新しい有機機能性材料 (ピセン)

ピセンは，ベンゼン環5個がW型に縮環した芳香族炭化水素化合物です。久保園らはピセンが有機電解効果トランジスタの活性層として極めて高い特性を示すことを報告しています。電解効果移動度は1.1 cm²V^-1s^-1と極めて高く，さらに70時間酸素暴露後には，移動度が1.75 cm²V^-1s^-1まで向上し，高性能トランジスタとしての応用が期待されています。また，カリウムを3.3等量注入した際，18 K (-255 °C)で超伝導転移が発現し，これは有機分子の転移温度としては最も高い温度です。今後もピセンを用いた応用研究が期待されます。

[1] “Air-assisted High-performance Field-effect Transistor with Thin Films of Picene”

H. Okamoto, N. Kawasaki, Y. Kaji, Y. Kubozono, A. Fujiwara, M. Yamaji, J. Am. Chem. Soc. 2008, 130, 10470. DOI: 10.1021/ja803291a

A field-effect transistor (FET) with thin films of picene has been fabricated on SiO2 gate dielectric. The FET showed p-channel enhancement-type FET characteristics with the field-effect mobility, μ, of 1.1 cm2 V−1 s−1 and the on−off ratio of >105. This excellent device performance was realized under atmospheric conditions. The μ increased with an increase in temperature, and the FET performance was improved by exposure to air or O2 for a long time. This result implies that this device is an air (O2)-assisted FET. The FET characteristics are discussed on the basis of structural topography and the energy diagram of picene thin films.

[2] “Superconductivity in alkali-metal-doped picene”

R. Mitsuhashi, Y. Suzuki, Y. Yamanari, H. Mitamura, T. Kambe, N. Ikeda, H. Okamoto, A. Fujiwara, M. Yamaji, N. Kawasaki, Y. Maniwa, Y. Kubozono, Nature 2010, 464, 76. DOI: 10.1038/nature08859

Efforts to identify and develop new superconducting materials continue apace, motivated by both fundamental science and the prospects for application. For example, several new superconducting material systems have been developed in the recent past, including calcium-intercalated graphite compounds1, boron-doped diamond2 and—most prominently—iron arsenides such as LaO1–xFxFeAs (ref. 3). In the case of organic superconductors, however, no new material system with a high superconducting transition temperature (Tc) has been discovered in the past decade. Here we report that intercalating an alkali metal into picene, a wide-bandgap semiconducting solid hydrocarbon, produces metallic behaviour and superconductivity. Solid potassium-intercalated picene (Kxpicene) shows Tc values of 7 K and 18 K, depending on the metal content. The drop of magnetization in Kxpicene solids at the transition temperature is sharp (<2 K), similar to the behaviour of Ca-intercalated graphite1. The Tc of 18 K is comparable to that of K-intercalated C60 (ref. 4). This discovery of superconductivity in Kxpicene shows that organic hydrocarbons are promising candidates for improved Tc values.