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

Abstract

We have performed a density functional theory study of the possible layered geometries occurring after dehydrogenation of a self-assembled monolayer (SAM) of biphenylthiol 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.

Classical molecular dynamics investigations of biphenyl-based carbon nanomembranes

“…Although these degrees of freedom constitute an enormous parameter space, we are able to summarize our numerical experience like follows: For local displacements of less than about ±1 Å always the graphene stripe configuration forms. This is in part also observed in DFT calculations [7]. Only for sufficient randomization of the initial state, which corresponds to a substantial excitation and to the breakup of sufficiently many phenyls, a true crosslinking is observed.…”

“…This is in part also observed in DFT calculations [8]. Only for a sufficient randomization of the initial state, which corresponds to a substantial excitation and to the breakup of sufficiently many phenyls, a true cross-linking is observed.…”

“…3. These states are not hypothetical or mere products of our classical molecular dynamics calculations: In experiment graphene forms indeed by heating the CNM [4,23], and the stripe order is also found in other calculations [8]. Therefore, we conjecture that the realistic CNM state is described by a local energy minimum, as in other disordered systems like for instance metal glasses [24–26], and this is an important conceptual ingredient of the modeling.…”

“…On the other hand, the quantum mechanical simulation of extended systems even by means of density functional theory (DFT) has to assume a regular lattice and can treat only small unit cells [6–8]. Consequently, the resulting structure is also regular [8]. If one, as in the present case, can expect that the structure is irregular, i.e., a lattice structure as in solids cannot be assumed, a quantum mechanical simulation is virtually impossible.…”

“…Nevertheless, the simulations of the very small system do not allow definite conclusions about the structure of the extended sheet. On the other hand, the quantum mechanical simulation of extended systems even by means of Density Functional Theory (DFT) has to assume a regular lattice and can treat only small unit cells [5][6][7]. Consequently, the resulting structure is also regular [7].…”