Organization of cyanobiphenyl liquid crystal molecules in prewetting films spreading on silicon wafers

We observe prewetting films of 8CB (4'-n-octyl-3-cyanobiphenyl) spreading at 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 ribbon geometry for the liquid crystal (LC) reservoir, allows us to determine the organization of the 8CB molecules in the homogenous film. It consists of a trilayer stacking with a smecticlike bilayer standing above a polar monolayer with tilted molecules. We show that the thickness of the bilayer is equal to the smectic periodicity in the bulk material and that the tilt angle of the molecules in contact with the solid surface is close to 60 degrees, in good agreement with second-harmonic generation studies reported by other groups. Such organization can be precisely determined using X-ray reflectivity because it induces a modulation of the electron density along the normal to the surface, Furthermore, a study of the ellispometric profile of 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 interface. In particular, the spreading behavior of the trilayer compared to the subsequent smecticlike bilayers indicates the existence of specific interaction between the trilayer and silicon wafer.