Covalent Functionalization by Cycloaddition Reactions of Pristine Defect-Free Graphene

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Covalent Functionalization by Cycloaddition Reactions of Pristine Defect-Free Graphene, ACS Nano, Decembre 2016

 

Graphene is presently considered for a large number of potential applications because of its fascinating properties. However, functionalizing this gapless highly non-reactive semiconductor with the appropriate molecules remains an important challenge. Combined with supramolecular chemistry, chemical functionalization, i.e., creating covalent bonds by converting sp2 into sp3 orbitals, represents a promising path, because it allows to selectively modify the surface of graphene with a high spatial control. Such a surface modification (hybridization) can be realized by Diels-Alder or other cycloaddition reactions. Graphene undergoes cycloaddition or Diels Alder (D-A) reactions mainly because of the degeneracy of the electronic states at the Dirac point. The states close to the Fermi level may give rise to either an anti-symmetric or symmetric graphene orbital. These orbitals allow graphene to function as both donor and acceptor within the Frontier Molecular Or-bital (FMO) theory.1–4 For exfoliated graphene, a D-A reaction and its reversibility have been first demonstrated by the team of Robert C. Haddon.1 The progress of the D-A reaction was followed by Raman spectroscopy and the ratio between the G and D bands which ascertained that graphene was modified. However, such global measurements did not allow to identify the locations where graphene was actually functionalized.

 

 

The results of our study provide a strong evidence that cycloaddtion reaction between graphene and fluorinated maleimide molecules is possible at room temperature. We observed that this reaction happened on a region of pristine graphene layer, which did not contain any pre-existing defect. The formation of covalent bonds was ascertained by the increase of the sp3 component of the XPS spectrum. Furthermore, using STM we have visualized marked standing-wave patterns on graphene, which were not observed for physisorbed molecules. These patterns, in particular their geometry and symmetry, indicated that the cycloaddition reaction occurred in the (1,2) or (1,4) configuration, as confirmed by the T-matrix approximation. DFT calculations clearly show that (1,2) configuration is stable on G/SiC(0001). We have interpreted the standing-wave patterns observed in direct space in analogy to standing-wave patterns observed in the vicinity of armchair and zig-zag step edges on graphene. ARPES measurements revealed a tendency for the opening of a gap and a slight decrease of the Fermi velocity. For bilayer graphene, the gap opening was more pronounced. Furthermore, we were able to distinguish which sub-lattice of graphene was perturbed by covalently grafting fluorinated maleimide molecules to graphene. In the future, in order to generate periodic molecular patterns chemically attached to graphene, we will incorporate the reactive group used here in larger molecules which, in addition,self assembly. We expect that, by creating covalent bonds, we will be able to form ribbons with a precise control of the nature (armchair or zigzag) of their edges.

 

 

Résumé: 

ACS Nano, Decembre 2016

 

 

Based on a low temperature scanning tunneling microscopy study, we present a direct visualization of a cycloaddition reaction performed for some specific fluorinated maleimide molecules deposited on graphene. Up to now it was widely admitted that such cycloaddition reaction can not happen without pre-existing defects. However, our study shows that the cycloaddition reaction can be carried out on a defect-free basal graphene plane at room temperature. In the course of covalently grafting the molecules to graphene, the sp2 conjugation of carbon atoms was broken and local sp3 bonds were created. The grafted molecules perturbed the graphene lattice, generating a standing-wave pattern with an anisotropy which was attributed to a (1,2) cycloaddition, as revealed by T-matrix approximation calculations. DFT calculations showed that while both (1,4) and (1,2) cycloaddition were possible on free standing graphene, only the (1,2) cycloaddition could be obtained for graphene on SiC(0001). Globally averaging spectroscopic techniques, XPS and ARPES, were used to determine the modification in the elemental composition of the samples induced by the reaction, indicating an opening of an electronic gap in graphene.

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Physico-chimie Théorique
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Références: 

Covalent Functionalization by Cycloaddition Reactions of Pristine Defect-Free Graphene

 

Lakshya Daukiya, Cristina Mattioli, Dominique Aubel, Samar Hajjar-Garreau, Francois Vonau, Emmanuel Denys, Guenter Reiter, Jonas Fransson, Elsa Perrin, Marie-Laure Bocquet, Cristina Bena, André Gourdon, and Laurent Simon

 

ACS Nano, Decembre 2016

 

doi : 10.1021/acsnano.6b06913