National Institute Of Materials Physics - Romania

The effect of fullerene on the radical polymerization of N-vinylpyrrolidone with lauroyl peroxide in toluene was investigated kinetically. C-60 was found to act both as inhibitor and as retarder because the polymerization rate and the molecular weight of resulting poly(vinylpyrrolidone) is decreasing with the increase of the fullerene concentration (0-6.94 x 10(-4) mol l(-1)). The water-soluble poly(vinylpyrrolidone)-modified fullerene C-60 compound was characterized by differential scanning calorimetric, Infrared and Raman spectroscopy, UV absorption and photoluminescence. Based on the results obtained by optical measurements, it is argued that by the covalent attachment of the polymeric radicals to fullerene cage the extended electronic conjugation system of the C-60 is broken leading to the appearance of a polyene structure.

The Stoner instability of the paramagnetic state, yielding to the occurence of ferromagnetism, is reviewed for electron density of states reflecting changes in the dimensionality of the system. The situations treated are one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) cases and also a special case where the density of states has a parabolic shape near the Fermi level, and is half-filled at equilibrium. We recover the basic results obtained in the original work of E.C. Stoner [Proc. Roy. Soc. London A 165, 372 (1938)]; also we demonstrate that in 1D and 2D case, whenever the Stoner criterion is satisfied, the system evolves spontaneously towards maximum polarization allowed by Hund's rules. For the 3D case, the situation is that: (i) when the Stoner criterion is satisfied, but the ratio between the Hubbard repulsion energy U and the Fermi energy epsilon(F) is between 4/3 and 3/2, the system evolves towards a ferromagnetic state with incomplete polarization (the polarization parameter is between 0 and 1); (ii) when U/epsilon(F), > 3/2, the system evolves towards maximum polarization. This situation was also recognized in the original paper of Stoner, but with no further analysis of the obtained polarizations nor comparison with experimental results. We apply the result of calculation in order to predict the Hubbard interaction energy. Finally, for the case of half-filled parabolic density of states a result quite similar with the case of the free electron 3D DOS is obtained, but here the Hubbard energy has to be compared with the band half-widh 6, whereas the absolute value of the Fermi energy has no relevance in this model. The Stoner criterion is U/delta > 4/3 and for U/delta < 2 the total energy is minimized for incomplete polarization. We compare the results obtained from the present theoretical study with the newest available experimental data.

Single-walled carbon nanotubes (SWNTs) may be used in technological applications as individual entities instead of bundled tubes. The Raman signature of these two forms is still a controversial subject. The profile, peak position and relative intensity of the Raman band associated with the tangential vibration modes (G band similar to 1600cm(-1)) and radial vibration modes (RBM similar to 100-200cm(-1)) are the main parameters which can reveal the dispersion state of nanotubes. Resonant Raman scattering and absorption spectroscopy on SWNTs furnish complementary information concerning the efficiency of the nanotube dispersion in individual entities. (C) 2008 Elsevier B.V. All rights reserved.

The experimental studies on III-V semiconductor compounds surface passivation phenomena are mainly dedicated to solve some technological problems as those regarding the ways to keep the chemical stability of native oxides on surfaces. Self-assembled monolayers (SAMs) provide a simple way to produce relatively ordered structures at a molecular scale, which seems to be capable to protect the clean surface against the evolution of oxidation process. In this respect, thin films of SAMs of aliphatic thiol (dodencanthiol-CH3(CH2)(11)SH) and aromatic thiol (4, 4' tiobisbenzenthiol-S (C6H4SH)(2) have been deposited on the surface of Gap (111) samples. The electrical properties measurements of some structures based on GaP compound was performed. There were recorded current-voltage (I-V) characteristics for complex structures AuGeNi/R-SH/GaP and AuGeNi/Ar-SH/GaP in darkness and also exposed to a Xe lamp. In dark and in "reverse bias" way, the I-V characteristics present the feature of a Zenner diode for GaP/Ar-SH and a gradual increase of current for GaP/R-SH. In dark and "in forward bias" way, the current increases as for a normal diode for both GaP/Ar-SH and GaP/R-SH structures. The complex structures (e.g.: In/AuGeNi/R-SH/GaP/R-SH/AuGeNi/In) are less sensitive to light. The SEM analysis performed on a GaP/R-SH surface shows a continuous packed up layer while GaP/Ar-SH looks like an inhomogeneous deposition of layers with different thickness regions. The diodes' ideality factors determined from I-V characteristics are unusually high (n >> 2) as a possible result of inhomogeneous Schottky contacts or due to ageing effects, in the field of degradation. (C) 2009 Elsevier Ltd. All rights reserved.

The Compact Muon Solenoid (CMS) detector is described. The detector operates at the Large Hadron Collider (LHC) at CERN. It was conceived to study proton-proton (and lead-lead) collisions at a centre-of-mass energy of 14 TeV (5.5 TeV nucleon-nucleon) and at luminosities up to 10(34)cm(-2)s(-1) (10(27)cm(-2)s(-1)). At the core of the CMS detector sits a high-magnetic-field and large-bore superconducting solenoid surrounding an all-silicon pixel and strip tracker, a lead-tungstate scintillating-crystals electromagnetic calorimeter, and a brass-scintillator sampling hadron calorimeter. The iron yoke of the flux-return is instrumented with four stations of muon detectors covering most of the 4 pi solid angle. Forward sampling calorimeters extend the pseudo-rapidity coverage to high values (vertical bar eta vertical bar <= 5) assuring very good hermeticity. The overall dimensions of the CMS detector are a length of 21.6 m, a diameter of 14.6 m and a total weight of 12500 t.

Carbon Nanotubes are exotic low-dimensional systems, and due to their remarkable properties, are promising components for technological applications. The electronic and vibrational signatures raising from their existence as individual entities or associated in bundles, is still a controversed subject. A careful analysis of the G (similar to 1600 cm(-1)) and RBM (100-200 cm(-1)) band features as: profile, peak position and intensity in the Raman spectra may be a valuable indication about their dispersion state. The UV-VIS-NIR absorption spectrum comes to support this assumption. Observing the absorption spectra associated to resonant excitation of semiconducting and metallic nanotubes, may reveal complementary informations concerning the individualization procedure efficiency.

Angle-resolved X-ray Photoelectron Spectroscopy (ARXPS) has been performed on (100) Si surfaces in different conditions as regards the oxides nature - native oxides, thermally formed oxides, and also after a cleaning procedure by Ar+ ion sputtering (e.g. E= 2 keV). The most sensitive angle to surface oxidation states (Si1+, Si2+, Si3+, Si4+) was the take-off-angle (TOA): 25 degrees. Native oxides and thermally oxide phases on Si surfaces consists in a mixture of Si2O, SiO, Si2O3 and SiO2. The XPS measurements have putted into evidence a higher concentration of suboxides in high oxidation state (Si3+) on naturally oxidized Si substrates. The thermally oxidized Silicon surfaces contain suboxides in a low oxidation state (Si1+). The Ar+ ion sputtering removed rapidly the superficial silicon oxides. The analysis and computation of SiO2 film thickness on crystalline Silicon was done in the frame of modified Bethe equation for Electron inelastic mean free path (IMFP) and taking into account the intensity of SiO2 peak and Si peak together with the TOA in the XPS measurement. Native oxide thickness for SiO2 was d(oxy) similar to 24 angstrom and for thermally oxidized sample was d(oxy) similar to 27 angstrom. Due to the slightly difference in the nature oxidation states it is suggested that the silicon dioxide appeared from two different kinetics in the oxidation processes.

TiAlZrCN superhard coatings, in which Ti and Al were additions to ZrCN base compound, were prepared by the cathodic arc method. The films were deposited on Si, plain carbon steel and high-speed steel substrates in a mixture of N-2 and CH4 gases. Elemental and phase composition, chemical bondings, texture, surface roughness, hardness, adhesion, tribological characteristics and erosive-corrosive resistance were investigates as a function of gas composition and total gas flow rate, using XPS, GDOES and XRD techniques, surface profilometry, microhardness and scratch adhesion measurements, tribological and erosive tests. It was shown that the properties and performance of the coatings were strongly dependent on the CH4/(CH4+N-2) flow rate and also on the total gas flow. The tribological and anti-erosive characteristics of the TiAlZrCN coatings were found to be superior to those of reference films (ZrN, ZrC, ZrCN, and TiAlCN). Maximum hardness values (42-45 GPa) were obtained for the films prepared at CH4/(CH4+N2) ratios ranging from 0.2 to 0.4. (c) 2008 Elsevier B.V. All rights reserved.

Composites based on zinc oxide (ZnO) nanoparticles and single-walled carbon nanotubes (SWNTs) were investigated by photoluminescence (PL) and Raman scattering. The room temperature PL spectrum of ZnO at the excitation wavelength of 335 nm displays a narrow excitonic band, peaking at 368 nm (3.36eV), and a wide one, disclosing two components with maxima at about 444 nm (2.79eV) and 560 nm (2.21eV) which are associated to the local oxygen vacancies and oxygen excess, respectively. Regardless the preparation method of the ZnO/SWNTs compound (electrochemical, mechanicochemical and hydrothermal synthesis), PL spectra reveal an increased quenching tendency with the concentration of nanotubes in the composite mass. As distinct feature in the PL spectra of ZnO/SWNTs composites are two bands, one at 450 nm (2.75 nm) and another one attributed to the formation of a donor-acceptor complex at 680 nm (1.82eV). The role of the dispersion of carbon nanotubes in the ZnO/SWNTs composite is observed by Raman scattering with the excitation wavelength of 676 nm. (C) 2007 Elsevier B.V. All rights reserved.

Angle-resolved X-ray photoelectron spectroscopy (ARXPS) analysis has been performed on GaAs (10 0) surfaces in different conditions as naturally oxidized, Ar+ ion sputtering (E = 1-5 keV) and chemical etching in H2SO4/H2O2/H2O(3: 1: 1). The most sensitive angle to the surface compositional changes was the take-off angle (TOA): 25 degrees. Native oxide phases on GaAs consist of a mixture of Ga2O3, As2O3 and As2O5. Ar+ ion sputtering procedure modifies the surface composition, in the altered layer where the concentration ratio C-Ga/C-As tends to 1.5-1.6. Wet chemical etching removes the oxide layer and the As-rich region from the surface. In the experiment combining chemical etching with Ar+ ion sputtering for cleaning purpose, the native oxides are removed from the surface and C-Ga/C-As tends to stoichiometry. The experiment on native oxide reconstruction after storage in high-vacuum conditions (P similar to 10(-8) Torr) provides evidence of the high reactivity of GaAs (10 0) surfaces. We have observed the presence of an As oxide (BE = 43 eV) within a concentration range of 2-3%. (C) 2007 Elsevier B.V. All rights reserved.