Reversing Molecular Dilution Effect by Varying the Bottom Electrode

This paper describes charge tunneling across mixed self-assembled monolayers (SAMs) and investigates the effect of bottom electrode on molecular dilution and tunneling function in large-area molecular junctions. Binary mixed SAMs are formed with organic molecular diode (2,2′-bipyridyl-terminated n-undecanethiol (SC₁₁BIPY)) and nonrectifying n-alkanethiolate diluent (SC_n; n = 8, 10, 16, 18) via coadsorption on different electrodes, namely Au and Ag. By taking advantage of self-referencing feature of rectification, the tunneling–molecular dilution relationship is reliably examined. Notably, rectification ratio of mixed SAMs on the Ag electrode decreases with increasing the diluent's length, whereas that of analogous SAMs on the Au electrode oppositely increases. Simulation of rectification using a parallel circuit model and surface analysis with contact angle goniometry evidence that such an opposite trend is dominated by the supramolecular structural order, which arises from the difference in interthiolate distance between Au and Ag. This work demonstrates that the molecular dilution effect on tunneling can be reversed by varying the bottom electrode.