National Institute Of Materials Physics - Romania

The local configurations and related hyperfine interactions of the tetrahedrally coordinated Fe2+ in two natural spinels were investigated. A special fitting procedure of the Mossbauer spectra is proposed in order to cover the involved mechanisms over the whole temperature range. The behavior of the quadrupole splitting vs temperature was successfully explained in terms of the split of the electronic 3d ground state doublet under a non-cooperative Jahn-Teller effect also providing estimates of the splitting energy for each configuration.

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.

We solve numerically the time-dependent self-consistent random phase approximation Dyson equations for a system of electrons and holes of a direct gap semiconductor excited by an ultrashort laser pulse interacting with LO phonons in equilibrium. Special care is devoted to the proper treatment of the prehistory of the system by including the polaron effects. We show that for weak Frohlich coupling (alpha=0.1), the generalized Kadanoff-Baym relationship, which enables a one-time formulation, is excellently verified while for intermediate coupling(alpha=1) the deviations become important. [S0163-1829(99)03943-0].

Al doped TiO2 anatase films epitaxially grown by Pulsed Laser Deposition (PLD) on MgO single crystal substrates have been studied by cross-section transmission electron microscopy. The main structural features of such films are the columnar morphology of the anatase grains and the formation of a spinel buffer layer at the TiO2/MgO interface. The spinel layer is Al rich and displays a steep gradient in Mg composition. Correlation between the microstructure and the optical properties of the films is presented also. (C) 1999 Kluwer Academic Publishers.

Thin Cu-C(60) films were prepared by vacuum co-deposition and investigated by Raman spectroscopy. Evidence was found for alteration of the intramolecular vibrational modes of C(60) due to the presence of Cu and also for activation of silent modes. The effects were attributed to the interaction at the interface between the metal and fullerene and to the related formation of interstitial metallic ions. This hypothesis is supported by structural evidence derived from x-ray diffraction and EXAFS, Copyright (C) 1999 John Wiley & Sons, Ltd.

The effect of tin impurities on optical spectra of thermally deposited AsSe films doped with Sn (1 to 10 at%) was studied in a wide energy interval from 0.8 to 6.2 eV by combination of reflection, absorption, photoresponse and photocapacitance spectroscopies. Most changes have been detected at the fundamental absorption edge over which a correlation between the band tail width and optical gap is demonstrated for various tin concentrations. The tin induced absorption band associated with a localized energy level at about 1.6 eV in the gap was revealed. Distinct variations in the reflectivity spectra of the fundamental absorption region suggest a tin assistance in the formation of the valence band states of the material.

We present the application of the inverse scattering method to the design of semiconductor heterostructures having a preset dependence of the (conduction) electrons' reflectance on the energy. The electron dynamics are described by either the effective mass Schrodinger equation or by the (variable mass) BenDaniel and Duke equations. The problem of phase (re)construction for the complex transmission and reflection coefficients is solved by a combination of Pade approximation techniques, obtaining reference solutions with simple analytic properties. Reflectance-preserving transformations allow bound state and reflection resonance management. The inverse scattering problem for the Schrodinger equation is solved using an algebraic approach due to Sabatier. This solution can be mapped unitarily onto a family of BenDaniel and Duke type equations. The boundary value problem for the nonlinear equation which determines the mapping is discussed in some detail. The chemical concentration profile of heterostructures whose self consistent potential yields the desired reflectance is solved completely in the case of Schrodinger dynamics and approximately for BenDaniel and Duke dynamics. The Appendix contains a brief digest of results from the scattering and inverse scattering theory for the one-dimensional Schrodinger equation which is used in the article. (C) 1999 Elsevier Science Ltd. All rights reserved.

Nitrate powder with cation composition Bi:Pb:Sr:Ba:Ca:Cu = 1.8: 0.4:1.8:0.2:1.2:2.0 was obtained by spray-frozen, freeze-drying technique. Samples of the nitrate precursor powder were placed in a heated furnace (heating rate similar to 100 degrees C/min) and extracted in air when temperature of the powdered samples attained values of 439, 495, 550, 600, 640, 647, 717, 766, 814, and 850 degrees C. Samples have been investigated by X-ray diffraction analysis. The obtained data allow us to propose and discuss phase formation and decomposition processes and reactions that occur in non-isothermal conditions at different temperatures during thermal decomposition of the nitrate powder.

A new model is proposed for conduction in thick-film SnO2 gas sensors, which offers a good explanation for the ageing effect. in this model, electrical resistance is governed by necks and it results from three interdependent resistances of three different regions of the sample, connected in parallel: a surface resistance which is that of a gas-sensitive, predominantly n-type semiconductor, a resistance, belonging to a different region of the sample, which, due to its interaction with water vapour, behaves like a mixed n-type and p-type semiconductor, and the n-type bulk resistance of the neck, slightly affected by gases and/or water vapour. (C) 1999 Elsevier Science S.A. All rights reserved.