A. Conan

XPS study of conducting polypyrrole-poly(vinyl alcohol) composites

Abstract Conductive polypyrrole-poly(vinyl alcohol) composites have been chemically prepared by exposing poly(vinyl alcohol) (PVA) films containing FeCl 3 to vapours of pyrrole. The obtained films have been characterized by photoelectron spectroscopy analysis (XPS), optical absorption, scanning electron microscopy and electron microprobe analysis. XPS measurements show that Fe 3+ in PVA is reduced to Fe 2+ during the polymerization reaction, confirmed also by optical measurements. A detailed analysis of the films with a scanning electron microscope equipped with an electron microprobe shows that PVAPPyFeCl 3 compounds are homogeneous composites. It is also shown by XPS that covalently bonded chlorine is presumably present in the films and diffuses with time towards the surface of the sample. This chlorine release has been checked by electron microprobe analysis.

Transport properties of MoTe2−x and MoSe2−x compounds between 130 and 300°K

Abstract Thermoelectric power and Hall coefficient values were measured on compact polycrystalline samples from the MoTe 2− x and MoSe 2− x systems. It appears that qualitatively the temperature dependence of the Seebeck coefficient is not very different from that of crystalline semiconductors. The Seebeck coefficient is positive indicating hole conduction whilst Hall coefficient measurements on the same samples show negative values implying that conduction is by electrons. Singularities in the graphs plotted as a function of the composition x are found for both MoTe 2− x and MoSe 2− x systems. These results are discussed in terms of a quasi amorphous semi-conductor model.

Electrical properties of molybdenum ditelluride thin films

Abstract Molybdenum ditelluride thin films deposited by d.c. diode sputtering of a water-cooled target made of pressed MoTe 2 powder. The composition of the deposited films was investigated as a function of the substrate temperature T s . Layers with a marked tellurium deficiency were obtained. The best Mo/Te ratio is obtained at T s = 400 K. After sputtering deposition the films were put with a small amount of tellurium in a vacuum-sealed pyrex tube and heated at an annealing temperature T a = 850 K for 24 h. Then the films were put in tepid H 2 SO 4 for 12 h to eliminate the small amount of tellurium condensed at the surface of the film during the cooling. Scanning electron microscopy (SEM), electron microprobe analysis, X-ray photoelectron spectroscopy (XPS), and X-ray analysis investigation show that after this treatment the thin films exhibit the MoTe 2 structure whilst they are not exactly stoichiometric (tellurium deficiency). Electrical conductivity and thermoelectric power measurements are interpreted on the basis of a compensated p -type semiconductor model. The total electrical conductivity results mainly from: —the conduction in the extended states of the valence band; and —hopping conduction in an acceptor narrow band. The model adopted here is in good agreement with the band structure model which has been found for non-stoichiometric crystals.

Semiconducting properties and band structure of polycrystalline Bi4Mo20O62 compounds

Abstract Transport coefficient measurements (electrical conductivity, thermoelectric power and Hall effect) have been performed on compacted bars of Bi4Mo20O62 in polycrystalline form over the temperature range (130–500 K) Experimental results are discussed in comparison to those obtained recently on Sb4Mo20O62 They are interpreted on the basis of the same p-type semiconductor model with two inverted deep levels near the midgap Conduction mechanisms are governed by acoustical-phonon scattering of the carrriers The top of the valence band is assumed to be formed from the dxy orbitais of some of the Mo atoms leading to narrow bonding bands, while the donor and acceptor levels may be formed from the nonbonding dxy orbitais of some of the Mo atoms of the distorted octahedron framework EPR measurements are discussed on the basis of this model and in comparison to the EPR results for Sb4MO20O62 It is assumed that more than one narrow energy level (localized d states located below the valence band) contributes to the area of the line The XPS spectrum obtained on Bi4Mo20O62 is also discussed.

XPS and transport studies of PVA-PPy and Pth-PST composites

Summary form only given. Polypyrrole-(polyvinyl alcohol- FeCl/sub 3/ or CuCl/sub 2/) and polythiophene-polystyrene-(FeCI/sub 3/) conducting polymer composites have been synthezised above the percolation threshold. Compounds reach conductivity as high as 5 S/cm and 2 S/cm respectively, at RT. In the temperature range 80-300 K, d.c. electrical conductivity is weakly activated while thermoelectrical power variations are quasi linear. A major contribution of bi-polaronic hopping between the polymeric chains is shown to take account for all these experimental results. Conductivity measurements extended from 4K to 300K are compared to those recorded for PPy powders doped at different levels. All results will be discussed in view of the existing theoretical models. The stability of the samples stored in ambiant atmosphere has then been studied by microprobe analysis, X-ray photoelectron spectroscopy (XPS) and conductivity. In the case of PPy -PVA composite, it is observed that the atomic percentage of oxygen increases with time. The existence of a Cl motion towards the surface of the film followed by a release from the composite has been put in evidence. It is shown that the N 1s high binding energy component can be attributed to positvely charged nitrogen induced by doping.

Electrical properties of iodine-doped selenium films

Abstract Electrical resistivity and thermoelectric power (T.E.P.) measurements were performed on iodine-doped selenium thin films between 80 and 450 K. The films, whose thicknesses ranged from 500 to 15000nm, are found to be p -type. The electrical resistivity is governed by hopping conduction in the low temperature range (80–190 K) and by grain boundary scattering mechanisms at higher temperatures. A local resistivity maximum occurs at about 380 K. This effect increases, together with the room temperature conductivity, with the annealing temperature. These phenomena can be explained by a structural change of the grain boundaries induced by the iodine content.

Electrical properties and band structure of the transition metal dichalcogenide MoTe/sub 2-e/ doped with bromine

Abstract Electrical conductivity and thermoelectric power measurements have been performed on MoTe 2-x -single crystals doped with bromine in a wide temperature range (125–770 K). The semiconductor is compensated and the random potential due to the charged lacunar sites and bromine impurities induces a broadening of the bromine level into a narrow band. This band contributes to the conduction in the form of thermally activated hopping.