National Institute Of Materials Physics - Romania

We report the formation of stable C-70 aggregate forms in o-dichlorobenzene/N-methyl-2-pyrrolidinone (DCB/NMP) and o-dichlorobenzene/acetonitrile (DCB/ACN) mixtures and their characterization by UV-VIS absorption spectroscopy and photoluminescence. As NMP acts on C-70 both as a solvent and reactant due to the unpaired electrons of the amidic nitrogen atom, molecular complexes (C-70/NMP) that aggregate slowly are formed by the binding of the NMP molecule to the fullerene cage. This interaction is seen in the UV-VIS absorption spectra of C-70 in DCB/NMP solvent mixtures by a gradual modification of the absorption spectra when increasing the NMP concentration. On the other hand, DCB and ACN combined form it typical solvent/non-solvent mixture favoring an intense aggregation of fullerenes. This yields to the C-70 clusters formations, having different size, similar to a precipitation process. Their characteristic is an abrupt variation in the absorbance appearing when the ACN is used in a greater extent. This different behavior of C-70 aggregation in DCB/NMP and DCB/ACN mixtures is also seen in their luminescence spectra recorded at different excitation wavelengths. While the PL spectra of C-70 solvated in DCB/NMP solution keeps almost the same modified shape irrespective the NMP concentration, the luminescence spectra Of C-70 in DCB/ACN mixture partially regains the powder C-70 PL signature as the ACN concentration is increased.

Hybrid material based on PbI2 intercalated with 2,2'-bypiridine (BIPY) was investigated by correlated studies of photoluminescence. infrared absorption and Raman spectroscopy. The PbI2(BIPY) intercalated compound has been synthesized by the chemical reaction of K1 and Pb(NO3)(2) in aqueous BIPY solution The optical studies reveal different properties for the hybrid material in comparison with those of pure PbI2 and BIPY. In the photoluminescence spectrum of the intercalated compound. recorded at liquid nitrogen temperature a new intense hand emission with maximum at 2 07 eV is observed. The excitation spectrum reveals it broad hand featured by several maxima at 2.77, 3.34 and 3 70 eV. New absorption bands at about 1589, 1489, 1435, 1312, 1009 cm(-1) are observed in the IR spectrum of PbI2(BIPY). The Raman spectrum of intercalated compound discloses new lines at 67, 83, 127 cm(-1) and the shift of two Raman lines from 994 cm(-1) to 1010 cm(-1) and from 1045 cm(-1) to 1060 cm(-1). A charge transfer process, leading to the formation of lead-BIPY coordination complexes. is considered its responsible for the strong host-guest interaction revealed by almost all experimental data

Functionalization of PbI2 with polyaniline-emeraldine base (PANI-EB) or polyaniline-emeraldine salt (PANI-ES) is demonstrated by Raman spectroscopy. Two functionalization methods were used: electrochemical polymerization of aniline onto the PbI2 modified Pt electrode and the mechanico-chemical reaction between PANI and PbI2. The functionalization induces changes in the Raman spectrum of PbI2 that consist in the appearance of new Raman lines with the peaks at 80, 144 and 170 cm(-1) The first line is the signature of the "stacking faults" that disrupt the stacking sequence of layers I-Pb-I atomic layers along the c crystalline axis by the intercalation of polymer molecules The bands at 144 and 170 cm(-1) are attributed to a vibrational mode associated to the Pb-NHR"(2) (R" = C6H4) bond.

TiO2:Fe thin films prepared by pulsed laser deposition exhibit in some case light dependent saturation magnetization, as determined from Kerr magnetometry measurements performed in dark or by illuminating the sample. This phenomenon is Studied in correlation with local atomic Structure investigated by extended X-ray absorption fine structure, composition and chemical state analyzed by X-ray photoelectron spectroscopy and by Xray absorption near-edge structure. It is found that light-controllable magnetism is a property of a mixture of Fe and oxidized Fe clusters embedded in the anatase TiO2 matrix.

Nanostructured intrinsic and doped titania materials have long been subject to both basic and applied research, in connection with environment and energy related applications. Apart from beneficial effects. such as widening the wavelength range for surface photocatalytic activation, doping TiO2 materials with 3d ferromagnetic cations is a main route to develop ail important class of diluted semiconductors, with potential applications in spintronics. In all the previous investigations the concentration of the ferromagnetic dopant was limited to a few percents. Here, we initiate ail extended investigation of Ti1-xFexO2 within a, wider iron composition range (x = 0 - 0.55) and discuss the magnetic and optical properties of these materials in thin films in relation with their elemental composition, nano-structure and local atomic ordering.

Iron- and europium-doped (<= 1 at.%)TiO2 nanoparticles powders have been synthesized by a hydrothermal route at 200 degrees C, starting with TiCl4, FeCl3 center dot 6H(2)O and EuCl3 center dot 6H(2)O. The structure, morphology and optical peculiarities were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS), Mossbauer spectroscopy and UV-vis measurements. The photocatalytic performance was analysed in the photodegradation reaction of phenol. Rietveld refinements of XRD patterns reveal that the as-prepared samples consist in iron- and europium-doped TiO2 in the tetragonal anatase structural shape, with particle size as low as 15 nm. By means of Mossbauer spectroscopy on both Fe-57 and Eu-151 isotopes as well as by EXAFS analyses, the presence of Fe3+ and/or Eu3+ ions in the nanosized powders has been evidenced. It was found that iron and europium ions can substitute for titanium in the anatase structure. From the UV-vis reflection spectra, by using the transformed Kubelka-Munk functions, the band gap energy (E-g) of the hydrothermal samples has been determined in comparison with that of Degussa P-25 photocatalyst. A decrease of E-g from 2.9eV found for Degussa photocatalyst to 2.8 eV for the titania doped with 1 at.% Fe has been evidenced, indicating a valuable absorption shift (similar to 20 nm) towards visible light region. However, the best photocatalytic activity in the photodegradation reaction of phenol was evidenced for the hydrothermal sample, TiO2: 1 at.% Fe, 0.5 at.% Eu, in both UV and visible light regions. The photocatalytic activities of iron-doped and iron-europium-codoped samples are high and practically the same only in visible light. The photocatalytic properties in Correlation with the structural and optical peculiarities of the hydrothermal samples are discussed. (C) 2008 Elsevier B.V. All rights reserved.

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.