jp5b07120_si_001.pdf (6.46 MB)
Kinetically Trapped Two-Component Self-Assembled Adlayer
journal contribution
posted on 2015-11-12, 00:00 authored by Abdolreza Jahanbekam, Bhaskar Chilukuri, Ursula Mazur, K. W. HippsA 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.