Advanced Materials Research Vols. 652-654

Abstract: Manganite perovskite La_1-xCa_xMnO₃ (x=0.33, 0.5 and 0.9) have been prepared by chemical co-precipitation method. Ammonium carbonate was used to coprecipitate lanthanum, calcium and manganese ions as carbonates under basic condition. This precursor on calcining at 900°C yields La-Ca-Mn-O perovskite phase. Follow by sintering at 1200°C after the powders were pressed into pellets gave La_1-xCa_xMnO₃ (LCMO) polycrystalline ceramics. The crystal phases of the resulting powders and ceramics were examined by X-ray diffraction (XRD) technique. The morphology of the powders was observed by scanning electron microscopy (SEM) and electrical transport properties of ceramics were measured by conventional four-point probe technique.

Abstract: The red long persistent phosphors (Sr _1-x Ca _x) S: Eu ²⁺ , Dy 3+ (0≤x≤0.4) were prepared by sol-gel thermolysis method under reducing atmosphere. The results of XRD indi^{Superscript text}cated that the phosphor of the major phase was SrS (0≤x≤0.4) and the diffraction peak positions shift to higher angles with increase in x. The spectral properties are measured, and it found that the emission wavelength and decay properties of the phosphor varied with x. With increasing x, the luminescence intensity increased at beginning ,then decreased, and at x=0.3, the initial brightness and the afterglow time of the phosphor became maximal.

Abstract: The discovery of ferromagnetism (FM) in wide band-gap semiconductors doped with transition metals (TM), known as diluted magnetic semiconductors (DMSs), has attracted much interest. These materials are applicable to spin-based optoelectronic devices working at room temperature (RT). Among DMSs, the system of Co-doped ZnO is considered as the most promising candidate, which was expected to robust magnetism. This paper focuses primarily on the recent progress in the experimental studies of ZnO:Co DMSs. The magnetic properties and possible mechanism of ZnO:Co DMSs prepared by different methods are summarized and reviewed.

Abstract: Structure stability and electronic properties of Cu-doped FeS2 were studied using the first principle calculations based on plane wave pseudo-potential theory. The calculated results revealed that the band-gap Eg of Cu-doped FeS₂ was 0.47 eV. The valence band of the density of state (DOS) was mostly due to the Cu 3d and S p orbitals. The bottom part of conduction band was mostly due to the Fe 3d orbitals. The calculated covalent character of the Fe–S bonds gave large delocalization of the spin resulting in smaller values. The Cu, Fe and S had the spin compensated leading to configuration s0.47 p0.61d9.78, Fe s0.27p0.58d7.03, S s1.83p4.23, respectively. The tetrahedral environment of the Fe and Cu and the relatively weak field of the S2− ligand were consistent to the Fe3+ and Cu+.

Abstract: To enhance the quality of ZnO films on Si(111) substrate, single layers of low-temperature ZnO (LT-ZnO) and AlN, as well as a combination of AlN and LT-ZnO layer, were used as intermediate layers by atmospheric pressure metalorganic chemical vapor deposition system. Only polycrystalline ZnO film was formed when a LT-ZnO single buffer was used. Crystal quality was enhanced when LT-ZnO was replaced by 20 nm AlN as the single buffer. The full width at half maximum (FWHM) of ZnO(0002) x-ray diffraction ω-rocking curve was 642 arcsec. Cracks began to appear on the film surface as crystallinity was enhanced. A ZnO mosaic single-crystal film with a mirror-like surface was successfully fabricated when a combined AlN and LT-ZnO served as buffer, and its FWHM of ZnO(0002) ω-rocking curve peak was only 460 arcsec. The film surface was smoother but cracks were still evident on the film. Contrary to the three-dimensional growth mode of samples with a single buffer, a quasi-two-dimensional growth mode was realized for the double-buffered high-temperature ZnO layer. Calculated film thickness was 2.14 μm, and the growth rate reached 4.3 μm/h based on the laser in situ laser reflectance trace.

Abstract: Ca(MoO₄)_x(WO₄)_(1-x) solid solution microcrystallines were synthesized by hydrothermal method at 120 °C; the crystal structure, surface morphology and room temperature photoluminescence properties of the as-synthesized microcrystallines were investigated by through X-ray diffraction (XRD), scanning electron micrograph (SEM), Fourier transform infrared spectroscopy (FT-IR) and fluorescence analysis (FA), respectively. Our results show that the obtained Ca(MoO4)x(WO4)(1-x)microcrystallines are single-phase scheelite structure with tetragonal symmetry. The grain size of the Ca(MoO4)x(WO4)(1-x) microcrystallines gradually increases with the increasing x (except x=0) and their agglomeration also becomes serious with x increasing. Under excited by 245 nm or 280 nm ultraviolet light (UV-light) at room temperature, the emission spectra of the Ca(MoO4)x(WO4)(1-x) microcrystallines vary slightly from 410nm to 490nm with increasing x. Whereas the emission intensity of Ca(MoO4)x(WO4)(1-x) microcrystallines changed markedly, and it increases with the increasing x (x＞0).

Abstract: Regular rare earth permanent magnet measurement systems are only able to measure standard samples with a fixed size. But NdFeB materials are usually processed into various kinds of shapes. Bonding magnets are even often formed together with the shaft or other parts. So there will be a distinct error in conventional measurement. The magnetic field intensity distribution between two poles of the system was analyzed. Because of the existence of air gap between the magnet and the poles of the measurement system, the measurement magnetic circuit is open in this experiment. By changing the height of the magnet and air gap, the measurement error of the intrinsic coercivity Hcj of NdFeB magnets is researched under this condition.

Abstract: We present the simulation results of the InGaAs/InP avalanche photodiode (APD). In the structure a 70 nm InGaAsP grade charge layer and a 70 nm InP charge layer between absorption and multiplication layer have been used for reducing the dark current and achieving higher avalanche gain. A 50 avalanche gain around 35 V breakdown voltages has obtained, which has enhanced by nearly 4 times than that of the conventional InGaAs/InP APD. It has been also shown that the dark current in the device can be significantly reduced nearly one order compared to the corresponding conventional one. The numerical simulation means may design the high gain and low breakdown voltage InGaAs/InP APD.

Abstract: Single crystal of Lu2Si2O7 (LPS):0.5%Ce,0.1%Ca was grown by the Czochralski method. The X-ray excited luminescence (XEL), photoluminescence excitation (PLE), photoluminescence (PL) and transmittance spectra were measured and discussed. The as grown LPS:Ce,Ca sample presents excellent optical quality with 81% transmittance. Two absorption peaks locate at 300 and 350 nm, corresponding to the electron transition of Ce3+ from 4f ground to 5d1 and 5d2 respectively. According to the Gaussian fitting, the XEL curve of LPS:Ce,Ca can be fitted into two peaks centering at 378 and 407 nm respectively. It is found that the addition of Ca2+ in LPS:Ce introduces more oxygen vacancies, leading to the decreases of the luminescence efficiency of LPS:Ce. Through the thermally stimulated luminescence (TSL) measurement, two kinds of charge trap are found in LPS:Ce,Ca, whose energy depths are 1.20 and 1.47 eV. The trap at 1.20 eV is intrinsic electron trap induced by Ce3+ doping in LPS host and the other trap is formed by the both impact of and defects.

Abstract: The Eu-doped TiO₂ nanocrystals with different content of Eu³⁺ and the Eu-doped TiO₂-ZnO composite powders with different content of ZnO were prepared by sol-gel method. The X-ray diffraction (XRD) spectra and photoluminescence(PL) spectra indicated that all of the Eu-doped TiO₂ have anatase structure. It indicated that the incorporation of Eu³⁺ can inhibit the transformation of TiO₂ from anatase to rutile phase. With the increase of the content of Eu³⁺, the luminous intensity first increased and then decreased, and luminescent properties were best when the content of Eu³⁺ is 1.1%. Considering the Eu-doped TiO₂-ZnO composite powders, with the increasing of the proportion of ZnO, a Zn₂TiO₄ phase has been gradually generated. When the content of ZnO up to 40%, the luminescent properties are the best.