Theory of STM junctions for \pi-conjugated molecules on thin insulating films

TL;DR

This paper develops a microscopic density matrix theory for electron transport in STM setups involving c-conjugated molecules on insulating films, highlighting the impact of geometry and tunnelling rates on transport properties.

Contribution

It introduces a detailed theoretical framework incorporating energy-dependent tunnelling rates to analyze transport in STM experiments with c-conjugated molecules.

Findings

Tunnelling rates encode geometrical differences between tip and substrate.

Calculated STM current-voltage characteristics for benzene show angular momentum channel effects.

Transport characteristics are sensitive to molecular symmetry and coupling geometry.

Abstract

A microscopic theory of the transport in a scanning tunnelling microscope (STM) set-up is introduced for \pi-conjugated molecules on insulating films, based on the density matrix formalism. A key role is played in the theory by the energy dependent tunnelling rates which account for the coupling of the molecule to the tip and to the substrate. In particular, we analyze how the geometrical differences between the localized tip and extended substrate are encoded in the tunnelling rate and influence the transport characteristics. Finally, using benzene as an example of a planar, rotationally symmetric molecule, we calculate the STM current voltage characteristics and current maps and analyze them in terms of few relevant angular momentum channels.