Investigating Single-Molecule Near-Resonant Charge Transport in Electrochemical Environments

Electrochemical methods have been used to realize a number of functions in single-molecule devices such as rectification and gating. Here, we perform scanning tunneling microscope-based break-junction measurements near the resonant tunneling regime of a family of methyl-sulfide terminated oligomers bridged with n units of thiophene-1,1-dioxide (TDOn). We produce current-voltage (IV) curves measured in an electrochemical environment and fit them using a single-level model of transport within the Landauer formalism. Unlike previous works, we do not make the low-temperature approximation, which we find introduces significant errors at room-temperature near the resonant-tunneling regime. We show that one can model the effect of the polar solvent by introducing a bias-dependent parameter into the level alignment. We demonstrate that junction rupture is strongly correlated with the level alignment, suggesting rupture mechanisms stimulated by resonant tunneling. Finally, we compare two-electrode and three-electrode gating measurements to characterize the voltage profile of the electrochemical environment around a single-molecule junction. These advances show that creative use of junction-by-junction IV curve-fitting can uncover a variety of interesting physics in nanoscale devices.