x Fe4+y Se5 originates from the Fe-vacancy order-to-disorder transition. Thus, the long-pending question as to whether magnetic and superconducting state are competing or cooperating for cuprate superconductors may also apply to the Fe-chalcogenide superconductors. It is believed that the iron selenides and related compounds will provide essential information to understand the origin of superconductivity in the iron-based superconductors, and possibly to the superconducting cuprates.

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

Ultrafast dynamics of quasiparticles and coherent acoustic phonons in slightly underdoped (BaK)Fe2As2.

We have utilized ultrafast optical spectroscopy to study carrier dynamics in slightly underdoped (BaK)Fe2As2 crystals without magnetic transition. The photoelastic signals due to coherent acoustic phonons have been quantitatively investigated. According to our temperature-dependent results, we found that the relaxation component of superconducting quasiparticles persisted from the superconducting state up to at least 70 K in the normal state. Our findings suggest that the pseudogaplike feature in the normal state is possibly the precursor of superconductivity. We also highlight that the pseudogap feature of K-doped BaFe2As2 is different from that of other iron-based superconductors, including Co-doped or P-doped BaFe2As2.

Structural, compositional, and photoluminescence characterization of thermal chemical vapor deposition-grown Zn3N2 microtips

The catalytic growth of Zn3N2 using guided-stream thermal chemical vapor deposition has been investigated within the parameter range of acicular growth to obtain uniform microtips with a high crystalline quality. The cubic anti-bixbyite crystal structure of Zn3N2 microtips and its related phonon mode are revealed by X-ray diffraction and Raman spectroscopy, respectively. The surface morphologies of pure and surface-oxidized Zn3N2 microtips are depicted by scanning electron microscopy and show the crack formation on the surface-oxidized Zn3N2 microtips. The spatial element distribution map confirms the VLS growth mechanism for Zn3N2 microtips and reveals the depth profile of zinc, nitrogen, oxygen, and nickel elements. Photoluminescence (PL) spectra of Zn3N2 microtips show a sharp infrared band-to-band emission peak at 1.34 eV with a full width at half maximum of ∼100 meV and a very broad oxygen-related defect band emission peak centered at ∼0.85 eV.

Highly Reliable and Low-Cost Fabrication of Warm-White LEDs Using Composite Silica Photonic Crystals

We demonstrated a technique requiring little phosphor that used white light-emitting diodes (WLEDs) containing composite silica colloidal photonic crystals (c-SCPhCs) for developing the warm-WLEDs (w-WLEDs). WLEDs containing c-SCPhCs enhanced luminous efficacy 5.6% more than commercial w-WLEDs did. We used a UV adhesive curing method to improve the adhesion properties of the c-SCPhCs. A reliability analysis (RA) test was performed on the WLEDs containing c-SCPhCs, applying a high temperature and high relative humidity (85°C/85 RH) during WLEDs operation at 120 mA. During a RA test of 2500 h, no visible degradation in optical performance was observed. We implemented a novel, inexpensive technique for producing high luminous flux w-WLEDs that can be used in residential light.

Structural characteristics and phase separation of superconducting Fe1+ySe1−xTex nanowires

Superconducting Fe1+y Se1?x Te x nanowires with considerable differences in the diameter and aspect ratio were synthesized via annealing thin films with particular composition prepared by pulsed laser deposition. Using transmission electron microscopy and related techniques, we found they were all crystallized in a PbO-type tetragonal structure with a growth direction of [010] (equivalent to [100]). All the nanowires were covered by a thin amorphous oxide layer (?5 nm) determined by electron energy loss spectroscopy. Chemical analysis and the calculated lattice constants reveal the fact that these nanowires are different from chemical compositions. High-resolution electron microscopy images show that the two thick nanowires (with diameters ?80 nm) are composed of multiple phases, while the thin nanowires (with diameters of 14 and 22 nm) are single phase. Digital moir? analysis reveals that the thin nanowires contain surface defects, as well as dislocations, inside the nanowires.