National Institute Of Materials Physics - Romania

Publications

839 articles found

361. Thin films of amorphous Ga2S3 and rare-earth sulphides

Authors: Popescu, M; Lorinczi, A; Sava, F; Velea, A; Simandan, ID; Badica, P; Burdusel, M; Galca, AC; Socol, G; Jipa, F; Zamfirescu, M

Published: MAR 1 2015, MATERIALS LETTERS, 142, 231, DOI: 10.1016/j.matlet.2014.12.028

The aim of this research is to prepare amorphous thin films of undoped gallium sulphide and doped with rare-earth sulphides, of rare-earth sulphides and to investigate their physical properties. We have prepared thin amorphous films of Ga2S3, EuS, Er2S3, Gd2S3, and Ga2S3 doped with rare-earth sulphides (Ga2S3:EuS, Ga2S3:Er2S3, Ga2S3:Gd2S3) by Pulsed laser Deposition (PLD). The corresponding targets for preparation of amorphous thin films were obtained by Spark Plasma Sintering (SPS) from commercially available powders of binary sulphides. The structural results for the undoped and doped Ga2S3 thin films indicate a packing of disordered layers similar to that of amorphous As2S3. Femtosecond laser irradiation of the Ga2S3 thin films shows a photoexpansion effect at low laser power (85-100 mW) and an ablation effect at higher laser power (above 105 mW). The threshold between low power and high power pulses is situated at higher value for Ga2S3 (100 mW) in comparison with the case of As2S3 thin films (20 mW). (C) 2014 Elsevier B.V. All rights reserved.

362. Structural and optical properties of fluorescent BaFBr-Eu2+@SiO2 core/shell phosphor heterostructure

Authors: Secu, M; Cernea, M; Secu, CE; Vasile, BS

Published: FEB 1 2015, MATERIALS CHEMISTRY AND PHYSICS, 151, 86, DOI: 10.1016/j.matchemphys.2014.11.038

Core-shell composites BaFBr:Eu2+@SiO2 were synthesized starting from BaFBr:Eu2+ nanophosphor powder prepared by precipitation method, followed by a silica gel coating process. Electron microscopy investigations have shown an average BaFBr:Eu2+ grains size of about 30 nm and 11 nm SiO2 shell thickness. The core-shell heterostructure exhibits Eu2+ luminescence at 385 nm that is improved during subsequent calcination at high temperatures up to 700 degrees C; the core-shell morphology (size and thickness) structure is preserved during the calcination. (C) 2014 Elsevier B.V. All rights reserved.

363. Photochemical processes developed in composite based on highly separated metallic and semiconducting SWCNTs functionalized with polydiphenylamine

Authors: Baibarac, M; Baltog, I; Smaranda, I; Magrez, A

Published: JAN 2015, CARBON, 81, 438, DOI: 10.1016/j.carbon.2014.09.075

The electrochemical polymerization of diphenylamine (DPA) onto electrodes of Pt coated with highly separated metallic (98%) or semiconducting (99%) single-walled carbon nanotubes (SWCNTs) in the presence of H3PW12O40 was performed by cyclic voltammetry in order to obtain composite materials based on polydiphenylamine (PDPA) doped with heteropolyanions of H3PW12O40 and carbon nanotubes. Our data demonstrate that the photoluminescence quenching effect of the PDPA doped with H3PW12O40 heteropolyanions in the presence of SWCNTs is due to the metallic component. Under UV irradiation of SWCNTs highly separated in metallic and semiconducting tubes functionalized with PDPA doped with heteropolyanions of H3PW12O40 new photochemical reactions are evidenced by photoluminescence studies. These reactions lead to a shortening of the macromolecular chain of PDPA. The photochemical process is more intense in the case semiconducting SWCNTs functionalized with PDPA doped with heteropolyanions in comparison to metallic SWCNTs functionalized with PDPA in doped state, it being a consequence of an additional chemical interaction between the DPA dimer doped with H3PW12O40 heteropolyanions and semiconducting SWCNTs. (C) 2014 Elsevier Ltd. All rights reserved.

364. Anti-Stokes Raman spectroscopy as amethod to identify the metallic and semiconducting configurations of double-walled carbon nanotubes

Authors: Baibarac, M; Baltog, I; Matea, A; Mihut, L; Lefrant, S

Published: JAN 2015, JOURNAL OF RAMAN SPECTROSCOPY, 46, 38, DOI: 10.1002/jrs.4597

Although Raman spectra reveal, as a signature of double-walled carbon nanotubes (DWCNTs), two radial breathing mode (RBM) lines associated with the inner and outer tubes, the specification of their nature as metallic or semiconducting remains a topic for debate. Investigating the spectral range of the RBM lines, we present a new procedure of the indexing of the semiconducting or metallic nature of the inner and outer shell that forms the DWCNT. The procedure exploits the difference between the intensities of recorded anti-Stokes Raman spectrum and the anti-Stokes spectrum calculated by applying the Boltzmann formulae to the recorded Stokes spectrum. The results indicate that the two spectra do not coincide with what should happen in a normal Raman process, namely, that there are RBM lines of the same intensity in both spectra, as well as RBM lines of higher intensity that are observed in the calculated spectrum. This discrepancy results from the surface-enhanced Raman scattering mechanism that operates differently on metallic or semiconducting nanotubes. In this context, the analysis of the RBM spectrum can reveal pairs of lines associated with the inner/outer shell structure of DWCNT, and when the intensities between the recorded and calculated spectra coincide, the nanotube is metallic; otherwise, the nanotube is semiconducting. Copyright (c) 2014 John Wiley & Sons, Ltd.

365. Anti-Stokes Raman spectroscopy as a method to identify metallic and mixed metallic/semiconducting configurations of multi-walled carbon nanotubes

Authors: Baibarac, M; Matea, A; Ilie, M; Baltog, I; Magrez, A

Published: 2015, ANALYTICAL METHODS, 7, 6230, DOI: 10.1039/c5ay01281c

SERS studies were performed on films of two families of multi-wall carbon nanotubes (MWCNTs) under an excitation light of 514.5 nm and 647.1 nm. These included Aldrich-MWCNTs, which alternate semiconducting and metallic tubes and M-MWCNTs that contain only metallic tubes obtained by water assisted catalytic chemical vapour deposition (CCVD). The two families of MWCNTs reveal similar spectra in the Stokes branch, which feature an increasing Raman intensity when the glass substrate is replaced with an Au or Ag substrate, indicating a (surface enhanced Raman scattering) SERS mechanism. In the anti-Stokes branch, despite an enhancement of approximately 100 times compared to the predictions of the Boltzmann law, only Aldrich-MWCNTs exhibit a Raman spectrum with an intensity that increases as a result of the change in the glass substrate to Au or Ag, a fact that is revealed by the signature of the SERS process. The invariance of the Raman intensity in the anti-Stokes branch as a result of the change of the substrate is characteristic of M-MWCNTs and results from a Raman light scattering process that takes place only within the skin depth of the metallic structure.

366. Dielectric Properties of LDH-Type Layered Materials Containing Zn or Mg Ions : On the Non Monotonous Temperature Dependence of Relaxation Times

Authors: Frunza, L; Frunza, S; Ganea, CP; Zgura, I; Neatu, F; Parvulescu, VI

Published: 2015, ROMANIAN JOURNAL OF INFORMATION SCIENCE AND TECHNOLOGY, 18, 328, DOI:

The electrical behavior of layered double hydroxides containing Al or Ga as the trivalent ions and Mg or Zn as bivalent ions was evaluated using broadband dielectric spectroscopy in a wide temperature range. Besides conduction effects a relaxation peak is observable at high frequencies which is assigned to the reorientational fluctuations of water molecules adsorbed on the oxide surface or in the interlayer voids. The Maxwell-Wagner-Sillars peak, superimposed to the conductivity phenomenon is observable at low frequencies. A non-monotonous temperature dependence of the relaxation rates of the relaxation process has been found. A quantitative description of this dependence was possible based on a model assuming two competing processes: rotational fluctuation of water molecules and formation of additional defects. Reasonable values for the characteristic parameters were obtained. Thus the water behavior in the studied layered oxides is similar to that observed for water in other porous materials. However, the activation energy of the rotational fluctuation, the pre exponential factor and the number of defects are higher whereas the value of the energy of defect formation is lower in the layered oxide materials than for water confined to nanoporous molecular sieves, porous glasses or in bulk ice.

367. Optical Properties of Single-Walled Carbon Nanotubes Functionalized with Poly(2,2 '-bithiophene-co-pyrene) Copolymer

Authors: Smaranda, I; Baibarac, M; Ilie, M; Matea, A; Baltog, I; Lefrant, S

Published: 2015, CURRENT ORGANIC CHEMISTRY, 19, 661, DOI: 10.2174/1385272819666150311231454

The photoluminescent (PL) properties of composites based on single-walled carbon nanotubes (SWNTs) and poly(2,2'-bithiophene-co-pyrene) (PBTh-Py), prepared by in situ chemical polymerization of the two monomers in the presence of carbon nanotubes, are reported. We demonstrate that the functionalization of SWNTs with PBTh-Py copolymer is revealed through a gradual quenching process of PL with the increase of SWNT content (semiconducting component) in the composite mass. FTIR spectroscopy indicates the existence of several steric hindrance effects that originate in the covalent functionalization of SWNTs with PBTh-Py copolymer. The film deposition of PBTh-Py copolymer and PBTh-Py/SWNTs composite onto rough Au supports induces changes in the FTIR spectrum, which originate in an adsorption mechanism caused by the preferential orientation of molecules on the metallic support. Surface-enhanced Raman scattering (SERS) spectroscopy reveals the side-wall functionalization of SWNTs with PBTh-Py copolymer by changes in the shapes, peak position and relative intensities of different Raman lines.

368. AB INITIO INVESTIGATION OF OPTICAL PROPERTIES IN TRIANGULAR GRAPHENE - BORON NITRIDE CORE-SHELL NANOSTRUCTURES

Authors: Nila, AA; Nemnes, GA; Manolescu, A

Published: 2015, ROMANIAN JOURNAL OF PHYSICS, 60, 700, DOI:

We calculate the optical properties of atomic-sized core-shell graphene - boron nitride nanoflakes with triangular shaped cross-section using the density functional theory. The optical properties can be tuned by using different sizes and proportions of the core-shell materials. Anisotropic effects manifested in the absorption of unpolarized light with different orientations of the optical vector are pointed out.

369. Organic-inorganic solar cell based on ZnO nanorod arrays

Authors: Sima, M; Vasile, E; Sima, M

Published: SEP-OCT 2015, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 17, 1313, DOI:

In this study, we communicate an investigation on perovskite solar cell based on ZnO nanorod array and spiro OMeTAD film as electron and hole selective contacts. It was found that large thickness or small electrical conduction of the Li doped spiro OMeTAD layer and recombination processes at the interfaces contributed to the reduced efficiency of the solar cell.

370. Layered materials of LDH-type containing Zn ions: dielectric measurements show rotational fluctuations of water molecules

Authors: Frunza, L; Ganea, P; Zgura, I; Frunza, S; Neatu, F; Parvulescu, VI

Published: 2015, 2015 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), 70, DOI:

Layered double hydroxide (LDH) materials containing Zn ions were studied by broadband dielectric spectroscopy to follow up rotational molecular dynamics of confined water. One main relaxation process was observed in the experimental window; it was assigned to fluctuations of water molecules forming a strongly adsorbed thin layer upon the oxide surface. The temperature dependence of the relaxation rates has an unusual shape characterized by the presence of a maximum. Despite the characteristic parameters, this non-monotonous dependence is shown to be a rather general feature for water confined to LDHs.