Effect of Solution Temperature on the Structure of Thioacetyl-terminated Tolane Self-assembled Monolayers on Au(111)

Abstract

Self-assembled monolayers (SAMs) formed by organosulfur compounds on metal surfaces offer a simple and powerful route for the fabrication of functional ultrathin films that can be employed in various technical applications such as corrosion inhibition, chemical sensors, biosensors, nanopatterning, and molecular electronic devices. Recently, SAMs derived from conjugated aromatic thiols on gold have attracted much attention as a result of their interesting electrical and optical properties as well as their potential for use in molecular electronics applications. Oligo(phenylene ethylene) (OPE) molecules have been considered as an extremely promising molecular system for device applications because they have a π-conjugated and rod-like molecular structure. The formation of OPE SAMs and their electrical properties have been extensively characterized by various surface characterization tools. Thioacetyl-protected conjugated molecules with a high chemical stability have been frequently used for electronic device applications. It has been reported that these molecules can form chemisorbed SAMs on gold in acidor base-catalyzed mediums or without any catalyzed medium. Prior to the development of SAM-based devices, it is essential to understand the formation and structure of SAMs on gold. However, although several macroscopic measurements for SAMs formed by thioacetyl-terminated molecules have been carried out to gain insight into SAM formation, there have been only a few nanometer-scale reports on these SAMs. Among these previous reports, one study showed that the conductance switching behavior in single molecules is due to conformational changes of molecular backbones, rather than electrostatic effects of charge transfer. Therefore, for SAM-based device applications, it is very important to control the two-dimensional structure of conjugated SAMs as well as to obtain high-quality SAMs. As far as processing conditions go, it is well known that the thermal annealing of pre-covered alkanethiol SAMs at 70-100 oC results in the formation of large ordered domains and healing of vacancy islands (VIs). It was also found that alkanethiol SAMs with large well-ordered domains and few VIs can be formed by liquid and vapor phase deposition methods at high temperature as a one-step process to obtain high quality SAMs. Tolanemethylthioacetate (TMTA) with a π-conjugated, rigid molecular backbone is one of interesting molecules for electronic device application. Figure 1 shows the structural formula of TMTA molecule. In this study, to obtain highquality ordered TMTA SAMs on Au(111), we examine the effect of solution temperature on the formation and structure of TMTA SAMs using STM. We report herein the first STM results showing the phase transition from the disordered phase to the ordered phase of TMTA SAMs as solution temperature increased. TMTA was synthesized by modifying a previously reported method. Au(111) substrates on mica were prepared by vacuum deposition as previously described. The SAMs were prepared by immersing Au(111) substrates in 0.5 mM N,N'-dimethylformamide (DMF) solutions of TMTA at room temperature, 50 °C, and 80 °C for 24 h. STM measurements were carried out using a NanoScope E with a commercial Pt/ Ir (80:20) tip under ambient conditions. The STM images in Figure 2 show the surface structures of TMTA SAMs on Au(111) formed after immersion as a function of solution temperature. Clearly, the solution temperature markedly affects the formation and structure of TMTA SAMs on Au(111). Although acetyl-protected TMTA molecules on Au(111) form chemisorbed SAMs via the spontaneous deprotection of acetyl group during the adsorption of TMTA in DMF solution, TMTA molecules form disordered phases containing partially ordered domains and molecular aggregates at room temperature, as shown in Figure 2a. Differently from the formation of well-ordered SAMs by alkanethiols at room temperature, TMTA molecules do not form highly ordered SAMs. The unusual structural characteristics of TMTA SAMs can be ascribed to the lower chemical activity of sulfur headgroups against the gold atoms, which is due to the existence of an electron withdrawing acetyl group attached to the sulfur atom. In contrast, more ordered SAMs with a uniform surface morphology were formed in the medium-temperature solution of 50 °C, as shown in the STM image of Figure 2b. We observed a large number of