National Institute Of Materials Physics - Romania

A method to produce composition modulated Bi2-xSbxTe3 alloys by electrodeposition has been developed. The electrolyte, which consists in a HCl 6M (pH = 0) aqueous solution, allows the codeposition of bismuth, antimony and tellurium to be accomplished at room temperature on glassy carbon, platinum or nickel electrode. The composition of the films. their crystal structures and morphology were studied as function of electrochemical parameters and bath composition. It is shown that the electrodeposits are monophasic and exhibit a polycrystalline state. The electrical resistivity was of the order of 7-10 mOmegacm, whereas the electrodeposition rate value was 30-50 mum/h for the films with good thermoelectric properties.

Structure of polyaniline (PAN)-C-60 nanocomposites was investigated by X-ray diffraction. Raman Spectroscopy was also used in order to find evidence for the C-60 - polymer interface interaction.

A new method for the preparation of BaFCl:Eu2+ has been developed. A coprecipitation chemical reaction between BaCl2 and NaF acidified aqueous solution has been used. Doping with Eu2+ was carried out by adding EuF3 during preparation time. A thermal treatment in vacuum similar to those used in the sintering process of supraconductive ceramics was used in order to accomplish the chemical reaction and to improve the homogeneity of europium ion distribution. Finally, a fine powder consisting of microcrystalline, 4-5 mu m grains was obtained. The product has been checked by X-ray diffractometry and characterised by optical methods. Photoluminescence (PL) measurements attest the europium impurification and oxygen contamination during preparation, which has a great importance for the photostimulability properties of the compound. Photostimulated luminescence (PSL) of the X-irradiated samples shows good performance as X-ray storage phosphor. (C) 2000 Elsevier Science B.V. All rights reserved.

Surface-enhanced Raman scattering (SERS) is a process with origins, electromagnetic and chemical. The electromagnetic enhancement consists of the excitation of surface plasmons in the metallic support of the thin film. With only the electromagnetic enhancement mechanism, the surface spectra should not differ from volume Raman spectra. However, between SERS and volume Raman spectra, there are differences resulting from the chemical reactions taking place at the polymer/metal interface, intermediated by solvent molecules, that finally depend on the types of polymers and metallic supports. Polyaniline (PAN) is an excellent material to emphasize the chemical component of SERS. This is due to its particular structure with a repeating unit that contains two entities at different weights-a reduced state and an oxidized state-that, in turn, react differently with a metallic substrate. SERS spectra depend on the oxidizing properties of the metal surface, which involves an intermediate compound of the types Ag2O and Au2O3 when N-methyl-2-pyrrolidinone is used as the solvent. This article presents new results concerning the surface chemical effects that produce variations of the PAN SERS spectra. The SERS spectra of the PAN emeraldine base (PAN-EB) layered on Au support are characterized by a semiquinoid structure that we believe is induced on the intermediate compound Au2O3. In the presence of H2SO4, the SERS spectra change gradually as the degree of acid protonation doping increases. The SERS spectra of the fully protonated PAN-EB are identical to those obtained on PAN emeraldine salt (PAN-ES) synthesized by cyclic voltammetry in an acid medium and are invariable with the type of metallic support. The SERS spectra show that the emeraldine salt can be partially or totally deprotonated with water or NH4OH. The deprotonation is complete for the Ag support and partial for the Au support. The SERS spectra of the fully protonated PAN-EB are characterized by a double band with maxima at about 1330 and 1370 cm(-1). Although the generation process of positive charge on the macromolecular chain of PAN-EB doped in the presence of (C4H9)(4)NBF4 is similar to that due to protonic acid doping, involving cation addition (C4H9+ or H+ ions, respectively) in SERS spectra, the complex band situated at about 1330-1370 cm(-1) no longer appears. The doping of PAN-EB with FeCl3 produces two polymer cm forms: a salt type characterized by a protonated structure similar to that found for PAN-ES and a base type similar to the leucoemeraldine form. (C) 2000 John Wiley & Sons, Inc.

The polarization behaviour of Armco iron in aqueous solutions of HCl with and without inhibitors has been studied by the potentiostatic method. At lower overvoltage values, the dissolution process is controlled by activation, while at higher overvoltage values, the dissolution process is controlled by diffusion. The inhibition of this metal corrosion in the aqueous solutions of HCl has been studied using four surfactants. Under the critical micelle concentration (CMC), the inhibition of these four surfactants is negligible. At a concentration higher than the CMC, the inhibiting action of tetradecyl trimethyl ammonium iodide (TTAI), tetradecyl trimethyl ammonium bromide (TTAB), hexadecyl trimethyl ammonium bromide (HTAB) and dodecyl trimethyl ammonium bromide (DTAB) increases rapidly. The process of inhibition was attributed to the formation of adsorbed film on the metal surface, that protects the metal against corrosive agents. Protection efficiency improved when the inhibitor concentration and the length of the alkyl chain were increased. The four quaternary ammonium salts tested were adsorbed on the Armco iron surface according to a Langmuir isotherm. (C) 2000 Elsevier Science S.A. All rights reserved.

Photostimulated luminescence (PSL) and diffuse reflectance measurements on X-ray irradiated BaFCl:Eu2+ microcrystalline powders were performed. We have investigated the nature of radiation induced defects involved in the photostimulation effect and its recombination mechanism. The experimental data sustain that X-ray irradiation gives rise to (F(Cl-) - O-F(-) - EU2+) photostimulable complexes and we have proposed several geometrical configurations of these complexes. (Received January 27, 2000; accepted February 16, 2000).

When a magnetic field is applied on a narrow gap semiconductor with high hole mobility, a substantial rise in the figure of merit can be observed due to Ethinghausen effect. The most important proof of this effect was achieved by using Bi-Sb alloys. The present paper is devoted to performance of the electrochemical synthesis of Bi-Sb alloys from acid aqueous solutions and to the investigations of physico-chemical. properties of these alloys. Thick films of Bi-Sb alloy were electrodeposited from both chloride and perchlorate solutions. The electro-deposited films obtained from perclorate solutions are thicker, more compact and more uniform. The Bi/Sb ratio in the solution bulk was found to be approximately the same to the Bi/Sb ratio in the electrodeposited alloy. The Hall effect was measured by a home-made apparatus. The range of the magnetic field (B) determined by a standard induction coil was 0.05-1 T. Carrier concentration, n is 10(18) cm(-3) and increases slow in the choosen magnetic range.

ITO thin films were deposited by spin coating method starting from indium and tin propoxide in the frame of a sol-gel process. Films with the thickness corresponding to 68 nm for one layer deposition were produced and characterized by X-ray diffraction, transmission electron microscopy and infrared reflection spectroscopy. Low conductivity polycrystalline films exhibiting cubical structure (bixbyite-type) and good adherence were obtained.

In this letter, we present a study of surface-enhanced Raman spectra of thin layers of phthalocyanine (Pc) complexes deposited on Ag and Au rough surfaces. The materials of this work are MgPc (monophthalocyanine) and the asymmetrical lutetium bisphthalocyanine (tBu)(4)PcLuPcCl8 (Pc'LuPc ") under three oxidation states. Enhancement of specific bands is observed depending on the surface and the oxidation state. SERS signatures of neutral, oxidised and reduced Pc'LuPc " are identified.

Surface-enhanced Raman scattering (SERS) has proved to be an effective technique for studying the structural properties of conducting polymer thin films. The enhancement process has a twofold origin, electromagnetic and chemical, The electromagnetic enhancement, which is the dominant mechanism in SERS generation, consists in the excitation of localized and delocalized surface plasmons (SPs) in the metallic support of the thin film, The Raman emission of the adsorbed molecules on the metal surface (the most efficient being Ag, Au and Cu) is due to the intense evanescent electromagnetic field located at the interface between the metal and the surrounding medium. The second enhancing mechanism for SERS is of chemical origin, involving the formation of new chemical bonds between the molecules and the metal surface, with the polarizability thus becoming considerably higher than that of the free molecules. This mechanism is as a rule accompanied by a metal-molecule or molecule-metal charge transfer, which partly accounts for the success of the SERS studies on conducting polymers. Unfortunately, these studies have revealed that the chemical effects at the polymer-metal interface varied substantially depending on the various types of polymer and metallic support. In this context, polyaniline containing two different entities (a reduced and an oxidized state) in its repeating units exhibits specific alterations in its SERS spectra depending on the type of metallic support. This paper presents new results concerning the structure of emeraldine-base and emeraldine-salt polyaniline thin films deposited on rough Ag and Au supports. The effect on the SERS spectra of the polymer-metal and polymer-ambient interface chemical reactions is also described. The presence of an interface compound depending on the oxidizing properties of the metallic support has a strong influence on the SERS spectra, no matter how the PAN films were deposited on the support (whether by solvent evaporation or by an electrochemical process, i.e. cyclic voltammetry). When an emeraldine base is doped with HSO4- ions, it turns into an emeraldine salt, which exhibits a disordered state in its macromolecular chain, leading to a modified profile of the similar to 1162 cm(-1) Raman line which is associated with the C-H bond of the quinoid ring. The Lorentzian profile is altered by the addition of a Gaussian profile component. For a rough Ag support, the SERS spectra show that the transformation of emeraldine base films into emeraldine salt films is a reversible process, Copyright (C) 1999 John Wiley & Sons, Ltd.