SCANNING TUNNELING MICROSCOPE CONTRAST OF PERYLENE-3,4,9,10-TETRACARBOXYLIC-DIANHYDRIDE ON GRAPHITE AND ITS APPLICATION TO THE STUDY OF EPITAXY

Epitaxial films of perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) on graphite (0001) were investigated by scanning tunneling microscopy. Molecular image contrast of PTCDA was found to depend strongly upon the molecular orientation and the position on graphite. In particular, the periodic discrepancy between PTCDA and graphite lattice points results in a modulation of contrast, which can be used to determine the epitaxial relation of PTCDA relative to the substrate accurately. By analyzing this modulation of contrast, we determined two kinds of epitaxial orientation of PTCDA. These orientations have no exact commensurate relation with graphite, but every lattice point of PTCDA lies on a lattice line parallel to the a axis (or b axis) of graphite. This specific feature contributes to decreasing the interfacial energy. The contrast mechanism of adsorbed molecules is also discussed.