Large-Ordered Striped Phase of Didodecyl Sulfide Self-Assembled Monolayers on Au(111)

Abstract

Alkanethiol self-assembled monolayers (SAMs) on metal surfaces are key elements for the fabrication of functional organic layers and devices in the broad fields of nanotechnology and biotechnology. However, it was found that alkanethiol SAMs were usually composed of structural defects such as domain boundaries and vacancy islands (VIs), which make them more amenable to oxidation. Compared to alkanethiols, thioethers (RSR') are more robust to oxidation, and their chemical structures with various alkyl chains can be modified readily by a simple synthetic method. Therefore, SAMs prepared using thioethers provide a very useful means for tuning the characteristics of metal surfaces. Despite these advantages, there have only been a limited number of papers involving the formation and structure of thioether SAMs on gold surfaces. It has been revealed that dioctadecyl sulfides (DODS, CH3(CH2)17S(CH2)17CH3) at the initial growth stage form SAMs with striped phases in which the molecular backbones are oriented parallel to the surface, whereas DODS SAMs formed after long immersion have two mixed phases containing closely packed and loosely packed standing-up phases where the molecular backbones are oriented perpendicular to the surface. High-resolution scanning tunneling microscopy (STM) observation revealed that the SAMs of dimethyl sulfides (DMS, CH3SCH3) with the shortest alkyl chains on Cu(111) have a herringbone-like packing structure, whereas dibutyl sulfides (DBS, CH3(CH2)3S(CH2)3CH3) with slightly larger alkyl chains form striped phases. From these results, we assumed that the formation of dialkyl sulfide SAMs strongly depends on van der Waals interactions between alkyl chains. So far, there no data has been reported on the formation and structure of didodecyl sulfides (DDS, CH3(CH2)11S(CH2)11CH3) with medium alkyl chains from a molecular-scale perspective. In this paper, we report the first STM results showing that the adsorption of DDS molecules at 70 oC generating long-range ordered SAMs with a 7.5 × √3 striped phase with VIs-free surfaces. Dodecanethiol (DDT, CH3(CH2)11SH) and DDS were purchased from Tokyo Chemical Industry (Tokyo, Japan). DDT and DDS SAMs were prepared by dipping the Au(111) substrate in 1 mM ethanol solutions of corresponding compounds at room temperature for 24 h. To understand the effect of solution temperature on the formation of DDS SAMs, the SAMs were prepared at 70 C for 1 h. STM images were obtained using a NanoScope E (Veeco, Santa Barbara, CA, USA) with a commercial Pt/Ir (80:20) tip under ambient conditions. The STM images in Figure 1 show remarkable structural differences in the formation of ordered domains and VIs (dark holes) for DDT and DDS SAMs on Au(111) formed after 24 h immersion at room temperature. Figure 1(a) shows a typical packing structure of DDT SAMs with a c(4 × 2) superlattice formed at saturation coverage. DDT SAMs were mainly composed of ordered phases with three domain orientations (Regions A, B, and C) separated by domain boundaries. The proportion of the VI areas to the total surface area was measured to be approximately 8%-12%, values which are similar to those observed from other alkanethiol SAMs. Unlike DDT SAMs, Figure 1(b) shows that the DDS SAMs have two mixed phases: the ordered phase with three domain orientations (Regions A, B, and C) and the disordered phase (Region D). The existence of three domain orientations for DDS SAMs on Au(111) means that the formation of SAMs was influenced by interactions between the sulfide atoms and gold surfaces. On the other hand, DDS SAMs contained few VIs comprising a small (23%) area fraction of the total surface area. The presence of disordered phases and a smaller VI fraction for DDS SAMs can be attributed to the weaker interactions between the sulfide atoms and gold surfaces compared to those between the sulfur atoms of thiols and gold surfaces for DDT SAMs. This finding is consistent with a previous result for the SAMs of DODS with long alkyl chains of 18 carbon units. The ordered domains have a row structure with an inter-row distance of 1.59 ± 0.03 nm, which is nearly half of the entire