Influence of Substrate Roughness on Molecular Orientation in Copper Phthalocyanine Thin Films

One strategy to control the molecular orientation on a metallic substrate is changing the substrate roughness. Planar small molecules, like copper phthalocyanine, can align on gold surfaces in two ways: either standing up, where the central ions of the molecules interact strongly with each other, or lying down, where the interaction with the substrate is more dominant. The roughness of the gold surface plays an important part in the molecular stacking. Here, we prepared thin films of gold with varying surface roughness and deposited copper phthalocyanine concurrently on all samples. The molecular arrangements were measured with x-ray diffraction and the roughness was analyzed from atomic force microscopy images. The emergence of the 27.7° diffraction peak and the reduction in height of the 6.8° peak with varying gold thickness indicate that the molecular orientation in thin films is influenced by surface roughness.