Ultra-high resolution imaging of thin films and single strands of polythiophene using atomic force microscopy

Abstract

Real-space images of polymers with sub-molecular resolution could provide valuable insights into the relationship between morphology and functionality of polymer optoelectronic devices, but their acquisition is problematic due to perceived limitations in atomic force microscopy (AFM). We show that individual thiophene units and the lattice of semicrystalline spin-coated films of polythiophenes (PTs) may be resolved using AFM under ambient conditions through the low-amplitude (≤ 1 nm) excitation of higher eigenmodes of a cantilever. PT strands are adsorbed on hexagonal boron nitride near-parallel to the surface in islands with lateral dimensions ~10 nm. On the surface of a spin-coated PT thin film, in which the thiophene groups are perpendicular to the interface, we resolve terminal CH ₃-groups in a square arrangement with a lattice constant 0.55 nm from which we can identify abrupt boundaries and also regions with more slowly varying disorder, which allow comparison with proposed models of PT domains.

Abstract

Semiconducting polymers are widely used in optoelectronic devices, in which their microstructure informs function. Here, the authors are able to resolve the molecular and sub-molecular ordering of polythiophene strands and thin films using atomic force microscopy, a significant step towards correlating polymer structure with device performance.