Simulating STM transport in alkanes from first principles

TL;DR

This paper uses first-principles simulations to explore how close-proximity STM measurements affect molecular electronic structure and charge transfer in alkane molecules on gold, revealing effects beyond traditional perturbative models.

Contribution

It introduces an ab initio quantum transport approach to simulate STM measurements at close tip-molecule distances, capturing electrostatic interactions and charge transfer effects.

Findings

Electrostatic interactions significantly alter molecular electronic properties.

Charge transfer asymmetry depends on end-group chemistry.

Close-proximity STM effects differ from traditional perturbative assumptions.

Abstract

Simulations of scanning tunneling microscopy measurements for molecules on surfaces are traditionally based on a perturbative approach, most typically employing the Tersoff-Hamann method. This assumes that the STM tip is far from the sample so that the two do not interact with each other. However, when the tip gets close to the molecule to perform measurements, the electrostatic interplay between the tip and substrate may generate non-trivial potential distribution, charge transfer and forces, all of which may alter the electronic and physical structure of the molecule. These effects are investigated with the ab initio quantum transport code SMEAGOL, combining non-equilibrium Green's functions formalism with density functional theory. In particular, we investigate alkanethiol molecules terminated with either CH3 or CF3 end-groups on gold surfaces, for which recent experimental data are available. We discuss the effects connected to the interaction between the STM tip and the molecule, as well as the asymmetric charge transfer between the molecule and the electrodes.