Highly Oriented Hexacene Molecules Grown in Thin Films on Cu(110)-(2 x 1)O

Larger acenes, such as hexacene, are increasingly considered as promising materials for applications in optoelectronic devices. We studied electronic interface properties and the molecular orientation of hexacene on Cu(110)-(2 X 1)O using X-ray absorption spectroscopy (XAS) and photoelectron spectroscopy. Interactions between hexacene and the substrate are weak. The detailed investigation of the orientation greatly benefits from a combination of polarization-dependent XAS and angle-resolved photoemission. The angular dependence of valence band features indicates that the molecules grow highly oriented with their long axes parallel to the oxygen rows of the Cu(110)-(2 X 1)O substrate, whereas XAS reveals that the short axis of the molecule is distinctly tilted with respect to the substrate surface. This orientation is maintained up to a film thickness of at least 16 nm, indicating that Cu(110)-(2 X 1)O acts as a template for the hexacene film growth.