Influence of the covalent grafting of organic radicals to graphene on its magnetoresistance

Graphene was obtained by direct exfoliation of graphite in o-dichlorobenzene (oDCB) or benzylamine, and further functionalized with 4,4′-[(1,3-dioxo-1,3-propanediyl)bis(oxy)]bis[2,2,6,6-tetramethyl-1- piperidinyloxy] (1-TEMPO) organic radicals by using the Bingel-Hirsch cyclopropanation reaction. Here, the use of different solvents permits variation of the density of radicals anchored to the carbon layers. Covalent grafting is unambiguously demonstrated by TGA, μ-Raman, XPS and EPR measurements, which also rule out spurious physisorption. Our transport measurements indicate that the conduction mechanism varies as a function of the density of radicals grafted to the carbon layers. Moreover, the presence of paramagnetic moieties influences the magnetoresistive behaviour of the system by the appearance of a low field magnetoresistance (LFMR) effect. Finally, the derivatization of graphene with diamagnetic ethylmalonate molecules, by following an analogous route, permits us to discard the chemical derivatization, which is responsible for the observed differences in the MR response that must be rather ascribed to the grafting of organic spin carriers.