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 (SC11Fc) in SAM-based tunneling junctions with ultra-flat Ag bottom electrodes and liquid metal (Ga2O3/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 ≈ 1.0 × 102 (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 Ga2O3/EGaIn top electrode, and separated from the Ag electrode by the SC11 moiety) and energetically below the Fermi levels of both electrodes to achieve rectification. The HOMO follows the potential of the Fermi level of the Ga2O3/EGaIn electrode; it overlaps energetically with both Fermi levels of the electrodes only in one direction of bias.

Model of self assembled monolayer based molecular diodes made of ferrocenyl-alkanethiols

“…Figure 1(b) shows the energy band diagram of the diode. In accordance with the experimental work by Nijhuis, Reus and Whitesides, 10 we consider metallic Ag and Ga 2 O 3 -EGaIn electrodes with work functions of W Ag ¼ 4.67 eV and W Ga2O3-EGaIn ¼ 4.23 eV, respectively. The LUMO level of the alkyl chains is set to À0.67 eV.…”

“…The image potential in the potential barriers is also included in our model. We compare the computed current versus bias voltage I(Va) of the diodes under darkness with experimental measurements obtained by Nijhuis, Reus, and Whitesides, 10 and we give an insight into the physical phenomena occurring in ferrocenyl-alkanethiols based molecular diodes. Especially, we investigate through simulation how bound and quasi-bound states of the electronic levels of the molecules can participate in the charge transport.…”

“…It is generally assumed that the terminated Fc moieties of the SAM form a Van der Walls contact with the Ga 2 O 3 -EGaIn electrode. 10,11 Figure 1(b) gives the theoretical band diagram (with respect to vacuum) of the resulting Ag-SC 11 Fc-Ga 2 O 3 -EgGaIn molecular junction before the alignment of the Fermi levels. Both electrodes are defined by their work functions, allowing us to determine the Fermi level of the electrons in the metals.…”

Rectification in Tunneling Junctions: 2,2′-Bipyridyl-Terminated n-Alkanethiolates

“…Equation 3 (which we have shown to be compatible with rectification ratios reported in the literature and summarized in Figure 5) in conjunction with the simplified Simmons model is compatible with the hypothesis that most of rectifiers previously reported use a mechanism similar-at least in part-to that proposed for the SC 11 Fc system. 21 In this approximation, for SAMs terminated in redox-active groups, the magnitude of rectification is related to the size or shape (by some metric, or combination of metrics) of the terminal group and to the accessibility of a one-electron redox process allowing hopping to or from the Ga 2 O 3 /EGaIn electrode. (The approximations in this estimatethe geometry of the T group, the value of β characterizing it, and the details of the shape of the tunneling barrierwere sufficient that this agreement indicates only mechanistic compatibility, and not mechanistic proof.…”

“…These junctions give rectification ratios of |J(−1.0 V)|/|J(+1.0 V)| = ∼150 for Fc 20 and ∼500 for Fc 2 . 21 The higher current is at negative polarity (e.g., the electrode close to the Fc group is oxidizing). The mechanism of this rectification is welldefined (detailed energy diagrams for rectification are discussed elsewhere 20,21 ): it involves a change in mechanism, from tunneling across the entire molecule at one bias, to hopping (from the Fc to the electrode due to energetic proximity of HOMO of Fc (−5.0 eV) to the Fermi level of the Ga 2 O 3 / EGaIn electrode (−4.3 eV) at zero bias) 29 followed by tunneling at the opposite polarity.…”