K W Hipps - Kinetically Trapped Two-Component Self-Assembled Adlayer

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      Publication Details (including relevant citation   information):

      Jahanbekam, Abdolreza, Chilukuri, Bhaskar, Mazur, Ursula, Hipps,   K. W. The Journal of Physical Chemistry C  2015 119 (45) 25364-25376

      Abstract: 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.

      Address (URL): http://dx.doi.org/10.1021/acs.jpcc.5b07120