OVERLAYERS OF ORGANIC-MOLECULES ON GRAPHITE STUDIED BY STM

Using the scanning tunnelling microscope (STM) to investigate molecular materials requires either that the material itself is conductive or that it forms a very thin layer on a conductor. STM images of the molecular material of the latter kind is considered here. Investigations of 1,2-di-(4-octadecyloxyphenyl)-1,2-dibromoethane show that the molecules form well ordered monolayers when adsorbed on graphite from solution. We have developed a Fourier transform technique, similar to that of X-ray crystallography, to determine the crystallographic properties. Two coexisting domains has been found and analysed using this technique. Those domains display close to ortho-rhombic symmetry which is contrary to previous results, and they do not seem to reflect the symmetry of the underlying graphite. Both domains contain one molecule per unit cell but the unit cell of one domain is turned approximately 90-degrees with respect to the other.