A novel approach for the synthesis of edge-functionalized graphene nanoribbons

by Virginia Greco

A technique for the synthesis of edge-fluorinated graphene nanoribbons that guarantees the stability of the functional group throughout the whole process has been developed by a team of researchers led by ICREA Prof. Aitor Mugarza, head of the ICN2 Atomic Manipulation and Spectroscopy group, and Prof. Diego Peña, from the University of Santiago de Compostela (Spain).

Due to their quasi one-dimensional nature, graphene nanoribbons (GNRs) ̶ which are narrow strips of graphene ̶ display advantages that make them even more attractive than 2D sheets of the same material for various applications. The structural, chemical and electronic properties of graphene nanoribbons can be effectively tailored, according to the needs, by edge-functionalization: in practice, functional moieties (fractions of molecules) are selectively attached to the edges of the strips. The effectiveness of this chemical process, though, is challenged by the difficulty to preserve the integrity of the functional groups throughout the synthesis pathway. Edge fluorination, which attaches fluorine groups to the nanoribbons, is a particular critical case in this sense, since, despite the carbon-fluorine bond being among the strongest in chemistry, previous attempts of synthesizing fluorinated nanoribbons have failed.

A team of researchers from the Catalan Institute of Nanoscience and Nanotechnology (Barcelona, Spain), the University of Santiago de Compostela (Spain) and the Donostia International Physics Center (Bilbao, Spain) has developed a technique for the on-surface synthesis of edge-fluorinated graphene nanoribbons that guarantees the survival of fluorine functionalization throughout the entire process. The results of this study have been recently published in ACS Nano, in a scientific article signed by Dr. Mirco Panighel and group leader ICREA Prof. Aitor Mugarza, from the ICN2 Atomic Manipulation and Spectroscopy group, and Prof. Diego Peña, from the University of Santiago de Compostela, as corresponding authors.

As explained in the paper, the intermediate structures obtained in the synthetic path can destabilize the fluorinated edges, so that these are partially or totally dissociated by the thermal energy supplied for the reactions. In particular, the hydrogen released by neighbor carbon atoms in the so called cyclodehydrogenation step can migrate to fluorine bonded carbons and induce dissociation by severely weakening the carbon-fluorine bond, as found in previous experiments.

According to the authors of this research, though, this can be avoided by designing a suitable precursor compound for the synthesis. The functional groups, in fact, are introduced with atomic precision in the original molecular building blocks, which are later coupled in a LEGO-like fashion to form the graphene nanoribbons. The key is to attach these functional groups in peripheral positions of the precursor structure, separating the fluorinated carbon atoms from those involved in the thermally activated reactions. In order to demonstrate the efficacy of this protocol, the researchers monitored the formation of the graphene nanoribbons and the stability of the functional groups during the synthesis process by advanced measurement techniques; namely, scanning tunneling microscopy and X-ray photoelectron spectroscopy.

The proposed approach is indeed independent of the functional group; therefore, different types of nanoribbons can be produced by designing a specific precursor. This represents a breakthrough in the synthesis of edge-functionalized graphene nanoribbons.

Reference article:

Mirco Panighel, Sabela Quiroga, Pedro Brandimarte, Cesar Moreno, Aran Garcia-Lekue, Manuel Vilas-Varela, Dulce Rey, Guillaume Sauthier, Gustavo Ceballos, Diego Peña, and Aitor Mugarza, Stabilizing Edge Fluorination in Graphene Nanoribbons. ACS Nano 2020 14 (9), 11120-11129.DOI: 10.1021/acsnano.0c01837