Acetylene adsorption on Si(111)(7×7): A scanning-tunneling-microscopy study

The microscopic adsorption state of acetylene (${\mathrm{C}}_2$ ${\mathrm{H}}_2$) on Si(111)(7×7) at room temperature has been studied by using scanning tunneling microscopy. The center adatoms are more reacted than the corner adatoms in a ratio of about 2:1 upon the adsorption of acetylene. It is predominantly the adatoms in the faulted subunit that are reacted by acetylene, rather than those in the unfaulted subunit, indicating that acetylene is initially in a mobile precursor state. No significant restructuring of surface Si atoms is observed. It is proposed that acetylene is di-σ bonded to the pair of adjacent Si adatom and Si rest atom, saturating Si dangling bonds of the Si(111)(7×7) surface.