Covalent functionalization by cycloaddition reactions of pristine, defect-free graphene

TL;DR

This study visualizes covalent cycloaddition reactions on pristine, defect-free graphene using microscopy and theoretical calculations, revealing new pathways for functionalization without pre-existing defects.

Contribution

It demonstrates that cycloaddition reactions can occur on defect-free graphene's basal plane, breaking sp2 bonds and creating local sp3 bonds, with specific reaction pathways identified.

Findings

Cycloaddition occurs on defect-free graphene's basal plane.

Reaction induces local sp3 bonding and lattice perturbation.

Electronic gap opening observed after functionalization.

Abstract

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. These studies showed that the cycloaddition reactions can be carried out on the basal plane of graphene, even when there are no pre-existing defects. 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.