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

Charge transport across molecular junctions can be described by G = G contactexp­(−βL), envisioned as sequential propagation through electrode-molecule contacts (G contact) and the molecular backbone (exp­(−βL)). How G contact and exp­(−βL) are modulated by the chemical potentials of the electrodes (E F), although essential, remains relatively unexplored because E F is typically driven by the applied V bias and hence limited to a small range in that a large V bias introduces complicated transport pathways. Herein, the interrelated E F and V bias are electrochemically disentangled by fixing V bias at a small value while potentiostatically positioning the electrode E F in a 1.5 V range. The results show that E F affects G contact more pronouncedly than the molecular backbone. For the covalently anchored acetylene-electrode (CC−Au) junctions, the energy level of the frontier molecular orbital (E FMO) is found to shift nonlinearly as E F changes; |E FMO – E F| is independent of E F in the range of −0.25 to 0.00 V (vs E Ag/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).