Initial adsorption configurations of acetylene molecules on the Si(001) surface

Abstract

Initial adsorption configurations of acetylene molecules on the Si(001) surface are investigated by scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. Three distinctive features are observed in the STM images at low coverage. The comparison of the atomic resolution STM images with the simulation reveals that there are basically two kinds of di-$\ensuremath{\sigma}$ bonding configurations: (i) on-top over a single dimer and (ii) end-bridge between two successive dimers in the same dimer row. The third feature is identified to be a pair of end-bridge di-$\ensuremath{\sigma}$ configurations occupying the same two dimers. The tetra-$\ensuremath{\sigma}$ bonding configurations are not found. The existence of the two kinds of di-$\ensuremath{\sigma}$ bonding configurations can resolve the apparently conflicting results of the existing studies.