Molecular Rectification in Metal-SAM-Metal Oxide-Metal Junctions

This Article compares the ability of self-assembled monolayers (SAMs) of alkanethiolates with ferrocene (Fc) head groups (SC11Fc), and SAMs of alkanethiolates lacking the Fc moiety (SC10CH3 and SC14CH3), to conduct charge. Ultraflat surfaces of template-stripped silver (AgTS) supported these SAMs, and a eutectic alloy of gallium and indium (EGaIn), covered with a skin of gallium oxide (presumably Ga2O3), formed electrical top-contacts with them. EGaIn is a liquid at room temperature, but its spontaneously formed surface oxide skin gives it apparent non-Newtonian properties and allows it to be molded into conically shaped tips; these tips formed soft electrical contacts with SAMs and formed stable SAM-based tunneling junctions in high (70-90%) yields. Measurements of current density, J, versus applied voltage, V, showed that tunneling junctions composed of SAMs of SC11Fc rectify current with a rectification ratio R ≈ 1.0 × 102 (R ) |J(-V)|/|J(V)| at (1 V and with a log-standard deviation of 3.0). In contrast, junctions lacking the Fc moiety, that is, junctions composed of SAMs of SCn-1CH3 (with n ) 11 or 15 and presenting terminal CH3 groups), showed only slight rectification (R ) 1.5 (1.4) and 2.1 (2.5), respectively). A statistical analysis of large numbers (N ) 300-1000) of data gave detailed information about the spread in values and the statistical significance of the rectification ratios and demonstrated the ability of the experimental techniques described here to generate SAM-based junctions in high yield useful in physical-organic studies.