Abstract

To understand the intrinsic electrical properties of conjugated polymers in general and of oligothiophenes in particular, ultra-pure and highly ordered thin films or single crystals are required. In this context, studies on thin films of quinquethiophene (α5T) prepared by evaporation in UHV were performed. As substrates insulating and conducting oxides such as different single crystal surfaces of TiO₂ were chosen. The latter are particularly interesting because TiO₂ can be doped to be n-conducting. These substrates may therefore be used for hybrid inorganic–organic thin film pn-diodes. Here, we report on SFM-measurements under ambient conditions, which show that the deposition of α5T from a Knudsen cell under UHV conditions leads to cluster growth on TiO₂(011) and to ordered layers on undoped TiO₂(001). On doped TiO₂(001) the growth mode is largely influenced by the surface roughness, i.e. the presence of pinning centers. It could be shown that the α5T molecules on TiO₂(001) are oriented with their long axis mainly perpendicular to the substrate plane and form two complete monolayers (Frank–Van der Merwe growth mode) before the growth mode changes to a simultaneous multilayer growth, in line with earlier experiments using other oxidic substrates.