Influence of the tip-induced electric field on the STM contrast of chemisorbedC2H4on the Si(001) surface

We present a first-principles calculation for the Si(001) surface on which ethylene molecules (${\mathrm{C}}_2$ ${\mathrm{H}}_4$) are adsorbed. The calculations apply the recently developed projector-augmented-wave method together with the Car-Parrinello scheme to determine the ground-state atomic and electronic structures. It is shown that the scanning tunneling microscopy (STM) contrast observed experimentally for this surface cannot be understood by considering only the surface wave functions obtained in zero electric field. The tip-induced electric field strongly modifies the surface electronic structure. A detailed study of the polarization of the Si(001) surface with and without adsorbed ${\mathrm{C}}_2$ ${\mathrm{H}}_4$ molecules is presented. Given the direction of the electric field, the electronic charge is ``pulled out'' from the surface into the vacuum. The response of the electronic states to the electric field is greater above the clean silicon dimers than above the molecules. Constant-current STM scans are obtained from a nonperturbative approach to the calculation of the tunneling current. In the presence of the tip-induced electric field, the resulting STM contrast is found to be in qualitative agreement with the experiments.