Supramolecular Tunnelling Junctions with Robust High Rectification Based on Assembly Effects

The performance (R) of a self-assembled monolayer (SAM) based molecular diode is dependent on the molecular length, the bias voltage range, and the type of metal used for the bottom electrode. This work focused on a systematic approach to achieve high rectification ratio (R) in bisferrocenyl-based molecular diodes, HSC_nFc–C≡C–Fc (n = 9-15) immobilised on three different metal surfaces (Ag, Au and Pt). The data shows that the molecular length and property of bottom electrode material influences the SAM packing which then influences the breakdown voltage (V_BD), the associated maximum R (R_max), and the bias at which the R_max is achieved (V_sat,R). The electrical characterisation of the most stable Pt–SC_nFc–C≡C–Fc//GaO_x/EGaIn junctions showed that V_BD, V_sat,R, and R_max all scale linearly with spacer length of C_n, and that R_max for all the SAMs consistently exceeds the “Landauer limit” of 10³. The experimental results agreed with both molecular dynamics simulation and single-level Landauer formulism. From the data and theory, it’s observed that the asymmetry across the junctions increases with the molecular length which then varies the molecule-electrode coupling strength resulting in a high rectification ratio.