Epitaxy at Non-60° Angles: An STM Study of 7CB on HOPG

Studies of homologues are often useful for correlating material properties to molecular structure. The liquid crystal 4-heptyl-4‘-cyanobiphenyl (7CB) has received very little study, although the homologous 8CB has been studied extensively by STM. Here we report several unusual properties of 7CB monolayers on graphite, particularly the fact that angles between contacting domains of this liquid crystal were observed to favor angles of 17° rather than the expected 60° angles. This and other observations, coupled with an experimentally determined unit cell of A = 3.85 ± 0.37 nm, B = 1.10 ± 0.10 nm, and ϑ AB = 92 ± 6°, lead us to propose an unusual monolayer structure for 7CB, where only half of the molecules make the expected alkyl−graphite contact in any domain. The 7CB monolayer shows great complexity, including chirality effects and domain−domain interactions.