National Institute Of Materials Physics - Romania

Double perovskite Sr2FeMoO6 was prepared by two ways consisting in sol gel technique and solid-state reaction method. The resulting powders from gel and mixed oxides precursors showed microstructures consisting of very fine grains (0.5-0.8 mu m) and a crystalline perovskite structure. The structural and microstructural properties of the double perovskite Sr2FeMoO6 powders as-prepared and ceramics were compared. Tetragonal Sr2FeMoO6 pellets were prepared from the two powders by spark plasma sintering at: 1000, 1100 and 1200 degrees C and then annealing at 1200 degrees C, 2 h in 5%H-2/Ar. The pellets presented different magnetic characteristics. The saturation magnetization of the samples prepared by sol-gel is close to those prepared by conventional synthesis method. (C) 2013 Elsevier Ltd. All rights reserved.

Sol-gel chemistry within the pores of a polycarbonate template membrane was used for the preparation of Eosin Y-SiO2 hybrid nano- and microrods, using tetraethylorthosilicate [TEOS, Si(OC2H5)(4)] as the precursor in the presence of trifluoroacetic acid (TFA) catalyst. The ethanolic solution of Eosin-Y was added to the silica sal to trap dye molecules inside the SiO2 gel network during the gelation. Structural and morphological characterization using scanning electron microscopy (SEM) and luminescence microscopy have shown the formation of rods with 200 nm and 1.2 mu m diameter and about 30 mu m length, exhibiting luminescence properties. Spectroscopic characterization has shown that the luminescence is due to Eosin-Y molecule in the xerogel porous network, surrounded by a solvation shell given mainly by the water. (C) 2013 Elsevier B.V. All rights reserved.

This work presents a systematic investigation by X-ray photoelectron spectroscopy of the mechanisms of interface formation and band bending for Au/Pb(Zr,Ti)O-3 (PZT) layers grown on SrTiO3(001) with a SrRuO3 buffer layer, as function on the initial state of the PZT surface. After isolating the chemical effects, such as the formation of metal Pb at some surfaces, the evolution of the core levels with Au deposition allows one to simultaneously investigate the Schottky barrier formation and the built-in potential effects (charging induced by the static ferroelectric polarization). Areas of the sample with outwards P(+) and no polarization perpendicular to the surface P-(0) are identified for all samples. Only the freshly prepared sample exhibited inward polarization areas P(-). The built-in potential is on the order of 0.9 eV, while the Schottky band bending ranges from 0.2 to 0.6 eV towards lower absolute energies, therefore indicating that the work function of PZT exceeds that of Au deposited. We report also a chemically differentiate value of the built-in potential, manifested by a preferential distribution of the charge accumulated at the surface on Ti and O atoms. The O 1s and Ti 2p core levels manifest quite strong variations with the Au thickness for freshly prepared samples, resulting in shifts on the order of 2 eV towards lower binding energies. Au deposited on areas with an outward polarization is positively charged by the same potential as atoms from the PZT film (0.8-0.9 eV), whereas Au deposited on areas with an inward polarization forms a continuous grounded layer, which progressively pumps the accumulated charge and removes the polarization of these areas. (C) 2013 Elsevier B. V. All rights reserved.

As-grown n-4H-SiC/SiO2 capacitors exhibit anomalous capacitance-vs.-voltage (C-V) characteristics at low temperatures. Abrupt minima appear in the C-V curves at specific values of the gate voltages independent of the sample temperature, strongly suggesting the presence of resonant electron tunneling. We put forward a qualitative model where neutral donor states present at the SiC/SiO2 interface enable electron tunneling into distinct energy levels in the oxide. Numerical simulations based on this model show close agreement with the anomalous C-V characteristics observed experimentally. The model implies that under given conditions, i.e., the existence of a sufficient density of neutral donors at the semiconductor/oxide interface and empty electron states in the oxide layer, abrupt minima are in general to be expected during C-V measurements of metal-oxide-semiconductor capacitors. (C) 2013 Elsevier B. V. All rights reserved.

Electric and magnetic properties of symmetric and asymmetric Pb(Zr-0.2 Ti-0.8)O-3-CoFe2O4 (PZT/CFO) heterostructures, grown by pulsed laser deposition on SrTiO3 (100) substrates with a 25 nm SrRuO3 (SRO) buffer layer as bottom electrode, were investigated by using hysteresis and capacitance measurements. X-ray diffraction, and transmission electron microscopy investigations reveal the high quality crystalline structure and the epitaxial relationship between SRO, PZT and CFO. The electric polarization-voltage hysteresis reveals that the remnant polarization and the coercive field are significantly affected by the CFO layer. The frequency dependence of capacitance suggests a Maxwell-Wagner type relaxation at low frequencies and is also affected by the presence of the PZT/CFO interface(s). The magnetic hysteresis measurements infer the possible presence of another spinel phase (Co3O4) in the CFO film, due to the lattice mismatch at the PZT/CFO interfaces, and with direct influence on the magnetic response of the structure. According to the electric and magnetic characterization, better room temperature multiferroic properties would be expected for the symmetric heterostructure. (C) 2013 Published by Elsevier B. V.

Thermally induced changes in the structure and composition of the shell of tightly aggregated cubic ZnS/Zn(OH)(2) core shell quantum dots of 1.9 nm average core size were investigated by multifrequency electron paramagnetic resonance of Mn2+ probing ions. The observed three-steps temperature induced transformation of the Mn2+ surface centers in the 80-450 degrees C temperature range Zn(OH)(2) shell into ZnO, with the formation of the Zn2O(OH)(2) and Zn4O3(OH)(2) intermediate nanocompounds. The presence of a 0.3 to 1.9 nm thick surface layer of disordered nanomaterial separating the cubic ZnS cores and its shrinking to a few atomic layers by mass loss after annealing up to 350 degrees C was observed by high resolution transmission electron microscopy. Unlike the single step dehydration around 120 degrees C of the bulk epsilon-Zn(OH)(2), the complex decomposition of the epsilon-Zn(OH)(2) shell is attributed to its nanosized, disordered structure.

The improvement of graphene derivates' fluorescence properties is a challenging topic and very few ways were reported up to now. In this Letter we propose an easy method to enhance the fluorescence of highly reduced graphene oxide (rGO) through non-covalent binding to a molecular fluorophore, namely the riboflavin (Rb). While the fluorescence of Rb is quenched, the Rb - decorated rGO exhibits strong blue fluorescence and significantly increased fluorescence lifetime, as compared to its pristine form. The data reported here represent a promising start towards tailoring the optical properties of rGOs, having utmost importance in optical applications. (C) 2013 Elsevier B.V. All rights reserved.

Cobalt ferrite was obtained by the thermolysis of two new cobalt ferrioxalate coordination compounds, namely [Fe2Co(C2O4)(3)(OH)(2)(OH2)(4)].2H(2)O (I) and [Fe2Co(C2O4)(4)(OH2)(6)] (II). The precursors were characterized by chemical analysis, electronic and vibrational spectra and thermal analysis. The corresponding oxides obtained after a heating treatment of the precursors at 350 degrees C/h were extensively investigated by various methods such as XRD, IR, SEM, TEM coupled with EDX, TPR, Mossbauer spectroscopy, magnetic investigations and N-2 adsorption-desorption isotherms. The mesoporous oxides, consisting of homogeneous particles as shape and size with mean crystallites size values of 111(I) and 86 (II) angstrom, exhibit high surface areas (77.6 (I) and 55.7 (II) m(2) g(-1)). The cobalt ferrites' catalytic performances for methane combustion are close to that of a platinum-based catalyst, the catalytic activity being analyzed and interpreted considering ferrites distinctive properties. (C) 2013 Elsevier B.V. All rights reserved.

Ferrofluid samples consisting of magnetite nanoparticles (NPs) coated with oleic acid and dispersed in a non-polar organic solvent have been synthesized by chemical routes. Different volume fractions, phi, of magnetic NPs were considered. The overall structural characterization of NPs has been performed by x-ray diffractometry, with lattice parameters and average coherence lengths evaluated via Rietveld refinements. The magnetic properties of different samples have been analysed by SQUID magnetometry and temperature-dependent Mossbauer spectroscopy and finally explained by adequate magnetic relaxation mechanisms. Zero field cooling-field cooling protocols provided useful information about specific volume fraction dependent magnetic relaxation and de-freezing processes, the lack of the Verwey transition and stronger dipolar interactions at higher volume fractions. Anisotropy energies as obtained by both temperature dependent Mossbauer spectroscopy and magnetometry data are compared and a new procedure for a quantitative characterization of the dipolar interactions is proposed.

Electroless deposition was successfully applied in developing crystalline particles of zinc oxide onto polyester textile materials; this deposition is here presented in comparison with other materials made from poly(lactic acid), polyamide or hemp. Structural and spectroscopic characterization of the raw and deposited samples has been performed. The structure of zinc oxide particles was that of wurtzite type as indicated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Crystallites were 20-500 nm in diameter and up to 1 mu m in length. The grown particles cover the fibers not only on the fabric surface but in the textile depth. Contact angle measurement by the sessile drop method was used to study the wettability behavior of the investigated composite systems. The hierarchical roughness structure generates superhydrophobic properties onto polyester fabrics, for which water contact angles exceed 150 degrees. The other functionalized samples also become more hydrophobic after deposition. Cassie-Baxter model was found suitable to describe the behavior, though the fraction of surface occupied by the water-solid interface is high enough. The electroless deposition technique applied previously for cotton fabrics was once more proven to be highly reproducible, easy scalable, and cheap, allowing a wide range of applications. (c) 2013 Wiley Periodicals, Inc.