Morphology-dependent luminescence behavior of Y<Subscript>2-x</Subscript>Gd<Subscript>x</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> thin-film phosphors grown by a laser ablation

Y_2-xGd_xO₃:Eu³⁺ luminescent thin films have been grown on Al₂O₃(0001) substrates using pulsed laser deposition. Films grown under different deposition conditions have been characterized using microstructural and luminescence measurements. The crystallinity, surface morphology and photoluminescence (PL) of the films are highly dependent on the amount of Gd present. The photoluminescence (PL) brightness data obtained from Y_2-xGd_xO₃:Eu³⁺ films grown under optimized conditions have indicated that Al₂O₃(0001) is one of the most promising substrates for the growth of high-quality Y_2-xGd_xO₃:Eu³⁺ thin-film red phosphors. In particular, the incorporation of Gd into the Y₂O₃ lattice could induce a remarkable increase of PL. The highest emission intensity was observed with Y_1.35Gd_0.60Eu_0.05O₃, whose brightness was increased by a factor of 3.1 in comparison with that of Y₂O₃:Eu³⁺ films. This phosphor may be promising for application in flat-panel displays. PACS 78.20.-e; 78.55.-m; 78.66.-w     

LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> hollow nano-micro hierarchical microspheres as advanced cathode for lithium ion batteries

The high-voltage spinel-type LiNi_0.5Mn_1.5O₄ (LNMO) is a promising cathode material for next-generation lithium ion batteries. In this study, hollow LNMO microspheres have been synthesized via co-precipitation method accompanied with high-temperature calcinations. The physical and electrochemical properties of the materials are characterized by x-ray diffraction (XRD), TGA, RAMAN, CV, scanning electron microscope (SEM), transmission electon microscopy (TEM), electrochemical impendence spectroscopy (EIS), and charge-discharge tests. The results prove that the microspheres combine hollow structures inward and own a cubic spinel structure with space group of Fd-3m, high crystallinity, and excellent electrochemical performances. With the short Li⁺ diffusion length and hollow structure, the hierarchical LNMO microspheres exhibit 138.2 and 108.5 mAh g^?1 at 0.5 and 10 C, respectively. Excellent cycle stability is also demonstrated with more than 98.8 and 88.2 % capacity retention after 100 cycles at 1 and 10 C, respectively.     

Effects of Gd<Superscript>3+</Superscript> modifications on the photoelectrochemical properties of TiO<Subscript>2</Subscript>-based dye-sensitized solar cells

In this paper, TiO₂ particles (~30 nm) modified with Gd₂O₃-coating layer (~2 nm) for dye-sensitized solar cells (DSSCs) were fabricated via the hydrothermal method. Among the solar cells based on the Gd³⁺-doped TiO₂ photoanodes, the optimal conversion efficiency was obtained from the 0.025Gd³⁺-modified TiO₂-based cell, with a 17.7% improvement in the efficiency as compared to the unmodified one (7.18%). This enhancement was probably due to the improved UV radiation harvesting via a down-conversion luminescence process by Gd³⁺ ions, enhancement of visible light absorption and improved dye loading capacity. In addition, after Gd modification, a thin coating could be formed on the TiO₂ nanoparticles, which worked as an energy barrier and resulted in a lower charge recombination.     

Energy levels of rare-earth ions in Gd<Subscript>2</Subscript>O<Subscript>2</Subscript>S

The energies of the ground 4f ⁿ levels of tri- and divalent rare-earth ions with respect to the conduction and valence bands of Gd₂O₂S crystal has been determined. It is shown that the Pr³⁺, Tb³⁺, and Eu³⁺ ions can be luminescence centers in Gd₂O₂S. The levels of the Nd³⁺, Dy³⁺, Er³⁺, Tm³⁺, Sm³⁺, and Ho³⁺ ions lie in the valence band; therefore, these ions cannot play the role of activators. The ground 4f level of the Ce³⁺ ion is near the midgap, due to which Ce³⁺ effectively captures holes from the valence band and electrons from the conduction band and significantly decreases the afterglow level of the Gd₂O₂S:Pr and Gd₂O₂S:Tb phosphors.     

An improved method for preparation of Ce<Subscript>0.8</Subscript>Pr<Subscript>0.2</Subscript>O<Subscript>Y</Subscript> solid solutions with nanoparticles smaller than 10 nm

Ce_0.8Pr_0.2O_Y solid solutions with ultrafine crystalline sizes and high specific surface area were prepared by an improved citrate precursor method, where a nitrogen treatment was added prior to calcinations in air. The samples were characterized by TG-DSC, Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and BET nitrogen adsorption. XRD and Raman results show that the formation of Ce_0.8Pr_0.2O_Y solid solutions typical of the fluorite-like cubic structure with oxygen vacancies occurs when the Ce–Pr citrate precursors are heated at high temperature in the nitrogen atmosphere. Subsequent calcinations at a low temperature in air to remove carbon species have no apparent effects on the formed solid solutions. Ce_0.8Pr_0.2O_Y solid solution prepared by the improved citrate precursor method at 800°C has ultrafine nanoparticles of less than 10 nm and high specific surface area of 92.1 m²/g, while the sample prepared by the conventional citrate precursor method has mean particle size of 62.1 nm and specific surface area of 18.1 m²/g. Furthermore, Ce–Pr solid solution by the improved method have the mesoporous structure, more lattice defects and oxygen vacancies, which will have a promising application in the catalyst region as well as SOFC field.     

Giant positive magnetoresistance in heterostructure (La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>) coated with YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript> composites

(La_0.7Sr_0.3MnO₃) _x /(YBa₂Cu₃O₇) _y composites were prepared by mixing La_0.7Sr_0.3MnO₃ powders and the sol–gel-derived YBa₂Cu₃O₇ matrix, followed by high-temperature calcinations. Their structural, magnetic properties and magnetoresistance effect have been investigated systematically. A giant positive magnetoresistance (PMR) at low magnetic field is observed at low temperatures. In the case of (La_0.7Sr_0.3MnO₃)₁/(YBa₂Cu₃O₇)₉ composite, the PMR achieves 260% under a magnetic field of 5800 Oe. However, the PMR value sharply decreases with increasing temperature and no magnetoresistance effects are found above metal-insulator transition temperature. The enhancement of spin-dependent scattering at the grain boundaries should be responsible for the observed PMR. In addition, the temperature dependence of resistance under magnetic field could be explained by the competition between diamagnetism and paramagnetism in YBCO phase. At low temperature, the diamagnetism is predominant over paramagnetism and the interface scattering between LSMO grains is enhanced correspondingly. As a result, the low-temperature resistance increases and large PMR appears.     

Improved photoluminescence of pulsed-laser-ablated Y<Subscript>1-x</Subscript>Gd<Subscript>x</Subscript>VO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> thin film phosphors by Gd substitution

Gd-substituted Y_1-xGd_xVO₄:Eu³⁺ luminescent thin films have been grown on Al₂O₃(0001) substrates using pulsed-laser deposition. The films grown under different deposition conditions have been characterized using microstructural and luminescent measurements. The crystallinity, surface morphology, and photoluminescence (PL) of the films are highly dependent on the amount of Gd. The photoluminescence (PL) brightness data obtained from Y_1-xGd_xVO₄:Eu³⁺ films grown under optimized conditions have indicated that the PL brightness is more dependent on the surface roughness than the crystallinity of the films. In particular, the incorporation of Gd into the YVO₄ lattice could induce a remarkable increase of PL. The highest emission intensity was observed with Y_0.57Gd_0.40Eu_0.03VO₄ thin film whose brightness was increased by a factor of 2.5 and 1.9 in comparison with that of YVO₄:Eu³⁺ and GdVO₄:Eu³⁺ films, respectively. This phosphor have application to flat panel displays. PACS 78.20.-e; 78.55.-m; 78.66.-w     

Red,Yellow, Blue and Green Emission from Eu<Superscript>3+</Superscript>, Dy<Superscript>3+</Superscript> and Bi<Superscript>3+</Superscript> Doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript>nano-Phosphors

Rare earth elements (RE = Eu³⁺& Dy³⁺)and Bi³⁺ doped Y₂O₃ nanoparticles were synthesized by urea hydrolysis method in ethylene glycol, which acts as reaction medium as well as a capping agent, at a low temperature of 140 °C,followed by calcination of the obtained product. Transmission electron microscope (TEM) images reveals that ovoid shaped Y₂O₃ nanoparticles of around 22–24 nm size range were obtained in this method. The respective RE and Bi³⁺ doped Y₂O₃ precursor nanoparticles when heated at 600 and 750 °C, retains the same shape as that of the as-synthesized Y₂O₃ precursor samples. From EDAX spectra, the incorporation of RE ions into the host has been studied. XRD pattern reveals the crystalline nature of the heated nanoparticles and indicate the absence of any impurity phase other than cubic Y₂O₃.However, the as-synthesized nanoparticles were highly amorphous without the presence of any sharp XRD peaks. Photoluminescence study suggests that the synthesized samples could be used as red (Eu³⁺), yellow (Dy³⁺), blue and green (Bi³⁺)emitting phosphors.     

Deposition and stoichiometry control of Nd-doped gadolinium gallium garnet thin films by combinatorial pulsed laser deposition using two targets of Nd:Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> and Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>

We have demonstrated pulsed laser deposition of Nd-doped gadolinium gallium garnet on Y₃Al₅O₁₂ by the simultaneous ablation of two separate targets of Nd:Gd₃Ga₅O₁₂ (GGG) and Ga₂O₃. Such an approach is of interest as a method of achieving stoichiometry control over films whilst the growth parameters are kept constant and optimal for high quality crystal growth. We show here how the stoichiometry and resultant lattice parameter of a film can be controlled by changing the relative deposition rates from the two targets. Films have been grown with enough extra Ga to compensate for the deficiency that commonly occurs when depositing only from a GGG target. We have also grown crystalline GGG films with an enriched Ga concentration, and this unconventional approach to film stoichiometry control may have potential applications in the fabrication of films with advanced compositionally graded structures.     

Controlled synthesis of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles by chemical reducing method in ethylene glycol

Antimony trioxide (Sb₂O₃) nanoparticles with particle size range from 2 to 12 nm were successfully synthesized by chemical reducing method. Antimony trichloride was reduced by hydrazine with the presence of sodium hydroxide (NaOH) as catalyst in ethylene glycol at 120 °C for 1 h. Effects of hydrazine concentration ([N₂H₅OH]/[Sb³⁺] = 0.75, 5, 10, 20, and 30, when concentration of NaOH was fixed [NaOH]/[Sb³⁺] = 3) and NaOH concentration ([NaOH]/[Sb³⁺] = 0, 1, 3, and 5, when concentration of hydrazine was fixed [N₂H₅OH]/[Sb³⁺] = 10) on the particle size and shape of the Sb₂O₃ nanoparticles were investigated. Transmission electron microscope, selected area electron diffraction pattern, and high resolution electron microscope were employed to study the morphology and crystallinity of the nanoparticles. It was observed that the particle size decreased and remained constant when [N₂H₅OH]/[Sb³⁺]) ≥ 10 and [NaOH]/[Sb³⁺] = 3. Further study on the crystallinity and phase of the nanoparticles was assisted by X-ray diffractometer (XRD). XRD revealed a cubic phase of Sb₂O₃ (ICDD file no. 00-043-1071) with preferred plane of (622) and lattice spacing of 1.68 Å. Correlation between UV–visible absorption wavelengths of the nanoparticles and their sizes was established.     

The synthesis of (Y<Subscript>1-x</Subscript>Gd<Subscript>x</Subscript>)<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu phosphor particles by flame spray pyrolysis with LiCl flux

(Y_1-xGd_x)₂O₃:Eu phosphor particles with dense morphology were prepared by flame spray pyrolysis and the effect of LiCl flux on the crystallinity, morphology, and photoluminescence characteristics of the particles was investigated. All as-prepared particles had monoclinic phase regardless of flux and had different luminescence characteristics from those of commercial Y₂O₃:Eu particles of cubic phase. The addition of LiCl flux reduced the post-treatment temperature by 300 °C for phase transformation from the monoclinic phase to the cubic phase. The post-treatment temperature of (Y_0.75Gd_0.25)₂O₃:Eu particles for phase transformation decreased from 1100 °C to 700 °C when LiCl flux was used. The morphology of the particles was also influenced by the Y/Gd ratio and the LiCl flux. The as-prepared particles had spherical shape and non-aggregation characteristics regardless of Y/Gd ratio and flux. The sphericity of the as-prepared particles prepared without flux was maintained after post-treatment for phase transformation in all Y/Gd ratios. However, LiCl addition promoted the aggregation between product particles. The prepared particles had high photoluminescence intensities comparable to that of the commercial product. PACS 64.70.Kb; 78.55.-m; 81.20.Rg; 75.50.Tt     

Magnetic properties of the Gd<Subscript>2</Subscript>V<Subscript>0.67</Subscript>Mo<Subscript>1.33</Subscript>O<Subscript>7</Subscript> and Y<Subscript>2</Subscript>VMoO<Subscript>7</Subscript> pyrochlore compounds

It is demonstrated that 50% substitution of vanadium for molybdenum in the pyrochlore lattice of the complex oxide Y₂(V _x Mo_{1 ? x} )₂O₇ results in a transition from the spin-glass ground state (at x = 0) to the ferromagnetic state in Y₂VMoO₇ (a = 10.1645(2) Å, T _C = 55 K). The Gd₂V_0.67Mo_1.33O₇ compound (a = 10.2862(3) Å) is a ferromagnet with T _C (84 K) exceeding that of undoped Gd₂MnO₂O₇.     

Phase stability and elasticity of Sc<Subscript>2</Subscript>O<Subscript>3</Subscript> at high pressure

An investigation into the high-pressure behavior of Sc₂O₃ was conducted using first-principle calculations based on density functional theory within the generalized gradient approximation. Nine candidate phases were considered and simulated, C-, B-, A-RES, Rh₂O₃(II), Gd₂S₃, U₂S₃, Al₂Er₃, corundum and PPv respectively. Our results demonstrate phase transitions of Sc₂O₃ from C- to B-RES phase at 15 GPa, then to Gd₂S₃ phase at 18 GPa. Elastic constants of Sc₂O₃ present softening from about 270 GPa to 337 GPa, associated with a semiconductor-to-metal crossover. The Gd₂S₃-type Sc₂O₃ is both mechanically and dynamical stable at least up to 302 GPa supported by the mechanically stable criteria and the phonon spectrum.     

Synthesis of LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> nano/microspheres with adjustable hollow structures for lithium-ion battery

High-voltage spinel LiNi_0.5Mn_1.5O₄ nano/microspheres with adjustable hollow structures have been fabricated based on the Kirkendall effect. The main characteristic is that the wall thickness of the hollow structure as well as the cavity size of the hollow structure can be adjusted by the different ratio of mixed precipitation agents. Especially, the diagrammatic sketch for the formation process of various LiNi_0.5Mn_1.5O₄ materials with adjustable hollow structures is discussed. Besides, the results of electrochemical performance test show that LiNi_0.5Mn_1.5O₄ obtained from 10:1 Na₂CO₃/NaOH (in mole) ratio is worth looking forward to, owing to its special hierarchical nano/microsphere and moderate hollow structures.     

Preparation of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles for superthermites by the detonation of an explosive nanocomposite material

This article reports on the preparation of chromium(III) oxide nanoparticles by detonation. For this purpose, a high explosive—hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)—has been solidified from a solution infiltrated into the macro- and mesoporosity of Cr₂O₃ powder obtained by the combustion of ammonium dichromate. The resulting Cr₂O₃/RDX nanocomposite material was embedded in a cylindrical charge of pure explosive and detonated in order to fragment the metallic oxide into nanoparticles. The resulting soot contains Cr₂O₃ nanoparticles, nanodiamonds, amorphous carbon species and inorganic particles resulting from the erosion by the blast of the detonation tank wall. The purification process consists in (i) removing the carbonaceous species by an oxidative treatment at 500 °C and (ii) dissolving the mineral particles by a chemical treatment with hydrofluoric acid. Contrary to what could be expected, the Cr₂O₃ particles formed during the detonation are twice larger than those of initial Cr₂O₃. The detonation causes the fragmentation of the porous oxide and the melting of resulting particles. Nanometric droplets of molten Cr₂O₃ are ejected and quenched by the water in which the charge is fired. Despite their larger size, the Cr₂O₃ nanoparticles prepared by detonation were found to be less aggregated than those of the initial oxide used as precursor. Finally, the Cr₂O₃ synthesized by detonation was used to prepare a superthermite with aluminium nanoparticles. This material possesses a lower sensitivity and a more regular combustion compared to the one made of initial Cr₂O₃.     

Luminescence thermometry using Gd<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript>:Eu<Superscript>3+</Superscript>

In this paper we study the possibility of using the synthesized nanopowder samples of Gd₂Zr₂O₇:Eu³⁺ for temperature measurements by analyzing the temperature effects on its photoluminescence. The nanopowder was prepared by solution combustion synthesis method. The photoluminescence spectra used for analysis of Gd₂Zr₂O₇:Eu³⁺ nano phosphor optical emission temperature dependence were acquired using continuous laser diode excitation at 405 nm. The temperature dependencies of line emission intensities of transitions from ⁵D₀ and ⁵D₁ energy levels to the ground state were analyzed. Based on this analysis we use the two lines intensity ratio method for temperature sensing. Our results show that the synthesized material can be efficiently used as thermographic phosphor up to 650 K.     

Photoluminescence of nanoparticles of (Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>)<Subscript>0.95</Subscript>(Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.05</Subscript> solid solution

We have ground bulk samples to obtain nanoparticles of (Ga₂S₃)_1–x (Eu₂O₃) _x solid solutions, the sizes of which were determined using an atomic force microscope. The photoluminescence spectra of the nanoparticles were studied in the temperature interval 77–300 K. We have established the mechanisms for emission and transfer of energy from the matrix to the rare-earth ion, and we determined the Stokes shift (ΔS = 0.7 eV), the Huang–Rhys parameter (S = 16), and the optical phonon energy (ħ−ω = 23 meV).     

Shape controlled quantum dot (QD)-decorated-mesoporous SiO<Subscript>2</Subscript> (m-SiO<Subscript>2</Subscript>/QD) hollow particles for photodynamic therapy

This work reported a novel kind of CdTe quantum dot (QD) decorated mesoporous SiO₂ (m-SiO₂/QD) hybrid hollow nanoparticles for carrying photodynamic therapy (PDT) reagent. Both rod-like and spherical nanoparticles were prepared by using different shaped templates. Due to the porous shell and hollow interior, the hybrid m-SiO₂/QD hollow nanorod with 360 nm long and 120 nm in diameter was selected for carrying zinc(II) phthalocyanine (ZnPc) photosensitizing molecules (61 mg/g) since the generated reactive ¹O₂ could be easily delivered out of the hollow particles through the porous shell (BET area 251 cm²/g). It was found that the m-SiO₂/QD-ZnPc hollow nanorods exhibited a good PDT activity and showed effective photocytotoxicity for the cancer cells. Because of the porous nature, fluorescence characteristic, and excellent storage ability, the m-SiO₂/QD hybrid hollow particles possessed broad potential in the fluorescent labeled PDT.

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Graphic abstract m-SiO₂/QD hybrid hollow particles with different morphologies could be successfully synthesized by using the templating method and they could be used as carriers for photodynamic therapy reagents.

     

Magnetic dynamics of phase separation domains in GdMn<Subscript>2</Subscript>O<Subscript>5</Subscript> and Gd<Subscript>0.8</Subscript>Ce<Subscript>0.2</Subscript>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

Specific features of the magnetic properties and magnetic dynamics of isolated phase separation domains in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ have been investigated. These domains represent 1D superlattices consisting of dielectric and conducting layers with the ferromagnetic orientation of their spins. A set of ferromagnetic resonances of separate superlattice layers has been studied. The properties of the 1D superlattices in GdMn₂O₅ and Gd_0.8Ce_0.2Mn₂O₅ are compared with the properties of the previously investigated RMn₂O₅ (R = Eu, Tb, Er, and Bi) series. The similarity of the properties for all the RMn₂O₅ compounds with different R ion types is established. Based on the concepts of the magnetic dynamics of ferromagnetic multilayers and properties of semiconductor superlattices, a 1D model of the superlattices in RMn₂O₅ is built.     

Optical absorption by Nd<Superscript>3+</Superscript> and Gd<Superscript>3+</Superscript> ions in epitaxial films grown on Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> substrates from a lead-containing solution melt

Epitaxial films of composition (Gd,Nd)₃Ga₅O₁₂ or (Gd,Y,Nd)₃Ga₅O₁₂ with a neodymium content varying from 0.3 to 15 at. % are grown by liquid-phase epitaxy from a supercooled PbO-B₂O₃-based solution melt on Gd₃Ga₅O₁₂(111) substrates. The optical absorption spectra of the epitaxial films grown are measured in the wavelength range 0.2–1.0 µm. The results of interpreting the absorption bands observed in the spectra are used to construct the energy level diagrams of Nd³⁺ and Gd³⁺ ions in the matrices of the epitaxial films.