National Institute Of Materials Physics - Romania

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.

Thin films of phosphate glasses were prepared by pulsed laser deposition in oxygen atmosphere on ITO-coated glass and silicon substrates respectively, using a Nd:YAG laser (266 nm). The targets were Li(2)O-Al(2)O(3)-P(2)O(5) bulk glasses doped with Pr and Nd. Infrared transmission-, energy dispersive X-ray- and X-ray photoemission spectroscopy investigations carried out on targets as well as on the deposited films proved a satisfactory compositions transfer on both substrates. Scanning electron microscopy and atomic force microscopy show continuous films with glassy spheres. Processes where substrates were positioned perpendicularly to targets produced discontinuous drops free films.

Thin films and crystalline powder of PbI2 intercalated with ammonia or pyridine have been studied by optical absorption, Raman scattering, photoluminescence, FTIR spectroscopy and thermo-gravimetric analysis. Ammonia intercalated PbI2 shows an increased optical band gap, of about 0.6 eV, with an intense broad emission band peaking at about 2.3 eV as signature in the photoluminescence spectrum. The intercalation of PbI2 with ammonia is noticed in the Raman spectrum by the appearance of new lines situated in low frequency range. The D-3d coordination geometry of Pb2+ in the PbI2 crystal is reduced by compression to orthorhombic one, the distribution of the electronic states in valence band is also changed. Thus, the top of the valence band undergo a deformation inducing a weakness of the interaction between the lead and iodine ions within a layer. Also the PbI2 intercalated with pyridine is featured by optical and vibration properties different that of pure crystalline powder. Finally, the intercalation of PbI2 with different molecules changes the basic semiconducting properties of the crystal. Thermal analyses and infrared absorption spectra have been used to study the desorption of guests species from PbI2 in order to evaluate the stability range and morphological changes with temperature.

This paper presents a study of two monomeric structures containing functional groups with different electronegativities [NH/NH-NH and NO2] and two different polymeric structures prepared by the copolymerisation of maleic anhydride and methylmethacrylate/maleic anhydride and vinyl benzyl chloride, and subsequent functionalization with 2,4 dinitroaniline. Thin films have been prepared by Matrix Assisted Pulsed Laser Evaporation (MAPLE) and UV-VIS, FTIR, Raman and Photoluminescence Spectroscopy have been used to comparatively investigate the properties of the synthesised compounds. We have evidenced the second harmonic emission (SHG), process sustained by the asymmetry of the chemical structure, which is correlated with the crystallization in non-centrosymmeric structures.