Two-probe theory of scanning tunneling microscopy of single molecules: Zn(II)-etioporphyrin on alumina

TL;DR

This paper develops a two-probe theoretical framework for understanding scanning tunneling microscopy of single molecules, emphasizing how electron flow and STM images depend on probe positions and molecule-substrate interactions.

Contribution

The paper introduces a novel two-probe model for STM of single molecules, accounting for non-uniform electron transmission through insulating layers and explaining experimental image variations.

Findings

STM images vary significantly with probe position.

Electron flow is strongly influenced by molecule-substrate coupling.

The model explains experimental STM topographs of Zn(II)-etioporphyrin I.

Abstract

We explore theoretically the scanning tunneling microscopy of single molecules on substrates using a framework of two local probes. This framework is appropriate for studying electron flow in tip/molecule/substrate systems where a thin insulating layer between the molecule and a conducting substrate transmits electrons non-uniformly and thus confines electron transmission between the molecule and substrate laterally to a nanoscale region significantly smaller in size than the molecule. The tip-molecule coupling and molecule-substrate coupling are treated on the same footing, as local probes to the molecule, with electron flow modelled using the Lippmann-Schwinger Green function scattering technique. STM images are simulated for various positions of the stationary (substrate) probe below a Zn(II)-etioporphyrin I molecule. We find that these images have a strong dependence on the substrate probe position, indicating that electron flow can depend strongly on both tip position and the location of the dominant molecule-substrate coupling. Differences in the STM images are explained in terms of the molecular orbitals that mediate electron flow in each case. Recent experimental results, showing STM topographs of Zn(II)-etioporphyrin I on alumina/NiAl(110) to be strongly dependent on which individual molecule on the substrate is being probed, are explained using this model. A further experimental test of the model is also proposed.