Are Asymmetric SAM‐Induced Work Function Modifications Relevant for Real Molecular Rectifiers?

Abstract

Due to the fabrication architecture employed—flat s(ubstrate) versus sharp t(ip), chemisorption versus physisorption, etc–, self assembled monolayers (SAMs) break the forward‐backward invariance even in case of homometal molecular junctions; they induce changes in electrodes' work function different from each other (normalΔΦs≠normalΔΦt$ \Delta \Phi _{\text{s}} \ne \Delta \Phi _{\text{t}}$). Because differences normalΔΦs>normalΔΦt$ \Delta \Phi _{\text{s}} > \Delta \Phi _{\text{t}}$ translate into internal (Volta) electric fields often exceeding those created by the highest applied biases that real junctions can withstand, one may expect effects relevant for current rectification (RR). Here, they theoretically analyze asymmetric raw I‐V data of benchmark junctions. Notwithstanding the large values |ΔΦs−normalΔΦt|≈1$ |\Delta \Phi _{\text{s}} - \Delta \Phi _{\text{t}}|\approx 1$ eV, they found that the impact on rectification is completely negligible. So, the present theoretical results conveys a word of caution: not any effect that strongly breaks the inversion symmetry necessarily leads to current rectification. While their analysis indicates that per se unequal SAM‐induced normalΔnormalΦ$\Delta \Phi$'s can hardly affect RR‐values in general, it does not rule out an indirect effect for junctions where normalΔnormalΦ$\Delta \Phi$'s affect the charge transfer between the metal layer at interface and the bulk. From this perspective, using interposed narrow‐band metal adlayers may be an appealing route toward improving rectification via interface engineering.

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Exact Analytic Formula for Conductance Predicting a Tunable Sommerfeld–Arrhenius Thermal Transition within a Single‐Step Tunneling Mechanism in Molecular Junctions Subject to Mechanical Stretching

“…which also give rise to a smeared Lorentzian-shaped transmission of finite half-width Γ a [26,28,29]…”