Orientation of substituted phthalocyanines on polycrystalline gold: distinguishing between the first layers and thin films

We have studied the molecular orientation of different metal (Mg, Zn) phthalocyanines (Pcs) grown as ultra-thin films on a polycrystalline gold substrate. For 10-20 nm thick films of Pcs with small substituents, a high degree of molecular orientation was observed. Similar to unsubstituted PcCu, the orientation of the molecules is perpendicular to the sample surface. Importantly, however, the molecular orientation is different for low coverages, the molecules lie on the substrate surface. The growth mode is discussed in terms of different molecule-molecule and molecule-substrate interactions.