Infrared spectroscopic characterization of orientation and order of thin oligothiophene films

Thin films (2–100 nm) of oligothiophenes (ST, 6T) were prepared on semiconductors (ZnSe, Si/SiO₂) by Knudsen evaporation. The orientation and order of these films were determined by FTIR spectroscopy. The dichroism of the (C=C) and (C-H)-stretching vibration demonstrates, that the films are highly orientated with a fixed tilt angle to the surface normal. This tilt angle y can be calculated from the dichroitic ratio of the polarized spectra. The order of these films was derived from the factor group splitting of the γ-C-H-out-of-plane mode. Vapor deposition on a substrate at 380 K results in a crystalline phase. A liquid crystal phase was obtained by evaporating on substrates at room temperature (300 K). In 30–100 nm films of 6T two different liquid crystalline phases were observed. By annealing 5T films the liquid crystal phase changes into the crystalline phase. After annealing 6T only the smectic phase was obtained. The intensity ratios of the factor group splitted bands give the herringbone angle τ.