Charge Polarity Control in Organic Transistors of Mixed and Segregated Complexes Based on Diaminonaphthalene and Pyrene.

Organic cocrystals of diaminonaphthalene (DAN) and diaminopyrene (DAP) with bromanil (BA) and tetracyanoquinodimethane (TCNQ) are an exemplar system for understanding the charge-transport process, where from the viewpoint of partition theory, orbital symmetry plays a crucial role in controlling the carrier charge polarity of transistors. In the mixed-stack complexes of BA and other p-quinone acceptors, a comparatively weak donor, 1,5-DAN, shows p-channel characteristics owing to the counteractive contribution of the next highest occupied molecular orbital for electron transport. This characteristic behavior occurs because the BA molecule, situated on top of the amino group, overlaps with half of the DAN molecule. By contrast, the BA and TCNQ complexes of a stronger donor, 1,6-DAP, exhibit n-channel transport due to the cooperative path and orthogonal orbitals. Similarly, TCNQ complexes of variously substituted DAN show n-channel transport, where the TCNQ molecules are located on top of the DAN molecules. However, when the carbon electrodes of (1,5-DAN)(BA) are replaced by silver, electron transport appears due to the competitive effect of the Schottky barriers. In a highly conducting segregated complex of (1,6-DAP)(TCNQ), ambipolar transistor characteristics are observed without subtracting the bulk current by using carefully prepared thin-film transistors.

Organic Molecular Conductors Based on Tetramethyl-TTP: Structural and Electrical Properties Modulated by the Anion Size and Shape.

The tetramethyl derivative of bis-fused tetrathiafulvalene, 2,5-bis(4,5-dimethyl-1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene (TMTTP), has been successfully synthesized. Most of the radical cation salts consisting of TMTTP feature a high electrical conductivity of σrt = 101 to 102 S cm-1 on a single crystal. The anion shape and size of TMTTP conductors modulate the electrical properties. The PF6- and AsF6- salts exhibit metallic conductivity down to 10 K, while the ReO4- and AuI2- salts show metal-to-insulator (MI) transition at 126 and 11 K, respectively. Single-crystal X-ray structure analysis of (TMTTP)2X (X = PF6, AsF6, BF4, and ReO4) and (TMTTP)3AuI2 reveals that the donor packing motif is categorized as the so-called ß-type. The tight-binding band calculations of (TMTTP)2X (X = ReO4, BF4, PF6, and AsF6) suggest these salts are characterized by quasi-one-dimensional Fermi surfaces. The band calculation based on X-ray structure analysis of (TMTTP)2ReO4 at 100 K demonstrates that the MI transition of this salt is associated with the charge ordering with the zigzag stripe pattern.

Charge injected proton transfer in indigo derivatives.

In analogy with excited-state proton transfer, proton transfer is significantly facilitated in cationic and anionic molecules of indigo derivatives generated in field-effect transistors. We have prepared extended and truncated indigo derivatives and investigated their ambipolar transistor properties. Since the proton transfer reduces the energy gap from 2.2 to 0.4 eV, the proton transferred states are stabilized in the charge injected cationic and anionic states; the energy increase is as small as 0.5 eV, which is half of that in the neutral state. The intermolecular proton transfer enlarges the equilibrium N-H distance typically by 0.03 Å, and improves the donor and acceptor abilities by 0.2-0.4 eV, though the reorganization energy is practically unchanged. In addition, the transfer integrals along the hydrogen bonds are as large as one third of the columnar transfers, to facilitate the two-dimensional carrier conduction. The influence of proton transfer is most significant in indigo and truncated indigo derivatives, though isoindigo and quinacridone exhibit similar properties. Accordingly, indigo derivatives show much better donor and acceptor abilities than those expected from isolated molecules.

Transistor Characteristics of Charge-Transfer Complexes Observed across a Neutral-Ionic Transition.

For basic understanding of transistor properties of doped organic semiconductors, 3,3',5,5'-tetramethylbenzidine charge-transfer complexes are investigated, which change from neutral to ionic by varying the acceptors. When going into the ionic state, the bulk conduction increases more rapidly than the mobility, but sufficiently thin devices exhibit transistor properties. The resulting ambipolar characteristics are analyzed in the linear regions at small drain voltages in analogy with graphene transistors. The model is further extended to include partial overlap of electron and hole transport regions. In the temperature dependence, the activation energy loses gate voltage dependence when the neutral-ionic transition takes place by 0.1-0.2 eV above the equal acceptor and donor levels; the difference comes from the typical trap (polaron) depth or the Madelung energy.

Electronic engineering of a tetrathiafulvalene charge-transfer salt via reduced symmetry induced by combined substituents.

A 1 : 1 metallic charge-transfer salt is obtained by cosublimation of (Z,E)-(SMe)2Me2TTF and TCNQ. X-ray diffraction studies confirm the formation of segregated stacks comprising donor and acceptor molecules in [(E)-(SMe)2Me2TTF](TCNQ). The crystal packing features lateral SS interactions between TTF stacks, which is in sharp contrast to that in (TTF)(TCNQ). Structural analysis and theoretical studies afford a partial charge-transfer (ρ ≈ 0.52), leading to a system with the electronic structure close to quarter-filled. Resistivity measurements reveal that this material behaves as a metal down to 56 K and 22 K at 1 bar and 14.9 kbar, respectively. The thermopower is negative in the metallic regime, indicating the dominant role of the acceptor stacks for the observed conducting behavior. Analysis of single-crystal EPR spectra shows the remaining spin susceptibility at 4.3 K, suggesting the importance of the Hubbard U correction. These results highlight the judicious engineering of electronic and geometrical effects on the TTF core; the combined use of methyl and thiomethyl groups has decreased the TCNQ bandwidth while maintaining the segregated stacks, converting the metal to insulator (M-I) transition to more 4kF like. In addition, the enhanced SS contacts between the TTF stacks lead to more rapidly decreasing M-I transition temperature under various pressures.

Carrier Charge Polarity in Mixed-Stack Charge-Transfer Crystals Containing Dithienobenzodithiophene.

Dithieno[2,3- d;2'3'- d']benzo[1,2- b;4,5- b']dithiophene forms mixed-stack charge-transfer complexes with fluorinated tetracyanoquinodimethanes (F nTCNQs, n = 0, 2, and 4) and dimethyldicyanoquinonediimine (DMDCNQI). The single-crystal transistors of the F nTCNQ complexes exhibit electron transport, whereas the DMDCNQI complex shows hole transport as well. The dominance of electron transport is explained by the superexchange mechanism, where transfers corresponding to the acceptor-to-acceptor hopping ( teeff) are more than 10 times larger than the donor-to-donor hopping ( theff). This is because the donor orbital next to the highest occupied molecular orbital makes a large contribution to the electron transport owing to the symmetry matching. Like this, inherently asymmetrical electron and hole transport in alternating stacks is understood by analyzing bridge orbitals other than the transport orbitals.

Successive Dimensional Transition in (TMTTF)_{2}PF_{6} Revealed by Synchrotron X-ray Diffraction.

A quasi-one-dimensional organic charge-transfer salt (TMTTF)_{2}PF_{6} undergoes a multistep phase transition as the temperature decreases. One of these transitions is called a "structureless transition," and these detailed structures were unknown for many years. With synchrotron x-ray diffraction, we observed a slight structural difference owing to the effect of charge-order transition between two TMTTF molecules in a dimer, which corresponds to the charge transfer δ_{CO}=0.20e. The two-dimensional Wigner crystallization was determined from an electron density analysis using core differential Fourier synthesis. Furthermore, we found that the ground state due to tetramerization, called the spin Peierls phase, is a three-dimensional transition with interchain correlation.

Benzothienobenzothiophene-Based Molecular Conductors: High Conductivity, Large Thermoelectric Power Factor, and One-Dimensional Instability.

On the basis of an excellent transistor material, [1]benzothieno[3,2-b][1]benzothiophene (BTBT), a series of highly conductive organic metals with the composition of (BTBT)2XF6 (X = P, As, Sb, and Ta) are prepared and the structural and physical properties are investigated. The room-temperature conductivity amounts to 4100 S cm(-1) in the AsF6 salt, corresponding to the drift mobility of 16 cm(2) V(-1) s(-1). Owing to the high conductivity, this salt shows a thermoelectric power factor of 55-88 µW K(-2) m(-1), which is a large value when this compound is regarded as an organic thermoelectric material. The thermoelectric power and the reflectance spectrum indicate a large bandwidth of 1.4 eV. These salts exhibit an abrupt resistivity jump under 200 K, which turns to an insulating state below 60 K. The paramagnetic spin susceptibility, and the Raman and the IR spectra suggest 4kF charge-density waves as an origin of the low-temperature insulating state.

A Single-Component Conductor Based on a Radical Gold Dithiolene Complex with Alkyl-Substituted Thiophene-2,3-dithiolate Ligand.

Alkyl-substituted thiophene-2,3-dithiolate ligands are prepared through a Thio-Claisen rearrangement of 4,5-bis(propargylthio)-1,3-dithiole-2-thione derivatives. The two novel dithiolate ligands, namely, 4,5-dimethyl-thiophene-2,3-dithiolate (α-Me2tpdt) and 4-ethyl-5-methyl-thiophene-2,3-dithiolate (α-EtMetpdt), are engaged in anionic Au(III) square planar complexes formulated as [Au(α-Me2tpdt)2](-) and [Au(α-EtMetpdt)2](-), isolated as Ph4P(+) salts. Monoelectronic oxidation gives the neutral radical complexes [Au(α-Me2tpdt)2](•) and [Au(α-EtMetpdt)2](•). The latter crystallizes into uniform stacks with limited interstack interactions, giving rise to a calculated half-filled band structure. It exhibits a semiconducting behavior with room temperature conductivity of 3 × 10(-3) S cm(-1), indicating that this single-component conductor can be described as a Mott insulator. The different structures observed in [Au(α-EtMetpdt)2](•) and the known [Au(Et-thiazdt)2](•) complex (Et-thiazdt: N-ethyl-thiazoline-2-thione-4,5-dithiolate), despite their very similar shapes, are tentatively attributed to differences in the electronic structures of the ligand skeleton.

A highly conducting organic metal derived from an organic-transistor material: benzothienobenzothiophene.

BTBT ([1]benzothieno[3,2-b][1]benzothiophene) is an organic semiconductor that realizes high mobility in organic transistors. Here we report that the charge-transfer (CT) salt, (BTBT)2PF6, shows a high room-temperature conductivity of 1500 S cm(-1). This compound exhibits a resistivity jump around 150 K, but when it is covered with Apiezon N grease the resistivity jump is suppressed, and the metallic conductivity is maintained down to 60 K. Owing to the very high conductivity, the ESR signal shows a significantly asymmetric Dysonian lineshape (A/B ≅ 3) even at room temperature. Since most organic conductors are based on strong electron donors, it is remarkable that such a weak electron donor as BTBT realizes a stable and highly conducting organic metal.

Interlayer charge disproportionation in the layered organic superconductor κ(H)-(DMEDO-TSeF)2[Au(CN)4](THF) with polar dielectric insulating layers.

We report the molecular dipole effect on conduction electrons in the title superconductor. The angular-dependent magnetoresistance has a peak for fields nearly parallel to the conducting layer, and the peak width scales as the field component perpendicular to the layer, indicating incoherent interlayer transport. However, two closed Fermi surfaces are observed in quantum oscillation. Accordingly, crystallographically independent layers have different charge densities in a bulk single crystal. The electric dipole of tetrahydrofuran gives rise to interlayer charge disproportionation.

Structural transitions from triangular to square molecular arrangements in the quasi-one-dimensional molecular conductors (DMEDO-TTF)2XF6 (X = P, As, and Sb).

A series of quasi-one-dimensional molecular conductors (DMEDO-TTF)(2)XF(6) (X = P, As, and Sb), where DMEDO-TTF is dimethyl(ethylenedioxy)tetrathiafulvalene, undergo characteristic structural transitions in the range of 130-195 K for the PF(6) salt and 222-242 K for the AsF(6) salt. The dramatic structural transition is induced by the order of the ethylenedioxy moiety, and the resulting anion rotation leads to the reconstruction of the H···F interaction between the methyl groups and the anions. The unique hydrogen bonds play a crucial role in the transition. As a result, the molecular packing is rearranged entirely; the high-temperature molecular stacks with an ordinary quasi-triangular molecular network transforms to a quasi-square-like network, which has never been observed among organic conductors. Nonetheless, the low-temperature phase exhibits a good metallic conductivity as well, so the transition is a metal-metal (MM) transition. The resistivity measured along the perpendicular direction to the conducting ac-plane (ρ(⊥)) and the calculation of the Fermi surface demonstrate that the high-temperature metal phase is a one-dimensional metal, whereas the low-temperature metal phase has considerable interchain interaction. In the SbF(6) salt, a similar structural transition takes place around 370 K, so that the quasi-square-like lattice is realized even at room temperature. Despite the largely different MM transition temperatures, all these salts undergo metal-insulator (MI) transitions approximately at the same temperature of 50 K. The low-temperature insulator phase is nonmagnetic, and the reflectance spectra suggest the presence of charge disproportionation with small charge difference (0.14).

Growth and structure analysis of tungsten oxide nanorods using environmental TEM.

WO3 nanorods targeted for applications in electric devices were grown from a tungsten wire heated in an oxygen atmosphere inside an environmental transmission electron microscope, which allowed the growth process to be observed to reveal the growth mechanism of the WO3 nanorods. The initial growth of the nanorods did not consist of tungsten oxide but rather crystal tungsten. The formed crystal tungsten nanorods were then oxidized, resulting in the formation of the tungsten oxide nanorods. Furthermore, it is expected that the nanorods grew through cracks in the natural surface oxide layer on the tungsten wire.

Stabilization of organic field-effect transistors by tert-butyl groups in dibenzotetrathiafulvalene derivatives.

For a material for organic thin-film transistors, not only high mobility but also low threshold voltage and long-term stability are important requirements. In order to realize these properties, materials with relatively large oxidation potentials, namely weak donors, have been designed as p-channel organic semiconductors. Here we propose a different strategy; transistor properties of dibenzotetrathiafulvalene (DBTTF) are significantly improved by the introduction of tert-butyl groups. Although this chemical modification does not much change the ionization potential, small threshold voltage and stability over several months are attained together with the improved mobility, probably due to some kind of passivation effect of the bulky tert-butyl groups. In contrast, the systematic fluorine substitution rapidly diminishes the transistor performance. There are two kinds of herringbone structures with much different dihedral angles of about 50° and 130°, and the tert-butyl compound falls into the former category.

Organic superconductors with an incommensurate anion structure.

Superconducting incommensurate organic composite crystals based on the methylenedithio-tetraselenafulvalene (MDT-TSF) series donors, where the energy band filling deviates from the usual 3/4-filled, are reviewed. The incommensurate anion potential reconstructs the Fermi surface for both (MDT-TSF)(AuI2)0.436 and (MDT-ST)(I3)0.417 neither by the fundamental anion periodicity q nor by 2 q , but by 3 q , where MDT-ST is 5H-2-(1,3-dithiol-2-ylidene)-1,3-diselena-4,6-dithiapentalene, and q is the reciprocal lattice vector of the anion lattice. The selection rule of the reconstructing vectors is associated with the magnitude of the incommensurate potential. The considerably large interlayer transfer integral and three-dimensional superconducting properties are due to the direct donor-donor interactions coming from the characteristic corrugated conducting sheet structure. The materials with high superconducting transition temperature, Tc, have large ratios of the observed cyclotron masses to the bare ones, which indicates that the strength of the many-body effect is the major determinant of Tc. (MDT-TS)(AuI2)0.441 shows a metal-insulator transition at TMI=50 K, where MDT-TS is 5H-2-(1,3-diselenol-2-ylidene)-1,3,4,6-tetrathiapentalene, and the insulating phase is an antiferromagnet with a high Néel temperature (TN=50 K) and a high spin-flop field (Bsf=6.9 T). There is a possibility that this material is an incommensurate Mott insulator. Hydrostatic pressure suppresses the insulating state and induces superconductivity at Tc=3.2 K above 1.05 GPa, where Tc rises to the maximum, Tcmax=4.9 K at 1.27 GPa. This compound shows a usual temperature-pressure phase diagram, in which the superconducting phase borders on the antiferromagnetic insulating phase, despite the unusual band filling.

Giant nonlinear conductivity and spontaneous current oscillation in an incommensurate organic superconductor.

Remarkable nonlinear conductivity is observed in the organic conductor (MDT-TS)(I2Br)_{0.420} (MDT-TS: 5H-2-(1,3-diselenol-2-ylidene)-1,3,4,6-tetrathiapentalene) below the metal-insulator transition at 30 K. This compound is characterized by the incommensurate donor and anion columns, and the non-Ohmic behavior is associated with the collective excitation at the antiferromagnetic insulating state. We have observed spontaneous current oscillation analogous to the organic thyristor found in the theta-phase organic conductor.

Current-induced metallic state in an organic (EDT-TSF)2GaCl4 conductor.

A newly prepared organic conductor, (EDT-TSF)(2)GaCl(4), shows considerable nonlinear conductance in the insulating state below 20 K, and a metallic state is restored by the application of moderate currents. This conductor has a stacking structure with a quasi-one-dimensional open Fermi surface, similar to the sulfur analogues. Since the interchain interaction is enhanced by the selenium substitution, the static magnetic susceptibility as well as ESR does not show any anomaly around 20 K, and low-temperature X-ray investigation does not show any extra spots. Isostructural (EDT-TSF)(2)FeCl(4) shows similar conducting properties, although the magnetic interaction of the anion is weak. Like this, nonlinear conductance is a versatile tool to restore a metallic state when the metal-insulator transition is almost suppressed.

A new organic superconductor beta-(meso-DMBEDT-TTF)2PF6.

A newly synthesized donor meso-DMBEDT-TTF [DMBEDT-TTF = 2-(5,6-dihydro-1,3-dithiolo[4,5-b][1,4]dithiin-2-ylidene)-5,6-dihydro-5,6-dimethyl-1,3-dithiolo[4,5-b][1,4]dithiin] afforded a superconducting salt beta-(meso-DMBEDT-TTF)2PF6, with a transition temperature at 4.3 K (onset) under a hydrostatic pressure of 4.0 kbar.

Ferromagnetic anomaly associated with the antiferromagnetic transitions in (donor)[Ni(mnt)2]-type charge-transfer salts.

Three newly prepared [Ni(mnt)2] complexes, (HMTTF)[Ni(mnt)2], (ChSTF)[Ni(mnt)2], and (DBTTF)2[Ni(mnt)2], are reported (DBTTF = dibenzotetrathiafulvalene, ChSTF = 2,3-cyclohexylenedithio-1,4-dithia-5,8-diselanafulvalene, HMTTF = bis(trimethylene)-tetrathiafulvalene, and mnt = maleonitrile dithiolate). The former two compounds have usual DA-type (D = donor, A = acceptor) mixed stacks, whereas the DBTTF complex has DDDDAA-type 6-fold columns. These compounds are electrical insulators, but the HMTTF and ChSTF complexes exhibit chiT minima at 16 and 55 K, respectively, followed by chiT peaks at 8 and 16 K. Below these temperatures the ESR signal disappears, indicating antiferromagnetic transitions. The origin of the ferromagnetic interaction is explained either from the difference of the g values between the donor and the anion or from the intrinsic ferromagnetic interaction of the [Ni(mnt)2] anions.