National Institute Of Materials Physics - Romania

Publications

5714 articles found

1941. Influence of morphology on the emissive properties of dye-doped PVP nanofibers produced by electrospinning

Authors: Enculescu, M; Evanghelidis, A; Enculescu, I

Published: DEC 2014, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 75, 1371, DOI: 10.1016/j.jpcs.2014.07.008

Dye-doped polymer micro- and nanofibers with tailored light emission properties have great potential for applications in optical, optoelectronic, or photonic devices. In this study, these types of structures were obtained by electrospinning rhodamine 6 G-doped polyvinylpyrrolidone (PVP) using a polymer solution of 10% (mass) concentration in ethanol. Polymer nanofibers with different morphologies (smooth and beaded) and diameters of about 500 nm were obtained using different electrospinning conditions with the same solutions. Fluorescence optical microscopy observations showed that the dye was distributed uniformly in the doped PVP nanofibers. Different shifts were observed when we compared the wavelength of the dye emission band peak of the smooth nanofibers (566 nm) and the wavelength of the dye emission band peak of the beaded fibers (561.5 nm) produced by electrospinning in different conditions with the wavelength of the emission band peak for transparent thin films produced by spin coating (558 nm) using the same polymer solution. This demonstrates that it is possible to tune the optical properties of electrospun dye-doped polymer nanofibers simply by modifying the morphology of the material, i.e., the parameters of the electrospinning process. (C) 2014 Elsevier Ltd. All rights reserved.

1942. Origin of the stabilization of the metastable tetragonal high-pressure phase in SrCuO2 thin films grown on SrTiO3 substrates by pulsed laser deposition

Authors: Mihailescu, CN; Pasuk, I; Straticiuc, M; Nita, CR; Pantelica, D; Giapintzakis, J

Published: NOV 30 2014, APPLIED SURFACE SCIENCE, 320, 857, DOI: 10.1016/j.apsusc.2014.09.116

In this work we have systematically investigated the evolution of structure and stoichiometry in SrCuO2 films grown on TiO2-terminated SrTiO3 substrates as a function of the substrate temperature. Depending on the growth temperature SrCuO2/SrTiO3 films can exhibit either a pure tetragonal high-pressure phase, or a pure orthorhombic low-pressure phase, or a mixed phase. Our results indicate that at low substrate temperatures the non-equilibrium state of the growth process is responsible for the stabilization of the metastable tetragonal high-pressure structure in SrCuO2 thin films grown on ( 0 01) SrTiO3 substrates, whose lattice matches the metastable structure. In addition, at higher substrate temperatures thermodynamics become dominant over other factors and the SrCuO2 thin films are stabilized in the thermodynamically stable orthorhombic phase. (C) 2014 Elsevier B.V. All rights reserved.

1943. Electrochemical Grafting of Reduced Graphene Oxide with Polydiphenylamine Doped with Heteropolyanions and Its Optical Properties

Authors: Smaranda, I; Benito, AM; Maser, WK; Baltog, I; Baibarac, M

Published: NOV 6 2014, JOURNAL OF PHYSICAL CHEMISTRY C, 118, 25717, DOI: 10.1021/jp507324r

Electropolymerization of diphenylamine (DPA) onto a reduced graphene oxide (RGO) film was carried out to obtain a corresponding RGO/polydiphenylamie (PDPA) composite doped with phosphotungstic acid (H3PW12O40) heteropolyanions (PT). The synthesis was performed in the absence of light, since UVvis spectroscopy and photoluminescence (PL) studies on RGO/DPA blends irradiated by UV light revealed a partial transformation of the DPA monomer into oligomers of PDPA. Raman scattering demonstrates that the electropolymerization of DPA in the presence of H3PW12O40 (PTA) and RGO leads to the formation of PDPA covalently bonded to the RGO sheets (RGO/PDPA:PT). The presence of heteropolyanions in the PDPA matrix (PDPA:PT) is detected by FTIR spectroscopy. Comparing the PL excitation spectra of PDPA:PT and the RGO/PDPA:PT composite highlights an upshift of the band gap that is accompanied by a change in the composition of the PL spectrum in the spectral range of 2.253.54 eV. These changes originate in a charge transfer that takes place at the interface of nongrafted RGO and PDPA:PT. The gradual increase of the PL intensity of RGO covalently grafted with PDPA:PT reveals photochemical reactions under UV irradiation. These involve the CC stretching vibrational mode in the benzene ring of PDPA and indicate the transformation of an RGO/PDPA:PT composite containing HPW12O402 anions into an RGO/PDPA:PT composite stabilized by PW12O403 anions. These results not only provide important insights on the interactions between RGO, conjugated polymers, and stabilizing dopant ions but also impact on the synthesis conditions.

1944. Transport in a hybrid normal metal/topological superconductor Kondo model

Authors: Chirla, R; Dinu, IV; Moldoveanu, V; Moca, CP

Published: NOV 5 2014, PHYSICAL REVIEW B, 90, DOI: 10.1103/PhysRevB.90.195108

We investigate the equilibrium and nonequilibrium transport through a quantum dot in the Kondo regime, embedded between a normal metal and a topological superconductor supportingMajorana bound states at its end points. We find that the Kondo physics is significantly modified by the presence of theMajorana modes. When the Majorana modes are coupled, aside from the Kondo scale T-K, a new energy scale T* << T-K emerges, that controls the low-energy physics of the system. At lowtemperatures, the ac conductance is suppressed for frequencies below T*, while the noise spectrum acquires a similar to omega(3) dependence. At high temperatures, T >> T-K, the regular logarithmic dependence in the differential conductance is also affected. Under nonequilibrium conditions, and in particular in the {T, B}-> 0 limit, the differential conductance becomes negative. These findings indicate that the changes in transport may serve as clues for detecting the Majorana bound states in such systems. In terms of methods used, we characterize the transport by using a combination of perturbative and renormalization-group approaches.

1945. Thermal behaviour and characterisation of new biologically active Cu(II) complexes with benzimidazole as main ligand

Authors: Badea, M; Vlaicu, ID; Olar, R; Constand, M; Bleotu, C; Chifiriuc, MC; Marutescu, L; Lazar, V; Grecu, MN; Marinescu, D

Published: NOV 2014, JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 118, 1133, DOI: 10.1007/s10973-014-3745-z

Four coordination compounds of copper(II) were synthesised and characterised in solid state by elemental analysis, infrared, electronic and EPR spectroscopy, as well as by thermal analysis (TG/DTA). The complexes were formulated on the basis of experimental data as: [Cu(BzIm)(2)(H2O)]center dot H2O (1), [Cu-2(Acr)(4)(HBzIm)(2)] (2), [Cu(Acr)(2)(HBzIm)(2)] (3) and [Cu(Acr)(2)(HBzIm)(2)(H2O)]center dot H2O (4). IR data are in accordance with the unidentate nature of benzimidazole, in complexes (2), (3) and (4), and bridge bidentate nature of benzimidazole, in complex (1), while acrylato acts as uni- or bridge/chelate ligand. The electronic spectra display the characteristic pattern of square planar, square pyramidal, or octahedral stereochemistry, also confirmed by EPR spectra. Thermal decomposition evidenced several well-defined steps as dehydration of complexes (1) and (4), benzimidazole molecule releases for all complexes and acrylate decomposition in carbonate for complexes (3) and (4). In all four cases, the final residue after thermal treatment in air flow is copper(II) oxide, formed during the decomposition steps for complexes (3) and (4), and, respectively, after the oxidation of the metallic copper for complexes (1) and (2). Antimicrobial activities of the complexes have been determined by in vitro assays, against various Gram-negative and Gram-positive bacterial and fungal strains. Copper(II) complexes were also evaluated for their cytotoxicity on eukaryotic cells.

1946. Forward current enhanced elimination of the radiation induced boron-oxygen complex in silicon n(+)-p diodes

Authors: Makarenko, LF; Lastovskii, SB; Yakushevich, HS; Moll, M; Pintilie, I

Published: NOV 2014, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 211, 2562, DOI: 10.1002/pssa.201431315

Using forward current injection with densities in the range 15-30A/cm(2) we can effectively eliminate the radiation-induced boron-oxygen complex, which is the main compensating center in irradiated Si solar cells. It was found that for a given forward current density the elimination rate is decreasing with increasing irradiation dose. Additionally, some evidences have been obtained on the negative-U properties of the radiation-induced boron-oxygen complex.

1947. Synthesis, physico-chemical characterization and thermal behavior of new complexes with N4O2 donor set

Authors: Olar, R; Scaeteanu, G; Vlaicu, ID; Marutescu, L; Badea, M

Published: NOV 2014, JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 118, 1202, DOI: 10.1007/s10973-014-3809-0

Three new coordinative compounds of the type [Co(en)(2)CO3]center dot 0.75H(2)O (1) and [M(en)(2)(H2O)(2)]CO3 ((2) M:Ni, (3) M:Cu; en: ethylenediamine) were synthesized and characterized. The IR and UV-Vis spectral data indicate that ethylenediamine acts as chelate, while carbonate ions act as bidentate chelate ligand for (1)/counter ion for (2) and (3) generating complexes with octahedral stereochemistry. The thermal behavior provided confirmation of the complexes composition, as well as the number and the nature of water molecules and the intervals of thermal stability. The biological assays revealed a good activity against Enterococcus faecium for copper complex.

1948. Pulse annealing electron paramagnetic resonance with probing transition ions

Authors: Nistor, SV; Stefan, M; Ghica, D

Published: NOV 2014, JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 118, 1031, DOI: 10.1007/s10973-014-3743-1

The analysis of the sequence of electron paramagnetic resonance (EPR) spectra of trace amounts of substitutional probing paramagnetic ions incorporated in (nano)crystalline samples submitted to isothermal and isochronal pulse annealing treatments can offer a wealth of information on the thermally induced compositional and structural changes of the host material. The potential of this new thermal analysis method is illustrated here with results of such investigations on the thermal decomposition of crystalline zinc hydroxide (Zn(OH)(2)) and anhydrous zinc carbonate basic (Zn-5(CO3)(2)(OH)(6)) precursors containing trace amounts of substitutional Mn2+ probing ions into nanostructured zinc oxide-ZnO. The quantitative analysis of the sequence of isochronal pulse annealing EPR spectra could provide, besides the thermal decomposition curves of the two precursors, additional information about the structure of the resulting nanostructured ZnO, some of it hard to get by standard structural diffraction techniques. The analysis of both isochronal and isothermal pulse annealing EPR data was further used to investigate the crystallization mechanism of the initially formed nanostructured disordered ZnO and to quantitatively describe the further growth of the resulting ZnO nanocrystals with the increasing annealing temperature and duration.

1949. The Influence of the Microstructure and Morphology of CeO2 Buffer Layer on the Properties of YBCO Films PLD Grown on Ni Tape

Authors: Mihalache, V; Stefan, N; Enculescu, I; Mihailescu, IN; Socol, M; Miroi, M

Published: NOV 2014, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 27, 2485, DOI: 10.1007/s10948-014-2679-9

YBa2Cu3O7-delta films were deposited on CeO2-buffered nickel substrates, with different buffer thickness. Full width at half maximum of rocking curve, Delta(omega), of CeO2 and yttrium barium copper oxide (YBCO), as well as the critical temperature, T-c, of YBCO were shown to be strongly dependent on buffer thickness. They behave similarly but not proportional to the buffer thickness increase. This and the fact that Delta(omega) vs. buffer thickness and T-c vs. buffer thickness for YBCO behave similar with RMS roughness vs. thickness of CeO2 indicates that the surface peculiarity of buffers is responsible for YBCO properties. More precisely, the surface of CeO2 films prepared by the chemical solution route based on propionic acid is prone to agglomerate (de-wet) and the degree of agglomeration depends in an intricate way on buffer thickness. We showed that nor RMS roughness neither (00l) texture degree can define alone the surface suitable for c-axis YBCO nucleation. The {111} faceted grains (even in the case of high (00l) texture) and other defects generated by agglomeration supply a low fraction of (00l) flat terminations of buffer that affect the nucleation of c-axis-oriented YBCO phase. Moreover, the thermal instability of the surface morphology of CeO2 buffers (further development of de-wetting process, {111} faceted grains, etc. during superconducting layer deposition) influence the quality of YBCO films.

1950. The evanescent-wave cavity ring-down spectroscopy technique applied to the investigation of thermally grown oxides on Si(100)

Authors: Cotirlan-Simioniuc, C; Ghita, RV; Negrila, CC; Logofatu, C

Published: NOV 2014, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 117, 1365, DOI: 10.1007/s00339-014-8556-3

We report about the contribution of thermally grown SiOx overlayer on the SiOx/Si interface (with oxidation states Sin+, where n = 1, 2, 3, 4) to the optical losses of a resonant spectroscopic cavity. The experiments on Si oxide thin films were performed in evanescent wave for Si samples in contact with a total internal reflection surface of a BK7 prism. The evanescent field can be exploited to investigate properties and processes such as the absorption of thin film or solid/air interface reactions. The results show that the oxide overlayer thickness grows with the thermal exposure time and is limited after more than 7 h of treatment. Transmission electron microscopy has been used for the native oxide thickness measurement and angle-resolved X-ray photoelectron spectroscopy used to determine the thermal oxide thickness. A change of absorption coefficient Delta alpha in the range 100-200 cm(-1) is obtained by evanescent-wave cavity ring-down spectroscopy (EW-CRDS) for thermal silicon oxide overlayer, in agreement with the general trend from literature. The evaluation from the EW-CRDS experiments presents the used setup as a competitive method for measuring the absorption properties of thin overlayer.