Self-assembled monolayers of oligophenylenes stiffer than steel and silicon, possibly even stiffer than Si$_3$N$_4$

TL;DR

This study theoretically calculates the Young's modulus of oligophenylene dithiol self-assembled monolayers, revealing they are significantly stiffer than steel, silicon, and even Si3N4, explaining experimental measurement challenges.

Contribution

The paper provides the first microscopic calculations of the Young's modulus for OPDn SAMs, showing they are much stiffer than previously estimated and surpass common materials.

Findings

Calculated E for OPDn SAMs is approximately 240 GPa.

OPDn SAMs are stiffer than steel and silicon.

OPDn SAMs are even stiffer than Si3N4.

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 the key quantity for this approach. The experimental determination of $E$ for SAMs of CP-AFM junctions fabricated using oligophenylene dithiols (OPDn, $1 \leq n \leq 4$) 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 ($E \approx 58\,$GPa). It is this state of affairs that motivated the present theoretical investigation. Our microscopic calculations yield values $E \approx 240 \pm 6\,$GPa for the OPDn SAMs of the aforementioned experimental study, which are larger than those of steel ($ E \approx 180 - 200\,$GPa) and silicon ($E \approx 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 $E \approx 337 \pm 8\,$GPa, OPDn SAMs with herringbone arrangement adsorbed on fcc (111)Au are even stiffer than Si$_3$N$_4$ ($E \approx 160 - 290\,$GPa).