Closed pi-electron Network in Large Polyhedral Multi-shell Carbon Nanoparticles

Abstract

High Resolution Transmission Electron Microscopy (HRTEM), X-ray Diffraction (XRD) and Raman spectroscopy reveal a polyhedral multi-shell fullerene-like structure of astralen carbon nanoparticles. The polyhedra consist of large 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 ~ 40 nm and ~ 15 nm, respectively. The astralen particles are suggested to have defect-free sp^2 flat faces and all defects condense at their polyhedral edges. Electron Paramagnetic Resonance (EPR) spectra of polycrystalline astralen samples reveal two components: a very broad signal with DHpp > 1 T and an asymmetric narrow one centered close to g = 2.00. The latter consists of two overlapping Lorentzian lines. All spectral components are independent of 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. Electron spin-lattice relaxation times T1e remain very short within the same temperature range. Temperature dependent 13C Nuclear Magneti? Resonance (NMR) measurements yield nuclear spin-lattice relaxation times T1n ~ T^-0.612. The exponent in the T1n(T)-dependence for astralen falls between the metallic behavior, T1n ~ T^-1 (Korringa relation), and the semiconductor behavior, T1n ~T^-0.5. All unusual magnetic resonance features of astralen are attributed to delocalized charge carriers which amount considerably exceeds that of spins localized in defects on multi-shell polyhedra edges.