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

Abstract

The adsorption, assembly, and growth mechanism of the optoelectronic titanyl phthalocyanine (TiOPc) molecules on Au(111) were carefully investigated by low-temperature and highresolution 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.

Evolution of TiOPc Films on Au(111): From Seed Layer to Crystalline Multilayers

“…Figure 2) and looks similar to that observed by Zhu et al, which they denoted as domain B. 22 We can confirm, that this phase appears to be thermally metastable, as it disappears upon annealing at 420− 450 K, yielding again the long-range ordered film structure that resembles those prepared at 450 K.…”

“…31 Using STM, Zhu et al reported a particular phase for TiOPc bilayer films at RT on Au(111), characterized by a molecular inclination of 30°according to their analysis. 22 Since such a structure could not be confirmed by our results and appears quite unusual; we reexamined their data. Using proper values for the van-der-Waals dimensions of the molecular size (instead of distances between diametrical carbon ring centers), we found significant inconsistency in their analysis (see Figure S9), which challenges their interpretation.…”

“…Interestingly, that micrograph of the bilayer film prepared at elevated temperature shows great similarity to the I-domain reported before by Zhu et al for TiOPc bilayers grown at room temperature. 22 As shown in Figure 2b the molecular appearance sometimes differs notably from the characteristic cloverleaf shape, which can hamper a clear identification of individual molecules and their lateral arrangement in the films. In our case, the presence of characteristic cloverleaf-shaped vacancies allows an unambiguous identification of the molecules and their lateral arrangement, which is rather similar to the one discussed before.…”

“…In the literature, there is an ongoing discussion if the TiOPc molecules adsorb in a different orientation when TiOPc is deposited at room temperature (RT). 22 On Ag(111) it was found that they still adopt a lying orientation, while showing only short-range order, when deposited at RT. 41 In order to investigate if this finding is also true on Au(111), where the interaction with the metal surface is supposed to be weaker, we have analyzed TiOPc films that were prepared at RT.…”

Long-Range Ordered Structures of Corannulene Governed by Electrostatic Repulsion and Surface-State Mediation

“…Construction of two-dimensional (2D) structures on surfaces has attracted great attention due to their unique properties and robust preparation methodology. Most 2D materials can be achieved by short-range interactions which typically function within a short distance of less than 5 Å, including covalent bonds, − ionic bonds, halogen bonds, − hydrogen bonds, dispersion force, − metal-coordinated interaction, and organic-ionic compound interaction . Among all these interactions, the dispersion force is very weak and decreases rapidly with separation which is therefore taken as a short-range interaction on the surface.…”