Self-assembly (SA) of organic monolayers onto gold surfaces has
been known to be influenced by the
surface conditions prior to monolayer adsorption. A new procedure
is introduced for obtaining clean,
reproducible Au surfaces, appropriate for monolayer self-assembly.
The two-step procedure involves (i)
exposure of the Au surface to UV/ozone (or O2 plasma)
treatment and (ii) immersion in pure ethanol. The
organic contaminants present on the (aged) gold surface are oxidized in
the first step to volatile products
such as carbon dioxide and water, followed by the second step where
gold oxide, formed on the Au surface
during the UV/ozone treatment, is reduced (to Au) by reaction of the
oxidized gold with ethanol (the most
frequently used solvent for SA of alkanethiols). It is shown by
scanning force microscopy (SFM)
measurements that gold oxide which is not reduced prior to monolayer SA
is encapsulated by the closely
packed alkanethiol monolayer adsorbed onto the gold oxide. This
enables convenient imaging of the
(otherwise unstable) gold oxide on the surface. The efficiency of
the pretreatment procedure is demonstrated
by complete removal of an octadecanethiol (C18SH) monolayer (a
simulated stubborn contamination) from
a gold surface using the above procedure, followed by reconstitution of
a similar monolayer. Ellipsometry,
contact-angle measurements, and grazing-incidence FTIR spectroscopy
show that the removed and the
reconstituted C18SH monolayers are indistinguishable. The
formation of a high density of small depressions
in such pretreated Au surfaces, as well as in aged (unpretreated) Au
surfaces, is shown by STM imaging
and discussed in light of similar morphologies induced by alkanethiol
self-assembly.
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