IR spectroscopy evidence for a substrate-dependent organization of sexithiophene thin films vacuum evaporated onto SiH/Si and SiO₂/Si

The orientation of sexithiophene (6T) thin films vacuum deposited on SiO₂/Si and SiH/Si is studied by IR spectroscopy. IR dichroic spectra in the (C=C) and (C-H) stretching region show a strong dependence on the nature of the substrate. With SiO₂/Si, a large proportion (70-80%) of molecules is nearly perpendicular to the substrate plane. Annealing, or deposition on a heated substrate, increases this proportion up to 90%. By contrast, around 60% of the molecules lie on the SiH/Si surface. IR calculations allowed us to determine the proportion of the perpendicular molecules, and also the relation between the tilt angle γ of the molecules and the herringbone angle α, which agree with X-ray diffraction data.