STM Observation for Self-Assembled Monolayers of Organic Thiols Containing Aliphatic Cyclic Ring or Aromatic Ring as an End Group on Au(111)

Abstract

The discovery of molecular assemblies by the spontaneous adsorption of organic thiols on gold surfaces has proven to be a major advance for chemists and surface scientists in the research fields ranging from biomedicine, wetting, corrosion inhibition, molecular sensors, and nanopatterning.1-3 SAMs prepared from n-alkanethiols and -functionalized alkanethiols have been extensively studied to date. However, there have been a few studies on the formation of cyclic thiol or thioacetate SAMs on gold surfaces. In order to extend the application of SAMs, it is essential to develop and characterize a new SAM system. It is also very important to understand the nature of the self-assembly processes and the SAM structure. In this study, we have characterized various SAMs of organic thiols containing aliphatic cyclic ring or aromatic ring as an end groups on Au(111) using scanning tunneling microscopy. The spontaneous adsorption of cyclohexanethiol containing a flexible cyclic ring on Au(111) in a 1 mM ethanol solution at room temperature leads to long-range two-dimensional ordered SAMs, which can be a (5 2 10)R48 superstructure with a low density surface compared to alkanethiol SAMs with a ( 3 3)R30 structure. Recently, we found that SAMs made by the adsorption of cyclohexylmethanethiol (CHMTs) or cyclohexylethanethiol (CHETs) containing one or two methylene groups between the sulfur headgroups and aliphatic ring exhibit totally different SAM structure. We obtained the first molecular-scale STM image of well-ordered CHMT SAMs formed at solution temperature of -20 °C, and the two-dimensional (2D) ordered SAMs can be described as a (√3ⅹ2)R57° structure. In addition, we observed that benzeneethanethiols having two methylene group between the sulfur headgroups and benzene ring form very uniform SAMs containing a few vacancy islands at 50°C. We found that 2D SAM structure can be easily tuned by modifying the chemical structure of molecular backbone.