Nuclear spin-lattice relaxation and paramagnetic defects in carbon nanomaterials

We review recent nuclear spin-lattice relaxation studies of carbon nanomaterials, such as nanodiamonds, carbon onions, activated carbon fibers, graphene and carbon nanoscrolls. We show that a significant reduction in the relaxation time in nanoparticles compared with that in the bulk compounds and a stretched exponential magnetization recovery are caused by the interaction of nuclear spins with unpaired electron spins of paramagnetic defects, which creates an effective channel for the nuclear relaxation. We present a theoretical approach for such kind of relaxation in static and magic angle spinning regimes and explain a difference in the data received in these regimes. We also extend our approach for the case of additional contributions to the relaxation resulting from interaction of nuclear spin with conduction electrons and with adsorbed paramagnetic oxygen molecules. The developed approach allows correct interpreting of the NMR relaxation data and receiving useful information on properties of nanomaterials from the NMR measurements.