Ordered structures of a nonplanar derivative of PTCDI on Ag(110): STM and LEED investigations

The adsorption behavior of large, nonplanar molecules on single silver (110) crystals has been studied by means of scanning tunneling microscopy and low-energy electron diffraction. The chosen adsorbate perylene-3,4.9,10 -tetra-carboxylic-diimide-di(2,6-isopropylphenyl) is a derivative of the organic semiconductor PTCDI: The chosen adsorbate [perylene-3,4)8,10-tetra-carboxylic-diimide-di(2,6-isopropy(phenyl] is a derivative of the organic semiconductor PTCDI. Two additional isopropylphenyl rings are antisymmetrically oriented perpendicularly to the perylene core, which gives the molecule its three-dimensional structure. The adsorbate exhibits high mobility on the Ag(110) surface at room temperature. In the submonolayer regime well-ordered structures have been observed. Moreover, two different structures have been distinguished in the monolayer regime depending on the preparation procedure. The first is a coincidence structure whereby the molecules (relative to the perylene core) lie nearly flat on the surface. The second one is a closer-packed commensurate structure where the molecules form stacks with a much larger tilt of their perylene core. For the second, more densely packed structure the overlap of flat-lying molecular functional groups is noted. The differences of submolecular image contrast in the two structures are considered to be a consequence of the different tilt angles of the adsorbed molecules.