Precursor Structures for Polymorphic Titanyl Phthalocyanine Crystal Phases on Au(111): A High-Resolution STM Study

The adsorption, assembly, and growth mechanism of the optoelectronic titanyl phthalocyanine (TiOPc) molecules on Au(111) were carefully investigated by low-temperature and high-resolution scanning tunneling microscopy (STM). Three domains, i.e., Y, I, and II, were formed on Au(111) with its coverage increased up to six monolayers (MLs), as structurally scrutinized at the submolecular level. The TiOPc molecules were adsorbed with their Pc planes parallel to the substrate surface in domains Y and I, while the Pc planes tilted against the surface with two lobes pointed toward the vacuum in domain II. A growth mode transition from layer-by-layer to bilayer-by-bilayer was experimentally observed in domains Y and I when the TiOPc coverage exceeded 2 ML, while domain II remained unchanged. The stable double-layer island growth, i.e., O-up and O-down in odd- and even-numbered layers, respectively, was maintained as the coverage increased in domains Y and I. Detailed structural analyses based on the submolecular resolution STM imaging indicated that domains Y, I, and II corresponded to polymorphic crystal phases Y-TiOPc, I-TiOPc, and deformed II-TiOPc grown on Au(111), which were frequently missed in previous studies due to the lack of subtle structural details by high-resolution images. Such a structural clarification of the grown TiOPc films should help in the understanding of the relationship between their optoelectronic device performance and polymorphism of the polycrystalline TiOPc thin films.