National Institute Of Materials Physics - Romania

We have studied the light emission properties of aged glassy (g-) and crystalline (c-) As2S3 and freshly fractured glassy As2S3 with photosensitive realgar inclusions in the 1.5-4.5 eV spectral range and applying different excitation photon energies. Additionally, Raman scattering measurements were performed on g-As40S60 to investigate structural changes caused by corrosion of the glass in air. Major features in the luminescence spectra were observed at 1.65,1.87, 2.04, 226, 2.80 and 325 eV photon energies. The 1.65 eV band in photoluminescence spectra of g-As2S3 was identified to be related to the As side by comparison of the photoluminescence spectrum with literature data for As4S4 realgar crystal. A newly observed band at 1.87 eV was assigned to the As side in pararealgar occurring due to the light induced transformation from realgar to pararealgar phase. An oxidation process was found to take place during the aging and photo-aging accompanied with formation of As oxides, indicated by the emission bands at 2.04 and 226 eV. A rapid photo-oxidation process was observed in freshly fractured surfaces at room temperature which was connected with the light stimulated realgar-pararealgar transformation. (C) 2016 Elsevier B.V. All rights reserved.

The crystallization process of Yb3+/Er3+ codoped LiYF4 nanoparticles obtained by sol-gel method was studied using differential scanning calorimetry and X-ray diffraction. Thermal data obtained from differential scanning calorimetry measurements under non-isothermal conditions at different heating rates were examined using both model-free and model fitting approaches. It was shown that LiYF4 phase crystallization occurs at around 383 degrees C as a result of the thermal decomposition of trifluoracetates and is shifted to higher temperatures as the heating rate increases. Both Friedman and Ozawa-Flynn-Wall analyses have indicated that the activation energy and pre-exponential factor decrease slightly as the crystallization proceeds. According to the model-fitting approach (by using extended Prout-Tompkins nth-order, ath autocatalysis reaction), LiYF4 nanocrystalline phase is the result of an autocatalytic process where a second metastable phase YF3 acts as a catalyst that accelerates the crystallization process of the LiYF4 phase. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Optical properties are reported for composites based on single-walled carbon nanotubes (SWNT5) and copolymer poly(3,4-ethylenedioxythiophene-co-pyrene) (PEDOT-Py) prepared by chemical polymerization of two monomers in the presence of carbon nanotubes. A charge transfer between SWNT5 and the PEDOT-Py copolymer was demonstrated by Raman scattering. The increase in the relative intensity of the Raman lines peaked at 440-577 cm(-1), which were assigned to the ethylenedioxy ring vibrational modes, indicated a significant hindrance steric in the case of the composites based on the PEDOT-Py copolymer and metallic SWNTs. The increase in the absorbance of IR band peaked at 984 cm(-1) occurred simultaneously with the disappearance of the IR band at 1639 cm(-1). This finding was a consequence of the formation of new covalent bonds between SWNTs and the thiophene and benzene rings of the repeating units of the PEDOT-Py copolymer. The photoluminescence (PL) quenching process of the PEDOT-Py copolymer was induced by semiconducting SWNTs. The PL quenching of PEDOT-Py copolymer in the presence of SWNT5 was. demonstrated based on the energy level diagrams of the two constituents of the PEDOT-Py/SWNTs composite material. (C) 2016 Elsevier B.V. All rights reserved.

Neodymium and gallium codoped barium titanate (Ba1-x-yNdxGayTiO3, x = 0.03, 0.05, 0.07, 0.1 and y = 0.03) were prepared by sol-gel method in order to investigate its dielectric and photoluminescence properties. The structure and morphology of Ba1-x-yNdxGayTiO3 powders calcined at 1100 A degrees C and sintered ceramics by spark plasma sintering (SPS) technique were analyzed using X-ray diffraction and scanning electron microscope, respectively. The photoluminescence and dielectric properties of Nd doped Ba0.97Ga0.03TiO3, (y = 0.03) as a function of Nd3+ concentration and sintering temperature were also investigated. The influence of Ga3+ and Nd3+ dopants on the crystalline phases, sintering process by SPS and macroscopic properties of Ba1-x-yNdxGayTiO3 ceramics was analyzed, as well as a weak hot emission from the 4F5/2 level. The sintered ceramics showed high dielectric constant and moderate dielectric losses. The highest value of the permittivity obtained for Ba1-x-yNdxGayTiO3 ceramics was E >(r) = 5890 at room temperature, 1 kHz and x = y = 0.03. This study shown that Nd3+ decreases the density of the pellets, increases the dielectric losses and decreases the dielectric constant of Ba0.97Ga0.03TiO3 ceramics. Therefore, the values of dielectric properties, at room temperature, of BaTiO3 doped with 3 at.% Ga and aecurrency sign7 at.% Nd are higher than those of undoped BaTiO3 ceramic.

Thin amorphous As2S3 films show a giant photoexpansion of similar to 5% upon femtosecond laser illumination and this expansion remains after switching off the laser beam. To understand the structural modifications which occur during the photoexpansion process we assumed a molecular cluster structure of the amorphous As2S3. The focused femtosecond laser beam induces electrical charge redistribution on the sulfur atoms of each cluster, which increases the electrical repulsion between sulfur atoms, and thus induces an expansion of the clusters, by network reconfiguration, without breaking the bonds. An increase of the van der Waals distance between molecular clusters also takes place. (C) 2016 Elsevier B.V. All rights reserved.

The phenomena occurring at the interface between the electrodes and a liquid with ionic conductivity is manifested specifically in the spectrum of the dielectric permittivity. The theoretical model of the electrode polarization (charge space polarization) is met in the dielectric spectroscopy measurements in which electrodes are conductors (allow transfer of electric charge) and the adsorption-desorption of ions occurs on their surface. A pair of ions with equal and opposite charge is considered. The ions are produced by the association dissociation of neutral molecules. We propope general boundary conditions (BCs) with charge transfer current which contains the adsorption-desorption parameters. The BCs and the basic equations of the Poisson-Nernst Planck model, i.e. transport equations of charge carriers and Poisson equation, are re-formulated through new variables. The mathematical problem is solved analytically. The results allow finding a criterion for choosing a general and simple admittance expression, thus using a unified approach for different cases. We show that the small" signal approximation is no longer needed, at least for modern dielectric spectroscopy equipment. The ions can be point-like or may have finite dimensions. For the last case a model of Stem compact layer with charge transfer is proposed, which allows to calculate the compact layer impedance. Finally we obtain the equivalent admittance of the physical system composed from metal electrodes and ionic liquid in the presence of the Stern layer and the space charge polarization. The limitations of the model are given by the hypothesis of "flat band" and the non-selective adsorption of ions. (C) 2016 Elsevier B.V. All rights reserved.

The effects of 800 keV Ar ion irradiation on thin nanocrystalline SiC films grown on (100) Si substrates using the pulsed laser deposition (PLD) technique were investigated. On such PLD grown films, which were very dense, flat and smooth, X-ray reflectivity, glancing incidence X-ray diffraction and nanoindentation investigations were easily performed to evaluate changes induced by irradiation on the density, surface roughness, crystalline structure, and mechanical properties. Results indicated that the SiC films retained their crystalline nature, the cubic phase partially transforming into the hexagonal phase, which had a slightly higher lattice parameter then the as-deposited films. Simulations of X-ray reflectivity curves indicated a 3% decrease of the films density after irradiation. Nanoindentation results showed a significant decrease of the hardness and Young's modulus values with respect to those measured on as-deposited films. Raman and X-ray photoelectron spectroscopy investigations found an increase of the C-C bonds and a corresponding decrease of the Si-C bonds in the irradiated area, which could explain the degradation of mechanical properties. (C) 2015 Elsevier B.V. All rights reserved.

This paper presents some studies about the preparation by matrix-assisted pulsed laser evaporation (MAPLE) of mixed layers based on two arylenevinylene oligomers, 1,4-bis [4-(N,N'-diphenylamino)phenylvinyl] benzene (L78) and 3,3'-bis(N-hexylcarbazole)vinylbenzene (L13) as donor and buckminsterfullerene (C-60) as acceptor, blended in three different weight ratios: 1:1, 1:2 and 1:3. The optical, morphological, structural and electrical properties of these mixed layers have been investigated emphasizing the effect of the layer composition and of the significant degree of disorder. I-V characteristics have revealed typically solar cell behaviour for the heterostructures prepared with mixed layers containing L78 (L13) and fullerene blended in a weight ratio of 1:2. The solar cell structure glass/ITO/L13:C-60/Al has shown the best parameters. (C) 2015 Elsevier B.V. All rights reserved.

Quasi-amorphous titanium oxynitride (TiON) films were obtained by annealing sol-gel anatase TiO2 films in NH3 atmosphere at 600 degrees C. These films were irradiated with 50 laser pulses using the fourth harmonic (266 nm) radiation of the Nd-YAG laser, with an average fluence of 20 mJ/cm(2). HRTEM observations of the pulsed laser irradiated films evidenced the rutile TiO2 nanocrystallites formation. The rutile structure was not present either in the TiON films before the laser irradiation, or in the initial sol-gel anatase TiO2 films. During the laser irradiation, the film structure remains in the solid state phase, as it results from the temperature estimation and microscopic observations. For the rutile nanocrystals formation, the atomic diffusion length of the oxygen and titanium atoms should be in the nanometric range during the laser pulse action, which implies a diffusivity close to the values observed in the liquid phase. We consider that the rutile phase formation is a proof of the fast atomic diffusion in the solid amorphous matrix, during the laser irradiation. (C) 2015 Published by Elsevier B.V.

Heterostructures based on zinc phthalocyanine (ZnPc), magnesium phthalocyanine (MgPc) and 5,10,15,20-tetra(4-pyrydil)21H,23H-porphine (TPyP) were deposited on ITO flexible substrates by Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique. Organic heterostructures containing (TPyP/ZnPc(MgPc)) stacked or (ZnPc(MgPc):TPyP) mixed layers were characterized by X-ray diffraction-XRD, photoluminescence-PL, UV-vis and FTIR spectroscopy. No chemical decomposition of the initial materials was observed. The investigated structures present a large spectral absorption in the visible range making them suitable for organic photovoltaics applications (OPV). Scanning electron microscopy-SEM and atomic force microscopy-AFM revealed morphologies typical for the films prepared by MAPLE. The current-voltage characteristics of the investigated structures, measured in dark and under light, present an improvement in the current value (similar to 3 order of magnitude larger) for the structure based on the mixed layer (Al/MgPc:TPyP/ITO) in comparison with the stacked layer (Al/MgPc//TPyP/ITO). A photogeneration process was evidenced in the case of structures Al/ZnPc:TPyP/ITO with mixed layers. (C) 2015 Elsevier B.V. All rights reserved.