Scanning Tunneling Microscope Study on C2H2 Adsorption on a Ge(100) Surface

Adsorption configurations of C2H2 on the Ge(001) surface at room temperature were investigated using scanning tunneling microscope (STM). Two adsorption features, occupying one dimer and two dimers, respectively, were observed as major species. Both of them appear in rounded or elongated bean shapes in filled-state images. In empty-state images, they reveal atomically resolved details: two and four local maxima for the one-dimer and the two-dimer features, respectively. The empty-state double maximum of the one-dimer feature confirms the adsorption of a C2H2 molecule on top of a dimer. Based on the empty-state quadruple maximum of the two-dimer feature, we propose the adsorption of a pair of C2H2 molecules across the ends of two adjacent dimers in the same dimer row (paired end-bridge). This differs from the previous interpretation of the two-dimer feature as a single C2H2 molecule adsorbed in a tetra-sigma-bonding configuration. We also observed C2H2 molecules adsorbed in the unpaired end-bridge configuration, but none in tetra-sigma-bonding configurations. These observations and interpretations are consistent with the energetics predicted by using theoretical calculations.