Role of the substrate thickness for the structural properties of organic-organic hetero structures

F(16)CuPc deposited on pentacene is characterized by the coexistence of two different configurations: F(16)CuPc is found in the standing up phase ("s-configuration") on top of pentacene terraces and in a lying down phase ("l-configuration") at pentacene step edges. By combining AFM and grazing incidence X-ray diffraction we show that the ratio between F(16)CuPc in l- and s-configurations increases with thickness of the pentacene substrate film, demonstrating the role of the pentacene steps as nucleation centers for the F(16)CuPc l-configuration. Experiments performed with ultra-thin pentacene thicknesses disclose that the F(16)CuPc l-configuration does not grow on top of the first and second pentacene layers, pointing to the action of long-range interactions with the substrate. (C) 2007 Elsevier B.V. All rights reserved.