Structural and Thermodynamic Properties of M3W3N （M=Fe,Co, Ni）

Ternary transition metal nitrides, Fe3 W3N, Coa W3N, and Nia WaN~ are studied by the use of interatomic potentials acquired from lattice inversion. The study indicates that Fe3 WaN would be more stable than the other compounds in the family of intermetallic tungsten nitrides. The investigation of phonon density of states indi- cates that the lower frequency modes are mostly excited by the metal atoms, and the higher frequency modes are mostly excited by the nitrogen atoms. A qualitative analysis is carried out with the relevant potentials for the phase stability and vibrational modes.     

Properties of AlyGa1-yN/AlxGa1-xN/AlN/GaN Double-Barrier High Electron Mobility Transistor Structure

Electrical properties of Al_yGa_1-yN/Al_xGa_1-xN/AlN/GaN structure are investigated by solving coupled Schrödinger and Poisson equation self-consistently. Our calculations show that the two-dimensional electron gas (2DEG) density will decrease with the thickness of the second barrier (Al_yGa_1-yN) once the AlN content of the second barrier is smaller than a critical value y_c, and will increase with the thickness of the second barrier (Al_yGa_1-yN) when the critical AlN content of the second barrier y_c is exceeded. Our calculations also show that the critical AlN content of the second barrier y_c will increase with the AlN content and the thickness of the first barrier layer (Al_xGa_1-xN).     

Effects of Different Plasma Energy Treatments on n-Type Al0.4Ga0.6N Material

El3ctronic properties, surface chemistry and surface morphology of plasma-treated n-Al0.4Ga0.6N material are studied by electrical contact measurements, atomic force microscopy and x-ray photoemission spectroscopy. Although excessive etching can cause the surface roughness to significantly increase, the nitrogen vacancies VN produced by the excessive etching can be compensated for by the negative effects of the rougher surface. Thus, VN produced by excessive etching plays a key role in Ohmic contact of high-A1 content AIGaN and it can reduce Ohmic contact resistance. The effect of rapid thermal annealing on the performance of n-Al0.4Ga0.6N can significantly reduce the etching damage caused by excessive etching.     

Laser Molecular Beam Epitaxy of Multilayer Heterostructure SrNb0.05Ti0.95O3/La0.9Sr0.1MnO3 in 10000 Unit-Cell Layers

Ten thousands of unit-cell multilayer heterosturctures, [SrNb0.05 Ti0.95O3/La0.9 Sr0.1MnO3]3 （SNTO/LSMO）, have been epitaxial grown on SrTiO3 （001） substrates by laser molecular beam epitaxy. The monitor of insitu. reflection high-energy electron diffraction demonstrates that the heterosturctures are layer-by-layer epitaxial growth. Atomic force microscope observation indicates that the surface of the heterosturcture is atomically smooth. The measurements of cross-sectional low magnification and high-resolution transmission electron microscopy as well as the corresponding selected area electron diffraction reveal that the interfaces are of perfect orientation, and the epitaxial crystalline structure shows the orientation relation of SNTO（001）//LSMO（001）, and SNTO[100]//LSMO[100].     

Superconductivity in Hole-Doped （S1-xKx）Fe2As2

A series of layered （Sr1-xKx）Fe2As2 compounds with nominal x = 0-0.40 are synthesized by solid state reaction method. Similar to other parent compounds of iron-based pnictide superconductors, pure SrFe2As2 shows a strong resistivity anomaly near 210 K, which was ascribed to the spin-density-wave instability. The anomaly temperature is much higher than those observed in LaOFeAs and BaFe2As2, the two prototype parent compounds with ZrCuSiAs- and ThCr2Si2-type structures. K-doping strongly suppresses this anomaly and induces superconductivity. Like in the case of K-doped BaFe2As2, sharp superconducting transitions at Tc ～ 38 K is observed. We perform the Hall coefficient measurement, and confirm that the dominant carriers are hole-type. The carrier density is enhanced by a factor of 3 in comparison to F-doped LaOFeAs superconductor.     

（Na0.52K0.44Li0.04）Nb0.9-xSbxTa0.1O3 Lead-Free Piezoelectric Ceramics with High Performance and High Curie Temperature

（Na0.52K0.44Li0.04）Nb0.9-x Sbx Ta0.1O3 lead-free piezoelectric ceramics are prepared by a solid-state reaction method. With increasing Sb content, the transition temperature from orthorhombic to tetragonal polymorphic phase decreased. A composition （Na0.52K0.44Li0.04）Nb0.863Sb0.037Ta0.1O3 is found to possess excellent piezo- electric and electromechanical performances （d33 = 306pC/N, kp =48%, and kt=50%）, and high Curie temperature （Tc = 320 ℃）. These results indicate that （Na0.52K0.44Li0.04）Nb0.863Sb0.037 Ta0.1O3 is a promising lead-free piezoceramics replacement for lead zirconate titanate.     

Properties of a New Nonlinear Optical Crystal CdZn2B2O6

Cadmium dizinc diborate （CdZn2B2O6） single crystals have been grown for the first time. The crystal structure of CdZn2B2O6 is the same as that of the Cd3Zn3B4O12. The x-ray diffraction, infrared and Raman spectra, differential scanning calorimetry analysis and density indicate that the physical and chemical properties of both crystals are very similar. Especially, the nonlinear optical coefficients of CdZn2B2O6 and Cd3Zn3B4O12 crystals are 2.6 and 2.4 times as large as that of KH2PO4 crystal respectively. Chemical etching experiments indicated that these crystals are very stable in neutral solution and not hygroscopic in air at room temperature.     

Preparation and Luminescence Characteristics of Ca3Y2（BO3）4：Eu^3＋ Phosphor

Ca3Y2 （BO3）4：Eu^3＋ phosphor is synthesized by high temperature solid-state reaction method, and the Iuminescence characteristics are investigated. The emission spectrum exhibits two strong red emissions at 613 and 621 nm corresponding to the electric dipole ^5 Do- ^7F2 transition of Eu^3＋ under 365 nm excitation, the reason is that Eu^3＋ substituting for Y^3＋ occupies the non-centrosymmetric position in the crystal structure of Ca3 Y2 （BO3）4. The excitation spectrum for 613 nm indicates that the phosphor can be effectively excited by ultraviolet （UV） （254 nm, 365nm and 400nm） and blue （470nm） light. The effect of Eu^3＋ concentration on the emission intensity of Ca3 Y2 （BO3）4 ：Eu^3＋ phosphor is measured, the result shows that the emission intensities increase with increasing Eu^3＋ concentration, then decrease. The CIE colour coordinates of Ca3Y2 （BO3）4：Eu^3＋ phosphor is （0.639, 0.357） at 15mol% Eu^3＋.     

Air Stability of Cs2CO3:Ag/Ag Cathode for Organic Light-Emitting Diodes

We report the superior stability of the composite Cs2CO3 ：Ag/Ag cathode structure, which can be used in efficient organic light-emitting diodes （OLEDs）. Devices with the Cs2CO3：Ag （1：10, 5nm）/Ag （95nm） cathode show a considerably improved lifetime compared with the control device with the Cs2CO3 （0.5 nm）/Ag （100 nm） cathode. The composite Cs2CO3 ：Ag/Ag film is proved to be stable in the atmosphere. X-ray diffraction （XRD） is applied to analyze the crystalline structure of the Cs2CO3：Ag film, and it is demonstrated that CsAg alloy is formed, leading to the improved stability of the thin film and the devices.     

Effects of Li Substitution and Sintering Temperature on Properties of Bi0.5（Na,K）0.5TiO3 Lead-Free Piezoelectric Ceramics

Bi0.5 （Na0.72K0.28- x Lix ）0.5 TiO3 （BNKLT- 100x） lead-free piezoelectric ceramics are synthesized by conventional solid state sintering techniques. The dielectric and piezoelectric properties of the BNKLT-100x ceramics as a function of Li content are systematically investigated. It is found that not only Li content but also the sintering temperature has a strong effect on the piezoelectric properties of BNKLT. The piezoelectric constant d33 Of BNKLT varies from 120 to 252pC/N in the Li content range from 0.03 to 0.16. In the sintering temperature range from 1080 to 1130℃, the d33 value of BNKLT-6 changes from 200pC/N to 252pC/N. The BNKLT-6 sample sintered at 1100℃ has the highest piezoelectric constant d33 of 252pC/N, with the electromechanical coupling factors kp of 0.32 and kt of 0.44.     

Effect of TiO2 Nanotubes on Polymer-Fullerene Bulk Heterojunction Solar Cells

We investigate the photovoltaic properties of hybrid organ/c solar cell based on the blend of poly[2-methoxy-5-（2- ethylhexoxy-l,4-phenylenevinylene） （MEH-PPV）, C60 and titanium dioxide （TiO2） nanotubes. In comparison of the composite devices with different TiO2：[MEH-PPV ＋C60] weight ratios of lw$.% （D1-1）, 2wt.% （D1-2）, 3wt.% （D1-3）, 5wt.% （D1-4）, 10wt.% （D1-5） and 20wt.% （D1-6）, it is found that the device Dl-a exhibits the best performance. The conversion efficiency is improved by a factor of 3 compared with the MEH-PPV：C60 device.     

Broadband Infrared Luminescence from Bismuth-Doped GeS2--Ga2S3 Chalcogenide Glasses

Near-infrared luminescence is observed from bismuth-doped GeS2-Ga2Sa chalcogenide glasses excited by an 808 nm laser diode. The emission peak with a maximum at about 1260 nm is observed in 80GeS2-2OGa2 Sa：O.fBi glass and it shifts toward the long wavelength with the addition of Bi gradually. The full width of half maximum （FWHM） is about 200 nm. The broadband infrared luminescence of Bi-doped GeS2-Ga2Sa chalcogenide glasses may be predominantly originated from the low valence state of Bi, such as Bi＋. Raman scattering is also conducted to claxify the structure of glasses. These Bi-doped GeS2 Ga2Sa chalcogenide glasses can be applied potentially in novel broadband optical fibre amplifiers and broadly tunable laser in optical communication system.     

Vacuum Ultraviolet Excited Photoluminescence Properties of Novel Na3Y9O3（BO3）8：Tb^3＋ Phosphor

The novel vacuum ultraviolet （VUV） excited Na3 Y9O3 （BO3）8：Tb^3＋ （NYOB：Tb^3＋） green phosphor is prepared. Strong VUV photoluminescence and high quenching concentration of Tb^3＋ （20 wt%） are observed in NYOB： Tb^3＋ and the strong emission are correlated with the unique layer-type structure of NYOB. All the characteristic 4 f - 5d transitions of Tb^3＋ and the host absorption band in VUV region are identified in the excitation spectrum. Based on the results, the energy levels scheme of Tb^3＋ in NYOB：Tb^3＋ is first established. This newly developed NYOB：Tb^3＋ phosphor shows excellent optical properties when compared with the commercial Zn2SiO4：Mn^2＋ and would be a potential VUV-excited green phosphor.     

Improvement in Dielectric Tunability of Ba0.6Sr0.4TiO4-Mg2TiO4 Composite Ceramics via Heterogeneous Nucleation Processing

Dielectric tunable composite ceramics Ba0.6Sr0.4 TiO4-Mg2 TiO4 （BST-MT） are prepared with a heterogeneous nucleation sol-gel approach. The Mg2 TiO4 powders are synthesized by the conventionM solid-state reaction method. The micro-sized MT powders with dispersant Ciba-4010 are introduced into Ba-Sr-Ti sol to obtain uniform and homogeneous mixture compounds with nano-sized BST particles synthesized via heterogeneous nucleation （HN） in the sol-gel process. Thus, the mierostructural and dielectric properties can be tailored. The dielectric constants of BST-MT composite ceramics can be adjusted in a larg＇e range from 294 to 1790, and the dielectric tunability can be adjusted from 29.4% to 37.0% with different MT contents from 60wt% to 20wt%. Compared to the samples prepared by the conventional solid-state （SS） process, the BST-MT composite ceramics by the heterogeneous nucleation sol-gel process exhibit a more uniform microstructure, and improve dielectric properties.     

Improvement of Atomic-Layer-Deposited Al2O3/GaAs Interface Property by Sulfuration and NH3 Thermal Nitridation

Fermi level pinning at the interface between high-h gate dielectric and GaAs induced by unstable native oxides is a major obstacle for high performance GaAs-based metal-oxide-semiconductor （MOS） devices. We demonstrate the improved Al2O3/GaAs interracial characteristics by （NH4）2S immersion and NH3 thermal pretreatment prior to A1203 deposition. X-ray photoelectron spectroscopy （XPS） analysis confirms that sulfuration of GaAs surface by （NH4 ）2S solution can effectively reduce As-O bonds while Ga-O bonds and elemental As still exist at Al2O3 /GaAs interface. However, it is found that N incorporation during the further thermal nitridation on sulfurated GaAs can effectively suppress the native oxides and elemental As in the sequent deposition of Al2O3. Atomic force microscopy （AFM） shows that the further thermal nitridation on sulfurated GaAs surface can also improve the surface roughness.     

Optical Dispersion Behavior and Band Gap Energy of Relaxor Ferroelectric 0.92Pb(Mg1/3Nb2/3)O3-0.08PbTiO3 Single Crystal

Refractive indices and extinction coefficients of 0.92Pb(Mg_1/3Nb_2/3)O₃-0.08PbTiO₃(PMN-0.08PT) single crystal are investigated by variable angle spectroscopic ellipsometry (VASE) at different wavelengths. The parameters relative to the energy band structure are obtained by fitting to the single-oscillator dispersion equation, and the band gap energy is also deduced from the Tauc equation. Similar to most oxygen-octahedra ferroelectrics, PMN-0.08PT has the same dispersion behavior described by the refractive-index dispersion parameters.     

Electroluminescence from Multilayered Diamond/CeF3/SiO2 Films

We report a thin film electroluminescent device with a three-layer structure （diamond/CeF3/SiO2 films）, which has a luminance of 1.5 cd/m^2 at dc voltage 215 V. The electroluminescence spectrum at room temperature shows that the main peaks locate at 527 and 593nm, which are attributed to isolated emission centers of Ce^3＋ ions.     

VUV/UV/X-Ray Excited Luminescent Properties of Eu^3＋ and Pr^3＋ Doped BiSbO4

Absorption spectra of BiSbO4 are studied. The electronic structure calculated by the DFT shows that BiSbO4 is a semiconductor, with direct band gap 2.96 eV, which is consistent with UV-visible diffuse reflectance experiment. The host lattice emission band is located at 440 nm under VUV excitation. Eu^3＋ and Pr^3＋ doped samples have high luminescence efficiency in emitting red and green light, respectively. From the partial density of states, Eu^3＋ doped emitting spectrum, and the host crystal structure parameters, the relationship between structure and optical properties is discussed. It is found that the Eu^3＋ ions occupied Bi^3＋ sites, and there could be an energy transfer from Bi^3＋ ions to RE^3＋ ions.     

Electronic Structure and Optical Properties of La-Doped SrTiO3 and Sr2TiO4 by Density Function Theory

The effect of La doping on the electronic structure and optical properties of SrTiO₃ and Sr₂TiO₄ is investigated by the first-principles calculation of plane wave ultrasoft pseudopotential based on the density function theory (DFT). The calculated results reveal that the electron doping in the case of Sr_0.875La_0.125TiO₃ and Sr_1.875La_0.125TiO₄ can be described within the rigid band model. The La³⁺ ions fully acts as electron donors in Sr_0.875La_0.125TiO₃ and Sr_1.875La_0.125TiO₄ systems and the Fermi level shifts further into the conduction bands (CBs) for Sr_1.875La_0.125TiO₄ compared to Sr_0.875La_0.125TiO₃. The two systems exhibit n-type degenerate semiconductor features. At the same time, the density of states (DOS) of the two systems shift towards low energies and the optical band gaps are broadened. The Sr_1.875La_0.125TiO₄ is highly transparent with the transmittance about 90% in the visible range, which is larger than that of Sr_0.875La_0.125TiO₃(85%). The wide band gap, small transition probability and weak absorption due to the low partial density of states (PDOS) of impurity in the Fermi level result in the optical transparency of the films...     

Phase-Transition and Magnetic Moment of the Gd3+ Ion in the Gd2Fe17 Compound

The structure and magnetic phase transitions of the Gd2Fe17 compound are investigated by using a differential thermal/thermogravimetric analyzer, x-ray diffraction, and magnetization measurements. The result shows that there are two phase structures for the Gd2Fe17 compound： the hexagonal Th2Nilr-type structure at high temperatures （above 1243℃）, and the rhombohedral Th2Zn17-type structure, respectively. A method to measure the magnetic moments of the Gd-sublattice and the Fe-sublattice in the Gd2Fe17 compound is presented. The moments of the Gd-sublattice and the Fe-sublattice in the Gd2Fe17 compound from 77 to 500 K are measured in this way with a vibrating sample magnetometer. A detailed discussion is presented.