Schottky-barrier formation in conducting polymers

Thin films of poly(3-hexylthiophene) and polypyrrole on highly oriented pyrolytic graphite have been studied with scanning tunneling microscopy. We observed semicrystalline order in the form of micro-islands and parallel strands of polymer. The orientation of these strands is determined by the substrate. Also, a new and until now unreported effect has been observed. The corrugation measured perpendicular to the strands in constant-current mode is different for positive and negative bias voltages. This difference can be attributed to the formation of a Schottky barrier between the metallic tip and the semiconducting polymer.