Modeling structure and conductivity of atomic-scale break junctions

A large number of single-molecule conductance measurements rely on the break junction approach in which a metal wire (typically Au) is stretched to the breaking point in the presence of the molecule of interest. As the wire breaks, a single molecule can bridge the gap between the electrodes and its conductance can be measured. This is a complex process during which many atomic rearrangements occur, resulting in complicated current versus distance traces that are difficult to interpret. We employ density functional theory (DFT) structure relaxations to investigate the possible geometries during a break junction experiment with and without the presence of molecules. Our focus is on Au electrodes, but we also consider other elements such as Ag and Pt. Next, we the relaxed structures and study their conductance properties with the non-equilibrium Green’s function technique coupled with DFT (NEGF-DFT). Our results reveal an intriguing relationship about the evolution of conductance as the junction is stretched, which can be attributed to atomic rearrangements and orbital alignment. These computational analyses can provide guidance in interpreting experimental data.