Molecular Arrangements in the First Monolayer of Cu-Phthalocyanine on In$_2$O$_3$(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 In$_2$O$_3$, 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 In$_2$O$_3$(111) surface. Increasing the coverage leads to densely-packed 1D chains oriented along $\langle1\bar{1}0\rangle$ directions, which dissolve into a highly ordered (2$\times$2) superstructure upon increasing the CuPc density to 3/4 per surface unit cell. At a coverage of one CuPc per surface unit cell, a densely packed (1$\times$1) superstructure fully covers the surface. The molecules still assume the same site and orientation as before, but they partially overlap to accommodate the high packing density, leading to a bending of the molecules. These results are compared to the behavior of CoPc on In$_2$O$_3$(111). In summary, we demonstrate that a uniform first layer of metal-phthalocyanine molecules can be realized on the In$_2$O$_3$(111) surface when using the proper metal atom in the molecule.