Preparation of large-area molecular junctions with metallic conducting Langmuir–Blodgett films

Metallic conducting Langmuir–Blodgett (LB) films were used as soft electrodes to fabricate molecular junctions with self-assembled monolayers (SAMs) of alkanethiols (CH3(CH2)n−1SH) on an Au surface. Alkanethiols can form highly ordered, stable dielectric SAMs on metal surfaces over large areas. However, it is difficult to establish electrical contacts on such SAMs, which has limited their application. In this work, we used metallic conducting LB films composed of bis(ethylenedioxy)tetrathiafulvalene and stearic acid as a soft electrode onto alkanethiol SAMs (Cn-SAM, n=12, 14, 16, 18) to prepare Au/SAM/metal junctions of relatively large size (~15.6×103μm2). The current density–voltage (J–V) characteristics across the junctions exhibited rectifying behavior with a ratio R of ~5 (R=|J(V)|/|J(−V)| at ±1V). The lower transfer rate corresponding to the electron transport from Au to the LB films exhibited nonlinear J–V characteristics, while the higher transfer rate of electrons from the LB film to Au showed linear J–V characteristics. Kelvin probe force microscopy revealed that the work function of the metallic LB films was smaller than that of Au. The observed rectification behavior is probably caused by different electron transport mechanisms between the two current directions.