Self-assembled monolayers of CH3S from the adsorption of CH3SSCH3 on Au(111)

Abstract

By performing density functional theory calculations, we have examined the adsorption of dimethyl disulfide (CH₃SSCH₃) on Au(111). In addition, we have also charted the reaction paths leading to dissociation of the S–S bond and the formation of thiolate-adatom species. Further, we have simulated the scanning tunneling microscopic (STM) images of the key adsorption intermediates and products and compared them with the experimental data available in the literature. In summary, our computations show that CH₃SSCH₃ adsorbs on Au(111) non-dissociatively with a moderate adsorption energy 0.42 eV for the trans-conformation which is 0.24 eV more stable than its cis-isomer. When the adsorbed trans-CH₃SSCH₃ dissociates at low temperature, the two CH₃S fragments are retained by the closest neighboring sites and the trans-conformation of them prior to S–S dissociation can be preserved. The activation barrier to rotating the CH₃S fragments on Au(111) is 0.17 eV so a moderate temperature rise can facilitate the transformation of the trans-conformation and generate a mixture of the trans- and cis-conformation. Our calculations also show that intrinsic defects such as gold adatoms (Au_ad) are active in CH₃SSCH₃ dissociation in two mechanisms: a direct mechanism in which CH₃SSCH₃ dissociation drives Au_ad formation and CH₃S–Au_ad formation simultaneously, with leftover vacancies as by-products; a sequential mechanism in which Au_ad is present in advance and subsequently interacts with a nearby CH₃S fragment to form CH₃S–Au_ad. The respective activation barriers are 0.15 and 0.32 eV for the trans- and cis-CH₃S–Au_ad–SCH₃ complex along the direct mechanism, and 0.21 and 0.18 eV along the sequential mechanism.