Abstract
To quantify charge transport through molecular junctions fabricated using the conducting probe atomic force microscopy (CP-AFM) platform, information on the number of molecules N per junction is absolutely necessary. N can be currently obtained only via contact mechanics, and the Young’s modulus E of the self-assembled monolayer (SAM) utilized in a key quantity for this approach. The experimental determination of E for SAMs of CP-AFM junctions fabricated using oligophenylene dithiols (OPDn, 1≤n≤4) and gold electrodes turned out to be too challenging. Recent measurements (Z. Xie et al, J. Am. Chem. Soc. 139 (2017) 5696) merely succeeded to provide a low bound estimate (E≈58 GPa). Supplying this missing experimental information is the aim of the present theoretical investigation. Our microscopic calculations yield values E≈240±6 GPa for the OPDn SAMs of the aforementioned experimental study, which are larger than those of steel (E≈180−200 GPa) and silicon (E≈130−185 GPa). The fact that the presently computed E is much larger than the aforementioned experimental lower bound explain why experimentally measuring E of OPDn SAM’s is so challenging. Having E≈337±8 GPa, OPDn SAMs with herringbone arrangement adsorbed on fcc (111)Au are even stiffer than Si3N4 (E≈160−290 GPa).
Highlights
conducting probe atomic force microscopy (CP-AFM) junctions consist of bundles containing a number N of molecules trapped between the metal-coated cantilever (AFM tip) and substrate covered by a self-assembled monolayer (SAM)
While the aforementioned nuclear methods enable the direct determination of the surface coverage Σ, models developed in contact mechanics pose certain problems to reliably estimate the contact area A between the AFM tip and the self-assembled monolayer (SAMs) under investigation
We found that OPDn SAMs are much stiffer than the lower bound estimate (E ≈ 58 GPa) deduced in recent experiments [24] may suggest
Summary
Among the various platforms to fabricate molecular junctions [1,2,3,4,5,6,7,8,9,10], conducting probe atomic force microscopy (CP-AFM) [11,12,13,14,15,16,17,18,19,20,21,22,23] is an approach pioneered by Frisbie’s group [11] that offers a series of advantages [24]. While the aforementioned nuclear methods enable the direct determination of the surface coverage Σ, models developed in contact mechanics pose certain problems to reliably estimate the contact area A between the AFM tip and the self-assembled monolayer (SAMs) under investigation. We found that OPDn SAMs are much stiffer than the lower bound estimate (E ≈ 58 GPa) deduced in recent experiments [24] may suggest. They are stiffer than steel and silicon, possibly stiffer than Si3N4
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