Structure at Homeotropic Liquid Crystal Surfaces

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A nematic liquid crystal consists of elongated molecules which have no regularly repeated positions. They do have orientational order in that the long axes of the molecules tend to align parallel. This direction of alignment is called the director. There are other liquid crystal phases such as the smectic A phase which can form at temperatures below the nematic. In the smectic A phase the molecules are arranged in layers which form a regular stack with the long axes perpendicular to the layers. The alignment of the director by surfaces is crucial to the operation of many nematic liquid crystal devices. The ubiquitous twisted nematic device has the director lying parallel to the surface. More recent devices such as those used in TV displays use homeotropic alignment where the director is perpendicular to the surface. The structure of nematic liquid crystals at homeotropic surfaces is the principal topic of this proposalThe structure of a nematic liquid crystal is modified in the region close to a surface because there is a tendency for the molecules to form into smectic layers. This has been observed using grazing incidence X-ray reflection at the air/nematic interface. However the important influence of the substrate interaction with the nematic has not been explored because of limitations to the X-ray method. This proposal is to use neutron reflection which is better suited to solid substrates. Neutron reflection will determine the smectic-like order and give a quantitative measure of the smectic order at the surface. Ellipsometry will quantify any enhancement of the orientational order of the nematic or isotropic phase near the surface. Thus a complete picture of the near surface structure will be obtained. The influence of the interaction with the substrate will be studied by using different alignment layers. The layer forming tendency of the liquid crystal material will also be varied by using molecules with different phase behaviour in order to test theoretical predictions. In addition, the possibility of an anomalous first layer and preferential layer formation by one component in a mixture will be studied to explore how homeotropic alignment may be controlled and adjusted. The same experimental methodology will be used to elucidate the nature of electrically addressed command surfaces. These are surfaces where the orientation of the molecules in one layer is switched electrically and controls the orientation in an adjacent bulk film of nematic.