Modeling Single Molecule Junction Mechanics as a Probe of Interface Bonding

TL;DR

This study combines atomic force microscopy measurements with density functional theory calculations to analyze the mechanics of single molecule junctions, revealing how interface bonds influence rupture behavior and enabling accurate bond energy extraction.

Contribution

It introduces a combined experimental and theoretical framework to model and interpret the force extension characteristics of single molecule junctions, highlighting the effectiveness of scaled models and bond potential analysis.

Findings

Force extension characteristics can be effectively scaled using maximum force and rupture distance.

Two-parameter bond potential models fit well to N-Au and S-Au bonds in scaled form.

Single-bond potential forms, including relaxation effects, accurately describe interface bond energies.

Abstract

Using the atomic force microscope based break junction approach, applicable to metal point contacts and single molecule junctions, measurements can be repeated thousands of times resulting in rich data sets characterizing the properties of an ensemble of nanoscale junction structures. This paper focuses on the relationship between the measured force extension characteristics including bond rupture and the properties of the interface bonds in the junction. A set of exemplary model junction structures have been analyzed using density functional theory based calculations to simulate the adiabatic potential surface that governs the junction elongation. The junction structures include representative molecules that bond to the electrodes through amine, methylsulfide and pyridine links. The force extension characteristics are shown to be most effectively analyzed in a scaled form with maximum sustainable force and the distance between the force zero and force maximum as scale factors. Widely used, two parameter models for chemical bond potential energy versus bond length are found to be nearly identical in scaled form. Furthermore, they fit well to the present calculations of N-Au and S-Au donor-acceptor bonds, provided no other degrees of freedom are allowed to relax. Examination of the reduced problem of a single interface, but including relaxation of atoms proximal to the interface bond, shows that a single-bond potential form renormalized by an effective harmonic potential in series fits well to the calculated results. This allows relatively accurate extraction of the interface bond energy. Analysis of full junction models show cooperative effects that go beyond the series inclusion of the second bond in the junction, the spectator bond that does not rupture.