Effect of edge-functionalization on the ease of graphene nanoribbon aggregation in solvent

Edge-terminating oligomers are frequently used to aid in the dispersion of solution-processed, “bottom-up” graphene nanoribbons (GNRs). However, the choice of edge-termination is typically made in a trial-and-error process. In this study, Molecular Dynamics was used to contrast the aggregational tendencies of GNRs with a simple hydrogen edge-termination to those with polyethylene glycol (PEG) and n-alkoxy chains substituted along the GNR edges. Unlike similar previous work on graphene sheets, this study focuses the effects on aggregation of adding chains to the side of the GNR that are commonly used in bottom-up synthesis routes. We used Thermodynamic Integration to determine energetic barriers to face-to-face and sliding pre-determined aggregational paths. In contrast, Steered Molecular Dynamics simulations offered an unbiased window into GNR aggregation, allowing us to assess to what extent the real aggregational mechanism might be a combination of those two paths. We found that the sensitivity of the potential of mean force to different edge-terminations was highly dependent on the aggregational path chosen. Along the lowest energy barrier path (GNRs sliding together), PEG edge-terminations increased the barrier to aggregation, whereas n-alkoxy chains not only decreased the energy barrier significantly, but also increased the range at which GNRs begin to attract each other.

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