J. Kastner

Raman spectra and ground state of the new low bandgap polymer poly(thienopyrazine)

Abstract Raman modes of the new processable polymer poly(2,3-alkyl-thieno[3,4-b]-pyrazine) with a bandgap of about 0.9 eV were measured and calculated. Raman spectroscopy revealed two strong lines at about 1520 cm −1 and 1560 cm −1 in the CC stretching mode region. By changing the exciting laser line these modes exhibit a dispersion of 16 and 25 cm −1 /eV, respectively. Calculations on the ground state revealed that the quinonoid structure is by about 13 kcal/mol per repeat unit more stable than the aromatic form. By using the scaled quantum mechanical oligomer force field method we calculated the Raman spectrum of the quinonoid form and identified the mode at 1520 cm −1 as the inter-ring A g vibration.

Electrochemical carbyne from perfluorinated hydrocarbons: Synthesis and stability studied by Raman scattering

Abstract The synthesis of electrochemical carbyne from perfluorinated hydrocarbons was optimized. The relative yield of carbyne was monitored by Raman spectroscopy using the CC stretching mode. The position and relative intensity of this band depend on the type of fluorocarbon and reducing agent used. Optimum conditions were attained for electrochemical reduction of poly(tetrafluoroethylene) with K- or Na-amalgams. Poly(tetrafluoroethylene) oriented by mechanical stretching gives partly oriented polyyne chains with a conjugation length of 16 carbon atoms. Electrochemical carbyne is stabilized against interchain crosslinking (graphitization) by interspersed microcrystalline alkali metal fluoride (reaction byproduct). Carbyne decomposes spontaneously if the alkali metal fluoride is extracted from the composite by leaching out with water. The final salt-free product is a highly disordered carbon material resembling diamond-like amorphous carbon; it graphitizes after thermal and pressure treatments.

Conduction mechanisms in undoped thin films of C60 and C60/70

Abstract Undoped thin films of C 60 and C 60/70 mixture have been electrically and optically characterized over a wide temperature range T=10…475 K. In this paper the results of the field-effect conductance, conductivity, thermoelectric power, thermoabsorption and electroabsorption measurements will be presented. The transittor studies gave an n-type behaviour, and the thermally activated field-effect conductance obeyed the Meyer-Neldel rule. The conductivity showed an activated temperature dependence above 250 K, too, but turned into a nonactivated dependence below 200 K. The results may be explained by assuming a distribution of gap states. Furthermore, we have deposited and characterized Langmuir-Blodgett films of C 60/70 and its heterostructures with poly(3-hexylthiophene).

Fullerene Field-Effect Transistors

Undoped C60 and C60/C70fullerenes have been used as the active material in thin-film field-effect transistors (FET). All FETs turned out to be n-channel devices operating in the accumulation mode with a field-effect mobility around 10−4 cm2/Vs. The conductance-activation energy is a function of the gate voltage. Like in amorphous silicon localized states have a strong influence on the transport properties. These localized states can either originate from disorder or from a self-trapping of the induced carriers in the fullerene.

UV-Vis absorption of thin electrochemical carbon layers on poly(tetrafluoroethylene-co-hexafluoropropene)

Abstract The surface of poly(tetrafluoroethylene- co -hexafluoropropene) foil was carbonized by a reaction with liquid alkali metal (Li, Na, K) amalgam at 25 °C. The carbonized layer is composed from n-doped elemental carbon and a stoichiometric amount of alkali metal fluoride. UV-Vis absorption spectra of the carbonized layers show features typical for carbons with supposed carbynoid structure. The plot of absorbance versus the preparation time is interpreted in terms of a two-step reaction mechanism, i.e. a fast chemical initialization followed by a slow electrochemical propagation. Long-term changes in the UV-Vis spectra are elucidated by a preconception of slow carbyne-to-graphene crosslinking taking place in the solid product. A phenomenological mathematical model of this process, similar to that describing long-term changes in electronic conductivity, is proposed.

In situ spectroscopic investigation of the doping process in poly(3-octylthiophene)

Abstract Electrochemical doping of poly(3-octylthiophene) was studied by in situ optical absorption and in situ resonance Raman scattering. Absorption of polarons and, at higher doping levels, of bipolarons was observed. A strong decrease of luminescence and of main Raman lines was found particularly for the initial part of the doping process. The amplitude mode shifts about 30 cm −1 from the nearly undoped to the heavily doped state. This behaviour is attributed to the quenching of the Raman line of the undoped state and at higher doping levels to the simultaneous generation of the Raman line for the bipolaron configuration at lower wavenumbers.

Resonance raman scattering from spincoated and langmuirblodgett poly(3-alkylthiophene) films

Abstract Spincoated poly(3-hexylthiophene) and Langmuir-Blodgett films of poly(3-hexylthiophene)/ arachidic acid with a different number of layers have been studied by resonance Raman scattering. In both kinds of samples a clear dispersion was observed for the Raman line of the amplitude mode. This dispersion effect is much higher for spincoated films as compared to LB films. We interpret this behaviour by the conjugation length model using the A-term of the Albrecht theory. An average conjugation lenght of about four thiophene rings was found for spincoated samples and a higher value for the LB films.

Buried K-Fullerides by Ion Implantation

We report the formation of buried K-fulleride layers by high dose implantation of 30 keV K+ at different implant temperatures. Raman scattering showed a transformation of fullerene molecules to amorphous carbon (a-C) within the range of the implanted ions. The microscopic structure of the a-C is discussed. At elevated implant temperatures potassium accumulates underneath the a-C layer. This behaviour is explained in terms of the formation process of a K-fulleride. For comparison, results obtained on Ar+ implanted fullerene films are presented.

Formation of air stable buried K-fulleride layers by ion implantation

Abstract The formation process of buried K-fulleride layers by high dose implantation of 30 keV K + at an implant temperature of 300°C has been studied as a function of dose. Raman scattering showed a transformation of the fullerene molecules to amorphous carbon (a-C) within the range of the implanted ions. At the elevated implant temperatures used most of the K diffuses into the depth where a doped layer is formed. The buried K-fulleride layer is stable on air due to a passivation by the a-C. The formation process is discussed on the basis of the K-C 60 phase diagram and various thermodynamic processes like segregation, phase formation and diffusion.

Raman spectra of the new low bandgap polymer poly(2,3-substituted thieno [3,4-b]-pyrazine)

Summary form only given. Vibrational modes of a new processable polymer with a bandgap as low as 0.95 eV have been measured by resonance Raman scattering and calculated by a quantum chemical method. Raman spectroscopy of the pristine polymer revealed two strong lines at about 1520 cm/sup -1/ and 1563 cm/sup -1/ in the C=C stretching mode region. By changing the exciting laser line these modes exhibit a dispersion of 14 and 25 cm/sup -1//eV, respectively. Substitution at the 2,3-positicns with alkyl groups has only a weak influence on the vibrational properties. The mode at 1520 cm/sup -1/ shifts moderately towards lower frequencies with increasing chain length. However, the utilization of thiophene rings as substituents results in a much higher mode softening. In addition, the Raman response of FeCl/sub 3/-doped samples is presented. A change in relative intensities and different line shifts were observed. In order to determine the structure of the ground state Raman spectra were calculated for the quinonoid and aromatic form and compared with the experimental data. It turned cut that the two strong lines experimentally observed originate from an interring vibration and from a mode which mainly comes from the thiopbene rings along the polymeric backbone.