National Institute Of Materials Physics - Romania

Surface enhanced Raman scattering (SERS) is being increasingly used for the study of the structural properties of conducting polymer thin films. It is generally accepted that the enhancement process has an electromagnetic origin, arising from the excitation of surface plasmons in the metal support on which the polymer film is deposited. However, the electromagnetic enhancement is also accompanied by a chemical process, for which available experimental data are scarce. The chemical process originates from the increase in the polarizability of the molecules at the metal surface under the action of the incident radiation, which leads to the formation of new chemical bonds with the atoms of the metal support. The present work was devoted to the study of the SERS spectra of poly(3-hexylthiophene) deposited on rough Ag and Au supports by evaporating the solvent from a solution of known concentration. The experiments revealed the existence of a chemical surface effect. The results obtained show that the SERS spectra depend on the oxidizing properties of the metal surface and on the nature of the solvent. This dependence is explained by the existence of some interfacial reactions that lead to the formation of interface compounds of the type MeX (Me = Ag or Au, X = Cl or O). The SERS measurements reported here reveal an increase in the intensities of the Raman lines, accompanied by a modification of the corresponding intensity ratios, when the degree of doping is increased. It was observed for the first time by SERS spectroscopy that the doping of 3-PHT with FeCl3 leads to the appearance of a state of disorder in the structure of the macromolecular chain, as a result of steric hindrance effects. (C) 1998 John Whey & Sons, Ltd.

A theoretical study of the radiation effects, from the point of view of the non-ionising energy loss and of the concentration of primary defects, in some A(3)B(5) semiconductors, has been performed. These effects have been analysed for charged pions, in the energy range 50 MeV-50 GeV. The investigated materials have been GaAs, InP, GaP, InAs and InSb, and the results have been compared with silicon, as a reference material, keeping into account the peculiarities of the pion-nucleus interaction, and the recoil energy redistribution in the lattice. The results of the calculations have put in evidence the higher radiation resistance of Si, GaP and GaAs in the whole energy range investigated, with respect to the other analysed materials: InP, InAs and InSb, that proved to be of interest from this point of view only in the intermediate energy region. (C) 1998 Elsevier Science B.V. All rights reserved.

The Mossbauer spectra of the melt-spun and subsequently intensively ball-milled Fe65Rh35, Fe50Rh50 and Fe26Rh74 alloys with fee structure are characterized at 4.2 K by broad hyperfine field distributions. The magnetic critical temperatures are 60(5) for the first one and, respectively, 85(5) K for the second and the latter composition, in good agreement with recent magnetic susceptibility studies. The distributions arise as result of the chemical disorder, local strains and atomic displacements, owing to the drastic ball-milling process. The values of the Fe magnetic moments at the peaks of the hyperfine field distributions reached 1.1, 1.6 and 1.9 mu(B) with decreasing iron concentration in semi-quantitative agreement with earlier experimental and recent theoretical evaluations of Fe moments in gamma-Fe and fee Fe50Rh50. Above 400 K an irreversible transformation to the bcc structure occurs for the two Fe-rich alloys with three types of Fe sites in the Fe richest alloy and two different Fe sites in the partially disordered Fe50Rh50. Our analysis allowed us to describe the nn atomic arrangements around these sites. (C) 1998 Elsevier Science S.A. All rights reserved.

The persistent extinction of fluorescence emission of Eu(3+) in glasses activated with europium and cerium is reported for the first time to the authors' knowledge. The glass samples containing EU(3+) and Ce(3+) were initially colorless and transparent and exhibited intense emission peaks at 592 ana 612 nm assigned to the (5)D0-(7)F(1,2) transitions of EU3+. Th, complete extinction of the Eu(3+)-ion emission was obtained as an effect of multipulse excimer-UV-laser (lambda = 248 nm, tau(FWHM) greater than or equal to 20 ns) irradiation of the glass samples. Fluorescence microscopy, Mossbauer spectrometry, and electron spin resonance were applied for investigation of the modifications induced by the laser treatment. As a decisive proof of the extinction of fluorescence me succeeded in recording three-dimensional fluorescent photographic patterns within the activated samples. (C) 1998 Optical Society of America.

The crystal structure of (Bi1.82Pb0.18) (ST1.84Pb0.16) CuO6 was determined from single-crystal intensity data measured on a four-circle diffractometer. The structure is orthorhombic, space group Cccm (No. 66); the lattice parameters: a=5.392(3) Angstrom, b=24.603(5) Angstrom, c=5.300(3) Angstrom, V=703.2 (5) Angstrom(3) (the space group can be denoted as Amaa by a cyclic transformation of the cell constants). The R value was 0.048 for 418 unique reflections with I>3.00 sigma(I). One of the O atoms bonded to Bi ions is disordered at two sites with a half occupancy and B-eq=3.6(6) Angstrom(2); although the thermal parameter is not so large as that reported by Torardi et al. [Phys. Rev. B 38, 225 (1988)], the disorder suggests that the crystal has a lower symmetry. The valence state of the Cu ion in the present compound was evaluated to be trivalent based on the Cu-O distances [Asbrink and Norrby, Acta Crystallogr. B 26, 8 (1970)] and the valence of Bi was estimated in comparison with that in other compounds. The O(3) sublattice dynamics in these types of superconductors is discussed based on the structural investigations.

The improvement of intragranular critical current density, j(c), and flux pinning obtained by thermal neutron irradiation (fluences between 10(16) - 10(18) neutrons/cm(2)) in YBa2Cu3O7,(-x) doped with 8 mol% lithium fluoride (LiF) (YBCO-LIF) is explained by the occurrence of various irradiation defects (point defect clusters, dislocation loops, stacking faults) observed by transmission electron microscopy (TEM), A twin fading process is also revealed accompanied by a lowering of the orthorhombicity index. At higher irradiation fluences (similar to 5 x 10(17) 10(18) neutrons/cm(2)), a new mechanism related to the dispersion of fine Cu2O precipitates in the superconducting YBCO-LiF matrix could explain the enhancement of up to eight times obtained for j(c) values. A good agreement between the dependence of the measured j(c) values on the applied magnetic field and the calculated curves, j(c) = f(B), derived from a theoretical model, previously proposed for the pinning contribution of fine particles dispersed in YBCO matrix, is obtained. The role of thermal neutron irradiation for the flux pinning improvement by introducing of strong pinning centers (irradiation defects and Cu2O microdispersoids) is also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.

Bi4Ti3O12 (BiT) ferroelectric thin films were used as photoconductive or photovoltaic ultraviolet detectors. The maximum of the spectral distribution lies around 370 or 390 nm, depending on whether the incident light is continuous or modulated. The maximum current responsivity obtained for the films annealed at 700 degrees C is about 0.01 A/W. The specific detectivity is 5.5 x 10(8) cm W-1 Hz(1/2) at a modulation frequency of 20 Hz. The frequency characteristics of the photovoltaic signal measured in modulated light suggests a 1/f frequency dependence of this signal. (C) 1998 American Institute of Physics.

We have measured I-V and C-V characteristics, the temperature dependence of dark currents, and thermally stimulated depolarisation currents on fresh and stored samples of photoluminescent porous silicon, By storage in ambient, the low rectifying I-V curves become strong rectifying, and C-V curves become MIS-like. I-T characteristics for fresh samples have only one activation energy, in the 0.49-0.55 eV range. After storage, a slightly modified value, of about 0.50-0.60 eV is observed at low temperatures only. At about 280 K, the activation energy suddenly changes to 1.20-1.80 eV. Also, both the number and the positions of maxima in thermally stimulated depolarisation currents change by storage. The annealing at about 50 degrees C induces small reversible changes in I-T characteristics and strong irreversible ones in thermally stimulated depolarisation currents, both for fresh and stored samples. A simplified quantum confinement model is proposed to explain the main aspects of the electrical behaviour of porous silicon films. The surface and/or interface contributions are observed especially in thermally stimulated depolarisation currents. The changes induced by storage are attributed to the oxidation process of the internal surface of porous silicon films. (C) 1998 Elsevier Science S.A. All rights reserved.

Thin carbon nitride films have been deposited on silicon(100) substrates downstream of a nitrogen plasma beam generated in a combined RF (13.56 MHz, 40-50 W) and DC (voltage +/-200 V, power 1-10 W) discharge between a graphite electrode and a graphite nozzle. By combining the RF and DC sources the capability of rf field to create extended plasmas is used together with the enhanced sputtering and biasing effect of the DC source. The plasma characteristics (electron temperature, presence of molecular species) have been studied by optical emission spectroscopy. Deposition rates of 2.5-3 nm/s are obtained at the centre of the plasma beam and at a few centimetres distance from the nozzle. The films have been investigated by X-ray photoelectron spectroscopy, spectroscopic ellipsometry, scanning and transmission electron microscopy, and microhardness measurements. The films have an overall N:C ratio of 0.28 but the distribution of different nitrogen bonds depends upon the DC bias conditions. In the spectral range 0.3-0.7 mu m the refractive index increases slightly from 1.5 to 2.2. The films are amorphous, with morphology consisting of a columnar structure. The columns have a diameter of about 20 nm. A hardness of 24 GPa has been measured. (C) 1998 Elsevier Science S.A. All rights reserved.

Surface-enhanced Raman scattering (SERS) has lately proved to be an effective technique which has led to its increasing application for studying the structural properties of conducting polymer thin films. It is generally accepted that the enhancement process has an electromagnetic origin arising from the excitation of surface plasmons (SPs) in the metal support on which the polymer film is deposited. However, the electromagnetic enhancement is also accompanied by a chemical process, on which only scarce experimental data are available at present. The chemical process originates in an increased polarizability of the molecules at the metal surface under the action of an incident radiation, as a result of which new chemical bonds are formed with the atoms of the metal support. This paper is devoted to the study of the SERS spectra under 1064 nm excitation for two forms of polyaniline, one of the emeraldine base type and the other of the emeraldine salt type. Our purpose has been to identify the chemical effects at the polymer/metal interface that are involved in specific modifications of the SERS spectra. The following results concerning SERS spectrum variation with metal support type (AE, Au and Cu) have been obtained. An oxidation of emeraldine base taking place at the polymer/metal interface has been revealed. In the case of emeraldine salt, an oxidized form of polyaniline, the SERS spectra remain unchanged regardless of the changes in the metal support, which proves the stability of this form. A new result has been obtained as SERS spectroscopy revealed an electron delocalization process taking place in the emeraldine salt molecule as a result of an electron transfer from metal to the polymer molecule occurring on the level of quinoid rings. The concept of a rough Raman-active surface has been clarified by using a parameter Wa, similar to the grating groove depth/grating periodicity ratio which occurs when a diffraction grating is used as optic coupler. (C) 1998 Elsevier Science S.A. All rights reserved.