Abstract
The ability to control the packing orientation in an organic molecular crystal is of great interest in fabricating an organic field effect transistor (OFET) because of the anisotropic nature of charge transport in organic semiconducting materials. The conductance behaviors of highly oriented pentacene nano crystals with their long molecular axes parallel to the Au surface or perpendicular to the organic self-assembled monolayer (SAM) modified Au surface were measured by conductive atomic force microscopy (CAFM). Rectifier current of conduction pathway along pentacene’s □-□ stacking direction was found four orders of magnitude larger than that perpendicular to the □-□ stacking direction. Phase images by electrostatic force microscopy show that pentacene with different orientation has different response to the applied electric field from Au tip. Nanocrystals of p-sexiphenyl or pentacene with high in-plane alignment were obtained by thermally depositing these molecules on a rubbed polymethylene film surface. The structures of the films were characterized by using grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM), reflection absorption IR spectroscopy (RAIRS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The polymethylene chains near the surface were markedly aligned after the rubbing treatment, with the carbon-carbon backbone chains aligned parallel to the rubbing direction. p-Sexiphenyl formed nanometer-scale, rod-like molecular crystals, with their long crystal axes perpendicular to the rubbing direction. Nevertheless, the molecules in the crystals lay “flat” on the rubbed polymethylene surface, with their long molecular axes parallel to the rubbing direction. Large scale alignment of highly oriented molecular crystals can be achieved in this manner. In contrast, pentacene crystals deposited on the rubbed polymethylene at room temperature show a preferred orientation in that the [110] plane aligns perpendicular to the rubbing direction, whereas the [120] plane aligns parallel to the rubbing direction when deposited at 50°C. Oriented growth of polycrystalline rubrene thin film on an oriented pentacene buffer layer was also investigated. The oriented pentacene buffer layer was created by thermal evaporation of pentacene on a rubbed polyvinyl alcohol (PVA) surface. The pentacene layer in turn induced the oriented growth of rubrene crystals upon thermal deposition. With the aligned pentacene/rubrene film as the active layer of a field-effect transistor, anisotropic mobilities were observed. The highest field-effect mobility (0.105 cm2/Vs) was observed along the direction 45□ away from the rubbing direction and was ~four times higher than that for similar device prepared on unrubbed PVA. The direction was consistent with the GIXD observation that a large number of rubrene crystallites are having their [002] axes aligned in this direction.