National Institute Of Materials Physics - Romania

Epitaxial thin films of NiFe2O4 are fabricated by pulsed laser deposition on SrTiO3 substrate. Symmetrical capacitor-like structures are formed using SrRuO3 as bottom and top electrodes. Electrical characterizations, including current-voltage, capacitance-voltage and capacitance-frequency measurement, reveal a hysteresis-like behaviour for current and capacitance as function of voltage. This could be assigned to a resistive and/or capacitive switching. A "degradation" process takes place after repeated voltage cycling or after heating the sample to 400 K, leading to the stabilization of different resistive states. These features can be related to the changes observed in the capacitance-frequency characteristics, suggesting the presence of a relaxation mechanism at low frequencies, and can be associated with the presence of a deep donor-type level in the band-gap of the NiFe2O4 layer.

A dynamic electrical model is introduced to investigate the hysteretic effects in the J-V characteristics of perovskite based solar cells. By making a simple ansatz for the polarization relaxation, our model is able to reproduce qualitatively and quantitatively detailed features of measured J-V characteristics. Pre-poling effects are discussed, pointing out the differences between initially over- and under-polarized samples. In particular, the presence of the current overshoot observed in the reverse characteristics is correlated with the solar cell pre-conditioning. Furthermore, the dynamic hysteresis is analyzed with respect to changing the bias scan rate, the obtained results being consistent with experimentally reported data: the hysteresis amplitude is maximum at intermediate scan rates, while at very slow and very fast ones it becomes negligible. The effects induced by different relaxation time scales are assessed. The proposed dynamic electrical model offers a comprehensive view of the solar cell operation, being a practical tool for future calibration of tentative microscopic descriptions. (C) 2016 Elsevier B.V. All rights reserved.

Additives to MgB2 can improve the superconducting functional characteristics, such as critical current density (J (c)) and irreversibility field (H (irr)). Recently, we have shown that repagermanium (C6H10Ge2O7) is an effective additive, enhancing both J (c) and H (irr). To look into details of the processes taking place during the reactive sintering, a thermal analysis study (0.167 K s(-1), in Ar) is reported. We used differential scanning calorimetry between 298 and 863 K and simultaneous thermogravimetric-differential thermal analysis between 298 and 1233 K. Samples were mixtures of powders with composition 97 mol% MgB2 and 3 mol% C6H10Ge2O7. Up to 863 K, repagermanium decomposes by multiple steps and forms amorphous phases. A reaction with MgB2 is not observed. Above this temperature, partial decomposition of MgB2 occurs. Crystalline Ge and MgO are detected before formation of Mg2Ge and MgB4, when temperature approaches the melting point of Ge (1211 K). Carbon substitution for boron in the crystal lattice of MgB2 is observed for samples heated above 863 K. The amount of substitutional C does not significantly change with temperature.

A multi-component reaction involving metal ion, amines and formaldehyde has been used for a series of decaaza bismacrocyclic complexes M2L(CH3COO)(4)center dot nH(2)O [(1) M: Ni, n = 2.5; (2) M:Cu, n = 1; (3) M:Zn, n = 10; L:1,3-bis(N,N-1,3,6,9,12-pentaazacyclotridecane)-benzene] preparation. Elemental analyses, ESI-MS, IR, UV-Vis-NIR, NMR and EPR spectra, magnetic susceptibility at room temperature, molar conductivities, as well as thermogravimetric analysis, provided data concerning complexes features. The macrocyclic ligand behaves as bischelate, resulting in either a square planar or an octahedral stereochemistry. The in vitro screening of the antimicrobial activity was performed against both reference and clinical isolates multi-drug-resistant strains. The overall antimicrobial potency of complexes was enhanced in comparison with the free ligand, against both planktonic and biofilm-embedded pathogenic strains. Complexes exhibit no cytotoxicity on the HCT 8 tumour cells. Thermogravimetric curves (TG, DTG and DTA) evidenced in air processes as water elimination, acetate into carbonate transformation as well as oxidative degradation of the bismacrocyclic ligand. The powder X-ray diffraction data indicate MO (M: Ni, Cu, Zn) as final product.

The research describes the synthesis, optical, photocatalytic, electrochemical and photoelectrocatalytical properties of highly defected, low surface area, tubular SiO2. High surface density of light-absorbing defects is obtained by conducting the synthesis at low temperature (0 degrees C) and by optimizing the ratios between reactants. The calcined tubular SiO2 is characterized by intense and broad absorption band centered at approximate to 320 nm, spanning from deep UV to green. The formation of Si3+ surface defects by calcination is accompanied by a substantial intensification of light absorbing characteristics. The annealing leads also to appearance of intense photoluminescence bands centered at around 433 and 533 nm for excitation wavelengths 320-380 nm. The photocatalytic generation of H-2 and CO2 from water-methanol mixture under solar (AM 1.5) and visible (lambda > 390 nm) irradiation is performed to probe the capability of light-absorbing defects of SiO2 to work as active sites and to gather indirect information on nature of active sites. The electrochemical and photoelectrochemical activity of tubular SiO2 are highlighted by employing [Ru(bpy)(3)]Cl-2 complex as electron transfer witness. According to our results, highly defected nanotubular SiO2 exhibit uncommon optical, light harvesting, electronic and electrochemical properties with great potential for practical applications in numerous fields. (C) 2016 Elsevier B.V. All rights reserved.

The phase shift of the electron's wave function after a tunneling event (i.e. the transmission phase) was at first measured for its fundamental or applicative relevance for quantum circuitry, but later the phase study self-motivated due to a number of unexpected results. One such result was the reduced increment of the phase on some resonances - with only fractions of pi - in the few-electrons "mesoscopic" regime. In this paper we address such a regime for a rectangular quantum dot and compute the total phase increase on the first four resonances by means of accurate configuration-interaction method and a generalized Friedel sum rule as proposed by Rontani (2006) [17]. Our findings confirm that the electronic correlations reduce the on-resonance phase growth which is also found to decrease quasi-linearly with the dot size, the decrease being more pronounced as the number of electrons on the dot is raised. Sudden jumps (of small amplitude) of the phase are found to accompany ground states spin transitions. (C) 2016 Elsevier B.V. All rights reserved.

Un-doped and Tb3+-doped calcium and strontium aluminates (CaAl2O4, CaAl2O4:Tb3+, SrAl2O4, SrAl2O4: Tb3+) were synthesized by a new soft chemical method the precursor route via the thermal decomposition of tartarate compounds. The complex precursors have been characterized by elemental chemical analysis, infrared spectroScopy (IR), luminescence spectroscopy and thermal analysis. X-ray diffraction (XRD), scanning electron microscopy (SEM), IR and Raman spectroscopy (RS) were used for the structural and morphological characterization of un-doped and doped alkaline-earth aluminate powders. Photoluminescence (PL) and Thermoluminescence (TL) properties of the aluminates were also investigated. The XRD patterns confirmed the formation of low-temperature structures: hexagonal structure for CaAl2O4 and monoclinic structure for SrAl2O4, with average crystallite size of 64 nm and 46 nm for CaAl2O4 and SrAl2O4, respectively. Microstructural analysis by SEM showed the homogeneous microstructure and fine particle size of the powder obtained by precursor method. (C) 2016 Elsevier Ltd. All rights reserved.

Synthesis and characterization of four new Ni(II) complexes with mixed ligands have been reported in the present paper. As interest ligands, acrylate (acr) ion together with one N-donor heterocyclic ligand, namely benzimidazole (HBzIm), 2-methylbenzimidazole (2-MeBzIm), 5-methylbenzimidazole (5-MeBzIm) or 5,6-dimethylbenzimidazole (5,6-Me(2)BzIm), was chosen. The syntheses afforded the complexes formulated as [Ni(HBzIm)(2)(acr)(2)(H2O)]center dot 3H(2)O (1), [Ni(2-MeBzIm)(2)(acr)(2)(H2O)]center dot 1.5H(2)O (2), [Ni(5-MeBzIm)(2)(acr)(2)(H2O)] (3) and [Ni(5,6-Me(2)BzIm)(2)(acr)(2)] (4). Complexes (1) and (2) contain crystallization water molecules and coordinated water molecules, both their nature and presence being confirmed on the basis of IR spectra and thermal analysis. In contrast to above-mentioned complexes, (3) contains only coordinated water molecule, while (4) is an anhydrous compound. Based on electronic spectra and magnetic measurements, a distorted octahedral stereochemistry was proposed for all Ni(II) complexes. Acrylate ions act both as unidentate and chelate ligands in complexes (1)-(3), while in complex (4) act only as chelate ligands. All used N-donor ligands function as monodentate in all Ni(II) complexes. Biological properties of complexes (1)-(4) were evaluated against several Gram(+), Gram(-) bacterial strains and against fungus Candida albicans.

Photoactive europium doped titanium dioxide (Eu: TiO2) thin films were grown by a matrix assisted pulsed laser evaporation technique. TiO2 and Eu cation-adsorbed TiO2 nanoparticles (NPs) sols were used as starting materials. The sols were synthesized by a colloidal sol-gel route. In order to obtain solid targets, the sols were cooled down until solidification in liquid nitrogen. The irradiation of the solid targets was performed in a controlled oxygen atmosphere using an UV KrF* (lambda = 248 nm, tau(FWHM) nu 25 ns, n = 10 Hz) excimer laser source. The NPs were transferred and deposited onto solid substrates placed in the front of the frozen targets, forming continuous thin films. The as-grown undoped and Eu: TiO2 thin films were submitted to post-deposition thermal treatments. The surface morphology, crystalline structure, and chemical composition of the thin films were characterized by field emission scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Functional optical properties of the films were investigated by UV-VIS-NIR spectroscopy, spectroscopic ellipsometry, and photoluminescence as a function of dopant concentration in the TiO2 host and annealing temperature. A direct correlation was established between the processing conditions, films structure, chemical composition and oxidation states studied both at the top surface and in-depth as well as their functional, optical properties. Eu doping induces a blue-shift of the absorption edge with respect to the as-grown samples. Under visible light excitation (lambda = 488 nm, emitted by an Ar+ laser source), the Eu: TiO2 samples show characteristic photoluminescence corresponding to the D-5(0) -> F-7(i) (i = 0, 1, 2, 3, 4) transitions of Eu+ ions in the visible spectral range. The samples treated at the highest annealing temperature are characterized by strong photoluminescence emission, high transmittance in the VIS-NIR spectral regions, and high refractive index.