Molecular structure of self-assembled bimolecular films on gold and silicon surfaces studied by external reflection infrared spectroscopy

Self-assembled bimolecular films were prepared by coupling homologous n-alkylsiloxane overlayers (CH3(CH2)(n)SiOx, RSiO, n=10, 13-17) to HO-terminated primer layers of HO(CH2)(16)SH (HHDT) on Au(111) and HO(CH2)(11)SiOx (HUDS) on oxidized Si(100). The structure and surface orientation of the hydrocarbon chains in the RSiO overlayers were investigated by external reflection infrared spectroscopy (ERIRS) and ellipsometry, and were compared to the molecular structure of alkanethiolate monolayers (CH3(CH2)(n)S, RS) on Au(111) and RSiO monolayers on oxidized Si(100). Similar structural properties were found for the RSiO overlayers on gold and silicon, despite the different structure of the primer layers HHDT and HUDS. The long-chain RSiO overlayers (n>15) show a highly ordered structure consisting of predominantly all-trans hydrocarbon chains with a uniform, close to perpendicular surface orientation of the chain axes, which is identical to the structure of the corresponding RSiO monolayers, but is markedly different to RS monolayers on Au(111), where the hydrocarbon chains are tilted by similar to 30 degrees with respect to the surface normal. Shorter-chain RSiO overlayers (n<15) exhibit increasing disorder with decreasing chain length caused by a gradual increase of the chain tilt angles associated with a destruction of the all-trans chain geometry, which results in an isotropic, liquid-like film structure for the shortest-chain molecules in this series (n=10). Similar order-disorder transitions also occur in the monolayer films RS/Au and RSiO/Si, whereby the transition region is shifted by at least five CH2 groups toward shorter chains.