Organization of cyanobiphenyl liquid crystal molecules in prewetting filmsspreading on silicon wafers

We observe prewetting films of 8CB (4'-n-octyl-3-cyanobiphenyl) spreading a t room temperature on silicon wafers by ellipsometry and X-ray reflectivity . Ellipsometry indicates the formation of a nondense monolayer spreading in front of a 45-Angstrom-thick film. X-ray reflectivity, performed using a r ibbon geometry for the liquid crystal (LC) reservoir, allows us to determin e the organization of the 8CB molecules in the homogenous film. It consists of a trilayer stacking with a smecticlike bilayer standing above a polar m onolayer with tilted molecules. We show that the thickness of the bilayer i s equal to the smectic periodicity in the bulk material and that the tilt a ngle of the molecules in contact with the solid surface is close to 60 degr ees, in good agreement with second-harmonic generation studies reported by other groups. Such organization can be precisely determined using X-ray ref lectivity because it induces a modulation of the electron density along the normal to the surface, Furthermore, a study of the ellispometric profile o f a drop heated in the nematic phase, where He observe a complete spreading of the LC, shows the complex structuration of the LC close to the solid in terface. In particular, the spreading behavior of the trilayer compared to the subsequent smecticlike bilayers indicates the existence of specific int eraction between the trilayer and silicon wafer.