National Institute Of Materials Physics - Romania

Publications

830 articles found

761. Structure of bulk and thin films of poly-methyl-methacrylate (PMMA) polymer prepared by pulsed laser deposition

Authors: Sava, F; Cristescu, R; Socol, G; Radvan, R; Savastru, R; Savastru, D

Published: DEC 2002, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 4, 970, DOI:

Homogeneous thin films have been obtained by UV-excimer pulsed laser deposition from a PMMA bulk material. The structure of films is at variance with the structure of the bulk polymer. The size of the ordered domains, as well as the structural features, evidenced by X-ray diffraction in the deposited films, depend on the fluence of the ultraviolet laser pulse used in the ablation process and on the temperature of the substrate. The radial distribution functions of the bulk PMMA material and thin films show differences in the short range order ascribed to the breaking of the polymer chains and monomer dissociation during deposition process.

762. Thin films of langasite (La3Ga5SiO14) prepared by pulsed laser deposition

Authors: Popescu, M; Sava, F; Georgescu, S; Gheorghe, L; Mihailescu, IN; Cristescu, R; Socol, G; Bradaczek, H

Published: SEP 2002, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 4, 818, DOI:

Thin films of langasite (La3Ga5SiO14) have been prepared from langasite crystal targets by pulsed laser deposition. The films deposited at room temperature on silicon wafers are amorphous. After annealing at 700 degreesC, in normal atmosphere, the films crystallize as polycrystalline langasite oriented with the plane (001) parallel to the surface of the silicon substrate.

763. X-ray diffraction analysis of the vitreous alloy Ag-As2S3

Authors: Popescu, M; Sava, F; Cornet, A; Broll, N

Published: JUL 2002, JOURNAL DE PHYSIQUE IV, 12, 49, DOI: 10.1051/jp4:20020211

The structure of several silver alloyed arsenic chalocgenide has been determined by X-ray diffraction. For low silver doping the disordered layer structure, characteristic to the glassy As2S3; is retained as demonstrated by the well developed first sharp diffraction peak in the X-ray diffraction pattern. For high amount of silver introduced in the As2S3; matrix, the disoredered layer configurations disappear, as shown by the diminishing and even disappearance of the first sharp diffraction peak in the X-ray patterns. A three-dimensional structure based on Ag2S-type configuration is formed.

764. Modification of surface-enhanced Raman scattering spectra of single-walled carbon nanotubes as a function of nanotube film thickness

Authors: Lefrant, S; Baltog, I; Baibarac, M; Schreiber, J; Chauvet, O

Published: JUN 15 2002, PHYSICAL REVIEW B, 65, DOI: 10.1103/PhysRevB.65.235401

In this paper we report modifications of the surface enhanced Raman scattering (SERS) spectra with film thickness of carbon single-walled nanotubes (SWNTs). The decrease of film thickness reveals an increased degree of disorder, which is interpreted in terms of degradation of the single-walled nanotubes and the formation of particles similar to highly oriented pyrolytic graphite (HOPG), C-60 and amorphous carbon. The degradation of the nanotubes, dependent on the metal substrate type and its morphology, is explained on the basis of interactions between the nanotubes and the metal substrate. The D band, which is an indicator of the degree of disorder, increases its intensity when the film thickness is decreased. SERS spectra recorded at different excitation wavelengths reveal that metallic nanotubes are primarily affected. A mutual correlation was found between the variations in the relative intensity of the D band and the Breit-Wigner-Fano component due to metallic tubes displayed by Raman bands situated in the 1500-1600 cm(-1) range. A direct interaction between C-60 and nanotubes is revealed by comparative SERS spectra on SWNTs and (SWNTs+C-60) films. At lambda(exc)=676.4 nm the narrowing in the lower energy side of the broad band associated to tangential vibration modes indicates that the metallic tubes are involved in the formation of a [(SWNT)C-+(60)-] complex type.

765. Substrate influence on the X-ray diffraction patterns of amorphous chalcogenide thin films deposited on silicon wafers

Authors: Lorinczi, A

Published: APR 3 2002, THIN SOLID FILMS, 408, 285, DOI: 10.1016/S0040-6090(02)00065-2

In the case of AsSe:Sn and Ge27Sb13Se60 amorphous thin films deposited on Si(100) wafer, it was observed that the diffraction pattern is distorted by the escape-peak, which is very sensitive in intensity to the wafer's position in its own plane, This peak was found to be superimposed on the first sharp diffraction peak, which characterises the medium range order in the amorphous materials. A method for the minimisation of the escape-peak effect is presented. A different method is suggested to avoid this peak. (C) 2002 Elsevier Science B.V. All rights reserved.

766. Bi2-xSbxTe3 thick thermoelectric films obtained by electrodeposition from hydrochloric acid solutions

Authors: Nedelcu, M; Sima, M; Manea, AS; Lazarescu, MF; Ghita, RV; Craciunoiu, F; Visan, T

Published: MAR 2002, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 4, 106, DOI:

A method to produce composition modulated Bi2-xSbxTe3 alloys by electrodeposition has been developed. The electrolyte, which consists in a HCl 6M (pH = 0) aqueous solution, allows for the co-deposition of bismuth, antimony and tellurium to be accomplished at room temperature on glassy carbon, platinum or nickel electrode. The composition of the films, their crystal structures and morphology were studied as function of electrochemical parameters and bath composition. It is shown that the electrodeposits are monophasic and exhibit a polycrystalline state. The electrical resistivity was of the order of 7-10 mOmegacm, whereas the electrodeposition rate value was 30-50 mum/h for the films with good thermoelectric properties.

767. SERS studies on single-walled carbon nanotubes submitted to chemical transformation with sulfuric acid

Authors: Lefrant, S; Baltog, I; Baibarac, M; Mevellec, JY; Chauvet, O

Published: 2002, CARBON, 40, 2211, DOI: 10.1016/S0008-6223(02)00089-1

Surface-enhanced Raman scattering (SERS) at 676.44 nm and 1064 nm excitation wavelengths was used to investigate chemical transformation of single-walled carbon nanotubes (SWNTs) deposited on a gold support. Sulfuric acid was used as the chemical reagent. Special attention was paid to the changes in the Raman bands associated to radial and tangential vibration modes. Partial restoration of the Raman spectra by a Subsequent alkaline treatment indicates a transformation with a certain degree of reversibility. The recovery reaction achieved with a 0.5 M KOH solution showed that the variations of tangential and radial band groups are not correlated. The intensity changes of the radial bands is a principal indicator for the chemical transformation of the SWNTs. Particular attention was paid to radial bands at 164 and 176 cm(-1), observed with 1064 nm and 676.44 nm excitation wavelength, respectively, and their 14 cm(-1) up-shifted replicas i.e. the bands at 178 and 190 cm(-1). A different behavior of these bands in the anti-Stokes side was observed. (C) 2002 Elsevier Science Ltd. All rights reserved.

768. Electrosynthesis and properties of some substituted polyanilines in phosphate buffer

Authors: Sima, M; Buda, M; Vasile, E; Visan, T

Published: JAN-FEB 2002, REVUE ROUMAINE DE CHIMIE, 47, 193, DOI:

The electropolymerizations of 2-aminohippuric acid methyl ester and 2-aminobenzoic acid methyl ester monomers in phosphate buffer (pH = 6.5) were studied. The formation of thin films at the platinum electrode surface is observed as main product and the electrosynthesis process of the two substituted polyanilines shows the voltammetric characteristics of the polyaniline film electrosynthesis. The necessary charge to deposit 0.05 mum poly(2-aminohippuric acid methyl ester) or poly(2-aminobenzoic acid methyl ester) films is about 60 mC/cm(2). The polymer films were investigated by electrochemical impedance spectroscopy. On the basis of the obtained results we propose the equivalent circuit for these polymer films.

769. Structure, properties and modelling of As25Te35Si40 glass

Authors: Popescu, MA; Lorinczi, AA; Sava, FA; Iovu, MB; Leonovici, MC; Stegarescu, MA; Halm, TD; Hoyer, WD

Published: 2002, XIIITH INTERNATIONAL SYMPOSIUM ON NON-OXIDE GLASSES AND NEW OPTICAL GLASSES PTS 1 AND 2, 84, DOI:

Glassy composition As25Te35Si40 was investigated by microhardness, differential scanning calorimetry, X-ray diffraction, electrical conductivity and Raman spectroscopy measurements. A continuous random network model of the glass structure has been developed.

770. Raman and SERS studies of carbon nanotubes

Authors: Lefrant, S; Baibarac, M; Baltog, I; Buisson, JP; Chauvet, O

Published: 2002, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 374, 334, DOI: 10.1080/10587250210472

Since the discovery of carbon nanotubes, Raman Scattering Spectroscopy and transmission Electron Microscopy have been proved to be powerful for a detailed investigation of their properties. In TEM, a direct observation can be made, whereas Raman Scattering is used extensively to probe the different samples via their vibrational properties. In this paper, we report on recent results obtained in both single-walled carbon nanotubes (SWNTs), multiwalled-carbon nanotubes (MWNTs) and PMMA/nanotubes composites. In addition, in the case of SWNTs, we present studies carried out in Raman scattering when SERS (Surface Enhanced Raman Scattering) conditions are used. In particular, we put in evidence an increased state of disorder at the interface nanotubes/metallic support when the nanotube film thickness is decreased. Strong degradations can be observed primarily on metallic tubes with a concomitant formation of C-60-like molecules. In the case of MWNTs, calculations of interactions between concentric tubes lead to the occurrence of low frequency vibrational modes, in good agreement with experiments.