The electron induced damage in self-assembled monolayers (SAMs) of n-dodecanethiolate on Au(111) and Au(100) single-crystalline surfaces is investigated in situ by X-ray photoelectron spectroscopy. The same irradiation dose produced different adsorbed groups. The damage at the headgroup-substrate interface leads to find dialkyl sulfide (RS-R') on Au(111), while dialkyl disulfide (RS-SR) and/or thiol (RSH) were produced on Au(100). With regards to C species, significant amounts of C=C are generated on Au(111) but not on Au(100), showing that the double bond formation is not triggered through the same pathways on these surfaces. A detailed analysis of a variety of mechanisms, which involved cationic (RS + ), anionic (RS − ) or thiyl radical (RS • ) species, in combination with ab-initio DFT calculation leads to the conclusion that the radical pathways successfully explain the experimental results. Molecular dynamics simulations show that the n-dodecanethiolate-SAMs on both surfaces are equivalent with regard to the van der Waals interactions. The breakage of the S-Au bonds is studied by means of DFT calculations. The thiyl radical would form close to the Au(100) surface, making it likely to react with another thiyl radical or thiolate to form the RS-SR species. On the other hand, for Au(111) the thiyl radical would form farther from the surface, reacting with the alkyl chains of neighboring molecules to form RS-R' species. The mechanistic framework here proposed is very useful to explain the behavior of related systems.
Self-assembled monolayers (SAMs) of ω-carboxylic acid thiols are very important in the surface modification of metals, especially on gold surfaces. Indeed, the 3mercaptopropanoic acid (MPA) and its ester or amide derivatives are widely used for SAMs-based sensor design. It was already shown that MPA does not suffer C-S bond scission when adsorbed on Au. On the other hand, in this work we demonstrate that its simplest derivative, methyl 3-mercapto propionate (Me-MPA), is prone to form significant amounts of atomic sulfur when adsorbs on Au. The MPA-derivatives are more sensible than MPA itself to alkaline solutions and its SAM-based sensors will rapidly degrade given atomic sulfur. In this work, we study the simplest MPA-derivative Me-MPA SAMs on preferentiallyoriented Au(111) surfaces by XPS and electrochemical measurements. It was found that the desulfuration of Me-MPA depends on its preparation conditions (grown from ethanol or toluene solution) and on its post-treatment with alkaline solution. In order to explain the S-C bond scission on Me-MPA SAMs we discuss different reaction mechanism. We concluded that the reaction mechanism involves an E1cB elimination pathway (β
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