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'-CyanoBiphenyl (8CB) on graphite and beta-Nb3I8 were studied by Scanning Tunneling Microscopy (STM), and the molecular arrangements at the liquid-solid interface were examined. Large-scale STM images show that the adsorbates possess complex multilayered structures, and that molecular ordering at the liquid-solid interfaces occurs primarily in the immediate 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 topmost overlayer. The STM images of the adsorbates on graphite show that the atomically flat surface of graphite leads organic molecules to form 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 cycloalkanes cannot assemble on the grooved surface due to a mismatch between the molecular shape and surface topography. The layers of 6CB and 8CB adsorbed on beta-Nb3I8 exhibit two types of domains, which may be related to how the grooves of the beta-Nb3I8 surface are occupied by the organic molecules. The STM images of decanol adsorbed on beta-Nb3I8 show two domains of different brightness. The relative brightness of these domains switches reversibly as the gap resistance is changed in the region around - 60 MOMEGA.