STM observation of sulfur dimerization in alkanethiol monolayers self-assembled on Au{111}

We present for the first time, direct microscopical observation by STM of sulfur dimer formation on alkanethiol self-assembled monolayers (SAM) on sputtered Au substrates. The sulfur dimers are observed when imaging at a bias where the tip-molecule interaction occurs, and are formed by displacement of sulfur atoms from their normal three-fold hollow site residence of the (4 × 2) superlattice to nearest-neighbor bridge-site residence between two Au atoms. The displacement is believed to occur due to defects induced in the alkyl chain of the monolayer due to the proximity of the STM tip. Only one of the sulfur atoms forming the dimer is bound to the surface and they are commensurate with the Au{111} adlattice along its [112] directions.