Are Asymmetric SAM‐Induced Work Function Modifications Relevant for Real Molecular Rectifiers?. Issue 7 (5th April 2022) - Bâldea, Ioan
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 ( Δ Φ s ≠ Δ Φ t $ \Delta \Phi _{\text{s}} \ne \Delta \Phi _{\text{t}}$ ). Because differences Δ Φ s > Δ Φ t $ \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 | Δ Φ s − Δ Φ t | ≈ 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 Δ Φ $\Delta \Phi$ 's can hardly affect RR‐values in general, it does not rule out an indirect effect for junctions where Δ Φ $\Delta \Phi$ '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. Abstract : Highly asymmetric changes in electrodes' work function due to self assembled monolayers translating into internal (Volta) electric fields often exceed those created by the highest biases that real molecular junctions withstand but the impact on current rectification is negligible. Thus, the present results emphasize that an effect that strongly breaks the inversion invariance does not necessarily lead to significant rectification.