Scanning tunneling microscope investigations of organic heterostructures prepared by a combination of self-assembly and molecular beam epitaxy

We report the realization of organic-organic heteroepitaxy by combining liq uid-phase self-assembly with ultrahigh vacuum (UHV) gas-phase molecular bea m epitaxy. As a model system, we have used self-assembled monolayers (SAMs) prepared by exposing an Au(111)-mica substrate to a dilute solution of dec anethiol in ethanol, with subsequent evaporation of 3,4,9,10-perylene-tetra carboxylic-dianhydride (PTCDA) dye molecules. The well-known ( root 3 x roo t 3)R30 degrees superstructure of almost upright standing molecules after c hemisorption is replaced by the 11.5 x root 3 (33.2 Angstrom) pin-stripe ph ase with flat lying molecules when the samples are annealed in UHV and cove rage decreases. The deposition of PTCDA induces reordering and displacement in the decanethiol SAM. Additional to the 33.2 Angstrom periodicity, the p reviously reported 22 Angstrom thiol stripe phase can be observed by scanni ng tunneling microscopy. Several PTCDA structures are observed: single and double rows of PTCDA that grow along decanethiol stripes on top of the Aat- lying alkane chain groups, as well as densely packed PTCDA monolayer domain s embedded into the thiol layer. These exhibit the well-known herringbone s tructure or a novel square lattice structure. We have also investigated thi cker PTCDA islands and observed molecular resolution for a thickness of sev eral monolayers. The structure of the three-dimensional islands can be iden tified as the alpha bulk modification. (C) 2000 Elsevier Science B.V. All r ights reserved.