Theory of tip‐dependent imaging of adsorbates in the STM: CO on Cu(111)

Abstract

AbstractThe processes of local electron injection or extraction in the scanning tunneling microscopy (STM) and spectroscopy (STS) lead to the creation of short‐lived excited states localized at the electrode surfaces. The dynamic relaxation of the transient negative or positive ion resonances, due to both local and long‐range interactions, is the clue to the understanding of numerous phenomena in STM/STS ranging from the “anomalously” large tip height corrugation amplitudes on clean metal surfaces to the observation of quantum mirages and features in the STS, which are not observed with the help of other surface spectroscopies. Quantum nanodynamics theory (QND) has been applied to calculate the interaction potential of a single CO molecule with the Cu(111) surface, with a transient negative ion resonance formed when an electron is injected from the tip, and the tunneling conductance on the clean and CO covered Cu(111) surface using a clean metal tip Al/Al(111) and a Pt(111) tip with an adsorbed CO molecule at the apex. Within QND and three‐dimensional scattering theory, regarding the tunneling as an excited‐state problem, we provide the explanation of the tip‐dependent STM image of a single CO molecule on Cu(111). The appearance of the CO molecule as an indentation, using a clean metal tip and as a protrusion with a tip terminated by a CO molecule, is understood as a result of tunneling through two competing channels. Tunneling via adsorbate‐induced ion resonances enhances the tunneling conductance. In contrast, tunneling via metal ion resonances only leads to attenuation of the conductance in the presence of the adsorbate. The current in the vicinity of the adsorbed CO molecule is reduced when a clean metal tip is used; i.e., CO appears dark in the STM image, because metal ion resonances on Cu(111) derive from the surface states with image state components coupling to plasmons and are therefore very diffuse. With a CO‐terminated tip, the major current channel is, for symmetry reasons, from the 2π‐derived orbital of the tip CO molecule, via the diffuse 2π‐derived orbital of the CO molecule on the sample, hence adsorbed CO appears bright. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006