Molecules–Oligomers–Nanowires–Graphene Nanoribbons: A Bottom-Up Stepwise On-Surface Covalent Synthesis Preserving Long-Range Order

Abstract

We report on a stepwise on-surface polymerization reaction leading to oriented graphene nanoribbons on Au(111) as the final product. Starting from the precursor 4,4″-dibromo-p-terphenyl and using the Ullmann coupling reaction followed by dehydrogenation and C-C coupling, we have developed a fine-tuned, annealing-triggered on-surface polymerization that allows us to obtain an oriented nanomesh of graphene nanoribbons via two well-defined intermediate products, namely, p-phenylene oligomers with reduced length dispersion and ordered submicrometric molecular wires of poly(p-phenylene). A fine balance involving gold catalytic activity in the Ullmann coupling, appropriate on-surface molecular mobility, and favorable topochemical conditions provided by the used precursor leads to a high degree of long-range order that characterizes each step of the synthesis and is rarely observed for surface organic frameworks obtained via Ullmann coupling.

On‐Surface Synthesis of Atomically Precise Graphene Nanoribbons

“…Modeling this process for the sizeable precursor Catalogue of molecular precursors for the on-surface synthesis of atomically precise graphene nanoribbons. a) Armchair GNRs, including widths N = 3, [ 13,44 ] 5, [ 17 ] 7, [ 15 ] 7 (B-doped), [ 25,26 ] 9, [ 19 ] and 13. [ 18 ] b) Chevron GNRs, including substitutional doping with zero, [ 15 ] one, [ 23 ] two, [ 23,83 ] and four [ 24 ] nitrogen atoms per precursor molecule.…”

“…[ 43 ] Only annealing at signifi cantly higher temperatures eventually leads to fusion of GNRs via cross-dehydrogenative coupling and thus provides access to GNRs with multiples of the original width, e.g., N = 7 → N = 14, 21. [44][45][46] This strategy has been employed to obtain 6-AGNRs from lateral fusion of 3-AGNRs (poly( para -phenylene)) on Cu(111). [ 44 ] Since the synthesis of 3-AGNRs does not involve cyclodehydrogenation, the improved separation between polymerization and cross-dehydrogenative coupling may contribute to the comparatively large lengths of the GNRs observed.…”

“…[44][45][46] This strategy has been employed to obtain 6-AGNRs from lateral fusion of 3-AGNRs (poly( para -phenylene)) on Cu(111). [ 44 ] Since the synthesis of 3-AGNRs does not involve cyclodehydrogenation, the improved separation between polymerization and cross-dehydrogenative coupling may contribute to the comparatively large lengths of the GNRs observed. Yet, precautions need to be taken in order to direct the random fusing of GNRs.…”

“…Yet, precautions need to be taken in order to direct the random fusing of GNRs. While parallel pre-alignment of GNRs favors lateral fusing, [ 44 ] it does not provide control over the width of the product. Further strategies, such as substrates with terraces of specifi c widths, will likely be required in order to produce monodisperse GNRs in this manner.…”

Low-temperature synthesis of few-layer graphene

“…Kim et al produced graphene nanoribbons of various widths using 4,5-dibromo-1,2-benzenediol by the cyclodehydrogenation reaction [3]. Basagni et al also performed a stepwise on-surface polymerization reaction to product graphene nanoribbons using 4,4″-dibromo-p-terphenyl as the precursor via Ullmann coupling [7]. All these methods producing graphene significantly contributed to the development of the carbon material, inspiring many researchers to explore further efforts to study the production methods of the graphene.…”