Molecular Alignment in Monolayers in LC with Scanning Tunneling Microscopy

We are achieving the systematic observations for the molecular alignments of the monolayers of homologous series in cyanobiphenyle (CB) liquid crystals such as 6CB to 12CB and homologous series in n-alkyloxy-cyanobiphenyle (nOCB) liquid crystals from n=7 to 10 on both molybdenum disulfide (MoS$_2$) and highly oriented pyroritic graphite (HOPG) substrates using a scanning tunneling microscope(STM). For the monophase of 6CB and two phase mixtures from 8CB to 12CB, the STM image of the molecular alignments were obtained on molybdenum disulfide substrates (MoS$_2$). In nCB with n=odd number, only the single row property of the monolayer alignments was observed. The monolayer alignments of nCB with n=even on the other hand showed double row characteristics. Thus all the observed aspects clearly indicated the odd-even effect of carbon numbers in alkyl chains. Futhermore the morphology of the molecular alignments due to the odd-even effect in the monolayers of cyanobiphenyl liquid crystals on HOPG and MoS$_2$ were systematically determined. According to our observation for nOCB, similarly for odd numbers of n of carbon atoms in alkylchains such as 7OCB and 9OCB, the molecular alignments on both substrates (MoS$_2$ and HOPG) are mainly due to the interaction between alkylchains and substrates. For even n, on the other hand, the alignments are basically due to the dipole-dipole interaction between liquid crystal molecules on the substrates. These differeces come from the situation of an end-carbon in an alkylchain. All clearly indicate the odd-even effect. Based on these experimental observation a model for molecular alignments of liquid crystal monolayers has been proposed and confirmed by computer simulation too.



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