Characterization of ultra-thin polymer films by polarization modulation FTIR spectroscopy

Self assembly monolayers of octadecyltrichlorosilane Cl3‐Si‐(CH2)17‐CH3 and 17‐cyanopentadecyltrichlorosilane Cl3‐Si‐(CH2)17‐CN on silicon wafers have been prepared by adsorption from solution. The molecular orientation within the monolayers was investigated by using Polarisation Modulation FTIR spectroscopy. Quantitative analysis reveals that both types of silanes – monofunctionalised and bifunctionalised – form highly ordered monolayers. A high degree of ordering as well as a small tilt angel of the molecular backbones with respect to the surface normal are indicated by the strength of the Si‐O‐Si stretching modes and the weakness of the CH2 stretching modes. The decomposition of the terminal nitrile group of the substituted silane into a carboxyl group could be identified. The decomposition is caused by a high local HCl concentration, which develops upon binding of 17‐cyanopentadecyltrichlorosilane to the OH groups of the silicon surface.