National Institute Of Materials Physics - Romania

Publications

830 articles found

741. Structure and electrical properties of electron irradiated CdSe thin films

Authors: Ion, L; Antohe, S; Popescu, M; Scarlat, F; Sava, F; Ionescu, F

Published: MAR 2004, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 6, 119, DOI:

Structural and electrical properties of polycrystalline CdSe thin films irradiated with high-energy electrons are analyzed. The samples were prepared by vacuum deposition onto optical glass substrate. Their structure and the temperature dependence of the electrical resistance were determined, both before and after irradiation with 6 MeV electrons and fluencies up to 10(16) electronS/cm(2). There were no measurable changes in the crystalline structure of the films after irradiation. Electrical properties are controlled by a defect level of donor type, possibly a selenium vacancy, with two charge states having ionization energies of about 0.40 eV, respectively 0.22 eV. Irradiation increases significantly the concentration of those defects.

742. Modelling of the complex carbon structure: Fullerene-nanotubule

Authors: Lorinczi, A; Popescu, M; Sava, F; Anghel, A

Published: MAR 2004, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 6, 352, DOI:

The complex structure fullerene(C-60)-nanotubule has been built from special plastic units and relaxed by a computer Monte-Carlo - Metropolis procedure. The purpose was to show that interconnection fullerene-nanotubule is possible from the energetical point of view. The results open the way towards the assessing the possibility to create nano-conductors and connections between different parts of nano-devices, using nano-carbon configurations filled by metal atoms.

743. Electrodeposition of CdTe nanorods in ion track membranes

Authors: Sima, M; Enculescu, I; Trautmann, C; Neumann, R

Published: MAR 2004, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 6, 125, DOI:

CdTe nanowires were electrodeposited in ion track membranes with pore diameters in the range 100-2000 nm. Acidic and basic baths were tested for obtaining materials with good stoichiometry. The cyclic voltammograms were compared for the case of deposition on a carbon rotating disc and on the porous membrane substrates, the differences being attributed to the additional resistance induced by the pores. For larger pores a tendency of growing hollow structures was observed.

744. Fluorescence of substituted aromatic derivatives crystalline materials for optical non-linear applications

Authors: Stanculescu, A; Tugulea, L; Stanculescu, F; Socol, M

Published: 2004, ROMOPTO 2003: SEVENTH CONFERENCE ON OPTICS, 5581, 607, DOI: 10.1117/12.583028

Our fluorescence study gave information on the presence of chemical defects and irregularities in crystal structure, offering a good tool for quality evaluation of organic crystalline materials. We have analysed the correlation between the bulk or thin film samples' quality determined by experimental parameters (thermal gradient, moving speed of the growth interface, crucible configuration, slow or fast solidification rates) and the fluorescence characteristics for pure meta-dinitrobenzene and meta-dinitrobenzene doped with 8 hydroxyquinoline or 1,3 dihydroxybenzene. The comparative study of the position and shape of the fluorescence emissions for pure and controlled doped materials nave us information on the chemical defects.

745. Covalent functionalization of single-walled carbon nanotubes by aniline electrochemical polymerization

Authors: Baibarac, M; Baltog, I; Godon, C; Lefrant, S; Chauvet, O

Published: 2004, CARBON, 42, 3152, DOI: 10.1016/j.carbon.2004.07.030

Electrochemical polymerization of aniline in an HCl solution on a single-walled carbon nanotubes (SWNTs) film has been studied by Raman and FTIR spectroscopy. It is shown that this method leads to a covalent functionalization of SWNTs with polyaniline (PANI). A careful study in Raman scattering shows that the increase in the intensity of the band at 178cm(-1) associated with radial breathing modes of SWNTs bundles suggests an additional nanotubes roping with PANI as a binding agent. A post chemical treatment with the NH4OH solution of polymer-functionalized SWNTs involves an internal redox reaction between PANI and carbon nanotubes. As a result, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one. (C) 2004 Elsevier Ltd. All rights reserved.

746. C-60 aggregates in pyrrolidine and N-methyl-2-pyrrolidinone evidenced by surface-enhanced Raman scaterring spectra

Authors: Baibarac, M; Preda, N; Mihut, L; Velula, T; Baltog, I; Mevellec, JY; Lefrant, S

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 417, 587, DOI: 10.1080/15421400490478434

At room temperature, soluted C-60 in pyrrolidine and N-methyl-2-pyrrolidinone associates slowly in stable aggregates. The process originates in the charge transfer from the solvent molecules as electron donors towards fullerene. In the aggregate phase the intermolecular interactions lead to a large reduction in parent I-h C-60 symmetry such that a modified phonon spectrum is observed. Using the surface-enhanced Raman scattering we demonstrate that the Raman range below 800 cm(-1) is most diagnostic for aggregate assignment. In particular, the strong decrease of the radial breathing mode and appearance of new bands at 110 and 93 cm(-1) due to the interball interactions must be noticed.

747. Sers spectra of polyaniline/carbon nanotubes and polyaniline/fullerene composites

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

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 415, 155, DOI: 10.1080/15421400490481269

SERS and FTIR spectra of M and S composites, provided by adding of single-walled carbon nanotubes (SWNTs) to the polyaniline (PANI) solution and chemical polymerization of aniline, respectively are different. The difference originates in the irreversible transformation of SWNTs in the polymerization medium. This explains the similarity of SERS and FTIR spectra of PANI/SWNTs and PANI/C-60 composites, chemically synthesized. For M composites, strong hindrance effects are evidenced by band at 773-755 cm(-1) associated to the vibration of deformation of the benzene and quinoid ring. The charge transfer is illustrated for all compounds by absorption band at 1144 cm(-1).

748. Photoluminescence studies on [C60] fullerene solutions in N-methyl-2-pyrrolidinone and pyrrolidine

Authors: Mihut, L; Preda, N; Baibarac, M; Baltog, I; Lefrant, S; Wery, J

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 415, 21, DOI: 10.1080/15421400490478209

Photoluminescence (PL) spectra of C-60 solutions in N-methyl-2-pyrrolidinone (NMP) show a drastic increase of the PL intensity if the solutions are stored longer periods of time. Two distinct spectral components, one in the red (1.92-2.00 eV) and another in the green (2.18-2.3 eV) were identified in the whole PL emission. Their weights depend on C-60 concentration and on storage time. The two emissions come from two successive processes running with different kinetics. Charge transfer, leading to (C-60, NMP) loose molecular complexes and a slow aggregation process of these entities seems to originate the red and green emission, respectively. Similar PL properties are found when pyrrolidine (P) is used as solvent.

749. Electrochemical deposition of PBSe1-xTex nanorod arrays using ion track etched membranes as template

Authors: Sima, M; Enculescu, I; Visan, T; Spohr, R; Trautmann, C

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 418, 755, DOI: 10.1080/15421400490478885

Lead chalcogenides (PbS, PbSe, PbTe) are narrow band gap semiconductors which have been studied in the field of IR detection and thermoelectric devices. The template method is a general approach for synthesizing nanomaterials within the pores of membranes. The membranes employed contain cylindrical pores with monodisperse diameters, and corresponding cylindrical nanostructures are obtained. The aim of the present study was to prepare PbSe1-xTex nanorod arrays using electrodeposition. The process was investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The resulted PbSe1-xTex nanostructures were characterized using scanning electron microscopy (SEM) as well as energy dispersive X-ray analysis (EDAX).

750. Raman studies of carbon nanotubes and polymer nanotube composites

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

Published: 2004, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 415, 132, DOI: 10.1080/15421400490482844

In this paper, we present Raman studies of different carbon nanotubes systems like single-walled nanotubes (SWNTs), multi-walled nanotubes (MWNTs) or composites prepared with different conjugated or non-conjugated polymers. We have focused our studies on the interactions that can take place in these systems and in general modify the characteristics of individual tubes. By introducing Van der Waals interactions between tubes, we are able to interpret experimental Raman spectra. In the case of SWNTs, we have put in evidence the so-called "bundle'' effect that consists of an upshift of the frequency of the RBM of 10 to 16 cm(-1), depending on the size of the bundles. In MWNTs, one can calculate the low frequency Raman modes observed experimentally. We found that they originate from the radial breathing modes of isolated tubes with an upshift that depends on the number of shells constituting the tube and we were then able to calculate the intensity of such modes. Finally, in polymer/nanotubes composites, we demonstrate that interactions take place between the two constituents in two different cases: PPV/SWNTs and PANI/SWNTs. The role of the method of preparation of composites is particularly put in evidence.