Abstract
This paper describes the performance of junctions based on self-assembled monolayers (SAMs) as the functional element of a half-wave rectifier (a simple circuit that converts, or rectifies, an alternating current (AC) signal to a direct current (DC) signal). Junctions with SAMs of 11-(ferrocenyl)-1-undecanethiol or 11-(biferrocenyl)-1-undecanethiol on ultraflat, template-stripped Ag (Ag(TS)) bottom electrodes, and contacted by top electrodes of eutectic indium-gallium (EGaIn), rectified AC signals, while similar junctions based on SAMs of 1-undecanethiol-SAMs lacking the ferrocenyl terminal group-did not. SAMs in these AC circuits (operating at 50 Hz) remain stable over a larger window of applied bias than in DC circuits. AC measurements, therefore, can investigate charge transport in SAM-based junctions at magnitudes of bias inaccessible to DC measurements. For junctions with SAMs of alkanethiols, combining the results from AC and DC measurements identifies two regimes of bias with different mechanisms of charge transport: (i) low bias (|V| < 1.3 V), at which direct tunneling dominates, and (ii) high bias (|V| > 1.3 V), at which Fowler-Nordheim (FN) tunneling dominates. For junctions with SAMs terminated by Fc moieties, the transition to FN tunneling occurs at |V| ≈ 2.0 V. Furthermore, at sufficient forward bias (V > 0.5 V), hopping makes a significant contribution to charge transport and occurs in series with direct tunneling (V ≲ 2.0 V) until FN tunneling activates (V ≳ 2.0 V). Thus, for Fc-terminated SAMs at forward bias, three regimes are apparent: (i) direct tunneling (V = 0-0.5 V), (ii) hopping plus direct tunneling (V ≈ 0.5-2.0 V), and (iii) FN tunneling (V ≳ 2.0 V). Since hopping does not occur at reverse bias, only two regimes are present over the measured range of reverse bias. This difference in the mechanisms of charge transport at forward and reverse bias for junctions with Fc moieties resulted in large rectification ratios (R > 100) and enabled half-wave rectification.
Highlights
The field of molecular electronics applies the techniques and principles derived from studying inorganic electronic devices to investigating charge transport in organic molecules
We found that two characteristics of the self-assembled monolayers (SAMs) of SC11Fc inside the junctions cause the large observed rectification ratios (R ≈ 1.0 × 102, measured at ± 1.0 V, direct current (DC) measurements): i) the potential drop across the SAM is non-uniform because the SAM is asymmetric,[3] and ii) the mechanism of charge transport changes from tunneling to hopping in only one direction of bias, and not in the other.[31]
We have described the J(V) characteristics obtained with DC measurements, and detailed discussions of the possible defects in our tunneling junctions and their influence on the J(V) characteristics previously.[1,2,3,31,46]
Summary
The Harvard community has made this article openly available. Please share how this access benefits you. Journal of the American Chemical Society 133, no. Siegel,[2] and George M. Whitesides2,* 1 Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 2 Department of Chemistry and Chemical Biology, Harvard University Cambridge, MA 02138 U.S.A
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