Structure and binding energies of unsaturated hydrocarbons on Si(001) and Ge(001)

The adsorption of acetylene, ethylene, and benzene on the Si(001) and Ge(001) surfaces is investigated by first-principles density-functional calculations within the generalized-gradient approximation. We find that the adsorption energies of the three hydrocarbons containing a triple bond, a double bond, and a pi-conjugated aromatic ring decrease as the sequence of C2H2>C2H4>C6H6. We also find that the bondings of acetylene, ethylene, and benzene to Ge(001) are much weaker than those to Si(001). As a result, benzene is weakly bound to Ge(001) while it is chemisorbed on Si(001), consistent with temperature-programmed desorption data.