Resolving charge transfer mechanisms in molecular tunnel junctions using dynamic charge transfer and static current-voltage measurements

Understanding charge transfer (CT) dynamics is important for controlling the tunneling mechanism in molecular junctions. Synchrotron-based core-hole clock (CHC) spectroscopy can quantify the femtosecond-scale CT time τ_CT across the metal-molecule interface, which affects the current density (J) produced with applied bias (V) in the junctions. However, directly determining the tunneling behavior from a comparison of the CHC τ_CT and the J(V) measurement of a junction requires prior knowledge of the molecular orbitals involved. To solve this problem, we examined CT dynamics across self-assembled monolayers (SAMs) based on oligophenylene ethynylene (OPE) wires with ferrocene (Fc) terminal groups with Au, Ag and Pt bottom electrodes. Density functional theory (DFT) helped identify the donor and acceptor levels, which are typically the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). The measured J(V) response of the SAM junctions with gallium-indium (EGaIn) alloy as the top electrode demonstrates that the tunneling decay coefficient β provides an intensive parameter to assess CT efficiency. We find that more delocalized molecular wavefunctions (in this case, LUMO+2, with contributions from Fc and OPE) facilitate faster and more efficient CT than more localized acceptor levels (here, the more iron-centered LUMO+1). These orbital-specific effects explain why we measure comparable β values for CT via LUMO+1 and J via HOMO and LUMO at −1 V bias. Our study highlights the utility of τ_CT measured by CHC in experimentally confirming the orbitals participating in charge transport measurements and shows that higher-lying delocalized orbitals can in some instances dominate over frontier orbitals despite larger energy offset (or increase in tunneling barrier height).