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

Charge transport across molecular junctions can be described by G = G(contact)exp(-beta L), envisioned as sequential propagation through electrode-molecule contacts (G(contact)) and the molecular backbone (exp(-beta L)). How G(contact) and exp(-beta L) 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 EF 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 similar to 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, Sigma).