First Principles Study of the Adsorption Structure of Ethylene on Ge(001) Surface

The adsorption of ethylene on the Ge(001) Surface is investigated by the first principles density-functional calculations. Our total energy calculations and reaction path investigations clarify the relative importance of various adsorption coil figurations at 0.5 and 1.0 monolayer adsorption coverage. The results are consistent with the experimental data in the literature in that both the di-sigma and paired-end-bridge configurations are the favorable structures on Ge(001). In addition, Our calculation results clarify that having di-sigma-bound ethylene and end-bridge-bound ethylene next to each other is unfavorable. Such new results imply that although di-sigma-bound ethylene and end-bridge-bound ethylene may coexist on Ge(001), phase separation will occur to form adsorbate domains. The electronic structures have also been studied, and the band structure calculations show that both the di-sigma and end-bridge models at 0.5 monolayer are semiconducting with a small band gap of similar to 0.4 eV, which is slightly larger than the band gap of the virgin Ge(001) p(2 x 2) surface. Increasing the coverage to 1.0 monolayer further widens the band gap in both cases. The results thus rule out the past postulation that ethylene adsorption may turn Ge(001) to metallic.