Resolution of EPR Signals in Graphene-based Materials from Few Layers to Nanographites

The focus of this chapter is set on the application of EPR methods to carbon-based materials, from nanographites to graphene-based materials, for the resolution and characterization of the different signals, related to the presence of specific species, or structures. Because of the intrinsic heterogeneity of the samples, this goal is not simple: most of the signals coming from different types of structures have similar spectroscopic features and are overlapping in the cw-EPR spectra with very different relative intensities. It is then necessary to use all possibilities that EPR offers, from the cw-EPR techniques to pulse EPR methods, to disentangle ideally all contributions. Our analysis of the EPR spectra considers the presence of three types of paramagnetic contributions: conduction electrons, edge states and molecular states. This interpretation framework has been shown to be effective for the considered materials, characterized by the presence of finite-dimension graphene layers, eventually stacked one above the other. In our analysis, we investigated different experimental parameters, like the variation in the temperature of the EPR intensity, the values of the g-tensors and the homogeneous lineshapes of the spectra to obtain further structural information. Pulse EPR methods were used to study and characterize species with long relaxation times (molecular states). Echo-detected EPR enabled to obtain their spectral lineshapes. Hyperfine spectroscopies, ESEEM, ENDOR and HYSCORE, determined the electron hyperfine couplings of unpaired electrons with magnetic nuclei, thus allowing the evaluation of the extent of the π-system and the presence of different types of nuclei.

Keywords: Conduction electrons, Dysonian lineshape, Edge states, Echodetected EPR (ED-EPR), ENDOR, EPR, ESEEM, Expanded graphite, FT EPR, Graphene defects, Graphite, Hyperfine interaction, HYSCORE, Lorentzian lineshape, Molecular states, Nanographite, Natural graphite, π-systems, Reduced graphene oxide (RGO), Synthetic graphite.