Monolayer Structure of Arachidic Acid on Graphite

Abstract

The self-assembly of arachidic acid (C19H39COOH) at the liquid solid interface between 1-phenyloctane (C6H5(CH2)(7)CH3) and highly oriented pyrolytic graphite (HOPG) is studied by scanning tunneling microscopy (STM) to identify the structure of the monomolecular film. We observe the formation of highly ordered domains with molecules oriented in three different orientations compatible with the symmetry of the HOPG substrate, a spontaneous enantiomeric separation of the pro-chiral molecules, and reveal structural details with submolecular resolution. To determine the surface unit cell with an intrinsic calibration to the substrate atomic structure, the intermolecular distance is precisely determined from the analysis of a SIN image exhibiting a moire pattern created by the superposition of current contributions from the molecular structure with contributions from the graphite atomic lattice. The dimensions of the unit cell accommodating two molecules are vertical bar a vertical bar 0.94 nm and vertical bar b vertical bar = 2.83 nm with an angle of 85 degrees between unit cell vectors a and h. The respective molecular arrangement allows hydrogen bonding between carboxylic groups with an unrelaxed O-O bond distance of 0.31 nm.