Growth and structure of pentacene films on graphite: Weak adhesion as a key for epitaxial film growth

The microstructure of pentacene films grown on the basal plane of graphite has been investigated. By combining various complementary techniques including scanning tunneling microscopy, atomic force microscopy, x-ray diffraction, thermal desorption spectroscopy, and x-ray absorption spectroscopy the molecular orientation, crystalline structure, and morphology of the films as well as their thermal stability have been characterized in detail as a function of the film thickness. Initial film growth leads to the formation of a commensurate monolayer consisting of flat-lying molecules while upon subsequent deposition epitaxially ordered (022)-oriented pentacene films are formed which adopt the Siegrist phase. The detailed analysis shows that this epitaxial growth of films with an essentially recumbent molecular orientation is brought about by a slight rotation of the molecules in the first layer around their long molecular axis upon deposition of overlying molecular layers. Such a structural modification is unusual and becomes possible by the rather weak adsorption energy on graphite. In contrast, a very different film structure including an upright orientation of molecules even in the first layer is found on nonperfect but rough graphite surfaces leading to the formation of (001)-oriented films which initially reveal the thin-film phase and continue to grow in the Campbell phase of pentacene.