Mechanism of Rectification in Tunneling Junctions Based on Molecules with Asymmetric Potential Drops

Abstract

This paper proposes a mechanism for the rectification of current by self-assembled monolayers (SAMs) of alkanethiolates with Fc head groups (SC(_{11})Fc) in SAM-based tunneling junctions with ultra-flat Ag bottom electrodes and liquid metal (Ga(_2)O(_3)/EGaIn) top electrodes. A systematic physical-organic study based on statistically large numbers of data (N = 300−1000) reached the conclusion that only one energetically accessible molecular orbital (the HOMO of the Fc) is necessary to obtain large rectification ratios (R \approx 1.0 \times 10^{2} (R = |J(−V)|/|J(V)|) at (±1) V()). Values of R are log-normally distributed, with a log-standard deviation of 3.0. The HOMO level has to be positioned spatially asymmetrically inside the junctions (in these experiments, in contact with the Ga(_2)O(3)/EGaIn top electrode, and separated from the Ag electrode by the SC({11}) moiety) and energetically below the Fermi levels of both electrodes to achieve rectification. The HOMO follows the potential of the Fermi level of the Ga(_2)O(_3)/EGaIn electrode; it overlaps energetically with both Fermi levels of the electrodes only in one direction of bias.