Temperature-induced epitaxial growth modes of para-sexiphenyl on Au(111)

We focus on the epitaxial growth of para-sexiphenyl on a Au(111) single-crystal surface. The films are prepared by molecular beam epitaxy at different temperatures (300  K, 330  K, and 430  K), and their thicknesses are 30  nm. X-ray diffraction investigations of these films show that the growth is strongly dependent on the substrate temperature. Lying molecules are always observed and lead to sexiphenyl crystals with their ±(21-3) plane parallel to the surface which corresponds to flat-on molecules with respect to the substrate. The in-plane alignment of the crystallites is determined by pole figure measurements, at which it is shown that it varies with the growth temperature. While standing molecules, which are only observed at 430  K, are aligned arbitrarily, lying molecules lead to preferred in-plane alignments. It is shown that the type of epitaxy changes from a molecule alignment (along distinct gold directions) on samples grown at 430  K and 330  K to an alignment of crystallites on the 300-K film, which is obviously driven by a lattice match. In the latter case the molecule long axes are tilted by 3° to the 11-2 direction of the gold surface. This experimentally observed in-plane alignment could be verified by a lattice mismatch calculation. The excellent agreement between experiment and calculated results reveals that in the latter case the bulk structure of the film is retained down to the interface and that its alignment is affected by the lattice match of the interface surface lattices.

©2006 The American Physical Society