Structure formation and dynamics in molecularly thin smectic liquid crystal films

Abstract

This doctoral thesis presents a study of thin films of smectic liquid crystals. This class of material shows the property to form stable films of only a few molecules thickness both on substrates and freely suspended in air. This feature together with the anisotropy of the material offers the possibility to prepare and investigate samples that cannot be formed by other materials. As the dimensions of the films are reduced, the physical properties are expected to change with respect to the observed surface structures and the dynamics in the film. For freely suspended films, the presented work analyses the translational dynamics in films with a thicknesses ranging from 22 down to only 2 smectic layers, using single molecule tracking. For all the studied compounds it was observed that thinner films show a faster diffusion. The measured effect is quantitatively larger than predicted from theory. However, molecular dynamics simulations support the finding of drastic changes for ultrathin films. Within the smectic A phase (i.e. not close to phase transitions), the temperature dependence is observed to show the classical Arrhenius behaviour. Contrary to that, in the vicinity of the phase transition to a smectic phase with in-plane ordering, the diffusion coefficient scales similarly to the one of a glass transition. Also heterogeneous free-standing film are studied concerning the molecular behaviour. It is shown that the molecules have a strong tendency to avoid crossing phase boundaries. For liquid crystal films on substrates, this work presents thin films of 8CB on silicon wafers. Despite the numerous studies of this material-substrate combination, no controlled creation of films with a small number of layers was reported so far. Such homogeneous films are created and studied with respect their inner structure using atomic force microscopy and ellipsometry. The dynamics is analysed using single molecule tracking, showing a slowdown of the diffusion when going to thinner films. These homogeneous films can also be used to create droplets with a volume below 10 femtolitres, which in turn can be used to study smectic layering on the surface with the main volume of the droplet in the nematic phase. It is also shown how the homogeneous films can be employed as a base material for the writing of stable structures on the nanoscale.