Calculating electron current in a tight-binding model of a field-driven molecular wire: Application to xylyl-dithiol

A recently developed Floquet theory-based formalism for computing electron transport through a molecular bridge coupled to two metal electrodes in the presence of a monochromatic ac radiation field is applied to an experimentally relevant system, namely a xylyl–dithiol molecule in contact at either end with gold electrodes. In this treatment, a nondissipative tight-binding model is assumed to describe the conduction of electric current. Net current through the wire is calculated for two configurations of the electrode–wire–electrode system. In one, symmetric, configuration, the electrodes are close (∼2 Å) and equidistant from the bridge molecule. In the other, asymmetric configuration, one electrode is farther away (∼5 Å), representing the tip of a scanning tunneling microscope located at this distance from the bridge molecule (the other end being chemisorbed to a gold substrate). For both configurations, electron current is calculated for a range of experimental inputs, including dc bias and the intensity and frequency of the laser. Via absorption/emission of photons, resonant conditions may be achieved under which electron transport is significantly enhanced compared to the unilluminated analog. Calculations show that this can be accomplished with experimentally accessible laser field strengths.