A $^{13}$C NMR Spectroscopic Investigation of Carbon Nanohorns

$^{13}$C NMR spectroscopic investigation of carbon nanohorn aggregates (CNH) shows that they consist of two components, characterized by different chemical shifts and spin-lattice relaxation (T$_{1})$ behavior. The first component with a chemical shift of 124 ppm and faster T$_{1}$ is assigned to the nanotube-like horns on the particles' surface. The second component with a chemical shift of 116 ppm and much slower T$_{1}$ is assigned to the graphite-like part of the CNH. Integrated peak area measurements indicate a 1:2 ratio of nanohorns to the graphite-like substrate. The lack of a Knight shift and the absence of a clear Korringa relaxation for either component of T$_{1}$ ruled out any metallic behavior and indicate a relaxation behavior characteristic of semiconducting materials with paramagnetic centers arising from structural defects. We also observed an anomalous change in T$_{1}$ in the nanohorn domains near 17 K, suggesting the development of an antiferromagnetic correlation between localized electron spins.