1,6-Hexanedithiol (HSC 6 SH) 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. HSC 6 SH 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 (C 6 SH) on Au(111) for comparison. Linear sweep voltammograms showed that a full monolayer of thiolate (7.6 ((0.2) × 10 -10 mol -2) chemisorbed on Au(111) with both HSC 6 SH and C 6 SH, 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 HSC 6 SH SAMs on Au(111), i.e. a thiolate and a thiol. The STM image could not be resolved with HSC 6 SH SAMs, while it was atomically well-defined with the C 6 SH case. SFG results showed that alkyl chains within the HSC 6 SH SAMs were in an all-trans conformation. Thus the experimental results concluded that densely packed and higly oriented HSC 6 SH SAMs could be successfully formed on Au(111) with the incubation conditions in the practical solution. MMC presented that HSC 6 SH SAMs were more stable than C 6 SH ones because of the additional end-end interaction among the thiol groups faced up to the air, albeit both SAMs were tilted by 28°from the surface normal.

The structures of 1,n-alkanedithiol (n = 2, 4, 6, 8, 10) self-assembled monolayers (SAMs) on a Au(111) substrate were investigated by electrochemical measurements and theoretical calculations. The results of the experimental techniques indicated that the dithiols, except n = 2, showed an upright molecular structure in the SAMs, in which alkanedithiols were bound to the Au surface via only one thiol functionality and the other one faced up to the air. The results also suggested that the formed dithiol SAMs were densely packed and highly oriented. Except ethanedithiol, which was thought to form a bilayer, the reductive desorption peak potentials of 1,n-alkanedithiol (n = 4, 6, 8, 10) SAMs were more negative than those of the corresponding monothiol ones in 0.1 M KOH solutions. This illustrates that the dithiol SAMs had higher stability than the corresponding monothiol ones. The major part of the high stability may be attributed to the van der Waals interaction among the sulfur atoms on top of the dithiol SAMs. The molecular modeling calculation showed that the structures of dithiol SAMs were similar to those of the corresponding monothiol SAMs and that all the dithiol SAMs, except ethanedithiol, were more stable than the corresponding monothiol SAMs. The calculated energy differences between dithiol and monothiol SAMs decreased with the increment of alkyl-chain length.