We have developed a novel and convenient method of crystal growth in a liquid phase. This method produces directly onto a substrate well-defined polygon organic thin crystals with uniform thickness. The thin crystals are found to be single crystals of high quality. We show various illustrations including thiophene/phenylene co-oligomers and an oligophenylene. The organic crystal transistors based on these crystals showed good device performance. These thin single crystals are expected to be suitably applied to electronic devices.

We grow single crystals of a thiophene/phenylene co-oligomer (TPCO) composed of seven alternating thiophene and phenylene rings (AC'7). These crystals are grown in a liquid phase directly onto silicon substrates covered with a silicon dioxide layer. The grown crystals are well-defined hexagons and are composed of a molecular layered structure along the molecular length. Top-contact field-effect transistors are fabricated by depositing metal electrodes on the AC'7 crystals grown on the substrates. We estimate the field effect mobilities in the transistor device configurations. The mobility data are associated with the crystallographic results of the material. The devices produce a maximum mobility of ∼0.1 cm2·V-1·s-1. This value is higher than that obtained for another TPCO composed of five alternating rings (AC5). The present results indicate that AC'7 crystals are potentially useful for electrical device applications.

We have investigated the laser oscillation and its polarization feature of a thin polygon crystal of a thiophene/phenylene co‐oligomer grown by the novel liquid‐phase method. The emission spectrum from the crystal ca ‐plane (along the b‐axis) narrows with increasing energy of the pulsed laser excitation, which is incident perpendicularly to the crystal plane (the ab ‐plane). The multi‐mode laser oscillation was observed around 515 nm with the excitation over 5 mJ/cm2 per pulse. A mode separation and a full width at half maximum of the peak were estimated to be ∼0.2 nm and ∼65–85 pm, respectively. The laser emission was confirmed to be strongly polarized along the c‐axis. Its polarization ratio of the c‐axis to the a‐axis was estimated to be ∼550. This large ratio is due to the upright configuration of the molecular long axis that is almost perpendicular to the ab‐plane. These results indicate that the present polygon thin crystal is suitable for laser device application. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We have improved performance of organic field-effect transistors (OFETs) composed of organic nanomolecular single crystals of a thiophene/phenylene co-oligomer. A poly(tetrafluoroethylene) thin layer was applied with friction-transfer technique to an insulator layer of silicon dioxide covering a silicon substrate. The crystals were grown in a liquid phase on the friction-transferred substrate such that the bottom-contact device was completed through depositing the crystals in firm contact with the premade metal electrodes. This technique ensures an excellent electrical contact between the crystal and the electrodes. The device shows the carrier mobility up to 0.26 cm2/Vs. The linear increase in the drain currents is clearly noted around the origin of the drain current-drain voltage action diagram. Thus we have achieved a high performance with the OFETs whose fabrication is based upon the friction-transfer technique.

Fig. 2 Polarizing micrographs of an AC'7 crystal. These are taken under (a) the extinction position and (b) the diagonal position of the crossed Nicols. The white dotted line is a guide to eye.