Scanning tunneling microscopy. II. Calculation of images of atomic and molecular adsorbates

Abstract

A theory of the scanning-tunneling-microscopy (STM) current of adsorbates with focus on effects of coherence in electron motion in the STM junction has been developed in the preceding paper [Phys. Rev. B 51, 11 074 (1995)]. That theory is illustrated through an application to the computation of several realistic STM images. The tip, the substrate, and the adsorbate are treated as finite clusters of atoms. Extended Hückel theory is used to calculate the STM current. The substrate used is Au (111). Images that can be compared directly with experimental observations are constructed for the bare Au (111) surface as well as for atomic and molecular adsorbates. The atomic adsorbates treated are Au, Na, H, C, and O. The molecular adsorbates treated are ${\mathrm{O}}_2$, CO, and benzene. A comparison is made with observations reported in the literature and agreement with a number of features is found. Our calculated images suggest that observations of giant corrugation in STM of metals might be arising from nonperturbative electron transport between localized surface states. Nontopographical artifacts due to adsorbates and the dependence of the image on the tip-sample separation are discussed.