DISSERTATION

______________________________________________________________________ Introduction
Figure 1.1. Self-assembly on an electrode surface.
In the beginning of the self-assembly process, while the density of molecules at the surface is
small, molecules either form a disordered mass or an ordered lying phase at the surface 1 (Figure
1.2, a). When the density increases, molecules start forming ordered three-dimensional
structures. Even though the Au-S bond is reasonably strong, the adsorbed thiols can still move
around on the electrode to heal gaps of exposed gold 12 . This so-called healing process occurs
in a second phase, where thiols slowly reorganize and order on the surface 13-15 (Figure 1.2, b).
Various parameters affect alkylthiol SAM formation such as the architecture and cleanliness of
the gold surface, temperature, thiol concentration, functional groups and solvent composition.
Commonly, organic solvents such as ethanol or DMSO are used for adsorption of alkylthiols
on gold.
Preparation of SAMs is standardly performed by immersing a gold substrate into an alkylthiol
solution. Thus, the adsorption occurs at open circuit potential (OCP) and the kinetics of the
adsorption roughly follows the Langmuir adsorption curve 1,9 . According to the Langmuir
kinetic model, the rate of adsorption is proportional to the free space on the surface:
dθ
dt = k(1 − θ) (1.3)
where θ is the partial coverage and k is the rate constant. Nevertheless, this model makes strong
assumptions such as the homogeneity of the surface (perfectly flat surface) and the absence of
interactions between adjacent molecules. In reality, surface roughness of polycrystalline gold
and thiol island formation affect the SAM formation kinetics.
1.1 Thiol self-assembling 4