TTF TCNQ Research Articles

Relations between the morphology and conductivity of thin films of tetrathiofulvalinium-tetracyanoquinodimethane

The techniques of electron and oprical microscopy were used to establish correlations between the morphology and the electrical properties of tetrathiofulvalinium-tetracyanoquinodimethane (TTFTCNQ) thin films. Glass, monocrystalline silicon and the (100) cleavage planes of LiF, KBr and NaCl were used as the substrates. The TTFTCNQ films are polycrystalline but are to some degree oriented depending on the substrate material. Doubly oriented areas diminishing with decreasing substrate temperature during deposition were found on KBr and NaCl. The electrical conductivity depends on the film thickness. Particularly high conductivities are measured at a thickness of about 150 nm. Above 450 nm the disordered growth of a porous film takes place with a corresponding decrease in conductivity. The thin film conductivity was found to be a nearly linear function of the predominant crystallite length.

Conditions for Reticulated Crystalline Doping of Polymer with Charge Transfer Complexes

Crystallization of the conductive charge transfer (CT) complex forming a reticulated, dendritic morphology within a polymeric matrix during film casting is a new way for obtaining conductive polymers. Subject to proper crystallization conditions during film casting one can obtain materials with high conductivity at a concentration of additives as low as 1 wt%. This paper describes a model concept allowing for the selection of proper crystallization conditions leading to suitable morphology of crystalline additives. It is based on an analysis of morphologies from evaporating solutions containing polymer and low molecular weight CT complexes-forming additives. Considerations are based on the results obtained for TTF–TCNQ CT complexes crystallized in polycarbonate and polystyrene matrices.

Partial ionicity, cohesion, and charge correlations in narrow-band solids

A many-electron, correlated-state representation is introduced for narrow-band solids of donors and acceptors. The electrostatic energy for partial ionicity 0⩽q⩽1 is related to zero bandwidth Wigner lattices and to wide-band Hartree–Fock results for both simple orthorhombic lattices of point charges and for the organic conductor TTF–TCNQ at q = 1/2. Partial ionicity requires repulsive as well as attractive Coulomb contacts in the first coordination sphere, a condition that is met by organic conductors. The configuration interaction among correlated crystal states washes out the fixed neutral and ionic sites of a Wigner lattice, while retaining most of its electrostatic binding. Short-range charge correlations in partly ionic lattices substantially improve the Hartree–Fock cohesion and rationalize the shallow minimum (relative to neutral solids) in representative partly ionic organic solids.

Lattice energy, structure and stability of three-dimensional tetrathiafulvalenium 7,7,8,8-tetracyanoquinodimethanide (TTF–TCNQ) models with segregated and mixed stacks

The lattice energy of tetrathiafulvalenium 7,7,8,8-tetracyanoquinodimethanide (C6H4S4.C12H4N4) with space group P21/c and segregated stacks was minimized for four interstack parameters; the unit-cell parameters a, c and β, and the angle, ϕ, between the projection of the longitudinal molecular axis on the ac plane and a. The lattice energy of a model with space group C2/m and mixed stacks was minimized for the unit-cell parameters a, b and β and the intrastack molecular distance, c. Van der Waals, repulsive and electrostatic interactions were calculated from two sets of atom-atom potentials and atomic charges. The minimum of the P21/c lattice was achieved at a structure with parameters deviating 0.2-2.5% from their observed values. The minimum of the C2/m lattice was achieved at a structure with parameters reasonably close to the observed values of related TCNQ compounds. The latter structure provided to be 9.2-16.8 kJ mol-1 more stable than the P21/c modification, indicating the possibility of the preparation of mixed-stack structures.

Microwave Measurements of the Thermal Expansion of Organic Metals

AbstractThe thermal expansion of organic metals is measured by the Buravov‐Shchegolev method (microwave cavity at 9.5 GHz). A relation is found between the frequency shift of the cavity and the length of the sample, independent of the skin depth in the sample. The linear thermal expansion coefficients αb are measured along the organic chains in charge transfer complexes (TTF–TCNQ, HMTTF–TCNQ) and ion radical salts (TMTTF2–I, TMTSeF2–ClO4, TTT2–I3 and TSeT2–I). Organic metals with TTF and its selenium analog TSeF show large coefficients for bi‐chain compounds (like TTF–TCNQ) as well as for monochain compounds (like TMTSeF2–ClO4). These metals are found to exhibit strong anharmonic effects leading to a T2 law for the electrical resistivity (Weger's model). On the contrary metals with TTT or TSeT have weak coefficients αb and their electric properties are more similar to those of inorganic metals (their resistivity follows a Bloch‐Grüneisen law with ϱ ∝ T).

Brownian Motion of a Kink in Sine-Gordon System and Diffusion Constant

In a one-dimensional sine-Gordon system, a propagating vibration with small amplitude collides with a kink to produce a translation of the kink, giving rise to its Brownian-like motion, since incoming vibrations are excited thermally. The diffusion constant of the kink is obtained, at low temperatures, using the fluctuation-dissipation theorem and the thermal average over the vibrations. It turns out to be one eighth as large as that obtained previously by Fesser. This reduction would be due to the fact that wave packets of the incident vibrations overlap each other. Assuming the validity of the Einstein relation, we use the result to calculate the low field conductivity of quasi-one-dimensional substances at low temperatures. The magnitudes of the observed conductivity prefactors are reproduced for TaS 3 , but not for TTF–TCNQ and TSeF–TCNQ.

Room- and high-pressure neutron structure determination of tetrathiafulvalene-7,7,8,8-tetracyano-p-quinodimethane (TTF–TCNQ). Thermal expansion and isothermal compressibility; errata

Room- and high-pressure neutron structure determination of tetrathiafulvalene-7,7,8,8-tetracyano-p-quinodimethane (TTF–TCNQ). Thermal expansion and isothermal compressibility; errata

Study of mode Grüneisen parameters in various quasi-one-dimensional crystals

This work reports the pressure dependence of the Raman spectra of TCNQ0, TTF0, K(TCNQ), Cs2(TCNQ)3, perylene–TCNQ, and phenazine–TCNQ in the range from 0 to 70 kbar. By using the average compressibility of ∼0.01 kbar−1 of TTF–TCNQ, approximate mode Grüneisen parameters are derived spanning a range of several orders of magnitude. In the gas phase spectrum of TCNQ, low lying internal modes are observed at 40, 76, and 145 cm−1, lowering the so far assumed demarcation between internal and external modes from about 200 cm−1 down to less than 50 cm−1. From the pressure and gas phase data it is concluded that all of the observed low frequency modes in this class of compounds actually are modes of mixed internal and external character. In TTF0 and TCNQ0 a continuous change in color from yellow to dark red is observed when pressure is applied until the samples become opaque at about 80 kbar. Furthermore, TCNQ shows a phase transition at 12 kbar.

The cohesive energy of tetrathiafulvalenium 7,7,8,8-tetracyanoquinodimethanide (TTF TCNQ) as a function of charge transfer

The Madelung, polarization, charge–dipole, and dispersion contributions to the theoretical lattice energy of tetrathiafulvalenium 7,7,8,8-tetracyanoquinodimethanide (TTF TCNQ) were computed for the uniform lattice as a function of charge transfer ρ and for the ρ = 1/2 Wigner lattice. The atom-in-molecule charges, local hybrid dipole moments, and polarizabilities used in this study were obtained from a finite-electric-field perturbation MINDO/3 calculation. Some improvements were obtained over the Madelung- only case, but the charge–dipole term tends to cancel the polarization energy term (or the Madelung energy term). The dispersion energies calculated with atom-in-molecule polarizabilities and molecular and ionic ionization energies are smaller than the dispersion energies obtained by using parametrized van der Waals coefficients for neutral hydrocarbons. The lattice energy terms at intermediate charge transfer were obtained by linear interpolation of the charges, local moments, and polarizabilities between their ρ = 0.0 values and their ρ = 1.0 values. The McConnell, Hoffman, and Metzger criterion for ionicity used along with all of the cohesive energy terms included here fails to yield the desired cohesive energy minimum at fractional charge transfer. Using the Soos estimate of the cost of ionization does provide a minimum at intermediate charge transfer, but not significantly far from the minimum obtained from using the Madelung energy alone.

High-pressure induced commensurability effects on the Peierls transitions in an organic metal: TTF–TCNQ

High-pressure induced commensurability effects on the Peierls transitions in an organic metal: TTF–TCNQ

Density of States in TTF-TCNQ Measured by Inelastic Neutron Scattering

Abstract The scattering law ζ(ω) of TTF—TCNQ at 77K has been measured between 35 and 455 meV (280-3670cm−1) by inelastic neutron scattering. The data have been corrected for background then deconvoluted by a resolution function established for the spectrometer INI-B (I.L.L.) and for a powder sample. They are finally compared to the infrared and Raman measurements available.

Room- and high-pressure neutron structure determination of tetrathiafulvalene-7,7,8,8-tetracyano-p-quinodimethane (TTF–TCNQ). Thermal expansion and isothermal compressibility

Room- and high-pressure neutron structure determination of tetrathiafulvalene-7,7,8,8-tetracyano-p-quinodimethane (TTF–TCNQ). Thermal expansion and isothermal compressibility

The Physics and Chemistry of Low Dimensional Solids

Luis Alcacer (ed) 1980 Dordrecht: D Reidel x + 436 pp price $50 This volume contains the proceedings of the Nato–ASI summer school at Tomar, Portugal which took place in August 1979. It is primarily concerned with the organic quasi-one-dimensional conductors of the TTF–TCNQ family, although there are reviews of NbSe3 and doped polyacetylene, the other two quasi-one-dimensional systems that are currently much studied.

Requirements for an “Organic Metal”

Abstract Current theories on the origin of high electrical conductivity in TCNQ-based materials emphasize the need for incomplete charge transfer as well as segregated stacks of donor and acceptor molecules and uniform spacing within the stacks. The requirements for an “organic metal” are discussed in terms of partial charge transfer. An examination of 61 structurally related TTF–TCNQ, based complexes suggests that the combinations with the redox potential −0.02≤E1(D)−E1(A)≤0.34 V have a high possibility of being “organic metals.” A method to arrange the complex by the difference between the electrochemical half-wave potentials of the donor and the acceptor molecule is proposed to predict electrical properties of the complex.

Investigations on a new series of organic conductors: The tetraselenotetracene iodides (TSeT-Ix)

A new series of radical-cation salts, the tetraselenotetracene iodides TSeT-Ix, have been synthesized by a cosublimation process. Three different stoichiometries have been obtained on which the structural and physical properties have been investigated. Phases B and C, TSeT-I0, 71 and TSeT-I0, 77, respectively, are semiconducting materials with incommensurate iodine sublattices; phase B exhibits a strong hysteresis effect detected by dc electrical conductivity and EPR linewidth measurements. The existence of elastic microdomains is postulated to explain this behavior. Concerning phase A, which is an ordered structure (TSeT-I0,5) we show that it is a molecular metal (the quality of which is sensitive to the preparation conditions). By using a general diagrammatic representation we are able to compare this salt to other one-chain compounds. We conclude that systems, the conductivity of which is dominated by radical cation chains, are organic metals potentially as good as charge transfer complexes belonging to TTF TCNQ series.

ChemInform Abstract: ORGANIC METALS. VAN DER WAALS DONOR STACKING IN TTF‐TCNQ (TETRATHIAFULVALENE‐TETRACYANOQUINODIMETHANE)

The slipped stacking geometry of TTF in TTF–TCNQ and in neutral TTF is shown to be consistent with the close packing of hard spheres having atomic van der Waals radii. Slipped or eclipsed stacking in TTF compounds is suggested to result from a competition between closed shell interactions and those interactions that arise from partially emptying the highest occupied π‐molecular orbital. This contrasts with TCNQ stacking in TCNQ salts which has been previously argued to be a consequence of only those interactions that result from filling the π‐molecular‐orbital affinity level.

Relation between the anharmonic properties of TTF–TCNQ and those of naphthalene and anthracene

Data on the pressure and temperature dependence of the frequency of some translational and librational modes in naphthalene and anthracene are used to estimate the properties of analogous modes in TTF–TCNQ.

Effects of controlled disorder on the phase transitions and charge density wave state in tetrathiofulvalene–tetracyanoquinodimethane

AbstractTransport and magnetic studies of the phase transitions and low temperature charge density wave state in TTFTCNQ as a function of irradiation induced defect concentration are presented. The transport and magnetic results imply a suppression and broadening of the phase transition and incomplete 3d ordering for T < 34 K. Although the TCNQ susceptibility decreases toward zero at T = 0 in irradiated samples, the contribution from the TTF chains remains finite.

Organic metals. van der Waals donor stacking in TTF–TCNQ (tetrathiafulvalene-tetracyanoquinodimethane)

The slipped stacking geometry of TTF in TTF–TCNQ and in neutral TTF is shown to be consistent with the close packing of hard spheres having atomic van der Waals radii. Slipped or eclipsed stacking in TTF compounds is suggested to result from a competition between closed shell interactions and those interactions that arise from partially emptying the highest occupied π-molecular orbital. This contrasts with TCNQ stacking in TCNQ salts which has been previously argued to be a consequence of only those interactions that result from filling the π-molecular-orbital affinity level.

Tetrathiafulvalene S-oxide: a potential ‘donor impurity’ in the organic metal TTF–TCNQ

Tetrathiafulvalene S-oxide, which because of its size similarity with tetrathiafulvalene is a potential ‘donor impurity’ in the organic metal TTF–TCNQ, was prepared and characterized spectroscopically. Experiments in which tetrathiafulvalene S-oxide was purposely doped into TTF–TCNQ indicate, however, that the S-oxide is not of major importance for the electrical conductivity of the TTF–TCNQ crystals. The surface of TTF–TCNQ crystals, which had been exposed to air, was analysed by means of ESCA spectroscopy. The results strongly indicate the presence of a totally oxidized surface