First-principles investigation of electron-induced cross-linking of aromatic self-assembled monolayers on Au(111)

TL;DR

This study uses density functional theory to explore how electron irradiation induces cross-linking in biphenyl-thiol monolayers on gold, revealing formation of graphene-like nanoflakes that could explain experimental observations.

Contribution

It provides a detailed theoretical analysis of the atomic-scale mechanisms behind electron-induced cross-linking in aromatic SAMs on Au(111), including substrate effects and nanoflake formation.

Findings

Dehydrogenated BPT molecules form covalent bonds spontaneously.

Graphene-like nanoflakes are identified as building blocks.

Cross-linking leads to entangled nanostructures resembling carbon layers.

Abstract

We have performed a density functional theory study of the possible layered geometries occurring after dehydrogenation of a self-assembled monolayer (SAM) of biphenyl-thiol molecules (BPTs) adsorbed on a Au(111), as it has been experimentally observed for low energy electron irradiated SAMs of 4'-nitro-1,1'-biphenyl-thiol adsorbed on a Au(111) surface. [Eck, W. et al., Advanced Materials 2000, 12, 805] Cross-link formation between the BPT molecules has been analyzed using different models with different degrees of complexity. We start by analyzing the bonding between biphenyl (BP) molecules in a lineal dimer and their characteristic vibration frequencies. Next, we consider the most stable cross-linked structures formed in an extended free-standing monolayer of fully dehydrogenated BP molecules. Finally, we analyze a more realistic model where the role of the Au(111) substrate and sulphur head groups is explicitly taken into account. In this more complex model, the dehydrogenated BPT molecules are found to interact covalently to spontaneously form "graphene-like" nanoflakes. We propose that these nanographenes provide plausible building-blocks for the structure of the carbon layers formed by electron irradiation of BPT-SAMs. In particular, it is quite tempting to visualize those structures as the result of the cross-link and entanglement of such graphene nanoflakes.