Structural investigations of nitrided Nb2O5 and Nb2O5–SiO2 sol–gel derived films

Sol–gel derived xNb₂O₅–(100 ? x)SiO₂ films (where x = 100, 80, 60, 50, 40, 20, 0 mol%) were nitrided at various temperatures (800 °C, 900 °C, 1000 °C, 1100 °C and 1200 °C). The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The XRD results have shown that the temperatures below 1100 °C were too low to obtain a pure NbN phase in the samples. The AFM observations indicate that the formation of the NbN phase and the size of NbN grains are related to the silica content in the layer. NbN grains become more regular and larger as the niobium content increases. The maximum grain size of about 100 nm was observed for x = 100. Preparation of the Nb₂O₅–SiO₂ sol–gel derived layers and the subsequent nitridation is a promising method of inducing crystalline NbN in amorphous matrices. It follows from the XPS results that a small amount of Nb₂O₅ remains in the films after nitridation at 1200 °C and that nitrogen reacted not only with Nb₂O₅ but also with SiO₂.     

X‐ray diffraction analysis of powder and thin film of (Gd1‐x Yx)2O3 prepared by sol‐gel process

The mixed oxide (Gd_1‐xY_x)₂O₃ (0.0 ≤ x ≤ 1.0) were synthesized, as powder and thin film, by a sol‐gel process. X‐ray diffraction data were collected and crystal structure and microstructure analysis were performed using Rietveld refinement method. All samples were found to have the same crystal system and formed solid solutions over the whole range of x. The cationic distribution, Gd³⁺ and Y³⁺, over the two non‐equivalent sites 8b and 24d of the space group Ia3 is found to be random for all values of (x). The lattice parameter is found to vary linearly with the composition (x). Replacing Gd³⁺ and Y³⁺ by each other introduces a systemic decrease in the x‐coordinate of cation position (24d) and slight changes in the oxygen coordinates. Crystallite size and microstrain analysis is performed along different crystallographic directions and anisotropic changes are found with the composition parameter (x). The average crystallite size ranges from 75 to 149 nm and the r.m.s strain from 0.027 to 0.068 x10^‐2. Textured Gd_1.841Y_0.159O₃ (400) buffer layers, with a high degree of alignment in both out‐plane and in‐plan, are successfully grown on cube textured Ni (001) tape substrates by sol–gel dip coating process. The resulting buffer layers are crack‐free, pinhole‐free, dense and smooth. YbBa₂Cu₃O_7‐x (YbBCO) thin film could be (00l) epitaxially grown on the obtained buffer layer using sol–gel dipping technique. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effective electro‐optic coefficient of (1–x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 single crystals

Refractive indices and effective electro‐optic coefficient γ_c of (1–x)Pb(Zn_1/3Nb_2/3)O₃–xPbTiO₃ (PZN‐xPT, x = 0.05, 0.09 and 0.12) single crystals were measured at 532 nm wavelength. Orientation and temperature dependences of the electro‐optic coefficient were investigated. Large electro‐optic coefficient (γ_c = 470 pm/V) was observed in [001]‐poled PZN‐0.09PT crystal. More importantly, γ_c of tetragonal PZN‐0.12PT is almost unchanged in a temperature range −20 ∼ 80 °C. The γ_c of PZN‐xPT single crystals are much higher than that of widely used electro‐optic crystal LiNbO₃ (γ_c = 20 pm/V). These results show that PZN‐xPT single crystals are very promising materials for electro‐optic modulators in optical communications.     

Effects of tantalum doping on microstructure and ferroelectric properties of Bi4Ti3O12 thin films prepared by a sol–gel method

Tantalum-substituted Bi₄Ti₃O₁₂ (Bi₄Ti_3-x/5Ta_x/5O₁₂, BTTO) thin films were fabricated on Pt(111)/Ti/SiO₂/Si(100) substrates by sol–gel technology. The effects of various processing parameters, including Ta content (x=0～0.08) and annealing temperature (500～800 °C), on the growth and properties of thin films were investigated. X-ray diffraction analysis shows that the BTTO thin films have a bismuth-layered perovskite structure with preferred (117) orientation. With the increase of Ta content, the grain size of film decreased slightly, and highly (117)-oriented BTTO films were obtained in the composition of x=0.06. Ta doping on the B-site of Bi₄Ti₃O₁₂ could induce the distortion of oxygen octahedral and decrease the oxygen vacancy concentration by a compensating effect. The highly (117)-oriented BTTO thin films with x=0.06 exhibits the maximum remanent polarization (2P_r) of 50 μC/cm² and a low coercive field (2E_c) of 104 kV/cm, fatigue free characteristics up to ≧ 10⁸ switching cycles.     

Nanostructural and optical features of amorphous AlxSi0.45−xO0.55 (0 ⩽ x ⩽ 0.05) thin films prepared by RF-magnetron sputter techniques

The nanostructural, chemical, and optical features of Al_xSi_0.45−xO_0.55 (0 ⩽ x ⩽ 0.05) thin films were investigated in terms of Al concentration and post-deposition annealing conditions; the films were prepared by co-sputtering a Si main target and Al-chips, and the annealing was carried out at temperatures of 400–1100 °C. The a-Si_0.45O_0.55 films prepared without Al-chips and annealed at 800 °C contain ∼3.5 nm-sized Si nanocrystallites. The photoluminescence (PL) intensity as well as the volume fraction of Si nanocrystallites increased with increasing the concentration of Al to a certain level. In particular, the intensity of the PL spectra of the Al_0.025Si_0.425O_0.550 films which were annealed at 800 °C increased significantly at wavelengths of ∼580 nm. It is highly likely that the observed increase in the PL intensity is caused by the raise in the total volume of the ∼3.5 nm-sized nanocrystallites in the films. The addition of Al as well as the post-deposition annealing allow adjustment and control of the nanostructural and light-emission features of the a-SiO_x films.     

Crystal structure and magnetic properties of nanosized Mg(Fe0.8Ga0.2)2O4-δ films on Si substrates

Film samples of nominal Mg(Fe_0.8Ga_0.2)₂O_4-δ composition were obtained on Si(100) substrates by oxygen-ion-beam sputtering of a Mg(Fe_0.8Ga_0.2)₂O_4-δ ceramic target. The film thicknesses were ～200 and ～400 nm. Field dependences of the specific magnetization of ～200-nm films annealed at different temperatures (800–1000°C) have been measured. The crystal structure, surface morphology, and magnetic characteristics of films of different thicknesses (～200 and ～400 nm) are investigated. The reasons for the discrepancy between the specific magnetizations of the films obtained and their ceramic analog are discussed.     

Czochralski growth of Lu1‐xScxBO3:Ce solid solution crystals

Single crystals of Lu_1‐xSc_xBO₃:Ce (x=0.2, 0.3, 0.5, 0.7) were grown by Czochralski method. Continuous solid solution with calcite structure and a linear compositional dependency of crystal lattice parameter in the system Lu_1‐xSc_xBO₃:Ce are formed and their symmetry belong to hexagonal system with R3c space group checked by X‐ray powder diffraction. The electron probe micro‐analysis measurements show that the main inclusions in Lu_1‐xSc_xBO₃:Ce crystals are in the form of Sc rich oxide and Ce rich oxide. The ICP‐AES tests show that the more Sc ion content in Lu_1‐xSc_xBO₃:Ce, the smaller effective segregation coefficient of Ce in crystal will be. The X‐ray excited luminescence spectra of Lu_1‐xSc_xBO₃:Ce crystals all present a double peaked emission band with maxima round 370 and 400 nm corresponding to Ce³⁺ emission and a self trapped excitons (STE) band peaking at 269 nm. In addition, due to high density, high relative light yield, fast decay time and no‐hygroscopic property, Lu_0.8Sc_0.2BO₃:1 at%Ce crystal could be a good candidate material for scintillation application by improving the crystal quality and cerium concentration. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of compounds LixMn1+xFe2–2xO4 with spinel structure in the quasiternary system “LiO0,5 – MnOx – FeOx“

The thermal decomposition of freeze‐dried Li‐Mn(II)‐Fe(III)‐formate precursors was investigated by means of DTA, TG and mass spectroscopy. By the thermal treatment of the prefired precursors between 400 and 1000°C, single phase solid solutions Li_xMn_1+xFe_2–2xO₄ (0 ≤ x ≤ 1) with cubic spinel structure were obtained. To get single phase spinels, special conditions concerning the temperature T and the oxygen partial pressure p(O₂) during the synthesis are required. Because of the high reactivity of the freeze‐dried precursors, in comparison with the conventional solid state reaction, the reaction temperature can be lowered by 200°C. The cation distribution and the properties of the Li‐Mn‐ferrites were studied by chemical analysis, X‐ray powder diffraction and magnetization measurements. It was found that for high substitution rates, almost all lithium occupies the tetrahedral coordinated A‐sites of the spinel lattice AB₂O₄, while at small x‐values, lithium ions are distributed over the tetrahedral and octahedral sites. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis,morphology and structure of the dense (Y1‐xEux)2O3 spherical shape particles

The method to decrease of the porosity (densification) of crystalline spherical particles of the solid substitution solution, obtained by the method of precipitation from aqueous solution followed by low temperature crystallization of the amorphous intermediate product was proposed. The comparative analysis of morphology and structure of the particles before and after densification have been carried. It has been established that porosity of (Y_1‐xEu_x)₂O₃ particles has decreased 5 times compared to their initial state. It has been shown that densification process of the (Y_1‐xEu_x)₂O₃ spherical particles changes their morphology and structure: the size of the crystals doubles, the number and area of crystalline boundaries decrease, the intercrystalline spaces, which forming pores, are almost absent.     

Dielectric relaxation of Al/Lu2O3/Al thin film structures from 10 μHz to 10 MHz

Results of dielectric studies of lutetium sesquioxide layers examined in Al/Lu₂O₃/Al thin film sandwiches are reported. The dielectric measurements were carried out in the frequency range 10⁻⁵–10⁷ Hz and for temperatures from 292 K to 500 K. Results are presented as plots of frequency functions: the capacitance, the dielectric loss factor, tan δ(f) and also on the complex plane as Cole–Cole plots and Nyquist plots. The influence of the external voltage on C(U) and tan δ(U) was examined. Experimental data were analyzed taking into account thin insulating Lu₂O₃ film, near-electrode regions of Al/Lu₂O₃ and Lu₂O₃/Al interfaces and series resistance of electrodes and leads. The parameters of Lu₂O₃ film, near-electrode regions and resistance of contacts and leads were determined.     

X-ray photoelectron spectroscopy studies on the bonding properties of oxygenated amorphous carbon nitride thin films synthesized by pulsed laser deposition at different substrate temperatures

Oxygenated amorphous carbon nitride thin films (a-CN_xO_y) were deposited by pulsed laser deposition of camphoric carbon (CC) target at various substrate temperatures (ST). The influence of ST on the bonding properties of a-CN_xO_y films was investigated. The nitrogen to carbon (N/C) atomic ratio and oxygen to carbon (O/C) atomic ratio, bonding state and microstructure of the deposited a-CN_xO_y films were characterized by X-ray photoelectron spectroscopy (XPS) and been confirmed using standard measurement techniques. The bonding states between the C and N, and C and O in the deposited films are found significantly influenced by the ST during deposition process. The N/C and O/C atomic ratio of the a-CN_xO_y films reached the maximum value at 400 °C. The XPS C 1s shows the bonding state of a-CN_xO_y films changes from diamond-like tetrahedral (sp³) carbon and carbon (C–C) bonding to graphite-like trihedral (sp²) CC bonding with the increase of ST. While, the XPS N 1s shows the sp³ C–N bonds increases with higher rates compared with sp² CN bonds up to 400 °C, after which it decreases with higher ST. The C–N bonding of C–N, CN and CN were observed in the deposited a-CN_xO_y films.     

Liquid phase epitaxial growth of Ca,Ge-Substituted garnet films having magnetic bubbles 1.0–3.0 μm in diameter

Ca, Ge-substituted, magnetic garnet films which would support bubbles of 1.0–3.0 μm diameter were grown by isothermal deposition frm supersaturated PbO-B₂O₃ fluxes upon garnet films were yttriumm iron garnet Y₃Fe₅O₁₂, in which rare earths substituted for yttrium and Ge substituted for Fe. Ca was added to charge compensate the Ge. A typical composition of a 2 μm bubble film might be Y_{1.35Lu0.3Sm0.6Ca0.75Fe4.25O12}, but the compositions of the films were varied by modifying the composition of the growth solution and the growth conditions. The magnetic properties of the films were dependent upon film perfection and composition. Bubble mobilities ranged ≈300–1000 cm/sec-Oe. The films, whose thickness varied from 1.0–3.0 μm to match buble diameters, were uniform in thickness to less than 0.30 μm across a 2.54 cm diameter slice. The properties of the films were sufficiently stable with temperature from 0–100°C to indicate that these materials could be used in memory devices.     

Synthesis and photoluminescence of Eu3+-doped ZnO–Ga2O3–SiO2 nano-glass-ceramics

Transparent rare-earth Eu³⁺-doped ZnO–Ga₂O₃–SiO₂ nano-glass-ceramics were obtained by a sol–gel method. X-ray diffraction and transmission electron microscopy were used to characterize the as-synthesized materials. Results showed that ZnGa₂O₄ nanocrystals with the size of 5 nm were precipitated from ZnO–Ga₂O₃–SiO₂ system and dispersed in the SiO₂-based glass when the heat-treatment temperature was up to 800 °C. Photoluminescence characterization of Eu³⁺-doped ZnO–Ga₂O₃–SiO₂ nano-glass-ceramics was carried out and the results show that the as-synthesized material display intense emission at 615 nm belonging to ⁵D₀ → ⁷F₂ transition.     

Intense luminescence of amorphous Eu2O3 prepared by aqueous sol–gel method

Amorphous Eu₂O₃ was prepared by an aqueous sol–gel method. Emission due to the ⁵D₀ → ⁷F_J (J = 0, 1, 2) transitions of Eu³⁺ ions were observed. The dominant transition was the ⁵D₀ → ⁷F₂ red emission of Eu³⁺. The properties of the as-prepared samples were different with changes in the annealing temperature. To investigate the luminescence properties of the amorphous Eu₂O₃, the temperature-dependent photoluminescence (PL) spectra of samples annealed at 600 °C were measured in the temperature range 77–300 K. PL peak positions were unchanged with the change of temperature.     

Enhancement of photoluminescence in SiCxNy nanoparticle films by addition of a Ni buffer layer

This work reports the effect of the presence of a Ni buffer layer on the photoluminescence (PL) of SiC_xN_y nanoparticle films prepared by RF plasma magnetron sputtering process in a reactive N₂ + Ar + H₂ gas mixture. An introduction of a Ni buffer of 80 nm or thicker remarkably improves the PL of the films. Annealing in a temperature range of 400–1100 °C is found to significantly affect the PL intensity. Optimal PL is achievable at 600 °C. X-ray photoelectron and Fourier-transform infrared spectroscopy suggest that the strong PL is directly related to the composition of the SiC_xN_y nanoparticle and the concentration of Si–O, and Si–N bonds. The results are relevant to the development of wide bandgap optoelectronic devices.     

The growth and spectral analysis of mixed crystal of Yb‐doped Lu0.5Y0.5PO4 mixed crystals

A new kind of 5 at% Yb‐doped Lu_0.5Y_0.5PO₄ crystals were firstly grown by spontaneously nucleated high‐temperature solution method using lead pyrophosphate (Pb₂P₂O₇) as the solvent. The X‐ray powder diffraction (XRPD) patterns recorded at room temperature showed the crystals possessed tetragonal xenotime structure. The polarized absorption spectra and the fluorescence spectra of Yb:Lu_xY_1‐xPO₄ were measured at room temperature, respectively. The results show that Yb:Lu_xY_1‐xPO₄ mixed crystal will be a promising laser material if the crystal size and quality is further improved.     

Highly enhanced f–f transitions of Eu3+ in La2O3 phosphor via citric acid and poly (ethylene glycol) precursor route

The luminescent material europium-activated La₂O₃ have been prepared by the citric acid and poly (ethylene glycol) (PEG) precursor route. Their structures and optical properties were characterized by FT-IR spectrum, X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), UV–vis spectroscopy, and photoluminescence (PL) spectra, respectively. The results show considerable enhancement of the photoluminescence, especially the Eu³⁺ f–f transition excitation lines and the charge transfer band (CT). The samples can exhibit strong red emission centered at 626 nm excited at either the CT band (300 nm) or the Eu³⁺ f–f transition (396 nm), suggesting the potential application as the red phosphors for ultraviolet light-emitting diodes (LEDs), which can be attributed to the ⁵D₀→⁷F₂ transition of Eu³⁺. The remarkable enhancement of color purity of red emission and the concentration quenching of Eu³⁺ in La₂O₃ were also observed with increasing Eu³⁺ doped concentration.     

Investigation of the structure of nanocrystalline refractory oxides by X-ray diffraction,electron microscopy,and atomic force microscopy

The structures of nanocrystalline fibrous powders of refractory oxides have been investigated by different methods: determination of coherent-scattering regions, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic-force microscopy (AFM). The sizes of nanograins of different crystalline phases of refractory metal oxides have been determined during the formation of these nanograins and the dynamics of their growth during heat treatment in the temperature range 600–1600°C has been studied. The data on the structure of nanocrystalline refractory oxide powders, obtained by different methods, are in good agreement. According to the data on coherent-scattering regions, the sizes of the ZrO₂ (Y₂O₃) and Al₂O₃ grains formed are in the range 4–6 nm, and the particle sizes determined according to the TEM and AFM data are in the ranges 5–7 and 2–10 nm, respectively. SEM analysis made it possible to investigate the dynamics of nanoparticle growth at temperatures above 1000°C and establish the limiting temperatures of their consolidation in fibers.

     

A facile route to synthesize luminescent YVO4:Eu3+ porous nanoplates

Porous nanoplates made of yttrium orthovanadate doped with europeum (YVO₄:Eu³⁺) have been successfully synthesized via a chemical co-precipitation method using NH₄VO₃, Y₂O₃, Eu₂O₃ and ethylene glycol. To investigate the effect of temperature on the pore size and morphology of the final product, the as-synthesized YVO₄:Eu³⁺ nanoplates were subjected to heat treatment at 300 and 600 °C for 2 h. The obtained samples were characterized by X-ray diffractometion, Fourier-transform infrared spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy and photoluminescence spectroscopy. The experimental results showed that the luminescent properties were significantly enhanced with increasing pore size of the YVO₄:Eu³⁺ porous nanoplates.     

Luminescence of Y2O3:Eu and Gd2O3:Eu Depending on Precursor and Activation Temperature

The activation of Y₂O₃, Gd₂O₃and (Y_0.7,Gd_0.3)₂O₃ with Eu³⁺ ions at temperatures lower than 1000 °C is studied using different starting compounds. The activator ions are introduced during the crystallization or precipitation of the precursor. Phosphors prepared from hydroxides and activated at 900 °C exhibit luminescence with high efficiency under 254 nm Hg-line excitation. Strong emission is observed even in samples activated at 700 °C. Luminescence intensity, emission and excitation spectra are compared to these of Y₂O₃:Eu produced by the industry.