Synthesis of phenacene incorporating tetradecyl chains in the axis positions and its application in field-effect transistors†
Field-effect transistors (FETs) were fabricated using a new type of phenacene molecule, 3,12-ditetradecylphenacene ((C14H29)2-phenacene), and solid gate dielectrics or an electric double layer (EDL) capacitor with an ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate (bmim[PF6])). The new molecule, (C14H29)2-phenacene, was efficiently synthesized via the Mallory photoreaction. Its crystal structure and electronic properties were determined, using X-ray diffraction, scanning tunneling microscopy/spectroscopy (STM and STS), absorption spectroscopy, and photoelectron yield spectroscopy, which showed a monoclinic crystal lattice (space group P21 (no. 4)) and an energy gap of ∼3.0 eV. The STM image clearly showed the molecular structure of (C14H29)2-phenacene, as well as the closed molecular stacking, indicative of a strong fastener effect between alkyl chains. The X-ray diffraction pattern of thin films of (C14H29)2-phenacene formed on a SiO2/Si substrate suggested that the molecule stood on the surface with an inclined angle of 30° with respect to the normal axis of the surface. The FET properties were recorded in two-terminal measurement mode, showing p-channel normally-off characteristics. The averaged values of field-effect mobility, μ, were 1.6(3) cm2 V−1 s−1 for a (C14H29)2-phenacene thin-film FET with a SiO2 gate dielectric and 6(4) × 10−1 cm2 V−1 s−1 for a (C14H29)2-phenacene thin-film EDL FET with bmim[PF6]. Thus, higher FET performance was obtained with an FET using a thin film of (C14H29)2-phenacene compared to parent phenacene. This study could pioneer an avenue for the realization of high-performance FETs through the addition of alkyl chains to phenacene molecules.