Supramolecular tunnelling junctions with robust high rectification based on assembly effects

The performance of large-area molecular diodes can in rare cases approach the lower limit of commercial semiconductor devices but predictive structure-property design remains difficult as the rectification ratio (R) achieved by self-assembled monolayer (SAM) based diodes depends on several intertwined parameters. This paper describes a systematic approach to achieve high rectification in bisferrocenyl-based molecular diodes, HSCFc-CC-Fc (n = 9-15) immobilised on metal surfaces (Ag, Au and Pt). Experiments supported by molecular dynamics simulations show that the molecular length and bottom electrode influence the SAM packing, which affects the breakdown voltage (V_BD), the associated maximum R (R_max), and the bias at which the R_max is achieved (V_sat,R). From the electrical characterisation of the most stable Pt-SCFc-CC-Fc//GaO/EGaIn junctions, we found that V_BD, V_sat,R, and R_max all scale linearly with the spacer length of C, and that R_max for all the SAMs consistently exceeds the "Landauer limit" of 10³. Our data shows that the robust switching of M-SCFc-CC-Fc//GaO/EGaIn junctions is the result of the combined optimisation of parameters involving the molecular structure, the type of metal substrate, and the applied operating conditions (bias window), to create stable and high-performance junctions.