Formation and Characteristic of Functionalized Organic Thiol Self-Assembled Monolayers on Au(111)

Abstract

The formation and surface structure of pentafluorobenzenethiols (PFBT) self-assembled monolayers (SAMs) on Au(111) formed from various experimental conditions were examined by means of scanning tunneling microscopy (STM). Although it is well known that PFBT molecules on metal surfaces do not form ordered SAMs, we clearly revealed for the first time that adsorption of PFBT on Au(111) at 75 °C for 2 h yields long-range, well-ordered self-assembled monolayers having a (2 × 5√13)R30° superlattice. Through STM observation of 3,4-difluorobenzenethiol (3,4-DFBT) and 2,4-difluorobenzenethiol (2,4-DFBT), we have found that electrostatic intermolecular interactions have a strong influence on the formation of 3,4-DFBT and 2,4-DFBT SAMs. In the case of the 3,4-DFBT SAMs, the surface structure and thermal stability were similar with the benzenethiol (BT) SAMs despite of the presence of the fluorinated aromatic ring. On the other hand, because of the stronger electrostatic attraction interaction between the 2,4-DFBT molecules compared the 3,4-DFBT SAMs, 2,4-DFBT SAMs have long-ranged well ordered surface structure having a (√6 × 3√3)R80° superlattice and present the dimer compounds when the 2,4-DFBT SAMs on Au(111) were heated for thermal desorption properties. SAMs formed by the adsorption of 4-fluorobenzenethiol (4-FBT) and 4-fluorobenzenemethanethiol (4-FBMT) on Au(111) were examined to understand the effect of a flexible methylene spacer between the sulfur headgroup and phenyl group and the effect of solution temperature on the formation and structure of the SAMs. Although the adsorption of 4-FBT on Au(111) at room temperature for 24 h generated only disordered phase SAMs containing gold adatom islands, 4-FBT at 75 °C for 2 h formed mixed SAMs: disordered phases and ordered (2 × 12√2)R10° superlattice with a rectangular unit cell containing six adsorbed molecules. On the other hand, SAMs formed from 4-FBMT, with a methylene spacer, at room temperature for 24 h on Au(111) had irregularly ordered phases containing uniformly distributed VIs with lateral dimensions of 2 – 5 nm; SAMs formed from 4-FBMT at 75 °C for 2 h were composed of slightly improved ordered phases and larger VIs with lateral dimensions of 5 – 12 nm as a result of Ostwald ripening. The structural transitions of PCBT SAMs from the mixed phase containing disordered and ordered domains to the uniform ordered domains were observed at 50 ºC depending on immersion time. The ordered packing structures of PCBT SAMs are incommensurate (√3 × √10)R45°, a type of structure that differs from those of PFBT SAMs with a (2 × 5√13)R30 structure. And we also found that the surface structure of PCBT SAMs was strongly affected by the solution concentration. In the case of tetrafluorobenzenethiol (TFBT) SAMs formed at 50 °C, the surface structure was dramatically changed and the quality of molecular ordering was greatly improved compared to that formed at room temperature. High-resolution STM images clearly revealed for the first time that TFBT SAMs formed at 50 °C were composed of highly, 2D ordered domains, which can be described as a (2√3 × 8√2)R45° structure. Varying ratio of vertical and parallel component of SAM dipole moment by 2D structural control of SAMs opens up the new possibility for modification of the electrode work function deposited with SAMs. As the number of aromatic ring was increased from one to two or three (from BT to BPT or TPT), the effective work function of gold was dramatically changed. STM was used to monitor changes in surface structure as the number of aromatic ring was increased from one to three. Even though the BT SAMs have mainly disordered phases, the BPT and TPT SAMs showed similar surface structure each other and contained unique ordered bright rows. Interestingly, BPT SAMs formed after 30 min have unique ordered domains containing well-ordered (3 × 3) R30º structures and bright rows that are connected by small aggregated domains with a periodicity of approximately 10 Å, results that have never been observed for other thiol SAM systems. STM observation of TPT SAMs formed in 0.01 mM ethanol solution at 60 °C showed the change in morphology as function of immersion time. To compare the surface structure and adsorption condition between the normal alkanethiol and PFAT SAMs, the surface structure of hexanethiol and F9HT SAMs were investigated under the high-solution temperature condition by using the STM and XPS. When the immersion time was longer over 2 h, the hexanethiol molecules were began to be desorbed and disordered phases were increased. On the other hand, for F9HT SAMs, the amount of chemisorbed F9HT molecules on Au(111) was increased as the immersion time was increased in spite of the high-temperature conditions. And the at 10 min immersion time, very well ordered row structure was shown, which have not been found ever. Even though the immersion time was increased at room temperature, the surface structure of heptanethiol SAMs was not almost changed. However, when the immersion time of MEET SAMs was increased, the quality of surface structure of SAMs was poor because the solvent molecules affect the surface structure of MEET SAMs such as insertion between the MEET molecules within the MEET SAMs. We clearly revealed for the first time that the adsorption of azide-terminated SAMs on Au(111) prepared in 3 mM ethanol solution for 24 hr represents the densely packed and highly well-ordered monolayers with (√3 × 2)R60° lattice structure, while the SAMs prepared in 1 mM ethanol solution have the row structure and many structural defects compared with that formed by 3 mM solution. Based on this surface structural information of azide-terminated SAMs which were used as a platform for functionalizing surfaces, we confirmed that the surface structure of SAMs have a lot of influence on the chemical reactions proceeded by click reaction (Huisgen 1,3-dipolar cycloadditions) for ferrocene immobilization on the azide-terminated SAMs.