R. Mertens

Optical properties of amorphous As-Se and Ge-As-Se thin films

Abstract Amorphous As–Se and Ge–As–Se thin films were prepared by thermal evaporation. From parameters of the Wemple–DiDomenico model, the values of the third-order non-linear susceptibility were estimated using the generalized Millers rule. Measurements of photodarkening at 78 K showed that the optical gap and the slope of the optical absorption edge are intercorrelated. The kinetics of the photoinduced shift of the gap are described using a stretched exponential law. A possible role of the concentration of Se atoms on the rate of photodarkening on one hand and of the network rigidity (measured by the mean coordination number) on the other are briefly discussed.

Characterization and d.c. conductivity of poly(p-phenylene vinylene) films doped with FeCl3

Films of poly(p-phenylene vinylene) (PPV) were synthesized via pyrolysis of a water-soluble sulphonium polyelectrolyte, derived from p-xylylene bis(tetrahydrothiophenium chloride). The thermal elimination reaction of the sulfide group was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The structure of the completely converted, unoriented PPV films was characterized using IR spectroscopy and X-ray diffraction. Doping of PPV films with FeCl3 in nitromethane was studied. Films of different dopant concentrations y = (FeClx/C8H6) were obtained by varying the molarity of the doping solutions and by changing the immersion time. The electrical conductivity of FeCl3-doped PPV films was measured as a function of concentration and doping time. The d.c. conductivity reached a maximum value of σdc = 35 ohm−1 cm−1.

Application of Poly(thienylene vinylene) as a chemiresistor for organic vapours

Results are reported about the application of iodine doped Poly(thienylene vinylene) (PTV) as a conductimetric sensor. A description is given of the layout of the sensor, formed by spin coating the soluble precursor polymer on an alumina substrate with gold contacts. The set-up for generating continuous vapour flows over the sensor and the monitoring of the electrical resistance are briefly discussed. Responses (%Ra) of the sensor to a series of nine saturated vapours are reported. Recovery (%Rb) from the response is in general not complete and characteristic for the vapour. The partial recovery is not a poisoning effect and does not affect the reproducibility; on the contrary, graphical combination of %Ra and %Rb shows additional information.

Low-temperature photodarkening of the AsxSe100-x system prepared by PECVD

Abstract Films of the amorphous As x Se 100− x system were prepared by plasma-enhanced chemical vapor deposition (PECVD) using different gas ratios of the hydrides, AsH 3 and H 2 Se. In the Se-rich region the optical bandgap decreased with increasing As content, in agreement with measurements made on films prepared by evaporation. The temperature dependence of the optical gap (77–300 K) could be described by Fans one-phonon approximation, indicating that electron–phonon interaction (thermal disorder) plays the most important role. Measurements of photodarkening at 77 K showed that the decrease of the optical gap and the slope of the Tauc plot are intercorrelated. The kinetics of the photoinduced shift could be best described by a stretched-exponential law.

Electrical properties of Cu(II)-N, N'-dimethyldithiooxamide polymers

Abstract Cu(II)-dithiooxamide polymers were prepared by a direct deprotonation of the ligand with a Cu(II) salt. The polymers had a chain length of about 13 units. The undoped polymers showed low electrical conductivity with an Arrhenius-type temperature dependence. Doping of Cu(II)- N , N ′-dimethyldithiooxamide was performed by iodine in the vapour phase and by FeCl 3 in solution. In both cases, the oxidative doping resulted in highly increased conductivities. A thermopower measurement indicated p-type conduction. The infrared spectra of the undoped material and the one doped with FeCl 3 were completely identical, indicating that charge transfer from the ligand to the dopant is small. The conductivity curves of the doped polymer did not show an Arrhenius-type behaviour. In order to find an explanation for the conduction mechanism different models were tested. The best fit of the experimental conductivity data of the FeCl 3 -doped polymers was obtained with the theoretical prediction of a variable-range hopping model.

Electrical conductivity of poly(paraphenylene vinylene) films doped with FeCl3

Abstract Highly oriented films of poly(p-phenylene vinylene), PPV, were prepared by uniaxial stressing during the thermal conversion of a sulphonium polyelectrolyte precursor polymer. The molecular orientation is studied by infrared dichroism as a function of the stretch ratio. Electrical conductivity data of FeCl 3 -doped PPV films obtained between 80 and 300 K provides evidence that conduction occurs by variable-range hopping in unstretched films and by fluctuation induced tunneling in highly stretched films. The room temperature conductivity parallel σ ∥ and perpendicular σ ⊥ to the stretch direction shows an anisotropy σ ∥ / σ ⊥ equal to 13.

D.C. conductivity and thermopower of FeCl3-doped poly(p-phenylene)

Abstract Poly( p -phenylene) was prepared by the oxidative cationic polymerization of benzene according to the Kovacic method (P. Kovacic and J. Oziomek, J. Org. Chem., 29 (1964) 100). The d.c. conductivity and thermopower were measured as a function of temperature after FeCl 3 -doping. For low doping levels the results can be best described by inter-polaron hopping using Kivelsons theory (S. Kivelson, Mol. Cryst. Liq. Cryst., 77 (1981) 65). For high doping, variable-range hopping in three dimensions seems to be most appropriate to explain the data.

Reversible photodarkening in amorphous AsxS100−x films prepared by thermal evaporation and plasma-enhanced chemical vapour deposition

Amorphous AsxS100 � x films were prepared by thermal evaporation and plasma-enhanced chemical vapour deposition (PECVD). Reversible photodarkening upon illumination with white light at 300 and 77 K was studied for both types of films. The magnitude of photodarkening increased with increasing As content, being somewhat larger for the evaporated films than for the PECVD ones. In agreement with our previous observations in the amorphous As–Se and Ge–Se systems, the shift of the optical absorption edge was not parallel but was always accompanied by a decrease in the slope of the Tauc curve, indicating an increase in disorder. The kinetics of the photoinduced optical shift could be best described by a stretched exponential law. This can be explained within Tanaka’s ‘‘single–double well model’’ with the addition that the process is most probably a kind of self-limiting process, whereby excitation to a quasi-stable state (photodarkening) and annealing (thermal and photoinduced) are simultaneously operative. D 2002 Elsevier Science B.V. All rights reserved.

Structural Characterization of Amorphous GexSe100-x by Infrared and Raman Spectroscopy

Amorphous chalcogenides are usually prepared in two ways: as bulk glasses by quenching from the melt or as thin films by thermal evaporation. A difference in structure between thin films prepared by thermal evaporation and bulk glasses was demonstrated experimentally. Tanaka and coworkers [1] found for the Ge-S system relatively large differences between bulk materials and evaporated films. They concluded that the films contain more homopolar and dangling bonds.

The effect of partial elimination of methoxy groups in a precursor polymer on the optical absorption and electrical properties of poly(thienylene vinylene)

Abstract Films of poly(thienylene vinylene) (PTV) were prepared by thermal elimination of methoxy groups in a precursor polymer in the presence of gaseous hydrochloric acid as a catalyst. The growth of the conjugation length of the polymer during elimination was studied by means of on-line UV-Vis spectroscopy under various conditions of reaction temperature, reaction time and HCl concentration. The optical absorption gap of the polymerś π-π* transition, determined from transmission and reflectivity data, was found to decrease with increasing conjugation length. Measurements of dc electrical conductivity on iodine-doped samples showed a direct connection between chain structure and charge carrier transport. From the temperature dependence of the electrical conductivity (90–300 K) we concluded that the electrical transport in the iodine-doped polymers can be described by a variable-range hopping model proposed by Schafer-Siebert and Roth.