Optimum IV-group ion concentrations in ZnGa2O4−xMx (M=S, Se, Te) phosphors for enhancement of luminescence

The ZnGa₂O_4−xM_x (M=S, Se, and Te ) samples with varying S, Se, and Te concentrations are synthesized through solid-state reactions. The X-ray diffraction patterns of ZnGa₂O_4−xM_x (M=S, Se, and Te) show that the positions of the (4 0 0) diffraction peak gradually shift to lower angles due to the doping of VI-group ions (S, Se, and Te) with larger ionic radius than oxygen. For ZnGa₂O_4−xS_x samples, the solubility limit is found to be about x=0.30. The cathodoluminescence measurements on ZnGa₂O_4−xM_x samples show that the optimized S, Se, and Te concentrations with the highest cathodoluminescence intensities are 0.10, 0.05, and 0.03, respectively. The luminous intensity of ZnGa₂O_3.95Se_0.05 is four times higher than that of ZnGa₂O₄. Thus, ZnGa₂O_3.95Se_0.05 can be a promising candidate phosphor for FED applications.     

Synthesis and photoluminescent properties of mesoporous (MgO)x(ZnO)1−x materials

(MgO)_x(ZnO)_1−x materials have been synthesized using mesoporous carbon as template. By increasing the MgO content in the materials greater than 25%, the (MgO)_x(ZnO)_1−x materials began to form the mesoporous structure. Pore size distribution curves indicated that the BJH pore diameter decreased with increasing MgO content. In photoluminescence spectra, all the samples except pure ZnO showed both the band-edge emission and the deep-level emission (green band). It was interesting to note that the UV emission peak energy (E_UV) had a red-shift of about 48 meV at the low MgO content range of 0-25%, while when the MgO content varied from 25 to 75%, the E_UV displayed a blue-shift of about 36 meV to the higher energy direction. The optical band gap (E_g) of the (MgO)_x(ZnO)_1−x calculated from the absorption spectra was far smaller than that in literature, and this may be related to the formation of mesoporous structure.     

Synthesis and characterization of M1−xNdxF2+x (M = Sr, Ca; 0.00 ≤ x ≤ 1.00)

Series of the mixed fluorides with the general composition M_1−xNd_xF_2+x (0.00≤x≤1.00; M=Sr²⁺ and Ca²⁺) were prepared by a vacuum heat treatment of the appropriate mixtures of MF₂ (M=Ca and Sr) and NdF₃. The products obtained were analyzed by powder X-ray diffraction, which revealed the phase relations in these systems. At the MF₂-rich compositions a fluorite-type solid solution was observed in both systems. The typical solid solution limits of NdF₃ in the SrF₂ and CaF₂ lattice are about 40 and 45 mol%, respectively. The incorporation of NdF₃ in the fluorite lattice of CaF₂ causes an expansion of the unit cell volume. The unit cell volume of the fluorite lattice in Sr_1−xNd_xF_2+x system surprisingly remains constant throughout the solid solution range. The NdF₃ (tysonite) type phase separates out beyond the solid solution limit, in both systems. Also, it was observed that about 15 and <10 mol% of CaF₂ and SrF₂, respectively, could be retained in the NdF₃, maintaining the tysonite lattice. No fluorite or tysonite-related ordered phases were observed in these systems.     

Tunable properties of wide-band gap p-type BaCu(Ch1 − xChx′)F (Ch = S, Se, Te) thin-film solid solutions

Thin-film solid solutions of BaCu(Ch_1 − xCh_x′)F (Ch, Ch′ = S, Se, or Te) wide-band gap p-type semiconductors are obtained by pulsed laser deposition at elevated substrate temperatures from alternating layers of BaCuChF and BaCuCh′F. Adjusting the thickness of the component layers varies the relative chalcogen content, which allows tunability of the film transparency and results in a conductivity change of more than three orders of magnitude. The tunability of the physical properties makes these chalcogen-based semiconductors potentially useful for optoelectronics applications. Lattice parameters of BaCuChF calculated using density functional theory agree with those previously reported for the powders. Deviations from Vegard's law are observed in BaCu(S_1 − xSe_x)F thin films with large sulfur content.     

Nonlinear optical diagnostic of semimagnetic semiconductors Pb1−xYbxX (X = S, Se, Te)

Nonlinear optical measurements were performed to elucidate the influence of magnetic ions on the behavior of charge carriers in magnetic semiconductors—Pb_1−xYb_xX (X = S, Se, Te at x = 1-3%). It was shown that nonlinear optical methods could be used as sensitive tools for investigations of electron-phonon anharmonicity near low-temperature semiconductor-insulator phase transitions. There exists a difference between surface and bulk-like contributions to the nonlinear optical effects. It was shown that only low-temperature Two Photon Absorption (TPA) oscillator may be related to the number of the electron-phonon anharmonic modes responsible for the observed phase transformation. The explanation of the anomalous temperature dependences is given in accordance with dipole momentum's behaviors determined by low-temperature spin-spin interactions and by electron-phonon anharmonic interactions. We have discovered that low-temperature dependence of specific heat of Pb_1−xR_xTe (R = Yb, Pr with x = 3% and 1.6%, respectively) exhibits a non-magnetic order caused by large electron-phonon contributions and structural disorder effects.     

Impurity induced band-gap narrowing in p-type CuIn1 − xGax(S,Se)2

Green's functions modelling of the impurity induced effects in p-type CuIn_1 − xGa_xS₂ and CuIn_1 − xGa_xSe₂ (x = 0.0, 0.5, and 1.0) reveals that: (i) the critical active acceptor concentration for the metal non-metal transition occurs at N_c ≈ 10¹⁷-10¹⁸ cm^− 3 for impurities with ionization energy of E_A ≈ 30-60 meV. (ii) For acceptor concentrations N_A > N_c, the hole gas of the metallic phase affects the band-edge energies and narrows the energy gap E_g = E_g⁰ − ΔE_g. The energy shift of the valence-band maximum ΔE_v1 is roughly twice as large as the shift of the conduction-band minimum ΔE_c1. (iii) ΔE_v1 depends strongly on the non-parabolicity of the valence bands. (iv) Sulfur based compounds and Ga-rich alloys have the largest shifts of their band edges. (v) A high active acceptor concentrations of N_A = 10²⁰ cm^− 3 implies a band-gap narrowing in the order of ΔE_g ≈ 0.2 eV, thus E_g = E_g⁰ − 0.2 eV, and an optical band gap of E_g^opt ≈ E_g⁰ − 0.1 eV.     

Hydrothermal synthesis of nanocrystalline MxZn1−xFe2O4 (M=Ni, Mn, Co; x=0.40-0.60) powders

A series of nanocrystalline M_xZn_1−xFe₂O₄ (M=Ni, Mn and Co; x=0.40-0.60) powders have been successfully prepared via hydrothermal process and characterized by XRD, TEM and IR techniques. The effects of reaction temperature and the initial pH value of the starting suspension solution on the particulate properties such as the particle size and morphology are discussed. IR spectra indicate that there are no hydroxyl in as-prepared Ni_xZn_1−xFe₂O₄ and Co_xZn_1−xFe₂O₄ powders, while there are obvious hydroxyl adsorption on the IR spectrum of Mn_xZn_1−xFe₂O₄ powder.     

Synthesis, structure and dielectric characterization of Ln2Ti2−2xM2xO7(Ln=Gd, Er; M=Zr, Sn, Si)

The progress in wireless communications and information access has demanded the use of electronic ceramics exhibiting desired properties. To further our understanding of these properties, compounds in the Ln₂Ti_2-2xM_2xO₇ (Ln=Gd, Er; M=Zr, Sn, Si) systems were synthesized by ceramic methods and characterized by powder X-ray diffraction. The ZrO₂-doped Gd₂Ti_2−2xZr_2xO₇ compounds adopt the pyrochlore structure type and form a complete solid solution. Er₂Ti_2−2xZr_2xO₇ forms a pyrochlore solid solution for x<0.1. However, stoichiometric Er₂Zr₂O₇ does not form; instead Er₄Zr₃O₁₂ forms a with defect fluorite structure. The Sn-doped Ln₂Ti_2−2xSn_2xO₇ (Ln=Gd, Er) compounds form complete solid solutions, and the Si compounds adopt the pyrochlore structure up to x=0.05. At ambient temperature, dielectric constants range from 10 to 61 for Er₂Ti_2−2xZr_2xO₇ and 16-31 for Gd₂Ti_2−2xZr_2xO₇ with low dielectric loss (1×10⁻³) at 1 GHz.     

Composition dependence of optical constants in ferroelectric Ba(Ti1 − x,Nix)O3 thin films by optical transmittance technique

Ba(Ti_1 − x,Ni_x)O₃ ferroelectric thin films with perovskite structure are prepared on fused quartz substrates by a sol-gel process. Optical transmittance measurements indicate that Ni-doping has an obvious effect on the energy band structure of BaTiO₃. It has been found that the refractive index n, extinction coefficient k, and band gap energy E_g of the films are functions of the film composition. The E_g of Ba(Ti_1 − x,Ni_x)O₃ decreases approximately linearly as the Ni content increases, which is attributed to the decline of conduction band energy level with increasing the Ni content. On the other hand, n and k both increase linearly with increasing the Ni content because of the increase of packing density. These results indicate that thin films might have potential applications in BaTiO₃-based thin-film optical devices.     

Synthesis and crystal structures of Ba6Mg11F34 and the solid solutions Ba6Mg11−xMxF34 (M = Mn, Fe) and luminescence of Ba6Mg11F34: Eu

Ba₆Mg₁₁F₃₄, a new compound of the pseudobinary BaF₂-MgF₂ system, has been synthesized by solid state techniques from stoichiometric amounts of BaF₂ and MgF₂ and its crystal structure determined by single crystal X-ray diffraction (space group P, a=7.5084(6), b=9.9192(8), c=10.0354(8) Å, α=81.563(2), β=72.402(2), γ=71.198(1)°, 3899 structure factors, 233 parameters, R(F²>2σ(F²))=0.018, wR(F² all) = 0.046). It is isotypic with the copper(II) analogue, Ba₆Cu₁₁F₃₄. The main features of the structure are a network of [MgF₆] octahedra and three different [BaF_x] polyhedra with x=12, 11+1 and 13. Ba₆Mg_11−xFe_xF₃₄ and Ba₆Mg_11−xMn_xF₃₄ solid solutions were prepared and their composition determined by single crystal structure analyses. The luminescence properties of Ba₆Mg₁₁F₃₄ doped with Eu²⁺ were studied using fluorescence spectroscopy. The observed luminescence was pale blue with a maximum at 465 nm.