Preparation of highly ordered quinquethiophene thin films on TiO2 substrates

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 (alpha 5T) 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 alpha 5T 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 alpha 5T 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.