1,6-Hexanedithiol Self-Assembled Monolayers on Au(111) Investigated by Electrochemical, Spectroscopic, and Molecular Mechanics Methods

Abstract

1,6-Hexanedithiol (HSC6SH) self-assembled monolayers (SAMs) on Au(111) were formed in a 1 mM ethanolic solution and were investigated by voltammetry in 0.1 M aqueous KOH solution, HSC6SH SAMs on Au(111) were further probed by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), sum frequency generation spectroscopy (SFG), and molecular mechanics calculation (MMC). An identical investigation was performed with the self-assembly of hexanethiol (C6SH) on Au(111) for comparison. Linear sweep voltammograms showed that a full monolayer of thiolate (7.6 (+/- 0.2) x 10(-10) mol(-2)) chemisorbed on Au(111) with both HSC6SH and C6SH, while the peak potential of the electroreduction of the former assembly was more negative than that of the latter. XPS revealed that two different forms Of Sulfur existed at HSC6SH SAMs on Au(111), i.e. a thiolate and a thiol. The STM image could not be resolved with HSC6SH SAMs, while it was atomically well-defined with the C6SH case. SFG results showed that alkyl chains within the HSC6SH SAMs were in an all-trans conformation. Thus the experimental results concluded that densely packed and higly oriented HSC6SH SAMs could be successfully formed on Au(111) with the incubation conditions in the practical Solution. MMC presented that HSC6SH SAMs were more stable than C6SH ones because of the additional end-end interaction among the thiol groups faced up to the air, albeit both SAMs were tilted by 28 degrees from the surface normal.