Covalent attachment of bent-core mesogens to silicon surfaces

Two vinyl-terminated bent core-shaped liquid crystalline molecules that exhibit thermotropic antiferroelectric SmCPA phases have been covalently attached onto a hydrogen-terminated silicon(111) surface. The surface attachment was achieved via a mild procedure from a mesitylene solution, using visible light at room temperature. AFM measurements indicate that a smooth monolayer has been formed. The thickness of the monolayer was evaluated with ellipsometry and X-ray reflectivity. Although the molecules differ in length by four carbon atoms, the thickness of the resulting monolayers was the same. The measured thicknesses correspond quite well with the smectic layer thickness in the bulk liquid crystalline material, suggesting a similar self-organization within the monolayer. From attenuated total reflectance infrared (ATR-IR), which clearly shows the C-H and C-O vibrations, a tilt angle of the mesogens is deduced that also corresponds well with the tilt angle in the liquid crystalline state. X-ray photoelectron spectroscopy (XPS) measurements confirm the high quality of the monolayers, with only marginal silicon oxide formation. The elemental composition and amounts of different O and C atoms deduced from the high-resolution XPS correspond very well with the calculated compositions.