Structural and magnetic resonance study of astralen nanoparticles

Using HRTEM, XRD and Raman spectroscopy we demonstrated a polyhedral multi-shell fullerene-like structure of astralen particles. The polyhedra consist of large defect-free flat graphitic faces connected by defective edge regions with presumably pentagon-like structure. The faces comprise a stacking of 20-50 planar graphene sheets with inter-sheet distance of ~ 0.340 nm. Average sizes of the particles and their flat faces are of ~ 40 nm and ~ 15 nm, respectively. EPR spectra of astralen powder reveal two components: a very broad signal with ΔH_pp > 1 T and an asymmetric narrow one centered close to g = 2.0. The latter consists of two overlapping Lorentzian lines. All spectral components are independent of the ambient pressure. The intensities of all EPR signals show no changes on decreasing temperature from T = 300 K down to 4 K demonstrating the Pauli paramagnetism. Temperature dependent ¹³C NMR measurements yield nuclear spin-lattice relaxation times T_1n ~ T^{- 0.612}. The exponent in the temperature T_1n(T)-dependence for astralen falls between the metallic behavior, T_1n ~ T^{- 1} (Korringa relation), and the semiconductor behavior, T_1n ~ T^{- 0.5}. The unusual magnetic resonance features are attributed to delocalized charge carriers which amount considerably exceeds that of spins localized in defects on astralen edges.