Transition from epitaxial to nonepitaxial ordered monolayers in pyrolyzed 8CB studied by STM

摘要

To design long-lived liquid crystal devices, it is important to understand how liquid crystal properties change as the material degrades. The liquid crystal 4'-octyl-4-cyanobiphenyl (8CB) is known to form well-ordered, epitaxial monolayers on graphite. We have found that adding two oxidation products (to mimic the effects of air oxidation or pyrolysis) changes the surface ordering of 8CB, causing it to form monolayers on graphite which possess good short-range order but only moderate long-range order, indicating the existence of many nonepitaxial domains. Although the unit cell dimensions of domains within these monolayers are identical to the unit cell dimensions of one of the oxidation products, both pure oxidation products were found to form epitaxial monolayers on graphite, as did all binary mixtures of these materials. Thus the loss of epitaxy seen in the 8CB mixtures is a consequence of interactions between all three components (8CB and two oxidation products). The observed behavior of these 8CB mixtures is consistent with segregation of two components at domain walls separating domains of the pure third component. [References: 18]