Molecular Electronics: Toward the Atomistic Modeling of Conductance Histograms

Reliability in molecular electronics break-junction experiments has come from statistically sampling thousands of repeat measurements. Here we discuss the computational challenges in reproducing the experimental conductance histograms and introduce a computational strategy to model molecular electronics experiments with statistics. The strategy combines classical molecular dynamics (MD) of junction formation and evolution, using a reactive force field that allows for bond-breaking and -making processes, with steady-state electronic transport computations using Green's function methods in the zero-bias limit. The strategy is illustrated using a molecular junction setup where an octanedimethylsulfide (C8SMe) connects to two gold electrodes. To attempt to reproduce the statistics encountered in experiments, we performed simulations using (1) a single MD trajectory of junction formation and evolution; (2) several MD trajectories with identical initial geometry for the electrodes; and (3) several MD trajectories each with a different geometry for the electrodes (obtained by separately crushing the electrodes and breaking the gold-gold contact). We find that these three classes of simulations can exhibit an apparent agreement with the experimental conductance histograms. Nevertheless, these simulations miss the time-averaging of the current that is inherent to the experiment. We further examined the simulated time averaged currents for an ensemble of trajectories with crushed electrodes and found that such simulations recover the width of the experimental conductance histograms despite the additional averaging. These results highlight the challenges in connecting theory with experiment in molecular electronics and establish a hierarchy of methods that can be used to understand the factors that influence the experimental conductance histogram.