DEPENDENCE OF STRUCTURAL ORDER OF 4-N-ALKYL-4'-CYANOBIPHENYL LAYERS ON THE NATURE OF THE SUBSTRATE - GRAPHITE AND BETA-NB3I8

Comparative scanning tunneling microscopy (STM) studies of 4-n-hexyl-4 '-cyanobiphenyl (6CB) and 4-n-octyl-4'-cyanobiphenyl (8CB) layers on g raphite and on beta-Nb3I8 demonstrate the dependence of the adsorbate structure on the substrate nature. Whereas areas with differently orie nted linear features assigned to crystal-like arrays of extended nCB m olecules are formed on graphite, domains of two types-including one wi th a large hexagon structure (a = 13 - 14 nm) -were observed in the ST M images of 6CB and 8CB layers on iodide surface of the beta-Nb3I8 cry stal. The orientations of the hexagon- and triangle-shaped domains of these types were different with respect to the crystallographic lattic e of the substrate. These findings were explained by the formation of two orientational phases of n-alkyl-cyanobiphenyls. The phases were in dependent of the temperature in the range from + 15 to + 50-degrees-C, where transitions in the bulk state occur. The molecular-scale STM im ages of 6CB and 8CB adsorbates on beta-Nb3I8 also reveal periodic feat ures, which were assigned to moire patterns associated with a superpos ition of the adsorbates and substrate lattices.