Luminescence characteristics of Pb centres in undoped and Ce-doped Lu3Al5O12 single-crystalline films and Pb→Ce energy transfer processes

At 4.2-350 K, the steady-state and time-resolved emission and excitation spectra and luminescence decay kinetics were studied under excitation in the 2.5-15 eV energy range for the undoped and Ce³⁺-doped Lu₃Al₅O₁₂ (LuAG) single-crystalline films grown by liquid phase epitaxy method from the PbO-based flux. The spectral bands arising from the single Pb²⁺-based centres were identified. The processes of energy transfer from the host lattice to Pb²⁺ and Ce³⁺ ions and from Pb²⁺ to Ce³⁺ ions were investigated. Competition between Pb²⁺ and Ce³⁺ ions in the processes of energy transfer from the LuAG crystal lattice was evidenced especially in the exciton absorption region. Due to overlap of the 3.61 eV emission band of Pb²⁺ centres with the 3.6 eV absorption band of Ce³⁺ centres, an effective nonradiative energy transfer from Pb²⁺ ions to Ce³⁺ ions takes place, resulting in the appearance of slower component in the luminescence decay kinetics of Ce³⁺ centres and decrease of the Ce³⁺-related luminescence intensity.     

Crystal structure and photoluminescence of (Y1−xCex)2Si3O3N4

Oxonitridosilicate phosphors with compositions of (Y_1−xCe_x)₂Si₃O₃N₄ (x=0−0.2) have been synthesized by solid state reaction method. The structures and photoluminescence properties have been investigated. Ce³⁺ ions have substituted for Y³⁺ ions in the lattice. The emission and excitation spectra of these phosphors show the characteristic photoluminescence spectra of Ce³⁺ ions. Based on the analyses of the diffuse reflection spectra and the PL spectra, a systematic energy diagram of Ce³⁺ ion in the forbidden band of sample with x=0.02 is given. The best doping Ce content in these phosphors is ∼2 mol%. The quenching temperature is ∼405 K for the 2 mol% Ce content sample. The luminescence decay properties were investigated. The primary studies indicate that these phosphors are potential candidates for application in three-phosphor-converted white LEDs.     

Reactivity and XAFS study on (1−x)CeO2-xNiO (x=0.025-0.3) system in the two-step water-splitting reaction for solar H2 production

The redox reaction of Ce⁴⁺-Ce³⁺ promoted by the catalytic function of nickel ions in a (1−x)CeO₂-xNiO solid solution was investigated for solar H₂ production by the two-step water-splitting reaction. By irradiation using an infrared imaging lamp as a solar simulator, the O₂-releasing reaction with (1−x)CeO₂-xNiO solid solution proceeded at 1673-1873 K, and its reduced form was produced. The amounts of H₂ gas evolved by the reduced form were 1.2-2.5 cm³/g and the evolved gases amounts ratio of H₂/O₂ was nearly 2, which is equal to the stoichiometric value of the water-splitting reaction (H₂O=H₂+1/2O₂). The maximum amounts of evolved H₂ and O₂ gases were obtained at the Ce:Ni mole ratio of 0.95:0.05 (x=0.05) in the (1−x)CeO₂-xNiO system. The X-ray absorption fine structure (XAFS) measurement showed that the O₂-releasing and H₂-generation reactions with (1−x)CeO₂-xNiO solid solution were repeatable with the redox system of Ce⁴⁺-Ce³⁺, which was enhanced by the catalytic function of Ni²⁺-Ni⁰.     

Magnetooptical evidence of exchange interactions in zero-gap Hg1−xFexTe mixed crystals

Interband magnetoabsorption is carried out on zero gap Hg_1-xFe_xTe alloys of x ～ 0.015.Γ₆ → Γ₈ magnetooptical spectra for σ⁺, σ^-, γ 6 H polarization are quantitatively interpreted within the “quasi Ge” model modified by the inclusion of exchange contributions. The field dependence of the magnetization provides evidence of antiferromagnetic interactions between localized spins.     

Reaction of water with Ce-Au(1 1 1) and CeOx/Au(1 1 1) surfaces: Photoemission and STM studies

This article reports photoemission and STM studies for the adsorption and dissociation of water on Ce-Au(1 1 1) alloys and CeO_x/Au(1 1 1) surfaces. In general, the adsorption of water at 300 K on disordered Ce-Au(1 1 1) alloys led to O-H bond breaking and the formation of Ce(OH)_x species. Heating to 500-600 K induced the decomposition or disproportionation of the adsorbed OH groups, with the evolution of H₂ and H₂O into gas phase and the formation of Ce₂O₃ islands on the gold substrate. The intrinsic Ce ↔ H₂O interactions were explored by depositing Ce atoms on water multilayers supported on Au(1 1 1). After adsorbing Ce on ice layers at 100 K, the admetal was oxidized immediately to yield Ce³⁺. Heating to room temperature produced finger-like islands of Ce(OH)_x on the gold substrate. The hydroxyl groups dissociated upon additional heating to 500-600 K, leaving Ce₂O₃ particles over the surface. On these systems, water was not able to fully oxidize Ce into CeO₂ under UHV conditions. A complete Ce₂O₃ → CeO₂ transformation was seen upon reaction with O₂. The particles of CeO₂ dispersed on Au(1 1 1) did not interact with water at 300 K or higher temperatures. In this respect, they exhibited the same reactivity as does a periodic CeO₂(1 1 1) surface. On the other hand, the Ce₂O₃/Au(1 1 1) and CeO_2−x/Au(1 1 1) surfaces readily dissociated H₂O at 300-500 K. These systems showed an interesting reactivity for H₂O decomposition. Water decomposed into OH groups on Ce₂O₃/Au(1 1 1) or CeO_2−x/Au(1 1 1) without completely oxidizing Ce³⁺ into Ce⁴⁺. Annealing over 500 K removed the hydroxyl groups leaving behind CeO_2−x/Au(1 1 1) surfaces. In other words, the activity of CeO_x/Au(1 1 1) for water dissociation can be easily recovered. The behavior of gold-ceria catalysts during the water-gas shift reaction is discussed in light of these results.     

The effect of Te doping on the electronic structure and thermoelectric properties of SnSe

SnSe_1−xTe_x (x=0, 0.0625) bulk materials were fabricated by melting Sn, Se and Te powders and then hot pressing them at various temperatures. The phase compositions of the materials were determined by X-ray diffraction (XRD) and the crystal lattice parameters were refined by the Rietveld method performed with DBWS. XRD analysis revealed that the grains in the materials preferentially grew along the (l 0 0) directions. The structural behavior of SnSe_1−xTe_x (x=0, 0.0625) was calculated using CASTEP package provided by Materials Studio. We found that the band gap of SnSe reduced from 0.643 to 0.608 eV after Te doping. The calculated results were in good agreement with experimental results. The electrical conductivity and the Seebeck coefficient of the as-prepared materials were measured from room temperature to 673 K. The maximum power factor of SnSe is ∼0.7 μW cm⁻¹K⁻² at 673 K.     

Sn interaction with the CeO2(1 1 1) system: Bimetallic bonding and ceria reduction

A tin layer 0.8 nm thick was deposited onto the CeO₂(1 1 1) surface by molecular beam deposition at a temperature of 520 K. The interaction of tin with cerium oxide (ceria) was investigated by X-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS) and resonant photoelectron spectroscopy (RPES). The strong tin-ceria interaction led to the formation of a homogeneous bulk Ce-Sn-O mixed oxide system. The bulk compound formation is accompanied by partial Ce⁴⁺ → Ce³⁺ reduction, observed as a giant 4f resonance enhancement of the Ce³⁺ species. CeO₂ and SnO₂ oxides were formed after oxygen treatment at 520 K. The study proved the existence of strong Ce-Sn interaction and charge transfer from Sn to the Ce-O complex that lead to a weakening of the cerium-oxygen bond, and consequently, to the formation of oxygen deficient active sites on the ceria surface. This behavior can be a key for understanding the higher catalytic activity of the SnO_x/CeO_x mixed oxide catalysts as compared with the individual pure oxides.     

Study of Zn1−xCoxO (0.02≤x≤0.08) dilute magnetic semiconductor prepared by mechanosynthesis route

Single-phase Zn_1−xCo_xO (0.02≤x≤0.08) dilute magnetic semiconductor is prepared by mechanical milling process. The shift of XRD peaks towards the higher angle and a redshift in the band gap compared to the undoped ZnO ensure the incorporation of Co²⁺ ions in the semiconductor host lattice. Pure Zn_xCo_1−xO phases show the paramagnetic behavior in the temperature range 80 K≤T≤300 K. The room temperature volume magnetic susceptibility (χ_v) estimated in case of Zn_0.96Co_0.04O is ∼10⁻⁵ emu/Oe cm³. The temperature dependence of susceptibility χ_v can be fitted well with Curie law confirming the paramagnetic interaction. The observed crystal-field splitting of 3d levels of Co²⁺ ions inside Zn_1−xCo_xO has been successfully interpreted using Curie law.     

High resolution laser spectroscopy of the titanium monohalides, TiCl and TiBr

The electronic spectra of the titanium monohalides, TiCl and TiBr, have been investigated in the blue-violet region using a laser ablation molecular beam spectrometer. Five subbands assigned as ⁴Γ_5/2-X⁴Φ_3/2 (0, 0), ⁴Γ_5/2-X⁴Φ_3/2 (1, 1), ⁴Γ_5/2-X⁴Φ_5/2 (0, 0), ⁴Γ_7/2-X⁴Φ_5/2 (0, 0) and ⁴Γ_7/2-X⁴Φ_7/2 (0, 0) were observed and recorded at both low and high-resolution for titanium monochloride. A Hund’s case (a) rotational analysis has been carried out for the ⁴⁸Ti³⁷Cl and ⁴⁸Ti³⁵Cl isotopic species, and polynomial analyses for these, as well as the ⁴⁶Ti³⁵Cl, ⁴⁷Ti³⁵Cl, ⁴⁹Ti³⁵Cl, and ⁵⁰Ti³⁵Cl isotopologues have been completed. The same spectral region yielded several molecular transitions for titanium monobromide, 10 of which were recorded at high resolution. Six of these have been attributed to a ⁴Γ-X⁴Φ electronic transition at 23 484 cm⁻¹, while the remaining four have been assigned to a second ⁴Γ-X⁴Φ electronic transition at 23 613 cm⁻¹. A Hund’s case (a) global analysis has been carried out for the ⁴⁸Ti⁷⁹Br and ⁴⁸Ti⁸¹Br isotopologues.     

Parallel syntheses and thermoelectric properties of Ce-doped SrTiO3 thin films

Thermoelectric properties of single crystalline Ce_xSr_1−xTiO₃ films (0 ≤ x ≤ 0.5) have been studied by using combinatorial pulsed-laser deposition. Temperature gradient method was used for identifying an optimum growth temperature for SrTiO₃ homoepitaxial growth, at which both oxygen stoichiometry and persisting layer-by-layer growth mode could be accomplished. Electrical conductivity (σ) and Seebeck coefficient (S) were measured at room temperature for the composition-spread films grown at the optimized temperature and found to be considerably higher than those reported for bulk poly-crystalline compounds. Hall measurement revealed that carrier density linearly increased with increasing x, suggesting that a trivalent Ce ions substituted divalent Sr ions to supply electrons. A maximum power factor (S²σ) was obtained for the x = 0.2 film, being 7 and 14 μW/K² cm at 300 and 900 K, respectively.     

Luminescence properties of LiPrxCe1−xP4O12

LiPr_1−xCe_xP₄O₁₂ (x=0, 0.002, 0.02; 0.1) powder samples were prepared using the melt solution technique. Luminescent parameters of LiPr_1−xCe_xP₄O₁₂ phosphors have been investigated under ultraviolet-vacuum ultraviolet (3-12 eV) synchrotron radiation and X-rays excitation at room and near liquid He temperatures. Excitation luminescence spectra of Ce³⁺ emission, luminescent spectra and decay curves from the lower excited state levels of the 4f¹5d¹ and 5d¹ electronic configuration of the Pr³⁺ and Ce³⁺, respectively, clearly indicate energy transfer from Pr³⁺ to Ce³⁺. Energy migration proceeds via the Pr-sublattice followed by nonradiation transfer from Pr³⁺ to Ce³⁺ ions.     

Ca2BO3Cl:Ce,Eu: A potential tunable yellow-white-blue-emitting phosphors for white light-emitting diodes

Polycrystalline Ca₂BO₃Cl:Ce³⁺,Eu²⁺ phosphors were synthesized by a solid-state reaction and which could display tunable color emission from blue to yellow under an ultraviolet (UV) source by adjusting the ratio of Ce³⁺ and Eu²⁺ appropriately. The mechanism of resonance-type energy transfer from Ce³⁺ to Eu²⁺ was established to be electric dipole-dipole natured, and the critical distance was estimated to be 31 Å based on the spectral overlap and concentration quenching model. A white light was obtained from Ca₂BO₃Cl:0.06Ce³⁺,0.01Eu²⁺ phosphor with chromaticity coordinates (x=0.31, y=0.29) and relative color temperature of 7330 K upon excitation with 360 nm, which is potentially a good candidate as an UV-convertible phosphor for white light-emitting diodes (LEDs).     

Enhanced luminescent properties of Y3Al5O12:Tb,Ce phosphor prepared by spray pyrolysis

Yttrium aluminum garnet (YAG) particles doped with Tb³⁺ or double doped with Tb³⁺ and Ce³⁺ were prepared by spray pyrolysis and characterized by photo- and cathode-luminescence. It was tried to incorporate a broad band of Ce³⁺ activator into the line peaks of Tb³⁺ in YAG host without the reduction of emission intensity. Ce-codoped YAG:Tb particles showed a broad band emission due to the d-f transition of Ce³⁺ and a reduction in the intensity of emission peaks due to ⁵D₃-⁷F_j (j=3, 4, 5, 6) transition of Tb³⁺ when they were excited by the ultraviolet light of 270 nm. These results supported that an effective energy transfer occurs from Tb³⁺ to Ce³⁺ in YAG host. Codoping Ce³⁺ ions greatly intensified the excitation peak at 270 nm for the emission at 540 nm of Tb³⁺, which means that more lattice defects, involving in the energy absorption and transfer to Tb³⁺, are formed by the Ce³⁺ codoping. The finding gives a promising approach for enhancing the luminescence efficiency.     

Energy transfer and luminescent properties of Ce, Cr co-doped Y3Al5O12

The phosphors in the system Y_3−xAl_5-yO₁₂:xCe³⁺,yCr³⁺ were synthesized by solid-state reactions and their photoluminescence properties were investigated. These phosphors have absorption in the visible light region and give luminescence in the far-red region (∼688 nm), which are suitable for the application in the device of luminescent solar concentrator (LSC). In these phosphors, Ce³⁺ located at Y³⁺ site can effectively transfer its absorbed energy to Cr³⁺ at Al³⁺ site.     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

Chemical pressure effect in CeFeAs1−xPxO0.95F0.05

We investigate the chemical pressure effect due to P doping in the CeFeAs_1−xP_xO_0.95F_0.05(0≤x≤0.4) system. The compound CeFeAsO_0.95F_0.05 without P doping is on the boundary between antiferromagnet (AFM) and superconductor. The AFM order of Ce³⁺ local moments causes a significant reentrance behavior in both resistivity and magnetic susceptibility. Upon P doping, T_c increases and reaches a maximum of 21.3 K at x=0.15, and then it is suppressed to lower temperatures. Meanwhile, the AFM order of Ce³⁺ ions remains nearly the same in the whole doping range (0≤x≤0.4). Our experimental results suggest a competition between superconductivity and Kondo effect in the Ce 1111 system.     

Synthesis, structure and luminescence of LaSi3N5:Ce phosphor

In this work, new LaSi₃N₅:Ce³⁺ phosphors have been synthesized by solid-state reaction. Rietveld refinement of the crystal structure of La_1−xCe_xSi₃N₅ reveals that Ce atoms substituted for La atoms occupy 4a crystallographic positions. Broad emission and excitation bands observed were attributed to the transitions between the doublet ground state of the 4f¹ configuration and the crystal field components of the 5d¹ excited state. At 77 K, the centroid and crystal field splitting ε_cfs of the 5d levels of Ce³⁺ in LaSi₃N₅:Ce³⁺ compounds were valuated at 33.4×10³ and 11.3×10³ cm⁻¹, respectively. The zero-phonon line and the Stokes shift were measured to be 26.0×10³ and 5.0×10³ cm⁻¹, respectively.     

Electronic and magnetic properties of layered LnFePO (Ln=La and Ce)

Effects of the replacement of La with Ce on the electronic and magnetic properties of a layered superconductor LaFePO (T_c=∼5 K) were studied. Polycrystalline samples of CeFePO, prepared by a solid-state reaction, showed metallic conduction down to 2 K without exhibiting superconducting transition, although the resistivity decreased largely at temperatures below 30 K. Further, they showed an apparent positive magnetoresistance (MR) below ∼2 K, superposed on a negative MR. Temperature dependence of magnetic susceptibility is decomposed to a temperature-sensitive Curie-Weiss component presumably due to the Ce³⁺ ions with a magnetic moment of 1.98μ_B and a less temperature-sensitive component attributable to itinerant electrons. The magnetic interaction between Ce³⁺ ions and itinerant electrons in CeFePO likely suppresses the superconducting transition observed in LaFePO.     

Spectral studies on Cr ions doped in sodium-lead borophosphate glasses

Electron paramagnetic resonance (EPR), optical absorption and emission spectra of Cr³⁺ ions doped in (30−x) (NaPO₃)₆+30PbO+40B₂O₃+xCr₂O₃ (x=0.5, 2.0, 3.0, 4.0 and 5.0 mol%) glasses have been studied. The EPR spectra exhibit resonance signals with effective g values at g≈4.55 and g≈1.97. The EPR spectra of x=3.0 mol% of Cr₂O₃ in sodium-lead borophosphate glass sample were studied at various temperatures (295-123 K). The intensity of the resonance signals increases with decrease in temperature. The optical absorption spectrum exhibits four bands characteristic of Cr³⁺ ions in octahedral symmetry. From the analysis of the bands, the crystal-field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. The emission spectrum exhibit one broad band characteristic of Cr³⁺ ions in octahedral symmetry. This band has been assigned to the transition ⁴T_2g (F)→⁴A_2g (F). Correlating EPR and optical data, the molecular bonding coefficient (α) has been evaluated.     

Valence band spin-orbit splitting in CdTe and Cdf−xMnxTe and giant zeeman effect in the Γ7 band of Cd1−xMnxTe

Optical transitions from spin-orbit split-off valence band Γ₇ to the conduction band Γ₆ were directly observed in transmission measurements on thin samples of CdTe and Cd_1?xMn_xTe. Precise values of spin-orbit splittings were determined. Giant Zeeman splitting of Γ₇ - Γ₆ exciton in Cd_1?xMn_xTe was observed and found coherent with corresponding splitting of Γ₈-Γ₆ exciton.