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 gamma of the molecules and the herringbone angle alpha, which agree with X-ray diffraction data.