Quantum chemical studies of the adsorption of acetylene on the Ge (001) surface

Abstract

The reconstruction of the Ge (001) surface and the adsorption geometry of acetylene on the Ge (001) surface have been explored through Hartree Fock (HF) and density functional theory (DFT) methods. Several cluster models have been tested to build an accurate representation of p(2×2) and c(4×2) Ge (001) surface and different binding arrangements of an acetylene molecule on the surface that are believed to be energetically most stable. The formation of dimers on the surface is characterized by the dimer bond length of 2.47~2.51Å, considerably shorter than the distance of unbonded Ge-Ge (4.0Å) in bulk. In comparison, the bond length of Ge-Ge is 2.44Å in bulk. The calculated bond lengths of Ge dimer depend on the size of cluster and the level of subsurface layers allowed to relax, but only weakly depend upon DFT functionals. The dimer formation strongly affects the position of atoms down to the fourth-layer, while the most fifth- or deeper-layer atoms are modified only slightly. The c(4×2) reconstruction on the Ge (001) surface is calculated to be energetically most stable in agreement with experiment. From HOMO and HOMO-1 of small clusters, one can qualitatively construct the isopotent map which can be correlated to scanning tunneling microscopy (STM) images of the p(2×2), c(4×2) Ge (001) surface, and the $C_2H_2$ adsorptions on the Ge (001) surface. The comparison of the atomic resolution STM images with the simulation and calculated energy values reveals that there are di-σ (on-top) and tetra-σ (p-bridge) bonding configurations for adsorbed acetylenes. Most acetylene molecules are di-σ bonded to the first-layer Ge dimers with the adsorption energy of 53.34 kcal/mol in DFT calculation. It is elucidated that the CC bond is essentially double bond and the Ge dimer bonds are not cleaved.