Theoretical study on mechanical evolution process of interface between gold electrode and pyridyl anchor group

Pyridyl-ended molecular junctions show high and low breaking forces successively during formation processes and at the same time present intriguing conductance switching behaviors. To understand the forming processes of pyridyl-ended molecular junctions, the interactions between 4,4′-bipyridine molecule and gold electrodes are studied by applying ab initio-based adiabatic simulation method. The processes that the molecule moves away from electrode tip with different contact configurations are simulated, and the molecule-electrode interface evolutions, energy of the molecule-electrode systems and the force between molecules and electrodes are calculated in the simulations. The numerical results show that, during the molecule moving away from the pyramid-shaped electrode, the pyridyl is easy to vertically adsorb on the second gold layer of the electrode tip. In this contact configurations, the tip Au atom deviates from the original position due to the lateral pushing force of the pyridyl. It needs about 1.3-1.5 nN stretching force for the pyridyl breaking from the second gold layer and switching to the tip Au atom, which is evidently larger than the force of 0.8-1.0 nN for the molecule breaking from the tip Au atom. This result is well consistent with the experiment observations, which thus decodes the relationship between the interface structures and the stretching force during the formation processes of bipyridyl molecular junctions in the experiments. The interactions between 4,4′-bipyridine molecule and plane-shaped gold electrodes are very weak. It needs not more than 0.5 nN for the molecule breaking from the plane-shaped gold electrode. However, when the molecule adsorbs on the single Au atom which is adsorbed on the surface of plane-shaped electrode, the molecule can sustain 1.7 nN stretching force. Our study shows that the ab initio-based adiabatic stretching simulation method can not only reveal the geometric evolution process of molecule-electrode systems, but also identify the specific contact configurations between molecule and electrode.