Surface structure of ultrathin smectic films on silicon substrates: pores and islands

We present an atomic force microscopy (AFM) and ellipsometry study of ultrathin smectic films on silicon substrates. By controlling the amount of the liquid-crystal material that is spin coated on the substrate, we are able to prepare films consisting of a defined small number (ranging from 1 to 4) of smectic layers. AFM measurements show that the films possess a specific surface structure with a lateral feature size of a few microns and steplike height variations of 3.3 nm. The height of the steps corresponds to the smectic layer spacing of the material used, indicating that the surface structure is the result of a partial formation of the topmost smectic layer of these films. The pattern of the surface structure either corresponds to isolated islands (regions in which the film thickness is enhanced by one smectic layer) or consists of pores (film thickness decreased by one layer). A smooth surface is only obtained if the amount of the liquid-crystal material is precisely tuned to certain values, indicating the formation of a complete smectic top layer. A well-defined relation exists between the liquid crystal concentration in the spin-coating solution and the obtained structure, enabling the controlled generation of island structures, pore structures, or smooth surfaces. The two-dimensional island or pore structure is stable on the time scale of a few days. Preliminary results concerning the thermal stability are reported. Our study highlights the usefulness of AFM measurements for the study of smectic liquid-crystal surfaces.