Kinetically Trapped Two-Component Self-Assembled Adlayer

A two-component self-assembly process that results in three different compositional phases is observed and explained. Solutions containing various molar ratios of cobalt octaethylporphyrin (CoOEP) and coronene in phenyloctane were brought into contact with a clean Au(111) surface. At no relative concentration was coexistence of the CoOEP and coronene phase observed. Rather, at intermediate concentrations a new 1–1 surface structure (1 coronene to 1 CoOEP) is formed with sharp compositional boundaries. This behavior is unusual in that only weak van der Waals forces, slight variations in surface charge exchange, and steric effects stabilize the 1–1 structure and that it occurs where the solution concentration ratio is an order of magnitude different than the surface concentration. The nature of this 1–1 structure and the transitions between the three phases (pure CoOEP, 1–1, and pure coronene) were studied by variable temperature STM and by density functional theory (DFT). Adsorption energies for CoOEP and coronene, both in their separate phases, and in the 1–1 structure were determined by DFT. The desorption energy into vacuum of CoOEP was found to be ∼1.8 times that of coronene, and the desorption energy of each component in the 1–1 composition was found to be greater than in the corresponding pure monolayer. Measured by energy per nm2, pure CoOEP is predicted to be the most strongly adsorbed, coronene is predicted to be the least strongly adsorbed, and the 1–1 structure holds an intermediate position. By heating the sample to 50 °C it is possible to observe the transformation of the kinetically stabilized 1–1 structure into the pure CoOEP monolayer under the same solution from which it is formed at 22 °C. The existence of these three surface structures is shown to be a kinetic phenomenon rather than due to thermodynamics. We attribute the existence of the three structures at various growth concentrations to changes in nucleation and growth rate with relative impingement rates of each component. A critical element is the fact that one component (CoOEP) is irreversibly adsorbed. The new 1–1 structure is found to have lattice constants of A = (1.73 ± 0.04) nm, B = (1.56 ± 0.04) nm, and α = 90° ± 2° and appears to be commensurate with the Au(111) surface.