OVERLAYERS OF ORGANIC-MOLECULES ON GRAPHITE STUDIED BY STM

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