Iodine versus Bromine Functionalization for Bottom-Up Graphene Nanoribbon Growth: Role of Diffusion

TL;DR

This study compares iodine and bromine functionalization in bottom-up graphene nanoribbon growth, revealing that both precursors polymerize at similar temperatures due to diffusion-limited processes, despite differences in bond strength.

Contribution

It demonstrates that C-I bonds enable lower-temperature GNR synthesis and shows that polymerization is diffusion-limited for both halogenated precursors.

Findings

Polymerization occurs at similar temperatures for both C-Br and C-I precursors.

Diffusion limits the polymerization process on Au(111).

C-I bonds allow for lower-temperature GNR synthesis.

Abstract

Deterministic bottom-up approaches for synthesizing atomically well-defined graphene nanoribbons (GNRs) largely rely on the surface-catalyzed activation of selected labile bonds in a molecular precursor followed by step growth polymerization and cyclodehydrogenation. While the majority of successful GNR precursors rely on the homolytic cleavage of thermally labile C-Br bonds, the introduction of weaker C-I bonds provides access to monomers that can be polymerized at significantly lower temperatures, thus helping to increase the flexibility of the GNR synthesis process. Scanning tunneling microscopy (STM) imaging of molecular precursors, activated intermediates, and polymers resulting from stepwise thermal annealing of both Br and I substituted precursors for chevron GNRs reveals that the polymerization of both precursors proceeds at similar temperatures on Au(111). This observation is consistent with diffusion-limited polymerization of the surface-stabilized radical intermediates that emerge from homolytic cleavage of either the C-Br or the C-I bonds.