Atomic Force Extrema Induced by the Bending of a CO-Functionalized Probe

TL;DR

This study demonstrates how a CO-functionalized atomic force microscope tip exhibits force extrema due to molecular bending, revealing new insights into single-molecule interactions at the nanoscale.

Contribution

It introduces a model explaining force extrema caused by molecular bending in AFM with a CO tip, linking experimental data with theoretical predictions.

Findings

Force dip-hump evolution depends on initial tilt angle

Model accurately reproduces experimental force data

Molecular bending influences vertical-force extrema

Abstract

The control and observation of reactants forming a chemical bond at the single-molecule level is a longstanding challenge in quantum physics and chemistry. Using a single CO molecule adsorbed at the apex of an atomic force microscope tip together with a Cu(111) surface, the molecular bending is induced by torques due to van der Waals attraction and Pauli repulsion. As a result, the vertical force exhibits a characteristic dip-hump evolution with the molecule-surface separation, which depends sensitively on the initial tilt angle the CO axis encloses with the surface normal. The experimental force data are reproduced by model calculations that consider the CO deflection in a harmonic potential and the molecular orientation in the Pauli repulsion term of the Lennard-Jones potential. The presented findings shed new light on vertical-force extrema that can occur in scanning probe experiments with functionalized tips.