Conduction Pathways of Quinoxalinyl Molecules in the STM-BJ-Fabricated Nanogap

Quinoxaline (Qx) terminated with two mercaptomethyl (-SMe) anchoring ligands demonstrated two conductance values when studied using the scanning tunneling microscope-based break-junction (STM-BJ) technique. Further research showed that the observed low and high conductances (termed GL and GH) resulted from two electron transfer pathways of different lengths with distinct molecular binding configurations. GL arises from the two terminal -SMe groups attached to the Au electrodes, and GH appears when one of the two Au-S linkages is replaced by an Au-N linkage where N of Qx is anchored to the electrode. This is one of the few instances where a single molecule can independently exhibit two different conductance states without an external stimulus, thereby offering a desired molecular prototype for developing conductance-dependent molecular electronics, such as molecular switches and other functional molecular devices.

Microscopic theory, analysis, and interpretation of conductance histograms in molecular junctions

Molecular electronics break-junction experiments are widely used to investigate fundamental physics and chemistry at the nanoscale. Reproducibility in these experiments relies on measuring conductance on thousands of freshly formed molecular junctions, yielding a broad histogram of conductance events. Experiments typically focus on the most probable conductance, while the information content of the conductance histogram has remained unclear. Here we develop a microscopic theory for the conductance histogram by merging the theory of force-spectroscopy with molecular conductance. The procedure yields analytical equations that accurately fit the conductance histogram of a wide range of molecular junctions and augments the information content that can be extracted from them. Our formulation captures contributions to the conductance dispersion due to conductance changes during the mechanical elongation inherent to the experiments. In turn, the histogram shape is determined by the non-equilibrium stochastic features of junction rupture and formation. The microscopic parameters in the theory capture the junction's electromechanical properties and can be isolated from separate conductance and rupture force (or junction-lifetime) measurements. The predicted behavior can be used to test the range of validity of the theory, understand the conductance histograms, design molecular junction experiments with enhanced resolution and molecular devices with more reproducible conductance properties.