Studies of thin smectic C∗ films by X-ray reflectivity

The smectic order in thin and ultra thin films (150–600Å) of the chiral ferroelectric liquid crystal mixture ZLI-3654 is studied using the X-ray reflectivity technique. The spin cast films on various substrates (float glass, Si wafer, polymer coated glass, etc.) order spontaneously with smectic layering parallel to the substrate surface. A simple model which assumes a sinusoidal density modulation can describe well the experimental reflectivity profiles. The X-ray reflectivity provides a method to evaluate the phases of the structure factor. We demonstrate, for the first time, that is possible to extract the molecular tilt angle, α, in ferroelectric liquid crystals from X-ray reflectivity measurements of ultra thin films. The temperature dependence of the tilt angle in the smectic C∗ phase are almost independent of the film thickness (down to ∼200 Å) and are similar to those in the bulk.