National Institute Of Materials Physics - Romania

Multiwall carbon nanotubes, as indicated by electron microscopy and spectroscopy, were grown over a novel Co(5 wt.%)/ZnO catalyst by using a cold wall radio-frequency catalytic chemical vapor deposition method. The nanotubes were found to have a rather narrow diameter distribution, which can be explained by the formation over the ZnO support particles of uniform Co clusters, as shown by high resolution transmission electron microscopy studies. The growth of carbon nanotubes over such a support could represent a fast and reliable method to develop in one step 3D ZnO-CNTs heterogeneous composites of any size or shape, for use in advanced applications. (c) 2009 Elsevier B.V. All rights reserved.

Functionalization of PbI2 with conjugated polymers (polyaniline-emeraldine base (PANI-EB) or polyaniline-emeraldine salt (PANI-ES)) is demonstrated by Raman and luminescence studies. PbI2/PANI hybrid material was prepared by electrochemical polymerization of aniline onto the PbI2 modified Pt electrode and mechanico-chemical reaction between the two constituents. PANI interacting with the PbI2 gives rise to new Raman bands at 80,144 and 170 cm(-1). First line reveals the formation of "stacking faults" that disrupt the I-Pb-I layers stacking along the c axis by the insertion of polymer molecules. The bands at 144 and 170 cm(-1) are attributed to the vibrational mode associated with Pb-NHR ''(2) (R '' = C6H4) bond. The functionalization of PbI2 with PANI-EB brings forth the PANI-ES form. Depending on the semiconducting (PANI-EB) or conducting (PANI-ES) properties of the polymer in the PbI2/PANI intercalated material, a partial or total collection of the charges generated under band to band irradiation is revealed by photoluminescence studies. (C) 2009 Elsevier Inc. All rights reserved.

Electrochemical polymerization of 2,2'-bithiophene (BTh) on single-walled carbon nanotube (SWCNT) films has been studied by Raman scattering and infrared absorption spectroscopy. Covalent functionalization of SWCNTs with poly(bithiophene) (PBTh) in its un-doped and doped states is demonstrated. The occurrence of a charge transfer process at the interface of PBTh and SWCNTs, is shown by: (i) an up-shift of the Raman lines associated with the radial breathing modes of SWCNTs that reveals both a doping process and an additional twisting together as a rope with the conducting polymer as binding agent; (ii) a new Raman band in the range 1430-1450 cm(-1) indicating the functionalization of SWCNTs with PBTh in doped and un-doped states; (iii) strong absorption bands situated in the interval 600-800 cm(-1) resulting from steric hindrance produced by the nanotube binding to the polymeric chain. Treatment of the PBTh/SWCNT composite with aqueous NH4OH Solution forms un-doped PBTh covalently functionalized SWCNTs. At the resonant excitation of the metallic tubes, an additionally enhanced Raman process is generated by plasmon excitation in the metallic nanotubes. It is evidenced by a particular behavior in the Stokes and anti-Stokes branch of the PBTh Raman line at 1450 cm(-1). (C) 2009 Elsevier Ltd. All rights reserved.

This paper presents investigations on the electrical transport phenomena in sandwich type inorganic/single (double) organic layer/inorganic such as silicon; ITO/TPyP, Alq3, alpha-NPD; ZnPc; perylene" PTCDA/silicon and inorganic (metal)/organic/metal such as silicon (A/)/ZnPc; alpha-NPD; TPyP/copper, Al, structures, prepared by vacuum evaporation. For most of these heterostructures the IN characteristics in the static regime have a near ohmic behaviour only at low voltages (<1 V) that changes in a power dependence with a coefficient n=2 or n=3 at higher voltages because of the space charge or trapped charge phenomena. For the same applied voltage, the n type Si electrode injects more charge carriers in TpyP than in Alq3. For higher voltages (>10V) an increased number of charge carriers are injected from n type Si in Alq3 compared to TpyP. The injection properties of the interface ITO, Cu, Al electrode/organic layer have also been investigated. The best injection has been obtained through ITO/ZnPc interface. For a grid configuration of the Al electrodes the transport phenomena are determined by the resistivity of the structure.

ZnO nano-needle arrays and films have been prepared by anodic etching of ZnO foil at constant current in potassium zincate solution and sodium hydroxide alcoholic solution, respectively. Photoluminescence (PL) spectra recorded on ZnO nano-needles have shown a broad band at 585 nm due to the native oxygen defects incorporated during the synthesis processes; in a ZnO film-type sample this band is replaced by another one at 432 nm. Thermoluminescence (TL) glow curves recorded on ZnO nano-needle arrays have shown a broad peak at about 360 degrees C similar to that observed in ZnO polycrystalline powder. A very strong TL peak at about 325 degrees C superimposed on a broad one has been observed in a ZnO film-type sample. TL recorded on ZnO nano-needles has tentatively been attributed to the recombination of charge carriers released from the surface states associated with oxygen defects, mainly interstitial oxygen ion centres; the TL mechanism is supported by a recovery effect of the PL and TL after storage in air. (C) 2008 Elsevier B.V. All rights reserved.

ZnO thin films were deposited by pulsed laser deposition (PLD) technique on optical glass substrate. Structural analysis by X-ray diffraction revealed a wurtzite phase ZnO, highly textured, with (002) planes perpendicular to the growth direction. They were optically characterized by absorption and transmission spectrometry. The wavelength dependence of the refractive index was extracted from transmission data by using a numerical fitting procedure.

This paper presents a study of some functionalised copolymers thin films prepared on silicon and quartz substrates by matrix assisted pulsed laser evaporation ( MAPLE) method. Two polymeric structures have been synthesised by the copolymerisation of maleic anhydride and methyl methacrylate, respectively, maleic anhydride and vinyl benzyl chloride and their subsequent functionalisation with 2,4-dinitroaniline. UV-Vis, FTIR, Raman and photoluminescence spectroscopy have been used to investigate the influence on the properties of the films of different substrate temperature ( 150 degrees C and 250 degrees C), background N-2 pressure (5-30 Pa) and polymer concentration in target (2% and 3%). We have evidenced that this deposition process does not damage the chemical structure of the polymers. SEM investigations revealed the droplets type morphology of the polymeric films with thickness between 41 nm and 105 nm calculated from ellipsometric measurements. (C) 2008 Elsevier B. V. All rights reserved.

Thin films of complex oxides have been obtained by pulsed-laser deposition (PLD) from glass targets belonging to the system Li2O-Al2O3-P2O5-(RE)(2)O-3, with RE = Nd, Pr, Er. The films were deposited on quartz, silicon and ITO/glass substrates using a F-2 laser (lambda = 157 nm, iota approximate to 20 ns) for ablation in vacuum. The structural, morphological and optical properties of the oxide films were investigated through IR and UV-VIS spectroscopy, Atomic Force Microscopy (AFM), Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy (SEM-EDX) and Spectroscopic Ellipsometry. The laser wavelength was found to be the key parameter to obtain thin films with very smooth surface. In this way new possibilities are opened to grow multilayer structures for photonic applications. (C) 2008 Elsevier B. V. All rights reserved.

Mossbauer spectroscopy studies of Fe-Co/MgO catalysts proved that a high dispersion degree of Fe may be achieved for catalysts containing Fe/Co in a ratio close to 1. During materials processing. the presence of hydrogen at high temperatures, even for short time, induced an increased particle size of the catalytic Fe. The evaluation of various iron species from their areas in the Mossbauer spectra revealed the dependence of the nanotube amount on (FeCo alloy + Fe2+) content in the catalyst, in good agreement with literature mechanisms. The Raman spectra show that carbon nanotubes synthesized from ethanol in argon flow on 4.5%Fe:4.5%Co/MgO are richer in low diameter (0.7-1.0nm) tubes than those obtained at low ethanol pressure, in agreement with the effects of hydrogen reported in literature. The close similarity with the Raman spectra of Aldrich double-walled carbon nanotubes and from literature, recommends the syntheses in argon flow as better conditions for double-walled carbon nanotube growth.

On subjecting a bulk 2H-PbI2 crystal to vacuum annealing at 500 K followed by a sudden cooling at liquid nitrogen temperature stacking faults are generated that separate distinct layers of nanometric thickness in which different numbers of I-Pb-I atomic layers are bundled together. Such structures, containing two, three, four, five etc I-Pb-I atomic layers, behave as quantum wells of different widths. The signature of such a transformation is given by a shift towards higher energies of the fundamental absorption edge, which is experimentally revealed by specific anisotropies in the photoluminescence and Raman spectra. The quantum confining effect is made visible by specific variations of a wide extra-excitonic band (G) at 2.06 eV that originates in the radiative recombination of carriers (electrons and holes), trapped on the surface defects. The excitation spectrum of the G band, with p polarized exciting light, reveals a fine structure comprised of narrow bands at 2.75, 2.64, 2.59 and 2.56 eV, which are associated with the PbI2 quantum wells formed from two, three, four and five I-Pb-I atomic layers of 0.7 nm thickness. Regardless of the polarization state of the laser exciting light of 514.5 nm (2.41 eV), which is close to the band gap energy of PbI2 (2.52 eV), the Raman scattering on bulk as-grown PbI2 crystals has the character of a resonant process. For p polarized exciting light, the Raman scattering process on vacuum annealed PbI2 becomes non-resonant. This originates from the quantum well structures generated inside the crystal, whose band gap energies are higher than the energy of the exciting light.