Dipole-Induced Rectification Across AgᵀS/SAM//Ga₂O₃/EGaIn Junctions

This Article describes the relationship between molecular structure, and the rectification of tunneling current, in tunneling junctions based on self-assembled monolayers (SAMs). Molecular dipoles from simple organic functional groups (amide, urea, and thiourea) were introduced into junctions with the structure AgᵀS/S(CH₂)ₙR(CH₂)ₘCH₃//Ga₂O₃/EGaIn. Here, R is an n-alkyl fragment (−CH₂−)₂ ₒᵣ ₃, an amide group (either −CONH– or −NHCO−), a urea group (−NHCONH−), or a thiourea group (−NHCSNH−). The amide, urea, or thiourea groups introduce a localized electric dipole moment into the SAM and change the polarizability of that section of the SAM, but do not produce large, electronically delocalized groups or change other aspects of the tunneling barrier. This local change in electronic properties correlates with a statistically significant, but not large, rectification of current (r⁺) at ±1.0 V (up to r⁺ ≈ 20). The results of this work demonstrate that the simplest form of rectification of current at ±1.0 V, in EGaIn junctions, is an interfacial effect, and is caused by a change in the work function of the SAM-modified silver electrode due to the proximity of the dipole associated with the amide (or related) group, and not to a change in the width or mean height of the tunneling barrier.