DISSERTATION

______________________________________________________________________ Introduction
1.1 Thiol self-assembling
The formation of self-assembled monolayers (SAMs) was first reported by Zisman et al. in the
late 1940s 6,7 . Later, in the 1980s, Nuzzo and Allara made initial studies on alkylthiols selfassembly
on gold 8 . Since then, a tremendous number of studies were devoted to understanding
and improving the formation of alkylthiol SAMs for numerous applications 1 – tailoring surface
wetting behavior (hydrophilicity/hydrophobicity), protective coatings, corrosion inhibition and
surface functionalization.
Self-assembling occurs from either a vapor or liquid phase 9 , where head groups assemble close
to the surface, while tail groups orientate away from the surface (Figure 1.1). Self-assembly of
thiols on gold surfaces is characterized by its simplicity of preparation and a high number of
available functional groups, even though it has a limited stability due to the relatively small safe
potential region of the Au-S bond.
Alkylthiols consist of an alkyl chain with a thiol head group at one end, which has a high affinity
towards gold, and a functional group of choice at the other end. Adsorbed thiols lose their
hydrogen atom from the head group and form a strong thiolate-gold bond at the interface. The
formed chains are in a trans-conformation obtaining an angle of 60-70° with respect to the
surface. The sulfur atom is in sp3 hybridization and therefore bound to three Au atoms. Au-S
bond formation occurs through the reduction of alkylthiols to alkylthiolates 10 and the following
equation presents one of the proposed mechanisms for this reaction:
R − SH + Au(0) n → R − S − Au(I) • Au(0) n + 1 2 H 2
(1.1)
Disulfides are believed to chemisorb on gold via S-S bond cleavage:
RS − SH + 2Au → 2RS − Au (1.2)
The thickness of the monolayer is controlled by the length of the alkyl chains. During selfassembly,
alkyl chains align themselves by van der Waals forces. Therefore, longer alkylthiols
result in more stable and densely packed monolayers 7 . Furthermore, longer alkylthiols adsorb
preferentially over shorter ones 11 .
1.1 Thiol self-assembling 3