Molecular arrangements in the first monolayer of Cu-phthalocyanine on In₂O₃(111)

Abstract

Well-ordered organic molecular layers on oxide surfaces are key for organic electronics. Using a combination of scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) we probe the structures of copper phthalocyanine (CuPc) on In₂O₃, a model for a prototypical transparent conductive oxide (TCO). These scanning-probe images allow the direct determination of the adsorption site and distortions of the molecules, which are corroborated by DFT calculations. Isolated CuPc molecules adsorb in a flat, slightly tilted geometry in three symmetry-equivalent configurations on the stoichiometric In₂O₃(111) surface. Increasing the coverage leads to densely packed 1D chains oriented along 〈11̄0〉 directions, which dissolve into a highly ordered (2 × 2) superstructure upon increasing the CuPc density to ¾ per surface unit cell. At a coverage of one CuPc per surface unit cell, a densely packed (1 × 1) superstructure fully covers the surface. The molecules still assume the same site and orientation as before, but they partially overlap to accommodate the high packing density, leading to bending of the molecules. These results are compared to the behavior of CoPc on In₂O₃(111). In summary, we demonstrate that a uniform first layer of metal-phthalocyanine molecules can be realized on the In₂O₃(111) surface when using the proper metal atom in the molecule.