Self-Assembled Monolayers on Oxidized Metals. 2. Gold Surface Oxidative Pretreatment, Monolayer Properties, and Depression Formation

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.