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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Self-assembly (SA) of long-chain alkanethiols on copper was studied. Two factors were found to have substantial influence on the SA process:  (i) the chemical reactivity of Cu toward substances present in the adsorption solution, particularly the solvent, and (ii) surface pretreatment, which influences the amount of oxide and the surface morphology. Both factors are less important in the case of SA onto gold because of its chemical inertness. Monolayers of octadecanethiol (C18SH) were adsorbed from different solvents (ethanol, toluene, and bicyclohexyl) at various thiol concentrations onto Cu surfaces pretreated in several different ways. The monolayers were characterized by contact-angle measurements, grazing-incidence Fourier transform infrared spectroscopy, and scanning force microscopy. Ethanol, the most common solvent for alkanethiol SA, is found to have a negative effect on monolayer SA apparently because of its chemical reactivity toward copper. With toluene as a solvent, better oriented and more crystalline monolayers are obtained provided that a higher thiol concentration is used to compensate for the higher solubility of thiols in toluene. Treatment of the Cu surface prior to SA is shown to significantly improve the SA by reducing the amount of surface oxide and the surface corrugation. The effect of the solvent is more critical than surface oxidation; hence, high-quality monolayers are formed in the presence of thin oxide layers on Cu surfaces. Superior C18SH monolayers, in terms of organization and crystallinity, are obtained by SA from toluene onto Cu surfaces sputtered-annealed in high vacuum, even when the Cu surface is subjected to short exposure to air before SA.

Alkylthiol monolayers were assembled in ethanol solutions onto gold surfaces held at positive potentials. The developing monolayer introduces a barrier to electron transfer; hence, measurement of the current corresponding to ethanol oxidation at the applied potential provides a convenient means for real-time monitoring of the self-assembly (SA) process and its completion. Monolayers produced by the new method are formed considerably faster than similar monolayers prepared by the common procedure (no applied voltage). Two other processes which occur under the same applied potential include gold surface oxidation and oxidative desorption of the monolayer, both related to the presence of small amounts of water in the ethanol solution. It is shown that the interplay between the combined processes allows considerable control over the SA process before, during, and after monolayer formation, such as the possibility to perform multiple adsorption−desorption cycles for wettability and surface control. An important consequence of understanding the mechanism of alkylthiol SA onto oxidized gold concerns alkylthiol vs dialkyl disulfide adsorption. While the two types of molecules produce similar monolayers on reduced gold surfaces, a totally different result is obtained with oxidized gold, namely, alkylthiols form compact monolayers whereas dialkyl disulfides do not. This, together with the possibility to determine the extent of gold surface preoxidation, opens the way to rational design of mixed monolayers.