Ab-initio calculation of the real contact area on the atomic scale

TL;DR

This paper introduces an ab-initio method using Bader's quantum theory to accurately determine the real contact area at the atomic scale during tip-surface contact, based on electronic density and atomic relaxations.

Contribution

The novel approach calculates contact area without external parameters by analyzing zero flux surfaces between Bader atoms, relying solely on electronic structure and atomic relaxations.

Findings

Contact area depends exponentially on tip-surface distance.

Method successfully identifies initial contact point via atomic relaxations.

Approach is fully ab-initio, parameter-free, and applicable to nanoscale systems.

Abstract

We present a novel approach to determine the onset of contact between a tip and a surface. The real contact area depending on the distance is calculated using Bader's quantum theory of atoms in molecules. The jump to contact, which is often observed in atomic force microscopy experiments, is used as an indicator for the initial point of contact, which in turn is defined by atomic relaxations and thus without the need of external parameters. Within our approach the contact area is estimated by evaluating the zero flux surfaces between the touching Bader-atoms, where the necessary electronic density cutoff for the Bader-partitioning is calculated to depend on the initial point of contact. Our proposed approach is therefore completely ab-initio and we are able to define and calculate the real area of contact without imposing restrictions or free parameters. As a prototype system we choose a tip made of a ten atom tungsten pyramid above a moir\'{e} layer of graphene on an fcc iridium (111) substrate. We find that the contact area depends exponentially on the effective distance between the tip apex and the surface atom directly below within the atomically relaxed nanosystem.