Ordered structures of tetracene and pentacene on Cu(110) surfaces

The adsorption of tetracene on a Cu(110) surface has been studied with high-resolution energy loss spectroscopy, low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Vibrational spectra confirm that at monolayer (ML) coverage the adsorbed molecules are in a flat-lying geometry with their molecular plane parallel to the substrate. At low coverage, 0.7 ML, LEED shows an oval pattern, which is, related to a slightly disordered surface, while STM indicates molecules in different azimuthal orientations. At saturation coverage, a c(10 x 2) structure is formed, which gradually transfers into a p(5 x 2) structure on annealing at 340 K. STM reveals the molecular arrangement within both these unit cells and clearly indicates that the long molecular axis is aligned along (110). Molecular rows show a long-range wavelike behavior related to the interdigitation of C-H bonds of adjacent molecules. Similar structures, c(12 x 2) and p(6 x 2), are formed for pentacene on Cu(110).