Effect of the Chemical Potentials of Electrodes on Charge Transport across Molecular Junctions

Charge transport across molecular junctions can be described by G = Gcontactexp(−βL), envisioned as sequential propagation through electrode-molecule contacts (Gcontact) and the molecular backbone (exp(−βL)). How Gcontact and exp(−βL) are modulated by the chemical potentials of the electrodes (EF), although essential, remains relatively unexplored because EF is typically driven by the applied Vbias and hence limited to a small range in that a large Vbias introduces complicated transport pathways. Herein, the interrelated EF and Vbias are electrochemically disentangled by fixing Vbias at a small value while potentiostatically positioning the electrode EF in a 1.5 V range. The results show that EF affects Gcontact more pronouncedly than the molecular backbone. For the covalently anchored acetylene-electrode (CC−Au) junctions, the energy level of the frontier molecular orbital (EFMO) is found to shift nonlinearly as EF changes; |EFMO – EF| is independent of EF in the range of −0.25 to 0.00 V (vs EAg/AgCl) and is narrowed by ∼32% at 0.00–0.75 V. These findings are elucidated by the refined Simmons model, Newns-Anderson model, and single-level Breit–Wigner formula and quantitatively shed light on the influence of electrodes on the molecular orbitals (viz., the self-energy, Σ).