S. Söderholm

Fabrication and characterization of Schottky barrier diodes with tetracyanoquinodimethane doped with bis(β-naphthyl)-tetrathiafulvalene

Abstract Stable Schottky barrier diodes with tetracyanoquinodimethane doped with 5% by weight bis(β-naphthyl)-tetrathiafulvalene as active ndoped semiconductor have been fabricated by physical vapor deposition. Gold and aluminum were used as ohmic and Schottky contacts, respectively. An ideality factor of 3.5 was deduced from current-voltage measurements. Several cycles of voltage sweeping were necessary before the diodes showed optimal characteristics. Charge migration is most likely the dominating mechanism behind this phenomenon. From capacitance-voltage measurements, at different frequencies, a charge carrier concentration of 2.2 × 10 16 cm −3 was determined. A high charge carrier mobility of 14 cm 2 V −1 s −1 was deduced. These devices showed no degradation after a month of storage in a laboratory environment.

A photoemission study of c-axis-oriented thin films of α-(BEDT-TTF)2I3

Abstract Highly c -axis-oriented thin films of α-(BEDT-TTF) 2 I 3 have been prepared by physical vapour deposition. Structural and morphological characterization are performed by X-ray diffraction and scanning electron microscopy. In situ photoemission experiments on the films reveal five distinct features in the valence-band regime; the two structures with the lowest binding energies are assigned to molecular orbitals present in the TTF backbone. None of the valence-band structures shows dispersion along the Γ-Z ( k z ) direction. The spectral intensity is zero at, and to within 0.5 eV of the Fermi level. Core-level spectroscopy on the I 4d level indicates that all iodine is present as I 3 − ions and that the surface consists of BEDT-TTF molecules.

The electrical and magnetic properties of TMDBFBF4, ClO4, AsF6 and PF6, the cation radical salts of tetramethoxydibenzofuran

The cation radical salts (crs) of tetramethoxydibenzofuran, (TMDBF)X, form non-stoichiometric salts with different anions (X = BF4−, ClO4−, AsF6− and PF6−). All investigated salts contained the solvent used in the electrocrystallization (CH2Cl2). The X-ray investigation showed that the crs were disordered. Conductivity measurements showed that the crs were thermally activated with Δ < 110 meV at high temperatures; at lower temperatures there are different deviations from this behaviour. All crs have σrt ∼ 10−2 (Ω cm)−1. The susceptibilities are large, χ ∼ 10−4 emu/mole, and barely temperature dependent between 300 and 50 K. Below 50 K, χ increases less than 1/T. These results are interpreted using a common model for σ and χ. Due to the quasi one-dimensionality of the crs, the disorder present causes all states to be localized. This leads to charge transport by phonon-assisted hopping between localized states near the Fermi level, and a paramagnetic susceptibility due to antiferromagnetic order in one dimension. Interacting chain segments exist with weak exchanges ϵJ at a random concentration among uniform nearest-neighbour exchanges J. The narrow e.s.r. lines (about 100 mG at room temperature) are caused by exchange narrowing, with an exchange frequency of about 1012 Hz.

The electronic properties of cation radical salts of tetramethoxynaphthalene, TMN, a comparative study of (TMN)3(ClO4)2 and (TMN)3(AsF6)2

Abstract In this communication we present the electronic properties of (TMN)3(ClO4)2 and (TMN)3(AsF6)2. The properties were studied by ESR and conductivity measurements on single crystals and absorption measurements on powder. The cation stacks are trimerized causing the charge transport to be thermally activated. The spin properties are described by triplet spin excitations from a diamagnetic ground state. Differences in the electronic properties can be related to differences in the structures, caused by the different sizes of the anions.

New donors for cation radical salts: 2,6-dimethoxy-3,7-bis(methylthio)-naphthalenes

Abstract The high yield, one-pot synthesis of three new donors for cation radical salts is described. These are 2,6-Dimethoxy-3,7-bis(methylthio)naphthalene 2a , 2,6-Dimethoxy-3,7-bis(methylseleno)naphthalene 2b , and 2,6-Dimethoxy-3,7-bis(methyltelluro)naphthalene 2c . They show a quasireversible halfwave potential with the values 1.05, 0.85, and 0.53 Volts respectively, the one for 2c being irreversible. 2a gives 3:2 salts when electrocrystallized in the presence of perchlorate and hexafluoroarsenate anions. The perchlorate salt is a semiconductor with a fairly high room temperature conductivity, 0.4–0.7 S/cm. This higher conductivity than the corresponding salt of 1a (2,7-Dimethoxy-3,6-bis(methylthio)-naphthalene) can be attributed to a high mobility, due to fewer stacking defects.

2,3,6,7-tetrakis(methylthio)naphthalene; synthesis and cation radical salt

Abstract The three step synthesis of 2,3,6,7-Tetrakis(methylthio)naphthalene (TMTN) 1 is described. The title donor gives a 1:1 salt with hexafluoroarsenate anion. The crystal structure consists of a dimerized donor stack, with alternate stacking. The conductivity is activated, room temperature conductivity; ≈10 −3 ·10 −4 Scm −1 . The susceptibility is quite low, χ (300K) = 7·10 −7 emu/mol, and the ESR linewidth is Δ B pp (300K)≈5 G.

Partially deuterated tetramethoxydibenzofuran-hexafluorophosphate: An electron spin resonance study

Abstract The susceptibility of the title compound is large, χ (300 K) = 3.8 × 10 −4 emu/mol, and the temperature dependence of the susceptibility is explained by the random exchange Heisenberg antiferromagnetic chain model. The localization of the electrons in an antiferromagnetic order is proved by the angular dependence of the e.s.r. linewidth, which corresponds to dipole-dipole interaction. A comparison of the spin properties between the title compound and the non-deurated compound exhibits only minor differences.

Influence of additives on the electrochemical oxidation of polyacetylene

Abstract The result of the electrochemical oxidation of polyacetylene is improved by additives in the electrolyte, which suppress overoxidation of polyacetylene. The temperature dependence of the conductivity is semiconductor-like and is interpreted by Shengs fluctuation-induced tunnelling model for tunnelling between metallic regions. Polyacetylene oxidized in the presence of an additive ( e.g. , bithiophene) has a smaller intrinsic resistance of the metallic strands.