Magnetic coupling in cm-scale buckypapers of self-organized Fe/Fe₃C-filled carbon nano-onions: A controlled chemical vapour deposition approach

We report the selective nucleation and growth of cm-scale thin buckypaper-films of carbon nano-onions (CNOs) filled with nanocrystalline Fe₃C and α-Fe ferromagnetic phases. The presented result was achieved by sublimation and pyrolysis of 20 to 100 mg of ferrocene in an Ar flow of 10 ml/min under conditions of low-carbon-supply pyrolysis. The high reproducibility of the presented method opens-up new directions for industrial applications of these nanomaterials. We highlight the observation of an unusual D-band splitting, indicative of a defect-induced structural transition within the graphitic shells of the CNOs. The morphological and structural properties of the buckypapers were investigated by employing scanning and transmission electron microscopy (SEM, TEM, HRTEM) together with X-ray diffraction (XRD). The magnetic properties were investigated by employing electron paramagnetic resonance (T-EPR) at T ~ 300 K, 200 K, 150 K, 100 K and 77 K in the field range from 0 to 500 mT. We highlight the presence of broad π-electron signals, with significant shift of the g-factor parameter as the temperature was decreased from T ~ 300 K to ~77 K. A possible coupling between the iron-based nanocrystal phases was found at T ~ 300 K, at g-factor ~ 20.