Scanning tunneling microscopy investigation of self-assembled poly(3-hexylthiophene) monolayer

Poly(3-hexylthiophene) (P3HT) monolayer has been investigated by scanning tunneling microscopy (STM). The monolayer was dominated by three kinds of ordered structure (I, II, and III), where the thiophene main chains lied parallel to one another, but high resolution STM images revealed that the arrangement of the hexyl side chains was different. In structure I, the hexyl side chains tilted at ~60° with respect to the main chain, and the interchain distance (distance between two parallel neighboring backbones) was ~1.41 nm. In structure II, the interchain distance was significantly larger at ~1.52 nm, and the hexyl side chains were liquid-like. Structure III exhibited similar interchain distance as structure II, but the hexyl side chains were perpendicular to the main chain and were interdigitated. In addition to these ordered domains, individual poly(3-hexylthiophene) chains in various special configurations were observed, and their unfolding into more stable structures was tracked by dynamic STM, which provides evidence that P3HT is a relatively flexible polymer.