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 liquid-phase self-assembly with ultrahigh vacuum (UHV) gas-phase molecular beam 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 decanethiol in ethanol, with subsequent evaporation of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) dye molecules. The well-known (3×3)R30° superstructure of almost upright standing molecules after chemisorption is replaced by the 11.5×3 (33.2 Å) pin-stripe phase with flat lying molecules when the samples are annealed in UHV and coverage decreases. The deposition of PTCDA induces reordering and displacement in the decanethiol SAM. Additional to the 33.2 Å periodicity, the previously reported 22 Å thiol stripe phase can be observed by scanning tunneling microscopy. Several PTCDA structures are observed: single and double rows of PTCDA that grow along decanethiol stripes on top of the flat-lying alkane chain groups, as well as densely packed PTCDA monolayer domains embedded into the thiol layer. These exhibit the well-known herringbone structure or a novel square lattice structure. We have also investigated thicker PTCDA islands and observed molecular resolution for a thickness of several monolayers. The structure of the three-dimensional islands can be identified as the α bulk modification.