ternary sulfides. Neutron diffraction data from powders was collected on the 3T2 diffractometer at the Laboratoire LØon Brillouin, Saclay, France, to follow the crystal and magnetic structures of the title compound as a function of temperature. Rietveld refinements made use of the FullProf Rietveld program [18].Variable-temperature Mössbauer spectra with good statistics were measured from 300 K down to 4.2 K using a source of 57 Co in rhodium. The data were collected with the sample in a helium cryostat. The program MOSFUN [19] was used to analyze the data.First principles calculations of the electronic structures were performed using the LMTO-ASA method. A detailed description of the LMTO-ASA method, including its applications, can be found elsewhere [20]. The scalar± relativistic Kohn Sham equations were solved, taking all relativistic effects into account except for spin±orbit coupling. The basis set included the s, p, and d wavefunctions for Fe, Cu, Cr, and S. The S d wavefunctions were treated using the down-folding procedure. The k integrated functions were evaluated by the tetrahedron method on a grid of 735 k points in the irreducible part of the Brillouin zone (1/48 of the BZ). We used lattice constants from the neutron diffraction refinement and constructed a supercell with an ordered arrangement of Fe and Cu on the tetrahedral sites in the space group F43m. For comparison, we have also performed calculations on the end members in the series, that is FeCr 2 S 4 and CuCr 2 S 4 .Because of magnetic ordering, the calculations were performed (using the von Barth±Hedin [21] form of the exchange correlation within the local spin density approximation) with the assumption of a magnetic ground state (spin-polarized calculation) as these, in agreement with experiment, yielded more stable total energies.

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A green approach for the preparation of carbon black (CB) and electrochemically reduced graphene oxide composite (ERGO) is described based on screen printed carbon electrodes (SPCEs) fabricated on poly(ethylene terephthalate) (PET) as electrochemical sensors. This approach leads to a heterogeneous hydrophilic surface with high concentration of defect sites according to scanning electron microscopy, contact angle and Raman spectroscopy measurements. The SPCE/CB-ERGO sensor was tested with dopamine (DA), epinephrine (EP) and paracetamol (PCM), exhibiting an enhanced electrocatalytic performance compared to the bare SPCE. It displayed a wider linear range, lower limit of detection and a remarkably higher analytical sensitivity, viz. 1.5, 0.13 and 0.028 A L mol for DA, EP and PCM, respectively, being also capable of simultaneous determination of the three analytes. Such high performance is demonstration that SPCE/CB-ERGO may serve as generic platform for cost-effective flexible electrochemical sensors.

Recebido em 1/8/01; aceito em 8/11/01 This paper describes the use of ellipsometry as a precise and accurate technique for characterizing substrates and overlayers. A brief historical development of ellipsometry and the basic principles necessary to understand how an ellipsometer works are presented. There are many examples of studies performed in addressing materials science issues, and several are presented here: measurements of thickness, optical properties, and modeling of surface roughness. These selected results obtained in our laboratory for substrates, Si/SiO 2 interfaces, and polymers provide evidence that ellipsometry can play a critical role in characterizing different types of materials.Keywords: ellipsometry; polarization; films. BRIEF HISTORICAL OVERVIEWEllipsometry (reflection ellipsometry) measures the changes in the state of polarization of light upon reflection from a surface. The fact that changes are measured rather than the absolute intensity of light renders ellipsometry sensitive to submonolayer surface coverage. As a non-invasive and non-destructive tool, ellipsometry requires only a low-power light source and, consequently, it does not affect most processes, which renders ellipsometry a convenient tool for in situ studies 1,2 . P. Drude 3 was the first to build an ellipsometer even before Rothen has coined the word 'ellipsometry ' in 1945 4 . The equipment built by Drude is surprisingly very similar to many types of instruments in use today. Except for sparse papers, this technique received little attention for several decades after Drude, but in the 1970's and 1980's, ellipsometry became widely utilized, and relevant technical papers [5][6][7] and a classical book 8 were produced during this period. In 1991, Azzam compiled a considerable number of papers on ellipsometry 9 . As a search of the literature indicates, the number of papers on experimental improvement of ellipsometers has been decreasing in the last years, because many of the problems that existed before the advance of computers have been solved, including measurement speeds and spectral range. Most of the equipment used today is automated with wide band light sources, scanning monochromators, optical multichannel analyzers, and interfaces with acquisition and analysis software. Early studies on ellipsometry were performed using single wavelength equipment, usually a wide band visible light source with a filter, and later lasers. However, spectroscopic ellipsometers are commonly used nowadays in different research fields. BASIC CONCEPTSA description of ellipsometry as an optical technique would not be complete without mentioning J.C. Maxwell (1831-1879). Maxwell's theory predicts that light is a wave represented by two mutually perpendicular vectors: E, the amplitude of the electric field strength, and B, the amplitude of the magnetic field strength, and both E and B are also perpendicular to the direction of propagation, z. The electromagnetic wave is described by its amplitude and frequency in complex form:where E ...

Langmuir-Blodgett (LB) films of a poly(p-phenylenevinylene) (PPV) precursor were prepared using an amphiphilic precursor, poly(xylylidenetetrahydrothiophenium chloride) (PTHT), where the counterion chloride was replaced by a long chain dodecylbenzenesulfonate (DBS) ion. Stable PTHT-DBS monolayers were transferred onto quartz substrates in the form of LB films. Using DBS allowed PPV films to be converted under atmospheric pressure at temperatures as low as 80 °C, with conjugation length and optical properties better than for standard films converted at temperatures above 200 °C under controlled atmospheres. These LB-PPV films are highly anisotropic as demonstrated by linear dichroism experiments using linearly polarized optical absorption and emission. The dichroic ratio between the intensity of the emitted light parallel and perpendicular to the dipping direction was ca. 17, which is much higher than the dichroic ratio of 3.6 found in the polarized absorption experiment. This difference indicates an efficient excitation transfer between the low conjugated segments (disordered) and the highly conjugated (ordered) ones after excitation.

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