National Institute Of Materials Physics - Romania

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.

CdxPb1-xS (0 less than or equal to x less than or equal to 1) thin films were prepared on glass substrates using the chemical bath deposition method. The structural, electrical, and photoelectrical properties of the films were investigated, and all were dependent on the x value. For x > 0.5 (high Cd content), the films show a poor crystallinity and weak photoconductivity. For x < 0.5 (high Pb content), the films exhibit a good crystallinity, with an average size of the crystallites of 100 nm, and good photoconductivity. The threshold voltage of photoconductivity is shifted to lower wavelengths with increasing Cd content. The experimental results point toward possible ternary deposition on some composition ranges (near pure PbS or CdS); whereas, for the rest, the deposited film is rather a mixture of phases [not CdxPb1-xS but (CdS)(x)(PbS)(1-x)]. (C) 2004 The Electrochemical Society.

The glassy composition As25Te35Si40 was investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), Raman spectroscopy, and infrared absorption (IR) measurements. X-ray diffraction patterns at room temperature and at temperatures around T-g were recorded and the radial distribution functions have been calculated. A continuous random network model for the ideal homogeneous glass structure has been proposed.

Using surface-enhanced Raman scattering (SERS) and Fourier transform infrared (FTIR) spectroscopy, we show that composites based on polyaniline (PANI) and single-walled carbon nanotubes (SWNTs) are different when they are prepared by two different methods: (1) by adding dispersed SWNTs to the polymer solutions and (2) by chemical polymerization of aniline in the presence of SWNTs. The difference originates from the irreversible chemical transformation of SWNTs in the polymerization medium. The synthesis medium used for the preparation of PANI transforms SWNTs into fragments of shorter length like closed-shell fullerenes. This explains the similarity of SERS and FTIR spectra of the composites PANI/SWNTs and PANI/C-60 chemically prepared. All compounds exhibit an absorption band at 1144 cm(-1) in their FTIR spectra, increasing with the carbon nanoparticules content, as a signature of a charge transfer between the constituents. Besides, the FTIR spectrum of the compounds obtained by adding SWNTs to the polymer solution display an intense absorption band with two components at 773 and 755 cm(-1), which are associated with the vibration of deformation of the benzene and the quinoid ring, respectively. This indicates a strong hindrance effect produced by the binding on the polymer chain of voluminous carbon particles as nanotubes and large fragments of nanotubes.

We present results obtained on carbon nanotube systems by means of Raman scattering and surface enhanced Raman scattering (SERS). Such systems consist of single-walled nanotubes (SWNTs), multi-walled nanotubes (MWNTs), or polymer/nanotubes composites. In particular, we have focused our studies on the interactions taking place in these compounds which lead to an upshift of the radial breathing mode (RBM) in the case of SWNTs, either when they are arranged in bundles or embedded in polymers. Similar calculations has allowed us to interpret the low frequency Raman modes observed in purified MWNTs. We show that they come from the RBM of individual tubes. From intensity calculations performed to simulate Raman spectra, we demonstrate the importance of two experimental parameters, namely the diameter of internal tubes and the number of shells which constitute the MWNTs, such parameters being extracted experimentally from high resolution transmission electron microscopy images. (C) 2003 Elsevier Science B.V. All rights reserved.

The 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. (C) 2003 Elsevier B.V. All rights reserved.

Electron spin resonance investigations on polyethyleneterephtalate irradiated with a YAG-Nd laser are reported. The dependence of electron spin resonance spectra on the incident laser beam intensity (power) is studied. It is noticed that the concentration of free radicals has a sigmoidal dependence on the crystal current, presenting saturation at high incident powers. The nature of free radicals induced by laser irradiation is discussed. (C) 2003 Elsevier B.V. All rights reserved.

Electron spin resonance investigations on collagen irradiated with protons are reported. The resonance spectra were assigned to a convolution of lines due to similar toCH(2)-CH2. residues and peroxy radicals. Raising the temperature of the sample (in air), the concentration of similar toCH(2)-CH2. residues decreases while, the concentration of peroxy radicals increases due to the denaturation induced enhancement of the oxidation process. An increase in the apparent intensity of the recorded spectrum has been observed around 100 degreesC and ascribed to the release of adsorbed water molecules. (C) 2003 Elsevier Science Ltd. All rights reserved.

Thin organic films based on poly-methyl-methacrylate (PMMA) polymer have been obtained by pulsed laser deposition (PLD) on silicon substrates. The films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy (RS). We observed that the film composition and structure depend on the laser fluence and on the temperature of the substrate during deposition. (C) 2002 Elsevier Science B.V. All rights reserved.

The simulation of interstitial void evolution in a model of crystalline silicon subjected to topological disordering revealed that the void size distribution broadens and shifts towards smaller mean diameter size, when the lattice disordering increases. The size distribution is not gaussian and the mean diameter of the voids is lower than the crystalline value. The calculation of the structure factor corresponding to void configuration evidences the absence of the short and long range order and point out to the presence of the medium range order.