Surface Structures of Self-Assembled Monolayers Formed by Tolane Molecules on Au(111)

Abstract

Surface structures and electrochemical properties of self-assembled monolayers (SAMs) formed by tolane compounds were examined by scanning tunneling microscopy (STM) and electrochemical quartz crystal microbalance (EQCM). We found that tolane compounds i.e., tolanethioacetate (TTA), tolane disulfide (TDS), and tolane methyl sulfide (TMS), and tolane methyl thioacetate (TMTA), on gold form only liquid-like disordered phases in 0.5 mM solutions of corresponding tolane compounds at room temperature, whereas all these molecules can form ordered SAMs at 50 ºC. This result indicates that two-dimensional ordered tolane SAMs on gold can be achieved by increasing the surface mobility of these molecules.

I. Introduction Self-assembled monolayers (SAMs) formed by conjugated aromatic molecules on gold have drawn much attention as a result of their potential applications in molecular electronics [1,2]. Recently, the conjugated molecules with a thioacetate group were often used for SAM fabrication because aromatic thiols can easily undergo oxidation to form disulfide or other oxidized products in the presence of a small amount of oxygen. The thioacetate group was deprotected in situ to the corresponding thiolate group by reacting with small amount of the acid or base, and then the molecules with a thioacetate group can form ordered self-assembled monolayer (SAM) on a gold surface. In this study, we have characterized organic thin films formed by various tolane molecules having the thioacetate, disulfide, and monosulfide groups at room temperature and 50 ºC, using scanning tunneling microscopy (STM) and electrochemical quartz crystal microbalance (EQCM). II. Experiment Au(111) substrates were prepared by the vacuum evaporation of gold onto freshly cleaved mica, as described previously [3]. In this study, we used three different tolane molecules, i.e., tolanethioacetate (TTA), tolane disulfide (TDS), and tolane methyl sulfide (TMS), and tolane methyl thioacetate (TMTA). The organic thin films were prepared by dipping the Au substrates in a 0.5 mM ethanol solutions of corresponding the tolane compounds at room temperature and 50 ºC. The thin films were examined by STM and EQCM.

III. Results and Discussion It was found that the adsorption of tolane compounds on Au(111) leads to only liquid-like disordered phases at room temperature, whereas these molecules can form ordered SAMs at 50 ºC, as shown in Fig. 1. However, the ordered SAM structures were completely different from each other. It is suggested that such totally different SAM structures may be due to a difference in adsorption ability of sulfur atoms, because the sulfur atoms are attached to different functional groups. As a result, the surface mobility of molecules on Au(111) would be different during the self-assembly processes, resulting in the difference in their final SAMs structures.

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