SCANNING-TUNNELING-MICROSCOPY STUDY OF MOLECULAR ORDER AT LIQUID-SOLID INTERFACES

Adsorbates of normal alkane C36H74, cycloalkanes (CH2)48 and (CH2)72, decanol C10H21OH, 4-hexyl-4'-CyanoBiphenyl (6CB) and 4-octyl-4'-CyanoB iphenyl (8CB) on graphite and beta-Nb3I8 were studied by Scanning Tunn eling Microscopy (STM), and the molecular arrangements at the liquid-s olid interface were examined. Large-scale STM images show that the ads orbates possess complex multilayered structures, and that molecular or dering at the liquid-solid interfaces occurs primarily in the immediat e vicinity of the substrate. Molecular-scale STM images are primarily determined by the electronic contributions of the most protruded atoms of the topmost overlayer. The underlying overlayers and the substrate affect the images indirectly by perturbing the topography of the topm ost overlayer. The STM images of the adsorbates on graphite show that the atomically flat surface of graphite leads organic molecules to for m lamella-like structures, while on the grooved surface of beta-Nb3I8, long chain-like molecules are trapped in the grooves. We were unable to image the cycloalkanes on beta-Nb3I8, which suggests that the cyclo alkanes cannot assemble on the grooved surface due to a mismatch betwe en the molecular shape and surface topography. The layers of 6CB and 8 CB adsorbed on beta-Nb3I8 exhibit two types of domains, which may be r elated to how the grooves of the beta-Nb3I8 surface are occupied by th e organic molecules. The STM images of decanol adsorbed on beta-Nb3I8 show two domains of different brightness. The relative brightness of t hese domains switches reversibly as the gap resistance is changed in t he region around - 60 MOMEGA.