Refilling strategy of crosslinked aromatic SAMs for enhancing the molecular packing density

The stability and uniform coverage of molecular self-assembled monolayers (SAMs) on metal substrates is of crucial importance for device integration. One strategy to enhance the stability of the SAMs is based on electron irradiation-induced intermolecular crosslinking. During this process, the coverage area of the SAMs decreases due to the formation of carbon–carbon covalent bonds between the neighboring molecules, which leaves empty spaces at the interfaces. Therefore, to increase molecular coverage, the empty spaces can be refilled either by the same or different types of molecules. In this article, we demonstrate a multistep method of creating densely packed SAMs of biphenyl-4-4-thiol molecules after electron radiation. 4′-Mercapto-[1,1′-biphenyl]-4-carbonitrile molecules are used for refilling because of the distinct signature of nitrogen atoms as a marker in the X-ray photoelectron spectroscopy (XPS) spectra. We used these molecules to estimate the reduction of the area of SAM coverage after electron radiation. As a result, 28.9% ± 5.9% of the total surface area was released. The experimental results are supplemented by density functional theory calculations to estimate the area of the empty regions caused by intermolecular crosslinking. The theoretical predictions align closely with the experimental results. These findings can be of practical importance in creating stable molecular SAM devices using the proposed alternating irradiation and refilling procedure.