Takeshi Terauchi

Carrier generation and electronic properties of a single-component pure organic metal

Metallic conduction generally requires high carrier concentration and wide bandwidth derived from strong orbital interaction between atoms or molecules. These requisites are especially important in organic compounds because a molecule is fundamentally an insulator; only multi-component salts with strong intermolecular interaction-namely, only charge transfer complexes and conducting polymers-have demonstrated intrinsic metallic behaviour. Herein we report a single-component electroactive molecule, zwitterionic tetrathiafulvalene(TTF)-extended dicarboxylate radical (TED), exhibiting metallic conduction even at low temperatures. TED exhibits d.c. conductivities of 530 S cm-1 at 300 K and 1,000 S cm-1 at 50 K with copper-like electronic properties. Spectroscopic and theoretical investigations of the carrier-generation mechanism and the electronic states of this single molecular species reveal a unique electronic structure with a spin-density gradient in the extended TTF moieties that becomes, in itself, a metallic state.

Ionic semiconductor: DC and AC conductivity of anilinium tetrathiafulvalene-2-carboxylate

A single crystal of anilinium tetrathiafulvalene-2-carboxylate exhibits a characteristic electrical conduction; it is a semiconductor with activation-type transport above 200 K; σ(rt) = 0.16 S cm(-1) with an activation energy of 0.11 eV. On the other hand, below 200 K, it does not obey the Arrhenius relation but is conductive even at 4 K with 2.1 × 10(-4) S cm(-1) at a frequency of 2 MHz. Its behavior exhibits strong frequency dependence and suggests a particular conduction coupled with dielectric relaxation, reflecting its ionic nature. The crystal structure of the salt shows that conducting molecules are assembled supramolecularly with multiple nonbonding interactions, such as the hydrogen bond, and the π/π and CH/π interactions. The hydrogen bond and CH/π interactions have a short bond length, which is similar to the charge-assisted-type interaction observed in organometallics.

Room-temperature proton transport and its effect on thermopower in a solid ionic semiconductor, TTFCOONH4

Ammonium proton in a solid ionic semiconductor, TTFCOONH4, is shown to be mobile under anhydrous conditions at room temperature by the hydrogen concentration cell method. Isotope substituted TTFCOOND4 exhibits a 2.2 H/D isotope effect in ion carrier mobility with TTFCOONH4. First-principles calculations reveal that an efficient proton-transfer pathway via low-barrier N⋯H+⋯N hydrogen bonds reduces the activation energy to 0.12 eV, which is quite small and comparable to that reported in a bulk water system. The ac conductivity of TTFCOONH4 and TTFCOOND4 is similar at room temperature, reflecting similar hole carrier concentrations. In sharp contrast, the thermopower exhibits a large isotope effect: TTFCOONH4 shows 260 μV K−1, which is twice as large as that predicted by the hole carrier concentration and the value of TTFCOOND4, with 138 μV K−1. The 1.9 H/D isotope effect in thermopower closely relates to the 2.2 H/D isotope effect in ion carrier mobility. Proton carriers in the temperature gradient enhance thermopower without cancelling out the effect of holes in the solid state owing to possession of the same positive charge.

Corrigendum: Carrier generation and electronic properties of a single-component pure organic metal

Nature Materials 16, 109–114 (2017); published online 10 October 2016; corrected after print 9 August 2017 In the version of this Article originally published, the sign of each x-axis value in Fig. 2d and its inset was incorrect. This has been corrected in the online version and the correct panel isshown here.

Transport properties of single-component organic conductors, TED derivatives

Narrow-gap semiconductors with high conductivity and mobility are an important class of materials for various applications, especially for thermoelectric and optical device applications. Herein, we designed and synthesized novel organic narrow-gap semiconductors, which are modified forms of the main skeleton of a single-component pure organic metal, tetrathiafulvalene-extended dicarboxylate (TED). Molecular design of the TED derivatives with substituent groups on the skeleton led to highly-conducting semiconductors even when powder crystalline samples were used. Their thermopower is greater than that of metallic TED without a substituent group, demonstrating the successful tuning of carrier concentration in the TED system by molecular design. Near-/middle-infrared (IR) diffuse reflectance measurements revealed each band gap, and optical parameters extracted from the spectra evaluated the carrier concentration and mobility of the TED derivatives with fitting calculations on the basis of a Drude–Lorentz dielectric function.